To: Dhaval Shah Per your request, below is a description of various coating methods currently used in microelectronic fabrication. If you would prefer to receive this text in a file format such as Word, please let me know. I received a considerable number of inquiries for information regarding alternative coating methods, so I have written a general summary shown below. Should you require any further information, such as names of specific equipment vendors or assistance running coating evaluations, please feel free to contact me again. I am an independent consultant in the field of microelectronics processing and work with a wide range of applications including semiconductors, multichip modules, flat panel displays and printed circuit boards. Best Regards, Susan Bagen, P.E. Advanced Process Concepts (972) 527-5028 Phone (972) 517-7928 Fax SUMMARY OF COATING METHODS A question was posted on the Internet regarding a precision coating method that would yield good coverage into deep features. The following is a general response to this issue. Adequate flow of a coating material into high aspect ratio features is dependent on several different factors: 1) Coating method. 2) Surface cleanliness and preparation - relates to wetting and flow of the coating solution. 3) Coating solution: viscosity, solvent system, wetting, surface tension (leveling agents). COATING METHODS: The coating methods considered here are typically capable of achieving high degrees of uniformity (+/- 2-5%) on flat, planar surfaces. Depending on the method used, coatings over deep topographical features can often be more planarized than conformal due to the flow of material into deep trenches. Thus, the total thickness variation in the film can be greater than that achieved on smooth surfaces. Below is a discussion of some of the current coating techniques used in the microelectronics industry. 1) Spin Coating: Spin coating can be used successfully for coating high degrees of topography if programmed appropriately. In a standard spin coat process, the resist material is puddled onto the center of the substrate, then spun at a high rpm to spread it over the substrate surface. When the substrate has deep topographical features, several factors work against a continuous coating. First, the high rpm promotes rapid drying of the resist solution, therefore, not allowing flow of the material into deep trenches. Secondly, the features themselves cause a physical obstruction to the solution flow, preventing incomplete coverage and often causing striations. Variations on the spin coating technique, however, can help to overcome these issues. A closed-bowl configuration and/or programming slow acceleration and spin speeds help to reduce the evaporation rate of the solvent in the coating solution. This allows time for the solution to flow and spread prior to drying which sets the film. Variations on the dispense method to first deposit the solution over the entire substrate prior to spinning also greatly helps. This technique also allows for fluid flow prior to drying the film. Programming the dispense arm to travel the radius of the substrate while the substrate is slowly rotated is one way to achieve this. A pause after the dispense step allows additional time for the solution to flow into the deep features. The subsequent spin step then achieves the desired thickness uniformity and promotes drying of the film to set it in place. 2) Spray Coating: Some spray coating systems are capable of producing highly uniform coatings of thicknesses ranging from less than 1000 Angstroms to greater than 100 microns. In the spray coating process, there is direct perpendicular impingement of the coating solution that promotes coverage into deep trenches. For thicker films, the solutions used in spray coating are often diluted as compared to solutions used to achieve a similar spin-coated film thickness. A reciprocating spray nozzle coats the substrate in multiple passes to buildup the total desired film thickness. This lower solution viscosity may facilitate fluid flow into deep features. In addition, the degree of atomization during spray is set to control how "wet" the coating is deposited onto the substrate, thus controlling the subsequent flow of the solution once deposited and prior to bake. 3) Meniscus Coating: In this process, a substrate is inverted and passed over a laminar flow of coating material. The result is highly uniform coatings, even on substrates with relatively poor flatness. The degree of coverage into deep topographical features is likely dependent on surface wettability. This aspect of the coating process has yet to be thoroughly evaluated. 4) Roller, Curtain and Extrusion Coating: These are all variations of directly applying the coating solution across the topside of the substrate. There is no forced drying during coating other than evaporation, therefore, the coating material has time to flow and planarize over surface features. The degree of coverage into deep features is highly dependent on the surface wettability and the solution viscosity. 5) Plasma-Deposited Photoresist: Ionic Systems, Inc. manufacturers a unique plasma- deposited photoresist system. This system is only capable of depositing relatively thin coatings (< 0.5 microns), but the coatings are very conformal over topography. Depending on the intended use of the resist, the thinner coatings may be more than adequate. E.g., the photoresist material used in this process has a very high selectivity (> 300:1) in chlorinated etch chemistries. 6) Electrophoretic (electrodeposited) Photoresist: This type of photoresist has typically only been used in the printed circuit board industry. Both positive and negative resist chemistries are available. Typical coating thicknesses are in the range of 5 - 10 microns, but specific resist systems can be deposited up to about 35 microns. Electrophoretic resist films tend to be conformal over features. The coating process performance over very high aspect ratio features, however, has not been fully characterized. SURFACE CLEANLINESS AND PREPARATION Regardless of which coating method is used, the wettability of the coating solution to the substrate surface is key to the flow of the coating solution into deep topography. The surface must be free of contaminants that would inhibit wetting and adhesion of the coating material. The type of contaminants present are dependent on the prior processing of the substrate. E.g., substrates which have previously undergone plasma processing with fluorinated gases are prone to fluorine-containing contaminants on the processed surface. These contaminants can form from reactions with photoresists or other organic materials, and can pose significant problems with wetting and adhesion of subsequent layers. Adequate stripping/cleaning operations (such as solvent, acid or oxygen plasma treatments) prior to subsequent coatings are essential. In addition, dehydration bake operations to remove surface moisture can greatly improve adhesion and surface wettability. Adhesion promoter solutions should be used where appropriate. PHOTORESIST COMPOSITION There are often options available regarding a photoresist's solvent system, viscosity, and additives (surfactants, leveling agents, etc.). Discuss this with your photoresist manufacturer and ask them to supply you with several different formulations to try. Often the addition or omission of an additive, or a change in the primary solvent carrier can greatly affect the surface tension and wettability of a photoresist solution. If you are currently working with a very high solids content, high viscosity solution to achieve thick coatings, you may want to consider using a diluted solution and accomplish the total thickness through multiple coats. The lower viscosity may flow better into high aspect ratio features, and is also less susceptible to entrapping air bubbles which may cause voids.