Title of Invention	AN IMPROVED PROCESS FOR THE PREPARATION OF HIGHLY SENSITIVE LEAD SULPHIDE INFRARED DETECTORS.
Abstract	An improved process for the preparation of highly sensitive lead sulphide infrared detector by depositing lead sulfide film on a glass substrate by immersing the substrate in a bath containing 30 - 40% lead acetate, 8 - 12% thiourea and water, adjusting the pH 10 - 11 by adding sodium hydroxide at a temperature 25-30°C, sensitising the said film characterized in that by insitu oxidation using an organic or inorganic oxidising agent, heating the said sensitised film at a temperature ranging between 450 - 550°C in oxygen atmosphere for a period of 1 - 45 minutes, providing gold with ohmic contact on the said film by known methods, soldering tinned copper leads to the above said gold film using an indium solder followed by lacquering to obtain the desired detector.

Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF HIGHLY SENSITIVE LEAD SULPHIDE INFRARED DETECTORS.

Abstract

An improved process for the preparation of highly sensitive lead sulphide infrared detector by depositing lead sulfide film on a glass substrate by immersing the substrate in a bath containing 30 - 40% lead acetate, 8 - 12% thiourea and water, adjusting the pH 10 - 11 by adding sodium hydroxide at a temperature 25-30°C, sensitising the said film characterized in that by insitu oxidation using an organic or inorganic oxidising agent, heating the said sensitised film at a temperature ranging between 450 - 550°C in oxygen atmosphere for a period of 1 - 45 minutes, providing gold with ohmic contact on the said film by known methods, soldering tinned copper leads to the above said gold film using an indium solder followed by lacquering to obtain the desired detector.

