HfO2 Etching Research Articles

Selective atomic layer etching of Al2O3, AlNx and HfO2 in conventional ICP etching tool

The atomic layer etching process of Al2O3, AlNx and HfO2 in conventional plasma etching tool was investigated. The etching process is based on surface modification by fluorocarbon film deposition from Ar/CF4/H2 plasma and subsequent activation of etching by Ar ion bombardment from plasma. The study of deposition process showed that varying plasma composition surface kinetics significantly change from deposition to etching. This is associated with the rise of fluorine concentration both in plasma and in deposited fluorocarbon film. Plasma mixture with low CF4 fraction was chosen for modification step of ALE process because it provides low fluorinated film which does not etch SiO2 and TiN improving selectivity to the potential mask material in etch process. The ALE process shows self-limiting characteristics providing the ALE process window at duration of modification step. The etch rate is 0.16 nm/cycle for Al2O3, 0.20 nm/cycle for AlNx and 0.11 nm/cycle for HfO2, which is close to monolayer mode. Relatively short cycle duration provides feasible etch rate in terms of time consumption. The significant selectivity over TiN (more than 20) was achieved allowing employing TiN as mask material for high resolution selective etching.

Plasma enhanced atomic layer etching of high-k layers on WS2

The etching of HfO2 and ZrO2 high-k dielectrics is studied using plasma enhanced atomic layer etching. The etching method relies on a continuous argon inductively coupled plasma discharge in which reactive gases are pulsed, followed by substrate biasing; both steps are separated by purge periods. It is found that pure BCl3 is too chemically active while a Cl2–BCl3 allows a high process synergy; in addition, the latter gives a high selectivity to SiO2. The optimal etch conditions are applied to high-k layers deposited on top of WS2 transition metal dichalcogenide. Postetch analysis shows negligible tungsten and sulfur depletion as well as negligible change in optical (Raman) response of the 2D layer, indicating that atomic layer etching concepts allows us to prevent WS2 material loss or damage.

Kinetics of HfO2 etching and impurity elimination with atomic hydrogen beams

Modification of the surface composition, stoichiometry, morphology, and structure of HfO2 ultra-thin layers upon exposure to atomic hydrogen beams has been investigated by a combination of time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profile analysis, angle-resolved x-ray photoelectron spectroscopy (ARXPS), atomic force microscopy (AFM) imaging, and transmission electron microscopy (TEM). The etching reaction has been found to be thermally activated (Ea = 96 kJ/mol) and governed by the expression Retch(Å/h) = 3.03 × 108e-1.15x104T within the 350–400 °C temperature range. The rate law determined experimentally, i.e., Retch(Å/h) = 1.07 × 10−1[H2]1/2, is consistent with a reaction mechanism that involves the formation of a volatile hafnium dihydroxide, Hf(OH)2 in two successive elementary steps. Precise control of the etching conditions allows to reach atomically smooth HfO2 surfaces having r.m.s. roughness below 0.2 nm, with no evidence of Hf metal formation derived from the reaction with atomic H. Elimination of C, Cl, F, and S contaminants from the HfO2 surface through the formation of the corresponding volatile hydrides has proved to be generally most effective at high temperatures and H2 flow rates.

Selective atomic layer etching of HfO2 over silicon by precursor and substrate-dependent selective deposition

