- Email: [email protected]
High resistivity and low dielectric constant amorphous carbon nitride films: application to low-k materials for ULSI
Diamond and Related Materials 11 (2002) 1219–1222
High resistivity and low dielectric constant amorphous carbon nitride films: application to low-k materials for ULSI Masami Aono, Shoji Nitta* Department of Electrical Engineering, Gifu University, 1-1 Yanaido, Gifu 501-1193, Japan
Abstract Amorphous carbon nitride films, a-CNx, have been candidates for interlayer insulator materials on ultra large-scale integration (ULSI). The most important property of insulators for ULSI application is low dielectric constant, i.e. low-k. It is reported in this paper the success in preparing carbon nitride films with dielectric constant less than 2. The sample films are prepared by a layerby-layer method. The preparation consists of two cyclic processes. First process is a preparation of a-CNx thin layer by a nitrogen radical sputtering. Second process is a treatment of a-CNx layer by atomic hydrogen. To understand the characteristic low-k by atomic hydrogen treatment and by a layer-by-layer process, we have studied the effect of hydrogen radicals to a-CNx to get lower dielectric constant materials. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous; Carbon; Nitrides; Insulator; Low-k
1. Introduction In very large-scale integrated circuit (VLSI) and ultra large-scale integrated circuit (ULSI), the dielectric constant of the insulation membrane is becoming an important parameter related with interconnection delay w1–3x. The interconnection delay caused by parasitic capacitance has been getting more attention. The performance of ULSI requires a new wiring material with low resistance and a new interlayer insulator with low dielectric constant which is called low-k materials. Silicon oxide films have been used as interlayer films and gate oxides. Dielectric constant of silicon dioxide SiO2 is approximately 3.9–4.2. New low-k materials to replace with SiO2, especially k-2.3 are expected to be found, but not yet w4x. We have studied the mechanism that governs the reduction in the dielectric constant of amorphous carbon nitride a-CNx films. Previously, we have found that aCNx films with low-k can be prepared by adding a hydrogen- or oxygen-plasma treatment process w5x. We have also found the reduction in both the orientation and electronic polarization components, by the hydrogen plasma treatment, in the dielectric constant of a-CNx films. *Corresponding author. Tel.: q81-58-293-2675; fax: q81-58-2301894. E-mail address: [email protected] (S. Nitta).
In this work, we studied the contribution of atomic hydrogen treatment to the dielectric constant of these films. Low dielectric constant a-CNx films were prepared by a layer-by-layer method (LL). 2. Sample preparation and experiments The effect of atomic hydrogen treatment is mainly the etching of surfaces on a-CNx films. The main effects of the atomic hydrogen etching are changing C–N bonding states and increasing the sp3-bonding fraction w5x. Atomic hydrogen etches and affects the bonding states in a limited depth from the surface of a-CNx films. To obtain atomic hydrogen processed a-CNx in almost whole films, we prepared LLa-CNx by a layerby-layer method. Fig. 1 shows a schematic diagram of LLa-CNx preparation method that consists of two cyclic processes. First process is a preparation of a-CNx thin layer by a nitrogen radical sputtering of a graphite target. Second process is the treatment of a-CNx layer by atomic hydrogen. The substrate temperature was set at room temperature. a-CNx films were prepared by a magnetron sputtering operated at a radio frequency (RF), of 13.56 MHz and power of 85 W. RF source of hydrogen etching process was under the same conditions as that for a-CNx. The reactive gas was N2 purity of 99.9999%. N2 gas was kept at a pressure of 0.12 torr. Hydrogen
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 7 1 8 - X
M. Aono, S. Nitta / Diamond and Related Materials 11 (2002) 1219–1222
1220
3. Results and discussions
Fig. 1. A schematic diagram of a layer-by-layer method.
