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Improved etching method for microelectronic devices with supercritical carbon dioxide
Microelectronic Engineering 86 (2009) 128–131
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Improved etching method for microelectronic devices with supercritical carbon dioxide Jae Hyun Bae a, Md. Zahangir Alam a, Jae Mok Jung a, Yeong-Soon Gal b, Hyosan Lee c, Hyun Gyu Kim d, Kwon Taek Lim a,* a
Division of Image System Science and Engineering, Pukyoung National University Busan 608-739, Republic of Korea Polymer Chemistry Laboratory, College of Engineering, Kyung Il University, Republic of Korea c Process Development Team, Memory Division, Samsung Electronics, Republic of Korea d Korea Basic Science Institute, Busan Centre, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 16 April 2008 Received in revised form 26 August 2008 Accepted 11 October 2008 Available online 18 October 2008 Keywords: Etching Sacrificial oxide SiN Supercritical carbon dioxide MEMS
a b s t r a c t Aqueous etchants used in traditional wet etching for the production of integrated circuits and MEMS devices hinder the processes and pose environmental difficulties. Therefore, we developed an improved dry etching method with HF/Pyridine (7:3) in supercritical carbon dioxide. Etch rates of BPSG, P-TEOS, Thermal SiO2 and SiN with dry etching method were several times higher than those in wet etching. Etch rates were found to be a function of temperature, HF concentration, and the kind of co-solvents. The presence of alcoholic co-solvents, especially IPA with HF/Pyridine etchant greatly increased the etch rate of BPSG. Etch selectivity could be controlled with the etchant concentration. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction As minimum feature sizes for integrated circuits (ICs) continue to shrink into the deep sub-half micrometer region, nanoscale structures will come to have a much higher aspect ratio (height/ width). Wafer processing in the microelectronics industry by wet isotropic etching in aqueous hydrogen fluoride (HF) is a fundamental step in the fabrication of integrated circuits (e. g. dynamic random access memory, DRAM capacitor fabrication in front end of the line, FEOL and post-etch residue removal in back end of the line, BEOL cleaning) [1–5]. Wet etching of sacrificial layers followed by common drying methods often causes sticking of microstructures [6,7] and ineffective deep cleaning and induces damage in high aspect ratio structures, which are the most critical obstacles to next generation semiconductor devices. Other major disadvantages of wet etching are corrosive and toxic materials that may incur high costs in handling and disposal [8]. Anhydrous HF gasphase etching and supercritical carbon dioxide drying has been developed to remove the sticking problem [9–12]. Recently, compressed carbon dioxide known as supercritical carbon dioxide (scCO2) has received a great attention of the scientists as a prospective alternative to current aqueous and organic
* Corresponding author. Tel.: +82 51 629 6409; fax: +82 51 629 6408. E-mail address: [email protected] (K.T. Lim). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.10.003
solvents in microelectronics fabrication facilities (FABs) [1,13]. The utilization of scCO2 has been demonstrated in several applications including drying [14], photoresist stripping [15], removal of post-ash and post-etch residues [16], low-k repair [17–21], metal deposition [22], chemical mechanical planarization [23], and dry lithography [24]. Supercritical CO2 is considered as a ‘‘dry” environmentally benign solvent because it is a gas at normal temperature and pressure and therefore, evaporates immediately from any surface or device that is being processed. Jones et al. developed HF/ Pyridine in scCO2 as an etchant [1]. The applications of scCO2based etching method addressed to microelectro mechanical systems (MEMS) device fabrication also have been reported [14,25]. In this work, we studied dry etching processes with non-aqueous HF/Pyridine using supercritical carbon dioxide. The effect of process variables and co-solvent on the etching performance such as etching rate and selectivity was investigated. scCO2 possesses zero surface tension and tunable solvent property: its density can be controlled via manipulation of both temperature and/or pressure. An enhanced etching rate resulting from low viscosity and high diffusivity is another distinct advantage. Again, scCO2 allows a variety of constituents to be dissolved at low temperature and also provide a high flux of etchant to support etching [13,26]. scCO2-Based solutions in wafer etching would eliminate waterbased drying step, and thus prevent structural damage to materials in microelectronic devices.
