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1、 1 SEMI MF28-0707 SEMI 2003, 2007 SEMI MF28-0707 TEST METHODS FOR MINORITY CARRIER LIFETIME IN BULK GERMANIUM AND SILICON BY MEASUREMENT OF PHOTOCONDUCTIVITY DECAY This standard was technically approved by the global Silicon Wafer Committee. This edition was approved for publication by the global Au
2、dits and Reviews Subcommittee on April 25, 2007. It was available at www.semi.org in June 2007 and on CD-ROM in July 2007. Originally published by ASTM International as ASTM F 25-63T; previously published November 2006. 1 Purpose 1.1 Minority carrier lifetime is one of the essential characteristics
3、of semiconductor materials. Many metallic impurities form recombination centers in germanium and silicon; in many cases, these recombination centers are deleterious to device and circuit performance. In other cases, the recombination characteristics must be carefully controlled to obtain the desired
4、 device performance. 1.1.1 If the free carrier density is not too high, minority carrier lifetime is controlled by such recombination centers; however, because it does not distinguish the type of center present, a measurement of minority carrier lifetime provides only a non-specific, qualitative tes
5、t for metallic contamination in the material. 1.1.2 When present in sufficient quantity, free carriers control the lifetime; thus, these test methods do not provide a reliable means for establishing the presence of recombination centers due to unwanted metallic or other non-dopant impurities when ap
6、plied to silicon specimens with resistivity below 1 cm. 1.2 These test methods are suitable for use in research, development, and process control applications; they are not suitable for acceptance testing of polished wafers since they cannot be performed on specimens with polished surfaces. 2 Scope
7、2.1 These test methods cover the measurement of minority carrier lifetime appropriate to carrier recombination processes in bulk specimens of extrinsic single-crystal germanium or silicon. 2.2 These test methods are based on the measurement under low level conditions of the filament lifetime of the
8、decay of the specimen conductivity after generation of carriers with a light pulse. If the decay is exponential from the initial time, the filament lifetime is the time (in s) between the peak or saturation of the photoconductivity voltage, V0, and the time where the photoconductivity voltage is equ
9、al to V0 divided by e. For an exponential decay curve, this lifetime is equal to the (1/e) lifetime of SEMI MF1535. However, as noted in that standard and in 3.5 of this standard, the initial part of the decay curve is frequently not exponential in nature so under these circumstances, the filament l
10、ifetime determined from the initial portion of the decay curve cannot be used to determine the minority carrier lifetime, which must then be determined from an exponential portion of the curve (see 11.7) occurring some time into the decay. Under these circumstances, the filament lifetime is the same
11、 as the primary mode lifetime of SEMI MF1535. 2.3 The following two test methods are described: 2.3.1 Test Method A Pulsed Light Method,1 that is suitable for both silicon and germination. 2.3.2 Test Method B Chopped Light Method,2 that is specific to silicon specimens with resistivity 1 cm. 2.4 Bot
12、h test methods are nondestructive in the sense that the specimens can be used repeatedly to carry out the measurement, but these methods require special bar-shaped test specimens of size (see Table 1) and surface condition (lapped) that would be generally unsuitable for other applications. 1 This te
13、st method is based in part on IEEE Standard 225. 2 DIN 50440/1 is an equivalent test method. SEMI MF28-0707 SEMI 2003, 2007 2 Table 1 Dimensions of Three Recommended Bar-Shaped Specimens Type Length, mm Width, mm Thickness, mm A 15.0 2.5 2.5 B 25.0 5.0 5.0 C 25.0 10.0 10.0 2.5 The shortest measurabl
14、e lifetime values are determined by the turn-off characteristics of the light source while the longest values are determined primarily by the size of the test specimen (see Table 2). Table 2 Maximum Measurable Values of Bulk Minority Carrier Lifetime, B, s Material Type A Type B Type C p-type german
15、ium 32 125 460 n-type germanium 64 250 950 p-type silicon 90 350 1300 n-type silicon 240 1000 3800 2.6 Because special test specimens are required, it is not possible to perform this test directly on the material to be employed for subsequent device or circuit fabrication. Furthermore, the density o
16、f recombination centers in a crystal is not likely to be homogeneously distributed. Therefore, it is necessary to select samples carefully in order to ensure that the test specimens are representative of the properties of the material being evaluated. NOTE 1: Minority carrier lifetime may also be de
