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The magnetization frequency dependence of enhanced inductively coupled plasma
Surface and Coatings Technology 146 – 147 (2001) 528–531
The magnetization frequency dependence of enhanced inductively coupled plasma O. Beom-hoan*, Chin-Woo Kim, Soo-Beom Jo, Se-Geun Park School of Information & Communication Engineering, 253 Yonghyun-dong, Inchon, 402-751, South Korea
Abstract The enhanced ICP (E-ICP) etcher with various magnetization frequency of external coil current has shown improved etch characteristics in silicon dioxide etch with CF4 gas, and it improves the etch rate of silicon dioxide with the C4 F8 gas also. It has better improvement in etch rate at low pressure process for C4 F8 gas; approximately 80% at 1.33 Pa (10 mtorr) and 30% at 2.66 Pa (20 mtorr). We compared the magnetization frequency dependence of etch rates for three different types of gas parameters, C4F8 gas with 60% of Ar gas, C4F8 gas only, and CF4 gas. Time-resolved measurements were accomplished for Ar plasma to have better perspective of the E-ICP operation. Improvement of etch characteristics is expected when the E-ICP frequency resonates to a lifetime of some desirable states. Further refined analysis of E-ICP with time-resolved measurement would explain the reasons of improvement in etch characteristics. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Simple ICP; E-ICP; Time-resolved measurement; Silicon dioxide; Etch
1. Introduction The control of plasma etch characteristics is very important for the fabrication of submicron semiconductor devices. As the degree of integration density increases and the pitch size of DRAM and other critical feature sizes decreases, the process of contact hole etching requires high selectivity between silicon dioxide and Si or photoresist with low damages as well as high etch rate of silicon dioxide to get higher aspect ratio w1x. While various etching equipments of capacitively coupled plasma(CCP) type has been commonly used for the process of contact hole etching, the inductively coupled plasma(ICP) etcher w2x is also tested to use its merit of high plasma density at low pressure and a novel ICP etcher, the enhanced ICP(E-ICP) etcher, has been proposed to improve etch characteristics further w3x. This E-ICP controls the plasma characteristics with changing its E-ICP magnetization frequency of external coil current, and it has shown improved etch characteristics in silicon dioxide etch with CF4 gas w4x, as well as in photoresist with oxygen plasma. * Corresponding author. Tel.: q82-32-860-7438; fax: q82-32-8607438. E-mail address: [email protected] (O. Beom-hoan).
As high ionization may degrade the selectivity due to the excessive ionization of F atom from the C–F chain of process gas, the formation and control of passivation layers on Si or photoresist is important for sub-half micron process. A process gas with high CyF ratio chemistry, such as C4F8, would help control this, and various process control methods have been shown to be effective w1,5,6x. In this work, we compare the magnetization, frequency and dependence of etch rates for three different types of gas parameters, C4F8 gas with 60% of Ar gas, C4F8 gas only, and CF4 gas. Time-resolved measurements of plasma characteristics were accomplished for Ar gas to get better perspective of the EICP operation. 2. Experimental details The chamber with two external coils for E-ICP operation is schematized in the Fig. 1. The outer diameter is 326 mm, and the planar 4-turn radiofrequence (RF) antenna is a spiral type, located over the quartz plate with a diameter of 260 mm and a thickness of 17 mm. The RF frequency for both main and bias power is 13.56 MHz, and the plasma diagnosis was done without the bias power. Two external 200-turn
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 6 4 - 5
O. Beom-hoan et al. / Surface and Coatings Technology 146 – 147 (2001) 528–531
529
Fig. 1. Schematics of experimental set-up of E-ICP.
copper coils of Helmholtz structure have a radius of 200 mm with a simple dc current on the lower coil and an ac current on the upper coil from a frequency controlled power supply for the operation of ‘E-ICP’. The operation of this system without any current on this coil system is just a normal ICP, named as ‘simple ICP’, and the operation with a dc current on both coils is the case of a ‘Magnetized-ICP’ (M-ICP) with constructive magnetic fields from each coils. The magnitude of magnetic fields produced by this coil system at the wafer position is approximately 1.5 mTesla for constructive superposition and below 0.5 mTesla for destructive superposition. While the magnetic field during the constructive (additive) superposition time period helps plasma to absorb RF power more efficiently, and so to
Fig. 3. The variation of etch rate (a) for three types of operation (b) as a function of additive gas percentage for simple ICP operation Source power: 1 kW, Bias power: 120 W, Pressure: 2.66 Pa (20 mtorr), Total Flow Rate: 50 sccm, E-ICP frequency: 10 Hz, Etch time: 60 s.
