GB2181456A

GB2181456A - Chemical vapour deposition of tungsten on dielectrics

Info

Publication number: GB2181456A
Application number: GB08622272A
Authority: GB
Inventors: David Winfield Woodruff
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1985-10-07
Filing date: 1986-09-16
Publication date: 1987-04-23
Also published as: DE3631758C2; JPS6248752B2; GB8622272D0; DE3631758A1; GB2181456B; FR2588277A1; JPS62109973A; FR2588277B1

Abstract

Silicon dioxide or silicon nitride dielectrics are sputter coated with molybdenum prior to coating with tungsten by chemical vapour deposition in order to improve the adhesion of the tungsten layer.

Description

SPECIFICATION Depositing metal films on dielectric substrates The present invention relates generally to the formation of thin tungsten films upon substrates.

More specifically, it relates to a method for depositing adherent tungsten films over dielectric surfaces such as silicon dioxide and silicon nitride.

In the manufacture of semiconductor devices, it is often desirable to deposit thin layers or films of metal upon various dielectric surfaces in order to form the components of integrated circuits. The process of depositing metal films is often referred to as metallization. Because the layers resulting from the metallization have varying stresses associated with them, and because the interfaces between these layers have varying degrees of chemical bonding, it has been seen that certain metals will not adhere very well to dielectrics. Such is the case of tungsten deposited by chemical vapor deposition (CVD) upon silicon dioxide, as reported by C.M. Melliar Smith, et al. in "Chemical Vapor Deposited Tungsten for Semiconductor Metallizations", Journal of The Electrochemical Society, Vol. 121, No. 2, pp. 298-303.

It is desirable, however, to form adherent tungsten films on dielectric surfaces in that tungsten exhibits high conductivity and does not migrate at areas of high current density where the film is thin, as does aluminum. This phenomenon is often referred to as "electro-migration" and can cause portions of the metallization layer to separate. In addition, silicon is soluble in aluminum and where they are in contact, silicon will penetrate the aluminum surface, leaving a void between the two surfaces. This phenomenon is often referred to as "spiking".

Silicon is not soluble in tungsten and due to its high conductivity and resistance to migration, efforts have been made to fill integrated circuit contact holes and vias with tungsten, such as the method disclosed in commonly assigned copending application Serial No. 733,445, filed May 1985. In such processes, tungsten is selectively deposited on the metal and semiconductor surfaces, where adhesion is satisfactory. Deposition on the dielectric surfaces is not desired in these processes.

It would be advantageous to deposit tungsten in a manner allowing simultaneous filling of the contact holes and vias with formation of a metallization layer. Manufacturing efficiency would be enhanced and the number of contact interfaces would be reduced improving the performance of the device.

Problems with metallization layer adhesion have commonly been overcome by the use of an adhesion promoter or "glue layer" between the non-adherent materials. However, adhesion promoters suitable for tungsten films deposited by chemical vapor deposition have been difficult to obtain.

Tungsten silicides are known adhesion promotors for CVD tungsten. However, layers of tungsten silicide (WSi2) are difficult to deposit. Special equipment is required to provide these layers by reaction of SiH4 and WF6. Sputter-deposited glue layers are much simpler to form and do not require complex equipment. Both sputtered titanium and chromium have been suggested as adhesion promoters for CVD tungsten. However, they have been found to be less than satisfactory. In "A Study of the Adherence of Tungsten and Molybdenum Coatings", J. I.

Federer, et al, Proceedings of the International Conference on Chemical Vapor Deposition, 3rd Edition, pp. 591-599, the authors cite a displacement reaction between chromium and the chemical vapor deposition gases as a reason for poor adhesion. The chromium layer is etched away by this displacement reaction.

A method for depositing sputtered tungsten films on a dielectric surface is disclosed by M.L.

