Plasma can be also called the fourth state of matter. Plasma is distinct from the other three states of matter in that it comprises free disassociated ions and electrons in a balanced steady state condition.
Features of Plasma
Certain specific features of plasma are listed below:
- In a disassociated state, plasma consists of ions, free radicals, unexcited molecules and electrons.
- The ion density of plasma is defined as the ratio of ions to the remaining molecules
- The plasma comprises an equal number of positive and negative charges, hence neutralizing the charge making it zero.
- Plasma processing may be described as a method of converting non-reactive molecules into electrically charged reactive molecules
- The electron energy of the plasma is based on the ionization rate.
- Normal plasmas have an ionization rate of 0.001% while high-density plasmas (HDPs) have an ionization rate of around 1%.
The Plasma Creation Process
Plasma is formed by introducing energy into matter. This is done in several ways as listed below:
- Electric Field
The following illustration explains how a neutral atom or molecule collides with an electron to produce an ion and another electron.
e- + A ---> A+ + 2e-
When an atom or a molecule collides with an electron it is excited to a higher energy state and remains in that state for a brief period and then returns to its original relaxed state. Energy gets released in the form of a photon. As different molecules or atoms emit light at different wavelengths, different gases show unique plasma glow colors. Hence the use of spectrometers is extremely useful for end point detection since wavelength peaks determine when a specific layer has been removed.
e- + A ---> A* + e-
A* ---> A + hv (photon)
Plasmas that find application in semiconductor processing are highly specific in nature. Processing semiconductor devices needs comparatively low temperature plasma. Hence the plasma is created by applying an electric field to conductive gases. Gases are electrically conductive and plasma are easily achieved atconsiderably low pressures around 1 Torr.
History of Plasma Reactors
Plasma processing was first introduced to the semiconductor industry in the 1960s. Initially barrel type systems were used to strip photoresists. Before this, wet chemical solvents that were carcinogenic and costly to dispose were used. Plasma processing is comparatively more efficient in removing positive resist, uses less chemicals and is more eco-friendly.
Types of Plasma Reactors
The different types of plasma reactors are listed below:
- Barrel Reactors
- Parallel Plate Reactors
- Reactive Ion Etching
- Hybrid Reactors
The salient features of barrel reactors are listed below:
- The initially built barrel reactors were inductively coupled and comprise a quartz bell jar turned on its side, with a coil around it as shown in Figure 1. As quartz chambers etch in fluorinated gas, these reactors were used only to strip photoresist with oxygen.
- More recent versions were capacitively coupled and comprise a cylindrical aluminum chamber with a concentric cathode inside as shown in Figure 2. Since aluminum is inert to fluorinated etch gas, these reactors can be used for etching
Fig 1. Inductively Coupled Barrel.
Fig 2. Capacitively Coupled Barrel.
Parallel Plate Reactors
Certain features of parallel plate reactors are listed below:
- Parallel plate reactors are capacitively coupled either top or bottom powered.
- Etching done in a top-powered reactor is called plasma mode or PE mode
- Etching done in a bottom powered reactor is called as reactive ion etching
A parallel plate reactor is shown in Figure 3.
Fig 3. Parallel Plate Reactor.
The Reinburg reactor shown in Figure 4 is a parallel plate reactor first developed in 1972. It is basically a bottom powered electrode system that is big enough to accommodate twenty five 100 mm wafers.
Fig 4. Reinburg Reactor.
Tegal introduced the first fully automated, single wafer parallel plate system to semiconductor production lines in 1979. Since single wafer systems are highly efficient, all production etch systems have the same configuration.
Reactive Ion Etching
The term Reactive Ion Etching is misleading. A nore accurate name for the technique would be Ion Assisted Etching. The ion percentage in a plasma is very small, and if only ions were participating then the total etch rate would also be very small.
The actual mechanism takes place in three steps as listed below:
- Chemical adsorption of reactive molecules or free radicals on the surface.
- Impact of an ion on the surface either reactive or not.
- Physical disassociation of many reaction by-products from the surface.
Until the late 1980s, reactive ion etch processes as shown in Figure 5 were the only plasma processing techniques available that created an anisotropic etch. This phenomenon is possible because of the chamber geometry, plasma physics and operating pressures of the systems. The two main things that define reactive ion etching are a bottom powered reactor and an operating pressure below 100 mTorr.
Fig 5. Reactive Ion Etch Configuration.
Hybrid Reactors (Triode, ECR and ICP)
The most recent type of reactors introduced for plasma processing include Triode, ECR (or downstream microwave) and ICP reactors. These are known as Hybrid reactors, since they introduce power into the reactor through a secondary source. This is beneficial since power can be added to the plasma without coupling it through the sample. These reactor types or secondary power sources excite the plasma to obtain high-density plasma (HDP). Hybrid reactors have comparatively less charge damage, sputtering, operating temperature; higher etch rate and selectivity.
This information has been sourced, reviewed and adapted from materials provided by Trion Technology.
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