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Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops
MR-12430; No of Pages 10 Microelectronics Reliability xxx (2017) xxx–xxx
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Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops Yunna Sun a, Dongwoo Kang b, Yazhou Zhang a, Jiangbo Luo a, Yanmei Liu a, Yan Wang a,⁎, Guifu Ding a,⁎ a b
National Key Laboratory of Micro/Nano Fabrication Technology, Shanghai Jiao Tong University, Shanghai 200240, China Test&Package (TP) Center, Samsung Electronics Co., Ltd., Asan-City 31489, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 March 2017 Received in revised form 6 June 2017 Accepted 7 June 2017 Available online xxxx Keywords: Etching defect Triangular-teeth and scallops Initial thermal stress and strain Thermo-mechanical reliability
a b s t r a c t Through silicon via (TSV) with triangular-teeth and scalloped side wall (TSSW) not only lowers down the electrical performance but also alters the mechanism of thermal mechanical stability. By considering the realistic etching defects on TSV side wall, a more reasonable and reliable three-dimension (3D) TSV model with TSSW is built. The thermo-mechanical issues induced by the unsmooth side wall of the TSV are studied by finite element method (FEM) in this work. With the presence of TSSW, much more tips in interfaces of the TSV-Cu and SiO2 and the SiO2 and Si are brought and the shear stress and normal stress are distinct comparing with the smooth side wall. Therefore, the thermo-mechanical mechanism of the TSV is changed greatly. By investigating the normal stress, shear stress and strain energy density (SED) of the triangular-teeth local region, it has been found that severe normal stress variation (−200–70 MPa) and multiple variation of shear stress τxy contributes the peeling, slip and crack issues. The effective plastic strain, displacement and von Mises stress, and shear stress and normal stress in the X, Y and Z direction are studied in detail. The effect of the TSSW on the thermal stress and keep-out zone (KOZ) size is discussed. © 2017 Published by Elsevier Ltd.
1. Introduction With the advantages of lower power consumption, higher integration density and shorter interconnection length, TSV has been used as one kind of high efficiency and reliable technology for stacking wafers, [1–2]. The application of TSV is more and more widely from 2.5D to 3D. Through silicon vias (TSVs) is formed by ion etching [3–4], and filled with good conductive Cu commonly. As an advanced packaging technology in the semiconductor industry, TSVs have attracted many scholars' attention. There exists some main factors for TSVs' rapid development, such as, high thermo-mechanical reliability, reliable electrical performance, mass production with low cost. Drilling via is one of the main processes in TSV fabrication which will affect the insulating layer forming, seed sputtering, and TSV-Cu electroplating. The deep reactive ion etch or Bosch etching was a common and convenient way for obtaining via. Etch rate played an important role in getting high-quality TSVs, such as smoother sidewall, [5– 6]. However, the through via was unable to be straight and the sidewall was rough (large scallops) when the etch cycle time was too long and passivation cycle time was too short, [6–8]. Since higher thermo-mechanical stress, leakage currents, slower response time and lower signal
⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (G. Ding).
transmission speed, and Cu diffusion might be brought by the large scallops, the reliability of the integrated circuit (IC) was cut down, [9–12]. Some methods have been proposed for scallop free TSV etching [3,13], but the Bosch etching is still adopted regularly in the TSV etching [4,5, 14]. The scallop free etching method merits future research [15]. After Bosch etching, some triangular-teeth appeared near the port of the via and connected with smother scallops. Stress concentration points were formed by the triangular-teeth and scallops. Due to the existence of the TSSW the thermo-mechanical reliability of the TSV encounters new challenges during adding thermal loads. Electrical performance is also related to the thermal stress distribution for avoiding capacitor formed between the adjacent TSVs and Cu diffusion aggravation [9]. The interplay of Cu diffusion, TSSW dimension and thermal stress will decrease the reliability of the microsystem and aggravate the TSV failure. 2. Problem statement and model building The triangular-teeth (about 8–11) may appears near the top opening of the TSV shown in Fig. 1. A 3D symmetric TSV model with TSSW is built (Fig. 2), just as [16–19]. To simplify, a 1/8 symmetric model is set up, which have 10 triangular-teeth at the bottom of the interface of TSV Cu and SiO2, and the interface of SiO2 and Si. This model mainly considers three parts, TSV-Cu, SiO2 and Si. The diffusion barrier layer
http://dx.doi.org/10.1016/j.microrel.2017.06.013 0026-2714/© 2017 Published by Elsevier Ltd.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 1. SEM cross section view of TSVs.
Fig. 3. Geometry graphic for triangular-tooth and scallop.
(related to thermo-mechanical but not affected much since this layer is much thinner than other layers) is neglected for simplify, [20–21]. The above mentioned three layers are assumed to be perfectly interconnected in the interfacial layers. The geometry graphics for triangular-tooth and scallop are shown in Fig. 3. The scallop structure can be modeled by the modified Quadratic Bezier Curve, see formula (1) (detailed in [17,22]). BðtÞ ¼
ð1‐tÞ2 W 1 P 1 þ 2t ð1−t ÞW 2 P 2 þ t 2 W 3 P 3 ð1‐tÞ2 W 1 þ 2t ð1−t ÞW 2 þ t 2 W 3
; 0≤t ≤1
ð1Þ
where, W1, W2, and W3 are the weight factors; P1, P2 and P3 are the vertex points, shown in Fig. 3. Different with [17], the above fit curve is much smoother and actual. Firstly the scallop's height and width, P1, P2, and P3 are obtained from the measurement, Fig. 3(b). And then point P2 is fund by two tangent lines L1 and L2. With the fixed weight factors, W1 = W3 = 1, the best fit Quadratic Bezier Curve can be achieved by adjusting the weight factor W2. In this paper we set W2 = 0.707 to describe the scallop and the according dimensions of triangular-tooth and scallop are listed in Table 1. The initial thermal stress is not set stress-free at 293.15 K, but related to the thermal residual stress of under-fill filling (UFF) stage shown in Fig. 4, [23]. The thermo-mechanical properties are shown in Table 2. By COMSOL software the meshing on the model is shown in Fig. 5. Firstly, free quad with three or four layers is meshed on the lateral
surface of SiO2 layer. Next the adjacent lateral surfaces, Si and partial of TSV-Cu are meshed with free quad. After that, the domains of Si, SiO2 and partial of TSV Cu are swept with the meshed lateral surfaces. Once more, bottom surface of the remainder TSV-Cu is meshed with free quad. Finally, the remainder domain of TSV-Cu is swept with the meshed bottom surface of TSV-Cu. The meshes of the SiO2 are much finer than those of the TSV-Cu, TSV-Pad and Si for the barrier SiO2 is the thinnest films in this 1/8 TSV model. The boundary conditions of the TSV model are set as that three surfaces are symmetric with X, Y and Z coordinates respectively, and point e is fixed at X, Y and Z direction (Fig. 2). The other surfaces is set free.
3. Numerical analysis 3.1. Equivalent von Mises stress After UFF, the von Mises stress on the interface of TSV-Cu and SiO2 has occurred distinct changes from stress-free to close to the yield strength of Cu, especially on the edge and triangular teeth region (scallop region is much smaller), shown in Fig. 6. This is mainly related to the special structure of TSV with TSSW. The interfacial lines L1, L2 experience higher stress. Thus, the triangular teeth region and TSV pad will suffer severe thermal deformation and plastic accumulation during the working stage. With the initial stress and strain, TSV deformation goes on in working stage (293.15 K ➔ 423.15 K). The total deformation and von Mises stress resulting from heating up to 423.15 K are shown in Fig. 7(a). Maximal displacement appears at the TSV-pad similar to the normal TSV model. The interfacial lines of TSV-Cu and SiO2, and SiO2 and Si sustain higher von Mises stress, Fig. 7(b). Since the TSSW bring many tips, lots of stress concentration points turn up at the TSSW interfacial lines, especially on the triangular-teeth. Therefore, the interfacial lines L1, L2 (Fig. 7(b)) emerge splitting and cracks with high probability.
