Query: Electron transport mechanism through ultrathin Al2O3 films grown at low temperatures using atomic–layer deposition

Candidate papers:

abstract: Alumina (Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt; films of different thicknesses have been grown at different low temperatures (100 250 alt;supagt;oalt;/supagt;C) by atomic layer deposition on n type Si substrate. The robustness of the Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt; film as a barrier has been investigated based on Al/Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt;/Si metal insulator semiconductor structures. The electron transport through the Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt; layer was fitted well by the Fowler Nordheim tunneling mechanism, from which the barrier heights (conduction band offset between Si and Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt; were deduced. It was discovered that the growth temperature and film thickness both influenced the carrier transport and barrier height. The Al/Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt;/Si structure with an ultrathin 3 nm Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt; fabricated at 150 alt;supagt;oalt;/supagt;C showed the largest barrier height, the lowest tunneling current density (4.9 x 10alt;supagt; 8alt;/supagt; A/cmalt;supagt;2alt;/supagt; at 5 MV/cm) and the highest breakdown field strength of 18 MV/cm. Using Au to replace Al as the electrode could suppress the tunneling current significantly. The Alalt;subagt;2alt;/subagt;Oalt;subagt;3alt;/subagt; films were also examined by X ray photoelectron spectroscopy to determine their chemical constituents.
author_names: Pengfei Ma

Wenhao Guo

Jiaming Sun

Jia-cheng Gao

Guanqun Zhang

Qian Xin

Yuxiang Li

Aimin Song
title: Electron transport mechanism through ultrathin Al2O3 films grown at low temperatures using atomic layer deposition
venue: Semiconductor Science and Technology
year: 2019

abstract: Abstract This paper presents a systematic study of HfAlO alloy films, which were prepared by doping HfO2 with different amounts of Al using atomic layer deposition at low growth temperature (150 degC) The chemical constituents of the various HfAlO films were determined by X ray photoelectron spectroscopy. Carrier transport through the HfAlO layer presented a good fit with the Poole Frenkel emission and Fowler Nordheim tunnelling mechanisms in different voltage ranges. Barrier heights (conduction band offset) between the HfAlO films and Si substrate, which were derived from the Fowler Nordheim fittings, increased with increasing Al content in the alloy films, and the leakage current was suppressed well owing to the higher barrier. The 10 nm HfAlO dielectric layer, which was grown by the repeated deposition of 1 cycle each of HfO2 and Al2O3 (defined as H1A1) showed a high gate capacitance of 723 nF/cm2, a high breakdown voltage of 8.0 V, a small leakage current, and a smooth surface. HfAlO films doped with different amounts of Al2O3 in HfO2 were employed to study their effects on the operation of amorphous indium gallium zinc oxide (a IGZO) thin film transistors (TFTs) The a IGZO TFTs based on the H1A1 gate dielectric showed desirable properties including a high Ion/off ratio of 1.7 x 107, a small subthreshold swing of 176 mV/decade, and low operating voltage of 2.0 V.
author_names: Pengfei Ma

Jiaming Sun

Guanqun Zhang

Guangda Liang

Qian Xin

Yuxiang Li

Aimin Song
title: Low temperature fabrication of HfAlO alloy dielectric using atomic layer deposition and its application in a low power device
venue: Journal of Alloys and Compounds
year: 2019

