Frieder Bigl

Ion beam and plasma jet etching for optical component fabrication

Ion beam figuring (IBF) using inert gas (e.g. Ar) and (Reactive) ion beam etching [(R)IBE] gain growing interest in precision optical surface processing, RIBE mainly for proportional transfer of 3D-resist masks structures in hard optical materials and IBF for finishing and nanometer precision surface figuring in high performance optics technology. Ion beam and plasma jet etching techniques related to different optical surface figuring requirements have been developed at IOM during the last decade. Some of these techniques have been proven to be mature for application in industrial production. The developmental work include material related process tuning with respect to enhance the processing speed and to improve surface roughness and waviness, further various processing algorithms related to different surface figure requirements and processing related equipment modification. Plasma jet assisted chemical etching is under development with respect to efficient machining techniques for precision asphere fabrication. The paper gives an overview of precision engineering techniques for optical surface processing focusing on the status of ion beam and plasma techniques. The status of the proportional transfer of 3D micro-optical resist structures (e.g. micro-lens arrays, blazed fresnel lens structures) into hard optical and optoelectronic materials by (reactive) ion beam etching will be summarized.

Precision optical asphere fabrication by plasma jet chemical etching (PJCE) and ion beam figuring

We develop a Plasma Jet Chemical Etching (PJCE) technique for high rate precision machining of optical materials aiming in a technology mature for precision asphere and free-form surface topology fabrication. The present contribution summarizes the achievements after about twelve months experience with a prototype production tool facility. PJCE is performed with the help of a microwave driven reactive plasma-jet working in a broad pressure range (10-600 mbar). We developed a moveable lightweight microwave plasma jet source for dwell time techniques performed in a roughly pumped process chamber equipped with a six axis system for precision workpiece and plasma source movement. Volume etch rates of some 10 mm3/min have been achieved for fused silica and silicon, respectively, using reactive (CF4,SF6,O2) and inert (Ar,He) gas mixtures and applying a microwave (2.45 GHz) power in the 100-200 W range. Large quartz plates (80-160 mm) have been figured using dwell time methods to achieve aspheric deformations of some 10 micrometers . The figured surfaces show shape errors of 1-2 micrometers and a microroughness of 50-100 nm RMS but no sub-surface damage enabling a small tool shape conserving post polishing up to the sub-nanometer roughness level. Thus, surface shaping to the nanometer error range can be done by ion beam finishing.

Fabrication of surface gratings in GaAs and AlGaAs by electron beam lithography and chemically assisted ion beam etching

Optical surface gratings for surface emitting semiconductor lasers require extremely smooth functional surfaces, vertical step edges, a low sidewall roughness, and a high accuracy cover a large are. We report on the fabrication of binary patterns with a period of 1300 nm in GaAs and AlGaAs by electron beam lithography and different dry etching techniques, such as IBE, CAIBE and CARIBE. In order to improve the selectivity of the etch technique as well as the sidewall roughness three different etching mask techniques were applied. For grating depths of about 150 nm the best result with respect other geometrical grating characteristics have been achieved by using the e-beam exposed and developed PMMA as a direct etching mask. Comparing the different etching techniques, CAIBE on GaAs with chlorine shows the best etching behavior. A step edge angle of 90 degrees was achieved at an energy of 200 eV and a chlorine flow of 1.25 sccm. Optical diffraction measurements reveal a high efficiency for the gratings with vertical sidewalls and a line/space ratio of approximately 1.

Transparent three-dimensional microstructures for the trapping of living cells

The simultaneous manipulation and observation of morphological features of single living cells along with the recording of functional changes related to cellular metabolism, interaction and communication in real time are of growing interest. With the advent of atomic force microscopy (AFM) structural studies of native cells became possible which in combination with adequate light microscopy give a much better resolution than light microscopy alone. However, the motion of the cell, the softness of the cell membrane and the two-dimensional growth of cells in culture limit applicability and resolution of this technique. A good mechanical fixation of living cells in a structure could be achieved by embedding cells into partially covered grooves produced in Si/SiO2. However, for additionally optical microscopy studies a transparent material is essential. In this study we present the fabrication of different transparent three-dimensional structures in a three- layer system (Si3N4, SiO2, Si3N4) on quartz specially sized for the trapping of living neural cells under physiological conditions. For the fabrication of the structures we utilized a combination of e-beam lithography (EBL), laser ablation, reactive ion beam etching (RIBE) and different wet etching techniques. The structures consist of a nano-net of Si3N4 covering a micro-cavity in SiO2 confining enough to prevent the cell body from escaping, but not so constraining that it hinders normal growth and development. Another type of presented structures consists of cavities which are connected by covered micro-channels, hence, an observation of cell-cell interactions is also possible. Advantages of these microstructures are the trapping of cells, the stabilization of the cell membrane and the precise placement of the cells for a multitude of biological investigations.