Fluorocarbon Discharges Research Articles

Differences in erosion mechanism and selectivity between Ti and TiN in fluorocarbon plasmas for dielectric etch

Metallic masking materials are promising candidates for plasma-based pattern transfer into low-k materials for fabricating integrated circuits. Improving etching selectivity (ES) between the low-k and hardmask material requires a fundamental understanding of material erosion in fluorocarbon (FC) plasmas. The authors have previously reported on the erosion mechanism and plasma parametric dependencies of Ti etch in FC discharges. The present work focuses on elucidating differences in the erosion behavior between Ti and TiN hardmasks. The authors studied erosion of Ti, TiN, and organosilicate glass (OSG), a reference low-k material, in CF4/Ar and C4F8/Ar plasmas. Changes in surface composition, FC surface reaction layer thicknesses, erosion rates, and corresponding ES were established by x-ray photoelectron spectroscopy and in situ ellipsometry. The authors found that the erosion stages and plasma parameter dependent surface compositions were similar for Ti and TiN. The previously established dependence of T...

Plasma Polymerization Behavior of Fluorocarbon Monomers in Low‐Pressure AF and RF Discharges

AbstractThe plasma polymerization behavior of fluorocarbon monomers was investigated using C3F6, C3F6O, C4F10 and C2F4H2, which were compared to hydrocarbons such as methane and butane. The plots of the normalized thickness growth rates of plasma polymers, GR/FM, versus the energy input composite parameters, W/FM, revealed the energy‐deficient and monomer‐deficient domains for each monomer, along with the critical energy input, (W/FM)c, representing the transition between the two domains. The luminous gas phase in fluorocarbon discharges does not contain much polymer‐forming species and thus, the monomer‐deficient domain shifted to a much lower W/FM and GR/FM values than in the case of hydrocarbon systems. For hydrocarbon systems, deposition rates were greater in RF than in AF discharges, which is opposite to the situation observed for the fluorocarbon systems. These differences were attributed to the locations of the dissociation glows and to the nature of the luminous gas phase in respect to polymer‐forming species.magnified image

Kinetics of etching in inductively coupled plasmas

A modified Langmuir type surface kinetics model was developed in our previous work for interpreting the overall etching behavior of silicon based materials in fluorocarbon discharges using inductively coupled plasma (ICP) reactors. This kinetics model was tested for the global applicability to the ICP etching, either for the etching of non-silicon-based materials or for the etching in non-fluorocarbon-based discharges. Though this model has some limitations, e.g. not applicable for the etching in which the etch rate decreases with the bias voltage at some higher bias voltages, it successfully describes the behavior of etching of a wide variety of materials in various ICPs. To meet the expectation of the model, the inverse etch rate presented in the literature was fairly linearly dependent on (the bias voltage) −3/2, which allows to evaluate some kinetics parameters for the ICP etching, and thus to characterize the etch behavior in a quantitative manner.

Etching of Si, SiO2 , and Si3 N 4 in Fluorocarbon Discharges : Interpretation Based on a Langmuir Surface Kinetics Model

Deposition/etching rates of fluorocarbon films measured as a function of the bias-voltage applied to the substrate in fluorocarbon discharges of an inductively coupled plasma reactor are analytically interpreted using a modified Langmuir surface kinetics model. The data for various fluorocarbon discharges given in the literature are well described by the modified surface kinetics model. Plotting as a function of yield a straight line, as expected, giving the value of the pure chemical etch rate and the relative value of sputter yield by incident ions. Based on this interpretation, etching behaviors of and Si in fluorocarbon discharges are approached and thereby the selective etching of over other materials is discussed. © 2003 The Electrochemical Society. All rights reserved.

Kinetics of Deposition in Fluorocarbon Discharges: Interpretation Based on an Analytical Plasma Chemistry Model

An analytical plasma chemistry model developed in our previous work was adopted for the interpretation of polymer deposition in fluorocarbon discharges. The rate of deposition of polymer films was expressed in terms of the three plasma variables (discharge power, flow rate of feed gas, and pressure). If the film precursors are generated directly from feed gas molecules in the plasma, the model predicts that the inverse deposition rate is linearly proportional to the inverse discharge power and, at enough low flow rates of feed gas, in the inverse flow rate. Some data in the literature followed this prediction fairly well, which allowed some plasma parameters such as the dissociation efficiency to be estimated. It is also expected that the pressure divided by the deposition rate is linearly proportional to the pressure squared. This expectation agreed well with data in the literature for several fluorocarbon discharges. From the line fit of the plotting (pressure/deposition rate vs. pressure squared) the relative yields of film precursors and etchants dissociated from a fluorocarbon molecule in the plasma were estimated. The etch behavior in fluorocarbon discharges can be approached in terms of these yields. © 2003 The Electrochemical Society. All rights reserved.

