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Investigating
the use of spray-coating technology in MEMS applications
Chad Brubaker, Markus Wimplinger, Paul Lindner, and Stefan Pargfriender, EV Group
While spin-coating technology is used to perform most photoresist deposition processes in the chipmaking area, it does not perform well on three-dimensional MEMS structures.1 The severe topography of MEMS devices, such as V-grooves, mesas, and etch cavities that pass entirely through the wafer, impedes or prevents the smooth flow of resist from the center to the edge of the wafer. Spray coating was developed to overcome these challenges. Because it deposits resist from above, spray coating works well on surfaces with severe topographies. Moreover, it consumes less material than spin coating. In addition to performing MEMS lithography applications, spray coating can be used to coat irregularly shaped or heavy substrates, coat many small substrates simultaneously, deposit a protective coating on top of fragile structures, and perform underfill steps. This article, basedon work performed by EV Group (Schärding, Austria), compares spin-coating with spray-coating technology.
Spray-Coating Technology The key to spray-coating technology is its use of an ultrasonic spray nozzle. This nozzle oscillates to produce microscopic resist droplets. The mean distribution of the droplets' diameter is typically around 20 µm. To achieve the proper droplet size distribution, the viscosity of the material introduced into the nozzle must be below 20 centistokes (cSt).2 The most common chemicals used in spray coating can attain that viscosity by being diluted with solvents that are compatible with the chemicals' base solvents and do not react with the chemicals before or during exposure. Table I lists resists and other materials that are used in spray-coating processes. Once the droplets have been produced by the ultrasonic nozzle, they are propelled toward the surface of the substrate using a stream of clean dry air (CDA) or nitrogen, which can be adjusted by a software preset. During the spray process, the substrate is spun at a slow speed of between 50 and 100 rpm, minimizing the influence of centrifugal force at the outer substrate area. This procedure prevents the formation of coating defects and nonhomogeneous areas on the final coated surface. The dynamics of the process are shown in Figure 1. One of the most important attributes of the spray-coating process is the variable velocity profile of the resist nozzle as it sweeps over the wafer. Because the substrate area requiring resist or chemical coverage shrinks as the nozzle nears the center of the wafer, a constant sweep speed would create a thicker resist layer toward the center of the substrate. To simplify the deposition process, the sweep path of the spray-coating system is divided into multiple parts, or indices, each with its own travel velocity. By measuring the relative area of each index, the system calculates an optimal velocity. Accordingly, the velocity profile of the nozzle near the center of the wafer is faster than at the edge.
The thickness, uniformity, and roughness of the resist film depend on the following parameters: • Solids content of the resist (or dilution). • Angle of the spray nozzle. • Resist dispense rate. • Scanning speed of the atomizer (velocity profile). • Spinner speed. All critical process parameters can be adjusted and stored in a software preset. Resist dilution and the angle of the atomizer are normally kept constant for one substrate or experiment. A recipe with optimized parameters has been developed to provide reasonable resist uniformity across the wafer. With these features it is possible to achieve good repeatability and a good run-to-run standard deviation for films with thicknesses between 0.1 and 100 µm. Table II shows the uniformity results of a 4.2-µm film layer spray-coated on a series of blank 150-mm silicon wafers.
Spray coating can be used on a wide variety of surfaces, including silicon, oxide, gallium arsenide, Pyrex, and an array of metal surfaces. Additionally, except for the deposition method, the spray process does not differ greatly from standard resist-coating processes. Coating Topographically Challenged Substrates Initially, the spray-coating technique was developed to coat substrates with severe topographies. In contrast to spray coating, spin coating is generally performed on substrates with nearly flat surfaces, or at least on surfaces with topographies that are much lower than the desired thickness of the resist film. In such cases, the resist can flow smoothly to the edge of the wafer, providing a highly uniform layer suitable for the production of fine print geometries. Most MEMS applications, however, are different.
For example, an anisotropically etched cavity is a typical MEMS profile that is created by etching the device in a bath of potassium hydroxide. The very distinctive etch follows the <111> plane of the silicon, creating a 54.7° sidewall angle, as illustrated in Figure 2. In its most extreme form, this etch structure forms a V shape, and is consequently known as a V- groove. Spin coating a substrate with a series of V-grooves produces films that are anything but uniform. The resist tends to form pools inside the V-grooves, resulting in "shadows" of thin or no resist coverage on the side of the feature opposite the center of the wafer. Additionally, a surface-tension effect will be seen at the edges of the features, commonly resulting in increased film thickness at the edges closest to the center of the wafer. Figure 3 presents images of V-groove structures that have been covered with film from a spin coater versus a spray coater.
