- Email: [email protected]
Surface modification on low dielectric constant material—methylsilsesquioxane
Thin Solid Films 441 (2003) 248–254
Surface modification on low dielectric constant material— methylsilsesquioxane Chung-Hsien Chena, Fon-Shan Huangb,* a
Department of Electrical Engineering, National Tsing Hua University, Hsin Chu 300, Taiwan, ROC b Institute of Electronics Engineering, National Tsing Hua University, Hsin Chu 300, Taiwan, ROC
Abstract The physical properties and thermal stability of surface modified methylsilsesquioxane (MSQ) were studied. Various posttreatments, such as thermal oxygen, thermal N2 O and oxygen plasma, were adopted on the cured MSQ film as the surfacemodification process. The CuyTaNyMSQySi metal-insulation-semiconductor capacitors with various surface modified MSQ films were prepared to measure the dielectric constant, capacitance–voltage and current–voltage characteristics. X-Ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were performed in order to understand the chemical composition of the modified film. From the above measurements, we find the best surface treatment condition for MSQ in Cu metallization. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Thermal stability; Oxygen thermal treatment; Space-charge-limited current; Poole–Frenkel mechanism
1. Introduction As device geometries continue to move into the deep sub-micron regime, the circuit performance, power dissipation and crosstalk depend rather strongly on the interconnect resistance–capacitance (RC) constant delay w1,2x. Recently, reducing the RC delay was achieved by introducing low dielectric constant (low-k) isolation and low resistivity conductors into novel ultra-large scale integration technology. Among several developed low dielectric constant materials, the spin-on material, methylsilsesquioxane (MSQ), has a low moisture content, little absorbability of humidity and a low dielectric constant of approximately 2.7 at 1 MHz. MSQ also provides excellent adhesion to silicon dioxide, gap-fill ability, low stress and high resistance to cracking w3x. The observed shrinkage is less than 2% for curing temperature up to 400 8C w4x. However, MSQ is an organic low dielectric constant material and contains 22% organic resulting from the CH3 group w3x. Bearing resemblance to other organic low dielectric material, carbon content, perhaps, creates problems in subsequent processes. For instance, H.D Jeong et al. mentioned that peeling problem resulting from plasma-enhanced TEOS *Corresponding author. Tel.: q886-3-5742586; fax: q886-35752120. E-mail address: [email protected] (F.-S. Huang).
SiO2 prepared on MSQ during the chemical mechanical polishing process that was solved by using NH3 yN2 plasma treatment w5x. This adhesion improvement is due to the decrease in Si–CH3 and formation of Si–NH on MSQ film. Besides, T.C. Chang et al. reported that the quality of organic MSQ film was degraded by the damage of oxygen plasma during photoresist stripping due to broken Si–CH3 bonds and hygroscopic behavior w6x. They proposed that the H2 or N2O plasma treatment performed at temperature of 300 8C at pressure of 300 mTorr for 9 min passivates the surface of MSQ and reduces the probability of moisture uptake. In another papers, C.F. Lin and J.C.M. Hui et al. described an electron-beam curing technique used to modify MSQ films w7,8x. The resulting films offered a good surface planarity and did not show water absorption. However, these modification schemes have various limitations related to cost, process complexity and reproduction. In this work, we propose four processes, including thermal treatment in oxygen, N2O ambient, O2 plasma treatment and capping by an oxide obtained by plasma enhanced chemical vapor deposition (PECVD) to perform the surface modification on MSQ films. X-Ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were measured in order to understand the chemical composition of the modified film. The capacitance–voltage (C–V) and current–volt-
0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)00951-9
C.-H. Chen, F.-S. Huang / Thin Solid Films 441 (2003) 248–254
249
Table 1 Various surface modification recipes Treatment
Surface modification recipes
Capped PECVD oxide O2 thermally treated N2O thermally treated O2 plasma treated None
MSQ capped with 30 nm PECVD oxide heated at 300 8Cy10 min in O2 ambient heated at 300 8Cy10 min in N2O ambient treated in O2 plasma at a powerytime of 50 Wy3 min no treatment, used as reference
age (I–V) measurements were performed on metalinsulator-semiconductor (MIS) capacitors to realize the effects of post-treatments on electrical conduction mechanisms. From above measurements, we find the best surface treatment condition for MSQ in Cu metallization.
(6 gyl), ethylenediaminetetraacetic (EDTA) (15 gyl) and formaldehyde (10 gyl). The pH of the solution was controlled by addition of NaOH (9 gyl). Then, a MIS capacitor structure was patterned and fabricated for C– V, I–V and bias-temperature stress measurements.
