New techniques for laser micromachining MEMS devices

Transcription

1 New techniques for laser micromachining MEMS devices Charles Abbott, Ric Allott, Bob Bann, Karl Boehlen, Malcolm Gower, Phil Rumsby, Ines Stassen- Boehlen and Neil Sykes Exitech Ltd, Oxford Industrial Park, Yarnton, Oxford OX5 1QU, England ABSTRACT Two new laser mask projection techniques Synchronized Image Scanning (SIS) and Bow Tie Scanning (BTS) have been developed for the efficient fabrication of dense arrays of repeating 3D microstructures on large area substrates. Details of these techniques are given and examples of key industrial applications are demonstrated. Key Words: Laser micromachining, Inkjet printer nozzles, plasma display panels 1. INTRODUCTION Pulsed laser micromachining techniques using mask projection methods are now widely used for the creation of miniature structures in both massive and thin substrates (1). MEMS devices are frequently prototyped by excimer laser ablation of polymer substrates using motion of the workpiece and/or mask to create complex 2.5D and 3D structures (2). The submicron thick inorganic, metallic and organic films intrinsic to solar panels, sensors and display devices are often patterned using mask projection methods with both YAG and excimer laser sources (3). Over many years such processes have become well established as production techniques with improvements limited mostly to enhancements in laser drive technology rather than changes to the basic mask projection, beam handling and motion control techniques. In this paper we describe two new pulsed laser micro machining strategies we call Synchronized Image Scanning (SIS) and Bow Tie Scanning (BTS). These techniques take advantage of the recent improvements made in speed and accuracy of modern stage and galvanometer mirror scanner systems. Both SIS and BTS allow major improvements to be made in the accuracy, speed and efficiency with which large area complex repeating arrays of miniature 2D and 3D patterns can be created by laser ablation. They will find immediate use in the laser manufacturing of devices such as super-long ink jet printer nozzles (e.g page-wide arrays), display enhancement films and plasma display panels (PDPs). 2. SYNCHRONISED IMAGE SCANNING (SIS) Synchronised Image Scanning (SIS) is similar to the Mask Dragging (4) process with the major difference being the substrate moves continuously during pulsed laser triggering such that coincident with each laser pulse the image projected onto the substrate has moved by exactly one image repeat pitch. Figure 1 shows diagrammatically how this is accomplished for the case of excimer laser ablation of a long repeating structure such as an ink jet printer nozzle array. To demonstrate the SIS process the beam from a 248nm KrF excimer laser (Lambda Physik LPX210i) is homogenized and shaped to illuminate a rectangular area at the mask (typically up to 75mm long x 5mm wide) with a sequence of circular apertures on a regular pitch. The image is projected and reduced by typically 5x to create a 15mm long image containing over 100 circular spots at a density of 180 dots per inch (dpi). If the workpiece were to be held stationary during laser firing then eventually a single nozzle plate of 15mm length would be created. However if the substrate is moved continuously during drilling at a velocity given by the product of the image pitch and the laser repetition rate then images overlap exactly and superlong nozzle plates can be created.

2 The fundamental requirement for the SIS process is that images are exactly superimposed on each other with a displacement exactly equal to the image pitch. This is accomplished by triggering the firing of laser pulses from the stage position encoder output with suitably fast jitter-free electronics and using stages with high resolution. To produce ink jet printer nozzles with input aperture roundness requirements of <±0.5µm then the velocity control and resolution of the stages should be between nm. Laser beam Encoder pulses: y Trigger level for laser firing: z Static mask Projection Lens z = p x Encoder res: x Pitch: p Pitch: p Moving work piece Figure 1. Diagram of SIS technique and schematic of the laser pulse triggering As illustrated in Fig 1, during SIS each pass of the substrate under the stationary beam creates a long image in which each nozzle location has been illuminated sequentially by each of the projected spots in the full image field. The substrate is repeatedly passed under the beam to accumulate sufficient shots on each area to drill to the required depth in the substrate. For the case of an ink jet printer nozzle plate fabricated in 50µm thick polyimide typically 300 shots are required to drill through so 3 passes are needed for an image containing 100 nozzles. Using SIS to fabricate inkjet printer nozzles has several major advantages compared to the Step and Repeat methods used currently. Since each nozzle is no longer subjected to an individual unchanging illumination pattern and is created by the superposition of images from all areas of the mask pattern, all nozzle telecentricity errors (due to e.g. lens nontelecentricity) and exit nozzle diameter size errors (due to e.g. non-uniformity of mask illumination) are dramatically reduced. Figure 2. SEM picture of nozzles drilled by SIS technique: Entrance = 42 µm, Exit = 32 µm

