|Title of Invention||
A PROCESS FOR DEPOSITING ELECTRONIC CERAMIC OXIDE THIN FILMS WITH CONTROLLED OXYGEN STOICHIOMETRY ON DIFFERENT SUBSTRATE MATERIALS
|Abstract||1. A process for coating electronic ceramic oxide thin films of materials on substrates such as, Si, Si with buffered layers (Pt/TiCVSiCVSi), lanthunum aluminate, platinized MgO and ytteria stabilized zirconia comprising, (a), providing a single phase pure film forming material in a dense disc form at the target position for laser ablation and the said substrate at appropriate distance from the said target in a pulse laser deposition chamber; (b). depositing a thin film of the said film forming material on the said substrate kept in the said pulse laser deposition chamber maintaining the said substrate temperature at 400 - 750 °C and oxygen at 0.1 to 1.0 torr; (c). flooding the said pulse laser deposition chamber with oxygen and allowing to cool it to room temperature on attaining the required thickness of the said thin film.|
|Full Text||FORM 2
THE PATENTS ACT 1970
(See Section 10)
Tata Institute of Fundamental Research,
Homi Bhabha Road, Colaba, Mumbai 400 005, Maharashtra, India.
An aided autonomous institution under the administrative purview of the
Department of Atomic Energy, Government of India,
Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg,
Mumbai 400 005, Maharashtra, India.
The following specification particularly describes the nature of the invention and the
manner in which it is to be performed
This invention relates to a process for the manufacture of ceramic oxide thin films with controlled oxygen stoichiometry. This invention particularly relates to a process for the manufacture of electronic ceramic oxide thin films of materials such as, ferroelectrics (like BiFe03, PbTi03, PbTixZrx.]03), colossal magneto resistance materials (like LaMnCb with different additives) and high Tc oxide superconductors (like YBa2Cu307, LaxSri.xCu045 Bi2CaSr2Cu208 etc.) where oxygen stoichiometry controls the physical properties.
The present invention more particularly relates to a process for the manufacture of thin films of ferroelectric bismuth-iron-oxide (BiFe03) on Si with buffered layers (Pt/Ti02/Si02/Si) substrate by Pulsed Laser Deposition (PLD) technique. The BiFe03 thin films prepared according to the invention may have diverse uses such as in non-volatile memories, piezoelectric and pyroelectric sensors and significantly as ethanol sensor with high stability and selectively.
Background and Prior Art:
Ferroelectric thin films are widely used in memory devices, pyroelectric and piezoelectric sensors etc. In the modern electronic industry there are applications which require alteration of magnetic and electric field in a material. The materials also need to withstand high temperatures during device operations in certain cases.
BiFeC>3 is claimed to have coexistence of antiferromagnetic and ferroelectric ordering with ferroelctric Tc as high as 830°C. [S. V. Kiselev, R. P. Ozerov and G. S. Zhdanov, Soviet Physics Dokl. 7, 742 (1963) C. and Tabares-Munoz, J. P. Rivera, A. Monnier and H. Schmid, Jp. J. Appl. Phys. 24, 1051 (1985). Though ferroelectric / ferroelastic single domain was observed by polarized light microscopy, the possibility of observing a ferroelectric hysteresis loop at room temperature was difficult due to the high conductivity of the sample.
Teague et.al. [J. R. Teague, R. Gerson and W. J. James, Solid State Communications 8, 1073 (1970)] carried out hysteresis loop measurements on a single crystal of BiFe03 at 80 K in order to enhance its resistivity. They could not observe saturation of the loop at fields as high as 55kV/cm, other than a loop with a spontaneous polarization of 3.5 u£/cm' in the
(100) direction. This indicates only a partial alignment of the ferroelectric domains in the sample.
