Master Thesis Kaustav Ghose TU Darmstadt

Elektrotechnik und Informationstechnik

Fachgebiet Optische Nachrichtentechnik Prof. Dr.-Ing. Peter Meißner

Merckstraße 25

64283 Darmstadt

Institut für Angewandte Physik

Fachgebiet Atoms Photons Quanta Prof. Dr.rer.nat. Gerhard Birkl

Schlossgartenstraße 7

64289 Darmstadt

Technische Universität Darmstadt

Master Thesis

Fabrication and Characterization of Micro-Electro-

Mechanical System (MEMS) Mirror Arrays

Author: Kaustav Ghose (Mat. Nr. 1254460)

Supervisors: Prof. Dr.-Ing. Peter Meißner

Prof. Dr.rer.nat. Gerhard Birkl

Dipl.-Ing. Sandro Jatta

Date of Submission: October 25, 2007

ii

Acknowledgements

I would like to express my deep thanks to Prof. Dr.-Ing. Peter Meißner for giving me the

opportunity to work towards this thesis at his institute. His support towards the beginning of the

thesis was critical towards establishing the groundwork for the project to be started. My thanks

naturally go to Prof. Dr.rer.nat. Gerhard Birkl, for all his guidance during the project, right from

its conceptualization. His help was invaluable in sorting out various practical and theoretical

issues with the design and deployment of the mirror arrays.

My sincerest thanks also go to my supervisor, Dipl.-Ing. Sandro Jatta, who has always given me

the assistance I needed, despite the several interruptions in his work to support my thesis. Apart

from helping to resolve various issues with the fabrication of the membranes, he also tracked my

project progress, while going out of his way to organize the equipment I needed. Any problems

with the fabrication facilities at the institute were resolved, thanks to Sandro.

My thanks also to Dipl.Phys. Andre Lengwenus from the group of Prof. Dr.rer.nat. Birkl at the

Institut fuer Angewandte Physik for all his help in providing me with the equipment and

assistance I needed to build the characterization setup for the mirror arrays.

.

iii

Abstract

To perform large-scale quantum computation based on neutral atoms, methods of trapping and

preparing large ensembles of atoms are necessary along with a method for entanglement of

individual sets of trapped atoms. In this work, arrays of quasi-spherical micro-mirrors for this

purpose are evaluated. The material for the mirrors is deposited using Plasma Enhanced

Chemical Vapor Deposition. The mirror arrays are designed and the masks for fabrication are

created. The arrays are then fabricated using a three mask process and packaged. The focal plane

of the mirrors is optically reimaged to form a red-detuned dipole trap for laser cooled Rubidium

atoms at each focus. Each mirror can be individually thermally actuated, resulting in a shift of the

point of focus. This is due to a change of the radius of curvature, when the supporting beams of

the mirror are symmetrically heated. We examine the sharpness of the focal points and their

stability after actuation of the mirror. The response of individual mirrors to actuation through

thermal heating is also characterized.

iv

Contents

1 Introduction ................................................................................................................................... 1

1.1 Motivation .............................................................................................................................. 2

1.2 Objective ................................................................................................................................ 3

2 Theoretical Basis for the Dipole Trap ......................................................................................... 5

2.1 Introduction ............................................................................................................................ 5

2.2 Doppler Cooling..................................................................................................................... 5

2.3 Magneto-Optical Trap (MOT) .............................................................................................. 7

2.4 The Dipole Trap ..................................................................................................................... 8

2.5 Mirror Requirements ............................................................................................................. 9

3 Plasma Enhanced Chemical Vapor Deposition ........................................................................ 13

3.1 Introduction .......................................................................................................................... 13

3.2 Deposition Sequence ........................................................................................................... 15

3.3 Deposition Characterization ................................................................................................ 16

3.4 Variation of Pressure to produce different intrinsic stress ................................................ 17

3.5 Change in Deposition Characteristics................................................................................. 21

3.6 Obtaining Improved SiO layers .......................................................................................... 22

4 Processing of the Micro-Mirror Arrays ..................................................................................... 27

4.1 Mesa Structuring .................................................................................................................. 27

4.1.1 Lapping (Chemical Mechanical Polishing) ................................................................ 27

4.1.2 Gold (Au)/Chromium (Cr) Evaporation ..................................................................... 28

4.1.3 Photoresist (PR) hard bake .......................................................................................... 29

4.1.4 Nickel (Ni) Sputtering.................................................................................................. 29

4.1.5 Lithography and Nickel Etching ................................................................................. 29

4.1.6 Photoresist Removal by O2 Reactive Ion Etching (RIE) ........................................... 30

4.1.7 Au/Cr wet etching ........................................................................................................ 30

4.1.8 SF6 plasma etching ....................................................................................................... 32

4.1.9 Nickel and Photoresist cleaning .................................................................................. 32

4.1.10 Au/Cr etching to define contact pads .......................................................................... 33

v

4.2 Bulk Micro-Machining ........................................................................................................ 34

4.3 Packaging ............................................................................................................................. 36

5 Characterization of Micro-Mirror Arrays ................................................................................. 38

5.1 Measurement of Curvature using a Confocal system ........................................................ 38

5.2 DC measurements ................................................................................................................ 46

5.3 Imaging the focal plane ....................................................................................................... 50

6 Conclusion and Future Scope .................................................................................................... 56

6.1 Conclusion ............................................................................................................................ 56

6.2 Future Work ......................................................................................................................... 57

vi

List of Figures Figure 1-1 Array of Traps formed by a Microlens Array [4] ............................................................. 2

Figure 1-2 Quasi-Spherical Suspended Membrane for a Tunable Laser [5] ..................................... 3

Figure 1-3 Concept of Multiple Quasi Spherical Mirrors for Dipole Traps...................................... 3

Figure 2-1 Two cycles in the Doppler Cooling of an Atom ............................................................... 6

Figure 2-2 Simple Schematic of a Magneto-Optical Trap (MOT) .................................................... 7

Figure 2-3 Simple Dipole trap at the focus of a laser beam ............................................................... 9

Figure 2-4 Simulation to determine spot size formed by mirror ...................................................... 10

Figure 2-5 Region of Trapping for a focused beam .......................................................................... 10

Figure 2-6 System Setup for the Micro-mirror array ........................................................................ 11

Figure 3-1 Schematic of the process chamber of the Plasmalab 100 system [7] ............................ 13

Figure 3-2 Characterizing Wafers with Deposition .......................................................................... 16

Figure 3-3 Stress for SiO deposition with variation in Pressure ...................................................... 18

Figure 3-4 Stress for SiN deposition with variation in Pressure ...................................................... 19

Figure 3-5 SiO Deposition rate with Variation in Pressure .............................................................. 19

Figure 3-6 SiN Deposition rate with variation in Pressure............................................................... 20

Figure 3-7 Stress for SiN/SiO deposited under different pressures at different times ................... 21

Figure 3-8 Deposition Rate for SiN/SiO deposited under different pressures at different times .. 22

Figure 3-9 SiO Deposition Uniformity and Deposition Rate with RF Power variation ................. 23

Figure 3-10 Etching Rate for SiO with variation of RF power ........................................................ 24

Figure 3-11 Comparison of SiO Etching Rates ................................................................................. 24

Figure 3-12 Stress Characterization with Pressure variation with RF Power 75W ........................ 25

Figure 4-1 Wafer Preparation for Lapping ........................................................................................ 28

Figure 4-2 Ni, PR and Gold layers above Dielectric ........................................................................ 30

Figure 4-3 After Nickel Etching ........................................................................................................ 30

Figure 4-4 After Photoresist Removal ............................................................................................... 30

Figure 4-5 After Gold/Chromium Etching ........................................................................................ 30

Figure 4-6 Chip after O2 Reactive Ion Etching ................................................................................. 31

Figure 4-7 Chip after Gold/Chromium Etching ................................................................................ 31

Figure 4-8 After SF6 Plasma Etching ................................................................................................ 32

vii

Figure 4-9 After Gold/Chromium Etching ........................................................................................ 33

Figure 4-10 Backside Opening after Spray Etching ......................................................................... 34

Figure 4-11 Freed Mirror array after Spray Etching ......................................................................... 35

Figure 4-12 Wire Bonding to the Gold Pads ..................................................................................... 36

Figure 4-13 Bonding between Chip, Glass carrier and Printed Circuit Board ................................ 37

Figure 4-14 Packaged chip on Holding Plate .................................................................................... 37

Figure 5-1 3-D Image showing Curvature of the Mirror .................................................................. 39

Figure 5-2 Profile View across two mirrors ...................................................................................... 39

Figure 5-3 Measurement axes for the radius of curvature ................................................................ 40

Figure 5-4 Front side of SiO/SiN array I ........................................................................................... 41

Figure 5-5 Plot of ROC measurements for SiO/SiN array I (Table 6) ............................................ 41

Figure 5-6 Front side of SiO/SiN array II.......................................................................................... 42

Figure 5-7 Plot of ROC measurements for SiO/SiN array I (Table 7) ............................................ 42

Figure 5-8 Front side SiN only array I ............................................................................................... 43

Figure 5-9 Plot of ROC measurements for SiN only array I (Table 8) ........................................... 43

Figure 5-10 Front Side SiN only Array II ......................................................................................... 44

