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Comparative study of PECVD SiOCH low-k films obtained at different deposition conditions
Microelectronic Engineering 64 (2002) 361–366 www.elsevier.com / locate / mee
Comparative study of PECVD SiOCH low-k films obtained at different deposition conditions D. Shamiryan a,e , *, K. Weidner b , W.D. Gray c , M.R. Baklanov d , S. Vanhaelemeersch a , K. Maex a,e a
IMEC, Leuven, Belgium Dow Corning at IMEC, Leuven, Belgium c Dow Corning Corporation, Midland, MI, USA d XPEQT at IMEC, Leuven, Belgium e Electrical Engineering Department, Katholieke Universiteit, Leuven, Belgium b
Abstract Four CVD SiOCH films deposited at various conditions were used for comparative evaluation. The films were evaluated by RBS, spectroscopic ellipsometry, and ellipsometric porosimetry. Oxygen plasma resistance was studied by spectroscopic ellipsometry and TOF-SIMS analysis after exposure of the films to downstream oxygen plasma. The different deposition conditions result in different carbon content and different porosity. The film with the highest carbon content has the lowest porosity and vice versa. As carbon content of films increases and their porosity decreases, the SiOCH films become more resistant to oxygen plasma. 2002 Elsevier Science B.V. All rights reserved. Keywords: Low-k dielectric; Porosity; Silicon oxycarbide
1. Introduction Minimisation of RC-based electrical delay is one of the issues in ULSI manufacturing. Copper is introduced to reduce resistance while dielectrics with low permittivity (low-k dielectrics) are used instead of conventional SiO 2 aiming at capacitance reduction. Silicon oxycarbide materials (SiOCH) obtained by plasma enhanced chemical vapor deposition (PECVD) are currently being evaluated as low-k dielectrics for advanced interconnections. These films are glassy alloy materials consisting of Si, O, C and H atoms. In this work the effect of deposition conditions on properties of SiOCH low-k dielectrics has been studied. * Corresponding author. E-mail address: [email protected] (D. Shamiryan). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 02 )00809-2
2002 Elsevier Science B.V. All rights reserved.
D. Shamiryan et al. / Microelectronic Engineering 64 (2002) 361–366
362
2. Experimental Using Dow Corning Z3MS as the precursor, the original PECVD SiOCH process is [1,2]: (CH 3 ) 3 SiH 1 N 2 O 1 hn → SiOCH In this work, studies were performed on two variations of this process [3]. The first is the use of oxygen as a replacement for nitrous oxide. This provides an alternative SiOCH process, which mitigates the deep ultra-violet (DUV) resist interaction by elimination of trace nitrogen in the film. The second variant is the addition of a low frequency bias excitation component to the plasma. The films will be referred to as follows: Z3MS for the reference process; Z3MS-SF for the process with N 2 O and single frequency biasing; Z3MS-DF for the process with N 2 O and double frequency biasing; and Z3MS-O2 for the process with O 2 (N-free) and single frequency biasing. Indeed, it was shown that replacement of N 2 O by O 2 as oxidiser results in the absence of nitrogen in the film [3]. The N 2 O process incorporates 1–2 at.% nitrogen into the film, regardless of the plasma bias frequency [3]. Porous structure of the films was analysed by ellipsometric porosimetry [4], where porosity of a film is determined by solvent absorption; resistance to oxygen plasma was evaluated in a Mattson downstream plasma reactor equipped with a Sentech-801 in situ spectroscopic ellipsometer. The samples were exposed to oxygen plasma at 450 mTorr at room temperature for 5 min. Applied power was 420 W. After plasma exposure Z3MS sample was analysed by time-of-flight secondary ions mass-spectroscopy (TOF-SIMS). The samples were also treated in 2% HF solution for 4 min at room temperature. Such treatment is known to increase porosity and pore size of the Z3MS film [5].
3. Results and discussion The difference in deposition conditions results in different carbon / oxygen ratios as was measured before [3] by RBS (Table 1). The silicon and hydrogen contents remain rather stable and are not shown. Fig. 1 shows open porosity and refractive index of the films as a function of carbon content. The open porosity is the volume of all interconnected pores available for solvent penetration. It should be noted that total porosity could be higher than open porosity due to the presence of pores disconnected from the top surface. It was shown that for Z3MS film total porosity could be double that of open Table 1 Film composition and density measured by RBS Film type
Carbon content (at.%)
Oxygen content (at.%)
Film density (g / cm 3 )
Z3MS-O2 Z3MS Z3MS-DF Z3MS-SF
12 20 28 32
36 30 18 15
0.97 – 1.08 1.17
Si and H show almost no difference and are not included in the table.
