Title of Invention	A PROCESS FOR THE MANUFACTURE OF IMPROVED MICRO THIN FILM USEFUL AS LOW RELATIVE HUMIDITY SENSOR
Abstract	A process for the manufacture of improved micro thin film useful as low relative humidity sensor, characterized by anodizing a pre-cleaned aluminum substrate having a protective coating on one side, in an acidic electrolyte consisting of 5-20 wt% of sulfuric acid, at a current density in the range of 10-20 mA/ cm2, maintaining the bath temperature in the range of 20-30°C, removing the protective coating by known methods.

Title of Invention

A PROCESS FOR THE MANUFACTURE OF IMPROVED MICRO THIN FILM USEFUL AS LOW RELATIVE HUMIDITY SENSOR

Abstract

A process for the manufacture of improved micro thin film useful as low relative humidity sensor, characterized by anodizing a pre-cleaned aluminum substrate having a protective coating on one side, in an acidic electrolyte consisting of 5-20 wt% of sulfuric acid, at a current density in the range of 10-20 mA/ cm2, maintaining the bath temperature in the range of 20-30°C, removing the protective coating by known methods.

Full Text	The present invention relates to a process for the manufacture of improved micro thin film useful for low relative humidity sensor, a low relative humidity sensor made thereof and a humidity measuring device made therefrom. Humidity is a variable environmental parameter. The measurement and control of humidity are important not only for human comfort but also for many industries and process technologies. The requirement of humidity varies with application. A dry atmosphere is required for the production of integrated circuits whereas a wet atmosphere is required for the growth of plants. Similarly for industries such as textile and agriculture specific humid environment is needed. The use of humidity sensor for moisture content measurement helps in improving the quality of production processes. Thin film micro humidity sensors are preferred for industrial applications because such sensors are reliable and compatible with process control instrumentation. The hitherto known processes for fabrication of humidity sensors are based on electrolytes, organic polymers and porous ceramics. Reference may be made to," Humidity sensors: principles and applications", N. Yamazoe and Y.Shimizu, Sensors and Actuators, 10 (1986) 379-398,wherein the materials used for humidity sensor based on variation of electrical parameters are described. The electrolyte humidity sensor using LiCl was the first electrical moisture sensing device. The drawbacks of electrolyte sensors are that the sensor shows slow response and is unable to operate in low and very humid environment. Reference may be made to, "Humidity sensors based on polymer thin films" , Y. Sakai, Y.Sadaoka and M. Matsuguchi, Sensors and Actuators B 35-36 (1996) 83-90, wherein various types of organic polymers based humidity sensors are described. The major drawbacks are that these sensors can not operate at high temperatures and they show poor sensitivity at low values of humidity. Reference may be made to, "Some properties and limitations of the aluminum oxide hygrometer", Humidity and Moisture, Vol I, A.C. Jason, A. Waxier (ed.), Reinhold Publishing Corp. New York (1961) 372-390 wherein bulk porous sintered ceramic materials have been used to fabricate humidity sensors . They show advantages in terms of their mechanical strength, resistance to chemical attack and thermal stability. However, their drawbacks are related to the large size and degradation of the performance with time. Reference may be made to, "Ceramic sensors for humidity detection : the state of the art and future developments" , E. Traversa, Sensors and Actuators B 23 (1995)135-156 , wherein humidity sensor employing porous aluminum oxide film grown by anodic oxidation of aluminum is described. The humidity sensor is made small in size with fast response. The drawback is that the sensitivity of the sensor is very low in the lower range (0 -45 %RH) of relative humidity. The hitherto known processes for the growth of porous aluminum oxide film are based on thermal evaporation and electrochemical anodisation methods . Reference may be made to "Humidity sensors with reactively evaporated A1203 films as porous dielectrics", Z.Chen, M.C. Zin and C.Zhen, Sensors and Actuators B2 (1990) 167-171, wherein porous aluminum oxide thin films were deposited by reactive thermal evaporation. The draw back of the process is that the film growth rate is very slow and the conductivity along porous in low relative humidity is low. Reference may be made to "Self Ordering of Cell arrangement of anodic porous alumina formed in sulfuric acid solution", H.Masuda, F. Hasegwa/and S.Ono, J. Electrochem. Soc.144 (1997) L127-L130, wherein the growth of porous aluminum oxide films by electrochemical process formed under different conditions is discussed. The limitation of this process is that although different porosity of the film can be obtained the effect on pore conductivity is not investigated. Reference may be made to "Growth kinetics and morphology of porous anodic alumina films formed using phosphoric acid", A.T.Shawaqfeh and R.E.Baltus, J. Electrochem. Soc. 145 (1998) 2699-2706 wherein porous aluminum oxide thin films formed by anodic oxidation of aluminum in phosphoric acid is described. The limitation of this process is that the pore diameter is large resulting in low porosity and the conductivity of the pore is not studied. The main object of the present invention is to provide a process for the