以離子植入選擇性增加碳化矽氧化速率暨溝槽式接面位障蕭基二極體之研究

Abstract

碳化矽材料是屬於寬能隙材料，它具有高崩潰電場以及高導熱係數的特性，因此非常適合用於高溫高壓的功率元件上，而碳化矽中應用最廣泛的功率元件為蕭基位障二極體、PN接面二極體、接面位障蕭基二極體。為了改良接面位障蕭基二極體的特性，出現了許多不同的結構變化，其中溝槽式接面位障蕭基二極體是碳化矽中最新的功率元件結構。 本篇論文提出新穎的溝槽式接面位障蕭基二極體，同時具有溝槽頂部與溝槽側壁的蕭基接面。由於二者的晶面並不相同，為了區別出兩者的蕭基位障高度，我們提出在溝槽底部成長厚氧化層的方式。但4H-SiC的氧化速率與晶面有很大的關係，因此我們發展出利用離子植入並搭配高溫濕氧化製程藉以得到局部厚氧化層的技術。 從過去的文獻，得知氧化速率的提升主要是被離子植入的SiC區域轉變為非晶態。因此我們分別用Ar與As離子植入，搭配相同的植入能量與劑量，再進行高溫濕氧化而得到厚氧化層。利用此技術成功製出單純側壁的蕭基二極體元件。藉由不同的溝槽角度，得到不同晶面的蕭基位障二極體。實驗結果顯示同一族對稱晶面有相同I-V特性，而蕭基位障高度與晶面的關係為(1 2 ̅1 0)>(1 1 ̅0 0 )>(0 0 0 1)。 在溝槽式接面位障蕭基二極體的實驗，利用Sentaurus TCAD模擬溝槽側壁為蕭基接面的TJBS結構，得出理想下溝槽深度越深，導通電流密度越大的結果。但實驗結果顯示，溝槽越深，導通電流密度卻越小。主要原因為側壁晶面的蕭基位障較溝槽頂部高，因此正偏操作下，溝槽側壁並不提供電流。另外加深溝槽將提升電流導通路徑的電阻(Rch)，造成特徵導通電阻上升。從實驗了解在相同的P+間距下，當溝槽深度等比例增加時，特徵導通電阻的變化量將相同。而在逆偏壓結果中，TJBS結構能得到比JBS低數十倍的漏電流密度，且溝槽深度越深，漏電流密度會越接近PN二極體的漏電流密度。 總體而言，離子植入可加速碳化矽氧化並得到厚氧化層，利用此技術可製出單純側壁蕭基二極體。藉由不同溝槽角度，可得不同晶面的蕭基二極體。在溝槽式接面位障蕭基二極體研究中，由於溝槽側壁蕭基位障較溝槽頂部高，因此正偏操作下側壁蕭基接面並不提供電流，但在逆偏壓下卻有更低的漏電流密度，因此溝槽式接面位障蕭基二極體應適用於低蕭基位障的金屬，而得到低導通電壓與低漏電流密度特性。如果要進一步降低此二極體的特徵導通電阻，必須開發出使側壁蕭基位障高度等於或低於頂部蕭基位障高度的技術。

Silicon Carbide (SiC) is a wide bandgap semiconductor. Due to its high critical electric field and high thermal conductivity, it is suitable for fabricating high power devices especially those devices operating under high voltage and high temperature conditions. SiC power devices such as Schottky Barrier Diode (SBD), PN Junction Diode, Junction Barrier Schottky Diode (JBSD) have been widely accepted in the commercial market. In order to improve the performance of Junction barrier Schottky diode, many different device structures have been proposed. In recent year the newest device structure is the Trench Junction Barrier Schottky Diode (TJBSD). In this thesis, we propose a novel TJBSD which has both top and trench sidewall Schottky junction. In order to distinguish the characteristics of the top and trench sidewall Schottky barrier diode, we propose to form a thick oxide layer at the trench bottom. Since the oxidation rate of 4H-SiC depends on crystal orientation strongly and the (0001) orientation has the lowest oxidation rate, we develop a process technology of using ion implantation and high temperature wet oxidation to selectively grow thick oxide layer at trench bottom. It is known that high dose ion implant can change SiC from crystal to amorphous phase and then enhances SiC oxidation rate. In this thesis, we used Ar and As ion implantation with the same energy and dose to form a thick amorphous layer. Then, wet oxidation was performed to grow thick oxide layer. Using this method, we can fabricate sidewall Schottky diode with different orientation. It is found that the Schottky barrier height of different crystal orientation form high to low is (1 2 ̅1 0)>(1 1 ̅0 0 )>(0 0 0 1). In the TJBS diode, we use the Sentaurus TCAD tool to simulate the TJBS characteristics. When the trench depth becomes deeper, the forward bias current density becomes higher. However, the experiment results show different trend. The reason is that the Schottky barrier height at trench sidewall is higher than that top surface. Therefore, the sidewall Schottky junction does not provide current under forward bias. In this case, the deep trench will increase channel resistance (Rch) and the specific on resistance. In reverse bias, our TJBS diode has much lower reverse leakage current density than the SBD and JBS diode. We also find that as the trench depth becomes deeper and the P+ junction spacing becomes narrower, the reverse leakage current density will be close to that of the PN diode. In conclusion, using ion implant can obtain thick oxide layer locally so that the trench sidewall Schottky diode can be realized. Then we can fabricate trench sidewall Schottky diode with different crystal orientation by pattering trenches with different directions. The TJBS diode experimental results show that the sidewall Schottky junction does not provide current at forward bias condition because the Schottky barrier height at trench sidewall is higher than that at top surface. On the other hand, TJBS diode has much lower reverse leakage current density than SBD and JBSD. To further improve the TJBS diode performance, technologies to make the Schottky barrier at the trench sidewall is equal to or lower than that at top surface should be developed.