D. Shamiryan et al. / Microelectronic Engineering 64 (2002) 361–366
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Fig. 1. Porosity (open circles) and refractive index (filled squares) at 633 nm of the SiOCH films as a function of carbon content. Higher porosity corresponds to lower refractive index.
porosity [5]. Although total porosity cannot be determined by ellipsometric porosimetry, measuring open porosity is also important. Open pores mostly influence the properties of porous material during processing (e.g. by absorbing chemicals or water). Porosity of the films increases with decrease of carbon content. Increase of refractive index might be a result of two factors: (i) increase of the skeleton refractive index due to replacement of SiO 2 (RI51.46 at 633 nm) by SiC (RI|2.0 at 633 nm); and (ii) increase of the film density (or decrease of porosity). Porosity data are consistent with the film density data obtained by RBS—lower carbon content corresponds to lower density (Table 1) and higher porosity. During solvent adsorption, which is used in ellipsometric porosimetry, swelling of the films has been observed. Fig. 2 demonstrates good agreement between swelling percentage and the hardness data obtained earlier [3]. The harder the film the less it swells during solvent adsorption.
Fig. 2. Correlation between film hardness and film swelling during solvent adsorption. Adsorption of solvent is used for porosity measurement [4].
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Table 2 Change of the film refractive index after treatment in 2% HF solution for 5 min Film type
RI decrease
Z3MS-O2 Z3MS Z3MS-DF Z3MS-SF
n/a 0.145 0.005 0
Decrease in refractive index (determined as the difference between refractive indices before and after HF treatment) reflects increase of the film porosity. Z3MS-O2 film was completely removed by such treatment.
Table 2 summarises the results of film treatment in 2% HF. Increase in Z3MS porosity results in substantial refractive index decrease (refractive index decrease of 0.145 corresponds to porosity increase from 7 to 35% [5]) while thickness of the film remains stable. In the case of the highest oxygen content (Z3MS-O2) the film was completely removed due to Si–O bonds breaking by fluorine. No change in refractive index was observed for the film with the highest carbon content (Z3MS-SF) due to lack of Si–O bonds to be broken by fluorine. Refractive index decrease of Z3MS-DF (which has carbon content between Z3MS and Z3MS-SF) is very small. Apparently, Z3MS has an oxygen / carbon ratio, which is optimal for such modification. Exposure of the Z3MS films to oxygen plasma results in oxidation of the top layer. Fig. 3 shows concentration profiles of carbon and oxygen obtained by TOF-SIMS after oxygen plasma treatment of the Z3MS film. The carbon concentration (Fig. 3(a)) drastically decreased in the top layer while the oxygen concentration (Fig. 3(b)) increased. The silicon concentration (not shown) did not change. One can conclude that the top layer of the film is oxidised and converted to some kind of silicon oxide. This oxidised layer was shown to be hydrophilic [6]. The thickness of the top oxidised layers has been measured by spectroscopic ellipsometry using a two-layer model. The optical constants of the remaining SiOCH film were supposed to be unchanged. The results are presented in Fig. 4. The total height of the bar represents the thickness of the film before oxygen treatment, the black part represents
Fig. 3. Oxygen (a) and carbon (b) concentration profiles of the Z3MS film obtained by TOF-SIMS. The thin curves marked with open circles represent the pristine film and the bold curves with filled squares represent the film after 5-min oxygen plasma treatment. Changes in the top layer of the film indicate conversion to an SiO 2 -like material.
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365
Fig. 4. Thickness of the different parts of the films after oxygen plasma treatment measured by spectroscopic ellipsometer. Total column height corresponds to the film thickness before oxygen plasma treatment, black part is unchanged SiOCH, and hatched part represents the top modified SiO 2 -like layer.
bulk SiOCH, while the hatched part shows the oxidised layer (designated as SiO 2 ). The refractive index of the oxidised layer was found to be 1.32–1.35 at 632.8 nm. The lowered refractive index (as compared to 1.46 for SiO 2 ) could indicate decreased density of the oxide. The thickness of the top oxidised layer versus carbon content of the film is plotted in Fig. 5. Films with higher carbon content show thinner oxidised layer or, in other words, higher resistance to oxygen plasma. However, films
Fig. 5. Thickness of the SiO 2 -like layer formed after oxygen plasma treatment as a function of carbon content of the film. Films with higher carbon content are more resistant to such treatment.
