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Revision of SEMI MF1048-1105
TEST METHOD FOR MEASURING THE EFFECTIVE SURFACE ROUGHNESS OF OPTICAL COMPONENTS BY TOTAL INTEGRATED SCATTERING
This test method was technically approved by the Global Silicon Wafer Committee and are the direct responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by the North American Regional Standards Committee on date tbd. Initially available at www.semi.org date tbd; to be published date tbd. Original edition published by ASTM International as ASTM F 1048-87. Last previous edition SEMI MF1048-1105.
1 Purpose
1.1 The performance of optical components is affected by the degree to which these components scatter optical radiation.
1.2 The effective rms microroughness, Rq, may be computed from the total integrated scattering (TIS).
1.3 This test method may be used in manufacturing, quality control, and research.
2 Scope
2.1 This test method covers the measurement of the effective surface roughness of an opaque reflecting surface as determined by the TIS.
2.2 This test method is particularly applicable to metal mirrors or smooth dielectrics covered with an opaque reflecting surface.
2.3 This test method is applicable to specimens ranging in size from 5 mm in diameter to as large as the supporting components will accommodate. The sampling area is approximately 1 mm in diameter.
2.4 This test method is limited to specimens with optical surfaces that are flat or that are spherical with a radius of curvature greater than 10 m.
2.5 This test method determines the integrated scattering from an angle approximately 2.5° from the surface normal to an angle approximately 70° from the surface normal.
2.6 The test method is performed with a helium-neon laser operating at the wavelength 632.8 nm.
2.7 Repeated use of this test method on different areas of the specimen permits mapping of the surface TIS.
2.8 This test method is nondestructive.
2.9 The maintenance of a control chart to monitor the stability of the measurement process is discussed in Appendix 1.
NOTICE: This standard does not purport to address safety issues, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health guides and determine the applicability of regulatory or other limitations prior to use.
3 Limitations
3.1 Variations in detector response as a function of the angle of incidence can affect the precision of the measurement.
3.2 FlawsDefects, such as scratches or pits, that are not characteristic of the surface finish or contaminants on the surface can produce scattering which that is not characteristic of the microroughness of the surface.
3.3 Stray background radiation affects the precision of measurement.
3.4 Detector offset voltages affect the precision of measurement.
3.5 Degradation of the diffuse reflectance standard degrades the precision of measurement.
3.6 Degradation of the specular reflectance standard can cause the control chart to indicate lack of experimental control.
4 Referenced Standards
4.1 SEMI Standard
SEMI M59 — Terminology for Silicon Technology
4.2 ASTM Standard
E691 — Practice for Conducting an Interlaboratory Test Program to Determine the Precision of Test Methods [1]
4.3 ANSI Standard
Z136.1 — Safe Use of Lasers[2]
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 Acronyms, terminology, and symbols used in silicon technology including those used in this test method may be found in SEMI M59.Definitions
5.1.1 rms microroughness, Rq — the root mean square of the short range surface profile height deviations Z(x) from the mean line taken within the evaluation length L. {Note. in SEMI M1 and now in Doc 3906, microroughness is defined as having a spatial wavelength less than 100 mm; do we want to do this here?}
5.1.2 surface flaws — defects such as scratches or pits which are not typical of the surface finish.
5.1.3 total integrated scatter, TIS — for the purposes of this test method, the radiation scattered by an opaque reflecting surface into a 2p solid angle.
6 Summary of Test Method
6.1 A beam of radiation from a helium-neon laser is made to impinge on the surface of the specimen. First, the specular reflectance is measured.
6.2 Next, the power of the radiation scattered by the specimen is measured by focussing the scattered radiation onto a silicon photovoltaic detector by means of a hemispherical mirror.
6.3 The specimen is then replaced by a diffuse reflectance standard and the measurement of the scattered power is repeated.
6.4 The effective surface roughness is computed from the two scattered radiation measurements, the specular reflectance of the specimen, and the total reflectance of the standard.
6.5 A control chart is established and maintained to monitor the stability of the measurement process over an extended period of time.
