An optical interconnection system is being developed to provide vertical, digital data channels for stacked multichip modules. A key component of the system is an array of individually addressable vertical-cavity surface-emitting lasers with diffractive lenses integrated into the substrate to control beam divergence and direction. The lenses were fabricated by direct-write e-beam lithography and reactive ion beam etching into the GaAs substrate. Preliminary device performance data and the design and fabrication issues are discussed

Carson, R. F.

Du, T. C.

Kilcoyne, S. P.

Lear, K. L.

Schneider, R. P.

Vawter, G. A.

Warren, M. E.

Wendt, J. R.

Zolper, J. C.

UNT Digital Library

? Integration of Diffractive Lenses with Addressable Vertical-Cavity Laser Arrays M.E. Warren, T.C. Du, J.R. Wendt, G.A. Vawter, R.F. Carson, K.L. Lear, S.P. Kilcoyne, R.P. Schneider and J.C. Zolper Sandia National Laboratories Albuquerque, NM 87 185-0603 ABSTRACT An optical interconnection system is being developed to provide vertical, digital data channels for stacked multichip modules. A key component of the system is an array of individually addressable vertical- cavity surface-emitting lasers with diffractive lenses integrated into the substrate to control beam divergence and direction. The lenses were fabricated by direct-write e-beam lithograpliy and reactive ion beam etching into the GaAs substrate. Preliminary device performance data and the design and fabrication issues are discussed. Keywords: diffractive optics, binary optics, vertic& cavity lasers, semicortductor lasers, optical interconnects, multichip modules, e-beam lithography, fabrication of diffractive optical elements 1. INTRODUCTIQN Vertical-Cavity Surface-Emitting lasers (VCSELs) are very attractive for a variety of integrated photonic system concepts. Two dimensional arrays are required to provide the compact geometry needed by many parallel optical interconnect systems’. However, without a discrete optical element such as a focusing lens, the diverging gaussian beam can introduce cross-talk if the adjacent receivers are within the spread of the beam diameter. In addition, the direction of the output beam limits many applications to on-axis system configurations. Integrating diffractive optical elements on the transparent substrate of a substrate- emitting VCSEL provides a method for controlling the direction and minimizing the divergence of the exiting beam2. We present the development of integrated, high-efficiency diffractive optics on an array of substrate-emitting VCSELs. Two types of diffractive optical elements have been fabricated so far; on- axis and off-axis lenses. These lenses are produced by etching the GaAs siibstrate to form the surface relief profiles needed for multiple phase-level diffractive lenses. The basic deviceconcept is illustrated in Figure 1. 2. SYSTEM APPLICATION A major motivation for this effort is the need for compact, two-dimensional arrays of surface-emitting sources with well controlled beam properties for optical interconnects in multichip module$. Multichip modules (MCMs) are a high density, high speed electronic packaging tech:nology that allows the mounting and interconnection of a number of bare integrated circuit die on a single substrate. Additional packaging density can be obtained by vertically stacking the modules. Routing signals from one vertical level to another then becomes important. We are working on a vertical photonic interconnect scheme based on two-dimensional emitting and receiving arrays that can be included in the MCM technology. VCSELs have very desirable attributes as source arrays, due to recent improvements in VCSEL device efficiencies and total power consumption for this type of application4. We have concentrated on 980 nm DISCLAIMER This report was prepared as an account of work sponsored by an agency of t he  United States Government. Neither t he  United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disctosed, or represents that its use would not infringe privately owned rights;. Reference herein. to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the  United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the  United States Government or any agency thereof. . I DlSCLAlMER Portions of this document may be illegible in electronic image produced from the document. products. Images are best available original I .  It is important to note that the prototype depicted in figure 2 is not compatible with a specific MCM technology. The implementation of a MCM technology requires a consistent method of interconnecting the die on the same module. This electrical interconnection may be provided by a metallization on the MCM substrate that the die are bonded to or may be a separate interconnect structure that is bonded to the tops of the die.5 In either case, it is impractical to mix the two mounting techniques i n  the same assembly as is shown in the prototype depicted in figure 2. A practical implementation of the vertical photonic interconnect will require more advanced optical routing concepts to be compatible with an existing MCM architecture. For example, bottom-emitting VCSEL arrays may be required with the light routed through the top of the device to maintain compatibility with an electrical interconnect to the tops of each die. Top-emitting VCSEL arrays may not be suitable because integration of a lens for divergence control directly on top of the emitter would not be practical. Such optical routing schemes provide motivation for investigating reflecting and off-axis optical elements. Aligning Bump Bond - Self- Glass Fiber Aligned Spacer Figure 2. Depiction of prototype vertical photonic intercomiect demonstration for MCMs. VCSEL emitter and HBT receiver modules with integrated diffractive lenses communicate through via holes in the MCM substrates. I .  