Technology for Reducing HEMT T-Gate Length via Formation of Silicon Nitride-Based Sidewall Dielectric Spacers for Mass Production of GaN MMICs

The paper presents an experimental validation of a technology for forming sidewall dielectric spacers based on silicon nitride to reduce the T-shaped gate length of GaN high electron mobility transistors for application in mass production of monolithic microwave integrated circuits. TCAD process simulations using the Synopsys environment showed that achieving a gate length of less than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.25 \mu ~\mathrm{m}$</tex> is possible when the thickness of the second <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Si}_{3} ~\mathrm{N}_{4}$</tex> layer is at least 200 nm and the initial mask window width is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.4 \mu ~\mathrm{m}$</tex>. Experimental results demonstrated that increasing the thickness of the second silicon nitride layer reduces the gate length, confirming the effectiveness of the sidewall spacer approach. However, excessive etching time can degrade the passivation layer and increase the resulting gate length. The minimum gate length achieved was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.35 \mu ~\mathrm{m}$</tex>. It was also found that increasing the thickness of the second <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Si}_{3} ~\mathrm{N}_{4}$</tex> layer decreases the sidewall angle of the mask, which may negatively impact parasitic capacitance and breakdown voltage. To implement this technology, projection photolithography with a resolution of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.5 \mu ~\mathrm{m}$</tex> is required, necessitating the use of a 240 nm wavelength stepper. The results confirm the feasibility of this approach for industrial application without the need for electron beam lithography.