使用者:Schenad/氣態探測器

氣體式探測器（英語：gaseous detection device，GDD）泛指在環境電子顯微鏡或類似的掃描式設備中能夠在一定的氣壓下對電子信號進行測量的儀器。

傳統的電子探測器（例如埃弗哈特-索恩利探測器）需在較高的真空（低於10^-3Pa或10^-5Torr)才能工作。氣體式探測器應用了不同的原理探測電子，工作環境可以覆蓋到較低的真空（50Pa或0.38Torr）。氣體式探測器的出現使得通過電子顯微鏡技術觀測一些在真空下無法維持其自然形態的樣品（例如生物組織、膠體和某些液體等）成為可能。如今，占有電子顯微鏡市場上超過50%的份額的可變壓電子顯微鏡（VP-SEM）或環境電子顯微鏡（ESEM）的系統內都可以見到氣體式探測器的應用。

歷史

傳統掃描電子顯微鏡內部的電子探測器需要在真空下才能工作；這大大限制了其可觀測的樣品。為了對真空下無法維持自身原本形態的樣品（例如生物組織、膠體和某些液體等）進行觀測，一類被稱作「環境電子顯微鏡」的新型電子顯微鏡應運而生。這一類電子顯微鏡能夠在低真空，甚至是在一個大氣壓的環境下對樣品進行觀測。搭載在環境電子顯微鏡上的電子探測器繼承了傳統掃描電子顯微鏡使用的一部分電子探測器並進行了一定的改造，使其在低真空下也能工作。例如，通過設計合適的幾何位置來優化電子束傳輸、背散無線電子分布和光波導傳輸可以改進背散無線電子探測器。然而，傳統掃描電子顯微鏡常用的二次電子探測器（或埃弗哈特-索恩利探測器）上採用了高電勢的設計；即使在低真空的條件下，該探測器也會出現災難性的擊穿，因此無法簡單地直接改造。1983年，Danilatos設計了基於完全不同原理的探測器，解決了這一難題。藉助不同信號對氣體的電離作用，環境中的氣體被當作了探測器的一部分。若對電極的配置和偏壓進行合適的控制，即可實現對二次電子的探測。氣體式探測器工作原理的已被發表為一篇綜述；下述的大部分內容都來源於此篇綜述。

原理

氣體式探測器的原理

氣體式探測器由核物理和天文學中對粒子的觀測手段改造而來。成像所需的參數需要考慮樣品腔（specimen chamber）的電子顯微鏡，以及腔內氣體存在時的情況。雖然由電子束-樣品相互作用產生的信號與周圍氣體會發生氣態電離和激發，但是信號-氣體相互作用的類型、強度和分布各不相同。幸運的是，這些相互作用通常能夠兼容環境電子顯微鏡成像所需的恆定時間（time-constant）的要求。確立這種兼容性促成了氣體式探測器的發明以及從粒子物理學到電子顯微鏡領域實現跨越的基礎。參與信號-氣體相互作用的主要包括背散無線電子和二次電子。

實現方式

高場強下的對比度變化

低場強下的對比度變化

右圖所展示的氣體式探測器原理不僅有二次電子模式，還包括了背散無線電子模式以及二者的組合。即使只想單獨使用SE信號，也建議至少再使用一個同心電極來幫助分離BSE干擾以及其他噪音源，比如氣體散射出的裙邊電子。這個附加部分可以作為「守護」電極，並且通過獨立調節其偏置與SE電極相比，可以有目的地控制圖像對比度。還可使用其他控制電極，例如陽極和陰極之間的網格。 Even if only the SE signal is desirable to use alone, at least one additional concentric electrode is recommended to employ in order to help in the separation from interference of BSE and also from other noise sources such as the skirt electrons scattered out of the primary beam by the gas. This addition may act as a 「guard」 electrode, and by varying its bias independently from the SE electrode, the image contrast can be controlled purposefully. Alternative control electrodes are used such as a mesh between anode and cathode. A multipurpose array of electrodes below and above the specimen and above the pressure limiting aperture of the ESEM has also been described elsewhere.

