1、Designation: F 1404 92 (Reapproved 1999)Test Method forCrystallographic Perfection of Gallium Arsenide by MoltenPotassium Hydroxide (KOH) Etch Technique1This standard is issued under the fixed designation F 1404; the number immediately following the designation indicates the year oforiginal adoption
2、 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 is used to determine whether an ingotor wafer of gallium a
3、rsenide is monocrystalline and, if so, tomeasure the etch pit density and to judge the nature of crystalimperfections. To the extent possible, it follows the corre-sponding test method for silicon, Test Method F 47. TestMethod F 47 also presents the definition of many crystallo-graphic terms, applic
4、able to this test method.1.2 This procedure is suitable for gallium arsenide crystalswith etch pit densities between 0 and 200 000/cm2.1.3 Gallium arsenide, either doped or undoped, and withvarious electrical properties, may be evaluated by this testmethod. The front surface normal direction of the
5、sample mustbe parallel to the within 6 5 and must be suitablyprepared by polishing or etching, or both. Unremoved process-ing damage may lead to etch pits, obscuring the quality of thebulk crystal.1.4 This standard does not purport to address all of thesafety problems, if any, associated with its us
6、e. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and to determine theapplicability of regulatory limitations prior to use. Specifichazard statements are given in Section 8.2. Referenced Documents2.1 ASTM Standards:2D 1125 Test Methods for
7、Electrical Conductivity and Re-sistivity of WaterE 177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsF 26 Test Methods for Determining the Orientation of aSemiconductive Single CrystalF 47 Test Method for Crystallographic Perfection of Siliconby Preferential Etch Techniques33.
8、Summary of Test Method3.1 The determination of the etch pit density is onlymeaningful for monocrystalline material. After a mechanical orchemical polish, or both, of the sample surface, the sample isetched in molten KOH. This agent preferentially attacks thegallium arsenide surface in regions of cry
9、stal imperfections,such as low angle grain boundaries, twin lamellae, precipitates,slip lines, and dislocations. The etched surface is examinedmicroscopically to characterize these imperfections, and deter-mine their density.3.2 Viewed through an optical microscope, etch pits appearas dark elongated
10、 hexagonal pits. The etch pit density (EPD) isdetermined by counting these pits at nine different standardizedlocations across the sample along and directions.A lens micrometer or a grid installed in the microscope is usedto define the sampling area. The reported EPD is obtained byaveraging the EPD
11、values in the nine counted areas.3.2.1 The orientation of the elongated KOH etch pits mayalso be used to determine the crystal orientation prior to theaddition of flats to gallium arsenide (GaAs) wafers or crystals.4. Significance and Use4.1 The use of GaAs for semiconductor devices requires aconsis
12、tent atomic lattice structure. However, lattice or crystalline defects of various types and quantities are always present,and rarely homogeneously distributed. It is important todetermine the mean value and the spatial distribution of theetch pit density.5. Characteristics of Revealed Imperfections5
13、.1 The KOH etch of the specimen surface reveals patternsthat are characteristic for several of the crystalline defectsdescribed in detail in Test Method F 47.5.1.1 Dislocations on 100 GaAs surfaces are character-ized by microscopic anisotropic six-sided etch pits. The size ofthe pits depends on the
14、consistency of the etch and the etchingtime and will be typically 25 to 50 m for the procedure1This test method is under the jurisdiction of F-1 on Electronics and is the directresponsibility of Subcommittee F01.15 on Gallium Arsenide.Current edition approved May 15, 1992. Published July 1992.2For r
15、eferenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Discontinued; see 1997 Annual Book of ASTM Standards, Vol 10.05.1C
16、opyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.described in Section 9. Because the sides of these pits are notnormal to the incident light, they appear dark under verticalfield illumination. The use of a Nomarski microscope isoptional
17、.5.1.2 Lineage, a precursor to a low-angle boundary, appearsas a linear array of etch pits with a density greater than 25pits/mm. For this test method, linear arrays less than 0.5 mm inlength are not considered lineage. The individual etch pits arealigned end to end, or side to side. The lineage doe
18、s notnecessarily follow a direction.5.1.3 Slip is evidenced by a pattern of one or more straightlines of etch pits that do not necessarily touch each other. Theends of the anisotropic etch pits will be on a common line. Thisline of etch pits will be in a direction.5.1.4 A grain boundary appears as a
