EMT 432/4

MEMS DESIGN AND FABRICATION

PENGUMUMAN

Taklimat Final Year Project

Tarikh: 17 Julai 2006 (Isnin)Masa: 2:00 PetangTempat: DKW1 (Dewan Keikhlasan KWSP)

Text books and references

Text book1. Tai-Ran Hsu, MEMS and Microsystems; Design and Manufacture, Boston, Mc Graw Hill 20022. Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Prentice Hall, 2001

Other References1. Nadim Maluf, An Introduction to Microelectromechanical System Engineering, Artech House MEMS Library, 2nd edition 19992. Peter Van Zant, Microchip Fabrication: A Practical Guide to Semiconductor Processing, Mc Graw Hill 20003. Hwaiyu Geng, Semiconductor Manufacturing Handbook, Mc Graw Hill, 2005

1st QUIZZ (out of 5 QUIZZES) – 3% of overall course evaluation

Write an essay titled “MEMS Industry in Malaysia”.

Free format, based on facts and figures. There are a lot of informationon the internet ! Please include;

• The industry background, present status • Major players• Products, with special attention to at least 2 products• Technology involved• Future trend (market and technology)

1. Font: Arial narrow size 11, single spacing2. Language: Preferable in English3. Length of the essay: around 5 pages (A4)4. To be submitted on 27 July 2006 (Ramzan)

Lecture 2:

Fundamental of Microsystem Design and Fabrication

1. Atomic Structure and Periodic Table2. Ion and Ionization3. Doping of Semiconductors4. Diffusion Process5. Plasma Physics6. Quantum Physics

ATOMIC STRUCTURE AND THE PERIODIC TABLE

protonnucleus

neutron

electron

• atom is the basic of substance• everything on earth is made of from 96 stable and 12 unstable elements• each element has a different atomic structure• solution to Schrodinger equation for the hydrogen atom leads to Bohr model• basic atomic structure consists of;

• nucleus (proton and neutron)• orbiting electron(s) or valence or free e

• the difference in atomic structure result in different properties of the elements.• neutrons carry no electrical charge• protons are positively charged, electrons are negatively charged• in neutral state, the number of protons and electrons are equal• ATOMIC NUMBER of element is the total number of electrons in respective atom

Hydrogen atom

nucleus

Lithium atom

Si atom• atoms normally contain more than 1 electron• electrons in atoms can exist in more than 1 energy level or orbit (concept of quantum, orbits n, l, m & s)• electrons could move from one orbit to another to keep it in natural state e will be excited to a higher energy state when given enough energy, return to a lower state while releasing the energy in the form of rays (LED)• materials with highly mobile electrons are called CONDUCTOR• materials with “immobile” electrons are called INSULATOR (or dielectrics)

General Rules based on The Periodic Table of Elements

• each element contains a specific number of protons, and NO two elements have the same number of protons• elements with the same number of electrons at the outer orbit have similar properties (electron valence)• elements are stable with 8 electrons in the outer orbit (inert gas)• atoms seek to combine with other atoms to reach a stable condition

ION AND IONIZATION

• ion is an electrically charged atom or molecules• negative ion is an atom that contains more electrons than in its natural state• positive ion is an atom that contains less electrons than in its natural state• IONIZATION is the process of creating ions.• ionization energy is defined as the energy required to remove the outermost electron from an atom.

• there are 2 common ionization techniques;• electrolysis process• electron beams

Electrolysis Process

• involves the production of chemical changes in a chemical solution by oppositely charged ions moving in opposite directions under an electric potential different.• A solution that conducts electric current is called ELECTROLYTE and the vessel that holds the electrolyte is called ELECTROLYTIC CELL.• Passing electric current through the fluid will produced ions in the electrolyte• The free electron in the current will alter the atomic structures in the fluid molecules, thus producing atoms with unbalanced electrons.

