Microelectronic Engineering 149 (2016) 145–152 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee Accelerated publication Measurement-based electrical characterization of through silicon vias and transmission lines for 3D integration Xin Sun a, Runiu Fang a, Yunhui Zhu a, Xiao Zhong a, Yuan Bian a, Yong Guan a, Min Miao b, Jing Chen a, Yufeng Jin c,a,⁎ a b c National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, China Information Microsystem Institute, Beijing Information Science and Technology University, Beijing 100101, China Peking University Shenzhen Graduate School, Shenzhen 518055, China a r t i c l e i n f o Article history: Received 2 February 2015 Received in revised form 3 September 2015 Accepted 19 October 2015 Available online 20 October 2015 Keywords: 3D integration Through silicon via (TSV) Transmission line Electrical measurement RF characterization a b s t r a c t Measurement-based electrical characterization of through silicon via (TSV) and redistribution layer (RDL) is of great importance for both fabrication process and system design of 3D integration. This paper presents the electrical measurements and analysis of TSV and double-sided RDL test structures, from DC to high frequency up to 40 GHz. TSV shows great dependence of DC resistance and leakage current on fabrication process. An inverse V-shaped C–V curve is presented between adjacent TSVs in N-type silicon substrate, from − 10 V to 10 V. In the high frequency characterization, two methods are proposed and applied to extract resistance and inductance of a single grounded TSV. Individual transmission loss of TSV, RDLs on top and bottom surface of silicon substrate are calculated, and corresponding circuit parameters thereof are extracted to characterize their electrical properties precisely. © 2015 Elsevier B.V. All rights reserved. 1. Introduction With increasing demands for smaller, multi-functional electronic devices, 3D integration with through silicon via (TSV) has attracted substantial attention and has been already on the way to commercialization. It provides vertical interconnects with high integrated density, greatly reduced interconnect length, low power consumption, heterogeneous integration, high system speed and small form factor [1–4]. Many 3D integration architectures have been proposed to address different applications, including MEMS, CMOS image sensor (CIS), stacked memories, and so on. Nevertheless with these sophisticated 3D integration technique, TSV and redistribution layer (RDL) remain the key components for signal transmission. According to the functionality requirements, a wide range of different signals (digital, analog, RF etc) exist in the electrical system. It is mandatory to thoroughly investigate the electrical behavior of TSV and redistribution layers, both at DC and RF/microwave frequencies. So far, electrical modeling and simulation of TSV have been done in lots of previous works. Several equivalent circuit models have been proposed for TSV, and analytical expressions of its RLCG parameters were derived. Kim et.al [5] proposed an equivalent circuit model for TSVs arranged in a ground-signal configuration. The values of RLCG ⁎ Corresponding author at: Peking University Shenzhen Graduate School, Shenzhen 518055, China. E-mail address: yfjin@pku.edu.cn (Y. Jin). http://dx.doi.org/10.1016/j.mee.2015.10.010 0167-9317/© 2015 Elsevier B.V. All rights reserved. parameters were tied to the geometrical and physical parameters of TSV based on two-conductor transmission line theory. Salah et al. [6] used a π-type equivalent circuit model for a single TSV inside grounded silicon substrate, and the dimensional analysis method was used to derive the expressions of RLCG parameters. Liu et al. [7] applied magneto-quasi-static theory and developed a rigorous resistanceinductance model of TSV, considering the skin effect and proximity effect of TSV at high frequency. Impact of physical parameters of TSV on its transmission performance were also widely studied, such as TSV height, diameter, pitch, liner thickness, cross-section shape, substrate resistivity, etc. [8–11]. Fundamental design rules can be drawn based on these parametric analyses. DC resistance, leakage current and C–V measurement of