PRELIMINARY CONTROL TECHNOLOGY SURVEY
on
MICRO POWER SYSTEMS, INC.
Santa Clara, California
to
U.S. Environmental Protection Agency
Industrial Environmental Research Laboratory
26 West St. Clair Avenue
Cincinnati, Ohio 45268
and
National Institute for Occupational Safety and Health
Division of Physical Sciences and Engineering
4676 Columbia Parkway
Cincinnati, Ohio 45226
July 11, 1984
by
Gary J. Mihla« and Robert D. Willson
BATTELLE
Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
Report No. 115-26a
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PLANT SURVEYED:
SIC CODE:
SURVEY DATE:
SURVEY CONDUCTED BY:
EMPLOYER REPRESENTATIVES CONTACTED;
Micro Power Systems, Inc.
3100 Alfred Street
Santa Clara, California 95050
3674
January 15, 1982
Gary J. Mihlan
Battelle Columbus Laboratories
Robert D. Willson, CIH
PEDCo Environmental, Inc.
Paul E. Caplan, CIH
NIOSH
Mark Bridley, Facilities Engineering
Debbie Bush, Personnel Training
James E. Byrne, Vice President, Wafer
Fabrication Operations
Bill Glaskell, Process Engineering
Manager
Jerry S. Olson, Senior Process
Engineer
David Steck, Personnel Training
Edward J. Sawicki, Industrial Hygiene
Consultant
EMPLOYEE REPRESENTATIVES CONTACTED:
None (No Union)
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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 5
3.4 Monitoring System 6
3.5 Ventilation System 6
3.6 Waste Management System 7
4.0 PROCESS DESCRIPTION 8
5.0 DESCRIPTION OF PROGRAMS 12
5.1 Industrial Hygiene 12
5.2 Education and Training 12
5.3 Respirators and Other Personal Protective Equipment 13
5.4 Medical 13
5.5 Housekeeping and Maintenance 14
6.0 DESCRIPTION OF CONTROL STRATEGIES FOR PROCESS
OPERATIONS OF INTEREST 14
6.1 Thermal Oxidation 15
6.2 Photolithography 16
6.2.1 Substrate Preparation 17
6.2.2 Substrate Exposure 17
6.2.3 Substrate Developing 18
6.3 Wet Chemical Cleaning and Etching 19
6.4 Plasma Etching 20
6.5 Diffusion 21
6.6 Ion Implantation 22
6.7 Epitaxial Silicon Deposition 24
6.8 Chemical Vapor Deposition 25
6.9 Metalization 27
7.0 CONCLUSIONS AND RECOMMENDATIONS 27
8.0 REFERENCES 28
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1.0 ABSTRACT
A preliminary control technology assessment survey was conducted by
Battelle Columbus Laboratories at Micro Power Systems, Inc., Santa Clara,
California, on January 15, 1982. The survey was conducted under a U. S.
Environmental Protection Agency contract funded through an Interagency Agree-
ment with the National Institute for Occupational Safety and Health. The
facility manufactures complementary metal oxide semiconductor (CMOS) and bi-
polar integrated circuits.
The process operations performed at Micro Power Systems, Inc.
include: 1) thermal oxidation of purchased silicon wafers; photolithography
processes for defining circuit patterns, including photoresist application,
substrate exposure, and photoresist development; 3) wet chemical etching and
cleaning; 4) plasma etching; 5) doping, including diffusion and ion implanta-
tion; 6) epitaxial silicon deposition (for bipolar fabrication only); chemical
vapor deposition of silicon nitride and silicon dioxide; and metalization by
electron beam evaporation and direct current (DC) sputtering.
The process operations for integrated circuit fabrication are per-
formed in a clean room environment. The supply air is passed through a
neutron filter to a bag filter and then distributed to the fabrication area
through ceiling diffusers. High efficiency particulate air (HEPA) filters are
only used in laminar flow work stations for specific process operations.
Engineering controls used at the facility vary by process operation
and process equipment. Several process operations are performed in sealed
reaction chambers that isolate the processes from the workers. This isolation
technique is used in plasma etching, ion implantation, chemical vapor deposi-
tion, epitaxial silicon deposition, and metalization. Shielding is used in
ion implantation units to control X-ray radiation emissions, in plasma etching
and epitaxial silicon deposition to control radio frequency radiation emis-
sions, and in substrate exposure to control ultraviolet emissions. Local
exhaust ventilation removes vapors, process gases and byproducts in wet chemi-
cal cleaning and etching, in photolithography processes, and in diffusion fur-
naces used for thermal oxidation and diffusion. Local exhaust ventilation is
also used for gas storage cabinets. For some operations permanently installed
inclined manometers monitor the ventilation system. Several process
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operations are controlled automatically. These operations include electron
beam evaporation, DC sputtering, plasma etching, ion implantation, chemical
vapor deposition, and epitaxial silicon deposition.
No continuous area monitoring systems are present for evaluating
emissions or operator exposures to chemical or physical agents. The plant
plans to install a combustible-gas monitoring system. Previous monitoring
activities include colorimetric, direct-reading tubes for dopants, and a sur-
vey meter for X-ray radiation. Film badges are also used to monitor operator
exposures to X-ray radiation at the ion implantation unit.
Personal protective equipment is used by operators, maintenance and
repair technicians, and chemical technicians to control worker exposures to
chemical and physical agents. This equipment varies by the job title and task
performed and includes safety glasses, face shield, acid-resistant coveralls,
apron, gloves with gauntlets, unspecified dust respirators, and self-contained
breathing apparatus (for emergencies). Operators are also required to wear
product-protective equipment to control product quality. Micro Power Systems,
Inc. has established a health and safety program that includes worker training
in safety, materials handling, personal protective equipment, emergency
response, and hazard reporting. The facility also employs consultants in in-
dustrial hygiene, safety, and health care.
Process operations that should be considered for detailed investiga-
tion whether by Battelle or Micro Power Systems, include plasma etching, ion
implantation, and epitaxial silicon deposition. The effectiveness of the
local exhaust ventilation system in controlling chromic acid, antimony
trioxide, and phenol should be evaluated. Work practices that may affect
emissions or operator exposures to chemical and physical agents could not be
addressed for all operations during the preliminary survey. The facility has
developed work practice policies such as special handling of antimony trioxide
and the associated housekeeping practices that may be effective in controlling
exposures. These work practices should be documented during a detailed
survey.
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2.0 INTRODUCTION
A preliminary survey was conducted at Micro Power Systems, Inc.,
Santa Clara, California, on January 15, 1982 as part of a control technology
assessment of the semiconductor manufacturing industry. The study was per-
formed under U. S. Environmental Protection Agency Contract No. 68-03-3026
through an Interagency Agreement with the National Institute for Occupational
Safety and Health. The survey was conducted by Battelle Columbus Labora-
tories, Columbus, Ohio.
