Electron Backscatter Diffraction Characterization of Inlaid Cu Lines for Interconnect Applications

$Electron Backscatter Diffraction Characterization of Inlaid Cu Lines for Interconnect Applications$

SCANNING VOL. 25, 309–315 (2003) Received: June 20, 2003 © FAMS, Inc. Accepted: September 9, 2003

D. P. FIELD, T. MUPPIDI, J. E. SANCHEZ, JR.* School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington; *Unity Semiconductor, Sunnyvale, California, USA

Summary: Automated electron backscatter diffraction Introduction (EBSD) techniques have been used to characterize the microstructures of thin films for the past decade or so. The re- Aluminum metallization has been widely used for inte- cent change in strategy from an aluminum-based inter- grated circuit (IC) interconnects over the last 40 years, connect structure in integrated circuits to one based on and a large knowledge base has been established on how copper has necessitated the development of new fabrica- to control the microstructure to expand manufacturing tion procedures. Along with new processes, complete char- yield and reliability performance limits. It is well known acterization of the microstructures is imperative for im- that an aluminum alloy containing a small fraction of cop- proving manufacturability of the Cu interconnect lines and per and a microstructure consisting of a uniform bamboo in-service reliability. Electron backscatter diffraction has grain structure and strong {111} texture is best suited for been adopted as an important characterization tool in this withstanding electromigration under high current densities. effort. Cu microstructures vary dramatically as a function It is also generally known that certain adhesion layers pro- of processing conditions, including electroplating bath vide growth conditions for fine grained films that allow chemistry, sublayer material, stacking sequence of sub- smooth sidewall formation during subsequent metal etch. layers, annealing conditions, and line widths and depths. The shift in strategy from Al to Cu interconnects, and from Crystallographic textures and grain size and grain bound- conventional metal etching to a damascene, or in-laid ary character distributions, all of which may influence metal, fabrication technique presents new challenges to un- manufacturability and reliability of interconnect lines, are derstand how manufacturing conditions control the mi- ideally characterized using EBSD. The present discussion crostructure of copper interconnect features. The final mi- presents some results showing structural dependence upon crostructure of inlaid copper interconnects forms within the processing parameters. In addition, the authors identify an confined space of the prepatterned dielectric, rather than in-plane orientation preference in inlaid Cu lines {111} nor- within a two-dimensional thin film. Epitaxial substrate ef- mal to the line surface and <110> aligned with the line di- fects from the trench bottom and sidewalls influence the de- rection. This relationship tends to strengthen as the line veloping microstructure depending on the interface en- width decreases. ergy with the barrier or adhesion layer, the feature width and depth, and the thermally induced stress conditions in Key words: thin films, copper, electron backscatter dif- subsequent processing. Electron backscatter diffraction fraction, texture has been used in a number of investigations of thin Cu films and lines to determine grain size and crystallographic tex- PACS: 85.40.Ls, 81.05.Bx, 68.37.Hk, 68.37.-d, 61.14.-x ture. The present discussion summarizes some of the information gleaned using EBSD about structural evolution in Cu films and lines. Also, the texture and grain size in single-level Cu lines as a function of line width are presented and compared with the structures previously observed in This work was supported in part by the Foundation for Advances in Al lines processed by the more conventional reactive ion Medicine and Science (FAMS, Inc.). etching (RIE) technique.

Address for reprints: Materials and Methods David Field 201 Sloan Hall Box 642920 All EBSD measurements and analysis were performed Washington State University using a TexSEM Laboratories (Draper, Ut., USA) orienta- Pullman, WA 99164-2920, USA tion imaging microscopy (OIM 3.0) system attached to ei- e-mail: [email protected] ther a FEI XL-30 FE-SEM or an FEI XL-40 FE-SEM (FEI 310 Scanning Vol. 25, 6 (2003)

