Coating Materials News Vol 7 Issue 3

Introduction:
Along with the positive feedback we get on a regular basis from the readers of Coating Materials News often come topic suggestions for future issues. These requests routinely cover a broad spectrum of interests pertaining to all different types of coating applications, processes, equipment and materials.

We have made a commitment to address these inquiries at least annually in special, "reader response" issues of CMN. We pool all of the suggestions that are not featured in individual editions and select the most popular topics for review.

If you would like to submit a suggestion, you may do so by calling Nora Bauer, Marketing Administrator at CERAC at 414-289-9800 x206, or by sending e-mail to marketing@cerac.com. If you would like to request additional information or view any of the on-line product data sheets or back issues of CMN , check out the CERAC web site at www.cerac.com. Keep those suggestions coming!

Metal Deposition:
The vacuum deposition of metal films has been discussed briefly in several previous issues of Coating Materials News (CMN) . Here, we will go into some detail concerning surface preparation and deposition parameters for the more commonly used metals.

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:
Aluminum adheres well to oxide surfaces such as glass or thin film oxide compounds. It has properties very compatible with silicon electronic circuits and forms ohmic contact. Sheet resistances near 0.1 ohm/sq. are produced for thicknesses between 3000 and 4000Å. Process temperatures must be kept below the Al-Si eutectic temperature of 577° C and low enough to prevent diffusion. Aluminum can be evaporated at a temperature near 1100° C, so resistance heated tungsten boats, coated tungsten wires and e-beam from graphite or BN crucibles can be used. The pressure must be in the 10 -6 Torr range and the rate high (>20 Å/s) to reduce oxidation in the deposited film. The substrate temperature should be below 150° C to obtain small grain size and smooth film surfaces. Aluminum can also be sputter deposited at rates exceeding 1000Å / min.

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:
Silver has the highest visible (>500 nm) to Far-IR reflectance of any metal while gold has the highest reflectance for use above 800 nm. Silver requires extraordinary protection from environmental exposure. With care, dielectric overcoatings provide protection, but are ultimately limited to diffusion times for moisture and reactive environmental gasses. Gold surfaces are very soft, and do not form adherent bonds, so protection by overcoating is not an option.

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.

Silicon Monoxide:
SiO has been discussed in several CMN articles, and a CERAC Product Data sheet is available upon request. Films have excellent barrier properties to the diffusion of water vapor and other gasses because they grow without grain boundaries, and are essentially amorphous. A major application is in the web coating production of food packaging, where a suboxide, SiO x (x=1.5 to 1.8) provides good barrier properties.

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.

Oxide Compounds:
The high temperatures required to evaporate oxide compounds is responsible for partial decomposition that generally results in the deposition of a sub-oxide layer. Reaction with partial pressure of oxygen at the substrate restores stoichiometric composition. This re-oxidation requires energy which can be provided in-situ by high substrate temperature, energetic ion impact, or reactive plasma conditions. The process is substrate dependent. For example, while deposition onto polymer substrates requires activated species rather than high temperatures, deposition onto silicon circuitry must exclude damaging plasma exposure.

Source Material Purity Considerations:
The question is often asked about the influence of residual impurities on optical film quality. Must the starting material be 5-9's, or is 3-9's sufficient? With the exception of coatings designed to survive high laser fluences, most other coatings are little affected by the presence of impurities to the ~1% level. In the UV, however, metal impurity levels must be low to reduce absorption due to electronic transitions.

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:
Improvements in laser damage resistance has been the topic of many papers and conferences covering wavelengths from UV excimer lasers to IR CO₂ lasers. In the simplified view, damage arises from heating due to absorption or from defects, so coatings must possess low optical absorption and very low point-defect density. The relative impacts of these problems pertains to the laser wavelength, exposure duration (pulse length), power density, etc. Absorption outside of fundamental atomic or molecular transitions is most often due to stoichiometric errors, as in an incompletely oxidized compound, or from contaminants. Particulate defect sites are created by an uncontrolled evaporation process or source.

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 [2]. 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 [1]. A solution for some oxide compounds is to provide an excess of oxygen during deposition.

Conclusion:
In this issue, we have discussed some of the key topics of interest to the thin film coating community. The technologies of coating materials and deposition processes are advancing at an ever increasing rate. There are some fundamental issues, however, that are still incompletely explored, and we hope that these pages help inspire questions that lead to further research in these and other areas.

References:
1. Karl H. Guenther, SPIE V 1782, 344, (1992).

2. Erich Hacker, Hans Lauth, Peter Weissbrodt, and Rudolph Wolf, SPIE V1782, 447 (1992).

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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