Electrical and Structural Properties of the Partial Ternary Thin-Film

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Electrical and Structural Properties of the Partial Ternary Thin-Film System Ni-Si-B Matthias Wambach, Nam T Nguyen, Sven Hamann, Mitsuaki Nishio, Shinjiro Yagyu, Toyohiro Chikyow, and Alfred Ludwig ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.8b00175 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Combinatorial Science

Electrical and Structural Properties of the Partial Ternary Thin-Film System Ni-Si-B Matthias Wambach1, Nam Nguyen2, Sven Hamann1, Mitsuaki Nishio3, Shinjiro Yagyu2, Toyohiro Chikyow2, Alfred Ludwig1* 1

Chair of MEMS Materials, Institute for Materials, Faculty of Mechanical Engineering, Ruhr-

University Bochum, Universitaetsstrasse 150, D-44801 Bochum, Germany 2

Nano-Elecronics Materials Unit, International Center for Materials Nanoarchitectonics MANA,

National Institute for Materials Science NIMS, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan 3

Materials Analysis Station, National Institute for Materials Science NIMS, 1-2-1 Sengen,

Tsukuba, Ibaraki 305-0047, Japan Keywords: Thin Films, Sputtering, Work Function, Silicides, Structure-Property Relationships

ABSTRACT High-throughput and combinatorial materials science methods were used to investigate the dependence of the work function in the Ni-Si system on the B content (0 -30 at.%). Alloying of NiSi is used to adapt its properties to suit the needs as a gate electrode material. Thin-film materials libraries were fabricated and investigated with respect to their structural and electrical properties. Further the work function values of selected samples in the region of interest were analyzed. The

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results show that the work function can be adjusted between 4.86 eV (B = 4.2 at.%) and 5.16 eV (B = 29.2 at.%) for (NiSi)Bx.

INTRODUCTION The limitations of SiO2 concerning further downscaling of gate oxides in state-of-the-art transistors lead to the investigation of alternative materials, so-called high-k dielectrics. 1,2 Before the 65 nm node, the prototype gate electrode material was polycrystalline Si (poly-Si) in combination with a Si oxide gate. High-κ gate oxides were proposed to substitute for Si oxide due to high leakage currents with decreasing gate thickness. However, when using this material, the threshold voltage of the transistor cannot be controlled by doping due to Fermi level pinning. 3 This obstacle could be overcome by changing the gate electrode material from poly-Si to a metal silicide in combination with a high-κ gate oxide. A promising candidate is NiSi due to its low resistivity and Si consumption. 4 However, to build suitable nMOSFET and pMOSFET devices, the work function of the gate electrode needs to be in the range of Φm = 4.0 - 4.2 eV for nMOSFETs and Φm = 5.0

- 5.2 eV for pMOSFETs. 5 A method to adjust the work function of NiSi via the self-aligned silicide

(SALICIDE) process,

4,6

is doping the substrate with impurities. 7,8 A further way is alloying of

different metal silicides 9,10 or doping / alloying silicides after formation. 11,12 Promising elements for alloying the NiSi system can be found in literature. Investigated dopants include P, As, Sb, N, Cl and B. 13,14,7 Using B as doping element is supported by Nakatsuka et al. who proposed a variation of the work function in NixSiy by B-doping, depending on the occupation of B atoms on Ni-sites and Si-sites respectively. 15 In this paper structural and electrical properties of Ni-Si-B thin film materials libraries covering a wide range of B content (4 at.% - 29 at.% Bc are presented.


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EXPERIMENTAL METHODS Thin-film composition spreads of the Ni-Si-B system were fabricated on thermally oxidized 100 mm SiO2/Si wafers using a magnetron sputter system (AJA Int.). The materials libraries were cosputtered from 4-inch diameter elemental targets. The Ni target (KJL Company) had a thickness of 3 mm and a purity of 99.995%. The Si target (AJA Int.) and the B target (AJA Int.) both had a thickness of 3.175 mm with a purity of 99.999% and 99.5% respectively. The Si and B targets were bonded to a Cu backing plate to reduce thermal shock. The mean thickness of the thin films was 300 nm. The materials libraries were annealed ex-situ at 800°C for two hours in vacuum (1∙109 - 2∙10-9 Torr). Annealing conditions were selected based on data from the ASM Phase Diagram DatabaseTM. 16 The composition of each measurement area on the materials library was measured using an Electron Probe Micro Analysis (EPMA) system (Jeol JXA-8900R). EPMA measurements were conducted at an acceleration voltage of 4 kV, which limits the accuracy of the results, observable as scattering in figure 1. As both the thin film and the substrate contain Si, it was necessary to limit the excitation range of the electron beam to the thin film. The sheet resistance of the thin films was measured using a 4-point resistance probing system (RT-3000/RG-80N, Napson Corporation). Film thickness was measured using a profilometer (Ambios XP2). Resistivity was calculated using sheet resistance and thickness data. Diffraction patterns of the materials library were measured using an X-ray diffractometer (XRD, Bruker D8 Discover). A Photoelectron Yield Spectroscopy (PYS) system (details can be found in 17) was used to measure the work function of the thin films. 18,19


