Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges
(Color online) Schematic representation of thermal ALD and plasma-assisted ALD. During the co-reactant step of the cycle (the 2nd half-cycle), the surface is exposed to a reactant gas or vapor such as NH3 or H2O, or to species generated by a plasma.
(Color online) Number of publications per year on the subject of plasma-assisted ALD, between 1991 and 2011 (status May 31, 2011). The search was run in published abstracts using Web of Science ® (Ref. 23). The search terms included “plasma-assisted ALD,” “plasma-enhanced ALD,” “radical enhanced ALD,” “remote plasma ALD,” “direct plasma ALD,” and “plasma ALD.” The first report of a plasma-assisted ALD process by De Keijser and Van Opdorp (Philips Research Laboratories, Eindhoven), published in 1991, is also included.
Reactor layout as used in the first plasma-assisted ALD experiments (Philips Research Laboratories, Eindhoven) reported in the literature Ref. 111. An H2 plasma was generated by means of a remote microwave-induced plasma source in a quartz tube. The H radicals assisted in the atomic layer epitaxy (ALE) process of GaAs. Reprinted from M. de Keijser and C. van Opdorp, Appl. Phys. Lett. 58, 1188 (1991). Copyright 1991, American Institute of Physics.
(Color online) Ion energy distribution as measured by a retarding field energy analyzer (RFEA) in O2, H2 and N2 plasmas (operating pressure: 8 mTorr; plasma power: 100 W) used for remote plasma-assisted ALD. The RFEA was positioned at the substrate stage. Measurements were performed in the home-built ALD-I reactor installed at Eindhoven University of Technology. Due to non-ideal effects such as capacitive coupling, the ion energies measured are higher than those measured in the Oxford Instruments FlexAL reactor, which are reported elsewhere (Ref. 303).
(Color online) Ion-surface interactions during plasma processes with respect to ion flux and ion energy (Ref. 345). The typical operating windows for remote plasma ALD and other plasma-based processes are indicated. Reprinted with permission from T. Tagaki, J. Vac. Sci. Technol. A 2, 382 (1984). Copyright 1984 American Vacuum Society.
(Color online) Optical emission spectra of plasma radiation in (a) an O2 plasma, (b) an H2 plasma, and (c) a N2 plasma as used for plasma-assisted ALD (operating pressure: 8 mTorr; plasma power: 100 W). The emission in the (vacuum) ultraviolet region was measured by means of a VUV monochromator and the emission in the visible by a simple spectrometer (Refs. 302 and 303). Emission peaks were identified using the literature (Refs. 373–377). The insets show photographs of the corresponding plasmas.
(Color online) Various reactor configurations for plasma-assisted ALD (Ref. 136): (a) radical-enhanced ALD, (b) direct plasma-assisted ALD, (c) remote plasma ALD, and (d) direct plasma reactor with mesh. The reactor layouts and plasma sources shown serve only as examples. Reprinted with permission from S.B.S. Heil et al., J. Vac. Sci. Technol. A 25, 1357 (2007). Copyright 2007 American Vacuum Society.
(Color online) Growth per cycle of Al2O3 films as a function of the substrate temperature. The films were deposited by plasma-assisted ALD (O2 plasma) and thermal ALD (H2O). Two different ALD reactors were used; one operating at 15 mTorr and the other at 170 mTorr (Refs. 43 and 323). From S.E. Potts et al., J. Electrochem. Soc. 157, P66 (2010). Reproduced by permission of ECS—The Electrochemical Society.
(Color online) Growth per cycle of TiO2 films as a function of the substrate temperature. Plasma-assisted ALD was carried out using Ti(O i Pr)4, Ti(CpMe)(O i Pr)3, Ti(Cp*)(OMe)3, and Ti(CpMe)(NMe2)3 as precursors in combination with an O2 plasma (Refs. 56 and 248). Data for thermal ALD with H2O (Ref. 325) and O3 (Ref. 324) using the widely employed Ti(O i Pr)4 precursor are given for comparison. From S.E. Potts et al., J. Electrochem. Soc. 157, P66 (2010). Reproduced by permission of ECS—The Electrochemical Society.
