Silicon_Wafer_Processing
To provide an overview for manufacturing systems students of the steps and processes required to make intergrated circuits from blank silicon wafers.
Silicon Wafer Processing
Dr. Seth P. Bates
Applied Materials
Summer, 2000
Objective
To provide an overview for manufacturing systems students of the steps and processes required to make
integrated circuits from blank silicon wafers.
Goals
The Transfer Plan provides a curriculum covering the process of manufacturing integrated circuits from
the silicon wafer blanks, using the equipment manufactured by Applied Materials, Lam Research, and
others of its competitors. The curriculum will be modular, with each module covering a process in
sequence. This curriculum will be developed for internet access.
Outline
Introduction
Preparation of the Silicon Wafer Media
Silicon Wafer Processing Steps
Silicon Wafer Processing
Outline of Contents
Introduction ................................................................................................................ 2
Preparation of the Silicon Wafer Media...................................................................... 3
Crystal Growth and Wafer Slicing Process
Thickness Sorting
Lapping & Etching Processes
Thickness Sorting and Flatness Checking
Polishing Process
Final Dimensional and Electrical Properties Qualification
Silicon Wafer Processing Steps ................................................................................. 8
Fabrication
Diffusion
Coat-Bake
Align
Develop
Dry etch
Wet etch & clean
Photolithography
Implant / Masking Steps
Die Attach / Wire Bond
Encapsulation
Lead Finish / Trim and Form
Final Testing / Shipping
Seth Bates SJSU / Applied Materials / IISME-ETP page 1
Introduction
The processing of Silicon wafers to produce integrated circuits involves a good deal of chemistry and
physics. In order to alter the surface conditions and properties, it is necessary to use both inert and toxic
chemicals, specific and unusual conditions, and to manipulate those conditions with both plasma-state
elements and with RF (Radio Frequency) energies. Starting with thin, round wafers of silicon crystal, in
diameters of 150, 200, and 300mm, the processes described here build up a succession of layers of
materials and geometries to produce thousands of electronic devices at tiny sizes, which together
function as integrated circuits (ICs). The devices which now occupy the surface of a one-inch square IC
would have occupied the better part of a medium-sized room 20 years ago, when all these devices
(transistors, resistors, capacitors, and so on) were only available as discreet units.
The conditions under which these processes can work to successfully transform the silicon into ICs
require an absolute absence of contaminants. Thus, the process chambers normally operate under
vacuum, with elemental, molecular, and other particulate contaminants rigorously controlled. In order to
understand these processes, then, we will begin the study of semiconductor processing with an overview
of vacuum systems and theory, of gas systems and theory, as applied specifically to these tools, and of
clean room processes and procedures
The semiconductor industry reflects and serves an extraordinary revolution in both materials science and
in data processing and storage. As recently as 1980, most individuals had no idea that computers would
ever impact their personal lives. Today, many families own one or two computers, and use many other
computers and dedicated processor systems in their appliances and automobiles. The intrusion of
electronics and computer technology into our lives and the devices we use daily is growing at an
exponential rate, and Moore’s Law still applied in the computer world. This is one of the few markets in
which, as time passes, the power and capacity of the products grows steadily, while the cost of that
power and capacity drops.
Today, only twenty years later, we are continually pushing the envelope of capabilities of the data
processing and storage systems that are now in the mainstream. Ingenuity and creativity, along with
great strides in quality control, process control, and worker productivity, are leading daily to new ideas
about how to further reduce device size and data density. On the horizon are visions of biochemically-
based devices which will be far smaller, work faster, and generate less heat than current devices. It is
worth spending some time imagining where this evolving technology will take us, and the society we
live in.
Seth Bates SJSU / Applied Materials / IISME-ETP page 2
Preparation of the Silicon Wafer Media
From: http://www.ade.com/employment/silicon_wafer.html
Wafer products are measured at various stages of the
Silicon Thermal Properties
process to identify defects inducted by the manufacturing
Thermal Conductivity (solid) 1.412 W/cm-K
process. This is done to eliminate unsatisfactory wafer Thermal Conductivity (liquid) 4.3 W/cm-K
materials from the process stream and to sort the wafers Specific Heat 0.70 J/g-K
Thermal Diffusivity .9 cm**2/s
into batches of uniform thickness and at a final inspection
Melting Point 1683 K
stage. These wafers will become the basic raw material for Boiling Point 2628 K
new integrated circuits. The following is a summary of the Critical Temperature 5159 K
Density (solid) 2.33 g/cm**3
steps in a typical wafer manufacturing process. Density (liquid) 2.53 g/cm**3
Vapor pressure
at 1050C 1e-7 Torr
at 1250C 1e-5 Torr
Crystal Growth and Wafer Slicing Process
Molar heat capacity 20.00 J/mol-K
The first step in the wafer manufacturing
process is the formation of a large, perfect silicon crystal. The
crystal is grown from a ‘seed crystal’ that is a perfect crystal.