Table 1 Basic size of the TSV model with TSSW.
Fig. 2. 1/8 symmetric model with triangular-tooth and scallop. A, B, C, D and a, b, c, d are the interfacial points; L1, L2 are the interfacial lines; TT1 indicates the first triangulartooth and TT2 is the second triangular-tooth and so to TT3… and TT10.
Parameters
Unite (μm)
Height of the substrate Width of the substrate Diameter of TSV-Cu Depth of the silica barrier Height of TSV-pad Cu Width of TSV-pad Cu Weight of TSSW (w) Height of the triangular-tooth (h) Height of h1 Height of the schallop (h2)
250 150 250/6 2.0 6.0 60 2.000 0.360 0.540 ~ 0.224
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 5. 3D symmetric model with meshing.
Fig. 4. Thermal load during under-fill filling stage.
3.2. Normal stress, shear stress and effective plastic strain The normal stress σx is equal to σy with a 90 degrees rotation. Almost all the Si and SiO2 layers suffer higher normal stress (compressive and tensile). The interfaces with TSSW still suffer higher thermal stress (Fig. 8(a) and (b)). The TSV-Cu has greater deformation and higher normal stress in Z direction (Fig. 7). The normal stress σx (or σy) stress map around the TSV forms a larger zone composites with tensile stress and compressive stress and no zone by σz shown in Fig. 8(c) and (d) respectively. The thermal stress induced delamination and cracks is discussed as the TSV design criteria, [25]. When the stress level of the interface (Cu and SiO2) of the TSV exceeds yield strength of Cu (200 MPa), it is expected to crack at any weak site on the unsmooth sidewalls, [26]; and the influence of the alternating compressive stress and tensile stress. The size of the KOZ may be increased by the stress interaction. The KOZ size is evaluated through a criterion of 5% change in the carrier mobility. Normal stress in X or Y direction is much higher than that in Z direction, such as and σx on L2 and σy on L1 (Fig. 9(a)). The triangular-teeth region experiences sharping and huge thermal stress. Due to the limitation of Si and SiO2 on the TSV-Cu, the TSV-Cu is compressed in X and Y direction and stretched in Z direction. With the existence of TSSW, the normal stress on the interfaces of TSV-Cu and SiO2, and SiO2 and Si are enlarged. As shown in Fig. 9(b) the interfacial line of TSV-pad and SiO2 (LBC) bears very higher compressive stress σx (− 389 MPa), however, σx on LAD is beyond 50 MPa (L1, L2, LAB, LBC, LCD, LAD, Lab and Lcd have defined in Fig. 2). It should be pointed out Lcd endures compressive and tensile stress (−150–125 MPa). Due to the special rotational structure, normal stress σz on these lines are very lower, only a few MPa. With the rotational structure the shear stress τxz and τxy with 90 degrees carry opposite signs. As the special structure of TSV-Cu, higher shear stress τxy and τxz are in the interfacial layers of TSV-Cu and SiO2, and SiO2 and Si. After heating up to 423.15 K, τxy is [−39.5, 200] MPa and τxz is [− 175, 305] MPa (Fig. 10). τxz is much larger than τxy for the interfacial lines exist in the Z direction, thus, sliding issues often
appear at these region. These are related to the special structure of TSV fixed by SiO2 and Si in X and Y direction, and having free space in Z direction for relieving thermal stress. Due to the τxy in XY plane (Fig. 10(a)), the interplay of the adjacent TSVs may be enlarges the KOZ size. As shown in Fig. 11(a), the shear stress on the interfacial line of TSVCu and SiO2 (L1) is much sharper than that on the SiO2 and Si (L2). The sharpest shear stress regions appear at the Triangular-teeth connected with the bottom TSV-Pad. Higher shear stress on Lab, Lcd, LAB, LBC, LCD and LAD may lead interfacial slip. Fig. 11(b) shows that the maximal shear stress τxy turns up at Lcd due to elastic deformation on Lcd, and elastic-plastic deformation on Lab, LAB, LBC, LCD and LAD. Since the shear stress of LBC is much higher, the interfaces of TSV-pad and SiO2 suffer slipping with higher probability. LBC and LAD suffer relative higher shear stress τxz in Fig. 11(c). That's, higher shear stress τxz arises at the TSV-pad and the adjacent regions. τxz and τyz on interface of are closed to the yield strength of Cu which leads plastic deformation and accumulation on the interface. τxz and τyz on interface of SiO2 and Si are over 305 MPa (Fig. 10). The shear stress of τxz and τyz are much higher than τxy. Thus, the sliding issues may occur in Z direction. The effective plastic strain on L1 accumulates greatly, shown in Fig. 12(a). As the distance (to the bottom TSV-pad) becoming larger, the effective plastic strain on triangular-teeth is lowered down and then smoothened. The effective plastic strain on scallops is flat (only about 0.0002). Thus, the triangular-teeth thermo-mechanical reliability is going to suffer great challenge. The effective plastic on the interfacial curves of TSV-Cu in XY plane is much smaller compared with L1 in Fig. 12. Therefore, the accumulation of plastic strain is mainly in Z direction.
Table 2 Thermo-mechanical properties [24]. Name
Cu
SiO2
Si
CTE (ppm/K) Young's modulus (GPa) Poisson's ratio Yield strength (MPa) Tangent modulus (MPa)
17 110 0.35 200 357
0.5 70 0.17 – –
2.6 130 0.28 – –
Fig. 6. Initial von Mises stress induced by under-fill filling.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 7. Distribution of (a) total displacement and (b) von Mises stress, after heating up.
3.3. Thermal stress and strain energy density on the triangular-teeth The thermal stress and SED [25,27] of the triangular-teeth of L1 and L2 are investigated and analyzed. Fig. 13 exhibits the thermal stress of each TT1, TT2–TT10 (Fig. 2) and Fig. 14 shows the average thermal stress and SED of each TT1, TT2–TT10. The normal stress on Si and SiO2 interfacial line L2 is much complicated, vibrated and lager than on L1. Due to the special location connected with TSV-pad and bounded by SiO2 and Si, triangular-teeth TT1–TT2 mainly experience compressive stress in X and Y direction, and tensile stress in Z direction. The normal stress becomes stable after TT6 or TT7, that is, the normal stress of TT6–TT10 on L1 and TT7–TT10 on L2 are changed pretty small. The normal stress σx are basically compressive press (Fig. 13(a) and (b)). While the normal stress σz on L1 is tensile stress (Fig. 13 (c)) and on L2 are alternative compressive stress and tensile stress (Fig. 13(d)). Higher compressive stress in X and Y direction arising near the tips of TT6–TT10 (Fig. 13(a) and (b)) may reduce the peeling phenomenon. However, these tips easily generate cracks and the σx related KOZ will be enlarged. TT1 and TT2 experience higher irregular thermal stress variation resulting from the special location closely connected with TSV-pad (more freedom for releasing stress) shown in
Fig. 9. Normal stress on L1, L2, LAB, LBC, LCD, LAD, Lab and Lcd.
