Physical Vapor Deposition: the Technical Process with a Relatively Short Past & a Long, Exciting Future

Physical Vapor Deposition: The Technical Process With a Relatively Short Past & a Long, Exciting Future

By Donald M. Mattox

When it comes to physical vapor deposition processes, Don Mattox is P&SF’s resident expert. Normally, he serves readers monthly as the columnist for “SVC Topics,” but he responded to giving an extended overview in this issue. As the technical director of the Society of Vacuum Coaters in Albuquerque, NM, Mattox is highly qualified to give special industry insight on where PVD has been—and where it’s going. Q. What are some of the recent developments in PVD processing technologies? A. This is one thing that never ceases to amaze me. Every few years there is a new technique or equipment that comes available and allows new processes and products to be developed. In the last couple of years, the use of pulsed DC sputtering and biasing has allowed dielectric films to be deposited at very high rates. Also, films formed by sequential deposition and reaction every few atomic layers are being used. In the past 10 years, magnetron sputtering has become the premiere PVD vaporization technology, even though I am sure that thermal evaporation still leads the way in the total film-area deposited. Many of these technologies depend on the development of new equipment, Vacuum coatings can enhance beauty and wear in many decorative applications, such as so the equipment manufacturers are door hardware. Photo courtesy of Baldwin Hardware Corporation, Reading, PA. often in the lead in introducing new technologies and equipment to the industry. sional functional articles has really looking titanium-nitride- (TiN)-coated increased. These coatings provide not products that have a high wear PVD Coating Markets only a decorative coating, but are resistance. Zirconium-nitride (ZrN) protective as well. For example, in the provides a wear-resistant, brass- What are some of the mar- high-end market for door hardware looking coating. Q. and plumbing fixtures, the item was The coating of cutting tools with kets for PVD coatings that have gold-plated with a lacquer topcoat. PVD processing is a large new opened up recently? The problem was that the lacquer was market. Even simple drills are now often removed with cleaning, and the coated to improve their performance, A. In the past few years, the gold was easily scratched. Now there as well as to increase their sales decorative coating of three-dimen- are several companies offering gold- appeal. The increased wear life is very

32 PLATING & SURFACE FINISHING important in applications where the example is the deposition of a mixed New technologies bring new object to be coated has a high cost, titanium-nitride-carbide layer, where requirements. PVD coating of such as an injection mold for plastics, the color can range from gold to violet plastics, for example, has allowed the or where failure creates product loss to gray to black, depending on the use of plastics to reduce the weight of or a long down-time, such as in composition. automobiles while still retaining the computer-controlled machining of a PVD has a place as an alternative to appearance of a metallic surface. The complicated surface. The thin nature electroplating in depositing simple packaging industry has some major of PVD coatings allows dimensional films. Its advantage is pollution requirements for new PVD coatings. tolerances to be maintained after control associated with the deposition In the past, aluminum films have coating. provided a good In the past 20 years, moisture permeation the coating of architec- barrier film for tural glass for energy flexible packaging. control has become a The consumer, major industry. Ini- however, can’t see tially, the coatings were the contents, and applied by vacuum from a marketing evaporation, but now standpoint, that is not they are deposited by desirable. What sputter deposition. The would be desirable is coating of plastic films a transparent barrier (webs) with transparent layer. heat-reflecting coatings Also, in microwave has allowed the heating, using development of very aluminum film energy-efficient packaging, the windows, in which the aluminum film is coated web is posi- heated and acts as an tioned between two oven for the contents. panes of glass. In many cases, it would be desirable to Q. What are some heat the contents of the future markets directly, which for PVD coatings? requires a dielectric barrier layer, such as A. The decorative an oxide. and protective coating In many cases, markets are opening up companies are being forced into consider- rapidly. This is particu- This door hinge exhibits a decorative functional finish. (Photo courtesy of Gold Star larly true where the two Coatings, Inc.) ing PVD processing objectives can be because of the combined to give a possibility that their higher quality, longer-lasting product. is not a major problem. It is interest- competitors will develop new prod- Protective coatings applied by PVD ing to note that PVD-coated products ucts and take over market share, either processing will become an important can pose a problem with waste because of lower cost, better perfor- market. For example, the use of disposal and recycling. Aluminum- mance or an environmental concern. aluminum as a corrosion-protective coated plastics, for example, cannot There are a number of applications in coating is a steadily growing market. be recycled as easily as uncoated the medical field, for example, where The use of passive and active films plastics. a vacuum must be maintained in an in devices such as flat panel displays optically transparent container for a and photochromic windows and Future Developments long period of time. This is now done mirrors is going to be a growing with a glass container, for which market. The PVD processes allow a Q. What do you see in the disposal is difficult. If a plastic wide range of designs. These include future for PVD processing? container with a gas barrier layer multilayers of materials and the could be developed, it would probably gradation of material properties in a A. The demands on PVD process- take over a lot of the market share for single layer. For example, one gold ing seem to be increasing all the time. this type of container, because it decorative coating application uses 25 Both new requirements and new could be easily incinerated. alternating layers of gold and tita- markets are driving the development nium-nitride to form a decorative and of equipment and processes. The Q. What are some areas for protective layer. The item is sold as ability of PVD processing to form development of PVD coatings? gold-plated. Composition control complex single- and multicomponent allows a variety of products. An layers provides an unlimited opportu- A. A major goal in the rigid nity for application. packaging industry is to develop an

