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SEMI E43-0301 © SEMI 1995 , 2001 3 7.2.1.3 Ch ar ge the reference calibratio n c apacito r to 1 volt with a charging source (power su pply). Calculate the am o unt of charge on t he capacitor by multiplying the v o ltage…

SEMI E43-0301 © SEMI 1995, 2001 2

operation as their surfaces may be at elevated potentials

that represent a shock hazard to the operator.

7 Equipment and Performan ce Verification

Methods

7.1 Equipment

7.1.1 Electrostatic Locator/Field Sensor/Field Meter

— An electrostatic fieldmeter measures the value of the

electrostatic field at its sensor. Electrostatic fieldmeters

are calibrated and recommended for use at a particular

distance from the charged object. Fieldmeters are best

suited for making general surveys or audits, for making

measurements of surfaces at very high potentials

(charge levels), and for making measurements when

long-term stability is not important. They are not well

suited for measurements of surfaces with very low

potentials or when high spatial resolution of the surface

potential is needed.

7.1.1.1 The electrostatic locator/field sensor/field

meter will henceforth be referred to as “the fieldmeter.”

Note that for measurements to be taken in the presence

of air ionization, a chopper stabilized fieldmeter is

required. The fieldmeter must be capable of making

field measurements at a distance of 2.54 centimeters

(cm) = 1 inch or less, from the field source to the sensor

for this guide, as written. However, see Section 7.2.5

for fieldmeters that are operated at fixed distance(s),

and adjust values in this document where applicable.

7.1.2 Electrostatic Voltmeter — An electrostatic

voltmeter nulls the electrostatic field at its sensor

(probe). An electrostatic voltmeter indicates the

presence and approximate level of the charge(s)

creating the electrostatic field. Under appropriate

conditions, electrostatic voltmeters provide a better

approximation of the charge level as compared to

electrostatic fieldmeters. Electrostatic voltmeters are

relatively free of drift and more environmentally stable

as compared to fieldmeters.

7.1.2.1 Electrostatic voltmeters are well suited for

fixed installation in equipment. Electrostatic voltmeters

exhibit a high degree of accuracy that is independent of

the distance from the charge. Thus, they are considered

better suited for making more accurate and repeatable

measurements as compared to fieldmeters. The probe

can be located very close to a charged surface without

arc-over, and, under appropriate conditions, can resolve

a small spatial area on a surface.

7.1.2.2 Electrostatic voltmeters are best suited for

making measurements of surfaces at potentials below

20kV, or when a calibrated or fixed distance from the

probe to the surface cannot be maintained. They are

also best suited for measuring low surface potentials, or

when it is desired to resolve a small area on the surface.

Electrostatic voltmeters are unsuitable for measuring

surfaces at very high potentials, such as above 20kV.

7.1.2.3 The electrostatic voltmeter will henceforth be

referred to as “the voltmeter.”

7.1.3 Electrometer — An electrometer is a contact

voltmeter with a very high input impedance. Ideally,

this input impedance would be infinite. In practice, it is

limited by intrinsic physical materials properties of

insulators and by stray leakage paths between the input

terminals. Low voltage electrometers (below 200 Volt)

have typical input resistances of 10

ohms, accuracies

better than 0.1%, and can resolve microVolt type

potentials. High voltage electrometers (Kilovolts)

usually rely on resistive voltage dividers and have

typical input resistances in the 10

ohms range with

accuracies in the 1% range. It is important to evaluate

and understand the burden that the input impedance of

an electrometer represents when measuring voltage

potentials on very small charged structures.

7.1.4 Charged Plate Monitor — A charged plate

monitor is an instrument typically used to monitor the

performance of air ionization equipment. Monitoring is

done with an electrically isolated 15 cm × 15 cm (6

inches × 6 inches) metal plate, henceforth referred to as

“the plate.” The instrument typically provides a means

to charge the plate to a known voltage (1000 or 5000

volts of either polarity), a plate sensor to determine the

voltage on the plate, and timing circuitry to determine

the time required to discharge the plate to a percentage

of its initial charge. For the purposes of this guide, the

charged plate monitor, or a separate isolated plate

assembly, can be used for performance verification

purposes as explained in Section 7.2.

7.2 Equipment Performance Verification (Confidence

Test)

7.2.1 Performance Verification of a Coulombmeter —

Refer to Figure 1.

7.2.1.1 Zero the coulombmeter prior to each

measurement.

