Micromachined silicon substrates

J. Cerdk, A. Cirera, A. Vilk, A. Cornet, and J.R. Morante. Deposition on micromachined silicon substrates of gas sensitive layers obtained by a wet chemical route a CO/CH4 high performance sensor , Thin Solid Films 391 (2001), 265-269. 

Micro-hotplates are made using a combination of thin-fihn and silicon micromachining processes. There are two main kinds of micromachined silicon substrates closed-membrane and bridge-membrane. They consist of a suspended thin dielectric membrane, made of silicon nitride and/or silicon oxide, that is released using silicon micromachining on either the obverse or... 

Figure 23. Processing flow for 3-D electrode array fabrication using silicon micromachining with colloidal filling of the electrode material. The six steps are identified as the following (i) patterned photoresist (PR) on silicon substrate, (ii) PR removal after DRIB micromachining, (iii) insulate silicon mold by oxidation, (iv) colloidal electrode filling material centrifuged into the mold, (v) silver epoxy added to provide mechanical stability and electrical contact, (vi) the electrode flipped over and released from the mold by immersion in a TEAOH solution.

A micromachined CE device featuring a truly monolithically integrated detector has been recently reported by Webster et al. [79]. A semiconductor radiation detector was fabricated together with a separation channel on a silicon substrate in a 10-mask process. The preliminary results achieved with the detection of beta decay events of 32P-labeled DNA at 27 V/cm demonstrate the feasibility of the concept. 

If silicon technology is involved all thermal sensors suffer from the high thermal conductivity of silicon, which dramatically decrease their sensitivity [12]. However, by use of micromachining and integrated silicon technology a powerful thermal biosensor can be realized. Using a thermopile integrated on a thin micromachined silicon membrane reduces thermal loss due to the substrate and so excellent performance can be accomplished [13]. 

Lee and co-workers demonstarted that a micromachined parylene nozzle could serve as an electrospray emitter on a silicon chip. This hollow-needle structure extended more than a millimeter beyond the edge of the silicon substrate. MS/MS analysis of the mixture of peptides obtained from the trypsin digest of cytochrome c was demonstrated with this micromachined emitter, and... 

The term polysilicon arises from the structure of these silicon layers, which are essentially polycrystalline in contrast to single-crystalline silicon substrates, due to the growth of these films on amorphous starting layers (in silicon surface micromachining, a silicon oxide layer is usually used as both a seed layer and a sacrificial layer). Polysilicon is widely used in sensor technology it can be used as part of a membrane layer, as an electrical connector, or as a part of a thermopile structure. In this contribution, we focus on its most important function - as the functional layer in surface-micromachined structures. In surface micromachining basically two approaches for producing polysilicon films are used ... 

Polymer microfiuidic systems for chemical analysis are normally fabricated by transferring a patterned microstructure from a template to a substrate. Micromachined silicon templates are often... 

A diagram of a typical cross-sectional view of a silicon micromachined metal-oxide (MOX) sensor is presented in Fig. 6.2. Their development has evolved towards silicon substrates to produce devices suitable for commercialization due to their low-cost, low-power consumption and high reliability. To lower the resistivity of the gas sensitive film, as well as to improve the kinetics of the chemical reactions, the metal-oxide layer is heated with a micro-heater. The heated area is usually embedded in a thin dielectric membrane to improve the thermal insulation and to reduce the power consumption of the device, which is typically in the order of a few tens of milliwatts at 300°C, and its thermal time constant (few to tens of milliseconds). Thermal programming allows kinetically controlled selectivity. 

By adding small amount of O2 gas, the etch rate can be accelerated. In general, in this approach, the etch rate is relatively small and in the range of few nanometers to tens nanometers per minute. The RIE process is not applicable for bulk material micromachining, and therefore, it is suitable only to etch thin films with thickness of few micrometers. However, RIE is a suitable process to etch shallow microchannels in silicon substrate with good etch-depth control. 

Silicon Micromachining, Fig. 6 Fabrication steps of a polysilicon cantilever using surface micromachining. (a) Deposition of Si02 on silicon substrate, followed by... 

Microcharmels and holes can be micromachined using wet chemical etching. However, as geometries of micro-device components get smaller, the requirement to etch silicon microchannels with vertical profile becomes important. An increase in aspect ratio for microstructures (e.g., microcharmel) is desirable because more devices can be made from the same size of silicon substrate and also lead to enhanced device characteristics. 

Other substrates that are used for MEMS include quartz, glass, and polymers. Quartz is attractive primarily because it is piezoelectric, so that it may be used as a sensor and an actuator. Quartz is, however, more difficult to micromachine than silicon. Pyrex glass is used in conjunction with silicon wafers, primarily for packaging, because of its optical transparency and close match in coefficient of thermal expansion (GTE). Polymers such as polycarbonate have been adopted as substrates for microfluidics because of their low cost channels require large areas, which makes silicon too expensive, and channels can be fabricated in polymers inexpensively by hot embossing. The advantages offered by silicon substrates are not required because microfluidic devices typically do not have complex... 

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