High RF Performance GaAs Metamorphic HEMTs with 90 nm Sidewall Gate

Abstract

就高頻應用而言，為了增進射頻特性，閘極尺寸有越做越小的趨勢。本論文主要的目的是利用側壁堆積法來縮小線寬，利用氧化矽及氮化矽兩種不同介電材料間的連續堆積及蝕刻，製作出奈米尺寸的線寬。另一方面，由於氮化矽與氧化矽之間的高選擇比，讓製程有較佳的再現性及均勻性。因此，我們可以發展出一個易於控制的小線寬製程技術。

在此次的研究中，側壁堆疊閘極法搭配適當的閘極蝕刻深度可以得到汲極飽和電流密度為620 mA/mm，而電流在1.5 V的汲極偏壓下最大的互導係數可達930 mS/mm。元件的電流增益截止頻率以及最大震盪頻率分別為130 GHz 與 200 GHz。雜訊方面，160 μm的元件在16 GHz的頻率下，雜訊值為0.69 dB以及9.76 dB的增益值。此外，在功率特性方面，160 μm的元件於2.4 GHz下，最大輸出功率為17.76 dBm (365mW/mm)，功率增益為57.7%，其線性增益為27.83 dB。因此，已證實九十奈米側壁閘極應用於砷化鎵變異結構高電子遷移率電晶體具有相當好的高射頻特性。

For high frequency applications, the requirement of short gate length is necessary to improve the RF performance of the devices. The main purpose of this dissertation is to shrink the gate length with sidewall gate process. By sequentially depositing and selectivity etching two different dielectric layers (SiNx and SiOx), the nano-scale T-shaped gate was successfully fabricated. This technology can provide high reproducibility and uniformity due to the high etching selectivity between SiNx and SiOx. Finally, we developed a well-controlled gate technology for the fabrication of GaAs Metamorphic HEMTs with gate length below 100 nm.

The fabricated 90nm MHEMTs with optimum gate recess width has a saturation drain current of 620 mA/mm and a maximum transconductance of 930 mS/mm measured at Vds=1.5 V. The device demonstrates a cutoff frequency fT of 130 GHz and a maximum frequency of oscillation fmax up to 200 GHz. The noise figure of a 160μm device is 0.69 dB and the associated gain is 9.76 dB at 16 GHz. The device also exhibits an output power of 17.67 dBm which is corresponding to a very high power density of 365 mW/mm. The maximum power-added efficiency is 57.7%, and the linear power gain is 27.83dB measured at 2.4 GHz. In conclusion, the 90nm sidewall gate process is successfully applied on the fabrication of the high RF performance In0.52Al0.48As/In0.52Ga0.48As metamorphic HEMTs with good reproducibity and uniformity.