Figure 5-11 Plot of ROC measurements for SiN only array II (Table 9) ........................................ 44

Figure 5-12 Linear I-V plot for a SiN only mirror ............................................................................ 46

Figure 5-13 Change in curvature with current for SiO/SiN membrane.......................................... 47

Figure 5-14 Profile Images of SiO/SiN membrane showing slight upward beam bending ........... 47

Figure 5-15 Change in curvature with current for SiN only membrane .......................................... 48

Figure 5-16 Profile Images of SiN only membrane showing slight upward beam bending .......... 48

Figure 5-17 Air Gap for a SiO/SiN membrane ................................................................................. 49

Figure 5-18 Schematic of the Characterization Setup ...................................................................... 50

Figure 5-19 Optical Bench Setup for Imaging the focal plane ........................................................ 51

Figure 5-20 Simulation to validate Characterization Setup ............................................................. 51

Figure 5-21 Focal Spots for the 2×2 Mirror Array SiO/SiN I (Table 6) ......................................... 52

Figure 5-22 Actuation of a Membrane .............................................................................................. 53

Figure 5-23 Astigmatism in Focused Beams .................................................................................... 54

Figure 5-21 Focal Spots for the 2×2 Mirror Array SiN only I (Table 8) ........................................ 55

Figure 6-1 Method to achieve less astigmatism ................................................................................ 58

Figure 6-2 Heating elements for beams ............................................................................................. 58

viii

List of Tables

Table 1 Process parameters for SiN/SiO growth with Pressure Variation……………………….18

Table 2 Composition of SiO/SiN membrane……………………………………………………..20

Table 3 Deposition parameters for SiO with RF variation……………………………………….23

Table 4 Composition of SiO with RF/SiN membrane……………………………………………26

Table 5 Composition of SiN only membrane…………………………………………………….26

Table 6 Curvature measurements for SiO/SiN array I……………………………………………41

Table 7 Curvature measurements for SiO/SiN array II…………………………………………..42

Table 8 Curvature measurements for SiN only array I………………………………………….. 43

Table 9 Curvature measurements for SiN only array II………………………………………….44

1

Chapter 1

1 Introduction

Introduction

Quantum Computing is a rapidly evolving area of research, with new and varied attempts to

construct and manipulate qubits, which are the building blocks of a quantum computer. A qubit is

an unit of quantum information, the state of which can be described by a two dimensional unit

vector whose unit vectors represent the states of a two level quantum mechanical system [1].

Therefore, to represent a qubit, neutral atoms have been proposed as a candidate for quantum

computation, with individual trapped atoms forming qubits. To be able to do so, they must be

trapped and manipulated. The two level system can be formed using the ground state and the next

excitation level of the outermost electron, or between two hyperfine levels of the outermost

occupied electron orbital. One element suited for use as neutral atom qubits is Rubidium (Rb),

since it is an alkali element, with only one electron in its outermost orbital. This outermost orbital

has two hyperfine levels in its ground state.

Cooling and trapping of neutral atoms has become readily achievable using a variety of

techniques [2]. With the increasing ability to trap and manipulate such atoms, attempts to use

trapped atoms as qubits for quantum information processing have become widespread. To

successfully achieve this, there should be multiple qubits available, along with mechanisms that

enable selective preparation of single qubits and entanglement of pairs of qubits. There are

various methods of trapping cooled atoms; a dipole trap formed at the focus of a laser beam is

among them [2].

To have arrays of qubits at small distances, micro-optical structures which form traps are a

suitable candidate [3]. Micro optical components are one method to realize such arrays. Trapping

of atoms has been achieved using microlens arrays [4]. A collimated laser beam, which has a

very large Rayleigh length, is passed through the array and each lens forms a focal point. A

dipole trap is formed at each focal point. By focusing a second laser through the lens array a

2

second set of traps can be formed. By changing the angle of the two laser beams the two sets of

traps can be moved relative to each other [4] for the purposes of entanglement. This does not

allow individual trap sites to be moved relative to each other.

Figure 1-1 Array of Traps formed by a Microlens Array [4]

1.1 Motivation

A MEMS device consisting of a central circular portion with four beams suspended from a

Gallium Arsenide (GaAs) substrate has been used in optical components such as tunable filters

and lasers for Dense Wavelength Division Multiplexing (DWDM) communication [5].The

material for the suspended membrane consists of differentially stressed dielectric layers, which

results in the central circular portion having a quasi-spherical curvature. Arrays of such curved

membranes, with a reflective surface to form mirrors, can be used to focus an incident laser beam

to form multiple points of focus. If the focus is sharp enough, then a dipole trap is formed at each

focus.

Heating the beams of the membrane by electrical means causes the radius of curvature of the

membrane to change due to thermal expansion. As a result of the change in curvature, the focal

point of the mirror should change its position. In an array of mirrors, heating an individual mirror

would cause the focal point of only that mirror to move. Any atoms trapped at the focus of this

mirror would move along with the shifting focus. This MEMS array would then allow additional

3

functionality, which makes it possible to selectively move a single dipole trap containing atoms.

As compared to the lens array, this MEMS structure would allow preparation and entanglement

of individual traps.

Figure 1-2 Quasi-Spherical Suspended Membrane for a Tunable Laser [5]

1.2 Objective

Figure 1-3 Concept of Multiple Quasi Spherical Mirrors for Dipole Traps

4

The mirror arrays will be fabricated and various characterizations will be performed to determine

suitability for the above application. The material for the mirrors is comprised of layers of

dielectric with different intrinsic stresses. Dielectric materials Silicon Nitride (SiNx), and Silicon

Dioxide (SiO2) are deposited on a GaAs substrate with <100> crystal orientation through Plasma

Enhanced Chemical Vapor Deposition (PECVD). The deposited layers have different stresses

intended to produce curvature of the mirrors.

5

Chapter 2

2 Theoretical Basis for the Dipole Trap

Trapping and Cooling of Atoms

2.1 Introduction

This chapter describes some methods used to cool and trap atoms. Only atoms below a certain

kinetic energy can be held in a trap. The kinetic energy is specified in Kelvin (K) or mili-Kelvin

(mK) using

<E> = 𝜿B T (2.1)

Where E is the kinetic energy, 𝜅B is the Boltzmann constant and T the equivalent temperature in

Kelvin. The quality of a trap for atoms is usually indicated by specifying its depth in mK. If the

depth of a trap is 0.5mK, it means that atoms having a kinetic energy lower than 0.5mK will

remain confined in the trap for a long duration. Before atoms can be trapped, they first need to be

cooled down to a temperature that is lower than the depth of the intended trap. Room temperature

corresponds to atoms having a velocity of hundreds of m/s. The first stage is to cool them and

reduce their mean temperature to approximately 200 mK using Doppler cooling. At the same

time, the atoms must be confined in a small region of space using magneto-optical trapping

before they can be loaded into a dipole trap.

2.2 Doppler Cooling

Every atom has a transition frequency, where an electron in its outermost orbital absorbs a photon

of this frequency, to go from its ground state to an excited state. Before atoms can be trapped,

they must be slowed down. This can be effectively done using the Doppler Effect seen by atoms

moving in the opposite direction to a laser red shifted to the transition frequency of the atom. The

frequency seen by the atom is blue shifted which is closer to its transition frequency. When the

6

laser frequency matches the transition frequency the atom is excited when it absorbs a photon.

The momentum of the atom is reduced due to the law of conservation of momentum [8], since it

absorbs the momentum of the photon as well. The photon is re-radiated after some time in a

random direction and the atom returns to its ground state. Over time, with multiple absorptions

and emissions the momentum of the atom reduces, since the change in momentum due to

radiating photons in a random directions averages to zero, and the change in momentum due to

absorption is just in the direction of the laser beam. With every emission, the atom emits a photon

with a higher frequency than the one it absorbs, which reduces its energy as well. While cooling

atoms, one possibility to compensate for the changing Doppler shift seen by the slowing atoms, is

to vary the laser frequency. This cooling method has its limit; since even the slowest moving

atom cooled using this method will continually absorb and radiate photons. Although the time

average of the momentum change will still be zero, each absorption and radiation imparts a

change in momentum. When the increase in momentum due to radiation is balanced by the

cooling due to the laser, the Doppler limit is reached. It is given by

𝑻𝑫𝑶𝑷𝑷𝑳𝑬𝑹 ≅ ℏ𝜸 ∕ 𝟐𝜿B (2.2)

where ℏ is the reduced Planck constant, 𝛾 the natural linewidth of the atomic transition, and 𝜅Bthe

Boltzmann constant.

Figure 2-1 Two cycles in the Doppler Cooling of an Atom

7

2.3 Magneto-Optical Trap (MOT)

Figure 2-2 Simple Schematic of a Magneto-Optical Trap (MOT)

To cool atoms in three dimensions, multiple laser beams are used along with a magnetic field to

build a magneto optical trap. Six laser beams, each along one axis in 3-D Cartesian coordinates

are directed towards the origin. The region where the six beams intersect is termed as “Optical

Molasses”. All the beams are detuned, and any atom moving in a direction counter to a beam will

absorb a photon which reduces its momentum. To confine the atoms in 3-co-ordinate space as

well collisions and the Doppler limit, an additional mechanism to trap the atoms is created in the

form of the magnetic field.

