David W. Reicher

Surface Figuring Using Neutral Ion Beams

During recent years, economic and technological pressures have driven research for higher performance optical fabrication techniques. Among the candidate figuring technologies is ion beam sputtering in which material is removed from the optical surface by the kinetic interaction of ions and atoms or molecules of the surface. The first use of sputtering as a means for optical figuring occurred in the mid 1960s [1,2], and the technique has been investigated by several groups since that time. The prior work was done primarily with ion sources producing high energy (20KeV and above), low current (fraction of a milliampere), narrow (usually less than one millimeter) ion beams. The low current directly translates to low removal rates, while the high energy contributes to radiation damage, ion implantation, and other effects. In the present work the low current, high energy source is replaced with a Kaufman broad-beam ion source[3]. These sources produce higher ion currents at lower energies, thus giving faster removal with minimal surface damage. The ion beam produced by a Kaufman ion source consists of a number of ions traveling in a (typi-cally) slightly diverging beam, along with an equal flux of lower energy electrons. The electrons are injected into the ion beam to reduce electrostatic repulsion in the beam, but also to prevent the charging of dielectric targets.

Defect formation in hafnium dioxide thin films

Hafnium dioxide thin films were deposited by reactive electron-beam evaporation at six different substrate temperatures on fused-silica substrates. During the depositions, the scattering of light caused by the growth of defects in the films was recorded with in situ total internal reflection microscopy. After deposition the films were analyzed by angle-resolved scatterometery, spectrophotometric measurement of film reflectance and transmittance, atomic force microscopy, and x-ray diffraction. We explore the effects of film defect formation on film optical properties and film surface topography using these data.

INFLUENCE OF CRYSTAL STRUCTURE ON THE LIGHT SCATTER OF ZIRCONIUM OXIDE FILMS

The relationship of light scatter by a thin film to thin-film morphology is examined. Light scatter by reactively evaporated ZrO(2) thin films is analyzed by using in situ total internal reflection microscopy and angle-resolved scatterometry. Film crystal structure is analyzed by transmission electron microscopy and x-ray diffraction. Relations between film crystal structure and film scatter are established by using this information. Surface topography is analyzed by the use of scanning force microscopy. Results of a spectrophotometric determination of the film refractive index are reported. The film scatter is found to be sensitive to the crystal phase of the film, which is a function of substrate deposition temperature. A simple method of separating bulk from surface scatter is described.

High-sensitivity surface-photoacoustic spectroscopy

The sensitivity of surface-acoustic-wave detection is extended by several orders of magnitude to a surface-specific absorbance of alphal ~ 10(-9) for a power density of 1 GW/cm(2) using a narrow-bandwidth interdigitated surface-acousticwave detector and an optical irradiation pattern to provide a matched acoustic signal. Major advantages include narrow-bandwidth detection and a large irradiated area that permits more optical energy on the sample. A rapid, nondestructive, reproducible liquid-bonding technique, which permits the extension of these measurements to a wide variety of samples, is demonstrated. Results for AlN and ZrO(2) films (alphal ~ 10(-4)) and fused-silica substrates (alphal ~ 10(-6)-10(-7)) are reported.

Characterization of thin Al films using grating coupling to surface plasma waves

A detailed characterization of the optical, microstructural, and electrical properties of thin (5–50 nm) Al films grown by thermal evaporation, magnetron sputtering, and ion‐assisted sputtering (IAS), is reported. Dielectric‐function measurements were carried out by using grating coupling to surface plasma waves (SPW) and, for comparison, ellipsometric measurements were also performed. Scanning electron microscope (SEM) studies of film microstructure as well as dc electrical resistivity measurements were carried out and correlated with the optical data. Using the Bruggeman effective media approximation, good agreement was obtained for thicker films (30–50 nm), but not for thinner films (<30 nm). SEM and resistivity measurements suggest that conditions of film growth influence the behavior of individual grains, resulting in increased electron reflectance at the grain boundaries with increasing energy delivered to the substrate during deposition. This resulted in lower electrical resistivities for evaporate...

Hermetic Coatings on Fluoride Glass Fibers Using Cylindrical Magnetron Reactive Sputtering

The hollow cathode cylindrical magnetron is demonstrated to be capable of depositing uniform coatings on heavy metal fluoride fibers while maintaining the fiber temperature below 150°C. Pure metal coatings were deposited at typically 3000 A/min, while oxynitride coatings were deposited at roughly 300 A/min. We also demonstrated the hermeticity of AlN coatings for periods of several hundred hours even when immersed in water.

Contamination of surfaces prior to optical coating by in-situ total internal reflection microscopy

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.

Ion beam milling of fused silica for window fabrication

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.

Fabrication of optical surfaces with low subsurface damage using a float polishing process

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 characterization of surface absorption

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.