Fig. 8. Normal stress distribution in (a) X, (b) Z direction; (c) X, (d) Z direction of the whole middle cross section.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Except for the compressive stress and tensile stress, σz on L1, other thermal stresses on TT1–TT10 mainly are compressive stress based on the above analysis and Fig. 13. The average thermal stress and SED of each triangular-teeth is available to evaluate the triangular-tooth as a whole shown in Fig. 14. With the distance increase of the triangulartooth on L1 away the TSV port, the von Mises stress (vMS in Fig. 14) and τxz reduces and σx become large. With the distance increase of the triangular-tooth on L2 away the TSV port, the von Mises stress and σx raise and σy, σz, τxz and SED decrease (Fig. 14(b)). τxz and τyz are very small comparing with other thermal stresses. σy range on L1 is −32– 30 MPa. TT1 suffers the highest SED resulting from the larger accumulated strain and deformation for releasing stress. The average σz on L1 shows tensile stress owing to the TSV-Cu bumping in Z direction. 3.4. Effect of thermal stress on the carrier mobility and KOZ The absolute value of the relative mobility changes along the [100] channel direction is defined as Eq. (2), [28,29]. By considering the piezoresistivity of Si, specific stress characteristics are used to enhance the performance of Si MOSFETs, and such “strained-Si” technology has been proposed since the 90-nm technology node [28], such as, implanting Ge into the source/drain region or introducing a silicon nitride capping layer for the N-MOSFETs [30]. By considering the effect of device alignment, the mobility change induced by the anisotropic properties of Si along [110] becomes Eq. (3), [29]. The piezoresistance coefficients for N-type and P-type Si are listed in Table. 3.
Fig. 10. Shear stress (a) τxy and (b) τxz, distribution TSV model.
Fig. 13(c) and (d). TT1–TT10 of L1 bear great tensile stress σz (Fig. 13(c)). Normal stress σz on TT1 of L2 challenges the reliability of the TSV with great local change. In addition, the interaction of compressive stress and tensile stress will enhance the peeling and crack issues on the triangular-teeth interline (Fig. 13(d)). In summary, peeling and crack issues in X and Y direction easily occur at the TT5–TT10 of the TSV-Cu and SiO2 interfacial line ((b)), while in Z direction arise at TT1–TT4 of the TSV-Cu and SiO2 interfacial line (Fig. 13(d)). The tendency of shear stress is much more mutable and complicated than the normal stress. Shear stress τxy is very small (−4.5–0.5 MPa). The interfacial slip issue in XY plane is small probability of occurrence. Comparing Fig. 13(e) with (f), τxz of TT1–TT6 on L2 are much mutable and so the slip and cracks will turn up in these regions with higher probability. After the TT1–TT6, τxz is cushioned and becomes smooth.
Δμ ¼ jπ11 σ 11 þπ12 ðσ 22 þσ 33 Þj μ
ð2Þ
Δμ ¼ j0:5ðπ11 þπ12 Þðσ 11 þσ 22 Þþπ44 σ 12 j μ
ð3Þ
The mobility changes on the TSV substrate Si with no device and with MOSFET are shown in Fig. 15–Fig. 17. Fig. 15 shows KOZ size (lines, the 5% mobility changes) of 4 XY planes (with different via depth).The mobility change on the N-type Si is higher than the P-type with no device, and the mobility change on the Si with N-type MOSFET is lower than P-type MOSFET. The size of the KOZs of the N-type Si with no device (Fig. 15(a)) and with the P-type MOSFET (Fig. 15(d)) are much larger while on the other two the KOZ can be neglected. The larger boundary distance R of N-type Si with no device and with the P-type MOSFET are alone the dashed lines (cut line). Fig. 16 shows the maximal R alone the Z coordination. The KOZ size is much larger near the opening of TSV and decreases dramatically and then increases with via depth, Fig. 16(a) and (d). That is, the KOZ size of the opening regions and the middle via depth region is larger, thus the KOZ size of the device also effected greatly though the device is planted near the Si surface. The mobility change of different types of Si and MOSFET at different X and Y coordinates (1/4 partial of the via) are shown in Fig. 17, the mobility
Fig. 11. Shear stress on L1, L2, LAB, LBC, LCD, LAD, Lab and Lcd.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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change for the MOSFET is near the Si surface (the depth on Si is beyond 1 μm). The mobility change of the P-type Si is below 5%. The KOZ size of the N-type Si is largest. The N-type MOSFET suffers a larger mobility change but the KOZ size is not large.
3.5. Effect of the triangular-teeth number
Fig. 12. Effective plastic strain on L1, LAB, LBC, LCD, LAD and Lab.
The maximal thermal stress, mobility change, and KOZ size of the vertical TSV are listed in Table. 4–Table. 5. The maximal thermal stress, mobility change, and KOZ size of the TSV with different triangular-teeth number is shown in Figs. 18-20. Due to the existence of the scallop the shear stress τxy and τxz are enlarged, and the normal stress σx and equivalent von Mises stress are reduced. The great decrease of the dominant normal stress σx and equivalent von Mises stress show that the presence of scallops improves thermo-mechanical stress performance of the TSV due to pinning effect. It shall be pointed out that if the shear stress is enlarged greatly by the unsmooth sidewall that may induce peeling issues. However, the triangular-teeth lowers down the advantage of the pinning effect induced by the unsmooth sidewall. The maximal equivalent von Mises stress, normal stress (σx or σy) and shear stress τxz are all increasing with the triangular-teeth number, except a jagged equity during 10 triangular-teeth (16% coverage rate), after the coverage rate is 24% the stress almost keep constant. However, σz and τxy rise first and then fall. σx and τxz are larger than the according to σz and τxy. The von Mises stress and normal stress is affected much seriously than shear stress. The σx increases greatly (about 70 MPa) and the τxy changes much smaller. Comparing the mobility change, and KOZ size of the TSVs with unsmooth sidewall with the vertical TSV (Table 5, and Figs. 19–20), the KOZ size of P-type Si is zero for the mobility change are all below 5%; for the N-type Si the maximal mobility change is enlarged from 33.68% to ~40%, the KOZ size is not enlarged except the model with 5 triangular-teeth, but becomes smaller for the model only with scallop and with more than 20 triangular-teeth. The maximal mobility level of the MOSFET is similar. The KOZ size of the P-type MOSFET is enlarged more than 26.6%.
Fig. 13. Normal stress and shear stress on triangular-teeth.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 14. The average thermal stress and strain energy density on the different triangular-teeth on (a) L1 and L2. Table 3 Piezoresistance coefficients for N- and P-type Si [29]. Name (10−11 1/Pa)
π11
π22
π44
N-type Si P-type Si
−102.2 6.6
53.7 −1.1
−13.6 138.1
4. Experiment analysis The electrical leakage current of the TSV samples is tested under thermal load. For the weakening or cracking is expected to result in a
Fig. 15. Carrier mobility distribution on XY plane for (a) N-type Si substrate and (b) P-type Si substrate in [100] direction, and (c) N-type MOSFET and (d) P-type MOSFET in [110] direction. The curves are the 5% mobility change.
Fig. 16. Carrier mobility distribution for (a) N-type Si substrate, (b) P-type Si substrate in [100] direction, and (c) N-type MOSFET and (d) P-type MOSFET in [110] direction. The curves are the 5% mobility change.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 17. Carrier mobility distributions in [100] direction for different type Si and in [110] direction for different type MOSFET.
high electrical leakage current due to copper ion diffusion through the weak barrier and dielectric layers [16], the electrical leakage current of different thermal load cycles is used to assess the thermo-mechanical reliability of the TSV. The leakage current is tested with Keithley 2400 SourceMeter and Probe station (Semishare electronic Co. Ltd., SM-4). The test structure is shown in Fig. 21.