Synthesizing a single phase BiFeO3 bulk sample is a difficult task because of the narrow temperature range in which BiFe03 stabilizes. Number of other phases of Bi and Fe appear if the gradient in the temperature is not accurate. S. Komarnani, V. C. Menon, Q. H. Li, R. Roy and F. Ainger have reported in J. Am. Ceram. Soc. 79, 1409 (1990) synthesis of single phase BiFe03 by microwave-hydrothermal technique. However, the process is somewhat complicated. A recent report originating from work at TIFR describes a simple process for the synthesis of single phase BiFeCh by oxide mixing technique. [M. Mahesh Kumar, V. R. Palkar, K. Srinivas and S. V. Suryanarayana, Appl. Phys. Lett. 76 (19) 1, 2000)]. In this process Fe203 and B12O3 powders are mixed in stoichiometric proportions and calcined in two stages. First at -650 °C and after grinding, calcined once again at -810 °C/1 hr. The material is then leached with dilute nitric acid to eliminate the impurity phase having different composition of Bi and Fe like BiaFe4 or unreacted Bismuth oxide. Though the bulk samples of the BiFe03, thus prepared by using either of the techniques described above, are single phase to XRD, ferroelectric hysteresis loop measurements are not possible since samples are highly conducting. This high conductivity of the samples is assumed to be arising due to very small variation in oxygen content. Both the cations (Bi & Fe) present in the BiFeCb molecule are multivalent and the non-stoichiometry of oxygen content can lead to valance fluctuation. Consequently it can give rise to hole or electron carriers. It has been observed that annealing of BiFeCh bulk sample in oxygen atmosphere during thermal processing also fails to maintain exact oxygen stoichiometry. As a result, no ferroelectric hysteresis loop of BiFe03 has been yet reported in the literature.
R. T. Smith, G. D. Achenbach, R. Gerson and W. J. James, J. Appl. Phys. 39, 70 (1968), have reported that the formation of a solid solution of BiFe03 with different perovskite like PbTi03, LaA103 etc. helps to control the formation of impurity phases and also the conductivity of the samples. However, the ferroelectric transition temperature of this solid solution is less than that of the BiFe03. Moreover, the preparation of solid solution increases the process steps since parent compounds are to be synthesized initially.
M. Polomska, W. Kaczmarek and Z. Pajek, Phys. Stat. Sol. (a), 23, 567 (1974) have studied electric and magnetic properties of bulk samples of (Bii_xLax)Fe03. They could
observe the ferroelectric hysteresis loop at room temperature for the solid solutions where value of x is in the range of 0.15 - 0.19.
T. Fujji, S. Jinzenji, Y. Asahara, A. Kijima and T. Shino, J. Appl. Phys. 64(10), 5434 (1988) have synthesized thin films of solid solutions of BiFe03 with other perovskite (BaTi03, PbTiC>3, Pb(Tii.xZrx)03) by sputtering. Though magnetic properties of these solid solutions are studied, no ferroelectric measurements have been reported on the films. Moreover, the paper does not give any information about the conductivity of the films.
In US patent 6162293 (2000) a method for manufacturing ferroelectric thin film of Bi layered perovskites such as bismuth titanate (B14T13O12) has been described. The methodology, however, is based on deposition of the gaseous film material on the surface of a substrate such as that of a silicon single crystal wafer, by flowing one stream of starting material for Bi and another for Ti on the carrier gas argon and oxygen gas as reaction gas in a film formation chamber. The flow of oxygen gas is controlled to the value necessary for the formation of the ferroelectric thin film having a desired orientation.
In US patent 5766697(1998) a method of making a thin film of a ferroelectric composite
material using a pulsed laser deposition technique has been described. In this patent, thin
film formation method is restricted to Bai_xSrxTi03 in combination with additives like,
alumina, magnesia, silica and zirconia, has been prepared The composites are used as
target for ablation. The aim is to form a thin film composite having enhanced electronic
The method comprises the following steps,
• providing a ceramic disc of said composition as an ablation target;
• providing substrate (commercially available) at temperature between 400-700 °C;
• depositing a thin film of said composition on said substrate using a krypton excimer
pulsed laser (193 - 248 nm; 226 - 350 m/j pulse; 10-20 ns pulse width at 5 - 25 Hz pulse
Requirement of oxygen though not claimed, oxygen backfill pressure of the order of 50 mT -100 mT has been used. Moreover, no claims have been made to suggest a process to prepare oxide thin films with controlled oxygen stiochiometry.