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from the dry etching were found to be much lower than those from wet etching. 3.2. Effect of temperature and pressure on etch rate
Fig. 1. Schematic representation of scCO2 dry etching process. (A): CO2 cylinder; (B): ISCO syringe pump; (C): Mixing chamber; (D): Sample chamber; (E): View cell; (F): Wafer; (G): Electromagnetic stirrer; (H): NaOH solution; (J): Water bath.
Temperature is one of the most important parameters that affect etching behavior [27]. A temperature range of 45–75 °C was investigated in this study. Fig. 2 depicts etched thickness of BPSG, P-TEOS, Thermal oxide and SiN with HF/Pyridine in scCO2 as a function of temperature at 20.7 MPa. Etch rate was found to increase with an increase in temperature. The etch rate of BPSG is more sensitive to temperature compared to the other films increasing 10 times with an increase in temperature from 45 °C to 75 °C.
2. Experimental A HF-compatible two-chamber etching apparatus (e.g. prechamber and etching chamber) shown in Fig. 1 was used in this study. The pre-chamber was designed for the preparation of etchant solution with pre-mixing of co-solvent and HF/Pyridine in CO2. HF/Pyridine complex (70:30 wt/wt%; Aldrich) was used as a dry source of HF. HF/pyridine complex was further diluted with anhydrous pyridine. 1M NaOH solution was used to neutralize HF after etching. Carbon dioxide (99.99%, Dae Young Co.) was used as received. A wide variety of co-solvents (e.g., methanol, ethanol, isopropyl alcohol, butanol) were used to observe their effects on etching performance. Blanket wafers with three sacrificial oxides of borophosphosilicate glass (BPSG), p-tetraethylorthosilicate (PTEOS), and Thermal oxide thin films of 900 nm each and SiN thin film of 120 nm were cleaved into 1 1 cm2 samples before use. Thicknesses of wafer films before and after etching were measured using a multiwavelength ellipsometer(L2W16E.830, Gaertner Scientific Corp.) equipped with both a 633 nm HeNe gas laser and a 830 nm laser diode at an angle of incidence of 70°. Each sample was measured in five different locations, and an average value was calculated. A patterned wafer composed of sacrificial BPSG, SiN etch stop and TiN rod was also used for etching experiment.
Fig. 2. Etched thickness of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of processing temperature (HF/Pyridine, [HF] = 0.63 mM, 20.7 MPa, 3 min.).
3. Results and discussion 3.1. Wet etching versus dry etching Table 1 shows etch rates and selectivity ratios resulted from wet etching with aqueous HF and dry etching with HF/Pyridine in scCO2. Wet etching experiments were carried out with 5 mM HF solution while only 0.63 mM HF concentration was used in dry etching method. Etch rates were calculated by dividing etched thickness by etch time. Etch selectivity ratios of sacrificial oxides were measured with respect to the etch rate of SiN. It was found that both the etch rate and selectivity ratio of sacrificial oxides were in the order of BPSG > P-TEOS > thermal oxide from the wet etching. In contrast, the etch rates of P-TEOS and Thermal oxide were almost same from the dry etching. It is notable that the etch rates of sacrificial oxides in HF/Pyridine/scCO2 were several times higher compared to the wet etching while the selectivity ratios
Fig. 3. scCO2 dry etching of BPSG and SiN at different pressures (HF/py, [HF] = 0.58 mM, 40 °C, 5 min).
