Our core competencies have grown over the past decades to understand the properties of gas plasma applied to organic layers on substrates used for the making of Semiconductor, MEMS or LED devices.
Plasma (also referred to as the fourth state of matter) is created by leading considerable amount of energy into a given gas mixture. This energy will be absorbed partially by the atoms and molecules of the gas mixture resulting in ionization these atoms or molecules. At the same time; radicals will also be created, which are electrically neutral but chemically highly active. The ions and radicals are used to facilitate the desired process result. A simple way to create plasma uses electrical energy to be added to the gas. Depending on the frequency, the resulting plasma has different properties. Most commonly the excitation uses either RF with 13.56MHz or Microwave with 2.45GHz with distinct differences in the properties of the resulting plasma process.
The number of free electrons is a measure for the activity of the gas combination in the plasma and it is directly related to the excitation frequency by laws of physics. Higher excitation frequency results in a higher density of electrons. This higher number of electrons creates more ions and radicals from the gas mixture inside the plasma. From this relation the comparison of excitation frequency MHz to Microwave can be derived with the result that the electron density of a Microwave driven plasma is much higher compared to the electron density of a plasma with MHz excitation. Theoretical data and actual measurement is different plasma vessels are shown in the diagram on the right.
DC Bias Voltage
Another feature of plasma driven by high frequency is the self-biassing effect. This effect is caused by the difference in mass and mobility of electrons compared to ions. With the much heavier ions not being able to follow the alternating electrical field from the excitation voltage, a difference in the net charge of the plasma appears. The result is a DC potential, which develops between the driven electrode and ground potential. The value of this DC potential or DC Bias voltage depends on process parameters such as power and pressure, but also on the difference in surface area between cathode and anode. As the excitation frequency increases, the difference in mobility between ions and electrons becomes less prominent; hence the resulting DC Bias voltage also decreases.
Physical and Chemical Etch
The result of an etching process is always a combination of a physical effect (Ion bombardment due to DC Biassing) and a chemical reaction of ions and radicals with the surface material. Since the process using MHz excitation is dominated by the physical effect, the result is greatly anisotropic with a preferred etching direction along the electrical field of the biassing voltage. Using GHz excitation, the process result is very isotropic with a strong domination of the chemical reaction. So it is important to use the process combination of physical and chemical effect s to match the application at hand.