Fredrik K. Nikolajeff

Transfer of micro-optical structures into GaAs and diamond

Transfer of continuous-relief micro-optical structures from resist into GaAs, by use of direct-write electron-beam lithography followed by dry etching in an inductively coupled plasma, is demonstrated. A BCl3/Ar chemistry has been found to give satisfactory results, N2 and Cl2 have been added to change the selectivity between GaAs and e-beam resist. The transfer process generates smooth etched structures. Distortion of the diffractive structures in the transfer process has been examined. Blazed gratings with a period of 10 micrometers have been optically evaluated using a 940 nm VCSEL. The diffraction efficiency was 67% in the first order with a theoretical value of 87%. Also, simulations of the optical performance for the transfered diffractive elements have been made using Fourier transform of the grating profile. For integrating the optical element with VCSELs there are several possible alternatives. We have fabricated the optical structure on the same substrate that is used for the VCSEL and characterization is presently under way. We also show our initial results on transfer of micro-optical structures from resist into diamond using dry etching.

Investigation of optical properties of injection moulded subwavelength gratings

Surface reflections from optical transmission components are in many cases unwanted and cumbersome. Thin film coating is the conventional technique used for anti-reflection treatment of optical components. In recent years subwavelength gratings have been studied as a replacement for thin films. Subwavelength gratings are microstructures that can be formed on one or both sides of a substrate. Typically an optical component needs to be AR-coated on both sides. We have fabricated injection moulded subwavelength gratings superimposed upon a blazed grating structure in polycarbonate. The gratings are initially formed by electron-beam lithography and subsequently replicated using the same process which is used to manufacture standard plastic compact discs (CDs). There are several problems when trying to characterize a component such as a blazed transmittance grating. First of all there is the spread of internal reflections. Light that is reflected inside the substrate is shifted in lateral position due to the angle of the grating. We have thoroughly investigated the effects of decrease in grating efficiency due to internal reflections and also tried to minimize these effects by appropriately treating both sides of the plastic CD.

Diamond micro-optics

We have developed a method for fabricating almost any type of optical surfaces in diamond. The method consists of the following steps: First, a polymer film, spun onto diamond substrates of optical quality, is patterned by lithographic processes. Next, the surface relief is transferred into the underlying diamond by use of inductively coupled plasma dry etching in an oxygen/argon chemistry. Using this technique, we have successfully demonstrated the fabrication of diamond spherical microlenses, blazed gratings, Fresnel lenses, subwavelength gratings and diffractive fan-out elements. The spherical microlenses had apertures of 90 μm and f-number of 4. The phase error, measured with a Twyman-Green interferometer at 633 nm, was found to be less than 31 nm. The diffraction efficiency for the blazed grating was measured to be 68% at 400 nm, with a theoretical maximum of 71%. The subwavelength grating was designed for reducing surface reflections at a wavelength of 10.6 μm. Spectrophotometric results show that the optical transmission was increased from 70%, using a non-structured diamond substrate, to 97%, using our subwavelength structured diamond. Finally, the fan-out element has been tested with a 6 kW carbon-dioxide laser, to microstructure pieces of PMMA. The results are excellent, showing diffraction limited spots.

Diamond optics: fabrication and applications

We have developed a method for fabricating almost any type of optical surfaces in diamond. The method consists of the following steps: First, a polymer film, spun onto diamond substrates of optical quality, is patterned by lithographic processes. Next, the surface relief is transferred into the underlying diamond by use of inductively coupled plasma dry etching in an oxygen/argon chemistry. Using this technique, we have successfully demonstrated the fabrication of diamond spherical microlenses, blazed gratings, Fresnel lenses, subwavelength gratings and diffractive fan-out elements. Applications for diamond optics include space technology, high power lasers and optoelectronic devices. In a first real world application we have manufactured subwavelength antireflective gratings which will be tested for use with a future space telescope. The wavelength region of interest will be in the far-IR. Our fabricated antireflective gratings increased the transmitted radiation from 71% to 98% between wavelengths of 21.5 μm and 26.5 μm.

Monolithic integration of continuous-relief diffractive structures with vertical-cavity surface-emitting lasers

In this work, we have studied the transfer of diffractive optical elements, originally made in resist, into GaAs for monolithic integration with vertical-cavity surface-emitting laser.

DOE manufacture with the DUV SLM-based Sigma7300 laser pattern generator

This paper treats a for the semiconductor industry somewhat different application: The first-ever manufacture of Diffractive Optical Elements (DOE’s) as directly written multilevel diffractive micro-reliefs using the DUV SLM-based Sigma7300 Mask. The reliefs were manufactured in the DUV Chemically Amplified Resist (CAR) FEP-171. This particular application is of direct interest since DOE’s are already incorporated in the Sigma7300 system. The design and manufacture are demonstrated with (1.) A Fan-out element and (2.) A logotype generator. The first attempts, reported here, resulted in a Fan-out element with diffraction efficiency of 64% compared to the theoretical design of 88%.

Novel type of micromachined retroreflector

This paper reports on the manufacturing of a novel type of retroreflecting sheeting material. The geometry presented has high reflection efficiency even at large incident angles, and it can be manufactured at low cost through polymer replication techniques. The paper consists of two parts. A theoretical section outlining the design parameters and their impact on the optical performance, and secondly, an experimental part comprising both manufacturing and optical evaluation for a candidate retroreflecting sheet material in traffic control devices. Experimental data show that the retroreflecting properties are promising. The retroreflector consists of a front layer of densely packed spherical microlenses, a back surface of densely packed spherical micromirrors, and a transparent spacer layer with a thickness equal or not equal to the focal length of the lens. The master structures for the lens and mirror sides of the retroreflector were produced by thermal reflow of photoresist pads on silicon wafers. The silicon master structures were transferred into metallic counterparts by electroforming. The casting of the retroreflector was then done in a cavity being limited by the respective mould inserts for the lens and mirror sides.

Micro-optical elements fabricated and replicated in semiconductor materials

Refractive microoptical elements were originally fabricated by mass-transport smoothing in gallium phosphide. Mass- transport smoothing is based on surface diffusion at elevated temperatures and allows the generation of highly efficient semiconductor microoptics. Starting from a master element, we have developed a replication technique for transferring microoptical surface reliefs into other semiconductor materials, such as gallium arsenide (GaAs). The technique uses a cast and dry etch process. Two different refractive microoptical elements have been replicated into GaAs, a Fresnel biprism and a concave micromirror. The elements have been characterized and show the high fidelity of the replication process.