4.1. Leakage current
rent for the two models are increased by about 27.0 pA and 33.6 pA, respectively. With the cycling of thermal load, the leakage current enlarged, especially for the TSV with TSSW. After 10 cycles, the leakage current for the two models are increased by about 30.9 pA and 41.8 pA, respectively. After 15 cycles, the leakage current for the two models are increased by about 76.7 pA and 219 pA respectively. After 15 cycles, the barrier of the TSV with TSSW is weakening or cracking. The thermo-mechanical reliability of TSV with scallops is much more reliable than the TSV with TSSW for the TSSW easily leads weakening or cracking.
The electrical leakage current of the TSV with scalloped structure and the TSSW structure is shown in Fig. 22. The leakage current is tested at the initial state, after 5 cycles (293.15 K ➔ 423.15 K ➔ 293.15 K), after 10 cycles and after 15 cycles. At the initial state the leakage current of the Scallop and TSSW models are similar and below 68.2 pA and 72.0 pA, respectively. After 5 cycles of thermal cycling, the leakage curTable 4 Maximal thermal stress of the vertical TSV. Parameter
vM
σx
σz
τxy
τxz
(MPa)
606.81
337.71
225.92
171.80
297.68
Table 5 Maximal mobility change and KOZ size of the vertical TSV. Parameter
N-Si
P-Si
N-MOSFET
P-MOSFET
Mobility change (%) KOZ size (μm)
33.68 34.17
2.43 0
22.64 5.67
23.35 13.17
Fig. 18. Maximal thermal stress with triangular-teeth number.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 21. Cross-sectional view of the test structure.
Fig. 19. Maximal mobility change with triangular-teeth number.
5. Conclusions TSV with triangular-teeth and scallops was analyzed by FEM. The 1/8 symmetric model with initial stress and strain was studied with static temperature ramp up from 293.15 to 423.15 K. With consideration the actual packaging process, the initial stress and strain was not set stress-free at 293.15 K but related to under-fill filling process. Much more reliable and finer meshes were built on the 3D model by sweeping the surface. The TSV with triangular-teeth suffers higher thermal stress than only with scallops which would lead electrical and mechanical failure issues. 1. Much higher von Mises stress, normal stress and shear stress were raised in the triangular-teeth and the TSV-pad edges, thus the tips of the TSSW might generate cracks and failure issues with high probability. The normal stress and shear stress on the triangular-teeth varied greatly, especially on the TSV-Cu and SiO2 interfacial line and connected with TSV-pad closely.
2. The much sharper surface on the triangular-teeth regions induced higher thermal stress and effective plastic strain which would lead electrical and mechanical failure issues. 3. Due to the larger accumulated strain and deformation for releasing stress, the TT1 on L1 suffers higher von Mises stress, τxz and especially highest SED. The TT1 of L2 suffers lower von Mises stress and τxz, and higher σx, σy, σz, and SED. The average σz on L1 shows tensile stress owing to the TSV-Cu bumping in Z direction. 4. Since τxz of TT1–TT6 on L2 were much mutable, the slip and cracks would turn up in these regions more easily. 5. Although higher compressive stress in X and Y direction arising near the tips of TT6–TT10 might not increase the probability of the interfacial peeling phenomenon, these tips easily generated cracks. 6. With the incensement of triangular-teeth number, the von Mises stress and normal stress have been enlarged greatly, but the shear stress almost constant (τxy, 25 MPa; τxz 3 MPa). When the triangular-teeth coverage rate reached 24% (15 triangular-teeth), the von Mises stress and σx also kept stable and σx reduced somehow. 7. With the incensement of triangular-teeth number, the KOZ size of Ptype MOSFET is increased more than 26.6%. 8. Since the leakage current of the TSV with TSSW or with scallops after 15 cycles of thermal load cycling increased to 358 pA and 202.8 pA (increased by ~ 397.2% and 197.3% respectively), the thermo-mechanical reliability of TSV with scallops is much more reliable than the TSV with TSSW for the TSSW easily leads weakening or cracking. The influence the slope angle of triangular-tooth, the triangulartooth on different size of TSV, and the impact of the triangular-tooth on the growth and extension of the cracks on the tips of the
Fig. 20. Maximal KOZ size with different triangular-teeth number.
Fig. 22. Leakage current of the scalloped structure and the TSSW structure.
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triangular-teeth also shall be considered and studied. The more precise experiment shall be designed and tested in the future. Acknowledgments The authors would like to thank supports from Samsung Electronics Co., Ltd. APPLICATION. References [1] Eric Beyne, et al., Through-silicon via and die stacking technologies for microsystems-integration, Electron Devices Meeting, 2008. IEDM 2008. IEEE International, IEEE, 2008. [2] Yao Jen Chang, et al., Modeling and characterization of TSV capacitor and stable low capacitance implementation for wide-I/O application, IEEE Trans. Device Mater. Reliab. 15 (2) (2015) 1. [3] Yasuhiro Morikawa, et al., A novel scallop free TSV etching method in magnetic neutral loop discharge plasma, Electronic Components and Technology Conference (ECTC), 2012 IEEE 62nd, IEEE, 2012. [4] Qing Xu, et al., Enhanced etch process for TSV & deep silicon etch, Advanced Semiconductor Manufacturing Conference (ASMC), 2015 26th Annual SEMI, IEEE, 2015. [5] J. Kraft, et al., 3D sensor application with open through silicon via technology, 2011 IEEE 61st Electronic Components and Technology Conference (ECTC), 2011. [6] P.R. Lin, et al., Effects of silicon via profile on passivation and metallization in TSV interposers for 2.5 D integration, Microelectron. Eng. 134 (2015) 22–26. [7] Lado Filipovic, Siegfried Selberherr, The effects of etching and deposition on the performance and stress evolution of open through silicon vias, Microelectron. Reliab. 54 (9) (2014) 1953–1958. [8] Yu-Chen Hsin, et al., Effects of etch rate on scallop of through-silicon vias (TSVs) in 200 mm and 300 mm wafers, Electronic Components and Technology Conference (ECTC), 2011 IEEE 61st, IEEE, 2011. [9] Kangwook Lee, et al., Impact of Cu diffusion from Cu through-silicon via (TSV) on device reliability in 3-D LSIs evaluated by transient capacitance measurement, Reliability Physics Symposium (IRPS), 2012 IEEE International, IEEE, 2012. [10] J. Chen, et al., Physicochemical effects of seed structure and composition on optimized TSV fill performance, Electronic Components and Technology Conference (ECTC), 2015 IEEE 65th, IEEE, 2015. [11] Tomoji Nakamura, et al., Comparative study of side-wall roughness effects on leakage currents in through-silicon via interconnects, 3D Systems Integration Conference (3DIC), 2011 IEEE International, IEEE, 2012. [12] Wu Wei, et al., Interfacial stress analysis in TSVs by considering the sidewall scallop, Electronic Packaging Technology (ICEPT), 2014 15th International Conference on, IEEE, 2014.
