Post on 20-Dec-2015
Large Multilayer Diffraction Gratings: Coating UniformitySenior Student: Erik Krous
Project Advisor: Dr. Carmen MenoniCollaborators: Dr. D. Patel, Dr. J.J. Rocca, J. Jensen and K.J. HsiaoAbstract
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 HfO2 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 mm2
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 HfO2 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 compressorA 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 mm2
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)2O5 and SiO2. 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.