Ferroelectric nanostructures and their processing issues


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  • Ferroelectricity is a phenomen on that was discovered in 1921 in Rochelle salt or sodium potasium tartrate. Certainly one of the major “turning points” in ferroelectricity came in the very early 1940s with discovery of the unusual dielectric properties of a number of simple mixed oxides which crystallize with the perovskite structure . The first of them was barium titanate Perhaps the most significant theoretical development in ferroelectricity occurred in 1960 with the formulation of the soft-mode description of the ferroelectric transition In 80-s the age of integration, in this time Capacitors, transducers, and electrooptic switches made from ferroelectric ceramics and crystals are being incorporated into thick-film and thin-film circuits, or multilayer packages. At present we are in the age of miniaturi zation.
  • All crystals that are found in nature can be devided into 32 different classes or point groups. These 32 point groups are subdivisions of 7 basic crystal system that are in order of ascending symetry: triclinic, monoclinic, orthorombic, tetragonal, rhombohedral, hexagonal and cubic. Of the 32 points groups, 21 classes are noncentrosymetric (a necesary condition for piezoelectricity to exist) and 20 of these are piezoelectric These crystal develop an electrical charge (polarization) under an applied mechanical stress this is called direct effect and a mechanical movement is generated by the application of an electrical field – converse effect . 10 crystal clases out of possible 20 are pyroelectric. This group of materials posses the unusual characteristic of beeing permanently polarized within a given temperature range. This polarization changes in the temperature gradient. A subgroup of spontaneously polarized pyroelectrics is a very special category of materials known as ferroelectrics. These materials posses spontaneous dipole which can be reversed by an electric field of some magnitude less than dielectric breakdown of the materials itself. some examples of such materials are tungsten bronze, Oxigen octahedral compounds with ABO3 stucture, the most known of them are ceramic perovskites. Other types are pyrochlore and layer structure generation of electricity as a result of mechanical pressure - direct effect mechanical distortion in response to a voltage applied across a piezoelectric material – inverse effect spontaneous polarization whose amplitude changes under the influence of temperature gradients reversible, non-volatile macroscopic spontaneous electric dipole moment in the absence of an external electric field.
  • Perovskite structure can be ideally described as a cubic unit cell with a large cation A as Ba, Pb ocupying the corner sites, a smaller cation B Ti, Zr in the body center and oxigen ocupying the centers of the faces. The cubic structure is the high tempeature stable form and it is paraelectric . On cooling down, at a certain temperature, called Curie temperature the symmetry of the crystal breaks down, and a tetrgonal distorsion takes place. The tetragonal form is a ferroelectric one and ferroelectricity originates from a small displacement of the transition metal cation from the centre of the O6 octahedron. This leads to a net dipole moment per unit volume called polarization. Almost all the ferroelectric perovskites contain non-magnetic transition metal ions with an empty d -shell ( d 0 confi guration), for example Ti4+, Nb5+ and W6+. Apparently the presence of the d 0 plays an important role in formation of a ferroelectric state.. In this way a stronger covalent bond with one (or three) instead of six weaker bonds with neighbouring oxygen atoms is formed.
  • Ferroelectrics find three main technological applications based on their physical characteristics: Because ferroelectric materials possess spontaneous polarization, when an electrical field is applied they present a non-linear response in polarization which is known as hysteresis. By changing the direction of the applied field polarization is also changed. Polarization up can be associated to 1 or 0 and polarization down can be 0 or 1. the two states are used in binary data storage: memory of the mobile phone, digital camera and so on. The ferroelectric materials are also piezoelectric. actuators are devices that produce a displacement or movement when voltage is applied. Actuators are used for many functions, including cancelling vibration, tool adjustment and control, micro-pumps, mirror positioning, wave generation, structural deformation, inspection systems and scanning microscopes. Another direction of applications is based on pyroelectric effect, for pyroelectric detectors for infrared detection, imaging, thermometry, ... .
  • in 1940-1950 bariun titanate era, the only known form to prepare materials was bulk a tri dimensional structure. In the age of integration, a new shape become available - by reducing one of the dimensions of the bulk . At this point we can talk about the transition from 3D to 2 D or from bulk to thin film. Portability of electronic systems has become a key driver of advanced semiconductor technologies. To meet the demand, new sophisticated materials and technologies are being introduced and reducing the size is required. This age of miniaturization is driven by the transition from 2D to one or zero D In other words, a ferroelectric nanostructure is a structure with all dimensions negligable!
