WO1989001135A1

WO1989001135A1 - Sensor and method for detecting levels of conductive liquids

Info

Publication number: WO1989001135A1
Application number: PCT/US1988/002549
Authority: WO
Inventors: Devlin M. Gualtieri; Edward Porbansky; Dave M. Narasimhan
Original assignee: Allied-Signal Inc.
Priority date: 1987-07-30
Filing date: 1988-07-27
Publication date: 1989-02-09

Abstract

An inductance coil serves as a displacement sensor arranged in connection with a tuned circuit of an oscillator. Liquid level displacement in an LEC crystal growth environment is sensed as a frequency change in the oscillator circuit as compared to a set point value. The sensor is protected from the environment by an outer protective sheath of non-conductive, non-contaminating material such as boron nitride.

Description

Sensor and method for detecting levels of conductive liquids

Background of the Invention The invention relates to an inductive sensor for use in detecting the level of conductive liquids in a sealed high pressure, high temperature environment, for example, of the type used in liquid-encapsulated Czochralski (LEC) growth of crystals such as gallium arsenide or indium phosphide.

Inductive sensors have been known for use in a number of inspection and monitoring applications. The basic principles of inductive techniques are well known. More specifically, an oscillating current flowing in a coil causes the field of one winding to add to the field of the next winding. The fields pulsate, in turn generating a pulsating electromagnetic field surrounding the coil. Placing the coil a nominal distance from a conductive or metal target induces a current flow on the surface and within the target. The induced current produces a secondary magnetic field that opposes and reduces the intensity of the original field, and changes in the impedance of the exciting coil can be analyzed to tell something about the target or the distance from the target.

Examples of such inductive sensor systems are, for example, disclosed in "A General Method for Designing Low-Temperature Drift, High-Bandwidth, Variable- Reluctance Position Sensors" by R.L. Maresca; IEEE Transactions on Magnetics, Vol. Mag-22, No. 2, March 1986 and a brochure published by Kaman Instrumentation Corporation in 1982, application note number 108 "General Application Considerations Inductive Displacement Measuring Systems". These sensors, while generally working satisfactorily in detecting surface conditions of metallic objects in atmospheric conditions and the like, are generally not thought suitable for use in an environment such as Czochralski growth of crystals.

More specifically, in liquid encapsulated Czochralski growth (hereinafter LEC) of crystals, the environments are generally thought to be extremely hostile to such sensor systems in a manner such that detrimental effects of the environment on the sensor itself precludes reliable inductive measurements in such environments. It is often the case that long exposure to high temperatures will cause sensor measurements to drift despite the fact that there was no change in liquid level. Thus, accuracy is compromised. Further, in the growth of gallium arsenide especially, it is often the case that arsenic becomes deposited on portions of the coil of the sensor thereby shorting out the coil and making further position measurements unreliable. Thus, in the growth of such crystals it has generally been the practice to employ physical contact melt depth sensors.

A problem with physical probes in the field of crystal growth is that they tend to disrupt the surface of the melt. Typically, it is essential in the field of such crystal growth that conditions be maintained very stable inasmuch as such growth involves contacting a seed crystal to the melt and thereafter very delicately pulling the growing larger crystal being grown and pulled by the seed from the melt. Any disruptions in the surface of the melt can result in separation of the pulling seed crystal from the melt thereby disrupting and terminating the process of crystal growth. Another problem with electrical contact probes is the contamination of the melt with unwanted impurities from the contacts.

In accordance with the invention, these problems encountered by physical contact of crystal melt are avoided by providing an inductive type sensor system which can be employed in such hostile environment crystal growth techniques.

