High electron mobility transistor semiconductor device and fabrication method thereof

A method of forming a semiconductor device on a semiconductor substrate, the method comprising:

• forming a photoresist layer on the semiconductor substrate;
• forming a window in the photoresist layer;
• forming a conformal layer comprised of a dielectric material on the photoresist layer and in the window;
• and
• selectively removing substantially all of the conformal layer from the photoresist layer and a bottom portion of the window to form dielectric sidewalls in the window, wherein
• the forming of the photoresist layer further includes forming a polymethyl methacrylate (PMMA) layer on the semiconductor substrate and forming a polymethyl methacrylate-methacrylic acid (PMMA-MAA) copolymer layer on the PMMA layer, and further wherein
• the forming of the conformal layer further includes depositing the dielectric material at a temperature at which the PMMA layer and the PMMA-MAA copolymer layer do not flow.

A method of forming a semiconductor device on a semiconductor substrate, the method comprising:

• forming a photoresist layer on the semiconductor substrate;
• forming a window in the photoresist layer;
• forming a conformal layer comprised of a dielectric material on the photoresist layer and in the window;
• and
• selectively removing substantially all of the conformal layer from the photoresist layer and a bottom portion of the window to form dielectric sidewalls in the window, wherein
• the forming of the window in the photoresist layer further includes forming the window by electron beam lithography (EBL).

A method of forming a metal T-gate on a Group III-V semiconductor substrate, a polymethyl methacrylate (PMMA) layer deposited on the semiconductor substrate, a polymethyl methacrylate-methacrylic acid (PMMA-MAA) copolymer layer deposited on the PMMA layer, and a window formed in the PMMA-MAA copolymer layer and the PMMA layer, the method comprising:

• depositing a dielectric layer on the PMMA-MAA copolymer layer and over the window;
• selectively removing the dielectric layer to leave dielectric sidewalls in the window;
• depositing a metal film in the window by evaporation;
• and
• lifting off the PMMA layer and the PMMA-MAA copolymer layer to thereby form the metal T-gate from the metal film.