Electrochemical and Solid-State Letters, 12 共8兲 H292-H295 共2009兲

H292

1099-0062/2009/12共8兲/H292/4/$25.00 © The Electrochemical Society

Effect of Porogen Residue on Chemical, Optical, and Mechanical Properties of CVD SiCOH Low-k Materials Adam M. Urbanowicz,a,b,z Kris Vanstreels,a Denis Shamiryan,a Stefan De Gendt,a,b,* and Mikhail R. Baklanova a

IMEC, Heverlee, Belgium Department of Chemistry, Katholieke Universiteit Leuven, Heverlee, Belgium

b

The effect of He/H2 downstream plasma on chemical vapor deposition 共CVD兲 low-k films with different porosities was studied. The results show that this plasma does not reduce the concentration of Si–CH3 bonds in the low-k matrix and that the films remain hydrophobic. However, mass loss and reduction in bulk C concentration were observed. The latter phenomena are related to the removal of porogen residue formed during the UV curing of the low-k films. It is demonstrated that the porogen residue removal changes the films’ porosity and mechanical properties. The depth of the modification is limited by the penetration of H radicals into the porous low-k films. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3139741兴 All rights reserved. Manuscript submitted March 16, 2009; revised manuscript received April 29, 2009. Published May 22, 2009.

The plasma-induced damage of porous SiCOH-type low-k materials is one of the key problems in Cu/low-k integration.1 The most severe plasma damage occurs during photomask 共resist兲 removal.2 The reason for this phenomenon is the hybrid nature of the SiCOH materials. These materials contain a SiO2-like matrix where part of the terminating oxygen atoms is replaced by organic groups 共most often CH3兲. The organic groups provide the films’ hydrophobicity, which is important for the dielectric constant reduction. The hybrid nature is the reason for the different reactivities of the low-k components. For instance, the reactive species from plasma such as O radicals penetrate into the porous network of the low-k films and may result in substantial carbon depletion, thus leading to an increase in the k value. The depth of penetration is defined by the diffusion coefficient of active radicals into the pores and their recombination probability on the pore wall.3 Two commonly known approaches are used for the resist removal: 共i兲 a low temperature, low pressure anisotropic plasma, where the photoresist is removed by an ion-assisted process, oxidizing or reducing plasma chemistries at low temperatures and 共ii兲 hydrogen-based downstream plasma 共DSP兲 where the resist is removed at high temperatures by a thermally activated chemical process. According to recent publications, the option with He/H2 and Ar/H2 DSPs prevents carbon depletion from the low-k materials matrix.4-6 Therefore, the degradation of the dielectric constant is minimal, and these processes are considered the most attractive options for the microelectronic industry. The porosity in advanced chemical vapor deposition 共CVD兲 low-k films is created after deposition by the removal of a sacrificial phase 共porogen兲 by UV-assisted thermal curing. UV curing also results in formation of the Si–O–Si network with improved mechanical properties.7,8 The porogen molecules are normally cyclic hydrocarbons9 that are photodissociated by UV light with the formation of volatile hydrocarbons and nonvolatile carbon-rich residues.10,11 The effect of the porogen residues on the low-k properties and the plasma processing compatibility are largely unknown. Fourier transform infrared 共FTIR兲 spectrometry has a limited sensitivity to amorphous carbon 共CvC and C–C bonds兲, and this is the reason why it is difficult to monitor porogen residues with this technique. Recent studies using Raman spectroscopy and UV spectroscopic ellipsometry 共UVSE兲 allowed a quantitative evaluation of porogen residues.10,11 In this article, using FTIR spectroscopy, time-of-flight secondary-ion mass spectroscopy 共TOF-SIMS兲, UVSE, and mass measurements, we demonstrate that the porogen residues actively react with hydrogen radicals and are removed during the processing

