KR20090131313A - Spin cast ferroelectric beta poly(vinylidene fluoride) thin film via rapid thermal annealing - Google Patents

Abstract

PURPOSE: A beta crystallization method of a PVDF(Poly Vinylidene Fluoride) thin film is provided to obtain ferroelectric by changing alpha crystal or gamma crystal structure of a spin-cast PVDF thin film into a beta crystal structure. CONSTITUTION: A PVDF thin film is manufactured by spin-casting a PVDF solution on an Au substrate. Thickness of the PVDF thin film is 10~500nm. The PVDF solution is formed by melting PVDF to a polar solvent. The polar solvent is one of DMF(Dimethylformamide) and DMSO(Dimethyl sulfoxide). RTA(Rapid Thermal Annealing) about the spin-cast PVDF thin film is performed at a temperature of 80~160°C and a heating speed more than 30°C/minute.

Description

Β crystallization of spin cast PDF thin film through RTA {Spin cast ferroelectric beta poly (vinylidene fluoride) thin film via rapid thermal annealing}

The present invention relates to a method of converting a PVDF (poly (vinyliden fluoride)) thin film of α crystal or γ crystal spin cast on an Au substrate into a PVDF thin film of β crystal. Since the spincast PVDF thin film has a structure mainly composed of α crystal or γ crystal and does not express high ferroelectricity, the crystal is ferroelectric β crystallized before it can be used for a nonvolatile memory device.

PVDF or P due to the polymer ferroelectricity derived from the bistability of permanent electron dipoles along the backbone of the long macromolecules of PVDF or copolymer P (VDF-TrFE) of PVDF used in non-volatile memory applications (VDF-TrFE) is receiving industrial attention. One of the biggest advantages of the ferroelectric polymer is that it is easily mixed with a general organic solvent and can be easily applied to a memory unit such as a capacitor or a transistor by a spin casting process.

In particular, referring to FIG. 1, in order to manufacture a successful electronic device based on PVDF, among three types of crystal structures of PVDF configuration, an α crystal of TGTG structure and a γ crystal of TTTG structure may be used. It is important to effectively express the β crystal structure of the TTTT sequence showing excellent polarity. Copolymer P (VDF-TrFE) between PVDF and PTrFE changes the crystal structure into β structure by the addition of fluorine, but it is not suitable for mass production because it requires a very complicated and expensive process. Moreover, the P (VDF-TrFE) copolymer has a Curie temperature of about 100 ° C., which is too low than the Curie temperature of 170 ° C. of PVDF, and is not suitable for devices operating at high temperatures.

The present invention relates to a method of β crystallization of PVDF thin films having a thickness of several hundred nanometers. In order to manufacture PVDF films having several micron thickness in the prior art, mechanical stretching, electric polling, and pressure are applied. Although various studies have been conducted on high compression, hygroscopic salts, epitaxy, solvent evaporation, etc., PVDF <300 nm is suitable for memory devices operating at operating voltages below 30 V. There are only a few ways to change the crystal structure in thin films. The β crystallization method of the PVDF thin film having a micron thickness could not be applied to the β crystallization of the PVDF thin film having a nano unit thickness.

As an example of the study on the β crystallization method in the nano-thick PVDF thin film as in the present invention, Wang et al. (J. Wang, H. Li, J. Liu, Y. Duan, S. Jiang, S. Yan, J. Am. Chem. Soc. 125, 1496 (2003) proposed a method for manufacturing by melting and recrystallization under a carbon evaporated surface in order to make a very thin film of β crystals. Wang's method is difficult to apply to electronic devices such as capacitors because the film is not uniformly manufactured.

From an industrial point of view, there was a need for a method for producing a PVDF thin film having a β crystal structure with a large area uniformity by spin casting, but such a method has not been presented yet, leading to the present invention.

An object of the present invention is to provide a β crystallization method of spincasted PVDF thin film for expressing ferroelectricity by changing the α crystal or γ crystal structure in the spincast PVDF thin film to β crystal structure.

It is also an object of the present invention to provide a method of forming a constant pattern of β crystals in α crystals or γ crystal forms together with the β crystallization method of PVDF thin films.

The present invention to achieve the above object,

A method for converting a spincast PVDF thin film having an α or γ crystal structure into a ferroelectric β crystal structure, comprising the steps of: (I) manufacturing a PVDF thin film by spincasting a PVDF solution on an Au substrate under a controlled humidity; And rapidly increasing the temperature of the spincasted PVDF thin film to a set temperature below the melting point of the PVDF thin film, followed by RTA (Rapid Thermal Annealing) step (II) for maintaining a predetermined temperature. It provides a β crystallization method of spincast PVDF thin film through RTA.

