Etch Rate Enhancement Research Articles

How many vs which: On confined track selection criteria for apatite fission track analysis

Inter-laboratory and inter-analyst experiments on confined fission-track length measurements in apatite have reported considerable variation, which can be attributed primarily to differences in analyst decision-making. Here we explore how different etching sequences, track selection approaches, and analytical methods influence track length and density measurements. The first experiment focuses on analyst judgement across different track selection criteria and light sources, and variation in effective etching time, using moderately annealed induced tracks in Durango apatite. Tracks are selected either with the usual approach of only choosing well-etched tracks, which we term the sufficiently-etched criterion, or a more relaxed criterion of simply being measurable without judging how etched they are. For the sufficiently-etched criterion, we separately analyze tracks found using exclusively reflected or transmitted light. We then follow the evolution of all selected tracks through further etching steps, and compare results based on length, angle, and dip, and placed in the context of a variable along-track etching rate model. All step etches show evidence of along-track etching rate falling from core to tip, and tracks measured using the sufficiently-etched criterion are slightly under-etched compared to the full latent track zone with enhanced etching rate. Minimizing effective etch time variation, by only using tracks selected after 10 s of etching, produced tracks averaging ~0.3 μm longer after 20 s than tracks selected using the sufficiently-etched criterion. Ensuring a minimum track etching time by first selecting measurable tracks after 20 s and etching for 10 s more produced tracks ~0.4 μm longer, but correctable to the latent length. Utilizing exclusively transmitted light diminishes mean track dip, and likely reflects lower efficiency in finding dipping tracks. In the second experiment, we repeatedly measure dipping confined tracks at various stages of annealing with and without using a cross-sectional view of the track, finding that the latter avoids an increase in scatter with increasing dip. We propose a two-step etching procedure that may provide more plentiful and reproducible data by reducing the impact of human decision-making, while still being relatable to published annealing data sets.

Contribution of Ion Energy and Flux on High-Aspect Ratio SiO2 Etching Characteristics in a Dual-Frequency Capacitively Coupled Ar/C4F8 Plasma: Individual Ion Energy and Flux Controlled.

As the process complexity has been increased to overcome challenges in plasma etching, individual control of internal plasma parameters for process optimization has attracted attention. This study investigated the individual contribution of internal parameters, the ion energy and flux, on high-aspect ratio SiO2 etching characteristics for various trench widths in a dual-frequency capacitively coupled plasma system with Ar/C4F8 gases. We established an individual control window of ion flux and energy by adjusting dual-frequency power sources and measuring the electron density and self-bias voltage. We separately varied the ion flux and energy with the same ratio from the reference condition and found that the increase in ion energy shows higher etching rate enhancement than that in the ion flux with the same increase ratio in a 200 nm pattern width. Based on a volume-averaged plasma model analysis, the weak contribution of the ion flux results from the increase in heavy radicals, which is inevitably accompanied with the increase in the ion flux and forms a fluorocarbon film, preventing etching. At the 60 nm pattern width, the etching stops at the reference condition and it remains despite increasing ion energy, which implies the surface charging-induced etching stops. The etching, however, slightly increased with the increasing ion flux from the reference condition, revealing the surface charge removal accompanied with conducting fluorocarbon film formation by heavy radicals. In addition, the entrance width of an amorphous carbon layer (ACL) mask enlarges with increasing ion energy, whereas it relatively remains constant with that of ion energy. These findings can be utilized to optimize the SiO2 etching process in high-aspect ratio etching applications.

