Method of control of plasma stream and plasma apparatus

A plasma stream is formed by plural plasma-forming gas through which electric currents are passed and on which a magnetic field is superposed a physical parameter of the plasma stream is monitored. The magnitude of force acting on one of the jets is varied until a required result is obtained. Plural plasma burners arranged at an angle to each other are connected to a power supply and a plasma-forming gas source. Each burner incudes an open magnetic circuit with a solenoid connected to another power supply. The physical parameters of the plasma stream are recorded. The recorder is connected to a processor having connected to both power supplies and plasma-forming gas source. The burners include a drive also connected to the processing unit.

The present invention relates to plasma treatment technique and, more 
particularly, the invention relates to a method of control of a plasma jet 
and to a plasma apparatus. 
The invention can be used in the electronic industry, mechanical 
engineering, instrumentation and in other fields of science and technology 
where the plasma treatment is used. 
Known in the art is a method of control of a plasma stream, in which the 
stream is formed by a system of converging plasma jets, and characterized 
in that a magnetic system is used for superposing magnetic fields on the 
current-conducting plasma jets. This procedure makes it possible to change 
the characteristics of the plasma stream, such as its shape, size, and the 
position of the plasma jets, by varying the magnetic field intensity. This 
method, however, has disadvantages since it does not provide control of 
the characteristics of the total plasma stream which is very important for 
the final results of the plasma treatment, such as the radiation 
brightness distribution in the plasma stream cross-section, or the 
distribution of the density of ions and active atoms near the surface 
being treated. Furthermore, the prior art method does not provide a 
possibility of accurate reproduction of the plasma stream parameters 
having the same longevity (PCT 90/00286 of Dec. 26, 1990, IPC HO5 B 7/22). 
Also known in the art is a device for controlling a plasma stream (PCT 
90/00266 of Dec. 26, 1990, IPC HO5 B 7/22) comprising two plasma burners 
whose longitudinal axes are disposed at an angle to each other. The plasma 
burners are connected to an electric current supply and communicate with a 
source of a plasma-forming gas. Each plasma burner is provided with a 
magnetic system made in the form of an open magnetic circuit with a 
solenoid connected to a power supply source. This prior art device has all 
the disadvantages of the above-described method. 
A basic object of the invention is to provide a method for controlling a 
plasma stream formed by plasma-forming jets which would allow one to 
obtain preset physical parameters of the total plasma stream. 
This object is attained by providing a method of control of a plasma stream 
formed by at least two plasma-forming gas jets, through which an electric 
current flows and a magnetic field is superposed on each plasma jet; 
according to the invention, one of the physical parameters of the total 
plasma stream is monitored and, in the case of its change, an appropriate 
action is taken on at least one of the converging plasma jets until the 
preset or required physical parameters of the total plasma stream are 
attained. 
An advantage of the proposed method for controlling a plasma stream formed 
by plasma jets is a possibility of continuous monitoring of all physical 
parameters of the total plasma stream which affect the treatment of 
products. The continuous checking of the parameters and control of the 
plasma jets make it possible to change the plasma stream characteristics 
or, on the other hand, by continuously correcting these characteristics, 
the physical parameters can be kept constant over a certain period of 
time. Such a method of control allows one to use the same plasma apparatus 
for performing various operations of the treatment by presetting necessary 
values of the physical parameters of the total plasma stream. 
One of the physical parameters of the total plasma stream is its 
cross-sectional dimension. Therefore, the cross-sectional dimension of the 
total plasma stream is monitored and modified by changing the intensity of 
the magnetic field superposed on at least one plasma jet. The 
cross-sectional dimension of the total plasma stream determines the 
specific heat content at a given power transmitted to the plasma-producing 
electric discharge. The specific heat content, in turn, determines the 
result of treatment of the final product. The treatment result can be 
maintained constant due to the jet size reproduction. 
The cross-sectional dimension of the total plasma stream can be changed 
both by varying the superposed magnetic field and by varying the 
plasma-forming gas flow rate. A higher flow rate of the plasma-forming gas 
results in a decrease of the dimension of the total plasma stream since 
the higher dynamic head of the jet restrains an increase of the 
cross-sectional dimension of the stream. 
