0-7803-8906-9/05/$20.00 ©2005 IEEE 2005 Electronic Components and Technology Conference

10Gbps Multi-Mode Waveguide for Optical Interconnect

Yi-Ming Chen 1 , Cheng-Lin Yang 1 , Yao-Ling Cheng 1, Hsiu-Hsiang Chen 1, Ying-Chih Chen 1, Yen Chu 1 , Tsung-Eong Hsieh 2

1. Opto-Electronics & Systems Laboratories, Industrial Technology Research Institute, R919,Bld. 51,195, Sec.4, Chung Hsing Rd. Chutung, Hsinchu, Taiwan 310, R.O.C.

Tel: +886-3-5913726 Fax: +886-3-5820258 E-mail: ymchen@itri.org.tw 2. Nation Chiao Tung University, Hsinchu, Taiwan

Abstract In this paper, high-speed multi-mode waveguide array is

proposed. As a core material, a polymer called SU-8 is used. The index of SU-8 at a wavelength of 850 nm is 1.58. It is highly transparent for wavelengths > 800 nm, has shown heat resistance to temperature > 200 degrees Celsius and it is chemically and mechanically stable [1], so it is most suited for optical interconnect applications.

In order to reduce the coupling loss of the system, the dimensions of waveguides are taken to be 50x50µm2. The SU-8 is spin-coated on the glass substrate. By varying the spin speed, the height of the layer can be varied from 10 to 60 µm. The layer is pre-baked at 95 degrees Celsius to evaporate the solvent. Then after pre-heating, the film is exposed to UV-light with a mask. Following the UV-exposure, the film is post-baked at a temperature of again 95 degrees Celsius. In this post exposure baking step the cross-linking of the polymer takes place in all exposed areas. The SU-8 film is developed in RER 600 (PGMEA). Next, the defined structures are exposed to UV-light and hard-baked at 150 degrees Celsius [1]. Finally, the embedded waveguides are covered by cladding layer and measured by 850 nm LD.

The measurements demonstrate robust 10 Gbps data transmission exceeding 50mm using a new high-speed 50µm multimode planar waveguide circuit (PLC). The this indicates that experiments driving VCSEL-based 850nm LD optical links 231-1 PRBS signal at speeds up to 10 Gbps. The results can be used to broaden the component specification range for 10 Gigabit optical interconnects applications. Introduction

Optical multimode planar waveguide technologies are increasingly employed in shot distance communication applications such as optical interconnects. In this paper, we used photochemical delineation method to fabricate large cross sectional multimode waveguides by SU-8 photo resist. The hot embossing process shows in figure 1[2-4]. The 50x50 µm2 waveguide on substrate did not have enough friction force to resist the metal plate moved off. In other words, the waveguide process didn’t fix easy on the substrate when we removed the metal plate.

This paper provides a low-cost potential for mass fabrication of thick waveguides using very simple equipments. The optical simulation and fabrication procedure of multimode waveguide will describe more detail in the following section.

b Hot embossing a Metal plate

substrate

cladding

cRemoved metal plate d Core filling and

overcladding

Figure 1. Hot embossing process

Optical Simulation As show in Figure 2, the model of entire optical

interconnect system consist of VSCEL array, passive multimode polymer waveguide with two 450 TIR mirror plane, and the photodiode array. The optical transmitter is set to be a surface light source with Gaussian intensity distribution and 150 half divergent angles, and the diameter of the VSCEL is 10µm corresponding to actual aperture size. The rays emitted from VSCEL array are propagating through the air and glass substrate then launch into optical waveguides. In order to reduce the coupling loss of this system, the dimensions of polymer waveguides are taken as large as possible to be 50µm x50µm, while the index of core and cladding are 1.52 and 1.5 correspondently. The transmitting light in waveguide will hit the first 450 mirror plane and those that satisfy the total internal reflection (TIR) condition are reflected at the mirror plane and guided in the waveguide core. Further, the rays propagated through the waveguide and satisfy the TIR condition at the PD side mirror are reflected toward the receiver, which has diameter of 100µm, corresponding to actual size of PD. All other rays that don’t satisfy the TIR condition at two 450 mirror planes are loss terms.

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Polymer waveguide

Optical loss

450mirror1

VSCEL

Glass substrate

detector x

z

y

Optical loss

450mirror2

Figure 2. Structure of optical interconnect

The coupling efficiency between VSCEL and PD array is the most important issue in this optical interconnect system. In order to make sure PD can identify optical power emitted from the VSCEL, optical simulation is done in this study. Because of the large cross-sectional size of the multimode waveguide compared to the wavelength of light, there are more than 1000 modes propagating in the waveguide core simultaneously, and the optical path can be analyzed by applying methods of geometrical optics, well know as ray tracing methods [5-7].

