Oleg A. Ershov

Fabrication and testing of chemically micromachined silicon echelle gratings

We have fabricated large, coarsely ruled, echelle patterns on silicon wafers by using photolithography and chemical-etching techniques. The grating patterns consist of 142-microm-wide, V-shaped grooves with an opening angle of 70.6 degrees, blazed at 54.7 degrees. We present a detailed description of our grating-fabrication techniques and the results of extensive testing. We have measured peak diffraction efficiencies of 70% at lambda = 632.8 nm and conclude that the gratings produced by our method are of sufficient quality for use in high-resolution spectrographs in the visible and near IR (lambda approximately = 500-5000 nm).

Micromachined silicon diffraction gratings for infrared spectroscopy

Micromachined silicon gratings offer two great advantages to astronomical spectroscopy in the IR: (1) Photolithographic processing techniques permit the production of gratings with much larger groove constants than are possible with conventional wavelength coverage, despite the relatively small format of IR arrays. (2) One can use anisotropic etching to form gratings on dielectric wedges. By illuminating the grating through the dielectric, we can achieve higher spectral resolution for a given grating size or a smaller grating for a given desired resolution. We discuss the technical challenges involved in micromachining large grating grooves over large areas while holding positional accuracy to very tight tolerances. Manufacturing issues include material choices, surface preparation, and chemical and physical effects during processing. We also discuss our program for evaluation of the finished products, show result of measurements we have made on front-surface and immersion devices, and use these result to assess the potential of these devices for real-world astronomical applications.

Silicon grisms and immersion gratings produced by anisotropic etching: testing and analysis

Because they can vastly reduce the required collimated beamsize at a given diffraction-limited resolution, silicon immersion gratings and grisms are an enabling technology for high resolution infrared spectroscopy from space and are highly useful in a range of ground-based and airborne instruments. We have used anistropic etching techniques to produce diffraction gratings on bulky silicon substrates. These devices can serve as high resolution grisms (when used in transmission), as coarse front-surface gratings, or as very high resolution immersion gratings. We have been able to produce devices with high optical efficiency by insuring that their entrance faces and groove surfaces are optically flat and that the groove positions are correct to within tolerance appropriate to the wavelength where the gratings will be used. We report here on testing and evaluation of high resolution Si gratings both in transmission (grism) and in reflection (immersion) mode.

Si-based surface-relief polygonal gratings for 1-to-many wafer scale optical clock signal distribution

In contrast to volume holographic material where 1-to-many fanouts are realized using multiplexed volume holograms, we report in this paper the first Si-based surface-relief polygonal gratings aiming at optical clock signal distribution application. Surface-relief gratings with 1-/spl mu/m period (0.5 /spl mu/m feature size) were fabricated using reactive ion beam etching (RIE). Both hexagonal and square gratings were demonstrated for 1-to-4 and 1-to-6 fanouts. Surface-normal input and output coupling schemes were carried out with an combined coupling efficiency of 65%. Employment of substrate modes in silicon greatly releases the required grating spacing for the demonstrated two-way surface-normal coupling. 7.5 GHz 1-to-4 clock signal distribution operating at 1.3 /spl mu/m was demonstrated with a signal-to-noise ratio as high as 60 dB. The intensity fluctuation among fanout beams was measured to be within 1 dB. Generalization of 1-to-many fanout can be realized by implementing a polygonal grating with an equivalent number of facets.

Infrared grisms using anisotropic etching of silicon to produce a highly asymmetric groove profile

Grisms are an important tool for astronomical spectroscopy because they allow for very compact, straight-through spectrometer designs in systems that can double as imagers. In the infrared, silicon grisms offer the advantage of superior resolving power for a given beam size and opening angle, when compared to grisms made of low refractive index materials. Silicon grisms with symmetric profiles and a blaze angle of 54.7°, the natural result of anistropic etching of silicon substrates oriented with the (100) crystal plane exposed, are relatively easy to produce. Low-order grisms, however, must be blazed at much shallower angles and will therefore have highly asymmetric groove profiles. In order to achieve these shallow blaze angles, the silicon surface must be precisely oriented at a bias from the (100) plane before cutting and polishing the substrate. Production of gratings with blaze angles as small as 6° is more difficult than production of unbiased gratings because it is very sensitive to changes in the etching process parameters. In this paper, we discuss our techniques for etching highly biased surfaces in silicon wafers, along with the first results of our production and testing of highly biased silicon gratings, including SEM groove profile pictures and optical testing in reflection at 632 nm and in transmission at 1523 nm.