In addition, the PVDF solution of step (I) is preferably a solution in which PVDF is dissolved in a polar solvent.

In addition, the polar solvent is preferably any one of DMF and DMSO.

 In addition, it is preferable that the heating rate of the rapid heat treatment in step (II) is 30 ° C./min or more.

In addition, the rapid heat treatment temperature in the step (II) is preferably 80 ~ 160 ℃.

In addition, the thickness of the PVDF thin film prepared above is preferably 10 ~ 500 nm or less. In particular, it is preferable that it is 100 nm or less.

In addition, the humidity is preferably adjusted to 5 to 20% or less.

In addition, the RTA time in the step (II) is preferably 2 hours or more.

In addition, the present invention for producing a PVDF thin film having a patterned β crystal structure

A method for converting a spincast PVDF thin film having an? or? crystal structure into a? crystal structure in which? crystals have a constant pattern, wherein the self-assembled thin films (SAMs) are not formed on the surface of the Au substrate. Micro-patterning step (I) of dividing the region into unparted portions; (II) manufacturing a PVDF thin film by spincasting a PVDF solution on a uniformly micro-patterned Au substrate by the self-assembled thin film; And a rapid thermal annealing (RTA) step (III) of maintaining the temperature for a predetermined time by rapidly raising the temperature of the spincasted PVDF thin film to a set temperature below the melting point of the PVDF thin film. Provided is a method for patterning β crystallization of cast PVDF thin films.

In addition, the PVDF solution of step (II) is preferably PVDF dissolved in a polar solvent.

In addition, the polar solvent is preferably any one of DMF and DMSO.

In addition, it is preferable that the heating rate of the rapid heat treatment in step (II) is 30 ° C./min or more.

In addition, the rapid heat treatment temperature in the step (III) is preferably 80 ~ 160 ℃.

In addition, the thickness of the PVDF thin film prepared above is preferably 10 ~ 500 nm or less.

In addition, the humidity is preferably controlled to 5 ~ 0% or less.

In addition, the self-assembled thin film is preferably a self-assembled thin film having a polar group at the end.

The terminal group of the self-assembled thin film having a polar group at the terminal is preferably any one of a carboxyl group, a thiol group, and a hydroxyl group.

The present invention provides a method of converting a PVDF thin film of α crystal or γ crystal structure spin-cast onto a gold substrate (Au Substrate) into a ferroelectric β crystal PVDF thin film through rapid thermal annealing (RTA), Large area, which is an advantage of spin casting process, can be used to manufacture PVDF thin film. It is also possible to produce β crystal uniform ferroelectric PVDF thin films composed of typical needle-like microdomains. In the capacitor to which a ferroelectric PDVF thin film having a 160 nm-thick β crystal structure manufactured by the method of the present invention was used, the residual polarization was about 7.0 μC / cm ² , the coercive voltage was about 8 V, and 160 ° C. Thermally stable until When applied to FeFET, it also shows drain current double safety at sweep voltage of ± 20 V and zero gate voltage of 10 ² .

The capacitor including the ferroelectric PVDF thin film manufactured by the present invention can operate at a high temperature of 160 ° C, and when a sweep voltage of ± 20 V is applied, 8 V low coercive voltage and residual of 7 μC / cm ² Polarization is indicated.

In the present invention, after the PVDF solution is controlled in a controlled humidity state, the α crystal or γ crystal structure is formed by rapid thermal annealing (RTA) at a heating rate of 30 ° C./min to a set temperature below the melting point of the PVDF. The PVDF thin film is characterized by changing into a PVDF thin film having a β crystal structure.

Hereinafter, the present invention will be described with reference to specific examples.

Experimental Example  One : GIRAS  Experiment to confirm β crystal structure through measurement

Melting point and crystallinity index of PVDF pellets of Mw = 180000 g / mol and PDI = 2.5 were measured at a temperature increase rate of 2 ° C./min using PerkinElmer's DSC-7. The crystallinity was about 50%. The melting point and glass transition temperature of PVDF used in this experiment were 167 ℃ and -41 ℃, respectively. PVDF thin films were prepared by spin casting on Au substrates with dimethylformamide (DMF, Dimethylformamide) as a solvent and adjusting the solution concentration to 1% by weight and humidity to 20% or less through nitrogen purge. The thickness of the prepared PVDF thin film was about 60-70 nm.

First, a typical PVDF thin film before RTA treatment cast on an Au substrate is a crystal structure in which α and γ crystal structures are mixed (the mixing degree of the α and γ crystals vary depending on the solvent) and has a surface too rough for use in a capacitor. In this embodiment, DMF was used as a solvent, and the reason is that polar solvents such as DMF and DMSO are known to make γ-crystal thin films well, and γ-crystal is a starting point for producing thin films having β crystals. Suitable.

