Method for Fabricating a Structure on a Cobalt Silicide Oxide Metalization Semiconductor Substrate

1. The Field of the Invention

The present invention relates to a novel material which is useful in the fabrication of polysilicon gate MOS transistors. More specifically, this invention describes a novel target material for use in a physical vapor deposition (PVD) process that deposits a cobalt silicide film on amorphous, polycrystalline, or single crystalline silicon. The novel target material greatly improves the thermal stability of the as-deposited cobalt silicide film while retaining the properties of the cobalt silicide film which are desirable for its use as a polycide of gate electrodes as well as contact material in MOS transistor structures.

2. The Relevant Technology

As is well known in the art, polycrystalline silicon (polysilicon) is the preferred material for gate electrodes in MOSFET structures. Polysilicon is advantageous over metal gate electrodes as it can withstand much higher subsequent processing temperatures before eutectic temperatures are reached. Polysilicon is readily deposited on bulk silicon or SiO.sub.2 using low pressure chemical vapor deposition (LPCVD), and the resistivities of doped polysilicon films are less than those of doped epitaxial or bulk silicon layers.

As the drive toward integrating more active devices on a single integrated circuit necessitates the fabrication of increasingly small MOSFET structures, the resistance of the MOSFET gate, source and drain contacts become limiting factors in device performance. It is therefore beneficial to use materials with the lowest possible bulk resistivities for making contacts with the MOSFET gate electrodes. To this end, it is well known that refractory metal silicides can be readily formed on silicon using conventional semiconductor sputtering, deposition, and annealing techniques. The refractory metal silicides have low sheet resistivities after annealing and also form low resistance ohmic contacts with commonly used interconnect metals.

Titanium disilicide (TiSi.sub.2) is known to have a low potential sheet resistivity within the available refractory metal silicides, and is in widespread use as a metallization material in MOSFET structures. TiSi.sub.2 is often unsuitable for use in particular MOSFET integrated circuit designs, however, due to limitations which result from its behavior during high temperature annealing steps wherein agglomeration (decomposition of TiSi.sub.2) occurs. These limitations are well known to those skilled in the art.

Due to the limitations associated with TiSi.sub.2, other refractory metal silicides are being investigated in more detail. In particular, cobalt silicides (CoSi.sub.x) are being increasingly used in sub-micron MOSFET structures. The theoretical minimum resistivity of CoSi.sub.2 is as low as that for TiSi.sub.2 in a crystalline phase. Furthermore, CoSi.sub.x has other desirable properties, particularly those associated with epitaxial source/drain electrodes in self-aligned silicide (salicide) MOSFET structures. CoSi.sub.X is known to exhibit significantly smoother silicide formation during the formation of salicide, which allows for shallower source drain junctions and reduced spacing between the source/drain junctions and the depletion region. This in turn allows for additional reduction in scaling of MOS devices before leakage current problems begin to appear. Furthermore, CoSi.sub.x is known to have properties which make it a superior dopant diffusion source over TiSi.sub.x, particularly when boron or arsenic are used as dopants. CoSi.sub.x is also known as a fine grain material (well below 400.ANG.) that has very small line-width dependent sheet resistance. This puts CoSi.sub.x as a good candidate for deep submicron IC application beyond 0.18 .mu.m technology.

While CoSi.sub.x shows potential as a refractory metal silicide in MOSFET circuit fabrication, shortcomings are known to exist which limit its use. Specifically, the processing of CoSi.sub.2 /polysilicon layers at moderate temperatures has been shown to cause the layer structure to severely deteriorate due to silicon recrystallization and grain growth within the silicide, leading eventually to complete consumption of the polysilicon layer and inversion of the layer order.

CoSi.sub.x films can be readily deposited on silicon substrates using physical vapor deposition, also known as PVD or sputtering. Sputter deposition processes occur in vacuum chambers, and the source material used to sputter deposit onto the substrate is a solid slab known as a target. Sputtering has advantages in that no chemical compositional changes occur during the deposition process, and the exact stoichiometry of the as-deposited material can be precisely regulated by regulating the composition of the target material. Alternately, controlled impurities can be introduced in the chamber during the sputtering process to affect the composition of the as-deposited film. For other silicides such as TiSi.sub.x, much is known about the effect such modifications have on the behavior and interactions of the silicide during further processing stages. Much less is known about the behavior of deposited CoSi.sub.x films, however, so shortcomings in the application of CoSi.sub.x remain.

For CoSi.sub.X to be effective as a metallization material in submicron polysilicon MOSFET fabrication, and for its other superior characteristics to fully utilized, it is clear that improvements are needed in the fabrication and processing of CoSi.sub.x films. More specifically, improvements are needed which will better control the grain size of as-deposited CoSi.sub.x films, and thereby the stability of the films during annealing.

According to an aspect of the invention, the novel target material is created by the steps of: Blending a small amount of SiO.sub.z material and CoSi.sub.x material; and applying pressure to the blended material in a hot powder press to form a target material composed of the compound CoSi.sub.x O.sub.y. The novel target can then be used in a PVD application for forming semiconductor structures.

