Serhat Sakarya

Technology of reflective membranes for spatial light modulators

This paper describes the fabrication technology of two different implementations of a spatial light modulator. Both implementations are based on continuous reflective membranes that are deformed electrostatically by potentials applied on an underlying electrode structure. By using a continuous membrane we achieve a 100% optical fill factor. Additionally, the membrane can be coated with any metal, enabling it to use a uniform technology to fabricate devices able to handle a high optical load over a wide spectral range. The first implementation is based on a continuous thin nitride membrane that rests on a support grid structure, which subdivides the membrane into individually controllable pixels. The second implementation uses viscoelastic layers as a carrier material for the reflective membrane, making the device more robust, enabling a high resolution, but a slower speed of response. Experiments with fabricated devices have shown sufficient deflection at low control voltages, making the use of integrated electronics possible. Applications for these devices include projection displays, optical information processing systems, optical lithography, etc.

Silicon micro-optics for smart light control

We present an overview of the results of our recent research in the field of adaptive optical components based on silicon microtechnologies, including membrane deformable mirrors, spatial light modulators, liquid-crystal correctors, wavefront sensors, and both spherical and aspherical micro-optical components. We aim at the realization of adaptive optical systems using standard-technology solutions.

Micromachined SLM based on pixelated reflective membranes

We report on an on-chip projection-type spatial light modulator that consists of a continuous reflective surface formed by a thin stretched silicon nitride membrane supported by a grid structure providing an array of individually accessible deformable pixels. The curvature of each pixel can be controlled electrostatically by changing the potentials on underlying integrated electrodes. This type of modulator can be used directly as a phase modulator (possibly combined with a microlens array) and in dark schlieren projection scheme as a display device. The continuous surface of the membrane provides 100% optical fill factor and, being appropriately coated, can handle high optical loads over a wide spectral range, including the infrared region. The support grid structure and electronics can be scaled so as to achieve a wide range of resolutions with a uniform technology. Integrated control electronics and the deformable pixelated mirror structure are fabricated independently on different wafers, allowing optimized fabrication of each and ensuring higher yield and quality and lower costs.

Technological approaches for fabrication of elastomer-based spatial light modulators

We present novel approaches to the fabrication of spatial light modulators based on thin viscoelastic layers. These layers are formed against two chips: the bottom one carries an interdigitated electrode structure and the top one is a sacrificial chip coated with a metal layer or a stack of materials. By etching away the top chip with bulk silicon techniques, a directly coated and planarized elastic layer results with very high optical quality. The surface is deformed in a sinusoidal shape under electrostatic load when alternating potentials are applied on the underlying electrodes. With this effect, solid-state alternatives for Eidophor projectors can be fabricated. The top chip can contain either a 125nm gold layer or a 50nm nitride and 80nm aluminum layer. After curing, the chip is encapsulated in a flexible elastomer based etch holder and placed in a 33wt% KOH solution at 85°C. This etches away the silicon of the top chip and stops on either the nitride or gold. The surface has a 100% optical fill factor over the active region and can scale easily to various resolutions and spectral ranges. Measurements of the surface has shown local initial deformations below 0.10 λ. Experiments done with devices with 50-100μm electrode size and 5μm spacer distance have shown significant far-field scattering under application of 300V potential difference between the electrodes. Further development will include optimizations of the modulation efficiency. Applications can be found in high performance projection displays, optical lithography and optical communication networks.

Technology for integrated spatial light modulators based on reflective membranes

Two approaches toward the fabrication of a spatial light modulator are presented. The first approach uses a pixelated nitride membrane that is suspended by a grid structure over an electrode array. Deformation of each membrane segment is achieved by means of electrostatic attraction. By using a continuous reflective surface, we achieve a 100% optical fill factor. Since we can coat with any metal or combination of materials, these devices can, in principle, handle high optical loads over a wide spectral range. We have tried different spacer materials and dimensions with this approach. In the second approach, the membrane rests on a viscoelastic carrier layer. During fabrication we use the thin nitride membrane to ensure uniformity of the elastic layer after which it is used as an etch stop for the bulk silicon etchant. This type of device is more robust, can use smaller pixel sizes, but has less sensitivity at low voltages. To achieve an optimal, but simple fabrication procedure, the membranes and electronics are fabricated in different processes, ensuring a higher optical quality of the membrane and increasing yield at lower costs. Applications lie in the field of projection displays, optical lithography, optical communication networks, etc.

