白金選擇性沉積於矽奈米元件表面之氫偵測特性研究

Abstract

本研究主要利用焦耳熱效應選擇性沉積白金(Platinum)於元件上做為氫氣感測器使用，並成功利用焦耳熱效應行self-heating在室溫下做即時的不同濃度氫氣量測。首先利用離子佈值技術控制矽奈米元件參雜濃度，(n+/n-/n+)的元件結構設計可精準的控制元件放熱區域(低參雜濃度區域)，並利用低濃度參雜區域對閘極電位變化靈敏的特點作為感測區使用，先利用COMSOL多重物理偶合模擬不同電壓下表面溫度分布，與後續選擇性燒除PMMA以及不同電壓self-heating相互驗證。由原子力顯微鏡量測證實感測區表面之PMMA被完整燒除，進而進行選擇性沉積不同大小及厚度的金屬層。在處於不同濃度氫氣環境下的電性量測上，先在真空中找出對於偵測氫氣最佳的金屬層條件，並比較在不同操作電壓下對氫氣的反應時間變化，定出最佳的量測條件使得在真空中可靈敏的偵測50 ppm氫氣。另外發現在高溫且長時間的操作下元件的感測特性可得到顯著提升。最後在大氣環境的量測上，將元件置於利用PDMS製作出的小腔體內可使元件在大氣下擁有跟在真空狀態下一樣靈敏的吸附及脫附特性，最低偵測濃度可到達2.5 ppm。我們相信此充分利用焦耳熱效應技術作為氫氣感測器的應用可提升元件在室溫下的應用以及判斷不同濃度的氫氣準確性，我們可以預期此技術對於後續相關的氫氣感測有重大的幫助。

In this study, we report the experimental results of selective deposition of Platinum on nanodevices as real-time hydrogen sensors by localized joule heating. The active channels of nanobelt devices were consisted of n+/n-/n+ structure, and localize joule heating was induced at the local high resistance region. The surface temperature was estimated by COMSOL simulations for different biases. AFM was used to investigate the removal of PMMA, and monitored different lengths and thicknesses of Platinum deposition. First, responses of different hydrogen concentrations in vacuum were measured. The results show that the metal length on sensing region are proportional to the sensitivity, and metal thickness are inversely proportional to the sensitivity. Furthermore, different self-heating voltages were applied to accelerate the response and recovery time, and increase detection limit to 50 ppm. In addition, devices after high voltage and long-time stress exhibited improvement in sensing. Finally, in order to detect hydrogen under atmosphere, we design a small chamber made by PDMS to achieve the hydrogen detection. This modification functioned well and showed as almost the same sensitive as that in vacuum, proving by equilibrium dissociation rate constant calculation. The detection limit under atmosphere is about 2.5 ppm.