  • Due to the technical evolution and industrial requirements about miniaturization, the scientist involve in the ferroelectric material research has believed that the ferroelectricity disappear at the moment when the polarization cannot be measured anymore. That limit was around 75-50nm. The ferroelectric polarization results from the sum over all dipol moments within a unit cell and it is equivalent to a surface charge density. Considering a ferroelectric matrerial with charactristic values for bulk P=10 and eps=200 for a capacitor with 100nm 2 there are aproximately 60electrons to swich and only three for the 4nm 2 capacitor. Applicability also requires two stable ferroelectric groundstates to toggle between. However, as we compare the energy W C of a ferroelectric capacitor with volume V : W C = 1 / 2 V P 2 /εε 0 2 with the thermal energy of k B T ( k B Boltzmann constant, T absolute temperature) at room temperature, it becomes evident that for the above values and small capacitors thermal fluctuations will become noticeable (1 nm3 corresponds to 17 meV). To assure a low thermally induced switching probability, W C should therefore be well above k B T .
  • How to build these structures? What methods are available? The first approaches at which scientists looked were those available in microelectronics. "Top down" implies a continuation of present technology, building devices that are smaller, faster, and less expensive by refining current fabrication methods. Increasing our knowledge of the chemical and physical properties of molecules such that we can design systems in which a desired pattern or product assembles itself. They are called self Paterning Physical and Chemical Routes and they are known as "Bottom up“ approach. Life, from single-celled animals to humans, is the inspiration for bottom-up approaches. Nanostructures obtained by top–down approach often suffer from damage by ion bombardment, which impedes the distinction between real size effects and those induced by the patterning method. Non-invasive techniques avoid these damages either by patterning prior to the final crystallization (e.g. e-beam direct writing or by bottomup approaches: self-assembly or by using suitable templates.
  • The FIB equipment is conceptually similar to a scanning electron microscope (SEM), but instead of a beam of electrons, a highly focussed beam of gallium ions is used to scan the sample surface. This ion beam can be used for two purposes: imaging, and micromachining: Scanning a low-current beam over an area of the sample surface and simultaneously collecting the induced secondary electrons, one can produce a high-resolution image of the sample, whereas at high beam current, highprecision local sputtering and milling can be easily performed. FIB equipment also allows the fabrication of small structures by direct deposition of metals (such as tungsten, gold, or platinum)which can be used as electrdes with a minimum feature size down to 40 nm and a precision of 20 nm, allowing studies on quantum transport mechanisms Having the unique nanopatterning ability to add or remove features, the FIB method is currently used to modify integrated circuits and masks, or to fabricate cross section transmission electron microscopy specimens. The major drawback of the method is associated with the high damage that occurs during milling and imaging, While most of the structural defects can be healed by a high-temperature thermal annealing after milling, a gallium doping is basically unavoidable and in special cases might be relatively harmful to the final electrical properties. A thermal annealing of the cut-out structures recovers the ferroelectric properties, but in general a damaged layer up to 10 nm thick still remains, in which the ferroelectric properties are altered The Focused Ion Beam (FIB) tool can cut away (mill) material from a defined area with dimensions typically in square microns or deposit material onto it. Milling is achieved by accelerating concentrated gallium ions to a specific site, which etches off any exposed material, leaving a very clean hole or surface. By introducing gases or an organic gas compound, the FIB can selectively etch one material much faster than surrounding materials, or deposit a metal or oxide. The FIB is used for such tasks as site-specific cross-sectioning for interfacial microstructure studies, preferential removal of certain metals or oxides, semiconductor device editing or modifications, site-specific TEM sample preparation, and grain imaging.
  • Electron Beam direct method is a non-invasive aproach. It starts from a solution containing metalorganic compounds or metal colloids. A substrate is spin-coated with a thin precursor film that contains the required elements in a proportion corresponding to that of the target ferroelectric phase. For example, precursor films for SBT and PZT structures can be prepared from Sr-, Bi-, and Pb-ethylhexanoate, Ti- and Zr-isopropylene, and Ta-methoxide solutions, using xylene and 2- methoxyethanol, respectively, as solvents. The obtained precursor film is dried and then patterned by scanning an electron beam of about 3 nm diameter over selected areas of the film (“pattern exposure”), and immersing the exposed sample for 1 minute in toluene (“pattern development”), followed by dry-blowing with nitrogen. As an example for EBDW, this fig. shows scanning electron microscope (SEM) images of PZT arrays
  • Lithography means printing Photolithography - is used in semiconductor device applications– it consists of producing a mask carrying the requisite pattern information and subsequently thansfering that pattern using some optical technique into a photoactive polymer or photoresist.In principle the resolution is limited by the optical system. to improve the resolution to nanometer range this requires the use of an electromagnetic radiation with a smaller wave length: extreme UV light or X-ray. A more friendly method is nanoimprint lithography (NIL). This does not use any energetic beam, so the resolution is not affected by the optical system The method has been adapted to chemical solution deposition (CSD) derived PZT precursor films to generate ferroelectric PZT cells down to 300 nm In this approach the deposited gel layer is patterned by imprinting a SiO2-Si mold into the film at a typical pressure of 1 kbar. After this process the imprinted gel layer is pyrolized and subsequently crystallized in air. The method enables a relatively low cost and large area patterning. In the case of polimers, the resolution of this method goes down to 20nm. However lateral ferroelectric nanostructures in the range of 20 nm as demonstrated for polymers have not been obtained.
