Laser Profiling of 3-D Microturbine Blades

Transcription

1 Laser Profiling of 3-D Microturbine Blades Andrew S. HOLMES *, Mark E. HEATON *, Guodong HONG *, Keith R. Pullen ** and Phil T. Rumsby *** * Optical & Semiconductor Devices Group, Department of Electrical & Electronic Engineering, Imperial College London, Exhibition Road, London SW7 2BT, United Kingdom a.holmes@imperial.ac.uk ** Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2BX, United Kingdom *** Exitech Ltd, Oxford Industrial Park, Yarnton, Oxford OX5 1QU, United Kingdom We have used KrF excimer laser ablation in the fabrication of a novel MEMS power conversion device based on an axial-flow turbine with an integral axial-flux electromagnetic generator. The device has a sandwich structure, comprising a pair of silicon stators either side of an SU8 polymer rotor. The curved turbine rotor blades were fabricated by projection ablation of SU8 parts preformed by conventional UV lithography. A variable aperture mask, implemented by stepping a moving aperture in front of a fixed one, was used to achieve the desired spatial variation in the ablated depth. An automatic process was set up on a commercial laser workstation, with the laser firing and mask motion being controlled by computer. High quality SU8 rotor parts with diameters of 13 mm and depths of 1 mm were produced at a fluence of.7 J/cm 2, corresponding to a material removal rate of ~.3 µm per pulse. A similar approach was used to form SU8 guide vane inserts for the stators. Keywords: excimer laser micromachining; SU8; microturbines; mask-dragging; flow sensing; power generation 1. Introduction A key feature of laser micromachining processes is their ability to produce structures with complex shapes that cannot be realised using conventional microfabrication methods. This feature, together with the ability to process a wide range of materials, has led to many diverse applications of laser processing in recent years [1]. In this paper we report on the use of excimer laser micromachining in the fabrication of a new kind of micropower conversion device. The device, shown in cross-section in figure 1, consists of a mm-scale axial-flow microturbine with an integral axial-flux electromagnetic generator. An axial flow of air (or other gas) through an annular fluid channel causes the rotor to turn, while permanent magnets embedded in the rotor induce output voltage in planar coils on the stators. Axial-flow microturbines are difficult to realize by conventional microfabrication methods, because to produce the necessary curved profiles on the rotor blades and guide vanes (see Figure 2) it is necessary to fabricate structures where the sidewall angle evolves with depth in a controlled manner. We have used excimer laser micromachining with a variable aperture mask to produce suitable sidewall profiles in SU8 polymer parts formed by conventional UV lithography. Fig. 1 Schematic cross-section of microturbine. The use of microturbines for generation of electrical power has been investigated previously by a number of groups [2,3]. However, the emphasis has been mainly on radial flow devices designed to operate in combination with a compressor and burner. Our aim was to develop a new class of device that can extract power directly from an air stream rather than burning a fuel. The axial-flow geometry is the obvious choice for this kind of application, because it can operate at low pressure ratios [4]. Applications are envisaged in areas such as wireless sensing, where the

2 turbine could be used to power a remote sensor and a shortrange radio transmitter. In the case of a flow sensor or an airspeed sensor, the turbine could also perform the sensing function, with the advantage over other sensor types of generating a frequency output performs for these parts were formed on silicon substrates by UV photolithography in SU8 layers. A two-level process was used to allow for a stepped aperture at the rotor centre for locating a shaft (see Figure 1). The performs were released at the end of the process by wet etching of a copper sacrificial layer. Figure 3 shows a rotor preform made by this route. Fig. 2 Suitable rotor blade and guide vane profiles for the device in Figure 1. In the following sections, we give a brief description of the overall fabrication process for our device, followed by a detailed discussion of the laser micromachining process. Experimental data for the variation of rotation speed and output voltage with nitrogen flow rate for a prototype device are also presented. 2. Overall fabrication process Referring to Figure 1, the microturbine has a sandwich structure consisting of a pair of silicon stators either side of an SU8 polymer rotor. The guide vanes are formed in SU8 parts that are inserted into the stators during final assembly. The overall size of the first prototypes was set by the decision to use conventional 3 mm-dia ball-race bearings, and commercial 1 mm-dia rare earth permanent magnets. This led to a design with inner and outer radii of 2.1 mm and 3.7 mm for the electromagnetic generator section, and 4.5 mm and 6. mm for the turbine annulus. The rotor thickness was made equal to the permanent magnet length of 1 mm. The stators were fabricated by double-sided processing of 4 -dia thermally oxidised silicon substrates. First, cavities were etched in the back side by deep reactive ion etching (RIE), and filled with electroplated nickel to form softmagnetic pole pieces for the generator. Second, two-layer planar coils, embedded in SU8, were formed on the front side by multi-layer UV lithography and copper electroplating. Finally, the SU8 on the front side was used as a mask for through-wafer etching of the bearing aperture and fluid channel by deep RIE. Further details of this process are contained in [5]. The rotors and the guide vane inserts could have been laser machined from bulk SU8 material. However, this would have been unnecessarily slow and expensive. Instead, Fig. 3 SEM photograph of an SU8 rotor preform prior to laser micromachining. 3. Laser profiling of rotor blades and guide vanes The rotor blades and guide vanes for our device were designed using standard turbomachinery practice, assuming a nominal shaft speed of ~3, rpm and a pressure ratio of 1.5 [5]. For simplicity, the same cross-sectional shape, shown in Figure 4a, was used both for the rotor blades the inlet stator guide vanes, with straight guide vanes being used on the outlet stator. No radial variation in cross-sectional profile was allowed. The dashed line in Figure 4a shows the cross-sectional shape of the block of SU8 within the preform from which each blade or vane was machined. Fig. 4 (a) Blade profile for inlet stator guide vanes and rotor blades (dimensions in µm); (b) variable aperture laser machining.