increase ion density, the low magnetic field during the destructive superposition time period gives it a chance of density homogenizing with the confining effect of plasma by the magnetic field left at the circumference. This effect is partially recognized by the time-resolved plasma diagnosis. This measurement was accomplished by fast sweeping sawtooth-shaped voltage on the Langmuir probe. The fast current detection is done with a current probe. The RF noise is removed using the RF blocking low pass filter. 3. Results and discussion
Fig. 2. Comparison of etched profiles of (a) 0.35 micrometer test pattern and (b) 1.0 micrometer pattern with C4 F8 gas flow of 50 sccm at 2.66 Pa (20 mtorr).
The etched profiles of test patterns for simple ICP and E-ICP with 10 Hz-magnetization frequency for C4F8 gas flow are compared in Fig. 2. It reveals clear evidence of improved etch rate in E-ICP also for C4F8 gas. Note that the etched profiles look similar to each
530
O. Beom-hoan et al. / Surface and Coatings Technology 146 – 147 (2001) 528–531
Fig. 4. Comparison of the variation of etch rate as a function of EICP frequency for three different gas parameters at 2.66 Pa (20 mtorr) Source power: 1 kW, Bias power: 120 W, Pressure: 2.66 Pa (20 mtorr), Total Flow Rate: 50 sccm, E-ICP frequency: 10 Hz, Etch time: 60 s.
other for C4F8 gas here, while the etch characteristics of E-ICP for CF4 gas are much superior to simple ICPs, as reported previously w4x. This seems to be from different CyF ratios of process gases, and it is quite high for C4F8 gas already. So, it would be necessary to find optimized process window of E-ICP for C4F8 gas, and it is worth to note that E-ICP showed better improvement in etch rate at low pressure process for C4F8 gas; approximately 80% at 1.33 Pa (10 mtorr) and 30% at 2.66 Pa (20 mtorr). The effects of additive gases are explored and compared in Fig. 3 (a) for three different types of etcher operation, simple ICP, E-ICP, and M-ICP. Note that the etch characteristics, such as selectivity and uniformity for M-ICP operation are degraded, while the etch rates are quite high. The variations of etch rate of ICP operation as a function of each additive gases, Ar and oxygen, are shown in Fig. 3b. Especially, it is interesting that the addition of 20% of Ar reveals better etch rate than C4F8 gas only in E-ICP operation, while it is inferior to C4F8 gas’s etch rate for the 20%-addition of any Ar and oxygen gas. Note that the addition of 60% of Ar improves the etch rate for ICP operation, while the addition of oxygen decreases etch rate always. Now, it is worth to check the magnetization frequency (E-ICP frequency) dependence of etch rate for some gas parameters including 60% of Ar. The variations of etch rates for three gas systems of C4F8 gas with 60% of Ar gas, C4F8 gas only, and CF4 gas, respectively, as a function of E-ICP frequency at 2.66 Pa (20 mtorr) are compared in Fig. 4. While the C4F8 gas has a maximum etch rate at 10 Hz, the C4F8 gas with 60% of Ar and
the CF4 gas have different dependences. Note that the optimized E-ICP frequencies for other gas systems are different, as reported previously w3x. The fluctuation of ion density and electron temperature for Ar plasma with the E-ICP frequencys20 Hz is measured by the time-resolved method, as shown in Fig. 5. The ion density is obtained from the ion saturation current, which is quite reliable even for the existence of some noise signals. The ion density for simple ICP is nearly constant and that for E-ICP is varying periodically with the same period of the applied magnetization frequency, as expected. The maximum density for EICP is about double the minimum, which is the same as the density for simple ICP. The electron temperature also shows some periodic tendency for E-ICP and just a random fluctuation for simple ICP, although the behavior includes some noisy points. The noisy points are due to the difficulties in getting steady exponential fit of electron current for all data points through noisy
Fig. 5. Time-resolved measurement for Ar plasma as a function of time with E-ICP frequencys20 Hz (a) ion density variation (b) electron temperature.
