Abstract

Non-conventional MEMs Processes

Micro-stereo lithography

Micro-stereo lithography (mSL) has been developed to produce highly precise, three-dimensional (3D) microstructures from broad selection of functional materials, especially biocompatible materials. In principle, mSL utilizes focused light to scan over the surface of a photo-curable resin, which undergoes photo-polymerization and forms solid microstructures. The mSL fabricated devices, containing complex engineered microstructures which are covered with self-assembled functional groups, can work as a unique interface between the nanometer scale functional group and Marco-scale bio-medical samples, therefore can find applications in Bio-MEMS.

The MSL system and the principle of operation

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Sol-gel Coating

The sol-gel process is a wet-chemical technique for the fabrication of materials starting from a chemical solution that reacts to produce colloidal particles (sol). Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and poly-condensation reactions to form a colloid, a system composed of solid particles (size ranging from 1 nm to 1 μm) dispersed in a solvent. The sol then form an inorganic network containing a liquid phase (gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. Drying process serves to remove the liquid phase from the gel thus forming a porous material, then a thermal treatment may be performed in order to favor further polycondensation and enhance mechanical properties.

The applications for sol gel-derived products are numerous. One of the largest application areas is thin film, which can be produced on a piece of substrate by spin-coating or dip-coating. Other methods include spraying, electrophoresis, inkjet printing or roll coating. Optical coatings, protective and decorative coatings, and electro-optic components can be applied to glass, metal and other types of substrates with these methods.

Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles with novel properties can be formed that cannot be created by any other method.

Nanoimprint lithography

It is a novel method of fabricating nanometer scale patterns. It is a simple process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting. Adhesion between the resist and the template is controlled to allow proper release.

Thermoplastic nanoimprint lithography

Thermoplastic Nanoimprint lithography (T-NIL) is the earliest nanoimprint lithography developed by Professor Stephen Y. Chou's group. In a standard T-NIL process, a thin layer of imprint resist (thermoplastic polymer) is spin coated onto the sample substrate. Then the mold, which has predefined topological patterns, is brought into contact with the sample and they are pressed together under certain pressure. When heated up above the glass transition temperature of the polymer, the pattern on the mold is pressed into the melt polymer film. After being cooled down, the mold is separated from the sample and the pattern resist is left on the substrate. A pattern transfer process (Reactive Ion Etching, normally) can be used to transfer the pattern in the resist to the underneath substrate.

Photo nanoimprint lithography

In Photo Nanoimprint Lithography (P-NIL), a photo(UV) curable liquid resist is applied to the sample substrate and the mold is normally made of transparent material like fused silica. After the mold and the substrate are pressed together, the resist is cured in UV light and becomes solid. After mold separation, a similar pattern transfer process can be used to transfer the pattern in resist onto the underneath material.

Electrochemical nanoimprinting

Electrochemical nanoimprinting can be achieved using a stamp made from a superionic conductor such as silver sulfide. When the stamp is contacted with metal, electrochemical etching can be carried out with an applied voltage. The electrochemical reaction generates metal ions which move from the original film into the stamp. Eventually all the metal is removed and the complementary stamp pattern is transferred to the remaining metal.

Laser Micro machining

Laser micro machining provides non-contact machining of very high resolution, repeatability and aspect ratios and can be fully automated

It can be controlled to do localized heating and requires minimal re-deposition.

The product normally requires no pre/post processing of material and could be done in a wide range of materials: fragile, ultra-thin and highly reflective surfaces

Ultra fast or Long

Ultra fast means that the laser pulse has a duration that is somewhat less that about 10 Pico seconds - usually some fraction of a Pico second (femtosecond). "Long" means that the pulse is longer than about 10 Pico seconds, that is, longer than the heat-diffusion time. These long pulse lasers may be continuous, quasi-continuous, or Q-switched, but in any case they are generating long pulses compared to the heat-diffusion time.

Laser Micromachining of Silicon: Fabricating THz Imaging Arrays

During fabrication of THz imaging arrays using laser micro machining of Silicon, the silicon substrate is contained in a flat vacuum cell under a slowly flowing ambient of chlorine. The thermal micro reactions of silicon in chlorine are chosen because of their speed. The chlorine ambient reacts with the silicon at temperatures near the melting point to form volatile silicon chlorides, which are pumped away from the surface . A ~1 micron³ of silicon is brought to just above its melting point by a focused, CW argon-ion laser operating at 488 nm wavelength. The (circularly polarized) beam is deflected in the x,y plane with a pair of computer-controlled galvos. A field size of 256 x 256 pixel elements can be addressed in random access speeds up to 5 x 10⁴ pixels/s, or in a raster mode up to 2.5 x 10⁶ pixels/s. The chlorine gas pressure, laser power, and scan rate are adjusted to give optimum surface quality. Waveguide surface roughness values measured with atomic force microscopy are typically on the order of 200 nm RMS. This surface quality is already sufficient to provide low-loss waveguide performance to > 10 THz. The RMS surface roughness can be reduced even further, to under 25 nm using standard polishing etch solutions. When necessary, multiple fields are stitched using a 4 inch travel x,y stage driven with stepper motors. Once the micromaching process is completed, gold is sputtered on the micromachined structure to make it conducting.

Laser bonding/Welding

The process involves using an interlayer of a mixture of two materials and reacting the materials with laser irradiation to form a thermally stable compound suitable for bonding the bodies together.

During LB process, a focused laser beam is transmitted through a transparent glass wafer and absorbed by the surface of an opaque silicon wafer. The absorbed laser energy melts a thin layer of the opaque substrate as well as the transparent material near the interface. This melting results in the formation of a strong chemical bond. In the present study, a LB workstation with the necessary apparatus was developed for experimentally establishing the correlation of the bonding process and its material parameters with the resulting quality of the bond. The parameters of contact pressure, surface roughness, the thickness of the intermediate oxide layer and bonding geometry were examined for their influence on the bond strength. The typical bond strength achieved by LB technique is 10 MPa. This strength, as measured by a tensile test, is comparable to the strength of bonds obtained using other major wafer bonding techniques including fusion bonding and anodic bonding. The bonded interfaces were analyzed using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) to define the mechanism of the wafer bonding through evaluation of the migration and diffusion of different atoms among the glass and silicon substrates during the bonding process. The main advantage of using the LB technique is that the bonding process can be performed at room temperature with a relatively low contact pressure and a reduced bonding time. There is also no need of applying a high electrical potential. TLB can be easily integrated into an existing semiconductor production line without adding a vacuum environment.

Conclusion

Reference

www.wikipedia.org

Mae.fulton.asu.edu/files/shared/graduate/Jong-SuengParkSpr06.doc

Biomems.uta.edu/Research/LaserPoster_files/FemtoLaserPoster.ppt

http://soral.as.arizona.edu/micromachining.html