abstract: Abstract Preparation of ultrathin alumina (Al2O3) films through Plasma Enhanced Atomic Layer Deposition (PE ALD) at low substrate temperature is discussed. The present work aims to investigate the physical mechanism of the PE ALD deposition process and also the characteristics of the ultrathin alumina films on silicon <100> wafer deposited using the technique. The deposition was performed using trimethyl aluminum (Al (CH3)3) as the precursor and argon gas for purging. During deposition, the target temperature was kept constant at 80, 100 and 150 degC and the pressure was 1.3 x 10 2 Pa. Two deposition cycles were tested, 400 and 800 cycles. As for understanding the process, the films deposited with and without oxygen plasma were compared. Various thin film characterization techniques, including Atomic Force Microscope (AFM) ellipsometry, Raman spectrometry measurement, X ray diffraction (XRD) and indentation technique, were applied for investigating the film properties. A transmission electron microscope (TEM) equipped with high angle annular dark field imaging line scan modes and energy dispersive X ray spectroscopy acquisition was used for imaging thin film cross sections. We found that the number of deposition cycles did not affect the substrate surface roughness as evidenced by AFM images. The mechanical property, the hardness of the film deposited with 800 cycles and plasma was the best. Raman spectroscopy measurements showed that a Al O Si phase exists when the films were deposited at 100 degC and 150 degC for 400 and 800 cycles under oxygen plasma atmosphere. While no Al O Si phase existed after the same number of ALD deposition cycle without plasma. Results from XRD measurements indicated that the films deposited at 100 degC and 150 degC for 400 and 800 cycles under oxygen plasma atmosphere has an Al O structure. TEM images clearly displayed the interface between the thin films, SiO2 interface layers and Si substrates. As for the sample deposited at 80 degC, an Al2O3 film was hardly seen, but when increasing the deposition temperature to 100 degC and 150deg, films started to build on top of the substrate. However, for all deposition conditions, TEM revealed that the amounts of carbon atoms in the reaction site remained relatively high.
author_names: Duangkhae Bootkul

P Jitsopakul

S Intarasiri

Dheerawan Boonyawan
title: Qualifying ultrathin alumina film prepared by plasma enhance atomic layer deposition under low temperature operation
year: 2017

abstract: An ultrathin, 5 nm, Al2O3 film grown by atomic layer deposition was used as a gate dielectric for amorphous indium gallium zinc oxide (a IGZO) thin film transistors (TFTs) The Al2O3 layer showed a low surface roughness of 0.15 nm, a low leakage current, and a high breakdown voltage of 6 V. In particular, a very high gate capacitance of 720 nF/cm2 was achieved, making it possible for the a IGZO TFTs to not only operate at a low voltage of 1 V but also exhibit desirable properties including a low threshold voltage of 0.3 V, a small subthreshold swing of 100 mV/decade, and a high on/off current ratio of 1.2 x 107. Furthermore, even under an ultralow operation voltage of 0.6 V, well behaved transistor characteristics were still observed with an on/off ratio as high as 3 x 106. The electron transport through the Al2O3 layer has also been analyzed, indicating the Fowler Nordheim tunneling mechanism.
author_names: Pengfei Ma

Lulu Du

Yiming Wang

Ran Jiang

Qian Xin

Yuxiang Li

Aimin Song
title: Low voltage operation of IGZO thin film transistors enabled by ultrathin Al2O3 gate dielectric
year: 2018

abstract: ZnO has intrinsic semiconductor conductivity because of an unintentional doping mechanism resulting from the growth process that is mainly attributable to oxygen vacancies (VO) positioned in the bandgap. ZnO has multiple electronic states that depend on the number of vacancies and the charge state of each vacancy. In addition to the individual electron states, the vacancies have different vibrational states. We developed a high temperature precursor vapor mask technique using Al2O3 to pattern the atomic layer deposition of ZnO and Al:ZnO layers on ZnO based substrates. This technique was used to create a memristor device based on Al:ZnO thin films having metallic and semiconducting and insulating transport properties ZnO. We demonstrated that adding combination of Al2O3 and TiO2 barrier layers improved the resistive switching behavior. The change in the resistance between the high and low resistivity states of the memristor with a combination of Al2O3 and TiO2 was approximately 157% The devices were exposed to laser light from three different laser diodes. The 450 nm laser diode noticeably affected the combined Al2O3 and TiO2 barrier, creating a high resistivity state with a 2.9% shift under illumination. The high resistivity state shift under laser illumination indicates defect shifts and the thermodynamic transition of ZnO defects.
author_names: Rajeh Mundle

