background statement for semi draft document 4716 revision...

Background Statement for SEMI Draft Document 4716 Revision to SEMI MF391-0708 TEST METHODS FOR MINORITY CARRIER DIFFUSION LENGTH IN EXTRINSIC SEMICONDUCTORS BY MEASUREMENT OF STEADY-STATE SURFACE PHOTOVOLTAGE

Notice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in reaching an informed decision based on the rationale of the activity that preceded the creation of this document. Notice: Recipients of this document are invited to submit, with their comments, notification of any relevant patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the latter case, only publicly available information on the contents of the patent application is to be provided.

The two test methods for measuring minority charge carrier diffusion length by SPV (Surface Photo Voltage) as described in SEMI MF 391 date back to times when computer control of measurement equipment was not easily available. In a presentation at a workshop in conjunction with SEMICON Europe 2008 Vladimir Podshivalov presented a new method that would improve the accuracy of the SPV measurement significantly. Therefore the Silicon Wafer Committee approved a new activity within the Test Methods TF with the goal of revising MF391 by adding a third method for measuring SPV and a corresponding SNARF was approved in the meeting of this TC in Stuttgart in October 2008. Vladimir Podshivalov prepared a first version of a draft document that was discussed in the meeting of the Test Methods TF in Berlin in May 2009 and modifications were recommended. Subsequently Vladimir Podshivalov improved the draft document based on the TF’s recommendations and submitted the new version to the TF for discussion. This new version of the document was reviewed in the meeting of the Test Methods TF in San Francisco in July 2009 in conjunction with SEMICON West and found ready for ballot. The Silicon Wafer Committee approved the draft document for ballot, following the recommendation of the Test Methods TF, to be adjudicated in the meeting of the TC in Dresden in October 2009, in conjunction with SEMICON Europe.

The results of this document be reviewed at the Int’l Test Methods TF and will be adjudicated by the European Silicon Wafer committee during their meetings at SEMICON Europa in October 7, 2009 in Dresden, Germany. Please check www.semi.org/standards for the latest meeting schedule.

This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted standard. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

Page 1 Doc. 4716 © SEMI®

Semiconductor Equipment and Materials International 3081 Zanker Road San Jose, CA 95134-2127 Phone:408.943.6900 Fax: 408.943.7943

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DRAFTDocument Number: 4716

Date: 7/31/2009

SEMI Draft Document 4716 REVISION to SEMI MF391-0708 TEST METHODS FOR MINORITY CARRIER DIFFUSION LENGTH IN EXTRINSIC SEMICONDUCTORS BY MEASUREMENT OF STEADY-STATE SURFACE PHOTOVOLTAGE 1 Purpose

1.1 Minority carrier lifetime is one of the essential characteristics of semiconductor materials. In epitaxial layers and in thin single crystal wafers, the surface recombination corrections necessary to derive the minority carrier lifetime from the photoconductive decay (PCD) method covered by SEMI MF28 and SEMI MF1535 are excessively large.

1.2 Therefore, other test methods are required to cover the measurement of minority carrier diffusion lengths in specimens of extrinsic single-crystal semiconducting materials or in homoepitaxial layers of known resistivity deposited on more heavily doped substrates of the same type, provided that the thickness of the specimen or layer is greater than four times the diffusion length. TwoThree test methods are described:

1.2.1 Test Method A — Constant magnitude surface photovoltage (CMSPV) method. This test method circumvents the influence of surface recombination on the lifetime measurement by maintaining constant front surface conditions.

1.2.2 Test Method B — Linear photovoltage, constant photon flux (LPVCPF) method. This test method utilizes only conditions and data points that are not influenced by surface recombination and other non-linear effects.

1.2.3 Test Method C —Digital oscilloscope recording (DOR) method. This test method provides the direct control of relaxation processes of surface photovoltage (SPV) appearance and disappearance on minimum radiation intensity and eliminates any non-linear and other distorting effects, which is feasible on these conditions.

1.3 These test methods are suitable for use in research, process control, and materials acceptance.

1.4 These test methods are particularly useful in testing materials to be used in photovoltaic cells and other optical device applications since the diffusion length is derived by methods that are closely related to the functioning of the device.

1.5 Because carrier lifetime is directly influenced by the presence of metallic impurity contamination, these test methods can be interpreted to establish the presence of such contamination. However, such interpretation is beyond the scope of these test methods.

1.6 If a very thin surface region with long lifetime, such as an epitaxial layer or a denuded zone, is on a bulk region with very short lifetime, such as a heavily doped substrate or an internally gettered wafer with oxide precipitates, respectively, the intercept can not be interpreted as the diffusion length (see ¶ 3.2). Under certain circumstances, the intercept can be related to the layer thickness, providing a nondestructive means for determining the thickness of the layer.

2 Scope

2.1 These test methods are based on the measurement of surface photovoltage (SPV) as a function of energy (wavelength) of the incident illumination.

NOTE 1: The minority carrier lifetime is the square of the diffusion length divided by the minority carrier diffusion constant that is assumed or can be determined from drift mobility measurements. SPV measurements 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 the lifetimes measured by the PCD method because the photoconductivity can contain contributions from majority as well as minority carriers. When both majority and minority carrier lifetimes are the same, both the SPV and PCD methods yield the same values of lifetime1 provided that the correct values of absorption coefficient

1 Saritas, M., and McKell, H. D., “Comparison of Minority-Carrier Diffusion Length Measurements in Silicon by the Photoconductive Decay and Surface Photovoltage Methods,” J. Appl. Phys. 63 (1988): pp. 4562–4567.




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are used for the SPV measurements and that the contributions from surface recombination are properly accounted for in the PCD measurement.

2.2 BothAll three test methods covered are nondestructive.

2.3 The limits of applicability with respect to specimen material, resistivity, and carrier lifetime have not been determined; however, measurements have been made on 0.01–50 Ω·cm n- and p-type silicon specimens with carrier lifetimes as short as 2 ns.

2.4 These test methods were developed for use on single crystal specimens of silicon. They may also be used to measure an effective diffusion length in specimens of other semiconductors such as gallium arsenide (with suitable adjustment of the wavelength (energy) range of the illumination and specimen preparation procedures) and an average effective diffusion length in specimens of polysilicon in which the grain boundaries are normal to the surface.

2.5 These test methods also have been applied to the determination of the width of the denuded zone in silicon wafers.

2.6 These test methods measure diffusion lengths at room temperature (22°C) only. Lifetime and diffusion length are a function of temperature.

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 and determine the applicability of regulatory or other limitations prior to use.

