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Measurement of the thermomechanical strain of electronic devices by shearography
MICROELECTRONICS RELIABILITY
PERGAMON
MicroelectronicsReliability40 (2000) 1509-1514 www.elsevier.com/locate/microrel
Measurement of the thermomechanical strain of electronic devices by shearography S. Dilhaire a, S. Jorez a, A. Cornet b, L.D. Patifio Lopez a, W. Claeys a aLaboratoire de CaractOrisation de Composants Electroniques, CPMOH Universit~ de Bordeaux I - 33405 Talence cedex, France bLaboratoire FYA~, Universit~ de Louvain, 1348 Louvain la neuve, Belgium
Abstract
This paper deals with the optical measurement of strain in electronic power devices under nonnai operating conditions. This non-contact and non-invasive method called shearography is based upon speckle interferometry. The technique developed produces images of normal displacement gradient of devices. Numerical processing allows the determination of the surface displacement and its related strain. The main advantages of the measuring tool are to be a simple optical set-up, to be very robust with a good sensitivity and to measure directly strain. Therefore, it is well suited to fit into an industrial environment. © 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The localisation of thermomechanical stress induced by running in power devices, used for example in satellite and automotive electronics, is a crucial issue in terms of reliability. Powerful simulation tools exist, often based upon finite element computer work; they allow a good description of the thermomechanical effects induced in electronic devices. The validity of such simulation work has to be checked as it depends on the physical effects taken into account and upon the thermal and mechanical constant definitely used. Measurements upon real devices can therefore not be passed round.
The purpose of the present work is to propose a novel measuring technique of the thermomechanical strain in electronic devices. It is based upon a measuring method called shearography [1] which is based on speckle interferometry [2-3]. The principle is not new and is applied to study largescale devices (aircraft wings for example). We present here a new optical design allowing stress measurements upon millimetre size devices. Shearing interferometry is based on a special imaging of a device illuminated by coherent light. Two images of the same sample, slightly shifted one with respect to the other, are recorded upon a CCD camera. Each pixel intensity results from interferences between lights coming from two shifted points of the device.
0026-2714/00/$ - see frontmatter. © 2000 ElsevierScienceLtd.All rights reserved. PII: S0026-2714(00)00124-4
1510
S. Dilhaire et al./ Microelectronics Reliability 40 (2000) 1509-1514
Due to the running or to the mechanical stress, the surface of the device moves inducing changes upon the images recorded by the CCD camera. Image processing together with a calibration procedure allows visualising directly the strain of the device as a function of time. Nanometric resolution in surface displacement is easily achieved, indicating how powerful such a technique can be in microelectronics.
2.
A calibration procedure allows to vary, in a controlled way, the phase difference of the two images at the CCD camera and to record the corresponding shearographs. This is done when no action or running is put to the sample. When the sample undergoes deformation, due to running for example, points from the surface move in the z direction. The shearograph evolves and contains information upon the relative Az displacement between points separated by an amount Ax.
Method.
2. 2. Strain localisation. 2.1. Principle of the method.
incident wave Y
Z
The procedure to extract this phase difference information is the following. The intensity distribution before and after the deformation of the sample is recorded and an image process calculates the displacement gradient of the sample in the x direction for example. A second step of the process consists in measuring the gradient in the other direction. Under these conditions, the phase difference Aqb,programmed by computer, is the image of the displacement's derivative in the x direction [6] : 4n (0w(x,y)/Ax AqbX(x,Y)=-~-~ ~
a
image plane Fig. 1: schematic view of the method. Figure 1 shows the principle of the method A sample is illuminated by coherent light from a laser source. An optical system E produces an image of the sample upon a CCD camera. As coherent light is used, the image is characterised by a speckle pattern (intense dots appear due to interference related to the roughness of the surface at light wavelength scale) [4]. To obtain the shearing interferometry, the optical system projects upon the CCD camera two identical images of the sample, slightly shifted (by an amount Ax, see figure 1) one to another. A complete theory of shearography can be found in several books [4-5]. For a better understanding, the 2 sheared images have been put in different plane (see fig. 1). These two sheared images produce a new intensity distribution: the combination of the two images upon the CCD camera produces an interference pattern image where the intensity seen by a given pixel is related to the intensity of two points from the sample (distant by Ax) and to their phase difference Aqb.