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with higher carbon content also have lower porosity, which also can be a reason for increased oxygen plasma resistance.
4. Conclusions The different deposition conditions of the SiOCH film result in different properties. The film obtained with O 2 instead of N 2 O as an oxidiser has the highest oxygen content. The film obtained with single-frequency substrate bias has the lowest oxygen concentration. Properties of the films can be correlated with the carbon / oxygen ratio of the films. With increase of carbon / oxygen ratio, porosity as well as hardness of the films decreases and resistance to oxygen plasma increases. However, it is difficult to determine whether the cause of the increased plasma is increased carbon content or decreased porosity.
Acknowledgements We would like to thank Thierry Connard for performing TOF-SIMS measurements and Ivan Callant for technical support of the equipment.
References [1] M.J. Loboda, J.A. Seifferly, R.F. Schneider, C.M. Grove, in: P.C. Andricacos, J.O. Dukovic, G.S. Mathad et al. (Eds.), Deposition of Low-K Dielectric Films Using Trimethylsilane, 194th Electrochem. Soc. Proc. of Interconnect and Contact Metallization, Boston, 1998, p. 118. [2] M.J. Loboda, in: Proc. AMC, ULSI XV, MRS, 2000, p. 371. [3] M.J. Loboda, W.D. Gray, B.K. Hwang, J.A. Seifferly, R.F. Schneider, in: A.J. McKerrow, Y. Shacham-Diamand, S. Zaima, T. Ohba (Eds.), Oxidation and plasma RF bias frequency effects in the PECVD growth of Si w C x O y H z dielectric films, MRS Proc. of Advanced Metallization Conference, Montreal, 2001, p. 325. [4] F.N. Dultsev, M.R. Baklanov, Electrochem. Solid-State Lett. 2 (1999) 192. [5] D. Shamiryan, M.R. Baklanov, S. Vanhaelemeersch, K. Maex, Electrochem. Solid-State Lett. 4 (2001) F3. [6] D. Shamiryan, M.R. Baklanov, S. Vanhaelemeersch, K. Maex. Electrochem. J. Vac. Sci. Technol. B 20(5) (2002), to be published.
Comparative study of PECVD SiOCH low-k films obtained at different deposition conditions D. Shamiryan a,e , *, K. Weidner b , W.D. Gray c , M.R. Baklanov d , S. Vanhaelemeersch a , K. Maex a,e a
IMEC, Leuven, Belgium Dow Corning at IMEC, Leuven, Belgium c Dow Corning Corporation, Midland, MI, USA d XPEQT at IMEC, Leuven, Belgium e Electrical Engineering Department, Katholieke Universiteit, Leuven, Belgium b
Abstract Four CVD SiOCH films deposited at various conditions were used for comparative evaluation. The films were evaluated by RBS, spectroscopic ellipsometry, and ellipsometric porosimetry. Oxygen plasma resistance was studied by spectroscopic ellipsometry and TOF-SIMS analysis after exposure of the films to downstream oxygen plasma. The different deposition conditions result in different carbon content and different porosity. The film with the highest carbon content has the lowest porosity and vice versa. As carbon content of films increases and their porosity decreases, the SiOCH films become more resistant to oxygen plasma. 2002 Elsevier Science B.V. All rights reserved. Keywords: Low-k dielectric; Porosity; Silicon oxycarbide
1. Introduction Minimisation of RC-based electrical delay is one of the issues in ULSI manufacturing. Copper is introduced to reduce resistance while dielectrics with low permittivity (low-k dielectrics) are used instead of conventional SiO 2 aiming at capacitance reduction. Silicon oxycarbide materials (SiOCH) obtained by plasma enhanced chemical vapor deposition (PECVD) are currently being evaluated as low-k dielectrics for advanced interconnections. These films are glassy alloy materials consisting of Si, O, C and H atoms. In this work the effect of deposition conditions on properties of SiOCH low-k dielectrics has been studied. * Corresponding author. E-mail address: [email protected] (D. Shamiryan). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 02 )00809-2
