Micromachining for the Precision Fabrication of Microfluidic Devices

Micromachining for the Precision Fabrication of Microfluidic Devices Athulan Vijayaraghavan, Jeffrey Hartnett, David Dornfeld Laboratory for Manufacturing and Sustainability, University of California, Berkeley Abstract Microfluidic devices are gaining popularity in a variety of applications, ranging from molecular biology to bio-defense. However, widespread adoption of this technology is constrained by the lack of efficient fabrication methods. This paper reviews research on the application of micromachining technology in fabricating microfluidic devices, done using the Mori Seiki NV1500 DCG Vertical Milling Machine as part of the MTTRF machine loan program at UC Berkeley. Micromachining is applied in the precision manufacturing of process tooling for two fabrication methods: microinjection molding and roller imprinting. The versatility and capabilities of micromachining in enabling high-volume microfluidic device fabrication is discussed. Keywords: Microfluidics, Micromachining, Microinjection Molding, Roller Imprinting 1 INTRODUCTION High precision machining is an integral component in the development of new products and process technology. Even when not applied in the manufacture of the product itself, machining is extensively used in the manufacture of process equipment and tooling, and contributes significantly to the precision of the part ultimately being manufactured. This paper discusses the use of micro-machining technology in developing fabrication processes for microfluidic devices. Micromachining is being applied for creating injection molds for use in microinjection molding, and for creating imprint rolls for use in roller imprinting. In this paper, we discuss the suitability of micromachining for application in these two methods, and we highlight the challenges in high-precision fabrication of process tooling for microfluidic applications. This paper is motivated by two compelling reasons: the processes used for machining the process tooling for microfluidic device fabrication have a significant effect on the precision of the devices; and, the feature sizes possible in the devices are limited by the accuracy of the machining processes used. The paper begins by discussing the need for developing manufacturing processes to fabricate microfluidic devices. This is followed by a study on using micromachining to create injection molds for microinjection molding. Issues in selecting appropriate process parameters and toolpath strategies are discussed. Various types of form and finish errors are also highlighted. Following this, the roller imprinting process is introduced and requirements for the high-precision manufacturing of imprint rolls are discussed, along with preliminary investigations into the effect of process parameters on imprint roll precision. Specific requirements in toolpath planning and design are also discussed. The paper concludes by highlighting the need for integrated machining process development for microfluidic device manufacturing. 2 MICROFLUIDIC DEVICES Microfluidics deals with the manipulation of small amounts of fluids (in the pico-liter range) using channels with dimensions in the microns domain [1]. A major application of MFDs is in developing miniaturized lab-on-chip devices for chemical and biological analysis. Of particular interest is in applying microfluidic devices for fabricating low-cost medical diagnostic technology [2]. For these applications it is important that the manufacturing process is inexpensive so that the per-device cost is low. The microfluidics © 2008 The Proceedings of MTTRF 2008 Annual Meeting research community has identified effective manufacturing processes as being a critical component for the widespread adoption of microfluidic technology [1]. It is equally important that the processes create precise devices, as the functionality of the devices is determined by the manufacturing precision. Microfluidic devices are composed of networks of fluid flow pathways, and the precision is determined by the positional accuracy of the pathways, the form error in the pathway channels, and the profile of the channel surfaces. Figure 1: Example of Microfluidic Device [2] The most common process used for MFD fabrication is soft lithography using Polydimethylsiloxane (PDMS) [3]. Features are created by casting PDMS over silicon masters, which are fabricated by lithography. Lithography- based methods however, have significant equipment requirements and tend to have a long lead-time due to the multiple steps involved (mask-making, photo-resist preparation, baking etc.). Semiconductor processes are also sub-optimal for creating features larger then 100 µm due to long lead-times. Moreover semiconductor processes can only create 2.5-dimensional features due to the physics of the lithography process, resulting in fixed-depth channels in the devices. Imprinting processes have also been used in creating MFDs in the past, albeit to a lesser extent. Xu et al. [4] discussed the development of a room-temperature imprinting process for PMMA (Polymethyl methacrylate) and reported that this was successful in accurately creating micron-scale features. Since these methods also rely on lithography processes for creating the masters, they can only be applied in creating fixed-depth channels.