Patent ID: 7902991

Claim:
Apparatus for signaling when a variable-frequency RF power supply is supplying RF power to an abnormal RF impedance within a plasma chamber, comprising: a variable-frequency RF power supply having an output and having a frequency control input connected to receive a frequency control signal, wherein the RF power supply produces RF power at its output having a frequency equal to a frequency represented by the frequency control signal; a sample-and-hold circuit having a sample input, a control input and an output, wherein the sample input is adapted to receive the frequency control signal, wherein the control input is adapted to receive a sample control signal having at least a first state and a second state, and wherein the sample-and-hold circuit maintains at its output 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; and a first comparator circuit adapted to receive the frequency control signal, wherein the first comparator circuit repeatedly compares the frequency represented by the frequency control signal to a first lower frequency limit and a first upper frequency limit and produces a first alert signal when the frequency represented by the frequency control signal is less than the first lower frequency limit or greater than the first upper frequency limit; wherein the first comparator circuit is connected to receive the output of the sample-and-hold circuit; and wherein the first comparator circuit establishes the first lower frequency limit and the 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.