An Extractive Method for Determining Particulate Contamination Levels of Wafer Carriers Using Ultrasonic Extraction: SEMASPEC Technology Transfer 94052358A-STD
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An Extractive Method for Determining Particulate Contamination Levels of Wafer Carriers Using Ultrasonic Extraction: SEMASPEC October 8, 1994 Abstract: Keywords: This specification defines a quantitative measurement method for particulate contamination on the functional surfaces of wafer carriers. This method uses ultrasonic extraction to remove surface contamination and to suspend it in an aqueous solution. A portion of the solution is measured with a liquid-borne particle counter (LPC) to determine the concentration of particles with respect to various sizes. The particle size range that this specification addresses is 0.5 µm to 60 µm. This method will measure cleaning processes to a level that allows six sigma control. SMIF, Contamination, Wafer Transport, Measurement, Specifications, 200 mm Wafers, Particle Count Authors: Approvals: Dr. Nagarajan (IBM) Gary Knoth, Project Engineer Ray Martin, Project Manager Jackie Marsh, Manager, Standards Programs Venu Menon, Director, Contamination Free Manufacturing
iii Table of Contents 1 EXECUTIVE SUMMARY... 1 2 INTRODUCTION... 1 2.1 Purpose... 1 2.2 Scope... 1 3 Referenced Documents... 2 4 FACILITIES, EQUIPMENT, CHEMICALS... 2 4.1 Class 100 per Federal Standard 209 (or better) environment for performing the test(s)... 3 5 SAFETY... 3 6 PRECAUTIONS... 4 7 PRINCIPLES OF OPERATION AND EQUIPMENT OVERVIEW... 4 8 CALIBRATION... 5 9 TEST PROCEDURE... 5 9.1 Set-Up... 5 9.2 Determination of Background... 5 9.3 Sample Analysis... 6 10 DATA ANALYSIS... 7 10.1 Calculation... 7 10.2 Interpretation of Results... 8 11 DATA PRESENTATION... 8 12 PRECISION AND BIAS... 8 13 BIBLIOGRAPHY... 9
iv ACKNOWLEDGEMENTS The Project Engineer wishes to acknowledge the contributions of Dr. Nagarajan and Mr. Ron Coplen of IBM-Storage Systems Division, San Jose, CA. Dr. Nagarajan developed the measurement methodology detailed in this document. Dr. Nagarajan is also the author of this document. Mr. Coplen aided Dr. Nagarajan with the development of the measurement methodology and provided all of the equipment information, including the calibration information. The Project Engineer served as the document editor and the contact. He also facilitated the member company review of the draft document and incorporated the reviewer suggestions into the final document, where appropriate.
1 1 EXECUTIVE SUMMARY This specification defines a quantitative measurement method for particulate contamination on the functional surfaces of wafer carriers (200 mm standard mechanical interface [SMIF] pods, 200 mm cassettes, and 200 mm run boxes). The method uses ultrasonic extraction to remove surface contamination and to suspend it in an aqueous solution. A portion of the solution is measured with a liquid-borne particle counter (LPC) to determine the concentration of particles with respect to various sizes (for this method, 0.5 µm to 60 µm). This procedure will measure cleaning processes to a level that allows six sigma control 2 INTRODUCTION 2.1 Purpose This specification defines a quantitative measurement method for particulate contamination on the functional surfaces of wafer carriers. 2.2 Scope For the purposes of this specification, "wafer carriers" are 200 mm standard mechanical interface pods, 200 mm wafer cassettes, and 200 mm run boxes. This method uses ultrasonic extraction to remove surface contamination and to suspend it in an aqueous solution. A portion of the solution is measured with a liquid-borne particle counter (LPC) to determine the concentration of particles with respect to various sizes. The particle size range that this specification addresses is 0.5 µm to 60 µm.
