SEMASPEC Test Method for Determination of Surface Roughness by Scanning Tunneling Microscopy for Gas Distribution System Components

Transcription

1 SEMASPEC Test Method for Determination of Surface Roughness by Scanning Tunneling Microscopy for Gas Distribution System Components Technology Transfer B-STD

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3 SEMASPEC Test Method for Determination of Surface Roughness by Scanning Tunneling Microscopy for Gas Distribution System Components February 22, 1993 Abstract: This SEMASPEC defines a method of testing surface features in the nanometer size range. Threedimensional data can be obtained from a surface, which can then be used to produce a model of the surface texture or to measure surface morphology. The purpose of this procedure is to evaluate components considered for use in ultra-high purity gas distribution systems. Application of this test method is expected to yield comparable data among components tested for the purposes of qualification for installation. The purpose of this SEMASPEC is to provide a document that member companies can use to correlate Research Triangle Institute (RTI) test data with the test method that was used. This document is in development as an industry standard by Semiconductor Equipment and Materials International (SEMI). When available, adherence to the SEMI standard is recommended. Keywords: Surface Roughness, STM, Defect Sources, Facilities, Gas Distribution Systems, Specifications, Components, Component Testing Authors: Jeff Riddle Approvals: Jeff Riddle, Project Manager Venu Menon, Program Manager Jackie Marsh, Director of Standards Program Gene Feit, Director, Contamination Free Manufacturing John Pankratz, Director, Technology Transfer Jeanne Cranford, Technical Information Transfer Team Leader

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5 1 SEMASPEC # B-STD SEMASPEC Test Method for Determination of Surface Roughness by Scanning Tunneling Microscopy for Gas Distribution System Components 1. Introduction Semiconductor cleanrooms are serviced by high-purity gas distribution systems. This document presents a test method that may be applied for the evaluation of one or more components considered for use in such systems. 1.1 Purpose The purpose of this document is to define a method for testing components being considered for installation into a high-purity gas distribution system. Application of this test method is expected to yield comparable data among components tested for purposes of qualification for this installation. 1.2 Scope Scanning tunneling microscopy (STM) is a noncontact method of profilometry, which can measure surface features in the nanometer size range. It can obtain three-dimensional data from a surface, which can then be used to produce a model of the surface texture or to measure surface morphology. The subsequent numerical analysis is to be performed per other standards, such as ANSI B Limitations This method is limited to characterization of stainless steel surfaces that are smoother than Ra = 0.25 µm, as determined by a contact-stylus profilometer Cutting necessary to obtain access to the areas to be examined must not modify those areas from their condition as found in the component. Etching or conductive coatings of the surface are considered modifications of the gas-wetted surface and are not covered by this test method This test method does not cover steels that have an oxide layer too thick to permit tunneling under the test conditions outlined in Section This technique is written with the assumption that the STM operator understands the use of the STM at a level equivalent to six months of experience This method assumes that the images obtained are unperturbed by very thin, nonsolid oxide layers (e.g., hydrocarbons, water) on the surface The statistical analysis only governs the determination of the roughness parameters from one-dimensional surface profiles. It does not cover those obtained from areas of a surface Comparisons between roughness parameters are only valid if the parameters are obtained using the same profiling methods (that is, identical trace lengths, identical scaling of feature heights, and identical means of profile acquisition) Discussion of tip preparation techniques are outside the scope of this method. Tips can be prepared as described in the literature or as commercially available Discussion of artifacts due to tip irregularities, their characterization, and deconvolution from the (true) surface image is also found in the literature and outside the scope of this method.

6 2 2. Reference Documents 2.1 ANSI B Surface Texture (Surface Roughness, Waviness, and Lay), ANSI/ASME SEMASPEC 2 Test Method for Determination of Surface # B-STD Roughness by Contact Profilometry for Gas Distribution System Components 3. Terminology 3.1 artifact any contribution to an image from other than true surface morphology. Examples include contamination, vibration, electronic noise, and tip imperfections. 3.2 current tunneling current, expressed in nano-amps (na), that flows across the tip-surface gap. 3.3 drift movement of the surface with respect to the tip due to a lack of thermal equilibrium. 3.4 feature height the height of features on the surface profile. The distance in the z direction of any point in the scan area, relative to the lowest point in the scan area, as derived from tunneling current or tunneling voltage fluctuations during tip raster. 3.5 gold-ruled grating a gold surface with uniformly spaced grooves of known separation and depth, to be used as a calibration standard for µm scale measurements. 3.6 HOPG (highly-ordered pyrolytic graphite) a type of pure, highly laminar graphite used as an atomic-scale calibration standard for scanning tunneling microscopy. 3.7 image representation of surface topography obtained using the signal from the tip or surface during rastering and under tunneling conditions. 3.8 Pt/Ir platinum and iridium alloy wire used to make tunneling tips. 3.9 raster movement or area defined by repetitively scanning in the x direction while moving stepwise in the y direction SA index (surface area index) the area of a best fit plane (or ideal surface) subtracted from the actual area calculated for the surface, divided by the ideal area, and multiplied by 1,000. SA index = [(true surface area - ideal surface area)/ideal surface area]10 3. Ideal surface area = area of least squares fitted plane scan movement of tip relative to sample surface, continuously in one direction scan area area defined by successive, side by side scan lengths scan direction the direction in which the tip or sample is continuously scanned, orthogonal to the y direction, in the sample plane. Also called x direction scan length distance from start to end of a single scan scan rate the speed at which the tip moves relative to the surface scan width the length of one side of the square is scanned. 1 American National Standards Institute, 1430 Broadway, New York, NY , 2706 Montopolis Dr. Austin, TX

