WALK-THROUGH SURVEY REPJRT: COi\7TROL TECHNOLOGY
FOR INTEGRATED CIRCUIT FABRICATION
at
INTEL CORPORATION
Chandler, Arizona
Prepared by
Gary J. Mihlan, Russell K. Smith and Leslie J. Ungers
Battelle
Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
Report No. 115-lla
May 31, 1983
U.S. Environmental Protection Agency
Industrial Environmental Research Laboratory
Cincinnati, Ohio 45268
and
»..
National Institute for Occupational Safety and Health
Division of Physical Sciences and Engineering
Engineering Control Technology Branch
4676 Columbia Parkway
Cincinnati, Ohio 45226
-------
PLANT SURVEYED: Intel Corporation
5000 W. Williams Field Rd.
Chandler, Arizona 85224
SIC CODE: 3674
SURVEY DATE: August 12, 1981
SURVEY CONDUCTED BY: Mr. Gary J. Mihlan, Battelle-Columbus Laboratories
Dr. Ralph I. Mitchell, Battelle-Colur/ous Laboratories
Mr. Russell K. Smith, Battelle-Columbus Laboratories
Mr. Leslie J. Ungers, PEDCo Environmental
Mr. James H. Jones, National Institute for Occupational
Safety and Health
Mr. Robert Hartley, U.S. Environmental Protection Agency
Mr. Eugene Harris, U.S. Environmental Protection Agency
EMPLOYER REPRESENTATIVES CONTACTED:
Mr. Edward J. Sawicki, Corporate Safety Manager
Mr. Ed Boleky, Plant Manager
Mr. Jerry O'Neal, Senior Facilities Engineer
Mr. Bill Taylor, Safety Engineer
Mr. Steve Kopp, Site Safety/Security Manager
EMPLOYEE REPRESENTATIVES CONTACTED:
Mr. Steve Schuster; Safety Committee Chairman
-------
TABLE OF CONTENTS
Page
1.0 ABSTRACT 1
2.0 INTRODUCTION 3
3.0 PLANT DESCRIPTION 4
3.1. General 4
3.2. Chemical Storage 4
3.3. Gas Handling System 4
3.4. Monitoring System 5
4.0 PROCESS DESCRIPTION 6
5.0 DESCRIPTION OF PROGRAMS 9
5.1. Industrial Hygiene 9
5.2. Education and Training 11
5.3. Respirators and Other Personal Protective Equipment 11
5.4. Medical 12
5.5. Housekeeping 12
6.0 SAMPLE DATA FROM PRELIMINARY OR PREVIOUS PLANT SURVEYS 13
7.0 DESCRIPTION OF CONTROL STRATEGIES FOR PROCESS
OPERATIONS OF INTEREST 13
7.1. Chemical Vapor Deposition 14
7.1.1. Engineering Controls 16
7.1.2. Monitoring 17
7.1.3. Personal Protective Equipment 17
7.1.4. Work Practices 17
7.2. Thermal Oxidation 17
7.2.1. Engineering Controls 18
7.2.2. Monitoring 18
7.2.3. Personal Protective Equipment 13
7.2.4. Work Practices 18
7.3. Doping and Hydrogen Alloying 18
7.3.1. Engineering Controls 21
7.3.2. Monitoring: 22
-------
TABLE OF CONTENTS
(Continued)
Page
7.3.3. Personal Protective Equipment 23
7.3.4. Work Practices 23
7.4. Wet Chemical Cleaning Processes 24
24
7.4.1. Engineering Controls
7.4.2. Monitoring 25
7.4.3. Personal Protective Equipment 25
7.4.4. Work Practices 25
7.5. Photolithography 25
7.5.1. Engineering Controls 27
7.5.2. Monitoring 28
7.5.3. Personal Protective Equipment 28
7.5.4. Work Practices 28
7.6. Plasma Etching 28
7.6.1. Engineering Controls 30
7.6.2. Monitoring 30
7.6.3. Personal Protective Equipment 30
7.6.4. Work Practices 30
7.7. DC Sputtering 30
7.7.1. Engineering Controls 31
7.7.2. Monitoring 31
7.7.3. Personal Protective Equipnent 31
7.7.4. Work Practices 32
7.8. Electron-Beam Evaporation 32
7.8.1. Engineering Controls 32
7.8.2. Monitoring 32
7.8,3. Personal Protective Equipment 32
7.8.4. Work Practices 32
8.0 CONCLUSIONS AND RECOMMENDATIONS 33
9.0 REFERENCES 35
11
-------
1.0 ABSTRACT
A preliminary control technology survey was conducted at Intel
Corporation, Chandler, Arizona on August 12, 1981 by Battelle's Columbus
Laboratories, Columbus, Ohio. The survey was conducted as part of a project
under a U.S. Environmental Protection Agency contract funded through an
Interagency Agreement with the National Institute for Occupational Safety and
Health. Intel Corporation manufacturers n-channel metal oxide semiconductor
(N-MOS) integrated circuits. The Chandler facility opened in 1980 and is at
less than half production capacity.
The process operations in the manufacture of integrated circuits at
Intel are contained within a Class 100 clean room. Particulate control is
performed by filtration of inlet air through HEPA filters. Local air filtra-
tion is provided in the clean room by laminar flow benches with HEPA filtra-
tion. Process equipment is generally located in the laminar flow benches.
Process operations employed at Intel Corporation in MOS integrated
circuit manufacture include: 1) thermal oxidation of purchased pre-doped
silicon wafers, 2) chemical vapor deposition of silicon nitride, polycrystal-
line silicon and silicon dioxide, 3) photolithographic processes for defining
circuit patterns, including wafer cleaning, photoresist coating, soft-bake,
projection mask alignment exposure, wafer developing, hard bake, plasma
etching, and photoresist stripping, 4) doping, including diffusion and ion
implantation, 5) hydrogen alloying, and 6) metalization, including electron
beam evaporation and direct current (DC) sputtering.
Engineering controls used at the facility vary by process operation.
General engineering controls include gas storage in ventilated cabinets or in
secured covered areas outside of the building, and gas lines of welded stain-
less steel construction or stainless steel in PVC lines. Local exhaust
ventilation is provided for removing process gases by local exhaust take-offs
located at the source of the emission or by removal of process gases from
enclosed systems employing vacuum pumping systems. Shielding and electrical
interlocks are used to control X-ray emissions from ion implantation.
Environmental monitoring of toxic gases in the clean room area is
performed by a Miran's SOI multico-.ponent multistation system. Hydrogen
monitoring is performed by a Bacharacrr" Model CDS50 and by an 1ST* sensor.
-------
Process operations are generally automated and controlled by microprocessors.
The process sequences generally require that workers load and unload wafers
from the equipment.
Intel Corporation has developed a comprehensive worker training
program. Worker education and training is required of all new employees. The
programs cover equipment operations, safety practices, hazardous properties of
chemical agents and personal protective equipment. Plant safety program
personnel have the primary responsibility for worker health and safety. A
detailed emergency response plan has been developed for the facility-
Areas which warrant further study include: 1), review of ventilation
system design, testing, and maintenance, 2) review of data from environmental
monitoring systems, 3) documentation of work practices including worker
training programs, and A) evaluation of maintenance activities including
identification of hazards and controls employed during maintenance activities.
-------
2.0 INTRODUCTION
A preliminary survey was conducted as part of the control technology
assessment of the electronic components industry (SIC 3674). The study was
conducted for U.S. Environmental Protection Agency contract through an Inter-
agency Agreement with the National Institute for Occupational Safety and
Health. The preliminary survey was conducted to identify and evaluate the
control technology used to control emissions and work exposures. The informa-
tion obtained from the preliminary survey will be used to select sites for the
detailed control technology assessment.
The preliminary survey was conducted on August 12, 1981, at Intel
Corporation, 5000 W. Williams Field Road, Chandler, Arizona. The survey was
conducted by Battelle Columbus Laboratories with assistance from Mr. James
Jones, NIOSH. Mr. Bob Hartley and Mr. Gene Harris, U.S. EPA/Industrial
Environmental Research Laboratory, Cincinnati, Ohio, accompanied the research
team as observers.
