High Effective Work Function Research Articles

Advanced Atomic Layer Deposition: Ultrathin and Continuous Metal Thin Film Growth and Work Function Control Using the Discrete Feeding Method.

The ultrathin and continuous ruthenium (Ru) film was deposited through an improved atomic layer deposition (ALD) process with a discrete feeding method (DFM), called DF-ALD, employing a cut-in purge step during the precursor feeding. The excess precursor molecules can be physically adsorbed onto the chemisorbed precursors on the substrate during precursor feeding, which screens the reactive sites on the surface. Using DF-ALD, surface coverage of precursors was enhanced because the cut-in purge removes the physisorbed precursors securing the reactive sites beneath them; thus, nucleation density was greatly increased. Therefore, the grain size decreased, which changed the microstructure and increased oxygen impurity concentration. However, a more metallic Ru thin film was formed due to thermodynamic stability and improved physical density. Consequently, DF-ALD enables the deposition of the ultrathin (3 nm) and continuous Ru film with a low resistivity of ∼60 μΩ cm and a high effective work function of ∼4.8 eV.

Impact of chemical bonding difference of ALD Mo on SiO2 and Al2O3 on the effective work function of the two gate stacks

This study investigates molybdenum deposited by atomic layer deposition (ALD) as a potential gate metallization for flash memory devices. Polycrystalline (110)-oriented, with low-resistivity (∼16 μΩ cm) ALD Mo films were deposited on SiO2 and Al2O3 using hydrogen reduction of Mo-oxychloride precursor. On SiO2, an effective work function (EWF) of 4.75 ± 0.1 eV was obtained for as-deposited samples, and its value increased up to 4.9 ± 0.05 eV upon annealing at 600 °C, whereas on Al2O3, a stable EWF value of 5.05 ± 0.05 eV was observed. The EWF variation is correlated with changes in the composition and chemical bonding at the metal/dielectric interface. The latter were investigated by energy dispersive x-ray spectroscopy and electron energy loss spectroscopy performed using scanning transmission electron microscopy and x-ray photoelectron spectroscopy. This analysis revealed that the presence of Mo oxide at the Al2O3/Mo interface stabilizes the EWF, and the EWF increase on SiO2 is attributed to Si enrichment at the SiO2/Mo interface upon annealing. A theoretical model is suggested to explain the chemical bonding difference on SiO2 and Al2O3, based on the Mo-precursor reactions with the surface groups of the dielectric. This study emphasizes the importance of the precursor/substrate reactions in determining the compositional and, therefore, electrical properties of the metal/dielectric interface, and demonstrates that ALD Mo deposited directly on SiO2 and Al2O3 is a promising candidate for gate metallization of flash devices due to its high EWF.

Novel Band-Edge Work Function Performance Modulation via NPT with PMOS1st/NMOS1st Laminated Stack for PMOS Low Power Target

In this paper, the band-edge work function performance is systematically investigated and modulated via novel nitrogen plasma treatment (NPT) with the advanced PMOS1st (TiN/TiN/TiAlC) and NMOS1st (TiN/TiN) laminated stacks for the fabricated PMOS capacitors. The basic multi-VT performance is strongly modulated by controlling NPT process. 1) Flatband voltage (VFB) shifts towards band edge are obtained as +120 mV (undiluted), +430 mV (diluted) for PMOS1st and +80 mV (undiluted), +210 mV (diluted) for NMOS1st. 2) By manipulating the NPT process from undiluted and diluted case, it can provide significant high band-edge effective work function ranging from 4.89 eV (undiluted) to 5.21 eV (diluted) for PMOS1st and 5.22 eV (undiluted) to 5.35 eV (diluted) for NMOS1st laminated stack, respectively. 3) NPT diluted with hydrogen is observed to maintain ultralow bulk trap density (1.11 × 1011 cm−2 for PMOS1st and nearly zero for NMOS1st) and interface trap density (3.34 × 1011 eV−1 cm−2 for PMOS1st and 6.45 × 1011 eV−1 cm−2 for NMOS1st). The significant band-edge work function modulation and very low bulk and interface trap density demonstrate the novel NPT with PMOS1st/NMOS1st laminated stack is very promising to achieve the target of PMOS low-power application in the further technology node.

Dipole-induced modulation of effective work function of metal gate in junctionless FETs

In this work, a metal gate with a high effective work function (Weff) that is suitable for junctionless field-effect transistors (JLFETs) has been fabricated. Weff is modulated by inserting an Al interfacial layer with different thicknesses between the HfO2 dielectric layer and the Pt gate metal. Transmission electron microscopy together with capacitance–voltage (C–V) measurement is used to investigate the underlying mechanism of this phenomenon. It is suggested that the oxygen scavenging from HfO2 by the Al layer leads to the formation of interface dipoles, which effectively modulate Weff. By varying the Al layer thickness, it is found that Weff first increases and then decreases as the Al thickness increases above ∼2.5 nm, and the largest increase in Weff is about 0.56 eV. Based on these results, our work provides a useful method using interface dipoles to adjust Weff to the desired value for JLFETs, which is otherwise difficult to achieve using a single metal.