3. VCSEL ARRAYS The MOVPE-grown bottom-emitting VCSEL structures consist of a triple IhGaAs quantum well active region with graded AlGaAs/GaAs DBR mirrors on top of the n+ GaAs substrate. The wafer growth technique and process steps involved to produce this device are similar to that reported elsewhere4. The VCSELs are fabricated for enhanced backside emission with an unannealeti gold layer deposited over an aperture in the p contact metal aligned to the implant-defined active region. The VCSELs are currently mesa isolated devices with an active region defined by proton implantation. The active region diameters are 10,15 and 20 microns. The design wavelength of the devices described here is nominally 980 nm. The VCSELs are fabricated as 4 X 4 arrays on 500 micron center-to-center spacings with top-side n and p contacts. 4. LENS DESIGN AND FABRICATION' The choice of lens parameters for the VCSEL arrays is based on the dimensions of the planned prototype depicted in figure 2. The lens was designed to produce the smallest spot size on the surface.of the receiver substrate 850 microns away from the VCSEL substrate surface. The VCSEL substrate thickness was chosen to be 350 microns and the beam waist diameter of the VCSEL .assumed to be 8 microns, based on divergence measurements of previous devices. It is necessary to consider the gaussian beam properties of the laser in designing the lens. Calculations of beam size at the receiver as a function of lens focal length were performed with ABCD matrix techniques. The On-iuciS lens design is 110 microns focal length with an 80-micron lens diameter. The l/e2 beam diameter at the receiver substrate surface is predicted to be 60 microns. Note that the lens diameter is much larger than necessary as the beam diameter at the lens is predicted to be 18 microns for these design parameters. Small misalignments of the lens to the VCSEL are magnified by a factor of ten at the receiver plane. The misalignment tolerance of the lens to the VCSEL aperture is therefore approximately +/- 3 microns. The VCSEL array is fabricated first on a substrate already thinned and polished to the desired thickness. The polished backside surface is protected during the VCSEL fabrication steps with a coating of S N  The protective SiN coating is removed for the lens fabrication sequence and the wafer is mounted on a carrier substrate with the VCSEL side down for protection. The next step is to transfer the e-beam lithography alignment reference marks to the backside of the array from alignment features on the VCSEL side. This is done with a conventional contact mask aligner with an IR-sensitive CCD camera. IR light transmitted through the sample allows the photomask to be aligned to features on the laser side of the substrate. With care, this alignment can be held to one or two microns. A metal liftoff step then produces alignment features for the e-beam system on the substrate surface. The direct-write, (:-beam exposure of the lens patterns is made in PMMA using a JEOL JBX-SFE electron beam pattern generator. After development of the pattern, the etching of the GaAs substrate is done in a chlorine reactive ion beam etch W E )  system. The RlBE system allows very anisotropic, uniform etching. The etch depths in GaAs are quite shallow and it is necessary to use an in situ monitoring technique to accurately stop at the correct depth. This is accomplished by optically monitoring the etch with a multilayer GaAdAlGaAs semiconductor witness piece that is mounted beside the VCSEL sample. A laser beam reflected from the monitor is modulated as the layer structure is etched away and the modulation pattern can be calibrated to the actual etch depth in the GaAs sample. Three successive mask and etch processes are used in the well known "binary optics" process that results in an eight phase level lens.6 The smallest liiewidth in the on-axis lens patterns is approximately 0.3 microns. After completion of the lenses, the wafer can be unmounted for testing and packaging. A scanning electron micrograph of a finished lens is shown in figure 3. Figure 3. Scanning electron micrograph of the surface of a tiiffractive lens fabricated in GaAs. The 80 micron diameter lens has 8 phaye levels in each zone. 5. DEVICE PERFORMANCE We have measured preliminary characteristics for lenses fabricated on functioning VCSEL arrays. Figure 4 shows a direct before-and-after comparison of the light versus current &-I) curves for a 20 micron diameter device taken without a lens on the substrate and then taken again after a diffractive lens was fabricated. Both devices had uncoated substrates. Fabry-Perot resonances due to the optical cavity formed by the substrate can be seen in both L-I curves. The data shows some degradation of the device efficiency after fabrication of the lens. This may be attributable to change in the amount and type of feedback into the laser from the back reflections from the substrate after the diffracting lens pattern is present. Another possible factor may the added thermal cycling of the device in the lens fabrication . steps. The efficiency of power transmission through a 100 micron pinhole 850 microns from the lens was measured for devices with and without