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Electromigration in AlCu lines: comparison of Dual Damascene and metal reactive ion etching
Thin Solid Films 388 Ž2001. 303᎐314
Electromigration in AlCu lines: comparison of Dual Damascene and metal reactive ion etching R.G. Filippi a,U , M.A. Gribelyuk a , T. Josepha , T. Kane a , T.D. Sullivana , L.A. Clevenger b , G. Costrini b , J. Gambino b , R.C. Igguldenb , E.W. Kiewrab , X.J. Ning c , R. Ravikumar c , R.F. Schnabel c , G. Stojakovic c , S.J. Weber c , L.M. Gignac d , C.-K. Hud , D.L. Rathd , K.P. Rodbelld a IBM Microelectronics, 2070 Route 52, Hopewell Junction, New York 12533-6531, USA IBM Microelectronics, DRAM De¨ elopment Alliance, IBM Semiconductor Research and De¨ elopment Center, 2070 Route 52, Hopewell Junction, New York 12533-6531, USA c Infineon Technologies, DRAM De¨ elopment Alliance, IBM Semiconductor Research and De¨ elopment Center, 2070 Route 52, Hopewell Junction, New York 12533-6531, USA d IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA b
Received 12 September 2000; received in revised form 23 January 2001; accepted 9 February 2001
Abstract The electromigration behavior and microstructural features of AlCu Dual Damascene lines are compared to those of AlCu metal reactively ion etched ŽRIE. lines. Test structures consist of 0.18-, 0.35- and 1.33-m-wide lines terminated by W diffusion barriers, and are tested at 250⬚C. A remarkable finding for the 0.18-m-wide Damascene samples is a threshold-length product of nearly 40 000 Arcm. Different failure mechanisms are revealed for Dual Damascene and metal RIE structures by observing the resistance shift vs. time as well as the lifetime vs. current density behaviors. It is found that the Damascene structures exhibit a long resistance incubation period followed by a rapid increase in resistance, while the RIE structures show a short resistance incubation period followed by a gradual increase in resistance. The current density exponent is found to be close to 2 for the Damascene process and close to 1 for the RIE process. The Damascene samples show a significant lifetime improvement over the RIE samples for low levels of resistance change, while the relative lifetime improvement decreases as the maximum allowed resistance shift increases. In order to understand the electromigration performance of each metallization system, various physical analysis techniques are implemented. The average grain size, determined from transmission electron microscopy ŽTEM., is found to be significantly larger for Damascene lines due to a higher AlCu deposition temperature. Both TEM and scanning electron microscopy ŽSEM. analyses indicate that TiAl 3 intermetallic formation occurs in both Dual Damascene and RIE lines, but is much more prevalent in the RIE case. Electron backscatter analysis reveals a weak Al crystallographic texture in sub-micron Dual Damascene samples and a strong Ž111. fiber texture in RIE samples. Optical and SEM inspections illustrate different failure signatures for 0.18 and 0.35 m Dual Damascene and RIE lines. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Electromigration; Interfaces; Metallization
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Corresponding author. Tel.: q1-845-892-2906; fax: q1-845-892-3039. E-mail address: [email protected] ŽR.G. Filippi..
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1. Introduction In the increasingly competitive world of Dynamic Random Access Memory ŽDRAM. manufacturing, many factors need to be considered when implementing interconnection processes. These factors include cost, yield, defect level, tight pitch wiring and reliability. As feature sizes are reduced below 0.20 m, reliability concerns due to electromigration become extremely important as operating current densities increase in the metal interconnections. Reactive ion etching ŽRIE. of aluminum copper ŽAlCu. based interconnections has been the standard metal patterning technique for both Logic and DRAM. However, this process is significantly more expensive due to numerous processing steps, which makes it less than ideal for DRAM applications. The use of a Dual Damascene interconnection technology, on the other hand, is one way to achieve a low cost process, while at the same time meet the equally important requirements of low contamination levels, tight pitch wiring and high reliability w1᎐3x. The electromigration reliability of AlCu Dual Damascene structures has generally been found to be superior or at least comparable to that of AlCu RIE structures at 0.30-m dimensions w2᎐4x. The electromigration lifetime of 0.30-m bamboo RIE samples, however, was found to be superior to that of 0.30-m bamboo Damascene samples w5x. Recently, the electromigration performance was observed to be very dependent on the liner quality and the liner deposition technique used for the Damascene trench w6x. In this paper, we compare the electromigration performance and microstructural characteristics of 0.18-, 0.35- and 1.33-m AlCu Dual Damascene and RIE interconnections. 2. Experiment The process of forming the Dual Damascene structure is the same as described by Clevenger et al. w2x and Iggulden et al. w3x, in which both physical vapor deposition ŽPVD. and chemical vapor deposition ŽCVD. techniques are utilized for aluminum copper ŽAlCu., titanium ŽTi. and titanium nitride ŽTiN.. The first metal level ŽM1. and the underlying via are filled simultaneously. Planarized top metal surfaces are achieved by chemical mechanical polishing ŽCMP.. The M1 is composed of a 0.48-m stack of PVD TirCVD TiNrCVD AlrPVD AlCu, where the Cu concentration is 0.5 weight percent Žwt.%. and the AlCu is deposited at F 450⬚C. Note that some results in this study are also based on Damascene lines with an M1 thickness of 0.25 m. The results for an M1 thickness of 0.25 m are shown only in Section 3.1.1. The Dual Damascene
structure is surrounded and passivated by a tetraethylorthosilane ŽTEOS.-based oxide. The process of forming the RIE structure utilizes PVD techniques only. The metallization is composed of AlCu sandwiched between redundant layers of Ti and TiN. The M1 is a 0.40-m stack of TirTiNr AlCurTirTiN, where the Cu concentration is 0.5 wt.% and the AlCu is deposited at F 150⬚C. Following metal etch, the RIE samples were annealed at 400⬚C in forming gas Ž10% H 2r90% N2 . for 20 min. The RIE structure is passivated with a high density plasma ŽHDP. oxide. Underlying CVD tungsten ŽW. studs are connected to TirTiNrAlCurTirTiN lines. Two-level structures of 0.18, 0.35 and 1.33-m dimensions are tested. In the case of the 0.18-m-wide lines Žsee Fig. 1a., W local interconnections are located at a lower level at both ends of M1. The M1 stripe length, or distance between AlCu vias ŽDamascene case. or W studs ŽRIE case., is 200 m. There is one stud-via at the cathode end of the structure and three stud-vias at the anode end. For the 0.35-m-wide lines Žsee Fig. 1b., a W local interconnection is located at a lower level at the cathode end, while the M1 widens to 5 m at the anode end. There is one stud-via at the cathode end of the structure, the distance between the stud-via and the point at which M1 widens is 200 m, and the distance between the point where M1 widens and the test pad is ) 300 m. The 1.33-m-wide lines Žsee Fig. 1c. are similar to the 0.35-m-wide lines except that there are three stud-vias at the cathode end of the structure and the M1 widens to 10 m at the anode end. Each stud-via measures 0.225 m in diameter for the 0.18, 0.35 and 1.33-m-wide lines. The resistance of the 0.35-m-wide Damascene structure measures approximately 55᎐60 ⍀ at 25⬚C and 90᎐100 ⍀ at 250⬚C, while the resistance of the 0.35-m-wide RIE structure measures approximately 65᎐70 ⍀ at 25⬚C and 105᎐115 ⍀ at 250⬚C. The 0.18-m-wide Damascene lines require higher current densities than the corresponding RIE samples in order to cause failure. The 0.18-m-wide Damascene and RIE samples are tested at 2.5 MArcm2 Ž2.5= 10 6 Arcm2 . and 1.8 MArcm2 , respectively. For current densities ) 2.0 MArcm2 , the 0.18-m-wide RIE structure exhibits early failures and a bimodal failure distribution. The 1.33-m-wide Damascene and RIE samples are tested at 1.7 and 2.4 MArcm2 , respectively. The current density dependence of both processes is determined by testing the 0.35-m-wide structure at multiple current densities Ž1.3, 1.7 and 2.1 MArcm2 for the Damascene samples, and 1.2, 1.8 and 2.4 MArcm2 for the RIE samples.. All stated current densities are based on the M1 Al cross-sectional area and ignore the formation of TiAl 3 . The oven temperature is chosen such that the ambient plus Joule heating
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Fig. 1. Schematic top-down view of test structures used for the Dual Damascene and RIE processes: Ža. 0.18-m-wide line, Žb. 0.35-m-wide line and Žc. 1.33-m-wide line. AlCu vias are present in the Dual Damascene process while W studs are present in the RIE process.
equals 250⬚C for all structures. Each stress cell, which consists of a sample type ŽDual Damascene or metal RIE., current density and line width, contains 13᎐24 samples. Various physical analysis techniques are used to help explain the electromigration results. The Al grain structure is characterized by plan view transmission electron microscopy ŽTEM.. The extent of TiAl 3 intermetallic formation is determined by cross-sectional TEM as well as cross-sectional scanning electron microscopy ŽSEM. with selective wet etch. The crystallographic texture of micron and sub-micron lines is measured by electron backscatter diffraction Žalso referred to as backscatter Kikuchi diffraction w7x. with a spatial resolution of 0.1 m. Size and location of electromigration-induced voids are determined for 0.18, 0.35 and 1.33-m-wide structures using top-down optical inspection for Dual Damascene samples and top-down SEM inspection for RIE samples. 3. Results 3.1. Electromigration beha¨ ior 3.1.1. High current density requirement for 0.18- m-wide Damascene lines High current densities are required to cause failures in the 0.18-m-wide Damascene samples. Failures Ži.e. resistance increase ) 0.1%. were never obtained for a
current density - 2.0 MArcm2 . This result is independent of the line thickness. Fig. 2 shows SEM cross-sections of a 0.18-m-wide structure tested at 2.5 MArcm2 and 250⬚C. The sample, which has an M1 stack thickness of 0.25 m, was on test for 118 h and increased in resistance - 0.1%. Please note that not all samples from this stress cell behaved identically, as some samples increased significantly in resistance Ži.e. ) 100%. after the same period of time. No significant electromigration-induced voiding is seen at the cathode end ŽFig. 2a., while Cu pile-up, in the form of Al 2 Cu precipitates, is observed at the anode end ŽFig. 2b.. A very small void is seen in Fig. 2a at the base of the Damascene via. Additional cross-sectional SEM images, up to 50 m from the cathode end, were generated for this sample and reveal no evidence of voiding. It is interesting to note that the Cu pile-up seen in Fig. 2b occurs in the via closest to the cathode end but not in the other two vias, which is most likely related to the flow of electrons in that region of the structure. These observations for AlCu Dual Damascene are significantly different than that found for AlCu RIE w8,9x, in which resistance increases ) 10% were observed in the RIE case Žafter 100 h and at 250⬚C. even for stripe lengths equal to 100 m and current densities equal to 2.0 MArcm2 . 3.1.2. Resistance shift ¨ s. time beha¨ ior Fig. 3 shows a plot of the median resistance shift vs.