In contrast to spin coating, spray coating does not exhibit these flow dynamics because of the slow rotational speed of the substrate during processing. Uniform films can therefore be produced even when through-holes exist. In fact, because spray coating uses slow rotational speeds, it can be performed without the use of a vacuum chuck, avoiding the added complication of drawing resist through the wafer and into the vacuum (not to mention the difficulty of vacuum fixing a wafer that has through-holes). Topographically challenged wafers typically fall into two categories: those that require a uniform film only at the tops or bottoms of the cavities, and those that require conformal films—in other words, films that cover all device contours uniformly. In the case of films used in cavities, the coating dynamic is identical to that involved in spray coating a blank wafer—it creates similarly uniform structures. In contrast, spin coating the same type of wafer can produce uniformity variations in excess of ±50%. In the case of conformal films, spray coating is much more effective at achieving acceptable layers than spin coating.3 The former can generally create film layers with uniformity variations better than ±10% on substrates with significant degrees of topography, enabling reasonable print quality on the tops, bottoms, and even sidewalls of device features. Figure 4 shows images of various profiles from spray-coated devices.4
Another technology has been developed to address the issue of conformal film coating: electrodeposited resist.5 This method is limited; it requires a conductive seed layer to produce the film, limiting the technology primarily to back-end processing. Additionally, since electrodeposition requires that devices be immersed in an electroplating bath, the method tends to be messy. Furthermore, it achieves only limited film conformality, since transport issues come into play, affecting the growth of resist in convex and concave features. Finally, since photoresist is inherently a current resistor, the process of growing film is self-limiting: as film thickness increases, the deposition rate decreases. Coating Irregular Shapes and Heavy Substrates Another process that suffers from the use of standard spin coating is the coating of noncircular substrates. When a standard spin-coating process is used to deposit resist on a noncircular substrate, the center of the substrate, in the form of an inscribed circle, remains very uniform. However, as shown in Figure 5, a marked nonuniform pattern appears at the corners of the substrate. The primary cause of this phenomenon is increased air friction at the edges of the substrate, which increases turbulence.6 Turbulence, in turn, increases the evaporation of resist, causing the resist to dry out more quickly at the corners of the substrate. The continual flow of resist from the center of the substrate begins to overlap the dried resist at the corners, creating a buildup.
Round substrates (except for those with a flat) do not exhibit such behavior, since they do not contain surfaces that are orthogonal to the rotational direction. Therefore, they are not strongly affected by air turbulence and do not suffer from the problem of dried-out resist. In
the spray-coating method, the use of a low rotational speed means
that the relative movement between the substrate and the air is
nearly the same. The resist at the edges of the substrate does
not dry out appreciably faster than that Reducing Materials Waste In the spin-coating process, most of the resist dispensed onto the substrate spins off the wafer and into the bowl, eventually becoming waste. Even when the process is optimized to the highest degree, waste is unavoidable. In contrast, the spray-coating deposition method ensures that most of the material dispensed onto the substrate (>90%) remains on the substrate. While reducing waste is always desirable, it is especially important in the case of precious materials. A good example is benzocyclobutene (BCB), marketed under the name Cyclotene by Dow Chemicals (Midland, MI). This material is widely used for advanced packaging applications because of its mechanical properties and its utility as a low-k dielectric. When spray coated, it can be used in the manufacture of cost-sensitive devices, where standard coating methods are cost-prohibitive.
A test was performed to study how much BCB was consumed in an optimized spin-coating process versus a spray-coating process. In addition, the test investigated the effects of both processes on film uniformity. As illustrated in Figure 6, the amount of material consumed in the spin-coating process was approximately 2.5 times greater than that consumed in the spray-coating process for both 150- and 200-mm wafers and two resist formulations and film thicknesses. Table III presents cost data from work performed with a major manufacturer of power devices showing the direct cost benefits of spray coating.