manufacture of improved micro thin film useful for measurement of low relative humidity which obviates the disadvantages as detailed above. Another object of the present invention is to provide a sensor with improved porosity and conductivity to enable low relative humidity measurement. Still another object of the present invention is to provide flexibility in the process to tailor the sensitivity to cover a desired range of relative humidity for the measurement of low and high relative humidity. Yet another object of the present invention is to provide a humidity measuring device. Still another object of the present invention is to provide a process which is compatible with integrated circuit (IC) technology. In the present invention an improved process for fabrication of micro thin film humidity sensor for measurement of low relative humidity comprises of an improved electrochemical process for growing porous aluminum oxide thin film, thermal annealing of the film for stabilization, thin water permeable gold film deposition for top contact and packaging of the sensor in an appropriate package. The said sensor package being connected to a known electronic circuit capable of measuring capacitance to provide low humidity measuring device. Accordingly the present invention provides a process for the manufacture of improved micro thin film useful for low relative humidity sensor, characterized by anodizing a pre-cleaned and preformed aluminum substrate having a protective coating on one side, in an acidic electrolyte consisting of 5-20 wt% of sulfuric acid, at a current density in the range of 10-20 mA/ cm2, maintaining the bath temperature in the range of 20-30°C, removing the protective coating by known methods In an embodiment of the present invention the aluminum substrate used may be of high purity of the order of 99.99%. In another embodiment of the present invention the electrolyte may be prepared using deionised water (DI). Still another embodiment of the present invention the aluminum substrate may be preformed on a silicon substrate. Accordingly the present invention provides a low relative humidity sensor made using the improved micro thin film prepared by the process as described above, which comprises stabilizing the micro thin film by thermal annealing at a temperature in the range of 200-300°C in the presence of air for a period in the range 1-2 hours, depositing over the said stabilized aluminum oxide micro thin film a water permeable god film by known methods, sizing and packaging by known methods to obtain a low relative humidity sensor. In an embodiment of the present invention water permeable gold film any be of the order of 200-300 A°. In another embodiment of the present invention the deposition of the water permeable gold film may be effected by known methods such as thermal evaporation, sputtering. Accordingly the present invention provides a humidity measuring device using the sensor prepared above, which comprises connecting the sensor of the present invention to a known circuit capable of measuring capacitance. The following are the major process steps involved in the fabrication of humidity sensor. The starting material is a piece of high purity aluminum foil which is used as substrate. The substrate is prepared in flag shape for growing sensing film. The substrate is degreased in organic chemicals and dipped in an oxide removing chemical to clean the surface. The substrate is rinsed in deionised water (DI) and dried. The backside of substrate is protected by coating with a thin layer of resin or wax. The electrochemical anodisation of the substrate is done in an anodic cell using an acidic electrolyte at an appropriate constant current density . After anodisation the sensing film is rinsed in DI water and the protective film from back side of the substrate is removed . The film is stabilised by thermal annealing in air at 200-300 ° C. Finally on the anodised film , thin gold film is deposited to complete the fabrication. The sensor is packaged in an appropriate package with a cap . The conventional humidity sensors based on aluminum oxide thin film are less sensitive in the lower range (0- 45 %RH) of relative humidity. This is related to the film microstructure , the pore wall conductivity of the porous film. Due to the low surface conductivity of the pore at low relative humidity the sensitivity of the conventional sensor is low. The surface conductivity of the pores can be enhanced by increasing ion incorporation in the film. This is achieved by the novel and inventive step of increasing electrolyte concentration (5-20wt%) and current density (10-20mA/cm2) and maintaining bath temperature (20-30 °C ) in the present process . This increases the pore density and also ion incorporation in the film during the film growth process consequently the film surface area and pore wall conductivity increases at low relative humidity and thus the sensitivity of the sensor is improved. The thin film aluminum oxide humidity sensor is a volume effect device. Water vapour from the ambient passes through the top thin metal film and equilibrate on the pore walls changing the capacitance of the sensor structure (Au-porous A12O3-A1). The change in capacitance is calibrated with respect to relative humidity. For use the micro thin film humidity sensor is exposed to the environment where the relative humidity is to be measured. The change in capacitance is measured by an appropriate capacitance meter. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. EXAMPLE 1 The starting material for the fabrication of micro thin film humidity sensor is high purity (99.99%) thin aluminum foil used as substrate. The substrate is prepared in flag shape with long pole length. The substrate cleaning is done using semiconductor grade organic chemicals , trichloroethylene, acetone and methanol for degreasing. After degreasing the substrate is dipped in (1:10) H3PO4 :DI water solution for 10 seconds to remove any oxide from the surface of the substrate and the substrate is thoroughly washed in DI water and dried. The backside of the substrate is protected from oxidation by coating with thin layer of positive photo resist. The electrochemical anodisation of the substrate is done in the anodic cell . The electrolyte is an acidic solution of controlled concentration .The electrolyte bath temperature is maintained and the electrolyte is stirred using a magnetic stirrer during the film growth. The typical parameters for the growth of thin porous aluminum oxide sensing film are, Electrolyte concentration 15% H2SO4 ( Electronic grade ) Electrolyte bath temperature 25°C Current density 20 mA/cm2 Anodisation voltage 11 volts Anodisation time 30 minutes After anodisation the film is rinsed in DI water. The photoresist from back side of the substrate is removed in acetone. The film is subjected to thermal annealing at 200 °C for about an hour for stabilisation of microstructure of the sensing film. Thin water permeable gold film 200 A° is deposited by sputtering on the sensing film to complete the fabrication. The sensor is mounted on transistor header (TO-5) using conducting epoxy and sealed with a cap . EXAMPLE 2 The sensitivity of the sensor is tailored by adjusting the current density for the growth of sensing film in the electrochemical anodisation process. Higher current density is used for high sensitivity in low relative humidity ranges (0-45 %RH) whereas low current density is used for high sensitivity in high humidity ranges (45-95%RH ). The starting material for the fabrication of micro thin film humidity sensor is high purity (99.99%) thin aluminum foil used as substrate. The substrate is prepared in flag shape with long pole length .The substrate cleaning is done using semiconductor grade organic chemicals , trichloroethylene, acetone and methanol for degreasing. After degreasing the substrate is dipped in (1:10) H3PO4 :DI water solution for 10 seconds to remove any oxide from the surface of the substrate and the substrate is thoroughly washed in DI water and dried. The backside of the substrate is protected from oxidation by coating with thin layer of positive photo resist. The electrochemical anodisation of the substrate is done in the anodic cell. The electrolyte is an acidic solution of controlled concentration .The electrolyte bath temperature is maintained and the electrolyte is stirred using a magnetic stirrer during the film growth. The typical parameters for the growth of thin porous aluminum oxide sensing film are, Electrolyte concentration 15%H2SO4 (Electronic grade ) Electrolyte bath temperature 25°C Current density 10 mA/cm2 Anodisation voltage 14 volts Anodisation time 30 minutes After anodisation the film is rinsed in DI water. The photoresist from back side of the substrate is removed in acetone. The film is subjected to thermal annealing at 200 °C for about an hour for stabilisation of microstructure of the sensing film. Thin water permeable gold film 200 A° is deposited by sputtering on the sensing film to complete the fabrication. The sensor is mounted on transistor header (TO-5) using conducting epoxy and sealed with cap The main advantages of the present invention are: 1. High sensitivity in low humidity range. 2. Sensitivity can be tailored for low and high humidity range. 3. Miniature in size. 4. Compatible with IC process technology. 5. Simple fabrication technology and low cost. We Claim: 1. A process for the manufacture of improved micro thin film useful for low relative humidity sensor, characterized by anodizing a pre-cleaned and preformed aluminum substrate having a protective coating on one side, in an acidic electrolyte consisting of 5-20 wt% of sulfuric acid, at a current density in the range of 10-20 mA/ cm2, maintaining the bath temperature in the range of 20-30°C, removing the protective coating by known methods. 2. A process as claimed in claim 1 wherein the aluminum substrate used is of high purity of the order of 99.99%. 3. A process as claimed in claim 1 to 2 wherein electrolyte is prepared using deionised water, 4. A process as claimed in claim 1 to 3 wherein the aluminum substrate is preformed on a silicon substrate. 5. A process for preparing low relative humidity sensor using the improved micro thin film as claimed in claim 1, which comprises stabilizing the said micro thin film by thermal annealing at a temperature in the range of 200- 300°C in the presence of air for a period in the range 1-2 hours, depositing over the said stabilized aluminum oxide micro thin film a water permeable gold film by known methods, sizing and packaging by known methods to obtain a low relative humidity sensor. 6. A process as claimed in claim 5 wherein the water permeably gold thin film is of the order of 200-300 A°. 7. A process as claimed in claim 5 wherein the deposition of water permeably gold film is effected by known methods as thermal evaporation and sputtering. 8. A process for the manufacture of improved micro thin film useful for low relative humidity sensor substantially as herein described with reference to the examples.

