|Title of Invention||
AN IMPROVED SPRAY PYROLYSIS PROCESS FOR THE PREPARATION OF GOOD QUALITY THIN FILM SEMICONDUCTING COATINGS AND APPARATUS THEREFOR
|Abstract||A process of spraying a solution of a desired presursor salt continuiouly by an atomizer (nozzle ), which converts solution into fine droplts, onto preheated substrates wherein the said salt undergoes pyrolytic decompositon leading to teh formation of thin film coating of a desired semiconducting material onto the said substrate, characterized in that an aparatus is modified to obtain an intermittent spray of an aqueous or no aqueous solution prepared by dissolving an appropriate quantity of said salt usually mjetal chlorides, mitrates, sulfates and acetates, into the solvents limke methanbol, iethanol, propanol distilled water using microprocessor controlled electrromagnetic solemoids, onto the substrates heated at temperature within the range 200 0 c to 800 0 c at optimized conditions of nozzle to substrate distance concentrationa nd quantity of the said solution, solution and gas flow rates, wherein solution si siphoned by nozzle either downwards or upwards in presence of adequate ambient resulting in the porcess similar to chemical vapour deposition having ability to improve quality of the thin film coating.|
13 JAN 2004
THE PATENTS ACT, 1970 (39 of 1970)
COMPLETE SPECIFICATION (See section 10; rule 13)
An improved spray pyrolysis process for the preparation of good quality thin film semiconducting coatings and apparatus
(a) Dr. Patil Pramod Shankararao
(b) Thin Film Materials Laboratory, Department of Physics, Shivaji
University, Kolhapur- 416 004, Maharashtra, India.
(a) Mr. Sadale Shivaji Babaso
(b) Thin Film Materials Laboratory, Department of Physics, Shivaji
University, Kolhapur- 416 004, Maharashtra, India.
The following specification (particularly) describes the nature of this invention (and the manner in which it is performed): -
The present invention relates to the chemical spray pyrolysis process for the deposition of semiconducting thin films. Objective of the said invention is to develop an intermittent spray pyrolysis process for the deposition of semiconducting thin film coatings, suitable for an intended application.
Materials lie at the heart of modern technology. Progress in materials science over the past decade has yielded novel electronic, magnetic, optical, polymeric, ceramic, and biological materials with improved properties for their intended applications. Virtually every aspect of present-day life relies on smaller, more reliable, more efficient, and less expensive devices, which have been made possible by the development of these materials. Devices produced from advanced electronic materials have revolutionized the microelectronics industry, which, in turn, underlies fields such as communications, transportation, and medicine. Photosensitive materials have found applications as light activated dental composites and medical sutures, lithographic imaging media, and protective coatings. New composite ceramic materials are used for aircraft and automotive bodies and engines, turbine blades in electric power plants, and temperature resistant tiles for spacecraft. These examples underscore the diversity of applications for new materials in the form of thin film coatings required by our modern technological society.
There are various processes by which one can deposit thin film coatings. These processes can broadly be classified as physical and chemical. Among these the chemical processes are relatively economical and easier than the physical processes. There is no ideal process to prepare the compounds and alloys in semiconducting thin film form, which will satisfy all the possible requirements. Among the various chemical processes viz. Chemical Vapour Deposition (CVD), Chemical Bath Deposition (CBD), Sol-gel, etc. Spray Pyrolysis process is the most popular due to its simplicity, cost-effectiveness and ability to control the structure at molecular level. A large number of thin film coatings from a group of inorganic, organic, polymer, and ceramic materials can be deposited onto variety of substrates such as soda glass, quartz, pyrex, sapphire, silicon wafer, single crystalline wafers. There is virtually no
restriction on substrate material and its surface, any support having high temperature tolerance will be a suitable substrate. The detail description of the intermittent spray pyrolysis process is given as follows.