Full Text	This present invention relates to an improved process for the preparation of highly sensitive lead sulphide infrared detector. Lead sulfide (PbS), lead selenide (PbSe) and lead telluride (PbTe) are well known infrared detectors. The lead sulfide detectors most versatile, rugged one for use at room temperature operation and can be in the wavelength range of 1-3 microns. PbS. polycrystalline thin layers used for infrared detectors were prepared hitherto by vacuum evaporation and chemical bath deposition. Easy and cheap method of preparation of chemical bath deposition reported by Kicinski [Chem.Industry (1948) p.54] has not given many essential details for infrared detection use. The preparative method consist of chemical reaction beweeen lead acetate and thiourea in highly alkaline conditions. A mirror like deposit of PbS is obtained on the glass plate suspended in the solution. The rate and quantity of alkali addition, temperature, stirring rate are very critical parameters in the preparation of satisfactory films. The sensitivity of the films as described by Kicinski is low. The films prepared by this process are tested under a bias of 170V with a 15 w tungsten lamp at a distance of 25 cm from the source with a 90 Hz chopper under the match resistance conditions. The signal obtained was 20 mV. Spectral response - visible - 3 microns Dark resistance - 5-6 M.ohms Signal to Noise ratio - 10 sensitivity This method talks of no further steps of sensititation after deposition. Suryanarayana et al (Patent 851631) have modified the method of preparation the layers are subsequent sensitised by heat treatment in oxygen atmosphere and lacquered for protection against atmospheric action. The layers thus fabricated were found to be sensitive. These layers require more than 30 days ageing for stabilisation before use, further there is a drift during biasing for detection at higher voltages. These are the major drawbacks of the process by Suryanarayana et al. The main object of the present invention is to provide a modified process for obtaining highly sensitive lead sulfide infrared detector which obviates the drawbacks mentioned earlier. The sensititation process involving insitu oxidation during preparation is achieved by using oxidising agents such as N-Bromo Succinamide, N-Chloro succinamide, HaO2 and KMnO4. The addition of 0.01 - 0.2 wt% of oxidising agent in alkaline solution and or treating the chemically prepared PbS film in by dip treatment for durations ranging from 1 - 30 mts. which stabilises the detector against drift and can be used without ageing. Another object of the present invention is to have a drift free detector under high applied voltage conditions. Accordingly the present invention provides an improved process for the preparation of highly sensitive lead sulphide infrared detector which comprises depositing lead sulfide film on a glass substrate by immersing the substrate in a bath containing 30 - 40% lead acetate, 8 - 12% thiourea and water, adjusting the pH 10 - 11 by adding sodium hydroxide at a temperature 25-30°C, sensitising the said film characterized in that by insitu oxidation using an organic or inorganic oxidising agent such as herein described, heating the said sensitised film at a temperature ranging between 450 - minutes, providing gold with ohmic contact on the said film by known methods, soldering tinned copper leads to the above said gold film using an indium solder followed by lacquering to obtain the desired detector. In an embodiment of prevent invention the organic oxidising agent used is selected from N-chloro-succinamide and N-bromo succinamde. In an embodiment of prevent invention inorganic oxidising agent used is selected from Kl, KMnCXtand HaC^. In an embodiment of prevent invention wherein heating is done preferably at a temperature of 490 - 540°C. The deposition bath consists of a mixture of 30 ml of 40% lead acetate and 10 ml of 12% thiourea to which are added 50 ml of water and kept stirred for 5 minutes at a speed of 200 rpm. The glass plates attached to a perspex holder is kept immersed in the bath. After 5 minutes, 2 ml of 66% sodium hydroxide is added and after 5 minutes yet another of 8 ml of hydroxide is added and stirred for 15 minutes. The oxidising agent is added to the deposition bath after 15 min. from the complete alkali addition. In another case after the complete addition of the alkali the films were removed from the bath, washed well and dried and heat treated at 450 - 550°C for 5 - 10 minutes in oxygen atmosphere is treated with the oxidising agent. The following typical examples are given to illustrate the invention which describes the method to increase the stability and to have drift free operational conditions when in use in any device. Example 1 Deposition bath consists of a mixture of 30 ml of 40% lead acetate and 10 ml of 12% thiourea to which are added 50 ml of water and kept stirred for 5 minutes at a speed of 200 rpm. The glass plates attached to a perspex holder is kept immersed in the bath. After 5 minutes, 2 ml of 66% sodium hydroxide is added and after 5 minutes yet another of 8 ml of hydroxide is added and stirred for 15 minutes, then 0.01% solution of H2O2 is added and after 15 minutes complete alkali addition. The films were with distilled and dried. The dried films were heat treated at 500°C for 10 minutes in oxygen. The films after heat treatment were provided gold with ohmic contacts by vacuum evaporation unit using a suitable mask. Immediately after gold evaporation the films were given a protective coating to avoid exposure to moisture with an optical plastic. Tinned copper leads were soldered to the gold electrodes, using indium solder. The characteristics of the representative sample are given below: When tested under 170 V bias with a 15 W tungsten lamp at a distance of 25 cm from the source with 90 Hz chopper at the matched resistance conditions. Signal obtained after RG 1000 filter (for visible cut off) was 0.6V. Spectral response Visible to 3 microns Dark resistance • 1 mOhm to 2 Mohm Signal to noise ratio 400 Sensitivity ( When tested for heavy water purity the lab analysis and the on-line monitor analysis agreed well and no drift with time was observed indicating the stability of the cell. Example 2 Deposition bath consists of a mixture of 40m ml of 40% lead acetate 12 ml of 12% thiourea to which are added 50 cc of water and kept for stirring for 5 minutes at moderately high speed 1000 rpm. The glass plate attached to this perspex holder is kept immersed in the bath, 2 ml of 60% NaOH is added and after one minute 8 ml of 60% NaOH is added. 0.1% of N-Brumo Succinamide in 2 ml NaOH is added after 15 minutes. The films were removed after 5 minutes and washed. The films heat treated at 500°C for 10 minutes in oxygen atmosphere were provided with an ohmic contact by vacuum evaporation using a suitable mask and soldered the tinned copper using indium solder and lacquered. The characteristics of the representative sample are given below Spectral response Visible to 3 microns Dark resistance 0.3 - 1 Mohm Signal to noise ratio 70 When tested under 170 V bias with a 15 W tungsten lamp at a distance of 25 cm from the source with a chopper at 90 Hz under matched load condition the signal developed with RG 1000 filter was 5 - 6 V. Sensitivity ( When tested for heavy water purity, the lab analysis and on-line monitor analysis agreed well and no drift was observed indicating the stability of this cell as against the cells prepared without addition agent which had a drift of 0.5 - 0.7% with time under heavy bias of 170 V required in any industrial unit. Example 3 The deposition bath consists of a mixture of 30 ml of 40% lead acetate and 10 ml of 10% thiourea to which added 50 ml water and stirred for 5 minutes at a moderate high speed of 800 rpm. The cleaned glass plate attached to the perspex holder is kept immersed in the bath. 2 ml of 60% NaOH is added and after 1 min. 8 ml of 60% NaOH is added. The films were removed after 15 min. and washed well and dried. The films were heat treated at 525°C for 10 min. in oxygen atmosphere after cooling to room temperature the films were dipped in 0.1% KMnO4 for 3 min. The films were removed, washed and dried. The films were further heat treated at 550°C for 3 min. in oxygen atmosphere. The films were provided with gold ohmic contact by vacuum evaporation. The tinned copper leads were soldered with indium soldered and lacquered. The characteristics of the representative samples are given below: Spectral response Visible to 3 microns Dark resistance 1 Mohm to 2 Mohm Signal to noise ratio 600 Sensitivity ( When tested under 170 V bias with a 15 W tungsten lamp at a distance of 25 cm from the source with a 90 Hz chopper under a match load condition the signal developed with RG 1000 filter was 4.5V. No drift was observed when tested indicating the stability of the cell. The following are the main advantages of the invention: 1 Sensitising the layer before and after heat-treatment has been introduced as an easy step for highly sensitive infrared detectors. 2 Ageing of the films is minimized by the addition of the oxidising agent during deposition or dipping in the oxidising agent. 3 The infrared detector is stable when higher biasing voltages are applied. We claim: 1 An improved process for the preparation of highly sensitive lead sulphide infrared detector which comprises depositing lead sulfide film on a glass substrate by immersing the substrate in a bath containing 30 - 40% lead acetate, 8 - 12% thiourea and water, adjusting the pH 10 - 11 by adding sodium hydroxide at a temperature 25-30°C, sensitising the said film characterized in that by insitu oxidation using an organic or inorganic oxidising agent such as herein described, heating the said sensitised film at a temperature ranging between 450 - 550°C in oxygen atmosphere for a period of 1 - 45 minutes, providing gold with ohmic contact on the said film by known methods, soldering tinned copper leads to the above said gold film using an indium solder followed by lacquering to obtain the desired detector. 2 An improved process as claimed in claim 1 wherein organic oxidising agent used is selected from N-chloro-succinamide and N-bromo succinamde. 3 An improved process as claimed in claims 1 & 2 wherein inorganic oxidising agent used is selected from Kl, KMnO4and H2O2. 4 An improved process as claimed in clams 1 to 3 wherein heating is done preferably at a temperature of 490 - 540°C. 5 An improved process for the preparation of highly sensitive lead sulphide infrared detector substantially as herein described with reference to the examples.