The following plant representatives supplied information at Micro
Power Systems, Inc.:
1. Mark Bridley, Manager, Facilities Engineering,
2. Debbie Bush, Personnel Training,
3. James E. Byrne, Vice President, Wafer Fabrication Operations,
4. Bill Glaskell, Process Engineering Manager,
5. Jerry S. Olson, Senior Process Engineer,
6. Edward J. Sawicki, Consultant, and
7. David Steck, Personnel Training.
The study protocol was provided to Mr. Mark Bridley before the
survey. During an opening conference, the study objectives and methods were
described. Plant staff provided a detailed description of the facility's
health and safety programs, including a review of the plant construction,
workforce, health and safety programs, air supply and exhaust, chemical
storage, and waste management practices.
Following the opening conference, the research team surveyed the
wafer fabrication area and chemical and waste storage areas. A closing con-
ference was held following the survey and all survey notes were reviewed with
the plant staff. The facility later provided injury and illness reports for
1981.
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3.0 PLANT DESCRIPTION
3.1 General
Micro Power Systems, Inc. has been in business since 1971. The
facility consists of two buildings: 1) a 42,720 square feet building of con-
crete block construction used for administration, marketing and sales, and 2)
a 11,340 square feet single story building of tilt up wall construction used
for wafer fabrication. The wafer fabrication building was constructed in 1972
with a masking room added in 1973. The administration building was con-
structed in 1975 and has not undergone any major changes.
The facility manufactures bipolar and complementary metal oxide
semiconductor (CMOS) integrated circuits. Operations performed at the
facility include wafer fabrication and testing. The facility employs 313
people with approximately 89 individuals in wafer fabrication, 59 in wafer
testing, 133 in administrative and technical services, and 32 individuals
providing outside services. The wafer fabrication and testing staff includes
81 workers on the first shift, 48 on the second shift, and 19 on the third
shift. Administrative, technical, and other staff primarily work the first
shift (160 of 165). Workers in the test area may work overtime.
3.2 Chemical Storage
Liquid chemicals are segregated as acids or organics and stored in
separate locked rooms in different buildings. The acids are supplied in 1-
gallon glass containers that are stored in boxes. The boxes are placed on
epoxy-coated metal shelves or on wooden shelves. The room is vented by an
exhaust in the ceiling and has a sprinkler system for fire control. The room
is not diked nor is a floor drain present. Workers handling acids are
required to wear gloves with gauntlets, apron, leather shoes, and a face
shield. Spill kits containing vermiculite are available in the room along
with a fire extinguisher and a fire hose.
Organic solvents are supplied in 1-gallon glass containers that are
stored in boxes and placed on wooden shelves. The room is vented by an
exhaust take-off located at the floor level and has fire sprinklers and
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explosion-proof wiring. The room is not diked nor is a drain present. Spill
clean-up materials or personal protective equipment were not observed in the
room. The solvents are transported to the wafer fabrication area by a chemi-
cal technician. The technicians place the 1-gallon bottles into safety con-
tainers that are loaded onto a cart and transported into the fabrication area.
Only the chemical technician or the area supervisor pour the chemicals.
3.3 Gas Handling System
Gases used in wafer fabrication are supplied from cylinders and from
bulk tank (i.e., house) supplies. Liquid oxygen is stored in a tank located
outside of the facility at the building used only for chemical storage and
administrative services. Oxygen is distributed to the fabrication area in
copper lines. Liquid nitrogen is stored in a bulk tank supplied by a pipeline
from an area gas supply firm. The lines do not have earthquake valves or
flow-limiting valves.
Gases supplied in cylinders are stored outside in a covered area.
Full cylinders are not segregated by gas type. Cylinder gases piped from the
outside include diborane, hydrogen, phosphine, and hydrogen chloride. The
cylinders are stored in the enclosed area and under a canopy. The gas mani-
folds are mounted on an outside wall of the chemical storage building and the
cylinders are chained to the wall. All toxic gases and hydrogen are distri-
buted in stainless steel lines. The lines that run above the ceiling are
welded and use compression (Swage loWK) fittings for connections below the
ceiling. Solenoid valves are present in the epitaxial reactor systems to shut
off hydrogen supply to the unit during power failures. All gas lines are
labeled with arrows indicating the direction of flow.
All toxic, corrosive, or flammable gases in the wafer fabrication
area are stored in ventilated gas cabinets. The gas cabinets are vented to
the plant scrubber system described in Section 3.5. Gas cabinets containing
silane cylinders have galvanized exhaust ducts that extend from the cabinet
through the ceiling. The galvanized duct acts as a burn box to provide a con-
trolled reaction of the spontaneously combustible gas should a leak occur.
Special procedures for handling gas cylinders are described in Section 5.3.
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3.4 Monitoring Systems
At the time of the survey there was no permanently installed con-
tinuous monitoring systems for evaluating toxic or combustible gases. Micro
Power Systems, Inc. has now installed a combustible-gas monitoring system.
Monitoring of combustible (hydrogen) gas is presently performed with a por-
table J and W TLV Sniffer®. The instrument is used to detect hydrogen leaks
in the epitaxial reactor area. Direct-reading colorimetric detector tubes
(Draeger®) are used to detect dopant gas (phosphine and diborane) leaks in the
fabrication area.
Film badges monitor emissions and operator exposures to X-ray radia-
tion in the ion implantation area. The badges are collected and sent to a
laboratory for weekly readings. X-ray emissions are also monitored with a
survey meter (unspecified).
3.5 Ventilation System
Specific details of the air circulation system at Micro Power
Systems were not obtained during the preliminary survey. Wafer fabrication is
performed in a clean room environment. Air is supplied to the area through
two air handlers having a total flow rate of approximately 18,000 cfm. The
air intake is located at roof level on the south end of the building.
The supply air is treated by passing it through an electrostatic
filter and a bag filter to collect particulates. The air is distributed to
the fabrication area through ceiling registers. High efficiency particulate
air (HEPA) filtration units are located in the fabrication area above indi-
vidual work stations. The air is supplied to the HEPA filtration units
through a grille located on the front of the unit that draws in air from the
area surrounding the work station. The photolithography area is maintained at
a positive pressure with respect to the remainder of the fabrication area.
Air is removed from the fabrication area by local exhaust ventila-
tion and by exfiltration. Exhaust from all wet chemical benches, photoresist
application and developing stations, chemical vapor deposition systems, diffu-
sion furnaces, ion implantation unit, and gas storage cabinets are vented to
the house exhaust system and passed through a 13,000 cfm water scrubber. The
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scrubber exhaust is vented to the atmosphere through a 30-inch diameter, 14-
ft. stack located approximately 30 ft. from the nearest air intake. The ex-
haust plenum for the wet chemical benches incorporates a demister to control
acid mists. Exhaust from the epitaxial reactor is vented through a separate
water scrubber of unspecified capacity.
Monitoring of local exhaust ventilation is performed monthly to
ensure a minimum capture velocity of 150 fpm at wet chemical stations.