3 A second major advantage of SIS for nozzle plate production is that high accuracy, high quality nozzle arrays can be readily fabricated with lengths limited only by the workpiece stage travel. Figure 2 is an SEM photograph of a section of a 50mm long, 50µm thick Upilex TM plate with ~180dpi SIS-drilled nozzles each with entrance and exit apertures of 42µm±0.5µm and 32±0.5µm respectively. To produce the staggered hole pattern required in such nozzles plates the step size for laser firing is 3 times the nozzle pitch. Like all mask image projection workpiece moving processes, the SIS technique creates image length regions at each end of the full scanned image where progressively less laser pulses per area are incident. Just as in conventional synchronized mask - workpiece scanning methods (5), synchronized movement of an aperture close to the mask at the ends of the machined structure (see Figure 3) can be used to obscure these regions. While these over scanned regions at the ends of the image clearly represent inefficiencies of SIS compared to Step and Repeat processing, they can be minimised by keeping the projected image size short. Provided the length is much less than the final full image length then such inefficiencies are minor and are more than compensated for by the increase in nozzle quality and repeatability. Laser beam Static mask Lens Moving aperture 1) 2) n) Moving work piece Figure 3. Synchronized moving aperture technique to eliminate ramping effects at each end of the image. From the first to the n-th hole in the mask for one nozzle end is shown; the other end operates in the reverse sense. Most ink jet printer nozzles require full taper angles of between 25 and 40. Wobbling of the mask image on the workpiece by mask or stage movement or by positioning rotating plates or PZT oscillating mirrors in the beam line between mask and image are techniques usually used to achieve this controlled tapering. However wobbling of the image to create a high taper angle leads to a significant increase in the number of laser pulses needed to drill through the film and in order to achieve submicron accuracy requires a highly sophisticated wobbling unit. SIS machining methods eliminate the need for image wobbling devices since the mask can contain a full range of apertures within its illuminated area to create a nozzle of the required taper angle. Figure 4 illustrates an SIS mask design used for fabricating holes with a controlled taper. = = = = = = Figure 4. Principle of mask design for drilling tapered nozzles Figure 5 shows holes from different 180dpi nozzle plates SIS-drilled with full taper angle of 30 created without any image wobbling. The hole on the left was created with a sequence of images increasing in size while the one on the right with a sequence of images decreasing in size. As can be seen the sequence with which images of different size are

4 projected onto the substrate does not affect the final profile of the hole created by SIS. By eliminating wobbling fewer laser pulses are required to drill to a specific depth, more than compensating for any inefficiencies introduced by the over-scanned end regions created by SIS. Figure 5. Comparison of nozzles demonstrating the drilling sequence of increasing or decreasing image size is unimportant. The left and right nozzles are drilled by progressively decreasing and increasing image sizes respectively. SIS is an ideally suited for the creation of arbitrarily long high-resolution nozzle plates. For example using a 250Hz excimer laser with stage speeds up to 106mm/sec and an image length of 15mm, 75mm long nozzle plates in 50µm thick material can be drilled with 150dpi staggered nozzle structures in < 9 seconds and 9 passes. The performance of SIS is fully realized when machining densely packed areas with for example a single feature size of 20µm corresponding to 1270 dpi. SIS can be used to machine with very high performance any 2.5D shape that has no undercuts and allows a major step forward to be made in the high-speed production of microstructures having feature sizes down to a few microns. For example to machine a 10µm depth pyramid structure the number of laser pulses required is small (e.g 50) with a pitch that can also be small (e.g. 20µm). In this example a beam size in the scan direction of only 1mm is sufficient to allow all 50 different image sizes to be applied in one pass. Since the allowable beam area at the substrate set by the available laser energy is typically ~12mm 2, a beam length of 12mm allows 600 lines of structures to be processed simultaneously. With a standard excimer laser operating at 250 Hz it is possible to yield 1 Million features in < 7 seconds - no matter how complex the single image, so long as its aspect ratio is modest e.g. depth/size < 0.2. Figure 6 shows three different pyramid structures - up standing, inverted and widely separated. All these ~10µm deep surfaces were created directly by SIS with one pass of the beam. With SIS all the image information is stored in the mask and therefore the mask design is very important to achieve the accuracy and quality of the final device. We have developed software programs that can compute SIS mask designs from 3-D CAD drawings of the desired microstructure to be machined. Figure 6. Micro structured surfaces with up coming, down going and wider separated pyramids.

