Low Temperature PEALD of TiN from TDMAT: A … resistivity in TiN is a very ion driven process and...
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www.oxford-instruments.com Parameter Result Ti:N ~ 50:50 Growth Rate 0.5 Å/cyc Film Thickness 75 nm Thickness Uniformity (8”) ±2.6% Resistivity 180 μΩcm Deposition Temperature 200 ºC Carbon Contamination <2% Oxygen Contamination <4% The optimised process shows good ALD behaviour and lower resistivity than an equivalent temperature process with TiCl 4 as the Ti source. The table to the right shows the results from the optimised process giving the lowest resistivity References [1] S.B.S. Heil, J.L. van Hemmen, C.J. Hodson, N. Singh, J.H. Klootwijk, et al.; J. Vac. Sci. Technol. A, 25, (2007), 1357 [2] H.B. Profijt, P. Kudlacek, M.C.M. van de Sanden, W.M.M. Kessels; J. Electrochem. Soc., 158(4), (2011), G88 [3] T.R. Sharp, A.K. Peter, B. Mackenzie, H.C.M. Knoops, and R. Gunn, ALD-AVS 2013 Poster Submission [4] S. E. Potts, H.B. Profijt, R. Roelofs, W.M.M. Kessels; Chem. Vap. Deposition 19(4-6), (2011), 125 Introduction In this study we report trends and results of an optimised process for PEALD deposited TiN at 200 ºC using the metal-organic precursor TDMAT as the titanium source. Through careful control of the plasma conditions we are able to achieve a resistivity of 180 μΩcm for 74 nm of film. Because of thin film scattering and other effects, the resistivity increases to 250 μΩcm for 26 nm which is an improvement on the previously reported 361 μΩcm for TiCl 4 at 250 ºC [1]. This makes the TDMAT process viable for either when chlorine-containing by-products must be avoided or where thermal budget must be minimised. TiN layers were prepared by sequential dosing of tetrakis(dimethylamino)titanium (TDMAT) and a mixed N 2 /H 2 plasma in the Oxford Instruments FlexAL ® remote plasma ALD reactor, described in detail in Ref [1] and photographed in Figure 1. The TDMAT precursor was held in a stainless steel bubbler at 60 ºC and delivered into the reactor through bubbled doses with 200 sccm of Ar. It has been observed previously that low pressure plasmas produce higher ion energies [2] and promote higher film quality in both nitrides [3] and oxides [4]. We have therefore maintained the plasma pressure at 3 mTorr throughout and varied other plasma conditions to observe trends. Figure 1 photograph of the FlexAL ® reactor Experimental Set-up Results & Discussion Conclusion Summary It was also studied whether an increased flux of plasma species could improve the resistivity, where flux is controlled by total exposure time. As with most conductive nitrides a long exposure is needed in order to minimise resistivity. This is the case here where the shoulder is seen at ~30 seconds [Fig. 4]. Contrary to previous assumptions about the remote ICP source, the centre point of an 8” area does to saturate much faster than the outer area suggesting the distribution of plasma species is relatively uniform at the wafer surface, allowing a resistivity uniformity of around ±10% to be achieved. The low pressure plasma likely aids the distribution of plasma species here. As is typical for remote ICP sources, plasma power is inversely correlated with electron temperature and peak ion energy [Ref. 2, Fig. 2]. It has been suggested that achieving low resistivity in TiN is a very ion driven process and this appears to be the case here [Fig. 3], with the lowest resistivity seen at 100 W. Note that at 50 W the source began to operate in a dominantly capacitive mode which caused under-saturation. Unlike other studied films no defined plateau in thickness vs resistivity is seen where thin film scattering is eliminated and the film begins to behave as a bulk material. A shoulder is observed in the graphs [Fig. 5] for both 200 ºC and 350 ºC deposition. At thicknesses greater than 20 nm it is believed that a slow crystallisation effect is responsible for the decrease in resistivity. This effect has been observed strongly in hotter TiCl 4 processes. 0 100 200 300 400 500 600 700 0 10 20 30 40 50 60 70 80 Resistivity (μΩcm) Thickness (nm) 350 °C 200 °C 0 50 100 150 200 250 300 0 100 200 300 400 500 600 700 0% 20% 40% 60% 80% Thickness (Å) Resistivity (μΩcm) %N 2 (%) Average Res Centre Res Average Thk Centre Thk Gas Composition is a parameter that strongly influences resistivity [Fig. 6]. A few factors are potentially at work including the increased presence of H 2 increasing the ion energy for a given pressure and power [Fig. 7] or the reducing nature of H radicals aiding removal of carbon. 