The silicon is supplied in granular powder form, then melted in
a crucible. The seed is immersed carefully into the crucible of
molten silicon, then slowly withdrawn.
Step 1: Obtaining the Sand
The sand used to grow the wafers has to be a very clean and good form of silicon. For this reason not just
any sand scraped off the beach will do. Most of the sand used for these processes is shipped from the
beaches of Australia.
Step 2: Preparing the Molten Silicon Bath
The sand (SiO2)is taken and put into a crucible and is heated to about 1600 degrees C – just above its
melting point. The molten sand will become the source of the silicon that will be the wafer.
Step 3: Making the Ingot
A pure silicon seed crystal is now placed into the molten sand
bath. This crystal will be pulled out slowly as it is rotated. The
dominant technique is known as the Czochralski (cz) method.
The result is a pure silicon cylinder that is called an ingot. As
description or a variant on the Czochralski method is available at
http://www.ioi.co.uk/tech/dera/p0526.htm
The Czochralski method
Seth Bates SJSU / Applied Materials / IISME-ETP page 3
Examples of some completed ingots An epitaxial reactor.
Growth of Epitaxial Silicon
This step is done to provide a good clean surface for later processing. If a layer of Silicon is grown onto
the top of the wafer using chemical methods then that layer is of a much better quality then the slightly
damaged or unclean layer of silicon in the wafer. The
epitaxial layer is where the actual processing will be
done.
The diameter of the silicon ingot is determined by the
temperature variables as well as the rate at which the
ingot is withdrawn. When the ingot is the correct
length, it is removed, then ground to a uniform
external surface and diameter.
Each of the wafers is given either a notch or a flat
edge that will be used later in orienting the wafer into
the exact position for later procedures. In these two figures you can see a notch (above) and flats. Flats
in this image are exaggerated for clarity.
Step 4: Preparing the Wafers
After the ingot is ground into the correct diameter for the wafers, the
silicon ingot is sliced into very thin wafers. This is usually done with a
diamond saw.
A diamond saw for cutting wafers
Each of these wafers will then go through polishing until they are very smooth and just the right
thickness (see Polishing Process, below).
Thickness Sorting
Seth Bates SJSU / Applied Materials / IISME-ETP page 4
Following slicing, silicon wafers are often sorted on an automated basis into
batches of uniform thickness to increase productivity in the next process step,
lapping. During thickness sorting, the wafer manufacturer can also identify defect
trends resulting from the slicing process.
Lapping & Etching Processes
Lapping removes the surface silicon which has been cracked or otherwise damaged
by the slicing process, and assures a flat surface. Wafers are then etched in a
chemically active reagent to remove any crystal damage remaining from the
previous process step.
Thickness Sorting and Flatness Checking
Following lapping or etching, silicon wafers are measured for flatness to identify and control defect
trends resulting from the lapping and etching processes. Wafers are also often sorted on an automated
basis according to thickness in order to increase productivity in the next process step, polishing.
Polishing Process
Polishing is a chemical/mechanical process that smoothes the uneven surface left by the lapping and
etching processes and makes the wafer flat and smooth enough to support optical photolithography.
A wafer polishing machine Wafers in storage trays
Final Dimensional and Electrical Properties Qualification
The wafers undergo a final test, performed in order to demonstrate conformance with customer
specification for flatness, thickness, resistivity and type. Process induced defect and defect trend
information is used by the wafer manufacturer for yield and process management of the immediately
preceding steps. Information regarding surface defects, such as scratches and particles, and defect trend
information are used by the wafer manufacturer for yield and process improvement.