Table 1 Comparison of etch rates and selectivity ratios of BPSG, P-TEOS, thermal SiO2 and SiN by wet etching with aqueous HF and dry etching with HF/Pyridine in scCO2. Etching method
Wet etching with aqueous HF, [HF] = 5 mM Dry etching with HF/Pyridine in scCO2, [HF] = 0.63 mM
Etch rate, nm/min
Selectivity (against SiN)
BPSG
P-TEOS
Thermal SiO2
SiN
BPSG
P-TEOS
Thermal SiO2
SiN
21.9 89.6
9.0 52.9
5.8 55.1
0.1 6.2
219 14.4
90 8.5
58 8.9
1 1
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energy for BPSG was calculated to be 52.6 KJ/mol using the equation
Etch rate ¼ As expðDE=RTÞ
Fig. 4. Etched thickness of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of alcoholic co-solvents. ([HF] = 0.32 mM, 55 °C, 20.7 MPa, 3 min).
assuming no change in the pre-exponential factor which includes concentration terms [29]. The selectivity ratios of BPSG, P-TEOS, and Thermal SiO2 with respect to SiN decreased slightly with an increase in temperature from 45 °C to 55 °C and above this temperature they became almost unchanged. The effect of pressure on etch rate of BPSG and SiN with HF/Pyridine at 40 °C in scCO2 was shown in Fig. 3. The etch rate of BPSG increased slightly with an increase in pressure although the increase was negligibly small for SiN. The higher pressure is also helpful in producing isotropic etching to control the lateral recess offset as a higher pressure increases CO2 density and enhances the mass transport. The rate increase effect, however, was not as pronounced as that of temperature, which suggests that the etch reaction is surface reaction rate controlled rather than mass transport controlled. 3.3. Effect of the presence of co-solvents
Fig. 5. Change in etched thickness of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of HF concentration in scCO2 at 45 °C and 20.7 MPa for 1 min.
On the other hand, the dependencies of etch rates of P-TEOS and Thermal SiO2 on temperature were almost same. The higher etch rate of BPSG is mainly due to the presence of phosphorus. In addition, the @BOSi„ and @BOB@ bonds are more rapidly attacked by H+ ions [28]. Therefore, the etch rate of BPSG can be controlled easily by changing temperature. The apparent activation
A wide variety of organic solvents were investigated to determine the most suitable co-solvent for a high etch rate. Fig. 4 revealed etched thicknesses of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of etchant co-solvents. Methanol, ethanol, isopropyl alcohol (IPA) and butanol in the ratio of 1:10 were used for dry etching with HF/Pyridine in scCO2 at 55 °C and 20.7 MPa for 3 min. It was found that the etch rate of BPSG increased in the presence of alcoholic co-solvents. The increased rate is most likely due to the high dissolution of etching residues from etching sites. Otherwise, the residues would remain on the site and inhibit further access of fresh etchant to the sites. Owing to the absence of a dipole moment, scCO2 is a weak solvent for polar or large molecular species. Hence the addition of alcoholic co-solvents to scCO2 increases its solvent strength. The other important role of co-solvent is the facile formation of active HF2 species in the co-solvent mixture [26]. The etch rate depends on the chemical structure of alcoholic co-solvent. Fig. 4 reveals that in presence of IPA the etch rate of BPSG was surprisingly high compared to others. An etch rate as high as about 200 nm/min was obtained with IPA co-solvent. The better solvent power of IPA for BPSG residues than other alcohols may be one of the major reasons for the higher etch rate. In the case of P-TEOS, Thermal SiO2 and SiN, the reactivity showed the following order: methanol > ethanol > IPA > butanol, but the rate difference was quite small among the co-solvents.
Fig. 6. SEM images of patterned wafers etched with HF/Pyridine/IPA at 45 °C and 20.7 MPa for a period of 2 min (a), 3 min (b) and 5 min (c).
