WO2002019379A1

WO2002019379A1 - Device and process for producing dc glow discharge

Info

Publication number: WO2002019379A1
Application number: PCT/IN2000/000077
Authority: WO
Inventors: S. K. Karkari; Subroto Mukherjee; Pucadyil Ittoop John; S. K. Nema
Original assignee: Institute For Plasma Research
Priority date: 2000-08-28
Filing date: 2000-08-28
Publication date: 2002-03-07

Abstract

A device and process for producing a DC glow discharge at low pressure comprising a cylindrical stainless steel closed vacuum chamber (3) provided with means to create a low pressure in the range of 1X10-5 to 5X10-5 mbar preferably 1X10-5 mbar, an open ended hollow cylindrical cathode (2) fixed near the one end of the said cylindrical stainless steel closed vacuum chamber at a predetermined distance from the said closed end of the said vaccum chamber by insulating means, a constricted anode (1) placed axially outside the said cylindrical cathode, a floating D.C. power supply to cylindrical cathode through a current limiting resistor and to the said constricted anode through a rectifier resulting in the emissions of the secondary electrons from the said cylindrical cathode by ion bombardement to travel along the electric field lines which are normal to the said hollow cylindrical cathode; introducing the media for creating the plasma in side the vacuum chamber to a pressure preferably at 10-2 mbar to produce the DC glow inside the chamber followed by switching off the thermionic electron emitter once the plasma is produced. The plasma so generated breaks the atoms and molecules into ions and electrons, individual atoms and small collection of atoms (radicals) resulting in better nitriding, oxidizing, coatings, killing of micro-organisms and synthesizing powders.

Description

DEVICE AND PROCESS FOR PRODUCING DC GLOW DISCHARGE

FIELD OF THE INVENTION

The present invention relates to a device and process for producing a DC- glow discharge with a constricted anode and a hollow cylindrical cathode to enhance the ionization efficiency of the glow discharge at low pressures.

The object of the invention is to increase the processing area of the substrate in the vacuum chamber , as the entire cathode area is exposed to the ion bombardment and to suitably choose the electrode geometry in order to increase the ionization efficiency of the glow discharge at low pressures.

Another object of the invention is the formation of DC glow discharge working at a lower pressure range than conventional glow discharge and to increase the electric flux density near the anode by additional ionization near the anode tip .

The embodiment of the invention resides in using a anode and cathode area ratio where anode area being much smaller as compared to the cathode area.

PRIOR ART

Glow discharges have been used-.for more than a century as an effective source for ionizing a gaseous medium and plasma formation. A large number of applications in plasma processing, like glow discharge plasma nitriding, plasma assisted chemical vapour deposition and the like derive their effectiveness by manipulating the glow discharge.

Conventionally, the dc glow discharge with plane parallel electrodes are used, where the cold cathode emits electrons by secondary emission created by ions striking the cathode. These electrons are accelerated by the electric field existing between the cathode and the anode. When the accelerated electrons traverse through the gap between the cathode and the anode, they encounter natural gas molecules and ionize them, thereby forming an electrical discharge plasma column between them.

In a glow discharge conventionally used in plasma processing, the cathode has an area smaller or equivalent to that of the anode and hence there is a lower limit of pressure to which the glow can be sustained. When the pressure is very low (~10^"3 mbar; 1000 mbar = 1 atmosphere), the gas density is low and the ionization mean free path, that is, the average distance traveled by an electron between ionizing collisions can become larger than the distance between the electrodes. By using such cathodes, which can be thermoionically heated to emit electrons, electrical discharge can still be produced. However, at such low pressures, it is difficult to maintain a glow discharge.

In a conventional discharge, the anode area is much bigger than cathode area. Thus the secondary electrons emitted from the cathode because of ion bombardment, are fewer in number because of lower cathode area. Hence it requires a higher number of neutrals (and hence higher pressures) to maintain a discharge.