In addition, the study found that the humidity during PVDF spin coating affects not only the quality of thin films but also the crystal structure. As with other studies, humidity increased the polar β crystal structure, but the surface was rough enough to be unusable for the device. In order to minimize the influence of humidity, in the present invention, a thin film was prepared while purging nitrogen to keep the humidity at 20% or less. As a result, the surface of the thin film was flat, and on the Au substrate as shown in "As cast" of FIG. The results of the Grazing Incident Reflection Absorption Spectra (GIRAS) measurement in the PVDF thin film after spincasting and before the RTA process produced a PVDF thin film mainly composed of γ crystals without the α crystal structure. It can be seen that β crystals were formed from absorption peaks 840 and 1280 cm ⁻¹ of IR in PVDF thin films made under the same humidity control as in this experiment. The β crystal, calculated by the formula A ₁₂₈₀ / (A ₁₂₈₀ + A ₁₂₃₄ ), from the absorption peak of the 1234 cm ^-1 absorption and the β crystal of 1280 cm ^-1 , still occupies a very small fraction of about 40% or less. . Therefore, in order to express the ferroelectricity of the spincast PVDF thin film, the PVDF thin film mainly composed of gamma crystals is subjected to the RTA process described later so that the β crystal is mainly composed. As a specific setting temperature of 167 ° C. or less, which is the melting point of PVDF through RTA, in Experimental Example 1, the temperature was rapidly raised to 150 ° C. at a rate of 30 ° C./min to 150 ° C. and then maintained for about 2 hours.

The rapid thermal treatment (RTA) results in evaporation with the directivity of the DMF remaining in the PVDF thin film spin-cast in the vertical direction of the PVDF thin film. As shown in FIG. 2, the β crystal structure in the PVDF thin film occupies 90% or more by RTA.

In order to confirm the effect of rapid removal of the solvent due to rapid heat treatment on the formation of β crystals, as cast PVDF films were subjected to RTA treatment for each temperature section up to 150 ° C. As shown in FIG. 3, it was found that the fraction of β crystals in the PVDF thin film was increased almost linearly to the temperature of 150 ° C. just below the Tm of the PVDF. This means that the faster the solvent is removed, the larger the fraction of β crystals is formed.

In addition, heat treatment at temperatures above Tm of PVDF always produces γ crystal structures regardless of the heating rate, which is consistent with the findings of other researchers. The results suggest that the directional motion of DMF, a polar solvent in PVDF films, provides sufficient energy to transform the Gauche conformation of the γ crystal into the trans confromation of the β crystal. Experiments with different PVDF thin films showed that the thinner the thin film, the higher the fraction of β crystals.

Experimental Example  2 : GIXD  Wow AFM Surface Structure Experiment

A 160 nm-thick PVDF thin film, which had undergone the RTA process in the same manner as in Experimental Example 1, was prepared using a 3 wt% PVDF solution (solvent DMF). GIXD and AFM measurements were performed on the PVDF thin film.

The microstructure of the PVDF thin film on the Au substrate having β crystals by the RTA of the present invention can be visualized by GIXD and AFM, as shown in (a) to (d) of FIG. 4.

Referring to (a) of FIG. 4, it is confirmed by GIXD of FIG. 4 (a) that crystallization of a chain of molecules of PVDF occurs with β crystal structure due to the removal of the directional DMF during RTA treatment. The reflection of the PVDF crystals of (110) or (200) has a preferred form that is vertically aligned with respect to the substrate. Due to the similar lattice-spacing of (110) and (200), it is not easy to distinguish the two reflections, and similar hexahex when the incident X-ray is parallel to the c-axis of the PVDF crystal This leads to a pseudo hexagonal deflection pattern. However, strong reflection in the meridan means that the polymer chains are oriented in the desired direction parallel to the Au surface. Electric poling along the PVDF thin film with the desired crystal orientation results in effective rotation of the conformation along the chain axis, resulting in a fairly large residual polarization described below.

4 (b) shows the microstructure in the PVDF thin film as a result of AFM measurement of the PVDF thin film manufactured by rapid heat treatment. As shown in FIG. 4 (b), hemi-spherical caps have a diameter of 200 to 500 nm and are closely packed with each other. Typical needle-like microdomains with a length of 100 nm and a width of 20 nm are more clearly seen in the inset of FIG. 4 (b). This suggests that the α crystals, as demonstrated in other studies, differ from those of a few microns in spherical form, and are the same results as those reported by other researchers in the form of β crystals.