Computational approach for plasma process optimization combined with deep learning model

As semiconductor device structures become more complex and sophisticated, the formation of finer and deeper patterns is required. To achieve a higher yield for mass production as the number of process steps increases and process variables become more diverse, process optimization requires extensive engineering effort to meet the target process requirements, such as uniformity. In this study, we propose an efficient process design framework that can efficiently search for optimal process conditions by combining deep learning (DL) with plasma simulations. To establish the DL model, a dataset was created using a two-dimensional (2D) hybrid plasma equipment model code for an argon inductively coupled plasma system under a given process window. The DL model was implemented and trained using the dataset to learn the functional relationship between the process conditions and their consequential plasma states, which was characterized by 2D field data. The performance of the DL model was confirmed by comparison of the output with the ground truth, validating its high consistency. Moreover, the DL results provide a reasonable interpretation of the fundamental features of plasmas and show a good correlation with the experimental observations in terms of the measured etch rate characteristics. Using the designed DL, an extensive exploration of process variables was conducted to find the optimal processing condition using the multi-objective particle swarm optimization algorithm for the given objective functions of high etch rate and its uniform distribution. The obtained optimal candidates were evaluated and compared to other process conditions experimentally, demonstrating a fairly enhanced etch rate and uniformity at the same time. The proposed computational framework substantially reduced trial-and-error repetitions in tailoring process conditions from a practical perspective. Moreover, it will serve as an effective tool to narrow the processing window, particularly in the early stages of development for advanced equipment and processes.

(Digital Presentation) Stacked Nanosheet FETs and Beyond

Si FinFETs have been used in the industry for mass production of advanced technology nodes [1-3]. Recently, the horizontal gate-all-around (GAA) transistors are considered as promising candidates to replace FinFETs due to the excellent electrostatics and short channel control [4-6]. The GAA architecture with vertically channel stacking can further enhance the drive current for a given footprint to achieve high performance and area scaling [7]. Moreover, high mobility channel is an attractive option to boost the drive current thanks to its intrinsic higher mobility as compared with Si [8]. Furthermore, a novel TreeFET architecture [9] as a combination of stacked nanosheets and additional fin interbridges (IBs) between nanosheets can enhance ION per footprint.In this work, we demonstrate the 8 stacked Ge0.75Si0.25 nanosheets with high inter-channel uniformity and the 7 stacked Ge0.95Si0.05 nanowires with record ION per channel footprint for nFETs [10, 11]. The highly stacked 8 Ge0.9Sn0.1 nanosheet pFET with ultrathin bodies and thick bodies are demonstrated for low power and high performance [12, 13], respectively. Furthermore, TreeFETs can increase Weff per footprint for high performance devices beyond FinFETs and nanosheets.The epilayers with 8 undoped and fully strained Ge0.75Si0.25 layers sandwiched by 9 P-doped Ge sacrificial layers (SL) were grown on the Ge buffer by CVD epitaxy. The S/D and channels of the 8 stacked Ge0.75Si0.25 nanosheets are fabricated by the same epilayer structures without S/D regrowth. The doping of S/D is obtained by the heavily P-doped Ge SLs by annealing, while the channel is undoped to reduce the impurity scattering. H2O2 wet etching was used to etch the Ge SLs in channel region. The etching selectivity of Ge over Ge0.75Si0.25 is attributed to the Si content in GeSi layers and the enhanced etching rate of Ge SLs by heavily doped phosphorus [14]. To further improve the drive current, [Si] in GeSi channels is decreased to 5% to ensure the electrons populated in L4 valleys with small mt. In addition to channel stacking, TreeFET combining with FinFETs and stacked nanosheets is demonstrated to enhance Weff per footprint. IBs of TreeFETs were formed by well-controlled H2O2 wet etching. With adequate etching selectivity and optimized etching time, the SLs were partially removed to form the IBs between the nanosheets for extra conduction channels.Strained GeSn has higher hole mobility than Ge to increase ION for pFET [15] due to the hole effective mass reduction under compressive strain. Recently, the radical-based highly selective isotropic dry etching (HiSIDE) was reported to form the stacked GeSn nanosheets [16-18]. However, the challenge of Ge-based channel is the large IOFF due to the small bandgap [15-20]. Ultrathin body device can effectively reduce the IOFF [21], but the mobility degradation is observed as the channel thickness decrease below 5nm due to surface roughness scattering [22]. However, the high mobility GeSn can afford some mobility loss due to the ultrathin body. By combining the high mobility channel and the large number of stacked nanosheets, the ION can be further improved with the suppression of IOFF by the ultrathin body. The 11nm Ge0.9Sn0.1 channel layers sandwiched by 8nm Ge0.97Sn0.03 caps, 3nm Ge caps, and 24nm Ge:B SLs were grown repeatedly on the Ge buffer. The thin double Ge0.97Sn0.03 caps can provide sufficient etching selectivity and prevent the channel bending. The heavily B-doped Ge SLs are used to reduce the S/D resistance. The significant IOFF reduction of the GeSn ultrathin bodies is attributed to the quantum confinement effects. The quantum confinement energy of 170meV in 3nm Ge0.9Sn0.1 ultrathin body is simulated by TCAD. The enlarged bandgap results in the suppression of IOFF and yields the record high ION/IOFF~107 of 8 stacked GeSn ultrathin bodies among GeSn/Ge 3D pFETs.Simple wet etching can reach the sufficient selectivity to fabricate the highly uniform Ge0.75Si0.25 nanosheets and the Ge0.95Si0.05 nanowires. The 3nm Ge0.9Sn0.1 ultrathin bodies with high inter-channel uniformity are demonstrated to reduce the IOFF. Highly stacked Ge0.9Sn0.1 thick nanosheets can improve the ION. TreeFET with the process flow similar to stacked nanosheets are the promising candidates for technology node scaling. Even higher number of stacked channels (>8) and extremely scaled ultrathin bodies (<3nm) are expected to further enhance the ION and to increase the ION/IOFF for advanced CMOS scaling. Acknowledgements- This work is supported by MOST (110-2218-E-002-030-, 110-2622-8-002-014-, 110-2218-E-002-034-MBK, and 110-2218-E-002-042-MBK) and MOE (NTU-CC-111L892001), Taiwan. The support from Taiwan Semiconductor Research Institute is also highly appreciated. Figure 1

