23 MEMS/NEMS Fabrication

Dr. Ayushi Paliwal and Dr. Monika Tomar

Introduction to MEMS/NEMS

Microelectromechanical systems (MEMS) is a term given to the electrical devices that are designed with dimensions more than 1 µm but less than 1 mm. They are a combination of mechanical and electrical components and are fabricated using the integrated circuit batch-processing technologies.

On length scales, the magnitude of MEMS based structures is four times larger than the diameter of hydrogen atom, while four orders of magnitude smaller than the conventional day-to-day devices (as shown in Figure 1). Interestingly, Microdevices can be designed to have characteristic length comparable to the diameter of a human hair. Nanodevices (or more precisely, NEMS) can be considered to be an advancement in the field of electromechanical miniaturization.

Since the design of MEMS devices is based on the electromechanical actuations, it is important to know the mechanical properties of the material before device fabrication. The elastic properties of the material govern the amount of deflection which can be induced in the system as a function of applied force, or vice versa. Thus, the operating limits of the device depend crucially on the strength of the material. It is extremely important to understand the properties of the material and the subsequent processing treatments required during MEMS/NEMS fabrication.

Materials for MEMS Fabrication

MEMS devices are based on electrical and mechanical structures which function in conjunction with one another. Thus, the choice of materials can significantly influence the performance of the device. MEMS fabrication usually involves the use of certain structural, sacrificial and shadowing materials on a common platform (substrate). The other design considerations which affect the fabrication are selectivity towards material etching, adhesion, stress generation, microstructure etc. thus, it is important to understand the material systems that are employed for MEMS fabrication. Following is a list of commonly employed materials for MEMS structures, with a brief overview of their properties.

(a) Single crystal Silicon (Si)

Silicon is the most widely employed semiconducting material for MEMS fabrication. The use of Si can be dated back to as early as 1950s, when the piezo-resistive effect in Si was reported. Not only can Si be chemically and mechanically micromachined, but its cost effectiveness and natural abundance makes it an ideal choice for the researchers working in the field of microfabrication technology.

For MEMS fabrication, single crystal Si is the most versatile material for bulk micromachining, primarily due to the availability of well established anisotropic etches and etch-mask materials. The IC device fabrication benefits from the Si single crystals to a great extent. The bulk micromachining of Si wafer involves dry and wet etching techniques in correlation with etch masks and stops to fabricate electromechanical devices.

During etching of the material, directionality is one of the crucial factors. If the etch rate of the material is uniform in all directions, then it is termed as “isotropic” etching. But in practice, etching is usually direction dependent, or in other words “anisotropic”. The etch rate is faster in the direction perpendicular to the wafer surface compared to the lateral etching. The anisotropic Si etchants attack the (100) and (110) crystal planes appreciably faster than the (111) crystal planes.

Wet chemical etching of Si: The most commonly used etchants for isotropic etching of Si wafer, are mixtures of Hydrofluoric acid (HF) and nitric (HNO3) acid in water or acetic acid (CH3COOH), commonly known as the HNA etching system. MEMS structures which require long etching times in KOH, Si3N4 is considered as a preferred masking material, owing to its chemical durability.

Dry chemical etching of Si: The process of dry etching spectrum ranges from physical etching of Si wafer via sputtering and ion milling process to chemical plasma etching. Two commonly used processes namely, reactive ion etching (RIE) and reactive ion beam etching (RIBE) involve both chemical and physical etching. In principle, dry etching processes involve a plasma of ionized gases, plus neutral particles to etch out material from the targeted area. Reactive ion etching is the most commonly used dry etch process to pattern Si. Fluorinated compounds like CF4, SF6 and NF3 or chlorinated compounds such as Cl2 or CCl4 occasionally mixed with He, O2 or H2 are commonly employed. The process of RIE is highly directional, thus enables the transfer of pattern directly from the masking material to the etched Si surface.

(b) Polysilicon

The surface micromachined MEMS device fabrication predominantly utilizes polycrystalline Si (usually known as polysilicon) as the primary structural material, whereas SiO2 is used as the sacrificial material and Si3N4 is employed for electrical isolation of device structures. Analogous to single crystal Si, the Polysilicon can also be doped during or after the film deposition by employing standard IC processing techniques. Polysilicon is composed of a large number of small single-crystal domains (called grains), whose alignment and/or orientations vary with respect to one another (Figure 2). The roughness of polysilicon surfaces is generally due to the granular nature of polysilicon.

