MF002800 - SEMI MF28 - Test Method for Minority Carrier Lifetime in Bulk Germanium and Silicon by Measurement of Photoconductivity Decay
This Standard was technically approved by the global Silicon Wafer Technical Committee. This edition was approved for publication by the global Audits and Reviews Subcommittee on February 2, 2017. Available at www.semiviews.org and www.semi.org in March 2017; originally published by ASTM International as ASTM F25; previously published September 2012.
Minority carrier lifetime is one of the essential characteristics 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 device performance.<br>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 nonspecific, qualitative test for metallic contamination in the material.<br>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 applied to silicon specimens with resistivity below 1 Ω·cm.<br>This Test Method is suitable for use in research, development, and process control applications; it is not suitable for acceptance testing of polished wafers since it cannot be performed on specimens with polished surfaces.
This Test Method covers the measurement of minority carrier lifetime appropriate to carrier recombination processes in bulk specimens of extrinsic single-crystal germanium or silicon.<br>This Test Method is based on the measurement under low-level conditions of the filament lifetime of the 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 µs) between the peak or saturation of the photoconductivity voltage, D V0, and the time where the photoconductivity voltage is equal to D 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 lifetime 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 as the primary mode lifetime of SEMI MF1535.<br>The following two test methods are described:<br>Test Method A — Pulsed Light Method, that is suitable for both silicon and germination.<br>Test Method B — Chopped Light Method, that is specific to silicon specimens with resistivity ≥1 Ω·cm.<br>Both test methods are non-destructive 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.
Referenced 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 Methods 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 Noncontact Measurement of Photoconductivity Decay by Microwave Reflectance
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