O. Beom-hoan et al. / Surface and Coatings Technology 146 – 147 (2001) 528–531
1–5 curve from the fast voltage sweeping. Note that the electron temperature oscillates up and down to much lower than the average electron temperature of simple ICP. It would be, then, desirable to set the magnetization frequency to have the system resonate on the state with low electron temperature. 4. Conclusion and summary In this work, we revealed that E-ICP improves the etch rate of silicon dioxide with the C4F8 gas also. We measured the magnetization frequency dependence of the etch rate for several gas systems and found interesting behavior with Ar addition even in E-ICP as well as in simple ICP. The time-resolved analysis of Ar plasma shows the time-varying behavior of Ar plasma characteristics. Improvement of etch characteristics is expected when the E-ICP frequency resonates to a lifetime of some desirable states. Further refined analysis of E-ICP with time-resolved measurement would explain the reasons of improvement in etch characteristics.
531
Acknowledgements This work has been supported by the Ministry of Science and Technology and by the Ministry of Commerce, Industry, and Energy of the Republic of Korea through the Research Program for the Development of Advanced Technologies for Flat-Panel Display and System IC 2010, and in part by the 2000 research program of Inha University. References w1x S. Samukawa, T. Mukai, J. Vac. Sci. Technol. B 18 (1) (2000) 166. w2x T. Fukasawa, K. Kubota, H. Shindo, Y. Horiike, Jpn. J. Appl. Phys. 33 (1994) 7042. w3x O. Beom-Hoan, J. Jae-Song, P. Se-Geun, Surface Coatings Technol. 120–121 (1999) 752. w4x O. Beom-Hoan, P. Se-Geun, R. Sang-Ho, J. Jae-Song, Surface Coatings Technol. 133–134 (2000) 589. w5x W. Chen, T. Hayasi, M. Itoh, Y. Morikawa, K. Sugita, T. Uchida, Vacuum 53 (1999) 29. w6x M. Schaepkens, G.S. Oehrlein, J.M. Cook, J. Vac. Sci. Technol. B 18 (2) (2000) 856.
The magnetization frequency dependence of enhanced inductively coupled plasma O. Beom-hoan*, Chin-Woo Kim, Soo-Beom Jo, Se-Geun Park School of Information & Communication Engineering, 253 Yonghyun-dong, Inchon, 402-751, South Korea
Abstract The enhanced ICP (E-ICP) etcher with various magnetization frequency of external coil current has shown improved etch characteristics in silicon dioxide etch with CF4 gas, and it improves the etch rate of silicon dioxide with the C4 F8 gas also. It has better improvement in etch rate at low pressure process for C4 F8 gas; approximately 80% at 1.33 Pa (10 mtorr) and 30% at 2.66 Pa (20 mtorr). We compared the magnetization frequency dependence of etch rates for three different types of gas parameters, C4F8 gas with 60% of Ar gas, C4F8 gas only, and CF4 gas. Time-resolved measurements were accomplished for Ar plasma to have better perspective of the E-ICP operation. Improvement of etch characteristics is expected when the E-ICP frequency resonates to a lifetime of some desirable states. Further refined analysis of E-ICP with time-resolved measurement would explain the reasons of improvement in etch characteristics. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Simple ICP; E-ICP; Time-resolved measurement; Silicon dioxide; Etch
1. Introduction The control of plasma etch characteristics is very important for the fabrication of submicron semiconductor devices. As the degree of integration density increases and the pitch size of DRAM and other critical feature sizes decreases, the process of contact hole etching requires high selectivity between silicon dioxide and Si or photoresist with low damages as well as high etch rate of silicon dioxide to get higher aspect ratio w1x. While various etching equipments of capacitively coupled plasma(CCP) type has been commonly used for the process of contact hole etching, the inductively coupled plasma(ICP) etcher w2x is also tested to use its merit of high plasma density at low pressure and a novel ICP etcher, the enhanced ICP(E-ICP) etcher, has been proposed to improve etch characteristics further w3x. This E-ICP controls the plasma characteristics with changing its E-ICP magnetization frequency of external coil current, and it has shown improved etch characteristics in silicon dioxide etch with CF4 gas w4x, as well as in photoresist with oxygen plasma. * Corresponding author. Tel.: q82-32-860-7438; fax: q82-32-8607438. E-mail address: [email protected] (O. Beom-hoan).
