Pulsed Nucleation Layer of Tungsten Nitride Barrier Film and its Application in DRAM and Logic Manufacturing

Transcription

1 Pulsed Nucleation Layer of Tungsten Nitride arrier Film and its Application in DRAM and Logic Manufacturing Kaihan Ashtiani, Josh Collins, Juwen Gao, Xinye Liu, Karl Levy Novellus Systems, Inc. 4 N. First Street, San Jose, California 95134, USA kaihan.ashtiani@novellus.com Abstract Highly conductive and conformal tungsten nitride (WN) thin films were grown by pulsed nucleation layer (PNL) deposition from diborane ( 2 H 6 ), tungsten hexafluoride (WF 6 ), and ammonia (NH 3 ). The as-deposited WN films were polycrystalline β-w 2 N. The Step coverage was 1% on contacts with 16:1 aspect ratio. It has been shown that WN films have better performance, as liner and barrier films, than TiN/Ti stack on various devices and surfaces. Depositing an ultra thin layer of W prior to PNL WN deposition further improved performance of WN thin films on selected substrates. Effects of different preclean techniques on the performance of WN barriers were also evaluated. It was found that the preclean method was critical to overall performance of PNL WN thin films. All the results showed that PNL WN provided an excellent low cost solution for a wide range of semiconductor applications. Introduction Titanium nitride (TiN) thin films have been used to prevent metal diffusion in semiconductor device fabrication for many years. To ensure good contact properties, a layer of PVD or CVD titanium (Ti) has been needed under the TiN layer as the liner film. As dimensions of semiconductor devices continue to scale down, the Ti/TiN liner/barrier performance in contacts is degraded due to various factors such as I) poor step coverage of MOCVD or PVD based processes, II) overhang due to PVD Ti process, III) inhomogeneous barrier properties of TiN films, IV) high temperature (>4 C) deposition process making it incompatible with advanced nickel silicide (NiSi x ) processes, and ultimately V) high contact resistance (R c ). Pulsed nucleation layer tungsten nitride deposition, as part of the DirectFill TM technology, was developed to overcome those limitations. DirectFill TM is an integrated process consisting of a contact preclean, PNL WN barrier deposition, followed by PNL/CVD W fill. This approach has many advantages over the conventional W deposition scheme using a TiN/Ti stack as barrier/liner. Firstly, PNL is capable of depositing WN on ultra high aspect ratio (UHAR) features with very good step coverage, which is far beyond the capability of the current TiN/Ti technology in many applications. Secondly, since only a very thin conformal layer of PNL WN is needed, its volume is substantially smaller than that of the TiN/Ti stack, therefore the available volume for W fill is maximized., This in turn reduces the overall contact resistance (R c ). Thirdly, it combines preclean, barrier deposition, and W fill in one tool. There is no vacuum break causing surface oxidization of the barrier/liner layer. In addition, PNL WN replaces the TiN/Ti stack as both the diffusion barrier and the liner due to its good adhesion to dielectrics, silicon, and silicides. Thus, no separate PVD tool is needed and the cost of ownership will be significantly reduced while the fabrication process flow is simplified. Finally, the process temperature of PNL WN, as well as that of PNL/CVD W fill, is below 4 C, which provides excellent compatibility with advanced contact silicidation schemes based on NiSi x. In fact, it has been shown that PNL WN based DirectFill TM technology can provide better electrical performance and higher yields than conventional TiN/Ti based technologies. PNL WN thin films have a wide range of applications as diffusion barriers [1, 2, 3], metal capacitor electrodes in DRAM and embedded DRAMs [4], metal gate electrodes in logic devices [5], and storage nodes in flash memories [6]. PNL WN films have also been tested on a variety of contact surfaces including active Si, poly- Si, W, Al, Cu, NiSi, and CoSi, where they have shown significant improvement in device performance as well as technology extendibility over conventional TiN/Ti based technologies. In this work, we will summarize the characteristics of the PNL WN process, properties of PNL WN films, and their applications in these areas. Experimental 1. PNL WN deposition system WN x Preclean WTS Figure 1. A Novellus Concept 3 ALTUS system. oth WN and W modules are capable of processing 4 wafers simultaneously. W

2 The PNL WN deposition in this work was performed in a Novellus Concept 3, 3mm ALTUS system (Figure 1). It consists of a wafer transfer system (WTS), a preclean module, a PNL WN deposition module, and a W deposition module. The WN module, as well as the W module, is capable of processing 4 wafers simultaneously resulting in a high throughput. 2. Process conditions Preclean: A iased Ar plasma and a biased remote H 2 plasma were used as preclean processes. In some cases, H 2 and/or N 2 were also mixed with Ar in the plasma. The process pressure was 2 mtorr to 1 mtorr. The plasma power was 5 W to 2 W. The wafer bias power was to 15 W. PNL WN deposition: The PNL WN deposition process is a thermal ALD process. 2 H 6, WF 6, and NH 3 were sequentially introduced into the chamber. Each precursor exposure was followed by an Ar purge. The process temperature was 3 to 35 C. The process pressure was 2 to 4 Torr. A PNL WN film was typically 25-5Å thick. PNL /CVD W deposition: The PNL W deposition process is also a thermal ALD process used for nucleation of W on the WN barrier film. This film is typically deposited at C at a thickness of 5Å. The process is based on sequential introduction of SiH4 and WF6 reactive gases into the chamber in the first two stations. The PNL W process is followed by a conventional CVD W process based on the WF 6 -H 2 reaction at 395 C on the last two stations in the W module [7]. 3. Characterization Film thickness was measured by XRF, XRR, TEM, or SEM. Sheet resistance was measured with a 4-point probe. Resistivity was calculated from the measured thickness and sheet resistance. The Microstructure was determined by X- ray diffraction. The Step coverage was measured with TEM or SEM. The electrical properties were evaluated at Novellus or customer sites on blanket wafers as well as device wafers. Results 1. Deposition characteristics Saturation curves: Different amounts of precursors were delivered into the reactor by using different dose times. The film thickness increased gradually as the 2 H 6 dose time increased from.2s to 1s, indicating the non-self-limiting nature of the reaction of 2 H 6 on the surface. A straight line fitting of thickness gave 18 Å/s, or.375 Å/cyc/s. The thickness of WN films did not change significantly with WF 6 dose time, indicating the self-limiting nature of the reaction between the surface and WF 6 (Figure 2a). NH 3 showed a similar non-self-limiting surface reaction as 2 H 6 (Figure 2b), but the change of thickness was much slower, only 3 Å/s, or about 4% of the total thickness. We will come back to this point in the discussion section of the paper H6 WF Dose Time (s) NH Does Time (s) Figure 2. Saturation curves of a) 2 H 6 and WF 6, and b) NH 3, Deposition rate: The thickness of PNL WN film changed linearly with the number of cycles of PNL deposition, showing a stable growth rate of the WN film per cycle of deposition (Figure 3) Number of Cycles Figure 3. Thickness dependence on number of deposition cycles The slope of the line gives the deposition rate of PNL WN deposition, which was 2.1 Å/cycle, roughly about a monolayer per cycle of deposition. The negative intercept of the fitting line at the Thickness axis suggested that there was a nucleation delay (or incubation process) on the surface, which was given by the intercept on the Number of Cycles axis, 1.6