For the monolithic device fabrication in the present day microelectronic circuits, growth of the ferroelectric thin film on Si or Si based (Pt/TKVSiCVSi) substrate is highly desirable. Commonly accepted criterion, as evidence of ferroelectricity is the presence of hysteresis loop. However, earlier attempts to observe ferroelectric hysteresis loop in bulk or thin films of BiFeCh have been failed due to lossy nature of the samples. High conductivity arising due to non-stoichiometric oxygen content in the sample leads to high loss factor. It is very difficult to control stoichiometry of multi-cations systems while growing the thin films by the process of sputtering, whereas there is a scope to control the stoichiometry in pulse laser deposition process. The pulsed laser deposition is a highly energetic process and could be carried out in oxygen rich ambient. Hence it was thought that under optimized process conditions it could be possible to control oxygen stoichiometry in the films.
Ethanol sensor with high stability and selectivity has tremendous application potential as a device in traffic management, food ferment, wine making and medical process. The merit of the sensor is judged by its sensitivity, selectivity, response and recovery time.
Colorimetric indicators are commonly used for ethanol detection in breath gas. The indicators are generally composed of compounds of cobalt, nickel or iron and polyalkylpolyamine, amino acids or salen type legands. The manufacturing process has been patented by De Castro et.al. (De Castro, S. Emory, D. R. Malat , US Patent application No.758557,November 29, 1996). However, it has been observed that though the indicators are simple to use, the sensitivity is poor and unreliable.
Swette et.al. (L.L.Swette, A. E. Griffith, A.B. LaConti have patented a method for continuous tracking of blood alcohol which is comprised of a potential and diffusion controlled electro-chemical solid polymer electrolyte sensor (US Patent 5944661)
Semiconductor gas sensors offer the potential for developing portable, rugged and inexpensive gas detection instrumentation. Therefore it has become a major activity to develop alcohol sensor based on change of the resistance of a semiconductor in air by alcohol vapor. Conventional sensors based on materials like, SnO2, y-Fe203 etc. though have higher sensitivity, lack in selectivity
(T. Maekawa, J. Tamaki, N. Miura and N. Yamazoe, 9,61, (1992),
S.R.Morrison, Sensors and Actuators 2,329 (1982).
N. Yamazoe, Y.Kurakowa, T. Seiyama Sensors and actuators 4,283 (1983)
O. K. Varghese, L. K. Malhotra and G. L. Sharma, Sensors and Actuators B 55, 161 (1999)
Weber et.al. have determined correlation between microstructure and ethanol sensing behavior of system. It has been reported that 400 ppm of ethanol could be sensed at 200°C. (I. T.Waber, R. Andrade, E. R. Leite and E.Longo, Sensors and Actuators B, 72,180 (2001).
LaFe03 is another material which is reported to be ethanol sensitive (J.Wu, L.Sun, J.
Zhang, Y. S. Ji and B. H.Guo J. Transducer Tech. 1(6),1,(1988)
H. Obayoshi, Y. Sakurai and T. Gejo J. Solid State Chem. 17,299 (1976)
Though sensitivity and selectivity of LaFe03 has been observed to be convenient for practical use, the shortcoming is long response and recovery time. Huo et.al (H. Suo, J.Wang, F. Wu, G. Liu, B. Xu and M.Zhao J. Solid State Chem. 130, 152 (1997) have reported that these factors could be improved by using Sr doped LaFeC>3 Suo et.al. have tried further improvement in sensor properties by using nanocrystalline materials. (H. Suo, F.Wu, Q. Wang, G. Liu, F. Qiu. B.Xu and M.Zhao, Sensors and Actuators B 45, 245 (1997) Sol-gel derived thin films of Sr doped LaFeC>3 are reported to act as an effective ethanol sensor at 120 °C by Zhao et.al. (S.Zao, J.Kin,B. Xu, M. Zhao, Z. Peng and H Cai, Sensors and Actators B 64, 84 (2000)
To our knowledge, the operating temperature for the ethanol sensors reported in the published literature and patents, is above room temperature (120-300 °C). High temperature requirement for sensing ethanol complicates the device fabrication as heating arrangement need to be installed in the gadget.