2. Experimental details
2.2. Physical measurement
2.1. Film preparation and conductor capacitor fabrication
metal-insulator-semi-
MSQ, flowable spin-on polymer, was spun on (100) n-Si wafers by using spin-coater. The deposited MSQ films were subsequently heated on hot plate in air ambient at 180 8C for 120 s followed 250 8C for 60 s. The final curing was performed in nitrogen ambient at atmospheric pressure at 450 8C for 30 min. Several different post-treatments were applied to the MSQ films as listed in Table 1. For capped PECVD oxide sample, the PECVD oxide cap with a 30 nm thickness was deposited at 1 Torr that is higher than the conventional process. The k value of the PECVD oxide was approximately 2.7. For O2 and N2O thermal treatments, the samples were heated in O2 or N2O ambient at 300 8C for 10 min. For the O2 plasma treatment, the oxygen plasma was sustained at a pressure of 350 mTorr with a power of 50 W for 3 min. An untreated sample (without any post-treatment) was used as the reference. A 30nm-thick TaN film was deposited on the surface of MSQ treated films by reactive ion sputtering at 120 Wy 20 mTorr. A thin copper seed layer with thickness 50 nm was then sputtered. Blanket coverage copper film was deposited by immersing in an electroless-plating basin w9x. The plating solution contained cupric sulfate
Both scanning electron microscopy (SEM) and astep were used to determine the thickness of the MSQ and TaN films, respectively. The thickness of the different samples was varied in the range of 159–280 nm and is listed in Table 2. The depth profiling of chemical component of modified MSQ films was determined by XPS. The XPS measurements were performed with a physical electronics ESCA-5600 spectrometer. For the excitation of photoelectrons, the Mg Ka (1253.6 eV) X-ray source was operating at 400 W (15 kV–27 mA). The specimens were analyzed at an electron take-off angle of 708, measured with respect to the surface plane. A depth profile was measured by sputtering with 3 keV Arq and recording the binding energy spectra of O 1s, C 1s and Si 2p, until the modified MSQ was completely removed. The quantifying XPS measurement was utilizing peak area and peak height sensitivity factors. The chemical structure of the modified films was characterized by FTIR. For the adhesion test, the breaking point tester, Romulus III, is used with epoxy-coated pull studs bonded to TaN film to perform the peeling test. We found that the stress between surface modified MSQ and TaN films were approximately from 119.98 to 142.68 kgycm2, while for the reference sample with TaN film the stress was only ;102 kgycm2. The high frequency C–V characteristics were measured at 1 MHz
Table 2 The thickness, dielectric constant (k), flat-band voltage shift (DVfb) of samples after 300 8Cy30 min and 400 8Cy30 min sintering, effective oxide charge (Qeff) of samples after 400 8Cy30 min sintering and dielectric breakdown field (Ebd) of MIS capacitors Modified MSQ
Capped PECVD oxide O2 thermally treated N2O thermally treated O2 plasma treated MSQ Untreated MSQ
Dielectric thickness (nm) 280 190 159 165 225
k value 300 8C
400 8C
2.67 2.77 2.88 2.89 2.84
2.67 2.74 2.87 2.89 2.78
DVfb (V)
Qeff (Cycm2)
Ebd (mVycm)
1.06 0.45 1.35 1.53 y2.44
6.6038Ey12 1.9081Ey12 2.4034Ey11 1.7446Ey11 4.1663Ey11
)2 )2 )2 1.926 1.391
250
C.-H. Chen, F.-S. Huang / Thin Solid Films 441 (2003) 248–254
From XPS data, the effects of O2 and N2O thermal treatment on the surface of MSQ will be observed. They will be correlated to the electrical measurement results. 3. Results and discussion 3.1. The effects of various post-treatments
Fig. 1. XPS depth profile of the (a) capped PECVD oxide, (b) O2 thermally treated, (c) N2O thermally treated, and (d) no treatment MSQ films.
with HP4294. The dielectric constant of MSQ can be obtained by capacitance and thickness measurement. The inter-diffusion of metalydielectric can also be investigated from C–V measurement after heating the MIS capacitor at various sintering temperature 300, 400 and 450 8C for 30 min. The degradation of this capacitance was then studied after the capacitor was exposed to 40– 50% relative humidity air for 120 days. Meanwhile, the samples were heated to 200 8C for 30 min at the applied electric field 0.5 mVycm in N2 ambient. After this biastemperature stress, C–V shift were taken to understand the existence of copper ions. The fixed charges can be calculated from the flat-band voltage shift before and after the stress. From I–V characteristics analysis of MIS capacitor, the dielectric breakdown strength and the electrical conduction mechanisms can be understood.
Fig. 1a–d display the XPS depth profile of MSQ films with surface modulation that includes (a) PECVD oxide capped, (b) O2 thermally treated, (c) N2O thermally treated and (d) no treatment. Fig. 1a, b and c show the high oxygen content close to the surface while in Fig. 1d the oxygen content is lower. These data indicate that the surface of the MSQ film has been changed to an oxide-like layer after O2 and N2O thermal treatment. In Fig. 1a, the thickness of the deposited PECVD cap oxide is approximately 30 nm; the thickness of oxide-like layer is approximately 24 nm and 15 nm in Fig. 1b and c. Comparing the oxygen peak height near the surface of the samples prepared by using the two thermal treatment methods, the O2 thermal treatment appears more efficient in forming the oxide-like layer. In addition, the carbon concentration in Fig. 1b is less than that in Fig. 1a, c and d. It might lead to less effective oxide charges and might be related to electrical conduction mechanism. The reflectance FTIR spectra of various post-treatment MSQ films are shown in Fig. 2. The FTIR spectrum of the cured MSQ film exhibits absorption peaks at (i) 3000 cmy1, (ii) 1275 and 780 cmy1, which correspond to C–H and Si–C stretching vibrations in the polymer, respectively. Two absorption peaks at 1000–1200 cmy1 are associated with the Si– O stretching cage-like peak near 1133 cmy1 and the Si–O stretching network peak near 1030 cmy1 w4,10x. After the post-treatments, the intensities of H–OH peak disappeared. The absence of H–OH signal for the posttreated samples implies a decrease of water absorption near the surface. Such result indicates that the moisture resistance of MSQ films may be enhanced after the
Fig. 2. FTIR spectrum of MSQ films for various surface treatments.
