Lithography

Traditionally, lithography is a method for printing on a smooth surface.

In nanoscience, lithography can be used to print different patterns on the different layers of our devices.

The most known types of lithography are Optic lithography and Electron beam lithography.

Optic Lithography (Photolithography)

Optic Lithography (Photolithography) uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical (photoresist, or simply "resist") on the substrate.

For example, we want to print a square of 100um x 100um on a Si wafer.

The simplest way, is a 1 resin optic lithography.

We need :

-the Si substrate

-a spin coater

-a mask (containing the negative of the pattern we want to print)

-an UV insolating machine

-photo resistant resin (photoresist)

Step 1. Applying resin

We clean the Si substrate end we depose a layer of resin.

The simplest way to do this, is by using a spin-coater. The resine is applied in the centre ot the wafer, then the substrate is spun at high speed ( 3000rmp for an usual resin). The resin will distribute itself in a uniform layer.

Step 2. Exposure and developing

We use a mask containing the model we want to print , and we irradiate the sample.

The principle is that ultra-violets rays irradiate the resin in the regions where we want to deposit the patterns(the transparent sections in the mask).

The resin from the irradiated regions is removed using a developer agent, then rinsed with a solvent.

The result is squares of resin on a a Si Wafer.

Usually we don’t need to print some resin on our substrates, but we need to print metal patterns.

So another 2 steps are needed :

Step 3. Metal deposition .

Using (for example) a metal vapour deposition reactor, we can depose a layer of Ti on our resin covered wafer.

Step 4 . The lift-off process.

To remove the metal on the resin, we put the wafer into a solvent The acetone attacks the resin which is exposed at the separation lines. The resin is dissolved and the layer of metal which covers it is removed. The process is augmented by bathing the recipient in ultrasonic bath. After the excess metal is removed, the result is as shown bellow.