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| Magnification Calculations The fundamental magnification (M) calculation involves dividing a chosen image (I) dimension by its corresponding object (O) dimension, where M = I/O The standard must be perpendicular to the exciting beam to prevent image foreshortening. If an SEM image containing 5 cycles of the 2µm pitch pattern (10µm) is represented over a 90mm distance on the SEM CRT, by a vernier, or on film, the magnification would be (90/0.01) 9,000X. If the indicated magnification were 10,000X, the error would be –10%. Linearity Measurements Imaging distortion is characteristic of all types of imaging systems. It appears as a barreling, pincushion and skew, and is generally measured as a percentage deviation from linearity (using the straight edge of the MRS-3 or MRS-4, a 1% distortion would be a 1mm deviation in the straightness of a 100mm line). Tilt Angle Determination For SEM’s, the MRS-3/MRS-4 should be normal to the electron beam (0º tilt). Both the X and Y magnifications should be equal. If the specimen is then tilted about the X-axis the magnification will decrease along the Y-axis (due to foreshortening) by the cosine of the tilt angle. For example, if the (100µm)2 boxes measure 100µm in the X-axis and 50µm in the Y-axis, the tilt angle would be: Tilt angle = cos-1 (50/100) = 60º Traceability A CRM (certified reference material) guarantees dimensions. Most commercially available standards have unknown precision. Grids and spheres may change size under vacuum or are distorted. MRS-4 and MRS-3 are offered with or without traceability. The non-traceable standard differs only in documentation and cost. Traceability in the X and Y dimensions is established from "masters" that have been measured by NPL (National Physics Laboratory), the NIST counterpart in the U.K. The "Z" dimension (100nm) has been established on MRS "masters" by NIST. Recertification Program Annual recertification is required. This practice is common with devices such as gage blocks and electronic instruments. Over years, it has been found that several standards could not be recertified due to physical damage and excessive contamination. Physical damage is most often caused by optical microscope objectives being drawn across the pattern surface, or abuse. It is possible that electron beam damage and corrosion from storage in a hostile environment will require service and recertification, which will insure that your standard will perform its proper task. Consult your Quality Department for advice. They are most familiar with the company’s quality requirements as it relates to ISO-9000, QS9000 or ISO-Guide 25 or 17025. |
Operating Parameters Optical microscopes can use the MRS-4 in all imaging modes. The anti-reflective coating greatly reduces scattered light enabling high contrast images to be observed and photographed. Magnifications can be measured directly by viewing CRTs, in reticles mounted within the ocular, or directly on photomicrographs. For instruments with verniers or electronic calipers, distance measurements can be verified using a pitch pattern of appropriate size. If the application primarily consists of optical microscopy, we suggest ordering the MRS-4 without the conductive coating since it reduces the reflected optical contrast in the image. ASTM E766-98: Standard practice for calibrating the magnification of a Scanning Electron Microscope E1951-98 Standard Guide for Calibrating Reticles and Light Microscope Magnifications |