ULTRAPRECISION MICROMACHINING OF MICROFLUIDIC DEVICES BY USE OF A HIGH-SPEED AIRBEARING SPINDLE

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1 ULTRAPRECISION MICROMACHINING OF MICROFLUIDIC DEVICES BY USE OF A HIGH-SPEED AIRBEARING SPINDLE Chunhe Zhang 1, Allen Y. Yi 1, Lei Li 1, L. James Lee 1, R. Ryan Vallance 2, Eric Marsh 3 1 The Ohio State University 2 George Washington University 3 Pennsylvania State University INTRODUCTION Recently design and fabrication of functional micro components such as microfluidic devices have been the focus of increasing research activities. High performance microfluidic devices are used in a large number of biomedical and medical applications. Traditional fabrication techniques for these microfluidic devices include lithography, laser micro machining and micro mechanical machining. Compared to its counterparts, the micro mechanical machining has the advantages of lower fabrication cost, higher process flexibility, a broader selection of workable materials, and the possibility of creating true 3D microstructures as reported in some recent studies[1-2]. Therefore, application oriented research and technical development related to the micro mechanical machining are drawing increasing attention in micro and nano scale technology and manufacturing engineering [1-2]. Micro mechanical machining is derived from conventional machining process by downscaling the cutting tool sizes and hence the removal volume per cut. However as shown in recent studies, the established technical knowledge for macro mechanical machining cannot be readily transferred to micro mechanical machining due to the unique process characteristics introduced by the tool size and the removal volume in micro mechanical machining [1-2]. In micro mechanical machining, the cutting mechanics (i.e. the material removal mechanism) involved is different from its macro counterparts. Consequently process parameters such as cutting force ratios, specific energy, chip formation, burr formation, tool failure, material machinability and achievable surface roughness, differ considerably from those obtained in the macro mechanical machining [1-2]. Unfortunately, until recently majority of these issues are not well understood [1-2]. The lack of process knowledge and skills in micro mechanical machining has to some extent hindered its implementation in industry. Logically, machine tools with higher accuracy are required for micro mechanical machining. It is understandable that even a small amount of error motion in spindle rotation or slide movement can have a profound influence on the stability of a micromilling process using a small 25 µm diameter tool. Most of the existing studies associated with micro mechanical machining are based on conventional machine tools and micromilling bits with larger than 100 µm diameters [1-2]. In this study, an ultraprecision micro mechanical machining system is built both for micromilling and microgrinding. The configuration of the test system includes a 350 FG ultraprecision machine from Moore Nanotechnology and a high-speed airbearing spindle from Professional Instruments Company ( The engineering materials tested in this study include PMMA for microfluidic devices, and glass, silicon and stainless steel substrates used as either mold inserts or directly as devices. ULTRAPRECISION MICRO MECHANICAL MACHINING SETUP The test setup, as shown in Fig.1, comprises the nanotech 350 FG machine tool, a Professional Instruments high-speed airbearing spindle, micromilling tools made of diamond and carbide and microgrinding wheels made of PCD (polycrystalline diamond). A Kistler MiniDyn multicomponent dynamometer 9256C was used to collect cutting force information. The straightness of X and Z linear slides is 0.25 µm over 300 mm travel with a feedback resolution of 8.6 nm. The Professional Instruments airbearing spindle was designed to operate at a maximum speed of 60,000 rpm and has an extremely low rotational error motion (less than 25 nm). The diamond micromills have two V-shaped cutting edges, with 9 µm tip radius. The carbide micromills are designed with two spiral flutes, similar to a twist drill bit. Their diameters are 25 µm and 50 µm respectively. These diamond and