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time for the 0.18, 0.35 and 1.33-m-wide Dual Damascene ŽFig. 3a. and RIE ŽFig. 3b. samples. For multilayered metallizations, a resistance increase during electromigration testing is caused by Al depletion w10,11x. The first observation to make is that the resistance incubation period, defined as the time before which the resistance starts to increase, is much longer for Damascene than for RIE samples. The second is that the Damascene samples exhibit an abrupt increase in resistance while the RIE samples Ž0.18 and 0.35-m wide lines. gradually increase in resistance. Similar trends
Fig. 3. Median resistance shift Ž%. vs. time for the Ža. Dual Damascene samples and Žb. RIE samples.
were observed for 0.30-m-wide AlCu Damascene and RIE samples tested at 1᎐2 MArcm2 and 200⬚C w5x. Note that the median resistance shift is plotted vs. time in Fig. 3a,b rather than the mean resistance shift because the latter does not illustrate the typical behavior of samples that abruptly shift in resistance. Resistance saturation effects are observed for 0.18m-wide Dual Damascene samples, as seen in Fig. 4. The figure shows that the fractional resistance shift vs. time for three Damascene lines saturates at approximately 250%. The saturation level indicated by Fig. 4 is representative for this stress cell, although it should be noted that not all of the samples exhibit resistance saturation effects. Similar behavior is not observed for the metal RIE lines.
Fig. 2. SEM cross-sections of a 0.18-m-wide Dual Damascene sample tested at 2.5 MArcm2 and 250⬚C: Ža. cathode end of structure showing a small void at the bottom of the via and Žb. anode end of structure showing Cu pile-up. The sample was on test for 118 h and did not significantly increase in resistance. The M1 thickness of this particular sample measures 0.25 m. The bright regions at the anode end are identified as Al 2 Cu precipitates.
3.1.3. Lifetime beha¨ ior The current density dependence for the 0.35-mwide Dual Damascene and RIE samples is illustrated in Fig. 5 in terms of a q20% resistance increase. The plot shows that the current density exponent, n, is equal to 1.95 and 1.03 for the Damascene and RIE processes, respectively. Table 1 indicates that the current density exponent is close to 2 for the Damascene process and close to 1 for the RIE process, and is independent of the failure criterion for both cases.
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Fig. 4. Resistance shift Ž%. vs. time for 0.18-m-wide Dual Damascene samples tested at 2.5 MArcm2 . Only 3 out of 22 samples are shown.
Fig. 5. Log᎐log plot of t50 vs. j for the 0.35-m-wide Dual Damascene and RIE structures. The failure criterion is a q20% resistance increase. The error bars represent the 95% confidence interval.
A lognormal distribution is used for describing failure times. The median lifetimes Ž t50 . for both processes are consistent with the resistance shift vs. time data shown in Fig. 3a,b. Moreover, the variation in the failure times, as measured by the lognormal sigma Ž ., is fairly independent of width, failure criterion Ži.e. maximum resistance shift. and process. Table 2 lists the electromigration statistics in terms of a q20% resistance increase for some of the Dual Damascene and RIE stress cells. Fig. 6 shows a lognormal cumulative distribution function ŽCDF. plot in terms of a q20% resistance increase for the 0.35- and 1.33-m-wide Damascene and RIE lines. The plotted failure times are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE. For both line widths, the Dual Damascene samples show a 3 = lifetime improvement over the corresponding RIE samples. The 0.18-m-wide samples are not included in Fig. 6 due to the fact that normalization of the Damascene failure times to 2.0 MArcm2 using n s 2 is not justified since failures in these narrow lines are not observed when stressed below 2.0 MArcm2 . Fig. 7 illustrates the lifetime improvement of Dual Damascene relative to metal RIE. The figure shows a plot of the ratio of the Dual Damascene t50 to the RIE t50 vs. resistance increase for the 0.35- and 1.33-m-
wide structures. The failure times that determine the t50 values are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE. The relative lifetime improvement of Damascene over RIE decreases as the maximum allowed resistance shift increases. For example, at a 2% resistance change, the 0.35-m-wide Damascene samples show a 5.0= lifetime improvement over the corresponding RIE samples. But at a 100% resistance change, the relative lifetime improvement of Damascene over RIE is approximately 1 for this line width. It is interesting to note that the 1.33-m-wide Damascene samples show a 2.4= longer lifetime than the 1.33-m-wide RIE samples for a 100% resistance change. The 0.18-m-wide samples are not included in Fig. 7 for the same reason that they are excluded from Fig. 6; normalization of the Damascene failure times to 2.0 MArcm2 using n s 2 is not justified for the 0.18m-wide samples since failures in these narrow lines are not observed when stressed below 2.0 MArcm2 .
Table 1 Current density exponent for the 0.35-m-wide structure at different levels of resistance increase Failure criterion Ž%.
2 10 20 50 100
Current density exponent, n Dual Damascene
RIE
1.96 1.96 1.95 1.93 1.88
1.11 1.04 1.03 1.19 1.03
3.2. TEM analysis Fig. 8 shows plan view TEM micrographs of 0.35and 1.33-m-wide lines. The microstructure is characterized as bamboo Žsingle grain across line width for vast majority of line., multiple-grained Žat least two grains across line width for vast majority of line. or near bamboo Žmix of bamboo and multiple-grained, but mostly bamboo.. The 0.35-m-wide Dual Damascene ŽFig. 8b. and RIE ŽFig. 8d. lines indicate bamboo and near bamboo microstructures, respectively. Short segments of the bamboo-grained Damascene lines have two grains across the line. The 1.33-m-wide Dual Damascene ŽFig. 8a. and RIE ŽFig. 8c. lines reveal near bamboo and multiple-grained microstructures, respectively. Short segments of the multiple-grained RIE lines have a single grain across the line. Table 3 lists
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Table 2 Electromigration lifetime statistics for the 0.18-, 0.35- and 1.33-m-wide structures in terms of a q20% resistance increase Metallization
Line width Žm.
j ŽMArcm2 .
t50 Žh.
Dual Damascene Dual Damascene Dual Damascene RIE RIE RIE
0.18 0.35 1.33 0.18 0.35 1.33
2.5 1.7 1.7 1.8 2.4 2.4
24.3 64.6 48.3 12.3 13.0 9.1
0.33 0.34 0.29 0.22 0.26 0.22
the average grain sizes for the 0.35- and 1.33-m-wide samples. The grain size is obtained by applying the line intercept method to the TEM images, where a line Žof known length. is drawn along the line length and the number of grain intercepts are counted. The average grain size is calculated by dividing the line length by the number of intercepts and averaging several line measurements. Fig. 9 shows TEM cross-sections of 0.18-m-wide lines. The Dual Damascene sample ŽFig. 9a. does not reveal any TiAl 3 intermetallic formation, while the RIE sample ŽFig. 9b. shows significant TiAl 3 formation near the bottom of the line. For the magnification shown, it was not possible to distinguish the Ti and TiN layers due to the slight difference in contrast between Ti and TiN as compared to Al. The TiAl 3 was identified by energy dispersive X-ray spectroscopy ŽEDS..
Fig. 7. Ratio of the Dual Damascene t50 to the RIE t50 vs. resistance increase for the 0.35- and 1.33-m-wide structures. The failure times that determine the t50 values are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE.
Although TiAl 3 is also observed below the top TiN layer in other RIE samples, it is interesting to note that the reaction of Ti and Al occurs predominantly above the bottom TiN layer. In general, formation of TiAl 3 occurs in both Dual Damascene as well as metal RIE lines, and is not continuous in either case. However, TEM cross-sections indicate that the formation of TiAl 3 is much less frequent in Dual Damascene samples. Although not shown here, SEM cross-sections Žalong the line length. combined with a selective wet etch of Al to reveal TiAl 3 show that intermetallic particle formation occurs every 5᎐10 m in Dual Damascene samples compared to every 0.4᎐0.5 m in the RIE samples. This suggests that the CVD TiN layer is more effective at preventing Ti and Al from reacting during the PVD AlCu deposition than the PVD TiN layer is Table 3 Average grain sizes for the 0.35- and 1.33-m-wide structures
Fig. 6. Lognormal CDF plot for the 0.35- and 1.33-m-wide Dual Damascene and RIE lines in terms of a q20% resistance shift. The plotted failure times are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE.
Metallization
Line width Žm.
Average grain size along line length Žm.
Dual Damascene Dual Damascene RIE RIE
0.35 1.33 0.35 1.33
1.1 1.8 0.4 0.5
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Fig. 8. Plan view TEM micrographs showing the microstructure of AlCu test lines: Ža. one 1.33-m wide Dual Damascene line segment; Žb. two 0.35-m-wide Dual Damascene line segments; Žc. one 1.33-m wide RIE line segment; and Žd. two 0.35-m-wide RIE line segments.
during the post metal etch anneal. The size of the TiAl 3 regions is approximately 0.1᎐0.2 m for both Dual Damascene and metal RIE. 3.3. Texture analysis Fig. 10 illustrates the Al crystallographic texture for Dual Damascene and RIE lines. In Fig. 10a, 1r is plotted vs. line width, where is defined as the width
Žin degrees. of the portion of the main 111 peak that contains 95% of the total 111 peak intensity. The background is subtracted before the peak is evaluated. The is determined from pole figures that are based on the backscatter Kikuchi diffraction results. In Fig. 10b, the volume fraction of random grains is plotted vs. line width. Both plots show that the texture of the Dual Damascene lines decreases as the line width is reduced, while the texture of the RIE lines is always stronger than that of the Damascene lines and is essentially independent of line width. For line widths F 0.35 m, the Damascene samples exhibit a totally random texture. The fact that the texture of the Damascene samples decreases as the line width is reduced can be attributed to an increasing contribution of material nucleated at the sidewalls of the Damascene trench w12x. 3.4. Physical failure analysis
Fig. 9. TEM cross-sections of 0.18-m-wide lines: Ža. Dual Damascene and Žb. metal RIE.
