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An improvement of the RI Auger electron thickness gauge for the measurement of thin metal films
NUCLEAR
INSTRUMENTS
AND
METHODS
I35
(I976) 473-476;
©
NORTH-HOLLAND
PUBLISHING
CO.
AN I M P R O V E M E N T OF T H E RI A U G E R E L E C T R O N T H I C K N E S S G A U G E F O R T H E M E A S U R E M E N T OF T H I N M E T A L F I L M S NOBUHITO
1SHIGURE, CHIZUO
M O R I and T A M A K I
WATANABE
Department of Nuclear Engineering, Faculty of Engineering, Nagoya University, Nagoya, Japan Received 9 M a r c h 1976 T h e RI (Radio Isotope) A u g e r electron thickness gauge presented in a previous paper is improved so that the gauge can be applied to the m e a s u r e m e n t o f thin metal films and also the ion charge-up effect on a n o n - c o n d u c t i n g specimen film can be removed. A negative voltage o f a few volts against the specimen film is supplied to the nickel grid which is attached on the counter side o f the aperture o f the d i a p h r a g m as the window o f a G M counter. Secondary electrons produced by A u g e r electrons in Q gas (He 99%, isobutane 1%) between a specimen a n d the nickel grid are repulsed toward the specimen a n d are prevented f r o m entering the G M counter. Also ions produced by the electron avalanche in the G M c o u n t e r are prevented from charging up on the specimen. A l u m i n u m films o f m a s s per unit area 1 0 / t g / c m 2 can be m e a s u r e d with a 6% error.
I. Introduction A new method of thin film thickness measurement using RI Auger electrons 1) has been shown to be useful for thin organic films. Vinyl or collodion films of 10/~g/cm z were able to be measured with an 11% error. Thin metal films are used in various fields of technology or science and it is often necessary to know the thickness. It is rather difficult to find a nondestructive and precise method to measure the thickness of thin metal films. In this case the RI Auger electron thickness gauge is applied to thin metal films. But the gauge does not have good stability in this application. This arises from the difference of the structure of the holding on a specimen holder of a thin metal film from that of an organic film. In this paper, the cause of the instability, the improvement to solve it and the characteristic of this thickness gauge after the improvement are reported.
attachment of Q gas, there remain free electrons in SG space for a while. When these electrons enter the G M counter after the dead time of the counter from entrance of the K Auger electron which produced them or when only the secondary electrons enter the counter without the entrance of the related K Auger electron, they are
Source holder Pine
'~
J
Diaphragm ....
; ....
/
55Fe source
/ Specimen
i [-speo,o., ,,,m
; ............
.......
coon ory e l e c t r o n ~ P a = l K Aujer electron \
5
..........
d d~e ooitton P "" Nickel grid supplied with negative voltage
2. Cause of instability 2.1. INFLUENCE OF SECONDARY ELECTRONS The central part of the improved apparatus is shown in fig. 1. The number of secondary electrons produced due to the ionization of Q gas (He 99%, isobutane 1%) by a K Auger electron, in a space of 1 m m between a specimen film and the nickel grid (hereafter referred to as SG space), is calculated to be 10-20, using the mass thickness of Q gas in SG space, the stopping power for K Auger electrons 2) and the W value of Q gas3). Since the attachments of these secondary electrons to Q gas molecules or positive ions do not occur immediately on account of the small probability of
I
I
I mm
Anode loop
/
Fig. 1. T h e central part o f the a p p a r a t u s after the improvement.
474
NOBUHITO
counted independently. Therefore the count rate of the G M counter is affected by the number of secondary electrons produced in SG space and then subjected to the variation of the length of the space. This causes erroneous results. For metal film specimens of interest it is fairly difficult to keep this length constant, which shall be referred to the next section.
ISHIGURE
et al.