The magnetic field is devised such that its strength increases as the distance from the

center of the trap increases. Due to the coupling between the electron spin and the magnetic field,

the energy levels of the atom are split (Zeeman Effect). Further away the atom from the center of

the field, the more its energy levels are split and the atom is more likely to come into resonance

with one of the laser beams, which then kicks the atom back towards the center of the optical

molasses. Thus a space dependent force which acts towards the center of the trap is set up.

8

2.4 The Dipole Trap

The idea of a dipole trap is based on the phenomenon of optical tweezers. Using optical tweezers,

nano and micrometer size particles can be trapped by using a very sharply focused laser beam.

The method of focusing can vary widely, but the simplest optical component that can be used is a

spherical lens or mirror. When the beam is tightly focused, there is a large variation in the

electromagnetic field gradient at the beam waist. The trapping mechanism can be described

depending on the ratio of the size of the particle to be trapped, to the wavelength of the laser

being used to generate the trap. In the case where the diameter of the particle to be trapped is

much lower than the wavelength of light, the particle can be treated as a point dipole in an

electromagnetic field.

An atom such as Rubidium (Rb), whose size is of the order of angstroms, can be considered as

such. For a dipole in a rapidly oscillating electric field, such as an atom in a laser beam, there is a

shift in the ground state energy levels of electron when the frequency of the laser is detuned from

that of the electron transition frequency. The rotating wave approximation is used, which is valid

considering that the size of the atom is much less compared to the wavelength of light. So the

intensity of the electric field can be considered constant in the region of an atom. As a result of

this approximation higher frequencies can be discarded, and the shift in energy levels is

determined by the amount of detuning δ given by

δ = ω - ωO (2.3)

where ω is the laser frequency and ωO is the transition frequency of the atom. The shift in ground

state energy levels is governed by the intensity of the field. For an atom in a laser beam red-

detuned from the atomic resonance frequency, the energy is lowest at the point of highest

intensity, i.e. at the focus of a Gaussian beam. So an atom at the focus of a red-detuned laser

beam is drawn to the center. For a sharply focused Gaussian beam with peak Intensity Io and

waist 𝑤Ο , its change in intensity from the focus with the radial coordinate r is given by

𝑰 𝒓 = 𝑰𝑶𝒆−𝒓𝟐/𝒘𝑶

𝟐

(2.4)

The transverse force at the waist is given by

9

𝑭 ≅ ℏ𝜸𝟐

𝟒𝜹

𝑰𝚶

𝑰𝒔

𝒓

𝒘𝑶𝟐 𝒆−𝒓𝟐/𝒘𝑶

𝟐

(2.5)

where ℏ is the reduced Planck constant, δ is the detuning , Is (saturation intensity) is the intensity

at which the atoms in the trap would be in their excited state 25% of the time on an average, and

γ is the spontaneous decay rate of the atom from its excited state [2]. For a given atom (such as

Rubidium) and laser beam, the detuning, laser power, saturation intensity and the decay rate are

known. The force acting on an atom therefore depends on the waist of the beam. If the waist is

narrower, the confining force of the atoms is greater, and the depth of the dipole trap is larger.

Figure 2-3 Simple Dipole trap at the focus of a laser beam

2.5 Mirror Requirements

For a given detuning and beam intensity, the depth of the trap is ultimately determined by the

focusing of the laser. This depends on the radius of curvature of the mirror. To have a sufficiently

deep trap, the desired beam waist at the point of focus is a maximum of 5µm. The intended

neutral atom to be trapped is Rubidium with a red-detuned beam of 800nm. To characterize the

mirrors a laser of frequency 780nm will be used. Based on Gaussian beam simulations using a

780nm wavelength, the required radius of curvature for an individual mirror must be less than

5mm to achieve a waist of <5µm at the focus.

10

The mirrors that will be fabricated are suspended from four beams. As a result, there is a

possibility that the curvature will not be uniform if the mirror is not suspended with near perfect

symmetry. This can result in different focal lengths for different axes of the mirror. This

phenomenon is known as astigmatism.

Figure 2-4 Simulation to determine spot size formed by mirror

Figure 2-5 Region of Trapping for a focused beam

11

Some astigmatism can be tolerated if there is an area in the region of the focus where the beam

waist does not exceed 5µm in any orientation. Simulations are performed using the software

OSLO EDU 6.4.3. The spot size is examined for different radii of curvature. The distance from

the waist of the focused beam, at which the beam expands to a spot size of 5µm, is noted. This

distance is approximately 50µm for a mirror of radius of curvature 3mm. The focal length for a

mirror is half its radius of curvature; therefore, the difference between the minimum and

maximum radii of the mirror should be within 0.1mm to form a trap.

In an experiment involving neutral atoms, the atoms are first cooled down in a magneto-optical

trap. The lasers of the Magneto-Optical Trap (Figure 2-2) are then turned off. After this, the laser

for the micromirror arrays is turned on so that the dipole traps are formed, and the atoms are

loaded into them. After the magneto-optical trapping is switched off, the atoms stay in the region

of trapping for a duration of the order of a second. Since any experiment must be completed in

this time frame, the response time of the mirrors has to be on the order of 1ms.

For trapping, high laser power is used. The mirrors must therefore have a very high reflectivity so

that it does not heat up due to absorption of laser light.

Figure 2-6 System Setup for the Micro-mirror array

12

The dipole traps that are formed by the mirrors can only trap atoms that have been cooled down

to <1mK in a Magneto-Optical Trap (MOT) and optical molasses. Atoms within a MOT are in a

vacuum. But this does not mean that the mirrors need to be in the same vacuum chamber. The

atoms to be trapped are present inside a small vacuum cell with transparent walls placed at the

center of the MOT. The mirror array is at some distance. Light from the desired laser source is

incident on the mirrors, and the resultant focal plane is re-imaged using some transfer optics so

that it is formed inside the vacuum cell at the center of the MOT. The incident beam from the

laser and the reflected light are separated with a beam splitter. Since the mirrors do not have to be

in vacuum, no special considerations are needed with respect to fabrication or packaging.

For quantum information processing, atoms in neighboring traps need to be as close together as

possible to make eventual entanglement easier to carry out. However, from the fabrication point

of view, more space between the mirrors is desirable to make bulk micromachining easier. The

focal plane of the mirrors is re-imaged into the trapped atom cloud in the MOT center. The size

of this cloud then becomes a limiting factor to how far apart the mirrors can be. For the purposes

of this work, the 2×2 arrays have a pitch of 500µm, so that all four focal points fall inside the

MOT atom cloud.

13

Chapter 3

3 Plasma Enhanced Chemical Vapor Deposition

Plasma Enhanced Chemical

Vapor Deposition (PECVD)

3.1 Introduction

Figure 3-1 Schematic of the process chamber of the Plasmalab 100 system [7]

14

For the deposition of dielectric layers, a Plasma Enhanced Chemical Vapor Deposition (PECVD)

system (Plasmalab 100 from Oxford Instruments) is used. The system is monitored and

controlled using a PC. The control software displays all operational parameters of the system,

such as the gas flow rates, pressure, power being supplied to the reactor, temperature, etc. The

major process parameters are accessible from the recipe and process setup pages, including

definition of gases on each line and calculation of mass flow settings in sccm. The software

includes data logging to disk of user selectable run-time process parameters for post process

verification and analysis of process conditions. Process recipes can be formulated and stored in

the computer and the system can be run in automatic using the recipes.

The system consists of a main process chamber with a viewport and various inlets for gases. The

chamber is designed as a “downstream” reactor which keeps the plasma generation away from

the wafer surface. 2” wafers are used for deposition. The process chamber has the following

attachments:

a) Pumping port for pre-pump and the turbo-molecular pump

b) The wafer is clamped down using a clamping plate.

c) Process gas inlet port. The process gases (N2 for SiNx, N2O for SiO2) are fed in through

an inlet at the top.

d) SiH4 is fed in through a ring with multiple exit points around its circumference, which is

placed between the Inductively Coupled Plasma (ICP) coil and the wafer.

e) Two ports for the connection of vacuum measurement components.

The plasma is ignited and maintained by the ICP coil which is towards the top of the chamber. N2

and N2O enter from the top of the chamber and into the plasma. If required, RF power is supplied

from the 13.56 Mhz RF generator through a matching circuit. The plasma then travels

downwards towards the ring where it reacts with the SiH4 to form the dielectric layer that is

deposited onto the wafers. The remaining gaseous reaction products are pumped out of the

chamber. The reactions for SiO2 and SiNx are

SiH4 + N20 → SiOx + H2 + N2 + … (3.1)

SiH4 + N2 → SiNx + H2 + N2 + … (3.2)

15

To regulate the temperature of the wafer, helium backside cooling is used. The purpose of the

helium supplied to the back of the wafer is to maintain the temperature of the wafer close to that

of the table, which is maintained by controlled heat transfer by means of an external water heater.

Helium is fed from a few holes in the table underneath the wafer, which is clamped to the table,

from where it flows towards the edges of the wafer. Helium is used to its inertness and good heat

transfer capabilities.