The early work of John Coburn and Harold Winters revealed a synergistic effect in ion-enhanced Si etching by the concurrent irradiation of Ar+ and XeF2. This finding provided an important foundation for the development of plasma dry etching processes. The experimental results of Coburn and Winters also found effects that are useful for the development of atomic layer etching (ALE) processes characterized by a self-limited etch rate. ALE approaches are widely established and can be utilized in either directional or isotropic etching by employing proper surface modification and product removal steps. Nevertheless, the development of material selective ALE processes is still limited. An approach that combines substrate-selective deposition with etching opens a new processing window for selective ALE. The authors studied the deposition behavior of mixtures of methane (CH4) with trifluoromethane (CHF3) and mixtures of methane with octafluorocyclobutane (C4F8) on HfO2 and Si surfaces. The experimental results show that a CH4/C4F8 mixture produces a comparable fluorocarbon (FC) deposition thickness on both HfO2 and Si during the deposition step. In contrast, a CH4/CHF3 mixture deposits an FC film on Si, whereas it fluorinates the HfO2 surface with negligible FC deposition. Utilizing these behaviors allows for an ALE process based on CH4/CHF3 for selective removal of HfO2 over Si. Surface characterization data that provide mechanistic insights into these processes are also provided and discussed.

Thermal Atomic Layer Etching of Al2O3, HfO2, and ZrO2 Using Sequential Hydrogen Fluoride and Dimethylaluminum Chloride Exposures

Atomic layer etching (ALE) of Al2O3, HfO2, and ZrO2 was accomplished using sequential exposures with hydrogen fluoride (HF) as the fluorination reagent and dimethylaluminum chloride (DMAC, AlCl(CH3...

Thermal atomic layer etching of HfO2 using HF for fluorination and TiCl4 for ligand-exchange

Thermal atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF has been a typical fluorination reactant. Various metal reactants have been used for ligand-exchange, such as Sn(acac)2, Al(CH3)3, AlCl(CH3)2, and SiCl4. This study explored TiCl4 as a new metal chloride reactant for ligand-exchange. Thermal HfO2 ALE using HF and TiCl4 as the reactants was studied using in situ quartz crystal microbalance (QCM) measurements from 200 to 300 °C. The HfO2 films were etched linearly versus the number of HF and TiCl4 reaction cycles. The sequential HF and TiCl4 reactions were also self-limiting versus reactant exposure. The QCM studies observed a mass change per cycle (MCPC) of −10.2 ng/(cm2 cycle) at 200 °C and −56.4 ng/(cm2 cycle) at 300 °C. These MCPCs correspond to HfO2 etch rates of 0.11 Å/cycle at 200 °C and 0.59 Å/cycle at 300 °C. To explore the selectivity of thermal ALE using HF and TiCl4 as the reactants, spectroscopic ellipsometry (SE) measurements were also employed to survey the etching of various materials. The SE results revealed that HfO2 and ZrO2 were etched by HF and TiCl4. In contrast, Al2O3, SiO2, Si3N4, and TiN were not etched by HF and TiCl4. The etching selectivity can be explained by the reaction thermochemistry and the stability and volatility of the possible etch products. Al2O3 can also serve as an etch stop for HfO2 ALE.

Electronic Structure of Oxygen Deficient Noncentrosymmetric Orthorhombic Hf0.5Zr0.5O2

