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Difference between revisions of "Semiconductor recycling plant case study of GaAs photovoltaic manufacturing - case study"

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==Waste during Production==
 
==Waste during Production==
 +
===Preliminary Calculations===
 
Current Production methods of GaAs semiconductors produce a large amount of waste.  To determine the amount of waste produced during production of GaAs solar cells, we must first determine the amount of GaAs per Area/Watt<sub>peak</sub>.  The calculations are seen below.
 
Current Production methods of GaAs semiconductors produce a large amount of waste.  To determine the amount of waste produced during production of GaAs solar cells, we must first determine the amount of GaAs per Area/Watt<sub>peak</sub>.  The calculations are seen below.
  
===Amount of Material per Area===
+
====Amount of Material per Area====
 
Size of Panels = 10 inches x 9 inches<ref name="[4]">
 
Size of Panels = 10 inches x 9 inches<ref name="[4]">
 
GaAs Solar Panel; Product Data Sheet; 3554 Chain Bridge Road, Suite 103, Fairfax, VA 22030,
 
GaAs Solar Panel; Product Data Sheet; 3554 Chain Bridge Road, Suite 103, Fairfax, VA 22030,
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'''Amount of Material per Area = 3100 g/m<sup>2</sup>'''
 
'''Amount of Material per Area = 3100 g/m<sup>2</sup>'''
  
===Amount of Material per Watt===
+
====Amount of Material per Watt====
  
 
Peak Power per Cell = 880mW <ref name="[4]"></ref>
 
Peak Power per Cell = 880mW <ref name="[4]"></ref>
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===Metalorganic Chemical Vapour Deposition (MOCVD)===
+
===Metalorganic Chemical Vapour Deposition Waste===
  
  
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'''Waste Material Rate MOCVD per Watt<sub>peak</sub> = 26.6 g/W<sub>peak</sub>'''
 
'''Waste Material Rate MOCVD per Watt<sub>peak</sub> = 26.6 g/W<sub>peak</sub>'''
  
===Molecular Beam Epitaxy===
+
===Molecular Beam Epitaxy Waste===
  
  
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'''Waste As Rate MBE per Area = 9099 g/m<sup>2</sup>'''  
 
'''Waste As Rate MBE per Area = 9099 g/m<sup>2</sup>'''  
  
Grams of As per Watt Peak = 11.4 g/Watt<sup>peak</sup>*74.922/(69.723+74.922) [(As/Watt<sup>peak</sup>)*(Wt% As)]
+
Grams of As per Watt<sub>peak</sub> = 11.4 g/Watt<sup>peak</sup>*74.922/(69.723+74.922) [(As/Watt<sup>peak</sup>)*(Wt% As)]
  
Grams of As per Watt Peak = 5.9 g/Watt<sup>peak</sup>
+
Grams of As per Watt<sub>peak</sub> = 5.9 g/Watt<sup>peak</sup>
  
Waste As Rate MBE per Watt Peak = (5.9 g/W<sub>peak</sub>/.15)-5.9g/W<sub>peak</sub>  [(As/Watt<sub>peak</sub>)/(Percentage Waste)-(As/Watt<sub>peak</sub>)]
+
Waste As Rate MBE per Watt<sub>peak</sub> = (5.9 g/W<sub>peak</sub>/.15)-5.9g/W<sub>peak</sub>  [(As/Watt<sub>peak</sub>)/(Percentage Waste)-(As/Watt<sub>peak</sub>)]
  
'''Waste As Rate MBE per Watt Peak = 33.4 g/W<sub>peak</sub>'''
+
'''Waste As Rate MBE per Watt<sub>peak</sub> = 33.4 g/W<sub>peak</sub>'''
  
 
==Recycling==
 
==Recycling==

Revision as of 21:48, 7 October 2011

Asi.png This page was part of a project for MY3701 -- an MTU class on semiconductors.

This page is now open edit -- please fix mistakes or feel free to leave comments using the discussion tab.


Introduction

Gallium arsenide (GaAs) is a semiconductor material that is used in a wide variety of applications ranging from circuits to solar cells. Solar cells of GaAs can be produced using both bulk and thin film growth methods.