Description of Spray Pyrolysis Process:
Spray pyrolysis is a process in which a thin film is deposited by spraying a solution onto a pre-heated substrate, where constituents react to form a chemical compound. An atomizer transforms the solution into the spray of fine droplets, forming a cone. The size of the droplet and their distribution in the cone depends on geometry of an atomizer. The resulting solution droplets are rapidly heated by the huge temperature gradient while approaching towards the heated substrates and the following physical phenomena can occur simultaneously: evaporation of the solvent, diffusion of the solvent vapour away from the droplet in the gaseous phase, shrinkage of the droplet, change in the droplet temperature and diffusion of solute towards the center of the droplet. The type of reaction that takes place in the spray deposition process depends on various parameters and can be classified as,
A: Droplet resides on the substrate surface and the solvent
evaporates leaving behind a solute that further decomposes and react in the dry state.
B: The solvent evaporates and solute decomposes before the droplet reaches the substrate surface.
C: Solvent vaporizes well above the substrate surface; the solute then melts and vaporizes and the vapours diffuse to the substrate, for desired reaction to occur.
Despite the simplicity of spray pyrolysis process, it has number of advantages,
1) It offers an extremely easy way to dope the films with virtually any element in any proportion by merely adding dopent solution to the spray solution.
2) Unlike closed vapour deposition methods, spray does not require high quality targets and / or substrates nor does it require vacuum at any stage.
3) The deposition rate and the film thickness can be easily controlled over a wide range by changing the process parameters, thus eliminating the major drawbacks of other chemical methods such as sol-gel which produce films of limited thickness.
4) Unlike high power methods such as radio frequency magnetron sputtering (RFMS). It does not cause local overheating that can be detrimental to the materials to be deposited. There are virtually no restrictions on substrate materials, dimensions or on its surface profile.
5) By changing composition of the spray solution during the spray process, it can be used to make layered films and films having composition gradients throughout the film thickness.
In pneumatic spray pyrolysis process the significant process parameters are the ambient temperature, substrate temperature, gas and solution flow rates, nozzle to substrate distance, droplet radius, droplet residence time, solution concentration, solution flow rate and orifices and geometry of an atomizer.
However the present spray pyrolysis process still faces challenging difficulties like control of heating and cooling rate of substrates, control of spray rate, control of nature & concentration of ambient medium, control of droplet resident time, adequate exhaust for the toxic gases evolved during the deposition.
The object of the present invention is to obviate these drawbacks by modifying the present spray pyrolysis process to an intermittent one. Figure 1 illustrates the complete system it consists of,
1) Spray reactor (component 1 in the design)
2) An atomizer
3) Temperature controller
4) Timer controller
5) Gas regulator valves and flow meters
1) Spray reactor: (metallic chamber)
The environment is an important factor affecting the growth of thin films. The nature and concentration of the ambient medium is also important for the growth of thin films e.g. an inert medium is essential to grow non oxide (sulphide/selenide) semiconducting thin films viz. ZnS, ZnSe, CdS, CdSe. Some times, during the deposition process toxic gases are evolved, which are essentially needed to be removed from the spray reactor. Considering the above requirement an air tight spray reactor made up of metallic (Stainless Steel no.304) chamber of height 52 cm and diameter 22 cm is fabricated.
2) An atomizer:
The process of atomization is another critical factor in spray pyrolysis process as it involves the critical operation of preparation of uniform and fine droplets. The decomposition location and time of droplets before striking to the substrate depends (time of flight) on the size of the droplet. Smaller droplets (5 to 50 urn) cool very fast and impinge onto the substrate surface and get solidified. Larger droplets (50 to 500 um) contain higher amount of latent heat and thermal energy. Their impact takes place during the state of phase change (semisolid) or even in a completely liquid state (> 100 um). The thermal behaviour of droplet during their flight in the spray cone is important in the growth of thin films.
In the present invention glass nozzle is used as an atomizer. We have developed a set of glass atomizers with different dimensions producing various droplet sizes and spray profiles.
Depending upon the requirement one can employ any one of the nozzle from the set to atomize the feedstock (precursor) solution.
3) Temperature Controller and Heater:
One more important parameter in the spray process is the substrate temperature which should not vary during thin film deposition process and remain constant till the entire quantity of solution is sprayed.
In the present invention a fuzzy logic based temperature controller is used to control substrate temperature. The controller offers the facility to change the heating rate of the substrate as per the requirement. This system gives stable and uniform temperature up to 800 °C. Intermittent spray is employed to assure that the substrate temperature remains almost constant.
