Cu Wiring Fabrication by Supercritical Fluid Deposition for MEMS Devices

Process technologies that use supercritical CO2 fluids to fabricate high-aspect-ratio three-dimensional nano- and micro-components are described. Supercritical CO2 is a state of CO2 above the critical point. Supercritical CO2 fluids are used as alternatives to common media (gases and liquids) in MEMS device fabrication to both overcome the drawbacks of these materials and to realize a superior three-dimensional process opportunity. Supercritical fluids behave as both gases and liquids, offer many of the advantages of both, and have zero surface tension. Supercritical fluids are an ideal medium for fabricating very high-aspect-ratio features owing to their superior capability of diffusion transport. As MEMSs have complex and high-aspect-ratio structures, using a supercritical fluid as a process medium in MEMS fabrication provides ideal performance in film coating, plug filling of concave features, and the etching/cleaning of residues. In this chapter, the physicochemical properties of supercritical fluids are first described in terms of MEMS processing, but from a different point of view than that of the common literature on supercritical chemical processing. Next, various applications to thin film processing are described with a focus on interconnect/wiring fabrication of MEMS devices

Development and application of an experimental concept for surface characterization of semiconductor based substrates using scanning electrochemical microscopy

In this work, evaluation of the applicability of electrochemical scanning microscopy (SECM) for semiconductor industry-relevant thin film materials was carried out. These investigations were focused on the local electrochemical characterization of electrodeposited copper layers and their growth behavior on a variety of barrier materials such as Pt, Ru, TiN, TaN, Ta, Ti, W, and TiW. A special holding device for wafer-based samples was developed to be able to handle this combined task on a laboratory scale. This multipurpose cell provided electrical contact for the thin films deposited on the silicon substrate while simultaneously sealing the sample with only a small exposed area without any complex sample preparation. Based on preliminary studies, it could be shown that local surface characterization by means of SECM as well as electrochemical copper deposition in a commercial laboratory tool on the aforementioned materials with this cell was feasible. Consequently, deposition protocols for electroplating of dense Cu films with good adhesion on various barrier materials were developed for surface characterization studies with SECM. It became apparent that conventional commercial acidic copper electrolytes were only suitable for deposition of dense and adherent films on Cu or platinoids such as Ru or Pt. In contrast, direct electroplating on Ta-, Ti- and W-based barrier thin films had shown that the deposition of dense and adherent layers was heavily dependent on potential, on electrolyte composition and process handling and was therefore inappropriate for galvanic coating on a wafer scale. On the other side, implementation of the multipurpose cell into the SECM setup revealed that the local surface characterization of semi-precious metals in conventional feedback mode was strongly restricted. It could be shown, that oxide formation and corrosion of the thin film surface on the basis of surface interaction effects in aqueous solution had a strong influence on the measurement results. Moreover, the local resolution of this technique is limited, since materials with similar conductivity cannot be distinguished as shown by approach curves studies on different metallic thin films such as Ru, Pt, Cu, TiN, TiW, W and TaN. Based on these results a non-destructive measuring concept was developed which would ensure a high electrochemical contrast between different metallic materials without mediator-based surface interferences on the measured signal. It was demonstrated that the hydrogen evolution reaction had the necessary material selectivity according to the results of chronoamperometric studies on different barrier thin films. Therefore, a mediatorless SECM concept in SG/TC mode was characterized from these findings, which showed that the aforementioned requirements were achieved. In the next step, the measurement concept was used for the electrochemical characterization of the growth of direct electroplated Cu on Ru thin films. This model system was selected since Ru did not restrict the composition of the Cu electrolyte. Therefore, it was possible to study the influence of a grain refiner such as citric acid on the early electrocrystallization stage of Cu on Ru. In a preliminary SEM-supported study, it was shown that citric acid had a strong impact on the nucleation since it effectively inhibited grain growth in the deposition process. Thus, nanocrystalline and adherent Cu layers with a grain radius of 10 nm could be formed on pure Ru surface. Since the local resolution of the SECM is dependent on the probe size, ultramicroelectrodes with a size of rtip < 10 nm are required. The fabrication of electrodes of this dimensions cannot be accomplished with existing methods. In order to electrochemically characterize the nucleation behavior of Cu on foreign substrates electrochemically, new manufacturing processes for ultramicroelectrodes in the lower nanometer range have to be developed

