s) Optical Semiconductor Module ... - IEEE Xplore

Extremely-Compact and High-Performance (160Gbps = 20GB/s) Optical Semiconductor Module Using Lead Frame Embedded Optoelectronic Ferrule Hiroshi Uemura1, Hiroshi Hamasaki1, Hideto Furuyama1, Hideo Numata2, Chiaki Takubo2, and Hideki Shibata1 1 Center for Semiconductor Research & Development, 2 Process & Manufacturing Engineering Center, Semiconductor Company, Toshiba Corporation 1, Komukai Toshiba-Cho, Saiwai-Ku, Kawasaki-City, Kanagawa, 212-8583, Japan [email protected] Tel.: +81-44-549-2754, Fax: +81-44-549-2886 Abstract A high-performance 160 Gbps (=20 GB/s) optical semiconductor module using an optoelectronic (OE) ferrule with the ultra-compact size of 4.4 × 4.5 × 1.0 mm3 was developed for the first time in the world. The ferrule is used as the interface of OE converter in our developed optoelectronic LSI package of over 1 Tbps, POST LSI package (Post-reflow Optical-interface Stacking Technique LSI package). The feature of this OE ferrule is to have finepitch lead frame electrodes formed by insert molding method, which realized highly reliable electric contacts and excellent electric characteristics of the module. Moreover, the highdensity assembly of 12ch optical semiconductor module coupling an optical semiconductor device and an optical fiber array was realized by a simple assembly process with high accuracy and high reliability. Introduction With a significant advance in processing ability of LSI chips such as CPUs and GPUs for responding to a request for higher-quality multimedia content, it is required to transmit higher-capacity data signal between those LSI chips. System bandwidth of 50 GB/s class in consumer products is no longer exceptional [1]. There is an approach to the system with terabyte I/O bandwidth for next-generation memory [2]. However, these high-speed signal transmissions by electric interconnection have lots of difficulties; signal attenuation by large loss components of materials, noise problems such as interchannel crosstalk, and signal delay and timing skew. In the development of high-speed signal transmission system today, it is reality that these problems are solved at a great cost, for example, introduction of novel circuit technology such as repeater, equalizer, and phase adjuster, or addition of the system performance restriction. This situation is driving the motivation to utilize optical signal for signal transmission between LSI chips. With optical signal, it is possible to realize repeater-less transmission by low loss characteristics, skew-less transmission by high speed characteristics, and wiring density improvement by noise-free characteristics, which enables extremely simple transmission system with no complicated circuits or system restrictions. We had already proposed highly practical novel optoelectronic LSI package of over 1 Tbps, POST LSI package [3]. POST LSI package has an optical interface on which all the optical components are mounted, and the interface is attached onto a motherboard or an interposer after all the packaging processes for electric components have been finished. This feature leads to the greatest characteristics of

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POST LSI package. That is, optical interconnection is realized without any changes to packaging processes or structures of existing electric components, although other traditional optoelectronic LSI packages need alternate processes or structures because they can not take the heat processes for electric components. Data signal in POST LSI package is transmitted by an optical semiconductor module which has electric signal I/O. This module is composed of optical semiconductor devices (VCSEL or PIN-PD) which convert electric signal and optical signal with each other, optical fibers which transfer optical signals, and OE ferrules which couple the optical semiconductor devices and the optical fibers (Fig. 1). Highquality optical signal transmission is realized by applying desired electric signals through driver IC to the electrodes on which a VCSEL chip is mounted, and the electric signals are reproduced from a PIN-PD chip mounted electrodes. The optical semiconductor module using an OE ferrule with electrodes on which an optical semiconductor device chip is mounted is 7 times as small as a traditional MT connector which is an optical connector with no devices on it (Fig. 2). This extremely compact module is an epoch-making device which easily realizes highly reliable optical coupling only by flip chip (FC) mounting of an optical semiconductor chip and the insertion of an optical fiber array to the OE ferrule, without particular kind of optical components such as lenses and mirrors. electric signal

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Fig. 1: The structure of optical semiconductor module using OE ferrule which couples the optical semiconductor devices and the optical fibers. This module has electric signal I/O, and the data signal is transmitted by optical one.

In this work, we have developed the optical semiconductor module using a novel lead frame embedded OE ferrule. The module achieved high-capacity signal transmission of 13.5 Gbps/ch × 12 ch = 160 Gbps (20 GB/s)

1936 2008 Electronic Components and Technology Conference

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Fig. 2: The comparison of the optical semiconductor module using an OE ferrule with electrodes on which an optical semiconductor device chip is mounted and a traditional MT connector which is an optical connector with no devices on it.

