In situ high temperature synchrotron-radiation diffraction studies of silicidation processes in nanoscale Ni layers

In situ high temperature synchrotron-radiation diffraction studies of silicidation processes in nanoscale Ni layers

Microelectronic Engineering 70 (2003) 226–232 www.elsevier.com / locate / mee In situ high temperature synchrotron-radiation diffraction studies of s...

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Microelectronic Engineering 70 (2003) 226–232 www.elsevier.com / locate / mee

In situ high temperature synchrotron-radiation diffraction studies of silicidation processes in nanoscale Ni layers a, a a b a c J. Rinderknecht *, H. Prinz , T. Kammler , N. Schell , E. Zschech , K. Wetzig , d T. Gessner a

Materials Analysis Department, AMD Saxony LLC & Co. KG, Wilschdorfer Landstr. 101, D-01109 Dresden, Germany b Research Center Rossendorf, Project group ESRF-beamline FWE, PF 510119, D-01314 Dresden, Germany c Institute for Solid State and Materials Research, Helmholtzstr. 20, D-01069 Dresden, Germany d Center of Microtechnologies, Chemnitz University of Technology, D-09107 Chemnitz, Germany

Abstract The formation of nickel silicides has been studied by X-ray diffraction experiments using synchrotron radiation (SR). A high temperature chamber was used to investigate the phase formation and transition processes under quasi-static conditions at temperatures from 200 to 650 8C. Samples with different dopants, several metal layer thicknesses as well as samples with ˚ TiN capping layer on single-crystal (001) and polycrystalline silicon substrates were examined. While and without a 150 A n-type dopants like P and As had no significant impact on the silicidation processes, boron decreased the range of thermal stability of the low-resistivity phase NiSi. A TiN capping layer shifts both these formation and transition temperatures to higher values.  2003 Elsevier B.V. All rights reserved. Keywords: Nickel silicide; Phase formation; Transition temperatures; Dopants; Capping layer

1. Introduction Faster microprocessors require shrunken transistor features like gate length and gate dielectric thickness as well as interconnect structures at reduced dimensions. In many cases, new materials are necessary to increase the microprocessor performance and to ensure the product reliability. Transition metal silicides are commonly used in the CMOS manufacturing process as contact material for source and drain regions as well as on top of polycrystalline *Corresponding author. Tel.: 149-351-277-4114; fax: 149351-2779-4114. E-mail address: [email protected] (J. Rinderknecht).

silicon gates of MOSFETs [1]. They will be used at least as long as no high-k gate dielectrics and metal gates will have been introduced. Due to its low resistivity and thermal stability, CoSi 2 is currently used in advanced integrated circuits (ICs) [2]. Recently, NiSi has increasingly attracted attention since it has some advantages like the low silicon consumption of only 35% compared with cobalt disilicide at similar sheet resistance [3,4]. NiSi can be formed with a lower thermal budget compared with CoSi 2 processing. Since the resulting NiSi layer is thinner than respective disilicide films, the contact resistance to the active regions of the transistors is reduced [5]. Furthermore, the thinner silicide films enable reduction of silicon-on-insulator thickness resulting in lower junction capacitance. Because of its beneficial

0167-9317 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00419-2

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properties, NiSi is expected to substitute CoSi 2 in high performance ICs [6]. The known lower thermal stability of NiSi, i.e. agglomeration and transformation to NiSi 2 , gave reason to investigate the influence of various substrates present on CMOS wafers and the interaction with capping layers. In this study, silicidation processes in nanoscale Ni layers were investigated using synchrotron radiation X-ray diffraction (SR-XRD). For systematic isothermal phase analysis, both the thermal treatment and the in situ measuring are possible with a high temperature chamber. Due to the fact that there are only very few scattering centers present in nanoscale layer stacks, an X-ray source of very high primary intensity, like a SR source, is indispensable. Another unique feature of SR is the possibility to choose a suitable wavelength to maximize the signal to noise ratio. Combining these unique capabilities, the impacts of the deposited metal layer thickness, of a TiN capping layer, and of different types of dopants on silicide formation were investigated systematically. Capping layers of either Ti or TiN are used in the semiconductor manufacturing of microprocessors to protect the deposited metal layer from oxidation. These layers are usually stripped off after a first rapid thermal annealing (RTA) [7]. The phase formation sequence as well as the formation and transition temperatures were studied between 200 and 650 8C under quasistatic conditions and compared with previous results [8]. The in situ high temperature SR-XRD experiments were performed at the Rossendorf Beamline (ROBL) Bending Magnet 20 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

2. Experimental setup A detailed description of ROBL was given by Matz et al. [9]. A photon energy of 8.048 keV just below the Ni–K absorption edge was chosen in order to minimize fluorescence radiation. The integrated photon flux was ¯3.5310 11 photons per s. The angle of incidence ai was fixed at 0.58, and a detector scan was acquired using a scintillation detector. The conditions for all measurements were: step size 0.18, vacuum in the range of 10 28 Pa. The sample size was 939 mm 2 to fit smoothly into a recess of the

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boron-nitride (BN) sample holder and to be fully illuminated by the primary X-rays. All wafer pieces were cut with a dicing saw to ¯408 rotation angle relative to (100u010) silicon substrate orientation to avoid substrate X-ray diffraction peaks. The measuring time was ¯2.5–4 s per step, resulting in a total measuring time of ¯15 min per temperature. An accurate temperature control was achieved using an electronically controlled heating system. Several thermocouples were used to modulate the current through the Ta resistivity heaters of the sample holder and through the environmental heaters to reach and maintain a given temperature. The temperature was stable within the range of 62.5 K. The sample temperature was measured using a NiAl / NiCr thermocouple in direct contact with the surface of the sample. A steplike procedure was used to increase the temperature, i.e. the temperature was kept at a constant value during the XRD scans to ensure static conditions during data collection.