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RESULTS Structural Properties The compositional region that is covered by the Ni-Si-B materials library is shown in Figure 1. The Si content ranges from 23 at.% to 68 at.% and the Ni content from 20 at.% to 72 at.%. The B content ranges between 2 at.% and 32 at.%. The structural phases identified are also depicted in Figure 1. In the Si-rich region of the materials library the NiSi2 phase was identified. With increasing Ni content, the NiSi phase is detected alongside the NiSi2 phase. At around equal composition of Ni and Si, the NiSi phase is the only present silicide phase. With further increasing Ni content, the Ni3Si2 phase forms, first along with NiSi and then solely in a single-phase region. For higher Ni concentrations, ternary Ni4Si2B and Ni6Si2B phases form alongside Ni-rich silicides such as Ni2Si and Ni31Si12. With decreasing Si content, the Ni31Si12 phase becomes more pronounced for low B contents over the Ni2Si phase and the Ni6Si2B phase over the Ni4Si2B phase for high B contents respectively. It should be noted that the exact stoichiometry of the Ni-deficient ternary phase is disputed in literature. This phase has been labeled Ni4.6Si2B

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as well as

Ni4.29Si2B1.43. 21 In this work the measurement area close to the composition Ni4Si2B has shown the highest intensities of the corresponding XRD peaks. However due to the aforementioned uncertainty of the composition, it remains an open question what the stoichiometric composition of this phase is. In an effort to simplify the denotation, this phase is labeled Ni4Si2B in this publication.


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Figure 1: Phase assessment of the Ni-Si-B thin film materials library (annealed 800°C, 2 h). Red circles indicate the measurement areas of which the work function was measured. The sample areas close to a Ni:Si ratio of one (dotted ellipse) are highlighted in figure 3. Figure 2 shows the electrical resistivity of the materials library. Most measurement areas show a resistivity ≤ 20 μΩm. One of the exceptions is on the very Si-rich side, where the resistivity ranges

from ≈ 40 μΩm up to 131.4 μΩm for a composition of Ni20Si59B21. The other exception is an area

where the Ni:Si ratio is close to 60:40. Here the resistivity ranges from 20 μΩm to 40.1 μΩm at

Ni39Si31B30. The lowest values were found in a B-deficient area with a Ni:Si ratio close to 70:30.


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The lowest value is 2.35 μΩm for Ni67Si28B5. An area of comparatively low resistivity was detected

at Ni51Si47B2. A clear trend towards higher resistivity values for higher B content is visible. The resistivity values closest to the stoichiometric composition of Ni4Si2B and Ni6Si2B were 7.5 μΩm

(Ni57Si29B14) and 5.7 μΩm (Ni66Si23B11) respectively.

Figure 2: Electrical resistivity of the Ni-Si-B thin film materials library with contour lines to guide the eye (annealed 800°C, 2 h).


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Since NiSi is the most promising Ni-silicide to be used as a metal gate electrode material, the measurement areas close to the stoichiometric composition of this phase were studied in more detail. This region is highlighted by a dotted ellipse in figure 1. The corresponding XRD diffractograms are depicted in figure 3.

Figure 3: Diffraction patterns of a row of sample areas close to Ni:Si ratio of 1 (see figure 1). The B content increases with distance from the abscissa. The identified peaks belong to the NiSi phase. The materials library was annealed 800°C for 2 h.


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A broad peak in the range of 20 -30 ° is present with all patterns. This “glass hill”

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indicates

that these measurement areas are partially amorphous. The diffraction peaks of the (002), (102), (210), (112) and (211) planes of the NiSi phase can be clearly identified. With increasing B content, a broadening of the XRD peaks is observed, which indicates either a decrease in grain size or stress/strain in the material. The peak intensity of the (102) and (211) planes decrease while the intensity of the (002) plane remain the same with increasing B content. A film texture was not identified in the 2D XRD data. Figure 4 shows the lattice parameters (figure 4 a-c), calculated from the diffraction patterns using the peak positions, and the unit cell volume over the B concentration (figure 4 d) based on the data shown in figure 3. The data suggests an increase of the lattice constant a of about 1% and a decrease of the lattice constants b and c can be identified. Here the change is in the order of 2.5% and 0.5% respectively. The unit cell volume shrinks with increasing B content. Over the investigated range a decrease of 2% can be identified.