Lead Frame Embedded OE Ferrule One of the key components in optical semiconductor module is the device which couples an optical semiconductor device and an optical waveguide. In traditional optical coupling devices, optical components such as mirrors for changing light paths or lenses for controlling beam radius are used between an optical semiconductor device and an optical waveguide. These components make the coupling device structure complex, which increases the cost and decreases the reliability. On the other hand, our developed OE ferrule enables excellent optical coupling without using above mentioned particular kind of optical components by means of mounting an optical semiconductor chip directly on the side edge of the OE ferrule. In addition, this OE ferrule has superiority in cost and productivity because it is manufactured by optimized resin molding process, insert molding method, in which the OE ferrule is formed by the mold with the lead frame hold on [4].

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Fig. 4: The impedance measurement results between adjacent lead frame electrodes of the lead frame embedded OE ferrule.

Another feature of this OE ferrule is to have 125 µm diameter fiber holes with the precision of ±1 µm. These fiber holes are guides when an optical fiber array is inserted, and realize highly accurate optical fiber array insertion without using complicated mechanical structure. In addition, these holes are extremely accurate fiducial when an optical semiconductor chip is mounted on the OE ferrule. This alignment of optical fiber array insertion and optical semiconductor chip mounting to the accurate fiber holes enables excellent optical coupling of the OE ferrule.

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Fig. 3 shows 12 ch lead frame embedded OE ferrule used in this work. This OE ferrule provides sufficient rigidity and stability of the electrodes, and the surface of the lead frame is plated by Au/Ni with high-quality. These characteristics realize highly reliable and robust bonding between an optical semiconductor chip and lead frame electrodes, in contrast to OE ferrules fabricated by 3D MID (Molded Interconnection Device) [5]. The exposed edge faces of the lead frame on top surface of the OE ferrule is used as wire bonding pads, and electric wiring to a driver or a receiver IC chip is possible as shown in Fig. 1. The fine-pitch lead frame with 50 µm width and 75 µm space is embedded in the OE ferrule with the precision of ±15 µm. Enough margins are secured by such designs for electric junction of optical semiconductor chip bumps and the lead frame electrodes. 25 electrodes in total are aligned in GSG (Ground-Signal- Ground). The impedance between adjacent electrodes comprised almost only capacitive constituent, and the parasitic capacitance was less than 100 fF at from 10 MHz to 1 GHz (Fig. 4). The dielectric resistance of more than 5×1012 Ω was also observed under the applied voltage of 5 V. Parasitic capacitance small enough for highspeed operation and dielectric resistance large enough for driving voltage of an optical semiconductor device are achieved in this lead frame embedded OE ferrule.

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in the extremely compact size of 4.4 × 4.5 × 1.0 mm3 with GI 50/125 multimode optical fiber array by realizing highly reliable FC mounting of an optical semiconductor chip on rigid lead frame electrodes and optimizing assembly process of the module.

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Fig. 3: Lead frame embedded OE ferule with 4.4 mm width, 4.5 mm length, and 1.0 mm thickness. The OE ferrule has fiber holes with 125 µm diameter and lead frame electrodes with 50 µm width.

High-Precision Assembly Process and Electric / Optical Characteristics The assembly process flow scheme of the optical semiconductor module using the lead frame embedded OE ferrule is shown in Fig. 5. There are only 3 steps in the assembly process flow; (1) FC mounting of an optical


Fig. 6: Example of mounting displacement. Blue circle is fiber hole with 125 µm diameter, red circle is optical fiber core with 50 µm diameter, and black dot circle is position of VCSEL device, indicating the distance from the center of the fiber core. 3.0 ch01, ch12

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semiconductor chip, (2) an optical fiber array insertion, and (3) underfill of an optical semiconductor chip and solidifying of an optical fiber array to the OE ferrule. In this article, we focus on the optimization of (1) FC mounting of an optical semiconductor chip and (3) underfill of an optical semiconductor chip. Ultrasonic FC method was chosen for the mounting of an optical semiconductor chip onto the OE ferrule. This method has excellent productivity and reliability because it enables mounting with minimum area and ultrahigh throughput, and also enables mounting with little damage to an optical semiconductor device by low-temperature process. On the other hand, in order to improve the mounting displacement and shear strength of an optical semiconductor chip to a sufficient level, we had to pay attention to lots of parameters such as ultrasonic power, weight value, their time control, temperature of chips and substrates, ultrasonic direction, image recognition methods, and so on. Especially, because a 12 ch optical semiconductor chip has high aspect ratio, a little inclination leads to appear large mounting displacement.

