This invention relates to semiconductor wafer plasma processing and particularly to inductively coupled plasma (IPC) and other processes in which multiple sources of RF power are present.
In certain equipment used to deposit thin films on semiconductor wafers in the manufacture of semiconductor devices or to etch the films on, or surfaces of, such wafers, plasmas are often generated for processing the wafer. Such plasmas include inductively coupled plasmas (ICPs) and plasmas that are otherwise coupled into the processing space of a vacuum chamber of the equipment from a primary RF source. Often, too, a secondary RF source is used to apply bias to the semiconductor wafer being processed in the chamber by the plasma. Ions are extracted from the plasma energized by the primary RF source by a DC field within a sheath that forms between the wafer and the plasma as a result of the RF bias that is applied to it from the secondary RF source. These ions modify the surface of the wafer, for example, by coating it, by etching it or by modifying the properties of a film that has been or is being deposited. In some cases, the deposition material itself is ionized, for example, as in the case of ionized physical vapor deposition (iPVD), so that material being deposited on the wafer can be accelerated normal to the wafer surface.
In such equipment, it is helpful to control the DC bias on a substrate, particularly in a high density inductively coupled plasma system where the plasma of such systems can itself lead to the appearance of an otherwise unwanted DC bias on the wafer. The unwanted bias can lead to detrimental effects on process performance. Furthermore, where RF bias from a secondary source is used to deliberately create a bias on the wafer, the additional bias from the plasma makes the required bias power for an optimum process develop a dependence on the ICP behavior. Variations in ICP properties can lead to a complex dependence of the process on the relative ICP and bias powers as well as other process parameters, including operating pressure.
Primary plasma properties are dependent on certain process parameters. For example, increasing process pressure in the range below 100-200 mTorr will generally increase plasma ion density and lower electron temperature. Increasing the power applied to the plasma from the primary RF source usually increases the ion density and electron density of the plasma. The effects seen on the wafer are a function of these plasma properties combined with the effects of the secondary RF power that is applied to bias the wafer. This secondary power, referred to herein as the xe2x80x9cbias powerxe2x80x9d, generates a xe2x80x9cbias voltagexe2x80x9d on the wafer whose magnitude depends on the electron temperature and the plasma density, via the xe2x80x9cplasma potentialxe2x80x9d.
In the development of a process, it is desirable that the effects of changing process parameters be separable. That bias power should significantly affect the plasma is undesirable because this renders the system more complex and its understanding more difficult to master. That the primary RF power to the plasma and its excitation system have the effect of changing the effective bias voltage on the substrate is also undesirable. The results of process changes are easier to predict when the primary plasma power source can be regarded as an essentially passive reservoir of ions that are then extracted by the DC component of the sheath field, and it can be assumed that the primary power to the plasma is not itself a source of bias power to the substrate.
In most plasma systems of this type, however, this is not the case. Instead, the primary power source for the plasma leads to the appearance of RF voltage on the wafer and the wafer supporting chuck, at both the fundamental frequency of excitation of the primary plasma source (the xe2x80x9cprimary frequencyxe2x80x9d) and its harmonics. The coupling of the fundamental frequency from the primary plasma power source to the wafer often occurs by capacitive coupling, with the effect of transmission of the voltage variations of the exciting primary power electrode being through the plasma to the wafer on the support. Harmonics are transmitted from the primary power source to the wafer because of the non-linear nature of the sheath on capacitive coupling, as well as from effects such as the fluctuation in plasma density as a function of time, and the RF Hall effect that is present in cases where high RF magnetic fields exist. The combination of these effects leads to an effective RF source within the plasma that can affect parameters such as the DC wafer bias which are critical for process control.
Accordingly, there is a need to eliminate the mutual dependence of RF plasma properties on substrate bias potential, and for a technique by which the source of RF plasma power and substrate bias power can be decoupled, reducing the effect of plasma power and other plasma parameters on wafer bias, thereby improving process control.
A primary objective of the present invention is to improve process control in a biased substrate plasma processing system. A more particular objective of the invention is to provide a method and apparatus by which the effects of the plasma power RF source and substrate bias can be decoupled to reduce the effect of plasma power on the wafer bias.
According to principles of the present invention, unwanted parameter coupling between the plasma and the substrate is controlled, allowing for simpler process development and leading to more reliable and repeatable plasma processes.
According to certain embodiments of the invention, a technique is provided that includes establishing a high density plasma adjacent to a semiconductor wafer by excitation of the plasma at an RF frequency, preferably at between 50 kHz and 50 MHz and applying RF power from a bias power source to a wafer support or chuck that exhibits high capacitance between the RF feed of the bias power source and the wafer. The RF power to the substrate support is preferably applied through a matching unit at a frequency that is identical to or close to that of the frequency of the primary power to the plasma.
According to further embodiments of the invention, filters are added between the bias RF power source connection to the substrate support and ground to eliminate harmonics of the plasma power source. Preferably, the filters are series resonant filters or notch filters, the resonant frequencies of which are preferably set to harmonics of the plasma power source frequency to suppress voltage components on the substrate support at these frequencies.