Method for forming T-shaped gate structure

A method for forming a T-shaped gate is provided. The method includes providing a substrate. Then, a photoresist structure is formed over the substrate. The photoresist structure includes two development rates. Next, a mask with an opening is formed over the photoresist structure to pattern the photoresist structure. An angle exposure is applied to the photoresist structure, and the exposed photoresist structure is developed to form a T-shaped notch. A width of the T-shaped notch is gradually reduced from a top portion thereof to a bottom portion to expose a surface of the substrate. Then, a gate metal is deposited in the T-shaped notch. Thereafter, the patterned photoresist structure is removed to form the T-shaped gate.

RELATED APPLICATIONS

This application claims priority to Taiwan Application Serial Number 101101109, filed Jan. 11, 2012, which is herein incorporated by reference.

BACKGROUND

1. Field of Invention

The present invention relates to a method for forming a gate structure, and more particularly to a method for forming a T-shaped gate structure.

2. Description of Related Art

A GaAs device tends to have less noise than a silicon device, especially at a high frequency resulting in higher carrier mobilities and lower resistive device parasitics. These properties recommend applying a GaAs circuitry in a mobile phone, satellite communication, a microwave point-to-point link and a higher frequency radar system.

To reduce a gate length without increasing a gate resistance, a T-shaped gate structure is commonly used. The T-shaped gate is composed of an upper wide layer which increases a cross sectional area of the gate for reducing the gate resistance, and a small footprint with a reduced gate capacitance. In general, the T-shaped gate is fabricated using e-beam lithography. It is a reliable technology because the resolution of e-beam lithography can be very high. However, it also has a very low throughput which limits its applications on mass production.

Therefore, a new T-shaped gate manufacturing method is needed.

SUMMARY

An object of the present invention is to provide a method for forming a T-shaped gate structure.

The present invention provides a method for forming a T-shaped gate. The method includes providing a substrate. Then, a photoresist structure is to formed over the substrate. The photoresist structure includes two development rates. Next, a mask is formed over the photoresist structure. The mask has an opening. The mask is used to pattern the photoresist structure. An angle exposure is applied to the photoresist structure, and the exposed photoresist structure is developed to form a T-shaped notch. A width of the T-shaped notch is gradually reduced from a top portion to a bottom portion to expose a surface of the substrate. Then, a gate metal is deposited in the T-shaped notch. Thereafter, the patterned photoresist structure is removed to form the T-shaped gate.

In an embodiment, a lift-off process is performed to remove the photoresist structure.

In an embodiment, the height of the photoresist is 1 um and the width of the opening is 2 um.

In an embodiment, the step of applying an angle exposure to the photoresist structure further comprises: applying a first angle exposure to the photoresist structure; and applying a second angle exposure to the photoresist structure, wherein the first angle exposure and the second angle exposure are applied from opposite sides. The first angle exposure and the second angle exposure are applied to the photoresist structure at an angle between 25 degrees to 65 degrees relative to the substrate.

In an embodiment, the mask is formed from Ti, NI, AU, Al or Cu.

In an embodiment, the step of forming the photoresist structure over the substrate further comprises: forming a first photoresist layer in the substrate; forming a second photoresist layer over the first photoresist layer; and forming a third photoresist layer over the second photoresist layer, wherein, when being compared with the second photoresist layer, the first photoresist layer and the third photoresist layer have lower development rates. The material forming the first photoresist layer and the third photoresist layer is Polymethyl methacrylate (PMMA), and the material forming the second photoresist layer is Polymethyl isopropenyl ketone (PMIPK) or Copolymer.

In an embodiment, the T-shaped gate includes Ni/Au bilayers, wherein the thickness of the Ni layer is 200 angstrom and the thickness of the Au layer is 3000 angstrom.

The present invention applies an angle exposure method to a tri-layer photoresist structure with different development rates. Because of the different development rate of the tri-layer photoresist structure, a T-shaped notch is developed in the substrate after a development process is performed. Moreover, an improved top to bottom width ratio of the T-shaped gate can be also achieved by the angle exposure method.

DETAILED DESCRIPTION

Two photoresists with different development rates are used to form a T-shaped gate. According to the present invention, a tri-layer photoresist structure is formed in a substrate. In an embodiment, a first photoresist layer is formed in the substrate. A second photoresist layer that has a higher development rate than the first photoresist layer is formed in the first photoresist layer. A third photoresist layer that has a lower development rate than the second photoresist layer is formed in the second photoresist layer. Then, an angle exposure method is applied in the tri-layer photoresist structure. Because of the different development rate of the tri-layer photoresist structure, a T-shaped notch is developed in the substrate after development process is performed. Then, a gate metal is deposited in the T-shaped notch by electron beam evaporation. Finally, the remaining photoresists and undesired metals are removed to form the T-shaped gate.