Study of additives used in a copper via filling chemistry

An experimental study on the effect of additives used in a copper via-filling chemistry is carried out by electroanalytical techniques. These include potential or current pulse reversal deposition and cyclic voltammetry methods. Suppression of electrodeposition caused by polyethylene glycol (PEG) and by a commercial suppressor was examined. Effect of bis-(3-sulfopropyl)-disulfide (SPS) and a combination with the suppressor was also examined. A model based on free accelerant complex formation was used to design the experiments. Contrast in the chemical environment between the bottom and the surface of the vias was simulated on a rotating disk electrode (RDE) by variation of rotation speeds. The currents measured at low and high speeds of RDE simulate the bottom and top of the via respectively. The fill ratio, current at low speed divided by current at high speed, was used an effective screening tool to compare baths with different additive chemistries

Electrodeposition of copper using additive-containing low metal ion concentration electrolytes for EnFACE applications

In the past decade a new electrodeposition process called Electrochemical nano and micro Fabrication by flow and Chemistry (EnFACE) was developed which enabled mask-less pattern transfers onto a metallic substrate. EnFACE uses a novel acid-free, additive-free plating electrolyte containing low concentrations of metal salts (0.1 MCuSO4), as the process requires electroplating under conditions of fast kinetics and low electrolyte conductivity. However, for electronic applications,industry requires the use of additives, which improve deposit properties such as thickness uniformity, strength, ductility, and conductivity. The use of pulsed current is also known to improve deposit properties such as grain structure, mechanical strength and throwing power. Therefore, in order to use EnFACE for fabrication of industrially useful products, the effect of additives on the electrochemical behaviour and deposit properties of this process needs to be assessed. In addition, the influence of current modulation; i.e. direct current vs pulsed current, on deposit properties also warrants investigation. Potentiodynamic polarisation experiments were performed on additive-free and additive-containing EnFACE electrolyte (0.1 M CuSO₄). The additives tested were Copper Gleam A, Copper Gleam B, and chloride ions (Cl⁻). The effect of two parameters: (i) additive type and (ii) additive concentration, on cathode polarisation were studied.Copper films were electroplated on stainless steel substrates from electrolytes containing different concentrations of plating additives (0%, 17%, 33%, 50%, 100%,200% of the industry recommended additive concentration). Both direct current (DC)ABSTRACT|ii and pulsed current (PC) plating were used. The deposit was characterised using scanning electron microscope (SEM), electron back scattered diffraction (EBSD),tensile test machine (UTM), four-point probe and X-ray diffraction (XRD).Cathode polarisation occurred when the additives were used individually. Thecombination of Copper Gleam B and Cl- suggested synergistic inhibition, particularly in the diffusion-limited region. The addition of Copper Gleam A to the CopperGleam B-Cl⁻ mix increased the limiting current and suggested plating acceleration.These effects are interpreted in terms of the adsorption-desorption behavior of the additives on the cathode surface. SEM and EBSD images indicated that additives caused a concentration dependent decrease in the grain size of the deposit in both the DC and PC plated deposit. This grain refinement resulted in an increase in yield and tensile strength,but reduced the ductility and resistivity of deposits. The PC-plated copper from theEnFACE electrolytes generally possessed better mechanical properties than its DC-plated counterparts, though both plating modes created copper films that can meet industry standards. The optimum additive concentration for the EnFACE electrolyte was 50% of the recommended value when using DC plating; while the optimum was only 33% when using PC plating.In the past decade a new electrodeposition process called Electrochemical nano and micro Fabrication by flow and Chemistry (EnFACE) was developed which enabled mask-less pattern transfers onto a metallic substrate. EnFACE uses a novel acid-free, additive-free plating electrolyte containing low concentrations of metal salts (0.1 MCuSO4), as the process requires electroplating under conditions of fast kinetics and low electrolyte conductivity. However, for electronic applications,industry requires the use of additives, which improve deposit properties such as thickness uniformity, strength, ductility, and conductivity. The use of pulsed current is also known to improve deposit properties such as grain structure, mechanical strength and throwing power. Therefore, in order to use EnFACE for fabrication of industrially useful products, the effect of additives on the electrochemical behaviour and deposit properties of this process needs to be assessed. In addition, the influence of current modulation; i.e. direct current vs pulsed current, on deposit properties also warrants investigation. Potentiodynamic polarisation experiments were performed on additive-free and additive-containing EnFACE electrolyte (0.1 M CuSO₄). The additives tested were Copper Gleam A, Copper Gleam B, and chloride ions (Cl⁻). The effect of two parameters: (i) additive type and (ii) additive concentration, on cathode polarisation were studied.Copper films were electroplated on stainless steel substrates from electrolytes containing different concentrations of plating additives (0%, 17%, 33%, 50%, 100%,200% of the industry recommended additive concentration). Both direct current (DC)ABSTRACT|ii and pulsed current (PC) plating were used. The deposit was characterised using scanning electron microscope (SEM), electron back scattered diffraction (EBSD),tensile test machine (UTM), four-point probe and X-ray diffraction (XRD).Cathode polarisation occurred when the additives were used individually. Thecombination of Copper Gleam B and Cl- suggested synergistic inhibition, particularly in the diffusion-limited region. The addition of Copper Gleam A to the CopperGleam B-Cl⁻ mix increased the limiting current and suggested plating acceleration.These effects are interpreted in terms of the adsorption-desorption behavior of the additives on the cathode surface. SEM and EBSD images indicated that additives caused a concentration dependent decrease in the grain size of the deposit in both the DC and PC plated deposit. This grain refinement resulted in an increase in yield and tensile strength,but reduced the ductility and resistivity of deposits. The PC-plated copper from theEnFACE electrolytes generally possessed better mechanical properties than its DC-plated counterparts, though both plating modes created copper films that can meet industry standards. The optimum additive concentration for the EnFACE electrolyte was 50% of the recommended value when using DC plating; while the optimum was only 33% when using PC plating

