A combination of synchronized mask scanning and mask dragging techniques (in which the mask is held stationary .... excimer laser is mounted behind the tool.
Fabrication Techniques and their application to produce novel Micromachined Structures and Devices using Excimer Laser Projection Erol C. Harvey and Phil T. Rumsby Exitech Ltd, Hanborough Park Long Hanborough, Oxford, UK OX8 8LH ABSTRACT New techniques for 3D micromachining by direct laser ablation of materials using excimer lasers have been developed. Basic to all of these techniques is the use of image projection in which the laser is used to illuminate an appropriate pattern on a chrome-on-quartz mask. The mask is then imaged by a high-resolution lens onto the sample. Non-repeating patterns with areas of up to 150 x 150mm can be machined with sub-micron resolution and total accuracies of the order of a few microns by using synchronized scanning of the mask and workpiece. A combination of synchronized mask scanning and mask dragging techniques (in which the mask is held stationary and the workpiece moved during laser firing) enables patterns of up to 400 x 400mm to be produced; the limiting feature being the travel and accuracy of the precision air-bearing stages used to support the workpiece. This talk describes the synchronized mask scanning and mask dragging techniques and illustrates their application by presenting details of novel rnicromachined structures and devices so produced. These include rapid prototyping of bio-processor chips, fabrication of mechanical anti-reflection structures in CsI infra-red optical material, patterning films as frequency selective reflecting structures, Laser-LIGA and high aspect ratio machining using lamination techniques to produce an optical methane detector. Keywords: Excimer lasers, micromachining, laser-LIGA.
1. INTRODUCTION The pulsed ultra-violet beam of an excimer laser is used for micorfabrication by direct ablation of materials in ways which are unique to this type of laser. The beam from most industrial excimer laser sources is not used as a focused spot but almost always as a high-intensity illumination source for image projection systems. High resolution imaging systems and the development of parallel processing techniques such as those described in this paper make most efficient use of the beam and hence speed up processing times and thereby reduce fabrication costs. The precise depth of a laser cut can be easily controlled by counting the number of laser shots that are used enabling full 3D laser machining to be realised(l). When combined with machining techniques which control or manipulate the total laser dose at the workpiece full 3D structures can be rapidly formed.
2. EXCIMER LASER MASK PROJECTION The basic excimer beam homogenizer and mask projection system is shown schematically in figure 1, The input laser beam is typically 20mm x 10mm and passes through cylindrical telescope lenses to expand the beam to fill the homogenizing lens arrays. The lens arrays may be either spherical or cylindrical elements (depending upon the final beam shape required) and the act to break up the input beam into many overlapping beamlets.
Typically arrays between 6 x 6 and 9 x 9 elements are used and the 36 or 81 individual beamlets are combined by overlapping them at the mask plane to average out localised variations in beam intensity. It is important that the input laser beam has a roughly symmetric profile in order for the homogenizer to work correctly. If, for example, the rectangular input beam has a linear variation in intensity across its long axis this will be reproduced at the mask plane since none of the beamlets is rotated by the homogenizer. The homogenizer is effective in averaging localized intensity variations and can achieve mask illumination uniformities of around ±5% RMS deviation.
Figure 1. Schematic of an excimer laser beam homogenization and mask projection system.
The homogenizer also introduces a range of angles (e.g. ±20 mrad) onto the beam which are significantly higher than the inherent, and often varying, laser divergence (approx ±2 mrad) enabling greater control of the cutting angles at the workpiece when used with a projection lens with a suitably matched NA(1,2) (typically 0.1 to 0.3).
Maximum pulse energies of the order of a few 100 mJ are emitted from the excimer laser so that project lenses magnifications of between 3x to I Ox are typically used in order to achieve the fluences of 1 to 2 J/cm2 required to efficiently ablate thin films and most polymers. This means that the maximum area that can be machined at the workpiece in any single laser shot is typically limited to approximately 20 to 30 mm2. Ceramics and glasses require ablation fluences around 5 to 10 times greater than polymers and hence maximum areas that can machined per laser shot for these materials are 25 to 100 times smaller. In order for any micromachining technique to be commercially viable it must have the ability to process large areas of material at the lowest possible cost, or at least at costs and machining rates which are compatible with the other processes used to manufacture the device. This requirement drives the excimer laser user to use techniques which maximise the use of the laser energy and minimise the machining time. Parallel patterning techniques such as Synchronized Mask Scanning and Mask Dragging described below are such techniques.