In order to determine the size and location of electromigration-induced voids, top-down optical and SEM inspections were performed on Dual Damascene and RIE samples, respectively. The absence of a Ti or TiN cap layer allows voiding in Damascene lines to be optically observed. The Damascene lines do not require significant sample preparation for optical inspection, while the RIE lines require all material above M1 to be removed for SEM analysis. The RIE samples are pulled from the electromigration test after increasing in resistance by 20᎐40%. It should be noted, however, that
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due to rapid resistance shifts Žsee Fig. 3a., Damascene samples are not removed from the test until the resistance increases ) 100%. Samples for each line width and process are analyzed, while micrographs are provided in this paper only for the 0.35-m-wide lines. The 0.18-m-wide Damascene lines typically show voids in M1 that accumulate in several distinct groups as far as 30᎐40 m from the cathode end. The 0.35m-wide Damascene lines ŽFig. 11a. show similar voiding characteristics as the 0.18-m-wide lines. The damage observed in these narrow Damascene lines is quite severe and suggests that Al has melted. A single void forms in the 1.33-m-wide Damascene lines that originates at the cathode end of the structure and extends a few microns in length. It is unknown whether any of the inspected Damascene samples exhibit voiding in the cathode viaŽs.. Some of the Damascene samples show extrusions at the anode end of the lines. Some of the 0.18-m-wide RIE lines show a single void in M1 at the cathode end, while others show a single void in M1 as far as 10 m from the cathode end. The 0.35-m-wide RIE lines ŽFig. 11b. show similar voiding characteristics as the 0.18-m-wide lines, except that voids which form away from the cathode via
Fig. 11. Top-down micrographs showing electromigration-induced voids in 0.35-m-wide lines: Ža. Dual Damascene and Žb. metal RIE. The Damascene sample Žoptical micrograph. is tested at 1.7 MArcm2 , while the RIE sample ŽSEM micrograph. is tested at 2.4 MArcm2 . The parallel lines on either side of the extrusion monitors in Ža. are included on all of the test structures for lithography purposes.
tend to lie closer Žwithin 5 m. to the cathode end. A single void forms in the 1.33-m-wide RIE lines that originates at the cathode end of the structure and extends a few microns in length. No evidence of extrusions is observed for any of the RIE samples. 4. Discussion
Fig. 10. Linear-log plots illustrating the Al crystallographic texture in Dual Damascene and RIE lines: Ža. 1r vs. line width and Žb. random volume fraction vs. line width.
The results reported above demonstrate many differences between Dual Damascene and RIE lines in terms of electromigration behavior and microstructure. In the following discussion, we explore possible explanations for these differences. The resistance incubation period in two-level, multiple-grained AlCu metallization systems is defined as the time required for Cu to be swept away from the void nucleation region plus the time required for the void to grow to a sufficiently large size so as to cause a resistance increase w10x. Fig. 3a,b illustrates that the resistance incubation time is significantly longer for Dual Damascene than for metal RIE. It is this fact that
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contributes most to the superior electromigration performance of Damascene lines at low levels of resistance increase. Plan view TEM images show bamboo and near bamboo grain structures in 0.35-m-wide Dual Damascene ŽFig. 8b. and RIE ŽFig. 8d. lines, respectively. TEM images for the 1.33-m-wide Dual Damascene ŽFig. 8a. and RIE ŽFig. 8c. lines reveal near bamboo and multiple grained microstructures, respectively. Although TEM images were not produced for the narrowest lines tested, it is fairly certain that a bamboo microstructure exists for 0.18-m-wide Dual Damascene and RIE lines. For all line widths, it is clear that the average grain sizes are significantly larger for Dual Damascene than for metal RIE. Table 3 indicates that the average grain size for Damascene lines is approximately three times larger than for RIE lines. This result is not surprising given that the AlCu deposition temperature is much higher in the Damascene process ŽF 450⬚C. than in the RIE process ŽF 150⬚C.. Larger grains for the Damascene samples leads to a reduced number of grain boundaries, which in turn gives rise to fewer vacancy sources and fewer diffusion paths. Therefore, the rate of Al Žand Cu. migration is expected to be lower in Damascene samples compared to metal RIE samples, which may contribute to the superior electromigration performance of the Damascene lines at low levels of resistance increase. It is interesting to note that that the grain sizes for the 0.35- and 1.33-m-wide RIE lines are similar while the grain size of the 1.33-m-wide Damascene lines is larger than that of the 0.35-m-wide Damascene lines Žsee Table 3.. The fact that the RIE grain size is relatively independent of line width is to be expected since the RIE lines are derived from a blanket deposition. The grains in the 1.33-m-wide Damascene lines are probably larger than those in the 0.35-m-wide Damascene lines because there is more area for grain growth during the ‘hot’ AlCu deposition. TEM cross-sections Žand SEM cross-sections combined with a selective wet etch. reveal that TiAl 3 formation is much more prevalent in metal RIE lines than in Dual Damascene lines, but is not continuous in either case. Intermetallic TiAl 3 is observed every 5᎐10 m in Dual Damascene lines and every 0.4᎐0.5 m in RIE lines. Recent TEM analyses indicates that CVD TiN can completely suppress the formation of TiAl 3 in Dual Damascene structures if the PVD AlCu deposition temperature is less than 430⬚C w2,3,13,14x. It should be noted that the formation of TiAl 3 causes a volume decrease in the metal line, which leads to a tensile stress and possible enhancement of void formation during electromigration testing. The presence of TiAl 3 has been recently shown to promote electromigration in metal RIE lines by providing a fast diffusion path for Al migration along the
311
interface between Al and TiAl 3 w15,16x. This diffusion path, however, depends on a continuous TiAl 3 zone along the entire length or at least along extended segments of the conductor line. ‘Patchy’ or ‘partial’ formation of TiAl 3 was not found to impact the electromigration lifetime w15,16x. Therefore, although TiAl 3 is more prevalent in metal RIE lines than in Dual Damascene lines, the fact that it is not continuous in either case probably means that the electromigration lifetimes are not significantly affected by the frequency of intermetallic formation. The resistance vs. time behavior illustrated in Fig. 3a,b can be explained in terms of the TirTiN layers. For Dual Damascene samples, a void in the M1 line forces the current to flow through the redundant Ti and TiN layers. These layers are quite thin, as seen in Fig. 9a, and have high resistivities Žf 55 ⍀-cm for PVD Ti and f 300᎐400 ⍀-cm for CVD TiN based on vendor tool specifications. compared to that of AlCu Žf 3 ⍀-cm.. In addition, since Ti is deposited by physical vapor deposition, the Ti thickness is not likely to be very uniform along the sidewalls and the bottom of the trench. Therefore, the Ti layer may be thinner than expected in certain regions. Once a void is formed, Joule heating of Ti and TiN reaches a critical level that causes adjacent Al regions to melt and rapidly solidify in certain areas. The combination of electromigration during the time the Al melts and subsequent solidification of melted Al may explain why islands of Al material lie between voided regions, as illustrated in Fig. 11a. This process occurs very quickly, resulting in a rapid resistance increase for the Damascene samples Žsee Fig. 3a.. For metal RIE, on the other hand, the TirTiN layers are thicker Žsee Fig. 9b. than in the Damascene case, and the resistivity of the PVD TiN layer Žf 200 ⍀-cm based on vendor tool specifications. is significantly lower than for CVD TiN. Also, the Ti thickness is expected to be more uniform along the length of the RIE line because all metal layers are put down in a blanket deposition. Once a void is formed, current flows predominately through the thick TirTiN cap layer and Joule heating is less likely to reach a critical level. Therefore, Al does not melt and the resistance increase is more gradual for 0.18- and 0.35-m-wide RIE samples Žsee Fig. 3b.. The fact that the 1.33-m-wide RIE structures show a rapid increase in resistance suggests that the electromigration test currents used for these wide lines generate sufficient Joule heating so as to cause Al to melt. Another point to consider when comparing the electromigration lifetimes of Dual Damascene and metal RIE is the role of interfacial diffusion. The sidewalls of the RIE lines consist of an interface between oxidized Al and SiO 2 that is affected by residues and interface roughness as a result of dry etch and wet clean processes. For bamboo or near bamboo grain struc-
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tures, significant Al mass transport occurs along this interface w17x and can be accelerated if sidewall roughness contributes to additional void nucleation sites w18x. For Dual Damascene lines, the absence of an etched Al᎐SiO 2 interface results in a lower rate of diffusion, while the interface between the SiO 2 and the top Al surface is smooth, clean and well-bonded due to CMP. Also, the interface between the CVD TiN and Al is most likely a slow diffusion pathway due to good bonding between CVD TiN and CVD Al. This last point is supported by a recent study w6x, in which the electromigration lifetime of Dual Damascene samples with a CVD TiNrCVD Al interface was found to be much longer than that of Dual Damascene samples with a PVD TiNrCVD Al interface. The fact that resistance increases are not observed in the 0.18-m-wide Damascene samples for current densities - 2.0 MArcm2 leads one to conclude that a back stress effect has occurred. The accumulation of Al atoms at the anode end of the line creates a stress gradient that opposes the electromigration driving force w19x. The critical product of current density and line length Ž jL.c , above which the resistance increases is nearly 40 000 Arcm, more than 10 times higher than reported previously. For example, in unpassivated, polycrystalline lines critical products of 1770 Arcm have been reported for Damascene lines w20x, and 1020 Arcm w20x and 1260 Arcm w19x for RIE lines. The huge difference in the above values can be explained in terms of the ability to pack atoms into the line. Dual Damascene samples in the present study exhibit little tensile stress Žpartially due to minimal TiAl 3 formation., few grain boundaries and good bonding at the CVD TiNrCVD Al and SiO 2rPVD Al interfaces. There are not many places to move atoms and consequently a great deal of energy is required to add layers of atoms to grain boundaries or other interfaces. As long as this holds, the critical product of 40 000 Arcm describes the strength of the Dual Damascene metallization system. Resistance saturation in 0.18-m-wide Damascene samples, as seen in Fig. 4, can be explained by combining the above model with the melting of Al material. Aluminum melts due to excessive Joule heating and rapidly solidifies in certain areas, resulting in islands of solidified Al between voided regions Žsee Fig. 11a.. The melting of Al continues until a region is encountered in which the Joule heating is significantly reduced, such as a location along the line where the Ti liner is thicker. Additional voiding is prevented downstream from the last voided region because the jL product becomes comparable to the critical product of 40 000 Arcm. Here, L is the distance from the last voided region to the anode end of the structure. For example, if L s 160 m Žfor a voided region 40 m from the cathode end.
