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Simulation of the thermal stress induced by CW 1340 nm laser on 28 nm advanced technologies
MR-12540; No of Pages 6 Microelectronics Reliability xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Simulation of the thermal stress induced by CW 1340 nm laser on 28 nm advanced technologies M. Penzes a,⁎, S. Dudit a, F. Monsieur a, L. Silvestri b, F. Nallet b, D. Lewis c, P. Perdu d,e a
STMicroelectronics, Crolles, France Synopsys Switzerland LLC, 71597 Thurgauerstrasse 40, Switzerland IMS Laboratory, Talence, France d CNES, Toulouse, France e Temasek Lab at NTU, Singapore b c
a r t i c l e
i n f o
Article history: Received 21 May 2017 Received in revised form 21 June 2017 Accepted 5 July 2017 Available online xxxx Keywords: Electrical failure analysis Laser invasiveness Thermal stress simulation 28 nm FDSOI technology
a b s t r a c t Previous study on the invasiveness of the CW 1340 nm laser source used in failure analysis, pinpointed silicide diffusions issue and experimentally defined a safe experimental area. Nevertheless, experimentally defining a safe area is a very long process. So we bypassed it by a new approach based on thermal laser stress modelling for defect localization applications (LVI/OBIRCH, cw-LVP). The first target of this study is the 28 nm FDSOI technologies. The results of this simulation are also compared to experiments to check accordance with the temperatures of material diffusion. The model can be used to define safe and not safe areas of interaction between the laser and the IC (exposure time, laser power). Laser invasiveness issues for different technologies and geometries can also be addressed. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction
2. Laser heating model
During backside failure analysis, laser stimulation or laser probing are key techniques to localize a defect inside VLSI. Mandatory spatial resolution for recent nanoelectronic technology devices is achieved using solid immersion lens (SIL). The high laser power density at focus can create electrical and morphological defects. These degradations have been characterized [1] for a 1340 nm wavelength on 28 nm FDSOI test structure and have clearly demonstrated Ni, Pt and Ti diffusion after laser stress (Fig. 1). These degradations have underlined the need to estimate the acceptable laser dose on advanced technologies. It is related to both the raised local temperature and the duration of stress. The temperature reached at transistor level can be very different in function of the geometry and the technology. Moreover, the exposure time of transistor during LVI or OBIRCH [2,3] depends on the analysis. The range of exposure time is between a few minutes to several hours in the worst case. These parameters become difficult to study experimentally. The proposed study aims at the simulation of the temperature distribution under a CW 1340 nm laser stress in Electrical Failure Analysis (EFA) conditions.
The model strategy is to calculate the absorbed power in a device composed of different mediums. To do this, we have used Sentaurus Device Electromagnetic Wave (EMW) solver which is a simulation module for the numeric analysis of electromagnetic waves. It is a full-wave time-domain simulator based on the finite-difference time-domain (FDTD) method. It takes this absorbed power as input and transforms it in a heat rate to deduce the heat injected by the laser source. Finally, the heat transfer equation is solved [4].
⁎ Corresponding author. E-mail addresses: [email protected] (M. Penzes), [email protected] (L. Silvestri).
2.1. Thermal model The model relies on the well-known heat transfer Eq. 1, where κ, ρ and Cp are the thermal conductivity (W·m−1·K−1), the mass density (kg·m−3), and the specific heat capacity (J·kg−1·K−1), respectively. G (W·m−3) is the power density injected by the source. ρCp ∂T=∂t ¼ ∇½κ∇T þ G
ð1Þ
The spatial distribution of the temperature T (K) in the device as a function of time is computed. It is then possible to plot the temperature as a function of time in particular points.
http://dx.doi.org/10.1016/j.microrel.2017.07.027 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
2
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 1. STEM EDX mapping - 28 nm FDSOI pMOS - (left) reference - (right) stressed by a 1340 nm laser at 18 mW with a 220× SIL.
laser spot, to the focal point, to the substrate thickness through which the laser beam passes and to the wavelength of the laser source.
