應用鈦/氮化鎢/鈦/銅多層金屬以供砷化鎵低噪音假晶高電子遷移率電晶體中銅空氣橋的薄金屬系統之研究

Abstract

本實驗使用新的多層結構鈦/氮化鎢/鈦/銅做為薄金屬系統以供砷化鎵元件的銅金屬空氣橋之研究。其主要目的在於為了減少砷化鎵元件的生產成本與提供內導線較佳的電性，而找出一個具有良好黏著性可靠的阻障層結構以應用於砷化鎵元件的銅金屬化導線中。 在本實驗中，空氣橋製作上使用氮化鎢作為銅導線高電子遷移率電晶體之擴散障礙層，這是由於氮化鎢和傳統製程上有高度的相容性。在此鈦/氮化鎢/鈦/銅結構中，使用濺鍍氮化鎢作為擴散障礙層並使用鈦金屬作為附著層以避免在金與氮化鎢及銅與氮化鎢之間產生剝落問題。並且在空氣橋導線上鍍上一層氮化矽膜以防止銅表面氧化。接著，試片放入空氣中高溫退火以研究元件的熱穩定性。我們也完成了最佳選擇性蝕刻製程應用於銅空氣橋製造中的鈦/氮化鎢/鈦/銅多層結構薄金屬系統中。 本實驗成功發展了使用鈦/氮化鎢/鈦/銅多層薄金屬系統製作的銅空氣橋導線之砷化鎵低噪音假晶高電子遷移率電晶體。其元件電性優於僅使用氮化鎢/銅為薄金屬結構的元件，並可與傳統金導線砷化鎵元件媲美。 製作出來的低噪音砷化鎵假晶高電子遷移率電晶體在操作頻率為一萬七千兆赫時，元件雜訊指數最低可達零點九六分貝、相關增益值可達十點六八分貝在Vds=1.5 伏特和 Vg = -0.5伏特偏壓下。經過攝氏兩百度的高溫退火三個小時以後，銅金屬以及下層的接觸金屬間並沒有金屬原子交互擴散的現象；元件特性在經過熱退火以後並未產生明顯的變化。並且等效電路模型顯示出鈦黏著層減低了元件的源極阻抗。 最大的獲益在於鈦/氮化鎢/鈦/銅多層結構提高了元件在製程中的良率；主要是因為鈦層提供了良好的黏著性，並解決了銅空氣橋導線在砷化鎵PHEMT元件中剝落的問題。

In this work, a new multilayer Ti/WNx/Ti/Cu thin metal system was used for the study of copper-airbridged GaAs devices. The attempt is to find a reliable diffusion barrier structure with good adhesion for copper metallization of GaAs devices in order to reduce the production cost of the GaAs devices and to provide better thermal and electrical performances. For airbridge fabrication, tungsten nitride was chosen as the diffusion barrier for copper-airbridged pseudomorphic high electron mobility transistor (PHEMT) due to its compatibility with the conventional airbridge process. In Ti/WNx/Ti/Cu structure, sputtered WNx was used as the diffusion barrier and Ti was used as the adhesion layer to avoid the peeling problem between Au/WNx and Cu/WNx. And silicon nitride film was deposited on the copper airbridges to prevent copper surface from oxidation. Then, the samples were annealed at high temperature to investigate the thermal stability of the copper metallized devices. We also developed the optimal selective etching processes for etching the thin metal system of Ti/WNx/Ti/Cu multilayer scheme in the copper airbridge fabrication. A gallium arsenate (GaAs) PHEMT with copper airbridge using of Ti/WNx/Ti/Cu multilayer as the thin metal system was successfully developed and the electrical performance of these devices were better than those only using WNx/Cu scheme and are comparable to those with the conventional gold airbridges. When tested at 17 GHz, the fabricated low noise GaAs PHEMT had the lowest noise figure of 0.96 dB and the associated gain was 10.68dB under Vds=1.5 V and Vg = -0.5 V. After thermal annealing at 2000C for 3 hours there’s no atomic interdiffusion between copper and underlying contact metals and the devices showed little change in device performance. The equivalent circuit modeling shows the Ti adhesion layer decreases the source resistance of the devices. And the most benefit is that the Ti/WNx/Ti/Cu scheme improves the yield of the devices because the Ti layer provides good adhesion and solves the copper airbridge peeling problem in GaAs PHEMTs.