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26 March 2014

Diode Bonding Evolves to Meet Telecom Demand

Posted in Published Articles

by Edward Stephens, Industrial Microphotonics Co.
St. Charles, MO
by Don Moore, President, Semiconductor Equipment Corporation
Moorpark, CA
Optoelectronics World, 1999

Producing quality laser-diode bars and arrays with the yields needed to achieve cost-per-unit targets can be technologically challenging.  This is due to the small size and delicate nature of the materials used in the devices and the fact that some of the physical elements must not be touched during the assembly operation.

Fabricating semiconductor-based lasers for applications such as gigabit-speed fiberoptic communications devices calls for the use of specialized precision equipment during the assembly process to ensure effective launching of the light into the fiber.  For example, the crystals facets on the opposite ends of edge-emitting diodes must not be touched by the bonder's pickup tool, because touching can create flaws that cause faulty operation or even complete failure.  Similarly, the center top/bottom surfaces of the emitting implant on vertical-cavity diodes are off limits to the pickup tool. 

These restrictions require that the bonding machine be equipped with a vacuum pickup tool that engages the other two surfaces only (as opposed to the traditional flat-faced vacuum pickup tool).  When a solder preform is involved, a special preform pickup tool interconnected with the vacuum pickup tool can be useful.

The Assembly Challenge
Precise alignment of the laser diode to the submount is absolutely critical during packaging.  For example, if an edge-emitting laser diode has an active stripe on the bottom and it gets placed back too far from the edge of the submount, the conical beam shape of the emitter light will be partially cut off on the bottom by the submount's top surface.  By the same token, if the edge-emitting laser diode is placed so that it extends out over the submount's edge, then insufficient heat-sinking will create a hot spot, and the diode will fail/burn out almost immediately.

In the case of a vertical-cavity surface-emitting laser (VCSEL), alignment in some respects is not as critical because the laser diode is usually put down first, and the end of the optical fiber is then aligned over the cavity.  This assembly technique is becoming very popular in the telecommunications industry.  Still, a fair degree of placement precision is required, given the constraints of the package's specifications.

The bonding system's capability in safely placing the laser diode on the submount is another important consideration.  Here the amount of bond load exerted becomes a crucial determinant.  The fragile nature of the diode materials requires a repeatable, very low bond load to prevent crushing of the device.  Even if the diode is only stressed, the operating characteristics of the device may be adversely affected.

Equipment Evolution 
The increasing needs of R & D, anticipated production run volumes, and multiple designs to be run have all contributed to the development of more versatile and precise bonders.  For example, when Industrial Microphotonics Co. (IMC; St. Charles, MO) was first formed, it used a rigged bonder to meet relatively straightforward demand.  Today IMC manufactures packaged and unmounted continuous-wave and pulsed laser-diode bars, lensed bars and arrays, fiber-coupled modules, laser-diode drivers, and related accessories (see Fig.1).  Increasing prototype development activities, expanding and varied product lines, and production volumes varying from a few hundred to thousands of units created the need for a bonding system that could handle specialized laser-diode designs and one that offered as many processing capabilities as possible (see Fig. 2).


Laser-diode Bars/Arrays - Semicorp.com  
Figure 1: Due to the increased use of diode-pumped solid-state lasers in industrial applications, a wide variety of laser-diode bars and arrays are being produced.  

Flip Chip Die Bonder - Semicorp.com  
Figure 2: Increasing demand for flexibility in product design has created the need for increasingly versatile bonding systems, such as this S.E.C. computer-controlled flip-chip bonder.  

Until recently, when bonder manufacturers began to target laser diodes as a specialized market, there weren't many machine specifically designed for bonding laser-diode bars and arrays.  Most of the available machines had to be modified for specific applications.  However, with the telecommunications industry aggressively moving into fiberoptics technology to satisfy demand for faster communication speeds, more designs are being introduced that address the special needs of the laser-diode-bonding process. 

A key bonder performance capability is to allow the operator to simultaneously view the laser bar image and the site on the submount where it is to be placed.  One means of accomplishing this is with an extend - retract cube beam splitter that superimposes the two images during the alignment step (Fig. 3).  In addition, a direct viewing microscope will enable the operator to inspect the bonding process and check alignment in real time.

Edge-emitting laser diode submount - Semicorp.com  
Figure 3: View from a color monitor shows alignment of edge-emitting laser diode with its bonding site on the submount.  

To best appreciate the capabilities that have evolved for laser-diode bonding, the process for bonding edge-emitting laser diodes to a submount using gold/tin alloy preforms should be understood.  Two types of bonding materials are currently in favor- gold/tin alloy is the most popular and is specified for use in the telecommunications industry.  It frequently comes as a preform.  The preforms are smaller than the die, which usually measures about 100µm thick by 200 to 300µm wide by 400 to 600µm long, corresponding to the size of a typical edge-emitting laser diode.  The VCSELs tend to measure about 300 to 400µm square.

Use of sputtered-on indium for very small laser devices appears to be waning, even though it allows for differences in thermal expansion between the laser diode and its submount (a trait that is beneficial when relatively large or long devices such as laser bars/arrays are being bonded).  However, in the case of laser arrays/bars, indium is still very much in use.

Bonding Step by Step
The submount is placed on the machine's rapid heat-up stage, mounted on the motorized sliding table, and locked in place.  The table traverses by means of joystick control, foot-pedal activation, or other means (depending on bonder design) from the pickup to placement position.  The stage is capable of  motorized travel in x, y and Ø directions to aid in the subsequent alignment process.  Precisely heated cover gas flows over the submount. 

The die is picked from its carrier by a custom, heated collet, which assures that the top of the die and the facets are not touched.  If a preform is used, it is then picked from its carrier and presented.  As noted earlier, depending on the bonder design, this may be accomplished via a special unheated preform pickup tool on the bonder that is interconnected with the die pickup tool through the machine's computer logic.  Using the bonder's cube beamsplitter and color video monitor, the operator aligns the preform with the bond site on the submount, retracts the cube beam splitter, and places the preform.

Next, using the cube beam splitter and video monitor, the operator aligns the laser die with the preform on the submount and then retracts the cube beam splitter.  To optimize viewing, the operator can zoom in on the preform and die-alignment image at high magnification.  Also, the system should be equipped with separate optic illuminators - one for the die and one for the submount - to help assure optimum light levels on the die and submount.

The die is lowered to search height (just above the submount) and is then automatically placed with ±5µm precision at elevated temperature and under a precise amount of bond load.  The pickup tool is then retracted to its upper position.  For processes that involve gold, some system designs provide a precision, servo-controlled, voice-coildriven, ultrafine linear-scrub bond head to achieve the partial attachment.  This system provides repeatable scrubbing motion of the die on the targeted submount site with return within about 5µm of the starting point.  A precision scrubbing action is paramount so that the scrubbing does not cause material to gather on the emitting facets.  Fine adjustments (via a "nudge" feature available on many bonder designs) can be made as needed after placement and just prior to the scrub action.

To facilitate alignment checks when assembling several components one on top of the other, the bonder's cube beamsplitter provides the operator with simultaneous true straight view of the edges of the laser assembly from the bottom and top of the assembly.  This eliminates the need for additional alignment checks using the direct viewing microscope.

Next, the temperature is precisely ramped up to reflow, usually by means of retractable hot-gas spot-heating nozzles.  Die alignment and reflow can also be observed through a direct viewing microscope and illuminator.  The temperature is then lowered below the melting temperature of the solder, and the assembly is removed.


As appeared in Optoelectrics World - November 1999