High-fidelity diffractive surfaces have been generated with single-point diamond-turning techniques. A key to the success of this technique is the ability to shape the diamond tool tip to provide the optimum phase-relief profile, given manufacturing constraints. Replication technology is used to transfer the phase-relief surface into a thin epoxy or photopolymer layer on a glass substrate. Diffraction efficiency results for a wide range of zone widths are presented to provide the reader with a baseline of expected performance for replicated visible and near-infrared diffractive optical elements. In addition, a new method for analyzing diffractive surface structures is presented. The ray-trace algorithm quickly provides accurate results of predicted diffraction efficiency for arbitrary zone profiles, which is extremely valuable in predicting manufacturing errors.

Progress along the path towards smaller semiconductor feature sizes continually presents new challenges. 157nm technology is a promising new step along this path. The major challenges encountered to date include environmental purging for high transmission and beam alignment in a purged environment at this short wavelength. We present a simple shearing interferometer consisting of two Ronchi phase gratings in series, used on axis. The common path set-up and zero optical path difference between the interfering diffraction orders makes this device both robust and easy to align. Ease of alignment is an added benefit when working remotely in a purged environment with low light levels. If one grating is shifted relative to the other, a phase shift is introduced and phase measurement techniques can be employed for high accuracy characterization of the incident wavefront. Set-ups, measurements and characterization of wavefronts and spatial-coherence at 157nm made with this device are presented.

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Diffractive optics technology offers optical system designers new degrees of freedom that can be used to optimize the performance of optical systems. The zone spacing of a diffractive lens can be chosen to impart focusing power as well as aspheric correction to the emerging wavefront. The surface (or blaze) profile within a given zone determines the diffraction efficiency of the element, or in other words, determines how the incident energy is distributed among the various diffraction orders. Unless the zone profile is generated with high fidelity, incident energy will be distributed into extraneous diffraction orders, which generally reduces the optical system performance. Several diffractive optical components have been fabricated using replication techniques that provide high-efficiency and accurate wavefront generation. Typical minimum efficiency measurements at the design wavelength for diffractive zone spacings greater than 10 jim are 95% or above. For minimum zone features as small as 5 jim, the measured efficiency is greater than 85% at the design wavelength. The integrated diffraction efficiency, which is a weighted measure of the efficiency across the clear aperture, is typically 2-3% more than the efficiency measurement at the minimum feature.During the past several years the interest in diffractive optics technology has increased significantly. There has been a relatively steady progression of technical developments from early papers1 that first mentioned the potential benefits of diffractive elements to readily available commercial optical design software packages that provide the ability to easily incorporate diffractive elements in optical system designs. A very important detail that is not modeled in the commercially available lens design software is the impact of non-unity diffraction efficiency.2 Typically, phase coefficients or the Sweat model are used to design diffractive surfaces.3 However, as mentioned above, a designer unfamiliar with diffractive optics technology may not be aware of the potential system performance losses due to energy propagating in spurious diffraction orders.Section 2 provides a brief review of degradation in optical system performance due to non-unity diffraction efficiency. Specific examples of measured MTFs of diffractive elements with different integrated diffraction efficiency values are presented. The requirements to fabricate high fidelity zone structures are discussed in Section 3. In addition, specific examples of measured zone structures are presented to demonstrate the fidelity of the master fabrication and replication techniques. Imaging effects of non-unity diffraction efficiencyA rather strange characteristic of diffractive optical elements (DOEs) is that a diffraction-limited spot can be obtained in an optical system that has rather poor imaging properties. This phenomenon is due to Fakils is currently employed with PSC, Webster, NY. 50 ISPIE Vol. 2600 0-8194-1964-8/95/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/22/2016 Terms ...

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