vvEPA
United States
Environmental Protection
Agency
Research and Development
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
EPA/600/S-92/036 Sept. 1992
ENVIRONMENTAL
RESEARCH BRIEF
Waste Minimization Assessment for a Manufacturer of
Silicon-Controlled Rectifiers and Schottky Rectifiers
Harry W. Edwards, Michael Kostrzewa*
Phylissa S. Miller, and Gwen P. Looby"
Abstract
The U.S. Environmental Protection Agency (EPA) has funded
a pilot project to assist small- and medium-size manufacturers
who want to minimize their generation of waste but who lack
the expertise to do so. In an effort to assist these manufactur-
ers Waste Minimization Assessment Centers (WMACs) were
established at selected universities and procedures were
adapted from the EPA Waste Minimization Opportunity As-
sessment Manual (EPA/625/7-88/003, July 1988). The WMAC
team at Colorado State University performed an assessment at
a plant manufacturing devices for converting alternating current
into direct current (silicon-controlled rectifiers and Schottky
rectifiers) — approximately 2.5 million units per year. Rectifier
manufacture is a two step process: wafer fabrication and as-
sembly. Silicon wafers are doped, spin coated, cleaned, and
rinsed. Next, the wafers are etched and the resist is stripped to
produce a final groove pattern. Layers of polysilicate and silicon
nitride are deposited via chemical vapor deposition, silicon
glass is fused to the surface ground, and then the wafer is cut
into chips or dice. The dice are tested, sorted, and evaluated
and then transferred to assembly. The team's report, detailing
findings and recommendations, indicated that the majority of
waste was generated by the stack scrubbers used to remove
contaminants from exhausted plant air and that the greatest
savings could be obtained by redirecting reject water from the
reverse osmosis unit to the stack scrubbers to eliminate the
wastewater stream from the reverse osmosis unit.
This Research Brief was developed by the principal investiga-
tors and EPA's Risk Reduction Engineering Laboratory, Cin-
cinnati, OH, to announce key findings of an ongoing research
project that is fully documented in a separate report of the
same title available from University City Science Center
' Colorado State University, Department of Mechanical Engineering
" University City Science Center, Philadelphia, PA
Introduction
The amount of waste generated by industrial plants has become
an increasingly costly problem for manufacturers and an addi-
tional stress on the environment. One solution to the problem
of waste is to reduce or eliminate the waste at its source.
University City Science Center (Philadelphia, PA) has begun a
pilot project to assist small- and medium-size manufacturers
who want to minimize their formation of waste but who lack the
inhouse expertise to do so. Under agreement with EPA's Risk
Reduction Engineering Laboratory, the Science Center has
established three WMACs. This assessment was done by
engineering faculty and students at Colorado State University's
(Fort Collins) WMAC. The assessment teams have consider-
able direct experience with process operations in manufactur-
ing plants and also have the knowledge and skills needed to
minimize waste generation.
The waste minimization assessments are done for small- and
medium-size manufacturers at no out-of-pocket cost to the
client. To qualify for the assessment, each client must fall
within Standard Industrial Classification Code 20-39, have gross
annual sales not exceeding $75 million, employ no more than
500 persons, and lack inhouse expertise in waste minimization.
The potential benefits of the pilot project include minimization
of the amount of waste generated by manufacturers, and
reduction of waste treatment and disposal costs for participat-
ing plants. In addition, the project provides valuable experience
for graduate and undergraduate students who participate in
the program, and a cleaner environment without more regula-
tions and higher costs for manufacturers.
Printed on Recycled Paper
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Methodology of Assessments
The waste minimization assessments require several site visits
to each client served. In general, the WMACs follow the proce-
dures outlined in the EPA Waste Minimization Opportunity As-
sessment Manual (EPA/625/7-88/003, July 1988). The WMAC
staff locates the sources of waste in the plant and identifies the
current disposal or treatment methods and their associated
costs. They then identify and analyze a variety of ways to
reduce or eliminate the waste. Specific measures to achieve
that goal are recommended and the essential supporting tech-
nological and economic information is developed. Finally, a
confidential report that details the WMAC's findings and rec-
ommendations (including cost savings, implementation costs,
and payback times) is prepared for each client.
Plant Background
The plant produces silicon-controlled rectifiers (SCRs) and
Schottky rectifiers for converting alternating current into direct
current. The plant operates 2,100 hr/yr to produce approxi-
mately 2.5 million units.
Manufacturing Process
The raw materials used by the plant include 4-in. silicon wafers,
various metals, photolithographic chemicals, photoresist, nitric
acid, sulfuric acid, hydrochloric acid, silicon glass, resist stripper,
plating chemicals, solder, and isopropyl alcohol.