  • There is a considerable interest in other fabrication methods that are not based on removing the material of the thin films, but rather on building structures from the bottom, using atoms and molecules. The bottom up approches will in principle allow inexpensive fabrication of structures with size of 10-20nm. The primary disadvantage of the bottom up methods is the random positioning of the obtained structures, that will make a precise interconection of them virtualy imposible. Self –patterning via physical routes seem to be reliable and making use of simple physical processes.
  • Pulsed laser deposition is a technique for creating thin films. Pulsed laser deposition (PLD) and pulsed laser ablation are also potential methods to obtain nanoparticles deposited on a substrate or in the form of a dispersed powder, The PLD method involves evaporation of a solid target in an Ultra High Vacuum chamber by means of short and high-energy laser pulses. In a typical PLD process, a ceramic called target is placed in a vacuum chamber. A pulsed laser beam vaporizes the surface of the target, and the vapor condenses on a substrate. The main components are a laser, optics, and a vacuum system. Additionaly for preparing nanoparticles a charger, a furnace and a collector are required. The principle of pulsed laser deposition, is a very complex physical phenomenon. It does not only involve the physical process of the laser-material interaction at the impact of high-power pulsed radiation on solid target, but also the formation of the plasma plume with high energetic species and even the transfer of the ablated material through the plasma plume onto the heated substrate surface. The particle formation process in PLD generally can be divided into the following four stages. The fig presents PZT nanoparticles obtained by laser ablation after heat tretment at 900 o C.
  • Chemical Vapor Deposition (CVD) is a chemical process for depositing thin films of various materials. In a typical CVD process the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit.When the volatile precursors are metal-organic precursors, such as ...The method is named MOCVD. Frequently, volatile by products are also produced, which are removed by gas flow through the reaction chamber.
  • This possibility of creating nanostructure has been found during an effort of growing single-crystal epitaxial films by chemical solution deposition (CSD). Lange, Speck, and coworkers have theoretically and experimentally established that an ultrathin amorphous layer of PZT (or another perovskite oxide) deposited onto a single-crystal substrate such as SrTiO3, during high-temperature crystallisation breaks up into small islands due to a microstructural instability. The driving force of this process is the minimisation of the free energy of the film-substrate system, by lowering the interface area (forming islands) and by forming low-energy surfaces via faceting The process starts with the preparation of suitable precursors that are often salts, typically, carboxylates, or other metallo-organic compounds, usually, alkoxides. The precursors are dissolved in appropriate solvents and mixed in a stoichiometric ratio that yields the desired composition of the final film. In some cases, additives such as chemical stabilizers are included during solution synthesis and additional processing steps, such as refluxing, are employed to adjust the properties of the coating solution. The next processing step is deposition of the coating solution on the substrate by spin coating using a rotating substrate, spray coating the misted solution, or dip coating the substrate in a solution bath. Subsequently, the (wet) as-deposited film is dried, pyrolyzed, crystallized, and (optionally) post-annealed for further densification or microstructure manipulation.
  • Using simple chemistry it is possible to fabricate nanosize crystals or nanoparticles which can later be spead onto any substrate surface. Very simillar to the method described before, particles with sizes in nanometer range can be prepared . The starting materials are, lead acetate which is a metal carboxylate, zirconium end titanium butoxide which are metal alkoxide and butanol (parent alcohol ) as solvent. Similar as befor they are mixed, refluxed and by products are removed. in this approch water is added to hydrolyse the alkoxide groups and imediately a gel is formed. After drying this leads to a very fine powder .