Company, Hillsboro, Ore., USA). It was previously demon- the trenches were completely filled in all cases, with no strated that the lateral spatial resolution of EBSD meas- egregious voids near the trench floors. urements using the column for this scanning electron mi- Grain size analysis was performed by orientation imag- croscope (SEM) is on the order of 10–20 nm depending on ing for all specimens. In the orientation imaging study, a the material being analyzed (Field 1999, Humphreys et al. grain is defined as a set of contiguous measurements 1999, Nowell and Field 1998, Troost 1993) and is thus ad- bounded by a region of misorientation >2° from point to equate for analysis of Cu thin film microstructures. point over the area scanned. Electromigration is influenced Structure evolution in inlaid Cu lines is generally ac- by grain size because the grain boundaries offer a path of cepted to be relatively independent of that observed in the rapid diffusion to accommodate migrating atoms, in com- Cu overburden film (Besser et al. 2001). Because of this, parison with the bulk lattice. Coherent twin boundaries do interrogation of line microstructure requires either cross not offer a path of rapid diffusion similar to that expected sectioning to reveal the through-thickness structure, or from a random, high angle boundary and should therefore plan-view analysis after over polishing to remove the rem- not be included in grain size determinations for this pur- nant structure from the overburden. Electron backscatter pose. For those 0.3 µm wide lines shown in Figure 1, the diffraction analysis of the Cu lines discussed in the pres- average length of grain along the lines were 0.98, 0.26, and ent work was performed on plan-view sections after a 0.16 µm when twin boundaries were not included as grain slight over polish during the chemical-mechanical pla- boundaries, for the A, B, and C processes, respectively. narization (CMP) process. The observed structures possess Crystallographic textures were measured from all lines a distinct character that is a function of line width, indi- using the data obtained during orientation imaging. The cating that the overburden microstructure did not signifi- present study included four to six scans of the 0.3 µm lines cantly influence the observed structures. that were 15–30 µm in length, giving between 60 and 200 grains for each condition. To obtain a statistically reliable measure of such textures from individual measurements Results would require on the order of 5,000 individual crystallite lattice orientations (Engler et al. 1999). Obviously, the Previous investigations into the microstructure and tex- statistics of the determination are poor for such weakly texture of Cu films and lines by EBSD showed a strong de- tured films, but the results point to important conclusions. pendence of texture and grain size distribution on plating Figure 2a–c contains {111} pole figures showing the in-line bath chemistry and line height to width ratio (Besser et al. Cu textures from the A, B, and C bath chemistries, re- 2001, Field et al. 2001, Vanasupa et al. 1999). Copper spectively. The directions identified on the pole figures are lines were manufactured by the damascene technique in

SiO2 using a 30 nm thick physical vapor deposited (PVD) aN barrier layer and a PVD copper seed layer. Copper films were deposited with three different electroplating chemistries (commercially available solutions, herein de- noted A, B, and C) to form inlaid Cu line structures with 0.3 µm width and 0.5 µm height. All samples were allowed to self anneal at room temperature for 5 days before CMP. The resulting textures and grain sizes in the lines were rad- ically distinct from one another. Figure 1 contains representative orientation images from self-annealed 0.3 µm lines for each of the deposition conditions investigated. The images were taken from specimens manufactured using plating processes A, B, and C from left to right across Figure 1. These orientation images are shaded with crystallite lattice poles aligned with the specimen normal orientation according to the key presented in the unit triangle as part of Figure 1. (All orientation images shown in this paper follow this convention.) It is apparent that individual grains from the structure obtained using process A span the entire width of the trench, while those in the process C structure generally span only half the width, leaving a grain boundary seam down the center of the line. The majority of the grains in the process B line span the width of the trench, but a significant fraction of those observed do not. Scanning electron mi- FIG. 1 Test structures for 0.3 µm wide A, B, and C bath chemistries croscopy cross-sectional analysis of these lines shows that from left to right. The orientation color key is also shown. D. P. Field et al.: EBSD characterization of Cu lines 311

