氟離子佈植對低溫多晶矽薄膜電晶體之研究

Abstract

本論文主要在研究氟離子佈值對多晶矽薄膜電晶體特性的影響，在製程步驟上，我們並不沉積櫬墊氧化層(pad oxide)來使氟離子堆積在多晶矽通道與氧化層介面，而是直接利用熱退火步驟產生的氧化層來代替積櫬墊氧化層的角色，因而能在減少製程的步驟情況 下，同樣得到氟離子鈍化(passivation)缺陷的效果。 由實驗結果顯示在5x1013cm-2的劑量下，不管是利用固相結晶法(solid phase crystallization)，或是準分子雷射退火結晶法(excimer laser annealing)製作的元件，其特性將得到改善，包括較高的遷移率(mobility)、開關電流比(on/off ratio)、較陡峭的次臨界導通斜率(subthreshold swing)與較佳的可靠度(relibility)。這是由於氟離子可以打斷扭曲的鍵結，如矽-氧-矽或矽-矽的鍵結，形成強度較高的矽-氟鍵，使應力得到舒緩，以減少tail state的數目，氟離子也會和一些斷鍵(dangling bonds)鍵結，讓deep state的數目降低，此外，由於矽-氟鍵的鍵結強度比矽-氫鍵結高，因而能得到比一般薄膜電晶體更佳的可靠度。實驗結果也顯示當劑量增加時，元件的特性將反而劣化，由萃取缺陷密度的結果，我們發現過量的氟離子反而會導致較高的缺陷密度，因而使元件的電性劣化。 由於此製程只需外加一道氟離子佈植的製程，因此可以較低的製程成本與複雜度來達到元件效能的提升，此元件可被運用於高效能的複晶矽薄膜電晶體之運用，尤其是在主動式薄膜電晶體液晶顯示器(AMLCD) ，以及三維立體的金氧半場效電晶體電路。

In this thesis, Polycrystalline silicon thin film transistors (Poly-Si TFTs) with fluorine ion implantation was investigated. In the process step, we used the surface-oxidized Si generated during thermal annealing in place of the pad oxide to make the drive force for fluorine atoms segregated to the poly-Si/oxide interface. Compared to the conventional fluorine incorporated poly-Si TFTs technology, the method we proposed needed no extra thermal annealing step and additional process steps. From the experiment result, it is found that the electrical characteristics of solid phase re-crystallized (SPC) and excimer laser annealing (ELA) fluorine ion implanted poly-Si TFTs will be improved, such as a higher mobility (μFE), a higher on/off current ratio (Ion/Ioff), a steep subthreshold swing (S.S) and a improved reliability. It is believed that the fluorine atoms can break the stress induced strained bonds, likely the strained Si-O-Si bonds and Si-Si bonds to form stronger Si-F bonds, leading to local stress relaxation and thus decreasing the tail state density. Moreover, the fluorine atoms in the poly-Si channel can also passivate the dangling bonds to decrease deep state density. In addition, the Si-F bonds is stronger than Si-H bonds, result in the improved reliability compared to conventional TFT. Experiment result show that the over amount of fluorine ion implantation doses cause the degraded electrical characteristics, result from the increase of the trap state density. This process only needs to increase one additional fluorine ion implantation process. The device can be improved by low cost and complication. Such TFTs are thus highly promising for use in future high-performance poly-Si TFT applications, especially in AMLCD and 3D MOSFET stacked circuits.