Plasma-polymer interactions: A review of progress in understanding polymer resist mask durability during plasma etching for nanoscale fabrication
(Color online) Simplified schematic of a 30 nm wide line formed by resist molecules. On the top of the line, about 6 molecules ( diameter) cover the distance from one side of the line to the other. The minimum spatial extent of the plasma modified region—considering ions ( energy) and neutrals —is depicted. Plasma-generated UV irradiation is also shown and will penetrate the polymer to an energy dependent that is 10 s of nm. The level of control needed for nanoscale manufacturing is of the order of both macromolecular and ion and neutral length scales. Lack of control of the interaction of the plasma with the polymer molecules of the resist will produce important profile imperfections.
(Color online) Polymer structures used for (a) 193 nm and (b) 248 nm photoresists, respectively (from Ref. 21).
Changes of surface and sidewall morphology of resist materials and structures. (a) Morphological changes introduced in the surface of a 193 nm photoresist by a fluorocarbon plasma process, where the same process leaves the surface of a 248 nm photoresist (b) undamaged (from Ref. 7). The image in (c) shows a roughened polymethylmethacrylate resist mask used to produce 50 nm wide trenches in silicon dioxide (from Ref. 8). Side and top view SEM images of high-aspect-ratio contact-hole profiles produced by (d) or (e) plasma etching from the work of Negishi et al. (Ref. 112). The roughness of the resist mask introduced by the plasma is transferred into the trenches (c) or contact holes [(d) and (e)] formed in . (f) and (g) illustrate the formation of sidewall striations and wiggling of photoresist lines, respectively, during a plasma etching process [from the work of Kim et al. (Ref. 113)].
Striation evolution on the sidewall of 193 nm photoresist mask during pattern transfer using Ar discharges (from Ref. 116): (a) as received, (b) 1.6 s, (c) 7 s, (d) 15 s, (e) 29 s, and (f) 60 s. Discharges were generated using 1000 W source power, a pressure of 10 mTorr, and 50 SCCM as total gas flow rate with a fixed bias voltage −125 V.
Atomic force microscopy of photoresist mask roughness transfer into a underlayer during plasma etch of a dielectric (from Ref. 117). The letters B, C, and D denote the photoresist, bottom antireflection coating, and the layer, respectively. In (a), the principle of the AFM technique used to image multilayer sidewalls on cleaved silicon wafers is shown. In (b), the AFM sidewall image after plasma etching of the layer is shown, displaying clearly the striations that extend vertically across the etched layer.
(Color online) Selected modifications of 193 nm photoresist polymer materials to evaluate their influence on photoresist material responses in plasma and energetic beam environments (from Ref. 21). In (a), a 193 nm PR ter-polymer structure is shown; (b) the PAG and base are specified which when added to the polymer shown in (a) produces full formulation of a photoresist system; (c) replacement of MAMA by EAMA in leaving group; (d) replacement of RAMA by RADA in polar group; (e) p-MAMA homopolymer.
(Color online) Etching rates and AFM RMS roughness values of different photoresist-related materials vs Ohnishi parameter OP (from Ref. 21). The lowest OP is found for the 248 nm PR shown in Fig. 2(b) and shows low etching rate and surface roughness. The p-MAMA homopolymer shown in Fig. 6(e) has an intermediate OP value, with a low etching rate but very high surface roughness. The 193 nm PR polymer and variations [Figs. 6(a), 6(c), and 6(d)], along with the fully formulated 193 nm PR system [Fig. 6(b)], have comparable OP values and are shown on the right.
Etching yields vs neutral coverage of for 193 and 248 nm photoresist materials and p-MAMA model polymer in pure Ar and discharges (from Ref. 167). The surface coverage was obtained by x-ray photoemission spectroscopy.
Time-evolution of rms surface roughness measured by atomic force microscopy for 193 nm PR after plasma etching using (after Ref. 171). Experimental conditions: , 10 mTorr, 800 W source power, −100 V self-bias voltage, and a substrate temperature of . The images show corresponding scanning electron micrographs.