Combining Experimental and DFT Investigation of the Mechanism Involved in Thermal Etching of Titanium Nitride Using Alternate Exposures of NbF5 and CCl4, or CCl4 Only

AbstractThermally activated chemical vapor‐phase etching of titanium nitride (TiN) is studied by utilizing either alternate exposures of niobium pentafluoride (NbF5) and carbon tetrachloride (CCl4) or by using CCl4 alone. Nitrogen (N2) gas purge steps are carried out in between every reactant exposure. Titanium nitride is etched in a non‐self‐limiting way by NbF5–CCl4 based binary chemistry or by CCl4 at temperatures between 370 and 460 °C. Spectroscopic ellipsometry and a weight balance are used to calculate the etch per cycle. For the binary chemistry, an etch per cycle of ≈0.8 Å is obtained for 0.5 and 3 s long exposures of NbF5 and CCl4, respectively at 460 °C. On the contrary, under the same conditions, the etch process with CCl4 alone gives an etch per cycle of about 0.5 Å. In the CCl4‐only etch process, the thickness of TiN films removed at 460 °C varies linearly with the number of etch cycles. Furthermore, CCl4 alone is able to etch TiN selectively over other materials such as Al2O3, SiO2, and Si3N4. X‐ray photoelectron spectroscopy and bright field transmission electron microscopy are used for studying the post‐etch surfaces. To understand possible reaction products and energetics, first‐principles calculations are carried out with density functional theory. From thermochemical analysis of possible reaction models, it is found that NbF5 alone cannot etch TiN while CCl4 alone can etch it at high temperatures. The predicted byproducts of the reaction between the CCl4 gas molecules and TiN surface are TiCl3 and ClCN. Similarly, TiF4, NbFCl3, and ClCN are predicted to be the likely products when TiN is exposed to both NbF5 and CCl4. A more favorable etch reaction is predicted when TiN is exposed to both NbF5 and CCl4 (ΔG = −2.7 eV at 640 K) as compared to exposure to CCl4 only (ΔG = −2 eV at 640 K) process. This indicates that an enhanced etch rate is possible when TiN is exposed alternately to both NbF5 and CCl4, which is in close agreement with the experimental results.

