- E-Beam Patterning
- Ferroelectric domain patterning with electron beams
- SPM Patterning
- Ferroelectric Polymers
- Complex Structures
The ability to selective manipulate a surface, either at the micron scale or nano scale, is the at the heart of every lithography technique. Lithography techniques very greatly from top-down lithography techniques, such as standard optical lithography at the micron-scale to dip-pen lithography at the nano-scale, which allow for complex patterning to bottom-up lithography techniques, such as a self assembled monolayer, which allow for long range order but not always complex patterning. There is also the possibility of a hybrid technique that combines top-down and bottom-up techniques to allow for the advantages of both methods.
Our group has developed a lithography technique that has the ability to create features down to 20nm and the ability to deposit various materials to influence the eventual device characteristics. The properties of ferroelectrics and the ability to manipulate these properties is the main driving force of this lithography technique. Ferroelectric films contain electric dipoles that are intrinsic to the atomic structure of the compound. For example in a cubic perovskite, the displacement of the body center cation in the unit cell produces a dipole in the structure. Dipole-dipole interaction between cells cause polarization alignment resulting in ferroelectric domains. Polarization discontinuities in the vicinity of surfaces and interfaces result in polarization bound charge that significantly affects materials properties.
Below are a few examples of our ability to manipulate the ferrelectric surface of a PZT thin film. The patterns can be as simple as boxes or stars to parallel line to very complex patterns, such as the portrait of the Vice Provost of Penn. These patterns were created by scanning over the surface with an AFM tip and alternating the voltage between +10V and -10V to selective reorient the domains in the pattern that is desired. Our group has also shown that ferroelectric domains can be reoriented by selectively exposing the surface to the electron beam of a SEM. In this method the domain orientation is controlled by the imaging current of the beam.
Ferroelectric solids contain electric dipoles that are intrinsic to the atomic structure of the compound. For example in a cubic perovskite, the displacement of the body center cation in the unit cell produces a dipole in the structure. Dipole-dipole interaction between cells cause polarization alignment resulting in ferroelectric domains. Polarization discontinuities in the vicinity of surfaces and interfaces result in polarization bound charge that significantly affects materials properties.
Polarization induced band bending at the ferroelectric surfaces results in the domain-specific photochemical activity. The mechanism of this phenomenon is shown below:
Combined with the possibility to control local domain structure by AFM, this opens a wide range of possibilities for the controlled preparation of nanoscale structures.
This is an example of gold nanoparticles photochemically grown on BaTiO3 surface.
Ferroelectric solids contain electric dipoles that are intrinsic to the atomic structure of a compound. For example, in a cubic perovskite, the displacement of the body center cation in the unit cell produces a dipole in that structure. Dipole-dipole interactions between unit cells causes polarization alignment resulting in ferroelectric domains. Polarization discontinuities in the vicinity of surfaces and interfaces result in polarization bound charge that significantly affects materials properties. The orientation of polarization can be altered with the application of an electric field. It is expected that interactions on ferroelectric surfaces will be influenced by the domain orientation. Since it has been demonstrated that domain orientation can be controlled to produce 10-20 nm domains, if the fundamental relationship between atomic polarization, charge compensation and local reactivity can be understood, it could be utilized to direct assembly of nanostructures.
- Electron beam domain switching is an effect of sample charging
- Positive charging dominated by blue oxide curve
- Negative charging dominated by green oxide curve
Piezoresponse Force Microscopy image illustrates our ability to switch c+ and c- domains with e-beams
|Positive Poling||Negative Poling|
Ferroelectric polarization in polycrystalline PZT thin film can be reoriented in both positive and negative directions using electron beams. Reorientation of the ferroelectric domains is a response to the electric field generated by an imbalance of electron emission and trapping at the surface. When an insulator surface is irradiated by electrons with energy higher than 1 keV, elastic and inelastic collisions in the crystal lead to the excitation of secondary electrons and the backscattering of incident electrons. Secondary electrons that are sufficiently close to the surface (less than 50 nm) are emitted from the surface, while the other electrons are either trapped in defect sites or self-trapped as polarons in the crystal. When the number of incident electrons is not equal to that of the emitted electrons, charge develops and an internal local electrical field is established in the film. When the field generated by the trapped charges is stronger than the coercive field of the ferroelectric compound, domain reorientation at the surface should occur.
