Squeegee Technique

The next step of the integration process is to connect each pad on the chip with the bonding pad on the pocket to achieve functionality. Several models of the integration, such as muscle stimulator and RFID chips were presented in previous chapters. These IC chips had 2 to 5 bonding pads with some discrete components; connecting the chips to the parylene pocket only required manual painting of the conducting epoxy because the number of metal pads is small and the pads are far away from each other. However, the chip that is to be integrated in this section has a very dense array. The average pad size is 100 µm and are placed 40 µm away from each other (140 µm pitch), and it is impractical and extremely difficult to connect everything manually. With the development of a fully automated process machine in mind, we present here an intermediate solution that is able to globally bond a large number of pads with the pads on the parylene pocket. In this process, an epoxy squeegee setup is used. A commercially available conductive epoxy (118-15A/B, Creative Materials, Tyngsboro, MA, CA) is first mixed and applied on the surface of the edge of the parylene film. The parylene film has holes and wells that were etched during the fabrication process. These holes and wells act as the screen for this squeegee process after they are aligned to the bonding pad of the chip underneath. A

(both illustrated in figure 5.20).

However, this squeegee process with the current fabricated device is not 100% repeatable as the wells on the top parylene layer have low aspect ratio and cannot retain the conductive epoxy. Thus, an additional photoresist lithography step is considered to make this process even more reliable. SU-8, a negative photoresist is used to create a higher well after the device is released from the wafer. The well that is created from this step can retain all of the epoxy that is squeegeed on top of it. A comparison of this process is shown in figure 5.21. An experiment was conducted to determine the optimized aspect ratio for this well after evaluating the bonding efficacy. The results of this experiment are presented in table 5.2. The epoxy footprints with different SU-8 screen openings are presented in figure 5.22. Masks that are 30 µm thick with 60 µm opening produce the cleanest pattern for this technique.

In addition, this SU-8 has very high plasma etching resistance and can be used as a mask to etch through a very thick layer of parylene. This property can also be utilized to eliminate the thick photoresist cracking problem. The only trade-off is that SU-8 is not removable after photolithography patterning.

Figure 5.19. The squeegee is used to push the excess conductive epoxy away from the surface; heat is applied to cure the epoxy that remains in the trench to make the

connection between the pads of the parylene and the IC chip.

Figure 5.20. (top) The conductive epoxy may short circuit two neighboring metal pads on the chip if the viscosity of the epoxy is too low; (bottom) the conductive epoxy may not

provide electrical connection if the viscosity is too high.

Figure 5.21. (left) The well after the conductive epoxy squeegee process. The well is only 5 microns tall and cannot retain the epoxy, leaving empty spaces in the well. (right) After

SU-8 layer application, the wells are completely retained and filled. Squeegee

Figure 5.22. The conductive epoxy footprint on a testing substrate after squeegee process. This test is conducted to determine the optimized aspect ratio of the SU-8 well.

After the squeegee process, the conductive epoxy can be cured in two ways; global oven curing and local laser heat curing. In global oven curing, the surface first needs to be cleaned with a very light wipe with acetone to further eliminate shorts and unwanted residues. The entire structure is then placed in the oven at 80ºC for 90 minutes [154] to cure the conductive epoxy. This process is easier, less time consuming and is currently used in our fabrication process. The profile of the bonding epoxy bumps is examined; the maximum height of the applied conductive epoxy is ~25 µm (figure 5.23).

In the second method, ultraviolet pulsed annealing (New Wave Research QuikLaze 50SR Multi-wavelength laser trimming system, New Wave Research, Fremont, CA, USA) can be used to cure the epoxy connections locally. The power of the laser

needs to be tuned so that it is strong enough to cure the epoxy without etching the epoxy wells. The laser system can be automated by programming the software to step through each connection well to anneal. After this annealing, acetone is used to wipe away the uncured conductive epoxy. This step leaves the surface of the bonding pad relatively clean and the space between pads free of unwanted short circuit conductions, as shown in figure 5.24.

Ideally, a UV curable conductive epoxy should be used for this purpose, as the time to cure would be shorter and would make the process more efficient. However, UV curable conductive ink is difficult to make due to reflection of the UV light by the solid fillers inside the epoxy, causing the epoxy resin to not cure properly. Research is being done to develop a reliable UV curable conductive material with secondary reaction [155].

Figure 5.23. The surface profile of the bonded surface. The maximum height of the conductive epoxy bump is about 25µm

Figure 5.24. The surface of the bonding pads right after the squeegee. There is a significant amount of residue left.