(Color online) Resistivity (at room temperature) of TiN films obtained at 100–400 °C. The films were deposited by plasma-assisted ALD using TiCl4 in combination with an H2/N2 plasma. The resistivity was determined by in situ spectroscopic ellipsometry (thin films, ∼10 nm) and four-point probe measurements (thicker films, > 45 nm) (Ref. 237). From S.B.S. Heil et al., J. Electrochem. Soc. 153, G956 (2006). Reproduced by permission of ECS—The Electrochemical Society.
(Color online) Resistivity (at room temperature) of TaN films as a function of H2 plasma exposure time (Ref. 223). The data were obtained by in situ spectroscopic ellipsometry and four-point probe measurements. Reprinted with permission from E. Langereis et al., J. Appl. Phys. 102, 083517 (2007). Copyright 2007, American Institute of Physics.
(Color online) Thickness evolution of Pt and PtO2 films deposited on an Al2O3 substrate (Ref. 164). The precursor was Pt(Cp Me )Me3 and O2 gas or an O2 plasma were used as the oxidants. After 150 cycles, the plasma-assisted ALD process was stopped and film growth was continued using thermal ALD. The plasma exposure time for was 0.5 s for Pt and 5 s for PtO2. From H.C.M. Knoops et al., Electrochem. Solid-State Lett. 12, G35 (2009). Reproduced with permission of ECS—The Electrochemical Society.
(Color online) Results from Monte Carlo simulations investigating the influence of surface recombination of radicals during plasma-assisted ALD (Ref. 304). (a) Equivalent thickness profile in a trench of aspect ratio 10 for different deposition regimes, obtained for various combinations of values for the sticking probability, s, and surface recombination probability, r. The positions within the trench labeled 0 and 100% correspond to the trench opening and trench bottom, respectively. Note that both recombination-limited cases show almost perfect overlap. (b) The dose required to reach saturation in trenches with aspect ratios of 10 and 30 for nonzero values of r. This dose is normalized to the dose required to reach saturation in these trenches when r = 0. For the simulations s = 0.01 was assumed. From H.C.M. Knoops et al., J. Electrochem. Soc. 157, G241 (2010). Reproduced with permission of ECS—The Electrochemical Society.
(Color online) Experiments proving that VUV radiation from the plasma affects the surface passivation of crystalline Si by Al2O3 when deposited by plasma-assisted ALD (Refs. 43 and 303). After annealing, the wafers were exposed to an O2 plasma for various exposure times. (a) The effective charge carrier lifetimes degraded for increasing exposure times at a rate which increased with increasing VUV radiation present in the plasma (higher intensity for higher power and/or lower pressure). (b) Results are also given for the situation in which the substrate is covered by quartz and MgF2 windows that respectively block and do not block the VUV photons of 9.5 eV. From H.B. Profijt et al., ECS Trans. 33, 61 (2010). Reproduced with permission of ECS—The Electrochemical Society.
Surface coverage of Ti atoms as a function of the number of ALD cycles, as measured by a quartz crystal microbalance at room temperature (Ref. 232). The precursor employed was TiCl4 and the reactant was an H2 plasma. For deposition on the as-received crystal, the growth showed a linear trend after ∼5 cycles. Reprinted with permission from H. Kim et al., J. Vac. Sci. Technol. A 20, 802 (2002). Copyright 2002 American Vacuum Society.
Barrier failure temperatures for TaN and Ta films deposited by PVD and plasma-assisted ALD (Refs. 204 and 219). After deposition the TaN and Ta layers were capped by 200 nm thick Cu films using the PVD technique. Barrier failure temperatures were determined by monitoring the disappearance of the Cu (111) peak by X-ray diffraction measurements. The ALD films were deposited using an H2/N2 plasma for TaN and an H2 plasma for Ta. From H. Kim et al., J. Appl. Phys. 95, 5848 (2004). Reprinted with permission. Copyright 2004, American Institute of Physics.