In DC glow discharges, specially those used in plasma processing, the cathode is the active electrode and the vessel wall behaves as the anode, and the circuit gets completed through the ionized gas. Also the cathode area is much smaller than the anode.

Conventionally used DC glow discharges have indicated that in most of the times the cathode has dimensions of the order or smaller than the anode. This results in a loss of secondary electrons generated from the cathode and hence operating pressures are high (~ 1 mbar). In the case of using DC glow discharge, the cold cathode emits electrons by secondary emission created by ions striking the cathode. These electrons are accelerated by the electric field existing between the cathode and the anode. When the accelerated electrons traverse through the gap between the cathode and anode, they encounter neutral gas molecules and ionize them, thereby forming an electrical discharge plasma column between them.

When the pressure is very low (~10^"3 mbar), the gas density is low and the ionization mean free path, that is the average distance traveled by an electron between ionizing collisions can become larger than the distance between the electrodes. At such low pressures, a cold cathode glow discharge cannot be produced. However, by using cathodes, which can be thermoionically heated to emit electrons, electrical discharge can still be produced. It is also possible to produce a glow discharge by applying a radio-frequency (RF) voltage between the anode and the cathode. The use of radio-frequency (RF) makes it possible to produce and maintain a glow discharge at lower pressures than that of a cold cathode DC.

To overcome the aforementioned drawbacks, the subject invention is successfully using a DC glow discharge where the anode and the hollow cathode have dimensions ratio very different from that of a typical glow discharge used in plasma processing.

Here, the cathode is having a large cylindrical surface area resulting in increasing the net secondary emission of electrons as the surface area of cathode is increased. Furthermore the cylindrical hollow cathode acts as a trap for these secondary electrons. The secondary electrons from the cathode bounce back multiple times in the chamber till they finally strike the anode resulting in the increased efficiency of the discharge at low pressures.

The use of a constricted anode increase the electric flux density near it and in order to conserve the net electric current in the circuit, the electric filed near the anode gets modified. The significant increase in the electric flux density near the anode results with an additional ionization near the anode tip contributing to the added density in comparison using large anode.

The anode used in the subject invention is placed outside the cathode region thus providing more workspace area in the active hollow cathode region.

In the subject invention, there is self-electrostatic confinement of secondary electrons due to hollow cathode geometry and constricted anode.

As the anode is having very small dimensions than the cathode, the electrons do not strike the anode directly, rather they get reflected multiple times between the cathode, and thereby collide with the neutrals and ionize them. As the electron stay in the system is increased considerably, ionization is effective at very low pressures.

In the subject invention, the fundamental problem of the loss of ionizing electrons by modifying the discharge electrode geometry has been avoided.

The anode is a situated in the axial plane outside the cylindrical cathode. The outer diameter of the anode is from 3mm to 8mm, preferably 6mm and the length is 8mm to 12mm preferably 10mm.

The device of subject application is built around a cylindrical stainless steel closed vacuum chamber of length ranging between 1.15 to 1.35m, preferably 1.25mm and diameter of 0.3 to 0.8m preferably 0.5m . The chamber is provided with rotary and diffusion pumps to obtain a base pressure of 1x10^"5 to 5x10^"5 .

The cathode is stainless steel open ended hollow cylinder having outer diameter of about 300-450mm , preferably 400mm and length of about 100 to 1000mm preferably 300mm . The cathode is placed at least 5cm away from the vacuum chamber so that the cathode potential drops the plasma potential. The electrodes are electrically isolated form the grounded vacuum vessel and powered using a floating D.C power supply. For using the grounded vacuum vessel as the anode , the positive terminal of the power supply is connected to the vacuum chamber. The said vacuum chamber is provided with suitable ports for viewing and for introducing electrical probes.