As high ionization may degrade the selectivity due to the excessive ionization of F atom from the C–F chain of process gas, the formation and control of passivation layers on Si or photoresist is important for sub-half micron process. A process gas with high CyF ratio chemistry, such as C4F8, would help control this, and various process control methods have been shown to be effective w1,5,6x. In this work, we compare the magnetization, frequency and dependence of etch rates for three different types of gas parameters, C4F8 gas with 60% of Ar gas, C4F8 gas only, and CF4 gas. Time-resolved measurements of plasma characteristics were accomplished for Ar gas to get better perspective of the EICP operation. 2. Experimental details The chamber with two external coils for E-ICP operation is schematized in the Fig. 1. The outer diameter is 326 mm, and the planar 4-turn radiofrequence (RF) antenna is a spiral type, located over the quartz plate with a diameter of 260 mm and a thickness of 17 mm. The RF frequency for both main and bias power is 13.56 MHz, and the plasma diagnosis was done without the bias power. Two external 200-turn
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 6 4 - 5
O. Beom-hoan et al. / Surface and Coatings Technology 146 – 147 (2001) 528–531
529
Fig. 1. Schematics of experimental set-up of E-ICP.
copper coils of Helmholtz structure have a radius of 200 mm with a simple dc current on the lower coil and an ac current on the upper coil from a frequency controlled power supply for the operation of ‘E-ICP’. The operation of this system without any current on this coil system is just a normal ICP, named as ‘simple ICP’, and the operation with a dc current on both coils is the case of a ‘Magnetized-ICP’ (M-ICP) with constructive magnetic fields from each coils. The magnitude of magnetic fields produced by this coil system at the wafer position is approximately 1.5 mTesla for constructive superposition and below 0.5 mTesla for destructive superposition. While the magnetic field during the constructive (additive) superposition time period helps plasma to absorb RF power more efficiently, and so to
Fig. 3. The variation of etch rate (a) for three types of operation (b) as a function of additive gas percentage for simple ICP operation Source power: 1 kW, Bias power: 120 W, Pressure: 2.66 Pa (20 mtorr), Total Flow Rate: 50 sccm, E-ICP frequency: 10 Hz, Etch time: 60 s.
increase ion density, the low magnetic field during the destructive superposition time period gives it a chance of density homogenizing with the confining effect of plasma by the magnetic field left at the circumference. This effect is partially recognized by the time-resolved plasma diagnosis. This measurement was accomplished by fast sweeping sawtooth-shaped voltage on the Langmuir probe. The fast current detection is done with a current probe. The RF noise is removed using the RF blocking low pass filter. 3. Results and discussion
Fig. 2. Comparison of etched profiles of (a) 0.35 micrometer test pattern and (b) 1.0 micrometer pattern with C4 F8 gas flow of 50 sccm at 2.66 Pa (20 mtorr).
The etched profiles of test patterns for simple ICP and E-ICP with 10 Hz-magnetization frequency for C4F8 gas flow are compared in Fig. 2. It reveals clear evidence of improved etch rate in E-ICP also for C4F8 gas. Note that the etched profiles look similar to each
530
O. Beom-hoan et al. / Surface and Coatings Technology 146 – 147 (2001) 528–531
Fig. 4. Comparison of the variation of etch rate as a function of EICP frequency for three different gas parameters at 2.66 Pa (20 mtorr) Source power: 1 kW, Bias power: 120 W, Pressure: 2.66 Pa (20 mtorr), Total Flow Rate: 50 sccm, E-ICP frequency: 10 Hz, Etch time: 60 s.
