Months of October and November

Phase III: Make mould using SU-8

The procedures of this phase are exactly the same as in phase 2 but the main difference between the 2 stages is, in phase 3, there is no more testing of thickness of any trial and error. The layer of SU-8 spin coated on the glass piece is chosen at 1000rpm.

The only test we made was to draw up a few shapes to see if UV light was able to pass through which type of ink from 2 markers. This is important as the print job is not perfect and the ink coated on the transparency is uneven thus UV light is able to pass through these area. Having this problem, we can use the proper marker to draw over these ‘holes’.

The finalised mould design is shown below.



Problems we faced and solutions

When colouring the mould design after printing

After printing, the result of the mould design is not really black. A marker is needed to colour the blacked-region of the design so as to prevent UV-light to pass through.
The problem occurs when colouring, some of the ink smudges to the designed channel area.

Solution

We print out on another piece of paper then cut out the respective design and paste it behind the pattern being printed on transparency using double sided tape


Phase IV: Familiarize and Make chip using PDMS

What have we done?

1) Mixing the PDMS

-PDMS mixed in a 1:10 ratio of curing agent to PDMS monomers base.
-Curing agent will cure the viscous PDMS base.
-Mixture stirred for 3 minutes to ensure uniformity.
-After 3 minutes of stirring, many bubbles are formed.

2) Degassing of PDMS

-Degassing is done to remove bubbles in the PDMS.
-PDMS in cup is placed in the desiccator connected to the vacuum pump.
-Aluminium foil is placed below the PDMS cup to catch any PDMS that spills out of the cup.
-Vacuum pump set at low level (around -200) to allow bubbles to rise up and burst at the surface.
- * Too high vacuum cannot be used as it will cause the PDMS to boil off making the PDMS even more viscous.
-May damage vacuum pump.
-Degassing duration is around 1-2 hours.

3) PDMS Pouring

-After degassing, PDMS is taken out.
-The mould is placed on an aluminium foil on top of a plastic Petri-dish and wrapped the inner side of the Petri dish.
-The aluminium foil wrapped container will contain the glass mould piece so that the PDMS can be filled into it. The purpose of doing this is for easy removal of the PDMS after solidifying instead of sticking onto the Petri dish..
-After that, the PDMS is poured slowly and near to the glass mould piece surface to prevent formation of additional air bubbles.
-The amount of PDMS poured depends on the thickness we want for the PDMS mould. In our case, its about 1mm thick.
-After ensuring the PDMS is evenly spread out, the Petri-dish can then be placed in the oven for 4-6 hours minimum at 60°C. To achieve optimum results, it must be baked for 8 hours in the oven at 60°C.

4) PDMS Unmoulding

- After taking out the PDMS mould from the oven, it is left to cool for about 10 minutes.
- For better efficiency, excess aluminium foil at the side of the PDMS mould is removed.
- Using a pen-knife, aluminium foil at the bottom of the PDMS mould is slowly and completely removed.
- After that, the pen-knife is inserted in between the PDMS mould and the glass piece edge.
- Then, carefully and slowly, the knife is slit around the edge of the PDMS mould and occasionally is inserted deeper to allow the mould to detach from the glass piece.
- After the edges of the PDMS mould is detached from the glass piece, it can be slowly peeled off using hand.
- *Be gentle and extra careful as the PDMS mould is brittle and may break easily.

5) Cleaning

- After cutting out the wanted PDMS layer, wash the layer with soap.
- After washing the layer with soap, rinse it with De-Ionized water.
- For the final washing, rinse it with Isopropyl Alcohol (IPA).
*- After rinsing with IPA, dry it by putting inside the Petri dish and cover it so as to prevent any particles or dust to stick on it.
*note: For proper procedures, after rinsing with IPA, it must be dried using nitrogen gas. Since we do not have a nitrogen gas air gun, it was left in the Petri dish covered and evaporates by itself.

Problems Encountered

i) Zero Error in Weighing Scale. Inaccuracy in weight measurement.

Solution
To overcome this, zero error has to be minus off from reading measurement.

ii) Plastic cup too high to be placed in the desiccator.

Solution
To overcome this, before the PDMS is prepared, the plastic cup is shortened to a height which can fit into the desiccator.

iii) During the PDMS pouring, the PDMS is unevenly spread out on the glass piece. This will cause the PDMS mould layer to be uneven with one side thick the other side thin.

Solution
To overcome this, a suggestion will be to pour the PDMS from the centre of the glass piece to allow more even spread in all directions.

iv) During the PDMS un-moulding, the PDMS mould could easily be damaged or destroyed when not properly handled.

Solution
To prevent this from happening, the aluminium foiled area is removed to the table and a penknife is used to cut round the glass mould. By doing this, it minimizes the chance of the PDMS to be damaged. After cutting, fold the areas at the side of the glass and the wanted area can be taken off the aluminium foil easily and the PDMS on top of the glass mould can be peeled of slowly and easily.
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