FIG. 1toFIG. 6illustrate a method for forming a T-shaped gate according to an embodiment. As illustrated inFIG. 1, a first photoresist layer101, a second photoresist layer102and a third photoresist layer103are sequentially deposited in a substrate100to form a tri-layer photoresist structure104. A metal layer105is deposited in the tri-layer photoresist structure104. A photolithography process is applied on the metal layer105to form a through hole105a. The hole105adefines the position of the T-shaped gate in the substrate100. In an embodiment, the width of the hole105ais 2 um. The material forming the metal layer105is Ti. In another embodiment, the material forming the metal layer105is Ni, Au, Al or Cu. A wet-etching method is used to form the hole105a. Moreover, when being compared with the second photoresist layer102, the first photoresist layer101and the third photoresist layer103have lower development rates. In an embodiment, the material forming the first photoresist layer101and the third photoresist layer103is Polymethyl methacrylate (PMMA). The material forming the second photoresist layer102is Polymethyl isopropenyl ketone (PMIPK) or Copolymer. That is, the tri-layer photoresist structure104is PMMA/PMIPK/PMMA or PMMA/Copolymer/PMMA. In an embodiment, the height of the tri-layer photoresist structure104is 1 um.

InFIG. 2, the metal layer105acts as a photomask for the following first angle exposures process. First UV light110ailluminates the tri-layer photoresist structure104to pattern the tri-layer photoresist structure104. An included angle θ1exists between the first UV light110aand the substrate100. The included angle θ1can control the depressed depth of the second photoresist102relatively to the first photoresist layer101in the positive X direction. The included angle θ1also decides an inclination angle α1of a slant sidewall formed by the first photoresist layer101as illustrated inFIG. 4. In an embodiment; the included angle θ1is between 25 degrees to 65 degrees.

InFIG. 3, the metal layer105acts as a photomask for the following second angle exposures process. Second UV light110billuminates the tri-layer photoresist structure104to pattern the tri-layer photoresist structure104. An included angle θ2exists between the first UV light110band the substrate100. The included angle θ2can control the depressed depth of the second photoresist102relatively to the first photoresist layer101in the negative X direction. The included angle θ2also decides an inclination angle α2of a slant sidewall formed by the first photoresist layer101as illustrated inFIG. 4. In an embodiment, the included angle θ2is between 25 degrees to 65 degrees. The first and the second angle exposures are applied from the opposite sides. For each exposure, only half of exposure-dose is applied.

InFIG. 4, the metal layer105is removed. In an embodiment, the metal layer105is removed by DHF before development. Then, the tri-layer photoresist structure104is developed. In an embodiment, the tri-layer photoresist structure104is PMMA/PMIPK/PMMA. The tri-layer photoresist structure104is developed using a mixture of MIBK:IPA of 1:3 with the assistance of ultrasonic variation, so as to expose the surface of the substrate100. A descuming process is carried out by O2/Ar plasma, followed by a wet etching with HCl:water of 1:10. A T-shaped notch107is formed in the substrate as illustrated inFIG. 4. Two slant sidewalls with inclination angles α1and α2are formed by the first photoresist layer101to constitute a neck portion of a T-shaped gate. In an embodiment, a T-shaped gate includes two metal layers. The bottom metal layer is disposed between the slant sidewalls to form the neck portion. The top metal layer is disposed over the first photoresist layer101. The width of the T-shaped notch107is gradually reduced from the top opening107ato the opening107band exposes the surface of the substrate100. The top opening107adecides the gate resistance of the T-shaped gate. The opening107bdecides the gate length of the T-shaped gate.

InFIG. 5, a gate metal106is deposited in the T-shaped notch107by electron beam evaporation. In an embodiment, the material forming the gate metal106is Ni/Au. In another embodiment, the material forming the gate metal106selected from the group consisting of Mo, W, Ti, Pt, or any combination of the above. Thereafter, inFIG. 6, lift-off process is performed to remove the tri-layer photoresist structure104and undesired metals to form the T-shaped gate108. In an embodiment, the T-shaped gate108includes Ni/Au bilayers. The thickness of the Ni layer is 200 angstroms. The thickness of the Au layer is 3000 angstroms.

FIG. 7is an Electron Microscopy image of a T-shaped gate according to an embodiment of the present invention. The gate length of the T-shaped gate is 0.6 um. The inclination angle of the neck portion is 30 degrees.

Accordingly, an angle exposure method is applied to a tri-layer photoresist structure with different development rates. Because of the different development rate of the tri-layer photoresist structure, a T-shaped notch is developed in the substrate after development process is performed. Then, gate metal is deposited in the T-shaped notch to form a T-shaped gate. Moreover, an improved top to bottom width ratio of the T-shaped gate can be also achieved by the angle exposure method.