DESC:FIELD OF THE INVENTION
The present invention relates to crystallization of ferroelectric thin films. More particularly, the present invention relates to a laser-based method for crystallization of ferroelectric thin films at low temperature and system therefor and their application in tunable microwave devices.

BACKGROUND OF THE INVENTION
Laser annealing or localized heating by a laser is well studied for the silicon in the 1980s to make thin film transistor-liquid crystal displays at low temperatures [Khaibulin et al, Baeri et al]. However, this technique was found to be suitable and useful for some ferroelectric thin films like lead zirconate titanate (PZT). In general, high temperature crystallized ferroelectric thin films carried out at a temperature more than 600oC show the best functional properties like tunability and high dielectric constant [Kim et al, Muralt et al]. The integration of ferroelectric thin films onto polymers/IC’s requires crystallization at lower temperatures like 300oC [Fedder et al]. In this process, lasers act as a heat source (photothermal effects) and its intense and directed energy can induce lattice mending due to the absorption of photons [Bauerle et al]. Heating through laser is localized to the surface layer, which reduces the film-substrate interfacial interaction that usually occurs at a higher temperature, resulting in improved electrical properties.
To integrate BST thin film directly into system-on-chip (SoC), it is necessary to process the BST film below 350oC and based on previous studies laser annealing was found to be the best process. Many reports are available on the laser annealing on lead-based thin films, which are investigated for their ferroelectric and piezoelectric properties [Zhu et al and Rajashekhar et al].
Queralto et al. used a Nd:YAG laser (266 nm) for the annealing of BST thin films and explained it based on the photon energy of the laser and the bond energy for the Ba-O, Sr-O and Ti-O bonds. Kang et al reported high dielectric constant and low loss with low leakage current density for the laser annealed BST thin films at 300oC. Out of several drawbacks, crack formation and lack of full crystallization are the major problems associated with the laser annealing of ferroelectric thin films. Haldar et al. used a Kr-F excimer laser to crystallize chemical solution deposited BST thin films with a substrate temperature of 25-250oC and energy density of 100 mJ/cm2. Microstructural investigation shows reduced cracks for the laser annealed BST thin films. Baldus et al. has done a series of experiments on laser annealing for the BST system by changing the number of pulses and laser energy density to prevent the crack, which appears due to thermal stress. The PZT thin films were partially crystallized (120 nm of 600 nm) using a KrF excimer laser and to increase the penetration depth of the laser, longer laser wavelengths (Ar laser, 488 nm) were used, Lu et Al. In the recent report published by Queralto et al. BST8 thin films (BST/LNO/SiO2/Si) were crystallized with a Kr-F excimer laser in an oxygen ambient at fluences ranging from 50 to 75 mJ/cm2 at a substrate temperature of 500oC with full crystallization of thin films of 40 nm thickness. However, 160 nm thick films showed the crystallization only up to 70 nm due to the large temperature difference between the film surface and interface.
Therefore, there is a need of a technology that enables fabrication of thin films at low temperatures i.e. at 300oC which are usually obtained at 700oC.

OBJECT OF THE INVENTION
The main object of the present invention is to provide full vertical crystallization of thin films, layer-by-layer deposition of the film and laser annealing at low processing temperature after each stage of deposition.
Another object of the present invention is to provide a system for crystallization of thin films at low temperature i.e. at 300ºC which are usually obtained at 700ºC.

Yet another object of the present invention is to provide BST5 films deposited on amorphous fused silica and platinized silicon Pt/Si (111) and double-side polished platinum coated silicon (PCS) substrates by pulsed laser deposition.
Still another object of the present invention is to fabricate lead free barium strontium titanate (BST) ferroelectric thin film based tunable devices at relatively low temperatures.

