\

! NBSIR 77-1235

FILE CO P

DO NOT REMCV

Some Aspects of Using

A Scanning Electron Microsope

for Total Dose Testing

K. F. Galloway and P. Roitman

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

September 1977

Final

Prepared for Defense Nuclear Agency

Washington, D.C. 20305

NBSIR 77-1235

SOME ASPECTS OF USING

A SCANNING ELECTRON MICROSCOPE

FOR TOTAL DOSE TESTING

K. F. Galloway and P. Roitman

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

September 1977

Final

Prepared for Defense Nuclear Agency

Washington, D.C. 20305

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
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TABLE OF CONTENTS

Page

Abstract i

1. Introduction 1

2. Calculation of Total Absorbed Dose 2

3. Example Calculation 8

4. Consideration of SEM Parameters 10

5. SEM Radiation Testing 12

Acknowledgment

References

Appendix A 21

Appendix B 25

Distribution 29

LIST OF FIGURES

Page

1. Projected electron range versus electron beam energy from

the expression of Everhart and Hoff 4

2. Energy deposition versus penetration depth for electron beams

for four different energies based on the work of Everhart and

Hoff 5

3. The function Y(y) = 0.6 y + 3.105 y^ - 4.133 y^ + 1.425 y^

plotted as a function of y 7

4. Nomograph for converting aluminum, silicon dioxide, or alumi- num plus silicon dioxide thicknesses in micrometers to mass thickness in yg/cm^ 9

5. Relative radial dose distribution in the oxide layer for a point beam of 20-keV electrons incident on a 150-nm oxide

layer beneath a 500-nm aluminum layer ]

1

6. Beam "profiles" obtained by defocuslng measured with a 5-ym

aluminum stripe MOS capacitor 13

7. Schematic illustration of measurement arrangement for obtaining defocused "profiles" shown in figure 6 14

lii
List of Figures (continued)

8. Relative electron fluence across the rastered area for

three different beams with assumed Gaussian distributions

9. Schematic cross section through an SEM specimen chamber

illustrating probe card arrangement for applying bias to an individual chip on a wafer

Page

. . 15

. . 17

iv
Some Aspects of Using a Scanning Electron Microscope

for Total Dose Testing

K. F. Galloway and P. Roitman*

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Abstract

This report addresses a number of aspects involved in

using a Scanning Electron Microscope (SEM) for radiation testing of semiconductor devices. Problems associated with using

the low energy electron beam to simulate ^*^Co exposure and a method for estimating the total absorbed dose in critical device oxides are discussed. The method is based on the experimentally determined expression for electron energy dissipation

versus penetration depth in solid materials of Everhart and

Hoff. An appendix giving the method of estimating the total

absorbed dose in a form suitable for ASTM deliberations is

included.

1. Introduction

Low energy electron beams such as those used in a scanning electron

microscope (SEM) have been used in a number of experiments to explore

the effects of ionizing radiation on semiconductor devices. The SEM

has been suggested as an instrument which can be used to selectively irradiate devices directly at the wafer level and which can simulate the

effects of ^®Co gamma Irradiation.

This report addresses a number

of aspects involved in using an SEM for radiation testing of semiconductor devices. In particular, problems associated with using the low en- ergy electron beam to simulate ^®Co exposure and a method for estimating

the total absorbed dose^ in critical device oxides are discussed.

If the SEM irradiation is intended to simulate a ^°Co radiation ex- posure, at least three factors must be considered. 1) For a low energy

electron beam, the depth-dose distribution through the oxide may be

quite different from the assumed constant depth-dose distribution for

^°Co exposure. 2) An SEM properly adjusted for Imaging using secondary

electrons will not deliver a uniform electron flux to the specimen.

NBS-NRC Postdoctoral Research Associate.

^In this report, the terms total dose and total absorbed dose are used

to indicate the total energy divided by total mass. This is to be distinguished from the term absorbed dose which is generally defined as a point quantity.

1
3) The dose rate during a typical SEM exposure is considerably higher

than typical ^^Co dose rates.

