Diffusion Control

Control is the key to guarantee process quality and reproducibility.

The definition of the   control parameters for each manufacturing step is an important task. Knowing what you need to control will give to you the ability not only to choose the machinery to do it, but also the understanding about the importance and the influence of this specific production step in your product.

Semiconductor is a complex manufacturing industry. It relies not only in one engineer area, but in at least 5 different engineering areas. That´s why there are so many controls inside this industry. Everything is interconnected and to have a great device, everything needs to be done with high accuracy and well monitored.

If something goes wrong in one step, that will compromise all the other manufacturing process.

There are several ways to control the hole process of the semiconductor industries that are: visual, mechanical, electrical, that are nondestructive and there a few that are destructive testing.

The main objective of this post is to talk about diffusion control.

The diffusion control is always based on some parameters: wafer resistivity, diffusion depth, surface concentration, diffusion uniformity and defects.

The wafer resistivity is associated to the substrate characteristics. Introducing impurities inside a substrate, new structures with different resistivities will be created. Based on the impurity concentration and junction depth the substrate resistivity changes. This property is used to control the diffusion depth.

The diffusion depth is important to control the diffusion characteristics, to guarantee the depletion layer length once it is associated to the reverse voltage drawn in the project for deep diffusion, for shallow diffusion it is more difficult to get junction depth.

I would like to observe that there are effects, which are associated to diffusion quality, such as the wafer surface color after the diffusion due to phosphorous staining, the wafer bowing, the presence of cross diffusion. Of course, that can´t be used as a control but can visually indicate that something happened.

There are techniques used to control resistivity of the silicon wafer as the diffused layer and diffusion depth. The most used techniques are the Spread Resistance Probe and 4-point probe test. For diffusion depth we can use angle lapping and staining. For shallow junctions you may use SIMS (secondary ion mass spectrometry) that is the most accurate measurement but also the most expensive junction depth test.

The Spreading Resistance Probe is an equipment used to obtain quantitative measurement of the local resistivity of the semiconductor from the surface of the wafer to junction. It is used in mapping concentration of the diffused species in silicon wafers and in obtaining thickness of the diffused junction, epitaxial layer, and ion-implanted layers obtained during the semiconductor device processing.

The Four Point Probe purpose is to measure the resistivity of any semiconductor material. It can measure bulk or thin film V/I.

The angle lapping is a destructive test where the wafer is cut, glued in a stainless steel substrate, then lapped until reach a pre chosen angle which allow us to see the color difference between each diffused layer after the staining. By doing this it will be possible to measure the thickness of each layer.