[13] Yasuhiro Morikawa, et al., Total cost effective scallop free Si etching for 2.5 D & 3D TSV fabrication technologies in 300 mm wafer, Electronic Components and Technology Conference (ECTC), 2013 IEEE 63rd, IEEE, 2013. [14] Goon Heng Wong, et al., Through silicon via (TSV) scallop smoothening technique, Electronics Packaging Technology Conference (EPTC), 2014 IEEE 16th, IEEE, 2014. [15] Takahide Murayama, et al., High aspect ratio TSV etching process for high-capacitor, CPMT Symposium Japan (ICSJ), 2014 IEEE, IEEE, 2014. [16] N. Ranganathan, et al., A study of thermo-mechanical stress and its impact on through-silicon vias, J. Micromech. Microeng. 18 (7) (2008) 75018–75030 (13). [17] Anderson P. Singulani, et al., Effects of Bosch scallops on metal layer stress of an open Through Silicon Via technology, Reliability Physics Symposium (IRPS), 2013 IEEE International, IEEE, 2013. [18] O. Fursenko, et al., Through silicon via profile metrology of Bosch etching process based on spectroscopic reflectometry, Microelectron. Eng. 139 (2015) 70–75. [19] A.P. Singulani, H. Ceric, S. Selberherr, Stress evolution in the metal layers of TSVs with Bosch scallops, Microelectron. Reliab. 53.9–11 (2013) 1602–1605. [20] Cheryl S. Selvanayagam, et al., Nonlinear thermal stress/strain analyses of copper filled TSV (through silicon via) and their flip-chip microbumps, IEEE Trans. Adv. Packag. 32 (4) (2008) 720–728. [21] F.X. Che, et al., Study on Cu protrusion of through-silicon via, IEEE Trans. Compon. Packag. Manuf. Technol. 3 (5) (2013) 732–739. [22] Duncan Marsh, Applied Geometry for Computer Graphics and CAD, Springer Science & Business Media, 2006. [23] Yunna Sun, et al., The thermal mechanical reliability induced by the integrated circuits fabrication process, Electronic Packaging Technology (ICEPT), 2015 16th International Conference on, IEEE, 2015. [24] Yunna Sun, et al., Initial thermal stress and strain effects on thermal mechanical stability of through silicon via, Microelectron. Eng. 165 (2016) 11–19. [25] Kuan H. Lu, et al., Thermal stress induced delamination of through silicon vias in 3-D interconnects, Electronic Components and Technology Conference (ECTC), 2010 Proceedings 60th, IEEE, 2010. [26] Gleen B. Alers, et al., Interlevel dielectric failures in copper/low-k structures, IEEE Trans. Device Mater. Reliab. 4 (2) (2004) 148–152. [27] Q.M. Li, Strain energy density failure criterion, Int. J. Solids Struct. 38 (38) (2001) 6997–7013. [28] S.E. Thompson, et al., A 90-nm logic technology featuring strained-silicon, IEEE Trans. Electron Devices 51 (11) (2004) 1790–1797. [29] S.K. Ryu, et al., Effect of thermal stresses on carrier mobility and keep-out zone around through-silicon vias for 3-D integration, IEEE Trans. Device Mater. Reliab. 12 (2) (2012) 255–262. [30] Chia Yu Lu, et al., Impacts of SiN-capping layer on the device characteristics and hotcarrier degradation of nMOSFETs, IEEE Trans. Device Mater. Reliab. 7 (1) (2007) 175–180.
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Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops Yunna Sun a, Dongwoo Kang b, Yazhou Zhang a, Jiangbo Luo a, Yanmei Liu a, Yan Wang a,⁎, Guifu Ding a,⁎ a b
National Key Laboratory of Micro/Nano Fabrication Technology, Shanghai Jiao Tong University, Shanghai 200240, China Test&Package (TP) Center, Samsung Electronics Co., Ltd., Asan-City 31489, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 March 2017 Received in revised form 6 June 2017 Accepted 7 June 2017 Available online xxxx Keywords: Etching defect Triangular-teeth and scallops Initial thermal stress and strain Thermo-mechanical reliability
a b s t r a c t Through silicon via (TSV) with triangular-teeth and scalloped side wall (TSSW) not only lowers down the electrical performance but also alters the mechanism of thermal mechanical stability. By considering the realistic etching defects on TSV side wall, a more reasonable and reliable three-dimension (3D) TSV model with TSSW is built. The thermo-mechanical issues induced by the unsmooth side wall of the TSV are studied by finite element method (FEM) in this work. With the presence of TSSW, much more tips in interfaces of the TSV-Cu and SiO2 and the SiO2 and Si are brought and the shear stress and normal stress are distinct comparing with the smooth side wall. Therefore, the thermo-mechanical mechanism of the TSV is changed greatly. By investigating the normal stress, shear stress and strain energy density (SED) of the triangular-teeth local region, it has been found that severe normal stress variation (−200–70 MPa) and multiple variation of shear stress τxy contributes the peeling, slip and crack issues. The effective plastic strain, displacement and von Mises stress, and shear stress and normal stress in the X, Y and Z direction are studied in detail. The effect of the TSSW on the thermal stress and keep-out zone (KOZ) size is discussed. © 2017 Published by Elsevier Ltd.
1. Introduction With the advantages of lower power consumption, higher integration density and shorter interconnection length, TSV has been used as one kind of high efficiency and reliable technology for stacking wafers, [1–2]. The application of TSV is more and more widely from 2.5D to 3D. Through silicon vias (TSVs) is formed by ion etching [3–4], and filled with good conductive Cu commonly. As an advanced packaging technology in the semiconductor industry, TSVs have attracted many scholars' attention. There exists some main factors for TSVs' rapid development, such as, high thermo-mechanical reliability, reliable electrical performance, mass production with low cost. Drilling via is one of the main processes in TSV fabrication which will affect the insulating layer forming, seed sputtering, and TSV-Cu electroplating. The deep reactive ion etch or Bosch etching was a common and convenient way for obtaining via. Etch rate played an important role in getting high-quality TSVs, such as smoother sidewall, [5– 6]. However, the through via was unable to be straight and the sidewall was rough (large scallops) when the etch cycle time was too long and passivation cycle time was too short, [6–8]. Since higher thermo-mechanical stress, leakage currents, slower response time and lower signal
⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (G. Ding).
transmission speed, and Cu diffusion might be brought by the large scallops, the reliability of the integrated circuit (IC) was cut down, [9–12]. Some methods have been proposed for scallop free TSV etching [3,13], but the Bosch etching is still adopted regularly in the TSV etching [4,5, 14]. The scallop free etching method merits future research [15]. After Bosch etching, some triangular-teeth appeared near the port of the via and connected with smother scallops. Stress concentration points were formed by the triangular-teeth and scallops. Due to the existence of the TSSW the thermo-mechanical reliability of the TSV encounters new challenges during adding thermal loads. Electrical performance is also related to the thermal stress distribution for avoiding capacitor formed between the adjacent TSVs and Cu diffusion aggravation [9]. The interplay of Cu diffusion, TSSW dimension and thermal stress will decrease the reliability of the microsystem and aggravate the TSV failure. 2. Problem statement and model building The triangular-teeth (about 8–11) may appears near the top opening of the TSV shown in Fig. 1. A 3D symmetric TSV model with TSSW is built (Fig. 2), just as [16–19]. To simplify, a 1/8 symmetric model is set up, which have 10 triangular-teeth at the bottom of the interface of TSV Cu and SiO2, and the interface of SiO2 and Si. This model mainly considers three parts, TSV-Cu, SiO2 and Si. The diffusion barrier layer
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Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 1. SEM cross section view of TSVs.
Fig. 3. Geometry graphic for triangular-tooth and scallop.
