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Effect of low k dielectrics on electromigration reliability for Cu interconnects
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 7 (2004) 157–163
Effect of low k dielectrics on electromigration reliability for Cu interconnects Paul S. Hoa,, Ki-Don Leeb, Sean Yoona, Xia Lua, Ennis T. Ogawab a
Laboratory for Interconnect and Packaging, University of Texas at Austin, Austin, TX 78712, USA b Silicon Technology Development, Texas Instruments, Inc., MS 3737, Dallas TX 75243, USA Available online 11 August 2004
Abstract Multi-link statistical test structures were used to study the effect of low k dielectrics on EM reliability of Cu interconnects. Experiments were performed on dual-damascene Cu interconnects integrated with oxide, CVD low k, porous MSQ, and organic polymer ILD. The EM activation energy for Cu structures was found to be between 0.8 and 1.0 eV, indicating mass transport dominated by diffusion at the Cu/SiNx cap-layer interface, independent of ILD. Compared with oxide, the decrease in lifetime and (jL)c observed for low-k structures can be attributed to less dielectric confinement in the low k structures. An effective modulus B obtained by finite element analysis was used to account for the dielectric confinement effect on EM and found to correlate well with EM lifetime and the (jL)c product of low-k interconnects. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electromigration; Statistical test structure; Critical length; Confinement; FEM
1. Introduction Electromigration (EM), a major reliability concern for on-chip interconnects, has been extensively studied in Cu interconnect structures with oxide interlevel dielectrics (ILD) in the past several years [1–3]. Compared with Al/oxide interconnects, Cu/oxide structures have distinct EM characteristics due to the dual-damascene architecture introducing different transport path, flux divergence and damage mechanisms. Statistical studies have revealed multi-mode failures in the Cu/oxide structures with early failures dominated by void formation at the via bottom interface [4,5]. With device scaling continuing to the 90 nm node, ILD with a low dielectric constant (k) is being implemented to replace oxide in Cu Corresponding author. Tel.: +1-512-471-8961; fax: +1512-471-8969. E-mail address: [email protected] (P.S. Ho).
interconnects. Compared with oxide, low k dielectrics are softer, expand more and conduct less heat. The weak thermomechanical properties cause significant concern on EM reliability of Cu/low k interconnects. This has generated great interests recently to study the effect of low k dielectrics on EM reliability. Several basic questions arise concerning the effects of dielectric thermomechanical properties on EM reliability including the rate of mass transport, the failure mechanism and the back-flow stress effect. We have investigated these questions using statistical multilink line/via structures which were found to be effective for EM studies of Cu interconnects with multi-mode failure statistics. In this paper, we summarize some recent results from our laboratory comparing EM characteristics for Cu interconnects with oxide and several low k dielectrics including a porous material. First we discuss the effect of dielectric confinement on EM reliability of Cu line structures based on the concept of effective elastic
1369-8001/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2004.06.005
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modulus B [6]. This parameter B was evaluated using finite element analysis for Cu/oxide and Cu/low k damascene structures and found to correlate well with the measured EM lifetime and the threshold current density-length (jL)c product.
2. Effect of dielectric confinement on EM Consider a line/via element in a Cu dual-damascene structure as shown in Fig. 1. Under a current driving force, the drift velocity (vd) of Cu ions induced by EM can be expressed as [7] vd ¼ vEM þ vBF ¼ mðZ erj ODs=LÞ;
ð1Þ
where Z e is the effective charge, j the current density, O the atomic volume and Ds=L the EM induced stress gradient along the line. In a confined structure, the current induced mass transport vEM is opposed by a back flow vBF induced by the Blech stress gradient as a result of mass transport from the cathode to the anode. According to Eq. (1), a threshold product (jL)c can be defined when vd=0 as ðjLÞc ¼ ODs=Z er:
ð2Þ
With O and Z er being material constants, (jL)c is proportional to DsU For a Cu damascene structure, the value of Ds that can be sustained depends on the confinement on the Cu structure imposed by the ILD and the surrounding barrier and cap layers. The barrier and cap layers although much thinner than the low k ILD, have much higher mechanical strengths and can make a significant contribution to confine the Cu lines in low k damascene structures. The (jL)c product provides an effective measure to evaluate the back-flow stress and the dielectric confinement effect on EM. In our study, the (jL)c product was measured for Cu damascene lines with oxide and low k dielectrics. The confinement effect can be quantified using an effective elastic modulus B following an approach first formulated by Korhonen et al. for Al interconnects [6]. Accordingly, B was defined to account for the stiffness of the interconnect structure responding to the stress generated by mass transport under dielectric vEM
vBF
Fig. 1. Mass transport under EM in a confined metal structure. The drift velocity driven by the current VEM is opposed by a stress-induced back flow VBF.