Tarng et al in U.S. Patent No. 4,404,234. Tarng et al promote the adhesion of sputtered metallization layers by partially covering a silicon dioxide dielectric surface with CVD tungsten or CVD molybdenum. The partial tungsten or molybdenum deposits form small islands on the dielectric surface, roughening the surface to promote adhesion. Unfortunately, this process is unsuitable for producing integrated circuits with a high component density and high aspect ratios, as is required in very large scale integration (VLSI) and ultra large scale integration (ULSI) applications.

Sputtered metallization layers do not provide the uniformity obtained by CVD metallization layers and they do not conform as well to the underlying surface. This is illustrated by l.A. Blech and H.A. Vander Plas in their article "Step Coverage Simulation and Measurement in a dc Planar Magnetron Sputtering System", J. Appl. Phys., Vol. 54(6), June 1983, which is incorporated herein by reference. More particularly, Blech and Plas discuss the coverage of sputtered layers over non-planar regions, referred to as "step-coverage" in the article. This non-conformity in sputtered layers is due to the fact that sputtering is a line-of-site process where the source of metal is remove from the surface, making uniform deposition difficult.In chemical vapor deposition processes, the source of metal is a gas which contacts the surface of the underlying layer, permitting an equal opportunity for deposition at every point on the surface. As a result, nonplanar regions with high aspect ratios (having a height/width value greater than 1/2) are difficult to fill by sputtering processes.

The present invention has been developed in response to the need in the art for a convenient method which provides adherent tungsten metallization layers obtained by chemical vapor deposition suitable for use in VLSI and ULSI applications.

In its broadest sense, the present invention concerns a method for producing adherent tungsten films deposited by chemical vapor deposition over a dielectric surface. This is achieved through the use of a molybdenum adhesion promotor or glue layer. This molybdenum glue layer is not attacked by the reactive gases of tungsten chemical vapor deposition and promotes excellent adhesion of tungsten over silicon dioxide on both planar regions and nonplanar regions of high aspect ratios (height/width), particularly where the aspect ratio values are greater than 1.

More specifically, this invention relates to a method for depositing a continuous, conformal tungsten film over a dielectric surface comprised of silicon dioxide or silicon nitride which comprises the steps of: sputter-depositing molybdenum on a dielectric surface to form a continuous layer thereon; and thereafter depositing tungsten on the adherent molybdenum layer by chemical vapor deposition.

As described herein, there is provided a method for enhancing the adhesion of CVD tungsten films over silicon dioxide so as to provide a continuous, conformal metallization layer suitable for use in integrated circuits.

The dielectric surfaces which can be processes in accordance with this invention include surfaces of silicon dioxide and silicon nitride. Such surfaces are most common in the transistors and integrated circuits utilized by the electronics industry. These surfaces can be sputtered upon a substrate such as the semiconductor surface of a silicon wafer or chip to provide an insulating layer above the substrate. Alternatively, a silicon dioxide layer can be generated by oxidizing the silicon semiconductor surface of the substrate.In addition, the silicon dioxide layer can be deposited on a substrate via chemical vapor deposition or plasma enhanced chemical vapor deposition by the reaction of silane homologs, i.e., SiH4 and SiCl2H2, with an oxygen source, i.e., O2 and N2O. For plasma enhanced chemical vapor deposition, an RF generator creates a plasma which in turn enhances the activity of the gas, permitting deposition at lower temperatures.

The dielectric surfaces can be continuous, planar surfaces or they can be patterned, nonplanar surfaces, such as the surfaces of a silicon wafer having patterned contact holes and vias where the underlying substrate is exposed. These patterned dielectric surfaces can be obtained by methods well-known to the art, such as a photolithography process wherein the surface is treated with a photoresist, masked and exposed to radiant energy. Where the patterned dielectric surface is obtained by a photolithography process, the depth of the contact holes preferably range from about 10 Angstroms to 3 microns, and most preferably from about 500 Angstroms to 1 micron.

