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Mechanical stability of airgaps in nano-interconnects
Microelectronics Reliability 107 (2020) 113597
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Mechanical stability of airgaps in nano-interconnects ⁎
T
K. Vanstreels , H. Zahedmanesh, M. Gonzalez imec, Kapeldreef 75, 3001 Leuven, Belgium
A B S T R A C T
In this study the impact of airgaps on the mechanical stability of advanced nano-interconnects is investigated with the focus on chip package interaction related issues. To this aim, the shear microprobing technique is used in combination with post-mortem failure analysis. This study reveals that in the absence of airgaps, the failure is located at the interface where a large elasticity mismatch exists. Furthermore, it was found that the via density and the via distribution across the back-endof-line (BEOL) have a clear impact on the force to initiate and grow cracks. The introduction of airgaps into nano-interconnects leads to a shift in the fracture location towards the interface where the airgaps are located, while the relative changes in the amount of failures are explained in terms of the via density and the specific location and geometry of the airgap structures. This work may serve as a guideline to further enhance the mechanical integrity of BEOL airgap technology for future nodes.
1. Introduction For many years, the semiconductor industry has improved the performance of microelectronic devices and their functionality through the continuous scaling of integrated circuits and by maximizing the transistor density [1–2]. This encouraged the search for new materials, processes, packaging strategies and chip designs and introducing them into the final products. In this context, the implementation of ultralowk dielectrics has become essential for future technology nodes. During the last two decades, various materials and methods were developed for fabricating low-k dielectric films, among which the plasma enhanced chemical vapor deposition (PECVD) technology of porous organosilicate glasses (OSG) is the most popular due to its better compatibility with the technological needs [3–6]. The reduction in k for low-k dielectrics is being pursued through the introduction of controlled levels of porosity. However, finding a good low-k material has proven to be more challenging than first expected due to several integration and reliability issues [1]. Besides the need of being compatible with the different lithography, etching, stripping and cleaning processes that are used in state-of-the-art integration schemes, they must also have sufficient mechanical strength to withstand the high shear stresses as well as harsh chemical environments that are involved during the chemical mechanical polishing process without cohesive or adhesive failures occurring [7–13]. On top of that, since low-k dielectrics exhibit intrinsic tensile stresses and have increased coefficients of thermal expansion compared to SiO2, thin film cracking and delamination are serious thermal-mechanical reliability issues for low-k dielectric materials [14–15]. Replacing ultralow-k dielectrics by air is a viable alternative for future technology nodes, since airgaps are not only less prone to
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Corresponding author. E-mail address: [email protected] (K. Vanstreels).
https://doi.org/10.1016/j.microrel.2020.113597 Received 17 December 2019; Accepted 11 February 2020 0026-2714/ © 2020 Elsevier Ltd. All rights reserved.
integration issues such as plasma damage, but they also enable extremely low capacitances since the permittivity of air is close to 1. However, airgaps confront serious challenges concerning their structural integrity and mechanical stability. This paper presents the impact of airgaps on the mechanical integrity of advanced nano-interconnects with the focus on chip package interaction related issues. To this end, the shear microprobing technique is employed in combination with post-mortem failure analysis and crack inspections. 2. Experimental details A dedicated test chip design was used to study the impact of metal density (particularly in the via layers) and airgap density on the mechanical integrity of the BEOL. The test chip is a passive test chip, which means that there are no active devices (transistors) present. For a more detailed description of the test chip, we refer to [16–17]. Briefly, the BEOL architecture consists of four Cu metal layers (M1-M4) processed on top of a dummy Intermetal (IM) tungsten layer, where Metal 1 (M1) and Metal 2 (M2) are tight 90 nm pitch metals integrated in a porous low-k dielectric with a dielectric constant of 2.4. Metal 3 (M3) is an intermediate layer with 2× larger dimensions than M1 and M2 and is integrated in a non-porous low-k dielectric with a dielectric constant of 3.2. Metal 4 (M4) is the last metal layer with dimensions which are 12× the minimum features and is integrated in SiO2. On top of the BEOL, an 825 nm thick passivation (25 nm SiCN, 300 nm SiO2 and 500 nm SiN) and a 50 μm high copper pillar with a diameter of 50 μm is deposited. A typical cross section of the BEOL stack is shown in Fig. 1a. The metal layers are interconnected with four Cu via layers (V0–V3), where the vias are small volumes of metal
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Fig. 1. (a) Schematic overview of the test chip layer build-up and cross-section; (b) cross-section transmission electron microscopy image of the airgap morphology.
required in-plane shear direction. The open frame design and large X, Y and Z travel range of the stage (X: 370 mm, Y: 168 mm and Z: 168 mm) offer the capability to handle the widest range of sample sizes. Fig. 3b shows the mode of operation of the shear microprobing tests. First, the shear probe is engaged with a low contact force at several microns before the copper bump, defined as the start position. After this, the shear probe is retracted to a predefined shear height above the surface of the silicon chip (Zshear), which is kept constant during the shear microprobing test. Next, the test continues with a lateral movement of the sample until the shear probe touches the sidewall of the pillar and finally applies shear forces until fracture is induced in the area of tensile stress close to the bottom of the copper bump. The data acquisition rate is very high so that sudden crack events can be captured in the force signals, Fxy(t), Fz(t), respectively. A parametric study was performed to determine the optimal shear velocity and shear height that maximized the chances of BEOL fracture, i.e. 10 μm/s shear velocity and a shear height at 50% of the bump diameter. In total, 3 different wafers were tested (1 wafer without airgaps and 2 wafers with airgaps). For each wafer, a particular cell with certain via and airgap density was always measured 7 times from the center towards the edge of the wafer. Per cell, 16 copper bumps were selected at the edge of the cell for shear tests, resulting in a total of 112 tests per cell on each wafer.
