Quality Improvement of Environmental Secondary Electron Detector Signal Using Helium Gas in Variable Pressure Scanning Electron Microscopy

Abstract

The quality of the image signal obtained from the environmental secondary electron detector (ESED) employed in a variable pressure (VP) SEM can be dramatically improved by using helium gas. The signal-to-noise ratio (SNR) increases gradually in the range of the pressures that can be used in our modified SEM. This method is especially useful in low-voltage VP SEM as well as in a variety of SEM operating conditions, because helium gas can more or less maintain the amount of unscattered primary electrons. In order to measure the SNR precisely, a digital scan generator system for obtaining two images with identical views is employed as a precondition.

Feature evaluation of complex hysteresis smoothing and its practical applications to noisy SEM images

“…Hence, the present formula can measure standard deviation of noise N σ. Incidentally, since √ Cov is equivalent to standard deviation of desired signal S σ, SNR = S σ/ N σ of an SEM image can be also obtained. These have been utilized for several applications (Erasmus and Smith, ; Oho et al, , , ; Joy ; Oho and Suzuki, ).…”

“…Cov is equivalent to standard deviation of desired signal Sσ , SNR = Sσ /Nσ of an SEM image can be also obtained. These have been utilized for several applications (Erasmus and Smith, '82;Oho et al, 2000Oho et al, , 2004Oho et al, , 2007Joy 2002;Oho and Suzuki, 2012).…”

“…Of course, by using a field-emission SEM, which has a fairly long history, the signal-to-noise ratio (SNR) of SEM images was dramatically improved (as the early study, Komoda and Saito,'72;Welter and Coates,'74). In addition, the quality of SEM signal has been dramatically improved by using helium gas in variable pressure (VP) SEM (Oho et al, 2000(Oho et al, , 2007. Nevertheless, the long acquisition time and the existence of heavy noise in SEM image are difficult problems to be solved.…”

Image restoration for TV‐scan moving images acquired through a semiconductor backscattered electron detector

“…Under this SEM condition, most samples may be observed without a metal coating, and wet and/or oily samples may also be observed successfully, though we have to frequently submit to a low signal-to-noise ratio. Recently, the quality of a low vacuum SEM image has been dramatically improved by introducing helium gas into the specimen chamber (Oho et al 2000(Oho et al , 2007. However, detectors of this type have a serious disadvantage on the frequency characteristics in greater or lesser degrees since they were developed (Gedcke et al 1978).…”

A dynamic scanning method based on signal-statistics for scanning electron microscopy

“…Most approaches to the problem can be categorized into the two groups, posterior spatial filtering after complete image acquisition such as conventional median filtering (Oho et al ., ) or the method described by Oho (), and realtime ensemble averaging of the acquisition signal (Taylor et al ., ; Erasmus ). Other methods physically increase the primary signal (Oho et al ., ), or adjust the image pixel values to human optical perception (Oho et al ., ) to achieve similar effects.…”