5 SIS techniques are also very effective for manufacturing many other novel and complex microstructures. We have produced convex and concave spherical micro-lenses and micro-ramps and demonstrated SIS can efficiently structure large areas of material with complex miniature patterns such as those used to create molds for the manufacture of display enhancement films, sensors, optical devices. 3. BOW TIE SCANNING We have developed the Bow Tie Scanning (BTS) technique to allow the accurate, rapid regular patterning or scribing of thin metallic or inorganic layers on a variety of planar substrates. The main driving application for this work is the high-speed direct single pulse YAG laser patterning of the ITO or SnO 2 conductive layers on large >1m 2 area glass substrates for plasma display panel (PDP) manufacture. The BTS technique is also likely to be of major importance for many other applications in which regular lines or dense patterns of contact holes in a regular array are required - as for example in solar panel and solar cell laser scribing. As illustrated in Figure 7, the key feature of BTS is that a focused laser beam or a demagnified image of an aperture illuminated by the laser is scanned in a straight line at high speed across a section of the substrate by for example galvanometer-driven mirror deflection while the substrate is moved on a linear stage at constant speed in the orthogonal direction. After each transverse scan the galvanometer mirror decelerates, reverses and performs a scan in the opposite direction. The lines drawn on the substrate are parallel and separated by the required pitch. In order to track the motion of the substrate the galvo scanner unit has a second mirror unit that deflects the beam in the direction parallel to the stage travel. The combined motion of both galvo axes leads to a crossed beam trajectory resembling a bow tie shape. Scanner unit Absolute laser beam position Laser beam translation Laser off Laser on Laser on Laser off Substrate translation Figure 7. Principle of BTS with motion compensation through the scanner which provides straight lines with a typical bow tie shape of the scanner movement. This type of combined beam scanning and substrate motion technique is not new and has been used extensively for scribing and microvia drilling operations (6). We have now extensively developed the technique to allow for it to be used for the high-speed creation of accurately positioned dense cell patterns on large area PDP substrates. The innovations we have introduced that allows BTS to be used effectively for patterning of PDPs are: 1. Use of high (~200W) power, up to 10kHz repetition rate, multimode 1.06µm diode-pumped Nd:YAG lasers incorporating precisely synchronised Q-switch firing of pulses together with beam positioning on the substrate by the stage and scanner control system. 2. Use of beam homogenisation and beam shaping techniques to create a quasi-uniform top-hat rectangular intensity distribution at the plane of a mask and on the workpiece. 3. Use of mask projection techniques incorporating scanner and F-theta optics with robust free-standing transmissive masks to create individual PDP cell patterns with T-bar or other complex electrode geometries.

6 Figure 8 is a schematic diagram of the beam homogenization, shaping and projection system developed to create correctly a beam that matches the PDP pixel cell size and shape. Beam shaping & homogenising optics Mask projection optics Multimode High power Yag-laser Beam expander Field lens Mask Scanner unit Beam shaping & homogenising unit f-theta lens Substrate Laser output beam Beam at the mask Figure 8. Arrangement for beam shaping and mask projection used for BTS patterning For initial process tests a modified Cutting Edge Optronics Scimitar laser producing > 30mJ at 3kHz repetition rate was used. At 3kHz the laser pulse duration is ~ 80ns. Control of the firing sequences of the laser pulses was achieved by precision triggering the acousto-optic (AO) Q-switch. We estimate the laser had an M 2 ~ 15 with sufficient transverse modes to minimize any residual interference effects produced by the homogeniser optics. Fluctuations to the average of the top-hat fluence distribution at the mask were < ±10% peak to peak. Such variations are within the acceptable process window for ablative removal of thin films of ITO or SnO 2. Figure 9 shows a sequence of 0.5 x 0.5mm cells whereby a 30Ω indium-tin-oxide (ITO) layer has been cleanly removed in one pulse at a 1.06µm laser fluence of 12J/cm 2 using the optical system shown in fig 8 without an aperture mask. The picture shows the shape and high quality edge sharpness achieved at the mask plane using just the homogenizer alone. Many experiments have been carried out with Nd:YAG lasers for direct ablative removal of thin transmissive conductive oxide (TCO) films such as ITO. To avoid damage to the glass substrate by the high peak laser fluences required for complete clearing of the TCO film usually has necessitated the use of strongly apertured quasi- Gaussian beams. Such aperture vignetting greatly reduces the potential efficiency and speed of the process. Use of the homogenization techniques discussed here overcomes these problems and allows for the highly efficient use of nearly all the available laser power. Figure 9. Single pulse ablation of ITO at 1.06µm. 0.5 x 0.5mm cells at 12J/cm 2 Insertion of a mask allows complex electrode structures to be created in each pixel cell. Figure 10 shows an area of ITO film patterned with T bar structures on a PDP glass panel using the BTS process. We manufactured free-standing masks for this process by laser cutting apertures in silicon wafers to the shape and size required. The beam direction is