November 1996 33 optically transparent, low-permeation These techniques allow the non- A. To make that determination coating for plastics. This coating intrusive monitoring and control of requires a knowledge of published would keep gases out and vapors in. sputtering rates. information, a knowledge of what This would allow the use of plastic affects film properties and what others containers to replace heavy glass Film Properties are doing. In some cases, there is no ones. This coating would be deposited simple answer. In coating door on a molded three-dimensional object, Q. What determines the proper- hardware and plumbing fixtures, for such as a bottle. Uniformly coating a ties of a PVD film? example, some people use sputter bottle at a high rate and low cost will deposition, some use arc vaporization, require significant equipment and There are no handbook values and a few use a combination of both. process development. A. for the properties of PVD films. The In some cases, portions of the In the future, I think that we will film properties are determined by a decision may be simple. In all cases I see more in-line, one-at-a-time number of factors, such as substrate know of in the PVD deposition of processing lines where several surface roughness, the angle-of- high-performance hard coatings, some processes are integrated into a single incidence of the depositing atoms, the version of ion plating is used (that is, line. The aluminum coating of chemical reaction between the during deposition there is concurrent compact music discs (CDs), for substrate surface and the depositing bombardment by energetic ions to example, started out by molding the atoms (nucleation and interface promote chemical reaction and discs, then mounting them into a formation), the deposition process densification). Mostly, the decision on fixture that holds several hundred used, the deposition conditions, etc. In which process is best is based on discs and then coating the discs by addition, there may be postdeposition testing of components coated by vacuum evaporation. Now individual treatments that can be used to modify equipment manufacturers or contract discs are molded, transferred to the the film properties. coaters. coating chamber and coated by sputter To obtain specific properties in a Many equipment suppliers will deposition at a rate of one every three film, an understanding is required of provide turnkey operations, in which seconds or less in a continuous the bulk properties of the film they develop the process and the processing line. material and the factors that affect the equipment to meet your specifica- Cleaning and contamination control film growth. Where the film is tions, and tell you which button to are major factors in obtaining repro- deposited by reactive deposition, the push. This can lead to problems in ducible processing, and this is a properties are a function of the film manufacturing. In addition to know- continuing area of interest. In the composition. Probably the main film ing which button to push, one must semiconductor industry, for example, property that affects all the film understand the equipment, the process where particulate contamination is a properties is density. Generally, the and what affects film properties. major concern, it is proposed that deposited film is less than fully dense, most of the contamination occurs in and special techniques must be used Vacuum Coating & PVD the deposition system and not in the to densify the film, either during film processing ambient. This has inspired deposition or by subsequent process- What is meant by the terms a major effort on the part of equip- Q. ing, in order to approach bulk “vacuum coating” and physical ment manufacturers to develop properties. For example, the complex vapor deposition? contamination-free processing corrosion/erosion coatings that are equipment. New developments in applied to aircraft turbine blades by turbopumps, using magnetic levitation A. The term “vacuum coating” sputter deposition are shot-peened instead of mechanical bearings and means that the coating process is done after deposition to densify the film adding molecular drag stages, have in a vacuum (i.e., under a pressure and seal pinholes in the coating. allowed the development of virtually less than the local atmospheric Adhesion is affected by the interfa- oil-free, high vacuum pumping for pressure). This is not very definitive, cial reactions and the residual film situations in which contamination is a because just about every coating stress, which are dependent on the problem. Cleaning processes, such as technique has some version that is deposition parameters. Pinholes in the plasma cleaning, that can be inte- done in a vacuum. These include low- film are often determined by surface grated into the deposition equipment pressure plasma spray, low-pressure roughness and particulate contamina- are of interest, as is integrating the chemical vapor deposition, and even tion on the substrate surface. cleaning line with the deposition electroplating. The thickness for PVD films ranges system in order to avoid recontamina- Physical vapor deposition (PVD) from several hundred atomic layers tion in the processing ambient. processes are those film deposition (1000 Å) needed for optical reflec- The development of PVD process processes in which the coating tance and diffusion barriers to several monitoring and control equipment and material, or some portion of it, is microns (about 1/50 of the thickness techniques will be important in vaporized from a solid or liquid of the human hair) for wear-resistant utilizing PVD techniques for sophisti- surface, transported through a low- coatings. The properties of the film cated processing. Atomic adsorption pressure gaseous environment, and can vary with the thickness. spectroscopy techniques, for example, condensed on a surface that is called a developed in conjunction with fusion substrate. If the depositing material reactor technology, are just becoming Q. What is the best way to reacts with the gaseous ambient or co- available to the PVD community. determine what PVD process is the deposited species, the process is best for a specific application? called a “reactive deposition process.” Reactive deposition processes are

