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.