SAW DEVICES

1. INTRODUCTION

SAW means Surface Acoustic Wave. Electronic signal processing by means of

selective manipulation of surface acoustic wave on piezoelectric substrate was

initiated in 1965 with the invention of thin film interdigital transducer (IDT) by White

and Voltmeter at University of California at Berkeley.

Acoustic waves are type of longitudinal waves that propagates by means of

adiabatic compression and decompression. Longitudinal waves have same direction of

vibration as their direction of propagation.

SAW ( Surface Acoustic Wave ) devices have emerged as the most unique passive

components, having advantages of being small, rugged, lightweight and easily

reproducible. These advantages have made SAW devices a key component in RF

communication systems with innovative applications. Other emerging applications of

SAW resonators include gas sensors, biosensors, and chemical sensors. The

significance of these devices in industry is evidenced by their large worldwide

production, as an example approximately 3 billion acoustic wave filters are used

annually, primarily in mobile cell phones and stations. Recent fabrication

technological developments in the complementary metal-oxide semiconductor

(CMOS) technology have led to a marked improvement in the performance of these

CMOS devices in the high frequency range. In addition recently, a wide variety of

novel MEMS devices have been successfully implemented using CMOS fabrication

techniques. 

1

2. DEFINATIONS OF SAW

1. SAW is an acoustic wave which travels along the surface of material

exhibiting elasticity, with amplitude that decays exponentially with depth into

substrate.

2. Surface Acoustic Wave is a wave propagating along the surface of an elastic

substrate.

3. A Surface Acoustic Wave is a type of mechanical wave motion which travels

along the surface of solid material.

In 1985 Lord Rayleigh explained the surface acoustic mode of propagation. So, these

waves are also known as Rayleigh waves. Rayleigh waves have longitudinal and

vertical shear components that can couple with any media in contact with coupling

strongly affects the amplitude and velocity of wave, allow SAW sensors to directly

sense mass and mechanical properties.

With reference to the basic structure sketched in fig.1, metal thin-film IDTs are

fabricated on the surface of suitable piezoelectric substrate that would act as electrical

input and output ports. SAW devices consist of two IDTs on piezoelectric substrate

such as quartz. IDTs consist of interleaved metal electrodes which are used to launch

and receive the waves, so that electrical signal is converted into acoustic wave and

then back to electrical signal.

Any changes to the characteristics of the propagation path affect the velocity and/or

amplitude of the wave. Changes in velocity can be monitored by measuring the

frequency or phase characteristics of the sensor and can then be correlated to the

corresponding physical quantity being measured.

2

Figure 1: shows a basic diagram of SAW device

3. CONSTRUCTION

As far as construction is concerned, basic structure of SAW device consists of:

1. Piezoelectric crystal substrate

2. Interdigital transducers

PIEZOELECTRIC CRYSTAL SUBSTRATE : In piezoelectric material there

is a mechanism which offers coupling between electrical and mechanical

disturbances. Application of electric field setup mechanical stresses and strain.

3.1 Piezoelectric Substrate Materials

Among the piezoelectric substrate materials that can be used for acoustic wave

sensors and devices, the most common are quartz (SiO2), lithium tantalite (LiTaO3),

and, to a lesser degree, lithium niobate (LiNbO3). Each has specific advantages and

disadvantages, which include cost, temperature dependence, attenuation, and

propagation velocity. An interesting property of quartz is that it is possible to select

the temperature dependence of the material by the cut angle and the wave propagation

direction. With proper selection, the first order temperature effect can be minimized.

An acoustic wave temperature sensor may be designed by maximizing this effect.

This is not true of lithium niobate or lithium tantalite, where linear temperature

dependence always exists for all material cuts and propagation directions. Other

materials with commercial potential include gallium arsenide (GaAs), silicon carbide

3

(SiC), langasite (LGS), zinc oxide (ZnO), aluminum nitride (AlN), lead zirconium

titanate (PZT), and polyvinylidene fluoride (PVdF).

3.1.1. REQUIREMENT FOR PIEZOELECTICITY:

a) Primary requirement for piezoelectricity is that material must be anisotropic, so that

its properties depend on orientation relative to internal arrangement of atoms.

Anisotropic material does not have constant propagation in all direction of

propagation. When this direction changes, there will be more or less losses occurs,

which thus lead to beam steering and that can degrade the response of band pass filter.

b) As material is anisotropic, the SAW properties depend on the orientation at which

the substrate has been cut from original material.

Fig.2 shows interdigital transducer on piezoelectric substrate

3.2 Interdigital Transducer:

It is made up of thin-film of metal and is fabricated on the piezoelectric substrate. It is

having an interleaved structure as shown in fig.2, in which alternative electrodes are

connected to two pads and one pad is connected to RF power supply and other pad is

connected to ground. Two IDTs are fabricated on a piezoelectric substrate; one which

is use to transmit and another is used to receive the waves. Transmitter IDT converts

the electrical signal into mechanical signal and receiver IDT converts the mechanical

signal into electrical signal. Metallization film thickness for IDT fabrication normally

is in range 500Å to 2000Å. If thickness is less than 500Å, this may lead to poor

electrical conductivity in IDT and there will be more insertion losses.

4

3.3 Shielding Layer:

In sensor applications it is often necessary to cover the sensor surface or the

interacting area of the sensor with a shielding layer. We present a well reproducible

and easy method to shield a sensor surface with an aromatic polyimide layer. The

shielding capacity was tested with surface acoustic wave (SAW) devices which are

commercially available and furthermore meet the requirements for (bio) sensor

applications in aqueous media. All experiments described here were done with these

devices. One major advantage of this technique was the prevention of corrosion

processes on the sensor surface, especially the damage of interdigital transducers

which often consist of aluminium, because this material is known to have the best

acoustoelectric properties for SAW-devices. Besides, we show that a thin polyimide

film enhance the typical sensor characteristics in terms ‘of sensitivity and stability’.

3.2 Absorber:

The reflection of the surface acoustic waves at the edges of a SAW substrate causes

interference with the main signal and degrades the characteristics of the device.

Therefore, the edges of SAW substrates are often provided with wax or another

material which absorbs the surface waves. SAW devices are being fabricated by

sophisticated photolithographic processes, while the deposition techniques for the

absorber material are mainly screen printing or manual painting. The development of

new materials in IC technology gives opportunities for new acoustic absorption films.

A very useful material is polyimide, a viscous organic material which maintains its

absorptive acoustic properties after curing. Polyimide can be spin coated on the wafer

and patterned by photolithographic techniques. Not only thin (0.2–1 μm) but also

thick (up to 20 μm and more) polyimide layers can be fabricated, a feature which

makes the material very attractive for use on SAW devices in the lower frequency

range (80–200 MHz). The layers show good adhesion on the substrate and excellent

absorbing properties.

5

4.OPERATION

When RF signal is applied to the transmitter IDT, then interdigital transducer convert

it into the mechanical waves i.e., acoustic waves. And these waves travel along the

surface of piezoelectric substrate. As the wave passes, each atom of material traces

out an elliptical wave motion.

Fig.3 shows the path trace by particles in presence of

SAW

As shown in fig.3 practical traces an elliptical path as it is having the vibrations in

both horizontal and vertical plane. The atoms move by smaller amount as one look

farther into depth, away from the surface. Thus the wave guided along the surface.

Absorber absorbs the vibrations which degrade the RF signal. It absorbs the unwanted

vibration in various directions and it increases the efficiency.

Piezoelectricity is a great help for transduction. If an electric field is applied to the

surface, corresponding stresses are set up which travel away from the source in the

form of SAWs. The easiest method is to use a set of interleaved electrodes alternately

connected to two bus bars, as in (Figure-2). The left transducer is launching the

waves. When a voltage is applied, the gaps between electrodes have electric fields

and, via the piezoelectric effect, mechanical stresses.

