Etching silicon, characteristics

HNA (HF, HN03, CH3COOH, and water [8]) is a complex etch system with highly variable etch rates and etch characteristics dependent on silicon dopant concentrations, the mix ratio of the three acids, the presence or absence of water, and even the degree of etchant agitation. The latter is typical of a diffusion-limited chemical reaction. For the same reason, F1NA etches silicon isotropically. 

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. 

There are other, nonhydrogel, new materials for chromatographic and electrophoretic separations [7,8,103,164,199,214,377,407], Eor example, Volkmuth and Austin [407] proposed electrophoretic studies in microlithographic arrays of posts and channels etched into sihcon wafers. This material may be useful for studying fundamental transport characteristics of macromolecules in defined media, and many recent studies have been conducted to develop chromatography and electrophoresis on silicon wafers with micron-scale channels... 

Flockhart, S. M., Dhariwal, R. S., Experimental and numerical investigation into the flow characteristics of channels etched in (100) silicon, J. Fluids Eng. 120 (1998) 291-295. 

Etch rate and homogeneity and anisotropic characteristics are the predominant factors in determining the resulting micro system device properties. Temperature and concentration of the KOH solution as well as the doping concentration of the silicon material have the largest impact on these properties and have to be thoroughly controlled. 

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 electrochemical etch-stop technology that produces the silicon island is rather complex, so that an etch stop directly on the dielectric layer would simplify the sensor fabrication (Sect. 4.1.2). The second device as presented in Fig. 4.6 was derived from the circular microhotplate design and features the same layout parameters of heaters and electrodes. It does, however, not feature any sihcon island. Due to the missing heat spreader, significant temperature gradients across the heated area are to be expected. Therefore, an array of temperature sensors was integrated on the hotplate to assess the temperature distribution. The temperature sensors (nominal resistance of 1 kfl) were placed in characteristic locations on the microhotplate, which were numbered Ti to T4. 

Guijt et al. [69] reported four-electrode capacitively coupled conductivity detection in NCE. The glass microchip consisted of a 6 cm etched channel (20 x 70 pm cross-section) with silicon nitride covered walls. Laugere et al. [70] described chip-based, contactless four-electrode conductivity detection in NCE. A 6 cm long, 70 pm wide, and 20 pm deep channel was etched on a glass substrate. Experimental results confirmed the improved characteristics of the four-electrode configuration over the classical two-electrode detection set up. Jiang et al. [71] reported a mini-electrochemical detector in NCE,... 

FIGURE 3.6. (a) Cross-sectional schematics of a silicon wafer with a nanopore etched through a suspended silicon nitride membrance. SAM is formed between sandwiched Au eletrodes in the pore area (circled), (b) I(V) characteristics of a Au-2 -amino-4-ethynylphenyl-4-ethynylphenyl-5 -nitro-1 -benzenethiolate-Au (chemical structure shown below) molecular junction device at 60 K. The peak current density is 50 A/cm2, the NDR is 2400 pQ. cm2, the peak-to-valley ratio is 1030 1. [Adapted from Ref.30 Chen el al., Science 286, 1550-1552 (1999).]... 

FIGURE 8.6 The cross section of a weir-type filter (not to scale). The channels in the silicon substrate are anisotropically etched using EPW. This gives the characteristic V-shaped grooves. This profile is critical to preventing surface tension lock. The barrier or weir is etched in a different step from the channels and can be anywhere from 0.1 pm to a few micrometers from the lid. The lid is Pyrex glass and is attached to the substrate by anodic bonding [836]. Reprinted with permission from Elsevier Science. 

A pn-junction layer 1 of HgCdTe, which has a passivation layer 12, is bonded to a silicon multiplexer chip 2 by a bonding layer 4. The lower layer 9 of the heterojunction I is p-type and the upper layer 10 is n-type. Holes 8 are ion-etched right through the semiconductor layer to contact pads 6. The regions of the layer 9 forming the walls of the holes are converted to n-type by the ion beam. A homojunction is thus formed in parallel with the heterojunction. The compositions are selected so that the heterojunctions determine the infrared response and noise characteristics and the homojunctions act as noiseless isolation devices. 

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