NBSIR 77-1248

Arris ax*. #.

tlsfrz. Lf-^hA Procedure for Calibrating the

Magnification of an Scanning

Electron Microscope Using

NBS SRM 484

David B. Ballard

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

June 1977

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 77-1248

A PROCEDURE FOR CALIBRATING THE

MAGNIFICATION OF AN SCANNING

ELECTRON MICROSCOPE USING

NBS SRM 484

David B. Ballard

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

June 1977

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

A Procedure for Calibrating the Magnification of

A Scanning Electron Microscope Using NBS SRM 484

David B. Ballard

A procedure for calibrating the magnification of a scanning electron microscope (SEM) is described in detail.

The steps involved in specimen preparation and manipulation

within the SEM are outlined. The SEM parameters that may

have an effect on magnification are reviewed and techniques

for correcting or minimizing these are presented. The photographic material used, image distortions recorded and

technique of photograph measurement are discussed.

Keywords: Calibration; instrument parameters; magnification

correction; photograph measurement; Scanning Electron

Microscope; SRM 484 specimen preparation.

1. INTRODUCTION

Presently there are over 3,000 scanning electron microscopes (SEM)

located in U.S. laboratories where research involves such topics as the

study of air pollution particulates, corrosion of materials, wear of

surfaces, fabrication of microelectronics and examination of microstructure of metals used in electrical energy distribution. An error

of 10-30% within the magnification range of 1KX-20KX has been found to

exist in some of these SEM's. Use of Standard Reference Material (SRM)

484 to calibrate the SEM magnification scale should reduce uncertainty

to 5% or less in the range of 1 to 20 KX. This NBS Technical Note

complements information issued with the SRM 484. A future NBS- 260

series publication will contain details of the development and statistical evaluation of SRM 484.

2. NBS - SRM 484

2.1 Fabrication and Calibration

The initial step in fabrication of the SRM is to electroplate a layer of bright nickel onto the surface of a thin Monel sheet. Subsequently, alternate layers of bright gold (40-80 nm thick) and bright

nickel (nominal thickness of 1, 3, 5, and 20 ym) are deposited under

precise control. A long-time, low- temperature vacuum anneal relieves

stresses in the layers. Small, square (9x9 mm) samples are sheared

from the central portion of the composite sheet and mounted in copperfilled diallyl phthlate for metallographic polishing. The distances

between the observed images of the gold lines are used to calibrate

the magnification scale of the SEM (Fig. 1 and 2). The distance

between adjacent gold lines was determined by comparison with a previously calibrated Master Standard that was measured by the NBS

Dimensional Technology Section using a polarizing helium - neon laser

interferometer. The uncertainty of this calibration, based on 81

independent measurements is 0.004 ym for distances from 1 to 5 ym and

1
Fig. 1 SRM 484 is designed to calibrate the magnification scale of an SEM to an accuracy of 5% or better within the range of 1,000 to

20,000 X. The distance between gold line pairs (0-1, 0-2, 2-3,

3-4, and 0-6 nominally spaced at 1, 2, 3, 5 and 50 ym) located

in a nickel matrix are individually calibrated. The calibrated

area of 24 ym is centered opposite the Knoop indentation placed

at the mid- length of the specimen.

Fig. 2 Photomicrograph of SRM 484 showing images of line #0 through 6

at a nominal magnification of 800X.

2
0.009 ym for distances from 10 to 50 ym. The total uncertainty for

SRM 484 is 0.039 ym for distances from 1 to 5 ym and 0.71 ym for distances from 10 to 50 ym. The calibrated line length of 24 ym is

centered opposite a Knoop indentation placed at the mid- length of the

specimen.

2.2 Specimen Preparation

The SRM 484 specimen was mounted on an SEM stub with electrically

conductive cement and dried in a vacuum oven at 60°C for 15 minutes.