3.2 Deposition Sequence

The following series of steps are carried out for a typical deposition

a) The chamber is vented with N2.

b) The wafer is clamped inside using the clamping plate and screws. The temperature of the

table is set using the external water heater and monitored with the internal temperature

sensor.

c) After loading the wafer, the chamber is evacuated to a pressure of 9.45×10-4

Torr, then the

recipe is started. The first step of the recipe is to purge the chamber with N2 after which it

is pumped to 2 × 10-5 Torr.

d) The deposition sequences are set using a list of pre-defined steps. A typical sequence for a

single deposition layer is

a. Purge the chamber with the process gases for 1-2 minutes

b. Plasma Ignition for deposition in a specified time duration. When the deposition is

supposed to happen at low pressure, a “Low Pressure Strike” (LPS) is used for the

ignition of the plasma. For the plasma to ignite there should be an avalanche

breakdown due to the charged particles formed inside the chamber. If the pressure

is too low, then the gas density is not enough for the avalanche effect to take hold.

The pressure is raised to a higher value (usually 10mTorr), the plasma is ignited

and the pressure is rapidly pumped down to the deposition pressure.

c. Pumping out the remaining gases after the deposition is complete

d. Venting the chamber with N2.

e) After the list of recipes is over, the chamber is vented for final time with nitrogen and the

wafer is removed.

16

3.3 Deposition Characterization

Figure 3-2 Characterizing Wafers with Deposition

A number of parameters are measured for the deposited layers. By using an ellipsometer, the

refractive index of deposited dielectric, its uniformity over the wafer and thickness are measured.

Knowing the deposition time and thickness, the uniformity of deposition and average deposition

rate are calculated. For successful, reliable ellipsometry the thickness of the deposition must be

approximately chosen in advance. Measurements are made at the center of the wafer, and at four

points on the periphery (Figure 3-2). The thickness uniformity is calculated from the ellipsometer

readings by

𝑑𝑈𝑁𝐼 = 𝑀𝑎𝑥 .𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 −𝑀𝑖𝑛 .𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠

2×𝑚𝑒𝑎𝑛 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 × 100% (3.3)

To determine the quality of the deposited layers they are etched in a BHF solution with the

temperature maintained at 20˚ (BHF 50:1 for SiO, BHF 7:1 for SiN). The etching rate is then

compared with that of thermally grown SiO2. A higher relative etching rate to thermal SiO2

indicates that the quality of the deposition is correspondingly worse.

17

To measure the stress of the deposited material, a Frontier Semiconductor (FSM) 500 is used.

The measurement device uses a reflection from a laser to measure the surface profile of the

wafer. The thickness of each wafer is first measured, before deposition. Then the FSM is used to

measure the radius of curvature of the wafer along four axes (Figure 3-2). Deposition is then

performed, and the thickness of the layer measured by using ellipsometry. A second measurement

of the radius is then made along the same directions after the deposition. The FSM software then

calculates the bulk stress of the deposited layer based on the change in radius of curvature due to

the stress using the Stoney equation (Equation 3.4).

𝜎𝑏𝑢𝑙𝑘 ,𝑡𝑜𝑡𝑎𝑙 = 𝐸𝑠

6 1−𝑣𝑠

𝑡𝑠2

𝑡𝑓

1

𝑅𝐴𝑓𝑡𝑒𝑟−

1

𝑅𝐵𝑒𝑓𝑜𝑟𝑒 (3.4)

𝐸𝑠 and 𝑣𝑠are the Young’s modulus and the Poisson ratio of the GaAs substrate, 𝑡𝑠 and 𝑡𝑓 the

thicknesses of the substrate and the film, 𝑅𝐵𝑒𝑓𝑜𝑟𝑒 and 𝑅𝐴𝑓𝑡𝑒𝑟 the radii of curvature before and

after the deposition, respectively. For the purposes of this work, the important parameters are the

deposition rate, and intrinsic stress of the layers. A secondary consideration is the quality of the

layers, since long term reliability of the mirrors strongly depends on the constituent materials.

Silicon Nitride is referred to as SiN and Silicon Dioxide as SiO since it is unlikely that the

deposited layers have the exact stoichiometric compositions of Si3N4 and SiO2, as evidenced by

the varying refractive index.

3.4 Variation of Pressure to produce different intrinsic stress

The deposition pressure is varied to observe the effect on stress of the layers. All other deposition

parameters are also measured. However, only the stress and deposition rates are presented in this

section. Other parameters such as the refractive index are not important for the membranes in this

case since gold is used for reflective purposes. SiO and SiN are deposited with the following

parameters.

18

Silicon Nitride Silicon Dioxide

Temperature [˚] 80˚ 80˚

N2 flow rate [sccm] 6 0

SiH4 flow rate [sccm] 6.5 3

N2O flow rate [sccm] 0 12

Helium Backing [Torr] 2 2

ICP Power [W] 200 500 + LPS

RF Power [W] 0 0

Pressure [mTorr] 4 - 6 3 – 6

Table 1 Process parameters for SiN/SiO growth with Pressure Variation

For the layers grown with the above parameters, the measurement results for stress are presented

in Figures 3-3 and 3-4 and those for deposition rates in Figures 3-5 and 3-6

Figure 3-3 Stress for SiO deposition with variation in Pressure

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

2.0 3.0 4.0 5.0 6.0 7.0 8.0

Str

ess

[ M

pa]

Pressure [mTorr]

SiO Stress Characterization

19

Figure 3-4 Stress for SiN deposition with variation in Pressure

Figure 3-5 SiO Deposition rate with Variation in Pressure

-500.00

-450.00

-400.00

-350.00

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

2.0 3.0 4.0 5.0 6.0 7.0 8.0

Str

ess

[Mpa]

Pressure [mTorr]

SiN stress characterization

140

145

150

155

160

165

170

175

180

185

190

2.0 3.0 4.0 5.0 6.0 7.0 8.0

Dep

osi

tion R

ate

[Ä/m

in ]

Pressure[mTorr]

20

Figure 3-6 SiN Deposition rate with variation in Pressure

Based on the above characteristics a SiO/SiN membrane was deposited with the following

composition. Layer 1 is the first layer deposited on the GaAs <100> substrate. The stresses of the

layers are chosen in order to obtain an eventual radius of curvature < 5mm.

Layer Number 1 2,4,…14 3, 5,..13 15 16

Material SiN SiO SiN SiN SiO

SiH4 flow rate [sccm] 6.5 3 6.5 6.5 3

N2O flow rate [sccm] 0 12 0 0 12

N2 flow rate [sccm] 6 0 6 6 0

Temp [°C] 80 80 80 80 80

Pressure [mTorr] 4 3 5 6 6

ICP Power [W] 200 500 200 200 500

RF Power [W] 0 0 0 0 0

Thickness [nm] 300 200 300 300 200

Growth time 26’12” 11’30” 22’19” 20’10” 11’18”

Stress [MPa] -465.9 -4.8 -162.2 -1.8 17.5

Table 2 Composition of SiO/SiN membrane

100

110

120

130

140

150

160

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

Dep

osi

tion R

ate

[Ä

/min

]

Pressure [mTorr]

SiN Deposition Rate

21

3.5 Change in Deposition Characteristics

As seen in Figures3-3 to Figures 3-6 there is some variation in the deposition characteristics

when deposition is performed after a few weeks interval even though the process parameters are

kept the same. This can be due to a number of reasons, such as outgassing in vacuum from the

chamber walls from residue from previous depositions, which can alter the silicon and nitrogen /

oxygen ratios, or variations in ambient temperature. To study this variation, depositions were

performed approximately six months apart and with a recently cleaned chamber.

Figure 3-7 Stress for SiN/SiO deposited under different pressures at different times

The layers used to deposit the SiO/SiN membrane (Table 2: SiN deposition at 4mTorr, 5mTorr,

6mTorr, and SiO deposition at 3mTorr, 6mTorr) are re-deposited. As seen in Figure 3-7, the

stress is observed to be lower when the chamber is cleaner and then increases depending upon the

residue in the chamber walls. This indicates that for reliable reproduction of intrinsic stresses,

deposition should be preferably performed with a recently cleaned chamber.

-600.00

-500.00

-400.00

-300.00

-200.00

-100.00

0.00

100.00

200.00

Str

ess [

Mpa]

Stress Characterization Feb 2007

Stress Characterization July 2007

Stress Characterization July 2007 Deposition with a clean chamber

4.0 5.0 6.0 3.0 6.0

Pressure [mTorr]

SiN SiO

22

Figure 3-8 Deposition Rate for SiN/SiO deposited under different pressures at different

times

The deposition rate also varies depending on the condition of the chamber. As a consequence of

these variations, some test deposition needs to be performed for all individual layers comprising a

membrane before the membrane itself is deposited.

3.6 Obtaining Improved SiO layers

For the membrane deposited using the recipe shown in Table 2 the quality of the SiO deposited is

seen to be very inferior based on the observed etching rate (Figure 3-11). This might have

implications for the long term reliability of the membrane. During operation, the dielectric

membrane is actuated thermally and the temperature of the membrane rises significantly above

room temperature. This repeated heating and cooling can adversely affect the SiO over time and

lead to changes in the performance or the membrane. Previously, the SiO was grown with RF

power set to zero. Deposition with variation of the RF power (0-300W) is studied to examine the

100

110

120

130

140

150

160

170

180

190D

epostion R

ate

[Ä

/min

]

Characterization Feb 2007

Characterization July 2007

Characterization July 2007 Deposition with a clean chamber

4.0 5.0 6.0 3.0 6.0 a

Pressure [mTorr]

23

effect of RF power on the SiO quality. The remaining parameters are kept constant with the

following values.

