三族氮化物半導體材料因為優秀的導熱性及電性成為未來高頻率電子元件相當重 要的材料，其中氮化鋁(AlN)是三族氮化物材料當中晶格常數最接近碳化矽且能隙最大 的材料，這使得氮化鋁不僅可以做為基板與三族氮化物薄膜間的緩衝層，同時也是許多 半導體元件重要的材料。由於 AlN 的製程過程會產生大量奈米微粒，進而影響薄膜品 質，為了提升三族氮化物半導體元件的效能，利用脈衝法降低製程中的奈米微粒以及控 制薄膜成核方向是未來此類電子元件發展重要的一環。相較於連續金屬有機化學氣相沉 積(MOCVD)，脈衝式 MOCVD 製程中脈衝的開啟與關閉會導致腔體內的傳輸現象變得 相當複雜，難以找到最佳的製程條件。 本研究參考流場較穩定的單通道水平式腔體，建立包含流體力學、熱傳、質傳以及 化學反應的數值模型，藉此觀察脈衝式金屬有機化學氣相沉積製程中的傳輸現象，並分 析不同脈衝持續時間對傳輸現象及薄膜沉積的影響。結果顯示腔體中的速度邊界層會使 III 或 V pulse 的邊界凸向下游，而 III pulse 的凸出率大於 V pulse 是因為 TMAl 的擴散 速度較慢、速度邊界層內外的質傳速率差異較大所致，這是導致不同時間進入腔體的前 驅物依然會互相混合形成奈米微粒的主要原因。而載盤表面 TMAl 及 NH3 的質量分率會 在 III pulse 及 V pulse 開啟 0.15 秒後達到穩定，因此對於脈衝長度超過 0.15 秒之 III pulse 及 V pulse，持續時間改變對腔體中的傳輸現象影響不大，但腔體中 TMAl 及 NH3 的濃 度隨著 H2 pulse 持續時間增加而緩慢下降，這使得 H2 pulse 的持續時間對減緩微粒產生 效果顯著。 最後，在抑制奈米微粒的優勢與拉長製程時間的缺點相互競爭之下，為了找到最佳 脈衝持續時間取得最高沉積速率，在固定 TMAl 用量的情況下，首先參考低 V/III 比的 製程可以有效找出不同脈衝時間下的最高沉積速率，最後配合 V/III 比增加對沉積速率 的負面效應，便可預測不同 V/III 比下的最高沉積速率與相對應的脈衝持續時間。 ;Because of high thermal conductivity and outstanding electric properties, III nitride semiconductor materials become one of the most important materials for the application of the high-frequency device and deep ultraviolet LED. Among all III-nitride materials, AlN has the highest bandgap. It makes the AlN can be applied not only as an extraordinary buffer layer between group III nitride film and substrate but also as the active layer of UV LED and a barrier layer of HEMT. To improve the performance of III-nitride devices and reduce the cost of production, controlling the generation of nano-particle and the direction of the nucleation is crucial in the semiconductor industry. Compared to the traditional continuous MOCVD process, the transport phenomenon is more complicated in pulsed MOCVD because of the sequence of different pulses during the process. A numerical model is built to observe the relationship between the transport phenomenon, chemical reaction, and deposition rate. The result shows that a slower mass transportation rate in the velocity boundary layer near the wall or substrate can make the shape of III or V pulse convex downstream. Moreover, the convexity of the interface of the III pulse increases faster than that of the V pulse because of the lower mass diffusion coefficient of TMAl. On the other hand, the mass fraction of TMAl or NH3 becomes stable 0.15 s after the III pulse or V pulse starts. But it takes massive time to make the mass fraction of TMAl and NH3 near the substrate back to zero because the mass diffusion rate is slow. The pulsed time for the H2 pulse can affect the chemical reaction between the TMAl and NH3 more than the pulsed time of the III or V pulse. By increasing the pulsed time of the H2 pulse, the generation of AlN nano-particle can be suppressed and the usage of the TMAl source can be improved. However, it also makes the process time increase. A method to find the best compromise between suppressing the generation of AlN and the shortening process is proposed. The method to estimate the maximum deposition rate for different V/III ratios is also presented.