and j s 2.5 MArcm2 , then the jL product is 40 000 Arcm. The current density dependence of the 0.35-m-wide structure Žsee Fig. 5 and Table 1. indicates that n is close to 2 for Damascene lines and close to 1 for RIE lines. This suggests that different mechanisms are responsible for void formation in the two cases. In the past, a current density exponent of n s 2 has been attributed to electromigration lifetimes limited by void nucleation w21x while a value of n s 1 has been associated with lifetimes controlled by void growth w22x. More recently, however, n has been shown to vary from 2 to 1 as the lifetime progresses from an incubation period to a steady-state period in the case of two-level, multiple-grained AlCu structures w17x. The migration of Cu controls the incubation period and results in a value of n f 2 for failure times close to the incubation time w17x. Following this incubation period Ždefined as the time required for Cu to migrate beyond a critical distance from the cathode end. and a slow motion transient stage, a steady-state regime is obtained in which the drift velocity becomes constant and the electromigration-induced void grows at a steady rate. A value of n f 1 is obtained for failure times much greater than the incubation time w17x. The different values of n found for Damascene and RIE can be understood if the previous arguments are assumed to be valid for the case of two-level, bamboo Žor near bamboo. grained AlCu structures. A value of n f 1 that is obtained for the 0.35-m-wide RIE lines is consistent with a void growth failure mechanism. The fact that the RIE lines exhibit a short resistance incubation period followed by a gradual increase in resistance is also in agreement with lifetimes controlled by void growth, and further suggests that Cu migration is less significant in these samples. On the other hand, a value of n f 2 that is obtained for the 0.35-m-wide Damascene lines would seem to indicate that the lifetime of these samples is limited by Cu migration. Furthermore, the absence of a fast diffusion path along extended segments of the Damascene lines produces a long resistance incubation period by reducing the Cu migration rate. Once Cu has migrated away, the time until the resistance starts to increase is relatively short since melting of Al begins as soon as a void becomes large enough to cause significant current to flow through the redundant TirTiN layer. The assumption that Cu plays an important role in bamboo or near bamboo grained AlCu microstructures is supported by a recent study of the effect of Cu concentration on the electromigration lifetime. Domenicucci et al. found that the electromigration lifetime of near bamboo grained, two-level AlCu structures increases 2 = as the Cu concentration increases from 0.5 to 1.0 wt.% w23x.
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A larger value of n for Dual Damascene samples means that the relative lifetimes of these lines compared to metal RIE will improve as the current density decreases. Therefore, the ratio of the Damascene t50 to RIE t50 may be much higher at chip operating conditions Ži.e. low current densities. than indicated by Fig. 7. This most likely explains why a lifetime ratio of 3 is found at 2.0 MArcm2 in terms of a q20% resistance increase Žsee Fig. 7., while a ratio of 9 was previously reported at 0.9 MArcm2 for the same metallization systems and failure criterion w2,3x. Finally, it is interesting to note that Dual Damascene samples with a random Al grain orientation exhibit a longer electromigration lifetime compared to RIE samples with a strong Al Ž111. texture Žsee Fig. 10a,b.. In RIE lines, the metal is deposited at a low temperature, patterned and then annealed. The Al grains can orient during this subsequent anneal and the surface normal texture tends to improve. In contrast for Damascene lines, the metal is deposited at a high temperature into trenches where there is substantial growth on both the sidewalls and the bottom of the trench. The surface normal texture is poor, but the texture along the sidewalls is strong. Therefore, the film is textured in a manner with Al Ž111. grains present along all of the internal interfaces. This, on the one hand, gives rise to a weak surface normal texture, but, on the other hand, leads to a more perfect line by yielding fewer diffusion paths in the form of large angle grain boundaries. Combined with large grain size and smooth interfaces, the Damascene samples yield long lifetimes.
4.
5.
6.
7.
8.
5. Summary and conclusions The electromigration and microstructural characteristics of AlCu Dual Damascene lines have been compared to that of AlCu RIE lines. Two-level test structures of 0.18-, 0.35- and 1.33-m-wide lines have been tested at 250⬚C. The summary and conclusions of this study are as follows: 1. The 0.18-m-wide Dual Damascene lines, which are terminated by W diffusion barriers at both the cathode and anode ends, do not exhibit resistance increases at current densities - 2.0 MArcm2 . The Ž jL.c product above which the resistance increases is nearly 40 000 Arcm. 2. Dual Damascene samples exhibit a long resistance incubation period followed by an abrupt increase in resistance, while the RIE samples show a short resistance incubation period followed by a gradual increase in resistance. 3. Dual Damascene lines show a superior electromigration performance at low levels of resistance change, but the relative performance decreases as the level of resistance shift increases. In terms of
9.
10.
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a q20% resistance shift, 0.35- and 1.33-m-wide Damascene lines show a 3 = longer lifetime than metal RIE lines at 2.0 MArcm2 . The current density exponent is close to 2 for 0.35-m-wide Dual Damascene lines and close to 1 for 0.35-m-wide RIE lines. This suggests that the Damascene lifetimes are limited by Cu migration while the RIE lifetimes are controlled by void growth. The different values of n implies that the relative electromigration performance of Damascene compared to RIE will increase as the current density decreases. Intermetallic TiAl 3 formation is much less prevalent in Damascene than in metal RIE lines, but is not continuous in either case. The average grain size is approximately three times larger for Damascene lines compared to RIE lines due to a higher AlCu deposition temperature. The superior Dual Damascene electromigration lifetime at low levels of resistance shift can be attributed to Ža. large grain size; Žb. smooth, clean and well-bonded top interface; Žc. good bonding between CVD TiN and CVD Al; and Žd. small tensile stress due to a reduction in TiAl 3 formation. Each of Ža., Žb. and Žc. contributes to the absence of a fast diffusion path along extended segments of the Damascene conductor Žsuch as an etched Al᎐SiO 2 interface .. The abrupt change in resistance following the resistance incubation period for Damascene lines occurs because the Joule heating of PVD Ti and CVD TiN layers reaches a critical level and causes adjacent regions of Al to melt. Resistance saturation effects are observed in 0.18m-wide Dual Damascene lines at high current densities Ž2.5 MArcm2 . and for long stripe lengths Ž200 m.. Electron backscatter diffraction measurements reveal a random Al grain orientation in sub-micron Dual Damascene lines and a strong Al Ž111. texture in metal RIE lines. The random Al grain orientation in Damascene lines is caused by the presence of Al Ž111. planes along all of the internal trench interfaces. This may lead to a more perfect line in the Damascene case by yielding fewer diffusion paths in the form of large angle grain boundaries.
Acknowledgements The authors would like to thank R. Raviart and W. Malkus for sample preparation, R. Daddazio for sample preparation and SEM work, P.C. Wang and R. Rosenberg for reviewing the manuscript and the Ad-
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vanced Semiconductor Technology Center in East Fishkill for processing the samples used in this study. References w1x C.W. Kaanta, S.G. Bombardier, W.J. Cote, W.R. Hill, G. Kerszykowski, H.S. Landis, D.J. Poindexter, C.W. Pollard, G.H. Ross, J.G. Ryan, S. Wolff, J.E. Cronin, Proceedings of the VLSI Multilevel Interconnection Conference, June, 1991, p. 144. w2x L.A. Clevenger, G. Costrini, D.M. Dubuzinsky, R. Filippi, J. Gambino, M. Hoinkis, L. Gignac, J.L. Hurd, R.C. Iggulden, C. Lin, R. Longo, G.Z. Lu, J. Ning, J.F. Nuetzel, R. Ploessl, K. Rodbell, M. Ronay, R.F. Schnabel, D. Tobben, S.J. Weber, L. Chen, S. Chiang, T. Guo, R. Mosley, S. Voss, L. Yang, Proceedings of the International Interconnect Technology Conference, June, 1998, p. 137. w3x R. Iggulden, L. Clevenger, G. Costrini, D. Dobuzinsky, R. Filippi, J. Gambino, L. Gignac, C. Lin, R. Longo, G. Lu, J. Ning, J. Nuetzel, K. Rodbell, M. Ronay, F. Schnabel, J. Stephens, D. Tobben, S. Weber, Proceedings of the VLSI Multilevel Interconnection Conference, June, 1998, p. 19. w4x T. Licata, M. Okazaki, M. Ronay, W. Landers, T. Ohiwa, H. Poetzlberger, H. Aochi, D. Dobuzinsky, R. Filippi, D. Restaino, D. Knorr, J. Ryan, Proceedings of the VLSI Multilevel Interconnection Conference, June, 1995, p. 596. w5x J. Proost, H. Li, B. Brijs, A. Witvrouw, K. Maex, Proceedings of the International Interconnect Technology Conference, June, 1998, p. 110. w6x R.F. Schnabel, R. Filippi, L.M. Gignac, K.P. Rodbell, J.L. Hurd, C.-K. Hu, L.A. Clevenger, S.J. Weber, R.C. Iggulden, R. Ravikumar, E.W. Kiewra, T. Kane, T. Joseph, Y.Y. Wang, Fifth International Workshop on Stress-Induced Phenomena in Metallization, Conf. Proc. 491 AIP, New York, 1999, p. 27. w7x J.L. Hurd, K.P. Rodbell, D.B. Knorr, N.L. Koligman, Mat. Res. Soc. Symp. Proc. 343 Ž1994. 653.