Object of the Invention:
The main object of the present invention is to develop a process for manufacturing highly resistive, single phase thin films of ferroelectric bismuth-iron-oxide (BiFeC^) with controlled oxygen stoichiometry on Si or Si based substrate.
A further object of the invention is to provide pure and highly resistive BiFeOs being adapted for use as ethanol sensor.
Summary of the Invention:
Accordingly the present invention provides a process for manufacturing electronic ceramic oxide thin films of materials such as, Si & Si with buffered layers (Pt/TiCVSKVSi), lanthunum aluminate, platinized MgO, ytteria stabilized zirconia etc. comprising,
(a), providing a single phase pure film forming material in a dense disc form at the target position for laser ablation and the said substrate at appropriate distance from the said target in a pulse laser deposition chamber;
(b). depositing a thin film of the said film forming material on the said substrate kept in the said pulse laser deposition chamber maintaining said substrate temperature at 400 - 750 °C and oxygen at 0.1 to 1.0 torr;
(c). flooding the said pulse laser deposition chamber with oxygen and allowing to cool it to room temperature on attaining the required thickness of the said thin film.
The applicants have found that BiFe03 could ideally function efficiently as a selective sensor for ethanol, at room temperature. At the same time it is quick in, reversibility and reproducibility and response time. BiFeCh thin films are highly resistive since Fe is maintained at +3 oxidation state by precisely controlled oxygen pressure in the chamber during growth process when ethanol vapors come in contact with the surface of the film, probability of redox reaction occurring between Fe+3 and ethanol is very strong. If alcohol oxidizes to aldehyde, Fe+3 could reduce to Fe+2 state leading to increase in conductivity.
The change in conductivity if calibrated against ethanol vapor concentration, BiFeC>3 can be used as an ethanol sensor.
According to a preferred aspect the BiFeCh thin film obtained by the aforesaid steps is mounted on the holder as shown in sensor assembly (Figure 6);
the initial two probe Resistance (Ro) of the film is determined using pressure contacts made on silver electrode pads with the help of multimeter;
the film is then tested as a sensor for variety of reagents and gases using the arrangement shown in figure 6. The change in resistance (R) is detected when film came in contact with different gases, vapors of different volatile organic solvents carried by nitrogen gas.
According to a further preferred aspect of the invention, BiFeC}? thin films for use as ethanol sensor is selected from materials capable of undergoing redox reaction and resulting into change in resistivity.
Description of the Preferred Embodiment of the Invention:
The film forming material used in the process of the present invention is selected from ferroelectrics, colossal magneto resistance materials and high Tc oxide superconductors wherein oxygen stoichiometry controls the physical properties. The ferroelectrics is selected from BiFe03, PbTi03 and PbTixZrx-i03 and the like. The colossal magneto resistance materials is selected from LaMn03 and the like with different additives. The high Tc oxide superconductor is selected from YBa2Cu3C7, LaxSri.xCu04 and Bi2CaSr2Cu208 and the like.
A preferred embodiment of the invention is when the film-forming material is pure BiFeO3 and substrate is (Pt/TiO/VSiO2/Si). The preferred embodiment of the process of the invention is described below:
Preparation of phase pure BiFeO3:
A powder sample of phase pure BiFe03 was prepared as described by M. Mahesh Kumar
et al. High purity (99.9 %) B12O3 and Fe203 are carefully weighed in stoichiometric proportions (1:1 mole ratio) jind thoroughly mixed in agate and mortar for about half an hour using high purity volatile organic solvent like alcohol, isopropyl alcohol, acetone etc. as a medium. The powder-mi* is first calcined at 610 - 650 °C for 1-4 hours in alumina or platinum crucible. The calcined material is ground thoroughly and calcined again at 800 (±10)°C for 1-2 hours. The two stage calcined sample is allowed to cool to room temperature and then leached with dilute nitric acid (10-20%) to eliminate impurity phases. The nitric acid is then thoroughly washed off and powder sample is dried for further use.
