HOW TO CHOOSE THE RIGHT PLASMA CLEANER FOR THE PDMS BONDING OF YOUR MICROFLUIDIC CHIP?

Plasma cleaner and PDMS soft lithography

Plasma cleaners you may buy on the market are machines that haven’t been specifically developed to perform PDMS glass bonding. For that reason it is important to look carefully at some parameters to make sure that your machine has the correct characteristics to allow you to make it easy and reproducible. The aim of this guide is to tell you which important points have to be kept in mind by researchers who wish to acquire plasma to make their own PDMS chip(s).

A short introduction to plasma cleaner and PDMS bonding

The step consists of making a PDMS Plasma Bonding between a PDMS chip and a glass slide, it is a tricky step in the soft-lithography process. Plasma cleaner allows you to make a glass-PDMS microfluidic chip, while bonding the PDMS replica containing the molded microchannel on a glass slide. Besides, the plasma cleaner will also allow you to bond together two PDMS replica to manufacture a multi-layer microfluidic devices. In case this process should be badly carried out, your glass-PDMS microfluidic chip will leak or will be obstructed and you won’t be able to use it properly. Depending on the process, you will have to pay particular attention to some points when choosing your plasma cleaner. Making PDMS chips with plasma can be very easy if, from the start, you have made the correct choices in terms of manufacturing process and equipment, so that this step doesn’t become the main research topic of your PhD students.

How does plasma react when used in a glass PDMS bonding?

Plasma is a state of the matter where the gas molecules have lost electrons, in our case, following the excitement generated by a high frequency electromagnetic field. Deprived of electrons, the gas molecules react to the surrounding materials. Among those surrounding materials are your glass slide and your PDMS block. Ideally, the plasma reacting to the glass and the PDMS will create on the surface silicon atoms with one missing electron. After reacting to the plasma treatment, these silicon bonds will ideally combine with a hydrogen atom, thus creating on the PDMS and the glass surface SI-OH bonds, to replace the initial SI-CH3 bonds. Later on, when the PDMS block comes into contact with the glass slide, these two SI-CH3 bonds recombine to form a covalent connection (or bond) SI-O-SI between the glass and the PDMS. Your device is definitely sealed (or established). If your plasma works well, the strength of the surface bonding between the glass and the PDMS can reach 75psi(→≈5bar).

What is the wettability of the PDMS after plasma treatment?

As we have seen previously, the plasma creates SI-OH polar groupings on the surface of the PDMS. These groupings change the surface properties of the PDMS, making it hydrophilic. The contact angle with DI water can be measured after a plasma treatment and goes from 110° to about 20°. But, just as the bonding between the glass and the PDMS doesn’t last long, neither does this hydrophilic behaviour. And two hours later the original 110° are restored. However this hydrophilic behaviour can be preserved longer, if the chip is filled with water after the bonding process and maintained in the canals with water.

The precision of the pressure controller of the plasma cleaner

Whether it is manual or automatic (that is rare) the pressure control of your plasma cleaner must be precise enough to enable a reproductible process. In case you use a manual valve, check the micrometric screw that is provided and the fluidic resistance related to it. That will keep the process to become a regulation session for some degrees of angle. (Especially if you have a significant inertia of pressure change as in the case of a six inch plasma chamber). If your plasma cleaner is equipped with automatic regulators, you shouldn’t be confronted with the problem of precision of the regulation.

The drift of the vacuum sensing system of the plasma cleaner

The control of the pressure is one of the most important parameters to make a successful PDMS plasma bonding. Most of the space gauges of plasma cleaners that we checked, have derived in time and are a major source of error in the plasma process. Your plasma bonding becomes less efficient with time passing, because you think you are working with the same pressure as the one you used in the initial tests whereas this pressure has changed in time. Do not neglect this point, which is often invisible at first. Besides, all the parameters equal to a pressure change from 300 to 1000m Torr, will cause your bonding force to be reduced by 40%.

The precision of the plasma cleaner vacuum gauge

The measurement and control of vacuum inside your plasma chamber is of the utmost importance. You can choose as you like between digital or analog gauges but it is necessary to make sure that, within the range you are interested in, they will reach a high degree of precision (for a glass/PDMS bonding and a plasma using surrounding air, you need a pressure between 300 and 1000m Torr).

Uniformity of the plasma within the chamber

Most of the plasma cleaners have been designed to provide some relatively uniform plasma. Except for some rare and somewhat out of date models, most of the plasma cleaners make, on this particular point, some very good plasma when they work with a chamber totally deprived of any object. However, in your case, you will probably use transport plates for your devices (to keep your PDMS blocks from sticking to the borosilicate chamber of your plasma, for example). In this case, don’t forget to ask the manufacturer in which part of the chamber the plasma will be in optimal condition (back chamber, middle or front part of the chamber). If you use microfluidic devices with metal connectors, be very careful, because the presence of metal can have an influence on the uniformity of plasmas.

Power adjustment of the plasma cleaner

As it is available on most of our models, the possibility to adjust the power of the plasma will allow you to finalize stronger processes on the length of time you can use. Besides, with equal conditions, a good choice of power will allow you to multiply by two the adhesion strength between the glass and the PDMS.

Size of the plasma cleaner chamber

It depends on what you need. A bigger chamber will allow you to make more chips simultaneously and for example to make bilayer PDMS microfluidic device on bigger wafers. However a bigger chamber will give more inertia to the pressure change, thus making any change in manual adjustment a little tricky. In case of automatic plasma regulators, we always recommend to choose the biggest chamber since the PID will regulate the pressure by itself.

The vacuum pump and the filters linked to the plasma cleaner

The possible presence of pollutants such as residual fuel oils coming from the oil of your vacuum pump will make your micro manufacturing much more unpredictable. You have to make sure that your pump and your filters won’t send any fuel oils to your plasma chamber. For the comfort of your staff members, choosing a very silent pump can also be an important asset. Do not overlook the vacuum pump issues, most laboratories we met previously met a lot of tricky problems with their plasma only because of vacuum pump contamination of the plasma chamber.

Electromagnetic accounting of the plasma cleaner

If you or your neighbours are doing very accurate electronics, be careful with the type of electronics your plasma cleaner uses in order to tune its frequency. Because according to the type of tuners they integrate, the plasmas can emit on a wide range of frequencies and can disturb your measurements and experiences.

Ease of use of your plasma cleaner

Most of the plasmas have been designed for a variety of applications. Choose plasma whose adjustments correspond to your expectations and that won’t make the formation of new ones even more complicated. Normally, for a glass/PDMS bonding a plasma cleaner including a power selector, an ON/OFF button and a pressure regulator should be enough.

Type of gas to be used inside your plasma chamber

You plan to make oxygen plasma? surrounding air plasma? The possibility of having several gas inlets will allow you to change the process when you’ll need it. Anyway the possibility of having several gas inlets is present on almost all the plasma cleaners on sale.