Material Processing

Manufacturing process of GaAs and other semiconductors with the waste products produced

Bulk Growth

There are two common ways to produce GaAs using bulk growth, Liquid-Encapsulated Czochralski (LEC) growth, and Vertical Gradient Freeze (VGF) technology. [1]

Liquid-Encapsulated Czochralski Growth(LEC)

LEC growth is accomplished by melting high-purity arsenic and gallium in a high temperature vessel, and slowly cooling to produce a single crystal. The GaAs crystal produced using this method however has some impurities such as significant levels of carbon, and numerous dislocations. These impurities cause the semiconductor to be unusable for some applications.

Vertical Gradient Freeze Technology (VGF)

VGF growth works by placing high purity arsenic and gallium in an enclosed quartz ampoule with a crystal of GaAs. The arsenic and gallium are melted, and then brought into contact with the GaAs crystal. When cooled slowly, a single crystal of GaAs is formed. The single crystal formed has many of the same impurities as LEC growth crystals, which restricts the utility of the crystals.

Typical dimensions of the semiconductor crystals are 1-6 inches in diameter and 2-30 inches long. The usual rate of crystal growth is 1-5mm per hour. [2]

Cutting, Polishing, and Etching

The bulk crystals (boule) produced are cylindrical with conical ends, thus they need to be cut, polished and etched before they can be used. During processing the conical ends are cut and wasted, the boule is ground to ensure a uniform diameter over the length, the crystals are aligned using x-ray diffraction, and the boule is cut into wafers. Just forming the correct shape results in nearly one third of the total mass being wasted. Even further processing is required to etch off the damaged layer of semiconductors from sawing and to polish the semiconductor. As a result of processing, nearly 50% of the semiconductor is wasted. [2]

Thin Film Growth

Thin films of GaAs have many advantages over large single crystals of GaAs when it comes to being used in solar cells. Thin films lack some of the impurities found in large crystals, and are capable of being used without requiring extensive slicing. The rest of the case study will be dedicated to thin film GaAs semiconductors.

The most common thin film growth methods for producing GaAs films are Metalorganic Chemical Vapour Deposition (MOCVD) and Molecular Beam Epitaxy (MBE).

Metalorganic Chemical Vapour Deposition (MOCVD)

MOCVD is a chemical vapour deposition method of depositing epitaxial films using surface reactions of organic compounds, metalorganics or metal hydrides. The growth of the epitaxial film is the result of a chemical reactions, and not physical deposition. MOCVD of GaAs requires a reaction chamber with gas injection units, a temperature control system, and a pressure control system.[3] During MOCVD of GaAs, trimethylgallium Ga(CH3)3 and arsine AsH3 are consumed.[4]

Molecular Beam Epitaxy (MBE)

Diagram of MBE setup

MBE is the process of depositing epitaxial films on a substrate under ultrahigh vacuum (UHV) conditions using atomic or molecular beams. The atomic or molecular beams are generated from elemental feedstocks in Knudsen-type effusion cells. The beams travel in straight paths to the substrate where they condense and grow under kinetically controlled conditions. [2] During MBE, ultra pure gallium and arsenic are consumed.

Waste during Production

Preliminary Calculations

Current Production methods of GaAs semiconductors produce a large amount of waste. To determine the amount of waste produced during production of GaAs solar cells, we must first determine the amount of GaAs per Area/Wattpeak. The calculations are seen below.

Amount of Material per Area

Size of Panels = 10 inches x 9 inches[5]

Number of Cells per Panel = 18[5]

Area of Cell = (90 in2/18)*.00064516 m2/in2 [((Area Panel)/(Number Cells))*(conversion from in.2 to m2)]

Area of Cell = .0032258 m2

Amount of Material per Cell = 10 grams[5]

Amount of Material per Area = 10 g /.0032258 m2 [(Material/Cell)/(Area Cell)]

Amount of Material per Area = 3100 g/m2

Amount of Material per Watt

Peak Power per Cell = 880mW [5]

Peak Power per Area = 880mW / .0032258 m2 [(Power/Cell)/(Area Cell)]

Peak Power per Area = 272.8 W/m2

Amount of Material per Wattpeak = 3100 g/m2 / 272.8 W/m2 [(Material/Area)/(Power/Area)]

Amount of Material per Wattpeak = 11.4 g/Wpeak


Metalorganic Chemical Vapour Deposition Waste

Average Material Utilization Efficiency = 30%[6]


Waste Material Rate MOCVD per Area = (3100 g/m2/.3)-3100g/m2 [(Material/Area)/(Percentage Waste)-(Material/Area)]

Waste Material Rate MOCVD per Area = 7233 g/m2

Waste Material Rate MOCVD per Wattpeak = (11.4 g/Wpeak/.3)-11.4g/Wpeak [(Material/Wattpeak)/(Percentage Waste)-(Material/Wattpeak)]

