Figure 11. 3000-on-1 damage morphology of EG&G PIN photodiodes. a. 3000-shots for UV at 2.5 J/cm2 (M=140X) Active Area Dark Current (nA) Dark Current (μA) Figure 12. Reverse-biased IV characteristics of irradiated EG&G SGD-040 PIN photodiodes showing breakdown. The current is the sum of the active area dark current and the guard-ring current. The postdamage curves for device # 1, device # 2, and device # 3 and the initial predamage curve are shown. Figure 13. Reverse-biased IV characteristics of irradiated EG&G SGD-040 PIN photodiodes. The current is the active area dark current. The postdamage curves for sample # 1, sample # 2, and sample # 3 and the initial predamage curve are shown. Figure 14. Reverse-biased CV characteristics of EG&G SGD-040 device #1. The CV curves for the less severely damaged devices, # 2 and # 3, were not changed by the laser irradiation. Manuscript Received 1-18-89 Stress Reduction of Ion-Beam-Sputtered Mixed-Oxide Coatings by Baking B. J. Pond, J. I. DeBar, C.K. Carniglia and Tilak Raj Martin Marietta Astronautics Group, Laser Systems Technology P.O. Box 9316, International Airport, Albuquerque, New Mexico 87119 Thin films deposited by ion-beam sputter deposition (IBSD) typically have a high compressive stress. This stress can be reduced for certain materials by cosputtering with another material [1]. The stress can be further reduced by baking the films in air after coating. Films of zirconia (ZrO2) and silica (SiO2) were prepared by IBSD from hot-pressed oxide targets using argon as the sputter gas. Films consisting of a mixture of silica and zirconia were prepared by sputtering from both targets simultaneously. Calorimetry measurements at 351 nm showed that the absorption in the mixed-oxide films was lower than the absorption in the zirconia film. A compressive stress of 219 kpsi was observed for the zirconia film and of 112 kpsi for the silica film. All of the mixed-oxide films had lower stress. Those films with silica fractions between 10% and 50% had stresses in the range of 40-50 kpsi. This stress could be reduced even further by baking the coated parts for several hours at 300°C. For mixed-oxide films with a silica fraction less than 50%, the stress of the films after baking was tensile. In particular, the film with 10% silica was changed from a compressive stress of 46 kpsi to a tensile stress of 23 kpsi by the baking process. Similar results were observed for a mixed-oxide film consisting of zirconia and alumina (A1203). These results indicate that a stress-free multilayer coating may be achievable by IBSD. Key words: cosputtering; ion beam; optical coatings; refractive index; silica; Ion-beam sputter deposition (IBSD) has been shown to be a viable process for producing high-energy laser coatings [2-4]. Thin-film optical coatings made by IBSD have a higher density and lower impurity levels than conventionally evaporated films. This is due to the higher energy of the sputtered particles condensing on the substrates. The average kinetic energy of sputtered particles is approximately 5-10 electron volts, whereas the kinetic energy of conventionally evaporated species is approximately one-tenth of an electron volt [5]. These energetic particles dislodge adsorbed impurities from the substrate surface and from the coating as it is being deposited. One limitation of coatings produced by IBSD is the high compressive stress of the films [3,6-9]. Stress can be a significant problem with thick coatings, since the mechanical forces scale with thickness. These forces can distort the substrate and cause the film to delaminate. In mirrors for optical systems, the deformed surfaces can result in considerable beam distortion. Numerous studies have been conducted to investigate the cause of stress and to determine methods of modifying the stress in thin films. However, most of these studies have been conducted with sputtered metal films [7-16]. Recently, several studies have shown that the stress in dielectric thin films can be altered by coevaporating two materials [17-18]. One of these studies also showed a change in the grain structure of the coatings which correlated with the changes observed in the stress [17]. In the case of electron-beam (E-beam) evaporated zirconia, a change was observed in the crystalline phase, the microstructure, the grain size and the stress of the films when a glass former such as silica was added using coevaporation [18]. This paper reports on the cosputtering of zirconia (ZrO2) with either silica (SiO2) or alumina (A1203) using IBSD. The effects of cosputtering on the stress and the A total of 12 different compositions were optical properties of the films are presented. investigated. Eleven of the films were zirconia/silica with the silica fraction ranging from 0% to 100% in increments of approximately 10%, and one of the films was zirconia/ alumina with an alumina fraction of approximately 10%. The addition of either alumina or silica to the zirconia was found to reduce the stress observed in the zirconia films. The effect on the stress of post-deposition baking was investigated to determine whether the stress in the films could be reduced even further. The films were baked at 300°C for three hours, and a noticeable change was observed in the stress of the films. A multilayer coating was fabricated using zirconia/silica mixed-oxide material as the high-index material and silica as the low-index material. After the coating was completed, it was baked at 300°C for several hours, and a change was observed in the stress which corresponded to the change observed in the stress of the single-layer films. The following section discusses the experimental procedures used in depositing, analyzing, and baking the films. This is followed by a section discussing the results. The films were fabricated by IBSD in a cryopumped 20-inch bell jar using two Kaufmantype ion sources to sputter material simultaneously from two separate targets. The zirconia and alumina targets were in the form of hot-pressed oxide material, and the silica target was in the form of fused silica. All of the targets were 17.5-cm in diameter. The base pressure of the vacuum system was 5 x 10-7 torr or lower. The ion energy used with both sources was 1000 eV. The sputter gas used was argon, and a partial pressure of oxygen of 3.0 x 10-5 torr was supplied directly into the chamber to achieve stoichiometric films [19]. An equal pressure of argon was used in each ion source and the total chamber pressure was 3.1 x 10-4 torr. The substrate temperature rose above ambient during deposition due to radiant heating from the ion sources. The highest temperatures were in the range 40°-60°C. The films were deposited on substrates held in a circular rack which was rotated about its axis. The film thicknesses were monitored by an optical monitoring system using front surface reflection monitoring. The various compositions of the single-layer films were obtained by adjusting the ion beam current incident on each target. The system was calibrated by depositing separate single-layer films of alumina, silica and zirconia using a fixed ion beam current for a fixed time. The thicknesses of these layers (denoted dao, dso and do for the alumina, silica and zirconia films, respectively) were determined using the optical methods described below. To obtain a film with a given volume fraction f of silica, the currents Is and Iz for the silica and zirconia ion beams are related by Is = Iz (dzo/dso) f/(1 - f). (1) For compositions with silica fractions less than 0.5, Iz was set to its maximum value, and eq. (1) was used to determine Is. For f > 0.5, Is was set to its maximum value, and Iz was determined using eq. (1). A similar relation was used to obtain a film with a given volume fraction of alumina. This deposition technique was used to give a series of single-layer coatings with approximate silica volume fractions equally spaced from 0% to 100% in 10% increments. The actual composition of each film was determined separately using analytical techniques. The Single-layer films of two different thicknesses were fabricated sequentially under identical conditions for each of the zirconia/silica compositions. The thinner films were approximately 100 nm in physical thickness, and these films were used for analysis by Rutherford back scattering (RBS) [20] to determine the actual composition of the films. thicker films had an optical thickness of approximately 5 quarter waves at 550 nm. thicker films were used for stress analysis, measurements of optical properties, and absorption measurements by laser calorimetry. The |