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Low resistivity tungsten for contact metallization
Microelectronic Engineering 82 (2005) 261–265 www.elsevier.com/locate/mee
Low resistivity tungsten for contact metallization Steven Smith a,*, Khaled Aouadi b, Josh Collins c, Eric van der Vegt a, Marie-The´rese Basso b, Marc Juhel b, Simone Pokrant a a
Crolles2 Alliance, Philips Semiconductors, Crolles, France b Crolles2 Alliance, STMicroelectronics, Crolles, France c Novellus Systems, Inc., San Jose, CA, USA Available online 18 August 2005
Abstract A new pulsed nucleation tungsten process, called ‘‘PNL low-Rs W’’, has been developed that results in near-bulk resistivity even in ultra-thin tungsten films. This solution greatly diminishes the need to develop alternative films for contact fill, probably for several generations. The process is done in a conventional, commercially available 300 mm tool with standard hardware and operating conditions. Tungsten films deposited with this technique exhibit significantly larger grain size and a smoother surface compared with conventional PNL and ALD techniques. The larger grains account for lower resistivity due to less grain boundary electron scattering. The process results in higher fluorine content in the liner/barrier, but does not have an adverse effect on contact performance. A comparison of 6:1 aspect ratio contacts showed no difference in fill properties between the PNL low-Rs W and conventional PNL and ALD processes. This new film has been characterized and demonstrated on 65 nm technology node product on contacts to nickel silicide, and on 90 nm technology node product on eDRAM high aspect ratio contacts. Ó 2005 Published by Elsevier B.V. Keywords: Tungsten; Contacts; PNL; ALD; Low resistivity
1. Introduction It is well known from the early development of CVD tungsten that the introduction of a small amount of B2H6 during the nucleation step results *
Corresponding author. Tel.: +33 (0)4 38 92 21 44; fax: +33 (0)4 38 92 21 22. E-mail address: [email protected] (S. Smith). 0167-9317/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.mee.2005.07.032
in decreased tungsten resistivity [1,2]. Today, pulsed nucleation layer (PNL) and atomic layer deposition (ALD) for tungsten nucleation integrated with traditional WF6–H2 CVD are the current state-of-the-art processes for void-free tungsten plug fill of contacts at the 90 nm and lower technology nodes for logic and systemon-chip devices [3,4]. Both utilize B2H6 in order to reduce film resistivity. Tungsten resistivity is a
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S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
significant component of total contact resistance (Rc). As contact size shrinks and aspect ratios increase, the bulk resistivity of tungsten contacts using conventional PNL and ALD nucleation will be too high to meet the Rc targets of future generation technology nodes. Advanced features like high aspect ratio (HAR) contacts in eDRAM devices with stacked capacitors will require reduced Rc. Historically, resistivity of thin tungsten films has been well above bulk, and an integration worry has been that new thin films with lower resistivity may be needed for future technology nodes [5]. However, a new PNL-based nucleation technique called ‘‘PNL low-Rs W’’ has been developed that results in significantly lower resistivity tungsten films and thus, lower Rc. The purpose of this paper is to present this film and its physical characteristics that help explain the phenomenon of lower resistivity, and show electrical results of this technique on 90 and 65 nm technology node products.
deposition to achieve total desired thickness. Resistivity was calculated from sheet resistance (Rs, 4-point probe) and thickness (XRF) measurements from the equation ð1Þ
q ¼ Rs t;
where q is the tungsten resistivity, Rs is the tungsten sheet resistance, and t is the total tungsten thickness. Additional measurements were performed to characterize film properties on blanket depositions. AFM, SIMS, and XRD analysis were done to measure surface roughness, in-film chemical contamination, and crystal orientation on samples with PNL, ALD, and PNL low-Rs W nucleation and CVD tungsten with a total thickness of 50 nm. 300 mm product wafers were run to validate that lower resistivity on blanket films translates to lower Rc in contacts. In addition, TEM cross sections of contacts were obtained showing relative grain size differences, and SEM cross sections were done to verify fill performance for each type of nucleation method.
