What are microfabricated electrodes?
Microfabricated electrodes are devices characterized by having:
- a very thin electrode, normally in the 100 to 2000 nm range
- a high purity electrode composition
- small features than SPEs, normally from 1 micrometer
How are microfabricated electrodes made?
Microfabricated electrodes are manufactured using MEMS (Micro Electro-Mechanical Systems) processes.
The MEMS fabrication processes encompass a wide variety of techniques. For microfabricated electrodes, the most common one is photolithography.
Photolithographic fabrication process
Fabricating electrodes via photolithography takes 5 steps:
Step 1. Photoresin deposition
The first step to make microfabricated electrodes is to deposit a photosensitive resin on the substrate where we will fabricate the electrode.
This deposition is done via a process known as spin-coating, followed by baking to harden the resin. Thanks to this process, it is possible to deposti a thin and homogenous photoresin film whose thickness can be controlled from a few nanometers to several microns.
Step 2. Photoresin exposure
After depositing the photoresin on the substrate, this is exposed to ultra-violet (UV) light.
This UV exposure creates a chemical change within the photoresin. As a result, the photoresin’s solubility to organic solvents changes in the regions exposed to UV light.
This exposure is performed via a photomask that contains the electrode design. Thanks to this photomask, it is possible to block UV light on selected regions and transfer the electrode design to the photoresin-coated substrate.
Step 3. Photoresin development
Once the substrate has been exposed to UV light and the electrode design has been transferred to the photoresin it needs to be developed.
Photoresin development consists in immersing the exposed substrate in a solvent, known as developer. This developer will selectively dissolve the resin, revealing the electrode design. As a result, the substrate will now have some exposed areas throughout the photoresin layer.
Step 4. Metal deposition
After the development, it is time to deposit metal to create the electrode. There are several techniques suitable for this purpose, but for this post we will focus on one called sputtering.
Sputtering is a physical vapour deposition (PVD) method in which the metal of choice is vapourized in a vaccum chamber and deposited onto the substrate.
Sputtering can be performed on both conductive and non-conductive substrates. After the process, the developed substrate will have metal coated on the exposed areas as well as the remaining photoresin.
Step 5. Lift off
This is the last step.
Since the sputtering results in a substrate completely covered in a thin metal layer, the excess metal needs to be lifted off the device.
To lift off the excess metal, the remaining photoresin needs to be dissolved completely. This is done by immersing the substrate in a resin stripper, which is normally an organic solvent like acetone.
After lift off, the excess metal is removed and the microfabrication process for the electrode is finished.
Advantages of microfabricated electrodes
Microfabricated electrodes, while a more expensive option than SPEs, have the following advantages:
- greater miniaturization capability
- less sample volume
- micrometric dimensional control
- scalable fabrication
- lower energy consumption