SW for trench sidewall and LD indicating the line direc- our present study. The wafers were annealed at 400ºC and tion. The intensity scale of the pole figures shows a max- the overburden was removed by chemical mechanical pol- imum (black) at five times random. Processes A and B re- ishing (CMP) to reveal the Cu line structure. The cross-sec- sult in very weak textures. Process A shows no identifiable tional SEM images indicated no voids or observable defects trend in texture, while process B contains a small fraction in the lines along the thickness. of {111} planes aligned with the trench sidewalls. Process The lines from these different sections were analyzed C results in a texture of near six times random, with the pre- using orientation imaging to determine the texture and ferred orientation being the {111} plane aligned with the grain size. The analytical conditions were 25 keV acceler- trench sidewalls. ating voltage using a step size of 50 nm. The sections One potentially important piece of information obtained scanned were 20 × 15 (µm)2 to obtain about 100,000 data from spatially specific orientation measurements is that of points for each region. These data were used to calculate misorientations between neighboring crystallites. Specif- the grain size and the pole figures to determine the mi- ically, the fraction of twin boundaries in a Cu line may be crostructure and preferred orientation of the grain structure of some importance in characterizing the microstructure for in the lines. interconnect applications. All specimens analyzed contain The orientation maps for some the Cu lines used in the a relatively high fraction of Σ3 boundaries as determined experiment are shown in Figure 3, along with the orienta- by misorientation. These are the annealing twin boundaries tion color key. The results of the average grain diameter, prevalent throughout all structures and typical of face-cen- excluding and including twins as separate grains, and the tered cubic (FCC) metals with low-stacking fault ener- twin boundary fraction for the various line widths is shown gies. The fraction of grain boundaries classified as twin in Table I. The maps and the grain size values clearly show boundaries by orientation imaging for these structures are a distinct difference between the wider and the narrower 0.85, 0.49, and 0.27 for the self-anneal processes after lines. The narrower lines, that is, those <1 µm wide, were deposition using A, B, and C bath chemistries, respec- completely bamboo structured with the grains spanning tively. through the line width, while the lines with the larger In a separate experiment, Cu lines were prepared from widths had a multicrystalline nature. The average grain size damascene trenches in TEOS- based oxide consisting of a (diameter), as is expected in this case, increases with in- number of 180 × 180 (µm)2 sections, each with a different creasing line width. The average grain diameter value in the line width ranging from 0.20 to 10 µm and 0.5 µm deep. wider lines was in the order of the grain sizes for the blan- The metallization process consisted of 300 Å PVD Ta bar- ket films with a similar thickness. The constraint imposed rier layer + 800 Å PVD Cu seed layer and electroplated Cu by the sidewall on the grain growth is likely the reason for fill. The spacing between the lines was not the same in all the decrease in the average grain sizes in the narrow lines. sections and varied with the width of the lines and the sec- Crystallographic textures were calculated from the ori- tion in use; however, this is assumed to have no effect in entation measurements using a discrete binning procedure, as described by Matthies and Vinel (1999) with a bin size of 5º. The {111} and {110} pole figures were calculated 111 111 and compared for the different line widths. Figure 4 contains representative {111} and {110} pole figures for lines of 0.20 µm and 2.8 µm width. For all line widths, the grains predominantly had a {111} out of plane orientation, SW SW as generally observed in electroplated blanket films. The {111} out of plane component is explained by the surface energy minimization criteria in FCC blanket films. In lines, if the plating of the Cu takes place from the bottom of the LD LD 111 (a) (b) TABLE I Grain size as a function of width for analyses including twins as distinct grains and not including twins Line width Average grain dia (µm), Average grain dia (µm), (um) with twins without twins 0.20 0.21 0.20 0.26 0.27 0.24 0.36 0.36 0.32 LD 0.55 0.46 0.40 (c) 0.70 0.56 0.48 0.99 0.59 0.49 FIG. 2 (111) pole figures from the 0.3 µm lines shown in Figure 1 2.80 0.65 0.55 for room temperature processes A, B, and C. The gray scale shading 3.64 0.72 0.58 (white to black) indicates 0–5 times random for all plots. LD=line di- 5.04 0.64 0.55 rection, SW=side wall. 312 Scanning Vol. 25, 6 (2003)

Boundary levels: 15° Boundary levels: 15° Boundary levels: 15° 4.5 µm = 90 steps IPF [001] 4.5 µm = 90 steps IPF [001] 4.5 µm = 90 steps IPF [001] 0.20 µm 0.36 µm 0.99 µm