The following steps are involved in making the various rectifiers:
Wafer Fabrication
Fabrication of individual electronic chips varies slightly for each
of the two main products. In both cases, 4-in. silicon wafers
purchased from an external supplier are used as the sub-
strates. The wafer fabrication process generates p-n junctions
and circuitry for chips that are cut from the wafers. As many as
200 chips can be fabricated from each wafer.
Chips for SCRs are manufactured in a clean room. During the
initial production steps prior to clean-room processing, the
wafers are doped with boron, phosphorus, and other metals.
This process involves heating in electrical induction furnaces to
temperatures above 600 °C for 18 hr. The doped wafers are
then transported to the clean room. Photolithographic pro-
cesses are used in the clean room to generate a pattern of
grooves for each chip. The grooves extend into about 40% of
the wafer thickness and expose the electrically active portion of
the substrate. Negative-image liquid photoresist is applied to
the doped wafers by an automated spin coating machine. A
photomask is then placed over the wafer, and the resist is
exposed to UV radiation to polymerize the resist and generate
the groove pattern for each chip. The wafer is then developed
and etched with nitric, sulfuric, and hydrochloric acids to re-
move unwanted material. The specific acids used depend on
the product being manufactured. The resist is then removed by
placing the wafer in a tunnel oven at 480 °C to char the resist.
Residual material is removed with compressed air. The pro-
cesses of resist coating, exposure, etching, and resist stripping
may be repeated several times, depending upon the product
being manufactured. When the final groove pattern is com-
pleted, layers of polysilicate and silicon nitride are deposited
onto the wafer surfaces by chemical vapor deposition. Pre-
formed silicon glass is applied and fused to the wafers to
provide glass passivation. The wafers are then coated with
metal, scribed with a laser, and divided into individual chips or
dice. The dice are then tested, sorted, evaluated, and stored
prior to assembly.
In addition to the advanced processes used to fabricate chips
in the clean rooms, about 5% of the chips are manufactured
using traditional wet chemistry electroplating techniques to
deposit and etch metals. These techniques are used for certain
SCRs. The wet chemistry consists of electroless nickel plating
of the silicon wafer, acid etch, nickel plating, and gold plating.
Roofing tar is used as the plating resist. Passivation of the
silicon surface is accomplished with a potassium hydroxide
solution. When the process is complete, the wafers are scribed
with a laser, divided into dice, and sorted prior to assembly.
Dice testing must be performed after assembly.
Fabrication of Schottky rectifiers is also performed in a clean
room. The wafers are first cleaned and surface-oxidized. Spin
coating and photolithographic processes similar to those used
for SCRs provide a patterned mask on the wafer surface.
Boron is then diffused onto the surface with high temperature
induction furnaces. The resist is then removed with a phenol-
based liquid resist stripper. Other mask/etch/deposition/resist
strip processes follow for various elements required to build the
desired patterns for the Schottky circuitry. After the final patterns
have been deposited, the front sides of the wafers are coated
with a protective layer of photoresist and a circular adhesive
dot. The wafers are then removed from the clean room in semi-
sealed containers to a grinding room where the back sides of
the wafers are ground with porous ceramic abrasive to remove
about 10 mils. The wafers are transported back to the clean
room, where the adhesive dot and the photoresist are removed
and the back sides of the wafers are coated with metal and
cleaned. Completed wafers are transported from the clean
room to a wafer saw with a diamond-tipped blade. The saw
cuts the wafers into individual dice which are then tested and
stored prior to assembly.
Assembly
Initial assembly procedures involve fusing the die into a pellet
in a process called solder mountdown. A pellet consists of
layered components that begin with a preformed disk of solder
brazed onto a nickel-plated copper or steel stud, a molybdenum
disk, another solder preform, and a die. The pellet is as-
sembled by hand and placed in one of several tunnel ovens to
fuse the components together. The ovens use a 100% hydrogen
atmosphere, with nitrogen blankets at the entrances and exits
to prevent oxidation. Fusing seals the mounted pellet to prevent
arcing with RTV, a white caulk-like material that vulcanizes at
room temperature. External packaging is welded onto the pel-
let/stud assembly and then the top ends of the rectifiers are
crimped to seal off holes. Some devices require a solderable
top. These units are dipped in flux and in a molten solder pot
and then rinsed in alcohol. Automated and manual testing is
performed on the final products before packaging, storage, and
shipment to customers.
An abbreviated process flow diagram is shown in Figure 1.
Existing Waste Management Practices
This plant has already implemented the following techniques to
manage and minimize its wastes.
• High-purity waste solvents from wafer fabrication are re-
used in the assembly area before disposal.