Due to the variation in depth-dose profiles of low energy electrons

in device structures, careful attention must be given to the method used

for determining the total absorbed dose. The energy deposited by the

electron beam can be considered primarily as a mechanism for electronhole pair production in the device materials. Since an electron of approximately 170 keV or greater is necessary for displacement damage in

silicon, permanent bulk damage can be neglected for SEM electron irradiation. In metals and semiconductor materials, the pair formation will

only result in a transient effect. However, the trapping of holes in

the silicon dioxide and interface state build-up at the silicon-silicon

dioxide interface can result from low energy electron exposure. These

are also the effects usually associated with ^*^Co exposure^^ where the

total absorbed dose in the oxide is the radiation parameter which correlates with changes in device electrical parameters. For this reason,

the method given in this report will be for estimating the total absorbed dose in the oxide. The method is based on the experimentally determined expression for electron energy dissipation versus penetration

depth in materials with atomic numbers between 10 and 15 given by Everhart and Hoff.^^

In the following sections, the calculational method for estimating

the total absorbed dose and various graphs to facilitate the calculation

are given, an example calculation is presented, and techniques and problems relevant to using an SEM for radiation testing are discussed. An

appendix giving the method of estimating the total absorbed dose in semiconductor devices due to SEM electron radiation in a form suitable for

ASTM deliberations is Included.

2. Calculation of Total Absorbed Dose

Early work on the distribution of energy loss versus penetration

depth for kilovolt electrons was done by Griin.^® Griin experimentally

determined the electron energy absorption as a function of penetration

depth in air and demonstrated two Important points. First, he obtained

a relationship between the projected range of electrons, Rq, and the

electron beam energy »

Rg = 4.57 Egl-75 ^ (1)

where Rg is expressed in micrograms per square centimeter^ and Eg is ex- pressed in kilo-electron volts. This expression is valid for 5 keV <

Efi ^ 25 keV. Second, he showed that the shape of the depth-dose relation

The unit of length used here is mass thickness - the product of material density and thickness. For example, a layered structure of 800 nm

of aluminum and 200 nm of silicon dioxide would have a thickness of 260

yg/cm^.

2
is practically invariant if the penetration distance is expressed as a function of Rq and the energy is expressed as a fraction of Eg.

Everhart and Hoff^^ extended these general conclusions to solids

and obtained a generalized depth-dose curve for solid materials. They

determined experimentally a depth-dose function by taking the steadystate electron-beam-induced current through the insulating layer of a metal-oxide-semlconductor structure as a measure of the energy dissipation in that layer. For structures of aluminum, silicon dioxide, and

silicon, Everhart and Hoff found the projected range expression,

Rg = 3.98 , (2)

to be accurate for 5 keV Eg ^ 25 keV. Figure 1 is a plot of projected

range versus electron beam energy. They also found that for elements

with an atomic number in the range 10 to 15 the energy dissipation per

unit mass thickness is given by

dE

dx

(l-fg)EgX(y) — (3)

where fg is the fraction of incident energy backscattered, typically

taken as 0.1 (see Appendix A) , y

= x/Rq where x is the penetration depth

in micrograms per square centimeter, and

A(y) = 0.60 + 6.21y - 12.40y2 + 5.69y^ . (4)

Equation 3 is plotted in figure 2 for several beam energies.

The work of Everhart and Hoff provides the basis for calculating

the total absorbed dose in the oxide layers of semiconductor devices ex- posed in a scanning electron microscope.

If uniform electron flux over the rastered area (Ag in square cen- timeters) is assumed, the number of incident electrons per unit area

(electrons per square centimeter) is

IB 9^, (5)

where Ig is the electron beam current in amperes, t is^the exposure time

in seconds, and q is the charge per electron (1.6 x 10 coulombs per

electron) . Multiplying N by the area of the oxide layer of interest (Aq

in square centimeters) gives the number of electrons incident on the oxide.

The energy deposited in the oxide per electron can be calculated

from eq (3) by integrating from xi, the distance from the device surface

3
ELECTRON BEAM ENERGY |keV|

Figure 1. Projected electron range versus electron beam energy from the

expression of Everhart and Hoff.

4
(keV-cmV/xg)

O' ^ ^ ' ' ' 0 100 200 300 400 500 600

PENETRATION DEPTH (/^g/cm^)

Figure 2. Energy deposition versus penetration depth for electron beams

for four different energies based on the work of Everhart and

Hoff. Ten percent of the beam energy is assumed to be backscattered .

5
to the top of the oxide, to X2 , the distance to the bottom of the oxide.

Normal incidence for the electron beam is assumed.