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3.4. Effect of HF concentration Fig. 5 shows the change in thickness of BPSG, P-TEOS, Thermal SiO2 and SiN after etching for 1 min in HF/Pyridine/IPA mixture dissolved in scCO2 at 45 °C and 20.7 MPa as a function of HF concentration. Etch rate increased linearly with HF concentration. The etch rate of BPSG was 199 nm/min at an HF concentration of 0.32 mM and about 850 nm/min at an HF concentration of 1.00 mM. A significant amount of etching was found to occur at a concentration as low as 0.30 mM. It is noteworthy that the etch selectivity of BPSG increased from 83 to 121 as HF concentration varies from 0.30 to 1.00 mM. This reveals that etching performance with HF/Pyridine/IPA complex using scCO2 can be also controlled easily by changing the concentration of etchant. 3.5. Etching of a patterned wafer Fig. 6 shows the SEM images of a patterned wafer etched with HF/Pyridine/IPA at 45 °C and 20.7 MPa for a period of 2 min (a), 3 min (b) and 5 min (c), respectively. The 2 min-reaction resulted in incomplete etching (see Fig. 6a), whereas the complete etching of sacrificial oxides was took place at 3 min-etching as seen in Fig. 6b. However, excessive reaction was found at 5 min-etching. Fig. 6c reveals that the etching continues to the SiN etch stop layer. A rod-collapsed wafer structure was also found in the image, which means too much etching of the wafer at 5 min. Thus the precise control of etch time is very important to make perfect structures as the etch rate is so fast. 4. Conclusion Etching of sacrificial layers of BPSG, P-TEOS, and Thermal SiO2 and protective structural layer of SiN with HF/Pyridine in scCO2 could provide a more environmentally friendly method to meet future demands of semiconductor industries. Etch rates were found to increase with an increase in temperature, pressure, and concentration of etchant, among which the effects of temperature and HF concentration were remarkable. The presence of alcoholic co-solvents with HF/Pyridine complex improved the etch rate. The presence of IPA as a co-solvent showed significant influence on etching of BPSG with HF/Pyridine in scCO2. Based on the experimental results, it is concluded that the etching performance could be manip-
ulated by co-solvents and reaction parameters temperature, pressure and HF concentration.
such
as
Acknowledgements This work was supported by ‘‘System IC 2010” project of Korea Ministry of Knowledge Economy. References [1] C.A. Jones III, D. Yang, E.A. Irene, S.M. Gross, M. Wagner, J. DeYoung, J.M. DeSimone, Chem. Mater. 15 (2003) 2867. [2] D.J. Monk, D.S. Soane, R.T. Hower, Transducer 24 (1991) 647. [3] D.J. Monk, D.S. Soane, J. Electrochem. Soc. 140 (1993) 2339. [4] W. Kern, RCA Rev. 47 (1986) 189. [5] W.R. Runyan, K.E. Bean, Semiconductor Integrated Circuit Processing Technology, Addison-Wiley, Boston, MA, 1990. [6] R. Legtenberg, J. Elders, M. Elwenspoek, in: International Conference on Solid State Sensors and Actuators, Transducers ’93, Yokohama, 1993, p. 198. [7] C.H. Mastrangelo, C.H. Hsu, J. Microelectromech. Syst. 2 (1993) 33. [8] B. Livshits, O. Taher-Zahar, Solid State Technol. 40 (1997) 197. [9] I. Jafri, H. Busta, S.T. Walsh, in: Proceedings of the SPIE. The International Society for Optical Engineering, 1999, p. 51. [10] P.J. Resnick, P.J. Clews, in: Proceedings of the SPIE. The International Society for Optical Engineering, 2001, p. 189. [11] H. Watanabe, S. Ohnishi, I. Honma, H. Kitajima, H. Ono, J. Electrochem. Soc. 142 (1995) 237. [12] J.I. Wang, C.A. Choi, M.L. Lee, H.C. Jun, T.K. Youn, J. Micromech. Microeng. 12 (2002) 297. [13] G.L. Weibel, C.K. Ober, Microelectron. Eng. 65 (2003) 145. [14] K. Saga, T. Hattori, Solid State Phenom. 134 (2008) 97. [15] K. Saga, H. Kuniyasu, T. Hattori, K. Saito, I. Mizobata, T. Iwata, S. Hirae, Solid State Phenom. 