Self-Aligned Chip-to-Chip Optical Interconnections in Ultra-Thin 3D Glass Interposers

Transcription

1 Self-Aligned Chip-to-Chip Optical Interconnections in Ultra-Thin 3D Glass Interposers William Vis^, Bruce C. Chou, Venky Sundaram, and Rao Tummala 3D Systems Packaging Research Center, 813 Ferst Drive NW, Georgia Institute of Technology, Atlanta, GA, USA (404) Abstract This paper presents the modeling, design and demonstration of a three-dimensional polymer waveguide (3D WG) that couples two optical through-package vias (TPVs) in a 3D ultra-thin glass interposer for chip-to-chip optical communications. Coupling of the device is enabled using positive and negative sloped, 45 total internal reflection (TIR) micro-mirrors. The simulated coupling efficiency is within 0.5 db for 45±5. A novel inclined UV photolithography process is proposed to fabricate the microstructures simultaneously with self-alignment. The alignment is inherent because it is resolved prior to inclined photolithography during the planar patterning of double-sided metallization layers. The new process is experimentally demonstrated using commercially available PCB manufacturing technologies. The measured alignment tolerance between the optical via and the polymer waveguide is within 2.5 um across the entire panel. Fifty micron tall polymer WGs at 20 ~ 60 um width with 45 degree entry and exit turning surfaces are fabricated on 150um thick glass substrate. Rounded waveguide sidewalls and inadequate adhesion are observed, which requires further process development to allow high quality optical measurements. Keywords: 3D glass interposer, 3D polymer waveguide, micromirror, inclined lithography. I. Introduction Since the advent of the optical fiber, optical interconnections have been a viable alternative to their electronic counterparts due their high bandwidth potential. The extremely low loss of optical interconnections in glass compared to their electrical counterpart makes them the de facto candidate for long distance transmissions. Ever increasing bandwidth demands have pushed the need for optical interconnection at shorter and shorter transmission distances [1]. As optical interconnects transition from board-to-board [2-4] to chip-to-chip [5] applications, outof-plane turning continues to be an important issue. Out-of-plane turning is most commonly achieved with diffraction gratings or micro mirrors. The typical operating wavelength of light for photonic applications (λ = 850, 1350, 1550 nm) requires submicron resolution for diffraction gratings, which make them impractical at package level. On the other hand, turning micro mirrors operate at all length scales, making them the preferred choice at package level. To date, micro mirrors are fabricated serially using laser ablation or simultaneously using lithographic techniques [2-4]. The 3D Glass Photonics (3DGP) team at the Packaging Research Center at Georgia Tech aims to develop an innovative low cost, low loss, and high volume out-of-plane optical turning solution at the interposer level, specifically in ultra-thin glass for chip-to-chip optical interconnection. A simple schematic of this innovative approach is given by Figure 1. A system level integration of the 3D waveguide (3DWG) has been presented in past conferences [6, 7]. Figure 1. 3DWG for chip-to-chip optical interconnection Many intensive research efforts have been done to develop a process for the simultaneous manufacturing of multiple out-ofplane turning surfaces. Several photolithographic techniques have been reported, including the gradient mask method [8], the moving mask method [9], and inclined lithography [10, 11]. The gradient mask method generates gradient exposure intensity by a grayscale mask, while moving mask method generates the same gradient by a translation of the substrate or mask during exposure. However, commonly-available photosensitive polymers have a single optimal exposure intensity that defines a well-developed polymer structure. As a result, these methods may not be well-suited for development of high quality turning surfaces. Moreover, most of these polymers are in available published work as only positive-toned photosensitive polymers, defined by gradient or moving mask. As such, these two methods demand a positive-tone photosensitive material with consistent high resolution at