Characterization of m-GaN and a-GaN Crystallographic Planes after Being Chemically Etched in TMAH Solution

This paper proposes a new technique to engineer the Fin channel in vertical GaN FinFET toward a straight and smooth channel sidewall. Consequently, the GaN wet etching in the TMAH solution is detailed; we found that the m-GaN plane has lower surface roughness than crystallographic planes with other orientations, including the a-GaN plane. The grooves and slope (Cuboids) at the channel base are also investigated. The agitation does not assist in Cuboid removal or crystallographic planes etching rate enhancement. Finally, the impact of UV light on m and a-GaN crystal plane etching rates in TMAH has been studied with and without UV light. Accordingly, it is found that the m-GaN plane etching rate is enhanced from 0.69 to 1.09 nm/min with UV light; in the case of a-GaN plane etching, UV light enhances the etching rate from 2.94 to 4.69 nm/min.

Enhanced etch rate of deep-UV laser induced etching of diamond in low pressure conditions

Ultraviolet laser-induced etching is a method of machining and nanostructuring diamond surfaces in which carbon is removed from the surface via a photochemical process involving oxygen. We show here that using a dry source of oxygen at pressures in the range of 0.01–1 Torr leads to a 10-fold increase in the etch rate compared to etching in atmospheric air. The enhanced etch rate is also found to be accompanied by a marked change in the nanopatterned surface morphology. We developed a rate equation model for the etch rate that provides good agreement with measurements for pressures up to approximately 0.1 Torr. For higher pressures, the reduced etch rate and departure from the model are attributed to the contamination of the diamond surface by trace amounts of water vapor, introduced as an impurity from the gas sources. The results provide a method for markedly increasing the etch rate, as well as a better understanding of the role of gas impurities on the etch mechanism and emergent nanopattern formation.

Controlling surface chemical states for selective patterning of CoFeB

Controlling the surface chemical states of CoFeB thin films was employed to create a chemical contrast between the modified surface and the underlying intermetallic thin film, thereby enabling a selective etching process. The oxidation states of Co and Fe were tailored by reacting the CoFeB surface to chlorine or oxygen plasmas. This modified surface resulted in an enhanced etch rate as well as selectivity in an etchant such as H2 plasma or an organic vapor such as formic acid. Hydrogen plasma was found to be effective in removing metal chlorides and restoring the magnetic properties of CoFeB. For a halogen-free process, the formic acid vapor was found to be most effective in removing oxidized CoFeB. To realize atomic layer etch, a cyclical process was developed with alternating cycles of oxygen plasma with a −100 V substrate bias and formic acid vapor, resulting in a controlled etch rate at 1.8 nm/cycle and extremely high selectivity to the underlying intermetallic CoFeB.

Enhanced etching rate of black silicon by Cu/Ni Co-assisted chemical etching process

In this paper, a new low cost Cu/Ni co-assisted chemical etching (Cu/Ni MACE) method was presented for fabrication of black silicon on diamond-wire-sawn multicrystalline silicon (mc-Si). The etching rate of Cu assisted chemical etching (Cu MACE) at room temperature was greatly improved by adding Ni ion solutions. A lowest average reflectivity of 10.33% in the range of wavelength 400–900 nm was obtained, with a decrease about 50% compared with that by the etching without Ni ions. The thickness reduction of the sample obtained by the Cu/Ni MACE was 3.5 times of the one of Cu MACE when ρ at 40% (ρ = HF/(HF+H2O2) the one prepared by the Cu MACE although the ρ (ρ = HF/(HF+H2O2), volume ratio) was changed with the former being about 3.5 times of the latter. Then, a tilted inverted pyramid-like structure was successfully achieved by one-step Cu/Ni co-assisted chemical etching in Si/Cu(NO3)2/NiSO4/HF/H2O2 solution followed by a further nano structure rebuilding treatment. Finally, tilted structure was proved to have better light trapping properties by simulation using the Finite-Difference Time-Domain (FDTD) method.

