by Don Moore, President, Semiconductor Equipment Corporation
US Tech, July 2001
|When selecting a bonder for attaching laser diodes to submounts, the forward-looking OEM or contract assembler will want to choose a design that allows performing the attachment process using either gold/tin alloy or epoxy.
Whether gold/tin or epoxy is used, though, careful consideration must be given to the bonder’s capabilities for attaching the tiny, delicate laser diode devices. Protecting these devices from overheating during the bonding process is a major challenge for OEMs and contract assemblers who use gold/tin and requires, among other things, precise computer control of the temperature profiles and bond loads. Regardless of whether gold/tin or epoxy is used, the Z-motion control on the bonder must also allow a repeatable, very low bond load to be applied to the device in order to avoid crushing the device.
With epoxy bonding, a major requirement will be the capability to deposit a very small amount of the adhesive epoxy. The size of the epoxy deposit needs to be smaller than is possible with a standard needle dispensing system with a positive displacement valve – and it must deliver the Hershey™ candy kiss dot-shaped deposit. These dispensing systems are quite challenged to repeatable deposit the correct volume of material for VCSEL (Vertical-Cavity Surface-Emitting Laser) applications. What happens is, because of the volume deposited, the material flows out around the edges when the device is placed and frequently moves up onto and obstructs the laser’s emission area.
Another consideration that may impact the choice of a bonder design is its ability to stack parts. For example, epoxy may first be deposited on a TO header, then an isolating piece of ceramic may be placed in the epoxy, then epoxy may be put on top of the ceramic and then the VCSEL placed in this layer of epoxy.
Epoxy Tool Design
In operation, the epoxy is placed in a continuously rotating cup at the epoxy station located on the system’s sliding table. The surface of the epoxy is constantly leveled by a micrometer-adjusted doctor blade. A gauge is provided for measuring the thickness of the film in the rotating cup so that needed process adjustments may be made. The tip of the transfer tool is coated with the epoxy by simply aligning the tool tip with the epoxy cup and initiating the dipping cycle, which is then carried out automatically by the machine. The dwell time in the cup and withdrawl rate is pre-set in the bonder’s head to assure optimum material pickup – the precise amount of epoxy required for laser diode bonding.
To apply the acquired epoxy on the tip of the transfer tool to the desired location on the submount, the operator uses the system’s extend-retract cube beam splitter viewing system to align the image of the tool tip with that of the target site. Once alignment has been achieved, the operator initiates the placement cycle, the transfer tool lowers – and ever so lightly – the tool tip touches down on the site to deposit the material.
However, certain applications – particularly those involving the placement of epoxy or gold/tin on top of a round TO header and then attaching a VCSEL – requires more alignment/placement capability than is achievable using just the cube beam splitter and microscope. The problems that will be encountered for this type of application are virtually insurmountable without the use of special viewing features on the semiautomatic laser bonder, or the automatic pattern recognition capabilities as found on more expensive fully automated bonders. The problems relate to not being able to see the VCSEL’s emission point after it has been picked up out of its carrier because the pickup tool holding the die blocks the view, variances (as much as 10 to 15 µm) in the shape of the cut laser chip, out – of – roundness of the TO header’s shoulder, variances in the TO header’s diameter, etc.
In order to provide a means for memorizing where the emission point is and to compensate as best as is possible for the out – of – round shoulder/diameter variance problems, S.E.C. engineers recently incorporated a relatively inexpensive but effective video image marker attachment for the 860. Its use allows the user to achieve the desired +/-5µm placement precision of the emission position any TO header. 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 and stores a fixed video overlay of cross hairs in the form of a bit map for centering the VCSEL’s emission point as well as up to 10 different video patterns that can be superimposed on the submount on which the VCSEL is to be mounted. An example of the latter would be a circle overlay image of the outside diameter of the top of a TO header. Custom patterns also can be created.
In operation, the laser diode is picked from its carrier tray and placed on the bonder’s precision alignment 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. With the emission point and header both now being aligned properly, the operator is able to precisely place the laser diode on the header.
The bonder should feature Z – motion control that allows precisely controlled, repeatable die bond loads as low as 10 grams. In gold/tin bonding processes – for both edge – emitting lasers and VCSELs – the bonder used must have the capabilities to hold the die at a specific, consistent load during both the bond cycle and the cool – down so that the die is neither crushed or stressed. In the case of epoxy attachment, a specific, consistent load is necessary to assure that the epoxy thickness between the two surfaces is as uniform as possible.
With processes using gold/tin, it may be desirable to have the capability to scrub the laser diode to achieve partial attachment after the alloy has been placed on the submount that is secured to the bonder’s heated workstage. Scrubbing eliminates any voids that may be present between the interconnecting surfaces. The amount of scrub will be somewhat dependent on die size – the smallest die generally don’t need scrubbing.
The bonder’s software must be designed to be versatile enough to go back and forth between epoxy and eutectic applications. All operating functions need to be programmed and controlled with a real – time software system that provides process verification via closed loop feedback.
|As appeared in US Tech – July 2001|