JPH0621103A - Formation method of t-shaped gate structure - Google Patents

Abstract

PURPOSE: To provide a method of forming a T-type gate structure having sub- micron unit length by using an optical lithography technique. CONSTITUTION: A T-type gate structure 50, having a wide head part 46 and a narrow leg part 44, is provided. Far-infrared ray is irradiated through a first mask to a negative photo-resist lower layer 10. A material whose phase are shifted only by 180 deg. is arranged in the lower layer 10. Phase-shifted light beams and non-shifted light beams interference at the boundary part of this material, and an unexposed part is generated. Therefore, a narrow leg part is formed. A photoresist upper layer 30 is formed and developed. Therefore, a wide head part is formed. Then, conductive layers 46 and 48 are formed.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates generally to the manufacture of high performance field effect transistors, and more particularly to a method of forming a T-type gate structure for such transistors.

[0002]

2. Description of the Related Art In the design and manufacture of high performance field effect transistors (FETs), there are conflicting requirements related to gate length and gate resistance. On the one hand, reducing the gate length leads to a reduction in the transit (transition) time and thus an increase in the operating speed of the FET. On the other hand, the gate resistance increases as the size of the gate is reduced, thereby
The expected increase in speed is offset. One way of addressing these conflicting requirements is to provide a gate structure that is T-shaped in cross section, or mushroom shaped. In the gate structure, the narrow legs are FE
Forming a T gate, and the wide head provides a large contact surface giving low resistance connections.

Typically, a T-shaped gate is an electron beam
It is formed by using lithographic techniques.
For example, the upper and lower resistance layers may be formed on a compound semiconductor such as gallium arsenide. The upper resistance layer is
A relatively wide aperture is formed by being exposed to the electron beam. On the other hand, the second exposure with the electron beam forms a narrow opening in the lower resistance layer. Then
A gate material, such as gold, is deposited in these openings and over the top resistive layer. A lift-off process removes these resistive layers and the gate material on those layers, leaving behind the desired gate pattern.

US Pat. No. 4,700,462 discloses a process for making T-type gates by using electron beam lithography techniques. Electron beam exposure is a commonly used technique when sub-micron dimensions are desired. This is because optical lithography techniques impose dimensional limitations. U.S. Pat. No. 4,959,326 discloses that deep ultraviolet printing techniques are limited by the use of the mask, due to limitations on the size of the terrain (shape) on the mask. U.S. Pat. No. 4,959,326 also discloses the selection of optical and electron beam lithography techniques for making T-shaped gate structures. However, the method further complicates the manufacturing process. T-gate structures are also disclosed in US Pat. No. 5,006,478, US Pat. No. 4,997,778 and US Pat. No. 4,975,382.

While electron beam lithography techniques have many advantages, they also have the following disadvantages. Compared to optical lithography techniques, electron beam lithography techniques are slower because the electron beam has to be continuously redirected. "Photolithography technique" is defined as a lithographic process that utilizes irradiation by electromagnetic waves whose wavelengths are at or above the wavelength of ultraviolet light, so electron beam lithography techniques and ion・ Beam lithography technique, optoelectronic lithography technique, X
Differentiated from line lithography techniques and the like. Moreover, electron beam lithography techniques are more costly. Another disadvantage is that positive resists commonly used in electron beam processing (eg polymethylmethacrylate: PMMA) require the use of organic solvents, which are subject to environmental regulations. It makes things even more difficult.

[0006]

OBJECTS OF THE INVENTION The object of the present invention is to provide an electron beam
It is an object of the present invention to provide a method for forming a T-shaped gate structure, which eliminates the drawbacks in lithographic techniques and enables the production of gates with submicron lengths.

[0007]

SUMMARY OF THE INVENTION The above objective is a method for forming a T-shaped gate structure, which includes an optical lithography technique and
By using a mask for miniaturizing the legs of the mold gate, this is achieved without approaching the dimensional limits of the opaque material for patterning on the mask. On the contrary, the length of the leg is defined by using only transparent phase shift material. Rays passing through the phase shifting material are 180 degrees out of phase with rays passing through other paths without passing through the material. The destructive interference between these two sets of rays causes cancellation, thereby creating a sharp contrast contour with dark areas at locations corresponding to the edges of the phase shift material.

The method includes the step of providing a substrate, such as a gallium arsenide member, having a lower layer of negative resist. Irradiation with deep UV light is directed through a first transparent mask having a phase shift material. The mask may be a quartz mask. The quartz-based mask is used to define the electrical connections made on the substrate.
An opaque material such as patterned chrome can optionally be included. Mutual interference of light rays that pass through the phase-shifting material with those that simply pass through the quartz creates an unexposed line pattern at locations corresponding to the edges of the phase-shifting material. The layer of bottom negative resist is then developed to remove the unexposed line pattern as well as any resist material left unexposed due to the opaque material on the quartz-based mask. Be done.

