Plasma etching and ashing

What is a Plasma
A plasma is a partially-ionised gas consisting of equal numbers of positive and negative charges and a different number of unionised neutral molecules. When a gas is subjected to a DC or radio frequency (RF) potential at reduced pressure this is usually accompanied by glow, which is known as glow discharge. The words glow discharge and plasma tend to be used synonymously, although glow discharges are not perfect plasmas - but for the purposes of this text they will not be differentiated. The characteristic glow of these plasmas is due to electronically excited species producing optical emission in the ultraviolet or visible regions of the spectrum and is characteristic of the composition of the glow discharge gas. For example, argon gives a bright blue colour and air or nitrogen gives a pink colour that is due to excited nitrogen molecules.

Ionisation
In the context of plasma-enhanced chemistry reactors, the plasma is created in a vacuum chamber, which contains a constant flow of a gas at reduced pressure - typically in the order of 1mbar. This gas is exposed to a radio frequency (RF) potential, which results in the partial ionisation of the gas. In the ionisation process, a bound electron in an atom is ejected from that atom. For example, the ionisation of an argon atom is expressed as follows: - Ar -> Ar+ + e

Excitation
A less dramatic transfer of energy allows the electron to jump to a higher energy level within the atom. This process is known as excitation. The excited state of an atom is conventionally shown by an asterisk: e + Ar -> Ar* = e

Dissociation
A further process that can occur is the dissociation of a molecule. If oxygen, for example, is the gas subjected to the RF potential, the oxygen molecule can be dissociated into two oxygen atoms, whereas a monatomic gas such as argon cannot be dissociated at all: e + 02 -> e + 0 + 0

A normal result of dissociation is an enhancement of chemical reactivity, since the products are usually more reactive than the parent molecule. Dissociation may or may not be accompanied by ionisation, for example: e + CF4 -> e + CF3 + F (Dissociation) or e + CF4 -> 2e + CF3+ + F (Dissociation)

Summary
Exposing a gas to the RF potential at reduced pressure creates a plasma which contains active species - for example, in the case of oxygen, atomic oxygen. Oxygen atoms will oxidise organic molecules more readily than oxygen molecules. So typically a cellulose material can be converted to carbon dioxide, carbon monoxide and water at room temperature, rather than at elevated temperatures (eg burning) and furthermore the oxidation is more controllable.

Types of Reactor System
There are many types of reactors available. They are all glow discharge systems but vary considerably in terms of excitation frequency (5kHz - 5GHz), operating pressure (1mbar - atmospheric pressure) and electrode arrangement.

In addition to barrel systems there are parallel plate reactors; these usually consist of a grounded plate onto which the specimens are placed and an insulated parallel plate to which the RF power is applied. The reverse of this arrangement where the specimens are placed on the non-grounded electrode is generally known as ‘Reactive ion etching’ (RIE).

Etching in this type of reactor is inherently directional, whereas the former can be both directional (anisotropic) or isotropic. The barrel reactor usually etches isotropically and is favoured for most plasma applications.

The barrel reactor, as the name implies, is a cylindrical container, which can be evacuated. The RF power, usually at 13.56MHz frequency is applied to the system via internal or external electrodes by capacitive or inductive coupling. This type of reactor is used for the plasma ashing process and also for the plasma etching process, although the disadvantage in the latter for some users is that the process is not completely isotonic so that undercutting can occur.

A barrel system: K1050X
The K1050X is a barrel system using capacitive coupling of the RF power to the gas. Barrel reactor cylinders can be made of Pyrex (borosilicate) glass, but the K1050X is fitted with quartz. The use of the quartz barrel and window option is very important to microelectronics users where boron contamination might be a problem. Borosilicate glass contains boron which might be extracted in a CF4 flow.

The barrel chamber electrodes are on the outside of the chamber in a quadrant format (see barrel reactor diagram). The work chamber is evacuated to about 0.05mbar and then the process gas is allowed to bleed into the chamber to obtain an operating pressure of about 0.5mbar. The RF power is then applied and the gas is ‘ionised’.

Under these conditions the chemical reactions occur and the subsequent volatile products are removed by the vacuum pump. Forward and reflected power levels are monitored. Forward power is the power into the work chamber, whereas the reflected power is the power reflected back into the power supply. The reflected power is a result of mismatching the power supply impedance with the reaction chamber impedance and this level should always be kept at a minimum. In practice, balance is achieved by tuning so that the forward power reaches a maximum as the reflected power drops to a minimum.

The K1050X has a built in ‘auto tuning’ facility that ensures that RF power is automatically impedance-matched to any variation in the system or loading - this is important as it gives faster reaction times, reproducibility of results and protects the RF power supply.

The K1050X has a 100mm diameter x 150mm quartz chamber, 100W variable power supply, two gas flow channels and metering of vacuum and power. The system is automatic; one button initiates a sequence of pump-down and ingress of process gas, the plasma is then switched on (for a period pre-set by the timer). At the completion of the process the chamber is vented to atmospheric pressure.