3 which was 2.5. It means that 2.5 cycles of deposition were needed to start linear WN growth. Throughput: The throughput of the PNL WN process was defined as the number of wafers a PNL WN module can process in an hour. To obtain the correct throughput, the overhead time of the PNL process, including wafer transfer time and heating time, was taken into account. The calculated throughput was higher than 3 wafers per hour. 2. Film properties Composition: The composition of PNL WN films varied according to the deposition conditions and analysis methods[i would hope not!]. For example, the W to N ratio varied from 2:1 to.9:1 as the NH 3 dosage and process pressure varied. RS analysis usually showed a lower W:N ratio than XPS or SIMS because Ar sputtering was needed in XPS and SIMS data collection and N is more easily removed than W by Ar. All the analytical techniques showed low impurity (, F, and O) levels. In Figure 4, the and F concentration in the WN film was and atoms/cm 3, respectively. The W and N concentration was and atoms/cm 3, respectively. The atomic percentage of and F was less than.12% and.62%, respectively. The O counts were around 1392 in the film and on the surface, which suggested insignificant O content of the film. Figure 5. X-ray diffraction spectra of the W.9 N film (upper) and W 1.5 N film (lower). Step coverage: SEM images of 7 Å WN film on a 11:1 aspect ratio contact with 11 nm opening showed a 1% step coverage. A CVD W deposition following PNL WN on the same system showed an excellent contact fill (Figure 6). There was no visible void after W fill. A Figure 4. SIMS spectrum of a PNL WN sample Microstructure: Glancing angle XRD spectra of WN films showed that the film was polycrystalline β-w 2 N. It was found that by adjusting the N content in the WN films, the average crystal size of the film was changed (Figure 5). More interestingly, the work function of the WN film was also changed. The N rich WN film, W.9 N, showed a 5.1eV (PMOS) as deposited work function, while the W rich film, W 1.5 N, indicated a 4.3 ev (NMOS) work function. oth films were deposited on a SiON substrate. This provides a great technique in fabricating WN electrodes with the desired work function. Figure 6. SEM images of a) 7 Å PNL WN on a 11:1 aspect ratio via, and b) WN with subsequent CVD W deposition on the same system. To compare conventional TiN/Ti-based W fill and PNL WN-based W fill, 12 plane-view SEM images were taken through the contact depth (Figure 7). TiN/Ti-based W fill showed a large void through the bottom of the via. PNL WN- based W fill had a much smaller seam [2]. We believe that the improved fill capability of the DirectFill TM process is due to I) conformal and homogeneous surface properties of the WN film and II) the elimination of overhang due to PVD Ti in the conventional TiN/Ti process. 1 W 2 N (111) Intensity W.9 N W 2 N (2)

4 resistance and tighter R c distribution than the baseline process using a TiN/Ti stack. In another test, PNL WN was deposited on an AMD Athlon microprocessor test vehicle as diffusion barrier on CoSi film after the wafer was precleaned with different techniques (Figure 9) [3]. It was shown that WN samples precleaned by an Ar plasma ( ) or an Ar+H2 plasma ( ) had much lower contact chain resistance than the TiN/Ti ( ) sample. oth the examples above show the viability of the DirectFill application to DRAM and logic devices for PNL WN, as a single film, replacing the traditional TiN/Ti stack. Figure 7. Plane view SEM images of W filled vias with a) TiN/Ti barrier/liner, and b) PNL WN barrier (SH Kim et al., ref. 2) Resistivity: The resistivity of PNL WN thin films decreased with film thickness (Figure 8). The change became much slower after 75 Å of WN film was deposited. The resistivity was 343 µω-cm when the film thickness was 127 Å. Resistivity (µω-cm) Figure 9. Contact chain resistance of W on WN vs TiN/Ti barriers on N-active wafer region (Frohberg et al, ref. 3 ). Storage nodes in flash memories: Another interesting application of WN is using WN nanocrystals as storage nodes in flash memories [6]. The advantages of using PNL WN nanocrystals include large threshold voltage shift, good retention (> 1 years) and endurance (1 5 cycles) Figure 8. Resistivity of PNL WN films decreased with film thickness. 