A Novel Approach to Control Source/Drain Cavity Profile for Device Performance Improvement

We present a novel method to form the source/ drain (S/D) cavity for pFET performance improvement—we named this cavity as doping-assisted cavity as its profile is controlled by the lightly doped drain implantation. By utilizing the enhanced etch rate in chlorine-based dry etch process, the cavity profile is aligned to the arsenic dopant contour, enabling a lower tip height that mitigates short-channel effects. In this paper, doping-assisted S/D pFET cavity demonstrates 7% device performance improvement with matched tip CD. In addition, this new cavity enables a smaller tip CD that offers the opportunity for further device performance improvement.

Etching Kinetics of (100) Single Crystal Diamond Surfaces in a Hydrogen Microwave Plasma, Studied with In Situ Low‐Coherence Interferometry

A low‐coherence interferometry (LCI) was used to measure in situ the etch rate (ER) of synthetic single crystal (SC) diamonds in H2 microwave plasma, at substrate temperatures in the broad range of 800–1370 °C. The method allows the collection of the kinetic data on a single sample without switching off the plasma. (100)‐orientated SC plates of CVD and IIa type HPHT diamond were systematically etched in pure hydrogen at pressure p = 130 Torr and microwave power density of ≈300 W cm−3. The activation energies Ea of 42 ± 5 and 32 ± 4 kCal mol−1 have been determined for the CVD and HPHT substrates. An enhanced etching rate of a subsurface defected layer with thickness of ∼1 μm or less, formed upon polishing of the samples, is revealed. Surface morphology, roughness, and the shape of etch pits produced by a selective etching of defects, were characterized with optical profilometry. CH and dimer C2 radicals were detected in the H2 plasma with optical emission spectroscopy, as a result of the diamond etching, and the emission intensity of these species was linked to the substrate etch rate.

Ion Beam Etch for Patterning of Resistive RAM (ReRAM) Devices

ABSTRACTWe investigate the feasibility of inert ion beam etch (IBE) for subtractive patterning of ReRAM-type structures. We report on the role of the angle-dependent ion beam etch rates in device area control and the minimization of sidewall re-deposition. The etch rates of key ReRAM materials are presented versus incidence angle and ion beam energy. As the ion beam voltage is increased, we demonstrate a significant enhancement in the relative etch rate at glancing incidence (for example, by a factor of 2 for HfO2). Since the feature sidewall is typically exposed to glancing incidence, this energy-dependence plays a role in optimization of the feature shape and in sidewall re-deposition removal.We present results of SRIM simulations to estimate depth of ion-bombardment damage to the TMO sidewall. Damage is minimized by minimizing ion energy; its depth can be reduced by roughly a factor of 5 over typical IBE energy ranges. For example, ion energies of less than ∼250 eV are indicated to maintain damage below ∼1nm. Multi-angle and multi-energy etch schemes are proposed to maximize sidewall angle and minimize damage, while eliminating re-deposition across the TMO. We utilize 2-D geometry/3-D etch model to simulate IBE patterning of tight-pitched ReRAM features, and generate etched feature shapes.

Effect of UV and gamma rays on bulk etching of the Pokalon track detector

Solid-state nuclear track detectors (SSNTDs) find applications in many fields of science and technology. Polycarbonates like CR-39 or Lexan are most widely used for such applications. Pokalon is a relatively new member of the polycarbonate family of SSNTDs. It has the same monomer as that of Makrofol. In the present work we have studied the bulk etching characteristics of Pokalon at different temperatures and normalities of NaOH etchant with and without magnetic stirring. The effect of UV and gamma irradiation on the bulk etch rate has also been studied. From the bulk etch rate values at different temperatures, we have determined the corresponding activation energies. Measurable enhancement of the bulk etch rates with accompanying reduction in the activation energies following UV and gamma irradiation suggests chain scission and bond breaking as possible mechanisms for these changes in the etching characteristics.

