- Email: [email protected]
The use of S.E.M. for linewidth measurements
Microelectronic Engineering 6 (1987) 679-681 North-Holland
The USE of S.E.M.
CENTRE
679
for L I N E W I D T H
MEASUREMENTS
Daniel BURLET Herve M A R T I N N A T I O N A L d'ETUDES des T E L E C O M M U N I C A T I O N S 38243 M E Y L A N Cedex FRANCE
Abstract : By studying the c r o s s - s e c t i o n of the lines to be m e a s u r e d we show that the absolute m e a s u r e m e n t of Critical D i m e n s i o n is not provided by S,E.M. dedicated to m e t r o l o g i c a l applications. A c a l i b r a t i o n for each layer is needed.
INTRODUCTION The classical optical measurements are difficult for the dimensions b e l o w 1.2 m i c r o n as the line edges are o b v i o u s l y too small to be seen. The light interference resulting from the t r a n s p a r a n c y layers, w h i c h depend on the process falsifies the measurement. A calibration for each level is therefore necessary. Two years ago we bought a S,E,M, dedicated to m e t r o l o g i c a l applications (JEPAS i000 of JEOL) because of the better improvement in the linewidth m e a s u r e m e n t due to the best resolution and the topography information given by secondary electrons (Fig.l, Fig.2). So, can a S,E.M, linewidth m e a s u r e m e n t system be used with no calibration ?
OPTICAL
S.E.M.
PROF ILE
PROFILE
_J w i,I d
!
fig. i*
6
MICRON
4
8 fig. 2*
4 MICRON
EXPERIMENTAL METHODOLOGY The optical m e a s u r e m e n t s require a c a l i b r a t i o n for each type of t e c h n o l o g y layer, Then we use the same m e t h o d with our S.E.M. to check if the c a l i b r a t i o n is n e e d e d [i]. The m e t h o d consists in defining the S.E.M. m e a s u r e m e n t s and the real linewidth and defining the best settings of the S.E.M. The R E A L L I N E W I D T H It can be said that the real llnewldth is the w i d t h measurement for a given height on the topographic profile predetermined by the technologist. This profile is obtained by studying the c r o s s - s e c t l o n of the llne using a S.E.M. (Fig.3). and taking a contour measurement followed by a p r o g r a m to correct the distortion and to compute precise m a g n i f i c a t i o n (Fig.4).
0167-9317/87/$3.50 © 1987, ElsevierSciencePublishers B.V. (North-Holland)
D. Burlet, H. Martin / Use o f SEM for linewidth measurements
680
ACTUAL
PROFILE
CU Z 0 r~ U F-4 E
OD
Fig. 3*
m
a
B
)
2
MICROM
Fig. 4*
RESULTS The relation b e t w e e n S.E.M, and actual width is revealed by the m e a s u r e m e n t of 9 lines from 0.6 to 3 m i c r o n by S.E.M. both by JEPAS and by the previous m e t h o d for each level of t e c h n o l o g y concerned by the measurements. The mean of the difference b e t w e e n these two measurements is called the calibration value and the standard d e v i a t i o n gives the accuracy of the calibration (taking into account the number of m e a s u r e m e n t s and 99% p r o b a b i l i t y of success : Fig.6). The settings must be chosen in order to minimize v a r i a t i o n s in the S.E.M. profile caused by charge effect. The accuracy of each calibration is used to get the best setting.
S.E.M.
REAL-JEPAS C.D.
PROFILE
AUTOMATIC (THRESHOLD)
A U T O M A T I C
M NuA
--
H 03 __I
NJAL
~(CROSS) j
Fig. 5*
e
MANUAL
MICROM MODE with
4
Fig. 6*
8 S. E . M . C . D .
5 BOTTOM
S,E.M.
