Frequency Monitoring to Detect Plasma Process Abnormality

A method of signaling when a variable-frequency RF power supply is supplying RF power to an abnormal RF impedance within a plasma chamber, comprising the steps of:

• providing a sample-and-hold circuit having a sample input, a control input and an output;
• coupling to the sample input a frequency control signal that represents the frequency of RF power being supplied by an RF power supply;
• coupling to the control input a sample control signal having at least a first state and a second state;
• the sample-and-hold circuit maintaining at the output of the sample-and-hold circuit the value of the frequency control signal at the most recent time when the sample control signal changes from the first state to the second state;
• establishing a first lower frequency limit and a first upper frequency limit in response to the output of the sample-and-hold circuit such that the first lower frequency limit and the first upper frequency limit are respectively less than and greater than the frequency represented by the output of the sample-and-hold circuit;
• repeatedly comparing the frequency represented by the frequency control signal to the first lower frequency limit and the first upper frequency limit so as to determine whether the frequency represented by the frequency control signal is less than the first lower frequency limit or greater than the first upper frequency limit;
• and
• producing a first alert signal when the comparing step determines that the frequency represented by the frequency control signal is less than the first lower frequency limit or greater than the first upper frequency limit.

A method of signaling when a variable-frequency RF power supply is supplying RF power to an abnormal RF impedance within a plasma chamber, comprising the steps of:

• providing a plasma within a plasma chamber;
• providing a frequency control signal;
• supplying RF power from an RF power supply to the plasma, wherein the RF power supply establishes the frequency of the RF power in response to the value of the frequency control signal;
• adjusting the value of the frequency control signal in response to reflected RF power between the plasma and the RF power supply;
• providing a sample-and-hold circuit having a sample input, a control input and an output;
• coupling the frequency control signal to the sample input;
• coupling to the control input a sample control signal having at least a first state and a second state;
• the sample-and-hold circuit maintaining at the output of the sample-and-hold circuit the value of the frequency control signal at the most recent time when the sample control signal changes from the first state to the second state;
• establishing a first lower frequency limit and a first upper frequency limit in response to the output of the sample-and-hold circuit such that the first lower frequency limit and the first upper frequency limit are respectively less than and greater than the frequency represented by the output of the sample-and-hold circuit;
• repeatedly comparing the frequency represented by the frequency control signal to the first lower frequency limit and the first upper frequency limit so as to determine whether the frequency represented by the frequency control signal is less than the first lower frequency limit or greater than the first upper frequency limit;
• and
• producing a first alert signal when the comparing step determines that the frequency represented by the frequency control signal is less than the first lower frequency limit or greater than the first upper frequency limit.