Remote Plasma Atomic Layer Deposition Research Articles

Characteristics of low-κ SiOC films deposited via atomic layer deposition

The deposition of SiOC thin films via remote plasma atomic layer deposition was investigated. Octamethylcyclotetrasiloxane (OMCTS) and O2, Ar, H2 plasmas were respectively used as a precursor and reactants during the deposition process at 400°C. Plasma and deposition temperatures had a significant effect on the physical and electrical characteristics of the films. When Ar and H2 plasma was respectively used during the deposition process, films exhibited low dielectric constants while incorporating carbon; however, O2 plasma yielded carbon free SiO2 films. Low dielectric constants resulted in low film densities and the presence of carbon within the films. When Ar and H2 plasma was used as the reactant gas, pores within the films with loose structures and SiC bonds served to lower the dielectric constant. As a result, Ar and H2 plasma conditions exhibited low dielectric constants of 2.7 and 3.1 at 100°C, respectively. Meanwhile, the presence of carbon and low film densities caused leakage paths within the films. X-ray photoelectron spectroscopy supported analyses demonstrating the bonding characteristics of Si, C, O components.

Interfaces Formed by ALD Metal Oxide Growth on Metal Layers

Redox-based resistive switching random access memory (ReRAM) has the potential to fulfill the demands of future Green IT solutions for non-volatile memory circuits with high speed data access, high reliability, and low power consumption. A single valence-change-mechanism (VCM)-type switching cell is typically built from a transition metal oxide layer sandwiched between metal electrodes of different oxidation enthalpy. It is accepted in the scientific community that oxygen exchange and drift/diffusion processes play a major role in the resistive switching process. Thus, understanding oxidation/reduction reactions taking place already during the growth process is of utmost importance. For future three-dimensional (3D) integrated arrays of ReRAM cells atomic layer deposition (ALD) will become the method of choice to grow ultra-thin, conformal, homogeneous and dense oxide films at a thermal budget below 450 °C. For todays’ two dimensional stacked crossbar arrays, the metal oxide films can be grown on nearly inert Pt electrodes, while the oxygen exchange layer built from a reactive metal is deposited on top of the metal oxide. However, in the 3D ReRAM arrays the metal oxide might need to be grown on the reactive metal as well. In this study ultrathin ALD metal oxide layers are deposited on various metal electrodes by means of in-situ transfer from the sputtered metal into the ALD chamber. For the metal layers we utilized Pt, Hf, and Ta while for the metal oxides Al2O3, TiO2, and HfO2 were chosen. ALD metal oxide processes were performed by plasma assisted ALD in an Oxford FlexAlâ system. Due to the passivation property of the dense ALD metal oxide layer the metal oxide / metal interfaces could be analyzed ex-situ if the vacuum break was kept short. Angle resolved X-ray photoemission spectroscopy studies combined with X-ray reflectivity analysis reveal a clear picture of the interface formation and the oxygen gradient formed at the interface already during growth. The influence of process parameters and stack sequences on this ‘built-in’ oxygen gradient at metal / ALD metal oxide interfaces will be covered in this presentation. Among others, the possibility for PtOx formation during remote oxygen plasma ALD processes will be discussed.

Advanced surface passivation of epitaxial boron emitters for high-efficiency ultrathin crystalline silicon solar cells

In this work, we demonstrated an enhanced surface passivation of epitaxially grown boron-doped Si emitters by replacing thermal SiO2 as a passivation layer employed in a 15.9% efficient 21-µm Si solar cell (88 cm2) on stainless steel with a remote-plasma atomic layer deposition (ALD) of an Al2O3 film. A thin Al2O3 film deposited by remote-plasma ALD was very effective at reducing the emitter saturation current density (J0e) of epitaxial p+-emitter to 16.2 fA/cm2, compared to the J0e of 184.9 fA/cm2 by thermal SiO2 films. This reduction in J0e enables an increase in an implied open-circuit voltage (iVoc) from 630 to 688 mV. Quokka simulation shows that about a 1.1% absolute efficiency increase in the calculated baseline efficiency of a 15.1% of the ultrathin Si solar cell can be achievable by enhancing emitter surface passivation without changing the concentration in either the epi-emitter or epi-base. Finally, our results show that a high efficiency of 17.3% can be reached from the calculated baseline efficiency of 15.1% using the optimized conditions of an epitaxially grown emitter in combination with increasing the base doping concentration and improved base recombination lifetime.