A molybdenum glue layer or adhesion promoter is sputter deposited onto this dielectric surface. The term "sputtering" or "sputter-depositing", as used herein, refers to a process for depositing metallic films wherein metal atoms are removed from a source-material electrode by bombarding the electrode with the ions of an inert gas, such as argon. These argon ions are generated by radiating the argon with a radio frequency. A dc current is passed through the electrode to attract the argon ions. The bombarding argon ions then knock off metal atoms on the electrode and provide a metal atom vapor pressure. When the system is saturated with metal atoms, they condense onto cold surfaces, including those of the substrate. Generation of the plasma is continued until the metal atoms which condense on the substrate form a layer of a desired thickness.

Essentially any sputtering process known to the art is suitable for producing the molybdenum glue layer. This includes the glow discharge sputtering, cylindrical magnetron sputtering, planar magnetron sputtering, s-gun magnetron sputtering and ion beam deposition, more particularly described by Vossen and Kern in Thin Film Processes, Academic Press (1978), pp. 12-204, which is incorporated herein by reference. Essentially any apparatus suitable for sputtering the molybdenum glue layer is suitable. Such devices are more particularly described by Vossen and Kern. These devices generally vary by the magnetron utilized, which can be circular, planar, cylindrical, etc.

Spattering of the molybdenum layer does not expose the dielectric surface to aggressive reactant gases as in chemical vapor deposition. Displacement of the dielectric surface materials does not occur in the presence of the plasma and molybdenum vapor and therefore, the dielectric surfaces remain intact. In that the sputtering process is dependent on line-of-site geometry, present sputtering processes do not fill contact holes, vias and other regions of a high aspect ratio uniformly. However, sputtering will provide a continuous molybdenum glue layer on the dielectric surface.

The term "continuous" as used herein refers to a coating, which, at a minimum, completely covers the high profile surfaces of the dielectric, i.e. the planar surfaces other than those surfaces within contact holes, vias, and other regions of a high aspect ratio. The term "nonplanar regions of a high aspect ratio" refers to zones wherein the high profile surfaces are etched away, such as by photolithography, forming a hole or via having a high height/width value, which is generally above 1/2. Thus, the side walls and bottom of contact holes need not be covered by the molybdenum glue layer. As long as the molybdenum covers the high profile surfaces surrounding the contact holes, the result will be satisfactory as tungsten will fill the holes during a subsequent processing step.

Although the sputtered molybdenum need not cover the surfaces within the contact holes, vias, etc, the sputtered molybdenum layers may cover these surfaces without departing from the scope of this invention. In fact, when possible, depositing a uniform layer of molybdenum which conforms to all the surfaces of the dielectric, including nonplanar surfaces, is preferred.

The thickness of the molybdenum glue layer is chosen so as to minimize the stress of the layer while maintaining continuity of the layer on the surface of the dielectric. A glue layer of molybdenum having a thickness of about 250-1000 Angstroms is preferred with those of approximately 500 Angstroms being most preferred. However, where thinner, continuous layers can be deposited uniformly, these may be desired. The process of this invention does not have an upper limit for the thickness of the sputtered molybdenum glue layer; however, clogging of the contact holes and vias may occur where the sputtered molybdenum layer is too thick.

Following deposition of the continuous molybdenum layer, a tungsten metallization layer is applied to the substrate by chemical vapor deposition. The tungsten metallization layer will be both continuous and conformal. The term "conformal" refers to a layer which covers the entire underlying surface, including planar, high profile areas and nonplanar areas of a high aspect ratio.

Tungsten will fill contact holes, vias and other regions with high height/width values. This is due to the fact that chemical vapor deposition is a material synthesis process wherein the constituents are in the vapor phase and react to form a film at the surface.

Chemical vapor deposition is not dependent on line-of-site geometry. Every point on the surface which is in contact with the gas is a site for film growth. Thicknesses of tungsten metallization layers as high as 3 microns can be achieved when this invention is employed.

Thicker films are believed to suffer from too much stress. The preferred range is about 500 Angstroms to 2 microns.