Inclined manometers are installed at all wet chemical stations in the diffu-
sion area, on the wet chemical station containing sulfuric and chromic acid,
and on the chemical vapor deposition system used for silicon nitride
deposition. The manometers are checked daily. A weekly review of the
scrubber system is also performed to ensure proper fan performance (RPMs and
belt tension). Ventilated gas cabinets are monitored weekly to determine if
duct velocities are at least 300 fpm with access doors open. Diffusion
furnace scavenger box exhaust is also monitored to ensure a velocity of at
least 300 fpm in the exhaust duct.
3.6 Waste Management System
Liquid wastes are handled separately and may be categorized as
organics (including HMDS, photoresist, and developer), acids containing
fluorides, chromic acid, other acids without fluorides, and pump oils.
Organic wastes are collected by chemical technicians and transferred to drums
stored in a specified waste storage area. Chromic acid and acids containing
fluorides are each drained from the point of use to separate manifolds and
transferred into bottles by chemical technicians. The bottles are placed in
wooden boxes and stored on wooden shelves in a separate secured acid waste
room. The room is ventilated by a wall register and has a fire sprinkler
system. The drums and bottles are collected by a waste hauler for off-site
disposal.
Acids that do not contain fluoride are collected along with scrubber
blowdown by a central drain system and transferred to a neutralizing tank
where the pH is adjusted with ammonia. The neutralized acid is then piped to
the city sewer system. Pump oil from oil-sealed mechanical pumps used in pro-
cess equipment is drained by line maintenance workers. The waste is
transferred to drums and disposed by a waste hauler.
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4.0 PROCESS DESCRIPTION
The fabrication sequence used for metal oxide semiconductor (MOS)
and bipolar integrated circuits varies according to the specific type of
device manufactured. Process operations observed at the facility are dis-
cussed below. The specific sequence in which the process operations are per-
formed is not presented. A general processing sequence for MOS and bipolar
integrated circuit manufacturing is provided by Colclaser (1980) and should be
consulted for a more detailed review of the fabrication process. Several pro-
cess operations are employed more than once in the fabrication sequence and
some process equipment is used for more than one process operation. The sili-
con wafers used as a substrate for device fabrication are purchased.
In the thermal oxidation process, wafers are oxidized at a high tem-
perature (approximately 900 to 1200°C) in a diffusion furnace assembly with
water vapor present in the furnace tube atmosphere. Hydrogen chloride gas is
added to the furnace tube to clean ("gettering") the growing oxide and furnace
tube of sodium ion contamination (Colclaser, 1980). The wafers are loaded
into carriers that are inserted into the diffusion furnace. The furnace tube
is heated by electrical resistance to the operating temperature while the tube
is purged with nitrogen. The oxidation may also be performed with oxygen.
Gas flow into the furnace tube is controlled by a manual analog system that
requires the operator to monitor and adjust the gas flow.
Following thermal oxidation, the wafers are ready for photolitho-
graphy including: 1) wafer cleaning, 2) primer and photoresist coating, 3)
pre- or soft-bake, 4) mask alignment and exposure, 5) development, 6) post- or
hard-bake, 7) etching, and 8) photoresist stripping. The wafer is cleaned by
spin-on application of xylene. Hexamethyldisilizane (HMDS) and photoresist
are then applied to the wafer by spin application. The photoresist consists
of a proprietary mixture of photosensitive organic polymers in a xylene
carrier solution. The coated wafer is soft-baked in a resistance-heated oven.
The spin operation is controlled automatically, requiring manual operation
only to load and unload the spin platforms with wafers.
The mask pattern is transferred to the coated wafer by ultraviolet
light (unspecified wavelength) using either proximity or contact printing.
The operator aligns the wafer with the mask by viewing through a split-field
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binocular microscope. With both systems the wafer and mask are exposed to
ultraviolet light from a mercury lamp located behind the mask. Photomasks are
produced to plant specifications by an outside vendor.
The exposed wafers are developed by spin-on application of the
developer. A mixture of n-butyl acetate and xylene develops the negative
photoresist and an alkaline solution develops the positive photoresist. The
developed wafers are hard-baked in a resistance-heated oven.
The exposed underlying layer may be etched using either wet chemical
or plasma etching techniques. Wet chemical etching is performed by immersing
the wafers in an etching solution. The etching methods include: 1) hydro-
fluoric acid and ammonium fluoride (i.e., buffered oxide etch) for etching
silicon dioxide and cleaning wafers, 2) phosphoric/nitric/acetic acid or
ammonium hydroxide for etching metal, 3) phenol, sulfonic acid, chlorobenzene
and unspecified aromatic solvents for cleaning photoresist from wafers, 4)
sulfuric and chromic acid for cleaning photoresist from metalized wafers, 5)
hydrochloric acid and hydrogen peroxide for equipment cleaning, 6) sulfuric
acid/hydrogen peroxide for cleaning wafers and for stripping photoresist, and
7) phosphoric or sulfuric acid for cleaning wafers. The etching operations
are performed in tanks recessed in polypropylene benches.
Plasma etching is performed by placing wafers in a plasma gas formed
by a radio frequency power source operating at 13.56 MHz. The plasma gas con-
tains ions, free radicals, and free electrons that react with the layer to be
etched. The gas used for creating the plasma is selected based upon the indi-
vidual layer and is either 1) carbon tetrafluoride and oxygen for etching
silicon nitride, or 2) oxygen for stripping photoresist. The plasma is formed
in a sealed reaction chamber at a vacuum of 0.1 to 20 torr created by an oil-
sealed mechanical pump.
Doping introduces impurities into the wafer, altering the electrical
properties of the doped area. Wafers are doped at various stages of the pro-
cessing sequence either by diffusion or ion implantation. Diffusion is accom-
plished by exposing the wafer to a high temperature atmosphere containing the
dopant. The operation is performed in a diffusion furnace assembly using a
solid (antimony trioxide) or liquid (boron tribromide or phosphorus
oxychloride) dopant source. Hybrid control diffusion furnaces similar to
those used for thermal oxidation are employed for diffusion. The diffusion
process may also include oxidizing the wafer during the process.
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Wafers are also doped via ion implantation. A source material is
ionized and passed through an analyzing magnet where the desired ions are col-
lected, accelerated, and implanted into an individual wafer held in a vacuum
chamber. The ion source, the analyzing and accelerating chamber, and the
wafer exposure chamber are operated at vacuum conditions of approximately 10~&
torr. This vacuum is maintained by two sets of pumps; each set consists of an
oil-sealed mechanical pump and an oil diffusion pump. The dopant source is
either phosphorus pentafluoride or boron trifluoride. The process operation
requires the operator to load a cassette into the load station of the ion
implantation unit. Individual wafers are automatically removed from the
cassette to a load lock chamber which is pumped to vacuum by an oil-sealed
mechanical pump. The wafer is transferred to the exposure chamber where the
dopant ions are implanted. The dosage received by the wafer is controlled
automatically. The implanted wafer is transferred through a second load lock
chamber and then into a cassette.
