Step Coverage Research Articles

(Invited) Low Temperature Atomic Layer Deposition of Ru Thin Films with Enhanced Nucleation Using Various Ru(0) Metallorganic Precursors and Molecular O2

Ruthenium (Ru) thin films were grown on thermally-grown SiO2 substrates by atomic layer deposition (ALD) using a sequential supply of three kinds of novel zero valence Ru precursors, isopropyl-methylbenzenecyclohexadiene Ru(0) (IMBCHRu, C16H22Ru), ethylbenzen-cyclohexadiene Ru(0) (EBCHRu, C14H18Ru), and ethylbenzen-ethyl-cyclohexadiene Ru(0) (EBECHRu, C16H22Ru) and molecular oxygen (O2) as a reactant at substrate temperatures ranging from 140 to 350 oC. It was shown that little incubation cycle was observed at ALD temperature window, indicating of the improved nucleation as compared to the use of typical higher valence Ru precursors such as cyclopendadienyl-based Ru (II) or β-diketonate Ru (III) metallorganic precursors. The step coverage of ALD-Ru was excellent, around 100 % at ultra-small trench with an aspect ratio of ~ 4.5 (top-opening diameter: ~ 25 nm). The developed ALD-Ru films was successfully used as a seed layer for Cu electroplating.

Conformal Deposition of an Insulator Layer and Ag Nano Paste Filling of a Through Silicon Via for a 3D Interconnection

In this study, we reported the feasibility of filling a high-aspect-ratio through silicon via (HARTSV) with Ag nano paste for a 3D interconnection. TSVs with aspect ratios of 8:1 10:1 were fabricated in a deep reactive etching system by using the Bosch process. Then, SiO2 insulators were deposited by using various chemical vapor deposition (CVD) processes, including plasma enhanced CVD oxides, of which precursors were silane (PECVD Oxide) and tetraethoxysilane (PECVDTEOS), and sub-atmospheric CVD oxide (SACVD oxide). We succeeded in obtaining a SiO2 layer with good step coverage over 80% for all via CD sizes by using SACVD oxidation process. The thickness of SiO2 for the via top and the via bottom were in the range 158.8 161.5 nm and 162.6 170.7 nm, respectively. The HAR-TSVs were filled with Ag nano paste by using vacuum assisted paste printing. Then, the samples were cured on a hotplate at 80 C for 2 min. The temperature was increased to 180 C at a rate of 25 C/min and the samples were re-annealed for 2 min. We investigated the eects for the time of evacuation/purge process and of the vacuum drying on the filling properties. A field emission scanning electron microscope (FE-SEM), X-ray microscope and focused ion beam (FIB) microscope were used to investigate the filling profile of the TSV with Ag nano pastes. By increasing the evacuation/purge time and the vacuum drying time, we could fully fill the TSV was full filled with Ag nano paste and then form a metal plug.

Fluxless Bonding of Bismuth Telluride Chips to Alumina Using Ag–In System for High Temperature Thermoelectric Devices

Bismuth telluride (Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) and its alloys are the most commonly used materials for thermoelectric devices. In this paper, the fluxless bonding process was developed to bond Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chips to alumina substrates for high temperature applications. The silver-indium (Ag-In) system was chosen for the process development. To work with this system, the Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chips were coated with 100 nm titanium (Ti) and 100 nm gold (Au) as barrier layer and plated with 10 μm Ag layer. The Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> samples were annealed at 250 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 200 h. No interdiffusion between Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Ag was detected. The Ti/Au barrier layer was not affected either. It showed exceptional step coverage on the rough Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> surface even after the annealing process. To prepare for bonding, alumina substrates with 40 nm TiW and 2.5 μm Au were plated with 60 μm Ag, followed by 5 μm In and thin Ag cap layer for oxidation prevention. The Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chips were bonded to alumina substrates at 180 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. No flux was used. The resulting void-free joint consists of five regions: Ag, (Ag), Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In, (Ag) and Ag. (Ag) is Ag-rich solid solution. The joint has a melting temperature higher than 660 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. Due to the thick ductile Ag layer on alumina, the Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chip did not break after bonding despite its significant coefficient of thermal expansion mismatch with alumina.