Christian G Carvajal

Aswini K Pradhan
title: ZnO/Al:ZnO Transparent Resistive Switching Devices Grown by Atomic Layer Deposition for Memristor Applications.
venue: Langmuir the ACS journal of surfaces and colloids
year: 2016

abstract: Minimizing the recombination losses of charge carriers at Si and other semiconductor surfaces is an important topic for many electronic and photonic devices. It is a particularly important issue in the field of crystalline Si photovoltaics as recombination losses cannot be tolerated for efficient solar cells. Surface recombination can be reduced by passivating the surface by dielectric films that lower the (impact of the) density of surface defect states. Thermally grown SiO2 films have traditionally provided the best surface passivation on a variety of Si surfaces with a wide range of doping levels. However the high temperatures and the long oxidation times required have prevented their widespread use in industrial solar cells. State of the art passivation of such solar cells, in particular the n type emitter at the front side of p type cells, is currently provided by hydrogenated amorphous silicon nitride (a SiNx:H) a thin film material generally deposited by plasma enhanced CVD (PECVD) Advances in solar cell technology require however the introduction of new solar cell passivation schemes. The development and study of surface passivation films is therefore currently a very active field of research. In this contribution recent results will be presented which demonstrate that excellent surface passivation (on par or superior to thermal SiO2) can be obtained by Al2O3 prepared by atomic layer deposition (ALD) These films can be ultrathin when employing plasma ALD (down to 5 nm) with Al(CH3)3 and O2 plasma or thermal ALD (down to 10 nm) with Al(CH3)3 and H2O. The key distinguishing property of Al2O3 is the fact that the films can contain a very high density of negative fixed charges ~5x10 cm at the interfacial region (with typically 1 2 nm interfacial SiOx) between the Al2O3 and the Si [1] These fixed charges lead to a surface space region lowering the electron density at the surface causing "field effect" passivation. Together with a significant reduction of the defect state density <10 eV cm, "chemical" passivation) Al2O3 leads to ultralow recombination velocities on n pand p type Si surfaces [2] The merits of ALD are the fact that it yields extremely uniform films, its precise thickness control, and the possibility of processing at low substrate temperatures <400 oC) [3] Another benefit of ALD that can be of importance for solar cells is the fact that it leads to very conformal films on demanding 3 D topologies, including very rough and non planar surfaces for light trapping and wrap through technologies, respectively. To extend the applicability of ALD for crystalline Si photovoltaics, also for applications that not benefit from the field effect passivation by negative fixed charges, an ALD process has been developed for SiO2 employing SiH2(N(C2H5)2)2 (Air Liquide SAM.24) and O2 plasma. The ALD process with this easy to apply precursor (high vapor pressure, Fig. 1) leads to high quality SiO2 films over a wide temperature range (Fig. 2) The synthesis and passivation quality of this ALD SiO2 will be addressed (Fig. 3) and possible functionalities in surface passivation schemes will be discussed. Figure 1: Relationship between pressure and temperature of TMA [Al(CH3)3] and SAM.24 [SiH2(N(C2H5)2)2] precursors.
author_names: G Gijs Dingemans

Cristian A A van Helvoirt

M M Mandoc

Richard van de Sanden

Wilhelmus M M Kessels
title: Atomic Layer Deposition of Al2O3 and SiO2 for Passivation of Si Surfaces
year: 2011