The annealing effect on work function variation of WNxCy films deposited by remote plasma atomic layer deposition

Tungsten‐nitrogen‐carbide (WNxCy) thin films were investigated as the metal gate of complementary metal‐oxide‐semiconductor (CMOS) devices. WNxCy thin films were deposited by employing the remote plasma atomic layer deposition (RPALD) using a bis(tert‐butylimido) bis (dimethylamido) tungsten (BTBMW) precursor and hydrogen plasma as a reactant. The growth rate of the WNxCy films was about 0.12 nm/cycle. X‐ray diffraction (XRD) analysis indicated that the films consisted of a mixture of tungsten carbide and tungsten nitride phases. The atomic force microscope (AFM) analysis further confirmed that the WNxCy film surfaces deposited by RPALD were smooth. In addition, the chemical bonding state analysis showed that the WNxCy films consisted of WN, WC, and WO phases. To measure the work function of the WNxCy film, a MOSCAP (metal oxide semiconductor capacitor) stack was fabricated and the flat band voltage was measured by current–voltage (C–V) measurements. A WNxCy work function value of 4.91 eV was suitable for p‐MOS and the work function of the WNxCy films varied depending on the annealing treatment, and was higher than the work function of the as‐deposited WNxCy film.

Induction of ferroelectricity in nanoscale ZrO2 thin films on Pt electrode without post-annealing

Large stable ferroelectricity in nanoscale undoped zirconia (ZrO2) thin films prepared without post-annealing has been demonstrated for the first time. Remanent polarizations up to 12μCcm−2 were obtained in the as-deposited ZrO2 thin films prepared by remote plasma atomic layer deposition at 300°C substrate temperature on the Pt electrode. Ferroelectric crystallization of the films was achieved without post-annealing, which is highly beneficial to the application of the films in non-volatile memories and ultralow-power nanoelectronics. The existence of the ferroelectric orthorhombic phase with noncentrosymmetric space group Pbc21 in the as-deposited ZrO2 thin films was confirmed by high-resolution transmission electron microscopy.

Atomic Layer Deposition of Gallium Oxide Films as Gate Dielectrics in AlGaN/GaN Metal-Oxide-Semiconductor High-Electron-Mobility Transistors.

In this study, films of gallium oxide (Ga2O3) were prepared through remote plasma atomic layer deposition (RP-ALD) using triethylgallium and oxygen plasma. The chemical composition and optical properties of the Ga2O3 thin films were investigated; the saturation growth displayed a linear dependence with respect to the number of ALD cycles. These uniform ALD films exhibited excellent uniformity and smooth Ga2O3–GaN interfaces. An ALD Ga2O3 film was then used as the gate dielectric and surface passivation layer in a metal–oxide–semiconductor high-electron-mobility transistor (MOS-HEMT), which exhibited device performance superior to that of a corresponding conventional Schottky gate HEMT. Under similar bias conditions, the gate leakage currents of the MOS-HEMT were two orders of magnitude lower than those of the conventional HEMT, with the power-added efficiency enhanced by up to 9 %. The subthreshold swing and effective interfacial state density of the MOS-HEMT were 78 mV decade–1 and 3.62 × 1011 eV–1 cm–2, respectively. The direct-current and radio-frequency performances of the MOS-HEMT device were greater than those of the conventional HEMT. In addition, the flicker noise of the MOS-HEMT was lower than that of the conventional HEMT.

