具有氮化矽覆蓋之形變通道金氧半場效電晶體特性與相關可靠性問題研究

Abstract

在本論文中，我們針對氮化矽覆蓋層對元件特性與相關可靠性影響作一系列之研究，主要涵蓋內容包括具有薄緩衝層(buffer layer)之氮化矽覆蓋n型與p型通道金氧半場效電晶體之製作與特性分析；利用改變前趨物氣體流量與沉積溫度，來最佳化氮化矽薄膜；以及對具有氮化矽覆蓋之元件作漏電流機制分析與閃爍雜訊(flicker noise)之探討。此外，我們也對氮化矽覆蓋造成的能階窄化效應(bandgap narrowing effect)與熱載子測試(hot-carrier stress)後之界面缺陷橫向分佈及負偏壓溫度不穩定性(NBTI)之交流可靠性分析做詳細的探討。

我們發現，雖然氮化矽覆蓋能有效提升載子遷移率及驅動電流，但卻犧牲熱載子與負偏壓溫度不穩定之可靠性，其主要原因歸咎於沉積氮化矽的過程，使用含氫元素的反應氣體，如氨氣(NH3)、矽甲烷(SiH4)，使得大量的氫元素擴散進入通道區域，因而造成熱載子與負偏壓溫度不穩定性之劣化。為了消弭這項缺失，我們提出，利用在閘極與氮化矽覆蓋層間加入一層薄緩衝層來抑制氫的擴散，結果證實，熱載子與負偏壓溫度不穩定之可靠性均獲得顯著改善，而且不會犧牲因氮化矽覆蓋造成之元件電流提升。

因為氮化矽覆蓋層中的氫是劣化元件可靠性之主因，因此，接下來我們藉由改變前趨物氣體流量與沉積溫度，直接調整氮化矽薄膜的組成。從X光光電子能譜術(XPS)與傅立葉轉換紅外光譜儀(FTIR)及應力量測系統之分析結果，我們發現，增加氮氣流量會增加氮化矽薄膜之伸張應力與氮含量，這是有助於n型通道元件之載子遷移率提升，此外，增加氮氣流量與沉積溫度會消減氮化矽中的矽氫鍵結，因此，可提升熱載子或負偏壓溫度不穩定之可靠性。

接下來，雖然伸張應力能提升n型金氧半場效電晶體之特性，但卻造成元件關閉時漏電流增加。結果顯示，閘極引發汲極漏電(gate-induced drain leakage)為此漏電流增加之主要原因。伸張應力造成之能階窄化將增強能帶與能帶間的穿遂(band-to-band tunneling)，因此導致閘極引發汲極漏電增加。這也說明了為何伸張應力會增加元件之漏電流。

最後，我們探討氮化矽覆蓋元件對閃爍雜訊之影響。雖然在沉積氮化矽過程中增加氮氣流量能提升n型通道金氧半場效電晶體之特性與可靠性，但氮化矽中氫含量之減少將降低界面處缺陷與懸浮鍵(dangling bond)之修補，因為此缺陷為造成閃爍雜訊之主因，故導致閃爍雜訊劣化。

In this thesis, we have investigated the impacts of SiN capping layer on the device performance and the related reliability issues. This study includes the fabrication and characterization of SiN-capped n- and p-channel MOSFETs with a thin buffer layer over the gate, the optimization of SiN film by varying precursor gas flow rate and deposition temperature, and investigation of off-state leakage current mechanism and flicker noise characteristics on the SiN-capped devices. In addition, bandgap narrowing effect induced by SiN capping, lateral distribution of interface state after hot-carrier stress, and AC NBTI stress are also investigated.

We found that although the SiN capping can dramatically enhance the carrier mobility and thus the drive current, the robustness to hot-carrier and negative bias temperature instability (NBTI) degradation is compromised as well, owing to the large amount of hydrogen contained in the SiN layer by using the hydrogen-containing precursors, i.e., NH3 and SiH4, which may diffuse into the channel region during the SiN deposition process. To eliminate this shortcoming, the insertion of a thin buffer layer between the gate and the SiN capping layer was proposed to suppress the diffusion of hydrogen, and the result demonstrates that the hot-carrier and NBTI reliability of the SiN-capped devices can be restored without compromising the current enhancement by the SiN capping.

Since abundant hydrogen species contained in the SiN capping layer are the primary culprit for aggravated reliability, we have directly adjusted the composition of SiN film by varying precursor gas flow rate and deposition temperature. From the analysis of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometer (FTIR), and stress measurement system, we found that increasing N2 flow rate will increase tensile stress and nitrogen content in the SiN film, which is beneficial for mobility enhancement of n-channel devices. In addition, increase in N2 flow rate and deposition temperature tend to weaken the signal of Si-H bonds, which is helpful for the improvement of hot-carrier and NBTI reliability.

In addition, although tensile stress can boost NMOSFETs’ performance, we found that it also results in off-state leakage current increase. Our results indicate that gate-induced drain leakage (GIDL) current is the major reason for increased off-state leakage current. Bandgap narrowing induced by tensile stress will enhance the band-to-band tunneling process, resulting in GIDL current increase, which accounts for off-state leakage current increase by tensile stress.

Finally, we have also investigated the impacts of SiN-capped devices on the flicker noise characteristics. Although NMOSFETs’ performance and hot-carrier reliability can be improved by increasing N2 flow rate in the SiN deposition, the accompanying decrease of hydrogen content reduces the passivation of defects and dangling bonds near the interface, which is considered to be the main culprit for flicker noise, resulting in the degradation of flicker noise.