* Electrochemical Society Active Member. z

E-mail: [email protected]

in He/H2 DSP.4 The porogen residue removal has a significant impact on the mechanical properties of low-k films. The main purpose of this work is the evaluation of the amount and properties of porogen residues formed in low-k films with different pore sizes and porosities and also of the effects of UV-curing conditions. For this reason we evaluated four different low-k materials. Experimental Materials and experimental procedure.— SiOCH low-k films of 180 and 500 nm with porosities in the range of 23–36% were deposited on 300 mm Si wafers 共Table I兲. The k values were in the range of 2.2–2.5. The matrix material was codeposited with sacrificial porogen by plasma-enhanced chemical vapor deposition 共PECVD兲. Then, the films were UV cured at 430°C. A broad-band UV curing was performed for all films except Ea. The materials “B” were deposited with different porogen concentrations and different porosities, but they were cured using the same UV source. The materials “E” were deposited in the same conditions but were cured by two different UV sources. The He/H2 20:1 共DSP兲 treatments of the blanket low-k films were performed in a 300 mm asher. The films were treated with He/H2 DSP at 280°C using 20–700 s. Instrumentation.— The surface hydrophobic properties were evaluated using H2O goniometry. The chemical composition and bonding structure were analyzed using a N2-purged FTIR spectrometer Biorad QS2200 ME. The TOF-SIMS analysis was done using an IONTOF IV instrument in a noninterlaced dual-beam mode with a Xe sputtering beam 共1 keV, 80 nA, 500 ⫻ 500 ␮m兲 and a bunched 15 keV Ga analysis beam 共100 ⫻ 100 ␮m area兲 to detect secondary ions. The optical properties and the depth of the plasma modification were measured by a UVSE in the range of 150–895 nm by using an Aleris ellipsometer from KLA Tencor. The optical properties were determined by fitting models to the measured spectra of the ellipsometric polarization angles at 70° by single- and doublelayer optical models using the Marquardt–Levenberg algorithm.11,12 For double-layer spectroscopic ellipsometry modeling, the bottom

Table I. The summary of basic characteristics of the studied CVD low-k films. Open Young’s porosity Targeted Thickness modulus Film 共%兲 k value 共nm兲 共GPa兲 B2 B3 Ea Eb

23 36 32 34

2.5 2.2 2.3 2.3

180 500 180 180

7.43 5.54 4.48 3.8

UV curing source wavelength 共nm兲 ⬎200 共broad band兲 ⬎200 共broad band兲 ⬃172 共narrow band兲 ⬎200 共broad band兲

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Electrochemical and Solid-State Letters, 12 共8兲 H292-H295 共2009兲

Figure 1. 共Color online兲 The chosen magnification of FTIR spectra reflecting the absorbance band of Si–CH3 groups 共1285–1260 cm−1兲 of asdeposited and He/H2 plasma treated Eb films.

layer was assumed to have optical properties of the as-deposited film, while the optical characteristics of the top modified layer were determined by fitting. The mass change was measured by mass balance metrology 共Mentor SF3 from Metryx兲 on 300 mm wafers 共⫾0.04 mg accuracy兲. The open porosity and pore-size distributions were evaluated using ellipsometric porosimetry 共EP兲.13 The mechanical properties, the elastic modulus 共EM兲, and the hardness of the low-k dielectric films were measured using a nanoindenter 共NI兲 XP system 共MTS Systems Corporation兲 with a dynamic contact module and a continuous stiffness measurement option under the constant strain rate condition. A standard three-sided pyramid diamond indenter tip 共Berkovich兲 was pressed into each sample; both the depth of penetration and the applied load were monitored. Results and Discussion Change in composition.— The changes in the IR absorbance of the low-k materials before and after exposure in He/H2 plasma are close to the measurement errors. Therefore, only the magnifications of the selected absorbance bands are shown. There is neither reduction in Si–CH3 concentration 共Fig. 1兲 nor OH group incorporation 共Fig. 2兲. This shows that the films remain hydrophobic. Only discrete changes are observed: the redshift in the Si–CH3 absorbance and a minimal H2O amplitude increase 共around 3200 cm−1兲 presumably due to an increase in the low-k pore radii after the He/H2 DSP plasma exposure. The observations are typical of all studied films. The FTIR results are supported by H2O-goniometry measure-

Figure 2. 共Color online兲 The chosen magnification of FTIR spectra showing absorbance bands related to Si–OH, H2O groups 共3800 − 3000 cm−1兲, and C–H and CvC sp2 groups 共1700–1450 cm−1兲.

