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Study on the cleaning of silicon after CMP in ULSI
Microelectronic Engineering 66 (2003) 433–437 www.elsevier.com / locate / mee
Study on the cleaning of silicon after CMP in ULSI Yuling Liu a , Kailiang Zhang a , *, Fang Wang b , Yunpeng Han a a
Institute of Microelectronic Technology and Materials, Hebei University of Technology, Tianjin 300130, China b Department of Photoelectronics, Tianjin University of Technology, Tianjin 300191, China
Abstract In this paper, the cleaning of silicon after CMP (chemical mechanical polishing) in ULSI was studied utilizing preferential adsorption knowledge. On the basis of analyzing adsorption state of contaminated particles on polished silicon wafer and interrelated adsorption theory, the preferential adsorption model is put forward: r2 t 5 k ]] . Non-ion surfactant is chosen and acts as the second kind of adsorbate, which is preferentially ( f1 f2 S ) adsorbed onto the polished silicon wafer. The second kind of adsorbate can effectively depress the surface energy of new polished silicon wafer, form a layer of protective film and prevent the chemical adsorption and bonding of the contaminated particle near the surface of polished silicon wafer. In addition, a new kind of chelant, being a free metal ion, is also added in order to chelate and wipe out the metal ion on the polished silicon wafer. Consequently, the organic impurity, impurity particles and metal ion on the silicon surface were wiped out effectively and the silicon has a pure surface after cleaning. The results of cleaning experiments show that the chemical cleaning method in this paper can control the adsorption state of particle on the polished silicon wafer and cause the adsorption state to remain at the physical adsorption state for a long time: even after 168 h in the surfactant solution the wafers reach the SEMI standard after being washed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Cleaning; Preferential adsorption model; Surfactant; CMP; ULSI
1. Introduction With the high-speed development of ULSI [1,2], the influence of the surface quality of polished silicon wafer on quality and rate of production of devices becomes more and more important. There are three kinds of adsorbate, organic impurity, impurity particles and metal ion, on the silicon surface. At present the methods used to clean the silicon surface after CMP are mainly double-sided scrubbing or concentrated chemical washing. To some extent the double-sided scrubbing used by several advanced companies is able to obtain clean surface, but the method requires that the silicon wafers be * Corresponding author. E-mail address: kailiang][email protected] (K. Zhang). 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)00906-1
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scrubbed and washed within 2 h after polishing and before the chemical adsorption state develops. There are also some disadvantages: the expensive equipment (costing $300 000|400 000), the low efficiency of brushing wafers one by one, more secondary defects and lower output. In order to effectively resolve the difficult question of removal of contaminated particles on the polished silicon wafer, a special surfactant was studied. In this paper, on the basis of analyzing the adsorption state of the contaminated particles on the polished silicon wafer and interrelated adsorption theory, a non-ion surfactant was chosen and acted as the second kind of adsorbate, which is preferentially adsorbed onto the polished silicon wafer. The second kind of adsorbate can effectively depress the surface energy of new polished silicon wafer, form a layer of protective film and prevent the chemical adsorption and bonding of the contaminated particle near the surface of polished silicon wafer.
2. Theory and analysis [3–5] Every atom of the surface of polished silicon wafer has many new rupturing bonds, and in addition its force field is very strong. The force field makes the surface of a newly polished silicon wafer unstable, and it may spontaneously adsorb some substances from the surroundings to decrease the surface energy. The adsorption state of the contaminated particle on the surface of the polished silicon wafer varies over time. At the beginning, the adsorption state is physical so it is easy to wash the particle off. With passing time, the adsorption state begins to quickly change into weak chemical adsorption. Soon it becomes chemical adsorption and bonding occurs so it is difficult to rinse and wipe the particle off (see curve ‘a’ in Fig. 1). In order to ensure that the adsorption state of the contaminated particle on the surface of the polished silicon wafer remains physical adsorption for a long time, a particular type of highly pure non-ion surfactant is chosen. This kind of non-ion surfactant is adsorbed preferentially onto the polished silicon wafer first, and markedly decreases the surface energy of the polished silicon wafer. At the same time, the adsorbate must be in the physical adsorption state because the adsorbate is a non-ion organic polymer and difficult to transform into chemical adsorption and bonding (see curve ‘b’ in Fig. 1). r2 Concerning the preferential adsorption of the adsorbate, the equation t 5 k ]] ( f 1 f 2 S ) is based on the adsorption theory and adsorption kinetic process. From the mathematical model, the results can be
Fig. 1. A comparison of the usual method with the new one controlling the adsorption.