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Fig. 7: Electric and optical characteristics of the optical semiconductor module exemplified in Fig. 6. Optical power was VCSEL output through the optical fiber.

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Fig. 5: The assembly process flow scheme of the optical semiconductor module. (1) FC mounting of an optical semiconductor chip, (2) an optical fiber array insertion, (3) underfill of an optical semiconductor chip and solidifying of an optical fiber array to the OE ferrule.

Fig. 6 shows the example of VCSEL mounting displacement of ch01 and ch12 at the both ends of a 12 ch VCSEL chip observed by grinding the module from the fiber array insertion side and exposing VCSEL devices proximity. Although the ch01 VCSEL device is 15 µm away from the center of the fiber hole with 125 µm diameter (blue circle), it is still in the optical fiber core with 50 µm diameter (red circle). Meanwhile, the ch12 VCSEL device is 27 µm away from the fiber hole center, and is outside of the optical fiber core. This mounting displacement leads to large differences of optical output power between these channels (Fig. 7). The measured optical power here was VCSEL output through the optical fiber. The power from ch12 which is outside of the optical fiber core is extremely low, while there are no large differences of current-voltage characteristics between both channels and electric connection of the VCSEL chip and lead frame electrodes are thought to be favorable. Although the displacement of this example is induced by chip inclination of less than 0.5 degree, it is enough to make a large influence on the optical transmission characteristics of the module.

Even though an optical semiconductor chip is mounted with no displacement, there is another factor which leads to optical characteristic variation in the underfill process. The factor is bubble which arises during underfill resin coating and its curing. The amount of light coupling between a VCSEL and an optical fiber will be changed enormously depending on with or without the bubble and the position of the bubble. Especially, wrong setting of the amount of the underfill resin, the viscosity of the resin, curing temperature, and the distance between an optical semiconductor device and an optical fiber often made it difficult to suppress the bubble arising. Fig. 8 is the example of bubble arising near VCSEL devices; (A) large bubble covering all the optical output area of VCSEL, (B) small bubble covering half the optical output area of VCSEL, and (C) no bubble. Optical output powers from these VCSEL devices measured through the optical fibers are shown in Fig. 9. While there is no electric characteristics differences between those devices, optical output power (B) seriously decreased. The light beam from the VCSEL device is thought to be scattered by small bubble because the boundary of the bubble is just on the optical output area of VCSEL in (B). Since there is no influence of a little absorption by the underfill resin, the output power (A) is a little bit larger than (C). However, the change of the lasing threshold of (A) from that of (C) implies near-end reflection, and there is concern about noise increasing in the signal transmission.


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Fig. 8: Example of bubble arising. (A) large bubble covering all the optical output area of VCSEL, (B) small bubble covering half the optical output area of VCSEL, (C) no bubble.

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Fig. 9: Electric and optical characteristics of the optical semiconductor module exemplified in Fig. 8.

To the above mentioned issues, improvement of components, selection of materials, and optimization of each process with extreme caution resulted in mounting with the displacement less than 10 µm and underfill with no bubble at all. The completed module with no displacement and bubble arising through all the 12 channels is shown in Fig. 10, and its electric and optical characteristics are shown in Fig. 11. Excellent properties without large variation among all the channels in both the electric and optical characteristics were observed. Still remaining variation, especially in optical output power, is attributed to the VCSEL itself, which means optical semiconductor device improvement will leads to much less characteristic variation of the optical semiconductor module.

High-Speed Optical Transmission Characteristics The OE ferrule with superior electric characteristics, robust electric connection between an optical semiconductor chip and lead frame electrodes, and excellent optical coupling between an optical semiconductor device and an optical fiber realized extremely-favorable high-speed signal transmission characteristics of the optical transmission module. Circuit diagram and setup are shown in Fig. 12 and Fig. 13, respectively. In the measurement of optical transmission module composed of VCSEL / PIN-PD mounted OE ferrules and GI 50/125 multimode optical fiber, bias current from electric source and high-speed signal voltage from pulse pattern generator (PPG) (Agilent, N4901B) were applied directly to VCSEL mounted lead frame electrodes through a bias-T and a RF contact probe. The applied electric signal is converted to optical one by the VCSEL, and is converted to electric one again by PIN-PD after the signal is transmitted through the optical fiber. The electric signal from PIN-PD is regenerated by a receiver IC with transimpedance amplifier (TIA) and limiting amplifier (LA) circuit mounted on the high-speed signal evaluation board, then measured by sampling oscilloscope (Agilent, 86117A). Although the optical fiber length in this measurement was about 50 cm, the result does not change with much longer or shorter length.

