Plasma etching

Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral (atoms and radicals). During the process, the plasma will generate volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.

Mechanisms

Generation of the Plasma

A plasma is a high energetic condition in which a lot of processes can occur. These processes happen because of electrons and atoms. To form the plasma electrons have to be accelerated to gain energy. Highly energetic electrons transfer the energy to atoms by collisions. Three different processes can occur because of this collisions:

• Excitation
• Dissociation
• Ionization

There are different species in the plasma present such as electrons, ions, radicals and neutral particles. Those species are interacting with each other constantly. Plasma etching itself can be divided into two main ways of interactions:

• The generation of the chemical species
• The interaction with the surrounding surfaces

Without a plasma all those processes would occur at a higher temperature. There are different ways to change the plasma chemistry and get different kinds of plasma etching or plasma depositions. One of the excitation techniques to form a plasma is by using RF excitation of a power source of 13.56 MHz.

The mode of operation of the plasma system will change if the operating pressure changes. Also, it is different for different structures of the reaction chamber. In the simple case, the electrode structure is symmetrical, and the sample is placed upon the grounded electrode.

Influences on the process

The key to developing successful complex etching processes is to find the appropriate gas etch chemistry that will form volatile products with the material to be etched as shown in Table 1. For some difficult materials (such as magnetic materials), the volatility can only be obtained when the wafer temperature is increased. The main factors that influence the plasma process:

• Electron source
• Pressure
• Gas species
• vacuum

File:Halogen-, hydride- and methyl-compounds in plasma etching.png

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Surface Interaction

The reaction of the products depend on the likelihood of dissimilar atoms, photons, or radicals reacting to form chemical compounds. The temperature of the surface also affects the reaction of products. Adsorption happens when a substance is able to gather and reach the surface in a condensed layer, ranging in thickness (usually a thin, oxidized layer.) Volatile products desorb in the plasma phase and help the plasma etching process as the material interacts with the sample's walls. If the products are not volatile, a thin film will form at the surface of the material. Different principals that affect a sample's ability for plasma etching:

• Volatility
• Adsorption
• Chemical Affinity
• Ion-bombarding
• Sputtering

Plasma etching can change the surface contact angles, such as, hydrophilic to hydrophobic or vice versa. The Argon plasma etching has reported to enhance contact angle from 52 deg to 68 deg, and, Oxygen plasma etching to reduce contact angle from 52 deg to 19 deg for CFRP composites for bone plate applications. Similarly, the plasma etching has reported to reduce the surface roughness from hundreds of nanometers to as lower as 3 nm for metals.

Types of plasma etching

The pressure is one factor that influences a plasma etching process a lot. For plasma etching to happen, the chamber has to be under low pressure which means less than 100 Pa. In order to generate low pressure plasma the gas species which is used has to be ionized. The ionization happens by a glow charge. Those excitations happen by an external source, which can deliver up to 30 kW and frequencies from 50 Hz (dc) over 5–10 Hz (pulsed dc) to radio and microwave frequency (MHz-GHz).

Microwave plasma etching

Microwave etching happens with an excitation sources in the microwave frequency, so between MHz and GHz. One example of plasma etching is shown here.

File:Microwave plasma etching.png

A microwave plasma etching apparatus. The microwave operates at 2.45 GHz. This frequency is generated by a magnetron and discharges through a rectangular and a round waveguide. The discharge area is in a quarz tube with a inner diameter of 66mm. Two coils and a permanent magnet are wrapped around the quarz tube to create a magnetic field which directs the plasma.

Hydrogen plasma etching

On form to use gas is the plasma etching is hydrogen plasma etching. Therefore an experimental apparatus like this can be used:

File:Hydrogen plasma etching.png

A quarz tube with an rf excitation of 30 MHz is shown. It is coupled with a coil around the tube with a power density of 2-10 W/cm³. The gas species is H₂ gas in the chamber. The range of the gas pressure is 100-300 um.

Applications

Plasma etching is currently being used to process semiconducting materials for their use in the fabrication of electronics. Small features can be etched into the surface of the semiconducting material in order to be more efficient or enhance certain properties when used in electronic devices. For example, plasma etching can be used to create deep trenches on the surface of Silicon for uses in Microelectromechanical systems. This application suggests that plasma etching also has the potential to play a major role in the production of microelectronics. Similarly, research is currently being done on how the process can be adjusted to the nanometer scale.

Hydrogen plasma etching, in particular, has other interesting applications. When used in the process of etching semiconductors, hydrogen plasma etching has been shown to be effective in removing portions of native oxides found on the surface. Hydrogen plasma etching also tends to leave a clean and chemically balanced surface, which is ideal for a number of applications.

See also

• Plasma etcher
• Plasma cleaning
• List of plasma (physics) articles

External links

• Plasma Etch Fundamentals
• http://stage.iupac.org/publications/pac/pdf/1990/pdf/6209x1699.pdf