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SEMATECH ConfidentialTechnology Transfer 94022208A-XFR

Development of Standard OutgassingTechniques for Detecting Volatile

Organic Compounds from PolymerMaterials Used in Microenvironments

© 1994 SEMATECH, Inc.

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Authorized SEMATECH Representative

Development of Standard Outgassing Techniques for DetectingVolatile Organic Compounds from Polymer Materials Used in

MicroenvironmentsTechnology Transfer # 94022208A-XFR

SEMATECH ConfidentialMarch 31, 1994

Abstract: This document is a summary of the work performed in 1993 by the Contamination FreeManufacturing (CFM) Research Center to develop standard, high sensitivity (less than 1 ppm)techniques to detect outgassing of volatile organic compounds (VOCs) from polymers typically usedto construct microenvironments. These tests provide standard testing methods for making relativejudgments on the outgassing threat of polymeric materials. The tests may be proposed as aSEMASPEC standard. Results from using thermal desorption with cryofocussing collection, gaschromatography/ mass spectroscopy (TD/GC/MS) and thermal desorption with Flame IonizationDetection (TD/FID) techniques to evaluate VOCs from polycarbonate materials are reported. A briefreview of the literature on the cleanroom sources of VOCs and their impact is also provided.

Keywords: Contamination Free Manufacturing, Microenvironmetns, Polymers, Organic Compounds, TestMethods, Outgassing

Authors: S.M. Thornberg (Sandia), D.C. McIntyre (Sandia), A.Y. Liang (Sandia), S.F. Bender (Sandia), andR.D. Lujan (Sandia)

Approvals: Gene Feit, Author/Program ManagerVenu Menon, Director of Contamination Free ManufacturingJeanne Cranford, Technical Information Transfer Team Leader

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Technology Transfer # 94022208A-XFR SEMATECH Confidential

Table of Contents

1 EXECUTIVE SUMMARY.......................................................................................................... 1

2 INTRODUCTION:....................................................................................................................... 1

3 PROJECT OVERVIEW/BACKGROUND ................................................................................. 13.1 Project Objectives ............................................................................................................... 13.2 Project Strategy ................................................................................................................... 23.3 Motivation for Developing Outgassing Testing Techniques............................................... 23.4 Semiconductor Industry Association (SIA) Semiconductor Technology Roadmap:

Potential Future Impact of Organic Contamination.......................................................... 33.5 Sources and Impact of Organic Contamination in Clean Room Integrated Circuit

Manufacturing................................................................................................................... 43.5.1 Studies of Organic Contamination From Wafer Boxes and Pods............................ 43.5.2 Organic Contamination from The Clean Room....................................................... 53.5.3 Review of Studies of the Impact of Organic Contamination................................... 6

4 TEST DEVELOPMENT AND EXPERIMENTAL DATA......................................................... 74.1 Samples/Sample Preparation............................................................................................... 84.2 Cryofocussed Gas Chromatography/Mass Spectroscopy (GC/MS) Technique.................. 84.3 Flame Ionization Detector (FID) Technique....................................................................... 94.4 Results ............................................................................................................................... 10

4.4.1 Compounds identified, GC/MS ............................................................................. 104.4.2 Outgassing from Deep Within the Polymer........................................................... 104.4.3 Comparison of several polycarbonates .................................................................. 10

5 RECOMMENDED PROCEDURES ......................................................................................... 115.1 Proposed Analytical Method:Cryofocussed TD/GC/MS.................................................. 11

5.1.1 Sample Preparation ................................................................................................ 115.1.2 Instrument .............................................................................................................. 115.1.3 Analysis Conditions............................................................................................... 125.1.4 Calibration ............................................................................................................. 125.1.5 Limitations/Cautions.............................................................................................. 12

6 RECOMMENDATIONS FOR FUTURE WORK FOR DEVELOPING STANDARDOUTGASSING TESTING TECHNIQUES............................................................................ 126.1 Recommendation for Development of Sample Preparation Procedure............................. 136.2 Recommendation for Further Calibration Tests................................................................ 136.3 Enhancement of TD/FID Test Sensitivity Using Cryotrapping ........................................ 13

7 SUMMARY/CONCLUSIONS .................................................................................................. 14

8 REFERENCES........................................................................................................................... 14

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SEMATECH Confidential Technology Transfer # 94022208A-XFR

List of Figures

Figure 1 Schematic of the TD/GC/MS......................................................................................... 16

Figure 2 Schematic of the TD/FID System.................................................................................. 16

Figure 3 Typical TD/FID Data Scan Using a Constant Temperature Ramp Date....................... 17

Figure 4 Outgassing as a Function of the Number of Times a Sample was Run (TD/FIDTechnique) ................................................................................................................. 17

Figure 5 Typical Plot of the TD/FID Data Using a Stepped Temperature Program.................... 18

Figure 6 Polycarbonate Outgassing for Four Peaks in a Chromatogram..................................... 19

Figure A-1 Polycarbonate #4; 150° C Desorption Temperature .................................................. 21

Figure A-2 Polycarbonate #4; 50° C Desorption Temperature .................................................... 22








Figure A-10 Polypropylene; 150° C Desorption Temperature..................................................... 30

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1 EXECUTIVE SUMMARY

This document summarizes the work performed in 1993 to develop standard high sensitivity (lessthan 1 ppm) techniques to detect outgassing of volatile organic compounds (VOCs) from polymerstypically used to construct microenvironments. The objectives of developing these tests were toprovide standard testing methods that could be used to make relative judgments on the outgassingthreat of polymeric materials, which also could be proposed as a SEMASPEC standard. Thedocument reports results from using thermal desorption with cryofocussing collection and gaschromatography/mass spectroscopy (TD/GC/MS) and thermal desorption with flame ionizationdetection (TD/FID) techniques to evaluate VOCs from polycarbonate materials are reported.