Thermal stability of atomic layer deposited WCxNy electrodes for metal oxide semiconductor devices

This study is a thorough investigation of the chemical, structural, and electrical stability of W based organo-metallic films, grown by atomic layer deposition, for future use as gate electrodes in advanced metal oxide semiconductor structures. In an earlier work, we have shown that high effective work-function (4.7 eV) was produced by nitrogen enriched films (WCxNy) dominated by W-N chemical bonding, and low effective work-function (4.2 eV) was produced by hydrogen plasma resulting in WCx films dominated by W-C chemical bonding. In the current work, we observe, using x-ray diffraction analysis, phase transformation of the tungsten carbide and tungsten nitride phases after 900 °C annealing to the cubic tungsten phase. Nitrogen diffusion is also observed and is analyzed with time-of-flight secondary ion mass spectroscopy. After this 900 °C anneal, WCxNy effective work function tunability is lost and effective work-function values of 4.7–4.8 eV are measured, similar to stable effective work function values measured for PVD TiN up to 900 °C anneal. All the observed changes after annealing are discussed and correlated to the observed change in the effective work function.

Influence of Oxide Interlayer by TiN Metal Gate in High-k First CMOS Devices

Due to its good film stability and electric property of high effective work function, TiN film has been widely used as high-k gate dielectric capping layer or work function metal in advanced High-k/metal gate CMOS devices. In this work, different TiN films were prepared by atomic layer deposition (ALD) and plasma vapor deposition (PVD) respectively, their thermal stability and interaction with gate stack films were investigated. Furthermore, their impacts on interface properties of interfacial oxide/high-k and interfacial oxide/silicon were also studied. The results show that both ALD and PVD TiN films have good thermal stability, while they tend to gather oxygen from atmosphere during sequential process steps. After high temperature annealing, the gathered oxygen in TiN film is easy to diffuse upward into TiN/poly-Si interface to form a thin TiSixOy layer above TiN film, and diffuse downward through high-k dielectric film resulting in interfacial oxide layer (IL) regrowth.

Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2–RuO2 Alloy Schottky Contacts for Silicon Photoanodes

We synthesized nanoscale TiO2-RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions ≥ 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13-46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2-RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.

Improvement of the effective work function and transmittance of thick indium tin oxide/ultrathin ruthenium doped indium oxide bilayers as transparent conductive oxide

Ruthenium doped indium oxide (In1−xRuxOy) films fabricated using DC magnetron co-sputtering with In2O3 and Ru targets were investigated for use as transparent conductive oxides. The In1−xRuxOy films had an amorphous structure in the wide compositional range of x=0.3–0.8 and had an extremely smooth surface. The transmittance and resistivity of the In1−xRuxOy films increased as the Ru content increased. The transmittance of the In0.38Ru0.62Oy film improved to over 80% when the film thickness was less than 5nm, while the specific resistivity (ρ) was kept to a low value of 1.6×10−4Ωcm. Based on these experimental data, we demonstrated that thick indium tin oxide (In0.9Sn0.1Oy, ITO) (150nm)/ultrathin In0.38Ru0.62Oy (3nm) bilayers have a high effective work function of 5.3eV, transmittance of 86%, and low ρ of 9.2×10−5Ωcm. This ITO/In0.38Ru0.62Oy bilayer is a candidate for use as an anode for organic electroluminescent devices.

Work function tuning of plasma-enhanced atomic layer deposited WCxNy electrodes for metal/oxide/semiconductor devices

One of the main challenges facing the integration of metals as gate electrodes in advanced MOS devices is control over the Fermi level position at the metal/dielectric interface. In this study, we demonstrate the ability to tune the effective work function (EWF) of W-based electrodes by process modifications of the atomic layer deposited (ALD) films. Tungsten carbo-nitrides (WCxNy) films were deposited via plasma-enhanced and/or thermal ALD processes using organometallic precursors. The process modifications enabled us to control the stoichiometry of the WCxNy films. Deposition in hydrogen plasma (without nitrogen based reactant) resulted in a stoichiometry of WC0.4 with primarily W-C chemical bonding, as determined by x-ray photoelectron spectroscopy. These films yielded a relatively low EWF of 4.2 ± 0.1 eV. The introduction of nitrogen based reactant to the plasma or the thermal ALD deposition resulted in a stoichiometry of WC0.1N0.6–0.8 with predominantly W-N chemical bonding. These films produced a high EWF of 4.7 ± 0.1 eV.