Accordingly, the subject invention relates to a device for producing a DC glow discharge at low pressure comprising a cylindrical stainless steel closed vacuum chamber provided with means to create a low pressure in the range of 1x10^"5 to 5x10^"5 mbar preferably 1x10^"5 mbar having length of about 1.15 to 1.35m and the diameter of about 0.3 to 0.8 m, an open ended hollow cylindrical cathode having an outer diameter of 300-450mm preferably 400mm and a length of 100-1000mm preferably 300mm fixed near the one end of the said cylindrical stainless steel closed vacuum chamber at a predetermined distance from the said closed end of the said vacuum chamber by insulating means, means for limiting the current to the said hollow cylindrical cathode, a constricted anode placed axially outside the said cylindrical cathode, a floating D.C power supply connected to the said electrodes electrically isolated from the said vacuum chamber, a media for creating the plasma suitably depending upon the end use, wherein the plasma generated breaks the atoms and molecules into ions and electrons , individual atoms and small collection of atoms (radicals) resulting in better nitriding, oxidizing, coatings , killing of micro-organisms and synthesizing powders.

The subject invention also relates to a process for forming a DC glow discharge at low pressure, comprising the steps of creating partial vacuum in the vacuum chamber by pumping down the air inside the chamber supplying the floating DC power to said cylindrical cathode through a current limiting resistor and to the said constricted anode through a rectifier resulting in the emissions of the secondary electrons from the said cylindrical cathode by ion bombardment to travel along the electric field lines which are normal to the said hollow cylindrical cathode ; introducing the media for creating the plasma in side the vacuum chamber to a pressure preferably at 10^"2 mbar to produce the DC glow inside the chamber followed by switching off the thermionic electron emitter once the plasma is produced

The said cathode is preferably made up of stainless steel and is fixed to the said vacuum chamber by means of a pair of insulating supports. The insulating supports are in the form of insulated rods made up of preferably Teflon or ceramic.

The current supply to the said cathode is controlled by the current limiting resistor.

The anode is in the form of constricted point made up of non-sputtering permanent metal preferably of stainless steel.

The vacuum is created in the vacuum chamber by rotary or diffusion pump and the media for creating the plasma are selected from the group consisting of argon, nitrogen, hydrogen, oxygen, hexamethyl disiloxane, hydrogen peroxide or ozone or mixture thereof.

The cylindrical cathode is placed at a predetermined distance of 5cm from the closed end of the said vacuum chamber enabling the cathode potential to drop to the plasma potential.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts the schematic diagram of a conventional glow discharge having similar anode and cathode areas Figure 2 depicts the schematic diagram of the set-up used in the present invention

Figure 3 depicts the comparison of plasma densities using different anodes Figure 4 depicts the graph of breakdown voltage and gas pressure Figure 5 depicts the current-voltage characteristics of the discharge.

Figure 6 depicts the characteristics of the discharge using two different anodes.

DETAILED DESCRIPTION OF THE INVENTION In the schematic diagram of a conventional DC glow discharge as shown in Fig. 1. The cold cathode (1) emits electrons by secondary emission created by ions striking the cathode (1) . These electrons are accelerated by the electric field existing between the cathode (1) and the anode (2) . When the accelerated electrons traverse through the gap between the cathode (1) and anode (2) , they encounter neutral gas molecules and ionize them, thereby forming an electrical discharge plasma column between them.

In the subject invention, as shown in figure 2, the anode area (1) is extremely small as compared to the area of the cathode (2) . The subject invention is in making a dc glow discharge with a cold hollow cathode and a constricted anode to work at low pressures preferably ~10^"2 mbar. The anode is placed outside the cathode region at the axial plane, allowing more workspace area in the said active hollow cathode region.

The secondary electrons emitted from the said hollow cathode as a result of ion bombardment, travel along the electric field lines which are normal to the hollow cathode surface. As the anode is having very small dimensions than the cathode, the electrons do not strike the anode directly , rather they get reflected multiple times between the cathode and thereby collide with the neutrals and ionize them. As the electron stay in the system is increased considerably , ionization is effective at very low pressures. The plasma density is high in the case of constricted anode as depicted in Figure 3 where the effect of constricted anode over the larger anode was observed by making the chamber wall as anode.