confinement as: dC=C ¼ ds=B:
ð3Þ
Here dC/C is the volumetric strain induced by EM, and ds is the corresponding amount of stress generated as measured by an effective modulus B of the interconnect structure. Defined in this way, B depends on the elastic properties and geometry of the metal line and surrounding barrier and cap layer materials. It also depends on the mass transport mechanism because the mechanical response of the interconnect structure depends on how mass transport is distributed in different directions. In AlCu interconnects, mass transport is primarily through the grain boundaries, resulting in an isotropic mass distribution and a ds stress. In Cu interconnects, mass transport is dominated by diffusion at the cap layer interface [8], which results in an anisotropic mass distribution primarily along the normal to the line direction. Using a finite element analysis, Hau-Riege calculated B for AlCu interconnects under EM assuming an isotropic mass transport [9]. He found that as the elastic modulus of ILD decreases from 72 GPa for oxide to below 10 GPa for low k materials, the value of B is reduced but to a significantly lesser degree, from 25 GPa to about 10 GPa. This indicates that the Ti/TiN upper and lower layers in the Al line contribute substantially in addition to the ILD to metal confinement. When the AlCu line was embedded in a damascene structure with additional side-wall barriers, B of low k ILD was found to increase about 50%. This demonstrated that the barrier and cap layers in the damascene structure make significant contributions to metal confinement, particularly for low k dielectrics. The confinement effect on EM reliability can be examined from Eq. (1) where a weak confinement will decrease the back-flow stress Ds resulting in an increase of the net drift velocity and a reduction of the EM lifetime. To evaluate the effect on EM lifetime, not only the dielectric confinement on mass transport, but also the kinetics of stress evolution and void formation and their roles in controlling EM damage have to be considered. Compared with Al interconnects, these parameters for Cu interconnects have distinct characteristics and impact EM reliability differently [10,11]. In Fig. 2, we show the initial state of a Cu line with a uniform tensile stress at time t0. Upon EM, mass transport builds up a small tensile stress at the cathode at t1 with a corresponding compressive stress at the anode. With continuing EM at t2, the tensile stress at the cathode can reach a critical value to induce void formation. For Al interconnects, void formation at the interface is not commonly observed; but for Cu interconnects, voids can form readily at the Cu interface near the cathode end under a moderate tensile stress of about 100 MPa [11]. Upon void formation, the local tensile stress will relax quickly to zero while the
ARTICLE IN PRESS P.S. Ho et al. / Materials Science in Semiconductor Processing 7 (2004) 157–163
σ
tensile σc σo
t0
0
t1 VC
x
t2 t3 compressive t4
Fig. 2. Stress evolution in confined Cu line under EM as a function of time. s0 is the initial thermal stress.
compressive stress in the Cu line away from the void continues to increase as shown at t3 and finally reaches a steady state at t4 as shown. This increases the steadystate compressive stress at the anode end where the Cu line becomes more prone to failure due to metal extrusion. Based on void growth kinetics, Korhonen et al. deduced a critical void volume at the steady state as [10] Vc ¼
sT L JreZ L2 þ : B 2OB
ð4Þ
Here sT is the initial thermal stress and L is the length of the metal line and both the initial thermal stress and EM contribute to the void volume. Assuming that the EM lifetime is determined by the critical void volume Vc, Eq. (4) provides a relationship to correlate EM lifetime to the confinement effect, which is inversely proportional to the effective modulus B and dependent on the line length L under test conditions of j and sT . However, the stress effect due to void formation is more complicated and should be statistical in nature since the initial voids can occur randomly at the interface and the subsequent void growth leading to EM failure depends on local interfacial mass transport which may vary from grain to grain. Such statistical issues concerning EM reliability are not well understood at this time. For Cu/low k structures with weak adhesion strength at the cap layer interface, interfacial delamination can occur prematurely to cause line failure by metal extrusion before the steady state is reached. In this case, the EM lifetime will be lower than that estimated from B based on the confinement effect. Another factor affecting EM reliability concerns void evolution where recent Cu EM results showed that void evolution and
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morphology are important in determining the EM failure statistics [12]. This can be particularly important for early failures induced by void formation at via bottom as a result of void evolution from the line at the cathode end to the via bottom. Based on these discussions, EM reliability and the confinement effect for Cu/low k interconnect seem to be a complex phenomenon depending on material properties, interconnect geometry and processing defects. The effect of dielectric confinement on thermal stress has also been investigated using X-ray diffraction to measure Cu damascene line structures with oxide and low k dielectrics [13]. The results confirmed that the cap and barrier layers are important and have to be considered in order to account for the observed thermal stress characteristics. In the following sections, we will summarize the results of EM studies on lifetime and (jL)c for Cu damascene structures with oxide and low k ILDs using a statistical approach. The results show a good correlation between the effective elastic modulus B and EM characteristics for most of the Cu/low k structures investigated. The exception was the organic polymer material where the discrepancy can be traced to premature failure due to interfacial delamination.
3. Experimental details Experiments were performed using statistical structures to examine the effect of low k dielectrics on EM reliability of Cu interconnects. For this study, a critical length (LC) test structure was used consisting of a collection of serially connected line/via interconnects. As shown in Fig. 3a, this test structure contains six repeating sets of 14 interconnects, where the M2 length varies from 10 to 300 mm and with a line width of 0.5 mm. The length of M1 remains constant at about 5 mm in order to drive the failure to occur above the M1 level at either the via or the M2 trench to facilitate failure analysis. Designed in this way, the serial connection of test structures provides an ensemble of line/via elements with varying line lengths to measure the dielectric confinement effect on EM failure statistics and threshold (jL)c product. The low k ILDs used included CVD low k, porous MSQ and organic polymer and their EM behaviors are compared with that of oxide. The material properties of the dielectric materials are listed in Table 1. Test structures were designed at UT-Austin and fabricated at International Sematech and LSI Logic. The samples were prepared using 200 mm wafers and consisted of two-level interconnect structures based on a Ta/low temperature PVD seed Cu/electroplated (EP) Cu stack [14]. The metal lines in the test structures showed an apparent ‘‘near bamboo’’ microstructure with a significant amount of twinning that was associated with
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Anode -
e
10 15 20 25 100 150
35 40 30 75 200 300
e-
SiNx
45 1st Set 50 Mirror Image 2nd Set 3rd Set 4th Set 5th Set 6th Set
M2
Low k
M1
Oxide Si
Cathode
(a)
Oxide Etch Stop
(b)
Fig. 3. (a) Schematic overview of LC test structure, (b) stacking layers of dual-damascene Cu/low k interconnects.