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Figure 3. 共Color online兲 C-depth profile for as-deposited and He/H2 treated EB films as measured by TOF-SIMS. The embedded graph presents the sputtering time proportional to depth of modification 共C depletion兲 vs He/H2 exposure time.

ments showing that surface contact angles with H2O remain higher than 80° for all the studied low-k films. Therefore, both the surface and the bulk of the low-k films remained hydrophobic, and one can conclude that no plasma damage in the low-k matrix has occurred. Figure 2 shows a magnification of the FTIR spectra in the 1450–1700 cm−1 range that indicates a reduction in the 1600 cm−1 absorbance band related to the CvC sp2 bond with the time of the He/H2 plasma treatment and some changes in C–H groups 共1475–1550 cm−1兲.10 The presence of the CvC and C–H groups is described as the signature of porogen residues.10,14 A reduction in the absorbance bands of CvC and C–H groups might indicate a removal of the porogen residues after the He/H2 plasma exposure 共Fig. 2b兲. However, the CvC and C–H group amplitudes were very small, and it agrees with the literature.10,14 TOF-SIMS analysis 共Fig. 3兲 shows a reduction in the carbon concentration as a result of the He/H2 plasma exposure. The depth of carbon depletion depends on the exposure time 共zoomed area in Fig. 3兲. The sputtering time in TOF-SIMS in first approximation is proportional to the film thickness. Therefore, the depth of carbon depletion has a tendency to saturate with time, which allows us to assume that this process is limited by the diffusion of H radicals from the He/H2 DSP plasma. The penetration depth of the H radicals must be determined by the properties of low-k films such as pore size, open porosity, porogen content, and the recombination coefficient of H radicals on the pore walls.3 The reduction in carbon concentration is limited: The carbon concentration in the top part of the films stays the same after 35, 140, and 700 s 共Fig. 3兲. Therefore, this carbon depletion process is limited by the porogen residue content in the film matrix. The change in carbon concentration is in qualitative agreement with the mass measurements 共Fig. 4兲. The mass of low-k films is reduced as a result of the exposure to the plasma and also has a tendency to saturate at sufficiently long exposure time. This agrees with TOFSIMS results and suggests that the depletion of carbon concentration in TOF-SIMS and mass loss have the same nature. The phenomena observed in TOF-SIMS and mass balance measurements have a seeming contradiction with the FTIR results 共no Si–CH3 group depletion兲. Therefore, TOF-SIMS and mass balance reflect the concentration change of the carbon compounds, whose bonding structure is almost invisible in the FTIR spectra. FTIR has a limited sensitivity to an amorphous carbonlike porogen residue 共Fig. 2兲. Therefore, the most reasonable assumption is that the carbon depletion and mass loss are related to the removal of the porogen residue. To prove this assumption, we studied the plasma exposed samples by UVSE. The spectra of as-deposited 共solid lines兲 and 700

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Electrochemical and Solid-State Letters, 12 共8兲 H292-H295 共2009兲

H294

Figure 5. 共Color online兲 Optical properties in the range of 150–900 nm of as-deposited and 700 s He/H2 modified films as measured by UVSE. Figure 4. 共Color online兲 Mass loss after He/H2 plasma modification as measured by mass balance on 300 mm wafers. The numbers show percentage mass loss in modified layers 共700 s兲 assuming modification depths as measured by UVSE 共Table II兲.

defined by a large amount of porogen residue deposited on the pore wall. The reduction of the refractive index 共RI兲 suggests an increase in porosity.13 Therefore, the damage-free strip based on the He/H2 plasma could significantly change the films’ properties if the deposition and curing conditions were not sufficiently optimized 共i.e., leaves too much porogen residue兲. One of the most expected problems related to porogen residue removal can be a change in the mechanical properties that directly depend on porosity.

s He/H2-plasma-modified 共dashed lines兲 films are shown in Fig. 5. The refractive indexes and extinction coefficients are different from film to film. According to recent results reported by Marsik et al.,15,16 absorption bands located between 200 and 300 nm are related to the presence of amorphous carbonlike porogen residues. Therefore, the difference in the absorption spectra of these films is mainly related to the different amounts of porogen residues. The films Ea and Eb were deposited in exactly the same conditions, but they were cured by light with different wavelengths. It is clear from Fig. 5 that the curing by light with ␭ ⬎ 200 nm 共Eb兲 leaves less porogen residues. Plasma exposure completely removed the porogen residue from both Ea and Eb films, and the final absorption spectra became similar to the UV spectra of the low-k matrix material.15 The refractive indexes of these two films after the porogen residue removal are the same. This is additional proof that all porogen residues were removed independently on the wavelength of UV light used for curing. The low-k films B2 and B3 were deposited in the same PECVD chamber and cured by the same broad-band lamp with ␭ ⬎ 200 nm 共Table I兲. However, they have different porosities that were provided by different ratios of the matrix and porogen precursors. Both these films contain less porogen residues in comparison with the films Ea and Eb. The film B3 has a higher porosity 共Table I兲 and contains more porogen residues in comparison with B2. The disappearance of absorption bands between 200 and 300 nm for He/H2-treated films B2 and B3 indicates a complete removal of the porogen residues. However, the complete porogen residue removal for B2 and B3 films results in smaller changes in refractive indexes than that for Ea and Eb films. This fact suggests that the relatively low porosity and pore size of the films Ea and Eb 共see Table II兲 were