other for C4F8 gas here, while the etch characteristics of E-ICP for CF4 gas are much superior to simple ICPs, as reported previously w4x. This seems to be from different CyF ratios of process gases, and it is quite high for C4F8 gas already. So, it would be necessary to find optimized process window of E-ICP for C4F8 gas, and it is worth to note that E-ICP showed better improvement in etch rate at low pressure process for C4F8 gas; approximately 80% at 1.33 Pa (10 mtorr) and 30% at 2.66 Pa (20 mtorr). The effects of additive gases are explored and compared in Fig. 3 (a) for three different types of etcher operation, simple ICP, E-ICP, and M-ICP. Note that the etch characteristics, such as selectivity and uniformity for M-ICP operation are degraded, while the etch rates are quite high. The variations of etch rate of ICP operation as a function of each additive gases, Ar and oxygen, are shown in Fig. 3b. Especially, it is interesting that the addition of 20% of Ar reveals better etch rate than C4F8 gas only in E-ICP operation, while it is inferior to C4F8 gas’s etch rate for the 20%-addition of any Ar and oxygen gas. Note that the addition of 60% of Ar improves the etch rate for ICP operation, while the addition of oxygen decreases etch rate always. Now, it is worth to check the magnetization frequency (E-ICP frequency) dependence of etch rate for some gas parameters including 60% of Ar. The variations of etch rates for three gas systems of C4F8 gas with 60% of Ar gas, C4F8 gas only, and CF4 gas, respectively, as a function of E-ICP frequency at 2.66 Pa (20 mtorr) are compared in Fig. 4. While the C4F8 gas has a maximum etch rate at 10 Hz, the C4F8 gas with 60% of Ar and
the CF4 gas have different dependences. Note that the optimized E-ICP frequencies for other gas systems are different, as reported previously w3x. The fluctuation of ion density and electron temperature for Ar plasma with the E-ICP frequencys20 Hz is measured by the time-resolved method, as shown in Fig. 5. The ion density is obtained from the ion saturation current, which is quite reliable even for the existence of some noise signals. The ion density for simple ICP is nearly constant and that for E-ICP is varying periodically with the same period of the applied magnetization frequency, as expected. The maximum density for EICP is about double the minimum, which is the same as the density for simple ICP. The electron temperature also shows some periodic tendency for E-ICP and just a random fluctuation for simple ICP, although the behavior includes some noisy points. The noisy points are due to the difficulties in getting steady exponential fit of electron current for all data points through noisy
Fig. 5. Time-resolved measurement for Ar plasma as a function of time with E-ICP frequencys20 Hz (a) ion density variation (b) electron temperature.
O. Beom-hoan et al. / Surface and Coatings Technology 146 – 147 (2001) 528–531
1–5 curve from the fast voltage sweeping. Note that the electron temperature oscillates up and down to much lower than the average electron temperature of simple ICP. It would be, then, desirable to set the magnetization frequency to have the system resonate on the state with low electron temperature. 4. Conclusion and summary In this work, we revealed that E-ICP improves the etch rate of silicon dioxide with the C4F8 gas also. We measured the magnetization frequency dependence of the etch rate for several gas systems and found interesting behavior with Ar addition even in E-ICP as well as in simple ICP. The time-resolved analysis of Ar plasma shows the time-varying behavior of Ar plasma characteristics. Improvement of etch characteristics is expected when the E-ICP frequency resonates to a lifetime of some desirable states. Further refined analysis of E-ICP with time-resolved measurement would explain the reasons of improvement in etch characteristics.
531
Acknowledgements This work has been supported by the Ministry of Science and Technology and by the Ministry of Commerce, Industry, and Energy of the Republic of Korea through the Research Program for the Development of Advanced Technologies for Flat-Panel Display and System IC 2010, and in part by the 2000 research program of Inha University. References w1x S. Samukawa, T. Mukai, J. Vac. Sci. Technol. B 18 (1) (2000) 166. w2x T. Fukasawa, K. Kubota, H. Shindo, Y. Horiike, Jpn. J. Appl. Phys. 33 (1994) 7042. w3x O. Beom-Hoan, J. Jae-Song, P. Se-Geun, Surface Coatings Technol. 120–121 (1999) 752. w4x O. Beom-Hoan, P. Se-Geun, R. Sang-Ho, J. Jae-Song, Surface Coatings Technol. 133–134 (2000) 589. w5x W. Chen, T. Hayasi, M. Itoh, Y. Morikawa, K. Sugita, T. Uchida, Vacuum 53 (1999) 29. w6x M. Schaepkens, G.S. Oehrlein, J.M. Cook, J. Vac. Sci. Technol. B 18 (2) (2000) 856.