Full Text

The present invention relates to a process for the manufacture of improved micro thin

film useful for low relative humidity sensor, a low relative humidity sensor made thereof
and a
humidity measuring device made therefrom. Humidity is a variable environmental parameter. The measurement and control of humidity are important not only for human comfort but also for many industries and process technologies. The requirement of humidity varies with application. A dry atmosphere is required for the production of integrated circuits whereas a wet atmosphere is required for the growth of plants. Similarly for industries such as textile and agriculture specific humid environment is needed. The use of humidity sensor for moisture content measurement helps in improving the quality of production processes. Thin film micro humidity sensors are preferred for industrial applications because such sensors are reliable and compatible with process control instrumentation.
The hitherto known processes for fabrication of humidity sensors are based on electrolytes, organic polymers and porous ceramics. Reference may be made to," Humidity sensors: principles and applications", N. Yamazoe and Y.Shimizu, Sensors and Actuators, 10 (1986) 379-398,wherein the materials used for humidity sensor based on variation of electrical parameters are described. The electrolyte humidity sensor using LiCl was the first electrical moisture sensing device. The drawbacks of electrolyte sensors are that the sensor shows slow response and is unable to operate in low and very humid environment. Reference may be made to, "Humidity sensors based on polymer thin films" , Y. Sakai, Y.Sadaoka and M. Matsuguchi, Sensors and Actuators B 35-36 (1996) 83-90, wherein various types of organic polymers based humidity sensors are described. The major drawbacks are that these sensors can not operate at high temperatures and they show poor sensitivity at low values of humidity. Reference may be made to, "Some properties and limitations of the aluminum oxide hygrometer", Humidity and

