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Most of the early silicon products were fabricated using bulk micromachining where chemical etchants were used to attack different planes of a silicon crystal at different rates This approach is based on etching down into a surface, stopping on certain crystal faces, doped regions and etchable films to form the desired structure [16] This method is normally used in the production of microsensors and accelerometers The etching process, either the orientation-independent isotropic etching or the orientation-dependent anisotropic etching is well suited for substrate materials such as silicon, SiC, GaAs and quartz Wet isotropic etchants, plasma etchants and thermal oxidation methods are used to provide geometries that are independent of crystallographic orientations [9] However, isotropic etching or orientation-independent etching is less desirable because of the lack of control over the finished geometry of the workpiece [6] As most substrate materials are not isotropic in their crystalline structure, the etching process is usually orientation dependent The <110> orientation is often favoured because the wafer cleaves more cleanly Thermal oxidation is very common in the fabrication of microelectronics and microsystems There are four types of thin films that are frequently used in microelectronics and microsystems, namely, thermal oxidation for electrical or thermal insulation media, dielectric layers for electrical insulation, polycrystalline silicon for local electrical conduction and metal films for electrical (ohmic) contact and junctions [18] Silicon dioxide is used as an electric insulator as well as for etching masks for silicon and sacrificial layers in surface micromachining The silicon oxide layer is produced in an electric resistance furnace Etching is a process of the selective removal of materials by chemical means Wet etching which involves immersing the wafers in an acidic liquid solution is easy and inexpensive to perform However, it often produces poor quality surfaces due to the presence of bubbles and the flow patterns of the solution On the other hand, dry etching involves the removal of the substrate by gaseous etchants without wet chemicals Some dry etching techniques include plasma, ion milling and reactive ion etch Reactive plasma etching involves chlorine or fluorine ions that diffuse and chemically react with the substrate, forming a volatile compound that is removed by a vacuum system Figure 832 shows the illustration of reactive plasma etching and the product of deep reactive ion etching Deep reactive ion etching involves the use of a high-density plasma source which is capable of producing virtually vertical walls [6] Photolithography involves the use of an optical image and a photosensitive film to produce a pattern on a substrate It is used to set patterns for masks for cavity etching in bulk micromanufacturing or for thin-film deposition and etching of sacrificial layers in surface micromachining as well as for the primary circuitry of electrical signal transduction in sensors and actuators [6] The process of photolithography is clearly illustrated in Figure 833 The substrate is first coated with a layer of a photoresist It is then exposed to a set of lights through a transparent quartz mask.

Although there are many types of base materials, they all contain three components:

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Each of these components is important in its own right, and in combination they determine the properties of the base material as well as the relative cost of the material. Environmental legislation such as the European Union s Restriction of Hazardous Substances (RoHS) directive has a profound impact on all levels of the electronics supply chain, including these components. RoHS restricts the use of lead, which is an element in the solder used for component assembly onto printed circuits. The impact on the base materials and components is primarily the result of higher assembly temperatures that are associated with lead-free assembly. Table 7.1 summarizes the key issues for base material components. RoHS issues will be discussed further in Chap. 10. The RoHS directive also restricts specific halogen-containing flame retardants. However, most base materials for printed circuits do not contain the restricted flame retardants. Nevertheless, there is growing interest in halogen-free materials, and this will be discussed further in this chapter.

The most successful and widely used resin systems for printed circuit applications are epoxy resin systems. There are many types of epoxy resins, and this class of resin system continues to be the workhorse among printed circuit board materials. This is a result of the combination of good mechanical, electrical, and physical properties and the relatively low cost of epoxies in comparison to the higher-performance resins. In addition, epoxy systems are relatively easy to process, which aids in keeping manufacturing costs down.

I/O Data bit line 0 (LSB) I/O Data bit line 1 I/O Data bit line 2 I/O Data bit line 3 I/O Data bit line 4

TABLE 7.1 Lead-Free Assembly Impact on Base Material Components Component Resin System Lead-Free Assembly Impact 1. Peak assembly temperatures can reach the point where resin decomposition begins. 2. Higher temperatures result in increased thermal expansion and stress on plated holes as a result. 3. Vapor pressure of absorbed moisture much higher at lead-free assembly temperatures; can lead to blistering/delamination. 4. Phenolic lead-free compatible materials often not as good for electrical performance, especially Df. Potential Solutions 1. Formulate resin system with higher decomposition temperatures. 2. Formulate for lower coefficients of thermal expansion. 3. Evaluate materials for moisture absorption/ release characteristics; drying processes in PCB fabrication and/or assembly. 4. Evaluate non-dicy/ non-phenolic laminate materials. Related Considerations 1. Reformulation can adversely affect electrical properties and manufacturability. 2. Can also impact mechanical properties and manufacturability. 3. PCB storage conditions during manufacturing and prior to assembly are much more important, especially humidity conditions. 4. Cost/performance tradeoff. New materials now available that offer more choices. Loss of resin-to-glass adhesion through thermal cycling could impact CAF resistance. Roughness will impact signal attenuation, especially at high frequenciesi.

Microelectro-mechanical Systems (MEMS)

FIGURE 50.2 Stainless steel bolster plate (l) and insulator (r). (Courtesy of Hewlett-Packard.)

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