Process for the fabrication of optical microstructures

0001The invention relates to a process for preparing optically effective microstructures in which in at least one step an optically effective surface topography on a Substrate is produced by a mechanical process and in at least one further finishing step the Surface topography is smoothed.

0002The invention particularly relates to the preparation of Lenses, lens arrays and prismatic structures with large Aspect ratio, large structural heights and with a Cross-sectional area less than 1.5 mm²,

0003an inventive A typical example for the Method microstructure produced is a plano-convex aspherical cylindrical lens, the collimator as the fast axis is used in high-power diode lasers. to To achieve maximum power density, high need of divergent laser beam as much as possible focused (See. R. Diehl, high-power diode laser, Fundamentals, Technology, Applications, 78 / Topics in Applied Physics). Around To meet these requirements, must Cylindrical lens extremely accurately and with low Tolerances are manufactured. Typical dimensions of such Cylindrical lenses are at a width of 1 mm, a Sagitta (optically effective structure height) of 0.3 mm in a total height of 0.8 mm and a length of 12 mm.

0004The cylindrical lenses for high-power diode lasers consist of brittle materials, such as optical glasses or quartz glass. The hard and brittle nature of the materials and the large structural height of the cylindrical lens making rule a multi-stage manufacturing process required.

0005The chipping manufacturing processes are divided regularly in the machining steps roughing, fine grinding and optical polishing (cf.. Volker R. Sinhoff, finishing Glasses in No Show, WZL - reports from the Manufacturing Engineering, Vol 6/97, Shaker Verlag). By the Grinding as a mechanical removal method is the optically effective surface topography generated while the polishing as a finishing step to improve the Surface quality while maintaining the Shape precision aiming.

0006Due to mechanical removal methods such as grinding or Ultrasonic machining can indeed be the necessary large structural heights of cylindrical lenses produce, however, is the achievable surface quality in general, limited. Even with the use of ductile Grinding is in each case, a final machining of the Surface topography required. The known mechanical finishing steps, such as the mechanochemical polishing based on suspensions for the production of cylindrical lenses in terms of attainable form accuracy and cost only conditionally suitable; they are difficult to handle and can be economically only for the production of individual use lenses or small series. The handling problems express themselves, for example the fact that with the support of Lens voltages occur during polishing, the one to may cause deformation of the lens. The deformations cause geometric errors that the quality of the lens significantly affect. Furthermore, in the polishing due to the in relation to the dimensions of the lens large contact surface with the polishing tool a sufficient location-dependent control of the material removal nearly impossible. Consequently, the requirements to the shape accuracy the lenses do not meet satisfactory.

0007To finish the optically effective structure are still other methods are available. is EP 0664891 B1 For example, thermally induced smoothing means Electron beams described. The method is based on a targeted melting of the surface. This cure near-surface defects from (cracks). decisive disadvantage This finishing step is the change in Surface topography. Due to the viscous state of the material during the smoothing tried the material to energetically favorable state of a ball to take. This distorts the during the mechanical Preprocessing scored dimension of the optically active Structure.

0008In R. Voelkel, M. Eisner, C. Ossmann, Refractive micro lenses for ultra-flat photolithographic projection system is a Production method for microlens described in which a layer (about 1 micron to 50 microns thick) photoresist on applied a mutually polished substrate made of quartz glass becomes. Then, the resist through a mask developed and characterized an arrangement of cylinders from Photoresist formed on the substrate. The cylinder Photoresist are by plasma-assisted etching, transmitted namely reactive ion etching into the quartz glass. The Atoms from the photoresist surface, and the quartz glass are removed at the same time by the ions to the lens is completely etched into the substrate. Since the etching rate of the Photoresist coating and the substrate as well as the Refractive indices do not match exactly the shape the lens after ion etching may be slightly changed. This form changes one tries to avoid, in the Etching rate is changed during the etching. On Finishing step is in this process required, since the surface characteristics of both sides polished substrate by the reactive ion etching almost is not changed. This manufacturing method comprises over mechanical initially described Manufacturing method has the advantage that high Precision economically large quantities of optically active be prepared structures. However, a disadvantage is the low etch rate and the limitation of this method realizable structure height to a maximum of 0.1 mm. Aspherical Cylinder lenses in the collimator of the fast axis High-power diode be employed, can be therefore not produced with this method.

0009Starting from this prior art, the invention on the object of specifying a method which comprises industrial production of optically active Microstructures, in particular structured arrays with high Precision and tight tolerances allowed. The process are also particularly suitable structure heights of more than 0.1 can be manufactured industrially mm.

0010This object is in a process of the type mentioned above achieved in that the optically masked effective surface topography photolithographically and etching, in particular by plasma-assisted Dry etching is smoothed. Suitable dry especially plasma etching or reactive ion etching in Consideration.

