We report on the ongoing investigation of magnetron sputtered germanium on silicon for photonics applications. Direct current (DC) magnetron sputtering has been used to deposit germanium layers on silicon at low growth temperatures and medium range vacuum levels. Standard photolithography has been used to make germanium photodetectors for the 1550 nm wavelength range. Electrical characterization, more specifically current-voltage measurements indicate that the devices function as intended. Sputtered silicon waveguides have also been fabricated and evaluated for possible applications in photonics integration. The sputtering-based developments in our present research are expected to provide for a flexible and economically viable manufacturing process for such devices.
Optical gratings are becoming available with precision down to the 2 nm level, or below. Such gratings can be employed
to make highly accurate measuring tools such as optical encoders and coordinate measuring tools which can find
numerous applications in integrated circuit industry and elsewhere. Such tools are significantly less expensive than
interferometers because of relaxed mechanical tolerances required on the associated stages. However, the accuracy of
grating-based measurement tools is limited by optical imaging techniques for viewing the gratings. In this paper, a novel
illumination technique involving two parallel coherent beams is developed for grating-based measurement tools with
nanometer accuracy. The interference grating image was viewed under a microscope and the video image of the grating
was taken and processed. The location of the grating was determined with an error of 0.09 nm. The interference artifacts
generally present in laser illumination are eliminated under this new illumination technique. This provides a cleaner
starting point for data analysis and thus higher accuracy measurement. The techniques developed here provide levels of
accuracy limited only by the errors in the grating. This is comparable to accuracy of state of the art interferometers, but
at much lower cost.
We report on the continuing development of a grayscale lithography technique based on the use of a polymeric grayscale
photomask. Arbitrary three-dimensional (3D) microstructures can be realized in positive and negative photoresist
materials by the use of the polymeric grayscale photomask with standard ultra-violet (UV) lithography. The fabrication
of such 3D structures depends on the differential absorption of in photo-absorbing material. The photomask is made of
patterned polydimethylsiloxane (PDMS) polymer doped with a UV absorbing laser dye. The PDMS photomask contains
micro-patterns made by micro-molding the PDMS on a complementary silicon master mold. By adjusting the thickness
of the patterns on the polymer photomask, the dopant dye concentration in the photomask and UV exposure dose, a
multitude of unique 3D microstructures can be fabricated on the substrate with desired geometries and dimensions.
While the feasibility of grayscale lithography with such a polymeric mask has been reported by us earlier, this paper
describes the fabrication of master mold and polymeric grayscale photomask.
The inherently high resolution of imprint lithography has the promise of extending integrated circuit minimum feature sizes down to the 10 nm region. However, the main effort of companies building nanoimprint tools has been directed to the development of robust printing techniques, rather than to alignment. Consequently, no alignment system currently exists for nanoimprint lithography that is capable of the alignment accuracy required by the semiconductor industry. This paper proposes a solution to the problem of obtaining accurate alignment over an extended imprinted area. On the one hand, alignment is difficult to perform when the mold template and the substrate are in contact, or nearly in contact. On the other hand, if they are widely separated, the accuracy is limited by the difficulty of simultaneously imaging fine features on both of them. However, by using orthogonal polarizations, sharp images of the template and substrate can be obtained when they are separated by 30 to 40 μm. Previous experience with a dual focus x-ray alignment microscope indicates that this alignment technique will readily meet the 18 nm 3 σ tolerance required by the semiconductor industry at the 45 nm node. Here the technique is adapted for nanoimprint lithography by employing transparent bulk mold materials (e.g. quartz) with opaque alignment targets on the surface. In addition, a flexure stage capable of reproducibly bringing the mold template and the substrate into contact is integrated into the system, so that after the alignment is performed it can be maintained during imprinting.
It is well known that zone plates can print extremely small features in microlithography. However, the size and complexity of zone plates has limited their application. In this paper simulated and experimental results are presented for simple zone plates with very high performance. It has previously been shown that submicron diameter zone plates, with only 2 or 3 zones, can focus 1 nm wavelength X-rays to less than 40 nm FWHM. A zone plate with two zones is a ring, whose ratio of outer radius to inner radius is about 0.7. This implies dimensions that may be too small for easy control. However, simulations have demonstrated excellent focusing for both clear and opaque rings over a ratio of radii of at least 0.6 to 0.8. For high contrast, 1 nm wavelength zone plates are typically fabricated in 200-300 nm thick gold. This leads to high aspect ratios, which are difficult to pattern. However simulations have shown excellent focusing in much thinner gold. In addition, conditions were found in 30nm to 90 nm thick gold which generate narrow dark "foci." The focusing of linear zone plates was also simulated. Linear zone plates with 3 and 5 zones produced excellent line foci, although linear zone plates with 2 and 4 zones were much poorer. Scaled up experiments in visible light supported both the circular and linear simulation results.
Blazed gratings have been fabricated using gray-scale X-ray lithography. The gratings have high efficiency, low parasitic light, and high groove quality. They can be generated over a considerable depth for use anywhere in the ultraviolet to middle infrared range. They can also be recorded on substrates of considerable curvature.