# Photolithography: How to Make a Circuit in 7 Easy Steps

Level 3 (with higher mathematics)
Level 3 requires the basics of vector calculus, differential and integral calculus. Suitable for undergraduates and high school students.
Updated by Alexander Fufaev on
1. On a substrate (e.g. a silicon wafer) a material layer is applied using a deposition process (e.g. done by PVD, MBE or LPE). This layer of material will later serve as a conducting trace through which an electric current can flow.

2. In the next step, a layer of photoresist is applied to the substrate. The photoresist is a plastic that can be affected by light. There is a positive photoresist, where the irradiated areas are removed, and a negative photoresist, where the NON-irradiated areas are removed. In addition, the substrate is rotated to distribute the photoresist poured onto it evenly over the material layer.

3. Then a mask is placed in front of the photoresist with the traces cut out (so the mask contains the shape of the traces). The mask should be placed as close as possible to the photoresist to minimize light diffraction at the gaps. You can even place the mask completely on the photoresist to prevent light diffraction. However, this usually breaks the mask (expensive).

4. Now the mask is illuminated with a specific light wavelength. Visible light, UV light, synchrotron radiation or even electron radiation can be used for this purpose. After irradiation, only the areas of the photoresist that were not irradiated (positive photoresist) or only the areas that were irradiated (negative photoresist) remain.

5. Then the areas where the photoresist has been removed are etched away (wet etching, dry etching) so that only the resist layer on the traces behind it (material layer) remains.

6. In the final step, the remaining resist is removed by repeated irradiation. What remains is a finished integrated circuit - consisting of the substrate and the integrated conducting traces.

## Which parameters influence the integrated circuit size?

The resolution limit $$w$$, i.e. how close two traces can be placed next to each other, you determine with the following formula:

Formula: Resolution limit
Formula anchor

The resolution limit depends on the light wavelength $$\lambda$$ with which the mask is irradiated. In addition, the refractive index $$n$$ of the medium between the mask and the material layer plays a role, as does the aperture angle $$\alpha_{\text{max}}$$. However, the resolution limit is also influenced by the imaging system used (e.g. use of lens systems), which is taken into account in the factor $$k$$.