S.K. Karkari

Surface loss rates of H and Cl radicals in an inductively coupled plasma etcher derived from time-resolved electron density and optical emission measurements

A study is undertaken of the loss kinetics of H and Cl atoms in an inductively coupled plasma (ICP) reactor used for the etching of III-V semiconductor materials. A time-resolved optical emission spectroscopy technique, also referred to as pulsed induced fluorescence (PIF), has been combined with time-resolved microwave hairpin probe measurements of the electron density in a pulsed Cl2/H2-based discharge for this purpose. The surface loss rate of H, kwH, was measured in H2 plasma and was found to lie in the 125–500 s−1 range (γH surface recombination coefficient of ∼0.006–0.023), depending on the reactor walls conditioning. The PIF technique was then evaluated for the derivation of kwCl, and γCl in Cl2-based plasmas. In contrast to H2 plasma, significant variations in the electron density may occur over the millisecond time scale corresponding to Cl2 dissociation at the rising edge of the plasma pulse. By comparing the temporal evolution of the electron density and the Ar-line intensity curves with 10% of...

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.

Using the resonance hairpin probe and pulsed photodetachment technique as a diagnostic for negative ions in oxygen plasma

In this work the resonance hairpin probe technique has been used for detection of photoelectrons generated during photodetachment experiments performed to determine negative ion density in an inductively coupled oxygen plasma. An investigation of the temporal development of the photoelectron population was recorded with the hairpin probe located inside the laser beam region and at various points outside the beam. Varying the external microwave frequency used to drive the probe resonator allowed the local increase in electron density resulting from photoelectrons to be determined. At a fixed probe frequency, we observed two resonance peaks in the photodetachment signal as the photoelectron density evolved as a function of time. Inside the laser beam the resonance peaks were asymmetric, the first peak rising sharply as compared with the second peak. Outside the laser beam region the peaks were symmetric. As the external frequency was tuned the resonance peaks merge at the maximum electron density. The resonance peak corresponding to maximum density outside the beam occurs at a delay of typically 1–2 µs as compared with the centre of the beam allowing an estimate of the negative ion velocity. Using this method, negative ion densities were measured under a range of operating conditions inside and outside the beam.

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.

Characterization of the pulse plasma source

Characterization of the pulse plasma source through the determination of the local thermodynamic equilibrium (LTE) threshold is described. The maximum electron density measured at the peak in discharge current is determined by the width of the He II Paschen alpha spectral line, and the electron temperature is determined from the ratios of the relative intensities of spectral lines emitted from successive ionized stages of atoms. The electron density and temperature maximum values are measured to be 1.3 ? 1017?cm?3 and 19?000?K, respectively. These are typical characteristics for low-pressure, pulsed plasma sources for input energy of 15.8?J at 130?Pa pressure in helium?argon mixture.The use of LTE-based analysis of the emission spectra is justified by measurement of the local plasma electron density at four positions in the discharge tube using a floating hairpin resonance probe. The hairpin resonance probe data are collected during the creation and decay phases of the pulse. From the spatio-temporal profile of the plasma density a 60??s time-window during which LTE exists throughout the entire plasma source is determined.

Resonance hairpin and Langmuir probe-assisted laser photodetachment measurements of the negative ion density in a pulsed dc magnetron discharge

The time-resolved negative oxygen ion density n− close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O− density remains pretty constant during the driven phase of the discharge at values typically below 5×1014 m−3; however, in the off-time, the O− density grows reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) α=n−/ne being higher in the off phase (maximum value α∼1) than in the on phase (α=0.05–0.3). The authors also see higher values of α at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative elec...

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...

Microwave resonances of a hairpin probe in a magnetized plasma

The effect due to the electron cyclotron frequency on the microwave resonances of a hairpin probe is investigated in a moderate to strongly magnetized plasma. The magnetic field is independently varied over a wide range from 0.01–0.13 T while maintaining the local plasma density constant. At strong magnetic fields the resonance frequency is found to be lower than that measured in vacuum implying that the relative plasma dielectric permittivity, ep>1. It is proposed that the experiments reported here are consistent with a permittivity model that includes magnetic field.

Dielectric covered hairpin probe for its application in reactive plasmas

The hairpin probe is a well known technique for measuring local electron density in low temperature plasmas. In reactive plasmas, the probe characteristics are affected by surface sputtering, contamination, and secondary electron emission. At higher densities, the plasma absorbs the entire electromagnetic energy of hairpin and hence limits the density measurements. These issues can be resolved by covering the hairpin surface with a thin layer of dielectric. In this letter, the dielectric contribution to the probe characteristics is incorporated in a theory which is experimentally verified. The dielectric covering improves the performance of probe and also allows the hairpin tip to survive in reactive plasma where classical electrical probes are easily damaged.

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.