Table 1 Dielectric constant (k), coefficient of temperature expansion (CTE) and Young’s modulus (E) for different dielectric ILDs and simulated effective bulk modulus (B) and (jL)c for dualdamascene Cu interconnects are summarized
Oxide CVD low k Porous MSQ Org. polymer a
CTE (ppm/1C)
E (Gpa)
B (Gpa)
jLca (A/cm)
3.9 2.7 2.2
0.51 25 7.3
71.4 6 3.6
13.7 7.6 7.3
3700 2000 2500
2.7
66
2.5
7.2
1200
2
These values were determined at 1.0 MA/cm .
Cu-film growth. In Fig. 3b, we show the schematic structure of the Cu/low k dual-damascene interconnect where the low k material is fully implemented in all levels. EM experiments were performed in a high vacuum chamber filled with 20 Torr of N2 gas to reduce oxidation and to improve temperature uniformity for all test samples. More experimental details have been described previously [5,15].
4. Results and discussion 4.1. EM lifetime and threshold (jL)c product EM experiments were performed on LC test structures at temperatures between 280 and 400 1C. The first abrupt change or instability in resistance trace during EM stressing was considered as an ‘‘onset’’ EM failure and was used to determine EM lifetime. In Fig. 4, Arrhenius plots of t50 vs. 1/kT obtained at 1.0 MA/cm2 are plotted for Cu/oxide, Cu/CVD low k, Cu/porous MSQ, and Cu/organic polymer structures integrated with Ta barrier and SiNx cap layer. The activation energies of Cu structures were found to be between 0.8 and 1.0 eV. This is commonly associated with mass transport at the Cu/SiNx interface, suggesting that interfacial diffusion dominates EM mass transport [15]. The test structures had the same Cu/Ta and Cu/
102
Cu/Oxide Q = 0.81 eV j = 1.0 MA/cm2
t50 (hrs)
k
Cu/CVD Low k Q = 0.86 eV j = 1.0 MA/cm2
101 Cu/Org. Pol. Q = 0.97 eV j = 1.0 MA/cm2
17
18
19 20 1/KT ( eV-1)
Cu/Porous MSQ Q = 0.93 eV j = 1.0 MA/cm2
21
22
Fig. 4. Graph shows t50 vs. 1/kT for Cu/oxide, Cu/CVD low k, Cu/porous MSQ, and Cu/organic polymer. The activation energies determined from the slope of Arrhenius plot were found to be between 0.8 and 1.0 eV.
SiNx interfaces, so EM occurred via the same mechanism independent of ILD. In Fig. 4, the EM lifetimes of Cu/low k structures are generally shorter than that of Cu/oxide structures under similar test conditions, which can be attributed to a weaker dielectric confinement effect. As shown in Eq. (1), the weaker dielectric confinement by low k ILD reduces the back-flow term vBF, resulting in an increase in the net mass transport and a reduction of EM lifetime. To estimate the confinement effect, B was calculated using finite element analysis for the Cu test structures used in this study assuming mass transport only at the cap layer interface. The results are listed in Table 1. The values of B for all low k ILDs are very similar, equal to about half of that of oxide. The difference of B is significantly less than that of E indicating that the barrier and cap layers are important in addition to the ILD in confining the Cu lines. Except for the organic polymer, the EM lifetime seems to be proportional to B, which is consistent with void-induced failure at the steady state. For the organic polymer, the EM lifetime is lower than that estimated from B. Failure analysis showed that this is due to premature failures caused by
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4.2. Failure analysis
Number of Fails
Focused ion beam (FIB) microprobe was used to identify the EM failure characteristics together with FIB-induced contrast (FIBIC) technique to locate the locations of interconnect failures in the Cu lines as shown in Fig. 7a for the Cu/porous MSQ structure. 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 0
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Line Length (µm) jLc = 3000 A/cm CN = 70 Fig. 5. Failure distribution of Cu/Porous MSQ LC test structures tested at 190 1C and 3.0 MA/cm2 is shown as a function of line length. Failure probability drops as line length decreases. Lc=10 mm. (jL)c=3000 A/cm.
5000
Cu/Oxide Cu/Porous MSQ Cu/CVD Low k Cu/Org. Polymer
4000 jLc (A/cm)
interfacial delamination at the cap/etch stop interface and will be discussed later. After long EM stressing, individual lines in LC structures were examined using a focused-ion beam (FIB) microprobe and an optical microscope to identify EM induced damages. In this way, the failure statistics as a function of line length can be determined. As shown in Fig. 5, the failure probability drops sharply as the line length decreases. The critical length (Lc) for a given current density (j) can be determined from the intercept of the regression line generated by assuming that failure probability is proportional to the net drift velocity and thus the line length [16]. In Fig. 5, Lc was found from the intercept to be 10 mm giving an (jL)c of 3000 A/cm. The results of (jL)c measured for Cu/oxide and Cu/ low k structures are summarized in Fig. 6. Compared with oxide, low k ILDs have smaller threshold (jL)c products, corresponding to 3700, 2000, 2500 and 1200 A/cm with error limits of 250–500 A/cm for oxide, CVD low k, porous MSQ and organic polymer, respectively. In general, there is no temperature dependence for (jL)c under our test conditions. There is a good correlation between the lifetimes in Fig. 3 and (jL)c since both are correlated to Ds, reflecting the dielectric confinement effect on EM characteristics. Similar to the lifetime, the extrinsic effect of interfacial delamination reduces (jL)c for the organic polymer material.
