Contamination Issues for MEMS Fabrication
Many of the particle and contamination issues of concern in IC manufacturing are valid for MEMS fabrication. However, the effects of contaminants tend to be very much design specific.
By Christian A. Ball, Mark D. Walters, Robert L. Wood
Microelectromechanical Systems (MEMS) are micrometer-to-millimeter-scale devices that are primarily fabricated using silicon integrated circuit (IC) technology or other micro-fabrication techniques. These devices and systems have many uses, depending on the design, but generally, they fall into the category of sensors or actuators. An enabling technology, MEMS partially accounts for the projections of 10-20 percent annual growth in the industry and the potential of a greater-than-$8 billion market by the year 2000.1
Because of the many different types of devices and processes that encompass MEMS technology, the effects of contamination and particles during fabrication are very much process and design specific. However, the vast majority of MEMS are fabricated using one or more of three basic methods: polysilicon surface micromachining, bulk silicon micromachining, and LIGA (Lithography, Electroforming, and Injection Molding). This article will discuss these three methods and describe the particle and contamination issues associated with each.
Polysilicon surface micromachining
Polysilicon surface micromachining encompasses many of the same fabrication techniques as traditional silicon IC fabrication. Layers of chemical-vapor-deposited (CVD) films are deposited and subsequently patterned using photolithography and plasma etch techniques. Alternating layers of silicon dioxide and polysilicon are deposited. When all the layers have been patterned, the silicon dioxide can be etched away with hydrofluoric acid, leaving only the polysilicon structures behind. Devices fabricated in this way are typically several microns in thickness. This process is schematically represented in Figure 1. The chief benefit of this process is the ability to fabricate moving parts on a silicon substrate. Some surface-micromachined devices that are common today include micromotors (Fig. 2) and resonators. The resonator device is similar to the accelerometer devices now being used as sensors in automotive air bag deployment systems.
As the scale on the SEM photograph in Figure 2 indicates--as with IC devices--critical dimensions of these devices are to be found on the micron scale. Typical minimum lateral dimensions are 2 microns. As such, many of the same cleanliness and contamination considerations in the manufacture of ICs are relevant to surface micromachining. In general, cleanliness requirements are directly related to size of the geometries in the device design. The rule of thumb is that a particle 1/4 to 1/5 the size of the smallest geometry is capable of causing a fatal defect.
Surface micromachining is usually performed in a Class 100 or better environment, using high purity chemicals and gases. While contamination in process gases, chemicals, and materials is of concern in IC fabrication because of the detrimental effects on electrical properties, MEMS devices have the additional worry of its effects on mechanical properties. Contamination during processing could possibly lead to non-uniformities in the structure of films during growth or deposition, altering the stress and mechanical properties of these films. In addition, unlike ICs, these types of MEMS devices often employ moving parts. The cleanliness of surfaces and the absorption of gases can have a substantial effect on static friction in these devices.2
Bulk micromachining of silicon employs chemical and/or plasma etching techniques to sculpt three-dimensional structures from the silicon substrate. Masking and various etch stop techniques are used to make microstructures such as diaphragms, bridges, or beams. Bulk micromachining is used for the overwhelming majority of today`s commercial MEMS devices, including pressure sensors, acceleration sensors, and microvalves.
Bulk silicon micromachining incorporates many of the same fabrication techniques as IC fabrication and is usually performed in a better than Class 1,000 cleanroom environment. However, bulk micromachining does present some unique requirements. It usually requires processing of both sides of the silicon substrate. Typically, the front is processed to fabricate sensing or measurement structures (such as piezoresistors), and then the substrate is flipped over and patterned on the back to open up the etch holes for the bulk silicon etching.
Critical dimensions on the front of these wafers are typically at the micron scale, but can drop down to the submicron regime if embedded electronics are included. Thus, contamination standards for front side processing of the silicon substrate are often equivalent to those used in IC processing.
Dimensions on the backside of the wafers are much larger, due to the fact that most of the methods for chemically etching the silicon substrate are anisotropic, and preferentially etch single-crystal silicon along certain crystal planes. For the most common silicon substrates, this means that the opening on the backside is larger at the surface than the bottom, with a sidewall slope of 54.7°. For this reason, geometries on the back of the wafer are typically on the order of 100`s of microns to millimeters. Therefore, particle requirements are not nearly as stringent for backside processing, and in fact, any micron-sized particles present in the geometries to be etched can be readily undercut and do not typically affect the etched geometry.
The major contamination issues associated with backside processing of wafers are: ensuring the integrity of the backside of the substrate while performing frontside processing; and protecting frontside structures and devices while patterning and etching the backside of the substrate. Scratches, nicks, and residues on the back of the wafer originating from various frontside processes all have detrimental effects on backside- etched geometries. During backside processing, the frontside often must be well protected, because silicon etchants are very caustic and will readily etch many of the metals and thin films used in processing the front side. Because of these considerations, frontside processing is usually performed in an IC cleanroom environment, while backside processing can be done in a less clean environment. Special fixtures protect the front surface while the backside of the substrate is being etched.