A single crystal silicon layer is deposited on the wafer surface by
epitaxial growth in an enclosed chamber. Epitaxial silicon deposition is only
performed for bipolar integrated circuit fabrication. The single crystal
silicon layer is deposited by the reaction of silicon tetrachloride and hydro-
gen. A doped silicon layer is deposited by introducing phosphine. Epitaxial
silicon is deposited at high temperature (approximately 950 to 1250°C) in a
reaction chamber at atmospheric pressure and heated by radio frequency radia-
tion operating at a frequency of 450 to 550 kHz and a power of 50 kilowatts.
The operation sequence is automatically controlled and requires the operator
to load wafers onto a platen that is then sealed in a bell jar reaction
chamber. A description of epitaxial silicon deposition is provided by
Atherton (1981) and Hammond (1978) and should be consulted for more detailed
information.
Another process operation performed during the fabrication sequence
is the deposition of a thin film on the wafer surface by chemical vapor depo-
sition, i.e., where the solid products of a vapor phase chemical reaction are
deposited on the substrate. Atmospheric pressure chemical vapor deposition
(CVD) is used to deposit silicon nitride by the reaction of silane with
ammonia or nitrous oxide. The operation is performed in a sealed reaction
chamber. The process operation requires the operator to place wafers onto a
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flat plate susceptor that is loaded onto a loading platform. The operator
initiates the process sequence by push-button control and the susceptor is
automatically loaded into the reaction chamber where it is heated by electri-
cal resistance.
Silicon dioxide is deposited by the reaction of silane with oxygen
in either of two horizontal reactor systems. The wafers are placed onto
platens and either loaded into a reaction tube or placed on a conveyor where
they are transported into the deposition zone. Both operations require the
operator to load and unload cassettes and to monitor the process equipment
during operation.
A metal or metal alloy layer is deposited onto the wafer surface by
direct current (DC) sputtering or by electron beam evaporation. The metal is
deposited on the wafer surface in a sealed reaction chamber or bell jar that
is maintained at a vacuum of approximately 10~" torr by a set of pumps con-
sisting of an oil-sealed mechanical pump and an oil diffusion pump or an oil-
sealed mechanical pump and a cryogenic pump. Aluminum is deposited by elec-
tron beam evaporation and a proprietary metal alloy is deposited by DC sput-
tering. The process operation sequence requires the operator to place the
wafers in a planetary structure or metal platen that is loaded into the reac-
tion chamber. The process operation is then controlled automatically.
Process operations such as photolithography, doping, metalization,
and chemical vapor deposition may be repeated several times during wafer
fabrication. Between these processing steps, wafers may be cleaned with
sulfuric acid/hydrogen peroxide, hydrofluoric acid, sulfuric acid, or
phosphoric acid. These wet chemical cleaning operations are performed in
enclosed benches as previously described.
The final step in wafer fabrication is backside metalization. This
operation is not performed at the plant. Following metalization, the wafers
are then electrically tested and sent to an assembly plant for die separation,
assembly, and packaging. The plant does perform a limited amount of the
assembly and packaging operation for products that require a quick turn-
around. These operations include diamond scribing and cutting of the wafer,
assembly of the die in plastic, ceramic, or cerdip (ceramic dual in-line
package), and marking and packing. These operations were not observed at the
plant during the preliminary survey.
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5.0 DESCRIPTION OF PROGRAMS
5.1 Industrial Hygiene
Micro Power Systems, Inc. does not employ a full- or part-time
industrial hygienist or safety engineer. The facility engineering manager co-
ordinates all health and safety matters. The facility employs two consultants
in safety and industrial hygiene.
Monitoring of emissions or operator exposures to chemical agents has
been limited to the use of a portable combustible-gas meter and direct-
reading, colorimetric detector tubes to check for leaks of process gases.
Personal sampling of workers has previously been performed but the facility
has contracted with a consulting firm to conduct personal sampling in the
future. The facility also has plans to install a combustible-gas detection
system as described in Section 3.4, and to conduct area monitoring of the dif-
fusion and photolithography (masking) areas. Film badges monitor emissions
and operator exposures to X-ray radiation at the ion implantation unit. The
badges are collected weekly and sent to a contract laboratory for analysis. A
survey meter (unspecified) is used to monitor X-ray radiation from the ion
implantation unit following maintenance of the unit. Measurements of the ven-
tilation sytem are described in Section 3.5.
5.2 Education and Training
Training programs have been developed in the areas of safety,
materials handling, personal protective equipment, emergency procedures, and
hazard reporting. New employees undergo a safety orientation which reviews
personal protective equipment requirements, electrical safety, gas cylinder
and chemical safety, response procedures during earthquakes and fires, and
evacuation procedures. A manual has been developed that addresses each of
these areas in detail. Newly hired employees are reviewed for a period to
ensure safe work practices. The supervisor is responsible for training new
employees.
Wafer fabrication area employees are trained on the job by senior
line technicians. The training requires 2 to 4 weeks and covers the use of
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personal protective equipment and spill cleanup. Operators are required to be
able to perform any other operator's job functions. A safety committee, con-
sisting of employee representatives, direct-line supervisors, and management
meets weekly.
5.3 Respirators and Other Personal Protective Equipment
All workers in the wafer fabrication area are required to wear
safety glasses, chemically resistant aprons, and safety shoes. Sandals or
cloth type shoes are prohibited in the fabrication area. Chemical techni-
cians, responsible for mixing and transferring liquid chemicals and wastes,
are required to wear acid-resistant coveralls, boots, gloves with gauntlets,
face shields, and aprons. Heat-protective aluminized asbestos gloves (Nome)®
or Zytel®) are used by operators handling hot equipment in the diffusion fur-
nace area. Operators use a respirator (unspecified type) when handling anti-
mony trioxide during the diffusion operation.
Emergency equipment available at the facility includes self-
contained breathing apparatus. Emergency showers, eye wash stations, and
emergency breathing oxygen are also available in the fabrication area. The
emergency showers do not have drains.
Operators responsible for changing gas cylinders are required to
wear safety glasses. Cylinders are checked by a qualified person using a
Snoop® bubble test of welded seams and valves. A self-contained breathing
apparatus is readily accessible in the gas storage area.
5.4 Medical
The facility does not employ a full- or part-time nurse or physi-
cian. Medical services provided at the plant are limited to first aid and
cardiopulmonary resuscitation (CRR) provided by trained employees present on
all three shifts. An off-site industrial medical clinic provides emergency
care. Micro Power Systems does not require preplacement or periodic medical
examinations of any employees. No routine biological monitoring is performed.
OSHA injury and illness statistics for 1981 show only four reported injuries:
1) a cut on the thumb from a hand saw, 2) a broken right thumb from a fall,
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3) a muscle strain from lifting materials, and 4) a hematoma on the forearm
following blood donation.
5.5 Housekeeping and Maintenance
Housekeeping and maintenance activities are necessary parts of main-
taining product quality and equipment operations. General housekeeping proce-
dures include prompt cleanup of spills and placing all trash and scrap into
waste containers. Hazardous conditions that housekeeping can correct are
immediately reported to the supervisor when observed by employees.
