Micro-contamination has become a serious problem in the semiconductor-manufacturing process, as well as in other areas of high-precision optics. It has already had a significant impact on the cost of ownership and life expectancy of photomasks and optical elements for advanced lithography. The problem is particularly severe when the optical elements are exposed to high-energy radiation. The most commonly recognized form of micro-contamination on photomasks is often referred to as ‘haze.’ First reported in 1999, haze has become a ubiquitous problem in the semiconductor industry.1 Figure 1 shows two types of commonly reported haze or crystal growth.
Figure 1. Transmission (a) and reflected (b) images of haze and crystal growth on a photomask. The vertical rectangles are the chromium images. Note that the haze occurs on the chromium surface.
Crystal growth can be introduced onto the surface of an optical element—such as a lens, mirror, beam splitter, or photomask—by three possible methods or mechanisms. In one case, the contaminants result from the processes that the optical element has been exposed to during its manufacture or cleaning. Alternatively, it can occur simply due to the nonreactive deposition of environmental contaminants. Finally, the micro-contaminants could be deposited by photochemical reaction(s) of surface contaminants that are initiated by exposure to high-energy radiation. Regardless of the contaminant type, the result can significantly reduce or change the transmissive or reflective properties of an optical element. In many cases, this results in catastrophic failure.
The most commonly detected contaminants are ammonium sulfate, ammonium oxalate, cyanuric acid, hydrocarbons, and organo-silicon compounds.2 Figure 2 shows the typical levels of contaminants often found on photomasks and optical elements. We obtained these results using time-of-flight secondary-ion mass spectroscopy (ToF-SIMS).
Figure 2. Typical levels of surface contaminants found on most photomasks. NH4 is ammonium ion (NH4), silicones are organo-silicon compounds, and CxHy represents hydrocarbon contamination. Intensity counts under 10,000 are considered low concentrations for these contaminants.
While ammonium and sulfate ions have a significant effect on the performance of a transmissive photomask, a greater concern for extreme UV (EUV) reflective masks are the concentrations of organo-silicon and hydrocarbon compounds. We analyzed the effects of contaminants on the transmission and reflection of an optical element (see Figure 3). The transmission and reflectivity measurements were made through the quartz area and on the metal surface of a photomask, respectively. The loss in reflectivity shown in Figure 3 would have a significant effect on the performance of an EUV mask.
Figure 3. Effects of high concentrations of surface contaminants on transmission and reflectivity of an optical element. These effects vary depending on the type of optical element.
Our results indicate that haze and crystal growth have a significant impact on optical lithography. Analytical results obtained to date indicate that the type of surface contaminant found on photomasks and optical elements will most likely have a deleterious effect on EUV masks.
We continue to evaluate surfaces and subsurfaces of photomasks and other optical elements in an attempt to understand the effects of radiation on surface chemistry and the stability of optical coatings and mask films.
Brian Grenon is a chemist and consultant in the semiconductor industry who focuses on issues involving photomasks, advanced lithography, and micro-contamination on optical surfaces.