Fabrication Method for Integrated Circuits With Polysilicon Lines Having Low Sheet Resistance

1. Field of the Invention

The present invention relates to the fabrication and structure for integrated circuits and more particularly to the fabrication of a MOSFET polysilicon self-aligned gate structure including a silicide forming metal layer between two polysilicon layers.

2. Description of the Prior Art

In integrated circuit fabrication, particularly in the fabrication process for self-aligned gates, polysilicon is used in the prior art for the gate element because the gate element is subjected to high temperatures during the diffusion step in producing the source and drain electrodes and polysilicon can withstand high temperatures similar to refractory metals. Also, in the self-aligned gate process a reoxidation step is performed to provide an oxide over the gate to separate the gate from the metal lines which are later disposed on the top of the structure and the oxide is grown better on polysilicon than on a refractory metal. A prior art structure of this type using polysilicon gate material is shown in FIG. 1 of the drawing.

A disadvantage of the polysilicon gate relative to a refractory metal gate is that it is desirable in many applications to connect the gate to the top layer of metal lines to provide a two-dimensional degree of freedom for the distribution of signals. When polysilicon is used for the gate material there is a mismatch between sheet resistance (ohms per squre cm.) of the metal and the polysilicon which causes inefficiencies such as reduced circuit speed. This disadvantage has been recognized in the prior art and attmepts have been made to avoid the resistivity mismatch. In the IBM Technical Disclosure Bulletin, Vol. 17, No. 6, November 1974, a publication entitled "Reducing the Sheet Resistance of Polysilicon Lines In Integrated Circuits" by V. L. Rideout, is directed to the use of polysilicon lines in multilayer integrated circuits because of the polysilicon's high temperature stability and the fact that silicon dioxide can be deposited or grown on it. The publication teaches that the resistance of the polysilicon lines can be decreased by forming a high-conductivity silicide layer on the exposed surface of the line. That is, the silicide is formed where the metal and the polysilicon meet. More particularly, the polysilicon lines are formed on a silicon dioxide layer over a substrate, a metal such as hafnium is deposited over the entire structure, hafnium silicide is formed over the polysilicon lines but the hafnium remains unreacted on the silicon dioxide regions and is etched away. Then an insulating layer of silicon dioxide is deposited over the structure and an aluminum metalization layer is formed over the silicon dioxide.

The present invention is distinct from the prior art in that a gate structure is formed in the self-aligned gate process wherein a first layer of polysilicon is formed, a silicide forming metal layer is then formed over the first polysilicon layer, and a second polysilicon layer is formed over the silicide forming metal layer. After masking, and during the subsequent reoxidation process, the metal layer reacts at two surfaces with the polysilicon layers and a silicide region is formed. The silicide region is available for a matched connection to the later formed upper metal layer, however the gate structure has a temperature stability property and an upper polysilicon region which provides a good oxide growth during the reoxidation process.

An object of the present invention is to provide a gate structure for integrated circuits which provides high temperature stability and low sheet resistance.

Another object of the present invention is to provide a low sheet resistance gate structure upon which an oxide can be efficiently formed.

A further object of the present invention is to provide a gate structure for integrated circuits which maintains a work function difference relative to a silicon substrate.

Still another object of the present invention is to provide a gate structure for integrated circuits including a silicide forming metal layer sandwiched between two polysilicon layers wherein the metal reacts with the polysilicon to form a silicide.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 is an illustration of a gate structure for integrated circuits as known in the prior art.

FIG. 2 is an illustration of a gate structure for integrated circuits including a silicide forming metal sandwiched between two polysilicon layers in accordance with the principles of the present invention.

Referring to FIG. 1, an illustration of a typical polysilicon self-aligned gate is shown including a p type substrate 10 with n+ type source and drain regions 12 and 14. An n+ polysilicon gate 16 is buried in an insulating layer of silicon dioxide (SiO.sub.2) 18 and a metalization layer 20 which may be located at the top of the structure.

Polysilicon material is chosen for the layer of which gate 16 is a part, rather than a refractory metal, because it is stable at high temperatures and silicon dioxide can be thermally grown on it or chemically vapor deposited. However, the sheet resistance of polysilicon is orders of magnitude higher than the sheet resistance of the upper metal layer 20, producing a conductivity mismatch which leads to inefficiency such a reduced circuit speed when the polysilicon layer of which gate 16 is a part, also is used for interconnection lines together with the normal metal layer 20. On the other hand, the use of a refractory metal gate selected for its conductivity match has the disadvantage that a native oxide cannot be easily grown on it.

In FIG. 2, an improved structure is illustrated including a similar substrate 10, source and drain regions 12 and 14, and insulating layer 18. The gate however, includes a first layer of polysilicon 22 approximately half as thick as the layer shown in FIG. 1. After layer 22 is deposited a thin layer 24 of silicide forming metal such as molybdenum, tungsten or titanium is deposited on polysilicon layer 22 by a flash deposition and then a second layer 26 of polysilicon approximately the same thickness as layer 22 is deposited on metal layer 24. An important advantage of the invention is that the three layers 22, 24 and 26 can be deposited using the same fabrication steps as in the normal gate process, that is, the deposition of the three layers can be carried out without interrupting the vacuum environment for the deposition and therefore only one vacuum pump down is required.

The three layer structure is next delineated by normal masking steps to produce the gate structure (consisting of layers 22, 24 and 26) as illustrated in FIG. 2. The reoxidation process is then performed to form the silicon dioxide layer above layer 26, and during this process the elevated temperature causes the metal layer 24 to react with polysilicon layers 22 and 26 with two reaction fronts to form a silicide having metallic conductivity.

The preceding discussion related to a description of the present invention as used to fabricate a gate structure for a MOSFET. The invention however, is not limited to this application and may be used more generally, for example, in the fabrication of bipolar devices and for polysilicon conductors or lines in multilayer circuits. Thus polysilicon lines may be fabricated with a sandwiched silicide layer according to the principles of the present invention so as to incorporate the temperature stability and reoxidation properties of polysilicon and the low sheet resistance of the silicide.

Other integrated circuit applications of the present invention include one-device cell memories and logic arrays. In the one-device cell the present silicide structure described herein can be used to maintain a low sheet resistance for the bit line without increasing the bit line capacitance, thereby improving device sensitivity as a result of improved coupling.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.