Tungsten Metallization

This invention relates generally to metallizations used in semiconductor devices, and particularly to devices having such metallizations which use tungsten.

As the complexity of integrated circuits continues to increase, the dimensions of the components of the integrated circuits continue to decrease. Not only do device dimensions decrease, but the dimensions of the interconnects, that is, the lines and windows used to connect devices decrease. The windows are often termed vias by those skilled in the art. It is noted that the term window is sometimes applied only to the openings to the source, gate, or drain electrodes while the term via is applied to the opening between levels in multilevel metal structures. Vias may be conveniently visualized as cylinders which are filled with a metal so that, e.g., devices located on different metallization levels can be electrically connected. As it is difficult to completely fill a small diameter cylinder with sputtered Al, a metal commonly used in integrated circuit metallizations, a heating step is frequently used to cause an overlying dielectric layer to flow into a portion of the via. The heating step creates a more easily filled conical shape. The conical shape is, of course, truncated at the bottom so that an electrical contact can be formed to the underlying conductor.

As smaller devices typically use relatively shallower junction depths than do larger devices, the use of high temperature thermal processing to taper the vias by causing a dielectric material to flow eventually becomes impossible as the via dimensions decrease. Consequently, the vias that must be filled with metal not only have a high aspect ratio, that is, a high ratio of height to width, but their walls are also substantially vertical. Aluminum is difficult to deposit uniformly in such vias and poor step coverage results for conventional deposition techniques such as sputtering. This not only leads to possible discontinuities in the metal coverage but also makes planarization of the surface, frequently required for subsequent processing such as second level metallization, very difficult.

Al metallizations suffer from several drawbacks. Al has a low electromigration resistance and is susceptible to hillock formation. Also, because of its low melting point and tendency to react with Si, severe limitations are placed on the maximum temperature that can be used in post-metallization processing.

Accordingly, alternatives to aluminum have been sought for at least portions of the metallization. One commonly contemplated alternative metal is low pressure chemical vapor deposition (LPCVD) tungsten. LPCVD tungsten is a desirable alternative because it has a conformal step coverage. LPCVD tungsten also offers advantages for use as interconnects. Besides having conformal step coverage, it has high electromigration resistance, resistance to hillock formation and high temperature stability. Although many methods of depositing LPCVD tungsten have been proposed, they are all included within two generic categories which are conveniently termed selective and blanket.

Selective deposition typically relies upon the reaction of a gas, such as tungsten hexafluoride, with substrates, such as silicon, to leave tungsten on the silicon surface. Selective deposition is also possible on metals and silicides. A carrier gas, such as hydrogen, is commonly used. Tungsten hexafluoride does not react with other materials, such as a surrounding dielectric region of silicon dioxide, and in theory, tungsten is deposited only on the silicon surface. However, selective deposition is difficult to obtain in practice in the thicknesses which are needed to fill deep vias because of the loss of selectivity. This probably arises because some tungsten hexafluoride reacts with the hydrogen carrier gas thus forming HF as well as nucleating W. HF reacts with and etches the SiO.sub.2, which is obviously an undesirable result.

With blanket deposition, tungsten is deposited over the entire surface and then etched back so that tungsten ideally remains only in the vias or as, for example, interconnects. While this process is conceptually simple, tungsten does not adhere well to silicon dioxide and practical problems arise. For example, after the deposition has been completed, the tungsten film may simply peel off the silicon dioxide which is also an obviously undesirable result.

The use of a glue layer has been proposed to overcome the adhesion problem just discussed. A glue layer is a layer of material deposited prior to the tungsten and which has good adhesion both to the underlying dielectric layer and to the tungsten. Several substances have been proposed for glue layers. For example, the use of both elemental metals, such as Ti, and metallic silicides, such as WSi.sub.2, has been proposed. See, for example, Comparison of Two Contact Plug Techniques for Use with Planarized Oxide and A Contact Filling Process with CVD-Tungsten for Multilevel Metallization Systems, Proceedings of the V-MIC Conference, pp. 403-410, and pp. 443-449, Jun. 9-10, 1986, respectively. A common feature of the approaches adopted by these references is the use of relatively thick glue layers. Thicknesses of approximately 100 nm or more were used. It should be noted that the thickness of the Ti layer in the former reference is not explicitly given. However, FIG. 3 illustrates a plug after etching has been completed. It is evident that there is severe overetching of the glue layer which would not be visible if a thin glue layer had been used.

However, the use of a thick glue layer is undesirable because during the etch back step, severe undercutting of the tungsten layer occurs if, as is often the case, the glue layer etches more rapidly than does the tungsten. The undercutting may make subsequent processing very difficult. For example, voids may be left in the oxide and in subsequent metallizations after metal deposition.

Of course, the glue layer material should be electrically conducting. Only metals or silicides have been proposed as glue layer materials because of the rapid film growth in these materials. However, the use of some metals, such as aluminum, has not been seriously considered because a thick aluminum layer may cause spiking due to its rapid diffusion into the underlying material. Other conducting compounds have not yet been proposed as glue layers.

We have found that tungsten can be blanket deposited with good adhesion over a dielectric covering a portion of a silicon surface by first depositing a film comprising either Al or a conducting nitride such as TiN as a glue layer. The glue layer film may be deposited, through openings in the dielectric, directly on the silicon or on a conducting material, such as a silicide, overlying the silicon. Of course, the glue layer is also deposited on the dielectric. Both TiN and Al films provide good adhesion down to thicknesses as small as approximately 3 nm. The minimum thickness is determined primarily by the requirement that the entire wafer surface be covered adequately. The thinner glue layers are generally preferred as they minimize any problems that might be caused by either spiking of deposited metal or undercutting during reactive sputter etching (RSE). However, if interaction of W with the substrate is to be prevented, a thicker layer of TiN may be used. To prevent spiking, the Al layer is typically less than 25 nm thick although the maximum thickness will be determined by device design and processing parameters. Uses other than plugs are contemplated. In particular, interconnects and gate structures are also contemplated.