6

The fields and stresses alternate in sign because of the alternating connections of the

electrodes, and the stresses act as sources of surface waves. If the frequency is chosen

such that the saw wavelength equals the transducer pitch, the waves generated by

subsequent gaps are all in phase and therefore reinforce each other. For a given

voltage, a longer transducer will give larger wave amplitude. The transducer on the

right is the same structure but used to receive the waves, i.e. to give an output voltage

in response to an incident wave. It operates in a reciprocal manner to the launching

transducer, so a longer transducer will give a larger voltage for a given saw amplitude.

5.FREQUENCY RESPONSE OF SAW DEVICES

Fig.4hows the frequency response of SAW devices

Surface acoustic waves travels on the elastic surface with frequency given by

F=v/ λ

Where v= velocity of SAW≈ 3100 m/s

The basic advantage of these waves travels at very slow speed≈ 3100 m/s that large delay

can be obtained.

When stress- strain relations are applied to a non-piezoelectric dielectric elastic solid

then application of an electric field to such solid would have no effect on its

mechanical stress and strain characteristics.

7

SAW velocity v will be dependent on the elasticity, density and piezoelectric

properties of substrate used. These all parameters can change with temperature.

Temperature variation will lead to the phase shift around the loop therefore, reducing

stability of oscillator.

For stability, temperature coefficient should be less and for narrow band width and

delay line K² should be low.

K² is electromechanical coupling coefficient. It is a measure of efficiency of given

piezoelectric in converting an applied electrical signal into mechanical energy

associated with a SAW. Here K² and velocity v represents two most important

practical material parameters used in SAW filter design.

6. ACOUSTIC WAVE PROPAGATION MODES

Acoustic wave devices are described by the mode of wave propagation through or on

a piezoelectric substrate. Acoustic waves are distinguished primarily by their

velocities and displacement directions; many combinations are possible, depending on

the material and boundary conditions. The IDT of each sensor provides the electric

field necessary to displace the substrate and thus form an acoustic wave. The wave

propagates through the substrate, where it is converted back to an electric field at the

IDT on the other side. Figure 2 shows the configuration of a typical acoustic wave

device. Transverse, or shear, waves have particle displacements that are normal to the

direction of wave propagation and which can be polarized so that the particle

displacements are either parallel to or normal

to the sensing surface. Shear horizontal wave

motion signifies transverse displacements

polarized parallel to the sensing surface; shear

vertical motion indicates transverse

displacements normal to the surface.

A wave propagating through the substrate is

called a bulk wave. The most commonly used

bulk acoustic wave (BAW) devices are the

Figure 5 Although it is the oldest

acoustic wave device, the thickness

shear mode resonator is still used for

measuring metal deposition rates.

8

thickness shear mode (TSM) resonator and the shear-horizontal acoustic plate mode

(SH-APM) sensor.

If the wave propagates on the surface of the substrate, it is known as a surface wave.

The most widely used surface wave devices are the surface acoustic wave sensor

and the shear-horizontal surface acoustic wave (SH-SAW) sensor, also known as the

surface transverse wave (STW) sensor.

All acoustic wave devices are sensors in that they are sensitive to perturbations of

many different physical parameters. Any change in the characteristics of the path over

which the acoustic wave propagates will result in a change in output. All the sensors

will function in gaseous or vacuum environments, but only a subset of them will

operate efficiently when they are in contact with liquids. The TSM, SH-APM, and

SH-SAW all generate waves that propagate primarily in the shear horizontal motion.

The shear horizontal wave does not radiate appreciable energy into liquids, allowing

liquid operation without excessive damping. Conversely, the SAW sensor has a

substantial surface-normal displacement that radiates compression waves into the

liquid, thus causing excessive damping. An exception to this rule occurs for devices

using waves that propagate at a velocity lower than the sound velocity in the liquid.

Regardless of the displacement components, such modes do not radiate coherently

and are thus relatively undamped by liquids.

7. DELAY LINE

Delay line is basically a filter. Delay line is a SAW device without absorber between

transmitter and receiver IDT. Construction of SAW materials is done in such a way to

produce desired delay. Due to dispersive characteristics of SAW devices, lines offer

wide bandwidths but have high attenuation. Used in RF microwave processing e.g.

pulse compression, spread spectrum systems, etc. 

9

APODIZATION: multiplication of length to some mathematical function so that

desired delay can be achieved.

IF frequency was set at 2.45 GHz because, in that frequency range, many low-cost

components are available, and also to lower the relative bandwidth of the SAW delay

line because bandwidths up to 800 MHz, or even more, are needed. Using a standard

SAW technique with uniform sampled transducers would result in high-insertion

attenuation because the impedance of the interdigital transducers (IDTs) would obtain

bad matching conditions. Furthermore, a lot of small taps would occur from the

compensation procedure (leading to additional losses) because small taps are affected

by strong diffraction effects. The insertion loss of filters with high relative bandwidth

can be reduced using dispersive IDTs. We, therefore, decided to realize the SAW

delay line with two chirped and weighted IDTs.

Fig 6. shows frequency response of delay line

Standard solutions result in filters consisting of an apodized and uniform transducer,

with one or both transducers being dispersive. In this configuration, Fresnel ripple

originating from the uniform transducer leads to problems, therefore, the apodized

transducer has to compensate for this Fresnel ripple.

To overcome this situation, it is advantageous to use two dispersive and amplitude

weighted transducers. There are, however, some drawbacks. The linear design is

difficult because the transfer function of the filter is not even in first order equal to the

product of the transfer functions of both transducers. Compared to configurations

using only one uniform transducer, second order effects, e.g., diffraction, become

10

more severe. In order to compensate for these second-order effects, precise simulation

tools for all relevant second-order effects, e.g., diffraction,

The design and optimization algorithm for SAW linear phase delay lines consisting of

two chirped and weighted

IDTs will be discussed. For high-volume, low-cost, and high yield production of

SAW structures with line widths down to 0.3 μm, technological improvements in the

SAW production technique had to be achieved, especially in photolithography.

Fig7shows SEM photograph of part of the IDT (0.6-_m periodicity and 0.4-_m

Line width).

8. COMPARISION WITH L-C FILTER

Conventional linear phase L-C filters all have some inherent degree of phase

non-linearity .The degrees to which their linearity of phase response is

achieved over a prescribed frequency range increases with order of filter (i-e,

with the number of reactive components).The resultant overall size of passive

LC filter together with its complexity and cost, may render it suitable for many

application.

SAW filter with comparable performance are often tiny by comparison.

Frequency response is better than that of LC filter.

11

SAW devices can not be implement low pass filters but only as band pass one

because of alternative polarities of fingers in SAW IDT.

SAW filters can also be designed to operate very efficiently in harmonic

modes up to about ninth harmonic frequency. This can lead to a considerable

simplification of costly microelectronic lithographic processes required for

gigahertz device fabrication.

9. APPLICATIONS

SAW devices are used as filters, oscillators and transformers, devices that are based

on the transduction of acoustic waves. The transduction from electric energy to

mechanical energy (in the form of SAWs) is accomplished by the use

of piezoelectric materials.Electronic devices employing SAWs normally use one or

more interdigital transducers (IDTs) to convert acoustic waves to electrical signals

and vice versa by exploiting the piezoelectric effect of certain materials

(quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, etc.).[2] These

devices are fabricated by photolithography, the process used in the manufacture of

silicon integrated circuits.

SAW filters are now used in mobile telephones, and provide significant advantages in

performance, cost, and size over other filter technologies such as quartz

crystals (based on bulk waves), LC filters, and waveguide filters. Much research has

been done in the last 20 years in the area of surface acoustic wave sensors. Sensor

applications include all areas of sensing (such as chemical, optical,

thermal, pressure, acceleration, torque and biological). SAW sensors have seen

relatively modest commercial success to date, but are commonly commercially

available for some applications such as touch screen displays.

