MEMS: fabrication, assembling and packing

INTRODUCTION TO MEMS FABRICATION, ASSEMBLING, AND PACKAGING:


Two basic components of MEMS and microengineering are microelectronics (to fabricate ICs) and micromachining (to fabricate motion microstructures). Using CMOS or VLSI technology, microelectronics (ICs) fabrication can be performed. Micromachining technology is needed to
fabricate motion microstructures to be used as the MEMS mechanical subsystems. It was emphasized that one of the main goals of microengineering is to integrate microelectronics with micromachined mechanical structures in order to produce completely integrated monolithic
high-performance MEMS.

To guarantee low cost, reliability, and manufacturability, the following must by guaranteed: the fabrication process has a high yield and batch processing techniques are used for as much of the process as possible (large numbers of microscale structures/devices per silicon wafer and large number of wafers are processed at the same time at each fabrication step). Assembling and packaging must be automated, and the most promising avenues are auto- or self-alignment and self assembly. Some MEMS subsystems (actuator and interactive environment sensors) must be protected from mechanical damage, and in addition, protected from contamination. Wear tolerance, electromagnetic and thermo isolation, among other problems have always challenged MEMS. Different manufacturing technologies must be applied to attain the desired performance level and cost. Microsubsystems can be coated directly by thin films of silicon dioxide or silicon nitride which are deposited using plasma enhanced chemical vapor deposition. It is possible to deposit (at 7000C to 9000C) films of diamond which have superior wear capabilities, excellent electric insulation and thermal characteristics. It must be emphasized that diamond like carbon films can be also deposited.

Microelectromechanical systems are connected (interfaced) with realworld systems (control surfaces of aircraft, flight computer, communication ports, et cetera). Furthermore, MEMS are packaged to protect systems from harsh environments, prevent mechanical damage, minimize stresses and vibrations, contamination, electromagnetic interference, et cetera. Therefore, MEMS are usually sealed. It is impossible to specify a generic MEMS package. Through input-output connections (power and communication bus) one delivers the power required, feeds control (command) and test (probe) signals, receives the output signals and data. Packages must be designed to minimize electromagnetic interference and noise. Heat, generated by MEMS, must be dissipated, and the thermal expansion problem must be solved. Conventional MEMS packages are usually ceramic and plastic. In ceramic packages, the die is bonded to a ceramic base, which includes a metal frame and pins for making electric outside connections. Plastic packages are connected in the similar way.

However, the package can be molded around the microdevice. Silicon and silicon carbide micromachining are the most developed micromachining technologies. Silicon is the primary substrate material which is used by the microelectronics industry. A single crystal ingot (solid cylinder 300 mm diameter and 1000 mm length) of very high purity silicon is grown, then
sawed with the desired thickness and polished using chemical and mechanical polishing techniques. Electromagnetic and mechanical wafer properties depend upon the orientation of the crystal growth, concentration and type of doped impurities. Depending on the silicon substrate, CMOS processes are used to manufacture ICs, and the process is classified as n-well, p-well, or twin-well.

The major steps are diffusion, oxidation, polysilicon gate formations, photolithography, masking, etching, metallization, wire bonding, et cetera. To fabricate motion microstructures microelectromechanical motion devices), CMOS technology must be modified. High-resolution photolithography is a technology that is applied to produce moulds for the fabrication of
micromachined mechanical components and to define their three-dimensional shape (geometry). That is, the micromachine geometry is defined photographically. First, a mask is produced on a glass plate. The silicon wafer is then coated with a polymer which is sensitive to ultraviolet light
(photoresistive layer is called photoresist). Ultraviolet light is shone through the
mask onto the photoresist to build the mask to the photoresist layer. The positive photoresist becomes softened, and the exposed layer can be removed.

In general, there are two types of photoresist, e.g., positive and negative. Where the ultraviolet light strikes the positive photoresist, it weakens the polymer. Hence, when the image is developed, the photoresist is washed where the light struck it. A high-resolution positive image results. In contrast, if the ultraviolet light strikes negative photoresist, it strengthens the polymer. Therefore, a negative image of the mask results. Chemical process is used to remove the oxide where it is exposed through the openings in the photoresist. When the
photoresist is removed, the patterned oxide appears. Alternatively, electron beam lithography can be used. Photolithography requires design of masks. The design of photolithography masks for micromachining is straightforward, and computer-aided-design (CAD) software is available and widely applied. There are a number of basic surface silicon micromachining technologies
that can be used in order to pattern thin films that have been deposited on a silicon wafer, and to shape the silicon wafer itself forming a set of basic microstructures. Three basic steps associated with silicon micromachining are:

1.deposition of thin films of materials;

2.removal of material (patterning) by wet or dry techniques;

3.doping.

Different microelectromechanical motion devices (motion microstructures) can be designed, and silicon wafers with different crystal orientations are used. Reactive ion etching (dry etching) is usually applied. Ions are accelerated towards the material to be etched, and the etching reaction is enhanced in the direction of ion traveling. Deep trenches and pits of desired shapes can be
etched in a variety of materials including silicon, oxide, and nitride. A combination of dry and wet etching can be embedded in the process. Metal films are patterned using the lift off stenciling technique. A thin film of the assisting material (oxide) is deposited, and a layer of photoresist is put over and patterned. The oxide is then etched to undercut the photoresist. The
metal film is then deposited on the silicon wafer through evaporation process. The metal pattern is stenciled through the gaps in the photoresist, which is then removed, lifting off the unwanted metal. The assisting layer is then stripped off, leaving the metal film pattern. The anisotropic wet etching and concentration dependent etching are called bulk silicon micromachining because the microstructures are formed by etching away the bulk of the silicon wafer.

Surface micromachining forms the structure in layers of thin films on the surface of the silicon wafer or other substrate. Hence, the surface micromachining process uses thin films of two different materials, e.g., structural (usually polysilicon) and sacrificial (oxide) materials. Sacrificial layers of oxide are deposited on the wafer surface, and dry etched. Then, the sacrificial material is wet etched away to release the structure. A variety of different complex motion microstructures with different geometry have been fabricated using the surface micromachining technology.

Micromachined silicon wafers must be bonded together. Anodic (electrostatic) bonding technique is used to bond silicon wafer and glass substrate. In particular, the silicon wafer and glass substrate are attached, heated, and electric field is applied across the join. These result in extremely strong bonds between the silicon wafer and glass substrate. In contrast, the
direct silicon bonding is based upon applying pressure to bond silicon wafer and glass substrate. It must be emphasized that to guarantee strong bonds, the silicon wafer and glass substrate surfaces must be flat and clean. The MEMCAD™ software (current version is 4.6), developed by
Microcosm, is widely used to design, model, simulate, characterize, and package MEMS. Using the built-in Microcosm Catapult™ layout editor, augmented with materials database and components library, threedimensional solid models of motion microstructures can be developed.
Furthermore, customizable packaging is fully supported.