以多孔矽作為矽基射頻元件基材之研究

Abstract

隨著無線通訊系統的發展與進步，人們對訊息的需求量愈大，電磁波所載之訊息量勢必提高，跟著影響的便是電子產品的操作頻率，故提高操作頻率已經成為不可避免之趨勢，但是隨著操作頻率的提高，相對於矽基射頻元件卻帶來嚴重的影響，本研究首要之課題便是改善矽基射頻元件因為操作頻率提高所造成之能量損失。 矽基材仍是目前的半導體元件之主流，但是在高頻時，矽基材與半導體元件之間的寄生效應所造成之能量損失，使得元件之品質因子低落，此狀況一直都是矽基射頻元件最大的缺陷，雖然造成半導體元件之品質因子低落的原因，不單純僅為基材之寄生效應，尚有元件之形狀及金屬的導電率等等，但是由於半導體廠的製程持續地進步下，其製程上之因素已經不再是重點，所以基材之寄生效應儼然成為降低半導體元件之品質因子的主要原因。 矽基材造成半導體元件之品質因子低落的主要原因在於元件中流動的電流會引發磁場，而磁場貫穿矽基材後，在其中衍生反向的渦電流，使得品質因子降低，此種問題在高頻段時特別嚴重，因為渦電流的強度與磁場的變化率呈現線性關係。此外元件與基材之間的寄生電容效應亦會直接影響其自振頻率和導致電場耦合而消耗能量。本研究所提出之方法為利用CMOS相容之微機電製程，將矽基材改造成多孔矽，使得反向之渦電流不易產生於矽基材當中，直接地解決矽基材對半導體元件的不良影響。

With the rapid growth of wireless communication system, the operation frequency is raised again and again. Although raising the operation frequency could load more data, it is not good for CMOS passive devices because of its substrate-loss. Silicon a stable material is used for CMOS process but the parasitic effect which between silicon substrate and CMOS passive devices decreases the quality factor of CMOS passive devices at high frequency. The parasitic effect is induced at high frequency; the magnetic flux which penetrates the silicon substrate induces inverse eddy current. The qulity factor of CMOS passive devices will decrease because of the inverse eddy current. A high resistivity material can’t easily induce inverse eddy current so gallium arsenide is suitable for RF devices than silicon. Gallium arsenide maybe a good substrate but it can’t integrate all circuits on a chip. If we want to achieve SOC, we should only choice silicon. This thesis transforms silicon substrate into porous silicon substrate in order to improve the characteristics of RF CMOS devices. Electrochemistry etching is used to form a porous silicon layer on the silicon substrate, which provides a low-loss substrate and better quality factor. In this study, a novel CMOS-compatible porous silicon process is used to transform the silicon substrate and improve the characteristics of RF CMOS devices.