Jong Up Jeon

Principle of ferroelectric domain imaging using atomic force microscope

The contrast mechanisms of domain imaging experiments assisted by atomic force microscope (AFM) have been investigated by model experiments on nonpiezoelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O3] thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tip–sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (Vdc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. In addition, a small ac voltage (Vac sin ωt) was applied to the tip to measure the amplitude (Aω) and phase (Φω) of the first harmonic (ω) signal as a function of Vdc. By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180° phase shift in Φω was observed. These results imply that in the contact mode the cantilever buckl...

Read/write mechanisms and data storage system using atomic force microscopy and MEMS technology.

Information storage system that has a potentially ultrahigh storage density based on the principles of atomic force microscopy (AFM) has been developed. Micro-electro-mechanical systems (MEMS) technology plays a major role in integration and miniaturization of the standard AFM. Its potential application for ultrahigh storage density has been demonstrated by AFM with a piezoresponse mode to write and read information bits in ferroelectric Pb(Zr(x)Ti(1 - x))O3 films. With this technique, bits as small as 40 nm in diameter have been achieved, resulting in a data storage density of simply more than 200 Gb/in2. Retention loss phenomenon has also been observed and investigated by AFM in the piezoresponse mode. Finally, local piezoelectric measurements of PZT films by different processing technologies are discussed in detail.

DETECTION MECHANISM OF SPONTANEOUS POLARIZATION IN FERROELECTRIC THIN FILMS USING ELECTROSTATIC FORCE MICROSCOPY

Mechanism on the detection of spontaneous polarization in a Pb(Zr0.5Ti0.5)O3 (PZT) film using contact mode of electrostatic force microscopy (EFM) is investigated. Theoretical calculations are performed on deflections induced by electrostatic force (uωe) between the tip and the sample and electromechanical vibrations (uωp) of the ferroelectric materials, respectively. From the calculation, uωe and uωp are 3.73×10-9 and 1.77×10-13 m. Enhanced mode of EFM shows the complete cancellation of the EFM image induced by the electrostatic force between the tip and the film through controlling dc voltage. Hence, electrostatic force effect is a main contributor on the detection mechanism of spontaneous polarization using EFM in contact mode.

Formation of ferroelectric nano-domains using scanning force microscopy for the future application of memory devices

Abstract Applying voltage between the conductive tip in atomic force microscopy (AFM) and bottom electrode through Pb(Zr, Ti)O3 (PZT) films can cause switching of ferroelectric domains. Formation and imaging of ferroelectric domains in nanometer scale could be applied to develop the future ultrahigh-density memory device. Relevant issues, i.e. bit (induced ferroelectric domains) size dependence on applied voltage and pulse width, are discussed. The bit size showed a log-linear dependence on the pulse width and a linear dependence on the pulse voltage. Using the analysis of electric field distribution, the size of the induced bits under certain pulse voltage and width was estimated.

Construction of probe-based data storage with ultra-high areal density

Our probe-based data storage (PDS) system consists of four parts: signal processing module, multi-probe array, xy-planar actuator, and recording media. The signal-processing module has 2 layers attached to each other. The first layer has 3 D wiring, and it is assembled to the second layer (ASIC module) for the signal processing circuit. The multi-probe array is designed to increase recording and reading speed of the data bits. Each probe has cantilever and tip that are characterized by a tip height of 15 /spl mu/m, a tip radius of 15 nm, a natural frequency of 18.75 kHz and a DC sensitivity of 16.7 nm/V. Poly-silicon probe was fabricated by molding technique to produce high aspect ratio tips. The electrostatic planar actuator serves as a scanner that positions the probe to the right place by moving the media in xy-plane. The moving distance was /spl plusmn/40 /spl mu/m at 10/spl plusmn/10 V, DC gain was 3.04 /spl mu/m/V at 4 V, and the resonance frequency was 143 Hz. The: recording media stores the recorded information in the form of electric polarization in ferroelectric materials. Ferroelectric Pb(Zr,Ti)O/sub 3/ thin film is a candidate material for the media, and it can yield a memory density as high as 400 Gb/in/sup 2/ by recording bits as small as 35 nm in diameter. The bits are fully rewritable and retain the information for a generation at the temperature range below than 50 /spl deg/C.

