**MEASUREMENT OF CHARGE STORAGE AND LEAKAGE IN POLYIMIDES \***

A. R. Frederickson and C. E. Benson

California Institute of Technology Jet Propulsion Laboratory

5800 Oak Grove Drive

Pasadena, CA 91109

and

James F. Bockman

NASA Langley Research Center

Langley, VA

28 November 2001

I. ABSTRACT

Resistivity and electric charge storage properties of LaRC-SI and Kapton
polyimide samples were studied in a dark evacuated environment, both
before and after 5-megarad cobalt-60 gamma irradiation. The data is
intended for application to spacecraft insulators that may experience
charging induced by space radiations. A "new" measurement method is
developed. Effects with both copper and aluminum electrodes were
examined. In different tests, slow electrons, and 4-keV electrons were
used to charge surfaces of 50-micron polyimide to about 500 volts while
the opposite surfaces were metalized and grounded. The subsequent decay
of surface voltage can be interpreted as a current through the
resistivity of the dielectric. The same samples tested by classical ASTM
and IEC methods provided resistivities of order 1E16 ohm-cm, but when
tested by our evacuated charge decay method the resistivity was 5E19 to
5E20 ohm cm or more. Charging with low energy electrons produced
resistivity data reasonably similar to that produced by charging with
4-keV electron beams. The gamma irradiation, followed by one month of
rest under no bias, reduced the resistivity by a factor of ½ in both
materials. Raising temperature from the normal 20C to 50C reduced the
resistivity in Kapton, but the reduction was not observable in the
LaRC-SI material. Even after twenty days of "constant-bias" the rate of
fractional charge loss continued to decline indicating that polarization
current rather than ideal resistivity was the cause of most voltage
decay, and that the true resistivity is higher than the values
determined here, and much higher by several orders of magnitude than the
values determined in classical ASTM and IEC test procedures. The effects
of electron beams at 20 keV were also briefly studied. Under electron
beams the LaRC-SI material was considerably less leaky than was Kapton,
and the LaRC-SI produced fewer discharge pulses than did the Kapton.

\* Most of this work was performed at the California Institute of
Technology, Jet Propulsion Laboratory as part of a program managed at
NASA Langley Research Center, and under contract with NASA.

II\. INTRODUCTION

Electrical Insulators on spacecraft are irradiated with high-energy
electrons in the space environment, and subsequently charged, sometimes
to high voltages. Occasionally such charged-insulators generate
spontaneous electric partial-discharge pulses of current of order mA to
ten Amperes and lasting tens to thousands of nanoseconds. Such pulses
can disrupt spacecraft, or destroy microcircuits. The pulses are very
similar to the pulses developed in normal applications on Earth by a
walking person becoming charged and then touching an electronic circuit,
or a car door handle. These events are known to disrupt, and sometimes
destroy microcircuits.

When evaluating the threat to spacecraft, reliability analysts will
calculate the rate of charge introduced into the insulators from space,
and attempt to balance this against the rate of charge leakage from the
insulators. The charge leakage current density is assumed to be the
quotient of electric field in the insulator divided by the bulk
resistivity of the insulator. The bulk resistivity of the insulator is
typically found in the material handbooks and is based on IEC or ASTM
test methods \[1,2\]. As the insulator in space accumulates charge the
electric field rises until the leakage current equals the accumulation
current, or until the insulator "breaks-down" and generates a pulse.\
Thus, the design of spacecraft is critically dependent on the value for
insulator resistivity.

It is likely that ASTM and IEC methods are not applicable to this
spacecraft problem. ASTM and IEC methods occur in air, place conductors
on both surfaces of insulators, and measure the leakage current after
only a few minutes to hours of bias application. But spacecraft often
have only one surface of the insulator in contact with a conductor, are
in vacuum, and experience reasonably steady bias for a year or longer.

That the leakage current in metalized insulators stressed by constant
voltage continues to decay for days and weeks is well known by
Dielectricians, and is briefly mentioned in the IEC test guidelines.
Apparently normal applications for the resistivity data in handbooks
have, until now, not been interested in the fact that the resistivity
continues to increase well after it has been measured. But in spacecraft
we need to know how the leakage decays for as long as a year or more.

Therefore we develop a method for measuring the charge storage and
leakage in insulators over extended periods intended for spacecraft
applications. We have measured the surface voltage continuously on some
samples for up to 38 days. There is no limit to the length of time that
the measurement can proceed. The handbooks \[3,4\] indicate values for
resistivity in polyimides from 1E16 to 1E18 ohm-cm based on ASTM and IEC
methods. Based on our method the actual resistivity is found to exceed
5E19 ohm-cm for Kapton polyimide, and for the LaRC polyimide exceeds
5E20 ohm-cm.

Long-term storage of charge in spacecraft applications is a detriment.
It implies that unless a mechanism for leakage is provided, eventually
spacecraft insulators will accumulate sufficient charge to generate a
dangerous discharge pulse. It is known that polyimides can be made
conductive by pyrolosis. Irradiation by high-energy rays such as
electrons, protons or gamma rays produces some of the same changes in
solids as does pyrolosis. Polyimide is especially interesting because it
maintains thermal and mechanical properties under pyrolosis, yet reduces
resistivity by many orders of magnitude. If one sufficiently pyrolizes
polyimide it will not store dangerous charge in spacecraft applications.

Therefore, it may be possible to simply irradiate polyimide to reduce
its resistivity in order to prevent pulsing in space applications, yet
maintain other desirable properties. To investigate this possibility we
irradiated samples with cobalt-60 gamma rays and assessed the change in
resistivity after the irradiation.

III\. EXPERIMENTAL METHODS

Both LaRC-SI and Kapton (tm: DuPont Co.) samples were measured. Sample
surfaces were charged to high voltage using two methods: charging by
slow electrons and charging by multi-keV electrons. All tests were
performed at high vacuum, typically 1E-6 Torr. The surface voltages were
monitored with an electrostatic field probe that did not contact the
surfaces. Samples could be heated or cooled, but results were obtained
at 20 and 50 degrees Celsius only.

**III.A Charging the Samples**

***[III.A.1]{.underline} Technique To Charge Insulators To High Voltage
By Slow \~10 eV Electron Impact.***

The simplest arrangement for charging an insulated surface is defined by
the following arrangement in Fig. 1. The insulated surface of the sample
faces the electron source and/or the field probe. The other surface of
the sample is metalized and connected to the sample mount so that it can
be biased relative to Earth.

![](media/image1.png){width="3.729861111111111in"
height="2.609722222222222in"}

**Figure 1. Arrangement for Charging Using Slow Electrons, and
Subsequent Measurement of Sample Surface Voltage**. The picoammeter is
not necessary, but it sometimes provides useful diagnostic information
during charging, or if the sample is discharging across the vacuum due
to photoemission or other means.

The field probe is moved to face the sample when one wishes to measure
surface potential. The filament is spaced about 10 cm from the sample
and is powered by about 1.5 volts at 0.35 watts. The field probe is
spaced roughly two cm from the sample face when the potential is
measured. The apparatus is placed in a vacuum chamber that also contains
an electron beam that can alternately be used to charge the sample
surface.

Imagine that the sample is a perfect insulator. If the charging voltage
is set to, say, +100 volts the field probe will measure \"100-volts\".
The field probe is then moved away from the sample. One then turns on
the filament and the electrons emitted by the filament travel to the
surface of the sample and \"ground\" the surface of the insulating
sample. This establishes 100 volts across the sample. The filament is
then turned off and the 100 volts remains across the sample. The field
probe may then be moved to face the sample and confirm the fact that the
sample surface is grounded by measuring 0-volts.

One then grounds the rear electrode of the sample and instantly the
field probe should register --100 volts as the surface voltage on the
insulating sample. If the sample is leaky, the --100 volts decays
through the sample bulk and into the ground wire, and its decay time
constant is measured. The resistivity of the sample is determined from
its decay time constant, τ = RC.

This experiment is ideal because it does not produce radiation damage to
the sample and measures its pre-radiation property. It is simple and
direct. It does not require de-convolving electron depth-dose and charge
deposition phenomena. It truly measures the dark conductivity of the
sample material.

Tests with one-inch samples have determined the smallest samples that
can be conveniently studied, 1-inch diameter. The field probe becomes
both hard to calibrate and much less sensitive for smaller samples.

Use of other insulators in the chamber to isolate the actual sample
would, in effect, produce data characteristic of the isolating
insulator, and not of the sample. One must be careful that the only
insulator contributing to electric fields sensed by the probe is the
sample. Additionally, one must be certain that other insulators do not
affect the charging-up of the sample under test. Other insulators
exposed to the electrons emitted from the filament may charge and then
deflect subsequent electrons in the chamber, thereby affecting the
charging-up process.