lenses. Figure 5 shows a comparison for a 15 micron diameter VCSEL without a lens and a 15 micron diameter VCSEL with a lens that is located nearby on the same substrate. The efficiency through the pinhole increases from about 20% wjthout a lens to 80% with a lens. The decrease in the efficiency with increasing current for the device without a lens is attributable to the thermal lensing effect changing the laser's effective waist size and increasing the beam divergence. Because these devices had moderately high resistance, the thermal lensing effect is larger here than it may be in less resistive devices. These measurements have been made with CW operation of the lasers. Pulsed measurements, closer to the actual operating conditions of the interconnect system will be made soon. n 3 E v L 5 0 n 1.2 1 0.8 0.6 0.4 0.2 0 - = - - - Power Before Lens  Power After Lens 0 5 10 15 20 25 30 35 Current (mA) Figure 4. L-I curves for a 20-micron diameter VCSEL taken before and after fabrication of a diffractive lens on the substrate. Both curves were taken with an uncoated substrate. The ripples in the L-I curve are due to Fabry-Perot resonances in the substrate. Devices with lenses show more asymmetrical structure in the beam profile at 850 microns from the lens than the unlensed VCSELs. This is presumably due to the less homogeneous feedback from the structured substrate surface affecting the transverse mode profile of the VCSEL. At higher currents, these VCSELs are operating with multiple transverse modes. We have not yet studied what effect this may have on random intensity noise @IN) in the devices during pulsed operation and whether the feedback from the lenses will produce any additional noise. 1.2 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 40 Drive Current (mA) Figure 5. Comparison of coupling efficiency through a 100-micron pinhole located 850 microns away from the substrate surface for 15- micron diameter VCSELs with and without diffractive lenses on the substrate. The efficiency is plotted as a function of drive current. 6. OFF AXIS LENSES Off-axis lenses were also fabricated on VCSEL substrates by the same process. The off-axis lens design was the same diameter and focal length as the on-axis lens with a 20 degree tilt for a collimated beam. The off-axis lens is effectively decentered in the aperture, resulting in smaller zone widths at one edge of the lens pattern. The minimum linewidth in the off-axis lens pattern is 0.17 microns. A scanning electron micrograph of an off-axis lens is shown in figure 6. Efficiency rnieasurements through a 100- micron pinhole at 850 microns from the lens surface gave 75% coupling efficiency for a 15 micron diameter VCSEL. For this measurement it was necessary to align the pinhole and detector on the optical axis 20 degrees off from the normal to the VCSEL substrate. Integration of the off-axis lens demonstrates a capability to control the direction as well as the divergence of a VCSEL beam with integrated diffractive lenses. - .  I Figure 6. A scanning electron micrograph of an off-axis lens fabricated in GaAs. The 80 micron diameter lens has a focal length of 11.0 microns and is designed to produce a beam with an optical axis 20 degrees off from the normal to the substrate surface. 7. CONCLUSIONS We have successfully demonstrated integration of multilevel, high efficiency, diffractive lenses with 16 element arrays of substrate-emitting VCSELs for optical interconnect applications. The lenses were fabricated by e-beam direct-write lithography and chlorine RIBE. Coupling efficiencies through a 100- micron pinhole located at the system receiver plane 850 microns from the ].ens surface were 80%. Advanced designs, such as off-axis lenses for directional control have also been demonstrated. We are currently planning to implement reflective elements and multiple lens designs as well. Feedback effects on the VCSELs from the diffractive lenses will be investigated and antireflection coatings evaluated for improved efficiency and feedback reduction. 8. ACKNOWLEDGMENTS The authors would like to acknowledEe S. Samora, T.R. Carter and J. Escobedo for their exuert technical support. This work was supporte& by the Department of Energy under contract DE-Ad04- 94AL85000. 9. REFERENCES 1. G.R. Olbright, J.L. Jewell, RP. Bryan, W.S. Fu, “Micro-optic and Microelectronic Integrated Packaging of Vertical Cavity Laser Arrays,” Proc. SPIE, Vol. 1851, pp. 97-105, 1993. 2. K. Rastani, M. Orenstein, E. Kapon, A.C. Von Lehmen, “Integration of planar Fresnel microlenses with Vertical-Cavity Surface-Emitting Laser arrays”, Optics Letters, Vol. 16, No. 12, pp. 919-921,1991. 3. R.F. Carson, P.K. Seigal, D.C. Craft, M.L. Lovejoy, “Future Manufacturing Techniques for Stacked MCM Interconnections”, Journal of Metals, Vol. 46, pp. 5145,1994. 4. K.L. Lear, R.P. Schneider, K.D. Choquette, S.P. Kilcoyne, J.J. Figiel, J.C. Zopler, “Vertical Cavity Surface Emitting Lasers with 21% Efficiency by Metalorganic Vapor Phase Epitaxy”, IEEE Photonics Technology Letters, Vol. 6, pp.1053-1055, 1994. 5. M. Alder, “GE High Density Interconnect: A Solution to the System Interconnect Problem”, Proc. SPIE, Vol. 1390, p 504-508, 1990. 6. G.J. Swanson and W.B. Veldkamp, “Diffractive Optical Elements for Use in Infrared Systems”, Opt. Eng. Vol. 28, pp. 605-608, 1989. 

Integration of diffractive lenses with addressable vertical-cavity laser arrays

Integration of diffractive lenses with addressable vertical-cavity laser arrays

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