Moisture, Vol I, A.C. Jason, A. Waxier (ed.), Reinhold Publishing Corp. New York (1961) 372-390 wherein bulk porous sintered ceramic materials have been used to fabricate humidity sensors . They show advantages in terms of their mechanical strength, resistance to chemical attack and thermal stability. However, their drawbacks are related to the large size and degradation of the performance with time. Reference may be made to, "Ceramic sensors for humidity detection : the state of the art and future developments" , E. Traversa, Sensors and Actuators B 23 (1995)135-156 , wherein humidity sensor employing porous aluminum oxide film grown by anodic oxidation of aluminum is described. The humidity sensor is made small in size with fast response. The drawback is that the sensitivity of the sensor is very low in the lower range (0 -45 %RH) of relative humidity.
The hitherto known processes for the growth of porous aluminum oxide film are based on thermal evaporation and electrochemical anodisation methods . Reference may be made to "Humidity sensors with reactively evaporated A1203 films as porous dielectrics", Z.Chen, M.C. Zin and C.Zhen, Sensors and Actuators B2 (1990) 167-171, wherein porous aluminum oxide thin films were deposited by reactive thermal evaporation. The draw back of the process is that the film growth rate is very slow and the conductivity along porous in low relative humidity is low. Reference may be made to "Self Ordering of Cell arrangement of anodic porous alumina formed in sulfuric acid solution", H.Masuda, F. Hasegwa/and S.Ono, J. Electrochem. Soc.144 (1997) L127-L130, wherein the growth of porous aluminum oxide films by electrochemical process formed under different conditions is discussed. The limitation of this process is that although different porosity of the film can be obtained the effect on pore conductivity is not investigated. Reference may be made to "Growth kinetics and morphology of porous anodic alumina films formed using phosphoric acid", A.T.Shawaqfeh and R.E.Baltus, J. Electrochem. Soc. 145 (1998)
2699-2706 wherein porous aluminum oxide thin films formed by anodic oxidation of aluminum in phosphoric acid is described. The limitation of this process is that the pore diameter is large resulting in low porosity and the conductivity of the pore is not studied.
The main object of the present invention is to provide a process for the manufacture of improved micro thin film useful for measurement of low relative humidity which obviates the disadvantages as detailed above.
Another object of the present invention is to provide a sensor with improved porosity and conductivity to enable low relative humidity measurement.
Still another object of the present invention is to provide flexibility in the process to tailor the sensitivity to cover a desired range of relative humidity for the measurement of low and high relative humidity.
Yet another object of the present invention is to provide a humidity measuring device.
Still another object of the present invention is to provide a process which is compatible with integrated circuit (IC) technology.
In the present invention an improved process for fabrication of micro thin film humidity sensor for measurement of low relative humidity comprises of an improved electrochemical process for growing porous aluminum oxide thin film, thermal annealing of the film for stabilization, thin water permeable gold film deposition for top contact and packaging of the sensor in an appropriate package. The said sensor package being connected to a known electronic circuit capable of measuring capacitance to provide low humidity measuring device.
Accordingly the present invention provides a process for the manufacture of improved micro thin film useful for low relative humidity sensor, characterized by anodizing a pre-cleaned and preformed aluminum substrate having a protective coating on one side, in an acidic electrolyte consisting of 5-20 wt% of sulfuric acid, at a current density in the range of 10-20 mA/ cm2, maintaining the bath temperature in the range of 20-30°C, removing the protective coating by known methods
In an embodiment of the present invention the aluminum substrate used may be of high purity of the order of 99.99%.
In another embodiment of the present invention the electrolyte may be prepared using deionised water (DI).
Still another embodiment of the present invention the aluminum substrate may be preformed on a silicon substrate.
Accordingly the present invention provides a low relative humidity sensor made using the improved micro thin film prepared by the process as described above, which comprises stabilizing the micro thin film by thermal annealing at a temperature in the range of 200-300°C in the presence of air for a period in the range 1-2 hours, depositing over the said stabilized aluminum oxide micro thin film a water permeable god film by known methods, sizing and packaging by known methods to obtain a low relative humidity sensor.
In an embodiment of the present invention water permeable gold film any be of the order of 200-300 A°.
In another embodiment of the present invention the deposition of the water permeable gold film may be effected by known methods such as thermal evaporation, sputtering.
Accordingly the present invention provides a humidity measuring device using the sensor prepared above, which comprises connecting the sensor of the present invention to a known circuit capable of measuring capacitance.
The following are the major process steps involved in the fabrication of humidity sensor.
The starting material is a piece of high purity aluminum foil which is used as substrate. The substrate is prepared in flag shape for growing sensing film. The substrate is degreased in organic chemicals and dipped in an oxide removing chemical to clean the surface. The substrate is rinsed in deionised water (DI) and dried. The backside of substrate is protected by coating with a thin layer of resin or wax. The electrochemical anodisation of the substrate is done in an anodic cell using an acidic electrolyte at an appropriate constant current density . After anodisation the sensing film is rinsed in DI water and the protective film from back side of the substrate is removed . The film is stabilised by thermal annealing in air at 200-300 ° C. Finally on the anodised film , thin gold film is deposited to complete the fabrication. The sensor is packaged in an appropriate package with a cap .
The conventional humidity sensors based on aluminum oxide thin film are less sensitive in the lower range (0- 45 %RH) of relative humidity. This is related to the film microstructure , the pore wall conductivity of the porous film. Due to the low surface conductivity of the pore at low relative humidity the sensitivity of the conventional sensor is low. The surface conductivity of the pores can be enhanced by increasing ion incorporation in the film. This is achieved by the novel and inventive step of increasing electrolyte concentration (5-20wt%) and current density (10-20mA/cm2) and maintaining bath temperature (20-30 °C ) in the present process . This increases the pore density and also ion incorporation in the film during the film growth process consequently the film surface area and pore wall conductivity increases at low relative humidity and thus the sensitivity of the sensor is improved.
The thin film aluminum oxide humidity sensor is a volume effect device. Water vapour from the ambient passes through the top thin metal film and equilibrate on the pore walls
changing the capacitance of the sensor structure (Au-porous A12O3-A1). The change in capacitance is calibrated with respect to relative humidity.
For use the micro thin film humidity sensor is exposed to the environment where the relative humidity is to be measured. The change in capacitance is measured by an appropriate capacitance meter.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
EXAMPLE 1
The starting material for the fabrication of micro thin film humidity sensor is high purity
(99.99%) thin aluminum foil used as substrate. The substrate is prepared in flag shape with long
pole length. The substrate cleaning is done using semiconductor grade organic chemicals ,
trichloroethylene, acetone and methanol for degreasing. After degreasing the substrate is dipped
in (1:10) H3PO4 :DI water solution for 10 seconds to remove any oxide from the surface of the
substrate and the substrate is thoroughly washed in DI water and dried. The backside of the
substrate is protected from oxidation by coating with thin layer of positive photo resist. The
electrochemical anodisation of the substrate is done in the anodic cell . The electrolyte is an
acidic solution of controlled concentration .The electrolyte bath temperature is maintained and
the electrolyte is stirred using a magnetic stirrer during the film growth. The typical parameters
for the growth of thin porous aluminum oxide sensing film are,
Electrolyte concentration 15% H2SO4 ( Electronic grade )
Electrolyte bath temperature 25°C
Current density 20 mA/cm2
Anodisation voltage 11 volts
Anodisation time 30 minutes
After anodisation the film is rinsed in DI water. The photoresist from back side of the substrate is removed in acetone. The film is subjected to thermal annealing at 200 °C for about an hour for stabilisation of microstructure of the sensing film. Thin water permeable gold film 200 A° is deposited by sputtering on the sensing film to complete the fabrication. The sensor is mounted on transistor header (TO-5) using conducting epoxy and sealed with a cap .
EXAMPLE 2
The sensitivity of the sensor is tailored by adjusting the current density for the growth of sensing film in the electrochemical anodisation process. Higher current density is used for high sensitivity in low relative humidity ranges (0-45 %RH) whereas low current density is used for high sensitivity in high humidity ranges (45-95%RH ). The starting material for the fabrication of micro thin film humidity sensor is high purity (99.99%) thin aluminum foil used as substrate. The substrate is prepared in flag shape with long pole length .The substrate cleaning is done using semiconductor grade organic chemicals , trichloroethylene, acetone and methanol for degreasing. After degreasing the substrate is dipped in (1:10) H3PO4 :DI water solution for 10 seconds to remove any oxide from the surface of the substrate and the substrate is thoroughly washed in DI water and dried. The backside of the substrate is protected from oxidation by coating with thin layer of positive photo resist. The electrochemical anodisation of the substrate is done in the anodic cell. The electrolyte is an acidic solution of controlled concentration .The electrolyte bath temperature is maintained and the electrolyte is stirred using a magnetic stirrer
during the film growth. The typical parameters for the growth of thin porous aluminum oxide sensing film are,
Electrolyte concentration 15%H2SO4 (Electronic grade )
Electrolyte bath temperature 25°C
Current density 10 mA/cm2
Anodisation voltage 14 volts
Anodisation time 30 minutes
After anodisation the film is rinsed in DI water. The photoresist from back side of the substrate is removed in acetone. The film is subjected to thermal annealing at 200 °C for about an hour for stabilisation of microstructure of the sensing film. Thin water permeable gold film 200 A° is deposited by sputtering on the sensing film to complete the fabrication. The sensor is mounted on transistor header (TO-5) using conducting epoxy and sealed with cap The main advantages of the present invention are:
1. High sensitivity in low humidity range.
2. Sensitivity can be tailored for low and high humidity range.
3. Miniature in size.
4. Compatible with IC process technology.
5. Simple fabrication technology and low cost.