Cross-sectional high-resolution transmission electron microscopy (HRTEM) images of as-grown HfO2 films deposited by (a) remote-plasma ALD and (b) direct-plasma ALD (Refs. 124 and 125). The films were deposited on Si at a deposition temperature of 250 °C using Hf(NEt2)4 as the precursor and an O2 plasma as the reactant. A gradual transition from the interface layer to the HfO2 layer can be observed for the remote-plasma ALD film, whereas the film deposited using a direct-plasma shows an abrupt transition. The film deposited using a direct-plasma was partially crystallized, whereas using a remote plasma afforded an amorphous film. From J. Kim et al., Appl. Phys. Lett. 87, 53108 (2005). Reprinted with permission. Copyright 2005, American Institute of Physics.
(Color online) The spacer-defined double patterning process (Ref. 321), in which (a) a photoresist layer is deposited on top of the target layer (i.e. the layer to be patterned) and patterned by UV exposure and photoresist development. In the next step (b) a SiO2 spacer layer is deposited at a low temperature using plasma-assisted ALD, after which an anisotropic etch is carried out. Subsequently, the photoresist is removed in step (d) after which the pattern can be transferred into the target layer. Finally, in step (f) the SiO2 spacers are removed, after which narrow features at half-pitch are left. Between steps (a) and (b), an optional plasma-treatment can be carried out in order to additionally reduce the feature thickness of the photo resist. In (g) a scanning-electron microscopy image is shown, illustrating a photoresist pattern covered by a conformal SiO2 spacer layer (corresponding to the situation in (b)). Courtesy of ASM International N.V.
(Color online) Water vapor transmission rate (WVTR) of a 20 nm thick Al2O3 films on poly(2,6-ethylenenaphthalate) (PEN) substrates as a function of the deposition temperature (Ref. 54).The Al2O3 films were deposited by plasma-assisted ALD using AlMe3 as the precursor and an O2 plasma as the reactant. The inset shows the WVTR as a function of the film thickness for a film deposited at room temperature. The WVTR values were determined using a standard calcium test (Ref. 378) and include water permeation through pinholes which are possibly present. From E. Langereis et al., Appl. Phys. Lett. 89, 081915 (2006). Reprinted with permission. Copyright 2006, American Institute of Physics.
Overview of the materials deposited by plasma-assisted ALD. The material, the precursor, the plasma gas (only the reactant gas, not the carrier gas), the reactor type (“re” is radical-enhanced, “d” is direct-plasma ALD, “r” is remote plasma ALD, and “—” is not specified) and the references are given for processes reported up to May 31, 2011. The search was run in published abstracts using Web of Science® (Ref. 23). acac = acetylacetonate, amd = N,N′-diisopropylacetamidinate, cod = 1,4-cyclooctadiene, Cp = η5-cyclopentadienyl, Cp* = η5-pentamethylcyclopentadienyl, CpEt = η5-ethylcyclopentadienyl, Cp i Pr = η5-isopropylcyclopentadienyl, CpMe = η5-methylcyclopentadienyl, dmamb = 1-dimethylamino-2-methyl-2-butanolate, dme = dimethoxyethane, Et = ethyl, fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate, hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate, i Pr = isopropyl, Me = methyl, mp = 3-methyl-3-pentoxyl, n Bu = butyl, Ph = phenyl, t Bu = tertiary butyl, thd = 2,2,6,6-tetramethyl-3,5-heptanedionate, t Pn = tertiarypentyl, vtmos = vinyltrimethoxylsilane.
Overview of recombination loss probabilities, r, for H, N and O radicals on the surfaces of various materials (Ref. 304). Accuracies in the values are indicated where available. The data are taken from Refs. 305–310.
Densities of plasma species in an O2 plasma, as typically used in plasma ALD processes. Data are presented for two different pressures and the electron temperature, Te , and energy, E ion, of ions accelerated to the (grounded) substrate are also given. The data have been compiled from the modeling results described in Ref. 314 for an inductively-coupled plasma operated at a source power of 500 W. The excited species O* and O2 * correspond to the lowest metastable states being O (1D) and O2 (a 1Δg), respectively. Note that the calculated ion energy is lower than the measured ion energy reported on in Fig. 4, probably as a result of a different reactor geometry and capacitive-coupling of the plasma between the coil and the grounded reactor wall.
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