At one extreme, the high volume low cost TV component market, SAW filters to be

competitive in price and performance with L-C filters employed in IF circuit stages.

At opposite extreme, the low volume high cost components of RADAR signal

processing, maximum emphasis was given to efficient implement of SAW pulse

compression filters with large compression gain.

12

10. MERITS OF RAYLEIGH WAVE DEVICES

SAW devices can generally be designed to provide quite complex signal

processing function within a single package that contains only a single

piezoelectric substrate with superimposed thin film I/O IDT. E.g., band pass

filters with outstanding response that would require several hundred inductors

and capacitors in conventional L-C filters.

They can be mass produced using semiconductor micro fabrication technique.

As a result, they can be made to be cost-competitive in mass-volume

application with some products selling for less than $1.00.

They can have outstanding reproducibility in performance from device to

device. This is especially desirable for design of radio frequency (RF) and IF

filters for mobile phones, narrow band high resonator, as well as for

channelized receivers for spectral analysis or other system requiring precision

filtering.

Although Saw devices are analog variety, they can be employed in many

digital communication systems. e.g., Nyquist filter in quadrature- AM (QAM)

digital radio modem.

SAW filters can be made to operate in high harmonics mode. Some devices

for 2GHz band are now being fabricated in this manner in order to meet

demanding lithographic tolerances and constraints.

11. DEMERITS OF SAW DEVICES

By 1980, SAW technique had reached a plateau in research because of the following

reasons:

Inherently high insertion losses (>6db of existing SAW filter design).

Signal-to-noise limitations.

So, this field was abandoned by many companies and research lab’s around the world.

The preceding slow growth scenario has changed most dramatically in recent years,

13

with discovery and utilization of other types of SAW. These are pseudo-SAW which

made possible by discovery of new piezoelectric substrate and/or crystal cuts.

Pseudo-SAW further classified into:

LSAW (leaky SAW)

SSBW (surface skimming bulk waves)

STW (surface transverse wave)

DEMERITS OF SAW DEVICES AND MERITS OF PSEUDO-SAW DEVICES

Pseudo-SAW and Rayleigh wave devices both may be visually

indistinguishable from one another but pseudo-SAW devices posses a number

of attractive features over their SAW counterparts.

Velocities of pseudo-SAW devices can be much higher than Rayleigh wave

devices and they can be operated upto about 1.6 times higher frequencies than

for Rayleigh wave counterpart with same lithographic geometry.

Pseudo-SAW piezoelectric crystal cuts have much higher values of

electromechanical coupling efficiencies which corresponding increase in

operational band width capabilities in conjunction with lower attainable

insertion loss.

Some pseudo-SAW piezoelectric crystal cuts of quartz can have superior

temperature stability coefficient of delay (TCD) over their SAW counterparts.

LSAW and SSBW propagation is beneath the piezoelectric surface, such

devices can be significantly less sensitive to surface contamination than

Rayleigh wave devices.

As pseudo-SAW penetrates farther into substrate than Rayleigh waves,

corresponding acoustic power densities will be less. That means, pseudo-SAW

devices are capable of handling large powers before onset of IDT degradation

due to violent surface vibrations.

14

12. FABRICATION OF DELAY LINES:

In today’s world, we are going towards miniaturization of circuits so that more

number of circuit elements such as transistors fit in a smaller amount of space. So, we

need photons or electrons to resolve such small dimensions.

Delay lines are fabricated on a wafer by using lithography technique. Lithography is a

process based on the ability of photo resist to store the replica of photo mask that is

used for subsequent processing steps.

e.g., etching, deposition or implantation. Resist protects the oxide layer from etching.

12.1 Lithography

Lithography originally used an image drawn (etched) into a coating of wax or an oily

substance applied to a plate of lithographic stone as the medium to transfer ink to the

blank paper sheet, and so produce a printed page. In modern lithography, the image is

made of a polymer coating applied to a flexible aluminum plate. To print an image

lithographically, the flat surface of the stone plate is (slightly) roughened — etched —

and divided into hydrophilic regions that accept a film of water, and thereby repel the

greasy ink; and hydrophobic regions that repel water and accept ink because

the surface tension is greater on the greasy image area, which remains dry. The image

can be printed directly from the stone plate (the orientation of the image is reversed),

or it can be offset, by transferring the image onto a flexible sheet (rubber) for printing

and publication.

As a printing technology, lithography is different from intaglio printing (gravure),

wherein a plate is either engraved, etched, or stippled to score cavities to contain the

printing ink; and woodblock printing, and letterpress printing, wherein ink is applied

to the raised surfaces of letters or images. Most types of books of high-volume text

are printed with offset lithography, the most common form of printing technology.

Etymologically, the word lithography also denotes photolithography, a micro

fabrication technique used to make integrated circuits and microelectromechanical

systems, as such are more technologically akin to etching than lithography, printing

from a stone plate.

The principle of lithography is: Lithography uses simple chemical processes to

create an image. For instance, the positive part of an image is a hydrophobic, or

15

"water hating" substance, while the negative image would be hydrophilic or "water

loving". Thus, when the plate is introduced to a compatible printing ink and water

mixture, the ink will adhere to the positive image and the water will clean the negative

image. This allows a flat print plate to be used, enabling much longer and more

detailed print runs than the older physical methods of printing (e.g., printing,

Letterpress).

TYPES OF LITHOGREPHY:

LITHOGRAPHY is of various types but usually we use the following techniques of

lithography

ELECTRON BEAM LITHOGRAPHY

PHOTOLITHOGRAPHY

X-RAY LITHOGRAPHY

12.1.1 ELECTRON BEAM LITHOGRAPHY

Many light-based nanotechnology measuring and fabricating tools are limited by the

wavelength of light. However, the smaller the wavelength of light, the higher the

energy of the light, which can subsequently cause unwanted side effects. One way

scientists get around this is to use electrons instead of light. Enter E-beam

lithography. Basically, E-beam lithography consists of shooting a narrow,

concentrated beam of electrons onto a resist coated substrate. Electrons can induce

the deposition of substances onto a surface (additive), or etch away at the surface

(subtractive). E-beam lithography is particularly important in micro electronics,

which require extremely precise placement of

micro sized circuit elements. E-beam lithography allows scientists to design and

place elements at the smallest possible scale. Also, electrons can be used to etch a

“mask” whose patterns can be later transferred onto a substance using other

techniques (think of a stencil you used in grade school). However, with such

precision, components can only be made very slowly and only one at a time, greatly

increasing the time and cost and prohibiting mass commercial acceptance. Also,

because electrons are charged particles, it is necessary to perform E-beam lithography

inside a vacuum, further complicating the required equipment and process.

16

E-beam Components. The process of E-beam lithography is simple, however, the

schematics and the parts required are quite complex. Instead of understanding the

process of E-beam lithography, it is more efficient to understand some of the

important components required for E-beam lithography to work successfully. Electron

Gun: The centerpiece behind E-beam lithography is the electron gun. The specifics of

an electron gun could stretch pages, so it is sufficient to know that the electron gun is

an apparatus that is able to “shoot” a beam of electrons in a specific direction. Two

common E-beam emitters are lanthanum hexaboride crystal and a zirconium oxide

coated tungsten needle. The emitter is first heated to produce and excite electrons on

the surface. Then, when a high voltage is applied, the excited electrons accelerate

towards a structure called the anode. By varying this voltage, the trajectory and the

focus of the beam can be manipulated.

Electron Optical Column: The electron optical column is a system of lenses

that, by a combination of electromagnetism and optics, has the ability to focus the

electrons into a concentrated beam in a desired direction. Two parallel plates inside

the column can be electrostatically charged to a precise degree; the resulting electric

field is able to bend the beam in a desired direction.