2 3 Referenced Documents 3.1 Operating and Maintenance Manual for the PMS Micro Laser Particle Spectrometer, P/N 10146-4 3.2 Operating and Maintenance Manual for the PMS CLS-600 Sampler, P/N 10155-6 3.3 Operating and Maintenance Manual for the PMS IMOLV-.5 Sensor, P/N 10166-1 3.4 Instruction Manual for the Branson Ultrasonic Cleaner, Model DHA-1000 3.5 Federal Standard 209 3.6 MIL-STD-1246B, Product Cleanliness Levels and Contamination Control Program, September 4, 1987 4 FACILITIES, EQUIPMENT, CHEMICALS Note: For purposes of repeatability and reproducibility of this test method, the names of specific products used in test development are included. In each case, an equivalent product may be used.
3 4.1 Class 100 per Federal Standard 209 (or better) environment for performing the test(s) 4.2 Model DHA-1000 Ultrasonic Cleaner (or equivalent), Branson Cleaning Equipment Company, Shelton, CT 06484 4.3 Model Micro LPS-8/16 (Micro Laser Particle Spectrometer) (or equivalent), Particle Measuring Systems, Inc., 1855 South 57th Court, Boulder, Colorado 80301, (303) 443-7100 4.4 Model CLS-600 Sampler, (Corrosive Liquid Sampler), (or equivalent), Particle Measuring Systems, Inc., 1855 South 57th Court, Boulder, Colorado 80301, (303) 443-7100 4.5 Model IMOLV-.5 (LD), Integrated Micro-Optical Liquid Volumetric, Sensor - Solid State Laser Diode), (or equivalent), Particle Measuring Systems, Inc., 1855 South 57th Court, Boulder, Colorado 80301, (303) 443-7100 4.6 Distilled or deionized (DI) water, preferably filtered to 0.2 µm at point of use 4.7 Timer (Graylab Universal timer or equivalent is recommended) 4.8 Centering bracket for glassware in the ultrasonic tank 4.9 Printer Paper for the PMS Micro LPS, P/N PR-22 (or equivalent) Technical Equipment Sales (510) 656-5333 4.10 Stopwatch 4.11 Hydrophone Type 8103, Calibrator Type 4223, Sound Level Meter Type 2231, (or equivalent), Bruel & Kjaer Instruments, Inc. 185 Forest Street, Marlborough, MA 01752, (617) 481-7000 4.12 P/N 6944-23L Pyrex Glassware (12 1/8 in. X 12 1/8 in. X 12 in. deep), P/N 13912-681 Pyrex Glassware (4 l beaker), (or equivalent), VWR Scientific, San Francisco, CA 94120, (415) 330 4026 5 SAFETY The equipment listed does not present any unusual hazards. Good safety practice, however, should always be observed when handling glassware. Exercise caution with the ultrasonic cleaner and all electrical connections. Do not turn on the ultrasonic heater. The heating coil is above the water line and may cause injury if turned on. The sensor houses a Class 1 laser, which transmits output inaccessible to the user. This laser is accessible only during maintenance.