7 standard conditions kpa, 0.0 C (14.73 psia, 32 F) STM scanning tunneling microscopy (or microscope) tip a sharpened, conductive point at the end of an STM probe tunneling electron flow through, rather than over, an energy barrier between two regions voltage voltage bias, expressed in volts (V) or millivolts (mv), applied between the tip and the surface to sustain tunneling current x direction see scan direction y direction the direction over which successive scans are taken, orthogonal to the scan direction, in the sample plane z direction perpendicular to the sample plane, orthogonal to the x and y directions. Also called feature height direction.3.25 Z max maximum height difference over entire surface proportional, integral, and differential controls (PID)-three gain control settings of the instrument that are critical to the resultant image The roughness parameters, RMS (root mean square), RA (average roughness), and Z avg are given RMS = 1 N RA= 1 N N i= 1 N i= 1 ( zi Zavg) z i Z avg 2 12 / Z avg N = zi 1 N i = 1 where: z i = height at point (x,y) i = number of measurements N = number of data points RA = average roughness RMS = root mean square Software for calculating these roughness parameters from the data is commercially available. 4. Test Protocol 4.1 Test Conditions Precautions This test method may involve hazardous materials, operations, and equipment. This test method does not purport to address the safety considerations associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and determine the applicability of regulatory limitations before using this method.

8 Sampling must not modify the surface due to stress, heat, corrosion, or contamination by particulates. A conductive path for the sample shall be provided for voltage biasing of the sample with respect to the tip. Tunneling in either air or vacuum is permissible. Operation in air can be done under oil. However, the oil must not modify the surface topography or introduce artifacts into the image [CAUTION: The tip must not have previously touched any surface.] 4.2 Apparatus Instrumentation Any scanning tunneling microscope capable of the following may be used : Scanning lengths up to at least 50 µm. Applying at least two volts between the tip and sample. Detecting currents as low as 0.1 na between tip and sample. Traversing feature height variations as great as 2 µm without touching the tip to the surface Calibration of the instrument should be performed against HOPG for atomic-scale resolution and/or against a suitable µm scale standard, such as a gold-ruled grating of known dimensions, for µm scale resolution. As feature height standards become available, their use in calibration should be considered. Calibration shall be conducted regularly, according to manufacturer's instructions and verified after each tip change Setup and Schematic Refer to the instrument manufacturer's instructions Mount the sample so the tip will scan the area of interest or a representative area Bring the sample and microscope to a temperature equilibrium. Inert atmospheres, temperature controls, acoustic isolation, and vibration isolation are to be provided as necessary to obtain artifact-free images. 4.3 Test Procedures Initiate tunneling between the tip and the sample as per manufacturer's instructions, using those currents and voltages found to provide artifact-free images. Set the scan area to a small region (500 nm across) and set the scan frequency slow enough to prevent the tip from touching the surface during rastering Scan the selected 500 nm area for at least five full, successive rasters and then obtain (in memory or hard copy) an image of that scan area. Drift from scan to scan should be minimal or less than 5%. Inconsistent topography is a sign that the tip is dragging through a nonconductive layer on the surface, and perhaps that the tip is being damaged. Figure A3 shows an example of an image obtained after a tip-crash Demagnify by scanning at a wider area (2000 nm across) centered around the same region, as in Section 4.3.2, and immediately obtain an image of that area Observe the 2000 nm image for evidence of damage from scanning the previous 500 nm area. This generally appears as a 500 nm square of very different topography or height in the center of the 2000 nm image. If there is such evidence, the surface may be too oxidized to analyze, thus giving a false representation of the surface and damaging the tip (see Figure X1.4) If no scanning damage is apparent, continue to obtain two more images 10 µm and 50 to 100 µm across. Store the data to allow subsequent numerical analysis if necessary.

