JP3167221B2 - Inductively coupled plasma generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma emission source used in an analytical instrument such as a spectrometer. The plasma emission source supplies photons and ions that are used to excite the sample to produce an emission of light at a wavelength indicative of the atoms of the sample. The emitted light is then detected by a spectrometer to identify the sample.

[0002]

BACKGROUND OF THE INVENTION The apparatus used to generate or create a plasma emission source generally comprises a radio frequency (RF) generator whose output is inductively coupled to a load, such as a plasma torch. Such a device is disclosed in U.S. Pat. No. 4,629,940 issued to the inventor of the present invention.
And No. 4,935,596.

[0003] A problem associated with all such conventional devices is that they cannot properly regulate and control the output power for use in today's devices. The present invention substantially eliminates this problem.

[0004] An important feature of such devices is the efficient transfer of power from the RF generator to the load. Since the RF generator is inductively coupled to the load, impedance mismatch between the load and the RF generator is an important issue to be overcome. This can result in very poor transmission of power with impedance mismatch as well as reflected output that may damage or destroy the generator circuit.

[0005]

In a fixed frequency RF device, such as that described in U.S. Pat. No. 4,629,940, described above, the problem of mismatch is between the RF generator and the plasma torch or load. Is reduced by a rather sophisticated electromechanical hybrid device to continuously match the impedance of the

It is an object of the present invention to provide an inductively coupled plasma generator that solves the mismatch problem without the need for such cumbersome equipment.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an oscillator for generating RF power, a load connected to the oscillator for generating plasma, and an actual RF supplied to the load. Achieved by an inductively coupled plasma generator comprising detector means for measuring power, and circuit means connected between said load means and said oscillator means for maintaining said RF power at a commanded level. Is done.

In another innovation and improvement, the present invention has eliminated the need for such sophisticated and cumbersome devices. Since impedance and frequency are functionally correlated, the present invention overcomes the mismatch problem by having the changing RF frequency automatically adjust for impedance mismatch. As described more fully in the specification, the desired RF
The signal indicating the deviation of the actual RF power from the power is RF
Used to control the generator.

Conventional inductively coupled RF plasma generators have been used regardless of the plasma potential. Thus, in these conventional devices, the plasma potential can often be high relative to ground, which can be destructive parasitic emission to a glass torch in atomic emission spectroscopy, or erodible to a sampler cone in mass spectroscopy. Causes damage. The present invention overcomes this problem by maintaining the plasma at or near ground potential throughout operation.

The present invention also includes means for monitoring generator parameters during operation and providing means for shutting down the circuit, eg, to prevent damage to the inductively coupled plasma generator. Misfires or misfires in the plasma can be a serious problem as such conditions may cause damage to the circuit or glass torch. The present invention solves this problem by detecting such ignition problems and continuously monitoring the generator parameters to shut off the device before damage occurs.

The present invention relates to an inductively coupled plasma generator for generating a plasma for use in an analytical instrument, for example, an atomic emission or mass spectrometer. The generator comprises a circuit including a vacuum triode adapted to be used as a radio frequency oscillator to generate a plasma. A first object of the present invention is accurate adjustment and stable control of RF power to a load coil. To this end, the present invention comprises a closed loop feedback circuit in which the actual output generated is used to control the oscillator output power.
To accomplish this, means are provided for measuring and multiplying the RF voltage and current to provide an output indicative of the actual RF output power. This output is compared to the commanded power to generate an error signal for application to the grid of the oscillator whose output is to be controlled. This is achieved in a way that changes the efficiency of the generator as well as eliminates the need for dynamic impedance matching. The invention also includes means for maintaining the plasma at or near zero potential and means for monitoring operating conditions of the generator. Plasma conditions are continuously monitored by sensing the condition of circuit parameters to prevent damage caused by misfiring or misfiring of the plasma.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0013]

1, there is shown an inductively coupled plasma generator 10 of the present invention. The generator is an oscillator,
For example, it comprises a triode vacuum tube 11 designed to operate as a Colpitts oscillator as shown in FIG. 2 in the radio frequency RF range. Plate voltage is high voltage power supply 1
2 to the generator 10. The high voltage power supply 12 has the potential to supply voltage at several distinct values, for example, four distinct values corresponding to low, medium, high, and ignition output ranges. The grid bias voltage is supplied to generator 10 by grid control circuit 13.

