Measurement of Arsenic Emission from Doped Czochralski Silicon CrystalGrowing Operation
D. Sinha*, C. Carlson**, Andy Homyak***, Elias Hunter***
*Mitsubishi Silicon America (formerly Siltec Corporation), P.O. Box 7748, Salem, OR 97303.
** Air Products and Chemicals, 19955 S.W. Teton Avenue, Tualatin, OR 97062.*** Air Products and Chemicals, 7201 Hamilton Blvd, Allentown, PA. 18195
ABSTRACT
The level of arsenic from the effluent of a crystal grower was measured at different phases of thesilicon crystal growth operation which includes arsenic doping, crystal body growth, burn-in, andpost burn-in periods. Measurement of arsenic levels were made in the discharge water used as thesealing liquid for the liquid ring vacuum pump used for process effluent discharge. Gas phasemeasurements were also performed for both particulate arsenic as As 20 3 as well as forcompounds of arsenic in vapor phase as AsH 3. Results of these analyses provided importantinformation for developing strategy for emission treatment and control, reducing arsenicexposure to personnel and evaluating the feasibility for recycling argon used in the crystalgrowing process.
INTRODUCTION
Advancement of semiconductor technology has resulted in increased chemical usage by theelectronics industry. Air emissions constitute nearly 75% of all the chemical releases by theelectronic industries. While the quantity of emission from the electronic industry is smaller thanthat of other industries, the hazardous nature of some of these emissions poses potential risk tohuman health and the environment.I
A number of dopant materials such as phosphorous, boron, antimony and arsenic are used fordoping silicon crystals in the manufacture of silicon wafers. Although all of these materials andtheir byproducts are hazardous in nature, arsenic and its compounds are particularly importantdue to their high level of toxicity and carcinogenity. The level of total arsenic in dischargedwater containing arsenic, based on leachability regulation of heavy metals (TCLP), is 5 ppm.Maximum contaminant level (MCL) for arsenic in drinking water is expected to be set below 50ppb by EPA (US Environmental Protection Agency).The arsenic is added to the silicon crystal to obtain desired low resistivity. The low resistivitycombined with low diffusivity of arsenic in silicon are particularly important for fabricatingintegrated circuits (IC) used in power devices. Heavily doped silicon crystal has a targetedresistivity of 0.004 ohm-cm (101 9 atoms/cm3). Based on silicon charge and segregationcoefficient of the dopant, the arsenic is added to obtain the desired resistivity for the crystal.Thissegregation coefficient is also affected by temperature and rotation speed of the crystal duringcrystal growth. This in turn affects the arsenic emission from the crystal growth process.During the crystal growth operation, an argon purge is maintained in the furnace. It provides acontrolled environment for crystal growth in an atmosphere free from moisture, oxygen andpartially oxidized silicon. It also helps to control intrinsic properties of the crystal, such asoxygen concentration. The argon flow rate varies between 65-85 liters/min. The crystal is grown
at a temperature of 1400 0C and at a pressure less than 100 torr. In this paper we have measuredthe level of arsenic emission during the growth of a silicon crystal heavily doped with arsenic.
EXPERIMENT
The determination of the arsenic emission was performed in the exhaust line of an air separatortank located downstream of a liquid ring vacuum pump used for a particular crystal grower. Atwo-stage Kinney vacuum pump was used to remove exhaust from the grower. The pump usedliquid sealant water for production and maintenance of vacuum. A lower suction pressure wasachieved by an additional air injector which provided the motive force for compressing theprocess gas from system pressure to the liquid ring pump inlet pressure. The determination ofarsenic in the discharged air and water was obtained by collecting samples from the effluentstream located at the top and the bottom of the air separator unit. The schematic of theexperimental set up is shown in Figure 1. The results of the analyses of the liquid and gas phasesamples are provided below.
Sample Valve Effluent from
a• Vent Header Crystal Grower
Vacum Pump withExhaust Air Separator Assembly
Discharged Water
Metering Valve Sealant Water Supply
SFlowmeter Metal Bellows Pump
Charcoal Tube/Membrane Filter
Figure 1. The schematic of experimental set up for measurement of arsenic emission in theexhaust gas and water.
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Crystal Growth Steps Approximate ArsenicDuration Concentration in
Discharged WaterArsenic Doping - 5-10 min 0.84 ppmw
Body Growth (Steady State) 34 hrs 0.26 ppmwBurn-in start of burn-in 81 ppmw
Post burn-in 5 min. past 0.95 ppmwburn-in
Post bum-in 105 min. past 0.31 ppmwbum-in
RESULTS
Arsenic in Discharged WaterThe measurement of arsenic in water was performed by collecting a water sample from thebottom of the air separator unit at the desired stage of the crystal growing operation. Normallythe sealing water is recycled back to the vacuum pump following cooling via a heat exchanger.However, during this experiment, incoming city water was used without recycling. The sealingwater flow rate was maintained at 10 gpm through the vacuum pump. The collected watersamples were analyzed using the inductively coupled plasma (Code of Federal Registry 200.7)method for arsenic with a detection limit of 50 ppb. The analytical results were corrected for abackground arsenic level which was found to be 100 ppb. Table I lists the different stages ofoperation during the silicon crystal growing process. Only the stages of interest from an arsenicmeasurement point-of-view are indicated here with their approximate duration. Solid arsenicsublimed around 600 0 C and was added to polysilicon melt inside the crystal grower duringdoping operation. Following body growth of the crystal a small amount of air was introduced inthe grower to bum off the residual dopant at around 300 0 C during bum-in. The argon purge wasmaintained during the entire operation. The level of arsenic concentration found in thedischarged water, during each stage, is also included in the same table.
