Thor Bakke

One-Megapixel Monocrystalline-Silicon Micromirror Array on CMOS Driving Electronics Manufactured With Very Large-Scale Heterogeneous Integration

In this paper, we demonstrate the first high-resolution spatial-light-modulator chip with 1 million tilting micromirrors made of monocrystalline silicon on analog high-voltage complementary metal-oxide-semiconductor driving electronics. This device, as result of a feasibility study, shows good optical and excellent mechanical properties. The micromirrors exhibit excellent surface properties, with a surface roughness below 1-nm root mean square. Actuated micromirrors show no imprinting behavior and operate drift free. Very large-scale heterogeneous integration was used to fabricate the micromirror arrays. The detailed fabrication process is presented in this paper, together with a characterization of the SLM devices. Large arrays of individually controllable micromirrors are the enabling component in high-perfomance mask-writing systems and promising for high throughput deep-ultraviolet maskless lithography systems. The adoption of new materials with enhanced characteristics is critical in meeting the challenging demands with regard to surface quality and operation stability in the future. Very large-scale heterogeneous integration may enable virtually any solid-state material to be integrated together with CMOS electronics.

A novel ultra-planar, long-stroke and low-voltage piezoelectric micromirror

A novel piston-type micromirror with a stroke of up to 20 µm at 20 V formed out of a silicon-on-insulator wafer with integrated piezoelectric actuators was designed, fabricated and characterized. The peak-to-valley planarity of a 2 mm diameter mirror was better than 15 nm, and tip-to-tip tilt upon actuation less than 30 nm. A resonance frequency of 9.8 kHz was measured. Analytical and finite element models were developed and compared to measurements. The design is based on a silicon-on-insulator wafer where the circular mirror is formed out of the handle silicon, thus forming a thick, highly rigid and ultra-planar mirror surface. The mirror plate is connected to a supporting frame through a membrane formed out of the device silicon layer. A piezoelectric actuator made of lead–zirconate–titanate (PZT) thin film is structured on top of the membrane, providing mirror deflection by deformation of the membrane. Two actuator designs were tested: one with a single ring and the other with a double ring providing bidirectional movement of the mirror. The fabricated mirrors were characterized by white light interferometry to determine the static and temporal response as well as mirror planarity.

High-performance long wave infrared bolometer fabricated by wafer bonding

A novel microbolometer with peak responsivity in the longwave infrared region of the electromagnetic radiation is under development at Sensonor Technologies. It is a focal plane array of pixels with a 25μm pitch, based on monocrystalline Si/SiGe quantum wells as IR sensitive material. The novelty of the proposed 3D process integration comes from the choice of several of the materials and key processes involved, which allow a high fill factor and provide improved transmission/absorption properties. Together with the high TCR and low 1/f noise provided by the thermistor material, they will lead to bolometer performances beyond those of existing devices. The thermistor material is transferred from the handle wafer to the read-out integrated circuit (ROIC) by wafer bonding. The low thermal conductance legs that connect the thermistor to the ROIC are fabricated prior to the transfer bonding and are situated under the pixel. Depending on the type of the transfer bonding used, the plugs connecting the legs to the thermistor are made before or after this bonding, resulting in two different configurations of the final structure. Using a low temperature oxide bonding and subsequent plugs formation result in through-pixel plugs. Pre-bonding plugs formation followed by thermo-compression bonding result in under-pixel plugs. The pixels are subsequently released by anhydrous vapor HF of the sacrificial oxide layer. The ROIC wafer containing the released FPAs is bonded in vacuum with a silicon cap wafer, providing hermetic encapsulation at low cost. Antireflection coatings and a thin layer getter are deposited on the cap wafer prior to bonding, ensuring high performance of the bolometer.

CMOS-Integrated Si/SiGe Quantum-Well Infrared Microbolometer Focal Plane Arrays Manufactured With Very Large-Scale Heterogeneous 3-D Integration

We demonstrate infrared focal plane arrays utilizing monocrystalline silicon/silicon-germanium (Si/SiGe) quantum-well microbolometers that are heterogeneously integrated on top of CMOS-based electronic read-out integrated circuit substrates. The microbolometers are designed to detect light in the long wavelength infrared (LWIR) range from 8 to 14 μm and are arranged in focal plane arrays consisting of 384 × 288 microbolometer pixels with a pixel pitch of 25 μm × 25 μm. Focal plane arrays with two different microbolometer designs have been implemented. The first is a conventional single-layer microbolometer design and the second is an umbrella design in which the microbolometer legs are placed underneath the microbolometer membrane to achieve an improved pixel fill-factor. The infrared focal plane arrays are vacuum packaged using a CMOS compatible wafer bonding and sealing process. The demonstrated heterogeneous 3-D integration and packaging processes are implemented at wafer-level and enable independent optimization of the CMOS-based integrated circuits and the microbolometer materials. All manufacturing is done using standard semiconductor and MEMS processes, thus offering a generic approach for integrating CMOS-electronics with complex miniaturized transducer elements.