Mechanical properties.— The amount of the removed porogen residue and the corresponding change in the films’ properties after 700 s of He/H2 DSP plasma exposure are summarized in Table II. The change in mass related to porogen residue removal 共Fig. 4兲 was normalized to the thickness of the top modified layer 共MD in Table II兲. The degree of porogen removal 共DPR兲 was the highest for the Ea film 共0.0137 mg/nm兲 and approximately 3.7 times lower for the B2 film with the lowest porogen content 共0.0037 mg/nm兲. The porosity was calculated using the Lorentz–Lorenz 共LL兲 equation,13 assuming that the removed porogen has an RI value close to the film skeleton 共the first approximation兲. The porosity is increased in all cases 共for all films兲. The mechanical properties of low-k films were evaluated using NI.17 Because the film thicknesses were relatively small 共Table I兲, only a relative study was possible due to the Si substrate effect.18 NI showed a reduction in the EM for all films except B2 共Fig. 6兲. Film B2 has the lowest porogen residue content, as indicated by both UVSE and mass balance measurements 共Fig. 5 and 4兲. The cause of the lower level of porogen residue is the lower porosity 共less porogen was used during deposition兲 and optimized deposition and curing conditions in comparison with the films Ea and Eb. The film Ea with the highest porogen residue content shows the highest relative reduction in EM. The reduction in mechanical properties during the exposure to the He/H2 DSP plasma is proportional to the initial porogen content in the low-k material. Furthermore, the initial mechanical properties of films Ea and Eb are very much determined by

Table II. Summary of as-deposited and 700 s HeÕH2 plasma treated film properties. The porosity change was calculated using LL equation, and mean pore size was measured by EP.

Film B2 B3 Ea Eb

MD 共UVSE兲 共nm兲

DPR 共mg/nm兲

69 150 117 154

0.0037 0.0084 0.0137 0.0120

EM 共GPa兲 共mean value兲

RI632

nm

共top layer兲

LL porosity 共top layer兲 共%兲

Mean pore radii 共top layer兲 共nm兲

Before

After

Before

After

Before

After

Before

After

7.43 5.54 4.48 3.8

7.74 4.97 2.84 2.54

1.343 1.341 1.378 1.339

1.320 1.279 1.226 1.230

23 36 32 34

28 47 58 56

0.7 1.1 0.9 1.0

0.7–0.8 n.a. 1.6 1.6

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Electrochemical and Solid-State Letters, 12 共8兲 H292-H295 共2009兲

H295

more porous, which is reflected in the mass loss, and the optical properties change after the plasma treatment. The good mechanical properties of the blanket films can be provided by the presence of dense porogen residues. The removal of this residue by He/H2 plasma significantly reduces Young’s modulus, suggesting that the film matrix would not remain sufficiently stiff after a regular UV curing. The controlled degradation of the mechanical properties and change in pore size after He/H2 plasma presents additional challenges during the integration. Therefore, the problem of a damagefree strip must be solved in combination with the improvement of deposition precursors, conditions, and curing of low-k films. IMEC assisted in meeting the publication costs of this article.

References

Figure 6. 共Color online兲 Relative EM of the He/H2 modified layer for different low-k films as measured by NI.

porogen residues. Porogen residues provide relatively high EM, although the skeleton of these films is much softer that those in films B2 and B3. The mechanical strengthening of silica-based porous materials by C-based polymer additions was already discussed by Zhang et al.19 Furthermore, Maidenberg et al.20 found that controlled porogen decomposition during the UV-curing process improves the fracture energy of methylsilsesquioxane low-k materials. However, this article reports on the modification of the CVD low-k mechanical properties due to the removal of porogen residue during the damage-free plasma processing. Conclusions The time effect of the He/H2 downstream ash plasma at 280°C on CVD low-k dielectric films with different porosities is evaluated. All films contain different amounts of porogen residue. The porogen residue is a nonvolatile product of UV photochemical dissociation of porogen with a chemical composition close to amorphous carbon and carbon-rich hydrocarbons. The amount of porogen residue depends on 共i兲 the deposition conditions and porosity. If more porogen is used during the deposition 共to get more porous films兲, more porogen residue remains after the curing. It also depends on 共ii兲 the UV-curing conditions. The curing process with a wavelength of 172 nm generates more residues than the curing process with a wavelength longer than 200 nm. The porogen residue can be removed by He/H2 plasma. The depth of removal is determined by the penetration depth of the H radicals into low-k materials. The porogen depleted layer becomes