***III.A.2 The Preferred Charging Field-Geometry.***

The best geometry is one that allows only the front of the sample to be
irradiated and charged. In Fig. 2, the vacuum chamber walls and the
aluminum mounting structure prevent electrons from irradiating any but
the front surface of the insulator. None of the wiring is irradiated
either. Thus, the front of the sample is the only insulated surface to
be exposed to radiation. Also, note the copper guard ring electrode on
the rear of the sample. This prevents edge effects from affecting the
measurements at the center electrode of the sample. Guard rings are
recommended in the IEC Method \[2\] and are often good practice in
electrostatic studies.

![](media/image2.png){width="3.998611111111111in"
height="2.5694444444444446in"}

**Figure 2. The arrangement for the tests of Polyimides.**

Other arrangements may be satisfactory in some cases. The critical
issues are: possible effects of other insulators in the chamber, and
control of the electron currents in the vacuum to produce known charging
of the test sample. For example, samples may be simply mounted on a
back-plane of metal and connected to the electronics. Alternately, large
samples may be stretched on a frame that is mounted to metallic
underpinnings. The sample can be made of a larger area than its metallic
supports and thus avoid measurement error due to secondary electron
effects across the surface.

Note that care is taken to prevent insulators near the sample from being
charged by the electron beam and by scattered electrons. The Peltier
cooler has some insulation associated with its construction, and the
sample mount has insulators, but all insulators are electrostatically
decoupled from the space around the face of the sample. It is difficult,
not impossible, for electrons to charge these insulating structures, so
they should be electrostatically decoupled.

**III.A.3 *Method of Charging with kev Electrons in Vacuum***.

In order to measure resistivity under spacecraft conditions where
non-metalized surfaces of polyimide are exposed to vacuum one may use an
electron beam to charge the sample. It is, in fact, high-energy
electrons in space that charge the sample. Once charged, the surface of
the sample can discharge to ground through the bulk-conductivity of the
sample and onto the grounded copper electrode at the other (rear)
surface of the sample.

The test method is simple. First one charges the surface. Then the
surface voltage is monitored using an electrostatic field probe. The
decay of surface voltage is related to the conductivity of the sample.
It is assumed that the decay is a simple exponential decay where τ is
the 1/e-time decay constant. The decay of surface voltage is related to
the resistivity, ρ, by the equation τ = ρε = RC. The permittivity of the
dielectric is ε.

Based on published ASTM methods \[1\], previous measurements on LaRC SI
polyimide in this laboratory indicated that its (so-called) resistivity
is of order 1.4E16 ohm cm. Its permittivity is 2.76E-13 Farads/cm
(relative dielectric constant is 3.12). Thus, based on the ASTM
measurements, one expects that τ = ρε = 3.86E3 seconds = 64 minutes.
Therefore the experiment is arranged to measure decay constants of order
64 minutes efficiently. One must be certain that surface voltage does
not decay by currents passing through the vacuum and, therefore, the
measurements are performed in the dark, and without vacuum gages being
near the sample. (Some vacuum gauges have been seen to emit a small
amount of plasma and thereby discharge surfaces.)

[Second Crossover Point]{.underline}

One must charge the samples with electrons and not overcharge to the
point of breakdown. One is advised to know the energy at the second
crossover so that overcharging of the surface by keV electron beams does
not occur. Since the samples are 0.051 mm (two mils) thick, one must not
exceed 1-kV on the surface for long periods (20 kV/mm is typical
breakdown threshold). Additionally, one may be concerned to avoid high
field conductivity effects and stay below the level of 10 kV/mm. Our
measurements using the ASTM method applied 64 volts through the sample,
roughly 1/10 of the voltage where high-field conductivity effects begin
to be important. High-field conductivity greatly exceeds regular
conductivity. Therefore the electron beam tests should initially stay
well below 1-kV potential on the surface.

A beam of 1-keV electrons is not capable of charging most polymer
surfaces because for every electron incident more than one secondary
electron is emitted. In order to charge the surface one must raise the
energy above the energy of the so-called second crossover point, above
which secondary electron emission is less than the incident particle
flux. For higher energy beams the surface voltage will ultimately
achieve a value given by the electron accelerator voltage minus the
second crossover voltage. For example, a 10 keV beam incident normally
on a surface with 1.8 keV second crossover will eventually charge the
surface to --8.2 kV (provided the beam incidence remains normal). For
our samples such charging voltage should be avoided.

The second crossover point was measured in both samples by exposing the
samples to electrons over the range 0 - 5,300 keV and measuring the
voltage on the floating back copper electrode. Back electrode voltage
vs. beam energy was plotted to locate the second crossover point. The
crossover point was determined to be 1.8 keV. Above this energy the
electrons entering the film are greater than the number being knocked
off the surface, and negative charge begins to accumulate.

The second crossover point is measured with the apparatus diagrammed in
Fig. 3.

![](media/image3.png){width="3.9520833333333334in" height="2.9875in"}

**Figure 3. Apparatus for determining second crossover point, and for
charging the sample with keV electron beams.**

The grounded screen decreases the deflection of the electron beam by the
charged sample. This holds the incident electron trajectories so that
they are incident upon the sample close to normal (perpendicular to the
surface). The field probe is fixed in position near the floating metal
plate that is attached to the back copper electrode. The field probe is
calibrated by impressing a known voltage on the floating metal plate.
For these tests, the field probe was spaced so that its reading
indicated 0.75 Vplate. Thus a 400-volt plate indicated as 300 V on the
field probe.

During the irradiation at each beam energy, say 2keV, the irradiation
proceeds long enough to fully charge the sample. The potential on the
metal plate is measured only when further irradiation at 2 keV does not
further charge the sample, and steady state voltage has been achieved.
The results are shown in Fig.4.

![](media/image4.png){width="3.4458333333333333in"
height="2.9451388888888888in"}

**Figure 4. Ultimate Surface Voltage vs. Electron Accelerator Voltage.**

From the second crossover data in Fig. 4 one sees immediately that the
second crossover point occurs at 1.8 kV for both samples. The two slopes
are slightly different because the placement of the probe near the
Kapton sample was not identical to that of the LaRC-SI sample creating a
slightly different calibration factor. The slopes are not exactly equal
to 0.75 because the grid did not fully eliminate the effect of beam
deflection by the charged surface.

[Capacitance Voltage Divider Model]{.underline}

For this kind of test there is one further calibration factor that needs
to be determined. Capacitive voltage division causes the front surface
voltage to be different from the back electrode voltage.

The following capacitance model represents the electric field probe
experimental setup. The field probe monitors the voltage, V~BE~, on the
rear electrode. The field probe voltage was calibrated by applying a
known voltage, V~BE,~ and reading the field probe. The field probe
reading was 75% of the applied V~BE~. But when the voltage is caused by
a charge, Q, on the surface of the sample, there is a voltage drop in
the sample causing a smaller voltage to appear on the back electrode. We
need to determine a correction for this effect. Figure 5 illustrates the
effect.

![](media/image5.png){width="4.752083333333333in" height="2.10625in"}

**Figure 5. Equivalent Circuit for Field Distribution at Field Probe**.

C~FS~ - Front surface capacitance (sample surface opposite the center
electrode to mounting hardware and vacuum chamber walls).

C~S~ - Sample capacitance (front of sample to copper electrode at rear
of sample).

C~RS~ - Rear probe plate capacitance (connecting wire and plate to
grounded objects including the mounting hardware, guard ring, etc.).

Q - Charge injected into sample (electrons from electron beam are
stopped within the first micron which is essentially on the surface).

V~S~ - Front surface voltage on sample.

C~RS~ = 10.5pF

C~FS~ = \~1-2pF (estimate)

C~S~ = εA/d = (3.12) ( 8.85E-14 F/cm) (16.6cm^2^) / (.0051cm) = 899 pF

Dielectric constant of LaRC-SI = 3.12

Dielectric constant of Kapton = 3.4

$\frac{V_{\text{BE}}}{V_{S}} = \frac{C_{S}}{C_{S} + C_{\text{RS}}} = 0\text{.}\text{99}$The
solution to this system is

Thus the capacitance division requires one to make a small correction,
only 1%, which in this work can be ignored.

**III.B. Surface Voltage Measurement Techniques**

Samples KA001 (Kapton) and SI001 (LaRC-SI) were charged using the low
energy electrons with an applied bias on the rear electrode in the
configuration of Fig. 6. The only insulator that can be charged by the
electron beam, or by the slow electrons, is the sample surface itself.

![](media/image2.png){width="3.4965277777777777in"
height="2.2527777777777778in"}

**Figure 6. The Sample Mounting Configuration**. Usually the guard ring
is held at ground potential. The diameter of the center electrode on the
sample is 4.6 cm and its area is 16.6 cm^2^. The diameter of the opening
on the aluminum mount is 5.08 cm. The vacuum chamber is 73 cm long so
that two samples and a ground reference sample were simultaneously in
the chamber. By remote control the field probe is placed in front of
either a sample or the ground reference, or placed near the edge of the
chamber.