3 Limitations

3.1 The quality of the measurement depends on the accuracy with which the absorption coefficient is known as a function of photon energy (wavelength).

3.1.1 Surface stresses strongly influence the absorption characteristics. These test methods provide absorption coefficient data appropriate to unstressed surfaces typical of those found on epitaxial layers and stress-relieved chemically or chem-mechanically polished wafers.

3.1.2 In heavily doped wafers, the free carrier absorption may affect the SPV measurement at long wavelengths.

3.1.3 The absorption coefficient is temperature dependent; the data given in these test methods are appropriate to room temperature only (22°C).

3.2 For the most accurate measurements, the thickness of the region to be measured must be greater than four times the diffusion length. An estimate of the diffusion length is possible when the diffusion length exceeds twice the thickness. The thickness condition is assessed after the measurement is made.

3.2.1 For measurements on a surface layer (epitaxial layer or denuded region), the intercept may be interpreted as the diffusion length in the substrate if the layer thickness is less than one-half the intercept value.2

3.2.2 If the layer thickness is between one-half and four times the intercept value, estimates of the diffusion length in the surface layer may be made provided that the thickness of the layer is known;2 conversely, the layer thickness may be deduced if certain assumptions are made about the ratio of diffusion lengths in the surface layer and substrate regions.

3.3 Unless the total specimen thickness is greater than three times the reciprocal absorption coefficient of the longest wavelength (lowest energy) illumination used, the SPV plot is nonlinear. The upper wavelength limit can be calculated before the measurement is made.

3.4 Variations in long relaxation time surface states may cause a slow drift of the amplitude of the SPV signal with time. This interference can be minimized by (1) allowing sufficient time for the states to approach equilibrium under measurement conditions and (2) making all of the measurements as quickly as possible.

2 Phillips, W. E., “Interpretation of Steady-State Surface Photovoltage Measurements in Epitaxial Semiconductor Layers,” Solid-State Electron. 15 (1972): pp. 1097–1102.




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3.5 The SPV signal can be masked by a photovoltage produced by the illumination of non-ohmic back contact or of a junction in the specimen. A masking photovoltage of this type can be identified by its large amplitude, a reversal in polarity as the illumination energy changes from large to small, or by the decrease of signal amplitude with increase of illumination intensity at longer wavelengths (smaller energy). In case of method C this masking photovoltage may be detected by SPV signal shape analysis. A junction photovoltage can be eliminated by making the reference potential contact to an unilluminated region of the front surface.

3.6 Lack of spectral purity of the illumination adversely affects the measurements. Although spectral purity requirements have not been definitively established, a spectral bandwidth of 5 nm and (if a grating monochromator is used) an intensity of higher order spectral components of less than 0.1% are expected to provide satisfactory results.

3.7 In some materials the lifetimes and diffusion lengths depend on the intensity of illumination. This occurs even when the density of hole-electron pairs is still much less than the majority carrier density. The principal effect is to give a diffusion length larger than the dark value. This effect can be minimized by working in a linear SPV range in which the SPV signal is directly proportional to the illumination intensity.

3.8 For Test MethodMethods A and C, correction must be made for any differences in losses as a function of energy (wavelength) in the optical path to the specimen and the optical path to the detector. For example, any surface film or coating can introduce an energy dependent absorption or reflection.

3.9 Handling of the test specimens with metal tweezers may introduce metal contamination that can shorten the minority carrier lifetime and result in an erroneous determination of diffusion length. To eliminate the effect of handling on diffusion length measurements, use clean plastic tweezers or a plastic vacuum pick-up.

4 Referenced Standards and Documents

4.1 SEMI Standards

SEMI C23 — Specifications for Buffered Oxide Etchants

SEMI C30 — Specifications and Guidelines for Hydrogen Peroxide

SEMI C34 — Specification and Guideline for Mixed Acid Etchants

SEMI M59 — Terminology for Silicon Technology

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

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

SEMI MF95 — Test Method for Thickness of Lightly Doped Silicon Epitaxial Layers on Heavily Doped Silicon Substrates Using an Infrared Dispersive Spectrophotometer

SEMI MF110 — Test Method for Thickness of Epitaxial or Diffused Layers in Silicon by the Angle Lapping and Staining Technique

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

SEMI MF673 — Test Methods for Measuring Resistivity of Semiconductor Slices or Sheet Resistance of Semiconductor Films with a Noncontact Eddy-Current Gauge

SEMI MF1535 — Test Method for Carrier Recombination Lifetime in Silicon Wafers by Non-Contact Measure-ment of Photoconductivity Decay by Microwave Reflectance

4.2 ASTM Standard3

ASTM D5127 — Guide for Ultra Pure Water Used in the Electronics and Semiconductor Industry

4.3 JEITA Standard4

3 American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959, USA. Telephone: 610.832.9585; Fax: 610.832.9555; http://www.astm.org




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JEITA EM-3509 — Sample preparation method for minority carrier diffusion length measurement in silicon wafers by surface photovoltage method

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.

5.2 Definitions of other terms related to minority carrier lifetime are defined in SEMI MF28 and SEMI MF1535.

6 Summary of Test Methods

6.1 Test Method A

6.1.1 The specimen surface is illuminated with chopped monochromatic radiation of energy slightly greater than the band gap of the semiconductor sample. Electron-hole pairs are produced and diffuse to the surface of the specimen where they are separated by the electric field of a depletion region to produce the SPV. The depletion region can be created by surface states, surface barrier, p-n junction, or liquid junction.

6.1.2 The SPV signal is capacitively or directly coupled into a lock-in amplifier for amplification and measurement.

6.1.3 The photon intensity is adjusted to produce the same value of SPV for all energies of the illuminating radiation.

6.1.4 The photon intensity at each selected energy is plotted against the reciprocal absorption coefficient for the energy.

6.1.5 The resultant linear plot is extrapolated to zero intensity; the (negative) intercept value is the effective diffusion length.

6.1.6 By using feedback from the detector to the light source, and a stepping motor for the monochromator, the procedure may be automated.

6.2 Test Method B

6.2.1 A surface photovoltage produced by chopped white light illumination is first measured for two different photon fluxes to ensure that the SPV is linear in photon flux.

6.2.2 Next, using monochromatic light produced by a set of narrow band filters at constant photon flux within the linear SPV range, the SPV is measured for a series of selected photon energies larger than the band gap of the semiconductor sample.

6.2.3 The reciprocals of the values of SPV that increase monotonically with photon energy are plotted against the reciprocal of the absorption coefficients corresponding to the selected photon energies.