28um

(a)

26um

(b)

Figure 3. Simulation coupling efficiency of the optical path with misalignment alone the (a) x-axis and (b) y-axis.

Figure 3 shows the simulation result of coupling efficiency with misalignment of VSCEL along both the x-axis and the y-axis. At no misalignment, the coupling efficiency of optical waveguide with core size of 50µm x50µm is as high as -4dB. As the misalignment distance along the x-axis increase to 28µm, the coupling efficiency will decrease by 3dB. And in the same way, when the misalignment distances along the y-axis increase to 26µm, the coupling efficiency will decrease by 3dB. In brief, as show in our simulation, the multimode waveguide with size of 50µmx50µm is not only perform highly coupling efficiency but also can tolerate huge fabrication misalignment in this system.

Waveguide Fabrication SU-8 has shown highly transparent, excellent heat

resistance and chemically and mechanically stable. Because of these advantageous properties, it a very useful material for a wide range of applications include micro-machining, micro-fluidity, package, micro optics and especially in optical interconnecting.

Glass substrate

Core layer UV

(a) (b)

(d)(c)

Upper cladding

photomask

Figure 4. Illustration of the fabrication process of

multimode waveguides. (a) Film formation of the core layers. (b) Photolithography using a photomask. (c) Post-bake and development. (d) Upper cladding layer coating.

In order to fabricate multimode optical waveguide with 50-µm-thick, the high viscosity NANOTM SU-8-50 had been used in this paper. Figure 4 illustrates the fabrication process of polymer waveguides using photolithography. First, SU-8 was spin coated on a cleaned 4 inch glass substrate. A 50-µm-thick layer was spin coated using an experimentally determined curve of thickness versus spin speed. The coated film was then pre-baked to evaporate the solvent at 650C for 3min followed by 950C for 6 min. The waveguide patterns were defined by the hard contact photolithography using I -line, which act as catalyst for the polymerization. Following the UV exposure, the film was post-baked at 650C for 1min with further baking at 950C for 5 min on a hot plate to step the cross-linking of the exposed polymer area. The waveguide pattern was developed by propylene glycol methylether acetate (PGMEA) for 4 min followed by hard baking on a hot plate for 1 hr. Figure 5 shows the Near field of multimode waveguide with dimension of 50x50µm2.

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Figure 5. Near field pattern of multimode waveguide

fabricated by hard contact photolithography.

SSuubb.. WWaavveegguuiiddee

Dicing saw (a)

(b)

(c)

Figure 6. 450 TIR mirrors (a)TIR mirrors fabricated by 900 V-shaped diamond

blade. (b) SEM of 450 TIR mirror. (c) Roughness of 450 mirror measurement by AFM.

TIR Mirror Fabrication As illustrate in Figure 6 (a), 450 TIR mirrors on the

polymer waveguide were then formed using a dicing saw with a 900 V-shaped diamond blade. The SEM picture of 450 mirrors is shown in Figure 6 (b). The roughness of the 450 TIR mirror is measured by AFM, as shown in Figure 6(c), the root mean square (RMS) value is about 6 nm in this case.

SU-8 Multimode Waveguide Testing for 10Gbps

Figure 7. SU-8 multimode waveguide testing

Figure 7 shows a diagram of the testing setup. The 850nm VCSEL light is coupled through the SU-8 multimode waveguide to the photodiode using the index matching oil. The VCSEL is driven directly via a 10 GHz function generator. Note that the VCSEL has a 10 dB extinction ratio at 10 Gbps (see figure 8). This contributes to the ISI (intersymbol interference) and BER (bit error rate) of the measured output signal. On the experiments driving VCSEL-based 850 nm LD optical links 231-1 PRBS signal at speeds up to 10 Gbps. Figure 9 show the performance of the through multimode waveguide receiver at 10 Gbps. This data was measured by using the experimental setup shown in figure 7 and a maximum input power of 100mW per channel. The reference light was emitting pass through the SU-8 multimode waveguide and received by photodiode. Then the optical loss was from 0 dB to -10 dB. The receiver character was shown in figure 9. The power consumption of each receiver is 100 mW at 10 Gbps. Again the output buffer of this amplifier limits the performance and prevents the output swing from surpassing 200 mV at high data rates. We designed the receiver to achieve rail-to-rail outputs, but the circuit cannot drive a 50 ohm load (of the oscilloscope) without a multistage buffer which limits the headroom of the output, adds latency to the circuit, and limits the possible operating bandwidth. Based upon simulation results, if the receivers were to drive an internal circuit with high impedance such as a gate, it would drive the circuit up to 3.3 V. Finally, the SU-8 multimode waveguide can use be used to broaden the component specification range for 10 Gigabit optical interconnects applications.