The vacuum chamber was nearly light tight. The vacuum gauges were
usually left off so that they would not contribute plasma to the
chamber. The vacuum was nominally 1E-6 Torr or better during the
extended period of the test. Unless otherwise noted the temperature was
between 20 and 25 C.

Figure 7 describes the method for surface voltage measurement. This
method has the advantage of being insensitive to drift in the
calibration of the electrostatic field probe. First the field probe is
positioned in front of a grounded plate and its zero is established. A
voltage applied to the plate may be used to assess if the field probe
sensitivity is approximately normal. Then the field probe is placed in
front of the sample and its surface voltage, as indicated by the probe,
is noted for reference. However, the surface voltage is measured by
adjusting the voltage source until the field probe reads zero volts. The
surface voltage is then equal, and opposite in polarity, to the voltage
source.

![](media/image6.png){width="2.9965277777777777in"
height="2.797222222222222in"}

**Figure 7. Measurement of the surface voltage by adjusting the voltage
source so that the field probe reads zero volts.**

The samples were charged using a positive bias of 500 volts on the rear
electrode as the electron-emitting filament (Fig. 1) provided several
nanoamperes per cm^2^ to the sample surface. At incident energies below
500 volts, all of the electrons are stopped within the first 1000
Angstroms of the insulator sample. After about ten minutes, the filament
was turned off and the surface voltages were monitored for 38 days while
the rear electrode was held grounded (except during the voltage
measurement). If the samples were conductive, charge would flow to
ground through the rear electrode causing the surface voltage to
decrease.

[Pictures of the Apparatus]{.underline}

The polyimide samples were mounted in their circular mountings as shown
in the next picture.

![](media/image7.jpeg){width="3.417361111111111in"
height="2.5694444444444446in"}

As a piggy-back experiment, a piece of Mylar was mounted between the two
polyimide samples. The Mylar experiment will not be discussed in this
report. The field probe was placed on a transit stage before the array
of the three samples as shown in the next picture.

![](media/image8.jpeg){width="3.5729166666666665in"
height="2.6847222222222222in"}

The field probe is contained in the vertical copper tube near the
right-most end of the transit stage. As the probe moves towards the
left, it first passes in front of a diamond-shaped plate whose bias can
be set, thence in front of the kapton-001 sample, thence in front of the
Mylar, and finally in front of the LaRC-SI-001 sample. As indicated in
the following picture, the field probe is spaced approximately 5 mm from
the surfaces of the sample holders as it moves on the transit stage. The
complete apparatus is inserted into the vacuum chamber with the two
circular rings tightened against the walls of the cylindrical chamber.

![](media/image9.jpeg){width="3.873611111111111in"
height="2.9097222222222223in"}

The brass tubes contain resistive heater elements that heat the aluminum
plate upon which are mounted the circular polyimide sample holders. The
Peltier coolers are squeezed between the aluminum plate and a cooled
aluminum "T" beam.

[Temperature Effects]{.underline}

The samples are expected to demonstrate their worst charging behavior at
low temperatures. Low temperature will be achieved by the use of an
in-chamber liquid-cooled thermal sink aided by electric Peltier coolers
to achieve a larger range of temperature. The base plate, to which the
sample mounts, is cooled by the Peltier cooler, or is heated by the
resistor heaters that are enclosed in the brass tubes. The cooler/heater
was constructed as part of this task.

The Peltier coolers allow lower temperatures to be achieved, colder than
the circulating fluids in the base plate. The cold thermal sink cools
everything in the chamber, including the field probe. The field probe
works neither at very low temperatures, nor at very high temperatures.
Thus the Peltier coolers are used to locally cool the sample beyond the
range of the field probe.

By reversing current through the Peltier coolers, they become heaters.
By passing hot fluid into the cold plate it becomes a heater as well.
Thus the temperature of the samples can be raised well beyond that of
the field probe. Resistive heaters are also available to heat the
samples.

Because the samples stored charge for exceedingly long times at room
temperature (20 -- 25 C) there was no need to go to lower temperatures
in the testing.

[Experimental Measurement of Leakage Through the Vacuum
Itself]{.underline}

It is known that light from the electron-emitting filament also removes
electrons from the sample surface, and may induce conductivity through
the bulk of the samples. This effect may generate errors in the measured
data. Additionally, plasma in the vacuum can alter the surface
potential. Plasma is generated by the vacuum gauges and by the ion
vacuum pump. Usually, these sources remained off if there was the
possibility that they would generate false data. However, it is best to
actually measure the magnitude of the ion currents so-generated.

A square plate, 5x5 cm^2^, was connected to the vacuum tank BNC
feed-through with several feet of coaxial cable. The capacitance to
ground (tank walls) of the plate and cable was measured to be 130 pF. A
bias of +500 volts was maintained overnight on the system in order to
"fully" polarize the cable dielectric. With the plate at +500V the field
probe measured 277 V, thus providing a calibration factor of 1.805.

The bias was removed and the 130 pF system was allowed to slowly
discharge while both the ion gage and the ion pump were off. Vacuum was
maintained with the oil diffusion pump system which generates no plasma
or light. After 5-hours the field probe reading had decayed to 261V, or
471 volts on the surface of the sample. The decay rate was 29V/5 hr =
5.8 V/hr. Most of this decay probably occurs through the coaxial cable
dielectric.

The same test was performed while the ion pump alone pumped on the
chamber. It was pumping at a "rate" of 20 mA, near its maximum rate at
about 5E-6 Torr. At this rate it is generating the maximum amount of
ions and electrons that it can for extended times. After 5 hr 20 minutes
the surface voltage had decayed from 471V to 437V. The decay rate is
therefore 34V/5.333hr = 6.4 V/hr.

Perhaps the data can be interpreted as if the ion pump and ion gage
cause the 130 pF system to decay by 6.4 -- 5.8 = 0.6 V/hr. Considering
the errors in these measurements, the decay rate induced by the pump and
gage was definitely less than 1 V/hr on 130 pF. The discharge current
through the vacuum due to ion pump and ion vacuum gage is thus less than
C\*dV/dt = I = 3.6 E-15 Amperes. This when the ion pump is generating
its largest sustainable ionized gas level.

It is interesting to speculate on the present capability of the system
to measure small currents in the chamber. We can certainly measure a
change of 2-volts with an error of 1-volt. We could do this on a witness
plate suspended by long and thin, effectively non-conducting, polymer or
glass strands. Such a plate would have a capacitance of less than 50 pF
to the chamber walls, assume approximately 10 pF. Assume that we monitor
the voltage decay for 12-days = 1E6 seconds. Then CdV/dt = 2E-17 amperes
which is approximately 100 electrons/second.

Leakage through the vacuum is "in parallel" with leakage through the
insulator sample. The vacuum currents thus have the effect of making the
insulator sample appear more conductive than it really is. The vacuum
currents have been shown to be less than 3.6E-15 Amperes from a sample
area (two sides) of 50 cm^2^. As experiments are performed the actual
vacuum currents will become better known. For example, the samples being
tested have a capacitance of 9.0E-10 F. The smallest measured decay rate
was about 0.33 V/day, or 3.8E-6 volts/second. The current I = CdV/dt =
3.4E-15 Amperes onto the sample with a surface area of 16.6 cm^2^. Thus
it is possible that the leakage in our samples was partially provided by
currents in the vacuum!

After the irradiations at all energies have been completed, the surface
voltage can be discharged using a continuous irradiation beginning with
electrons at 1 keV above the surface voltage and continuing while very
slowly lowering the beam energy to less than the second crossover
energy. Although this removes the surface voltage, it does not remove an
internal dipole field just below (within about 1000 Angstroms) the
surface. The internal dipole field is a remnant of the stopping depths
of the electron beams (and other effects).

IV RESULTS OF CHARGE DECAY TESTS ON POLYIMIDE WITH COPPER ELECTRODE

**IV.A. After Charging by Slow 50-500 eV Electrons**.

The virgin Kapton 001 and virgin LaRC-SI 001 samples were first charged
with low energy electrons, 50-500 eV, on 17 April, 2001. This was
accomplished by biasing the rear copper electrodes at +500 volts and
lighting the electron-emitting filament in the vacuum spaced about 20 cm
from the sample\'s front faces. The un-metalized surfaces of the samples
were thereby charged to approximately minus 400 to 500 volts relative to
the copper electrodes on the rear faces of the samples. It takes more
than one minute to move the field probe into position before the sample
and take a surface voltage measurement. Because the operations of the
field probe occur slowly, one cannot know the exact starting voltage on
the samples, except that it cannot "exceed" (-)500 volts.

Five hundred volts was chosen for good reasons. Generally, conductivity
in dielectrics increases, resistivity decreases, with increasing
electric field strength. Five hundred volts is near to the maximum
electric field that can safely be placed on this material for many years
without generating partial discharges that could harm sensitive
electronics. Additionally, space radiation induces similar electric
fields in good insulators. One wishes to know the amount of conductance
that is available to remove radiation-induced space-charge at field
strengths where it is safe from partial discharge pulsing, but not
unreasonably safe.

