Fabrication, Apparatus Setup, Calibration, and Experimental

discussion, the entire fabrication procedures of SiOB and transmitter assembly, including the manufacturing of 45°micro-reflectors, V-groove arrays, high frequency transmission lines that connected with 4-channel VCSEL array and bonding pads are demonstrated and shown in Figure 5-12 [94]. It should be emphasized that the 45°micro-reflectors and V-groove arrays did not be employed in the simulation and model analysis due to they are not main components of paths of thermal flow and furthermore for the first structure simplification.

Additionally, since the fiber assembly of proposed modules used for the optical interconnection is passively aligned, the V-groove array is designed to assemble the multimode fiber (MMF) array. The SiOB is monolithically fabricated with a 45°

micro-reflector and the V-groove array. In order to etch the bench that can incorporate the optic fiber with the V- groove and provide the reflection surface for optical coupling between fiber and VCSEL simultaneously, a two-step anisotropic wet etching is developed using KOH solution. SiO2 film is deposited on a (100) silicon substrate and used as a hard mask for anisotropic wet etching. Dedicated patterns for etching trenches are formed using photo-lithography and dry etching on the SiO2 film to define patterns. The anisotropic wet etching using KOH solution mixed with IPA is applied to form the 45° micro-reflector and the V-groove array. After that, the photolithography is adopted again to fabricate transmission lines. Ti/Au (500/9500 Å ) layers are deposited on the SiO₂ layer by E-Gun Evaporator.

Transmission lines (TMLs) are then formed using lift-off process. For flip-chip bonding VCSEL onto the as-fabricated TMLs, Au-Sn patterns of 1μm are defined by photolithography and thermal evaporator deposition. Finally, once the 4-channel VCSEL array is flip-chip bonded onto the Au/Sn pads, the SiOB transmission module is fabricated with a position accuracy of 1μm.

Furthermore, InfraScope II, Thermal Imaging Microscope, (Quantum Focus Instruments

(a)

(b)

Figure 5-12: (a) Fabrication of SiOB and (b) Fabrication of High Frequency 4 Channel 2.5 GHz Transmission Lines [94].

Corp, Vista, CA USA) is employed for both thermal mapping and hot spot detection in the measurement. Two main effects, vibration coupling and thermal air current, that could degrade the measured accuracy have been carefully considered and controlled. The InfraScope is mounted on a vibration-isolated table and the measurement environment is located away from air flux. The entire 3-D VCSELs optical stack is fixed on the thermal stage of the InfraScope, where a bias temperature is set at 75^oC to imitate the conventional operation environment of typical optical transceiver systems. The optical stack starts with an unpowered state for measurement calibration. Radiance calibration is adopted to create the correlation between the output of infrared detector and the infrared radiance. The reference established in the unpowered state is used to calibrate the radiance units by accurately measuring the infrared radiation emitted by each pixel area of the optical stack. Then, the extracted emissivity map is utilized as a reference frame for each of the subsequent thermal images from the powered 3-D optical stack. Thus, the map can be processed immediately for the temperature acquisitions. At final, by means of zooming in the region of the heating source adequately, i.e. the region of the VCSELs, the temperature distribution of the hot spot can be definitely determined as well as the hottest temperature on the surface of the VCSELs.

Figure 5-13 is the detected infrared-ray (IR) thermal image which shows the temperature distribution of the SiOB heated by the singly operated VCSEL. The bottom of SiOB is constrained with a bias temperature of 75C for fair comparison. Only a laser diode is operated by probe B with 8 mA input current and 2 V bias voltage. The measured hottest temperature shown in Figure 5-14 validates the simulation and model predictions as shown in Figure 5-10 and 5-11, respectively. The comparison between the CoventorWare simulation results with and without air and SiOB, respectively, also verify the simplification indicated by the EETCM. In order to further show the model accuracy in dealing with the thermal cross-talk between two operated lasers, Figure 5-15 shows the thermal image of the optical transmitter where two lasers are operated simultaneously. Each laser diode is operated with 8

mA input current and 2 V bias voltage. Figure 5-16 shows the comparison between the measured data, simulation, and the EETCM. Excellent temperature matches within ~0.5C verify the EETCM and show the practicality of the simplified structure in which we can have 90% CPU operation time saving due to 80% mesh number reduction. In addition, the slight temperature mismatch should be caused by the thermal impedance mismatch between the interfaces and the phonon vibration. Further investigation and model improvement are still undergoing.

Figure 5-13: The measured temperature distribution of SiOB heated by the operating VCSEL using IR microscope. Only a laser diode is operated by probe B with 8 mA input current and 2 V bias voltage.

Figure 5-15: The measured temperature distribution of SiOB heated by the operating VCSELs using IR microscope. Two laser diodes are operated with 8 mA input current and 2 V bias voltage.

Figure 5-14: Comparison between the EETCM with single laser turned on, measurement data, and simulated results with and without air and SiOB, respectively.