134 (2008) 355. [16] J.A. Keagy, X. Zhang, K.P. Johnston, E. Busch, F. Weber, P.J. Wolf, T. Rhoad, J. Supercrit. Fluid 39 (2006) 277. [17] J.A. Keagy, Y. Li, P.F. Green, K.P. Johnston, J. Supercrit. Fluid 42 (2007) 398. [18] P.D. Matz, R.F. Reidy, Solid State Phenom. 103 (2005) 315. [19] B. Xie, L. Choate, A.J. Muscat, Microelectron. Eng. 80 (2005) 349. [20] B. Xie, A.J. Muscat, Microelectron. Eng. 76 (2004) 52. [21] B. Xie, A.J. Muscat, Microelectron. Eng. 82 (2005) 434. [22] E. Kondoh, K. Shigama, Thin Solid Films 491 (2005) 228. [23] C.A. Bessel, G.M. Denison, J.M. DeSimone, J. DeYoung, S. Gross, C.K. Schauer, P.M. Visintin, J. Am. Chem. Soc. 125 (2003) 4980. [24] E.N. Hoggan, K. Wang, D. Flowers, J.M. DeSimone, R.G. Carbonell, IEEE Trans. Semicond. Manuf. 17 (2004) 510. [25] K. Saga, H. Kuniyasu, T. Hattori, K. Yamada, T. Azuma, Solid State Phenom. 103– 104 (2005) 115. [26] B. Xie, C.C. Finstad, A.J. Muscat, Chem. Mater. 17 (2005) 1753. [27] J.S. Judge, J. Electrochem. Soc. 118 (1971) 1772. [28] G. Spierings, J. Mater. Sci. 28 (1993) 6261. [29] G.I. Parisi, S.E. Haszko, G.A. Rozgonyi, J. Electrochem. Soc. 124 (1977) 917.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Improved etching method for microelectronic devices with supercritical carbon dioxide Jae Hyun Bae a, Md. Zahangir Alam a, Jae Mok Jung a, Yeong-Soon Gal b, Hyosan Lee c, Hyun Gyu Kim d, Kwon Taek Lim a,* a
Division of Image System Science and Engineering, Pukyoung National University Busan 608-739, Republic of Korea Polymer Chemistry Laboratory, College of Engineering, Kyung Il University, Republic of Korea c Process Development Team, Memory Division, Samsung Electronics, Republic of Korea d Korea Basic Science Institute, Busan Centre, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 16 April 2008 Received in revised form 26 August 2008 Accepted 11 October 2008 Available online 18 October 2008 Keywords: Etching Sacrificial oxide SiN Supercritical carbon dioxide MEMS
a b s t r a c t Aqueous etchants used in traditional wet etching for the production of integrated circuits and MEMS devices hinder the processes and pose environmental difficulties. Therefore, we developed an improved dry etching method with HF/Pyridine (7:3) in supercritical carbon dioxide. Etch rates of BPSG, P-TEOS, Thermal SiO2 and SiN with dry etching method were several times higher than those in wet etching. Etch rates were found to be a function of temperature, HF concentration, and the kind of co-solvents. The presence of alcoholic co-solvents, especially IPA with HF/Pyridine etchant greatly increased the etch rate of BPSG. Etch selectivity could be controlled with the etchant concentration. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction As minimum feature sizes for integrated circuits (ICs) continue to shrink into the deep sub-half micrometer region, nanoscale structures will come to have a much higher aspect ratio (height/ width). Wafer processing in the microelectronics industry by wet isotropic etching in aqueous hydrogen fluoride (HF) is a fundamental step in the fabrication of integrated circuits (e. g. dynamic random access memory, DRAM capacitor fabrication in front end of the line, FEOL and post-etch residue removal in back end of the line, BEOL cleaning) [1–5]. Wet etching of sacrificial layers followed by common drying methods often causes sticking of microstructures [6,7] and ineffective deep cleaning and induces damage in high aspect ratio structures, which are the most critical obstacles to next generation semiconductor devices. Other major disadvantages of wet etching are corrosive and toxic materials that may incur high costs in handling and disposal [8]. Anhydrous HF gasphase etching and supercritical carbon dioxide drying has been developed to remove the sticking problem [9–12]. Recently, compressed carbon dioxide known as supercritical carbon dioxide (scCO2) has received a great attention of the scientists as a prospective alternative to current aqueous and organic
