Impedance matching option for Nova RF Plasma Generators

• Automatically Locates Best Operating Frequency for Optimum Power Transfer
• Eliminates Need for Complex Matching Network
• Improves Speed and Reliability

Automatic Frequency Tuning (AFT) is an option available for Spectrum, NOVA and other RF Plasma Generators. For processes which allow the frequency to be changed, AFT has the major benefits of both speed and reliability. This is due to the elimination of the movable tuning elements found in traditional matching networks. Nova generators include a Direct Digital Synthesizer (DDS) which allows the control system to adjust the output frequency of the generator. The DDS converts a crystal clock source to a digital Sinewave output using a sinewave lookup table. The crystal clock source ensures that the output frequency is extremely stable and repeatable.

AFT also has the benefit that the output has reduced harmonics (compared with a square wave) and requires no settling time when the frequency is changed, unlike phase locked loop schemes.

Nova generators also include a DSP-based control system which is tightly coupled to sensors such as temperature, forward power and reflected power. Since the DSP also controls generator frequency with the DDS, it is able to incorporate an algorithm which can adjust frequency in order to minimize reflected power.

The Tuning Process

To use AFT it is necessary to incorporate a fixed tuning element matching network between the generator and chamber. This network must be designed so that all process points can be tuned within a reasonable frequency range (typically ±10% for mid-frequency and +5% for high frequency).

Initial Tuning

The tuning process starts when the generator's RF is turned on. The algorithm simply moves in a direction which reduces the reflected power. When the reflected power reaches a pre-defined threshold (a typical value would be 50 Watts) the tuning algorithm changes modes to the minimum search mode.

Minimum Searching

Once below the pre-defined threshold, the tuning algorithm searches for the minimum reflected power, by watching for turning points. As the frequency is swept, the algorithm continuously updates a memory element which represents the lowest power "seen". If the reflected power increases by more than 3 Watts above this minimum, the algorithm assumes that it has moved through the minimum. Therefore, the frequency sweep direction is reversed in order to travel back to the minimum.

Halting

After the algorithm has reversed itself several times, tuning stops. At this point the algorithm monitors the reflected power to see if re-tuning is necessary.

Tracking

If the power rises by more than 3 Watts after the tuning process has been halted, the algorithm re-starts the minimum finding portion of the process to facilitate tracking as the chamber process gradually shifts.

Controllable Parameters

Several parameters are adjustable to allow optimization of the tuning process according to the different tuning situations that will be encountered.

Start Frequency defines the frequency at which the tuning process starts. Normally this frequency is chosen to give good ignition. To ensure the best tune time, the frequency should also be chosen so that tuning proceeds towards the ignited tune point.

Delay Time sets the time for which the algorithm remains at the start frequency. The delay time should be chosen to ensure that ignition is achieved reliably and the plasma is stable before the frequency starts to move.

Upper and Lower Frequency Limits set the bounds of the tuning range. These limits should be set to encompass all of the tune points that will be required for the processes being run. The limits should not be set wider than necessary because this may impact tuning time. In addition, the frequency limits should not include any FCC-regulated lockout frequencies.

Lockout Frequencies

To ensure that AFT does not interfere with emergency safety, search and rescue radio transmissions, the following frequency bands are blocked: 490kHz to 510kHz, 2170kHz to 2194kHz and 8354kHz to 8374kHz.

Need help?

Contact an Applications Specialist by sending an email to MKS ENI Products or call 585-427-8300.

© 2009 by MKS Instruments