A top layer of negative resist is then deposited over the bottom layer and within the openings formed by material removal from the bottom layer. A second exposure is then carried out by using a second mask whereby irradiation with deep UV light causes the second exposure with an opaque material such as chromium to form a pattern.
Directed through the mask. The chrome forms a pattern that defines the head of the T-shaped gate, and
The pattern is formed to define electrical connection locations along the substrate, as described above for the transparent material on the first mask. Except for the part corresponding to the above head,
No chromium is placed in the areas corresponding to the openings formed by the edges of the phase shift material. Therefore, the resist material of the top layer remains in its associated openings in the bottom layer as a result of the development of the top layer.

A gate material is then deposited over the lower negative resist layer and within the openings formed in the negative resist layer. Lift-off technology is used to remove unnecessary gate material,
This leaves a T-shaped gate structure and selectively leaves conductive connections along the surface of the substrate.

Preferably, a single type of negative photoresist material is used for both the bottom and top layers.
A suitable resist material is Shipley Company.
Inc.), which has a US federal trademark "SAL-601". The lower layer is irradiated and exposed to form a first image, and then the image is developed to form a first opening in the lower layer. The top layer is then deposited on top of the bottom layer and within the first opening. Since the lower layer is fully crosslinked, the upper layer can be of the same material as the lower layer, with the exposure and development steps of the upper layer providing a first image of the lower layer. There is no fear of adverse effects. A second image is formed in the upper layer, and development of the second image forms a second opening and removes material from at least one of the first openings. Thus, unlike the prior art, a single layer of photoresist material can be used to fabricate a two-layer structure, where the two-layer structure is used to form the T-gate and any other desired structure. It can be manufactured in any of the configurations. That is, this aspect of the invention extends to structures beyond the T-gate structure.

[0012]

EXAMPLE Referring to FIG. 1, lower photoresist layer
10 is shown on the compound semiconductor substrate 12. The substrate
12 is a gallium arsenide substrate, but for the present invention,
Material is not important. The bottom photoresist layer 10 is a negative, high resolution resist. A suitable candidate for the negative resist is SAL-601, where SAL-601 is Sipl.
It is a US federally registered trademark of ey Company Inc. The bottom photoresist layer 10 is spin coated by using conventional techniques. The thickness of the lower photoresist layer 10 is, for example, 1500 Å.

The use of the mask 14 creates intensity variations in light intensity across the upper surface of the lower photoresist layer 10. A transparent phase shift material 16 is present on the surface of the mask. The phase shift material is λ
It has a thickness equal to / (2 (n-1)). Where: λ
Is the wavelength of the light to be phase shifted and n is the material 16
Is the refractive index of. One of the materials that can phase shift the wave from the krF excimer laser is polymethyl
There is methacrylate (ie PMMA). A layer of PMMA having a thickness of about 2530 angstroms is coated over the mask 14 and then electron beam lithography techniques are used to pattern the material 16.
1990 IEEE Transaction, CH2865-4 90 0000-
In 0821 (pp. 821-824), as described by Watanabe et al., An alternative approach for making phase shift masks is to use SiN as the phase shift material.

A ray having a wavelength of 248 nanometers from a KrF excimer laser is shown as an arrow in FIG. The light beam passes through an opaque quartz-based substrate.
Similarly, the ray passes through the phase shift material 16. However, the wave emanating from the phase shift material is 180 degrees out of phase with the other waves. Waves passing through the mask 14, whether phase-shifted or not, expose the surface of the lower photoresist layer 10.

However, for each edge 18, 20 of the phase shift material 16, an unexposed line pattern is formed. The rays whose phase is shifted by 180 degrees because they have passed through the material 16 interfere with the rays that pass through the quartz-based substrate of the mask 14. As a result, cancellations occur and each edge 1
Dark areas are provided at positions corresponding to 8 and 20. Even higher resolutions and contrasts are obtained than those obtained by using conventional transmission masks.

Optionally, mask 14 can include a patterned opaque material 22. Desirably, the opaque material 22 is chrome. Chromium is patterned to form a substrate 12 on which conductive paths are to be manufactured.
Prevents KrF light from being sent to the upper region.

Although the present invention has been described using a KrF laser exposure, other light sources may be utilized. Desirably, the light rays are in the far ultraviolet light region. The wavelength of the ray plays a major role in terms of resolution. The exposure amount at the time of exposure is not so important.