Y. Liu et al. / Microelectronic Engineering 66 (2003) 433–437
435
obtained as follows: the shorter the adsorption time, the more the adsorbate is adsorbed preferentially. This is also true for the distance between adsorbate and adsorbent, the field strength of surface force for adsorbent or adsorbate and the area of adsorbed species. According to the above preferential adsorption model and adsorption mechanism of particulate on the surface of polished silicon wafer, a non-ion surfactant, which disperses in water, is wetted well with the silicon wafer and eliminated easily by washing procedure, was chosen and acted as the second kind of adsorbate. The non-ion surfactant molecules replacing the contaminated particle are adsorbed preferentially onto the surface of the polished silicon wafer, and become a molecular layer with inner hydrophilic groups and outer hydrophobic groups. The outer molecular layer also adsorbed another reversed molecular layer, which forms the protective film on the surface of silicon wafer. The situation is shown in Fig. 2(a). The protective film prevents the formation of chemical adsorption and bonding between impurity particle and silicon wafer. The oxygen molecules in water may traverse the film and arrive at the surface of silicon wafer, forming the amorphous silicon oxide layer without impurity particle. Also, the surfactant molecules may penetrate the interface between the silicon wafer surface and the contaminated particle adsorbed on the silicon wafer, and acting as a wedge in the interface, can split the silicon wafer and contaminated particle (Fig. 2(b)). In the case of weak chemical adsorption the contaminated particles are floated by the surfactant, and the adsorbed contaminated particles are replaced by the surfactant molecules. At the same time, a protective layer around the contaminated particles is formed by the adsorbed surfactant molecules (Fig. 2(c)), which become a barrier between the silicon wafer and contaminated particles. Because the adsorbate may remain in the physical adsorption state which, for a long time, is easy to wash out, and is not easily transformed into chemical adsorption and bonding which is difficult to wash out, the application of surfactant effectively resolves the contaminated particles’ chemical adsorption and bonding during the preparation of micro-electronics devices.
3. Experiment and discussion To test the effectiveness of the surfactant, the experiments were conducted as follows. After the silicon wafers were polished, they were put into the surfactant solution immediately and kept there for 2, 24, 48, 72, 120 and 168 h, and then washed out. The tests were applied to 100 pieces of n(100) and p(111) wafers. The results are listed in Table 1 and show that when the polished wafers are kept in pure water for 2 h, over half of the polished wafers are not suitable after being washed. But all the polished silicon wafers in the surfactant solution reached the SEMI standard after being washed, even after being kept in the surfactant solution for 168 h. That is to say, for each piece of silicon wafer the number of particles larger than 0.5 mm was less than 10, while the number of particles larger than 0.2 mm was also less than 10. In addition, the surfactant also plays an important role in washing organics on the polished silicon wafer, in which the amphipathic characteristic of surfactant is fully applied. Concerning removal of metal ion, one new kind of chelate free of metal ion and possessing 13 chelate rings, was chosen and added into cleaning liquid. As a result, metal ion on the polished silicon surface decreased markedly, which is very important when making high quality IC devices.
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Fig. 2. Control of the adsorption state and removal of the contaminated particle adsorbed on the polished silicon wafer.
