Uniform coating of large areas is a technically challenging aspect of physical vapor deposition. This investigation shows
that good film uniformity across large areas can be repetitively achieved by a DC magnetron sputtering process by use of
multiple sources. A unique feature of this technique is the ability to predict and control the film distribution using the
deposition rate, adding flexibility to the deposition system. A model for predicting the material distribution from
multiple sources is presented. It will also be demonstrated that this process yields efficient use of the vapor generated
from the sources, which results in higher deposition rates and less system maintenance.
In optical firing sets, laser light is used to supply power to electronics (to charge capacitors, for example), to trigger electronics (such as vacuum switches), or in some cases, initiate explosives directly. Since MEMS devices combine electronics with electro-mechanical actuators, one can integrate safe and arm logic alongside the actuators to provide all functions in a single miniature package. We propose using MEMS-activated mirrors to make or break optical paths as part of the safe and arm architecture in an optical firing set. In the safe mode, a miniature (~1 mm diameter) mirror is oriented to prevent completion of the optical path. To arm the firing set, the MEMS mirrors are deflected into the proper orientation thereby completing the optical path required for system functionality (e.g., light from a miniature laser completes the path to an optically triggered switch). The optical properties (i.e. damage threshold, reflectivity, transmission, absorption and scatter) of the miniature mirrors are critical to this application. Since Si is a strong absorber at the wavelengths under consideration (800 to 1064 nm), high-reflectivity, high-damage-threshold, dielectric coatings must be applied to the MEMS devices. In this paper we present conceptual MEMS-activated mirror architectures for performing arming and safing functions in an optical firing set and report test data which shows that dielectric coatings applied to MEMS-mirrors can withstand the prerequisite laser pulse irradiance. The measured optical damage threshold of polysilicon membranes with high-reflectivity multilayer dielectric coatings is ~ 4 GW/cm2, clearly demonstrating the feasibility of using coated MEMS mirrors in firing sets.
HfO2/SiO2 and ZrO2/SiO2 high reflectors at 1.064 microns were deposited by pulsed reactive DC magnetron sputtering. These dielectric thin film high reflectors were deposited with and without the use of an electron source. The electron source greatly decreased arcing of the magnetrons during the deposition process resulting in thin films with fewer defects. The high reflectors were laser damage tested at 1.064 microns. The optical properties of the thin film coatings were characterized prior to laser damage testing. Optical characterization techniques included angle resolved scatter (BRDF), total integrated scatter (TIS), and adiabatic calorimetry. The dependence of the laser damage threshold and optical properties on deposition conditions is reported.
An international round robin study was conducted on the absorption measurement of laser-quality coatings. Sets of optically coated samples were made by a reactive DC magnetron sputtering and an ion beam sputtering deposition process. The sample set included a high reflector at 514 nm and a high reflector for the near infrared (1030 to 1318 nm), single layers of silicon dioxide, tantalum pentoxide, and hafnium dioxide. For calibration purposes, a sample metalized with hafnium and an uncoated, superpolished fused silica substrate were also included. The set was sent to laboratory groups for absorptance measurement of these coatings. Whenever possible, each group was to measure a common, central area and another area specifically assigned to the respective group. Specific test protocols were also suggested in regards to the laser exposure time, power density, and surface preparation.
In this investigation a technique was developed for depositing high reflectors of alternating layers of TiO2 and SiO2 with varying levels of stress. The TiO2 layers were deposited by ion assisted electron beam evaporation, and the SiO2 layers by modulated reactive DC magnetron sputtering. The TiO2 layers were in tensile stress, and the stress depended on the ion beam current density. The SiO2 layers were in compressive stress. The total stress in the high reflector coating was controlled by the ion beam current density applied during deposition of the TiO2 films. The coatings were deposited at a substrate temperature of approximately 50 degrees Celsius. Coating stress levels were unaffected by changes in relative humidity. The stability of the coating over time depended on the density of the TiO2 layers.
Scatter from optical thin film coatings is a significant problem for high power laser optics. Theoretical aspects of the problem have ben well explored for scattering due to surface roughness. Over the past twenty years the surface roughness of optics has been significantly reduced. Improvements in optical surface fabrication and film deposition techniques have progressed to the point that even for complex coatings, surfaces of less than 1nm rms roughness are routinely achievable. As the surface roughness of optics decreases, bulk scatter, rather than topographic scatter, may be the major scatter source in these smooth surfaces. Atomic force microscopy can profile the surface of a coating with atomic resolution. By comparing the power spectral density (PSD) derived from the surface profile with the PSD derived from angle resolved scatter measurements, some conclusions can be reached on this question. Data from analysis of dual wavelength high reflectors deposited by reactive DC magnetron sputtered Nb2O3/SiO2 and ZrO2/SiO2 structures will be presented, allowing analysis of results for these film materials.
Scatter in thin film optical coatings may arise from a variety of sources. Our previous investigations have used total internal reflection microscopy (TIRM) to monitor scatter site generation during film growth. These studies have reported the effects of substrate cleaning techniques and certain deposition parameters on scatter site generation. The present investigation using TIRM to monitor the coating process has yielded new insight into defect generating mechanisms for films of HfO2 and ZrO2. Of particular interest is surface contamination apparently caused by electrostatic effects. Introduction of high electric potentials in the vacuum chamber has been observed to cause surface contamination prior to deposition, resulting in a significant increase in the number of scatter sites.
The attempt to eliminate subsurface damage in polished materials is a major objective in optical and semiconductor fabrication. The level of subsurface damage in optical components is proportional to the surface scatter and related to the laser damage threshold of the optic. The float polishing process has been shown to produce surfaces with low subsurface damage on ferrite materials. We have ground samples of rough cut Corning 7940 fused silica using synthetic polycrystalline diamond. These samples were then float polished on a precision machine manufactured by Toyoda Machine Works Limited. Our surfaces were characterized using differential phase interference microscopy, total internal reflection microscopy, and scatterometry. We will describe the fabrication process and report the results of the surface and subsurface characterization.
Photoacoustic spectrososcopy is used to characterize the surface absorption of polished fused silica substrates and thin films deposited on fused silica substrates. The extreme sensitivity of this technique allows measurement of surface absorptions of a few tenths of a part per million. Characterization of samples with surfaces finished using a variety of methods is reported. Key words: Photoacoustic ion beam absorption thin films. 1.
Ion beam figuring has been demonstrated to be a deterministic efficient flexible technique for removing material from optical surfaces. Recent interest in using this process to produce high quality optical components has driven the need to fully characterize the resulting surfaces. We have performed a polishing parameter matrix investigation to optimize fused silica (Corning 7957) surfaces for subsequent ion milling. Samples were characterized for surface scatter surface absorption surface roughness subsurface damage and laser damage as a function of mill depth. Small defects (pits) were evident on surfaces after milling a few microns with pit density dependent to some degree upon the surface preparation technique. The defects were often in lines apparently following a surface or subsurface scratch in the materiaL Surface scatter decreased significantly (up to lOX) and laser damage threshold increased in some cases by 400. Laser damage was not correlated with defects in the material. Key words: ion beam milling laser damage scatter fused silica absorption. 1.