Apparatus and method for measuring conductance including a temperature controlled resonant tank circuit with shielding

What is claimed is:

1. A method for measuring conductance of a sample comprising the steps of:

(a) selecting apparatus comprising:

(i) a resonant tank circuit having a tank coil for loading by the sample;

(ii) an oscillator incorporating the tank circuit;

(iii) a control circuit for the oscillator responsive to the loading of the tank coil by the sample, the control circuit producing a control signal proportional to the loading; and

(iv) means responsive to the control signal for indicating the sheet conductance;

(b) coupling a variable direct current to the tank coil;

(c) measuring the magnitude of the direct current;

(d) measuring an offset voltage produced in the tank coil by the direct current;

(e) dividing the measured offst voltage by the measured direct current for producing a resistance signal representing the resistance of the tank coil;

(f) controlling the direct current in response to the resistance signal for stabilizing the temperature of the tank coil;

(g) positioning the tank coil in a predetermined relation to the sample; and

(h) reading the sheet conductance from the indicating means.

2. Apparatus capable of measuring sheet conductance of a sample having unrestricted area and having a conducting surface and a supporting surface parallel to the conducting surface, the supporting surface being more than 0.15 inch from the conducting surface, the apparatus not touching the conducting surface, the apparatus comprising:

(a) a probe comprising:

(i) a tank coil in a resonant tank circuit, the tank coil having an axis;

(ii) means for locating the probe against the supporting surface of the sample with the axis of the tank coil perpendicular to the sample and a predetermined distance between the tank coil and the sample;

(iii) a conductive housing for the tank coil, the conductive housing being open at one end proximate the sample, the conductive housing shielding magnetic flux associated with the tank coil not directed toward the sample; and

(iv) a shield for preventing capacitive coupling between the tank coil and the sample, the shield comprising a plurality of separate, elongated narrow conductors on opposite sides of an insulating substrate covering the open end of the housing, the conductors crossing to form an intersecting pattern, the conductors being electrically connected to the housing at one location only on each conductor for preventing eddy current losses in the shield, for providing an electrostatic shield enclosing the coil;

(b) an oscillator incorporating the tank circuit in resonance, the oscillator further comprising a high frequency operational amplifier having optically coupled gain for external control;

(c) means for controlling the temperature of the tank coil by varying a direct current added to the tank coil in response to changes in measured resistance of the tank coil, the controlling means comprising:

(i) a DC coupling network for adding the direct current to the tank coil, the network having a coupler output connected to the tank coil, the network incorporating a circuit for sensing an offset voltage at the coupler output, the offset voltage resulting from the direct current in the tank coil, the sensing circuit including means for blocking oscillator signals;

(ii) a power amplifier for variably producing the direct current in response to an external signal;

(iii) meane for measuring the magnitude of the direct current; and

(iv) a circuit for producing the external signal for the power amplifier as a quotient proportional to the offset voltage divided by the magnitude of the direct current, the external signal being indicative of the measured resistance, the direct current being reduced in response to increased resistance of the tank coil;

(d) a control circuit for the oscillator responsive to loading of the tank coil by the conducting surface, the control circuit stabilizing the amplitude of oscillation of the oscillator and producing a control signal proportional to the loading; and

(e) means responsive to the control signal for indicating the sheet conductance.

3. Apparatus for measuring conductance of a sample comprising:

(a) a resonant tank circuit having a tank coil for loading by the sample, the tank coil having an axis;

(b) means for positioning the tank coil in a predetermined relation to the sample;

(c) an oscillator incorporating the tank circuit;

(d) a control circuit for the oscillator responsive to the loading of the tank coil by the sample, the control circuit stabilizing the amplitude of oscillation of the oscillator and producing a control signal proportional to the loading;

(e) means for controlling the temperature of the tank coil by coupling a variable direct current into the tank coil, the direct current varying in response to a circuit for measuring the resistance of the tank coil so that the direct current is reduced when the resistance of the tank coil increases; and

(f) means responsive to the control signal for indicating the conductance.

