We describe high-precision microlenses with excellent optical characteristics. The lenses are formed precisely at desired locations on a wafer using a polymer-jet system in which hydrophobic effects define the lens diameter and surface tension creates a high-quality optical surface. To make the lenses, we defined hydrophilic circular regions at desired locations using photolithography to pattern a 0.2-pm thick Teflon (hydrophobic) layer on a quartz substrate, as shown in Figures 1 and 2. Then, using a polymer-microjet printing system (Figure 3), we dispense an exact amount of UV-curable polymer within hydrophilic circles to obtain microlenses having desired optical properties [ 13. Figure 4 shows that adjusting the volume of the W-curable optical epoxy within a hydrophilic circle of a given diameter changes the curvature of the microlens. The step resolution of the microlens volume is determined by the average droplet size (-25pL) of the polymer-jet print head. This hybrid method enables us to define the locations and diameters of microlenses with a 21 pm precision as well as to control the curvatures of the microlenses accurately. We measured the quality of our lenses using several precise optical-characterization systems. Figures 5 and 6 are scanning-electron microscope pictures showing several of our microlenses. AFM measurements show that the surface roughnesses of our microlenses are lower than 5nm. We used a WYKO NT3300 to measure the curvature and volume of the microlenses. The maximum deviation of the surface profile from an ideal circle was approximately 0.15pm for most cases (range-0.05-0.23pm). The effective focal lengths measured are shown in Figure 7. Thef-numbers range from 1.5-2.1, 2.0-5.5, 3.4-6.3, and 2.9-7.4 for 200pm-, 400 pm-, 600 pm-, and 1 mm-diameter microlenses, respectively. The Seidel aberrations, root-mean-square wavefront errors (rms WFE), peak-to-valley optical-path differences (p-v OPD) of the microlenses were measured at h = 635 nm using a commercial Shack-Hartmann system with an accuracy of AI100 [2], [3]. The rms WFE values of our microlenses were between U5 and 2/80, depending on the aperture size, diameter, and volume of the microlenses. The average p-v OPD values were 0.14,0.25,0.33, and 0.46 p m for 200 pm-, 400 pm-, 600 pm-, and lmm-diameter microlenses, respectively. Decreasing the aperture size of the microlenses produced much smaller rms WFE and p-v OPD values, as shown in Figure 8. These values were sometimes as low as 2/80. The fabrication process showed good repeatability as well; twenty 400-pm-diameter microlenses showed-1.43% variations in measured volumes and effective focal lengths. The fabrication of microlenses using surface tension and/or hydrophobic effects has attracted considerable research because of this method's potential applicability to micro-optical systems. A previously reported dipping method produced lenses, but did not provide a reliable means to vary or control optical properties if, for example, several lenses at differing locations on a wafer wer...
