JPH07335671A - Manufacture of t-type gate electrode - Google Patents

Abstract

PURPOSE:To avoid the double exposure of a resist by performing a thin resist in the case of manufacturing a T-type gate electrode adapted for a high frequency field effect transistor. CONSTITUTION:In order to form the wide upper part of a T-type gate electrode, an opening 4 undercut in mask films 1, 2 is initially formed. Then, a resist thin layer 5 is provided, and patterned to form a narrow gate footprint. Since the resist is held along the opening 4 undercut in the films 1, 2, the shape of the resist is adapted for the lift-off of gate metal 8 provided by vacuum vapor- depositing. As a result of avoiding double exposure of the layer 5 and the resist 5, the T-type gate electrode (gate length < 0.150 micron) having the shorter gate length than that of the conventional T-type gate electrode can be easily manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a T-type gate electrode applied to a high frequency field effect transistor (FET).

[0002]

2. Description of the Related Art FET manufacturing methods are well known in the field of semiconductor technology. The use of these FETs is desirable in high frequency RF applications.

Short gate lengths are particularly desirable for high frequency RF applications because the maximum operating frequency is directly related to the electron transfer time under the gate. However, when the gate length is shortened and the gate resistance is increased, the power amplification factor is reduced. A T-shaped gate electrode is widely used as a method of reducing the gate resistance. This is because in the T-shaped gate electrode, the wide upper portion has a large cross-sectional area and thus the resistance is small, and the gate length is determined by the narrow lower portion.

An example of a method of manufacturing a T-type gate electrode is disclosed in Japanese Patent Laid-Open No. 3-60113, "Method of forming resist pattern for lift-off".

A conventional method for manufacturing a T-shaped gate electrode is shown in FIG.
Will be described. FIG. 2 is a diagram showing a conventional method for manufacturing a T-type gate electrode, and is a partial cross-sectional view of the FET in each step.

First, as shown in FIG. 2A, the semiconductor substrate 22 is covered with the electron beam resist 21.

Next, as shown in FIG. 2B, the footprint portion 23 of the resist 21 is exposed with a high-energy electron beam 24 to define a narrow footprint of the T-shaped gate electrode.

Next, as shown in FIG. 2C, the upper portion 26 of the resist 21 is exposed to the low energy electron beam 25 to define a wide upper portion of the T-shaped gate electrode.

Finally, as shown in FIG. 2D, the resist 21 is developed to form an opening. The upper portion of the formed opening has an undercut shape suitable for vacuum deposition and lift-off of gate metal.

[0010]

While the conventional method of making T-type gate electrodes is suitable for many applications, it has two drawbacks that increase the minimum achievable gate width.

The first drawback is that the electron beam resist must be thick enough (about 1 micron) to define the T-gate from the top to the footprint.
It is well known in the prior art that thin resists are suitable for defining fine features due to the spread of the electron beam.

The second drawback is that the resist is exposed twice.
That is, the first exposure is to expose a small area for the footprint, and the second exposure is to expose a wide area above the gate electrode. It is well known in the prior art that such double exposure increases the achievable minimum gate length dimension due to the reduced contrast.

Therefore, it is desired to realize a method for producing a T-type gate electrode which enables a thinner resist and avoids double exposure of the resist.

An object of the present invention is to provide a method of manufacturing a T-type gate electrode that meets such requirements.

[0015]

A method of manufacturing a T-type gate electrode according to the present invention comprises: a) providing a substrate; b) forming a mask film on the substrate; and c) undercutting the mask film. A first opening having a curved shape to expose the substrate, d) forming a resist layer on the upper portion of the mask film and on the exposed portion of the substrate, and e) a first opening in the first opening. 2 openings are formed in the resist layer, f) a metal film is vacuum-deposited, and g) the metal film portion outside the first opening is lifted off.

[0016]

Since the resist film is held along the first undercut opening in the mask film, the shape of the resist is suitable for lift-off of the gate metal provided by vacuum deposition. Thus, for proper lift-off of the gate metal, it is important that the resist maintain an undercut shape at the edge of the first opening. It is a feature of the present invention to use an undercut mask film to impart an undercut shape to the resist layer. The present invention allows the use of thinner resist layers than in the prior art and eliminates the need to expose the resist twice.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a method of manufacturing a T-type gate electrode according to the present invention, which is a partial cross-sectional view of an FET in each step.