Improvement of the thermal stability of nickel silicide using a ruthenium interlayer deposited via remote plasma atomic layer deposition

In this study, the effects of a thin Ru interlayer on the thermal and morphological stability of NiSi have been investigated. Ru and Ni thin films were deposited sequentially to form a Ni/Ru/Si bilayered structure, without breaking the vacuum, by remote plasma atomic layer deposition (RPALD) on a p-type Si wafer. After annealing at various temperatures, the thermal stabilities of the Ni/Ru/Si and Ni/Si structures were investigated by various analysis techniques. The results showed that the sheet resistance of the Ni/Ru/Si sample was consistently lower compared to the Ni/Si sample over the entire temperature range. Although both samples exhibited the formation of NiSi2 phases at an annealing temperature of 800 °C, as seen with glancing angle x-ray diffraction, the peaks of the Ni/Ru/Si sample were observed to have much weaker intensities than those obtained for the Ni/Si sample. Moreover, the NiSi film with a Ru interlayer exhibited a better interface and improved surface morphologies compared to the NiSi film without a Ru interlayer. These results show that the phase transformation of NiSi to NiSi2 was retarded and that the smooth NiSi/Si interface was retained due to the activation energy increment for NiSi2 nucleation that is caused by adding a Ru interlayer. Hence, it can be said that the Ru interlayer deposited by RPALD can be used to control the phase transformation and physical properties of nickel silicide phases.

Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs.

High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6” wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors.

The effect of plasma power on the properties of low‐temperature silicon nitride deposited by RPALD for a gate spacer

We investigated the effects of NH3 plasma power on characteristics of low‐temperature silicon nitride thin films for application of a gate spacer. SiNx thin film was deposited on a Si(100) substrate by remote plasma atomic layer deposition (RPALD) using trisilylamine (TSA) as a Si precursor and NH3 gas as a reactant. NH3 remote plasma was analyzed with optical emission spectroscopy (OES) and it largely consisted of NH and H. As the plasma power increased, more NH and H radicals were generated and a proportion of NH radicals in the plasma increased, which resulted in the slight increase of the high‐N content and low‐H content in SiNx thin film. The low‐H content with nearly stoichiometric SiNx thin films improve etch rate properties. The densities of RPALD SiNx thin film were 2.7 g cm−3 and almost the same regardless of plasma power. RPALD SiNx thin films showed a low leakage current density of 10−7 A cm−2 at 2 MV cm−1 and a breakdown voltage of approximately 8 MV cm−1.

Characteristics of WNxCy films deposited using remote plasma atomic layer deposition with (MeCp)W(CO)2(NO) for Cu diffusion barrier

Diffusion barrier characteristics of tungsten–nitride–carbide (WNxCy) thin films interposed between Cu and SiO2 layers were studied. The WNxCy films were deposited by remote plasma atomic layer deposition (RPALD) using a metal organic source, (MeCp)W(CO)2(NO), and ammonia. Auger electron spectroscopy analysis indicated the WNxCy films consisted of tungsten, nitrogen, carbon, and oxygen. X-ray diffraction (XRD) analysis showed that the film deposited at 350 °C was nanocrystalline. The resistivity of WNxCy film deposited by RPALD was very low compared to that in previous research because of the lower nitrogen content and different crystal structures of the WNxCy. To verify the diffusion barrier characteristics of the WNxCy film, Cu films were deposited by physical vapor deposition after WNxCy film was formed by RPALD on Si substrate. The Cu/WNxCy/Si film stack was annealed in a vacuum by rapid thermal annealing at 500 °C. Cu diffusion through the barrier layer was verified by XRD. Stable film properties were observed up to 500 °C, confirming that WNxCy film is suitable as a Cu diffusion barrier in microelectronic circuits.

