超薄氧化層元件中直接穿隧效應所引發之氧化層可靠性問題探討

Abstract

當製程推進到奈米(sub-100nm)元件世代時，超薄氧化層仍然在元件設計上扮演重要的角色。然而，直接穿隧效應所引發之氧化層可靠性問題仍然是奈米尺度元件設計的一大挑戰。 本篇論文將針對超薄氧化層的可靠性問題做一系列的探討。首先，吾人發現了正偏基極操作模式下所增強的熱載子衰退效應。此種衰退現象不能藉由傳統的熱載子理論解釋，吾人成功地利用一套以歐傑再復合(Auger recombination)幫助電子獲得能量之機制來加以解釋。為了更進一步驗證，吾人量測了熱載子閘極電流注入與熱載子光激發之特性來證明當施予正基板偏壓時，通道內的電子能量將被增強。相較於傳統熱載子效應，這種由歐傑再復合所引起的衰退現象具有正的溫度效應，不僅影響元件高溫操作下的可靠性，更進一步地將限制正偏基極在類比電路上之應用。 接下來，吾人在超薄氧化層的價電子穿隧區中也發現熱載子衰退效應有增強的趨勢，此退化情形可歸因於價電子穿遂所產生之通道電洞。這些電洞與通道中所流經的電子發生歐傑再復合，進而增強熱電子的能量而導致破壞。吾人也將模擬通道中的歐傑復合速率，並以此速率來研究元件退化的趨勢；次外，吾人亦發現隨著元件操作電壓的降低，此熱載子退化變得更加顯著。在超薄氧化層n型元件中，吾人的實驗顯示此退化情形與基極電壓具有正相關的特性，此相關性與傳統熱載子加壓所造成之退化是完全相反的，對於某些正偏基極或基底浮接的先進元件來說，此現象將造成元件操作上嚴重的可靠性問題。 再者，直接穿隧效應也會對超薄氧化層的崩潰及元件之毀壞產生影響。一般來說，元件的毀壞與否是由氧化層崩潰所造成破壞程度所決定，代表破壞程度較低的氧化層漏電流對實際電路應用而言，並不會造成任何操作上的影響。吾人在此針對各種基極偏壓對氧化層崩潰的破壞程度做完整之研究。在p型超薄氧化層元件中，吾人發現了正偏基極操作模式下所加速之崩潰破壞。當氧化層初崩潰時，高能量的通道電洞在正偏基極時產生較大的電洞加壓電流，進而使得氧化層產生更大的破壞。藉由熱載子光激發實驗及熱電洞在通道能階上的分佈分析，吾人成功地解釋出此基極偏壓相依性。吾人並預測此種崩潰破壞將對一些正偏基極的先進元件產生新的可靠性問題。 最後，吾人提出在超薄氧化層p型元件中的一種雜訊退化模式，此雜訊退化是由氧化層崩潰時於崩潰點附近所增強之負偏壓溫度效應(NBTI)所致。吾人發現當氧化層崩潰時，高能量的電洞不僅惡化了氧化層的阻絕性，更加速了崩潰點附近的負偏壓溫度反應。對元件寬度相近於崩潰點之奈米元件來說，此反應所帶來的氧化層電荷缺陷破壞元件正常的操作效率，也惡化了元件輸出之低頻雜訊。吾人並進一步分析及探討此種雜訊退化模式在正偏基極的類比元件上之重要性。

The use of gate oxides in direct tunneling regime is required for sub-100nm CMOS devices. While, a great reliability concern induced by direct tunneling in such thin oxides is being aroused. The objective of this dissertation is to investigate direct tunneling caused reliability issues in ultra-thin oxide devices. First of all, enhanced hot carrier degradation in nMOSFETs with a forward substrate bias is observed. The enhanced degradation cannot be simply explained by conventional hot carrier theory. In this work, an Auger recombination assisted electron energy gain mechanism is proposed to explain this phenomenon. Characterization of hot electron gate injection current and hot carrier light emission is performed to confirm the proposed theory. The role of the Auger effect in terms of operating voltages is also identified. As opposed to conventional hot carrier degradation, this Auger enhanced degradation exhibits positive temperature dependence. This new degradation mode may bring about a major reliability concern in high temperature operation and imposes a limitation on the applied substrate bias. Based on above understanding, we found that Auger recombination also plays an important role when an ultra-thin oxide is stressed in the valence-band tunneling regime. In ultra-thin oxide nMOSFETs, holes created by valence-band tunneling provide for Auger recombination with channel electrons, thus increasing hot electron energy and hot carrier degradation. The degradation features are discussed, and the numerical calculation for the Auger recombination rate is performed. Since valence holes can be swept out by a reverse substrate bias, the degradation exhibits positive substrate bias dependence. Such dependence is particularly obvious at a reduced drain bias, where the Auger effect becomes dominant. The valence-band tunneling induced degradation may cause a severe reliability issue in forward biased substrate or floating substrate devices. Furthermore, a large direct tunneling current has been known to decrease oxide time-to-breakdown and limit oxide further scaling. Actually in most circuits, the failure criterion is determined by the hardness of oxide breakdown (BD). In this part, forward substrate bias enhanced breakdown progression in ultra-thin oxide pMOS is proposed. The enhanced progression is attributed to the increase of hole tunneling current resulting from breakdown induced channel carrier heating. The carrier temperature extracted from the spectral distribution of hot carrier luminescence is around 1300K. The substrate bias dependence of post-breakdown hole tunneling current is confirmed through the calculation of channel hole distribution in sub-bands. This observed phenomenon is significant to ultra-thin gate oxide reliability in floating substrate and forward-biased substrate devices. Finally, a new flicker noise degradation mode in pMOSFETs caused by oxide breakdown is proposed. Breakdown (BD) induced channel carrier heating not only accelerates the BD progression, but also enhances the reaction for negative bias temperature instability (NBTI) in the local BD spot. The flicker noise degradation can be explained by the non-uniform threshold voltage distribution resulting from NBTI created positive and localized oxide charges. Substrate bias effect on the NBT degradation rate is also evaluated for analog applications.