Specific housekeeping procedures which were identified by the plant
to prevent worker exposures to chemical agents include a portable vacuum in
the diffusion area for cleaning antimony trioxide spills and for removing any
antimony that has condensed around the scavenger box exhaust.
Planned maintenance activities could not be identified for each pro-
cess operation at the facility due to time constraints. General maintenance
activities include periodic draining and replacement of pump oils from the
oil-sealed mechanical pumps used for ion implantation, plasma etching systems,
and metalization systems.
Components of the ion implantation unit are disassembled and cleaned
by bead blasting in a glove box using a silicon abrasive. Diffusion furnace
tubes may be cleaned by an in situ process using hydrogen chloride gas. The
bell jar chamber from the epitaxial reactor system is removed and cleaned two
times per month with hydrofluoric acid. The bell jar chambers from the
metalization systems are removed and cleaned with hydrochloric acid and
hydrogen peroxide three to four times per year. Quartzware from the silicon
nitride CVD operation is cleaned every 3 weeks with hydrofluoric acid.
6.0 DESCRIPTION OF CONTROL STRATEGY FOR PROCESS OPERATIONS OF INTEREST
A variety of control strategies are used at Micro Power Systems to
control emissions and worker exposures. These control strategies include
local and general exhaust ventilation, process isolation, process and environ-
mental monitoring, and personal protective equipment. Devices or work
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stations that contain toxic materials considered of potential danger to life
and health are controlled by local exhaust ventilation, whereas less
potentially hazardous areas are controlled by general exhaust ventilation.
Specific engineering control strategies for individual process operations are
described below. Monitoring systems are described in Section 3.4 and briefly
described below for each specific process operation. General personal
protective equipment requirements are described in Section 5.3; specific
requirements for some process operations are summarized below.
Process automation has influenced many work practices and, there-
fore, operator exposures to chemical and physical agents. Automated process
controls limit the time that operators are working with the equipment and only
require that the operator load and unload wafers, initiate the processing
sequence (with push-button controls), and perform routine cleaning operations.
The operator is then free to perform other tasks such as wet chemical cleaning
and etching or to operate other automated units as he or she is not required
to be at a specific unit for an entire work shift. Hence, any exposures to
chemical or physical agents would be limited to relatively short time periods
throughout the shift.
Specific descriptions are given below for control strategies
employed for thermal oxidation, photolithography, wet chemical etching and
cleaning, plasma etching, diffusion, ion implantation, epitaxial silicon depo-
sition, chemical vapor deposition (CVD), and metalization. Although wafer
testing and some scribing and assembly operations are performed at the plant,
these were not observed during the preliminary survey.
6.1 Thermal Oxidation
A diffusion furnace assembly is used to oxidize purchased silicon
wafers. The wafers are exposed to water vapor in a high temperature environ-
ment in the furnace tube. The water vapor is supplied by boiling water in a
flask that is connected to the furnace tube. Oxidation is also performed as
part of the diffusion operation using oxygen.
The diffusion furnace assembly consists of: 1) a load station where
carriers containing wafers are loaded and unloaded in the furnace tube; 2) a
furnace cabinet containing the furnace tubes and electrical resistance heat
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source; and 3) a source cabinet that encloses the furnace tube end. Process
gases enter the furnace tube through tubing which connects at the source cabi-
net end. The furnace cabinet acts as a protective barrier against the hot
contact surfaces of the furnace tube. Processing gases include nitrogen, oxy-
gen, and hydrogen chloride. The hydrogen chloride gas is used to clean or
getter both the growing oxide and the furnace tube of sodium ion contamination
(Colclaser, 1980).
The diffusion furnace assembly is ventilated at the furnace tube
opening by a scavenger box that encloses the opening. The air velocity at the
scavenger box is maintained at 300 fpm.
Nitrogen and oxygen are provided from house supplies described in
Section 3.3. Hydrogen chloride gas is supplied in cylinders stored in a
covered and secured area outside of the building.
The operation is performed by placing wafers in carriers that are
loaded into the furnace. The tube temperature is increased (the specific tem-
perature depends on the operation performed) and purged with nitrogen followed
by introduction of the water vapor. The oxide film is then cleaned by the
addition of two to six percent of HC1 gas. After completion of the process
sequence, the operator removes the carriers. The furnace operating parameters
use a manual analog control that is preset but requires operator observation
and adjustment as needed.
No permanently installed, continuous monitoring systems are present
in the area for evaluating combustible or toxic gases. General personal pro-
tective equipment requirements include safety glasses and heat-protective,
aluminized asbestos gloves.
6.2 Photolithography
The photolithography process may be repeated several times during
the processing sequence. The photolithography process consists of four basic
steps; 1) substrate preparation, 2) substrate exposure, 3) substrate
developing, and 4) photoresist stripping. Following wafer developing, the
exposed underlying layer may be etched using either a wet chemical etching or
plasma etching operation described in Sections 6.3 and 6.4 respectively. The
photoresist stripping operation is also performed either by wet chemical
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etching or plasma etching. Photomasks used for substrate exposure are pro-
duced to plant specifications by an outside vendor.
6.2.1 Substrate Penetration. The operations performed in substrate
preparation include: 1) cleaning of the wafer by spin application of xylene,
2) spin-on application of hexamethyldisilizane and a photoresist, and 3) soft-
bake of the coated wafer in a resistance-heated oven.
The operator places the wafers on a loading jig and places the jig
over a set of four spin platforms. The process is initiated by pushbutton and
the spin platform rises to lift the wafers above the jig. The loading jig is
removed by the operator, and the wafers are spun. Xylene is automatically
applied to the wafer followed by HMDS and either a negative or positive photo-
resist. The wafers are then removed from the platform with the loading jig.
Coated wafers are placed in a resistance-heated oven that is purged with
nitrogen. The oven is vented to the room air.
The photoresist is a mixture of proprietary photosensitive organic
polymers in a xylene carrier. Xylene, HMDS and the photoresist are supplied
in separate 5-gallon pressurized metal containers that are stored in a cabinet
beneath the spin platforms. The containers are electrically grounded.
The spin platforms are ventilated from below through the waste drain
into a collection tank that has an in-line blower to boost the exhaust into
the main duct. The exhaust is directed to the plant scrubber system as de-
scribed in Section 3.5.
No monitoring systems are present for evaluating emissions or expo-
sures to chemical or physical agents. Personal protective equipment used by
operators includes the normal fabrication area requirements described in
Section 5.3.
6.2.2 Substrate Exposure. A mask pattern is transferred to the
photoresist-coated wafers by a mask image with ultraviolet (UV) light using
either proximity or contact printing. The two methods differ in that the mask
is either in contact with the wafer (contact printing) or the mask is sepa-
rated from the wafer (proximity printing). The ultraviolet light source for
both systems is a mercury lamp producing UV light of unspecified wavelength.
The mercury lamp is enclosed to prevent direct viewing of UV light. The lamp
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enclosures are not vented. The exposure systems are located beneath vertical
laminar flow HEPA filtration units to control wafer contamination by
particulates.
The process is performed by an operator seated at the unit. A cas-
sette, containing photoresist-coated wafers, is loaded into the exposure unit.
