Electron-beam/ultraviolet hybrid exposure combined with novel bilayer resist system for a 0.15 μm T-shaped gate fabrication process

Abstract

Electron-beam/ultraviolet (UV) exposure technology to produce undercut T-shaped resist cavities with bottom openings as small as 0.15 μm is demonstrated with a novel bilayer resist system for AlInAs/InGaAs high electron mobility transistors operated at the millimeter-wave band. We employed an image reversal resist (AZ5206E) for the top layer and a polydimethyl glutarimide (PMGI) for the bottom layer. The top layer is delineated by UV exposure and the bottom layer is delineated by electron-beam direct writing. These resist layers are developed layer by layer in different content aqueous tetramethyl ammonium hydroxide solution. Resist profiles are extremely well controlled because exposure and development of both layers are completely independent. A reliable overhang structure for metal liftoff, with a 0.15 μm footprint, was obtained. Gate length variation of less than ±10% on a 3-in.-diam InP substrate was successfully accomplished. In addition, an interesting phenomenon was noted. The combination of these two resists leads to an electron-beam sensitivity decrease of PMGI. This phenomenon is closely related to the change of molecular weight distribution in PMGI. The novel bilayer resist system also allows the further improvement of resolution. This would provide a practical means for electron-beam lithography in the nanometer region.

High resolution electron beam lithography of PMGI using solvent developers

“…From Fig. 1 and Table 1 we can see that: (1) Among the three developers, MIBK gives the highest contrast comparable to that of PMMA, followed by MEK, while cellosolve gives the lowest contrast which is similar to that reported using base developer [2]; (2) the sensitivity is less dependent on developer and baking temperature, and it mostly lies between 900 lC/cm 2 and 1100 lC/cm 2 which is about four times that of PMMA; (3) higher baking temperature leads to higher contrast for MEK and cellosolve, but for MIBK the optimum baking temperature is 200°C and both the contrast and the sensitivity drops (by 40% for sensitivity) at 250°C; and (4) a residual layer is usually left for MIBK, but negligible for the other two developers.…”

“…PMGI is more sensitive than PMMA but with a much lower contrast. As a matter of fact, PMGI (designed as a liftoff resist) is supposed to be dissolvable by base developers even without exposure to e-beam, though the dissolution rate drops drastically with the decrease of base concentration [2]. Therefore, PMGI (or its derivative LOR) is employed more often as a liftoff resist in a bi-layer resist system such as with PMMA [4].…”

“…PMGI displays several advantages over PMMA: resistant to general solvents, less susceptible to intermix with other polymers or resists when stacked [2], slightly more resistant to acids, slower dry etch rate (by 30% in CHF 3 plasma), and higher thermal stability with a glass transition temperature of 189°C (vs. 105°C for PMMA).…”

“…PMGI (polydimethyl glutarimide) has also shown to be sensitive to e-beam exposure [2,3] and 150 nm wide lines have been achieved by EBL using (low concentration) aqueous base developer such as TMAH. PMGI is more sensitive than PMMA but with a much lower contrast.…”

“…In addition, the opening is widest for the first PMMA layer because of forward scattering of the incident beam (back scattering is less important for isolated lines). Certainly, using a single step development, a double layer of PMMA and PMGI could be employed to facilitate the liftoff process by creating an undercut (PMGI on top), or to fabricate a T-shaped gate structure (PMMA on top) [2]. Alternatively, 3D metal nanostructures with a zigzag profile can be generated by electroplating into the patterns created in the resist stack (Fig.…”