Demonstration of broadband radio frequency sensing: Empirical polysilicon etch rate estimation in a Lam 9400 etch tool

The sensitivity of a novel broad frequency band (1-2.25 GHz) radio frequenc y sensing system to plasma etching process conditions is demonstrated. This is accomplished by using the sensing system to estimate polysilicon etch r ate in a Lam 9400 etch tool. A designed experiment varying physical and che mical reactive ion etching regimes was performed with five repetitions at e ach experimental point. A model relating broadband sensor response to etch rate was regressed using four repetitions of the data and validated on the fifth. Two representations of the broadband data were considered separately when regressing the models, with subset selection used in each case to cho ose the best predictor variables. In one representation, the sensor data wa s considered as a vector of 402 real numbers corresponding to magnitude and phase of reflection coefficient at each of 201 frequencies, resulting in a n R-2 of etch rate estimate of 0.997. In the other, the broadband response was parameterized on the basis of a multimodal cavity resonance model. The inferred parameters of natural frequency, quality factor and resistance wer e then used as the predictor variables for regression, resulting in an R-2 of 0.962. (C) 2000 American Vacuum Society. [S0734-2101(00)10904-2].