was used to prepare atomic hydrogen and its pressure was kept at 0.5 torr. A target of graphite with purity at 99.999%, and 3 inches in diameter, was set below 43 mm from the substrate. The target was in-situ during the hydrogen etching process. However, no indication of hydrogen inclusion with atomic hydrogen etching to a-CNx was confirmed in the infrared absorption spectra w6x. The thickness of one layer of a-CNx was controlled by the nitrogen gas flow time, i.e. by a deposition time using a microcomputer system. The atomic hydrogen treatment time was fixed at 40 sylayer. The dielectric constants of the deposited films were obtained from the C–V characteristic using the metal– insulator–metal (MIM), structure at 1 MHz. The capacitance was measured with a Hewlett–Packard-4285 LCR meter. Vacuum evaporated aluminum films were used as the electrode metal in C–V measurements. Substrates were Corning 7059 glass. The nitrogen contents xsnitrogenycarbon was obtained from X-ray photoelectron spectroscopy using a Shimazu ESCA-850. sp3 fraction of carbon was estimated from Raman spectra w7,8x. The film thickness and refractive index were determined using a UV-vis transmittance spectroscopy. The value of the refractive index n was calculated by: nŽl.sNqyN2yn0ns
LLa-CNx films have higher resistivity and lower dielectric constant than that of a-CNx w9x. The highest resistivity of LLa-CNx obtained using gap electrodes was approximately 1018 V-cm, which was approximately 103 times greater than that of a-CNx. The resistivity with parallel electrodes of a-CNx obtained from the LCR meter is shown in Fig. 2. The etching by atomic hydrogen decreased a component of graphite-like carbon and increased the resistivity of a sample with small one layer thickness dl in LLa-CNx. A cause of different resistivity obtained from electrical conductivity and that from a LCR meter is the use of different shaped electrodes. For the electrical conductivity, gap electrodes were used on a sample surface, and for a LCR meter sandwich-type electrodes were used. In the sandwichtype electrodes case, there is a possibility of a decrease in resistivity by the penetration of aluminum used as electrodes. More time is required to study this point. The capacitances of LLa-CNx films were measured by a LCR meter at frequency of 1 MHz. The dielectric constant of LLa-CNx was lower than that of a-CNx. The dependence of dielectric constant ´ on one layer thickness dl in LLa-CNx at 1 MHz is shown in Fig. 3. We found that atomic hydrogen treatment decreased effectively the dielectric constant of LLa-CNx down to 1.9 at dl ;4 nmycycle. Three types of polarization contribute to capacitance in this frequency range: 1. Electronic polarization ae: dielectric constant ´e. 2. Ionic polarization ai: dielectric constant ´i. 3. Orientation polarization ao: dielectric constant ´o. It is well known that the dielectric constant is frequency-dependent w10x. The dielectric constant of electronic polarization ´e can be obtained from the refractive index in the visible
(1)
where, Ns
8n2s Žn0qns.3
yTmaxTmin
where n0 is the refractive index of air, and ns is the refractive index of substrate, respectively. The transmittance Tmax and Tmin of sample denotes the maximum and the minimum in the interference pattern. The value of film thickness d is given by: ds
Dl 4pn
(2)
where Dl is the difference of wavelengths at transmittance spectrum peaks.
Fig. 2. Dependence of the resistivity of LLa-CNx on one layer thickness dl.
M. Aono, S. Nitta / Diamond and Related Materials 11 (2002) 1219–1222
Fig. 3. Dependence of the dielectric constant at 1 MHz of LLa-CNx on one layer thickness dl.
1221
Fig. 5. Dependence of the dielectric constant of electric polarization ´e on a layer thickness dl.