Because both BCB materials used in this test have viscosities >300 cSt, they had to be diluted to reduce their viscosities to <20 cSt. Dilution offers an opportunity to consume less material. Because film thickness is a direct (linear) function of the solids content of the diluted material rather than its viscosity, materials with a higher solids content can be used to create a thinner film just by diluting them more. Thus, the 10-µm Cyclotene 4026-46 film in Figure 6 can be diluted to create a 5-µm film, resulting in even lower material consumption. Consumption can be reduced by using a syringe dispense system. That system lowers "dead volume" (the volume of material between the source and dispense point) to as low as a single milliliter. Furthermore, it minimizes the amount of sensitive material that must be stored at room temperature. (At room temperature, BCB expires within five days.) Finally, it allows users in an R&D environment to change resists (or dilutions) quickly. Coating Multiple Substrates Simultaneously Because of its uniform flow dynamics, spray coating can coat multiple substrates simultaneously, while maintaining the film uniformity required to perform photolithography. Known as array coating, this application is useful when the substrate size is small (approximately 25 X 25 mm). In addition, the slow rotational speed used in spray coating makes it unnecessary to vacuum fix each substrate to a spinner chuck. As pictured in Figure 7, substrates are placed into specialized 200-mm-diameter frames designed to hold 20 to 50 substrates, depending on their size. Then coating takes place as if the carrier were a 200-mm wafer. While spin coating can usually deposit photoresist on small substrates, its single-substrate processing configuration results in low throughput.
In array coating, uniformity variations of ±2% per substrate have been observed, while within-carrier substrate-to-substrate uniformity variations have been as low as ±1.5%.7 By using this application, throughput can be multiplied greatly, since the time required to spray coat dozens of substrates in a single carrier is similar to that required to spin coat a single substrate. Moreover, the array-coating method can be used to perform conformal coating and does not waste materials. In fact, the amount of resist consumed per substrate is lower in array coating than in single-substrate coating. For example, in one test, a 2-µm-thick film of S1813 positive photoresist from Shipley (Marlborough, MA) was applied using ~0.03 ml of resist per 25 X 25-mm substrate. Protective Coating of Fragile Structures Many MEMS designs include extremely fragile components, such as cantilevers or suspended structures. While wafer bonding is often used to cap substrates before further processing (such as dicing), processing sometimes must occur without capping. In such cases, unprotected fragile structures would be destroyed. For example, in dicing applications, the flow of water used to prevent dust during the sawing process would cause unprotected MEMS structures to shear off. Creating a protective layer by means of spin coating would have a similar effect, since the lateral motion of the resist would create a shear force that would ultimately damage or destroy delicate structures. The spray-coating technique can dispense a protective film layer without destroying unprotected structures. First, the protective resist film is deposited from above by a fairly low (<150 mbar) pressure stream of nitrogen or CDA, causing little stress to delicate structures. Second, the system's low rotational speed prevents the motion of the resist from exerting shear force on device components. Finally, film layers up to 75 µm thick can be deposited by the spray nozzle in a single pass, protecting features up to 75 µm tall. The process of depositing a protective coating differs from standard lithographic processing in that a protective coating does not require patterning. Consequently, a wide array of materials can be used, such as photoresists, dielectric materials, fluorinated polymers, and even spin-on adhesives. The material used must be compatible with the process to be performed on the layer after deposition. For example, the protective film deposited before dicing must be strong enough to resist being damaged (chipped) by the saw, sufficiently heat resistant so that the heat generated by the friction from the saw will not cause undue damage to the film, transparent enough for the dicing system to "see" through the film to the dicing streets (even if the film is >70 µm thick), and easily removable using a solvent immersion or similar process. By using the spray-coating process in conjunction with suitable materials, a protective layer can be created that can help to increase device yields. In some cases, the use of a protective coating may even provide an enabling process without which development of the device is not possible. Underfill Process Similar to the protective-coating process is the underfill process. However, they differ from each other in two critical respects. First, protection is not the only goal of the underfill process. Second, whereas a protective coating is deposited on essentially freestanding structures, the underfill process is intended to fill spaces under suspended structures. Suspended structures, a common feature in devices that perform high-power applications, reduce parasitic capacitance.8 By filling the space underneath a suspended structure with a low-k dielectric material, low parasitic capacitance can be maintained while minimizing the chance of damaging fragile structures and thus effectively increasing yield. While the underfill process can be performed using spin coating, there are distinct disadvantages to doing so. In many cases, the path from the top of the substrate to the area under the suspended region is relatively small (sometimes just a few microns), requiring the use of a relatively low-viscosity material (<20 cSt) so that capillary effects can draw the substance under the bridge. However, because structures are often suspended 5 µm or more above the bulk surface of the wafer, a significant amount of material must be placed underneath them. Since material with a viscosity of 20 cSt results in a film ~1 µm thick when spun on a blank wafer at ~2000 rpm, it takes multiple coats to achieve a deposited film that is thick enough (~5 µm) to completely fill the space under some suspended structures, resulting in a waste of time and materials.