500 mesh was attached on the paint. Gold was vacuumdeposited on the paint surface since the whole inner surface of the counter must have good conductivity. Then the nickel grid, which was connected with the gold deposition and was insulated from the diaphragm, was connected electrically with an external dry battery. 3.2. CONTRIBUTIONOF SECONDARYELECTRONS
2.2. STRUCTUREOF THIN METALFILM SPECIMENS Aluminum fihns were used as specimens here, since their preparation was comparatively easy and the authors needed to know the thickness to use them in other experiment. The preparation method of the film specimens is as follows. A vacuum-deposited aluminum film on the cleavage surface of a single crystal of sodium chloride was stripped off on a water surface and fixed on a sheet of nickel grid of 500 mesh. Then this grid was adhered to a ring as a specimen holder, which had a hole of an adequate diameter (3-15 ram). By this method, considerably thin films of 100/~ could be prepared. This adhesion causes an uneven surface of the specimen holder due mainly to the difference of the amount of adhesive agent daubed. The uneven surface varies the length of SG space from specimen to specimen and the difference was estimated to be 100 pro, which came out to a change of 10% in the length of SG space itself. Then the production of secondary electrons changes by 10% and the count rate of the G M counter will change.
The dependence of the count rate on the supplied voltage to the nickel grid is shown in fig. 2. The count rate decreased remarkably with increasing supplied voltage in comparison with that at zero volt which is the same to that before the improvement. Obviously this decrease is equal to the number of the secondary electrons which were counted before the improvement. It was found that the contribution of secondary electrons at zero volt was about 40% of the total counts and that at most a few volts were sufficient for the repulsion of nearly all secondary electrons. 3.3. E F F E C T OF THE IMPROVEMENT FOR THE CHANGE
OF SG SPACE To confirm the effect of the improvement, it is necessary to check the improvement of count rate on the change of the length of SG space. Spacers of various thickness, from 50 p m to 200 pm, were put between a specimen holder and the diaphragm I
I
o
3. Improvement 3.1. IMPROVEMENTOF THE APPARATUS In order to avoid the secondary electron effect, it was considered to prevent secondary electrons from entering the G M counter. There will be two ways for this purpose. One is the replacement of the nickel grid as the window of the counter by a conductive thin film so that secondary electrons can be stopped by the film. Another is the supply of a negative voltage to the nickel grid so that secondary electrons can be repulsed toward the specimen. The former has several faults: the difficulty of the exchange of air for Q gas in SG space, the reduction of the measurable thickness range due to the absorption of K Auger electrons by the film and the danger of the break of the conductive film. On the other hand the latter has not these faults and is comparatively simple. The practical method is as follows. The counter side of the diaphragm was coated with non-conducting paint and a sheet of nickel grid of
I
×1048 l
=E 6 i o ,.=,
a. 4
¢..)
o
0 SUPPLIED
[
I0
I
I
20
3o
VOLTAGE
TO
NICKEL
GRID
| V
Fig. 2. D e p e n d e n c e o f c o u n t rate on s u p p l i e d v o l t a g e t o the n i c k e l grid.
AUGER ELECTRON THICKNESS GAUGE in order to vary the length of SG space intentionally. The results are shown in fig. 3. Without any supplied 'voltage, the count rate changed with the increasing rate of 5%/100/zm. The change of 5% in count rate comes out to an error of 15% in thickness determined using the calibration curve in fig. 5. Obviously, this iincrease of 5% is due to the secondary electrons. With a supplied voltage of 1.5 or 6.0 V, the count rate was not dependent on the change of the length of SG space. :3.4. EFFECTFOR CHARGE-UP The improvement has another advantage. The time ,dependences of the count rate for VYNS film specimens were observed as shown in fig. 4. Before the iimprovement, the count rate decreased gradually for :fifteen minutes to the saturated value. After the improvement, there was no change in count rate. For a non-conducting film such as a VYNS film, when a negative voltage is not supplied to the nickel grid, positive ions produced by an electron avalanche in the G M counter are charged up on the film surface. 'Then an electric field appears in SG space, which I O -
O
I-I
V
( Before
--I.S
V
--6-0
V
o
uJ
I
I improvement
I
) ~
-
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
~ . oe -
. . . .
