Coating Materials News Vol 7 Issue 3
|July - September, 1997
Annual Reader Response Issue
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Metal films are used in electronic as well as optical applications. In the former, metals form ohmic contacts and interconnects on semiconductor devices. In optics, metals are used to make highly reflecting surfaces. Divergent processing and operating conditions require different deposition procedures. For semiconductors, the requirements are good adhesion along with the ability to pattern, and diffusion stability at high process temperatures. Optical applications require high reflection and low scatter as well as good adhesion and resistance to environmental degradation.
Aluminum has high reflectance over a wider wavelength range than any metal, ~200 nm to the Far-IR. In reflector applications, a protective overcoat is applied for two reasons: 1) to stabilize the surface oxidation experienced upon exposure to the atmosphere and 2) to provide abrasion resistance. Silicon monoxide one half-wave thick at 550 nm is typically used. SiO absorbs below 450 nm and in the 9 to 10 µm silicate vibrational region of the IR. Other dielectric overcoatings such as yttria can also be used. In UV applications, a layer of MgF2 is deposited immediately after the aluminum layer to prevent the formation of UV-absorbing aluminum oxide layers. Bare aluminum reflects about 90% in the visible region, except near 810 nm where the reflectance can dip by >5% depending on incidence angle. Overcoated aluminum reflects ~4% less, but maintains its reflectance in spite of environmental exposure and cleaning. Oxide compounds that do not absorb in the near-UV can also be used for protection. Yttria overcoating provides good abrasion resistance with transparency to at least 300 nm wavelength and low absorption in the IR on aluminum (and other reflecting metals).
Silver and Gold:
Gold and silver can be evaporated from resistance heated molybdenum or carbon crucibles at ~1100° C. Vacuum should be ~10 -6 Torr and substrate temperature ~100° C. Higher temperatures promote the formation of large grains and higher scatter. Both metals require a mutually soluble interface layer, generally chromium or titanium, to provide adherence to silicon, glass, polymer, and other substrates. The adhesion metal layer is 100 to 200 Å thick, and reacts with residual oxygen in the chamber to form a suboxide.
Adhesion of metal films to silicon surfaces is process dependent as demonstrated by adhesion problems encountered when titanium was sputter deposited on silicon wafers to make electrodes. The bond between Si and Ti was stronger than that to the Si-O surface layer, so it was important to remove the oxidized surface layer. Argon ion bombardment was used to remove the native oxide before deposition. Argon from the sputter deposition and the cleaning steps was found on the surface of the silicon, however, and resulted in poor adhesion. Replacement of the Argon cleaning step by buffered HF acid cleaning, and reduction of Ar ion energies produced adherent Ti films.
Evaporation is best done from baffled tantalum boxes heated to ~1200° C at pressure < 2="" x="" 10="" -6="" torr.="" substrate="" temperature="" can="" be=""><50° c="" to="">200° C, depending on the material. A high rate (>20Å/sec) is recommended to prevent reaction to a higher oxidation state material which will possess very different mechanical and optical properties. For example, at pressures near 1 x 10 -5 Torr and rate near 5 Å/sec, Si x O y is formed. By introducing a partial pressure of oxygen near 1x10 -4 Torr and reducing the rate, SiO2 is formed. Similarly, the presence of water vapor (especially in chambers whose walls have not been baked out under vacuum) can produce unstable films that tend toward Si x O y composition. Non-stoichiometric films can change stress from tensile to compressive upon exposure to moist air. Of the three silicon oxides mentioned, only SiO possesses low stress and low gas permeability while being relatively hard to abrasive wear.
Source Material Purity Considerations:
Rather, it is composition errors in the deposited film that are responsible for most high absorption and index inconsistencies. This is the case for oxide, fluoride, and sulfide/selenide compounds. Minor departures from stoichiometry can cause large absorption values. Of the compound classes mentioned, fluorides have the highest binding energies and therefore are the most stable during evaporation.
With the exception of silicon dioxide (and alumina under special conditions), oxide compounds require the presence of a partial pressure of oxygen during condensation to achieve a fully oxidized film. Condensation rate and energy also must be within particular ranges. Partial oxygen pressures in the low to mid-10 -5 Torr range, rates below 5 Å/sec, and substrate temperature above 200° C or ion bombardment are general parameters for oxides. Depending on the impurity species, some impurities have evaporation temperatures different enough from the main material to undergo fractionation, and either deposit first or not at all. For example, compounds with similar chemistries may be very difficult to refine to high purity. Thus, MgF2 can contain between 10 ppm and 1% CaF2 impurity, depending on grade. Similarly, HfO2 can contain between 0.5% and 2% Zr. In most cases, the impurity neighbor element does not produce a detrimental effect on film quality. Indeed, process variables as mentioned above introduce greater effects on film quality and reproducibility, and sometimes themselves introduce significant amounts of impurity. Again, the notable exception is for UV applications where more attention must be directed to the purity levels of specific contaminant elements.
Laser Damage Resistant Coatings:
Oxide and fluoride compounds can be deposited with very low absorption in the UV to mid-IR spectral region, except for water band absorption near 1.4 µm, 2.7 µm, 6.1 µm. Techniques that involve high energy deposition and produce bulk-like packing densities reduce or eliminate the absorption of water in the microstructure of the film layer. Specific techniques displaying this advantage are sputtering, reactive ion plating (RIP), ion-assisted deposition (IAD), ion beam sputtering (IBS), and to a lesser degree, e-beam on high temperature substrates. It is desirable to grow amorphous layers rather than microcrystalline or columnar microstructures for low water content, low impurity concentration, and low stress [1,2]. Amorphous films have low void volume preventing water inclusion and migration. Another result of water absorption is spectral shifting due to the increase in effective layer index as void volume is filled by water. IAD, RIP, and most forms of sputtering produce environmentally stable (humidity - vacuum invariant) films in that respect.
Impurities from process contamination often migrate to grain boundaries, which is one reason amorphous films exhibit higher damage thresholds than polycrystalline or columnar microstructures. Source material studies conducted by CERAC have produced materials that favor amorphous growth and are capable of providing the benefits described above, but with the use of conventional thermal evaporation processes. Specific fluoride and oxide compounds are modified by adding suitable mixants. The resultant tendency toward small grain sizes and amorphicity discourages the introduction of contaminanants such as carbon and water. This topic was examined in CMN Vol. 7, Issue 2, (April - June 1997): "Hard Films for Optics" (p2).
The microstructure exhibited by many refractory oxide materials fits a computer generated random arrival model. In the fractal growth structure of a film layer deposited by a low energy process such as thermal evaporation, a columnar structure can grow that is open down to the substrate. As thickness increases, the grain size changes, which can lead to an inhomogeneous index profile . Greater void volume is available for water absorption. When high deposition energies are applied, the open structure collapses on itself, leading to denser, less permeable structures. When adatom kinetic or bombarding species energies are excessively high, however, the benefit can be lost because of redox reactions, and can lead to higher absorption. This has been reported for RIP coatings . A solution for some oxide compounds is to provide an excess of oxygen during deposition.
2. Erich Hacker, Hans Lauth, Peter Weissbrodt, and Rudolph Wolf, SPIE V1782, 447 (1992).
Dr. Ervin Colton, Editor
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
Samuel Pellicori, Principal Contributor
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