In semiconductor manufacturing plasma ashing is the process of removing the photoresist (light sensitive coating) from a wafer. Using a plasma source, a monatomic (single atom) substance known as a reactive species is generated. Oxygen or fluorine is the most common reactive species. The reactive species combines with the photoresist to form ash which is removed with a vacuum pump.
Typically, monatomic oxygen plasma is created by exposing oxygen gas at a low pressure (O2) to high power radio waves, which ionise it. This process is done under vacuum in order to create plasma. As the plasma is formed, many free radicals are created which could damage the wafer. Newer, smaller circuitry is increasingly susceptible to these particles.
Two forms of plasma ashing are typically performed on wafers. High temperature ashing, or stripping, is performed to remove as much photo resist as possible, while the “descum” process is used to remove residual photo resist in trenches. The main difference between the two processes is the temperature the wafer is exposed to while in an ashing chamber.
Application of Plasma Ashing
The plasma asher was first used to remove photoresist, which is made of organic compounds, used in the microelectronics industries. The role of the plasma asher follows the process of the plasma etching of wafers. Plasma ashing systems are environmentally friendly due to the fact that they don’t require any harmful chemicals. As we are aware that in semiconductor technology to remove resist plasma process is being widely used across the globe but except of that plasma ashing system can be used in other important applications too.
- Surface treatment of Plastics – A number of applications of plasma involve the surface treatment of plastic materials, prior to a subsequent process. An example is the treatment of reinforcing fibres that are to be integrated into an epoxy structure. Treatment in an oxygen plasma for say, five minutes at 50-100W, increases surface roughness. These pitted fibres enhance adhesion and a good mechanical bond is produced with enhanced rigidity and strength. Plasma processing of plastics can also convert a hydrophobic surface to a hydrophilic surface. This type of treatment usually requires short exposure (3-5 minutes) at low power (50 watts). This sort of reaction has been applied to the assembly of ink pens to improve the speed of ink filling and transfer.
- Plasma Polymerization – Plasma polymerization refers to the polymerization of active species generated in a plasma. For example, the introduction of polysiloxanes onto hard contact lenses improves the hydrophilic nature of the surface. An application in the soft drinks industry using CF4, to create a fluorinated surface on PET and polypropylene bottles, making bottles less pervious to carbon dioxide.
- Ashing of Biological material food stuffs – Plasma ashing has also been successfully used to ash materials as varied as post-mortem lung tissue (for asbestos), bread (to determine type and distribution or iron) and specimens of prepared food (for asbestos and manmade mineral fibres). Specimens need to be dried prior to ashing and their size kept to a minimum.
- Detection of metals in Blood – Plasma ashing as a pre-treatment for atomic absorption analysis (AAS) is another well-established application. In this case one is normally looking for metals such as lead, cadmium, zinc and mercury in trace quantities in organic materials such as vegetables, dairy products or animal tissue. A specific example involves the treatment of multiple specimens of human blood exposed to CF4/02 plasma. The organic materials in this application can be removed in 15 minutes, leaving only the metallic contaminants to be analysed for cadmium.