光激發電子顯微術(photoemission electron microscopy ; PEEM)是一項接收光激發電子來成像的顯微術，一直以來被廣泛的應用在研究樣品表面之特性。其收集訊號的方式是在樣品與電子透鏡組之物鏡間建立一個達數千甚至上萬伏特的正電壓來收集被光激發後脫離樣品表面的電子訊號，這種將樣品作為陰極來接收電子的透鏡組可以有效的收集光激發光電子，因此也被稱為陰極透鏡(Cathode lens)。不過，陰極透鏡雖然能有效的收集電子訊號，它在物(object)與像(image)兩側建立的非對稱場也讓此透鏡的像差較能夠提供對稱場的離子透鏡，如Einzel lens，之像差來得大。除了電子透鏡所造成的像差，利用高亮度的脈衝式光源所激發出來的大量電子其原有的軌跡也會因為電子之間的庫倫作用力而扭曲，進而降低影像的解析度。為了提升PEEM影像所能達到的解析度，加入像差修正器或降低光源強度是目前已經獲得實驗證實有效的方式。
光激發電子顯微術可利用紫外光(ultraviolet ; UV)或Ｘ光(X-ray)作為激發光源，同步輻射光源包含廣泛波長、具有偏振性與高亮度的特性，所以對光激發電子顯微術來說是極良好的光源。隨著同步輻射設施的進步帶來亮度更高的光源，加上偵檢器敏感度的提升，光激發電子顯微術除了繼續使用光激發二次電子訊號來成像以外，也開始以強度較弱的一次電子訊號來成像。透過接收高能量一次電子的光激發電子顯微術，我們可以對樣品深層進行探測；從過去約1nm的探測深度(電子動能為數百電子伏特)提高至約10nm的探測深度(電子動能為數千電子伏特)。然而，在探討新技術所能帶來的儀器功能提升與研究機會之際，隨之而來的是另一個鮮少被討論的影像解析度限制；訊號源的深度。
當電子訊號來自較深層的樣品內部時，其從訊號源傳播到樣品表面的過程會因為非彈性散射而造成電子能量、表面發射位置及發射強度上的重新分佈，進而使最終逃脫樣品表面的訊號分佈與原始訊號分佈有所差異。有鑑於光激發電子顯微術的電子透鏡組所接收的影像訊號是源自於從樣品表面發射的最終分佈，因此在傳播過程中所帶來的訊號分佈重組應該視為X光PEEM影像的本質限制(intrinsic limitation)。本篇論文的研究是透過C語言以數值模擬的方式，計算出來自不同樣品深度的電子訊號，探討於樣品內部射出的原始訊號分佈與樣品表面發射之電子訊號分佈之間會如何隨原始訊號的深度而改變。論文的內容包含以二次電子和一次電子為訊號源的討論。
關鍵詞： X光激發電子顯微術，影像解析度，數值模擬

Photoemission electron microscope (PEEM) is a full field microscope that records the two-dimensional (2D) electron emission maps with a beam-acceleration lens that treats sample as a cathode. The cathode lens greatly improves the microscope’s electron collection efficiency, but its asymmetric electric fields introduces additional aberrations that are hard to compensate even with a state-of-the-art aberration corrector. Another uncorrectable image degradation in PEEM is the space charge effects triggered by intense pulsed photon illumination. So far, installing an aberration corrector or reduce the photon intensity are proven approaches to improve the resolution of PEEM images.
Photoemission electron microscope commonly uses the ultraviolet or X-ray as its photon source. When synchrotron radiation is adopted, its wide range of wavelength, variable polarization, and high photon intensity make it a great photon source for PEEM. In fact, with the establishment of bright synchrotron facility and the improvement of detector sensitivity, PEEM can imagine with not only secondary electrons but primary electrons as well. With an intensive hard X–ray as its photon source, the hard X-ray PEEM (HAXPEEM) is able to extend the collection of primary electrons to 10 nm deep inside the specimen. However, while instrumentation improvement does open up the new research opportunities, it comes with limitations that were rarely discussed before, namely, the depth dependence on image resolution.
In this thesis, I look into the fundamental constraints of PEEM imaging when the collected electrons are originated from objects buried underneath specimen’s surface. Because X-ray’s attenuation length is generally longer than the low-energy electron’s inelastic mean free path (IMFP), it is straightforward to conclude that IMFP is responsible to the limited probing depth in photoelectron detection. However, as the electron emission is isotropic, the electron emission distribution is expected to evolve along with electron’s propagation. As a result, the projection of electron emissions from buried objects to surface is expected to vary with objects’ depths. This depth dependence has nothing to do with the acceleration field applied in cathode lens nor the space charge effects. Here, we do some numerical simulation for this issue.
Keywords:X-ray photoemission electron microscope (PEEM), image resolution, numerical simulation

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