Silicon Dioxide

Temperature [˚C] 80

N2 flow rate [sccm] 0

SiH4 flow rate [sccm] 3

N2O flow rate [sccm] 12

Helium Backing [Torr] 2

ICP Power [W] 500 + LPS

Pressure [mTorr] 6

Table 3 Deposition parameters for SiO with RF variation

Figure 3-9 SiO Deposition Uniformity and Deposition Rate with RF Power variation

From Figure 3-9 it can be seen that the best uniformity is obtained with RF power set to 75W.

Hence, to examine the effect of pressure variation with RF power, 75W is the chosen value.

75

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

180.0

200.0

1.50

2.50

3.50

4.50

5.50

6.50

7.50

8.50

9.50

10.50

0 50 100 150 200 250 300 350

Dep

osi

tion R

ate

[Ä/m

in]

Dep

osi

tion U

nif

orm

ity [

%]

RF Power [W]

Deposition Rate

Deposition

Uniformity

24

Figure 3-10 Etching Rate for SiO with variation of RF power

Figure 3-11 Comparison of SiO Etching Rates

100.0

300.0

500.0

700.0

900.0

1100.0

0 50 100 150 200 250 300 350

Etc

hin

g R

ate

[Ä/m

in]

RF Power [W]

SiO Etching Rate

Etching with BHF 50:1 at 20˚C

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

1600.0

0 2 4 6 8 10 12

Etc

hin

g R

ate

[Ä

/min

]

Pressure [mTorr]

RF = 75W

Etching with BHF 50:1 at 20˚C

RF = 0W

25

From Figure 3-11 it is clearly observed that using RF power leads to a reduced etching rate, and

indicates that SiO deposited using RF power is of better quality. As seen in Figure 3-10 the

etching rate is relatively uniform over the range of RF power used. However, the best uniformity

is obtained with 75W and there is no advantage to using higher RF power.

Figure 3-12 Stress Characterization with Pressure variation with RF Power 75W

As compared to Figure 3-3, it is observed in Figure 3-12 that deposition carried out with RF

power leads to high intrinsic compressive stress. The nature of the plot in Figure 3-12 can be

explained since depositions at 5mTorr, 7mTorr, and 9mTorr were carried out at a different time

from the depositions at 4mTorr, 6mTorr and 8mTorr. This also demonstrates change of the

deposition characteristics at different times.

Ultimately, even though better quality SiO is obtained, the membrane should be stable and have

the desired ROC. Two membranes with different compositions were deposited as alternatives to

using low quality SiO. One is deposited with the improved SiO with RF along with SiN layers;

-140.0

-120.0

-100.0

-80.0

-60.0

-40.0

-20.0

0.0

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

Str

ess

[MP

a]

Pressure [mTorr]

Stress Characterization for RF power

26

the other membrane consists only of SiN layers. The membrane with the SiO with RF and SiN

layers peels off the wafer it is deposited on, possibly due to excessive stress. One method to avoid

peeling might be to use SiO with RF in conjunction with SiN layers having lower stress.

Layer Number 1 2,4,…16 3, 5,..15 17

Material SiN SiO SiN SiN

SiH4 flow rate [sccm] 6.5 3 6.5 6.5

N2O flow rate [sccm] 0 12 0 0

N2 flow rate [sccm] 6 0 6 6

Temp [°C] 80 80 80 80

Pressure [mTorr] 4 6 6 7

ICP Power [W] 200 500+LPS 200 200

RF Power [W] 0 75 0 0

Thickness [nm] 300 200 300 300

Growth time 26’35” 15’19” 20’43” 18’49”

Stress [MPa] -465.9 -95.8 17.6 46.9

Table 4 Composition of SiO with RF/SiN membrane

Layer Number 1 2,4,…12 3, 5,..15 14 16

Material SiN SiN SiN SiN SiN

SiH4 flow rate [sccm] 6.5 6.5 6.5 6.5 6.5

N2O flow rate [sccm] 0 0 0 0 0

N2 flow rate [sccm] 6 6 6 6 6

Temp [°C] 80 80 80 80 80

Pressure [mTorr] 4 6 5 7 7

ICP Power [W] 200 200 200 200 200

RF Power [W] 0 0 0 0 0

Thickness [nm] 300 200 300 200 300

Growth time 26’35” 13’49” 23’15” 12’32” 18’49”

Stress [MPa] -468.3 17.6 -207.2 46.9 46.9

Table 5 Composition of SiN only membrane

27

Chapter 4

4 Processing of the Micro-Mirror Arrays

Fabrication of the Micro-Mirror

Arrays

This chapter describes the fabrication process and steps used in the manufacture of the

micromirror arrays. After the deposition of the SiN and SiO layers with Plasma Enhanced

Chemical Vapor Deposition, the wafer is thinned, followed by mesa structuring and bulk

micromachining. The process uses three masks. The first mask is used to define the shape of the

membranes, the second to form the gold contact pads and conductors and the third for the

backside opening in the substrate. The final step is to mount the membrane chip onto a glass

carrier and hold it in place and connect the contact pads for individual membranes to external

electrical connections.

4.1 Mesa Structuring

4.1.1 Lapping (Chemical Mechanical Polishing)

After the deposition of the dielectric layers (see Tables 2,5) the wafer is cleaved into pieces of

roughly one-fourth of its surface area so that it fits into a 2” circumference glass plate with spare

area around the circumference. The wafer piece is glued with “Crystal Bond” wax onto the glass

plate with the substrate side up. Metal spacers of approximate thickness 150µm and 100µm, five

of each, are alternately glued with equidistant spacing around the periphery of the wafer piece

(Figure 4-1). The glass plate is then covered with a weight and placed onto a revolving polishing

28

cloth and counter-rotated to ensure maximum uniformity. A lapping fluid (Chemlox) is added to

aid the lapping process. The fluid acts as an oxidizer for GaAs and the amount of fluid added

controls the speed of the process. As the wafer approaches the target thickness of 100µm, the

flow of fluid is gradually curtailed. This makes the process more mechanical which is desirable

for better uniformity. There is usually a variation of 10µm in the thickness of the thinned wafer.

After lapping the wafer is cleaved into 8×9 mm2 chips. GaAs gets cleaved along very well-

defined planes, which are used for aligning masks.

Figure 4-1 Wafer Preparation for Lapping

4.1.2 Gold (Au)/Chromium (Cr) Evaporation

Chromium and gold are evaporated onto the dielectric surface. The gold layer acts as the

reflector, and also a conductor for the current by which the membrane is heated. The chromium is

evaporated first since it adheres better to the dielectric. After cleaving the chips are glued with the

dielectric side up onto glass carriers and cleaned with acetone. Care should be taken that there are

no air bubbles between the chip and the glass carrier as it can lead to the chip cracking in the high

vacuum of the evaporator. The chips are loaded into the evaporator along with granules of Au

and Cr. A glow discharge is performed within the evaporator to increase the surface roughness of

the dielectric for better adhesion of the metal that will be evaporated. Then the evaporation

chamber is pumped out to a vacuum of the order of 1×10-5

Torr. The Cr is then electrically heated

until the evaporation process starts. 3.5nm of Cr is deposited first followed by 70 nm of Au. The

skin depth of gold is 28nm at 780nm [6]. Therefore, the evaporated thickness of the gold is

29

chosen to be 70nm, more than twice the skin depth. After evaporation, its reflectance is measured

at 96% for incident light of wavelength 780nm. The thickness of the deposited metal is monitored

by observing the change in the oscillation frequency of a crystal within the evaporation chamber.

The change in frequency is directly proportional to the thickness of the metal deposited, with

different constants of proportionality for different metals.

4.1.3 Photoresist (PR) hard bake

The Au surface evaporated in the previous step has to reflect light and should be as optically

perfect as possible. If it becomes rough, then the incident laser light will be scattered instead of

being focused. To protect the reflective Au surface, PR AZ1505 is deposited on the Au at

6000rpm for 40s. Then it is hard baked at 90˚ for 5min and then put on a 120˚ hotplate for 5min.

The temperature is raised to 160˚ and kept there for two hours. Then the hot plate is switched off

and the chips are allowed to cool down to room temperature. Care should be taken since at 160˚

the glue bonding the chips to the glass carriers is liquid.

4.1.4 Nickel (Ni) Sputtering

Ni is used to form the etching mask when the dielectric is dry etched using SF6 plasma. It is

deposited using Argon sputtering for 4min 30s using the following parameters: Pressure 300×10-4

mBar, Power 2KW. A test sample is placed along with the chips so that the Ni thickness can be

determined later using a Dektak profiler. The thickness of the Nickel is measured to be around

160nm.The deposition parameters must be chosen such that the sputtered nickel layer does not

have very high intrinsic stress to avoid its cracking during later processing steps.

4.1.5 Lithography and Nickel Etching

PR AZ4533 is deposited on the Nickel at 6000rpm for 40s and then pre-baked at 5mins at 90˚.

Lithography is then done using the first mask to define the shapes of the membranes with an

exposure time of 20s. Then the PR is developed using 1:4 AZ400K developer with a

development time of 60s and then rinsed in DI H2O. After drying by blowing Nitrogen the Nickel

is etched in a HNO3 (65%):H2O = 1:30 solution heated to 60˚ (Figure 4-3). The PR AZ4533 used

in this step must be significantly thicker than the hard baked PR AZ1505 used in protecting the

gold surface. In the next step the hard baked PR is removed using O2 Reactive Ion Etching(RIE).