4. The apparatus of claim 3 wherein the means for controlling the temperature comprises:

(a) a D.C. coupling network for adding the direct current to the tank coil, the coupling network being connected in the tank circuit;

(b) a power amplifier for variably producing the direct current in response to a resistance signal, the power amplifier being connected to the coupling network;

(c) means for measuring a voltage offset in the tank coil in response to the direct current;

(d) means for measuring the magnitude of the direct current; and

(e) means for producing the resistance signal as a quotient proportional to the voltage offset divided by the direct current.

5. The apparatus of claim 4 in which the measuring means comprises a low-pass filter in the D.C. coupling network, the filter having an output connected to an input of a voltage amplifier, the voltage amplifier driving the producing means, and the current measuring means comprises a current-shunt resistor connected between the power amplifier and the coupling network, opposite ends of the current-shunt resistor being connected to corresponding inputs of a sensing amplifier, the sensing amplifier having an output connected to the producing means.

6. The apparatus of claim 3 further comprising a shield for preventing capacitive coupling between the tank coil and the sample, the shield being located in fixed relation to the tank coil and comprising a plurality of separate elongated narrow conductors, the conductors being electrically interconnected only at one location on each conductor for preventing eddy current losses in the shield.

7. The apparatus of claim 6 wherein the control circuit is in an electronics unit and the tank coil, the shield and the positioning means are in a probe remote from the electronics unit for convenient manipulation of the tank coil.

8. The apparatus of claim 7 wherein the probe is connected to the electronics unit by a coaxial cable more than about two feet long.

9. The apparatus of claim 3 wherein the oscillator further comprises a high frequency operational amplifier circuit having optically coupled gain control driven by the control signal.

10. The apparatus of claim 6 further comprising a conductive housing for the tank coil, the conductive housing being open at one end proximate to the sample, the conductive housing shielding magnetic flux associated with the tank coil not directed toward the sample, the open end of the housing being covered by the shield for preventing capacitive coupling, the interconnection of the conductors being electrically connected to the conductive housing for providing an electrostatic shield enclosing the coil.

11. The apparatus of claim 10 wherein the conductors of the shield for preventing capacitive coupling form at least two closely spaced surfaces, the conductors of the respective surfaces being oriented to form an intersecting pattern on opposite sides of an insulating substrate.

Description Submit all comments and votes

BACKGROUND

This invention relates to electrical conductance measurements, and more particularly to noncontact sheet conductance measurements.

Non-contact conductance measurements based on the loading effects of a sample on the "Q" of a nearby coil are known in the prior art. Sheet conductance (G.sub.s) in units of Siemens per unit square is the inverse of sheet resistance (R.sub.s) in units of ohms per unit square.

The analyses of these loading effects are quite complex, involving the concepts of alternating magnetic fields, magnetic coupling, mutual inductance, vector descriptions of induced currents and their interactions with their associated magnetic fields. When concerned with the interactions of time varying magnetic fields on conductive or semi-conductive objects or surfaces, the subject is generally lumped into the concept of "eddy currents". Eddy currents are induced in the object by the magnetic field. The resulting losses and interactive magnetic field are reflected back into the driving inductor. These are seen as the changes in the resistive and inductive components of the impedance of the coil. The figure of merit Q is defined as the ratio of the reactive to the resistive impedance components of a coil at a given frequency. Thus the reflection of eddy current losses from a sample conductor to a coil influences the Q of the coil.

The change in Q depends on the conductivity of the sample and the proximity of the sample to the coil. The magnitude of the eddy current loading effect is very sensitive to the distance between the sensor coil and a conductive surface. Therefore, means must be provided to establish a definite positional relationship between the sensor and the conductive surface of a sample.

The prior art includes devices for determining resistivities by measurements of Q in combination with the use of non-linear calibration curves of resistivity versus Q from samples of known resistivity. For example, U.S. Pat. No. 3,234,461 to Trent et al discloses the use of a commercial Q meter connected to a slotted coil with calibration curves relating actual and relative Q to the resistivity of samples positioned within the slot.

The prior art also includes U.S. Pat. No. 4,000,458 to Miller et al, disclosing a method of measuring sheet conductance of a sample as a linear function of the drive current required in a constant amplitude resonant circuit loaded by the sample. The Miller patent describes a resonant coil tightly coupled to both sides of the sample and magnetically shielded by aluminum cups for confining the magnetic field to a defined area of the sample. An electrostatic shield of conductive paper covers open ends of the cups between the coil and the sample for minimizing capacitive coupling to the sample.