* Corresponding author. Tel.: +82 51 629 6409; fax: +82 51 629 6408. E-mail address: [email protected] (K.T. Lim). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.10.003
solvents in microelectronics fabrication facilities (FABs) [1,13]. The utilization of scCO2 has been demonstrated in several applications including drying [14], photoresist stripping [15], removal of post-ash and post-etch residues [16], low-k repair [17–21], metal deposition [22], chemical mechanical planarization [23], and dry lithography [24]. Supercritical CO2 is considered as a ‘‘dry” environmentally benign solvent because it is a gas at normal temperature and pressure and therefore, evaporates immediately from any surface or device that is being processed. Jones et al. developed HF/ Pyridine in scCO2 as an etchant [1]. The applications of scCO2based etching method addressed to microelectro mechanical systems (MEMS) device fabrication also have been reported [14,25]. In this work, we studied dry etching processes with non-aqueous HF/Pyridine using supercritical carbon dioxide. The effect of process variables and co-solvent on the etching performance such as etching rate and selectivity was investigated. scCO2 possesses zero surface tension and tunable solvent property: its density can be controlled via manipulation of both temperature and/or pressure. An enhanced etching rate resulting from low viscosity and high diffusivity is another distinct advantage. Again, scCO2 allows a variety of constituents to be dissolved at low temperature and also provide a high flux of etchant to support etching [13,26]. scCO2-Based solutions in wafer etching would eliminate waterbased drying step, and thus prevent structural damage to materials in microelectronic devices.
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from the dry etching were found to be much lower than those from wet etching. 3.2. Effect of temperature and pressure on etch rate
Fig. 1. Schematic representation of scCO2 dry etching process. (A): CO2 cylinder; (B): ISCO syringe pump; (C): Mixing chamber; (D): Sample chamber; (E): View cell; (F): Wafer; (G): Electromagnetic stirrer; (H): NaOH solution; (J): Water bath.
Temperature is one of the most important parameters that affect etching behavior [27]. A temperature range of 45–75 °C was investigated in this study. Fig. 2 depicts etched thickness of BPSG, P-TEOS, Thermal oxide and SiN with HF/Pyridine in scCO2 as a function of temperature at 20.7 MPa. Etch rate was found to increase with an increase in temperature. The etch rate of BPSG is more sensitive to temperature compared to the other films increasing 10 times with an increase in temperature from 45 °C to 75 °C.
2. Experimental A HF-compatible two-chamber etching apparatus (e.g. prechamber and etching chamber) shown in Fig. 1 was used in this study. The pre-chamber was designed for the preparation of etchant solution with pre-mixing of co-solvent and HF/Pyridine in CO2. HF/Pyridine complex (70:30 wt/wt%; Aldrich) was used as a dry source of HF. HF/pyridine complex was further diluted with anhydrous pyridine. 1M NaOH solution was used to neutralize HF after etching. Carbon dioxide (99.99%, Dae Young Co.) was used as received. A wide variety of co-solvents (e.g., methanol, ethanol, isopropyl alcohol, butanol) were used to observe their effects on etching performance. Blanket wafers with three sacrificial oxides of borophosphosilicate glass (BPSG), p-tetraethylorthosilicate (PTEOS), and Thermal oxide thin films of 900 nm each and SiN thin film of 120 nm were cleaved into 1 1 cm2 samples before use. Thicknesses of wafer films before and after etching were measured using a multiwavelength ellipsometer(L2W16E.830, Gaertner Scientific Corp.) equipped with both a 633 nm HeNe gas laser and a 830 nm laser diode at an angle of incidence of 70°. Each sample was measured in five different locations, and an average value was calculated. A patterned wafer composed of sacrificial BPSG, SiN etch stop and TiN rod was also used for etching experiment.