vertically connecting long metal lines. The density of the four metals is kept constant at approximately 50%, while the number of the vias in different BEOL layers is varied to achieve the targeted density. To study the impact of via density on the mechanical stability of the BEOL, the densities of V0, V1 and V2 are varied, resulting in different BEOL architectures. The density of V0 is varied from 0.26%–4.15%, V1 from 0.26%–3.11% and V2 from 0.35%–3.46%. These are the typical ranges that can be found in contemporary devices. More detailed information about the via distribution within the chip can be found in [16]. In addition to different via densities, the test chip is designed to include airgaps, specifically located in the M1 layer, as illustrated on Fig. 1b. From this figure, the shape and dimensions with respect to the line dimensions are indicated. The airgaps are about 30 nm deeper with respect to the bottom of the copper lines, while the airgap height is 20 nm larger compared to the line height. It is found by Zahedmanesh and co-workers that when the airgaps are implemented with a stiff dielectric liner, as in this case, deeper airgaps with respect to the bottom of the M1 lines result in lower energy release rates, i.e. higher toughness of the BEOL [18]. On the other hand, it was found that increasing the airgap height versus line height increases the energy release rate under a given CPI stress, resulting in a lower toughness of the BEOL [18]. Therefore, the most vulnerable location in the BEOL when including airgaps is expected to be between the M1 and M2 layer near the top of the airgap structures. Two complementing masks are used to determine the impact of airgap density and airgap morphology on the mechanical integrity of the BEOL. Mask I varies the airgap density between 5 and 20%, while Mask II allows to vary the airgap density between 5 and 52.5% and mainly explores the impact of airgap morphology and the maximum possible airgap density on the mechanical integrity of the BEOL, while the via distribution within the tile remains unchanged. Fig. 2 shows an example of a BEOL with V0, V1 and V2 density of 0.28%, 1.21% and 1.38%, respectively, and a different airgap morphology for 10% of airgap density. Shear microprobing tests are performed on 15 cm × 15 cm coupons of fully integrated 300 mm wafers using a commercial Condor Sigma shear tool from XYZTEC equipped with a 200 μm wide shear tip (Fig. 3a). The Condor Sigma shear tool can perform shear tests with a submicron positioning accuracy and can apply shear forces up 9.81 N with an accuracy of 0.075%. The shear probe can be rotated with an accuracy of 1° into any preferred direction. In this way, the normal force (Fz) and the net in-plane force (Fxy) can be measured for any
3. Results and discussion Two different failure modes were observed after shear testing, i.e. BEOL fracture event or bump shear-off without opening of the BEOL. Fig. 4 illustrates the typical lateral force versus lateral displacement curves for the two observed failure modes. In both cases, the lateral force, Fxy, is normalized to the maximum lateral force, Fxy, max, while the lateral displacement, dxy, is normalized to the bump diameter, dbump. In case of BEOL fracture, the lateral force versus lateral displacement curve is composed out of three major regions: an initial region of almost perfect linearity (region 1), a second region of sudden lateral force relaxation (region 2) and a third region of damage evolution (region 3). In case of bump shear off, no sudden lateral force relaxation is observed during the shear test. The lateral force first increases linearly towards a maximum, after which it gradually decreases until the entire bump is sheared off. As shown on Fig. 4, the initial region is almost perfectly linear, it exhibits a large amount of plastic deformation, which may arise from 2
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Fig. 2. Example of different airgap morphology for the same airgap density (10%) in case of a BEOL with V0, V1 and V2 density respectively 0.28%, 1.21% and 1.38%.
surrounding region. Fig. 5a shows the percentage of BEOL fracture events, FOBEOL, as a function of the V1 and V2 density, while keeping the V0 density constant at 0.26%. Clearly, FOBEOL decreases with increasing V1 and V2 density. This agrees with the work of Zahedmanesh et al., who found that a larger via density increases the effective critical energy release rate of the via layer, therefore leading to better mechanical integrity of the BEOL [19–20]. However, although this figure already gives a first glance of the impact of via density on the mechanical integrity of the
the plastic deformation of the copper bump and the plasticity of metal layers under the bump. Since the BEOL below the bump is constrained by the surrounding material, the stress continues to build up during the initial linear region until a critical amount of stress has built up, enough to free the bump from the surrounding constraint. This sudden removal of constraint causes one end of the bump to pop out, leading to a sharp drop in stress. From this point on, the measured lateral force reflects the crack propagation across the area underneath the bump and in the
Fig. 3. (a) XYZTEC Condor Sigma setup with Resolving Measurement Unit, stage, light microscope and 200 μm shear tip; (b) mode of operation for shear microprobing. 3
Microelectronics Reliability 107 (2020) 113597
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Fig. 4. (a) Typical lateral force versus lateral displacement curves for the two observed failure modes; (b) post-mortem SEM image of a BEOL fracture event; (c) postmortem SEM image of a bump shear off event.
estimated by conducting a series of finite element simulations of a pull (mode I) test on the BEOL for each V0, V1 and V2 combination [16–17]. Simulating a pure mode I test on the BEOL is a good estimate since during shear microprobing a BEOL fracture is induced in the area of tensile stress close to the bottom of the copper bump. It must be noted that due to the bending moment of the bump when pushed laterally, there is also a contribution of normal forces (Fz), which in turn will also contribute to the stress levels that are found at the different via levels. By applying a 100 MPa tensile load to the BEOL tile, the change in stress in V0, V1 and V2 with respect to the BEOL architecture with V0 and V1 density equal to 0.26% and V2 density equal to 1.38%, can be calculated for different combinations of V0, V1 and V2 density. It is found that by changing the V1 density from 0.26% to 1.21%, while keeping V0 and V2 density constant at respectively 0.26% and 1.38%, no significant change in stress and FOBEOL was observed (Table 1). In contrast, when decreasing the V2 density from 1.38% to 0.35%, while keeping the V0 and V1 density constant at respectively 0.26% and 0.26%, a significant increase in FOBEOL is found. This can be explained in terms of the mean stress that is found at the observed fracture location, i.e. between M2-M3 near the V2 layers. By changing the V2 density, the biggest stress change in the V2 layer is found, resulting in a large increase in FOBEOL. On the other hand, by changing the surrounding via densities, the stress in the V2 layer can also be changed, which in turn will have an impact on FOBEOL. It must be noted that the conclusions that are made based on Table 1 are unchanged by changing the magnitude of the applied stress. It is worthwhile mentioning that the via density also seem to have an impact on the distribution of the critical force for BEOL fracture upon shear microprobing (Fig. 7a). When comparing two extreme cases of V1 and V2 density, it is found that higher V1 and V2 density (while keeping V0 density constant) results in a tighter distribution of the critical forces for BEOL fracture, compared to structures with a lower V1 and V2 density. This can be explained in terms of the spatial distribution of the V1 and V2 in the BEOL architectures and is confirmed by Zahedmanesh et al., who found that uniformly distributed vias reduce the energy release rates in the dielectric cracks more effectively than clumped vias [19–20]. Based on these observations, a schematic overview of the impact of via density and via distribution on the mechanical strength of the BEOL is shown on Fig. 7b. As shown, a certain change of BEOL strength leads to a higher fraction of BEOL failures when the via density is higher because it has a tighter distribution of the BEOL strength compared to lower via
Fig. 5. percentage of BEOL fracture events as a function of V1 and V2 density.