161
3000 2000 1000 0 200
240
280
320
360
400
Temperature (˚C) Fig. 6. Graph shows (jL)c vs. T. There was no temperature dependence in our test conditions. (jL)c data were obtained from EM tests at 1.0 MA/cm2, except for Cu/CVD low k which was tested at 0.5 MA/cm2.
Here we found that voiding at the cathode end was large enough to stop electric current flow causing a failure. In most cases, test structures were found to fail by cathode voiding in the Cu/porous low k structures. Some of the Cu/porous MSQ structures showed lateral Cu extrusion near the anode under the SiNx cap layer, followed by interfacial delamination, as shown in Fig. 7b. While CVD low k and porous MSQ structures failed mainly by cathode void formation, organic polymer structures failed by cathode void formation followed by anode extrusion [17]. In both cases, EM lifetime depends on the amount of void formation at the cathode, so regardless of anode extrusion, voiding seems to fail the line. If anode extrusion occurs prematurely due to low adhesion strength at the anode interfaces, such an extrinsic failure effectively reduces the back stress and accelerates the cathode void formation. This mechanism can reduce the EM lifetime significantly as observed in the organic polymer structures. FIB was used to examine the failure mode at the anode of organic polymer low k interconnects and the results are shown Fig. 8. Here the top view (Fig. 8a) shows an extrusion failure near the anode end. The cross-sectional FIB micrograph in Fig. 8b shows that the extrusion was initiated at the top corners of the Cu line beneath the oxide etch stop (see schematic drawing in Fig. 8c). This suggests that the corner at the barrier, low k ILD and cap/etch stop layer intersection is a mechanical weak point where failure can occur by interfacial delamination induced by a large compressive back-flow stress at the anode. This was confirmed by high resolution SEM observations which identified interfacial delamination and anode extrusion along the low k and oxide etch stop interface, as shown in Fig. 9. Additionally, atomic force microscopy (AFM) was used to examine the surface topology, revealing that SiNx at
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Extrusion
Extrusion Extrusion Monitor
Extrusion Cathode Voiding Extrusion (a)
Anode
Extrusion Monitor
Interfacial Delamination
(b)
Fig. 7. (a) Void formation at the cathode end, (b) Extrusion at the anode end with interfacial delamination observed in Cu/porous MSQ structures.
A′
A
A
A
A′
A′
Anode (a)
(b)
(c)
Fig. 8. (a) Top down view of anode extrusion, and (b) A–A0 cross section of anode of Cu/organic polymer test structures, (c) Schematic view of FIB cutting.
Anode Extrusion
Interfacial Delamination
SiN x
Org. Polymer Cu 0.5 SiC µm Extrusion SiO Anode 2 Monitor
Extrusion Monitor
Fig. 9. Cross section of Cu/organic polymer interconnect taken by high-resolution SEM showing anode extrusion and interfacial delamination due to high hydrostatic compressive stress at anode.
the anode end was lifted by EM by about 180 nm [17]. Thus the weak adhesion of low k/etch stop interface causes the interfacial breakdown and premature EM failure. In this case, the Cu/low k structure was able to sustain less back-flow stress than that estimated from the effective elastic modulus.
with oxide, CVD low k, porous MSQ, and organic polymer ILD. The EM activation energy for Cu structures was found to be between 0.8 and 1.0 eV, indicating mass transport is dominated by diffusion at the Cu/SiNx cap-layer interface, independent of ILD. Compared with oxide, the decrease in lifetime and (jL)c observed for low-k structures can be attributed to less dielectric confinement in the low k structures. An effective modulus B obtained by finite element analysis was used to account for the dielectric confinement effect on EM. For all the ILDs studied, (jL)c showed no temperature dependence. A number of interesting questions remain to be answered. These include the contribution of plastic deformation to the confinement effect, the effect of premature failure before reaching the steady state on failure statistics, and the effect of residual thermal stress and stress relaxation on EM lifetime and threshold product. EM experiments are being performed on Cu/ low k interconnects under various conditions in our laboratory to address these issues.
Acknowledgement 5. Summary In summary, multi-link statistical test structures were used to study the effect of low k dielectrics on EM reliability of Cu interconnects. Experiments were performed on dual-damascene Cu interconnects integrated
The authors would like to thank the International SEMATECH and LSI Logic Corporation for providing test structures for this study. They also gratefully acknowledge the partial support from the state of Texas and the SRC Center for Advanced Interconnect Science and Technology.
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References [1] Hu CK, Luther B. Mater Chem Phy 1995;41:1–7. [2] Hu CK, Harper JME. Mater Chem Phy 1998;52:5–16. [3] Ogawa ET, Blaschke VA, Bierwag A, Lee K-D, Matsuhashi H, Griffiths D, Ramamurthi A, Justison PR, Havemann RH, Ho PS. Mater Res Soc Symp Proc 2000;612:D2.3.1–6. [4] Ogawa ET, Lee K-D, Matsuhashi H, Ko K-S, Justison PR, Ramamurthi AN, Bierwag AJ, Ho PS, Blaschke VA, Havemann RH. 2001 IEEE international reliability physics symposium proceedings of the 39th Annual. 2001. p. 341–9. [5] Lee K-D, Lu X, Ogawa ET, Matsuhashi H, Blaschke VA, Augur R, Ho PS. 2002 IEEE international reliability physics symposium proceedings. Dallas, TX, April 2002. [6] Korhonen MA, Borgesen P, Tu KN, Li C-Y. J Appl Phys 1993;73:3790. [7] Blech IA. J Appl Phys 1976;47:1203. [8] Hu CK, Rosenberg R, Lee KY. Appl Phys Lett 1999;74:2945.