The following individuals were contacted at Intel:
1. Mr. Edward J. Sawicki, Corporate Safety Manager
2. Mr. Ed Boleky, Plant Manager
3. Mr. Jerry O'Neal, Senior Facilities Engineer
4. Mr. Bill Taylor, Safety Engineer
5. Mr. Steve Xopp, Site Safety/Security Manager
6. Mr. Steve Schuster, Safety Committee Chairman
The study protocol was provided to the corporate safety manager
prior to the preliminary survey. An opening conference was held with plant
representatives. The study objectives and methods were described. A detailed
description of the health and safety programs at the plant was provided by the
plant staff. The plant structural design and layout, process description,
unit operations, monitoring systems, gas handling systems, and chemical
storage facilities were reviewed.
Following the opening conference a detailed tour of the facility was
conducted. Production areas, gas and chemical storage areas, air handling
systems, waste management systems, and medical facilities at the plant were
reviewed. A closing conference was held following the survey. All information
-------
provided by the plant that was considered confidential was identified by pla:
representatives.
3.0 PLANT DESCRIPTION
3.1. General
The plant is an integrated circuit fabrication facility located in a
two-building complex. It consists of a 150,000 sq. ft. building, with a
34,000 sq. ft. Class 100 clean room, of tilt-up construction, Type 3N,
(Uniform Building Code). The plant was opened in 1980 and is at less than
half production capacity. The plant produces N-MOS type integrated circuits.
Approximately 300 people are employed on three shifts at the plant
as follows: 194 first shift, 84 second shift, and 22 third shift.
3.2. Chemical Storage
Chemicals are stored in a chemical warehouse and transported to the
facility via common carrier. Chemicals stored at the plant are segregated as
acids and solvents (organics). Storage roons are approximately 6 inches below
the surrounding floor level. Separate storage rooms are used for acids and
sol'/ents. Acids are received as boxed shipments and stored on wooden pallets.
Chemical spill kits are available in the storage rooms.
Solvents are received as boxed shipments or drums. The solvent
storage room is mechanically ventilated with ventilation take-offs located at
floor level. A trench is located _in the storage room. Drums are placed on
the grating above the trench, grounded, and connected to an automatic pumping
system.
3.3. Gas Handling System
Production gases are provided either in cylinders or as bulk
shipments. Gas cylinders are stored outside of the facility in a c;--erec
secured area. Cylinders are segregated into four major categories:
-------
flammable, toxic, pyrophoric, and cxidizer. Most cylinders have excess flow-
control valves to automatically close the line when gas flow exceeds a preset
flow limit. A purge gas cylinder is located beside the process gas cylinder.
Control assemblies for gas purging are mounted above the gas cylinders.
Bulk storage tanks are used for nitrogen and oxygen, and are located
outside of the building. Gases supplied by cylinders include arsine, Freon®,
phosphine, hydrogen chloride, silane, dichlorosilane, boron trifluoride, and
ammonia. Hydrogen chloride, hydrogen, and dichlorosilane are stored in
ventilated cabinets located indoors. Gas control systems have excess flow
valves. Cabinets have local exhaust ventilation and are located in fenced,
locked enclosures. Gases are piped to process operations using welded
stainless steel lines. Hydrogen is supplied to process operations in a
double-line of welded stainless steel tubing enclosed in PVC. The interstice
contains nitrogen, which is purged toward a combustible gas monitor. Process
gas for ion implantation is supplied in lecture bottles located in a ven-
tilated gas storage cabinet in the ion implantation unit.
Natural gas is piped through a seismic protection valve into the
plant. Cylinder gases are purchased in size K and T bottles and piped into
the building.
3.4. Monitoring Systems
Two types of monitors are used for monitoring gases. Hydrogen is
monitored by both a Bacharach® Model CD 850 and by an 1ST6 sensor. The
Bacharach® sensor is used to monitor lines for explosive concentrations. The
1ST® unit is employed near the ion implantation units. The alarm from this
system (10 ppci) will identify a toxic gas leak. This is a combustible mixture
alarm that will activate for combustible compounds, such as hydrogen and
alcohols. The Bacharach® sensor and the 1ST® sensor have been in use since
early 1981.
A second toxic gas monitoring system is the Mi ran® 801. This system
is a multistation, inulticomponent monitoring system. The system is used to
monitor for arsine, phosphine, hydrogen chloride, nitrogen dioxide, hydrogen
fluoride, xylene, n-butyl acetate, and acetic acid. Isopropanol, water, and
carbon monoxide are interference co-pounds wnich are also monitored. Each
-------
line is prepurged before it is sampled so that the gas analyzed is represen-
tative of that present at the sampling location during the period analyzed.
Local exhaust ventilation is monitored through visual alarms.
Velocity, temperature, humidity, and pressure are measured at key points, such
as ventilation ducts and upstream of the scrubber. Breathing zone monitoring
of workers for arsenic and organics is also performed.
Alnor velometers are used to measure ventilation flow rates on a
periodic basis. The ventilation system power is backed up for full flow-
capability on the emergency power system.
t
4.0 PROCESS DESCRIPTION
The plant purchases pre-doped p- and n-type silicon wafers. Photo-
masks used for defining circuit patterns in the wafer are produced for the
plant by another manufacturer. Process operations performed at the facility
include thermal oxidation, chemical vapor deposition (silicon nitride, poly-
crystalline silicon, and silicon dioxide), photolithography (wafer cleaning,
photoresist coating, soft-bake, wafer exposure, develop, hard bake, etching,
and photoresist stripping), doping (diffusion, ion implantation, and hydrogen
alloying), and metallization (electron beam evaporation and DC sputtering).
Several process operations utilize similar types of equipment for
different process steps. Thermal oxidation, silicon nitride and polycrystal-
line silicon deposition, hydrogen alloying, and doping (diffusion) are
performed in direct digital control furnaces. The processes vary in the types
of source gases used, furnace temperatures, processing tine, and sequencing of
operation.
Similar process steps may be performed with a variety of process
equipment. Chemical vapor deposition is performed in a direct digital control
furnace and a continuous vapor phase system. Plasma etching is performed ir.
both a planar etching system and a barrel or tunnel etching system.
The production area is designed as a Class IOC clean room with
temperature and humidity controlled. All operations, except electron bear
evaooration of gold, are performed in the clean room.
-------
The purcnased wafers are loaded into quartz boats and placed in an
open end tube called an elephant. The elephant is manually attached to the
direct digital control (DDC) furnace. The furance consists of a horizontal
quartz tube heated by electrical resistance. One end of the tube has a ground
glass joint with a removable cap.
The direct digital control furnace is operated by a microprocessor,
which controls gas flow, tube temperature, the rate of wafer loading, and
unloading and process time. Once the elephant is attached to the furnace, the
boats containing the wafers are automatically inserted into the furnace at a
programmed rate. Depending on the process step, the gases introduced into the
furnace include oxygen (for thermal dxidation), dichlorosilane and ammonia
(for silicon nitride deposition), silane (for polycrystalline silicon deposi-
tion), phosphorous oxychloride (for doping or junction formation), and hydro-
gen (for hydrogen alloying). The DDC furnace is used to deposit a layer on
the wafer (silicon dioxide, polycrystalline silicon, or silicon nitride), and
to promote diffusion of dopants into the wafer surface. Those processes in
which a layer is deposited on the wafer surface by a chemical vapor are known
as chemical vapor deposition or CVD. These CVD processes include silicon
nitride, polycrystalline silicon, and silicon dioxide deposition. Silicon
dioxide is deposited in a vapor phase reactor with silane and oxygen.
Wet chemical processes are used at various steps in integrated
circuit fabrication. Sulfuric acid and hydrogen peroxide are used to clean
wafers by iinoersion in the heated mixture. The immersion baths are located in
laminar flow hoods in molded plastic benches. Local exhaust ventilation of
the baths is provided by slots located at the top of the bath and at the rear
of the bench. Hydrofluoric acid is used to remove silicon dioxide.