3. SYNCHRONIZED MASK SCANNING
The beam delivery system shown in Figure 1 is useful if the structure to be machined can be done in a step-and-repeat mode. For useful lens resolutions in the range 1 - 5 µm the performance of affordable projection lenses limits the maximum magnification and field size which can be used. In order to machine larger non-repeating patterns either the beam must be scanned over the stationary mask and workpiece, or the mask and workpiece scanned through the stationary beam. Since the beam delivery optics form an imaging system with optimal performance at fixed optical distances it is often more convenient to move the mask and workpiece rather than the beam. This is illustrated schematically in fig. 2.
The motion of the mask and workpiece and the firing of the laser are co-ordinated by an Aerotech Unidex500 multi-axis CNC which is programmed to precisely accommodate the image reversal and demagnification factor of the projection lens. Mechanical stability and the tuning of the DC Servo drive motors of the translation stages currently limit the ultimate resolution of synchronized mask patterning to around 5µm so that in order to utilize the ultimate resolution of the projection lens static imaging. where both mask and workpiece are brought to a stop before firing the laser, must be used. Synchronized mask and workpiece scanning is also applicable to cylindrical workpieces as illustrated in fig.3. where a line-beam mask projection system is used to transfer the pattern information from the mask onto the workpiece. Devices such as biomedical catheters, antennas and micromotors are fabricated directly onto cylindrical work-pieces using this technique.
Figure 2. Synchronized mask and workpiece scanning enables large non-repeating patterns to be machined.
Figure 3. Synchronized scanning of a cylindrical workpiece using a line-beam mask projection system.
Figure 4. Series 8000 excimer laser mask scanning patterning tool.
The machine which we used to do this work is the Exitech Series 8000 patterning tool shown in fig. 4. The excimer laser is mounted behind the tool. The beam delivery system contains appropriate beam shaping and homogenization optics to create a uniform spot at the plane of a mask held on an open-frame CNC controlled X Y stage set. Projection lenses of various magnifications may be used to transfer the pattern of the mask onto the workpiece which is mounted on precision air-bearing X Y stages. An elevator stage with 0.1um resolution coupled to a diode laser-based height sensor system maintain the workpiece at the correct height during processing.
4. MASK DRAGGING
Mask dragging is a powerful technique for structuring surfaces to fabricate micro-channels, grooves, concave and convex cylindrical profiles. pyramids, cubes and many other surface relief features. The technique is shown schematically in fig. 5. Here a mask with a “T” aperture is held stationary and projected onto the workpiece. The workpiece moves while the laser fires repetitively, and since the laser pulse duration is around 20ns this motion causes no resoveable motion blurring of the projected image. Areas of the workpiece illuminated by the arms of the “T” will only receive half the total number of laser shots that the areas of the workpiece illuminated by the central portion of the mask receive. Since the ablation depth is proportional to the number of laser shots the resulting structure will be the two-level channel illustrated in fig. 5(b). The start and end sections of the channel will be ramps which will smoothly connect the bottom of the channels to the surface in a distance equal to the projected length of a single mask image. One technique for eliminating the ramps from the ablated tracks is to place two protective strips of metal tape across the workpiece and ablate the tracks by starting on one strip and dragging the image to the opposite strip. If the metal tape is thick enough it will not be damaged by the laser and can be removed after the tracks have been fabricated leaving perpendicular end walls.
(a)
(b)
Figure 5. Mask Dragging Technique. Fig. 5(a) shows a “T” aperture used as a mask. If, while the laser fires repetitively, the workpiece is moved beneath the stationary mask and laser beam, areas of the workpiece illuminated through a will receive half the total number of shots that area b will receive hence will cut to half the depth. For most applications many parallel structures will be required so that rather than having a single aperture in the mask it will usually have a series of apertures side-by-side, as seen in the static ablation pattern shown in fig. 6(a). The Exitech Series 8000 machine is able to hold a 6” square chrome-on-quartz masks which can contain more than 100 patterns each 10mm x 10mm. Therefore a mask designed to produce triangular grooves can be made to be versatile by writing diamond apertures of various pitches and aspect ratios enabling the engraved grooves to be easily tuned to the required shape simply by indexing the mask to alternative patterns. When “V” grooves are engraved in two orthogonal directions the resulting structures are a set of pyramids seen in fig. 6(b). The technique is not limited to straight-sided channels and can just as easily produce channels with concave or convex channels or mixed structures when used with an appropriately designed mask.