Summary of the Invention

In accordance with the 'invention there is provided a non-contacting displacement sensor for measuring levels of conductive liquids in high temperature and high pressure crystal growth. The sensor comprises a non-conductive cylindrically shaped tube having a spiral groove on the outer surface extending along the length thereof at a predetermined pitch. A conductive wire is wound about the cylindrically shaped tube, in the groove, and has two ends extending from one end of the tube. A protective sheath is received about the tube having the wire wound thereabout for preventing any conductive liquid from coming in contact with the wire when the sensor is in use. Further, the sheath is positioned such that the two ends of the wire extend out from the one end of the tube. A power supply is connected to the wire for supplying power thereto, with resonance means in line with the wire to cause current flowing through the wire to oscillate, whereby upon change in proximity of the tube to a conductive liquid, the current oscillation will change as a function of distance whereby distance, i.e., because of eddy currents induced which cause a change in inductance in the system, between the tube and a conductive liquid can then be determined. In a more specific aspect, the protective sheath is a boron nitride, silicon nitride, aluminum nitride or aluminum oxide tube, especially adapted for use in gallium arsenide crystal growth, closed at one end and open at the other end to permit the two ends of the wire to extend therefrom. Still further, preferably the tube having the wire wound around it is made of either boron nitride, silicon nitride, aluminum nitride or aluminum oxide. The sensor system further comprises an insulator plug mountable in an opening leading to a high pressure and high temperature crystal growth chamber of the type from which such crystals can be grown from a conductive liquid, for sealing such an opening. The insulator plug has two openings with conductive inserts therein with the inserts at one end having an opening of a size sufficient to receive, and receiving the respective ends of the conductive wires therein, and extending into passageways which lead into respective openings on the other end of the inserts in the plug, with the wires extending thereinto. The second openings in the plug inserts are of a size sufficient to receive "banana" plugs therein which serve to connect the wires to the power supply and to the detecting circuits for the sensor. Still yet further, the power supply is connected to the wires through the "banana" plugs. The power supply is of the type for supplying an oscillating current through the wires and having an output to supply a signal indicative of the oscillations per time of said oscillating current to a detector. A counter is connected to the output to count the oscillations per time of the current in the wires whereby as a result of varying oscillations from a predetermined value, proximity of the sensor to a conductive liquid in the chamber can be determined.

Stability of the power supply can be further enhanced by placing it in a thermostatic chamber, or using other commonly employed temperature-stabilizing techniques.

In yet still another aspect, the invention relates to a method of detecting the level of a conductive liquid by using the sensor described above. The method comprises the steps of placing the sheathed wire wound tube a predetermined distance above the level of a conductive liquid in a sealed crystal growth chamber. An oscillating current is supplied, of predetermined number of resonating oscillations per time, to the sheathed wire wound around the tube, and the oscillations are detected and counted. Thereafter, any change in the counted oscillations per time is detected and measured to determine a change in the liquid level. In addition, the method further comprises detecting the temperature in the sealed chamber and applying a correction factor to the power supply upon a change in temperature in the chamber to compensate for temperature change effects in the chamber.

Brief Description of the Drawings

Having briefly described the invention, the same will become better understood from the following detailed discussion thereof, taken in conjunction with the drawings wherein:

Figure 1 is a schematic diagram of the positioning of a sensor in accordance with the invention relative, for example, to a gallium arsenide melt in a Czochralski growth chamber; Figure 2 is a side schematic view of the base tube of the sensor in accordance with the invention;

Figure 3 is a bottom plan view schematically illustrating the bottom of the tube of the sensor of the invention; Figure 4 is an exploded partial side view showing details about the helical groove, having a conductive wire wound therearound, of the sensor of the invention;

Figure 5 is a partial side cross-sectional view illustrating the connecting arrangement for the sensor of the invention, received within the czochralski growth chamber, and showing the interconnection to exterior circuitry;

Figure 6 is a schematic diagram illustrating the power supply and oscillation detecting circuit of the sensor of the invention;

Figure 7 is a schematic view of a temperature compensating circuit employed with the sensor of the invention; and Figure 8 is a partial side cross-sectional schematic view showing the crystal growth chamber with which the sensor of the invention is employed illustrating how the sensor is mounted therin and the connecting leads to outside located electronic control circuitry.

Detailed Discussion of the Invention

The sensor in accordance with the invention is partially shown in Figure 1 as a wound coil of conductive wire 1, preferably of molybdenum, which is, when employed in sensing, positioned at a separation D from a conductive liquid melt 3 such gallium arsenide. The principle of operation of the sensor is by inductance changes which result in a variation of an oscillating current therein which can be detected and which is a function of the separation D from the surface of the conductive liquid melt 3.