2.2. Electromagnetic model The heat rate G coming from the laser is calculated with the Eq. 2, where I0 and g represent the given intensity and the normalized profile, respectively. g is directly related to the laser power, to the size of the
G ¼ I0 g
ð2Þ
Fig. 2. 3D representation of the geometry.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
3
Table 1 Thermal properties of materials at 300 K. Material
ρ (g·cm−3)
Cp (J·kg−1·K−1)
κ (W·cm−1·K−1)
Si SiO2 TiN NiSi W Cu
2.33 2.2 5.4 4.02 19.3 8.96
715 703 523 443 132 384.4
1.49 0.02 0.22 0.91 1.74 4.1
To get the laser stress in the entire device, g is calculated by means of the Finite-difference time-domain method. The EMW solver can numerically solve the Maxwell's electromagnetic equations for a given electromagnetic field. By this way, the temporal evolution of the electromagnetic waves in a device can be followed. The result of the EMW solver is the absorbed photon density (cm−3·s−1) for every nodes of the mesh. It is then directly translated in heat rate (K·s−1). 3. Test of the model in EFA conditions 3.1. Geometry The geometry (see Fig. 2) corresponds to a typical 28 nm FDSOI MOS with a TiN and Si-poly gate. The dimensions are the same than in [1]. The interconnections between the gate, the source, the drain and the contacts are in NiSi. Contacts are made of W and the metallization levels 1, 2, 3 and 4 are made of Cu. The pattern of metal layers is the same as the experiments [1]. The experiments show no degradation of the high-κ dielectric material as it is not take into account in the simulation.
Fig. 3. Boundary conditions.
3.2. Material properties The optical properties of materials can change with temperature, but this is not considered here. However, it is possible to use a dispersive model to address the laser stress simulation with different wavelengths. Tables 1 and 2 sum up respectively the thermal and optical properties at 300 K and 1340 nm of the main materials used in the simulation. Polycrystalline and monocrystalline Si are considered to have the same thermal and optical properties. We also do not take into account the doping of silicon. 3.3. Laser beam The model has been tested using experimental EFA conditions as inputs. To address this laser stress, the following parameters are set: – The wavelength is 1340 nm (usual wavelength for LVI or OBIRCH analysis). – The numerical aperture is 2.45.
Table 2 Optical properties of materials at 1340 nm. Material
Refractive index
Extinction coefficient
Si SiO2 TiN NiSi W Cu
3.49 1.45 0.92 0.92 3.14 0.6
≈0 0 4.18 4.18 4.45 9.44
Fig. 4. Cut of the simulated absorbed photon density induced by a CW 1340 nm laser in a 28 nm FDSOI MOS.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
4
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
– The laser spot is Gaussian. The diameter d at mid-level is calculated with Rayleigh's (see Eq. 4) where λ is the wavelength (m) and N.A is the numerical aperture. d equals 330 nm at 1340 nm.
d ¼ 0:61λ=ðN:A:Þ
ð4Þ
– The incident angle with respect to x-axis and to z direction and y-z plane are arbitrary at 0° (the light is perpendicular to the device surface). – The laser power is tested from 1 to 15 mW.
– The laser is focused at the transistor level. – The time of exposition is 128 μs and corresponds to the usual scan speed for LSM analysis (128 μs/pixel).
The laser power is calibrated in parked mode at the centre of the picture without SIL tip by using a standard optical power meter. Then, the value is weighted to take in account the optical properties of the SIL. Consequently, the value corresponds to the laser power at the output of the lens. The no-doped silicon is considered transparent at 1340 nm as we admit the value is also the laser power at transistor level. This is the laser power used in the simulation.