The heater is mounted on an adjustable stand, to vary the distance between substrate to nozzle. Depending on solution composition this distance is adjusted, e.g. for non-aqueous solution it is less and for aqueous it is more.
4) Timer Controller:
It is known that pyrolysis of the droplet and subsequent formation of grain structure, where atoms line up with particular orientation, depends on the droplet resident time (DRT).
In the present system DRT can be controlled by varying solution solenoid ON/OFF times. One can use any combination of the solution solenoid ON-OFF timings to get better control on the growth of thin films.
Electronic timer controller is used to control the ON/OFF switching times of both solution and gas solenoids.
Gas solenoid switching range : 1 to 10 minutes. Solution solenoid switching range: ON times: 1 to 9 seconds.
OFF times: 1 to 9 seconds.
5) Gas regulator Valves and Flow meters:
The gas regulator valve is used to control the pressure of the gas. Appropriate gas regulator and flow meters are employed to control and to measure the flow rate of the carrier gas. The use of flow meters allows us to control the spray rate more precisely.
Detailed specification of Figure 1:
Figure 1 illustrates a set up of an intermittent spray pyrolysis process for making of thin film coating onto a substrate. As shown in figure 1, the setup consists of a metallic spray reactor "1" of 52 cm height and 22 cm diameter having 5 ports, three are having 5 cm diameter, numbered as 13,14,15 and two of which are of diameter 9.5 cm numbered as 16 and 17 in figure. Port 13 is to illuminate the nozzle tip, port 14 is to inspect a spray profile at the tip of nozzle and 15 is to inspect the film formation process. Ports 16 and 17 are used to handle the substrates and to adjust the height of the heater. The height of the heater from the base can be varied by adjusting stand 20, between 5 to 20 cm, so as to adjust nozzle to substrate distance between 15 to 28 cm. The spray reactor "1", is closed with top flange, 11 and bottom flange, 21. Top flange has two more ports labeled as 9 and 10 to illuminate the spray reactor with halogen lamp, 8, powered by power supply and manual switch 27. A small aluminium ring is fitted to top flange to hold the nozzle, 12. Heater, 19 and substrate holder assembly, 18 are mounted on bottom flange. Bottom flange has a valve, which is connected to suction pump, 22 to expel the air inside the spray reactor before the experiment starts. The heater is robust and supported on a firm base, 23. The temperature of the heater is controlled by the fuzzy logic based programmable temperature controller, 25, having facility to vary the heating and cooling rates. The heater is of 3" diameter and maximum attainable temperature in the presence of spray is 800 °C. The solution container 1A contains solution of a precursor salt to be sprayed onto a substrate kept on heater at the bottom. The appropriate quantity of salt (solute) is dissolved in an adequate solvent to form a homogeneous solution, to be sprayed. Aqueous solutions are commonly used due to ease of handling, safety, low
cost and availability of a wide range of water soluble metal salts. The solute must have high solubility to increase the yield of the process.
In general metal chlorides, oxychlorides, nitrates, sulfates and acetates are used as solute. Hybrid system in which one of the component is added via a solution and remainder as particles (nano) are also used. Sparingly soluble salts e.g. Hydroxides, oxalates or carbonates are not very much suitable as the dissolved precursor salts precipitate during the initial stage of spraying process. The flow meters 4,5,6 (for 02, N2, and air respectively, range: 0 to 30 liters/min.) are used to control the rate of flow of carrier gas. Only one carrier gas is used at a time. 2 and 3 are solution and gas solenoids respectively, which are connected to electronic timer controller, 26. It is programmed so that solenoid no.2 flips between ON and OFF states. When it is switched ON, solution in the container 1A gets feeded to the glass nozzle, 12, solenoid 3 must simultaneously be ON, which provides impulse to the solution to flow swiftly towards nozzle, 12. Nozzle, 12 has a specific geometry (figure 2) and acts as an atomizer which transforms the solution into the fine droplets, forming a solid cone. The size of the droplet and their distribution in the cone depends on geometry of an atomizer. Air or gas flows through a tube labeled as 2a and solution enters through tube la. Due to air pressure and inner and outer orifices of the nozzle (shown in figure 2), solution is sucked and gets converted into a spray comprising of small droplets at the tip of nozzle. These droplets leave nozzle tip with specific velocity and get transported toward heated substrates. The diameter of the droplets and their distribution depends on orifice diameters. The substrates are heated at high temperature (300 to 800 °C), which causes rise in temperature of the air / gas in the vicinity of the heater. This heat gets transported towards relatively cooler air, (towards nozzle) due to convection, resulting in the formation of temperature gradient in the space between nozzle to substrate. When the droplets leave the tip of the nozzle and approach toward substrates, they encounter with increasingly higher temperatures. It is as if droplet is transported rapidly from a furnace at tow temperature to a furnace at high temperature. During in-flight time (transit time), droplets undergo solvent evaporation, solute condensation and
decomposition and finally deposition of a layer of the material onto the substrates takes place.