A Difference in Using Atomic Layer Deposition or Physical Vapour Deposition TiN as Electrode Material in Metal-Insulator-Metal and Metal-Insulator-Silicon Capacitors

In this work, metal-insulator-metal (MIM) and metal-insulator-silicon (MIS) capacitors are studied using titanium nitride (TiN) as the electrode material. The effect of structural defects on the electrical properties on MIS and MIM capacitors is studied for various electrode configurations. In the MIM capacitors the bottom electrode is a patterned 100 nm TiN layer (called BE type 1), deposited via sputtering, while MIS capacitors have a flat bottom electrode (called BE type 2-silicon substrate). A high quality 50-100 nm thick SiO2 layer, made by inductively-coupled plasma CVD at 150 degrees C, is deposited as a dielectric on top of both types of bottom electrodes. BE type 1 (MIM) capacitors have a varying from low to high concentration of structural defects in the SiO2 layer. BE type 2 (MIS) capacitors have a low concentration of structural defects and are used as a reference. Two sets of each capacitor design are fabricated with the TiN top electrode deposited either via physical vapour deposition (PVD, i.e., sputtering) or atomic layer deposition (ALD). The MIM and MIS capacitors are electrically characterized in terms of the leakage current at an electric field of 0.1 MV/cm (I leak) and for different structural defect concentrations. It is shown that the structural defects only show up in the electrical characteristics of BE type 1 capacitors with an ALD TiN-based top electrode. This is due to the excellent step coverage of the ALD process. This work clearly demonstrates the sensitivity to process-induced structural defects, when ALD is used as a step in process integration of conductors on insulation materials.

A Sub-Atmospheric Chemical Vapor Deposition Process for Deposition of Oxide Liner in High Aspect Ratio Through Silicon Vias

The formation of a Through Silicon Via (TSV) includes a deep Si trench etching and the formation of an insulating layer along the high-aspect-ratio trench and the filling of a conductive material into the via hole. The isolation of the filling conductor from the silicon substrate becomes more important for higher frequencies due to the high coupling of the signal to the silicon. The importance of the oxide thickness on the via wall isolation can be verified using electromagnetic field simulators. To satisfy the needs on the Silicon dioxide deposition, a sub-atmospheric chemical vapor deposition (SA-CVD) process has been developed to deposit an isolation oxide to the walls of deep silicon trenches. The technique provides excellent step coverage of the 100 microm depth silicon trenches with the high aspect ratio of 20 and more. The developed technique allows covering the deep silicon trenches by oxide and makes the high isolation of TSVs from silicon substrate feasible which is the key factor for the performance of TSVs for mm-wave 3D packaging.

Deposition of Thermal and Hot-Wire Chemical Vapor Deposition Copper Thin Films on Patterned Substrates

In this work we study the hot-wire chemical vapor deposition (HWCVD) of copper films on blanket and patterned substrates at high filament temperatures. A vertical chemical vapor deposition reactor was used in which the chemical reactions were assisted by a tungsten filament heated at 650 degrees C. Hexafluoroacetylacetonate Cu(I) trimethylvinylsilane (CupraSelect) vapors were used, directly injected into the reactor with the aid of a liquid injection system using N2 as carrier gas. Copper thin films grown also by thermal and hot-wire CVD. The substrates used were oxidized silicon wafers on which trenches with dimensions of the order of 500 nm were formed and subsequently covered with LPCVD W. HWCVD copper thin films grown at filament temperature of 650 degrees C showed higher growth rates compared to the thermally ones. They also exhibited higher resistivities than thermal and HWCVD films grown at lower filament temperatures. Thermally grown Cu films have very uniform deposition leading to full coverage of the patterned substrates while the HWCVD films exhibited a tendency to vertical growth, thereby creating gaps and incomplete step coverage.