In Figure 2 there Is shown a more detailed side view of a support tube 5 making up the sensor element 1. The support tube 5 includes a single helical groove 7 extending therearound to result in up to 17 levels at which a wire can be wound around the groove. Two notches 9 are located at the bottom 11 of the tube 5 to permit loop back of a wire 15 which is wound around the tube 5. As shown in Figure 3, preferably the notches 9 are spaced at an angle α from each other, which angle is equal to about 45°. The tube 5 as can be seen is hollow and has an inner space 13. As shown in Figure 4, the wire is preferably molybdenum wire of about 24

AWG. Of course as will be readily apparent to those of ordinary skill in the art other equivalents can be substituted.

With respect to the helical groove, it can be appreciated from Figure 4 that a precise spacing A between respective turns of the groove is maintained to ensure uniformity of current flow and inductance effects on the wire. With respect to the tube material 5 it is preferably boron nitride although a material such as silicon nitride, aluminum nitride or aluminum oxide can be substituted. When assembled, an outer sheath 12 as shown in Figure 2 can be assembled on the tube to isolate and protect the coils of wire from, for example, vapor deposition of arsenic which could short out the coils. Preferably the outer sheath 12 is also made of boron nitride, and as can be seen from Figure 2 fully encloses the bottom 11 of the inner tube 5. In Figure 5 there is shown the connection arrangement to a power supply and detection circuit employed with the tube. More specifically, the enclosure for this chamber 17 encloses and surrounds the inner chamber 19 wherein is contained the conductive melt as well as other crystal growing associated equipment. The chamber 17 includes an opening wherein is received an insert 21 made up of two parts. More specifically, a first part 23 is of an insulating material such as, for example, boron nitride. Two. openings are located in the insert 21 and received in the two openings are additional inserts 25 which are preferably of a conducting material such a stainless steel. The inserts 25 include a larger opening 27 at the top extending into a small bore with an opening at the bottom. The wires from the sensor, i.e., wires 15 extend through the openings in the insert 21 into the inserts 25 with the upper openings 27 of the inserts 25 being of a size sufficient to receive banana plugs 33. In this regard, it is noted that the banana plugs 33 are received in a stainless steel body 29 having insulating inserts 31, preferably of boron nitride, therein through which the banana plugs 33 extend up to outer protective contacts 35 and 37 to which can be connected associated circuitry. In this arrangement, due to the small size of the opening through which the wires 15 are received, there is the no possibility of arsenic diffusing as vapor up into the connecting arrangement and fouling up or shorting the contacts of the banana plugs 33 which are received in openings 27 in contact with wires 15 against the walls of the opening 27. More specifically, effective electrical isolations of the contacts from each other and the, e.g., arsenic is maintained, as is tautness of wires. Tautness of wires 15 is required, since moving wires will give spurious signals. By tautness is meant sufficiently tight such that the wires do not move without application of external force.

As shown in Figure 6, sensor coil 1 is connected through contacts 35 and 37 to a two-stage oscillating circuit. Field-effect transistors 56 and 65 are connected through resistive and capacitive components 45, 47, 55, 57, 59, 61, 63, 67, and 69 as an amplifier. This amplifier circuitry is powered by a 15 volt current supply 49, as filtered through the inductance/ capacitance filter elements 51 and 53. The resonant condition of the oscillator is set by the inductance of the sensor coil 1 and the parallel capacitors 39 and 41. The amplifier is arranged to oscillate at the resonance frequency by a feedback capacitance 43, and the output is buffered by a buffer amplifier comprised of amplifier circuit 73 and resistor 81, and powered by the 15 volt current supply 49 through the inductance/ capacitance filter elements 77 and 79. The buffered frequency output is then applied through resistor 83 and capacitor 85 to line-driver circuitry suitable for sending radio frequency signals of the resonant frequency through a 50 ohm coaxial cable. This line driver is comprised of the buffer amplifier stage formed by transistor 23 and its associated resistors and capacitors 13, 15, 17, 19, and 21, and the emitter- follower stage formed by transistor 25 and resistor 11. The line driver circuitry is powered by a 15 volt current supply 49, as filtered through the inductance and capacitance combination 27 and 29. The resonant frequency signal can then be supplied to a 50 ohm coaxial cable at terminals 3 (shield) and 5 (inner conductor) which complete the connection to the line driver through resistor 7 and capacitor 9. The purpose of the 50 ohm line driver is to allow simple connection to standard laboratory instruments for measurement of the resonant frequency and computation based on such a measurement. Such instruments are used to detect changes in the resonant frequency which result from movement of the liquid melt relative to the sensor coil, and thus, as a function of the change, the distance of the sensor coil from the conductive melt. The choices of frequency determining elements 1, 39, and 41 are such that the nominal oscillating frequency is 10,000,000 Hz (10 MHz).