0 50 100 150 200 250 300 0 100 200 300 400 500 0 100 200 300 400 500 Thickness (Å) Resistivity (μΩcm) Power (W) Centre Res. Centre Thk 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 0 20 40 60 80 Thickness (Å) Resistivity (μΩcm) Plasma Time (s) Average Res Centre Res Average Thk Centre Thk Surface and Composition AFM analysis [Fig. 8] shows a 65 nm film of TiN deposited from TDMAT at 200 ºC with an R a of 0.71 nm. A comparable thickness of film deposited from TiCl 4 at 550 ºC shows a much higher R a of 3.3 nm. This is in agreement with the idea that resistivity is linked to crystal grain size. Sputtered XPS [Fig. 9] shows the film to be stoichiometric TiN with oxygen content at 4% and carbon content close to the detection limit of 2%. 0 10 20 30 40 50 60 0 300 600 900 1,200 Atomic % (%) Etch Time (s) O1s Si2p C1s N1s Ti2p Deposition has been onto p-type silicon with resistivity between 1 and 10 Ωcm. Initial experiments were also made in tandem with deposition onto PECVD grown SiO 2 on silicon which was observed to give no more accurate resistivity measurements. For ease of ellipsometer modelling, all depositions were therefore straight onto native oxide-coated Si. Ellipsometer measurements were made ex-situ using a Woollam M2000-V and thickness determined using a GenOsc model with three Lorentz oscillators. Resistivity was measured via a Signatone four-point-probe connected to a Keithley 2410 source meter. Plasma Power Film Thickness PEALD of TiN using the metal organic precursor TDMAT has been demonstrated to give lower resistivity than equivalent process conditions using TiCl 4 as the Ti source. The lower surface roughness and lower process temperature could make this a viable process option when thermal budget must be controlled Plasma Exposure Time Plasma Gas Composition Figure 2 (left) Mean ion energy (top) and electron temperature (bottom) for FlexAL (<25 mTorr) and OpAL (>25 mTorr) Figure 3 (right) Resistivity and Thickness vs Plasma Power Figure 5 Resistivity vs Thickness for TiN deposited from TDMAT at 350 ºC (orange) and 200 ºC (blue). NB: 350 ºC is acknowledged to be above the decomposition temperature for TDMAT and a lage increase in growth rate is observed however the films still appear uniform and comparable to 200 ºC deposition Figure 4 Resistivity and Thickness vs Plasma Time Figure 6 (left) Resistivity and Thickness vs %N2 in mixed H2/N2 plasma Figure 7 (right) Ion energy distribution measured by RFEA on homebuilt TU/e system. Comparing to FlexAL, the O 2 the peak ion energy is ~4 eV lower but the trend is assumed the same Figure 8 (right) AFM image of TiN deposited from TDMAT at 200 ºC (top) and from TiCl4 at 550 ºC (bottom) Figure 9 (left) Sputtered XPS trace of TiN deposited from TDMAT at 200 ºC * [email protected] Low Temperature PEALD of TiN from TDMAT: A Chlorine-free Route to Low Resistivity T. R. Sharp*, A. K. Peter, B. MacKenzie, H. C. M Knoops, G. Proudfoot and R. Gunn Oxford Instruments Plasma Technology, North End Road, Yatton, Bristol, BS49 4AP, UK
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Parameter Result Ti:N ~ 50:50
Growth Rate 0.5 Å/cyc Film Thickness 75 nm
Thickness Uniformity (8”) ±2.6% Resistivity 180 µΩcm
Deposition Temperature 200 ºC
Carbon Contamination <2%
Oxygen Contamination <4%
The optimised process shows good ALD behaviour and lower resistivity than an equivalent temperature process with TiCl4 as the Ti source. The table to the right shows the results from the optimised process giving the lowest resistivity
References [1] S.B.S. Heil, J.L. van Hemmen, C.J. Hodson, N. Singh, J.H. Klootwijk, et al.; J. Vac. Sci. Technol. A, 25, (2007), 1357 [2] H.B. Profijt, P. Kudlacek, M.C.M. van de Sanden, W.M.M. Kessels; J. Electrochem. Soc., 158(4), (2011), G88 [3] T.R. Sharp, A.K. Peter, B. Mackenzie, H.C.M. Knoops, and R. Gunn, ALD-AVS 2013 Poster Submission [4] S. E. Potts, H.B. Profijt, R. Roelofs, W.M.M. Kessels; Chem. Vap. Deposition 19(4-6), (2011), 125
Introduction In this study we report trends and results of an optimised process for PEALD deposited TiN at 200 ºC using the metal-organic precursor TDMAT as the titanium source. Through careful control of the plasma conditions we are able to achieve a resistivity of 180 µΩcm for 74 nm of film. Because of thin film scattering and other effects, the resistivity increases to 250 µΩcm for 26 nm which is an improvement on the previously reported 361 µΩcm for TiCl4 at 250 ºC [1]. This makes the TDMAT process viable for either when chlorine-containing by-products must be avoided or where thermal budget must be minimised.