The easiest m e t h o d of m e a s u r i n g the linewidth is to find on the electronic profile the b e g i n n i n g of the transition caused by the edges of the p a t t e r n (Fig,5). This method, usable only in the manual mode, gives the e s t i m a t e d b o t t o m of the sample. Some layers are detailed b e l o w : Line Substrate Calibration Accuracy(i)
RESIST . . . . . . . . AI WSi2 Poly Si Si3N4 -0.084 -0.018 -0.053 -0.037 -0.049 0.060 0.060 0.040 0.090 0,040
Multi-level AI -0,180 0.060
Line Substrate Calibration Accuracy(i)
AI WSi2 L.T,O. Si02 -0.245 -0,125 0.060 0.060
W Cr -0.210 0.030
M e a n of all our calibrations
Poly O.N.O. SiO2 SiO2 -0.080 -0.233 0.060 0.050 : 0.12
Si3N4 Si02 -0.034 0.030
Standard
Deviation
(Micron) : 0,20
(Fig.7)
D. Burlet, H. Martin / Use o f SEM for linewidth measurements
AUTOMATIC
681
MODE
The JEPAS is used for m e a s u r i n g numerous dices on the wafer. The manucfacturer had developped an threshold m e t h o d wich selects the most plausibles events given by the intersection points b e t w e e n the profile and a t h r e s h o l d level (Fig.5). O b v i o u s l y the m e a s u r e m e n t depends on it. Some layers are detailed below : Line Substrate Calibration Accuracy(T)
RESIST AI 0.059 0.020
Line Substrate Calibration Accuracy(i)
AI LTO -0.122 0.040
" WSi2 0.136 0.120
" Poly 0.130 0.150
WSi2 SiO2 -0.043 0.050
Poly SiO2 0.067 0.025
Mean of all our calibrations
MANUAL
" Si 0.173 0.040
" Si3N4 0.379 0.060
O.N.O. Si3N4 Si02 SiO2 -0.034 -0.020 0.025 0.035
: 0.i0
Standard
MODE
Multi-level AI 0.035 0.040 W Cr -0.066 0.040 Deviation
(Micron) : 0.15
AUTOMATIC
(Fig.8)
MODE
¢--
[email protected] Fig.7
MICRON
@.G
CALIBRATION
-0. G Fig.8
MICRON
@.G
CALIBRATION
The absolute error of an u n c a l i b r a t e d m e a s u r e m e n t can reach -0.60!0.205 in manual mode and +0.i00!0.406 in automatic mode. As an incorrect threshold can produce u n r e a l i s t i c values we have d e v e l o p p e d an automatic inspection which rejects the estimated bad values by taking into account the statistics of the whole results. CONCLUSION The l i n e w i d t h m e a s u r e m e n t by S.E.M. requires individual and precise settings to avoid any bad measurements.
(*) Resist
on WSi2,
1.2 m i c r o n
calibrations
on the mask
[i] H. MARTIN, D. BURLET "Methode de Calibration d'un M i c r o d e n s i t o m e t r e Optique" Le VIDE, les COUCHES MINCES. Societe Francaise du Vide supplement au n u m e r o 233 (Aout Septembre Octobre 1986) p 43-46.
The USE of S.E.M.
CENTRE
679
for L I N E W I D T H
MEASUREMENTS
Daniel BURLET Herve M A R T I N N A T I O N A L d'ETUDES des T E L E C O M M U N I C A T I O N S 38243 M E Y L A N Cedex FRANCE
Abstract : By studying the c r o s s - s e c t i o n of the lines to be m e a s u r e d we show that the absolute m e a s u r e m e n t of Critical D i m e n s i o n is not provided by S,E.M. dedicated to m e t r o l o g i c a l applications. A c a l i b r a t i o n for each layer is needed.
INTRODUCTION The classical optical measurements are difficult for the dimensions b e l o w 1.2 m i c r o n as the line edges are o b v i o u s l y too small to be seen. The light interference resulting from the t r a n s p a r a n c y layers, w h i c h depend on the process falsifies the measurement. A calibration for each level is therefore necessary. Two years ago we bought a S,E,M, dedicated to m e t r o l o g i c a l applications (JEPAS i000 of JEOL) because of the better improvement in the linewidth m e a s u r e m e n t due to the best resolution and the topography information given by secondary electrons (Fig.l, Fig.2). So, can a S,E.M, linewidth m e a s u r e m e n t system be used with no calibration ?
OPTICAL
S.E.M.
PROF ILE
PROFILE
_J w i,I d
!
fig. i*
6
MICRON
4
8 fig. 2*
4 MICRON
EXPERIMENTAL METHODOLOGY The optical m e a s u r e m e n t s require a c a l i b r a t i o n for each type of t e c h n o l o g y layer, Then we use the same m e t h o d with our S.E.M. to check if the c a l i b r a t i o n is n e e d e d [i]. The m e t h o d consists in defining the S.E.M. m e a s u r e m e n t s and the real linewidth and defining the best settings of the S.E.M. The R E A L L I N E W I D T H It can be said that the real llnewldth is the w i d t h measurement for a given height on the topographic profile predetermined by the technologist. This profile is obtained by studying the c r o s s - s e c t l o n of the llne using a S.E.M. (Fig.3). and taking a contour measurement followed by a p r o g r a m to correct the distortion and to compute precise m a g n i f i c a t i o n (Fig.4).