Thermal atomic layer etching of crystalline aluminum nitride using sequential, self-limiting hydrogen fluoride and Sn(acac)2 reactions and enhancement by H2 and Ar plasmas

Thermal atomic layer etching (ALE) of crystalline aluminum nitride (AlN) films was demonstrated using sequential, self-limiting reactions with hydrogen fluoride (HF) and tin(II) acetylacetonate [Sn(acac)2] as the reactants. Film thicknesses were monitored versus number of ALE reaction cycles at 275 °C using in situ spectroscopic ellipsometry (SE). A low etch rate of ∼0.07 Å/cycle was measured during etching of the first 40 Å of the film. This small etch rate corresponded with the AlOxNy layer on the AlN film. The etch rate then increased to ∼0.36 Å/cycle for the pure AlN films. In situ SE experiments established the HF and Sn(acac)2 exposures that were necessary for self-limiting surface reactions. In the proposed reaction mechanism for thermal AlN ALE, HF fluorinates the AlN film and produces an AlF3 layer on the surface. The metal precursor, Sn(acac)2, then accepts fluorine from the AlF3 layer and transfers an acac ligand to the AlF3 layer in a ligand-exchange reaction. The possible volatile etch products are SnF(acac) and either Al(acac)3 or AlF(acac)2. Adding a H2 plasma exposure after each Sn(acac)2 exposure dramatically increased the AlN etch rate from 0.36 to 1.96 Å/cycle. This enhanced etch rate is believed to result from the ability of the H2 plasma to remove acac surface species that may limit the AlN etch rate. The active agent from the H2 plasma is either hydrogen radicals or radiation. Adding an Ar plasma exposure after each Sn(acac)2 exposure increased the AlN etch rate from 0.36 to 0.66 Å/cycle. This enhanced etch rate is attributed to either ions or radiation from the Ar plasma that may also lead to the desorption of acac surface species.

Exposure and analysis of microparticles embedded in silica aerogel keystones using NF3‐mediated electron beam–induced etching and energy‐dispersive X‐ray spectroscopy

AbstractIn 2006, NASA's Stardust spacecraft delivered to Earth dust particles collected from the coma of comet 81P/Wild 2, with the goal of furthering the understanding of solar system formation. Stardust cometary samples were collected in a low‐density, nanoporous silica aerogel making their study technically challenging. This article demonstrates the identification, exposure, and elemental composition analysis of particles analogous to those collected by NASA's Stardust mission using in‐situ SEM techniques. Backscattered electron imaging is shown by experimental observation and Monte Carlo simulation to be suitable for locating particles of a range of sizes relevant to Stardust (down to submicron diameters) embedded within silica aerogel. Selective removal of the silica aerogel encapsulating an embedded particle is performed by cryogenic NF3‐mediated electron beam–induced etching. The porous, low‐density nature of the aerogel results in an enhanced etch rate compared with solid material, making it an effective, nonmechanical method for the exposure of particles. After exposure, elemental composition of the particle was analyzed by energy‐dispersive X‐ray spectroscopy using a high spectral resolution microcalorimeter. Signals from fluorine contamination are shown to correspond to nonremoved silica aerogel and only in residual concentrations.

CBr4-based in-situ etching of GaAs, assisted with TMAl and TMGa

In-situ etching of GaAs with CBr4 in metalorganic vapor phase epitaxy has been investigated at different temperatures using in-situ reflectivity to measure the etch rates. Deep (150nm) etching of epitaxially-grown GaAs leads to the development of etch pits on the surface, corresponding to the emerging points of threading dislocations. Addition of trimethylgallium (TMGa) leads to a linear superposition of growth and etching. Trimethylaluminium (TMAl) added in moderate quantity enhances the etch rate and inhibits the development of etch pits. A model description for the enhanced etch rate is presented.

