Gilles Cunge

CF2 production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization

The study of CF and CF2 radical production and loss mechanisms in capacitively-coupled 13.56 MHz CF4 plasmas has been extended to CF4 plasmas with an Si substrate, and to C2F6 plasmas, conditions where the atomic fluorine concentration is lower and where more polymer deposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were investigated by time resolved axial concentration profiles obtained by laser induced fluorescence, combined with absolute calibration techniques. The results for CF were similar to those observed in the fluorine rich case, whereas the results for CF2 were strikingly different and more complex. This paper focuses on the CF2 radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface precursor. The results can only be explained if large polymeric ions and/or neutrals are produced by polymerization in the gas phase. The gas-phase CF2 concentration is high, causing the otherwise s...

CFx radical production and loss in a CF4 reactive ion etching plasma: Fluorine rich conditions

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 13.56 MHz plasmas in pure CF4 at 50 and 200 mTorr. Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions. This mechanism often predominates over the gas-phase...

A novel electrostatic probe method for ion flux measurements

A novel electrostatic probe method is described in which the ion flux is determined from the discharging of an RF-biased capacitance in series with the probe. By using a large-area planar probe, with a guard ring and located in or on other surfaces, edge effects and perturbations to the plasma volume can be kept small. The ion flux to the probe can be determined even when its surface is coated with insulating material from the plasma itself. Results are reported for ion fluxes in RF-excited plasmas in Ar and in in a RIE reactor. In Ar, ion fluxes to the earthed surfaces increase with pressure and power over the ranges 50 - 200 mTorr and 30 - 200 W. In , over the same ranges the ion fluxes to the surfaces decrease with increasing pressure.

Pulsed high-density plasmas for advanced dry etching processes

Plasma etching processes at the 22 nm technology node and below will have to satisfy multiple stringent scaling requirements of microelectronics fabrication. To satisfy these requirements simultaneously, significant improvements in controlling key plasma parameters are essential. Pulsed plasmas exhibit considerable potential to meet the majority of the scaling challenges, while leveraging the broad expertise developed over the years in conventional continuous wave plasma processing. Comprehending the underlying physics and etching mechanisms in pulsed plasma operation is, however, a complex undertaking; hence the full potential of this strategy has not yet been realized. In this review paper, we first address the general potential of pulsed plasmas for plasma etching processes followed by the dynamics of pulsed plasmas in conventional high-density plasma reactors. The authors reviewed more than 30 years of academic research on pulsed plasmas for microelectronics processing, primarily for silicon and conductor etch applications, highlighting the potential benefits to date and challenges in extending the technology for mass-production. Schemes such as source pulsing, bias pulsing, synchronous pulsing, and others in conventional high-density plasma reactors used in the semiconductor industry have demonstrated greater flexibility in controlling critical plasma parameters such as ion and radical densities, ion energies, and electron temperature. Specifically, plasma pulsing allows for independent control of ion flux and neutral radicals flux to the wafer, which is key to eliminating several feature profile distortions at the nanometer scale. However, such flexibility might also introduce some difficulty in developing new etching processes based on pulsed plasmas. Therefore, the main characteristics of continuous wave plasmas and different pulsing schemes are compared to provide guidelines for implementing different schemes in advanced plasma etching processes based on results from a particularly challenging etch process in an industrial reactor.

Absolute radical densities in etching plasmas determined by broad-band UV absorption spectroscopy

Broad-band UV absorption spectroscopy was used to determine radical densities in reactive gas plasmas generated in a 13.56 MHz capacitively coupled parallel plate reactor. Five radical species were detected: , CF, AlF, and . Absolute (line-integrated) densities were determined in and plasmas, as were the vibrational and rotational temperatures in the latter case. In plasmas the CF radical was also detected, along with the etch products AlF (from the Al powered electrode) and (when an Si substrate was present). The fraction that comprises of the total etch products was estimated. Finally, the dimer was detected in an plasma in the presence of an Si substrate. This simple technique allows absolute concentrations of many key reactive species to be determined in reactive plasmas, without the need to analyse the complex rotational spectra of these polyatomic molecules.