BEOL, it does not give information about which of these via densities is more dominant. To investigate this more in detail, one first must look to the fracture location after performing shear microprobing tests. Fig. 6a–c shows the fracture surface in case of a BEOL fracture after performing a shear microprobing test. The shear direction is indicated with a white arrow. By comparing high resolution top down SEM images of the fracture surface with the specific design underneath the copper bump, it was found that for all combinations of V0, V1 and V2 density, the failure location is mainly found at the M2-M3 interface. The failure location is expected at this location due to the large elasticity mismatch that is found at the M2-M3 interface and the rigidity of the upper layers, as demonstrated by Zahedmanesh and co-workers [19–20]. They found that via layers adjacent to an interface with a large elasticity mismatch endure a higher energy release rate and thus are most vulnerable to delamination. The fact that the failure location is found at the M2-M3 interface also suggests that FOBEOL is expected to be more sensitive to changes in V2 density compared to changes in V0 and V1 density. To further support this, the stresses that are found at the different via levels were 4
Microelectronics Reliability 107 (2020) 113597
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Fig. 6. (a) Typical fracture location of a typical fracture surface after shear microprobing; (b) high resolution SEM image of region A on panel a; (c) high resolution SEM image of region B on panel a; (d) typical fracture location of a typical fracture surface after shear microprobing BEOL structures with airgaps; (e) high resolution SEM image of region A on panel d; (f) high resolution SEM image of region B on panel d.
microprobing (shear direction is indicated with a white arrow) when airgaps are introduced into the BEOL stack. By comparing high resolution top down SEM images of the facture surface and compare it with the specific design of the BEOL that is located underneath the sheared copper bump, it was found that for BEOL architectures with airgaps the failure location was mainly found at the M1-M2 interface for all combinations of V0, V1 and V2 density. This failure location is different from the case without airgaps, where the failure location was determined at the M2-M3 interface (Fig. 6a–c). Potential reason for this shift in failure location towards the lower layers in the BEOL stack can be found when looking to the specific geometry and location of the airgaps as shown in Fig. 1b. The airgaps are located at the M1 layer, while in terms of geometry, the airgap height is 20 nm larger with respect to the liner at the side of the M1 lines. Moreover, the airgaps also extend about 30 nm towards the IM layers. As was demonstrated by Houman Zahedmanesh et al., deeper airgaps, when airgaps are implemented with a stiff dielectric barrier such as SiCN and the interlayer dielectric is a soft dielectric, such as in this case, result in lower energy release rates, i.e. higher toughness of the BEOL [18]. Moreover, it was shown that increasing the airgap height versus line height increases the
Table 1 Stress change in V0, V1, V2 and FOBEOL for different combinations of via density. V0, %
V1, %
V2, %
Stress change in V0, %
Stress change in V1, %
Stress change in V2, %
FOBEOL, %
0.26 0.26 1.56 0.26 0.26
0.26 1.21 0.26 0.26 3.11
1.38 1.38 1.38 0.35 3.46
0 −0.90 −1.56 3.12 −0.41
0 −2.86 −1.93 2.21 −11.33
0 0.35 2.09 11.50 −4.00
39.84% 42.86% 54.46% 66.07% 24.11%
densities. To study the impact of introducing airgaps to the BEOL architecture on the integrity of the BEOL, shear microprobing tests were conducted on two different wafers with two different airgap masks (Fig. 2), after which the failure locations were inspected by top down SEM imaging and compared to the specific design of the investigated structures. It was found that the introduction of airgaps into the BEOL stack clearly has a strong impact on the failure location after shear microprobing. Fig. 6d–f shows the typical fracture surface after shear 5
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the surrounding via densities (V0 and V2) in the BEOL stack, as was demonstrated in the previous section. Fig. 8 shows the FOBEOL as a function of the airgap density for different combinations of V0, V1 and V2 density. For the case without airgaps, the differences that are found in FOBEOL between BEOL stacks with different combinations of V0, V1 and V2 can be understood in terms of the actual stress that is applied to the V2 layer, as explained in previous section. On the other hand, the introduction of airgaps clearly has a detrimental impact on the FOBEOL. To understand this, first the impact of airgaps on the effective adhesion energy of the via layer (Gc-effvialayer) for a specific via density and airgap density is estimated based on the work of Zahedmanesh et al. [20]. By adapting the approach of Martson et al. which was originally developed for uniaxially reinforced composite materials [21], the effective adhesion energy of the via layer above airgaps when the airgaps protrude into the via layer and with uniformly distributed vias, can be expressed as
Gcvialayer = Vv Gcvia + (1 − Vv − VAG ) Gcdielectric + − eff
4Vv lpull − out d
Gcpull − out
where Gcvia, by assuming delamination at the ULK/dielectric barrier interface and bottom of the vias, is the adhesion energy at the bottom of vias, Vv is the volume fraction of vias in the via layer, d is the diameter of the via (Vv and d are not independent), lpull-out is the length of via pulled out of the dielectric barrier and Gcpull-out is the adhesion energy at the pull-out interface i.e. via/dielectric barrier. Assuming “ideally brittle” interfaces for the dielectric/SiCN interface, Gcdielectric = 3.7 J/ m2, for the TaNTa/SiCN interface, Gcpull-out = 14 J/m2 and for the TaNTa/Cu, Gcvia = 40 J/m2, the thickness of the dielectric barrier, lpullout = 5 nm and via diameter, d = 45 nm [19]. Results of the calculations are shown in Table 2. Clearly, the impact of introducing 10% of airgap density reduces the Gc-effvialayer with about 9.7% and 7.4% for respectively low and high via density. This means that by introducing airgaps in the BEOL stack, the BEOL strength curve would shift towards lower strength, hence leading to higher amount of failures when applying the same global stress to the BEOL (Fig. 8). To investigate the net impact of the introduction of airgaps on the mechanical strength of a specific BEOL stack of certain V0, V1 and V2 via density, and in order to be able to anticipate the influence of V1 density and the surrounding via densities (V0 and V2), one can define ΔFOBEOLAG as the relative change in FOBEOL due to the introduction of airgaps,
Fig. 7. (a) BEOL failure force distribution after shear microprobing as a function of V1 and V2 density and the corresponding spatial distribution of V0, V1 and V2; (b) schematic overview of the BEOL stack strength as a function of the cumulative % of failures.