a range of exposure dosages. Inclined lithography, however, does not require precise exposure gradient because a constant exposure is used to define the polymer microstructure. Further, inclined lithography has also been reported for both positive and negative photosensitive polymers. Inclined lithography is not without its own limitations. First, a 45 degree turning angle is not achievable using inclined lithography in air. The high index of refraction contrast between air (n 1 =1) and photosensitive polymers (typically 1.5 < n 2 < 1.6) does not allow a turning angle greater than the critical angle established by Snell s Law /15/$ IEEE Electronic Components & Technology Conference

2 (1) Arranging for the critical angle gives for light going from n 1 to n 2, sin (2) where is the incident angle which has a maximum of giving, sin (3) When fabricating a polymer waveguide with n 1 =1 and n 2 =1.5, the critical angle, or the maximum turning angle, calculated using equation (3) is Figure 2a illustrates the critical angle (red) at maximum incident. Fortunately, this limitation can be overcome by changing the medium during exposure. For example, immersion in water during the exposure is a simple solution to this limitation [12]. Second, the shape of the geometry cannot be achieved by inclined lithography unless there is a zero gap mask contact to the polymer [13]. Figure 2b illustrates the effect of gap δ on the shape of the waveguide. One solution to ensure good contact is by using a direct-coated mask. In fact, a high-quality polymer microstructure with 45 degree turning has been fabricated on a glass mask, without a substrate [10]. However, this process requires an additional transfer step to a substrate, typically by molding. Consequently, the alignment of the 3D WG to a light source assembled on the substrate is entirely dictated by the alignment precision in the transfer step. Lastly, the symmetrical 3D WG geometry as shown in Figure 1 is not easily achieved with a single exposure. Figure 2c shows the microstructure being achieved by a double exposure method. An alternative method to double exposure involves using a reflective substrate as shown in Figure 2d [13]. Again, this requires a secondary transfer step. c. d. e. Figure 2. Ray behavior for critical angle from air to polymer, gap δ, c. double exposure method, d. using reflective substrate, e. during novel inclined lithography by I-line exposure (λ=365nm). The index of refractions, n are given for λ=365nm. In this paper, an innovative new process that is compatible with PRC s 3D glass interposer technology is developed to address the above mentioned limitations in inclined lithography. The first limitation is overcome by immersion in water. The second limitation is overcome by using glass as both the mask and the substrate; therefore zero gap contact is ensured with no transfer step. To elucidate, the mask is created by planar patterning of double-sided copper seed layers. These seed layers are also used in the semi-additive process (SAP) for electrical buildup. Figure 2e shows the snapshot of the ray mechanics during exposure for the novel process. As shown in this figure, the last limitation described above is overcome by reintroducing an air gap to allow reflection to occur by TIR. sin (4) Using (4) when 45, the argument of the arcsine is greater than 1. Therefore, no refraction occurs and all of the light is reflected. In addition to the TIR, the double sided copper mask is necessary for the desired symmetry. The bottom side mask defines the onset of the entry turning surface (point A), and the topside mask defines the onset of exit turning surface (point B) with a calculated offset to be discussed in Section II. Ultimately, the resulting waveguide is self-aligned to these masks created by planar lithography. The rest of this paper will be organized as follows. Section II will show modeling of the turning surface by total internal reflection. Section II will show design of the test mask and design of the holder to achieve desired tilt and reintroduce the air gap. Section IV will go through the overall process in detail. Section V will show the characterization of the direct coated contact mask and the polymer microstructure. Section VI will provide a summary and conclusion. II. Modeling The turning surface is modeled using 2D