Chapter 2 OVERVIEW OF MEMS

Transcription

1 6 Chapter 2 OVERVIEW OF MEMS 2.1 MEMS and Microsystems The term MEMS is an abbreviation of microelectromechanical system. MEMS contains components ofsizes in 1 micrometer to 1 millimeter. The core element in MEMS generally consists oftwo principal components: a sensing or land actuating element and a signal transduction unit. Figure 2.1 illustrates the functional relationship between sensing element and the transduction unit in a microsensor. Input signal Transduction unit Output signal Figure 2.1 MEMS as a microsensor Microsensors have the advantages of being sensitive and accurate with minimal amount of required sample substance. They can also be mass produced in batches with large volumes. Figure 2.2 illustrates the functional relationship between the actuating element and the transduction unit in a microactuator. The transduction unit converts the input power supply into the form such as voltage for a transducer, which functions as the actuating element.

2 Output action Microactuating element Transduction unit Figure 2.2 MEMS as a microactuator A microsystem is an engineering system that contains MEMS components that are designed to perform specific engineering functions. Major components of a microsystem are microsensors, microactuators, and signal transduction and processing unit. Functional relationship of these three components is illustrated in figure 2.3. Power supply Sensor I Microsystem I Figure 2.3 Components ofa Microsystem 7

3 2.2 Materials for MEMS A well-rounded understanding of MEMS requires a mature knowledge of the materials used to construct the devices, as the material properties of each component can influence the device performance [9,14]. As the fabrication ofmems structures often depends on the use ofstructural, sacrificial and masking materials on a common substrate, issues related to etch selectivity, adhesion, microstructure and a host of other properties are important design considerations. The important materials used for silicon micromachining are presented below Single- Crystal Silicon The conjoining of Si IC processing with Si micromachining techniques during the 1980s marked the advent of MEMS, and positioned Si as the primary material for MEMS. There is little question that Si is the most widely known semicoducting material inuse today. Single-crystal Si has a diamond (cubic) crystal structure. It has an electronic band gap of 1.1 ev, and like many semiconducting materials, it can be doped with impurities to alter its conductivity. Phosphorus (P) is a common dopant for n-type Si and boron (B) is commonly used to produce p-type Si. A solid-phase silicon oxide (SiOz) that is chemically stable under most conditions can readily be grown on Si surfaces. Mechanically, Si is a brittle material with a Young's modulus of about 190Gpa, a value that is comparable to steel (210 Gpa). Si is among the most abundant elements on the Earth that can readily be refined from sand to produce electronic-grade material. Mature industrial processes exist for the low-cost production of single-crystal Si wafered substrates that have large surface areas (>8 in. diameter) and very low defect densities. Single-crystal Si is perhaps the most versatile material for bulk micromachining, owing to the availability of well-characterized anisotropic etches and etch-mask materials. For surface micromachining applications, single-crystal Si substrates are used as mechanical platforms on which device structures are fabricated, whether they are made from Si or other materials. In the case of Si-based integrated MEMS devices, single-crystal Si is the primary electronic material from which the IC devices are fabricated. 8

4 2.2.2 Polysilicon The most common material system for the fabrication of surface micromachined MEMS devices utilizes polycrystalline Si (polysilicon) as the primary structural material, Si0 2 as the sacrificial material and ShN 4 for electrical isolation of device structures. Heavy reliance on this material system stems in part from the fact that these three materials find uses in the fabrication of ICs and, as a result, film can be doped during or after film deposition and etching technologies are readily and widely available. Like single-crystal Si, polysilicon can be doped using standard IC processing techniques. Si0 2 can be grown or deposited over a broad temperature range (200 to I 150 C) to meet various process and material requirements. For surface micromachined structures, polysilicon is an attractive material because it has the mechanical properties that are comparable to single-crystal Si. The required processing technology has been developed for IC applications, and it is resistant to Si0 2 etchants. During the fabrication ofmicromechanical devices, polysilicon films typically undergo one or more high-temperature processing steps (e.g., doping, thermal oxidation, annealing) after deposition. Smooth surfaces are desired for many mechanical structures, as defects associated with surface roughness can act as initiating points of structural failure. Like single-crystal Si, oxidation of polysilicon can be modeled by using process simulation software. Minimizing the maximum required temperature and duration of hightemperature processing steps is important for the fabrication of micromechanical components on wafers that contain temperature-sensitive layers. In polysilicon micromechanical structures, the residual stress in the films can greatly affect the performance of the device. Annealing can be used to reduce the compressive stress in as-deposited polysilicon films Silicon Dioxide Si0 2 can be grown thermally on Si substrates as well as deposited using a variety of processes to satisfy a wide range of requirements. In polysilicon surface micromachining, Si0 2 is used as a sacrificial material, as it can be easily dissolved using etchants that do not attack polysilicon. In a less prominent role, Si0 2 is used as an etch mask for dry etching of thick polysilicon films, as it is chemically resistant to 9