The first step in the photolithographic process is wafer prepara-
tion. The photoresist layer, consisting of an organic polymer in a xylene or
cellosolve acetate solvent, is spun onto the wafer and soft-baked in an
electrical resistance heated oven. The soft-baked wafer is loaded into wafer
carriers and transferred to the projection mask aligper for pattern defini-
tion. The wafer is automatically removed from the carrier and counted against
a photomask. The circuit pattern is transferred to the wafer by exposure of
the wafer to ultraviolet light, The exposed wafer is returned to the carrier
and the process is repealed for the remaining wafers. The treated wafer is
-------
then submerged in a wet chemical bach to remove the photoresist. Depending or.
the type of photoresist used (positive or negative) the layer removed is
either the unexposed or exposed area. The exposed wafers are treated in a
post- or hard-bake oven.
A dopant may then be diffused into the wafer surface. Dopants are
introduced into the substrate by gas diffusion or ion implantation, or as a
solid dopant, which is then heat treated in a DDC furnace. Diffusion is
performed in the direct digital control furnaces. Wafers are inserted, and
heated, and a gas mixture of nitrogen and phosphorous oxychloride is intro-
duced to the furnace. Ion implantation uses a focused ion beam. The ions are
generated by an electrical arc discharge in a vacuum system containing the
doping gas (phosphine, arsine, or boron trifluoride). The ions are targeted
at the individual wafer and implanted into the substrate. The ion implanta-
tion occurs under high vacuum conditions.
The exposed underlying layer may then be etched using plasma etching
systems. Plasma etching is a dry chemical etching method using a plasma gas
containing reactive ions which remove material from the wafer surface. The
operation is performed under high vacuum conditions. Wafers are loaded into
the system, which is sealed and pumped to vacuum. A plasma containing
reactive fluoride ions is created by passing a reactant gas through a radio
frequency field created inside the sealed chamber. Planar plasma etching and
barrel or tunnel reactor plasma etching systems are used at Intel.
An aluminum layer, deposited on the wafer by DC sputtering under
high vacuum conditions, acts as a metallic connect. Wafers are loaded onto
platens and placed in a load chamber. The load c'naaber is pumped to a vacuum
and the platens are automatically transferred to the sputtering chamber. The
sputtering chamber consists of a bell jar and DC sputtering source. The bell
jar is lowered and a high vacuum seal is forced. The aluminum is contained in
a target material, which is in the bell jar assembly. The aluminum is
deposited on the wafer surface by removing surface molecules of the netal from
the source target. ' This deposition is accomplished by the ionizing energy
supplied by a DC power source, which is applied to the target. A gaseous glow
discharge is established between the anode (containing the wafers) and the
target (containing the material to be deposited). The alurinum atorr_3
are
-------
sputtered from the target to the wafer surface. After deposition of the
metal, the photolithographic sequence is repeated to define the ratal pattern.
Etching is used to remove the exposed aluminum and establish the contact
pattern.
The last step in the fabrication process is the deposition of a gold
layer on the wafer backside. This layer promotes bonding of the integrated
circuit chip during packaging and is deposited through electron beam evapora-
tion. The wafers are loaded into the planetary. The bell jar is lowered and
the unit is pumped to a high vacuum. Gold is evaporated from a target source
using an electron-beam field applied to the gold source material. High vacuum
conditions (10~- torr) are created with an initial rough pumping of the
bell jar using a mechanical roughing pump. Lower pressures are obtained using
a cryogenic or diffusion pump. The diffusion pumps utilize a low boiling
hydrocarbon fluid. Molecules of the hydrocarbon vapor are heated in the pump
bottom and ejected downward, sweeping gas molecules from the chamber. A cryo-
genic trap is used to remove water vapor present in the chamber air.
The above steps may be repeated several times to produce an inte-
grated circuit. The differences in electrical properties of the layers and
the'arrangement and sequence of the layers forms the transistors, capacitors,
resistors, and conductors, which constitute the integrated circuit.
5.0 DESCRIPTION OF PROGRAMS
5.1. Industrial Hvsiene
The plant employs a full-time safety engineer. An industrial
hygienist employed at corporate headquarters is also available. Consultants
are used as necessary in the areas of industrial hygiene, health physics,
medicine, and toxicology. Corporate staff were involved in the initial desig:
of the plant .
The work area is monitored with personal and area sampling methods.
A monitoring system has been developed which samples workroom air iron; 24
remote locations with analysis at a central location using the Mirar:'1' 801 for
-------
10
the agents and interference compounds listed in the discussion of monitoring
systems (Section 3.4).
The results from area sampling are reported on computer printouts.
Analytical results are reported as 8-'nour, time weighted averages and peak
concentrations by location of sampling stations. Sampling probes were located
based on results of smoke tests. A hydrogen monitoring system is also present
with sampling points near areas of hydrogen use (e.g., hydrogen alloying
furnace).
Personal monitoring has been conducted at the plant for production
and maintenance workers. Verbal and/or written results of personal monitoring
are provided to employees. Monitoring records were available, but were not
reviewed during the survey. Personal monitoring has been conducted for
numerous chemical agents, including arsenic. Plant personnel indicated that
personal breathing zone samples of workers exposed to arsenic have not been
above 1 ug/nH (limit of detection). Monitoring of exposures to physical
agents, including X-rays, radio frequency radiation, and ultraviolet
radiation, has been conducted.
Local exhaust ventilation is monitored with Alnor® velometers.
In-house systems are equipped with audible and visual alarums should a ventil-
ation failure occur. Ventilation systems are backed up with emergency
generators.
Standard operating procedures are defined for normal production and
maintenance operations. Work practices are listed for all job classifica-
tions. Chemical transport in-house is limited to individuals specifically
trained in chemical handling. Maintenance personnel are trained for specific
pieces of equipment. Written maintenance procedures are available. Material
safety data sheets are available for chemicals used at the facilility.
Detailed procedures have been developed for chemical spills, confined area
entry, chemical handling, earthquakes, electrical maintenance and lockout,
electrical safety, and gas cylinder storage.
Emergency response equipment is available at the facility. A loss
control team has been established to respond to emergencies, to facilitate
plant evacuation, and to safely shut-down building services and utilities as
needed. Written procedures for emergency response
-------
The facility has established s. radiation safety program, adrtini-
'stered by a radiation safety officer. The program requires controlled access
to high radiation areas. Permissible levels of exposure are adapted from
29CFR1910.96.
5.2. Education and Training
All new employees must complete a 20-hour new hire technical
orientation. The orientation includes sections on safety practices, tools,
equipment, and technology. General facility safety practices are described.
Personal protective equipment and emergency procedures are provided. Poten-
tial chemical and physical hazards in the plant, the effects from exposure to
these agents, and the appropriate first aid treatment are described. Main-
tenance procedures for equipment are also outlined. Gas handling procedures
and training in changing cylinders, calibrating cylinders, assembling regula-
tors, and leak testing cylinders are part of the training program. The
hazards of gases used at the facility are also described.
Training of employees occurs every 6 months. All employees are
continuously monitored for safety. The safety practices of new employees are
carefully monitored for the first 3 months to identify unsafe work practices.
5.3. Respirators and Other Personal Protective Equipment
Personal protective equipment required by the plant includes safety
glasses or goggles, bunny suits (coveralls) with hoods, safety shoes, and
latex gloves. Additional protective equipment used during the handling of
acids includes aprons with sleevelets and face shields. Self-contained
breathing apparatus with a full facepiece operated in a pressure demand code
used when changing lecture bottles for ion implantation. Self-contained
breathing apparatus (SCBA) are also accessible for emergencies within the
plant. Those performing maintenance requiring any type of respiratory
protection use a self-contained breathing apparatus described above.