(1)
where : - Aqb x is the phase variation between 2 sample dots (distant by Ax) along x-axis. - w is the normal displacement of the sample surface. To illustrate the procedure, a simple situation is now taken for clarity (see fig. 2a-2d). We assume a gaussian shape normal displacement w(x,y) of a square surface. Figure 2a shows a two dimensional map of displacement. Our experimental set-up measures both the x and y component of the displacement gradient, ( 0Wc~x'Y)-) and ( - 0 w ~ Y).). These two gradients are represented in figures 2b and 2c. As the displacement gradient is expressed in Cartesian coordinates, the revolution symmetry is lost. Nevertheless, the modulus of the gradient allows retrieving this symmetry as presented in figure 2d which localises the strongest strained surface. For simplicity, we have not taken into account in this example the gradient component in the z direction
S. Dilhaire et al./ Microelectronics Reliability 40 (2000) 1509-1514
1511
3. Optical set-up
a) l;tl~.IX
b}
c)
IT l =17"x
1 ;'1 ~:iX
The optical set-up we have developed is schematised in figure 3. The optical system E (see fig. 1) is made by a Wollaston prism (W), a sheet polariser (P) and a lens. The coherent light coming from a He-Ne laser illuminates the sample surface. The scattered light goes through the Wollaston which produces 2 sheared images on the CCD camera connected to a computer for image processing. A beam splitter and a mirror M have been added to the set-up to measure the normal surface displacement by ESPI [7-8]. In the shearing configuration, the mirror M is taken off. The Wollaston prism acts upon orthogonal polarisation components of the incoming light from the sample, this creates 2 laterally shifted images on the CCD camera. To make these 2 images to interference, a sheet polarizer after the Wollaston is used. The stability of this set-up is inversely proportional to the distance between the Wollaston and the sheet polarizer. This distance can be made very short. This set-up is more robust than classical shearograph set-ups [4-5] because there is a little room for parasitic noise to influence the interference pattern. Indeed the orthogonal polarisations separated by the Wollaston prism travel nearby over a very short distance before being mixed in the sheet polariser. mirror M i
i
sample l~l ~1"~
[ 7 HeNe laser
splitter
d}
lens
I':I3X
wollaston prism
~
sheet polarizer
lens c o m p u t e ~
CCD camera
Fig. 3 " Optical set-up. Fig. 2a-2d: Simulation of the surface displacement (2a), displacement gradient in the x and y-direction (2b-2c) and gradient modulus (2d)
The biggest advantage of this teclmique is to be a non-contact and non-destructive method. The resolution is of the order of tens of nanometers with a lateral resolution of the order of a few micrometers.
1512
S. Dilhaire et al./ Microelectlvnics Reliability 40 (2000) 1509--1514
This instrument is therefore well suited to study electronic devices. Sample dimensions can typically be of the order of5x5 mm 2. This set-up is also used to perform ESPI (Electronic Speckle Pattern Interferometry). This is another technique we have developed. It allows to measure the out-of-plane displacement of the surface sample (in the z direction). As explained before we have to add mirror M and to take the Wollaston prism off. Actually the shearography technique gives us information about the gradient displacement in both the x and y directions. To reconstruct the total gradient modulus, the information of the displacement gradient in the z-direction is needed. Therefore, with the assumption of constant strain in the z-direction, we deduce the gradient along the z axis from the displacement itself. Strain is the ratio of out-of-plane displacement to sample thickness. So with the combination of shearography and ESPI, (OW-~'Y)-), ( O W ~ Y).1 and (.Ow-~) Y)) , which are gradient displacement in the three directions, are measured. This allows us to calculate the total gradient modulus of the surface displacement : 0w(x, y) 2
Fig. 4: View of the sample. We can see (ba,(x. "~(_..~,y).)in figure 5a switching from positive to negative along the central direction showing 2 regions where the gradient is maximum. These regions reveal great inflection of the surface displacement whereas the middle of the chip depicts the maximum displacement as the derivative is zero. Therefore the deformation due to running bends the transistor surface as a cylindrical dome close to the centre. Wesee ( 0 w ~ y),) in figure 5b to show
and to derive a strain map.
constant values in the y-direction. This indicates a tilt of the transistor along this axis and shows greater on the right hand side than on the left. These interpretations are confirmed by the
4. Results
[t"Ow~, (x y ) j"~image We have studied the strain distribution in a MOS power transistor shown in figure 4. Calibration is made when the transistor is off. When the transistor is switched on with a steady power, thermal effects induce surface bending. The power dissipated by the transistor is 5W. Few seconds after the switching is on, its displacement stabilises. Shearograph acquisition is made continuously by the CCD camera during the entire thermal process. Results shown in figures 5a-c are maps of the gradient displacement in the x-direction (fig. 5a), y-direction (fig. 5b) and z-direction (fig. 5c) after computer processing is done. These results are steady state values.
