Eric C. Benck

Comparison of electron density measurements in planar inductively coupled plasmas by means of the plasma oscillation method and Langmuir probes

The absolute, spatially resolved electron densities in planar inductively coupled plasmas have been measured by two different plasma diagnostic techniques, the plasma oscillation method and Langmuir probes. In the plasma oscillation method a weak electron beam injected into the plasma excites electrostatic electron waves oscillating at the electron plasma frequency, which is proportional to the square root of the electron density. The plasma source is a modified Gaseous Electronics Conference RF Reference Cell. The results for both methods in electropositive, pure rare gas and molecular gas discharges (Ar, Ne, Kr and ) and in electronegative gases and gas mixtures like , , , , Ar:, Ar: and Ar: are presented and analysed. Over a wide parameter range (gas type, input power and gas pressure), the two techniques yield charge densities which agree within the experimental uncertainty. The electron densities inferred from the plasma oscillation method are generally between the electron and positive-ion density obtained from Langmuir probe measurements. Disagreement between the two techniques is found in molecular gases at higher pressures (p>3 Pa), where the plasma oscillation method yields electron densities of up to a factor of two higher than the Langmuir probe results.

Time-resolved measurements of ion energy distributions and optical emissions in pulsed radio-frequency discharges

In pulse-modulated inductively coupled plasmas generated in CF4:Ar mixtures, a transition between a capacitive coupling mode (E mode) and an inductive coupling mode (H mode) was observed. For a pulsed plasma in a 50%CF4:50%Ar volume mixture with a peak rf power of 200 W at 13.56 MHz and a modulation frequency at 500 Hz with a duty cycle of 95%, the E→H mode transition occurs repetitively 0.75 ms after each rf pulse is applied. This long delay in the mode transition allows us to perform not only time-resolved measurements of optical emission and electrical characteristics but also time-resolved measurements of ion energy distributions at the grounded electrode. These measurements show that a relatively short rf off period can severely perturb the equilibrium plasma state and its recovery may take much longer than the rf off period.

Electron density measurement in a rf helium plasma by laser‐collision induced fluorescence method

Laser‐collision induced fluorescence (LCIF) is the emission of light from states that have been populated by laser excitation and a subsequent collision. By simultaneously measuring the LCIF from two different states, it is possible to determine both the electron density and temperature of the low energy bulk electrons within a plasma. This method is described in detail and has been applied in the determination of the total, temporally averaged, and spatially resolved electron density in a rf (13.56 MHz) helium discharge in the Gaseous Electronics Conference reference cell. The rf discharge was operated at pressures P=33.3–133.3 Pa (0.25–1.0 Torr) and peak‐to‐peak voltages of Vpp=75–300 V were applied. We found the total electron density varied from 1.8×108 cm−3 at P=33.3 Pa and Vpp=75 V to 4.0×1010 cm−3 at P=133.3 Pa and Vpp=300 V. A comparison of results from different experiments has been made.

Transition from E to H mode discharge in pulse-modulated inductively coupled plasmas

Time-resolved measurements of pulse-modulated inductively coupled plasmas were carried out by using a Langmuir probe. It was found that under a certain set of conditions (a mixture of 20% O2 and 80% Ar, a pressure of 2.67 Pa, an rf power of 200 W, a pulse frequency of 500 Hz, and an rf-off time of 100 μs), a plasma transits from E mode (capacitive coupling mode) to H mode (inductive coupling mode) after the rf-off time. With a shorter rf-off time of 50 μs, the plasma returned to H mode without passing through E mode. Whether H mode or E mode appears after the rf-off time depends on the electron density at the end of the afterglow. Namely, the restoration to H mode after the rf-off time occurs if the plasma has an adequate electron density, and E mode occurs if electron density is not enough to sustain H mode. It was also found that electron temperature decreases and plasma potential increases gradually during E mode because of the change of the electron-energy distribution. The change of the plasma reduce...

