Luke J. Bissell and Dr. David Allred, Physics and Astronomy
Introduction
Computers are getting faster, and as they do, computer chips are getting smaller. Photolithography is a process used to design nanoscale patterns on a computer chip. In photolithography, the image of a circuit pattern is transferred from a mask to the semiconductor wafer. This is accomplished by exposing the mask to light and then reflecting the mask’s image through a series of image-reducing multilayer mirrors. These mirrors focus the reduced circuit pattern on to the semiconductor wafer. The wavelength of light incident on the mask determines the size of the resolvable features on the wafer. It has been predicted that by 2006, the dimension of transistors on commercial computer chips will be ~ 100 nm. Light with a wavelength of 11-13 nm is necessary to obtain this resolution [1].
Light in this wavelength regime (10-100 nm) is a part of the extreme ultraviolet (EUV) region of the electromagnetic spectrum [2]. In order for photolithography to work well, the mirrors used to transfer the circuit pattern from the mask to the wafer must reflect well at EUV wavelengths. The current industry goal is to have mirrors which reflect at least 70% of the light at a wavelength of 13.5 nm. [2]
The thin film optics research in the BYU Physics department has been investigating designs for high reflectance mirrors in the EUV for several years. Many materials have high absorption coefficients and low indices of refraction, so it is difficult to get high reflectances from a single surface. That is why multilayer mirror designs are used: light reflected from each surface in the mirror will combine in phase to increase total reflectance, provided the thicknesses of the layers are optimized for the wavelength of the incident light [3].
The standard mirrors being used by industrial researchers are silicon/molybdenum multilayers. The problem with these mirrors is that both silicon and molybdenum both oxidize over a period of days, cutting the reflectance of the mirror by as much as 5%. Several suggestions have been made to reduce this reflectance loss. One proposal has been to use a ruthenium capping layer on the top surface of the mirror, because ruthenium resists oxidation [2]. The research project, therefore, is to experimentally determine ruthenium’s optical constants—the index of refraction (n) and absorption coefficient (k)—in an effort to model the affect of ruthenium on a silicon/molybdenum multilayer. Methodology
The project can be divided into 4 parts:
· fabrication of ruthenium films,
· measurement of the films’ reflectance in the EUV,
· characterization of film properties such as roughness and thickness,
· calculation of n and k from the known reflectance and film parameters.
We chose to measure four different ruthenium/silicon multilayers which had already been fabricated by another student. Because silicon’s optical constants are well known, the desired ruthenium constants n and k can be extracted from the ruthenium/silicon reflectance data. The reflectance measurements were made at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The ALS houses a synchrotron generator, a machine capable of producing radiation of all wavelengths. We made reflectance measurements of the ruthenium/silicon multilayers in the range 11-14 nm. This wavelength region was chosen because it is the spectral range that current industrial EUV light sources are capable of. Characterization of the multilayers’ thickness and surface roughness is still being performed. This involves using X-ray Diffraction (XRD) analysis of the multilayers’ thicknesses, and Atomic Force Microscopy (AFM) measurements of the surface roughness.
Once these parameters have been determined, a model can be made in a computer program, IMD. With a correct model of the multilayer and the reflectance data, IMD can compute the optical constants of ruthenium (Figure 1). Once these constants have been calculated, we hope to be able to model the affect of a ruthenium capping layer on a molybdenum/silicon multilayer.
References
- David Atwood, Soft X-Rays and Extreme Ultraviolet Radiation (Cambridge, New York, 2000) pp. 111-112
- S. Bajt, J.B. Alameda, T.W. Barbee Jr., W.M. Clift, J.A. Folta, B. Kaufmann, E.A. Spiller, “Improved Reflectance and Stability of Mo-Si multilayers,” Opt. Eng. 41, 1797-1804 (2002).
- Shannon Lunt, M.S. Thesis, Brigham Young University, 2002. pp. 1,4