4 6 PRECAUTIONS 6.1 This test method may involve hazardous materials, operations, and equipment. This test method does not purport to address the safety considerations associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and to determine the applicability of regulatory limitations before using this method. 7 PRINCIPLES OF OPERATION AND EQUIPMENT OVERVIEW 7.1 An ultrasonic generator, located beneath the ultrasonic tank, converts 50/60 Hz line voltage to high frequency (40 khz) electrical energy. This energy is fed to transducers connected externally to the underside of the ultrasonic tank, where it is converted from electrical to mechanical energy. The transducers vibrate longitudinally and transfer this motion as a pressure wave to the cleaning, or extraction, medium. This causes the liquid to cavitate. Ultrasonic waves in the liquid establish a pattern of pressures below and above the vapor pressure of the liquid. Bubbles formed in the low pressure regions explode when subjected to high pressures. Shock waves emitted by exploding bubbles act to scrub particles from immersed surfaces. 7.2 The material removed by ultrasonic extraction and placed in solution is detected by the particle counter. To maintain a constant energy level of extraction, it is recommended that only the Branson ultrasonic unit called out in the equipment section be used. If other ultrasonic tanks are proposed for use instead, a B & K hydrophone or other ultrasonic energy measurement devices may be used to verify that the energy level in the proposed tank is 220 +/- 5 dbs (or equivalent in other units). It is necessary, however, that particle counts obtained be correlated with those obtained using the reference standard, Branson DHA-1000 ultrasonic cleaner, to confirm equivalence. Also to be noted is that the test vessel (or substrate) must always be centered in the ultrasonic tank. 7.3 The IMOLV sensor operates on the principle that laser light scattered by a particle is a direct function of its size. Particles produce pulses of radiant energy during transit through the laser beam. These light pulses are sensed by a detector and are then sized with a sixteen channel pulse height analyzer to determine particle size. 7.4 The Micro LPS accepts particle data from the sensor and the flowmeter, and controls the sampler. The sampler automatically draws solution from the extraction vessel, compresses it to remove any bubbles, passes it through the view volume of the sensor, then drains it into the user s drain or waste container. The test described in this document measures only a portion of the contamination found on the parts tested. Sixty ml is drawn from the sample container and the actual volume of solution tested is 45 ml. 7.5 The B & K hydrophone used to measure ultrasonic intensity in the tank is a piezoelectric transducer; i.e., it uses piezoelectric ceramics as sound-pressure sensing
elements. It has a flat frequency response and is omnidirectional over a wide frequency range. 5 8 CALIBRATION 8.1 The IMOLV sensor is calibrated using a standard fluid consisting of polystyrene latex microspheres suspended in water. This calibration is carried out by the manufacturer prior to shipment. The system must be recalibrated annually. To calibrate, follow the instructions in the Operating and Maintenance Manual for the sensor. 9 TEST PROCEDURE 9.1 Set-Up 9.1.1 At the initial start-up, pour one inch of water into the ultrasonic tank (the tank water depth is important and should be accurately measured). Then turn the ultrasonic on for five minutes minimum to degas the water. 9.1.2 Prior to using the system, the Micro LPS and sampler must be adjusted to the following parameters (refer to the operating manuals for details on how to do the adjustments). Micro LPS: Counts/N/mL Compression Delay = Counts (the word counts is displayed on screen) = 10 seconds Sampler: 1. Operating Pressure: The gauge on the front of the sampler displays the operating pressure when the sample solution is being compressed and when it is moving through the sensor during the sample cycle. The gauge must read 20 psig ± 2 psig during these cycles. Adjust the compression pressure knob if an adjustment is necessary. 2. Fill Cycle Time: (Follow the instructions in the operating manual) Rotate the vacuum knob if an adjustment is necessary. 3. Flow Rate: During the sample cycle the flow rate must be 80 ml/min +/- 10 ml/min. Adjust the flow control knob if an adjustment is necessary. 9.2 Determination of Background 9.2.1 All surfaces that contact the test fluid must be kept meticulously clean. Care must be exercised to eliminate possible sources of contamination to the sample since the technique is sensitive to environmental contributions. When glassware is used in the form of extraction vessels, clean the insides with a sponge and an unscented liquid detergent. Rinse with overflow water until the background particle count, obtained by the procedure outlined below, meets the stated criteria. Before using any beakers,