/

X2

dx

dx

/

Yl

Y2

A(y)dy

= (l-fg)Eg [Y(y2) - YCy^)]

( 6 )

where f^ is the fraction of incident electron energy deposited between

yj and y 2 and

Y(y) = 0.6y + 3.105y2 - A.133y3 + 1.425y^ . (7)

Figure 3 is a plot of the function Y.

The total energy deposited in the oxide in kilo-electron volts is

then

Et = . ( 8 )

The radiation dose in the oxide can be calculated by dividing by the mass of the oxide layer in grams

M = (x2 - xi) . (9)

The result, in kilo-electron volts per microgram, is

Dose = N*Ej^*(x2 - . ( 10 )

The commonly used unit of radiation dose, the rad, is defined as the

amount of radiation which deposits 100 ergs of energy per gram of irradiated material; the total absorbed dose in the oxide layer in rad(Si02 ) is

Dose [rad(Si02)] = 1.602 x 10“^ N*Ej^*(x2 - xj)"^ . (11)

The parameters used in determining N and Ej) can be substituted explicitly in eq (11) and the total absorbed dose in the oxide layer can be ex- pressed as

10^*+ I^E^t(l-f„)f

B B B' D

Ag (X2 - xi)

Dose [rad(Si02)] =

( 12 )
1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

y

Figure 3. The function Y(y) = 0.6 y + 3.105 y^ - 4.133 y^ + 1.425 y^^

plotted as a function of y.

7
The quantities appearing in eq (12) and their units are given in table I.

Table I. SYMBOLS AND UNITS

Symbol Parameter Units

beam current A

energy of beam electrons keV

t scan time s A

s area of scan 2 cm*^ X2-X1 oxide thickness yg/cm^

^B fraction of incident energy

backscattered from device

unitless

fraction of Incident energy

deposited in oxide

unitless

3. Example Calculation

Consider a critical oxide layer of 100 nm, for example the gate oxide of an MOS device, beneath 1 ym of aluminum which is in turn beneath

a silicon oxide overcoat 1 ym thick. Figure 4 is a nomograph which can

be used to convert aluminum, silicon dioxide, or aluminum plus silicon

dioxide thickness in micrometers to mass thickness in micrograms per

square centimeter. On a depth scale measured from the top of the overcoat, the critical oxide extends from 500 yg/cm^ to 523 yg/cm^ (xi and

X2 , respectively). For a 20-keV electron beam, R(; is 752.8 yg/cm2 (see

fig. 1). Thus

0.664

(13)

and from eq (7)

0.695

Y(yi) = 0.834

Y(y2) = 0.861 .

(14)

Thus, the energy deposited in the oxide expressed in kilo-electron volts

per electron is

= (1. 0-0.1) 20 [0.861-0.834]

= 0.486 . 8

( 15 )
THICKNESS

OF

SILICON

DIOXIDE

|/tm|

Figure 4

1.0

0.5

- 800

- 700

C>4E

o

bi)

CO

CO

LiJ

- 600

- 500

400

oc

LU

CO

oco

CO

00

300

200

L 0

- 1.5

E

1.0

05

0.0

Nomograph for converting aluminum, silicon dioxide, or aluminum plus silicon dioxide thicknesses in micrometers to mass thickness in pg/cm^. To use, draw a line from the silicon dioxide thickness in micrometers to the aluminum thickness in

micrometers and read the absorber thickness in yg/cm^.

9
For an electron beam of 100 pA scanning an area of 0.02 cm^ for 100 s, the number of incident electrons per square centimeter is

= (100 10~^^) ( 100 ) (1.6 X 10-19) (.02)

(16)

= 3.125 X 10l2 .

The total absorbed dose in the oxide for this case is then

Dose [rad(Si02)] (1.602 X iq-5) (3.125 X ( 0 . 436 ) 23

= 1.06 X 10^ . (17)

4. Consideration of SEM Parameters

If the procedure for estimating the total absorbed dose outlined in

the preceding sections is to yield reasonable results, the SEM should be

adjusted so that the assumptions made in the calculation are met and the

SEM parameters used in the calculation should be accurately determined.

The requirement of a uniform electron flux incident on the specimen

needs special attention.