SUMMARY OF THE INVENTION
The present invention provides a method for full vertical crystallization of ferroelectric thin films over a substrate at a low temperature and a system therefor.
In an embodiment, the present invention provides a method for crystallization of ferroelectric thin film comprising the steps of placing a target material for deposition and a substrate for laser annealing and focusing a pulsed laser beam inside the vacuum chamber to strike on the target material for layer wise deposition of amorphous thin film and annealing the deposited layer by irradiating excimer laser from a light emitting unit to said light emitting unit and repeating the focussing of pulsed laser beam for layer deposition and annealing the layer to obtain the ferroelectric thin film of desired thickness. Said ferroelectric thin film is crystalline and is useful in fabricating tunable microwave devices.
In another embodiment, the present invention provides a system for crystallization of ferroelectric thin film comprises of a vacuum chamber, a pump for creating vacuum in said chamber, a target placed at a position in the chamber, a laser emitting source for emitting excimer laser at said thin film deposited from target material for crystallization, a mirror for concentrating said laser on target unit; and a heating unit for maintaining temperature of said vacuum chamber at predefined value during crystallization.
Here, the target material includes but is not limited to barium strontium titanate (BST). A layer of target material is first deposited and then annealed by irradiating predefined amount of Kr-F excimer laser of predefined wavelength at a predefined temperature. The layer wise deposition and annealing continues till a vertically crystallized ferroelectric thin film of predefined thickness is obtained.

BRIEF DESCRIPTION OF THE DRAWING
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying images in which:
Figure 1 shows a schematic diagram of the system for crystallization of ferroelectric thin film.
Figure 2(a) shows a schematic diagram of cross-section of varactor fabricated in accordance with the present invention.
Figure 2(b) shows a top view of the varactor fabricated in accordance with the present invention.
Figure 3(i) shows XRD pattern of thin film deposited at 300ºC and 700ºC on fused silica.
Figure 3(ii) shows XRD pattern of thin film deposited layer-by-layer after laser annealing at 300oC on fused silica and platinized silicon.
Figures 4(i) to 4(iii) shows (a) capacitance and (b) dielectric constant of Ba0.5Sr0.5TiO3 (BST5) thin film deposited at (i) 300ºC (ADF 300C PS) (ii) 700ºC (ADF 700C PS) (iii) Layer by layer deposited and laser annealed at 300ºC on platinized silicon substrates (LLD 300C PS).
Figure 5(a) to 5(b) shows measured frequency spectrum of HBAR with and without biasing, S11 in broad band 500 MHz - 3 GHz and 2 GHz - 2.1 GHz in a narrow band for conventional deposited at ADF 700PCS.

Figure 5(c) to 5(d) shows measured frequency spectrum of HBAR with and without biasing, S11 in broad band 500 MHz – 3 GHz and 2 GHz - 2.1 GHz in a narrow band for LLD 300PCS.

DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be described in detail hereinafter with reference to the accompanying drawings in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiment set forth herein. Rather, the embodiment is provided so that this disclosure will be thorough, and will fully convey the scope of the invention to those skilled in the art.
The present invention discloses fabrication of crystalline Ba0.5Sr0.5TiO3 (BST5) thin films at lower temperatures using excimer laser annealing technique by PLD process.
In an embodiment, the present invention provides a method for crystallization of ferroelectric thin film comprising the steps of (a) placing a target of ferroelectric material at a position (3) in a chamber (1) for deposition and a substrate for laser annealing; (b) converging a laser emitting source (4) onto the chamber (1) to strike on target of the ferroelectric material placed at position (3) for vaporizing the ferroelectric material for deposition of a layer of amorphous thin film over a substrate, placed near to the target; (c) placing a substrate having the deposited amorphous thin film of the ferroelectric material at position (3) inside the chamber (1) for crystallization; (d) annealing the layer of thin film as obtained in step (b) by irradiating an excimer laser of predefined wavelength at an elevated temperature of chamber (1) from the laser emitting source (4) to substrate placed at position (3) by spreading the beam; and (d) repeating step (b) to (d) until the crystallized ferroelectric film of predefined thickness is obtained. Said ferroelectric thin film is crystalline and is useful in fabricating tunable microwave devices. Here, the elevated temperature is 300ºC or below and the predefined wavelength of excimer laser is preferably 248 nm. Said predefined thickness of ferroelectric thin film is 600 nm and said crystalline ferroelectric thin film shows a microwave tunabilty of approximately 34% at 1 GHz.
In another embodiment, the present invention provides a system for crystallization of ferroelectric thin film comprises of a chamber, a pump for creating vacuum in said chamber, a target with a film forming material, a predefined target position to place target material for deposition and substrate with deposited layer of target material for laser annealing in said chamber; a laser emitting source for emitting excimer laser beam at said target unit for crystallization, a mirror for directing said laser on target unit; a heating unit for maintaining an elevated temperature inside said chamber at predefined value during crystallization; and a quartz window to allow said excimer laser beam to pass through inside said chamber.
Here, the target material includes but is not limited to barium strontium titanate (BST). A layer of target material is first deposited and then annealed by irradiating Kr-F excimer laser of predefined wavelength at a predefined temperature. The layer wise deposition and annealing continues till a vertically crystallized ferroelectric thin film of predefined thickness is obtained.
Referring to Figure 1, discloses a laser annealing system for fabricating thin films. The system mainly comprises of an excimer laser emitting source (4), mirror (5), and vacuum chamber (1). The laser (Kr-F, ? = 248 nm) as a source, and the mirror (5) is positioned to direct the laser on the ferroelectric thin films inside the vacuum chamber (1).
Excimer laser: The Kr-F laser (? = 248 nm) is used as a laser emitting source (4) for annealing.
Mirror: Mirror (5) is used to bend and direct the laser beam on the ferroelectric thin films inside the vacuum chamber (1).
Chamber (1): PLD Chamber, in its usual form, has a substrate heating unit (6) with temperature controller and attachments like pressure gauges, pumps, etc. These help in maintaining sample purity and control over the atmosphere during the crystallization process.
Position (3): a predefined position (3) to place target of the ferroelectric material for deposition and substrate with deposited layer of ferroelectric material for laser annealing in said chamber (1);
Pump (2): a pump (2) is used to create a vacuum inside the chamber (1).
Heating unit (6): a heating unit (6) is used for the ferroelectric thin films to keep at desired elevated temperatures below 300°C inside the vacuum chamber (1).
Quartz window (7): a quartz window (7) is attached to the chamber (1) and laser beam passing inside the chamber (1) through this window. Quartz material is stronger than glass, and is useful at temperatures up to 1050°C and has high average transmission.
This chamber (1) is kept at desired elevated temperatures below 300°C. Thin films of various materials are directly exposed to the incoming laser with or without diverging to get the required energy density.
The chamber (1) is rotatable in order to heat the substrate including but limited to amorphous fused silica, platinized silicon (Pt/Si) (111), platinum-coated fused silica, double-side polished platinum-coated silicon and platinum-coated sapphire in controlled atmosphere since the nucleation of crystalline phases are significantly affected by incident laser energy density, repetition rate and the number of shots used for irradiation. The deposition of the films was carried out at 300ºC temperature and 8×10-3 mbar oxygen pressure that were used for laser annealing later.

EXAMPLE 1
Characterization of ferroelectric thin films
The BST5 films were deposited on amorphous fused silica and platinized silicon Pt/Si (111) and double-side polished platinum coated silicon (PCS) substrates by pulsed laser deposition (PLD) method. Here, the 20 ns pulse width KrF Excimer laser of wavelength 248 nm (Coherent-Complex Pro 102 F) with a 5 Hz repetition rate is used. The deposition of BST5 films are carried out in a spherical chamber (Excel Instruments, India). Initially, the chamber was evacuated to 5×10-6 mbar using Turbo and backing pumps.
Table 1: The conditions of films grown by layer by layer deposition and subsequent annealing process at the same temperature (300?C).

Table 1
Conditions of films grown by layer by layer deposition and subsequent annealing process at the same temperature (300?C)
BST5 thin films Deposition conditions Annealing conditions
Temp. (?C) Rep. Rate (Hz) Energy (J/cm2) No. of pulses Temp (?C) Rep. Rate (Hz) Energy (mJ/cm2) No. of pulses
300 5 2 2000 300 10 66 2000

The in-situ annealing was carried out in the presence of oxygen for 30 min after deposition. The thickness of the BST5 thin films for each layer is around 120 nm. Both the deposition and Laser annealing of BST5 thin films have been carried out in the presence of oxygen working pressure of 8×10-3 mbar. This same process of layer-by-layer deposition and subsequent annealing was repeated for each deposition layer to achieve full crystallization.