3. Sample preparation Samples were prepared to study phase formation / transition as a function of temperature and • substrate type: single crystal (001) versus polycrystalline substrate. ˚ • Ni layer thickness: 80, 100, 150 A. • dopant type and energy: As 1 @ 30 keV, P 1 @ 15 keV, B 1 @ 5 keV. ˚ TiN capping layer versus no • capping layer: 150 A capping layer. The implantation dose was 8310 15 at. / cm 2 for all doped samples and dopant species. The thin films were deposited using a physical vapor deposition process. The deposition temperature of the nickel layers was 150 8C. The thickness of the layer stacks, controlled by X-ray reflectivity, revealed a very good matching with the target values.

4. Results

4.1. Impact of dopants on the nickel silicidation 4.1.1. Single-crystal substrates The silicidation process on single-crystal substrates starts at 200 8C. While measuring at RT and at

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160 8C (not shown here) reveal no clear diffraction maxima which can be attributed to any silicide phases, so that metallic Ni could be confirmed as starting material. Ni 2 Si is detected in all samples at 200 8C, regardless of the doping. The transition to NiSi is completed between 300 and 325 8C. NiSi is stable up to 450 8C for undoped samples and at least up to 600 8C for all of the doped samples. This means, the transition to NiSi 2 takes place at significantly higher temperatures for any doped samples compared with undoped samples (see Fig. 1 lower part).

4.1.2. Polycrystalline substrates The As 1 doped and the undoped samples show NiSi diffraction peaks already at 200 8C. The first clear diffraction peaks of NiSi are observed at 300 8C on B 1 doped samples. On polycrystalline substrates, boron doping stabilizes the Ni 2 Si phase compared with undoped and arsenic doped samples. But all dopants have a stabilizing effect on the thermal stability of NiSi. On undoped polycrystalline substrates NiSi 2 can be detected at temperatures as low as 475 8C. The formation of NiSi 2 is retarded for the doped samples for at least 25–50 8C. NiSi 2 cannot be identified at temperatures lower than

500 8C in any of the doped samples compared with the undoped sample. Fig. 1 summarizes the SR-XRD results.

4.2. Impact of a TiN capping layer on the nickel silicidation 4.2.1. Single-crystal substrates In the case of the uncapped sample, Ni 2 Si is observed in the temperature range between 200 and 300 8C. At 300 8C, only NiSi is detected, whereas the TiN capped sample still shows clear evidence of Ni 2 Si at this temperature. At 350 8C, the transition from Ni 2 Si to NiSi is completed for the capped and uncapped samples. In both cases, NiSi is stable up to at least 600 8C, i.e. no indications of NiSi 2 formation are detected. Fig. 2 summarizes the SR-XRD results. 4.2.2. Polycrystalline substrates In the low temperature regime, no impact of the capping layer is obvious on polycrystalline substrates. Both samples show diffraction peaks of Ni 2 Si at 200 and 250 8C. At 300 8C, only NiSi is detected in the capped and in the uncapped sample. The transition from NiSi to NiSi 2 starts below 600 8C within the uncapped sample, whereas no

˚ nickel samples with several dopants. High bars are actual measuring temperatures. Fig. 1. Summary of 60 SR-XRD results of 100 A

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˚ nickel samples with and without a 150 A ˚ TiN capping layer on single and polycrystalline Fig. 2. Summary of 31 SR-XRD results of 100 A silicon substrates. All samples are B 1 doped. High bars are actual measuring temperatures.

NiSi 2 formation is detected in the capped sample up to 655 8C. Fig. 2 summarizes the SR-XRD results. Fig. 3 shows an example of diffraction patterns obtained at different temperatures for a capped, ˚ nickel sample on a single-crystal boron doped, 100 A silicon substrate.

4.3. Impact of layer thickness on the nickel silicidation 4.3.1. Single-crystal substrates The onset of Ni 2 Si formation cannot be determined for starting temperatures of 200 8C, since

˚ nickel sample with a 150 A ˚ TiN capping layer on a B 1 doped single-crystal substrate. On the right Fig. 3. Diffraction patterns of 100 A hand side, a magnified view of the low-temperature region is shown.

