Topic: Assembly of delicate and fragile semiconductor materials

By Howard Lasto, Product Manager, AIR-VAC Engineering, Seymour, CT

Semiconductor manufacturing can require the use of many different materials including gallium(Ga) base materials. Due to the sensitive nature of some of these materials, unique assemblymethods may be required to prevent chipping or fracturing from the assembly process. Inaddition, the dark mirrored surfaces must be manufactured in clean room environments tominimize airborne contamination.

Typically, devices are introduced into the micro-assembly machinesthrough the use of special polished stainless steel boats with themirrored surfaces facing up. There can be no contact within theimages on the mirrored surface. The delicate edges of the parts areextremely brittle and can be damaged easily. In the past,mechanical gripper nozzles (Fig. 1) were used to pick up and placedevices onto the wafer for bonding. While the gripping force islight, pressure is applied to the side of the chip. There is also theissue of uniform placement pressure while the bonding occurs (spotcuring). In either case, whether due to the localize gripper arm orthe different forces in the placement, breakage and chipping canoccur.

In some cases, there can be a “safe”area around the perimeter of thesemiconductor device where no imageis present. For this application,perimeter contact tools (PCT) weredesigned for safely picking up andplacing chips (Fig. 2). A raised 10 mil(0.010”) polished perimeter edge gentlymakes contact with the outside edge ofthe chip’s surface leaving the delicate,interior surface un-touched.

Also, a uniform contact area meansmuch more precise bonding andpressure control. A consistent bond lineis critical for most sensor manufacturing.

In addition to picking and placing of the devices, some components may require flipping toachieve the proper orientation of the dies to be bonded. Therefore, receiving tools or landingpads (LP’s) were created to capture the flipped devices now mirror side down (Fig. 3). Both thepick tool and flipper components incorporate an independent vacuum channel to secure the dieswhile in motion.

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Figure 2

Figure 1

Figure 4

The vacuum system is typically a venturibased design where there is instant on andinstant off capabilities. This reduces anychance of movement during motion events.

If the semiconductor chips cannot allow forperimeter contact areas, the use of invertedpyramid tools (IPT) can be utilized (Fig. 2).These tools allow for minimal contact of thedie while a gentle vacuum flow holds thecomponents without damage to the edges.IPT’s also provide a very uniform placementand bonding control.

Non-Contact

Future development will call for non-contact means for component handling particularly in thecomponent flipper area. A system that incorporates a universal holding station would eliminateseveral custom tooling pieces and reduce cost. A design using the Bernoulli Principle could beintegrated into the machine to allow for a non-contact part transfer of the delicate materials. Thisprinciple can be used to create a dynamic lifting force using standard air pressure to hold ormove objects around. One concern is the manufacturing size of the non-contact lifting units.Current suppliers have sizes ranging from 20 to 60 mm diameters. For the semiconductorindustry, however, these sizes will need to be reduced. As a mechanical device, there would beno physical reason a smaller unit could not be created. Another interesting possibility is theconcept of ultrasonic levitation where a thin gas film is produced offering enough repelling forcesto move the part without friction. Further studies are required to assess the feasibility ofintegration into the robot peripherals.

Flatness

Another critical area of concern is component and wafer flatness. To ensureproper bond lines, accurate control of perpendicularity of the robot heads to theworking surfaces must be maintained. While tight mechanical specifications areheld to provide good robot accuracy, the use of touch probes offers an excellentsolution for both adjusting and monitoring machine flatness (Fig. 4). Dependingon the touch probes used, an overall flatness within 2 microns can be achieved.A simple predetermined program can be activated where three positions areselected around the perimeter of the boats or wafer to verify true position. Oncethe measurement is taken at the 3 locations and the offset determined, a series offine spring-loaded setscrews are adjusted to correct the flatness. These customsetscrews have a calibration feature where one turn equals approximately 5microns.

Micro Force

While the need for high bonding pressures (2000 – 4000 grams) may be required in someapplications, there is also a need for feather-touch force placement. Micro-force load tables canutilize up to 4 load cells and provide a delicate pick and place possibility down to just a fewgrams. As with all sensitive materials, such as LED’s, it will be important to offer both a firm yetgentle bond control to prevent component fractures.

Conclusion

Figure 3

Flipper head

Landing Pad

Delicate component handling will be very important to produce higher yields with minimal loss.Sensor and semiconductor manufacturing will continue to utilize sensitive materials such as Gaand, with the increase in security and surveillance, this need will only continue to grow.Machines will need to be able to adapt quickly to changing technologies and specificapplications. Dark mirrored surfaces offer a unique challenge to laser distance sensors as anexample. Robot platforms should offer quick-change tool designs (Fig 5) so manufacturers canmove from process to process for the lowest possible cost. Companies should ask robot suppliersabout their internal manufacturing and software development capabilities to determine theirability to address customer needs quickly. Having the ability to pool in-house resources at amoments noticewill be vital tomeet thecustomerdemands andtimelines. Theissues ofhandlingsensitivematerials willchallengesuppliers toinvent newcapabilities. Inmany cases, thesmaller customrobot supplierwill be able torespond muchmore rapidly todifficultapplicationrequirements.

Figure 5