Coating Materials News Vol 8 Issue 3
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July - September, 1998 Recent Advances in Thin Film Deposition Improved Materials and Newly Developed Processes Unveiled in '98 Thin film layers are used on more optical and mechanical surfaces today than in the past because improvements in materials and deposition technology permit their advantages to be widespread. Applications range from mechanical abrasion reduction to highly sophisticated CD, DVD, and wavelength division multiplexing (WDM) optical and microelectronic devices where film quality is measured in sub-wavelengths. Thin film technology is used to replicate diamond, the hardest material known, and the subtle colors of insects. Digital recording and storage of information at densities unpredictable only a couple of years ago is commonplace today, and no limit is in sight. There has been much effort concentrated in recent years on deposition process and technique development. The number of acronyms for deposition of thin films continues to increase. Techniques range from thermal evaporation, ion sputtering and ion assisted film growth to laser evaporation, flash evaporation, MBE and others. Improvements in the quality of the thin films have come from refinements in the preparation of the compounds and in their composition, frequently including mixing with appropriate additives either during deposition or in the starting material. This is one of the primary themes evident at the June 1998 Optical Interference Coatings Conference sponsored by the Optical Society of America. In this issue of CMN, we summarize some of the latest work presented at that conference and in recent publications.
Materials Titania Titanium dioxide has the highest refractive index of the transparent evaporable materials in the wavelength range ~400 nm to ~1000 nm and is used in most multilayer coatings for the visible region. Many studies have been done to improve the evaporation and thin film properties of titania because the compound requires special attention to achieve films with low optical absorption and tensile stress, and high density and index. An essential ingredient in achieving optimum film properties is the preparation of the starting material. We have discussed the starting materials for e-beam evaporation, their forms, and required preconditioning in past issues. From the list of compounds, namely TiO, TiO2, Ti2O3, Ti3O5, Ti4O7, etc, it is generally agreed that Ti3O5 is the easiest and most successful starting compound. Since this form melts, outgassing and spitting during preconditioning is minimal, and reactive oxidation is more complete. A study concerned with stress and water absorption by T. Aoki and S. Ogura was reported at the Tucson Optical Interference Coatings conference in June 1998. Evaporation parameters for the different compositions were: 1 and 2 E-04 Torr oxygen, substrate temperature of 60° C and 300° C, and a rate of 5 Å/s. The interesting result was that lower stress is obtained under conditions of high pressure and low temperature, but that these layers exhibited greater water absorption than films deposited at high temperature and low pressure. Layers deposited from Ti3O5 as the starting compound exhibited the lowest stress levels. Another study at the same conference reported the improvement in the surface morphology of titania films by co-sputtering other materials. Films of pure titania have a columnar microstructure and thus a rough surface. J-C. Hsu, et. al. found that by co-sputtering Al, Si, or silica, the surface roughness could be reduced by a factor of 10. A target of Ti with pieces of these materials placed on it was ion beam sputtered. The ratio of added material to Ti was varied, and n, k, and rms roughness of the films were measured after baking at 275° C and 450° C. Normally a titania film converts from amorphous to crystalline at 450° C and its roughness increases dramatically. The addition of 5% silicon to the titania layer resulted in the best improvement in all properties, and did not lower the n value substantially. Silica also produced smoother surfaces, but 20% is needed to produce comparable smoothness, and this results in lowering the n of the film. Both "doped" films retained their smoothness at 450° C. We have discussed how adding foreign materials during layer growth can act to promote amorphous, dense films. This ion beam co-sputtering technique is another way to achieve this goal. Magnesium Fluoride It is well known that high substrate temperature is required to grow hard, scratch-resistance films of MgF2 (and other fluoride compounds). The explanation is that additional surface energy present on a hot surface improves adatom mobility and therefore encourages a denser two-dimensional microstructure. The packing densities of fluoride films can be as low as 70%, and only temperatures in excess of 250° C or energetic ion bombardment during growth (IAD) can increase the density above 90%. We often observe that permitting the coating to "mature" by exposure it to air for a day or two can result in improved mechanical properties and changes in optical properties. Mixing with up to 10% of a foreign material forces amorphous structures, reduces stress, and increases packing density by discouraging open microstructural growth (this process has been discussed previously, and is the basis for CERAC products CIROM®-IRX [U.S. Patent No. 5,262,196], CIROM-IRB, and doped MgF2 materials). These process variations can produce harder MgF2 films with more stable optical indices. Work presented at the Tucson conference [1] provides some insight on understanding the stress behavior and thus the hardness improvement of MgF2 layers. The presence of voids between microcrystalline grains is accompanied by tensile forces in the columnar microstructure. The energy transferred by IAD collapses the gaps, reducing tensile stress, and possibly introducing compressive stresses. Absorption of water vapor on the internal surfaces reduces the energy fields within the columnar structure and thus can reduce the tensile stress of the layer and improve its durability to abrasion. In conventionally deposited pure MgF2 films, changes in internal stress and refractive index can be reproduced by thermal cycling at temperatures below ~150° C. This is most likely caused by the desorption and re-absorption of water vapor in the void volume. When heated above 250° C, grain growth is observed, reducing void volume and consolidating and densifying the microstructure. Stress is reduced and hardness increases. This explains why the common