Self-Limiting Film Growth of Transparent Conducting In2O3 by Atomic Layer Deposition using Trimethylindium and Water Vapor

Conformal and self-limiting film growth of transparent conducting In2O3 is demonstrated with atomic layer deposition (ALD) using trimethylindium (TMIn) and water vapor. Various parameters, including pulsing times of TMIn and H2O and deposition temperature, are varied to confirm the ALD processing window of In2O3. It is found that an extremely large Langmuir exposure of H2O (∼2 Torr s) is required to fully react the surface-adsorbed In-(CH3)* groups with H2O. Self-limiting film growth is obtainable at the deposition temperatures between 200 and 251 °C where the film growth is controlled by surface adsorption of TMIn molecules. Within the ALD window, ALD-In2O3 exhibits a film growth rate of ∼0.039 nm/cycle at 217 °C. In addition, the film exhibits an excellent step coverage of ∼97% on a high-aspect-ratio (∼11) nanostructure. The resistivity of the ALD-In2O3 film deposited at 200 °C is about 2.8 × 10–3 Ω cm, and this is attributed to the high Hall mobility of 84 cm2/(V s). Finally, the reduction of the Hall mobility of the films with respect to increases in the deposition temperature is correlated to the evolution of microstructures.

Improving the quality of barrier/seed interface by optimizing physical vapor deposition of Cu Film in hollow cathode magnetron

The quality of physical vapor deposition (PVD) grown barrier/seed interface in Cu interconnect metallization was significantly improved by enhancing Cu nucleation on the Ta barrier surface. This was accomplished through filtering of nonenergetic species from the deposition flux, increasing the fraction of Cu ions, improving metal ion flux uniformity, and minimizing gas ion bombardment of the growing film. The self-sputtering ability of Cu was combined with a magnetically confined high-density plasma in the Novellus hollow cathode magnetron (HCM®) PVD source. Spatial profiles of plasma density and temperature, as well as ion flux, metal ion fraction, and ion energy, were measured by planar Langmuir probes, quartz crystal microbalances, and gridded energy analyzers, all located at the wafer level. Multiple criteria, such as seed step coverage and roughness, the seed layer’s resistance to agglomeration, and its stability in the plating bath, have been used to evaluate interface quality. As a result, a new and improved Cu PVD process which demonstrates superior stability during subsequent process steps and ensures successful electrofill performance with a more than 50 % reduction in minimal requirement of field thickness as well as sidewall thickness has been developed.

Conformal Polymeric Thin Films by Low-Temperature Rapid Initiated Chemical Vapor Deposition (iCVD) Using tert-Butyl Peroxybenzoate as an Initiator

Conformal poly(cyclohexyl methacrylate) (pCHMA) thin films were synthesized via initiated chemical vapor deposition (iCVD), with tert-butyl peroxybenzoate (TBPOB) as the initiator, representing the first time that TBPOB has been used as an initiator for iCVD synthesis. Using TBPOB instead of tert-butyl peroxide (TBPO), the rate of iCVD film growth increased by a factor of up to seven at comparable conformality and lower the filament temperature from 257 to 170 °C at a comparable deposition rate of 3 nm/min. The conformal deposition of functional thin films is desired for applications including microfluidics, medical devices and membranes. Lower filament temperatures reduce the heat load to the deposition surface and thus are advantageous for polymeric substrates that are temperature sensitive or monomers that decompose at high temperatures. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results demonstrate the similarity of the TBPOB- to the TBPO-initiated pCHMA main chains. However, the aromatic group in TBPOB provided a unique spectral signature of the polymer chain end group in the FTIR and the peak intensity increased with increase of filament temperature. Scanning electron micrographs (SEMs) revealed that the pCHMA coatings are conformal over non-planar structures; however, at identical process conditions, TBPO-initiated films showed a slightly better conformality due to the lower sticking coefficient of TBPO. At a monomer partial pressure of 0.45, TBPOB has a sticking coefficient value of 0.1188 ± 0.0092, which is ∼3 times as high as that of TBPO (0.0413 ± 0.0058). The step coverage is insensitive to filament temperature if the surface concentration of the monomer is fixed.

Plasma enhanced atomic layer deposition of SiNx:H and SiO2

As the nanoelectronics industry looks to transition to both three dimensional transistor and interconnect technologies at the &amp;lt;22 nm node, highly conformal dielectric coatings with precise thickness control are increasingly being demanded. Plasma enhanced chemical vapor deposition (PECVD) currently fills this role for most applications requiring low temperature processing but does not always meet step coverage and thickness precision requirements. The authors present results for a hybrid technique, plasma enhanced atomic layer deposition (PEALD), which utilizes typical PECVD process gases and tooling while delivering improved topography coverage and thickness control. Specifically, the authors show that alternating SiH4 gas/N2 plasma exposures applied in an atomic layer deposition sequence can be used to deposit SiNx:H films in a self-limiting fashion with improved conformality and superior performance as a moisture barrier. PEALD of SiO2 using alternating SiH4 and CO2 plasma exposures is further demonstrated.