6.2.4 The resultant linear plot is extrapolated to zero intensity; the (negative) intercept value is the effective diffusion length. The values outside the monotonic range are rejected from the analysis to eliminate interference from surface recombination effects.

6.2.5 A small area contact can be used to measure the SPV; by moving the test specimen under the probe, an area map of diffusion length can be made.

6.2.6 The procedure may be automated by using stepping motors for the filter wheel and stage; feedback to the light source is not required.

6.3 Test Method C

6.3.1 The specimen surface is illuminated with IR-radiation rectangular pulses.

6.3.2 The pulse SPV signal is capacitively coupled into a high resistively input amplifier and further into a digital oscilloscope. 4 Japanese Electronic and Information Technology Industries Association, 3rd Fl., Mitsui Sumitomo Kaijo Bldg. Annex, 11, Kanda Surugadai 3-chome, Chiyoda-ku, Tokyo 101-0062, Japan. http://www.jeita.or.jp




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6.3.3 The radiation pulse duration should exceed signal rising edge duration by a factor of three, and the time between pulses should exceed falling edge duration by a factor of three. For the correct registration of SPV impulse amplitude and shape the time constant of measuring circuit has to be significantly longer than the complete duration of this pulse by a factor of three.

6.3.4 Rising edge regression is performed by an exponential function to determine steady-state SPV.

6.3.5 The amplitude of this function is the steady-state SPV.

6.3.6 The steady-state SPV is determined for several values of IR-radiation irradiance.

6.3.7 The regression of dependency steady-state SPV on IR-radiation intensity by power function is performed and the derivative value of this function is determined at the point of origin.

6.3.8 The effective diffusion length is calculated from the derivative values at the point of origin for more that one wavelength.

6.3.9 The minority carrier diffusion length determination by method C allows highly automated measurements.

7 Apparatus

7.1 Light Source and Monochromator or Filter Wheel — Covering the wavelength range from 0.8–1.0 μm (energy range from 1.55–1.24 eV) with a means for controlling the intensity (variable ac or dc input, adjustable aperture, or neutral density filters). Both tungsten and quartz halogen lamps have been found to be suitable sources.

7.1.1 If a filter wheel is used (recommended for Test Method B), a minimum of six energies, approximately evenly spaced between 1.24 and 1.55 eV, is recommended. For Test Method B, the output photon flux (at the specimen) at each energy should be equal within ±3%. In addition, for Test Method B, provision must be made for two neutral density attenuators to provide white light at two photon flux values with a ratio φ1 to φ2 known to 1%.

7.1.2 If a grating monochromator is used, a sharp cutoff filter that attenuates at least 99% of the light with wavelength shorter than 0.6 μm is required. In this case, calibrated interference filters are required to verify the wavelength calibration of the monochromator.

7.2 Mechanical Light Chopper — To operate at a frequency that is low enough to permit a steady-state distribution of carriers to exist in the specimen, low enough to be compatible with the response time of the detector (see Note 2), and high enough to permit effective coupling of the SPV signal into the amplifier.

NOTE 2: A frequency of about 10 Hz is recommended for most applications with Test Method A. Because Test Method B does not require a detector, this condition does not apply for Test Method B and higher frequencies can also be used.

7.3 Optical Components — To couple the illumination to the specimen and photon detector. A system of mirrors (or quartz lenses or both) or a system of fiber optic cables can be used. If mirrors or lenses are used, they should be arranged to focus an image of the exit slit on the chopper blade and on the specimen and detector (see Figure 1). In this case a wavelength-independent beam splitter is used to direct some the illumination to the detector; alternatively, the detector signal can be obtained by using the reflection from the back of the chopper blades. Fiber optic cables are preferred for use with Test Method B (see Figure 2).




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Figure 1

Block Diagram of SPV Equipment and Schematic of Specimen Holder for Capacitively Coupled Contact Set Up for Test Method A

Figure 2

Block Diagram of Optical and Electronic Components of SPV Equipment for Test Method B




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Figure 3 Block Diagram of Optical and Electronic Components of SPV Equipment for Test Method С

7.4 Photon Counter or Detector — With known relative spectral sensitivity (for Test Method A only). Absolute calibration is not required. A thermopile capable of operating at the chopper frequency is satisfactory. A silicon photodiode or pyroelectric detector can also be used.

NOTE 3: The detector calibration is simplified if the optical path to the detector includes a duplicate of the front contact structure of the specimen holder so that the optical paths to the detector and specimen are similar (see ¶ 3.8).

7.5 Specimen Holder — To support the specimen and to provide a transparent capacitively coupled front contact (a glass plate with a tin oxide coating and a 50 μm thick mica dielectric layer have been found to be satisfactory) and a reference potential contact to the back surface or to an unilluminated region of the front surface. For a surface barrier, p-n junction, or liquid junction, direct electrical connection to the illuminated surface of the specimen can be made in place of the capacitively coupled front contact. The holder may provide lateral and rotational motion of the specimen if the front contact covers only a small area of the front surface and if information on the area dependence of the diffusion length is desired.

7.6 Lock-In Amplifiers — Two, to measure the amplitudes of the SPV and detector signals (see Figure 1). A sensitivity of 1 μV full scale and an output noise level of less than 0.1 μV are required. An input impedance of 10 MΩ or higher is needed to match the high source impedance of the capacitively coupled specimen. Alternatively, a single dual-input lock-in amplifier can replace the two amplifiers if care is taken to prevent interference between the two signals. This alternative configuration is particularly appropriate for Test Method B (see Figure 2) since the photon flux need not be measured during the SPV measurements.




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7.7 Conventional Laboratory Facilities — For cleaning, polishing, and etching specimens, if required.

7.8 Thermometer — Or other temperature measuring instrument, to determine the ambient temperature to ±0.5°C.

7.9 Computer Control System — (Optional) with appropriate stepping motors to perform the appropriate calculations and control the wavelength selection, stage motion, and (for Test Method A) feedback from the detector to the light source, as required.

7.10 Low Level Light Source — Coupled by fiber optic cable to the SPV system (see Figure 2), consisting of a variable dc voltage control, incandescent lamp, and 800 μm thick silicon filter.

7.11 SPV Equipment for the Test Method C — consists of a measuring unit, a control unit and a computer (see Figure 3). The measuring unit is located in a rack, enclosure and has to be guarded with respect to electrical noise. The irradiating light has to be blocked from leaving the set-up for safety reasons.

7.12 Input Amplifier — input resistance is about several GOhm, voltage amplification is 100 – 150. Noise and ripple free power supply is required.. The semiconductor surface – capacitive electrode – input amplifier measurement circuit must provide distortionless transmission of square voltage impulse with rising and falling edge ≤ 0.3 μs and a flat top decay of less than 0.7 over 0.3 seconds.