3. Device performance Diffusion barriers: PNL WN films have found their most common applications as diffusion barriers. In most of the device tests in this work, PNL WN was combined with preclean and W fill on the same tool, i.e., PNL WN was tested as part of the DirectFill TM technology. In a previously Figure 1. WN nanocrystals as storage nodes in flash published paper [2], we discussed the results of the DirectFill memories (SH Lim et al., ref. 6). process as applied to a sub 8 nm DRAM fabrication. In that study, an ultrahigh aspect ratio (~17:1) contact was used. The test showed that WN based technology had lower contact Metal gate electrodes: PNL WN can also be stacked with poly-si as metal gate on PMOSFET devices. The test results showed that the poly-si/wn stack had an improved thermal

5 stability, drive current, and transconductance than the p+ poly-si gate. Poly-Si/PNL WN also showed a 2 orders of magnitude lower off current than poly-si/pvd-wn [5]. We speculate that the improved performance of PNL WN vs. PVD WN on thin gate dielectrics is due to the thermal nature of the PNL process vs. the PVD plasma process. Contact materials: PNL WN has been tested with a wide variety of contact materials such as active Si, poly-si, W, Al, Cu, CoSi, and NiSi, and has resulted in better performance in almost all the cases. One example is WN on NiSi [8]. In Figure 11, WN showed similar or slightly better contact resistance than the baseline process using a TiN/Ti stack. More significantly, the DirectFill process using WN as the liner and barrier showed a tighter distribution than the conventional process. In all cases, the contact leakage to the substrate was similar to the baseline. and adhesion properties of PNL WN films were significantly enhanced. In Figure 12, it is shown that the overall resistivity of the WN/W stack was lower than that of WN with the same thickness. Similar results have been obtained on blanket wafers and device wafers [3, 8]. This layer of W is called flash W due to its extreme thin nature, in the order of 1-2Å. A TEM study on the cross section of WN/Si contact area showed that a very thin layer of SiO 2 exists between WN and Si when only PNL WN was deposited. The thin SiO 2 layer could not be seen if the flash W was deposited prior to the PNL WN deposition. In a later publication we will examine the thermodynamics of the W reaction with various interface oxides to indicate the possibly oxyphillic nature of W, similar to that of a titanium layer. 12 w/o flash W with flash W 1 Resistivity (µω-cm) Figure 12. Resistivity of WN with and without a flash W layer. Discussion Saturation curves of 2 H 6 and NH 3 : To test whether self- Figure 11. PNL WN results equal Rc and tighter decomposition of 2 H 6 caused a non-self-limiting surface distribution than TiN/Ti (S. Smith et al., ref. 8). reaction of 2 H 6 or not, a patterned wafer was exposed to 2% 2 H 6 in Ar at 4 Torr and 3 C for 1 minutes. An About 27 Å film was grown on the wafer, showing a.45 Å/s In Table 1, the leakage and contact resistance of the deposition rate. This deposition rate was higher than that in DirectFill process using PNL WN on various contacts are the PNL WN deposition (.375 Å/s), probably due to a higher summarized. It is clearly shown that PNL WN can potentially 2 H 6 partial pressure in the decomposition test (.8 Torr) than show improved electrical performance on all contacts of in WN deposition (<.3 Torr). Film growth from the 2 H 6 interest used in DRAM, Flash, and logic manufacturing. decomposition on the patterned wafer was highly conformal (Figure 13), suggesting that it would not cause poor step Table 1. Summary of electrical properties of PNL WN coverage in a PNL WN deposition. This explained why PNL with comparison to TiN/Ti stack WN films were highly conformal even though 2 H 6 showed a non-self-limiting surface reaction. In the actual deposition process, the growth of the boron-containing film on the substrate is controlled such that this film plays the role of a sacrificial layer for deposition of WN in the subsequent process steps. It is well known that NH 3 does not decompose appreciably at 3 C, thus the non-self-limiting behavior of NH 3 should be caused by insufficient NH 3 purge time due to strong adsorption of NH 3 molecules on the wafer surface. With a very long purge time (>3s), a decrease of the deposition rate Recent progresses was observed. Since the variation of NH 3 exposure time It was found recently that by depositing an ultra thin layer caused much less variation in the WN film thickness than a of W (1 to 5 Å) prior to the PNL WN deposition, electrical variation of the 2 H 6 exposure time, it was expected that it

