The following will attempt to describe the photolithography process. The creation of a MOSFET serves as an example. To this day, the photolithography process plays a crucial role in the production of integrated circuits.
Creation of a MOSFET
The goal is to build a MOSFET. This basically looks like this:
How can such structures, which are tiny, be made? This requires 20 steps, which are explained below.
The basis of a MOSFET is the substrate. In most cases this is a silicon crystal. Today, a silicon disk is used, which is called a wafer. It is pre-doped. In our case it is a p substrate
Step 1: OXIDATION
The surface is oxidized, creating a layer of non-conductive silicon dioxide
Step 2: NITRID
A layer of nitride is now vapor deposited onto the oxide layer. This layer will later serve as a so-called mask.
SCHRITT 3: FOTO resist
A layer of photo resist is applied to the nitride layer. This is exposed to light using a mask.
Step 4: Wash Out
The unexposed areas of the photo resist are washed out, similar to a photographic negativ.
Step 5: Etching
The nitride layer is etched away through the gaps around the photo resist
Step 5: Remove the Photo REsist layer
The photo resist layer is chemically removed. The nitride mask remains.
Step 6: Thickening the OXIDATION
Wet oxidation specifically increases the oxidation in the gaps in the nitride layer in order to later achieve insulation of the component.
Step 7: Removement of the Nitride Layer
The nitride layer is chemically removed
Step 8: Polycrystalline silicon
Polycrystalline silicon is vapor deposited. This material is conductive and later corresponds to the “metal gate” of the MOSFET
Step 9: application of a new Mask
As described in steps 3-4, a photo mask is applied at the position of the later gate.
Step 10: Etching
By etching away the polysilicon layer, the gate is created under the resist layer.
Durch wegätzen der Polysiliziumschicht entsteht das Gate unter der Lack Schicht.
Step 11: mask removal
The gate is now in the desired position, isolated from the p-doped base substrate.
Step 12: OXIDe Layer
Another layer of oxide (green) is vapor deposited which insulates the gate.
Step 13: DOping
N-doped regions are created with phosphorus ions.
Step 14: OXIDe Layer
A thick oxide layer is applied to insulate later metal layers.
Step 14: MASK
Another Mask Layer is applied.
Step 15: Etching
The Oxide Layer are etched away.
Step 16: REmoval of the Mask
Step 17: Metalization
Vapor deposition of aluminum is applied, to create the contacts to the doped areas and the gate.
Step 18: Application of a Mask
Application of another mask for etching
Step 19: Etching
Final Step 20: Removal of the mask
Damit ist der MOSFET fertig mit seinen drei Anschlüssen. Sie können entweder nach „außen“ geführt werden oder wie bei integrierten Schaltungen mit weiteren Metallschichten weiterverdrahtet werden.
The MOSFET is now complete with its three connections. They can either be routed “outside” or, as with integrated circuits, further wired with additional metal layers.