矽在絕緣層上N型金氧半場效電晶體在射頻應用之製程研究

Abstract

最近幾年，矽在絕緣層上(silicon-on-insulator, SOI)製程一直被認為是未來在製作超大型積體電路的主流技術。然而，在射頻的領域中，不同的閘極結構會影響到電晶體的射頻特性，而扭結效應(kink effect)在SOI上的互補式金氧半電晶體元件仍然是一個爭論的議題。而且，完全空乏與部分空乏SOI金氧半電晶體兩者熟劣熟優也是一個爭論的議題。 在我們研究中，六吋SIMOX晶圓被使用來製造射頻矽在絕緣層上N型金氧半電晶體。在射頻特性方面，當汲極與閘極電壓均為2伏時，對於0.3微米的N型金氧半電晶體，其截止頻率與最大震盪頻率為12.7 GHz與15 GHz。在固定總閘寬度的情況下，單一根閘的寬度對射頻SOI金氧半電晶體的特性影響是我們的研究重點之一。而研究結果指出，在具H閘結構的部分空乏SOI元件中，縮短單一根閘的寬度會使元件的射頻特性變差，這是因為較多的寄生閘源電容寄生在元件中。 而部分空乏與完全空乏元件之間特性比較亦是我們研究的目的之一，而以製程與元件特性的觀點來看，完全空乏的元件較優於部分空乏的元件。因為完全空乏的元件具有較高的轉導，以致於其射頻特性明顯地較佳，而且在製程步驟上也較簡單。對於部分空乏且基體懸浮的元件會有扭結現象的發生，而扭結現象會使轉導變大，同時使射頻特性變的較佳。然而，在部分空乏基體接地的元件中，扭結效應可以完全的被抑制，而其元件特性類似於塊材元件。然而，在完全空乏的元件中，我們發現其扭結效應並不明顯。

Recently silicon-on-insulator (SOI) technology has been considered as the mainstream technology of the future for ULSI circuits. However, in the RF field, different gate structure may influence RF characteristics of the MOS transistors and kink effect remains an issue for fabricating CMOS on the SOI substrate. Furthermore which one is superior between the partially-depleted (PD) and fully-depleted (FD) SOI MOSFETs is still a controversial issue. In our study, 6-inch n-type (100) SIMOX wafers were employed to fabricate RF SOI nMOSFETs. In RF characteristics, ft (cut-off frequency) = 12.7 GHz and fmax (maximum oscillation frequency) = 15 GHz at Vg = Vd = 2 V could be observed for 0.3 µm nMOSFET. Influence of the finger length at fixed total gate width on the RF characteristics of the SOI nMOSFETs was investigated. Results of our investigation indicate that in the PD H-gate devices, shortening the finger length degrades RF characteristics. That is due to more parasitic gate-source capacitance would exist in the device. And comparison between PD and FD devices is another object in our study. In the viewpoint of process and devices performance, FD devices are always superior to PD ones. In FD devices, due to higher gm, the RF characteristics could be improved significantly; furthermore the process of fabricating nMOSFETs is simpler. For PD devices, kink effect can be observed in floating-body devices. And kink effect may enhance gm and make RF characteristics better. But kink effect is still an issue in RF circuits. However in the PD body-tied devices, the kink effect can be suppressed entirely. And the characteristics are much like the bulk devices. In the FD devices, we found the kink effect can be unobvious.