7.13 Laser Diodes (Ld1 – Ld3, fig.3) — have pulse power > 10 mW, wavelength 800 ÷ 1040 nm and spectrum width ≤3 nm. Two or more laser diodes are used for method C.

7.14 Variable attenuator (fig. 3) — provides light intensity attenuation from 0 to 60 dB.

7.15 Fiber Optic Cable and Fiber Optic Collector Construction (fig. 3) — provides illumination equalization at the output.

7.16 Computer (fig. 3) –PC, clock rate ≥ 1 GHz.

7.17 Digital Oscilloscope (expansion card in PC) — double channel, eight-bit, sampling frequency 100 MHz.

7.18 Pulse Oscillator (expansion card in PC) — single channel, eight-bit, pulse duration 0.1 ÷ 20 ms, output voltage 0 ÷ 10 V. The pulse oscillator, the laser diodes and their supply circuit must provide IR-radiation pulse edges duration of no longer than 0.3 μs and pulse top drop no more than 3%. It is assumed an initial surge no more then 7% regarding amplitude and duration.

7.19 Relay Unit (fig. 3) — provides the laser diodes commutation.

7.20 Input-output Board (expansion card in PC) — provides commutation relay manipulation.

8 Reagents

8.1 Purity of Reagents — All chemicals for which such specifications exist shall conform to the assay and to impurity levels of Grade 1 SEMI Specifications for the appropriate chemical. Other grades may be used provided it is first ascertained that the reagent is of sufficiently high purity to permit use without lessening the accuracy of the determination.

8.2 Purity of Water — Reference to water shall be understood to mean Type E-3 or better water as described in ASTM Guide D5127.

8.3 Etching Solution CP4A — 5:3:3 mixed acid etchant in conformance with Grade 1 of SEMI C34, for chemical polishing of silicon specimen surfaces (if necessary). To prepare, mix 50 mL of concentrated nitric acid (HNO3), 30 mL of concentrated hydrofluoric acid (HF), and 30 mL of glacial acetic acid (CH3COOH).

8.4 Buffered Oxide Etchant — Mixture of 40% ammonium fluoride solution (NH4F) and concentrated hydrofluoric acid (HF) in conformance with Grade 1 of SEMI C23, for use in improving the SPV signal in p-type silicon specimen surfaces, if required.

NOTE 4: A mixture of 6 parts NH4F and 1 part HF, by volume, has been found to be satisfactory for this purpose.

8.5 Hydrogen Peroxide (H2O2) — 30%, unstabilized, in conformance with Grade 1 of SEMI C30, for use in improving the SPV signal in n-type silicon specimen surfaces, if required.




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9 Hazards

9.1 The acids in the etchants used for chemically preparing or treating the specimen surfaces (when such preparation or treatment is required) are extremely hazardous. All precautions normally used when handling these chemicals should be strictly observed.

9.2 WARNING — Hydrofluoric acid solutions are particularly hazardous. Make sure that the user is familiar with the specific preventive measures and first aid treatments given in the appropriate Material Safety Data Sheet.

9.3 The method C assumes using laser diodes. Laser radiation is hazardous for eyes. Looking at the open optical cable ends must be avoided and appropriate safety measures must be taken.

10 Specimen Preparation

10.1 Epitaxial and stress-relieved chem-mechanically polished wafers can usually be measured in the as-received condition.

10.2 Surfaces of stress-relieved sawed or lapped single crystal silicon specimens require either chemical polishing with an etch such as Etch Solution CP4A (see ¶ 8.3) or chem-mechanical polishing to remove any mechanical damage to the surface.

NOTE 5: If the SPV signal is low, it can often be increased by a treatment that enhances the depletion layer. For n-type silicon, a useful treatment consists of boiling the specimen in H2O2 for about 15 minutes. For p-type silicon, a suitable treatment is an etch in buffered HF (see ¶ 8.4) for 1 minute.

NOTE 6: For more information about surface treatments for silicon, JEITA EM-3509.4

10.3 Solar cells can be measured in the as-received condition provided that the top layer is thin enough that significant carrier generation is not induced by the illumination.

10.4 A liquid junction with a transparent electrolyte can be used.5

10.5 If a thin metal surface (Schottky) barrier is used as the front contact, the optical behavior of the metal must be characterized so that the appropriate correction can be made to obtain the relative internal photon flux.

11 Calibration

11.1 The wavelength (energy) of the illumination must be accurately known. Calibrated interference filters provide a convenient means of checking wavelength calibration of a monochromator; if a filter wheel is used, the wavelength of each filter must be known or determined.

11.2 For Test Method Methods A, C, the wavelength (energy) dependence of the photon detector response, if any, must be known or determined. However, absolute calibration of the photon detector is not required.

11.3 For Test Method C, the transfer constant of the semiconductor surface – capacity probe – amplifier input measurement path must be known (with taking into account illuminated spot area).

11.4 For Test Method C, the transfer constant of the laser diodes feed circuit (pulse oscillator output voltage – IR-radiation pulse magnitude for everyone laser diode) must be known.

11.5 For Test Method C, the wavelength dependence of the variable attenuator must be known.

12 Test Method A — Constant Magnitude Surface Photovoltage (CMSPV) Method 12.1 Procedure

12.1.1 Turn on the light source, chopper, and lock-in amplifiers, and align the optical system using visible light from the monochromator or filter wheel. If a grating monochromator is used, remove the sharp cut-off filter and use a higher order diffraction mode in the visible range.

5 Micheels, R. H., and Rauh, R. D., “Use of Liquid Electrolyte Junction for the Measurement of Diffusion Length in Silicon Ribbon,” J. Electrochem. Soc. 131 (1984): pp. 217–219.




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12.1.2 Set the monochromator or filter wheel to the shortest wavelength (highest energy) to be used, usually 0.8 μm (1.55 eV).

12.1.3 Adjust the illumination intensity to about half maximum power or 70% of maximum amplitude.

12.1.4 Mount the specimen in the specimen holder and bring the capacitative or other front contact into the measurement position.

12.1.5 Adjust the frequency and phase of the lock-in amplifier connected to the specimen for maximum signal. If the same amplifier is used for both the specimen and detector, adjust the phase as needed before each reading.

