Table 1.

Troubleshooting table.

STEP	PROBLEM	POSSIBLE REASON	SOLUTION
3	Spin-coated photoresist is not uniform and has traces of particulates and particles.	Amount of photoresist is not enough. Air bubbles are generated during dispensing. Wafer surface is not clean.	Make sure enough photoresist is dispensed in the middle of the wafer before spin coating. Make sure no air bubble trapped in photoresist before dispensing. Re-clean wafer with the Piranha solution. If still not clean, use a fresh wafer directly out of a new wafer box.
7	Photoresist pattern on the wafer doesn’t reproduce the photomask features with good fidelity.	Photoresist is under- or over-exposed. Photoresist is under- or over-developed. The photomask is contaminated.	The exposure dose and development time need to be better characterized. The thickness of the photoresist layer needs to be monitored after spin- coating for consistency. Gently clean the photomask with solvents and then blow dry with N2.
14, 17, 18	After RIE process, some black silicon appears on the etched area.	Photoresist residue is left after development. There is a thin layer of native oxide on the silicon wafer. RIE machine chamber is not clean and has an accumulation of organic residue.	Hard bake the wafer at 120°C for additional 30 min before the RIE step. Make sure to perform Steps 11-13. They are critical to ensure no black silicon etching to occur during the RIE process.
28	KOH etch rate for silicon is slow and not uniform across the entire wafer.	Temperature of the KOH solution is low; or the KOH concentration is not set right. There is a layer of native oxide on the silicon wafer that prevents KOH etching. The silicon nitride mask layer is not completely etched through during the RIE step. There exists a loading effect during KOH etching.	Double check the temperature and concentration of the KOH solution. Quickly dip the wafer into a diluted HF solution (DI:HF=50:1) for 10 s to remove native oxide on the wafer. The thickness of the nitride film and the RIE nitride etch rate need to be carefully characterized to ensure the nitride layer etched through. Step 27 is critical for this purpose. Rotate the wafer for 90o every hour during KOH etching.
35	Silicon wafer breaks during thermal oxidation process.	Buffer access holes are too large, and the wafer becomes fragile after KOH etching. The wafer edge is not fully protected and therefore is etched during KOH etching. The wafers become too fragile and cannot survive the thermal stress caused by thermal oxidation.	Buffer access holes can be designed to be as small as possible. Before RIE nitride etching, make sure the wafer edge is covered fully with photoresist. Make sure oxidation furnace cools down slowly. The quartz boat needs to be pulled out the furnace at a very slow speed.
38	Bonding strength between Pyrex and silicon wafers is not strong after anodic bonding, and the wafers can easily become delaminated.	Surface roughness of the polished Pyrex wafer is too high. Wafers are not clean, so there is no spontaneous hydrophilic bonding between the wafers. Temperature and voltage are not set at appropriate values during anodic bonding.	Pyrex wafer needs to be fully polished with a minimum surface roughness (ideally smaller than 5 nm across the wafer). Step 37 is critical for wafer surface cleanness and spontaneous hydrophilic bonding between the wafers. Slightly increase the bonding temperature and voltage, and monitor the current carefully.
41	TBE buffer is not spontaneously filling the entire ANA device.	During die saw cutting, some water leaks into the ANA device. The ANA device is kept under ambient condition for too long after die saw cutting.	The buffer access holes of the ANA need to be properly sealed with wafer dicing tape. The ANA device should be filled with TBE buffer immediately after the die saw cutting.
45	After application of the electric fields, the initial biomolecule stream is not injected from the sample injection channels.	Air bubbles are trapped in the sample injection channels. Oxide layer breaks, and there is a current leakage from buffer solution to the Si substrate.	Run electro-osmosis through the ANA device to remove air bubbles. Replace the current leaking ANA device with a fresh new one.
x	It is hard to determine the (111) planes.	Nitride mask layer is not completely open. KOH etching is either too short or too long to distinguish the level of undercuts between adjacent lines. The (111) planes are far off from the wafer flat (111) plane, and therefore it is beyond the detection limit of the alignment mark.	The nitride film thickness and the RIE nitride etch rate need to be carefully characterized to ensure the nitride layer etched through. KOH etching time needs to be carefully adjusted. The wafer should be positioned properly during photolithography. Pay attention to wafer specifications and make sure the alignment mark can account for the offset between the real (111) plane and wafer flat orientation.
xii	Irregular trenches (e.g., non-uniform width or depth) of the ANA narrow regions are generated after KOH etching.	The mask pattern is not reproduced with high fidelity during photolithography. There is a thin layer of native oxide on the wafer that prevents KOH etching. KOH etching condition is not set right. The alignment process to find the real (111) planes is not successful.	Refer to Troubleshooting for Step 7. Quickly dip the wafer into diluted HF solution (DI:HF=50:1) for 30 s to remove native oxide on the wafer. Double check the temperature and concentration of the KOH solution. Double check the alignment process.
xiv	Spin-coated photoresist can not uniformly cover the wafer.	The ANA deep trenches are not completely sealed by PECVD deposited oxide.	Strip the photoresist and deposit additional oxide layer to ensure complete sealing.
xviii	DRIE etching doesn’t reproduce the features on the mask with good fidelity.	Severe undercuts occur during DRIE process. The etch mask (i.e., the photoresist layer) is etched away during DRIE process.	Double check the DRIE etch recipe. If necessary, adjust the etching parameters to minimize undercuts. Use a thicker layer of photoresist or switch to an oxide mask layer if very deep Si etching is desired.
xxv	The final gap size of the ANA narrow regions is either too small or too large.	Thickness of the thermal oxide layer is not controlled well. Depth of the ANA wide channels is different from of the ANA narrow channels. Nitride mask layer is not completely removed, or there is some Teflon-like polymer left in the deep trenches after the DRIE process.	Double check thermal oxidation conditions such as temperature and gas flows. Run thermal oxidation again if more oxide deposition is desired. Take cross-sectional SEM of the ANA device to make sure the depths of the ANA narrow and wide regions are the same. Otherwise, further adjustment of the respective etching time is required. Immerse the wafer in the HF solution for a longer period to strip the nitride mask layer completely. Run oxygen plasma for a longer time to remove the polymer layer.