Spatial light modulators with monocrystalline silicon micromirrors made by wafer bonding

Spatial light modulators (SLMs) based on micromirrors for use in DUV lithography and adaptive optics require very high mirror planarity as well as mirror stability. The ideal mechanical properties of monocrystalline silicon make this material ideally suited for use in high precision optical MEMS devices. However, the integration of MEMS with CMOS poses certain restrictions on processing temperatures as well as on the compatibility of materials. The key to the successful fabrication of monocrystalline silicon micromirrors on CMOS is the silicon layer transfer process. Here, we discuss two carefully adapted wafer bonding processes that are CMOS compatible and that allow the transfer of a 300nm thick monocrystalline silicon thin film from a SOI donor wafer. One process is based on adhesive bonding using a patterned polymer layer, while the other process is based on direct bonding to a planarization layer of polished glass.

Large-scale drift-free monocrystalline silicon micromirror arrays made by wafer bonding

Large scale arrays of more than 67k micromirrors of monocrystalline silicon with underlying planar actuation electrodes have been fabricated. The mirrors were fabricated by transferring a 300nm thick silicon layer from a silicon-on-insulator (SOI) wafer to a wafer containing metal electrodes by adhesive wafer bonding in a thermo-compression bonding tool. The bonding was followed by grinding and spin-etching of the handle silicon and the buried oxide, which leaves only the thin device silicon on the electrode wafer. Mirrors and metal plugs were formed using standard micromachining techniques such as sputtering and dry etching. The arrays consist of 16μm×16μm mirrors with 0.7μm wide and 2μm long torsional hinges. Deflection is achieved by applying a voltage between the mirrors and one of two underlying electrodes. It was found that 15V is enough to deflect the mirrors 48nm, which is sufficient to create a black pixel in a diffractive deep UV application that involves modulation of 193nm light. Furthermore, no measurable instability due to plastic hinge deformation or charging could be determined by static deflection for more than one hour. The developed fabrication process is fully CMOS ompatible and can be directly applied to fabricate spatial light modulators (SLM) with mirror arrays in excess of one megapixels with individually addressable analog mirrors that are truly drift free. Application areas are photolithographic mask writers or systems for maskless lithography.

Fabrication of large-scale mono-crystalline silicon micro-mirror arrays using adhesive wafer transfer bonding

Today, spatial light modulators (SLMs) based on individually addressable micro-mirrors play an important role for use in DUV lithography and adaptive optics. Especially the mirror planarity and stability are important issues for these applications. Mono-crystalline silicon as mirror material offers a great possibility to combine the perfect surface with the good mechanical properties of the crystalline material. Nevertheless, the challenge is the integration of mono-crystalline silicon in a CMOS process with low temperature budget (below 450°C) and restricted material options. Thus, standard processes like epitaxial growth or re-crystallization of poly-silicon cannot be used. We will present a CMOS-compatible approach, using adhesive wafer transfer bonding with Benzocyclobutene (BCB) of a 300nm thin silicon membrane, located on a SOI-donor wafer. After the bond process, the SOI-donor wafer is grinded and spin etched to remove the handle silicon and the buried oxide layer, which results in a transfer of the mono-crystalline silicon membrane to the CMOS wafer. This technology is fully compatible for integration in a CMOS process, in order to fabricate SLMs, consisting of one million individually addressable mono-crystalline silicon micro-mirrors. The mirrors, presented here, have a size of 16×16 μm2. Deflection is achieved by applying a voltage between the mirrors and the underlying electrodes of the CMOS electronics. In this paper, we will present the fabrication process as well as first investigations of the mirror properties.

Technology development for micromirror arrays with high optical fill factor and stable analogue deflection integrated on CMOS substrates

At Fraunhofer IPMS Dresden micromechanical mirror arrays are developed and fabricated using a high-voltage CMOS process for applications such as lithographic mask writers and adaptive optics. Different approaches for the fabrication of micromechanical mirror arrays with up to 1 million analogue addressable pixels in a MEMS-on-CMOS technology are discussed: sacrificial layer technologies of 1-level actuators made from a single Al-TiAl-Al structural multilayer or 2-level actuators with an additional TiAl hinge layer respectively. Also the fabrication of single crystalline Si micro-mirrors using layer-transfer bonding is discussed.

Two-state Optical Filter Based on Micromechanical Diffractive Elements

We have designed a robust two-state filter for infrared gas measurement, where the filter transmittance alternates between a single bandpass function, and a double-band offset reference. The device consists of fixed and movable diffractive sub-elements, micromachined in the device layer of a bonded silicon on insulator (BSOI) wafer. Switching between the two states of the filter is obtained by actuation of the movable sub- elements between idle and pull-in positions, which affects the interference of reflected light The characteristics of the filter are defined by a diffractive microrelief pattern etched on top of the sub-elements and by the position of the movable sub-elements at pull-in, the latter mechanically defined by the buried oxide layer. Thus, no accurate electrical control is needed to operate the filter. The first test components operate at 2 mum wavelength using a displacement of 500 nm and an actuation voltage of 5 V. No sticking or change in filter characteristics have been observed after repeated pull-in operations. The simplicity of fabrication and operation is likely to make the two-state filter an attractive component for sensors such as non-dispersive infrared gas detector.

One Megapixel SLM with High Optical Fill Factor and Low Creep Actuators

A one megapixel micromirror based SLM with two layer torsional mirror actuators was monolithically integrated on CMOS. The mirrors have a high optical fill factor and a very low level of mechanical creep when actuated