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Figure 8. 10Gbps reference light source

Figure 9. Through multimode waveguide result

For Optical Interconnect Application

Figure 10. Evaluation setup

Electrical boards, modules, and integrated circuits are essentially planar, and thus, a SU-8 planar waveguide optical interconnection scheme matches the electrical systems from a topographical standpoint. There are a variety of approaches to the partitioning of optical and electrical signals in a mixed electrical/optical interconnection system. Figure 10 turn the optical beam out of the substrate into the optoelectronic active device keep the optical beam confined to the substrate. Optical beams can be turned 90 degree using TIR mirrors and can be turned into optical/optoelectronic devices, which may contain a combination of active and passive optical/ optoelectronic devices and circuitry. By employing diffractive optical elements, such as preferential gratings, high coupling efficiency and limited spectral selectivity can be achieved [8].

A great deal of research to date has focused upon the implementation of polymer optical waveguides with standard electrical interconnection substrates, and there have been demonstrations of polymer waveguides addressing photodetectors fabricated on SiO2 substrates. An integrated circuit is attached to the electrical interconnection substrate

and wire bonded to the photodetectors, as shown in Figure 10. This work represents steps toward chip to chip embedded optical interconnections integrated with electrical interface circuitry. The chip to chip optical interconnections described herein utilize optical signals which can originate and/or terminate in the waveguide directly on the board or module without optical beam turning. However, the use of polymer waveguides and low cost epoxy and polymer substrates is interesting for chip to chip optical interconnections in electrical interconnection systems, and thus the emphasis in this paper on polymer waveguides for low cost optical interconnection which can be integrated with substrates such as high temperature FR-4. Polymer waveguides integrated onto Si [9] or GaAs [10-11] electrical interconnection substrates which have photodetectors fabricated in the substrate have been reported. However, this approach excludes epoxy and polymer substrates since high performance photodetectors cannot be fabricated in these materials. The embedded waveguide approach reported herein uses thin film OE devices (with the OE device growth substrate removed) which can be bonded to any host substrate, including polymer and epoxy boards such as FR-4. The polymer waveguide material can then be deposited directly under or on top of the thin film active OE devices, which are thus embedded directly in the waveguide or cladding, and circuits can be interconnected to the photodectors using either bump or wire bonds, as illustrated in Figure 11, respectively. Actually, the SU-8 multimode waveguide and OE devices were integrated successful.

Figure 11. SU-8 for optical interconnect application

Conclusions The 10Gbps multimode waveguide by using SU-8

material was developed in this paper. It has the characters like highly transparent, excellent heat resistance and chemically and mechanically stable. Through this process, the waveguide can pass the 10Gbps testing by using SU-8 multimode waveguide. Finally, the experiment results solved that SU-8 has low optical loss and high speed optical transmission for optical interconnect application.

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[3] S. Lehmacher et al, “Polymer Optical Waveguides Integrated in Printed Circuit Boards,” Proceedings 27th European Conference on Optical Communications ECOC, 2001

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[5] Wasserman, Y, “Integrated Single-Wafer RP Solutions for 0.25-micron Technologies,” IEEE Trans-CPMT-A, Vol. 17, No. 3 (1995), pp. 346-351.

[6] Shu, William K., “PBGA Wire Bonding Development,” Proc 46th Electronic Components and Technology Conf, Orlando, FL, May. 1996, pp. 219-225.

[7] R. Yoshimura, M. Hikita, M. Usui, S. Tomaru, and S. Imamura, “Polymeric optical waveguide film with 45o-mirrors formed with a 90° V-shaped diamond blade,” Eletron. Lett., 33, pp. 1311-1312, 1997.

[8] S. K. Tewksbury and L. A. Hornak, “Optical clock distribution in electronic systems,” J. VLSI Signal Processing, vol. 16, pp. 225–246, 1997.

[9] C. H. Buchal, A. Roelofs, M. Siegert, and M. Loken, “Polymeric strip waveguides and their connection to very thin ultrafast metal-semiconductor-metal detectors,” Mat. Res. Soc. Symp. Proc. vol. 597, 2000, pp. 97-102.

[10] F. Gouin, L. Robitaille, C. L. Callender, J. Noad, C. Almeida, “ A 4x4optoelectronic switch matrix integrating an MSM array with polyimide optical waveguides,” SPIE, vol. 3290, 1997, pp. 287-295.

[11] C. L. Callender, L. Robitaille, J. P. Noad, F. Gouin, and C. Almeida, “Optimization of metalsemiconductor-metal (MSM) photodetector arrays integrated with polyimide waveguides,” SPIE, vol. 2918, 1997, pp. 211-221.

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