An alternative procedure was to clamp the specimen with set screws that

were threaded into a modified SEM stub. Every effort was made to prevent relative motion or vibration between the stage and the electron

beam. Lint and other loose debris were cleaned from the specimen sur- face with a blast of clean air before insertion into SEM specimen

chamber. If additional cleaning was required to remove fingerprints,

the specimen surface was gently swabbed with cotton saturated with a detergent solution. The detergent residue was removed by a water rinse,

followed by a rinse with ethyl alcohol and clean air-blast dried.

Stubborn stains were removed with a minimum polish using metallographic

grade MgO on microcloth, hand-held and stroking parallel to the gold

lines. Any abrasive grinding of the polished surface would remove

material in depth and subsequently change the calibrated line spacing.

A good vacuum of 10' 5 mm Hg was established before SEM examination to

reduce surface contamination rates. This was desirable as contamination

would decrease the contrast between the gold lines and the nickel matrix.

3. SEM PARAMETERS

The operational steps used to calibrate the magnification of each

manufacturer’s scanning electron microscope (SEM) are different and

often do not consider all the instrument parameters which may change

the resultant magnification (See Table) . The following procedure

details the use of NBS SRM 484, to calibrate one particular SEM*, but

may be used as a guide for calibration of other SEM's.

3.1. Outline of Procedure for Calibrating SEM Magnification Scale

1. Rigidly mount SRM 484, whose surface has been inspected for

cleanliness, on SEM stub with electrically conductive cement

or clamp onto stage of SEM.

2. Surface of SRM 484 should be normal to electron beam.

3. A clean vacuum of 10" 4 mm of Hg or better is necessary to keep

the contamination rate on the surface of the SRM as low as possible.

4. Adjust electron gun voltage (between 5 to 30kV) , saturate

filament, and check filament alignment.

5. Adjust final lens current at a resettable value. Cycle lens

circuit OFF-ON 3 times to minimize hysteresis effects.

* Cambridge Stereoscan MK II. This instrument is identified solely for

the purpose of describing the system used in this experiment.
6. Allow a 30 minute or more warm-up of electronic circuits to

achieve operational stability.

7. Adjust lens apertures and stigmator for optimum resolution

(minimum astigmatism)

8. SEM resolution should be a minimum of 0.05 ym.

9. Position the SRM-484 specimen at a nominal magnification of

1KX so that the image of the Knoop indentation is centered at one edge of the viewing CRT. The width of the gold- line

calibrated area extends 12 ym above and below this indentation.

10. a) The same working distance or magnification scale of the

SEM can be reproducibly obtained by focusing on the image of

the gold lines with Z axis control at highest possible magnification to minimize depth focus. An alternate focus method

is to use "y" mode and adjust Z axis for maximum signal.

b) To minimize the effect of distortions produced by the

recording system, the SRM-484 can be substituted and photographed for the unknown sample. Specific lines on SRM 484 to

be used in the calibration should be chosen so that the distance

between them corresponds as closely as possible to the length

of the object to be measured with both images positioned in

the same area on the cathode ray tube (CRT) . A millimeter

scale taped onto the edges of the CRT of the x and y directions

will assist in the relocation on the respective images.

11. Add "Black Level" for contrast if necessary, S/N ratio should

be 2:1 minimum.

12. Allow Polaroid prints to dry 15 to 20 minutes or more to

minimize effects due to emulsion and coating shrinkage.

13. Measure the perpendicular distance between the lines using

center of each line image on the photograph with a TEM

Diffraction Plate Reader or use an equivalent instrument the

precision of which is suitable for this purpose (about 0.2 mm).

14. Repeat measurements 3 times on each photograph to determine the

average spacing.

15. Magnification = Distance between lines as measured on photograph

Certified distance between same lines

16. To determine SEM stability and reproducibility, repeat all

steps at hourly or daily intervals or after adjustments and

repairs

Some of the above parameters that effect magnification will be

briefly reviewed and discussed with examples. It is assumed that all

electronic signal detection and processing components of the SEM

involved are within manufacturer's operating specifications.

