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Background Statement for Document 4726A
NEW STANDARD: TEST METHODS FOR MEASURING RESISTIVITY OR SHEET RESISTANCE WITH A SINGLE-SIDED NONCONTACT EDDY-CURRENT GAUGE
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.
Eddy-current gauges are widely used in the PV area for characterizing resistivity and resistivity related properties of silicon materials. SEMI MF673 is a long and widely used set of test methods for making resistivity and/or sheet resistance measurements on wafers and thin films of silicon and certain other semiconductor materials. These test methods utilize an instrument with two opposing eddy-current transducers and the relatively thin wafer or other test specimen is placed between them. An ingot or brick would not fit between the two transducers so only a single transducer is used in these methods. Note that a single-sided gauge can also be used to measure wafers.
Thin silicon and TCO (transparent conductive oxide) films on non-conducting substrates are also used in the PV industry. Hence, this method is written to include both silicon bricks and ingots and thin silicon or TCO films on non-conducting substrates.
This letter ballot was approved for issuance in cycle 4 of 2011. It was reviewed by the PV Electrical & Optical Properties MeasurementsTask Force and adjudicated by the PV Committee at their meetings in connection with Intersolar North America in San Francisco the week of July 10. One negative vote and one comment were found to have substantive issues so the document was returned to the task force for further work.
Document 4726A, which addresses these issues, was approved for balloting in cycle 5 of 2011. It will be reviewed during the European meetings in the Fall of 2011 and again by the PV Electrical & Optical Properties MeasurementsTask Force at their NA Fall meetings in San Jose, California the week of October 24th. It will be adjudicated by the PV Materials Committee. Check for the latest meeting schedule.
SEMI Draft Document 4726A
NEW STANDARD: TEST METHODS FOR MEASURING RESISTIVITY OR SHEET RESISTANCE WITH A SINGLE-SIDED NONCONTACT EDDY-CURRENT GAUGE
1 Purpose
1.1 Resistivity is a primary quantity for characterization and specification of material used for photovoltaic devices. Sheet resistance is a primary quantity for characterization, specification, and monitoring of thin films. An eddy-current gauge directly measures conductance of a specimen. Values of resistivity or sheet resistance are calculated from the measured conductance, with the resistivity values also requiring a measurement of specimen thickness.
1.2 These test methods outline the principles of eddy-current measurements as they relate to silicon bricks and ingots or silicon films on nonconductive substrates that are used in photovoltaic sensors. These test methods are very similar to those in SEMI MF673, which covers the principles of eddy-current measurements as they relate to silicon wafers and certain thin films fabricated on such substrates, but the nature of the apparatus is significantly different to accommodate the bricks and ingots. Such an instrument can also be used for measurements on thin silicon or other thin conducting films on nonconductive substrates. SEMI MF673 should be referred to for measurements on silicon wafers and certain thin films fabricated on such substrates.
NOTE 1: A single-sided noncontact eddy current gauge can also be used to measure silicon wafers.
2 Scope
2.1 In addition to providing the procedures for eddy-current measurements on both single crystalline and multi-crystalline silicon bricks and ingots as well as silicon or other thin conducting films on nonconductive substrates, these test methods cover the requirements for setting up and calibrating such instruments for use, particularly at a buyer-seller interface.
2.1.1 For measurements on thin conducting films, the sheet resistance of the film shall be in the nominal range from 0.04 to 3000 per square. The substrate on which the thin film is fabricated shall have a minimum edge to edge dimension of 25 mm, measured through the center point and an effective sheet resistance at least 1000 times that of the thin film.
NOTE 2: The effective sheet resistance of a bulk substrate is its bulk resistivity (in ·cm) divided by its thickness in cm.
2.2 These test methods require no specimen preparation. Measurements are not affected by specimen surface finish or crystallinity of the test specimen.
2.3 For measurements on silicon specimens, these test methods require the use of single crystal silicon resistivity standards to calibrate the apparatus (see ¶ 7.1 ), and a set of similar reference specimens for qualifying the apparatus (see ¶ 7.1.1 ). For measurements on TCO films, it is possible to use TCO film standards, calibrated by four-point probe sheet resistance measurements.
2.4 Two test methods are covered by this standard.
2.4.1 Method I ascertains the conformance of the apparatus to linearity and slope limits (±1 digit) over a broad range (2 decades) of calibration standard values. It qualifies apparatus for use over a wide range of sample values.
2.4.2 Method II, for use on silicon specimens only, assumes instrument linearity between calibration standards whose values are narrowly separated (typically ±25% of the anticipated sample range median point). Method II is particularly well suited to computer-based systems where all measurements can be quickly and automatically corrected for value offset and for the temperature coefficient of the resistivity of silicon.
2.4.3 These methods differ in calibration technique, sample measurement value range, data correction techniques, and suitability of instrumentation as indicated in Table 1. Either method may be applied to brick or ingot specimens with resistivity in the range from 0.1 to 10,000 Ω·cm or to silicon or other thin films of sheet resistance 2 to 3,000 /square provided that suitable calibration standards can be obtained.
NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their use. It is the responsibility of the users of the documents to establish appropriate safety and health practices, and determine the applicability of regulatory or other limitations prior to use.
Table 1 Characteristics of Eddy-Current MethodsFactors / Method I / Method II
Calibration / Ascertains linearity with five samples / Ascertains slope with two samples
Application range / Broad: 2 decades / Narrow: ± 25%
Sample data / Direct reading, then correct for temperature. / Direct reading, then correct for slope.
Suitable instruments / Manual/Automatic / Automatic: computer-controlled
3 Limitations
3.1 Because eddy-current measurements for semiconductor materials are made almost exclusively with commercial instrumentation from one of several suppliers, some details included in these test methods, such as specific range limits and manner of entering specimen thickness values to obtain resistivity values may not apply strictly to all instruments. In all such cases, the owner's manual for the particular instrument shall be considered to contain the correct information for that instrument.
3.2 Radial resistivity variations or other resistivity non-uniformities (such as those occurring in multi-crystalline materials) under the transducer are averaged by these test methods in a manner which may be different from that of other types of resistivity or sheet resistance measurements that are responsive to a finite lateral area. The results may therefore differ from those of four-point probe measurements depending on dopant density fluctuation and the probe spacing used.
3.3 Spurious currents can be introduced in the test equipment when it is located near high-frequency generators. If equipment is located near such sources, adequate shielding must be provided. Power line filtering may also be required.
3.4 Silicon has a significant temperature coefficient of resistivity. Temperature-correction factors for extrinsic silicon specimens are given in SEMI MF84 but these may not provide sufficient accuracy for use with multi-crystalline or heavily compensated material. Note that they should apply accurately to the single crystal reference wafers. Temperature differences between any of the silicon reference wafers or test specimens, during calibration or measurement, or both, introduce a measurement error in Method II.
3.5 High levels of humidity may affect the indicated value and should be avoided.
3.6 Photoconductive and photovoltaic effects can seriously influence the observed resistivity. Therefore, all determinations should be made in a dark chamber unless experience shows that the material is insensitive to ambient illumination.
4 Referenced Standards and Documents
4.1 SEMI Standards
SEMI M59 — Terminology for Silicon Technology
SEMI MF43 — Test Methods for Measuring Resistivity of Semiconductor Materials
SEMI MF84 — Test Method for Measuring Resistivity of Silicon Wafers with an In-line Four-point Probe
SEMI MF374 — Test Method for Sheet Resistance of Silicon Epitaxial, Diffused, Polysilicon, and Ion-Implanted Layers Using an In-Line Four-Point Probe with the Single-Configuration Procedure
SEMI MF533 — Test Method for Thickness and Thickness Variation of Silicon Wafers
SEMI MF1527 — Guide for Application of Certified Reference Materials and Reference Wafers for Calibration and Control of Instruments for Measuring Resistivity of Silicon
4.2 ASTM Standard
E2251 — Specification for Liquid-in-Glass ASTM Thermometers with Low-Hazard Precision Liquids[1]
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 Acronyms, terms, and symbols related to silicon technology, including those used in this practice, are listed and defined in SEMI M59.
5.2 The following term and symbol are also used in this standard:
5.2.1 transparent conducting oxide (tco) — a transparent conducting film, consisting of a metal-oxide compound such as indium-tin-oxide, used for electrically contacting the active semiconducting regions of a PV solar cell and for improving the spectral response of the cell.
6 Summary of Test Methods
6.1 Method I
6.1.1 The temperature of the apparatus during the measurement is determined.
6.1.2 The apparatus is first calibrated using standards of known resistivity or sheet resistance with the sensor located the same measured distance above each calibration specimen. Then the apparatus is subjected to a test for linearity that involves measuring a set of five reference specimens. As a part of the linearity test, a plot is made of the indicated values as a function of the known values; two limiting curves are also plotted on the same graph. If all the plotted points fall within the limit curves, the apparatus is regarded as satisfactory.
6.1.3 For subsequent measurements of bulk test specimens placed with the sensor at the same distance above the test specimen as used for the calibration specimens, the conductance of the specimen is then measured by the apparatus and converted to a resistivity value that is displayed. The resistivity is subsequently corrected to its 23°C equivalent.
6.1.4 For subsequent measurement of thin film specimens again placed with the sensor at the same distance above the test specimen as used for the calibration specimens, the sheet conductance is measured, converted to sheet resistance, and displayed.
6.2 Method II (for measurements on silicon test specimens only)
6.2.1 The temperature of the apparatus during the measurement is determined.
6.2.2 The apparatus is first calibrated using two single crystal silicon standards of known bulk resistivity with in each case the sensor located the same measured distance above the calibration specimen. These standards should be the same electrical type as the test specimens to be measured (Note 3). The apparatus is then subjected to a test that quantifies apparatus slope at two points, and provides a means of correcting subsequent sample measurements, for values between these two points, to a calibration line established between the two standards employed.
NOTE 3: The maximum error derived from calibrating on one conductivity type and measuring the other is less than or equal to 0.12%/°C in the resistivity range 0.01·cm to 1,000 ·cm.