Seth Bates SJSU / Applied Materials / IISME-ETP page 5
Seth Bates SJSU / Applied Materials / IISME-ETP page 6
Silicon Wafer Processing Steps
Semiconductor manufacturing
from http://www.micron.com/resources/semi_manufacture.htm
Today, most integrated circuits (ICs) are made of silicon. Turning silicon into
memory chips is an exacting, meticulous procedure involving engineers,
metallurgists, chemists and physicists. The first step from silicon to circuit is the creation of a pure,
single-crystal cylinder or ingot of silicon six to eight inches in diameter. These cylinders are sliced into
thin, highly polished wafers less than one-fortieth of an inch thick. Micron uses six- and eight-inch
wafers. The circuit elements (transistors, resistors, and capacitors) are built in layers on the silicon wafer.
Hundreds of memory chips are etched onto each wafer.
Pure single-crystal cylinders of silicon are sliced into thin, highly polished wafers less than one-fortieth
of an inch thick. Hundreds of memory chips are etched onto each wafer, while for processor chips,
perhaps only ten to 50 devices will fit on one wafer.
Most chip designs are developed with the help of computer
systems or computer-aided design (CAD) systems. Circuits are
developed, tested by simulation, and perfected on computer
systems before they are actually built. When the design is
complete, glass photomasks are made—one mask for each layer of
the circuit. These glass photomasks are used in a process called
photolithography.
Fabrication
Semiconductor memory chips are manufactured in cleanroom environments because the circuitry is so
small even tiny bits of dust can damage it. Class 1 and class 10 cleanrooms are typical. In a class 1
cleanroom, there is no more than 1 particle of dust in a cubic
foot of air. In comparison, a clean, modern hospital has about
10,000 dust particles per cubic foot. The air inside a
cleanroom is filtered and recirculated continuously, and
employees wear special clothing such as dust-free gowns,
caps, and masks to help keep the air particle-free.
The figure at the right shows a modern semiconductor etch
machine. At the top left is the wafer handling system, which
accepts wafers from the factory materials handling system,
aligns them for processing in the etch machine, and moves
them into the main part of the machine. While this system
normally operates at atmospheric pressure, it is usually at a
Seth Bates SJSU / Applied Materials / IISME-ETP page 7
Class 10 clean room level. When wafers move into the process part of the machine, they are contained
in a vacuum, with extremely low particle contamination levels (Class 1). Even the smallest particle can
ruin an entire wafer, with a large number of integrated circuits affected, and costing hundreds or
thousands of dollars.
The large circular part in the center of the machine is a transfer area, which moves the wafers between
process chambers. The machine shown has four process chamber locations, each one of which can be
individually configured depending on the process that is desired.
After processing, the wafers are moved back into the materials handling system and returned to the
factory floor. For further processing. A single wafer may have to undergo many succesive process steps
to achieve the complex
layers of conductor,
semiconductor, and
insulating material needed
to produce the desired
circuitry. This process is
described below, in
outline first, then in more
detail.
Layers on a semiconductor device.
Process Steps Outline
! Diffusion. A layer of material such as oxide is grown or deposited onto the
wafer.
! Coat / Bake. The resist, a light sensitive protective layer, is applied and
cured in place.
! Align. A reticule is positioned over the wafer. Ultraviolet light shines
through the clear portions of the reticule exposing the pattern onto the
photosensitive resist.
! Develop. The resist is developed and unwanted resist is washed away.
! Dry Etch. Dry etch removes oxide not protected by resist.
! Wet Etch and Clean. The remaining resist is removed in wet etch to reveal the patterned oxide layer.
Then the wafer is cleaned. The process is repeated up to 18 times to create the various layers necessary for
each part's circuitry.
Process Steps Details
In this sterile environment, the wafers are exposed to a multiple-step photolithography process that is
repeated once for each mask required by the circuit. Each mask defines different parts of a transistor,
capacitor, resistor, or connector composing the complete integrated circuit and defines the circuitry
pattern for each layer on which the device is fabricated.
Seth Bates SJSU / Applied Materials / IISME-ETP page 8
At the beginning of the production process, the bare silicon wafer is covered with a thin glass layer
followed by a nitride layer. The glass layer is formed by exposing the silicon wafer to oxygen at
temperatures of 900 degrees C or higher for an hour or more, depending on how thick a layer is required.
Glass (silicon dioxide) is formed in the silicon material by exposing it to oxygen. At high temperatures,
this chemical reaction (called oxidation) occurs at a much faster rate.