Role of point defects and HfO2/TiN interface stoichiometry on effective work function modulation in ultra-scaled complementary metal–oxide–semiconductor devices

We investigate the physical properties of a portion of the gate stack of an ultra-scaled complementary metal-oxide-semiconductor (CMOS) device. The effects of point defects, such as oxygen vacancy, oxygen, and aluminum interstitials at the HfO2/TiN interface, on the effective work function of TiN are explored using density functional theory. We compute the diffusion barriers of such point defects in the bulk TiN and across the HfO2/TiN interface. Diffusion of these point defects across the HfO2/TiN interface occurs during the device integration process. This results in variation of the effective work function and hence in the threshold voltage variation in the devices. Further, we simulate the effects of varying the HfO2/TiN interface stoichiometry on the effective work function modulation in these extremely-scaled CMOS devices. Our results show that the interface rich in nitrogen gives higher effective work function, whereas the interface rich in titanium gives lower effective work function, compared to a stoichiometric HfO2/TiN interface. This theoretical prediction is confirmed by the experiment, demonstrating over 700 meV modulation in the effective work function.

Effective work function control of metal inserted poly-Si electrodes on HfSiO dielectrics by in-situ oxygen treatment of metal surface

We fabricated novel poly-Si/TiN/HfSiO/SiO2 gate stacks by using an in-situ oxygen treatment process of a TiN electrode surface in order to suppress Si diffusion from poly-Si upper layer during post deposition annealing (PDA). The Si depth profile after PDA showed that no Si diffused into oxidized stacks due to the formation of TiON layers as a Si diffusion barrier. Furthermore, a high effective work function (EWF) of 4.94 eV was obtained from the oxidized stack even after PDA at 1000 °C. This high EWF is due not only to Si diffusion barrier formation but also to oxygen vacancy compensation by diffused-oxygen from the TiON layer. However, we observed significant growth of interfacial SiO2 after high temperature annealing. These results indicate a trade-off relationship between EWF control and equivalent oxide thickness (EOT) scaling, and imply that an additional method for EWF modulation is required for scaled high-k devices.

Impact of Si Diffusion Barrier Layer Formed on TiN Surface by In Situ Oxygen Treatment Process for Advanced Gate-First Metal/High-k Stacks

We investigated the formation of Si barrier layer by an in-situ oxygen treatment process of TiN electrode surface before poly-Si deposition. The Si depth profile after post deposition annealing (PDA) showed that no Si diffused into oxidized stacks due to the formation of TiON layers as a Si-diffusion barrier. Moreover, a high effective work function (EWF) of 4.94 eV was obtained from the oxidized stack even after PDA at 1000oC. However, we observed significant growth of interfacial SiO2 after high temperature annealing. These results indicate a trade-off relationship between EWF control and equivalent oxide thickness (EOT) scaling, and imply that an additional method for EWF modulation is required for scaled high-k devices.

(Invited) Band-Edge Effective Work Functions by Controlling HfO2/TiN Interfacial Composition for Gate-Last CMOS

Effective work function (EWF) changes of TiN/HfO2 annealed at low temperatures in different ambient environments are correlated to the atomic concentration of oxygen, nitrogen, and aluminum at the metal/dielectric interface. Low EWFs (4.0 eV) are obtained by allowing aluminum to migrate to the TiN/HfO2 interface during a forming gas anneal. High EWFs (5.1 eV) are achieved with anneals that incorporate oxygen throughout the TiN with [O] = 2.8x1021 cm-3 near the TiN/HfO2 interface. First-principles calculations indicate the exchange of O and N atoms near the TiN/HfO2 interface cause the formation of dipoles that increase the EWF.

Growth and band alignment of epitaxial Ni metal gate on crystalline LaAlO3 (001) dielectric film

An epitaxial Ni thin film is grown on a LaAlO3 single-crystalline thin film on Si (001). The p-type Schottky barrier height at the Ni/LaAlO3 interface is measured to be 2.88 eV by x-ray photoemission spectroscopy. The effective work function of the Ni film on LaAlO3 is determined to be 5.15 eV. Good epitaxial quality and a high effective work function suggest that Ni is a promising gate material integrated with LaAlO3 dielectric films for p-type metal oxide semiconductor field effect transistors.