Surface: After the beam is directed and concentrated by the optical column, it is

ready to be focused on the surface. As with most lithography techniques, a

substance called a photo resist covers the surface. However, E-beam photo resist are

not as specific as other types. Technically, high energy electron bombardment will

cause bond breakage in any polymer. When the beam hits the surface, either an

additive or subtractive reaction takes place. An additive writing method uses the

electrons to induce a deposition of a compound on the surface. Subtractive writing

methods use the e-beam to remove the sections of the resist and surface. This method

is common in creating masks for other lithographic techniques such as UV

lithography.

17

         

1. Negative

electron

beam resist,

a material

that becomes

insoluble in

developing

solutions

when

exposed to

radiation, is

spun on a

silicon-on-

insulator

(SOI) wafer.

Photonic

structures

are defined

by electron

beam

lithography.

2. The resist is

developed

leaving the

resist mask.

3. Reactive Ion

Etching (RIE)

is used to etch

the mask

pattern into the

top silicon

layer, down to

the buried

oxide layer.

4. The device is

covered with a

cladding of

oxide using for

example

Plasma

Enhanced

Chemical

Vapor

Deposition

(PECVD).

Fig.7 shows electron beam lithography process

18

SCANNING METHODS

Raster Scan: The e-beam is swept across the entire surface, pixel by pixel, with the

beam being turned on and off according to the desired pattern. This method is easy to

design and calibrate, however, because the beam is scanned across the entire surface,

sparse patterns take the same amount of time to write as dense patterns, making this

method inefficient for certain types of patterns.

Fig. 8 shows a) RASTER SCANNING b) VECTOR SCANNING

Vector Scan: The e-beam “jumps” from one patterned area to the next, skipping

unwanted areas. This makes the vector scan much faster than the raster scan for

sparse pattern writing. Adjustments to the beam can also be made relatively easily.

However, it takes longer for the beam to settle, making it more difficult to maintain

accurate placing for the beam.

Disadvantages:

Electron Backscattering and Proximity effects: When electrons are subjected directly

to a surface, they tend to “scatter” quickly. This phenomenon, known as electron

backscattering, causes unwanted reactions to take place outside of the focused

electron beam. As a result, the resolution of an E-beam is not limited to only the size

of the focused beam. In addition to backscattering, the focused Ebeam hitting the

surface produces secondary electrons, which can expose the resist as much as several

micrometers away from the point of exposure. These proximity effects can cause

19

critical variations when dealing with surfaces that need to be exact on the sub-micro

level.

Scattering of electron beam.

Serial processing. i.e., slow and small area processing.

Advantages:

Better resolution.

Direct writing, no mask needed.

Arbitrary size, shape and order.

Efficiency:

While E-beam lithography is perhaps the most accurate and precise of all the

lithographic techniques, perfection comes at a high price. The complex equipment

and slow exposure times makes E-beam lithography impractical as a mass production

micro manufacturing method. Also, because electrons are

charged particles, E-beam lithography must be performed in a vacuum. Steps are

being taken however, in customizing tools such as scanning electron microscopes into

having the ability to produce focused electron beams.

Because of some limitations as mentioned above electron beam lithography is not

preferred in mass fabrication. Hence, optical lithography is used for fabrication of

mask of SAW devices such as delay lines.

20

12.1.2. PHOTOLITHOGRAPHY

OPTICAL LITHOGRAPHY: It consists of a photo mask, an optical system and

photo resist spinned on the top of wafer. It is a process by which geometric shapes on

a mask or wafer are transferred onto a substrate using photons when it is exposed to

light.

PHOTOMASK: Photo mask is a precision glass plate coated with chrome having very

small features of electronic circuit. The substrate (transparent) type used may be

quartz, low expansion glass or soda lime. And several opaque materials are used to

block light and they may be chrome, emulsion, iron oxide.

Fig.9 shows photomasks

STEPS TO GENERATE MASK

First step in mask generation for IC fabrication is to draw large-scale

composite of set of masks, typically 100x to 2000x the final stage.

Then composite layout is converted into a set of oversized artwork with a

drawing for each masking level.

Then the artwork is reduced from 10x glass retical.

The final mask is made from 10x retical using another photo reduction system

that reduces the image to 1x.

Masks are made from glass emulsion plates like kodac high resolution plate or glass

covered with hard surface material. Emulsion masks are least expensive but are

usually only used with feature size in 5 μm region.

All the electron beam masks are made up with hard surface materials such as

chromium, chromium oxide, iron oxide or silicon. These masks are more expensive

than emulsion but feature size in 1 μm region can be defined on the surface of wafer.

21

So, often master mask is made on quartz; then the pattern is transferred to less

expensive glass where it is step and repeated to create several dies.

There are two polarities of mask which are commonly used:

Light field: This type of field is mostly clear.

Bright field: This type of field is mostly dark.

Fields of photomask

Fig.9 shows fields of photomask

PHOTO RESIST: A photo resist is a light-sensitive material used in several

industrial processes, such as photolithography and photoengraving to form a patterned

coating on a surface. It changes is chemical composition when exposed to light. Photo

resist are basically of two types:

Types of photoresist

Fig.10 shows types of photoresist

22

POSITIVE PHOTO RESIST: For positive resists, the resist is exposed with UV

light wherever the underlying material is to be removed. In these resists, exposure to

the UV light changes the chemical structure of the resist so that it becomes more

soluble in the developer. The exposed resist is then washed away by the developer

solution, leaving windows of the bare underlying material. In other words, "whatever

shows, goes." The mask, therefore, contains an exact copy of the pattern which is to

remain on the wafer.

NEGETIVE PHOTO RESIST: Negative resists behave in just the opposite manner.

Exposure to the UV light causes the negative resist to become polymerized, and more

difficult to dissolve. Therefore, the negative resist remains on the surface wherever it

is exposed, and the developer solution removes only the unexposed portions. Masks

used for negative photo resists, therefore, contain the inverse (or photographic

"negative") of the pattern to be transferred. The figure below shows the pattern

differences generated from the use of positive and negative resist.

Fig.11 shows processing of photoresist

23

DIFFERNCE BETWEEN POSITIVE AND NEGATIVE

PHOTORESIST

POSITIVE PHOTO RESIST

It becomes more soluble when exposed to light.

Solubility in developer is finite even at zero exposure energy.

At some threshold it becomes completely soluble.

Longer exposure time and throughput is less.

NEGETIVE PHOTO RESIST

It becomes less soluble when expose to light.

At low energies it remains completely soluble in the developer.

Exposure above threshold energy is increased, more the resist film remain

after development.

Exposure time is less and high throughput.

This figure shows the graph for film thickness versus exposure dose for a) positive

resist and b) negative resist. Contrast is defined as the slope of the linear portion of

falling (or rising) section of curve. Curves are affected by:

Initial resist thickness, spectral distribution of exposure radiation, prebake conditions,

developer chemistry, developing time and so on.

Fig.12 shows graph of positive and negative photoresist

24

COMPONENTS OF PHOTO RESIST

Matrix material (Novolac resin).

Sensitizer (or dissolution inhibitor) – diazoquinones.

Solvent ( n-butyl acetate, xylene)

MATRIX MATERIAL

The binder is a Novolac resin which has the following key properties:

Inert to radiation.

Provides good adhesion to substrate.

Etch resistant in wet and dry “etchers”.

Fig.13 shows chemical composition of matrix material

25

SENSITIZERS

Sensitizers are photoactive compounds (PAC) – diazoquinones.

Sensitizers absorb chemical reaction to dissolution properties in the resist.

The net result is differential (100:1) between areas that absorbed radiation.

Sensitizers and Developer resistant before they absorb radiation – typical resist

developer and hydroxides- KOH, NaOH, TMAH, etc.

Fig. 14 shows SENSITIZER REACTIONS after UV Exposure

26

SOLVENT

XYLENE

o Keeps photo resist in liquid state.

o Allows spin coating of photo resist.

o Solvent content determines viscosity and hence thickness.

