Optical Interference Coatings Research Articles

Optical Interference Coatings: Three Manufacturing Problems

How can one produce a thin-film filter that reproduces irregular spectral transmittance and reflectance curves for near-normal incident light? How about filters intended for use at oblique angles of incidence? These problems were part of a unique thin-film manufacturing competition at OSA’s Topical Meeting on Optical Interference Coatings.

Optical Society of America's 2007 Topical Meeting on Optical Interference Coatings: overview

The Optical Society of America's Topical Meeting on Optical Interference Coatings convenes every three years to survey and capture advancements in the broad area of optical coatings. This meeting serves as a focal point for global technical interchange in the field of optical interference coatings. It includes papers on research, development, and applications of optical coatings, such as fundamental and theoretical contributions in the field as well as practical techniques and applications.

Crystal phase transition of HfO_2 films evaporated by plasma-ion-assisted deposition

HfO(2) is a well-known high-refractive-index material for optical interference coatings from the infrared down to the ultraviolet (UV). Dense, homogeneous, and stoichiometric HfO(2) film is critical for low-loss UV optics where the spectral region is close to its fundamental absorption edge. We prepare HfO(2) films by plasma-ion-assisted deposition with various amounts of plasma-ion momentum transfer. The correlation between the HfO(2) film structure and the optical properties are evaluated by a variable-angle spectroscopic ellipsometry, indicating inhomogeneous to homogeneous to inhomogeneous film structural evolution with increasing momentum transfer during deposition. The HfO(2) film inhomogeneity, surface roughness, and crystal phase are confirmed by scanning electron microscopy cross-sectional, atomic force microscopy, and x-ray diffraction measurements. The results suggest that the HfO(2) film structural evolution corresponds to crystal phase transition from weak monocline amorphous to strong monocline depending on the amount of plasma-ion momentum transfer during film deposition.

Optical Interference Coatings Design Contest 2007: triple bandpass filter and nonpolarizing beam splitter

A triple bandpass filter (28 solutions received) and a nonpolarizing beam splitter (23 solutions received) were the subjects of the design contest held in conjunction with the 2007 Optical Interference Coatings topical meeting of the Optical Society of America. Fifteen designers participated using a wide spectrum of design approaches and optimization strategies to create the submissions. The results differ significantly, but all meet the contest requirements. Fabien Lemarchand wins both contests by submitting the thinnest (6254 nm) triple bandpass design and the widest (61.7 nm) nonpolarizing beam-splitter design. Michael Trubetskov is in second place, followed by Vladimir Pervak in both contests. The submitted designs are described and evaluated.

Multilayer antireflective and protective coatings comprising amorphous diamond and amorphous hydrogenated germanium carbide for ZnS optical elements

To effectively protect and improve the transmittance of ZnS optical elements in the far infrared band, combined amorphous diamond (a-D) and amorphous hydrogenated germanium carbide (a-Ge 1− x C x :H) films have been developed. The optical interference coatings were designed according to the layer optics theory. The a-D films, of which refractive index and film thickness were controlled by changing substrate bias and deposition time respectively, were deposited by filtered cathodic vacuum arc technology. The a-Ge 1− x C x :H films were prepared by radio frequency sputtering technology. During this process their refractive index was modulated by changing the gas flow rate ratio and their film thickness was controlled by the flow rate ratio and deposition time. It has been shown that the combined films are superexcellent antireflective and protective coatings for ZnS optical elements.

Fifty Years of Optical Interference Coatings at the National Research Council of Canada

For the past 50 years, the Thin Films Group of the National Research Council (NRC) in Canada has successfully developed optical interference coatings that have changed the fabric of our lives. The coatings have been used for everything from protecting paper currency against counterfeiting, to increasing the contrast and efficiency of organic LEDs, to forming components of artificial vision systems used in space shuttles. This article reflects on NRC's past successes, current challenges and future directions.

Oxidation of evaporated porous silicon rugate filters

Rugate filters are thin-film optical interference coatings with sinusoidal variation of the refractive index. Several of these filters were fabricated with glancing angle deposition, which exploits atomic competition during growth to create nanoporous materials with controllable effective refractive index. This method enables the fabrication of devices with almost arbitrary refractive index profiles varying between the ambient, 1.0, and the index of the film material, in this case silicon with an index of 4.0 (at 600 nm). As these filters are inherently porous, oxidation of the silicon can occur throughout the device layer, and here we study the intentional oxidation of silicon filters by high-temperature reaction with gaseous oxygen. We find that a significant portion of the silicon filter oxidizes in approximately 10 min when heated to 600 degrees C-650 degrees C in an oxygen environment; oxidation then continues slowly over several hours. The presence of water vapor has little apparent effect on the oxidation reaction, and attempts to oxidize with ozone at room temperature were unsuccessful. As silicon filters oxidize to become silica, spectral blueshifts and increased short-wavelength transmittance are observed. Measured and calculated transmittance spectra generally agree, although the lack of absorption and dispersion in the theoretical model limits detailed comparison.