4. Conclusions The above theory analysis and experimental results show that the application of surfactant may effectively control the adsorption state of particles on the polished silicon wafer: even after 168 h in the surfactant solution the wafers reach the SEMI standard after being washed. Because the adsorbate can be kept in the physical adsorption state for a long time which is easy to wash out, the polished silicon wafers may be washed out all together. Compared with double-sided scrubbing, this method may simplify the technology, economize on chemical reagents and present a series of advantages such as lower cost, higher efficiency and easy operation, etc. In addition, the adding of surfactant
Y. Liu et al. / Microelectronic Engineering 66 (2003) 433–437
437
Table 1 Comparison of qualified numbers for wafers treated with and without surfactant Time (h)
2 24 48 72 120 168
Particle size (mm)
.0.5 .0.2 .0.5 .0.2 .0.5 .0.2 .0.5 .0.2 .0.5 .0.2 .0.5 .0.2
Pass percentage (%) With surfactant
Without surfactant
n(100)
p(111)
n(100)
p(111)
100 100 100 100 100 100 100 100 100 100 100 67
100 100 100 100 100 100 100 100 100 100 100 75
47 32 0 0 0 0 0 0 0 0 0 0
51 36 0 0 0 0 0 0 0 0 0 0
For the SEMI standard, the requirements are particle size .0.5 mm, ,10 / wafer (F 200 mm). The last requirement of ULSI is particle size .0.2 mm, ,10 / wafer (F 200 mm).
effectively washes out the organics on the polished silicon wafer. A new kind of chelate free of metal ion also wiped out metal ion, which is very important when making high quality IC devices. In conclusion, in this paper a polished silicon wafer cleaning process and method, which is cheaper, more efficient and results in a cleaner surface, are put forward, and a new type of very effective chelate is proposed.
References [1] T. Hattori, V.J. Ruzyllo, R. Novak, Wafer cleaning, J. Electrochem. Soc. 97 (35) (1998) 1. [2] D. Hymes, I. Malik, Brush scrubbing emerges as future wafer-cleaning technology, Solid State Technol. 7 (1997) 209–214. [3] H. Gu, A study of the mechanism and removal techniques of the particle contamination on Si wafer in ULSI. Hebei University of Technology master’s degree dissertation, Tianjin, 2001. [4] Y. Liu, N. Liu, Y. Cao, Investigation on adsorption state of surface adsorbate on silicon wafer, Rare Metal 18 (2) (1999) 106–112. [5] K. Zhang, Y. Liu, F. Wang, Study on controlling the adsorption state of particle on the polished silicon wafer, in: B.-Z. Li, G.-P. Ru, P. Yu, H. Iwai (Eds.), The Sixth International Conference on Solid-State and Integrated-Circuit Technology Proceedings, 2001, People Post & Telecommunications, Publishing House, Beijing, pp. 464–467.
Study on the cleaning of silicon after CMP in ULSI Yuling Liu a , Kailiang Zhang a , *, Fang Wang b , Yunpeng Han a a
Institute of Microelectronic Technology and Materials, Hebei University of Technology, Tianjin 300130, China b Department of Photoelectronics, Tianjin University of Technology, Tianjin 300191, China
Abstract In this paper, the cleaning of silicon after CMP (chemical mechanical polishing) in ULSI was studied utilizing preferential adsorption knowledge. On the basis of analyzing adsorption state of contaminated particles on polished silicon wafer and interrelated adsorption theory, the preferential adsorption model is put forward: r2 t 5 k ]] . Non-ion surfactant is chosen and acts as the second kind of adsorbate, which is preferentially ( f1 f2 S ) adsorbed onto the polished silicon wafer. The second kind of adsorbate can effectively depress the surface energy of new polished silicon wafer, form a layer of protective film and prevent the chemical adsorption and bonding of the contaminated particle near the surface of polished silicon wafer. In addition, a new kind of chelant, being a free metal ion, is also added in order to chelate and wipe out the metal ion on the polished silicon wafer. Consequently, the organic impurity, impurity particles and metal ion on the silicon surface were wiped out effectively and the silicon has a pure surface after cleaning. The results of cleaning experiments show that the chemical cleaning method in this paper can control the adsorption state of particle on the polished silicon wafer and cause the adsorption state to remain at the physical adsorption state for a long time: even after 168 h in the surfactant solution the wafers reach the SEMI standard after being washed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Cleaning; Preferential adsorption model; Surfactant; CMP; ULSI
1. Introduction With the high-speed development of ULSI [1,2], the influence of the surface quality of polished silicon wafer on quality and rate of production of devices becomes more and more important. There are three kinds of adsorbate, organic impurity, impurity particles and metal ion, on the silicon surface. At present the methods used to clean the silicon surface after CMP are mainly double-sided scrubbing or concentrated chemical washing. To some extent the double-sided scrubbing used by several advanced companies is able to obtain clean surface, but the method requires that the silicon wafers be * Corresponding author. E-mail address: kailiang][email protected] (K. Zhang). 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)00906-1
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Y. Liu et al. / Microelectronic Engineering 66 (2003) 433–437
scrubbed and washed within 2 h after polishing and before the chemical adsorption state develops. There are also some disadvantages: the expensive equipment (costing $300 000|400 000), the low efficiency of brushing wafers one by one, more secondary defects and lower output. In order to effectively resolve the difficult question of removal of contaminated particles on the polished silicon wafer, a special surfactant was studied. In this paper, on the basis of analyzing the adsorption state of the contaminated particles on the polished silicon wafer and interrelated adsorption theory, a non-ion surfactant was chosen and acted as the second kind of adsorbate, which is preferentially adsorbed onto the polished silicon wafer. The second kind of adsorbate can effectively depress the surface energy of new polished silicon wafer, form a layer of protective film and prevent the chemical adsorption and bonding of the contaminated particle near the surface of polished silicon wafer.