Prior art non-contact sheet conductance and bulk conductivity meters are commonly used for process control of semiconductor wafers and chips in the microelectronics industry.

Another application for sheet conductance measurements is in the process control of large area substrates having an applied conductive surface. These conductive surfaces are often quite delicate and must not be touched. The substrates can be panels spanning several feet and having thicknesses of up to 3/4 inch (20 mm). The substrates can be made from a non-conductive material such as glass or plastic.

The term "non-contact" has been used in the prior art to distinguish from earlier prior art meter technology using a traditional four point probe to establish direct electrical contact with the conductive surface of the sample.

Usually, the sample is sandwiched in a slot within the coil or in a gap between portions of the core. Consequently, both sides of the sample are subject to being touched by the meter through normal handling procedures.

A successful commercial instrument identified as "M-gage 200" Metalization Monitor", manufactured by Tencor Instruments Company is designed specifically for semiconductor wafers. It provides for the positioning requirement by incorporating a transport mechanism to support the wafer and position it accurately between two sensor coils approximately 1/4 inch apart. The sample wafers can have a maximum thickness of 0.120 inch.

A disadvantage of prior art non-contact conductivity meters is a requirement for close proximity (less than 0.15 inch) of the conductive surface to the sensor (a coil or a high permeability core tightly coupled to the coil). In order to provide for exact and repeatable spacing of the sensor to the sample, prior art meters usually compromise the non-contact feature by requiring physical contact of some part of the sensor assembly with the conductive surface of the sample in order to establish the required exact spacing between the sensor and the sample.

A noncontact resistivity meter touching one side only of the sample is disclosed in U.S. Pat. No. 2,859,407 to Henisch. For samples comprising a conductive material on a thick substrate, the conductive surface must face the meter to be in close proximity to the coil for proper meter operation. Therefore, unless the sample is very thin, the meter must touch the conductive surface of the sample.

A further application for sheet conductance measurements is in the manufacturing of panels having a conductive surface between thick laminated substrates, such as for windshields. These panels are sometimes curved. The prior art resistance meters are not suitable for this application because the internal conductive surface cannot be in close proximity to the coil.

A problem encountered in locating the conductive surface a distance from the instrument is that an interposed conductive surface used as an electrostatic shield, as described above, interferes with magnetic coupling to the sample. The more remotely the sample is located from the coil and the shield, the more dominantly the shield loads the resonant circuit, rendering the instrument insensitive to the conductivity of the sample.

Another disadvantage of prior art non-contact sheet conductance and bulk conductivity meters is that they are not suitable for measurements of large samples. The non-contact meters of the prior art require samples for measurement to be placed within or on the instrument. This would be quite cumbersome, even for measurements near the edge of the sample. Measurements remote from the edge of the sample would require destructive cutting of the sample.

Thus there is a need for a noncontact sheet conductance meter that can be conveniently used on large area flat or curved samples having a variety of thicknesses and having interior or exterior conductive surfaces that must not or cannot be touched by the meter.

SUMMARY

The meter of the present invention meets these needs by providing an eddy current probe for use against a supporting surface of the sample that can be at a substantial predetermined distance from the conductive surface.

The meter comprises a tank coil used as an inductive element in a tank circuit, the coil being positioned at a predetermined distance from the sample, the tank circuit being connected in an oscillator amplifier circuit, the oscillator amplifier circuit being controlled in response to eddy current loading of the tank coil by the sample, a control voltage being produced proportional to sheet conductivity of the sample. The tank coil is capacitively shielded from the sample by a plurality of separate conducting fingers for preventing eddy current losses in the shield. The difference between the control voltage and an adjustable offset voltage can be amplified by an output amplifier and displayed by a digital voltmeter as the sheet conductance of the sample.