  • A microemulsion is defined as a dispersion of two immiscible liquids consisting of microdomains of one or both liquids stabilized by an artificial film of surface active molecules (surfactant). The water in oil (W/O) type of emulsion is formed from water droplets (5-25nm) which are dispersed in the oil phase. If two reactants A and B in the water phase of two different microemulsions are mixed,, reactants A and B are exchanged between the water droplets because of collision and coalescence. This interchange is very fast. As the reactants come in contact with each other, they react and form precipitate AB. The nucleation of the precipitate particles takes place inside the water droplets. The final particle size is controlled by the size of the water droplets, which in turn is controlled by the ratio of water to surfactant. When the size of the particles becomes comparable to the droplet size, the surfactant molecules are atached to the surface of the particles, stabilizing and protecting them against further growth.
  • Due to their small size, the characterisation of the cells or structures obtained cannot be achieved using conventional measurement techniques. For this purpose, scanning probe techniques, in particular scanning force microscopy (SFM), have proven to be most valuable. The principle of an SFM is quite simple and it is very similar to AFM. A sharp tip mounted on a cantilever is brought close to the sample surface. The (attractive or repulsive) force between the tip and the surface is detected using the bending of the cantilever. A feedback loop adjusts the z-position of the cantilever using a piezoelectric element, so that the interaction force is kept constant. By scanning the tip over the surface and recording the z-position of the cantilever, a map of the sample topography can be obtained . In the case of PFM a conductive tip is brought into contact with the with the surface and by applying an electrical field the piezoelectric response is detected. The fig presents three possible orientations of the polarization and electrical field. When the polarization and electrical field are in the same direction the material will expand (pozitive piezoelectric coefficient). Perpendicular – no piezoelectric deformation but a shear strain appears in ferroelectric, displacement of the surface parallel to itself, along the polarization direction.
  • Ferroelectric nanostructures and their processing issues

    1. 1. Ion Elena-Daniela Ferroelectric nanostructures and their processing issues
    2. 2. <ul><li>FE nanostructure s </li></ul><ul><li>Processing issues </li></ul><ul><li>Potential applications </li></ul><ul><li>Ferroelectricit y, Ferroelectric materials </li></ul><ul><li>F E nanostructure, Size limit in ferroelectricity </li></ul><ul><li>Invasive and Non-invasive approach </li></ul><ul><li>C haracterization of FE nanostructure </li></ul> Outline
    3. 3. Important Events 1921 Discovery of ferroelectricity in Rochelle Salt 40’s Barium titanate era 60’s Age of high science 80’s Age of integration 1990-present Age of miniaturization d ef = reversibility of the direction of the electric dipole by means of an applied electric field in a polar crystal Materials which exhibit ferroelectricity are called ferroelectric materials <ul><ul><li>Ferroelectricit y </li></ul></ul>
    4. 4. P i = d ijk  jk (Direct Effect)  ij  d kij    Converse Effect   P s =   T <ul><ul><ul><li>Ferroelectric materials </li></ul></ul></ul>32 Sym m etry Points groups 21 Noncentrosymetric 20 Piezoelectric Polarized under stress 10 Pyroelectric Spontaneously polarized Subgroup Ferroelectric Spontaneously polarized Polarization reversible Tungsten Bronze Ox y gen Octahedral ABO 3 Pyrochlore Layer structure 11 Centrosymetric Non-piezoelectric Ceramic Perovskite
    5. 5. e.g. Perovskite oxide – ABO 3 BaTiO 3 , PbTiO 3 , Pb(Zr x Ti 1-x )O 3 ,
    6. 6. Reversible spontaneous polarization- 1 or 0 data bits (binary data storage media) Piezoelectric effect- Piezoelectric actuators, sonar detectors Pyroelectric effect Pyroelectric detectors for infrared detection, imaging, thermometry, ... Applications
