Design and Fabrication of a Pre-aligned Free-space Optical Interconnection device

Traditional electrical interconnections for chip-to-chip transmission have been facing several critical challenges for years [1, 2]. First, the continuing scaling down of electronic device brings a lot of problems to conventional metal interconnects -the degradation of the wire performance, the power dissipation, and the signal integrity. Second, for data transmissions between chips, the performance is dominated by the interconnection medium rather than the device at either end [1]. Because of the electrical resistance of interconnect wires and interferences, the distance between chips affects data transmission rate significantly, and the interchip and chip-to-board data transmission rate became a bottleneck factor in computer industry. Optical chip-to-chip connections is a very attractive solution to maximize the data-transferring rate and permit longer interchip distance [3]. It also offers the advantages of lower signal loss and lower power consumption. Many different research efforts have been made in industry and academics [4-8]. These proposed optical approaches can be generally classified into guided wave and free space. Those approaches based on guided wave technology involve the use of waveguides to propagate the optical signals [7, 9]. Free-space optical interconnect (FSOI) is another approach for chip-to-chip level interconnection. In comparison with waveguide technology, it has the advantages of large interconnection density, lower power consumption, and better crosstalk performances [1, 5, 10]. One of the biggest challenges for FSOI approach has been the high-precision alignment of the optical components. Adaptive optical components are often used to compensate the misalignment [11]. However, fabrication and control of the adaptive components are complicated and costly. Based on our previous work, we proposed the design and fabrication of a pre-aligned integrated FOSI for chip-to-chip connection application. All optical components including microlens array and micro mirrors are pre-aligned during mask fabrication stage and positioned in an out-of-plane fashion. A fast replication method is then followed to provide good optical performance and lower the cost. The schematic diagram of our FSOI design is shown in Figure 1 with all the relevant parameters indicated. Our current work is mainly focused on design and fabrication of a FSOI with all the basic optical components integrated on it, no efforts will be made to fabricate optoelectronics. Since it is a prototype device, a symmetric design is used to simplify the analysis and fabrication process. On one end of the FSOI system, vertical-cavity surface-emitting lasers (VCSEL) will be attached to emit the signals while on the other end detectors will be attached to receive the signals. Fig.1 Schematic diagram of FSOI Figure 2 shows the replication process in details. First, all the optical components including the microlens array and micromirrors are fabricated by SU-8 tilted lithography on a silicon substrate (Fig. 2(a)). The distance between the microlens array and the reflection mirrors was carefully calculated to reach the optimal performance of Gaussian beam propagated in FSOI as stated before. Second, a negative mold is created with PDMS using the fabricated SU-8 master (Fig. 2(b)). Third, UV curable polymer such as NOA 73 is drop-dispensed on the PDMS mold and a transparent glass substrate is pressed onto the coated UV curable polymer (Fig. 2(c)). Subsequently, it is exposed to UV light ( nm 400 300 ~ − = λ ) for several minutes through the glass substrate. After the UV curing, the fabricated optical structure is removed from the PDMS negative mold by being peeled (Fig. 2(d)). In the last step, using E-beam evaporation method, a thin metal film (such as gold or aluminum) is deposited onto the micromirror to form the reflection coating (Fig. 2(e)). Fig. 2 Schematic of the fabrication process of FSOI based on fast replication method: (a) Fabrication of SU-8 master (b) Fabrication of PDMS negative mold (c) Peeling PDMS mold (d) UV curing of polymer (e) Deposition of metal film to form reflection mirror. Using the presented PDMS molding and curable polymer casting technique, out-of-plane microlens array and reflection mirror for a FSOI device have been successfully fabricated. Figure 3 shows the SEM pictures of the fabricated FSOI. The upper-left picture shows the microlens array L2 and L3 corresponding in Fig. 1. The upper-right picture is a close view of the fabricated microlens. The diameter and focal length of our designed microlens was supposed to be 450μm and 885μm, respectively. Although our preliminary results proved that the focal lengths and surface profiles of the microlens array are predominantly determined by the mask design and the development process only helps to smooth the surfaces. However, other facts as exposure dosage and SU-8 development time also affected the final surface profiles and the focal lengths. The measured focal lengths tend to be about 10-15% less than the designed ones. We are working on establishing a mathematical model to preciously control the profile of the microlens and its focal length. Fig. 3 SEM pictures of fabricated FSOI. ACKNOWLEDGMENTSThanks for the useful advices from Guocheng Shao aswell as from other lab members Weiping Qiu, Yuxuan Zhou,and Ziliang Cai. Thanks Kyung-Nam Kang in CAMD forthe E-beam evaporation work. REFERENCES1 D. A. B. Miller, “Rationale and Challenges for OpticalInterconnects to Electronic Chips,” Proceeding of The IEEE,88(6), 22 (2000).2 J. G. a. M. D. M. Forbes, “Optically interconnected electronicchips: a tutorial and review of the technology,” Electronics &Communication Engineering Journal, 12 (2001).3 J.W.Goodman, “Optical interconnections for VLSI systems,”Proceeding of The IEEE, 72(7), 17 (1984).4 F. B. McCormick, F. A. P. Tooley, T. J. Cloonan et al., “Opticalinterconnections using microlens arrays,” Optical andQuantum Electronics, 24(4), S465-S477 (1992).5 J. Xue, A. Garg, B. Ciftcioglu et al., [An intra-chip free-spaceoptical interconnect] ACM, Saint-Malo, France(2010).6 E. M. Strzelecka, D. A. Louderback, B. J. Thibeault et al.,“Parallel Free-Space Optical Interconnect Based on Arrays ofVertical-Cavity Lasers and Detectors with MonolithicMicrolenses,” Appl. Opt., 37(14), 2811-2821 (1998).7 X. Wang, W. Jiang, L. Wang et al., “Fully Embedded Board-LevelOptical Interconnects From Waveguide Fabrication to DeviceIntegration,” J. Lightwave Technol., 26(2), 243-250 (2008).8 M. J. McFadden, M. Iqbal, T. Dillon et al., “Multiscale free-space optical interconnects for intrachip globalcommunication: motivation, analysis, and experimentalvalidation,” Appl. Opt., 45(25), 6358-6366 (2006).9 S. Kopetz, D. Cai, E. Rabe et al., “PDMS-based opticalwaveguide layer for integration in electrical-optical circuitboards,” AEU International Journal of Electronics andCommunications, 61(3), 163-167 (2007).10 L. J. Camp, R. Sharma, and M. R. Feldman, “Guided-wave andfree-space optical interconnects for parallel-processingsystems: a comparison,” Appl. Opt., 33(26), 6168-6180 (1994).11 C. J. Henderson, B. Robertson, D. G. Leyva et al., “Control of afree -space adaptive optical interconnect using a liquid-crystalspatial light modulator for beam steering,” Optical Engineering,44(7), 075401-8 (2005).