Digital Printing of Optical Components

The use of direct write micro-printing for opto-electronic packaging is being accepted as a key enabling technology, specifically printing microlenses for optical interconnects and solders for electrical interconnects. This paper presents these two areas where micro-printing technology benefits opto-electronic packaging . For increasing light coupling efficiency, a microlens array is actively aligned and attached to a VCSEL array and photodiode array in smart pixel technology. A microlens array is also directly integrated with a VCSEL array in a wafer level fabrication. Solder jet is used for either bumping the VCSEL die or for directly connecting the VCSEL die with circuits on a substrate. By using micro-printing, components can have less part-count and smaller package size. The labor required for conventional optical alignment can be greatly reduced. Thus this technical advancement offers also cost effectiveness in manufacturing. Introduction A piezoelectric micro-printing device, operating at drop-ondemand (DOD) mode, can reproducibly dispense spheres of fluid with diameters of 15 -100 μm (volumes of 2pl 5nl) at rates up to 25,000 per second [1]. The deposition is non-contact and datadriven. In combination with precision stages, the drops can be delivered to designated positions accurately. This technology has been developed to dispense fluids like solder alloys, optical polymers, and other adhesives. The deposition of optical polymers can form microlenses, waveguides, or other optical elements for shaping or guiding lights from photonic devices. The deposition of solders or adhesives can be used for attachment or bonding.of opto-electronic components at different levels. Owing to the small and precise amount of material delivered, and also to the highly flexible process, this direct write micro-printing technology is being accepted as a key enabling technology for opto-electronic packaging. In this paper, we present the concept, process, and results of the applications of this technology in printing microlenses for optical interconnects and printing solders for electrical interconnects or structural features. The majority of examples in this paper are given in the area of packaging VCSEL arrays. These examples show the advantages of this technology in the integration of optical and electric performance, and in reducing package size and assembly cost. Background of Micro-Printing In a piezoelectric dispensing device the voltage pulse is applied to a piezoelectric transducer. The deformation of the transducer is coupled to the fluid at ambient pressure in a cavity and generates acoustic waves that result in the ejection of drops from the device orifice. In DOD operation, a drop is only ejected from the orifice when a voltage pulse is applied to the transducer [1]. The dispensed drop size is approximately equal to the orifice diameter. Figure 1 shows 50μm ethylene glycol drops from a DOD inkjet device at 2 kHz. The general fluid property requirements for using a piezoelectric DOD dispensing device are: viscosity in the range of 0.5 40 cps and surface tension in the range of 20-70 dy/cm. Solder alloys need to be heated to molten states and need to have inert gas protection to prevent them from oxidization. Fluid samples with properties outside these ranges need to be heated or cooled to have the required rheological properties at the orifice of the device. In many applications the solidified optical polymer or solder is required to have certain profiles (such as hemispherical lens profile), or to have certain spreading areas. Therefore the substrate surface treatment can be important for the control of drop wetting and spreading. Shown in Figure 2 is a JetLab II dispensing platform developed and assembled at MicroFab for various printing tasks. This compact platform consists of sub-systems that include: print head, temperature and pressure control, X-Y-Z stages with motion control, machine vision system, spot UV delivery, and N2 co-flow. Figure 2. A compact printing platform – JetLab II. Figure 1. Dispensing of 50 μm ethylene glycol drops at 2 kHz. Printing Micro-optics Using Polymer Jet UV-curing optical epoxies are a preferred class of material for microlens printing, because of their thermal and chemical durability, as compared to other optical-grade polymers, such as acrylics, photoresists, and thermoplastics [2]. Our in-house developed MRXH-series of optical epoxy is a 100%-solid formulation of prepolymers. Its viscosity can be reduced to below 40 cps at temperatures above 100°C to enable DOD printing. The dispersion curve of MRXH, along with that of other printable UVcuring acrylic polymers, is given in Figure 3. Printing the liquid prepolymer drops onto the designated locations, followed by UV Proc., IS&T's DF06, the International Conference on Digital Fabrication Technologies September 17 21, 2006 1.48 1.49 1.5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 0 50