The cryofocussed GC/MS technique is the recommended method for determining generalcontamination of raw polymer material and the cleanliness of existing or processed material.However, the major drawback of this analysis technique is that a sample must be taken from thefabricated item, which can be destructive.

The majority of the outgassing results are reported for polycarbonate materials. Tests of thismaterial were emphasized because the Project Technical Advisory Board (PTAB) for this taskconcluded that polycarbonates were most likely the cleanest class of materials thatmicroenvironment producers would use in the near term. The ASYST pod, the onlymicroenvironment on the market at the time of this report, is primarily polycarbonate. However,other materials (i.e., the Viton o-rings) are also used in the present pod design.

In addition to the results of the test development, a brief review of the literature on the cleanroomsources of VOCs and their impact is provided.

2 INTRODUCTION

This report is a summary of the technical work performed during 1993 to develop standard, lowtemperature outgassing tests by personnel associated with Task 4 (Microenvironments Task) of theContamination Free Manufacturing Research Center (CFMRC) located at Sandia NationalLaboratories-New Mexico. The document fulfills a year end deliverable of the project (listed in the1993 statement of work for Task 4) to issue a final report on the general results of in-depthoutgassing testing of microenvironment materials and recommendations for test methodsnominated for SEMASPEC standards.

3 PROJECT OVERVIEW/BACKGROUND

3.1 Project Objectives

The general objectives in 1993 of Task 4 of the CFMRC were to:

• Provide methods by which microenvironment users and producers may identifyairborne and condensable contaminants and their sources so that relativejudgments on potential microenvironment performance can be made,

• Develop SEMASPEC standard techniques that can be used by producers and usersby microenvironments to quantitatively assess the relative contaminant outgassingpotential and contamination sources in polymeric microenvironment materials ofconstruction, and

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• Provide a technique by which contamination within microenvironments can becontinuously measured so that microenvironment performance duringmanufacturing can be monitored.

This report is a summary of the work performed to address the first two objectives listed above.There is a separate report being issued on the in-situ sensor development effort undertaken by thisTask to address the third objective listed above. The sensor development report will be issued atapproximately the same time as this report (12/93) and is entitled "Real-time SAW Measurementsof NVR in Cleanrooms and Microenvironments" by A.Y. Liang, D.C. McIntyre, R.D. Lujan, S.M.Thornberg, and S.F. Bender (SEMATECH TT #94022209A-XFR).

3.2 Project Strategy

Two low temperature, high sensitivity outgassing techniques were developed to detect volatileorganic compounds (VOCs) outgassed from polymeric materials of construction formicroenvironments. The performance specifications for the test development to generate testingmethods that were sensitive to outgassed contamination from polymers at or below the parts-per-million level and to develop methods for monitoring outgassing at temperatures between 0 °C and70 °C.

The majority of outgassing results that are reported below are for polycarbonate materials that weresupplied to the Task by ASYST, EMPAK, and Fluoroware. The testing of polycarbonate materialswas emphasized in 1993 because of a consensus that was developed by this Task's ProjectTechnical Advisory Board that polycarbonates were most likely the cleanest class of materials thatmicroenvironment producers and potential producers would use in microenvironment production inthe near-term. The ASYST pod is the only microenvironment on the market at the time this reportis being written. The majority of the body of the pod is constructed of polycarbonate materials, butother materials (for example: Viton O-rings) are also used in the present pod designs. It was theconsensus of the PTAB that these other non-polycarbonate materials were more of potentialoutgassing threat.

3.3 Motivation for Developing Outgassing Testing Techniques

At the beginning of 1993 it was decided by personnel at the CFMRC, SEMATECH, and membercompany representatives that one of the two principle areas of effort for this Task was toconcentrate on developing techniques for evaluating the outgassing from microenvironmentmaterials of construction. Studies had already been reported that demonstrated thatmicroenvironments could be effective in reducing the amount of particulate contamination onwafers during manufacturing (1-6). However, there had also been some studies that indicated thatmicroenvironments or wafer boxes could be the source of molecular contamination (7-11). Themolecular contamination studies are discussed in more detail below. There is currently not aconsensus about the impact of molecular organic contamination in the integrated circuit (IC)manufacturing industry. One of the reasons for the lack of consensus is that there have been veryfew controlled experiments performed to evaluate the impact of organic contamination on variousmanufacturing processes, device performance, and device yield. In fact, most IC fabricationfacilities do not monitor airborne organic contamination. Therefore, there is only limitedknowledge of what organic contaminants could have potential detrimental processing impacts.

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One of the key elements missing in evaluating the impact of VOCs is a standard technique formeasuring outgassing from polymeric materials. A standard outgas testing technique can be used tomake comparisons between the contamination threat of different potential cleanroom materials,which in turn will lead to the development of "cleaner" materials. The use of cleaner materialscould also lead to a reduction in the number of cleaning steps and expand the acceptable timewindow between processing steps before wafer cleaning techniques must be applied.

This standard test development effort was undertaken to provide a tool for comparing theoutgassing characteristics of microenvironment materials. However, the test can also be used toevaluate the outgassing from many other clean room materials.

3.4 Semiconductor Industry Association (SIA) Semiconductor Technology Roadmap:Potential Future Impact of Organic Contamination

Several statements were made on the potential future impact of organic and other contamination inthe 1993 SIA Semiconductor Technology Roadmap. Specifically, the Materials and Bulk ProcessesWorking Group stated in their committee report (15),

1. "The removal of unintentional contaminants including particulates, adsorbedorganics, ions, and metals must be accomplished with unprecedented control(16),"

2. There is a need for a "better understanding of the mechanisms for the removal ofunintentional surface contaminants and maintenance of surface cleanliness(17),"

3. The upper acceptable limit for organic contamination in terms of number ofcarbon atoms per square centimeter will be 1E15 in 1992, 1E13 in 1995, and1E12 in 1998 (18).

