How to make a PDMS lithography replication from a SU-8 mold

The PDMS is a polymer widely used in microfluidics to make devices such as lab on chip. Every year many laboratories begin a microfluidic activity and sometimes not with the good equipment or methodology, this review aims to bring the basic knowledge on how to do the PDMS lithography replication, also called soft lithography process. We are going to give you here little tips and tricks to make the PDMS lithography replication as easy as pushing a button. Do not hesitate if you have specific questions we always like to help researchers, as we are some too, and we will try to answer as best as possible. If you are interested in doing PDMS replication you may look at our PDMS station.

1. The preparation of the mold with the silanization
2. The scaling and mixing of the PDMS and the curing agent
3. The degassing to remove bubbles
4. The PDMS pouring on the mold
5. The PDMS baking
6. The PDMS peeling off the mold
7. The PDMS cutting and piercing
8. The PDMS bonding

1.PDMS replication: Preparation of the mold with the silanization

For a first use of your mold you have to make it hydrophobic. You can buy a mold from a foundry or do it by yourself. The silicon from your SU-8 mold is normally quite hydrophilic and thus the PDMS will have a good affinity to it, strong enough to make the peeling impossible and you can only break your mold and go back from the beginning of your process so do not forget this step.
It exists several ways to make the silicon hydrophobic. We use silane function which once bonding on the surface quite permanently, allow an angle contact above 90°. You can use a gas or liquid treatment, it means you can bond the silane function on the surface from a gas or thanks to successive liquid bath. We are going to briefly see the two solutions.
The silanization use dangerous product for human beings and has to be used in proper conditions, namely under a fume hood. If you have any doubt do not hesitate to ask.

Silanization in liquid

You can use for example some OTS (octadecyltrichlorosilane) following the recipe:

1. Dilute 0,5ml of OTS in 50ml of hexadecane in a crystallizer.
2. If possible remove the air from it with a N2 air gun.
3. Put your wafer inside and close it with parafilm for 15 minutes.
4. Put the wafer in a bath of pure hexadecane.
5. Put the wafer in a bath of acetone.
6. Put the wafer in a bath of ethanol.
7. Dry your wafer.
8. Test the angle contact: your mold is ready!

Silanization in gas

To do silanization in gas you can use some TMCS (TriMethylChloroSilane) which is volatile at room pressure and temperature and follow this recipe:

1. Put your wafer inside a Petri dish.
2. Add few droplets of TMCS around the wafer, inside the Petri dish.
3. Close the Petri dish for 15 minutes.
4. Open the Petri dish and wait 10 minutes for all the TMCS to be evaporated.
5. Test the angle contact: your mold is ready!

2. PDMS replication: The scaling and mixing of the PDMS and the curing agent

Once your mold is ready, we can prepare the PDMS. PDMS is the name of the polymer. To make it harder you have to add a curing agent. The most used PDMS is clearly the Sylgard 184. For this PDMS the usual ratio between the curing agent and the PDMS is 1:10 (weight). For specific applications the ratio can be changed to have a PDMS that is softer or harder.
To prepare your PDMS follow the recipe:

1. Weight the PDMS (60g for example)
2. Add the curing agent (6g for example)
3. Mix them strongly

It seems easy but be really careful to put first the PDMS and then the curing agent, doing the opposite may lead to a bad cross linkage of the polymer.
To mix the both components you can also use an automatic PDMS mixer, if you are interested, please contact us.

3.PDMS replication: PDMS degassing to remove bubbles

Your preparation is filled with bubbles due to the mixing, but these bubbles have to be removed because if not they will be trapped inside your PDMS chip. There are different ways to degas your PDMS mix; you can use a centrifuge or use a desiccator and a vacuum pump. The second option, even if longer in time, is the simplest and the most used. Indeed the centrifuge is more adapted for smaller quantities of PDMS.
To degas your mix thanks to a desiccator, put your cup inside, close it and check it’s well sealed. Do the vacuum using a vacuum pump or a vacuum line of your lab, be careful if you do a vacuum too strong or too fast, the PDMS may overflow. It usually takes 30 minutes to remove all the bubbles. Once the bubbles are removed, you can switch off the vacuum and bake your PDMS. Be careful not to try to open the desiccator when the pressure is not stabilized, otherwise your PDMS may blow up.