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[9] Hau-Riege SP, Thompson CV. Mat Res Soc Symp Proc 2000;612:D10.2.1. [10] Korhonen MA, Borgesen P, Brown DD, Li C-Y. J Appl Phys 1993;74:4995. [11] Hau-Riege SP. J Appl Phys 2002;91:2014. [12] Kawasaki H. Proceedings of the advanced metallurgy conference. San Diego, December 2002, in press. [13] Rhee SH, Du Y, Ho PS. J Appl Phys 2003, in press. [14] Blaschke V, Mucha J, Foran B, Jiang QT, Sidensol K, Nelson A. In: 1998 proceedings of the Advanced metallization conference. 1998. p.43–9. [15] Lee K-D, Ogawa ET, Matsuhashi H, Justison PR, Ko K-S, Ho PS, Blaschke VA. Appl Phys Lett 2001;79:3236. [16] Ogawa ET, Bierwag AJ, Lee K-D, Matsuhashi H, Justison PR, Ramamurthi AN, Ho PS, Blaschke VA, Griffiths D, Nelsen A, Breen M, Havemann RH. Appl Phys Lett 2001;78:2652. [17] Lee K-D, Lu X, Ogawa ET, Matsuhashi H, Blaschke VA, Augur R, Ho PS. 2002 IEEE international reliability physics symposium proceedings 40th annual. 2002. p. 322–6.
Materials Science in Semiconductor Processing 7 (2004) 157–163
Effect of low k dielectrics on electromigration reliability for Cu interconnects Paul S. Hoa,, Ki-Don Leeb, Sean Yoona, Xia Lua, Ennis T. Ogawab a
Laboratory for Interconnect and Packaging, University of Texas at Austin, Austin, TX 78712, USA b Silicon Technology Development, Texas Instruments, Inc., MS 3737, Dallas TX 75243, USA Available online 11 August 2004
Abstract Multi-link statistical test structures were used to study the effect of low k dielectrics on EM reliability of Cu interconnects. Experiments were performed on dual-damascene Cu interconnects integrated with oxide, CVD low k, porous MSQ, and organic polymer ILD. The EM activation energy for Cu structures was found to be between 0.8 and 1.0 eV, indicating mass transport dominated by diffusion at the Cu/SiNx cap-layer interface, independent of ILD. Compared with oxide, the decrease in lifetime and (jL)c observed for low-k structures can be attributed to less dielectric confinement in the low k structures. An effective modulus B obtained by finite element analysis was used to account for the dielectric confinement effect on EM and found to correlate well with EM lifetime and the (jL)c product of low-k interconnects. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electromigration; Statistical test structure; Critical length; Confinement; FEM
1. Introduction Electromigration (EM), a major reliability concern for on-chip interconnects, has been extensively studied in Cu interconnect structures with oxide interlevel dielectrics (ILD) in the past several years [1–3]. Compared with Al/oxide interconnects, Cu/oxide structures have distinct EM characteristics due to the dual-damascene architecture introducing different transport path, flux divergence and damage mechanisms. Statistical studies have revealed multi-mode failures in the Cu/oxide structures with early failures dominated by void formation at the via bottom interface [4,5]. With device scaling continuing to the 90 nm node, ILD with a low dielectric constant (k) is being implemented to replace oxide in Cu Corresponding author. Tel.: +1-512-471-8961; fax: +1512-471-8969. E-mail address: [email protected] (P.S. Ho).
interconnects. Compared with oxide, low k dielectrics are softer, expand more and conduct less heat. The weak thermomechanical properties cause significant concern on EM reliability of Cu/low k interconnects. This has generated great interests recently to study the effect of low k dielectrics on EM reliability. Several basic questions arise concerning the effects of dielectric thermomechanical properties on EM reliability including the rate of mass transport, the failure mechanism and the back-flow stress effect. We have investigated these questions using statistical multilink line/via structures which were found to be effective for EM studies of Cu interconnects with multi-mode failure statistics. In this paper, we summarize some recent results from our laboratory comparing EM characteristics for Cu interconnects with oxide and several low k dielectrics including a porous material. First we discuss the effect of dielectric confinement on EM reliability of Cu line structures based on the concept of effective elastic
1369-8001/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2004.06.005
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modulus B [6]. This parameter B was evaluated using finite element analysis for Cu/oxide and Cu/low k damascene structures and found to correlate well with the measured EM lifetime and the threshold current density-length (jL)c product.
2. Effect of dielectric confinement on EM Consider a line/via element in a Cu dual-damascene structure as shown in Fig. 1. Under a current driving force, the drift velocity (vd) of Cu ions induced by EM can be expressed as [7] vd ¼ vEM þ vBF ¼ mðZ erj ODs=LÞ;
ð1Þ
where Z e is the effective charge, j the current density, O the atomic volume and Ds=L the EM induced stress gradient along the line. In a confined structure, the current induced mass transport vEM is opposed by a back flow vBF induced by the Blech stress gradient as a result of mass transport from the cathode to the anode. According to Eq. (1), a threshold product (jL)c can be defined when vd=0 as ðjLÞc ¼ ODs=Z er:
ð2Þ
With O and Z er being material constants, (jL)c is proportional to DsU For a Cu damascene structure, the value of Ds that can be sustained depends on the confinement on the Cu structure imposed by the ILD and the surrounding barrier and cap layers. The barrier and cap layers although much thinner than the low k ILD, have much higher mechanical strengths and can make a significant contribution to confine the Cu lines in low k damascene structures. The (jL)c product provides an effective measure to evaluate the back-flow stress and the dielectric confinement effect on EM. In our study, the (jL)c product was measured for Cu damascene lines with oxide and low k dielectrics. The confinement effect can be quantified using an effective elastic modulus B following an approach first formulated by Korhonen et al. for Al interconnects [6]. Accordingly, B was defined to account for the stiffness of the interconnect structure responding to the stress generated by mass transport under dielectric vEM
vBF
Fig. 1. Mass transport under EM in a confined metal structure. The drift velocity driven by the current VEM is opposed by a stress-induced back flow VBF.