Table 1. Concentration of arsenic in discharged water during various stages of the crystal growingoperation.
A high level of arsenic in the discharged water was observed during burn-in. This was followedby a rapid drop-off in the arsenic level as observed during the post bum-in. Based on theconcentration of arsenic in the water and the duration indicated above, an approximate materialbalance is performed to determine the level of arsenic in the discharged stream. The amount ofarsenic discharged with the water is about 26 wt% of total arsenic used for doping.Approximately 57wt% of the total arsenic used (140 gm) is incorporated in the crystal. Theremainder (17 wt%) of arsenic is discharged in to the air. These results are based on theassumption that the arsenic concentration in the water is assumed to be constant between theperiods of measurement. Any loss of arsenic or its oxide by condensation or due to anentrapment inside the pipe, connecting the crystal grower to the vacuum pump, is also assumedto be insignificant.
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Arsenic in discharged AirThe sample volume was determined by measuring the flow rate for a predetermined period oftime, depending on the length of the individual process step. The flow rate of the argon/airmixture, during sample collection, was maintained at 0.5 slpm during the sample collection. Aportion of the exhaust from the air separator located downstream of the process vacuum pumpwas connected to the sample tube via a sample valve. The sample flow rate was controlled by themetering valve located downstream of the flowmeter. A metal bellows pump was used toovercome the pressure drop of the discharged gas and maintain desired flow rate duringsampling.The samples of arsine were collected by National Institute of Occupational Safety andHealth (NIOSH) procedure 6001. The procedure involved purging the effluent gas mixturethrough charcoal sorbent tubes divided into three sections. The first section contained glass woolto remove particulates, the second section contained 120 mg. of charcoal and the third sectioncontained approximately 60 mg of charcoal to verify charcoal scrubbing efficiency. Highrecovery was observed between the first and second charcoal stages. The collected arsine andarsenic were recovered by soaking the charcoal and the glass wool in high-purity 10% nitric acidand subsequently analyzed for arsenic by electrothermal atomization atomic absorptionspectroscopy.Table II shows the relative amount of arsenic found at various stages of the crystal growingoperation based on an 80%/20% exhaust gas mixture of air and argon respectively. The analysisof trapped arsenic particulate and its oxide removed by the glass wool, located in front ofcharcoal bed, is presented as arsenic whereas the arsenic collected by the charcoal is determinedas arsine. Total arsenic content is also shown in the same table assuming all the arsenic capturedby the charcoal is arsine. However, analysis of the charcoal might include some arsenic bearingparticles that escaped from the glass wool. An extension to this experiment involved capturingthe particulate arsenic with 0.025 micron mixed cellulose acetate membrane filter manufacturedby Millipore Corporation. The air mixed argon gas was allowed to flow through the filter for apredetermined period of time during steady state and burn-in operations. The arsenic wasrecovered and analyzed by a procedure similar to the procedure indicated above. Theconcentration of the arsenic obtained by filter analysis is shown in the same table.
Table I1. Average concentration of arsenic (as particle) and arsine in the effluent gas streamduring silicon crystal growth operation.
Process Step Arsenic (ppbw) Arsine (ppbv) Total Arsenic(ppbw)
Doping* 97.4 218.6 578.4Steady State 29.5 72.2 189Burn-in 71.3 592 1376Post Burn-in <15 9.71 36.4**Steady State (with 57.7 57.7membrane filter)Burn-in (with 294.8 294.8membrane filter) II
* Average of two sets of measurements.** Maximum level.
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Analysis of these data reveals a lower level of arsenic as oxide or particulate compared to arseniccaptured by the charcoal. This is expected, since a significant portion of the arsenic escapes inthe gas phase by the process of sublimation during doping. The difference becomes significantduring bum-in, as an additional amount of air is introduced to purge and clean the furnace toremove undoped arsenic from the surfaces inside the grower. Also the level of arsenic capturedas particulate with the membrane filter indicates a higher concentration compared to thatcaptured by the glass wool. Part of this difference can be explained by a higher particle captureefficiency, particularly for the smaller particles, of the membrane filter. This difference can alsobe influenced by the errors caused due to the short burn-in period and the variation in the amountof arsenic incorporated in the crystal for these two different experimental runs.An analysis of the effluent gas following charcoal filtration was performed in order to determinethe quality of the effluent argon. A metal bellows pump was used to collect samples in tocylinders to a pressure of 50 psig. Results of the analysis are shown in Table III. Two of thesamples were collected during steady state and one was taken during bum-in operation of thecrystal growth process. A Trace Analytical process gas analyzer (RGA5), equipped with both areduction gas and a flame ionization detector was used to analyze for methane, hydrogen andcarbon monoxide impurities present in the effluent stream. The analysis of carbon dioxide wasperformed using a Thermo Environmental gas filter correlation (GFC) analyzer. Oxygen analysiswas performed using an Illinois Instrument model 2550 oxygen analyzer. The presence ofhydrogen in the effluent gas indicates the formation of hydrogen as a by-product during thecrystal growth process with a subsequent reaction with arsenic to form arsine.