Fig. 5. Cut of the simulated temperature induced by a CW 1340 nm laser inside a 28 nm FDSOI MOS at 128 μs.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
5
3.4. Heat boundary conditions As shown in Fig. 3, the limit at the bottom of the structure is an interface between air and silicon as it a thermal radiation. The radiative flux is calculated with the Stefan-Boltzmann law (see Eq. 5) hbottom ¼ −5:6703:10−12 :ε: T4 –T0 4
ð5Þ
In Eq. 5, ε is the emissivity of silicon, T is the temperature (K) and T0 is the ambient temperature (300K). The heat flux at the top htop is defined with the heat sink transfer equation. The simulation space sides are extended for the heat equation. 4. Results 4.1. Absorbed photon density Fig. 4 shows a 2D spatial scattering of the simulated absorbed photon density in a transistor stressed with CW 1340 nm laser source. This source is parked mode on the transistor. In this geometry, a maximum of 2.4.1024 cm−3·s−1 is found in the contacts. The other layers of metal M1, M2, M3 and M4 absorbed also the electromagnetic wave. 4.2. Heat diffusion The absorbed photon density results in a heating rate directly proportional to the absorbed photon density. Then, the heat spreads through the device. Fig. 5 shows the temperature at 15 mW at different moment of the simulation. At 10 ns, the hottest area (374 K) is located at the metal 1 connected to the gate. At 100 ns, the metal 2 becomes hotter (560 K). Then, the metal 2 heat spreads through the oxide until the transistor which heats up until 1040 K at 10 μs. Finally, at 36 μs the steadystate is reached and the maximum of temperature is located at the contact of the gate to a temperature of 1129 K. Fig. 6 shows the evolution of the maximum temperature in the NiSi gate for a 1 mW, 5 mW and 15 mW laser power. The temperature increases from 300 K (room temperature) to a maximum reached at 36 μs. 5. Discussion As we test this model in different configurations, we find a nonegligible effect of the metallization on the heat. Fig. 7 shows the
Fig. 7. Absorbed photon density and laser heating of the NiSi gate for different configurations of metallization.
absorbed photon density a and the laser heating ΔT (K·mW− 1) in the NiSi gate for different metal layer configurations. The geometries of layers are the same as Fig. 2. We can see when there are only the contacts, α and ΔT are 2.1022 cm−3·s−1 and 0.53 K·mW−1 respectively. Adding more layers, both these values increase. With all the metal layers (or more), α and ΔT tend to a constant of 7.1022 cm−3·s−1 and 1117 K. The copper has a reflexion coefficient of 0.97 at 1340 nm. Consequently, the several reflections on metals increase the laser stress on both the transistor and the metals themselves. It shows that the acceptable laser dose cannot be defined by a laser power but by a temperature threshold which depends highly on the environment of the transistor. For the first time, we can do a link between EFA observations, the simulation and the process. Gregoire et al. explain that during millisecond annealing for silicide formation, NiPt10%Si electrical degrades at 1200 K [5]. They also show that NiPt10%Si is more impacted with Sipoly than with Si-mono. In the Table 3, we can note a difference of 100 K between the simulation and the literature. Several hypotheses can explain this difference: – The simulation shows there is a strong effect of metallization which are not homogeneous in the design chosen for experiments. Then as the limit of 15 mW is arbitrary defined there is an uncertainty on this value due to the laser power step between stresses. – The temperature indicated in the literature corresponds to tests made on NiPtSi layers on wafer for process study. The hot spot at gate level predict by the simulation is in good agreement with the experiments which show damages only in this area (see Fig. 8). 6. Conclusion The interest of using laser annealing model for the simulation of heat generation by a CW 1340 nm laser exposure is demonstrated. The increasing of the temperature can be investigated for different laser beam power, geometry technology and wavelength. Table 3 Comparison of simulation and experiments.
Fig. 6. Temperature in the gate NiSi versus time for 1, 5 and 15 mW of laser power.
