As we enter the deep sub-wavelength lithography regime, using the 193 nm wavelength for the 130 nm node and beyond, the often overlooked mask material properties are beginning to have an impact on imaging performance. By analyzing properties such as index of refraction homogeneity and stress birefringence it was found that a wide variation exists within the available population of mask materials. The investigation of these materials demonstrated the considerable range in performance that can be achieved from the population of mask materials. We will show that due diligence in the selection of the mask material will provide the ability to image multiple features with a reduction in Mask Error Enhancement Factor and improved critical dimension uniformity on the wafer.
As optical lithographers push to extend optical lithography technologies to create smaller features with higher NA, lower k1 values and shorter wavelengths, transmitted wavefront specifications for HPFSR fused silica blanks continue to tighten. HPFSR fused silica blanks are typically certified for acceptance using an interferometer operating at a wavelength of 632.8 nm. As the market demands increasingly tighter homogeneity specifications, it has become critical to understand the sources of variation in wavefront measurements. Corning has recently initiated a study to identify those sources of variation. One glass attribute being studied is the impact of residual stress on the wavefront. It is known that residual stresses can alter the refractive index of fused silica. To obtain the residual stress measurements, birefringence measurements were obtained at 632.8 nm for comparison to wavefront measurements at 632.8 nm. The relationship between residual birefringence and transmitted wavefront measurements, at 632.8 nm on Corning HPFSR fused silica blanks, is explored in this paper.
Mask materials are the often-overlooked link in the lithography chain. Chipmakers in general, have no idea of the optical properties of the mask substrate material that is being used to build his critical layer mask. By analyzing properties such as stress birefringence, transmission uniformity, index of refraction homogeneity, and certain laser damage properties it was found that a wide variation exists within the available population of mask substrate materials. Testing the impact of materials on a lithography tool demonstrated the wide range of performance that can be obtained from a population of mask substrates. For example, dose to clear tests gave results that ranged from 3.15mJ/cm2 to 3.25mJ/cm2 with dose non-uniformity ranging from 2.00% to 5.25%. This paper will discuss the tests that were made and the results to date. Due diligence on the selection of material substrates is required.
A shift to shorter wavelength radiation sources along with improvements in the quality of lithography grade optical materials has driven the production of integrated circuits with smaller feature sizes. Optical characterization of these materials, in some cases, is performed at visible wavelengths due to the complexity level associated with measuring in the DUV. Birefringence measurements of stepper lens blanks, for example, are typically measured at 633nm using a Helium Neon laser source. However, knowledge of the correlating DUV birefringence values is needed for determining the acceptable magnitude of birefringence in the material and for predicting the magnitude of loss in CD contrast. In this paper we report results on how the birefringence in Corning's HPFSR synthetic silica glass changes at the DUV wavelengths used in lithography systems. An examination of the wavelength dependent stress-optic response that produces birefringence was performed and found to increase from 633nm to 193nm. Birefringence in lithography stepper lens elements degrades imaging performance so an understanding of its dispersion is important for system designs.
To further extend the lifetime of optical lithography, stronger requirements are placed on the optical materials used in stepper systems. For example, photomask requirements from mask makers and users include good transmission uniformity, low induced absorption, low defect density and low thermal expansion. However little emphasis and attention has been placed on the magnitude of birefringence. In this paper we present results on the dispersion of birefringence in fused silica photomask substrates measured from the visible to DUV wavelengths. An examination of the wavelength dependent stress-optic response that produces birefringence is performed and found to increase from 633 nm to the DUV. Measurements were performed on substrates with levels of birefringence that varied by an order of magnitude. Birefringence in photomasks results in a change in the polarization of an optical beam and for some lithography tools this can influence system performance and degrade image quality. Therefore, we believe an emphasis on low birefringence photomask substrates can possibly improve system performance.
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