7.2.1.2 Maintain a reference calibration capacitor. It

should be a polystyrene or polypropylene 10 nF

capacitor (Mallory SX-110 or equivalent). Measure the

value of the capacitor to better than 1%. It is important

to handle the reference calibration capacitor very

carefully. Do not hold the capacitor by its body or

discharge it by touching both leads with the fingers.

Hold the capacitor by one lead only. Use a clip lead

connected between ground and this lead of the capacitor

to maneuver the other lead of the capacitor between the

“hot” side of the charging source and the input terminal

of the coulombmeter.

SEMI E43-0301 © SEMI 1995, 20013

7.2.1.3 Charge the reference calibration capacitor to 1

volt with a charging source (power supply). Calculate

the amount of charge on the capacitor by multiplying

the voltage by the value of the capacitor. Example: 1V

x 10 nF = 10 nC of charge.

7.2.1.4 Disconnect the charging source from the

capacitor.

7.2.1.5 Connect the coulombmeter input probe to the

capacitor and discharge the capacitor into the

coulombmeter. The coulombmeter should indicate the

calculated value.

7.2.2 Performance Verification of Fieldmeters and

Voltmeters — Refer to Figure 2.

7.2.2.1 Choosing Test Voltage(s) — Choose one or

more test voltage(s) from Table 1, based upon the

electrostatic field level of concern:

Table 1 Test Voltages

Field of Concern Test Voltage

Under 4,000 volts/meter or

100 volts/2.5 cm

100 volts

Under 40,000 volts/meter or

1000 volts/2.5 cm

1,000 volts

Over 200,000 volts/meter or

5,000 volts/2.5 cm (See NOTE 1.)

5,000 volts

NOTE 1: If fieldmeter or voltmeter performance verification is

needed above 5,000 volts, it is left to the user to select values using

the table as guide.

7.2.2.2 Instrument Performance Verification —

Charge a conductive test plate to the desired

verification voltage. Use of a suitable power supply or a

charged plate monitor for test purposes is

recommended.

7.2.2.3 Assuring Meters and Operator Are Grounded

— Assure that the fieldmeter, voltmeter and operator

are grounded. Turn on the meter and zero it as required

according to manufacturer's instructions.

7.2.2.4 Directing or Pointing the Sense Head — Direct

or point the sense head of the fieldmeter or voltmeter at

the center and parallel to the surface of the plate at a

distance at least twice the manufacturer’s recommended

measurement. Slowly move the sense head toward the

center of the charged plate until a reading equal to the

voltage applied to the plate in Section 7.2.2.1 above is

displayed by the meter. Measure and record the

distance from the sense head to the surface to the plate.

Using the plate voltage from Section 7.2.2.1 above and

the recorded distance, compute the field strength for the

fieldmeter. See Figure 2, Fieldmeter and Voltmeter

Verification Check.

7.2.2.5 Alternative to Section 7.2.2.4 — Take

measurements at a specified/fixed distance per

manufacturer's instructions. Locate the sense head of

the fieldmeter or voltmeter as in Section 7.2.2.4, but, at

specified distance; reading displayed (on meter) should

be within 5% of applied voltage to plate.

NOTE 1: Section 7.2.2.4 or 7.2.2.5 should be applicable to

most meters. However, in every case, the electrostatic

fieldmeter or voltmeter manufacturer's instructions should be

read, understood, and followed.

7.2.2.6 Other Desired Test Voltages — Repeat

Sections 7.2.2.4 and 7.2.2.5 for any other desired test

voltages.

7.2.3 Performance Verification of an Electrometer —

It is good practice to occasionally check the

performance of the electrometer by connecting it to a

known voltage source, and comparing its readings with

readings taken by another reference voltmeter.

7.2.4 Meter Stability — All measure ment devices

should be turned on and pre-conditioned for as long a

warm-up period as recommended by the manufacturer

7.2.4.1 Reset (zero) the coulombmeter prior to each

measurement.

7.2.4.2 Check the zero on the fieldmeter or voltmeter

as specified by the manufacturer. Usually this is done

while the probe is positioned to view a grounded

surface. If the zero of the meter has drifted by more

than 5% of the test voltage for any range contained in

Table 1, the meter is not suitable for use for

measurements over that range. It may be suitable for

use over other ranges contained in Table 1, using other

test voltages. Reverify the meter’s calibration at the

selected test voltage.

7.2.4.3 Zeroing an Electrometer — Except on some

older analog models, there are usually no provisions to

zero an electrometer. Some electrometers with analog

or digital read-outs do allow offsetting of a reading, as

well as relative (delta) measurements. However, the

electronic zero of the electrometer is usually set by the

manufacturer, and should be part of the normal

calibration. It is good practice to occasionally check the

zero by shorting the input terminals together and

verifying that the zero reading is within the

manufacturer’s specifications.

