Acceptance characterization of electron detector in SEM using stainless steel sphere.

Although modern scanning electron microscope (SEM) possess several electron detectors, it is not clear what kind of information is contained in a SEM image taken by a certain detector. Especially the detectors installed in the objective lens are difficult to know their characters. Thus, we propose a simple method to assess the acceptance of electron detector using a stainless-steel sphere. After taking images under certain conditions, say electron beam energy, working distance etc., the image intensity of each pixel point, which is characterized by coordinate (θ, φ), is evaluated. The advantage of this method is the ease of implementation and the whole information of electron emission from the tilted surfaces is contained in the image. Using this information, the acceptance of the detector can be analyzed systematically. In this paper, the traditional Everhart-Thornley detector is analyzed with this method. It is demonstrated how the sphere image changes according to the measurement condition. The ET image quality is strongly governed by working distance but not so much by the electron beam energy. We propose an alternative method to avoid the ambiguity of working distance. Using a needle type specimen stage, the ET image does not vary so much with WD and the reliability of ET image significantly improves.