Applying drop-on-demand (DOD) inkjet printing technology to directly write photoresist or polymer patterns on a wafer surface is becoming a new and powerful tool for microprocessing. Compared with other lithography approaches, this maskless lithography method has the advantages of the low cost, the wide variety of materials it could pattern, as well as the versatility of the substrates that a circuit could be built on.
The existing inkjet systems could only form liquid droplets with a volume bigger than several picoliters, which limited their minimum printable size to tens of microns. To build a system that is capable of producing droplets in the micron to submicron regime, we first chose a thermal bi-membrane actuator structure to provide the high driving force for ejecting a small droplet. However, the bi-membrane system suffered problems such as fragility of the membranes and irregularity of droplet generation. Therefore, we rebuilt our system based on the most common actuation mechanism used nowadays in inkjet printers–thermal vapor bubble formation by applying an extremely high thermal flux to the liquid on top of a smooth heater surface. The pressure change generated by the rapid growth and collapse of a vapor bubble in a chamber will push liquid through a nozzle and then break it to form a single droplet.
We have been able to successfully fabricate monolithic thermal bubble inkjet printheads by epoxy stamp bonding, in which we bonded a pair of wafers with nozzle, chamber, and heater structures using a thin film of epoxy transferred to the high areas of the top wafer by a dummy. The test chips were then plugged into our experimental system (as described in abstracts in previous Research Summaries) and we observed stable and continuous generation of 13 µm scale water droplets from those chips (Figure 1). Our next step is to shrink the liquid size further by reducing the nozzle diameter and changing the electric driving conditions. We will also try to print patterns with different materials utilizing the new printhead.
Figure 1: Water droplets generation sequence viewed at the nozzle of our new monolithic thermal bubble printhead