A schematic illustration for the process of Si Wafers.

Materials Research

Recycling of Silicon-Wafers Production Wastes to SiALON based Ceramics with Improved Mechanical Properties.

Principal Investigator: James R Varner, Ph. D

Co-Principal Investigator: Vasantha R. W. Amarakoon, Ph. D

Industrial Partner: Refractron Technologies Corp. of Newark, NY

Academic Partner: NYS Center for Advanced Ceramic Technology (CACT) at Alfred University.

Project Period: September 01,2005 to December 31, 2006.

II. Abstract  

1) Description:

The construction of industrial metabolic systems is an emergent issue in the 21 st century. The output of highly pure semiconductor silicon for integrated circuits and memories is increasing year by year. It was 4000 ton in the year 2000. The silicon is produced as a single crystalline ingot. During wafer production process, about 60% (2400 tons) of silicon ingot after trimming are scraped with the waste water disposal due to cutting and polishing. This silicon sludge is left outside and dried, there is some fear of the pollution by diffusing out of fine powders in air, or fire. The recycling to highly pure silicon is very costly.

Most of the Silicon wafer producers in USA are located in California. California Integrated waste management policy requires to divert at least 25 percent of their solid waste from landfills by January 1, 2002 and to divert 50 percent by January 1,2004 through recycling. Per Title-14, California code of Regulations, section 18722(j)(8), “Silicon sludge” from Semiconductor industry is treated as “Special hazardous waste”. If the silicon sludge can be converted to nitride based structural ceramics (SiAlON), it is helpful for semiconductor industry and ecological problems. The objective of this project is to recycle the silicon sludge to SiAlON ceramics by using combustion synthesis process.

SiAlON ceramics have been successfully produced from industrial wastes such as silicon sludge, aluminum dross and fly ash from power plants. One of the major limitations for assuring the reliability and lifetime of such ceramic materials is their inherent brittle nature and low fracture toughness. We propose to demonstrate through this project that the fracture toughness of the SiALON based ceramics produced from silicon sludge can be improved by reinforcing secondary particles such as ZrO 2 into the SiAlON matrix. The anticipated benefits are high fracture toughness, low cost, reduced environmental pollution,significant energy saving due to microwave sintering and reduced emission due to the self-propagating exothermic synthesis reaction. The sintered products can be used for abrasives, corrosion resistant filters, and wear resistant materials below 1000° C.

Participants in this project are Alfred University, Refractron technologies Corp and NYS Center for Advanced Ceramic Technology (CACT). Several other structural ceramic components manufactures in the ceramic industry have been provided copies of the proposal for review and expected to participate. James R. Varner and Vasantha R. W. Amarakoon, project investigators, specializes in ceramic processing and microstructure evolution in advanced ceramics. One Doctoral graduate student will perform the bulk of the experimental work. Industrial partner will contribute raw materials and characterization facilities which are not available in Alfred University

2) Objective:

The objectives of the project are:

• Reduce the environment pollution due to silicon sludge produced in the semiconductor industry by recycling silicon sludge and convert into SiAlON ceramics by combustion synthesis process.

• Reduce the energy requirement of the SiAlON ceramic processing by Microwave heating.

• Demonstrate that fracture toughness of SiAlON ceramics can be improved by transformation toughening of ZrO 2 secondary particles reinforcing into SiAlON matrix.

3) Approach:

Dried and pulverized sludge will be ignited in a pressured nitrogen atmosphere. The synthesized SiALON powder will be coated with varying amounts of ZrO 2 and sintered using a 2.45 GHz microwave power without sintering aids over the temperature range of 1400-1600 oC for varying time of 30min to 5 hours in a nitrogen atmosphere. Sintering will also be performed in a conventional furnace for comparing experimental results. Polished sections will be used for microstructure characterization. The same polished specimens will be used for measurements of Vickers hardness and indentation fracture toughness. Once trends are established, bar specimens will be used to measure fracture toughness using a standard technique.

4) Expected Results:

The silicon sludge contains lots of ceramic abrasives such as Al 2O 3, ZrSiO 4, ZrO 2, coagulants (FeCl 2, polymers), grinding oils and water. It is disposed to reclaiming lands. If the silicon sludge is left outside and dried, there is some fear of pollution by diffusing out of fine powder in air. The recycling of this industrial waste to highly pure silicon is very costly. Our project can reduce the environmental pollution by recycling at least 1000 tons of the silicon sludge to low cost high performance SiALON ceramics. Nitriding combustion of silicon sludge leaves solid product of metal nitrides without discharging CO 2 like other combustion reaction resulting reduced environmental pollution.

5) Supplemental Keywords:

Transformation toughening, Self propagating Nitriding combustion, SiALON, Recycling, Wastes, Microwave sintering .