The quality of the thin film coatings depends on how efficient these processes take place within the droplets during its in-flight time. Ideally pyrolytic decomposition of the droplets should be completed just above the surface of the substrate, and before next droplet strikes the same place.
In the conventional spray pyrolysis process it is difficult to control above steps due to continuous spray, which forms and sprays the mist of droplets continuously, till entire solution quantity is exhausted. Additionally it causes surface temperature of the substrate to lower appreciably, leading to deform the temperature gradient formed between nozzle to substrate and finally resulting into incomplete decomposition of the solution. This compels the decomposition process to occur on the surface of substrate, which severely affects quality of the thin film coatings.
In the intermittent spray pyrolysis process droplet residence time (DRT) is sufficiently long, it is as if treating one batch of the droplets completely and then send another batch. Once solenoid 2 is ON (e.g. for 1 second) solution flows through the nozzle only for one second and then spray stops, leaving only one batch of droplets to undergo pyrolytic decomposition. Following droplets undergo similar treatment and resides swiftly onto the grains formed previously leading to the increment in grain size and improvement in the quality of the coatings appreciably. This is the crux of the present process.
Solution solenoid 2 flips between ON-OFF state till entire solution is sprayed onto the substrate and then after some time solenoid 3 is switched off.
Thus one can have better control over pyrolytic decomposition process by intelligent coupling of solenoid 2"s ON-OFF timings, substrate temperature and solution properties.
Operating Procedure for an intermittent Spray pyrolysis process:
1) The feedstock solution of desired material of required concentration is prepared.
2) The cleaned substrates were mounted on the heater and heater is fixed to an appropriate height.
3) The timer controller is set for the required
1) gas purging time for the gas solenoid.
2) ON/OFF time for solution solenoid.
4) The valve at the bottom is connected to suction pump and the spray reactor is evacuated. During evacuation, valve on top flange was kept closed.
5) The gas solenoid is switched ON to purge the inert gas.
6) During the purge time valve at the bottom flange was kept open while valve on the top flange was closed.
7) The required substrate (deposition) temperature and heating rate is set by using temperature controller and then the heater is switched ON.
8) The system can then be started with any one of the two modes,
1) Auto:-automatically starts the spray when purge time is over.
2) Manual:-spray begins by switching ON the solution solenoid manually.
9) Once the spray is over then,
1) The solution solenoid is switched OFF while gas solenoid is kept ON.
2) The heater is kept on for some desired time and then switched OFF.
10) The heater is allowed to cool to room temperature and then the substrates are taken out from the spray reactor through the two bottom side ports.
1) The system allows easy variation in nozzle to substrate distance as per the requirement.
2) It is possible to get desired droplet size by using appropriate nozzle dimension.
3) It is possible to control the velocity of spray particle by adjusting the carrier gas flow rate with the help of gas regulator valve and flow meter.
4) The system allows Intermittent spraying of solution.
5) View ports allow the observation of films formation process.
6) Upward spray (analogous to chemical vapour deposition (CVD) is possible, by inverting the places of substrate and nozzle.
7) System provides stable temperature up to 800 °C.
8) The nature and concentration of ambient can be controlled.
9) Adequate exhaust facility is available.
