High Density Plasma Deposition: Overview

In our discussion of high density plasma oxide films, we saw that in order to obtain the desired results of high deposition rate, and good gap filling and planarity (and thus high sputter rate), we need to build a reactor that provides very high plasma densities (exceeding 1010 electrons/cm3). High sputter yield also requires that we accelerate ions in the sheath to several hundred volts, so we need a high plasma potential (at least relative to the wafer). We'd like this all to happen at a well-controlled temperature, much less than 400 °C for intermetal dielectric applications, and we want to have well-controlled stoichiometry throughout the deposition. These requirements force us to move to radically different reactor designs from the showerhead plasma.

Much lower pressures (a few mTorr) are needed to minimize scattering in the sheath and obtain high-energy, directional ions for sputtering; low pressure also helps us achieve good uniformity at high gas flows. At these low pressures, showerhead reactors cannot achieve high plasma density, as the energetic sheath electrons pass right through the plasma; instead, we use more exotic approaches such as electron-cyclotron resonance or inductive excitation. Such methods often place severe demands on reactor materials and design.

The requirement for high ion flux at high ion energy means that hundreds or even thousands of watts of power are dissipated on the wafer, which is in a vacuum environment where heat transport due to conduction and convection is negligible. With radiative equilibrium alone, the wafer will rapidly heat up to above 600 °C. Some means of cooling the wafer is needed; generally, the approach adopted is to dispense several Torr of helium on the back of the wafer. This gas would cause the wafer to float away from the chuck, so a clamping mechanism is needed, but a mechanical clamp generates particles and disturbs the plasma. An electrostatic chuck is the solution, but involves more non-trivial material and procedural challenges.

Process control is of great importance: if the RF potential is turned on before the silane gas reaches the chamber, the edges of the metallization will be rapidly sputtered, causing dimensional loss and leakage from metal sputtered onto the oxide surface between the lines. If the silane is turned on before the RF potential is applied, the initial layers will not be sputtered and will have poor gap fill properties, perhaps making it impossible to fill a high-aspect-ratio gap. Run/vent plumbing configurations (in which gas from the mass flow controllers is directed into the exhaust until needed, and then immediately directed into the chamber with valves immediately adjacent to the gas dispense) can be useful.

Thus, there are many technological challenges to fabricating a working, practical HDP reactor. In the remainder of the discussion we will look in some more detail at the challenges and their solutions.

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