19、 grooved line of anylength in which individual etch pits cannot be resolvedmicroscopically at 2003 magnification. The grooved lines mayenclose an area of the etched surface or extend to the peripheryof the specimen.5.1.5 A twin boundary appears as a straight line at theintersection of a crystallogra
20、phic plane (usually a plane)and the etched surface under examination. Two parallel twinboundaries that are separated by only a few crystal latticeplanes form a twin lamella that appears as a straight groovedline.6. Apparatus6.1 Slicing EquipmentTypically an inside diameter (ID)saw. Such a saw produc
21、es a minimum amount of cuttingdamage.6.2 Wafer Preparation EquipmentThis equipment in-cludes lapping and polishing facilities capable of removing aminimum of 12 m from the surface to be characterized. Apolishing etch may be used in place of the wafer polisher, butwill require substantially more stoc
22、k removal (50 m mini-mum).6.3 Laboratory EquipmentNickel crucibles and tweezersare necessary to work with molten KOH. Platinum or zirco-nium have also been used successfully and can be substitutedfor the nickel tools.6.4 Device, capable of heating the crucible with the samplesto 500C.6.5 Microscope,
23、 provided with 103 and 203 magnificationobjective lenses, a 103 magnification eye piece, a 0.5-mmpitch micrometer, and a metric stage micrometer.7. Reagents and Materials7.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Where available, all reagents shall conform tothe specifi
24、cations of the Committee on Analytical Reagents ofthe American Chemical Society.4Other grades may be used,provided it is first ascertained that the reagent is of sufficientlyhigh purity that it will not reduce the accuracy of the test.7.2 Purity of Water Reference to water shall be under-stood to me
25、an either distilled or deionized water with aresistivity greater than 2 MVcm at 25C, as determined by thenonreferee method of Test Methods D 1125.7.3 Chemical Polish One of the following:7.3.1 Polishing Etch, (such as bromine/methanol, or sulfuricacid/hydrogen peroxide).7.3.2 Sodium Hypochlorite.7.4
26、 Lapping Abrasive Alumina, Size 5 (0.06 to 0.3 m).7.5 Degreasing ChemicalsAs required according to pre-vious process such as:7.5.1 1,1,1trichloroethane (TCA 1-1-1),7.5.2 Acetone,7.5.3 Isopropanol (2-propanol), and7.5.4 Other Wax-Removing Solvent.7.6 Defect Etch:7.6.1 Potassium Hydroxide (KOH), anhyd
27、rous.8. Hazards8.1 The chemicals used in this evaluation procedure arepotentially harmful and must be handled with the utmost careat all times. Read the most current copy of the Material SafetyData Sheet (MSDS) for each chemical used. Wear protectivegloves and a safety mask so that molten KOH cannot
28、 contactyour skin. Safety glasses must be worn at all times. Observecommon laboratory safety precautions. Dispose of all chemi-cals properly.9. Sample9.1 The wafer to be measured must be free of inclusions,large grains and twins. Those would interfere with the deter-mination of the average EPD value
29、.9.2 The procedure applies to crystals grown by any method,such as Liquid Encapsulated Czochralski (LEC), HorizontalBridgman (HB), and Vertical Gradient Freeze (VGF). Thesample surface must be oriented within 5 parallel to a plane.10. Procedure10.1 Orient the ingot so that the front surface normaldi
30、rection of the sample is parallel to the within 5.Either the X-ray or the optical method of Test Methods F 26can be applied. Cut a wafer at least 0.025 in.-thick from thecrystal. If the crystal has no flats, notch a 110 edge of thewafer. This will later permit locating areas for etch pitcounting. LE
31、C crystals grown on result in round wafers.HB wafers are D-shaped, unless processed into round wafers.10.2 Polish the wafer. Afterwards, the wafer must becleaned and dried. Make sure that a minimum of 0.0015 in. hasbeen removed from each side. If the wafer appears contami-nated or not fully polished
32、, repeat the polishing process.10.3 If the wafer was exposed to wax during previousprocesses, it must be fully degreased. Immerse the wafer forfive min in hot (60C) 1,1,1trichloroethane, followed by 5min in cold 1,1,1trichloroethane, followed by an acetone dipand by an isopropanol dip. Finally, imme
33、rse the wafer for fivemin in hot (60C) isopropanol; remove the wafer and allow toair dry.4“Reagent Chemicals, American Chemical Society Specifications,” Am. Chemi-cal Soc., Washington, DC. For suggestions on the testing of reagents not listed bythe American Chemical Society, see “Reagent Chemicals a
34、nd Standards,” by JosephRosin, D. Van Nostrand Co., Inc., New York, NY, and the “United StatesPharmacopeia.”F 1404 92 (1999)210.4 Place the wafer in the center of the bottom of a nickelcrucible. If several wafers are treated simultaneously, thewafers should not touch each other or the walls of the c