Threshold
Simulation
Experiments
Literature [5]
15 mW/1100 K
15 mW
1200 K
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
6
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 8. (Left) Simulated temperature at 15 mW with a hot spot in the gate – (right) TEM picture of MOS stressed at 15 mW.
Especially, the study highlight, a strong dependency of the local temperature in function of the density of metal under laser stress. Without copper layers the temperature increase to a few kelvin while it can reach 1100 K with 4 metal layers. A link with the literature and the experiments shows a good match. NiPt10%Si are degraded at 1200 K while the simulation shows this temperature is reached at about 15 mW. 15 mW is the laser power at which NiPt10%Si diffuses during experiments. The perspectives are the simulation of materials diffusion (NiSi, Pt, TiN) versus the laser power and the exposure time. Then, the shift in transistor; the estimation of electrical characteristics shifts (Vt, Ion, simple device. These results will be also compared to experiments (diffusion profil and nano-probing measurement) to ensure a good calibration of the model.
References [1] M. Penzes, S. Dudit, T. Parrassin, M. Vallet, P. Perdu, D. Lewis, Study of 1340 nm continuous laser invasiveness on 28 nm advanced technologies, ASM International Symposia for Testing and Failure Analysis (2016). [2] F. Beaudoin, Fault Location From the Back Side of the Integrated Circuits(PhD thesis) University of Bordeaux 1, 2002. [3] G. Celi, Study, Applications and Improvements on LVI Technical Faults Encountered in Advanced CMOS Technologies 45 nm and Below, (PhD thesis), University of Bordeaux 1, 2013. [4] SentaurusTM Process User Guide and SentaurusTM Device Electromagnetic Wave Solver User Guide, Version M-2016.12, Synopsys, Inc., 2016. [5] M. Gregoire, R. Beneyton, P. Morin, “Millisecond Annealing for Salicide Formation Challenge of NiSi Agglomeration Free Process” IEEE International Interconnect Technology Conference, 2011.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Simulation of the thermal stress induced by CW 1340 nm laser on 28 nm advanced technologies M. Penzes a,⁎, S. Dudit a, F. Monsieur a, L. Silvestri b, F. Nallet b, D. Lewis c, P. Perdu d,e a
STMicroelectronics, Crolles, France Synopsys Switzerland LLC, 71597 Thurgauerstrasse 40, Switzerland IMS Laboratory, Talence, France d CNES, Toulouse, France e Temasek Lab at NTU, Singapore b c
a r t i c l e
i n f o
Article history: Received 21 May 2017 Received in revised form 21 June 2017 Accepted 5 July 2017 Available online xxxx Keywords: Electrical failure analysis Laser invasiveness Thermal stress simulation 28 nm FDSOI technology
a b s t r a c t Previous study on the invasiveness of the CW 1340 nm laser source used in failure analysis, pinpointed silicide diffusions issue and experimentally defined a safe experimental area. Nevertheless, experimentally defining a safe area is a very long process. So we bypassed it by a new approach based on thermal laser stress modelling for defect localization applications (LVI/OBIRCH, cw-LVP). The first target of this study is the 28 nm FDSOI technologies. The results of this simulation are also compared to experiments to check accordance with the temperatures of material diffusion. The model can be used to define safe and not safe areas of interaction between the laser and the IC (exposure time, laser power). Laser invasiveness issues for different technologies and geometries can also be addressed. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction
2. Laser heating model
During backside failure analysis, laser stimulation or laser probing are key techniques to localize a defect inside VLSI. Mandatory spatial resolution for recent nanoelectronic technology devices is achieved using solid immersion lens (SIL). The high laser power density at focus can create electrical and morphological defects. These degradations have been characterized [1] for a 1340 nm wavelength on 28 nm FDSOI test structure and have clearly demonstrated Ni, Pt and Ti diffusion after laser stress (Fig. 1). These degradations have underlined the need to estimate the acceptable laser dose on advanced technologies. It is related to both the raised local temperature and the duration of stress. The temperature reached at transistor level can be very different in function of the geometry and the technology. Moreover, the exposure time of transistor during LVI or OBIRCH [2,3] depends on the analysis. The range of exposure time is between a few minutes to several hours in the worst case. These parameters become difficult to study experimentally. The proposed study aims at the simulation of the temperature distribution under a CW 1340 nm laser stress in Electrical Failure Analysis (EFA) conditions.
