Process for Forming Self-Aligning Cobalt Silicide T-Gates of Silicon MOS Devices

The invention is related to the field of silicon MOS devices and, in particular, to a process for forming self-aligning cobalt silicide T-gate structures on a silicon substrate.

Metal T-gates have been used for some time to improve the microwave performance of III-IV FET's by simultaneously providing low gate resistance and low parasitic capacitance. T-gates are not common in silicon MOS devices since gate resistance does not usually limit performance in digital circuits. Although MOS devices are rarely used in microwave applications, recent advances have created interest in MOS technology as a low cost alternative in the current gallium arsenide and silicon bipolar technologies. In conventional MOS devices, gate resistance is the dominant factor limiting the microwave performance of the device, thus a T-gate providing a low gate resistance and low parasitic capacitance is ideally suited for these applications.

A. K. Agarwal et al, in their article "MICROX--An Advanced Silicon Technology For Microwave Circuits Up To X-Band," published in the 1991 IEDM TECH DIGEST, pp. 687-690, discloses a microwave SOI process with T-gates. The T-gates are fabricated by first forming a conventional gate then using e-beam lithography to align and pattern a gold cap on the gate. However, both the use of e-beam lithography and gold are inconsistent with the goals of low cost technology.

Nasr, in U.S. Pat. No. 4,912,061, discloses an alternative for forming a cobalt silicide self-aligned metal oxide semiconductor device using disposable silicon nitride spacers.

The invention is a process for making a self-aligned cobalt silicide T-gate for silicon MOS devices. The process comprises the steps of forming a gate oxide layer on a top surface of a silicon substrate then forming a polysilicon layer on the gate oxide layer. A masking oxide layer is then formed on the polysilicon layer. An opening is formed in the masking oxide layer, exposing a gate region on the polysilicon layer. A layer of cobalt is then deposited over the masking oxide layer and the surface of the polysilicon layer exposed by the opening in the masking oxide layer and heated to a temperature in the range from 600.degree. C. to 700.degree. C. for a time sufficient for the layer of cobalt to react with the polysilicon in the gate region to form a cobalt silicide layer. The unreacted portion of the cobalt layer and the underlying masking oxide layer are removed by a selective etch as in a conventional self-aligned silicide process. The polysilicon is then reactive ion etched or etched using the cobalt silicide layer as a mask. The polysilicon underlying the cobalt silicide layer is only partially etched away leaving a self-aligned T-gate structure in the gate region of the MOS device.

One advantage of the cobalt silicide T-gate structure is that it automatically aligns the gate with subsequently formed source and drain regions.

Another advantage of the cobalt silicide T-gate structure is that it provides low gate resistance and low parasitic capacitance.

Another advantage is that the process allows a significant reduction in the thickness of the polysilicon layer without increasing gate resistivity.

Still another advantage is that the thinner polysilicon layer results in easier planarization of subsequent layers and easier etching of sub-micron gates.

These and other advantages of the process for forming self-aligned cobalt silicide T-gates will become more apparent from a reading of the specification in conjunction with the attached drawings.

FIG. 1 is a cross-sectional view of the MOS device showing the substrate, the added layers and the oxide mask;

FIG. 2 is a cross-sectional view showing the deposited cobalt layer;

FIG. 3 is a cross-sectional view showing the resultant cobalt silicide layer after heating and removal of the unreacted cobalt layer and the masking oxide layer; and

FIG. 4 is a cross-sectional view showing the resultant cobalt silicide T-gate structure after etching of the polysilicon layer.

The process for producing a self-aligned cobalt silicide T-gate for silicon MOS devices is illustrated in FIGS. 1 through 4. FIG. 1 shows a silicon substrate 10 on which is formed a silicon oxide gate layer 12. The substrate may be processed using any of the methods known in the art to form the silicon oxide gate layer 12. A polysilicon layer 14 about 4,000 .ANG. thick is deposited on the gate oxide layer 12. A silicon dioxide masking layer 16 is grown or deposited on the polysilicon layer 14 and is patterned to provide an opening 18 defining a gate region where a gate is to be formed.

A layer of cobalt 20 is then deposited on the masking layer 16 and on the top surface 22 of the polysilicon layer 14 within the opening 18 as shown in FIG. 2. This structure is then heated to a temperature in the range from 600.degree. C. to 700.degree. C. for a time sufficient for the cobalt layer 20 deposited on the polysilicon layer 14 within the opening 18 to react with the polysilicon to form a cobalt silicide layer 24 as shown in FIG. 3. The cobalt layer 20 deposited on the surface of the oxide masking layer 16 does not react with the masking oxide layer 16. This unreacted cobalt of the cobalt layer 20 remaining on the surface of the masking oxide layer 16 is removed by a selective wet etch leaving only the oxide masking layer 16 and the cobalt silicide layer 24.

Since the cobalt silicide layer 24 is largely impervious to reactive ion etching, the remainder of the masking oxide layer is removed using reactive ion etching. Subsequently, the remainder of the polysilicon layer 14 is removed using a conventional wet or reactive ion etching process. During this etching process, the cobalt silicide layer 24 becomes an etch mask, which prevents the etching of the polysilicon thereunder. During the etching process, a portion of the polysilicon under the cobalt silicide layer 24 will be etched from the sides forming a self-aligned T-gate structure standing upright on the substrate 10 and the gate oxide layer 12 as shown in FIG. 4.

This process provides the usual advantages of the T-gate structure in which the cobalt silicide layer acts as a self-aligned mask for the subsequent formation of source and drain regions on opposite sides of the gate. The process further provides a lower gate resistance and allows significant reduction in the thickness of the polysilicon layer 14 without increasing gate resistivity. The thinner polysilicon layer further results in easier planarization layers and easier etching of sub-micron gates.

Having disclosed the process for making cobalt silicide T-gates for silicon MOSFET's, is it recognized that others skilled in the art may make certain changes or improvements to the process within the scope of the invention as set forth in the appended claims.