A German acronym that translates to Lithography, Electroforming, and Injection Molding, LIGA was developed in Germany in the 1980s and has slowly gained widespread interest. The key to LIGA`s popularity is the ability to mass-replicate high-aspect ratio structures out of metals, polymers and ceramics, and fabricate structures that are very tall, with great precision of placement, size, and edge acuity. LIGA structures can be hundreds of microns to several millimeters tall, with lateral dimensions as small as a few microns. Figure 3 schematically represents the LIGA process. Figure 4 is an SEM micrograph that shows a structure fabricated with the LIGA technique. LIGA requires the use of an extremely energetic, highly collimated photon source, which restricts its practice to those facilities with access to X-ray synchrotrons.
Although LIGA is typically used to produce patterns on the order of tens to hundreds of microns in size, there are several points in the process that are vulnerable to microscopic contamination. The first of these is the X-ray mask fabrication process, in which a gold or other heavy metallic X-ray absorber is plated into a resist stencil. This stencil is typically produced on silicon wafers using conventional IC processing, equipment and levels of cleanliness. Owing to the extreme fidelity of X-ray lithography, any missing or extraneous absorber material will result in a corresponding pattern defect in the X-ray resist. Particles in the plating base layer, pinholes in the resist stencil, residues caused by contamination or incomplete resist removal, and dust particles all contribute to these types of defects. For example, a pinhole in the resist layer of only 5 mm in size will be reproduced as a pipe in the final LIGA structure, which may be several hundred microns tall. If located in a critical area, this could cause a fatal flaw. Because LIGA structures tend to have large active areas compared to their IC cousins, small defect densities can have major consequences.
Another area of contamination sensitivity is the X-ray resist process. To achieve thicknesses compatible with LIGA, Polymethylmethacrylate (PMMA) sheets are bonded to a substrate wafer using a solvent bonding technique.3 Any particles trapped between the sheet and the wafer can cause a pattern defect if they fall in or near an exposed area. It is therefore essential that surfaces of the wafers and sheets are extremely clean before bonding, and that the actual bonding operation be carried out in a cleanroom. Static charging is of particular concern, making particles particularly difficult to remove from the PMMA sheet. Chemical contamination of the surface can cause poor adhesion of the PMMA sheet. In patterned areas, this leads to lifted or missing images and plating beneath the resist (underplating), often producing disastrous results.
During electroplating of the final X-ray resist structure, it is important to keep the plating bath protected from the environment and continuously filter the electrolyte during the plating to prevent particles from becoming embedded in the plating deposit or from creating voids in the pattern. Particles are not only generated from external sources, but from degradation of the anode material, particularly if a soluble anode is used. The turbulent flow conditions required to achieve uniform plating provide an ideal medium for carrying these particles and contaminants to the substrate--thus the need for constant filtration. Plating is generally performed outside of a cleanroom but within a hood equipped with HEPA filters.
Finally, the use of the LIGA structure either as a mold or as a subassembly or standalone part introduces a number of contamination control challenges. LIGA assemblies having submicron fit tolerances have been produced. Particles or contaminants in such structures present obvious problems. The mechanical nature of LIGA structures suggests that they will need to interact with the environment at some level, unlike their IC counterparts, which stay snug and protected in their encapsulating package. This environmental interaction poses new questions and challenges--keeping contamination at bay not only during construction of the device, but during its useful life as well.
Many of the processes and materials employed in silicon micromachining are closely related to those used in IC fabrication. As such, many of the particle and contamination issues of concern in IC manufacturing are valid for MEMS fabrication. However, because MEMS technology encompasses a wide variety of devices, processes, and applications, the effects of contaminants tend to be very much design specific. Table 1 lists minimum geometries in devices fabricated by surface, bulk, and LIGA micromachining, and summarizes the special contamination issues associated with these processes. n
References available by contacting CleanRooms at (603) 891-9230.
Christian A. Ball, Mark D. Walters, and Robert L. Wood are members of the technical staff at the MCNC MEMS Technology Applications Center (Research Triangle Park, NC).
Figure 1. Shown is a schematic representation of the polysilicon surface-micromachining process. The main advantage of this process is the ability to fabricate moving parts on the silicon substrate.
Figure 2. A micromotor fabricated at MCNC is one of the most common surface micro machined devices.
Figure 3. A schematic representation of the LIGA micromachining process begins with (a) incident X-rays being absorbed or transmitted by regions in the X-ray masks. Then, regions of resist are exposed to X-rays removed during the wet chemical develop process. b) Metal is electrochemically deposited into a resist template formed by the lithography process c) The resist template is stripped, leaving the metal structure. This can either be the final part or it can be used as a primary transfer mold for replication processes. d) The metal mold can be used as the insert for casting, embossing, and injection-molding to produce mass replicated parts.
Figure 4. A SEM micrograph shows a motor stator and pin fabricated using the LIGA process. (Photo courtesy of the University of Wisconsin and MCNC.)