7.2.4.4 See Related Information 1 for notes on

equipment accuracy and limitations.

8 Sampling

8.1 Sampling methods for this guide should be

determined by the requirements of the user's appli-

cation. Electrostatic surveys can be repeated at different

times to make them more representative of actual static

charge conditions in the surveyed area. The results will

vary due to environment (e.g., humidity) and

workstation setup/conditions. However, any measure-

ment that is in excess of a (user) defined maximum or

that is a benchmark value, should be repeated more than

once, after performing a zero check of the measuring

equipment. This is to validate previous reading(s)

and/or establish a range/bounds in the case of varying-

moving fields on previous reading(s).

9 Test Methods & Measurem ents

9.1 Coulombmeter Measurements

9.1.1 Verifying the Coulombmeter — Verify the

performance of the coulombmeter as in Section 7

above. Check/reset the zero before each measurement

and/or per manufacturer's instructions. Assure that the

coulombmeter and operator are grounded.

9.1.2 Equipment Selection — Use a coulombmeter for

direct measurement of charge. A feedback-type

coulombmeter is recommended for charge

measurements for the most complete transfer of charge.

Shunt-type coulombmeters do not completely transfer

charge and are not as straightforward to use as

feedback-type coulombmeters. When using a Faraday

enclosure, the Faraday enclosure must be large enough

to hold the objects to be measured. The Faraday

enclosure is used to measure charge on insulating

materials as well as on conductors.

9.1.3 Measurements — Best results are achieved when

all surfaces surrounding the measurement area are

grounded (to minimize the effects of stray fields on the

measurement) and when a consistent, systematic

handling method is used during the measurement

process. The operator should be grounded using a

grounded wrist strap.

9.1.3.1 Isolated Conductors — To measure the charge

on an isolated conductor, touch the lead from the

coulombmeter to the isolated conductor.

9.1.3.2 Faraday Enclosure Measurements — Refer to

Figure 3. To measure the charge on an object, carefully

pick up the object with an insulated tool and place the

charged object into the Faraday enclosure. Special

handling considerations: Be careful not to add or

subtract any charge in the process of moving the

charged object into the Faraday enclosure. Don’t let the

charged object rub or slide against any other surface, as

this may add or subtract charge from the object.

9.1.4 Limitations — Do not attempt to measure

charges of magnitudes that are below the drift rate of

the coulombmeter.

9.2 Electrostatic Fieldmeter Measurements

9.2.1 Verifying the Fieldmeter — Verify the

performance of the fieldmeter as in Section 7 above.

Check/reset the zero periodically and/or per

manufacturer's instructions. Assure that the fieldmeter

and operator are grounded.

9.2.2 Measurements — Measurements made to this

guide should be taken/reported in units that conform to

the customer specifications. Most common fieldmeters

manufactured to date have operating instructions that

reflect the user doing calibration and taking

measurements in English units of volts/inch or volts at a

fixed distance in inch(es) and in these cases, raw data

are reported/listed directly. The international

community specifies that units shall be in SI (Standard

International) Metric units and the SI conversion factor

in Section 7.1.1.1 will apply. However, by definition,

electric field is expressed in volts per meter, and thus

would be expressed according to Table 2.

9.2.2.1 For instance, when using a meter calibrated

only at 100 volts, measurements under 4,000 volts/m

would be expressed to the nearest 400 volts/m.

Measurements over 4,000 volts/m would be expressed

as > 4,000 volts/m. For a meter calibrated to all three

voltages, measurements under 4,000 volts/m would be

expressed to the nearest 400 volts/m, measurements

between 4,000 and 40,000 volts/m would be expressed

to the nearest 4,000 volts/m, and measurements over

40,000 volts/m would be expressed to the nearest

40,000 volts/m.

Table 2 Measurement Units

Test Voltage For Readings of Express in Multiples of For Readings of Express as

100 V < 4,000 V/m 400 V/m > 4,000 V/m > 4,000 V/m

1,000 V < 40,000 V/m 4,000 V/m > 40,000 V/m > 40,000 V/m

5,000 V < 200,000 V/m 20,000 V/m > 200,000 V/m multiples of 200,000 V/m

Note: Measurements above 1000 volts/2.54 cm may be made based on verification of the meter at 1000 volts where less precision is acceptable

due to safety concerns with verification equipment/setup or availability of such equipment.