電漿氣相輔助沉積氮化矽覆蓋層之形變N型金氧半場效電晶體之電流增強方法與可靠度之研究

Abstract

在本論文中，我們主要探討在N型金氧半場效電晶體中，利用電漿氣相輔助沉積(PECVD)氮化矽覆蓋層及其相關沉積製程參數對元件特性與熱載子退化效應的影響。我們在沉積氮化矽覆蓋層時，固定矽甲烷(SiH4)與氨(NH3)的流量，而調整氮(N2)流量，使所沉積的氮化矽覆蓋層舒張應力增加，來造成元件通道的形變，進而增進元件載子遷移率。在實驗中，我們也利用一不同特性的電漿氣相輔助沉積系統，來沉積具壓縮應力的氮化矽覆蓋層，並證實了元件在舒張的應力下可得到較大電子遷移率，而在壓縮應力下則呈現相反的效果。另外，我們也證實在電漿氣相輔助沉積氮化矽的過程中，因為所施與的熱預算(thermal budget)並不算大，因此並未引起多晶矽空乏現象（poly-depletion）。除此之外，我們也發現氮化矽覆蓋會使元件熱載子退化效應更嚴重，但可藉由增加氮(N2)流量使其退化效應下降。因此在以電漿氣相輔助沉積系統沉積氮化矽覆蓋層時，我們可以藉由增加氮(N2)流量使N型金氧半場效電晶體電性變好，而避免熱載子退化效應，達到雙贏的效果。

Recently locally strained devices have emerged as the main technique for carrier mobility enhancement (e.g., SiN-capped devices). In this thesis, we investigated the impact of silicon nitride (SiN) capping layer and the associated deposition process parameters on the device characteristics and hot-electron degradation of strained NMOSFETs. The SiN layer used to induce channel strain for mobility enhancement was deposited by plasma-enhanced chemical vapor deposition (PECVD) by varying the flow rate of N2, while fixing the flow rate of SiH4 and NH3, to adjust the tensile stress of SiN film. For comparison, we also fabricated devices with compressive SiN capping layer using another PECVD system. We confirmed that the electrical characteristics of devices with tensile SiN capping layer will be improved. In contrast, the compressive SiN capping layer will degrade the device performance. We also found that the deposition of the SiN does not worsen the poly-depletion effect due to the light thermal budget. Furthermore, although the device hot-electron degradation is aggravated by the SiN capping layer, the degradation can be alleviated by increasing the flow rate of N2. We thus conclude that the electrical performance and reliability can both be improved by increasing N2 flow rate for NMOSFET with tensile SiN capping.