6 examine the insides visually for signs of chips, pits, cracks, or other physical defects. It is advisable to discard the glassware after about six months of regular use. 9.2.2 Perform the following steps to create a background solution: 1. Pour 4 L of filtered water into the beaker described in Step 3.12. 2. Place the suction tube from the sampler into the beaker. 3. Test the background with the PMS system a minimum of three times. The first test result is inaccurate because only a portion of the solution tested is from the contents of the beaker. The second and third tests must be within 10% of each other. The cumulative counts >/= 0.5 µm are to be used for this comparison. If the third reading is not within 10% of the second, then a fourth reading should be taken, and checked for ± 10% agreement with the third. This process may be repeated up to six times, after which the solution must be discarded, and the procedure repeated. 4. Average the two readings that are within 10% of each other. This represents the background count. 5. The background count must not exceed 1000 particles >/= 0.5 µm in 45 ml of solution. Note: If the background solution does not meet the above requirement, additional cleaning or replacement of the glassware, or purging of contaminants from the sampler by running fresh filtered water may be necessary. 9.3 Sample Analysis 9.3.1 200 mm SMIF pods 1. Pour DI water into a 4 L beaker to a level of 3.5 L. 2. Place the test pod into the ultrasonic tank. 3. Pour the DI water into the test pod. 4. Make up three more batches of 3.5 L of filtered water from the same 4 L beaker and pour the contents into the pod. The total volume will be 14 L. Note: Background testing of the latter batches of solution is unnecessary provided that the operator does not introduce any contamination to the solution. 5. Turn the ultrasonic on for two minutes ± two seconds. 6. Place the suction tube into the pod, and test the solution with the PMS system. Test the solution with the PMS system three times minimum. The first test result is inaccurate since only a portion of the solution tested is from the contents of the pod. The second and third tests must be within 10% of each other. The cumulative counts 0.5 µm are to be used for this comparison. If the third reading is not within 10% of the second, then a fourth reading should be taken, and checked for ±
10% agreement with the third. This process may be repeated until convergence to ± 10% agreement between any two readings is obtained. 7. Average the two readings that are within 10% of each other. This represents the reading for the pod. 8. Subtract the background count (Section 8.2) from the pod reading in each channel. Minus values should be recorded as zeros. 9. Record the results. 9.3.2 200 mm run boxes, 200 mm wafer cassettes 1. Use the 4 L beaker to fill a clean 12 1/8 in. X 12 1/8 in. X 12 in. deep Pyrex glassware with 20 L of DI water. 2. Take a background reading on the water in the Pyrex container. The cumulative count per 45 ml > 0.5 µm must not exceed 1,000. Note: Background counts less than 10% of the reading for the part are acceptable in most cases. 3. Immerse the 200 mm wafer cassette or either half of the 200 mm run box in the solution until it is fully covered. Note: It is recommended that if only one half of the run box is tested, it should be the top half since it is more likely to shed particles on to the product. 4. Ultrasonic clean, as before, for 2 minutes ± 2 seconds, and obtain an average particle count for the sample solution. 5. Subtract the background count from the averaged reading for the part, and record the data. 7 10 DATA ANALYSIS At the conclusion of the test, particle counts are obtained in the sampling volume of 45 ml. The monitor displays both differential and cumulative counts in the 0.5 to 60 µm size range. It is recommended that the cumulative count > 0.5 µm in 45 ml of solution be used as the primary cleanliness parameter. Distribution of particle counts in various size buckets may be examined when attempting process investigation or improvement. 10.1 Calculation The counts in 45 ml may be converted to particle counts per part by scaling the number to the total extraction volume. For example, in the case of the SMIF pods, use the following: Particles per SMIF pod > 0.5 µm = Particles per 45 ml > 0.5 µm * 14,000/45 As an additional refinement, the particles-per-part number may be divided by the total wetted surface area of the part to yield counts per square centimeter. The particle size
