電漿表面處理對n型氮化鎵歐姆接觸之影響

Abstract

本研究中，電感耦合電漿蝕刻是用來研究電漿表面處理對n型氮化鎵歐姆接觸的影響。研究工作分成兩個部分，首先探討乾蝕刻參數，包括偏壓大小、氯氣流量比例、氬氣流量比例，對於試片蝕刻速率、側壁蝕刻輪廓及表面型態的影響。結果顯示，大部分的試片有非常平滑的表面型態。越高的偏壓導致越高的蝕刻速率且對於試片側壁蝕刻輪廓的垂直性有加強的趨勢。另外，提高氬氣流量，也有助於側壁蝕刻輪廓的垂直性，但對於蝕刻速率並沒有明顯的變化。至於氯氣流量方面，流量越高，試片蝕刻速率也越快，且有傾斜的側壁蝕刻輪廓，這跟晶體方向有關。 然後，研究電漿表面處理對於n型氮化鎵歐姆接觸電性的影響。其中分為三種電漿氣體環境來討論，分別為Cl2氣與Ar氣混合環境、純Ar氣環境及純Cl2氣環境。我們發現試片在Cl2氣與Ar氣混合電漿及純Ar氣電漿處理後，接觸電阻有明顯的改善。對於n－-GaN試片，最低的接觸電阻分別為0.21和0.27Ω•mm。對於n＋-GaN試片，有更低的接觸電組約0.1Ω•mm左右。 除了室溫(25℃)量測外，我們將處理過後的歐姆接觸在高溫(100、200、300℃)下作電性的研究，觀察接觸電阻與溫度的關係。在高溫300℃下，n－-GaN試片，處理條件為Cl2/Ar=50/20sccm，蝕刻時間為1分鐘的試片仍然有非常低的接觸電阻(0.412 Ω•mm)與特性接觸電阻(9.3×10-7 Ω•cm2)，而且比未做電漿表面處理的試片在室溫下量測到電阻還要小。我們也在溫度400到600℃氮氣與空氣環境下，進行老化測試實驗，測試其熱穩定性的好壞。整體來看，在600℃、2個小時後的老化過程，並沒有明顯看到接觸電阻電性退化的現象。這說明了經過電漿處理後的歐姆接觸，其熱穩定性並不會受到試片上損害恢復的影響。

In this study, inductively coupled plasma (ICP) etching was used to investigate the influence of plasma surface treatment on n-GaN ohmic contacts. The research work is divided into two parts. First, the influences of dry etch parameters, including bias power, Cl2 flow rate, and Ar flow rate, on etching rate, sidewall profile and surface morphology were studied. As a result, most samples have very smooth surface morphology. Larger bias powers would lead to higher etching rates and vertical sidewall profiles. Samples etched with higher Ar flow rate also showed vertical sidewall profile. But little difference in etching rate was found in this condition. Plasma with higher Cl2/Ar ratio would result in higher etching rate and sloped sidewall profile, which is related to the crystal orientation. Then, the effects of plasma surface treatment on the electrical characteristics of n-GaN ohmic contacts were extensively studied. Three types of plasmas including Cl2, Cl2/Ar and Ar plasmas were used for this experiment. We found that samples treated with Cl2/Ar and Ar plasma could have substantial improvement in contact resistance. Their lowest contact resistances for n--GaN are 0.21 and 0.27 W×mm, respectively. For n+-GaN samples, much lower contact resistances of around 0.1 W×mm were obtained. In addition to room temperature (25℃) measurement, the electrical characteristics of plasma-treated ohmic contacts at high temperature (100, 200 and 300℃) were studied. At 300 °C, n--GaN samples treated with Cl2/Ar=50/20 sccm still have very low contact resistance (0.412 W×mm) and specific contact resistivity (9.3x10-7 W×cm2), which are lower than those measured at room temperature of samples without plasma treatment. Aging test at temperatures ranging from 400 to 600℃ in N2 and air ambient were performed to examine the thermal stability of plasma-treated ohmic contacts. No obvious degradation of contact resistance was observed after aging at 600 °C for 2 hrs in N2 and air ambient. This means the thermal stability of plasma-treated ohmic contacts would not be influenced by the recovery of plasma-induced damages on the wafer surface.