There is still another method of changing the cross-sectional dimension of 
the plasma stream in which the angle of convergence of the plasma jets is 
controlled. By increasing the angle between the directions of the 
outflowing jets, one can decrease the cross-sectional dimension of the 
total plasma stream and vice versa. 
The brightness distribution of the total plasma stream is also monitored 
and corrected by controlling the intensity of the magnetic field 
superposed on at least one plasma jet. The brightness distribution depends 
on the distribution of the plasma temperature and, therefore, the 
distribution of the excited atoms, molecules, ions and electrons in the 
plasma, i.e. the active particles in the reaction zone in the process of 
plasma treatment of a surface. Therefore, the results of the reaction 
between the plasma and the surface to be treated will depend on the 
brightness distribution. By presetting the magnitude of brightness 
distribution, one can change the intensity of the physical and chemical 
action on the surface being treated. If this distribution is reproduced in 
the process of the following treatments and kept at the same level, it is 
possible to stabilize the result of such a treatment. 
A more important characteristic of the plasma stream, compared to the 
brightness distribution in the stream cross-section, is the distribution 
of the spectral radiation factor of ions, atoms, radicals and molecules. 
In the first approximation, the radiation intensity is proportional to the 
concentration of the above particles. The surface plasma treatment rate 
and quality depend on the concentration of the active plasma components. 
In this connection, it is desirable to have information on the spectral 
radiation factor of a plasma jet, which enables one to determine the 
concentration of active particles in the total plasma stream, and, by 
changing the composition of the plasma-forming gas or its flow rate, to 
control the distribution of spectral radiation in the total plasma stream. 
It is reasonable to trace directly the concentration of ions in the plasma 
stream and, acting on the converging plasma jets by varying the 
composition of the plasma-forming gas or its flow rate in at least one 
jet, to change the concentration of ions in the total plasma stream, 
because during the interaction of the plasma stream with the surface being 
treated, the plasma properties suffer significant changes, and the plasma 
loses it equilibrium physically and chemically while the interpretation of 
the spectral data under these conditions is very difficult. 
It is also necessary to monitor the distribution of the heat flow in the 
plasma jet and to perform a physical action on the converging jets to 
obtain the preset values of the heat flow distribution in the total plasma 
stream. The physical action is effected by controlling the electric 
current flowing through the plasma jets. This is necessary because, in 
addition to the flows of active particles to the surface being treated, 
the plasma jet transfers a lot of heat to this surface. This heat warms up 
the surface being treated and affects the rate of the chemical reactions 
and, therefore, the uniformity and quality of the treatment. 
The proposed method can be carried into effect by means of a plasma 
apparatus comprising at least two plasma burners disposed at an angle to 
each other, connected to a power supply and communicating with a source of 
a plasma-forming gas. Each plasma burner is provided with a magnetic 
system made in the form of an open magnetic circuit with a solenoid 
connected to a power supply. The magnetic system has a unit for recording 
the physical parameters of the plasma stream connected to a processing 
unit whose outputs are connected to the power supply of the plasma burners 
and/or solenoid, and/or the plasma-forming gas source. 
Such as apparatus capable of checking the physical parameters of the plasma 
stream makes it possible to perform the above-described method in a simple 
manner, for example, when the unit for recording the physical parameters 
is made in the form of an optical system installed so that its optical 
axis intersects the longitudinal axis of the plasma stream and a 
light-sensitive cell is installed in the image plane of the optical 
system. 
The light-sensitive cell may be made of a string of photodetectors enabling 
one to check the brightness distribution in the plasma stream 
cross-section. 
If the above-described unit for recording the physical parameters is 
provided with a dispersing element installed between the optical system 
and the light-sensitive cell, it is possible to monitor the distribution 
of the spectral radiation factor in the plasma stream. 
In order to monitor the heat flow distribution, the unit for recording the 
physical parameters may be made as a thermocouple installed so that it is 
in contact with the plasma stream in its cross section. 
