Large Multilayer Diffraction Gratings: Coating Uniformity Senior Student: Erik Krous Project Advisor: Dr. Carmen Menoni Collaborators: Dr. D. Patel, Dr. J.J. Rocca, J. Jensen and K.J. Hsiao Abstract Chirped-pulse amplified (CPA) laser systems produce high-powered, short optical pulses and are being used in the development of Extreme Ultraviolet (EUV) science and technology here at the National Science Foundation Engineering Research Center (ERC). The output power of this type of laser is currently limited by gold-coated diffraction gratings used in the pulse stretcher/compressor stage of the laser system. These limitations motivate the development of large-area multilayer oxide diffraction gratings, which will allow for greater power extraction from the chirped-pulse amplified laser system. The multilayer gratings are of an area larger than the film deposition system (an ion beam deposition system) is designed for. This requires alterations to the deposition chamber to assure deposition uniformity. New methods of characterizing such large-area films are also required. Improving the CPA system will allow for continuing advancement in EUV science and technology. Ion Beam Deposition of Multilayer Oxides  The deposition process occurs in vacuo  ~99.9% metal material is housed in a rotating assembly  High energy ions impact and release target material  The released metal combines with injected oxygen  The resulting oxide deposits on a substrate  The substrate holder rotates as fast as 600 rpm  A shaped shadow mask controls the horizontal deposition uniformity  The oxide diffraction gratings will have to be 4” x 9” in area  This area is much larger than the Spector is designed to coat  The gratings’ optical properties must be uniform over the entire area  This requires uniform coating thickness over entire area  The grating uniformity must be characterized  This is done optically  Non-uniformities must be corrected  This is done, primarily, with shadow masks Project Challenges Veeco Spector® Ion Beam Deposition System Three Sided Target Ass'y (35 cm dia. Target) 107 cm dia. Chamber Shutter Door Cryo Pump Port Plenum High Speed Fixture with 30 cm dia. Substrate Holder Target Gas (oxygen) Optical Monitor Beam 16 cm RF Ion Beam Source (Deposition) 12 cm RF Ion Beam Source (Clean/Assist) Shadow Mask Substrate Optical Uniformity Measurements  First thickness measurements were made with profilometry  Optical thickness measurements gave more consistent measurements  Original optical measurements made on single HfO 2 layer  Transmission spectra can be fit using Essential MacLeod© software  Assume a substrate material and film material  Through iterative thickness modulations, the software fits spectra  The Transmission spectra originally obtained via spectrophotometry  The original spectrophotometer has disadvantages  Not enough spatial resolution  The optical beam area is ~10 mm 2  A 4” x 9” substrate will not fit in the measurement chamber  The attempted solution:  Build a spectrophotometer with an optical spectrum analyzer A transmission spectrum of a 20-layer multilayer design with 1% thickness variations, in all layers, in the two curves. MacLeod fit (black) to a measured HfO 2 spectrum (red). Optical Spectrum Analyzer (OSA) Chirped-Pulse Amplified Laser Systems  CPA systems have four stages  Short pulse oscillator (i.e. a Ti:Sapphire oscillator)  A pulse stretcher  An amplification stage  A pulse compressor A chirped-pulse amplified laser system schematic.  Currently, gold coated polymer gratings are used in the stretcher/compressor  The gold gratings limit the power extraction from the CPA system  Metallic films generally have a low laser induced damaged threshold (LIDT)  Oxide films have much higher LIDTs  Etched multilayer oxide structures can form highly reflective gratings with high LIDTs  An OSA measures optical power as a function of wavelength  The OSA used here is an Advantest Q8384  Wavelength range: 600 nm to 1700 nm  50 dB dynamic range  Up to 10 pm spectral resolution  A spectrophotometer can be made by measuring normalized power transmission, of a white light source, through a sample Spectrophotometer  This system has two advantages over the previous spectrophotometry device:  Spatial resolution ~1 mm 2  A 4” x 9” x 2” sample can be accommodated Photograph and schematic of the OSA spectrophotometer system. Transmission spectrum of a 20-layer stack of alternating (NbTa) 2 O 5 and SiO 2. The two curves are the design and the spectrum measured with the OSA spectrophotometer.  Single-layer transmission spectra do not have features to fit at λ > 800 nm  Multilayer “spike” designs will be used to calculate thickness uniformity  Based on the shift in the peaks (spikes) the local thickness is determined  This assumes a linear relationship between film thickness and peak position 952 nm “spike” design. Based on the shift in the 952 nm peak of the measured sample, the sample film thickness can be determined.