Preparation of BiFe03 Target Pellet:
The powder sample is then compacted into disc of the size suitable for the target for laser ablation (-0.75" - 1" diameter) under high pressure of the order of 1-2 tons/cm2. The pellets are sintered at 800 (±10)°C /1- 4 hr. to obtain dense target. The phase purity of the sintered pellet is checked by X-ray diffraction (XRD) prior to its use as a target for ablation to grow thin films. XRD patterns of BiFe03, before and after leaching are shown in Example I, Fig. 2. Fig. 2 also includes simulated pattern for BiFeOs for comparison.
Preparation of Thin Films of BiFe03:
BiFe03 target and substrate are mounted in the laser ablation chamber (as shown in Example I, Fig.l). Substrates are chosen from commercially available materials like Si, Pt/TiCySKVSi, LaA103; MgO, ytteria stabilized zirconia etc. Target to substrate distance is maintained in the range of 3 - 8 cm.
The target is scanned by using a stepper motor controlled target scanner to avoid local heating. Ablation is carried out in oxygen ambient at 0.1 -1 torr. The substrate temperature is maintained in the range of 400 - 750 °C controlled by an Eurotherm P808 controller. 195 - 306 nm UV range excimer laser pulses, with energy density in the range of 1.5 - 4 J/cm2, a pulse width of 25 ns and repetition rate of 1 -50 Hz are used for ablation process. During the process, BiFeCh is ablated from the target and deposited on the substrate and films with nominal thickness of 1000 - 4000 A are obtained by keeping the growth time between 10 - 60 min. At the termination of the ablation, the chamber is flooded with oxygen and the films are allowed to cool at the normal rate by switching off the heater.
Characterization of the Films :
The phase purity and orientation of the films is checked by x-ray diffractometer (XRD) using Cu (Ka) radiation (as shown in Example I, Figure 3).
Bi/Fe ratio is determined by energy despersive X-ray (EDX) analysis.
Films are observed under Scanning Electron Microscope (SEM) to determine microstractural homogeneity and continuity of the film (as shown in Example I, Fig. 4).
To carry out ferroelectric measurements, capacitors are made using parallel plate geometry. Pt is used as bottom electrode while top electrode is silver coated by thermal evaporation. Circular dots of 50-100 urn diameter are patterned by standard photolithography followed by etching of the silver film.
Saturation and remnant polarization (Ps and Pr, respectively) values, and resistivity of the film are determined with the help of ferroelectric loop tracer, RT66A Radiant Technology, USA (as shown in Example I, Fig. 5).
A preferred embodiment of the invention is when single phase, highly resistive, stiochiometric BiFe03 thin film grown on Pt/Ti02/Si02/Si substrate is used as sensor. The preferred embodiment of the process of invention is described below:
Preparation of BiFeO3 thin film
Phase pure BiFe03 thin film with controlled oxygen stiochiometry is grown on Pt/TiO2/SiO2/Si substrate using PLD technique described above. The film obtained is characterized by high resistance.
The film is then mounted on sample holder of the testing assembly (fig.6) used for the experiment. The initial resistance (R0) of the film is noted down.
Change in resistance (R) observed when different gases and vapors of volatile liquids
came in contact with the surface of the film was noted down at regular time intervals. It gave the idea regarding sensing capacity of BiFe03 thin film for different materials.
The quantitative results obtained are shown in fig. 7, 8,9.
The present invention will now be described by way of an example. The Example is for illustration only and not to restrict the scope of invention.
Example I: Preparation of Thin Film Of Ferroelectric BiFeC>3 on Si with Buffered Layers (Pt/TKVSiCVSi) as a Substrate
i. Preparation of BiFeO3 Bulk Sample
Samples of BiFe03 were prepared by the usual solid state reaction method. High purity Bi203 (99.999% pure, Koch Chemical Ltd., England) and Fe203 (99.99% pure, Johnson Mathey Chemicals Ltd., England), were carefully weighed in stoichiometric proportions (1:1 mole ratio) and thoroughly mixed in an agate mortar for about half an hour using high purity isopropyl alcohol as a medium. The sample was pre-calcined at 650 °C for 1 hour in an alumina crucible. The calcined material was once again ground thoroughly prior to heating at 810 °C for 1 hour. Furnace with a maximum gradient of 10 °C was used for calcination. The presence of Bismuth Oxide (B12O2.75) impurity phase was detected in the XRD pattern in addition to the major BiFe03 phase (Fig. 2). Leaching with dilute nitric acid helped to eliminate the impurity phase as indicated in Fig. 2 of the accompanying drawing.
