SEMI MF1535 - Test Method for Carrier Recombination Lifetime in Electronic-Grade Silicon Wafers by Noncontact Measurement of Photoconductivity Decay by Microwave Reflectance -

Abstract

If the free carrier density of an electronic-grade semiconductor is not too high, the carrier recombination lifetime is controlled by impurity centers that have energies located in the forbidden energy gap. Many metallic impurities form such recombination centers in silicon. In most cases, very small densities of these impurities (»1010 atoms/cm3) reduce the carrier recombination lifetime and adversely affect device and circuit performance. Such impurities may be introduced into the wafer during manufacture or during various processing steps, especially those that involve high temperatures.

 

In some cases, such as very fast bipolar switching devices and high power devices, the recombination characteristics must be carefully controlled to obtain the desired device performance, but for most electronic-grade silicon wafers in use today, the concern is simply with controlling the process so that such impurities are not present. Although this Test Method is generally nonselective, certain individual impurity species sometimes can be identified under very restricted conditions.

 

This Test Method is suitable for use in research and development, process control, and materials acceptance applications. However, the results obtained by this Test Method depend on many of the experimental conditions including degree of surface passivation, injection level, and so forth. Therefore, when this Test Method is used for materials specification or acceptance, the supplier and the purchaser must agree fully on the experimental conditions used.

In this Test Method, the decay of the wafer conductivity following generation of excess carriers with a light pulse is determined by monitoring the microwave reflectivity of the wafer. Since no contact is made to the specimen except for the mounting stage on which it rests, this Test Method is nondestructive. If wafer and stage cleanness is maintained, wafers may be further processed following testing by this Test Method.

 

Measurement of the carrier recombination lifetime in the absence of surface recombination results in the determination of the bulk recombination lifetime (tb) provided that the excess carrier density is much less than the majority carrier density. This determination is not included in the main part of the Test Method, but information is given in Related Information 1 for those users who may wish to extend it to include the reduction of surface recombination to allow the determination of tb at low excess carrier density levels.

 

In general, however, it is very difficult to completely suppress surface recombination, and the lifetime measurement is in many cases done without any surface passivation or without confirming that the surface passivation is perfect. Consequently, two parameters, the 1/e lifetime (τe) and/or the primary mode lifetime (τ1), which do not depend on surface passivation, are determined in this Test Method.

In addition, the initial injection level also affects the measured lifetime value. If the initial injection level is sufficiently low, the measured lifetime is not influenced by the injection level. However, to enhance the signal-to-noise (S/N) ratio, higher-level injection is often adopted. Therefore, this Test Method also allows measurement of these parameters when the injection level may influence the measured lifetime especially in the early stages of decay.

 

Referenced SEMI Standards (purchase separately)

SEMI M1 — Specification for Polished Single Crystal Silicon Wafers

SEMI M59 — Terminology for Silicon Technology

SEMI MF28 — Test Method for Minority-Carrier Lifetime in Bulk Germanium and Silicon by Measurement of Photoconductive Decay

SEMI MF42 — Test Method for Conductivity Type of Extrinsic Semiconducting Materials

SEMI MF84 — Test Method for Measuring Resistivity of Silicon Wafers with an In-Line Four-Point Probe

SEMI MF391 — Test Method for Minority Carrier Diffusion Length in Extrinsic Semiconductors by Measurement of Steady-State Surface Photovoltage

SEMI MF533 — Test Method for Thickness and Thickness Variation of Silicon Wafers

SEMI MF673 — Test Method for Measuring Resistivity of Semiconductor Wafers or Sheet Resistance of Semiconductor Films with a Noncontact Eddy-Current Gage

SEMI MF723 — Practice for Conversion Between Resistivity and Dopant Density for Boron-Doped, Phosphorus-Doped, and Arsenic-Doped Silicon

SEMI MF978 — Test Method for Characterizing Semiconductor Deep Levels by Transient Capacitance Techniques

SEMI MF1388 — Test Method for Generation Lifetime and Generation Velocity of Silicon Material by Capacitance-Time Measurements of Metal-Oxide-Silicon (MOS) Capacitors

SEMI MF1530 — Test Method for Flatness, Thickness, and Thickness Variation of Silicon Wafers by Automated Noncontact Scanning

 

Revision History

SEMI MF1535-1015 (Reapproved 1121)

SEMI MF1535-1015 (complete rewrite)

SEMI MF1535-0707 (technical revision)

SEMI MF1535-1106 (technical revision)

SEMI MF1535-1104 (technical revision)

SEMI MF1535-00 (first SEMI publication)

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