Reducing damage to Si substrates during gate etching processes by synchronous plasma pulsing

Plasma oxidation of the c-Si substrate through a very thin gate oxide layer can be observed during HBr/O2/Ar based plasma overetch steps of gate etch processes. This phenomenon generates the so-called silicon recess in the channel and source/drain regions of the transistors. In this work, the authors compare the silicon recess generated by continuous wave HBr/O2/Ar plasmas and synchronous pulsed HBr/O2/Ar plasmas. Thin SiO2 layers are exposed to continuous and pulsed HBr/O2/Ar plasmas, reproducing the overetch process conditions of a typical gate etch process. Using in situ ellipsometry and angle resolved X-ray photoelectron spectroscopy, the authors demonstrate that the oxidized layer which leads to silicon recess can be reduced from 4 to 0.8 nm by pulsing the plasma in synchronous mode.

Ultraviolet cavity ring-down spectroscopy of free radicals in etching plasmas

Abstract Many reactive species of interest in technological plasmas absorb light in the UV spectral region (200–300 nm). Measurement of these weak absorbances (typically 10 −2 –10 −4 for a single pass) allows us to determine their absolute concentration. Low-resolution absorption spectra of these systems have previously been obtained by broad-band absorption spectroscopy. Here we present spectra obtained using laser cavity ring-down spectroscopy, which has much higher spectral resolution, and potentially higher sensitivity. Spectra were obtained for CF, CF 2 , AlF and SiF 2 radicals in capacitively-coupled radio-frequency plasmas in fluorocarbon gases. This technique offers the possibility of real-time (1 s) absolute concentration measurements during wafer processing.

HIGH MASS POSITIVE IONS AND MOLECULES IN CAPACITIVELY-COUPLED RADIO-FREQUENCY CF4 PLASMAS

The positive ions and neutral radicals arriving at the earthed walls of a capacitively-coupled radio-frequency pure CF4 plasma were analyzed using a quadrupole mass spectrometer adapted for high masses. Experiments were performed at 50 and 200 mTorr, in an empty reactor and with Si and SiO2-coated Si substrates on the powered electrode. High mass ions and neutrals were detected, up to 500 and 300 amu, respectively. The abundance of high-mass species was greatest in the presence of silicon wafers and at higher pressure. The observed ion masses can be separated into distinct series, originating from different initial bases to which successive CF2 units have been added. We, therefore, propose that these high-mass species are the result of a gas phase polymerization process consisting of CF2 addition reactions, in agreement with a model proposed recently by our group. The influence of a silicon substrate derives primarily from the strong decrease that it induces in the concentration of F atoms, which otherwis...

Laser-induced fluorescence detection of as a primary product of Si and reactive ion etching with gas

The radical was detected in the gas phase during the etching of both Si and substrates under reactive ion etching conditions in steady state and in the afterglow of pulsed plasmas, by laser-induced fluorescence (LIF). Spatially and temporally resolved measurements show that there is a major source of these radicals at, or very close to, the etched substrates, and that desorption of as one of the primary products is the most likely explanation. As the absolute concentration was not determined, it is not currently possible to say whether is a major etch product, although the observed signals were large. With an Si substrate, is produced both under steady-state RIE conditions (i.e. in the presence of ion bombardment) and in the afterglow of a pulsed discharge (i.e. by pure chemical etching by F atoms). In contrast, with an substrate, is only produced in the steady-state plasma. The net surface reaction probability of was found to be close to unity on the reactor walls. Some possible gas-phase reactions of are also discussed. The fluorescence lifetime of the excited state of was measured for the first time, giving a value of ns, in good agreement with theoretical estimates.

Etching mechanisms of HfO2, SiO2, and poly-Si substrates in BCl3 plasmas

BCl3 based plasmas exhibit promising plasma chemistries to etch high-k materials and, in particular, HfO2, with a high selectivity over SiO2 and Si substrates. The authors report on the mechanisms involved in the etching of HfO2, SiO2, and poly-Si substrates in BCl3 plasmas. X-ray photoelectron spectroscopy analyses help in understanding the mechanism driving the high etch selectivity between HfO2 and silicon-containing substrates. The ion energy plays an important role in the etching mechanisms since it controls a transition between a BCl-like deposition on the substrate and its etching by ionic bombardment. The ion energy threshold above which etching occurs is different from one substrate to another, being lower for HfO2 than for Si substrates. Indeed, BClx deposition forms more easily on poly-Si or SiO2 rather than on HfO2 surfaces, because boron reacts with Si atoms to form Si–B bonds initiating the growth of BClx polymer on Si-containing surfaces, while on HfO2 surfaces, boron is directly involved i...