Diffusion is the uniform distribution of impurity atoms (dopants) into a silicon crystal lattice in a wafer. The purpose of introducing impurities is to change the electrical characteristics of silicon to make it either P-type silicon or N-type silicon. Some of the impurities used to produce N-type include: Antimony, Arsenic, and Phosphorus. Some of the impurities used to produce P-type include: Boron, Gallium, and Aluminum. Diffusion actually consists of two major steps, pre-deposition and the actual diffusion. Pre-deposition is the process of introducing a small amount of dopant into the surface of the silicon. Before this can be done, the surface of the silicon must be thoroughly cleaned to prevent any unwanted material from being added to the silicon crystal lattice with the dopant. After the cleaning, the silicon wafers are placed in a quartz boat into a pre-deposition furnace, this is quite similar to the oxidation furnace. There are three main methods to inject the dopant in the pre-deposition process. The dopant can be injected as a solid, liquid, or gas. When using a dopant in a solid state, nitrogen gas is used to carry a very fine dopant powder, that has been heated to a high temperature, into the furnace against the wafer. Sometime oxygen is added to partially oxidize the silicon when doping it. A typical furnace called a thermal reactor When using a dopant in a gas state, the dopant gas is mixed with nitrogen and send into the furnace against the wafer. This method is less favored since the dopant gas is harder to control than as a liquid or solid. When using a dopant in a liquid state, nitrogen gas carries a the dopant as a mist into the furnace against the wafer. In all three cases, the main controlling factor is the temperature of the furnace. The higher the temperature, the faster the dopant penetrates the silicon crystal lattice. The typical temperature used is between 700 to 1400 degrees Celsius. Other minor pre-deposition techniques include: spin-on dopants, a dopant in a chemical mixture is pored onto a wafer and the wafer is spun to allow for a uniform distribution; and low pressure chemical vapor, this is blowing a liquid dopant at a low temperature and pressure against the wafer. After the pre-deposition, the diffusion process occurs. This basically consists of growing a layer of silicon dioxide, about one micrometer, over the doped regions of silicon. This process helps the dopant to penetrate the silicon crystal lattice. To grow the layer of silicon dioxide, the wafer is placed on a quartz boat in a oxidation furnace. At a high temperature, oxygen is blown in the furnace at the wafer. After the silicon dioxide layer is grown, the wafer is cleaned and the doped regions are tested to how electrically resistant they are. |