2 carbide micromills are commercially available. The microgrinding wheels were made of sintered PCD in one of the authors lab (Vallance) using WEDM process (micro wire electrical discharge machining). Fig.2 is an SEM (scanning electron microscope) view of these tools used for our experiments. The MiniDyn dynamometer 9256C has a noise threshold of 2 mn, and is one of the most sensitive dynamometers commercially available. Its bandwith is around 5.0 khz. In our experiments, the dynamometer was used to test and identify the critical process parameters for micromilling and microgrinding processes. For microscopic analysis, an SEM (Hitachi S-4300 FESEM) was used to observe and analyze the tool edge sharpness and the surface quality. thus it was difficult to select the proper process conditions for the micromilling test, especially when using small tools such as the 25 µm and 50 µm carbide micromills. In our study, a tapered cutting (milling) technique was employed to determine the process parameters, as shown in Fig.3. During the tapered milling, the milling forces were recorded as an indicator of tool breakage. Cutting direction Rotating tool 0.35 degrees Workpiece Mills or wheels Workpiece Inclination angle Dyno 9256C Fig.3 Schematic of taper milling Dyno 9256C PI high speed airspindle Fig.1 View of ultraprecision micro mechanical machining setup By calculating the cutting distance, the safety conditions (no breakage) for the two carbide mills are obtained and shown in Table 1. For diamond mills, a feed rate of 10mm/min, and depths of cut of 10 µm ~ 200 µm were used in our experiments. Table 1: Recommended process conditions for PMMA micromilling Spindle speed Feed rate Depth of cut 25µm micromill diamond micromill PCD grinding wheel 10,000 and 20,000 rpm 3 mm/min for 50 µm mills 1 mm/min for 25 µm mills 1 µm ~ 150 µm for 50 µm mills 1 µm ~ 30 µm for 25 µm mills Fig.2 View of micromills and PCD grinding wheel MICRO MILLING OF PMMA FLUIDICS PMMA (polymethylmethacrylate) microfluidic components are widely used in biomedical devices as the replacement of glass counterparts in recent years due to its low process cost and biomedical compatibility. 3 PMMA has a density of 1.19 g/cm and has a relatively low water absorption rate. PMMA also has good mechanical strength and dimensional stability. Micromilling experiments were conducted on the test setup shown in Fig.1 using both diamond and carbide mills. Since available data on the machinability of PMMA are not readily available Fig.4 shows some examples of machined PMMA microfluidics. Fig.4 (a) is an 8-reservoir reactor, Fig.4 (b) is a 2-pillar reactor mold, Fig.4 (c) is part of a 25 µm wide 10 mm long channel, and Fig4 (d) is a micro-well array. Among these microfeatures, Fig.4 (a) and (b) are machined by use of diamond mills, and Fig.4 (c) and (d) are with a 25 µm carbide mill. These experimental results have confirmed the capability of micromilling for directly manufacturing of high quality micro channels and other geometrical microfluidics in PMMA. The achievable minimum width of the microchannels is around 25 µm (or the tool diameter), and the minimum width of the walls is less than 1 µm.

3 (a) 8-reservoir reactor (c) 25µm X 10mm channel (b) 2-pillar reactor mold (d) Micro-well array Fig.4 Micromilled PMMA microfluidics Fig.5 shows the edge properties of machined channels and wells, in which Fig.5 (a) is machined using a 25 µm carbide mill, and Fig.5 (b) is with a diamond mill. In micromilling of PMMA with diamond mills, no burrs were formed along the edges of the channel and the well, while with carbide mills, a number of burrs were left along the edges of the channel. For the carbide mills the sharpness of cutting edges was around 1 µm to 10 µm, while for diamond mills it is down to 50 nm. Therefore, in micromilling with carbide mills more plowing actions were involved in the chip formation process or the removal process. On the other hand, the extreme sharpness of diamond mills can easily cut the chip out without incurring too much plastic deformation and flow. It can be reasonably concluded that the edge sharpness was the main cause for the excessive burr formation, rather than the process parameter. Therefore, burr can be more effectively controlled or eliminated by sharpening the tool edge, rather than changing process parameters in micro machining. (a) Burr formed along channel edge (b) Clean edge, no burr Fig.5 Edge properties and burr formation MICRO GRINDING OF GLASS, SILICON, STAINLESS STEEL FLUIDICS AND MOLDS Although PMMA is becoming a popular material for microfluidics and biomedical devices, glass, silicon, and stainless steel are frequently selected for some specific applications. In addition, mass production of PMMA micro fluidics requires durable molds. Glass, silicon and stainless steel are mold materials for hot embossing or injection molding of PMMA microfluidics. Therefore, micromachining of these materials were tested and explored on the same setup. The microgrinding wheels were prepared using micro WEDM. The details of the process can be found elsewhere [3-4]. It was discovered that wheel preparation [3-4], i.e. the process parameters of the WEDM can significantly affect the wheel performance. Microgrinding mechanism and process design were not studied as much as micromilling, while this lack of know-how inadvertently hindered the development of high quality grinding wheels. To determine the proper process parameters, a tapered grinding test was conducted in a similar fashion as was shown in Fig.3. In this experiment, force measurements were used to analyze the interaction of the grinding wheel with the workpiece. The inclination angle of the workpiece was 0.15 degrees, and the diameter of the PCD grinding wheels was 100 µm. The test material was single crystal silicon. Fig.6 shows a microscopic view of the machined slot. The SEM scans of the ground silicon substrate revealed some interesting features in microgrinding of single crystal silicon with a 100 µm diameter grinding wheel. As shown in Fig.6, from the start of the grinding process to when the PCD tool failed, the process of the tapered grinding of silicon experienced four different steps, i.e. unstable cutting-in, initial stable cutting, unstable cutting II, and stable cutting II. Each of these four cutting steps was related to a different edge chipping status and a different level of surface roughness. In the unstable cutting-in zone, edge chipping could be seen clearly, although it was small. The bottom surface was smooth. In the initial stable cutting zone, edge chipping was reduced and nearly invisible but the quality of the bottom surface deteriorated, especially near the unstable zone II. Inside the unstable zone II, edge chipping increased and the machined channel lost its design geometry. In the stable cutting zone II, the minimum edge chipping and the best bottom surface were achieved. At the moment of tool breakage, the most serious chipping was observed, as shown in the figure.

