1452 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 11, NOVEMBER 1999 Two-Dimensional 45 Surface-Normal Microcoupler Array for Guided-Wave Optical Clock Distribution Jianhua Gan, Linghui Wu, Hongfa Luan, Bipin Bihari, and Ray T. Chen, Member, IEEE Abstract—Surface-normal couplers are indispensable parts of a guided-wave optoelectronic interconnects for the coupling of optical signals into and out of the waveguides while facilitating the packaging. In this paper, integration of the 45 surface-normal couplers at each fanout end of the H-tree waveguide structure is described. An optical clock signal distribution system is under development using polyimide based H-tree waveguide structure. The coupler is a 45 slanted end surface of the polyimide waveguide. The coupler works for a wide range of wavelength. The experimentally estimated output coupling efficiency is nearly 100%. To determine the optimized size and shape of the pho- todetector, near and far field diffraction patterns are evaluated. Experimental results conclude that the phenomenon is dominated by the fundamental mode of the highly multimode waveguide. Index Terms—45 microcoupler, guided-wave optical intercon- nects, H-tree polyimide waveguide, optical clock distribution. I. INTRODUCTION T HE CLOCK skew, bandwidth limitations, cross talk and severe power budget requirement due to the skin effect are serious problems for electrical interconnects when the clock rate in the computer is pushed toward the multigigahertz range. Guided-wave optical interconnects show great potential to overcome these bottlenecks [1]. Optical interconnects for interchip and interboard level are being vigorously investigated for high data rate applications [2]–[4]. All guided-wave opto- electronic interconnection configuration involve the coupling of the light signal from the source (e.g., VCSEL) to waveguide and the coupling of it from the waveguide to photodetectors. Thus surface-normal microcoupler is a key component in planar integrated optical systems. Employment of conventional coupling techniques utilizing prisms and lenses is costly, bulky and very inconvenient from the packaging point of view and often put restriction on planarization. The waveguide grating or waveguide mirror based coupler can overcome the above problems. However, a grating based approach requires precise control of grating parameters for efficient coupling and usually have low tolerance to wavelength variations. Unlike grating couplers [5], [6], 45 surface-normal microcouplers are easy to fabricate, reproducible and relatively insensitive to wavelength variations. Here, we describe the integration of 45 microcouplers in a guided-wave clock distribution system. This guided-wave optical interconnection network is designed Manuscript received February 16, 1999; revised July 6, 1999. This work was supported by DARPA, by the BMDO, by the AFOSR, by Cray Research, by the 3M Foundation, by the Texas ATP program, and by the OIDA Joint U.S.–Japan Optoelectronics Project. The authors are with the Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712 USA. Publisher Item Identifier S 1041-1135(99)08685-1. to be compatibly integrated onto the supercomputer boards to become an additional interconnection layer among many other electrical interconnection layers. This optical clock distribution system is capable of delivering clock signal to other chips at gigabits per second speed with a minimized clock skew. Polyimides are widely used in silicon CMOS processing. We have used Ultradel 9120D to fabricate an H-tree waveguide, a 1-to-48 fanout structure with equivalent optical paths. 9120D is a negative-acting, photosensitive polyimide. It is characterized by high optical transparency, high thermal stability and ease of fabrication. At the wavelength of 1.310 m, the refrac- tive indices are 1.5364 and 1.5073 for TE and TM waves, respectively. The optical losses of the material are 1.04 dB/cm at 633 nm, 0.13 dB/cm at 830 nm, 0.09 dB/cm at 1064 nm, 0.34 dB/cm at 1300 nm and 1.21 dB/cm at 1550 nm. The glass transition temperature is 390 C. The coefficient of thermal expansion at 300 C is 27 ppm/ C. The moisture uptake at 100% relative humidity is 3.0%. Polyimides with lower refractive indices are also available for cladding and buffer coatings. II. TWO-DIMENSIONAL 45 SURFACE-NORMAL MICROCOUPLER ARRAY FABRICATION A novel board-level optical clock distribution system based on polyimide waveguides, 45 surface-normal microcouplers and fast photodetectors will be the enabling technology to boost the clock rate in supercomputer and other high per- formance digital systems to multigigahertz range. One major concern that needs to be addressed is the Si-CMOS process compatibility [1]. The thermal stability of polyimide renders the polyimide waveguide and microcouplers compatible with Si-CMOS process. In order to minimize the clock skew in the clock distribution system, we use an H-tree configuration to equalize the paths to all the fanout points. The schematic of a 1-to-48 H-tree structure is shown in Fig. 1. The H-tree waveguides have been fabricated using pho- tolithography. A silicon wafer with 2- m-thick silicon dioxide is used as the substrate. A silicon dioxide layer acts as the buffer layer. As an alternative, other low index polyimide can also be used as cladding and buffer layers. About 10- m- thick polyimide 9120D is spin-coated on the clean substrate. The H-tree structure is patterned to the polyimide layer using UV photolithography, and the waveguide without 45 micro- couplers is obtained [7]. We use RIE to fabricate the 45 microcoupler at the end of each branch of the waveguide. The corresponding processing steps are shown in Fig. 2. A 0.3 nm- thick aluminum film is coated over the H-tree waveguides. Photoresist AZ5206E is coated over aluminum. A mask is 1041–1135/99$10.00  1999 IEEE