Dr Paul May, S103A, email: email@example.com
In this lecture we shall look a bit more closely at some of the fundamental physics and chemistry occurring within the low pressure plasmas used for semiconductor processing. We shall mainly be concerned with the energies of the various species, such as electrons and ions, within the plasma, and their effect upon etching characteristics. When considering RF plasmas, it is convenient to divide the system into two separate regions, the bulk plasma and the sheath (около электродная область). It is reasonable to do this, since the physics occurring in these 2 regions is very different. We shall also limit our discussion to parallel plate RF plasmas, since these are the most commonly used.
One of the most important concepts in the description of electrical discharges is the idea of a sheath region surrounding any surface in contact with the plasma. This includes the electrodes, the substrate (which normally sits on one of the electrodes), and any probe we may wish to insert into the plasma for diagnostic purposes. It is the difference between the electrical potential between the plasma region and this sheath region that directly leads to positive ion bombardment of the surface, and hence to etching.
If an electrically isolated substrate is inserted into a plasma, it will initially be struck by positive ions and electrons. The flux of each species, however, will be unequal as a result of the much higher speed of the electrons, and the substrate begins to charge negatively with respect to the plasma. This excess negative charge density around the substrate is called the space-charge, or sheath. The substrate will continue to charge negatively until the electron flux is reduced by repulsion just enough to balance the ion flux. The potential held by an isolated substrate in the plasma is known as the 'floating potential', Vf, since the potential 'floats' to a value sufficient to maintain an equal flux of positive and negative species.
Except around disturbances such as these, the remainder of the plasma is at equipotential. This potential is termed the 'plasma potential', Vp. For DC plasmas, Vp is constant, but for AC or RF plasmas, Vp oscillates with the voltage waveform applied to the powered electrode, V0(t). The potential difference between the floating potential and the plasma potential (Vp-Vf) is the 'sheath potential'. This is the magnitude of the energy barrier which an electron must surmount (преодолевать) in order to reach the substrate. It is also the potential through which a positive ion is accelerated onto the substrate.
Diagram illustrating the relationship between the applied RF voltage, V0, the plasma potential, Vp, and the sheath voltages developed at either electrode. Note these are shown for only 2 instants in time, when V0 is at its most negative (top) and most positive (bottom), and for the case of equal area electrodes (see later).
Since the sheath is a region over which a negative potential is dropped, electrons are rapidly expelled (вытеснять) from the sheath. This loss of electron density results in a reduction of electron impact excitation reactions in that region. Since such reactions lead to fluorescent emission from excited species, the sheath region does not glow as much as the plasma bulk. So the substrate is surrounded by a comparatively dark space, which may be a few µm to a few mm in size depending upon the plasma conditions.
All surfaces in contact with the plasma are surrounded by a dark space sheath, including the electrodes and walls of the chamber. Most of these surfaces are usually grounded (for safety!), and so their corresponding sheath potentials at any instant in time are equivalent to the plasma potential. The powered electrode however, is capacitively coupled to a generator which provides a potential which oscillates sinusoidally on an RF timescale. Therefore the powered electrode sheath potential can be very large (several hundred V) and will oscillate in magnitude with the applied RF. The sheath will also expand and contract with the RF.
The RF generator supplies a sinusoidal waveform to the powered electrode, and the plasma potential rises and falls in phase with this. Most RF systems have a variable capacitor between the RF generator and the plasma. This is because in order to ensure maximum power transfer
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