Micromachined silicon membranes

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]. 

Desai, T.A. Hansford, D. Ferrari, M. Characterization of micromachined silicon membranes for immunoisolation and bioseparation applications. 

Drug delivery concepts have been presented that are based on microfabrication. Possible applications include micromachined silicon membranes to create implantable biocapsules for the immimoisola-tion of pancreatic islet cells, as a possible treatment for diabetes and sustained release of injectable drugs needed over long time periods. 

The next step was the introduction of ion implantation to dope Si for thermometers. Downey et al. [66] used micromachining to realize a Si bolometer with an implanted thermometer. This bolometer had very little low-frequency noise. The use of thermometers doped by neutron transmutation instead of melt doping is described by Lange et al. [67], The evolution of bolometers sees the replacement of the nylon wires to make the conductance to the bath, used by Lange et al. with a micromachined silicon nitride membrane with a definite reduction in the heat capacity associated to the conductance G [68],... 

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... 

A micropump or active microvalve is said to use external actuators when the component responsible for opening or closing the valve or moving the fluid is added to the device after fabrication. Examples would be a separate miniature solenoid valve used to pressurize or depressurize air chambers in a pneumatic actuator or piezoelectric patches glued to a micromachined silicon or glass membrane. 

As a detailed example of a micromachined fuel cell, an alternative solution which does not use an ionomer for the proton-exchange membrane has also been reported. It consists in a porous silicon membrane with a proton-conducting silane grafted on the pores walls. With this membrane, performances as high as 58 mW cm have been achieved. This promising membrane can still be improved. Future work should focus on the reduction of the pores diameter to decrease the gas crossover, the control of the grafting density into the porous silicon, the replacement of the electrode carbon cloth by an ink and the use of a more proton-conducting silane (with SOg terminations). 

Torres N, Duch M, Santander J et al (2009) Porous Silicon Membrane for Micro Fuel Cell Application J. New Mater electrochem Syst 12(2-3) 93-96 Torres N, Duch M, Santander J et al (2009) Si micro-turbine by proton beam writing and porous silicon micromachining. Nucl Instr Meth Phys Res Sect B-Beam Interact Mater Atoms 267(12-13) 2292-2295... 

Desai TA, Hansford DJ, Ferrari M (2000a) Micromachined interfaces new approaches in cell immunoisolation and biomolecular separation. Biomol Eng 17(l) 23-36 Desai TA, Hansford DJ, Leoni L, Essenpreis M, Ferrari M (2000b) Nanoporous anti-fouling silicon membranes for biosensor applications. Biosens Bioelectron 15(9-10) 453-462 Desai TA, West T, Cohen M, Boiarski T, Rampersaud A (2004) Nanoporous microsystems for islet cell replacement. Adv Drug Deliv Rev 56(11) 1661-1673 Dunleavy M (1996) Polymeric membranes. A Rev Appl Med Dev Tech 7(4) 18-21... 

Porosified silicon membranes of defined thicknesses were first studied in the 1990s and have now been realized by electrochemical anodization, micromachining techniques, and the annealing of ultrathin deposited films. The three fabrication routes produce very different morphologies and levels of porosity. A variety of applications have been explored for both macroporous and mesoporous membranes and these are also surveyed. Wholly microporous membranes in silicon, where all pores have diameters less than 2 nm, have not been achieved to date. 

There are now three top-down techniques developed to realize porous silicon membranes from solid silicon electrochemical etching (anodization), micromachining, and thin film deposition/annealing. These techniques create different pore morphologies and are suited to different membrane thiek-nesses and porosity ranges. In this regard they are quite complementary. 

Table 1 collates the varied potential applications that have been explored for porous silicon membranes, both mesoporous and macroporous. Membrane thicknesses vary from submicron to tens of microns to the full thicknesses of wafers (hundreds of microns). Most studies have utilized anodization to realize the porosity. Notable exceptions highlighted in Table 1 are the micromachining and deposition/anneal techniques already mentioned. 

The readout are connected to a hydraulic restriction. The key processes of the device are a thermal bonding of a polyimide sheet to an already micromachined silicon wafer and the deposition of transducers onto the polyimide membrane. A schematic cross section is... 

Zhu J, Barrow D. 2005. Analysis of droplet size during crossflow membrane emulsification using stationary and vibrating micromachined silicon nitride membranes. / Membr Sci 261 136-144. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

C. Dilcso, E. Vazsonyi, M. Adam, I. Szabo, I. Bdrsony, J.G.E. Gardeniers, and A. van den Berg. Porous silicon bulk micromachining for thermally isolated membrane formation . Sensors and Actuators A60 (1997), 235-239. 

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