The development of this detector has required devoted electronics circuitry, especially when the signal is picked up by the anode at high bias, because the floating current amplified must be coupled at full bandwidth to the ground amplifier and video display circuits (developed by ElectroScan). An alternative is to bias the cathode with a negative potential and pickup the signal from the anode at floating ground without the need for coupling between amplifier stages. However, this would require extra precaution to protect users from exposure to a high potential at the specimen stage.

A further alternative that has been implemented at the laboratory stage is by the application of a high bias at the anode but by pickup of the signals from the cathode at floating ground, as shown in the accompanying diagram. Concentric electrodes (E2, E3, E4) are made on a copper-coated fiberglass printed circuit board (PCB) and a copper wire (E1) is added at the center of the disk. The anode is made again from the same PCB with a conical hole (400 micrometres) to act as a pressure limiting aperture in the ESEM. The exposed fiberglass material inside the aperture cone together with its surface above are coated with silver paint in continuity with the copper material of the anode electrode (E0), which is held at high potential. The cathode electrodes are independently connected to ground amplifiers, which, in fact, can be biased with low voltage directly from the amplifier power supplies in the range of ±15 volts without any further coupling required. On account of the induction mechanism operating behind the GDD, this configuration is equivalent to the previous diagram, except for the inverted signal that is electronically restored. While electrode E0 is held at 250 V, meaningful imaging is done as shown by a series of images with composition of signals from various electrodes at two pressures of supplied air. All images show part of the central copper wire (E1), exposed fiber-glass (FG, middle), and copper (part of E2) with some silver paint used to attach the wire. The close resemblance of (a) with (b) at low pressure and (c) with (d) at high pressure is a manifestation of the principle of equivalence by induction. The purest SE image is (e) and the purest BSE is (h). Image (f) has prevailing SE characteristics, whilst (g) has a comparable contribution of both SE and BSE. Images (a) and (b) are dominated by SE with some BSE contribution, whilst (c) and (d) have comparable contribution by both SE and BSE.

The very bright areas on the FG material result from genuine high specimen signal yield and not from erratic charging or other artifacts familiar with plastics in vacuum SEM. High yield of edges, oblique incidence, etc. can for the first time be studied from the true surfaces without obstruction in ESEM. Mild charging, if present, may produce stable contrast characteristic of material properties and can be used as a means for studies of the physics of the surfaces. The images presented in this series are reproductions from photographic paper with limited bandwidth, on which attempting to bring up detail in dark areas results in saturating the bright areas and vice versa, whilst a lot more information is usually contained on the negative film. Electronic manipulation of the signal together with modern computer graphics can overcome some old imaging limitations.

An example of the GDD operating at low voltage is shown with four images of the same field of view of a polished mineral containing aluminum, iron, silicon and some unknown surface impurities. The anode electrode is a single thin wire placed on the side and below the specimen surface, several mm away from it. Image (a) shows predominantly SE contrast at low pressure, whilst (b) shows BSE material contrast at higher pressure. Image (c) shows cathodoluminescence (CL) from the specimen surface by use of water vapor (which does not scintillate), whilst (d) shows additional photon signal by changing the gas to air which scintillates by signal electrons originating from the specimen. The latter appears to be a mixture of CL with SE, but it may also contain additional information from the surface contaminant charging to a varying degree with gas pressure.

The GDD at high voltage has clear advantages over the low voltage mode, but the latter may be used easily with special applications such as at very high pressures where the BSE produce a high ionization gain from their own high energy, or in cases when the electric field requires shaping to purposeful ends. In general, the detector should be designed to operate at both high and low bias levels including variable negative (electron retarding) bias with important contrast generation.

Further improvements have been envisaged, such as the use of special electrode materials, gas composition and shaping the trajectory of detection electrons by special electric and magnetic fields (page 91).

商業化

第一個成功商業化的氣體式探測器來自於ElectroScan公司。他們推出了首字母縮寫為ESD的「environmental secondary detector」（環境二次電子探測器）。之後其改進版本——「gaseous secondary electron detector」（氣體式二次電子探測器，GSED）也隨之發布。在電子顯微鏡的物鏡中使用磁場也可見於另一商業專利中。LEO公司（現在的Carl Zeiss SMT）開發了其環境電鏡氣體式探測器的閃爍模式（scintillation mode）和電離模式（ionization mode），能夠在低真空且較大範圍的真空條件下工作。

外部連結

• ESEM Development and its Future

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