1.0
o
I
I
50
I O0
POSITION
OF
ALUMINUM
I
I
150
200
FILM
O
E ¢L o --
V
-4.5
( Before
attracts secondary electrons to the film surface. The number of secondary electrons to enter the counter decreases with increase of the charge-up until the chargeup reaches the equilibrium state which depends on the arrival rate of ions, the rate of running away of ions through a film surface and the recombination rate of ions and electrons. When a negative voltage is supplied, almost all secondary electrons are repulsed and positive ions are not charged up on the film surface because the nickel grid catches them, and then the count rate is constant. 4.
Calibration
curve and error estimate
This gauge can not determine absolute thickness and needs a calibration curve or standard samples. Fig. 5 shows the ralation between count rate and absolute thickness measured by flame absorptiometry. The curve was obtained by the method of least squares, assuming that it was a part of a Gaussian distribution curve. The errors, expressed as standard deviation, in absolute thickness determination were estimated as follows. a) Standard deviation of the reproducibility of the count rate for the same specimen film: Ten measurements of 10 min each for replacing repeatedly the same specimen film, gave a deviation of _ 0 . 7 % , that is about +0.3 pg/cm z error for an aluminum film of 10/~g/cm z. The value of + 0.7% included the statistical deviation, +0.5%, of counts for 10 min. b) The error caused by atmospheric pressure and temperature change: A change of ___3m m H g and + 1.5 °C gave an error of +0.3/tg/cm 2 of aluminum as mentioned in ref. 1.
( p.m )
Fig. 3. Dependences of count rate on the position of aluminum film.
__
475
improvement
8
I
I
X 10 4
;8
)
V
o
\ 8000
'°°°t. " t
~
' ~,
.
u
,
T
o
,
o o
o
n
o
~ o
4
o 0
I-- 7 0 0 0
• For
o o
VYNS I 5
0 TIME
AFTER
• --e----
film f I0 SUPPLYING
I 15 HIGH
VOLTAGE
2 20 ( rain, )
iFig. 4. Time dependences of count rate for VYNS film to find the effect of the improvement for the ion charge-up phenomenon on specimen film.
,
I 10
ALUMINUM
FILM
0
, THICKNESS
I 20
30
(p.g / cm 2 )
Fig. 5. Calibration curve for aluminum film.
476
NOBUHITO
ISHIGURE
c) The error of the calibration curve, which depends on the above two errors (a), (b) and the error of the thickness determined by flame absorptiometry: Since the calibration curve in fig. 5 was obtained using a number of measured values, the error could be expressed as standard deviation of the discrepancies of the measured values from this curve. It was calculated to be +0.4/~g/cm 2. After all, the total error of thickness measurement was about +0.6/lg/cm 2 for 10/~g/cm 2 thickness of aluminum film. 5. Conclusion The RI Auger electron thickness gauge was improved in order to be applied to thin metal films. In this application secondary electrons produced by K Auger electrons in the space between a specimen and the nickel grid were the main cause of instability of the gauge. A negative voltage of a few volts against the specimen film was supplied to the nickel grid as the
et al.
window of the GM counter, so that these secondary electrons were repulsed toward the specimen and did not enter the GM counter. This improvement gave independence of the count rate on the change of the length of the space and removed the instability of counts due to charge-up of positive ions on the surface of a non-conducting specimen film. The authors are indebted to Dr N. Ishizuka of the Government Industrial Research Institute, Nagoya for the measurement of standard aluminum samples using flame absorptiometry.
References ~) C. Mori, J. Koike and T. W a t a n a b e , Nucl. Instr. and Meth. 121 (1974) 253. 2) H. Sugiyama, Bull. Electrotechn. Lab. J a p a n 38 (1974) 352. 3) W. P. Jesse and J. Sadaukis, Phys. Rev. 102 (1956) 389.