\
- -
obtained
from
ESPI
/
measurements. ESPI measures the out-of-plane /
displacement
w(x,y).
N
/Ow-~xzY)./ is \ oz j calculated dividing w(x,y) by the thickness of the chip. Results are presented in figure 5c. The modulus of the gradient (see eq. (2)) is represented in figure 5d. This image outlines areas of large strain and identifies qualitatively the loci of stress in the running device. surface
o
E
I
--I
l
m
~°
r~
o
&
N
~..*.
°.
° -...~
l
L
;
/
~J
?
,.j
a%
1514
X Dilhaire el al./ Microelectronics Reliability 40 (2000) 1509-1514
5. Conclusion
We have developed a very powerful optical method to visualise strain in electronic devices. It is a non-contact, non-destructive and fast method. Further work will be on simulation programs allowing translation of our data into strain and stress values. This implies, among others the knowledge of the mechanical constants of the device. It will be the subject of future publication. Besides steady state stress measurements, our method allows following as a function of time the stress built-up or relaxation as each time a CCD picture is taken, a strain map can be produced. With 25 images per second, real time stress relaxation can be done.
References
[1] Hung. "Shearography : a new optical method for strain measurement and non-destructive testing", Optical Engineering, (1982) vol. 21 n°3, pp 391395.
[2] Pfeifer, Mischo, Ettemeyer, "Strain/Stress Measurements using Electronic Speckle Pattern Interferometry", Proceeding of the SPIE, (1998) vol. 3520, pp 262-270. [3] B Sharp, "Electronic Speckle Pattern Interferometry (ESPI)", Optics and lasers in engineering, (1989)vol. 11, pp 241-255. [4] R Jones and C. Wykes. "Holographic and speckle interferometry". Cambridge University Press, Cambridge 1989 [5] T. Kreis. "Holographic Interferometry: Principles and Methods". Berlin, Akademie Verlag, 1996 [6] James and Tatam, "3D shearography for surface strain analysis", proceeding of the SPIE, (1999) vol. 3783, p247-256. 17] Dilhaire, Jorez, Cornet, Schaub, Claeys. "Optical method for the measurement of the thermomechanical behaviour of electronic devices", A4icroelectronics reliability, (1999) vol. 39, pp 981-985. [8] Nassim et al., "Thermomechanical deformation imaging of power devices by ESPI", Microelectronics reliability, (1998) vol. 38, pp 1341-1345.
PERGAMON
MicroelectronicsReliability40 (2000) 1509-1514 www.elsevier.com/locate/microrel
Measurement of the thermomechanical strain of electronic devices by shearography S. Dilhaire a, S. Jorez a, A. Cornet b, L.D. Patifio Lopez a, W. Claeys a aLaboratoire de CaractOrisation de Composants Electroniques, CPMOH Universit~ de Bordeaux I - 33405 Talence cedex, France bLaboratoire FYA~, Universit~ de Louvain, 1348 Louvain la neuve, Belgium
Abstract
This paper deals with the optical measurement of strain in electronic power devices under nonnai operating conditions. This non-contact and non-invasive method called shearography is based upon speckle interferometry. The technique developed produces images of normal displacement gradient of devices. Numerical processing allows the determination of the surface displacement and its related strain. The main advantages of the measuring tool are to be a simple optical set-up, to be very robust with a good sensitivity and to measure directly strain. Therefore, it is well suited to fit into an industrial environment. © 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The localisation of thermomechanical stress induced by running in power devices, used for example in satellite and automotive electronics, is a crucial issue in terms of reliability. Powerful simulation tools exist, often based upon finite element computer work; they allow a good description of the thermomechanical effects induced in electronic devices. The validity of such simulation work has to be checked as it depends on the physical effects taken into account and upon the thermal and mechanical constant definitely used. Measurements upon real devices can therefore not be passed round.
The purpose of the present work is to propose a novel measuring technique of the thermomechanical strain in electronic devices. It is based upon a measuring method called shearography [1] which is based on speckle interferometry [2-3]. The principle is not new and is applied to study largescale devices (aircraft wings for example). We present here a new optical design allowing stress measurements upon millimetre size devices. Shearing interferometry is based on a special imaging of a device illuminated by coherent light. Two images of the same sample, slightly shifted one with respect to the other, are recorded upon a CCD camera. Each pixel intensity results from interferences between lights coming from two shifted points of the device.
0026-2714/00/$ - see frontmatter. © 2000 ElsevierScienceLtd.All rights reserved. PII: S0026-2714(00)00124-4
1510
S. Dilhaire et al./ Microelectronics Reliability 40 (2000) 1509-1514
Due to the running or to the mechanical stress, the surface of the device moves inducing changes upon the images recorded by the CCD camera. Image processing together with a calibration procedure allows visualising directly the strain of the device as a function of time. Nanometric resolution in surface displacement is easily achieved, indicating how powerful such a technique can be in microelectronics.
