Eight Major Steps to Semiconductor Fabrication, Part 5: Etching a Circuit Pattern

In the previous part of the series, we covered the photolithography, or “photo,” process in which circuit patterns were drawn on the wafer surface. Now, the wafer is ready go through the etching process to have any unnecessary materials removed so that only the desired patterns remain on its exterior.

An art in itself

Etching is a method widely used in print-making for illustrations that require elaborate and detailed lines. Rembrandt, known as “the master of light and shadow,” and Goya, a leading artist of the late 18^th century, both used etching techniques in their artwork.

In etching, an anti-corrosive material is coated on a metal plate, then carved out with sharp tools like scribes and needles 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. The exposed area is etched away, while the remaining area remains intact.

The Etching Process

Similarly, the etching process in semiconductor fabrication uses a liquid or gas etchant to selectively remove unnecessary parts until the desired circuit patterns are left on the wafer surface. By repeating this process on multiple layers, a semiconductor chip is eventually born.

The two fundamental types of etchants are liquid-phase (“wet”) and plasma-phase (“dry”), and the method used depends on which material is to be etched. Whereas engraving utilizes sharp tools to physically carve out patterns, etchants are used in semiconductor manufacturing to remove the undesired portion of the wafer while the circuit patterns are protected with an anti-corrosive layer formed during the photo process.

Compared to the wet etching technique, dry etching is more costly and complicated. However, with continued innovation in semiconductor technology and the circuitry now in nanoscale, dry etching is the more widely used technique, as it produces a higher yield.

Dry etching to rid of unnecessary parts

So how does dry etching remove the unnecessary substrate materials?

Dry etching, also called plasma etching, starts with the generation of plasma. 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 a neutron or molecule has changed its state of electrical charge by either losing or gaining electrons.

The Generation of Plasma

When a magnetic field is applied to gas, its free electrons become energized and start bumping into neighboring neutrons or molecules. This collision, or ionization, of free electrons and neutrons produces even more free electrons.

This chain reaction of ionization, called the avalanche effect, causes the number of ions to increase exponentially, resulting in 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 that we need to pay special attention to during the dry etching process.

The first is to maintain uniformity of the etching speed throughout the wafer’s surface. If the time it took to etch varies in 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, which refers to the amount of surface material 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 reaction to occur.

In addition, selectivity and profile are also considered to be important elements of dry etching. All these factors must be closely controlled so as to improve the overall yield.

Now you know a bit more about the etching process that constructs the circuit patterns of semiconductors. In the next part of our series, we’ll take a look at how a semiconductor wafer gets its electrical properties.

In Korean, http://samsungsemiconstory.com/152.