According to another aspect of the invention, a method of fabricating a semiconductor device gate stack is provided, including the steps of: depositing an insulating film and a polysilicon film on a silicon substrate; patterning the insulating film and the polysilicon film into a gate oxide layer and a polysilicon gate on a selected portion of the silicon substrate while leaving another portion of the silicon substrate exposed; sputter depositing a film of the compound CoSi.sub.x O.sub.y onto the wafer, and forming an insulating spacer along the side walls of the gate stack. In the foregoing steps, the film of the compound CoSi.sub.x O.sub.y is formed into a metallization layer on the silicon substrate over the source/drain regions associated with a shallow junction region, and the film is also formed at the top of the gate stack to form a gate electrode. The depth of the shallow junction region will preferably be less than about 2000 .ANG.. Ion implantation can then be made into the film of co-deposited CoSi.sub.x O.sub.y over the source/drain regions where it will be used as a diffusion barrier, or the ion implantation can be made through and clear of the film of co-deposited CoSi.sub.x O.sub.y and into the silicon substrate. A conductive plug can then be formed on top of the metalization layer upon which a metal fill material such as aluminum can be positioned.

As will be shown, by proper selection of the layer depths and CoSi.sub.x O.sub.y stoichiometry, CoSi.sub.x O.sub.y polycide and salicide structures can be created that have superior thermal stability characteristics.

In order that the manner in which the above-recited and other advantages and objects of the invention are obtained may be more fully explained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments and applications thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments and applications of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 is a cross sectional illustration of a PVD target formed from the compound CoSi.sub.x O.sub.y material.

FIG. 2 is a cross sectional view of a layer of CoSi.sub.x O.sub.y deposited on a p-doped silicon substrate, and showing ion implantation into both the layer of CoSi.sub.x O.sub.y and the p-doped silicon substrate.

FIG. 3 is a cross sectional view of a stack that includes a layer of CoSi.sub.x O.sub.y deposited on a layer of polysilicon which is deposited on a gate oxide layer, the gate oxide layer being situated upon a p-doped silicon substrate.

FIG. 4 shows the stack of FIG. 3, with a layer of amorphous silicon interposed between the layer of polysilicon and the layer of CoSi.sub.x O.sub.y.

FIG. 5 is a simplified cross sectional schematic of a gate stack including the layer of the stack seen in FIG. 3, with a layer of oxide deposited on the layer of CoSi.sub.x O.sub.y, the gate stack being laterally covered by a TEOS side wall spacer positioned upon a shallow implanted source/drain junction, the shallow implanted junction also having deposited thereon a layer of CoSi.sub.x O.sub.y upon which a laterally isolated contact plug is positioned at an end thereof with an electrically conductive layer contacting an opposite end of the contact plug.

Additional objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein preferred embodiments of the invention are shown and described in the disclosure, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

The invention of a novel PVD target material compound which significantly increases thermal stability of annealed cobalt disilicide films is disclosed. The PVD target material compound is formed using conventional processing steps by combining a small amount of SiO.sub.z with CoSi.sub.x. In the preferred embodiment, x is chosen to be in a range of about 1.4 to about 2.2, and more preferably about 1.64. The variable z is chosen so that the resulting y will be in a range of about 0.010 to about 0.100 and, more preferably, about 0.039, although other values could be selected for a given z. The material is then blended into a mixture using known mixing techniques. The mixture of blended material is then placed in a hot powder press to be compressed and shaped at conventional hot powder press temperatures and pressures so as to form a target slab composed of the compound CoSi.sub.x O.sub.y. The shape of the target slab can be selected to be of appropriate dimensions for use in a sputtering chamber. The target slab, an example of which is seen at 10 in FIG. 1, may be used in conventional PVD sputtering process steps to deposit CoSi.sub.x O.sub.y films on a semiconductor wafers in standard sputtering chambers. Typically, thermal stability and bulk resistivity of CoSi.sub.x O.sub.y increase with value of y. There is a trade-off here to keep y small to maintain low bulk resistivity and achieve reasonable thermal stability. For gate electrode application, bulk resistivity up to 200 .mu.-Ohm-cm is acceptable. Preferably, a layer composed of CoSi.sub.x O.sub.y will have a bulk resistivity in a range of about 12 .mu.-Ohm-cm to about 200 .mu.-Ohm-cm.

The selected value of y in CoSi.sub.x O.sub.y determines the amount of oxide that will be within the resultant cobalt silicide film. The presence of oxide within the cobalt silicide film effectively restricts the migration of cobalt or silicon which is known to occur during high temperature annealing stages wherein polycide or salicide is formed. The oxide increases the overall sheet resistivity of the cobalt silicide film, but when used in small amounts as in the preferred embodiment, bulk resistivities of the cobalt silicide films have been observed to remain in a range of about 25 to about 26 .mu.Ohm-cm after annealing with no measurable change in resistivity value after repeated rapid thermal annealing steps at 1000.degree. C. This is only twice as much of the bulk resistivity (12-15 .mu.-Ohm-cm) of pure CoSi.sub.2.0, which is still considered to be a very low bulk resistivity. Furthermore, these results have been demonstrated for films as thin as 250 .ANG. that are deposited directly over amorphous silicon and polysilicon layers such as would be found in polysilicon MOSFET gate stacks. These results have been demonstrated for films deposited directly on epitaxial silicon such as would be the case for source/drain contacts.

