Blog: Controlling a Plasma Chamber with Advanced RF Generator Signals

Blog: Controlling a Plasma Chamber with Advanced RF Generator Signals

Reno Sub-Systems, Inc. (Reno) is recognized as a leading supplier of differentiated RF matching network and RF generator products that are enabling the semiconductor industry to move to sub-7nm process nodes. Reno uses a solid-state Electronically Variable Capacitor (EVCTM)-based matching network for which its technologies are backed by a broad patent portfolio. Reno has also patented RF generator technology that allows development of a complete solid-state solution including RF generators ideally suited to operate with EVCTM matching networks that delivers precise process control.

 

The Transformation of the RF Signal

The RF matching network is positioned between the RF generator and the plasma chamber to transform the plasma impedance to a value suitable for the RF generator. In the semiconductor fabrication processes, the RF generator has a nonreactive 50 ohm output impedance, and the RF power is transmitted through coaxial cables that also have a nonreactive 50 ohm characteristic impedance. The impedance of the plasma chamber varies based on RF power, gas chemistry, gas flow and gas pressure, among other things. This varying plasma impedance must be transformed to nonreactive 50 ohm impedance for maximum power transmission from the RF generator. The RF matching network performs this task of continuously transforming the plasma impedance to 50 ohm for the RF generator. Most RF matching networks use variable capacitors for transforming this impedance. The value and size of the variable capacitors are influenced by the power handling capability, frequency of operation and impedance range of the plasma chamber.

 

The predominant variable capacitor in use in RF matching networks is the vacuum variable capacitor (VVC). The VVC is an electromechanical device that consists of two concentric metallic rings that move in relation to each other to change the capacitance. In complex semiconductor processes, where the impedance changes are very rapid, the rapid and frequent movements put stress on the VVC’s mechanical parts, leading to their failure. VVC-based RF matching networks are one of the last electromechanical components in the semiconductor fabrication process.

 

As semiconductor devices shrink in size and become more complex, the processing time to fabricate these features becomes smaller, typically in the 5- to 6-second range. Current RF matching networks take 1-2 seconds to tune the process, and this results in unstable process parameters for a significant portion of the process time. Electronically variable capacitor (EVCTM) based solid-state matching network technology (see U.S. Pat. No. 7,251,121) enables a reduction in this semiconductor processing tune time from 1-2 seconds to less than 500 microseconds. Because an EVCTM is purely an electronic device, it is not a one-for-one replacement for a VVC. As such, advancements are being made to the RF generator to take full advantage of using EVCTMs as part of an RF matching network.

 

Reno’s Advances in RF Generator Technology Make for More Efficient and Reliable RF Generator Power

Reno’s RF generators are some of the most compact generators in the industry, and include several advanced features that improve the reliability of the RF generator as well as the precision of power delivery.

 

Reno’s generators dynamically change the load-line of the RF generator to reduce the power loss in the RF generator, thus improving the power delivery profile as well as improving the reliability of the generator. The load-line change is accomplished by maintaining the power set point while adjusting the drive and the DC rail that feed the RF amplifiers. This adjustment is transparent to the output and does not impact the power set point.

 

For semiconductor processes that use multiple RF frequencies, the plasma chamber mixes those frequencies and the detector circuit at the output of the RF generator samples this frequency mix. To precisely control the power, the generator must separate other frequencies from its operating frequency. The same circuit that we use to separate the operating frequency from this mixture also works when the RF signal is pulsed, like many of today’s most demanding processes are. Reno’s proprietary, dual-stage heterodyne circuitry enables a faster slew rate response from the detector stages of the control circuit. This provides a considerably more accurate measurement of pulsed RF signals.

 

Reno’s detector and control circuitry can also process those detected signals further, such as through fast Fourier transforms, and provide additional insight into the semiconductor process that can then be used to control the power using artificial intelligence and machine learning.

 

Reno’s RF generators, in combination with Reno’s EVCTM-based RF matching networks, can detect fast transients at the output of the RF matching network. These transients may represent micro-arcing in the plasma chamber that the etch or deposition tool would never identify. Reno’s RF generator can then control the power to the chamber to eliminate the micro-arcing.

