高頻寬頻表面聲波濾波器之研究

Abstract

本論文利用電子束微影系統，在絕緣體壓電材料(鈮酸鋰)及半導體壓電材料(砷化鎵)基板上製作出中心頻率約2.1 GHz等級的高頻表面聲波元件，並藉由指叉狀電極不同排列方式，分別製作出單頻、寬頻表面聲波濾波器。一開始設計了不同的波傳距離，目的在了解究竟多少的波傳距離，可以有較佳的通帶表現，發現距離500μm的波傳距離，會有較小的通帶漣波，符合通帶平坦化的條件。接下來再設計不同的指叉狀電極對數，發現對數較多之表面聲波濾波器會有較小之插入損失。並藉由蒸鍍不同種類之金屬，發現鋁由於質量密度小，於基板表面有較小之負載效應，因此只有約2.4MHz之頻率位移量，相當準確。單頻表面聲波濾波器，可以藉由指叉狀電極於壓電基板上所製作出，而寬頻表面聲波濾波器則是由斜交指叉狀電極於壓電基板上所製作出來。我們可以藉由設計指叉狀電極之寬度，來製作出多少中心頻率之表面聲波濾波器。在這邊，我們利用電子束微影系統，製作出高達GHz等級、頻寬約57MHz的砷化鎵寬頻表面聲波濾波器。並藉由不同的電子束曝光方式，成功地製作出頻寬約168MHz、通帶平坦化之鈮酸鋰寬頻表面聲波濾波器。

This work studied the fabrication of broadband surface acoustic wave (SAW) filters with the center frequency at 2.1GHz on GaAs and LiNbO3 piezoelectric substrates by electron beam lithography. By using the conventional interdigital transducer (IDT) and the slanted finger interdigital transducer (SFIT) on the piezoelectric substrate, we have fabricated the single-band and the broadband SAW filters, respectively. At first, I designed different propagation lengths to determine the best spacing between input and output transducers, in term of passband flatness. It turns out that the propagation length of 500μm was best because of its smooth passband. Secondly, the pair number of transducer was studied and the results showed that the insertion loss decreased as the pair number of transducers increased. By depositing different metals for the transducer fingers, I also found that the Ti/Al fingers gave the smallest frequency shift 2.4MHz. With these results, I fabricated the broadband SAW filters by using SFITs on the piezoelectric substrate. In the experiment, the GaAs broadband SAW filter with bandwidth 57MHz around 2GHz was realized. By correcting the problem of e-beam exposure, the LiNbO3 broadband SAW filter with flat bandwidth 168MHz around 2.1GHz was obtained successfully.