The potentials of the un-metalized faces were then periodically
monitored from 17 April to 7 May 2001 at room temperature, and from 8
May to 11 May at 50 C. Initially there was a steep decline of surface
voltage, about 20% on the Kapton sample and 10% on the LaRC-SI sample
during the first three days. The early decay is shown in Fig. 8.
Afterward, as shown in Fig. 9, the LaRC SI polyimide maintained nearly
all of its charge even during the application of 50 C temperature. Fifty
degrees C was maintained on both samples between 20.6 days and 23.6 days
when the increased rate of charge decay can be seen in the Kapton
sample.

![](media/image10.png){width="3.7090277777777776in"
height="2.9972222222222222in"}

**Figure 8. Decay of surface voltage during the first day**. It is
important to note that the ASTM D 257-99 method for measurement of
conductance takes its measurement after only ten minutes of bias
application. Although leakage current is still substantial at ten
minutes, it is significantly reduced after a few hours.

![](media/image11.png){width="3.936111111111111in"
height="3.1680555555555556in"}

**Figure 9. Surface Voltage Decay for Two Polyimide Samples**. The first
20-days were at room temperature, Days 20.6-23.6 at 50 C, and room
temperature thereafter. Electron beam charging occured at 23.7 days.

**IV.B. Electron Beam Discharging and Charging of the Sample Surfaces**

The next step in the test procedure is to generate the surface voltage
using 4 keV electrons instead of the \<500 eV electrons. This is done in
order to see how the charge storage properties compare for the two
methods of charging. The purpose of this comparison is only a matter of
careful analysis and verification of our modeling. We expect to see, at
most, a small difference between the two methods.

Consider the fact that the sample surfaces have been maintained at
roughly (-) 350 to 400 volts for several weeks. The polarizations of the
internal dipoles have become relatively stabilized by this time (a
presumption). The field internal to the region of penetration by the 500
eV to 50 eV electrons has also become stable. Now, the 4-keV electron
irradiation will penetrate more deeply into the faces of the samples and
produce a different charge depth profile than was produced by the \<500
eV electron charging case. Electrons and holes will be excited into
traps over the range of depths penetrated by the 4-keV electrons. Later
some of these trapped carriers will be thermally excited into the
conduction (valence) band and move so as to reduce the local electric
fields. The experimental data is taken in an attempt to see if this
happens in a measurable way.

[Discharging]{.underline}:

First, the sample surface is discharged to nearly zero volts.
Discharging is accomplished by irradiating the surface with electrons at
incident energies between the first and second crossover points for
secondary electron emission. Since the sample surfaces have been charged
to approximately minus 400 volts, an electron beam at 1 keV will hit the
surface with 600 eV energy. This will emit more than one secondary
electron per incident electron, thereby discharging the negative surface
potential. The samples were bombarded with roughly 0.5 nA/cm2 for
several minutes as the beam energy was continuously lowered from an
initial 1.5 keV to zero eV. The sample surfaces were then measured to be
within 3 volts of ground. The sample surfaces remained at zero voltage
for nearly 30 minutes before being recharged by the 4-keV beam. This
30-minute time span allowed their polarizations to partly relax towards
the zero-field condition.

[Charging]{.underline}:

The electron beam was raised in energy to 4 keV. The second crossover
energy had previously been determined to be roughly 1.8 keV. Therefore 2
keV is the minimum energy required to develop 200 volts surface
potential. A 4-keV electron beam is able to develop more voltage. The
samples were radiated for about 4 minutes at 4 keV. The radiation was
performed in a sequence of short irradiations over a time span of 20
minutes so as to monitor the voltage as charging progressed and thereby
develop surface voltages similar to those developed in the 0-500 eV
electron charging case.

The Kapton was charged to about (-)325 volts, and the LaRC-SI was
charged to about (-)439 volts. Again the Kapton exhibited a rapid decay
of surface voltage, from 325 to 293 volts in 2 hours 41 minutes. The
LaRC-SI surface voltage decayed very little, from 439 to 437 volts in
the same time. The open data points in Fig. 9 describe the results after
discharging and recharging with the electron beams.

If charging by the 4-keV electron beam is different than charging by
50-500 eV electron impact, Fig 9 does not indicate a dramatic
difference. Perhaps after the 4-keV charging the LaRC-SI sample is
slightly more "leaky". And perhaps the Kapton is less "leaky". The
differences are not dramatic.

The following table of data is provided so that one may see the results
in full detail.

**Table of data from which Fig. 9 is generated**.

Kapton sample #KA001

LaRC-SI sample #SI001

Data of april-may 2001.

Room temp., 20-24C

+----------------+----------------+---+---------------+----------------+
| Kapton         | Kapton         |   | LaRC-SI001    | LaRC-SI001     |
|                |                |   |               |                |
| KA001          | KA001          |   |               |                |
+----------------+----------------+---+---------------+----------------+
| DAYS           | VOLTS          |   | DAYS          | VOLTS          |
+----------------+----------------+---+---------------+----------------+
| 0              | 478            |   | 0             | 438.7          |
+----------------+----------------+---+---------------+----------------+
| 0.021          | 408            |   | 0.036         | 403.8          |
+----------------+----------------+---+---------------+----------------+
| 0.14           | 382            |   | 0.16          | 402            |
+----------------+----------------+---+---------------+----------------+
| 0.71           | 377            |   | 0.7           | 402            |
+----------------+----------------+---+---------------+----------------+
| 0.95           | 377            |   | 0.95          | 402            |
+----------------+----------------+---+---------------+----------------+
| 1.18           | 375            |   | 1.15          | 402            |
+----------------+----------------+---+---------------+----------------+
| 1.58           | 374            |   | 1.58          | 402            |
+----------------+----------------+---+---------------+----------------+
| 1.92           | 372            |   | 1.91          | 402            |
+----------------+----------------+---+---------------+----------------+
| 2.62           | 371            |   | 2.61          | 402            |
+----------------+----------------+---+---------------+----------------+
| 2.92           | 370            |   | 2.91          | 402            |
+----------------+----------------+---+---------------+----------------+
| 5.6            | 364            |   | 5.6           | 402            |
+----------------+----------------+---+---------------+----------------+
| 5.9            | 363            |   | 5.9           | 402            |
+----------------+----------------+---+---------------+----------------+
| 6.6            | 362            |   | 6.6           | 401            |
+----------------+----------------+---+---------------+----------------+
| 6.9            | 360            |   | 6.9           | 401            |
+----------------+----------------+---+---------------+----------------+
| 7.6            | 359            |   | 7.6           | 401            |
+----------------+----------------+---+---------------+----------------+
| 8.6            | 358            |   | 8.6           | 401            |
+----------------+----------------+---+---------------+----------------+
| 8.9            | 357            |   | 8.9           | 400            |
+----------------+----------------+---+---------------+----------------+
| 9.6            | 356            |   | 9.6           | 401            |
+----------------+----------------+---+---------------+----------------+
| 9.9            | 356            |   | 9.9           | 401            |
+----------------+----------------+---+---------------+----------------+
| 12.6           | 351            |   | 12.6          | 400            |
+----------------+----------------+---+---------------+----------------+
| 12.9           | 351            |   | 12.9          | 400            |
+----------------+----------------+---+---------------+----------------+
| 13.6           | 350            |   | 13.6          | 400            |
+----------------+----------------+---+---------------+----------------+
| 14.6           | 349            |   | 14.6          | 400            |
+----------------+----------------+---+---------------+----------------+
| 14.9           | 349            |   | 14.9          | 400            |
+----------------+----------------+---+---------------+----------------+
| 15.6           | 348            |   | 15.6          | 400            |
+----------------+----------------+---+---------------+----------------+
| 15.9           | 347            |   | 15.9          | 400            |
+----------------+----------------+---+---------------+----------------+
| 16.7           | 346            |   | 16.7          | 398            |
+----------------+----------------+---+---------------+----------------+
| 19.6           | 344            |   | 19.6          | 397            |
+----------------+----------------+---+---------------+----------------+
| 19.9           | 343            |   | 19.9          | 396            |
+----------------+----------------+---+---------------+----------------+
| 20             | 344            |   | 20            | 396            |
+----------------+----------------+---+---------------+----------------+

Change Temperature to 50 C.

  ---------------- ---------------- ---- ---------------- ----------------
  20.6             344                   20.6             397

  20.9             344                   20.9             397

  21.6             343                   21.6             397

  21.9             341                   21.9             396

  22.6             339                   22.6             396

  22.9             337                   22.9             396

  23.6             337                   23.6             396
  ---------------- ---------------- ---- ---------------- ----------------

Return to 22 C. Discharge the surfaces.

Then charge surfaces with 4 keV electrons.