Since the concentration of ions in the plasma stream influences the plasma 
electrical conductivity, at least one electric probe made in the form of a 
pair of electrodes may be used as a recording unit to monitor the ion 
concentration. This electric probe is installed so that some ends of the 
electrodes are in contact with the plasma stream while the other ends are 
connected to a power supply and a current meter. The whole unit is 
installed with a possibility of crossing the longitudinal axis of the 
plasma stream. 
The invention will be better understood from the following detailed 
description of some specific embodiments of the invention, which do not 
limit the scope of the same, and with reference to the accompanying 
drawings, in which: 
FIG. 1 is a general view of the apparatus; 
FIG. 2 shows the optical recording unit with light-sensitive cells; 
FIG. 3 is a simple diagram of the processing unit; 
FIG. 4 shows the optical recording unit with a string of photodetectors; 
FIG. 5 shows the unit for pre-processing the signal transmitted from the 
string of photodetectors; 
FIG. 6 is a diagram of the signal takes from the string of photodetectors; 
FIG. 7 shows the optical recording unit with a dispersing element; 
FIG. 8 a schematic view of the apparatus with a thermocouple as a recording 
unit; 
FIG. 9 is a very simple embodiment of the electric probe.

Referring to FIG. 1, consider the operation of the proposed plasma 
apparatus to clarify the essence of the proposed method. 
Shown in FIG. 1 is the simplest embodiment of the proposed apparatus. This 
apparatus comprises two plasma burners 1 arranged at an angle of 
90.degree. to each other and produce a total plasma stream 2. The burners 
are provided with an electric drive 3 allowing the angle and distance 
between them to be varied. Each burner 1 is equipped with a magnetic 
system consisting of open magnetic circuits 4 carrying solenoids 5 
connected to a current supply 6. The magnetic circuits 4 are made of 
electrical steel with a cross section of 0.3 cm.sup.2. The solenoids 5 
consist of 1000 turns of a copper wire. The plasma burners 1 are connected 
to a power supply 7, which is a d.c. voltage source; the positive terminal 
of the power supply is connected to one plasma burner and the negative 
terminal is connected to the other burner. In addition, each burner 1 is 
fed with a plasma-forming gas from a supply system 8. The apparatus 
comprises a recording unit 9 connected to a processing unit 10 whose 
outputs can be connected to the inputs of the electric drive 3, power 
supply 6 of the solenoids, power supply 7, and plasma-forming gas supply 
system 8. Let us consider the recording unit in the form of an embodiment 
with an optical detector shown in FIG. 2, where the elements similar to 
those in FIG. 1 have the same reference numerals. The recording unit shown 
in FIG. 2 is a single-element lens 11 whose optical axis intersects the 
longitudinal axis of the plasma stream 2 and has a string of photodiodes 
12 whose outputs are connected to the inputs of the processing unit 10. 
The simplest version of the processing unit 10 is shown in FIG. 3. This 
unit is a system of primary adders 13, one input of each adder receiving 
the data on the electric currents from the string of photodiodes 12 and 
the other input being fed with preset values of these currents. The 
outputs of the primary adders 13 are connected to the inputs of a common 
adder 14. In turn, the signal from the common adder 14 is applied to one 
of the inputs of multipliers 15, and weighting factors are applied to the 
second input of these multipliers. The outputs of the multipliers 15 are 
outputs of the processing unit 10 and, for example, are connected to the 
control inputs of the drive 3. The weighting factors are found 
experimentally. Each weighting factor reflects the degree of change of the 
observed parameters of the plasma stream in response to a given physical 
action. The factor value is lower, as the rate of change of the parameter 
of the plasma stream becomes greater for a corresponding unit of action. 
The installation operates as follows. 
The plasma burners 1 are supplied with nitrogen through the plasma-forming 
gas supply system 8, and an electric d.c. current of 100 A from the power 
supply 7 flows between the burners 1 through the plasma jets. The outflow 
plasma jets form a total plasma stream 2. The initial direction of the 
plasma jets is determined by setting a required position of the burners by 
means of the drive 3. A required size of the plasma stream 2 is 
established by changing the angle between burners 1. An increase of the 
angle between the burners 1 for one degree results in an increase of the 
cross-sectional dimension of the overall flow 2 in the cross section under 
discussion of 5 mm. 