Effects of voltage distribution along an induction coil and discharge frequency in inductively coupled plasmas

Recent etching processes often use low to middle plasma density in order to increase etching controllability or to reduce charging damages. In inductively coupled plasmas (ICPs) in low to middle plasma density, effects of a capacitively coupled discharge (E discharge) on the total plasma characteristic cannot be neglected. It is thus more difficult to understand the ICPs in low to middle density than to understand high-density ICPs which can be regarded as being generated by an inductively coupled discharge (H discharge) only. In this research, we changed the voltage distribution of the induction coil of an ICP-modified gaseous electronics conference reference cell by inserting a termination capacitor between the coil and the ground. We also changed the discharge frequencies (6.28 MHz, 13.56 MHz, and 20 MHz), and investigated their effects on Ar plasmas. As a result, it was observed that the structure of the E discharge was changed by the voltage distribution of the coil and, therefore, E-to-H mode transitions were dramatically altered. Although no difference between the electron energy distribution functions (EEDFs) was observed in a pure H discharge with an electrostatic shield for the experimental conditions studied (1.33 Pa and 150 W), it was observed that EEDFs were affected by the discharge frequency without an electrostatic shield, probably due to a decrease of electron density by the effect of an E discharge and an increase of the ratio of the E to H discharge with increasing discharge frequency.Recent etching processes often use low to middle plasma density in order to increase etching controllability or to reduce charging damages. In inductively coupled plasmas (ICPs) in low to middle plasma density, effects of a capacitively coupled discharge (E discharge) on the total plasma characteristic cannot be neglected. It is thus more difficult to understand the ICPs in low to middle density than to understand high-density ICPs which can be regarded as being generated by an inductively coupled discharge (H discharge) only. In this research, we changed the voltage distribution of the induction coil of an ICP-modified gaseous electronics conference reference cell by inserting a termination capacitor between the coil and the ground. We also changed the discharge frequencies (6.28 MHz, 13.56 MHz, and 20 MHz), and investigated their effects on Ar plasmas. As a result, it was observed that the structure of the E discharge was changed by the voltage distribution of the coil and, therefore, E-to-H mode transi...

Continuous-Wave Terahertz Spectroscopy of Plasmas and Biomolecules

Continuous-wave linear-absorption spectroscopy based on THz radiation generated by solid-state photomixers has been applied to the investigation of the dynamics of biomolecules in polyethylene matrices and to line shape studies of HF for diagnostics of semiconductor etching plasmas. The THz spectra of biotin and myoglobin have been obtained using a variable-temperature, cryogenic sampling system. The spectrum of biotin displays a small number of discrete absorptions over the temperature range from 4.2 K to room temperature while the spectrum of myoglobin has no obvious resonance structure at the >10% fractional absorption level. Spectral predictions from the lowest energy ab initio conformations of biotin are in poor agreement with experiment, suggesting the need to include condensed-phase environmental interactions for qualitative predictions of the THz spectrum. Vibrational anharmonicity is used to model the line shapes that result from drastic changes in vibrational sequence level populations of biotin over this temperature range. Anharmonicity factors (χeωe/ωe) at the levels of 0.1% to 0.8% are obtained from non-linear least squares fits of the observed resonances and illustrate their important for refining model predictions. Application of the photomixer system to line shape studies in etching plasmas has been used to study the formation efficiency and translational temperature of HF at 1.2 THz under different operating conditions. These results will aid in understanding the chemistry of industry-standard fluorocarbon and oxygenated fluorocarbon etching plasmas.

Submillimeter-wavelength plasma chemical diagnostics for semiconductor manufacturing

Submillimeter-wavelength linear-absorption spectroscopy has been applied to the chemical diagnostics of reactive-ion etching plasmas in a modified capacitively coupled gaseous electronics conference reactor. Approximately 1 mW of narrow-band (<10 kHz) submillimeter radiation between 450 and 750 GHz is produced using a backward-wave oscillator (BWO). The BWO is frequency stabilized to a harmonic of a 78–118 GHz frequency synthesizer. The submillimeter method offers high sensitivity for the ≈1 MHz linewidth, Doppler-broadened absorption lines typical of gas-phase molecules at a total pressure of less than 133 Pa (1 Torr). A large number of molecules can be detected, limited primarily by the need for a permanent electric dipole moment and for accurate line frequency predictions, the latter of which are often available in the literature. The capabilities of the diagnostic method have been demonstrated by the following three applications: (1) the measurement of water-vapor contamination in the reactor and in the precursor gas by monitoring a rotational transition of H2O in the reactor just prior to the initiation of the plasma; (2) the assessment of progress in the cleaning of the reactor by an O2/Ar plasma after a fluorocarbon plasma etch by monitoring the build up of the concentration of O3 and the depletion of the concentration of CF2O in the plasma; and (3) the determination of the endpoint in the etching of a SiO2 thin film on silicon by an octafluorocyclobutane/O2/Ar plasma by monitoring the decrease in the concentration of SiO in the plasma. The last observation is made possible by the large electric dipole moment for SiO of 1×10−29 C m (3.1 D), which gives a low minimum detectable number density for the radical of 2×107 cm−3 for an optical pathlength of 39 cm.Submillimeter-wavelength linear-absorption spectroscopy has been applied to the chemical diagnostics of reactive-ion etching plasmas in a modified capacitively coupled gaseous electronics conference reactor. Approximately 1 mW of narrow-band (<10 kHz) submillimeter radiation between 450 and 750 GHz is produced using a backward-wave oscillator (BWO). The BWO is frequency stabilized to a harmonic of a 78–118 GHz frequency synthesizer. The submillimeter method offers high sensitivity for the ≈1 MHz linewidth, Doppler-broadened absorption lines typical of gas-phase molecules at a total pressure of less than 133 Pa (1 Torr). A large number of molecules can be detected, limited primarily by the need for a permanent electric dipole moment and for accurate line frequency predictions, the latter of which are often available in the literature. The capabilities of the diagnostic method have been demonstrated by the following three applications: (1) the measurement of water-vapor contamination in the reactor and in t...