111

001 101

Boundary levels: 15° Boundary levels: 15° Orientation color key 4.5 µm = 90 steps IPF [001] 4.5 µm = 90 steps IPF [001] 3.64 µm 5.04 µm

FIG. 3 Orientation maps of the copper lines obtained using orientation imaging microscopy. The color code indicates the orientation normal to the surface. trench and not on the sidewall, then the argument of surface and interface energy is still valid giving a (111) out of plane texture in the grains. The {111} orientation intensi- 111 101 ties varied with the line width, showing that the side wall and stress differences become more of a factor with the decreasing width. A plot showing the maximum {111} intensity as a function of line width is shown in Figure 5. The SW chart shows a U-shaped curve with a minimum obtained around 1 µm. The {110} pole figure of the 0.2 µm lines shows a nonuniform in-plane intensity distribution and a maximum LD LD {110} orientation along the line length. Figure 6 contains 0.20 µm lines inverse pole figures giving the pole orientation intensity 111 101 aligned with the line directions. These texture plots show a {110} maximum along the line length, showing the preferred in-plane texture component. These texture values increased with the decreasing line width, and the highest SW value of 4× random intensity was observed in the case of max=16.20 the narrowest lines, 0.2 µm wide. This result confirms the 8.600 increasing effect of the sidewall on texture evolution as line 5.592 3.637 width decreases and height to width aspect ratio increases. LD LD 2.365 2.80 µm lines 1.538 1.000 Discussion 0.650

FIG. 4 (111) and (110) pole figures for the 0.20 and 2.8 µm wide Orientation dependence in thin films is controlled by lines showing (111) out of plane and, for the narrow line, (110) in- processing conditions including sublayer material, thick- plane textures. LD= line direction, SW= side wall. ness, and stacking sequence. Preferred orientation devel- D. P. Field et al.: EBSD characterization of Cu lines 313 oping during deposition, recrystallization, and grain growth texture development during grain growth since the grain is generally considered to be either surface energy domi- boundary area will tend to minimize, creating a bamboo- nated for thin films or strain energy dominated for thicker type grain morphology. Sanchez and Besser (1998) and films (Thompson and Carel 1995). When line microstruc- Hau-Riege and Thompson (1999) discussed structure evo- tures are considered, additional energetic and physical lution and the energetics associated with grain growth for constraints control texture evolution within the line. The in- free-standing and inlaid line microstructures. Two line laid Cu line is surrounded on three sides by a barrier layer width-dependent phenomena were observed in the tex- material. The constraint associated with energy minimiza- tures of the Cu lines. First, the {111} texture strength de- tion for these interfaces could be more dominant than sur- creased with line width down to about 1 µm, then began face energy considerations for the free surface. In addition, to increase as line width further decreased to 0.2 µm. Sec- grain boundary energy minimization could play a role in ond, an in-plane orientation preference was observed for the most narrow lines. 30 Decreasing {111} texture strength as the line width narrows can be attributed to the increasing influence of the 25 sidewalls. For wide lines, the metal is essentially in a state of biaxial stress, and the free-surface and bottom interface 20 energies of the lines dominate grain growth and texture de- 15 velopment. The low {111} surface energy dominates growth and results in a preferred {111} fiber texture with 10 little influence from sidewall nucleated growth. As the line width decreases, there is increased likelihood that the side- 5 walls will contribute significantly. When the height-to- 111 Peak intensity (x random) 111 Peak 0 width ratio of the lines becomes >1, the interface energy 0123456associated with the sidewalls could become a dominant fac- Line width (µm) tor in structure evolution and a {111} sidewall texture can develop (Besser et al. 2001, Lingk et al. 1999). There was FIG. 5 Chart showing the change in the (111) peak intensity with varying line width.