The model strategy is to calculate the absorbed power in a device composed of different mediums. To do this, we have used Sentaurus Device Electromagnetic Wave (EMW) solver which is a simulation module for the numeric analysis of electromagnetic waves. It is a full-wave time-domain simulator based on the finite-difference time-domain (FDTD) method. It takes this absorbed power as input and transforms it in a heat rate to deduce the heat injected by the laser source. Finally, the heat transfer equation is solved [4].
⁎ Corresponding author. E-mail addresses: [email protected] (M. Penzes), [email protected] (L. Silvestri).
2.1. Thermal model The model relies on the well-known heat transfer Eq. 1, where κ, ρ and Cp are the thermal conductivity (W·m−1·K−1), the mass density (kg·m−3), and the specific heat capacity (J·kg−1·K−1), respectively. G (W·m−3) is the power density injected by the source. ρCp ∂T=∂t ¼ ∇½κ∇T þ G
ð1Þ
The spatial distribution of the temperature T (K) in the device as a function of time is computed. It is then possible to plot the temperature as a function of time in particular points.
http://dx.doi.org/10.1016/j.microrel.2017.07.027 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
2
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 1. STEM EDX mapping - 28 nm FDSOI pMOS - (left) reference - (right) stressed by a 1340 nm laser at 18 mW with a 220× SIL.
laser spot, to the focal point, to the substrate thickness through which the laser beam passes and to the wavelength of the laser source.
2.2. Electromagnetic model The heat rate G coming from the laser is calculated with the Eq. 2, where I0 and g represent the given intensity and the normalized profile, respectively. g is directly related to the laser power, to the size of the
G ¼ I0 g
ð2Þ
Fig. 2. 3D representation of the geometry.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
3
Table 1 Thermal properties of materials at 300 K. Material
ρ (g·cm−3)
Cp (J·kg−1·K−1)
κ (W·cm−1·K−1)
Si SiO2 TiN NiSi W Cu
2.33 2.2 5.4 4.02 19.3 8.96
715 703 523 443 132 384.4
1.49 0.02 0.22 0.91 1.74 4.1
To get the laser stress in the entire device, g is calculated by means of the Finite-difference time-domain method. The EMW solver can numerically solve the Maxwell's electromagnetic equations for a given electromagnetic field. By this way, the temporal evolution of the electromagnetic waves in a device can be followed. The result of the EMW solver is the absorbed photon density (cm−3·s−1) for every nodes of the mesh. It is then directly translated in heat rate (K·s−1). 3. Test of the model in EFA conditions 3.1. Geometry The geometry (see Fig. 2) corresponds to a typical 28 nm FDSOI MOS with a TiN and Si-poly gate. The dimensions are the same than in [1]. The interconnections between the gate, the source, the drain and the contacts are in NiSi. Contacts are made of W and the metallization levels 1, 2, 3 and 4 are made of Cu. The pattern of metal layers is the same as the experiments [1]. The experiments show no degradation of the high-κ dielectric material as it is not take into account in the simulation.
Fig. 3. Boundary conditions.
3.2. Material properties The optical properties of materials can change with temperature, but this is not considered here. However, it is possible to use a dispersive model to address the laser stress simulation with different wavelengths. Tables 1 and 2 sum up respectively the thermal and optical properties at 300 K and 1340 nm of the main materials used in the simulation. Polycrystalline and monocrystalline Si are considered to have the same thermal and optical properties. We also do not take into account the doping of silicon. 3.3. Laser beam The model has been tested using experimental EFA conditions as inputs. To address this laser stress, the following parameters are set: – The wavelength is 1340 nm (usual wavelength for LVI or OBIRCH analysis). – The numerical aperture is 2.45.