The break down voltage for varying gas pressures for a given electrode configuration is depicted in figure 4. Plasma is characterized using single and double probes for different gas pressures and varying input power to the discharge electrodes. The minimum electron temperature is approximately 1- 3eV and a maximum of 10eV. The plasma shows hysterisis as shown in figure 5 with the constricted anode, where the breakdown at various anode dimensions is depicted in figure 6.

The vacuum is created by pumping down the air inside the chamber (3) as shown in figure 2 with the help of diffusion pumps and then the mixture of desired gases are introduced inside the chamber.

Initiation of the plasma is done by increasing the pressure. The plasma can be initiated by thermionic electron emission from a filament and making the plasma initiate at low pressures (~ 10^"2 mbar). Once the plasma is produced, the thermionic electron emitter may be switched off

Thus, the subject invention indicates that it is possible to maintain the D.C plasma having a significant density which can be utilized for plasma nitriding, plasma assisted chemical vapour deposition and the like, at low pressures by proper design of the anode-cathode areas and their positioning in the vacuum chamber .

The subject application is a statement of invention where various alterations and modifications are possible without, deviating from the scope of the application, hence the same should not be construed to restrict the scope of the invention.

Claims

1 A device for producing a DC glow discharge at low pressure comprising a cylindrical stainless steel closed vacuum chamber provided with means to create a low pressure in the range of 1x10^"5 to 5x10^"5 mbar preferably 1x10^"5 mbar having length of about 1.15 to 1.35m and the diameter of about 0.3 to 0.8 m, an open ended hollow cylindrical cathode having an outer diameter of 300-450mm preferably 400mm and a length of 100-1000mm preferably 300mm fixed near the one end of the said cylindrical stainless steel closed vacuum chamber at a predetermined distance from the said closed end of the said vacuum chamber by insulating means, means for limiting the current to the said hollow cylindrical cathode, a constricted anode placed axially outside the said cylindrical cathode, a floating D.C power supply connected to the said electrodes electrically isolated from the s.aid vacuum chamber, a media for creating the plasma suitably depending upon the end use, wherein the plasma generated breaks the atoms and molecules into ions and electrons , individual atoms and small collection of atoms (radicals) resulting in better nitriding, oxidizing, coatings , killing of microorganisms and synthesizing powders.

2. The device as claimed in claim 1 , wherein the said cathode is preferably made up of stainless steel and is fixed to the said vacuum chamber by means of a pair of insulating supports.

3 The device as claimed in claim 2, wherein the said insulating supports are insulated rods made up of preferably Teflon or ceramic.

4. The device as claimed in claim 2, wherein the said means for limiting the current to the said cathode is the current limiting resistor.

5. The device as claimed in claim 1 , wherein the said anode is in the form of constricted point made up of non-sputtering permanent metal.

6. The device as claimed in claim 5, wherein the said non-sputtering permanent metal preferably is stainless steel.

7. The device as claimed in claim 1 , wherein the said means for creating the vacuum in the said partial vacuum chamber is rotary or diffusion pump.

8 The device as claimed in claim 1 , wherein the said media for creating the plasma are selected from the group consisting of argon, nitrogen, hydrogen, oxygen, hexamethyl disiloxane, hydrogen peroxide or ozone or mixture thereof.

9 The device as claimed in claim 2, wherein the said cylindrical cathode is placed at a predetermined distance of 5cm from the closed end of the said vacuum chamber enabling the cathode potential to drop to the plasma potential.