w8x R.G. Filippi, G.A. Biery, R.A. Wachnik, J. Appl. Phys. 78 Ž1995. 3756. w9x R.G. Filippi, R.A. Wachnik, H. Aochi, J.R. Lloyd, M.A. Korhonen, Appl. Phys. Lett. 69 Ž1996. 2350. w10x C.-K. Hu, M.B. Small, P.S. Ho, J. Appl. Phys. 74 Ž1993. 969. w11x A.S. Oates, E.P. Martin, D. Alugbin, F. Nkansah, Appl. Phys. Lett. 62 Ž1993. 3273. w12x J.L. Hurd, K.P. Rodbell, L.M. Gignac, L.A. Clevenger, R.C. Iggulden, R.F. Schnabel, S.J. Weber, N.H. Schmidt, Appl. Phys. Lett. 76 Ž1998. 326. w13x L.M. Gignac, K.P. Rodbell, L.A. Clevenger, R.C. Iggulden, R.F. Schnabel, S.J. Weber, C. Lavoie, C. Cabral Jr., P.W. DeHaven, Y.-Y. Wang, S.H. Boettcher, Advanced Metallization and Interconnect Systems for ULSI Applications, Proceedings ULSI XIII MRS, Pittsburgh, PA, 1998, p. 79. w14x K.P. Rodbell, L.M. Gignac, J.L. Hurd, Y.-Y. Wang, R. Filippi, L.A. Clevenger, R.C. Iggulden, R.F. Schnabel, S. Weber, in ICOTOM-12, Proceedings of the International Conference on Textures of Materials, Montreal, Canada, August, 1999, p. 1287. w15x M. Hosaka, T. Kouno, Y. Hayakawa, 36th Annual Proceedings of Reliability Physics, Reno, Nevada, IEEE, New York, 1998, p. 329. w16x T. Kouno, M. Hosaka, H. Niwa, M. Yamada, J. Appl. Phys. 84 Ž1998. 742. w17x C.-K. Hu, Thin Solid Films 260 Ž1995. 124. w18x R. Ravikumar, H. Cichy, R.G. Filippi, E.W. Kiewra, D.L. Rath, G. Stojakovic, Proceedings of the International Interconnect Technology Conference, June, 1999, p. 203. w19x I.A. Blech, J.Appl. Phys. 47 Ž1976. 1203. w20x J. Proost, K. Maex, L. Delaey, Appl. Phys. Lett 73 Ž1998. 2748. w21x M. Shatzkes, J.R. Lloyd, J. Appl. Phys. 59 Ž1986. 3890. w22x R. Kirchheim, U. Kaeber, J. Appl. Phys. 70 Ž1991. 172. w23x A.G. Domenicucci, R.G. Filippi, K.W. Choi, C.-K. Hu, K.P. Rodbell, J. Appl. Phys. 80 Ž1996. 4952.
Electromigration in AlCu lines: comparison of Dual Damascene and metal reactive ion etching R.G. Filippi a,U , M.A. Gribelyuk a , T. Josepha , T. Kane a , T.D. Sullivana , L.A. Clevenger b , G. Costrini b , J. Gambino b , R.C. Igguldenb , E.W. Kiewrab , X.J. Ning c , R. Ravikumar c , R.F. Schnabel c , G. Stojakovic c , S.J. Weber c , L.M. Gignac d , C.-K. Hud , D.L. Rathd , K.P. Rodbelld a IBM Microelectronics, 2070 Route 52, Hopewell Junction, New York 12533-6531, USA IBM Microelectronics, DRAM De¨ elopment Alliance, IBM Semiconductor Research and De¨ elopment Center, 2070 Route 52, Hopewell Junction, New York 12533-6531, USA c Infineon Technologies, DRAM De¨ elopment Alliance, IBM Semiconductor Research and De¨ elopment Center, 2070 Route 52, Hopewell Junction, New York 12533-6531, USA d IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA b
Received 12 September 2000; received in revised form 23 January 2001; accepted 9 February 2001
Abstract The electromigration behavior and microstructural features of AlCu Dual Damascene lines are compared to those of AlCu metal reactively ion etched ŽRIE. lines. Test structures consist of 0.18-, 0.35- and 1.33-m-wide lines terminated by W diffusion barriers, and are tested at 250⬚C. A remarkable finding for the 0.18-m-wide Damascene samples is a threshold-length product of nearly 40 000 Arcm. Different failure mechanisms are revealed for Dual Damascene and metal RIE structures by observing the resistance shift vs. time as well as the lifetime vs. current density behaviors. It is found that the Damascene structures exhibit a long resistance incubation period followed by a rapid increase in resistance, while the RIE structures show a short resistance incubation period followed by a gradual increase in resistance. The current density exponent is found to be close to 2 for the Damascene process and close to 1 for the RIE process. The Damascene samples show a significant lifetime improvement over the RIE samples for low levels of resistance change, while the relative lifetime improvement decreases as the maximum allowed resistance shift increases. In order to understand the electromigration performance of each metallization system, various physical analysis techniques are implemented. The average grain size, determined from transmission electron microscopy ŽTEM., is found to be significantly larger for Damascene lines due to a higher AlCu deposition temperature. Both TEM and scanning electron microscopy ŽSEM. analyses indicate that TiAl 3 intermetallic formation occurs in both Dual Damascene and RIE lines, but is much more prevalent in the RIE case. Electron backscatter analysis reveals a weak Al crystallographic texture in sub-micron Dual Damascene samples and a strong Ž111. fiber texture in RIE samples. Optical and SEM inspections illustrate different failure signatures for 0.18 and 0.35 m Dual Damascene and RIE lines. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Electromigration; Interfaces; Metallization
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Corresponding author. Tel.: q1-845-892-2906; fax: q1-845-892-3039. E-mail address: [email protected] ŽR.G. Filippi..
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1. Introduction In the increasingly competitive world of Dynamic Random Access Memory ŽDRAM. manufacturing, many factors need to be considered when implementing interconnection processes. These factors include cost, yield, defect level, tight pitch wiring and reliability. As feature sizes are reduced below 0.20 m, reliability concerns due to electromigration become extremely important as operating current densities increase in the metal interconnections. Reactive ion etching ŽRIE. of aluminum copper ŽAlCu. based interconnections has been the standard metal patterning technique for both Logic and DRAM. However, this process is significantly more expensive due to numerous processing steps, which makes it less than ideal for DRAM applications. The use of a Dual Damascene interconnection technology, on the other hand, is one way to achieve a low cost process, while at the same time meet the equally important requirements of low contamination levels, tight pitch wiring and high reliability w1᎐3x. The electromigration reliability of AlCu Dual Damascene structures has generally been found to be superior or at least comparable to that of AlCu RIE structures at 0.30-m dimensions w2᎐4x. The electromigration lifetime of 0.30-m bamboo RIE samples, however, was found to be superior to that of 0.30-m bamboo Damascene samples w5x. Recently, the electromigration performance was observed to be very dependent on the liner quality and the liner deposition technique used for the Damascene trench w6x. In this paper, we compare the electromigration performance and microstructural characteristics of 0.18-, 0.35- and 1.33-m AlCu Dual Damascene and RIE interconnections. 2. Experiment The process of forming the Dual Damascene structure is the same as described by Clevenger et al. w2x and Iggulden et al. w3x, in which both physical vapor deposition ŽPVD. and chemical vapor deposition ŽCVD. techniques are utilized for aluminum copper ŽAlCu., titanium ŽTi. and titanium nitride ŽTiN.. The first metal level ŽM1. and the underlying via are filled simultaneously. Planarized top metal surfaces are achieved by chemical mechanical polishing ŽCMP.. The M1 is composed of a 0.48-m stack of PVD TirCVD TiNrCVD AlrPVD AlCu, where the Cu concentration is 0.5 weight percent Žwt.%. and the AlCu is deposited at F 450⬚C. Note that some results in this study are also based on Damascene lines with an M1 thickness of 0.25 m. The results for an M1 thickness of 0.25 m are shown only in Section 3.1.1. The Dual Damascene
structure is surrounded and passivated by a tetraethylorthosilane ŽTEOS.-based oxide. The process of forming the RIE structure utilizes PVD techniques only. The metallization is composed of AlCu sandwiched between redundant layers of Ti and TiN. The M1 is a 0.40-m stack of TirTiNr AlCurTirTiN, where the Cu concentration is 0.5 wt.% and the AlCu is deposited at F 150⬚C. Following metal etch, the RIE samples were annealed at 400⬚C in forming gas Ž10% H 2r90% N2 . for 20 min. The RIE structure is passivated with a high density plasma ŽHDP. oxide. Underlying CVD tungsten ŽW. studs are connected to TirTiNrAlCurTirTiN lines. Two-level structures of 0.18, 0.35 and 1.33-m dimensions are tested. In the case of the 0.18-m-wide lines Žsee Fig. 1a., W local interconnections are located at a lower level at both ends of M1. The M1 stripe length, or distance between AlCu vias ŽDamascene case. or W studs ŽRIE case., is 200 m. There is one stud-via at the cathode end of the structure and three stud-vias at the anode end. For the 0.35-m-wide lines Žsee Fig. 1b., a W local interconnection is located at a lower level at the cathode end, while the M1 widens to 5 m at the anode end. There is one stud-via at the cathode end of the structure, the distance between the stud-via and the point at which M1 widens is 200 m, and the distance between the point where M1 widens and the test pad is ) 300 m. The 1.33-m-wide lines Žsee Fig. 1c. are similar to the 0.35-m-wide lines except that there are three stud-vias at the cathode end of the structure and the M1 widens to 10 m at the anode end. Each stud-via measures 0.225 m in diameter for the 0.18, 0.35 and 1.33-m-wide lines. The resistance of the 0.35-m-wide Damascene structure measures approximately 55᎐60 ⍀ at 25⬚C and 90᎐100 ⍀ at 250⬚C, while the resistance of the 0.35-m-wide RIE structure measures approximately 65᎐70 ⍀ at 25⬚C and 105᎐115 ⍀ at 250⬚C. The 0.18-m-wide Damascene lines require higher current densities than the corresponding RIE samples in order to cause failure. The 0.18-m-wide Damascene and RIE samples are tested at 2.5 MArcm2 Ž2.5= 10 6 Arcm2 . and 1.8 MArcm2 , respectively. For current densities ) 2.0 MArcm2 , the 0.18-m-wide RIE structure exhibits early failures and a bimodal failure distribution. The 1.33-m-wide Damascene and RIE samples are tested at 1.7 and 2.4 MArcm2 , respectively. The current density dependence of both processes is determined by testing the 0.35-m-wide structure at multiple current densities Ž1.3, 1.7 and 2.1 MArcm2 for the Damascene samples, and 1.2, 1.8 and 2.4 MArcm2 for the RIE samples.. All stated current densities are based on the M1 Al cross-sectional area and ignore the formation of TiAl 3 . The oven temperature is chosen such that the ambient plus Joule heating
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Fig. 1. Schematic top-down view of test structures used for the Dual Damascene and RIE processes: Ža. 0.18-m-wide line, Žb. 0.35-m-wide line and Žc. 1.33-m-wide line. AlCu vias are present in the Dual Damascene process while W studs are present in the RIE process.