confinement as: dC=C ¼ ds=B:
ð3Þ
Here dC/C is the volumetric strain induced by EM, and ds is the corresponding amount of stress generated as measured by an effective modulus B of the interconnect structure. Defined in this way, B depends on the elastic properties and geometry of the metal line and surrounding barrier and cap layer materials. It also depends on the mass transport mechanism because the mechanical response of the interconnect structure depends on how mass transport is distributed in different directions. In AlCu interconnects, mass transport is primarily through the grain boundaries, resulting in an isotropic mass distribution and a ds stress. In Cu interconnects, mass transport is dominated by diffusion at the cap layer interface [8], which results in an anisotropic mass distribution primarily along the normal to the line direction. Using a finite element analysis, Hau-Riege calculated B for AlCu interconnects under EM assuming an isotropic mass transport [9]. He found that as the elastic modulus of ILD decreases from 72 GPa for oxide to below 10 GPa for low k materials, the value of B is reduced but to a significantly lesser degree, from 25 GPa to about 10 GPa. This indicates that the Ti/TiN upper and lower layers in the Al line contribute substantially in addition to the ILD to metal confinement. When the AlCu line was embedded in a damascene structure with additional side-wall barriers, B of low k ILD was found to increase about 50%. This demonstrated that the barrier and cap layers in the damascene structure make significant contributions to metal confinement, particularly for low k dielectrics. The confinement effect on EM reliability can be examined from Eq. (1) where a weak confinement will decrease the back-flow stress Ds resulting in an increase of the net drift velocity and a reduction of the EM lifetime. To evaluate the effect on EM lifetime, not only the dielectric confinement on mass transport, but also the kinetics of stress evolution and void formation and their roles in controlling EM damage have to be considered. Compared with Al interconnects, these parameters for Cu interconnects have distinct characteristics and impact EM reliability differently [10,11]. In Fig. 2, we show the initial state of a Cu line with a uniform tensile stress at time t0. Upon EM, mass transport builds up a small tensile stress at the cathode at t1 with a corresponding compressive stress at the anode. With continuing EM at t2, the tensile stress at the cathode can reach a critical value to induce void formation. For Al interconnects, void formation at the interface is not commonly observed; but for Cu interconnects, voids can form readily at the Cu interface near the cathode end under a moderate tensile stress of about 100 MPa [11]. Upon void formation, the local tensile stress will relax quickly to zero while the
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σ
tensile σc σo
t0
0
t1 VC
x
t2 t3 compressive t4
Fig. 2. Stress evolution in confined Cu line under EM as a function of time. s0 is the initial thermal stress.
compressive stress in the Cu line away from the void continues to increase as shown at t3 and finally reaches a steady state at t4 as shown. This increases the steadystate compressive stress at the anode end where the Cu line becomes more prone to failure due to metal extrusion. Based on void growth kinetics, Korhonen et al. deduced a critical void volume at the steady state as [10] Vc ¼
sT L JreZ L2 þ : B 2OB
ð4Þ
Here sT is the initial thermal stress and L is the length of the metal line and both the initial thermal stress and EM contribute to the void volume. Assuming that the EM lifetime is determined by the critical void volume Vc, Eq. (4) provides a relationship to correlate EM lifetime to the confinement effect, which is inversely proportional to the effective modulus B and dependent on the line length L under test conditions of j and sT . However, the stress effect due to void formation is more complicated and should be statistical in nature since the initial voids can occur randomly at the interface and the subsequent void growth leading to EM failure depends on local interfacial mass transport which may vary from grain to grain. Such statistical issues concerning EM reliability are not well understood at this time. For Cu/low k structures with weak adhesion strength at the cap layer interface, interfacial delamination can occur prematurely to cause line failure by metal extrusion before the steady state is reached. In this case, the EM lifetime will be lower than that estimated from B based on the confinement effect. Another factor affecting EM reliability concerns void evolution where recent Cu EM results showed that void evolution and
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morphology are important in determining the EM failure statistics [12]. This can be particularly important for early failures induced by void formation at via bottom as a result of void evolution from the line at the cathode end to the via bottom. Based on these discussions, EM reliability and the confinement effect for Cu/low k interconnect seem to be a complex phenomenon depending on material properties, interconnect geometry and processing defects. The effect of dielectric confinement on thermal stress has also been investigated using X-ray diffraction to measure Cu damascene line structures with oxide and low k dielectrics [13]. The results confirmed that the cap and barrier layers are important and have to be considered in order to account for the observed thermal stress characteristics. In the following sections, we will summarize the results of EM studies on lifetime and (jL)c for Cu damascene structures with oxide and low k ILDs using a statistical approach. The results show a good correlation between the effective elastic modulus B and EM characteristics for most of the Cu/low k structures investigated. The exception was the organic polymer material where the discrepancy can be traced to premature failure due to interfacial delamination.
