Lithography
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Materials
- Wafer
- Tweezers
- High-pressure air duster
- Petri Dish
- Heating plate
- Rotating chamber
- (Machine for exposure)
- Soft mask
- Cristallisoir
FIRST DAY IN THE CLEAN ROOM
Wafer preparation
To clean any organic or inorganic contaminations present on the wafer surface
- Clean the wafer under a fume hood.
- Grab the wafer with tweezers, pour ethanol, and then with a high-pressure air duster, quickly dry it.
- Check that the surface is completely cleaned, without any traces of dust.
We had trouble getting rid of the dust, maybe because the hood or the Petri dishes were not clean enough. Some of the plates may have had some scratches.
- Put the wafer in the petri dish (as clean as possible).
- Bring the wafers inside the room for photolithography.
- Place the wafer on the heating plate, at a sufficient temperature to get rid of any traces of alcohol left.
Adhesion layer
Photoresist Coating:
The photoresist sets the final coat thickness and allows solvent to evaporate, partially stabilizing the film.
We needed 2 μm of thickness of SU-8 2005, the photoresist.
- Set the machine to 3 000 RPM for 30 seconds.
- Wait for the wafer to be cold.
- Center the wafer on the Teflon plate
- Cover the sides of the rotating chamber with aluminium.
- Turn on the vacuum and make the wafer rotate for a few seconds to check if it is actually centred.
- Turn on the vacuum and add on the centre 4 ml of SU-8 using a disposable pipet.
- Close the chamber and start the machine.
Soft bake (prebake):
To drive off excess photoresist solvent and solidify the film.
- Grab the wafer with tweezers on the flat side and put it on a hotplate for 1 minute at 95°C.
- Take the wafer from the hotplate and let it cool down holding it in the air to prevent a heat shock.
- Put the wafer on the petri dish.
Exposure:
- Expose photoresist to light, with wavelengths at 365nm. The power of the machine is 10 mW/cm2. The layer needs 80 mJ/cm2 to harden, which makes it 8 seconds in the machine.
Post exposure bake:
To help reduce standing wave phenomena caused by destructive and constructive interference patterns of the incident light.
- Take the wafer and put it on the hotplate for 2 minutes at 95°C.
- Take the wafer and let it cool down in the air, then put it on the plate or desk under the hood.
First layer
Coating:
100 μm of thickness of SU-8 2050.
- Set the machine to 1750 RPM.
- Center the wafer, turn on the vacuum, and carefully add a medium amount of SU-8 directly from the bottle and without using a pipet since the compound is viscous.
- Close the chamber and turn on the device (pay attention that the vacuum is turned on).
Soft bake:
- Take the wafer with tweezers and put it on a hotplate for 5 minutes at 65°C.
- Move the wafer quickly to a hotplate of 95°C for 18 minutes.
- Let the wafer cool down holding it with tweezers up in the air before setting it on the table to prevent heat shock.
Exposure:
- Soft mask must be stuck with tape to the glass plate.
- Pay attention to the position of the shapes, so that they can fit without any problem into the circular window of the mask holder.
- Exposure of the wafer to light for 24 seconds with a wavelength of 365 nm since the wafer needs 240 mJ/cm2.
Post exposure bake:
- Put the wafer for 5 minutes on a 65°C hotplate.
- Quickly move the wafer to a 95°C heating plate for 10 minutes.
Development:
To wash away soluble photoresist.
- Bring the wafer out of the photolithography room and into a hood in the cleanroom.
- With 2 cristallisoir, one with the SU-8 developer and one for cleaning, let the wafer be immersed in the developer for 9 minutes.
- Shake the wafer gently every now and then to allow good convection.
- Take the wafer with tweezers and hold it over the cristallisoir for cleaning.
- Pour acetone and then ethanol in quick succession.
- Make sure there are no traces (white trail residues on the channels), otherwise, repeat the cleaning process.
- If you still see traces, put the wafer back in the developer for 30 seconds to 1 minute.
- Put the wafer on a chem wipe and dry it with a pressurized air blower.
- Put the wafer back on a petri dish.
- You can check with the bright field microscope if there are blue traces. If there are too many or too big you can put the wafer back in the developer for a few minutes.
