超薄電漿氮化氧化層與高介電常數氧化鉿於閘極介電層之研究

Abstract

在以超薄電漿氮化氧化層作為閘極介電層的N型金氧半場效電晶體中，由通道熱電子與基板熱電子注入而引起的元件劣化效應是本論文研究的主題之一。相較於一般傳統使用的熱氧化層，可以發現超薄氮化閘極介電層比較容易受到通道熱電子與基板熱電子效應的影響，導致更大的臨界電壓偏移與轉移電導值降低。劣化增加的嚴重性會隨著在閘極介電層因氮化時間增長所致的氮元素含量增加而增加。儘管對於傳統的熱氧化層或是電漿氮化氧化層二者而言，由基板熱電子所引起的劣化是與注入電子的能量強烈相關，但是氮化氧化層表現在臨界電壓偏移的明顯劣化情況卻是發生於較熱氧化層為低的基板偏壓值。在以電漿氮化氧化層作為閘極介電層的N型金氧半場效電晶體中，這種經由基板負偏壓致使的劣化增加現象，可歸因於在電漿氮化製程程序中導入的一個較高濃度的順磁性電子陷阱先驅物。另一方面，在以超薄電漿氮化氧化層作為閘極介電層的P型金氧半場效電晶體元件中，也可以發現類似的劣化趨勢。在通道熱電子壓迫測試之後，可以發現更嚴重的臨界電壓偏移與轉導值降低現象。不過，氮化氧化層作為閘極介電層的P型金氧半場效電晶體，會受到相較於傳統熱氧化層為大的負偏壓溫度不穩定效應。這種由於受偏壓與溫度壓迫測試導致的不穩定現象，在N型金氧半場效電晶體元件中是不顯著的。 本論文探討的另一個主題是氧化鉿，一個作為金氧半場效電晶體閘極介電層的最佳高介電常數材料。在金絕半電容器的製作中，採用原子汽相沉積氧化鉿介電層，搭配以濺鍍方式沉積銅或鋁金屬作為閘極所組成的。為了便於比較，同時也製作了以二氧化矽介電層所組成的金絕半電容器對照組。利用偏壓溫度壓迫測試與崩潰電荷測試方法，可以對電容器的穩定性與可靠性進行檢驗。相對於在二氧化矽介電層中的高漂移速率，銅金屬在氧化鉿介電層的組成中顯得相當穩定。銅金屬閘極的氧化鉿電容器也具有比鋁金屬閘極較高的電容值，而且沒有可靠性降低的問題。此實驗結果顯示具有高密度9.68 g/cm3的氧化鉿是一個很好的阻擋銅金屬擴散的阻障層。而這也意味著銅金屬在積體電路閘極介電層後續製程整合的可行性。 氧化鉿高介電常數材料的另一個重要應用是作為金絕金電容器的絕緣層。利用一標準的後段金屬層作為下電極，金絕金電容器已經成為微處理器、高頻電路與混合信號積體電路之一關鍵被動元件。為了增加電路密度與減少晶胞面積與成本，高電容密度是金絕金電容器一項很重要的考量因素。因此採用高介電常數材料像是氧化鉿，就是增加電容密度的一個很有效的方式。在金絕金電容器的實驗結果可以得到約5*10^9 A/cm2的低漏電流密度與約3.4 fF/µm2的高電容密度，達到很小的溫度係數與頻散效應。一些不同的金屬電極像是鉭、鋁、銅等也加以比較。最後氧化鉿金絕金電容器的電性傳導機制可被推導出來並歸類為Frenkel-Poole形式。

The degradation induced by channel hot electron (CHE) and substrate hot electron (SHE) injection in nMOSFETs with ultrathin plasma nitrided gate dielectric was studied in this thesis. Compared to the conventional thermal oxide, the ultrathin nitrided gate dielectric is found to be more vulnerable to CHE and SHE stress, resulting in enhanced threshold voltage shift and transconductance reduction. The severity of the enhanced degradation increases with increasing nitrogen content in gate dielectric with prolonged nitridation time. While the SHE-induced degradation is found to strongly relate to the injected electron energy for both conventional oxide and plasma-nitrided oxide, dramatic degradation in threshold voltage shift for nitrided oxide is found to occur at a lower substrate bias magnitude, compared to thermal oxide. This enhanced degradation by negative substrate bias in nMOSFETs with plasma-nitrided gate dielectric is attributed to a higher concentration of paramagnetic electron trap precursors introduced during plasma nitridation. On the other hand, similar degradation trend was also found in the pMOSFET devices with ultrathin plasma nitrided gate dielectric. Enhanced threshold voltage shift and transconductance reduction were observed after CHE stress for the nitrided devices. Nevertheless, the pMOSFETs with nitrided gate dielectric suffer larger negative bias temperature instability (NBTI), comparing to that with conventional thermal oxide. Such instability owing to bias-temperature stressing is inconspicuous in nMOSFET devices. The other subject included in this thesis is HfO2, a promising high-k material in gate dielectric of MOSFETs. Metal-insulator-semiconductor (MIS) capacitors were fabricated using atomic vapor deposition (AVD) HfO2 dielectric with sputtered copper and aluminum gate electrodes. The counterparts with SiO2 dielectric were also fabricated for comparison. Bias-temperature stress (BTS) and charge-to-breakdown (QBD) test were conducted to examine the stability and reliability of these capacitors. In contrast with the high Cu drift rate in SiO2 dielectric, Cu in contact with HfO2 seems to be very stable. The HfO2 capacitors with Cu-gate also depict higher capacitance without showing any reliability degradation, compared to the Al-gate counterparts. These results indicate that HfO2 with its considerably high density of 9.68 g/cm3 is acting as a good barrier to Cu diffusion, and it thus appears feasible to integrate Cu metal with the post-gate-dielectric ULSI manufacturing processes. Another application for HfO2 high-k dielectrics is metal-insulator-metal (MIM) capacitors. MIM capacitors using one of the standard back-end metal layers as bottom electrode have emerged as key passive components for microprocessors, high frequency circuits, and mixed-signal integrated circuits applications. A high capacitance density is important for a MIM capacitor to increase the circuit density and reduce the cell area and cost. Therefore, adoption of high-k material like HfO2 is a very efficient way to increase the capacitance density. Experimental results show low leakage current densities of ~5*10^9 A/cm2 and high capacitance density of ~3.4 fF/µm2 at 100 kHz in the MIM capacitors. The temperature coefficient and frequency dispersion effect for these MIM capacitors were very small. Different metal electrodes like tantalum, aluminum, and copper were also investigated and compared. Finally, the mechanism of electrical transport was extracted for the HfO2 MIM capacitors to be Frenkel-Poole type conduction mechanism.