Philip Werner Baumeister (1929-2003): in memoriam

Philip Baumeister pioneered the design of optical thin films by computer methods. He has published many important papers on various aspects of optical coatings, and he initiated the Optical Society of America's Topical Meetings on Optical Interference Coatings. Most important, during his time at the University of Rochester and thereafter, he trained and mentored generations of researchers and engineers in the theory and practice of thin-film interference coatings. Originally, a talk was prepared to celebrate his 75th birthday at the ninth Topical Meeting on Optical Interference Coatings. Because of his untimely demise on 22 October 2003, it was presented at the conference to honor his memory. This is a written version of the tribute that contains a comprehensive list of Philip Baumeister's various publications.

2004 Optical Society of America's Topical Meeting on Optical Interference Coatings: manufacturing problem

Results are presented for the second Optical Society of America's Optical Interference Coatings Manufacturing Problem. The participants were asked to produce multilayer coatings which, in the 450-650 nm spectral region and for light incident at 60 degrees, would have transmittances of 0.7 and 0.3 for p- and s-polarized light, respectively. Three different teams each submitted four solutions. Three different deposition processes were used to produce these coatings. The smallest average departure from the target transmission values was 0.79%. A number of interesting conclusions can be drawn from this exercise.

Alfred Josef Thelen--on the occasion of his seventy-fifth birthday

Alfred Thelen has made many very important contributions to the field of optical thin films. To honor him on the occasion of his 75th birthday, I prepared a bibliography of his many contributions for the ninth Topical Meeting on Optical Interference Coatings. Here an updated list of Alfred Thelen's books, chapters, papers, conference reports, and patents is presented.

Optical Interference Coatings Design Contest 2004

A manufacturable, broadband, broad-angle antireflection (AR) coating for the visible (13 designs submitted) and a minimum-shift immersed short-pass filter (12 designs submitted) were the subjects of the design contest held in conjunction with the 2004 Optical Interference Coatings topical meeting of the Optical Society of America. Under the specified constraints, the broadband, broad-angle AR coating could be made more than 65 nm wide. The statistical stability of manufacturing simulations is discussed. The short-pass filter could operate up to a +/- 5.5 degree angular range. The submitted designs are described and evaluated.

Optical characterization of an unknown single layer: Institut Fresnel contribution to the Optical Interference Coatings 2004 Topical Meeting Measurement Problem

We present the characterizations performed at the Institut Fresnel for the Measurement Problem of the Optical Interference Coatings 2004 Topical Meeting. A single layer coated on a fused-silica substrate of unknown composition and parameters is analyzed in terms of optogeometrical parameters, uniformity, and scattering. We determine the refractive index and the average thickness of the coating, then provide the localized determination of the thickness with a 2 mm spatial resolution. Topography measurements include atomic force microscopy and angle-resolved scattering measurements. These results are completed thanks to a Taylor Hobson noncontact 3D surface profiler.

Production of MF and DC-pulse sputtered anti-reflective/anti-static optical interference coatings using a large area in-line coater

This paper is on the deposition of antistatic and antireflective coatings (ARAS coatings) based on TiO 2, SiO 2 and ITO films on 300 × 300 mm 2 glass substrates using a production-like in-line sputter system with a cathode length of 750 mm. In order to achieve a reproducible and homogenous deposition process, we studied the deposition of single and multilayer systems for different process conditions. The optical constants and individual layer thicknesses have been determined by variable angle spectroscopic ellipsometry and spectroscopic photometry as well as by X-ray reflectometry (XRR). With this information we used sequent redesigns of the ARAS stack in order to compensate for film thickness deviations during the deposition of the multilayer stack. Approaches to minimize the drawbacks of depositing optical coatings using in-line sputter systems are shown.

Pulse magnetron sputtering in a reactive gas mixture of variable composition to manufacture multilayer and gradient optical coatings

Optical interference coatings have been produced by pulse magnetron sputtering a silicon target in a variable mixture of the reactive gases oxygen and nitrogen. Conventional H/L multilayer stacks with sharp transition of refractive index and gradient layer systems with inhomogeneous refractive index profile have been designed and deposited on glass and plastic substrates. Design examples and results of deposited films will be presented for multi-band rugate filters, edge and dichroic filters as well as for graded index antireflection coatings. All films were grown at a single sputtering station only, without interruption of the plasma process. The avoidance of substrate movement and process interruption, together with accurate process control, guarantees a very stable and highly efficient process with high reproducibility of the coating on 8″ substrates with a film thickness uniformity of ± 1%.