Effects of surface passivation on top-down ZnO nanowire transistors

We fabricated unpassivated and passivated zinc oxide (ZnO) nanowire field effect transistors (NWFETs) using conventional top-down method of remote plasma atomic layer deposition and anisotropic dry etch. This paper investigates the effect of Al2O3 passivation on the electrical characteristics of the ZnO NWFETs. Measured unpassivated ZnO NWFETs show a threshold voltage of 6.5V, drain current on/off ratio of 106 and field effect mobility of 31.4cm2/Vs. Passivated ZnO NWFETs demonstrate threshold voltage shift to −10V, drain current on/off ratio of 104 and improvement of mobility of 35.5cm2/Vs. The passivated device results indicate suitability for biosensing applications.

Characteristics of a nickel thin film and formation of nickel silicide by using remote plasma atomic layer deposition with Ni( i Pr-DAD)2

In this study, the characteristics of a thin nickel film deposited by using remote plasma atomic layer deposition (RPALD) on a p-type Si substrate and formation of nickel silicide by using rapid thermal annealing were determined. Bis(1,4-di-isopropyl-1,3-diazabutadienyl)nickel, (Ni( i Pr-DAD)2) was used as the Ni precursor and an ammonia plasma was used as a reactant. This was the first attempt to deposit a thin Ni film using by Ni( i Pr-DAD)2 as a precursor for the ALD process. The Ni film that was deposited by using RPALD at a growth rate of around 2.2 Å/cycle at 250°C showed a very low resistivity of 33 μΩ·cm with a total impurity concentration of around 10 at.%. The impurities in the thin film, carbon and nitrogen, were existed in the forms of C–C and C–N bonding states. The potential for removing impurities by comparing of experimental conditions, namely, the process temperature and pressure. The nitrogen impurity could be removed by using thermal desorption during each ALD cycle, and the carbon impurity could be reduced by optimizing the process pressure, which is directly related to the mean free path in the NH3 plasma. After Ni deposition, nickel silicide was formed by rapid thermal annealing (RTA) in a vacuum ambient for 1 minute. Nickel-silicide layers from obtained by used the ALD of Ni and obtained by used of the PVD Ni annealed at temperatures from 500 to 900°C. NiSi obtained by used the ALD of Ni showed better thermal stability due to the contributions of small amounts of carbon and nitrogen in the as-deposited Ni thin film. Degradation of the silicide layer was effectively suppressed by using the ALD of ALD Ni.

Growth behavior and structural characteristics of TiO2thin films using (CpN)Ti(NMe2)2and oxygen remote plasma

This study examined the effect of a novel precursor on the growth behavior and structure of titanium dioxide (TiO2) thin films. TiO2 thin films were deposited by remote plasma atomic layer deposition (RPALD). Dimethylamin-cyclopentadienyl titanium, (CpN)Ti(NMe2)2, was used as a Ti precursor and oxygen plasma was used as a reactant. (CpN)Ti(NMe2)2 is a newly synthesized precursor for TiO2 deposition. Characteristics of TiO2 thin films deposited by (CpN)Ti(NMe2)2 were compared with those of films deposited using a titanium tetrakis isopropoxide (TTIP) precursor. A wide processing window from 170 to 400 °C was observed, and the growth rate was around 0.44 Å/cycle at 200 °C. X-ray photoelectron spectroscopy (XPS) measurements revealed that nearly stoichiometric TiO2 films were deposited. The residual impurity concentrations inside the films were almost the same for (CpN)Ti(NMe2)2 and TTIP. TiO2 films deposited on ruthenium showed a pure rutile phase at a low deposition temperature and low power pre-treatment. Therefore, this TiO2 film is very promising for metal–insulator–metal (MIM) capacitor dielectrics for dynamic random access memory (DRAM) applications.