1. K. Maex, M. R. Baklanov, D. Shamiryan, F. Iacopi, S. H. Brongersma, and Z. S. Yanovitskaya, J. Appl. Phys., 93, 8793 共2003兲. 2. A. M. Urbanowicz, A. Humbert, G. Mannaert, Z. Tokei, and M. Baklanov, Solid State Phenom., 134, 317 共2008兲. 3. M. R. Baklanov, A. M. Urbanowicz, G. Mannaert, and S. Vanhaelemeersch, in International Conference on Solid-State and Integrated Circuit Technology, Chinese Institute of Electronics, p. 291 共2006兲. 4. I. L. Berry, Q. Han, C. Waldfried, O. Escorcia, and A. Becknell, in SEMI Technical Symposium: Innovations in Semiconductor Manufacturing, SEMICON WEST 共2004兲. 5. M. Darnon, T. Chevolleau, T. David, N. Posseme, J. Ducote, C. Licitra, L. Vallier, O. Joubert, and J. Torres, J. Vac. Sci. Technol. B, 26, 1964 共2008兲. 6. A. M. Urbanowicz, D. Shamiryan, P. Marsik, Y. Travaly, A. Jonas, P. Verdonck, K. Vanstreels, A. Ferchichi, D. De Roest, H. Sprey, et al., in 25th Advanced Methalisation Conference, UC Berkley, Vol. P.VI2.2 共2008兲. 7. F. Iacopi, Y. Travaly, B. Eyckens, C. Waldfried, T. Abell, E. P. Guyer, D. M. Gage, R. H. Dauskardt, T. Sajavaara, K. Houthoofd, et al., J. Appl. Phys., 99, 053511 共2006兲. 8. A. Zenasni, F. Ciaramella, V. Jousseaume, C. Le Cornec, and G. Passemard, J. Electrochem. Soc., 154, G6 共2007兲. 9. A. Grill and V. Patel, J. Appl. Phys., 104, 024113 共2008兲. 10. M. Matsuura, K. Goto, N. Miura, J. M. Haag, S. Hashii, and K. Asai, Mater. Res. Soc. Symp. Proc., 914, F01-06 共2006兲. 11. P. Marsik and M. Baklanov, in Ninth International Conference on Solid-State and Integrated-Circuit Technology, IEEE Beijing Section and Chinese Institute of Electronics, p. 765 共2008兲. 12. S. Eslava, G. Eymery, P. Marsik, F. Iacopi, C. E. A. Kirschhock, K. Maex, J. A. Martens, and M. R. Baklanov, J. Electrochem. Soc., 155, G115 共2008兲. 14. A. Zenasni, V. Jousseaume, P. Holliger, L. Favennec, O. Gourhant, P. Maury, and G. Gerbaud, J. Appl. Phys., 102, 094107 共2007兲. 15. P. Marsik, A. Urbanowicz, P. Verdonck, K. Ferchichi, D. De Roest, L. Prager, and M. R. Baklanov, Mater. Res. Soc. Symp. Proc., 1079E, N07-064 共2009兲. 16. P. Marsik, P. Verdonck, D. De Roest, and M. R. Baklanov, Thin Solid Films, Submitted. 13. M. R. Baklanov, K. P. Mogilnikov, V. G. Polovinkin, and F. N. Dultsev, J. Vac. Sci. Technol. B, 18, 1385 共2000兲. 17. D. J. Morris and R. F. Cook, J. Mater. Res., 23, 2429 共2008兲. 18. M. Gonzalez, K. Vanstreels, and A. M. Urbanowicz, in Seventh EUROSIM Congress, Czech and Slovak Simulation Society 共2009兲. 19. G. Zhang, A. M. M. Rawashdeh, C. Sotiriou-Leventis, and N. Leventis, Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 44, 35 共2003兲. 20. D. A. Maidenberg, W. Volksen, R. D. Miller, and R. H. Dauskardt, Nature Mater., 3, 464 共2004兲.

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Effect of Porogen Residue on Chemical, Optical, and ...

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