奈米級蕭特基金氧半場效電晶體之載子傳輸特性與通道背向散射研究

Abstract

在前瞻超大型積體電路元件中，為了提升元件的效能，許多新穎的元件結構已被廣泛的提出，例如：高介電係數介電層、應變矽通道、金屬閘極與金屬源/汲極結構。當元件微縮至奈米級尺寸時，通道背向散射理論已經成功的運用在預測元件微縮極限上。而今，由於蕭特基金氧半場效電晶體製作的最佳化方法已趨可行，其在前瞻元件演進的地位已大幅的提升。因此，蕭特基金氧半場效電晶體的載子傳輸特性的研究成為主要課題。 本論文中，我們首先著眼於利用活化能(Activation Energy Method)方法求得等效的蕭特基位障勢。蕭特基場效電晶體之汲極電流傳導機制與閘極電壓的關係式可利用等效蕭特基位障勢表示。另外，我們同時發現蕭特基金氧半場效電晶體在打開狀態時，產生一個負等效蕭特基位障勢，使通道背向散射原理可運用於此。以往，溫度相依法(Temperature Dependent Method)常被用來探討通道背向散射係數。但是，在蕭特基金氧半場效電晶體中，載子主要是透過熱場發射機制由源極入射制通道內。所以對此元件來說，溫度相依法是不可行的。為了要求得載子彈道入射的機率，我們導入了等效彈道遷移率(Effective Ballistic Mobility)的觀念，此原理是建立在載子遷移率(Mobility)會隨著通道縮小而下降的因素上。因此，我們可以透過等效彈道遷移率的方法得到載子在元件線性區的彈道入射係數與載子熱入射(Thermal Injection Velocity)速度。然後，我們運用當電晶體在負等效蕭特基位障勢發生時的載子平均傳輸速度(Carrier Average Velocity)與載子熱入射速度上，藉由這兩個速度的關係式，載子在打開狀態時的載子彈道入射機率即可求得。 由本文的研究，我們得到幾個結論： (1) 背向散射理論在蕭特基金氧半場效電機體中，因負等效位障勢的產生而再度的適用， (2) 載子由源極經通道到達汲極的背向散射機率因非局部的熱場穿遂機制而較傳統金氧半場效電晶體高， (3) 應變矽通道元件對背向散射係數影響較輕，但對載子熱入射速度影響較劇烈， (4) 遷移擴散(Drift-Diffusion)模型在quasi-ballistic區仍適用。因此，蕭特基金氧半場效電晶體加上高參雜隔離層(Dopant Segregation Implantation)與CESL(Contact-Etched Stoped Layer)技術，可達道元件高速操作的需求。

In advanced VLSI devices, a lot of new structures have been brought up for enhancing drain current such as strained-Si channel, high-κ dielectric, metal gate and metal source/drain. In the nanoscale channel length, the channel backscattering theory has been applied to predict the scaling-limitations of these structures successfully. Nowadays, the Schottky-barrier MOSFETs have aroused much more attention because some optimized processes become feasible. Hence, the carrier transport mechanism of Schottky-barrier MOSFETs from source to drain becomes the most popular topic in researches. In the thesis, first, we will focus on finding the effective Schottky-barrier height from the activation energy method. We can describe the effective Schottky-barrier height versus carrier transport mechanism relationship from this method. A negative effective Schottky-barrier height is found in the ON-state of the Schottky-barrier MOSFETs so that the channel backscattering theory can be used for extracting the carrier ballistic rate. In the past, the ballistic coefficient is extracted by temperature dependent method. However, the major carrier transport mechanism in the Schottky-barrier MOSFET is field emission, the temperature dependent method is failed. We practiced the effective ballistic mobility which is from mobility degradation in short channel devices. We may directly obtain the ballistic coefficient and thermal injection velocity in the linear region. Then, we derive the carrier average velocity versus thermal injection velocity relations in ON-state. By the two velocity components, the ballistic probability of the Schottky-barrier MOSFET can be extracted easily. Based on the results of this work, it was concluded that: (1) the backscattering theory is practicable from the negatively effective Schottky-barrier height, (2) the backscattering probability in the source side of Schottky-barrier is smaller than that in the conventional MOSFETs due to non-local tunneling, (3) the strained technology affects the backscattering coefficient lightly but it affects the thermal injection velocity drastically, (4) the drift-diffusion model is still workable in quasi-ballistic region. Thus, Schottky-barrier MOSFET with dopant segregation implantation and CESL(Contact-Etched Stoped Layer) can enhance the ballistic rate and thermal injection velocity that produced high speed operation in Schottky-barrier MOSFETs.