High-κ hafnia/zirconia solid solutions Hf0.5Zr0.5O2 are promising for advanced microelectronics. It is known that the noncentrosymmetric orthorhombic (Pbc21) Hf0.5Zr0.5O2phase exhibits ferroelectric properties. This new ferroelectric material is of significant interest due to a potential possibility of replacement of traditional perovskite-based ferroelectrics in ferroelectric random access memory (FeRAM) [1]. In the present study the electronic structure of oxygen vacancies (VO) in o-Hf0.5Zr0.5O2 was investigated with quantum-chemical simulations using the Quantum-ESPRESSO code. The simulations are based on the density functional theory with B3LYP hybrid exchange-correlation functional. We use the periodic (96 atom) supercells model. The structure was obtained by the replacement of half of the Hf atom to Zr in the orthorhombic HfO2 cell [2]. Three types of VO with fourfold «HHZZ» and threefold coordinated «HHZ», «HZZ» were investigated. 20 nm thick orthorhombic Hf0.5Zr0.5O2 were synthesized by ALD technique with following rapid thermal annealing at 400 °C. X-ray photoelectron spectra was obtained using synchrotron radiation with a photon energy of 720 eV for the analysis of the chemical composition and 200 eV to record the spectrum of the valence band. Film stoichiometry was calculated as ratios of the integrated intensities of the main XPS lines of the metal atoms show the stoichiometry close to Hf0.5Zr0.5O2. The calculated band gap of o-Hf0.5Zr0.5O2 is 5.65 eV. The Kohn–Sham levels of five VO charged states (q = -2, -1, 0, 1, 2) localized in the band gap indicate that any type of oxygen vacancies can capture both electrons and holes. Note, that the oxygen vacancies with a trapped electron is a magnetic defect. The added electron localizes itself in an energy well due to strong neighboring atom relaxation (polaron effect). The values of thermal (E th) and optical (E opt) ionization energies were calculated for different charge states as described in [3]. It was found that the oxygen vacancy can act as an amphoteric localization center (trap) for charge carriers, i.e. it can capture both electrons and holes. The calculated E th and E opt values are in satisfactory agreement with the values of thermal (W t=1.25 eV) and optical (W opt=2.5 eV) trap energies, obtained from experiments on charge transport. Charge localization on the oxygen vacancy is confirmed by the spatial distribution of the electron density in VO and its first coordination sphere. The positive charge is distributed approximately evenly between the metal atoms nearest to VO, while the negative charge is distributed unevenly, and only a couple of atoms are bounded. The position of each subsequent atom for removing was found by examining of all the possible variants and determining the lowest defect formation energy. The oxygen atoms in o-Hf0.5Zr0.5O2, which are removed, have an HHZZ type and are placed at a significant distance from each other. The forming energy of triply coordinated VO (about 6.4 eV) is substantially higher than that for fourfold coordinated VO (about 6.1 eV). The formation of closely spaced VO (i.e. polyvacancy) in o-Hf0.5Zr0.5O2 is not profitable energetically. On the contrary, for the cubic, tetragonal and monoclinic HfO2 and ZrO2 formation energies of three- and fourfold coordinated VO is not so much different from each other, the it is energy profitable to form oxygen vacancies close to each other (there is some clustering of vacancies). Probably, this is the reason why the o-Hf0.5Zr0.5O2 film etching with Ar+ ions does not lead to metal enrichment, whereas the etching of amorphous HfO2 and ZrO2 leads to the generation high concentration of VO in the surface layer of the films. As a result of Hf0.5Zr0.5O2 etching with Ar+ ions, the XPS lines corresponding to Hf 4f and Zr 3d in the metallic state are not observed, and theoretically predicted XPS peak does not appear above the edge of the valence band. It is concluded that the original Hf0.5Zr0.5O2 films are not enriched with metal, and etching with Ar+ ions does not lead to depletion of oxygen. However, the experimental data do not exclude the oxygen vacancies presence in the Hf0.5Zr0.5O2 film, since XPS technique is insensitive to defects with density less than 0.1%. The work was supported by the Ministry of Education and Science of the Russian Federation, project #RFMEFI57514X0027 (Hf0.5Zr0.5O2 films synthesis), and by Russian Science Foundation, grant #14-19-00192 (XPS measurements, calculations). The modeling was performed on a cluster of the Institute of Semiconductor Physics SB RAS. [1] J.Müller, T.S.Böscke, U.Schröder, et al.Nano Lett. 12, 4318 (2012). [2] Q.Zeng, A.R.Oganov, A.O.Lyakhov, et al.Acta Cryst. C 70, 76 (2014). [3] D.Muñoz Ramo, J.L.Gavartin, A.L.Shluger et al. Phys. Rev. B 75, 205336 (2007).

Prospects for Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions.