C.-H. Chen, F.-S. Huang / Thin Solid Films 441 (2003) 248–254
Fig. 3. C–V curves of MIS capacitors with (a) O2 thermally treated, (b) N2O thermally treated and (c) untreated, MSQ films stressed at 300 8C, 400 8C and 450 8C for 30 min sintering, respectively.
post-treatments. Comparing the untreated MSQ film with post-treated samples, there is no conspicuous difference except for the Si–C (1275 cmy1) (780 cmy1) dips and C–H (2978 cmy1) dip, which are reduced in the posttreated samples, confirming that post-treatments decrease the carbon intensity. Among the four samples, the O2 thermally, N2O thermally and O2 plasma treated samples have a lower carbon concentration than that the untreated sample. Those results are consistent with XPS data presented in Fig. 1. 3.2. C–V characteristics of CuyTaNysurface modified MSQySi capacitance Fig. 3a–c depict the C–V results of three MIS capacitors prepared with O2 thermally treated, O2 plasma-treated, and untreated MSQ films, and subjected to sintering at temperature 300, 400 and 450 8C for 30 min in N2 ambient. From Fig. 3a and b, we found that the C–V curves tend to shift upward with increasing sintering temperature (300 to 400 8C). Samples prepared using the PECVD oxide capped and N2O thermally treated MSQ films had the same behavior (not shown).
251
This phenomenon indicates that the effective oxide charges are reduced during sintering process. After sintering at 450 8C, the C–V curves shift to the left. It might be due to the penetration of Cu ions into MSQ films. For the reference sample (Fig. 3c), the negative shift is observed at 400 8C. This implies that this sample has a lower barrier resistance to Cu. This indicates the oxide-like surface formed on the post-treated surfaces has enhanced the resistance to Cu diffusion. The MIS sample prepared with O2 thermally treated MSQ shows the least flat-band voltage shift in the C–V curves between 300 and 400 8C sintering, evidencing that the O2 thermal treatment gives the least reduction of effective oxide charge. The results of dielectric constant k and flat-band voltage shift DVfb of all the MIS capacitors are summarized in Table 2. All of the samples have retained a k value of approximately 2.67 to 2.89. The k value varied little after sample sintering at temperature of 300 8C and 400 8C except for the MIS capacitor prepared with untreated MSQ. From these data, we believe sintering at a temperature of 400 8C for 30 min in N2 ambient is a good choice for the sintering process. The effective oxide charges inside the MSQ film can be determined from the flat-band voltage data for the samples sintered at 400 8C. The value of Fms was about y0.758 volt for our samples. The amount of effective oxide charges can be calculated w11x and are summarized in Table 2. The effective oxide charges are the sum of the interface states and fixed oxide charges. The MIS capacitor prepared from O2 thermally treated MSQ has the less effective oxide charges. This sample also showed the lower carbon content from XPS and FTIR data. These evidences are related to the good electrical characteristic of O2 thermally treated MSQ film in leakage current measurement described in Section 3.3. Fig. 4 displays C–V results for MIS sample prepared from untreated and O2 thermally treated MSQ and subjected to bias-temperature stress at 200 8C for 30 min with an electric field ("Es) of "0.5 mVycm. The C–V curve of the O2 thermally treated sample is weakly affected by the bias-temperature stress. The curve labeled (qEs), (yEs), and no bias aligned together which are an evidence that no copper ions has penetrated into the MSQ film after bias-temperature stress. The bias-temperature stress behaviors of the other modified samples are similar to this sample. For the reference sample, a 1.3 V shift of (qEs) curve toward the negative direction after q0.5 mVycm bias stressed was observed. The hysteresis loops of the C–V curves indicate the presence of mobile Cu ion. These results are consistent with those from Fig. 3. From this biastemperature stress experiment, the improvement of the interface of CuyMSQ and the enhancement of the Cu diffusion resistance by an O2 thermal treatment of the MSQ film is confirmed.
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C.-H. Chen, F.-S. Huang / Thin Solid Films 441 (2003) 248–254
Fig. 4. C–V curves of MIS capacitors of (a) O2 thermally treated and (b) untreated, MSQ films after heating at 200 8C for 30 min under positive (qEs), negative (yEs) gate and no bias.
3.3. I–V characteristics of CuyTaNysurface modified MSQySi capacitance Fig. 5 shows the leakage current density (JL) of the MIS capacitors as a function of electric field (E). The applied gate voltage is ramped at the slow rate of 0.1 Vys. The MIS capacitor prepared from O2 thermally treated MSQ has the lower leakage current density. This might be related to the small effective oxide trapped charges of the sample (Table 2). The dielectric breakdown fields (Ebd), defined as the electrical field at
Fig. 5. Leakage current density (JL) of MIS capacitors as a function of electric field (E) on a log JL vs. E plot.
which the capacitor’s leakage current density exceeds 8=10y6 Aycm2 w12x, are summarized in Table 2 for all samples. Except for samples prepared using the O2 plasma treated and the untreated MSQ films, the dielectric breakdown field of the samples was exceeding 2 mVycm. The JL –E curves of all samples were parabolic, except for the O2 thermally treated sample. In order to understand the carrier conduction mechanism in this sample, the I–V the plot of and two the fitted curves are presented in Fig. 6. The solid lines are fitted curves, IsaV2 where as0.780=10y10 for V)34.46 V and as 2.622=10y12 for V-34.46 V. The broken line is fitted with IsbV6 where bs7.937=10y18. The good fitting behavior between experimental data and bV6 curve implies that space charge limited current with trap dominates the conduction mechanism w13x. For the traps distribution given by NtsAey´ykTc, where ´ is the energy measured from the bottom of the conduction band and Tc is a characteristic temperature greater than the temperature at which the currents are measured, the analysis of space-charge-limited current will take the form V n where ns(TcyT)q1. For larger n values, the trap charge density, Nt, approaches a uniform distribution over most of the distance between plane parallel electrodes. These deep trap states lower the drift mobility of injected carriers, but cannot be thermal excited into conduction band. Fig. 7 shows the leakage current behaviors of the other samples plotted as JL vs. E 1y2. All samples but the O2 plasma treated exhibited a linear relationship between JL and the square root of the applied electric field. This linear relationship can be explained by Poole–Frenkel (P–F) mechanism, which is due to field-enhanced thermal excitation of a certain level of trapped electrons into the conduction band w14x. In Fig. 7, the O2 plasma treatment modified sample shows two different slopes. This implies that two different trap states exist in the MSQ film of the O2 plasmatreated sample. The enlarged leakage current at higher electric field might be due to trapped states of deeper energy level. Based on the analysis of dielectric conduction mechanism of the above leakage current, P–F
Fig. 6. The experimental data of leakage current (I) vs. gate voltage (V) (symbols) fitted with polynomial aV2 (solid lines) and bV6 (broken line) for O2 thermally treated MSQ film.
C.-H. Chen, F.-S. Huang / Thin Solid Films 441 (2003) 248–254
253
mechanism was dominated in sample with higher effective oxide charge density in MSQ, and space-chargelimited current conduction mechanism was observed in the O2 thermally treated sample with lower effective oxide charge density in MSQ. According to XPS depth profile data (Fig. 1), carbon concentration in the O2 thermally treated MSQ film is lower. Therefore, the carrier conduction mechanism should be related to effective oxide charge density that might come from the existence of the carbon impurity.
trical techniques including C–V, I–V characteristics measurements, XPS and FTIR. The surface modifications of the organic polymer lead to improvement of the barrieryMSQ interface near the surface by forming oxide-like layer. The dielectric constant varies from 2.67 to 2.89 for the surface modified samples sintered at 300 and 4008C. After surface post-treatments, O2 thermal treatment, our C–V data showed that fixed oxide charges residing in the MSQ film could be removed. From biastemperature stressing study, the C–V plot of O2 thermally treated sample shows the absence of a flat-band shift that indicates the absence of copper ion into MSQ films. The leakage current density of O2 thermally treated sample with copper electrode gate was approximately 2=10y10 Aycm2 at electric field strength of 1 mVycm. The O2 thermal treatment also reduced the carbon intensity in MSQ film and led to the spacecharge-limited mechanism of the leakage current conduction. Furthermore, the degradation of the various post-treated capacitors subjected to 40–50% relative humidity environment for 120 days was studied by measuring k values. In all post-treated MSQ films, the stable capacitance demonstrated the moisture resistance of capacitor fabricated. Therefore, based on all measurements, O2 thermal treatment at temperature 300 8C for 10 min represents a suitable method to improve the surface quality of MSQ film.
3.4. Degradation study
Acknowledgments
Stable dielectric constant will be an important reliability challenge for the Cuylow-k dielectric integration. Therefore, the moisture-induced dielectric degradation was also investigated. SEM cross-section view and C– V measurements were taken for the stacked MIS capacitors after expositing them to 40–50% relative humidity flowing air for 120 days. The degradations of k values of the samples are summarized in Table 3. All the k values of samples with surface modification almost remained constant within the error range. From this degradation study, the surface modification processes have improved the moisture resistance of MSQ film.
The authors would like to thank to Allied Signal Incorporated for providing the methylsilsesquioxane for evaluation. This work was supported by the National Science Council of the Republic of China under contract no. NSC89-2215-E-007-015
Fig. 7. Leakage current density (JL) of the MIS capacitors after PECVD oxide capped, N2O thermally treated, O2 plasma treated and untreated, MSQ films as a function of electric field (E) on a log JL vs. E 1y2 plot.
4. Conclusions The physical properties of MSQ after various posttreatments, namely MSQ coated with PECVD oxide, O2 thermal treatment, N2O plasma treatment and O2 thermal treatment, were investigated by sensitive elecTable 3 k values of fresh samples and samples after exposited them to 40– 50% relative humidity flowing air for 120 days Capped PECVD oxide O2 thermally treated MSQ N2O thermally treated MSQ
2.67"0.07 2.74"0.10 2.87"0.01
2.70"0.04 2.75"0.09 2.87"0.01
References w1x R.H. Havemann, J.A. Hutchby, Proc. IEEE 89 (2001) 586. w2x J.A. Davis, R. Venkatesan, A. Kaloyeros, M. Beylansky, S.J. Souri, K. Banerjee, K.C. Saraswat, A. Rahman, R. Reif, J.D. Meindl, Proc. IEEE 89 (2001) 305. w3x Allied Signal Advanced Materials, Accuspin 418 Flowable Spin-on Polymer, Product Bulletin, Sunnyvale, CA, 1996. w4x C.T. Chua, G. Sarkar, S.Y.M. Chooi, L. Chan, J. Mater. Sci. Lett. 18 (1999) 33. w5x H.D. Jeong, H.S. Park, H.J. Shin, B.J. Kim, H.K. Kang, M.Y. Lee, Proceeding of the IEEE 1999 International Interconnect Technology Conference, San Francisco, CA, May 24–26, 1999, IEEE, 1999, p. 190. w6x T.C. Chang, Y.J. Mei, Y.S. Mor, T.H. Perng, Y.L. Yang, S.M. Sze, J. Vac. Sci. Technol. B 17 (1999) 2325. w7x C.F. Lin, I.C. Tung, M.S. Feng, Jpn. J. Appl. Phys. 38 (1999) 6253. w8x J.C.M. Hui, Y. Xu, C.Y. Foong, L. Marvin, L. Charles, L.Y. Shung, A. Inamdar, J. Yang, J. Kennedy, M. Ross, S.Q. Wang, Proceeding of the IEEE 1998 International Interconnect Technology Conference, San Francisco, CA, June 1–3, 1998, IEEE, 1998, p. 217.
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w9x Y. Shacham-Diamand, V. Dubin, M. Angyal, Thin Solid Films 262 (1995) 93. w10x N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, 1990, Chap. 12. w11x S.M. Sze, Physics of Semiconductor Devices, John Wiley and Sons, New York, 1981, p. 395.
w12x S.U. Kim. T. Cho, P.S. Ho, 1999 IEEE International Reliability Physics Symposium Proceedings 37th Annual, San Diego, CA, March 23–25, 1999, IEEE, 1999, p. 277. w13x A. Rose, Phys. Rev. 97 (1955) 1538. w14x S.M. Sze, Physics of Semiconductor Devices, John Wiley and Sons, New York, 1981, p. 403.
Surface modification on low dielectric constant material— methylsilsesquioxane Chung-Hsien Chena, Fon-Shan Huangb,* a
Department of Electrical Engineering, National Tsing Hua University, Hsin Chu 300, Taiwan, ROC b Institute of Electronics Engineering, National Tsing Hua University, Hsin Chu 300, Taiwan, ROC
Abstract The physical properties and thermal stability of surface modified methylsilsesquioxane (MSQ) were studied. Various posttreatments, such as thermal oxygen, thermal N2 O and oxygen plasma, were adopted on the cured MSQ film as the surfacemodification process. The CuyTaNyMSQySi metal-insulation-semiconductor capacitors with various surface modified MSQ films were prepared to measure the dielectric constant, capacitance–voltage and current–voltage characteristics. X-Ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were performed in order to understand the chemical composition of the modified film. From the above measurements, we find the best surface treatment condition for MSQ in Cu metallization. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Thermal stability; Oxygen thermal treatment; Space-charge-limited current; Poole–Frenkel mechanism
1. Introduction As device geometries continue to move into the deep sub-micron regime, the circuit performance, power dissipation and crosstalk depend rather strongly on the interconnect resistance–capacitance (RC) constant delay w1,2x. Recently, reducing the RC delay was achieved by introducing low dielectric constant (low-k) isolation and low resistivity conductors into novel ultra-large scale integration technology. Among several developed low dielectric constant materials, the spin-on material, methylsilsesquioxane (MSQ), has a low moisture content, little absorbability of humidity and a low dielectric constant of approximately 2.7 at 1 MHz. MSQ also provides excellent adhesion to silicon dioxide, gap-fill ability, low stress and high resistance to cracking w3x. The observed shrinkage is less than 2% for curing temperature up to 400 8C w4x. However, MSQ is an organic low dielectric constant material and contains 22% organic resulting from the CH3 group w3x. Bearing resemblance to other organic low dielectric material, carbon content, perhaps, creates problems in subsequent processes. For instance, H.D Jeong et al. mentioned that peeling problem resulting from plasma-enhanced TEOS *Corresponding author. Tel.: q886-3-5742586; fax: q886-35752120. E-mail address: [email protected] (F.-S. Huang).
SiO2 prepared on MSQ during the chemical mechanical polishing process that was solved by using NH3 yN2 plasma treatment w5x. This adhesion improvement is due to the decrease in Si–CH3 and formation of Si–NH on MSQ film. Besides, T.C. Chang et al. reported that the quality of organic MSQ film was degraded by the damage of oxygen plasma during photoresist stripping due to broken Si–CH3 bonds and hygroscopic behavior w6x. They proposed that the H2 or N2O plasma treatment performed at temperature of 300 8C at pressure of 300 mTorr for 9 min passivates the surface of MSQ and reduces the probability of moisture uptake. In another papers, C.F. Lin and J.C.M. Hui et al. described an electron-beam curing technique used to modify MSQ films w7,8x. The resulting films offered a good surface planarity and did not show water absorption. However, these modification schemes have various limitations related to cost, process complexity and reproduction. In this work, we propose four processes, including thermal treatment in oxygen, N2O ambient, O2 plasma treatment and capping by an oxide obtained by plasma enhanced chemical vapor deposition (PECVD) to perform the surface modification on MSQ films. X-Ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were measured in order to understand the chemical composition of the modified film. The capacitance–voltage (C–V) and current–volt-
0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)00951-9
C.-H. Chen, F.-S. Huang / Thin Solid Films 441 (2003) 248–254
249
Table 1 Various surface modification recipes Treatment
Surface modification recipes
Capped PECVD oxide O2 thermally treated N2O thermally treated O2 plasma treated None
MSQ capped with 30 nm PECVD oxide heated at 300 8Cy10 min in O2 ambient heated at 300 8Cy10 min in N2O ambient treated in O2 plasma at a powerytime of 50 Wy3 min no treatment, used as reference
age (I–V) measurements were performed on metalinsulator-semiconductor (MIS) capacitors to realize the effects of post-treatments on electrical conduction mechanisms. From above measurements, we find the best surface treatment condition for MSQ in Cu metallization.
(6 gyl), ethylenediaminetetraacetic (EDTA) (15 gyl) and formaldehyde (10 gyl). The pH of the solution was controlled by addition of NaOH (9 gyl). Then, a MIS capacitor structure was patterned and fabricated for C– V, I–V and bias-temperature stress measurements.
2. Experimental details
2.2. Physical measurement
2.1. Film preparation and conductor capacitor fabrication
metal-insulator-semi-
MSQ, flowable spin-on polymer, was spun on (100) n-Si wafers by using spin-coater. The deposited MSQ films were subsequently heated on hot plate in air ambient at 180 8C for 120 s followed 250 8C for 60 s. The final curing was performed in nitrogen ambient at atmospheric pressure at 450 8C for 30 min. Several different post-treatments were applied to the MSQ films as listed in Table 1. For capped PECVD oxide sample, the PECVD oxide cap with a 30 nm thickness was deposited at 1 Torr that is higher than the conventional process. The k value of the PECVD oxide was approximately 2.7. For O2 and N2O thermal treatments, the samples were heated in O2 or N2O ambient at 300 8C for 10 min. For the O2 plasma treatment, the oxygen plasma was sustained at a pressure of 350 mTorr with a power of 50 W for 3 min. An untreated sample (without any post-treatment) was used as the reference. A 30nm-thick TaN film was deposited on the surface of MSQ treated films by reactive ion sputtering at 120 Wy 20 mTorr. A thin copper seed layer with thickness 50 nm was then sputtered. Blanket coverage copper film was deposited by immersing in an electroless-plating basin w9x. The plating solution contained cupric sulfate
Both scanning electron microscopy (SEM) and astep were used to determine the thickness of the MSQ and TaN films, respectively. The thickness of the different samples was varied in the range of 159–280 nm and is listed in Table 2. The depth profiling of chemical component of modified MSQ films was determined by XPS. The XPS measurements were performed with a physical electronics ESCA-5600 spectrometer. For the excitation of photoelectrons, the Mg Ka (1253.6 eV) X-ray source was operating at 400 W (15 kV–27 mA). The specimens were analyzed at an electron take-off angle of 708, measured with respect to the surface plane. A depth profile was measured by sputtering with 3 keV Arq and recording the binding energy spectra of O 1s, C 1s and Si 2p, until the modified MSQ was completely removed. The quantifying XPS measurement was utilizing peak area and peak height sensitivity factors. The chemical structure of the modified films was characterized by FTIR. For the adhesion test, the breaking point tester, Romulus III, is used with epoxy-coated pull studs bonded to TaN film to perform the peeling test. We found that the stress between surface modified MSQ and TaN films were approximately from 119.98 to 142.68 kgycm2, while for the reference sample with TaN film the stress was only ;102 kgycm2. The high frequency C–V characteristics were measured at 1 MHz
Table 2 The thickness, dielectric constant (k), flat-band voltage shift (DVfb) of samples after 300 8Cy30 min and 400 8Cy30 min sintering, effective oxide charge (Qeff) of samples after 400 8Cy30 min sintering and dielectric breakdown field (Ebd) of MIS capacitors Modified MSQ
Capped PECVD oxide O2 thermally treated N2O thermally treated O2 plasma treated MSQ Untreated MSQ
Dielectric thickness (nm) 280 190 159 165 225
k value 300 8C
400 8C
2.67 2.77 2.88 2.89 2.84
2.67 2.74 2.87 2.89 2.78
DVfb (V)
Qeff (Cycm2)
Ebd (mVycm)
1.06 0.45 1.35 1.53 y2.44
6.6038Ey12 1.9081Ey12 2.4034Ey11 1.7446Ey11 4.1663Ey11
)2 )2 )2 1.926 1.391
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From XPS data, the effects of O2 and N2O thermal treatment on the surface of MSQ will be observed. They will be correlated to the electrical measurement results. 3. Results and discussion 3.1. The effects of various post-treatments
Fig. 1. XPS depth profile of the (a) capped PECVD oxide, (b) O2 thermally treated, (c) N2O thermally treated, and (d) no treatment MSQ films.
with HP4294. The dielectric constant of MSQ can be obtained by capacitance and thickness measurement. The inter-diffusion of metalydielectric can also be investigated from C–V measurement after heating the MIS capacitor at various sintering temperature 300, 400 and 450 8C for 30 min. The degradation of this capacitance was then studied after the capacitor was exposed to 40– 50% relative humidity air for 120 days. Meanwhile, the samples were heated to 200 8C for 30 min at the applied electric field 0.5 mVycm in N2 ambient. After this biastemperature stress, C–V shift were taken to understand the existence of copper ions. The fixed charges can be calculated from the flat-band voltage shift before and after the stress. From I–V characteristics analysis of MIS capacitor, the dielectric breakdown strength and the electrical conduction mechanisms can be understood.
Fig. 1a–d display the XPS depth profile of MSQ films with surface modulation that includes (a) PECVD oxide capped, (b) O2 thermally treated, (c) N2O thermally treated and (d) no treatment. Fig. 1a, b and c show the high oxygen content close to the surface while in Fig. 1d the oxygen content is lower. These data indicate that the surface of the MSQ film has been changed to an oxide-like layer after O2 and N2O thermal treatment. In Fig. 1a, the thickness of the deposited PECVD cap oxide is approximately 30 nm; the thickness of oxide-like layer is approximately 24 nm and 15 nm in Fig. 1b and c. Comparing the oxygen peak height near the surface of the samples prepared by using the two thermal treatment methods, the O2 thermal treatment appears more efficient in forming the oxide-like layer. In addition, the carbon concentration in Fig. 1b is less than that in Fig. 1a, c and d. It might lead to less effective oxide charges and might be related to electrical conduction mechanism. The reflectance FTIR spectra of various post-treatment MSQ films are shown in Fig. 2. The FTIR spectrum of the cured MSQ film exhibits absorption peaks at (i) 3000 cmy1, (ii) 1275 and 780 cmy1, which correspond to C–H and Si–C stretching vibrations in the polymer, respectively. Two absorption peaks at 1000–1200 cmy1 are associated with the Si– O stretching cage-like peak near 1133 cmy1 and the Si–O stretching network peak near 1030 cmy1 w4,10x. After the post-treatments, the intensities of H–OH peak disappeared. The absence of H–OH signal for the posttreated samples implies a decrease of water absorption near the surface. Such result indicates that the moisture resistance of MSQ films may be enhanced after the
Fig. 2. FTIR spectrum of MSQ films for various surface treatments.
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Fig. 3. C–V curves of MIS capacitors with (a) O2 thermally treated, (b) N2O thermally treated and (c) untreated, MSQ films stressed at 300 8C, 400 8C and 450 8C for 30 min sintering, respectively.
post-treatments. Comparing the untreated MSQ film with post-treated samples, there is no conspicuous difference except for the Si–C (1275 cmy1) (780 cmy1) dips and C–H (2978 cmy1) dip, which are reduced in the posttreated samples, confirming that post-treatments decrease the carbon intensity. Among the four samples, the O2 thermally, N2O thermally and O2 plasma treated samples have a lower carbon concentration than that the untreated sample. Those results are consistent with XPS data presented in Fig. 1. 3.2. C–V characteristics of CuyTaNysurface modified MSQySi capacitance Fig. 3a–c depict the C–V results of three MIS capacitors prepared with O2 thermally treated, O2 plasma-treated, and untreated MSQ films, and subjected to sintering at temperature 300, 400 and 450 8C for 30 min in N2 ambient. From Fig. 3a and b, we found that the C–V curves tend to shift upward with increasing sintering temperature (300 to 400 8C). Samples prepared using the PECVD oxide capped and N2O thermally treated MSQ films had the same behavior (not shown).
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This phenomenon indicates that the effective oxide charges are reduced during sintering process. After sintering at 450 8C, the C–V curves shift to the left. It might be due to the penetration of Cu ions into MSQ films. For the reference sample (Fig. 3c), the negative shift is observed at 400 8C. This implies that this sample has a lower barrier resistance to Cu. This indicates the oxide-like surface formed on the post-treated surfaces has enhanced the resistance to Cu diffusion. The MIS sample prepared with O2 thermally treated MSQ shows the least flat-band voltage shift in the C–V curves between 300 and 400 8C sintering, evidencing that the O2 thermal treatment gives the least reduction of effective oxide charge. The results of dielectric constant k and flat-band voltage shift DVfb of all the MIS capacitors are summarized in Table 2. All of the samples have retained a k value of approximately 2.67 to 2.89. The k value varied little after sample sintering at temperature of 300 8C and 400 8C except for the MIS capacitor prepared with untreated MSQ. From these data, we believe sintering at a temperature of 400 8C for 30 min in N2 ambient is a good choice for the sintering process. The effective oxide charges inside the MSQ film can be determined from the flat-band voltage data for the samples sintered at 400 8C. The value of Fms was about y0.758 volt for our samples. The amount of effective oxide charges can be calculated w11x and are summarized in Table 2. The effective oxide charges are the sum of the interface states and fixed oxide charges. The MIS capacitor prepared from O2 thermally treated MSQ has the less effective oxide charges. This sample also showed the lower carbon content from XPS and FTIR data. These evidences are related to the good electrical characteristic of O2 thermally treated MSQ film in leakage current measurement described in Section 3.3. Fig. 4 displays C–V results for MIS sample prepared from untreated and O2 thermally treated MSQ and subjected to bias-temperature stress at 200 8C for 30 min with an electric field ("Es) of "0.5 mVycm. The C–V curve of the O2 thermally treated sample is weakly affected by the bias-temperature stress. The curve labeled (qEs), (yEs), and no bias aligned together which are an evidence that no copper ions has penetrated into the MSQ film after bias-temperature stress. The bias-temperature stress behaviors of the other modified samples are similar to this sample. For the reference sample, a 1.3 V shift of (qEs) curve toward the negative direction after q0.5 mVycm bias stressed was observed. The hysteresis loops of the C–V curves indicate the presence of mobile Cu ion. These results are consistent with those from Fig. 3. From this biastemperature stress experiment, the improvement of the interface of CuyMSQ and the enhancement of the Cu diffusion resistance by an O2 thermal treatment of the MSQ film is confirmed.
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Fig. 4. C–V curves of MIS capacitors of (a) O2 thermally treated and (b) untreated, MSQ films after heating at 200 8C for 30 min under positive (qEs), negative (yEs) gate and no bias.
3.3. I–V characteristics of CuyTaNysurface modified MSQySi capacitance Fig. 5 shows the leakage current density (JL) of the MIS capacitors as a function of electric field (E). The applied gate voltage is ramped at the slow rate of 0.1 Vys. The MIS capacitor prepared from O2 thermally treated MSQ has the lower leakage current density. This might be related to the small effective oxide trapped charges of the sample (Table 2). The dielectric breakdown fields (Ebd), defined as the electrical field at
Fig. 5. Leakage current density (JL) of MIS capacitors as a function of electric field (E) on a log JL vs. E plot.
which the capacitor’s leakage current density exceeds 8=10y6 Aycm2 w12x, are summarized in Table 2 for all samples. Except for samples prepared using the O2 plasma treated and the untreated MSQ films, the dielectric breakdown field of the samples was exceeding 2 mVycm. The JL –E curves of all samples were parabolic, except for the O2 thermally treated sample. In order to understand the carrier conduction mechanism in this sample, the I–V the plot of and two the fitted curves are presented in Fig. 6. The solid lines are fitted curves, IsaV2 where as0.780=10y10 for V)34.46 V and as 2.622=10y12 for V-34.46 V. The broken line is fitted with IsbV6 where bs7.937=10y18. The good fitting behavior between experimental data and bV6 curve implies that space charge limited current with trap dominates the conduction mechanism w13x. For the traps distribution given by NtsAey´ykTc, where ´ is the energy measured from the bottom of the conduction band and Tc is a characteristic temperature greater than the temperature at which the currents are measured, the analysis of space-charge-limited current will take the form V n where ns(TcyT)q1. For larger n values, the trap charge density, Nt, approaches a uniform distribution over most of the distance between plane parallel electrodes. These deep trap states lower the drift mobility of injected carriers, but cannot be thermal excited into conduction band. Fig. 7 shows the leakage current behaviors of the other samples plotted as JL vs. E 1y2. All samples but the O2 plasma treated exhibited a linear relationship between JL and the square root of the applied electric field. This linear relationship can be explained by Poole–Frenkel (P–F) mechanism, which is due to field-enhanced thermal excitation of a certain level of trapped electrons into the conduction band w14x. In Fig. 7, the O2 plasma treatment modified sample shows two different slopes. This implies that two different trap states exist in the MSQ film of the O2 plasmatreated sample. The enlarged leakage current at higher electric field might be due to trapped states of deeper energy level. Based on the analysis of dielectric conduction mechanism of the above leakage current, P–F
Fig. 6. The experimental data of leakage current (I) vs. gate voltage (V) (symbols) fitted with polynomial aV2 (solid lines) and bV6 (broken line) for O2 thermally treated MSQ film.
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mechanism was dominated in sample with higher effective oxide charge density in MSQ, and space-chargelimited current conduction mechanism was observed in the O2 thermally treated sample with lower effective oxide charge density in MSQ. According to XPS depth profile data (Fig. 1), carbon concentration in the O2 thermally treated MSQ film is lower. Therefore, the carrier conduction mechanism should be related to effective oxide charge density that might come from the existence of the carbon impurity.
trical techniques including C–V, I–V characteristics measurements, XPS and FTIR. The surface modifications of the organic polymer lead to improvement of the barrieryMSQ interface near the surface by forming oxide-like layer. The dielectric constant varies from 2.67 to 2.89 for the surface modified samples sintered at 300 and 4008C. After surface post-treatments, O2 thermal treatment, our C–V data showed that fixed oxide charges residing in the MSQ film could be removed. From biastemperature stressing study, the C–V plot of O2 thermally treated sample shows the absence of a flat-band shift that indicates the absence of copper ion into MSQ films. The leakage current density of O2 thermally treated sample with copper electrode gate was approximately 2=10y10 Aycm2 at electric field strength of 1 mVycm. The O2 thermal treatment also reduced the carbon intensity in MSQ film and led to the spacecharge-limited mechanism of the leakage current conduction. Furthermore, the degradation of the various post-treated capacitors subjected to 40–50% relative humidity environment for 120 days was studied by measuring k values. In all post-treated MSQ films, the stable capacitance demonstrated the moisture resistance of capacitor fabricated. Therefore, based on all measurements, O2 thermal treatment at temperature 300 8C for 10 min represents a suitable method to improve the surface quality of MSQ film.
3.4. Degradation study
Acknowledgments
Stable dielectric constant will be an important reliability challenge for the Cuylow-k dielectric integration. Therefore, the moisture-induced dielectric degradation was also investigated. SEM cross-section view and C– V measurements were taken for the stacked MIS capacitors after expositing them to 40–50% relative humidity flowing air for 120 days. The degradations of k values of the samples are summarized in Table 3. All the k values of samples with surface modification almost remained constant within the error range. From this degradation study, the surface modification processes have improved the moisture resistance of MSQ film.
The authors would like to thank to Allied Signal Incorporated for providing the methylsilsesquioxane for evaluation. This work was supported by the National Science Council of the Republic of China under contract no. NSC89-2215-E-007-015
Fig. 7. Leakage current density (JL) of the MIS capacitors after PECVD oxide capped, N2O thermally treated, O2 plasma treated and untreated, MSQ films as a function of electric field (E) on a log JL vs. E 1y2 plot.
4. Conclusions The physical properties of MSQ after various posttreatments, namely MSQ coated with PECVD oxide, O2 thermal treatment, N2O plasma treatment and O2 thermal treatment, were investigated by sensitive elecTable 3 k values of fresh samples and samples after exposited them to 40– 50% relative humidity flowing air for 120 days Capped PECVD oxide O2 thermally treated MSQ N2O thermally treated MSQ
2.67"0.07 2.74"0.10 2.87"0.01
2.70"0.04 2.75"0.09 2.87"0.01
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