finite-difference time-domain (FDTD) modeling without cladding or metallization on the turning surface. The FDTD setup has been described previously [6]. The purpose of the simulation is to determine the loss and angle tolerance of turning for the intended 3D WG structure. The simulation assumes that the light wave is entering from the optical via, then it incidents the 45 degree turning structure, which reflects the light wave into a rectangular optical waveguide. The light source is assumed to be a standard 850 nm VCSEL, with 5um beam radius, while the WG is a multimode rectangular waveguide with 50 um sides. The optical via through 100 um thick glass substrate has 60 um diameter, and is fully filled with the same material as the optical waveguide. Figure 3a shows the time step simulation performed for these three variations. There is acceptable loss at both 45 and 50 degree turning angles, but not at 55 degrees. In fact, the loss at ~45 ± 5 degrees is less than 0.5 db, as shown in Figure 3 A sharp increase in optical loss is observed at ~49 degrees due to the angle offset exceeding the critical angle needed for total internal reflection. 805

3 cases for the waveguide with respect to via to demonstrate the effect of the via on the inclined process, (1) waveguides over no vias, (2) waveguides over an entry via, (3) waveguides over an exit via, and (4) waveguides over both entry and exit vias. A holder with a stack up shown by Figure 4a is designed to allow immersion in water while reintroducing the air gap to ensure TIR at the exit end. The actual holder used is shown in Figure 4 The substrate and holder tilt angle is calculated by an extension of Equation 1 with variables described in the ray diagram in Figure 2e. The tilt is given by (6) where n1 =1.33 and n3 = With θ 1 = 45, 57. Figure 3. 2D FDTD time domain simulation of turning surface with no metallization or cladding at three different angles. Simulation result showing turning loss vs. offset. III. Design It is shown by the ray diagram in Figure 2e that bottom must have a calculated offset with respect to the top. One design can be used to pattern both sides. The offset can be calculated by the following equation, offset tan 2 tan (5) where is the thickness of the glass substrate and is the height of the polymer. The angles can be calculated using Equation 1. For the 150um thick glass used and a polymer thickness of 50um, the offset is calculated to be 239um. The design of experiments (DOE) for the test mask is given in Table 1. A range of widths are provided so the minimum waveguide feature size can be observed. The target waveguide width is 50um. The waveguide (WG) end spacing is varied to allow for various tilt angles. Table 1. DOE for WG structures Variable Range WG width 20 ~ 60 um WG length 0.25 ~ 15 mm WG end spacing 100 ~ 400 um Table 2. Variations for 3D waveguide and via integration The variation for the 3D waveguide integration with the polymer-filled optical vias is given in Table 2. There are four Figure 4. Schematic of holder for tilt and index of refraction control. Experimental carrier. IV. Fabrication A novel lithography process is designed and developed to fit within PRC s 3D glass interposer technology. Polymer via filling occurs before the 3D WG fabrication process. The detailed polymer via filling process has been discussed in [7]. Figure 5 shows the 3D WG fabrication process without any vias. Initially, 4 x 4 150um thick Corning Eagle XG glass panels are sputtered on both sides with a Ti/Cu seed layer. The panels are then laminated on both sides with 15um thick, Hitachi dry film photoresist. The top and bottom sides are exposed using a UX projection lithography tool by Ushio with a calculated offset of 239um with respect to each other. Upon development of the PR, the Ti/Cu seed layers are etched. Figure 6 shows the well-aligned copper masks. The exact alignment is characterized in Section V. After fabrication of the contact masks and sufficient surface cleaning (O2 plasma), an HDMS adhesion promoter for the waveguide material (LightLink by Dow Chemical) is deposited on the bottom side using spin coating. The bottom side refers to the directionality with respect to UV exposure, depicted in Figure 5. LightLink (LL), a siloxane-based photosensitive polymer with an index of refraction of n=1.511 is chosen as the waveguide material due its index of refraction similarity to the Corning glass (n=1.503) and other glass 806