abstract: Single and few layers of semiconducting transition metal dichalcogenides (MX2) hold promise as twodimensional (2D) channel material for future low power logic applications [1] Semiconducting MX2 have significant band gaps, as opposed to graphene where a band gap is absent. Besides their atomic scale thickness, the MX2 surface is chemically inert and lacks dangling bonds. Therefore, few charge traps and scattering centers are expected to inhibit charge transport. For these 2D materials to be integrated into large scale integrated circuits, a large area deposition technique becomes indispensable. The ALD principle ensures growth control at the atomic level, which is essential to control the deposition of single and few layered MX2. The feasibility of growing thin films of MX2 by atomic layer deposition (ALD) has been shown in limited reports [2,3] Nevertheless, growth control down to a single monolayer still has to be proven. Furthermore, to ensure a layered crystalline structure, high temperature anneal treatments are necessary [2] Finally, also the surface chemistry is not well understood, in particular the control of the partial reduction or oxidation of the metal in the MX2 ALD process [4] In this work, we demonstrate a plasma enhanced ALD (PEALD) process for WS2 films, using WF6 and H2S in combination with H2 plasma. Deposition is performed on 300mm Si substrates covered with either 20 nm TiN by physical vapor deposition (PVD) or 30 nm ALD grown and annealed Al2O3 layers. Deposition was first investigated on TiN substrates because of the known reactivity of WF6 and TiN [5] The initial WF6 reaction, forming metallic W and volatile TiF4, is confirmed to be self limiting at temperatures between 250 and 350oC (Rutherford backscattering Spectrometry, RBS) However, even though nucleation is enabled, sequential exposures of WF6 and H2S precursors do not yield a steady state WS2 growth component at temperatures between 300 and 450degC (RBS, Fig. 1a) The deposition stops after ~20 cycles, resulting in the formation of a tungsten subsulfide layer (WSx, with x=~0.7) with ~1.6E+15 W atoms/cm2. We propose that the reduction of W6+ (in WF6) does not occur through reaction with H2S. The steady state growth of tungsten disulfide (WS2) is enabled by PEALD, using reaction cycles consisting of WF6, H2 plasma and H2S exposures (RBS, Fig. 1a) After the initial growth enhancement, both W and S content increase linearly with the number of reaction cycles. The S content increases almost two times faster than the W content, indicating WS2 deposition with a better stoichiometry. The deposition occurs in a broad temperature window, between 250 and 450oC. Interestingly, the PEALD sequence also yields nearly stoichiometric WS2 when applied on Al2O3 substrates. On polycrystalline Al2O3, up to 6 monolayers of WS2 have been grown, with a S to W ratio of 1.8 2 and good thickness uniformity over the 300mm substrate. The WS2 signatures are clearly observed in the Raman spectrum (Fig. 1b) A polycrystalline layered 2D structure is indicated by X ray diffraction and Transmission Electron Microscopy. The WS2 planes are mainly oriented parallel to the substrate surface. The impact of the crystallinity and roughness of the Al2O3 layers on the nucleation, growth and 2D structure of WS2 is being further analyzed.
author_names: Benjamin Groven

M H Heyne

Karel P S Haesevoets

Johan Meersschaut

Thomas Nuytten

Pascal R Verdonck

Sven Van Elshocht

Marc M Heyns

Iuliana P Radu

Annelies Delabie

Matty Caymax
title: Plasma enhanced atomic layer deposition of two dimensional WS2 from WF6, H2S and H2 plasma
year: 2015

