How to make a spin-coated PDMS layer?

Introduction

A thin layer of PDMS is called a PDMS membrane. The PDMS membrane has several properties which make it quite valuable.

Thanks to the permeability of PDMS, the membrane can be used to exchange gas or small molecules between two liquids or a gas and a liquid without direct contact. What’s more, thanks to the softness of PDMS, a membrane can be used to create valves such as Quake valves.

Other applications can be imagined, everything will mainly depend on the thickness of the layer that defines the diffusion and the softness of the layer. We are going to see here how to make a PDMS membrane and manipulate it. The PDMS membrane can be done with few equipment that you can find in our full Soft-lithography station.

The process for PDMS membrane fabrication is composed of 4 main steps

1. The preparation of the substrate
2. The sacrificial layer realization
3. The PDMS membrane realization
4. The PDMS membrane release

1. PDMS membrane: The preparation of the substrate

The PDMS membrane is generally made on a wafer which will be used in a spin coater to realize the PDMS membrane. Before use the wafer has to be cleaned. A classic cleaning is recommended as described here.

If you are in a clean room, you can clean it with piranha solution (H2SO4+H2O2), outside a clean room you can use acetone. The cleaning can be optional if you are sure about the state of your wafer but strongly advised. In any case, you have to heat your wafer to remove all moisture from the surface. We advice 15min at 120°C in an oven for example. If needed, a plasma treatment (O2 or Air plasma works fine) during 5 minutes will increase the spreading.

Spin coating parameters:

2. PDMS membrane : The sacrificial layer realization

To be able to remove the PDMS from the substrate, the best way is to use a sacrificial layer that will be removed at the end of the process to release the PDMS.

 You can use several materials for the sacrificial layer, we use some AZ4562 photoresist because it is easy to handle and works perfectly.

To create the AZ4562, spin coat the photoresist (1ml per inch of the substrate), you will obtain a thin and planar layer. Then bake it at 100°C during 2 minutes. There is no need to have a temperature ramp for heating or cooling down, so you can directly remove the wafer from the hot plate after the 2 minutes.

There is no need to expose the photoresist, the wafer is already ready for the next step.

3. The PDMS membrane realization

The PDMS membrane is made by spin coating the PDMS on the substrate, so before that, the PDMS has to be prepared. It means, mixing of the monomer and of the curing agent (10/1) and degassing the PDMS. You can find more information on these steps thanks to our tutorial on “How to make a PDMS chip“.

The most relevant step is now, the spin coating of the PDMS since the speed, the acceleration, and the time of the spin coating will define the thickness of the layer you want.

You will be able to have some information on what kind of layer you might obtain according to the parameters you are using in the following graphic.

4. The PDMS membrane release

Now you have your membrane at the desired thickness but it is quite difficult to handle and remove from the wafer.

You just have to put the wafer in a acetone solution and wait the dissolution of the AZ4562. It will take few minutes (2-3min), then you will have your PDMS membrane floating on the surface.

Note that the best is to keep the PDMS membrane into water to easily manipulate it since it is really sticky.