Chapter 3 Bead Integration Technology and an LC-ESI/MS Chip
3.4 LC-ESI/MS Chip with Integrated Bead Column
3.4.3 Microchip Fabrication and Packaging
3.4.3.2 Photoresist Releasing Issue
The third 5 µm thick parylene layer is deposited to cover the wafer frontside (Figure 3-15 (c)), and patterned with a thick photoresist mask to open the nozzle. DRIE is performed again on the backside to open the access hole. The frontside substrate silicon underneath the nozzle and along a trench perpendicular to the nozzle is etched with BrF3
gas-phase silicon etching, in order to make the nozzle freestanding. Pre-patterned frontside oxide is used as the mask. The etching depth is 150 µm. Then, the channels are released by dissolving the channel sacrificial photoresist in warm acetone. The freed beads automatically pack to the nozzle end during drying, since the nozzle end is a much smaller opening compared to the column inlet. The column can be further packed with an off-chip pressurized flow. Finally, the nozzle is freed by detaching a piece of the chip along the across-chip trench (Figure 3-15 (e)). To further strengthen and passivate the device, an optional epoxy overcoating layer can be applied to the device.
Figure 3-14 shows various parts of a fabricated LC-ESI chip, both before (Figure 3-14 (a), (b)) and after photoresist removal (Figure 3-14 (c), (d), (e)). Both the multiple inlet filters and outlet filter/nozzle can be clearly seen in the pictures. After photoresist removal, it can be seen that the beads are released in the channel. Figure 3-14 (e) shows a fluorescent picture of the freestanding parylene ESI nozzle. The freestanding part is 600 µm long. The nozzle opening is 15 µm wide by 3 µm high.
photoresist. A detailed discussion of photoresist as a micro-channel sacrificial material by dissolution in acetone is given in [18]. One effective way to reduce the diffusion time is to increase the diffusion constant by increasing temperature of the acetone solution, which also increases the dissolution rate. However, heating temperature is limited by the acetone boiling point of 56.5 °C and safety concerns since acetone is flammable. It is found that ST-22 standard photoresist stripper is much less effective in the micro-channel releasing process than acetone, although ST-22 provides a cleaner surface than acetone for photoresist film stripping. It is because ST-22 is much more viscous and the diffusion constant is much lower. IPA (isopropyl alcohol), another common solvent for removing organic residues, is much less effective for dissolving photoresist than acetone even when heated close to its boiling temperature of 82.5 °C.
Secondly, since the photoresist is not etched away but rather only dissolved, it remains in the acetone solution in the channels. The only process taking photoresist out of the channel is diffusion, until the equilibrium is reached between the photoresist concentrations in the channel and in the bulk acetone solution. Therefore, frequent changing of the acetone solution can help lower the photoresist concentration inside the channel faster. Ultrasonic agitation can also speed up this equilibrium process, but it may damage freestanding parylene structures as well. Theoretically, the photoresist cannot be completely depleted from the channel, even if the acetone solution is refreshed periodically.
Thirdly, even if the channel has a very low photoresist concentration, which usually appears as a clear channel under optical microscope, the photoresist could still clog small openings when dried. Since acetone is highly volatile while photoresist is
basically non-volatile, photoresist is usually left at the channel exit when the acetone all evaporates. This could cause clogging when the exit is small. Sometime this is so serious that even repetitive acetone dissolving and drying cannot remove the clogs. One way to avoid this is to use large openings whenever possible. Based on past experiences, openings less than 10 µm wide, 3 µm high can be problematic.
Figure 3-16 Fluorescent diagnostic technique.
Since low-concentration photoresist could still be present in the channel even when the channel looks clear under optical microscope, a fluorescent diagnostic method is developed to confirm the end of the releasing process. Fluorescent detection is chosen because of its high sensitivity. Using a fluorescent microscope, the chip can be imaged to show the contents of the parylene channels and nozzle. Figure 3-16 shows two fluorescent pictures before and after photoresist releasing. Parylene appears to be blue, while photoresist appears brown (Figure 3-16 (a)) to red (Figure 3-17) depending on its concentration. Multiple layers of parylene, hollow parylene channels, and voids can also be easily identified.
Figure 3-17 The photoresist releasing issue in various forms.
Figure 3-17 (a) shows an optical picture of a typical photoresist clogging, which is caused by an in-channel post array greatly restricting flow. Figure 3-17 (b) to (f) are all fluorescent pictures showing a variety of photoresist releasing problems after drying.
Figure 3-17 (b) is typical where the photoresist either blocks the channel or just stays somewhere in the channel without blocking it. Figure 3-17 (c) shows a clog near nozzle tip. Figure 3-17 (d) and (f) show two forms of photoresist coatings on channel wall after drying. The photoresist in (f) is extremely hard to visualize with an optical microscope, since the photoresist layer is very thin and the concentration is very low (very light color in the picture). Figure 3-17 (e) shows photoresist residue in a bead-photoresist mixture column after incomplete releasing, which is also difficult to visualize with an optical microscope.