Part 5, Etching a Circuit Pattern

In the previous part, we covered the photolithography, or “photo” process, in which circuit patterns are drawn on the wafer surface. Now, the wafer is ready to go through the etching process to have any unnecessary materials removed so that only the desired patterns remain. Let’s take a look at the etching process this time, where patterns that form a semiconductor structure are made.

An Art in Itself

You may have all heard of etching as an art form. Shapes are carved on the surface of wood, copperplate, or stone and then printed out on paper or fabric using ink or paint. The etching process in semiconductor fabrication has principles that are similar to the etching technique in art. In etching, an anti-corrosive material is coated on a metal plate and then carved out with sharp tools to draw the desired design. The plate is then dipped in a corrosive material such as nitric acid, and the level of corrosion is controlled to form an image.

▲ The etching process

Similarly, the etching process in semiconductor fabrication uses a liquid or gas etchant to selectively remove unnecessary materials until the desired circuit patterns are left on the wafer surface. Materials in the areas without photoresist are removed by an etchant to form circuit patterns on the wafer. The photoresist is removed once the etching process is complete. This process of creating desired circuit patterns is repeated on multiple layers. Etching is categorized into wet etching and dry etching depending on the materials used in the etching process. Dry etching uses reactive gases and ions to selectively remove unnecessary materials, whereas wet etching uses chemical solutions. Compared to the wet etching technique, dry etching is more costly and complicated. However, with the latest advanced semiconductor technologies in nano-scale, circuits are getting finer and finer. Therefore, dry etching is more widely used, producing a higher yield.

Dry Etching to Remove Unnecessary Parts

So how does dry etching remove the unnecessary substrate materials? Dry etching is also called plasma etching. Plasma is generated by injecting gas into a vacuum chamber that has lower pressure than the air pressure and then by supplying electrical energy. Plasma is a state of matter—along with solid, liquid and gas—that consists of a large number of free electrons, ions, and neutrons or molecules in the form of ionized gas. When something is ionized, it means that a neutron or molecule has changed its state of electrical charge by either losing or gaining electrons.

▲ The generation of plasma

Plasma is generated when a sufficiently strong magnetic field created by electrical energy is applied to gas to make gas molecules collide and become ionized. In other words, free electrons are accelerated by the magnetic field, and the subsequent high-energy electrons collide with neutrons or molecules and ionize them. This chain reaction of ionization, called the avalanche effect, causes the number of ions to increase exponentially. And this is so-called a “plasma state”. A radical atom dissociated from this plasma state becomes volatile and moves itself away from the wafer surface, consequently peeling off surface material that was not previously coated and protected with photoresist. There are a couple of things to watch out for during the dry etching process. The first is to maintain uniformity. Uniformity shows the consistency of the etching speed throughout the wafer’s surface. If the etching speed varies at different areas of the wafer, then there will be inconsistencies in the etch depths. This could lead to malfunctioning chips or chips containing differing properties in certain areas. The second is etch rate. This refers to the amount of surface materials removed in a given amount of time. Etch rate can differ depending on the number of reactive atoms and ions, as well as the amount of energy the ions carry which causes the surface reactions to occur. Namely, we strive to control these factors to increase the overall yield. In addition, selectivity and profile are also considered to be important elements of dry etching. Now you know a bit more about the etching process that constructs the circuit patterns of semiconductors. Semiconductors, which are the epitome of integrated circuit technology, are manufactured by repeating the processes of applying a thin film of desired materials on the silicon substrate, applying a protective layer, and selectively etching the unnecessary parts. This entire process is conducted within a safely designed instrument. In the next part of our series, we’ll take a look at the “thin film” process, where a semiconductor wafer gets its electrical properties.