Acoustic Generation of Femtoliter to Picoliter Droplets Using Two-Dimensional Micromachined Microdroplet Ejector Arrays
There is growing demand in the fields of semiconductor manufacturing and biotechnology to reliably generate repeatable, uniform, femtoliter to picoliter-size fluid droplets. A novel ejector array design of acoustically actuated 2-D micromachined droplet ejector arrays can eject various solvents and other fluids ranging from femtoliter to picoliter droplet volumes. We propose microdroplet ejector arrays for environmentally benign deposition of photoresist and other spin-on materials, such as low-k and high-k dielectrics used in integrated circuit (IC) manufacturing. Direct deposition of these chemicals will reduce waste and 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. Moreover, these ejectors are attractive to biomedicine and biotechnology for droplet generation in applications such as printing of DNA or protein assays and drug testing. Two novel methods for generating millions of droplets per second using acoustically actuated 2-D micromachined microdroplet ejector arrays will be presented. First, membrane based 2-D micromachined ejector arrays will be introduced. Each element of a membrane based 2-D ejector array consists of a flexurally vibrating circular membrane on one face of a cylindrical fluid reservoir. The membrane has an orifice at the center. A piezoelectric transducer generating ultrasonic waves, located at the open face of the reservoir, actuates the membranes. As a result of this actuation, droplets are ejected through the membrane orifice. The ejectors operated most efficiently at 1.2 MHz and generated 3-7 um diameter droplets. Membrane based ejector arrays were built with either SixNy or single crystal silicon membranes using two different fabrication processes. We show that single crystal silicon membranes are more uniform in their thickness and material quality than those of SixNy membranes. The single crystal silicon membrane based devices showed thickness and material uniformity across all the membranes of an array. This improvement eliminated non-uniform membrane resonance frequencies across an array as observed with SixNy membrane based devices. Therefore, it should be possible to repeatably build devices and to predict their dynamic characteristics. Using the fabricated devices, we demonstrated water ejection at 470 kHz, 1.24 MHz and 2.26 MHz. The corresponding droplet diameters were 6.5 um, 5 um and 3.5 um, respectively. We ejected water at 1.06 MHz, isopropanol at 1.14 MHz, ethyl alcohol at 1.06 MHz, and acetone at 1.04 MHz fi-om a 20x20 single reservoir 2-D Micromachined Array with 160 um in diameter SixNy membranes and 10 um in diameter orifices. The performance of single reservoir flextensional membrane based ejector arrays was compared to flextensional membrane based ejector arrays with reservoirs. A 50 percent decrease in the required power per ejected droplet and a reduced design complexity was demonstrated over the 2-D Micromachined Arrays with individual reservoirs. In addition, we deposited Shipley SPR 3612 photoresist at 1.12 MHz in a dry lab environment. No spinning was done after deposition. We covered a 2mm x 2mm area on a wafer with a 5.5 um thick photoresist layer. The maximum thickness variation over the area was 0.4 um. Moreover, we present a directly written 1.6 um thick, 900 um wide and 8 mm long homogeneous photoresist line. The photoresist thickness variation along the line was 0.2 um and 0.4 um in vertical and horizontal directions, respectively. Second, acoustic focus based 2-D micromachined ejector arrays was demonstrated. The radiation pressure associated with the acoustic beam overcomes the surface tension force, and releases droplets into the air at every actuation cycle. The ejectors operated most efficiently at 34.7 MHz, and generated 28 um diameter droplets in both drop-on-demand and continuous modes of operation, as predicted by the finite element analysis. Photoresist, water, isopropanol, ethyl alcohol, and acetone were ejected from a 4x4 2-D micromachined ejector array. We covered fully a 4" silicon wafer with a 2.4 um thick photoresist layer. The maximum thickness variation over the area was ±0.02 um at the most uniform coated areas of the wafer and -0.3 um at the areas with the worst uniformity. The surface roughness was measured to vary from 68A to 600A over the wafer without spinning and without waste. When spinning is performed after droplet by droplet coverage, the uniformity and surface roughness results would be same as that of spin coating. In the case of spinning the waste would be still remarkable decreased down to less than 5 percent when compared to the amount of photoresist wasted with the spin coating method. Moreover, the theory of operation, fabrication and the experimental results obtained with novel acoustically actuated 2-D micromachined microdroplet ejector arrays are presented.