3. Experimental details Experiments were performed using statistical structures to examine the effect of low k dielectrics on EM reliability of Cu interconnects. For this study, a critical length (LC) test structure was used consisting of a collection of serially connected line/via interconnects. As shown in Fig. 3a, this test structure contains six repeating sets of 14 interconnects, where the M2 length varies from 10 to 300 mm and with a line width of 0.5 mm. The length of M1 remains constant at about 5 mm in order to drive the failure to occur above the M1 level at either the via or the M2 trench to facilitate failure analysis. Designed in this way, the serial connection of test structures provides an ensemble of line/via elements with varying line lengths to measure the dielectric confinement effect on EM failure statistics and threshold (jL)c product. The low k ILDs used included CVD low k, porous MSQ and organic polymer and their EM behaviors are compared with that of oxide. The material properties of the dielectric materials are listed in Table 1. Test structures were designed at UT-Austin and fabricated at International Sematech and LSI Logic. The samples were prepared using 200 mm wafers and consisted of two-level interconnect structures based on a Ta/low temperature PVD seed Cu/electroplated (EP) Cu stack [14]. The metal lines in the test structures showed an apparent ‘‘near bamboo’’ microstructure with a significant amount of twinning that was associated with
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Anode -
e
10 15 20 25 100 150
35 40 30 75 200 300
e-
SiNx
45 1st Set 50 Mirror Image 2nd Set 3rd Set 4th Set 5th Set 6th Set
M2
Low k
M1
Oxide Si
Cathode
(a)
Oxide Etch Stop
(b)
Fig. 3. (a) Schematic overview of LC test structure, (b) stacking layers of dual-damascene Cu/low k interconnects.
Table 1 Dielectric constant (k), coefficient of temperature expansion (CTE) and Young’s modulus (E) for different dielectric ILDs and simulated effective bulk modulus (B) and (jL)c for dualdamascene Cu interconnects are summarized
Oxide CVD low k Porous MSQ Org. polymer a
CTE (ppm/1C)
E (Gpa)
B (Gpa)
jLca (A/cm)
3.9 2.7 2.2
0.51 25 7.3
71.4 6 3.6
13.7 7.6 7.3
3700 2000 2500
2.7
66
2.5
7.2
1200
2
These values were determined at 1.0 MA/cm .
Cu-film growth. In Fig. 3b, we show the schematic structure of the Cu/low k dual-damascene interconnect where the low k material is fully implemented in all levels. EM experiments were performed in a high vacuum chamber filled with 20 Torr of N2 gas to reduce oxidation and to improve temperature uniformity for all test samples. More experimental details have been described previously [5,15].
4. Results and discussion 4.1. EM lifetime and threshold (jL)c product EM experiments were performed on LC test structures at temperatures between 280 and 400 1C. The first abrupt change or instability in resistance trace during EM stressing was considered as an ‘‘onset’’ EM failure and was used to determine EM lifetime. In Fig. 4, Arrhenius plots of t50 vs. 1/kT obtained at 1.0 MA/cm2 are plotted for Cu/oxide, Cu/CVD low k, Cu/porous MSQ, and Cu/organic polymer structures integrated with Ta barrier and SiNx cap layer. The activation energies of Cu structures were found to be between 0.8 and 1.0 eV. This is commonly associated with mass transport at the Cu/SiNx interface, suggesting that interfacial diffusion dominates EM mass transport [15]. The test structures had the same Cu/Ta and Cu/
102
Cu/Oxide Q = 0.81 eV j = 1.0 MA/cm2
t50 (hrs)
k
Cu/CVD Low k Q = 0.86 eV j = 1.0 MA/cm2
101 Cu/Org. Pol. Q = 0.97 eV j = 1.0 MA/cm2
17
18
19 20 1/KT ( eV-1)
Cu/Porous MSQ Q = 0.93 eV j = 1.0 MA/cm2
21
22
Fig. 4. Graph shows t50 vs. 1/kT for Cu/oxide, Cu/CVD low k, Cu/porous MSQ, and Cu/organic polymer. The activation energies determined from the slope of Arrhenius plot were found to be between 0.8 and 1.0 eV.
SiNx interfaces, so EM occurred via the same mechanism independent of ILD. In Fig. 4, the EM lifetimes of Cu/low k structures are generally shorter than that of Cu/oxide structures under similar test conditions, which can be attributed to a weaker dielectric confinement effect. As shown in Eq. (1), the weaker dielectric confinement by low k ILD reduces the back-flow term vBF, resulting in an increase in the net mass transport and a reduction of EM lifetime. To estimate the confinement effect, B was calculated using finite element analysis for the Cu test structures used in this study assuming mass transport only at the cap layer interface. The results are listed in Table 1. The values of B for all low k ILDs are very similar, equal to about half of that of oxide. The difference of B is significantly less than that of E indicating that the barrier and cap layers are important in addition to the ILD in confining the Cu lines. Except for the organic polymer, the EM lifetime seems to be proportional to B, which is consistent with void-induced failure at the steady state. For the organic polymer, the EM lifetime is lower than that estimated from B. Failure analysis showed that this is due to premature failures caused by
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4.2. Failure analysis
Number of Fails
Focused ion beam (FIB) microprobe was used to identify the EM failure characteristics together with FIB-induced contrast (FIBIC) technique to locate the locations of interconnect failures in the Cu lines as shown in Fig. 7a for the Cu/porous MSQ structure. 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 0
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Line Length (µm) jLc = 3000 A/cm CN = 70 Fig. 5. Failure distribution of Cu/Porous MSQ LC test structures tested at 190 1C and 3.0 MA/cm2 is shown as a function of line length. Failure probability drops as line length decreases. Lc=10 mm. (jL)c=3000 A/cm.