2002 Elsevier Science B.V. All rights reserved.
D. Shamiryan et al. / Microelectronic Engineering 64 (2002) 361–366
362
2. Experimental Using Dow Corning Z3MS as the precursor, the original PECVD SiOCH process is [1,2]: (CH 3 ) 3 SiH 1 N 2 O 1 hn → SiOCH In this work, studies were performed on two variations of this process [3]. The first is the use of oxygen as a replacement for nitrous oxide. This provides an alternative SiOCH process, which mitigates the deep ultra-violet (DUV) resist interaction by elimination of trace nitrogen in the film. The second variant is the addition of a low frequency bias excitation component to the plasma. The films will be referred to as follows: Z3MS for the reference process; Z3MS-SF for the process with N 2 O and single frequency biasing; Z3MS-DF for the process with N 2 O and double frequency biasing; and Z3MS-O2 for the process with O 2 (N-free) and single frequency biasing. Indeed, it was shown that replacement of N 2 O by O 2 as oxidiser results in the absence of nitrogen in the film [3]. The N 2 O process incorporates 1–2 at.% nitrogen into the film, regardless of the plasma bias frequency [3]. Porous structure of the films was analysed by ellipsometric porosimetry [4], where porosity of a film is determined by solvent absorption; resistance to oxygen plasma was evaluated in a Mattson downstream plasma reactor equipped with a Sentech-801 in situ spectroscopic ellipsometer. The samples were exposed to oxygen plasma at 450 mTorr at room temperature for 5 min. Applied power was 420 W. After plasma exposure Z3MS sample was analysed by time-of-flight secondary ions mass-spectroscopy (TOF-SIMS). The samples were also treated in 2% HF solution for 4 min at room temperature. Such treatment is known to increase porosity and pore size of the Z3MS film [5].
3. Results and discussion The difference in deposition conditions results in different carbon / oxygen ratios as was measured before [3] by RBS (Table 1). The silicon and hydrogen contents remain rather stable and are not shown. Fig. 1 shows open porosity and refractive index of the films as a function of carbon content. The open porosity is the volume of all interconnected pores available for solvent penetration. It should be noted that total porosity could be higher than open porosity due to the presence of pores disconnected from the top surface. It was shown that for Z3MS film total porosity could be double that of open Table 1 Film composition and density measured by RBS Film type
Carbon content (at.%)
Oxygen content (at.%)
Film density (g / cm 3 )
Z3MS-O2 Z3MS Z3MS-DF Z3MS-SF
12 20 28 32
36 30 18 15
0.97 – 1.08 1.17
Si and H show almost no difference and are not included in the table.
D. Shamiryan et al. / Microelectronic Engineering 64 (2002) 361–366
363
Fig. 1. Porosity (open circles) and refractive index (filled squares) at 633 nm of the SiOCH films as a function of carbon content. Higher porosity corresponds to lower refractive index.
porosity [5]. Although total porosity cannot be determined by ellipsometric porosimetry, measuring open porosity is also important. Open pores mostly influence the properties of porous material during processing (e.g. by absorbing chemicals or water). Porosity of the films increases with decrease of carbon content. Increase of refractive index might be a result of two factors: (i) increase of the skeleton refractive index due to replacement of SiO 2 (RI51.46 at 633 nm) by SiC (RI|2.0 at 633 nm); and (ii) increase of the film density (or decrease of porosity). Porosity data are consistent with the film density data obtained by RBS—lower carbon content corresponds to lower density (Table 1) and higher porosity. During solvent adsorption, which is used in ellipsometric porosimetry, swelling of the films has been observed. Fig. 2 demonstrates good agreement between swelling percentage and the hardness data obtained earlier [3]. The harder the film the less it swells during solvent adsorption.
Fig. 2. Correlation between film hardness and film swelling during solvent adsorption. Adsorption of solvent is used for porosity measurement [4].
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Table 2 Change of the film refractive index after treatment in 2% HF solution for 5 min Film type
RI decrease
Z3MS-O2 Z3MS Z3MS-DF Z3MS-SF
n/a 0.145 0.005 0
Decrease in refractive index (determined as the difference between refractive indices before and after HF treatment) reflects increase of the film porosity. Z3MS-O2 film was completely removed by such treatment.