2.
A calibration procedure allows to vary, in a controlled way, the phase difference of the two images at the CCD camera and to record the corresponding shearographs. This is done when no action or running is put to the sample. When the sample undergoes deformation, due to running for example, points from the surface move in the z direction. The shearograph evolves and contains information upon the relative Az displacement between points separated by an amount Ax.
Method.
2. 2. Strain localisation. 2.1. Principle of the method.
incident wave Y
Z
The procedure to extract this phase difference information is the following. The intensity distribution before and after the deformation of the sample is recorded and an image process calculates the displacement gradient of the sample in the x direction for example. A second step of the process consists in measuring the gradient in the other direction. Under these conditions, the phase difference Aqb,programmed by computer, is the image of the displacement's derivative in the x direction [6] : 4n (0w(x,y)/Ax AqbX(x,Y)=-~-~ ~
a
image plane Fig. 1: schematic view of the method. Figure 1 shows the principle of the method A sample is illuminated by coherent light from a laser source. An optical system E produces an image of the sample upon a CCD camera. As coherent light is used, the image is characterised by a speckle pattern (intense dots appear due to interference related to the roughness of the surface at light wavelength scale) [4]. To obtain the shearing interferometry, the optical system projects upon the CCD camera two identical images of the sample, slightly shifted (by an amount Ax, see figure 1) one to another. A complete theory of shearography can be found in several books [4-5]. For a better understanding, the 2 sheared images have been put in different plane (see fig. 1). These two sheared images produce a new intensity distribution: the combination of the two images upon the CCD camera produces an interference pattern image where the intensity seen by a given pixel is related to the intensity of two points from the sample (distant by Ax) and to their phase difference Aqb.
(1)
where : - Aqb x is the phase variation between 2 sample dots (distant by Ax) along x-axis. - w is the normal displacement of the sample surface. To illustrate the procedure, a simple situation is now taken for clarity (see fig. 2a-2d). We assume a gaussian shape normal displacement w(x,y) of a square surface. Figure 2a shows a two dimensional map of displacement. Our experimental set-up measures both the x and y component of the displacement gradient, ( 0Wc~x'Y)-) and ( - 0 w ~ Y).). These two gradients are represented in figures 2b and 2c. As the displacement gradient is expressed in Cartesian coordinates, the revolution symmetry is lost. Nevertheless, the modulus of the gradient allows retrieving this symmetry as presented in figure 2d which localises the strongest strained surface. For simplicity, we have not taken into account in this example the gradient component in the z direction
S. Dilhaire et al./ Microelectronics Reliability 40 (2000) 1509-1514
1511
3. Optical set-up
a) l;tl~.IX
b}
c)
IT l =17"x
1 ;'1 ~:iX
The optical set-up we have developed is schematised in figure 3. The optical system E (see fig. 1) is made by a Wollaston prism (W), a sheet polariser (P) and a lens. The coherent light coming from a He-Ne laser illuminates the sample surface. The scattered light goes through the Wollaston which produces 2 sheared images on the CCD camera connected to a computer for image processing. A beam splitter and a mirror M have been added to the set-up to measure the normal surface displacement by ESPI [7-8]. In the shearing configuration, the mirror M is taken off. The Wollaston prism acts upon orthogonal polarisation components of the incoming light from the sample, this creates 2 laterally shifted images on the CCD camera. To make these 2 images to interference, a sheet polarizer after the Wollaston is used. The stability of this set-up is inversely proportional to the distance between the Wollaston and the sheet polarizer. This distance can be made very short. This set-up is more robust than classical shearograph set-ups [4-5] because there is a little room for parasitic noise to influence the interference pattern. Indeed the orthogonal polarisations separated by the Wollaston prism travel nearby over a very short distance before being mixed in the sheet polariser. mirror M i
i
sample l~l ~1"~
[ 7 HeNe laser
splitter
d}
lens
I':I3X
wollaston prism
~
sheet polarizer
lens c o m p u t e ~
CCD camera
Fig. 3 " Optical set-up. Fig. 2a-2d: Simulation of the surface displacement (2a), displacement gradient in the x and y-direction (2b-2c) and gradient modulus (2d)
The biggest advantage of this teclmique is to be a non-contact and non-destructive method. The resolution is of the order of tens of nanometers with a lateral resolution of the order of a few micrometers.