5000
Cu/Oxide Cu/Porous MSQ Cu/CVD Low k Cu/Org. Polymer
4000 jLc (A/cm)
interfacial delamination at the cap/etch stop interface and will be discussed later. After long EM stressing, individual lines in LC structures were examined using a focused-ion beam (FIB) microprobe and an optical microscope to identify EM induced damages. In this way, the failure statistics as a function of line length can be determined. As shown in Fig. 5, the failure probability drops sharply as the line length decreases. The critical length (Lc) for a given current density (j) can be determined from the intercept of the regression line generated by assuming that failure probability is proportional to the net drift velocity and thus the line length [16]. In Fig. 5, Lc was found from the intercept to be 10 mm giving an (jL)c of 3000 A/cm. The results of (jL)c measured for Cu/oxide and Cu/ low k structures are summarized in Fig. 6. Compared with oxide, low k ILDs have smaller threshold (jL)c products, corresponding to 3700, 2000, 2500 and 1200 A/cm with error limits of 250–500 A/cm for oxide, CVD low k, porous MSQ and organic polymer, respectively. In general, there is no temperature dependence for (jL)c under our test conditions. There is a good correlation between the lifetimes in Fig. 3 and (jL)c since both are correlated to Ds, reflecting the dielectric confinement effect on EM characteristics. Similar to the lifetime, the extrinsic effect of interfacial delamination reduces (jL)c for the organic polymer material.
161
3000 2000 1000 0 200
240
280
320
360
400
Temperature (˚C) Fig. 6. Graph shows (jL)c vs. T. There was no temperature dependence in our test conditions. (jL)c data were obtained from EM tests at 1.0 MA/cm2, except for Cu/CVD low k which was tested at 0.5 MA/cm2.
Here we found that voiding at the cathode end was large enough to stop electric current flow causing a failure. In most cases, test structures were found to fail by cathode voiding in the Cu/porous low k structures. Some of the Cu/porous MSQ structures showed lateral Cu extrusion near the anode under the SiNx cap layer, followed by interfacial delamination, as shown in Fig. 7b. While CVD low k and porous MSQ structures failed mainly by cathode void formation, organic polymer structures failed by cathode void formation followed by anode extrusion [17]. In both cases, EM lifetime depends on the amount of void formation at the cathode, so regardless of anode extrusion, voiding seems to fail the line. If anode extrusion occurs prematurely due to low adhesion strength at the anode interfaces, such an extrinsic failure effectively reduces the back stress and accelerates the cathode void formation. This mechanism can reduce the EM lifetime significantly as observed in the organic polymer structures. FIB was used to examine the failure mode at the anode of organic polymer low k interconnects and the results are shown Fig. 8. Here the top view (Fig. 8a) shows an extrusion failure near the anode end. The cross-sectional FIB micrograph in Fig. 8b shows that the extrusion was initiated at the top corners of the Cu line beneath the oxide etch stop (see schematic drawing in Fig. 8c). This suggests that the corner at the barrier, low k ILD and cap/etch stop layer intersection is a mechanical weak point where failure can occur by interfacial delamination induced by a large compressive back-flow stress at the anode. This was confirmed by high resolution SEM observations which identified interfacial delamination and anode extrusion along the low k and oxide etch stop interface, as shown in Fig. 9. Additionally, atomic force microscopy (AFM) was used to examine the surface topology, revealing that SiNx at
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Extrusion
Extrusion Extrusion Monitor
Extrusion Cathode Voiding Extrusion (a)
Anode
Extrusion Monitor
Interfacial Delamination
(b)
Fig. 7. (a) Void formation at the cathode end, (b) Extrusion at the anode end with interfacial delamination observed in Cu/porous MSQ structures.
A′
A
A
A
A′
A′
Anode (a)
(b)
(c)
Fig. 8. (a) Top down view of anode extrusion, and (b) A–A0 cross section of anode of Cu/organic polymer test structures, (c) Schematic view of FIB cutting.
Anode Extrusion
Interfacial Delamination
SiN x
Org. Polymer Cu 0.5 SiC µm Extrusion SiO Anode 2 Monitor
Extrusion Monitor
Fig. 9. Cross section of Cu/organic polymer interconnect taken by high-resolution SEM showing anode extrusion and interfacial delamination due to high hydrostatic compressive stress at anode.
the anode end was lifted by EM by about 180 nm [17]. Thus the weak adhesion of low k/etch stop interface causes the interfacial breakdown and premature EM failure. In this case, the Cu/low k structure was able to sustain less back-flow stress than that estimated from the effective elastic modulus.
with oxide, CVD low k, porous MSQ, and organic polymer ILD. The EM activation energy for Cu structures was found to be between 0.8 and 1.0 eV, indicating mass transport is dominated by diffusion at the Cu/SiNx cap-layer interface, independent of ILD. Compared with oxide, the decrease in lifetime and (jL)c observed for low-k structures can be attributed to less dielectric confinement in the low k structures. An effective modulus B obtained by finite element analysis was used to account for the dielectric confinement effect on EM. For all the ILDs studied, (jL)c showed no temperature dependence. A number of interesting questions remain to be answered. These include the contribution of plastic deformation to the confinement effect, the effect of premature failure before reaching the steady state on failure statistics, and the effect of residual thermal stress and stress relaxation on EM lifetime and threshold product. EM experiments are being performed on Cu/ low k interconnects under various conditions in our laboratory to address these issues.
Acknowledgement 5. Summary In summary, multi-link statistical test structures were used to study the effect of low k dielectrics on EM reliability of Cu interconnects. Experiments were performed on dual-damascene Cu interconnects integrated
The authors would like to thank the International SEMATECH and LSI Logic Corporation for providing test structures for this study. They also gratefully acknowledge the partial support from the state of Texas and the SRC Center for Advanced Interconnect Science and Technology.
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