Superposed on the current-carrying portions of the plasma jets is a 
magnetic field which is produced between the poles of the open magnetic 
circuit 4 by passing an electric current of 100 A from the power supply 6 
through the solenoids 5. 
A required cross-sectional dimension of the plasma stream 2 is determined 
by the processing unit 10 by setting the values of the currents I.sub.1 
-I.sub.6 at the inputs of the primary adders 13. If the dimension of the 
total plasma stream 2 diverge from the preset value, the primary adders 13 
produce output error signals .increment.I.sub.1 -.increment.I.sub.6 
proportional to the difference between the observed and preset values of 
the currents of the photodiodes 12. The error signals .sub..increment. 
I.sub.1 -.sub..increment. I.sub.6 are summed up by the common adder 14 
whose output signals are applied to the inputs of the multipliers 15. The 
outputs of the multipliers 15 are outputs of the processing unit 10 and 
control inputs of the drive 3. In the presence of a signal at the output 
of the multipliers 15 and appearance of this signal at the input of the 
drive 3 of the plasma burners, the drive 3 will change the angle between 
the burners 1 until the signal from the common adder 14 is equal to zero, 
i.e. a preset dimension of the total plasma stream is established. In a 
similar way, one can change the cross-sectional dimension of the plasma 
stream 2 by controlling the flow rate of the plasma-forming gas with the 
same value of the magnetic field of the open magnetic circuit 4 or, on the 
contrary, by varying the magnetic field of the magnetic circuit 4 with a 
constant flow rate of the plasma-forming gas. In these cases, the signals 
of the processing unit 10 are control signals for the plasma-forming gas 
supply system 8 or for the source 6 to supply electric current to the 
solenoids. The control signal applied to the system 8 for supply of 
plasma-forming gas decreases or increases its flow rate thereby affecting 
the cross-sectional dimension of the plasma stream 2. If the control 
signal is sent to the source 6 supplying an electric current to the 
solenoids, the magnetic field superposed on each of the plasma jets also 
leads to a change of the cross-sectional dimension of the plasma jets. 
From the above it is clear that the essence of the proposed method of 
control of a plasma stream formed by at least two plasma-forming gas jets 
consists in that these jets are acted on by electric currents flowing 
through them and by a magnetic field superposed on each jet. One of the 
physical parameters of the total plasma stream is monitored end controlled 
by acting on at least one plasma jet and the magnitude of this action is 
varied to obtain the preset values of physical parameters of the total 
plasma stream. 
The apparatus shown in FIG. 1 enables one to monitor and modify the 
cross-sectional dimension of the total plasma stream 2. However, one of 
the most informative physical parameters of the plasma stream is 
distribution of its radiation brightness over the cross-sectional area of 
this stream. The brightness helps to estimate the size of the flow, its 
symmetry, temperature distribution and enthalpy, i.e. the flow 
characteristics determining the result of the surface treatment. Shown in 
FIG. 4 is an optical recording unit for tracing the brightness 
distribution in the total plasma stream 2 including a lens 11 and a 
photodetector based on a string 16 of photosensitive cells. The image of 
the plasma stream 2 is projected by the lens 11 onto the string 16 of 
photosensitive cells. The photodetector may be made in the form of a 
series of photodiodes or a unit based on charge-coupling devices having 
100 or more photosensitive cells. 
The signal from the string 16 of photosensitive cells is transmitted to a 
pre-processing unit whose circuit diagram is shown in FIG. 5. 
In this specific circuit use is made of a string based on charge-coupling 
devices. The principle of operation of this circuit is based on comparison 
of the signal from each string with a reference signal. The coordinate of 
the jet center is the middle of an interval within which the level of the 
signal from the string 16 of charge-coupling devices exceeds the reference 
signal level. The circuit operates as follows. 
Following the commands from the generator 17, the signals from the elements 
of the string 16 are transmitted through a switch 18 to a comparator 19. 
In the comparator 19 these signals are compared with the reference value 
and, when a signal from any element of the receiving string 16 reaches the 
reference value of the comparator 19, the latter is set to the "one" state 
thereby rendering the switch 20 conductive. 