abstract: The method of atomic layer deposition (ALD) is considered one of the primary candidates for the uniform and conformal deposition of ultrathin films vital for the continuous miniaturization in the semiconductor industry and related high technology markets. By the virtue of two selflimiting surface reactions, the ALD technique yields an ultimate control of film growth in the sense that a submonolayer of material is deposited per so called ALD cycle. With established materials being at the verge of industrial implementation, efforts are continuously undertaken to optimize and develop new ALD configurations and processes. So far, the main emphasis within the field of ALD has been on the materials characterization of the films by means of ex situ analysis. The research described in this thesis aims at the development of the relatively new configuration of plasma assisted ALD and at in situ diagnostics studies of the (plasmaassisted) ALD processes. In plasma assisted ALD, a plasma is used to activate the reactants in the gas phase in order to supply additional reactivity to the ALD chemistry. Plasma assisted ALD is researched to provide benefits in the development of new ALD processes and materials. In particular, the opportunities to improve and tune the film properties, and to deposit films at reduced substrate temperatures have been addressed in this thesis. This work has been accompanied by studies using various in situ diagnostics, from which fundamental insight into the reaction mechanisms governing the ALD processes can be obtained. Moreover, in situ techniques provide the opportunity to monitor, optimize, and control the ALD process. In this work the use of in situ spectroscopic ellipsometry, transmission infrared spectroscopy, mass spectrometry, and optical emission spectroscopy has been demonstrated in studies of the plasma assisted ALD processes of metal nitrides and metal oxides. The results of the film characterization obtained by these techniques have been corroborated and complemented by extensive ex situ analysis. In particular, the combination of in situ spectroscopic ellipsometry and the layer by layer ALD growth has been explored comprehensively. The merits of this in situ technique during ALD have been demonstrated by addressing various aspects relevant to ALD processes and materials. A large part of the work has concentrated on the plasma assisted ALD process of the metal nitrides TiN and TaN. The merits of plasma assisted ALD were observed in the deposition TiN films with excellent conductivity and low impurity content, even at low deposition temperatures. Furthermore, it was shown that by variation of the plasma condition in the ALD process of TaN, the film properties could be tailored from conductive, cubic TaNx;x??1 to semiconductive, amorphous Ta3N5. These aspects were clearly demonstrated by in situ spectroscopic ellipsometry, where the transition in TaNx phase could be distinguished by monitoring the energy dispersion in the optical constants. For the conductive films, the light absorption by free conduction electrons could be probed and that enabled extraction of the electrical film properties from the ellipsometry data. The latter was valuable to demonstrate electron impurity scattering and finite size effects in TiN films. Furthermore, fundamental insight into the reaction mechanisms of plasma assisted ALD process of TaN was obtained by detection of the volatile reaction by products by mass spectrometry and optical emission spectroscopy. The possibilities for plasma assisted ALD to improve the material properties and to deposit at reduced temperatures have been demonstrated for the process of Al2O3. The Al2O3 films were deposited at substrate temperatures down to room temperature and these films yielded good moisture permeation barrier properties as relevant for encapsulation purposes. The fundamental reaction mechanisms of this plasma assisted ALD process were elucidated by transmission infrared spectroscopy in order to understand and further improve the film properties obtained at these reduced deposition temperatures. It was established that the surface chemistry is ruled by CH3 and OH surface groups created by the Al(CH3)3 precursor adsorption and the combustionlike reactions during the O2 plasma step, respectively. Moreover, infrared spectroscopy provided insight into the influence of deposition temperature on the material properties. It was shown that by prolonging the plasma exposure, i.e. by supplying more plasma reactivity to the ALD process, the surface chemistry at low temperatures was enhanced and the impurity content in the Al2O3 was reduced. In conclusion, the knowledge gained through the in situ diagnostic studies in this work is relevant to further develop the ALD technique. The insight obtained into the reaction mechanisms and the material properties of the ALD films in this work are particularly useful to further exploit the possibilities and opportunities of the plasma assisted ALD technique in the synthesis of novel (complex) materials.
author_names: E Erik Langereis
title: Plasma assisted atomic layer deposition an in situ diagnostic study
year: 2008