Uniform GaN thin films grown on (100) silicon by remote plasma atomic layer deposition

The growth of uniform gallium nitride (GaN) thin films was reported on (100) Si substrate by remote plasma atomic layer deposition (RP-ALD) using triethylgallium (TEG) and NH3 as the precursors. The self-limiting growth of GaN was manifested by the saturation of the deposition rate with the doses of TEG and NH3. The increase in the growth temperature leads to the rise of nitrogen content and improved crystallinity of GaN thin films, from amorphous at a low deposition temperature of 200 °C to polycrystalline hexagonal structures at a high growth temperature of 500 °C. No melting-back etching was observed at the GaN/Si interface. The excellent uniformity and almost atomic flat surface of the GaN thin films also infer the surface control mode of the GaN thin films grown by the RP-ALD technique. The GaN thin films grown by RP-ALD will be further applied in the light-emitting diodes and high electron mobility transistors on (100) Si substrate.

Temperature dependence of silicon nitride deposited by remote plasma atomic layer deposition

We investigated the characteristics of silicon nitride (SiNx) thin films deposited by remote plasma atomic layer deposition (RPALD) using trisilyamine (TSA) and ammonia (NH3) plasma at low temperatures. Although the process window of SiNx thin films is 150–350 °C, considering the refractive index (RI), SiNx thin films deposited at 250–350 °C were focused on for analyses. All of the SiNx films were nearly stoichiometric, regardless of the deposition temperature. As the deposition temperature increased, the RI increased, while the hydrogen content decreased. The defect density also changed at higher deposition temperatures; as the deposition temperature increased, all of the trap densities increased because of the low-hydrogen content in the SiNx thin films. The characteristics of the SiNx thin film deposited by RPALD could be controlled to adjust the defect density for charge trap flash memory applications by changing the deposition temperature.

Al2O3 multi-density layer structure as a moisture permeation barrier deposited by radio frequency remote plasma atomic layer deposition

Al2O3 films deposited by remote plasma atomic layer deposition have been used for thin film encapsulation of organic light emitting diode. In this study, a multi-density layer structure consisting of two Al2O3 layers with different densities are deposited with different deposition conditions of O2 plasma reactant time. This structure improves moisture permeation barrier characteristics, as confirmed by a water vapor transmission rate (WVTR) test. The lowest WVTR of the multi-density layer structure was 4.7 × 10−5 gm−2 day−1, which is one order of magnitude less than WVTR for the reference single-density Al2O3 layer. This improvement is attributed to the location mismatch of paths for atmospheric gases, such as O2 and H2O, in the film due to different densities in the layers. This mechanism is analyzed by high resolution transmission electron microscopy, elastic recoil detection, and angle resolved X-ray photoelectron spectroscopy. These results confirmed that the multi-density layer structure exhibits very good characteristics as an encapsulation layer via location mismatch of paths for H2O and O2 between the two layers.

Permeation barrier properties of an Al2O3/ZrO2 multilayer deposited by remote plasma atomic layer deposition

We report the permeation barrier properties of Al2O3/ZrO2 multi-layers deposited by remote plasma atomic layer deposition. Electrical Ca degradation tests were performed to derive the water vapor transmission rate (WVTR) of Al2O3, ZrO2 and Al2O3/ZrO2 multi-layers at 50 °C and 50% relative humidity (RH). Al2O3/ZrO2 multi-layers exhibit better barrier properties than Al2O3 and ZrO2 layers, and when more individual layers were deposited in the same total thickness, the WVTR value was reduced further, indicating a better barrier property. The WVTR of the Al2O3 and ZrO2 layers were 9.5 × 10−3 and 1.6 × 10−2 g/m2 day, respectively, but when deposited alternatively with 1 cycle of each layer, the WVTR decreased to 9.9 × 10−4 g/m2 day. X-ray diffraction results indicated that ZrO2 has a monoclinic structure but Al2O3 and Al2O3/ZrO2 multi-layers show an amorphous structure. Cross sectional Al2O3/ZrO2 multi-layer structures and the formation of a ZrAlxOy phase are observed by transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) results indicate that Al2O3 and ZrO2 contain 33.7% and 37.8%, respectively, Al–OH and Zr–OH bonding. However, the ZrAlxOy phase contained 30.5% Al–OH and Zr–OH bonding. The results of transmittance measurement indicate that overall, Al2O3, ZrO2 and Al2O3/ZrO2 multi-layers show high transmittance greater than 80% in the visible region.