III. Research Plan

Background :

High performance silicon nitride and SiALON ceramics have an advantage over competing materials because of their excellent properties such as high temperature strength, high oxidation and corrosion resistance. SiALON ceramics that are iso-structural with silicon nitride offer the advantage of incorporating some of the sintering additives into the silicon nitride lattice, thus reducing the overall amount of secondary phase and potentially improving high temperature properties. They offer advantages of easier fabrication compared with silicon nitride ceramics because of the lower viscosity of the M-Si-Al-O-N liquid phase, where M is one of the cations Li, Mg, Ca, Y, Sc and most rare-earth, which facilitate easier densification at sintering temperatures. One of the major limitations for assuring the reliability and lifetime of such ceramic materials is their inherent brittle and low fracture toughness. The toughness enhancement of SiALON ceramics has been a hot research topic in recent years. 1The fracture toughness of SiALON can be improved by incorporating fibers, particles and whiskers with in the ceramics matrix. The toughening mechanisms are explained in terms of crack bridging, crack deflection and grain pullout mechanism.

SiALON powder can be synthesized by nitriding combustion of silicon sludge produced in semiconductor industries. Nitriding combustion was discovered by A.G. Merzhanov and his coworkers in 1967 as a solid-gas combustion mode of self-propagating high temperature synthesis (SHS). Many other compounds such as carbides, borides, silicides, aluminides, and other compounds are produced from the mixture of metal and non-metal elements by SHS. 2 Nitriding combustion is similar to oxidation combustion since it involves a highly exothermic reaction, but different in that it leaves solid products of metal nitrides without discharging carbon dioxide.Nitriding combustion is regarded as an energy-saving process to produce various nitride ceramics because the synthesis reaction propagates spontaneously after the initiation of combustion. However, the preparation of raw metal powders and pressurized nitrogen is costly. It is important, therefore, to understand how to produce high performance materials or how to prepare metal powders with low cost. Our idea is to use reclaimed or by-product metal powders as the combustion agent to assist the nitriding combustion.

Gilbert and Mosset described the attractive possibilities of SiALON powder preparations from fly ashes by carbothermal reduction. Even though fly ashes from various plants were used in their work, no real SiALON composite ceramic can be made out of any of the available fly ashes.

The output of semiconductor silicon for large-scale integrated circuits and memories in USA is about 2000 tons/year in recent years. 3 It is produced as a single crystalline ingot and processed to wafers through cutting, polishing and washing. Large edges of a silicon ingot cut by trimming (~10% of an ingot) are used as a source material for polycrystalline silicon solar batteries. However, about 60% of an ingot after trimming is scrapped with the waste water disposal in cutting and polishing processes as shown in fig(1). 4

This silicon sludge contains a lot of ceramic abrasives (Al 2O 3, ZrSiO 4 or ZrO 2), coagulants (CaOH) 2 , FeCl 2 , polymers), grinding oils, and water. It is added to the source material of cement or disposed in land reclamation. If the silicon sludge is left outside and dried, there is some fear of pollution by diffusing out of fine powders in air or fire. Recycling to high purity silicon is very difficult and costly.

Figure-1. A schematic illustration for the process of Si Wafers. 4

Objectives:

The objectives of this project are to reduce the environment pollution due to silicon sludge produced in the semiconductor industry by recycling silicon sludge and convert into SiAlON ceramics by combustion synthesis process, reduce the energy requirement of the ceramic processing by Microwave heating, demonstrate that fracture toughness of SiAlON ceramics can be improved by transformation toughening of ZrO 2 secondary particles reinforcing into SiAlON matrix.

A doctoral graduate research assistant will be used to conduct majority of the laboratory work over sixteen months. The research will be communicated to a national audience of industrial and academic personnel at the annual American Ceramic Society meetings. In addition, at least two papers on the work will be published in refereed journals. The sintered products can be used for abrasives, corrosion resistant filters, and wear resistant materials below 1000° C. 5

Approach:

The nitriding combustion which we will investigate is based on the following reaction. 6

3Si + 2N 2 = Si 3N 4 -748 kJ/mol

These exothermic reactions propagate spontaneously and rapidly when the reactant is charged with a powder form in a pressurized nitrogen atmosphere over 0.5MPa. At lower nitrogen pressures, the nitrogen is not sufficiently supplied to sustain the combustion reaction. The combustion is initiated by passing a current of ten amperes through an ignition heater as shown in Fig (2).