TSV have been widely used for process evaluation in previous works [12–15]. Gu et al. [16] evaluated the loss and crosstalk properties of RDL transmission lines on silicon substrate. A TSV–RDLbump test vehicle was measured up to 20 GHz in [5], and the measured S parameters agreed well with the ones calculated from the equivalent circuit model. Ndip et al. [17] extracted the inductance and capacitance of GSG–TSV from single-port open and short TSV test vehicles, in the 100 MHz to 40 GHz frequency range. Dual-port TSV chain structures were measured in [18], and RLCG parameters of TSV were extracted based on empirical formulas and 3D full wave electromagnetic simulations. Four-port test vehicle with two grounded TSVs were applied in [19]. Coupling capacitance and conductance were extracted. In the above-mentioned studies, some only dealt with experimental results without sufficient analysis. As measurement analysis is of great concern 146 X. Sun et al. / Microelectronic Engineering 149 (2016) 145–152 dependent properties of TSV's resistance and inductance. Accuracy is improved with a new de-embedding method. Another test vehicle with GSG–TSV and RDL transmission lines is measured to show the signal integrity of the interconnect path for 3D integration. The contributions of TSV, RDL transmission line, and test pad to the total transmission loss of the test vehicle are calculated respectively, and RLCG parameters of TSV and RDL transmission line are extracted and analyzed for design recommendations. 2. TSV–RDL structure description Fig. 1. SEM cross section of the fabricated structures with TSV and metal lines on both sides of the silicon substrate. to both fabrication process and system design, there is still room for improvements in the studies of measurement-based characterization of TSV and RDL. In this paper, electrical measurement and analysis of TSV and RDL structures are carried out, from DC to high frequency up to 40 GHz. Test structures are designed and fabricated based on an in-house fabrication process [20–21]. DC resistance, leakage current, and C–V curves are measured to evaluate the basic properties of TSV and RDLs. A test vehicle with a grounded TSV is applied to study the frequency- Several test structures are fabricated to analyze the electrical properties of TSV and RDLs, which consist of TSVs and RDLs on both top and bottom surface of silicon substrate. Fig. 1 is the scanning electron microscope (SEM) image of the fabricated TSV and double-sided RDL structures. TSVs are etched with DRIE (deep reactive ion etching) equipment and filled with electroplated copper. The fabricated TSVs show a tapered cross-section, with 17.6 μm in diameter and approximately 99 μm in depth. TSVs are isolated with SiO2 from Si substrate. The SiO2 liner is no thicker than 0.5 μm due to the difficulty of high aspect ratio deposition. As shown in Fig. 1, copper RDLs on the top and bottom surface of the silicon substrate are electroplated with a thickness of 6.8 μm and 5.8 μm respectively. RDLs on the top side are lined with a 9.2 μm thick BCB (benzocyclobutene) layer from Si substrate, and bottom RDLs are deposited on a SiO2 layer, which is less than 2 μm thick. N-type silicon substrate is thinned to 87.6 μm, of which the average bulk resistivity is 3.1 Ω cm by four-point probe measurement. Fig. 2. Four-point Kelvin structure for TSV resistance measurement, (a) 3D model, (b) SEM image in top view, (c) distribution of measured single TSV resistance, the inset shows resistance distribution below 200 mΩ. X. Sun et al. / Microelectronic Engineering 149 (2016) 145–152 Fig. 3. Resistance of daisy chains with different TSV number. 