Plasma processing systems have long been employed to process substrates, such as silicon substrates or other substrates, to form semiconductor devices, nano-machines, etc. Plasma processing is particularly effective in etching applications, wherein a plasma formed from an etchant source gas mixture is employed to selectively etch a layer on the substrate. To etch a substrate in a plasma processing system, the substrate is disposed inside a plasma processing chamber, and an etchant source gas is flowed into the chamber. The etchant source gas is ignited to form a plasma, which then exposed etch areas of the target layer on the substrate, i.e., areas not protected by a mask. Plasma is ignited and sustained using RF energy, which is provided to one or more electrodes in the plasma processing chamber.
In a capacitively coupled plasma processing system, one or more electrodes may be provided in each processing chamber. RF energy having various frequencies may be provided to one or more of the electrodes of the capacitively coupled plasma processing system. For example, if there are provided an upper and a lower electrodes, 2 MHz, 27 MHz and 60 MHz RF signals may be provided to one or both of the electrodes.
Some etch applications may require that the upper electrode be grounded with respect to one or more of the RF frequencies. To clarify, the upper electrode in a plasma processing chamber represents the electrode that is opposite to the substrate and separate from the substrate by a plasma cloud during plasma processing. The lower electrode represents the electrode on which the substrate is disposed for processing. For example, certain etch applications require the upper electrode to be grounded with respect to the lower frequency RF signal (e.g., 2 MHz). Another etch application may require the upper electrode to be grounded with respect to the higher frequency RF signal (e.g., 27 MHz and/or 60 MHz). Still another etch application may require the upper electrode to be grounded with respect to all of the RF signal frequencies (e.g., 2 MHz, 27 MHz, and 60 MHz). Since a given chamber may be employed for different etch steps, each of which may have a different grounding requirement, the ability to selectively ground the upper electrode with respect to one or more or all of the RF frequencies is highly desirable.
In the prior art, a relay and an inductor combination may be employed to ground one or more of the RF frequencies. FIG. 1 shows such an example wherein relay R1 is employed to control the current flow through an inductor L1. When relay R1 is closed, current flows through L1. L1 functions as a filter that, depending on the value of L1, effectively filters out certain frequencies from the output OUT.
It has been found that the use of the relay/inductor is unsuitable for certain processing systems due to the space constraint imposed by the relay/inductor combination. Further, it has been found the relay/inductor arrangement for grounding RF signals tends to be more effective for RF signals at the lower frequency ranges and tends to be less effective for RF signals at the higher frequency ranges. This is because the high impedance presented by the inductor L1 renders the combination less effective for grounding high frequency RF signals (e.g., 60 MHz RF signal). Yet, in some etch applications, effective grounding of selected or all RF signals is a requirement for good process results (e.g., good plasma uniformity and good etch rate uniformity).
What is desired, therefore, is an arrangement for effectively grounding, in a selectable manner, all the RF signals that are supplied to one or more electrodes of the plasma processing chamber. More preferably, an arrangement is desired to effectively ground all RF signals supplied to the upper electrode in a plasma processing chamber.