8 distribution (in the 1-60 µm size range) obtained in this manner may then be plotted on a log-normal graph according to MIL-STD-1246B [1]. MIL-STD-1246 levels of surface cleanliness may then be defined. These levels are useful when comparing different materials or different processes for producing the same product. 10.2 Interpretation of Results When cleanliness criteria are being established, a minimum of 30 parts should be measured for cleanliness. [Note: The actual number of parts to be tested will vary depending on the normality and tightness of the cleanliness distribution.] From this population, a mean and a standard deviation may be established. The upper control limit for the process is typically set as mean + 3 X standard deviation; the acceptance criterion is typically set as mean + 4.5 X standard deviation. This should yield a process that is 6 sigma capable and should be acceptable provided the criterion established according to this procedure satisfies functional requirements. 11 DATA PRESENTATION Cleanliness data may either be presented as differential or cumulative counts relative to individual sizes or as size-concentration distributions, depending on product requirements. Once the critical parameters have been defined, they should be charted using statistical process control techniques. At a minimum, the raw data, calculated averages, standard deviations, and ranges must be charted and used to establish and to respond to trends. Presenting the > 1.0 µm data in MIL-STD-1246B plots offers the added benefit of being able to compare against a reference ( normal ) population. 12 PRECISION AND BIAS The gauge capability of the indirect surface cleanliness measurement method involving ultrasonic extraction and the use of a liquid-borne particle counter has previously been demonstrated to be acceptable. The natural variability in the ultrasonic field is the primary limiting factor in determining the repeatability and reproducibility of this method. A cumulative count of 2,000 particles > 0.5 µm in 45 ml sample volume represents the approximate threshold limit above which the procedure outlined in this specification is clearly valid. Sizing and counting accuracy of the particle counter apparatus are documented [2]. Although ultrasonic extraction will tend to lose some efficiency in the sub-µm size range, the efficiency is nevertheless repeatable and reproducible the primary requirements for a measurement method.
9 13 BIBLIOGRAPHY 1. MIL-STD-1246B, "Product Cleanliness Levels and Contamination Control Program, September 4, 1987 2. Welker, R.L. Weaver and R.P. Coplen, "Gauge Capability, Correlation, And Other Analyses of Particle Cleanliness Measurement Methods," IBM San Jose Internal Report, 1991 (unclassified) 3. Nagarajan, "Estimating Surface Cleanliness and Cleanability, in Proceedings, CleanRooms 93 West, September 27-30, 1993, Santa Clara, CA, pp. 134-146 4. Nagarajan, " Cavitation Erosion of Substrates in Disk Drive Component Cleaning: An Exploratory Study," Wear J., 152 (1992) 75-89
10 A. APPENDIX: ALTERNATIVE EXTRACTION PROCEDURES A.1 Low pressure spray extraction may be used in troubleshooting or investigative mode to selectively measure portions of the surface instead of the entire surface. While also offering the advantage of less sample dilution, the method is more subject to operatorinduced variability and is therefore not as capable a gauge as ultrasonic extraction. A.2 Multiple ultrasonic extractions may be performed to assess the cleanability of the surface being measured. The procedure and the definition of cleanability indices are contained in [3]. It should be noted that multiple extraction analyses of the plastic materials involved in this study did not indicate the onset of cavitation-induced erosive damage [4]. While prolonged exposure to sustained sonication can cause many polymers to degrade, a single two-minute extraction or 6-8 stages of extraction that are spaced to allow relaxation of the material will not, in this case, cause detectable surface deterioration. A.3 This procedure uses DI water without surfactant as the extraction medium. This is based on data indicating that fewer particles are extracted with a surfactant than without. Surfactants promote particle removal by wetting the particle/surface interface, but simultaneously lower the ultrasonic energy level in the tank by introducing microbubbles and by lowering the surface tension of water. The interaction between these two conflicting effects is complex and must be understood on a case-by-case basis. If an application requires the use of a surfactant, a low concentration (0.02 percent by volume) of a non-foaming, non-ionic surfactant, (e.g., Alfonic 610-50R made by Vista Chemical Co., Chicago, IL) is recommended. The background count in this case must be limited to 2000 particles per 45 ml 0.5 µm, or 10% of the reading for the part, whichever is achievable. NOTICE: DISCLAIMS ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. MAKES NO WARRANTIES AS TO THE SUITABILITY OF THE METHOD FOR ANY PARTICULAR APPLICATION. THE DETERMINATION OF THE SUITABILITY OF THIS METHOD IS SOLELY THE RESPONSIBILITY OF THE USER.
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