Surface roughness reduction for AlCu-based ...

Microelectronic Engineering 50 (2000) 311–319 www.elsevier.nl / locate / mee

Surface roughness reduction for AlCu-based metallization: implications from an integrative point of view S. Spinler*, S. Schmidbauer, J. Klotzsche ¨ ¨ Str. 180, D-01099 Dresden, Germany Siemens Microelectronics Center GmbH & Co. OHG, Konigsbrucker Abstract For an AlCu-based metallization utilized in a state-of-the-art production environment we demonstrate that, beyond the more traditional electromigration properties, also surface roughness needs to be considered carefully in order to achieve optimal yields. Through experiments carried out on blanket wafers, mechanisms are proposed which account for a reduced surface roughness of aluminum grown on titanium rather than on a sandwich of titanium and titanium nitride. The evolution of aluminum grain growth and thus roughness is studied for the two different metal stacks. The practical implications of a reduced surface roughness for integration purposes is subsequently shown by considering etch behavior, short yield as well as signals depending on the metal’s reflectivity.  2000 Elsevier Science B.V. All rights reserved. Keywords: Surface roughness; Film growth; Texture; Reflectivity; Electromigration endurance; Metal shorts

1. Introduction The metallization layers of present generation integrated circuits (0.25 mm and below) generally consist of a Ti / TiN (or Ti only, respectively) barrier layer, followed by an AlCu layer forming the actual current path. On top, a TiN layer acts as an anti-reflective coating (ARC) for subsequent lithography. Starting large-scale production of DRAM circuits with a Ti / TiN bottom layer we found that, for AlCu thicknesses equaling or exceeding 1000 nm, the AlCu layer exhibits a substantial surface roughness which in turn leads to an imperfect coverage by the top TiN layer. As a consequence, the developer may creep through the TiN layer causing, through chemical reaction with the AlCu, residues that cannot be etched in the subsequent metal etch step. This kind of remainders eventually leads to metal shorts. To overcome this issue the top TiN layer may be ‘stuffed’ with an ozone flash prior to resist coating which reduces but, however, does not fully eliminate the shorts. Also, it is possible to remove the etch inhibitors by increasing the physical component of the etch process, i.e., enhance sputter power in the breakthrough step. *Corresponding author. 0167-9317 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 99 )00297-X

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A remedy against the root cause though can only be a reduction of the AlCu surface roughness which we achieved by substituting the Ti / TiN sandwich by a single Ti layer. Indeed, a smoother AlCu surface not only suppresses efficiently the formation of ring shaped defects, but also impacts positively signals depending on the metal’s reflectivity, e.g., overlay measurement. In the present study we provide data on roughness evolution as a function of underlayer as investigated on blanket wafers and demonstrate the above-mentioned benefits from reduced surface roughness on fully integrated wafers. Also, reliability data are presented for the two stacks. 2. Experimental All sputter experiments were done on the AMAT Endura HP5500 platform.

2.1. Blanket wafer Blanket film studies were carried out by depositing AlCu (0.5 wt% Cu) films to a variety of thicknesses on two different substrate materials. Thermal oxidized wafers were lamp degassed at around 1808C, preclean sputter etched in argon atmosphere (etch removal of about 50 nm). Subsequently, either 25 nm Ti / 25 nm TiN or 25 nm Ti only films were deposited. On these films sequentially thicker films of AlCu were deposited without air exposure. The base pressure of the aluminum chamber was kept lower than 3 3 10 28 Torr with an electrostatic chuck (MCAE) at a temperature of 3508C. Before starting the aluminum deposition the wafer was chucked and a wafer backside pressure of 6 Torr was applied. Target power was held at 1 kW to get control over very thin films deposited with 2, 4, 8, 12 s, assuming a deposition rate of about 1.77 nm / kW s. Thus, except for the target power for the AlCu deposition, all other parameters were identical to standard settings to get information about film growth behavior which reflects the standard deposition conditions for a metal last application utilized in production. All sequentially deposited films were analyzed by SEM (Hitachi S5000, 20K magnification, tilt angle of 308) to study nucleation / growth characteristics. Fully deposited wafers with Ti / TiN or Ti as underlayers 1 1000 nm AlCu were characterized by SEM, focus ion beam (FIB Hitachi S5000), AFM (10 3 10-mm scan, AFM Vecco D9000, tip radius 30 nm), Reflectivity (UV1050, 49-point measurement, 6-mm edge, excl. at 236 nm wavelength) and XRD (Siemens D500, Rocking curve scan with Cu Ka anode) measurements to get information about the final status of blanket film properties of the different metal stacks directly after sputtering.

2.2. Fully integrated wafer For fully integrated wafers, the metal last layer consisted of a stack of 25 nm Ti (vs. 25 nm Ti / 25 nm TiN) / 1000 nm AlCu / 37.5 nm TiN, deposited on TEOS acting as interlevel dielectric. For lithography, a 1500-nm thick JSR IX920G resist was used, the developer utilized was JSR TMA 248 WA. Metal etch was performed on a LAM TCP 9600 tool. 3. Results and discussion Fig. 1 shows the characteristics of aluminum grain growth from initial nuclei form to larger and larger islands on both Ti and Ti / TiN underlayers. As more material is deposited the islands begin to


Fig. 1. AlCu island / grain growth on Ti vs. Ti / TiN.