The plant respiratory protection program requires employees using
respirators to undergo a medical examination to determine their ability to use
-------
12
this equipment. The prograa administrator is the employees' supervisor. Trie
correct respirator is selected by the supervisor, and the employee is trained
in the use, inspection, assembly, disassembly, cleaning, and sanitizing of the
respirators. Respirators are tested by the employee using a negative pressure
test. A logbook is provided for each respirator which details its use,
cleaning, and inspection. Only self-contained breathing apparatuses were used
at the plant. Engineering controls preclude the need to use other respiratory
protection.
The facility is equipped with acid suits, disposable acid resistant
coveralls and SCBA with full facepiece operated in a pressure demand mode for
emergencies.
5.4. Medical Program
The plant employs a full-time registered nurse during the first and
second shifts, and a physician is available from the corporate headquarters.
Furthermore, security personnel are trained in first aid and cardiopulmonary
resuscitation. Medical treatment is available at a nearby hospital.
The medical program requires only a medical history with visual
tests of employees also being performed. Baseline tests of urinary arsenic
levels are required of all employees working with arsenic compounds. A pre-
placement medical examination of all new employees began on September 1, 1981
and physical examinations will be required on an annual basis. This program
includes a medical history questionnaire, vital signs, routine urir.alysis,
blood tests, and back analysis (Xraus-Weber fitness exam and X-ray).
Emergency equipment available at the plant includes showers, eye
wash stations, and oxygen supply. --
5.5. Housekeeping
As noted, the production area is a Class 100 clean room (100 parti-
cles per cubic foot). Dust levels in the rooms are controlled by passing roor.
air through HEPA filters. Also, as stated earlier, production workers are
required to wear bunny suits with hoods, booties, gloves and safety glasses or
goggles. Individuals with facial hair are required to wear -asks. These
controls are designed to limit particulate levels in the fabrication area.
-------
13
Additional engineering controls ha-"e been included in the design of
the facility which eliniinates many housekeeping problems . These engineering
controls are defined in detail in Section 7.0 and include distribution lines
ior pumping process chemicals from a central storage area to the specific unit
operation, and suction lines and waste drains for transporting waste liquids
from wet chemical areas to waste storage and treatment facilities. Pump oils
and filtration media from roughing pumps are replaced by reversing the pump
flow and pumping wastes into a container. The pumping oil and filtration
media are changed every 1 to 4 months. Written maintenance procedures have
been established for process equipment. Equipment maintenance operations were
not evaluated during the survey.
Hazardous wastes generated at the facility are transported to a
Class I hazardous waste landfill for disposal. Hydrofluoric acid wastes are
transported to an acid treatment facility. Other acid wastes are diluted to
an acceptable pH level and disposed of in the sewer system for treatment by
the public sewage treatment plant.
6.0 SAMPLE DATA FROM PRELIMINARY OR PREVIOUS PLANT SURVEYS
Sample data from previous plant surveys and equipment evaluations
conducted by Intel staff have been compiled by plant staff but were not
reviewed at the preliminary survey. Monitoring data available include results
of the plant area monitoring network, personal monitoring of worker exposures,
and equipment . evaluations.
7.0 DESCRIPTION. OF CONTROL STRATEGIES FOR
PROCESS OPERATIONS OF INTEREST
A variety of strategies are used at Intel to control emissions and
work exposures. Control strategies used include local and general exhaust
ventilation, process modification, process substitution, process isolation,
process and environmental monitoring, personal protective equipment, and work
-------
14
practices. The rollowing is a detailed description of each process operation
and the control strategies applied.
7.1. Chemical Vapor Desositic
on
Chemical vapor deposition (CVD) is the formation of a stable com-
pound on a heated substrate by the thermal reaction or decomposition of
gaseous compounds. Examples of CVD observed at the plant include silicon
nitride deposition, polycrystalline silicon deposition, and silicon dioxide
deposition. The deposition of silicon nitride is performed in a direct digi-
tal control furnace. Silicon dixoide is deposited in a continuous vapor phase
system. A general description of the direct digital control furnace is given
below. The description is applicable to those operations which use the DDC
furnace (silicon nitride deposition, thermal oxidation, diffusion, and
hydrogen alloying).
The direct: digital control furnace system includes a microprocessor,
which organizes the overall furnace processing using feedback control loops.
The control loops are used to insert and withdraw wafer carriers at a speci-
fied rate, to raise or lower the furnace temperatures at a specified rate, to
adjust the various gas flows as a function of time, and to monitor the actual
temperature profile inside the furnace as a function of time (Douglas, 1981).
The microprocessor can automatically clean the furnace, perform an automatic
calibration cycle, and tailor the dynamic performance of the furnace to a
given process step. The advantage of the direct digital control furnace is
the high degree of process reproducibility possible with the system. A dis-
advantage is that all control is lost if the computer fails.
The primary components of"the system include an electronics
enclosure, jungle cabinet, load station, furnace modules, and source cabinet.
The source cabinet is used for the diffusion furnace bubbler system, source
dopant system, and as a location where the gas systems interfaces with the
furnace tube.
Wafers are received in fused silica boats. The boats are loaded in
queue onto a carrier and the carrier is placed into a silica glass tube. The
entire task is performed by a single operator at a laminar flov work bench
-------
15
close to the furnace. A glass plug is removed fron the furnace tube and the
elephant is manually lifted into the furnace loading station and attached to
the furnace. The loading station is enclosed with movable panels, which
allows access during loading and unloading of the elephant.
The silica glass elephant is ground at one end to promote a fairly
tight seal with the furnace tube. A small round opening at the opposite end
of the elephant receives a silica glass tube called a boat puller. The boat
puller is used to advance the boats into the furnace at a programmed rate.
The boat puller also retrieves the boats upon completion of the cycle.
Local exhaust ventilation consists of a local exhaust take-off at
the furnace tube opening. The face velocity at the ventilation take-off was
reported.to be 600 to 1300 lineal feet per minute.
The chemical vapor deposition of silicon nitride is used in both
bipolar and MOS technologies. In the production of bipolar integrated
circuits the silicon nitride deposition provides for passivation. In MOS
integrated circuits it is used for multilayered insulators.
Silicon nitride is formed on silicon wafers in a direct digital
control furnace. The deposition of silicon nitride on silicon wafers is
similar to that described above. The wafers are loaded into boats, placed in
a carrier, and inserted into the elephant. The elephant is attached to the
furnace and the boats are automatically loaded into the furnace. The pro-
cessing sequence is controlled by the system microprocessor, as described
above. The wafers are heated in an atmosphere of dichlorosilane and ammonia.
An amorphous silicon nitride film grows on the wafer surface and hydrogen gas
is liberated during the reaction.
Silicon dioxide deposition is a form of chemical vapor deposition
(CVD) where a stable Si02 layer is _formed on a heated substrate (single
crystal silicon wafers) by the thermal reaction or decomposition of gaseous
compounds. The Si02 layer is deposited by the oxidation of silane with
oxygen. The SiO? glass deposition system observed at the Intel plant was a
continuous vapor-phase oxidation system. The system is a low-temperature cold
wall reactor operating at atmospheric pressure. It includes a reaction cham-
ber, gas control system, time and sequence control system, heat source, and
effluent handling system. Time and sequence of gas input (SiH^, Oo and
POT) are computer controlled.
-------
16
Prior to loading wafers into the system, the wafers are cleaned with
a degreasing solvent, followed by acid cleaning (sulfuric hydroxide -
H2S04 and H202) and drying. The wafers are manually loaded in the
Si02 glass deposition system. Wafers are automatically transported through
a nitrogen purge to the preheat zone and into the reaction chamber. The
Si02 film is deposited by the oxidation of silane (SiH4) with oxygen.
Phosphine is added to the reaction chamber gases and is deposited with the
Si02 as a dopant. After the deposition sequence is completed, the wafers
are removed and placed in wafer carriers.
7.1.1. Engineering Controls. Local exhaust ventilation of the DDC
furnace consists of a takeoff located at the furnace tube opening. Air flow
is directed from the source cabinet through the furnace tube to the furnace
opening. A nitrogen purge of the furnace tube opening provides dilution of
the reactant gases released from the furnace.
