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TechEmergent

Filling contacts using a pulsed nucleation layer of tungsten nitride

Kai Frohberg, Katja Huy, and Hartmut Ruelke, AMD; and
Kaihan Abidi Ashtiani, Josh Collins, and Juwen Gao, Novellus Systems

As device dimensions shrink to <45 nm, the traditional method of fil-
ling contacts and vias using titanium/titanium nitride (Ti/TiN) liner/barrier films may no longer be extendable. PVD Ti liners create overhangs in the contact opening that can result in keyhole formation during plug fill and coring during chemical-mechanical polishing (CMP), ultimately affecting the contact resistance of very small features. Plasma-assisted MOCVD TiN deposition processes suffer from inhomogeneous film properties and limited step coverage in high-aspect-ratio features. While titanium tetrachloride (TiCl4)–based Ti and TiN processes provide improved step coverage, they do so at deposition temperatures much greater than 400°C, which are incompatible with future contact silicidation processes such as nickel silicide (NiSi).

To overcome these limitations, a method has been developed to directly fill contacts without the use of a traditional Ti/TiN film stack. DirectFill is an integrated process that consists of a contact preclean step, pulsed nucleation layer (PNL) tungsten nitride (WN) barrier deposition, and tungsten fill. A thin PNL WN barrier is deposited using thermal atomic layer deposition (ALD) at a wafer temperature that does not exceed 400°C. Then a PNL layer of tungsten is deposited on the WN barrier, after which the contact or via is filled with tungsten using chemical vapor deposition (CVD). The entire sequence takes place in an integrated fashion. Integrating tungsten nucleation and fill with PNL WN avoids oxidation of the thin (~50Å) WN nucleation layer. Moreover, the temperature of the entire contact fill process does not exceed 400°C, making it compatible with advanced silicides.

The PNL WN film adheres to dielectrics, silicon, and silicides. It also has demonstrated barrier properties for the subsequent tungsten plug fill process. Therefore, the PNL WN film can act as a liner and barrier film, replacing Ti/TiN films and thereby maximizing the contact volume for deposition of tungsten in the contact and reducing contact resistance (Rc). DirectFill’s sputter- and/or reactive-based preclean step is similar to that used in traditional fill applications. In addition, PNL W nucleation is a thermal ALD process with demonstrated extendability to very small and high-aspect-ratio features.1 Successful demonstration of this process flow for contacts to tungsten bit lines has been reported previously.2

This article, which is based on experiments performed at AMD’s Fab 30 in Dresden, Germany, investigates PNL-WN film characteristics. It also demonstrates that for cobalt silicide (CoSi) contacts, which are widely used in logic and flash memory circuitry, the DirectFill method results in much lower resistance than the traditional Ti-TiN-W contact metallization approach.

Experimental Procedure

PNL WN films were deposited using a 200-mm Altus DirectFill system from Novellus Systems (San Jose). The tool is equipped with an argon sputtering and/or reactive preclean module, a PNL WN liner/barrier deposition module, and a PNL tungsten and CVD tungsten module in which both PNL tungsten nucleation and CVD tungsten fill steps are performed simultaneously in a multistation sequential deposition chamber. The preclean module includes a 400-kHz inductively coupled plasma (ICP) source and a biased wafer pedestal that operates at 13.56 MHz. It uses argon, H22, and N2 as sputtering and/or etching gases. The WN module performs multistation sequential deposition using five deposition
stations. The module’s process temperature is fixed at 300°C, and it operates at 3 Torr.

The contact preclean step and initial contact metal deposition are the keys to successful contact metallization and electrical performance. As shown in Table I, this study considered four different precleans. Preclean characterization was performed on blanket wafers. The film removal amount measured on the blanket wafers was then used to determine how much film should be removed on patterned wafers.

Table I: PNL WN/tungsten experimental procedure.

In addition to a preclean step, an optional tungsten-first (tungsten flash-layer) deposition step was performed, in which a 10-Å layer of PNL tungsten metal was deposited before PNL WN deposition. A tungsten flash layer was evaluated because it can improve the interface between the WN stack and the CoSi film, improving electrical performance. In addition, it can help produce a more stoichiometric WN film at the interface.

The PNL WN deposition process is a thermal ALD process that uses B2H6, WF6, and NH3 as reagent gases. To drive the surface-saturated thermal ALD process, short doses of the gases are introduced sequentially into each deposition station in rapid succession and then purged. The deposition rate of WN film was ~1.0–1.4 Å/cycle.

PNL tungsten nucleation and CVD tungsten plug fill were performed in a standard Altus PNL module. The first two stations in the module deposit an ALD-like tungsten nucleation layer at 300°C using SiH4 and WF6 as reagent gases. Argon and H2 are used as background gases. In the module’s remaining stations, low-temperature CVD tungsten deposition is performed simultaneously to complete the tungsten plug fill process using H2 and WF6 as reagent gases.