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Figure 4: Lattice constants a, b and c as well as the unit cell volume of the orthorhombic NiSi phase in dependence of the B content. The selected measurement areas where taken of measurement areas in the materials library that are close to Ni:Si ratio of 1 (see figure 1). The materials library was annealed 800°C for 2 h. To get further insight into the microstructural impact of the addition of B to NiSi, the coherence length was estimated from XRD measurements using the Scherrer equation. 23 The peaks of the diffraction plot where fitted using a Pearson VII function in order to gain the FWHM. It was assumed that the effect of microstrain on the peak broadening is small compared to the broadening due to the crystallite size. This assumption is supported by the fact that the addition of B is known to result in finer crystallites in Ti- and Al-alloys

24,25,26

and the profile shape factor of the fitted


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peaks was close to m = 1 indicating the Lorentzian nature of the peak shapes. These assumptions in addition to the application of the already simplified Scherrer equation as well as peak broadening contribution of the measurement system, lead to a rough estimate. However, a qualitative conclusion of the change in coherence length over the B content can be drawn. In figure 5 a strong decrease of the coherence length from 16 nm to 9 nm in the range of 7 at.% to 12 at.% B was identified. However, an additional increase of the B content yielded only a comparatively small decrease. In the range from 12 at.% to 30 at.% the coherence length only decreases from 9 nm to around 7 nm.

Figure 5: Coherence length in dependence of the B content of samples close to a Ni:Si ratio of 1 (see figure 1). The materials library was annealed 800°C for 2 h.


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Functional Properties of (NiSi)-B The work function was measured selectively for samples highlighted by grey circles in figure 1. The error of these measurements was estimated to be 0.05 eV. Figure 6 shows the results, where the Ni:Si ratio is plotted over the B content. The color-coding of the measurement points corresponds to the work function. The solid horizontal lines represent the nominal composition of the NixSiy phases and serve as a visual guide. For almost all measurement areas the work function increases with increasing B content, with the exception of the measurement area at Ni:Si = 1.54 and a B content of 29 at.%. This may be due to the formation of the ternary Ni4Si2B-phase in this area. Focusing on the area close to Ni:Si = 1, an increase in work function from 4.86 eV to 5.16 eV was detected for a B content of 4.2 at.% to 29.2 at.% respectively.


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Figure 6: Plot of the Ni:Si ratio over the B content. The color code refers to the work function of the individual measurement areas. Solid lines represent the Ni:Si ratios at which phases of the NixSiy are expected. The measurement areas are taken from a materials library annealed at 800°C for 2 h. One of the most relevant properties of gate electrode materials is the electrical resistivity as it influences the hot carrier degradation and the gate electrode RC delay (GERDE) effect. 27 Figure 7 shows the resistivity and work function as a function of the B content for measurement areas close to Ni:Si = 1. The plot shows an increase of the resistivity from around 6 μΩm to 14 μΩm. From 20 at.% B onwards, the electrical resistivity increases more strongly with increasing B concentration. The work function of measurement areas in this region is plotted in dependence of the B content in figure 7. The range of the work function in this area spans from 4.86 to 5.16 eV and increases with increasing B content.


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Figure 7: Plot of the resistivity and the work function over the B content of selected measurement areas with Ni:Si ≈ 1. The materials library was annealed 800°C for 2 h.

DISCUSSION Comparing the phase diagrams of Lugscheider et al. 16 and Chaban et al. 28 to the one derived from our experiments, the SixBy phases that were identified by Lugscheider et al., but not by Chaban et al., are also not present in our phase diagram. The NixSiy-phases NiSi2, NiSi, Ni2Si and Ni31Si12 as well as the ternary phase Ni4Si2B and Ni6Si2B were identified in all three experimental works. In contrast to this work and the publication of Chaban et al., Lugscheider et al. identified the high-


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temperature Ni1.5Si phase instead of the Ni3Si2 phase. This might be due to the fact that the phase diagram by Lugscheider et al. was determined at a higher temperature (850°C). In general, the phase boundaries match with the ones published by Chaban and co-workers. This is mainly due to the absence of the SixBy phases. The phase boundaries shift towards higher Sicontent with increasing B content between NiSi and Ni2Si. The stability regions of single-phase areas, e.g. NiSi or Ni4Si2B, are broader than in the phase diagrams published by Lugscheider or Chaban. This might be attributed to higher solubility of the species in the amorphous matrix at room temperature. The electrical resistivity values of the B-alloyed Ni-Si measurement areas presented in this work are significantly higher than the published data for Ni-Si. Colgan et al. published resistivity values for NiSi2, NiSi and Ni2Si: 0.34 μΩm, 0.14 μΩm and 0.24 μΩm respectively. 29 Gas et al. measured