5 dry polysilicon etch chemistries. Thermal Si0 2, Low Temperature Oxide (LTD) and Phospho Silicate Glass (PSG) are electrical insulators suitable for many MEMS applications Silicon Nitride ShN4 is widely used in MEMS for electrical isolation, surface passivation, etch masking and as a mechanical material. Two deposition methods are commonly used to deposit ShN4 thin films: Low Pressure Chemical Vapour Deposition (LPCVD) and Plasma Enhanced CVD (PECVD). LPCVD ShN4 is commonly used as an insulating layer to isolate the device structures from the substrate and from the other device structures, because it is a good insulator with a resistivity of Q-cm and a field breakdown limit of 10 7 V/cm. Low-stress silicon nitride has been successfully used as a structural material in a surface micromachining process that use polysilicon as the sacrificial material Metals Metals are used in many different capacities, ranging from hard etch masks and thin film conducting interconnects to structural elements in microsensors and microactuators. Metallic thin films can be deposited using a wide range of deposition techniques, the most common being evaporation, sputtering, CVD and electroplating. Such a wide range of deposition methods makes metal thin films one of the most versatile classes ofmaterials used in MEMS devices. 2.3 MEMS Fabrication processes Many of the fabrication processes used in producing integrated circuits have been adopted to create the complex three-dimensional shapes of many MEMS / Microsystems Photolithography Photolithography process involves the use of an optical image and a photosensitive film to produce a pattern on a substrate. Photolithography is one of the most important steps in microfabrication. In Microsystems, however, photolithography is used to set patterns for masks for cavity etching in bulk micromachining, or for thin film deposition and etching of sacrificial layers in surface 10

6 micromachining, as well as for the primary circuitry of electrical signal transduction in sensors and actuators. A photoresist is first coated onto the flat surface of the substrate. The substrate with photoresist is then exposed to a set oflights through a transparent mask with the desired patterns. Masks used for this purpose are often made of quartz. Photoresist materials change their solubility when they are exposed to light. Photoresists that become more soluble under light are classified as positive photoresists, whereas the negative photoresists become more soluble under the shadow Ion implantation Ion implantation involves "forcing" free atoms, such as boron or phosphorus, with charged particles into a substrate, thereby achieving imbalance between the numberofprotons and electrons in the resulting atomic structure Diffusion The diffusion process is often used in microelectronics for the introduction of a controlled amount offoreign materials (dopants) into the selected regions ofanother material (the substrate). Unlike ion implantation, diffusion is a slow doping process. Diffusion takes place at elevated temperatures Oxidation Oxidation IS a very important process ill both microelectronic and microsystem fabrication. Materials for dielectric films involve ceramics those grown over the substrate's surface such as silicon dioxide and silicon nitride Chemical Vapor Deposition (CVD) Depositing thin films over the surface of substrates and other MEMS and microsystem components is a common and necessary practice in micromachining. Unlike the diffusion and thermal oxidation processes, deposition adds thin films to, instead ofconsuming, the substrates. In general, Low Pressure Chemical Vapor Depositions (LPCVD) provides a means for depositing thick (>2 ).lm) Si0 2 films at temperatures much lower than thermal oxidation. An advantage of the LPCVD process is that dopant gases can be included in the flow ofsource gases in order to dope the as-deposited Si0 2 films. 11

7 2.3.6 Physical Vapor Deposition (PVD) - Sputtering Sputtering is a process that is often used to deposit thin metallic films of o 0 the order of 100 A thick (1 A = m) on substrate surfaces. The sputtering process is carried out with plasma under very low pressure (i.e., in high vacuum at around 5xlO- 7 torr). This process involves low temperature. Plasma is made ofpositively charged gas ion, and plasma can be produced by either high-voltage dc sources or RF (Radio-Frequency) sources. The positive ions of the metal in an inert argon gas carrier bombard the surface ofthe target at such a high velocity that the momentum transfer on impingement causes the metal ions to evaporate. condensation. The metal ion is then led to the substrate surface and is deposited after Deposition By Epitaxy Epitaxy is the extension of a single-crystal substrate by growing a film of the same single-crystal material Etching Etching is one of the most important processes in microfabrication. involves the removal of materials in desired areas by physical or chemical means. In micromachining, etching is used to shape the geometry of microcomponents in MEMS and microsystems. Of the two common types of etching techniques mentioned above, the physical etching is usually referred to as dry etching or plasma etching, whereas the chemical etching is referred to as wet etching. The chemical solutions used in etching, or etchants, attack the parts of the substrate that are not protected by the mask. The mask used in micromachining may be either the photoresists or Si0 2 for substrates in HF solutions. A large number of dry etch processes are available to pattern single-crystal Si. Reactive ion etching is the most commonly used dry etch process to pattern Si. The RIE process is highly directional, thereby enabling direct pattern transfer from the masking material to the etched Si surface. For MEMS applications, photoresist and Si0 2 thin films are often used as masking materials. Recent development of deep reactive ion etching processes has extended Si etch depths well beyond several It 12