First of all, as shown in FIG.
The lower mask film 2 of silicon dioxide with a thickness of 00 nm is
Low pressure chemical vapor deposition (LPC) in the temperature range of 90 to 450 ° C.
VD) is deposited on the substrate 3. Next, a silicon nitride upper mask film 1 having a thickness of 100 nm is formed on the upper surface of the film 260 to 350
Deposit on the substrate 3 by low pressure chemical PECVD in the temperature range of ° C.

Next, a photoresist / etching mask is formed on the SiN film by using a general photolithography technique, and the mask films 1 and 2 are etched with buffered hydrofluoric acid, as shown in FIG. As shown, an undercut first opening 4 is formed to expose the substrate 3. Next, the photoresist / etching mask is removed. Since the etching rate of silicon dioxide is higher than that of silicon nitride, which is about 10 times higher in this embodiment, an undercut is formed. Undercut is
As explained below, it is important for proper lift-off of the gate metal. The first opening 4 defines a wide upper portion of the T-shaped gate electrode.

Next, as shown in FIG.
A layer 5 of 150 nm thick Tokyo Ohka OEBR1000 electron beam resist is coated on the substrate and exposed with an electron beam 6 to define the narrow footprint of the T-shaped gate electrode. The thickness of this resist is equal to the height of the narrow footprint forming part of the T-shaped gate electrode. For proper lift-off of the gate metal, it is important that the resist keeps the undercut shape at the edge of the first opening 4. Using an undercut mask film to give the resist an undercut shape is
This is a feature of the present invention. This technique allows the use of thinner resist layers than in the prior art, eliminating the need to expose the resist twice.

Next, as shown in FIG. 1D, the resist 5 is developed to form a second opening 7 which defines the footprint of the narrow gate. Then a gate metal 8 of 5-15 nm titanium and 100-300 nm gold is deposited by vacuum evaporation. The vacuum deposited metal outside the undercut mask layer opening is lifted off by melting the electron beam resist layer 5, leaving only the vacuum deposited metal in the undercut opening. . The remaining metal forms a T-shaped gate electrode.

Although a mask film having silicon nitride and silicon dioxide deposited by LPCVD was used in this embodiment, other films (eg, titanium nitride, titanium oxide, tungsten nitride, and aluminum oxide), and other films are used. Deposition methods (eg vapor deposition, sputtering, chemical vapor deposition, photochemical vapor deposition) can be used. The etching was performed with buffered hydrofluoric acid in this example, but other etching methods such as plasma assisted etching can also be used. In this embodiment, OEBR1000 electron beam resist 5 manufactured by Tokyo Ohka was used, but various resists such as PMMA can be used. Although the resist is exposed by the electron beam in this embodiment, focus ion beam exposure can also be used. Further, in this embodiment,
The gate electrode was made of Ti / Au, but Ti / Pt
/ Au or other metal systems such as Mo / Ti / Pt / Au.

[0023]

The present invention avoids the problems of double exposure and electron beam divergence in thick resist layers of the prior art. Therefore, according to the present invention, it is easy to manufacture a T-shaped gate electrode having a gate length smaller than 0.150 μm.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of an FET illustrating a method of manufacturing a T-type gate electrode according to the present invention.

FIG. 2 is a partial cross-sectional view of an FET illustrating a method of manufacturing a T-type gate electrode according to a conventional technique.

[Explanation of symbols]

 1 Upper Mask Film 2 Lower Mask Film 3,22 Substrate 4 First Opening 5,21 Resist 6,24,25 Electron Beam 7 Second Opening 8 Gate Electrode 23 Footprint Part 26 Top

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location H01L 29/78 H01L 29/78 301 G

Claims (3)

[Claims] 1. A substrate is provided, b) a mask film is formed on the substrate, and c) a first opening having an undercut shape is formed in the mask film to expose the substrate. D) forming a resist layer on the mask film and on the exposed portion of the substrate, e) forming a second opening in the first opening in the resist layer, and f) a metal. A method for manufacturing a T-type gate electrode, comprising: vacuum depositing a film; and g) lifting off the metal film portion outside the first opening. 2. The mask film comprises a first film deposited on the substrate and a second film deposited on the first film, and the etching rate of the first film is the first film. The method for producing a T-type gate electrode according to claim 1, wherein the etching rate of the second film is higher than that of the second film. 3. The first film is a film whose composition is mainly silicon and oxygen, and the second film is a film whose composition is mainly silicon and nitrogen. Item 2
A method for producing the T-shaped gate electrode described.