Referring now to FIG. 2, if SAL-601 is used for the negative bottom photoresist layer 10,
Aqueous alkaline developers can be used. Substrate 12 and bottom layer 10
Is baked for 1 minute at a temperature of about 115 degrees. Due to the exposure to the light energy and the heat treatment of the next step, the lower photoresist layer is cross-linked between the portion where the exposure is not blocked and the portion where the light rays do not cancel each other due to the interference. To be done. Aqueous (bridgeless, cross-linked) alkaline developers make complying with environmental regulations easier when compared to the use of organic solvents commonly used in electron beam processes. Moreover, the negative bottom photoresist layer 10 is less susceptible to damage during SEM inspection and deposition of gate material to form the conductive gate structure.

Development of the lower photoresist layer 10 forms narrow gaps 24, 26 and long areas 28 for depositing conductive pathways. As mentioned above, these narrow gaps 24,26 align with the edges 18,20 of the phase shifting material 16 in FIG. Lower photoresist layer 10 to 24
It has been found that the gaps 24, 26 can be maintained at a length of 1000 angstroms when exposed to 8 nanometer KrF radiation. Therefore, photolithographic techniques can be used to fabricate T-shaped gates with lengths less than half the wavelength of the light used for exposure irradiation. As described above, the wavelength of the light beam used for exposure irradiation plays an important role in determining resolution. This is because scattering occurs from defects in the phase shift material 16. The dimensions of the long area 28 are generally
It corresponds to the dimensions of the opaque material 22 of the mask 14 of FIG.

The negative photoresist top layer 30 shown in FIG. 3 is applied over the photoresist layer 10 and the substrate 12. The material selected to form the bottom layer 10 is
It must be a material that is fully crosslinked in the above steps. By doing so, the lower layer 10 is made insoluble in the casting solution used when the upper layer 30 is molded, whereby the pattern in the lower layer 10 is applied to the upper layer 30. Sometimes disturbed is prevented. The upper layer 30 fills the gaps 24,26. The thickness of the upper layer 30 is 5000 Å. Here again, SAL-601 is a suitable material for forming a resist layer. Therefore, a single type of material can be used to form both two photoresist layers. The second mask 32 is a quartz substrate and is made of an opaque material 3
Including 4, 36. The opaque material may be chrome. The deep UV light, eg, KrF radiation, exposes the photoresist top layer 30 at locations where the radiation was not blocked by the opaque materials 34, 36. The opaque material 36 corresponds to the chromium present on the first mask of FIG. The photoresist strongly absorbs the exposing radiation, which inevitably results in a frustoconical shape, which is why the opaque material 36 on the second mask should be narrower than the opaque material on the first mask. is there.

The opaque material 34 determines the dimensions of the head of the T-shaped gate structure to be manufactured. The gap 24 is the T
Determine the length and height of the legs of the mold gate structure. The narrow gap 26 is only due to the fact that a line pattern is created wherever the edges of the phase shift material are located.
Is formed. However, here, the narrow gaps 26 not used in forming the gate structure are simply filled with the material left in the next step for depositing the conductive gate material. Therefore, the undesired narrow gap is removed without the need for complicated patterning steps.

Referring now to FIG. 4, the second masking step is followed by removal of uncrosslinked (exposed) photoresist material. Two frustoconical gaps
38, 40 are formed in the photoresist top layer 30.
The downwardly expanding shape of the gaps 38, 40 is common in the manufacture of T-gate structure heads. The point to be noted here is that the narrow gap 26, which was additionally formed when the narrow gap 24 for the leg was formed, is not formed in the upper layer 30.
It is not exposed while the photoresist material is removed from the. Therefore, the narrow gap formed incidentally does not play any role in the next step.

A layer 42 of gate material is then deposited, as shown in FIG. The gate layer 42 can be manufactured by using processes that are conventional in the manufacture of T-type gate structures. For example, a film having a thickness of 5000 Angstroms is deposited by evaporating gold. The evaporation creates a conductive path 48 of the material, legs 44 of the T-shaped gate structure, and a head 46 on the substrate 12.

Lift-off (peeling) in the final process
The technique is used to remove unwanted portions of the gate layer 42. Negative photoresist top layer 30
And the negative lower photoresist layer 10 is stripped off,
The portion of the gate layer overlying the photoresist top layer 30 is removed. Therefore, the structure shown in FIG. 6 remains. The T-shaped gate structure 50 is composed of a head portion 44 and a leg portion 46. Furthermore,
The conductive path 48 is formed without requiring additional manufacturing steps.

As described above, the photoresist layers 10, 30
A single type of photoresist material can be used in forming both. In addition to forming a T-shaped gate,
Aspects of the invention can be utilized in making other two-layer structures. The first negative tone photoresist layer is exposed to radiation to form an image, which is then developed to create the first opening. Post-exposure bake provides sufficient cross-linking, which allows the formation of a second layer of the same negative working photoresist material. An image is created in the second negative tone photoresist layer. The photolithographic technique for forming the image in the layer is not essential to the invention, but is preferably a photolithographic technique. The image in the second negative photoresist layer is developed to provide a second opening and remove material from at least some of the first opening. A conductive material such as gold is then deposited in the first and second openings to form the desired two layer structure.

[0026]

An advantage of the present invention is that optical lithography techniques are less expensive than electron beam lithography techniques commonly used in the fabrication of T-gate structures. Moreover, optical lithography techniques can achieve higher manufacturing efficiency because they do not require the continuous exposure steps of electron beam lithography techniques.

Another advantage of the present invention resides in the ability to choose a negative resist that can be developed with an aqueous alkaline solution. That is, this is
It means that environmental regulations can be easily addressed when compared to the organic solvents commonly used in electron beam lithography techniques. Moreover, the preferred negative resist is less susceptible to damage during SEM inspection or during the gate material deposition process.

The present invention solves all the incidental image removal problems formed along the edges of the phase shift material without incurring cost. The top layer of the negative resist is
After filling the openings in the lower negative resist layer, the unwanted image locations in the lower negative resist layer are left unprotected during the exposure step of the upper layer. Optionally, the phase shift material can be dimensioned to eliminate dark areas along selected edges of the material.
For example, a small area of quarter-wave phase shift material can be provided at selected edges where the image is not needed. However, this complicates the manufacturing process and makes the focus control during the exposure process more stringent.
The present invention avoids these complications.

Another advantage resides in that the invention leads to higher transistor performance. Commercially available equipment is 1/5 the size of the terrain (shape) on the mask
Can be printed. This reduces the line edge roughness when compared to that obtained by using an electron beam process. The reduction in roughness reduces gate length variations that adversely affect transistor performance. The present invention uses the phase shift technique only for terrain that requires a minimum length, and uses opaque material for larger patterns, so that sub-resolution grating ( s
The complexity of patterning ub-resolution gratings is avoided.

[Brief description of drawings]

FIG. 1 is a diagram showing a first mask process of exposing a photoresist layer on a substrate in the present invention.

2 is a diagram showing the substrate after developing the photoresist layer after the step of FIG. 1; FIG.

FIG. 3 is a second exposure of the photoresist layer on the substrate of FIG.
It is the figure which showed the mask process.

FIG. 4 is a diagram showing the substrate after developing the photoresist layer after the step of FIG. 3;

5 is a diagram in which a conductive layer is formed on the substrate of FIG.

FIG. 6 is a diagram of a structure in which a lift-off process is performed after the step of FIG. 5 to leave a T-shaped gate structure and a conductive path.

[Explanation of symbols]

12: Substrate, 10, 30: Photoresist layer, 16: Phase shift material 36: Opaque material, 50: T-type gate

Claims (6)

[Claims] 1. A method of forming a T-shaped gate structure having narrow legs and a wide head, which comprises providing a substrate having a first resist layer, irradiating the first region with a first light beam, and The area is exposed to a second light ray having a phase difference of 180 ° from the first light ray, and the leg portion is exposed at the boundary between the first and second areas due to the interference of the first and second light rays. Forming a pattern in the first resist layer, providing a second resist layer on the substrate, irradiating the second resist layer to define the head, and defining from the second resist layer A method of forming a T-shaped gate structure comprising: removing a resist layer from the head portion and the leg portion defined in the first resist layer; and forming a gate material on the head portion and the leg portion. 2. The T-shaped gate structure according to claim 1, wherein in the step of forming the exposed pattern in the first resist layer, light rays are irradiated through a mask provided with a 180 ° phase shift material. Forming method. 3. The step of forming an exposed pattern in the first resist layer uses a mask provided with an opaque material to prevent light irradiation to selected areas of the first resist layer. The method for forming a T-shaped gate structure according to claim 1, wherein a conductive tangent portion is defined on the substrate. 4. The T-shaped gate structure of claim 1, wherein the step of exposing the second resist layer comprises using a mask provided with an opaque material to prevent irradiation of the head. Forming method. 5. The method for forming a T-type gate structure according to claim 1, wherein the second resist layer is a negative resist layer made of the same material as the first resist layer. 6. The method for forming a T-shaped gate structure according to claim 1, wherein the step of forming the exposed pattern and the step of exposing the second resist layer are performed by an optical lithography technique.