3 3.1 Laser machining strategy The problem of laser machining a structure such as the one in Figure 4a can be approached in several ways. One method would be to direct-write the structure by scanning a focused laser spot over the resist surface. However, this approach is more applicable to solid-state laser machining, where the pulse energy is too low to process larger areas in parallel. In the case of excimer laser machining, where mask projection methods are normally used, several techniques are available, including indexed mask projection (i.e. sequential exposure using multiple static masks), mask- and workpiecedragging, and half-tone masking [6,7]. We adopted a variant of the mask-dragging technique in which a moving mask is used in conjunction with a fixed mask as shown in Figure 4b. Note that the masks are shown in proximity to the workpiece in this illustration, although in practice mask projection was used. The role of the fixed mask was simply to ensure that the laser exposure was confined to a single blade, regardless of the position of the moving mask. The fixed mask therefore comprised a single rectangular aperture corresponding to an exposed area slightly larger than one blade (15 µm long x 6 µm wide). The moving mask was used to shield parts of the blade surface from laser exposure once they had been machined to the desired depth. Two laser micromachining operations were performed for each blade, one for the concave surface and one for the convex surface. In each case, the blade surface was machined in a number of steps, each involving moving the edge of the fixed mask to a specified lateral position X n (see Figure 4b), and firing the laser until the machined depth of the exposed material increased to the desired value Y n. The number of pulses at each step of the process was therefore (Y n Y n-1 )/d, where d was the machined depth per pulse. The sample stage was elevated at each step to ensure that the exposed surface of the sample remained in focus. This was found to give better results in terms of machining quality than maintaining focus at the top surface of the workpiece. To reduce the number of alignment operations required, no attempt was made to round the leading and trailing edges of the blades as in Figure 4a; the outline of each blade was therefore defined by machining two 6 µm-radius arcs, and a 36 µm-high vertical section on the concave side. 3.2 SU8 ablation characteristics Preliminary tests were carried out to determine the ablation characteristics of cross-linked SU8 at 248 nm wavelength. Figure 5 shows an ablation curve obtained from a single 5 pulse exposure at 1 J/cm 2 fluence using a halftone calibration mask. The material was found to have an almost ideal Beer s Law ablation characteristic above about 2 mj/cm 2 fluence, and to etch relatively cleanly, without excessive ablation debris. Based on these initial measurements, a fluence of 7 mj/cm 2 was selected for subsequent experiments, corresponding to a material removal rate of ~.3 µm/pulse. Etch depth per pulse (µm) Fig Fluence (mj/sq.cm) Measured variation of ablation depth per pulse with fluence at 248 nm wavelength for SU Experimental setup Figure 6 shows the key elements of the Exitech Series 7 laser micromachining workstation used for this work. This comprised a KrF excimer laser, beam delivery optics, a projection lens (4X reduction), and a set of precision translation stages. Motion of the stages and the laser firing were under computer numerical control. The workpiece (rotor or stator insert) was held on a specially designed rotational mount that allowed it to be rotated about its axis. This meant that multiple blades or guide vanes could be processed sequentially without lateral re-alignment of the workpiece to the masks. The mount was additional to the standard XYZ sample stage of the workstation. The moving mask was mounted on a vertical translation stage that was also under computer control. Fig. 6 Laser machining setup in Exitech Series 7 workstation. The elevation of the sample stage was adjusted to produce a focused image of the moving mask at the machining surface. This resulted in the fixed mask being

