氮化鎵功率元件之電性可靠度與壽命評估研究

Abstract

AlGaN/GaN功率元件，相較於傳統以矽為主體的功率元件，具有高的電子遷移率、高崩潰電壓的特徵，適合操作在高功率、高頻率下，是未來半導體的趨勢之一。但因其製程上的缺陷與操作時的失效機制，必須探討其可靠度問題，並進一步快速有效地提升與改善元件性能。若此元件的緩衝層(buffer layer)絕緣性不佳，部分電子可經由緩衝層流至汲極(Drain)，其電流不受閘極(Gate)電壓所控制視為元件漏電流(Leakage current)。因此，緩衝層的品質好壞、絕緣特性便會影響元件的截止特性。由文獻得知GaN在操作過程中會有閘極漏電流，降低功率效益。 為了減少漏電流，本研究之元件在閘極與AlGaN層之間加入絕緣層(Si3N4)形成MIS-HEMT(Metal-Insulator-Semiconductor High Electron Mobility Transistor)結構。為了探討失效與結構性的退化，本研究定義關鍵電壓(Critical voltage, VC)為發生顯著電性退化時的電壓值。在探針量測設備下直接量測晶圓，利用Step-stress加速性實驗量測電性參數，並搭配文獻紀載之事實，定義其失效之可能性與原因。再藉由Dynamic-RON研究，探討元件在開關條件下的暫態失效情況為何。而功率元件之使用壽命也是值得探討的議題，其壽命預估與失效時間之判斷，將利用TDDB(time-dependent dielectric breakdown)進行可靠度量測，利用統計學之方法，對量測資料進行分析，運用韋伯圖(Weibull Plot)以及曲線擬合(Curve fitting)等後處理動作，可得到預期使用20年之安全電壓範圍。

AlGaN/GaN power transistor, compared to conventional silicon-based transistors, have characteristics of high electron mobility, high breakdown voltage, suitable for operating at high power and high frequency conditions. Due to the defects in the manufacturing process and the failure mechanism while the operation, it is necessary to discuss the reliability to improve the performance of the device effectively. If the buffer layer of the device is poorly insulated, some of the electrons can flow through the buffer layer to the Drain, the current which is not controlled by the Gate voltage is regarded as the leakage current. Therefore, the quality of buffer layer and the property of insulation would affect the cut-off characteristic of the device. It is known from the paper that GaN device will have Gate leakage current during operation and it would lead to the reduction of the efficiency. In order to reduce the leakage current, the device of this study insert the insulator (Si3N4) between the Gate and AlGaN layer to form MIS-HEMT (Metal-Insulator-Semiconductor High Electron Mobility Transistor) structure. In order to discuss the failure mechanism and structural degradation, this study defines the Critical Voltage as the voltage while the occurrence of significant electrical degradation. The 4-inch wafer was measured directly by probe station, and the electrical parameter were measured by Step-stress method, and the possibility and the reasons of failure mechanism will be defined. Additionally, the failure mechanism of Deep-Level-Transient of Dynamic-RON have been proposed by switch mode. The lifetime of the power transistor is also worth discussing, the lifetime evaluation and the choise of failure time, will use TDDB (time-dependent dielectric breakdown) method for the final step of the reliability experiment. Using the methods of statistics to analyze the data. The use of Weibull plot, curve fitting and other post-processing, can find the safely operating voltage for twenty years.