An individual wafer is automatically removed from the cassette and rotated
into position. The operator aligns the mask and wafer with a split-field
binocular microscope, then exposes the wafer to UV light. The wafer is auto-
matically removed and the process is repeated.
The intensity of the ultraviolet light is monitored for quality con-
trol. The lamp is replaced when the intensity decreases below a predetermined
limit. Bettes (1982) provides a detailed review of controlling ultraviolet
exposure systems and should be consulted for additional information.
Operators are required to wear safety glasses which may be effective
in protecting workers from UV exposure. However, removal of the glasses
during alignment of the wafers may result in ocular exposure should UV light
emissions occur. Monitoring systems for evaluating emissions or operator
exposures to physical or chemical agents were not present in the area.
6.2.3 Substrate Developing. Photoresist-coated wafers that have
been exposed to the mask pattern are developed by spin-application of the
developer solution onto the wafer surface. Negative photoresist is developed
with xylene, and n-butyl acetate. Positive photoresist is developed with an
alkaline solution. The developers are applied to the wafer in a spin-on
operation similar to that used for photoresist application described in
Section 6.2.1. The spin system used for developing differs by the presence of
a hinged plastic enclosure that is placed over the spin platforms.
Following application of the developer solution, the wafers are
inspected visually. The wafers are then hard-baked in a resistance-heated
oven that is purged with nitrogen and vented to the room atmosphere.
Engineering controls for the developer operation are similar to
those described in Section 6.2.1. No monitoring systems are present for
evaluating emissions or operator exposures to chemical or physical agents.
Personal protective equipment requirements include safety glasses and safety
shoes.
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6.3 Wet Chemical Cleaning and Etching
Wet chemical operations are used to clean wafers, to etch deposited
layers, and to clean process equipment. Polypropylene benches are used for
acid cleaning and etching operations. Chemicals used in wet benches include:
1) hydrofluoric acid and ammonium fluoride (buffered oxide etch) for etching
silicon dioxide; 2) sulfuric acid/hydrogen peroxide, sulfuric acid, hydro-
fluoric acid, buffered oxide etch, or phosphoric acid for cleaning wafers
before specific process operations; 3) phosphoric acid/nitric acid/acetic acid
or ammonium hydroxide for etching metal; 4) phenol, sulfonic acid, chloro-
benzene, and unspecified aromatic solvents for cleaning photoresist from
wafers; 5) sulfuric acid and chromic acid for cleaning photoresist from
metalized wafers; 6) hydrochloric acid and hydrogen peroxide for cleaning the
metalization system bell jar; and 7) sulfuric acid/hydrogen peroxide for
photoresist stripping. Cleaning and etching operations are performed by
placing carriers containing wafers into the appropriate etching bath. The
tank contents and concentrations are labeled for all tanks.
Engineering controls used on wet chemical benches are similar for
all operations, although some variations in the use of local exhaust ventila-
tion do exist. The tanks containing the etching or cleaning solutions are
recessed in the benches. For some benches the work surface is perforated with
a ventilated spill plenum located below the perforated deck. A slot across
the rear panel of the bench provides additional ventilation of the tank. In
tanks without a perforated deck, ventilation is provided by slots located
around the tank perimater and by a slot across the rear panel of the bench.
In addition to the perforated deck and rear panel slot exhaust, a
slot across the top of the hood enclosure provides additional control in the
bench containing chromic acid. The spill plenum and slot exhausts enter a
demister plenum located at the rear of the bench. The tanks containing the
etching or cleaning solutions are located across the rear of the bench in
front of the slot exhaust. Additional tanks containing deionized water are
located in each bench to rinse wafers before further processing. A recircula-
ting, water-cooled lid is placed over the phosphoric acid tank to control
vapors. The lid is removed to immerse the wafers. The exhausts from all wet
chemical benches are vented to the plant scrubber system described in Section
3.5.
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Permanently installed inclined pitot-static manometers monitor the
performance of the local exhaust ventilation for the majority of benches. The
manometers display a duct velocity (fpm) and are checked daily by plant
engineering. Hinged, clear-plastic splash shields are mounted across the
front of each bench.
Wastes are either drained to a central system for pH neutralization
or they are drained into containers below the benches and collected by chemi-
cal technicians for disposal by a waste contractor. A detailed description of
the waste management practices is provided in Section 3.6.
Operators are required to wear safety glasses, aprons, and chemical-
resistant gloves. Chemical technicians who are responsible for mixing, trans-
porting, and pouring chemicals and for removing wastes wear safety glasses,
face shields, acid-resistant coveralls, aprons, safety shoes or boots, and
gloves with gauntlets. No routine monitoring is performed to evaluate emis-
sions or operator exposures to chemical agents.
All acids are transported in polypropylene carts. Written proce-
dures for using the carts are posted and require that the bottom shelf of the
cart should be filled before the top shelf to prevent the cart from tipping.
Emergency showers and eyewash stations are available throughout the fabrica-
tion area. Spill kits containing vermiculite are also available in the area.
6.4 Plasma Etching
Plasma etching is a chemical etching method using a plasma gas con-
taining ions, free electrons, and free radicals to remove a specific material
or layer from the wafer surface. The plasma is created by ionizing a gas in a
radio frequency field at 13.56 MHz. The gases and types of reactors (barrel
and planar) vary according to the layer to be etched. A barrel reaction
chamber is used to strip photoresist from the wafer with an oxygen plasma. A
planar reaction chamber, operating at 100 watts, is used to etch silicon
nitride with a carbon tetrafluoride and oxygen plasma. A detailed description
of plasma etching technologies is provided by O'Neill (1981) and Bersin (1976)
and should be consulted for additional information.
Plasma etching is performed with the reaction chamber pressure
negative to the room pressure. The vacuum is approximately 0.1 to 20 torr and
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is maintained by an oil-sealed mechanical pump. The plasma gases, containing
the volatile species formed by the plasma ions reacting with the substrate,
are exhausted from the unit by the pump and go to a scrubber. Carbon tetra-
fluoride/oxygen is supplied from a cylinder stored adjacent to the unit.
Oxygen is provided from house supplies.
Task? performed by the operator depend on the system used. Silicon
nitride etching is performed in an automated cassette-to-cassette unit that
processes individual wafers. Operators load and unload cassettes containing
wafers into the unit and initiate the process by pushbutton. Photoresist
stripping is performed as a batch operation in an automated unit. The
operator places carriers containing wafers into the reaction chamber. The
chamber door is closed and the process sequence is initiated by pushbutton.
Personal protective equipment requirements consist of safety
glasses.
Periodic maintenance includes draining and replacement of pump oil
by line maintenance technicians. The oil is considered a hazardous waste and
the disposal practices are described in Section 3.6.
The plasma etching systems were not observed in operation during the
preliminary survey.
6.5 Diffusion
Diffusion of dopants into the wafer is performed in a diffusion
furnace assembly similar to that described in Section 7.1 for thermal oxida-
tion. The wafers are heated to a high temperature (approximately 600 to
1200°C depending on the source used) and a dopant is introduced that diffuses
into the wafer. The dopant is supplied as either a solid (antimony trioxide)
or a liquid (boron tribromide or phosphorus oxychloride).