1512
S. Dilhaire et al./ Microelectlvnics Reliability 40 (2000) 1509--1514
This instrument is therefore well suited to study electronic devices. Sample dimensions can typically be of the order of5x5 mm 2. This set-up is also used to perform ESPI (Electronic Speckle Pattern Interferometry). This is another technique we have developed. It allows to measure the out-of-plane displacement of the surface sample (in the z direction). As explained before we have to add mirror M and to take the Wollaston prism off. Actually the shearography technique gives us information about the gradient displacement in both the x and y directions. To reconstruct the total gradient modulus, the information of the displacement gradient in the z-direction is needed. Therefore, with the assumption of constant strain in the z-direction, we deduce the gradient along the z axis from the displacement itself. Strain is the ratio of out-of-plane displacement to sample thickness. So with the combination of shearography and ESPI, (OW-~'Y)-), ( O W ~ Y).1 and (.Ow-~) Y)) , which are gradient displacement in the three directions, are measured. This allows us to calculate the total gradient modulus of the surface displacement : 0w(x, y) 2
Fig. 4: View of the sample. We can see (ba,(x. "~(_..~,y).)in figure 5a switching from positive to negative along the central direction showing 2 regions where the gradient is maximum. These regions reveal great inflection of the surface displacement whereas the middle of the chip depicts the maximum displacement as the derivative is zero. Therefore the deformation due to running bends the transistor surface as a cylindrical dome close to the centre. Wesee ( 0 w ~ y),) in figure 5b to show
and to derive a strain map.
constant values in the y-direction. This indicates a tilt of the transistor along this axis and shows greater on the right hand side than on the left. These interpretations are confirmed by the
4. Results
[t"Ow~, (x y ) j"~image We have studied the strain distribution in a MOS power transistor shown in figure 4. Calibration is made when the transistor is off. When the transistor is switched on with a steady power, thermal effects induce surface bending. The power dissipated by the transistor is 5W. Few seconds after the switching is on, its displacement stabilises. Shearograph acquisition is made continuously by the CCD camera during the entire thermal process. Results shown in figures 5a-c are maps of the gradient displacement in the x-direction (fig. 5a), y-direction (fig. 5b) and z-direction (fig. 5c) after computer processing is done. These results are steady state values.
\
- -
obtained
from
ESPI
/
measurements. ESPI measures the out-of-plane /
displacement
w(x,y).
N
/Ow-~xzY)./ is \ oz j calculated dividing w(x,y) by the thickness of the chip. Results are presented in figure 5c. The modulus of the gradient (see eq. (2)) is represented in figure 5d. This image outlines areas of large strain and identifies qualitatively the loci of stress in the running device. surface
o
E
I
--I
l
m
~°
r~
o
&
N
~..*.
°.
° -...~
l
L
;
/
~J
?
,.j
a%
1514
X Dilhaire el al./ Microelectronics Reliability 40 (2000) 1509-1514
5. Conclusion
We have developed a very powerful optical method to visualise strain in electronic devices. It is a non-contact, non-destructive and fast method. Further work will be on simulation programs allowing translation of our data into strain and stress values. This implies, among others the knowledge of the mechanical constants of the device. It will be the subject of future publication. Besides steady state stress measurements, our method allows following as a function of time the stress built-up or relaxation as each time a CCD picture is taken, a strain map can be produced. With 25 images per second, real time stress relaxation can be done.
References
[1] Hung. "Shearography : a new optical method for strain measurement and non-destructive testing", Optical Engineering, (1982) vol. 21 n°3, pp 391395.
[2] Pfeifer, Mischo, Ettemeyer, "Strain/Stress Measurements using Electronic Speckle Pattern Interferometry", Proceeding of the SPIE, (1998) vol. 3520, pp 262-270. [3] B Sharp, "Electronic Speckle Pattern Interferometry (ESPI)", Optics and lasers in engineering, (1989)vol. 11, pp 241-255. [4] R Jones and C. Wykes. "Holographic and speckle interferometry". Cambridge University Press, Cambridge 1989 [5] T. Kreis. "Holographic Interferometry: Principles and Methods". Berlin, Akademie Verlag, 1996 [6] James and Tatam, "3D shearography for surface strain analysis", proceeding of the SPIE, (1999) vol. 3783, p247-256. 17] Dilhaire, Jorez, Cornet, Schaub, Claeys. "Optical method for the measurement of the thermomechanical behaviour of electronic devices", A4icroelectronics reliability, (1999) vol. 39, pp 981-985. [8] Nassim et al., "Thermomechanical deformation imaging of power devices by ESPI", Microelectronics reliability, (1998) vol. 38, pp 1341-1345.