abstract: Ultra thin Al2O3 films were deposited by plasma enhanced atomic layer deposition (PEALD) as Cu diffusion barrier on low k (k=2.5) material. The thermal stability and electrical properties of the Cu/low k system with Ta, Ta/Al2O3 and Ru/Ta layers with different thickness were compared after annealing. The TEM and EDX results reveal that the ultrathin Al2O3 films are thermally stable and have excellent Cu diffusion barrier performance. The I V and TDDB electrical measurements further confirm that the ultra thin Al2O3 film is a potential Cu diffusion barrier in copper/low k interconnect. Corresponding Author: Xin Ping Qu, Professor, With the continuing scaling down of the line pitch in very large scale integrated circuits, ultrathin Cu diffusion barriers are necessary to mitigate the increase in the effective resistivity of the Cu interconnects due to grain boundary scattering and interface scattering. Conformal deposition of thin films in high aspect ratio trenches using traditional physical vapor deposition (PVD) technique is becoming difficult [1] Atomic layer deposition (ALD) is considered as a promising method to get excellent conformal films with thickness control at atomic level owing to its intrinsic self limited growth feature [2] ALD grown metals or nitrides such as Ru [3 4] and TaN films[5,6] have been widely studied as Cu diffusion barriers or adhesion layers. ALD grown oxides, such as TiO2[7] Ta2O5 [8] and Al2O3 [9] have also been reported as Cu diffusion barriers due to their excellent uniformity and thermal stability. Qi Xie et.al reported the excellent thermal stability of ALD TiO2.[7] P. Majumder et. al. reported that the ultrathin Al2O3 layer deposited by thermal ALD as diffusion barrier between Cu and Si substrate had excellent thermal stability.[9] However, these oxide layers were all directly deposited onto Si and the very primary properties of diffusion barriers to Cu were measured. Systematic study of ALD oxide films as Cu diffusion barrier on the low k dielectric has been rarely reported. In this work, ultra thin Al2O3 films were deposited by plasma enhanced atomic layer deposition (PEALD) together with Ta adhesion layer, its properties as a Cu diffusion barrier on the low k dielectric (k=2.5) were studied. The low k films (Novellus CORAL, k=2.5) were deposited on the 12" p type Si wafers with a thickness of 220 nm. The wafers were cut into small pieces and then were cleaned in an ultrasonic cleaner with 2 propanol solution before loading to the loadlock chamber. After in situ annealing at 250 deg C for 20 minutes in the high vacuum ALD chamber, 11 or 21 cycles ~1.2 A/cycle) Al2O3 films were grown by plasma enhanced atomic layer deposition (PEALD) using trimethylaluminium (TMA) and remote O2 plasma (200W) as precursors. The growth temperature was 250deg C. After deposition, samples were taken out and sent into a PVD chamber. The details of the ALD growth can be seen elsewhere.[7] Then the Cu(50 nm)/Ta(5 nm) film stacks were deposited by DC magnetron sputtering. The thin Ta film was used as a glue layer to promote adhesion between the Cu and Al2O3 barrier layers. To form the patterned metal insulatorsemiconductor (MIS) structures, a shadow mask was used when depositing the Cu/Ta stacks. For comparison, the Cu/Ta stack was also directly deposited onto the low k substrates. A Ti(40 nm)/Pt(30 nm) bi layer was deposited as backside electrode by sputtering. Furnace annealing was conducted in the N2 atmosphere at 400 degC for 30 minutes. After annealing, energy dispersive X ray (EDX) transmission electron microscope (TEM) time dependent dielectric breakdown (TDDB) and dielectric breakdown measurements were used to characterize the thermal stability and electrical reliability of the Cu/barrier/low k/Si system. The capacitance voltage (C V) measurements were carried out using an HP 4192A impedance analyzer to evaluate the effective k value of the system. The XTEM images of the as deposited Cu/Ta/Al2O3 (22cycle) /low k/Si samples and the one annealed at 400 degC/30min were shown in Fig. 1(a) and (b) For the asdeposited sample, the thickness of the Ta and Al2O3 layer is about 5 nm and 2.5 nm, respectively. The interface between the Ta and Al2O3 layer is clear. The contrast between the Al2O3 layer and the low k dielectric is not obvious due to the amorphous oxide structure for both Al2O3 and low k. For the annealed sample, the Ta and Al2O3 layer become mixed and the thickness of the barrier is 7.1 nm. The barrier is continuous but the interfaces of Cu/barrier and barrier/low k are obscure. Fig.2 shows the corresponding EDX depth profiles of these two samples. It can be seen that here is no Cu signal in the low k dielectric after annealing. For comparison, we also prepared other two kinds of samples, one is sputtered Cu/Ta (5 nm) and the other is sputtered Cu/ALD Ru (7 nm)/ALD TaN(5 nm) on the low k dielectrics. All the samples were annealed at 400 degC for 30min. The experimental detail of ALD growth of the