Modern fabrication processes frequently require depositions of layers of one or more materials onto a receptor. For example, most semiconductor fabrication processes require that a resist (such as photoresist or electron resist) be deposited on a substrate. Then by using photolithographic techniques, selected areas of the resist layer can be removed and the underlying substrate can be subjected to subsequent processing (such as metal film deposition) to form complex semiconductor devices.
While semiconductor fabrication is an important fabrication process which requires the deposition of material layers, it is certainly not the only one. Other examples include the deposition of molten solder onto printed circuit boards, the deposition of catalysts, such as those used in the production of flex cables, and the deposition of materials, such as hot melt waxes, to create three dimensional structures.
While some fabrication processes, such as creating three dimensional structures, require depositions of materials over controlled areas of a receptor, in other processes, such as semiconductor fabrication, the deposited layer can simply be uniformly formed over the entire surface of the receptor. While forming a uniform layer over the entire surface of a receptor is usually much simpler than depositing a layer over selected areas of a receptor, even the simpler case has its problems. The depositing of resist onto a substrate will illustrate some of those problems.
Resist is usually applied to a substrate using a process called spin coating. A typical spin coating process begins with the placement of a relatively thick layer of liquid resist over the substrate. This may be accomplished by pouring liquid resist on the center of a slowly rotating substrate. As the substrate rotates, the resist spreads over the substrate. After the substrate is fully coated with the resist, the substrate is quickly accelerated, causing excess resist to flow over the substrate toward its edges. Finally, the substrate is rotated at high speed so that droplets of the excess resist are spun off of the substrate, leaving only a thin, even layer of resist. After the resist's solvent sufficiently evaporates, the resist is solid and the spinning stops.
While spin coating is a highly developed process, it has the problem of being wasteful of resist since most of the resist is spun off the substrate. Another problem is the great care that must be taken to prevent the large droplets which are spun off of the substrate from returning to the substrate and disturbing the existing resist layer. A third problem is a result of the chemicals which comprise modern resists. Many of those chemicals are toxic and hazardous to both the health of workers and to the environment. Thus, the less resist used, the better. Additionally, spin coating cannot easily apply layers in selected locations of the substrate. This is a significant drawback since multilevel depositions (which are three dimensional structures)are not easily formed, if at all, using spin coating.
In other applications in which a material is deposited over a wide area, such as in some color filter fabrication techniques, the deposited material is expensive. Techniques which reduce the amount of material that is deposited are beneficial.
As previously mentioned, the difficulty of depositing material layers over selected areas of a receptor are greater then when simply fully coating a surface. For example, one cannot easily use spin coating. Furthermore, once a particular technique of depositing a layer over selected areas of a receptor is developed, any change in the area being covered may be difficult to implement.
Therefore, a new method of depositing one or more material layers onto a receptor, particularly a method capable of saving material and which permits the deposition of material only on selected areas of the receptor, would be beneficial.