(Color online) Spatial roughness distributions obtained by fast Fourier transform analysis of AFM data used for Fig. 9 for different plasma etching times (after Ref. 171). The FFT calculation was performed in horizontal direction of AFM images like shown on the top and averaging over the scanned area.
Schematic showing plasma-polymer interactions for two process conditions that differ in polymer etching yield and produce differing energy densities at the surface (from Ref. 166). The differences in dissipated energy are assumed to produce dissimilar polymer surface modifications.
Measured surface roughening rates of 193 and 248 nm photoresist materials and p-MAMA model polymer vs energy density during plasma processing for a wide range of plasma operating conditions (from Ref. 166). The universal behavior seen as a function of energy density for each polymer structures indicates the overriding importance of both deposited energy density and polymer structure in determining observed surface roughening rates.
(Color online) Thickness reduction of a 400 nm thick 193 nm PR film as a function of time of exposure to different photon energies produced in Ar plasma (from Ref. 195). The data were obtained by real-time ellipsometry for different radiation wavelength ranges selected by using optical filters between the plasma and 193 nm PR material. The thickness reduction is due to radiation induced evolution of specific groups (e.g., adamantyl and ) at wavelength dependent depths of the polymer, along with densification of the modified polymer depth. Irradiation with visible radiation (glass filter) results in a thickness reduction of about 1.5 nm. Irradiation with more energetic photons (alumina and filters) reduces PR film thickness to a greater photon energy dependent extent. The thickness changes saturate for extended exposures to the plasma.
(Color online) Correlation of film thickness reduction as estimated by ellipsometry measurements and bulk material modifications of 193 nm PR and 248 nm PR as observed by FTIR (from Ref. 17). Thickness reduction is directly correlated with absorbance loss in the spectra and in the characteristic region for lactone in the spectra for 193 nm PR. Changes observed for exposure using the glass filter are significantly smaller than for exposure using the filter. The 248 nm PR shows high structural stability and only small thickness changes.
Relationship between measured line edge roughness for 193 nm photoresist trenches and surface roughness on top of the photoresist lines for discharges (after Ref. 168).
(Color online) Schematic of chemical structures of model polymers used by Bruce et al. (Refs. 9 and 178) including PS, P4MS, , and P4VP.
Ellipsometric plot (a) and optical model (b) of P4MS etched in Ar plasma with −100 V bias (from Ref. 178). Also shown are simulated trajectories of unmodified P4MS (i) and P4MS with a 1.58 nm thick damaged layer with a complex refractive index (ii). Ar discharges were generated using 300 W source power, 10 mTorr pressure, 40 SCCM gas flow, −100 V self-bias, and 5 min plasma exposure time.
Difference in UV response for P4MS and exposed to Ar discharges (from Ref. 178). Experimental plots of P4MS and exposed to Ar discharges are shown, along with simulated curves. The simulated trajectories of unexposed P4MS (i) and (ii) with varying thickness and constant refractive index are shown for comparison. The simulated trajectory for a constant 30 nm UV modified layer on top of a varying thickness of unexposed is shown as curve (iii). Argon discharges were generated using 300 W source power, 10 mTorr pressure, 40 SCCM gas flow, and no rf bias.
(Color online) Schematic of the buckling model of surface roughening proposed by Bruce et al. (Ref. 9). The figure shows the highly stressed, modified layer formation and roughening that occurs simultaneously at the surface of a polystyrene film under Ar plasma exposure. The symbols used to represent important materials and morphological properties are also shown.
(Color online) Comparison of observed and calculated values of a roughness model based on buckling of the polymer surface (from Ref. 9). (a) AFM images showing surface roughness in PS after ion bombardment at varying maximum ion energies. Also shown are calculated and experimental values of (b) and (c) vs maximum ion energy of the nanoscale roughness measured after plasm processing at .
(Color online) Dependence of plasma-induced surface roughness on polymer structure (from Ref. 212). In (a), AFM images are shown for PS, , and P4VP after 60 s inductively coupled plasma treatment in Ar or discharges (10 mTorr pressure, −100 V self-bias in each case). In (b), the time evaluation of surface roughness during plasma treatment is plotted vs either plasma exposure time or thickness of polymer etched.
