Sources of Substrate Potential

The most common versions of ion plating use a potential on the substrate surface to accelerate ions to the surface. The potential on the surface can be applied by several methods.

Applied Bias (dc). A dc potential can be applied directly to an electrically conducting surface. Bombardment will be relatively uniform over flat surfaces where the equipotential field lines are conformal to the surface, but it will vary greatly if the field lines are curved, because ions are accelerated normal to the field lines. The dc discharge that is generated will fill the chamber volume if the pressure is sufficiently high. For pressures too low to establish a dc discharge, a magnetron configuration can be used to enhance the plasma over the surface of a web strip passing over the magnetron surface.

In the application of a dc potential, often the applied voltage and measured current (power expressed in watts/cm2) to the surface is used as a process parameter and control variable. However, it must be realized that the bombarding ions generally have not been accelerated to the full applied potential due to the position of their formation, charge exchange collisions, and the physical collisions in the gas. The measured current consists of the incident ion flux (the ions may be multiply charged) and the loss of secondary electrons from the surface. The cathode power is a useful process parameter to maintain reproducibility only if parameters such as gas composition, gas pressure, system geometry, and so on are kept constant.

Applied Bias (rf). An rf potential (e.g., 13.56 MHz) must be applied to a surface if the surface is an insulator. Otherwise, charge buildup on the surface will result in arcing over the surface or through the insulating layer if it is thin (Ref 62). When applying an rf potential, the potential of the surface in contact with the plasma will vary continuously, although it will always be negative with respect to the plasma. The dc component of the bias will depend on the presence of blocking capacitance in the circuit and whether a dc bias supply is present. The energy of the ions that bombard the surface will depend on the frequency of the rf source and the gas pressure. Maximum bombardment energy will be attained at low frequencies and low gas pressures. When using rf sputtering as a vapor source, a different rf frequency and power may be used on the substrate (Ref 63).

The rf bias has the advantage that it can establish a discharge in the space between the electrodes at a pressure lower than that required in a dc bias. It has the limitation that the rf electrode is like a radio antenna, and the plasma density formed over the surface depends on the shape of the substrate/fixture system. In some cases, the substrate/fixture should be surrounded by a "cage" to smooth out the electric field and give a more uniform plasma density. In all cases, ground shields should be kept well away from the rf electrode, and in the case of an insulator, the insulator should completely cover the rf electrode.

Applied Bias--dc plus rf. A dc potential and an rf potential can be applied at the same time if an rf choke is used in the dc circuit to prevent the rf from entering the dc power supply. By applying a dc bias, the insulating surface is exposed to bombardment for a longer period of time during the rf cycle.

Self-Bias. A negative self-bias is induced on an insulating or floating surface due to the higher mobility of the electrons compared to that of the ions. The higher the electron energy, the higher the negative self-bias generated. Figure 6 shows a technique for inducing a high self-bias by accelerating electrons away from a source and magnetically confining them so that they must bombard a substrate surface. It is possible to generate a positive self-bias if the electrons are prevented from bombarding the surface by using a magnetic field, because ions can reach the surface by scattering and diffusion. For example, substrates in a postcathode magnetron sputtering system can have a positive self-bias.

Fig. 6 Generation of a high self-bias and a plasma using accelerated electrons, an electrically isolated substrate holder, and a confining magnetic field. The vaporization source is a differentially pumped e-beam evaporator.
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