Polarization-switching D/A converter

This paper describes a novel digital-to-analog (D/A) conversion technique, which uses the analog quantity polarization as a D/A conversion medium. It can be implemented by CMOS capacitors or by ferroelectric capacitors, which exhibit strong nonlinearity in charge versus voltage behavior. Because a ferroelectric material inherently has spontaneous polarization and generally has a large dielectric constant, the effective capacitance of a ferroelectric capacitor is much larger than that of a CMOS capacitor of the same size. This ensures less influence of bottom-electrode parasitic capacitance on a ferroelectric capacitor. Furthermore, a data converter based on ferroelectric capacitors possesses the potential nonvolatile memory function owing to ferroelectric hysteresis. Along with the architecture proposed for polarization-switching digital-to-analog converter (PDAC), its circuit implementation is introduced. Described is implementation of two 9-bit bipolar PDACs: one is based on CMOS capacitors and the other on off-chip ferroelectric capacitors. Experimental results are presented for the performance of these two prototypes.     

Effect of UV radiation on slow relaxation processes in ferroelectric thin-film capacitors

The influence of UV radiation with wavelengths of about λ = 340 nm and power within 0.001–0.15 mW on the suppression of slow relaxation of residual polarization in ferroelectric capacitors based on thin (Ba,Sr)TiO₃ (BSTO) films in a paraelectric state has been studied. Experimental data on the behavior of capacitance relaxation time and conduction current in ferroelectric capacitors under the action of UV radiation are presented. A mechanism of relaxation with a characteristic time on the order of 1 s is found that is not affected by UV radiation.     

Electrical and optical modulation on ferroelectric properties of P(VDF-TrFE) thin film capacitors

Modulating the ferroelectric properties of P(VDF-TrFE) polymers both electrically and optically could open up new opportunities for their applications in non-volatile memories and sensors. Here by using the Nb:SrTiO₃ semiconductor as electrode compared with metal Au electrode, we report on the modulation of ferroelectric properties of P(VDF-TrFE) thin film capacitors both by electric field and UV light. A ferroelectric hysteresis loop shift together with the asymmetric switching behavior has been observed when using semiconducting electrode, which could be explained by the band alignment model based on interfacial charge screening. On the basis of band bending near the ferroelectric/semiconductor interface, we could further modulate the ferroelectric switching behaviors reversibly by UV light illumination. Our research provides a new route to engineer the ferroelectric properties of P(VDF-TrFE) polymer thin film capacitors, promising their applications in optoelectronic devices.     

A method and instrument for measuring the capacitance switching time of nonlinear capacitors

A device and procedure for measuring the switching time of the capacitance of nonlinear microwave capacitors, based on ferroelectric films and bulk ceramics (variconds) when acted upon by controlling electric field pulses, are considered. The method can be used to measure high-speed semiconductor varactors, microelectromechanical components and the residual polarization of capacitors based on a linear ceramic.     

Development and characterization of a high dielectric constant paste based on BaTiO3 for use in thick-film capacitors

Results of measurements on a thick-film capacitor based on a paste of mixed ferroelectric phases made out of crystallization from a glassy phase are presented. This material can be used as a high dielectric constant paste for designing capacitors in hybrid circuits. The tanδ is observed to decrease with rise in temperature.     

A survey of behavioral modeling of ferroelectric capacitors

Six different behavioral models for ferroelectric capacitors are surveyed with an emphasis on their usefulness in the transient circuit simulation of integrated nonvolatile memories. These models can be broadly classified into two categories, namely, those that rely on the hysteresis loop and those that rely on the switching current of a ferroelectric capacitor. The former often involves a continuous cycling of a ferroelectric capacitor with a sinusoidal waveform. The latter employs a pulse measurement technique to capture the switching current of the capacitor. The pulse waveform applied to the ferroelectric capacitor in the latter approach resembles the actual waveform encountered in a typical ferroelectric memory access. This resemblance makes switching-current based models more suitable for use in high-speed-memory circuit simulations     

Integrated ferroelectric stacked mim capacitors with 100 nF/mm2and 90 V breakdown as replacement for discretes

This paper shows for the first time integrated thin film ferroelectric metal-insulator-metal capacitors on silicon with a record high capacitance density above 100 nF/mm² combined with a breakdown voltage of 90 V and a lifetime exceeding 10 years at 85degC and 5 V. The high capacitance density was obtained by a combination of material optimizations resulting in a dielectric constant of 1600, and stacking of capacitors. The reliability of these ferroelectric capacitors was studied in detail with accelerated lifetime testing. The high performance of the integrated capacitors in this paper shows great potential for applications demanding high capacitance densities combined with electrostatic discharge protection.     