4. PDMS replication: PDMS pouring on the mold

Always clean your mold before using it, use N2 or clean compressed air to remove all dust and particules on your SU-8 mold surface.
Once the PDMS is degassed, you can pour it on the SU-8 mold previously silanized. Use a container to put your wafer inside such as a Petri dish. The size of the container will define the quantity of PDMS you need for the wanted thickness of your devices. For the size of a 4 inches substrate, 40g of PDMS will give you about 5mm-thick devices.
If some bubbles appear during the pouring you can remove them with a needle or putting back your wafer under vacuum. If you choose the second option be careful, the wafer can get up and the PDMS will go under your substrate, thus it will be more difficult to release it later.

5. PDMS replication: PDMS baking

Once the PDMS and the curing agent are mixed, the cross linkage has begun but alone it will take around 24 hours to get a solid enough device. That’s why the mold and the PDMS have to be baked. The time and temperature of the baking vary depending on the laboratory and the user, as well as the tools used. We will try to see here what can be done and some advice to succeed at this step.

People use hot plates or an oven to bake the PDMS, there is no best tool here. Inside an oven you can bake more than one mold at once. For setting the temperature and time, it depends, you can bake hotter during a shorter time or colder during a longer time. We did experiments and measure the angle contact with water on the surface of PDMS layer with different baking and no variation have been measured so the baking will not influence the chemicals of your PDMS, but it will influence its mechanical properties. Indeed, after baking, your PDMS can be more or less soft and so can be more or less easy to manipulate. Your PDMS will develop with quite the same mechanical properties at the end but can take a longer time. Nevertheless, be careful not to bake your PDMS too much, it will become “old”, too hard and it will be really difficult to pierce it later. The same phenomena can be seen with an old PDMS sheet (after several months).
The temperature has to be chosen according to the mold container, indeed if you use a Petri dish do not heat over 90°C. We advise you to bake at 80°C in an oven during 2 hours. You will be able to manipulate your PDMS just after it cools down.

6. PDMS replication: The PDMS peeling off the mold

Wait for the mold and the PDMS to cool down, then you can cut around your substrate and peel off the PDMS to release your devices. You can use a scalpel to easily cut the PDMS. During cutting be careful to keep the blade tangent to your mold to avoid breaking it.

During the peeling, just pull the PDMS gently – it should be removed easily. If the PDMS sticks too much to your wafer, it’s perhaps because you missed the silanization or it was not made well enough.
Make sure to use gloves during this step to avoid adding grease to your PDMS, otherwise it will be difficult to clean and can be problematic for the process.

7. PDMS replication:  PDMS cutting and piercing

At this time you have your PDMS with all the devices, you have to cut it to release the different chips. Like before you can use a scalpel to do it.
In this step you also have to make some holes inside your PDMS if you want to add and remove fluids later. To do so you can use a punch, choose the right size depending on your tubing. Since the PDMS is a soft material, choose to make the hole a little smaller than your tubing, this way the tubing will be strongly maintained in your chip and it will decrease the leak problems.
Make sure to use gloves during this step to not add grease on your PDMS, otherwise it will be difficult to clean which can be problematic for the process.

8. PDMS replication:  PDMS bonding

The last step is to bond your device to close your channels. The PDMS can be bound on another piece of PDMS or on glass but the protocol is the same. Before anything, each part has to be well cleaned to remove any dust or particles from the surface. You can use isopropanol to clean your PDMS and your glass, you can even use an ultrasonic bath to better clean the surface and inside your PDMS holes.

To bond the PDMS you need to activate the surface, transforming the Si-CH3 function of the PDMS to a Si-OH and when you will press against the same function you will create strong and permanent Si-O-Si link. To do so the most used tool is a plasma cleaner working with O2 or air. To make a good plasma you have to control the pressure inside the chamber. We advise a pressure at 500mTorr, some equipment can help you to achieve a perfect plasma treatment such as the Equinox. To functionalize the surface, the treatment doesn’t have to be long, 2 minutes is a good time to create strong bonds with glass or PDMS.
To get more information about the bonding you can read our specific review on how to get the best PDMS plasma bonding process.