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3.5 Sources and Impact of Organic Contamination in Clean Room Integrated CircuitManufacturing

3.5.1 Studies of Organic Contamination From Wafer Boxes and Pods

As part of a study to assess the condensation of organic residue on wafer surfaces, Suzuki (10) andcoworkers demonstrated that the contact angle of water with oxidized Si surfaces increased less ina SMIF-Pod with a replaceable PVC inner liner than in the cleanroom the SMIF-Pod was located.The contact angle increased from 5° to 20° for the oxidized wafers that were exposed to thecleanroom for 150 hours while the contact angle remained at approximately 5° for the wafersstored in the pod.

Miller and co-workers (11) studied the influence of wafer storage in pods on the surface chemistryof the wafers. In that study, wafers with approximately a 4 nm thick oxide were stored in used,used and cleaned, and new polycarbonate pods for between 4 and 20 hours. Control wafers werealso stored in clean (flamed) glass containers. The contamination on the wafers was characterizedby heating the wafers to 250 °C in a stream of nitrogen. The volatilized organics were thencharacterized using plasma chromatograph mass spectroscopy (PCMS.) Results of the testsincluded an observation that BHT (butyl-hydroxy-toluene), a polymer anti-oxidant, was found intrace amounts on some wafers stored in used, cleaned, and new pods, but not always detected on allwafers stored under these three conditions. In addition NMP (n-methyl pyrrolidone), a solvent usedin processing, was absent on wafers exposed to new pods but generally found on wafers stored in(1) used and (2) used and cleaned pods. In all cases, the total amount of surface contamination onthe wafers was less than a monolayer thick.

In this same study, Miller and coworkers also conducted tests in which the water contact angle onoxidized (oxide 4-8 nm thick) wafers was measured as a function of wafer storage time inlaboratory air, new pods, new and used storage boxes, and flamed glass containers (controlsamples.) Wafers stored for 288 hours in lab air, pods, and boxes had contact angles between 35°and 39° while the control samples had contact angles of approximately 21°. The larger contactangle for the laboratory air exposed wafers and pod and box stored wafers indicates that thesurfaces of these wafers were more hydrophobic than the control wafers, which in turn is consistentwith a higher concentration of organic concentration on the non-control wafers.

In an early study of wafer box materials, Budde and Holzapfel (9) showed that there was a largevariation in outgassing at 160 °C from different lots of polyamide and polypropylene from the samemanufacturers. These authors also demonstrated that plasticizers and antioxidents condensed on Sisamples stored in used polypropylene wafer boxes. In addition, this study found solvents from alithography process on the samples, indicating that the Si pieces had been indirectly contaminatedfrom wafers previously stored during processing in the wafer box. In both studies, the authors usedion mobility spectroscopy (IMS) to detect and characterize the outgassed species.

In a recent study Budde and coworkers (7) analyzed outgassing from two different brands of carbonfilled polypropylene wafer boxes using gas chromatography/mass spectroscopy techniques. In thatstudy, 50 mg of polymeric sample was heated at 100 °C in a sample tube with He flowing throughthe tube. The condensables from the He gas stream were condensed in a TENAX filled trap thatwas cooled to -30 °C. After a 30 minute sampling time, the TENAX trap was rapidly heated to 250°C and contaminants were released into a methyisilicone GC column for separation. Thecontaminants released from the column were detected using mass spectroscopy with electron-

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impact ionization. Budde and coworkers also characterized the contamination on Si wafer piecesthat were held in the polypropylene boxes for two days. Following exposure, contam-inated Sisamples were placed in clean sample tubes, heated to 160 °C in a He gas stream and thecontaminants characterized as outlined above for the polymer materials.

Budde and coworkers found that the principle outgassed constituents from both boxes werepolypropylene oligomers. One of the box materials also outgassed significant concentrations ofisopropanol while the other box material outgassed toluene. Additional analysis of the constituentsthermally desorbed from the two box materials found oxidation and decomposition products ofpolypropylene, the antioxidant butyl-hydroxy-toluene (BHT), and in the case of one of the boxmaterials, a anisol-type oxidation stabilizer. There was a factor of two difference in the amount ofoutgassed material from the two different box materials. In contrast to measurements made on thebox materials, Si wafers stored in the box that outgassed the most had a smaller, thermally-desorbed outgassed flux than the Si pieces that were stored in the box composed of a polymer thatoutgassed the least. However, this study did not analyze the residue remaining on the Si wafers thatwas not thermally desorbed during testing.

3.5.2 Organic Contamination from The Clean Room

Although the subject of this report is the analysis of organic contamination originating from pods, itshould be noted that many other sources of potentially detrimental organic contamination have beenidentified in the literature. Judgments on the relative contribution of pod storage to on-waferorganic contamination must be made with knowledge of other possible sources of organiccontamination in the cleanroom. The information that follows is not an exhaustive review of theliterature pertaining to this issue, but it does illustrate other potential sources of on-wafercontamination.

In a study of the impact of common cleaning processes used during integrated circuit (IC)production, Budde and Holzapfel (8) found that several chemicals were significantly contaminatedby the polymeric components they had come in contact with during transport and storage. Buddefound polymer-additive, antioxidants and polymeric degradation products in HCL and HF acids, thebase NH4OH, and deionized H2O. In one experiment, Budde and Holzapfel demonstrated that theorganic residue on Si surfaces increased by a factor of 4 as a result of an RCA "cleaning" treatment.In other work (9), Budde and Holzapfel have demonstrated that significant contamination can bedeposited during Piranha bath treatments and buffered oxide etchings. In the latter treatment, thecontamination was traced to residue left by the surfactant used in the etching solution.

