In addition to cleaning, the substrate surface can be modified in other ways to improve film properties. Surface modification can entail changing the chemistry, morphology or the mechanical properties of the surface.

Changing the surface chemistry can be accomplished by modification of the existing surface or by the addition of a layer that can be called the "glue layer." An example of a glue layer is the thin (50 nm) chromium or titanium layer deposited on glass before the deposition of gold or silver. The titanium reacts with the glass to give good adhesion to the surface and alloys with the gold to give good adhesion to the gold film material. The glue layer can be formed by grading the composition of the depositing film material. For example, when depositing TiN, the initial material deposited can be pure titanium by limiting the availability of the nitrogen. The film composition can then be "graded" to Tin by increasing the nitrogen availability during deposition.

An example of chemical modification of an existing surface is the oxygen or nitrogen plasma treatment of polymer surfaces. The plasma can be in the form of a flame or a corona discharge at atmospheric pressure or a low-pressure plasma discharge. The plasma treatment of most polymers form carbonyl (oxygen plasma) or amine (nitrogen or ammonia plasma) radicals on the polymer surface, which can then react with the depositing atoms to create an increased nucleation density and increased adhesion. The figure on the next page shows the effect of plasma treatments on the adhesion of metal films to various polymer surfaces. The plasma treatment can also increase the cross-linking in the surface region of a polymer and its mechanical strength. Plasma treatment is not effective in treating fluorocarbon polymer surfaces for increased film adhesion.

"Basecoats" are materials that are applied to substrate surfaces to change the surface properties, such as smoothness, or to act as a barrier for materials diffusing out of the bulk, or to prevent reaction of the film material with the surface. For example, a polymer basecoat can be flowed over a rough metal surface to provide a smooth surface for depositing a decorative coating on the metal, or an epoxy coating can be applied to a molded polymer surface to prevent plasticizers from diffusing to the surface during film deposition. In semiconductor processing, layers of sputter-deposited TiN and TiW and thermally nitrided titanium films are used as "basecoat" materials to prevent the reaction of aluminum or tungsten metallization with the silicon substrate. Another example is the planarization (smoothing) of a patterned semiconductor surface with a liquid spin-on glass (SOG) such as siloxane.

Topcoats are materials generally applied to film surfaces to protect the surfaces. For example, aluminized headlight reflectors are dip-coated with a five micron-thick heat-cured polysiloxane coating to impart abrasion and corrosion resistance to the aluminum surface, and ion-plated aluminum coatings for corrosion protection can have a chromate conversion coating applied to increase corrosion resistance of the coating.

Polymer materials used for basecoats and topcoats can be applied by painting, powder spraying, electrostatic spraying, dip-coating, electrophoretic deposition or plasma polymerization. Liquid polymer

coatings can be cured (i.e., cross-linked) by heat, ultraviolet (UV) radiation or electron beam irradiation. A major problem with liquid polymers is the volatile organic content (VOC) that has to be disposed of in a pollution-!free manner. Generally, the heat-curable polymers have the highest VOC content, with the UV-curable materials having less and the plasma-deposited polymers the least. Some polymer solutions are water-based and do not have the VOC problem but are often not the best polymer for the job.

Increasing the surface roughness can increase the adhesion of a deposited film by "mechanical interlocking" of the film material with the substrate if the deposition process results in "filling-in" the surface roughness. The rough interface causes a circuitous fracture path on adhesion failure. The roughness of surfaces can be increased by mechanical abrasion or chemical etching. The surface roughness can affect the film growth and the film properties. Therefore, substrate surface roughness can be one of the critical parameters in obtaining reproducible film properties.

The mechanical properties of a surface can be modified without changing the chemical composition of the surface. For example, the surface of a brittle material can be strengthened (increased fracture toughness) by putting it in compressive stress. A compressive stress in the surface region can be attained by "show peening" or "work hardening" a ductile material or by "thermal tempering" a brittle material such as a glass. In many cases the strength of a surface can be increased by removing the flaws from the surface. For example, machined brittle surfaces such as carbon, ceramics or glass are chemically etched to blunt surface cracks caused by the machining operation. If the surface flaws are not removed before the film is deposited, they can act as fracture initiation sites in the interfacial region, which results in poor adhesion of the film. Surfaces that are porous and friable can be densified and strengthened by impregnation with a bonding material.

Changing the chemical composition of the surface can be used to change its mechanical properties. For example, reacting the surface region of a metal with carbon, nitrogen or oxygen can form a compound or composite metal-compound region that is mechanically stronger than the original surface. One technique for increasing the surface hardness of an alloy steel is by "plasma nitriding" the surface region to form metal nitrides in a metal matrix to a depth of several microns. Plasma nitriding can be combined with the reactive deposition of a TiN film in the same processing equipment in a single processing cycle to create a combination of surface reaction and film deposition to enhance the wear resistance of the steel surface.

Surface modification can also be in the form of "activation" of a surface by the removal of a surface layer such as an oxide. This effectiveness of the activation can be very time-dependent because the surface layer can reform with time. For example, an oxide-covered surface can be mechanically brushed in a vacuum to disrupt the oxide layer and roughen the surface just prior to film deposition. This surface modification technique has been termed "mechanical activation" and is used in the large-scale coating of aluminum on strip steel. The corona or plasma surface treatment of a polymer is often called activation, and the effect of the surface oxidation decreases with time as the high-surface-energy-oxidized surface converts to a low-energy surface by the diffusion of low-energy material to the surface from the bulk.

Surface modification can also be in the form of "sensitization," where a small amount of material is added to the surface to act as a "nucleating agent." For example, when oxygen plasma cleaning an oxide surface, the ozone formed in the plasma is adsorbed on the oxide surface and is available to react with a depositing oxygen-active material. The adsorbed oxygen increases the nucleation density of the depositing atoms of metals that do not have strong oxygen bonding, such as silver. When adsorbed oxygen is on the oxide surface, the silver atoms do not have to react with the substrate material and compete for oxygen bonding. The ozone desorbs with time, so the sensitization is time-dependent. An oxygen active surface such as uranium can be sensitized by acid stripping the oxide and, at the same time, depositing tin from the acid bath by chemical displacement. The deposited tin will then act as a nucleating agent for the subsequent deposition of the film material.

Other sensitization treatments include deposition of sub-stoichiometric compound films and the excess film constituent is available for reaction with a depositing atom. For example, when evaporating an oxide material such as Bi203 or PbO, oxygen is lost from the deposited film and the excess bismuth or lead is available to alloy with the depositing film material, such as gold. High energy radiation such as ultraviolet or X-ray radiation can be used to sensitize insulator surfaces by creating charge sites that act as nucleating sites for the depositing atoms.

References

1. R.W. Burger and L.J. Gerenser, "Understanding the formation and properties of metal/polymer interfaces via spectroscopic studies of chemical bonding," Proceedings of the 34th Annual Technical Conference of the Society of Vacuum Coaters,1991, p. 162 (ISBN 1 -878068-10-5,1991).

2. D. Pramanikand V.Jain,"Barriermetals for ULSI: Deposition and Manufacturing," Solid State Technol36(1), 73 (1993)

3. S. Dressier, "Single cycle plasma nitridingóTiN deposition for alloy steer parts," Industrial Heating 59(10), 38 (1992).