Stability of this circuitry is enhanced by placement in a thermostatic oven. The temperature of this oven is set at about 35 degrees Centigrade, which is far enough above the ambient room temperature to allow a constant circuit temperature regardless of room temperature variations. The oven is constructed from a cylinder of poly-vinyl chloride 48 mm outside diameter, 100 mm length and 4 mm wall thickness. This cylinder is wrapped with an evenly spaced bifilar winding of 160 turns of 36 AWG copper wire, 80 feet in length, of approximately 35 ohms resistance. The bifilar winding serves to reduce the net inductance. The twelve volt current supply to this heater is provided by temperature regulating circuitry, described below, and provides about 4 watts of power. A temperature sensor, described below, is fixed to the outside of this cylinder at an equal distance from the ends with poly-vinyl chloride tape to ensure good thermal contact. The outside of the cylinder is then wrapped in two layers of 0.002 inch aluminum foil, 5 mm thick plastic foam insulation, and poly-vinyl chloride tape. After placement of the resonating circuit, the ends of the cylinder are capped by 10 mm thick plastic foam insulation. The necessary connecting wires to the sensor coil, current supply, and the coaxial output are routed through holes in the foaminsulation of the end caps. Figure 7 shows the circuit for control of the oven temperature. Operational amplifier 113, which is a portion of an LM324N integrated circuit, forms a precision voltage reference source with a band gap reference integrated circuit 107. A 12 volt current supply 101 provides a biasing current to the band gap reference integrated circuit 107 through resistor 103. The temperature stable reference voltage from the band gap reference integrated circuit is applied to a non- inverting amplifier comprised of operational amplifier 113 and its associated resistors 109, 111, and 115. The amplified reference voltage is then buffered by operational amplifier 143 and its associated resistors 139, 141, and 145, and it is then applied through a filter comprised of inductor 147 and capacitors 149 and 151 to power the temperature sensing integrated circuit 155. This temperature sensor is mounted on the thermostatic oven in close thermal contact with the heating element 181, as described above, to provide necessary feedback for temperature control purposes. This temperature sensor is biased by resistor 153 to provide a temperature signal of approximately 10 mV/°C at about 3 volts at room temperature across resistor 157. The voltage signal across resistor 157 is filtered by capacitor 159 and buffered by operational amplifier 161 and its associated resistor 163. The buffered temperature signal is then compared by operational amplifier 127 and its associated resistors 123 and 125 with a set-point voltage provided by the voltage divider comprised of resistors 117, 119, and 121. During operation of the oven, this set-point voltage is adjusted to obtain the desired oven temperature of 35°C. The output of the operational amplifier comparator 127 is a square-wave signal which cycles between low and high voltages of about 2 and 10 volts with respect to circuit ground.

This high/low output state is a heat demand signal which indicates whether the oven temperature is above or below the set-point temperature. This temperature demand signal is applied through the voltage divider comprised of resistors 129 and 131 to the trigger input of the monostable multivibrator integrated circuit 133, the cycle time of which is set by resistor 135 and capacitor 137. A temperature demand signal indicating a need for more heat will cause the output at pin 3 of integrated circuit 133 to be at 12 volts with respect to circuit ground for at least as long as the timing elements 135 and 137 allow, which is about 1-2 seconds for the component values shown. A constant demand for more heat, as when the circuit is first activated at room temperature, will cause the output of integrated circuit 133 to remain at 12 volts continuously, allowing maximum heating effect. At oven temperatures near the set- point, infrequent output pulses of about 1-2 seconds duration will appear as the oven temperature drifts just above and below the set-point.

