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1、 SEMI MF1723-1104 SEMI 2004 1 SEMI MF1723-1104 PRACTICE FOR EVALUATION OF POLYCRYSTALLINE SILICON RODS BY FLOAT-ZONE CRYSTAL GROWTH AND SPECTROSCOPY This guide was technically approved by the Global Silicon Wafer Committee and is the direct responsibility of the North American Silicon Wafer Committe
2、e. Current edition approved for publication by the North American Regional Standards Committee on August 16, 2004. Initially available at www.semi.org September 2004; to be published November 2004. Original edition published by ASTM International as ASTM F 1723-96. Last previous edition SEMI MF1723-
3、02. 1 Purpose 1.1 The concentration of acceptor and donor impurities in polycrystalline silicon (polysilicon) is used by the grower of monocrystalline silicon ingots to calculate the additional dopant needed to produce the required ingot resistivity or to predict the resistivity of undoped ingots. 1
4、.2 The concentration of acceptor and donor elements and carbon in the polysilicon is used by the crystal grower to determine material acceptance. 1.3 The concentration of impurities in the polysilicon is used for monitoring source gas purity, polysilicon production processes, development of new proc
5、esses, and materials acceptance purposes. 1.4 This practice describes the sampling system and float-zone crystal growth procedures used to prepare polysilicon core samples for analysis of acceptor, donor, and carbon content. 2 Scope 2.1 This practice covers procedures for sampling polycrystalline si
6、licon rods and growing single crystals from these samples by the float-zone technique. The resultant single crystal ingots are analyzed by spectrophotometric methods to determine the trace impurities in the polysilicon. These trace impurities are acceptor (usually boron or aluminum, or both), donor
7、(usually phosphorus or arsenic, or both), and carbon impurities. 2.2 The useful range of impurity concentration covered by this practice is 0.002 to 100 parts per billion atomic (ppba) for acceptor and donor impurities, and 0.02 to 15 parts per million atomic (ppma) for carbon impurity. These impuri
8、ties are analyzed in the ingot samples by infrared or photoluminescence spectroscopy. 2.3 This practice is applicable only to evaluation of polysilicon ingots grown by a method that utilizes a slim silicon rod (filament) upon which the polycrystal- line silicon is deposited. NOTICE: This standard do
9、es not purport to address safety issues, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health guides and determine the applicability of regulatory or other limitations prior to use. 3 Limitations 3.1 Polysilicon rods that a
10、re cracked, highly stressed, or have deep dendritic growth cannot be sampled due to shattering or breaking during the coring process. 3.2 Polysilicon cores with fractures, cracked surfaces, or voids in the surface are difficult to clean. Impurities are not completely etched out of the cracks or void
11、s, or etch residues may remain in the cracks, thus contrib- uting contamination. Cracked or highly stressed cores may shatter or break during the zoning process. Cores must be cleaned after fabrication to remove any oil, grease, or handling contamination. 3.3 The purity of the acids and deionized wa
12、ter (DI) is critically important. Impurities in the acids, etching apparatus, or water may interfere with accurate, reproducible analysis. Etching and zoning should be done in a clean room to minimize impurities from the ambient air, walls, floors, and furniture. The specific acid mixture, acid etch
13、 temperature, silicon removal rate, number of etch-rinse cycles, and exposure time are other factors that must be monitored and controlled to prevent impurity interferences. Any materials that contact the etched cores, such as boats and containers, must be cleaned before use and monitored to prevent
14、 contamination. Gloves or other materials used to wrap the etched cores must be tested and monitored to prevent contamination. 3.4 The zoner itself, especially the preheater, can introduce impurities into the growing silicon ingot. The walls, preheater, coil, and seals of the zoner are usual sources
15、 of contamination. Maintaining a clean zoner is very important to the procedures covered by this practice. SEMI MF1723-1104 SEMI 2004 2 3.5 Any variation from the prescribed float-zoning procedures that can affect the distribution of the volatile impurities in the gas, liquid, and solid phases will
16、alter the results. Variations in core diameter, zone dimen- sions, pull rate, seal purity, or ambient conditions may alter the effective distribution coefficient or evaporation rate and thus change the amount of impurity incorpor- ated into the crystal. 3.6 Each acceptor or donor element and carbon