17、duced from the diffusion length as measured by the surface photovoltage (SPV) method made in accordance with SEMI MF391. The minority carrier lifetime is the square of the diffusion length divided by the minority carrier diffusion constant which can be calculated from the drift mobility. SPV measure
18、ments are sensitive primarily to the minority carriers; the contribution from majority carriers is minimized by the use of a surface depletion region. As a result lifetimes measured by the SPV method are often shorter than lifetimes measured by the photoconductivity decay (PCD) method because the ph
19、otoconductivity can contain contributions from majority as well as minority carriers. In the absence of carrier trapping, both the SPV and PCD methods should yield the same values of lifetime3 providing that the correct values of absorption coefficient are used for the SPV measurements and that the
20、contributions from surface recombination are properly accounted for in the PCD measurement. NOTICE: This standard does not purport to address safety issues, if any, associated with its use. It is the responsibility of the users of this standard to establish appropriate safety and health practices an
21、d determine the applicability of regulatory or other limitations prior to use. 3 Limitations 3.1 Carrier trapping may be significant in silicon at room temperature and in germanium at lower temperatures. If trapping of either electrons or holes occurs in the specimen, the excess concentration of the
22、 other type of carrier remains high for a relatively long period of time following cessation of the light pulse, contributing a long tail to the photoconductivity decay curve. Measurements made on this portion of the decay curve result in erroneously long time constants. 3.1.1 Trapping can be identi
23、fied by increases in the time constant as the measurement is made further and further along the decay curve. 3.1.2 Trapping in silicon may be eliminated by heating the specimen to a temperature between 50 and 70C or by flooding the specimen with steady background light. 3.1.3 The minority carrier li
24、fetime should not be determined from a specimen in which trapping contributes more than 5% to the total amplitude of the decay curve (Test Method A) or in which the decay curve is non-exponential (Test Method B). 3 Saritas, M., and McKell, H. D., “Comparison of Minority-Carrier Diffusion Length Meas
25、urements in Silicon by the Photoconductive Decay and Surface Photovoltage Methods,” J. Appl. Phys. 63, 45624567 (1988). -,-,- 3 SEMI MF28-0707 SEMI 2003, 2007 3.2 The measurement is affected by surface recombination effects, especially if small specimens are used. The specified specimen preparation
26、results in an infinite surface recombination velocity. Corrections for surface recombination for specimens with infinite surface recombination velocity and specific recommended sizes are given in Table 3. A general formula for establishing the correction is also provided in the calculations section;
27、 use of this correction is especially important when the ratio of the surface area to volume of the specimen is large. 3.2.1 If the correction for surface recombination is too large, the accuracy of the minority carrier lifetime determination is severely degraded. It is recommended that the correcti
28、ons applied to the observed decay time not exceed one-half of the reciprocal of the observed value of decay time. Maximum bulk lifetimes that can be determined on the standard bar-shaped specimens are listed in Table 2. 3.3 The conductivity modulation in the specimen must be very small if the observ
29、ed decay, that is actually the decay of the potential across the specimen, is to be equal to the decay of the photoinjected carriers. Table 3 Surface Recombination Rate, Rs, s 1 Material Type A Type B Type C p-type germanium 0.03230 0.00813 0.00215 n-type germanium 0.01575 0.00396 0.00105 p-type sil
30、icon 0.01120 0.00282 0.00075 n-type silicon 0.00420 0.00105 0.00028 3.3.1 Test Method A allows the use of a correction when the maximum modulation of the measured direct current voltage across the specimen, V0/Vdc, exceeds 0.01. 3.3.2 Test Method B does not permit the use of this correction. In this
31、 test method, the condition for low-level photoinjection is that the ratio of the density of injected minority carriers in the specimen that exists in the steady state under constant illumination to the equilibrium majority carrier density be less than 0.001 (see 12.10). If the photoinjection cannot
32、 be reduced to a low-level value, the specimen is not suitable for measurement by this test method. 3.4 Inhomogeneities in the specimen may result in photovoltages that distort the photoconductivity decay signal. Tests for the presence of photovoltages are provided in both test methods (see 11.5 and
33、 12.6). Specimens that exhibit photovoltages in the absence of current are not suitable for minority carrier lifetime measurement by these test methods. 3.5 Higher mode decay of photoinjected carriers can influence the shape of the decay curve, particularly in its early phases.4 This phenomenon is m