ii. Pelletization and Sintering
Pellets of 0.75" size in diameter were prepared under the pressure of the order of ~2 tons/cm using hydraulic press. Poly Vinyl Alcohol (PVA) is used as a binder. The pellets were then sintered at 800°C (±10°C) / 4hr. so as to obtain dense targets suitable for laser ablation.
iii. Depositing thin film of BiFe03 on Si with buffered layers (Pt/Ti02/Si02/Si) Substrate
The method of deposition and the system used for laser ablation process is shown in Fig.l of the accompanying drawing.
Laser energy was set to 250 mJ and 12mm x 4mm-size aperture was used. The final spot size on the target was set to 2.7mm x 0.75mm by adjusting the lens position (11). The laser energy incident on the target was 40 mJ.
Si with buffered layers (Pt/TiCVSiCVSi) substrate (obtained from Radiant Technology, USA) was cleaned and vapour degreased using trichloroethylene and mounted onto the substrate heater / holder assembly (7) using silver paste. The heater assembly was then mounted in the laser ablation / deposition chamber (1).
The target (5) was mounted in the chamber opposite to the substrate (7) and the target to substrate distance was adjusted to 5 cm. The chamber (1) was evacuated to a vacuum of ~ 10"5 torr.
The heater (7) was heated to temperature of 600 °C and oxygen was introduced in the chamber through gas inlet (15) and the pressure in the chamber was adjusted to 3.0 x 10 _1 torr by controlling the gas flow.
The target (5) was preablated to expose fresh surface. During this process a shutter (8) was used to prevent the deposition of the film on the substrate (7). After removing the shutter (7) ablation was carried out for 30 min. to deposit the thin film on the substrate (7).
At the end of the ablation the laser (13) was stopped, the vacuum pumps (3 and 4) were switched off and the chamber (1) was flooded with oxygen. Heater (7) was put off and the entire chamber (1) was allowed to cool at normal rate.
Some characteristics of the starting material and of the product obtained in the Example 1 are shown in figures 2 to 5 in which,
Fig. 2. XRD pattern of BiFe03 target used for Pulsed Laser Deposition (PLD).
Fig. 3. XRD pattern of BiFe03 thin film grown at 600°C.
Fig. 4. SEM picture of BiFe03 thin film grown at 600°C.
Fig. 5. Ferroelectric hysteresis loop for BiFe03 thin film grown at 600°C.
iv. Examination of the Thin Films
Phase and orientation of the thin film was determined by X-ray diffraction (XRD) as shown in Fig. 3 in the accompanying drawing. It indicates that the films are single phase and oriented. The continuity and texture of the thin film were studied by Scanning Electron Microscopy (SEM) as shown in Fig.4 in the accompanying drawing. The SEM picture reveals that the films are continuous and granular in texture.
Chemical homogeneity and the chemical composition .were.. determined by Energy Dispersive X-ray fluorescence (EDX). Bi/Fe atomic ratio was observed to be 1:1 as per requirement.
Ferroelectric properties were measured using ferroelelctric loop tracer (Radiant Technology, USA). Ferroelelctric hysteresis loop with saturation polarization (Ps) of ~2 uC/cm2 and remnant polarization (Pr) of -0.85 uC/cm2 obtained for BiFe03 thin film is shown in the accompanying drawing (Fig. 5).
EXAMPLE - 2
Evaluation of the performance of phase pure BiFeO3 thin film characterized by high resistance (in MOhm range)
The selected film is then mounted on the sensor holder (5) of the testing assembly shown in figure 1 and initial resistance (R0) is noted down.