Observations on as-deposited and annealed films which have been fabricated from the compound CoSi.sub.x O.sub.y reveal the average grain sizes to remain well under 10 nanometers. This fine grain material is an excellent candidate for deep submicron IC application because it has very small line-width dependent changes on bulk resistivity. The small grain sizes also make for a smooth surface, and serves to limit the migration of silicon into and through the CoSi.sub.x O.sub.y layer. In addition to enhancing the overall stability of the CoSi.sub.x O.sub.y layer, the limited migration of silicon into and through the CoSi.sub.x O.sub.y layer reduces consumption of silicon when a CoSi.sub.x O.sub.y layer has been deposited on an epitaxial silicon substrate. This in turn enables the fabrication of source/drain contacts to sufficiently shallow junction to minimize channel shorting effects, even when the MOSFET structures are scaled down to the deep submicron level.

While a preferred embodiment has been disclosed having specific values of x=1.64 and y=0.039 for CoSi.sub.x O.sub.y and SiO.sub.z, other values for x and z could be used, depending the optimal trade-off between sheet resistivity, thermal stability, and grain size for a given application. The amount of SiO.sub.z cab be varied depending upon sheet resistivity, thermal stability, and dry etch selectivity requirements. As such, a series of compounds of CoSi.sub.x O.sub.y can be made as "z" varies.

Examples of the use of the novel target material of the invention are describe herein with respect to FIGS. 2 though 5.

In FIG. 2, a CoSi.sub.x O.sub.y layer 12 has been sputter deposited onto a p-doped silicon substrate 14 of a semiconductor wafer, although an n-doped silicon substrate could be similarly treated. CoSi.sub.x O.sub.y layer 12 can be used as a diffusion barrier, and ions 16 can be implanted through CoSi.sub.x O.sub.y layer 12 into p-doped silicon substrate 14 prior to an anneal step. Alternatively, ions 18 can be implanted into CoSi.sub.x O.sub.y layer 12 prior to the anneal step.

In FIG. 3, a stack has been formed that includes CoSi.sub.x O.sub.y layer 12 deposited on a polysilicon layer 20 upon a gate oxide layer 22, the gate oxide layer being deposited on a p-doped silicon substrate 14 of a semiconductor wafer.

FIG. 4 shows the stack seen in FIG. 3 but also includes an interposed layer of amorphous silicon 23 in between CoSi.sub.x O.sub.y layer 12 and polysilicon layer 20.

A MOSFET polysilicon gate stack is shown in FIG. 5, which features the gate stack of FIG. 3 plus a layer 24 of oxide (or an insulating layer) deposited on CoSi.sub.x O.sub.y layer 12. Alternatively, FIG. 5 can also be interpreted to represent the gate stack of FIG. 4 plus a layer 24 of oxide deposited on CoSi.sub.x O.sub.y layer 12. The gate stack of FIG. 5 is laterally surrounded by TEOS side wall spacers 26, which may optionally also be formed of silicon nitride. TEOS side wall spacers 26 are positioned upon source/drain regions 30 which have been implanted with ions 32 and are covered by a layer 28 of CoSi.sub.x O.sub.y. By way of example, ions 32 can be elements such as P, N, Co, W, and Mo, although others are also contemplated. The purpose of the implantation of ions 32 is to further improve CoSi.sub.x O.sub.y thermal stability at the Si substrate surface where the dose of ions 32 should be smaller and shallower than typical source/drain ion implantations such that the presence of ions 32 does not affect MOS device performance.

Source/drain regions 30 can be implanted with ions through CoSi.sub.x O.sub.y layer 28 into p-doped silicon substrate 14 prior to an anneal step. Alternatively, ions can be implanted directed into layer 28 of CoSi.sub.x O.sub.y, similar to what is depicted in FIG. 2. An anneal step can then follow.

Preferably, source/drain regions 30 have a depth of less than about 2000 .ANG.. An electrically conductive plug 34 is positioned at an end thereof upon layer 28 and is contained at an opposite end thereof with an electrically conductive material 36, such as aluminum. Plug 34 can be composed, for example, of tungsten or polysilicon. Each plug 34 is electrically isolated from another plug 34 by insulative layers 38, 40.

The structures seen in FIGS. 2-5 can be formed by convention processing steps using the novel compound CoSi.sub.x O.sub.y as a PVD target in a sputtering deposition.

As described herein, the present invention takes advantage of SiO.sub.z mixed with CoSi.sub.x to form a compound material of CoSi.sub.x O.sub.y. The compound of CoSi.sub.x O.sub.y is formed into a target slab that is sputter deposited as a CoSi.sub.x O.sub.y film having superior grain structure. The grain structure of the CoSi.sub.x O.sub.y film restricts the movement of cobalt and silicon during high temperature processing, and produces layers and structures incorporating cobalt silicide oxide that have greatly improved thermal stability and reliability over conventional CoSi.sub.x film for use in fabrication of submicron MOSFET devices.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.