3. 2 Specimen Stage Controls

The controls of the SEM stage should allow substitution of SRM 484

for the unknown specimen without changing or interruption of the

electronic circuits. Thus, it is necessary to mechanically refocus onto

the surface of each specimen using the z axis (working distance) stage

micrometer. Calibration is most easily accomplished with the specimen

4
oriented at right angles to the electron beam. However, if a tilted

surface is necessary for sufficient secondary electron signal, only

the central portion of the photo with the gold lines parallel to the

tilt axis of the stage can be considered as the calibrated area. Magnification values on other axes are subject to error due to foreshortening, particularly at low magnifications and at high tilt angles.

Of course, SRM 484 could be used to measure the amount of foreshortening

if the specimen surface orientation is known.

3.3 Image Drift

Apparent electronic circuit instability may be observed even after

a 30 minute warm-up period. This may be due to three parameters of

the SEM that can have a major short term (less than one hour) effect on

the magnification. These are (1) lens hysteresis, (2) the electron-gun

high voltage power supply, and (3) scan coils including related scan- generator circuits. To stabilize the magnetic lenses, it is recommended

that they be cycled ON-OFF at least twice or until the observed image

remains in focus and image displacement is minimized at the highest

magnification to be calibrated. Some electron tube circuits of parameters (2) and (3) have less electronic stability than solid state

circuits.

Other sources of image instability are specimen surface

contamination, aperture contamination, stage creep, and continuous

thermal drift of electron column parts. All of these parameters should

be considered, controlled if possible, and compensated for by the

operator. For example, an image position drift of 7 ym/hour, using one

line of the SRM 484 as an index, has been observed at 20,000 X in the

SEM used in this work. This drift is not serious if the scan line

direction is perpendicular to the gold line axis and for the following

reasons. The exposure time for one photograph was 40 seconds. The

total height of the image field was 90 mm, but only a 3 mm segment of

the image line length was used for reference measurements. This segment

represented -1.3 seconds of scan time equivalent to -2.6 nm drift. A

displacement of this magnitude was below the SEM resolution and was

therefore considered negligible.

3.4 Magnification Drift

A separate magnification correction may be necessary for any

change in operation parameters. As an example, each magnification

range must be examined where a change of fixed resistors is involved.

However, over a long term (more than 1 hour) magnification calibration

of the SEM can change. For example, a change was documented by locating

a precise area on the SRM 484 specimen and photographing this area at

three different magnifications over an extended time period. This

technique was applied at frequent daily intervals over approximately

five months. The results of these observations showed a maximum of

0.6% change for a nominal 2 ym and a 5 ym space recorded at 20 and 10KX.

The amplitude and sign of change in magnification varied in a

5
nonsystematic fashion with time. The instrument operation covered the

normal work period hours and included many operators using various modes

of3.5SEM operation.

Resolution

At least 50 nm resolution was found necessary to have a sufficiently

sharp image of the gold lines of SRM 484 so that measurements could be

made from line-center to line-center. This procedure allowed a precision

of ± 0.2 mm on measurements of the spacing on photomicrographs between

lines #0, 1 and 2. A typical photo recorded at nominal magnification

of 20,000 X demonstrates this quality of image and is shown in Fig. 3.

Available resolution test materials such as NBS RM-100 (aluminum - tungsten dendrites), magnetic tape, copy paper and etched gold grid may

be employed to assist in the adjustment of minimum astigmatism, alignment of apertures, selection of lens currents, and signal processing,

prior to magnification calibration.

3.6 Recording Photographic Material

The photographic material used here was Polaroid* type #52 film.

The ASA rating of 400 and contrast latitude of this film produced good

contrast images of the gold lines. The short development time after

exposure was a convenience that allowed images to be measured. After

exposure, the prints were processed and coated as specified by the

manufacturer. The photo emulsion was allowed to dry and equilibrate

for a minimum of 20 minutes before measurements were made. No significant difference in line spacing was noted due to a longer drying time

or temperature and humidity changes normally experienced in laboratory

room conditions. Other fine grain photographic products could be used

if appropriate care is exercised.