INSTRUMENTS
AND
METHODS
I35
(I976) 473-476;
©
NORTH-HOLLAND
PUBLISHING
CO.
AN I M P R O V E M E N T OF T H E RI A U G E R E L E C T R O N T H I C K N E S S G A U G E F O R T H E M E A S U R E M E N T OF T H I N M E T A L F I L M S NOBUHITO
1SHIGURE, CHIZUO
M O R I and T A M A K I
WATANABE
Department of Nuclear Engineering, Faculty of Engineering, Nagoya University, Nagoya, Japan Received 9 M a r c h 1976 T h e RI (Radio Isotope) A u g e r electron thickness gauge presented in a previous paper is improved so that the gauge can be applied to the m e a s u r e m e n t o f thin metal films and also the ion charge-up effect on a n o n - c o n d u c t i n g specimen film can be removed. A negative voltage o f a few volts against the specimen film is supplied to the nickel grid which is attached on the counter side o f the aperture o f the d i a p h r a g m as the window o f a G M counter. Secondary electrons produced by A u g e r electrons in Q gas (He 99%, isobutane 1%) between a specimen a n d the nickel grid are repulsed toward the specimen a n d are prevented f r o m entering the G M counter. Also ions produced by the electron avalanche in the G M c o u n t e r are prevented from charging up on the specimen. A l u m i n u m films o f m a s s per unit area 1 0 / t g / c m 2 can be m e a s u r e d with a 6% error.
I. Introduction A new method of thin film thickness measurement using RI Auger electrons 1) has been shown to be useful for thin organic films. Vinyl or collodion films of 10/~g/cm z were able to be measured with an 11% error. Thin metal films are used in various fields of technology or science and it is often necessary to know the thickness. It is rather difficult to find a nondestructive and precise method to measure the thickness of thin metal films. In this case the RI Auger electron thickness gauge is applied to thin metal films. But the gauge does not have good stability in this application. This arises from the difference of the structure of the holding on a specimen holder of a thin metal film from that of an organic film. In this paper, the cause of the instability, the improvement to solve it and the characteristic of this thickness gauge after the improvement are reported.
attachment of Q gas, there remain free electrons in SG space for a while. When these electrons enter the G M counter after the dead time of the counter from entrance of the K Auger electron which produced them or when only the secondary electrons enter the counter without the entrance of the related K Auger electron, they are
Source holder Pine
'~
J
Diaphragm ....
; ....
/
55Fe source
/ Specimen
i [-speo,o., ,,,m
; ............
.......
coon ory e l e c t r o n ~ P a = l K Aujer electron \
5
..........
d d~e ooitton P "" Nickel grid supplied with negative voltage
2. Cause of instability 2.1. INFLUENCE OF SECONDARY ELECTRONS The central part of the improved apparatus is shown in fig. 1. The number of secondary electrons produced due to the ionization of Q gas (He 99%, isobutane 1%) by a K Auger electron, in a space of 1 m m between a specimen film and the nickel grid (hereafter referred to as SG space), is calculated to be 10-20, using the mass thickness of Q gas in SG space, the stopping power for K Auger electrons 2) and the W value of Q gas3). Since the attachments of these secondary electrons to Q gas molecules or positive ions do not occur immediately on account of the small probability of
I
I
I mm
Anode loop
/
Fig. 1. T h e central part o f the a p p a r a t u s after the improvement.
474
NOBUHITO
counted independently. Therefore the count rate of the G M counter is affected by the number of secondary electrons produced in SG space and then subjected to the variation of the length of the space. This causes erroneous results. For metal film specimens of interest it is fairly difficult to keep this length constant, which shall be referred to the next section.
ISHIGURE
et al.
