Two grating mask optical schemes were investigated corresponding to Figs. Patterning with Lloyd’s mirror configuration For mask 2, the difference between the 1st-order and 0th-order intensities was about 8%, with only 11% reflected. For the present experiments, a 0-order power of ∼8% was achieved.
32 For mask 1, an ideal phase grating would have no power in the transmitted 0 order and ∼41% of the power in each of the ☑ diffracted orders. The etch depth for mask 2 was estimated before fabrication using simulation done in Lumerical™ FDTD. The required etching depth for minimizing the 0-order intensity for mask 1 and equalizing the 0th and −1st transmission intensities for mask 2 was obtained empirically after a few attempts by iteratively checking the optical performance and changing the etching time. Following the SOG etch, the photoresist pattern was removed using an acetone spray gun and the remaining layer of BARC was etched away with an oxygen plasma. The photoresist patterned glass substrate was then etched using CF 4 to transfer the grating pattern into the SOG layer, at a gas flow rate of 21 sscm, a pressure of 15 mTorr, an RF power of 100 W, and an ICP power of 300 W. Gas flow rate, process pressure, RF power, and inductively coupled plasma (ICP) power were 30 sscm, 15 mTorr, 25 W, and 300 W, respectively. An oxygen plasma was used to etch the exposed BARC layer between the PR lines. The substrate was further hard-baked after the development at 100 ☌ for 60 s in order to improve the mechanical stability of the photoresist pattern prior to etching. The exposed PR-BARC-substrate was baked and developed as described above. The next step was to make use of IL in Lloyd’s mirror arrangement with a single mode, frequency-tripled YAG laser source 1 (Coherent Infinity 40-100) operating at 355 nm to make a one-dimensional pattern of period 600 nm on the photoresist coated substrate. The soft bake temperature and time were 150 ☌ and 180 s, respectively. This was followed by spin coating of a layer of I-line negative photoresist (NR7-500™) to a thickness of about 500 nm. The sample was hotplate baked at a temperature of 190 ☌ for 180 s. EXPERIMENTĪ 150-nm thick layer of back-anti-reflection-coating (BARC) ICON-16™ was applied by spin coating atop the SOG layer. This is a common mode region insensitive to vibrations, but, of course, differential variations in the refractive properties of the air or any other immersion medium across the 1st-order diffracted beams have to be minimized. 1(c), the spacing between the first and second grating masks serves to allow blocking of any 0-order transmission of the first grating mask. This makes this configuration particularly suitable for large-area manufacturing operations. Importantly, in this configuration, the gap between the beam splitter, the second grating mask and the sample, the region where the two interferometric beams propagate independently, can be very small (<1 mm), significantly reducing the sensitivity to vibrations and air currents. The width of the in-phase regions is X Λ Lloyd ( σ d 2 cos θ / 2 Δ λ ) ] − 1 ]. The gratings from each longitudinal mode are in phase and reinforce each other only in the narrow bands indicated by dark vertical lines in the figure. Each mode produces a grating pattern at a period X k = λ k/(2sin θ 0), resulting in a moiré pattern I ( x, z ) = ∑ k ( A k / 2 ) , where A k is the intensity in the kth mode. Each longitudinal mode interferes only with its counterpart from the opposite direction since only pairs at the same frequency are time stationary, ⟨ e i ω k t e − i ω j t ⟩ = δ k, j. 1 The angles are all fixed at θ 0 by the geometry. Figure 1(a) is Lloyd’s mirror arrangement often used with single-frequency lasers.
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