12.1.6 Note the approximate SPV signal amplitude, VSPV.

12.1.7 Set the monochromator or filter wheel to the longest wavelength (lowest energy) for which data are desired (usually 1.04 μm (1.19 eV) in bulk silicon specimens and 1.0 μm (1.24 eV) in (epitaxial silicon). For specimens with short diffusion length (<20 μm), the longest wavelength may be 1.0 μm (1.24 eV) or less.

12.1.8 Change the illumination intensity to obtain a convenient value of VSPV for the series of measurements. The preferred value is near that noted in ¶ 12.1.6.

12.1.9 Reset the monochromator or filter wheel to the shortest wavelength (highest energy) and reset the intensity to produce the chosen value of VSPV.

12.1.10 Record the value of wavelength,λ, and VSPV.

12.1.11 Read and record the signal level, VD, from the photon detector.

12.1.12 Increase λ (decrease photon energy) in increments to the maximum λ (minimum energy) desired. For silicon, suitable steps are 0.85, 0.90, 0.95, 0.97, 0.99, 1.00, 1.01, 1.02, 1.03, and 1.04 μm (1.46, 1.38, 1.31, 1.28, 1.25, 1.24, 1.23, 1.22, 1.20, and 1.19 eV). For short diffusion lengths, use only the shorter values of λ (higher values of energy).

12.1.13 Adjust the illumination intensity at each wavelength (energy) to produce the chosen value for VSPV.

12.1.14 Read and record λ (or photon energy), VSPV, and VD for each value of λ (or photon energy).

12.1.15 Measure and record the specimen thickness in accordance with SEMI MF533. For an epitaxial layer, also measure and record the layer thickness in accordance with SEMI MF95 or SEMI MF110.

12.1.16 Optionally, measure the specimen resistivity in accordance with SEMI MF84 or SEMI MF673.

12.2 Calculations

12.2.1 Determine the reciprocal absorption coefficient, α−1, either by direct measurement or from the following relation:6

2

1 417.76732.84)(−

− ⎟⎠⎞

⎜⎝⎛ −=

λλα (1)

where absorption coefficient, α, is in cm−1, and wavelength, λ, is in μm.

12.2.2 Determine the relative photon intensity, I0, in arbitrary units, from VD for each value of wavelength (energy), correcting for any wavelength dependent losses.

12.2.3 If a thermopile of constant energy sensitivity at all wavelengths is used as the photon detector, and if there are no wavelength dependent components in the detector path, determine I0 as follows:

DVkI λ=0 (2)

where k is an arbitrary constant that can be taken as unity and VD is the signal level from the photon detector.

6 Nartowitz, E. S., and Goodman, A. M., “Evaluation of Silicon Optical Absorption Data for Use in Minority-Carrier-Diffusion-Length Measurements by the SPV Method,” J. Electrochem. Soc. 132 (1985): pp. 2992–2997.




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12.2.3.1 If the photon detector has a wavelength-dependent sensitivity, apply the appropriate correction factor at each wavelength used.

12.2.4 Calculate the term (1 − R) from the following relation that is valid for the case of polished silicon surfaces with thin native oxide in the wavelength range from 0.7–1.05 μm:7

21 03149.003565.06786.0)(1 −− −+=− λλλR (3)

where λ is the wavelength in μm.

12.2.4.1 For other materials or if any surface films are present on silicon, (Equation 3) is not valid; determine the term (1 − R) by direct measurement.

12.2.5 Calculate the product I0(1 − R) at each wavelength used and plot this product against α−1 .

12.2.6 Fit a straight line to the points visually or calculate a least squares fit to the data. Extrapolate the line to the (negative) abscissa and measure and record the magnitude of the intercept on the negative abscissa (see Figure 34).

12.2.7 The magnitude of this intercept is the effective diffusion length, L0. If L0 is less than one fourth the specimen (or epitaxial layer) thickness, L0 can be taken to be equal to the diffusion length, LD.

12.3 Report

12.3.1 Report the following information:

12.3.1.1 Specimen identification,

12.3.1.2 Data (λ, α−1, VSPV, and VD) for each wavelength used,

12.3.1.3 Ambient (room) temperature, and

12.3.1.4 Effective diffusion length, L0, and a statement as to whether this value is equal to the diffusion length, LD, in the layer or specimen.

12.3.2 If desired, also report the following information:

12.3.2.1 Specimen thickness,

12.3.2.2 Epitaxial layer thickness (if appropriate), and

12.3.2.3 Specimen resistivity (if determined).

7 A mathematical representation of the data of Philip, H. R., and Taft, E. A., “Optical Constants of Silicon in the Region 1 to 10 eV,” Phys. Rev. 120 (1960): pp. 37–38.




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SPV Data

Wavelength, μm (λ) Thermopile Voltage,μV (VD) Reciprocal Absorption Coefficient, μm (α-1) Intensity(I0)

1.020 8.40 206.329 5.854 1.010 6.85 164.864 4.725 1.000 5.90 134.306 4.028 0.990 5.20 111.159 3.513 0.980 4.55 93.225 3.041 0.970 4.10 79.063 2.711 0.950 3.35 58.450 2.168 0.930 2.90 44.494 1.835 0.900 2.40 30.797 .467 0.850 1.95 18.032 1.122 0.800 1.85 11.269 0.997

L0 = 28.9236 μm, Sigma = 0.9716 μm, Specimen: 20P850-32, Date: 12/27/71 SPV Signal = 2.5 mV. NOTE: This plot is based on a previously used analytic approximation for the absorption coefficient (see Related Information 1).

Figure 4 Typical Plot and Print Out of SPV Data Obtained Using Test Method A

12.4 Precision and Bias

12.4.1 An interlaboratory test was conducted in which six laboratories performed three determinations of effective diffusion length on each of six silicon specimens. Two of the specimens were chem-mechanically polished wafers with diffusion length in the 100–200 μm range and four were epitaxial layers on more heavily doped substrates of the same conductivity type. In the case of the epitaxial specimens, the layer thickness was less than four times the measured effective diffusion length so that L0 ≠ LD. Data from one laboratory was excluded from the analysis because of deviations from the measurement procedure. Results are summarized in Tables 1 and 2.

12.4.2 Although this test was based on earlier versions of Test Method A (1978 and 1984 editions) that utilized a different analytic approximation for the absorption coefficient (see Related Information 1) it is not expected that this would affect the estimate of precision.




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Table 1 Summary of Single-Laboratory Results of Interlaboratory Experiment to Evaluate Test Method A

Bulk Specimens Epitaxial Layers

Sample Standard Deviation of Least-Squares Fit

Single-Laboratory Sample Standard Deviation

Sample Standard Deviation of Least-Squares Fit

Single-Laboratory Sample Standard Deviation

0.6–12 μm 0.4–14.4 μm 0.3–2.2 μm 0.2–20 μm All 30 values were <10% of

L0avg

All 10 values were <12.5% of L0avg

One value of 60 was >10% of L0avg

Three values of 20 were >50% of L0avg

#1

#1 If these three values are excluded from the analysis, the upper limit of the range was 4.3 μm.