In a clean room we produced a primary mold made with SU-8 on a silicon wafer. This was done using a photolithography machine and a soft mask during half a day of lab work.
Because of troubleshooting with the wafer and polymer, we did not have time to use the second layer of soft mask (to produce the Herringbone grooves) which will have to be done at a later date.
SECOND DAY IN THE CLEAN ROOM
Wafer preparation
"To ensure there is no water at the surface."
- Put the brand new wafers and the wafer with already one layer of SU-8 on a heater at 120°C.
Second layer
Coating:
100 μm of thickness of SU-8 2050.
- Set the machine to 1900 RPM.
- Center the wafer, turn on the vacuum, and carefully add a medium amount of SU-8 directly from the bottle and without using a pipet since the compound is viscous.
- Successively use a vacuum chamber and a heater at 35°C to fluidify the SU-8.
It will prevent the formation of bubbles due to the design already deposited on the wafer from the first layer. This will help the bubbles float to the surface, detaching from our design, so they can be eliminated once the spinning starts.
- Close the chamber and turn on the device (pay attention that the vacuum is turned on).
Soft bake:
- Take the wafer with tweezers and put it on a hotplate for 5 minutes at 65°C.
- Move the wafer quickly to a hotplate of 95°C for 18 minutes.
- Let the wafer cool down while holding it with tweezers up in the air before setting it on the table to prevent heat shock.
Exposure:
The cross alignment was difficult to do and the result was not perfect as the crosses were too close to each other making it hard to check if the mask may be tilted. In the future, crosses should be put at the opposite end of our mask so that setting the photolithograph is be easier.
- Soft mask must be stuck with tape to the glass plate.
- Pay attention to the position of the shapes, so that they can fit without any problem into the circular window of the mask holder.
- Exposure of the wafer to light for 23 seconds with a wavelength of 365 nm since the wafer needs 230 mJ/cm2.
Post exposure bake:
- Put the wafer for 5 minutes on a 65°C hotplate.
- Quickly move the wafer to a 95°C hotplate for 10 minutes.
Development:
To wash away soluble photoresist.
- Bring the wafer out of the photolithography room and into a hood in the cleanroom.
- With 2 cristallisoir, one with the SU-8 developer and one for cleaning, let the wafer be immersed in the developer for 9 minutes.
- Shake the wafer gently every now and then to allow good convection.
- Take the wafer with tweezers and hold it over the cristallisoir for cleaning.
- Pour acetone and then ethanol in quick succession.
- Make sure there are no traces (white trail residues on the channels), otherwise, repeat the cleaning process.
- If you still see traces, put the crystallizer in a sonic bath.
- Put the wafer on a chem wipe and dry it with a pressurized air blower.
- Put the wafer back on a petri dish.
Hot embossing with the Sublym Machine
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Materials
- Sublym Machine
- Flexdym
- Wafer with design
- Counter mold
- Protecting gloves
- Spacers
- Metal hole punch pliers
- Tweezers
- Petri dish
Sumblym machine set up:
- Open the Sublym machine and check that the inner deformable membrane is well attached and not stuck together .
- Close the Sublym machine and pre-heat it to 150°C (no need to go higher than 180°C) for a few minutes.
- Once the temperature is reached, press on "vacuum" twice to turn on the sound and stop pumping.
Outside the hood
- Cut 2 pieces of flexdym sheet and and place them on top of each other, on the wafer right over your design. Do not forget to remove the protective PTFE sheets from the Flexdym.
- Stick your wafer + flexdym on a counter mold so that your flexdym is between the wafer and the counter mold. The wafer+flexdym+countermold is called "Sandwich". It is possible to clean the counter mold beforehand with ethanol.
- Once the Sublym machine is set up, open it using protecting gloves.
- Check that the deformable membrane is well placed and not stuck together.
- Remove the Lid from the molding chamber.
- Place the spacers you need.
- With the tweezer, place your Mold-Flexdym-Counter mold sandwich in the molding chamber inside the machine.
- Replace the Lid in the molding chamber over your sandwich.
- Close the Sublym machine and set the process to 120 seconds.
- Press "start process" to start pumping & molding.
- After 120 seconds, press "vacuum" twice to turn on the sound and stop pumping.