practice of depositing MgF2 on substrates heated to near 300° C produces hard films. Different TiOxNy Compositions Often the replacement of some of the oxygen with nitrogen to form a tertiary compound will result in a film material with superior properties compared with the oxide. This is the case with TiOxNy. Reactive sputtering is the process in which one can control the composition of the oxynitride. TiN is used as a tribological coating, as we discussed in earlier CMN issues. When the oxynitride form is produced, lower stress and better adhesion result. In addition, the resistivity can be altered while retaining hardness and low coefficient of friction. Researchers using dual ion beam sputtering (DIBS) sputtered a Ti target with one ion source and varied the ratio of O to N while bombarding the film with the second ion source [2]. Above a 14 % O to N composition, the films are amorphous, smooth, and semi-transparent. It is clear that this process has the potential of producing an improved material for mechanical and optical coating. Techniques Coatings on Plastics Several papers discussing this topic emphasized the importance of surface preparation on the mechanical properties of the coating. Anyone challenged with producing optical coatings with good optical and mechanical properties on polymer surfaces has had the experiences of adhesive failure, crazing, poor abrasion resistance, stress deformation, etc. Many of these problems can be traced to the surface treatment prior to coating deposition, regardless of deposition method. The weak bond strengths characteristic of polymers (contrary to glass materials) make them susceptible to disruption by energetic radiation such as UV or ions. On the other hand, removal of contaminating layers is difficult because of the fragile chemical and mechanical natures of many polymers. So surface preconditioning can make matters worse or better depending on the process. Techniques are being developed to restore a virgin surface and to alter the surface and create a favorable interface layer between the polymer and subsequently deposited thin film layer. Often the interface layer is composed of crosslinked O, C, or CH groups, depending on the polymer, and is <100 å="" thick.="" the="" simplest="" and="" traditional="" way="" to="" make="" an="" interface="" layer="" is="" to="" use="" a="" glow="" discharge="" plasma.="" however,="" some="" polymers="" are="" detrimentally="" affected="" by="" the="" forms="" or="" the="" energies="" of="" the="" radiation="" in="" a="" typical="" glow="" discharge="" and="" specific="" processes="" must="" then="" be="" developed.="" large="" areas="" of="" polymer="" films="" are="" coated="" web="" machines="" by="" sputtering.="" the="" composition="" and="" energy="" of="" the="" plasma="" must="" be="" designed="" for="" the="" plastic="" in="" question,="" and="" this="" technology="" is="" still="" in=""> Processes Radical-Activated CVD (RACVD) While we generally concentrate on PVD technology for optical and tribological coatings, chemical vapor deposition is widely used in semiconductor manufacturing industries. CVD requires the management of organometallic materials in their vapor phase and typically requires high temperatures to decompose and react to form solid films. The temperatures are above 300° C, so the process is restricted to substrates that can tolerate these conditions. In the Tucson meeting, R. H. Bennett, J. Simpson, and K. L. Lewis reported on a low temperature CVD technique for oxide deposition known as Radical-Activated CVD. The principle of operation is based on the reaction of oxygen radicals with the precursor material gas. Oxygen radicals are created in a R. F. field and drawn into the low vacuum region (< 1="" torr)="" where="" they="" decompose="" the="" metal="" alkoxide="" precursors.="" oxide="" compounds="" condense="" the="" substrate="" and="" are="" densified="" during="" condensation="" by="" argon="" atom="" bombardment.="" low="" absorption="" titania,="" hafnia,="" and="" silica="" have="" been="" deposited="" by="" this="" process,="" but="" control="" of="" layer="" thickness="" was="" not=""> Reactive Ion-Assisted Co-Deposition (RIACD) Reactive Ion-Assisted Co-Deposition employs e-beam evaporation and an argon and oxygen ion plasma discharge. Deposition of normally unlikely partner compounds has been achieved with this process. For example, the mechanical, chemical and optical properties of composite CaF2 films have been improved over pure CaF2 with the co-evaporation of 10% TiO2 [3,4], and similar results have been reported for MgF2 composites [5]. CaF2 has the desirable properties of low index (1.4), wide transparency range (200 nm to >10 um), and low solubility (0.002). However, the films are soft and grow with a coarse polycrystalline microstructure which is susceptible to moisture absorption resulting in crazing. We have found this behavior to occur with other fluoride compounds such as BaF2 [6,7]. CaF2 films of good durability have been deposited on cold substrates with IAD [8]. The researchers [3] evaporated CaF2 at rates up to 20 Å/s and simultaneously Ti2O3 at 2 Å/s at 2 E-04 Torr pressure and 100 eV ion energy. With the TiO2 relative content increasing from 10%, the films become less crystalline and smoother. The hardness and adhesion of the composite films increase dramatically by nearly 10 times at this composition. Above 10% titania, hardness and adhesion continue to increase, but optical absorption and index values also increase. Water absorption, indicated by spectral shifts due to index increases, and solubility are significantly lower than pure CaF2 films. Another important benefit is that this energetic deposition process is able to produce these improved properties at low substrate temperature. CERAC IRB™, a low index fluoride mixture, exhibits comparable improvements and does not require co-evaporation. Thus, we see that many properties of thin films can be improved by deliberately depositing impure rather than pure materials. The negative aspect of this particular process, characteristic of mixing dissimilar compounds, is that two evaporation sources are required. Conclusion We have summarized here some of the latest work in areas including deposition materials, processes and techniques. We anticipate more complete reports will be published in the usual journals dealing with thin films and coatings. References
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Dr. Mitchell C. Colton, Editor Samuel Pellicori, Principal Contributor |
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