Spectroscopic ellipsometry applications in photomask technology

Development and manufacture of advanced photomasks requires understanding the optical properties of mask materials and the ability to carefully monitor and control the film thickness, refractive index, and absorption of each film layer. Spectroscopic Ellipsometry (SE), a nondestructive optical analysis technique for determination of film thickness, refractive index, absorption, and film microstructure has been applied to many applications in current and emerging areas of photomask technology. This work surveys a variety of applications including analysis of fused silica mask blanks, imprint lithography templates, free-standing pellicles, deposited films of chromium, chromium oxide, MoSi, and multiple layers of these materials. For fused silica substrates we determine the optical properties (n & k) versus wavelength as well as the surface roughness. The near-surface index and surface roughness are used as indicators of surface polishing quality, which is an issue in the use of photomask substrates as template stamps in imprint lithography. Pellicles are free-standing films stretched over a metal frame. Because these thin pellicles are much thinner than the light coherence length, spectroscopic ellipsometry can determine the thickness, refractive index, and absorbance of the pellicle versus wavelength via interference between light reflecting from the upper and lower surfaces of the film. It is also possible to measure pellicles suspended over an underlying mask. The optical constants (n & k) of MoSi films vary widely with deposition conditions due to both composition and microstructure. Films optimized for 248 nm and 193 nm wavelengths have been analyzed. Spectroscopic ellipsometry shows good sensitivity to surface roughness and index gradients through the MoSi films. Multilayer masks such as NTAR have multiple absorbing layers such as chromium and chrome oxide. It is possible to simultaneously analyze multiple data sets acquired from the front side, from the back side through the substrate, and transmitted data through the stack. This increases the information content in the data, allowing more details from the multilayer to be determined. It is also possible to measure the birefringence in mask substrates and coated masks by using ellipsometry in transmission mode. By establishing a known polarization state incident on the mask it is possible to analyze the polarization after transmission to determine the phase difference, which is used to measure the birefringence of the sample. For patterned structures such as gratings it is possible to measure polarization-dependent diffraction effects versus angle. Measurement can be made in either reflection or transmission mode. It is possible to independently measure both s- and p-polarization components and compare to theory. As photomasks become more complex, with increasing numbers of layers (such as multilayer mirrors for EUV masks) in-situ ellipsometry shows great promise, providing the ultimate characterization of each film by allowing measurement in real-time as each layer is added to the mask structure.