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Ni 2 Si is already present at 200 8C in all samples. Up to 250 8C, all samples show diffraction peaks of Ni 2 Si. The starting point of NiSi formation seems to be independent of the deposited Ni layer thickness. ˚ sample the NiSi phase is formed at For the 80 A about 275 8C. The transition to NiSi is completed at 300 8C and NiSi 2 can be already be detected at ˚ Ni sample. 550 8C for the 80 A The transition from Ni 2 Si to NiSi is also com˚ Ni sample. However, pleted at 300 8C for the 100 A NiSi is stable in this sample up to at least 600 8C. ˚ Ni samples, Ni 2 Si is still found at In the 150 A 325 8C, although diffraction peaks of NiSi can be identified at 275 8C. No indication for NiSi 2 forma˚ Fig. 4 tion is detected up to 650 8C for the 150 A. summarizes the SR-XRD results.

4.3.2. Polycrystalline substrates In the temperature range between 200 and 250 8C, ˚ Ni only Ni 2 Si is detected within 100 and 150 A samples. ˚ Ni sample the transition to NiSi is For the 100 A completed at 300 8C. NiSi 2 is found at 600 8C in this sample. ˚ Ni sample shows diffraction At 275 8C, the 150 A peaks of NiSi. Only NiSi can be detected at 350 8C ˚ Ni sample. NiSi is stable up to at least in the 100 A

550 8C in this sample. Fig. 4 summarizes the SRXRD results.

5. Discussion The doping of single crystal substrates accelerates the transition from the metal-rich silicides to NiSi, i.e. the transition is completed at slightly lower temperatures compared with undoped samples. Although the formation of NiSi is known to be diffusion controlled, any doping atoms seem to promote the initial formation of nuclei that are necessary for the subsequent growth of NiSi, thus leading to a completed NiSi formation at 300 8C. Once a NiSi layer has been formed, it is stabilized by the dopants, causing an enhanced stability range of the low-resistivity phase NiSi. Since the transformation to NiSi 2 is nucleation dominated, all dopants seem to suppress the nucleation of NiSi 2 . But, both the formation and the transition temperatures are significantly lower for polycrystalline substrates compared with single crystal substrates. The overall higher density of crystal defects, i.e. of nucleation sites, diffusion paths, e.g. grain boundaries, and unsaturated bonds account for this phenomenon. In case of cobalt silicides, this observation is in agreement with Cabral et al. [10] who showed the impact

Fig. 4. Summary of 32 SR-XRD results of B 1 nickel samples without a capping layer. High bars are actual measuring temperatures.

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of substrate doping type for the silicidation process. The TiN capping layer retards the silicidation process due to the interaction of the deposited metal with the cap since it reduces the mobility of Ni during silicidation [1]. The TiN cap leads to a reduction of heterogeneous nucleation of NiSi 2 and therefore, mainly the transition from NiSi to NiSi 2 is affected. This effect of a TiN capping layer on the thermal stability of NiSi was also reported by Chamirian et al. [11]. A Ti cap has the same effect on CoSi 2 formation from Co or Co–Ni layers [12], and can even lead to the formation of ternary Co–Ti silicides [7]. However, a TiN capping layer is usually stripped off after thermal treatment, so the capping layer effect might be corrupted on CMOS devices. A reduced metal layer thickness reduces the phase formation and the transition temperatures. Other authors suggest that agglomeration of the NiSi film is a prerequisite for the transformation of NiSi to NiSi 2 [11]. Agglomeration is known to depend on the layer thickness, so the observed thickness effect can be explained. This observation was also reported by Besser et al. [13]. Using RTA conditions, the onset temperature for NiSi formation of 275 8C is shifted to higher values [11]. Only after the complete layer has been transformed into a specific nickel silicide, e.g. NiSi, the formation of the next less metal-rich silicide starts. This behavior is in agreement with the subsequent growth of silicides [14,15] and is typical for interdiffusion controlled growth processes of welded samples or layer stacks.

6. Summary and conclusion The temperature range for the existence of the target phase NiSi is between 300 and 500–525 8C for doped samples. The n-type doping species, i.e. As 1 or P 1 , do not have such a significant impact on the formation and the transition temperatures as B 1 has. This result confirms previous investigations [8]. The p-type dopant B 1 increases the formation temperature of NiSi and, therefore, it decreases the stability range of the low-resistivity NiSi phase. Further backend-of-line processing has to consider the limited high-temperature stability of this phase, especially on polycrystalline substrates, for which the

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formation temperature of NiSi 2 is generally lower than for single-crystal substrates. Higher temperatures cause an epitaxial growth of the high-resistivity NiSi 2 phase on single-crystal substrates, as confirmed by TEM [8]. The reduced formation temperature of NiSi 2 in very thin films has to be taken into account for the next technology nodes of microprocessor manufacturing, i.e. the 90 and 65 nm technology nodes. In general, the doping stabilizes the NiSi phase, especially in the high temperature regime. The TiN capping layer shifts the formation temperatures to higher values on both types of substrates for Ni 2 Si and for NiSi. The contributions to the increase of the formation and the transition temperatures of the nickel silicide phases caused by the doping and the capping layer add up (compare Figs. 1 and 2).

Acknowledgements The authors wish to thank Wolfgang Matz and Johannes von Borany, Research Center Rossendorf, for their administrative support to enable the synchrotron radiation experiment at the ESRF, Grenoble.

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