不同氧化層厚度次微米 LDD MOS 元件中熱載子導致元件的退化分析

Abstract

本論文著重於研究熱載子對不同氧化層厚度之 LDD N 型金氧半場效電晶 體 (LDD nMOSFET)所造成的特性退化。研究顯示，在相同的操作偏壓下， 氧化層較薄的 LDD nMOSFET有較大的特性退化。然而，根據以往的報告， 氧化層較薄的傳統金氧半場效電晶體 (conventional MOSFET)卻有較小的 特性退化。既然 LDD 元件的主要退化機制與傳統元件不同，導因於 LDD nMOSFET's 有 spacer-induced degradation ，我們發展出一套改良 的汲極電流特性退化模型以研究在 LDD nMOSFET's中的特性退化機制。研 究發現，移動率退化不是 LDD nMOSFET's的主要退化機制。一個新的退化 機制將被引入以解釋在 n- 區域中因界面狀態 (interface state)的產生 而增加之電阻。再者，因 LDD元件的等效通道長度 (effective channel length) 和源–汲極串聯電阻 (source-drain series resistance) 與閘 極電壓有關，我們使用 paired Vg method 來萃取出隨閘極電壓而變的等 效通道長度與源–汲極串聯電阻。結果顯示這個一般化的 MOSFET 汲極電 流特性退化模型可以獲得一與實驗結果相近的值。根據此一模型，吾人可 以明確的瞭解隨氧化層厚度不同而產生的退化特性的差異。 In this thesis, gate-oxide thickness dependecnce of hot- carrier-induced degradation is investigated for LDD nMOSFET' s. It is shown that a thinner gate oxide LDD nMOSFET's causes larger drain current degradation under the same bias stress condition. However, it has been reported that a thinner gate oxide conventional nMOSFET shows smaller degradation. Since the dominant degradation mechanism for the LDD device differ from the conventional device, due to the spacer-induced degradation, an improved drain linear-current degradation model is developed in order to investigate the degradation mechanism in LDD MOSFET. A new degradation mechanism is introduced to account for the increasing of resistance in the n- region due to the generation of interface states. Further, since the effective channel length and the source-drain series resistance of an LDD device are gate-voltage dependent, the paired Vg method is used to extract the effective channel length and the series resistance. It can be found that this generalized drain current degradation model gives a good agreement to the measured data for different gate oxide thickness. Based on this model, the gate-oxide thickness dependence of degradation can be well analyzed.