3. DC analysis of TSV and RDL In this section, resistance, leakage current and C–V curves are measured to emphasize the DC/low frequency properties of TSV and RDL. Several 3 μm-thick, 500 μm-long BCB lined metal lines are used for resistance measurement of RDLs, which are fabricated on the top surface of silicon substrate, with line width of 10 μm, 20 μm and 30 μm respectively. The calculated sheet resistance is 8.87 mΩ/□, and the calculated conductivity of copper is about 3.6 × 107 S/m, lower than the theoretical value of 5.8 × 107 S/m. Lithography resolution, BCB shrinkage, surface roughness of metal lines may all contribute to the fluctuations of RDL resistance. DC resistance of a single TSV is evaluated with four-point Kelvin structures, as show in Fig. 2(a). Electric current flows through test pads marked as I + and I −, and voltage between test pads V + and V − is recorded, so that the accurate resistance of the centered TSV is measured. Fig. 2(b) shows the surface SEM image of the Kelvin test structure. Cumulative distribution of single TSV resistance in Fig. 2(c) shows that, TSV resistance gets a relatively wide distribution, ranging from 4.3 mΩ to 7 Ω, with over 75% lying between 4.3 mΩ to 200 mΩ, as in the inset curve. Contact resistance between TSV and RDL may be the major cause of wide spreading of resistance. Besides, straight-line daisy chain with maximum 42 TSVs, and two serpentine daisy chains with 54 and 108 TSVs are also measured, which consist of TSVs, RDLs on top and bottom surface of Si substrate. 147 The pitch between adjacent TSVs in line or in array is 50 μm. Fig. 3 shows the resistance of daisy chains as a function of TSV number, which shows fine connectivity between TSV and RDLs. The calculated slope of curve segment in Fig. 3 indicates that, the resistance of TSV and surface RDLs in series is 14 mΩ for straight-line daisy chain, and 33 mΩ for serpentine ones. Dense TSV distribution leads to higher resistance. Interdigital comb structure of top metal lines is designed and fabricated for leakage current test, measured with voltage varying from 0 V to 50 V. Each electrode has seven 700 μm-long RDL lines in parallel. The width and space of top RDL lines are 20 μm/30 μm. Leakage current between top RDLs is less than 3 pA up to 50 V, indicating great insulation of metal lines. Open-ended TSV pair is fabricated for leakage current measurement, as graphed in Fig. 4(a). Two adjacent TSVs are of 30 μm in pitch with each connected by test pad. It is observed from Fig. 4(b) that the leakage current between TSVs is tens of nA below 2 V, and reaches 150 nA up to 30 V. Leakage current between the top metal pad pair stays below 1 pA up to 30 V. Top metal lines shows pretty good insulation properties, while TSV dominates the leakage current performance. As mentioned above, the sidewall SiO2 liner of TSV is no thicker than 0.5 μm, causing a large leakage current as voltage increases. Isolation of sidewall liner is still of great importance for the electrical reliability of TSV. To investigate the MOS capacitance effect of TSV, TSV pairs similar to Fig. 4(a) but with pitch of 40 μm/50 μm between TSVs are fabricated and measured from −10 V to 10 V at 5 MHz. A sharp inverse V-shaped C–V curve is formed, as shown in Fig. 5. The peak capacitance value is around 0 V, which is ten times larger than that near ±10 V. The capacitance between metal pads is very small and is neglected here. As TSVs are buried in a low-resistivity N-type silicon substrate with thin SiO2 liner around, this coupling structure with two adjacent TSVs is similar to a twoterminal partially-depleted SOI MOS capacitor, a Metal-InsulatorSemiconductor-Insulator-Metal (MISIM) structure. As the applied voltage varies, the semiconductor surface around one of these two TSVs changes from inversion mode to accumulation mode, and the other operates reversely [22]. 4. High frequency analysis of TSV and RDL transmission lines Regarding high frequency and high speed applications of 3D integration, high frequency measurement and analysis of TSV and RDL are also necessary. In this section, scattering parameters of RDL and TSV–RDL chain structures are measured with microwave probe station and vector network analyzer up to 40 GHz. High frequency transmission performances of TSV and double-sided RDLs are investigated, and their electrical parameters are also extracted and analyzed. Fig. 4. Leakage current test of TSV pair, (a) SEM image of TSV pair for leakage current test in top view, (b) current–voltage curves of TSV pair up to 30 V, compared with that of pad pair without TSV. 148 X. Sun et al. / Microelectronic Engineering 149 (2016) 145–152 Fig. 5. Capacitance-voltage curves of TSV pair with pitches of 40 μm and 50 μm. 4.1. Resistance and inductance extraction of single TSV The dual-port GSG (Ground-Signal-Ground) structure