0011In at least a first step, by a mechanical process, in particular imaging loops, Ultrasonic machining or laser ablating the optically effective surface topography produced on the substrate; in at least one further finishing step, the incurred by the mechanical removal methods Surface defects by etching, in particular by plasma-assisted dry etching smoothed. As Dry particular through the plasma or reactive ion etching into consideration.

0012In plasma-dry material is a removed gaseous etching medium, the attack in a plasma generated by particles ätzaktiven Büttgenbach, Stephen: micromechanics: Introduction to Technology and Applications - 2nd Edition - Stuttgart:. Teubner, 1994 physicochemical chemical, physical or mixed may be natural.

0013For the novel process, the plasma have and reactive ion etching found to be advantageous. Both methods are based on a mixed physical / chemical Etching mechanism. In plasma, the ätzaktiven particles reactive radicals, weak ion assisted. In reactive ion etching, the ätzaktiven particles reactive radicals, highly ion-assisted reactive ion.

0014In the finishing step is no shape correction required. In contrast to the in R. Voelkel, M. Eisner, C. Ossmann, Refractive micro lenses for ultra-flat photolithographic projection systems described method, is reactive ion etching or plasma etching in which The method according to the invention for smoothing a misappropriated already structured surface topography in a Substrate and not for transmitting a Surface topography in a smoothed by polishing Substrate used.

0015The etching is preceded by photolithographic masking, that preferably includes the following two steps:<ul><li>uniform application of radiation sensitive resist on the optically effective surface topography and </li><li>Baking the with radiation sensitive resist coated, optically effective surface topography, wherein the baking, evaporation of the solvent in Paint effects.</li></ul>

0016To the radiation-sensitive coating evenly on the applying an optically active surface topography, it is according to one feature of the invention that the Lacquer by means of the known spin-coating process is applied by spraying or.

0017The inventive method is particularly suitable for substrates suitable, which are characterized by plasma-assisted etching process can be processed, such as quartz and silicon. With the process can be particularly cylindrical lenses for produce high power diode lasers, because of the Emission characteristics of the diodes have a high numerical aperture need to collimate the light.

0018The method of the invention based on the will Mimic diagram further illustrated in Figure 1:

0019First, a grinding tool 1 is produced whose Abrasive surface 1a as a negative mold of a onto a substrate 2 structure to be produced a surface topography 2a equivalent. For the preparation of the abrasive tool 1 Process into consideration, which in metallic Materials contours with form accuracies of less than 1 .mu.m can be produced. This form accuracy is required, so that the lens to be produced in the required array Tolerance. Good example is the Micro spark erosion or a combination of Diamond turning and galvanic coating of Grinding tool. 1

0020In the first process step, the negative mold 1a of the The tool 1 by means of grinding (3) in a plurality of rows on a Substrate 2 transferred from quartz glass. The usage of Grinding surfaces 1a with fine abrasive media, For example, a diamond grit with an average Graining of 7 microns, ensuring little damage during the preparation of optically active Surface topography 2a.

0021In the finishing step is the first step produced surface topography 2 a smoothed, in which they photolithographically masked first (4) and then by reactive ion etching (5) is smoothed.

0022The masking comprises applying the radiation-sensitive photoresist coating material 4a in the spin-coating process (4) to the substrate 2. Subsequently, the Photoresist 4a at in an oven not shown a temperature of about 150 ° - 200 ° C baked. If necessary, the photoresist is exposed. A local exposure the photoresist coating and a subsequent Development step can be used to adapt or optimize Profile shape of the microstructure can be used.

0023The applied photoresist layer as a mask designated.

0024to the reactive ion etching Adjoining the masking (RIBE) (5) on to the optically effective surface topography smooth, without changing their contours. In contrast to Ion milling is the reactive ion the substrate 2 edited the entire surface, so that during the reactive Ion etching (5) covers the entire optically active Surface topography (2 a) is irradiated simultaneously.

0025The atoms of the photoresist layer and the underlying substrate by ion milling removed and any defects in the surface topography the substrate smoothed.

0026Particularly challenging is the inventive method in the preparation of optically active Surface topographies with large structural heights of 0.3 mm. In such a structure heights of even order is the radiation-sensitive resist crucial as uneven paint distribution uneven Smoothing entails. The spin-coating process favors the uniform distribution of the paint in the Layer thicknesses is applied 1-10 microns, crucial. In the spin-coating method, the coating is by a rotation of the substrate 2 (see (4)) from the center of the edge thrown.

0027To further improve the distribution of the lacquer, in particular at a surface topography with large Structure heights, the surface topography can 2 a a have star-shaped structure (ie, the individual Structures extend radially from the center to the edge of Substrate). This is the high structural height counteracted that with an arrangement of structures corresponding to Figure 1 of a uniform distribution of the varnish can counteract. An alternative solution consists in spray paint.