Table 2 Summary of Multi-Laboratory Results of Interlaboratory Experiment to Evaluate Test Method A

Sample Number Diffusion Average Length,μm Sample Standard Deviation,μm Relative Sample Standard Deviation, %

Bulk 1 132 25 19 Bulk 2 97 37 38 Epi 1 10.9 4.8 44 Epi 2 7.8 6.0 77 Epi 3 31.8 10.5 33 Epi 4 22.3 13.8 62

12.4.3 Because there are no reference standards for diffusion length in silicon or other semiconducting materials, no statement regarding bias can be made.

13 Test Method B — Linear Photovoltage, Constant Photon Flux (LPVCPF) Method

13.1 Procedure

13.1.1 Turn on the light source, chopper, and lock-in amplifier, and adjust the chopping frequency to the preselected value (e.g., 13 Hz).

13.1.2 Place the test specimen on the specimen holder with the capacitative contact raised, and center the specimen on the holder.

13.1.3 Set the filter wheel to the high-intensity white light position. Verify that white light is passing through the system.

13.1.4 Lower the capacitative contact to contact the specimen surface.

13.1.5 Adjust the lock-in amplifier for maximum signal.

13.1.6 Using an attenuator in the optical path, adjust the magnitude of the SPV signal to about 2 mV. Turn on the low level light source and adjust its intensity so that the SPV signal is about 1 mV.

13.1.7 Read and record the SPV signal amplitude as V1.

13.1.8 Set the filter wheel to the low-intensity white light position and read and record the SPV signal amplitude as V2.

13.1.9 Verify that the response is linear by taking the ratio of V1 to V2. If this ratio is not within 5% of the known ratio of φ1 to φ2, reset the filter wheel to the high-intensity white light position and reduce the SPV signal amplitude to one half of its previous value by means of the attenuator and repeat ¶¶ 13.1.7–13.1.9.

13.1.10 Set the filter wheel to the position that gives the highest energy (shortest wavelength) illumination and reset the intensity to produce an SPV value approximately equal to V2.

13.1.11 Reset the filter wheel to the position that gives the lowest energy (longest wavelength). Read and record the resulting value of SPV as V3 and record the corresponding known wavelength as λ3.




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13.1.12 Set the filter wheel to the next positions in order of increasing photon energy. Read and record the value of SPV at each position as V4, V 5, V6, etc., which correspond to the known values of wavelength, λ4, λ5, λ 6, etc.

13.1.13 Measure and record the specimen thickness in accordance with SEMI MF533. For an epitaxial layer, also measure and record the layer thickness in accordance with SEMI MF95 or SEMI MF110.

13.1.14 Optionally, measure the specimen resistivity in accordance with SEMI MF84 or SEMI MF673.

13.2 Calculation

13.2.1 Examine the sequence of SPV values and reject from further analysis any value Vn that is not greater than Vn−1.

13.2.2 If the specimen thickness is less than 500 μm, reject any SPV values obtained for photon energy less than 1.24 eV (λ > 1.00 μm).

13.2.3 For each wavelength used, determine the reciprocal absorption coefficient, α−1, either by direct measurement or from Equation 1 (see ¶ 12.2.1).

13.2.4 Plot the reciprocal of each SPV value against the value of α−1 for the wavelength of the illumination used to obtain the SPV value. Fit a straight line to the points visually or calculate a least squares fit to the data. Extrapolate the line to the (negative) abscissa and measure and record the magnitude of the intercept on the negative abscissa (see Figure 45).

NOTE: The numbers at the top of the graph refer to positions of the filter wheel that provides illumination at the appropriate energies and output photon flux (see ¶ 6.1.1).

Figure 5 Typical Plot of SPV Data Obtained Using Test Method B

13.2.5 The magnitude of this intercept is the effective diffusion length, L0. If L0 is less than one fourth the specimen (or epitaxial layer) thickness, L0 can be taken to be equal to the diffusion length, LD. If L0 is greater than or equal to the specimen thickness, then find Lreal according to Table 3, interpolating for non-standard wafer thicknesses if necessary. Lreal can be taken to be equal to diffusion length, LD.




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Table 3 Diffusion Length Conversion Table

Measured Values of LD for Various Sample Thicknesses Lreal

t = 725 μm t = 675 μm t = 625 μm t = 525 μm

2000 550.6 505.7 460.8 371.3 1950 549.9 505.2 460.3 371.1 1900 549.1 504.5 459.9 370.9 1850 548.2 503.9 459.4 370.6 1800 547.3 503.2 458.8 370.3 1750 546.3 502.4 458.2 370.0 1700 545.2 501.5 457.6 369.7 1650 544.0 500.6 456.9 369.3 1600 542.7 499.6 456.1 368.9 1550 541.3 498.5 455.3 368.5 1500 539.8 497.3 454.4 368.0 1450 538.1 496.0 453.4 367.4 1400 536.2 494.5 452.2 366.8 1350 534.2 492.9 451.0 366.2 1300 531.9 491.1 449.7 365.5 1250 529.4 489.1 448.1 364.6 1200 526.5 486.9 446.4 363.7 1150 523.4 484.5 444.5 362.7 1100 519.9 481.7 442.4 361.6 1050 515.9 478.5 440.0 360.2 1000 511.4 475.0 437.2 358.7 950 506.3 470.9 434.0 357.0 900 500.5 466.2 430.4 355.0 850 493.8 460.9 426.2 352.7 800 486.2 454.7 421.4 349.9 750 477.3 447.5 415.7 346.7 700 467.0 439.1 408.9 342.8 650 454.9 429.1 401.0 338.2 600 440.8 417.4 391.5 332.6 550 424.2 403.4 380.1 325.7 500 404.6 386.8 366.3 317.2 450 381.5 366.8 349.5 306.5 400 354.3 342.9 329.0 293.0 350 322.5 314.4 304.1 275.7 300 285.8 280.6 273.9 253.6 250 244.1 241.5 237.8 225.4 200 198.3 197.4 195.9 190.1 150 149.8 149.6 149.2 147.6 100 100.0 99.98 99.96 99.82 50 50.0 50.0 50.0 50.0




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13.3 Report



13.3.1.2 Data (λ, α−1, VSPV, and VD) for each wavelength used,








13.4.1 A single operator has made single center-point measurements on five silicon specimens using four different systems of the same type. The measurements were made in accordance with this test method; in particular, the surface treatments listed in Note 5 were employed. These results (see Table 4) provide an estimate of the intralaboratory repeatability that might be expected from this test method when it is used by competent operators.

