Structured light is a robust and accurate method for 3D range imaging in which one or more light patterns are projected onto the scene and observed with an off-axis camera. Commercial sensors typically utilize DMD- or LCD-based LED projectors, which produce good results but have a number of drawbacks, e.g. limited speed, limited depth of focus, large sensitivity to ambient light and somewhat low light efficiency.
We present a 3D imaging system based on a laser light source and a novel tip-tilt-piston micro-mirror. Optical interference is utilized to create sinusoidal fringe patterns. The setup allows fast and easy control of both the frequency and the phase of the fringe patterns by altering the axes of the micro-mirror. For 3D reconstruction we have adapted a Dual Frequency Phase Shifting method which gives robust range measurements with sub-millimeter accuracy.
The use of interference for generating sine patterns provides high light efficiency and good focusing properties. The use of a laser and a bandpass filter allows easy removal of ambient light. The fast response of the micro-mirror in combination with a high-speed camera and real-time processing on the GPU allows highly accurate 3D range image acquisition at video rates.
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.
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.
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.
This paper describes charging effects on spatial light modulators (SLM). These light modulators consist of up to one
million mirrors that can be addressed individually and are operated at a frame rate of up to 2 kHz. They are used for
DUV mask writing where they have to meet very high requirements with respect to accuracy.
In order to be usable in a mask-writing tool, the chips have to be able to work under DUV light and maintain their
performance with high accuracy over a long time. Charging effects are a problem frequently encountered with MEMS,
especially when they are operated in an analog mode.
In this paper, the issue of charging effects in SLMs used for microlithography, their causes and methods of their
reduction or elimination, by means of addressing methods as well as technological changes, will be discussed. The first
method deals with the way charges can accumulate within the actuator, it is a simple method that requires no
technological changes but cannot always be implemented. The second involves the removal of the materials within the
actuator where charges can accumulate.
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.
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.
Design details and performance data are presented for (Al,Ga)As and polymeric monolithic tapered rib waveguides achieving modal spot-size transformation for mode-matching from a variety of devices to single-mode optical fiber. 2D expanded output modes of waveguide modulators and lasers are achieved using 1D and 2D tapers between non-critical initial and final widths well suited for optical lithography.
Optical switches based on deflection of a waveguide element offer low crosstalk, low polarization dependency, low power consumption, and high degree of integration. Such switches made by post processing of polymeric waveguides onto MEMS structures of silicon-on-insulator (SOI) efficiently combine low loss waveguides with the exceptional mechanical properties of single crystalline silicon. An important aspect of this concept is that it allows independent optimization of the mechanical and optical structures by efficiently separating the two. Well established, high yield methods exist for structuring silicon based on deep reactive ion etching (DRIE), which allows the formation of mechanical structures with high aspect ratio. The mechanical structure can then be planarized for further processing by utilizing spin coating properties of certain polymers. This allows post processing of high-resolution passive polymeric waveguide networks that can fulfil a variety of functions depending on the application, including spot-size transformers for low loss coupling to optical fibers. These waveguides can also potentially be integrated with CMOS or active optoelectronic elements into forming highly functional hybrid photonic integrated circuits, partly facilitated by the low temperatures required for processing of polymers. This paper highlights key process technologies and specifically discusses issues related to an optical switch that was developed for proof of concept. This switch was made of 5μm thick SOI with 3μm wide, high optical confinement polymeric waveguides. Switching times were down to 30μs, switching voltages 20 to 50V, and crosstalk was -32dB. The paper further outlines possible applications of the switch to state-of-the-art problems in photonics.
The Fraunhofer IMS in Dresden is developing and fabricating spatial light modulators (SLMs) for micro lithography with DUV radiation. The accuracy of analog modulation is very important for the resulting accuracy of the generated features. On the other hand, fabrication tolerances create variations for example in spring constant, zero voltage deflection, and reflectivity. The slightly different response curves of the individual pixels therefore require an individual calibration. The parameters of these are stored in a look-up table so that the proper addressing voltage for the required optical response can be selected. As the deflection angle as well as the size of the SLM pixels are quite small, a direct measurement of the pixel response is not straightforward. An optical system similar to the one in the lithography machine has been set up, where the SLM is operating as a phase grating and the image is generated by a spatial filter. The pixel deflection can be calculated from the aerial image for isolated deflected pixels. The background pixels, that are not calibrated yet, contribute some error to this calculation. However, this error is not very large. Simulations regarding the accuracy of this measurement are discussed, and experimental results are shown.
A micro-mechanical optical switch based on lateral deflection of a waveguide is described. The switching element is a suspended silicon beam formed by deep reactive ion etching and release of silicon-on-insulator (SOI). Actuation is implemented using integrated electrostatic comb drives. Dry etched polymeric optical waveguides are post processed onto the mechanical structure. We present design, simulations, and preliminary experimental results.
New methods for integration of dissimilar components and optical inputs/outputs are expected to mass-produce photonic micro-systems at reduced levels of difficulty and therefore reduced cost. These methods involve monolithic and hybrid approaches, the latter at both wafer-to-wafer and chip-to- wafer levels. Broadly, these are called 'heterogeneous integration' and encompass technologies as diverse as wafer- fusion and DNA-assisted micro-assembly. This review summarizes the associated micro-assembly techniques and discusses their possible influence upon cost- and yield-benefits to industry.
High optical loss due to mode mismatch at the interfaces of different components in a hybrid photonic integrated circuit (PIC) poses a major challenge in the implementation of such devices. Increased coupling efficiency can be achieved by incorporating an optical mode converter at the interface. This converter basically consists of a tapered waveguide section adapting different modal spot-sizes. Optimized coupling requires total control of the transverse optical field. This can be achieved by the ability to shape both the vertical as well as the lateral waveguide dimension. We present design, simulations, and fabrication considerations for a 3D tapered waveguide structure for low loss mode conversion. Our mode converter concept is based on polymeric optical waveguides on silicon substrate. A gradually deeper trench is formed in the silicon substrate, using diffusion limited wet etching with a laterally tapered mask pattern. The structure is then planarized with a polymer and patterned laterally. Our method thus allows control of both the lateral and vertical waveguide dimensions. The concept is consistent with low-loss coupling to singlemode fibers as well as between laser and amplifier arrays and single mode waveguides in a low-cost hybrid PIC solution.
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