Figure-2. Experimental set-up for the nitriding combustion for Si wastes. 7

The silicon sludge used contains Si(26 wt%), Al 2O 3(14 wt), ZrSiO 4(31 wt), Fe 2O 3(27 wt), and CaO(2 wt) after removal of volatile species at 200°C. 8 The silicon content is too low to sustain the nitriding combustion. When the reclaimed aluminium is added as much as 10 wt% to the mixture of 70 wt% silicon sludge and 20 wt% reclaimed silicon, the nitriding reaction can occur at 1 MPa nitrogen pressure. The products will be pulverized and sintered using 2.45 GHz microwave energy without sintering aids over the temperature range of 1400 -1600 oC for varying time of 30min to 5hours in nitrogen atmosphere. Sintering will also be performed in a conventional furnace for comparing experimental results. The feasibility of this concept will be determined by a nested series of experiments designed to demonstrate the degree of improvement in fracture toughness of SiALON. The polished specimens used for microstructure analysis will also be used for determining Vickers hardness and indentation fracture toughness. Once trends are established, bar specimens will be prepared for determining fracture toughness using one of the standard methods.

The chemical compatibility of ZrO 2 with in a nitride matrix will be studied because t->m transformation may not occur if ZrO 2 reacts with the matrix to form ZrN, especially when fine zirconia particles are used. 9 These compounds have tetragonal or cubic structure, structurally similar to t- or c-ZrO 2 but do not undergo normal transformation; more over just as ZrN they are easily oxidized into m-ZrO 2 in air at low temperatures(600°C-800°C) resulting in a large volume increase and stress build up. This cannot occur in ZrO 2 toughened oxide ceramics, however, the volume increase resulting from the formation of m-ZrO 2 by oxidation here could improve the fracture toughness by generating surface stress within the material which then opposes crack growth. The reaction between the ZrO 2 and SiALON is believed to occur qualitatively as follows: 10

ZrO 2 + Li 1.3Si 9.5 Al 2.5O 1.2 N 14.8 → Zr 7O 8N 4/Zr 7O 11N 2 +Si 2N 2O + LiAlSiO 4

Expected Results or Benefits:

The output of semiconductor silicon for large-scale integrated circuits and memories in USA is about 2000 tons/year in recent years. The silicon sludge contains lots of ceramic abrasives such as Al 2O 3, ZrSiO 4 or ZrO 2, coagulants (FeCl 2, polymers), grinding oils and water. It is disposed to reclaiming lands. If the silicon sludge is left outside and dried, there is some fear of pollution by diffusing out of fine powder in air. The recycling of this industrial waste to highly pure silicon is very costly. Our project can reduce the environmental pollution by recycling the silicon sludge to low cost high performance SiALON ceramics. Nitriding combustion of silicon sludge leaves solid product of metal nitrides without discharging carbon dioxide like other oxidation combustion resulting reduced environmental pollution.

General Project Information:

Industrial Support: We are contacting semiconductor IC manufacturers to obtain silicon waste and manufacturers of abrasives and corrosion resistant filters( Refractron and Vesuvius Hi-Tech Ceramics) for matching industrial support. Also, it may be possible to leverage funds from NYSERDA.

References:

[1] Hirao K, Miyamoto Y, Koizumi Y. Combustion synthesis of nitride powders under high nitrogen pressure. Adv Ceram, American Ceram Soc 1987;12:289-300.

[2] Kanehira S, Miyamoto Y, Hirota K, Yamaguchi O. Recycling of aluminium dross to ALON ceramics by combustion synthesis. In:PACRIM4: Int Conf on Advanced Ceramics and Glasses.Wailea, Maui, Hawaii: American Ceram Soc.; 2001.

[3] U. S. Environmental Protection Agency: Advanced Search for Silicon producers for semiconductor Industries in USA.

[4] Zheng J, Miyamoto Y, Yamada O. Combustion synthesis of sialon powders. J Am Ceram Soc 1991;73: 3700-2.

[5] Hirao K, Miyamoto Y, Koizumi M. Synthesis of silicon nitrides by a combustion reaction under high nitrogen pressure. J Am Ceram Soc 1986;69: C60-1.

[6] Miyamoto Y, Kanehira S,Yamaguchi O, Kajiyama K. Recycling of silicon-wafer production wastes to sialon-based ceramics by nitriding combustion. Ceram Trans, Am Ceram Soc 2000;107:57-64.

[7] Merzhanov Ag. History of and new developments in SHS. Ceram Trans, Am Ceram Soc 1995;56:3-25.

[8] Miyamoto Y, Li Z, Tanihata K. Recycling process of Si waste to advanced ceramics using SHS reaction. Ann Chim Fr 1995;20:197-2003.

[9] Washburn ME, Hartline SD. Lightweight silicon oxynitride. US Pat.No.4043823, August 23, 1977.

[10] Washburn ME. High density silicon oxynitride. US Pat. No.4331771,May 25, 1982.

CEER is funded in large part by the United States Environmental Protection Agency.

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