Improving the etching performance of high-aspect-ratio contacts by wafer temperature control: Uniform temperature design and etching rate enhancement

A novel wafer temperature control system using direct expansion cycles is developed to improve etching performance. This system enables rapid temperature control of a wafer with low power consumption. In a previous report, we confirmed that the etching rate and mask selectivity of high-aspect-ratio contact etching could be increased by around 6% and 14%, respectively, by controlling the temperature of the wafer during the etching process. In this study, an advanced wafer temperature control system that realizes not only rapid response but also uniform wafer cooling is developed, and a new etching process that controls O2 gas flow rate as well as wafer temperature during etching is evaluated to decrease the etching rate depression of high-aspect-ratio contact etching. As a result, a rate of wafer temperature change of 1°C/s and uniformity of ±0.7% with a coefficient of performance exceeding 3 is achieved over a wafer with a diameter of 300mm during the etching process. Furthermore, etching rate depression in C4F6/Ar/O2 plasma is decreased from 14.4% to 7.8% for a sample with a diameter of 100nm and aspect ratio of 30.

Single and multilayered materials processing by argon ion beam etching: study of ion angle incidence and defect formation

Ion beam etching (IBE) is a very promising technique in microelectronics because of its capability to etch small patterns with a high resolution and inert materials. In this study, the angular incidence of an argon ion beam on the etch rate and uniformity is discussed in the case of several materials often used in microelectronics. The capability of the IBE technique to etch multilayered stack samples with positive anisotropic profiles was demonstrated on TiNiAu, TiNiCuAu, BST and PZT. Two typical defects involved in IBE processing (fences and not etched pattern foots) due to shadow masking and redeposition effect, are explained and solutions are presented to avoid them. Deep IBE was performed on GaN with an etch depth as high as 10 μm, using a 8 μm thick SiO2 mask. The etching of other mask materials, such as TiN, was investigated in order to improve the selectivity. Using a TiN mask, a selectivity to GaN as high as 5 is reported. Finally, the etch rate enhancement needed for deep etching was studied.

Viable chemical approach for patterning nanoscale magnetoresistive random access memory

A reactive ion etching process with alternating Cl2 and H2 exposures has been shown to chemically etch CoFe film that is an integral component in magnetoresistive random access memory (MRAM). Starting with systematic thermodynamic calculations assessing various chemistries and reaction pathways leading to the highest possible vapor pressure of the etch products reactions, the potential chemical combinations were verified by etch rate investigation and surface chemistry analysis in plasma treated CoFe films. An ∼20% enhancement in etch rate was observed with the alternating use of Cl2 and H2 plasmas, in comparison with the use of only Cl2 plasma. This chemical combination was effective in removing metal chloride layers, thus maintaining the desired magnetic properties of the CoFe films. Scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy showed visually and spectroscopically that the metal chloride layers generated by Cl2 plasma were eliminated with H2 plasma to yield a clean etch profile. This work suggests that the selected chemistries can be used to etch magnetic metal alloys with a smooth etch profile and this general strategy can be applied to design chemically based etch processes to enable the fabrication of highly integrated nanoscale MRAM devices.