  ---------------- ---------------- ---- ---------------- ----------------
  23.871           325                   23.871           439

  23.875           323                   23.873           439

  23.88            319                   23.88            439

  23.89            308                   23.89            438

  23.905           302                   23.905           438

  23.95            295                   23.95            437

  23.98            293                   23.98            436

  24.11            289                   24.11            434

  27.9             288                   27.9             426

  28.6             287                   28.6             426

  28.9             286                   28.9             426

  29.6             286                   29.6             426

  29.9             286                   29.9             425

  30.6             286                   30.6             425

  30.9             286                   30.9             424

  33.6             286                   33.6             423

  33.9             286                   33.9             423

  34.6             285                   34.6             423

  34.9             285                   34.9             423

  35.6             285                   35.6             423

  35.9             285                   35.9             423

  36.6             285                   36.6             422

  36.9             285                   36.9             422

  37.6             285                   37.6             422
  ---------------- ---------------- ---- ---------------- ----------------

**IV.C. Alteration Of Charge Storage By Irradiation With Cobalt 60 Gamma
Rays**

An experiment was performed to see if radiation can be used to
permanently make polyimides more conductive, and thus less able to store
charge. Irradiation generates traps, radicals and free carriers that can
contribute conductivity to polyimides. It is well known that extreme
heating (pyrolosis) causes polyimides to become conductive.

[Irradiation]{.underline}

The same two samples were irradiated to 5-megarads with Co-60 Gamma
rays. The Cobalt-60 radiation chamber consists of the stack of materials
surrounding the sample as shown in figure 10.

![](media/image12.png){width="3.5006944444444446in"
height="2.113888888888889in"}

**Figure 10. Sample Irradiation System**. The entire system is located
in vacuum adjacent to the cobalt gamma ray source.

We want to keep the internal charging generated by the cobalt and
scattered photons to a minimum. The interface dose-enhancement effects
should be minimized, knowing that they cannot be eliminated. Thus we
require the lead (Pb) in order to attenuate the broad low-energy peak of
scattered photons below 300 keV that is typically generated by Co-60
sources. A factor of ten attenuation is more than sufficient. At 300
keV, the mass attenuation coefficient, A, for Pb is 0.38 cm2/g. If T is
thickness, then TA=2.3 for an attenuation factor of ten. Thus, T = 6
g/cm2. The density of Pb is 11.3 so that the lead need not be thicker
than 5 mm, but should exceed 2 mm. In a similar line of reasoning, the
aluminum is used to attenuate the fluorescent photons that the lead
generates, as well as its Auger-electrons and photoelectrons.
Additionally, the Al comprises the support structure and protective
encasement for the samples. Finally, the thin polymer foils protect the
sample from being coated by aluminum while under compression in the
encasement.

Although one might worry about the effects of oxygen, in this
arrangement there is a limited amount of oxygen available to the sample
during cobalt irradiation. Never the less, we have procured a small
vacuum chamber in which the irradiations can proceed. The radiations
proceeded in an evacuated air chamber at less than 10^-4^ atmospheres
(100 microns Hg), but more than 10-6 atmospheres (1 micron Hg).

The radiation intensity was measured with an air-ionization chamber
temporarily placed between the aluminum sheets where the samples are
normally placed. The probable error is +/- 20%. The dose rate was 22.6
rads(Si)/second and the samples were exposed for 3670 minutes starting
at 17:20 on 17 August 2001, and ending on 20 August, thus providing 5.0
megarads to the samples. The sample irradiation temperature is estimated
to have been between 25 and 30 degrees C, and storage temperature varied
from 20 to 25 degrees C.

[Testing After the Cobalt-Irradiation]{.underline}

The samples were subsequently stored for one month: unbiased in the dark
in air until 31 August, and then in vacuum unbiased until testing began.
Thus, the effects of the cobalt irradiation had approximately one month
to decay before any charge storage measurements were performed. The rest
was scheduled in order to simulate the fact that in actual application
of radiation treatments, the irradiated material would not be
immediately pressed into service.

Sample KA001 was then first charged using +500 volts on its electrode
and the filament electron emitter at 09:00 on 14 September. It alone was
monitored until 09:00 on 17 September when sample SI-001 was similarly
charged. Testing of charge storage occurred in the dark at a vacuum of
roughly 3E-6 Torr at room temperature, 20 -- 25 degrees C.

The post-radiation data for the polyimide samples is provided in the
following table.

  --------------------------- --------------------- ----------------------
  SEPT, 2001 data             post-rad              post-rad

  DAYS                        Kapton-001            LaRC-SI-001

                              volts                 volts

  0                           496                   473

  0.0007                      494                   

  0.0014                      488                   

  0.0042                      473                   

  0.007                       461                   

  0.0104                      453                   

  0.0146                      446                   

  0.0222                      428                   

  0.0257                      425                   

  0.0306                      421                   

  0.0521                      409                   

  0.0604                      407                   

  0.0868                      402                   

  0.159                       398                   

  0.17                                              471

  0.22                        397                   

  0.23                                              467

  0.254                       396                   

  0.96                        384                   468

  2.06                        377                   467

  2.37                                              466

  2.88                        371                   

  3.05                        345                   467

  3.125                       345                   

  3.26                        344                   

  4                           340                   467

  4.3                         338                   466

  5                           334                   

  5.3                         334                   

  6                           332                   

  6.3                                               

  7                           329                   465
  --------------------------- --------------------- ----------------------

*Evaluation of Resistivity*, ρ.

Assume that the sample is a simple uniform parallel-plate leaky
capacitor. Apply a voltage, Vo, until time t=0. At t=0 disconnect the
voltage source from the capacitor so that the following functions hold.

V = Vo exp\[-t/ρε\] . ρ is the resistivity of the sample material, ε is
its permittivity.

For small t/ρε, exp\[-t/ρε\]= 1 - t/ρε .

Our experiments always deal in the regime where t \<\< ρε and the decay
is only a small percentage of the initial surface voltage.

Then (1/Vo)(dV/dt) = (-1/ρε) exp\[-t/ρε\] = (-1/ρε) \[1 - t/ρε\] . Note
that this function is very nearly equal to (--1/ρε) .

Therefore, ρ = -1/\[ε(1/Vo)(dV/dt)\] .

Thus a tabulation of (1/Vo)(dV/dt) provides the simplest evaluation of
the resistivity, ρ, of the insulator sample. The dielectric
permittivity, ε, for the polyimide samples is here taken to be a
constant approximately equal to 3.2 x 10^-13^ F/cm. However, the reader
should not presume that ε really is a constant. Our data proves that the
dielectric constant is changing (getting larger) throughout the process
of measuring charge decay. (In fact, most of the apparent charge decay
current has been a polarization current which equates to a changing
dielectric constant as frequency goes lower.) It does not, however,
change by more than a factor of two during our measurements. So, the
instantaneous value of dV/dt will be measured, and the evaluation of
resistivity will proceed with the presumption of a constant dielectric
constant, ε. There are practical reasons for making this simple
assumption that may be discussed elsewhere. For the time being, we want
to stay with the normal ideas that are generally used in this field,
even if they are incorrect.

*Kapton Resistivity, post 5-megarads*.

The measured surface voltage decay rate for the Kapton sample is
provided in Fig. 11. After six or seven days of being charged, the decay
rate in the irradiated Kapton is approximately 1E-7 per second.

![](media/image13.png){width="4.623611111111111in"
height="2.8569444444444443in"}

**Figure 11. Normalized Voltage Decay Rate in 5-Megarad Irradiated
Kapton as a Function of Time Since Charge Was Established**.

We arbitrarily choose to compare the pre-rad and post-rad resistivities
after one week under constant DC voltage stress when the post-rad decay
rate was approximately .95E-7 (fraction/sec). Thus the resistivity of
the Kapton post-rad is 1/(3.2E-13)(.95E-7) = 3E19 ohm-cm. Pre-rad
resistivity at one week is about 5E19 ohm cm. The radiation has reduced
the resistivity by nearly a factor of ½.

*LaRC-SI Resistivity, post 5-megarads*

In the LaRC-SI sample the voltage decay is too small for a graph like
Fig. 11 to be meaningful because with small voltage decay, voltage
differentials cannot be determined. Instead one simply evaluates the
decay for a fixed time, for both pre- and post-rad. Prior to
irradiation, the sample decayed by only one volt, from 402 to 401 volts,
from day-2 through day-7 after initial charging. After irradiation, the
LaRC-SI sample surface-voltage decayed by two volts, from 467 volts to
465 volts from day-2 through day-7 after initial charging. Perhaps the
radiation has increased the conductivity in this sample by a factor of
two as it did in the Kapton. The error in a measured voltage difference
between two of the readings is approximately one-volt. Because of this
error bar, the evidence for radiation-enhanced conductivity is weak in
the LaRC-SI.