Full Text

This present invention relates to an improved process for the preparation of highly sensitive lead sulphide infrared detector.
Lead sulfide (PbS), lead selenide (PbSe) and lead telluride (PbTe) are well known infrared detectors. The lead sulfide detectors most versatile, rugged one for use at room temperature operation and can be in the wavelength range of 1-3 microns. PbS. polycrystalline thin layers used for infrared detectors were prepared hitherto by vacuum evaporation and chemical bath deposition. Easy and cheap method of preparation of chemical bath deposition reported by Kicinski [Chem.Industry (1948) p.54] has not given many essential details for infrared detection use. The preparative method consist of chemical reaction beweeen lead acetate and thiourea in highly alkaline conditions. A mirror like deposit of PbS is obtained on the glass plate suspended in the solution. The rate and quantity of alkali addition, temperature, stirring rate are very critical parameters in the preparation of satisfactory films.
The sensitivity of the films as described by Kicinski is low. The films
prepared by this process are tested under a bias of 170V with a 15 w tungsten
lamp at a distance of 25 cm from the source with a 90 Hz chopper under the
match resistance conditions. The signal obtained was 20 mV.
Spectral response - visible - 3 microns
Dark resistance - 5-6 M.ohms
Signal to Noise ratio - 10
sensitivity This method talks of no further steps of sensititation after deposition. Suryanarayana et al (Patent 851631) have modified the method of