Photolithography
Next, the wafer is uniformly coated with a thick light-sensitive
liquid called photoresist. The coating is applied while the
wafer is spinning.
Portions of the wafer are selected for exposure by carefully
aligning a mask between an ultraviolet light source and the
wafer. In the transparent areas of the mask, light passes
through and exposes the photoresist.
Direct Wafer Stepping (DWS)
In this method the mask is quite a bit farther
away from the wafer and through a series of
optics the image is placed onto the wafer. The
main advantage of this method is that the mask
can be quite a bit larger then the final pattern and
through optical and mechanical manipulations a
better resolution can be exposed onto the
photoresist. This method is currently the number
one method used in industry.
Photoresist hardens and becomes impervious to
etchants when exposed to ultraviolet light. This
chemical change allows the subsequent developer solution to remove the unexposed photoresist while
leaving the hardened, exposed photoresist on the wafer.
Etching the Wafer Surface
The etching process is used immediately after photolithography to etch the unwanted material from the
wafer. This process is not selective and that is why the pattern had to be traced onto the wafer using
photoresist. There are two main methods of etching, wet etching and dry etching. This leaves a pattern
on the wafer in the exact design of the mask. The hardened photoresist is then removed (cleaned) with
another chemical.
Seth Bates SJSU / Applied Materials / IISME-ETP page 9
Wet Etching
Wet etching is done with the use of chemicals. A batch of wafers is dipped into a higly concentrated pool
of acid and the exposed areas of the wafer are etched away. Wet etching is good in that it is fairly cheap
and capable of processing many wafers quickly. The disadvantage is that wet etching does not allow the
smaller critical geometries that are needed for todays chips.
Acid being poured onto a wafer A closer look at acid and the wafer.
Dry Etching
Dry etching refers to any of the methods of etching that use gas instead of chemical etchants. Dry etching
is capable of producing critical geometries that are very small.
Plasma Etching
Plasma etching uses a gas that is subjected to an intense electric field to generate the plasma state of
matter. The electric field is produced with coils that are wrapped around the chamber and exposed to a
high level RF source. There are two different versions of this type of etching based on the shape of the
chamber used. One consists of a barrel type chamber where the wafers are placed sitting up while the gas
is flowed over the wafers and out through an exhaust pipe. The second process uses a parallel plate
reactor. There are two plates that are used to give the gas the electric field rather than the coil that is
wrapped around the barrel chamber. In plasma form, the gases used are very reactive, providing effective
etching of the exposed surface. Plasma etching provides good critical geometry but the wafer can be
damaged from the RF radiation.
Reactive Ion Etching
This method works at a lower pressure and uses a combined physical and chemical method to etch the
wafer.
Ion Milling
Ion milling uses electric and magnetic fields to cause the plasma ions to form a beam that is used to do
the etching. This method is extremely accurate and has the ability to reach very small critical geometries.
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Anisotropic Etching vs. Isotropic Etching
Isotropic etching is a problem that results from chemical etching and some forms of dry etching. The
result is that the etchant material will etch to the side (laterally) as well as straight down. This can cause
some of the material under the patterned resist to be etched away, resulting in undercutting and poor
image accuracy. Anisotropic etching occurs in most forms of dry etching. In this process there is no
lateral etching so an exact representation of the pattern is etched onto the wafer. Anisotropic etching is
more desirable because there can be problems in maintaining the desired electrical characteristics if there
is lateral etching as well as vertical etching.
Implant / Masking Steps: Diffusion & Ion Implant
Electrical characteristics of selected areas on the developing integrated circuit are changed by implanting
energized ions (dopants) in the form of specific impurities into areas not protected by resist or other
layers. The dopants come to rest below the wafer's surface, creating the positive and negative areas on
the wafer which encourage or discourage the flow of electrical current throughout the die. These basic
steps are repeated for additional layers of polysilicon, glass, and aluminum. Typical dopants include:
! Boron
! Arsenic
! Phosphorous
These processes can be damaging to the wafer, so a heating process known as annealing is used to reduce
any damage to the wafers.
Diffusion
Diffusion is done in a furnace with a flow of gas running over the wafers. This step, like etch, is not
selective so the photoresist and patterning need to be done before this step. The best way to understand
the processes of this step is to imagine oxidation. Diffusion is very similar to oxidation except using a
different gas other than oxygen.