Table 4 Results of Intralaboratory Experiment to Evaluate Test Method B

Diffusion Length, μm, as Measured by System Number

Standard Deviation Wafer

Number Dopant Resistivity,Ω·cm

1 2 3 4

Mean, μm

μm %

A1 Phosphorus 10 140 141 134 139 138.5 3.11 2.2 T2 Phosphorus 0.1 15.9 14.9 16.1 15.4 15.6 0.54 3.5 G2 Phosphorus 10 280 280 282 277 279.8 2.06 0.7 D1 Boron 10 229 231 220 224 226.0 4.97 2.2 T4 Boron 10 306 309 308 302 306.3 3.10 1.0

13.4.1.1 Two n-type and two p-type specimens with resistivity at room temperature of about 10 Ω·cm and minority carrier diffusion length (as determined by this test method) of between 130 and 310 μm were measured. Sample standard deviations ranged from 2.1–5.0 μm (0.7%–2.2%). Neither the sample standard deviation nor the relative sample standard deviation showed a correlation with minority carrier diffusion length.

13.4.1.2 The fifth specimen was n-type with room temperature resistivity of about 0.1 Ω·cm and minority carrier diffusion length of about 16 μm. The sample standard deviation of the four measurements was 0.54 μm (3.5%).

13.4.2 This test method has not yet been evaluated by interlaboratory experiment to determine its interlaboratory reproducibility.

13.4.3 Because there are no reference standards for diffusion length in silicon or other semiconducting materials, no statement regarding bias can be made.




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14 Test Method C — Digital Oscilloscope Recording (DOR) Method 14.1 Procedure

14.1.1 Turn on the supply of the measuring unit and the control unit. Turn on the computer; activate pulse oscillator, digital oscilloscope, laser diode commutation systems.

14.1.2 On the pulse oscillator set up pulse duration 1 ms and output pulse voltage ~ (3÷5) V.

14.1.3 On the digital oscilloscope set up sweep length – ≥ 10 ms, sweep start lead – 10 times quantization step.

14.1.4 Mount the specimen in the specimen holder.

14.1.5 On the laser diodes commutation panel select laser diode Ld1 and trigger IR-radiation pulse from laser diode Ld1. Estimate the value of SPV pulse by using the digital oscilloscope. Replicate this action for Ld2, Ld3 and other diodes.

14.1.5.1 On the pulse oscillator set up initial amplitude pulse value; it should provide amplitude of SPV from ~2 to ~10 mV for each laser diode. Decrease (or increase) IR-radiation intensity by using variable optical attenuator in case of emergency. Perform the spectral calibration of the variable attenuator after every change of the attenuation value (¶ 11.5).

14.1.5.2 Check the SPV pulses shape. The pulse should have monotonously rising leading edge and monotonously decaying trailing edge; any surges are not allowed.

14.1.5.3 Determine the durations of rising and falling SPV pulse edges. On the pulse oscillator set up pulse duration more than three times of rising SPV pulse edge duration for each laser diode. Set up period between pulses to be longer than three full SPV pulse durations.

14.1.5.4 Set up digital oscilloscope sweep length to be longer than 10 pulses

14.1.6 On the laser diodes commutation panel select laser diode Ld1. Start 10 IR-radiation pulses from laser diode Ld1. Record SPV signal and signal from laser power control photodiode and store it in the computer .

14.1.6.1 Perform these operations for Ld2 and Ld3 laser diode.

NOTE 7: IR-radiation pulses number may be increased to 100 and more for reducing the error of steady state SPV value. This number may be decreased in accordance with specimen properties and the level of measurement procedure optimization.

14.1.7 On the pulse oscillator decrease output voltages and set the amplitudes of SPV ~0.6 initial values Repeat steps ¶¶ 14.1.5 and, 14.1.6.

14.1.8 On the pulse oscillator decrease output voltages and set the amplitudes of SPV ~0.3 initial values. Repeat steps ¶¶ 14.1.5 and 14.16.

NOTE 8: ¶¶ 14.1.5 – 14.1.8 should be preferably executed according to early development program for the shortest possible measurement time. In any case the measurement time must be not longer than 1 min.

NOTE 9: The number of pulse oscillator output pulse voltage values, which are used during the measurement, may be increased (or decreased) in accordance with specimen properties and the level of measurement procedure optimization.

NOTE 10: The number of laser diodes used may be increased (or decreased) in accordance with specimen properties and the level of measurement procedure optimization. In any case it must be not less than 2.

14.2 Calculation

14.2.1 For every wavelength λi used determine the reciprocal absorption coefficient, α-1, either by direct measurement or from the following relation6:

Li = α-1(λ) = (84.732/λ – 76.417)-2 (4)

where absorption coefficient α is given in cm -1, and wavelength λ in μm.




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14.2.2 For every wavelength λi used calculate the term (1-R) from the following relation that is valid for the case of polished silicon surface with thin native oxide in the wavelength range from 0.7 to 1.05 μm7:

(1-R(λ)) = 0.6786 + 0.03565λ-1 – 0.03149λ-2 (5)

14.2.3 For other materials or if any surface films are present on silicon, (Eq 5) is not valid; determine the term (1-R) then by direct measurement.

14.2.4 Determine photon flux averaged intensities(photons per sec per m2) which correspond to the radiation pulses as described in ¶¶ 14.1.6 – 14.1.8 by the following procedure:

14.2.4.1 Average the signal from laser power control photodiode, which corresponds to radiation pulse, as described in ¶¶ 14.1.6 – 14.1.8, and determine the averaged amplitudes Āi,j of all pulses.

14.2.4.2 Determine photon flux averaged intensities

Īi,j = K·(1-R(λi))·K0(λi)·λi ·Āij , (6)

where K is an arbitrary constant, K0(λi) is the laser power control photodiode spectrum sensitivity (W per A).

14.2.5 Determine averaged steady state SPV values over all radiation pulses generated in ¶¶ 14.1.6 – 14.1.8 by averaging pulse signals from input amplifier for all radiation pulses and then perform leading edges fit by the function Vi,j·(1 – e-t/τ). An example of an averaged SPV signal is presented in Fig. 6, and the signal fit in Fig.7.