4 substrates (~1.5). The index of refraction values are given for the eventual application wavelength, λ = 850nm. By matching the index of refractions, the substrate is able to act as a cladding layer for the waveguide, simplifying the process by removing a cladding step. Process optimization is performed on LightLink XP-6701A Core on ultra-thin glass. The target thickness is 50um, a standard for multimode waveguides. However, the viscosity of LL does not permit a thickness of 50um with a single spin coat. Thus, a double spin coat process is adopted to achieve the target thickness. For the first spin coated layer, the substrate is soft baked on a hot plate to ensure maximum bonding at the glasspolymer interface. Adhesion is the primary challenge here due to the ultra-smooth glass surface. Surface roughing (i.e. plasma etching) can be employed to improve adhesion, however, doing so will increase planar waveguide loss. Due to a much lower thermal conductivity of the glass substrate with respect to silicon (1000x), a higher hot plate temperature is necessary for soft baking. For the second spin coated layer, the substrate is oven baked to minimize thermal processing to the first layer. Further, this ensures a high quality sidewall, which is imperative for a planar turning surface. During the optimization of the double spin coat method, SEM was used to highlight the discrepancies in the thermal profiles experienced by the individual polymer layers. Figure 7a shows a step in the sidewall as a result of the nonequivalent thermal processing. The two soft bake steps were optimized until the step was minimized. Figure 7b shows final minimized step by polished cross sectioning. Figure 5. Process flow for the turning waveguide. Figure 7. Planar waveguide highlighting thermal processing difference between the two layers during double spin coat optimization, (before) characterized by SEM and (after) characterized by polished cross sectioning. Figure 6. Experimental mask offset by UX Ushio tool After soft baking, the substrate is loaded in the holder illustrated in Figure 4. In addition to the tilt angle calculated above ( 57 ), the holder is designed to allow multiple angles of exposure to confirm the calculation. The air gap is successfully introduced, allowing the total internal reflection of the UV rays at the exit end. The optimized exposure time, 807

5 determined from a planar exposure ladder, is used in the oblique case. Following exposure, a post exposure bake (PEB) is optimized on a hot plate. After PEB, the waveguide is developed using LightLink XP-3636 Developer, a sodium hydroxide based solution. The development is done using a 40 second followed by a 20 second double puddle process at 40C. Lastly, the waveguide is cured at 145C in an oven for one hour. V. Characterization: A multipoint alignment error analysis is performed to assess the alignment accuracy of the directly coated copper masks to the optical vias. Sixteen points are inspected as illustrated in Figure 8 At each point, two circles are drawn, (1) the circle equidistant from the mask edge and end, and (2) the circle defining the optical vi A misalignment vector, Δ is defined from the center of the via to the center of the other circle. Figure 8c illustrates the alignment analysis done at a single point. The misalignment vector is found for each point and the average of the vector magnitudes is, Δ avg = 2.5um with a standard deviation of 1.5um. Table 3 documents all of the misalignment vectors and their associated magnitude. Figure 8b shows the calculated vectors at their respective position. is proportional to the exposure intensity. Therefore even with TIR, some additional exposure dosage will be necessary to fully define the exit end. The undercutting along the cross section, which is also shown in Figure 7b is likely a result of interfacial stress induced between the glass and polymer upon processing. A polishing cross section is used to determine the turning angle. Figure 9b shows ~ 45 entry and exit turning. Improvement of the turning surface co-planarity can be achieved with further sidewall optimization, specifically in the PEB step. Table 3. Misalignment vector data at each point ID x y Δ x y Δ ID (um) (um) (um) (um) (um) (um) Average 2.3 St. Dev. 1.6 c. Figure 8. Via to mask misalignment analysis: point mapping across entire 4x4 panel, associated misalignment vectors, c. individual point analysis. Figure 9a shows the 3D WG microstructure. The undercutting along the length localized at the reflected end is a result of underexposure. The distance of travel for the ray throughout the LL is increased by 2 everywhere and by 2 2 at the reflected end relative to the 50um thickness assumed in the planar case. The polymer crosslinks and absorbs some of the energy as the ray advances, so the ray s distance of travel in LL Figure 9. 