ΔFOBEOL AG = (FOBEOL AG–FOBEOL no AG)/FOBEOL AG Table 3 shows ΔFOBEOLAG for different combinations of V0, V1, V2 and airgap (AG) density. From this table one can conclude that increasing the V1 density while keeping V0 and V2 density constant leads to a larger ΔFOBEOLAG, i.e. a larger relative difference in BEOL failure occurrence between interconnects with and without airgaps. This can be explained by the fact that on the one hand, a higher V1 density results in a more uniform distribution of the vias, which in turn leads to a tighter distribution of the BEOL strength compared to lower via densities. On the other hand, from Table 3, it is found that a higher via density has no significant impact on the reduction of Gc-effvialayer when introducing 10% of airgap density. Therefore, also a similar shift in the BEOL strength curve towards lower strength values is expected for both BEOL architectures with high and low via densities (Fig. 9). Table 2 Effective adhesion energy of the via layer for different combinations of via and airgap density.
Fig. 8. FOBEOL as a function of the airgap density for different combinations of V0, V1 and V2 density.
energy release rate, resulting in lower toughness of the BEOL. Therefore, the fracture location is expected between the M1 and M2 layer near the top of the airgap structures and FOBEOL is expected to be sensitive to the V1 density. However, FOBEOL can also be influenced by
6
Vv, %
VAG, %
Gcvia, J/m2
Gcdielectric, J/m2
Gc-effvialayer, J/m2
0.3 0.3 3 3
0 10 0 10
40 40 40 40
3.7 3.7 3.7 3.7
3.828 3.458 4.976 4.606
ΔGc-effvialayer
–9.7% –7.4%
Microelectronics Reliability 107 (2020) 113597
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Table 3 ΔFOBEOLAG for different combinations of V0, V1, V2 and airgap (AG) density. V0, %
V1, %
V2, %
AG, %
ΔFOBEOLAG
Comment
0.26 0.26 0.26 1.56 0.26 0.26 0.26 0.26 0.26 4.15 0.26 4.15
0.26 1.21 0.26 0.26 0.26 0.26 0.26 3.11 0.26 3.11 0.26 0.26
1.38 1.38 1.38 1.38 0.35 1.38 0.35 3.46 0.35 0.35 0.35 3.46
20 10 52.5 52.5 52.5 52.5 5 10 5 10 52.5 52.5
0.636 0.646 0.837 0.705 0.148 0.837 0.045 0.963 0.045 0.002 0.148 0.778
Impact of V1 density Impact of V0 density Impact of V2 density Impact of V1 and V2 density Impact of V0 and V1 density Impact of V0 and V2 density
When applying the same global stress to the BEOL in both cases, one can therefore expect that the introduction of airgaps results in a bigger increase in failures for structures which have a higher V1 density, as observed. To see the impact of the other via densities V0 and V2 on the relative change in FOBEOL due to the introduction of airgaps, one can first compare the case where V0 density is changed, while keeping the V1 and V2 density constant. It is found that a higher V0 density leads to a lower relative change in FOBEOL. Since the failure location is expected between the M1 and M2 layer near the top of the airgap structures, replacing the lower layers near the bottom of the airgaps by more robust layers (higher V0 density) would reduce the stress intensity factor at the failure location [22], which in turn would lead to lower failure rates, hence less impact of introducing airgaps. In contrast, by increasing the V2 density, while keeping the V0 and V1 density constant, a much higher relative change in FOBEOL is observed. This can be explained by the specific location and geometry of the airgaps and the fact that by reinforcing the layers above this failure location in the BEOL (by increasing the V2 density) under the displacement controlled bump shear tests condition, this would increase the local stress that is applied to the failure location, which in turn would lead to larger failure rates. These observations were confirmed in cases where V1 and V2, V0 and V1 and V0 and V2 are changed together. The impact of the airgap distribution on the BEOL strength is shown on Fig. 10, where FOBEOLAG for two different airgap distributions (Fig. 2) are compared for BEOL structures with low via densities and high via densities. In both cases, it is found that when the airgaps are more uniformly distributed (Airgap Mask I, Fig. 2), FOBEOLAG is lower compared to the case where the airgaps are more clumped (Airgap Mask II, Fig. 2).
Fig. 10. Comparison of ΔFOBEOLAG for two different morphologies of airgaps. Both the case of a BEOL architecture with low and high via densities are compared.
4. Conclusions The impact of airgaps on the mechanical strength of advanced nanointerconnects was investigated using shear microprobing in combination with post-mortem failure analysis and crack inspections. In absence of airgaps, the failure location was found at the interface where a large elasticity mismatch was found. Moreover, it was found that the amount of BEOL failures scales with the stress that is applied to the vias near the fracture location. The latter is mainly influenced by the via density but can also be influenced by changing the via densities of the surrounding layers or by changing the type of integrated dielectric materials. Moreover, it was shown that the via density and the via distribution across the nano-interconnects have a clear impact on the force to initiate and grow cracks inside the nano-interconnects. The introduction of airgaps leads to a shift in the fracture location towards the interface where the airgaps are located, while the relative changes that are found in the number of BEOL failures can be explained in terms of the via density and the specific location and geometry of the airgap structures. Finally, it was found that more uniformly distributed airgaps in nanointerconnects lead to a lower amount of BEOL failures. This work may serve as a guideline to further enhance the mechanical integrity of airgaps in advanced nano-interconnects for future technology nodes.
Fig. 9. (a) Schematic overview of the BEOL stack strength as a function of the cumulative % of failures in the case of BEOL stacks with or without airgaps; (b) schematic overview of the BEOL stack strength as a function of the cumulative % of failures in the case of BEOL stacks with different V1 density before and after introducing airgaps. 7
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Declaration of competing interest
[10] K. Vanstreels, H. Li, J. Vlassak, Advanced Interconnects for ULSI Technology, Wiley, 2012, pp. 339–368. [11] K. Vanstreels, C. Wu, M.R. Baklanov, ECS Journal of Solid State Science and Technology 4 (1) (2015) N3058–N3064. [12] K. Vanstreels, M. Krishtab, L. Garcia Gonzalez, S. Armini, Appl. Phys. Lett. 111 (2017) 161906. [13] K. Vanstreels, I. De Wolf, H. Zahedmanesh, H. Bender, M. Gonzalez, J. Lefebvre, S. Bhowmick, Appl. Phys. Lett. 105 (2014) 213102. [14] T.-S. Kim, R.H. Dauskardt, IEEE Trans. Device Mater. Reliab. 9 (4) (2009) 509. [15] G.T. Wang, P.S. Ho, S. Groothuis, Microelectron. Reliab. 45 (7–8) (2005) 1079. [16] L. Kljucar, M. Gonzalez, I. De Wolf, K. Croes, J. Boemmels, Z. Tokei, Microelectron, Rel. 56 (2016) 93–100. [17] L. Kljucar, M. Gonzalez, K. Croes, I. De Wolf, G. Murdoch, J. De Vos, J. Boemmels, E. Beyne, Z. Tokei, IEEE Electronic Components and Technology Conference, Las Vegas, (2016). [18] H. Zahedmanesh, M. Gonzalez, I. Ciofi, K. Croes, T.Z. Bömmels, Microelectr. Reliability 59 (2016) 102–107. [19] H. Zahedmanesh, K. Vanstreels, Q. Toan Le, P. Verdonck, M. Gonzalez, Theor. Appl. Fract. Mech. 95 (2018) 194–207. [20] H. Zahedmanesh, K. Vanstreels, Micro and Nano Engineering 2 (2019) 35–40. [21] T.U. Marston, A.G. Atkins, D.K. Febeck, J. Mat. Sci. 9 (1974) 447–455. [22] H. Zahedmanesh, K. Vanstreels, M. Gonzalez, Microelectron. Eng. 156 (2016) 108–115.
