Transcript a joint production of

EMT 432/4
MEMS DESIGN AND FABRICATION
PENGUMUMAN
Taklimat Final Year Project
Tarikh: 17 Julai 2006 (Isnin)
Masa: 2:00 Petang
Tempat: DKW1 (Dewan Keikhlasan KWSP)
Text books and references
Text book
1. Tai-Ran Hsu, MEMS and Microsystems; Design and Manufacture,
Boston, Mc Graw Hill 2002
2. Hong Xiao, Introduction to Semiconductor Manufacturing
Technology, Prentice Hall, 2001
Other References
1. Nadim Maluf, An Introduction to Microelectromechanical System
Engineering, Artech House MEMS Library, 2nd edition 1999
2. Peter Van Zant, Microchip Fabrication: A Practical Guide to
Semiconductor Processing, Mc Graw Hill 2000
3. 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 information
on 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.
2.
3.
4.
Font: Arial narrow size 11, single spacing
Language: Preferable in English
Length of the essay: around 5 pages (A4)
To be submitted on 27 July 2006 (Ramzan)
Lecture 2:
Fundamental of Microsystem Design and Fabrication
1.
2.
3.
4.
5.
6.
Atomic Structure and Periodic Table
Ion and Ionization
Doping of Semiconductors
Diffusion Process
Plasma Physics
Quantum Physics
ATOMIC STRUCTURE AND THE PERIODIC TABLE
nucleus
proton
• atom is the basic of substance
neutron
• 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;
electron
• nucleus (proton and neutron)
• orbiting electron(s) or valence or free e
Hydrogen atom
• 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
nucleus
Lithium atom
• atoms normally contain more than 1 electron
Si atom
• 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.
electrolyte
NaCl solution
Flow of e
cathode
anode
Cl-
Na+
NaCl solution
• 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 the
materials to the movement of electrons)
1. Conductor
2. Insulator
3. semiconductor
INTRINSIC / EXTRINSIC SEMICONDUCTOR MATERIALS
• 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
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.
WHY WE NEED TO DOPE SEMICONDUCTOR MATERIALS ?
• 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
DONORS AND ACCEPTORS
DONORS (contribute free electrons)
B
C
N
Al
Si
P
Ga
Ge
As
In
Sn
Sb
Pb
Bi
Ti
ACCEPTORS (contribute free holes)
CARRIERS IN INTRINSIC MATERIALS
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
e-
Si
Si
Valence bond: 2 electrons of
opposite spins
Si
Si
• some bonds to break due to
thermal agitation
• each broken bond creates one
electron and one hole
• free and mobile e released
• In intrinsic semiconductor, electron and holes are generated in pairs
n = p = ni
n – electron concentration
p – hole concentration
ni – 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 electrons
without creating holes
Donors – P, As, Sb, Bi
Si
Si
Si
P
+
Si
Si
Si
Si
Si
Si
eSi
Si
Fixed, positively charged donor
Si
Si
Si
Si
N-TYPE DOPING: substitutional donors create free electrons
without 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 hole
without creating free electron
Acceptors – B, In
Si
Si
Si
B
Si
Si
Si
Si
-
Si
Si
Si
Si
Si
Si
hole
Si
Si
Valence bond 2 electrons of
Opposite spins. 2nd electron
borrowed from neighbor.
Fixed, negatively charged
acceptor
P-TYPE DOPING: substitutional acceptors create holes
without 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 of
Foreign 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
Si + O2
SiO2
SiO2
Si
In microfabrication, diffusion is applied in;
•
•
Deposition of thin film materials in the CVD process
Epitaxy
SiO2, metals, nitride, silicate glass, etc
Si
Mathematically, the diffusion process is governed Fick’s Law:
The law stated that the concentration of a liquid A in a liquid B with distinct
concentration is proportional to the difference of the concentrations of two
liquids, but inversely proportional to the distance over which the diffusion
effect take place.
Ca,x0 – C a, x


Ca
or Ca  - C
 x
x0 - x
Where;
C1 > C2
Ca The concentration of liquid A at a distance x away
From the initial contacting surface per unit area, time
Liquid A, C1
x0 Position of the initial interface of the two liquids
Liquid B, C2
Ca,x0 and C a, x
X
Respective concentrations of
liquid A at x0 and x
C
can be expressed in other way for a continuous
 x
variation of the concentration Ca along the x axis as;
Equation Ca  -

Ca = - D


C
x
where the constant D is the diffusivity of liquid A
Diffusivity is treated as a material property and in most cases it increases with
temperature
Foreign material with concentration Cs
Mask with opening
Diffused material with concentration C (x, t)
Substrate material
• 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 t
can be determined by solving the Fick’s Law equation in the form of;
 C (x,
t)t
=
2

D C (x,
t)
 x2
The solution for the above equation can take the following form;
C (x, t) = Cs erfc
Where
x
2 Dt
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) where
erf (x) is error function
Example
Phosphorus 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 the
substrate 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
x
C (x, t) = Cs erfc
We have Cs = 4.5 x 1020 atoms / cm3
We have 2
Dt
=
2
2 Dt
(0.085)2 x 30/60
= 0.1202 um
Therefore, the concentration of P at the depth x after 30 minutes into diffusion can be calculated
as;
x
x
C (x, 0.5) = Cs erfc
= 4.5 x 1020 erfc
0.1202
2 Dt
Using table of complimentary error function, we can estimate the concentration of P at the
depth of 0.075 um to be;
0.075
= 4.5 x 1020 erfc (0.624)
C (0.075, 0.5) = 4.5 x 1020 erfc
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 positive
Ions 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