9 After obtaining the above specified images, disengage tunneling, change to a new tip, move to a new area at least 10 µm from the previous region, and repeat steps in Sections (The old tip can be reused if it was not damaged or did not produce artifacts.) Repeat the process a third time if a comparison between equal magnifications (scan widths) reveals any obvious artifacts for one particular tip, such as each feature's having one or several nearby identical twins (also see Figure X1.4). If uncertainty remains after a third set of images is obtained, all images are to be treated as real representations of a surface having a variable topography. 4.4 Data Analysis Data Presentation Statistical figures of merit utilizing the three dimensional models obtained allow the representation of both transverse and longitudinal roughness features. These figures will be presented in a data table, such as the one in Section 5, Figure 1. The software utilized for numerical reduction will report Ra and RMS values as defined and calculated from the data files obtained. For purposes of consistency, the 95% confidence interval should always be stated along with the Ra value An additional figure of merit is the surface area index (SA index). This indexes the increase in surface area resulting from surface morphology. The SA index is defined as the area of a best fit plane (or ideal surface), subtracted from the actual area calculated for the surface, divided by the ideal area, and multiplied by 1, The data table summarizing the measurements should list: (1) the scan length; (2) the SA index; (3) the maximum and average Z-values; (4) the RMS value; (5) the RA value and its confidence interval; and (6) the proportional, integral, and differential (PID) gain control settings The choice of height scale must be consistent as magnification changes from one image to another. For example, the full height scale value for the 2000 nm image must be four times that of the full height scale value for the 500 nm image, since there is a four-fold difference in the magnification between the two. Otherwise an artificial enhancement or suppression of the feature height appearance will result. This height scale relationship between magnifications must be conserved for the 10 µm and µm images (or others) as well Profiles taken from the images should be in the x direction only (in the continuous scan direction) and not across successive scans. They should be taken from a consistent starting point from sample to sample. Profiles taken from sequential images of decreasing magnification (increasing width) should be taken from a point no more than 1/4 up from the bottom of the raster, or 1/4 down from the top of the raster. Thus, profiles taken from the 2000 nm images, for example, will not contain a portion of the area previously scanned at 500 nm across.

10 6 5. Illustrations Surface Roughness Parameters for Stainless Steel Samples Stainless Steel Sample SA Index (nm) Z max Z avg RMS (nm) (nm) (nm) RA (nm) Bright annealed pipe Electropolished tubing Oxygen passivated tubing Figure 1 Surfaces of Stainless Steel Samples

11 7 6. Additional References Binnig, G. and Rohrer, H. IBM Journal of Research and Development, v :355.

12 8 APPENDIXES (Supplementary Information) X1. Application Notes for Scanning Tunneling Microscopy X1.1 The images in Figures X1.1 and X1.2 were obtained for 0.25 inch stainless steel tubing, with tip voltage at 1800 mv, current levels monitored at 0.5 na, and scan speed set at about 2 µm/sec. The tip was Pt/Ir with a radius of about 1000 nm. The tubing was cut, first longitudinally and then transversely, to a length of 0.5 centimeters. The sample was imaged in air. The longitudinal direction of the tubing sample is in the y direction in the image. A plane fitting filter is applied to the image to correct for tilt (but not curvature). An example of an image containing tip artifacts is in Figure X1.4. Each feature on the surface has several nearby twins. This is likely due to roughness on the end of the tip giving multiple images. An example of surface damage due to the tip touching the surface is shown in Figures X1.3 and A1.4. Figure A1.3 shows an instance of tip crashing, and Figure X1.4 shows damage done at 500 nm area imaging when viewed at the lower magnification of 2000 nm. This is likely due to the tip dragging through a nonconductive layer on the surface.

13 A.1.2 Illustrations APPENDIXES (Supplementary Information) 9 Figure X1.1 Tilted Surface Representation of Electropolished 316L SS, 500 nm Across

14 10 APPENDIXES (Supplementary Information) Figure X1.2 Tilted Surface Representatoin of Chemically Polished 316L SS, N00 nm Across

15 APPENDIXES (Supplementary Information) 11 The lower half of the image shows the surface before crashing and the upper half shows the surface after crashing. Figure X1.3 Top View Representation of Stainless Steel Surface Showing Tip Crashing

16 12 APPENDIXES (Supplementary Information) Figure X1.4 Top View Representation 2000 nm Across Showing Damage from Previous 500 nm Wide Scan Note dark square area in center of image showing the damage from the previous scan. Note also repeating, hook-shaped tip artifact.

17 APPENDIXES (Supplementary Information) 13 X2. Alternative Methods X2.1 The method for Fractal Based Surface Roughness Characterization may be applicable to STM. See SEMASPEC # B-STD, Section A2 for details. NOTICE: DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. MAKES NO WARRANTIES AS TO THE SUITABILITY OF THIS METHOD FOR ANY PARTICULAR APPLICATION. THE DETERMINATION OF THE SUITABILITY OF THIS METHOD IS SOLELY THE RESPONSIBILITY OF THE USER.

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