Waste Material Rate MOCVD per Wattpeak = 26.6 g/Wpeak

Molecular Beam Epitaxy Waste

Material utilization efficiency for Ga = 40-70%[6]

Average Material utilization efficiency for Ga = 55%

Grams of Ga per Area = 3100 g/m2*69.723/(69.723+74.922) [(Material per Area)*(Wt% Ga)]

Grams of Ga per Area = 1494.3 g/m2

Waste Ga Rate MBE per Area = (1494.3 g/m2/.55)-1494.3g/m2 [(Ga/Area)/(Percentage Waste)-(Ga/Area)]

Waste Ga Rate MBE per Area = 1222.6 g/m2

Grams of Ga per Wattpeak = 11.4 g/Wattpeak*69.723/(69.723+74.922) [(Material/Wattpeak)*(Wt% Ga)]

Grams of Ga per Wattpeak = 5.495 g/Wattpeak

Waste Ga Rate MBE per Wattpeak = (5.495 g/Wpeak/.55)-5.495g/Wpeak [(Ga/Wattpeak)/(Percentage Waste)-(Ga/Wattpeak)]

Waste Ga Rate MBE per Wattpeak = 4.496 g/Wpeak

Material utilization efficiency for As = 10-20%[6]

Average Material utilization efficiency for As = 15%


Grams of As per Area = 3100 g/m2*74.922/(69.723+74.922) [(Material/Area)*(Wt% As)]

Grams of As per Area = 1605.7 g/m2

Waste As Rate MBE per Area = (1605.7 g/m2/.15)-1605.7g/m2 [(As/Area)/(Percentage Waste)-(As/Area)]

Waste As Rate MBE per Area = 9099 g/m2

Grams of As per Wattpeak = 11.4 g/Wattpeak*74.922/(69.723+74.922) [(As/Wattpeak)*(Wt% As)]

Grams of As per Wattpeak = 5.9 g/Wattpeak

Waste As Rate MBE per Wattpeak = (5.9 g/Wpeak/.15)-5.9g/Wpeak [(As/Wattpeak)/(Percentage Waste)-(As/Wattpeak)]

Waste As Rate MBE per Wattpeak = 33.4 g/Wpeak

Recycling

Collecting Waste

Semiconductor waste from MBE and MOCVD occurs in two primary methods, solid waste coating the reactor walls and parts, and non solid waste in the form of exhaust vapors drawn off of the epitaxial reactors.

Solid Waste

The coating of the reactor walls and parts forms a solid waste material, which can be collected by simply scraping it off off reactor components. The solid waste material collected in this manner may be contaminated with dopants such as Si, Zn, C and Cr, as well as with GaAsP, arsenic oxides, and phosphorous oxides. These dopants however usually only have concentration levels of only 1018[2] atoms/cc.

The energy collection cost for this solid waste is assumed to be negligible, as the solid waste needs to be removed in current production processes.

Non-solid Waste

The collection of semiconductor waste from exhaust vapors is not as straight forward as the collection of solid waste material. In order to collect the waste from the exhaust, the exhaust vapors would have to undergo a series of "scrubbing" processes, in which cooled water would be sprayed into the vapor, cooling it off until the relative temperature of the desired components in the exhaust reach a temperature where it changes from a gas to either liquid or solid phase.[7] This material would then be collected and, depending upon the contaminants (which would include the dopants used to create the thin films), would undergo multiple purification process in order to reach the desired purity for reuse.

Process of recycling and re-purification

Proposed recycling process of GaAs semiconductor material

The recycling of GaAs is a relatively new idea and the current methods are deemed "satisfactory". The recovery process of Gallium is also practiced more than that of Arsenic, likely due to the toxicity of Arsenic gas. The issue with the common thermal processing of gallium is that it develops gallium and arsenic oxides, thus increasing the number of steps required to obtain a purified amount of recycled gallium.[2] The prototype process pictured is one that could be operated for a relatively low cost and return the recovered materials directly back into the crystal growing process. This process consists of only three steps and three pieces of equipment, a thermal separation furnace, arsenic condenser (for purification), and a zone refiner (for gallium purification).

