Nanoindentation probing of high-aspect ratio pillar structures on optical multilayer dielectric diffraction gratings
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Nanoindentation probing of high-aspect ratio pillar structures on optical multilayer dielectric diffraction gratings K. Mehrotra, 1,3,a) H.P. Howard, 2,3 S.D. Jacobs, 2,3 and J.C. Lambropoulos 1,2,3 1
Department of Mechanical Engineering, Materials Science Program and, 3 Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627 a Electronic mail: [email protected] 2
ABSTRACT We measure the mechanical response of optical multilayer dielectric (MLD) diffraction gratings, geometries which are constrained in only one transverse direction but free in the other, using nanoindentation. The results are explained using a stress-strain model, which reveals a uniaxial yield stress of 4.1- 4.6 GPa and predicts a similar dependence of yield stress on loads for both fully-elastic and fully-plastic solutions. Following R. Hill’s model of an expanding cavity under internal pressure, we show that the indentation response of the high-aspect ratio “pillar” geometry can be expressed in terms of uniaxial yield stress rather than material hardness.
INTRODUCTION As work in inertial confinement fusion (ICF) and the fast ignition concept have expanded and evolved over the past few decades, the ultra-short pulse, high power laser systems that support ICF research have placed stringent requirements on the optical components. At the Laboratory for Laser Energetics (LLE), the peak power capability – and thus the overall performance of the petawatt-class OMEGA EP laser system is limited by the laser damage resistance of diffraction gratings in the chirped-pulse amplification (CPA) 1 pulse compressors for each beamline. Increasing the damage thresholds of these components is therefore an important objective. The multilayer dielectric (MLD) gratings used in OMEGA EP’s pulse compressors are surface relief gratings, composed of an MLD mirror with a periodically grooved top diffraction layer. The MLD high reflector is a modified quarter-wave stack 2 of alternating low and high refractive index layers on a glass substrate, typically hafnia (HfO2) and silica (SiO2) coated onto BK7 glass. The grating is patterned by small-beam interference lithography (SBIL) 3 and etched into the top silica MLD layer at Plymouth Grating Laboratory (PGL) 4-5. During the final step, aggressive chemical cleaners such as acid piranha (mixture of H2O2 and H2SO4) are used to strip away residual photoresist, antireflective coating (ARC), and other debris from the grating surface. 5 There is some concern that this cleaning step mechanically weakens the fragile grating pillars, possibly affecting the grating’s optical performance as well as its resistance to laser damage. The development of a methodology for monitoring a grating’s mechanical properties could enable a better understanding of the fabrication and cleaning process, and point to appropriate modifications that will preserve the grating’s integrity.
MATERIALS & EXPERIMENTAL METHODS The surface pattern of the grating (Figure 1) consists of periodic parallel grating lines or “pillars” app
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