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Coincidence Technique in Scanning Electron Microscopes

IP.com Disclosure Number: IPCOM000060699D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Jenkins, KA: AUTHOR

Abstract

This publication describes a technique for the suppression of accidental noise in very low intensity Scanning Electron Microscopes (SEM).

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Coincidence Technique in Scanning Electron Microscopes

This publication describes a technique for the suppression of accidental noise in very low intensity Scanning Electron Microscopes (SEM).

The standard SEM uses the analogue signal of a photomultiplier (PM) tube to modulate a video signal. At very low elec beam intensities, this method may introduce photomultiplier noise (dark current) into the signal.

The time coincidence technique can be used to suppress this noise. This can be done as a modification of the Everhart- Thornley detector as shown in the drawing. The basis of the method is that the signal from the PM tube is fed to a discriminator circuit which puts out a logic pulse if a threshold i crossed. Two pulses, from independent PM tubes, are tested for coincidence and form a signal pulse if they arrive simultaneously. A noise pulse in only one PM tube does not become a signal pulse. The number of coincidence pulses per unit time is proportional to the rate of detected electrons, and may be converted to an analog signal for video viewing.

The amount of noise suppression depends on the widths of the discriminator outputs and the noise rates: Accidental coincidences = 2t RARB where t is the discriminator pulse width, and RA and RB are the noise rates of the PM tubes. For example, if RA = RB = 105/sec, and t = 10 ns, then the accidental rate is 200/sec, which is .2% of the single PM noise rate.

The technique could be used in a variety of ways. One could us...