Downcycling

Large scale downcycling for GaAs waste currently does not exist, and will most likely not be examined in the near future. The reason that downcycling does not exist for GaAs is the limited utility of downcycled gallium and arsenic. GaAs semiconductor components account for 98% of the current gallium consumption[8]. With the narrow usage of gallium in other fields, there is no real market behind downcycling gallium. Arsenic, although more widely used in other fields than gallium, also does not have a market drive behind downcycling. Arsenic can be used to treat wood, preserve leather, kill insects, and harden lead for use in bullets, however it is extremely toxic to humans. There is a large push to reduce the usage of arsenic in industry, especially because companies that produce arsenic products are often responsible for the environmental clean up[2].


Producing a Recycling Plant

Part of the goal of this case study is to determine the technical viability of recycling GaAs. This can be accomplished by designing a Semiconductor Recycling Plant capable of supporting a 1 GW thin-film soar photovoltaic manufacturing facility.

Plant Capacity

Using the material waste values produced through various methods of GaAs thin-film production, we can determine the necessary capacity of a feasible recycling plant.

Supporting a MOCVD plant

The amount of material per Watt Peak wasted using MOCVD was determined to be 26.6 g/Wpeak. Using this value, and the size of the supported manufacturing plant, we can determine the daily required capacity of a recycling plant.

Required Recycling Capacity = (26.6 g/Wpeak *1 GW)/(1000 *365)[((Waste/Wpeak)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

Required Recycling Capacity = 72876.7 kg/day

Supporting a MBE plant

The amount of material per Watt Peak wasted using MBE was determined to be 4.496 g/Wpeak for Ga and 33.4 g/Wpeak for As. Using these values, and the size of the supported manufacturing plant, we can determine the daily required capacity of a recycling plant.

Required Ga Recycling Capacity = (4.496 g/Wpeak *1 GW)/(1000 *365)[((Waste/Wpeak)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

Required Ga Recycling Capacity = 12318 kg/day

Required As Recycling Capacity = (33.4 g/Wpeak *1 GW)/(1000 *365)[((Waste/Wpeak)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

Required As Recycling Capacity = 91506.8 kg/day

Safety

MSDS file for Gallium

MSDS File for Arsenic

MSDS File for Trimethylgallium

MSDS File for Arsine

The manufacturing of semiconductors using gallium and arsenic does result in the handling of hazardous chemicals and needing to dispose of them. These waste materials can come on solid and liquid form posing many issues of containment and disposal.[6] Many PV manufacturers have a system for the management, handling, and disposal of the hazardous waste. Examples of these are secondary enclosures, ventilation systems, chemical detection, and neutralization systems, and automating delivery and process systems as much as possible.[6] The administration also deals with implementing employee training programs, and ensuring that proper emergency procedures are in place. Remotely operated cylinder valves have helped remove workers from possible contact with hazards by allowing them to operate cylinders from a distance, and also allowing them to shutdown remotely in case of an emergency.[6] Redundancy is also implemented in the critical systems that control pumps, flow regulators, valves, exhaust pumps, and any other equipment that could be critical in a spill prevention or hazard detection.

References

  1. R.L. Adams, Growth of high purity GaAs using low-pressure vapour-phase epitaxy, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 395, Issue 1, 1 August 1997, Pages 125-128, ISSN 0168-9002, 10.1016/S0168-9002(97)00624-4. (http://www.sciencedirect.com/science/article/pii/S0168900297006244) Keywords: Low-pressure vapour-phase epitaxy; LPVPE; GaAs
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Swartzbaugh Joseph, Sturgill Jeffery. Reduction of Arsenic Wastes in the Semiconductor Industry, 1998 (http://www.epa.gov/nrmrl/pubs/600r02089/600R02089.pdf)
  3. http://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy
  4. http://en.wikipedia.org/wiki/Gallium_arsenide
  5. 5.0 5.1 5.2 5.3 GaAs Solar Panel; Product Data Sheet; 3554 Chain Bridge Road, Suite 103, Fairfax, VA 22030, http://www.spacequest.com/products/SP-X.pdf , (accessed September 2011).
  6. 6.0 6.1 6.2 6.3 6.4 6.5 V.M. Fthenakis, B. Bowerman, Environmental Health and Safety (EHS) Issues in III-V Solar Cell Manufacturing, National PV EHS Assistance Center (http://www.bnl.gov/pv/files/pdf/art_168.pdf)
  7. Potter, G. U.S. Patent 4,008,056, 1977 (http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F4008056)
  8. Deborah A. Kramer, Gallium, Mineral Resources Program USGS (http://minerals.usgs.gov/minerals/pubs/commodity/gallium/460303.pdf )