Semiconductor Integrated Circuit Interconnections

The invention is in the field of semiconductor integrated circuits. At the present state of the art, integrated circuit structures involve a plurality of devices made in a semiconductor crystal with multilevel interconnecting lines running over the top of each other and over the top of isolating regions in the structure. There have been a number of ways in the art such as gaseous diffusion, ion implantation doping and the use of doped polycrystalline semiconductor material to provide the lines. However, the scaling down of device dimensions causes the resistance to increase to levels which seriously impair device performance. This operates to cause the wiring interconnection delay of arrays to assume an intolerably large portion of the circuit delay. In addition, as the resistance becomes larger, voltage drops become larger, making power distribution problems and signal sensing more difficult.

The advantages of doped polycrystalline silicon in interconnections in semiconductor integrated circuits have been set forth in a wide variety of art of which IBM Technical Disclosure Bulletins, Vol. 20, No. 11A, p. 4286, April 1978; Vol. 20, No. 2, p. 539, July 1977 and Vol. 18, No. 11, p. 3840, April 1976 are examples.

The use of a metal silicide such as molybdenum silicide has been employed in the semiconductor art in discrete devices as set forth in U.S. Pat. No. 3,381,182. In each of these applications, however, the problem of acquiring high conductivity while at the same time controlling capacitance associated with conductor overlap have not been addressed.

FIG. 1 is an illustration of an improved semiconductor integrated interconnecting conductor.

FIGS. 2A, 2B and 2C illustrate the application of the conductor to the fabrication of a field effect transistor device.

FIGS. 3A, 3B, 3C, 3D and 3E illustrate the application of the invention to the fabrication of a one device semiconductor memory device.

The invention involves a technique of fabricating integrated circuits wherein the interconnections of the integrated circuits are made up of composite conducting lines of doped polycrystalline silicon and metal silicide, the combination hereafter refered to as polycide, which yield the simultaneous advantages of high conductivity and reduced parasitic capacitance.

Referring to FIG. 1, a semiconductor body 1, for example silicon, is provided with a doped polycrystalline silicon line 2. Integrally associated with the doped polycrystalline silicon region 2 is a silicide region 3 wherein a metal is caused to combine with silicon and to form a region of silicide. The region 3 may be formed by conversion to silicide of some of the polycrystalline silicon region 2 or it may be provided from independent sources of silicon and metal. This composite of regions 2 and 3 will be referred to hereinafter as "polycide".

The polycide 2, 3 interconnecting member provides the simultaneous advantages of very high conductivity interconnections accompanied by a reduced parasitic capacitance effect when the interconnecting polycide 2, 3 crosses over other semiconductor device areas separated by insulators. One reason this takes place is that with the polycide interconnecting member 2, 3 there is no longer any need to rely on the conduction of doped regions in the silicon substrate 1. The conductor line is separated from the substrate semiconductor by a layer of oxide, not shown. A significant advantage of the polycide member 2, 3 also is that it may be oxidized like polycrystalline silicon allowing other interconnection lines to cross it.

It will be apparent to one skilled in the art that, as the packing density and the number of layers involved in making the semiconductor integrated circuit go up, the particular advantages gained by the polycide member 2, 3 of this invention will become increasingly valuable and provide great device interconnecting flexibility.

The improved integrated circuit interconnection technique of the invention may be employed in a wide variety of semiconductor integrated circuit arrays. The illustrations in connection with FIGS. 2 and 3 are directed to the application of interconnecting member 2, 3 to two typical types of semiconductor structures.

Referring next to FIG. 2 and its illustrations 2A, 2B and 2C, the technique of fabrication of an insulated gate field effect transistor involving recessed oxidation isolation is illustrated.

In FIG. 2A, in a p-conductivity type semiconductor crystal 1, around a region in which the field effect transistor is to be formed, recessed oxide 4 is provided generally by forming around an oxidation mask. The oxide 4 operates to isolate the device to be formed from other devices in the semiconductor crystal. A layer 2 of polycrystalline silicon semiconductor material is placed over the region 4 and over the exposed portion of the crystal 1.

In the formation of the polycrystalline material a dopant of n-conductivity type is included that can be caused later to diffuse into the crystal 1 to form the source 18 and drain 19 regions of an insulated gate Field Effect Transistor (IGFET). A layer 3 of silicide is next formed over the polycrystalline layer 2. The silicide is formed or deposited by the techniques of coevaporation, sputtering, or vacuum deposition such that in the process either the metal combines with the polysilicon to form the silicide or in the alternative the layer 3 is made up in toto of a mixture of codeposited metal and silicon. One satisfactory technique is set forth in U.S. Pat. No. 4,180,596. The metals that operate best are those that form passivating and insulating oxides with silicon such as tungsten, molybdenum, tantalum, titanium and rhodium. A layer of silicon dioxide labelled element 5 is formed over the layer 3. The layer 5 may be either formed in situ or partially grown, then partially deposited or completely deposited.

Referring next to FIG. 2B, an etching operation is then undertaken through a mask and an opening 7 is provided in the layers 2, 3 and 5 exposing the region 6 which can serve as the channel of a field effect type device. After the etching, the structure is taken through a heating cycle to form the source 18 and drain 19 regions by out-diffusion from the doped polycrystalline silicon 2. At this point threshhold tailoring may be done in region 6. This procedure minimizes redistribution of channel 6 impurities. An alternative procedure is that during the thermal cycle in which the source 18 and 19 regions are out-diffused from the polysilicon 2, a thin gate oxide region as set forth as 10 in FIG. 2C may be grown. Following this threshhold adjustment, tailoring may be done using ion implantation techniques.

Referring to FIG. 2C, the gate of the field effect device is next applied. A first layer of gate oxide 10 is formed in contact with the crystal 1. Next, a second polycide layer 20, 21 is applied over element 10 thereby forming the gate structure 11.

A polycide layer 20, 21 is applied so that it overlaps the source 8 and drain 9 electrodes separated by the insulator 10. The dopants and metals employed in the layer 20, 21 may be different from polycide layer 2, 3 to impart different properties, particularly suitable for the gate 11.

It will also be apparent to one skilled in the art that the addition of silicon nitride layers which facilitate self-alignment techniques may be employed in connection with the fabrication of the improved interconnection member of applicants' invention.

While the above description has been directed to a simplified approach to the fabrication of IGFETS, other devices such as bipolar transistors may readily be fabricated using the polycide concepts described herein.

Referring next to FIGS. 3A through 3E, the technique of the invention is shown as applied to the fabrication of a field effect type of device known as a one device memory cell. An example of such a device is shown in U.S. Pat. No. 3,387,286.

Referring to FIG. 3A, in the crystal 1 recessed oxide 4 is provided that defines a device area. The layer of oxide 12 is then grown over the surface of the crystal within the area bounded by 4. A resist 13 is then positioned to cover a portion of the oxide 12 and a portion of the recessed oxide 4 and leaving exposed a portion of the oxide 12.

In FIG. 3B, a portion of the oxide 12 is then etched away which exposes a portion of the surface of the crystal 1 and the resist layer 13 is removed.

In FIG. 3C, a polycide member 2, 3 is now applied over the remains of the oxide 12 and in contact with the crystal 1.

Referring next to FIG. 3D, an etching step re-exposes the crystal and the channel 6. At this point the interconnections 8 and 14 are defined with 8 serving also as the source contact.

Referring next to FIG. 3E a gate structure is formed by growing an oxide layer 15 corresponding to element 10 of FIG. 2C. During this oxidation the polycide structure 2, 3 is also oxidized and the source region 17 is formed by out-diffusion from the doped polycrystalline silicon 2 portion of the polycide member ohmically connected to interconnection contact 8. A second polycide member 16 is then formed completing the gate structure. The resulting structure provides the FET transistor and plate capacitor structure of U.S. Pat. No. 3,387,286.

What has been described is a technique of fabricating improved interconnecting devices employing members composed of a polycrystalline silicon portion associated with a silicide portion both as contacts and as conductor lines which permit simplified FET gate structures to be made therewith.

It should be noted that in the improved field effect type structures of the invention involving both polycide contacts and polycide gates in addition to the improved conductivity and reduced parasitic capacitance structural features, the structures are produced without detrimental processing steps such as heavy ion implantation and diffusion damage.

While the invention has been shown applied to two semiconductor structures, it will be apparent to one skilled in the art in the light of the principles set forth that the application of the teaching will be applicable to many semiconductor structures and to many variations, for example, the substitution of other insulating materials such as silicon nitride for the oxide.