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Fig. 2. Calculated island size of AlCu on Ti vs. Ti / TiN.

coalesce. In the case of deposition on Ti the islands are larger and flatter, whereas on Ti / TiN there are more hemispherical islands which are smaller and higher. Figs. 2 and 3 show the difference of island size and island density of aluminum depending on substrate material. In the case of Ti / TiN underlayer, values of more than 100 islands per unit area can be observed, whereas for Ti underneath values of less than 30 islands per unit area can be obtained. These results are in agreement with fractal analysis for Al onto Ti [1]. The final grain distribution of the 1000-nm AlCu film after sputtering under standard conditions is shown in Fig. 1 (last column), which indicates that the grain size is substantially smaller for a Ti underlayer (a few microns versus tens of microns for Ti / TiN underlayer). In addition, the grains tend to be flatter, indicating that a strong Alk111l crystal orientation can be achieved: the rocking curves based on diffraction analysis exhibits a narrow and strong k111l peak when aluminum is deposited onto Ti with FWHM of only 1.678, while the FWHM for AlCu grown on Ti / TiN is 7.098 (see Fig. 4). Indeed, the AlCu ‘inherits’ its texture from the underlayer which is more favorable in case of Ti rather than TiN.

Fig. 3. Calculated island density of AlCu on Ti vs. Ti / TiN.


Fig. 4. Rocking curve of Al k111l for different underlayers.

Fig. 5. AFM scan for 1000 nm AlCu on Ti vs. Ti / TiN.

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Fig. 6. Reflectivity for 1000 nm AlCu on Ti vs. Ti / TiN.

Hence a dramatic improvement in texture can be obtained for an AlCu layer deposited on Ti thus minimizing height differences between adjacent grains, which we demonstrated to be favorable for integration aspects. The preferred orientation of AlCu along k111l can be improved further by using IMPE Ti instead of standard Ti most likely due to an improvement of the Ti texture itself because of the inheritance effect.

Fig. 7. A typical ring-shaped defect observed in SEM due to isotropic Alx O y formation.


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Fig. 8. Metal short yield comparison for three process alternatives concerning underlayer of the AlCu.

The deeper grain boundaries or even collapsed grains in the AlCu film on Ti / TiN underlayers observed on FIB images (see Fig. 1 last column) correlate with AFM measurements. Fig. 5 shows an AFM scan of aluminum with 1000 nm on both Ti and Ti / TiN underlayer indicating increased ˚ whereas for Ti / TiN the value is about 15 roughness. RMS for Ti-based metallization is about 12 A, ˚ i.e., deeper grain boundaries, in the case of a Ti / TiN substrate. Hence pinholes in the TiN ARC A, layer, which is directly sputtered onto the aluminum film, may be caused by the AlCu surface roughness. This observation can be confirmed by reflectivity measurement on the same set of wafers. The result is a narrow distribution for Ti only underlayer at higher absolute values (mean at 0.87) compared to a wider distribution at lower value (mean 0.76) for Ti / TiN underlayer (Fig. 6). The conclusion is that the weakest spots are triple points where three grains with different crystal orientations intersect thus leading to a maximum of height variation. These spots can be assumed to be most prone to the attack from the developer. Where the developer penetrates through the TiN, a compound of Al x O y is formed through chemical reaction of the

Fig. 9. Overlay measurement structure for Ti / TiN (left) and Ti only (right) underlayer.

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Fig. 10. EM data comparing Ti / TiN and Ti only underlayer.

developer / rinse with the AlCu. This compound turns out be etch-resistant. Due to the isotropic nature of the attack, ring shaped defects can be observed post metal etch (see Fig. 7) in the case in which a Ti / TiN layer is used instead of a single Ti layer underneath the aluminum. Since the defect contains conducting material and since it occurs in a large number, i.e., . 20 within an area of a few cm 2 , it is very likely to cause metal shorts. Fig. 8 confirms this observation: the metal short yield can be improved substantially by using a Ti-only stack, hence by achieving a smoother AlCu surface. The short yield was measured on a serpentine structure with metal lines 640 nm wide, the size corresponding to a 640K cell array. Note also that, as indicated in Fig. 8 in the second column, a ‘stuffing’ of the top TiN surface by applying an O 3 flash does improve the situation; however, not as efficiently as reducing the AlCu surface roughness by using Ti only as an underlayer. Fig. 9 highlights the fact that a signal depending on surface roughness, here shown for an overlay measurement structure, can be interpreted much more accurately and reliably on a smooth surface formed by AlCu grown on Ti rather than AlCu grown on Ti / TiN. Electromigration endurance is generally believed to be enhanced by a strong k111l orientation of the AlCu film [2]. The EM data obtained for the two different stacks reflect this hypothesis, as can be seen in Fig. 10. An improved mean time to failure can be achieved with the Ti /AlCu-based metallization. The EM test structure consisted of a 640-nm wide and 400-mm long metal line.

4. Conclusions The deposition of AlCu films onto a thin Ti layer rather than on a Ti / TiN sandwich favorably affects the nucleation and growth of the aluminum film, such that the resulting surface roughness of the aluminum is superior as compared to AlCu grown on a Ti / TiN underlayer. Our experiments on blanket wafers as well as the results from fully integrated wafers highlight that, in particular for thick metallization layers, in addition to traditionally considered EM endurance, the surface properties of the metallization layer are a crucial characteristic.


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From an integrative point of view, a minimum AlCu surface roughness is the key to achieve a dense TiN ARC layer on top of the AlCu layer, which widens the process window for subsequent process steps and improves metal short yields dramatically. Acknowledgements ¨ The authors would like to thank Mrs Prufer for SEM and Mrs Jobst for XRD measurement. References [1] E. Kusano, Y. Kuroda, A. Satoh, M. Kitagawa, H. Nanto, A. Kinbara, Fractal analysis of island structures of Al thin films, Mater. Sci. Forum 287 / 288 (1998) 479–482. [2] T. Sasaki, H. Dohnomae, M. Imafuku, S. Takebayashi, Development of (111) Al texture grown on Ti and TiN underlayer for electromigration lifetime improvement, in: AIP Conference Proceedings, 1998, pp. 224–229.