Etching silicon nitrides

Fig. 11. SFM pictures of etched GaAs surface obtained by using (a) the MWCNT probe and (b) standard silicon nitride probe [36],...

Fig. 7 Schematics of a nanometer scale M-A-M diode (not drawn to scale in relative thickness). Top schematic is the cross section of a silicon wafer with a nanometer scale pore etched through a suspended silicon nitride membrane. Middle and bottom schematics show a Au/SAM/Au junction formed in the pore area. (Reprinted with permission from [30])...

The etch rate of CVD silicon nitride in HF is sensitive to the details of the deposition process. Values measured for the etch rate of Si3N4 deposited at 850°C are shown in Fig. 2.8. The best fit to these etch rates is found to be (for r in nm min-1 and cHf in % of aqueous HF) ... 

Fig. 6.6 PS layer thickness inhomogeneities as a result of different kinds of masking layers and doping densities, (a) While underetching is minimal for a silicon nitride mask, (b) a resist mask shows severe under-etching.

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 steam reformer is a serpentine channel with a channel width of 1000 fim and depth of 230 fim (Figure 15). Four reformers were fabricated per single 100 mm silicon wafer polished on both sides. In the procedure employed to fabricate the reactors, plasma enhanced chemical vapor deposition (PECVD) was used to deposit silicon nitride, an etch stop for a silicon wet etch later in the process, on both sides of the wafer. Next, the desired pattern was transferred to the back of the wafer using photolithography, and the silicon nitride was plasma etched. Potassium hydroxide was then used to etch the exposed silicon to the desired depth. Copper, approximately 33 nm thick, which was used as the reforming catalyst, was then deposited by sputter deposition. The reactor inlet was made by etching a 1 mm hole into the end... 

Mass spectrometry is also extremely useful as a process monitor. Less sophisticated residual gas analyzers (RGA) operating on the principles of mass spectrometry are available for these purposes and for end point detection. For the etching of Si 128-130), poly-Si 130), silicon nitride 130), and Si02 (729), SiF (m/e=85) has been shown to be effective for end-point detection. In addition, (m/e=14) is useful for nitride 129,130) in leak tight systems, while O (m/e =16), CO (m/e =44) and Si" " (m/e=29) are useful for oxide (757). Because of the general nature of mass spectrometry as a diagnostic tool, it should be applicable to etching studies of metals and other semiconductor materials. 

Cantilevers are usually microfabricated from silicon by using conventional pho-tomasking and etching techniques. Typical dimensions of a cantilever are 100 pm in length, 20 pm in width, and 1 pm in thickness. Silicon and silicon nitride cantilevers and cantilever arrays that utilize optical beam deflection for signal transduction are commercially available. Piezoresistive cantilever arrays are also commercially available. Piezoresistive cantilevers are 120 pm in length, 1 pm in thickness, and 40 pm in width. 

The etching rate of PECVD silicon nitride is comparable to PECVD TEOS oxide. Ti, TiN, and W present an acceptable etching rate whatever the pH. In the presence of copper, only HF-based chemistries can be used. For Al/Cu none of the tested mixtures are suitable. 

Both silicon oxide and alumina slurries can be efficiently removed on PECVD TEOS oxide or silicon nitride substrates in a conventional SCI or in a SCI without any water peroxide in the case of outcropping tungsten (see Fig. 5). When water peroxide is not present to continuously regrow a protective oxide layer, OH species can etch the silicon. In the latter case, the backside of the wafer must therefore be protected with a nitride or oxide layer to avoid a severe silicon roughening effect. Nevertheless to achieve the same particle removal efficiency obtained with a scrubber, power mega-sonics also have to be used (see Fig. 18). 

Silicon Dioxide and Silicon Nitride. Silicon dioxide can also be etched by F atoms in a downstream discharge configuration. However, because of the strength of the Si-O bond, etch rates (equation 29) are low without particle bombardment (95). 

Some reviews [5-7] have appeared on NCE-electrospray ionization-mass spectrometry (NCE-ESI-MS) discussing various factors responsible for detection. Recently, Zamfir [8] reviewed sheathless interfacing in NCE-ESI-MS in which the authors discussed several issues related to sheathless interfaces. Feustel et al. [9] attempted to couple mass spectrometry with microfluidic devices in 1994. Other developments in mass spectroscopy have been made by different workers. McGruer and Karger [10] successfully interfaced a microchip with an electrospray mass spectrometer and achieved detection limits lower than 6x 10-8 mole for myoglobin. Ramsey and Ramsey [11] developed electrospray from small channels etched on glass planar substrates and tested its successful application in an ion trap mass spectrometer for tetrabutylammonium iodide as model compound. Desai et al. [12] reported an electrospray microdevice with an integrated particle filter on silicon nitride. 

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,... 

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