7 Apparatus
7.1 Laser — He-Ne type, operating in the TEM00 mode at wavelength 632.8 nm, having a beam diameter of 1 mm or less, a beam divergence of 5 mrad or less and an output power of approximately 2 mW.
7.2 Detector — Silicon photovoltaic type having a dynamic range of 105, with a sensitive area of 2 mm2 or greater, and in a housing less than 10 mm in diameter. The detector shall be equipped with a diffuser or other means to minimize variation of signal with the position of the image on the detector, with angle of incidence, and with polarization of the radiation.
7.3 Filters — Attenuation type to prevent detector saturation.
7.4 Mirror — Spherical type having a radius of curvature 10 cm or greater and having an aperture at the center that shall subtend an angle of 5° or less at the specimen. The outer diameter of the mirror shall allow for a collecting angle of 70° or greater relative to the specimen normal. The optical quality of the mirror shall be such that the irradiated portion of the specimen is imaged fully within the detector area.
7.5 Aperture Plate — Opaque type approximately 10 cm by 10 cm in size containing an aperture 3 mm in diameter and a target symbol located 10 mm from the aperture center. See Figure 1 for schematic of aperture plate.
Figure 1
Schematic Diagram of Aperture Plate
7.6 Optical Chopper, provided with a reference output signal.
7.7 Amplifier, Lock-In Type, equipped with a preamplifier with a range of gain settings appropriate for accommodating the range of signals originating from the detector. The amplifier-detector combination shall have a linear response within ±10% and reproducibility of ±5% over the required range of measurement.
7.8 Diffuse Reflectance Standard — A diffuse reflector of known total reflectance, rRS.
NOTE 1: Commonly used diffuse reflectance standards are fabricated from barium sulfate or polytetrafluoroethylene powders. The total reflectance of reflectors freshly prepared from these materials is typically greater than 0.98; therefore, a reflectance value of 0.98 for such reflectors is adequate for the purposes of this test method.
7.9 Polished Reflectance Standard — An optically polished metallic mirror to be maintained in a protected environment to prevent degradation of polish.
NOTE 2: A polished stainless steel mirror preserved in a covered glass petri dish would be adequate for this test method.
8 Hazards
8.1 Warning: Lasers with an output power on the order of 1 mW or greater can cause injury to personnel. The provisions of ANSI Z136.1 should be followed.
8.2 Warning: Do not let the unexpanded laser beam impinge on the skin. In no case should the laser beam be viewed directly. Avoid looking directly into the beam or into a specularly reflected beam.
9 Sampling
9.1 The location of each of the M measurement positions on the specimen and the number of measurements, N, to be performed at each of these positions shall be agreed to by the parties to the test. Record M and N together with the location of each agreed upon measurement position.
9.2 The number of test specimens and their manner of selection from a given lot shall be agreed to by the parties to the test.
10 Procedure
10.1 Align laser beam, aperture plate, specimen, spherical collection mirror, and detector in accordance with Figure 2 such that the scattered radiation is sharply imaged on the detector. Adjust the specimen angle such that the specularly reflected beam makes an angle of less than 1.50° with respect to the incident beam and such that it is directed through the center of the entrance aperture in the collection mirror onto the aperture plate.
Figure 2
Outline of TIS Apparatus
10.2 Remove the specimen and position the detector at the specimen location.
10.3 Observe the detector reading.
10.3.1 If reading indicates detector saturation, insert attenuation filters into the beam until the reading is within the linear range of the detector and near the upper limit of the linear range.
10.3.2 Record the signal as Vo.
NOTE 3: A representative data sheet is given in Figure 3.
10.4 Remove the detector and reposition the specimen in the beam such that the beam impinges on a specified measurement location on the specimen. Tilt the specimen such that the specularly reflected beam is directed through the center of the entrance aperture of the collection mirror onto the aperture plate, but approximately 10 mm away from the opening in the aperture plate.
NOTE 4: This implies that the incident laser beam deviates slightly from the center of the entrance aperture of the collection mirror.