0167-9317/87/$3.50 © 1987, ElsevierSciencePublishers B.V. (North-Holland)
D. Burlet, H. Martin / Use o f SEM for linewidth measurements
680
ACTUAL
PROFILE
CU Z 0 r~ U F-4 E
OD
Fig. 3*
m
a
B
)
2
MICROM
Fig. 4*
RESULTS The relation b e t w e e n S.E.M, and actual width is revealed by the m e a s u r e m e n t of 9 lines from 0.6 to 3 m i c r o n by S.E.M. both by JEPAS and by the previous m e t h o d for each level of t e c h n o l o g y concerned by the measurements. The mean of the difference b e t w e e n these two measurements is called the calibration value and the standard d e v i a t i o n gives the accuracy of the calibration (taking into account the number of m e a s u r e m e n t s and 99% p r o b a b i l i t y of success : Fig.6). The settings must be chosen in order to minimize v a r i a t i o n s in the S.E.M. profile caused by charge effect. The accuracy of each calibration is used to get the best setting.
S.E.M.
REAL-JEPAS C.D.
PROFILE
AUTOMATIC (THRESHOLD)
A U T O M A T I C
M NuA
--
H 03 __I
NJAL
~(CROSS) j
Fig. 5*
e
MANUAL
MICROM MODE with
4
Fig. 6*
8 S. E . M . C . D .
5 BOTTOM
S,E.M.
The easiest m e t h o d of m e a s u r i n g the linewidth is to find on the electronic profile the b e g i n n i n g of the transition caused by the edges of the p a t t e r n (Fig,5). This method, usable only in the manual mode, gives the e s t i m a t e d b o t t o m of the sample. Some layers are detailed b e l o w : Line Substrate Calibration Accuracy(i)
RESIST . . . . . . . . AI WSi2 Poly Si Si3N4 -0.084 -0.018 -0.053 -0.037 -0.049 0.060 0.060 0.040 0.090 0,040
Multi-level AI -0,180 0.060
Line Substrate Calibration Accuracy(i)
AI WSi2 L.T,O. Si02 -0.245 -0,125 0.060 0.060
W Cr -0.210 0.030
M e a n of all our calibrations
Poly O.N.O. SiO2 SiO2 -0.080 -0.233 0.060 0.050 : 0.12
Si3N4 Si02 -0.034 0.030
Standard
Deviation
(Micron) : 0,20
(Fig.7)
D. Burlet, H. Martin / Use o f SEM for linewidth measurements
AUTOMATIC
681
MODE
The JEPAS is used for m e a s u r i n g numerous dices on the wafer. The manucfacturer had developped an threshold m e t h o d wich selects the most plausibles events given by the intersection points b e t w e e n the profile and a t h r e s h o l d level (Fig.5). O b v i o u s l y the m e a s u r e m e n t depends on it. Some layers are detailed below : Line Substrate Calibration Accuracy(T)
RESIST AI 0.059 0.020
Line Substrate Calibration Accuracy(i)
AI LTO -0.122 0.040
" WSi2 0.136 0.120
" Poly 0.130 0.150
WSi2 SiO2 -0.043 0.050
Poly SiO2 0.067 0.025
Mean of all our calibrations
MANUAL
" Si 0.173 0.040
" Si3N4 0.379 0.060
O.N.O. Si3N4 Si02 SiO2 -0.034 -0.020 0.025 0.035
: 0.i0
Standard
MODE
Multi-level AI 0.035 0.040 W Cr -0.066 0.040 Deviation
(Micron) : 0.15
AUTOMATIC
(Fig.8)
MODE
¢--
[email protected] Fig.7
MICRON
@.G
CALIBRATION
-0. G Fig.8
MICRON
@.G
CALIBRATION
The absolute error of an u n c a l i b r a t e d m e a s u r e m e n t can reach -0.60!0.205 in manual mode and +0.i00!0.406 in automatic mode. As an incorrect threshold can produce u n r e a l i s t i c values we have d e v e l o p p e d an automatic inspection which rejects the estimated bad values by taking into account the statistics of the whole results. CONCLUSION The l i n e w i d t h m e a s u r e m e n t by S.E.M. requires individual and precise settings to avoid any bad measurements.
(*) Resist
on WSi2,
1.2 m i c r o n
calibrations
on the mask
[i] H. MARTIN, D. BURLET "Methode de Calibration d'un M i c r o d e n s i t o m e t r e Optique" Le VIDE, les COUCHES MINCES. Societe Francaise du Vide supplement au n u m e r o 233 (Aout Septembre Octobre 1986) p 43-46.