PR is removed all over the sample, but the PR AZ4533 covering the non-etched Nickel must

30

remain after the RIE process. It is needed to protect the Nickel when the chromium is wet etched

since the chromium etching solution can damage the nickel.

4.1.6 Photoresist Removal by O2 Reactive Ion Etching (RIE)

The hard baked PR lying over the gold is removed using Reactive Ion Etching (RIE) performed

for 5min with the following parameters: RF power 200W, O2 flow rate 200 sccm, Pressure

150mTorr. RIE is used so that vertical side-walls are obtained (Figure 4-4).

4.1.7 Au/Cr wet etching

To expose the dielectric, the gold is wet etched for 30s using a gold etching solution (H2O 200ml,

KI 20g, I2 5g) followed by chromium etching for 10s using a chromium etching solution (41.15g

(NH4)2[Ce(NO3)6], 22.5ml HNO3(65%), 250ml H2O) (Figure 4-5).The nickel should be protected

from the Chromium etching solution by PR AZ4533 that remains after O2 RIE.

Figure 4-2 Ni, PR and Gold layers above Dielectric Figure 4-3 After Nickel Etching

Figure 4-4 After Photoresist Removal Figure 4-5 After Gold/Chromium Etching

31

Figure 4-6 Chip after O2 Reactive Ion Etching

Figure 4-7 Chip after Gold/Chromium Etching

32

4.1.8 SF6 plasma etching

To realize the membrane, the dielectric is removed using SF6 plasma etching for 40min with the

following parameters: DC voltage 100V, Pressure 125 mTorr, SF6 flow rate 35 sccm, RF power

150W. There is a significant under etching during this step and the effective thickness of the

beams is reduced as a result. The process should be periodically monitored; otherwise excessive

etching makes the beams too thin for the mirror to be useful. The nickel acts as a mask in this

step.

Figure 4-8 After SF6 Plasma Etching

4.1.9 Nickel and Photoresist cleaning

The Nickel is then cleaned by etching it again in HNO3 (65%):H2O = 1:30 for around 90s. Liftoff

is not effective in removing the nickel. During liftoff, extremely small particles of nickel can

break off and adhere to the gold surface, thus increasing its roughness. The hard baked PR

33

AZ1505 is then cleaned by immersing the chips in N-Methyl Pyrrolidone (NMP) at 90˚ for 2

hours. Before putting them in NMP the samples must be cleaned of all bonding wax.

4.1.10 Au/Cr etching to define contact pads

PR AZ1505 is deposited at 6000rpm for 40s and then pre-baked at 90˚ for 2 mins. The

lithography is performed using the second mask with an exposure time of 5s. Then the PR is

developed using 1:4 AZ400K developer with a development time of 20s and then rinsed in DI

H2O. The gold and chromium are then etched away using the respective etching solutions. This

completes the surface micromachining steps. The chips are then placed in acetone for a long time

to remove them from the glass carrier so they can be bulk micro-machined.

Figure 4-9 After Gold/Chromium Etching

34

4.2 Bulk Micro-Machining

The chip is first glued with the substrate facing upwards to a glass carrier. Then PR AZ4562 is

deposited at 6000 rpm for 40s and pre-baked at 90˚ for 10 mins. The lithography is done in two

steps since the PR at the edge of the chip is much thicker and makes accurate alignment very

difficult. In the first lithography step a mask with rectangles is used to expose the PR at the edges

which is then developed away. This leaves the more uniform PR on which a second lithography

step is performed using the third mask to define the shape of the backside openings. For both

lithography steps the exposure time is 100s and the development is done using 1:3 AZ400K

developer with a development time of 60s and then rinsed in DI H2O. After the backside

openings are developed the edges are covered again with AZ4562. The PR is then baked at 90˚

for 90mins to harden it for spray etching.

Figure 4-10 Backside Opening after Spray Etching

35

Figure 4-11 Freed Mirror array after Spray Etching

Spray etching is then performed using H2O2 (30%):NH4OH (25%) = 100:10 for a time

duration varying between 10-12 minutes depending on the substrate thickness and etching rate.

During and after the spray etch process the chip is cleaned with 32% HCl for 5s to remove oxides

of Gallium and Arsenic. During the etching, the chip is rotated periodically to obtain uniform

etching. Afterwards, the chip is placed in acetone for a long time to remove the PR and bonding

wax, followed by a final cleaning with immersion in N-Methyl Pyrrolidone (NMP) at 90˚ for 2

hours. Other than the backside openings the spray etching opens up gaps between the mirror

arrays on every chip which allows easy separation of the individual arrays. The individual arrays

are cleaved into smaller chips for mounting. This must be performed with great care since the

membranes are now freed from the substrate and very little force can result in breaking of their

beams.

36

Figure 4-12 Wire Bonding to the Gold Pads

4.3 Packaging

The chip with the mirror is glued to a glass carrier using adhesive 4061T which is hardened by

exposure to blue light for 60s. The chip is glued to a glass carrier at only a single corner which

allows expansion of the substrate when the mirrors are heated. The glass carrier is glued to the

Printed Circuit Board (PCB) using the same adhesive (Figure 4-13). Then sixteen copper filament

wires are glued to the glass carrier, each for one contact pad among the sixteen of the mirror

array. Each pad is linked to a single beam so there is full control over which particular beams can

be individually heated. Conductive epoxy H20E is then used to bond the wires to the pads. The

epoxy is formed by mixing a conductive polymer with a hardener. It is cured at 100˚ for two

hours (Figure 4-12).

Before the chip is mounted, a hole is drilled in the PCB at the point beneath the point the

mirror array. For characterization the mirror array is illuminated with a laser. The light that isn’t

reflected from the mirrors passes through the hole. Otherwise it would be scattered from the PCB

which would make taking images of the reflected light very difficult.

37

Figure 4-13 Bonding between Chip, Glass carrier and Printed Circuit Board

Figure 4-14 Packaged chip on Holding Plate

38

Chapter 5

5 Characterization of Micro-Mirror Arrays

Characterization of Micro-

Mirror Arrays

The mirrors are characterized in a number of ways. The curvature is measured using a confocal

system to determine astigmatism. Current is passed through the membrane and the change in

curvature is measured. Finally, optical characterization is performed using an optical bench setup

to examine the focal spot formed by the mirrors.

5.1 Measurement of Curvature using a Confocal system

A Plµ Confocal Imaging System is used to measure the curvature of the micromirrors. The

sample moves in height increments of 0.5-2µm. The system then takes an image of the tunable

membrane at its focal plane, for each position of the sample. Then, it performs signal processing

to build a 3-D image of the mirror. Images are taken at two magnifications, 10x to measure the

characteristics of the 2x2array as a whole, and 20x to measure the curvature of individual mirrors.

The radius of curvature is measured by taking a set of 1-D measurements along profiles of the

mirrors along certain axes (Figure 5-3). The software tries to fit the curvature onto the boundary

of a circle to determine the radius of curvature along a particular axis. The measurements made

along selected axes allow the astigmatism of the mirror to be determined. The radius of curvature

is measured along various axes for individual mirrors to determine whether the curvature is

similar for individual mirrors in a set and to study the nature of the astigmatism of the mirrors by

39

comparing measurements across a wide set. It should be noted that radius of curvature

measurements made by the system is use is precise only to 0.1 mm.

Figure 5-1 3-D Image showing Curvature of the Mirror

Figure 5-2 Profile View across two mirrors

40

Studies of the curvature are presented for four mirror arrays with different configurations and

backside openings. Two arrays have been fabricated using the membrane comprising of SiN and

SiO without RF layers (for composition see Table 2). These arrays are denoted by SiO/SiN array

I (Results presented on page 41) and SiO/SiN array II (Results presented on page 42). The other

two arrays comprising of SiN layers only (for composition see Table 5) are denoted as SiN only

array I (Results presented on page 43) and SiN only array II (Results presented on page 44).

Photos of the freed membranes are included to show the particular backside array used in the

measurement.

To determine the severity of astigmatism, the standard deviation is computed for the set of

measurements for a membrane. The standard deviation gives an idea about the variation in the

measured ROC for different membranes. If the standard deviation is less, the variation in radius

of curvature is lower. For a set of four mirrors, the range of the measurements along each axis

(Figure 5-3) is averaged to determine the variation within a given array of mirrors. These two

figures of merit are used to compare different mirror arrays.

Figure 5-3 Measurement axes for the radius of curvature

41

Figure 5-4 Front side of SiO/SiN array I

Radius of Curvature X [mm] Y [mm] Beam I [mm] Beam II [mm]

a 2.597 2.865 2.571 2.53

b 2.622 2.938 2.499 2.544

c 2.616 2.895 2.494 2.579

d 2.631 2.79 2.711 2.453

Range (max-min) 0.034 0.148 0.217 0.126

Range Average 0.13125

Standard Deviation 0.1506

Table 6 Curvature measurements for SiO/SiN array I

Figure 5-5 Plot of ROC measurements for SiO/SiN array I (Table 6)

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3

a b c d

Rad

ius

of

Cu

rvat

ure

[m

m]

Mirrors in 2x2 Array

curvature along x axis

Curvature along Y axis

Curvature along beams I

Curvature along beams II

The centers of the mirrors are 500µm apart

with each mirror having a diameter of ~250µm.