During MEMS fabrication, polysilicon films usually undergo one or more high-temperature processing treatments like doping, thermal oxidation or annealing, post deposition. These high-temperature steps can lead to recrystallization of the polysilicon grains, thus leading to a reorientation of the film and a significant increase in average grain size. Consequently, the polysilicon surface roughness increases with the increase in grain size, which is an undesirable outcome from a fabrication point of view because surface roughness limits pattern resolution.

(c) Silicon Dioxide

SiO2 can be grown thermally on Si substrates as well as also deposited using a variety of processes to satisfy a wide range of different requirements. In polysilicon surface micromachining, SiO2 is used as a sacrificial material, as it can be easily dissolved using etchants that do not attack polysilicon. The SiO2 growth and deposition processes most widely used in polysilicon surface micromachining are thermal oxidation and LPCVD. Thermal oxidation of Si is performed at high temperatures (e.g., 900 to 1000°C) in the presence of oxygen or steam. Because thermal oxidation is a self-limiting process (i.e. the oxide growth rate decreases with increasing film thickness) the maximum practical film thickness that can be obtained is about 2μm, which for many sacrificial applications is sufficient. SiO2 films for MEMS applications can also be deposited using an LPCVD process known as low temperature oxidation (LTO). In general, LPCVD provides a means for depositing thick (>2 μm) SiO2 films at temperatures much lower than thermal oxidation.

Recent report documents the development of another low-pressure process, known as plasma enhanced chemical vapor deposition (PECVD), for MEMS applications. The objective was to deposit low-stress, very thick (10 to 20 μm) SiO2 films for insulating layers in micromachined gas turbine engines. PECVD was selected, in part, because it offers the possibility to deposit films of the desired thickness at a reasonable deposition rate.

The dissolution of the sacrificial SiO2 to release free-standing structures is a critical step in polysilicon surface micromachining. Typically, 49% (by weight) HF is used for the release process.

(d) Silicon Nitride

Si3N4 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 Si3N4 thin films: LPCVD and PECVD. PECVD Si3N4 is generally nonstoichiometric and may contain significant concentrations of hydrogen. Use of PECVD Si3N4 in micromachining applications is somewhat limited because its etch rate in HF can be high (e.g., often higher than that of thermally grown SiO2) due to the porosity of the film. However, PECVD offers the potential to deposit nearly stress-free Si3N4 films, an attractive property for many MEMS applications, especially in the area of encapsulation and packaging.

The residual stress in stochiometric Si3N4 is large and tensile, with a magnitude of about 1010 dyne/cm2. Such a large residual stress limits the practical thickness of a deposited Si3N4 film to a few thousand angstroms because thicker films tend to crack. Nevertheless, stoichiometric Si3N4 films have been used as mechanical support structures and electrical insulating layers in piezoresistive pressure sensors

(e) Metals

Metals are an integral part of MEMS fabrication, where they are commonly employed as hard etch masks and as interconnects to various structural elements such as microactuators, microsensors etc. Metal thin films can be grown using a number of deposition techniques like RF/DC Sputtering, e-beam/thermal evaporation, electroplating and CVD. Thus, the growth of good quality metallic thin film contacts or interconnects is crucial for the development of high performance MEMS based devices. Some of the most commonly employed metals in MEMS fabrication are as follows:

a) Aluminum (Al): It is one of the most extensively exploited metal in MEMS devices. In MEMS, Al metal thin films can be used in combination with different polymers like polyimide because the metal films are generally sputtered at low temperatures. Al is usually employed as a structural layer in MEMS devices; but it can also be used as a sacrificial layer during the fabrication.

b) Tungsten (W) is also used as a structural material in many devices, whereas Silicon dioxide (SiO2) is employed as a sacrificial layer during micromachining. In such cases, HF is used as an etchant for removing the sacrificial oxide layer.

c) For high-aspect-ratio processes, the structural layers of nickel and copper are employed with polyimide, while other metals (e.g., Chromium) are used as the sacrificial layers.

d) An Alloy of Titanium (Ti) and Nickel (Ni), commonly called as TiNi, is among the most common shape-memory alloy due to its large actuation work density and bandwidth (up to 0.1 kHz). TiNi is considered as an attractive material also because of the fact that it can be deposited via conventional sputtering techniques.

(B) MEMS Fabrication

(i) Bulk Micromachining

Bulk micromachining is the most established and commonly employed technology for MEMS Fabrication. In this technique, the mechanical components are fabricated within the bulk of a material, like silicon wafer (Figure 3). This is done by selectively removing out certain parts of the wafer material, by employing orientation-dependent etchants. The etch-stopping and masking films methods are very important during the bulk micromachining of a wafer. The etching of the material can be either isotropic or anisotropic, or it can also be a combination of both. During isotropic etching, the rate of etching is equal in all the directions, while during anisotropic etching, the rate depends on the crystallographic orientation of the wafer. Ethylene diamine and pyrocatechol (EDP) and an aqueous solution of potassium hydroxide (KOH) are the two commonly used etchants. The etchant should be effective in removing the respective material at precise location, and for the said process, etch-stopping techniques have been established over the years.