Developments in ferroelectric ceramics for capacitor applications

A brief overview of the materials and processes for making ceramic capacitors based on BaTiO₃ and relaxor ferroelectric compositions is presented with special emphasis on more recent developments.     

Tunable microwave filters with controlled ferroelectric capacitors

Microwave microstrip resonators and filters controlled by ferroelectric capacitors have been simulated and experimentally studied. Control at room temperature and under cryogenic conditions has been ensured by capacitors based on BSTO and STO films, respectively. Cooled devices based on YBa₂Cu₃O_7-δ high-temperature superconducting films have been fabricated. The developed filters are efficiently tuned by applying a control voltage, are small, and have a low level of introduced losses.     

A 1.5 GHz phase-locked loop with leakage current suppression in 65 nm CMOS

In the nanoscale CMOS process, the problem of leakage current causes the performance of the analog circuits to degrade. The leakage current of a loop filter, which is realised by MOS capacitors, significantly degrades the jitter performance of a phase-locked loop. A leakage suppression circuit is presented by using a combination of switchable varactors and current sources to compensate the leakage of MOS capacitors in a loop filter. This PLL has been fabricated in a 65 nm CMOS process and the core area is 0.4 X 0.5 mm². With the leakage suppression circuit, the peak-to-peak jitter and the rms jitter are 43 and 5.36 ps, respectively.The power is 17 mW for a 1.2 V supply.     

Ferroelectric Thin-Film Capacitors and Piezoelectric Switches for Mobile Communication Applications

Thin-film ferroelectric capacitors have been integrated with resistors and active functions such as ESD protection into small, miniaturized modules, which enable a board space saving of up to 80%. With the optimum materials and processes, integrated capacitors with capacitance densities of up to 100 nF/mm>² for stacked capacitors combined with breakdown voltages of 90 V have been achieved. The integration of these high-density capacitors with extremely high breakdown voltage is a major accomplishment in the world of passive components and has not yet been reported for any other passive integration technology. Furthermore, thin-film tunable capacitors based on barium strontium titanate with high tuning range and high quality factor at 1 GHz have been demonstrated. Finally, piezoelectric thin films for piezoelectric switches with high switching speed have been realized.     

Examination of the possibility of negative capacitance using ferroelectric materials in solid state electronic devices

We show here, using fundamental energy storage relationships for capacitors, that there are severe constraints upon what can be realized utilizing ferroelectric materials as FET dielectrics. A basic equation governing all small signal behavior is derived, a negative capacitance quality factor is defined based upon it, and thousands of carefully measured devices are evaluated. We show that no instance of negative capacitance occurs within our huge database. Furthermore, we demonstrate that highly nonlinear biasing behavior in a series stack could be misinterpreted as giving a negative capacitance.     

Special features in the pyroelectric properties of PZT films containing excess lead oxide

Pyroelectric hysteresis in unipolar lead zirconate titanate (PZT) films was studied upon preliminary poling in a strong external electric field and immediately in the applied external field. The asymmetry of the observed hysteresis loops is considered within the framework of an electromechanical approach to the phenomenon of natural unipolarity (spontaneous polarization) in thin-film ferroelectric capacitors formed on silicon and glass-ceramic composite substrates.     

Grain-size dependence of electrical properties in (Na0.5Bi0.5)0.94Ba0.06TiO3 films by PLD

Journal of Materials Science: Materials in Electronics - Grain size has significant effects on dielectric and ferroelectric properties of ferroelectric thin film for capacitors applications. In...     

Key integration technologies for nanoscale FRAMs

We discuss key technologies of 180-nm node ferroelectric memories, whose process integration is becoming extremely complex when device dimension shrinks into a nano scale. This is because process technology in ferroelectric integration does not extend to conventional shrink technology due to many difficulties of coping with metal-insulator-metal (MIM) capacitors. The key integration technologies in ferroelectric random access memory (FRAM) comprise: etching technology to have less plasma damage; stack technology for the preparation of robust ferroelectrics; capping technology to encapsulate cell capacitors; and vertical conjunction technology to connect cell capacitors to the plate line. What has been achieved from these novel approaches is not only to have a peak-to-peak value of 675 mV in bit-line potential but also to ensure a sensing margin of 300 mV in opposite-state retention, even after 1000 hour suffering at 150 degrees C.     

