Creating shaped piezo silicon micropumps using MEMS-based manufacturing
10/01/2005
Microelectromechanical systems (MEMS) technology is driving progress in the manufacturing of new “inkjetting” products called shaped piezo silicon micropumps. This technology provides the capability for the deposition of a whole new generation of organic and inorganic functional fluids on a variety of substrates. The result is a highly flexible design and a manufacturing space capable of servicing a broad range of printing applications in addition to pushing “inkjet technology” into the more generalized precision liquid-dispensing applications market.
 Dimatix’s MEMS-based printhead architecture and its M-300 printhead jet module |
Typical electronic materials deposition applications include display printing, conductive traces, and interconnects that ultimately lead to transistor functions, electronic circuits, or backplane applications. The technology also provides the precision placement and flow control needed for the deposition of DNA and proteins for applications in the life sciences field.
Stringent drop-placement accuracy and overall jet-to-jet uniformity are key requirements to expanding into materials deposition applications. At the same time, increased printing speeds call for higher firing rates with greater overall uniformity across the operating range. These requirements can only be met by using MEMS technology in manufacturing processes to generate precision printheads.
MEMS processing
Figure 1 shows the major steps in the fabrication process of a shaped piezo silicon micropump. The final wafer is fabricated from a three wafer stack-up: two silicon wafers and one lead zirconate titanate (PZT) wafer. Each die is then singulated from this wafer stack and contains the entire fluid pumping chamber as well as the nozzle orifice.
 Figure 1. Process flow of an M-Class jetting printhead, from the starting wafer to the final PZT layer and individual jets. |
Silicon-on-insulator (SOI) wafers are used as the basis layer on which the MEMS process is applied. All the non-PZT processes are standard MEMS/IC processes: metal sputtering, wafer grinding, chemical mechanical polishing (CMP), deep reactive-ion etching (DRIE), and silicon fusion bonding. Silicon and metal planar geometries are defined through photolithography.
Advantages to using MEMS processes for inkjet applications are twofold: 1) MEMS processes are statistically capable of meeting the dimensional tolerances suitable for current and future inkjet technology functionality, and 2) design dimensions are set with standard photomasks that can easily be changed across a wide range of dimensions and configurations.
Chemical and mechanical robustness
Silicon’s material properties are well suited for the construction of liquid dispensing applications. As a bulk material, silicon has demonstrated superior resistance to a wide range of jetting formulations including aqueous inks, solvents, and both highly acidic and basic fluids. Furthermore, the fusion bonds used in the processing are no less resistant to chemical attack than the bulk material. Additionally, silicon is mechanically tough and is sufficiently abrasion-resistant to permit frequent wiping and allow the jetting of abrasive suspensions.
A thin silicon membrane with excellent mechanical properties - i.e., its strength and stiffness - is combined with the piezo material and creates the deformations needed to generate sufficient acoustic energy to fire drops at high frequencies (up to 100kHz) through the nozzle orifice. With a single-crystal structure, silicon will not exhibit creep or fatigue, or deform over extended cycling at very high stress levels. Additionally, the thermal expansion coefficient is close to that of PZT, which lets the entire unit expand relatively uniformly without stress under thermal loads.
Processing and scaling
The micrographs in Fig. 2 show the variety and precision of MEMS DRIE processing. In the architectural approach, all critical dimensions are photolithographically defined. Thus, nozzle spacing distances and diameters are controlled on the submicron level to improve jet straightness and reduce overall drop-placement error. These gains enable jet-to-jet and die-to-die functional uniformity. Design thickness dimensions exhibit 0.5μm capability.
The manufacturing processes are extremely flexible. Different products with different jet dimensions such as nozzle dimensions, jet-packing densities, and jets-per-die can be produced using existing processes with new mask sets. Similarly, devices with different thickness dimensions can be produced over a wide range by simply setting different target dimensions.
Because the critical dimensions of the jetting structure are lithographically defined, a very large design space is approachable by simply running the core process with a different photomask set. Additionally, since the physics of drop formation are complex and are difficult to model, fluidic dimensional scaling can be used as part of the product design process to reduce design task complexity.
It is well known that fluidic systems maintain functional similarity across dimensional scaling when the appropriate fluidic, nondimensional numbers are maintained. The fluidic flow “similarity” achieved by fluidic scaling is the result of maintaining the relative size of the forces involved in determining the flows for different physical dimensions. If the viscous, inertial, surface-tension, and compressibility forces are kept proportional, the flow will be similar. This approach maintains the drop formation physics even as drop sizes change. Using fluidic scaling makes it possible to scale performance to predict corresponding performance for any other sized device.
Module functional implications of scaled devices
The implication of using MEMS manufacturing techniques for scaling dispensing devices is that drop sizes can be substantially reduced and drop placement accuracy can be improved. Given the strength of the relationship between drop size and dimensions, drop sizes as low as sub-picoliters and corresponding linewidths <10μm are possible through dimensional scaling.
As nozzle dimension shrinks and drop mass decreases, the drop velocity increases substantially. This combination is important for drop placement accuracy. When drop velocity decreases, all other things being equal, the accuracy of drop placement goes down. Therefore, drop velocity needs to be maintained. For uniform scaling, the increase in speed at lower drop mass allows higher stand-off distances or greater accuracy.
These higher drop velocities can be used either to increase firing rates or to enable complex firing schemes. One can design complex pulses capable of generating different-sized drops from the same nozzle by generating multipulse configurations.
MEMS-based processing is an enabling tool for advancing the performance of precision microfluidic dispensing devices for applications ranging from printing to electronic materials deposition.
Key features of devices processed using MEMS technology are dimensional control, process flexibility, and the excellent application-specific properties of silicon. Shaped piezo silicon, employing MEMS fabrication techniques, can meet the challenges of current and emerging materials deposition applications.
Fred Stevens has more than 20 years of experience as a technical writer and editor, and is the technical publications editor for Dimatix’s Spectra Printing Div., 109 Etna Road, Lebanon, NH 03766; ph 603/443-8306, e-mail [email protected].