Essentially, any tungsten chemical vapor deposition process is suitable for use in this invention. The most common CVD processes for depositing tungsten are those which reduce WF6 or WCI6. Those tungsten CVD processes which perform the reaction of WF6+3H2#Wi+6Hfl are particularly suitable for this invention. A more detailed description of a chemical vapor deposition process is given by Kirk-Othmer in Encyclopedia of Chemical Technology, 3rd Edition, Vol. 13, p. 636 and by Vossen and Kern in Thin film Processes (1978), pp. 257-319, referred to above. Both of these publications are incorporated herein by reference.

Chemical vapor deposition processes which are suitable provide for (1) the transport (meter and timing) of the reactant and diluent gases into the reactor to contact the substrate surface and permit adsorption of the reactants onto the surface, (2) the heat necessary for reaction of the reactant gases and diffusion of reactant gases onto the surface and (3) the transport of by-product gases from the substrate which desorb from the surface.

The suitable CVD processes include low-temperature CVD, high temperature CVD, low pressure CVD, plasma enhanced CVD and atmospheric pressure CVD. The preferred process is low pressure CVD.

Essentially any apparatus suitable for providing chemical vapor deposition of tungsten is suitable for use in this invention. These include both hot-wall and cold-wall reactors of a horizontal, pedestal, barrel and pancake configuration, as described by Vossen and Kern in Thin Film Processes.

Preferred conditions for the chemical vapor deposition process utilize temperatures in the range of about 300" to 7000C and pressures of approximately 0.1 Torr to about 1 atmosphere.

Preferred temperatures fall within the range of about 400-500"C and preferred pressures are of about 1-2 Torr. The ratio of hydrogen to tungsten hexafluoride may range between approximately 3:1 and 1000:1. The preferred ratios fall within the range of 5:1 to 20:1, with ratios approaching 20:1 being most preferred.

Upon performing this process, the article produced is suitable for further processing. When tungsten is deposited upon the substrate by the methods disclosed herein, it will yield a metallization layer with good adhesion, excellent coverage of contact holes and vias with little or no degradation of the dielectric substrate material.

The following examples are provided to illustrate particular embodiments of this invention. It is not intended to limit the scope of this invention to the embodiments disclosed.

EXAMPLE 1-18 In these examples a cold-walled reactor produced by Materials Research Corp. was utilized to sputter the adhesion layer onto the silicon wafer. Three types of metal adhesion layers were tried, including molybdenum, titanium and chromium. In addition, adhesion to polysilicon obtained by chemical vapor deposition was tested. The metals above were deposited at a thickness of about 500 Angstroms while the polysilicon was deposited at about 1000 Angstroms. Tungsten was deposited at varying thicknesses over the metal adhesion layers and polysilicon. The thicknesses range from about 0.5 to about 2.5 microns. Tungsten was deposited in a coldwalled reactor operating at about 450"C with a mixture of hydrogen and tungsten in a ratio of about 10.20:1. Deposition was carried out for approximately 10-25 minutes.The laminates produced were tested for adhesion in accordance with the tests described below under the heading "Experimental". Details as to the tungsten thickness, the adhesion layer identity, and the adhesion rating are provided in Table I below.

Example 1 illustrates the level of adhesion obtained for tungsten to a semi-conductor surface (non-dielectric) and Example 2 illustrates the level of adhesion obtained for tungsten to a dielectric surface, silicon dioxide. Examples 1 and 2 are not within the scope of this invention.

They are provided to calibrate the level of adhesion in the articles produced by this invention.

