Surface Roughness Measurement Using Lasers
in collaboration with Coherix, inc.
Why does it matter? The measurement of surface roughness and other statistical properties of surfaces are important for a wide range of applications in various industries. For example, in the semiconductor industry, the reliability and stability of thin film components depend in large part on the smoothness of substrates on which they are deposited [1]. In the automotive industry, the surface roughness plays an important role in controlling friction and influences the performance and fuel economy of engines[2,3]. Current technologies use a either a contact based profilometer or a white light interferometer for roughness measurements. However, these techniques often require samples to be taken offline and is either contact based or has a very small stand-off distance.
What is our solution? We utilize the spectral dependence of reflected light to develop a surface roughness measurement system using our near infrared supercontinuum (SC) laser ( ~1-2.5 microns; ~20 mW average power)[4]. The theory is based on the Beckmann Kirchhoff scattering model, which provides the relationship between the wavelength of light incident on a surface and surface roughness, which predicts that, as the wavelength of the incident light increases, the same surface begins to look smoother [5]. Thus, if we were to plot the reflected light intensity from a surface against the wavelength of the incident light, we can expect to see a unique signature, from which the surface roughness parameter can be accurately extracted. Our current system is capable of measuring an RMS roughness range of ~0.05-0.35 microns at ~ 1 m stand-off distances. An additional advantage of performing measurements in the infrared, especially for metallic samples is that we don’t need a reference of the same material as the sample. This is possible because, in the infrared, unlike the visible wavelengths, most metals have fairly flat reflectance signatures. Thus, the normalized reflectance signature in the infrared is determined almost entirely by the surface roughness features alone and is not influenced much by the metal properties itself.
[1] R. M. Anderson, and G.W. Neudeck, Flatness and surface roughness of some common thin film substrate materials. J Vac Sci Technol 8 (1971) 454-457.
[2] S. Adachi, K. Horio, Y. Nakamura, K. Nakano, and A. Tanke, Development of Toyota 1ZZ-FE Engine. SAE Technical papers 981087 (1998).
[3] S. Tung, and M.L. McMillan, Automotive tribology overview of current advances and challenges for the future. Tribology Int 37 (2004) 517-536.
[4] V.V. Alexander, H. Deng, M.N. Islam, J.F.L. Terry, R.B. Pittman, and T. Valen, Surface roughness measurement of flat and curved machined metal parts using a near infrared super-continuum laser. Opt. Eng 50 (2011).
[5] P. Beckmann, and A. Spizzichino, The scattering of electromagnetic waves from rough surfaces, Pergamon, New York 1963.