In contrast, spray coating simplifies the underfill process. Since it already requires the use of low-viscosity materials, spray coating can easily place sufficient material under suspended structures in a single pass. Accordingly, the amount of material required to coat a substrate fully is 70% less than that used in a single spin coat. In addition, the spin-coating process tends to limit the planarity of the top surface of the film (above the suspended structure), while the spray-coating method results in a highly planar surface, as shown in the images in Figure 8. Conclusion Although
spray coating was originally developed to perform lithography
processes on MEMS devices with challenging topographies, it can
be used in applications spanning many fields. For example, it
can be useful in the advanced packaging area because it results
in materials savings. It can be used to apply a protective coating
on potentially fragile wafer bumps in processes such as back grinding
or dicing. Because it can perform the underfill process efficiently,
it is appropriate for compound semiconductor and power-device
applications. Spray coating's versa- References 1. N Pham, P Sarro, and J Burghartz, "IC-Compatible Process for Pattern Transfer in Deep Wells for Integration of RF Components," in Proceedings of the SPIE, vol. 4174, Micromachining and Microfabrication Process Technology VI (Bellingham, WA: SPIE, 2000), 390–397. 2. B Wieder et al., "Spray Coating for NEMS, MEMS, and Microsystems" (paper presented at the Pacific Rim Workshop on Transducers and Micro/Nano Technologies, Xiamen, China, July 22–24, 2002). 3. T Luxbacher and A Mirza, "Spray Coating for MEMS and Advanced Packaging," HDI Magazine no. 5 (1999): 36–41. 4. N Pham et al., "Direct Spray Coating of Photoresist for MEMS Applications," in Proceedings of the SPIE, vol. 4557, Micromachining and Microfabrication Process Technology VII (Bellingham, WA: SPIE, 2001), 312–319. 5. S Linder et al., "Fabrication Technology for Wafer Through-Hole Interconnections and Three-Dimensional Stacks of Chips and Wafers," in Proceedings of the IEEE, Micro Electro Mechanical Systems (Piscataway, NJ: IEEE, 1994), 349–354. 6. G Carcano, M Ceriani, and F Soglio, "Spin Coating with High Viscosity Photoresist on Square Substrates," Hybrid Circuits, vol. 32 (1993): 12 7. B Wieder et al., "Spray Coating: New Coating Method on MEMS with Metal Surfaces" (paper presented at Sensors Expo, Boston, September 23–26, 2002). 8. J Park and M Allen, "Packaging Compatible High Q Microinductors and Microfilters for Wireless Applications," IEEE Transactions on Advanced Packaging 22, no. 2 (1999): 207–213. Chad Brubaker is a process engineer specializing in lithographic process development at EV Group (Phoenix). He has been with the company since 2000 and has been instrumental in the development of thick resist and spray-coating processes. He received a BS in chemical engineering from the University of Arizona in Tucson. (Brubaker can be reached at 602/437-9492, ext. 119, or c.brubaker@evgroup.com.) Markus Wimplinger is EV Group's director of technology for North America. In 2001 he began working as project manager at the company's group headquarters in Schärding, Austria. Previously, Wimplinger was involved in design, development, and many other aspects of capital equipment production both at EV Group and other companies. He received an electrical engineering degree in 1997 from the Höhere Technische Lehranstalt in Braunau, Austria. (Wimplinger can be reached at 602/437-9492, ext. 118, or m.wimplinger@evgroup.com.) Paul Lindner is CTO at EV Group headquarters in Schärding, Austria. He began working as a CAD designer for various semiconductor processing systems and tooling systems used for custom applications. With more than 10 years of experience in the semiconductor industry, Lindner has been involved in design, development, process technology management, customer support, and other IC equipment manufacturing areas. He received a mechanical engineering degree from the Höhere Technische Lehranstalt in Wels, Austria. (Lindner can be reached at +43 7712 5311 or p.lindner@evgroup.com.) Stefan Pargfrieder began working at EV Group's headquarters in Schärding, Austria, as technology manager in 2002. He is active in the areas of lithography, bonding, nanoimprinting, and hot embossing. He received a degree in technical physics from the University of Linz, Austria, receiving a master's degree in collaboration with Infineon Technologies in the field of semiconductor metrology. (Pargfrieder can be reached at +43 7712 5311 or s.pargfrieder@evgroup.com.)
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Fabricating
the many different varieties of microelectromechanical systems
(MEMS) poses complex processing challenges, particularly in the
lithography area. Since MEMS devices rely on mechanical elements,
they incorporate three-dimensional microstructures.
