<title>Protecting the DUV process and optimizing optical transmission</title>

Abstract

It has been well documented that DUV lithographic processes are sensitive to airborne contamination such as ammonia and n-methyl-2-pyrrolidone (NMP). Chemical filtration technologies have aided in minimizing the problems associated with these contaminants in the photolithographic process. As the demand for smaller features increases, so will the need to operate within even cleaner environments than are available today. One such area where airborne contamination has proven to be of significant concern, is within the lensing system of the tool. With decreasing feature size, the lithographic process has proven to be more sensitive to contamination of the lens itself, and within the environment surrounding the lens.Condensation on the lens (hazing) and the presence of contamination between the lens and substrate can result in poor optical transmission. To minimize these problems, a purge gas is typically employed. Even though high purity gases are used, contamination within the gas still is an issue. This work describes our efforts directed at understanding the purge gas and lens environments. In addition, we will address our efforts that have focused on the development of chemical filters that provide environments for optimized optical transmission in lithographic applications.

New concerns with the design of filters for the protection of lithography optics

“…2,11,12,16 It has been reported that the photolithographic process and the optical elements in the exposure tool can be sensitive to contaminant levels below 1 ppb. [9][10][11][12]26 Therefore, the chemical filter media must be capable of reducing the contaminant concentration below this level at the conditions expected within each area of the tool. The chemical filter must be capable of maintaining these exceptionally low levels for extended periods of time; hopefully, this meets, or exceeds the life of the tool.…”

“…8 POU chemical filters must be capable of reducing molecular contamination in gas streams to levels (typically <1 ppb) that do not compromise the photolithographic process and do not degrade the exposure tool optical elements. [9][10][11][12] These levels must be achieved while still maintaining the required environmental specifications at each location within the tool. For example, within the wafer stage a low level of moisture, acidic, basic and organic contaminants is required in order to both protect optical coatings and allow maximum light transmission.…”

“…9,[10][11][12][16][17][18][19][20] Many of these studies have focused on the outgassing (including photo-induced outgassing) of organic contaminants from photoresists. 10,12,[16][17][18] These organic contaminants reduce system transmittance by partially absorbing the light energy at the specific radiation wavelength by being in the beam path or adsorbed to the lens surface (see inset Figure 1).…”

“…Other specific contaminants of importance, hexamethyldisiloxane (HMDSO) and trimethylsilanol, are effectively removed by activated carbon and modified activated carbon. 1,9,19 Basic and acidic contaminants are generally removed using the mechanism of chemical adsorption (chemisorption). This mechanism involves a strong interaction between the contaminant and specific active sites on the surface of the substrate.…”