2. Experimental 3. Results and discussion 3.1. Resistivity and contact resistance Fig. 2 is a comparison of resistivity vs. total tungsten thickness for blanket films deposited 30
PNL 45 nm
28
Resistivity (µohm-cm)
Blanket tungsten was deposited on 300 mm wafers to compare the effect of various nucleation techniques on the resistivity of tungsten films (nucleation + CVD). The total thickness deposited corresponds to the amount needed to completely fill a contact, or approximately 1/2 of the contact diameter after barrier deposition, as shown in Fig. 1. The range of thicknesses chosen includes technology nodes 120 nm and below. A nucleation layer of about 5 nm was deposited using PNL, ALD, or PNL low-Rs W, followed by CVD
26
ALD Low Rs
65 nm
24
90 nm
22 20
120 nm
18 16 14 12
110
100
90
80
70
60
50
40
30
20
10
XRF Thickness (nm)
Fig. 1. Tungsten thickness required to fill 1/2 of the contact for several technology nodes.
Fig. 2. Total tungsten thickness (5 nm nucleation thickness + CVD) vs. resistivity, with total thicknesses required to fill contacts at various technology nodes also noted.
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
3.2. Physical analysis AFM surface roughness measurements and top–down SEMs, both shown in Fig. 5(a), and
P+ Wide N+ Wide
12
P+ Narrow N+ Narrow
10
Rc %Decrease
8 6 4 2
0.15
0.14
0.13
0.12
0.11
0.1
0 0.09
using 5 nm PNL, ALD, or low-Rs W nucleation. Also indicated is the thickness of tungsten required to fill contacts at various technology nodes. Note that the PNL low-Rs W nucleation results in a significant overall decrease in resistivity compared to the others for all thicknesses measured: 35% lower compared to PNL, and 25% lower compared to ALD. Lower resistivity results are mirrored by improved contact resistance on product. Fig. 3 shows Rc vs. nucleation method for high aspect ratio contacts (6:1) on 90 nm product. Once again, PNL low-Rs W results in significantly lower Rc than the other nucleation methods: 15% lower compared to PNL, and 10% lower compared to ALD. Similar results were seen on all types of contacts (N + /P+, active/poly) and validate that the resistivity trend of the nucleation techniques translates directly to Rc on product. Leakage currents and breakdown voltages were statistically the same for all splits (not shown). The chart in Fig. 4 indicates the percent decrease in Rc for PNL low-Rs W vs. PNL on 65 nm product. For various types of contacts measured, Rc was always improved using PNL low-Rs W nucleation. Also validated is the important phenomenon that as the size of the contact decreases, the benefit of using PNL low-Rs W increases.
263
Contact Size (µm)
Fig. 4. Contact resistance %decrease comparison of low-Rs W vs. PNL nucleation on various contacts measured on 65 nm product. In all cases, low-Rs W improves performance as contact size decreases.
TEM analysis of tungsten filled contacts on product, Fig. 5(b), reveal a significant physical difference between nucleation methods. Tungsten grain size and size distribution are dependent on the type of nucleation used. This is the underlying physical phenomenon that accounts for the differences in resistivity. It was generally observed on both blanket films and in contacts that PNL nucleation produces many small grains of tungsten, ALD nucleation results in a few large grains inter-dispersed among many small grains, and PNL low-Rs W nucleation has many large, flat grains mixed with small grains. The larger the grain size, the fewer the grain boundaries and
Fig. 3. Probability plot of Rc (x-axis) vs. yield (y-axis) for 90 nm node contacts with 6:1 aspect ratio showing significant Rc reduction for the low-Rs process (10% lower than ALD, 15% lower than PNL).
264
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
Fig. 5. Comparison of physical properties of tungsten films deposited with different nucleation techniques. Left column = PNL, center column = ALD, right column = low-Rs W. (a) SEM photos showing roughness and grain size differences of 50 nm blanket tungsten films deposited with different nucleation techniques. (b) TEMs of high aspect ratio contacts comparing tungsten grain size using different nucleation techniques. (c) SIMS analysis results comparing fluorine (F) and boron (B) concentrations on 50 nm tungsten films as a function of nucleation technique. (d) SEM cross sections comparing tungsten fill properties in high aspect ratio contacts on 90 nm product.
the lower the grain boundary resistance. This can explain the difference in resistivity between samples.
PNL low-Rs W process results in the smoothest films 70 nm and thinner, as measured by AFM, suggesting that the nucleation process enables
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
more of a two-dimensional growth compared to the other methods, since the surface is smooth and the grains are large. The PNL film is slightly rougher, while the ALD film is twice as rough, suggesting more three-dimensional growth for these films. This phenomenon is important from the aspect of conformally filling contacts or other holes and minimizing seam voids. The smoother the film, the less significant the seam. Samples of films deposited using each type of nucleation were measured by SIMS analysis for chemical contamination, namely fluorine and boron. Historically, fluorine penetration of the barrier TiN into the Ti liner has been responsible for the formation of volcanoes. It has also been observed that too much boron saturation of PNL and ALD type tungsten films can result in poor adhesion of the tungsten layer. No volcanoes were observed, and scribed tape tests proved good adhesion of tungsten to the barrier. SIMS analysis results are shown in Fig. 5(c). Note that the fluorine levels in the TiN/Ti layers of the PNL low-Rs W film are elevated with respect to the other two samples. This is likely due to the fact that the process temperature is different and facilitates fluorine penetration. Another theory to explain this could be that nucleation that results in more two-dimensional grain growth may be a more open network of nucleation sites that more easily allow fluorine exposure to the barrier. The SIMS analysis shows that boron concentration for the PNL low-Rs film is between that of the other two samples, an acceptable level. Fig. 5(d) shows cross-sectional SEMs of high aspect ratio (6:1) contacts on 90 nm product. There is no observable difference between samples.
265
Similar results were seen on standard contacts (4:1) on 65 nm product with contact to NiSi (not shown).
4. Conclusions In summary, the resistivity of tungsten using PNL low-Rs W nucleation is significantly lower than those using conventional PNL and ALD nucleation processes. The underlying reason for lower resistivity is that the process results in much larger grains, and thus less grain boundary electron scattering. Although the tungsten grains are larger, the surface is also smoother at thicknesses required to fill contacts, suggesting a more twodimensional film growth. It was shown that contacts with 6:1 aspect ratio on 90 nm product could be adequately filled with this process. It was also shown that contact resistance on 90 and 65 nm product was significantly improved using the PNL low-Rs W nucleation process.
References [1] P. Smith, J. Fleming, R. Lujan, E. Roherry-Osmun, D. Roberts, in: Proceedings of Advanced Metallization for ULSI Conference, October 1993. [2] T. Hara, T. Ohba, H. Tsuchikawa, in: Proceedings of ULSIIX Conference, 1994. [3] S.-H. Lee, L. Gonzalez, J. Collins, K. Ashtiani, K. Levy, in: Proceedings of AMC2001, 2001, pp. 649–654. [4] M. Yang et. al., in: Proceedings of AMC2001, 2001, pp. 655–660. [5] S.M. Rossnagel, Semiconductor Fabtech, Edition Number 24, fourth edition, 2004, pp. 13–17.
Low resistivity tungsten for contact metallization Steven Smith a,*, Khaled Aouadi b, Josh Collins c, Eric van der Vegt a, Marie-The´rese Basso b, Marc Juhel b, Simone Pokrant a a
Crolles2 Alliance, Philips Semiconductors, Crolles, France b Crolles2 Alliance, STMicroelectronics, Crolles, France c Novellus Systems, Inc., San Jose, CA, USA Available online 18 August 2005
Abstract A new pulsed nucleation tungsten process, called ‘‘PNL low-Rs W’’, has been developed that results in near-bulk resistivity even in ultra-thin tungsten films. This solution greatly diminishes the need to develop alternative films for contact fill, probably for several generations. The process is done in a conventional, commercially available 300 mm tool with standard hardware and operating conditions. Tungsten films deposited with this technique exhibit significantly larger grain size and a smoother surface compared with conventional PNL and ALD techniques. The larger grains account for lower resistivity due to less grain boundary electron scattering. The process results in higher fluorine content in the liner/barrier, but does not have an adverse effect on contact performance. A comparison of 6:1 aspect ratio contacts showed no difference in fill properties between the PNL low-Rs W and conventional PNL and ALD processes. This new film has been characterized and demonstrated on 65 nm technology node product on contacts to nickel silicide, and on 90 nm technology node product on eDRAM high aspect ratio contacts. Ó 2005 Published by Elsevier B.V. Keywords: Tungsten; Contacts; PNL; ALD; Low resistivity
1. Introduction It is well known from the early development of CVD tungsten that the introduction of a small amount of B2H6 during the nucleation step results *
Corresponding author. Tel.: +33 (0)4 38 92 21 44; fax: +33 (0)4 38 92 21 22. E-mail address: [email protected] (S. Smith). 0167-9317/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.mee.2005.07.032
in decreased tungsten resistivity [1,2]. Today, pulsed nucleation layer (PNL) and atomic layer deposition (ALD) for tungsten nucleation integrated with traditional WF6–H2 CVD are the current state-of-the-art processes for void-free tungsten plug fill of contacts at the 90 nm and lower technology nodes for logic and systemon-chip devices [3,4]. Both utilize B2H6 in order to reduce film resistivity. Tungsten resistivity is a
262
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
significant component of total contact resistance (Rc). As contact size shrinks and aspect ratios increase, the bulk resistivity of tungsten contacts using conventional PNL and ALD nucleation will be too high to meet the Rc targets of future generation technology nodes. Advanced features like high aspect ratio (HAR) contacts in eDRAM devices with stacked capacitors will require reduced Rc. Historically, resistivity of thin tungsten films has been well above bulk, and an integration worry has been that new thin films with lower resistivity may be needed for future technology nodes [5]. However, a new PNL-based nucleation technique called ‘‘PNL low-Rs W’’ has been developed that results in significantly lower resistivity tungsten films and thus, lower Rc. The purpose of this paper is to present this film and its physical characteristics that help explain the phenomenon of lower resistivity, and show electrical results of this technique on 90 and 65 nm technology node products.