The furnace loading station is enclosed by movable panels. The
elephant containing the wafer carriers is placed in the loading station and
attached to the furnace. The panels are closed and the load station is auto-
matically purged with nitrogen. The source cabinet containing the bubbler
system, source dopant system, and gas interface systems is enclosed.
The continuous vapor phase reactor consists of a preheat zone, main
Si02 deposition zone, and a post heat zone. The wafers are manually fed to
the reactor. Wafers enter a nitrogen purge in the preheat zone and pass
through a nitrogen curtain prior to the main deposition zov.e. Phosphorus-
doped silicon dioxide is deposited on the wafer surface in the main zone. The
system consists of a heater block, dispersion tubes, gas controls, and local
exhaust ventilation of the reactor". Wafers with a deposited silicon dioxide
layer pass through the nitrogen curtain and nitrogen purge. Loading and
unloading stations are normally covered during operations. The metal covers
are manually removed when loading or unloading wafers.
The reactor is located in a protective enclosure. The gas control
systems for silane, oxygen, nitrogen, and phosphine are located at the tront
of the process unit. Wafer loading and unloading is from the side of the
equipment .
-------
17
7.1.2. Monitoring. The DDC furnace is nicroprocessor controlled.
The microprocessor controls the process cycle directing loading and unloading
of the furnace, gas flow, temperature, calibration, and cleaning. A specific
"recipe" is programmed for the individual process step.
Environmental monitoring of the workroom is performed with multi-
station, multicomponent sampling, and central analysis by the Miran® 801.
Sampling ports are located in the clean room area above the gas jungle
cabinets. Specific details of the gas monitoring system are described in
Section 3.4.
7.1.3. Personal Protective Equipment. Personal protective equip-
ment requirements include normal clean room attire consisting of hoods,
booties, clean suit, latex gloves, chemical safety goggles or safety glasses,
and safety shoes. Heat protective gloves are used by workers unloading wafers
from the continuous vapor phase reactor. A heat resistant handle is used to
remove the ground glass cover from the furnace tube
7.1.4. Work Practices. Computer control of the DDC furnace results
in limited contact of the worker with the furnace. After loading of the
elephant, the process is completely automated, requiring only that the worker
initiate the program.
Detailed written maintenance procedures have been established for
the continuous vapor phase reactor.
7.2. Thermal Oxidation
The oxidation of silicon to silicon dioxide is an important process
in both bipolar and MOS monolithic, integrated circuit technology. Thermal
oxides are used in both technologies as barriers to the diffusion of doping
agents. Oxide areas, which have been defined by photolithography, are used t
establish the boundaries of the circuit pattern. Silicon dioxide is also used
in MOS technology as the dielectric gate material.
Silicon wafers are oxidized by a wet oxidation process. Deionized
water vapor in a nitrogen carrier gas is introduced into the furnace as the
o
-------
18
oxidant. Silicon dioxide is formed on the wafer in a direct digital control
furnace. The furnace design and processing steps are similar to those
described for chemical vapor deposition of silicon nitride. The wafers are
loaded into boats placed on a carrier, and inserted into the elephant. The
elephant is attached to the furnace and the boats are automatically inserted
into the furnace. The processing sequence is controlled by the furnace system
microprocessor.
7.2.1. Engineering Controls. Thermal oxidation is performed in DDC
furnaces using a wet oxidation process of deionized water vapor. Engineering
controls for the DDC furnace are summarized in Section 7.1.1.
7.2.2. Monitoring. General clean room monitoring is performed with
remote sampling ports and analysis by the Miran® 801. The monitoring system
is described in Section 3.4. The process control of the DDC furnace is
described in Section 7.1.
7.2.3. Personal Protective Equipment. Personal protective equip-
ment requirements for thermal oxidation include chemical safety goggles or
safety glasses, safety shoes, and heat protective gloves. General clean room
attire is also required.
7.2.4. Work Practices. Work practices for operation of the DDC
furnace are described in Section 7.1.4.
7.3. Doping and Hydrogen Alloying
Doping is the process of introducing impurities into the substrate
to produce changes in its electrical properties. These impurities are
referred to as dopants. Two specific doping methods observed at the plant are
diffusion and ion implantation. Hydrogen alloying of the substrates is also
performed to remove radiation damage from the metalization process and to
promote good electrical contact between the metal and silicon.
-------
19
Gaseous dopants used at Intel include boron trifluoride, arsine
phosphine. Doping of substrates with 'gaseous dopants is performed in direct
digital control diffusion furnaces. These dopants are used for diffusion and
ion implantation. The dopant gases are introduced to the diffusion furnace at
atmospheric pressure. The gas flow is controlled by the system
microprocessor.
Arsine, phosphine, and boron trifluoride dopant gases are used in a
high vacuum ion implantation system. Gas flow and other process variables are
controlled by the system microprocessor. Ion implantation gases are stored in
ventilated gas storage cabinets located within the ion implantation unit.
Diffusion operations introduce impurities into the substrate to
produce changes in the _electrical properties of the substrate. Dopants used
at the plant include n-type electron donors (POCl^). Doping of the sub-
strate is performed in a direct digital control furnace.
Prior to doping, a layer of silicon dioxide is deposited on the
substrate. Photolithographic processes are used to define the mask pattern.
The wafers with the silicon dioxide mask are loaded onto a carrier and placed
into a silica glass tube or elephant. The elephant is placed in the load
station of the furnace and connected to the furnace with a ground glass seal.
A small round opening in the opposite end of the elephant receives a silica
glass tube which is used to insert the carrier into the furnace at a pro-
grammed rate. The doping agents, introduced into the furnace in nitrogen,
diffuse into areas of the substrate where the mask layer has been removed from
earlier photolithographic processes.
Diffusion of the dopant into the substrate is determined by the
temperature, gas flow, time sequence, and type of dopant. The dopant
(POC13) is contained in a quartz bubbler placed in the source cabinet of the
furnace. Nitrogen is bubbled through the liquid. The resulting gas contains
sufficient POC13 for doping. The POC13 reacts with oxygen in the furnace
to produce ?2®5'
Ion implantation is used in both bipolar and MOS technologies to
introduce selected impurities into semiconductor wafers. This technique
allows greater control over the amount of dopant being introduced and operates
at much lower temperatures than diffusion processes.
-------
20
Ion implantation uses the focused ion bean of a source gas to dope
semiconductor material. The beam is generated at the ion source. The source
consists of a Freon cooled arc chamber (anode) surrounding a tungsten filament
source (cathode). An electrical arc discharge is maintained by passing a
source gas (or vaporized liquid) through the chamber. The ion beam is draw-n
from the arc chamber by an extraction electrode and directed to the analyzing
magnet. The magnet analyzes, resolves, and focuses the beam and selects only
the desired species of ions required for implantation.
The,selected ions are then targeted through the acceleration tube in
the direction of the wafer target. The acceleration tube optimizes both the
focusing and transmission of the selected ion beam. The selected beam enters
the lens and scavenger box, where it is further focused and deflected toward
the wafer target. The scanner has the capacity to move the ion bean in a
raster pattern to cover the entire target area with the smaller ion beam
diameter. The focused beam finally enters the target chamber, where a silicon
wafer has been automatically removed from a standard wafer cassette (boat) and
positioned for impact. Each target wafer enters the chamber via an input
vacuum lock. The locks are sealed and vented before a wafer enters or leaves
the target chamber. The ion iraplanter scans the beam across the target wafer,
implanting the desired ion species at a preferred concentration.
Three types of source gases are commonly used during ion implant-
ation: boron trifluoride, BF3; phosphine in hydrogen; or arsine in
hydrogen. The source gases are supplied in lecture bottles located in the
nitrogen purged gas box.
The ion implanter maintains three independent vacuums: the source,
beam line, and end station (or target). Typically, each vacuum is produced by
a mechanical roughing pump and a diffusion pump. The input and output vacuum
locks are serviced independently by two roughing pumps. Cryogenic pumps may
be used in the source and end station regions. The beamline and end station
are isolated from the beam regions of the instrument using liquid nitrogen
locks. Each pumping system is fully automatic and will shutdown the implanta-
tion process in event of a vacuum leak.