Preferably the tank coil is in a probe remote from an electronics package housing the control electronics. The probe is constructed for convenient alignment of the tank coil perpendicular to the supporting surface at a predetermined distance from the sample, which surface can be flat or uniformly curved parallel to the conductance surface. Preferably the probe is remotely connected to the electronics package by a coaxial cable more than two feet long for convenient manipulation of the tank coil.

Preferably the oscillator comprises a high frequency operational amplifier with optically coupled gain control driven by the control signal.

Preferably means are provided for controlling the temperature of the tank coil to avoid changes in Q caused by the resistance of the tank coil changing with temperature. Direct current for heating is coupled to the tank coil. A control circuit varies the direct current in response to measured changes in the tank coil resistance for stabilizing the resistance and temperature of the tank coil. Preferably a divider circuit measures the tank coil resistance as a ratio of tank coil voltage to the corresponding direct current.

Preferably the probe is provided with a shield connected to the coaxial cable and surrounding the tank coil, the portion of the shield between the tank coil and the sample forming closely spaced long narrow conductors interconnected at one location only on each conductor. Preferably the conductors form two closely spaced surfaces, the conductors of each surface crossing at an angle of about ninety degrees. The interconnections of the conductors are electrically connected to a conductive housing completing an electrostatic shield for the coil. The conductive housing provides magnetic shielding for magnetic flux associated with the tank coil not directed toward the sample. The combination of the conductive housing and the conductors connected at one location only confines the magnetic field of the tank coil to an area of the sample substantially corresponding to an area only slightly larger than that circumscribed by the diameter of the housing, and electrostatically shields the tank coil from the sample without interfering with magnetic coupling between the tank coil and the sample.

The shield configuration of the probe permits meaningful sheet conductance measurements to be made at very high gain sensitivities so that, when necessary, the tank coil can be located a substantial distance of at least 0.75 inch from the conductive surface of the sample. In addition, the remotely connected probe can be conveniently positioned on a large sample to measure selected areas of a conductive surface without touching the conductive surface.

Thus a noncontact sheet conductance meter is provided that can measure the sheet conductance of selected areas within a large flat or curved panel having substantial thickness and having an interior or exterior conductive surface that must not or cannot be touched by the meter.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a fragmentary elevational view showing a probe of the meter of the present invention positioned against a supporting surface of a sample;

FIG. 2 is a fragmentary plan view of the probe taken along line 2--2 in FIG. 1;

FIG. 3 is a sectional elevational view of the probe taken along the line 3--3 in FIG. 2;

FIG. 4 is a block diagram of the meter of the present invention showing the probe connected to circuitry of an electronic package; and

FIG. 5 is a fragmentary simplified schematic diagram of the meter of FIG. 4 showing a D.C. coupling network and a probe temperature control in the electronic package.

DESCRIPTION

The present invention is directed to a noncontact sheet conductance meter that can be used on a variety of samples having conductive surfaces that must not or cannot be touched by the meter. With reference to FIG. 1, a conductive sheet or sample 10 rests on a support 12. The sample 10 comprises a subsrate 14 having a conducting surface 16 and an opposite supporting surface 18, the supporting surface 18 being in contact with the support 12. A probe 20 is positioned against the supporting surface 18 for measurement of the conductivity of the conducting surface 16 of the sample 10. Although shown beneath the sample 10 for illustrative purposes, the probe 20 can be used in any position to provide convenient proximity to the sample.

MEASUREMENT ELECTRONICS

With reference to FIG. 4, the probe 20 is connected by a coaxial cable 22 to an electronic package 24. The probe 20 includes an inductive element for a tank circuit, described more fully below. A capacitive element for the tank circuit is a tank capacitor 25 located within the electronic package. An oscillator circuit 26 having an oscillator output 27 is conected to the probe 20 together with the tank capacitor 25 at a tank node 23.

Preferably the oscillator circuit 26 is a high frequency operational amplifier circuit having optically coupled gain control for controlling the voltage amplitude of oscillation. The tank node 23 can be connected with positive feedback in the operational amplifier circuit to a non-inverting input of an operational amplifier. Optically coupled gain voltage amplitude control of operational amplifier circuits are known in the prior art. For example, a Raytheon CK 2142 optically controlled variable resistor can be used as a gain control element for the operational amplifier circuit.