TiN layers were prepared by sequential dosing of tetrakis(dimethylamino)titanium (TDMAT) and a mixed N2/H2 plasma in the Oxford Instruments FlexAL® remote plasma ALD reactor, described in detail in Ref [1] and photographed in Figure 1. The TDMAT precursor was held in a stainless steel bubbler at 60 ºC and delivered into the reactor through bubbled doses with 200 sccm of Ar. It has been observed previously that low pressure plasmas produce higher ion energies [2] and promote higher film quality in both nitrides [3] and oxides [4]. We have therefore maintained the plasma pressure at 3 mTorr throughout and varied other plasma conditions to observe trends.
Figure 1 photograph of the FlexAL® reactor
Experimental Set-up
Results & Discussion
Conclusion
Summary
It was also studied whether an increased flux of plasma species could improve the resistivity, where flux is controlled by total exposure time. As with most conductive nitrides a long exposure is needed in order to minimise resistivity. This is the case here where the shoulder is seen at ~30 seconds [Fig. 4]. Contrary to previous assumptions about the remote ICP source, the centre point of an 8” area does to saturate much faster than the outer area suggesting the distribution of plasma species is relatively uniform at the wafer surface, allowing a resistivity uniformity of around ±10% to be achieved. The low pressure plasma likely aids the distribution of plasma species here.
As is typical for remote ICP sources, plasma power is inversely correlated with electron temperature and peak ion energy [Ref. 2, Fig. 2]. It has been suggested that achieving low resistivity in TiN is a very ion driven process and this appears to be the case here [Fig. 3], with the lowest resistivity seen at 100 W. Note that at 50 W the source began to operate in a dominantly capacitive mode which caused under-saturation.
Unlike other studied films no defined plateau in thickness vs resistivity is seen where thin film scattering is eliminated and the film begins to behave as a bulk material. A shoulder is observed in the graphs [Fig. 5] for both 200 ºC and 350 ºC deposition. At thicknesses greater than 20 nm it is believed that a slow crystallisation effect is responsible for the decrease in resistivity. This effect has been observed strongly in hotter TiCl4 processes.
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Gas Composition is a parameter that strongly influences resistivity [Fig. 6]. A few factors are potentially at work including the increased presence of H2 increasing the ion energy for a given pressure and power [Fig. 7] or the reducing nature of H radicals aiding removal of carbon.
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Surface and Composition
AFM analysis [Fig. 8] shows a 65 nm film of TiN deposited from TDMAT at 200 ºC with an Ra of 0.71 nm. A comparable thickness of film deposited from TiCl4 at 550 ºC shows a much higher Ra of 3.3 nm. This is in agreement with the idea that resistivity is linked to crystal grain size. Sputtered XPS [Fig. 9] shows the film to be stoichiometric TiN with oxygen content at 4% and carbon content close to the detection limit of 2%.
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Deposition has been onto p-type silicon with resistivity between 1 and 10 Ωcm. Initial experiments were also made in tandem with deposition onto PECVD grown SiO2 on silicon which was observed to give no more accurate resistivity measurements. For ease of ellipsometer modelling, all depositions were therefore straight onto native oxide-coated Si. Ellipsometer measurements were made ex-situ using a Woollam M2000-V and thickness determined using a GenOsc model with three Lorentz oscillators. Resistivity was measured via a Signatone four-point-probe connected to a Keithley 2410 source meter.
Plasma Power
Film Thickness
PEALD of TiN using the metal organic precursor TDMAT has been demonstrated to give lower resistivity than equivalent process conditions using TiCl4 as the Ti source. The lower surface roughness and lower process temperature could make this a viable process option when thermal budget must be controlled
Plasma Exposure Time
Plasma Gas Composition
Figure 2 (left) Mean ion energy (top) and electron temperature (bottom) for FlexAL (<25 mTorr) and OpAL (>25 mTorr)
Figure 3 (right) Resistivity and
Thickness vs Plasma Power
Figure 5 Resistivity vs Thickness for TiN deposited from TDMAT at 350 ºC (orange) and 200 ºC (blue). NB: 350 ºC is acknowledged to be above the
decomposition temperature for TDMAT and a lage increase in growth rate is observed however the films still appear
uniform and comparable to 200 ºC deposition
Figure 4 Resistivity and Thickness vs Plasma Time
Figure 6 (left) Resistivity and Thickness vs %N2 in mixed H2/N2 plasma
Figure 7 (right) Ion energy distribution
measured by RFEA on homebuilt TU/e system.
Comparing to FlexAL, the O2 the peak ion energy is ~4 eV lower but the trend
is assumed the same
Figure 8 (right) AFM image of TiN deposited from TDMAT at 200 ºC (top) and from TiCl4 at
550 ºC (bottom)
Figure 9 (left) Sputtered XPS trace of TiN deposited from TDMAT at 200 ºC
Low Temperature PEALD of TiN from TDMAT: A Chlorine-free Route to Low Resistivity T. R. Sharp*, A. K. Peter, B. MacKenzie, H. C. M Knoops, G. Proudfoot and R. Gunn
Oxford Instruments Plasma Technology, North End Road, Yatton, Bristol, BS49 4AP, UK
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