The oscillator or generator 11 is a glass torch 1
8 is connected to a plasma load coil 17 arranged around. The output power to the load coil 17 is the load coil 1
7 is supplied through an oscillator resonance circuit 16 which is one of the components. Resonant circuit 16 is shown in schematic detail in FIG. Argon or a similar gas is introduced, for example, through a glass torch through the opening 18a, and the load coil 17 is ionized by the electromagnetic field generated by the RF power circulating through the resonant output circuit 16, its inductive part. The generated plasma is symbolically indicated as flame 18b. However, it should be noted that the plasma consisting of photons and ions is used differently depending on its use in an atomic emission spectrometer or mass spectrometer.

The capacitance of the oscillator resonance circuit 16 is divided as shown in FIG. 2 to act as a voltage divider so that the actual ground appears at the center of the load coil 17. This allows the plasma to be operated at or near zero potential due to its attendant advantages of eliminating parasitic emissions when used in an atomic emission spectrometer and emissions that erode the sampler when used in a mass spectrometer.

The capacitive voltage divider circuit 14 and the current transformer circuit 15 are connected between the generator 11 and the radio frequency multiplier circuit 19. The radio frequency multiplier circuit 19 is a comparator circuit 2
Supply input to 1. Capacitive voltage divider circuit 14 provides an output to RF multiplier circuit 19 indicating the RF voltage output of generator 11, while current transformer 15 provides an signal to RF multiplier circuit 19 indicating the RF current output of generator 11. Supply. The voltage divider circuit 14 and the current transformer circuit 15 are connected to the RF supplied to the load.
Connected so as not to interfere with power.

The RF voltage and current are effectively multiplied in an RF multiplier circuit 19 to provide an output signal indicative of the RF power actually being supplied to the plasma 18b through the load coil 17. In an embodiment of the present invention, R
The F multiplier circuit 19 is a 1990/91 edition of Analog Devices, Inc., "Linear Products Databook".
The same logarithmic response amplifier and adder circuit as the AD8344 quadrant (for quadrant) multiplier as described on pages 6-65. RF voltage signal and RF
Since the current signals are logarithmically amplified and then added together, they are arithmetically equivalent to being multiplied together. Since the arithmetic product of the RF voltage and the RF current is RF power, the output signal from RF multiplier circuit 19 indicates the RF power being supplied to the plasma through load coil 17.

The signal indicating the RF power is continuously applied to the comparator circuit 21. Microprocessor 23 provides a second input to comparator circuit 21 indicating the desired or commanded RF power. A digital-to-analog converter
The digital data from the microprocessor is inserted between the microprocessor 23 and the comparator circuit 22 to transmit the RF data to the RF circuit.
The signal is converted into an analog form that can be compared with the analog signal from the multiplier circuit 19.

The comparator circuit 21 supplies an error signal to the grid control circuit 13. This error signal indicates a positive or negative deviation of the actual RF power going to the load coil 17 from the desired or commanded power provided by the microprocessor 23. In particular, this value is supplied by a host computer having a keyboard input device or the like and a display / printer. The signal from the comparator circuit 21 is fed back to the grid control circuit 13 and maintains the RF power to the load coil 17 precisely and accurately regulated and stably at the commanded power.

The grid control circuit includes a power transistor,
For example, it includes a FET transistor to which an error signal is supplied. The transistor controls the grid bias of the triode vacuum tube 11 according to the error signal.

In order to extend the range of the tube 11 of the oscillator, its efficiency changes the grid current of the tube 11 according to the actual output power, eg low efficiency for low grid currents and high efficiency for high grid currents. Can be varied between about 40% and 60%.

