Temperature surface micromachining

Deposition in a conventional LPCVD reactor by thermal decomposition of silane (SiH4) at temperatures between 550°C and 630°C. The growth rate at this temperature is around 5 nm min-1. The typical thickness of such thin films is about 2 pm, as used, for example, by Analog Devices for their surface-micromachined... 

The stability of the properties of polysilicon films to processing is not ideal. The structure and intrinsic stress of LPCVD-poly films created by different organizations but with the same deposition recipe were different [21]. The properties of LPCVD-poly can differ even on wafers from the same deposition run, due to small temperature fluctuations in the deposition furnace [25]. However, since several LPCVD-poly surface-micromachined sensors are in fact produced in high volumes for the automotive industry (e.g., by Analog Devices, Infineon, and Motorola), these literature data may not represent the current state of the art. 

The CVD method is very versatile and can work at low or atmospheric pressure and at relatively low temperatures. Amorphous, polycrystalline, epitaxial, and uniaxially oriented polycrystalline layers can be deposited with a high degree of purity, control, and economy. CVD is used extensively in the semiconductor industry and has played an important role in past transistor miniaturization by making it possible to deposit very thin films of silicon. CVD also constitutes the principal building technique in surface micromachining (see below). 

Table 3.9 Critical temperatures of often-used CVD-deposited surface micromachining materials. 

Surface micromachined electromechanical devices on silicon substrates represent a popular fabrication route to micro-electromechanical systems (MEMS), due to the in tructure and process knowledge available from the semiconductor industry. Although the number of materials compatible with silicon surface micromachining is large, material selection may be constrained by process temperatures or chemical compatibility, particularly in cases where the micromachine is to be integrated vnXh CMOS logic. Surface treatments can be used to improve interfacial properties where the composition of the structural elements is constrained. 

A bolometer is a thermally isolated, temperature-sensitive element that can be used to measure absorbed energy. Either surface micromachining or bulk silicon micromachining can be used to fabricate a structure with low thermal mass that is thermally isolated from the substrate by long, thin suspension arms. Here we consider a bolometer that is fabricated in the Poly2 layer of the PolyMUMPS process, as shown in Figure 5.14. If the device consists of a 100 (pm) platform suspended by four suspension arms that are 5 pm wide, we can calculate the thermal conductance of the arms, the thermal capacitance of the platform, and the thermal time constant for the sensor. 

The process uses the implantation of inert gases in the mirror surface to create a thin layer of compressively stressed material that acts to flatten the mirror segment. This approach is similar to the stress reduction that occurs due to phosphorous dopant diffusion from the PSG layers during the PolyMUMPS high-temperature anneals, as described in Section 1.2.1 on surface micromachining. In contrast to the anneal step, where dopants diffuse in from both sides, the ion bombardment process only occurs on the top side of the mirror surface. 

In the positive branch of the i/V graph, anodic dissolution process will remove material from silicon crystals. The conditions for optimal etching of silicon have been extensively explored for micromachining or surface polishing in the fabrication of electronic devices. Most generally, the etch rate of silicon in HE solutions is isotropic among the various crystalKne orientations. The etch rate of silicon at room temperature at the open-circuit potential (OCP) is very low, on the order of 10 nm s , which is equivalent to 100 nA cm , in aqueous HE solutions. 

Zinc oxide (ZnO, wurtzite structure) eliminates oxygen on heating to form nonstoichio-metric colored phases, Zni+xO with x < 70 ppm. ZnO is almost transparent and is used as white pigment, polymer stabilizer, emollient in zinc ointments, creams and lotions, as well as in the production of Zu2Si04 for TV screens. A major application is in the rubber industry to lower the temperatures and to raise the rate of vulcanization. Furthermore, it is an n-type semiconductor (band gap 3.37 eV) and shows piezoelectric properties, making zinc oxide useful for microsensor devices and micromachined actuators. Other applications include gas sensors , solar cell windows and surface acoustic devices. ZnO has also been considered for spintronic application because of theoretical predictions of room-temperature ferromagnetism . 

Microstructured surfaces, as well as micromachined substrates and devices discussed in Sects. II, III, and iy are suitable for a number of applications. They include reflective and absorbing surfaces, wavelength-sensitive filters, multiaperture lens arrays and Fresnel microoptics, field emitter arrays, precision apertures, or molds for microstructured surfaces of other materials. Microstructured alumina ceramics can also be used for tuned broadband infrared emitters. In addition, due to the robustness at high temperatures and well-developed and controlled porosity, the freestanding, heat-treated micromachined anodic alumina substrates can be used for the fabrication of sensors that incorporate a high temperature microheater with low power consumption. 

The mechanical properties of thin films used as membranes or moving structures and produced either by bulk or surface silicon micromachining are of crucial importance. In either type of application, the mechanical stress and the fracture properties of the applied thin films determine the behavior and long-term reliability of the device. In automotive applications, the environmental constraints (e.g., high ambient temperatures and temperature variations) and the required long life expectancy of the sensors require the use of high-quality thin films with well-controlled properties. 

Determination of the temperature distribution induced by laser, electron, or plasma beam sources is relevant in operations such as surface transformation hardening of metals, drilling, cutting, annealing, shaping, and micromachining. Descriptions of beam-generating devices as well as discussions of applications are available [7-15]. 

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