We Claim:
1. A process for the manufacture of improved micro thin film useful for low
relative humidity sensor, characterized by anodizing a pre-cleaned and
preformed aluminum substrate having a protective coating on one side, in
an acidic electrolyte consisting of 5-20 wt% of sulfuric acid, at a current
density in the range of 10-20 mA/ cm2, maintaining the bath temperature
in the range of 20-30°C, removing the protective coating by known
methods.
2. A process as claimed in claim 1 wherein the aluminum substrate used is of
high purity of the order of 99.99%.
3. A process as claimed in claim 1 to 2 wherein electrolyte is prepared using
deionised water,
4. A process as claimed in claim 1 to 3 wherein the aluminum substrate is
preformed on a silicon substrate.
5. A process for preparing low relative humidity sensor using the improved
micro thin film as claimed in claim 1, which comprises stabilizing the said
micro thin film by thermal annealing at a temperature in the range of 200-
300°C in the presence of air for a period in the range 1-2 hours, depositing
over the said stabilized aluminum oxide micro thin film a water permeable
gold film by known methods, sizing and packaging by known methods to
obtain a low relative humidity sensor.
6. A process as claimed in claim 5 wherein the water permeably gold thin film
is of the order of 200-300 A°.
7. A process as claimed in claim 5 wherein the deposition of water permeably
gold film is effected by known methods as thermal evaporation and
sputtering.
8. A process for the manufacture of improved micro thin film useful for low
relative humidity sensor substantially as herein described with reference to
the examples.