Mori and coworkers (12) analyzed the impurities on wafers exposed to an open-air clean roomenvironment for 24 hours. Following exposure, the wafers were heated to 275 °C in a He gasstream. The volatilized organic contaminants were characterized using gas-chromatography withflame-ionization, flame-photometric, and mass-selective detection. Test results indicated that sub-monolayer concentrations of triethylphosphate (TEP) and other organics were present on thewafers. The primary source of the TEP and organic contaminants was found to be components ofthe HEPA-filtration units. In particular, the polyurethane sealant used in securing the filter media tothe HEPA-filter modules was found to be the primary source of TEP. Further investigations byMori and coworkers found that the level of phosphorus contamination originating from sealantsvaried by three orders of magnitude between three different vendors. The presence of phosphorusas a contaminant is of potential concern because it can unintentionally dope the Si wafers during

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high temperature processing. Similar concerns about unintentional doping have been raised aboutboron-containing species detected in cleanrooms (13).

In a comprehensive study of organic contamination in cleanrooms, Muller and coworkers (14) usedcharcoal collection badges to sample organic contamination levels in multiple locations in fourseparate AT&T manufacturing cleanrooms. The badges were used as passive samplers todetermine airborne concentrations and arrival rates of organic vapors at surfaces. After four weekduration exposures the badges were extracted for an hour in carbon disulfide. A portion of theextracted liquid from the column was injected into a GC/MS for analysis. The results of this workshowed that many of the organic compounds detected in the cleanrooms are generally found inindoor environments and are from sources such as paint, sealants, structural plastics, and otherbuilding materials. Other sources of airborne organic contamination included HEPA grid sealants,conductive flooring, cleanroom clothing, and chemicals used in integrated circuit manufacturing.Significant differences in the organic contamination species and concentrations were found betweenthe cleanrooms and at different locations within the same cleanroom. Muller and coworkers pointout that on a mass basis, the concentration of potential contaminant species in a cleanroom istypically several orders of magnitude greater than particle concentrations. In addition, it was statedthat the surface arrival rate of these molecular contaminants was expected to greatly exceed particlearrival rates because of the much larger diffusivities of the contaminant species.

3.5.3 Review of Studies of the Impact of Organic Contamination

Organic contamination has been traced to several problems encountered in integrated circuitmanufacturing (14.) Much of the work connected with evaluating the influence of organiccontamination on defects has been connected to native and thermally grown silicon oxide.

Okada and coworkers (19) compared the rate of room temperature growth of silicon oxide ofwafers stored in a polypropylene boxes and wafers exposed to a cleanroom. They found that theoxide thickness on wafers stored in the cleanroom was approximately 0.6 nm, while there was lessthan a monolayer of oxide on wafers stored in the polymer box. Measurements of VOCs in thecleanroom using gas chromatography indicated that acetone, ethanol, and toluene were abundant.Oxide growth was monitored for wafers stored in a box and supplied with ethanol saturated air andit was found that the native oxide thickness was the same as when wafers were stored in thecleanroom. When polypropylene boxes were saturated with ethanol and toluene, the oxide growthrate was significantly less than with acetone containing atmospheres.

Licciardello and coworkers (20) observed inhibited growth of native oxide on Si wafers pretreatedby an HF etching treatment. They traced the reduction in oxide growth to organic contaminationoriginating from the polymer container used to store the HF acid. Licciardello and coworkerssuggested that the organic contamination was probably due to the leaching of plasticizers from thestorage bottle.

Studies have shown that the surface energy of a Si wafer can influence the rate of condensation ofVOCs in a cleanroom environment. Olsen and coworkers (21) used infrared internal reflectionspectroscopic techniques to characterize the rate of VOC condensation on hydrophobic andhydrophilic Si surfaces. They concluded that the rate of VOC condensation on hydrophobicsurfaces was significantly faster than for hydrophilic surfaces. The condensed organic compoundswere not identified.

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The surface energy of Si surfaces has been shown to affect influence of organic contamination onthe rate of thermally grown oxide (22). Silicon wafers with hydrophobic and hydrophilic surfaceswere dipped in isopropyl alcohol and then oxidized in dry oxygen at 1000 °C. It was demonstratedthat alcohol contamination of hydrophilic surfaces had no measurable affect on thermal oxidegrowth rate. In contrast, alcohol contamination of hydrophobic surfaces enhanced the initial oxidegrowth rate. Hussian and coworkers concluded that rather than volatilize in the oxidizingatmosphere, the alcohol residue played an important role in surface reactions during oxide growth.

In their evaluation of the influence of organic contamination on capacitor performance, Kasi andcoworkers (23) found that the adsorption of approximately 0.3 of a monolayer of valeric acid onwafers before the thermal growth of 12 nm of oxide at 900 °C, resulted in a 50% reduction in thebreakdown field of capacitors fabricated from the oxide. They concluded from this and other workthat the extent of degradation of capacitor breakdown strength due to organic contaminationdepended on (1) the hydrocarbon sticking probability, (2) the peak adsorption temperature, and (3)the ease of molecular fragmentation of the contaminant.

Studies of the interaction of VOC's and Si surfaces have shown that the interaction of organiccompounds and Si can be complex, particularly at high temperatures. Using thermal desorption,Göbel and coworkers (24) demonstrated that the interaction of organics (condensed on wafers fromcleanroom air) and the Si wafer surface resulted in the transformation of the initial contaminantsinto volatile fragments that desorbed from the surface and a nonvolatile carbonaceous deposits thatremained on the wafer even at temperatures of 1000 °C. This work is consistent with other work(25,26) that has shown that residual carbon contamination can only be removed when Si surfacetemperatures exceed 1200 °C in vacuum.

Based on the studies summarized above, it can be expected that the VOC's originating frompolymeric materials used to construct microenvironments can have an affect on manufacturingprocesses and device properties. The extent of the influence will depend on the rate ofcontamination accumulation on the wafer surface and the chemical properties of both the outgassedcontaminants and the Si wafer.