35、rucible.A large, flat crucible may be necessary.10.5 Cover the wafers with KOH completely. Use KOHsparingly and avoid skin contact; remember to wear eyeprotection.10.6 Preheat the heating device to 450C. Place the lid onthe crucible. Place the crucible on the heating device. Checkafter 3 min to veri
36、fy that KOH is completely molten; if not,increase the heat-up time. Leave the crucible on the heatingdevice for additional 7 min.10.7 Using long tongs, remove the crucible from the heatingdevice and place it on a hot pad nearby. Remove the lid.10.8 Pour the molten KOH into a second nickel crucible.T
37、he molten KOH can be used once more for a second batch ofwafers.10.9 Using the nickel tweezers, place the wafer(s) nearlyupright along the wall of the crucible so that most of the moltenKOH drips off. This will also allow for easy wafer removalonce the KOH freezes. Allow the wafers to cool for 5 min
38、.10.10 Place the wafer(s) under running dionized water untilthe remaining solid KOH is fully removed. Any KOH remain-ing in the crucible may be removed the same way.10.11 Examine the wafer. The previously polished surfaceshould now have a dull, matte appearance. It may exhibit somecellular structure
39、s. Examine it under the microscope with a103 objective lens to determine if the proper development ofetch pits has occurred. If no pits have developed, repeat 10.4through 10.11 with an adjustment of the heating period, forexample, to a total time of 15 min.10.12 Adjustments for Overetched Wafers:10.
40、12.1 If large and overcrowded pits are present the samplemay have been etched too long, or at too high a temperature.10.12.2 Change the objective lens to 203 magnification andcheck again. If the pits are still too difficult to count, repeat10.1 to 10.11 for a shorter etch time, for example, to a tot
41、altime of 7 min.10.13 For etch pit densities less than 500/cm2, choose a fieldof view that results in a minimum of 20 pits and a maximumof 150 pits in each counting area. The field selection shouldremain as representative as possible.10.14 The etch pit density is to be established in 9 locations.The
42、se locations are defined in Table 1 for round wafers such asproduced by the LEC process and in Table 2 for D-shapedwafers produced by the HB method. Table 2 has been takenfrom Table 1 of Test Method F 47.10.14.1 For LEC GaAs the EPD distribution does not havecircular symmetry. The average EPD does n
43、ot fully describethe condition of the entire wafer.10.14.2 For specimens with EPD greater than 500/cm2,a103 objective lens is recommended. A203 objective is rec-ommended when counting EPDs in the range of 30 000 to200 000/cm2.10.15 Round LEC Wafers To count the EPD, place theetched specimen on the m
44、icroscope stage so that the major flatfaces towards the operator. If the sample has no flats, orient itso that the long axis of the pits point toward the operator. Notethe location of the reference notch from 10.1 for test records.10.15.1 Measure the diameter of the wafer using the Vernierscale of t
45、he microscope stage. Determine the nine countingpositions according to Table 1 and Fig. 1.NOTE 1The order of the counting locations differs from Test MethodF 47 to avoid interference with the flats on GaAs wafers.10.15.2 With the wafer flat facing the operator, move thewafer so that the 0.5-mm micro
46、metre disk is centered inPosition 1. Count the etch pits and record the results as well asthe microscope objective magnification. Repeat the procedurefor Position 2 etc. Upon reaching Position 5, rotate the wafer45 clockwise and continue for Positions 6 through 9. Incalculating the average EPD, be s
47、ure to count Position 3 onlyonce.10.16 100 Oriented D-Shaped Wafers From BoulesGrown by the Bridgman Method:10.16.1 The etch pit density of these wafers will be countedat 9 locations that are different from the locations used for LECwafers. Because of the wafer shape asymmetry, and becausethe KOH et
48、ch pits on D-shaped wafers have a more uniformdistribution, the two counting axes are chosen at 90 to oneanother as shown in Fig. 2.10.16.2 To count the etch pits, construct the two axes of Fig.2. The first axis is perpendicular to the flat part of the wafer.The second axis is perpendicular to the f
49、irst axis, and bisects it.10.16.3 Measure the length of each axis to the nearestmillimetre.10.16.4 Locate the nine counting positions along both axesaccording to Table 2. With the flat side of the wafer pointingaway from the operator, begin with Location 1 and continue toPosition 5. Rotate the wafer 90 counterclockwise and proceedwith Positions 6 through 9. Record results and conditions asdescribed in 10.15.2.10.17 Very High and Very Low Etch Pit Density Procedure:10.17.1 If for an LEC sample the total number of pitscounted for the 9 positions is less