Nanostructured ferroelectrics: fabrication and structure-property relations

With the continued demand for ultrahigh density ferroelectric data storage applications, it is becoming increasingly important to scale the dimension of ferroelectrics down to the nanometer-scale region and to thoroughly understand the effects of miniaturization on the materials properties. Upon reduction of the physical dimension of the material, the change in physical properties associated with size reduction becomes extremely difficult to characterize and to understand because of a complicated interplay between structures, surface properties, strain effects from substrates, domain nucleation, and wall motions. In this Review, the recent progress in fabrication and structure-property relations of nanostructured ferroelectric oxides is summarized. Various fabrication approaches are reviewed, with special emphasis on a newly developed stencil-based method for fabricating ferroelectric nanocapacitors, and advantages and limitations of the processes are discussed. Stress-induced evolutions of domain structures upon reduction of the dimension of the material and their implications on the electrical properties are discussed in detail. Distinct domain nucleation, growth, and propagation behaviors in nanometer-scale ferroelectric capacitors are discussed and compared to those of micrometer-scale counterparts. The structural effect of ferroelectric nanocapacitors on the domain switching behavior and cross-talk between neighboring capacitors under external electric field is reviewed.     

Solid-state cooling line based on the electrocaloric effect

In this paper, the feasibility of using ferroelectric materials as a cooling device or solid-state refrigerator based on the electrocaloric effect has been considered. The electrocaloric responses of two different ferroelectric capacitors (PMN-25PT, PZN-4.5PT) are studied. In this study, the dynamics of temperature variation at the edges of a layered structure comprised of an electrocaloric material with heatconducting elements on its sides to an applied periodic electric field have been studied. Electrocaloric elements can generate directed heat flux as a thermal pump by alternative switching. A temperature reduction of 6° is obtained in an electric field of 1 KV/mm at a frequency of 1 Hz for PMN-25PT material after applying 80 cycles. It is shown that the type of applied electric pulses and ferroelectric material affect the results.     

Tunable Molecular Electrodes for Bistable Polarization Screening

The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, screening is done by the electrodes, while in bare ferroelectric surfaces it is typically accomplished by atmospheric adsorbates. Although chemisorbed species can have even better screening efficiency than conventional electrodes, they are subject to unpredictable environmental fluctuations and, moreover, dominant charged species favor one polarity over the opposite. This paper proposes a new screening concept, namely surface functionalization with resonance-hybrid molecules, which combines the predictability and bipolarity of conventional electrodes with the screening efficiency of adsorbates. Thin films of barium titanate (BaTiO₃) coated with resonant para-aminobenzoic acid (pABA) display increased coercivity for both signs of ferroelectric polarization irrespective of the molecular layer thickness, thanks to the ability of these molecules to swap between different electronic configurations and adapt their surface charge density to the screening needs of the ferroelectric underneath. Because electron delocalization is only in the vertical direction, unlike conventional metals, chemical electrodes allow writing localized domains of different polarity underneath the same electrode. In addition, hybrid capacitors composed of graphene/pABA/ferroelectric have been made with enhanced coercivity compared to pure graphene-electode capacitors.     

The commutation quality factor of electrically controlled microwave device components

Various circuit components based on electrically controlled capacitors can be compared using an integral criterion, called the commutation quality factor and determined by the efficiency of capacitance tuning and the losses in the control device. The value of commutation quality factor is independent of the physical nature of controlled components, which ensures objective comparative analysis of the efficiency of electrically tunable capacitive components operating in the microwave frequency range: p-i-n diodes, microelectromechanical switches, diodes with variable p-n junction capacitance (varactors), and ferroelectric capacitors.     

Organic Nonvolatile Memory Devices Based on Ferroelectricity

A memory functionality is a prerequisite for many applications of electronic devices. Organic nonvolatile memory devices based on ferroelectricity are a promising approach toward the development of a low‐cost memory technology. In this Review Article we discuss the latest developments in this area with a focus on three of the most important device concepts: ferroelectric capacitors, field‐effect transistors, and diodes. Integration of these devices into larger memory arrays is also discussed.