Ru/TaN bi layer barrier can be seen elsewhere [4] Fig.3 (a) and (b) show the XTEM images of the Cu/Ta/low k/Si and Cu/Ru/TaN/low k/Si samples annealed at 400 degC for 30min. For the Cu/Ta/low k/Si sample, after annealing at 400 degC, there appeared a diffusion region between the Ta layer and low k. The EDX results, not shown here, revealed that there were strong Cu signals, about 10% in the diffusion region. For the Cu/ low k sample with ALD Ru/TaN bilayer as diffusion barrier (Fig.3(b) after annealing at 400 degC for 30 minutes, the Ru/TaN barrier was markedly destroyed and there was a diffusion region around 50 nm thick between the barrier layer and low k. The EDX results (not shown here) revealed that Cu signals almost appeared in every detect point, including the outside of the diffusion region. The failure of the ALD Ru/TaN stack is believed to be due to the diffusion of the Ta precursor to the porous low k dielectric during the ALD process. The diffusion of Ru precursor into the ULK during the ALD process was also observed by Heo et.al. [3] Although in the ALD deposition of Al2O3, the TMA precursor may also diffuse into the porous low k, but Al2O3 is very easy to form and it can act as a barrier to prevent the further penetration of precursor. Our results demonstrate that the thermal stability of the Ta(5 nm) /Al2O3(2.5 nm) barrier stack is not only better than the Ta film itself, but also the thick ALD Ru(7nm)/TaN(5 nm) The surface morphologies of Cu on the Ta/low k and Ta/Al2O3 (2.5 nm)/low k substrates are observed by SEM. As shown in Fig.4, after annealing at 400 degC for 30 min in N2, the Cu on the Ta/low k substrate experiences severe agglomeration; but for the samples with Ta/Al2O3 stack, there are no pinholes on the Cu surface. Agglomeration of the Cu films can be caused by reaction of barrier with moisture outgassed from the low k substrate, as well as the degraded Cu adhesion with barrier due to the oxidation of the barrier. [10 12] Thin Al2O3 films deposited by ALD used as superior moisture and oxygen barriers for organic devices has been reported. [13] Moisture barrier is also important for low k dielectrics, since the oxidation of Cu lines induced by the absorption and outgas of moisture in dielectric may degrade the electrical properties of the Cu interconnects. [14 15] Therefore, in our case, the ALD oxide layer can also suppress the moisture outgassed from the low k dielectric and enhance the Cu adhesion on Ta. Since Al2O3 is a kind of high k dielectrics (k ~9.0) [16] its effect on the effective dielectric constant of the Cu/low k system was evaluated. The C V measurements were carried out for the Cu/Ta/Al2O3(1.3 nm)/low k/Si, Cu/Ta/low k/Si and Cu/Ta/TaN(5 nm)/low k/Si MIS capacitor. The original k value was 2.55 after IPA cleaning. After sputtered deposition of the metal stacks, the measured effective k values (ke) of all the capacitors were all increased. The ke values for the Cu/Ta/low k/Si and Cu/Ta/Al2O3(1.3 nm) /low k samples were the same (2.8) and lower than that of the Cu/Ta/TaN(5 nm)/low k/Si (3.0) The increase of the effective k value is mainly due to the damage of the low k dielectric during the deposition and annealing process as well as the oxidation of the TaN barrier. These results indicate that using an ultra thin Al2O3 films as Cu diffusion barriers doesn't affect much on the k value of the low k dielectric. The electrical properties of the Cu gated MIS structures were measured. Figure 5(a) shows the leakage current densities of the the as deposited MIS capacitors with Ta(5 nm) with Ta(5 nm)/Al2O3(1.3 nm) and Ta(5 nm)/Al2O3(2.5 nm) as diffusion barriers =and those after annealing at 400 degC for 30 minutes in N2. The leakage current densities for all the as deposited and annealed samples were similar under the applied electric field of 1MV/cm. Further we compared the breakdown voltages for these samples. Figure 5(b) shows the cumulative failure distribution of breakdown electric field (Eb) of the capacitors with different diffusion barriers after annealing at 400 degC for 30 minutes in N2. For capacitors with the Ta(5 nm)/2.5 nm Al2O3 stack, Eb is mainly around 9 MV/cm; for those with 5 nm Ta layer and Ru/TaN, Eb is lower than 3 MV/cm. Although the Eb is similar for the samples with 1.3 nm or 2.5 nm Al2O3 layer, but the TDDB measurements (figure not shown here) show that the TDDB of the annealed MIS capacitor with 1.3 nm Al2O3 is about 2.3x10 4 s under a stressing field of 7.7 MV/cm; for the MIS capacitor with 2.5 nm Al2O3 layer, the TDDB exceeds 1.7x10 5 s under an higher EM of 8.6 MV/cm. Although under RT, copper is difficult to migrate under low electric field, it can move very rapidly under a high electric filed through defects in the diffusion barrier. The presence of Cu in the dielectrics would greatly reduce dielectric life time. [17] Our results further indicate that the Al2O3 film is an excellent barrier for Cu metallization and the thickness of the Al
author_names: S F Ding