7 along the continuous track with a cell pitch along the tracks of 0.25mm. While some evidence for residual traces of ITO at the boundary between cells can be seen, since it lies under the cell separation ribs and has a high resistivity it is not expected to affect overall PDP operation. Figure 10. ITO removed T-bar structures created by the BTS technique Experiments have shown that all TCO films appropriate for manufacturing PDPs can be structured effectively with single Nd:YAG laser pulses at fluences between 6-16J/cm 2. Transfer of these concepts to real PDP production requires full understanding of a few key constraints. 42 VGA PDPs typically have around 1.2 million cells each with an area of about 0.4mm 2 so using the BTS technique will require a laser producing 60-65mJ/pulse. On the other hand 42 XGA PDPs have over 3 million cells of smaller area (e.g 0.15mm 2 ) that will require only about 25mJ/pulse from the laser. Since one laser pulse processes one cell the laser repetition rate is a function of the cell pitch and the beam speed. Constraining the scanner turnaround time to a value significantly less than the scan time and with a scan length set to a reasonable value over which accuracy can be readily maintained (e.g. 30mm) limits the scan speed to less than 2-3m/sec. These beam speeds then set the maximum laser repetition rate at about 15kHz for a typical 42 XGA display necessitating an average laser power requirement close to 400W. Such high power multimode Nd:YAG lasers are now being offered by some manufacturers. Because such Nd:YAG lasers are currently unable to deliver such high powers at low repetition rates, for BTS processing of 42 VGA displays the repetition rate will be limited to about 3kHz (200W average power) in order to maintain the requisite pulse energy. A 400W, 15kHz Nd:YAG laser is able to process a single 30mm wide band down a 42 XGA PDP in about 12sec. To cover the entire area of a 42 PDP requires 8 such parallel optical scribing heads each processing 4 adjacent bands sequentially with the beams stepped sideways between passes. When alignment and loading times are included such a tool can process one 42 PDP within 1min. Our discussions here have concentrated on the use of BTS techniques for TCO structuring for PDP manufacture since these are the most demanding in terms of beam position control, laser timing control, accuracy of position over very large substrates with the added requirements of high process reliability, enhanced tool functionality while maintaining tool simplicity. In addition this application is one in which introduction of laser processing is likely to have the most dramatic economic impact since the present lithographic techniques used for TCO patterning are become increasingly expensive and yields reduce as panel sizes continue to enlarge. Of course BTS techniques are also applicable to many other application areas where lower accuracy and smaller substrates will ease control issues. Scribing parallel grooves

8 and bus bars in crystalline silicon solar cells as well as the formation of solar batteries by scribing interconnect lines in the ITO, α Si and metal thin film layers on glass substrates are examples of other industrial areas that will benefit by the use of BTS processing. 4. CONCLUSIONS Two new pulsed laser micromachining methods Synchronized Image Scanning (SIS) and Bow Tie Scanning (BTS) which incorporate mask projection techniques have been developed. Both enable the high-speed and high-accuracy manufacture of dense repeating microstructures in a diverse range of devices to be produced. Areas demonstrated for which these techniques have been implemented include the manufacture of jet printer nozzles, plasma display panels, micro-optical devices and arrays, solar cells, batteries and related devices. 5. REFERENCES 1. M C Gower, Excimer Laser Micromachining: A 10 year perspective in Laser applications in Microelectronic and Optoelectronic Manufacturing IV. SPIE 3618, 251 (1999). 2. P T Rumsby, E C Harvey and D W Thomas: Laser Microprojection for Micromechanical Device Fabrication in Experimental Mechanics: Advances and Applications, SPIE, 2921, 684 (1996). 3. M C Gower, Industrial Applications of Pulsed Lasers to Materials Processing in High Power Laser Ablation, SPIE 3343, 171 (1998). 4. E C Harvey and P T Rumsby Fabrication techniques and their application to produce novel micromachined structures and devices using excimer laser projection in Micromachining and Microfabrication Process Technology III, SPIE 3223, 26 (1997) 5. N H Rizvi, P T Rumsby and M C Gower New Developments and Applications in the Production of 3D Microstructures by laser Micro machining in Photonic Systems and Applications in Defense and Manufacturing SPIE, 3898, 240 (1999). 6. C Dunsky Beam shaping applications in laser micromachining for the microelectronics industry in Laser Beam Shaping II, SPIE 4443, 135 (2001)