500 mesh was attached on the paint. Gold was vacuumdeposited on the paint surface since the whole inner surface of the counter must have good conductivity. Then the nickel grid, which was connected with the gold deposition and was insulated from the diaphragm, was connected electrically with an external dry battery. 3.2. CONTRIBUTIONOF SECONDARYELECTRONS
2.2. STRUCTUREOF THIN METALFILM SPECIMENS Aluminum fihns were used as specimens here, since their preparation was comparatively easy and the authors needed to know the thickness to use them in other experiment. The preparation method of the film specimens is as follows. A vacuum-deposited aluminum film on the cleavage surface of a single crystal of sodium chloride was stripped off on a water surface and fixed on a sheet of nickel grid of 500 mesh. Then this grid was adhered to a ring as a specimen holder, which had a hole of an adequate diameter (3-15 ram). By this method, considerably thin films of 100/~ could be prepared. This adhesion causes an uneven surface of the specimen holder due mainly to the difference of the amount of adhesive agent daubed. The uneven surface varies the length of SG space from specimen to specimen and the difference was estimated to be 100 pro, which came out to a change of 10% in the length of SG space itself. Then the production of secondary electrons changes by 10% and the count rate of the G M counter will change.
The dependence of the count rate on the supplied voltage to the nickel grid is shown in fig. 2. The count rate decreased remarkably with increasing supplied voltage in comparison with that at zero volt which is the same to that before the improvement. Obviously this decrease is equal to the number of the secondary electrons which were counted before the improvement. It was found that the contribution of secondary electrons at zero volt was about 40% of the total counts and that at most a few volts were sufficient for the repulsion of nearly all secondary electrons. 3.3. E F F E C T OF THE IMPROVEMENT FOR THE CHANGE
OF SG SPACE To confirm the effect of the improvement, it is necessary to check the improvement of count rate on the change of the length of SG space. Spacers of various thickness, from 50 p m to 200 pm, were put between a specimen holder and the diaphragm I
I
o
3. Improvement 3.1. IMPROVEMENTOF THE APPARATUS In order to avoid the secondary electron effect, it was considered to prevent secondary electrons from entering the G M counter. There will be two ways for this purpose. One is the replacement of the nickel grid as the window of the counter by a conductive thin film so that secondary electrons can be stopped by the film. Another is the supply of a negative voltage to the nickel grid so that secondary electrons can be repulsed toward the specimen. The former has several faults: the difficulty of the exchange of air for Q gas in SG space, the reduction of the measurable thickness range due to the absorption of K Auger electrons by the film and the danger of the break of the conductive film. On the other hand the latter has not these faults and is comparatively simple. The practical method is as follows. The counter side of the diaphragm was coated with non-conducting paint and a sheet of nickel grid of
I
×1048 l
=E 6 i o ,.=,
a. 4
¢..)
o
0 SUPPLIED
[
I0
I
I
20
3o
VOLTAGE
TO
NICKEL
GRID
| V
Fig. 2. D e p e n d e n c e o f c o u n t rate on s u p p l i e d v o l t a g e t o the n i c k e l grid.
AUGER ELECTRON THICKNESS GAUGE in order to vary the length of SG space intentionally. The results are shown in fig. 3. Without any supplied 'voltage, the count rate changed with the increasing rate of 5%/100/zm. The change of 5% in count rate comes out to an error of 15% in thickness determined using the calibration curve in fig. 5. Obviously, this iincrease of 5% is due to the secondary electrons. With a supplied voltage of 1.5 or 6.0 V, the count rate was not dependent on the change of the length of SG space. :3.4. EFFECTFOR CHARGE-UP The improvement has another advantage. The time ,dependences of the count rate for VYNS film specimens were observed as shown in fig. 4. Before the iimprovement, the count rate decreased gradually for :fifteen minutes to the saturated value. After the improvement, there was no change in count rate. For a non-conducting film such as a VYNS film, when a negative voltage is not supplied to the nickel grid, positive ions produced by an electron avalanche in the G M counter are charged up on the film surface. 'Then an electric field appears in SG space, which I O -
O
I-I
V
( Before
--I.S
V
--6-0
V
o
uJ
I
I improvement
I
) ~
-
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
~ . oe -
. . . .