deposition to achieve total desired thickness. Resistivity was calculated from sheet resistance (Rs, 4-point probe) and thickness (XRF) measurements from the equation ð1Þ
q ¼ Rs t;
where q is the tungsten resistivity, Rs is the tungsten sheet resistance, and t is the total tungsten thickness. Additional measurements were performed to characterize film properties on blanket depositions. AFM, SIMS, and XRD analysis were done to measure surface roughness, in-film chemical contamination, and crystal orientation on samples with PNL, ALD, and PNL low-Rs W nucleation and CVD tungsten with a total thickness of 50 nm. 300 mm product wafers were run to validate that lower resistivity on blanket films translates to lower Rc in contacts. In addition, TEM cross sections of contacts were obtained showing relative grain size differences, and SEM cross sections were done to verify fill performance for each type of nucleation method.
2. Experimental 3. Results and discussion 3.1. Resistivity and contact resistance Fig. 2 is a comparison of resistivity vs. total tungsten thickness for blanket films deposited 30
PNL 45 nm
28
Resistivity (µohm-cm)
Blanket tungsten was deposited on 300 mm wafers to compare the effect of various nucleation techniques on the resistivity of tungsten films (nucleation + CVD). The total thickness deposited corresponds to the amount needed to completely fill a contact, or approximately 1/2 of the contact diameter after barrier deposition, as shown in Fig. 1. The range of thicknesses chosen includes technology nodes 120 nm and below. A nucleation layer of about 5 nm was deposited using PNL, ALD, or PNL low-Rs W, followed by CVD
26
ALD Low Rs
65 nm
24
90 nm
22 20
120 nm
18 16 14 12
110
100
90
80
70
60
50
40
30
20
10
XRF Thickness (nm)
Fig. 1. Tungsten thickness required to fill 1/2 of the contact for several technology nodes.
Fig. 2. Total tungsten thickness (5 nm nucleation thickness + CVD) vs. resistivity, with total thicknesses required to fill contacts at various technology nodes also noted.
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
3.2. Physical analysis AFM surface roughness measurements and top–down SEMs, both shown in Fig. 5(a), and
P+ Wide N+ Wide
12
P+ Narrow N+ Narrow
10
Rc %Decrease
8 6 4 2
0.15
0.14
0.13
0.12
0.11
0.1
0 0.09
using 5 nm PNL, ALD, or low-Rs W nucleation. Also indicated is the thickness of tungsten required to fill contacts at various technology nodes. Note that the PNL low-Rs W nucleation results in a significant overall decrease in resistivity compared to the others for all thicknesses measured: 35% lower compared to PNL, and 25% lower compared to ALD. Lower resistivity results are mirrored by improved contact resistance on product. Fig. 3 shows Rc vs. nucleation method for high aspect ratio contacts (6:1) on 90 nm product. Once again, PNL low-Rs W results in significantly lower Rc than the other nucleation methods: 15% lower compared to PNL, and 10% lower compared to ALD. Similar results were seen on all types of contacts (N + /P+, active/poly) and validate that the resistivity trend of the nucleation techniques translates directly to Rc on product. Leakage currents and breakdown voltages were statistically the same for all splits (not shown). The chart in Fig. 4 indicates the percent decrease in Rc for PNL low-Rs W vs. PNL on 65 nm product. For various types of contacts measured, Rc was always improved using PNL low-Rs W nucleation. Also validated is the important phenomenon that as the size of the contact decreases, the benefit of using PNL low-Rs W increases.
263
Contact Size (µm)
Fig. 4. Contact resistance %decrease comparison of low-Rs W vs. PNL nucleation on various contacts measured on 65 nm product. In all cases, low-Rs W improves performance as contact size decreases.
