Die Bonding In An Optoelectric World

This “other world” consists of odd substrates and package designs with tiny, delicate photonic devices inside them.  Frequently, the photonic devices are housed in hermetically sealed packages for stability; for example, laser diode inside transistor outline (TO) headers.  The mounting structures to which the optoelectronic devices are connected to form circuits may be flex circuitry, FR-4 boards or several other interconnect materials.  One example is a telecom array amplifier on am FR-4 board that is then over-molded.  Another example is an optoelectronic transmitter mounted on a PCB (Fig 1).

Figure 1: Typical elements of an optoelectric transmitter mounted on a PCB.

Because of the nature of today’s optoelectronics market, PCB-oriented OEMs and EMS providers must become familiar with a wide variety of substrates as well as both active and passive devices such as transmitters, receivers, lenses, individual laser diodes, laser diode arrays/bars, photodetectors, fiber optic cables, microelectronic-mechanical system (MEMS) mirror arrays, and even micro-optoelectronic-mechanical system (MOEMS) fluid switches.  Their involvement will be at every level, from dies to packages to transmitting/receiving fibers (Fig 2).

Figure 2: Multi-channel optoelectric packaging.

Now and in the future, the heart of a photonic circuit is a laser diode.  Some examples are edge emitting laser diodes and vertical – cavity surface-emitting laser diodes (VCSELs).  These devices require a bonder with capabilities and design features that most traditional PCB assemblers are not accustomed to using.

Bonding Material
Currently, gold/tin alloy is the most specified bonding material for attaching VCSELs and edge-emitters to submounts in telecom applications.  This alloy is applied either via plating onto the submount or via placement of a gold/tin preform that is then heated to reflow to make the attachment.

However, some packages are using epoxy, particularly for computer communications applications, because it simplifies and speeds up the attachment process and is less expensive.  For example, a laser diode can be epoxy-attached to each TO header sitting in a tray on the bonder’s placement station, and then all of the epoxy can be cured simultaneously.

Once standard processes and packages for optoelectronic assembly are implemented, epoxy bonding should gain even more favor because it can facilitate the automation process that follows the development and startup low-volume production phases for new designs.  The message: Choose a bonder that can run both alloy and epoxy attachment processes so that the bonder’s use can be maximized.

Laser Diode Bonding
A laser diode’s grain-of-salt size and delicate nature create unusual challenges for OEMs and EMS providers.  Precise computer control of the bond loads and temperature profiles are absolutely essential to protect these devices from crushing or overheating during the bonding process.  The bonder’s Z motion must apply a repeatable, very low bond load to the device to avoid crushing or even stressing it.

Another consideration that may affect the choice of a bonder is its ability to stack components.  For example, the application may call for epoxy to be deposited first on a TO header, then an isolating piece of ceramic placed in the epoxy, then epoxy put top on the ceramic, and finally a VCSEL placed into it.

If epoxy is to be used, the size of the epoxy deposit must be smaller than is achievable with a standard needle dispensing system with a positive displacement valve.  This type of system delivers a Hershey “candy kiss” deposit, which is too much material for VCSEL applications.  Because of the large deposit, the material flows out around the edges when the device is placed, moves up onto the emission area of the laser device and obstructs the emitted beam of light (Figure 3).

Figure 3: Excessive epoxy deposited via a standard needle dispenser.

Today, many optoelectronic assemblers attempt the epoxy attachment process using a needle dispensing system.  However, these assemblers typically lack extensive semiconductor packaging experience.  To succeed, the assembler must have a bonder equipped with epoxy transfer tool technology, which was commonly used in the semiconductor industry 20 to 25 years ago.  This technology allows the bonder operator to precisely deposit extremely small volumes of epoxy on the submount.

To perform laser diode epoxy attachments, certain features are useful on the bonder.  For example, the bonder’s pickup tool may be equipped with a dual head; one picks the laser diode from its carrier and the other is for the epoxy transfer tool.

Vision capabilities are needed to achieve the alignment and placement accuracy required to precisely attach the tiny laser devices.  Several design features should be considered for the viewing system.  First, a beam splitter that simultaneously superimposes the transfer tool tip image and site landing image at high magnifications during the alignment step is useful.  Second, a stereo zoom microscope with fiber optic illumination allows the operator to inspect the attachment process and check alignment in real time.

If VCSELs are being placed, misalignments of these chips may occur.  For example, the VCSELs emission point can become misaligned when the bonder’s pickup tool comes down to pick the VCSEL up out of the carrier because the vacuum on the tool head is turned off.  However, if the vacuum were turned on, it could cause the VCSEL to jump up and become misaligned.  Also, VCSELs in gel packs can become misaligned when the pickup tool pulls them away from the carrier’s sticky bed.