Antimony trioxide is supplied in a powdered form. The dopant is
placed into a quartz spoon that is loaded into a source furnace. The source
furnace is located in the source cabinet and attached to the furnace tube.
The antimony trioxide is stored in a sealed, nitrogen-purged cabinet at the
back of the diffusion furnace assembly.
The liquid dopants are supplied in bubblers that are placed in
cooling flasks located in the source cabinet. The flasks use ethylene glycol
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to maintain a stable temperature. The bubbler is attached to the furnace
tube. Nitrogen or oxygen is introduced into the bubbler and becomes
saturated. The saturated gas is vented into the furnace tube.
Engineering controls present in the diffusion operation are outlined
in Section 6.1 for diffusion furnace assemblies. These controls include local
exhaust ventilation of the furnace tube opening by a scavenger box. The
source cabinet containing the liquid bubblers or source furnace is not
ventilated or enclosed. A vacuum cleaning system (a portable shop vacuum
cleaner) is used to clean the scavenger box and the vacuum nozzle is located
next to the furnace tubes. The vacuum cleaning system is also used to clean
up powder that may be spilled during transfer.
Operators wear a respirator (unspecified) when transferring antimony
trioxide into the furnace tube. Aluminized asbestos gloves are used for
handling hot quartzware.
Detailed written procedures for handling liquid bubblers have been
developed. These procedures include: 1) the use of acid resistant gloves and
a chemical apron with sleevelets when handling the bubblers, 2) instructions
for unpacking the bubblers, and 3) instructions for attaching the bubblers to
the diffusion furnace. Workers replacing bubblers are required to wear safety
glasses, face shield, gloves with gauntlets, and an apron. A self-contained
breathing apparatus is available for cleaning up the liquid dopant if a spill
should occur. Written spill cleanup procedures have been developed.
6.6 Ion Implantation
Ion implantation introduces impurities or dopants into the wafer
surface. The impurities are p- or n-type ions created by a confined electric
discharge sustained by a dopant gas. The ion beam is drawn from the arc
chamber by an extraction electrode and directed toward the analyzing magnet.
The magnet resolves and focuses the ion beam and selects only the desired ion
species for wafer implantation. The selected ions are targeted through an
acceleration chamber and focused to produce a uniform dose to the substrate.
The ion implantation is performed in a sealed chamber at vacuum.conditions of
approximately 10~^ torr. The ion implantation unit operates at a power of 200
keV.
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Phosphorus pentafluoride and boron trifluoride, both at 100 percent
concentrations, are used as the dopant source gases. The gases are stored in
a ventilated cabinet located within a lead shield enclosure at the source end
of the unit.
The power source and ion source are contained in a lead-shielded
cabinet to control X-ray radiation emissions. The cabinet is electrically
interlocked to the power source to shut down if the cabinet access door is
opened. The beam exits the cabinet and is directed to a deflection magnet
which directs the beam to one of two implant chambers located in the room.
Controls for the unit are located near the implantation chambers.
Two sets of pumps maintain vacuum conditions in the unit. The pump
sets consist of an oil-sealed mechanical (roughing) pump and an oil diffusion
pump. One set evacuates the ion source and the second set evacuates the ion
beamline. A separate oil-sealed mechanical pump evacuates the implantation
chamber where wafers are processed. The pump exhausts are vented to the house
scrubber system described in Section 3.5.
The operation is a batch process where wafers are loaded by the
operator onto one of two carousels. Each carousel is contained in a separate
implantation chamber. After loading the wafers, the chambers are sealed and
the operator initiates the process sequence by push-button control. The
control panel is located between the two implantation chambers.
Scheduled maintenance of the unit includes draining and replacement
of pump oils for the mechanical pumps with a silicon-based oil. Every 2 years
parts of the implantation unit are disassembled and cleaned by blasting with
silica beads in a glove box. The maintenance operation is performed by an
engineer or maintenance technician.
Radiation film badges are used for monitoring worker exposure to X-
ray radiation. The badges are analyzed weekly by an outside laboratory. A
portable survey meter (unspecified type) is used by the facility engineer to
monitor X-ray radiation emissions. Previous surveys have indicated detectable
X-ray emissions at the deflection magnet. The magnet is not shielded and is
in an area near the operator that is also accessible to other workers in the
area.
Personal protective equipment requirements include safety glasses.
Personal protective equipment requirements for handling compressed gas
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cylinders are described in Section 5.3. The ion implantation unit was not in
use at the time of the survey. Therefore, work practices for controlling
emissions or operator exposures were not observed.
6.7 Epitaxial Silicon Deposition
A single crystal silicon layer is deposited on the silicon wafer in
an epitaxial reactor system consisting of a reactor assembly cabinet and sys-
tem control console. The process sequence is controlled automatically but
specific operating parameters, such as gas flow, are controlled by the
operator. The components of the reactor assembly cabinet include a gas
distribution system, radio frequency power supply, reactor chamber, and system
exhaust.
The reactor chambers consist of two quartz bell jars that are con-
tained within separate enclosures in the assembly cabinet. Access to the bell
jars is through a sliding panel on the front of the unit. The control console
is also located in the cabinet next to the bell jars.
The first step in epitaxial silicon deposition is the etching of
wafers with hydrofluoric acid. The operation is identical to that described
in Section 6.3. The quartz bell jar is mechanically raised to expose a flat
plate graphite susceptor. The operator places wafers onto the susceptor and
the bell jar is lowered into place. The enclosure containing the bell jar is
closed with a sliding panel and the process sequence is initiated. The
chamber is heated to operating temperature by a 450 to 550 KHz radio frequency
power source operating at 50 kilowatts. The wafers are cleaned in the chamber
with hydrogen chloride followed by deposition of the epitaxial silicon layer
through the introduction of silicon tetrachloride into the chamber. The
chamber is cooled, purged, and the wafers are unloaded by the operator for
inspection. The wafers are inspected with laser ellipsometry using an ultra-
violet laser at 273 ran wavelength. The power level and operating mode were
not specified.
Process gases include 1) hydrogen chloride for wafer cleaning, 2)
silicon tetrachloride and phosphine (300 ppm in hydrogen) for deposition of a
phosphorus-doped epitaxial layer, and 3) hydrogen and nitrogen for purging the
reactor chamber. The process gases are supplied in cylinders stored outside
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of the building in a covered area described in Section 3.3. The epitaxial
reactor is located in a room with an outside wall contiguous with the gas
storage area. Gases are distributed in stainless steel lines that are labeled
with the direction of flow and gas content.
The reactor chamber bell jars are contained in separate enclosures
in the cabinet. Cooling coils are mounted on the walls of each enclosure for
heat control. The walls of the enclosure also act as exhaust plenums with air
entering through slots located at the base of the walls, level with the
susceptors. The exhaust plenums provide heat control to the enclosure and
removes process gases that may escape from the bell jar chamber. The sliding
panel that provides access to the bell jars is interlocked with the radio
frequency power source and will shut off power if the panel is opened. The
bell jars are ventilated by a separate exhaust system through stainless steel
tubing to a wafer scrubber separate from the plant scrubber system.