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Mechanical stability of airgaps in nano-interconnects ⁎
T
K. Vanstreels , H. Zahedmanesh, M. Gonzalez imec, Kapeldreef 75, 3001 Leuven, Belgium
A B S T R A C T
In this study the impact of airgaps on the mechanical stability of advanced nano-interconnects is investigated with the focus on chip package interaction related issues. To this aim, the shear microprobing technique is used in combination with post-mortem failure analysis. This study reveals that in the absence of airgaps, the failure is located at the interface where a large elasticity mismatch exists. Furthermore, it was found that the via density and the via distribution across the back-endof-line (BEOL) have a clear impact on the force to initiate and grow cracks. The introduction of airgaps into nano-interconnects leads to a shift in the fracture location towards the interface where the airgaps are located, while the relative changes in the amount of failures are explained in terms of the via density and the specific location and geometry of the airgap structures. This work may serve as a guideline to further enhance the mechanical integrity of BEOL airgap technology for future nodes.
1. Introduction For many years, the semiconductor industry has improved the performance of microelectronic devices and their functionality through the continuous scaling of integrated circuits and by maximizing the transistor density [1–2]. This encouraged the search for new materials, processes, packaging strategies and chip designs and introducing them into the final products. In this context, the implementation of ultralowk dielectrics has become essential for future technology nodes. During the last two decades, various materials and methods were developed for fabricating low-k dielectric films, among which the plasma enhanced chemical vapor deposition (PECVD) technology of porous organosilicate glasses (OSG) is the most popular due to its better compatibility with the technological needs [3–6]. The reduction in k for low-k dielectrics is being pursued through the introduction of controlled levels of porosity. However, finding a good low-k material has proven to be more challenging than first expected due to several integration and reliability issues [1]. Besides the need of being compatible with the different lithography, etching, stripping and cleaning processes that are used in state-of-the-art integration schemes, they must also have sufficient mechanical strength to withstand the high shear stresses as well as harsh chemical environments that are involved during the chemical mechanical polishing process without cohesive or adhesive failures occurring [7–13]. On top of that, since low-k dielectrics exhibit intrinsic tensile stresses and have increased coefficients of thermal expansion compared to SiO2, thin film cracking and delamination are serious thermal-mechanical reliability issues for low-k dielectric materials [14–15]. Replacing ultralow-k dielectrics by air is a viable alternative for future technology nodes, since airgaps are not only less prone to
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Corresponding author. E-mail address: [email protected] (K. Vanstreels).
https://doi.org/10.1016/j.microrel.2020.113597 Received 17 December 2019; Accepted 11 February 2020 0026-2714/ © 2020 Elsevier Ltd. All rights reserved.
integration issues such as plasma damage, but they also enable extremely low capacitances since the permittivity of air is close to 1. However, airgaps confront serious challenges concerning their structural integrity and mechanical stability. This paper presents the impact of airgaps on the mechanical integrity of advanced nano-interconnects with the focus on chip package interaction related issues. To this end, the shear microprobing technique is employed in combination with post-mortem failure analysis and crack inspections. 2. Experimental details A dedicated test chip design was used to study the impact of metal density (particularly in the via layers) and airgap density on the mechanical integrity of the BEOL. The test chip is a passive test chip, which means that there are no active devices (transistors) present. For a more detailed description of the test chip, we refer to [16–17]. Briefly, the BEOL architecture consists of four Cu metal layers (M1-M4) processed on top of a dummy Intermetal (IM) tungsten layer, where Metal 1 (M1) and Metal 2 (M2) are tight 90 nm pitch metals integrated in a porous low-k dielectric with a dielectric constant of 2.4. Metal 3 (M3) is an intermediate layer with 2× larger dimensions than M1 and M2 and is integrated in a non-porous low-k dielectric with a dielectric constant of 3.2. Metal 4 (M4) is the last metal layer with dimensions which are 12× the minimum features and is integrated in SiO2. On top of the BEOL, an 825 nm thick passivation (25 nm SiCN, 300 nm SiO2 and 500 nm SiN) and a 50 μm high copper pillar with a diameter of 50 μm is deposited. A typical cross section of the BEOL stack is shown in Fig. 1a. The metal layers are interconnected with four Cu via layers (V0–V3), where the vias are small volumes of metal
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Fig. 1. (a) Schematic overview of the test chip layer build-up and cross-section; (b) cross-section transmission electron microscopy image of the airgap morphology.
required in-plane shear direction. The open frame design and large X, Y and Z travel range of the stage (X: 370 mm, Y: 168 mm and Z: 168 mm) offer the capability to handle the widest range of sample sizes. Fig. 3b shows the mode of operation of the shear microprobing tests. First, the shear probe is engaged with a low contact force at several microns before the copper bump, defined as the start position. After this, the shear probe is retracted to a predefined shear height above the surface of the silicon chip (Zshear), which is kept constant during the shear microprobing test. Next, the test continues with a lateral movement of the sample until the shear probe touches the sidewall of the pillar and finally applies shear forces until fracture is induced in the area of tensile stress close to the bottom of the copper bump. The data acquisition rate is very high so that sudden crack events can be captured in the force signals, Fxy(t), Fz(t), respectively. A parametric study was performed to determine the optimal shear velocity and shear height that maximized the chances of BEOL fracture, i.e. 10 μm/s shear velocity and a shear height at 50% of the bump diameter. In total, 3 different wafers were tested (1 wafer without airgaps and 2 wafers with airgaps). For each wafer, a particular cell with certain via and airgap density was always measured 7 times from the center towards the edge of the wafer. Per cell, 16 copper bumps were selected at the edge of the cell for shear tests, resulting in a total of 112 tests per cell on each wafer.