Table III. Analysis of the effluent gas from the crystal grower.
Impurity Steady State 1 Steady State 2 Burn-in PeriodOxygen 15.4 % 16.1% 16.8%N itrogen* ----.-.......-Methane 1.6 ppmv 1.6 ppmv 1.7 ppmvCarbon Monoxide 1.4 ppmv 1.9 ppmv 15.0 ppmvCarbon Dioxide > 1000 ppmv 710 ppmv 720 ppmvHydrogen 11 ppmv 8.1 ppmv 8.4 ppmv* Analysis not performed.From the analysis of the exhaust gas, it is clear that the majority of oxygen is introduced by theair injector of the vacuum pump used for the grower. Based on this information, we can infer thatthe effluent is a mixture of primarily air (80%) and argon (20%).
CONCLUSIONS
Measurement of arsenic and its compounds revealed significant variation in the amount ofrejected arsenic during different stages of the silicon crystal growing operation. Theconcentrations indicated here are average concentrations observed during various stages of thecrystal growing process. The error in measurement is partially due to the non-continuousmeasurement methods used, where overlapping steps in the crystal growth process affected themeasured arsenic concentration. The rapid drop in arsenic level following bum-in allowed us toimprove productivity by reducing the cycle time between each crystal growing operation. Theinjection of air to the liquid ring vacuum pump resulted into dilution of arsenic discharged in theair. During normal operation the sealant water is recycled back to vacuum pump following
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cooling to 60 F. This in turn causes build-up of arsenic in the recycled water. The testing of thevacuum pump recycled water revealed a number of heavy metals including arsenic andantimony. The level of As (V) was found to be six times the level of As (III) in the dischargedwater. This ratio is important as the toxicity of As(1II) is an order of magnitude higher thanAs(V).2 An arsenic reduction plan using pretreatment followed by membrane separationtechnology is currently under consideration to remove the arsenic from the recycled water.3 Dueto the dilution of the discharged air stream, wet scrubbing with a multistage scrubber using anoxidizer in an alkaline environment is being considered in addition to dry scrubbing and thermaltreatment for removing arsenic and its compounds from the discharged air.4 The argonconcentration in the discharged air stream was found to be low due to air injection to the pump asindicated above. A subsequent analysis of the argon downstream of the grower prior to thevacuum pump indicated more than 90%(v/v) argon in the grower exhaust. Recovering andrecycling of this purge argon is currently under investigation. Appropriate safety protocols havebeen developed for the operator working in the crystal growing area as well as the personnelhandling grower vacuum pump water in order to prevent arsenic exposure during routinemaintenance operations and/or during an accidental release. Most of the residual elementalarsenic, in the crystal grower, is expected to be converted to arsenic trioxide (As 20 3) and arsinefollowing burn-in period. Arsenic trioxide is easily suspended as small particles in air due to itslow sublimation temperature (- 193 °C). 5 A wet wipe down method has been developed tomonitor the arsenic level in the dust particles settled on the exposed surfaces around the crystalgrower. This measurement in combination with an arsine detector will provide continuedprotection to the workers from unwarranted arsenic exposure.
ACKNOWLEDGMENTS
Authors gratefully acknowledge the support of the silicon operation division of MitsubishiSilicon America Corporation. We also want to thank Gil Garcia and Jeff Biondi of Air Productsand Chemicals for their help in collection of the gas samples.
REFERENCES
1. D. Sinha and K. Fuller in Reduction and Control of air emissions from the SemiconductorIndustry. (Proceedings of Institute of Environmental Sciences 38th Annual Meeting, Nashville,Tennessee, 1992), 2, pp. 518-524.
2. P. Frank and D. Clifford, U.S. EPA Project. EPA/600/S-2-86/021, Water EngineeringResearch Laboratory, April 1986, Cincinnati, OH. 45268.
3. G. Amy, P. Brandhuber, Steve Dundorf, S. Kang and P. Westerhoff, (Proceedings ofAmerican Water Works Association, Membrane Technology Conference, Reno, Nevada,1995).pp. 375-381.
4. M. Hayes and K. Woods, Solid State Technology, 39 (10), 141(1996).
5.CRC Handbook of Chemistry and Physics, Edited by D. R. Lide, 74th Ed. (CRC Press Inc.,Florida, 1993-1994.), p.4 - 41.