preparation the layers are subsequent sensitised by heat treatment in oxygen atmosphere and lacquered for protection against atmospheric action. The layers thus fabricated were found to be sensitive. These layers require more than 30 days ageing for stabilisation before use, further there is a drift during biasing for detection at higher voltages. These are the major drawbacks of the process by Suryanarayana et al.
The main object of the present invention is to provide a modified process for obtaining highly sensitive lead sulfide infrared detector which obviates the drawbacks mentioned earlier. The sensititation process involving insitu oxidation during preparation is achieved by using oxidising agents such as N-Bromo Succinamide, N-Chloro succinamide, HaO2 and KMnO4. The addition of 0.01 - 0.2 wt% of oxidising agent in alkaline solution and or treating the chemically prepared PbS film in by dip treatment for durations ranging from 1 - 30 mts. which stabilises the detector against drift and can be used without ageing.
Another object of the present invention is to have a drift free detector under high applied voltage conditions.
Accordingly the present invention provides an improved process for the preparation of highly sensitive lead sulphide infrared detector which comprises depositing lead sulfide film on a glass substrate by immersing the substrate in a bath containing 30 - 40% lead acetate, 8 - 12% thiourea and water, adjusting the pH 10 - 11 by adding sodium hydroxide at a temperature 25-30°C, sensitising the said film characterized in that by insitu oxidation using an organic or inorganic oxidising agent such as herein described, heating the said sensitised film at a temperature ranging between 450 -

minutes, providing gold with ohmic contact on the said film by known methods, soldering tinned copper leads to the above said gold film using an indium solder followed by lacquering to obtain the desired detector.
In an embodiment of prevent invention the organic oxidising agent used is selected from N-chloro-succinamide and N-bromo succinamde.
In an embodiment of prevent invention inorganic oxidising agent used is selected from Kl, KMnCXtand HaC^.
In an embodiment of prevent invention wherein heating is done preferably at a temperature of 490 - 540°C.
The deposition bath consists of a mixture of 30 ml of 40% lead acetate and 10 ml of 12% thiourea to which are added 50 ml of water and kept stirred for 5 minutes at a speed of 200 rpm. The glass plates attached to a perspex holder is kept immersed in the bath. After 5 minutes, 2 ml of 66% sodium hydroxide is added and after 5 minutes yet another of 8 ml of hydroxide is added and stirred for 15 minutes. The oxidising agent is added to the deposition bath after 15 min. from the complete alkali addition. In another case after the complete addition of the alkali the films were removed from the bath, washed well and dried and heat treated at 450 - 550°C for 5 - 10 minutes in oxygen atmosphere is treated with the oxidising agent.
The following typical examples are given to illustrate the invention which describes the method to increase the stability and to have drift free operational conditions when in use in any device.

Example 1
Deposition bath consists of a mixture of 30 ml of 40% lead acetate and 10 ml of 12% thiourea to which are added 50 ml of water and kept stirred for 5 minutes at a speed of 200 rpm. The glass plates attached to a perspex holder is kept immersed in the bath. After 5 minutes, 2 ml of 66% sodium hydroxide is added and after 5 minutes yet another of 8 ml of hydroxide is added and stirred for 15 minutes, then 0.01% solution of H2O2 is added and after 15 minutes complete alkali addition. The films were with distilled and dried. The dried films were heat treated at 500°C for 10 minutes in oxygen. The films after heat treatment were provided gold with ohmic contacts by vacuum evaporation unit using a suitable mask.
Immediately after gold evaporation the films were given a protective coating to avoid exposure to moisture with an optical plastic. Tinned copper leads were soldered to the gold electrodes, using indium solder.
The characteristics of the representative sample are given below: When tested under 170 V bias with a 15 W tungsten lamp at a distance of 25 cm from the source with 90 Hz chopper at the matched resistance conditions. Signal obtained after RG 1000 filter (for visible cut off) was 0.6V.
Spectral response Visible to 3 microns
Dark resistance • 1 mOhm to 2 Mohm
Signal to noise ratio 400
Sensitivity ( When tested for heavy water purity the lab analysis and the on-line monitor analysis agreed well and no drift with time was observed indicating the stability of the cell.

