Insulating layers silicon nitride

The design of the Pd-membrane reactor was based on the chip design of reactor [R 10]. The membrane is a composite of three layers, silicon nitride, silicon oxide and palladium. The first two layers are perforated and function as structural support for the latter. They serve also for electrical insulation of the Pd film from the integrated temperature-sensing and heater element. The latter is needed to set the temperature as one parameter that determines the hydrogen flow. 

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. 

The main goal of another microhotplate design was the replacement of all CMOS-metal elements within the heated area by materials featuring a better temperature stability. This was accomplished by introducing a novel polysilicon heater layout and a Pt temperature sensor (Sect. 4.3). The Pt-elements had to be passivated for protection and electrical insulation, so that a local deposition of a silicon-nitride passivation through a mask was performed. This silicon-nitride layer also can be varied in its thickness and with regard to its stress characteristics (compressive or tensile). This hotplate allowed for reaching operation temperatures up to 500 °C and it showed a thermal resistance of 7.6 °C/mW. 

Figure 15-29 Operation of a chemicalsensing field effect transistor. The transistor is coated with an insulating Si02 layer and a second layer of Si3N4 (silicon nitride), which is impervious to ions and improves electrical stability. The circuit at the lower left adjusts the potential difference between the reference electrode and the source in response to changes in the analyte solution such that a constant drain-source current is maintained.

In addition, flame propagation is not possible in stainless-steel channels owing to the high heat conductivity and affinity to radicals. But even for materials which do not adsorb radicals, a minimum channel width exists below which no homogeneous combustion is possible any longer. Insulating materials such as silicon nitride and inert layers such as alumina are required to maintain the homogeneous reaction. 

Silicon devices covered with insulating layers of thermal oxide and CVD nitride have been shown to suffer from breakdown problems which limit the applicable voltages [34]. For that reason, very few experimental studies on highspeed CE (requiring high field strengths) have appeared in the literature where silicon substrates were used, in contrast to its successful use in other separation techniques as discussed in Sects. 4 and 5. 

Infrared rays which are incident to regions 25 penetrate into the silicon nitride layer and ZnS layer are thereby partly attenuated and reflected, and are finally reflected from the metal layer. The incident rays and the reflected rays cause a complex interference with each other, and an apparent overall reflectance of the light shield layer is reduced if proper thickness and refraction index are chosen for each layer of the insulation layer. Preferable, the refractive index of the first layer 8 is less than the refractive index of the second layer 9. 

Passivation is needed to insulate the backplane from the OLED stacks everywhere except the ITO and bonding contact areas. Unlike poly-Si and a-Si H backplanes, on which both organic and inorganic passivation layers can easily work, the device passivation technique needs extra consideration for pentacene TFTs. We explored several different materials for passivation of pentacene TFTs, including poly(vinyl alcohol) (PVA), room temperature plasma-enhanced chemical vapor deposition silicon nitride (RT PECVD SiN), and vapor-deposited parylene. 

Two major improvements in the fabrication of an ion-sensitive FET that avoid most of the tedious polymer encapsulauon process have been reported. Matsuo and his coworkers (4, 37) fabricated a probe-type FET with a three-dimensional silicon nitride passivation layer around most of its surface, as shown in Fig. 2. The probe-type FET has one disadvantage Its fabrication requires a three-dimensional process that is uncommon for semiconductor construction facilities. An alternative approach utilizes a silicon-on-sapphire (SOS) wafer for FET fabrication (38, 39). The structure of a SOS-FET is depicted in Fig. 3. It has an island-like silicon layer on a sapphire substrate, in which an ion-sensitive FET is fabricated. The bare lateral sides do not need encapsulation because of the high insulation property of sapphire. 

There are other insulating materials that can be used instead of silicon dioxide. Silicon nitride, alumina, and aluminum nitride are a few that are often used. The selection of a proper insulation layer is based on the specific needs and the properties of selected materials. 

The sacrificial oxide layer is initially formed on top of the silicon substrate by thermal or plasma processing (Fig. 5.3.1a). If insulation of the functional layer from the silicon substrate is required, the sacrificial layer may be preceded by deposition of an insulating film, for example, silicon nitride. The thickness of the sacrificial oxide layer determines the height of the released cantilever above the silicon substrate. It is typically a few micrometers thick, depending on the specific requirements and application. 

Silicon oxides (SiOx) are the most widely used thin films in silicon microelectronic and micromechanical devices. Similar to silicon nitride (Section 5.5.4), these amorphous films exhibit dielectric properties. Silicon oxide is often utilized as part of a dielectric membrane, as a passivation or insulating layer, or as a sacrificial layer, which can be etched with hydrofluoric acid (HF)-containing etchants. Two different approaches to forming a silicon oxide thin film are... 

A Pt layer was deposited by means of high vacuum evaporation, patterned via a lift-off process and insulated with PECVD silicon nitride. The working electrodes... 

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