equals 250⬚C for all structures. Each stress cell, which consists of a sample type ŽDual Damascene or metal RIE., current density and line width, contains 13᎐24 samples. Various physical analysis techniques are used to help explain the electromigration results. The Al grain structure is characterized by plan view transmission electron microscopy ŽTEM.. The extent of TiAl 3 intermetallic formation is determined by cross-sectional TEM as well as cross-sectional scanning electron microscopy ŽSEM. with selective wet etch. The crystallographic texture of micron and sub-micron lines is measured by electron backscatter diffraction Žalso referred to as backscatter Kikuchi diffraction w7x. with a spatial resolution of 0.1 m. Size and location of electromigration-induced voids are determined for 0.18, 0.35 and 1.33-m-wide structures using top-down optical inspection for Dual Damascene samples and top-down SEM inspection for RIE samples. 3. Results 3.1. Electromigration beha¨ ior 3.1.1. High current density requirement for 0.18- m-wide Damascene lines High current densities are required to cause failures in the 0.18-m-wide Damascene samples. Failures Ži.e. resistance increase ) 0.1%. were never obtained for a
current density - 2.0 MArcm2 . This result is independent of the line thickness. Fig. 2 shows SEM cross-sections of a 0.18-m-wide structure tested at 2.5 MArcm2 and 250⬚C. The sample, which has an M1 stack thickness of 0.25 m, was on test for 118 h and increased in resistance - 0.1%. Please note that not all samples from this stress cell behaved identically, as some samples increased significantly in resistance Ži.e. ) 100%. after the same period of time. No significant electromigration-induced voiding is seen at the cathode end ŽFig. 2a., while Cu pile-up, in the form of Al 2 Cu precipitates, is observed at the anode end ŽFig. 2b.. A very small void is seen in Fig. 2a at the base of the Damascene via. Additional cross-sectional SEM images, up to 50 m from the cathode end, were generated for this sample and reveal no evidence of voiding. It is interesting to note that the Cu pile-up seen in Fig. 2b occurs in the via closest to the cathode end but not in the other two vias, which is most likely related to the flow of electrons in that region of the structure. These observations for AlCu Dual Damascene are significantly different than that found for AlCu RIE w8,9x, in which resistance increases ) 10% were observed in the RIE case Žafter 100 h and at 250⬚C. even for stripe lengths equal to 100 m and current densities equal to 2.0 MArcm2 . 3.1.2. Resistance shift ¨ s. time beha¨ ior Fig. 3 shows a plot of the median resistance shift vs.
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time for the 0.18, 0.35 and 1.33-m-wide Dual Damascene ŽFig. 3a. and RIE ŽFig. 3b. samples. For multilayered metallizations, a resistance increase during electromigration testing is caused by Al depletion w10,11x. The first observation to make is that the resistance incubation period, defined as the time before which the resistance starts to increase, is much longer for Damascene than for RIE samples. The second is that the Damascene samples exhibit an abrupt increase in resistance while the RIE samples Ž0.18 and 0.35-m wide lines. gradually increase in resistance. Similar trends
Fig. 3. Median resistance shift Ž%. vs. time for the Ža. Dual Damascene samples and Žb. RIE samples.
were observed for 0.30-m-wide AlCu Damascene and RIE samples tested at 1᎐2 MArcm2 and 200⬚C w5x. Note that the median resistance shift is plotted vs. time in Fig. 3a,b rather than the mean resistance shift because the latter does not illustrate the typical behavior of samples that abruptly shift in resistance. Resistance saturation effects are observed for 0.18m-wide Dual Damascene samples, as seen in Fig. 4. The figure shows that the fractional resistance shift vs. time for three Damascene lines saturates at approximately 250%. The saturation level indicated by Fig. 4 is representative for this stress cell, although it should be noted that not all of the samples exhibit resistance saturation effects. Similar behavior is not observed for the metal RIE lines.
Fig. 2. SEM cross-sections of a 0.18-m-wide Dual Damascene sample tested at 2.5 MArcm2 and 250⬚C: Ža. cathode end of structure showing a small void at the bottom of the via and Žb. anode end of structure showing Cu pile-up. The sample was on test for 118 h and did not significantly increase in resistance. The M1 thickness of this particular sample measures 0.25 m. The bright regions at the anode end are identified as Al 2 Cu precipitates.
3.1.3. Lifetime beha¨ ior The current density dependence for the 0.35-mwide Dual Damascene and RIE samples is illustrated in Fig. 5 in terms of a q20% resistance increase. The plot shows that the current density exponent, n, is equal to 1.95 and 1.03 for the Damascene and RIE processes, respectively. Table 1 indicates that the current density exponent is close to 2 for the Damascene process and close to 1 for the RIE process, and is independent of the failure criterion for both cases.
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Fig. 4. Resistance shift Ž%. vs. time for 0.18-m-wide Dual Damascene samples tested at 2.5 MArcm2 . Only 3 out of 22 samples are shown.
Fig. 5. Log᎐log plot of t50 vs. j for the 0.35-m-wide Dual Damascene and RIE structures. The failure criterion is a q20% resistance increase. The error bars represent the 95% confidence interval.
A lognormal distribution is used for describing failure times. The median lifetimes Ž t50 . for both processes are consistent with the resistance shift vs. time data shown in Fig. 3a,b. Moreover, the variation in the failure times, as measured by the lognormal sigma Ž ., is fairly independent of width, failure criterion Ži.e. maximum resistance shift. and process. Table 2 lists the electromigration statistics in terms of a q20% resistance increase for some of the Dual Damascene and RIE stress cells. Fig. 6 shows a lognormal cumulative distribution function ŽCDF. plot in terms of a q20% resistance increase for the 0.35- and 1.33-m-wide Damascene and RIE lines. The plotted failure times are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE. For both line widths, the Dual Damascene samples show a 3 = lifetime improvement over the corresponding RIE samples. The 0.18-m-wide samples are not included in Fig. 6 due to the fact that normalization of the Damascene failure times to 2.0 MArcm2 using n s 2 is not justified since failures in these narrow lines are not observed when stressed below 2.0 MArcm2 . Fig. 7 illustrates the lifetime improvement of Dual Damascene relative to metal RIE. The figure shows a plot of the ratio of the Dual Damascene t50 to the RIE t50 vs. resistance increase for the 0.35- and 1.33-m-
wide structures. The failure times that determine the t50 values are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE. The relative lifetime improvement of Damascene over RIE decreases as the maximum allowed resistance shift increases. For example, at a 2% resistance change, the 0.35-m-wide Damascene samples show a 5.0= lifetime improvement over the corresponding RIE samples. But at a 100% resistance change, the relative lifetime improvement of Damascene over RIE is approximately 1 for this line width. It is interesting to note that the 1.33-m-wide Damascene samples show a 2.4= longer lifetime than the 1.33-m-wide RIE samples for a 100% resistance change. The 0.18-m-wide samples are not included in Fig. 7 for the same reason that they are excluded from Fig. 6; normalization of the Damascene failure times to 2.0 MArcm2 using n s 2 is not justified for the 0.18m-wide samples since failures in these narrow lines are not observed when stressed below 2.0 MArcm2 .
Table 1 Current density exponent for the 0.35-m-wide structure at different levels of resistance increase Failure criterion Ž%.
2 10 20 50 100
Current density exponent, n Dual Damascene
RIE
1.96 1.96 1.95 1.93 1.88
1.11 1.04 1.03 1.19 1.03
3.2. TEM analysis Fig. 8 shows plan view TEM micrographs of 0.35and 1.33-m-wide lines. The microstructure is characterized as bamboo Žsingle grain across line width for vast majority of line., multiple-grained Žat least two grains across line width for vast majority of line. or near bamboo Žmix of bamboo and multiple-grained, but mostly bamboo.. The 0.35-m-wide Dual Damascene ŽFig. 8b. and RIE ŽFig. 8d. lines indicate bamboo and near bamboo microstructures, respectively. Short segments of the bamboo-grained Damascene lines have two grains across the line. The 1.33-m-wide Dual Damascene ŽFig. 8a. and RIE ŽFig. 8c. lines reveal near bamboo and multiple-grained microstructures, respectively. Short segments of the multiple-grained RIE lines have a single grain across the line. Table 3 lists
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Table 2 Electromigration lifetime statistics for the 0.18-, 0.35- and 1.33-m-wide structures in terms of a q20% resistance increase Metallization
Line width Žm.
j ŽMArcm2 .
t50 Žh.
Dual Damascene Dual Damascene Dual Damascene RIE RIE RIE
0.18 0.35 1.33 0.18 0.35 1.33
2.5 1.7 1.7 1.8 2.4 2.4
24.3 64.6 48.3 12.3 13.0 9.1
0.33 0.34 0.29 0.22 0.26 0.22
the average grain sizes for the 0.35- and 1.33-m-wide samples. The grain size is obtained by applying the line intercept method to the TEM images, where a line Žof known length. is drawn along the line length and the number of grain intercepts are counted. The average grain size is calculated by dividing the line length by the number of intercepts and averaging several line measurements. Fig. 9 shows TEM cross-sections of 0.18-m-wide lines. The Dual Damascene sample ŽFig. 9a. does not reveal any TiAl 3 intermetallic formation, while the RIE sample ŽFig. 9b. shows significant TiAl 3 formation near the bottom of the line. For the magnification shown, it was not possible to distinguish the Ti and TiN layers due to the slight difference in contrast between Ti and TiN as compared to Al. The TiAl 3 was identified by energy dispersive X-ray spectroscopy ŽEDS..