(Invited) Gate-All-Around Ge FETs

- High performance Ge inversion (INV) and junctionless (JL) GAAFETs are demonstrated. The (111) sidewall-enhanced Ge GAA nFETs show 2x enhanced Ion of 110 μA/μm at 1V with respect to the devices with near (110) sidewalls and subthreshold characteristics of 94 mV/dec. The JL Ge GAA pFETs with fin width (Wfin) down to 27 nm and channel doping (Nch) of 5×1018 cm-3 has Ion/Ioff = 1.5×106, SS = 95 mV/dec, and Ion = 194 μA/μm at VGS - VT = -2V and VDS= -1 V. Introduction For the technology nodes of 10 nm and beyond, the high mobility channels (Ge, and III-V) are required to enhance drive current, and new device architectures (FinFET, tri-gates [1], and GAA FETs [2]) are desired to reduce power. Since the Ge epichannel directly on Si can be a low-cost solution, Ge attracts the most interesting for emergent device integration. The misfits of the bottom Ge can be removal by anisotropic etching. The nearly defect-free Ge channel can be formed for GAA transistors. Experiment The intrinsic and heavily doped p-type epi-Ge layer were grown on SOI by the RTCVD for INV and JL devices, respectively. The Ge fins were patterned and formed by anisotropic etching with Cl2/HBr-based plasma to etch away the high defective Ge near Ge/Si interface. The floating Ge fin on SOI was made due to the higher etching selectivity of Ge than Si and the enhanced etching rate by the defects. After the Al2O3/GeO2 gate stack formation, the contact electrodes were deposited and defined. For INV devices, the S/D were implanted with P (1×1015, 18 keV) and activated at 550°C for 30s. For JL devices, The Nch of 5×1018 cm-3 was in-situ doped by PH3 flow. Results and Discussions Since the mobility of (111) Ge is 2 times than (100) Ge, the Ge fin with (111) sidewalls are fabricated to take the advantage of the sidewall enhanced mobility. Thetransfer characteristics of the INV Ge GAA nFETs with the Wfin of 58 nm and the Lg of 350 nm are shown in Fig. 1. Thanks to the GAA structure and the removal of the defect region, the SS of ~ 94 mV/dec and Ion/Ioff of 1.6× 104 at VDS = 0.05V is achieved. The 2x enhancement of the Ion is also observed for the Ge GAA nFET with (111) sidewalls relatively to the Ge FinFET with ~(110) sidewalls. The EOT of 5.5nm obtained from planar devices, and Dit of 1×1012cm-2eV-1 extracted from the simulation. The TEM image of Ge fin with (111) sidewall is shown in the inset of Fig. 1.Thetransfer characteristics of the JL Ge GAA pFETs with the Wfin of 27 nm and the Lg of 250 nm are shown in Fig. 2. The SS of ~ 95 mV/dec and Ion/Ioff of 1.5×106 is achieved for the Nch of 5×1018 cm-3. The Dit of 1×1012 cm-2eV-1 and the EOT of 10 nm are responsible for the SS, confirmed by numerical simulation of GAA FETs. The drain current at VGS - VT = -2V and VDS = -1 V can reach 290 μA/μm for the device with the Nch of 8×101 9 cm-3 and the Wfin of 9 nm, ~1.4X enhancement of the Ion as compared to Nch = 5×1018 cm-3. Note that the Ion of JL GAA nFETs with the Nch of 1×1019 is compatible with the JL GAA pFETs. As compared to INV Ge GAA pFETs, the device with 27-nm Wfinexhibits 49% mobility improvement at high overdrive voltage. The roll-off of mobility of the INV devices with the increasing inversion carrier density is mainly due to the surface roughness scattering (SRS) [3]. As a result, the JL GAA devices have the better immunity of SRS as compared to the INV devices. Conclusion With the removal of defective Ge near Ge/ Si interface, a defect-free Ge GAAFETs can be integrated on the Si platform. With the (111) sidewall-enhanced mobility, INV Ge GAA nFET is demonstrated to have ~2x enhanced Ion. The JL Ge GAA pFETs are also demonstrated with the SS of 95 mV/dec for the device with Nch of 5×101 8 cm-3 and Ion of 290 μA/μm for the device with Nch of 8×101 9 cm-3. Our results show that Ge GAAFETs is a promising candidate for future CMOS applications. Reference [1] C. Auth et al., 2012 Symp. on VLSI Tech., (2012) p. 131.[2] Shu-Han Hsu et al., 2011 International Electron Devices Meeting (IEDM), (2011) p. 825.[3] H. -S. Lan et al., Appl. Phys. Lett., 99 112109 (2011).