Drop-by-drop Polymer Deposition by Acoustic Picoliter Droplet Generators for Applications in Semiconductor Industry and
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Drop-by-Drop Polymer Deposition by Acoustic Picoliter Droplet Generators for Applications in Semiconductor Industry and Biotechnology Grace C. Lee1,2, Jeremiah R. Cohen1,2, and Utkan Demirci1,2 1 Harvard-MIT Division of Health Sciences and Technology, Bio-Acoustic-MEMS, Massachusetts Institute of Technology, Cambridge, MA, 02139 2 Center for Biomedical Engineering, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139
ABSTRACT We ejected photoresist droplets onto a wafer surface using an acoustic two dimensional micromachined ejector array. We also studied the spread of single droplets on a silicon wafer surface at varying droplet speeds. Series of photoresist droplets were printed periodically dropon-demand on a silicon wafer surface, and profiles of a single droplet and two droplets overlapping with varying distances of 25 µm and 1 µm on a silicon wafer were demonstrated. Moreover, we demonstrated 3.4 µm-thick spinless full coverage of a 4 inch wafer with photoresist, which indicates a potential for coating wafers in less than a few seconds. INTRODUCTION This reliable and rapid method of dispensing picoliters of fluids emerged as an attractive technology enabling various MEMS (Microelectromechanical Systems) fabrication methods and bioengineering applications 1-4. One example is high throughput arrays for drug testing, where arrays of cells placed on a surface could be tested with picoliter droplets of drugs. DNA arrays could be also written by drop ejection 4,5. Moreover, deposition of organic polymers is the most employed process in semiconductor fabrication 6,7. Among various reported organic polymer deposition techniques, the spin coating method dominates current industrial applications 7,8. However, this method has disadvantages: up to 95 % of the expensive chemicals are wasted; the cost of disposing this hazardous waste is high; and there is edge bead formation at the wafer edges due to spinning 8-10. A drop-by-drop polymer deposition method has the potential to minimize hazardous photoresist waste and disposal costs, and to remove the edge bead formation problem 11,12. THEORY The basic building block of a 2D ejector array is an interdigital ring transducer on a piezoelectric substrate as shown in Figure 1. These devices launch surface acoustic waves that leak into the medium in contact with the piezoelectric substrate and interfere constructively, forming a focus. If the force exerted by radiation pressure at the focal point can overcome the surface tension forces of the fluid, then a droplet will be ejected 12.
Figure 1. Geometry of a fluid loaded unit cell of a 2D micromachined ejector array. The droplet diameter can be modeled by fluid cylinder geometry as shown in Figure 1. The diffraction limited acoustic beam expressions are ‘ d = 1.02λ F ’ and ‘ h = 7.1λ F 2 ’, where ‘F’ is the focal number 12-16. The speed of surface acoustic waves in a substrate ‘vpiezo’ is larger than that of an acoustic wave in water ‘vfluid’. The circular acoustic wave
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