3.7 Recorded Image Distortion

As a result of distortions within the recording CRT and the camera

photo lens, the magnification of an image near the print edges or at

other locations on the photo may be erroneous as compared to that in the

calibrated area. This magnification distortion is independent of

specimen tilt angle or signal processing. The amount and location of

distortion can be surveyed and measured by displacing, at 5 mm intervals

on the SEM recording screen, the appropriate gold line space of SRM 484

at magnifications of 1 kX to 20 kX. A photo recording is made at each

interval. The same calibration space between the gold lines is

measured and the correction is applied to the indicated instrument scale

or photo area.

*Polaroid - This product is identified solely for the purpose of

describing the material used in this procedure.

6
Fig. 3 Photomicrograph of SRM 484 showing the images of lines #0, 1 and

2 at a nominal magnification of 20,000 X.

Fig. 4. Photo-reader used to measure space between line images.

7
3.8 Photographic Measurements

To reduce errors which may be as much as ± 0.5 mm due to photo

placement onto the photo reader, a mark was placed on the face of the

CRT so that on each recorded photo this fiduciary could be used as an index (Fig. 4). Measurements of the space between line-centers on the

photo must be made perpendicular to the line length.

An average value for each line spacing was determined from a minimum

of three measurements made on each photograph. This value was then

corrected for photo-field distortion, which could be as much as 15% and

is shown in line #6 of Fig. 2. The correction applied to the measured

data was of the form f(x) = a + bx + cx

2 (a parabola). For the SEM used

in this study, the image area having least distortion was found to be

offset 5 mm to the right of the photograph center.

4. CONCLUSIONS

An independent (closed Loop) check on all the above magnification

correction procedures was made. The line spacings of one SRM 484

specimen were measured with the same accurate polarized laser interferometer that was used to calibrate the Master Standard. The values

determined for line spacings of 1, 2, 3, 5 and 50 ym agreed within 2.5%

(probable error) of those obtained using the SEM.

If the above SEM parameters were properly controlled and a specimen

of SRM 484 used, the magnification scale of this SEM could be calibrated

to an uncertainty of 5% or less between the range of lkX and 20kX.

Extension of this method should be applicable to other SEM instruments.

Disclaimer:

Certain trade names and company products are identified in order to

adequately specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National

Bureau of Standards nor does it imply that the products are necessarily

the best available for the purpose.

8
Table

SOME OF THE PARAMETERS THAT MAY EFFECT THE MAGNIFICATION OF AN SEM

1. Photographic material - shrinkage/expansion, photographic enlarging.

2. Recording camera-lens distortions, and change of CRT image size.

3. Recording CRT - distortion of faceplate, and beam deflection.

4. Scan generator - nonlinearity, and aging components.

5. Scan coils - nonorthogonal

6. Magnification switch resistor chain - different for each

magnification.

7. Zoom control on magnification - resettability

8. Working distance - long (small angle beam deflection) vs. short

(large angle beam deflection) for same magnification.

9. Specimen tilt - not perpendicular to beam.

10. Specimen tilt correction - relative to tilt axis.

11. Specimen image drift - charging or stage drift.

12. Depth of focus

13. Reproducibility of specimen height control.

14. Lens strength combinations - particularly for three lens instruments.

15. Lens astigmatism and hysterisis.

16. Electron gun stability.

17. Extraneous magnetic and electrostatic fields.

9
HlgHTU (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 77 1248

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

A Procedure for Calibrating the Magnification of a Scanning Electron Microscope Using NBS SRM 4&4

5. Publication Date June 1977

6. Performing Organization Code

7. AUTHOR(S)

David B. Ballard

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report 8t Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or leas factual summary of most significant information, tf document includes a significant

bibliography or literature survey, mention it here.)

A procedure for calibrating the magnification of a scanning electron

microscope (SEM) is described in detail. The steps involved in specimen

preparation and manipulation within the SEM are outlined. The SRM parameters that may have an effect on magnification are reviewed and techniques

for correcting or minimizing these are presented. The photographic material

used, image distortions recorded and technique of photograph measurement are discussed.

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

)

Calibration; instrument parameters; magnification correction; photograph

measurement; scanning electron microscope; SRM 484 speciment preparation.

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