Choosing a tool to monitor ESD isn’t necessarily a trade-off between low cost and accuracy. What if you could have both?
A new technique for noncontacting electrostatic voltage measurements, implemented in the AC feedback electrostatic voltmeter, is an alternative that may give you the best of both worlds. But how does this instrument differ from traditional tools?
Typically, electrostatic fieldmeters are used as ESD monitoring tools. Fieldmeters, while low in cost, cannot provide an accurate measurement. They are very sensitive to variations in spacing and measurements are not very repeatable.
In applications where better accuracy is essential, electrostatic voltmeters are used. Present electrostatic voltmeter technology requires that the voltmeter generate a voltage equivalent to the voltage on the surface under test. Generating this high voltage requires circuitry that is more complex and expensive than that of fieldmeters. As the voltage range becomes very high, so does the cost of the voltmeter. For applications that require several voltmeters to monitor different points throughout a process, it can become a substantial investment.
The new voltmeter uses a low-amplitude AC feedback voltage, so its cost is similar to that of fieldmeters. An AC signal is fed back to the probe to generate a probe electrode current equal and opposite to the current generated by the modulated capacitive coupling between the surface and the probe electrode. Since feedback is used to provide a nulling condition, the new meter offers spacing-independent measurements with better accuracy and repeatability than fieldmeters.
How Does It Work?
A block diagram of the technology is shown in Figure 1. As in the traditional voltmeter, a sensitive electrode is mechanically vibrated relative to the surface-under-test to produce capacitive modulation dC/dt of the physical capacitance between the surface and the electrode. The equation of the detector circuit with point X grounded, as shown by the dotted line, is:
(1)
where Qc is the charge stored in capacitance C and Vc is the voltage between the detector electrode and the test surface. Differentiating with respect to time produces the relationship:
(2)
The electrode voltage cannot change due to its connection to the summing node of amplifier A1; therefore, dVc/dt = 0, leaving:
(3)
Letting dQc/dt = I1 and Vc = VS, then:
(4)
I1 is a capacitive displacement current that flows through C due to the voltage difference between the electrode and surface and the change in capacitance between the two. I11, at the output of amplifier A1 equal to I1R. will flow through feedback resistor R to produce a voltage, V
Amplifier A2 monitors the voltage across resistor R. If the dC/dt term is a sinusoid, then A2 output voltage, V2, will also be a sinusoid whose frequency is identical to the vibration frequency of the electrode and whose amplitude is proportional to the surface voltage and the magnitude of dC/dt.
If the ground connection at point X is removed and the sinusoidal generator G is connected, any voltage applied to point X and the noninverting terminal of amplifier A1 will appear at the inverting input of amplifier A1. This is due to the operational characteristics of an amplifier of this type.
With the introduction of the sinusoidal voltage signal VG at the inverting input of A1 due to generator G, a new current, I2, is generated in the capacitance C between the electrode and the surface under test. I2 is represented as:
(5)
If the frequency of VG is identical to the signal from the modulator that is producing dC/dt, an amplitude and phase (either 0 or 180º relative to the modulator) can be found which will produce an I2 that will exactly cancel current I1 in capacitance C due to the surface voltage VS and the modulation dC/dt.
The resulting equation is:
(6)
or
(7)
If dQc/dt = Itotal = 0, then:
(8)
or
(9)
or
(10)
The ratio between dVG/dt, an AC signal, and the surface voltage VS, a DC level, is fixed by:
(11)
which is the ratio of the capacitance C of the detector electrode and the surface and the change in that capacitance due to the motion of the electrode.
Over a wide range of electrode-to-surface spacing, Equation 11 remains constant to better than 1% for any fixed peak-to-peak motion of the electrode. So the ratio between the generator G voltage (VG) and the measured surface voltage (VS) remains constant to better than 1% over a wide spacing range to achieve the sum of I1 and I2 equal to zero.
In Figure 2, the generator G is replaced by an AC amplifier to provide the necessary current nulling signal. The AC amplifier has a bandpass characteristic centered around a frequency equal to the rate of capacitance modulation. The feedback signal closes the loop to create an automatic spacing-independent measurement.
VM is an indicator used to show the value of test surface voltage VS over a wide electrode-to-surface spacing variation by monitoring the value of VFB, the feedback voltage generated by the AC amplifier.
Performance Characteristics
Due to the simple design and the fact that no high voltage is generated, the AC feedback electrostatic voltmeter can be scaled to virtually any voltage range. A measurement range of 50 kV is no more complicated to achieve than one of 5 kV.
The probe containing the sensing electrode can be located at considerable distance from the instrument to monitor remote points in a process. If necessary, the probe can be positioned close to the surface-under-test to provide good spatial resolution.
The distance between the probe and the surface depends on the voltage level to be measured. When introducing the probe (which has a low-voltage AC signal on it) near a high-voltage surface, there is the risk of arcing between the two. With higher voltage ranges, the probe must be located farther away from the surface-under-test, resulting in a larger area being measured on the surface.
The sensing electrode “views” a spot on the surface-under-test that is approximately five times the probe-to-surface spacing to produce a 1% error due to spatial resolution. For example, at a probe-to-surface spacing of 2 mm, the electrode sees a spot 10 mm in diameter on the surface with a 1% error. This can affect the readings when the field of view is larger than the surface being measured. In these cases, charged devices near the surface will have an effect on the measurement.
Conclusion
The AC feedback electrostatic voltmeter provides a new method for ESD monitoring. By eliminating any high-voltage circuitry and inducing a current to exactly cancel the current produced by the electrode modulation, a low-cost, accurate and spacing-insensitive measurement can be obtained.
Reference
1. Williams, B.T., “High Voltage Electrostatic Surface Potential Monitoring System Using Low Voltage AC Feedback,” U.S. Patent 4,797,620, Jan. 10, 1989.
About the Author
David M. Zacher is a Sales Engineer at Trek. He received a B.S. degree in electrical engineering from the University of Rochester and an M.S. degree in electrical engineering from the State University of New York at Buffalo. Trek Inc., 3932 Salt Works Rd., Medina, NY 14103, (716) 798-3140.
Copyright 1995 Nelson Publishing Inc.
November 1995