vertically connecting long metal lines. The density of the four metals is kept constant at approximately 50%, while the number of the vias in different BEOL layers is varied to achieve the targeted density. To study the impact of via density on the mechanical stability of the BEOL, the densities of V0, V1 and V2 are varied, resulting in different BEOL architectures. The density of V0 is varied from 0.26%–4.15%, V1 from 0.26%–3.11% and V2 from 0.35%–3.46%. These are the typical ranges that can be found in contemporary devices. More detailed information about the via distribution within the chip can be found in [16]. In addition to different via densities, the test chip is designed to include airgaps, specifically located in the M1 layer, as illustrated on Fig. 1b. From this figure, the shape and dimensions with respect to the line dimensions are indicated. The airgaps are about 30 nm deeper with respect to the bottom of the copper lines, while the airgap height is 20 nm larger compared to the line height. It is found by Zahedmanesh and co-workers that when the airgaps are implemented with a stiff dielectric liner, as in this case, deeper airgaps with respect to the bottom of the M1 lines result in lower energy release rates, i.e. higher toughness of the BEOL [18]. On the other hand, it was found that increasing the airgap height versus line height increases the energy release rate under a given CPI stress, resulting in a lower toughness of the BEOL [18]. Therefore, the most vulnerable location in the BEOL when including airgaps is expected to be between the M1 and M2 layer near the top of the airgap structures. Two complementing masks are used to determine the impact of airgap density and airgap morphology on the mechanical integrity of the BEOL. Mask I varies the airgap density between 5 and 20%, while Mask II allows to vary the airgap density between 5 and 52.5% and mainly explores the impact of airgap morphology and the maximum possible airgap density on the mechanical integrity of the BEOL, while the via distribution within the tile remains unchanged. Fig. 2 shows an example of a BEOL with V0, V1 and V2 density of 0.28%, 1.21% and 1.38%, respectively, and a different airgap morphology for 10% of airgap density. Shear microprobing tests are performed on 15 cm × 15 cm coupons of fully integrated 300 mm wafers using a commercial Condor Sigma shear tool from XYZTEC equipped with a 200 μm wide shear tip (Fig. 3a). The Condor Sigma shear tool can perform shear tests with a submicron positioning accuracy and can apply shear forces up 9.81 N with an accuracy of 0.075%. The shear probe can be rotated with an accuracy of 1° into any preferred direction. In this way, the normal force (Fz) and the net in-plane force (Fxy) can be measured for any
3. Results and discussion Two different failure modes were observed after shear testing, i.e. BEOL fracture event or bump shear-off without opening of the BEOL. Fig. 4 illustrates the typical lateral force versus lateral displacement curves for the two observed failure modes. In both cases, the lateral force, Fxy, is normalized to the maximum lateral force, Fxy, max, while the lateral displacement, dxy, is normalized to the bump diameter, dbump. In case of BEOL fracture, the lateral force versus lateral displacement curve is composed out of three major regions: an initial region of almost perfect linearity (region 1), a second region of sudden lateral force relaxation (region 2) and a third region of damage evolution (region 3). In case of bump shear off, no sudden lateral force relaxation is observed during the shear test. The lateral force first increases linearly towards a maximum, after which it gradually decreases until the entire bump is sheared off. As shown on Fig. 4, the initial region is almost perfectly linear, it exhibits a large amount of plastic deformation, which may arise from 2
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Fig. 2. Example of different airgap morphology for the same airgap density (10%) in case of a BEOL with V0, V1 and V2 density respectively 0.28%, 1.21% and 1.38%.
surrounding region. Fig. 5a shows the percentage of BEOL fracture events, FOBEOL, as a function of the V1 and V2 density, while keeping the V0 density constant at 0.26%. Clearly, FOBEOL decreases with increasing V1 and V2 density. This agrees with the work of Zahedmanesh et al., who found that a larger via density increases the effective critical energy release rate of the via layer, therefore leading to better mechanical integrity of the BEOL [19–20]. However, although this figure already gives a first glance of the impact of via density on the mechanical integrity of the
the plastic deformation of the copper bump and the plasticity of metal layers under the bump. Since the BEOL below the bump is constrained by the surrounding material, the stress continues to build up during the initial linear region until a critical amount of stress has built up, enough to free the bump from the surrounding constraint. This sudden removal of constraint causes one end of the bump to pop out, leading to a sharp drop in stress. From this point on, the measured lateral force reflects the crack propagation across the area underneath the bump and in the
Fig. 3. (a) XYZTEC Condor Sigma setup with Resolving Measurement Unit, stage, light microscope and 200 μm shear tip; (b) mode of operation for shear microprobing. 3
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Fig. 4. (a) Typical lateral force versus lateral displacement curves for the two observed failure modes; (b) post-mortem SEM image of a BEOL fracture event; (c) postmortem SEM image of a bump shear off event.
estimated by conducting a series of finite element simulations of a pull (mode I) test on the BEOL for each V0, V1 and V2 combination [16–17]. Simulating a pure mode I test on the BEOL is a good estimate since during shear microprobing a BEOL fracture is induced in the area of tensile stress close to the bottom of the copper bump. It must be noted that due to the bending moment of the bump when pushed laterally, there is also a contribution of normal forces (Fz), which in turn will also contribute to the stress levels that are found at the different via levels. By applying a 100 MPa tensile load to the BEOL tile, the change in stress in V0, V1 and V2 with respect to the BEOL architecture with V0 and V1 density equal to 0.26% and V2 density equal to 1.38%, can be calculated for different combinations of V0, V1 and V2 density. It is found that by changing the V1 density from 0.26% to 1.21%, while keeping V0 and V2 density constant at respectively 0.26% and 1.38%, no significant change in stress and FOBEOL was observed (Table 1). In contrast, when decreasing the V2 density from 1.38% to 0.35%, while keeping the V0 and V1 density constant at respectively 0.26% and 0.26%, a significant increase in FOBEOL is found. This can be explained in terms of the mean stress that is found at the observed fracture location, i.e. between M2-M3 near the V2 layers. By changing the V2 density, the biggest stress change in the V2 layer is found, resulting in a large increase in FOBEOL. On the other hand, by changing the surrounding via densities, the stress in the V2 layer can also be changed, which in turn will have an impact on FOBEOL. It must be noted that the conclusions that are made based on Table 1 are unchanged by changing the magnitude of the applied stress. It is worthwhile mentioning that the via density also seem to have an impact on the distribution of the critical force for BEOL fracture upon shear microprobing (Fig. 7a). When comparing two extreme cases of V1 and V2 density, it is found that higher V1 and V2 density (while keeping V0 density constant) results in a tighter distribution of the critical forces for BEOL fracture, compared to structures with a lower V1 and V2 density. This can be explained in terms of the spatial distribution of the V1 and V2 in the BEOL architectures and is confirmed by Zahedmanesh et al., who found that uniformly distributed vias reduce the energy release rates in the dielectric cracks more effectively than clumped vias [19–20]. Based on these observations, a schematic overview of the impact of via density and via distribution on the mechanical strength of the BEOL is shown on Fig. 7b. As shown, a certain change of BEOL strength leads to a higher fraction of BEOL failures when the via density is higher because it has a tighter distribution of the BEOL strength compared to lower via
Fig. 5. percentage of BEOL fracture events as a function of V1 and V2 density.