A level comparator 28 compares the voltage at the oscillator output 27 with a reference voltge 29. The level comparator 28 is connected to drive an error amplifier 30 to produce a control voltage 32. The control voltage 32 is connected as a filtered feedback control signal 34 optically coupled to the oscillator circuit 26 for controlling the gain of the oscillator circuit 26 to stabilize the voltage at the oscillator output 27. The control voltage 32 can be nominally 6.5 volts.

The magnitude of the control voltage 32 required to sustain oscillation is an inverse function of the Q of the tank circuit. The control voltage 32 is also directly related to the loading effect imposed on the inductive element of the probe by eddy currents induced in a lossy conductive surface placed in the field of the probe. Ignoring certain non-linearities inherent in the electronics implementation, the net result is a control voltage 32 which is a linear function of the conductance per unit square (Gs) of the conductive surface.

Preferably the tank capacitor 25 is adjustable for tuning the resonant frequency of the tank circuit to the frequency of maximum Q of the tank coil 64.

An output amplifier 36, balanced by an offset potentiometer 38, is driven by the control voltage 32 for producing an output voltage 39 which is read by a voltmeter 40.

Temperature Control

The electronic package 24 is equipped with an electronic temperature control 42 connected to a heater 44 for stabilizing the temperature of circuitry within the electronic package 24, thereby reducing variations in the output voltage 39 caused by thermal drift of the circuitry.

The electronic package 24 also provides temperature control of the probe 20 to prevent changes in the Q of the inductive element of the probe 20 which would otherwise result from changes in probe inductive element resistance as a function of temperature. A DC coupling network 46 is connected in series between the probe 20 and the tank node 23 for superimposing heat-producing direct current on the inductive element of the probe 20. A probe temperature control 48 is connected to the DC coupling network 46 to control the magnitude of the direct current within the probe 20 for stabilizing the temperature of the inductive element.

The oscillator circuit 26, the tank capacitor 25, and the DC coupling network 46 of the electronic package 24 can be enclosed in a thermal shield 47, the thermal shield 47 being thermally coupled to the heater 44. The thermal shield 47 can have separate cavities, one enclosing the oscillator circuit 26 and the tank capacitor 25, the other enclosing the DC coupling network 46. Thus the temperature stabilization by the electronic temperature control 42 is concentrated where it is most needed.

With reference to FIG. 5, the DC coupling network 46 includes a coupling capacitor 461 and a grounded bypass inductor 462 connected to the tank node 23 for coupling the tank node 23 to a coupler output 463, the coupler output 463 being connected by the coaxial cable 22 to the probe 20.

The coupler output 463 is coupled to the probe temperature control 48 by a current inductor 464 and a sensing inductor 465, each having a grounded bypass capacitor 466 for blocking signals at the oscillator frequency. A grounded power return 467 and a grounded voltage return 468 separately reference the probe temperature control 48 to the DC coupling network 46.

The probe temperature control 48 has a power amplifier 481, referenced to the power return 467, for driving the power inductor 464 through a shunt resistor 482. Direct current from the power amplifier 481 is coupled to the probe 20 by the current inductor 464, producing a proportional voltage across the shunt resistor 482. A current sensing amplifier 483 is connected across the shunt resistor 482 for providing a current signal 484 to an analog divider circuit 485. A voltage sensing amplifier 486 is connected to the sensing inductor 465 and the voltage return 468 for providing a voltage signal 487 to the analog divider circuit 485. The analog divider circuit 485 is connected for providing a resistance signal 488 proportional to the resistance of the probe 20 by dividing the voltage signal 487 by the current signal 484. The resistance signal 488 is processed by a control amplifier 489 for driving the power amplifier 481 so that current coupled to the probe 20 is reduced by any increase in the resistance signal 488, stabilizing the temperature and resistance of the probe 20.

The control amplifier 489 has an adjustable temperature set point and proportional, integral and derivative (PID) control modes for dynamic compensation.

Preferably the power amplifier 481 can provide from approximately 0.1 to 1.5 amperes DC to the probe 20 for temperature control. The current minimum provides for continuous resistance monitoring while the maximum provides circuit protection and limits the required dynamic range of control.

In summary of the above,