However, misalignments can be completely avoided by choosing a bonder with a precising station.  The precising station allows the VCSELs to be removed from the gel paks with an unheated tool without damaging the gel paks.  Subsequently, the VCSELs can be handled with a preheated tool before placing them on the submount.  The precising station also holds the VCSELs firmly in place using vacuum, so, when the pickup tool lifts each VCSEL. its emission point is not moved off-center from its alignment with the bonding system’s overlay image.

VCSEL Viewing Challenges
To properly handle all forms of laser diode bonding, another vision feature is helpful on the bonder.  Certain applications require more alignment/placement capability than is achievable using just a beam splitter and scope.  One example is placing epoxy or gold/tin on top of a round TO header and then attaching a VCSEL.  The problems that will be encountered in this type of application are virtually insurmountable without using either a special viewing feature on semiautomatic laser bonders or the automatic pattern recognition capability found on more expensive fully automatic bonders.

Specifically, the operator cannot see the VCSEL’s emission point after the VCSEL has been picked up out of its carrier because the pickup tool holding the die blocks the view.  To align the emission point to a specific feature on the submount, the operator must know where the emission point is on the laser diode.  Normally, the diode’s emission  point is aligned to the center of the TO header.  In the case, a reference mark must be provided that can be superimposed over the header’s shoulder so that the positions of the shoulder and the emission point on the VCSEL can be viewed simultaneously.

However, sometimes the emission point is located off-center on the laser diode because space is needed on the laser chip for the landing/bonding pad.  A further complication is that, when the laser chip on the wafer is cut, a 10- to 15- micron variance in the shape of the cut chip may occur.

In addition, the TO header on which the laser is to be located is a machine-stamped piece of metal, so its tolerances can vary around the TO header’s shoulder.  If the TO header’s shoulder is out of round by some amount, the emission point placement accuracy will be adversely affected.  Even if the TO header is perfectly round, its diameter may vary substantially.

One solution would be a bonder that “remembers” where the emission point is before the VCSEL is picked up out of the carrier and that also compensates for the out-of-round shoulder/diameter variance.  Expensive automatic bonders have pattern recognition systems that look at the chip while it is sitting in the carrier, memorize the emission point’s location and coordinate that with the landing site in the submount.  Many PCB assemblers venturing into optoelectronics are not going to want to learn the ropes in this new filed using an automatic machine and adding such capability to a semiautomatic bonder may not be a viable option.

To solve these problems, a relatively inexpensive video image marker could be added to a laser bonder.  The video image marker consists of a keyboard, a control box and x-y knobs for adjusting the system’s magnified electronic images.  The video image marker creates a fixed video overlay of crosshairs for centering the VCSEL’s emission point.  Different video patterns can then be superimposed on the submount on which the VCSEL is to be mounted.  An example would be a circle overlay image of the outside diameter of the top of a TO header.

In operation, the laser diode is picked from its carrier tray and placed on the bonder’s precising station.  The image marker’s crosshairs are aligned to the diode’s emission point, and the diode is picked up in preparation for placement on the TO header.  The marker’s circle overlay is aligned to the header (Figure 4).  With the emission point and header both now properly aligned, the operator can precisely place the laser diode on the header.

Figure 4: VCSEL on TO header with alignment measurement aids.

Platform/Gantry Considerations
Laser diode bonding requires a sturdy bonder platform/gantry and precise slides and rails.  Z-motion control should allow precise, repeatable die bonds at low loads.  For gold/tin bonding processes involving either edge-emitting lasers and VCSELs, the bonder must be able to hold the die at a specific, consistent load during both bonding and cool-down cycles so the die is not crushed or stressed.  In epoxy attachments, a specific, consistent load is necessary to ensure that the epoxy thickness between the two surfaces is as uniform as possible.

If gold/tin alloy is used, a scrubbing capability may be helpful.  Scrubbing eliminates any voids that may be present between the interconnecting surfaces.  The amount of scrubbing needed depends on the diode size; the smallest diodes generally do not need any.  The bonder should be equipped with a head that returns the scrubbing action precisely to the starting position to maintain the original alignment.  Depending on die size, the amplitude and frequency of the scrub may need to be varied.  When working with edge-emitters, great care must be taken not to scrub in such a way that the bonding material piles up on the diode’s front and back crystal facets, or they will become contaminated.

The bonder’s software program should be versatile enough for both eutectic and epoxy applications.  All operating functions should be controlled with a real-time software system that provides process verification via closed-loop feedback.

Conclusion
To take on the challenges and reap the rewards offered by the exploding optoelectronics market. PCB-oriented OEMs and EMS providers need to rethink the traditional assembly process.  the substrates and package design in today’s optoelectronic assembly call for uncommon bonding equipment capabilities.  With an appropriate bonder, OEMs and EMS providers can gain experience and compete in this new and exciting industry.

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