Magnification Calibration Interlaboratory SEM Study: Part 1

How well are SEMs calibrated ?

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

List of Instruments Surveyed

Magnification Ranges

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 Calibration Interlaboratory 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 Compensation Interlaboratory 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 Calibration Interlaboratory 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.

Linewidth Metrology Interlaboratory SEM Study: Part 5

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

Conclusion

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.

The End