17、have unique segregation coefficients. By growing several ingots with lengths up to 30 times the zone length, the effective segregation coefficient can be measured. These should agree with published values.1,2 Wafers are cut from this ingot at equilibrium positions corresponding to the segregation co
18、efficient. Wafers cut from other locations may not accurately represent the amount of impurity in the polysilicon. If ingots can not be grown to sufficient length to achieve the flat portion of the axial concentration profile, wafers can be cut from the ingot, and the measured values corrected for t
19、he effective segregation coefficient, based on repeated measurements of control rods. 3.7 In the conversion of the core to a monocrystal during zoning, it is possible to lose structure and have a zoned rod that is not monocrystalline. Ingots with excessive crystallographic defects give photolumines-
20、 cence or infrared spectra with excessive noise; such spectra are difficult to interpret accurately. In extreme cases, it is not possible to obtain acceptable spectra. 4 Referenced Standards 4.1 SEMI Standards SEMI C3.42 Specification for Argon SEMI C28 Specifications and Guidelines for Hydro- fluor
21、ic Acid SEMI C35 Specifications and Guideline for Nitric Acid SEMI MF26 Test Methods for Determining the Orientation of a Semiconductive Single Crystal SEMI MF42 Test Methods for Conductivity Type of Extrinsic Semiconducting Materials SEMI MF397 Test Method for Resistivity of Silicon Bars Using a Tw
22、o-Point Probe SEMI MF723 Practice for Conversion Between Resistivity and Dopant Density for Boron-Doped, Phosphorus-Doped, and Arsenic-Doped Silicon SEMI MF1241 Terminology of Silicon Technology 1 Pfann, W., Zone Melting, John Wiley and Sons, New York, 1958. 2 Keller, W., et al., Floating Zone Silic
23、on, Marcel Dekker, Inc., New York, 1981. SEMI MF1389 Test Methods for Photoluminescence Analysis of Single Crystal Silicon for III-V Impurities SEMI MF1391 Test Method for Substitutional Carbon Content of Silicon by Infrared Absorption SEMI MF1630 Test Method for Low Temperature FT-IR Analysis of Si
24、ngle Crystal Silicon for III-V Impurities SEMI MF1725 Guide for Analysis of Crystallo- graphic Perfection of Silicon Ingots 4.2 ASTM Standard3 D 5127 Guide for Ultra Pure Water Used in the Electronics and Semiconductor Industry 4.3 Federal Standard4 209-E Airborne Particulate Cleanliness Classes in
25、Cleanrooms and Clean Zones 4.4 ISO Standard5 ISO 146441 Cleanrooms and associated controlled environmentsPart 1: Classification of airborne particulates NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions. 5 Terminology 5.1 Definitions 5.1.1 Terms related t
26、o semiconductor technology are defined in SEMI MF1241. 5.1.2 Other Definitions 5.1.2.1 control rod, n a cylinder of polysilicon taken from a polysilicon rod with a uniform deposition layer, having known amounts of boron, phosphorus, and carbon from repeated analysis. 5.1.2.2 core, n a cylinder of po
27、lysilicon obtained from a larger piece of polysilicon by drilling with a hollow diamond drill. 3 Annual Book of ASTM Standards, Vol 11.01, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone: 610-832-9500, Fax: 610-832-9555, Website: www.astm.org 4 Standardization Docum
28、ents Order Desk, Bldg. 4 Section D, 700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS. (This standard has been superseded by ISO 14644-1 and may no longer be available.) 5 International Organization for Standardization, ISO Central Secretariat, 1, rue de Varemb, Case postale 56, CH-1211 Gen
29、eva 20, Switzerland. Telephone: 41.22.749.01.11; Fax: 41.22.733.34.30 Website: www.iso.ch; also available in the US from American National Standards Institute, New York Office: 11 West 42nd Street, New York, NY 10036, USA. Telephone: 212.642.4900; Fax: 212.398.0023 Website: www.ansi.org, and in othe
30、r countries from ISO member organizations. -,-,- SEMI MF1723-1104 SEMI 2004 3 5.1.2.3 deposition layer (growth layer), n the layer of polysilicon surrounding the filament, extending to the outer diameter of the poly rod. 5.1.2.4 filament, slim rod, n a small diameter silicon rod, assembled into a U-
31、shape, used to provide a substrate or seed for the deposition of polycrystalline silicon. 6 Summary of Practice 6.1 One or more core samples, selected according to a prescribed plan, are taken from the polysilicon rod to be evaluated. Cores can be taken parallel or perpendicular to the filament at b
32、oth ends of the rod. The preparation and zoning process is the same for both types, but the data calculation and carbon analysis are different. 6.2 After inspection for damage, the polysilicon cores are identified and scheduled for etching and crystal growth. Cores are etched in acid, rinsed clean,
33、mounted into a float-zone crystal growth apparatus, and converted to single crystal ingots. Cores must be float- zoned as soon as possible after being etched to avoid surface contamination. Studies in one laboratory in an ISO Class 5 (Note 1) clean room indicated that surface contamination can occur