8 hundred microns, thereby enabling a multitude of new designs for bulk micromachined structures. 2.4 Micromachining techniques The three-dimensional microstructures can be produced by removing part of the base material by physical or chemical etching process, whereas thin-film deposition techniques are used to build layers of materials on the base materials. The boom ofthe microelectromechanical systems industry in recent years would not have been possible without the maturity of microelectronics technology. Despite the fact that micromanufacturing evolved from IC fabrication technologies, there are several other science and engineering disciplines involved in today's microsytems design and manufacturing. It is fair to say that microsystems are a major step towards the ultimate miniaturization ofmachines and devices. Generally speaking, there are two distinct micromachining techniques used by current industry. These are (l) bulk micromachining, (2) surface micromachining. Aforementioned two principal micromanufacturing techniques are discussed below Bulk micromachining Bulk micromachining is widely used in the production of microsensors and accelerometers. Bulk micromachining or micromanufacturing involves the removal of materials from the bulk substrates, usually silicon wafers, to form the desired threedimensional geometry of the microstructures. The technique is thus similar to that used by sculptors in shaping sculptures. Shaping of microsystem components of the size between 1 /lm and 1 mm made of tough materials such as silicon is beyond any existing mechanical means. Bulk micromachining of Si uses wet and dry etching techniques in conjunction with etch masks and etch stops to sculpt micromechanical devices from the Si substrate. From the materials perspective, two key capabilities make bulk micromachining a viable technology: (l) the availability of anisotropic etchants such as ethylene-diamine pyrocatecol (EDP) and potassium hydroxide (KOH), which preferentially etch single-crystal Si along select crystal plane; and (2) the availability of so-compatible etch-mask and etch-stop materials that can be used in conjunction with the etch chemistries to protect select regions of the substrate from removal. Substrates that can be treated this way involve silicon, SiC, GaAs and quartz. Etching, either the orientation-independent isotropic etching or the 13

9 orientation-dependent anisotropic etching is thus the key technology used in bulk micromanufacturing. Wet Etchants Popular anisotropic etchants for silicon include potassium hydroxide (KOH), ethylene-diamine and pyrocatecol (EDP), tetramethyl ammonium hydroxide (TMAH), and hydrazine. Typical ranges for etching rates for common substrate materials with these etchants are given in Table 2.1. Table 2.1 Typical etching rates for silicon and silicon compounds Material Etchant Etch Rate Silicon in <100> KOH Joun/min Silicon in <100> EDP 0.75 j.lm/min Silicon dioxide KOH nm/h Silicon dioxide EDP 12 nm/h Silicon nitride Silicon nitride KOH EDP 5nm/h 6nm/h Consequently, the high selectivity ratio of silicon dioxide and silicon nitride makes these materials suitable candidates for the masks for etching silicon substrates SURFACE MICROMACHINING In contrast to bulk rnicromanufacturing in which substrate material is removed by physical or chemical means, the surface micromachining technique builds microstructure by adding materials layer by layer on top ofthe substrate. Deposition techniques, in particular the low pressure chemical vapor deposition (LPCVD) 14

10 technique are used for such buildups, and polycrystalline silicon (polysilicon) is a common material for the layer material. Sacrificial layers, usually made of Si0 2, are used in constructing the MEMS components but are later removed to create necessary void space in the depth. Wet etching is the common method used for that purpose. Layers that are being added in surface micromachining are typically 2 to 5 11m thick each. In special applications, this range can be extended to 5 to 20 11m. Figure 2.4 illustrates the difference between bulk micromanufacturing and surface micromachining. In Figure 2.4a, we see a microcantilever beam. The cantilever beam is made of single-crystal silicon with a significant amount of material etched away as illustrated in Figure 2.5. The same cantilever beam structure can be produced by polysilicon with a surface micromachining technique as illustrated in Figure 2.6. Silicon cantilever beam Die attac~ /,..._-...:_ , Constraint base (a) By bulk micromachining I Polysilicon cantilever beam Constraint base (b) By surface micromachining Figure 2.4 Microcantilever beams produced by two micromachining techniques TStandard silicon wafer thickness Figure 2.5 Waste ofmaterial in bulk micromachining 15

11 1 Silicon constraint base r PSG sacrificial layer ~- 2 Mask 1 for etching 3 4 P/ L _ I Mask 2 for deposition L / 5 r:-:zzzlzzzlzlzzlzzlzzzlzlzzj /// / / / / / / / / / /I '? 6 Silicon constraint base After etching of sacrificial layer Figure 2.6 Surface micromachining process 16