The area of the specimen exposed to the electron beam or the area

scanned, Ag, is usually related to the area of the recording CRT, Acj^-p,

and the SEM magnification by

A =

s

A CRT

Mag (18)

For this reason, the magnification needs to be accurately determined.

The magnification is a function of many different variables and is usually determined using a calibration artifact. The electron beam current,

Ig, is usually measured using a Faraday cup. The beam energy. Eg, is

probably best determined from the x-rays emitted from a known target.

Techniques for determining these and other critical parameters are discussed in a paper by Joy.^^

In order that the assumption of uniform electron exposure be met,

a number of factors must be carefully considered. The goal, of course,

is a uniform dose deposited in the oxide layer. An SEM electron beam

properly adjusted for secondary imaging is approximately circular in

projection on the specimen with about 80 percent of the electrons in a circle 10 to 25 nm in diameter. As these electrons penetrate to the oxide layer of interest a radially varying dose distribution in the oxide

results, primarily from multiple scattering of the electrons. Figure 5,

taken from the work of Chadsey,^° illustrates the radial dose distribution in the oxide for a point beam of 20-keV electrons incident on a 150—nm silicon dioxide layer on silicon beneath a 500-nm aluminum layer.

Extrapolating from the data in this figure, it is obvious that when us10
RELATIVE

DOSE

DISTANCE FROM BEAM CENTER ||jm)

Figure 5. Relative radial dose distribution In the oxide layer for a point beam of 20-keV electrons incident on a 150-nra oxide

layer beneath a 500-nm aluminum layer.

11
ing a well focused beam the scan lines must be on the order of 0.5 vim or less apart to achieve a uniform dose when irradiating typical chips.

This is impractical since a typical chip to be exposed is on the order

of 2500 ym on a side and the number of scan lines per frame is usually

between 500 and 2000. Therefore, an SEM operated in its normal imaging

mode will not deliver a uniform dose to typical device oxides.

This problem can be solved by defocusing the electron beam in order

to obtain a uniform electron exposure. This is accomplished by decreasing the objective lens current. Beam diameters as large as 50 to 100 ym are easily attainable. Figure 6 illustrates beam "profiles” obtained by

defocusing. The beam "profiles" shown in figure 6 were measured using

an MOS induced current technique schematically shown in figure 7. An

MOS capacitor with a gate 5 ym wide and several hundred micrometers long was oriented perpendicular to the scan direction and biased to accumulation. The current induced by the beam in the oxide was amplified and re- corded on an x-y plotter. Figure 6a shows the profile of the gate at

focus (beam diameter much less than gate width) and can be used to estimate the beam widths of the other traces. Figures 6b and 6c show the

profiles obtained as the beam is progressively defocused. The amplitude

is arbitrary as the beam current changes with objective lens setting.

The beam current used to calculate the dose must be measured with the

beam defocused. The profiles obtained in this way are not true beam intensity profiles as the gate integrates the electron distribution in one dimension. However, the full width of the measured profile, from where

the current rises from zero to where it returns to zero, is exactly the

full width of the beam plus the width of the gate stripe. Figure 8 represents the uniformity of exposure across a chip for electron beams with

assumed Gaussian distributions of 0.025, 5.0, and ^10.0 ym FWHM. If,

for example, a 50-ym diameter beam is scanned across a chip on lines 5 ym apart, the resulting dose will be uniform.

Another factor to be considered is the time of exposure. If the

time per frame is tp and the time of exposure is t, the assumption of

uniform exposure of the specimen is most nearly met if t is a rational

multiple of t^ or if t is very much greater than t^.

5. SEM Radiation Testing

This final section is devoted to a discussion of a number of other

important details which must be considered when using an SEM for the

radiation testing of semiconductor devices. Practical problems associated with device positioning, device biasing, and possible damage to adjacent devices are briefly addressed. Also, the effects of differences

in depth-dose distribution and in dose rate between the low energy electrons from SEM exposure and the gamma-rays from ^°Co radiation testing

are pointed out.

Positioning the device to be exposed in the SEM chamber may present a problem. This is particularly true if it is desired to expose

only one or a few devices on a wafer. Some systems have optical viewing systems which are useful in positioning. It is also possible to

12
RELATIVE

CURRENT

Figure 6. Beam "profiles" obtained by defocusing measured with a 5-um

aluminum stripe MOS capacitor. A. Focused beam; the width

of the peak is approximately equal to the width of the 5-ym

stripe. B. Beam width ym. C. Beam width '^>18 ym.