The results for both samples are summarized in the following table. For
reference, the estimated value for resistivity is provided in
(brackets).

Table of Decay Rates and Resistivities

+----------------+--------------------------+--------------------------+
| SAMPLE         | Pre-Rad Decay            | Post-5MRad Decay         |
|                |                          |                          |
|                | Fraction/day             | Fraction/day             |
|                | (resistivity)            |                          |
+----------------+--------------------------+--------------------------+
| Kapton (#001)  | 0.005 (\>5E19 ohm-cm)    | 0.009 (\>3E19 ohm-cm)    |
+----------------+--------------------------+--------------------------+
| LaRC-SI (#001) | 0.0005 (\>5E20 ohm-cm)   | 0.0009 (\>3E20 ohm-cm)   |
+----------------+--------------------------+--------------------------+

The following table of surface voltage data was used to generate the
graph in Fig. 11. One can see that although the resolution of absolute
voltage is about one volt, one could obtain better resolution by fitting
a smooth function to the data.

Table of Surface Voltage Decay for Figure 11.

  -------- ---------------- ----------- -- --------- ---------------- -----------
  Kapton   pre-rad          pre-rad        Kapton    post-rad         post-rad

  DAYS     (1/V)\*(dv/dt)   Surf Volts     DAYS      (1/V)\*(dv/dt)   Surf Volts

  0                         478            0                          496

  0.021    1.7              408            0.0007    11.6             494

  0.14     0.12             382            0.0014    12.3             488

  0.71     0.016            377            0.0042    10.2             473

  0.95                      377            0.007     7                461

  1.18     0.013            375            0.0104    4.4              453

  1.58                      374            0.0146    4.7              446

  1.92     0.0093           372            0.0222    4.4              428

  2.62                      371            0.0257    1.96             425

  2.92     0.0063           370            0.0306    1.44             421

  5.6      0.0063           364            0.0521    1.15             409

  5.9                       363            0.0604    0.5              407

  6.54     0.0069           362            0.0868    0.23             402

  6.9      0.0056           360            0.159     0.094            398

  7.6                       359            0.22      0.044            397

  8.6                       358            0.254     0.044            396

  8.9      0.0037           357            0.96      0.044            384

  9.6      0.0045           356            2.06      0.017            377

  9.9      0.0049           356            2.88      0.02             371

  12.6     0.0043           351            2.95                       345

  12.9     0.004            351            3.125     0.015            345

  13.6     0.004            350            3.26      0.018            344

  14.6     0.0035           349            3.92      0.017            340

  14.9     0.0038           349            4.29      0.016            338

  15.6     0.0029           348            5         0.0104           334

  15.9     0.0029           347            5.3                        334

  16.7     0.0029           346            6         0.0075           332

  19.6     0.0024           344            6.5       0.0091           

  19.9                      343            7         0.0084           329

  20                        344            8.5       0.0082           

                                           10                         321
  -------- ---------------- ----------- -- --------- ---------------- -----------

V RESULTS OF CHARGE DECAY TESTS ON POLYIMIDE WITH ALUMINUM ELECTRODE

Samples with aluminum electrodes were studied because one of us (ARF)
had made a mental note at an IEEE Dielectric Conference (nearly ten
years previously) that some data in a poster paper for Kapton samples
with aluminum electrodes on both sides behaved strangely. The poster
paper appeared to misinterpret the data, but it could be reinterpreted
to mean that aluminum electrodes actually inject holes into Kapton, and
that the holes drift as much as 5-microns before being stably trapped.
Such an effect would have excellent consequences for the spacecraft
charging problem because holes injected into Kapton would reduce the
effect of electron charging.

LaRC-SI and Kapton samples were prepared at NASA Langley with Aluminum
electrodes in the same arrangement as described for copper electrodes in
Fig 6, above.

**V.A. LaRC-SI Sample with Aluminum Electrode**

On September 27 at 15:38 hours the sample was initially measured in
grounded configuration to have a pre-existing surface voltage of +13
volts. Note that this initial condition could be caused by any of a
number of events, but is not in disagreement with the thesis that
aluminum electrodes inject holes into polyimides.

From 16:00 to 16:07 hours on September 27 the sample electrode was then
biased to +500 volts and the electron-emitting filament turned on to
inject electrons onto the free polyimide surface. The first surface
voltage measurement (with grounded electrode) was quickly completed
within two minutes to indicate -501 volts (+/- 1 volt) had been achieved
on the surface.

The surface voltage then decayed to --499 volts at 17:55 hours,
approximately 108 minutes after charging the surface. By September 28 at
10:00 hours the surface voltage had decayed to --494 volts, a decay rate
of 1% in 16 hours.

Contrast this with the LaRC-SI001 sample with copper electrode where the
surface voltage dropped dramatically in the first half hour by nearly
8%. (The data is shown in a table earlier in this report.) But, in the
next 16 hours its surface voltage dropped by about 0.5% (from 404 to 402
volts). For the first hour the copper electrode appears to provide
dramatic voltage decay. For the succeeding 16 hours the aluminum
electrode provides the superior decay, by a factor of two.

Perhaps the aluminum injects holes, more than copper, over the long run.
But long run data is not available for this sample.

Later the same day, on September 28, the LaRC-SI02 sample with aluminum
electrode was exposed to light for a few minutes from the
electron-emitting filament (while the Kapton sample with aluminum
electrode was being charged with electrons) during which time its
surface voltage decayed to only --80 volts. Light from a yellow tungsten
filament causes rapid decay of surface voltage on our charged samples.
Therefore the experiment with this sample effectively ended at this
time.

Because of the very different results for the Kapton sample, below, it
must be noted that this LaRC-SI02 sample with aluminum electrode
produced stable readings with a clear interpretation, as did the two
samples, above, with copper electrodes.

**V.B. Kapton Sample with Aluminum Electrode**

This sample was difficult to characterize. There may have been problems
with the sample, or problems with our understanding of the measurements
in general.

[Initial Conditions]{.underline}

On September 27 at 13:55 hours, prior to any treatment, it appeared to
have approximately +23 volts surface charge. Again one might find this
result consistent with injection of holes by aluminum electrodes. But it
was difficult to calibrate the geometric factor for the field probe
because the surface voltage drifted rapidly when bias was applied to the
electrode.. At 14:00 the electrode was lifted from ground and biased to
+500 volts. The field probe initially read +195 volts and rapidly
drifted to 208 volts. This again is consistent with injection of holes
(and with other explanations). Assuming an approximate calibration
factor of 2, this is a drift of over one volt per minute. (On Oct. 1 at
9:00 the sample surface was stable and negatively charged to --228 volts
while the field probe read --128 volts, thus providing a geometry
calibration factor of 1.78, a reasonable number.)

Later, at 16:00 on 27 Sept., the electron filament was on while charging
the other sample. The electrode of this sample was grounded at this time
as it was general practice to not allow the potential of any metal to
float. Light from the filament, and perhaps electrons, would change the
charge distribution in the sample.

On September 28 at 10:03 the sample appeared to be biased at
approximately --21 volts (-12 volts on the Field Probe). Again the
geometry calibration factor was made difficult by drifting surface
voltage. Application of +500 volts to the electrode produced a rapid
shift in field probe reading from --12 volts (with electrode grounded)
to +195 volts indicating an approximate calibration factor of 500/207 =
2.42. Such a geometric factor is difficult to believe because it implies
that the field probe is much further from the sample than usually occurs
for the arrangement of sample mount and field probe transit stage.
Beginning at 10:04 with initial application of the constant +500 volt
bias, the field probe reading drifted as shown in Fig. 12.

The cause for the behavior in Fig. 12 has not been determined. Perhaps
the sample is loose in its mounting and drifting closer to the field
probe. Perhaps the electrode is not well-attached to the Kapton. Perhaps
there is something very special about the sample such as a very large
drifting internal or transverse dipole field.

![](media/image14.wmf)

**Figure 12. Drift of Kapton Surface Voltage Upon Application or +500
Volts to the Aluminum Electrode (Sept 28, 2001).** Theoretically, one
multiplies the field probe voltage by 1.78(?) to obtain actual surface
voltage, but for this sample it is uncertain.

[Charging the Sample]{.underline}

On Sept. 28, within minutes of obtaining the last data point (at 261
minutes) in Fig. 12, the electron-emitting filament was activated while
the +500 volt bias was maintained for the purpose of negatively charging
the Kapton surface. Note the picoammeter in Fig. 1 which can be viewed
while charging the sample. In a normal charging test, the picoammeter
indicates that the charging current flowing from the filament decays
smoothly to near-zero as the sample becomes charged. For this sample the
digital picoammeter bounced wildly as if the sample was occasionally
pulse-discharging. Normally it takes one or two minutes to charge the
sample, but in this case the picoammeter had not settled even after ten
minutes, so the filament was turned off at ten minutes to see what had
been achieved.