3D waveguide characterized by SEM, exit (left) and entry (right) cross-section of 3D waveguide illustrating 45 degree entry and exit turning. VI. Conclusion: A 3D optical waveguide (3D WG) with 45 degree entry and exit turning with respect to optical TPVs was demonstrated using a novel inclined lithography technique on an ultra-thin 3D glass substrate. Furthermore, the process enabled inherent alignment of the optical microstructure and the integration for chip-to-chip optical communication. The FDTD modeling of the 3D WG 808

6 microstructure indicated TIR turning with a tolerance of 45 ± 5. A 4in x 4in and 150um thick glass panel was fabricated to demonstrate this new proposed process. Fifty micron tall polymer WGs at 20 ~ 60 um width with 45 degree entry and exit turning surfaces were achieved. The alignment tolerance across the panel, as measured by the misalignment between the via and the 3D WG was 2.5 um. While the proof of concept was demonstrated, additional development of the 3D waveguide geometry is necessary for implementation in ultra-low loss applications. Acknowledgment The authors would like to thank Abhishek Thumaty and Tim Fleck for their generous fabrication help. Furthermore, the authors would like to thank Fuhan Lui and Daniel Guidotti for their guidance and Ryuta Furuya of Ushio for operating the UX tool. The authors would also like to thank Jason Bishop for the construction of the experimental holder. References 1. D. Miller, "Device Requirements for Optical Interconnects to Silicon Chips", Proc. IEEE, vol. 97, no.7, pp , M. Immonen et al., Fabrication and Characterization of Polymer Optical Waveguides With Integrated Micromirrors for Three-Dimensional Board-LevelOptical Interconnects, IEEE Trans. Compon. Packag. Manuf. Technol., vol. 28, no. 4, pp , F. Doany et al, 160 Gb/s Bidirectional Polymer-Waveguide Board-Level Optical Interconnects Using CMOS-Based Transceivers, IEEE Trans. Adv. Packag., vol. 32, no. 2, pp , L. Brusberg et al., Glass Carrier Based Packaging Approach Demonstrated on a Parallel Optoelectronic Transceiver Module for PCB Assembling in Proc. IEEE Electronic Components and Technol. Conf. (ECTC), Las Vegas, NV, June 1 4, 2010, pp P. Shen et al., On-Chip Optical Interconnects Integrated with Laser and Photodetector Using Three-Dimensional Silicon Waveguides, in Optical Fiber Communication Conference, San Francisco, CA, Mar 9 13, B. Chou et al., Modeling, Design, and Fabrication of Ultrahigh Bandwidth 3D Glass Photonics (3DGP) in Glass Interposers, in Proc. IEEE Electronic Components and Technol. Conf. (ECTC), Las Vegas, NV, May 28 31, 2013, pp B. Chou et al, Modeling, Design, and Demonstration of Ultra-miniaturized and High Efficiency 3D Glass Photonic Modules, in Proc. IEEE Electronic Components and Technol. Conf. (ECTC), Orlando, FL, May 27 30, 2014, pp A. Chen et al., Vertically tapered polymer waveguide mode size transformer for improved fiber coupling, Opt. Eng., vol. 39, no. 6, pp , Y. Hirai et al, Moving mask UV lithography for threedimensional structuring, J. Micromech. Microeng., vol. 17, pp. 199, X. Lin, X. Dou, A. X. Wang, and R. T. Chen, "Polymeric waveguide array with 45 degree slopes fabricated by bottom side tilted exposure," Proc. of SPIE, X. Dou, X. Wang, H. Huang, X. Lin, D. Ding, D. Z. Pan, et al., "Polymeric waveguides with embedded micromirrors formed by Metallic Hard Mold," Opt. Express, vol. 18, pp , F. Wang et al., 45 Degree Polymer Micromirror Integration for Board-Level Three-Dimensional Optical Interconnect, Optics Express, vol. 17, no. 13, pp , Z. Zhu et al., Modeling, simulation and experimental verification of inclined UV lithography for SU-8 negative thick photoresists, J. Micromech. Microenig., vol. 18, no. 12,