Qi Xie

Fei Yu Chen

Hai-Sheng Lu

Shao-Ren Deng

Christophe Detavernier

Xin-Ping Qu
title: Excellent thermal stability by using PEALD ultra thin Al2O3 film with Ta as Cu diffusion barrier on low k dielectrics
year: 2011

abstract: Understanding conduction mechanisms (CMs) through a thin dielectric oxide is important in state of the art Complementary Metal Oxide Semiconductor (CMOS) technology. This is because when an Electric field (E) is applied across the dielectric film; an undesired current appears which degrades the behavior of the device, thus shortening its life span. One of the most widely used dielectric in microelectronics is Aluminum Oxide (Al2O3) because it presents a large bandgap ~8.8 eV) and relatively high dielectric constant (k~9) it also presents great thermal stability and can passivate important interfaces with other electronic materials thus enhancing the performance of electron devices. This work reviews the conduction mechanisms in Metal Insulator Semiconductor (MIS) devices fabricated using ultra thin Al2O3 (5nm and 10 nm) deposited by Atomic Layer Deposition (ALD) at relatively low processing temperatures. After gate patterning, thermal treatment was performed using Forming Gas (H2/N2) at 450deg C for 30 minutes. MIS devices were then electrically characterized using standard I V and C V measurements, taking the devices to dielectric breakdown; afterwards, experimental measurements were compared with semi empirical tunneling models like Direct tunneling (DT) Ohmic Conduction (OC) Poole Frenkel emission (PF) and Fowler Nordheim tunneling (FN) in order to determine the precise conduction mechanisms and their corresponding physical parameters. After dielectric breakdown, Schottky diode conduction is obtained, thus demonstrating the nature of the post breakdown event. Once these well known semi empirical models were validated through standard equations, SILVACO simulations were made in order to further validate the proposed models. Afterwards, physical parameters before and after thermal annealing such as effective mass (m) barrier height (before and after breakdown) and trap energy level were extracted. Finally, only one self consistent equation containing all physical parameters and conduction models is obtained.
author_names: Molina Reyes Joel

Uribe Vargas Hector Manuel

Carlos Zuniga

Wilfrido Calleja
title: COMPARISON OF THE EXPERIMENTAL CARRIER TRANSPORT IN MIS DEVICES FABRICATED WITH ULTRA THIN Al 2 O 3 WITH STANDARD CONDUCTION MODELS AND SIMULATIONS
year: 2018