Method for producing a field effect transistor

Claim 2

A process for producing a field effect transistor comprising the steps of depositing a bottom layer of a first photoresist on said wafer, depositing a top layer of a second photoresist on said bottom layer, exposing an image of said pattern on said top layer, developing said top layer to remove exposed portions thereof, exposing the bottom layer beneath the removed portions of said top layer under conditions undercutting the bottom layer beneath the edges of the remaining portions of said top layer, developing the bottom layer to remove the exposed portions thereof, and forming T-bar shaped remaining unexposed portions of overlying top and bottom layers, depositing metal on the upper surface of said T-bar shaped portions and on the exposed surface of said wafer to form a drain and a source, lifting off the metal on said T-bar shaped portions together with the underlying remaining top layer portions, depositing an inorganic film on the remaining portions of the bottom layer and over the metal on the surface of the wafer, lifting off the inorganic film on the remaining bottom layer portions together with said bottom layer portions, and depositing the gate in the space between remaining portions of the inorganic film on said wafer defining the gate.

Claim 3

The process of claim 1, wherein said bottom layer is a polymethylmethacrylate layer and said top layer is a positive photoresist.

Claim 4

The process of claim 2, wherein said bottom layer is exposed to deep UV flood exposure, scattering of light beneath the edges of said remaining portions of said top layer to provide said undercutting beneath the edges of said remaining top layer portions.

Claim 5

The process of claim 1, wherein a space is left between the outer edges of the metal deposited on the surface of the wafer and the adjacent bottom layer portions of the T-bar shaped portions, defined by the undercut formed beneath the top layer portions of said T-bar shaped portions.

Claim 6

The process of claim 4, wherein said inorganic film deposited on the metal on the surface of the wafer envelopes around and covers the outer edges of the metal in the space left between the outer edges of the metal and the adjacent bottom layer portions of the T-bar shaped portions.

Claim 7

The process of claim 1, wherein the first lift-off step is carried out in a solvent capable of removing the remaining top layer portions containing the metal, and the second lift-off step is carried out in a solvent capable of removing the remaining bottom layer portions containing the inorganic film.

Claim 8

The process of claim 1, said metal comprising a first layer of gold, germanium eutectic alloy and a second layer of nickel over said first layer.

Claim 9

The process of claim 1, said inorganic film being a SiO x film, where x is 1 or 2.

Claim 10

The process of claim 1, including subjecting the metal on the surface of the wafer to elevated temperature sufficient to cause alloying of said metal.

Claim 11

A self-aligned process for production of high electron mobility transistors having good transconductance and good subthreshold characteristics, which comprises the steps of: depositing a bottom layer of a first polymethylmethacrylate photoresist on a high electron mobility GaAs/GaAlAs substrate, depositing a second top layer of positive photoresist on said bottom layer, exposing the top layer under a pattern, developing said top layer to remove exposed portions thereof, exposing the bottom layer beneath the previously exposed portions of the top layer, with deep UV flood exposure, scattering of light beneath the edges of said remaining portions of said top layer to undercut the edges of the remaining portions of said top layer, developing the bottom layer to remove the exposed portions thereof, and forming T-bar shaped remaining unexposed portions of overlying top and bottom layers, one of said T-bar shaped portions forming a T-bar dummy gate, depositing a first metal on the upper surface of said T-bar shaped portions and on the exposed surface of said substrate to form a source and a drain, and leaving a space between the outer edges of the metal deposited on the surface of the substrate and the adjacent bottom layer portions of the T-bar shaped portions, defined by the undercut formed beneath the top layer portions of said T-bar shaped portions, lifting off the metal on said T-bar shaped portions including said T-bar dummy gate, together with the underlying top remaining layer portions by treatment in a solvent for said positive photoresist, depositing a SiO film on the remaining portions of the bottom layer and over the on the surface of the substrate, said film covering the outer edges of the metal in the space left between the outer edges of the metal and the adjacent bottom layer portions of the T-bar shaped portions, lifting off the SiO film on the remaining bottom layer portions together with said bottom layer portions, by treatment in a solvent for said polymethylmethacrylate bottom layer portions, and depositing gate metal on the exposed surface of said substrate in the space between remaining portions of the SiO film to define the gate.

Claim 12

A self-aligned process for production of high electron mobility transistors having good transconductance and good subthreshold characteristics, which comprises the steps of: depositing a bottom layer of a first polymethylmethacrylate photoresist on a high electron mobility GaAs/GaAlAs substrate, depositing a second top layer of positive photoresist on said bottom layer, exposing the top layer under a pattern, developing said top layer to remove exposed portions thereof, exposing the bottom layer beneath the previously exposed portions of the top layer, with deep UV flood exposure, scattering of light beneath the edges of said remaining portions of said top layer to undercut the edges of the remaining portions of said top layer, developing the bottom layer to remove the exposed portions thereof, and forming T-bar shaped remaining unexposed portions of overlying top and bottom layers, one of said T-bar shaped portions forming a T-bar dummy gate, depositing a first metal on the upper surface of said T-bar shaped portions and on the exposed surface of said substrate to form a source and a drain, and leaving a space between the outer edges of the metal deposited on the surface of the substrate and the adjacent bottom layer portions of the T-bar shaped portions, defined by the undercut formed beneath the top layer portions of said T-bar shaped portions, lifting off the metal on said T-bar shaped portions including said T-bar dummy gate, together with the underlying top remaining layer portions by treatment in a solvent for said positive photoresist, depositing a SiO film on the remaining portions of the bottom layer and over the on the surface of the substrate, said film covering the outer edges of the metal in the space left between the outer edges of the metal and the adjacent bottom layer portions of the T-bar shaped portions, lifting off the SiO film on the remaining bottom layer portions together with said bottom layer portions, by treatment in a solvent for said polymethylmethacrylate bottom layer portions, and depositing gate metal on the exposed surface of said substrate in the space between remaining portions of the SiO film to define the gate.

Claim 13

The process of claim 10, said first metal comprising a first layer of gold, germanium eutectic alloy and a second layer of nickel over said first layer.

Claim 14

The process of claim 11, including subjecting the first metal on the surface of the substrate to elevated temperature sufficient to cause alloying of said metal and form ohmic contacts.

Claim 15

The process of claim 12, including depositing metal on the silica imprinted portions on the substrate to form interconnections to said source and drain.