Fig.15 shows chemical composition of solvent

PHOTORESIST METRICS

Resolution

Contrast

Sensitivity

Spectral Response Curve

RESOLUTION: How a fine line the resist can produce from an areal image.

Resolution of resist is determined by

o Contrast, Thickness, Proximity effects

o Swelling and contraction after development.

27

CONTRAST: It is an ability of resist to distinguish between light and dark

regions.

o Measured by exposing the resist of given thickness to varying

radiation dose and measuring dissolution rate.

Fig.16 shows contrast curve

SENSITIVITY:

o Incident energy necessary to produce the photochemical

reactions required for defining patterns.

o Related to quantum yield = ( # of photon- induced events)

# of photon absorbed

o Higher sensitivity required at shorter wavelength because

of limited brightness of UV sources and optics efficiency.

o Trade-off between exposure time and brightness.

28

SPECTRAL RESPONSE: Many photo resist types exist to suit a large

variety of applications, and the action spectra can vary from one to the next. It is

critical to control the action spectra exposure settings, both the intensity of the light

and the time of exposure, to avoid wasteful under or overexposure of the photo

resist during processing and production.

To maintain these settings, a light meter with a spectral response as close to the photo

resist’s action spectrum as possible is required to get an accurate idea of how well

the photo resist is being exposed. Further complicating matters is the processing

equipment where photo resists are exposed are frequently very compact and have

minimal room for taking these measurements.

International Light Technologies is committed to providing a variety of unique, photo

resist-specific instruments to assist our customers in making these often difficult

measurements.

Fig.17 shows spectral response of photoresist

The typical emission spectrum of a mask aligner or stepper with Hg light source and

without optical selective mirrors/filters contains g- (wavelength 436 nm), h- (405 nm)

29

and i-line (365 nm), with an i-line intensity approx. 40 % of the total emission

between 440 and 340 nm.

Especially for exposure dose sensitive processes (image reversal-, thick resist

processing, high resolution) a calibration of the illumination intensity (changing with

bulb operating time) is strongly recommended. A measurement of the lateral intensity

distribution should reveal less then 10 % deviation over the substrate size in order to

allow a proper exposure dose for central and edge-near regions of the resist film.

HOW PHOTOMASKS ARE MADE

CIRCUIT DESIGN:

The customer designs the circuit and digitizes the information. The customer then

sends us the digitized data containing the design for each layer. The data can be sent

on a floppy disk, magnetic tape, cassette or via modem.

CUSTOMER DATA FRACTURED DATA

Fig. 18 shows types of design

DATA PREPARATION:

Photronics takes the customer's data and formats it for the write (lithography)

tools or systems. This includes fracturing the data, sizing the data if needed,

rotating the data if needed, adding fiducials and internal reference marks, and

30

making a job deck (instructions for the placement of all the different patterns on the

mask).

Fracturing the data means translating the customer data into a language the write tool

can understand. The write system uses rectangles and trapezoids - so the customer

data is divided up (fractured) into these shapes.

The job deck with the fractured data is put on a magnetic tape and sent to the write

area or pulled directly to the machines using network software.

PROCESSING OF PHOTO RESIST

Photo resist layers have two basic functions: 1) precise pattern formation; and

2) protection of the substrate from chemical attack during the etch process. Typical

resists consist of three components: 1) the resin, which serves as the binder of the

film; 2) the inhibitor or sensitizer, which is the photoactive ingredient; and 3)

the solvent, which keeps the resist in liquid state until it is processed. The pattern

formed by the resist layer actually results only after the unwanted portions of the

erstwhile uniformly distributed resist have been removed, as explained in the

following discussion on photo resist processing.

Photos resist processing, or simply resist processing, basically consists of six steps:

1) dehydration and priming; 2) resist coating; 3) soft baking; 4) exposure; 5)

development; and 6) post-development inspection.

   

Prior to the application of resist to a wafer, the wafer must be free of moisture and

contaminants, both of which cause a multitude of resist processing

problems. Dehydration baking is performed to eliminate any moisture adsorbed by

substrate surfaces, since hydrated substrates result in adhesion failures. The bake is

usually performed between 400 deg C to 800 deg C.  Convection ovens may be used

for baking up to 400 deg C, while furnace tubes are used for 800 deg C baking. After

dehydration baking, the wafer is coated with a pre-resist priming layer designed to

enhance the adhesion properties of the wafer even further. One of the most common

primers used for this purpose is hexamethyldisilazane (HMDS). Resist coating must

follow as soon as possible after priming (within an hour after priming).

31

   

Resist coating, or the process itself of producing a uniform, adherent, and defect-free

resist film of correct thickness over the wafer, is usually performed by spin-

coating. Spin-coating consists of dispensing the resist solution over the wafer surface

and rapidly spinning the wafer until it becomes dry. Most spin-coating processes are

conducted at final spin speeds of 3000-7000 rpm for duration of 20-30 seconds.

 Resist coating is followed by a soft bake, which is done to: 1) drive away the solvent

from the spun-on resist; 2) improve the adhesion of the resist to the wafer;  and 3)

anneal the shear stresses introduced during the spin-coating. Soft baking may be

performed using one of several types of ovens (e.g., convection, IR, hot plate). Soft-

bake ovens must provide well-controlled and uniformly distributed temperatures and

a bake environment that possesses a high degree of cleanliness.  The recommended

temperature range for soft baking is between 90-100 deg C, while the exposure time

needs to be established based on the heating method used and the resulting properties

of the soft-baked resist.

 After a wafer has been coated with photo resist and subjected to soft baking, it has to

undergo exposure to some form of radiation that will produce the pattern image on

the resist.  The pattern is formed on the wafer using a mask, which defines which

areas of the resist surface, will be exposed to radiation and those that will be

covered.  The chemical properties of the resist regions struck by radiation change in

a manner that depends on the type of resist used.  Irradiated regions of positive photo

resists will become more soluble in the developer, so positive resists form a positive

image of the mask on the wafer.  Negative resists form a negative image of the mask

on the wafer because the exposed regions become less soluble in the developer.

 Development, which is the process step that follows resist exposure, is done to leave

behind the correct resist pattern on the wafer which will serve as the physical mask

that covers areas on the wafer that need to be protected from chemical attack during

subsequent etching, implantation, lift-off, and the like.  The development process

involves chemical reactions wherein unprotected parts of the resist get dissolved in

the developer. A good development process has a short duration (less than a minute),

results in minimum pattern distortion or swelling, keeps the original film thickness of

protected areas intact, and recreates the intended pattern faithfully. 

                    

32

Development is carried out either by immersion developing, spray developing, or

puddle developing. Regardless of method used, it should always be followed by

thorough rinsing and drying to ensure that the development action will not continue

after the developer has been removed from the wafer surface. 

 Post-development inspection, as the name implies, is an inspection conducted after

development to ensure that the resist processing steps conducted earlier have

produced the desired results.  This is typically done using an optical microscope,

although SEM and laser-based systems are also used in some post-development

inspection tasks. Items that this inspection step checks for include the following: 1)

use of the correct mask; 2) resist film quality; 3) adequate image definition; 4)

dimensions of critical features; 5) defects and their densities; and 6) Pattern

registration.

HOW ARE PHOTOMASKS USED?

The steps in making (fabricating) devices like ICs include deposition,

photolithography, and etching. During deposition, a layer of either electrically

insulating or electrically conductive material (i.e. metal, polysilicon, oxide) is

deposited on the surface of a silicon wafer. This material is then coated with a

photosensitive resist. A photomask is then used much the same way a photographic

negative is used to make a photo. Photolithography involves projecting the image on

the photomask onto the wafer. (If the image on the photomask is projected several

times side by side onto the wafer, this is known as stepping and the photomask is

called a reticle.)

33

The wafer is developed and then etched to remove material from the areas

exposed with the photomask image.