4 TEST DEVELOPMENT AND EXPERIMENTAL DATA

The goal for this work was to identify a technique for comparing outgassing characteristics ofvarious volatile organic compounds (VOCs). The techniques chosen were gaschromatography/mass spectroscopy (GC/MS) for impurity detection/identification, and GC with aflame ionization detector (FID) for total organics detection. These techniques were chosen aspotential test methods for measuring polymer outgassing since they are very sensitive to VOCs, arewidely available at most sites, they are easy to implement, and use common, relatively inexpensive(cost <$100,000) analytical equipment. Early in this program, other techniques such as solventextraction, supercritical fluid extraction, and pyrolisis with GC/MS were explored as possiblecandidates for standard tests. However, these latter tests were not pursued because it wasdetermined that the conditions by which contaminants were being measured were not representativeof the polymer use environments in IC manufacturing.

Although the original charter was to develop a method for analyzing various polymer types and didnot cover identifying unknowns, during the course of method development, some out-gassedcompounds were identified and will be discussed in the Results section.

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4.1 Samples/Sample Preparation

Samples were used as received from the manufacturers who provided samples for the tests. Someof the first samples were shipped in plastic bags and it is acknowledged that some detectedcontamination probably originated from the bags. Some samples were resin pellets (raw materialused in polymer molding), while others were obtained from shavings generated by slow drilling ofprocessed polymer sheets. Before use, all sample tubes (approx. 0.25 inches (dia.) x 6 inches,stainless steel) and associated analysis end-caps were heated to the same temperature as theexperimental run and analyzed on the GC/MS until no detectable impurities remained (thebackground levels have not been quantified accurately but are estimated to be less than 10 ppb).Approximately 0.1 grams of sample was weighed and placed in the sample tube. One end of thesample tube was equipped with a stainless steel screen to keep the sample in the tube. Both ends ofthe tubes were then fitted with analysis caps for the autosampler.

4.2 Cryofocussed Gas Chromatography/Mass Spectroscopy (GC/MS) Technique

A block diagram of the instrument is shown in Figure 1. The sample was placed in the sample tubein the temperature controlled oven. Sample temperatures ranged from 50°C to 150 °C. Theoutgassed VOCs from the sample were swept from the sample tube to the cold finger by a heliumcarrier gas. The cryofocussing unit used in this test is the PE ATD 50 with a multisamplingcapability. The cold finger was filled with TENAX 60/80 adsorbent resin. The cold fingercollected the VOCs at -30 °C for a specified amount of time, 30 minutes typically. Then, the coldfinger was heated rapidly (up to 300 °C) sending the collected sample to the Perkin Elmer 8500GC/MS where the various constituents are separated by the GC and analyzed by the massspectrometer. The GC column is a DB-5, 30 meter, .25 mm diameter with a .25 mm stationaryphase, and the oven program included a starting temperature of 45 °C, a 10 °C/min ramp up to 300°C. This combination of column type, diameter, ramp rate, etc., were found to provide goodseparation of outgassed compounds. The upper temperature value is somewhat arbitrary but wasfound to work well for cleaning out the column before the next run. Background runs were madebefore and after each sample scan using the same parameters as the samples to ensure nocontamination was present in the cold finger, GC column, or the MS from the previous sample.

The temperatures for the test represent high temperature conditions (150 °C) where mostpolycarbonate outgassing and other studies (e.g., thermal degradation) have been performed. Initialstudies used temperatures up to 150 °C as a starting point to develop the analytical method in aregion where outgassing is known to occur in easily measurable amounts. As the analyticalmethods were refined, results at temperatures as low as 50 °C were achieved which is a much closerapproximation to normal environmental conditions the wafer carriers will experience.

The gas chromatograms and spectra shown in Appendix 1 show the very good resolution capabilityof this technique. The samples were acquired at temperatures very close to room temperature (50°C) up to 150 °C. The VOCs spectra collected from some polymers were very similar, while othersspectra were very different from each other. The chromatogram of polypropylene is included toshow the large significant difference in VOCs between polycarbonate and polypropylene. Most ofthe peaks shown in the polypropylene spectra appear to be fragments from the polymer, whereasmost of the peaks appearing with polycarbonate are some type of impurity or additive.

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4.3 Flame Ionization Detector (FID) Technique

The block diagram for the GC/FID technique is shown in Figure 2. The sample was placeddirectly in a sample tube (the same size as for the cryofocussed GC/MS technique). The sampleholder is located in the oven of the gas chromatograph (HP5890II) instead of the chromatographiccolumn. The sample tube is connected to the helium carrier and the FID using a four-way valvewhich allows the isolation of the sample loop while keeping the carrier flowing through the FIDduring sample loading. During data acquisition, the valve is turned to the other (Analyze) positionwhere the carrier flows through the sample tube and to the FID.

Two types of sample loading/preparation techniques were tried. In the first method, the sampletube was evacuated for a short period (approximately 15 seconds) to remove any room air from thetube. In the second method, no evacuation was performed and the room air was analyzed with theoutgassed gases. The first method was not as repeatable as the second. The exact reason for thelack of repeatability has not been determined but could arise from many factors includingcontaminants from the vacuum pump and hardware, and variable pump-down pressures result invariable off-gassing of the polymer.

Two types of temperature ramps were tried: a constant rate temperature ramp (40 °C per minute),and a stepped ramp. A typical chromatogram for the constant temperature ramp is shown in Figure3. The constant rate ramp produced one large peak with a peak width on the order of 5 minuteswide. The maximum in the FID signal corresponded to a temperature of approximately 100 °C.The area under the peak is proportional to the amount of VOCs emitted from the sample. The peakcorresponds to the total organic outgassing of the sample for that run. The peak is not a separationof the compounds since there was not a GC column after the sample tube. Consequently, the FIDmeasured the total hydrocarbon content of the outgassed constituents. The peak is very wide(approximately 5 minutes wide) due to the large sample volume, the "slowness" of the heating ratesfor the sample, and the bulk outgassing characteristics of the polymer.

In preliminary sensitivity studies, this technique could detect approximately 10 ng. of toluene,though further work is needed to determine the sensitivity precisely. First, a polymer sample wasspiked with a standard solution of toluene in acetone by injecting it directly on the polymer. Thesample was then sealed in the tube and analyzed using the TD-FID. The increase in peak heightwas difficult to distinguish from the off-gassing of the polymer and the dissolving of the polymer bythe acetone. As a result, the spike was added to a cleaned, empty sample tube to ensure the peaksmeasured resulted from the VOCs being added to the tube.

There is evidence that the polymer retains many impurities in the bulk and off-gas them slowly asthey diffuse to the surface. Fig. 4 shows a plot of repeated runs, where the temperature was rampedto 150 °C on the same polycarbonate sample. The amount of outgassing is low at first, butincreases to a maximum after about 2 or 3 runs, and finally decreases as the impurities are depletedfrom the polymer.

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4.4 Results

4.4.1 Compounds identified, GC/MS

Compounds tentatively identified from the mass spectral fingerprint for each compound using theGC/MS data included benzene, toluene, chlorobenzene, phenol, phenol derivatives, phenylderivatives, and various long chain polymers. Some compounds were positively identified, whileothers could not be matched with any spectra within the mass spectral library (Perkin-Elmerlibrary).

4.4.2 Outgassing from Deep Within the Polymer

A very significant finding is that the polycarbonate outgassing components are primarily impuritiesthat were either part of the polymerizing process, or impurities picked up from the atmosphere,package, etc. The results showed little or no polymeric fragments being emitted. This demonstratesthat at the conditions for the test (approximately 50 °C) polycarbonate is stable and couldpotentially be cleaned through thermal desorption, vacuum baking, etc. However the GC/FID dataindicate the impurities are deeply imbedded in the polymer (or strongly adhering to the surface)since a single sample can outgas significant quantities for 5 to 7 runs. Therefore, any cleaningprocess must be able to address the deeply residing impurities, and not just the surfacecontamination.

4.4.3 Comparison of several polycarbonates

A large amount of information can be obtained from looking at the GC/MS data for variouspolycarbonates at various temperatures. Three primary differences in the spectra were seen as thetemperature was changed for a given polymer:

1. The quantity outgassed for some compounds increased as temperature increased.

2. Others appeared only briefly at specific temperatures.

3. Some compounds decreased as the temperature increased.

Figure 6 shows a plot of the signals from 4 compounds versus outgassing temperature selected fromchromatographs from the same polymer which demonstrate these three cases. Impurities arisingfrom the analytical system were found by noting a constant level of some compounds as thetemperature increased. The majority of the outgassed compounds fell into the first category.Compounds that decreased as the temperature was increased were typically the more volatilecompounds which may undergo some thermal degradation or reactions.

The type of polymerization process used for making the polycarbonate can sometimes bedetermined by the presence of chlorobenzene (used in one polymerization process), or toluene (usedin another polymerization process). Some of the polycarbonates were found to be very clean, whileothers had a large number of impurity peaks.

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5 RECOMMENDED PROCEDURES

The cryofocussed GC/MS technique is the recommended method for determining generalcontamination of raw polymer material and for the cleanliness of existing or processed material.The major drawback of this analysis technique is that a sample must be taken from the fabricateditem which can be destructive.

The TD/FID technique is intended to be a screening tool for overall organic outgassing of eitherpolymer resin or processed polymers. Adequate sensitivity has been demonstrated, however, thesensitivity can be greatly enhanced with some further development. Plans for further work toenhance this technique will be presented below in Section 6.

5.1 Proposed Analytical Method:Cryofocussed TD/GC/MS

5.1.1 Sample Preparation

Samples should be taken in such a manner as to not induce contamination during sampling, storage,or transportation to the laboratory. Materials such a glass or stainless steel can be cleaned very welland are appropriate for sample containers. Lids for these containers should be free fromcontamination and should be carefully chosen. A smaller surface area for the lid provides lessoutgassing area, but makes sample introduction into the container more difficult. All samplesshould be stored in the same type of container before testing.

Samples must be weighed to ensure the same quantity of polymer is used from sample to sample.A proposed sample size is 100 mg which gives adequate sensitivity for most instruments. If moresample is needed, consistent sample sizes must be used for the data to be useful in comparisons.Care must be taken to keep sample thicknesses and surface area to volume fractions consistent fromsample to sample. Methods for sampling processed polycarbonate sheet and other samplegeometries will be investigated in the first quarter of 1994.

5.1.2 Instrument

A commercially available GC/MS with a thermal desorption/cryofocussing capability is needed forthis method. The cryofocussing unit should be capable of reaching cryotrapping temperatures atleast as low as -30 °C. The GC column should be a 30 meter DB-1 or DB-5 (or equivalent). Eitherof these columns will provide good separation of most outgassed compounds. The massspectrometer (typically, a quadrapole or ion trap detector) must be of a suitable quality to be able todetect the quantities outgassed by the samples.

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5.1.3 Analysis Conditions

The sample is cold-purged at room temperature for 1 minute to remove room air from the sampletube. The sample is raised to the analysis temperature (typically 50°C or 75°C) and all compoundsoutgassed are collected for 15 minutes on the cryotrap (held at -30 °C or less). The time can beincreased if more sensitivity is desired, or decreased if sample peaks are easily detected at lowertimes. However, when comparing results, the same amount of time must be used for all samples.The trap is heated rapidly (1000°C/min or greater) and the compounds are sent to the GC. All splitratios (the amount of gas entering the instrument versus the amount exhausted from the instrument)must be adjusted on the instrument such that the appropriate quantity of sample is presented to theGC. The GC oven program includes a starting temperature of 45°C and a 10°C/min ramp up to300°C.

5.1.4 Calibration

Spiking the sample with a known amount of two or three hydrocarbons provides internal peaks withwhich to compare the analyte on a chromatogram. Compounds currently suggested forpolycarbonate evaluation include deuterated phenol and dodecane. Outgassed phenol was typicallyseen in the polycarbonate outgassing tests and is therefore representative of compounds that areexpected for this class of materials. Dodecane is a typical long chain, non-polar, alkane compoundthat is also typical of compounds observed in the chromatographs for polycarbonate materials.

5.1.5 Limitations/Cautions

As with any analytical method, the results are only as good as the sample. Samples used in this testwere received in various types of packaging which will all probably contribute in some way to thecontamination levels in the polycarbonate. Care must be taken to ensure the sampling and samplestorage process does not introduce measurable levels of contamination in the sample.

Great care is required to ensure the instrument backgrounds are not mistaken for sample peaks.Furthermore, the gas supply for the GC must be very clean since a large volume of gas is sampledwhen using a TD-cryofocussing unit. A small impurity in the gas, not detectable in the gasnormally, can accumulate into a measurable contaminant after 30 minutes of accumulation.

Sample sizes must be large enough to get a good average of the bulk and large enough to provideenough outgassed mass to be detectable. All samples must be representative of the bulk and freefrom sampling biases.

6 RECOMMENDATIONS FOR FUTURE WORK FOR DEVELOPINGSTANDARD OUTGASSING TESTING TECHNIQUES

Two techniques were pursued as possible standard outgassing techniques for evaluating outgassingfrom polymeric materials of construction for microenvironments. Of these techniques theTD/GC/MS technique is the most fully developed at the time this report is being written. However,Task 4 personnel feel that there are two areas important to the standard test that need furtherdevelopment. These areas are sample preparation and calibration procedures. Our plans to addressthese issues are given below. We also recommend further development of the TD/FID Techniqueto enhance its sensitivity.

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6.1 Recommendation for Development of Sample Preparation Procedure

The outgassing testing techniques developed for this task measure outgassing at low temperaturesand are therefore very sensitive to the condition of the surfaces of the sample pieces. It is importantto provide sample pieces with approximately the same size dimensions and surface area to samplevolume ratios. This requirement is particularly challenging to meet in those testing situations werethe sources of the samples are significantly different. For example, outgassing testing of resinpellets and processed resin sheet material to assess the influence of processing on VOCs presents asample preparation problem because of the significantly different dimensions of the sample sources.

Following a recommendation of Dr. B. Bachman of EMPAK, we will pursue the development of acryogenic grinding technique for sample preparation. This technique utilizes an inexpensive, highspeed, analytical grinder with liquid nitrogen cooling of the sample to prepare the sample. Theground sample is passed through sieves so that a limited size distribution of particles can bedefined. We will operate this grinder under different temperature conditions and with differentpolycarbonate source materials to assess the affect of grinding on particle shape and size. Theduration of this effort will be three months.

6.2 Recommendation for Further Calibration Tests

To date, only toluene has been used to establish sensitivity of the outgassing testing techniques. Wewill also evaluate the sensitivity of the technique for compounds such as deuterated phenol anddodecane. These measurements are important to establish the variation in sensitivity withcompound type. The estimated duration of this effort will be three months and it will proceedconcurrently with the sample preparation procedure development outlined in Section 6.1

6.3 Enhancement of TD/FID Test Sensitivity Using Cryotrapping

The GC/FID data as described in Section 4.3 resulted in a peak approximately 5 minutes wide. Ifthe peak can be narrowed significantly, the resultant peak height would be greatly increased.Consequently, the addition of a cryofocussing unit between the sample and the FID should greatlyincrease the sensitivity of the technique. A typical cryofocussing unit can boil off a collectedsample in a matter of seconds. In this way, a two or three order of magnitude improvement insensitivity could be realized while keeping the technique very simple. The effort to improve theTD/FID test sensitivity will have a six month duration and will be finished by the end of the secondquarter in calendar year 1994.

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7 SUMMARY/CONCLUSIONS

The work conducted by Task 4 of the CFMRC at Sandia National Laboratories to develop standardoutgassing techniques for detecting VOCs from polymeric materials has been reported in thisdocument. The technique utilizing low temperature desorption from the polymer samples withcryofocussing collection and Gas Chromatography with Mass Spectroscopy has been shown to besensitive enough to detect outgassing from polycarbonate materials at low temperatures. Signalpeaks in the chromatograms were shown to have good signal to noise ratio and resolution, and thusprovided data essential for VOC identification. Thermal desorption with flame ionization detectionwas also shown to have good sensitivity for total outgassed VOCs at low temperatures (<75°C.).Data collected using this technique indicated that subsurface migration of VOCs to the surface ofthe polymer samples may play a role in outgassing, even at low temperatures. Recommendationsfor future work to improve both outgassing techniques were presented.

At the time this report is being written (12/93), Sandia National Laboratories' Gas Analysis Labs,EMPAK, and Balazs Laboratories are engaged in testing three polycarbonate samples from thesame source using the gas chromatography/ mass spectroscopy technique recommended in Section5. Preliminary reports indicate that the laboratories are getting very similar results.

8 REFERENCES

1.) Inbody, J. and B. Van Eck. “Upgrading an Existing Wafer-Fab Facility With SMIFTechnology.” Microcontamination, September, p. 25 (1990).

2.) Sayre, S., T.-H. Lin, P. Sanchez, and S. Bhola. “SMIF Reduces Defect Density in Class 100Production Facility.” Semiconductor International, September, p. 104 (1990).

3.) Tolliver, D.L.“Manufacturing Paradigm Shift Underscores Need for Microenvironmental Tack.”Microcontamination, July, p.28 (1991).

4.) Harada, H. and Y. Suzuki. “SMIF System Performance at 0.22 micron Particle Size.” SolidState Technology, December, p.61 (1986).

5.) Hughs, R.A., G.B. Moslehi, and E.D. Castel. “Eliminating the Cleanroom: Experiences With anOpen-Area Smif Isolation Site (OASIS).” Microcontamination, April, p.31 (1988).

6.) Shu, C.Y. and L.C. Tu. “Designing, Operating A Submicron Facility With IsolationTechnology.” Microcontamination, March, p.29 (1992).

7.) Budde, K.J., W.J. Holzapfel, and M.M. Beyer. “Detection of Volatile Organic Contaminants inSemiconductor Technology-A Comparison of Investigations by Gas Chromotography and by IonMobility.” Proc. of the 39th Annual Technical Meeting of the IES, Los Vegas, NV, April 1993, p.366.

8.) Budde, K.J. and W.J. Holzapfel. “Detection of Volatile Organic Surface ContaminationsArising From Wafer Boxes and Cleaning Processes.” in Proc. of the First International Symposiumon Semiconductor Wafer Bonding: Scienmce, Technology, and Applications, Proceedings Volume92-7, The Electrochemical Society, P. 271.

9.) Budde, K.J. and W.J. Holzapfel. “Measurements of Organic Contaminations From SiliconSurfaces.” Proc. of the 38th Annual Technical Meeting of the IES, Nashville, TN, May 1992, P.483.

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10.) Suzuki,Y., T. Kajima, T. Sekiguchi, C. Watanabe, K. Kiuchi, and C. Hashimoto. “GasAdsorption on Wafers.” Microelectronics Manufacturing Technology, December, 1991, p. 38.

11.) Miller, R.J, L.B. Rothman, J.T. Yeh, J. Hoffman, C. Smith, E. Spaulding, J. Todoroff, J.J. Wu,F. Kern, S. Silverman, D. Chen, J. Chu, F. Kao, C.C. Lin, A. Peng, R. Tsai, L.C. Tu, H,C, Yu, andW.D. Wang, “Mini-environments and SMIF Part II: Pod Cleanliness.” The 11th InternationalConference of the Contamination Control Society Proceedings, September, 1992, p 164.

12.) Mori, E.J., J.D. Dowdy, and L.W. Shive. “Correlating Organophosphorus Contamination onWafer surfaces with Hepa-Filter Installation.” Microcontamination, November 1992, p. 63.

13.) Stevie, F.A, E.P. Martin, P.M. Kahora, J.T. Cargo, A.K. Nanda, A.S. Harris, A.J. Muller, andH.W. Krautter. “Boron Contamination of Surfaces in Silicon Microelectronic Processing:Characterization and Causes.” J. Vac. Sci. Technol. A, 9(5), p.2813 (1991).

14.) Muller, A.J., L.A. Psota-Kelty, H.W. Krautter, J.D. Sinclair, P.A. Stevie, E.P. Martin, P.M.Kahora, and N.K. Nanda, “Detection and Sources of Volatile Clean Room Contaminants,” inSymposium on Contamination Control and Defect Reduction in Semiconductor Manufacturing II,ECS Proceedings, May 16-21, 1993.

15.) Materials and Bulk Processes Committee Report, in Semiconductor Technology Workshop:Working Group Reports,published by the Semiconductor Industry Association, pp. 61-80 (1993).

16.) Materials and Bulk Processes Committee Report, in Semiconductor Technology Workshop:Working Group Reports, published by the Semiconductor Industry Association, p. 76 (1993).



19.) Okada, C., H. Kobayashi, I. Takahashi, J. Ryuta, and T. Shingyouji. “Growth of Native Oxideand Accumulation of Organic Matter on Bare Si Wafer in Air.” Jpn. J. Appl. Phys. 32, p. L1031(1993).

20.) Licciardello, A., O. Puglisi, and S. Pignataro. “Effect of Organic Contaminants on theOxidation Kinetics of Silicon at Room Temperature.” Appl. Phys. Lett 48(1), p.41 (1986).

21.) Olsen, J.E. and F. Shimura. “Infrared Analysis of Film Growth on the Silicon Surface inRoom Air.” J. Vac. Sci. Technol. A7(6), p. 3275 (1989).

22.) Hossain, S.D., C. G. Pantano, and J. Ruzyllo. “Removal of Surface Organic Contaminantsduring Thermal Oxidation of Silicon.” J. Electrochem. Soc. 137(10), p. 3287 (1990).

23. Kasi, S.R., M. Liehr, P.A. Thirty, H. Dallaporta, and M. Offenberg. “Hydrocarbon Reactionswith HF-Cleaned Si(100) and Effects on Metal-Oxide-Semiconductor Device Quality.” Appl.Phys. Lett. 69(1), p.108 (1991).

24.) Göbel, U., M. Wesemann, W. Bensch, and R. Schlögl. “Analysis of Organic Contaminationson Si(100) by Thermal Desorption Spectroscopy.” Fresenius J. Anal. Chem. 343, p. 582 (1992).

25.) Henderson, R.C., R.B. Marcus, and J. Polito, J. Appl. Phys. 42, p.1208 (1971).

26.) Henderson, R.C. J. Elechtrochem. Soc. 124, p.1795 (1977).

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Figure 1 Schematic of the TD/GC/MS

Figure 2 Schematic of the TD/FID System

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Figure 3 Typical TD/FID Data Scan Using a Constant Temperature Ramp Date

Figure 4 Outgassing as a Function of the Number of Times a Sample was Run(TD/FID Technique)

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Figure 5 Typical Plot of the TD/FID Data Using a Stepped Temperature Program

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Figure 6 Polycarbonate Outgassing for Four Peaks in a Chromatogram

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APPENDIX: SAMPLE GC/MS CHROMATOGRAMS

Figure A-1 Polycarbonate #4; 150° C Desorption Temperature

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Figure A-10 Polypropylene; 150° C Desorption Temperature

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