EXAMPLE 2
Tunable Capacitor and Fabrication of Varactor
Referring to Figure 2(a) and 2(b) shows the Schematic (cross-section) and (b) the top view of the varactor. It had a configuration Au/BST/Pt/Si as shown in Figure 2(a). Circular patch capacitor (CPC) as shown in Figure 2(b) was chosen to design the MIM (metal-insulator-metal) capacitor structure. The outer circular patch establishes virtual grounding with the bottom electrode when the DC bias is applied as the area of the patch is large. The CPC structure directly facilitates on wafer probing.
The active ferroelectric thin film layer, i.e. BST5 was deposited on the Pt/Si substrate using the pulsed laser deposition (PLD) system. Further, photoresist is spin coated and patterned using photolithographic techniques. Then, by using RF sputtering Au film is deposited upon the sample. Finally, by lift-off the required pattern of the capacitor (CPC) top electrode is obtained.
Results: Phase and structural properties:
Figure 3(i) and 3(ii) disclose phase and structural properties of the films studied by X-ray diffraction (GI-XRD-Bruker D8 Discover). Figure 3(i) and (ii) shows XRD pattern of films deposited at 300ºC, 700ºC as well as films deposited layer-by-layer and laser annealed at 300oC. The XRD of BST5 thin films deposited at 700oC exhibit polycrystalline nature and films that are deposited at 300oC shows amorphous nature as shown in Figure 3(i). Further, Figure 3(ii) shows layer-by-layer deposited and laser annealed BST5 thin films at 300oC on amorphous fused silica and platinized silicon substrates. The XRD pattern of films deposited at 700oC as well as layer-by-layer deposited and laser annealed at 300oC both matches well with the standard data, JCPDS card no (file # 39-1395) and is indexed to a cubic structure.
Further, the present invention discloses microwave dielectric response of the Ba0.5Sr0.5TiO3 (BST5) thin films deposited at processing temperature of 300oC, 700oC along with films that are layer-by-layer deposited and laser annealed at 300oC.
Figure 4(i) - 4(iii), shows capacitance & dielectric constant of BST5 thin films deposited at 300ºC, 700ºC as well as layer by layer deposited and laser annealed at 300ºC on platinized silicon substrates. Figure 4(i) shows deposition of thin film at a temperature of 300ºC without laser annealing. The measurement is done in microwave frequency range (0.5 to 4 GHz) while varying the voltages. The BST thin films show almost no tunability with variation of voltages. This is the typical property of the amorphous ferroelectric thin films.
Figure 4(ii) shows BST5 thin films deposition by using conventional annealing technique at a temperature of 700oC (higher temperature deposition, no laser annealing). The measurement is done in microwave frequency range (0.5 to 4 GHz) while varying the DC bias voltages. The high temperature deposited BST thin films show higher tunabilty which changes as the voltage increases. This change in tunability with voltages is a typical nature of the crystalline ferroelectric thin films.
Figure 4(iii) shows layer by laser deposition and laser annealing of Ba0.5Sr0.5TiO3 (BST5) thin films on platinized silicon substrate for fabrication of thin film at a low temperature of 300ºC. The measurements were performed in microwave frequency range (0.5 to 4 GHz) at different DC bias voltages. The capacitance of BST5 thin films changes as a function of voltage and show the high tunabilty at the highest applied voltage. After laser annealing, the change in capacitance with voltage shows the laser induced crystallinity in BST5 thin films.
The Ba0.5Sr0.5TiO3 thin films have a very high dielectric permittivity and their strong dependence on external DC bias provides an additional feature called tunability defined as given in equation (A):
Tunability (%) = (A)
and are the dielectric capacitance values at zero and non-zero dc electric bias field respectively. Using equation (A), the microwave tunability is calculated at a frequency of 1GHz and given in Table 2 for films deposited on platinized silicon substrates.
Table 2 in accordance with Figure 3(i) and 3(ii) compares microwave dielectric properties of BST5 thin films prepared by conventional methods and by laser annealing after layer-by-layer deposition.
Table 2
Comparative analysis
Ba0.5Sr0.5TiO3 (BST5) thin film
Prepared by:
Microwave tunability at
1 GHz
(i) As-deposited at 300oC (conventional- ADF 300C PS) No response
(ii) As-deposited at 700oC (conventional- ADF 700C PS) 54%
(iii) Layer-by-layer deposited and laser annealed at 300oC on platinized silicon
(LLD 300C PS) 34%