Thermal atomic layer etching (ALE) of Al2O3 and HfO2 using sequential, self-limiting fluorination and ligand-exchange reactions was recently demonstrated using HF and tin acetylacetonate (Sn(acac)2) as the reactants. This new thermal pathway for ALE represents the reverse of atomic layer deposition (ALD) and should lead to isotropic etching. Atomic layer deposition and ALE can together define the atomic layer growth and removal steps required for advanced semiconductor fabrication. The thermal ALE of many materials should be possible using fluorination and ligand-exchange reactions. The chemical details of ligand-exchange can lead to selective ALE between various materials. Thermal ALE could produce conformal etching in high-aspect-ratio structures. Thermal ALE could also yield ultrasmooth thin films based on deposit/etch-back methods. Enhancement of ALE rates and possible anisotropic ALE could be achieved using radicals or ions together with thermal ALE.

(Invited) Atomic Layer Etching Using Thermal Reactions: Atomic Layer Deposition in Reverse

Atomic layer etching (ALE) is a thin film removal technique based on sequential, self-limiting surface reactions. ALE is the reverse of atomic layer deposition (ALD). ALD has developed rapidly over the last 10-15 years to meet many technological needs such as the miniaturization of semiconductor devices. In contrast, ALE is relatively new and is only starting to receive serious attention. Initial work in ALE has focused on plasma ALE using halide adsorption and energetic ion bombardment to obtain anisotropic etching. There is a need for thermal approaches to ALE based on spontaneous reactions that would produce isotropic etching. This talk will present a new thermal approach to ALE based on sequential, self-limiting thermal surface reactions. Examples will be presented for Al2O3 [1, 2], HfO2 [3] and AlF3 ALE. The ALE is achieved using sequential, self-limiting thermal reactions. For the metal oxides, the ALE process is based on fluorination and ligand-exchange reactions. Using HF and Sn(acac)2 as the reactants, Al2O3 and HfO2 ALE are examined using quartz crystal microbalance, x-ray reflectivity and Fourier transform infrared spectroscopy measurements. These studies show that controlled, atomic level removal of Al2O3 and HfO2 is possible at temperatures from 150-250°C [1-3]. The surface reaction mechanism involves the formation of a metal fluoride reaction intermediate from fluorination of the metal oxide by HF. The HF reactant also allows H2O to leave as a reaction product. The Sn(acac)2 reactant then accepts fluorine from the metal fluoride and donates acac to the substrate. The donated acac ligand can produce Al(acac)3 or Hf(acac)4 as volatile reaction products. AlF(acac)2 and HfF(acac)3 may also be volatile reactions products. Sequential exposures of HF and Sn(acac)2 produce Al2O3 ALE etching rates of 0.14 Å/cycle- 0.61 Å/cycle per cycle at temperatures from 150- 250°C [1, 2]. Sequential exposures of HF and Sn(acac)2 produce HfO2 ALE etching rates of 0.070 Å/cycle - 0.117 Å/cycle per cycle at temperatures from 150- 250°C [3]. This ALE reaction mechanism based on fluorination and ligand-exchange should be applicable to other materials such as metal nitrides, metal phosphides, metal arsenides and elemental metals. This ALE reaction mechanism is also general and works with other fluorine metal acceptors such as Al(CH3)3 and fluorination precursors such as XeF2. In addition, metal fluorides are candidates for ALE. Fourier transform infrared spectroscopy reveals that Al2O3 is converted to AlF3 during the HF exposure. Sn(acac)2 can then remove the AlF3 by the ligand-exchange process. If AlF3 is a reaction intermediate during Al2O3 ALE, then AlF3 ALE should also be possible using AlF3 substrates. We have demonstrated AlF3 ALE using AlF3 ALD thin films grown using Al(CH3)3 and HF. The AlF3 ALE was performed using Sn(acac)2 and HF as the ALE reactants. 1. Younghee Lee and Steven M. George, “Atomic Layer Etching of Al2O3 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF”, ACS Nano 9,2061 (2015). 2. Younghee Lee, Jaime W. DuMont and Steven M. George, “Mechanism of Thermal Al2O3 Atomic Layer Etching Using Sequential Reactions with Sn(acac)2 and HF” Chem. Mater. (In Press). 3. Younghee Lee, Jaime W. DuMont and Steven M. George, “Atomic Layer Etching of HfO2 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF”, ECS J. Solid State Sci. Technol. 4, N5013 (2015).

