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How to succeed your PDMS bonding?
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Introduction to the PDMS bonding
The bonding step enables to finish your chip fabrication. It is done thanks to a plasma treatment that is going to modify the surface chemicals and allow the sticking the PDMS with the channels against on other substrate (PDMS or glass). If this step is doing wrong, your chip will leak and couldn’t be use normally. You will have to be carful on different points in the choice of the plasma equipment but in the protocol too.
The relevant parameters to succeed the PDMS bonding
The plasma is a device that influences the surface; all contamination will highly affect the final results of the treatment. Contrary to popular thinking, for a PDMS bonding, a longer treatment will not improve the surface except for really particular cases. For example the presence of fat such as a fingerprint will lead to a treatment failure on the concerned surface.
The air pollution inside the plasma room (oil)
The gaz composition inside the plasma room will change the kind of chemical links that are created on the surface of your glass or PDMS. Some impurities even in very low quantity will pollute your surface. The most common polluting is oil coming from the vacuum pump or compressors around. Because of the oil inside the chamber, you will probably see the same plasma that you used to see but the chemical will be different and the PDMS will not bound.
The plasma stability and color
A good indicator of the quality of the plasma is generally its color/luminosity (which depends on the pressure and the gas used). But it is not always the case, what you have to remember is if your color plasma change with the same parameters, it may fail.
The plasma time
The timing is a good key of the success of your treatment and so of your bounding. A too short plasma treatment will not functionalized the whole surface and a plasma treatment too long will modify too strongly your PDMS surface. Longer the plasma is activated and rougher is your surface and the bounding prosperities will be affected. With plasma usually used for softlithography, the perfect time to have the most powerful bound is generally between 20 and 60 secondes.
The timing after the plasma
Just after the plasma treatment the chemical bound of the surface begin to recombine and after few minutes the surface becomes not enough activated for bounding. For this statement, you have to do the bounding just after the treatment, do not let your sample inside the plasma room after venting, and put them rapidly together.
A bake to improve bounding quality
To make easier the chemical link after the contact between the PDMS and the glass or PDMS, it is recommended to heat the set. The time, the temperature and the device can change between the laboratory and the user, baking at 80-90°C during 15-30 minutes is usually enough to have a great bounding.
The choice of the gas
The surface state will depend on the gas used. A room air plasma will work really well in most of the case. Some people will prefer pure O2 to have a total controlled atmosphere but ask for more equipment and strictness during the process.
The presence of dust on the surface will prevent the bounding where the dust is but also on a disc around and the size will depend on the rigidity of the PDMS. A first cleaning with, at least, a clean dry air jet is required. There are other methods to remove dust, you can use 3M Scotch tape to remove particles on the surface or more efficient you can dive your chip in Isopropanol and use sound wave to detach all unwanted particles on the surface and inside the holes in the PDMS.
To clean the glass use successively acetone, isopropanol, water and dry it.
Some wrong common idea
You need to press strongly on the PDMS chip to correct the bounding
Press strongly on the PDMS to urge a bounding is tempting to correct a bad plasma treatment. However it is not working, what you really risk to do it is to collapse and deform your channel irreversibly. Keep in mind that the bounding should have to be fast and easy if the contact is not good between your two parts it is perhaps because of dust. In order to correct the bounding, the only try to do, it’s to heat your set and press gently again the chip.
We can move the PDMS chip a second time after a first contact
If you put your chip at the wrong place the first time or if the plasma treatment is not working everywhere it’s useless to try to reposition your chip a second time. The best to do it’s one you throw out the chip or you do again the plasma treatment but it can be sure to work and it’s certainly not reproducible.
How to check the quality of your plasma treatment during the calibration steps of your process?
First test: the angle contact measure
The plasma treatment modifies the surface properties and namely the hydrophobicity of the glass and the PDMS. A good treatment makes the surface hydrophilic. A first test consists in put a water droplet (about 20µl) on each surface and measure the angle contact with the surface. A contact angle below 20° will lead generally to an adhesion strength upper than 2.5bar.
Photo angle de contact plasma OK et plasma pas OK sur verre PDMS.
Second test: the spreading of the sticking front
When you bond your chip, at the contact the part become darker so you can follow the contact. After the plasma treatment drop off gently the PDMS on the glass or on an other piece of PDMS the contact front should progress of itself quickly and easily.
Third test: no leaking at high pressure
A third test consists in injecting a liquid inside your device and tests its behavior with high pressure. You can use a simple syringe and push with fingers it will be enough to have several bar.
Photo fuite a l’entrée (bon plasma) , fuite sur le coté ( mauvais plasma)
Fourth test: the wrenching
This last test is destructive, and consists in wrenching the chip from its shelf. The chip should break and not be removable, it will remain PDMS on both part. Normally a good bonding will enables to use your device for pressure until around 3 at 5 bars.