Experimental Results

WN Film Properties. The adhesion properties of WN films were tested using a scribed-tape test with a 50-Å WN film stack followed by a 2500-Å tungsten film. After a dry adhesion test was performed, the films were soaked in water and tape-tested again in the same scribed area. The WN film adhered excellently to blanket dielectric and CoSi films. It also adhered to the films in the electrical test structures that were used in this work. In previous four-point bend studies of WN adhesion to undoped silicate glass (USG) samples, an adhesion energy of >10 J/m2 was recorded, indicating the film’s excellent adhesion to the dielectric film.

Glancing-angle x-ray diffraction (XRD) measurements show that PNL WN has a b-W2N microstructure, as illustrated in Figure 1. Both the film’s degree of crystallinity and preferred orientation can be modulated significantly by changing the WN deposition recipe. For example, the use of a higher-pressure recipe or changes in NH3 dosage can modulate the preferred orientation and stoichiometry of the WN film, as will be discussed in future papers. The WN used in this study had a polycrystalline structure without a strong preferred orientation. Rutherford backscattering spectroscopy analysis indicates that the atomic ratio of the WN film is roughly 1.3:1 (W:N), indicating that excess nitrogen is located in the grain boundaries of the WN film.

Figure: 1: Glancing-angle XRD analysis of a WN film indicates that W2N(111) is the preferred orientation, while other peaks of W2N are present.

Figure 2 presents a focused-ion-beam scanning electron microscopy (FIB-SEM) image of tungsten-filled contacts that were produced using the fill scheme described in this article. The contact fill area was free of keyhole formations, indicating that the contacts would not form exposed seams after CMP processing. The seam was also very smooth. A WN film stack on blanket wafers with a USG substrate had a root mean square value of film roughness, as measured by atomic force microscopy, of <8% of the total film thickness. The absence of keyholes in the contacts partially resulted from the elimination of physical vapor deposition (PVD) titanium and its associated overhang. The smoothness of the seam was attributed to the ALD nature of the PNL WN and tungsten films, creating an optimum template for the growth of CVD tungsten in the contacts.

Figure: 2: A FIB-SEM image of tungsten contacts shows good fill, good post-CMP performance, and a very smooth seam.

Electrical Test Results

To test the metallization of contacts filled with WN and tungsten films, 90-nm logic contacts on CoSi films on AMD’s Athlon microprocessor were evaluated. The electrical characteristics of this test vehicle are stable and well understood.

Electrical test results are shown in Figures 3–6. Control wafers (indicated by Ti/TiN in the legends) were processed using the conventional approach in which Ti/TiN layers are deposited as liner and barrier films. In this case, the titanium layer was deposited using ionized PVD, after which TiN was deposited using metal-organic CVD. In Figure 3, which shows contact chain resistance in negative (N)-active wafer regions, the PNL WN/W process resulted in better contact Rc than the Ti/TiN process when either a 100-Å argon sputter preclean or a 100-Å Ar-H2 preclean preceded WN deposition. However, both types of Ar-H2-N2 precleans increased contact Rc significantly, which may indicate that these preclean methods led to the formation of a nitrided interface layer on the CoSi contact.

Figure: 3: Contact chain resistance results from the N-active wafer region.

In contrast to the performance of the standard PNL WN/W process, the DirectFill process in conjunction with the tungsten flash-deposition scheme increased contact Rc and resulted in poor yields, although the opposite result had been expected. Failure analysis of the WN/CoSi interface in the affected contacts using transmission electron microscopy and electron-energy-loss spectroscopy is required to understand this result.

Kelvin contact resistance to CoSi films on both N- and positive (P)-active silicon regions (Figures 4 and 5, respectively) was similar to contact chain resistance. The argon and Ar-H2 sputter preclean processes produced the best resistance values, while the two Ar-H2-N2 precleans produced significant increases in Rc and the tungsten-flash/WN process resulted in poor yields.

Figure: 4: Kelvin contact resistance results from the N-active wafer region.
Figure: 5: Kelvin contact resistance results from the P-active wafer region.

In order to study the extendability of the PNL WN/W process, undersized and misaligned contacts were utilized to determine contact chain resistance. Undersized contacts (with 20%-smaller critical dimensions than those used in 90-nm technology) and intentionally misaligned contacts had Rc and yield results that were similar to those of normal-sized and properly aligned contacts. As shown in Figure 6, good Rc results and high yields were obtained from misaligned contacts fabricated using the PNL WN/W process preceded by an argon or Ar-H2 preclean. Because of the conformal nature of PNL WN and PNL W in addition to the reduced number of process steps in the DirectFill approach, the results achieved with the current contact generation are expected to be maintained or improved as contacts are scaled down.

Figure: 6: Contact chain resistance of misaligned contacts on the N-active wafer region.