1.0
o
I
I
50
I O0
POSITION
OF
ALUMINUM
I
I
150
200
FILM
O
E ¢L o --
V
-4.5
( Before
attracts secondary electrons to the film surface. The number of secondary electrons to enter the counter decreases with increase of the charge-up until the chargeup reaches the equilibrium state which depends on the arrival rate of ions, the rate of running away of ions through a film surface and the recombination rate of ions and electrons. When a negative voltage is supplied, almost all secondary electrons are repulsed and positive ions are not charged up on the film surface because the nickel grid catches them, and then the count rate is constant. 4.
Calibration
curve and error estimate
This gauge can not determine absolute thickness and needs a calibration curve or standard samples. Fig. 5 shows the ralation between count rate and absolute thickness measured by flame absorptiometry. The curve was obtained by the method of least squares, assuming that it was a part of a Gaussian distribution curve. The errors, expressed as standard deviation, in absolute thickness determination were estimated as follows. a) Standard deviation of the reproducibility of the count rate for the same specimen film: Ten measurements of 10 min each for replacing repeatedly the same specimen film, gave a deviation of _ 0 . 7 % , that is about +0.3 pg/cm z error for an aluminum film of 10/~g/cm z. The value of + 0.7% included the statistical deviation, +0.5%, of counts for 10 min. b) The error caused by atmospheric pressure and temperature change: A change of ___3m m H g and + 1.5 °C gave an error of +0.3/tg/cm 2 of aluminum as mentioned in ref. 1.
( p.m )
Fig. 3. Dependences of count rate on the position of aluminum film.
__
475
improvement
8
I
I
X 10 4
;8
)
V
o
\ 8000
'°°°t. " t
~
' ~,
.
u
,
T
o
,
o o
o
n
o
~ o
4
o 0
I-- 7 0 0 0
• For
o o
VYNS I 5
0 TIME
AFTER
• --e----
film f I0 SUPPLYING
I 15 HIGH
VOLTAGE
2 20 ( rain, )
iFig. 4. Time dependences of count rate for VYNS film to find the effect of the improvement for the ion charge-up phenomenon on specimen film.
,
I 10
ALUMINUM
FILM
0
, THICKNESS
I 20
30
(p.g / cm 2 )
Fig. 5. Calibration curve for aluminum film.
476
NOBUHITO
ISHIGURE
c) The error of the calibration curve, which depends on the above two errors (a), (b) and the error of the thickness determined by flame absorptiometry: Since the calibration curve in fig. 5 was obtained using a number of measured values, the error could be expressed as standard deviation of the discrepancies of the measured values from this curve. It was calculated to be +0.4/~g/cm 2. After all, the total error of thickness measurement was about +0.6/lg/cm 2 for 10/~g/cm 2 thickness of aluminum film. 5. Conclusion The RI Auger electron thickness gauge was improved in order to be applied to thin metal films. In this application secondary electrons produced by K Auger electrons in the space between a specimen and the nickel grid were the main cause of instability of the gauge. A negative voltage of a few volts against the specimen film was supplied to the nickel grid as the
et al.
window of the GM counter, so that these secondary electrons were repulsed toward the specimen and did not enter the GM counter. This improvement gave independence of the count rate on the change of the length of the space and removed the instability of counts due to charge-up of positive ions on the surface of a non-conducting specimen film. The authors are indebted to Dr N. Ishizuka of the Government Industrial Research Institute, Nagoya for the measurement of standard aluminum samples using flame absorptiometry.
References ~) C. Mori, J. Koike and T. W a t a n a b e , Nucl. Instr. and Meth. 121 (1974) 253. 2) H. Sugiyama, Bull. Electrotechn. Lab. J a p a n 38 (1974) 352. 3) W. P. Jesse and J. Sadaukis, Phys. Rev. 102 (1956) 389.