Documents:

534-del-2000-abstract.pdf

534-del-2000-claims.pdf

534-del-2000-correspondence-others.pdf

534-del-2000-correspondence-po.pdf

534-del-2000-description (complete).pdf

534-del-2000-form-1.pdf

534-del-2000-form-19.pdf

534-del-2000-form-2.pdf

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Patent Number

218369

Indian Patent Application Number

534/DEL/2000

PG Journal Number

13/2009

Publication Date

27-Mar-2009

Grant Date

31-Mar-2008

Date of Filing

25-May-2000

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110 001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	RAJENDRA KUMAR NAHAR	CENTRAL ELCTRONICS ENGINEERING RESEARCH INSTITUTE, PALANI 333031 RAJASTHAN, INDIA
2	RAM PRATAP GUPTA	CENTRAL ELCTRONICS ENGINEERING RESEARCH INSTITUTE, PALANI 333031 RAJASTHAN, INDIA
3	NIRANJAN MANEKLAL DEVASHRAYEE	CENTRAL ELCTRONICS ENGINEERING RESEARCH INSTITUTE, PALANI 333031 RAJASTHAN, INDIA
4	RAJENDRA KUMAR NAHAR	CENTRAL ELCTRONICS ENGINEERING RESEARCH INSTITUTE, PALANI 333031 RAJASTHAN, INDIA
5	RAM PRATAP GUPTA	CENTRAL ELCTRONICS ENGINEERING RESEARCH INSTITUTE, PALANI 333031 RAJASTHAN, INDIA
6	NIRANJAN MANEKLAL DEVASHRAYEE	CENTRAL ELCTRONICS ENGINEERING RESEARCH INSTITUTE, PALANI 333031 RAJASTHAN, INDIA

PCT International Classification Number

G01N 27/12

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA