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Repairing plasma-damaged low-k HSQ films with trimethylchlorosilane treatment
Materials Science and Engineering B 127 (2006) 29–33
Repairing plasma-damaged low-k HSQ films with trimethylchlorosilane treatment S.K. Singh, A.A. Kumbhar, R.O. Dusane ∗ Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai 400076, India Received 28 July 2005; received in revised form 6 September 2005; accepted 9 September 2005
Abstract Low-density materials, such as the commercially available hydrogen silsesquioxane (HSQ) offer a low dielectric constant. Thus HSQ with a low value of k (∼2.85) can be spin-coated if the density of Si H bonding is maintained at a high level and the formation of OH bonds and absorption or creation of water in the film is minimized. O2 plasma exposure on HSQ film properties increases leakage current of metal/HSQ/Si/metal structures. Also the dielectric constant shows a significant increase after O2 plasma exposure. Another important consequence of the O2 plasma exposure is the large decrease in the contact angle of the HSQ surface. In this paper, we demonstrate first damage repair process involving trimethylchlorosilane (TMCS) treatment for 10 min at atmospheric pressure leads to a regain of a leakage current density and dielectric which approach values very near to the as-deposited film. These results show that the TMCS treatment is a promising technique to repair the damage even in the commercially available and highly applicable low-k material and increase the visibility of its use at the 0.1 m technology. The increase of the hydrophilic nature of the surface after O2 plasma exposure leads to increase absorption of moisture with a subsequent increase in the dielectric constant. © 2005 Elsevier B.V. All rights reserved. Keywords: HSQ; RF plasma; Low-k dielectrics; Contact angle; Trimethylchlorosilane (TMCS)
1. Introduction The shrinking in dimensions of devices in ULSI circuits to achieve faster performance has led to a very large device density in today’s ULSI circuits. Subsequently interconnects are becoming narrower (increases their resistance, R) and are coming nearer (increases the capacitance, C). This leads to the increase of the RC delay, which increases with scaling down and leads to various problems. This RC delay can be reduced by using Cu instead of Al (by approximately 35%), and by using inter-metal dielectric with dielectric constant k lower than that of SiO2 (∼4), so called low-k dielectrics [1–3]. Replacement of SiO2 by air (k∼1) and Al by Cu for the same circuitry can reduce RC delay by 75%. The general way of decreasing the dielectric constant is to increase the porosity in the low-k films. Dielectric materials presently under investigation are HSQ, porous silica, MSQ and even materials like silicon–carbon and fluorinated carbon [5–7]. Hydrogen silsesquioxane (HSQ) is one of the
∗
Corresponding author. Fax: +91-22-25723480. E-mail address: [email protected] (R.O. Dusane).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.09.042
promising candidates with a highly porous three-dimensional network structure. The general formula for HSQ is (HSiO3/2 )2n where n = 3–8. Hydrogen silsesquioxane has many advantages, including a low dielectric constant, has no carbon, non-etch back processing, excellent gap filling, good planarization, and low moisture uptake. Multilevel wiring schemes are now routinely used and would become more demanding with the increase in device density in the circuits. The dielectric between the wiring levels must have high quality and reliability, low stress, prepared through a simple process, and should be compatible to different processes employed during integration [4]. Plasma exposure is widely used for post-etch treatment, such as resist strip or residue etch cleaning [8]. Oxygen plasma is commonly used for this purpose. However, oxygen plasma oxidizes most of the low-k dielectrics, removing hydrophobic Si H bonds, and increases the hydrophilicity of these films. In other words, the oxygen plasma destroys the low-k properties of HSQ films. Thus the dielectric properties of the HSQ will deteriorate during photo-resist stripping process [9]. In this work, we have studied the effect of O2 plasma exposure on the structural and electrical properties of the spin-coated HSQ films. The effect of O2 plasma exposure is investigated to understand the impact
S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
30
of this process on the dielectric film quality. We also studied the damage recovery by TMCS vapor exposure at atmospheric pressure. 2. Experimental The standard RCA cleaned p-type Si1 0 0 wafers are spincoated with a single layer of HSQ, and baked sequentially on the hot plate at 150, 250 and 350 ◦ C for 1 min and followed by furnace curing at 400 ◦ C for 1 h in nitrogen ambient. The films were exposed to a 300 W radio frequency (13.56 MHz) capacitively coupled O2 plasma for 3, 6 and 9 min at a pressure of 500 mTorr and oxygen gas flow rate of 10 sccm. The as cured HSQ films and the O2 plasma-exposed films were treated with TMCS vapor generated by heating TMCS solution to 60 ◦ C for 10 min. The samples were then annealed at 400 ◦ C in N2 ambient for 1 h. The chemical bonding of the HSQ films before and after various treatments was investigated by Fourier transform infrared (FT-IR) spectroscopy (Nicolet Model Magna 550). The film thickness and refractive index were measured by Ellipsometry (Scentech Model SE 800). The film surface hydrophobicity is studied by contact angle measurements (Reme Hardt, Model 100-00-230). MIS capacitors were fabricated by evaporating aluminum (0.25 mm dots for front contact) and also as the back contact to the silicon wafer. A Model 82 CV (Agilent) meter was used to measure the dielectric constant of HSQ. The capacitance was measured at 100 KHz. The dc current–voltage (I–V) characteristics were measured with a Keithly (Model SMU 2400) setup to evaluate the insulating property of HSQ film. 3. Results and discussion Fig. 1(a) and (b) shows the FT-IR spectra of HSQ films before and after O2 plasma exposure (for different times) in the range 4000–500 and 1300–600 cm−1 respectively. The FT-IR spectrum of HSQ gives the following absorption peaks after O2 plasma exposure: 1. 2. 3. 4. 5. 6.
Si Si Si Si Si Si
Fig. 1. (a) FT-IR spectra of HSQ before and after a series of O2 plasma exposure; (b) the IR spectra in the range 1300–600 cm−1 depicting the variations in the bending modes.
H stretch mode (2250 cm−1 ), O stretch cage-like peak (1130 cm−1 ), O stretch network (1070 cm−1 ), O bending cage-like (860 cm−1 ), O bending network peak (830 cm−1 ), OH (∼3350 cm−1 ) (this corresponds to moisture).
The appearance of the Si OH bonds in the plasma-exposed films at 3350 cm−1 is due to the conversion of the Si H bonds (2250 cm−1 ) into Si OH (3350 cm−1 ) bonds when O2 plasma interacts with the HSQ films. The intensity of the Si H bond decreases and the intensity of the Si OH bond increases with the increase in O2 plasma treatment time. The increase of the Si OH bond signal has two possible causes. One is that when the O2 plasma breaks Si H bonds, HSQ immediately absorbs O radicals to convert Si H bonds into Si OH bonds. The other is that the dangling bonds absorb water immediately when the HSQ film is exposed to air after the O2 plasma treatment. Fig. 2
Fig. 2. Integrated area of the Si H and Si OH vibrational peaks of HSQ film as a function of O2 plasma exposure time.
S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
Fig. 3. Contact angle of HSQ film as a function of O2 plasma exposure time.
31
Fig. 4. Refractive index of HSQ film as a function of O2 plasma exposure time.
shows the variation of the intensities of the Si H and Si OH bonding configurations. The decrease in the integrated intensity of Si H bond and the subsequent increase in the Si OH bonds with increase in O2 plasma exposure time. The possible reaction is 2Si H + O2 = 2Si OH Another prominent observation is in the bending mode at 800–900 cm−1 (Fig. 1b). The O2 plasma exposure retransforms the HSQ network into cage-like structure. This change gets reflected only in the Si O bending modes. In order to confirm that the hydrophilicity of the film surface does increase after the oxygen plasma exposure, we carried out the contact angle measurement with water drop. These observations are depicted in Fig. 3. The contact angle for the as-deposited film is ∼64◦ while it decreases to ∼20◦ after O2 plasma exposure. This is because the O2 plasma causes many of the Si H bonds to break, leaving many dangling bonds in the film [10]. Some of those dangling bonds form Si O bonds and others become Si OH bonds through moisture absorption. As exposure time of O2 plasma increases the HSQ film absorbs more water molecules and this is also reflected in the FT-IR spectra shown in Fig. 1a and in the refractive index (Fig. 4). As plasma exposure time increases, the refractive index of the HSQ film increases indicating a more SiO2 like character of the film after plasma exposure. The most significant effect of concern is the increase in the dielectric constant of the films after plasma exposure. Fig. 5 shows that the dielectric constant of HSQ film increases with increasing O2 plasma exposure time. The reason of this again is the structural transformation of Si H bonds to Si OH and Si O bonds [11]. The high polarity water molecules (dielectric constant ∼76) present in the HSQ film give rise to an increase in both leakage current and dielectric constant. Fig. 6 shows the leakage current density of HSQ after the O2 plasma exposure for 3–9 min. The leakage current increases by two orders of magnitude after a 9 min plasma exposure.
Fig. 5. Dielectric constant of HSQ film as a function of O2 plasma exposure time.
Fig. 6. Leakage current density of HSQ film for O2 plasma exposure for 3, 6 and 9 min.
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S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
Fig. 9. Water contact angle of the as cured, plasma-exposed and TMCS-treated HSQ film. Fig. 7. FT-IR spectra of the HSQ film before and after O2 plasma-exposed and TMCS treatment.
4. O2 plasma damage recovery by TMCS treatment Now we consider the effect of a TMCS treatment on the plasma-damaged HSQ films. From Fig. 7, the band corresponding to Si OH (3400 cm−1 ) reduces. Another important observation is that there is significant increase in the Si H bond density. By O2 plasma exposure there was a change from network to cage structure (which was seen in the bending modes of Si O). The TMCS treatment reverse transformation, i.e. from cage to network takes place. This restores the dielectric property of the damaged HSQ films. The TMCS treatment leads to a decrease in the refractive index (Fig. 8). Also, the TMCS treatment changes the film surface from hydrophilic to hydrophobic (Fig. 9). Regarding the leakage current density, we have seen that it increases by nearly two orders of magnitude when HSQ sample undergoes O2 plasma exposure. As shown in Fig. 10, subsequent TMCS treatment, the leakage current density decreases by three orders of magnitude. Thus the electrical properties of the films
Fig. 8. Refractive index of the as cured, plasma-exposed and TMCS-treated HSQ film.
Fig. 10. Leakage current density of the HSQ film before and after O2 plasma and TMCS treatment.
are also regained after TMCS treatment. It is also interesting to see, that the dielectric constant which increased significantly with plasma exposure can be regained by the after TMCS treatment, as shown in Fig. 11. After a sequential TMCS treatment,
Fig. 11. Dielectric constant of the HSQ film before, after O2 plasma and TMCS treatment.
S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
the dielectric constant reduced to 3. These observations clearly indicate that the TMCS treatment can repair the damage of HSQ films. The possible reasons of the repair of the plasma damage has also been discussed by Chang et al. [10]. In their case where they have studied the repair for porous silica (MSQ) by TMCS, the C H bond which was damaged by the plasma got replenished and also the CH3 from the TMCS got attached to sites generated by the plasma thus rendering the surface more hydrophobic. In our case there is no C H. However we think that the attachment of CH3 to form Me3 Si O Si in the HSQ at the sites generated by the plasma repairs the damage and also makes the surface hydrophobic thus leading to the damage repair. Secondly there is a clear increase in the Si H related absorption after TMCS treatment, which is responsible for the improvement of the electrical performance of the device. 5. Conclusions In this study, we have shown that even HSQ films are prone to damage when exposed to oxygen plasma. As we know such plasma exposure is unavoidable during the O2 plasma ashing step in a VLSI fabrication process. Effects similar to those observed in porous silica such as increase in dielectric constant and leakage current occur in the HSQ films. However, these changes involve the breaking of the Si H bonds in the case of HSQ leading to the deterioration of the film properties. It was also seen that the TMCS repaired the damage, which is occurred during this plasma exposure. The Si OH bonds could be eliminated and transformed into Si H bonds by TMCS treatment. The Si H bonds possess hydrophobic properties and thus the surface
33
the HSQ can be changed from hydrophilic to hydrophobic. This hydrophobic surface would also prevent moisture uptake keeping the leakage current and the dielectric constant to the desired values. In conclusion therefore, TMCS treatment is an effective method to repair the damage caused during O2 plasma ashing even in the case of HSQ films. Acknowledgement The work was carried out with financial support from the Department of Science and Technology, Govt. of India under the project No. SR/FTP/PS-41/2000. References [1] S. Bothra, B. Rogers, M. Kellam, C.M. Osburn, IEEE Trans. Electron Dev. 40 (1993) 591. [2] T. Sakurai, IEEE Trans. Electron Dev. 40 (1993) 118. [3] B.K. Liew, P. Fang, N.W. Cheung, C. Hu, IEEE Trans. Electron Dev. 39 (1992) 2472. [4] P.T. Liu, T.C. Chang, Y.L. Yang, Y.F. Cheng, S.M. Sze, IEEE Trans. Electron Dev. 47 (2000) 1733. [5] P.T. Liu, T.C. Chang, K.C. Hsu, T.Y. Tseng, L.M. Chen, C.J. Wang, S.M. Sze, Thin Solid Films 414 (2002) 1. [6] A. Gill, Diam. Relat. Mater. 10 (2001) 234. [7] K. Maex, M.R. Baklanov, D. Shamiryan, F. Iacopi, S.H. Brongersma, Z.S. Yanovitskaya, J. Appl. Phys. 93 (2003) 8793. [8] M.J. Loboda, C.M. Grove, R.F. Schneider, J. Electrochem. Soc. 145 (1998) 2861. [9] S.W. Hwang, G.R. Lee, J.H. Min, S.H. Moon, Surf. Coat. Technol. 174–175 (2003) 835. [10] T.C. Chang, Y.S. Mor, P.T. Liu, T.M. Tsai, C.W. Chen, C.J. Chu, F.M. Pan, W. Lur, S.M. Sze, J. Electrochem. Soc. 149 (2002) F145. [11] T.C. Chang, Y.S. Mor, P.T. Liu, T.M. Tsai, C.W. Chen, Y.J. Mei, S.M. Sze, Thin Solid Films 398–399 (2001) 523.
Repairing plasma-damaged low-k HSQ films with trimethylchlorosilane treatment S.K. Singh, A.A. Kumbhar, R.O. Dusane ∗ Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai 400076, India Received 28 July 2005; received in revised form 6 September 2005; accepted 9 September 2005
Abstract Low-density materials, such as the commercially available hydrogen silsesquioxane (HSQ) offer a low dielectric constant. Thus HSQ with a low value of k (∼2.85) can be spin-coated if the density of Si H bonding is maintained at a high level and the formation of OH bonds and absorption or creation of water in the film is minimized. O2 plasma exposure on HSQ film properties increases leakage current of metal/HSQ/Si/metal structures. Also the dielectric constant shows a significant increase after O2 plasma exposure. Another important consequence of the O2 plasma exposure is the large decrease in the contact angle of the HSQ surface. In this paper, we demonstrate first damage repair process involving trimethylchlorosilane (TMCS) treatment for 10 min at atmospheric pressure leads to a regain of a leakage current density and dielectric which approach values very near to the as-deposited film. These results show that the TMCS treatment is a promising technique to repair the damage even in the commercially available and highly applicable low-k material and increase the visibility of its use at the 0.1 m technology. The increase of the hydrophilic nature of the surface after O2 plasma exposure leads to increase absorption of moisture with a subsequent increase in the dielectric constant. © 2005 Elsevier B.V. All rights reserved. Keywords: HSQ; RF plasma; Low-k dielectrics; Contact angle; Trimethylchlorosilane (TMCS)
1. Introduction The shrinking in dimensions of devices in ULSI circuits to achieve faster performance has led to a very large device density in today’s ULSI circuits. Subsequently interconnects are becoming narrower (increases their resistance, R) and are coming nearer (increases the capacitance, C). This leads to the increase of the RC delay, which increases with scaling down and leads to various problems. This RC delay can be reduced by using Cu instead of Al (by approximately 35%), and by using inter-metal dielectric with dielectric constant k lower than that of SiO2 (∼4), so called low-k dielectrics [1–3]. Replacement of SiO2 by air (k∼1) and Al by Cu for the same circuitry can reduce RC delay by 75%. The general way of decreasing the dielectric constant is to increase the porosity in the low-k films. Dielectric materials presently under investigation are HSQ, porous silica, MSQ and even materials like silicon–carbon and fluorinated carbon [5–7]. Hydrogen silsesquioxane (HSQ) is one of the
∗
Corresponding author. Fax: +91-22-25723480. E-mail address: [email protected] (R.O. Dusane).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.09.042
promising candidates with a highly porous three-dimensional network structure. The general formula for HSQ is (HSiO3/2 )2n where n = 3–8. Hydrogen silsesquioxane has many advantages, including a low dielectric constant, has no carbon, non-etch back processing, excellent gap filling, good planarization, and low moisture uptake. Multilevel wiring schemes are now routinely used and would become more demanding with the increase in device density in the circuits. The dielectric between the wiring levels must have high quality and reliability, low stress, prepared through a simple process, and should be compatible to different processes employed during integration [4]. Plasma exposure is widely used for post-etch treatment, such as resist strip or residue etch cleaning [8]. Oxygen plasma is commonly used for this purpose. However, oxygen plasma oxidizes most of the low-k dielectrics, removing hydrophobic Si H bonds, and increases the hydrophilicity of these films. In other words, the oxygen plasma destroys the low-k properties of HSQ films. Thus the dielectric properties of the HSQ will deteriorate during photo-resist stripping process [9]. In this work, we have studied the effect of O2 plasma exposure on the structural and electrical properties of the spin-coated HSQ films. The effect of O2 plasma exposure is investigated to understand the impact
S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
30
of this process on the dielectric film quality. We also studied the damage recovery by TMCS vapor exposure at atmospheric pressure. 2. Experimental The standard RCA cleaned p-type Si1 0 0 wafers are spincoated with a single layer of HSQ, and baked sequentially on the hot plate at 150, 250 and 350 ◦ C for 1 min and followed by furnace curing at 400 ◦ C for 1 h in nitrogen ambient. The films were exposed to a 300 W radio frequency (13.56 MHz) capacitively coupled O2 plasma for 3, 6 and 9 min at a pressure of 500 mTorr and oxygen gas flow rate of 10 sccm. The as cured HSQ films and the O2 plasma-exposed films were treated with TMCS vapor generated by heating TMCS solution to 60 ◦ C for 10 min. The samples were then annealed at 400 ◦ C in N2 ambient for 1 h. The chemical bonding of the HSQ films before and after various treatments was investigated by Fourier transform infrared (FT-IR) spectroscopy (Nicolet Model Magna 550). The film thickness and refractive index were measured by Ellipsometry (Scentech Model SE 800). The film surface hydrophobicity is studied by contact angle measurements (Reme Hardt, Model 100-00-230). MIS capacitors were fabricated by evaporating aluminum (0.25 mm dots for front contact) and also as the back contact to the silicon wafer. A Model 82 CV (Agilent) meter was used to measure the dielectric constant of HSQ. The capacitance was measured at 100 KHz. The dc current–voltage (I–V) characteristics were measured with a Keithly (Model SMU 2400) setup to evaluate the insulating property of HSQ film. 3. Results and discussion Fig. 1(a) and (b) shows the FT-IR spectra of HSQ films before and after O2 plasma exposure (for different times) in the range 4000–500 and 1300–600 cm−1 respectively. The FT-IR spectrum of HSQ gives the following absorption peaks after O2 plasma exposure: 1. 2. 3. 4. 5. 6.
Si Si Si Si Si Si
Fig. 1. (a) FT-IR spectra of HSQ before and after a series of O2 plasma exposure; (b) the IR spectra in the range 1300–600 cm−1 depicting the variations in the bending modes.
H stretch mode (2250 cm−1 ), O stretch cage-like peak (1130 cm−1 ), O stretch network (1070 cm−1 ), O bending cage-like (860 cm−1 ), O bending network peak (830 cm−1 ), OH (∼3350 cm−1 ) (this corresponds to moisture).
The appearance of the Si OH bonds in the plasma-exposed films at 3350 cm−1 is due to the conversion of the Si H bonds (2250 cm−1 ) into Si OH (3350 cm−1 ) bonds when O2 plasma interacts with the HSQ films. The intensity of the Si H bond decreases and the intensity of the Si OH bond increases with the increase in O2 plasma treatment time. The increase of the Si OH bond signal has two possible causes. One is that when the O2 plasma breaks Si H bonds, HSQ immediately absorbs O radicals to convert Si H bonds into Si OH bonds. The other is that the dangling bonds absorb water immediately when the HSQ film is exposed to air after the O2 plasma treatment. Fig. 2
Fig. 2. Integrated area of the Si H and Si OH vibrational peaks of HSQ film as a function of O2 plasma exposure time.
S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
Fig. 3. Contact angle of HSQ film as a function of O2 plasma exposure time.
31
Fig. 4. Refractive index of HSQ film as a function of O2 plasma exposure time.
shows the variation of the intensities of the Si H and Si OH bonding configurations. The decrease in the integrated intensity of Si H bond and the subsequent increase in the Si OH bonds with increase in O2 plasma exposure time. The possible reaction is 2Si H + O2 = 2Si OH Another prominent observation is in the bending mode at 800–900 cm−1 (Fig. 1b). The O2 plasma exposure retransforms the HSQ network into cage-like structure. This change gets reflected only in the Si O bending modes. In order to confirm that the hydrophilicity of the film surface does increase after the oxygen plasma exposure, we carried out the contact angle measurement with water drop. These observations are depicted in Fig. 3. The contact angle for the as-deposited film is ∼64◦ while it decreases to ∼20◦ after O2 plasma exposure. This is because the O2 plasma causes many of the Si H bonds to break, leaving many dangling bonds in the film [10]. Some of those dangling bonds form Si O bonds and others become Si OH bonds through moisture absorption. As exposure time of O2 plasma increases the HSQ film absorbs more water molecules and this is also reflected in the FT-IR spectra shown in Fig. 1a and in the refractive index (Fig. 4). As plasma exposure time increases, the refractive index of the HSQ film increases indicating a more SiO2 like character of the film after plasma exposure. The most significant effect of concern is the increase in the dielectric constant of the films after plasma exposure. Fig. 5 shows that the dielectric constant of HSQ film increases with increasing O2 plasma exposure time. The reason of this again is the structural transformation of Si H bonds to Si OH and Si O bonds [11]. The high polarity water molecules (dielectric constant ∼76) present in the HSQ film give rise to an increase in both leakage current and dielectric constant. Fig. 6 shows the leakage current density of HSQ after the O2 plasma exposure for 3–9 min. The leakage current increases by two orders of magnitude after a 9 min plasma exposure.
Fig. 5. Dielectric constant of HSQ film as a function of O2 plasma exposure time.
Fig. 6. Leakage current density of HSQ film for O2 plasma exposure for 3, 6 and 9 min.
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S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
Fig. 9. Water contact angle of the as cured, plasma-exposed and TMCS-treated HSQ film. Fig. 7. FT-IR spectra of the HSQ film before and after O2 plasma-exposed and TMCS treatment.
4. O2 plasma damage recovery by TMCS treatment Now we consider the effect of a TMCS treatment on the plasma-damaged HSQ films. From Fig. 7, the band corresponding to Si OH (3400 cm−1 ) reduces. Another important observation is that there is significant increase in the Si H bond density. By O2 plasma exposure there was a change from network to cage structure (which was seen in the bending modes of Si O). The TMCS treatment reverse transformation, i.e. from cage to network takes place. This restores the dielectric property of the damaged HSQ films. The TMCS treatment leads to a decrease in the refractive index (Fig. 8). Also, the TMCS treatment changes the film surface from hydrophilic to hydrophobic (Fig. 9). Regarding the leakage current density, we have seen that it increases by nearly two orders of magnitude when HSQ sample undergoes O2 plasma exposure. As shown in Fig. 10, subsequent TMCS treatment, the leakage current density decreases by three orders of magnitude. Thus the electrical properties of the films
Fig. 8. Refractive index of the as cured, plasma-exposed and TMCS-treated HSQ film.
Fig. 10. Leakage current density of the HSQ film before and after O2 plasma and TMCS treatment.
are also regained after TMCS treatment. It is also interesting to see, that the dielectric constant which increased significantly with plasma exposure can be regained by the after TMCS treatment, as shown in Fig. 11. After a sequential TMCS treatment,
Fig. 11. Dielectric constant of the HSQ film before, after O2 plasma and TMCS treatment.
S.K. Singh et al. / Materials Science and Engineering B 127 (2006) 29–33
the dielectric constant reduced to 3. These observations clearly indicate that the TMCS treatment can repair the damage of HSQ films. The possible reasons of the repair of the plasma damage has also been discussed by Chang et al. [10]. In their case where they have studied the repair for porous silica (MSQ) by TMCS, the C H bond which was damaged by the plasma got replenished and also the CH3 from the TMCS got attached to sites generated by the plasma thus rendering the surface more hydrophobic. In our case there is no C H. However we think that the attachment of CH3 to form Me3 Si O Si in the HSQ at the sites generated by the plasma repairs the damage and also makes the surface hydrophobic thus leading to the damage repair. Secondly there is a clear increase in the Si H related absorption after TMCS treatment, which is responsible for the improvement of the electrical performance of the device. 5. Conclusions In this study, we have shown that even HSQ films are prone to damage when exposed to oxygen plasma. As we know such plasma exposure is unavoidable during the O2 plasma ashing step in a VLSI fabrication process. Effects similar to those observed in porous silica such as increase in dielectric constant and leakage current occur in the HSQ films. However, these changes involve the breaking of the Si H bonds in the case of HSQ leading to the deterioration of the film properties. It was also seen that the TMCS repaired the damage, which is occurred during this plasma exposure. The Si OH bonds could be eliminated and transformed into Si H bonds by TMCS treatment. The Si H bonds possess hydrophobic properties and thus the surface
33
the HSQ can be changed from hydrophilic to hydrophobic. This hydrophobic surface would also prevent moisture uptake keeping the leakage current and the dielectric constant to the desired values. In conclusion therefore, TMCS treatment is an effective method to repair the damage caused during O2 plasma ashing even in the case of HSQ films. Acknowledgement The work was carried out with financial support from the Department of Science and Technology, Govt. of India under the project No. SR/FTP/PS-41/2000. References [1] S. Bothra, B. Rogers, M. Kellam, C.M. Osburn, IEEE Trans. Electron Dev. 40 (1993) 591. [2] T. Sakurai, IEEE Trans. Electron Dev. 40 (1993) 118. [3] B.K. Liew, P. Fang, N.W. Cheung, C. Hu, IEEE Trans. Electron Dev. 39 (1992) 2472. [4] P.T. Liu, T.C. Chang, Y.L. Yang, Y.F. Cheng, S.M. Sze, IEEE Trans. Electron Dev. 47 (2000) 1733. [5] P.T. Liu, T.C. Chang, K.C. Hsu, T.Y. Tseng, L.M. Chen, C.J. Wang, S.M. Sze, Thin Solid Films 414 (2002) 1. [6] A. Gill, Diam. Relat. Mater. 10 (2001) 234. [7] K. Maex, M.R. Baklanov, D. Shamiryan, F. Iacopi, S.H. Brongersma, Z.S. Yanovitskaya, J. Appl. Phys. 93 (2003) 8793. [8] M.J. Loboda, C.M. Grove, R.F. Schneider, J. Electrochem. Soc. 145 (1998) 2861. [9] S.W. Hwang, G.R. Lee, J.H. Min, S.H. Moon, Surf. Coat. Technol. 174–175 (2003) 835. [10] T.C. Chang, Y.S. Mor, P.T. Liu, T.M. Tsai, C.W. Chen, C.J. Chu, F.M. Pan, W. Lur, S.M. Sze, J. Electrochem. Soc. 149 (2002) F145. [11] T.C. Chang, Y.S. Mor, P.T. Liu, T.M. Tsai, C.W. Chen, Y.J. Mei, S.M. Sze, Thin Solid Films 398–399 (2001) 523.