region. ´e is equal to the square of the refractive index n. The refractive index of a-CNx decreased with nitrogen contents x w9x. The nitrogen contents x of a-CNx can be increased with the substrate temperature w9x. As shown in Fig. 4, the refractive index of LLa-CNx drops sharply when nitrogen content x is approaching is 0.7. Contrary to LLa-CNx, refractive index n of a-CNx decreased gradually with x. As shown in Fig. 5, ´e of LLa-CNx decreased with dl. The atomic hydrogen may etch the highly polarized bonds such as C_C, C^C, C_N and C^N, for which results have been estimated from XPS spectra w5x. Dielectric constants of ionic polarization ´i can be obtained by using the Kramers–Kronig relationship from the measured FT-IR spectra w10,11x. The dielectric constant in the infrared region is divided into two components, i.e. ionic and electronic polarization. In brief, we used the measured absorbance and film thickness to obtain the damping factor, k. Then we used the following Eq. (3) to calculate the refractive index n
where P is the principle value of the integral and subscript i indicates the data point. These values of n and k were then used to calculate the complex dielectric constants. The FT-IR spectra used in our calculations were in the range of 350–6500 cmy1, because it is not possible to measure the absorption spectra from 0 to 8. Fig. 6 shows that ´i decreased with the thickness dl of one a-CNx layer in LLa-CNx. Figs. 5 and 6 show that electronic and ionic components of dielectric constant are decreased with the decrease of dl. Therefore, the atomic hydrogen treatment and a layer-by-layer method are the effective technique for the processing in order to get low dielectric constant a-CNx films, though the hydrogen radicals do not diffuse into a-CNx films.
Fig. 4. Dependence of the refractive indices of a-CNx and LLa-CNx on nitrogen contents x.
Fig. 6. Dependence of the dielectric constant of ionic polarization ´i on a layer thickness dl.
2 nisn`q P p
`
|
´risni2yki2
0
vkŽv. v2yvi2
dv
(3) (4)
1222
M. Aono, S. Nitta / Diamond and Related Materials 11 (2002) 1219–1222
4. Summary We found that a layer-by-layer method using atomic hydrogen treatment of a-CNx decreased the dielectric constant at 1 MHz down to 1.9. The atomic hydrogen treatment modifies C–N bonding states and increases sp3 fraction in a-CNx, producing a stereophonic structure. As a result, the refractive index and the dielectric constant of ionic polarization decrease with decreasing one layer thickness dl of a-CNx. The resistivity of LLaCNx is larger by three orders than that of a-CNx. Thus, atomic hydrogen etching is an effective technique for the processing of a-CNx films to get lower dielectric constant materials. Acknowledgments The authors would like to thank Mr H. Horibata and Mr T. Katsuno for their help in the sample preparation. This work was supported in part by the Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References w1x C.Y. Chang, S.M. Sze, ULSI Technology, McGraw-Hill Companies, INC, 1996. w2x S. Wolf, Silicon Processing for the VLSI Era,, The submicron MOSFET, vol. 3, Lattice Press, Sunset Beach, California, 1995. w3x R.D. Miller, Science 286 (1999) 422. w4x Examples: ‘Low-Dielectric Constant Materials I;V’, Material Research Society Symposium Proceedings, Vols. 384, 443, 476, 511 and 565 (1995–1999). w5x M. Aono, Y. Naruse, S. Nitta, T. Katsuno, Diamond Relat. Mater. 10 (2001) 1147. w6x S. Nitta, N. Takada, K. Sugiyama, T. Itoh, S. Nonomura, J. Non-Cryst. Solids 227–230 (1998) 655. w7x A.C. Ferrari, B. Kleinsorge, G. Adamopoulos, J. Robertson, W.I. Milne, V. Stojolan, L.M. Brown, A. LiBassi, B.K. Tanner, J. Non-Cryst. Solids 266–269 (2000) 765. w8x S.E. Rodil, A.C. Ferrari, J. Robertson, W.I. Milne, J. Appl. Phys. 89 (10) (2001) 5425. w9x M. Aono, S. Nitta, T. Iwasaki, H. Yokoi, T. Itoh, S. Nonomura, ‘Low-Dielectric Constant Materials’, VJ. Hummel, K. Endo, W.W. Lee, M. Mills, S.-Q. Wang, (Eds) Mat. Res. Soc. Symp. Proc., Vol. 565 (MRS, Pennsylvania, 1999) 291. w10x B.K.P. Scaife, Principles of Dielectrics, Clarendon Press, Oxford, 1998. w11x S.W. Lim, Y. Shimogaki, Y. Nakano, K. Tada, H. Komiyama, Jpn. J. Appl. Phys. 35 (1996) 1468.