Example 2
Deposition bath consists of a mixture of 40m ml of 40% lead acetate 12 ml of 12% thiourea to which are added 50 cc of water and kept for stirring for 5 minutes at moderately high speed 1000 rpm. The glass plate attached to this perspex holder is kept immersed in the bath, 2 ml of 60% NaOH is added and after one minute 8 ml of 60% NaOH is added. 0.1% of N-Brumo Succinamide in 2 ml NaOH is added after 15 minutes. The films were removed after 5 minutes and washed. The films heat treated at 500°C for 10 minutes in oxygen atmosphere were provided with an ohmic contact by vacuum evaporation using a suitable mask and soldered the tinned copper using indium solder and lacquered.
The characteristics of the representative sample are given below
Spectral response Visible to 3 microns
Dark resistance 0.3 - 1 Mohm
Signal to noise ratio 70
When tested under 170 V bias with a 15 W tungsten lamp at a distance of 25 cm from the source with a chopper at 90 Hz under matched load condition the signal developed with RG 1000 filter was 5 - 6 V.
Sensitivity ( When tested for heavy water purity, the lab analysis and on-line monitor analysis agreed well and no drift was observed indicating the stability of this cell as against the cells prepared without addition agent which had a drift of 0.5 - 0.7% with time under heavy bias of 170 V required in any industrial unit.

Example 3
The deposition bath consists of a mixture of 30 ml of 40% lead acetate and 10 ml of 10% thiourea to which added 50 ml water and stirred for 5 minutes at a moderate high speed of 800 rpm. The cleaned glass plate attached to the perspex holder is kept immersed in the bath. 2 ml of 60% NaOH is added and after 1 min. 8 ml of 60% NaOH is added. The films were removed after 15 min. and washed well and dried.
The films were heat treated at 525°C for 10 min. in oxygen atmosphere after cooling to room temperature the films were dipped in 0.1% KMnO4 for 3 min. The films were removed, washed and dried. The films were further heat treated at 550°C for 3 min. in oxygen atmosphere. The films were provided with gold ohmic contact by vacuum evaporation. The tinned copper leads were soldered with indium soldered and lacquered.
The characteristics of the representative samples are given below:
Spectral response Visible to 3 microns
Dark resistance 1 Mohm to 2 Mohm
Signal to noise ratio 600
Sensitivity ( When tested under 170 V bias with a 15 W tungsten lamp at a distance of 25 cm from the source with a 90 Hz chopper under a match load condition the signal developed with RG 1000 filter was 4.5V. No drift was observed when tested indicating the stability of the cell.

The following are the main advantages of the invention:
1 Sensitising the layer before and after heat-treatment has been
introduced as an easy step for highly sensitive infrared detectors.
2 Ageing of the films is minimized by the addition of the oxidising agent
during deposition or dipping in the oxidising agent.
3 The infrared detector is stable when higher biasing voltages are
applied.

We claim:
1 An improved process for the preparation of highly sensitive lead
sulphide infrared detector which comprises depositing lead sulfide film
on a glass substrate by immersing the substrate in a bath containing 30
- 40% lead acetate, 8 - 12% thiourea and water, adjusting the pH 10 -
11 by adding sodium hydroxide at a temperature 25-30°C, sensitising
the said film characterized in that by insitu oxidation using an organic
or inorganic oxidising agent such as herein described, heating the said
sensitised film at a temperature ranging between 450 - 550°C in
oxygen atmosphere for a period of 1 - 45 minutes, providing gold with
ohmic contact on the said film by known methods, soldering tinned
copper leads to the above said gold film using an indium solder
followed by lacquering to obtain the desired detector.
2 An improved process as claimed in claim 1 wherein organic oxidising
agent used is selected from N-chloro-succinamide and N-bromo
succinamde.
3 An improved process as claimed in claims 1 & 2 wherein inorganic
oxidising agent used is selected from Kl, KMnO4and H2O2.
4 An improved process as claimed in clams 1 to 3 wherein heating is
done preferably at a temperature of 490 - 540°C.
5 An improved process for the preparation of highly sensitive lead
sulphide infrared detector substantially as herein described with
reference to the examples.

Documents:

410-del-2001-abstract.pdf

410-del-2001-claims.pdf

410-del-2001-correspondence-others.pdf

410-del-2001-correspondence-po.pdf

410-del-2001-description (complete).pdf

410-del-2001-form-1.pdf

410-del-2001-form-18.pdf

410-del-2001-form-2.pdf

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Patent Number

230535

Indian Patent Application Number

410/DEL/2001

PG Journal Number

11/2009

Publication Date

13-Mar-2009

Grant Date

27-Feb-2009

Date of Filing

29-Mar-2001

Name of Patentee

See attached documents

Applicant Address

See attached documents

Inventors:

#	Inventor's Name	Inventor's Address
1	See attached documents	See attached documents

PCT International Classification Number

H01L 27/14

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA