ASTM_F_1724_-_96.pdf

Designation:F 1724 96Standard Test Method forMeasuring Surface Metal Contamination of PolycrystallineSilicon by Acid Extraction-Atomic AbsorptionSpectroscopy1This standard is issued under the fixed designation F 1724;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the quantitative determinationof surface trace metal contamination on the surface of poly-crystalline silicon using an acid to extract the metals from thesurface.The metals content of the acid is then analyzed bygraphite furnace atomic-absorption spectroscopy.1.2 This test method can be used for various rod,chunk,granule and chip sizes,for polycrystalline or single crystalsilicon,to determine surface metal contaminants.Since thearea of irregularly-shaped chunks,chips,or granules is difficultto measure accurately,values are based on sample weight.Using a sample weight of 300 g allows detection limits at the0.1 ppbw(parts per billion weight)level.1.3 The strength,composition,temperature,and exposuretime of the acid determine the depth of surface etching and theefficiency of the extraction of the contaminants from thesurface.Less than 1%of the sample weight is removed in thistest method.1.4 This test method is useful for determining the alkalielements,alkali earth,and first series transition elements,suchas sodium,potassium,calcium,iron,chromium,nickel,copper,zinc,as well as other elements such as aluminum.The recoveryof these elements from the silicon surface is measured asgreater than 90%,using control standards intentionally addedto the polysilicon surface.1.5 This test method suggests a particular sample size,acidcomposition,etch cycle,testing environment,and instrumentprotocol.Variations in these parameters may be used,but mayeffect the recovery efficiency or retention of metals duringprocessing.In practice,this test method is used for sampleweights of 25 to 5000 g.For referee purposes,this test methodspecifies a sample weight of 300 g.This test method includesguidelines to alert the analyst to the interferences and resultantvariations in this test method,and includes standard methodsfor quantifying and reporting these variations.1.6 This test method specifies the use of graphite furnaceatomic-absorption spectroscopy to analyze trace metals contentof the acid extract.Other instruments of equivalent sensitivity,such as inductively-coupled plasma/mass spectrometry,may beused.1.7 The detection limit and method variation depend on theefficiency of the acid extraction procedure,sample size,themethod interferences,the absorption spectrum of each element,and the instrumental sensitivity,background,and blank value.1.8 This test method uses hot acid to etch away the surfaceof the silicon.The etchant is potentially harmful and must behandled in an acid exhaust fume hood,with utmost care at alltimes.Hydrofluoric acid solutions are particularly hazardousand should not be used by anyone who is not familiar with thespecific preventive measures and first aid treatments given inthe appropriate Material Safety Data Sheet.1.9 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.Specific precau-tionary statements are given in Section 9.2.Referenced Documents2.1 ASTM Standards:D 5127 Guide for Electronic Grade Water2E 122 Practice for Choice of Sample Size to Estimate aMeasure of Quality of a Lot or Process32.2SEMI Standards:C 7 Specification for Reagents42.3Federal Standard:209E Airborne Particulate Cleanliness Classes in Clean-rooms and Clean Zones53.Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 acid blanka sample of acid used to establish thebackground spectrum and trace metal contamination of theextraction acid used in the procedure.3.1.2 carrousel protocolthe order and function of1This test method is under the jurisdiction of ASTM Committee F-1 onElectronics and is the direct responsibility of Subcommittee F01.06 on SiliconMaterials and Process Control.Current edition approved July 10,1996.Published September 1996.2Annual Book of ASTM Standards,Vol 11.01.3Annual Book of ASTM Standards,Vol 14.02.4Available from Semiconductor Equipment and Materials International,805 E.Middlefield Road,Mountain View,CA 94043.5Available from Superintendent of Documents,U.S.Government PrintingOffice,Washington,DC 20402.1Copyright ASTM,100 Barr Harbor Drive,West Conshohocken,PA 19428-2959,United States.samples,standards,and blanks loaded into the sampling tray ofthe atomic absorption spectrograph(AAS).3.1.3 digestionholding the polysilicon chunks in the acidmixtur