FIG. 1 is a schematic representation of a exemplary metallization according to this invention;

FIG. 2 is a schematic representation of a via filled with tungsten after the etching step has been completed; and

FIGS. 3 and 4 are schematic representations useful in explaining interconnects according to this invention.

For reasons of clarity, the elements of the devices depicted are not drawn to scale.

Our invention will be described by reference to FIG. 1 which is a schematic representation of a tungsten metallization according to this invention. Depicted are silicon layer 1, dielectric region 3, glue layer 5 and tungsten metallization 7. As can be seen, the glue layer and metallization extend into a via 9. The glue layer covers the interior surface of the via as well as the underlying silicon layer 1. The glue layer also covers the dielectric region 3. It will be readily appreciated that the individual components of the integrated circuit are not depicted for reasons of clarity.

The vias are formed by conventional VLSI processing steps which deposit, pattern and etch the dielectric, etc. These steps are well known to those skilled in the art and need not be described in detail. The dielectric region depicted comprises SiO.sub.2 although it will be appreciated by those skilled in the art that other dielectric materials may be used. For example, B and P doped SiO.sub.2 and Si.sub.3 N.sub.4 might be used. It will also be appreciated that although layer 1 is described as silicon, other conducting materials might be used. For example, conducting silicides might be used. Such silicides are formed on the silicon surface. Thus, while the glue layer is described as being deposited over a silicon surface, the presence of other materials between the glue layer and the silicon surface is contemplated.

The glue layer is expediently deposited by well known techniques such as sputtering. The glue layer comprises at least one material selected from the group consisting of Al and conducting nitrides such as TiN. It will be appreciated that minor amounts of other materials may be present in the glue layer. For example, either or both Si and Cu may be present in Al. These materials are metallurgically stable with respect to the tungsten film. Good adhesion, as evidenced by an inability to remove the film by pulling on an attached adhesive tape, is obtained down to glue layer thicknesses of approximately 3 nm.

The native oxides on TiN and Al.sub.2 O.sub.3 are thin. Although the successful use of Al might seem surprising in view of its strong susceptibility to oxide formation, it is believed that during the tungsten deposition most of the aluminum oxide present is removed although residues of non-conductive fluorides may remain. To prevent spiking, the Al layer should be less than approximately 25 nm thick. However, as will be readily appreciated by those skilled in the art, a precise maximum thickness cannot be given as it depends on several factors. These factors include the desired junction depth and the post Al deposition processing parameters. Given these factors, one skilled in the art can readily determine the maximum Al thickness.

The thinness of the glue layer is surprising but it must be noted, in retrospect, that pinholes in the glue layer need not be a significant problem. That is, the presence of some pinholes will not necessarily adversely affect the adhesive properties sought. It must also be remembered that, within the vias, pinholes are not significant as the electrical contact will still be formed if W goes through a pinhole at the bottom of the via provided that there is no chemical interaction with the substrate. For example, W interaction with, e.g., TiS.sub.2, is not a problem. However, pinholes may be a problem for the W/Si interface although the interaction of W with a small amount of silicon can generally be tolerated. No adverse results occurs if the W touches the dielectric through a pinhole.

The growth of W films on the nitrides is surprising because it is hypothesized there are no displacement reactions which provide W nuclei for further film growth, i.e., the nitrides do not reduce WF.sub.6. The Ti-N bonds are sufficiently strong so that the interaction of Ti with WF.sub.6 is not energetically favored. The growth of W films on the nitrides is believed to occur by the reduction of WF.sub.6 with H.sub.2, solely. Upon exposure to WF.sub.6 and H.sub.2, H and F atoms react and nucleate W on the surface. After the resulting islands coalesce, the growth rate of films is similar to the rates on Si or Al coated surfaces. The disadvantage of using a displacement reaction is that fluorides are formed at the interface.

FIG. 2 depicts a via after etching. Conventional etching techniques can be used for the etching steps. It should be remembered that the tungsten layer is typically overetched to guarantee its complete removal from areas not covered with resist in case there are non-uniformities in layer thickness. The overetch not only removes the tungsten but removes the thin glue layer contacting layer 1 without the need for any additional etching chemistry.

The absence of significant amounts of O and F at the W/TiN interface makes TiN ideally suited for use as a glue layer with etch-back plugs and interconnects.

The use of a thin layer also minimizes undercutting problems. It is apparent that a relatively planar surface is obtained in preparation for further processing. It should be noted that at most only a very thin layer of A1 contacts the Si. This eliminates or minimizes the possibility of spiking which occurs with thicker A1 films.

Other uses for our deposition process are contemplated. For example, it may be used to form interconnects or gate structures. The strong adhesion provided by thin glue layers is useful for patterning W interconnects. FIG. 3 is useful in explaining interconnect fabrication. Numerals identical to those used in FIG. 1 represent identical elements. Also depicted is patterned resist 11. To form interconnects, the tungsten and glue layers depicted are etched and the resist removed to yield the structure shown in FIG. 4. An A1 layer under W interconnects should provide low resistance contacts. The use of A1 is particularly suited for upper levels in multilevel metallization schemes where junction spiking is not a consideration. Also, it is suitable for very thin A1 glue layer on Si contacts, as well as silicided junctions, where junction spiking may not occur.