SUMMARY OF THE INVENTION
The present invention is related to spray pyrolysis process for depositing semiconducting thin film coatings and the fabrication of an apparatus therefor. This process is simple, cost effective and suitable for the depositions of wide variety of semiconducting thin film coatings. The facility of an intermittent spraying of a solution offers an easy way to control the quality of the thta film coatings precisely. The structural, microstructural, electrical, optical, iono-optical and electrochemical properties of the materials are tailored by optimizing the preparation conditions namely nozzle to substrate distance, substrate temperature, solution concentration, solution flow rate, solution quantity, flow rate of carrier gas and ambient for depositions. The recipes for the preparation of thin film coatings of various semiconducting materials with desired properties for intended applications are generated.
An apparatus used to accomplish an intermittent spray pyrolysis process consists of a spray reactor, nozzle assembly, substrate heater, electromagnetic gas and solution solenoids, timer controller for solenoids, temperature controller and air/gas flow meters. The electromagnetic solenoids are programmed to change the operating state from OFF to ON allowing solution spray and from ON to OFF ceasing the spray at a predetermined optimized ON-OFF timings. This causes prolongation in droplet residence time (DRT), leading to complete pyrolytic decomposition of a precursor solution and the improvement in the film formation process.
Examples: Having described the basic aspects of the invention, the
following examples are given to illustrate the specific embodiments thereof.
The intermittent spray pyrolysis process involves number of preparative parameters viz. substrate temperature, nozzle to substrate distance, carrier gas flow rate, solution concentration etc. The role of preparative parameters in the process is important, more precisely many properties of the coatings are governed by the preparative parameters. The substrate temperature affects the stoichiometry and structural properties of the film. The structural and morphological properties are dependent on thermal gradient in vapor space which in turn decide the location and time at which the spray droplet vaporizes entirely in the spray reactor. Hence one requires to have adequate thermal gradient in the spray reactor by optimizing the nozzle to substrate distance (NSD). The spray droplet size, mass and velocity are mainly dependent on carrier gas flow rate and concentration of the feedstock solution.
A systematic investigation of these thin film coating parameters is carried out. The structural and morphological studies are carried out by using x-ray diffraction (X-RD) and scanning electron microscope (SEM) respectively. The compositional study of the compounds is carried out by employing energy dispersive x-ray analysis (EDS).
Example 4: THE DEPOSITION OF sno2 THIN FILMS FROM
TRIBUTYL TIN ACETATE fTBTA).
Example 5: THE DEPOSITION OF IRIDIUM OXIDE THIN FILMS.
Example 6: THE DEPOSITION OF NICKEL OXIDE THIN FILMS
Example 7: THE DEPOSITION OF COBALT OXIDE THIN FILMS.
Example 8:THE DEPOSITION OF NIOBIUM PENTAOXIDE (Nb2o5)
These are some of the common examples illustrating formation of transition metal oxide (TMQ) and metal oxide (MO) thin film coatings. Many other examples may be illustrated without departing from the spirit of this invention.
1. A process of spraying a solution of a desired precursor salt continuously by an atomizer (nozzle), which converts solution into fine droplets, onto preheated substrates wherein the said salt undergoes pyrolytic decomposition leading to the formation of thin film coating of a desired semiconducting material onto the said substrate, characterized in that an aqueous or non-aqueous solution, prepared by dissolving an appropriate quantity of said salt, usually metal chlorides, nitrates, sulfates and acetates, into the solvents like methanol, ethanol, propanol, distilled water, using microprocessor controlled electromagnetic solenoids is sprayed intermittently onto the substrates heated at temperature within the range 250°C to 800°C at optimized conditions of nozzle to substrate distance, concentration and quantity of the said solution, solution and gas flow rates, wherein solutionis siphoned by nozzle either downwards or upwards in presence of adequate ambient resulting in the process similar to chemical vapour deposition having ability to improve quality of the thin film coating.
