Part 4, Drawing Structures in Nano-scale

In the previous part, we introduced the oxidation process and integrated circuits. Now, let’s move on and find out about the photolithography process, through which electronic circuit patterns are transferred onto the wafer. The photolithography process is quite similar to taking a photo using a film camera. But how?

Photolithography, Similar to Printing a Black and White Photo

Photolithography is called “photo” for short. The photolithography process got its name for transferring the circuit design onto a wafer by exposing the patterned mask to light. Making a replica on a wafer is like printing a black and white negative on light-sensitive paper. As the circuit patterns become increasingly denser, the chip components also need to be made smaller using high-precision nano-scale processes. Making these finer circuit patterns is entirely up to the photolithography process, and therefore, high-precision and advanced photolithography technology is essential as the chips become smaller.

Preparing to Draw a Circuit Pattern on a Wafer

Let’s take a closer look at the photolithography process. The first step is to use computer-aided design (CAD) software and devise the circuits to be drawn onto the wafer. On this canvas of an electronic circuit pattern, a precisely designed pattern from the engineers will end up, determining the density of semiconductor chips.

Creating a Photomask, like a Negative Film

▲ Photomask

The circuit pattern design is recreated as a photomask with the microscopic circuit embodied using chromium onto a glass substrate made of ultra-pure quartz. A patterned substrate called a photoreticle, more commonly known as a photomask, which works like a negative film, is created. The photomask is designed much larger than the size of the circuits for more precise patterning, and is optically shrunk onto the wafer using a lens. The photolithography process consists of photoresist application, stepper exposure, and development. Let’s see what they are.

Making the Wafer Surface into a Photographic Print

Now we are ready to draw some patterns on the wafer. First, photoresist (PR, highly sensitive to light) is evenly applied on the wafer surface. This makes the wafer like photographic paper for printing photos. The PR layer needs to be thin, even, and highly sensitive to ultraviolet rays to produce high-quality and precise circuit patterns.

Transferring Circuit Design to the Wafer by Exposure to Light

After a wafer is prepared with the PR layer, it then goes through the stepper where the circuit design on the patterned mask is projected and transferred onto it with ultraviolet light. This process is called “stepper exposure.” Due to the scale in semiconductor manufacturing, the area exposed to light is highly controlled and selective.

Development to Form Circuit Patterns

The last step of the photolithography process, development, is comparable to the process for developing photographs. This step is critical because it determines the final shape of the circuit patterns on the wafer. As developing solutions are sprayed onto the wafer, certain areas are selectively removed to create the final patterns. The photoresist can either be positive or negative depending on its reaction to light. Areas with positive photoresist are removed during the developing process when exposed to light, while those with negative photoresist remain. The development step concludes the photolithography process. Wafers are carefully inspected using various instruments, including an optical microscope, and only wafers that pass all the inspections can move on to the next step. We looked into the photolithography process where precise circuit designs are traced onto the wafer. Stay tuned for the next part of the series, where we will see how the desired circuit patterns are created on the wafer by selectively removing unnecessary materials in the etching process.