Laser diodes are commonly bonded to headers, heat spreaders and submounts. For this discussion, the term “submount” will be used. The process of bonding edge emitting laser diodes includes placing a gold/tin solder preform onto the submount, raising the temperature of the submount above the preform’s melting temperature of approximately 310°C, precisely aligning the edge of the diode to the edge of the submount, placing the diode on the submount, optionally mechanically scrubbing the diode against the submount to assure complete void-free wetting of the diode and submount by the melted preform, and cooling the assembly until a solid bond is achieved.
A crucial, high-precision step in this process is to properly align the emitting facet of the diode with the edge of the submount. If the diode is placed too far back on the submount, the light beam becomes partially blocked and inefficient. If the diode is placed too far forward on the submount, so that it overhangs the edge of the submount, a hot spot will develop and cause the diode to burn out.
The distance between the edge of the diode and the edge of the submount is especially critical with diodes that are mounted with their emitting stripe face down. In this case, the diode’s emitting edge must be placed within a few microns of the submount’s edge or the light will be blocked. Diodes that are placed with their emitting stripe face up are more tolerant of position and can be placed further back on the submount without blocking the emitted light beam.
Another important step to insure prolonged life of the diode is to produce a void-free bond. Mechanically scrubbing diodes in melted preforms using ultrasonic energy helps prevent void formation. To avoid causing a buildup of material on the emitting facets during scrubbing, the direction of scrubbing must not be perpendicular to the light emitting edge of the diode.
The submount must be held securely on a stage that is capable of ramping from a starting temperature of 100° – 150° C. up to the melting temperature of the gold/tin preform. Forced cooling may be required to quickly solidify the bond. Heated cover gas (N2 or N2+H2) should flood the bond site to help prevent oxidation.
A holder is required to present the diode and preform parts for pick-up. Diodes are presented in a gel pack or waffle pack. Preforms are usually presented on a mirror or in a waffle pack.
Laser die are usually picked up with a two-sided vacuum collet that touches only the upper edges on the ends of the die, not on the edges of the emitting facets. If ultrasonic scrub is not required, a flat-faced tool that touches the top surface may be used instead of the two-sided collet. In either case, the emitting facet must remain visible for a clear view of the diode’s alignment with the submount as the die is placed and bonded. The pick-up tool is usually heated to about 150° C. to preheat the die and reduce thermal shock during assembly.
A vision system is required that provides a clear view of the preform and the laser diode relative to the submount’s bond site. A video system with a bidirectional cube beam splitter offers a simultaneous view of the die and the submount for rapid alignment. A direct viewing microscope is a helpful to the video system. It allows observing the actual bond for making final adjustments.
Die placement on the submount requires a precisely controlled load, typically of 20 to 50 grams. The ultrasonic scrub system must be linear and must return accurately to its starting point.
The following capabilities are desireable for rapid high-yield laser diode mounting:
- A rapid heat cycle workstage that securely holds the submount and quickly ramps up to preform melting temperatures.
- A stage cover gas manifold which can dispense heated gas at a preset idle temperature.
- A cooling gas manifold mounted on the stage to help solidify the bond.
- A hot gas heating source with a nozzle to provide a localized flow of hot gas for spot heating of the die.
- A die tray holder holding waffle packs for die and preform presentation.
- A dual tool bond head with a heated pick-up tool and a preform tool.
- A linear ultrasonic scrub programmable in amplitude and frequency, mounted to the bond head.
- A cube beam splitter viewing system giving a clear simultaneous view of the submount and die for the alignment process.
- A direct view microscope mounted for a close view of the bonding process.
Based upon an application note by
Scott Schafer, Semiconductor Equipment Corporation.