13
ELECTRON BEAM

Figure 7. Schematic illustration of measurement arrangement for obtaining defocused "profiles" shown in figure 6.

14

H'lHl'
RELATIVE

ELECTRON

FLUENCE

1.0

0.9

FWHM-.02^m

|A|

1

I

SCAN LINE SPACING

Figure 8. Relative electron fluence across the rastered area for three

different beams with assumed Gaussian distributions. A. FWHM

0.2 ym. B. FWHM = 5 ym. C. FWHM ^ 10 ym.

15
design and construct a fixture which will hold a wafer and provide

shielding for those devices which are not to be exposed to the beam.

In general, a very low energy electron beam (Eg 1 keV) can be used to

locate and align the device to be exposed. Electrons of this energy

usually do not penetrate to critical oxide layers. However, a small

but potentially significant number of continuum x-rays, generated by

the electrons in the material covering the critical oxide layers, may

penetrate to the oxide. If this technique is to be used, the exposure

during set-up should be as short as possible. For a particular SEM system, it may be necessary to explore a number of techniques to discover

the best method.

It is generally accepted that ionizing radiation effects are accentuated by applying bias to the device during the radiation exposure.

Provisions for applying biases during SEM exposure to a single device

mounted on a header are available in most instruments. However, SEM

systems equipped with multiple probes for IC probing are not currently

commercially available. A group interested in doing on-wafer failure

analysis has designed a fixture which was mounted in an SEM chamber so that individual devices on a wafer could be biased during SEM irradlation.^^j^^ The fixture, containing a probe card with the required number of probes, was rigidly mounted in the SEM chamber and aligned so that the region to be probed was centered on the electron optic axis.

Figure 9 is a schematic illustration of this arrangement. The wafer is

fixed in a specimen holder on the moveable stage of the SEM, and in

operation, the chip to be investigated is adjusted relative to the

probes and the wafer raised in the Z-direction until the probes mate

with the pads. A system such as this would permit pre- and postradiation electrical characterization and irradiation under bias of se- lected chips at the wafer level.

Another concern during wafer level irradiations is the possible

damage to devices adjacent to the target device due to scattering of the

electron beam in the target device or due to stray radiation in the SEM

chamber. Using Monte Carlo techniques to examine the problem of scattering in the target device, Chadsey has shown that this effect is negligible in neighboring devices.^® The magnitude of stray radiation in the

SEI4 chamber is more difficult to predict. This background is due to

electrons backscattered from the sample rescattering from the pole-piece

and walls of the sample chamber. Measurements by Lipman et indicated no effect on the gain of neighboring devices when the target device received a dose of approximately 1 Mrad(Si02). However, Ma et

in experiments on MOS capacitors observed an effect where the dose

due to stray radiation can be estimated to be 10 ^ to 10 ^ times the

dose in the target device.

In order to most closely simulate a ^°Co exposure with an SEM electron beam, the electron beam energy should be selected such that the

energy dissipated per unit mass thickness (dE/dx) across the critical oxide is nearly constant. Exposure to ^®Co gamma-rays results in almost

uniform energy deposition throughout a typical device. This is not the case for a low energy electron beam. Consider, for example, a critical

16
Figure 9. Schematic cross section through an SEM specimen chamber illustrating probe card arrangement for applying bias to an individual chip on a wafer.

17
oxide located between 200 and 250 pg/cm^ in figure 2. A 5-keV beam will

deposit no energy in this oxide layer. A 10-keV beam or a 20-keV beam

could deposit the same amount of energy in this oxide layer if the individual times of exposure and beam currents were appropriately adjusted.

However, the 20-keV beam deposits its energy more uniformly throughout

the oxide. For this reason, a beam energy of 20 keV would be the better

choice for simulating a ^*^Co exposure for this particular device configuration .

Substantial differences in dose rate can exist between an SEM ex- posure and a ^^Co exposure delivering the same total dose to a device.

