Bonding Edge Emitting Laser Diodes

Lasers are commonly bonded to headers, heat spreaders and submounts.  For the purpose of this discussion, the term “submount” is used.

A vital part of this process is the proper alignment of the emitting facet of the diode to the edge of the submount.  If the diode is placed too far back on the submount, the light beam becomes partially blocked and inefficient.  The diode cannot be placed too far forward either: if it hangs over the edge of the submount, a hot spot will develop and burn out the diode.  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 several microns of the submount’s edge or the light will be blocked.  Diodes that are placed with their emitting stripe face up have a bit more tolerance and can be placed further back on the submount without causing blockage of the light beam.

Another important factor in the process is that the bond be void free to insure prolonged life of the diode.  Scrubbing diodes in melted preforms helps deter voids from occurring.  The scrubbing motion must not be perpendicular to the light emitting edge of the diode or a buildup of material on the emitting facets will occur.

The submount needs to 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 (310° C.).  A cooling feature is also helpful to quickly solidify the bond.  Heat 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 collet that only touches the upper edges on the ends of the die (not on the edges of the emitting facets) or with a flat-faced tool that touches the top surface.  The flat-faced tool cannot be used if scrub is used.  The emitting facet remains visible for a clear view of the alignment with the submount as the die is placed and bonded,  The tool is usually heated to about 150° C. to prevent thermal shock to the die.

The machine needs a vision system that gives a clear view of the preform and laser diode in relation to the submount’s bond site.  A video system that gives a view of the die and a view of the submount as with a cube beam splitter is best.  A direct viewing microscope is helpful to observe the actual bond and make final adjustments.

The die placement on the submount needs a precisely controlled load (20-50 grams) and scrub system (if used) needs to be linear and return accurately to the starting point.

A rapid heat cycle workstage that securely holds the submount and quickly ramps up to preform melting temperatures is available for both machines.  The stage’s cover gas manifold dispenses heated gas at a set idle temperature.  A cooling gas manifold option mounts on the stage’s heat shield and helps to solidify the bond.

A single hot gas spot heating option features an adjustable quartz nozzle that mounts on the machine.  The nozzle applies a localized flow of hot gas to the die.

Both bonders come equipped with a die tray holder that holds waffle packs for die and preform presentation.

A dual tool bond head is available, which includes a heated pick-up tool and a preform tool.  Linear scrub with programmable amplitude and frequency can be mounted to the bond head.

A cube beam splitter viewing system gives a clear view of the submount and die for the alignment process.  A direct view microscope mounts on both machines for a close view of the bonding process.

• Model 410 Flip Chip Die Bonder or Model 860 Omni Bonder
• Dual Tool Bond Head
• Rapid Heat Cycle Workstage
• Direct View Microscope

• Cool Gas Manifold
• Linear Scrub
• Single Hot Gas Spot Heating