Radio frequency plasma power dependence of the moisture permeation barrier characteristics of Al2O3 films deposited by remote plasma atomic layer deposition

In the present study, we investigated the gas and moisture permeation barrier properties of Al2O3 films deposited on polyethersulfone films (PES) by capacitively coupled plasma (CCP) type Remote Plasma Atomic Layer Deposition (RPALD) at Radio Frequency (RF) plasma powers ranging from 100 W to 400 W in 100 W increments using Trimethylaluminum [TMA, Al(CH3)3] as the Al source and O2 plasma as the reactant. To study the gas and moisture permeation barrier properties of 100-nm-thick Al2O3 at various plasma powers, the Water Vapor Transmission Rate (WVTR) was measured using an electrical Ca degradation test. WVTR decreased as plasma power increased with WVTR values for 400 W and 100 W of 2.6 × 10−4 gm−2day−1 and 1.2 × 10−3 gm−2day−1, respectively. The trends for life time, Al-O and O-H bond, density, and stoichiometry were similar to that of WVTR with improvement associated with increasing plasma power. Further, among plasma power ranging from 100 W to 400 W, the highest power of 400 W resulted in the best moisture permeation barrier properties. This result was attributed to differences in volume and amount of ion and radical fluxes, to join the ALD process, generated by O2 plasma as the plasma power changed during ALD process, which was determined using a plasma diagnosis technique called the Floating Harmonic Method (FHM). Plasma diagnosis by FHM revealed an increase in ion flux with increasing plasma power. With respect to the ALD process, our results indicated that higher plasma power generated increased ion and radical flux compared with lower plasma power. Thus, a higher plasma power provides the best gas and moisture permeation barrier properties.

Investigation of the Flatband Voltage (<I>V</I><SUB>FB</SUB>) Shift of Al<SUB>2</SUB>O<SUB>3</SUB> on N<SUB>2</SUB> Plasma Treated Si Substrate

The relationships between the physical and electrical characteristics of films treated with N2 plasma followed by forming gas annealing (FGA) were investigated. The Si substrates were treated with various radio frequency (RF) power levels under a N2 ambient. Al2O3 films were then deposited on Si substrates via remote plasma atomic-layer deposition. The plasma characteristics, such as the radical and ion density, were investigated using optical emission spectroscopy. Through X-ray photoelectron spectroscopy, the chemical-bonding configurations of the samples treated with N2 plasma and FGA were examined. The quantity of Si-N bonds increased as the RF power was increased, and Si--O--N bonds were generated after FGA. The flatband voltage (VFB) was shifted in the negative direction with increasing RF power, but the VFB values of the samples after FGA shifted in the positive direction due to the formation of Si--O--N bonds. N2 plasma treatment with various RF power levels slightly increased the leakage current due to the generation of defect sites.

Moisture Barrier Properties of Al2O3 Films deposited by Remote Plasma Atomic Layer Deposition at Low Temperatures

We report the effect of process temperature on moisture permeation barrier properties of Al2O3 films deposited by remote plasma atomic layer deposition (RPALD) at various low temperatures from 50 to 200 °C. XPS analysis of O 1s peak reveals that the O–H ratio decreases with process temperature from 38.1% at 50 °C to 25.8% at 200 °C. The water transmission rates using electrical Ca degradation test indicates that the 100 nm Al2O3 film enhances the moisture barrier performance from 2.0×10-2 to 5.0×10-4 g m-2 day-1 with increasing the process temperature. This result indicates that increasing the process temperature improves the moisture permeation barrier properties significantly even in RPALD process. It is attributed to the increase in the Al2O3 mass density due to the decrease of relatively O–H ratio with increase in temperature as revealed by XPS O 1s peak deconvolution and FTIR analysis in the Al2O3 films.