Fig. 7. Ratio of the Dual Damascene t50 to the RIE t50 vs. resistance increase for the 0.35- and 1.33-m-wide structures. The failure times that determine the t50 values are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE.
Although TiAl 3 is also observed below the top TiN layer in other RIE samples, it is interesting to note that the reaction of Ti and Al occurs predominantly above the bottom TiN layer. In general, formation of TiAl 3 occurs in both Dual Damascene as well as metal RIE lines, and is not continuous in either case. However, TEM cross-sections indicate that the formation of TiAl 3 is much less frequent in Dual Damascene samples. Although not shown here, SEM cross-sections Žalong the line length. combined with a selective wet etch of Al to reveal TiAl 3 show that intermetallic particle formation occurs every 5᎐10 m in Dual Damascene samples compared to every 0.4᎐0.5 m in the RIE samples. This suggests that the CVD TiN layer is more effective at preventing Ti and Al from reacting during the PVD AlCu deposition than the PVD TiN layer is Table 3 Average grain sizes for the 0.35- and 1.33-m-wide structures
Fig. 6. Lognormal CDF plot for the 0.35- and 1.33-m-wide Dual Damascene and RIE lines in terms of a q20% resistance shift. The plotted failure times are generated by normalizing the actual failure times to 2.0 MArcm2 using a current density exponent of n s 2 for Dual Damascene and n s 1 for metal RIE.
Metallization
Line width Žm.
Average grain size along line length Žm.
Dual Damascene Dual Damascene RIE RIE
0.35 1.33 0.35 1.33
1.1 1.8 0.4 0.5
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Fig. 8. Plan view TEM micrographs showing the microstructure of AlCu test lines: Ža. one 1.33-m wide Dual Damascene line segment; Žb. two 0.35-m-wide Dual Damascene line segments; Žc. one 1.33-m wide RIE line segment; and Žd. two 0.35-m-wide RIE line segments.
during the post metal etch anneal. The size of the TiAl 3 regions is approximately 0.1᎐0.2 m for both Dual Damascene and metal RIE. 3.3. Texture analysis Fig. 10 illustrates the Al crystallographic texture for Dual Damascene and RIE lines. In Fig. 10a, 1r is plotted vs. line width, where is defined as the width
Žin degrees. of the portion of the main 111 peak that contains 95% of the total 111 peak intensity. The background is subtracted before the peak is evaluated. The is determined from pole figures that are based on the backscatter Kikuchi diffraction results. In Fig. 10b, the volume fraction of random grains is plotted vs. line width. Both plots show that the texture of the Dual Damascene lines decreases as the line width is reduced, while the texture of the RIE lines is always stronger than that of the Damascene lines and is essentially independent of line width. For line widths F 0.35 m, the Damascene samples exhibit a totally random texture. The fact that the texture of the Damascene samples decreases as the line width is reduced can be attributed to an increasing contribution of material nucleated at the sidewalls of the Damascene trench w12x. 3.4. Physical failure analysis
Fig. 9. TEM cross-sections of 0.18-m-wide lines: Ža. Dual Damascene and Žb. metal RIE.
In order to determine the size and location of electromigration-induced voids, top-down optical and SEM inspections were performed on Dual Damascene and RIE samples, respectively. The absence of a Ti or TiN cap layer allows voiding in Damascene lines to be optically observed. The Damascene lines do not require significant sample preparation for optical inspection, while the RIE lines require all material above M1 to be removed for SEM analysis. The RIE samples are pulled from the electromigration test after increasing in resistance by 20᎐40%. It should be noted, however, that
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due to rapid resistance shifts Žsee Fig. 3a., Damascene samples are not removed from the test until the resistance increases ) 100%. Samples for each line width and process are analyzed, while micrographs are provided in this paper only for the 0.35-m-wide lines. The 0.18-m-wide Damascene lines typically show voids in M1 that accumulate in several distinct groups as far as 30᎐40 m from the cathode end. The 0.35m-wide Damascene lines ŽFig. 11a. show similar voiding characteristics as the 0.18-m-wide lines. The damage observed in these narrow Damascene lines is quite severe and suggests that Al has melted. A single void forms in the 1.33-m-wide Damascene lines that originates at the cathode end of the structure and extends a few microns in length. It is unknown whether any of the inspected Damascene samples exhibit voiding in the cathode viaŽs.. Some of the Damascene samples show extrusions at the anode end of the lines. Some of the 0.18-m-wide RIE lines show a single void in M1 at the cathode end, while others show a single void in M1 as far as 10 m from the cathode end. The 0.35-m-wide RIE lines ŽFig. 11b. show similar voiding characteristics as the 0.18-m-wide lines, except that voids which form away from the cathode via
Fig. 11. Top-down micrographs showing electromigration-induced voids in 0.35-m-wide lines: Ža. Dual Damascene and Žb. metal RIE. The Damascene sample Žoptical micrograph. is tested at 1.7 MArcm2 , while the RIE sample ŽSEM micrograph. is tested at 2.4 MArcm2 . The parallel lines on either side of the extrusion monitors in Ža. are included on all of the test structures for lithography purposes.
tend to lie closer Žwithin 5 m. to the cathode end. A single void forms in the 1.33-m-wide RIE lines that originates at the cathode end of the structure and extends a few microns in length. No evidence of extrusions is observed for any of the RIE samples. 4. Discussion
Fig. 10. Linear-log plots illustrating the Al crystallographic texture in Dual Damascene and RIE lines: Ža. 1r vs. line width and Žb. random volume fraction vs. line width.
The results reported above demonstrate many differences between Dual Damascene and RIE lines in terms of electromigration behavior and microstructure. In the following discussion, we explore possible explanations for these differences. The resistance incubation period in two-level, multiple-grained AlCu metallization systems is defined as the time required for Cu to be swept away from the void nucleation region plus the time required for the void to grow to a sufficiently large size so as to cause a resistance increase w10x. Fig. 3a,b illustrates that the resistance incubation time is significantly longer for Dual Damascene than for metal RIE. It is this fact that
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contributes most to the superior electromigration performance of Damascene lines at low levels of resistance increase. Plan view TEM images show bamboo and near bamboo grain structures in 0.35-m-wide Dual Damascene ŽFig. 8b. and RIE ŽFig. 8d. lines, respectively. TEM images for the 1.33-m-wide Dual Damascene ŽFig. 8a. and RIE ŽFig. 8c. lines reveal near bamboo and multiple grained microstructures, respectively. Although TEM images were not produced for the narrowest lines tested, it is fairly certain that a bamboo microstructure exists for 0.18-m-wide Dual Damascene and RIE lines. For all line widths, it is clear that the average grain sizes are significantly larger for Dual Damascene than for metal RIE. Table 3 indicates that the average grain size for Damascene lines is approximately three times larger than for RIE lines. This result is not surprising given that the AlCu deposition temperature is much higher in the Damascene process ŽF 450⬚C. than in the RIE process ŽF 150⬚C.. Larger grains for the Damascene samples leads to a reduced number of grain boundaries, which in turn gives rise to fewer vacancy sources and fewer diffusion paths. Therefore, the rate of Al Žand Cu. migration is expected to be lower in Damascene samples compared to metal RIE samples, which may contribute to the superior electromigration performance of the Damascene lines at low levels of resistance increase. It is interesting to note that that the grain sizes for the 0.35- and 1.33-m-wide RIE lines are similar while the grain size of the 1.33-m-wide Damascene lines is larger than that of the 0.35-m-wide Damascene lines Žsee Table 3.. The fact that the RIE grain size is relatively independent of line width is to be expected since the RIE lines are derived from a blanket deposition. The grains in the 1.33-m-wide Damascene lines are probably larger than those in the 0.35-m-wide Damascene lines because there is more area for grain growth during the ‘hot’ AlCu deposition. TEM cross-sections Žand SEM cross-sections combined with a selective wet etch. reveal that TiAl 3 formation is much more prevalent in metal RIE lines than in Dual Damascene lines, but is not continuous in either case. Intermetallic TiAl 3 is observed every 5᎐10 m in Dual Damascene lines and every 0.4᎐0.5 m in RIE lines. Recent TEM analyses indicates that CVD TiN can completely suppress the formation of TiAl 3 in Dual Damascene structures if the PVD AlCu deposition temperature is less than 430⬚C w2,3,13,14x. It should be noted that the formation of TiAl 3 causes a volume decrease in the metal line, which leads to a tensile stress and possible enhancement of void formation during electromigration testing. The presence of TiAl 3 has been recently shown to promote electromigration in metal RIE lines by providing a fast diffusion path for Al migration along the
311
interface between Al and TiAl 3 w15,16x. This diffusion path, however, depends on a continuous TiAl 3 zone along the entire length or at least along extended segments of the conductor line. ‘Patchy’ or ‘partial’ formation of TiAl 3 was not found to impact the electromigration lifetime w15,16x. Therefore, although TiAl 3 is more prevalent in metal RIE lines than in Dual Damascene lines, the fact that it is not continuous in either case probably means that the electromigration lifetimes are not significantly affected by the frequency of intermetallic formation. The resistance vs. time behavior illustrated in Fig. 3a,b can be explained in terms of the TirTiN layers. For Dual Damascene samples, a void in the M1 line forces the current to flow through the redundant Ti and TiN layers. These layers are quite thin, as seen in Fig. 9a, and have high resistivities Žf 55 ⍀-cm for PVD Ti and f 300᎐400 ⍀-cm for CVD TiN based on vendor tool specifications. compared to that of AlCu Žf 3 ⍀-cm.. In addition, since Ti is deposited by physical vapor deposition, the Ti thickness is not likely to be very uniform along the sidewalls and the bottom of the trench. Therefore, the Ti layer may be thinner than expected in certain regions. Once a void is formed, Joule heating of Ti and TiN reaches a critical level that causes adjacent Al regions to melt and rapidly solidify in certain areas. The combination of electromigration during the time the Al melts and subsequent solidification of melted Al may explain why islands of Al material lie between voided regions, as illustrated in Fig. 11a. This process occurs very quickly, resulting in a rapid resistance increase for the Damascene samples Žsee Fig. 3a.. For metal RIE, on the other hand, the TirTiN layers are thicker Žsee Fig. 9b. than in the Damascene case, and the resistivity of the PVD TiN layer Žf 200 ⍀-cm based on vendor tool specifications. is significantly lower than for CVD TiN. Also, the Ti thickness is expected to be more uniform along the length of the RIE line because all metal layers are put down in a blanket deposition. Once a void is formed, current flows predominately through the thick TirTiN cap layer and Joule heating is less likely to reach a critical level. Therefore, Al does not melt and the resistance increase is more gradual for 0.18- and 0.35-m-wide RIE samples Žsee Fig. 3b.. The fact that the 1.33-m-wide RIE structures show a rapid increase in resistance suggests that the electromigration test currents used for these wide lines generate sufficient Joule heating so as to cause Al to melt. Another point to consider when comparing the electromigration lifetimes of Dual Damascene and metal RIE is the role of interfacial diffusion. The sidewalls of the RIE lines consist of an interface between oxidized Al and SiO 2 that is affected by residues and interface roughness as a result of dry etch and wet clean processes. For bamboo or near bamboo grain struc-
