Fabrication of ultra-short T gates by a two-step electron beam lithography process
Introduction
T gates with short feet and broad heads are important for ultra high working frequency, low noise and low gate resistance in high electron mobility transistors (HEMTs) for micro/millimetre wavelength communication. T gate process by one-step electron beam lithography (EBL), i.e., exposing the foot and the head in one lithography step on a bi-layer resist stack, has achieved foot width as short as 25–30 nm [1], [2], which is limited by the beam spread due to the forward scattering of incident electrons in the top resist layer [3]. For the same reason, there is also a limitation to the resist thickness of the head layer, resulting in difficulty in lift off and achieving large head cross-section. In this paper, we propose a two-step electron beam lithography technique, which avoids forward scattering of the electron beam in the top layer and opens the prospect for even shorter T gates with vertically large head size.
Section snippets
The two-step EBL process
It is well known that the ultimate resolution in electron beam lithography is limited by the forward scattering of electron in resists for a given fine beam. In T gate patterning on a bi-layer resist stack by a one-step lithography process, there is a trade off between the foot width and the head layer thickness: on the one hand, the head layer resist has to be thick enough to assure a sufficiently deep head trench for a good lift off and enough head mass for low resistance; on the other hand,
The experimental results and discussions
Using the two-step lithography process, resist profiles of T shape have been achieved. Fig. 3 shows a SEM micrograph of a T shape profile in resists with foot width below 30 nm, patterned by the two-step lithography process. This process requires very accurate alignment. The VB6 used in this work has the capability to achieve alignment accuracy better than 25 nm, which is less than 1/10 of the head width. From the SEM micrograph in Fig. 3 it can be seen that the foot trench was aligned
Conclusions
A two-step EBL process has been proposed for fabricating ultra-short T gates. Both computer simulation and experiment have proved that the new process can pattern T gates with foot width shorter than that given by the traditional one-step lithography technique. Short foot width with deeper head trench becomes possible by this process since there is no limitation to the thickness of the top layer. The lower limit of the LOR thickness was found to be around 20 nm. The key to this process is the
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Nanofabrications of T shape gates for high electron mobility transistors in microwaves and THz waves, a review
2021, Micro and Nano EngineeringCitation Excerpt :So far, a big variety of processes have been developed for generating the above-mentioned T shape profiles in resists by EBL. Notably they are bilayer resist technique [35,116–121], trilayer process with a thin dielectric film in the middle [122–125], two step exposure techniques for replicating the head and the foot separately [121,126,127], and multiple step exposures combined with a dielectric film on the bottom [40,128–130], etc. This part will review these developed techniques for different T shape gates, trying to give brief comments on each technique in regards of the process reliability, real application prospects and the impacts on the HEMTs.
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2015, Microelectronic EngineeringCitation Excerpt :Metamorphic HEMTs had a gm of 1500 mS/mm and ft of 350 GHz, which were the fastest transistors of their kind in the world in 2003 [57]. Starting from PMMA/Al/UVIII process for T shape gates in 1999 [2,10], further advance was made to replace the thin Al layer by a 20 nm LOR, saving a vacuum evaporation process [57,58], foot-width was further reduced from 100 nm down to as short as 30 nm, as presented in Fig. 12. To further reduce the foot-width below 30 nm and enhance the mechanical reliability of the ultra short T shape gates, a 60 nm SiNx layer was added to the sandwiched layer structure to define the foot-width, sub-30 nm foot-width was achieved thanks to the tapering effect in the reactive ion etch on the thin SiNx film, which was one of the shortest T shape gates in early 2000s [59].
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