Dose rate can be calculated from eq (12) using the raster scan time and

the raster area or, equivalently, using the area of the beam spot and

the time the beam spends on each spot if the electron exposure is uniform. A typical MOS gate oxide might be 100 nm thick under 1 pm of aluminum covered by 1 pm of glass. For a beam energy of 30 keV, a beam current of 100 pA, a raster area of 0.1 cm^ (a chip of approximately 125

mils by 125 mils), and raster scan time of 1 s, the dose rate is 2.7 x 10^ rad(Si02)/s. The beam current in the SEM may be varied conveniently

from 1 pA to 10 nA, thereby varying the dose rate in a range of approximately 10 to 10^ rad(Si02)/s. The lower limit is set by the reliability

of the current-measuring electronics, assuming an image is not required

during irradiation. The upper limit is set by the apertures of the SEM

optics; beam currents of 10 pA or greater are obtainable if these apertures are removed (resolution will be lost). For comparison, typical

dose rates for ^®Co exposures are 20 to 200 rad(Si02 )/s.

Some dose rate effects have been reported for very high dose

rates. At the lower limits of SEM beam current the dose rate is com- parable with ^*^Co sources so those effects are clearly not a problem.

In general, a consideration of the physics of device response would indicate that rate effects should not be significant at 10^ to 10^ rad/s.

Above this rate, space charge effects may be important. Thus, radiation

testing in the SEM offers the potential advantage of depositing significant doses in only a few minutes.

SEM radiation testing has been shown to yield results similar to

^®Co exposure for both blpolar^^ and MOS devices. This technique has

a unique feature in that the radiation sensitivity of different regions

of an integrated circuit can be separately investigated.^^ When planning

a program which is to include SEM radiation testing, reasonable simulation of ^*^Co total dose exposure can be obtained if the various facets

of SEM low energy electron irradiation are accounted for.

Acknowledgment

W. J. Keery, R. L. Pease, E. A. Wolicki, S. Othmer, J. R. Srour,

and K. 0. Leedy have offered suggestions and comments useful in the

preparation of this report.

18
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383-386 (1974).

19
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20
Appendix A

Analysis of the Fraction of Energy Backscattered

In order to utilize the energy deposited versus penetration results

of Everhart and Hoff [Al] to calculate the energy deposited in aluminum,

silicon dioxide, and silicon structures by a low energy electron beam,

knowledge of the fraction of incident energy backscattered from the specimen (fg) is necessary. This fraction is usually taken to be 0.1 from

the work of Bishop [A2] at 30 keV. A study was undertaken to examine

the validity of using this value at lower electron beam energies.

The fraction fg depends on p, the fraction of incident electrons

backscattered, and the fractional mean energy of the backscattered electrons : f B

9

where Egj,j^ is the rae^an energy of backscattered electrons and Eg is the

beam energy. Both Egck/^B depend on the incident energy, specimen composition, the incident beam angle, and the scattering angle at

which they are measured. The data reviewed here are for normal incidence and are integrated over all possible scattering angles.

There have been several experimental determinations of p using a variety of experimental techniques [A3-A7]. In the energy range of interest here (usually Eg ^ 30 keV) , the fraction of electrons backscattered from aluminum or silicon is almost independent of the beam energy.

Eg, as shown in figure Al. Data on the fractional mean energy of backscattered electrons are scarce [A2,A8,A9]. Figure A2 illustrates the

variation of Eg^k/Eg with beam energy for electrons backscattered from

aluminum. The values given by Thomas [A8] were measured at 138 deg with

respect to the beam direction; the average value over all backscattering

angles would be greater. The values_of fg for an aluminum specimen can be calculated using these values of Eg(,|^/Eg and values of p from figure

Alb Interpolated when necessary to obtain values at the same energies.

The results, with error bars estimated on the basis of scatter in the

reported data, are shown in figure A3. It is apparent that taking the

value of fg to be 0.1 in the range 5 to 30 keV makes no more than a 2 percent contribution to the error in calculating the energy deposited.

This contribution is small in comparison to the other possible sources

of error. To a first approximation for silicon specimens, values of fg can be taken to be the same as aluminum. The results are also expected

to be applicable in general to devices consisting of silicon, silicon

dioxide, and aluminum.

References

Al. Everhart, T. E., and Hoff, P. H. , Determination of Kilovolt Electron Energy Dissipation vs. Penetration Depth in Solid Materials,

J. Appl. Phys. 5837-58A6 (1971).

21
A2 . Bishop, H. W. , Electron Scattering in Thick Targets, Brit. J. Appl.

Phys. 703-715 (1967).