Fig. 2. Etched thickness of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of processing temperature (HF/Pyridine, [HF] = 0.63 mM, 20.7 MPa, 3 min.).
3. Results and discussion 3.1. Wet etching versus dry etching Table 1 shows etch rates and selectivity ratios resulted from wet etching with aqueous HF and dry etching with HF/Pyridine in scCO2. Wet etching experiments were carried out with 5 mM HF solution while only 0.63 mM HF concentration was used in dry etching method. Etch rates were calculated by dividing etched thickness by etch time. Etch selectivity ratios of sacrificial oxides were measured with respect to the etch rate of SiN. It was found that both the etch rate and selectivity ratio of sacrificial oxides were in the order of BPSG > P-TEOS > thermal oxide from the wet etching. In contrast, the etch rates of P-TEOS and Thermal oxide were almost same from the dry etching. It is notable that the etch rates of sacrificial oxides in HF/Pyridine/scCO2 were several times higher compared to the wet etching while the selectivity ratios
Fig. 3. scCO2 dry etching of BPSG and SiN at different pressures (HF/py, [HF] = 0.58 mM, 40 °C, 5 min).
Table 1 Comparison of etch rates and selectivity ratios of BPSG, P-TEOS, thermal SiO2 and SiN by wet etching with aqueous HF and dry etching with HF/Pyridine in scCO2. Etching method
Wet etching with aqueous HF, [HF] = 5 mM Dry etching with HF/Pyridine in scCO2, [HF] = 0.63 mM
Etch rate, nm/min
Selectivity (against SiN)
BPSG
P-TEOS
Thermal SiO2
SiN
BPSG
P-TEOS
Thermal SiO2
SiN
21.9 89.6
9.0 52.9
5.8 55.1
0.1 6.2
219 14.4
90 8.5
58 8.9
1 1
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energy for BPSG was calculated to be 52.6 KJ/mol using the equation
Etch rate ¼ As expðDE=RTÞ
Fig. 4. Etched thickness of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of alcoholic co-solvents. ([HF] = 0.32 mM, 55 °C, 20.7 MPa, 3 min).
assuming no change in the pre-exponential factor which includes concentration terms [29]. The selectivity ratios of BPSG, P-TEOS, and Thermal SiO2 with respect to SiN decreased slightly with an increase in temperature from 45 °C to 55 °C and above this temperature they became almost unchanged. The effect of pressure on etch rate of BPSG and SiN with HF/Pyridine at 40 °C in scCO2 was shown in Fig. 3. The etch rate of BPSG increased slightly with an increase in pressure although the increase was negligibly small for SiN. The higher pressure is also helpful in producing isotropic etching to control the lateral recess offset as a higher pressure increases CO2 density and enhances the mass transport. The rate increase effect, however, was not as pronounced as that of temperature, which suggests that the etch reaction is surface reaction rate controlled rather than mass transport controlled. 3.3. Effect of the presence of co-solvents
Fig. 5. Change in etched thickness of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of HF concentration in scCO2 at 45 °C and 20.7 MPa for 1 min.