The output of the monostable multivibrator integrated circuit 133 is applied through resistor 165 to transistor 167 which switches current from the 12 volt current supply 101 through the oven heater resistance coil 181. Light emitting diode 169, powered through resistor 171, is wired to show the on/off status of the oven heater. The inductance and capacitance elements 173, 175, 177, and 179 filter the current pulses applied to the oven heater resistance coil.

The component oven, as controlled by the circuitry in Figure 7, provides 0.1° C temperature stability for the resonator circuitry of Figure 6, allowing accuracy of distance measurement. Those skilled in the art will discern other methods of temperature compensation, such as replacement of resistive elements with elements of specific temperature coefficient of resistance, or sensing the temperature of the circuit and deriving a correcting biasing voltage for transistors 56 and 65 of Figure 6. Circuitry of Figure 6 can be integrated onto a single substrate with integral substrate heater and temperature controller. These are given as examples and are not intended to limit the scope of possible temperature compensation measures.

In Figure 8, is shown a typical chamber for use in crystal growth wherein a sensor of the type of the invention would be shown. The chamber 19 Includes a lead 17 with the sensor 1 extending therethrough. Wires 35, 37 extend out in the direction shown towards the electronic circuitry which is housed outside of containment vessel 101 Having described the invention in detail, the scope thereof is set for in the appendent claims which are not intended to be limiting in any way whatsoever.

Claims

What is claimed is:

1. A non-contacting displacement sensor for measuring levels of conductive liquids in high temperature and high pressure crystal growth, the sensor comprising: a non-conductive cylindrically shaped tube having a spiral groove on the outer surface extending along the length thereof at a predetermined pitch; a conductive wire wound about said cylindrically shaped tube, in said groove, and having two ends extending from one end of said tube; protective sheath means received about said tube, said tube having said wire wound thereabout, for preventing any conductive liquid from coming in contact with said wire when said sensor is in use, and positioned such that two ends of the wire extend out from the one end of said tube; and power supply means connected to said wire for supplying power thereto, with resonating means in line with said wire to cause current flowing through said wire to oscillate whereby upon change in proximity of the tube to a conductive liquid, the current oscillation will change as a function of distance whereby distance between the tube and a conductive liquid can thus be determined.

2. A sensor as in claim 1 wherein said protective sheath means is one of a boron nitride, silicon nitride, aluminum nitride and aluminum oxide tube, closed at one end, and open at the other end to permit the two ends of the wire to extend therefrom.

3. A sensor as in claim 2 wherein said tube having said wire wound around it is made of one of boron nitride, silicon nitride, aluminum nitride and aluminum oxide.

4. A sensor as in claim 1 further comprising as insulator plug mountable in an opening leading to a high pressure and high temperature crystal growth chamber of the type from which a crystal can be grown from a conductive liquid, for sealing such opening, said insulator plug having two openings with conductive inserts therein, said inserts, at one end having an opening of a size sufficient to receive, and receiving, said respective ends of said conductive wires therein, and extending as passage ways into respective openings on the other ends of said inserts, with the wires extending thereinto, of a size sufficient to receive banana plugs therein to thereby connect the wires to said sensor power supply means.

5. A sensor as in claim 4 wherein said power supply means is connected to said wires through banana plugs, said power supply means being of the type for supplying an oscillating current through said wires, and having an output to supply a signal indicative of the oscillations per time of said oscillating current to a detector.

6. A sensor as in claim 5 further comprising a counter connected to said output to count the oscillations per time of current in said wires, whereby as a result of varying oscillations from a predetermined value, proximity of said sensor to a conductive liquid is determined.

7. A method of detecting the level of a conductive liquid using the sensor of claim 6 comprising the steps:

(a) placing the sheathed wire wound tube a predetermined distance above the level of a conductive liquid in a sealed crystal growth chamber;

(b) supplying an oscillating current of predetermined number of oscillations per time to said sheathed wire wound tube and detecting and counting said oscillations; and

(c) measuring any change in said counted oscillations per time to determine a change in liquid level.

8. A method as in claim 7 further comprising detecting the temperature in said sealed chamber and applying a correction factor to said power supply upon a change in temperature in the chamber to compensate for temperature change effects in the chamber.