Atomic Layer Etching of HfO2 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF

The atomic layer etching (ALEt) of HfO2 was performed using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac)2) and HF as the reactants. The HF source was a HF-pyridine solution. The etching of HfO2 was linear with atomic level control versus number of Sn(acac)2 and HF reaction cycles. The HfO2 ALEt was measured at temperatures from 150–250°C. Quartz crystal microbalance (QCM) measurements determined that the mass change per cycle (MCPC) increased with temperature from −6.7 ng/(cm2 cycle) at 150°C to −11.2 ng/(cm2 cycle) at 250°C. These MCPC values correspond to etch rates from 0.070 Å/cycle at 150°C to 0.117 Å/cycle at 250°C. X-ray reflectivity analysis confirmed the linear removal of HfO2 and measured an HfO2 ALEt etch rate of 0.11 Å/cycle at 200°C. Fourier transform infrared (FTIR) spectroscopy measurements also observed HfO2 ALEt using the infrared absorbance of the Hf-O stretching vibration. FTIR analysis also revealed absorbance features consistent with HfF4 or HfFx surface species as a reaction intermediate. The HfO2 etching is believed to follow the reaction: HfO2 + 4Sn(acac)2 + 4HF → Hf(acac)4 + 4SnF(acac) + 2H2O. In the proposed reaction mechanism, Sn(acac)2 donates acac to the substrate to produce Hf(acac)4. HF allows SnF(acac) and H2O to leave as reaction products. The thermal ALEt of many other metal oxides, as well as metal nitrides, phosphides, sulfides and arsenides, should be possible by a similar mechanism.

On the etching characteristics and mechanisms of HfO2 thin films in CF4/O2/Ar and CHF3/O2/Ar plasma for nano-devices.

The study of etching characteristics and mechanisms for HfO2 and Si in CF4/O2/Ar and CHF3/O2/Ar inductively-coupled plasmas was carried out. The etching rates of HfO2 thin films as well as the HfO2/Si etching selectivities were measured as functions of Ar content in a feed gas (0-50% Ar) at fixed fluorocarbon gas content (50%), gas pressure (6 mTorr), input power (700 W), bias power (200 W), and total gas flow rate (40 sccm). Plasma parameters as well as the differences in plasma chemistries for CF4- and CHF3-based plasmas were analyzed using Langmuir probe diagnostics and 0-dimensional plasma modeling. It was found that, in both gas systems, the non-monotonic (with a maximum at about 15-20% Ar) HfO2 etching rate does not correlate with monotonic changes of F atom flux and ion energy flux. It was proposed that, under the given set of experimental conditions, the HfO2 etching process is affected by the factors determining the formation and decomposition kinetics of the fluorocarbon polymer layer. These factor are the fluxes of CF(x) (x = 1, 2) radicals, O atoms and H atoms.

Ion beam etching for multilayer dielectric pulse compressor gratings with top layers of HfO2

Multilayer dielectric grating (MDG) is one of the key optical elements of high-power laser systems. To meet the need of MDGs for high-power laser systems, experimental investigation on MDG with a top layer of HfO2 has been carried out using Kaufman-type ion beam etcher. The optimal ion source conditions have been obtained by etching of HfO2 in pure Ar and Ar/CHF3 mixture plasmas. Compared with pure Ar plasma etching, better selectivity was achieved with Ar/CHF3. The redeposition of sidewalls effects are quite obvious during etching, which results in the increase in duty cycle of etched grating. As there is a distribution of etch rate along the direction normal to the scan movement, a special-shaped mask was made to be used as a substrate holder, which increases uniformity of the etched profile. In order to process repeatability, the ion source should be cleaned up, the cathode and neutralizer filament should be changed after etching process to full completion. Based on the above techniques, a number of MDGs have been achieved, each of which has a mean diffraction efficiency greater than 95%, a line density 1480 lines/mm, and on aperture up to 80 mm×150 mm. Experimental results agree fairly well with the designed, which provides a good reference for the large aperture MDGs ion beam etching.