2. An apparatus for carrying out a process of spraying a precursor solution intermittently as claimed in claim 1 comprising;
a cylindrical air-tight metallic (stainless steel) spray reactor, housing nozzle assembly at the top/bottom and substrates placed onto the heater at the bottom/top, depending on predetermined spray direction, either downwards or upwards;
a solution solenoid, capable of changing an operating state from ON to OFF automatically after predetermined switching time, wherein said time is fixed at any value between 1 seconds to 10 seconds, using microprocessor;
a gas solenoid, capable of changing an operating state from ON to OFF automatically/manually after predetermined switching time, wherein said time is fixed at any value between 1 minutes to 10 minutes, using microprocessor;
a non-corrosive glass nozzle, fitted to its top flange with an aluminium O-ring;
a substrate holder fitted onto the top surface of the heater on which substrates are placed;
a height adjustable heater, wherein the nozzle to substrate distance is varied between 23 cm to 28 cm;
a PID temperature controller, which controls the temperature of the heater and maintain it at the predetermined value during the spray process, wherein temperature value is set between 250°C to 800°C;
a oxygen gas flow meter, wherein the gas flow rate is fixed at any value between 1 liter/minute to 30 liters/minute;
a nitrogen gas flow meter, wherein the gas flow rate is fixed at any value between 1 liter/minute to 30 liters/minute;
an air flow meter, wherein the gas flow rate is fixed at any value between 1 liter/minute to 30 liters/minute;
Circular view ports and nozzle and substrate replacement ports, fitted with toughened glass with O-rings;
a valve, attached to the suction/vacuum pump at the bottom flange, which evacuates the spray reactor prior to the spray deposition;
a valve, attached to the exhaust fan, at the top flange, which expels toxic gases and water vapours generated during spray deposition.
3. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein the desired precursor solution is atomized,by means of a glass nozzle selected from a series of specially designed glass nozzles
glass nozzles having inner orifice between 0.2 mm to 0.6 mm and outer orifice between 1 mm to 6 mm;
4. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein substrate or deposition temperature remains uniform at predetermined value, between 250°C to 800°C and constant within ± 1°C due to intermittent spraying of a solution, facilitating stress-free good quality thin film coatings;
5. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein deposition of thin film coatings is carried out in any desired medium viz. oxygen or ambient air medium for formation of oxide thin films, nitrogen or argon medium for formation of non-oxide thin films, as the spray reactor is air tight and has facility of expelling the inside air and marginally evacuating and then metering of desired gas.
6. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein crystallinity of the material to be deposited is controlled by varying substrate temperature, which must be lower then 300°C for getting amorphous films, above 300°C for microcrystalline and at about 500°C for polycrystalline coatings, the phase and structure of the coating material is also tailored viz. Ir203 below 600°C, Ir02 below 800°C and IrO at 800°C onto a quartz substrate.
7. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein thickness of the coating can be varied from as low as 10 nm to 3 \xm by varying quantity from 8 ml to 200 ml or by varying concentration of spraying solution from 10 mM to 50 mM.
8. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein good-quality thin film is deposited onto metallic, insulating or semiconducting substrates.
9. The process of spraying a precursor solution intermittently as claimed
in claim 1, wherein the thin film coating is made up of more than one
material by sequentially spraying the solutions of different precursors.
10. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein nanoparticles or nanostructured particles of a guest material, like Ti02, are suspended in a spraying solution of host material to embed nanoparticles in the host matrix, thereby improving properties of the host material.
11. The process of spraying a precursor solution intermittently as claimed in claim 1, wherein fabrication of thin film coating is accomplished in a single step and post annealing treatment (required, if any).
Dated 4th March, 2005
Dr. Patil Pramod Shankararao
Mr. Sadale Shivaji Babaso
|Indian Patent Application Number||63/MUM/2004|
|PG Journal Number||13/2008|
|Date of Filing||23-Jan-2004|
|Name of Patentee||1) DR. PATIL PRAMOD SHANKARARAO 2) MR. SADALE SHIVAJI BABASO|
|Applicant Address||THIN FILM MATERIALS LABORATORY, DEPARTMENT OF PHYSICS, SHIVAJI UNIVERSITY, KOLHAPUR 416 004|
|PCT International Classification Number||B05D1/02|
|PCT International Application Number||N/A|
|PCT International Filing date|