Long nanoscale gaps on III–V substrates by electron beam lithography

Abstract

Deep submicron gaps between metal pads of around 100 μm in size are difficult to fabricate on III-V substrates using electron beam lithography, polymethyl methacrylate (PMMA) resist, and metal lift-off. In device fabrication, gold is commonly used to form metal contacts. However, gold etch methods are not well suited to high resolution patterning and lift-off methods are frequently employed. In this paper, the authors investigate a number of different methods for realizing long 100 nm scale gaps for structures fabricated on III–V substrates. Initially, they explain why the fabrication of long narrow gaps between metal pads using PMMA resist and metal lift-off is difficult. The authors show that 15 nm gaps can be fabricated using a double patterning method and discuss the limitations of this technique. They show that good undercut profiles for metal lift-off can be realized by the controlled etching of a sacrificial polymer layer placed beneath the resist. The authors demonstrate that reliable 100 nm scale gaps can be fabricated between 100 μm square metal pads using these methods.

Terahertz oscillations in an In0.53Ga0.47As submicron planar Gunn diode

“…The fabrication technique for a submicron separation of anode-cathode contact with several tens of micron width is difficult to achieve. There are many reports of submicron field-effect transistors [23][24], however, all nanometer gate field-effect transistors have drain-source separation larger than one micron while sub-micron drain-source separation is achieved using a T-gate shadow. In planar Gunn diodes there is no gate electrode to create a shadow mask for submicron anode-cathode separation; therefore, a new fabrication technique has been developed using two layers of electron beam resist separated by Si3N4 to achieve submicron separation for up to 120μm wide anode and cathode contacts.…”

“…In this paper, a new fabrication process similar to one used for making field effect transistors [23] is presented to realise the first submicron planar Gunn diode using an epitaxial layer of In0.53Ga0.47As on a semi-insulating InP substrate. The measured spectrum show an oscillation at 307GHz.…”

Investigation of High-Frequency Fine Structure in the Current Output of Shaped Contact Planar Gunn Diodes

“…The silicon nitride layer is not etched by the plasma and so enables the PMGI layer to be etched back by a controlled amount to give profiles optimal for metal liftoff. We have demonstrated the fabrication of planar Gunn diodes using this technique [25]- [28] down to 100-nm channel gaps though we were only able to achieve 600-nm channel gap planar Gunn diodes which oscillated. Therefore, we have demonstrable techniques using a combination of several processes to achieve 100-nm channels, which will offer the possibility of a low cost and fully integrated THz source.…”

“…One other possibility could be deposit the gold and then etch small gaps, but it is relatively difficult to etch high-resolution gold patterns, and because of this, small gaps between metal contacts are difficult to fabricate using the conventional subtractive processing flow of metal deposition followed by resist patterning and metal etch. Thoms and Macintyre [25] previously showed that 100-nm gaps between 100-μm square metal contacts can be realized using a trilayer of polydimethylglutarimide (PMGI) silicon nitride and UVIII resist. The UVIII resist layer is patterned by electron beam lithography.…”

Fabrication of submicron planar Gunn diode

“…Unfortunately, it is relatively difficult to etch high resolution gold patterns, and because of this, small gaps between metal contacts are difficult to fabricate using the conventional subtractive processing flow of metal deposition followed by resist patterning and metal etch. Thoms et al [1] previously showed that 100 nm gaps between 100 µm square metal contacts can be realized using a tri-layer of polydimethylglutarimide (PMGI) silicon nitride and UVIII. The UVIII resist layer is patterned by electron beam lithography.…”