Fabrication Methods for Adaptive Deformable Mirrors

Previously, it was difficult to fabricate deformable mirrors made by piezoelectric actuators. This is because numerous actuators need to be precisely assembled to control the surface shape of the mirror. Two approaches have been developed. Both approaches begin by depositing a stack of piezoelectric films and electrodes over a silicon wafer substrate. In the first approach, the silicon wafer is removed initially by plasmabased reactive ion etching (RIE), and non-plasma dry etching with xenon difluoride (XeF2). In the second approach, the actuator film stack is immersed in a liquid such as deionized water. The adhesion between the actuator film stack and the substrate is relatively weak. Simply by seeping liquid between the film and the substrate, the actuator film stack is gently released from the substrate. The deformable mirror contains multiple piezoelectric membrane layers as well as multiple electrode layers (some are patterned and some are unpatterned). At the piezolectric layer, polyvinylidene fluoride (PVDF), or its co-polymer, poly(vinylidene fluoride trifluoroethylene P(VDF-TrFE) is used. The surface of the mirror is coated with a reflective coating. The actuator film stack is fabricated on silicon, or silicon on insulator (SOI) substrate, by repeatedly spin-coating the PVDF or P(VDFTrFE) solution and patterned metal (electrode) deposition. In the first approach, the actuator film stack is prepared on SOI substrate. Then, the thick silicon (typically 500-micron thick and called handle silicon) of the SOI wafer is etched by a deep reactive ion etching process tool (SF6-based plasma etching). This deep RIE stops at the middle SiO2 layer. The middle SiO2 layer is etched by either HF-based wet etching or dry plasma etch. The thin silicon layer (generally called a device layer) of SOI is removed by XeF2 dry etch. This XeF2 etch is very gentle and extremely selective, so the released mirror membrane is not damaged. It is possible to replace SOI with silicon substrate, but this will require tighter DRIE process control as well as generally longer and less efficient XeF2 etch. In the second approach, the actuator film stack is first constructed on a silicon wafer. It helps to use a polyimide intermediate layer such as Kapton because the adhesion between the polyimide and silicon is generally weak. A mirror mount ring is attached by using adhesive. Then, the assembly is partially submerged in liquid water. The water tends to seep between the actuator film stack and silicon substrate. As a result, the actuator membrane can be gently released from the silicon substrate. The actuator membrane is very flat because it is fixed to the mirror mount prior to the release. Deformable mirrors require extremely good surface optical quality. In the technology described here, the deformable mirror is fabricated on pristine substrates such as prime-grade silicon wafers. The deformable mirror is released by selectively removing the substrate. Therefore, the released deformable mirror surface replicates the optical quality of the underlying pristine substrate

An experimental study of stress singularities at a sharp corner in a contact problem

The photoelastic method was used to investigate the nature of the local stress field at a sharp corner of a wedge that was compressed against a larger body. Planar wedge specimens made of photoelastic material were compressed against a half plane (larger body) of identical material at various load levels. Several wedge angles were studied. The nature of the singular stress field postulated by linear elastic analysis was verified and the strength of the singularity was obtained by plotting the variation of fringe order as a function of radial distance from the sharp corner on a logarithmic scale. The experimental results were found to be in good agreement with the theoretical predictions. The effect of interface friction and the effect of rounding off the sharp edge are brietly discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43931/1/11340_2006_Article_BF02322823.pd

Stress singularity at a sharp edge in contact problems with friction

The paper discusses the nature of the singularity that arises at a sharp edge in contact problems with friction. The theoretical treatment is based on the Mellin transform of the elastic fields. The results regarding the power singularities confirm the previous work of Gdoutos and Theocaris, but it is shown that logarithmic singularities are always present. Some experimental observations in photoelasticity are also presented. Die Art der Spannungssingularität, die an einer scharfen Ecke in Berührungsproblemen erscheint, ist für den Fall mit Reibung untersucht. Die theoretische Behandlung stützt sich auf die Mellin-Transformation der elastischen Felder. Die Ergebnisse bezüglich der Potenzsingularitäten bestätigen die früheren Resultate von Gdoutos und Theocaris. Es wird jedoch gezeigt, daß logarithmische Singularitäten stets anwesend sind. Auch einige Beobachtungen von photoelastischen Versuchen sind dargestellt.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43357/1/33_2005_Article_BF01594906.pd

A lightweight tile structure integrating photovoltaic conversion and RF power transfer for space solar power applications

We demonstrate the development of a prototype lightweight (1.5 kg/m^3) tile structure capable of photovoltaic solar power capture, conversion to radio frequency power, and transmission through antennas. This modular tile can be repeated over an arbitrary area to forma large aperture which could be placed in orbit to collect sunlight and transmit electricity to any location. Prototype design is described and validated through finite element analysis, and high-precision ultra-light component manufacture and robust assembly are described

Some consequences of the inequality conditions in contact and crack problems

The importance of the inequalities and related side conditions that must be incorporated in contact and crack problems is emphasized and the ensuing consequences explored. An asymptotic analysis of the transitions from slip to separation, stick to slip, and stick to separation is carried out. The inequalities in contact problems make the contact pressure continuous for all levels of friction. They also make a direct transition from stick to separation impossible, unless the combination of materials is special. The inequalities in crack problems are less stringent, but they preclude certain singularities that appear to have flourished in the literature previously. On appuie sur l'importance d'inéquations et d'autres conditions auxiliaires qui doivent être comprises dans les problèmes de contact et de fissure, et on en explore les conséquences. On emploie une analyse asympotique sur les transitions entre les zones glissement-décollement, adhérence-glussement et adhérence-décollement. Il s'ensuit que la contrainte normale du contact doit être continue pour toutes les valeurs du frottement. De plus, la transition directe adhérence-décollement est impossible, à moin que la combinaison des matériaux ne soit exceptionelle. Les inéquations dans les problèmes de fissure sont moins fortes, mais elles sont cependant suffisantes pour empêcher l'existence de certaines singularités qui apparaissent souvent dans les études précedentes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42689/1/10659_2004_Article_BF00040981.pd

A lightweight tile structure integrating photovoltaic conversion and RF power transfer for space solar power applications

We demonstrate the development of a prototype lightweight (1.5 kg/m^3) tile structure capable of photovoltaic solar power capture, conversion to radio frequency power, and transmission through antennas. This modular tile can be repeated over an arbitrary area to forma large aperture which could be placed in orbit to collect sunlight and transmit electricity to any location. Prototype design is described and validated through finite element analysis, and high-precision ultra-light component manufacture and robust assembly are described

A flexible phased array system with low areal mass density

Phased arrays are multiple antenna systems capable of forming and steering beams electronically using constructive and destructive interference between sources. They are employed extensively in radar and communication systems but are typically rigid, bulky and heavy, which limits their use in compact or portable devices and systems. Here, we report a scalable phased array system that is both lightweight and flexible. The array architecture consists of a self-monitoring complementary metal–oxide–semiconductor-based integrated circuit, which is responsible for generating multiple independent phase- and amplitude-controlled signal channels, combined with flexible and collapsible radiating structures. The modular platform, which can be collapsed, rolled and folded, is capable of operating standalone or as a subarray in a larger-scale flexible phased array system. To illustrate the capabilities of the approach, we created a 4 × 4 flexible phased array tile operating at 9.4–10.4 GHz, with a low areal mass density of 0.1 g cm^(−2). We also created a flexible phased array prototype that is powered by photovoltaic cells and intended for use in a wireless space-based solar power transfer array