111 111 111

001 101 001 101 001 101 0.20 µm 0.26 µm 0.36 µm

111 111 111 max=4.88 4.000 3.031 2.297 1.741 1.320 1.000 0.758

001 101 001 101 001 101 0.55 µm 0.70 µm 0.99 µm

FIG. 6 The inverse pole figure texture plots with line length as the reference direction showing a preferred orientation of (110) along the line length. 314 Scanning Vol. 25, 6 (2003) no evidence of sidewall-oriented textures in any of the minimization would require that the {211} plane aligned lines investigated in this study, indicating again that addi- with the trench sidewalls is the low-energy orientation tional factors influence texture evolution and grain growth plane for Cu grains in contact with the Ta barrier (for in electroplated inlaid Cu lines (such as bath chemistry and planes normal to the {111} surface orientation). Grain sublayer material and thickness). The observed increase in boundary energy minimization would require that the texture strength at the most narrow line widths is also an {110} plane aligned with the grain boundary plane is the indication that sidewall nucleation and grain growth in- low-energy position for [111] tilt boundaries. While inter- fluenced by the sidewall interface energy is not a dominant face energy data are not available, the energy of free sur- mechanism of texture development. faces shows that the {112} plane is the lowest energy plane In addition to potential sidewall nucleation, the stress- normal to the {111} surface orientation (Sundquist 1964). state in the Cu line changes to one of tri-axial stress when Perhaps not coincidentally, a similar preferred structure the height-to-width ratio is near ≥1.0. Because of the phys- was observed in heavily annealed, free-standing, narrow Al ical constraints on three sides of the lines, this effect con- lines. Figure 8 is adapted from Field and Wang (1998), tinues to increase as the line width narrows. Another struc- showing the preferred structure in the Al lines. The narrow tural feature observed from the EBSD imaging of the Cu line Cu microstructures discussed in this paper tend to the lines is the relatively high frequency of twin boundaries in same orientation preference as shown for the narrow Al lines having widths near 1 µm. Figure 7 shows the fraction lines. This comparison suggests that the {211} plane in the of twin boundaries as a function of line width. In this case, {111} oriented Cu grains creates a low energy interface the twins were identified as those having misorientations with the Ta barrier in a manner similar to the Al grains within 2° of the ideal twin misorientation (60º about a where it is presumed that free surface energy minimization <111> axis). It is apparent by comparison of Figures 5 and results in the observed texture. 7 that the high twin fractions exist in line widths where the weaker textures were present. It should be pointed out that this observation was not an anomaly that could be attributed to inhomogeneity of the microstructure. The weaker 0.45 textures and higher fractions of twin boundaries were con- 0.40 µ sistently present in lines of approximately 1 m in width 0.35 as observed over several different and widely separated re- 0.30 gions. When a large number of twin boundaries exists in a microstructure, there is a concomitant weakening of the tex- 0.25 ture. A single grain twinned repeatedly over the various 0.20 twin variants, and including twins of twins, results in a rel- 0.15 atively random distribution of orientation components after 0.10 Twin boundary fraction Twin only three twinning generations (Humphreys and Hatherly 0.05 1995). Therefore, the weaker texture in the regions where 0.00 a large number of twin boundaries exists is not surprising. 0123 456 The growth of annealing twin is dependent on several fac- Line width (µm) tors, including stress in the material, but is not generally FIG. 7 Twin boundary fraction in the Cu lines as a function of line well understood. It is apparent from the data presented width. herein that, for the given processing conditions, twin boundaries are most likely to develop in lines having widths near one micron. Since these lines were side by side with those of other widths, it is reasonable to conclude that the differences in stress state are responsible for the higher fraction of twin boundaries; these boundaries, in turn, are responsible for the weaker textures observed. The observed in-plane orientation preference for the narrow inlaid Cu lines shows that the annealed structure in (111) the line, for the processing conditions used, tends to a {111} surface normal and a [110] direction along the line length. The {111} surface orientation is the low-energy surface in FCC materials and provides evidence of bottom-up growth of the Cu during electroplating. Side-wall growth (112) would result in a preferred orientation having the {111} (110) pole aligned with the trench side wall. The [110] in-plane FIG. 8 Schematic showing the preferred orientation for both free- orientation component must be a result of either interface standing narrow Al lines (Field and Wang 1998) and the narrow Cu or grain boundary energy minimization. Interface energy lines analyzed in this investigation. D. P. Field et al.: EBSD characterization of Cu lines 315

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