Improved Step Coverage of Cu Seed Layers by Magnetic-Field-Assisted Ionized Sputtering

The relationship between the ion flux of self-ionized sputtering and the static magnetic field supplied by DC coils was investigated by ion current measurements. The ion flux was clarified in order to increase the substrate ion current. The additional magnetic field perpendicular to the substrate increases the substrate ion current and decreases the side electron current. The suppression of Cu ion diffusion increases the substrate ion current of self-ionized sputtering and the current distribution depends on the magnetic field configuration. A stronger magnetic field in the region of the wafer edge is necessary to increase the substrate ion current. This condition results in an increase in bottom step coverage and any asymmetry at the wafer edge is also corrected.

Improved Step Coverage of Cu Seed Layers by Magnetic-Field-Assisted Ionized Sputtering

Improved Step Coverage of Cu Seed Layers by Magnetic-Field-Assisted Ionized Sputtering

Atomic Layer Deposition of Ta-doped TiO2 Electrodes for Dye-Sensitized Solar Cells

Ta-doped TiO2 inverse opals were obtained by selective etching of a silica template after atomic layer deposition (ALD) of Ta-doped TiO2 films and were applied as an electrode for dye-sensitized solar cells (DSSCs). Ta content in the Ta-doped TiO2 film was controlled by the Ta/(Ta+Ti) unitcycle ratios in the Ta–TiO2 supercycle of ALD. Also, excellent step coverage of nearly 100% in the inverse opal structure was confirmed by field-emission scanning electron microscopy (FE-SEM). Maximum photoconversion efficiency of 1.56% was achieved with the Ta (3.4 atom %)-doped TiO2 inverse opal electrode due to increased photocurrent density. However, further Ta doping (> 4.9 atom %) decreased the Jsc and photoconversion efficiency.

Direct Copper Electrochemical Deposition on Ru-Based Substrates for Advanced Interconnects Target 30 nm and 1/2 Pitch Lines: From Coupon to Full-Wafer Experiments

Extending copper electrochemical deposition to 3x nm nodes and beyond requires a new plating approach that is not constrained by typical PVD copper seed step coverage performance. To this purpose, we propose a copper direct plating process on Plasma Enhanced Atomic Layer Deposition (PEALD) Ru -based resistive substrates, where the Cu seed is deposited in-situ during the front propagation from the edge to the center of the wafer. In order to understand the full-wafer copper direct plating process that occurs on these liners, the effect of plating tool advanced features, applied waveform, plating chemistry and substrate surface activation on the subsequent plated copper nucleation behavior are studied.

Thermal Atomic Layer Deposition (ALD) of Ru Films for Cu Direct Plating

Ruthenium (Ru) thin films were grown on thermally-grown SiO2 substrates by thermal atomic layer deposition (ALD) using a sequential supply of a zero metal valence precursor, isopropyl-methylbenzene-cyclohexadiene Ru(0) (IMBCHRu, C16H22Ru) and molecular oxygen (O2) at substrate temperatures ranging from 185 to 310°C. The growth rate at 185°C was approximately 0.059 nm/cycle but its resistivity was >3000 μΩ cm. When the deposition was done at 200°C, the resistivity was decreased drastically to ∼100 μΩ cm, and the film growth rate increased to 0.075 nm/cycle. An ALD temperature window from 225 to 270°C was observed. A high growth rate of 0.086–0.089 nm/cycle was obtained at this ALD temperature window. The film deposited at 270°C showed a minimum resistivity of ∼30 μΩ cm and a high density of 11.7 g/cm3 and with no impurities in the film, such as oxygen and carbon. At 310°C, the growth rate increased to 0.136 nm/cycle due to a partial decomposition of the precursor. In addition, the film resistivity increased slightly to ∼40 μΩ cm with the incorporation of carbon and the formation of a less-dense film. The step coverage of the ALD-Ru film was dependent on the dimensions of the contact and deposition temperature. At the contact with an aspect ratio of ∼4.6 (top opening diameter: 80 nm), the step coverage was excellent irrespective of the deposition temperature. However, at the contact with an aspect ratio of ∼25, the step coverage of the film deposited at 310°C (or above ALD temperature window) was degraded, even though those prepared within ALD temperature window were ∼100%. Finally, ALD-Ru film was used successfully as a seed layer for Cu electroplating.