Electroplating of Nanostructures

The electroplating was widely used to electrodeposit the nanostructures because of its relatively low deposition temperature, low cost and controlling the thickness of the coatings. With advances in electronics and microprocessor, the amount and form of the electrodeposition current applied can be controlled. The pulse electrodeposition has the interesting advantages such as higher current density application, higher efficiency and more variable parameters compared to direct current density. This book collects new developments about electroplating and its use in nanotechnology

Fabrication and optimization of three-dimensional metamaterials for terahertz energy-harvesting

Metamaterials typically consist of metallic and dielectric repeating structures. Electrodeposition of copper is the preferred approach to fabricating the metallic part of the metamaterials of interest in this study. The highly-variant topography requires chemical additives, like chloride ions, 3-mercapto-1-propanesulfonic acid (MPSA), polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) to enhance bottom-up superfilling while maintaining terrace flatness. This study focuses on both experimental and computational investigations of the degradation potential of the additives and their adsorption mechanism in a highly-acidic copper electrolyte in order to optimally parameterize the copper electrodeposition process. Results show Cl-MPSAPEG-PVP additives perform well, but substitution of PVP with Janus Green B provides better terrace leveling. Additionally, NMR data show a quick and complete conversion of MPSA to bis(3-sulfopropyl) disulfide (SPS) in the acidic copper bath. Finally, FEM simulations further show that the accelerator species may initially accumulate and be transported vertically until overplating, whereby they are transported laterally. Our study also show that the electromagnetic performance of metamaterial is dependent on SU-8 related parameters, including pillar height, bottom side length and spacing between pillars. The metamaterial geometry is successfully optimized and it can localize the surface plasmon with the peak frequency of 1.2 THz to the nano-antenna and the highest terahertz absorption is 30%.Includes bibliographical reference

Micromachining

To present their work in the field of micromachining, researchers from distant parts of the world have joined their efforts and contributed their ideas according to their interest and engagement. Their articles will give you the opportunity to understand the concepts of micromachining of advanced materials. Surface texturing using pico- and femto-second laser micromachining is presented, as well as the silicon-based micromachining process for flexible electronics. You can learn about the CMOS compatible wet bulk micromachining process for MEMS applications and the physical process and plasma parameters in a radio frequency hybrid plasma system for thin-film production with ion assistance. Last but not least, study on the specific coefficient in the micromachining process and multiscale simulation of influence of surface defects on nanoindentation using quasi-continuum method provides us with an insight in modelling and the simulation of micromachining processes. The editors hope that this book will allow both professionals and readers not involved in the immediate field to understand and enjoy the topic