Change in resistance was determined for different gases (nitrogen, oxygen, forming gas and carbon dioxide) by passing the gas from the cylinder (1) over the film. Calibrated gas flow meter (3) was used for fine control of the flow rate. Identical flow rate (0.335 It./min.) was maintained in all the cases for quantitative results useful for reliable
comparison. Resistance of the film (R) was measured at the regular interval of time (1 min.). using digital multimeter (8). Pressure contacts were made on silver contact pad (7) deposited on the film
In case of volatile solvents, (ethanol, propanol, acetone, trichloro-ethylene and water vapour) nitrogen gas was used as a carrier gas to carry the vapors. Nitrogen gas was passed at the rate of 0.3 lit./min. through a bubbler (4) containing reagent under test. Change in resistance (R) when vapors come in contact with the film surface, was measured with time. Vapor concentration of different solvents carried by the nitrogen gas at the interval of 1 minute was estimated separately in each case.
RO/R values obtained for different gases and vapors are plotted against time as well as vapor concentration to compare detection sensitivity of BiFeCh thin film. To check the reproducibility and shelf life the experiments were repeated in regular intervals for 4 months.
The results obtained in Example 2 are shown in figures 7 to 9.
Fig. 7. RO/R of BIFe03 Sensor in Contact with Different Gases and Ethanol Vapor
Fig. 8. RO/R of BiFeO3 Sensor in Contact with Solvent Vapors
Fig. 9. RO/R Vs % Vapor Concentration of Solvents in Nitrogen Gas.
Advantages of the Invention :
The process of present invention shows following advantages of growing ferroelectric BiFe03 thin films on Si with buffered layers (Pt/Ti02/Si02/Si) substrate.
The thin films prepared by the process of the present invention are highly resistive. For the first time saturated ferroelectric hysteresis loop has been observed for BiFeO3 sample. It confirms the ferroelectric nature of BiFeO3. Hence, it could find applications in non¬volatile memories, piezoelectric and pyroelectric sensors etc.
Since the Curie temperature of BiFe03 is as high as 830°C the device can withstand very high temperatures during operations.
The ferroelectric thin films prepared by the process of the present invention since grown on silicon based substrate, useful in integrating monolithic devices. As a result, external coupling of the device to microelectronic circuit could be avoided.
The thorough investigations of the physical properties of the material like BiFe03, having coexistence of electric and magnetic ordering opens up area of research, which could lead to new findings in physics.
It confirms that BiFeO3 thin films could be selectively and effectively used as a sensor for ethanol for concentration as low as 0.005 % (v/v) in nitrogen carrier gas.
As the sensor works at room temperature unlike the sensors reported in the literature, the sensing device fabrication could be simpler since no heating element will be required in the gadget.
The performance of the sensor seems to be excellent as far as the detection sensitivity, reversibility, reproducibility, and shelf life is concerned.
Industrial applications of the invention
Ethanol sensor is highly useful for traffic police to control the drunken driving. Detection
and control of alcohol consumption is necessary for the well being of the society.
It could be used for monitoring fermentation and other chemical processes in chemical
The sensor could be made at low cost since it functions at room temperature and hence has commercial value.
The coated material produced according to the present invention finds applications in preparation of non-volalile memories, piezo and pyroelectric sensors and Micro-Electro-Mechanical Systems (MEMS) Devices as well as ethanol sensors. This material has further
1. A process for coating electronic ceramic oxide thin films of materials on
substrates such as, Si, Si with buffered layers (Pt/TiCVSiCVSi), lanthunum
aluminate, platinized MgO and ytteria stabilized zirconia comprising,
(a), providing a single phase pure film forming material in a dense disc form at the
target position for laser ablation and the said substrate at appropriate distance from
the said target in a pulse laser deposition chamber;
(b). depositing a thin film of the said film forming material on the said substrate
kept in the said pulse laser deposition chamber maintaining the said substrate
temperature at 400 - 750 °C and oxygen at 0.1 to 1.0 torr;
(c). flooding the said pulse laser deposition chamber with oxygen and allowing to
cool it to room temperature on attaining the required thickness of the said thin
2. A process for coating electronic ceramic oxide thin films as claimed in claim 1, wherein the film forming material is selected from ferroelectrics, colossal magneto resistance materials and high transition temperature oxide superconductors wherein oxygen stoichiometry controls the physical properties.
3. A process for coating electronic ceramic oxide thin films as claimed in claim 2, wherein the ferroelectric is selected from BiFe03, PbTi035 PbTixZri.x03 and like.