4 slightly defocused, because the two masks were separated along the optic axis by ~27 mm. This defocus was acceptable because the edges of the fixed mask were not projected onto the machined structure; three of the four edges of the exposure field were defined by the moving mask, while the fourth, which was defined by the fixed mask, was positioned to fall in the gap between adjacent blades. Alignment of the workpiece to the projected image of the masks was achieved using an off-axis camera. Dummy samples were exposed, and translated off-axis to lie in the microscope field. Alignment markers were then placed on the screen to identify the boundaries of the exposed region. The workpiece was aligned to these markers, and then translated back to the laser axis. This kind of secondary alignment is widely used in laser micromachining, and typically achieves overlay accuracy of several microns. 4. Results Figures 7 and 8 show SEM view of laser machined rotors viewed from the outlet and inlet sides respectively. Good matching to the target profiles, and a high degree of blade-toblade uniformity was achieved. A small amount of ablation debris is evident on the surfaces of the rotor hub and rim, but apart from this the structures are clean. Figure 9 shows a close-up SEM view of a convex blade surface. This image clearly shows the fine structure that arises from moving the mask in a number of discrete steps. The values (Xn, Yn) were chosen to make the step size, defined as [(X n X n-1 ) 2 + (Y n Y n-1 ) 2 ] 1/2, uniform over the curved surface. This was achieved by making X n and Y n take the forms: X n = Rsin(n θ) : Y n = R[1 - cos(n θ)] where R = 6 µm is the radius of curvature. A value of 1 was chosen for θ, giving a total of 9 steps over the complete quadrant. Fig. 8 Fig. 9 SEM photograph of laser profiled SU8 rotor, viewed from inlet side. Close-up SEM photograph of convex blade surface, showing fine structure in laser micromachined surface. Fig. 7 SEM photograph of laser profiled SU8 rotor, viewed from outlet side. Some surface defects are evident at the trailing edges of the blades in Figure 9. These were introduced by the laser micromachining process, and are thought to result from localised melting towards the end of the laser micromachining process that forms the concave surface. If so, this effect could be difficult to eliminate, since thinning of the blade tip will tend to inhibit cooling by conduction to the bulk material. Figure 1 is a more oblique view of the trailing edges of the blades, showing the stepped structure on the concave surfaces. These surfaces were machined in a series of 4 constant-height steps, each with a step height of 15 µm. Note that there is some tapering of the blade thickness in the radial direction, the cause of which is unclear at present. However, the average thickness along the length of the blade is close to the design value of 75 µm.

5 conventional UV lithography. High quality parts were produced, with good reproduction of the designed blade profiles, and low levels of ablation debris. The process was relatively fast, with a machining time of around 2 minutes for each blade at a laser repetition rate of 5 Hz. Nevertheless, for routine manufacture laser micromachining would most probably be used to define polymer masters from which production parts would be produced by replication. Prototype devices have been tested with compressed nitrogen, and shown to deliver output powers in the mw range at flow rates in the range 3 to 6 litres per minute. Future work will be focused on increasing the turbine efficiency to allow operation at lower pressure ratios. Smaller devices incorporating micro-engineered bearings and permanent magnets deposited by, for example, printing or electroplating will also be investigated. Fig. 1 Close-up SEM photograph of convex blade surface, showing trailing edges of blades. Prototype microturbines with laser micromachined rotors and stator inserts have been assembled and tested with compressed nitrogen. Figure 11 shows the measured variation of rotation speed and generator output voltage per stator for a typical device. At a rotation speed of 4, rpm, each stator on this device can deliver a power of approximately 1.1 mw into a matched (~4 Ω) load. This level of power is sufficient for many remote sensing applications. However, the pressure ratios required to generate these power levels are currently rather high as a result of low turbine efficiency. Rotation speed (krpm) Left axis Right axis Flow rate (LPM) Fig. 11 Measured variations of rotation rate and peak-to-peak output voltage per stator with nitrogen flow rate for a prototype device. 5. Discussion Excimer laser micromachining with a variable aperture mask has been used to define the curved rotor blades and guide vanes for an axial-flow microturbine device. To minimise the laser machining time, the blades and guide vanes were machined into SU8 preforms made by Output voltage (V) Acknowledgements This work was supported by the UK Engineering and Physical Sciences Research Council under Grant No. GR/N18895 Microengineered Axial-Flow Pumps and Turbines. The authors are grateful to Karl Boehlen and Jason Dallimore, both of Exitech Ltd, for their assistance with the laser micromachining work. References [1] M.C. Gower, Industrial applications of pulsed lasers to materials processing, Proc. SPIE 3343, (1998) 171. [2] T.G. Wiegele, Micro-turbo-generator design and fabrication: a preliminary study, Proc. IECE 96, vol. 4, (1996) [3] A. Mehra, A.A. Ayon, I.A. Waitz, M.A. Schmidt, Microfabrication of high-temperature silicon devices using wafer bonding and deep reactive ion etching, IEEE J. Microelectromech. Syst., 8(2), (1999) [4] B.S. Massey: Mechanics of fluids, 6 th edition, Routledge, [5] G. Hong, A.S. Holmes, M.E. Heaton, K.R. Pullen: Design, fabrication and characterization of an axialflow turbine for flow sensing, Proc. Transducers 3, Boston MA, 8-12 June, (23) [6] N.H. Rizvi, P.T. Rumsby, M.C. Gower, New developments and applications in the production of 3D micro-structures by laser micro-machining, Proc. SPIE 3898, (1999) [7] F. Quentel, J. Fieret, A.S. Holmes, S. Paineau, Multilevel diffractive optical element manufacture by excimer laser ablation using halftone masks, Proc. SPIE 4272, (21)