Production of high-order micromachined silicon echelles on optically flat substrates

Infrared spectrometers using silicon immersion gratings and prisms can have substantial performance advantages over conventional instruments. The immersion gratings and grisms share a common geometry: prism-shaped pieces of silicon with blazed grooves along one side. The grooves can either be machined directly into substrates or the grooves can be machined into thin wafers which are then bonded to flat-surfaced prisms. Chemical micromachining currently is the best method of ruling grooves directly into silicon surfaces. The tolerances for near-IR diffraction gratings make direct machining of the grooves onto one surface of a bulky, prism-shaped substrate very difficult. We encountered a number of issues that we had to resolve when we tried to etch precisely positioned grooves into massive pieces of silicon: silicon substrate purity, lithography mask alignment, photoresist thickness uniformity, temperature control, wet etching vs. reactive-ion etching. We have successfully manufactured 7 line / mm gratings on 15 mm thick substrates. We performed optical tests with these gratings used as front-surface devices to determine efficiency and diffraction limited performance. Our echelle gratings have 70\% efficiency in 365th-368th order at 632.8 nm. Testing shows that the grating preserves a diffraction-limited point-spread function making them good dispersing elements for applications requiring high spectral resolving power.

Large-area silicon immersion echelle gratings and grisms for IR spectroscopy

We present design examples for instruments making use of micromachined silicon grisms and immersion gratings. The capabilities of high index grisms, transmission grating-prism hybrids, open up new possibilities in compact IR spectrograph design with spectral resolving power, R~500-5000. Coarsely grooved immersion gratings will provide for unique high resolution spectrograph designs in the near and mid-infrared (resolving power, R~104-105). The high refractive index of silicon shortens the required grating depth, to produce a given resolving power, by up to a factor of 3.4. Alternatively, at a given resolving power, an immersion grating can allow a spectrograph slit to be widened by this factor relative to an instrument using a grating illuminated in air or vacuum; this increases the instrument sensitivity without degrading the spectral resolution. Our analysis here illustrates the potential of these devices to improve spectrograph throughput, spectral resolution, and wavelength coverage while reducing the required instrument volume relative to similar instruments using non-immersed diffraction gratings and low index prisms and grisms.

Grating-based surface-normal optoelectronic interconnects on Si substrate

We report in this paper the surface-normal input and output grating couplers for 1-to-many fanouts aiming at optical clock signal distribution application. For wafer-scale interconnects, surface-relief polygonal gratings with a 1 micrometer period (0.5 micrometer feature size) were fabricated using reactive ion beam etching (RIE). Surface- normal input and output coupling schemes were carried out with a combined coupling efficiency of 65%. Employment of substrate modes in silicon greatly releases the required grating spacing for the demonstrated two-way surface-normal coupling. Seven and five-tenths GHz 1-to-4 clock signal distribution operating at 1.3 micrometer was demonstrated with a signal to noise ratio as high as 60 dB. Generalization of 1-to-many fanout can be realized by implementing a polygonal grating with an equivalent number of facets. For board level optical clock signal distribution system, a preliminary result using a parallelogramic grating to couple surface-normal input light into polyimide waveguide is also reported.

Novel Si substrate mode based wafer scale optical clock distribution architecture

In this paper we demonstrate a novel Si wafer based optical clock distribution technique operating 1.3 micrometers and based on a central polygonal input coupling grating structure and surrounding rings of linear output coupling gratings. In this arrangement, both the central polygonal and linear output gratings have a period of 1 micrometers , allowing light to be efficiently coupled into and out of the Si wafer substrate mode in the surface normal direction. A double side polished Si wafer is used to limit the surface scattering losses as the signal travels through the bulk of the Si wafer. One of the major advantages of this technique is that, since the gratings can be written onto the Si surface using optical contact lithography and reactive ion etching, an array of grating shapes and depths can be selected to optimize the diffraction efficiency and focus the output beams onto the associated multi-chip module (MCM). This helps to reduce the optical power requirements that a future system would have and also allows for greater flexibility in system packaging design.