34、 after 36 h. For each laboratory, maximum holding times and handling- packaging procedures must be determined. Cores must be reetched if the maximum holding period is exceeded. To extend the holding period, cores may be wrapped and sealed in a suitably clean material and stored in a clean environmen
35、t until use. NOTE 1: ISO Class 5 as defined in ISO 14644-1 is about the same as Class 100 as defined in Federal Standard 209E. 6.3 A control rod is etched and float-zoned along with the sample rods to monitor any contamination inter- ferences from the sample preparation and float zoning process. 6.4
36、 The polysilicon cores are converted to single crystal ingots by the float-zone technique, using one zone pass in an argon atmosphere. After crystal growth is completed, the ingots are checked for monocrystalline character, diameter, and length. 6.5 Sections of the ingot are selected for measurement
37、 of acceptor, donor, and carbon content, according to the individual segregation coefficients for these elements. 6.6 From the selected sections of the ingot, wafers are cut and prepared for analysis by spectrophotometric techniques described in SEMI MF1389, SEMI MF1391, and SEMI MF1630. 7 Apparatus
38、 7.1 Coring Equipment 7.1.1 Drill Press With water cooling capability. 7.1.2 Diamond Core Drill Bit sized to produce a 20 mm diameter (approximate) polysilicon core at least 100 mm in length for parallel cores and a length suitable to drill completely through the rod diameter for perpendicular cores
39、. Drill diameters of 3 mm or 5 mm are used for seed preparation. 7.2 Etching Equipment 7.2.1 Etch Bench Located in an ISO Class 6 Clean Room, as defined in ISO 14644-1, to minimize ambient contamination, with adequate exhaust for acid fumes, tanks for etching acid and DI water rinsing, and facility
40、for drying samples in a clean environment. NOTE 2: ISO Class 6 is about the same as Class 1000 as defined in Federal Standard 209E. 7.2.2 Quartz Boats Or other acid-resistant material, such as polytetrafluoroethylene, designed to hold poly- silicon rods of the specified diameter and length, during t
41、he etching, rinsing, and drying process. 7.3 Float Zone Crystal Growth Equipment 7.3.1 Float Zone Crystal Growth Furnace With an inert gas atmosphere, and water-cooled chamber of sufficient size to accommodate growth of ingots of specified diameter and length, located in a clean room of ISO Class 6
42、or better. The apparatus shall allow relative vertical motion of the work, with respect to the coil, with no significant lateral motion. This vertical motion may be accomplished by screw, cable, or hydraulic mechanisms. In addition, there shall be a shaft to support the core sample and a shaft to su
43、pport the seed. At least one shaft shall be capable of vertical displacement relative to the other. The seed shaft shall be rotated about its longitudinal axis as a precaution against thermal and solute asymmetries in the molten zone. Either the sample or seed chuck shall be free to slip with respec
44、t to the rotation in the event of freezing of the molten zone. The sample and seed chucks shall be of molybdenum, tantalum, tungsten, or quartz to minimize contamination of the silicon. The coil design and power control shall maintain a stable, completely molten zone during the entire growth process
45、. Materi- als used in the apparatus shall have vapor pressures less than 1 106 torr under operating conditions. The susceptor (preheater) shall be about the same diameter as the sample core and made of tantalum, or other material that minimize the contamination of the silicon. -,-,- SEMI MF1723-1104
46、 SEMI 2004 4 7.3.2 Scale Calibrated in mm, suitable for accurate measurement of ingot length and marking locations in the ingot for cutting. 7.3.3 Wire Brush Made of stainless steel, suitable for cleaning the inside of the chamber of the vacuum zoner, with a handle long enough to reach the length of
47、 the chamber. 7.3.4 Vacuum Cleaner Suitable for clean room use, with flexible hose and narrow nozzle. 7.3.5 Clean Room Gloves Gowns, Masks, Hoods, Wipes, and other clean room materials. 7.3.6 Wafering Saw Suitable for cutting wafer samples, about 2-mm thick, from the ingot. 8 Reagents 8.1 Nitric Aci
48、d (HNO3) In accordance with Grade 2 of SEMI C35. 8.2 Hydrofluoric Acid (HF) In accordance with Grade 2 of SEMI C28. 8.3 Acid Etching Mixture Typically 4 to 1 to 8 to 1 HNO3 to HF. 8.4 Deionized Water With a purity equal to or greater than that specified for Type E-2 in ASTM D 5127. 8.5 Argon Purge G
49、as In accordance with SEMI C3.42. 9 Hazards 9.1 It is required that the user have a working knowledge of fabrication techniques, acid handling practices, and crystal growth furnaces. Good laboratory practices also must be understood. 9.2 This practice uses hot acid to etch away the surface of the polysilicon rod. The etchant is potentially harmful and must be handled in an acid exhaust fume hood with utmost care at all