Table 2 Optical properties of materials at 1340 nm. Material
Refractive index
Extinction coefficient
Si SiO2 TiN NiSi W Cu
3.49 1.45 0.92 0.92 3.14 0.6
≈0 0 4.18 4.18 4.45 9.44
Fig. 4. Cut of the simulated absorbed photon density induced by a CW 1340 nm laser in a 28 nm FDSOI MOS.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
4
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
– The laser spot is Gaussian. The diameter d at mid-level is calculated with Rayleigh's (see Eq. 4) where λ is the wavelength (m) and N.A is the numerical aperture. d equals 330 nm at 1340 nm.
d ¼ 0:61λ=ðN:A:Þ
ð4Þ
– The incident angle with respect to x-axis and to z direction and y-z plane are arbitrary at 0° (the light is perpendicular to the device surface). – The laser power is tested from 1 to 15 mW.
– The laser is focused at the transistor level. – The time of exposition is 128 μs and corresponds to the usual scan speed for LSM analysis (128 μs/pixel).
The laser power is calibrated in parked mode at the centre of the picture without SIL tip by using a standard optical power meter. Then, the value is weighted to take in account the optical properties of the SIL. Consequently, the value corresponds to the laser power at the output of the lens. The no-doped silicon is considered transparent at 1340 nm as we admit the value is also the laser power at transistor level. This is the laser power used in the simulation.
Fig. 5. Cut of the simulated temperature induced by a CW 1340 nm laser inside a 28 nm FDSOI MOS at 128 μs.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
5
3.4. Heat boundary conditions As shown in Fig. 3, the limit at the bottom of the structure is an interface between air and silicon as it a thermal radiation. The radiative flux is calculated with the Stefan-Boltzmann law (see Eq. 5) hbottom ¼ −5:6703:10−12 :ε: T4 –T0 4
ð5Þ
In Eq. 5, ε is the emissivity of silicon, T is the temperature (K) and T0 is the ambient temperature (300K). The heat flux at the top htop is defined with the heat sink transfer equation. The simulation space sides are extended for the heat equation. 4. Results 4.1. Absorbed photon density Fig. 4 shows a 2D spatial scattering of the simulated absorbed photon density in a transistor stressed with CW 1340 nm laser source. This source is parked mode on the transistor. In this geometry, a maximum of 2.4.1024 cm−3·s−1 is found in the contacts. The other layers of metal M1, M2, M3 and M4 absorbed also the electromagnetic wave. 4.2. Heat diffusion The absorbed photon density results in a heating rate directly proportional to the absorbed photon density. Then, the heat spreads through the device. Fig. 5 shows the temperature at 15 mW at different moment of the simulation. At 10 ns, the hottest area (374 K) is located at the metal 1 connected to the gate. At 100 ns, the metal 2 becomes hotter (560 K). Then, the metal 2 heat spreads through the oxide until the transistor which heats up until 1040 K at 10 μs. Finally, at 36 μs the steadystate is reached and the maximum of temperature is located at the contact of the gate to a temperature of 1129 K. Fig. 6 shows the evolution of the maximum temperature in the NiSi gate for a 1 mW, 5 mW and 15 mW laser power. The temperature increases from 300 K (room temperature) to a maximum reached at 36 μs. 5. Discussion As we test this model in different configurations, we find a nonegligible effect of the metallization on the heat. Fig. 7 shows the
Fig. 7. Absorbed photon density and laser heating of the NiSi gate for different configurations of metallization.