Routine monitoring is performed with a TLV Sniffer® to check for
hydrogen leaks. Direct reading colorimetric tubes are used to check for phos-
phine leaks. At the time of the survey the installation of a combustible-gas
(hydrogen) monitoring system was planned within 45 to 60 days. The system is
now in operation. Personal protective equipment requirements include safety
glasses and heat-protective, aluminized asbestos gloves for handling hot
materials.
6.8 Chemical Vapor Deposition
Chemical vapor depositon (CVD) is the process of depositing a film
on the wafer surface by a chemical reaction or pyrolytic decomposition in the
gas phase. Silicon nitride and silicon dioxide are deposited using two dif-
ferent CVD systems described below.
Silicon nitride is deposited by the reaction of silane with ammonia
or nitrous oxide. The system consists of a cabinet containing the reaction
chamber, a loading station, and a control console. The operation is performed
at atmospheric pressure with the reaction chamber vented to the plant scrubber
system. The reaction chamber is heated by an infrared heat source. Ammonia
and silane are supplied in cylinders stored in a ventilated cabinet located in
the room. A galvanized metal duct is used to exhaust the storage cabinet. A
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cylinder containing nitrous oxide is chained to a wall adjacent to the unit.
A small canopy hood encloses the gas regulator attached to the cylinder to
provide ventilation in case of a leak.
The wafers are placed on a flat plate susceptor and automatically
loaded into the horizontal reaction chamber. The process sequence is
initiated by push-button control. The chamber is sealed, purged, and the
wafers are heated by electrical resistance. The process gases are introduced
and react to deposit a silicon nitride layer. The chamber is cooled and the
susceptor is automatically unloaded from the reaction chamber. The operator
removes the wafers from the susceptor.
Additional engineering controls include thermal shielding of the
reaction chamber provided by the cabinet to prevent worker contact with hot
surfaces. The reaction chamber access door is interlocked to the gas flow
control to prevent opening before the 3 minute purge cycle is completed.
Routine maintenance of the unit includes an jti situ cleaning with
hydrogen chloride once every shift. Once every 3 weeks the chamber components
are cleaned with hydrofluoric acid at a wet chemical station similar to those
described in Section 6.3.
Silicon dioxide is deposited by the reaction of silane with oxygen
in either of two horizontal reactor systems. The first system consists of a
sealed reaction chamber operating at atmospheric pressure. The wafers are
placed on a susceptor and loaded into the chamber tube. The operation
sequence is initiated by push-button control and the tube is automatically
closed. The reaction chamber is vented to the plant scrubber system. The
second system uses a gas distribution/deposition head mounted above a con-
veyor. The wafers are loaded onto platens and placed on the conveyor. The
wafers are heated on the conveyor and transported to the deposition head. The
deposition head consists of parallel metal plates that are mounted vertically
and perpendicular to the conveyor path. The plates provide uniform distribu-
tion of the process gases across the wafer surface. The deposition head is
ventilated at both ends where the wafers enter and exit. The exhaust is
vented to the plant scrubber system.
Process gases for both silicon dioxide deposition systems are
supplied in cylinders stored in a ventilated cabinet located in the room. The
gas cabinet is vented to the plant scrubber system.
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6.9 Metalization
A metal or metal alloy is deposited onto the wafer surface as an
electrical contact with the circuit components. Metals are deposited by: 1)
direct current (DC) sputtering of a proprietary metal alloy, and 2) electron
beam evaporation of aluminum. The metalization process is performed in a
sealed reaction chamber under low pressure (approximately 10~6 torr) main-
tained by a set of pumps including an oil-sealed mechanical pump and oil dif-
fusion pump or an oil-sealed mechanical pump and a cryogenic pump. The pump
exhausts are vented to the plant scrubber system.
Both metalization systems are automatically controlled. Operators
are required to mount the wafers onto a planetary structure or a platen that
is then placed inside the reaction chamber. The process sequence is initiated
by push-button control, the chamber is sealed and evacuated by the pumping
system. The power is applied to the metal source and the metal is evaporated
or sputtered onto the wafer. Following metal deposition the chamber is
cooled, vented, and the wafers are unloaded.
There appeared to be little potential for operator exposure to
chemical or physical agents during normal process operation. Personal
protective equipment requirements include safety glasses.
Routine maintenance operations include draining and replacement of
the pump oil as described in Section 5.5. The reaction chamber bell jar from
the electron beam evaporation unit is removed from the unit and cleaned with
hydrochloric acid and hydrogen peroxide at a wet chemical station similar to
that described in Section 6.3. The cleaning operation is performed three to
four times per year. The reaction chambers are also cleaned in place by
vacuum cleaning.
7.0 CONCLUSIONS AND RECOMMENDATIONS
Components of the integrated circuit fabrication process at Micro
Power Systems, Inc. are representative of the present state-of-the art in
wafer processing. Process operations observed that may also be indicative of
future processing trends include the use of plasma etching. The plant has
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28
included a variety of engineering controls to limit emissions or operator
exposures to chemical and physical agents. These controls consist of local
exhaust ventilation of all operations involving potentially hazardous agents,
monitoring of some exhaust systems with permanently installed inclined mano-
meters, and special housekeeping and work practices that control emissions or
operator exposures. The facility staff recognizes the potential hazards that
exist at the facility and has developed training programs, engineering con-
trols, work practices, personal protective equipment, and monitoring systems
to control the hazards. The engineering controls are frequently included in
the original design of the purchased equipment or are an integral part of the
process operation. Specific recommendations are outlined below.
1. Preplacement and periodic medical examinations should be esta-
blished, especially for those individuals potentially exposed to
toxic agents such as antimony trioxide and chromic acid.
2. The deflection magnet of the ion implantation unit should be
shielded to control X-ray radiation emissions.
A detailed survey should evaluate radio frequency emissions from
plasma etching and epitaxial silicon deposition and X-ray radiation from ion
implantation. The effectiveness of the local exhaust ventilation system to
control emissions of chemical agents such as chromic acid, phenol, and
antimony trioxide should also be evaluated.
8.0 REFERENCES
Atherton, R. W., Microelectronics: Processing and Device Design. John Wiley
and Sons. New York. 1980.
Bersin, R. A., A Survey of Plasma Etching Processes. Solid State Technology.
19(5):31-36. 1976.
Bettes, T. C., UV Exposure, Systems and Controls. Semiconductor
International. 5(4):83-96. 1982.
Colclaser, R. A., Microelectronics: Processing and Device Design. John Wiley
and Sons. New York. 1980.
Hammond, M. L., Silicon Epitaxy. Solid State Technology. 21(11):68-75. 1978,
O'Neill, T. G., Dry Etching Systems for Planar Processing. Semiconductor
International. 4(4):67-89. 1981.
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