in Fig. 6 is fabricated to extract the resistance (noted as RTSV) and inductance (noted as LTSV) of a single TSV. The peripheral 36 TSVs connect the backside ground plane and the ground ring on the top surface, while the centered TSV connects the backside ground plane and a topside metal line in the middle. Unlike DC measurement, high frequency characterization can capture the frequency-dependent properties of TSV. Two equivalent models are applied for this dual-port test structure, T-model based on Z parameter, and cascaded model in terms of ABCD matrix, shown in Fig. 7(a) and (b) respectively. For the T-model in Fig. 7(a), RTSV and LTSV can be extracted from the real and imaginary part of Z3 component respectively [23]. In this T-model extraction method, the dual-port structure in Fig. 6 is considered as a lumped circuit, which neglects distributed parasitic parameters of metal lines connecting TSV. To improve the accuracy of RTSV/LTSV extraction, the cascaded model in Fig. 7(b) is proposed here. An additional “THRU” structure similar to Fig. 6 but without the centered TSV is used to de-embed the parasitic parameters of metal lines. As shown in Fig. 7(b), both metal lines noted as ‘TL’ and the centered TSV are represented by distributed ABCD parameters. The ABCD matrix of TSV can be solved with the “THRU” which consists of only two TL metal lines. Using the cascaded model, the shunt admittance of TSV only includes RTSV and LTSV in series, in terms of ABCD matrix [24]. The extracted resistance and inductance of the centered TSV are illustrated in Fig. 8(a) and (b). It is shown that, the resistance (RTSV) and inductance (LTSV) extracted from the T-model with Z parameters Fig. 7. Equivalent models for resistance and inductance extraction of single TSV in Fig. 6, (a) T-shaped model with Z parameter, (b) cascaded model with ABCD parameter. are higher than that from the cascaded model, especially as the frequency increases. Both RTSV and LTSV are frequency-dependent due to skin effect [17]. Based on the cascaded model extraction method, RTSV is about 1 Ω, and LTSV 46 pH at 40 GHz. Comparison with simulation results are also illustrated in these two figures. The extracted RTSV is higher than the simulated one, because RTSV has a strong variation with electrodeposition quality of TSV, tapered shape of TSV, contact resistance between TSV and RDL, and so on. As LTSV is correlated with fewer variables, only the shape and size of TSV, it matches better with the simulation results. 4.2. High frequency characterization of TSV–RDL path Critical signal path used in 3D integration may consist of RDLs on the top and bottom surface of silicon substrate, and TSV. It is of great significance to evaluate the performance of a typical TSV–RDL signal path, and to find out the dominant part of the transmission loss. Aiming at this issue, test structures shown in Fig. 9 are fabricated and measured, including two transmission line structures noted as TL1, TL2, and two TSV–RDL chain structures noted as DUT1 and DUT2. Dimensions of these test structures are listed in Fig. 9(b). TL1 and TL2 consists of CPW-type top RDL lines with two test pads on the ends, and the CPW line of TL2 is two times as long as the one of TL1. DUT1 and DUT2 has the similar configuration including test pads, top RDLs, GSG–TSV sets (50 μm TSV–TSV pitch) and bottom RDLs. The backside RDLs of DUT2 are also two times as long as the ones of DUT1. Fig. 6. Dual-port TSV structure for extraction of TSV resistance and inductance, (a) 3D model, (b) SEM image in top view. X. Sun et al. / Microelectronic Engineering 149 (2016) 145–152 149 Fig. 8. Extracted resistance and inductance of single TSV with test structures in Fig. 6, dark line for simulation results, red line for T model extraction method, and blue line for cascaded model extraction method, (a) resistance, (b) inductance. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 9. TSV test structures in ground-signal-ground (GSG) type, TL1 and TL2 denote 200 μm-long and 400 μm-long GSG metal lines on the top side of silicon substrate, DUT1 and DUT2 denote chain structures of TSV and metal lines with 200 μm-long and 400 μm-long backside metal lines, (a) 3D model, (b) SEM image in top view. All of the four structures are schematically divided into small transmission segments, noted as TL (top RDL line, 200 μm long), BL (Bottom RDL line, 200 μm long), PAD and TSV (GSG–TSV set), as shown in Fig. 10. Then those four test structures can be represented by these transmission segments cascading in terms of ABCD matrices, so that the transmission parameters of these segments can be obtained by matrix operations [25–26]. Fig. 11(a) and (b) shows the measured S11 and S21 parameters of TSV–RDL test structures in Fig. 9, along with calculated S11 and S21 parameters of test pad, top RDL line (TL), bottom RDL line (TL) and GSG– TSV set. It is observed that, structure DUT2 in Fig. 9 shows large insertion loss, up to −7.7 dB at 40 GHz, while the loss of structure TL2 with only top RDLs is much lower, −2.2 dB at 40 GHz. Curves of PAD covers the slight loss caused by probe contact, discontinuity, impedance mismatch, and so on. Top transmission line (TL) benefits from the 9 μm thick BCB liner, with S21 magnitude of just − 0.54 dB at 40 GHz. As for bottom transmission line (BL), thin SiO2 liner helps to attenuate S21 magnitude to −0.96 dB at 40 GHz. Comparing with PAD, TL, BL segments, GSG–TSV shows much larger loss in 0–40 GHz range, −1.89 dB at 40 GHz, and a steep drop of S21 magnitude below 1.5 GHz. Generally, it is indicated that TSV dominates the transmission loss of TSV–RDL signal path. Low-resistivity (3.1 Ω cm) silicon substrate and very thin sidewall SiO2 liner could be the causes of TSV's large loss. To validate this conclusion, RLCG parameters (resistance, inductance, capacitance, and conductance) of TSV are extracted from the calculated ABCD matrix of GSG–TSV structure. Both π-model and T-model are applied as the equivalent circuit model of GSG–TSV, as shown in Fig. 12(a) and (b). Then conversion from ABCD parameters to Y parameters is carried out for π-model, and conversion to Z parameters for T-model [27]. RLCG parameters of circuit models in Fig. 12, can be extracted from Y parameters and Z parameters of GSG–TSV, respectively. Fig. 13(a) and (b) illustrate the extracted RLCG values of GSG–TSV, of which ‘π’ is short for π-model in Fig. 12(a), and ‘T’ for T-model in Fig. 12(b). Except for the slight deviation of resistance values, RLCG parameters derived from π-model and T-model are almost identical. Because of skin effect, the resistance of TSV increases with frequency, and inductance decreases with frequency. Comparing with single TSV structure in Fig. 6, GSG configuration leads to more obvious proximity effect between signal TSV and grounded TSV, which results in higher series resistance values. Mutual inductance between signal TSV and grounded TSV helps to reduce the total inductance of GSG–TSV model. Fig. 10. Decomposition diagram of the four test structures shown in Fig. 9, each structure consists of test pad (PAD), 200 μm-long topside metal line (TL), 200 μm-long bottom metal line (BL), and/or GSG–TSV (TSV) segments. Brackets [] denote the ABCD parameters of corresponding transmission segment. 150 X. Sun et al. / Microelectronic Engineering 149 (2016) 145–152 Fig. 11. S parameter of TSV test structures and transmission segments (test pad, top/bottom metal lines and GSG–TSV set). (a) S11 magnitude, (b) S21 magnitude. The inductance values of single TSV and GSG–TSV are approximately 46 pH and 27 pH above 5 GHz respectively. Back to the concerns of low-resistive Si substrate and SiO2 liner, from Fig. 13(b), it is shown that, extracted capacitance of GSG–TSV excesses 400 fF below 1.5 GHz, causing a sharp increase of transmission loss in lower frequency range as shown in Fig. 11(b). As frequency increases, capacitance value tends to steady around 25 fF, while the conductance of TSV rises to 8–10 mS, which equals a shunt resistor of 100 Ω or so, and dominates the transmission loss of GSG–TSV in a broad frequency range above 5 GHz. Distributed RLCG parameters of top and bottom RDL transmission lines are also graphed in Fig. 14 (a)–(d), with black lines for top RDL, and blue lines for bottom RDL [28]. Telegrapher's equation model is applied and illustrated in these figures. As above-mentioned, both top RDL and bottom RDL transmission lines are CPW-type. The width of signal line in the middle is 28 μm, and