(related to thermo-mechanical but not affected much since this layer is much thinner than other layers) is neglected for simplify, [20–21]. The above mentioned three layers are assumed to be perfectly interconnected in the interfacial layers. The geometry graphics for triangular-tooth and scallop are shown in Fig. 3. The scallop structure can be modeled by the modified Quadratic Bezier Curve, see formula (1) (detailed in [17,22]). BðtÞ ¼
ð1‐tÞ2 W 1 P 1 þ 2t ð1−t ÞW 2 P 2 þ t 2 W 3 P 3 ð1‐tÞ2 W 1 þ 2t ð1−t ÞW 2 þ t 2 W 3
; 0≤t ≤1
ð1Þ
where, W1, W2, and W3 are the weight factors; P1, P2 and P3 are the vertex points, shown in Fig. 3. Different with [17], the above fit curve is much smoother and actual. Firstly the scallop's height and width, P1, P2, and P3 are obtained from the measurement, Fig. 3(b). And then point P2 is fund by two tangent lines L1 and L2. With the fixed weight factors, W1 = W3 = 1, the best fit Quadratic Bezier Curve can be achieved by adjusting the weight factor W2. In this paper we set W2 = 0.707 to describe the scallop and the according dimensions of triangular-tooth and scallop are listed in Table 1. The initial thermal stress is not set stress-free at 293.15 K, but related to the thermal residual stress of under-fill filling (UFF) stage shown in Fig. 4, [23]. The thermo-mechanical properties are shown in Table 2. By COMSOL software the meshing on the model is shown in Fig. 5. Firstly, free quad with three or four layers is meshed on the lateral
surface of SiO2 layer. Next the adjacent lateral surfaces, Si and partial of TSV-Cu are meshed with free quad. After that, the domains of Si, SiO2 and partial of TSV Cu are swept with the meshed lateral surfaces. Once more, bottom surface of the remainder TSV-Cu is meshed with free quad. Finally, the remainder domain of TSV-Cu is swept with the meshed bottom surface of TSV-Cu. The meshes of the SiO2 are much finer than those of the TSV-Cu, TSV-Pad and Si for the barrier SiO2 is the thinnest films in this 1/8 TSV model. The boundary conditions of the TSV model are set as that three surfaces are symmetric with X, Y and Z coordinates respectively, and point e is fixed at X, Y and Z direction (Fig. 2). The other surfaces is set free.
3. Numerical analysis 3.1. Equivalent von Mises stress After UFF, the von Mises stress on the interface of TSV-Cu and SiO2 has occurred distinct changes from stress-free to close to the yield strength of Cu, especially on the edge and triangular teeth region (scallop region is much smaller), shown in Fig. 6. This is mainly related to the special structure of TSV with TSSW. The interfacial lines L1, L2 experience higher stress. Thus, the triangular teeth region and TSV pad will suffer severe thermal deformation and plastic accumulation during the working stage. With the initial stress and strain, TSV deformation goes on in working stage (293.15 K ➔ 423.15 K). The total deformation and von Mises stress resulting from heating up to 423.15 K are shown in Fig. 7(a). Maximal displacement appears at the TSV-pad similar to the normal TSV model. The interfacial lines of TSV-Cu and SiO2, and SiO2 and Si sustain higher von Mises stress, Fig. 7(b). Since the TSSW bring many tips, lots of stress concentration points turn up at the TSSW interfacial lines, especially on the triangular-teeth. Therefore, the interfacial lines L1, L2 (Fig. 7(b)) emerge splitting and cracks with high probability.
Table 1 Basic size of the TSV model with TSSW.
Fig. 2. 1/8 symmetric model with triangular-tooth and scallop. A, B, C, D and a, b, c, d are the interfacial points; L1, L2 are the interfacial lines; TT1 indicates the first triangulartooth and TT2 is the second triangular-tooth and so to TT3… and TT10.
Parameters
Unite (μm)
Height of the substrate Width of the substrate Diameter of TSV-Cu Depth of the silica barrier Height of TSV-pad Cu Width of TSV-pad Cu Weight of TSSW (w) Height of the triangular-tooth (h) Height of h1 Height of the schallop (h2)
250 150 250/6 2.0 6.0 60 2.000 0.360 0.540 ~ 0.224
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Fig. 5. 3D symmetric model with meshing.
Fig. 4. Thermal load during under-fill filling stage.
3.2. Normal stress, shear stress and effective plastic strain The normal stress σx is equal to σy with a 90 degrees rotation. Almost all the Si and SiO2 layers suffer higher normal stress (compressive and tensile). The interfaces with TSSW still suffer higher thermal stress (Fig. 8(a) and (b)). The TSV-Cu has greater deformation and higher normal stress in Z direction (Fig. 7). The normal stress σx (or σy) stress map around the TSV forms a larger zone composites with tensile stress and compressive stress and no zone by σz shown in Fig. 8(c) and (d) respectively. The thermal stress induced delamination and cracks is discussed as the TSV design criteria, [25]. When the stress level of the interface (Cu and SiO2) of the TSV exceeds yield strength of Cu (200 MPa), it is expected to crack at any weak site on the unsmooth sidewalls, [26]; and the influence of the alternating compressive stress and tensile stress. The size of the KOZ may be increased by the stress interaction. The KOZ size is evaluated through a criterion of 5% change in the carrier mobility. Normal stress in X or Y direction is much higher than that in Z direction, such as and σx on L2 and σy on L1 (Fig. 9(a)). The triangular-teeth region experiences sharping and huge thermal stress. Due to the limitation of Si and SiO2 on the TSV-Cu, the TSV-Cu is compressed in X and Y direction and stretched in Z direction. With the existence of TSSW, the normal stress on the interfaces of TSV-Cu and SiO2, and SiO2 and Si are enlarged. As shown in Fig. 9(b) the interfacial line of TSV-pad and SiO2 (LBC) bears very higher compressive stress σx (− 389 MPa), however, σx on LAD is beyond 50 MPa (L1, L2, LAB, LBC, LCD, LAD, Lab and Lcd have defined in Fig. 2). It should be pointed out Lcd endures compressive and tensile stress (−150–125 MPa). Due to the special rotational structure, normal stress σz on these lines are very lower, only a few MPa. With the rotational structure the shear stress τxz and τxy with 90 degrees carry opposite signs. As the special structure of TSV-Cu, higher shear stress τxy and τxz are in the interfacial layers of TSV-Cu and SiO2, and SiO2 and Si. After heating up to 423.15 K, τxy is [−39.5, 200] MPa and τxz is [− 175, 305] MPa (Fig. 10). τxz is much larger than τxy for the interfacial lines exist in the Z direction, thus, sliding issues often
appear at these region. These are related to the special structure of TSV fixed by SiO2 and Si in X and Y direction, and having free space in Z direction for relieving thermal stress. Due to the τxy in XY plane (Fig. 10(a)), the interplay of the adjacent TSVs may be enlarges the KOZ size. As shown in Fig. 11(a), the shear stress on the interfacial line of TSVCu and SiO2 (L1) is much sharper than that on the SiO2 and Si (L2). The sharpest shear stress regions appear at the Triangular-teeth connected with the bottom TSV-Pad. Higher shear stress on Lab, Lcd, LAB, LBC, LCD and LAD may lead interfacial slip. Fig. 11(b) shows that the maximal shear stress τxy turns up at Lcd due to elastic deformation on Lcd, and elastic-plastic deformation on Lab, LAB, LBC, LCD and LAD. Since the shear stress of LBC is much higher, the interfaces of TSV-pad and SiO2 suffer slipping with higher probability. LBC and LAD suffer relative higher shear stress τxz in Fig. 11(c). That's, higher shear stress τxz arises at the TSV-pad and the adjacent regions. τxz and τyz on interface of are closed to the yield strength of Cu which leads plastic deformation and accumulation on the interface. τxz and τyz on interface of SiO2 and Si are over 305 MPa (Fig. 10). The shear stress of τxz and τyz are much higher than τxy. Thus, the sliding issues may occur in Z direction. The effective plastic strain on L1 accumulates greatly, shown in Fig. 12(a). As the distance (to the bottom TSV-pad) becoming larger, the effective plastic strain on triangular-teeth is lowered down and then smoothened. The effective plastic strain on scallops is flat (only about 0.0002). Thus, the triangular-teeth thermo-mechanical reliability is going to suffer great challenge. The effective plastic on the interfacial curves of TSV-Cu in XY plane is much smaller compared with L1 in Fig. 12. Therefore, the accumulation of plastic strain is mainly in Z direction.