Instrument circuit 25 has an input from generator 11. The instrument circuit 25 has an output connected to a comparator circuit 26 and an analog-to-digital converter circuit 29 which itself provides an input to the microprocessor 23. Monitor circuit 27 provides a second input to comparator circuit 26, the output of which is provided as an input to the protection circuit. Protection circuit 28
Are connected to a high voltage power supply.

One purpose of this device is to provide a plate voltage,
Generator 11 such as plate current and grid current
Monitoring the operating parameters of the Therefore,
When one of the parameters exceeds the threshold value set by the monitor circuit 27, the protection circuit 28 shuts off the high voltage power supply 12. This feature is particularly important when determining the operating conditions of the plasma by monitoring the grid current.
Prior to ignition of the plasma emission, the grid current is extremely high. This is because very little power is being absorbed from the load coil and most of the power supplied by the RF generator 11 is, as mentioned above, essentially a triode vacuum generator. Feed back to the grid. If the grid current remains high for more than a predetermined time, it may indicate an ignition problem. The protection circuit 28 cuts off the high voltage from the power supply 12 to the generator 11 to prevent damage to the circuit and / or the glass torch 18. Similarly, the grid current can indicate good ignition by returning from a high pre-ignition value to a stable low operating condition. Changes in the grid current may also indicate a "bad" plasma, one of which is a breakdown discharge. This may also be detected in a timely manner so as to shut off the high voltage to generator 11 before damage occurs.

Depending on the predetermined RF power range, eg, ignition or various operating ranges, the microprocessor 23
Controls the power level output of generator 11 by adjusting the amount of power provided by power supply 12. This means, for example, that the actual high voltage level is
This can be achieved by turning on one of the four triacs or similar devices that control the plate.

Referring particularly to FIG. 2, the oscillator of the present invention is shown. As can be seen, the oscillator consists of a triode tube 11 connected as a somewhat modified Colpitts oscillator. Resonant output circuit 16 is essentially a capacitor C ₁ whose junction is connected to a grounded cathode.
And it consists of C ₂ load coil 17 connected in parallel to the. This Korupitsutsu oscillator circuit is not equal those in the device of conventional on the other hand unlike the circuit of conventional for C ₁ and C ₂ are connected to the equal and opposite values to each other in the device. In the present device, the ratio of C ₂ to C ₃ controls the grid drive power while C
Ratio of ₁ to C ₂ to perform this function.

By having capacitors C ₁ and C ₂ connected equal and opposite to the reactance, the voltage across the load coil 17 is equal in magnitude but opposite in phase to ground. The function of this device is to operate the plasma without an indirect DC potential, i.e. at ground potential, with the attendant advantage of eliminating damage to the glass torch in atom emission spectroscopy or damage to the sampler cone in mass spectroscopy. The actual grounding occurs at 17 electrical midpoints.

Microprocessor 23 obtains information about predetermined or commanded RF power and other operating parameters, such as the level of plate voltage from high voltage power supply 12 from host computer 24.

The commanded RF parameter signal is supplied to a comparator circuit 22 via a digital-to-analog converter 22 while a high voltage (low, medium, high and ignition) at which the oscillator can be activated is a digital input. Output circuit 3
1 via a high voltage power supply 12.

An inductively coupled plasma generator with novel means for accurate regulation and stable control of RF output power has been described. Because the circuit measures the true RMS power, it takes into account the different plasma operating conditions and the phase shift of the RF current and RF voltage due to the change in load impedance caused by the change in plate impedance of the vacuum tube by changing the grid bias.

In addition to maintaining the plasma at ground voltage, the tube grid current is constantly monitored to prevent the damage problem described above.

Finally, as noted above, the power adjustment range of the generator is extended by changing the efficiency of the tube generator of the RF generator.

[0032]

As described above, the present invention relates to oscillator means for generating RF power, load means connected to the oscillator for generating plasma, and actual RF power supplied to the load means. Detector means for measuring
Mismatch problems without the need for cumbersome devices, as the circuit consists of circuit means connected between the load means and the oscillator means to maintain the RF power at a commanded level. Can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an inductively coupled plasma generator of the present invention.