NaCl solutionelectrolyte

NaCl solution

Flow of e

Cl- Na+

cathodeanode

• electrodes are placed at the two ends of the electrolytic cell and then electric potential is established to a dc source.• this source will push electrons into one electrode (cathode), and pulling electrons from the other electrode (anode).• this action will decompose the NaCl solution into positively charged Na ions and negatively charged Chlorine ions. • Na+ ions will be driven into cathode, collected an electron there and become Na atom (deposited there). The same thing happened to Cl- at the anode.• this method is very useful technique in separating and extracting chemical compound which found many applications in microelectronics and micromachining fabrication

HOMEWORKS

• To study the electron beams technique for ionization• To study the principle and application of Electrohydrodynamics

DOPING OF SEMICONDUCTORS

3 types of materials based on the electrical conductivity (resistance of thematerials to the movement of electrons)

1. Conductor2. Insulator3. semiconductor

INTRINSIC / EXTRINSIC SEMICONDUCTOR MATERIALS

Intrinsic Materials – Semiconductor materials with a very low impurity level i.e. high resistivity due to low carriers concentration

Extrinsic Materials – Semiconductor materials with high impurity level i.e. low resistivity due to a high carriers concentration.

• the most important material to MEMS and microsystem is semiconductor• it can conduct electricity but not as efficient as the conductors• however, they can make to be a conductor by introducing certain foreign atoms.• the method of adding impurities into the semiconductor materials is called DOPING and normally done by a process called ion implantation and diffusion

• to control the intensity and path of electric current flow through the semiconductor materials• to alter material’s resistance to chemical and physical etching• to act as the etching stop• to produce p-n junction

WHY WE NEED TO DOPE SEMICONDUCTOR MATERIALS ?

DONORS AND ACCEPTORS

C

Si

Ge

Sn

Pb

B

Al

Ga

In

Ti

N

P

As

Sb

Bi

DONORS (contribute free electrons)

ACCEPTORS (contribute free holes)

CARRIERS IN INTRINSIC MATERIALS

Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Si

• some bonds to break due to thermal agitation• each broken bond creates one electron and one hole • free and mobile e released

Valence bond: 2 electrons ofopposite spins

e-

• In intrinsic semiconductor, electron and holes are generated in pairs n = p = ni

n – electron concentration p – hole concentrationni – intrinsic concentration

• For low to moderate dopant concentration

ni = 2.63 x 1016 T1.5 e-6885/T cm-3

CARRIERS IN EXTRINSIC MATERIALS

N-TYPE DOPING: substitutional donors create free electronswithout creating holes

Si Si Si Si

Si P Si Si

Si Si Si Si

Si Si Si Si

Donors – P, As, Sb, Bi

+

e-

Fixed, positively charged donor

N-TYPE DOPING: substitutional donors create free electronswithout creating holes

• Donor has 5 valence electrons• 4 electrons form 4 bonds with 4 adjacent Si• 5th electron is “free” to move • Each donor creates one free electron• By losing e, donor becomes fixed positive ion• At equilibrium, the pn product is constant ~ 2 x 1020 cm-6 (at room temperature)• When n increases, p decreases: silicon is n-type• Electrons are majority carriers, holes are minority carriers

P-TYPE DOPING: substitutional acceptors create holewithout creating free electron

Si Si Si Si

Si B Si Si

Si Si Si Si

Si Si Si Si

Acceptors – B, In

-

Fixed, negatively charged acceptor

Valence bond 2 electrons of Opposite spins. 2nd electron borrowed from neighbor.

hole

P-TYPE DOPING: substitutional acceptors create holeswithout creating free electrons.

• Acceptors have 3 valence electrons• 4 electrons needed to form 4 bonds with 4 adjacent Si• 4th electron “borrowed from neighbor”, creates hole • Each acceptor creates one hole• By gaining e, acceptor becomes fixed negative ion• At equilibrium, the pn product is constant ~ 2 x 1020 cm-6 (at room temperature)• When p increases, n decreases: silicon is p-type• Holes are majority carriers, electrons are minority carriers

DIFFUSION PROCESS

The diffusion process is defined as the introduction of a controlled amount ofForeign material into selected regions of another material.• The spread of dark ink in a pot of a clear water• The spread of perfume• The oxidation of metal in a natural environment