Piezoelectric hysteresis measurement using atomic force microscopy

Abstract Local piezoelectric hysteresis of Pb(Zr,Ti)O3 thin films were measured using atomic force microscopy and lock-in amplifier. PZT films were prepared by sol-gel processing and rf-sputtering. PZT films exhibit inverse piezoelectric response on application of a voltage. To detect such a small response of the thin (less than 100 nm in thickness) PZT films, ac modulation technique was used. Actual displacements were obtained by the calibration of the first harmonic signal (Acosθ) from X-cut quartz crystal (piezoelectric coefficient of 23pm/V). Coercive voltages and the maximum displacements were measured, respectively, from the deconvoluted phase and amplitude. As calibrated, the effective local piezoelectric coefficient were 5.8 and 11.5 in arbitrary unit from sol-gel processed and sputtered films. In case of the PZT films processed by sol-gel technique showed a larger local variation of the piezoelectric response and smaller displacements than the sputtered PZT films with similar film thickness.

Formation and observation of ferroelectric domains in PbZr1-xTixO3(PZT) thin films using atomic force microscopy

Very small-sized ferroelectric domains were induced and observed using a modified atomic force microscopy (AFM). Bias voltage between a conductive AFM tip and a sol-gel processed PZT film caused the switching of small ferroelectric domains. ELectrostatic forces between the polarized area and the tip provide the imaging of the polarized small domains. Applying voltage with the opposite sign can depolarize the polarized area and the formation of a series of data dots was demonstrated. In addition, the retention phenomena of micron size domains in PZT films were investigated. The polarized images disappeared within a few days even without an application of voltage - often called the retention loss or failure. An empirical relationship between relaxation time, bit size and poling time is established and verified. Two operative processes for the retention loss are either the stray charge accumulation on the polarized surfaces or the stress relaxation of the piezoelectric films. An effective way of improving the retention characteristics is suggested. The experimental results obtained in this study provide substantial insight into the mechanism for the retention failure of the polarized domains as well as the polarization behavior in PZT films with a nanometer scale.

Fabrication of in situ patterned iron oxide films using micro contact printing and selective deposition

Abstract In situ patterning of iron oxide thin layers were fabricated via microcontact printing (μCP) and selective area deposition. μCP is used to pattern two different surface moieties of self-assembled organic monolayers (SAMs) on Au/Cr/Si substrates. An elastomeric stamp is used to transfer hydrophobic (hexadecanethiol (HDT)) SAMs that are to sustain deposition of iron oxide precipitates. Hydrophilic (dithiothreitol, DTT) SAMs were used to induce crystalline iron oxide films. 0.05 M of iron nitrate (Fe(NO3)3·9H2O) aqueous solutions containing urea under nitric acid (pH ≤ 2) were used. Selective depositions were realized through precipitation of crystalline iron oxides at ambient temperature (80°C) onto the mixed SAM surfaces.

Effect of metal-insulator-semiconductor structure derived space charge field on the tip vibration signal in electrostatic force microscopy

The role of metal–insulator–semiconductor structure derived space charge field in the tip vibration signal of electrostatic force microscopy was studied using boron doped Si tip and Pt film sputtered on Si substrate. The ohmic contact between the tip and the film was confirmed by current–voltage characterization. Then the tip was held at a position so that an air gap of 100 nm existed between the tip and the film. The tip deflection and the tip vibration signals were examined by applying dc voltage to the film and ac voltage to the tip. The asymmetry in both signals supports the existence of the space charge field, and the direction of the field at zero dc bias field is from the tip to the sample as expected from the band bending theory.

An application of polarized domains in ferroelectric thin films using scanning probe microscope

The feasibility of utilizing PZT films as future data storage media has been investigated using a modified AFM. Applying voltages between a conductive AFM tip and the PZT films causes the switching of ferroelectric domains. The domains are observed using an EFM imaging technique. The experimental results and calculations revealed that the electrostatic force generated between the polarized area and the tip is a main contributor for the imaging of the polarized domains. The written features on ferroelectric films were less than 100 nm in diameter, implying the possibility of realizing data storage devices with ultrahigh areal density. The disappearance of the polarized images without any applied voltage spontaneous reversal polarization was observed, which is a drawback in this application of PZT thin films.