Fluorocarbon-based plasma etching of SiO2: Comparison of C4F6/Ar and C4F8/Ar discharges

A gas phase and surface chemistry study of inductively coupled plasmas fed with C4F6/Ar and C4F8/Ar intended for SiO2 etching processes was performed. Adding Ar to those fluorocarbon gases results in a strong increase of the ion current, by up to a factor of 5 at 90% Ar relative to the pure fluorocarbon gases. The fluorocarbon deposition rate is higher for C4F6/Ar than for C4F8/Ar, whereas the fluorocarbon etching rate is lower, and both quantities decrease as the amount of Ar is increased. For both C4F6/Ar and C4F8/Ar, the CF2 density is more than an order of magnitude greater than the CF density. The CF2 partial pressure decreases as more Ar is added to the C4F6/Ar plasmas. A comparison of these data with corresponding results obtained with C4F8/Ar shows that the CF2 partial pressure in C4F6 is higher for Ar-lean gas mixture than for C4F8/Ar. This remains true up to 40% Ar. Above 40% Ar the CF2 partial pressure in C4F8 is higher than for C4F6. The CF and COF2 partial pressures in C4F8 are higher than for C4F6. The SiO2 etch rate is higher for C4F8/Ar than for C4F6/Ar. This may be attributed in part to the higher F/C ratio of the steady-state fluorocarbon film formed on SiO2 surfaces for C4F8/Ar which was determined by x-ray photoemission spectroscopy (XPS). The etching selectivity of SiO2 over resist and silicon is increased by the addition of Ar to the fluorocarbon gases. Overall, the SiO2/resist and SiO2/Si etching selectivity are higher for C4F6/Ar (i.e., 4 and 9, respectively) at 90% Ar than for C4F8/Ar (i.e., 2 and 5, respectively) at 90% Ar and otherwise identical conditions. Both ellipsometry and XPS measurements show that the steady-state fluorocarbon layer thickness is greater for C4F6/Ar (∼4 nm) than for C4F8/Ar (∼2.8 nm). Argon addition leads to a strong decrease of the fluorine content of the steady-state fluorocarbon layers on both Si and SiO2 surfaces relative to films produced in pure fluorocarbon discharges, and this effect is related to the increase of the SiO2/Si and SiO2/resist etching selectivity.

CF 2 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 slow gas-phase concatenation reactions CXFY(CF2)n+CF2→CXFY(CF2)n+1 to occur. These processes produce high-mass neutrals (and ions) which are the real polymer precursors. The CF2 radical therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine atom concentration, which simultaneously controls the concentrations of CF2, of chain initiating species such as CF3 and of dangling bonds on the growing oligomers. This model appears to apply to fluorocarbon discharges in general, and agrees well with other results presented in the literature.

‘‘Self-thickness-limited’’ plasma polymerization of an ultrathin antiadhesive film

Ultrathin (<5 nm) fluorinated polymer films of homogeneous thicknesses have been deposited from a CF4/H2 microwave discharge. The films have an extremely low surface energy of 4.2 mJ/m2, which is more than four times lower than that for polytetrafluoroethylene. The deposition was carried out in a type of ‘‘self-thickness-limited’’ mode, in which the thickness of the deposited polymer film is limited by the plasma parameters. The deposition can be separated into two phases, a growth and a treatment phase. During the treatment phase, the deposited film is fluorinated, which results in a dramatic decrease of the surface energy. The thickness limiting behavior of the plasma is explained by the dualism of etching and polymerization occurring in fluorocarbon discharges. The film was tested as antiadhesion film for the replication of micro-optical structures, using a Ni shim with a very fine surface relief grating (400 nm periodicity) to hot emboss (at 180 °C) polycarbonate sheed. The tests demonstrate the excellent antiadhesive property of the film with the polycarbonate and reveal a good adhesion of the film with the Ni substrate.

Fluorocarbon high density plasmas. VII. Investigation of selective SiO2-to-Si3N4 high density plasma etch processes

A comparison of the plasma etching characteristics of SiO2 and Si3N4 in high-density fluorocarbon discharges with the goal of identifying an etching chemistry with a very high SiO2-to-Si3N4 etch selectivity has been initiated. High-density plasmas were excited in an electron cyclotron resonance apparatus equipped with a cooled rf powered electrostatic chuck. Gas mixtures of either CF4/H2, CHF3/H2 (low carbon/fluorine ratio fluorocarbon gases), or C2F4/H2, C2F6/H2, and C3F6/H2 (high C/F ratio) were used in this work. We will show that a carbon-rich fluorocarbon gas like C2F4 and a modest amount of H2 are useful in achieving high SiO2/Si3N4 etch selectivity (≳28). On the other hand, hydrogen-rich fluorocarbon gas mixtures which contain less carbon, e.g., CHF3/H2, are not useful for achieving SiO2 over Si3N4 etch selectivity although they will enable SiO2/Si etch selectivity. The gas phase and surface chemical aspects of the different gas mixtures were studied by optical emission spectrometry and line of sight mass spectrometry and by post plasma x-ray photoelectron spectroscopy. The results of these measurements can explain the etch rate data by selective deposition of fluorocarbon films on Si3N4 and Si surfaces.

Study of plasma - surface interactions: chemical dry etching and high-density plasma etching

We report results of the characterization of the plasma - surface interactions of silicon and silicon dioxide in fluorocarbon discharges using real-time ellipsometry and post-plasma multi-technique surface analysis for chemical dry etching (CDE) and high-density plasma etching (HDPE). We show that changes of the gas composition in CDE causes major changes in silicon surface chemistry and etching behaviour. For low-pressure HDPE we investigate the influence of power deposition into the discharge and bias voltage and bias power at the wafer on the surface chemical changes of silicon and .

Measurements of F*, CF, and CF2 formation and decay in pulsed fluorocarbon discharges

The temporal population profiles of F*, CF, and CF2 in a sharp-edged, pulsed (500 μs), fluorocarbon discharge are examined. F* population rises and falls with the discharge current suggesting that electron impact of the parent fluorocarbon is the primary source of emitting fluorine atoms. Ground-state CF and CF2, monitored by laser-induced fluorescence, show noticeably slower formation and decay, but a simple kinetic model assuming that each arises from direct electron impact of the parent gas fits the data. It is shown that CF can be conveniently monitored by exciting the B̃(v′=2)−X̃(v″=0) transition with a 193 nm ArF excimer laser.