Referring to Figure 4 (c), in order to determine the effect of the chemical surface properties of the Au substrate on the crystal formation of the PVDF thin film, as Self Assembled Monolayers (SAMs) having a carboxyl group (COOH) terminal, The result of AFM measurement after chemically linking 16-mercaptohexadecanoic acid to an Au substrate and performing RTA.

In this way, the chemical properties of the substrate surface can be varied in various ways without modifying the physical topology of the surface of the Au substrate. Au substrates treated with SAMs with carboxyl end groups transformed the spincast PVDF thin films into α-crystal dominated structures. The microstructure of the PVDF thin film prepared on the Au substrate surface-treated with SAMs shows that the α crystal of the spherulite remains after RTA in FIG. 4 (c), which is caused by the energy due to the rapid heat treatment. This means that the Gauche conformation of the crystal could not be converted to the trans conformation of the β crystal. In general, polarized substrates are easily spin-casted into films, but thin films are always produced with? Crystals. The same is true for Au treated with Al ₂ O ₃ , SiO ₂ and oxygen plasma. The Au substrate surface-treated with SAMs having a non-polar methyl group terminal group was excluded from the experiment because of the difficulty in film formation. Effective surface energy suitable for the substrate is a very important requirement for the production of PVDF thin films having β crystal structures.

Referring to (d) of FIG. 4, the formation of the β crystal structure depending on the surface of the substrate is performed by rapid thermal treatment after spin casting of PVDF on variously patterned Au substrates with SAMs to form β crystal structures having various micropatterns. It can be produced. A 250 nm thick PVDF thin film prepared on an Au substrate patterned by SAMs with COOH end groups shows a micropattern clearly contrasted by the crystal density difference between the α crystal and the β crystal (insertion of FIG. 4 (d)). See also). The AFM image of FIG. 4 (d) shows a thin film having a micropattern having a periodic line of 20 μm, and has a very sharp boundary between the α crystal of the spherical crystal and the β crystal of the sphere cap. .

Experimental Example  3: D-E Hysteresis curve  Measurement experiment

Experimental Example 3 was carried out on the same RTA-treated PVDF thin film as Experimental Example 2. That is, FIG. 5 shows a hysteresis loop of a 160 nm thick PVDF thin film spin-cast on an Au substrate and then maintained for 2 hours after rapid heat treatment to 150 ° C. Due to the β crystal formed by RTA, a nearly saturated loop appeared at the sweep voltage ± 20 V. Electropolling applied during voltage sweeping at low electric fields is sufficient to rearrange molecular dipoles between hydrogen and fluorine by rotation of the chain, resulting in 7 μC / cm ² It has a large residual polarization (Pr), a response voltage (Vc) of 8V, which corresponds to a coagulation field (Ec) of about 50 MV / m.

Experimental Example  4 : FeFET Property measurement experiment

Ferroelectric properties were measured using a virtual ground circuit. After the PVDF was spin-coated on the Au substrate as in Experimental Example 2, a ferroelectric field effect transistor (FeFET) was manufactured using a 150 gate RTA treated as a bottom gate. PVDF thin film about 50 nm in thickness with poly (4-vinyl phenol) (PVP) on top is spin-coated, again was then deposited over the pentacene (pentacene) is open to 60nm thickness (thermal evaporation) (during the deposition pressure it is 10 ^{- 6} mB, 0.1 to 0.2 μs / s). Source and drain electrodes were fabricated by thermal evaporation on an Au substrate through a shadow mask. Aluminum upper electrodes were prepared by evaporation in the same way as in other studies.

In the above, the PVP layer not only controls the growth of pentacene through the reduction of the roughness of the substrate, but also enables low electric leakage. The drain current (-I _DS ) vs. gate voltage (V _G ) curve scanned at the sweeping voltage (± 20 V) and V _SD -5V is shown in FIG. 6.

Experimental Example  5: Thermal stability  Experiment

The biggest advantage of using PVDF thin film as ferroelectric is that the Curie temperature is higher than that of PVDF copolymers, which increases the operating temperature in memory devices using polymer thin films. Compared to the copolymer of PVDF, which exhibits a Curie temperature of 60-100 ° C., depending on the content of TrFE, the pure PVDF thin film transitions from ferroelectric to paraelectric at melting temperature 167 ° C. In order to measure the thermal stability of the PVDF thin film prepared according to the present invention, a GIRAS measurement experiment was performed at different temperatures.