Generally, diffusion process can take place between;• Liquids to solids• Gases to solids• Liquids to liquids

In microfabrication, diffusion is applied in;

• Oxidation of wafer surface (dry and wet oxidation)• Wafer is heated in the furnace at a very high temperature (800-1000C) in the O2 environment

SiSiO2

Si + O2 SiO2

In microfabrication, diffusion is applied in;

• Deposition of thin film materials in the CVD process• Epitaxy

SiSiO2, metals, nitride, silicate glass, etc

Mathematically, the diffusion process is governed Fick’s Law:

The law stated that the concentration of a liquid A in a liquid B with distinctconcentration is proportional to the difference of the concentrations of twoliquids, but inversely proportional to the distance over which the diffusion effect take place.

Liquid A, C1

Liquid B, C2

X

Ca

Ca,x0 – C a, x

x0 - x

Where;

Ca -

Cx

CaThe concentration of liquid A at a distance x awayFrom the initial contacting surface per unit area, time

or

x0 Position of the initial interface of the two liquids

Ca,x0 and C a, x

Respective concentrations of liquid A at x0 and x

C1 > C2

Ca -

Cx

Equation can be expressed in other way for a continuous

variation of the concentration Ca along the x axis as;

Ca - D

Cx

where the constant D is the diffusivity of liquid A

Diffusivity is treated as a material property and in most cases it increases withtemperature

=

Mask with opening

Substrate material

Foreign material with concentration Cs

Diffused material with concentration C (x, t)

• In the doping of semiconductor by diffusion, the semiconductor substrates usually are heated to a carefully selected temperatures.• the dopant is applied at the required region while the other areas were covered by other material called MASK. • the dopant is allowed to diffuse into the substrate at the mask opening until the required concentration is achieved.• the maximum concentration of dopant through diffusion is called solid solubility.• solid solubility is a very important material parameter in semiconductor technology

The concentration C (x, t) in example at given depth x in the substrate and time tcan be determined by solving the Fick’s Law equation in the form of;

C (x, t)

t= D C (x, t)

2

x2

The solution for the above equation can take the following form;

C (x, t) = Cs erfc x

Dt2

Where C (x, t) is the concentration of foreign material at the depth x into the substrate at time t

Cs is the solid solubility at the diffusion temperature

erfc (x) is complimentary error function, erfc (x) = 1 – erf (x) whereerf (x) is error function

ExamplePhosphorus is to be doped into a silicon wafer substrate by a diffusion process.The substrate is heated at 1000C for 30 minutes in the presence of the dopant.Find the concentration of the dopant with the depth x = 0.075um beneath thesubstrate surface. Given the solid solubility of Phosphorus at 1000C is 4.5 x 1020 atoms / cm3

and (D)1/2 = 0.085 um / (h)1/2 . Both Cs and (D)1/2 values were obtained from the tables

Solution

We have Cs = 4.5 x 1020 atoms / cm3

We have 2 D t = 2 (0.085)2 x 30/60 = 0.1202 um

C (x, t) = Cs erfc x

Dt2

Therefore, the concentration of P at the depth x after 30 minutes into diffusion can be calculatedas;

C (x, 0.5) = Cs erfc x

Dt2= 4.5 x 1020 erfc x

0.1202

Using table of complimentary error function, we can estimate the concentration of P at the depth of 0.075 um to be;

C (0.075, 0.5) = = 4.5 x 1020 erfc (0.624)4.5 x 1020 erfc0.075

0.1202

4.5 x 1020 x 0.38 = = 1.71 x 1020 atoms / cm 3

PLASMA PHYSICS

Plasma is a gas that carries electrical charges. Approximately contains equal numbers of electrons and positively charged ions. As a result, plasma is a mixture of neutral ionized gas.

It is very important in the microfabrication it contains a large number of positiveIons with extremely high kinetic energy to perform the following tasks;• Assist in depositing foreign materials onto the base materials as in CVD

and PVD• Assist in penetrating desirable foreign substances into base material such as in implantation• Remove a portion of base material such as in the RIE process (Reactive Ion Etching)

To be discussed further in Microelectronics Fabrication Technology