Hot wire chemical vapor deposition of germanium selenide thin films for nonvolatile random access memory applications

Thin films of germanium selenide (GexSe100−x with 0&amp;lt;x&amp;lt;57) were deposited on different substrates by hot wire metalorganic chemical vapor deposition using tetraallylgermanium and di-tert-butylselenide. The growth kinetics of the deposition process as well as the properties of the films were investigated. The growth rate was found to decrease with increasing temperature and decreasing pressure. The conformal step coverage was demonstrated. Germanium selenide films covered and subsequently diffused by silver were used as programmable metallization cells for a verification of the electrical switching properties.

Fabrication of coaxial p-Cu 2O/n-ZnO nanowire photodiodes

The authors report the deposition of Cu 2O onto vertically well aligned ZnO nanowires by DC sputtering. The average length, average diameter and density of these VLS-synthesized ZnO nanowires were 1 μm, 100 nm and 23 wires/μm 2, respectively. With proper sputtering parameters, the deposited Cu 2O could fill the gaps between the ZnO nanowires with good step coverage to form coaxial p-Cu 2O/n-ZnO nanowires with a rectifying current–voltage characteristic. Furthermore, the fabricated coaxial p-Cu 2O/n-ZnO nanowire photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratio and the fast responses.

Second-harmonic microscopy for fluence-dependent investigation of laser-induced surface reactions

Spatially resolved optical second-harmonic generation (SHG), that is, SHG microscopy, is shown to be a versatile tool for the investigation of femtosecond laser-induced surface processes such as adsorbate diffusion and desorption and their dependence on absorbed laser fluence. As an example, time-resolved measurements on the diffusion of O from the steps onto the terraces of a vicinal Pt(111) surface at low substrate temperature were performed. Using the sensitivity of SHG on step coverage and analyzing the signal change across the laser beam profile via SHG microscopy during laser-induced diffusion, the strong nonlinear fluence dependence of the diffusion rate could be determined. Using SHG microscopy in combination with a two-pulse correlation scheme, time-resolved information on the surface reaction was obtained as a function of absorbed laser fluence.

Quantitative study on the enhancement of sidewall coverage of sputter-deposited film by partially tapering the sidewall of via holes

As the aspect ratio of a via increases, the film sputter-deposited inside the via suffers from poor step coverage. In this study, the authors introduced a partially tapered via and simulated the thickness profile of sputter-deposited film inside it. For the simulation, the directionality factor k was introduced to the Monte Carlo method to consider the angular directionality of depositing atoms. The optimum partially tapered via, which has a maximum sidewall coverage, was obtained for various via dimensions (i.e., aspect ratios and bottom-to-entrance size ratios) and directionality factors of depositing atoms. The enhancement effect of the sidewall coverage by introducing an optimum partially tapered via was investigated quantitatively. The enhancement factor of an optimum partially tapered via is always greater than that of a fully tapered via. To achieve high sidewall coverage for high aspect ratio vias, it is suggested to deposit a film inside an optimum partially tapered via under the condition with high directionality.

Barrier/bonding layers on bismuth telluride (Bi2Te3) for high temperature thermoelectric modules

In this research, a fundamental study is conducted to identify the materials and develop the processes for producing barrier/bonding composite on Bi2Te3 for high temperature thermoelectric applications. The composite must meet four basic requirements: (a) prevent inter-diffusion between the electrode material, for our design, silver(Ag) and Bi2Te3, (b) bond well to Bi2Te3, (c) bond well to Ag electrode, and (d) do not themselves diffuse into Bi2Te3. The composites investigated include palladium (Pd), nickel/gold (Ni/Au), Ag, and titanium/gold (Ti/Au). After annealing at 250 °C for 200 h, only the Ti/Au design meets all four requirements. The thickness of Ti and Au, respectively, is only 100 nm. Other than meeting these four requirements, the Ti/Au layers exhibit excellent step coverage on the rough Bi2Te3 surface even after the annealing process.