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

Abstract

The sensitivity of a novel broad frequency band (1–2.25 GHz) radio frequency sensing system to plasma etching process conditions is demonstrated. This is accomplished by using the sensing system to estimate polysilicon etch rate in a Lam 9400 etch tool. A designed experiment varying physical and chemical reactive ion etching regimes was performed with five repetitions at each 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 choose the best predictor variables. In one representation, the sensor data was considered as a vector of 402 real numbers corresponding to magnitude and phase of reflection coefficient at each of 201 frequencies, resulting in an R2 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 were then used as the predictor variables for regression, resulting in an R2 of 0.962.

Oxide etch rate estimation using plasma impedance monitoring

“…Very few publications have described the use of plasma impedance data for the prediction or estimation of the plasma etch rate. Garvin and Grizzle describe their broadband radio frequency sensing technology to empirically and parametrically estimate the etch rate using a multiple regression technique and a stepwise approach to subset selection to reduce multicollinearity [1]. The biased coefficient of multiple determination was used to quantify the performance of regression models in a 5 run experiment in which the 5 th run was estimated from the first four yielded a R 2 equal to 0.997 and 0.962 for the empirical and parametric regression models, respectively.…”

A transmission-line microwave interferometer for plasma electron density measurement

“…It is therefore desirable to develop a microwave sensor which uses only one vacuum port and has a transmission-line structure to avoid the multipaths problem. Resonant-type microwave sensors based on the principle of plasma oscillation excitation [6], plasma surface wave absorption (plasma absorption probe (PAP)) [7], transmission-line resonator (hair-pin probe) [8] or cavity resonance [9] have been developed and even employed to monitor the electron density of plasma-based processes [10] or as the sensor for feedback control [11]. In contrast to the reflectiontype measurements in these sensors, a transmission version of the PAP [12], the plasma transmission probe (PTP) [13], as well as a transmission-type technique detecting the cutoff of the microwave propagating through the plasmas (wave cutoff method), has also been demonstrated recently [14].…”