- With protecting gloves and tweezer, take your sandwich and bring it under the hood.
Figure1: Sublym Hot Embossing Machine, taken from Eden Tech webpage (1)
Inside the hood
- Place your sandwich on the table until it cools down at room temperature.
- De-assemble gently the Flexdym from the mold and counter mold by pouring ethanol on the Mold-Flexdym-Counter mold sandwich and using tweezers.
- Cut the flexdym around your design.
- Use metal hole punch pliers to create holes in your chambers.
- Put your flexdym on a Petri dish so that the design is on the side of the petri dish and the flexdym flat surface is turned towards the ski.
- Gently apply pressure with the tweezer on the flexdym to remove any bubbles.
Ressources:
(1) Eden Tech webpage on the Sublym Machine
(2) Sublym Machine protocol by Eden Tech
Chip Preparation
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Materials
- Chip
- sheet of aluminium
- slate of glass or polystyrene
- heater
- 2kg flat weight
- ethanol 70%
- chemwipes
Preparation
Chip Preparation is done after hot embossing or after use of the chip.
- Clean the chip and its polystyrene or glass support with ethanol
- Remove the ethanol with a chemwipe
- The chip is placed (channel facing up) on an aluminium sheet along with its support and then on a heater at 80°C for 10 min to evaporate the rest of the ethanol.
- Use a piece of tape to remove potential dust from the chip and its support.
- Place the chip (channel facing down) on its support (glass for active polystyrene for passive). Make sure that no bubble stays trapped between the flexDym and the glass or polystyrene.
- On the heater, place in that order, the sheet of aluminium, the assembled chip, a Teflon film and a 2kg weight. Let it heat up at 80°C for 1 hour.
According to the Eden website, glass sticks more effectively to flexDym when kept at 95°C for 1 hour. However, we found that at this temperature, we risk melting the more delicate parts of our design such as the sharp edges.
We were hesitant to increase the temperature for polystyrene support as softening of polystyrene may start at 100°C
Ressources:
https://eden-microfluidics.com/wp-content/uploads/2021/03/Application-Notes-Flexdym-Bonding.pdf
Experiments
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Materials
- syringe pusher
- 2X5mL syringes
- 2,5 mL syringe
- 10 mL syringe
- 1,5 mm diameter tubing (?)
- eppendorfs
- eppendorf support
- assembled chip
- fluorescent microscope
- NdFeB Magnets 4mmx2,5mm
- tubing adapter
- tape
- tweezers
Set Up
For Active Chip:
- set the syringe pusher to the diameter of the 5 mL syringes
- Fill one syringe with your fitC PBS solution and fill the other syringe with pure PBS, make sure to chase the bubbles afterwards.
- using the tubing adapter, plug two tubes onto the two syringes then place the syringes in the syringe pusher.
- Use magnets to secure the input and output of the chip and to fix the piezo buzzer against the glass.
- fix the chip on the plate of the microscope using tape
- carefully insert the tubing at the input and output using tweezers, the outlet leading to one eppendorf.
the tubes need to be inserted perfectly vertical to the chip to avoid damaging the inputs which could cause leakage.
For Passive Chip:
This protocol was also used to produce 2 liposomes samples :
SAMPLE I
with an aqueous solution of PBS and fitC 1mg/mL in the 10 mL syringe
wit an organic solution of PBS and Lipids 1mg/mL in the 5 mL syringe
SAMPLE II
with an aqueous solution of PBS and fitC 1mg/mL in the 5 mL syringe
wit an organic solution of PBS and Lipids 1mg/mL in the 2,5 mL syringe
- set the syringe pusher to the diameter of the 5 mL syringes
- Fill one syringe with your fitC PBS solution and fill the other syringe with pure PBS, make sure to chase the bubbles afterwards.
- using the tubing adapter, plug two tubes onto the two syringes then place the syringes in the syringe pusher.
- fix the chip on the plate of the microscope using tape
- carefully insert the tubing at the input and output using tweezers, the outlet leading to one eppendorf.
Syringes of different volumes were used to have different flows for the two solutions while using the same syringe pusher. Different flow ratio between the aqueous and organic phase was linked to size control of liposomes in the literature. see reference.