10. A process for forming a DC glow discharge at low pressure, comprising the steps of - creating partial vacuum in the vacuum chamber by pumping down the air inside the chamber supplying the floating DC power to said cylindrical cathode through a current limiting resistor and to the said constricted anode through a rectifier resulting in the emissions of the secondary electrons from the said cylindrical cathode by ion bombardment to travel along the electric field lines which are normal to the said hollow cylindrical cathode ; introducing the media for creating the plasma in side the vacuum chamber to a pressure preferably at 10^"2 mbar to produce the DC glow inside the chamber followed by switching off the thermionic electron emitter once the plasma is produced.

11 The process as claimed in claim 10, wherein the said media for creating the plasma is selected from the group consisting of argon, nitrogen, hydrogen, oxygen, hexamethyl disiloxane, hydrogen peroxide or ozone or mixture thereof

12. The process as claimed in claim 10, wherein initiation of said glow discharge is done by thermionic electron emission from a filament at low pressure and subsequently switching off the said thermionic electron emitter after the said glow discharge is produced.

AMENDED CLAIMS

[received by the International Bureau on 26 June 2001 (26.06.01); original claims 1-12 replaced by amended claims 1-11 (3 pages)]

1. [AMENDED] A device for producing a DC glow discharge at low pressure comprising a cylindrical stainless steel closed vacuum chamber provided with means to create a low pressure in the range of 1x10^"5 to 5x10^"5 mbar preferably 1x10^"5 mbar having length of about 1.15 to 1.35m and the diameter of about 0.3 to 0.8 m, an open ended hollow cylindrical cathode having an outer diameter of 300-450mm preferably 400mm and a length of 100-1000mm preferably 300mm fixed near the one end of the said cylindrical stainless steel closed vacuum chamber placed at a distance of 5cm from the closed end of the said vacuum chamber by insulating means enabling the cathode potential to drop to the plasma potential, means for limiting the current to the said hollow cylindrical cathode, a constricted anode placed axially outside the said cylindrical cathode, a floating D.C power supply connected to the said electrodes electrically isolated from the said vacuum chamber, a media for creating the plasma suitably depending upon the end use, wherein the plasma generated breaks the atoms and molecules into ions and electrons , individual atoms and small collection of atoms (radicals) resulting in better nitriding, oxidizing, coatings , killing of micro-organisms and synthesizing powders.

9 A process for forming a DC glow discharge at low pressure, comprising the steps of creating partial vacuum in the vacuum chamber by pumping down the air inside the chamber supplying the floating DC power to said cylindrical cathode through a current limiting resistor and to the said constricted anode through a rectifier resulting in the emissions of the secondary electrons from the said cylindrical cathode by ion bombardment to travel along the electric field lines which are normal to the said hollow cylindrical cathode ; introducing the media for creating the plasma in side the vacuum chamber to a pressure preferably at 10^"2 mbar to produce the DC glow inside the chamber followed by switching off the thermionic electron emitter once the plasma is produced.

10. [AMENDED] The process as claimed in claim 9, wherein the said media for creating the plasma is selected from the group consisting of argon, nitrogen, hydrogen, oxygen, hexamethyl disiloxane, hydrogen peroxide or ozone or mixture thereof.

11. [AMENDED] The process as claimed in claim 9, wherein initiation of said glow discharge is done by thermionic electron emission from a filament at low pressure and subsequently switching off the said thermionic electron emitter after the said glow discharge is produced.

STATEMENT UNDER ARTICLE 19(1)

In the application of "FACILITATION CENTER FOR INDUSTRIAL PLASMA TECHNOLOGIES" filed on August 28, 2000.

For: "DEVICE AND PROCESS FOR PRODUCING DC GLOW DISCHARGE"

The subject invention relates to a device for producing a DC glow discharge at low pressure where a constricted anode and hollow cylindrical cathode is used to enhance the ionization efficiency of the glow discharge.

The device of subject invention is used to produce and extract plasma through the large opening, which is used to extract an electron beam through foils covering the small holes in the cathode.

The claim 1 has been amended to make the invention more explicit and definitive, claim 9 as originally filed is canceled and dependency of claims 10 and 11 have been amended.