Fredrik Nikolajeff

Proximity-compensated blazed transmission grating manufacture with direct-writing, electron-beam lithography

Proximity-compensated, as well as uncompensated, blazed transmission gratings with periods of 4, 8, and 16 µm were manufactured with direct-writing, electron-beam lithography in positive resist. The compensated gratings performed better than the uncompensated ones. For the 4-µm compensated grating the measured diffraction efficiency was 67%. It was 35% for the uncompensated grating. The compensation was made by repeated convolutions in the spatial domain with the electron-beam point spread function. We determined this function by retrieving the phase from the measured diffraction pattern of the uncompensated gratings.

Replication of continuous-relief diffractive optical elements by conventional compact disc injection-molding techniques

Continuous-relief diffractive optical elements have been replicated by use of conventional compact disc injection-molding techniques. Two continuous-relief microstructures, a blazed grating and a fan-out element, were chosen to evaluate the replication process. Original elements were fabricated by direct-write electron-beam lithography. Optical measurements and atomic force microscopy were used for investigating the replication fidelity.

Diffractive microlenses replicated in fused silica for excimer laser-beam homogenizing

A diffractive beam homogenizer, based on an array of square, off-axis, continuous-relief diffractive microlenses, for use with an excimer laser has been studied. We originally fabricated the homogenizer by direct-write electron-beam lithography, from which we made replicas in UV-grade fused silica by hot embossing and reactive ion etching. Atomic force microscopy measurements of original and replicated elements showed the accuracy of the replication fidelity. One of the replicated homogenizers was evaluated together with a KrF excimer laser. The homogenized beam had a flat-top profile with 31% of the beam energy contained within an area where the beam intensity was above a threshold level of 90% of the maximum intensity.

Fabrication and simulation of diffractive optical elements with superimposed antireflection subwavelength gratings

With the aim of reducing surface reflections and increasing the diffraction efficiency we investigated the superposition of subwavelength phase gratings onto blazed phase gratings. With direct-write electron-beam lithography bare blazed gratings and blazed gratings carrying subwavelength gratings were fabricated and their optical performances compared. For TE polarization the subwavelength-carrying gratings showed a maximum diffraction efficiency of 90.6%, whereas the corresponding maximum value for the bare grating was 86.3%. The experiment was simulated with rigorous diffraction theory.

Successive development optimization of resist kinoforms manufactured with direct-writing, electron-beam lithography

It is shown that multilevel SAL 110 resist kinoforms can be developed stepwise. Measurements of the kinoform diffraction pattern, performed between the development steps, permitted correct final developments to be made. No significant relief shape degradation was observed for development times as high as 25 min. The results imply that the electron-beam exposure doses, and hence the exposure time, can be reduced by a factor of 3 compared with doses used currently.

Spatial-mode control of vertical-cavity lasers with micromirrors fabricated and replicated in semiconductor materials

Micromirrors were fabricated in gallium phosphide by mass transport to provide spatial-mode control of vertical-cavity surface-emitting lasers (VCSELs). The concave mirrors were used in an external-cavity configuration to provide spatial filtering in the far field. Single-mode cw lasing was demonstrated in 15-microm-diameter VCSELs with currents as high as 6 times threshold. The fabrication process was extended to micromirrors in gallium arsenide by use of a replication and dry-etch transfer process.

Measuring and modeling the proximity effect in direct-write electron-beam lithography kinoforms

The proximity effect in successively developed direct-write electron-beam lithography gratings is measured. The grating relief shapes are obtained from the measured power in several of the gratings diffraction orders. Describing the proximity effect by a convolution with a double Gaussian point-spread function, we determine the parameters of the point-spread function. The writing part of the point-spread function is found to increase significantly with increasing development time, the background part much less.