Pulse compression with time-domain optimized chirped mirrors

Dispersive optical interference coatings (chirped mirrors - CMs) are designed by computer optimization of an analytically calculated initial multilayer. Traditionally, the relevant properties of the CM (reflectance and the frequency-dependence of the phase shift upon reflection) are optimized to match frequency-domain targets. We propose a novel target function that quantifies directly the capability of a multilayer to control the temporal shape of the reflected optical pulse. Employing this time-domain analysis/optimization one can design dispersive multilayers having air as medium of incidence and supporting the generation of pulses with durations in the sub-5-fs-range, as demonstrated in a proof-of-principle compression experiment.

Deposition of optical coatings with real time control by the spectroellipsometry

In-situ spectroscopic ellipsometry is known to be very sensitive, non-invasive technique for monitoring and control of thin film growth. In the production of optical interference coatings the refractive index of the material is usually assumed to remain constant within a single layer. Under such assumption only the optical thickness of the layer can be efficiently controlled. For modern complex structures, however, even insignificant deviation from the design, due to the shift in the refractive index, can be very detrimental to the resulting performance of the coating. Simultaneous real-time determination of refractive index and growth rate is necessary in order to comply with strict specifications. If the index departs from the target value, one has to adjust process parameters and, ultimately, perform re-calculation of the filter structure to compensate for an error. We will review the work performed in our laboratory during last few years. Development of several different control strategies will be discussed and their application to the control of PECVD of optical interference coatings demonstrated. Latest work is concerned with the advances in the closed-loop control of the fabrication of optical thin films by in-situ multi-wavelength phase-modulated kinetic ellipsometry using both, direct numerical inversion algorithm for the real-time reconstruction of refractive index and layer thickness and real-time least-squire fitting-based approach. These techniques have been tested on quarter-wave index optical filters as well as on inhomogeneous refractive index profiles and demonstrated efficiency and robustness.

Topical meeting on optical interference coatings (OIC'2001): manufacturing problem.

Measurements are presented of the experimental filters submitted to the first optical thin-film manufacturing problem posed in conjunction with the Topical Meeting on Optical Interference Coatings, in which the object was to produce multilayers with spectral transmittance and reflectance curves that were as close as possible to the target values that were specified in the 400- to 600-nm spectral region. No limit was set on the overall thickness of the solutions or the number of layers used in their construction. The participants were free to use the coating materials of their choice. Six different groups submitted a total of 11 different filters for evaluation. Three different physical vapor deposition processes were used for the manufacture of the coatings: magnetron sputtering, ion-beam sputtering, and plasma-ion-assisted, electron-beam gun evaporation. The solutions ranged in metric thickness from 758 to 4226 nm and consisted of between 8 and 27 layers. For all but two of the samples submitted, the average rms departure of the measured transmittances and reflectances from the target values in the spectral region of interest was between 0.98% and 1.55%.

Optical interference coatings: eighth topical meeting

A brief account is given of the Eighth Optical Society of America Topical Meeting on Optical Interference Coatings held 15–20 July 2001 in Banff, Alberta, Canada. Photographs of some attendees are appended.

Topical meeting on optical interference coatings (OIC'2001): design contest results.

A gain-flattening filter (GFF) for minimum manufacturing errors (12 designs submitted) and dense wavelength-division multiplex (DWDM) filters for low group-delay (GD) variation (9 designs submitted) was the subject of a design contest held in conjunction with the Optical Interference Coatings 2001 topical meeting of the Optical Society of America. Results of the contest are given and evaluated. It turned out that the parameter space for GFFs with optimum performance when manufacturing errors are not considered is much different from that when manufacturing errors are considered. DWDM filter solutions with low GD variation are possible.

Design of a robust thin-film interference filter for erbium-doped fiber amplifier gain equalization.

Gain-flattening filters (GFFs) are key wavelength division multiplexing components in fiber-optics telecommunications. Challenging issues in the design of thin-film GFFs were recently the subject of a contest organized at the 2001 Conference on Optical Interference Coatings. The interest and main difficulty of the proposed problem was to minimize the sensitivity of a GFF to simulated fabrication errors. A high-yield solution and its design philosophy are described. The approach used to control the filter robustness is explained and illustrated by numerical results.