CN101718810B - Method for measuring ferroelectric hysteresis loop of leakage ferroelectric film - Google Patents

Abstract

The invention belongs to the technical field of solid-state dielectric performance testing, in particular to a method for measuring the hysteresis loop of a leakage ferroelectric thin film. In the method of the present invention, firstly, an applied voltage of the same polarity is applied on the drain polarized ferroelectric thin film, the leakage current passing through the ferroelectric thin film under different voltages at both ends of the thin film is measured and recorded, and the leakage current and the applied voltage are obtained through curve fitting. The function equation is used to calculate the variation relationship of the polarization current of the domain flipping with time. Finally, the displacement current is integrated with time, and the displacement charge density generated by the ferroelectric film under the applied voltage is calculated. Repeat the above measurement and calculation process to obtain the corresponding electric displacement of the ferroelectric thin film under different voltages, so as to measure the hysteresis loop of the entire leakage ferroelectric thin film. The invention solves the difficult problem of electrical characterization of the leakage ferroelectric thin film, and especially provides a powerful means for the research on the electrical properties of the ultra-thin ferroelectric leakage thin film.

Description

A Method for Measuring the Hysteresis Loop of Leakage Ferroelectric Thin Film

technical field

The invention belongs to the technical field of solid-state dielectric performance testing, and in particular relates to a testing method for a hysteresis loop of a leakage ferroelectric film.

Background technique

Ferroelectric thin film materials have high spontaneous polarization and large dielectric constant and can be applied to non-volatile random access memory (FRAM), dynamic random access memory (DRAM), uncooled infrared detectors, thin film dielectric capacitors , microwave devices modulated by electric field, AC electroluminescent devices and thin film sensors, etc. With the improvement of device integration density, the device unit size has been greatly reduced, approaching the atomic or molecular level. It is expected that the storage density of FRAM and DRAM in the future will reach the order of ~Tb/in ² , which is close to the current magnetic recording hard disk manufactured by vertical technology. level.

The difficulty in the electrical measurement of leakage ferroelectric thin films stems from the defects, cavities, or discontinuous growth of the thin film in the local area of the thin film, such as islands, rods, etc., which lead to the leakage of the entire thin film or the short circuit between the upper and lower electrodes of the parallel plate capacitor, making Currently typical commercial ferroelectric testers such as Radiant PremierI/II and aixACCT TF2000analyzer are powerless for the electrical characterization of these leaky thin films. It is especially urgent to develop the necessary new ferroelectric test technology for the electrical research of ultra-thin thin films. With the development of the market and the improvement of the integration of microelectronic devices, the size of the device is developing at a speed of 1 times every 2 years according to Moore's law, gradually approaching the level of atoms or molecules, such as the 16nm microelectronics that Intel is currently researching and developing. The MOS devices below the size and the research plan of functional devices below 22nm in the major special project of "very large scale integrated circuit equipment and complete process" launched during the "Eleventh Five-Year Plan" period of our country are all moving towards this goal. Therefore, it is very urgent to actively develop advanced thin film testing technology from the perspective of market application. The research on the growth of ferroelectric thin films at the atomic level is also of positive significance for the development of the kinetic theory of domain nucleation and growth. Theoretically, the flipping of the electric domain starts from the formation of the reverse sub-nuclei of the critical size, which initially originates from the surface of the film or the accumulation area of defects. The latest density functional (DFT) calculations show that the size of this critical sub-nucleus is on the order of three unit cells, that is to say, only those sub-nuclei larger than this size will contribute to the flipping of electric domains. However, when the thickness of the film is close to or lower than the critical dimension, the flip mechanism must be different, and the understanding of the mechanism is still unclear, which affects the further development of device functions. In addition, as the film thickness decreases, the quantum tunneling current increases accordingly. On the one hand, this tunneling current will change with the reorientation of the electrical domain, which can be applied to the non-destructive reading of logic signals in high-density FRAM memory cells, which is of positive significance; on the other hand, it also further improves Leakage current is reduced, which deepens the difficulty of thin film electrical measurement. That is, even if we were able to grow flawless thin-film crystals, it would be impossible to complete the electrical characterization of these thin films with existing commercial equipment. Therefore, the leakage ferroelectric thin film test method designed in the present invention and the leakage ferroelectric thin film electrical performance test system designed by applying this principle provide a powerful tool for the research and application of ferroelectric thin film materials.

Contents of the invention

The purpose of the present invention is to provide a method for measuring the hysteresis loop of the leakage ferroelectric thin film, so as to study the performance of the leakage ferroelectric thin film.