PARTS OF PHOTOMASK

DIE

A die is a single complete device image. A primary die (also called the primary

pattern) has the device design that will be used to make the circuit. The primary die or

dies will make up most of the array or fields. A test die contains a simplified

functional device (of the same process type as the primary die). The test die is used

for process control and monitoring during wafer fabrication and sometimes to test

new design ideas.

Fig.19.1 shows die of photomask

34

SCRIBE LINES

Scribe lines (scribes) are the lines forming a border around each die separating the

dies from one another. There may be small patterns placed within these

scribes,usually alignment marks or test patterns used in wafer fabrication.

Fig. 19.2 shows scribe lines

ARRAY

The array is the area made up of the rows and columns of dies on 1X masters, 5X

reticles and other reduction reticles.

Fig. 19.3 shows array of photomask

35

FIELD

Fields are the blocks of dies on a UT1X reticle. Primary fields contain primary dies. A

test field(s) contains test dies and/or patterns and usually some primary dies.

ROW

A row consist of a horizontal line of fields on a UT1X reticle

36

FIDUCIALS

Fiducials are patterns on reticles used for alignment on wafer steppers. Each

brand and model of stepper has specific types of fiducials. The fiducials are located

outside of the array or fields.

Fig. 19.4 shows fudicials in photomask

KLA/KLARIS REFERENCE MARKS

KLA/KLARIS reference marks are internal marks placed in the corners of the

mask. These marks are used as a points of reference when setting up inspections on an

inspection system (such as a KLA system) and when repairing defects found in such

inspections. The design and placement of these reference marks may vary from one

manufacturing site to another.

AUTO-INSPECTION MARKS (KLA MARKS)

Auto-Inspection Marks (KLA Marks) are used on UT1X reticles and located

outside each field. These marks are crosses defining the area to be inspected on the

KLA or other inspection systems.

37

CLOSURE CHECKS

Closure Checks are sets of patterns used to monitor the performance and

accuracy of the lithography equipment. These marks are located outside the array. The

closure check consists of several patterns; half of a pattern is written at the beginning

of the write process and the second half of a pattern is written at the end of the write

process. Comparing the two halves of the patterns can show any shifts or

discrepancies that have occurred while the mask was being written. The actual

patterns used in a closure check may vary from one manufacturing site to another.

BLANKS and AUXILIARIES (AUX.'S)

Blanks and Auxiliaries (Aux.'s) are used on 1X masters and UT1X reticles.

A blank is a pattern, containing no circuitry, used to clear out a specified window

area.

An auxiliary is a pattern, containing no circuitry, which surrounds a smaller pattern in

order to clear out a specified window area and isolate the smaller pattern.

TYPES OF PHOTOMASK

Masters (used in MultiMEMS):

►Used on projection aligners.

►Patterns are projected: only once onto the wafer, with no reduction (1X).

►The repetitions of the device are on the photomask.

38

Reticles (not used in MultiMEMS):

►Used on steppers.

►Patterns are projected: many times onto the wafer, with reduction (2X, 4X, 5X,

10X).

►The repetitions of the device are done by stepping the projections

QUALITY INSPECTION ROUTINE

In order for the circuits being made from the photomask imagery to function, the

photomasks must meet certain quality standards. The masks are tested for critical

dimensions, defects, registration, and contamination before being sent to the wafer

fab.

Checking the Critical Dimensions (CD).

Defect Inspection.

Data Verification.

Registration.

Contamination.

CRITICAL DIMENSIONS

Customers have very specific requirements for geometry sizes and specify

certain geometry on the mask to be used as a gauge. These geometries are called

critical dimensions or CDs. The customer indicates the target size or spec of the CDs

in microns (a thousand microns equals a millimeter) and the acceptable variance from

that target (the tolerance).

39

The amount of time a mask is processed (developed and etched) after the mask is

written is important because it affects the size of the geometry. The longer a mask is

processed the smaller the chrome geometry will be and the larger the clear areas will

be. CDs are measured on a microscope during the processing of a mask to determine

process times and to make sure the CD size is acceptable. (Sometimes CDs will also

be measured again on registration equipment).

Fig. 20.1 shows critical dimension geometry

DEFECT INSPECTION

A defect is any flaw affecting the geometry. This includes chrome where it should not

be (chrome spots, chrome extensions, chrome bridging between geometry) or

unwanted clear areas (pin holes, clear extensions, clear breaks). A defect can cause

the customer's circuit not to function. The customer will indicate the size of defects

that will affect their process (defect spec). All defects that size and larger must be

repaired, or if they can not be repaired the mask must be rejected and rewritten.

There is also a class of defects known as cosmetic defects. These are defects that may

not affect the circuitry geometry but still may not be acceptable to the customer.

Cosmetic defects include scratches on the chrome outside the array, damaged or

partially removed AR coating, contamination on the chrome, glass chips on the edge

of the mask, etc.

Fig. 20.2 shows defects of photomask

40

A die-to-die inspection system (KLA) is used to inspect for defects. These

machines use two objectives and transmitted (bottom) light to compare similar die

patterns to one another.

The image seen through the objectives is divided into pixels. The information in each

pixel is digitized and compared to the information found in the other objective. If the

pixels do not match the machine registers a discrepancy. This process is known as

image processing.

After the inspection is completed, an operator must view each discrepancy and

determine which kind of defect was found and give the defect a code number or

classification.

DATA VERIFICATION

A die-to-database (KLARIS or Orbot) inspection is similar to a die-to-die

inspection, except instead of comparing a die to another die it is compared to a

database. This inspection is used to insure that the geometry on the mask matches the

customer's design.

The image seen through the objective is compared to the digitized image on the

database. If the images do not match the system registers a discrepancy. Again when

the inspection is complete, an operator must review each discrepancy and classify it.

Database image Right objective

Fig. 20.3 shows images for data verification

41

REGISTRATION

Registration equipment (LMS 2000 and Nikon 2i) is used to measure the positioning

of patterns on a mask in relation to other layers in the device or to a design grid. This

can include checking the size, placement, and rotation of dies, fields, and arrays. This

equipment can also be used to measure the placement of fiducials and measure CDs.

Fig. 20.4 shows types of registration inspections:

DIE FIT ARRAY REGISTRATION

a) comparing the size, b) comparing the size,

distortion and rotation distortion and rotation

of the die on mask of the die on the array

to the design grid to the design grid

CONTAMINATION

Contamination on a mask will have the same ill effect as a chrome defect when

projected onto the wafer. That is why photomasks are made and used in cleanroom

environments. Before being shipped to the customer the photomask must be carefully

cleaned and then inspected for contamination. Pellicles (metal frames with a

protective membrane) are often attached to the masks to help keep them clean.

Photronics uses four ways to inspect for contamination:

42

1) Gross Light Inspection - the mask is inspected under a high intensity light for gross

contamination (contamination that can be seen with the naked eye).

2) Microscope Inspection - an operator manually inspects the mask on a

microscope using a high power (50 to 200 times) magnification .

3) Q.C. Optics or KLA Starlight Inspection - an automated inspection on a Q.C.

Optics or a Starlight. These machines inspect for contamination using deflected light

to detect any particles on the surface of the mask or pellicle.

4) Post-pell die-to-die or die-to-database inspections - an automated inspection on a

KLA or Orbot defect inspection system. Though the KLA and Orbot systems are used

to detect defects they will also detect contamination in the clear areas on the mask.

Post-pell (post-pellicle) inspections are performed after the mask has been cleaned

and has had a pellicle attached to it.

OTHER QUALITY INSPECTIONS

Some other quality inspections are performed by operators. These inspections include

verifying that the mask has the correct titles, fiducials, field tone, data parity, array

rotation and has no cosmetic defects. Manual microscope inspections are used to look

for defects in any areas that could not be inspected on automated equipment, this

includes test patterns, scribes, and fiducials. Sometimes a microscope is also used to

take array and fiducial placement measurements.