Conclusion

Based on the experiments discussed in this article, the integrated PNL WN/W approach for filling contacts is a promising alternative to the traditional Ti-TiN-W scheme. The results of this work indicate the importance of performing a preclean step to modulate the measured Rc and thus obtain good electrical contact behavior. The argon and Ar-H2 precleans in conjunction with the PNL WN/W process resulted in lower contact Rc than the Ti-TiN baseline process. This result would appear to be a consequence of the reduced number of interfaces in the WN/W contact as well as the thin and conformal nature of the ALD WN barrier, which allows the contact to have a wider opening for tungsten deposition. These characteristics of the PNL WN/W technique should prove beneficial for advanced devices with smaller geometries.

While the tungsten-flash integration scheme for depositing a 10-Å tungsten layer before WN deposition did not produce favorable results, it may require additional consideration in future work. Analysis of this scheme is required to understand the failure mechanisms involved and to undertake corrective action.

Acknowledgments

The authors wish to thank Karl Levy and S. H. Lee from Novellus Systems for their contributions to this article.

References

1. SH Lee et al., “Pulsed Nucleation for Ultra-High Aspect Ratio Tungsten Plugfill,” in Proceedings of the Advanced Metallization Conference ULSI XVII (Warrendale, PA: Materials Research Society, 2002), 649–654.

2. SW Lee et al., “Pulsed Deposition of Tungsten Nitride and Its Application to Direct Fill of Tungsten Vias,” in Proceedings of the Advanced Metallization Conference ULSI XVIII (Warrendale, PA: Materials Research Society, 2004).


Kai Frohberg is a senior process integration engineer at AMD’s Fab36 in Dresden, Germany. His responsibilities include research, development, and implementation of new processes in the contact module for the 90- and 65-nm nodes. Before joining the company in 2000, he was a research associate at the Fraunhofer Institute for Microelectronic Circuits and Systems in Dresden and worked on a fully CMOS-compatible process technology for micromirror-array spatial light modulators. He received an MS in physics from the Technical University of Dresden in 1996. (Frohberg can be reached at kai.frohberg@amd.com.)

Katja Huy is a senior process engineer in the thin-films group at AMD’s Fab30 in Dresden, where she works on advanced patterning films, shallow-trench isolation, and spacer and contact materials. She has been with the company since 1997. She received an MS in electrical/automation engineering from the Technical University of Chemnitz, Germany, in 1996. (Huy can be reached at katja.huy@amd.com.)

Hartmut Ruelke works at AMD’s Fab36. An AMD fellow, he also leads the CVD and PVD projects in the thin-film module at AMD’s 300-mm fab in Dresden. From 1996 to 2004, he was the lead CVD engineer on the company’s 200-mm fabrication line, where he was responsible for PECVD, HDP, and tungsten CVD processes. Before joining AMD in 1996, Ruelke worked for System Microelectronic Innovation as a senior process engineer for CVD and epitaxy processes. From 1991 to 1992, he headed the group doping processes/CVD area in the technology department of Halbleiter Elektronik. He received an MS in physics in 1979 from the Technical University of Magdeburg, Germany, and completed postgraduate studies in the area of semiconductor technology in 1982 at the Technical University of Karl-Marx-Stadt, Germany. (Ruelke can be reached at hartmut.ruelke@amd.com.)

Kaihan Abidi Ashtiani, PhD, is a director of technology at Novellus Systems (San Jose). From 1997 to 2000, he led development efforts in the design of hollow-cathode magnetron sources for Ti(N), Ta(N), and copper seed deposition processes. Since then, he has worked on tungsten and WN ALD processes. Before joining Novellus, Ashtiani was a project leader for the development of advanced preclean and ionized-PVD hardware and processes at Materials Research Corp. He received a PhD in plasma physics from the University of Wisconsin–Madison. (Ashtiani can be reached at 408/570-2530 or kaihan.ashtiani@novellus.com.)

Josh Collins, PhD, is the new product development manager for the direct metals business unit at Novellus Systems. During his 10 years with the company, he has led process development efforts for CVD-TiN, PNL tungsten, low-resistivity tungsten, and DirectFill WN-tungsten. Before joining the company, Collins was a hardware design engineer at GE Lighting. He received a BS in mechanical engineering from the University of Michigan in Ann Arbor and a PhD in chemical engineering from Yale University in New Haven, CT. (Collins can be reached at 408/922-4874 or josh.collins@novellus.com.)

Juwen Gao, PhD, is a process development engineer in the direct metals business unit of Novellus Systems. During his five years with the company, he has made significant contributions toward the development of PNL (ALD) tungsten and DirectFill WN-W processes. He received a BS in thermal science from the University of Science and Technology of China in Hefei and a PhD in mechanical engineering from Pennsylvania State University. (Gao can be reached at 408/545-3538 or juwen.gao@novellus.com.)


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