單軸應變矽奈米尺寸金氧半場效電晶體對於載子遷移率之各種散射機制的實驗性研究

Abstract

此研究針對單軸應變對於載子遷移率中不同散射機制的影響做一個全面性的探討。首先，我們引進一個以柏克萊短通道IGFET 模型(BSIM)的寄生電阻萃取方法。這個方法比傳統的通道電阻法還有偏移比例法(Shift & Ratio Method)還要準確，因為它考慮了因為單軸應變跟水平方向非均勻通道摻雜所導致的遷移率變化.我們利用了各種不同的製程條件來驗證這個方法而且跟實驗數據都能夠一致性的吻合。這個以柏克萊短通道IGFET 模型為基礎的寄生電阻萃取方法也透過半導體工藝模擬以及器件模擬工具(Technology Computer Aided Design)得到驗證。 除此之外，我們利用分離式電容電壓量測 (Split-CV Method) 的方式萃取短通道載子遷移率進而探討單軸應變對短通道載子遷移率的影響。接著對於N型及P型金氧半場效電晶體，我們在不同溫度下針對庫倫散射遷移率與應力之間的關係作實驗性的探討。我們的研究結果發現應力對於庫倫散射遷移率的敏感度跟溫度呈現顯著關係，這是因為基板電荷跟介面電荷散射機制互相抗衡的結果。因此，為了要將應力對於庫倫散射遷移率的效益極佳化 ，必須要將介面電荷加以抑制。 除此之外，透過低溫系統量測，我們也探討了單軸應變對於P型金氧半場效電晶體表面粗糙散射遷移率的影響。我們更進一步比較了應力對於表面粗糙散射遷移率跟聲子散射遷移率的敏感度。我們量測數據指出應力很明顯的提升表面粗糙散射遷移率，同時表面粗糙散射遷移率的敏感度比聲子散射遷移率的敏感度還要高。我們的實驗結果證實了之前發表過的模擬結果。除此之外，本文也從波函數穿透的觀點來解釋應力改變表面粗糙散射遷移率的可能原因。 此外， 我們在奈米尺寸P型金氧半場效電晶體實驗性地評估單軸應變對於載子遷移率的溫度效應。研究結果指出電洞遷移率對於應力的改變量隨著溫度增加而減少，這個結果跟之前利用單軸機械彎曲實驗研究結果一致.此現象是因為溫度增加的時候，更少的電洞會聚集在能使等效載子質量變輕的能帶邊緣，進而使得應力效率降低。同時，藉由低溫量測系統分開萃取表面粗糙散射遷移率跟聲子散射遷移率,我們更進一步在P型金氧半場效電晶體探討應力對於聲子散射遷移率的溫度效應。而從萃取出的表面粗糙散射遷移率跟聲子散射遷移率數據中，它們跟垂直電場以及溫度的關係跟已經發表文獻的數據也是一致的。而聲子散射遷移率跟溫度的關係會因為壓縮單軸應力的增加而變強，其原因是因為壓縮單軸應力增加會使得光熱子能量增加，進而增加了聲子散射遷移率對於溫度的敏感度，而這新發現也可以解釋為何汲極電流的溫度效應在單軸應變金氧半場效電晶體會變的更加敏感。

This dissertation provides a comprehensive study on the impact of process-induced uniaxial strain on the carrier mobility considering various scattering mechanisms. First, we introduce a BSIM-based method for the Rsd extraction. This BISM-based method is more accurate than the conventional Channel-Resistance and Shift & Ratio method because it considers the gate-length dependence of mobility caused by local uniaxial stress and laterally non-uniform channel doping. This method was verified using samples with different process conditions and good agreement with experimental data has been obtained. The accuracy of BSIM Rsd extraction method has also been verified by TCAD simulations. In addition, the short channel mobility extraction method by using split-CV is introduced to investigate the strain impact on short channel mobility. Then the uniaxial strain dependence of Coulomb mobility extracted by Matthiessen’s rule is experimentally investigated for both nMOSFETs and pMOSFETs under various temperatures. Our study indicates that the stress sensitivity of the Coulomb mobility shows strong temperature dependence. It is due to the competition result of the stress sensitivity between bulk charge scattering and interface charge (Nit) scattering. Therefore, in order to optimize the strain efficiency on Coulomb mobility, it is necessary to suppress the formation of Nit. Besides, through He-based low temperature measurement, the uniaxial strain dependence on surface roughness mobility (mSR) of pMOSFETs is also studied. Moreover, we compare the strain sensitivity between mPH and mSR. Our measured data indicates that mSR can be significantly enhanced by the uniaxial compressive strain. Furthermore, the mSR has higher strain dependence mPH. Our experimental results confirm the previously reported simulation results. In addition, a wavefunction penetration perspective is proposed to explain the possible physical origin of the uniaxial strain dependence of mSR. Moreover, we experimentally assess the impact of process-induced uniaxial strain on the temperature dependency of carrier mobility in nanoscale pMOSFETs. Our study indicates that the strain sensitivity of hole mobility becomes less with increasing temperature and it is consistent with previous uniaxial mechanical bending result. It is because the less hole repopulations at energy band edge induce less strain sensitivity as temperature increases. Furthermore, through decoupling mSR and mPH, we investigate the impact of uniaxial strain on the temperature dependence of phonon-scattering limited mobility in nanoscale PMOSFETs. The vertical electric field dependence (EEFF) and temperature dependence of the extracted mSR and mPH are consistent with the reported data in the literature. The temperature sensitivity of the extracted phonon mobility becomes higher when compressive strain is applied. It is contributed by the higher optical phonon energy induced by uniaxially-compressive strain. Our new findings also explain the higher temperature sensitivity of drain current presented in uniaxial strain PMOSFETs.