34、ore significant when a pulsed light source is used because the initial density of injected carriers is less uniform than when a chopped light source is used. Consequently, Test Method A requires the use of a filter (to increase the uniformity of the injected carrier density) and measurement of the d
35、ecay curve after the higher modes have died away to establish the filament lifetime. 3.6 If minority carriers are swept out of an end of the specimen by the electric field generated by the current, they do not contribute to the decay curve. Both test methods require the use of a mask to shield the e
36、nds of the specimen from illumination and have tests to ensure that sweep-out effects are not significant. 3.7 The recombination characteristics of impurities in semiconductors are strongly temperature dependent. Consequently, it is essential to control the temperature of the measurement. If compari
37、sons between measurements are to be made, both measurements should be made at the same temperature. 3.8 Different impurity centers have different recombination characteristics. Therefore, if more than one type of recombination center is present in the specimen, the decay may consist of two or more e
38、xponentials with different time constants. The resulting decay curve is not exponential; a single minority carrier lifetime value cannot be deduced from photoconductivity decay measurements on such a specimen. 4 Blakemore, J. S., Semiconductor Statistics (Dover Publications, New York, 1987) 10.4. SE
39、MI MF28-0707 SEMI 2003, 2007 4 4 Referenced Standards and Documents 4.1 SEMI Standards SEMI M59 Terminology for Silicon Technology SEMI MF42 Test Method for Conductivity Type of Extrinsic Semiconducting Materials SEMI MF43 Test Method for Resistivity of Semiconductor Materials SEMI MF391 Test Method
40、s for Minority Carrier Diffusion Length in Extrinsic Semiconductors by Measurement of Steady-State Surface Photovoltage SEMI MF1535 Test Method for Carrier Recombination Lifetime in Silicon Wafers by Non-Contact Measure- ment of Photoconductivity Decay by Microwave ReflectanceASTM Standard 4.2 ASTM
41、Standard5 D 5127 Guide for Ultra Pure Water Used in the Electronics and Semiconductor Industry 4.3 DIN Standard6 DIN 50440/1 Measurement of Carrier Lifetime in Silicon Single Crystals by Means of Photoconductive Decay: Measurement on Bar-Shaped Test Specimens 4.4 IEEE Standard7 IEEE Standard 225 Mea
42、surement of Minority-Carrier Lifetime in Germanium and Silicon by the Method of Photoconductive Decay NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions. 5 Terminology 5.1 Terms relating to silicon and other semiconductor technology are defined in SEMI M59
43、. 6 Summary of Test Methods 6.1 Test Method A 6.1.1 By means of ohmic contacts at each end, direct current is passed through a bar-shaped homogeneous monocrystalline semiconductor specimen with lapped surfaces. 6.1.2 The voltage drop across the specimen is observed on an oscilloscope. 6.1.3 Excess c
44、arriers are created in the specimen for a very brief time by a short pulse of light with energy near the energy of the forbidden gap. 6.1.4 An oscilloscope trace is triggered by the light pulse and the time constant of the voltage decay following cessation of the light pulse is measured from the osc
45、illoscope trace. 6.1.5 If the conductivity modulation of the specimen is very small, the observed voltage decay is equivalent to the decay of the photoinjected carriers. Thus the time constant of the voltage decay is equal to the time constant of excess carrier decay. 6.1.6 The minority carrier life
46、time is determined from this time constant; trapping effects are eliminated and corrections are made for surface recombination and excess conductivity modulation, as required. 5 Annual Book of ASTM Standards, Vol 11.01, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephon
47、e: 610-832-9500; Fax: 610-832-9555; http:/www.astm.org 6 Available in both German and English editions from Deutches Institut fr Normung e.V., Beuth Verlag GmbH, Burggrafenstrasse 4-10, D 10787 Berlin, Germany; http:/www.din.de 7 Proceedings IRE 49, 12921299 (1961) 5 SEMI MF28-0707 SEMI 2003, 2007 6
48、.2 Test Method B 6.2.1 This test method, that is specific to silicon, is similar to Test Method A except that the excess carriers are generated by a chopped rather than a pulsed light source. 6.2.2 The wavelength of the light is specified to be between 1.0 and 1.1 m. In addition, it is required that
49、 low- injection-level conditions are employed so that excess conductivity modulation effects are avoided. Special contacting procedures are given to ensure the formation of ohmic contacts, and signal conditioning may be employed before the oscilloscope. 6.2.3 Correction for surface recombination is required. 6.2.4 Test specimens that yield non-exponential signals under the conditions of the test are deemed to be unsuitable for the measurement. 7 Apparatus (see Figure 1) 7.1 L
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