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• The potassium hydroxide passivation process has been
eliminated since the initial visit by the WMAC team. The
result of this action is that methanol use and disposal have
been reduced 50%.
• Recent changes in material specifications have resulted in
the elimination of an oil coating on the copper and steel
studs received for assembly. 1,1,1-trichloroethane (TCA)
used onsite to remove this oil has therefore been eliminated.
Waste Minimization Opportunities
The type of waste currently generated by the plant, the source
of the waste, the quantity of the waste, and the treatment and
disposal costs are given in Table 1.
Table 2 shows the opportunities for waste minimization that the
WMAC team recommended for the plant. The type of waste,
the minimization opportunity, the possible waste reduction and
associated savings, and the implementation cost along with the
payback time are given in the table. The quantities of waste
currently generated by the plant and possible waste reduction
depend on the production level of the plant. All values should
be considered in that context.
It should be noted that, in most cases, the economic savings of
the minimization opportunities result from the need for less raw
material and from reduced present and future costs associated
with waste treatment and disposal. Other savings not quantifi-
able by this study include a wide variety of possible future
costs related to changing emissions standards, liability, and
employee health. It should also be noted that the savings given
for each opportunity reflect the savings achievable when
implementing each waste minimization opportunity indepen-
dently and do not reflect duplication of savings that would
result when the opportunities are implemented in a package.
This research brief summarizes a part of the work done under
Cooperative Agreement No. CR-814903 by the University City
Science Center under the sponsorship of the U.S. Environmental
Protection Agency. The EPA Project Officer was Emma Lou
George.
Wafer Fabrication
- Doping
- Spin Coating
- Cleaning/Rinsing
- Etching
- Resist Stripping
- Chemical vapor Deposition
- Passivation
- Grinding
- Wafer Cutting
Solvent Emissions
to Scrubber
Dice
Assembly
• KOH Passivation
- Pellet Assembly
- Thermal Fusion
- Sealing
- Welding
- Crimping
- Soldering
- Cleaning
- Inspection/Testing
SCRs and Schottky
Rectifiers Packaged
and Shipped
Spent Acids and
Bases and
Rinse Water
to Sewer
Solvent
Wastes to
Offsite
Combustion
Figure 1. Abbreviated process flow diagram.
•U.S. Government Printing Office: 1992 — 648-080/60063
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Table 1. Summary of Current Waste Generation
Waste Generated Source of Waste
Annual Quantity
Generated (gal)
Annual Waste
Management Cost
Resist stripper
Xylene
Acetone
Mixed solvent wastes
including n-butyl acetate,
xylene, and spent
photoresist waste
Isopropyl alcohol
Waste acids and bases
including ammonium
hydroxide, hydrogen
peroxide,sulfuric acid,
hydrochloric acid,
hydrofluoric acid,and
nitric acid
Rinse water
Methyl alcohol
1,1,1-Trichloroethane
Freon
Stack scrubber water
Reverse osmosis reject water
Wafer fabrication process for
Schottky rectifiers
Removal of residual resist stripper
from the wafers following stripping
Removal of the protective adhesive
dot and photoresist coating from the
wafers after grinding
Removal of excess photoresist from
the wafers in the spin coating
equipment
Removal of xylene from the wafers
Etching and cleaning between process
steps
Wafer fabrication
Cleaning and drying
Ultrasonic degreaser and vapor
degreaser
Vapor degreaser
Scrubber system
Reverse osmosis unit
730
1,795
620
420
315
11,618
$13,630
12,450
5,160
7,120
3,570
71,360
'Includes raw material costs
1,908,816
2,295
840
55
4,032,000
818,064
27,130
1 1,220
7,980
14,620
14,860
3,850
Table 2. Summary of Recommended Waste Minimization Opportunities
Waste Generated Minimization Opportunity
Annual Waste Reduction
Quantity Percent
Net
Annual Savings
Implementation
Costs
Payback
Years
RO reject water
Resist strip
Isopropyl alcohol
Xylene
1,1,1-Trichloroethane
Reuse reject water from
the reverse osmosis unit
for use in the stack
scrubber system.
Replace resis t s trip
with a water miscible,
biodegradable, nontoxic
product containing no
aromatic hydrocarbons.
This product will be used
with a supplemental
stripper rinse solution.
Replace 1,1,1-
trichloroethane vapor
degreasing with a
nontoxic, nonhazarclous,
low volatility aqueous
cleaner and cleaner
rinse.
8 18,064 gal
730 gal
300 gal
200 gal
100
100
100
100
100
$2,142
1,184
$5,111
2.4
immediate
604
1,550
2.6
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/S-92/036
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