Ion Implantation
Ion implantation is different from diffusion. Diffusion uses the natural state of gas going to where there
is no gas, while ion implantation shoots the desired dopant ions into the wafer. Ion implantation has been
best equated with firing a machine gun into a wall.
In this analogy the wall is the wafer and the bullets are
the ions. The main disadvantage of ion
implantation is that it can only process a single wafer at a
time while a diffusion chamber is capable of
handling many wafers.
Seth Bates SJSU / Applied Materials / IISME-ETP page 11
The magnets used to control the ion beam A wafer handling tray in ion implantation
Drive In
This is the next step after the ions have been placed into the wafer. In this step the wafer is heated to
make the ions go deeper into the wafer.
Annealing
Due to the incredible damage that these processes (especially ion implantation) can cause to the wafer an
additional stage of heating is required. During this final stage the wafer is heated so that the crystal
lattice structure of the wafer will repair itself.
The finished wafer is an intricate sandwich of n-type and
p-type silicon and insulating layers of glass and silicon
nitride.
All of the circuit elements (transistor, resistor, and
capacitor) are constructed during the first few mask
operations. The next masking steps connect these circuit elements together.
An insulating layer of glass (called BPSG) is deposited and a contact mask is used to define the contact
points or windows of each of the circuit elements. After the contact windows are etched, the entire wafer
is covered with a thin layer of aluminum in a sputtering chamber. The metal mask is used to define the
aluminum layer leaving a fine network of thin metal connections or wires.
The entire wafer is then covered with an insulating layer of glass and silicon nitride to protect it from
contamination during assembly. This protective coating is called the passivation layer. The final mask
and passivation etch removes the passivation material from the terminals, called bonding pads. The
bonding pads are used to electrically connect the die to the metal pins of the plastic or ceramic package.
While still on the wafer, every integrated circuit is tested and functional and nonfunctional chips are
identified and mapped into a computer data file. A diamond saw then cuts the wafer into individual
chips. Nonfunctional chips are discarded and the rest are sent on to be assembled into plastic packages.
Seth Bates SJSU / Applied Materials / IISME-ETP page 12
Die Attach / Wire Bond
Before the die are encapsulated, they are mounted on to lead frames, and
thin gold wires connect the bonding pads on the chip to the frames to
create the electrical path between the die and lead fingers.
Product samples are taken out of the normal product flow for
environmental and reliability assurance testing. These quality assurance
test push chips to their extreme limits of performance to ensure high-
quality, reliable die and to assist engineering with product and process
improvements.
Encapsulation
During Encapsulation, lead frames are placed onto mold plates and heated. Molten plastic material is
pressed around each die to form its individual package. The mold is opened, and the lead frames are
pressed out and cleaned.
Lead Finish / Trim and Form
Electroplating (illustrated at left) is the next process where the encapsulated lead frames are "charged"
while submerged in a tin/lead solution. The tin/lead ions are attracted to the electrically charged leads
create a uniform plated deposit which increases the conductivity and provides a clean consistent surface
for surface mount applications.
In Trim & Form, lead frames are loaded into trim-and-form machines where the leads are formed step by
step until finally the chips are severed from the frames. Individual chips are then put into anti-static tubes
for handling and transportation to the test area for final testing.
Various Memory Chip Packaging (the thinnest being the most recent)
The various positions and shapes of the leads and the package size and shape depend on the final
application and the customer's packaging requirements.
Seth Bates SJSU / Applied Materials / IISME-ETP page 13
Final Testing / Shipping
Each memory chip is tested at various stages in the manufacturing process to see how fast it can store or
retrieve information, including the high temperature burn-in in Micron's proprietary AMBYX® ovens
which test the circuitry of each chip, ensuring the quality and reliability. This monitored burn-in provides
feedback throughout the process, allowing identification and correction of manufacturing problems.
The completed packages are inspected, sealed, and marked with a special ink to indicate product type,
date, package code, and speed. The finished goods area ships the chips to computer, peripheral,
telecommunications, and transportation customers throughout the world.
In the last 30 years semiconductors have become virtually indispensable in many aspects of daily life.
Even people who do not own or use a computer are likely to use semiconductor memory in one way or
another. Many of the fantastic capabilities of our modern world are possible thanks to the semiconductor
memory chip.
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