TABLE I Comparative Levels of Adhesion Obtained From Various Glue Layers Example CVD W Adhesion No. Layer Thickness Rating 1 Si wafer 2.63 ,u 6.0 2 none 1.4 ,u 2.0 3 Mo 2.5 jt 6.0 4 Mo 0.78 8 6.0 5 Mo 1.9 ,u 6.0 6 Mo 1.46 8 6.0 7 Mo 1.65,u 6.0 8 Ti 2.1 8 6.0 9 Ti 0.65 2.0 10 Ti 1.3 , 2.0 11 Ti 1.01 it 2.0 12 Cr 2.4 ii 2.0 13 Cr 0.54 8 2.0 14 Cr ** 15 Cr ** 4* 16 Poly 1.7 ,ìx 2.0 17 Poly 1.4 4.0 18 Poly 1.3 8 2.0 * Deposition of tungsten directly on SiO2 ** No deposition was achieved This Table illustrates that molybdenum is a superior adhesion layer for all thicknesses of CVD tungsten. Only one combination of a 500 Angstroms titanium adhesion layer and a 2.1 micron tungsten metallization layer matched the adhesion rating obtained for all molybdenum-tungsten combinations. With chromium, the adhesion was always poor and occasionally no deposition took place.

Experimental To evaluate the adhesion of the glue layers for CVD tungsten, the graded adhesion tests described below in Table II were utilized. The "tape" utilized was 3/4" wide, cellophane tape No. 6100, distributed by LePages, Inc., Pittsburgh, PA.

Where "etched", the tungsten layer was masked with black wax by placing a row of dots on the surface. A wet etch was applied to the masked surface to remove the uncovered tungsten.

The wet etch comprises 30 gm. K3Fe(CN)6, 10 gm. NaOH and 100 ml. H2O. The wax was removed and adhesion of the remaining tungsten was monitored.

Where "scribed", a diamond tipped pen was utilized to cut 6-8 lines within the tungsten layer. Adhesion at the scribe marks was monitored.

Adhesion Ratings If it passes... It is rated..

Nothing, spontaneous delamination 0 No spontaneous delamination 1 Brass Wire Brush (Brush) 2.0 Tape pull fails, survives Brush 2.2 over edges Survives Tape Pull, no etch 3.0 ETCH; black wax, wet etch 3.1 Brush 2 over etch lines 4.0 Tape Pull over etch lines 5.0 SCRIBE 5.1 Brush 6.0 Tape pull over scribe lines 7.0 The thickness of the molybdenum glue layer and tungsten CVD layer can vary widely. These similar variations of the process will be obvious to those skilled in the art from this disclosure and are deemed within the scope of the invention.

Claims

1. A method for depositing a continuous, conformal tungsten film over a dielectric surface comprised of silicon dioxide or silicon nitride said process comprising: sputter-depositing molybdenum to form a continuous layer on said dielectric surface and thereafter depositing tungsten on the continuous molybdenum layer by chemical vapor deposition to form a continuous, conformal layer of about 3 microns or less.

2. A method as in claim 1 wherein the dielectric surface has non-planar regions of a high aspect ratio having a height/width value greater than 1/2.

3. A method as in claim 2 wherein the dielectric surface is patterned by photolithography and the underlying layer is exposed in the non-planar regions, said underlying layer being selected from the group consisting of silicon, tungsten, aluminum and molybdenum.

4. A method as in claim 3 wherein the patterned dielectric surface is part of an integrated circuit and the regions having a high aspect ratio are contact holes and vias.

5. A method as in claim 1 wherein the molybdenum is deposited to a thickness of between approximately 250 and 1000 Angstroms.

6. A method as in claim 1 wherein the tungsten is deposited to a thickness of between approximately 500 Angstroms and 2 microns.

7. A method as in claim 1 wherein the chemical vapor deposition of tungsten is achieved by the reaction of a gaseous mixture of WF6 and H2.

8. A method as in claim 1 wherein the molybdenum is deposted to a thickness of about 500 Angstroms, the tungsten is deposited to a thickness of between about 500 Angstroms and 2 microns and the chemical vapor deposition of tungsten is performed at a temperature of about 400"-500"C, a pressure of about 1-2 Torr and a ratio of H2 to WF6 of about 10:1 to 25:1

9. A method of depoditing a tungsten film over a dielectric surface, the method being substantially as herein described with reference to any one of the examples.