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tures, significant Al mass transport occurs along this interface w17x and can be accelerated if sidewall roughness contributes to additional void nucleation sites w18x. For Dual Damascene lines, the absence of an etched Al᎐SiO 2 interface results in a lower rate of diffusion, while the interface between the SiO 2 and the top Al surface is smooth, clean and well-bonded due to CMP. Also, the interface between the CVD TiN and Al is most likely a slow diffusion pathway due to good bonding between CVD TiN and CVD Al. This last point is supported by a recent study w6x, in which the electromigration lifetime of Dual Damascene samples with a CVD TiNrCVD Al interface was found to be much longer than that of Dual Damascene samples with a PVD TiNrCVD Al interface. The fact that resistance increases are not observed in the 0.18-m-wide Damascene samples for current densities - 2.0 MArcm2 leads one to conclude that a back stress effect has occurred. The accumulation of Al atoms at the anode end of the line creates a stress gradient that opposes the electromigration driving force w19x. The critical product of current density and line length Ž jL.c , above which the resistance increases is nearly 40 000 Arcm, more than 10 times higher than reported previously. For example, in unpassivated, polycrystalline lines critical products of 1770 Arcm have been reported for Damascene lines w20x, and 1020 Arcm w20x and 1260 Arcm w19x for RIE lines. The huge difference in the above values can be explained in terms of the ability to pack atoms into the line. Dual Damascene samples in the present study exhibit little tensile stress Žpartially due to minimal TiAl 3 formation., few grain boundaries and good bonding at the CVD TiNrCVD Al and SiO 2rPVD Al interfaces. There are not many places to move atoms and consequently a great deal of energy is required to add layers of atoms to grain boundaries or other interfaces. As long as this holds, the critical product of 40 000 Arcm describes the strength of the Dual Damascene metallization system. Resistance saturation in 0.18-m-wide Damascene samples, as seen in Fig. 4, can be explained by combining the above model with the melting of Al material. Aluminum melts due to excessive Joule heating and rapidly solidifies in certain areas, resulting in islands of solidified Al between voided regions Žsee Fig. 11a.. The melting of Al continues until a region is encountered in which the Joule heating is significantly reduced, such as a location along the line where the Ti liner is thicker. Additional voiding is prevented downstream from the last voided region because the jL product becomes comparable to the critical product of 40 000 Arcm. Here, L is the distance from the last voided region to the anode end of the structure. For example, if L s 160 m Žfor a voided region 40 m from the cathode end.
and j s 2.5 MArcm2 , then the jL product is 40 000 Arcm. The current density dependence of the 0.35-m-wide structure Žsee Fig. 5 and Table 1. indicates that n is close to 2 for Damascene lines and close to 1 for RIE lines. This suggests that different mechanisms are responsible for void formation in the two cases. In the past, a current density exponent of n s 2 has been attributed to electromigration lifetimes limited by void nucleation w21x while a value of n s 1 has been associated with lifetimes controlled by void growth w22x. More recently, however, n has been shown to vary from 2 to 1 as the lifetime progresses from an incubation period to a steady-state period in the case of two-level, multiple-grained AlCu structures w17x. The migration of Cu controls the incubation period and results in a value of n f 2 for failure times close to the incubation time w17x. Following this incubation period Ždefined as the time required for Cu to migrate beyond a critical distance from the cathode end. and a slow motion transient stage, a steady-state regime is obtained in which the drift velocity becomes constant and the electromigration-induced void grows at a steady rate. A value of n f 1 is obtained for failure times much greater than the incubation time w17x. The different values of n found for Damascene and RIE can be understood if the previous arguments are assumed to be valid for the case of two-level, bamboo Žor near bamboo. grained AlCu structures. A value of n f 1 that is obtained for the 0.35-m-wide RIE lines is consistent with a void growth failure mechanism. The fact that the RIE lines exhibit a short resistance incubation period followed by a gradual increase in resistance is also in agreement with lifetimes controlled by void growth, and further suggests that Cu migration is less significant in these samples. On the other hand, a value of n f 2 that is obtained for the 0.35-m-wide Damascene lines would seem to indicate that the lifetime of these samples is limited by Cu migration. Furthermore, the absence of a fast diffusion path along extended segments of the Damascene lines produces a long resistance incubation period by reducing the Cu migration rate. Once Cu has migrated away, the time until the resistance starts to increase is relatively short since melting of Al begins as soon as a void becomes large enough to cause significant current to flow through the redundant TirTiN layer. The assumption that Cu plays an important role in bamboo or near bamboo grained AlCu microstructures is supported by a recent study of the effect of Cu concentration on the electromigration lifetime. Domenicucci et al. found that the electromigration lifetime of near bamboo grained, two-level AlCu structures increases 2 = as the Cu concentration increases from 0.5 to 1.0 wt.% w23x.
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A larger value of n for Dual Damascene samples means that the relative lifetimes of these lines compared to metal RIE will improve as the current density decreases. Therefore, the ratio of the Damascene t50 to RIE t50 may be much higher at chip operating conditions Ži.e. low current densities. than indicated by Fig. 7. This most likely explains why a lifetime ratio of 3 is found at 2.0 MArcm2 in terms of a q20% resistance increase Žsee Fig. 7., while a ratio of 9 was previously reported at 0.9 MArcm2 for the same metallization systems and failure criterion w2,3x. Finally, it is interesting to note that Dual Damascene samples with a random Al grain orientation exhibit a longer electromigration lifetime compared to RIE samples with a strong Al Ž111. texture Žsee Fig. 10a,b.. In RIE lines, the metal is deposited at a low temperature, patterned and then annealed. The Al grains can orient during this subsequent anneal and the surface normal texture tends to improve. In contrast for Damascene lines, the metal is deposited at a high temperature into trenches where there is substantial growth on both the sidewalls and the bottom of the trench. The surface normal texture is poor, but the texture along the sidewalls is strong. Therefore, the film is textured in a manner with Al Ž111. grains present along all of the internal interfaces. This, on the one hand, gives rise to a weak surface normal texture, but, on the other hand, leads to a more perfect line by yielding fewer diffusion paths in the form of large angle grain boundaries. Combined with large grain size and smooth interfaces, the Damascene samples yield long lifetimes.
4.
5.
6.
7.
8.
5. Summary and conclusions The electromigration and microstructural characteristics of AlCu Dual Damascene lines have been compared to that of AlCu RIE lines. Two-level test structures of 0.18-, 0.35- and 1.33-m-wide lines have been tested at 250⬚C. The summary and conclusions of this study are as follows: 1. The 0.18-m-wide Dual Damascene lines, which are terminated by W diffusion barriers at both the cathode and anode ends, do not exhibit resistance increases at current densities - 2.0 MArcm2 . The Ž jL.c product above which the resistance increases is nearly 40 000 Arcm. 2. Dual Damascene samples exhibit a long resistance incubation period followed by an abrupt increase in resistance, while the RIE samples show a short resistance incubation period followed by a gradual increase in resistance. 3. Dual Damascene lines show a superior electromigration performance at low levels of resistance change, but the relative performance decreases as the level of resistance shift increases. In terms of
9.
10.
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a q20% resistance shift, 0.35- and 1.33-m-wide Damascene lines show a 3 = longer lifetime than metal RIE lines at 2.0 MArcm2 . The current density exponent is close to 2 for 0.35-m-wide Dual Damascene lines and close to 1 for 0.35-m-wide RIE lines. This suggests that the Damascene lifetimes are limited by Cu migration while the RIE lifetimes are controlled by void growth. The different values of n implies that the relative electromigration performance of Damascene compared to RIE will increase as the current density decreases. Intermetallic TiAl 3 formation is much less prevalent in Damascene than in metal RIE lines, but is not continuous in either case. The average grain size is approximately three times larger for Damascene lines compared to RIE lines due to a higher AlCu deposition temperature. The superior Dual Damascene electromigration lifetime at low levels of resistance shift can be attributed to Ža. large grain size; Žb. smooth, clean and well-bonded top interface; Žc. good bonding between CVD TiN and CVD Al; and Žd. small tensile stress due to a reduction in TiAl 3 formation. Each of Ža., Žb. and Žc. contributes to the absence of a fast diffusion path along extended segments of the Damascene conductor Žsuch as an etched Al᎐SiO 2 interface .. The abrupt change in resistance following the resistance incubation period for Damascene lines occurs because the Joule heating of PVD Ti and CVD TiN layers reaches a critical level and causes adjacent regions of Al to melt. Resistance saturation effects are observed in 0.18m-wide Dual Damascene lines at high current densities Ž2.5 MArcm2 . and for long stripe lengths Ž200 m.. Electron backscatter diffraction measurements reveal a random Al grain orientation in sub-micron Dual Damascene lines and a strong Al Ž111. texture in metal RIE lines. The random Al grain orientation in Damascene lines is caused by the presence of Al Ž111. planes along all of the internal trench interfaces. This may lead to a more perfect line in the Damascene case by yielding fewer diffusion paths in the form of large angle grain boundaries.
Acknowledgements The authors would like to thank R. Raviart and W. Malkus for sample preparation, R. Daddazio for sample preparation and SEM work, P.C. Wang and R. Rosenberg for reviewing the manuscript and the Ad-
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