34 PLATING & SURFACE FINISHING used to deposit oxide, nitride and A. In ion plating, the growth of the About the Author carbide films of materials. depositing film is modified by Donald M. Mattox concurrent bombardment by atomic- is technical Q. What are the most common sized energetic particles. This director of the vaporization techniques in PVD bombardment densifies the film and Society of Vacuum processing? promotes chemical reaction, in the Coaters, 440 Live case of reactive deposition. The Oak Loop, Albu- A. The highest volume vaporiza- bombardment can also induce querque, NM tion technique is thermal evaporation, compressive stresses that can be used 87122; phone 505/ usually from a liquid surface. There is to offset the tensile stresses that are 856-7188. He is the probably more aluminum evaporated often introduced into high-modulus regular columnist than any other material. Physical films during growth. The ions used for P&SF’s “SVC Topics.” He is an sputtering, where atoms are ejected for bombardment are usually ex- instructor for SVC-sponsored courses, from a solid surface by momentum tracted from a low pressure by a has many publications to his credit transfer from a high-energy bombard- negative bias on the substrate surface. and is in demand as a guest speaker ing ion, is used extensively. Arc In some cases, the ions are formed in on physical vapor deposition. vaporization, which uses a high- an “ion gun.” P&SF current, low-voltage arc to evaporate and ionize material, is becoming an important vaporization technique. Q. What gas pressures are used for PVD processing? A. PVD processes typically use gas pressure (vacuum) below about 10-4 atmospheres. At that vacuum, atoms can travel from the course to the substrate without a significant number of collisions and deposit with little contamination from residual gases. At pressures above about 10-4 atmospheres, the atoms will combine in the gas during the transport process to form “soot,” which gives a poor film. The lower the pressure in the system, the less the possible effects of gaseous contamination on the depositing film material. Typical vacuum evaporation processes will use a gas pressure of about 10-8 atmospheres. Q. What is a “plasma” used in PVD processing? A. A plasma is a gaseous atmo- sphere that contains an appreciable number of ions and electrons so that it acts as an electrical conductor, even though often the neutral atoms far outnumber the ions (a “weakly ionized plasma”). The plasma supplies the ions that can be acceler- ated in an electric field to cause sputtering, and also activates reactive gas species in order to enhance reactive deposition processes. Plasmas used in PVD processes are typically at a gas pressure of about 10-6 to 10-5 atmospheres. Q. What is the ion plating process? Free Details: Circle 130 on postpaid reader service card. November 1996 35