In the wet bulk micromachining, the electro-mechanical structures are shaped inside the bulk of materials like silicon, SiC, quartz, InP, GaAs, Ge or glass by anisotropic and/or by isotropic wet etchants. A schematic of the typical structure fashioned using a wet bulk micromachining method is shown in Figure 4.

(ii) Surface micromachining

Contrary to the process of Bulk micromachining, where three-dimensional structures are etched out from the bulk of crystalline or non-crystalline materials, the surface micromachined structural features are fabricated, layer by layer, over the surface of a substrate (such as a single crystal silicon wafer). Dry etching process creates the surface features in the x,y plane of the material, whereas the wet etching process forms them from the same plane by undercutting. During surface micromachining, the shapes in the x,y plane are unobstructed by the crystallographic planes of the substrate material. Thin films employed in surface micromachining must stand out a large number of rigorous structural, chemical, mechanical as well as electrical requirements. Some of the important features like adhesion, low pinhole/dislocation density, low residual stress, good mechanical strength and chemical resistance to certain etchants, all may be required simultaneously. Some of these essential features are discussed in detail, as follows:

(a) Adhesion: Adhesion is very crucial in the micromachining of a material since mechanical forces are usually involved during actuation. If the films tend to lift from the substrate upon application of a repetitive, applied mechanical force, the fabricated device will not operate over a long time. It is extremely beneficial to incorporate a layer of oxide-forming elements between the metal and an oxide substrate. Metals like Cr, Ti, Al etc., provide a good adhesion for subsequent metal deposition. Intermediate film formation allowing a continuous transition from one lattice to the other results in the best adhesion.

(b) Stress: Delamination, cracking of film, and void/pin-hole formation may be attributed to stress generated in the film. Most of the films are found in a state of residual stress, due to in the thermal expansion coefficient mismatch, non-uniformity in plastic deformation, substitutional or interstitial impurities and lattice mismatch. Thermal stresses, the most common type of extrinsic stresses that develop in a material. They arise either due to inhomogeneous thermal expansion coefficients exposed to a uniform temperature change or in a homogeneous material subjected to a thermal gradient.

Basic Process Sequence of surface micromachining

A surface micromachining process sequence for the creation of a simple free-standing poly-Si bridge is illustrated in figure

The steps adopted during the surface micromachining of a material can be broadly summarized as follows:

(A) Deposition of a spacer layer (the thin dielectric insulator layer is not shown in Figure 5).

(B) Patterning of base with mask 1.

(C) Deposition of Microstructure layer.

(D) Patterning of microstructures using mask 2.

(E) Selective etching of the spacer layer

The surface micromachining technique is applicable to combinations of thin films and lateral dimensions where the sacrificial layer can be etched without significant etching or attack of the microstructure, the dielectric or the substrate.

(iii) LIGA

LIGA is a german acronym coined for X-ray lithography (X-ray lithographie), electrodeposition (galvanoformung), and molding (abformtechnik) process. LIGA process employs a thick layer of X-ray resist—which may vary from a few microns to centimeters, exposure to highly energetic X-ray radiations, followed by development to arrive at a three-dimensional resist structure. The subsequent electrodeposition is used to fill the developed resist mold using metallization and, after removing the resist, a free-standing metal structure is obtained. Figure 6 shows the basic steps involved in the LIGA process.

Figure 6: Basic steps involved in the LGA process

The metal shape obtained may be a final product or may also serve as a mold insert for precise plastic injection molding. The injection-molded plastic parts may then be the final products or lost molds. The so formed plastic mold retains the shape, size and form as that of the original resist structure. Not only is it produced quickly, but is also an inexpensive method to replicate patterns. Furthermore, the plastic lost mold could generate metallic parts in a second electroforming method or ceramic parts in a slip casting process.

Mold inserts, depending on the dimensions of the microparts, the accuracy requirements and the fabrication costs are realized by e-beam writing, deep UV resists, excimer laser ablation, electro-discharge machining, laser cutting and X-ray lithography as involved in the LIGA technique.

Summary

Introduction to MEMS/NEMS and various materials employed for MEMS Fabrication like Single crystal Silicon, Polysilicon, Silicon Dioxide, Silicon Nitride, and Metals were discussed Bulk and surface Micromachining.LIGA process was discussed