Wet-Chemical Etching of IGZO and HfO2

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Feedback control of plasma electron density and ion energy in an inductively coupled plasma etcher

Here the authors report the development of a fuzzy logic based feedback control of the plasma electron density and ion energy for high density plasma etch process. The plasma electron density was measured using their recently developed transmission line microstrip microwave interferometer mounted on the chamber wall, and the rf voltage was measured by a commercial impedance meter connected to the wafer stage. The actuators were two 13.56 MHz rf power generators which provided the inductively coupled plasma power and bias power, respectively. The control system adopted the fuzzy logic control algorithm to reduce frequent actuator action resulting from measurement noise. The experimental results show that the first wafer effect can be eliminated using closed-loop control for both poly-Si and HfO2 etching. In particular, for the HfO2 etch, the controlled variables in this work were much more effective than the previous one where ion current was controlled, instead of the electron density. However, the pressu...

Dilute hydrogen plasma cleaning of boron from silicon after etching of HfO2 films in BCl3 plasmas: Substrate temperature dependence

The authors have investigated the effects of elevated substrate temperature (Ts) on cleaning of boron residues from silicon substrates in 1%H2–Ar plasmas, following etching of HfO2 in BCl3 plasmas. Vacuum-transfer x-ray photoelectron spectroscopy (XPS) provided a measure of total B removal rates, as well as information on individual BClxOy moities. B cleaning rates increased with Ts in an Arrhenius manner, with an apparent activation energy of 1.7kcal∕mol. Conversely, the Si etching rate decreased with increasing substrate temperature with an apparent activation energy of −0.8kcal∕mol. Therefore, when considering selectivity with respect to Si etching, it is advantageous to remove B at higher Ts. For example, at Ts=235°C, ∼90% of B is cleaned from Si in 10s, while <1.5nm of Si is removed. An apparent diffusion of H into the near-surface region of Si at higher temperatures, detected indirectly by a shift and broadening of the Si(2p) XPS peak, may limit the maximum optimum substrate temperature, however. It was also found that Si does not etch in 1%H2∕Ar plasmas if an oxide layer is present.

Selective Etching of HfO2 by Using Inductively-Coupled Ar/C4F8 Plasmas and the Removal of Etch Residue on Si by Using an O2 Plasma Treatment

HfO2 thin lms were etched using an Ar/C4F8 inductively-coupled plasma (ICP) for high etch selectivity of HfO2/Si and the uorocarbon remaining on the silicon surface after the HfO2 etching was removed by using an oxygen ICP and its e ect was investigated. The etching of HfO2 using Ar/C4F8 magnetically-enhanced ICP (MEICP) improved the etch selectivity of HfO2/Si by more than three times, possibly due to the di erences in the thicknesses of the uorocarbon polymer layers formed on the surfaces of HfO2 and Si. In addition, the oxygen ICP treatment after the HfO2 thin lm etching by using Ar/C4F8 ICP removed the polymer layer on the silicon surface e ectively, so for the HfO2-nMOSFET (n-type metal-oxied-semiconductoreld-e ect-transistors) devices, an improvement in drain current of more than 60 % could be observed after the O2 ICP treatment.