4 The profile of the grinding force in the tapered grinding with a 100 µm PCD tool is plotted in Fig.7. It is evident that the force profile reflected clearly each of the 4 grinding steps illustrated above. During the period of the initial cutting-in, grinding force was relative small, but did increase with increasing depth of cut. In the stable zone I, the grinding force increased significantly, and the band of force variation became wider. However as the process progressed into unstable zone II, the grinding force decreased to a very low level, but was dotted with occasional peaks. This represented that fracture breakage and serious edge chipping were dominating the grinding process. In the stable zone II, the grinding force again steadily increased with increasing depth of cut. In the last step, both fracture breakage and ductile removal contributed to the removal of the material, while the latter may play a larger role. Grinding force, N Grinding time, sec. Fig.7 Force profile in taper grinding of silicon Based on the tapered grinding experiment, the recommended conditions for micro grinding of silicon with a 100 µm PCD tool are derived and summarized in Table 2. For glass and stainless steel, the proper process parameters can be determined using similar setup. Table 2: Recommended for silicon microgrinding Spindle speed Feed rate Depth of cut 40,000 rpm 0.5 mm/min 2.5 µm ~ 4 µm for stable cutting zone I 10 µm ~ 20 µm for stable cutting zone II Fig.8 shows some examples of the machined microchannels and microwalls in Pyrex glass and stainless steel by use of micro grinding with 100 µm diameter PCD tool. The microscopic images of the machined microchannels demonstrated the feasibility of using PCD microgrinding to fabricate microfluidics on advanced materials. In our experiments, the achievable minimum width of the wall is less than 0.5 µm, while the width of microchannels is limited by the wheel diameter. For glass, by selecting the proper process parameters, the extent of edge chipping can be reduced to the required level. For stainless steel, burr formation is difficult to eliminate by changing the process parameters alone. As shown in Fig.9, three depths of cut were tested to evaluate the effect on the grinding-induced burr formation. No obvious improvement was observed. Therefore, a post polishing or other burr-removing process should be used to clean the surface and the edge of the fluidics or molds made by

5 microgrinding with 100 µm PCD tools before use. (a) Pyrex glass (b) Stainless steel Fig.8 Glass and stainless steel microchannels Depth of cut 1µm Depth of cut 2µm Depth of cut 3µm Fig.9 Effect of depth of cut on burr formation CONCLUSIONS A precision high speed airbearing spindle was integrated into an ultraprecision machine for both micromilling and microgrinding tests. Micromilling of PMMA and microgrinding of silicon, glass, and stainless steel were carried out on the system using three different micro tools, i.e. diamond mills, carbide mills and PCD grinding tools. Micromilling is a capable and cost-effective method to fabricate PMMA microfluidics. The minimal width of microwalls that was achieved is less than 1µm, while the width of microchannels is limited by the tool diameter, around 25 µm. In micromilling, burr formation can be reduced or eliminated using extremely sharp tools, such as diamond micromills. Microgrinding is capable of producing micro features in glass, silicon and stainless steel. The minimal achievable width of micro walls is less than 0.5 µm, while the width of microchannels is limited by the tool size similarly to micromilling. In microgrinding of glass and silicon, the extent of edge chipping can be reduced significantly by properly selecting the process parameters. Furthermore, it was discovered that in microgrinding of silicon with a 100 µm PCD grinding wheel, 4 distinctive stages were observed as the depth of cut increased, i.e., from totally unstable, to slightlystable, to seriously-unstable, and finally to stable. For stainless steel, no major impact was observed on the burr size by changing process parameters alone. ACKNOWLEDGEMENT The authors would like to thank Professional Instruments for providing the high speed airbearing spindle for this research. This material is also based upon work partially supported by National Science Foundation under Grants No. EEC and CMMI Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. REFERENCES [1] Masuzawa T. State of the art on Micro Machining. CIRP Annals. 2000; 49; [2] Zhang C, Brinksmeier E, Rentsch R. MicroUSAL Technique for the Manufacture of High Quality Microstructures in Brittle Materials. Precision Engineering. 2006; 30; [3] Vallance R R, Morgan C J, Shreve S M, Marsh E R. Micro-tool Characterization Using Scanning White Light Interferometry. Journal of Micromechanics and Microengineering. 2004; 14; [4] Morgan C J, Vallance R R, Marsh E R. Micro-machining and Micro-grinding with Tools Fabricated by Micro Electro-discharge Machining. International Journal of Nanomanufacturing. 2006; 1;