FIG. 2 is a schematic diagram showing an oscillator according to the present invention.

[Explanation of symbols]

 Reference Signs List 10 inductively coupled plasma generator 11 oscillator means (triode vacuum tube) 12 high voltage power supply 13 grid control circuit 14 first means (capacity voltage divider) 15 second means (current transformer) 16 resonance circuit 17 load means (load coil) Reference Signs List 18 glass torch 19 third means (radio frequency multiplier circuit) 21 comparator circuit 22 digital-analog converter circuit 23 microprocessor 28 protection circuit

Continuation of the front page (72) Inventor Peter Jay Morislow Connecticut, United States 06776, New Milford, Cornwall Drive 27 (56) References JP-A-59-171838 (JP, A) JP-A-60- 205241 (JP, A) JP-A-62-145700 (JP, A) JP-A-4-72599 (JP, U) JP-A-63-135799 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H05H 1/00 H05H 1/46 G01N 21/73 G01N 27/62 H01J 49/10

Claims (11)

(57) [Claims] 1. Oscillator means for generating RF power
When, maintaining a load means connected to said oscillator for generating a plasma, detector means for measuring actual RF power delivered to said load means, the level of the commanded RF power and a circuit means connected between said oscillator means and said load means to said detector means, indicates the RF voltage output of said oscillator means
Connected to the output side of the oscillator means for supplying
First means and before supplying a current indicative of the RF current output of the oscillator means;
A second means connected to the output side of the oscillator means, and a second means connected to the inductive load means by the oscillator means.
The first and second power supply circuits supply an output voltage indicating the RF power at the time of
To effectively multiply the outputs of the first and second means.
Inductively coupled plasma generator characterized in that it comprises a third means connected to the pre-second means. 2. The comparator circuit means , wherein said circuit means is connected to said third means for receiving said voltage indicative of said actual RF power, said comparator circuit means indicative of said commanded RF power. Connected to the comparator circuit to provide an input to the fourth
Means, the comparator circuit means for providing an error signal output indicative of a difference between the actual RF power and the commanded RF power.
Stage, induction of claim 1, characterized in that it consists of said oscillator means and control circuit means connected to said comparator circuit for controlling the RF power of said oscillator means has but I and the error signal Combined plasma generator. 3. An inductively coupled plasma generator according to claim 2 , wherein said oscillator means comprises a triode vacuum tube having a plate, a cathode and a grid. 4. A grid control connected to said grid of said triode vacuum tube to apply said error signal to said grid to maintain RF output power at said commanded RF power. The inductively coupled plasma generator according to claim 3 , comprising a circuit. 5. The grid control circuit according to claim 1, further comprising:
5. An inductively coupled plasma generator according to claim 4 , including output transistor means for controlling grid current to vary the efficiency of said oscillator means between limits determined by F output power. 6. The glass torch, a load coil disposed around the glass torch, the load to form a resonant circuit connected between the plate and the cathode of the triode with the load means. The inductively coupled plasma generator according to claim 5 , comprising first and second series-connected capacitors connected in parallel to the coil. 7. An inductively coupled plasma generator according to claim 6 , wherein said capacitors are of equal value and are connected to ground at their junction, thereby maintaining the plasma voltage at zero potential. 8. The inductively coupled plasma generator according to claim 7 , wherein said fourth means is a microprocessor programmed to provide said commanded RF power. 9. In addition, the plate voltage supply means connected between said plate and said microprocessor of said triode for the there was the commanded RF power <br/> changing the plate voltage I The inductively coupled plasma generator according to claim 8 , comprising: 10. An inductively coupled plasma generator according to claim 9 , wherein said first means and said second means each comprise a logarithmic response amplifier, and said third means comprises an adder circuit. And further comprising protection circuit means responsive to significant changes in grid current of the triode to disconnect the power supply from the triode to prevent damage to the glass torch. Claim 1.
0. The inductively coupled plasma generator according to 0 .