Thermally stable high effective work function TaCN thin films for metal gate electrode applications

TaCN layers were deposited using metal-organic chemical-vapor deposition for applications as metal gate electrodes in p-type metal-oxide-semiconductor (pMOS) devices. The films were formed by thermal decomposition of tertiary-amylimido-tris(dimethylamido)tantalum (TAIMATA®) between 400 and 600 °C. The composition was dependent on the growth temperature with increasing C and decreasing N content at higher temperature. Films grown below 500 °C were nearly amorphous and became weakly polycrystalline with a cubic structure at higher growth temperature. The layer density was ∼8.1 g/cm3, about half of the TaCN bulk density. Grazing-incidence x-ray diffraction and transmission electron microscopy showed that the films consist of small polycrystalline grains in an amorphous matrix. The resistivity was found to decrease with increasing growth temperature. Lowest resistivity values were around 1 mΩ cm for films grown at 600 °C. The films formed an ∼4 nm thick insulating surface oxide, which leads to a thickness dependence of the film resistivity. Oxygen was also found to diffuse slowly into the bulk metal, which leads to a resistivity aging effect. The effective work function of the TaCN films was found to be 4.8 eV on HfSiO4 and HfSiON and to shift weakly by high thermal budget annealing toward the Si valence band, reaching 4.9 eV on HfSiO4.

Work-Function Engineering for 32-nm-Node pMOS Devices: High-Performance TaCNO-Gated Films

We have demonstrated p-type field effect transistors (p-FETs) devices using a TaCNO metal gate for the first time. These p-FETs have threshold voltage values of - 0.4 and - 0.25 V for HfSiON and HfSiO gate dielectrics, respectively, with equivalent oxide thickness of 1.6-1.7 nm. The TaCNO metal shows a high effective work function (eWF) of 4.89 eV on thick SiO2 interface layer, although the eWF rolls off with reducing EOT. Excellent transistor characteristics are achieved, with Ion of 375 muA/mum at Ioff = 60 nA, for Vdd = 1.1V .

Thermally Unstable Ruthenium Oxide Gate Electrodes in Metal/High-k Gate Stacks

RuO2 films were investigated as a work-function-determining metal in metal/high-k gate stacks required for scaled complementary metal–oxide–semiconductor (CMOS) devices. After a low-thermal-budget processing below 400 °C, RuO2 on HfSiON exhibits a high effective work function of about 5.5 eV, which is applicable in p-channel field-effect transistors. However, RuO2 is easily reduced to Ru metal in metal/high-k gate stacks by annealing above 400 °C. Consequently, the high effective work function is unintentionally lowered. This is because RuO2 is thermodynamically unstable when in contact with W and polycrystalline silicon, which are both common components of a metal gate electrode structure. The insertion of TiN and TaN between RuO2 and W does not suppress the reduction. It is concluded that RuO2 is difficult to employ as a metal gate in metal/high-k gate stacks.

High-Work-Function Ir/HfLaO ${\rm p}$-MOSFETs Using Low-Temperature-Processed Shallow Junction

We report a high effective work function (Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m-eff</sub> ) and a very low Vt Ir gate on HfLaO p-MOSFETs using novel self-aligned low-temperature shallow junctions. This gate-first process has shallow junctions of 9.6 or 20 nm that are formed by solid phase diffusion using SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -covered Ga or Ni/Ga. At 1.2-nm effective oxide thickness, good Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m-eff</sub> of 5.3 eV, low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> of +0.05 V, high mobility of 90 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s at -0.3 MV/cm, and small 85degC negative bias-temperature instability (NBTI) of 20 mV (10 MV/cm for 1 h) are measured for Ir/HfLaO p-MOSFETs.

Decoupling the Fermi-level pinning effect and intrinsic limitations on p-type effective work function metal electrodes

A review of literature combined with recent experimental results addressing the intrinsic and extrinsic factors controlling the effective work function (EWF) of metal gate electrodes on Hf-based high-K dielectrics is discussed. Through a systematic study including accurate extraction of EWF, our observations suggest, unlike popularly perceived, intrinsic E f-pinning does not limit the EWF tuning on high-K. Also, a critical issue challenging the maintenance of high EWF metals at low effective oxide thicknesses (EOT), due to a new phenomena described as the “ V fb roll-off”, is reported for the first time.

Work Function Tuning Via Interface Dipole by Ultrathin Reaction Layers Using AlTa and AlTaN Alloys

This letter presents a route for tuning the metal gate effective work function via interface dipoles formed using AlTa and AlTaN alloys. It was found that the AlTa alloy has a higher effective work function (4.45 eV) compared to either Al (~ 4.1 eV) or Ta (4.2 eV) gates on SiO 2 at 400 degC. This increase in effective work function was attributed to interface dipoles formed at the gate electrode and dielectric interface. The origin of this dipole is attributed to a reaction between the AlTa alloy and the dielectric layer. Similar AlTa effective work function tuning was also observed on high-k dielectrics. However, since the AlTa alloy is not thermally stable on SiO2, nitrogen was added to stabilize the electrode. The addition of N stabilizes the equivalent oxide thickness while still allowing for work function tuning under high temperatures. AlTaN alloys were deposited by reactive sputtering and resulted in an effective work function of ~ 5.1 eV after a 1000 degC anneal, making them suitable for PMOS gate applications