The output of the generator 17 is connected through the switch 20 to the 
input of a counter 21. As soon as the "1" signal appears at the output of 
the comparator 19, the switch 20 closes the circuit and the digital code 
at the counter 21 corresponds to the number of the element of the 
receiving string 16 whose output signal has coincided with the reference 
signal. This digital code is recorded in a register 22. 
After the switch 20 has broken the circuit to the counter 21, the signals 
from the generator 17 are sent through an element 25 to a counter 24 until 
the signal from the elements of the receiving string 16 becomes lower than 
the reference signal. After that, the comparator 19 is put to the "zero" 
state and the switch 20 is closed. Therefore, the counter 24 acquires a 
code corresponding to the amount of cells with the signal whose level 
exceeds that of the reference signal. 
The code of the counter 24 is applied to a shift register 25 performing an 
operation of division by shifting the code to the right for one position. 
Then, this code and the code of the register 22 are summed up in the adder 
26 and sent to a digital-analog converter 27 and are applied to the input 
of the processing unit 10 through a switch 28. 
The trailing edge of the signal passes through a delay line 28 and resets 
the counters 21 and 24. 
The trailing edge of the signal passes through a delay line 30 and opens 
switches 20 and 28. 
After the trailing edge of the second signal "1" has passed the delay line 
30, the counter 31 produces a signal applied to the inputs of the 
processing unit 10 (FIG. 3). 
Therefore, the inputs of the primary adders 13 of the processing unit 10 
are supplied with information on the position of the centers of the jets. 
In a simple case, when the total plasma stream is formed of two converging 
jets, the projection data from the string 16 represent a double-hump curve 
shown in FIG. 6. The maximum position corresponds to the coordinate of the 
converging jets in the considered cross section of the total plasma stream 
2. 
In this example, the outputs of the multipliers 15 are connected to the 
inputs of the current supply sources 6 of the solenoids 5. In accordance 
with the Ampere law, the interaction of the magnetic field with the 
electric current flowing through the current-conducting portions of the 
jets initiates a force which deflect the plasma jets. If the current is 
changed by 10 mA, the jet center in the cross section under consideration 
is deflected for 3 mm. Thus, the size and shape of the overall flow 2 are 
controlled by varying the current flowing through the solenoid 5. 
A required brightness distribution in the plasma jet is assigned in the 
processing unit 10 by setting the values of the currents of the primary 
adders 13, the information on the position of the centers obtained from 
the charge-coupling devices of the string 16 being applied to the same 
unit 10. 
The operation of this unit is generally performed similarly to that 
described in the above example shown in FIG. 1, however, in this case the 
jets are acted on by controlling the magnetic field. In the presence of a 
signal at the output of the multipliers 15 and its appearance at the input 
of the current supply 6 of the solenoids 5, this signal will change the 
current flowing through the solenoids 5 of the magnetic system until the 
voltage at the output of the multipliers 15 is equal to zero indicating 
the brightness distribution in the total plasma stream 2 coincides with a 
preset value. 
The brightness distribution in the total plasma stream 2 can also be 
controlled by varying the angle of convergence of the plasma jets, i.e. by 
changing the mutual position of the of the burners 1 by means of the 
electric drive 3 (FIG. 1), or by varying the flow rate of the 
plasma-forming gas in the jets. In these cases the control signals of the 
processing unit 10 are sent either to the electric drive 3 or to the 
plasma-forming gas supply system 8. 
Consider now an example of controlling the total plasma stream 2 by the 
results of tracking the distribution of the spectral radiation factor. 
This makes it possible to form and maintain very accurately a preset 
plasma composition, which determines the plasma treatment rate and 
quality. FIG. 7 illustrates an embodiment of the optical recording unit 
including a single-element lens 12 (similarly to the optical unit of FIG. 
4) which helps to project the plasma stream image onto a slot 32 which 
cuts off a required projection of the flow. Installed behind the slot 32 
is a dispersing element or a lens 33. The prism 33 is capable of turning 
about an axis normal to the optical axis of the lens 12. The radiation 
flux formed by the lens 12 and slot 32 passes through a prism 33 and is 
dispersed into a spectrum which is recorded by the coupling-charge 
elements of the string 16. The radiation of a definite wavelength is 
projected onto the coupling-charge elements of the string 16 by turning 
the prism 33. In so doing, a necessary value of the distribution of the 
radiation spectral factor on a definite wavelength is put in the 
processing unit 10 of the unit shown in FIGS. 1, 3. The signal taken from 
the coupling-charge elements of the string 16 is applied to the input of 
the processing unit 10 producing an output control signal applied to the 
input of the plasma-forming gas supply system 8, varying the gas 
composition, for example, by increasing the quantity of oxygen in the 
plasma-forming gas (nitrogen). 
