In this chapter we will see various deposition techniques that are used in microelectronic fabrication. If a material like copper or tungsten has to be deposited onto the silicon wafer, there are multiple ways to achieve that goal. The deposition methods used in semiconductor industry can be divided into four groups.
- Physical Vapor Deposition (PVD)
- Chemical Vapor Deposition (CVD)
- Electrochemical Deposition (ECD)
- Spin-on coating
Among these four processes, PVD and spin-on coating are purely physical processes. CVD and ECD are chemical processes. That means in CVD and ECD, chemical reactions occur.
In IC manufacturing, the actual quantity of material to be deposited in any single step is very low. The deposited material will be in the form of a thin film, almost like the painting on a surface and in reality much thinner than the paint coating.
Thin film requirements: Any deposition method must satisfy certain requirements:
- The deposition must be uniform throughout the wafer
- A very good control is necessary. If we want 1000 nm of the material deposited on the wafer, then the variation must be within one or two percentage. i.e. the thickness of the film after deposition must be at the minimum 980 nm and at the most 1020 nm.
- In the places where trenches or vias are made, the side wall coverage must be good. This is explained in the later section (figure 3.11)
- The material should adhere to the wafer well and should not peel off
- Dust particles should not fall onto the wafer during the deposition process
- The crystal structure of the film deposited must be of sufficient quality because it will affect the properties of the film. For example, when copper is deposited, we need large grain size since it will result in less electrical resistance.
- If we are depositing alloys, then the composition must be uniform throughout the process
In general, PVD is used to deposit titanium, titanium nitrate, tantalum, tantalum nitrate, aluminum and a very thin film of copper called seed layer. CVD is used to deposit tungsten, titanium, again copper seed layer, silicon di-oxide, silicon nitride, etc. Electrochemical deposition is used for depositing copper. Spin-on coating is used to deposit insulators or dielectric materials, which are usually organic in nature.
Historically, some of the metals were evaporated and then the vapor was deposited onto the wafer. The wafer would be kept at lower temperature and the vapor will cool down and form the solid metal. This methods used large amount of energy and had other difficulties. It could not be used for all materials, and particularly for alloys. The material will also deposit on other places like the chamber walls. Hence, it is not currently used for IC manufacturing.
If the raw material is taken in gaseous form and the final material is deposited on the wafer after reaction, it is called chemical vapor deposition (CVD). If the material is taken in solid state, but sent as very small particles or atoms and deposited on the wafer without any reaction, it is called physical vapor deposition (PVD). We will first see the details of the PVD. |
