HfO2高介電材料在動態與靜態應力作用下可靠度之分析

在本論文中，我們研究二氧化蛤 (HfO2) 薄膜在靜態與動態應力下之可靠度之行為。研究的主題有二，為: (1)表面前處理的影響；(2)閘極電極面積的影響。首先，我們使用原子層沉積 (ALD) 沉積不同厚度和不同表面前處理的HfO2薄膜來量測金氧半電容 (MOSC) 的電性及可靠度。實驗結果指出有臭氧 (O3) 前處理的HfO2薄膜在漏電流、平帶電壓、遲滯及電荷捕捉上有最佳的性質。另一方面，沒有RCA標準清洗流程的HfO2薄膜則呈現最差的電性特性。在可靠度方面，不論薄膜沉積的厚度，相較於其它表面前處理的方式，有O3前處理的HfO2薄膜有較好的崩潰電場和失效時間，這是由於在施加應力中有較低的介面陷阱增生率。此外，HfO2薄膜電容在動態應力下其失效時間改善度也隨前處理而有所不同。有O3前處理的HfO2電容在動態應力下其元件壽命改善度最高，可達6.65倍，而沒有RCA標準清洗流程的HfO2電容其元件壽命其改善度則為5.97倍。此差異性是由於在施加動態應力的過程中，經過O3前處理的HfO2電容其薄膜內電荷累積量較少。其次，我們比較不同電極面積的HfO2薄膜電容其電性及可靠度。實驗結果指出，較大面積的HfO2薄膜電容在施加應力時產生較多的電荷量和陷阱，導致較差的電性表現。因而，較大面積的HfO2薄膜電容則在動態應力下則呈現較佳的電性改善度，主要由於較為有效的電荷脫阱 (charge detrapping) 效應。相較於靜態應力而言，大面積的HfO2電容器在動態應力下能使漏電流改善2.14倍，且隨著閘極面積的縮小，此改善度會縮減。在可靠度方面，當HfO2電容的面積縮小時，其軟崩潰的現象就隨之增加且導致較長的元件壽命，這是由於軟崩潰代表在閘極介電材料裡有電荷掉阱-脫井 (trapping-detrapping) 現象的發生。同時，在動態應力下，較高的頻率和較小的工作週期 (duty cycle) 可得到較長的壽命提升度。不論靜態或動態應力，不同面積的HfO2電容其崩潰分佈皆可被合併成一條單一的韋伯分佈圖，表示在這兩種電應力下介電材料為本質的崩潰。最後，在動態應力其相反極性的偏壓上，增加施加應力的時間和電壓大小皆可增加介電材料的失效時間。我們也同時觀察到相反極性應力施加的時間在10-3秒以上時，其介電材料的失效時間才會增加，表示此為增強charge detrapping的一個關鍵時間。

關鍵字

原子層沉積；高介電質材料；二氧化鉿；表面前處理；動態應力；可靠度

並列摘要

In this thesis, we studied the reliability of HfO2 film under static stress and dynamic stress. The research topics are: (1) Effects of the surface pre-treatment； (2) Effects of the gate-electrode area. First, we used atomic layer deposition (ALD) HfO2 film with different deposition thicknesses and various surface pre-treatments to measure the electrical properties and reliability of the metal-oxide-semiconductor capacitor (MOSC). Experimental results indicate that HfO2 film with ozone (O3) pre-treatment has best performance in the leakage current, flat-band voltage, hysteresis, and charge trapping. On the other hand, the HfO2 films without RCA standard clean present the worst electrical characteristics. For the reliability performance, irrespective of the deposition thickness, the HfO2 film with O3 pre-treatment has better breakdown field and lifetime due to a lower generation of interface traps during stress as compared with other surface pre-treatments. In addition, the improvement in lifetime of the HfO2 capacitors with various pre-treatments under dynamic stress is different. The HfO2 capacitor with O3 pre-treatment under dynamic stress has the best improvement of lifetime. It can improve the lifetime by 6.65 times. While the HfO2 capacitor without RCA standard clean only improve the lifetime by 5.97 times. This difference is due to the less accumulated charges of the HfO2 capacitors with O3 pre-treatment during dynamic stress. Second, we compared the electrical properties and reliability of HfO2 capacitors with different gate-electrode areas. Experimental results indicate that the HfO2 capacitor with a larger area has more charge and traps during stress and results in the worst electrical performance. However, the HfO2 capacitor with a larger area under dynamic stress behaves the better electrical improvement because the effect of charge detrapping becomes effective. Compared to static stress, the HfO2 capacitor with a larger area under dynamic stress can improve the leakage current by 2.14 times and this improvement is reduced as the gate area decreases. For the reliability performance, as the area of HfO2 capacitor decreases, the phenomenon of soft breakdown is increased and the breakdown time increases. This is due to the soft breakdown represents the charge trapping-detrapping in the gate dielectrics. Additionally, a higher frequency and lower duty cycle in the dynamic stress resulted in a longer lifetime enhancement. Irrespective of the static and dynamic stress, the breakdown distributions of HfO2 capacitors with various areas can be merged to a single Weilbull plot, suggesting that the dielectric breakdown is intrinsic for both cases. Additionally, increasing the stress time and voltage of the opposite polarity in the dynamic stress enhanced the dielectric breakdown lifetimes. We also found that an increase in the dielectric breakdown time is observed as the stress at the opposite polarity above 10-3 s, indicating that there is a critical time for enhancement in charge detrapping.