The concrete steps of the inventive method are as follows:

(1) Use a pulse generator to generate a pulse first, so that the reversible electric domain in the leakage ferroelectric film under test is completely reversed under the applied voltage.

(2) After the current is stable, measure the current passing through the ferroelectric film with the same polarity as the leakage current. Since the polarization direction of the ferroelectric domain is consistent with the direction of the external electric field at this time, the measurement process is carried out after the capacitor is fully charged, and the measured current only includes the leakage current. Part, that is, the measured leakage current value through the leakage ferroelectric film.

(3) Change the pulse amplitude (including the direction), and measure the leakage current value passing through the leakage ferroelectric film under different voltages.

(4) According to the above measured data, and by the formula

V _F =VI _L R

(Wherein R is the total resistance of the system, V is the measurement pulse amplitude, I _L is the leakage current through the leakage ferroelectric film and the series resistance, and V _F is the voltage applied to the ferroelectric film.) Make the leakage current I _L and the curve of the voltage V _F at both ends of the film, and perform polynomial fitting to obtain the equation _IL = F(V _F ) of the leakage current I _L with the voltage V _F at both ends of the film as an independent variable.

(5) The polarities of the applied voltage and domain orientation are opposite. Record the change I(t) of the total current on the entire series resistance with time by an oscilloscope, and obtain the change V _F (t) of the real voltage in the ferroelectric film with time. From the above leakage current-voltage function relationship, calculate the ferroelectric The relationship of the leakage current changing with time in the electric film I _L =F[V _F (t)], and deducted from I(t), to obtain the relationship of the displacement current changing with time I(t)-F[V _F (t )].

(6) Integrate the displacement current curve with respect to time and divide it by the electrode area to obtain the polarized charge density (electrical displacement) caused by the voltage across the leakage ferroelectric film under the applied voltage.

(7) Change the applied voltage, repeat the above measurement and calculation, and obtain the electric displacement of the ferroelectric film under different voltages, that is, the hysteresis loop of the entire leakage ferroelectric film.

In the above method, the pulse width generated by the pulse generator in step (1) should be sufficient to completely turn all the reversible ferroelectric domains, and the width should be determined by the specific test sample.

In the above method, the measurement current in step (2) should be carried out after the capacitor is charged. Since the direction of the ferroelectric domain polarization is consistent with the direction of the external electric field at this time, the current passing through the system does not contain the component of the displacement current. The measured current is Leakage current through a leaky ferroelectric film.

In the above method, the magnitude of the pulse amplitude in step (3) should include all voltages from 0V to ±V (considering that the divided voltage of the internal resistance is V>V _s , the specific value depends on the actual measurement system, V _s is The voltage to saturate the sample polarization depends on the electrical characteristics of the ferroelectric thin film sample itself.)

In the above method, the relationship between the voltage V _F at both ends of the ferroelectric thin film and the total current I in step (4) is written as the formula V _F = VI _L R because the displacement current is zero I = I _L at this time, and in the whole current relaxation process It should be written as V _F =V-IR, V is the pulse amplitude.

In the above method, the relationship between the leakage current of the ferroelectric thin film and the voltage across the ferroelectric thin film in step (5) is applied to the whole current relaxation process based on a generally applicable assumption, that is, the leakage current through the ferroelectric thin film is only related to the ferroelectric thin film. The voltage at both ends of the electric film is related, and the polarization state of the electric domain has no effect on the leakage.

Description of drawings

Figure 1 is a schematic diagram of the measurement system for leakage ferroelectric thin film hysteresis loops.

Figure 2 is a schematic diagram of deducting the leakage current from the total test current.

Fig. 3 is the relationship curve between the measured leakage current and the voltage across the leakage ferroelectric film.

Fig. 4 is the function curve of the leakage current and the voltage across the leakage ferroelectric film obtained by fitting a high-order polynomial.

Fig. 5 is the test result of the simulated leakage capacitance (connecting the standard capacitance and any resistance in parallel to form the simulated leakage capacitance) by using the leakage ferroelectric film test system.

Fig. 6 is the test result of a simulated leakage ferroelectric film (a standard ferroelectric capacitor connected in parallel with an arbitrary resistor to form a simulated leakage ferroelectric film) using a leakage ferroelectric film test system.

Detailed ways

The pulse signals required for the test are all edited by the Agilent 81150A arbitrary waveform signal generator, the current is recorded by the LCWR6200A oscilloscope, and the internal resistance of the system is 50 ohms.