absorbed photon density a and the laser heating ΔT (K·mW− 1) in the NiSi gate for different metal layer configurations. The geometries of layers are the same as Fig. 2. We can see when there are only the contacts, α and ΔT are 2.1022 cm−3·s−1 and 0.53 K·mW−1 respectively. Adding more layers, both these values increase. With all the metal layers (or more), α and ΔT tend to a constant of 7.1022 cm−3·s−1 and 1117 K. The copper has a reflexion coefficient of 0.97 at 1340 nm. Consequently, the several reflections on metals increase the laser stress on both the transistor and the metals themselves. It shows that the acceptable laser dose cannot be defined by a laser power but by a temperature threshold which depends highly on the environment of the transistor. For the first time, we can do a link between EFA observations, the simulation and the process. Gregoire et al. explain that during millisecond annealing for silicide formation, NiPt10%Si electrical degrades at 1200 K [5]. They also show that NiPt10%Si is more impacted with Sipoly than with Si-mono. In the Table 3, we can note a difference of 100 K between the simulation and the literature. Several hypotheses can explain this difference: – The simulation shows there is a strong effect of metallization which are not homogeneous in the design chosen for experiments. Then as the limit of 15 mW is arbitrary defined there is an uncertainty on this value due to the laser power step between stresses. – The temperature indicated in the literature corresponds to tests made on NiPtSi layers on wafer for process study. The hot spot at gate level predict by the simulation is in good agreement with the experiments which show damages only in this area (see Fig. 8). 6. Conclusion The interest of using laser annealing model for the simulation of heat generation by a CW 1340 nm laser exposure is demonstrated. The increasing of the temperature can be investigated for different laser beam power, geometry technology and wavelength. Table 3 Comparison of simulation and experiments.
Fig. 6. Temperature in the gate NiSi versus time for 1, 5 and 15 mW of laser power.
Threshold
Simulation
Experiments
Literature [5]
15 mW/1100 K
15 mW
1200 K
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027
6
M. Penzes et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 8. (Left) Simulated temperature at 15 mW with a hot spot in the gate – (right) TEM picture of MOS stressed at 15 mW.
Especially, the study highlight, a strong dependency of the local temperature in function of the density of metal under laser stress. Without copper layers the temperature increase to a few kelvin while it can reach 1100 K with 4 metal layers. A link with the literature and the experiments shows a good match. NiPt10%Si are degraded at 1200 K while the simulation shows this temperature is reached at about 15 mW. 15 mW is the laser power at which NiPt10%Si diffuses during experiments. The perspectives are the simulation of materials diffusion (NiSi, Pt, TiN) versus the laser power and the exposure time. Then, the shift in transistor; the estimation of electrical characteristics shifts (Vt, Ion, simple device. These results will be also compared to experiments (diffusion profil and nano-probing measurement) to ensure a good calibration of the model.
References [1] M. Penzes, S. Dudit, T. Parrassin, M. Vallet, P. Perdu, D. Lewis, Study of 1340 nm continuous laser invasiveness on 28 nm advanced technologies, ASM International Symposia for Testing and Failure Analysis (2016). [2] F. Beaudoin, Fault Location From the Back Side of the Integrated Circuits(PhD thesis) University of Bordeaux 1, 2002. [3] G. Celi, Study, Applications and Improvements on LVI Technical Faults Encountered in Advanced CMOS Technologies 45 nm and Below, (PhD thesis), University of Bordeaux 1, 2013. [4] SentaurusTM Process User Guide and SentaurusTM Device Electromagnetic Wave Solver User Guide, Version M-2016.12, Synopsys, Inc., 2016. [5] M. Gregoire, R. Beneyton, P. Morin, “Millisecond Annealing for Salicide Formation Challenge of NiSi Agglomeration Free Process” IEEE International Interconnect Technology Conference, 2011.
Please cite this article as: M. Penzes, et al., Simulation of the thermal stress induced by CW 1340nm laser on 28nm advanced technologies, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.07.027