Table 2 summarises the results of film treatment in 2% HF. Increase in Z3MS porosity results in substantial refractive index decrease (refractive index decrease of 0.145 corresponds to porosity increase from 7 to 35% [5]) while thickness of the film remains stable. In the case of the highest oxygen content (Z3MS-O2) the film was completely removed due to Si–O bonds breaking by fluorine. No change in refractive index was observed for the film with the highest carbon content (Z3MS-SF) due to lack of Si–O bonds to be broken by fluorine. Refractive index decrease of Z3MS-DF (which has carbon content between Z3MS and Z3MS-SF) is very small. Apparently, Z3MS has an oxygen / carbon ratio, which is optimal for such modification. Exposure of the Z3MS films to oxygen plasma results in oxidation of the top layer. Fig. 3 shows concentration profiles of carbon and oxygen obtained by TOF-SIMS after oxygen plasma treatment of the Z3MS film. The carbon concentration (Fig. 3(a)) drastically decreased in the top layer while the oxygen concentration (Fig. 3(b)) increased. The silicon concentration (not shown) did not change. One can conclude that the top layer of the film is oxidised and converted to some kind of silicon oxide. This oxidised layer was shown to be hydrophilic [6]. The thickness of the top oxidised layers has been measured by spectroscopic ellipsometry using a two-layer model. The optical constants of the remaining SiOCH film were supposed to be unchanged. The results are presented in Fig. 4. The total height of the bar represents the thickness of the film before oxygen treatment, the black part represents
Fig. 3. Oxygen (a) and carbon (b) concentration profiles of the Z3MS film obtained by TOF-SIMS. The thin curves marked with open circles represent the pristine film and the bold curves with filled squares represent the film after 5-min oxygen plasma treatment. Changes in the top layer of the film indicate conversion to an SiO 2 -like material.
D. Shamiryan et al. / Microelectronic Engineering 64 (2002) 361–366
365
Fig. 4. Thickness of the different parts of the films after oxygen plasma treatment measured by spectroscopic ellipsometer. Total column height corresponds to the film thickness before oxygen plasma treatment, black part is unchanged SiOCH, and hatched part represents the top modified SiO 2 -like layer.
bulk SiOCH, while the hatched part shows the oxidised layer (designated as SiO 2 ). The refractive index of the oxidised layer was found to be 1.32–1.35 at 632.8 nm. The lowered refractive index (as compared to 1.46 for SiO 2 ) could indicate decreased density of the oxide. The thickness of the top oxidised layer versus carbon content of the film is plotted in Fig. 5. Films with higher carbon content show thinner oxidised layer or, in other words, higher resistance to oxygen plasma. However, films
Fig. 5. Thickness of the SiO 2 -like layer formed after oxygen plasma treatment as a function of carbon content of the film. Films with higher carbon content are more resistant to such treatment.
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with higher carbon content also have lower porosity, which also can be a reason for increased oxygen plasma resistance.
4. Conclusions The different deposition conditions of the SiOCH film result in different properties. The film obtained with O 2 instead of N 2 O as an oxidiser has the highest oxygen content. The film obtained with single-frequency substrate bias has the lowest oxygen concentration. Properties of the films can be correlated with the carbon / oxygen ratio of the films. With increase of carbon / oxygen ratio, porosity as well as hardness of the films decreases and resistance to oxygen plasma increases. However, it is difficult to determine whether the cause of the increased plasma is increased carbon content or decreased porosity.
Acknowledgements We would like to thank Thierry Connard for performing TOF-SIMS measurements and Ivan Callant for technical support of the equipment.
References [1] M.J. Loboda, J.A. Seifferly, R.F. Schneider, C.M. Grove, in: P.C. Andricacos, J.O. Dukovic, G.S. Mathad et al. (Eds.), Deposition of Low-K Dielectric Films Using Trimethylsilane, 194th Electrochem. Soc. Proc. of Interconnect and Contact Metallization, Boston, 1998, p. 118. [2] M.J. Loboda, in: Proc. AMC, ULSI XV, MRS, 2000, p. 371. [3] M.J. Loboda, W.D. Gray, B.K. Hwang, J.A. Seifferly, R.F. Schneider, in: A.J. McKerrow, Y. Shacham-Diamand, S. Zaima, T. Ohba (Eds.), Oxidation and plasma RF bias frequency effects in the PECVD growth of Si w C x O y H z dielectric films, MRS Proc. of Advanced Metallization Conference, Montreal, 2001, p. 325. [4] F.N. Dultsev, M.R. Baklanov, Electrochem. Solid-State Lett. 2 (1999) 192. [5] D. Shamiryan, M.R. Baklanov, S. Vanhaelemeersch, K. Maex, Electrochem. Solid-State Lett. 4 (2001) F3. [6] D. Shamiryan, M.R. Baklanov, S. Vanhaelemeersch, K. Maex. Electrochem. J. Vac. Sci. Technol. B 20(5) (2002), to be published.