C. Gaman

A floating hairpin resonance probe technique for measuring time-resolved electron density in pulse discharge

We present an automated hairpin resonance probe for obtaining time-varying plasma electron density in a pulsed-magnetron discharge, operated with a 13.56 MHz radio-frequency source. When the resonator is placed in plasma, its characteristic resonance frequency in vacuum shifts to a higher value. From the frequency shifts, electron density is easily determined. By applying a fixed microwave frequency, the probe immersed in plasma resonates only at a specific time of the pulse waveform. At a different time of the pulse, the probe resonates at a different frequency. The procedure is automated using a Labview™ program, which increments the applied microwave frequency in small steps of the prescribed value and reads the corresponding resonance peak from an oscilloscope. The spatial and temporal electron density measured using this technique shows a sharp drop in density during the first few microseconds in the on-phase, followed by an increase in density as the discharge develops in the steady-state on-phase. The off-phase shows that decay in electron density at different rates is faster in the region where the magnetic field lines intersect the target. A quantitative model is described to explain different features observed in the experiment.

Direct measurement of spatial electron density oscillations in a dual frequency capacitive plasma

The spatio-temporal electron density oscillation in a narrow gap dual frequency (27.12 and 1.937 MHz) capacitive discharge has been measured for the first time by using a floating microwave hairpin resonance probe. By measuring the probe’s resonance frequency in a space and phase-resolved manner, we observe significant oscillation in electron density at both drive frequencies throughout the region between the parallel plate electrodes. The observed phenomenon is attributed to the influence of presheath electric fields of the opposing electrodes in alternate fashion.

Electron density modulation in an asymmetric bipolar pulsed dc magnetron discharge

This paper investigates the spatial and temporal variation in plasma electron density over a region between 5 and 10cm above the race-track region of a pulsed magnetron sputtering target. The pulse operation is performed using an asymmetric bipolar pulsed dc power supply, which provides a sequence of large negative “on-phase” voltage (−350V) and a small positive “reverse-phase” voltage (+10V) for 55% of the pulse duration (10μs). The electron density is measured using a floating microwave hairpin resonance probe. The results show electron expulsion from the target in the initial on phase, which propagates with a characteristic speed exceeding the ion thermal speed. In the steady state on phase, a consistent higher density is observed. A quantitative model has been developed to explain the resultant density drops in the initial on phase. While in the reverse phase, we observed an anomalous growth in density at a specific location from the target (d>7cm). The mechanism behind the increase in electron densit...

Real-time plasma control in a dual-frequency, confined plasma etcher

The physics issues of developing model-based control of plasma etching are presented. A novel methodology for incorporating real-time model-based control of plasma processing systems is developed. The methodology is developed for control of two dependent variables (ion flux and chemical densities) by two independent controls (27 MHz power and O2 flow). A phenomenological physics model of the nonlinear coupling between the independent controls and the dependent variables of the plasma is presented. By using a design of experiment, the functional dependencies of the response surface are determined. In conjunction with the physical model, the dependencies are used to deconvolve the sensor signals onto the control inputs, allowing compensation of the interaction between control paths. The compensated sensor signals and compensated set–points are then used as inputs to proportional-integral-derivative controllers to adjust radio frequency power and oxygen flow to yield the desired ion flux and chemical density...

Measurement of electron density in a laser produced plasma using a hairpin resonance probe

A floating hairpin resonance probe has been used for the first time to measure the spatial and time evolution of local electron density in a laser produced plasma expanding in vacuum. The measured variation in electron density agrees closely with the variation of ion charge density as measured with a time-of-flight planar Langmuir ion probe confirming the reliability of Langmuir probe in the laser produced plasma.

Observation of Electron Density Oscillations in Confined Plasma with Two Radio-Frequency Capacitive Sheath

Summary form only given. The spatial electron density oscillation in a symmetric, two radio-frequencies, 27.12 MHz and 1.937 MHz, confined- capacitive-coupled discharge has been measured using a floating hairpin resonance probe. By measuring ne in a space and phase-resolved manner, we observe oscillations of bulk electron density at both (1.937 MHz and 27.12 MHz) drive frequencies. With the probe placed within the region of the maximum low-frequency sheath extent, the expulsion of the electrons by the large low-frequency voltage is observed. When the probe is placed in the opposing sheath, the phase of the electron expulsion is shifted by half-cycle as expected. Near the mid-plane of the parallel plate electrodes, the plasma density oscillates twice per low-frequency cycle. The observed electron density oscillations are attributed to the expansion and the collapse of the radio-frequency sheaths, resulting in the spatial electron density variation between the discharge electrodes. The 27.12 MHz oscillation in electron density is observed to be higher in magnitude during the phase when 1.937 MHz sheath voltage has maximum in amplitude.

Influence of External Perturbations on a Real Time Plasma Control

Summary form only given. Influences of perturbers, like RF power and gas flow, on dual-radio frequency capacitive coupled discharge are reported. A control of plasma parameters is the key for plasma etching of dielectric films include ion-flux and gas-density of oxygen containing species in the plasma. The ion-flux is measured with an isolated collection area built into the electrode surface, biased to -18 Volts. Densities of chemical species are measured using mass-spectrometry technique. The response-surface of the sensors in the process-space was collected over the process space. In this paper we demonstrate real-time control of ion-electrode-flux independent of plasma chemistry in a modified Exelan chamber (Lam Research).