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There are three categories of thin film silicon deposition: chemical reaction, physical vapor, and hybrid processes. Physical vapor deposition involves evaporating material from a source and transferring it into thin-film silicon layers on a target substrate. Chemical vapor deposition is more accurate but slower and uses toxic gases. Each method has its own benefits, costs, and risks. New products like dye-sensitized solar cells offer a less dangerous and less expensive approach to producing thin films.
There are dozens of different methods for deposition of thin film silicon, but they can generally be divided into three categories. There are chemical reaction deposition processes, such as chemical vapor deposition, molecular beam epitaxy, and electrodeposition. Physical vapor deposition is a deposition process in which only one physical reaction takes place. There are also hybrid processes using physical and chemical means, including sputter deposition and gas or glow discharge methods.
Physical vapor deposition is related to the variety of sputtering technologies used and involves evaporating material from a source and transferring it into thin-film silicon layers on a target substrate. The source material is evaporated in a vacuum chamber, causing the particles to disperse equally and coat all surfaces of the chamber. The two methods used for physical vapor deposition are electron beams, or e-beams, to heat and evaporate the source material, or resistive evaporation using a high electric current. Sputter deposition uses a partial vacuum charged with an inert but ionized gas, such as argon, and the charged ions are attracted to the target materials used, which break apart the atoms which then deposit on the substrate as thin film silicon. There are many different types of sputtering, including reactive ion, magnetron, and cluster beam sputtering, all of which are variations on how ion bombardment of the source material is performed.
Chemical vapor deposition is one of the most common processes used to produce thin film silicon and is more accurate than physical methods. A reactor is filled with a variety of gases, which interact with each other to produce solid by-products which condense on all surfaces of the reactor. Thin-film silicon produced in this way can have extremely uniform characteristics and very high purity, making this method useful for the semiconductor industry as well as in the production of optical coatings. The downside is that these types of deposition methods can be relatively slow, often require reactor chambers operating at temperatures up to 2,012° Fahrenheit (1,100° Celsius), and use very toxic gases, such as silane.
Each of the dozens of different deposition processes must be considered when producing thin-film silicon, as each has its own unique benefits, costs, and risks. The first reactive ion chambers were suspended from the floor of the laboratory to insulate them, as they had to be charged to 50,000 volts and could short out computer equipment even if they simply sat on concrete nearby. The twelve-inch diameter copper pipes that ran from these reactors into the bedrock beneath the production floor, were colloquially known as “Jesus sticks” by lab workers, in reference to the fact that anyone who touched them would speak to Jesus. as it would have killed him or her. Products such as dye-sensitized solar cells offer a new, less dangerous and less expensive approach to producing thin films, as they do not require precise silicon semiconductor substrates and can be produced at much lower temperatures of about 248° Fahrenheit (120° Celsius).
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