The beams are ~40-45µm wide. The central

diameter and beam widths reduce slightly due to

SF6 plasma under etching (4.1.8) during

fabrication. Some membranes in this set show the

effects of excessive heating.

42

Figure 5-6 Front side of SiO/SiN array II


with each mirror having a diameter of

~300µm. The beams are ~40-45µm wide.

The central diameter and beam widths

reduce slightly due to SF6 plasma under

etching (4.1.8) during fabrication


a 2.313 2.318 2.323 2.471

b 2.229 2.328 2.302 2.632

c 2.158 2.205 2.241 2.612

d 2.448 2.282 2.306 2.499

Range (max-min) 0.29 0.123 0.082 0.161

Range Average 0.164


Table 7 Curvature measurements for SiO/SiN array II

Figure 5-7 Plot of ROC measurements for SiO/SiN array I (Table 7)

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3

a b c d

Rad

ius

of

Cu

rvat

ure

[m

m]






43

Figure 5-8 Front side SiN only array I


with each mirror having a diameter of

~250µm. The beams are ~40-45µm wide.

The central diameter and beam widths

reduce slightly due to SF6 plasma under

etching (4.1.8) during fabrication.


a 3.116 3.642 3.438 3.397

b 3.282 3.678 3.556 3.391

c 3.107 3.544 3.438 3.381

d 3.222 3.788 3.537 3.443

Range (max-min) 0.175 0.244 0.118 0.062



Table 8 Curvature measurements for SiN only array I

Figure 5-9 Plot of ROC measurements for SiN only array I (Table 8)

2.75

2.95

3.15

3.35

3.55

3.75

3.95

4.15

a b c d

Rad

ius

of

Curv

ature

[m

m]






44

Figure 5-10 Front Side SiN only Array II

The centers of the mirrors are 500µm

apart with each mirror having a diameter

of ~300µm. The beams are ~40-45µm

wide. The central diameter and beam

widths reduce slightly due to SF6 plasma

under etching (4.1.8) during fabrication


a 3.596 4.09 3.841 3.803

b 3.493 4.081 3.819 3.766

c 3.47 4.014 3.746 3.809

d 3.537 4.067 3.723 3.756

Range (max-min) 0.126 0.076 0.118 0.053



Table 9 Curvature measurements for SiN only array II

Figure 5-11 Plot of ROC measurements for SiN only array II (Table 9)

2.75

2.95

3.15

3.35

3.55

3.75

3.95

4.15

a b c d

Rad

ius

of

Curv

ature

[m

m]






45

From the plots of the radii of curvature, it is observed that for a given axis within the four mirrors

in each 2×2 set, the variation in measured ROC is usually within or close to 0.1 mm of variation,

particularly SiN only Array II (Figure 5-11). This indicates that the natures of curvature of all

four mirrors are similar. However, for measurements along different axes among the four mirrors

a large difference is seen in the radii of curvature which is significantly more than 0.1 mm. This

indicates that the shape of the backside hole created by wet etching is the dominating factor

determining the astigmatism of the mirrors, since the shape is common to all four mirrors in an

array. Wet etching of Gallium Arsenide is highly anisotropic and non-uniform. Another cause of

astigmatism can be misalignment during lithography. The front lithography for the mesa

structuring and the back lithography for the bulk micromachining is done by manual alignment at

the edge of the chip. This can lead to some offset between the front and back sides.

Other causes of astigmatism can be the variation in substrate thickness as a byproduct of lapping

which would cause backside openings of different dimensions. Local fluctuations in the intrinsic

stress of the dielectric layers comprising the membrane can also be a contributing factor to the

astigmatism. However, these two factors may be minor contributors to the astigmatism at best,

since the pitch of the array is 500µm, and not much variation is expected over this range.

Compare SiN only arrays I and II (Tables 8 and 9). The range average of array II is much lower

than that of array I. However, its standard deviation is larger than that of array I, indicating that

its astigmatism is higher. Array I will form sharper focal spots than array II.

Compare SiO/SiN arrays I and II (Tables 6 and 7). The standard deviation of array II is lower

than that of array I. However, its range average is greater than that of array I, indicating that the

similarity between the curvature of individual mirrors is lower. Array II will form sharper focal

spots, but these focal spots will have a greater separation from each other in the direction of

propagation of the laser.

The standard deviation indicates total variation in curvature among the mirrors in an array and

can be taken as a measure of the astigmatism. The range average indicates the similarity between

the mirrors in an array. Both figures should be as low as possible. Just the range average being

low or just the standard deviation being low is not sufficient.

46

The overall radius of curvature for the SiN only arrays is higher (~3.5-4mm) (Tables 8, 9) as

compared to the SiO/SiN arrays (~2.5 mm) (Tables 6, 7) despite having a similar stress profile.

This can be attributed to the higher Young’s Modulus of silicon nitride.

From Tables 6, 7,8 and 9 it is seen that the range variation is lower for the mirrors for the

measurements along the beams (Beam I, Beam II directions).

5.2 DC measurements

Current is passed through the membrane and the resulting change in curvature is measured using

the confocal system. The current and voltage values are recorded for each step. Destructive

testing performed on one SiO/SiN mirror showed that currents up to 75mA can be sustained

before the membrane fails. This is well within the current required to produce a 5-10% change in

the radius of curvature.

Figure 5-12 Linear I-V plot for a SiN only mirror

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20

Volt

age

[V]

Current [mA]

Direction of

current flow

47

Figure 5-13 Change in curvature with current for SiO/SiN membrane

(a) at 0 mA (b) at 30 mA

Figure 5-14 Profile Images of SiO/SiN membrane showing slight upward beam bending

2.5

2.55

2.6

2.65

2.7

2.75

2.8

2.85

2.9

2.95

3

0 5 10 15 20

Radiu

s of

curv

atu

re [

mm

]

Current [mA]

Curvature along X axis


48

Figure 5-15 Change in curvature with current for SiN only membrane

(a) at 0 mA (b) at 16 mA

Figure 5-16 Profile Images of SiN only membrane showing slight upward beam bending

For the SiN/SiO membrane, whose change in curvature is shown in Figure 5-13 the radius of

curvature initially decreases and then increases again. This can be due to the packaging method

used, since the chip was bonded to the glass carrier at more than one point using UV hardened

3.7

3.8

3.9

4

4.1

4.2

4.3

4.4

4.5

0 5 10 15 20

Radiu

s of

curv

atu

re [

mm

]

Current [mA]

Curvature along X axis


49

adhesive. If the adhesive does not allow the substrate to expand freely, then it is likely to bend

upwards which would cause the membrane to stretch. This would result in a reduction in the

apparent radius of curvature.

However, the SiN only membrane was packaged with precaution and care was taken to glue the

chip to the glass carrier at only one corner. But the same behavior is seen; with the radius of

curvature first decreasing and increasing as more heating current is supplied (Figure 5-15).

Therefore, packaging may not be the cause of this behavior. In these membranes the supporting

beams are much thinner (~40 µm after plasma under-etching) than the central circular portion

(250µm – 300µm). Hence the gold over the beams has a significantly higher resistance than the

central portion. The current density in the beams is also correspondingly higher. Moreover, the

heating is from one surface of the membrane only. It is supposed that, as the heating current

increases, and the heat supplied increases proportional to the square of the current, the beams are

heated far more due to the higher current density in them and will try to expand more than the

large central portion whose mass is much greater than that of the beams. This non-uniform

heating results in the beams bending upwards which then reduces the net radius of curvature.

This can be observed in the profile view of the membranes, where the beams are seen to have a

slight upwards movement at higher heating currents. However, the upwards deflection is very

slight and the profile must be examined with care to see the movement. A better measure of this

is the increase, then decrease in the air gap of the membrane with increasing current.

Figure 5-17 Air Gap for a SiO/SiN membrane

7.46

7.48

7.5

7.52

7.54

7.56

7.58

7.6

7.62

7.64

7.66

7.68

0 2 4 6 8 10 12 14 16

Air

Gap [

µm

]

Current [mA]

50

5.3 Imaging the focal plane

The setup is created on an optical bench. A 780nm semiconductor laser is coupled into an optical

fiber. The fiber output is coupled out through a lens which is used to collimate the beam to the

greatest extent possible. The beam then passes through a beam splitter adjusted at 45˚ and falls on

the mirror array. The reflected beam falls back onto the splitter and is turned through 90˚ and

passed through two lenses to re-focus the focal plane onto a Charge Coupled Device (CCD)

camera. The lenses are on a sliding mount along with the CCD camera so the distances between

the lenses and camera can be adjusted for a sharper image. Some modeling was performed to

determine the validity of the setup before setting it up on the optical bench using the optical

simulation OLSO Edu 6.4.3. Different combinations of lenses with varying focal lengths are used

to obtain different magnifications at the camera sensor.

The fiber collimator, beam splitter and chip holder are all mounted on stages movable in the X-Y

plane, the Z direction being the direction of propagation of the light. This gives sufficient degrees

of freedom to align the beam through the entire setup.