(Color online) Proposed mechanism to explain the dependence of plasma-induced surface roughening on polymer structure seen in Fig. 21, and consistent with the buckling model of surface roughness initiation (from Ref. 212). Changes in mechanical properties of the polymer near-surface layers are produced by both ion bombardment and VUV irradiation. For the latter, scission or cross-linking reactions in the near surface region of the polymer can either reduce (b) or increase (c) the elastic modulus of that region, and change the magnitude of the stress due to the ion-damaged layer.
(a) Chemical modifications of 193 nm PR after remote argon plasma UV/VUV exposure including loss of bonds. (b) Loss of peak at corresponds to modification of top of material. (c) Loss of bonds is also observed. (d) and stretching region is nearly unaffected.
(Color online) Surface roughness of 193 nm PR after ion only [(a)–(d)] and simultaneous exposure [(e)–(h)] from 50 to as observed with AFM images. Ion bombardment results in “pebbling” of the surface and is only weakly dependent on temperature. Simultaneous exposure is highly dependent on substrate temperature resulting in very rough surfaces at . Color scale is 2.5 nm unless noted on the image. All line plots are on the same scale with a 5× exaggerated vertical axis.
(Color online) Surface roughness of 193 nm PR (a) and 248 nm PR (b) after simultaneous UV/VUV and 150 eV ion bombardment as observed with AFM images. Surface roughness of 193 nm PR (c) and 248 nm PR (d) after 20 s argon plasma exposure. In both cases, 193 nm PR is considerably rougher than equivalent exposures of 248 nm PR. Color scale noted on each image.
SEM images taken at 75 K magnification of simultaneous UV/VUV and ion bombarded 193 nm PR at (a) and (b). SEM image of argon plasma-exposed 193 nm PR (c). Nanoscale roughness observed during argon plasma has the same lateral scale of roughness developed during simultaneous UV/VUV, ion bombardment, and heating in the beam system.
(Color online) Side view images of (a) virgin polymer and (b) steady-state surfaces after fluence PMMA polymers. To form image (b) from image (a), 16 nm of materials were removed. (c) Depth profiles of C, H, and O atom density. C—red, H—black; O—green.
SY (Eq. C removed per Ar ion) vs ion fluence from MD simulation of p-MAMA polymer and experimental data from 193 nm photoresist. ion energy is 100 eV. .
(a) Logarithm of height-height correlation functions vs logarithm of distance, calculated from AFM topography maps for etched in ; etching times are indicated to the right of each curve (from Ref. 240). The slope in the small distance regime gives twice the scaling exponent, i.e., . Also indicated are the correlation length , and twice the square of the rms roughness, i.e., . (b) Roughness exponent estimated by fitting the height correlation functions to the form suggested by Sinha et al. (Ref. 241) Open circles are for ; closed circles are for PS; open squares are for P4MS; all are etched in . Dashed line shows , as predicted by the flux reemission model.228 (c) Logarithm of rms amplitude vs logarithm of etching time for etched in . Based on scaling theory, the exponent is given by the long-time slope of this plot, , compared with a value of approximately 1 predicted by the flux reemission model (Ref. 228). (d) Logarithm of correlation length vs logarithm of etching time for etched in . Based on scaling theory, the exponent is given by the long-time slope of this plot, , compared with a value of approximately 1 predicted by the flux re-emission model. (e) Comparison of measured values of and for PS (left bars), (middle bars), and P4MS (right bars) etched in .
Etch rates of various polymers in using a capacitively coupled plasma system (from Ref. 62). Conditions: Barrel-type capacitively coupled plasma etching system, electrodes located at the top and bottom of the chamber, 200W of 13.56 MHz power, chamber pressure of 0.55 Torr, and 15 ml/min gas flow rate.
UV emission lines identified in the spectrum of ICP discharges produced using (from Ref. 191).
Summary of photon radiation induced material modifications for 248 and 193 nm photoresists in Ar or discharges (from Ref. 17).
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