the gap between signal line and GND line aside is 22 μm. Top RDL lines are about 1 μm thicker than RDL lines on the backside of Si substrate, as shown in Fig. 1, so that the series resistances of top RDL are lower than the ones of bottom RDL, and resistance curves in Fig. 14(a) are expected, increasing with frequency. Fig. 14(b) shows that inductance is relatively constant over frequency, but presents a rapid drop below 1.5 GHz. The inductance of top RDL transmission line is higher than that of the bottom RDL, due to the thicker metal as well. Benefiting from the 9 μm thick BCB liner, top RDL transmission line gets shunt capacitance of only 0.12 pF/mm over frequency. As for the bottom RDL transmission line with thin SiO2 liner, the capacitance is much larger at low frequency, and decreases as frequency rises, as shown in Fig. 14(c). In Fig. 14(d), it can be seen that, top RDL transmission line exhibits a slow increase of conductance over frequency, while the conductance of bottom RDL transmission line increase rapidly below 20 GHz, and goes flat beyond 20 GHz. With these analyses, it is shown that thicker insulation liner for TSV and RDL helps decrease the frequency dependent loss, and a highresistivity silicon substrate is recommended for better high frequency performance. MOS capacitance effect of TSV, the measured capacitance between two adjacent TSVs shows a sharp inverse V-shaped curve under bias voltage from −10 V to 10 V. Peak capacitance value is around 0 V, and is ten times larger than that near ± 10 V. Mechanism of this MISIM structure will be further studied in the future. As for the high frequency characterization up to 40 GHz, two methods are presented to extract resistance and inductance of a single grounded TSV. A “THRU” structure is used to improve the accuracy of this extraction. Four ground-signalground (GSG) type TSV–RDL chain structures are measured, and calculations are operated to make clear the contributions of GSG–TSV, pad, RDL transmission lines to the transmission loss of those TSV-RDL test structures. It is found that TSV dominates the transmission loss of three-dimensional TSV–RDL signal path. RLCG parameters of both GSG–TSV and RDLs are extracted and analyzed. Thicker insulation layer for TSV and RDLs, and high-resistivity silicon substrate are recommended for better high frequency performance of TSV-RDL signal path. Acknowledgments The work presented is funded by Major State Basic Research Development Program of China (Project No. 2015CB0572), the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (Great Wall Scholar, No. CIT&TCD20150320), 5. Conclusions In this paper, electrical measurement of TSV and RDL test structures are carried out and analyzed, from DC to high frequency up to 40 GHz. The measured sheet resistance of 3 μm-thick RDLs on the silicon substrate is 8.87 mΩ/□, and great insulation is observed between metal lines with less than 3 pA leakage current up to 50 V. The measured DC resistance of a single TSV shows a wide distribution, ranging from 4.3 mΩ to 7 Ω, requiring process optimization of TSV in the future. Resistance measurement results of daisy chains show good electrical connectivity between TSVs and RDLs. Leakage current between two adjacent TSVs reaches 150 nA up to 30 V without breakdown. Due to Fig. 12. Equivalent Circuit model for GSG–TSV set, (a) π-model, (b) T-model. X. Sun et al. / Microelectronic Engineering 149 (2016) 145–152 151 Fig. 13. Extracted electrical parameters of GSG–TSV with circuit models in Fig. 12, (a) resistance and inductance, (b) capacitance and conductance. Fig. 14. Extracted electrical parameters per millimeter of metal lines on top and bottom sides of silicon substrate, (a) resistance, (b) inductance, (c) capacitance, (d) conductance. National Science and Technology Major Project of China (Project No. 2009ZX02038), and National Natural Science Foundation of China (Project No.61176102). The authors would also like to thank Huawei Technologies for supporting this research. References [1] J.U. Knickerbocker, P.S. Andry, E. Colgan, B. Dang, T. Dickson, X. Gu, C. Haymes, C. Jahnes, Y. Liu, J. Maria, R.J. Polastre, C.K. Tsang, L. Turlapati, B.C. Webb, L. Wiggins, S.L. 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