Method of forming a gate contact

A method is provided for forming a gate contact for a compound semiconductor device. The gate contact is formed from a gate contact portion and a top or wing contact portion. The method allows for the tunablity of the size of the wing contact portion, while retaining the size of the gate contact portion based on a desired operational frequency. This is accomplished by providing for one or more additional conductive material processes on the wing contact portion to increase the cross-sectional area of the wing contact portion reducing the gate resistance, while maintaing the length of the gate contact portion to maintain the operating frequency of the device.

TECHNICAL FIELD

The present invention relates to semiconductors, and more particularly to a method of forming a gate contact.

BACKGROUND

Group III-V semiconductor transistors, such as high electron mobility transistors (HEMTs), require smaller gate contact lengths (Lg) (less than 40 nm) and low gate contact resistance (Rg) for high frequency operation. However the smaller gate contact size normally associates with higher gate resistance (Rg) due to the smaller physical dimensions of gate contact length. Historically, fabrication of a T-shaped gate contact (T-gate contact) approach has been implemented in the industry for group III to V semiconductor transistors. The T-shaped gate contact employs a small gate contact portion and a larger top contact portion, referred to as a wing. The purpose of the larger top contact portion (wing) is to increase the cross sectional area of gate (to lower the Rg) while maintaining the smaller gate length (lower Lg). However, when the T-gate is scaled down to smaller gate contact sizes (less than or equal to 40 nm), the wing contact portion size scales down proportionally and has smaller dimensions.

Some techniques have been employed to form gate contacts with smaller gate contact portion size with larger wing contact portions. One of the techniques employed to form gate contacts with smaller gate contact portion sizes with larger wing contact portions involves using two separate exposure/development steps and two separate metalization steps. The gate contact is formed in the first resist exposure, development, and metallization processing step, and the wing contact portion in the second resist exposure, development, and metallization processing step. This process would allow the wing to be sized to many selective wing sizes. However the disadvantages are that the registration of the wing contact portion to the gate contact portion can be misaligned. Furthermore, the increased number of processing steps to achieve the results will lower the total gate yield when dealing with gate sizes of 40 nm or smaller. The number of processing steps will also effectively increase the length of processing time and cost.

Another technique is to use a two step resist exposure process of exposing and developing a resist of a wing contact portion first and then exposing and developing a resist of a gate contact portion. However there can be misalignment of wing contact portion to gate contact portion registration, and the gate size control/uniformity can degrade by wing dose contribution to the gate dose.

A third technique is to use multiple layers of resist, and use a set of developers. Each developer specifically develops each resist layer individually. This allows the wing size to be developed for longer time resulting in a larger wing. However, each of the resist layers needs to be developed completely, so that the next developer can develop the next layer correctly. Any resist not completely developed away will prevent the next developer from opening the resist for the next layer. Additionally, the different resists can form an inter-mixed layer which can be difficult to develop out completely by any of the developers.

SUMMARY

In accordance with an aspect of the invention, a method is provided for forming a gate contact for a compound semiconductor device. The method comprises depositing a first photoresist material layer over a substrate and depositing a second photoresist material layer over the first photoresist material layer. The second photoresist material layer has a higher sensitivity to a photoresist development process than the first photoresist material layer. The method further comprises performing a photoresist development process to form a first via in the first photoresist material layer and a second larger via, overlying the first via, in the second photoresist material layer, performing a first conductive material deposition process to form a gate contact having a gate contact portion formed in the first via in contact with the substrate and a wing contact portion disposed over and in contact with the gate contact portion in the second larger via, and stripping a conductive material layer that results from the first conductive material deposition process from the second photoresist material layer. The method also comprises performing etching process to remove additional portions of the second photoresist material layer from the second larger via to laterally extend the second larger via, and performing a second conductive material deposition process to form an outer wing contact portion to increase the cross-sectional area of the wing contact portion, while maintaining the length of the gate contact portion.

In another aspect of the invention, a method for forming a compound semiconductor device is provided. The method comprises depositing a first photoresist material layer over a substrate, and depositing a second photoresist material layer over the first photoresist material layer. The second photoresist material layer has a higher sensitivity to a photoresist development process than the first photoresist material layer. The method further comprises performing an electron beam lithography process to form a first via in the first photoresist material layer and a second larger via, overlying the first via, in the second photoresist material layer, depositing a first conductive material to form a gate contact having a gate contact portion formed in the first via in contact with the substrate and a wing contact portion located over and in contact with the gate contact portion in the second larger via, and stripping a conductive material layer that results from the depositing the first conductive material from the second photoresist material layer. The method further comprises performing an oxygen O2plasma process to remove additional portions of the second photoresist material layer from the second larger via to laterally extend the second larger via, and evaporating and depositing a second conductive material to form an outer wing contact portion to increase the cross-sectional area of the wing contact portion, while maintaining the length of the gate contact portion, wherein the length of the gate contact portion is selected to provide a desired operating frequency and the cross-sectional area of the wing contact portion is selected to provide a desired gate contact resistance without deleterious effects on the desired operating frequency.

In accordance with yet another aspect of the invention, a compound semiconductor device is provided that comprises a substrate and a gate contact having a gate contact portion in contact with the substrate and a wing contact portion disposed over and in contact with the gate contact portion. The wing contact portion has an inner wing contact portion and an outer wing portion, wherein the outer wing contact portion increases the cross-sectional area of the wing contact portion, while maintaining the length of the gate contact portion. The length of the gate contact portion is selected to provide a desired operating frequency and the cross-sectional area of the wing contact portion is selected to provide a desired gate contact resistance without deleterious effects on the desired operating frequency.

DETAILED DESCRIPTION

The present disclosure provides for a method of forming a gate contact for a compound semiconductor device, such as a high electron mobility transistor (HEMT). A compound semiconductor device includes two different atomic elements in each layer of the semiconductor device. For example, the semiconductor device can have semiconductor layers formed from Group III-V semiconductor materials, such as Gallium Nitride (GaN), Gallium Arsenide (GaAs), Indium Phosphide (InP) or other compound semiconductor. The gate contact is formed from a gate contact portion and a top or wing contact portion. The method allows for the tunablity of the size of the wing contact portion, while retaining the size of the gate contact portion based on a desired operational frequency. This is accomplished by providing for one or more additional conductive material processes on the wing contact portion to increase the cross-sectional area of the wing contact portion reducing the gate resistance, while maintaing the length of the gate contact portion to maintain the operating frequency of the device.

FIG. 1is a schematic cross-sectional illustration of a portion of a compound semiconductor structure10in accordance with the present invention. The compound semiconductor structure10includes a gate contact14that overlies a substrate12. The substrate12can be a dielectric layer overlying a channel region (not shown) of a transistor. The transistor can be, for example, a high electron mobility transistor (HEMT) formed from various compound semiconductor material layers. The gate contact14has a generally pine tree shape or “T” shape to minimize resistance which provides high device operating performance. The gate contact12can be formed from one or more conductive materials, such as gold, aluminum, copper, platinum, or other conductive material layer.

The gate contact12is formed of a gate contact portion16that is in contact with the substrate12and a top or wing contact portion18in contact with and overlying the gate contact portion16. The gate contact portion16has a length that is selected based on a desired operating frequency, while the wing contact portion18is selected to have a cross-sectional area that minimizes the resistance of the gate contact14, which also contributes to high operating frequency. The wing contact portion18includes an inner wing contact portion20and an outer wing contact portion22. The inner wing contact portion20and the outer contact portion22were formed from different conductive material deposition processes to increase the cross-sectional area of the wing contact portion14without increasing the length of the gate contact portion16, as will be described below.

It is to be appreciated that when the gate contact is scaled down to smaller gate size (≦40 nm), the wing size scales down proportionally and has smaller dimensions. The wing size can decrease to as much as 30% of the original wing size when the gate size is in this range. By increasing the wing size using the disclosed methodology, the gate resistance can be reduced as shown in chart30ofFIG. 2.

Turning now toFIGS. 3-13, process blocks in connection with fabrication of a portion of compound semiconductor structure (e.g., HEMT) in accordance with an aspect of the present invention are described. A first photoresist material layer54is deposited over a substrate52(e.g., compound semiconductor substrate, dielectric layer) and a second photoresist material layer56is deposited over the first photoresist material layer54. The first photoresist material layer54and the second photoresist material layer56may be deposited using any suitable means. For example, the first photoresist material layer54may be deposited over the substrate52utilizing spin-coating or spin casting deposition techniques. Once the first photoresist material layer54dries, then the second photoresist material layer56may be deposited over the first photoresist material layer54utilizing spin-coating or spin casting deposition techniques. The second photoresist material layer56is selected to have a higher sensitivity to a photoresist development process than the first photoresist material layer54, such that the second photoresist material layer56provides a larger via than the first photoresist material layer54upon completion of the photoresist development process.

FIG. 4illustrates the structure ofFIG. 3undergoing a photoresist development process100in which an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the first and second photoresist material layers54and56through an intervening mask, for a particular pattern, to become either more or less soluble (depending on the coating) in a particular solvent developer. Next, as represented inFIG. 5, solvent developer is applied to the exposed first and second photoresist material layers54and56to open a first via58in the first photoresist material layer54and a second larger via60in the second photoresist material layer56that overlies the first via58. In one aspect of the invention, the photoresist development process employs electron beam lithography. However, any suitable photolithographic techniques can be performed to form the patterned first and second photoresist material layers54and56.

FIG. 6illustrates the structure ofFIG. 5undergoing a first conductive material deposition process110. Any suitable technique for depositing the conductive material may be employed such as metal evaporation, sputter evaporation, plating, Low Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Atomic Layer Deposition (ALD), sputtering or spin on techniques. The conductive material can be one or more conductive materials and/or conductive material layers, such as gold, aluminum, copper, platinum, or other conductive materials.

FIG. 7illustrates the structure ofFIG. 6after the conductive material deposition process110in which a gate contact61has been formed. The gate contact61has a generally pine tree shape or “T” shape with a gate contact portion62that is in contact with the substrate52and a top or wing contact portion64overlying the gate contact portion62. Additionally, a conductive material layer66has formed overlying the second photoresist material layer56as a result of the conductive material layer process110. The conductive material layer66includes portions that overly the second larger via60, which prohibits further forming of the cross-sectional area of the wing contact portion64.

Next, the conductive material layer66is stripped from the second photoresist material layer56and the resultant structure is subjected to an isotropic oxygen O2plasma process120, as illustrated inFIG. 8. The isotropic oxygen O2plasma process120removes additional portion of the second photoresist material layer56and laterally widens the second larger via60. The amount of lateral widen depends on the length of time of the plasma process and/or the plasma rate. It is to be appreciated that other etching techniques could be employed to laterally widen the second larger via60.

It was discovered that the stripping of the conductive material layer66results in the top surface of the second photoresist material layer56becoming more resistant to the isotropic plasma process120than the photoresist material in the second larger via60. Therefore, the photoresist material in the second larger via60etches at a faster rate than the top surface of the second photoresist material layer56.FIG. 9illustrates the resultant structure ofFIG. 8with a laterally extended second via68.

FIG. 10illustrates the structure ofFIG. 9undergoing a second conductive material deposition process130. The second conductive material deposition process130can be a non-conformal conductive material deposition and employ a conductive material evaporator which deposits conductive material only on the top of the wing contact portion64. The resultant structure is illustrated inFIG. 11, where an outer wing contact portion72is formed over the wing contact portion64, which is now an inner wing contact portion. Again, a conductive material layer74has formed overlying the second photoresist material layer56as a result of the conductive material layer process130. Next, the conductive material layer74is stripped from the second photoresist material layer56and the resultant structure is illustrated inFIG. 12.

The process can be repeated to form additional outer wing contact portions to increase the cross-sectional area of the wing contact portion64within practical limitations. The first and second photoresist material layers54and56illustrated inFIG. 12can be stripped employing a wet chemical strip to provide the resultant structure illustrated inFIG. 1.