6.2.3 For subsequent measurement of test specimens placed with the sensor at the same distance above the test specimen (see ¶6.2.2 ), the conductance is measured, converted to Resistivity and displayed. These measurements are subsequently referred to the standard reference plot. The corrected values are known to a greater precision than those obtained following Method I, and in most instances are also 23°C equivalents.
7 Apparatus
7.1 Electrical Measuring Apparatus — with instructions for use and consisting of the following assemblies:
7.1.1 Eddy-Current (Transducer) Sensor — placed above the specimen. The assembly shall include a support on which the specimen rests, a device for locating the specimen on the support, a mechanism for adjusting and determining the height of the sensor above the specimen (see Note 4) and a high-frequency oscillator to excite the sensing element. For measurements on bricks or ingots, the frequency of the oscillator shall be chosen to provide a typical skin depth at around 2 mm or greater. The skin depth is a function of the resistivity of the specimen and the frequency of the oscillator as discussed in the paper by Miller, Robinson, and Wiley It should also be noted that even with a skin depth greater than 2 mm, the sensing capabilities are limited by the size and geometry of the sensor. This assembly and associated apparatus are shown schematically in Figure 1.
NOTE 4: Alternatively, the apparatus may include a compensation circuit for correcting for variations in the distance between the coil and specimen surface. When this is done, it may be possible to make the measurements over a range of distances between the coil and specimen surface.
7.1.2 Analysis System — Means for electronically converting, by analog or digital circuitry, the measured conductance signal to a resistivity or sheet resistance value. The processor shall incorporate a means for displaying resistivity, a means of zeroing the conductance signal in the absence of a specimen and a means for calibrating the instrument with known calibration specimens.
NOTE 5: A typical apparatus operates as follows. When a specimen is inserted onto the support below the eddy-current sensor or transducer, in a special oscillator circuit, eddy currents are induced in the specimen by the alternating field applied to the transducer. The current needed to maintain a constant voltage in the oscillator is determined internally; this current is a function of the specimen conductance. The specimen conductance is obtained by monitoring this current. Sheet resistance or resistivity values are obtained from the specimen conductance by analog or digital electronic means; calculation of resistivity values also requires knowledge of specimen thickness.
Figure 1
Schematic of Single-Sided Eddy-Current Sensor Assembly
7.2 Precision Thermometer — having a range from 8 to +32°C and conforming to the requirements for Thermometer S63C as specified in ASTM Specification E2251.
7.3 Thickness Gauge — as specified in ¶7.1 of SEMI MF533.
NOTE 6: Calibration and linearity checking must be done in consistent units, whether resistivity, sheet resistance or sheet conductance, according to the requirements of the given instrument. If resistivity values are used, knowledge of the specimen thickness is also required. For bulk calibration or linearity-check specimens, the thickness is the as-measured thickness in centimeters. For thin film specimens, the total thickness of the thin film plus substrate should be measured and used; if this cannot be done, an effective thickness of 0.0508 cm may be used.
8 Resistivity Standards
8.1 Resistivity Standards — Single crystal silicon reference specimens to check the accuracy and linearity of the instrument.
8.1.1 For measurements on silicon bricks and ingots, the reference standards are bulk single crystal silicon wafers at least 15 mm thick’ for each of which the resistivity at the reference temperature of 23ºC is determined in accordance with the four-point probe method of SEMI MF43 with temperature control and temperature correction in accordance with SEMI MF84.
8.1.2 For measurements on silicon wafers, the reference standards are single crystal silicon wafers with thickness within ±25% of the wafers being measured. For each of these reference standards, the resistivity is measured in accordance with SEMI MF84.
8.1.3 For measurements on thin conducting films, the reference standards are measured for sheet resistance by four-point probe in accordance with SEMI MF374, and shall have a sheet resistance of the substrate that is at least 1000 times the sheet resistance of the thin film. For measurements on thin silicon films, use implanted silicon films (see SEMI MF374), For measurements on TCO films, use uniform TCO films of thickness within ±25% of the films being measured.
NOTE 7: Such reference specimens can be constructed in accordance with procedures given in SEMI MF1527.
8.1.4 The standards and other reference specimens for Method I shall be at least five in number and have a range of values that spans the full range of the instrument. Table 2 gives a list of values recommended for checking the full range of a typical instrument for Method I application; these values should be met within ±25%. For Method II, where the specimens to be measured have a narrow range of resistivity values, the standards and other reference specimens shall be two in number. Their values shall span a resistivity range at least as large as that of the specimens to be measured.
Table 2 Resistivity Values for Apparatus Qualification Test
For Test-EquipmentMeasurementRange, ·cm / Use These Reference Specimens, ·cm / Implanted Specimen Sheet Resistance Equivalent to a 50.8-m Thick Bulk Resistivity Specimen, /square0.001–0.999 / 0.002 / 0.04
0.01 / 0.2
0.03 / 0.6
0.10 / 2
0.30 / 6
0.90 / 18
0.1–99.9 / 0.9 / 18
3 / 60
10 / 200
30 / 600
75 / 1500
9 Sampling