The field probe reading was monitored for three hours as it slowly
varied between -125 and -132 volts while the sample electrode remained
at ground. During the first 13 minutes it started at --125 V and drifted
monotonically to --132 volts, and afterward slowly drifted monotonically
to --128.5 V. Assuming a calibration factor of 1.78 this corresponds to
surface voltage range of -222 to --235 volts. The sample was then left
in the dark in vacuum with grounded electrode until Oct. 1.

October 1 at 9:00 hours the field probe indicated --128 volts, right
where it was last measured on 28 Sept. Perhaps no holes had been
injected into the Kapton from the aluminum during the nearly three days
of storage. At 9:02 hours +228 volts was applied to the electrode which
brought the field probe reading to 0 volts, and showed that the
geometric calibration factor is (hopefully) 228/128 = 1.78. The sample
was then grounded for nearly five hours until 14:00. At 14:01 it was
determined that +237 volts on the electrode produced zero volts on the
field probe indicating that the sample surface (or at least the part of
the surface to which the field probe was most sensitive) had drifted to
--237 volts.

Interpretation (or understanding) of this sample is problematic. Beyond
the problems relative to Fig. 12 there are the problems related to
charging and probable pulsed discharging. One can imagine strong
transverse electric fields and charge density gradients on this sample,
especially after the charging procedure. The field probe senses the
electric field over a limited region of the sample surface, and does not
average over the entire surface. It does not appear advisable to
continue this study until the existing data can be explained.

Future work should bring the apparatus into air and perform the
electrode bias study alone, without the electron-emitting filament. In
air one can observe the sample to see that it is mechanically stable.
With the exception of transverse field effects, the drifting problem in
Fig. 12 should not occur. When the causes for the drifting problem have
been removed, or understood, only then may in-chamber studies be
meaningful.

VI\. ELECTRON BEAM-INDUCED DISCHARGE PULSES AND CHARGE STORAGE

In anticipation of developing a "leaky sample" by either Cobalt
Irradiation, or hole injection at the aluminum electrode, some
preliminary discharge pulsing tests were performed on 25 May 2001 on the
samples with copper electrodes. A leaky sample would provide the
opportunity to demonstrate reduced pulsing by the leaky sample relative
to the standard sample. Because a leaky sample was not achieved, the
pulsing tests will not be presented in detail.

In anticipation that LaRC-SI would provide superior immunity from
radiation-induced arcing due to its superior electrical arc-tracking
performance, the statistics for pulse rate and pulse amplitude created
by the 20 keV electron irradiation are compared between LaRC-SI and
Kapton.

Samples KA-001 and SI-001 were irradiated with 20 keV electrons at 0.2
nA/cm^2^ for about 30 minutes and developed 44 pulses. Rough measurement
of surface voltage found SI-001 at roughly 6000 volts, and KA-001 at
roughly 4000 volts, on average during the test. These surface voltages,
as well as the internal electric fields, far exceed those to be expected
in space, or in normal applications by a factor of roughly 5 to 10.

At one hour after beam charging started, and with roughly 4-kV on its
surface, the voltage on sample KA-001 was decaying at roughly 50
V/minute, and on SI-001 the 6-kVsurface voltage was decaying less than
5V/min. If we had developed leaky samples, these numbers would be
important to study as a function of electric field and temperature in
the sample. Without examining it in detail, the Kapton surface voltage
was decaying 50/4000 = 1.25% per minute. With only 400 volts on its
surface and roughly one hour after bias application, this same sample
decayed by 26 volts during a 3-hour time span, or at 0.036% per minute
(see Fig. 9 and its associated table of data). As usual for any
material, the resistivity of the Kapton insulator is dramatically
reduced at high electric field. The electric field strength with 4-kV
and 6-kV on the 50-micron thick samples (roughly 100 volts/micron) was
sufficient to produce frequent breakdown pulses, more than one per
minute.

Mono-energetic electron irradiations produce severe over-voltage
conditions unless they actually penetrate the sample or unless the
sample is exceedingly thick. Our test definitely produced over-voltage
conditions that would not occur in actual space radiations. A caution is
in order here before one places confidence in the findings. The electric
field inside insulators actually developed in space conditions depends
upon a parameter that we cannot determine from these tests; a parameter
called the Coefficient of Radiation-induced Conductivity. Unless LaRC-SI
is unusual in this regard, it is probable that radiation-induced
conductivity effects would cause the LaRC-SI material to be even better
than our limited tests found. Further testing would be required to prove
this.

Although one might assume that the extra leakiness of Kapton would help
to prevent radiation charging in Kapton compared to that in LaRC-SI,
this is not true in general. It would be true only for a narrow range of
radiation intensities where the radiation-induced conductivity is
comparable to the material dark conductivity. The extra leakiness in
Kapton did provide our Kapton sample with a smaller voltage stress
during the electron beam tests, yet the LaRC-SI performed better in
spite of this. The leakiness advantage of Kapton is expected to be less
of an advantage in real space radiations.

The following table clearly shows the superiority of LaRC-SI. It pulses
less frequently, its pulses are not larger, and it withstands larger
voltage stress. In real space radiations the superiority of LaRC-SI is
expected to improve further.

Table of Pulsing Performance During 20 keV Electron Irradiation

  ---------------------------------- -- ----------------------------------
  Sample: LaRC-SI-001 (copper)          Sample: Kapton-001 (copper)

  Electric field stress, 120            Electric field stress, 80 V/micron
  V/micron                              

  \# of pulses during test = 14         \# of pulses during test = 29

  Average peak pulse amplitude, 1.7V    Average peak pulse amplitude, 2.9V
  ---------------------------------- -- ----------------------------------

VII\. CONCLUSIONS

When a material is used in space applications the material properties
appropriate for that environment must be taken into account. Contrary to
this advice, resistivity measurements for insulators are usually
performed in the earth atmosphere, 1 atm, 20C, with some humidity
present and with electrodes attached to the sample. We have repeated
such atmospheric tests over a year ago to find resistivity in polyimide
of order 1.4E16 ohm-cm. Such a resistivity predicts a charge decay time
constant of ρε = (1.4E16 ohm-cm)(8.85E-14F/cm)(3.12) = 64 minutes. Thus,
charging by space radiation may be incorrectly assumed by an analyst or
spacecraft designer to quickly (1-hour) dissipate.

For the space environment we have measured the resistivity of Kapton
polyimide to be \>5x10^19^ ohm cm, and of LaRC-SI polyimide to be
\>5x10^20^ ohm cm. Inappropriate resistivity values of various
magnitudes are commonly assumed for space applications. The Guide to
Plastics by the Editors of Modern Plastics Encyclopedia tabulate the
resistivity of polyimides as \" \>1E16.\" W. Tillar Shugg in [Handbook
of Electrical and Electronic Insulating Materials, 2nd Ed.]{.underline}
tabulates polyimide resistivity as "E18". The units are ohm-cm. The
charging test methods introduced here are the proper ways to test any
space material.

We have performed the measurement of resistivity in a simulated space
environment, 1E-6 torr, 25C in our experiment lasting many days. There
was only small measured charge decay even with heating the samples to 50
degrees C. At the minimum, charge will be stored for several months so
that Earth and Jovian radiations may strongly charge up polyimide
insulators. This result clearly demonstrates the need to take into
account the proper sample conditions in a space environment.

While air molecules, humidity, and other effects help dissipate charge
buildup in earth applications, in a space environment the
radiation-induced charge will continue to build until harmful discharges
occur. If these characteristics are overlooked and precautions not
taken, using materials such as LaRC-SI and Kapton for flex circuit
material or packaging in space could result in serious ESD damage to
electronic circuitry. The material that is least likely to pulse,
probably LaRC-SI, would be preferred.

However, it is believed that there are ways to lower the resistivity of
Kapton or LaRC-SI material to dissipate charge, and yet not cause
circuit cross talk. Irradiation with Cobalt-60 to 5 megarads has been
shown in this work to lower the effective resistivity by a factor ½.
However, our limited test did not induce enough conduction to provide a
complete solution to radiation-induced charging in space applications.
Irradiation to perhaps 100 megarads or more might provide relief.

The generation of conductivity by hole-injection at aluminum electrodes
on polyimides was investigated. The Kapton-aluminum sample produced
confusing results and acted anomalously. More study would be necessary
to learn how this material combination behaves. Aluminum on LaRC-SI
material showed some promise at inducing charge injection with resulting
conduction in the polyimide, but a significant (useful) effect did not
occur. Because some positive effect was seen on these samples, where
care was not taken to deposit the aluminum with complete adhesion, study
of improved samples with tight-bonded aluminum might show improved
properties. But a hoped-for dramatic effect was not observed.

The LaRC-SI provides superior performance in a radiation-induced
charging environment. It generates fewer arcs and sparks than does
Kapton even when stressed to higher electric field than Kapton.