Figure 5-18 Schematic of the Characterization Setup

51

Figure 5-19 Optical Bench Setup for Imaging the focal plane

Figure 5-20 Simulation to validate Characterization Setup

52

If the mirror array is used for trapping atoms, the camera would be replaced by a vacuum cell

containing Rubidium atoms, placed at the center of a Magneto-Optical Trap.

Figure 5-21 Focal Spots for the 2×2 Mirror Array SiO/SiN I (Table 6)

53

The image in Figure 5-21 were obtained from the CCD camera. They show the four focal spots of

SIO/SiN array I (Curvature measurements on page 41). The spot sizes are estimated to be 20µm

based on features visible in the background, whose size is known, such as the membrane

dimensions. Due to astigmatism of the mirrors, the focal spots are seen to be elliptical. White

areas within the images indicate the points of highest intensity, and blue the lowest. Between the

focal spots of the membranes, some light is seen to be reflected from the gold conductors

between the beams of the membranes and the contact pads. This very low intensity reflection

does not interfere with the traps that might be formed at the focal points.

a) Left membrane with no current

b) Left membrane with 9.5mA heating current

c) Left membrane with heating current switched off

Figure 5-22 Actuation of a Membrane

54

The series of images in Figure 5-22 show the actuation of a membrane (part of SiO/SiN Array I)

that forms the focal spot on the left hand side of the image. In a) no current is being passed

through the membrane and its focal spot is shown along with that of the adjacent membrane. In b)

9.5mA of heating current is passed through the left membrane. The diagram of the membrane in

Figure 5-12 shows the way in which the current is passed. A change is seen in the observed

intensity profile. The heating causes a change in the radius of curvature, which results in a shift of

the focal spot which manifests itself as a change in the observed profile. In c) the heating current

is turned off, and the intensity profile becomes similar to that in a). There is no discernible

sideways tilt of the focal spot when the membrane is heated.

Figure 5-23 Astigmatism in Focused Beams

The image in Figure 5-23 is taken by moving the camera slightly away from the point at which

the sharpest focus is obtained. The distorted images of the beam are a result of the astigmatism of

the mirrors. Comparing the axes of the membrane, shown in Figure 5-3, to the orientation of the

distorted focal spots, it can be observed that the astigmatism is severe along the X and Y

directions, and relatively less along the Beam I and Beam II directions. Faint rings are seen in

some areas of the image. This might be due to an interference effect caused by the lens surface

not being perfectly clean, or a coma effect produced by a slight misalignment of the reflected

laser light with the optical axis of the lenses [9].

55

Figure 5-24 Focal Spots for the 2×2 Mirror Array SiN only I (Table 8)

Figure 5-21 shows the focal plane formed by the SiN only Array I. (Curvature measurements

page 43). The images of the focal spots are even more elliptical as compared to those in Figure 5-

21 for the SiO/SiN array. This can be explained due to the higher standard deviation measured in

the curvature of the SiN array (Table 8) as compared to that of the SiO/SiN array (Table 6),

which indicates higher astigmatism.

56

Chapter 6

6 Conclusion and Future Scope

Conclusion and Future Work

6.1 Conclusion

Micromirror arrays for the purposes of forming multiple dipole traps were fabricated successfully

with a yield ranging from 50-75%. Various issues encountered during deposition, fabrication and

packaging were resolved. The mirrors were mounted on glass carriers and electrical connections

were formed. The focal plane of the mirrors was imaged using a 780 nm laser on an optical bench

setup.

The material of the mirrors was a SiN/SiO membrane deposited using PECVD. To

achieve the required radius of curvature, the layers comprising the membrane have a range of

intrinsic stress values. Extensive characterizations were done by varying the deposition pressure

both for SiN and SiO. To obtain better quality SiO, characterization was done using RF power

and pressure variation. SiO grown with RF power was observed to have a significantly improved

etching rate. Three depositions of membranes were made using combinations of the characterized

layers:

SiO without RF with SiN layers with different stresses

Only SiN layers with different stresses

RF SiO with SiN layers with different stresses

The radius of curvature for the SiO/SiN membranes was around 2.5mm and that for the

SiN membranes around 3.5-4 mm. The RF SiO/SiN membrane was found to be unstable due to

excessively high cumulative stress.

57

Current was passed through the membranes to determine their actuation characteristics.

The voltage –current relationship is linear. The change in curvature due to heating is measured

and found to be on the order of 5-10%. At higher heating currents the expansion is not uniform

and found to be occurring more in the beams as compared to the central portion of the mirrors. A

heating current of up to 75mA can be tolerated by the membranes.

The mirror arrays have high astigmatism. The astigmatism is less for the SiO/SiN

membranes as compare to the SiN only membranes. Based on the imaged focal spots formed by

individual mirrors, the waist size is estimated to be 20µm, which is higher than the requirement

of 5µm.

6.2 Future Work

Astigmatism in the mirrors is seen to be the most significant issue in obtaining a sharp focal spot,

which is required for deeper traps. The material of the membrane is all amorphous in nature, so

the stress tensor is expected to be symmetrical. If the mirror is supported symmetrically, then the

curvature should be nearly uniform along all axes. Using spray etching on a GaAs <100>

substrate, it is not possible to achieve perfectly symmetrical holes on the backside. Wet etching of

GaAs is highly anisotropic.

Dry etching is required. Dry etching using plasma or RIE is usually isotropic. Hence, using dry

etching, backside holes of arbitrary shape and symmetry can be formed. GaAs dry etching

facilities were not available, hence Silicon was considered for the substrate material. SF6 plasma

etching is available, which can be used to dry etch Silicon. A new mask was designed that

included patterns for dry etching the backside hole. Due to time constraints, and inexperience

with Silicon processing, this idea could not be explored.

Significant difficulties were encountered in thinning the Silicon wafer with respect to uniformity

of the wafer and cracking of the wafer surface. Another potential issue is that SF6 plasma etching

does not have a high selectivity between Silicon and SiO/SiN. Thus, there is chance that the

membrane will be damaged if SF6 plasma etching is utilized. If GaAs dry etching facilities are

made available, then a better option is to dry etch the GaAs instead of trying Silicon.

58

Another way to tackle the astigmatism is to use membranes with more supporting beams, since

the curvature along the beams is seen to be more uniform. If more beams were to be used, then

the curvature could be more uniform.

Figure 6-1 Method to achieve less astigmatism

When the membranes are heated the change in curvature is not uniform or monotonic. This might

be due to imprecise measurements, since the confocal imaging software is accurate only till 0.1

mm. However, heating the beams more uniformly might result in better, more monotonic change

in curvature. Another solution to the non-monotonic change in radius would be to actuate the

membranes electro-statically.

Figure 6-2 Heating elements for beams

59

Bibliography

[1] Definition of a Qubit: http://en.wikipedia.org/wiki/Qubit

[2] H. Metcalf and P. van der Straaten, “Laser Cooling and Trapping”, Springer, New York.

1999

[3] R. Dumke, M. Volk, T. Müther, F.B.J. Buchkremer, W. Ertmer, G. Birkl, “Quantum

Information Processing with Atoms in Optical Micro-Structures”, Quantum Information

Processing, pp. 246 – 256, Wiley-VCH, Weinheim (2003)

[4] R. Dumke, M. Volk, T. Müther, F. B. J. Buchkremer, G. Birkl, and W. Ertmer, "Micro-

optical Realization of Arrays of Selectively Addressable Dipole Traps: A Scalable

Configuration for Quantum Computation with Atomic Qubits", Phys. Rev. Lett. 89,

097903

[5] A. Tarraf, F. Riemenschneider, M. Strassner, J. Daleiden, S. Irmer, H. Halbritter, H.

Hillmer, and P. Meissner, “Continuously Tunable 1.55µm VCSEL Implemented by

Precisely Curved Dielectric Top DBR Involving Tailored Stress”, IEEE Photonics

Technology Letters, Vol. 16, No. 3, March 2004

[6] R. Qiang, R.L. Chen and J. Chen, “Modeling Electrical Properties of Gold Films at

Infrared Frequency Using FDTD Method”, International Journal of Infrared and

Millimeter Waves, Volume 25, Number 8 / August, 2004

[7] Oxford Instruments Plasmalab 100:

http://www.oxinst.com/wps/wcm/connect/Oxford+Instruments/Products/Etching+&+Dep

osition/Plasmalabsystem100/Plasmalab%C2%AESystem100

[8] Law of Conservation of Momentum:

http://en.wikipedia.org/wiki/Momentum#Conservation_of_momentum

[9] Coma Effect: http://en.wikipedia.org/wiki/Coma_%28optics%29

http://en.wikipedia.org/wiki/Momentum#Conservation_of_momentum

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A. Appendix

Figure A-1 Image of the AutoCAD drawing of mask used for SiO/SiN membranes

61

Figure A-2 Image of the AutoCAD drawing mask used for SiN only membranes

The second mask was designed based on feedback obtained from processing done using the first

mask used to fabricate the SiO/SiN membrane arrays. The most significant difference between

the first and second mask is that the second mask contains shapes intended for dry etching the

backside hole to reduce the astigmatism. It also allows for a Silicon substrate to be used instead

of Gallium Arsenide. Another improvement is the provision for larger chip sizes so that the

external contacts to the arrays can be formed more easily during packaging.

Master Thesis Kaustav Ghose TU Darmstadt

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