Materials and processes for advanced lithography applications

textStep and Flash Imprint Lithography (S-FIL) is a high resolution, next-generation lithography technique that uses an ambient temperature and low pressure process to replicate high resolution images in a UV-curable liquid material. Application of the S-FIL process in conjunction with multi-level imprint templates and new imprint materials enables one S-FIL step to reproduce the same structures that require two photolithography steps, thereby greatly reducing the number of patterning steps required for the copper, dual damascene process used to fabricate interconnect wirings in modern integrated circuits. Two approaches were explored for the implementation of S-FIL in the dual damascene process: sacrificial imprint materials and imprintable dielectric materials. Sacrificial imprint materials function as a pattern recording medium during S-FIL and a three-dimensional etch mask during the dielectric substrate etch, enabling the simultaneous patterning of both the via and metal structures in the dielectric substrate. Development of sacrificial imprint materials and the associated imprint and etch processes are described. Application of S-FIL and the sacrificial imprint material in a commercial copper dual damascene process successfully produced functional copper interconnect structures, demonstrating the feasibility of integrating multi-level S-FIL in the copper dual damascene process. Imprintable dielectric materials are designed to combine the multi-level patterning capability of S-FIL with novel dielectric precursor materials, enabling the simultaneous deposition and patterning of the interlayer dielectric material. Several candidate imprintable dielectric materials were evaluated: sol-gel, polyhedral oligomeric silsesquioxane (POSS) epoxide, POSS acrylate, POSS azide, and POSS thiol. POSS thiol shows the most promise as functional imprintable dielectric material, although additional work in the POSS thiol formulation and viscous dispense process are needed to produce functional interconnect structures. Integration of S-FIL with imprintable dielectric materials would enable further streamlining of the dual damascene fabrication process. The fabrication of electronic devices on flexible substrates represents an opportunity for the development of macroelectronics such as flexible displays and large area devices. Traditional optical lithography encounters alignment and overlay limitations when applied on flexible substrates. A thermally activated, dual-tone photoresist system and its associated etch process were developed to enable the simultaneous patterning of two device layers on a flexible substrate.Chemical Engineerin

The development of microfabricated ion traps towards quantum information and simulation

Trapped ions within Paul traps have shown to be a promising architecture in the realisation of a quantum information processor together with the ability of providing quantum simulations. Linear Paul traps have demonstrated long coherence times with ions being well isolated from the environment, single and multi-qubit gates and the high fidelity detection of states. The scalability to large number of qubits, incorporating all the previous achievements requires an array of linear ion traps. Microfabrication techniques allow for fabrication and micron level accuracy of the trap electrode dimensions through photolithography techniques. The first part of this thesis presents the experiential setup and trapping of Yb+ ions needed to test large ion trap arrays. This include vacuum systems that can host advanced symmetric and asymmetric ion traps with up to 90 static voltage control electrodes. Demonstration of a single trapped Yb+ ion within a two-layer macroscopic ion trap is presented. with an ion-electrode distance of 310(10) μm. The anomalous heating rate and spectral noise density of the trap was measured, a main form of decoherence within ion traps. The second half of this thesis presents the design and fabrication of multi-layer asymmetric ion traps. This allows for isolated electrodes that cannot be accessed via surface pathways, allowing for higher density of electrodes as well as creating novel trap designs that allow for the potential of quantum simulations to be demonstrated. These include two-dimensional lattices and ring trap designs in which the isolated electrodes provide more control in the ion position. For the microfabrication of these traps I present a novel high-aspect ratio electroplated electrode design that provides shielding of the dielectric layer. This provides a means to mitigate stray electric field due to charge build up on the dielectric surfaces. Electrical testing of the trap structures was performed to test bulk breakdown and surface flashover of the ion trap architectures. Results showed sufficient isolation between electrodes for both radio frequency and static breakdown. Surface flashover voltage measurements over the dielectric layer showed an improvement of more than double over previous results using a new fabrication technique. This will allow for more powerful ion trap chips needed for the next generation of microfabricated ion trap arrays for scalable quantum technologies

Studies of block copolymer stabilizers for dispersion polymerization

Anionic polymerization techniques have been used to prepare AB block copolymers of polystyrene and poly[dimethyl siloxane] having well-defined molecular weight and composition, and narrow molecular weight distribution. Block copolymers prepared over a range of molecular weights and compositions were characterized by gel permeation chromatography, osmometry and silicon analysis. Such block copolymers have been used as stabilizers for non-aqueous dispersion polymerizations of styrene and methyl methacrylate in aliphatic hydrocarbon. The polymer particles thus produced were stabilized by well-defined surface layers of poly[dimethyl siloxane]. The effects of varying the polymerization conditions, and the type and concentration of stabilizer present, were studied. Both radical and anionic polymerization mechanisms have been considered, and methods of preparing polymer particles of a narrow size distribution were developed. [Continues.