Low-cost technological approaches to micromachined spatial light modulators

Approaches toward the fabrication of low-cost integrated micromachined spatial light modulators are presented. An optimized fabrication procedure minimizes requirements on integrated electronics and mechanical layers. This surface can rest on a viscoelastic carrier material under which an electrode array is placed. Under application of appropriate potentials on the underlying electrodes, localized sinusoidal phase gratings can be produced. The depth of modulation can be converted to an intensity value by using Schlieren bars and integrating properties of the projection lens. The pixel sizes can vary from 20μm to 1mm. In the fabrication procedure, a top chip is used, which is coated with a 50nm nitride layer and an 80nm Al layer. A droplet of the carrier layer is placed on the bottom chip, on which the top chip is then pressed, planarizing the surface. A 33 wt% KOH solution is used to bulk-etch the silicon of the top chip, using the thin nitride membrane as an etch stop, thus transferring the metal layer to the carrier substrate, a technique potentially also usable for adaptive deformable mirrors. A special elastomer based etch holder technology was developed to provide low stress protection of the sidewalls and aluminum bond pads during etching. Applications of these devices lie in the field of projection displays, optical lithography and optical communication networks.

Technology of spatial light modulators based on viscoelastic layers

We present a novel technology for spatial light modulators based on electrostatic deformation of viscoelastic layers. To fabricate the modulator structure, we bond two silicon chips using an intermediate viscoelastic layer and then etch away the top chip. This results in a very high optical quality viscoelastic layer deposited directly on top of the bottom chip. Light modulation is achieved by deforming the deposited viscoelastic layer using electrodes integrated into the bottom chip. Before bonding, the top chip is coated with a 80nm layer of aluminum and 50nm layer of nitride, that serves as the etch stop and reflector at the silicon. The thin nitride layer functions as the etch stop. Special technology was developed for low-stress side and back etch protection of contact pads of the devices. The continuous reflective membrane results in a 100% optical fill factor, enabling the modulator to handle relatively high optical loads. Also, given a sufficient bias voltage, the voltages on the electrodes should be in the range of 15-30V, making integrated solutions possible. Applications lie in the field of optical communication networks and projection displays.

Fabrication technology for micromachined spatial light modulators

This paper presents novel approaches on fabrication technology for micromachined spatial light modulators that are based on thin deformable viscoelastic layers. These layers are formed between two chips. The bottom chip contains an array of interdigitated electrode structures, where each structure represents one pixel. The top chip contains the mechanical layers which are transferred to the elastic layers by means of bulk etching techniques. This results in a high quality reflective surface with a 100% optical fill factor over the active region. Flexibility in choice of coatings gives the devices the potential to operate in specific spectral ranges with high load handling capability. The top chip is coated with a 50nm nitride layer onto which a 80nm aluminum layer is deposited. After curing of the intermediate viscoelastic layer, the entire device is placed in an elastomer holder and the bulk silicon is etched away in a 33wt% KOH solution. Devices were fabricated with electrode sizes in the range of 10 to 100μm and a 5μm thick viscoelastic layer. Experiments have shown far-field scattering as a result of 300V potential difference applied between the electrode. Biasing the membrane will lower this potential requirement to make integrated electronics possible. Applications can be found in high-end projection displays, optical lithography and optical communication networks.

Technology for Spatial Light Modulators Based on Reflective Silicon Structures

Spatial light modulators are used for spatial modulation of optical wavefronts in terms of intensity, phase and polarization. These devices are typically used for projection displays, information processing systems, communication networks, optical lithography, etc.

ADAPTIVE OPTICAL SYSTEM BASED ON A MICROMACHINED ADAPTIVE MIRROR

An implementation of an adaptive optical system based on a 15-mm 37-channel silicon micromachined adaptive mirror [1] is reported. The control loop consists of a beam quality sensor-in our case a brightness sensor placed in the focus of a positive lens-and electronics which includes a single 12-bit ADC to input the quality parameter into the control computer and a 37-channel high-voltage 8-bit DAC to control the shape of the mirror.