A3. Heinrich, K. F. J., Electron Probe Microanalysis by Specimen Current

Measurement, Prooeedings of the Fourth International Congress of

X-Ray Optics and Microanalysis

R. Castaing, P. Deschamps, and J.

Philibert, Eds., pp. 159-167 (Hermann, Paris, 1966).

A4 . Colby, J. W. , Wise, W. N. , and Conley, D. K. , Quantitative Microprobe Analysis by Means of Target Current Measurements, Advan. XRay Anal. 447-461 (1967).

A5 . Bishop, H. E., Electron Scattering and X-Ray Production, Ph.D. Dissertation, University of Cambridge (1966).

A6. Weinryb, E., and Philibert, J., Mesure du Coefficient de Retrodlffusion des Electrons de 5 ^ 30 keV, Compt. Rend. 258 , 4535-4538

(1964)

.

A7 . Drescher, H. , Reimer, L. , and Seidel, H., Riickstreukoef f izient und

Sekundarelektronen-Ausbeute von 10-100 keV-Elketronen und Beziehungen zur Raster-Elektronenmikroskopie, Z. ang. Phys. 331-336

(1970).

A8. Thomas, R. N. , Scattering of 5-20 keV Electrons in Metallic Films,

Ph.D. Dissertation, University of Cambridge (1961).

A9 . Darlington, E. H. , Electron Backscattering at Low Energies, Ph.D.

Dissertation, University of Cambridge (1970).

22
0.2

0.1

0.2

0.1

r

SILICON

6

BISHOP

<K HEINRICH

A DRESCHER et al

10 20

BEAM ENERGY |keV|

30

) 10 20 30

BEAM ENERGY (keV)

(a)

' 1 ^ 1

^ 1

<s ALUMINUM

- A

-

X - 0 ^ 0

- 0 0

o COLBY et al 0

BISHOP

0 WEINRYB AND PHILIBERT

0 HEINRICH

A DRESCHER et al

1 1 1 1

(b)

Al. Ratio of backscattered to Incident electrons, n, as a function of

beam energy. (a) Silicon specimen. (b) Aluminum specimen.

23
10 20

BEAM ENERGY |keV|

A2. Fractional mean energy backscattered , from aluminum as a function of beam energy, E . D

A3. Fraction of incident energy backscattered, f^, from aluminum as a function of beam energy.

24
Appendix B

Draft of Recommended Practice

This appendix gives a method of estimating the total absorbed dose

in semiconductor devices due to SEM electron irradiation in a form suitable as a first draft for presentation to Subcommittee F-1.11 on Quality

and Hardness Assurance of ASTM Committee F-1 on Electronics.

Recommended Practice for Estimating the Total

Absorbed Dose in Semiconductor Devices from SEM Electron Irradiation^

1. Scope

1.1 This recommended practice covers a method for calculating an estimation of the total absorbed dose in critical semiconductor device oxides resulting from exposure to the low energy

electron beam available in a scanning electron microscope

(SEM). The calculation is based on the experimental work on energy dissipation versus electron penetration depth of Everhart and Hoff (1).^

1.2 The calculation requires knowledge of the geometry and composition of the device structure and the parameters associated with

the scanning electron microscope exposure: the electron beam

energy, the electron beam current, the duration of the exposure, and the area scanned by the electron beam.

1.3 This method is limited to devices fabricated from materials

with atomic numbers between 10 and 15. Thus, it is applicable

to devices consisting of silicon, silicon oxides, silicon nitrides, and aluminum.

1.4 The experimental measurements of Everhart and Hoff were limited to electron energies between 5 and 25 keV. An extrapolation of these results to 40 keV is expected to incur only a small error.

1.5 This method assumes that the scanning electron microscope is

adjusted so that the electron fluence incident on the device

is uniform, that the electron beam is incident normally on the

device, and that 10% of the incident energy is backscattered

from the surface of the device (2)

.

^Reserved for ASTM jurisdictional footnote.

^The bold face numbers in parentheses refer to the list of references

appended to this practice.

25
2. Significance

2.1 Knowledge of the effects of a total ionizing dose on the electrical characteristics of a semiconductor device is a requirement for many applications. Total absorbed dose testing is

typically accomplished using ^®Co irradiation; however, it is

often more convenient to simulate the exposure of a device to

^*^Co gamma rays with an SEM than to use a ®*^Co source.