43

13.PHOTOLITHOGRAPHY PROCESSING

Fig. 21 shows the different process used for PHOTO RESIST PROCESSING

CLEAN: Before the processing, the surface of resist is made clean in which

moisture and oxides are removed from the surface of photo resist. To drive off the

moisture, it is baked at temperature between 150- 200 degree Celsius.

SPIN COATING: Spin coating is the preferred method for application of thin,

uniform films to flat substrates. An excess amount of polymer solution is placed on

the substrate. The substrate is then rotated at high speed in order to spread the fluid

by centrifugal force. Rotation is continued for some time, with fluid being spun off

the edges of the substrate, until the desired film thickness is achieved. The solvent is

usually volatile, providing for its simultaneous evaporation.

SOFT BAKE: After coating, the resist film has a thickness-dependant remaining

solvent concentration (e.g. PGMEA). Soft baking minimizes the solvent concentration

in order to:

- avoid mask contamination or sticking to the mask,

- prevent bubbling or foaming by N2 during exposure,

- improve the resist adhesion to the substrate,

- minimize the dark erosion during development,

- prevent dissolving one resist layer by a following during multiple coating, and

- prevent bubbling during subsequent thermal processes (coating, dry etching).

44

After spin coating, the typical average remaining PGMEA concentration in the resist

film is between 20% (thin films) and 40% (thick films). Soft baking reduces the

remaining solvent concentration in the resist by thermally activated solvent diffusion

in the resist bulk, and evaporation as a function of the temperature and the resist

surface solvent concentration. A good starting point for a sufficient softbake is

100°C hotplate for 1 minute/μm resist film thickness.

Insufficiently soft baked resist films reveal high dark erosion during development

(resist thinning), with the remaining resist structures too small and less sharp than

desired.

Beside solvent reduction, a long/hot softbake thermally decomposes a part of

the photo active compound (DNQ-sulfonate) thus reducing the development rate.

Therefore, despite lower dark erosion, the longer necessary development rate may

increase the total dark erosion.

Compared to a hotplate, the much more distinct temperature hysteresis of an oven

as well as the different heat transfer mechanism (convection in stead of heat

conduction) causes - especially for short (few minutes) - baking steps (softbake,

reversal bake, hard bake) or substrates with high heat capacity (thick glasses and

ceramics) different effective temperatures in the photo resist and time intervals for

the required final temperature.

EXPOSE:

During exposure with matched UV-light, the photo active compound

DiazoNaphtoQuinone- (DNQ-) sulfonate (left) releases a N2 molecule and converts

into indene carboxylic acid hereby requiring a H2O molecule. Compared to unexposed

DNQ-sulfonate, the carboxylic acid yields a resist development rate (alkaline

45

solubility) several orders of magnitude higher. In order to i) improve the DNQ

solubility in the resist, ii) to increase the inhibitor property (dark erosion reduction),

and iii) to improve the thermal stability, generally several DNQ-sulfonate molecules

are bonded to a so-called backbone-molecule.

The photo reaction quantum efficiency defines the number of above-mentioned

reactions in relation to the photons absorbed in the resist film. Using a sufficiently

transparent resin, suited photon energy (g, h, i-line with respect to the specific DNQ)

and a sufficient H2O-concentration in the resist (rehydration!), the quantum efficiency

in DNQ-based positive-tone photo resists achieves values of typically 20-30%.

If the resist lacks a minimum concentration of water, the ketone formed during the

photoreaction may perform various side reactions (e.g. esterify with the resin or

polymerize accompanied by CO2 separation). In both cases, the development rate

solely increases by the reduction of the inhibitor (DNQ-sulfonate) concentration.

POST EXPOSURE BAKES:

The post exposure bake PEB (performed after exposure, but before development) can

be applied above the softening point of the resist without destroying the structures to

be developed due to the still closed resist film. Possible reasons for a PEB (typically

performed at 110°C for 1-2 min on a hotplate) are:

In chemically amplified resists, the PEB catalytically performs and completes the

photo reaction initiated during exposure. The AZ® and TI resists distributed by Micro

Chemicals® do not belong to chemically amplified resists, and therefore do not require

a PEB for this purpose.

A PEB performed near the softening point of the photo resist reduces mechanical

stress formed during softbake and exposure of especially thick resist films due to the

expanding nitrogen and therefore improves resist adhesion and reduces under etching

in subsequent wet chemical etching. However, a certain delay between exposure and

PEB is required to outgas N2. Otherwise, during PEB the N2 in the resist will expand

and increase mechanical stress in the film!

The PEB promotes the thermally activated diffusion of carboxylic acid formed during

exposure from the photo active compound. This diffusion step smoothens the spatial

periodic pattern of carboxylic acid having their origin in standing light waves during

monochromatic exposure especially in case of highly reflective substrates. These

46

patterns otherwise would transfer to the resist profile thus e.g. reducing the spatial

resolution of the resist and the desired aspect ratio.

In many cases, for processes without high (<1 micron) resolutions required, the PEB

is not necessary. However, for certain (negative tone) resists such as the AZ® 2000

nLOF family, a PEB is inevitable for the crosslinking induced by the exposure.

DEVELOP: The carboxylic acid formed during exposure moves from the

hydrophobic to the hydrophilic part of the cresol chain and promotes the

deprotonisation of the OH-group increasing the resist solubility in aqueous alkaline

developers.

Beside the carboxylic acid, also acetic acid (formed by alkaline developers from the

resist solvent PGMEA) increases the development rate of exposed and unexposed

resist. This explains the higher dark erosion of resist with a remaining solvent

concentration too high (e.g. in case of an insufficient softbake).

HARD BAKE:

The hard bake sometimes performed after development intends to increase

the thermal, chemical, and physical stability of developed resist structures for

subsequent processes (e.g. electroplating, wet-chemical and dry-chemical etching).

Hereby the following mechanisms have to be considered:

Coated photo resists react with atmospheric oxygen and embrittle above approx.

130°C. The different thermal expansion coefficient of resist/substrate can lead to the

cracking of the resist making it useless as mask for wet or drying-chemical etching or

electroplating. If the hard bake cannot be waived, nor the hard bake temperature

reduced, the cracking can be suppressed by a slow cooling (e.g. -3°C/min ramp, or by

keeping the substrate in/on the switched-off oven/hotplate for soft cooling, if

feasible).

ETCH: ETCHING is done by two methods:

WET ETCHING:

Inferior resist adhesion causes lifting of the resist starting from the resist edges, with

subsequent under etching. In exothermic etching solutions, this resist lifting is

promoted by local heating and may further be promoted by mechanical forces if e.g.

H2 evolution occurs. Resist adhesion can be improved by i) using a suited resist, ii) a

47

proper substrate pretreatment (e.g. TI PRIME), and iii)

optimized processing (sufficient softbake, image reversal bake). On glass and

ceramics, a hard bake is not generally recommended due to the different heat

expansion coefficients of resist and substrate, which might cause crackles in the resist.

In isotropic etch media, even optimized resist adhesion results in under etching. If

convection and diffusion of the etched medium is not the etch rate bottleneck, the

lateral under etching near the substrate/resist interface is comparable to the etched

depth.

DRY ETCHING:

In plasma, UV radiation with spectral lines within the absorption range (g-, h-, and i-

line) of photo resists might expose the resist during etching causing N2 at a high rate

forming bubbles and foaming at elevated temperatures. In this case, a flood

exposure (without mask, with subsequent delay to outgas N2) or/and the usage of

image reversal resists in the reversal mode will help. Also a good alternative is the

usage of resists with a high thermal stability and without gas formation during

exposure like AZ® nLOF 2000 series, which can be developed in aqueous alkaline

solutions, and removed in O-plasma or, alternatively, in organic solvents such as

acetone or NMP.

STRIPPING: Photo resist stripping, or simply 'resist stripping', is the removal of

unwanted photo resist layers from the wafer.  Its objective is to eliminate the photo

resist material from the wafer as quickly as possible, without allowing any surface

materials under the resist to get attacked by the chemicals used.  Resist stripping can

be classified into: 1) organic stripping; 2) inorganic stripping; and 3) dry stripping. 