We measured GIRAS with varying temperatures for a 160nm-thick PVDF thin film heat-treated at 150 ° C for 2 hours. The ratio of 1280 cm ⁻¹ β absorption peak measured at room temperatur to absorption peak measured at each measurement temperature is shown in FIG. 7. As the temperature increased, the amount of β crystals decreased slowly, and about 160% of β crystals remained at 160 ° C. compared with those measured at room temperature. This is because β crystals collapse when GIRAS is measured near the melting point of PVDF.

In addition, the experimental results show that the PVDF thin film has a β crystal structure of about 70% after RTA when the thickness of the PVDF thin film is 500 nm, and has a beta crystal structure of about 90% after RAT when the thin film thickness is 60 to 70 nm. The thickness of also affected β crystallization.

1 is an explanatory diagram showing α, β, and γ crystal structures of PVDF.

Figure 2 is a graph of the results of Experimental Example 1, a PVDF thin film manufactured without the RTA treatment, and GIRAS spetra of the RDF treated PVDF thin film for each temperature.

3 is a result graph showing the fraction of β crystals for each RTA temperature.

4 is the results of Experimental Example 2, the measurement results through GIXD and AFM.

5 is a D-E hysteresis curve of the PVDF thin film of the present invention as a result of Experimental Example 3. FIG.

6 is a performance curve of FeFET to which the PVDF thin film of the present invention is applied as a result of Experimental Example 4. FIG.

7 is a result of Experimental Example 5, GIRAS measurement results for each temperature.

Claims (17)

A method for converting a spincast PVDF thin film of α or γ crystal structure into a ferroelectric β crystal structure, (I) preparing a PVDF thin film by spincasting a PVDF solution on an Au substrate in a humidity controlled atmosphere; And Rapid thermal annealing (RTA) step of rapidly raising the spin cast PVDF thin film to a set temperature below the melting point of the PVDF thin film and maintaining the temperature for a predetermined time (II). Β crystallization method of spincast PVDF thin film through RTA comprising a. The method of claim 1, wherein the PVDF solution of step (I) is a crystallization method of β crystals of PVDF thin film spin-cast through RTA, characterized in that the solution dissolved PVDF in a polar solvent. The β crystallization method of spincasting PVDF thin film through RTA according to claim 2, wherein the polar solvent is one of DMF and DMSO. The β crystallization method of spincasting PVDF thin film through RTA according to claim 1, wherein the heating rate of the rapid heat treatment in step (II) is 30 ° C / min or more. The β crystallization method of spincasting PVDF thin film through RTA according to claim 1, wherein the rapid heat treatment temperature is 80 to 160 ° C in the step (II). [4] The β crystallization method of spincasting PVDF thin film through RTA, wherein the thickness of the prepared PVDF thin film is 10 to 500 nm or less. The β crystallization method of spincasting PVDF thin film through RTA, characterized in that the humidity is adjusted to 5 ~ 20% or less. The β crystallization method of spincasting PVDF thin film through RTA, characterized in that the RTA time in step (II) is 2 hours or more. A method for converting a spincast PVDF thin film having an α or γ crystal structure into a patterned β crystal structure such that the β crystal has a constant pattern,  Micro-patterning step (I) for dividing the region into portions in which self-assembled thin films (SAMs) are formed and portions not formed on the surface of the Au substrate (II) manufacturing a PVDF thin film by spincasting a PVDF solution on a uniformly micro-patterned Au substrate by the self-assembled thin film; And Rapid Thermal Annealing (RTA) step of maintaining the temperature for a predetermined time by rapidly raising the temperature of the spincasted PVDF thin film to a set temperature below the melting point of the PVDF thin film (III) Patterned β crystallization method of a spincast PVDF thin film comprising a. 10. The method of claim 9, wherein the PVDF solution of step (II) is a solution in which PVDF is dissolved in a polar solvent. 12. The method of claim 10, wherein the polar solvent is one of DMF and DMSO. 10. The method of claim 9, wherein the heating rate of the rapid heat treatment in step (II) is at least 30 ° C / min. 10. The method of claim 9, wherein the rapid heat treatment temperature in step (III) is 80-160 ° C. 10. The method of claim 9, wherein the thickness of the PVDF thin film prepared above 10 ~ 500 nm, characterized in that the patterned β crystallization method of spincast PVDF thin film. 10. The method of claim 9, wherein the humidity is controlled to 5-20% or less. 10. The method of claim 9, wherein the self-assembled thin film is a self-assembled thin film having a polar group at its end. 17. The patterned β of the spincasted PVDF thin film according to claim 16, wherein the self-assembled thin film having a terminal group of any one of a carboxyl group, a thiol group, and a hydroxyl group as an end group of the self-assembled thin film having a polar group at the end thereof. Crystallization method.

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