Figure 3
Representative Report for Specimen with Three Measurement Locations (M = 3) and Four Measurement Sets (N = 4) at Each Location
10.5 Position the detector in the specularly reflected beam just before the aperture plate.
10.6 Record the signal as Vr.
10.7 Reposition the detector such that the scattered radiation is sharply imaged on the detector.
10.8 Record the signal as Vd.
10.9 Replace the specimen with the diffuse reflectance standard. Position the standard such that the scattered radiation is sharply focused on the detector.
10.10 Record the signal as VRS.
10.11 Repeat the procedure N times at each agreed upon measurement position on the specimen as determined in 9.1 until N measurements have been completed at each of the agreed upon measurement positions.
11 Calculations
11.1 For each set, j, of measurements at each agreed upon measurement position, i, on the specimen:
11.1.1 Calculate the specular reflectance, rs, from:
(1)
Record as (rs)ij, where i identifies the position number and j identifies one of the N sets of measurements at position i..
11.1.2 Calculate the TIS from:
(2)
Record as (TIS)ij, where i identifies the position number and j identifies one of the N sets of measurements at position i.
11.2 Calculate the average (TIS)i at each specified position on the specimen as follows:
(3)
where N equals the number of measurements at position i. Record each of the values of (TIS)i.
11.3 Calculate the average TIS for each specimen as (TIS)ave as follows:
(4)
where M equals the number of locations measured. Record (TIS)ave.
11.4 Calculate the effective rms microroughness of the test surface as follows:
(5)
12 Report
12.1 The report shall contain the following information as appropriate (see Figure 3):
12.1.1 Date of test.
12.1.2 Operator identification.
12.1.3 Description of each specimen including:
12.1.3.1 Specimen number,
12.1.3.2 Lot number,
12.1.3.3 Substrate material,
12.1.3.4 Coating material(s),
12.1.3.5 Description of polish, and
12.1.3.6 Vendor.
12.2 For each of the M measurement locations on each specimen the report shall contain the following:
12.2.1 Specified measurement location number, i.
12.2.2 The number of measurement sets at location i, N.
12.2.3 The value of each of the following parameters for each measurement set at location, i:
12.2.3.1 Measurement set number, j,
12.2.3.2 Vo
12.2.3.3 Vr
12.2.3.4 Vd
12.2.3.5 rs
12.2.3.6 VRS,
12.2.3.7 rRS, and
12.2.3.8 (TIS)ij.
12.2.4 The average TIS at position i, (TIS)i.
12.3 The average TIS for each specimen (TIS)ave.
13 Precision and Bias
13.1 The precision of this test method for specimens with an rms surface microroughness within the range from 0 to 10 nm is ±5%.
13.2 The bias of this test method is ±15%.
14 Keywords
14.1 integrated scatter; rms microroughness; surface microroughness; TIS
APPENDIX 1
CONTROL CHART
NOTICE: The material in this appendix is an official part of SEMI MF1048. Approval was by full letter ballot procedures with publication authorized by the NA Regional Standards Committee on date tbd.
A1.1 After initial assembly of apparatus, each day during sustained operation, or at least every 60 days, perform the procedure of this test method using the polished reflectance standard as the specimen.
A1.1.1 Upon initial assembly, compute (TIS)ave for the data obtained during the first five working days in accordance with ASTM Practice E691. Record these values on the control chart. Compute the (TIS)ave for each measurement set. Record on the control chart the date and the (TIS)ave value. A representative control chart is `shown in Figure A1-1. After a baseline of 10 values for (TIS)ave has been established, compute the cumulative average of (TIS)ave and the standard deviation for the data. Record these values on the control chart.
A1.2 On each day, before and after a set of test measurements, perform the procedure on the polished reflectance standard. Calculate the (TIS)ave and record on the control chart together with the date.
A1.2.1 If no test measurements are being performed over an extended period of time, perform the procedure on the polished reflectance standard at least once every 60 days. Record the computed (TIS)ave and the date.
A1.3 Compare the measured (TIS)ave with the previous cumulative average of (TIS)ave on the control chart.
A1.3.1 Compute the deviation of the measured (TIS)ave from the cumulative average (TIS)ave.
A1.3.2 If the deviation is less than or equal to three standard deviations, then the measurement process is considered to be under control. Recompute the average (TIS)ave and the standard deviation using all previous data plus the newest data. Record on the control chart.