Organic stripping employs organic strippers, which are chemicals that break down

the structure of the resist layer. The most widely-used commercially available

organic strippers used to be the phenol-based ones, but their short pot life and

difficulties with phenol disposal made low-phenol or phenol-free organic strippers

the more popular choice nowadays.

Wet inorganic strippers, which are also known as oxidizing-type strippers, are used

for inorganic stripping, usually to remove photo resist from non-metalized wafers, as

well as post-baked and other hard-to-remove resists.  Inorganic strippers are

48

solutions of sulfuric acid and an oxidant (such as ammonium persulfate), heated to

about 125 deg C.

 Dry stripping pertains to the removal of photo resist by dry etching using plasma

etching equipment.  Its advantages over wet etching with organic or inorganic

strippers include better safety, absence of metal ion contamination,

decreased pollution issues, and less tendency to attach underlying substrate layers. 

    

13.1 Mask Alignment and Exposure

One of the most important steps in the photolithography process is mask alignment. A

mask or "photomask" is a square glass plate with a patterned emulsion of metal film

on one side. The mask is aligned with the wafer, so that the pattern can be transferred

onto the wafer surface. Each mask after the first one must be aligned to the previous

pattern.

Once the mask has been accurately aligned with the pattern on the wafer's surface, the

photo resist is exposed through the pattern on the mask with a high intensity

ultraviolet light. There are three primary exposure methods: contact, proximity, and

projection. They are shown in the figure below.

49

Fig. 22 shows types of PHOTOLITHOGRAPHY

Contact Printing

In contact printing, the resist-coated silicon wafer is brought into physical contact

with the glass photomask. The wafer is held on a vacuum chuck, and the whole

assembly rises until the wafer and mask contact each other. The photo resist is

exposed with UV light while the wafer is in contact position with the mask. Because

of the contact between the resist and mask, very high resolution is possible in contact

printing (e.g. 1-micron features in 0.5 microns of positive resist). The problem with

contact printing is that debris, trapped between the resist and the mask, can damage

the mask and cause defects in the pattern.

Proximity Printing

The proximity exposure method is similar to contact printing except that a small gap,

10 to 25 microns wide, is maintained between the wafer and the mask during

exposure. This gap minimizes (but may not eliminate) mask damage. Approximately

2- to 4-micron resolution is possible with proximity printing.

50

Projection Printing

Projection printing, avoids mask damage entirely. An image of the patterns on the

mask is projected onto the resist-coated wafer, which are many centimeters away. In

order to achieve high resolution, only a small portion of the mask is imaged. This

small image field is scanned or stepped over the surface of the wafer. Projection

printers that step the mask image over the wafer surface are called step-and-repeat

systems. Step-and-repeat projection printers are capable of approximately 1-micron

resolution.

13.2 Exposure of Photoresists

When using a laser beam as light source for photoresist exposure in stead of the

usually used Hg bulbs, one has to consider two main points:

The light intensity is quite different: While in laser interference lithography, the light

intensity is rather low, laser scribing causes intensities many orders of magnitude

beyond the intensity of a mask aligner or stepper. The exposure wavelengths of the

laser often differs from the 365, 405, or 435 nm Hg lines which are matched to the

spectral sensitivity of photoresists. The spectral sensitivity of photoresists does not

abruptly end at a certain wavelength, but smoothly drops to zero over few 10 nm.

Therefore, using an adjusted exposure dose (e. g. laser scribing), exposure with

wavelengths outside the sensitivity range given in the technical data sheet is also

possible to a certain extent.

51

16. FABRICATION OF DELAY LINE:

Fabrication of delay line can be done by using photolithography process. And which

can be achieved by these following processes:

Wafer(LiTaO3 or LiNbO3, or SiO2)

Al deposit (sputtering) 0.15 to 1.5 micron.

Photo resist (PR coating)

Exposure

Develop

Al etching (wet etching)

PR removal

QC check + Probing

Sieving (Scribing)

QC check (chips, cracks)

Mounting Ag/ UV bond

Wire bounding

Seam sealing

Marking

Final test and inspections

52

Fig. 23 shows fabrication of SAW device

Optical Absorption and Spectral Sensitivity

The optical absorption of most unexposed photoresist ranges from the approx. 440 nm

in the VIS to near UV. This spectral sensitivity is matched to the emission spectrum

of Hg lamps (i-line = 365 nm, h-line = 405 nm, g-line = 435 nm) in mask aligners

left-hand) and causes the typical reddish-brownish color of many photoresists.

53

18. CONCLUSION

The fabrication of Saw delay line is done successfully on a piezoelectric elastic

substrate. Mask was prepared and complete design was transferred on piezoelectric

bulk. The structure can be used for calibration of SAW DELAY LINE.

54

Table of Contents

SAW DEVICES.............................................................................................................1

1. INTRODUCTION.....................................................................................................1

2. DEFINATIONS OF SAW........................................................................................2

3. CONSTRUCTION.....................................................................................................3

3.1 Piezoelectric Substrate Materials..................................................................................33.1.1. REQUIREMENT FOR PIEZOELECTICITY.......................................................................4

3.2 Interdigital Transducer................................................................................................4

3.3 Shielding Layer:.............................................................................................................5

3.2 Absorber:.......................................................................................................................5

4.OPERATION..............................................................................................................6

5.FREQUENCY RESPONSE OF SAW DEVICES....................................................7

6. ACOUSTIC WAVE PROPAGATION MODES.......................................................8

7. DELAY LINE..........................................................................................................10

8. COMPARISION WITH L-C FILTER...............................................................12

9. APPLICATIONS....................................................................................................12

10. MERITS OF RAYLEIGH WAVE DEVICES.................................................13

11. DEMERITS OF SAW DEVICES.....................................................................14

12. FABRICATION OF DELAY LINES:..................................................................15

12.1 Lithography................................................................................................................15

TYPES OF LITHOGREPHY:..........................................................................................16

12.1.1 ELECTRON BEAM LITHOGRAPHY...........................................................16

SCANNING METHODS...................................................................................................19

Disadvantages:............................................................................................................19

Advantages:.................................................................................................................20

Efficiency:....................................................................................................................20

12.1.2. PHOTOLITHOGRAPHY...............................................................................21

DIFFERNCE BETWEEN POSITIVE AND NEGATIVE PHOTORESIST...............24

COMPONENTS OF PHOTO RESIST............................................................................25

Fig.15 shows chemical composition of solvent..........................................................27

PHOTORESIST METRICS..............................................................................................27

HOW PHOTOMASKS ARE MADE...............................................................................30

CIRCUIT DESIGN:...........................................................................................................30

DATA PREPARATION:...................................................................................................30

55

PROCESSING OF PHOTO RESIST.........................................................................31

HOW ARE PHOTOMASKS USED?...............................................................................33

PARTS OF PHOTOMASK...............................................................................................34

TYPES OF PHOTOMASK...............................................................................................38

QUALITY INSPECTION ROUTINE..............................................................................39CRITICAL DIMENSIONS............................................................................................................39DEFECT INSPECTION................................................................................................................40DATA VERIFICATION................................................................................................................41REGISTRATION...........................................................................................................................42

CONTAMINATION...........................................................................................................42

OTHER QUALITY INSPECTIONS................................................................................43

13.PHOTOLITHOGRAPHY PROCESSING............................................................44

13.1 Mask Alignment and Exposure..........................................................................49Contact Printing..............................................................................................................................50Proximity Printing..........................................................................................................................50Projection Printing..........................................................................................................................51

13.2 Exposure of Photoresists............................................................................................51

16. FABRICATION OF DELAY LINE:...................................................................52

Optical Absorption and Spectral Sensitivity...............................................................53

18. CONCLUSION.....................................................................................................54

56