2. Theory and analysis [3–5] Every atom of the surface of polished silicon wafer has many new rupturing bonds, and in addition its force field is very strong. The force field makes the surface of a newly polished silicon wafer unstable, and it may spontaneously adsorb some substances from the surroundings to decrease the surface energy. The adsorption state of the contaminated particle on the surface of the polished silicon wafer varies over time. At the beginning, the adsorption state is physical so it is easy to wash the particle off. With passing time, the adsorption state begins to quickly change into weak chemical adsorption. Soon it becomes chemical adsorption and bonding occurs so it is difficult to rinse and wipe the particle off (see curve ‘a’ in Fig. 1). In order to ensure that the adsorption state of the contaminated particle on the surface of the polished silicon wafer remains physical adsorption for a long time, a particular type of highly pure non-ion surfactant is chosen. This kind of non-ion surfactant is adsorbed preferentially onto the polished silicon wafer first, and markedly decreases the surface energy of the polished silicon wafer. At the same time, the adsorbate must be in the physical adsorption state because the adsorbate is a non-ion organic polymer and difficult to transform into chemical adsorption and bonding (see curve ‘b’ in Fig. 1). r2 Concerning the preferential adsorption of the adsorbate, the equation t 5 k ]] ( f 1 f 2 S ) is based on the adsorption theory and adsorption kinetic process. From the mathematical model, the results can be
Fig. 1. A comparison of the usual method with the new one controlling the adsorption.
Y. Liu et al. / Microelectronic Engineering 66 (2003) 433–437
435
obtained as follows: the shorter the adsorption time, the more the adsorbate is adsorbed preferentially. This is also true for the distance between adsorbate and adsorbent, the field strength of surface force for adsorbent or adsorbate and the area of adsorbed species. According to the above preferential adsorption model and adsorption mechanism of particulate on the surface of polished silicon wafer, a non-ion surfactant, which disperses in water, is wetted well with the silicon wafer and eliminated easily by washing procedure, was chosen and acted as the second kind of adsorbate. The non-ion surfactant molecules replacing the contaminated particle are adsorbed preferentially onto the surface of the polished silicon wafer, and become a molecular layer with inner hydrophilic groups and outer hydrophobic groups. The outer molecular layer also adsorbed another reversed molecular layer, which forms the protective film on the surface of silicon wafer. The situation is shown in Fig. 2(a). The protective film prevents the formation of chemical adsorption and bonding between impurity particle and silicon wafer. The oxygen molecules in water may traverse the film and arrive at the surface of silicon wafer, forming the amorphous silicon oxide layer without impurity particle. Also, the surfactant molecules may penetrate the interface between the silicon wafer surface and the contaminated particle adsorbed on the silicon wafer, and acting as a wedge in the interface, can split the silicon wafer and contaminated particle (Fig. 2(b)). In the case of weak chemical adsorption the contaminated particles are floated by the surfactant, and the adsorbed contaminated particles are replaced by the surfactant molecules. At the same time, a protective layer around the contaminated particles is formed by the adsorbed surfactant molecules (Fig. 2(c)), which become a barrier between the silicon wafer and contaminated particles. Because the adsorbate may remain in the physical adsorption state which, for a long time, is easy to wash out, and is not easily transformed into chemical adsorption and bonding which is difficult to wash out, the application of surfactant effectively resolves the contaminated particles’ chemical adsorption and bonding during the preparation of micro-electronics devices.
