利用質子佈植之高阻值矽基板上被動元件射頻/微波特性及其應用

Abstract

隨著行動無線通訊系統需求日益增加，對高性能、低成本、低功率、以及小體積之射頻/微波電路之需要更顯得迫切。而有著整合低成本射頻積體電路(RFICs)與數位電路可能性的矽晶片就成為目前可應用於微波領域炙手可熱的基板選擇之一。因為CMOS的微縮化使得其最大震盪頻率已可超過100 GH (0.13 □m)。然而，對CMOS製程所使用之標準低阻值矽基板(□ ~10 □-cm)而言，其上的傳輸線以及被動元件有著相當高的訊號損失。這些損失不但造成元件本身特性變差，更會破壞CMOS射頻/微波電路的效能，而這些低性能(相對於三五族砷化鎵晶片而言)的被動元件，正是目前CMOS射頻電路最大的致命傷之一。因此，如何克服此問題，對未來CMOS射頻電路的研究與發展將有著關鍵性的影響。本論文中提出了一種與CMOS製程相容之質子佈值法，此法可將標準矽晶片之低阻值選擇性地轉變成高阻值，如此便可降低其上元件之訊號損失，提升射頻/微波性能，以期改善CMOS射頻電路系統之特性。 本論文以射頻/微波電路中最常見之收發器(transceiver)電路出發，將其中會使用到的被動元件作一系統性的探討。包括：共平面(CoPlanar Waveguide， CPW)傳輸線、射頻電感、微波帶通(band-pass)與帶止(band-stop)分散式(distributed)濾波器，以及積體天線(integrated antenna)，最後亦探討了晶片內之無線傳輸。研究內容主要在於基板阻值對元件特性的影響，包括功率耗損、雜訊比較、Q值高低，和傳輸消耗。在積體天線中，並討論了矽基板對輻射場型(radiation pattern)之影響。若能結合這些被動元件於收發器電路中，應可作出高性能高度整合之單晶片CMOS收發器。 本論文中將先就高能量質子佈植作一簡單介紹，此法可將10□□□cm之標準低阻值矽基板轉變成擁有106□□□cm、近似於半絕緣砷化鎵特性、且極適用於高頻應用之基板。此點可由製作於其上的CPW傳輸線之低損耗特性獲得驗證。此外，此傳輸線之雜訊值經量測也比一般標準矽晶片來得小。探討其因是由於質子撞擊於矽基板上產生許多陷阱(traps)，這些陷阱不斷捕捉帶電載子而使得矽基板阻值增加，又因其生命期(life time)短，故不易產生相對低頻之雜訊。此低耗損低雜訊傳輸線特性可維持於400攝氏度以下，110 GHz，相當高頻的區段。接著探討的是射頻電感特性。量測後發現經過質子佈值過後之電感感值並未改變，然而Q值卻提升近60%，此外，高Q值頻段亦增加，表示電感之效能獲得改善，可應用之頻率獲得拓展。帶通與帶止微波濾波器，採用的是分散式設計，此設計適合於高頻操作。濾波器震盪頻率分別設計於22-91 GHz，然而不管在哪個頻段，其傳輸消耗接相當低(~<2 dB)。不但挑戰了矽製程被動元件的極限，也證明此基板可應用於微波領域。最後探討的是積體天線，天線頻率分布於10-40 GHz。從高頻參數與場型的量測結果皆可看出，質子佈植對天線訊號之損耗有相當的改善。而從晶片間無線傳輸之結果，也可得到同樣的結論。

The possibility of low-cost RF integrated circuits (RFICs) integrated with digital circuitry is creating strong interest in silicon as microwave substrate. This is driven by the scaling down of CMOS technology with a maximum frequency of oscillation fmax of >100 GHz (for 0.13 □m RF CMOS). However, transmission lines and passive circuit components on standard low-resistivity silicon substrates such as used in CMOS processing have high loss. These high losses degrade the characteristics of devices and further damage the performance of CMOS RF/Microwave circuits, which has become one of the primary impairments for Si RF applications. To overcome this problem, several approaches have been used for Si RFICs. In this work, we develop a proton implantation with compatibility with VLSI technology, which is able to convert the standard low-resistivity Si to high-resistivity for reducing the substrate loss. In this work, we investigate the most passive components used in a common transceiver structure. Including: coplanar waveguide (CPW) transmission lines, RF inductors, microwave band-pass and band-stop distributed filters and integrated antennas, and finally the inter-chip wireless transmission is also studied. We probe the effect of substrate resistivity to the device characteristics and measure the power loss, noise figure, Q-factor, and insertion loss for the devices, and radiation patterns for integrated antennas. These passive devices can be ultimately modified and integrated into a fully-integrated single-chip transceiver. In this work, we first introduce proton implantation process, which converts the standard low-resistivity Si substrate (10□□□cm) to a high resistivity of 106□□□cm, close to semi-insulating GaAs. The CPW transmission lines on the proton implanted Si have extremely low loss, indicating this technology is suitable for high frequency applications. Besides, low-noise characteristics of transmission lines are also obtained. The reason for high resistivity may be a result that the traps generated by proton bombardment catch the electronic carriers which are responsible for conductivity originally. On the other hand, because the life time of carriers becomes so short, the relatively low frequency noise is also reduced. These low-loss and low-noise performance for transmission lines keep under 400oC, up to 110 GHz, a very high frequency range. Next, a RF inductor is measured after proton implantation. The Q-factor is increased for 60 % with an extended high Q frequency range, while the inductance maintains. Distributed band-pass and band-stop filters are suitable for high operation frequencies. They are designed from 22-91GHz and the measured insertion losses are <~2 GHz on the proton implanted Si, indicating the substrate can be used even at such higher frequencies. Finally several integrated antennas were designed and fabricated from 10-40 GHz. From the measured S-parameters and radiation patterns, we can see the improved substrate loss from proton implantation. For the wireless inter-chip transmission, the same trend is observed.