The present invention discloses a system and method to fabricate the crystalline Ba0.5Sr0.5TiO3 (BST5) thin films at lower temperatures using excimer laser annealing technique by PLD process.

EXAMPLE 3
High overtone bulk acoustic resonator (HBAR)
High overtone bulk acoustic wave resonator (HBAR) are realized with piezoelectric thin film sandwiched between two electrodes supported by a substrate having low acoustic attenuation. HBAR has vast application in designing filters and even sensors of various type by giving an added advantage of tunability and flexibility.
Microwave measurements of the fabricated HBAR were done using an on-wafer probe station with a 250 µm pitched Ground-Signal-Ground probe. Calibration was done with standard open, short and load. The one port S-parameters (S11) were measured for different frequency ranges and with varying dc bias voltages. The absolute of the complex input impedance extracted from the measured reflection coefficient has been plotted from the frequency range of 500 MHz to 3 GHz with and without biasing as shown in Figure 5(a) to 5(d) for both the BST5 as-deposited film at 700ºC (ADF 700 PCS) and layer-by-layer deposited and laser annealed at 300ºC on platinum coated silicon (LLD 300 PCS) cases. It is clear from this figure that the LLD 300 PCS is responding to the bias applied and acts like a piezoelectric thin film due to electrostriction as in the case of ADF 700 PCS. This induced piezoelectric in the film translates as resonances in the frequency spectrum, and multiple resonance peak occur due to creation of standing waves inside the double side polished silicon substrate. From this result we can infer that, such laser annealed processed films can be useful for designing other acoustic resonators like film bulk acoustic resonator, solidly mounted resonators in addition to HBAR. These devices have vast applications in designing filters and even sensors of various type by giving an added advantage of tunability and flexibility.
The BST5 thin films were fully crystallized by layer-by-layer deposition and laser annealing at 300oC. In this process, one layer of ~120 nm BST thin films were deposited and subsequently annealed by using high and low intensity of the laser. The temperature of 300oC was kept constant for both the processes. The phase formation and full vertical crystallization of BST5 thin films was confirmed by XRD patterns, Raman and UV-Vis-NIR study as well as by cross sectional SEM. The band gap values show a systematic decrease after each laser annealing step. These layer by layer deposited and laser annealed BST5 thin films show the microwave tunability of 34% at 1 GHz, which is close to the tunability shown by conventionally deposited BST5 films (at 700oC) and useful for fabricating ferroelectric thin film based tunable devices like microwave varactors and resonators at low temperatures thereby making the method compatible with polymers and flexible electronics.
For a better understanding, below is glossary of terms and their definitions: Ferroelectrics: A ferroelectric material is a non-linear dielectric that exhibits a remanent polarization in the absence of an external electric field and an applied electric field can switch its direction.
Pulsed laser deposition (PLD): Pulsed laser deposition is a thin film deposition technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target (in a plasma plume) which deposits it as a thin film on a substrate.
Barium Strontium Titanate thin films (BST): Barium Strontium Titanate (BST) ceramics undergoes a ferroelectric to paraelectric phase transition at room temperature. In the paraelectric phase, BST shows a high voltage-dependent dielectric constant (termed as tunability) and relatively low dielectric loss at microwave frequencies.
Laser crystallization: Laser Induced Crystallization (LIC) is the phase transformation of materials from amorphous to crystalline phase at a lower temperature using a laser of high-power density and short pulse width.
Laser Annealing: It is a method by which a material is made to undergo an amorphous to crystalline phase transition under the influence of an irradiated laser beam. It is done after the film is formed.
Tunable microwave devices: Some ferroelectric materials show a voltage dependent dielectric constant even at microwave frequencies with low loss. It results in variable electrical length of a transmission line fabricated over this film which is DC bias field dependent. Using such films microwave devices whose properties can be altered by applying a DC voltage can be made.
Flexible Electronics: So far electronic devices are fabricated on Silicon substrates which are inflexible. The next wave of Electronics that is coming up is achieving electronic devices on substrates like polymers which are flexible, wearable and conformal.
The present invention reports the laser induced low temperature crystallization of whole ferroelectric thin film (~600 nm) by layer by layer deposition of each layer of 120 nm thick thin film and subsequent laser annealing of each layer to obtain the full crystallization of whole films of final thickness of ~600 nm (repeated 5 times). Thus, by this method fully vertical crystallized films of desired thicknesses is obtained.
Therefore, the present invention is useful for fabricating ferroelectric thin film based tunable devices like microwave varactors and resonators at low temperatures thereby making the method compatible with polymers and flexible electronics.
Many modifications and other embodiments of the invention set forth herein will readily occur to one skilled in the art to which the invention pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
,CLAIMS:CLAIMS