a resistivity value of 0.68 μΩm for Ni3Si2 while their value for NiSi and Ni2Si are 0.15 and 0.25

respectively. 30 Zhang et al. report a resistivity of 0.60 μΩm for Ni31Si12 nanowires. 31 Their values for NiSi and Ni2Si, 0.13 μΩm and 0.25 μΩm respectively, for nanowires match the thin film values

mentioned before very well. No experimental data on the electrical resistivity of the Ni4Si2B and Ni6Si2B phases was found in literature. The resistivity values in this work are an order of magnitude higher compared to the values for pure Ni-Si phases in literature. This is attributed to the addition of B, resulting in a grain refinement and partial amorphization of the microstructure, which leads to an increased resistivity. However, the general tendency of NiSi2 and Ni3Si2 being the phases of higher resistivity while NiSi and Ni2Si show lower resistivity is in line with other publications. The resistivity values for measurement areas containing Ni31Si12 do not appear to increase significantly as would be expected for the comparatively high values for Ni31Si12 in literature.


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The work function values obtained in this study are also higher than the literature values in the NiSi system or for B-doped NiSi. Kittl et al. published the work function of undoped NiSi, Ni2Si, Ni31Si12 on SiO2. 32 The values are 4.66 eV, 4.77 eV and 4.73 eV respectively. Biswas et al.

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reported the work functions of thin film gate stacks W/NixSiy/SiO2/Si. For a composition of Ni40Si60, Ni50Si50 and Ni60Si40 they published a work function of 4.32 eV, 4.36 eV and 4.47 eV respectively. For B-doped NiSi films reported values range from 4.75 eV 34 and 4.8 eV 35 to around 4.95 eV.7 The work function can be increased with both B-doping, as shown in literature, and the alloying of B, as shown here. The higher work function values in the presented data compared to B-doped samples may be attributed to the higher amounts of B as well as the semi-amorphous microstructure. The decrease in unit cell volume (figure 4) suggest that B atoms substitute in the NiSi crystals rather than exist as interstitials. However, the shift is not very pronounced. Both the work function and electrical resistivity values show a feature at around 20 at.% B as can be seen in figure 7. For this B content neither the lattice parameters nor the coherence lengths show a significant change. Also, no phase boundary can be identified from the XRD results at this composition. Since the resistivity and work function can be influenced by many factors, further studies need to be conducted to illuminate the effect on these properties.

CONCLUSION A Ni-Si-B material library was fabricated and investigated using high-throughput methods. A partial phase diagram in the composition range 0 at.% - 30 at.% B, 25 at.% - 70 at.% Si and 20 at.% - 75 at.% Ni, was established and the electrical resistivity was measured in this region. For selected compositions the work function was measured, which was found to increase for Ni-Si


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with increasing B content. For a more detailed analysis, measurement areas with a Ni:Si ratio close to unity were selected. The analysis indicates that the structure is not fully crystalline and mainly the NiSi phase is present in all areas. The grain refinement effect of the B addition is most significant up to 12 at.% of B while a further increase leads to no further decrease of the coherence length. The electrical resistivity as well as the work function increase with increasing B content from 6 to 14 μΩm and from 4.86 to 5.16 eV respectively. Both plots show a significant change in

slope at a B content of around 20 at.%. It was shown that the work function can be increased within the range of 5.0 – 5.2 eV by alloying B into NiSi.

AUTHOR INFORMATION Corresponding Author * [email protected] Funding Sources The work has been partially funded by the NIMS internship program (MW).

ACKNOWLEDGMENT The authors would like to thank C. Long and I. Takeuchi for permission to use their CombiView software. REFERENCES

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(33) Biswas, N.; Gurganus, J.; Misra, V. Work function tuning of nickel silicide by co-sputtering nickel and silicon. Appl. Phys. Lett. 2005, 87, 171908. (34) Pawlak, M. A.; Lauwers, A.; Janssens, T.; Anil, K. G.; Opsomer, K.; Maex, K.; Vantomme, A.; Kittl, J. A. Modulation of the workfunction of Ni fully silicided gates by doping: Dielectric and silicide phase effects. IEEE Electron Device Lett. 2006, 27, 99–101. (35) Kedzierski, J.; Nowak, E.; Kanarsky, T.; Zhang, Y.; Boyd, D.; Carruthers, R.; Cabral, C.; Amos, R.; Lavoie, C.; Roy, R. et al. Metal-gate FinFET and fully-depleted SOI devices using total gate silicidation. In IEEE International Electron Devices Meeting; pp 247–250.


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Electrical and Structural Properties of the Partial Ternary Thin-Film

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