The instrument and gas box are vacuum vented using dry nitrogen.
The target chamber and wafer handling mechanisms are ventilated by a v
laminar flow hood at 600 cfra.
-------
21
Hydrogen alloying is the process of heating the substrate in a
hydrogen or hydrogen/nitrogen atmosphere. It is performed to remove radiation
damage from netalization and to minimize contact resistance between aluminum
and silicon. Hydrogen alloying is performed in direct digital control
furnaces identical to those used for chemical -vapor deposition of silicon
nitride.
The wafers are loaded into carriers, placed in an elephant, and
connected to the furnace. The wafers are automatically inserted into the
furnace. Wafer loading/unloading, temperature, gas flow, and process time are
automatically controlled by a microprocessor control unit. Following inser-
tion into the furnace, the wafers are heated in a pure hydrogen atmosphere for
a specified time. The furnace is purged with nitrogen and the wafers are
automatically removed.
The elephant containing the wafers is attached to the furnace at a
loading station that is enclosed by movable panels for access during loading
and unloading. The furnace tube is sealed with a removable quartz cap when
not in production. A ground glass seal on the tube end provides the connec-
tion for attaching the elephant or cap to the furnace. The system is purged
with nitrogen during the alloying process to dilute hydrogen gas escaping from
the seal.
Local exhaust ventilation is provided at the furnace opening. A
ventilation take-off is located adjacent to the furnace tube opening. Hydro-
gen sensors are located in the source cabinet, at the load station, and
exhaust duct, and above the jungle cabinet. Hydrogen gas is supplied to the
furnace in a double jacketed stainless steel/?VC line. This line is purged
with nitrogen from the furnace back to the storage cabinet. A hydrogen
monitor is located in the storage Cabinet.
7.3.1. Engineering Controls. As described, diffusion and hydrogen
alloying are performed in a DDC furnace. Engineering controls for the furnace
are described in Section 7.1.1. DDC furnaces used for diffusion require the
use of liquid dopant sources. Quartz bubblers containing POC13 are mounted
in the enclosed source cabinet. Air flow froa the cabinet is directed to the
-------
22
rusion furnace- Lccal exhaust ventilation of the DDC furnace is located at
the furnace tube opening near the loading station.
DDC furnaces used for hydrogen alloying do not require the quartz
bubbler system. Hydrogen is supplied to the furnace from cylinders stored in
ventilated gas storage cabinets located in the basement. The hydrogen is
piped to the furnace entering at the source cabinet. Hydrogen lines are
double jacketed stainless steel in PVC.
Ion implantation is performed in high vacuum conditions (10~->
torr). The vacuum is established by a mechanical roughing pump followed by a
cryogenic trap and a diffusion pump. The vacuum creates negative pressure
conditions which limit the release of the ions into the workroom air. Indi-
vidual wafers are loaded into the ion implantation unit through load locks
which are evacuated with a mechanical pump. Prior to loading or unloading
wafers, the lock is purged with nitrogen. Ion implantation gases (boron
trifluoride, phosphine, and arsine) are stored in a ventilated gas cabinet
located inside the ion implantation unit.
The~iofi beam source is lead shield1 ed^t'o" p "rev ent X-ray leakage. The
ion source is contained within two lead shielded cabinets. Access to the
source is through panels which are electrically interlocked to the system.
The cabinets are electrically grounded.
7.3.2. Monitoring. Interlocks on the ion implantation vacuum
system will shut down the unit if leaks occur in the vacuum system. Process
monitoring is performed by a microprocessor, which controls wafer cycling and
determines the implanted dose.
Environmental monitoring of the workroom environment is provided by
a sampling port located above the ion implanter. The workroom air is sampled
and analyzed by the Mir an® 801 system described in Section 3.4.
Hydrogen alloying performed in the DDC furnaces is monitored using
combustible gas detectors placed in the load station, source cabinet, and heat
exhaust duct. A detector is located in the ventilated gas cylinder cabinet
where the hydrogen cylinder is stored.
Hydrogen is monitored at the ion implantation unit. The hydrogen is
used as a carrier for the source gas. Hydrogen is monitored to determine if a
-------
23
-eak condition exists for the more toxic gases. A hydrogen sensor is located
in the exhaust stack of the gas storage cabinet located in the ion icplanter.
7.3.3. Personal Protective Equipment. Personal protection require-
ments for workers operating the ion implantation unit, and DDC furnace consist
of standard clean room attire. This attire includes hood, clean suit,
booties, chemical safety goggles or safety glasses, latex gloves, and safety
shoes.
Individuals responsible for changing gas bottles for the ion
implanter are required to wear pressure demand self-contained breathing
apparatus.
Workers handling quartz bubblers containing POC13 must wear acid-
resistant gloves, chemical aprons, and face shields.
7.3.4. Work Practices. Handling of lecture bottles used in ion
implantation is limited to specially trained workers. Workers are required to
wear positive pressure SCBA when handling ion implant lecture bottles.
Pump oils for the oil roughing pump are changed by reversing the
flow of the pump and pumping out oils into a container. The oil is manually
replaced. Freon for the diffusion pump is manually charged.
Phosphorous oxychloride is supplied in sealed quartz bubblers.
Standard operating procedures have been established for handling the bubblers.
Bubblers are only changed by trained personnel. The bubblers are received in
metal containers packed in wooden boxes. The containers are rezcved from the
boxes and transferred to a laboratory hood. The quartz bubbler containing the
liquid POC13 is removed from the metal can. The used bubbler is removed
from the source cabinet, placed in"a chemical carrier bucket, and transported
to the laboratory hood, where it is packed into the metal can, sealed, and
stored for hazardous waste disposal. The replacement bubbler is transferred
to the furnace in a chemical carrier bucket. The bubbler is placed in the
source cabinet and attached to the furnace.
Emergency procedures for corrosive spill cleanup of the bubbler
solution have been established.
-------
24
7 • 4 • Wet Chemical Cleaning Processes
The silicon wafer cleaning process is performed to obtain a clean,
uniform, and stable surface. The cleaning operations are performed in Class
100 enclosures using ultrapure, electronic grade chemicals.
The initial step during the cleaning process is the immersion of a
cassette of wafers in a solution of sulfuric acid and hydrogen peroxide,
commonly referred to as sulfuric peroxide. The exact proportions of sulfuric
acid to hydrogen peroxide were not specified at Intel. However, typical
solutions used in industry contain approximately nine parts concentrated
H2S04 to one part ^02 . Nitric or hydrochloric acid may be added in
small amounts. The initial solution is designed to remove organic residue,
which may have accumulated on the wafers during handling.
A second wet chemical cleaning process is designed to remove
unwanted silicon dioxide formed during the cleaning processes. The clean
wafers (in cassettes) are submerged in a solution of one part hydrofluoric
acid to ten parts deionized water. When oxide etching is completed, the
wafers are rinsed in pure deionized water.
Following the deionized ^water rinse, the wafers are removed from the
cassettes and dried by spinning under a nitrogen blow-off. This portion of
the process is automated.
The cleaning baths, oxide etch, rinse baths, and nitrogen blow-off
system are located under laminar flow ventilation. Baths or etching tubs used
in the cleaning process are equipped with take-otf vents around the car..-;
perimeter. The vents are used to maintain a constant down draft across the
face of each tub.
Spent solutions used in the cleaning process are aspirated out of
the tubs. New solutions are manually added.
7. A.I. Engineering Controls^. Wet chemical benches are of plastic
construction. The benches are located under laminar flow hoods. Air is
passed through HEPA filters. Acid tanks and deionized water tanks are
recessed in the wet chemical benches. The acid tanks are heated by electrical
resistance. Local exhaust ventilation of the tank is through slots located
-------
25
inside the tank around the top perimeter. Local exhaust ventilatio-. of the
entire bench is provided by slot ventilation with take-offs located at the
rear of the bench. Air flow for the bench is directed down from the HEPA
filters across the work bench to the rear ventilation take-off.