We claim:
1. A method for the crystallization of ferroelectric thin film comprising the following steps:

(a) placing a target of the ferroelectric material at a position (3) in a chamber (1) for deposition and a substrate for laser annealing;

(b) converging a laser emitting source (4) onto the chamber (1) to strike on target of the ferroelectric material placed at position (3) for vaporizing the ferroelectric material for deposition of a layer of amorphous thin film over a substrate, placed near to the target;

(c) placing a substrate having the deposited amorphous thin film of the ferroelectric material at position (3) inside the chamber (1) for crystallization;

(d) annealing the layer of thin film as obtained in step (b) by irradiating an excimer laser of predefined wavelength at an elevated temperature of chamber (1) from the laser emitting source (4) to substrate placed at position (3) by spreading the beam; and

(e) repetition of step (b) to (d) is carried out, until the crystallized ferroelectric film of predefined thickness is obtained;

wherein;
said elevated temperature is 300ºC or below;

said predefined thickness of ferroelectric thin film is 600 nm; and

said crystalline ferroelectric thin film shows a microwave tunabilty of approximately 34% at 1 GHz.

2. The method as claimed in claim 1, wherein said target material is including but not limited to barium strontium titanate.

3. The method as claimed in claim 1, wherein said substrate includes but is not limited to amorphous fused silica, platinized silicon (Pt/Si) (111), platinum-coated fused silica, double-side polished platinum-coated silicon and platinum-coated sapphire.

4. The method as claimed in claim 1, wherein said predefined wavelength of excimer laser is preferably 248 nm.

5. A system for crystallization of ferroelectric thin film comprising of:

a) a chamber (1) having a substrate and a substrate holder;

b) a pump (2) for creating vacuum in said chamber (1);

c) a predefined position (3) to place target of the ferroelectric material for deposition and substrate with deposited layer of ferroelectric material for laser annealing in said chamber (1);

d) a laser emitting source (4) for emitting an excimer laser beam at said position (3) for crystallization;

e) a mirror (5) for directing said excimer laser beam on said position (3);

f) a heating unit (6) for maintaining an elevated temperature inside said chamber (1); and

g) a quartz window (7) to allow said excimer laser beam to pass through inside said chamber (1);
wherein,
said film forming material includes but is not limited to barium strontium titanate;
said film forming material is layer wise deposited and annealed by irradiating excimer laser of predefined wavelength to obtain vertical crystallization of ferroelectric thin film of desired thickness;
said elevated temperature is 300ºC or below; and
said crystalline ferroelectric thin film shows a microwave tunabilty of approximately 34% at 1 GHz.
6. The system as claimed in claim 5, wherein said substrate includes but is not limited to amorphous fused silica, platinized silicon (Pt/Si) (111), platinum-coated fused silica, double-side polished platinum-coated silicon and platinum-coated sapphire.
7. The system as claimed in claim 5, wherein said chamber (1) is rotatable for heating the substrate in controlled atmosphere.