Quantitative aspects in terms of beam dosage, energy and current density effects on polarization reorientation have been determined. A threshold of 500 mC/cm2 and a saturation of 1500 mC/cm2 were identified. Regardless of beam energy, the polarization is reoriented negatively for beam currents less than 50 pA and positively for beam currents greater than 1 nA. Due to the length scale accessible to conventional e-beam tools, this method provides the opportunity for domain engineering at the nanometer scale.
Figure shows the illustration of charging and polarization reorientation by an electron beam. Accumulation of positive charges causes the domains to point downwards (yielding the c- domains shown on left). In contrast, negative charging leads to positive domain orientation (c+ domains on right).
Figure above (a) PFM phase image showing negative domain polarization (dark area) switched by E=10 keV, I=30 pA, and dosages ranging from ~500 to 5000 mC/cm2. The exposure is increased from left to right and from bottom to top in the figure. (b) The fraction of c- domains switched perpendicularly to the surface, increases with electron dosage. (c) Model of domain switching with electric field (E) as dashed arrows. For E greater than Ecritical the domains begin switching. The fraction of switched domains increases with electron dose until the reorientation saturates at 1500 mC/cm2, as in (b).
Figure above shows the polarization reorientation dependence on beam energy and current. Dots represent experimental determination of domain polarity superimposed on a schematic representation of QT, which should follow the expected trend of the total electron emission yield s for an insulator. At high beam current (>1 nA), a net negative charge accumulates in the film, which switches the underlying domains in the positive direction (c+, bright areas in PFM phase image). It is reversed for low beam current (<50 pA). For beam currents between 50 pA and 1 nA, the sign of net charge depends on beam energy.
The ferroelectric surface provides bound charge which allow for the photo-deposition of metal salts to the surface. The orientation of the ferroelectric domains influence how much charge is available to drive the reduction reaction and hence provide differing deposition rates. This is seen in the two images to the left. The top image shows a PFM image of a selectively poled PZT surface, while the image below that shows the deposition of silver nano-particles on the surface created above it and the preferred deposition of the metal on the up domains compared to the down domains.
Now this can be expanded to create devices besides simple resistor lines. The bottom image shows the deposition of silver nano-particles to create a micro RF tag. The electrical characteristics of the RF tag can be customized not just by changing the dimensions of the tag but by depositing another metal from gold to iron to platinum or attaching customized molecules to the metal nano-particles.
Ferroelectric Nano-Lithography is a versatile and exciting new lithography technique that has great potential to allow the deposition of many different layers of materials to control and manipulate device characteristics on the nano-scale.
On an oriented perovskite single crystal, such as BaTiO3 (100) surface or PZT films, the atomic polarization may be directed perpendicular to the surface in the positive or negative direction or in the plane of the surface. On a randomly oriented surface the polarization vector may have intermediate orientation. In order to exploit atomic polarization assemble complex structures, the local orientation of the domains must be controlled. The figures below illustrate how atomic polarization can be oriented with a conductive SPM tip and applied voltages in the range of 5 to 10V.
Atomic polarization of the organic ferroelectric material polyvinylidene fluoride (PVDF) can be manipulated at the nanometer scale in order to influence the local electronic structure and reactivity at the surface. A DC voltage applied through a conductive scanning probe tip is used to pattern ferroelectric domains in a PVDF thin film, and the polarization direction of these domains influences the kinetics of electron exchange at the surface. By means of a surface photoreduction reaction, which occurs in a metal ion solution under ultraviolet irradiation, metal nanoparticles can be deposited in a predetermined configuration on the polymer surface. It was determined that the photoexcited electrons that take part in this surface reaction likely generate from the thermionic emission of electrons in defect states within the energy gap of the material and that these gap states are found not only at the interface, but throughout the polymer bulk.