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Fig. 10: Completed module with no displacement and bubble arising through all the 12 channels.

Fig. 12: Circuit diagram of the high-speed transmission measurement.


fiber, for (A). It is thought that the impact of external optical feedback effects caused by the reflection or scattering is small in this module. That is, this optical transmission module is able to not only be assembled without characteristic variation but also absorb a little variation caused by the assembly process, and is extremely-superior in productivity, cost, and performance.

Fig. 13: Setup of the high-speed transmission measurement of the completed optical transmission module.

Fig. 14 shows the eye diagram of the output of the completed optical transmission module operated at 13.5 Gbps NRZ PRBS (2E31-1) data waveform, and the eye is clearly open at such high-speed transmission. Although this diagram was affected by the transmission characteristics of SMA connector and evaluation board wiring because of aforementioned setup of the measurement, nevertheless clear eye opening of this diagram implies much higher potential of this completed optical transmission module as an elementary substance. Fig. 15 shows the example of bit error rate map of differential signal output from the receiver IC. We have observed an error rate less than 1E-12 in actual error rate measurement, and have confirmed that this completed optical transmission module can be applied to practical use.

Fig. 14: The eye diagram of the output of the completed optical transmission module operated at 13.5 Gbps. Vertical axis is 100 mV/div, and horizontal axis is 20 ps/div.

Fig. 15: The example of bit error rate map of differential signal output from the receiver IC.

Interestingly, both the eye diagrams of the channel (A) and (C) exemplified at Fig. 8 were almost the same, although there is concern about the influence of the bubble, near-end reflection or beam scattering at the end face of an optical

Conclusions In the circumstances that electric high-speed signal transmission is approaching to its bandwidth limitations, we have realized the optical semiconductor module with ultra high-speed of 13.5 Gbps/ch × 12 ch = 160 Gbps (20 GB/s) by using the lead frame embedded OE ferrule with extremelycompact size of 4.4 × 4.5 × 1.0 mm3. This OE ferrule with the excellence in cost, productivity, and electric characteristics realized highly reliable electric contacts between an optical semiconductor chip and the OE ferrule by adopting lead frame embedded electrodes and also realized extremely favorable optical coupling between an optical semiconductor device and an optical fiber without using complicated traditional optical components. Assembly process optimization of the optical semiconductor module using this OE ferrule resulted in excellent electric and optical characteristics with little interchannel variation. The completed module has the actual potential of high-speed operation of 13.5 Gbps with low bit error rate less than 1E-12, and has the robustness which can absorb a little characteristic variation caused by the assembly process. Acknowledgments The authors wish to thank Masakazu Shiosaki of Process & Manufacturing Engineering Center and Tohru Furuyama of Center for Semiconductor Research & Development, both of Semiconductor Company, Toshiba Corporation for their firm support throughout the research and development. The authors would also like to pay special thanks Wataru Sakurai, Ken-ichiro Ohtsuka, Mitsuaki Tamura of Sumitomo Electric Industries, Ltd. for fabricating the OE-ferrule. References 1. Pham. D et al., “The Design and Implementation of a FirstGeneration CELL Processor”, Digest of Technical Papers. ISSCC., (2005), pp. 184-185 2. Rumbus Developer forum Japan 2007, Tokyo, Japan, Nov. 2007 3. H. Hamasaki et al., "An Ultra High Performance 1Tbps bandwidth Optoelectronic LSI Package using Post-reflow Optical-interface Stacking Technique", IMAPS 40th International Symposium on Microelectronics, (2007), pp.87-93 4. W. Sakurai, et al., “A Novel Optoelectronic Ferrule and Easy Ribbon Fiber Splicer for Cost-effective Optical Interconnection”, Electronic Packaging Technology Conference, (2006), pp. 367-372 5. H. Hamasaki, et al., “Novel Optoelectronic LSI Packaging Suitable for Standard FR-4 Printed Wiring Board with Bandwidth Capability of Over 1Tbps”, Electronic Components and Technology Conference, (2006), pp. 298-302