14.2.6 Plot for every wavelength λi the steady state SPV value versus photon stream intensity and perform regression of values Vi, by function

Vi (Īi,j) = Bi · Īi,j + Ci ·( Īi,j)2 . (7)

The derivatives of plots Vi (Īi,j) at the origin are Вi. These plots are illustrated in Fig. 8.

14.2.7 Calculate the effective diffusion length L0 by performing a linear regression of |1/Bi| versus 1/α(λi). L0 is determined by the intercept of the regression line with the abscissa. An example is displayed in Fig. 9.

Figure 6 SPV Signal Recording from a p-type Boron Doped Silicon Slab with a Resistivity ρ = 12 Ohm.cm, puls

Duration 3 ms, Wavelength 0.92 micrometer. Dotted line - initial signal, Solid line - averaging for 10 impulses




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Figure 7 Regression of SPV Signal for P-Type Boron Doped Silicon Slab

0 10 20 30 40 50 60 70 80 90 100 1100.013

0.012

0.011

0.01

0.009

0.008

0.007

0.006

0.005

0.004

0.003

0.002

0.001

0

I, rel unt

Vst

, v

Figure 8

Example of SPV Signal quasi-stationary Value Dependence on Radiation Intensity I. Sample: p-type Boron Doped Silicon, Resistivity 0.6 Ohm·cm; Wavelength 0.92 (dashed line), 0.977(dotted line), 1.008 micrometer

(solid line).




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Figure 9 Plot of Dependence |1/B| versus L(λ); L – radiation Penetration Depth (L = 1/α(λ)). Sample: p-type Boron

Doped Silicon, Resistivity 0.6 Ohm·cm. Diffusion Length is 81 microns

14.3 Report



14.3.1.2 Data (λ, α−1, VSPV, and B/1 ) for each wavelength used,








14.4.1 In Table 5, the results of control measurements of 11 wafers are given with ρ - samples resistivity, L – diffusion average length, ΔL/L - standard deviation. Measurements were carried out 3-10 times within 10-30 minutes for each sample. Each measurement time was ≈ 1 min. Sample No 8 was measured twice, with an interval of two days. No chemical treatment of the samples was performed.

14.4.2 In Table 6 the results of the measurements of epitaxial layers are presented.




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Table 5 Results of Intralaboratory Experiment to Evaluate Test Method C – Wafers

Sample

number

Dopant ρ,

Ohm·cm

Thickness,

μm

L,

μm

Number of measurements

ΔL/L,

%

Note

1 Boron 12 1400 421 6 13 2 Boron 12 1400 110 8 20 3 Boron 12 1400 458 6 20 4 Boron 12 1400 237 5 7 5 Boron 12 1400 489 10 16 5 Boron 12 480 3 12

Sample 8 was measured twice,

with 2 days interval 6 Phosphorus 4.5 460 227 4 2.4 7 Phosphorus 4.5 460 247 4 4.3 8 Antimony 0.01 460 56.8 4 8 9 Antimony 0.01 460 53.5 4 6 10 Boron 0.01 460 4.6 4 24 11 Boron 0.01 460 15.3 4 29

Table 6 Results of Intralaboratory Experiment to Evaluate Test Method C – Epitaxial Layers

14.4.3 This test method has not yet been evaluated by interlaboratory experiment to determine its interlaboratory reproducibility.

14.4.4 Because there are no reference standards for diffusion length in silicon or other semi conducting materials, no statement regarding bias can be made.

14 15 Keywords

14.1 15.1 diffusion length; minority carriers; polysilicon; silicon; single crystal silicon; surface photovoltage

Sample

number

Polarity of conductivity Layer Thickness,

μm

ρ,

Ohm·cm

L,

μm

Number of measurements ΔL/L,

%

12 n-n++ 80 10 50.4 4 2.6 13 n-n++ 80 1.5 22.2 4 2.7 14 p-p++ 80 10 50.4 4 3 15 p-p++ 50 1 23.4 4 13




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RELATED INFORMATION 1 PREVIOUSLY USED ANALYTIC EXPRESSIONS FOR ABSORPTION COEFFICIENT OFSTRESS-RELIEVED SILICON NOTICE: This related information is not an official part of SEMI MF391. It was developed as a non-mandatory annex during the 1990 revision of the standard within ASTM Committee F-1. This related information was approved for publication by full letter ballot on August 24, 2006.

R1-1 Earlier editions of these test methods utilized an analytic expression for the reciprocal absorption coefficient developed by Phillips2 from the data of Runyan8 as follows:

13211 )039958.0585368.014425.1526367.0( −−−−− ++−= λλλα μm (R1-1)

where λ is the wavelength of the incident illumination in μm.

R1-2 This expression has been widely employed in constant magnitude SPV measurements because it yielded a much more linear curve than the previous data of Dash and Newman9 that appears to have been based on measure-ments made on non-stress-relieved specimens.

R1-3 However, since this expression was developed, a number of investigators have measured the absorption coefficient in the wavelength range relevant to SPV measurements. These results have been critically reviewed by Nartowitz and Goodman.6

R1-4 This work and subsequent work by Saritas and McKell10 have shown that the expression in Equation R1-1 overestimates α, especially in the region of the wavelength range above 0.9 μm. Thus when this expression is used to determine α as a function of λ, the test method yields diffusion length values that are too low.

R1-5 Consequently, the use of Equation R1-1 in SPV measurements is no longer recommended.

NOTICE: SEMI makes no warranties or representations as to the suitability of the standard(s) set forth herein for any particular application. The determination of the suitability of the standard(s) is solely the responsibility of the user. Users are cautioned to refer to manufacturer’s instructions, product labels, product data sheets, and other relevant literature respecting any materials or equipment mentioned herein. These standards are subject to change without notice.

By publication of this standard, Semiconductor Equipment and Materials International (SEMI) takes no position respecting the validity of any patent rights or copyrights asserted in connection with any item mentioned in this standard. Users of this standard are expressly advised that determination of any such patent rights or copyrights, and the risk of infringement of such rights are entirely their own responsibility.

8 Runyan, W. R., “A Study of the Absorption Coefficient of Silicon in the Wave Length Region Between 0.5 and 1.1 Micron,” Southern Methodist University Report SMU 83-13 (1967). Also available as HASA CR 93154 from the National Technical Information Service (N68-16510). 9 Dash, W. C., and Newman, R., “Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77 K and 300 K,” Phys. Rev 99 (1955): pp. 1151–1155. 10 Saritas, M., and McKell, H. D., “Absorption Coefficient of Si in the Wavelength Region Between 0.80–1.16 μm,” J. Appl. Phys. 61 (1987): pp. 4923–4925.