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This study examines various aspects of SEM calibration and metrology, including magnification calibration, photographic CRT calibration, accelerating voltage compensation, X and Y squareness calibration, and linewidth metrology.
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SEM Calibration Study • Several areas of SEM Calibration were studied in an Interlaboratory Study. • Magnification Calibration • Adjustment of the X and Y column scans • Photographic CRT adjustment • Adjustment of the visual CRT • Linewidth measurement
Magnification Calibration • Tolerance limits established at +5% and -5% for plotting purposes. • SEM magnification traditionally considered to be good to 10% • SRM 484 has an uncertainty of 5% of the 1μm pitch (on the sample used)
Photographic CRT CalibrationInterlaboratory SEM Study: Part 2
Photographic CRT Calibration • In laboratory instruments, the photo field size requires calibration. • This procedure sets the length of the alphanumerics. • Therefore, this also sets the length of the LINESCALE on the photograph.
Photographic CRT Calibration • Measurements made directly from the micrographs can be incorrect because of either alphanumeric error or scan/magnification error. • This is important because the laboratory instrument is often used to calibrate the in-line instruments.
Accelerating Voltage CompensationInterlaboratory SEM Study: Part 3
Accelerating Voltage Compensation • When the accelerating voltage is changed the instrument must make corrections. • Otherwise the instrument focus is altered. • Correction results in an undesirable change in magnification. • kV compensation can either be done in hardware or software. • Relates directly to hysteresis in the magnetic lenses of the instrument.
Lens Hysteresis Compensation • Lens Materials • Degaussing procedures • Monitoring • Current monitor • Hall probe in the lens
“X” and “Y” Squareness CalibrationInterlaboratory SEM Study: Part 4
“X” and “Y” Squareness Calibration • Calibration must be done in both the “X” and the “Y” directions. • The magnification ratio of X/Y should equal 1. • Otherwise circular objects will appear oval and square objects will appear rectangular.
Dimensional Metrology • In the Interlaboratory Study, width measurements were made of the finest lines (0.2μm). • This was considered to be a “best-guess” measurement using the participants standard methodology. • Comparison measurements were made using the NIST laser interferometer instrument.
Dimensional Metrology • The NIST Metrology instrument measured an average pitch of 401 nm and an average width of 204 nm using the BSE image and an arbitrary (negative) 50% threshold crossing algorithm. • These measurements compared within 3 nm of another metrology instrument in a commercial lab. • These numbers were used as the “standard nominal” to which the participants data were compared.
Dimensional Metrology • Participant variability was quite large. • One participant (working under presumably the same operating conditions) reported a difference of 31 nm between two accelerating voltages: • 1 kV 315 nm • 2 kV 284 nm • Causes of the measurement variability: • Electron beam interaction • Electron beam diameter • Sample contamination • Sample edge variations • Measurement algorithm differences
Reminder • Do not Forget: • That where SEM metrology is concerned, the computer providing you and your management with the answers has not taken this course. • Therefore, it is ignorant of all the major points I presented regarding the potential instrument problems • Believe these data only after you have confirmed them
Conclusion • Using the SEM for metrology is now commonplace. • Obtaining GOOD data is not. • The SEM metrologist must continually think of the places where errors can enter into the data. • These areas must then be eliminated or at least minimized in order for good metrology to result.
Conclusion • SEM Metrology is a viable technique. • There are potential pitfalls, but they can be avoided with care and understanding of the tool.
