Micro-bonding laser chips using arrayed beams

As individual computer chips become more powerful, communication between chips becomes more congested. For several years, researchers have been investigating the idea of chip-to-chip communications that use optical interconnects instead of the electrical copper lines on printed circuit boards (PCBs) that are used today. Vertical-cavity surface emitting lasers (VCSELs) have been vigorously studied as a light source for optical interconnects over the last several years due to their low-threshold current, low manufacturing costs, and two-dimensional integration of the laser chips. To make advanced optical interconnection modules, it is desirable to flip-chip-bond VCSELs directly onto PCBs, or onto integrated circuits.¹ Flip-chip bonding (FCB) would simplify not only the bonding process but module structures as well. Because of this expectation, several FCB methods have been proposed thus far, including thermocompression, self-alignment,² and conductive adhesion. None of these methods has yet been used in manufacturing environments.

Consider FCB of VCSELs using a thermopressing method as an example to clarify underlying issues. The bonding process requires a VCSEL to be pressed against a PCB using a heated bonding tool. VCSELs fabricated on a brittle substrate like GaAs crystal, however, can be readily deformed with dislocations grown in the crystal under a mechanical stress and/or a thermal stress. Such deformation in the cavity of the VCSEL strongly affects its performance.

Shown in Figure 1 is a conceptual drawing of laser-based FCB for VCSELs. The difference from nonlaser FCB is that the VCSEL need not be preheated. A load is put on the back side of the VCSEL, and the electrodes printed on the board are laser-irradiated through the PCB— which is transparent at the laser wavelength— just long enough to join the chip to the board. Figure 2 shows a schematic layout of the laser-bonding system that we have constructed. We opted to use a continuous wave (CW) infrared fiber laser as the heat source. This laser's compact fiber cavity has a small footprint, is capable of direct modulation, and is intense enough for our application, with an output power of 15W. In addition, the beam quality is excellent— with M² close to 1.0— which enables the beam to be focused to a small spot.

Figure 1. A focused laser beam melts the solder, bonding the chip (pink) that contains the vertical-cavity surface emitting laser (VCSEL) to the electrode (orange) on a printed circuit board (PCB). This protects the VCSEL from damage due to heat and pressure.

To connect multiple joints simultaneously, we created an array of beam spots. This was provided by a diffractive optical element (DOE) that incorporates both a focusing and a splitting function. Because of its multifunctionality, we call it a hybrid DOE. Figure 3(a) shows a topographic view of part of a hybrid DOE. The maximum depth of the DOE is 2.3μm and the minimum pitch 21μm. Curved surfaces within the periods are quite smooth because of the gray-scale capability of the laser writer that we use to draw DOE patterns.³ Figure 3(b) shows the intensity distribution of the diffracted beams from the hybrid DOE. The measured efficiency is greater than 82%, and the measured uniformity among the four fan-out beams is higher than 0.90 (out of a maximum of 1.0), both of which meet the application requirements. The infrared heating beams are directed to the anode and cathode electrodes of the PCB.

Figure 2. Schematic layout of the laser-bonding system for VCSELs. DOE: Diffractive optical element.

Figure 3. Hybrid DOE. (a) Surface topography. (b) Intensity distribution of the split and focused beams.

The laser-bonding system is equipped with the 1110nm fiber laser for heating, the hybrid DOE for multibeam parallel processing, and a mounting head for precise alignment and translation. Pictured in Figure 4 is an 850nm VCSEL laser-bonded on a PCB. Using this system, we can achieve a practical joint strength (76gf per two bumps) and a satisfactory output power (maximum 7.7mW) from the laser-bonded VCSELs. These results, among other obtained characteristics, are acceptable when the VCSELs are considered as a light source for optical interconnection modules.

Figure 4. A VCSEL in contact with a glass PCB under irradiation by the split beams from the DOE.

The laser-based micro-bonding process described here has great potential to overcome the existing problems that have hindered FCB from being used in a variety of packaging and assembly applications. This process is promising, although it requires further examination of VCSEL emission life as well as performance reliability.