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SEMI E43-0301 © SEMI 1995 , 2001 2 operation as their s urfaces may be at elevated potentials that represent a sh ock hazard to the operator. 7 Equipment and Performa n c e Verification Methods 7.1 Equi pment 7.1.1 Elect…

SEMI E43-0301 © SEMI 1995, 20011

SEMI E43-0301

GUIDE FOR MEASURING STATIC CHARGE ON OBJECTS AND

SURFACES

This guide was technically approved by the Global Metrics Committee and is the direct responsibility of the

North American Metrics Committee. Current edition approved by the North American Regional Standards

Committee on November 22, 2000. Initially available at www.semi.org December 2000; to be published

March 2001. Originally published in 1995.

This document was entirely rewritten in 2001.

1 Purpose

1.1 The purpose is to establish a guide for

reproducible measurement of electrostatic charge(s) on

any surface or object, consistent with the scope and

limitations set forth below.

2 Scope

2.1 The measurement methods described herein can be

applied to characterize the general electrostatic charge

level(s) on objects and surfaces in all environments.

Acceptable instrumentation, calibration, and measure-

ment techniques are described in this document.

Appendices include background information on the

equipment specified and calibration procedure, as well

as information and advice on performing a useful

general static survey.

2.2 This standard does not purport to address safety

issues, if any, associated with its use. It is the

responsibility of the users of this standard to establish

appropriate safety health practices and determine the

applicability or regulatory limitations prior to use.

3 Limitations

3.1 Direct measurement of charge usually requires the

use of a coulombmeter. Charges on an isolated

conductor can be measured by transferring the charge

into the coulombmeter by contacting the isolated

conductor with the coulombmeter input probe. Charges

on isolated conductors and insulators can be measured

by transferring the charged object into a Faraday

enclosure that is connected to the coulombmeter. These

measurements can be relatively precise if care is taken

in the transfer process to avoid changing the charge

level when making the measurements.

3.2 Direct measurement of charge is often impractical.

In these instances, charge is indirectly evaluated by

detecting the electrostatic field from a charged surface

using an electrostatic fieldmeter or an electrostatic

voltmeter.

3.3 This guide does not describe instrumentation and

techniques capable of making highly precise

measurement of electrostatic charge. It is not suitable

for measurement of electrostatic charge on small

objects, such as packaged devices (i.e., reading(s)

obtained are indicative/general area and not precise/

minute). No methods of preconditioning the surface

prior to measurements and no methods of character-

izing the basic electrostatic performance of materials,

such as tribocharging, resistance, and decay rate are a

part of this document. Measurements made using this

guide on the same surface or object may differ due to

differences in the environment or history of the surface

or object between the times any two measurements are

made.

4 Referenced Standards

4.1 None.

5 Terminology

5.1 electrostatic discharge (ESD) — the rapid

spontaneous transfer of electrostatic charge induced by

a high electrostatic field.

5.2 ground — a conducting connection between an

object, electrical equipment, and earth, such as the

portion of an electrical circuit of the same electrical

potential as earth.

5.3 grounded — connected to earth or some other

conducting body that serves in the place of earth.

6 Safety

6.1 Measurements of Very High Static Potentials (>

30,000 Volts) — Measurements of very high static

potentials (> 30,000 V) may need to be done at larger

distances to avoid exceeding the measurement range of

the meter and/or an ESD event to the meter.

6.2 Measurements on Moving Objects or Surfaces —

Care should be taken, when attempting to read

electrostatic charges on moving objects or surfaces, to

maintain correct distance and avoid any contact; this is

to assure “good” readings with no mechanical damage

or personal injury.

6.3 Measurements Using Electrostatic Voltmeters —

Avoid handling electrostatic voltmeter probes during

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