Waste chemicals are removed from the tanks by aspirators located in
the bench. Waste acid lines are plumbed from the aspirator to a central waste
acid tank located outside of the building.
7.4.2. Monitoring. Sampling ports for the MI RAN7® 801 monitoring
system described in Section 3.4 were located at ceiling height (approximately
8 feet). The sampling ports in the wet chemical bench areas were located
based on smoke tests conducted by Intel.
7.4.3. Personal Protective Equipment. Workers are required to wear
normal clean room attire consisting of booties, clean suit, hood, latex
gloves, chemical goggles or safety glasses, and hard toe shoes. Workers
replacing chemical solutions are required to wear face shields and chemical
aprons.
7.4.4. Work Practices. Wafers are contained in carriers of plastic
construction. Plastic handles are attached to the carrier and the unit is
dipped into the bath. Release of the handle is by hand squeezing of the grip.
Individuals mixing solutions for the wet chemical areas are specially trained
in chemical handling.
7.5. Photolithography
Photolithography includes the following process operations: 1)
photoresist application, 2) substrate exposure, and 3) a photoresist develop-
ment. These operations are outlined below.
Microelectronic technology uses photosensitive organic compounds to
delineate the circuit patterns on silicon wafers. Exposure to light, in
particular light at ultraviolet (UV) wavelengths, alters the chemical resis-
tive characteristics of these photoresist compounds. The photoresists ar;
-------
26
grouped into two separate classes depending on their reaction to UV light
exposure. Negative photoresists become insoluble or resistant to etching
chemicals on exposure. Positive photoresists become soluble or nonresistent
when exposed. Negative UV sensitive photoresists are most frequently used
during the production of integrated circuits.
Intel uses both positive and negative photoresists during integrated
circuit production. Since negative photoresists were used during the survey,
they will be the model for this process description. The application is
actually a series of individual "job shop" operations presented as one process
for simplicity.
The first operation is the actual application of photoresist
material to each clean, dehydrated wafer. A uniform coat of photoresist is
spun onto the wafer. The use of either positive or negative photoresist
depends on the requirements of the circuit pattern. In either case the
procedures for application will be the same. The photoresist is deposited in
a uniform layer by spinning at speeds of several thousand rpm's. Chemical
exposures for this operation are most often associated with the photoresist
solvent; at Intel the negative photoresist solvent was xylene, the positive
was cellosolve acetate. During the spin process excess photoresist will be
expelled from the wafer surface, Redeposition of the expelled material is
prevented through the maintenance of a downdraft at the wafer perimeter. In
addition to the local downdraft, laminar flow ventilation is used to establish
a clean area around the application/spinning equipment.
The photoresist coated wafers are transfered in cassettes to a pre-
or soft-baking oven. The purpose of the pre-baks is to drive off the remain-
ing photoresist solvent prior to wafer exposure. Laminar air flow is provided
at the pre-bake work station. Intel used a nitrogen atmosphere, resistance
heated oven for the pre-bake.
Substrates coated with a soft-baked photoresist layer are exposed
using a projection mask alignment system. Cassettes containing substrates are
manually loaded into the alignment system. Single wafers are automatically
removed from the cassette. The substrate is held in a vacuum chuck, which is
part of an x-y table with rotational adjustment. A single wafer is aligned
split field optics, which permit simultaneous viewin- of
-------
27
the wafer and mask. After alignment the substrate is clamped to the mask, the
microscope is moved away, and the substrate is exposed to ultraviolet light.
A mercury vapor lamp is used as the UV source. The alignment system exposes
the substrate from a 1:1 mask by successive sweeps of an exposure slit.
Exposed substrates are automatically returned to the cassette.
An operator is seated at the alignment system to load and unload
cassettes and to check mask alignment. The mask alignment system has internal
environmental controls, which heat the air and filter particulates.
The development process consists of rinsing away the dissolvable
portions of the exposed photoresist film from the surface of the silicon
I
wafer. The dissolvable portion of negative photoresist is the area covered by
the mask which has not been exposed to UV light. In the case of positive
photoresist, the portion removed is the exposed film surface. The rinsing
process usually consists of chemical dissolution of the soft photoresist and
finish rinsing in deionized water. The developed photoresist is finally heat
treated in a post- or hard-bake oven to improve its hardness and adhesion
characteristics.
The photoresist development process at Intel receives the exposed
wafers in a standard cassette carrier. The cassettes are manually submerged
in the dissolution chemical and deionized water baths. A mixture of n-butyl
acetate and various mixed isodecanes are used to remove unexposed negative
photoresist. Exposed positive photoresist is removed using a solution of
sodium hydroxide. Both dissolution baths are followed by rinses in deionized
wa t e r .
Post- or hard baking at Intel is performed using a resistance heated
batch oven. Laminar air flow is present at the oven and work station. No
additional local ventilation was observed.
7.5.1. Engineering Controls. Wafer cleaning, heating, application
of photoresist, soft-bake, projection mask alignment, and hard bake are done
under laminar flow hoods with HE?A filtration. The spin-on process for
application of photoresist is automatically controlled. Local exhaust or the
operation directs air downward around the perimeter of the spinning platform
to a local exhaust take-off located at the base of the platform. Air enters
the enclosure through openings in the rear of the enclosure.
-------
28
The mask alignment system contains internal environmental controls.
The wafer exposure area is enclosed to prevent contamination of the cask and
wafer and to limit ultraviolet light emission. Interlocks of the projection
mask aligner are also designed to prevent ultraviolet light emissions.
7.5.2. Monitoring. Sampling ports for the MIRAN® 801 monitoring
system are located in the photolithography area. The system is described in
Section 3.4.
Monitoring of process operations is through automated control of the
process cycle. The process cycle for application of the photoresist is
automatically controlled to provide uniformity.
7.5.3. Personal Protective Equipment. Personal protective
equipment used in the photolithography area consists of the normal clean room
attire of hood, clean suit, latex gloves, booties, chemical safety goggles or
glasses, and safety shoes.
7.5.4. Work Practices. The handling of individual wafers requires
the use of vacuum wands or tweezers. Removable handles are used to place the
wafer cassettes in the developing and rinse tanks. Paper towels vetted with
isopropanol are used by workers for general equipment cleaning.
7.6. Plasma Etching
Plasma etching is a dry chemical etching method used to remove a
specific material or layer from the wafer surface. It is used in the
fabrication of semiconductor devices where fine line widths are required. The
wafer to be etched may be a thermal Si02 layer, aluminum or aluminum alloy
thin films, silicon nitride or silicon. A photoresist layer is spun onto the
wafer, baked, and exposed. The wafer is then developed and hard-baked. The
wafer at this point contains a baked photoresist layer with areas of the
underlying substrate exposed from the developing process. The substrate is
then ready for etching. The gas used in plasma etching is selected based on
the specific substrate material to be etched.
-------
29
As noted in Section 4.0, two types of plasma etching systems are
used at Intel, planar plasma etching and barrel reactor plasma etching. Both
etching processes are located in Class 100 clean rooms under laaiinar flow-
hoods. The planar plasma etching system consists of a reaction chamber with
parallel electrode plates. The top electrode acts as a cathode to establish a
radio frequency (RF) field in the chamber, while the lower electrode (anode)
holds the wafers. Wafers are loaded on platens and inserted into the reaction
chamber. The chamber is sealed, purged with nitrogen, and evacuated to
approximately"0.1 to 10 torr. A plasma is created between the plates by
passing a reactant gas through the radio frequency field. Ultraviolet
radiation generated from the plasma may be released from the reaction chamber
through the glass viewing port.
The plasma consists of a variety of ions and free radicals. The
free radicals chemically attack the substrate but do not appreciably attack
the protective photoresist. The reaction products are removed from the
chamber by the exhaust system. A Freon gas is used as the reactant gas for
the planar plasma etching system. The fluoride ions produced are reactive
with silicon dioxide.
