This work presents droplet volume adjustable microinjectors for applications to high-resolution inkjet printheads, which can adjust the ejected droplet volumes using the digital operation of a microheater array. Previous studies on inkjet printheads have focused on the improvement of image quality by removing satellite droplets or by increasing the uniformity of ejected droplet volumes. However, if we adjust the droplet volume during printing, we can improve the printing speed of high-resolution inkjet printers while maintaining image quality. In this work, we present the droplet volume adjustable, thermal type microinjectors using a microheater array.
The present digital microinjectors have a 4-bit digital microheater array between the fluid inlet and nozzle exit. If an electrical input signal is applied to a single microheater, the smallest microbubble is generated on the microheater, thus ejecting the smallest droplet. When the identical electrical signals are applied to multiple microheaters, a larger microbubble is generated, thus ejecting larger microdroplet. In the theoretical analysis, we establish a 1-dimentional analytical model to estimate microheater temperature, microbubble pressure as well as ejected droplet volume.
In this paper, we design, fabricate and test two types of digital microinjectors. We design the first prototype microinjector having 8-channel array of microinjectors. In the fabrication process of the first prototype microinjector, we use a 1000A-thick TaAl layer for the microheater array, a 5000A-thick Al layer for the electrical interconnections and a 20μm-thick SU8 layer for the microchannel barrier. We use an epoxy adhesive to bond the SU8 to the Pyrex glass cap. The size and the resistance of microheater is 30μm×30μm and 40±0.4Ω, respectively. The size of fabricated device is 7,640μm×5,260μm.
In the experimental test, we observe the generation, growth and collapse of microbubbles. The maximum size of the microbubbles appears at the same time of 2μsec after turn-on when a 10.0V, 1kHz, 1μsec pulse-width electrical input signal is applied to the selected microheaters. The collapse time of the generated microbubbles is gradually increased as the number of operating microheaters is increased. For example, the collapse time of the one-microheater operation is 4μsec and that of the four-microheater operation is 8μsec. We also extract the ejected droplet volumes from the size of dots printed on the paper. The measured droplet volumes gradually increase from 1.41pl to 11.04pl as the number of operated microheaters increases from one to four.
The first prototype microinjectors have several problems to be modified in the design, fabrication and the experimental methods. In the modification of design process, we insert the micronozzle structure to minimize the backflow and reduce the ratio of microchannel width to the microheater size to increase the microdroplet volume. We also design the comparative test structures having a variation of microheater sizes, inter-microheater gaps and microchannel widths in order to experimentally characterize the effect of each design parameters affecting on the volumes and velocities of the ejected droplets. In the modification of fabrication process, we use a 30μm-thick DFR (Dry Film PhotoResist) for the top layer to improve the sealing between the neighboring microchannels. We also insert the SiNx layer for the electrical passivation between microheaters and fluid.
In the experimental test of the modified microinjector, we use a stroboscope and CCD camera to take an instantaneous picture of ejected droplets. We apply the fixed input signals of 10.0V, 1kHz, 1.5μsec pulse-width for all experimental tests. In the 4-bit digital operation test, we can find that the volumes and velocities of ejected droplets are adjusted from 12.1pl to 55.6pl and 2.3m/s to 15.7m/s, respectively. We also find that the ejected droplet volumes are decreased 0.042pl/μm for the single microheater operation, 0.218pl/μm for the double microheater operation and 0.429pl/μm for the triple microheater operation as a center of the operating microheater area moves in the direction to the micronozzle exit. We also measure the volumes and velocities of ejected droplets in terms of the microheater size (Test H), the inter-microheater gap (Test G), and the microchannel width (Test W), respectively. From the test, we find that the microheater size is the most dominant design parameters affecting on the volumes and velocities of ejected droplets.
On the basis of the theoretical analysis and experimental test, we can conclude that the present droplet volume adjustable microinjectors have potentials to improve the printing speed of high-resolution inkjet printers while maintaining the image quality.
