反射電子検出器

backscattered-electron detector

電子ビームの照射により試料表面からの反射電子を検出する検出器。TEM機ではマイクロチャンネルプレートが使われている(SEM専用機ではp-nジャンクションを利用した半導体検出器が使われている)。検出効率を上げるために円環状にした検出器を試料直上に置く。マイクロチャンネルプレートは反射電子を検出し電気信号に変換する。反射電子のエネルギーは高いので、二次電子の場合のように付加的な加速は必要ない。反射電子像は、電子ビームで試料表面上を走査し、それに同期して反射電子の強度を輝点列としてコンピュータモニタ上に表示して得られる。円環状の検出器を2分割して用いて、組成(COMPO)像および凹凸(TOPO)像を得る手法が一般化している。すなわち、分割して得た二つの信号をプリアンプで増幅したあとメインアンプで演算処理し、組成像は二つの信号を加算して、凹凸像は減算して得られる。4分割した検出器を用いて、組成信号と凹凸信号を合成して立体(SHADOW)像を得ることもある。

A detector that is used for detecting backscattered electrons from the specimen surface by electron-beam illumination. In a TEM, a micro-channel plate (MCP) is used as the "backscattered-electron detector." (In a dedicated SEM instrument, a semiconductor detector that uses the p-n junction mechanism is adopted.) To improve the detection efficiency, an annular detector is placed just above the specimen. The MCP detects backscattered electrons and converts the electron signal into an electric signal. Since the energy of backscattered electrons is high, additional electron acceleration that is applied to secondary electrons is not required. A backscattered-electron image is obtained by displaying the intensity of the backscattered electrons on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe. A two-segment annular detector is often used for obtaining a composition (COMPO) image and a topographic (TOPO) image. That is, two signals acquired from each segment are amplified by with the preamplifier, and then processed with the main amplifier in such a way that COMPO image is obtained by addition of the two signals and TOPO image is obtained by subtraction of them. Furthermore, the use of a four-segment detector enables us to obtain a stereoscopic (SHADOW) image. To obtain SHADOW image, the composition signal from two segments and the topography signal from other two segments are synthesized.