2.2 The variation of dose with depth through the device for the

SEM electron beam is dependent on the device structure and the

beam energy; this variation may be quite different from the es sentially constant depth-dose distribution for ^°Co exposure.

2.3 This practice takes account of the variations in depth-dose

profiles of low energy electrons in device structures in the

calculation of the total absorbed dose in critical device oxides .

3. Calculation

3.1 Calculate the number of incident electrons per unit area

N

where

N = electron fluence, electrons/cm^

= electron beam current, A,

B

t = exposure time, s, A = area scanned, cm^ , and

sq = 1.6 X 10“^^ C/electron.

3.2 Determine the projected range of the incident electrons

= 3.98

(j B

where

R = electron projected range, yg/cm^, and

G

E„ = electron beam energy, keV.

B

3.3 Using knowledge of the device structure, determine xj, the dis

tance from the device surface to the top of the oxide of inter

est, and X2 , the distance to the bottom of the oxide both in

micrograms per square centimeter.

26
xi = Zp . d . 3 3

X2 = XI + p d

o o

where

Pj = density of layer j above oxide, yg/cm^,

dj = thickness of layer j above oxide, cm,

p^

= density of oxide layer, yg/cm^, and

d^ = thickness of oxide layer, cm.

3.4

Calculate yi and y£

yi

xi

rT

X2

72 =

3.5 If X2

> Rg> 72 “ 1.0. The electron beam is not penetrating

the oxide layer. The results may be anomalous. Reconsider

beam energy being used.

3.6 If X2

> Rq, the dose in the oxide layer equals zero; stop the

calculation.

3.7 If xi < R_, continue with the calculation.

3.8 Calculate the fraction of incident electron energy deposited

between y^ and y 2

= Y(y2) - Y(yi)

where

fp = fraction of incident energy deposited and

Y(y) = 0.6 y + 3.105y2 - 4.133y3 + 1.425y'+

3.9

Calculate the energy deposited in the oxide layer per incident

electron

E

D 0-5 Vd

where

= energy deposited per electron, keV/electron.

3.10

Calculate the total absorbed dose in the oxide

D[rad(Si02)] = 1.602 x 10"5 N-Ep-(x2 - xi)~^

27
References

1. Everhart, T. E. , and Hoff, P. E., Determination of Kilovolt Energy

Dissipation vs. Penetration Distance in Solid Materials, J. Appl,

Phys. 5837-5846 (1971).

2. Galloway, K. F. , and Roitman, P., Some Aspects of Using a Scanning

Electron Microscope for Total Dose Testing, NBSIR 77-1235 (1977).

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NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. FUBUC ATION OR RKPORT NO. NBSIR 77-1235

2. Gov't Acce.ssion No.

4. TITLE AND SUBTITLE

Some Aspects of Using an SEM for Total Dose Testing

7. AUTHOR(S)

K. F, Galloway and P. Roitman

3. Re» ipicm’s At cession No.

5. Publication Datc- September 1977

6. Performing Organization ( «ul«-

8. Performing rertorming Organ. W Report h N(» NBSIR-77-1235

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projec t /Task /Work Unit Ni:

11. (iomract/Grant No. DNA lACRO 77-809

12. Sponsoring Organization Name and Complete Address /Street, City, Stale, ZIP)

Defense Nuclear Agency

Washington, D.C. 20305

13, Type of Report & Period Covered

14. Sponsoring Agency C'ode

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200~word or less {actual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) This report addresses a number of aspects involved in using a Scanning Electron Microscope (SEM) for radiation testing of semiconductor devices. Problems associated

with using the low energy electron beam to simulate exposure and a method for

estimating the total absorbed dose in critical device oxides are discussed. The

method is based on the experimental ly determined expression for electron energy

dissipation versus penetration depth in solid materials of Everhart and Hoff. An appendix giving the method of estimating the total absorbed dose in a form suitable

for ASTM deliberations is included.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Electron beam energy deposition; ionizing radiation effects; radiation dose;

radiation testing; semiconductor devices; scanning electron microscope.

18. AVAILABILITY [X Unlimited

[T: For Official Distribution. Do Not Release to NTIS

|X ' Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

[_n Order From National Technical Information Service (NTIS)

Springfield, Virginia 221^1

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

40

22. Price

$4.50

USCOMM-DC ;>904<>-P74
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