The barrel reactor system consists of a cylindrical chamber. The
wafers are vertically mounted in a fused silica carrier. A plasma is created
by an RF coil outside the reactor chamber. A perforated cylinder surrounds
the substrates which shunts the RF field and confines the plasma between the
reactor wall and the cylinder (Colclaser, 1930). The reacting species pass
through the perforated cylinder and chemically etch the substrate. The chemi-
cally active free radicals in the plasma react with the wafer surface causing
etching through a reduced-pressure adsorption-reaction-desorption process
(Douglas, 1981). The sequence of events prior to the etching is similar to
that described above. A Freon gas is introduced into the barrel reactor and a
radio frequency field is established which generates a plasraa that is reactive
with silicon nitride, silicon dioxide, and polycrystalline silicon. Tie etch
is believed to be the result of atomic fluorine, which diffuses to the silicon
surface, forming a volatile Si?4 that diffuses away from the surface.
The advantage of the planar plasma etch system over the barrel
reactor plasma etch system is its ability to produce
-------
30
wnere Che etch is primarily in one direction (generally perpendicular to the
substrate surface). Wet chemical etching methods and barrel reactor plasma
etching systems produce isotropic etching profiles where the etch rate is
equal in all directions. Isotropic etching results in undercutting of the
resist layers.
7.6.1. Engineering Controls. Plasma etching processes operate
under vacuum conditions. The vacuum is created usin^ a mechanical rou^hin^
o o o
pump. The system pressure is negative to room pressure. The plasma gases are
exhausted from the pump through local exhaust. Pump o'ils are continuously
filtered.
The plasma etching systems are located under laminar flow hoods.
They may be considered as substitutes for wet chemical acid etching where
hydrofluoric acid is used to etch the wafers. Plasma etching uses rreon as a
source of fluoride atoms. Px.eactive fluoride ions are formed in an RJ field.
7.6.2. Monitoring. No specific environmental monitoring systems
for plasma etching are used. Process monitoring of the plasma etching systems
is by automatic control of the equipment.
7.6.3. Personal Protective Equipment. Personal protective
equipment used at the plasma etching systems consist of normal clean room
attire. Specific equipment used for the process operation was not identified.
7.6.4. Work Practices. Wafers are mounted on platens or in
carriers and manually placed in the reaction chamber. Specific practices used
for the operation of the equipment were not observed.
Mechanical pump oil is changed by reversing the pump flow and
pumping the oil into a waste container. Oil is manually added from portable
containers.
7.7. DC Sputtering
Aluminum is deposited on the wafer surface by removing surface
molecules of the raet-il from a source material (target) by bonbard-.ent with
-------
31
energetic ions. The ionizing energy is supplied by a direct current (DC)
power supply. The sputtering process occurs in a vacuum system. The system
operates under high vacuum conditions (approximately 30 - 70 x 10"^ torr).
The high vacuum conditions are created in the bell jar by punping first with a
roughing pump, then a cryogenic pump.
The wafers are cleaned and dried prior to loading on the anode,
which is done through a loadlock attached to the sputtering chamber. The
loadlock is closed and pumped to vacuum. The wafers are automatically loaded
into the bell jar, where a high vacuum is formed. Argon is introduced to the
chamber and the DC current is applied. A gaseous discharge of metal atoms is
sputtered from the target to the wafers, producing a thin film of aluminum on
the wafer. The chamber is then automatically unloaded and the wafers are
transferred to the loadlock, where they are ready for photoresist application
and further processing. The sputtering sequence is automatically controlled.
7.7. 1. Engineering Controls. The DC sputtering system is located
in the general clean room environment. The process operates at a pressure
which is negative to that of the room. The pump exhaust is vented through the
local exhaust ventilation system. The metalization system employs two
chambers, a loading chamber and a bell jar chamber. The separate loading
chamber minimizes contamination of the sputtering chamber bell jar. The
process is automatically controlled.
7.7.2. Monitoring. Specific environmental monitoring systems to
evaluate emissions from the process are not present. The process is monitored
by automatic control of the process cycle. Interlock is provided to stop the
cycle if the hood chamber door is obstructed. A vacuum interlock system will
shut down power if the chamber vacuum is not achieved.
7.7.3. Personal Protective Equipment. Personal protective equip-
ment consists of normal clean room attire. Specific equipment used for the
process operation includes gloves used during chamber cleaning with potassium
hydroxide.
-------
32
7.7.4. Work Practices. ±.quipr.er.t maintenance requires removal of
the bell jar and internal reactor assembly for cleaning. The bell jar is
cleaned with potassium hydroxide. Metal sputtering parts are bead-blasted for
cleaning.
7.8. Electron—Beam Evaporation
Gold is deposited on the substrate by heating the gold in a high
vacuum and condensing the vapor on a cooler substrate. A focused electron
beam provides the heat necessary to vaporize the metal. The wafers are placed
above the source in a movable fixture called a planetary. During evaporation
the wafers are rotated to ensure maximum uniformity of the deposited layer. A
glass bell jar is lowered over the fixture and a high vacuum is pumped.
The gold deposition process occurs in a basement area separated from
other fabrication activities but not in a Class 100 clean rocn. Equipment
ventilation is limited to exhaust for the vacuum pumping system. The vacuum
system is similar to that previously described. The vacuum is created by an
initial rough pumping followed by a liquid cold trap and diffusion pump.
7.8.1. Engineering Controls. The system operates under high vacuum
conditions, resulting in a chamber pressure which is negative to that of the
room.
7.8.2. Monitoring. No specific environmental monitoring systems
are used to evaluate work room emissions from the process. The process cycle
is monitored through microprocessor control. The entire vacuum cycle is
automatically controlled.
7.8.3. Personal Protective Equipment. Personal protective
equipment used during electron beam evaporation was not identified. The unit
was not in operation during the preliminary survey-
7.8.4. Work Practices. Work practices specific to the operation
were not identified.
-------
33
8.0 CONCLUSIONS AND RECOMMENDATION'S
The production of integrated circuits at Intel is performed in an
environment established to control product quality and provide a safe work
environment. Individual engineering controls are used within this clean room
environment to control product quality and protect the workforce. Individual
process operations may be located in the clean room within a laminar flow
bench with HEPA filtration. These process operations have associated
engineering controls. These process specific engineering controls include
operation at negative pressure (vacuum conditions), local exhaust ventilation,
automated process control (microprocessor), enclosure (other than vacuum
conditions), shielding, and interlocks of the operation..
Work practices, environmental and process monitoring systems, and
personal protective equipment are used by Intel to complement engineering
controls. Intel appears to have a well-engineered, state-of-the-art facility,
with a comprehensive health and safety program to ensure protection of the
health and safety of the workforce.
Areas that warrant further study include:
1. Ventilation System Design—It would be beneficial to document
how the overall system is designed, how balance is achieved in
the system, and how the adequacy of the ventilation system is
determined. Procedures and work practices in maintenance of the
system should be evaluated in more detail. The general design
for adding processes and capacity should also be discussed.
2. Monitoring Systems—The calibration and testing of the Mi ran®
801 and the combustible gas monitoring systems should be
detailed. The analy.sis of output data trends should be
evaluated. Results of personal monitoring conducted by Intel
should be evaluated since the data may be useful in documenting
the effectiveness of the control technology.
3, Work Practices—Intel appears to excel in establishing safe work
practices. Documentation of the overall training philosophy,
training methods, and cost of training (administrative burden
and employee time) would be beneficial.
-------
-Maintenance Activities--The liaintenance activities of the
integrated circuit manufacturing processes should be
investigated. Work practices should be identified. Methods
used in controlling emissions and worker exposures during
maintenance activities should be detailed.
-------
35
9.0 REFERENCES
Colclaser, R. A. Microelectronics: Processing and Device Design. John Wiley
and Sons. New York. 1980.
Douglas, E. C. Advanced Process Technology for VLSI Circuits. Solid State
Technology. 24(5):65-72. 1981.
------- |