On the other hand, the dependencies of etch rates of P-TEOS and Thermal SiO2 on temperature were almost same. The higher etch rate of BPSG is mainly due to the presence of phosphorus. In addition, the @BOSi„ and @BOB@ bonds are more rapidly attacked by H+ ions [28]. Therefore, the etch rate of BPSG can be controlled easily by changing temperature. The apparent activation
A wide variety of organic solvents were investigated to determine the most suitable co-solvent for a high etch rate. Fig. 4 revealed etched thicknesses of BPSG, P-TEOS, Thermal SiO2 and SiN as a function of etchant co-solvents. Methanol, ethanol, isopropyl alcohol (IPA) and butanol in the ratio of 1:10 were used for dry etching with HF/Pyridine in scCO2 at 55 °C and 20.7 MPa for 3 min. It was found that the etch rate of BPSG increased in the presence of alcoholic co-solvents. The increased rate is most likely due to the high dissolution of etching residues from etching sites. Otherwise, the residues would remain on the site and inhibit further access of fresh etchant to the sites. Owing to the absence of a dipole moment, scCO2 is a weak solvent for polar or large molecular species. Hence the addition of alcoholic co-solvents to scCO2 increases its solvent strength. The other important role of co-solvent is the facile formation of active HF2 species in the co-solvent mixture [26]. The etch rate depends on the chemical structure of alcoholic co-solvent. Fig. 4 reveals that in presence of IPA the etch rate of BPSG was surprisingly high compared to others. An etch rate as high as about 200 nm/min was obtained with IPA co-solvent. The better solvent power of IPA for BPSG residues than other alcohols may be one of the major reasons for the higher etch rate. In the case of P-TEOS, Thermal SiO2 and SiN, the reactivity showed the following order: methanol > ethanol > IPA > butanol, but the rate difference was quite small among the co-solvents.
Fig. 6. SEM images of patterned wafers etched with HF/Pyridine/IPA at 45 °C and 20.7 MPa for a period of 2 min (a), 3 min (b) and 5 min (c).
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3.4. Effect of HF concentration Fig. 5 shows the change in thickness of BPSG, P-TEOS, Thermal SiO2 and SiN after etching for 1 min in HF/Pyridine/IPA mixture dissolved in scCO2 at 45 °C and 20.7 MPa as a function of HF concentration. Etch rate increased linearly with HF concentration. The etch rate of BPSG was 199 nm/min at an HF concentration of 0.32 mM and about 850 nm/min at an HF concentration of 1.00 mM. A significant amount of etching was found to occur at a concentration as low as 0.30 mM. It is noteworthy that the etch selectivity of BPSG increased from 83 to 121 as HF concentration varies from 0.30 to 1.00 mM. This reveals that etching performance with HF/Pyridine/IPA complex using scCO2 can be also controlled easily by changing the concentration of etchant. 3.5. Etching of a patterned wafer Fig. 6 shows the SEM images of a patterned wafer etched with HF/Pyridine/IPA at 45 °C and 20.7 MPa for a period of 2 min (a), 3 min (b) and 5 min (c), respectively. The 2 min-reaction resulted in incomplete etching (see Fig. 6a), whereas the complete etching of sacrificial oxides was took place at 3 min-etching as seen in Fig. 6b. However, excessive reaction was found at 5 min-etching. Fig. 6c reveals that the etching continues to the SiN etch stop layer. A rod-collapsed wafer structure was also found in the image, which means too much etching of the wafer at 5 min. Thus the precise control of etch time is very important to make perfect structures as the etch rate is so fast. 4. Conclusion Etching of sacrificial layers of BPSG, P-TEOS, and Thermal SiO2 and protective structural layer of SiN with HF/Pyridine in scCO2 could provide a more environmentally friendly method to meet future demands of semiconductor industries. Etch rates were found to increase with an increase in temperature, pressure, and concentration of etchant, among which the effects of temperature and HF concentration were remarkable. The presence of alcoholic co-solvents with HF/Pyridine complex improved the etch rate. The presence of IPA as a co-solvent showed significant influence on etching of BPSG with HF/Pyridine in scCO2. Based on the experimental results, it is concluded that the etching performance could be manip-
ulated by co-solvents and reaction parameters temperature, pressure and HF concentration.
such
as
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