Feedback control of HfO2 etch processing in inductively coupled Cl2/N2/Ar plasmas

The etch rate of HfO2 etch processing has been feedback controlled in inductively coupled Cl2/N2/Ar plasmas. The ion current and the root mean square rf voltage on the wafer stage, which are measured using a commercial impedance meter connected to the wafer stage, are chosen as controlled variables because the positive-ion flux and ion energy incident upon the wafer surface are the key factors that determine the etch rate. Two 13.56 MHz rf generators are used to adjust the inductively coupled plasma power and bias power which control ion density and ion energy, respectively. The adopted HfO2 etch processing used rather low rf voltage. The ion-current value obtained by the power/voltage method is underestimated, so the neural-network model was developed to assist estimating the correct ion-current value. The experimental results show that the etch-rate variation of the closed-loop control is smaller than that of the open-loop control. However, the first wafer effect cannot be eliminated using closed-loop control and thus to achieve a constant etch rate, the chamber-conditioning procedure is required in this etch processing.

Effect of O2 Gas during Inductively Coupled O2/Cl2 Plasma Etching of Mo and HfO2 for Gate Stack Patterning

In this study, we investigated the etching characteristics of Mo and HfO2 single layers and Mo/HfO2 stacked structure for metal electrode/high-k gate stack patterning in O2/Cl2 inductively coupled plasmas and the effects of O2 addition on the etch rates and etch selectivity of the Mo to the HfO2 layer. By controlling the process parameters such as the O2/Cl2 flow ratio, the top electrode power and the dc self-bias voltage (Vdc), the Mo/HfO2 etch selectivity as high as \\cong67 could be obtained. Addition of O2 gas to the O2/Cl2 chemistry improved the Mo/HfO2 etch selectivity because the O2 gas in a certain flow ratio range reduced the HfO2 etching reactions due to less chlorination of Hf but enhanced the Mo etch rate presumably due to effective formation of highly volatile Mo–O–Cl etch by-products.

Mechanisms and selectivity for etching of HfO2 and Si in BCl3 plasmas

The authors have investigated plasma etching of HfO2, a high dielectric constant material, and poly-Si in BCl3 plasmas. Etching rates were measured as a function of substrate temperature (Ts) at several source powers. Activation energies range from 0.2to1.0kcal∕mol for HfO2 and from 0.8to1.8kcal∕mol for Si, with little or no dependence on source power (20–200W). These low activation energies suggest that product removal is limited by chemical sputtering of the chemisorbed Hf or Si-containing layer, with a higher Ts only modestly increasing the chemical sputtering rate. The slightly lower activation energy for HfO2 results in a small improvement in selectivity over Si at low temperature. The surface layers formed on HfO2 and Si after etching in BCl3 plasmas were also investigated by vacuum-transfer x-ray photoelectron spectroscopy. A thin boron-containing layer was observed on partially etched HfO2 and on poly-Si after etching through HfO2 films. For HfO2, a single B(1s) feature at 194eV was ascribed to a heavily oxidized species with bonding similar to B2O3. B(1s) features were observed for poly-Si surfaces at 187.6eV (B bound to Si), 189.8eV, and 193eV (both ascribed to BOxCly). In the presence of a deliberately added 0.5% air, the B-containing layer on HfO2 is largely unaffected, while that on Si converts to a thick layer with a single B(1s) peak at 194eV and an approximate stoichiometry of B3O4Cl.

Ion-radical synergy in HfO2 etching studied with a XeF2/Ar+ beam setup

To gain more insight into fundamental aspects of the etching behavior of Hf-based high-k materials in plasma etch reactors, HfO2 films were etched in a multiple-beam setup consisting of a low energy Ar+ ion beam and a XeF2 radical beam. The etch rate and etch products were monitored by real-time ellipsometry and mass spectrometry, respectively. Although etching of HfO2 in XeF2/Ar+ chemistry is mainly a physical effect, an unambiguous proof of the ion-radical synergistic effect for the etching of HfO2 is presented. The etch yield for 400 eV Ar+ ions at a substrate temperature of 300 °C was 0.3 atoms/ion for Ar+ sputtering and increased to 2 atoms/ion when XeF2 was also supplied. The etch yield proved to follow the common square root of ion energy dependence both for pure sputtering and radical enhanced etching, with a threshold energy at room temperature of 69±17 eV for Ar+ ions and 54±14 eV for Ar+ ions with XeF2.