As a final note it might help the reader to realize that there is
on-going discussion in the insulator community to explain the results of
applied voltage in the classical manner of a parallel plate capacitor
with metallic electrodes. Evidence is mounting that conductive
electrodes introduce charge into the insulator sample, usually as mobile
ions, which continue to move for many days and cause the applied voltage
to generate a current flow due to the mobile ions (often in IEEE Trans.
Dielectrics and Elec. Insulation). That this effect is suppressed in the
evacuated electron beam case where only one electrode (or perhaps none)
is on the sample may provide a partial explanation for the strong
difference between our measurements and the classical ASTM methods. This
difference must no longer be ignored by the spacecraft analyst.

VIII\. REFERENCES

1.) ASTM D 257-99, [Standard Test Methods for DC Resistance or
Conductance of Insulating Materials]{.underline}, (1999)

2\) IEC 93, International Electrotechnical Commission Publication 93,
[Methods of Test for Volume Resistivity and Surface Resistivity of Solid
Electrical Insulating Materials]{.underline}, Second Edition, 1980.

3\) W. Tillar Shugg, [Handbook of Electrical and Electronic Insulating
Materials, 2nd Ed.]{.underline}

4\) . The Guide to Plastics by the Editors of Modern Plastics
Encyclopedia, McGraw Hill, Inc., N.Y., 1970.

IX\. APPENDIX

**IX.A. Comments On Resistivity And Permittivity**

It is extremely important to comment on polarization current versus real
current, and on dielectric constant versus resistivity. The concept of
resistivity is generated by the human condition of desiring to apply
Ohm's Law or comparable ideas to the analysis of such a system. In
reality, Ohm's law is not physical, it is imagined, and our data proves
that it does not apply to our case. Consider that if Ohm's law applied,
and the sample had constant resistance, then the fractional voltage
decay per unit time would be a constant. In such a case the data in Fig.
12 would constitute a horizontal line. Instead, throughout the data, out
to 20 days, the fractional decay is decreasing. It is very likely that
this process is due to polarization currents throughout the dielectric,
possibly assisted by injection and subsequent trapping of real charge
near the metal-dielectric and vacuum-dielectric interfaces. These
processes seem to dominate the total current out to 20 days. The
fractional voltage decay out to 20 days appears to be due mostly to a
change of dielectric constant (polarization currents) rather than to
resistivity (free carriers effectively transiting the entire sample with
a constant current density).

A full mathematical analysis of this quasi-static problem could be
developed, but would consist of a number of terms that would require
further experimental investigation. One would not obtain closure for the
problem at hand. So, we continued with the contrivance whereby the data
is interpreted as a change in resistivity. For real problems, one must
remember to relate the experimental voltage decay and its time scale to
the situation on spacecraft where the insulator must withstand the
charging processes for the lifetime of the spacecraft, typically years.
The resistivity values mentioned in this report cannot be used with
Ohm's Law alone to estimate the charging processes on spacecraft.
Instead, one must consider that the voltage decay is due to changing
polarization currents, and the actual resistivity values after long time
in space are an unknown value. In effect, we have determined a minimum
value for the resistivity (see Table of Decay Rates and Resistivities)
where the real value is larger by an unknown amount.

**IX.B. Comment on Data Presentation**

Simple numeric data is used because it is closest to the actual
experiment. It would have been interesting to fit the data to a
theoretical function, or to a spline fit, in order to provide smoother
derivatives, dV/dt for Fig. 11. But there is little to be gained by
doing so in this brief study.

The electrostatic field probe usually, but not always, has a resolution
of about 1 volt. Thus, in order to measure the decay, one should let the
decay proceed for 3 volts or more. Sometimes the act of charging the
samples can also charge the insides of the field probe causing it to be
a little unstable and reducing its resolution to 2 volts. Because we
plot a differential of voltage, these effects are the cause for the wide
scatter in the data points in Fig. 11. In practice, the data
presentation, and its accuracy, is improved by placing a smoothed line
fit to the data. But it serves no purpose to do so here. The reader
should be aware that the scatter should not be interpreted to mean that
the post-rad and pre-rad data are almost equal. They really are
different by a factor of almost two.

**IX.C. Comment on Application of the New Information to Spacecraft**

Spacecraft experience variable environments. In typical Earth orbits,
the spacecraft environment varies both because the Solar wind modulates
the magnetosphere on time scales from minutes to weeks, and because the
spacecraft moves into and out of radiation zones on timescales of
minutes to days. Consider a case where the spacecraft experiences
elevated radiation for a few days, followed by many days of benign
environments. If the insulators have charge decay time constants of a
few days, then the charging threat consists only of the amount of charge
delivered to the spacecraft in a few days. But if the insulators have
time constants of a year, then all the charge accumulated in a year
becomes the threat.

Figure 12 plots the voltage decay time constant as a function of
insulator resistivity for dielectrics with typical permittivity. The
period for one orbit in Low Earth Orbit (LEO) is also noted. It is clear
that our new values for the resistivities of polyimides imply that one
must integrate the spacecraft charging threat for at least one year, not
1 to 10 hours as has often been done for spacecraft.

![](media/image15.png){width="3.4972222222222222in"
height="3.1166666666666667in"}

**Figure 12. Theoretical Voltage Decay Time Constant as a Function of
Dielectric Resistivity**.

**IX.D. Comment on Copper Electrode Preparation**

Injection of mobile carriers at the electrode is always a possibility
until proven otherwise. Our attempt to compare aluminum electrodes to
the copper electrodes was motivated by this consideration. The
measurement of conductivity or resistivity would be changed in the event
of injection by the electrode interface. Therefore it may be important
to record the manner in which the electrodes were applied to the
insulating material in order to compare results from these tests to
other tests that may be performed in the future. The copper electrodes
were prepared as follows:

**COPPER LAMINATED LARC-SI FILM SAMPLES**

by Gregory K. Draughon, NASA/Langley, EATB/MTSS, September 6, 2000

The basic procedures for producing the JPL test samples are casting a
fully cured LaRC-SI polyimide film, copper laminating, photolithography
and etching. Test samples are composed of LaRC-SI films made of 30%
solids, 3% offset poly (amic) acid gel material. All films are cast
using a doctor blade at a 15-mil blade height in a glove box. After
casting, the films are left in the box approximately 16 to 20 hours to
become stable and tack free before being moved. The temperature of the
glove box is kept around 77 degrees Celsius, at approximately 14%
humidity. The films are then placed in an oven, under an aluminum cover
at 65 degrees Celsius for approximately 5 hours to further dry tack free
before final curing. Once curing is to begin, the full cover is replaced
with a three-sided cover to allow solvents to boil off and vent during
high temperature processing. It also helps protect the film from air
current disturbances and any loose particulates that may be blown around
in the oven. The film is then fully cured using a slow, ramping and
soaking cycle up to 325 degrees Celsius under air. Once the chamber is
cooled, the glass substrate that the film was cast on is then soaked in
boiling water until the film separates from the glass. The final film
size is usually 12\" x 18\" in size and range from 1 to 3 mils
thickness.

**LAMINATING PROCEDURES**

In laminating the copper to the LaRC-SI, the copper foil that is
provided with an adhesion promoter on the back provides the best
results. The copper is first dipped in a 2% sulfuric acid solution to
remove any oils or contaminants. After drying, one side of the copper is
sprayed with a minimum of three coats of a LaRC-SI solution as an
adhesion promoter and dried at 65 degrees Celsius for 20 minutes between
each coat. The LaRC-SI film and copper are then layered and vacuum
bagged for autoclaving. In the lay up, upilex is used as a release film,
and alzac, a finely polished aluminum is used directly outside of the
release film to prevent any scratches, which are common in stock
aluminum, from being transferred to the surface of the sample. Because
of its softness the alzac must have a second layer of regular stock
aluminum on its outer sides to prevent the pattern of the fiberglass
used for bagging from being transferred to the work piece. The lay up is
vacuum bagged and processed at 300 degree Celsius, 100 psi and held for
1 hour. The result is a strong bond of LaRC-SI film to copper, which can
now be patterned and etched.

# PHOTOLITHOGRAPHY AND ETCHING

The pattern transfer is done using normal photolithography procedures.
The copper film samples are coated using Shipley 1805 photo-resist, and
pre baked at 90 degree Celsius for 30 minutes. Samples are exposed using
the Nu-Arc plate maker exposure unit at 30 to 40 light units. Samples
are then developed using a 50/50 mixture of Shipley photo resist
developer and deionized water. Samples are then post baked at 140
degrees Celsius for 30 minutes. After post baking, the pattern is then
ready to be etched.

Etching of the copper is done using a mixture of 2/3 water and 1/3
sulfuric acid heated to 190 degrees Fahrenheit. The sample is submerged
into the mixture until the copper pattern is cleanly etched. The sample
is then thoroughly rinsed with deionized water and dried. The last step
is to remove the remaining photo-resist using preheated photo-resist
remover, and thoroughly rinse the sample again with deionized water.