Figure 6. Static ablation of a mask with a series of diamond-shaped apertures produces the pattern shown in figure (a). When such a mask is dragged in two 90°-crossed directions the resultant structure is a set of double-pyramids shown in (b). The pyramid pitch is set by the mask imaging system, and their depth by the number of laser shots and the aspect ratio of the apertures in the mask. Black bars at the bottom of the images represent 40um lengths. Another advantage of both mask dragging and synchronized mask scanning over step-and-repeat machining is that both techniques provide inherent dose homogenization. Providing several laser shots per site are required to complete the machining it can be arranged that all parts of the workpiece see all parts of the laser beam - a fundamental definition of beam homogenization, and so great uniformity of machining over large areas can be achieved.
5. APPLICATIONS The development of the dielectrophoretic bioparticle sensor(3) is a good illustration of the use of three different excimer laser fabrication techniques; step-and-repeat, synchronized mask workpiece scanning, and mask dragging. The basic mask set required to fabricate the three layers of a straight-section linear electrode array for the sensor is shown in figure 7. Electrically conducting tracks are patterned into a 100nm evaporated gold layer using the mask shown in fig. 7(a). The electrode pitch at the workpiece is Mum and one laser shot is needed at 0.5 J/cm2 to produce the full pattern. Wider breakout tracks fan out from the fine electrode array and are used to connect to the external drive electronics and, since a 10x lens is used, synchronized mask scanning is required in order to fully pattern this section which covers several 10’s of square cm at the mask. The mechanical stability of the CNC stages limits the resolution of the ablated structure in synchronized mask scanning to around 5 - 10µm, so the fine pitch electrode array must be imaged in a step-and-repeat fashion with all mechanical systems stationary during laser firing. Using step-and-repeat patterning for this step means that the linear section can be made as long as necessary during the printing process simply by repeating the section of patterning as many times as is required.
A 2um thick spun-on polyimide layer is used as the dielectric and the mask shown in fig. 7b is used to drill via holes to connect to the lower electrodes. It has been found that step-and-repeat patterning is most useful here. Optimum via half-taper angles of around 20° are needed in order to produce holes which are easy to plate to form a reliable electrical connection to the lower layer. The workpiece is “wobbled” or oscillated under CNC control during laser machining to open out the vias, enlarging the holes and producing the correct wall angles. This type of machining is frequently used to control the taper angle of ink jet printer nozzles and other similar fluid injection apertures.
Gold is deposited over the device and, after alignment, patterned using the mask in fig. 7c in a combination of mask dragging for the straight sections and step-and-repeat for the ends. Finally a thicker polymer layer is deposited over the electrode and a fluidic channel cut using mask dragging. The biosample is admitted into the sensor in aqueous solution and tagged bioorganisms may be driven and separated through the sensor. A completed electrode section is shown in figure 8. The methods described here are a very flexible development technique; the electrode length can be easily adjusted by changing the CNC program without the need to fabricate a new mask each time the sensor design changes. Figure 8. Completed linear electrode section of the dielectrophoretic biosensor. The fabrication of anti-reflection structures is another application of mask dragging. Here surface features with a pitch significantly smaller than the wavelength of light for which they are designed to operate create a gradual change in refractive index and can be used to produce an anti-reflection layer(4). Mask dragging diamond apertures has been used to produce the Laser Ablated Anti-Reflection Structures (LAMARS) shown in fig. 9. These CsI pyramids have a pitch of 12um and function as AR structures for radiation centred around 25µm.
Figure 9. Laser Ablated Anti-Reflection Structures (LAMARS) in CsI
Similar structures have very general applications. Controlled roughening of materials to increase surface area is useful for enhancing chemical or catalytic reactions. The microgrooves which are produced half-way through forming the pyramids are important in fluid flow by improving boundary layer adhesion and thereby delaying the onset of turbulence.