BEOL, it does not give information about which of these via densities is more dominant. To investigate this more in detail, one first must look to the fracture location after performing shear microprobing tests. Fig. 6a–c shows the fracture surface in case of a BEOL fracture after performing a shear microprobing test. The shear direction is indicated with a white arrow. By comparing high resolution top down SEM images of the fracture surface with the specific design underneath the copper bump, it was found that for all combinations of V0, V1 and V2 density, the failure location is mainly found at the M2-M3 interface. The failure location is expected at this location due to the large elasticity mismatch that is found at the M2-M3 interface and the rigidity of the upper layers, as demonstrated by Zahedmanesh and co-workers [19–20]. They found that via layers adjacent to an interface with a large elasticity mismatch endure a higher energy release rate and thus are most vulnerable to delamination. The fact that the failure location is found at the M2-M3 interface also suggests that FOBEOL is expected to be more sensitive to changes in V2 density compared to changes in V0 and V1 density. To further support this, the stresses that are found at the different via levels were 4
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Fig. 6. (a) Typical fracture location of a typical fracture surface after shear microprobing; (b) high resolution SEM image of region A on panel a; (c) high resolution SEM image of region B on panel a; (d) typical fracture location of a typical fracture surface after shear microprobing BEOL structures with airgaps; (e) high resolution SEM image of region A on panel d; (f) high resolution SEM image of region B on panel d.
microprobing (shear direction is indicated with a white arrow) when airgaps are introduced into the BEOL stack. By comparing high resolution top down SEM images of the facture surface and compare it with the specific design of the BEOL that is located underneath the sheared copper bump, it was found that for BEOL architectures with airgaps the failure location was mainly found at the M1-M2 interface for all combinations of V0, V1 and V2 density. This failure location is different from the case without airgaps, where the failure location was determined at the M2-M3 interface (Fig. 6a–c). Potential reason for this shift in failure location towards the lower layers in the BEOL stack can be found when looking to the specific geometry and location of the airgaps as shown in Fig. 1b. The airgaps are located at the M1 layer, while in terms of geometry, the airgap height is 20 nm larger with respect to the liner at the side of the M1 lines. Moreover, the airgaps also extend about 30 nm towards the IM layers. As was demonstrated by Houman Zahedmanesh et al., deeper airgaps, when airgaps are implemented with a stiff dielectric barrier such as SiCN and the interlayer dielectric is a soft dielectric, such as in this case, result in lower energy release rates, i.e. higher toughness of the BEOL [18]. Moreover, it was shown that increasing the airgap height versus line height increases the
Table 1 Stress change in V0, V1, V2 and FOBEOL for different combinations of via density. V0, %
V1, %
V2, %
Stress change in V0, %
Stress change in V1, %
Stress change in V2, %
FOBEOL, %
0.26 0.26 1.56 0.26 0.26
0.26 1.21 0.26 0.26 3.11
1.38 1.38 1.38 0.35 3.46
0 −0.90 −1.56 3.12 −0.41
0 −2.86 −1.93 2.21 −11.33
0 0.35 2.09 11.50 −4.00
39.84% 42.86% 54.46% 66.07% 24.11%
densities. To study the impact of introducing airgaps to the BEOL architecture on the integrity of the BEOL, shear microprobing tests were conducted on two different wafers with two different airgap masks (Fig. 2), after which the failure locations were inspected by top down SEM imaging and compared to the specific design of the investigated structures. It was found that the introduction of airgaps into the BEOL stack clearly has a strong impact on the failure location after shear microprobing. Fig. 6d–f shows the typical fracture surface after shear 5
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the surrounding via densities (V0 and V2) in the BEOL stack, as was demonstrated in the previous section. Fig. 8 shows the FOBEOL as a function of the airgap density for different combinations of V0, V1 and V2 density. For the case without airgaps, the differences that are found in FOBEOL between BEOL stacks with different combinations of V0, V1 and V2 can be understood in terms of the actual stress that is applied to the V2 layer, as explained in previous section. On the other hand, the introduction of airgaps clearly has a detrimental impact on the FOBEOL. To understand this, first the impact of airgaps on the effective adhesion energy of the via layer (Gc-effvialayer) for a specific via density and airgap density is estimated based on the work of Zahedmanesh et al. [20]. By adapting the approach of Martson et al. which was originally developed for uniaxially reinforced composite materials [21], the effective adhesion energy of the via layer above airgaps when the airgaps protrude into the via layer and with uniformly distributed vias, can be expressed as
Gcvialayer = Vv Gcvia + (1 − Vv − VAG ) Gcdielectric + − eff
4Vv lpull − out d
Gcpull − out
where Gcvia, by assuming delamination at the ULK/dielectric barrier interface and bottom of the vias, is the adhesion energy at the bottom of vias, Vv is the volume fraction of vias in the via layer, d is the diameter of the via (Vv and d are not independent), lpull-out is the length of via pulled out of the dielectric barrier and Gcpull-out is the adhesion energy at the pull-out interface i.e. via/dielectric barrier. Assuming “ideally brittle” interfaces for the dielectric/SiCN interface, Gcdielectric = 3.7 J/ m2, for the TaNTa/SiCN interface, Gcpull-out = 14 J/m2 and for the TaNTa/Cu, Gcvia = 40 J/m2, the thickness of the dielectric barrier, lpullout = 5 nm and via diameter, d = 45 nm [19]. Results of the calculations are shown in Table 2. Clearly, the impact of introducing 10% of airgap density reduces the Gc-effvialayer with about 9.7% and 7.4% for respectively low and high via density. This means that by introducing airgaps in the BEOL stack, the BEOL strength curve would shift towards lower strength, hence leading to higher amount of failures when applying the same global stress to the BEOL (Fig. 8). To investigate the net impact of the introduction of airgaps on the mechanical strength of a specific BEOL stack of certain V0, V1 and V2 via density, and in order to be able to anticipate the influence of V1 density and the surrounding via densities (V0 and V2), one can define ΔFOBEOLAG as the relative change in FOBEOL due to the introduction of airgaps,
Fig. 7. (a) BEOL failure force distribution after shear microprobing as a function of V1 and V2 density and the corresponding spatial distribution of V0, V1 and V2; (b) schematic overview of the BEOL stack strength as a function of the cumulative % of failures.