Table 2 Thermo-mechanical properties [24]. Name
Cu
SiO2
Si
CTE (ppm/K) Young's modulus (GPa) Poisson's ratio Yield strength (MPa) Tangent modulus (MPa)
17 110 0.35 200 357
0.5 70 0.17 – –
2.6 130 0.28 – –
Fig. 6. Initial von Mises stress induced by under-fill filling.
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Fig. 7. Distribution of (a) total displacement and (b) von Mises stress, after heating up.
3.3. Thermal stress and strain energy density on the triangular-teeth The thermal stress and SED [25,27] of the triangular-teeth of L1 and L2 are investigated and analyzed. Fig. 13 exhibits the thermal stress of each TT1, TT2–TT10 (Fig. 2) and Fig. 14 shows the average thermal stress and SED of each TT1, TT2–TT10. The normal stress on Si and SiO2 interfacial line L2 is much complicated, vibrated and lager than on L1. Due to the special location connected with TSV-pad and bounded by SiO2 and Si, triangular-teeth TT1–TT2 mainly experience compressive stress in X and Y direction, and tensile stress in Z direction. The normal stress becomes stable after TT6 or TT7, that is, the normal stress of TT6–TT10 on L1 and TT7–TT10 on L2 are changed pretty small. The normal stress σx are basically compressive press (Fig. 13(a) and (b)). While the normal stress σz on L1 is tensile stress (Fig. 13 (c)) and on L2 are alternative compressive stress and tensile stress (Fig. 13(d)). Higher compressive stress in X and Y direction arising near the tips of TT6–TT10 (Fig. 13(a) and (b)) may reduce the peeling phenomenon. However, these tips easily generate cracks and the σx related KOZ will be enlarged. TT1 and TT2 experience higher irregular thermal stress variation resulting from the special location closely connected with TSV-pad (more freedom for releasing stress) shown in
Fig. 9. Normal stress on L1, L2, LAB, LBC, LCD, LAD, Lab and Lcd.
Fig. 8. Normal stress distribution in (a) X, (b) Z direction; (c) X, (d) Z direction of the whole middle cross section.
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Except for the compressive stress and tensile stress, σz on L1, other thermal stresses on TT1–TT10 mainly are compressive stress based on the above analysis and Fig. 13. The average thermal stress and SED of each triangular-teeth is available to evaluate the triangular-tooth as a whole shown in Fig. 14. With the distance increase of the triangulartooth on L1 away the TSV port, the von Mises stress (vMS in Fig. 14) and τxz reduces and σx become large. With the distance increase of the triangular-tooth on L2 away the TSV port, the von Mises stress and σx raise and σy, σz, τxz and SED decrease (Fig. 14(b)). τxz and τyz are very small comparing with other thermal stresses. σy range on L1 is −32– 30 MPa. TT1 suffers the highest SED resulting from the larger accumulated strain and deformation for releasing stress. The average σz on L1 shows tensile stress owing to the TSV-Cu bumping in Z direction. 3.4. Effect of thermal stress on the carrier mobility and KOZ The absolute value of the relative mobility changes along the [100] channel direction is defined as Eq. (2), [28,29]. By considering the piezoresistivity of Si, specific stress characteristics are used to enhance the performance of Si MOSFETs, and such “strained-Si” technology has been proposed since the 90-nm technology node [28], such as, implanting Ge into the source/drain region or introducing a silicon nitride capping layer for the N-MOSFETs [30]. By considering the effect of device alignment, the mobility change induced by the anisotropic properties of Si along [110] becomes Eq. (3), [29]. The piezoresistance coefficients for N-type and P-type Si are listed in Table. 3.
Fig. 10. Shear stress (a) τxy and (b) τxz, distribution TSV model.
Fig. 13(c) and (d). TT1–TT10 of L1 bear great tensile stress σz (Fig. 13(c)). Normal stress σz on TT1 of L2 challenges the reliability of the TSV with great local change. In addition, the interaction of compressive stress and tensile stress will enhance the peeling and crack issues on the triangular-teeth interline (Fig. 13(d)). In summary, peeling and crack issues in X and Y direction easily occur at the TT5–TT10 of the TSV-Cu and SiO2 interfacial line ((b)), while in Z direction arise at TT1–TT4 of the TSV-Cu and SiO2 interfacial line (Fig. 13(d)). The tendency of shear stress is much more mutable and complicated than the normal stress. Shear stress τxy is very small (−4.5–0.5 MPa). The interfacial slip issue in XY plane is small probability of occurrence. Comparing Fig. 13(e) with (f), τxz of TT1–TT6 on L2 are much mutable and so the slip and cracks will turn up in these regions with higher probability. After the TT1–TT6, τxz is cushioned and becomes smooth.
Δμ ¼ jπ11 σ 11 þπ12 ðσ 22 þσ 33 Þj μ
ð2Þ
Δμ ¼ j0:5ðπ11 þπ12 Þðσ 11 þσ 22 Þþπ44 σ 12 j μ
ð3Þ
The mobility changes on the TSV substrate Si with no device and with MOSFET are shown in Fig. 15–Fig. 17. Fig. 15 shows KOZ size (lines, the 5% mobility changes) of 4 XY planes (with different via depth).The mobility change on the N-type Si is higher than the P-type with no device, and the mobility change on the Si with N-type MOSFET is lower than P-type MOSFET. The size of the KOZs of the N-type Si with no device (Fig. 15(a)) and with the P-type MOSFET (Fig. 15(d)) are much larger while on the other two the KOZ can be neglected. The larger boundary distance R of N-type Si with no device and with the P-type MOSFET are alone the dashed lines (cut line). Fig. 16 shows the maximal R alone the Z coordination. The KOZ size is much larger near the opening of TSV and decreases dramatically and then increases with via depth, Fig. 16(a) and (d). That is, the KOZ size of the opening regions and the middle via depth region is larger, thus the KOZ size of the device also effected greatly though the device is planted near the Si surface. The mobility change of different types of Si and MOSFET at different X and Y coordinates (1/4 partial of the via) are shown in Fig. 17, the mobility
Fig. 11. Shear stress on L1, L2, LAB, LBC, LCD, LAD, Lab and Lcd.
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change for the MOSFET is near the Si surface (the depth on Si is beyond 1 μm). The mobility change of the P-type Si is below 5%. The KOZ size of the N-type Si is largest. The N-type MOSFET suffers a larger mobility change but the KOZ size is not large.
3.5. Effect of the triangular-teeth number
Fig. 12. Effective plastic strain on L1, LAB, LBC, LCD, LAD and Lab.
