Acoustic picoliter droplets for emerging applications in semiconductor industry and biotechnology

This paper presents the theory of operation, fabrication, and experimental results obtained with a new acoustically actuated two-dimensional (2-D) micromachined microdroplet ejector array. Direct droplet based deposition of chemicals used in IC manufacturing such as photoresist and other spin-on materials, low-k and high-k dielectrics by ejector arrays is demonstrated to reduce waste contributing to environmentally benign fabrication and lower production cost. These ejectors are chemically compatible with the materials used in IC manufacturing and do not harm fluids that are heat or pressure sensitive. A focused acoustic beam overcomes the surface tension and releases droplets in air in every actuation cycle. The ejectors were operated most efficiently at 34.7 MHz and generated 28 mu m diameter droplets in drop-on-demand and continuous modes of operation as predicted by the finite element analysis (FEA). Photoresist, water, isopropanol, ethyl alcohol, and acetone were ejected from a 4 x 4 2-D micromachined ejector array. Single photoresist droplets were printed onto a silicon wafer by drop-on-demand and continuous modes of operation. Parallel photoresist lines were drawn and a 4-in wafer was coated by Shipley 3612 photoresist by using acoustically actuated 2-D micromachined microdroplet ejector arrays.