Deformation-free form error measurement of thin, plane-parallel optics floated on a heavy liquid

We describe a novel method for measuring the unconstrained flatness error of thin, plane-parallel precision optics. Test parts are floated on high-density aqueous metatungstate solutions while measuring the flatness error with an interferometer. The support of the flat optics by the uniform hydrostatic pressure at the submerged face of the flat optic eliminates flatness errors caused by mounting forces. A small, well characterized flatness error results from the bending of the floating flat by the hydrostatic pressure gradient at the edges. An equation describing the bending of thin, flat plates floating on a liquid is derived, which can be used to correct the flatness measurements of arbitrarily shaped plates. The method can be used to measure flatness errors of both nontransparent and transparent parts, and it is illustrated with flatness measurements of photomask blanks and substrates for extreme ultraviolet lithography. The refractive index of a saturated aqueous lithium metatungstate solution was measured at 632.8 nm and was found to be close to the refractive indices of several low thermal expansion optical materials.

Birefringence issues with uniaxial crystals as last lens elements for high-index immersion lithography

We discuss the birefringence issues associated with use of crystalline sapphire, with uniaxial crystal structure, as a last lens element for high-index 193 nm immersion lithography. Sapphire is a credible high-index lens material candidate because with appropriate orientation and TE polarization the ordinary ray exhibits the required isotropic optical properties. Also, its material properties may give it higher potential to meet the stringent optical requirements compared to the potential of the principal candidate materials, cubic-symmetry LuAG and ceramic spinel. The TE polarization restriction is required anyway for hyper-NA imaging, due to TM-polarization contrast degradation effects. Further, the high uniaxial-structure birefringence of sapphire may offer the advantage that any residual TM polarization results in a relatively-uniform flare instead of contrast degradation. One issue with this concept is that spatial-dispersion-induced effects should cause some index anisotropy of the ordinary rays, in a way similar to the intrinsic birefringence (IBR) effects in cubic crystals, except that there is no ray splitting. We present the theory of this effect for the trigonal crystal structure of sapphire and discuss its implications for lithography optics. For this material the spatial-dispersion-induced effects are characterized by eight material parameters, of which three contribute to index anisotropy of the ordinary rays. Only one gives rise to azimuthal distortions, and may present challenges for correction. To assess the consequences of using sapphire as a last element, neglecting any IBR effects, we use lithography simulations to characterize the lithographic performance for a 1.7 NA design, and compare to that for LuAG.

Time-resolved measurements of the E-to-H mode transition in electronegative pulse-modulated inductively coupled plasmas

Time-resolved measurements of electronegative pulse-modulated inductively coupled plasmas (ICPs) were carried out using various measurement techniques. In order to explain the experimentally observed results, it is proposed that the structure of an Ar∕CF4 plasma during an afterglow dynamically changes, passing through three stages when the period of the afterglow is long enough: (1) The first stage is the initial afterglow where the electron temperature suddenly decreases due to inelastic collision with CF4. Electron density decreases and the density of fluorine negative ions increases by electron attachment, but the sheath potential still exists and the negative ions are confined in the bulk plasma region. Since charge neutrality should be maintained, the density of positive ions is almost constant during this stage. (2) The second stage is the intermediate afterglow where the plasma consists mainly of negative and positive ions but sheath potentials remain, reducing the negative ion flux from the plasma...