The maximal thermal stress, mobility change, and KOZ size of the vertical TSV are listed in Table. 4–Table. 5. The maximal thermal stress, mobility change, and KOZ size of the TSV with different triangular-teeth number is shown in Figs. 18-20. Due to the existence of the scallop the shear stress τxy and τxz are enlarged, and the normal stress σx and equivalent von Mises stress are reduced. The great decrease of the dominant normal stress σx and equivalent von Mises stress show that the presence of scallops improves thermo-mechanical stress performance of the TSV due to pinning effect. It shall be pointed out that if the shear stress is enlarged greatly by the unsmooth sidewall that may induce peeling issues. However, the triangular-teeth lowers down the advantage of the pinning effect induced by the unsmooth sidewall. The maximal equivalent von Mises stress, normal stress (σx or σy) and shear stress τxz are all increasing with the triangular-teeth number, except a jagged equity during 10 triangular-teeth (16% coverage rate), after the coverage rate is 24% the stress almost keep constant. However, σz and τxy rise first and then fall. σx and τxz are larger than the according to σz and τxy. The von Mises stress and normal stress is affected much seriously than shear stress. The σx increases greatly (about 70 MPa) and the τxy changes much smaller. Comparing the mobility change, and KOZ size of the TSVs with unsmooth sidewall with the vertical TSV (Table 5, and Figs. 19–20), the KOZ size of P-type Si is zero for the mobility change are all below 5%; for the N-type Si the maximal mobility change is enlarged from 33.68% to ~40%, the KOZ size is not enlarged except the model with 5 triangular-teeth, but becomes smaller for the model only with scallop and with more than 20 triangular-teeth. The maximal mobility level of the MOSFET is similar. The KOZ size of the P-type MOSFET is enlarged more than 26.6%.
Fig. 13. Normal stress and shear stress on triangular-teeth.
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Fig. 14. The average thermal stress and strain energy density on the different triangular-teeth on (a) L1 and L2. Table 3 Piezoresistance coefficients for N- and P-type Si [29]. Name (10−11 1/Pa)
π11
π22
π44
N-type Si P-type Si
−102.2 6.6
53.7 −1.1
−13.6 138.1
4. Experiment analysis The electrical leakage current of the TSV samples is tested under thermal load. For the weakening or cracking is expected to result in a
Fig. 15. Carrier mobility distribution on XY plane for (a) N-type Si substrate and (b) P-type Si substrate in [100] direction, and (c) N-type MOSFET and (d) P-type MOSFET in [110] direction. The curves are the 5% mobility change.
Fig. 16. Carrier mobility distribution for (a) N-type Si substrate, (b) P-type Si substrate in [100] direction, and (c) N-type MOSFET and (d) P-type MOSFET in [110] direction. The curves are the 5% mobility change.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Y. Sun et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Fig. 17. Carrier mobility distributions in [100] direction for different type Si and in [110] direction for different type MOSFET.
high electrical leakage current due to copper ion diffusion through the weak barrier and dielectric layers [16], the electrical leakage current of different thermal load cycles is used to assess the thermo-mechanical reliability of the TSV. The leakage current is tested with Keithley 2400 SourceMeter and Probe station (Semishare electronic Co. Ltd., SM-4). The test structure is shown in Fig. 21.
4.1. Leakage current
rent for the two models are increased by about 27.0 pA and 33.6 pA, respectively. With the cycling of thermal load, the leakage current enlarged, especially for the TSV with TSSW. After 10 cycles, the leakage current for the two models are increased by about 30.9 pA and 41.8 pA, respectively. After 15 cycles, the leakage current for the two models are increased by about 76.7 pA and 219 pA respectively. After 15 cycles, the barrier of the TSV with TSSW is weakening or cracking. The thermo-mechanical reliability of TSV with scallops is much more reliable than the TSV with TSSW for the TSSW easily leads weakening or cracking.
The electrical leakage current of the TSV with scalloped structure and the TSSW structure is shown in Fig. 22. The leakage current is tested at the initial state, after 5 cycles (293.15 K ➔ 423.15 K ➔ 293.15 K), after 10 cycles and after 15 cycles. At the initial state the leakage current of the Scallop and TSSW models are similar and below 68.2 pA and 72.0 pA, respectively. After 5 cycles of thermal cycling, the leakage curTable 4 Maximal thermal stress of the vertical TSV. Parameter
vM
σx
σz
τxy
τxz
(MPa)
606.81
337.71
225.92
171.80
297.68
Table 5 Maximal mobility change and KOZ size of the vertical TSV. Parameter
N-Si
P-Si
N-MOSFET
P-MOSFET
Mobility change (%) KOZ size (μm)
33.68 34.17
2.43 0
22.64 5.67
23.35 13.17
Fig. 18. Maximal thermal stress with triangular-teeth number.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
Y. Sun et al. / Microelectronics Reliability xxx (2017) xxx–xxx
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Fig. 21. Cross-sectional view of the test structure.
Fig. 19. Maximal mobility change with triangular-teeth number.
5. Conclusions TSV with triangular-teeth and scallops was analyzed by FEM. The 1/8 symmetric model with initial stress and strain was studied with static temperature ramp up from 293.15 to 423.15 K. With consideration the actual packaging process, the initial stress and strain was not set stress-free at 293.15 K but related to under-fill filling process. Much more reliable and finer meshes were built on the 3D model by sweeping the surface. The TSV with triangular-teeth suffers higher thermal stress than only with scallops which would lead electrical and mechanical failure issues. 1. Much higher von Mises stress, normal stress and shear stress were raised in the triangular-teeth and the TSV-pad edges, thus the tips of the TSSW might generate cracks and failure issues with high probability. The normal stress and shear stress on the triangular-teeth varied greatly, especially on the TSV-Cu and SiO2 interfacial line and connected with TSV-pad closely.
2. The much sharper surface on the triangular-teeth regions induced higher thermal stress and effective plastic strain which would lead electrical and mechanical failure issues. 3. Due to the larger accumulated strain and deformation for releasing stress, the TT1 on L1 suffers higher von Mises stress, τxz and especially highest SED. The TT1 of L2 suffers lower von Mises stress and τxz, and higher σx, σy, σz, and SED. The average σz on L1 shows tensile stress owing to the TSV-Cu bumping in Z direction. 4. Since τxz of TT1–TT6 on L2 were much mutable, the slip and cracks would turn up in these regions more easily. 5. Although higher compressive stress in X and Y direction arising near the tips of TT6–TT10 might not increase the probability of the interfacial peeling phenomenon, these tips easily generated cracks. 6. With the incensement of triangular-teeth number, the von Mises stress and normal stress have been enlarged greatly, but the shear stress almost constant (τxy, 25 MPa; τxz 3 MPa). When the triangular-teeth coverage rate reached 24% (15 triangular-teeth), the von Mises stress and σx also kept stable and σx reduced somehow. 7. With the incensement of triangular-teeth number, the KOZ size of Ptype MOSFET is increased more than 26.6%. 8. Since the leakage current of the TSV with TSSW or with scallops after 15 cycles of thermal load cycling increased to 358 pA and 202.8 pA (increased by ~ 397.2% and 197.3% respectively), the thermo-mechanical reliability of TSV with scallops is much more reliable than the TSV with TSSW for the TSSW easily leads weakening or cracking. The influence the slope angle of triangular-tooth, the triangulartooth on different size of TSV, and the impact of the triangular-tooth on the growth and extension of the cracks on the tips of the
Fig. 20. Maximal KOZ size with different triangular-teeth number.
Fig. 22. Leakage current of the scalloped structure and the TSSW structure.
Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013
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Please cite this article as: Y. Sun, et al., Plastic analysis for through silicon via with actual etching defect of triangular-teeth and scallops, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.013