6 would contribute much less to a non-ideal step coverage behavior. SiN Cr Conclusions WN thin films deposited by PNL were highly conformal and conductive. They were found to be polycrystalline β- W 2 N. The deposition rate was around 2 Å/cycle, which provided an overall throughput of 3 wafers per hour. In combination with an Ar or H 2 preclean and a PNL/CVD W deposition, PNL WN showed better performance than the conventional TiN/Ti stack in applications such as diffusion barriers and metal gate electrodes. Tests on various surfaces have demonstrated these advantages over the TiN/Ti stack. y depositing an ultra thin layer of flash W prior to the WN deposition, the performance of PNL WN was significantly improved, which made PNL WN even more promising in < 45 nm technologies. Figure 13. SEM image showed excellent step coverage of film grown from 2 H 6 decomposition Composition of WN films: XRD showed a dominating β- W 2 N phase of the WN films while all the compositional analysis showed a lower W:N ratio. The excess N from compositional analysis should be mostly located at grain boundaries so that it was not detectable by XRD. It is also possible that W 2 N crystals were embedded in an amorphous WN x network where x>.5. The N at the grain boundaries could be the N source in annealing after WN deposition. High resolution TEM and degree of crystallinity studies should be helpful in giving us a better understanding of this issue. Flash W layer: Thick W film has been known for its poor adhesion properties on the surface of some very common materials such as Si and SiO 2. Poor adhesion will directly result in higher contact resistance and poor yield. The PNL WN film, in contrast, shows excellent adhesion properties to many substrates. However, if extra N species exist at the grain boundaries of PNL WN films, they could diffuse along the grain boundaries to potentially form a layer of nitride at the interface. If no flash W is deposited prior to PNL WN deposition, the material that contacts WN is Si, so it could be nitrided to form an insulating layer of Si3N4, which will result in a higher contact resistance. However, if a thin layer of flash W (1-2Å) was deposited prior to WN deposition, the formation of this insulating layer is avoided while the adhesion properties of the stack is not compromised, resulting in improved Rc. In addition, during the initial WF 6 exposure, surface SiO 2 may react with WF 6 to form volatile SiF 4. This reaction acts like a preclean, thus improving Rc. However, WF 6 also reacts with Si, which means he Si contact can be easily over etched by WF 6 if too much WF 6 was used in the flash W deposition. This could degrade device performance or cause increased contact leakage to the substrate. Therefore, it takes delicate process tuning to obtain a positive effect of the flash W. Acknowledgments The authors wish to acknowledge many contributions of Dr. SW Lee, Dr. SH Lim (and colleagues) at Samsung Electronics, Co., Dr. SH Kim (and colleagues) at Hynix Electronics, Co., Dr. K. Forhberg (and colleagues) at AMD, Dr. DG Park of IM, Dr. S. Smith and Dr. E. Gerritsen at Crolles II, and our dedicated team at Novellus Systems, Inc, W. Yan, E. Liu, P. Wongsenakhum, D. Pisharoty, and J. Guo. References 1 SW Lee et al., AMC XIX 24, SH Kim et al, AMC 25, 3 K. Frohberg, et al., Micro, October/November 25, 39 4 Unpublished internal data 5 DG Park et al., 24 Symposium on VLSI Technology Digest of Technical Papers, SH Lim et al., 25 Symposium on VLSI Technology Digest of Technical Papers, 19 7 J. Collins et al., SEMI Technology Symposium, SEMICON Korea 23, S. Smith, Advanced Metallization Conference 25