High resistivity and low dielectric constant amorphous carbon nitride films: application to low-k materials for ULSI Masami Aono, Shoji Nitta* Department of Electrical Engineering, Gifu University, 1-1 Yanaido, Gifu 501-1193, Japan
Abstract Amorphous carbon nitride films, a-CNx, have been candidates for interlayer insulator materials on ultra large-scale integration (ULSI). The most important property of insulators for ULSI application is low dielectric constant, i.e. low-k. It is reported in this paper the success in preparing carbon nitride films with dielectric constant less than 2. The sample films are prepared by a layerby-layer method. The preparation consists of two cyclic processes. First process is a preparation of a-CNx thin layer by a nitrogen radical sputtering. Second process is a treatment of a-CNx layer by atomic hydrogen. To understand the characteristic low-k by atomic hydrogen treatment and by a layer-by-layer process, we have studied the effect of hydrogen radicals to a-CNx to get lower dielectric constant materials. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous; Carbon; Nitrides; Insulator; Low-k
1. Introduction In very large-scale integrated circuit (VLSI) and ultra large-scale integrated circuit (ULSI), the dielectric constant of the insulation membrane is becoming an important parameter related with interconnection delay w1–3x. The interconnection delay caused by parasitic capacitance has been getting more attention. The performance of ULSI requires a new wiring material with low resistance and a new interlayer insulator with low dielectric constant which is called low-k materials. Silicon oxide films have been used as interlayer films and gate oxides. Dielectric constant of silicon dioxide SiO2 is approximately 3.9–4.2. New low-k materials to replace with SiO2, especially k-2.3 are expected to be found, but not yet w4x. We have studied the mechanism that governs the reduction in the dielectric constant of amorphous carbon nitride a-CNx films. Previously, we have found that aCNx films with low-k can be prepared by adding a hydrogen- or oxygen-plasma treatment process w5x. We have also found the reduction in both the orientation and electronic polarization components, by the hydrogen plasma treatment, in the dielectric constant of a-CNx films. *Corresponding author. Tel.: q81-58-293-2675; fax: q81-58-2301894. E-mail address: [email protected] (S. Nitta).
In this work, we studied the contribution of atomic hydrogen treatment to the dielectric constant of these films. Low dielectric constant a-CNx films were prepared by a layer-by-layer method (LL). 2. Sample preparation and experiments The effect of atomic hydrogen treatment is mainly the etching of surfaces on a-CNx films. The main effects of the atomic hydrogen etching are changing C–N bonding states and increasing the sp3-bonding fraction w5x. Atomic hydrogen etches and affects the bonding states in a limited depth from the surface of a-CNx films. To obtain atomic hydrogen processed a-CNx in almost whole films, we prepared LLa-CNx by a layerby-layer method. Fig. 1 shows a schematic diagram of LLa-CNx preparation method that consists of two cyclic processes. First process is a preparation of a-CNx thin layer by a nitrogen radical sputtering of a graphite target. Second process is the treatment of a-CNx layer by atomic hydrogen. The substrate temperature was set at room temperature. a-CNx films were prepared by a magnetron sputtering operated at a radio frequency (RF), of 13.56 MHz and power of 85 W. RF source of hydrogen etching process was under the same conditions as that for a-CNx. The reactive gas was N2 purity of 99.9999%. N2 gas was kept at a pressure of 0.12 torr. Hydrogen
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 7 1 8 - X
M. Aono, S. Nitta / Diamond and Related Materials 11 (2002) 1219–1222
1220
3. Results and discussions
Fig. 1. A schematic diagram of a layer-by-layer method.