3. Experiment and discussion To test the effectiveness of the surfactant, the experiments were conducted as follows. After the silicon wafers were polished, they were put into the surfactant solution immediately and kept there for 2, 24, 48, 72, 120 and 168 h, and then washed out. The tests were applied to 100 pieces of n(100) and p(111) wafers. The results are listed in Table 1 and show that when the polished wafers are kept in pure water for 2 h, over half of the polished wafers are not suitable after being washed. But all the polished silicon wafers in the surfactant solution reached the SEMI standard after being washed, even after being kept in the surfactant solution for 168 h. That is to say, for each piece of silicon wafer the number of particles larger than 0.5 mm was less than 10, while the number of particles larger than 0.2 mm was also less than 10. In addition, the surfactant also plays an important role in washing organics on the polished silicon wafer, in which the amphipathic characteristic of surfactant is fully applied. Concerning removal of metal ion, one new kind of chelate free of metal ion and possessing 13 chelate rings, was chosen and added into cleaning liquid. As a result, metal ion on the polished silicon surface decreased markedly, which is very important when making high quality IC devices.
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Y. Liu et al. / Microelectronic Engineering 66 (2003) 433–437
Fig. 2. Control of the adsorption state and removal of the contaminated particle adsorbed on the polished silicon wafer.
4. Conclusions The above theory analysis and experimental results show that the application of surfactant may effectively control the adsorption state of particles on the polished silicon wafer: even after 168 h in the surfactant solution the wafers reach the SEMI standard after being washed. Because the adsorbate can be kept in the physical adsorption state for a long time which is easy to wash out, the polished silicon wafers may be washed out all together. Compared with double-sided scrubbing, this method may simplify the technology, economize on chemical reagents and present a series of advantages such as lower cost, higher efficiency and easy operation, etc. In addition, the adding of surfactant
Y. Liu et al. / Microelectronic Engineering 66 (2003) 433–437
437
Table 1 Comparison of qualified numbers for wafers treated with and without surfactant Time (h)
2 24 48 72 120 168
Particle size (mm)
.0.5 .0.2 .0.5 .0.2 .0.5 .0.2 .0.5 .0.2 .0.5 .0.2 .0.5 .0.2
Pass percentage (%) With surfactant
Without surfactant
n(100)
p(111)
n(100)
p(111)
100 100 100 100 100 100 100 100 100 100 100 67
100 100 100 100 100 100 100 100 100 100 100 75
47 32 0 0 0 0 0 0 0 0 0 0
51 36 0 0 0 0 0 0 0 0 0 0
For the SEMI standard, the requirements are particle size .0.5 mm, ,10 / wafer (F 200 mm). The last requirement of ULSI is particle size .0.2 mm, ,10 / wafer (F 200 mm).
effectively washes out the organics on the polished silicon wafer. A new kind of chelate free of metal ion also wiped out metal ion, which is very important when making high quality IC devices. In conclusion, in this paper a polished silicon wafer cleaning process and method, which is cheaper, more efficient and results in a cleaner surface, are put forward, and a new type of very effective chelate is proposed.
References [1] T. Hattori, V.J. Ruzyllo, R. Novak, Wafer cleaning, J. Electrochem. Soc. 97 (35) (1998) 1. [2] D. Hymes, I. Malik, Brush scrubbing emerges as future wafer-cleaning technology, Solid State Technol. 7 (1997) 209–214. [3] H. Gu, A study of the mechanism and removal techniques of the particle contamination on Si wafer in ULSI. Hebei University of Technology master’s degree dissertation, Tianjin, 2001. [4] Y. Liu, N. Liu, Y. Cao, Investigation on adsorption state of surface adsorbate on silicon wafer, Rare Metal 18 (2) (1999) 106–112. [5] K. Zhang, Y. Liu, F. Wang, Study on controlling the adsorption state of particle on the polished silicon wafer, in: B.-Z. Li, G.-P. Ru, P. Yu, H. Iwai (Eds.), The Sixth International Conference on Solid-State and Integrated-Circuit Technology Proceedings, 2001, People Post & Telecommunications, Publishing House, Beijing, pp. 464–467.