    7. 7. <ul><li>3D -> 2D transition : bulk to thin film </li></ul><ul><li>2D -> 1D transition : thin film to wire </li></ul><ul><li>2D -> 0D transition : thin film to nanostructures </li></ul><ul><ul><ul><li>Ferroelectric nanostructure s </li></ul></ul></ul>F. D. Morrison et al. Rev. Adv. Mater. Sci. 4 (2003) 114 Alexe et al. APL- 75, 1793, (1999) Ma et al. APL, 83, 3770 (2003) Luo et al. APL, 83, 3, 440, (2003) Yun et. al. Nano Letters, S1530, (2002 )
    8. 8. <ul><li>Switching @ nanoscale </li></ul><ul><li>P=10 µC/cm2, ε = 200 </li></ul><ul><li>a = 10 nm  Q = P·S ≈ 6 0 e </li></ul><ul><li>a = 2 nm  Q = P·S ≈ 3 e </li></ul><ul><ul><ul><li>Size limit in ferroelectricity </li></ul></ul></ul>Rudinger et. al. Appl. Ph y s. A, 80, 1247 (2005) Factor s that influence the ferroelectric properties in nanostructure: Grain size, Mechanical bondary conditions, ... W c > k B T
    9. 9. Electron Beam Direct Writing Focussed Ion Beam Patterning Lithography Methods NANOSTRU C TURE S Top-Down Focussed Ion Beam Patterning Electron Beam Direct Writing Lithography Methods Self-Patterning Chemical Routes Self-Patterning Physical Routes Bottom-Up Self-Patterning Physical Routes Self-Patterning Chemical Routes <ul><ul><li>Invasive and Non-invasive appro a ch </li></ul></ul>
    10. 10. Focussed Ion Beam Patterning <ul><li>FIB equipment </li></ul><ul><li>similar to SEM </li></ul><ul><li>a highly focussed beam of gallium ions </li></ul><ul><li>purposes: imaging, and micromachining </li></ul><ul><li>nanopatterning - resolution ~ 20 nm </li></ul><ul><li>gallium doping, damaged surface layer </li></ul>C.S. Ganpule et.al. APL 75, 409 (1999)
    11. 11. Electron Beam Direct Writing Alexe, Harnagea and Hesse, J. Electroceram. 12, 69 (2004) <ul><li>solution: metalorganic compounds (Sr-, Bi-, Pb-ethylhexanoate, Ti-, Zr-isopropylene and Ta-methoxide ) or metal colloids and solvent: xylene and 2-methoxiethanol </li></ul><ul><li>patterning by scanning an electron beam </li></ul><ul><li>Powerful method to prepare arrays of ferroelectric cells with lateral sizes down to 75nm </li></ul><ul><li>Expensive equipment and time </li></ul>
    12. 12. Lithography Methods <ul><li>Photolithography </li></ul><ul><li>Soft lithography  Nanoimprint </li></ul>Alexe, Harnagea and Hesse, J. Electroceram. 12, 69 (2004) Large-area and low-cost ferroelectric cells below 100nm in lateral size
    13. 13. Self-Patterning Physical Routes Pulsed Laser Deposition Met.Org. Chem.Vap. Deposition Chemical Solution Deposition
    14. 14. Pulsed Laser Deposition 1. Laser radiation interaction with the target 2. Dynamic of the ablation 3. Transport of the ablated material to a charger and a furnace 4. Nucleation and growth Seol et al. - Appl. Phys. Lett., 81, 1894, 2002 Crystalline nanoparticles ~ 4-20nm Complex experimental set-up, low yield
    15. 15. Met.Org. Chem.Vap. Deposition Large surface coating area Used in combination with FIB, EBDW Expensive equipement Metal organic precursor: Tetraethyl lead - Pb(C 2 H 5 ) 4 Titanium isopropoxide: Ti(i-OC 3 H 7 ) 4 Zirconium tert-butoxide: Zr (t-OC 4 H 9 ) 4 M. Shimizu et.al.-Jpn. J. Apl. Phys, 33, 5168 (1994)
    16. 16. R.W. Schwartz et al. / C. R. Chimie 7 (2004) 433–461 Metal carboxylate: R-COOM M: Pb R: CH 3 -, C 2 H 5 - Metal alkoxide: M(OR) x M: Ti, Zr, ( OR ): (OC 3 H 7 ), (OC 4 H 9 ) Solvent: CH 3 OC 2 H 5 OH Chemical Solution Deposition (Sol-gel)
    17. 17. Self-Patterning Chemical Routes Pb(O 2 C 2 H 3 ) 2 R= [ H2O ]/ [Pb] Zr(OC 4 H 9 ) 4 pH=11 Ti(OC 4 H 9 ) 4 C 4 H 10 O BET: 58nm BET: 109nm BET: 144nm Cost-effective, various shapes Agglomeration (Sol-gel) 11 -PT- 15 11 -PZ- 15 11 - PZT- 15
    18. 18. Self-Patterning Chemical Routes Synthesis, Functionalization and surface treatment of Nanoparticles Marie-Isabelle Baraton/ ASP 2003 Alexe, Harnagea and Hesse, J. Electroceram. 12, 69 (2004) Microemulsion Removal of the surfactant Uniforme particles in nm range
    19. 19. Characterization of ferroelectric nanostructures <ul><li>P iezoresponse Scanning F orce M icroscopy (PFM) </li></ul>M. Alexe, C. Harnagea and D. Hesse, J. Electroceram. 12, 69 (2004) C.H. Ahn, K.M. Rabe, J.M. Tiscone, Science, 303, 488, ( 2004 )
    20. 20. Potential applications Many others to come!!!
    21. 21. Thank you for your attention!