 

Reno’s RF generators and EVCTM-based matching networks can work together in several ways. One of them is with the generator set to sweep the frequency to tune the reactive portion of the plasma impedance, for which the EVCTM-based matching network adjusts the capacitance values to tune the resistive portion of the plasma impedance. Reno’s approach to combine the frequency tuning option with its solid-state EVCTM technology provides an option to fully tune the plasma impedance, compared to standard frequency tuning approaches that can only tune limited plasma impedances, and then only the reactive part of the plasma impedance. Reno’s RF generators and EVCTM-based matching networks provide additional tuning options, such as sequential tuning, where the frequency tuning and EVCTM tuning are done sequentially for faster, repeatable and more stable tuning of the plasma impedance.

 

Combined RF Generators and EVC Match

Reno has developed RF generators (PrecisTM), which are precisely integrated with our EVCTM matching networks.   These are available with multiple powers and frequencies.

 

Reno has also developed a totally integrated RF generator & matching network unit (GenMatchTM), which is all contained in one compact housing for the ultimate RF system. This unit can perform multiple matches, powers and frequencies. This unit is the industry’s first complete solid-state RF delivery system with 1.25kW, 13MHz RF generator and matching network all in one box.

 

This provides for precise control of process power with higher efficiency and better control of chamber transients, such as micro-arcing. The GenMatchTM unit is the same size as our existing EVCTM-based RF matching network unit, which consequently saves tool space and requires fewer I/Os and utility interfaces to the tool.

 

Reno’s RF Generator Patents

This series of patents has pioneered the technically superior methodology of using control circuitry to integrate the RF generator with the matching network.

 

  • US 9,345,122 HIGH EFFICIENCY OPERATI0N MODE FOR RF AMPLIFIERS AND RF GENERATORS VIA VARIABLE DC RAIL, Priority May 2014

The RF generator has a control unit that alters the DC voltage to decrease power dissipation and enables output power to be equal to or greater than the power set point.

 

  • US 9,543,122 METHOD FOR CONTROLLING AN RF GENERATOR, Priority May 2014

Determine a power dissipation at the RF generator and alter the DC voltage to a final DC voltage that decreases the power dissipation at the RF generator while enabling the output power at the RF output to be substantially equal to the power set point.

 

  • US 9,728,378 METHOD FOR CONTROLLING AN RF GENERATOR, Priority May 2014

This patent incorporates the role of an RF control algorithm to keep the output power at the power set point.

 

  • US 9,591,739 MULTI-STAGE HETERODYNE CONTROL CIRCUIT, Priority May 2014

A method for processing and detecting RF signals for purposes of providing feedback to an RF generator using a detection stage wherein a control circuit is configured to mix a filtered heterodyne signal to control the RF signal in response to the DC signal.

 

  • US 9,745,660 METHOD FOR CONTROLLING A PLASMA CHAMBER, Priority May 2014

Using information provided by a sensor at plasma chamber input, the RF generator can be protected from abrupt rate of change in RF voltage, RF current and phase angle of load impedance. Upon detection of a repetitive change condition, the system regulates the power of RF signal provided to chamber.

 

  • US 2019/0326094 IMPEDANCE MATCHING USING ELECTRONICALLY VARIABLE CAPACITANCE AND FREQUENCY CONSIDERATIONS, Priority July 2018

This patent application describes a method for matching using independent EVC and frequency control.

 

  • US 10,727,029 IMPEDANCE MATCHING USING INDEPENDENT CAPACITANCE AND FREQUENCY CONTROL, Priority July 2018

This patent describes separate control circuits for controlling EVCTM capacitance and frequency.

 

  • US 2020/0066488 IMPEDANCE MATCHING NETWORK AND METHOD, Priority Nov 2018

This patent application describes a method for frequency and EVCTM tuning with VVC adjustment to keep the frequency within range.

 

  • US 2020/0168439 FREQUENCY BASED VACUUM VARIABLE CAPACITOR ADJUSTMENT, Priority Feb 2019

This patent application describes a frequency-based VVC adjustment with control of the RF frequency separately done by the RF source.

 

Why Is Reno’s Approach Better?

Reno’s patented PrecisTM and GenMatchTM RF generators, combined with our EVCTM matching networks, provide enhanced processing capabilities, unrivaled plasma stability, with on-plasma repeatability of 0.5ms or less. This reduces match time, widens the process window, increases plasma stability and delivers the exact amount of RF power to every wafer, every lot, on every tool. Also, this run-to-run repeatability and accuracy of Reno’s match technology enables the precise, high-aspect-ratio, selectively anisotropic sharp-edge plasma processing required for today’s and tomorrow’s devices, including 3D structures.