4. A process for coating electronic ceramic oxide thin films as claimed in claim 2, wherein the colossal magneto resistance materials is LaMn03 and the like with different additives.
5. A process for coating electronic ceramic oxide thin films as claimed in claim 2, wherein the high transition temperature superconductor is selected from YBa2Cu307, LaxSri.xCu04, Bi2CaSr2Cu208 and the like.
6. A process for coating electronic ceramic oxide thin films as claimed in claim 1 to 5, wherein the said ferroelectric is pure BiFeCb and substrate is (Pt/TiCVSiCVSi).
7. A process for coating electronic ceramic oxide thin films as claimed in claim 6 wherein, BiFe03 as a dense target material is obtained from a pure, single phase BiFe03 prepared from Bi203 (99,9 % pure powder) and Fe2O3 (99.9 % pure powder) taken in 1:1 mole proportion, by through mixing the said Bi203 and the said Fe2O3 and calcining the mixture followed by nitric acid leaching to eliminate impurity.
8. A process for coating electronic ceramic oxide thin films as claimed in any claim 7 wherein, the said thorough mixing of powders of the said BI2O3 and the said Fe203 is brought about by using high purity volatile, organic reagent such as ethyl alcohol, isopropyl alcohol and acetone, as a medium.
9. A process for coating electronic ceramic oxide thin films as claimed in any claim 7 or 8 wherein, the said mixture of powders is first calcined at 650 °C for 1 hour in an alumina or platinum crucible then cooled, ground and recalcined in a furnace at 800°C (±10°C) for 1 hour with a maximum gradient of 10°C and cooling at normal furnace cooling rate;
10. A process for coating electronic ceramic oxide thin films as claimed in claim 9 wherein, calcined material is leached in dilute nitric acid (10-20 %) till XRD pattern does not show presence of impurity phase of Bismuth Oxide (Bi202.75) in BiFeC^ phase.
11. A process for coating electronic ceramic oxide thin films as claimed in claim 10 wherein, the said calcined, leached BiFeO3 powder is compacted into pellets of about 3/4 to 1 inch diameter and 2-3 mm thickness under high pressure of 1 -2 ton/cm2 in a hydraulic press, using polyvinyl alcohol (PVA) as a binder.
12. A process for coating electronic ceramic oxide thin films as claimed in claim 11, wherein, the said pellets of BiFeO3 are sintered at 800 ±10°C / 1- 4 hrs to obtain the said dense discs.
13. A process for coating electronic ceramic oxide thin films as claimed in claim 1 wherein, the said laser used for ablation process is KrF excimer laser operating in 195 -306 nm UV range.
14. A process for coating electronic ceramic oxide thin films as claimed in claim 13 wherein, the said laser with pulse energy density in the range of 1.5-4 J/cm2 with a pulsed width of 25 ns and repetition rate of 1 -10 Hz.
15. A process for coating electronic ceramic oxide thin films as claimed in claim 13 wherein, said laser ablation is carried out in oxygen ambient at pressure of 0.1-1 torr.
16. A process for coating electronic ceramic oxide thin films as claimed in claim 13 wherein, target to substrate distance is maintained from 3 to 8 cm.
17. A process for coating electronic ceramic oxide thin films as claimed in claim 13 wherein, thin films are deposited at substrate temperature in the range of 450-650°C.
18. A process for coating electronic ceramic oxide thin films as claimed in any claim 13 to 17 wherein, thin films are deposited for the duration of 10-30 min.
19. A process for coating electronic ceramic oxide thin films substantially as herein described in the text and in the example.
20. Devices incorporating the ceramic oxide thin film obtained by the process of any one of the preceding claims.
Dated this 23rd dav of February 2002.
of S. MAJUMDAR & CO.
|Indian Patent Application Number||210/MUM/2001|
|PG Journal Number||28/2007|
|Date of Filing||27-Feb-2001|
|Name of Patentee||TATA INSTITUTE OF FUNDAMENTAL RESEARCH|
|Applicant Address||HOMI BHABHA ROAD, COLABA, MUMBAI|
|PCT International Classification Number||H01L 23/29|
|PCT International Application Number||N/A|
|PCT International Filing date|