In the preceding discussion Mask dragging has been considered only in the context of a stationary mask and a moving workpiece. The reverse situation can of course also be used, and is especially useful in the case of a rotating mask. The nozzle shown in figure 10 was ablated using a static workpiece and a mask rotating under CNC control. By programming the controller to synchronize the firing of the laser to the angular position of the mask the riblets seen in figure 10 could be formed. The pitch and depth of the riblets may be altered by changing the synchronization parameter in the CNC program Figure 10. A nozzle with rifling holes ablated using a rotating mask.
Figure 11. A methane gas sensor fabricated by ablation of laminated layers of polymer. A partial view of the ablated channels in (a) shows one of the re-inforcement layers. The completed device is shown in (b). Lamination techniques have been used to create 3D structures with enclosed sealed cavities or buried pathways by excimer laser micromachining(5). The method is analogous to stereo lithography however the critical difference is that macro stereo lithography is an additive process where material is added layer by layer to form a closed structure, whereas 3D prototyping by ablative excimer laser machining is a subtractive process with material being removed from each added layer. For deep (e.g. l mm) 3D devices lamination involves application of polymer layers of 50-100 microns thickness. In this case each layer has a UV curing adhesive layer backing applied to it and is laminated on to the previously patterned layer. Mask projection using either mask and workpiece scanning or step and repeat processing depending on device size is then used to pattern each sequential layer. If all level mask designs are on one mask plate then switching between patterns is rapid.
Figure 11 shows a picture of a prototype of a 12 level gas filled diffractive optical filter made by this method. The filter is part of a low cost methane gas sensor device and consists of a 1mm thick grating type structure with 25 micron wide channels filled alternately with methane or air. The active filter area is 5mm wide, 20mm long and 1mm thick with CaF2 windows at top and bottom. Appropriate gas manifolds and inlet and outlet ports are incorporated into the structure as laser fabrication proceeds layer by layer. Reinforcing structures can be seen connecting the channels in Fig l la and are used in every second layer to give support to the side walls enabling them to withstand the shock of laser ablation to the upper layers.
6. CONCLUSION Synchronized mask and workpiece scanning overcomes most of the limitations of step-and-repeat machining enabling large areas to be machined with unique non-repeating patterns. Patterning resolutions of around 5µm can be achieved using modern PC based high speed CNC controllers and suitable mask and workpiece translation stages however this does not achieve the ultimate resolution of the laser projection system which is usually around 1µm. Mask dragging is a unique method for parallel scribing 3D structures into surfaces creating an almost limitless variety of different features and shapes. The method is flexible, simple to apply and is limited in the area it can process only by the travel of the workpiece stages used. There are tremendous opportunities for the creative application of this technique to both micro and macro-system technologies where boundary layers or surface interface layers need to be precisely controlled or structured.
7. ACKNOWLEDGEMENTS The work on the LAMAR Structures has been conducted with the co-operation and guidance of Didier Dubreuil, CEA, Dapnia Service d'Astrophysique, France. We would also like to acknowledge the support of all workers at Exitech in the development, demonstration and application of these techniques.
8. REFERENCES 1. Erol C. Harvey, Phil T. Rumsby, Malcolm C. Gower and Jason L. Remnant "Microstructuring by Excimer Laser" in "Micromachining and Microfabrication Technology", SPIE Vol 2639, pp266-277, (1995) 2. H.J. Kahlert, U. Sarbach, B. Burghardt and B. Klimt "Excimer laser illumination and imaging optics for controlled microstructure generation" in "Excimer Lasers: Applications, Beam Delivery Systems and Laser Design", SPIE Vol 1835, pp110-118, (1992) 3. M. S. Talary, JPH Burt, JA Tame and R Pethig, Electro manipulation and separation of cells using travelling electro fields J Phys D, Appl Phys 29, pp 2198-2203 (1996). 4. Daniel H. Raguin and G. Michael Morris "Antireflection structured surfaces for the infrared spectral region", Appl. Optics, 32, No 7,pp 1154- (1993) 5. P. T. Rumsby, E. C. Harvey and D. W. Thomas "Laser Microprojection for Micromechanical Device Fabrication", SPIE Vol 2921, pp684-692, (1996)