Proximity-compensated kinoforms directly written by e-beam lithography

We report on direct-writing EBL manufactured, proximity compensated blazed transmission gratings. The proximity compensation is made using a non-linear iterative process in the spatial domain. The diffraction efficiency for a compensated 8 micron period grating was 84%, almost twice as that of an uncompensated grating. Electron beam lithography (EBL) is a powerful method to manufacture computer generated holograms (CGH:s). Of particular interest is the manufacture of phase-only CGH:s of relief-type, since their diffraction efficiencies may approach 100%. Generally it takes several phase levels in the hologram to achieve such high efficiencies, though. To manufacture phase-only CGH:S either repeated binary or direct writing techniques can be used. With the former method several EBL-manufactured binary masks are used sequentially in a photolithographic process. The advantage with that method is high phase level accuracy and the main drawback is mask alignment inaccuracy. With direct writing EBL the situation is reversed: the lateral accuracy is excellent, the main problem is reaching the intended relief heights. The problem is caused by electron scattering in the resist during exposure termed the proximity effect which tends to smear out small relief features. Here we describe our attempts to compensate for the proximity effect with the particular aim to manufacture directly written, high quality Fresnel lenses. If one exposes a Fresnel lens without any compensation at all one finds that the relief amplitude decreases with increasing distance from the centre. Since a Fresnel lens is essentially a blazed grating with continuously varying period we chose to analyze this particular manufacturing problem by manufacturing and studying linear blazed gratings with different periods, both proximity compensated and uncompensated. (An advantage with such gratings is their simple diffraction pattern, ideal to analyze.) The proximity effect can be described as a convolution between the electron dose exposure profile and the electron point-spread function (PSF). For the PSF we use the the following model12: f(r) = 1 [ —i-e()2 + it(l+r) I a2 where r = lateral distance from primary electron beam a = Gaussian l/e-radius of forward scattered electrons 13 = Gaussian l/e-radius of backward scattered electrons 11 = ratio of backto forward scattered exposure. Proximity compensation is a problem of inverse filtering. This can readily be performed in the Fourier domain3. Now, a is known to increase with t, the resist thickness2. Therefore, since a is the sharpest 0819408832/93/

Self-aligned optofiber/waveguide connector with integrated V-grooves and diffractive optical elements

4.00 SPIE Vol. 1718 Workshop on Digital Holography (1992) / 115 writing feature of the PSF, t should be small. Thus the relief should be positioned close to the resist top surface. Then truncation effects appear, however, and that can not be handled by the (linear) Fourier domain compensation scheme. Neither can it take into account possible dose dependence of a. Therefore we chose to perform the inverse filtering in the spatial domain, although not as convenient mathematically: repeated convolutions are made on successively modified dose profiles in an iterative process. The resulting dose profiles of the blazed gratings come out markedly non-symmetric and zero doses appear. By manufacturing and then measuring the diffraction from blazed gratings we try to find realistic values of a, and r for our specific exposures, specified below: Electron energy: 25 kV Incident spot radius: 0. 1 tm Beam current: 1-3 nA Resist material: SAL 1 10 polyimide, positive Resist thickness: 1 .4 im Substrate: fused silica Grating periods: 4, 8, 16 m Ideal relief height: 1 .17 im Design wavelength: 633 nm Best optical performance so far was obtained for a proximity compensated transmission grating with an 8 micron period: 84% of the transmitted light power appeared in the first diffraction order. The corresponding number for the uncompensated grating was 45%. For a compensated 4 micron period blazed grating the diffraction efficiency was 66%. These values are in reasonable agreement with the design value obtained with a = 0.3 tim, 3 = 5.0 m and r = 0.5. These results are not final; better performance can be expected when more accurate proximity parameters have been determined. Improved resolution should further be obtained by increasing the electron acceleration voltage from 25 to 50 kV, since a decreases with increasing voltage2. Finally we like to point out that our spatial domain proximity compensation method can readily be applied to two-dimensional, arbitrary CGH reliefs. Acknowledgements The exposures were made using the EBL facility at the National Nanometer Laboratory (NNL), Chalmers University of Technology, Sweden. The research was supported by grants from the Swedish Board for Technical Development.

A novel technique for intégration of optical elements onto silicon

We demonstrate a novel type of self-aligned optofiber/waveguide connector with integrated V-grooves and diffractive optical elements. The self-alignment is achieved by connecting microstructures which have originally been formed in silicon and later replicated in concave and convex forms. V-grooves hold the optical fibers and the light is coupled out through diffractive optical elements (DOEs). A manufacturing process has been developed which allows both deep microstructures (V-grooves and alignment structures) and shallow surface reliefs (diffractive elements) to be realized on the same substrate. The self-alignment using microstructures allows a positioning accuracy of about /spl plusmn/5 /spl mu/m. Two different fan-out DOEs have been optically characterized.