NanoEngineering World Forum Micro-Printing System for MEMS Packaging Using Nano-Structured Materials

Ink-jet microdispensing methods can be used to address a number of MEMS packaging challenges. Inkjet microdispensing is data-driven, non-contact, and is capable of precise deposition of picoliter volumes at high rates, even onto non-planar surfaces. Being data-driven, it is highly flexible and can be readily automated into manufacturing lines. It does not require application-specific tooling such as photomasks or screens, and, as an additive process with no chemical waste, it is environmentally friendly. As high performance nanostructured materials evolve, ink-jet technology can be used to deploy these materials in high value applications, such as MEMS packaging. Author(s) Biography David B. Wallace is currently Vice President, Technology Development for MicroFab Technologies, inc.. He received his BSE and MSME from Southern Methodist University, and a Ph.D. from the University of Texas at Arlington. He has 30 years of industrial experience in complex fluid flow phenomena, has published over 80 papers & articles, and has been awarded 28 patents. Donald J. Hayes is President of MicroFab Technologies, Inc. He holds a B.S. and M.S. in Physics from Louisiana State University and a Ph.D. in Materials Science from Rice. He has over 30 years experience managing research and development of process driven manufacturing for ink-jet printers, semiconductor devices, and electronic assemblies. Dr. Hayes has been awarded 52 patents. INTRODUCTION Due to the variety of functions that need to be integrated into a single package, MEMS devices represent a major challenge to the packaging industry. A single package may contain a variety of technologies: optics, electronics, motion, chemistry, biology, etc. In some cases the package needs to be hermetic. In others, small volumes of fluid need to flow in and out of the device. Also, many MEMS packages require both optical and electrical I/O and require the fabrication of non-planar structures not readily accomplishable by photolithographic processes. The ability to use ink-jet processes in MEMS packaging applications is dependent on formulation of materials that can both be jetted and perform the desired function on the substrate, the latter includes creating the feature with the correct shape. New developments in nanostructured materials give material formulators a number of degrees of freedom not previously available to them, and thus have opened up many new potential application areas for using ink-jet processes. In this paper we will address a variety of MEMS packaging solutions based upon ink-jet printing technology, which fundamentally is a precision microdispensing technology. First, the general capabilities of the ink-jet printing technology will be discussed. Next, specific hardware implementations applicable to MEMS packaging will be described. Finally, we will show results of precision microdispensing technology applied to: optical and electrical interconnects, package sealing, and other assembly processes. BACKGROUND ON INK-JET TECHNOLOGY Ink-jet printing technology is familiar to most people in the form of desktop office printers. Actually, there is a broad range of diverse technologies that fall into the ink-jet printing category. The physics and the methods employed within this group may differ substantially, but the end effect is repeatable generation of small droplets of fluid. Only demand mode technology will be discussed in this paper, since it is the most widely applicable to MEMS manufacturing applications. Demand Mode Ink-Jet Technologies In a drop-on-demand ink-jet printer, the fluid is maintained at ambient pressure and a transducer is used to create a drop only when needed. The transducer creates a volumetric change in the fluid that creates pressure waves. The pressure waves travel to the orifice, are converted to fluid velocity, which results in a drop being ejected from the orifice. Figure 1: Schematic of a drop-on-demand ink-jet printing system. The transducer in demand mode ink-jet systems can be either a structure that incorporates piezoelectric materials or a thin film resistor. In the latter, a current is passed through this resistor, causing the temperature to rise rapidly. The ink in contact with it is vaporized, forming a vapor bubble over the resistor. This vapor bubble creates a volume displacement in the fluid in a similar manner as the electromechanical action of a piezoelectric transducer. Figure 1 shows a schematic of a drop-on-demand type ink-jet system, and Figure 2 shows an image of a drop-on-demand type ink-jet device generating 60μm diameter drops of butyl carbitol (an organic solvent) from a device with a 50μm orifice at 4,000 drops per second. Demand mode ink-jet printing systems produce droplets that are approximately equal in diameter to the orifice diameter of the droplet generator. Droplets less than 20μm are used in photographic quality printers, and drop diameters up to 120μm have been demonstrated. Figure 2: Drop-on-demand type ink-jet device generating 60μm diameter drops at 4kHz. Sequence from left to right spans 130μs. Discussion As a non-contact printing process, the volumetric accuracy of ink-jet dispensing is not affected by how the fluid wets a substrate, as is the case when positive displacement or pin transfer systems “touch off” the fluid onto the substrate during the dispensing event. In addition, the fluid source cannot be contaminated by the substrate, as is the potential during pin transfer touching. Finally, the ability to freefly the droplets of fluid over a millimeter of more allows fluids to be dispensed into wells or other substrate features (e.g., features that are created to control wetting and spreading). In general, piezoelectric demand mode technology can be more readily adapted to fluid microdispensing applications. Demand mode (both piezoelectric and thermal) does not require recirculation or wastage of the working fluid, as does continuous mode. It is easier to achieve small drop diameters with demand mode (again, both piezoelectric and thermal). It is easier to achieve lower drop velocities with piezoelectric demand mode. Piezoelectric demand mode does not create thermal stress on the fluid, which decreases the life of both the printhead and fluid. Piezoelectric demand mode does not depend on the thermal properties of the fluid to impart acoustic energy to the working fluid, adding an additional fluid property consideration to the problem. Printing System Configurations All desktop ink-jet printers have the same configuration: the printhead is translated on one axis (nominally 100 mm/s), and the paper is indexed ninety degrees to the printhead motion. For manufacturing applications of ink-jet printing technology, the substrate to be printed upon determines the machine configuration. In most cases, the substrate will be, or will be similar to a silicon wafer, a circuit board, circuit board panel, or other relatively flat, rigid substrate. In addition, manufacturing equipment using ink-jet dispensing will have setup, alignment, and control functions not generally found in desktop printers. To expand the range of materials that can be jetted, the printhead may require elevated or depressed temperature operation. Also, the substrate may need to be heated or cooled to control the pattern formed by the dispensed fluid after it has landed on the substrate. Environmental control requirements may include particle control, oxygen control, or water vapor control. Curing immediately after printing may require injection of reactive gasses or UV illumination. Figure 3 shows a block diagram for a typical ink-jet based printing system. In this case, the workpiece is shown mounted onto an X-Y stage, so that the printhead assembly is stationary. A stationary printhead does not have to be designed to account for acceleration effects on the contained fluid, or the motion of service lines if a remote reservoir is utilized. Figure 4 shows MicroFab’s Jetlab II research printing platform for ink-jet dispensing application development, which is based on the system configuration of Figure 3. Figure 3: Block diagram for ink-jet based printing system. Figure 4: Printing platform for development of ink-jet dispensing applications. High Temperature Printhead Configurations Operation at temperatures above 150°C is usually considered infeasible for piezoelectric systems. However, MicroFab has developed the capability to operate piezoelectric demand mode ink-jet devices at up to 240°C indefinitely, at temperatures of up to 320°C for several days, and briefly at temperatures as high as 370°C. Both the device construction and the drive waveform utilized during operation are critical to this capability. Operation at elevated temperatures allows high viscosity polymers, solders, and other materials to be dispensed using demand mode ink-jet technology. Figure 5 shows a printhead designed for operation at up to 370°C, mounted onto a system designed for depositing onto silicon and gallium arsenide wafers. This system includes the ability to inject an inert gas around the jetting device and into the local region which is being printing on. Figure 5: Printhead for operation at up to 370°C