Measure a simulated leakage ferroelectric thin film system consisting of a standard ferroelectric capacitor and resistor in parallel.

(1) Use a pulse generator to first generate a pulse with a sufficient width so that the reversible electric domain in the measured leakage ferroelectric film is completely reversed under the applied voltage.

(2) After the current is stable, measure the current passing through the ferroelectric film with the same polarity as the leakage current. Since the polarization direction of the ferroelectric domain is consistent with the direction of the external electric field at this time, the measurement process is carried out after the capacitor is fully charged, and the measured current only includes the leakage current. Part, that is, the measured leakage current value through the leakage ferroelectric film.

(3) Change the pulse amplitude (0 to ±10V), and measure the leakage current value passing through the leakage ferroelectric film under different voltages.

(4) According to the above measured data, and by the formula

V _F =VI _L R

(Wherein R is the total resistance of the system, V is the measurement pulse amplitude, I _L is the leakage current through the leakage ferroelectric film and the series resistance, and V _F is the voltage applied on the ferroelectric film.) Make the leakage current I and Polynomial fitting is performed on the curve of the voltage V _F across the film, and the equation _IL = F(V _F ) of the leakage current _IL with the voltage V _F across the film as an independent variable is obtained.

(5) The polarities of the applied voltage and domain orientation are opposite. Record the change I(t) of the total current on the entire series resistance with time by an oscilloscope, and obtain the change V _F (t) of the real voltage in the ferroelectric film with time. From the above leakage current-voltage function relationship, calculate the ferroelectric The leakage current in the electric film changes with time I _L =F[V _F (t)], and is deducted from I(t) to obtain the relationship of displacement current with time I(t)-F[V _F (t)] .

(6) Integrate the displacement current curve and divide it by the electrode area to obtain the polarization charge density (electric displacement) caused by the voltage across the leakage ferroelectric film under the applied voltage.

(7) Change the applied voltage, repeat the above measurement and calculation, and obtain the electric displacement of the ferroelectric film under different voltages, that is, the hysteresis loop of the entire leakage ferroelectric film.

Figure 4 and Figure 5 respectively show the capacitance polarization curve and the hysteresis loop of the ferroelectric film obtained by measuring the simulated leakage capacitance and the simulated leakage ferroelectric film by using the leakage ferroelectric film test system. It can be proved by the comparison of the simulated leakage situation and the ideal situation in the two figures that the method of the present invention can effectively measure the electrical properties of the leakage ferroelectric thin film sample.

Claims (1)

1. method of measuring ferroelectric hysteresis loop of leakage ferroelectric film is characterized in that concrete steps are following:

(1) utilize pulse producer at first to produce a pulse, the width of this pulse can make the institute the electric ferroelectric thin film that leaks hunting under institute's making alive reversible electricdomain overturn fully;

(2) after current stabilization, measure the electric current that passes through electric leakage same polarity ferroelectric thin film, record leakage current value through the electric leakage ferroelectric thin film;

(3) change pulse amplitude, measure different electric and depress leakage current value through the electric leakage ferroelectric thin film;

(4) according to the above data of surveying, and by formula

V _F＝V-I _LR

Make leakage current I _LWith film voltage V _FCurve, carry out fitting of a polynomial, obtain leakage current I _LWith film voltage V _FEquation I for independent variable _L=F (V _F), wherein R is system's all-in resistance, V is for measuring pulse amplitude, I _LFor passing through the leakage current on electric leakage ferroelectric thin film and the resistance in series, V _FFor being applied to the voltage on the ferroelectric thin film;

(5) polarity of impressed voltage and electric domain orientation is opposite, and by the I (t) over time of the total current on the whole resistance in series of oscillograph recording, the real voltage of obtaining in the ferroelectric thin film changes V in time _F(t); By above leakage current I _LWith voltage V _FRelation curve, calculate the time dependent relations I of leakage current in the ferroelectric thin film _L=F [V _F(t)], and from I (t), deduct, obtain displacement current variation relation I (t)-F [V in time _F(t)];

(6) the displacement current curve is carried out integration about the time, and just obtain the electric displacement that electric leakage ferroelectric thin film voltage causes under this impressed voltage divided by electrode area;

(7) change impressed voltage, repeat above measurements and calculations, obtain the electric displacement that different electric is depressed ferroelectric thin film, be the ferroelectric hysteresis loop of whole electric leakage ferroelectric thin film.

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