ΔFOBEOL AG = (FOBEOL AG–FOBEOL no AG)/FOBEOL AG Table 3 shows ΔFOBEOLAG for different combinations of V0, V1, V2 and airgap (AG) density. From this table one can conclude that increasing the V1 density while keeping V0 and V2 density constant leads to a larger ΔFOBEOLAG, i.e. a larger relative difference in BEOL failure occurrence between interconnects with and without airgaps. This can be explained by the fact that on the one hand, a higher V1 density results in a more uniform distribution of the vias, which in turn leads to a tighter distribution of the BEOL strength compared to lower via densities. On the other hand, from Table 3, it is found that a higher via density has no significant impact on the reduction of Gc-effvialayer when introducing 10% of airgap density. Therefore, also a similar shift in the BEOL strength curve towards lower strength values is expected for both BEOL architectures with high and low via densities (Fig. 9). Table 2 Effective adhesion energy of the via layer for different combinations of via and airgap density.
Fig. 8. FOBEOL as a function of the airgap density for different combinations of V0, V1 and V2 density.
energy release rate, resulting in lower toughness of the BEOL. Therefore, the fracture location is expected between the M1 and M2 layer near the top of the airgap structures and FOBEOL is expected to be sensitive to the V1 density. However, FOBEOL can also be influenced by
6
Vv, %
VAG, %
Gcvia, J/m2
Gcdielectric, J/m2
Gc-effvialayer, J/m2
0.3 0.3 3 3
0 10 0 10
40 40 40 40
3.7 3.7 3.7 3.7
3.828 3.458 4.976 4.606
ΔGc-effvialayer
–9.7% –7.4%
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Table 3 ΔFOBEOLAG for different combinations of V0, V1, V2 and airgap (AG) density. V0, %
V1, %
V2, %
AG, %
ΔFOBEOLAG
Comment
0.26 0.26 0.26 1.56 0.26 0.26 0.26 0.26 0.26 4.15 0.26 4.15
0.26 1.21 0.26 0.26 0.26 0.26 0.26 3.11 0.26 3.11 0.26 0.26
1.38 1.38 1.38 1.38 0.35 1.38 0.35 3.46 0.35 0.35 0.35 3.46
20 10 52.5 52.5 52.5 52.5 5 10 5 10 52.5 52.5
0.636 0.646 0.837 0.705 0.148 0.837 0.045 0.963 0.045 0.002 0.148 0.778
Impact of V1 density Impact of V0 density Impact of V2 density Impact of V1 and V2 density Impact of V0 and V1 density Impact of V0 and V2 density
When applying the same global stress to the BEOL in both cases, one can therefore expect that the introduction of airgaps results in a bigger increase in failures for structures which have a higher V1 density, as observed. To see the impact of the other via densities V0 and V2 on the relative change in FOBEOL due to the introduction of airgaps, one can first compare the case where V0 density is changed, while keeping the V1 and V2 density constant. It is found that a higher V0 density leads to a lower relative change in FOBEOL. Since the failure location is expected between the M1 and M2 layer near the top of the airgap structures, replacing the lower layers near the bottom of the airgaps by more robust layers (higher V0 density) would reduce the stress intensity factor at the failure location [22], which in turn would lead to lower failure rates, hence less impact of introducing airgaps. In contrast, by increasing the V2 density, while keeping the V0 and V1 density constant, a much higher relative change in FOBEOL is observed. This can be explained by the specific location and geometry of the airgaps and the fact that by reinforcing the layers above this failure location in the BEOL (by increasing the V2 density) under the displacement controlled bump shear tests condition, this would increase the local stress that is applied to the failure location, which in turn would lead to larger failure rates. These observations were confirmed in cases where V1 and V2, V0 and V1 and V0 and V2 are changed together. The impact of the airgap distribution on the BEOL strength is shown on Fig. 10, where FOBEOLAG for two different airgap distributions (Fig. 2) are compared for BEOL structures with low via densities and high via densities. In both cases, it is found that when the airgaps are more uniformly distributed (Airgap Mask I, Fig. 2), FOBEOLAG is lower compared to the case where the airgaps are more clumped (Airgap Mask II, Fig. 2).
Fig. 10. Comparison of ΔFOBEOLAG for two different morphologies of airgaps. Both the case of a BEOL architecture with low and high via densities are compared.
4. Conclusions The impact of airgaps on the mechanical strength of advanced nanointerconnects was investigated using shear microprobing in combination with post-mortem failure analysis and crack inspections. In absence of airgaps, the failure location was found at the interface where a large elasticity mismatch was found. Moreover, it was found that the amount of BEOL failures scales with the stress that is applied to the vias near the fracture location. The latter is mainly influenced by the via density but can also be influenced by changing the via densities of the surrounding layers or by changing the type of integrated dielectric materials. Moreover, it was shown that the via density and the via distribution across the nano-interconnects have a clear impact on the force to initiate and grow cracks inside the nano-interconnects. The introduction of airgaps leads to a shift in the fracture location towards the interface where the airgaps are located, while the relative changes that are found in the number of BEOL failures can be explained in terms of the via density and the specific location and geometry of the airgap structures. Finally, it was found that more uniformly distributed airgaps in nanointerconnects lead to a lower amount of BEOL failures. This work may serve as a guideline to further enhance the mechanical integrity of airgaps in advanced nano-interconnects for future technology nodes.
Fig. 9. (a) Schematic overview of the BEOL stack strength as a function of the cumulative % of failures in the case of BEOL stacks with or without airgaps; (b) schematic overview of the BEOL stack strength as a function of the cumulative % of failures in the case of BEOL stacks with different V1 density before and after introducing airgaps. 7
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Declaration of competing interest
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