was used to prepare atomic hydrogen and its pressure was kept at 0.5 torr. A target of graphite with purity at 99.999%, and 3 inches in diameter, was set below 43 mm from the substrate. The target was in-situ during the hydrogen etching process. However, no indication of hydrogen inclusion with atomic hydrogen etching to a-CNx was confirmed in the infrared absorption spectra w6x. The thickness of one layer of a-CNx was controlled by the nitrogen gas flow time, i.e. by a deposition time using a microcomputer system. The atomic hydrogen treatment time was fixed at 40 sylayer. The dielectric constants of the deposited films were obtained from the C–V characteristic using the metal– insulator–metal (MIM), structure at 1 MHz. The capacitance was measured with a Hewlett–Packard-4285 LCR meter. Vacuum evaporated aluminum films were used as the electrode metal in C–V measurements. Substrates were Corning 7059 glass. The nitrogen contents xsnitrogenycarbon was obtained from X-ray photoelectron spectroscopy using a Shimazu ESCA-850. sp3 fraction of carbon was estimated from Raman spectra w7,8x. The film thickness and refractive index were determined using a UV-vis transmittance spectroscopy. The value of the refractive index n was calculated by: nŽl.sNqyN2yn0ns
LLa-CNx films have higher resistivity and lower dielectric constant than that of a-CNx w9x. The highest resistivity of LLa-CNx obtained using gap electrodes was approximately 1018 V-cm, which was approximately 103 times greater than that of a-CNx. The resistivity with parallel electrodes of a-CNx obtained from the LCR meter is shown in Fig. 2. The etching by atomic hydrogen decreased a component of graphite-like carbon and increased the resistivity of a sample with small one layer thickness dl in LLa-CNx. A cause of different resistivity obtained from electrical conductivity and that from a LCR meter is the use of different shaped electrodes. For the electrical conductivity, gap electrodes were used on a sample surface, and for a LCR meter sandwich-type electrodes were used. In the sandwichtype electrodes case, there is a possibility of a decrease in resistivity by the penetration of aluminum used as electrodes. More time is required to study this point. The capacitances of LLa-CNx films were measured by a LCR meter at frequency of 1 MHz. The dielectric constant of LLa-CNx was lower than that of a-CNx. The dependence of dielectric constant ´ on one layer thickness dl in LLa-CNx at 1 MHz is shown in Fig. 3. We found that atomic hydrogen treatment decreased effectively the dielectric constant of LLa-CNx down to 1.9 at dl ;4 nmycycle. Three types of polarization contribute to capacitance in this frequency range: 1. Electronic polarization ae: dielectric constant ´e. 2. Ionic polarization ai: dielectric constant ´i. 3. Orientation polarization ao: dielectric constant ´o. It is well known that the dielectric constant is frequency-dependent w10x. The dielectric constant of electronic polarization ´e can be obtained from the refractive index in the visible
(1)
where, Ns
8n2s Žn0qns.3
yTmaxTmin
where n0 is the refractive index of air, and ns is the refractive index of substrate, respectively. The transmittance Tmax and Tmin of sample denotes the maximum and the minimum in the interference pattern. The value of film thickness d is given by: ds
Dl 4pn
(2)
where Dl is the difference of wavelengths at transmittance spectrum peaks.
Fig. 2. Dependence of the resistivity of LLa-CNx on one layer thickness dl.
M. Aono, S. Nitta / Diamond and Related Materials 11 (2002) 1219–1222
Fig. 3. Dependence of the dielectric constant at 1 MHz of LLa-CNx on one layer thickness dl.
1221
Fig. 5. Dependence of the dielectric constant of electric polarization ´e on a layer thickness dl.