The apparatus shown in FIG. 1 makes it possible to control the total plasma 
stream by measuring the heat flow in the total plasma stream 2. The heat 
flow warms up the surface being treated and this affects the speed of the 
chemical reactions occurring in the process of treatment and can lead to 
non-uniform processing of the surface or to a poor quality of the treated 
surface. 
The elements of the apparatus shown in FIG, 8 and identical to those in 
FIG. 1 have the same reference numerals. 
In the apparatus shown in FIG. 8 the unit for recording the physical 
parameters of the total plasma stream 2 is made in the form of a drive 34 
with holders 35 carrying a thermocouple 36. The drive 34 allows the holder 
35 with the thermocouple 36 to move in a vertical direction along the 
plasma stream 2 and to move in a horizontal plane to cross the plasma 
stream 2. The thermocouple 36 is installed on the holder 35 so that its 
sensing area comes in contact with the plasma stream 2 when crossing it. 
The magnitude of the electromotive force appearing across the thermocouple 
is used for estimation of the heat flow in the cross section of the plasma 
stream 2 being measured. In a way, similar to that described above, the 
signal from the thermocouple 56 is transmitted to the processing unit 10 
and the output signal of this unit is applied to the power supply 7 of the 
plasma burners 1 varying the electric current flowing therethrough. 
The vertical motion of the holder 35 makes it possible to determine the 
heat flow at any cross section of the plasma stream 2. 
During the interaction of the plasma stream with the surface being treated 
the plasma properties are changed considerably; the plasma acquires a 
state of non-equilibrium physically and chemically. Under these conditions 
it is reasonable to check the ion concentration in the plasma stream. The 
electrical conductivity of the plasma stream depends on this 
concentration. The higher the ion concentration, the higher the electrical 
conductivity. 
Therefore, to measure the electrical conductivity of the plasma stream in 
the apparatus shown in FIG. 8, it is sufficient to install so additional 
counter, i.e. an electrostatic probe 37 on the holder 35. The construction 
of the electrostatic probe 37 is shown in FIG. 9. The electrostatic probe 
has an insulation plate 38, on which two conductors 39 are mounted. The 
lower ends of the conductors are connected to the unlike poles of a 
battery 40, and a current meter 41 is inserted in this circuit. The signal 
from the current meter 41 is applied to the input of the processing unit 
10 (FIG. 8). The upper ends of the conductors are in contact with the 
plasma stream 2 as soon as the holder 35 starts moving in a horizontal 
plane. When the holder 35 crosses the plasma stream 2, the ions and 
electrons of the plasma start moving from one energized conductor to the 
other. As a result, the electric circuit is closed and an electric current 
starts flowing through this circuit, the value of this current being 
indicated by the current meter 41. The magnitude of the measured current 
allows one to estimate the ion concentration in the plasma stream 2. The 
concentration of ions can be varied similarly to the abovedescribed 
examples by changing the composition of the plasma-forming gas or by 
varying the flow rate of this gas. 
In order to change the distribution of the ion concentration in the flow, 
several conductors 39 must be installed on the insulator 38. One conductor 
is connected to one terminal of the battery 40 while the rest are 
connected to the other terminal of the battery 40. From the output of each 
probe 39 a current signal is taken and sent to the input of the processing 
unit 10. In this case, the plasma stream is controlled in a manner similar 
to that described above. 
Described above are preferred embodiments of the invention. It is obvious 
that those skilled in the art may make changes and modifications in the 
method and apparatus without departing from the scope of the present 
invention. 
For example, using a more complex processing unit, one can check not only 
individual parameters of the plasma stream but also a set of these 
parameters to make them stable in time and, therefore, to attain good 
reproducibility of the high-quality treatment.