Physical and Chemical Properties

Metals are inorganic substances that are composed of one or more metallic elements.

Examples of metallic materials with one element are iron, aluminum, copper, and cobalt.

When a metallic material is composed of two or more metallic elements, it is called an alloy. Some metallic materials may contain nonmetallic elements that are added inten-tionally to improve the material's engineering qualities. An example of such a metallic material is steel, in which the nonmetallic element carbon is added to iron. Metals and alloys are commonly divided into two types: ferrous metals and alloys that contain high concentrations of iron and nonferrous metals and alloys that contain no or very low concentrations of iron.

Single-crystal metals are mostly found in the three simple types of cells: BCC, FCC, and HCP. Under different conditions of temperature and pressure, different crystal structures (that is, different unit cells) or phases for the same metal are formed. For example, a bar of iron at room temperature has a BCC structure. However, if the bar is heated above 900 °C, the structure changes to FCC4. The BCC iron and the FCC iron are called the a-phase and y-phase, respectively.

4 The phase change of a material is sometimes used as the sensing or actuating principle of a microdevice. One example is a shape-memory alloy.

50 MEMS MATERIALS AND THEIR PREPARATION

Table 3.4 The atomic properties and crystal structures of selected metals

Atomic Symbol Atomic radius Lattice Interatomic number (Z) (A) structure distance (A)

13

Metals are, in general, good thermal and electrical conductors. They are somewhat strong and ductile at room temperature and maintain good strength both at room and elevated temperatures. Table F.1 in Appendix F gives some important physical properties of metals that are commonly used in microelectronics and MEMS.

Table 3.4 provides atomic and crystal structure information on 12 selected metals, and these illustrate the three principal lattice structures described earlier.

3.2.2 Metallisation

Metallisation is a process in which metal films are formed on the surface of a substrate.

These metallic films are used for interconnections, ohmic contacts, and so on5. Metal films can be formed using various methods, the most important being physical vapour deposition (PVD). PVD is performed under vacuum using either the evaporation or the sputtering technique.

3.2.2.1 Evaporation

Thin metallic films can be evaporated from a hot source onto a substrate, as shown in Figure 3.17. An evaporation system consists of a vacuum chamber, pump, wafer holder, crucible, and a shutter. A sample of the metal to be deposited is placed in an inert crucible, and the chamber is evacuated to a pressure of 10-6 to 10-7 torr. The crucible is then heated using a tungsten filament or an electron beam to flash-evaporate the metal from the crucible and condense it onto the cold sample. The film thickness is determined

5 Copper-based printed circuit board and other interconnect technologies are discussed in Section 4.5.

METALS 51

Vacuum enclosure

Molten evaporated material

Heated crucible

Sample

Shutter

Figure 3.17 Schematic view of a thermal evaporation unit for depositing materials by the length of time that the shutter is opened and can be measured using a quartz microbalance (QMB)—based film thickness monitor. The evaporation rate is a function of the vapour pressure of the metal. Therefore, metals that have a low melting point Tmp

(e.g. 660 °C for aluminum) are easily evaporated, whereas refractory metals require much higher temperatures (e.g. 3422 °C for tungsten) and can cause damage to polymeric or plastic samples. In general, evaporated films are highly disordered and have large residual stresses; thus, only thin layers of the metal can be evaporated. In addition, the deposition process is relatively slow at a few nanometres per second.

3.2.2.2 Sputtering

Sputtering is a physical phenomenon, which involves the acceleration of ions through a potential gradient and the bombardment of a 'target' or cathode. Through momentum transfer, atoms near the surface of the target metal become volatile and are transported as a vapour to a substrate. A film grows at the surface of the substrate through deposition.

Figure 3.18 shows a typical sputtering system that comprises a vacuum chamber, a sputtering target of the desired film, a sample holder, and a high-voltage direct current (DC) or radio frequency (RF) power supply. After evacuating the chamber down to a pressure of 10^–6 to 10^–8 torr, an inert gas such as helium is introduced into the chamber at a few millitorr of pressure. A plasma of the inert gas is then ignited. The energetic ions of the plasma bombard the surface of the target. The energy of the bombarding ions (~keV) is sufficient to make some of the target atoms escape from the surface. Some

52 MEMS MATERIALS AND THEIR PREPARATION

e- Primary electron

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Substrate Anode

Figure 3.18 Basic components in a physical sputtering unit for depositing materials of these atoms land on the sample surface and form a thin film. Sputtered films tend to have better uniformity than evaporated ones, and the high-energy plasma overcomes the temperature limitations of evaporation. Most elements from the periodic table, including both inorganic and organic compounds, can be sputtered. Refractory materials can be sputtered with ease, whereas the evaporation of materials with very high boiling points is problematic. In addition, materials from more than one target can be sputtered at the same time. This process is referred to as cosputtering.

The structure of sputtered films is mainly amorphous, and its stress and mechanical properties are sensitive to specific sputtering conditions. Some atoms of the inert gas can be trapped in the film, causing anomalies in its mechanical and structural characteristics.

Therefore, the exact properties of a thin film vary according to the precise conditions under which it was made. Consequently, values given for the bulk material, such as those given in Appendix F, serve only as an approximate guide to the film values.

3.3 SEMICONDUCTORS