region. ´e is equal to the square of the refractive index n. The refractive index of a-CNx decreased with nitrogen contents x w9x. The nitrogen contents x of a-CNx can be increased with the substrate temperature w9x. As shown in Fig. 4, the refractive index of LLa-CNx drops sharply when nitrogen content x is approaching is 0.7. Contrary to LLa-CNx, refractive index n of a-CNx decreased gradually with x. As shown in Fig. 5, ´e of LLa-CNx decreased with dl. The atomic hydrogen may etch the highly polarized bonds such as C_C, C^C, C_N and C^N, for which results have been estimated from XPS spectra w5x. Dielectric constants of ionic polarization ´i can be obtained by using the Kramers–Kronig relationship from the measured FT-IR spectra w10,11x. The dielectric constant in the infrared region is divided into two components, i.e. ionic and electronic polarization. In brief, we used the measured absorbance and film thickness to obtain the damping factor, k. Then we used the following Eq. (3) to calculate the refractive index n
where P is the principle value of the integral and subscript i indicates the data point. These values of n and k were then used to calculate the complex dielectric constants. The FT-IR spectra used in our calculations were in the range of 350–6500 cmy1, because it is not possible to measure the absorption spectra from 0 to 8. Fig. 6 shows that ´i decreased with the thickness dl of one a-CNx layer in LLa-CNx. Figs. 5 and 6 show that electronic and ionic components of dielectric constant are decreased with the decrease of dl. Therefore, the atomic hydrogen treatment and a layer-by-layer method are the effective technique for the processing in order to get low dielectric constant a-CNx films, though the hydrogen radicals do not diffuse into a-CNx films.
Fig. 4. Dependence of the refractive indices of a-CNx and LLa-CNx on nitrogen contents x.
Fig. 6. Dependence of the dielectric constant of ionic polarization ´i on a layer thickness dl.
2 nisn`q P p
`
|
´risni2yki2
0
vkŽv. v2yvi2
dv
(3) (4)
1222
M. Aono, S. Nitta / Diamond and Related Materials 11 (2002) 1219–1222
4. Summary We found that a layer-by-layer method using atomic hydrogen treatment of a-CNx decreased the dielectric constant at 1 MHz down to 1.9. The atomic hydrogen treatment modifies C–N bonding states and increases sp3 fraction in a-CNx, producing a stereophonic structure. As a result, the refractive index and the dielectric constant of ionic polarization decrease with decreasing one layer thickness dl of a-CNx. The resistivity of LLaCNx is larger by three orders than that of a-CNx. Thus, atomic hydrogen etching is an effective technique for the processing of a-CNx films to get lower dielectric constant materials. Acknowledgments The authors would like to thank Mr H. Horibata and Mr T. Katsuno for their help in the sample preparation. This work was supported in part by the Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References w1x C.Y. Chang, S.M. Sze, ULSI Technology, McGraw-Hill Companies, INC, 1996. w2x S. Wolf, Silicon Processing for the VLSI Era,, The submicron MOSFET, vol. 3, Lattice Press, Sunset Beach, California, 1995. w3x R.D. Miller, Science 286 (1999) 422. w4x Examples: ‘Low-Dielectric Constant Materials I;V’, Material Research Society Symposium Proceedings, Vols. 384, 443, 476, 511 and 565 (1995–1999). w5x M. Aono, Y. Naruse, S. Nitta, T. Katsuno, Diamond Relat. Mater. 10 (2001) 1147. w6x S. Nitta, N. Takada, K. Sugiyama, T. Itoh, S. Nonomura, J. Non-Cryst. Solids 227–230 (1998) 655. w7x A.C. Ferrari, B. Kleinsorge, G. Adamopoulos, J. Robertson, W.I. Milne, V. Stojolan, L.M. Brown, A. LiBassi, B.K. Tanner, J. Non-Cryst. Solids 266–269 (2000) 765. w8x S.E. Rodil, A.C. Ferrari, J. Robertson, W.I. Milne, J. Appl. Phys. 89 (10) (2001) 5425. w9x M. Aono, S. Nitta, T. Iwasaki, H. Yokoi, T. Itoh, S. Nonomura, ‘Low-Dielectric Constant Materials’, VJ. Hummel, K. Endo, W.W. Lee, M. Mills, S.-Q. Wang, (Eds) Mat. Res. Soc. Symp. Proc., Vol. 565 (MRS, Pennsylvania, 1999) 291. w10x B.K.P. Scaife, Principles of Dielectrics, Clarendon Press, Oxford, 1998. w11x S.W. Lim, Y. Shimogaki, Y. Nakano, K. Tada, H. Komiyama, Jpn. J. Appl. Phys. 35 (1996) 1468.