TEM analysis of tungsten filled contacts on product, Fig. 5(b), reveal a significant physical difference between nucleation methods. Tungsten grain size and size distribution are dependent on the type of nucleation used. This is the underlying physical phenomenon that accounts for the differences in resistivity. It was generally observed on both blanket films and in contacts that PNL nucleation produces many small grains of tungsten, ALD nucleation results in a few large grains inter-dispersed among many small grains, and PNL low-Rs W nucleation has many large, flat grains mixed with small grains. The larger the grain size, the fewer the grain boundaries and
Fig. 3. Probability plot of Rc (x-axis) vs. yield (y-axis) for 90 nm node contacts with 6:1 aspect ratio showing significant Rc reduction for the low-Rs process (10% lower than ALD, 15% lower than PNL).
264
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
Fig. 5. Comparison of physical properties of tungsten films deposited with different nucleation techniques. Left column = PNL, center column = ALD, right column = low-Rs W. (a) SEM photos showing roughness and grain size differences of 50 nm blanket tungsten films deposited with different nucleation techniques. (b) TEMs of high aspect ratio contacts comparing tungsten grain size using different nucleation techniques. (c) SIMS analysis results comparing fluorine (F) and boron (B) concentrations on 50 nm tungsten films as a function of nucleation technique. (d) SEM cross sections comparing tungsten fill properties in high aspect ratio contacts on 90 nm product.
the lower the grain boundary resistance. This can explain the difference in resistivity between samples.
PNL low-Rs W process results in the smoothest films 70 nm and thinner, as measured by AFM, suggesting that the nucleation process enables
S. Smith et al. / Microelectronic Engineering 82 (2005) 261–265
more of a two-dimensional growth compared to the other methods, since the surface is smooth and the grains are large. The PNL film is slightly rougher, while the ALD film is twice as rough, suggesting more three-dimensional growth for these films. This phenomenon is important from the aspect of conformally filling contacts or other holes and minimizing seam voids. The smoother the film, the less significant the seam. Samples of films deposited using each type of nucleation were measured by SIMS analysis for chemical contamination, namely fluorine and boron. Historically, fluorine penetration of the barrier TiN into the Ti liner has been responsible for the formation of volcanoes. It has also been observed that too much boron saturation of PNL and ALD type tungsten films can result in poor adhesion of the tungsten layer. No volcanoes were observed, and scribed tape tests proved good adhesion of tungsten to the barrier. SIMS analysis results are shown in Fig. 5(c). Note that the fluorine levels in the TiN/Ti layers of the PNL low-Rs W film are elevated with respect to the other two samples. This is likely due to the fact that the process temperature is different and facilitates fluorine penetration. Another theory to explain this could be that nucleation that results in more two-dimensional grain growth may be a more open network of nucleation sites that more easily allow fluorine exposure to the barrier. The SIMS analysis shows that boron concentration for the PNL low-Rs film is between that of the other two samples, an acceptable level. Fig. 5(d) shows cross-sectional SEMs of high aspect ratio (6:1) contacts on 90 nm product. There is no observable difference between samples.
265
Similar results were seen on standard contacts (4:1) on 65 nm product with contact to NiSi (not shown).
4. Conclusions In summary, the resistivity of tungsten using PNL low-Rs W nucleation is significantly lower than those using conventional PNL and ALD nucleation processes. The underlying reason for lower resistivity is that the process results in much larger grains, and thus less grain boundary electron scattering. Although the tungsten grains are larger, the surface is also smoother at thicknesses required to fill contacts, suggesting a more twodimensional film growth. It was shown that contacts with 6:1 aspect ratio on 90 nm product could be adequately filled with this process. It was also shown that contact resistance on 90 and 65 nm product was significantly improved using the PNL low-Rs W nucleation process.
References [1] P. Smith, J. Fleming, R. Lujan, E. Roherry-Osmun, D. Roberts, in: Proceedings of Advanced Metallization for ULSI Conference, October 1993. [2] T. Hara, T. Ohba, H. Tsuchikawa, in: Proceedings of ULSIIX Conference, 1994. [3] S.-H. Lee, L. Gonzalez, J. Collins, K. Ashtiani, K. Levy, in: Proceedings of AMC2001, 2001, pp. 649–654. [4] M. Yang et. al., in: Proceedings of AMC2001, 2001, pp. 655–660. [5] S.M. Rossnagel, Semiconductor Fabtech, Edition Number 24, fourth edition, 2004, pp. 13–17.