O2 Plasma Damage and Dielectric Recoveries to Patterned CDO Low-k Dielectrics

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1 O2 Plasma Damage and Dielectric Recoveries to Patterned CDO Low-k Dielectrics H. Huang 1, J. Bao 1, H. Shi 1, P. S. Ho 1, M L McSwiney 2, M D Goodner 2, M Moinpour 2, and G M Kloster 2 1 Laboratory for Interconnect and Packaging, Microelectronics Research Center, The University of Texas at Austin, Austin, TX, Intel Corporation, Logic Technology Development, Hillsboro, OR, ABSTRACT This paper investigated the mechanism of oxygen plasma damage to patterned low-κ structures and dielectric recovery by silylation. Plasma damage was induced to patterned structures using a remote hybrid plasma source with separate ions and atomic radicals. In addition, the oxygen plasma damage to blanket low-κ films and its angular dependence were studied. The damage was characterized by a combination of analytical techniques and the results indicated that damage by ions was anisotropic while damage by radicals was isotropic. The carbon depletion depth was found to be controlled by diffusion of radical species. Sidewall recovery by trimethylchlorosilane (TMCS) vapor silylation was performed and it was found to be more effective for recovery of the sidewall carbon loss induced by O 2 radicals compared to that by hybrid O 2. INTRODUCTION With continuing scaling beyond the 45nm technology node, implementation of ultra lowκ (ULK) dielectrics into Cu interconnects becomes necessary [1]. Plasma processing can degrade low-κ dielectrics by depleting the methyl groups leading to moisture uptake and increases in the effective dielectric constant and leakage current of the low k structure [2, 3]. For ULK dielectrics, the incorporation of porosity can enhance the plasma damage due to increase in the penetration of the plasma species. So far, few reports are available on plasma damage to patterned low-κ dielectrics due to the limitations of metrology techniques [4]. This study investigates the mechanism of O 2 plasma damage and dielectric recovery for patterned low-κ structures. Multiple analytical techniques were used to analyze the damage by oxygen plasma on low κ films at different incident angles (0 to 90 ). The results enabled us to understand the angular dependence of damage by ions and radicals on surface chemistry, composition, and density of the low-κ film. This was supplemented by the use of electron energy loss spectroscopy (EELS) in transmission electron microscope to examine the oxygen plasma damage at the sidewall of patterned low-κ structures. Finally, dielectric recovery by trimethylchlorosilane (TMCS) vapor silylation was performed on the patterned samples and it was found to be more effective for recovery of the sidewall carbon loss induced by O 2 radicals than that by hybrid O 2. EXPERIMENT DETAILS The pristine CDO films were deposited in a standard PECVD chamber to yield a film with about 25% porosity, dielectric constant of 2.5, density of 1.25 g/cm3 and refractive index at 633 nm of The patterns were formed by plasma etching and subsequent DHF clean. The

samples were treated by remote plasmas using an atom/ion hybrid source which can generate hybrid ions and neutrals or atomic radical only.

$XPS surface analyses were performed using a PHI 5700 XPS system. TEM/EELS analysis was performed using an FEI TECNAI G2 F20 Analytical TEM. Film thickness and refractive index were measured by a J. A. Woollam VASE Spectroscopic Ellipsometer.$ Water contact angle was measured by a CA100 Ramé-Hart goniometer. Dielectric recovery was performed in a vapor silylation chamber.

Water contact angle was measured by a CA100 Ramé-Hart goniometer. Dielectric recovery was performed in a vapor silylation chamber.

2 samples were treated by remote plasmas using an atom/ion hybrid source which can generate hybrid ions and neutrals or atomic radical only. The operation of the plasma source has been described elsewhere [5]. The typical ion flux in hybrid beam was estimated to be of the order of atoms/ (cm 2 s) by a Langmuir probe. XPS surface analyses were performed using a PHI 5700 XPS system. TEM/EELS analysis was performed using an FEI TECNAI G2 F20 Analytical TEM. Film thickness and refractive index were measured by a J. A. Woollam VASE Spectroscopic Ellipsometer. Chemical bond structures were investigated by a Magna 560 FTIR spectrometer. Film density was extracted from x-ray reflectivity measured by an X Pert MRD system. Water contact angle was measured by a CA100 Ramé-Hart goniometer. Dielectric recovery was performed in a vapor silylation chamber. TMCS was stored in the bubblers in the liquid phase at room temperature. They were carried into the silylation chamber by CO 2 gases. An optimized experimental condition was reached using a high CO 2 flow rate while maintaining the temperature of the chamber wall, tube, and substrate around 100 C. The damaged samples were treated by the silylation agents for identical 60 minutes at 250 torr. Detailed description of using vapor silylation agents for dielectric recovery can be found in ref. 6. RESULT and DISCUSSION Damage to patterned low-κ structures 0.50 CPS (a.u.) Hybrid 400 ev Hybrid 900 ev Radical After 400 ev hybrid O 2 C After radical O 2 C-H/Si Hybrid O Radical O Position (nm) 0.20 Incident Angle ( 0 ) Figure. 1. EELS line scan of the carbon Figure. 2. Relation between C-H/Si and concentration on the sidewall after the treatment the beam incident angles after 400 ev of 400 ev hybrid O 2, 900 ev hybrid O 2 and hybrid O 2 and radical O 2. radical O 2. Insets are TEM cross section images of trenches after hybrid and radical O 2 damage. The damage to patterned low-κ structures after hybrid beam treatment was different from that after radical beam treatment. TEM and EELS analyses were performed and the results are shown in Fig.1. To avoid contamination during TEM sample preparation, the damaged patterned low-κ films were coated with a protective layer of ~30 nm Cr and ~20 nm Au. The TEM images (insets in Fig.1) showed more distortion in the trench geometry by the radical beam. The EELS

3 line scans across the middle of the trench sidewall (Fig. 1) indicated plasma treatment induced a gradient carbon depletion region from the surface into the bulk. Compared with hybrid beams, the radical beams are isotropic and thus interact more effectively with the carbon atoms on the sidewall. This led to a carbon depletion depth after radical O 2 treatment as large as 140 nm, as compared with a depletion depth of 100nm after hybrid beam treatment at 400 ev. Increasing the hybrid beam energy to 900 ev decreased the depletion depth to 70 nm, which was still less than that of the radical beam. These results indicate that hybrid beams are more directional and penetrate less and cause less carbon loss on the sidewall. Angular dependence of damage to low-κ films Previous studies performed in our laboratory showed that hybrid oxygen beam treatment of blanket low-κ films produced more CO 2 and H 2 O uptake than radical oxygen beam treatment. XPS and FTIR analysis indicated that the energetic ions in the hybrid beam enhanced the formation of C=O and C-O bonds on the low-κ surface and released subsequently more CO 2 gases in the reaction byproducts [7]. In the patterned low-κ structure, the sidewall can be regarded as a surface that interacts with ions and neutrals in the plasma at different incident angles. To study the nature of the directional effect, we investigated plasma damage on low-κ films as a function of incidence angles. The blanket low-κ film was rotated from 0 to 90 corresponding to the beam normal to the sample surface at 0 and parallel at 90. To examine the surface chemistry change, we focused on the loss of surface carbon concentration due to methyl depletion. For this purpose, the XPS carbon 1s peak after hybrid beam treatment was deconvoluted into three sub-peaks of C-H ( ev), C-O ( ev) and C=O ( ev). Here the C-H bond represents the characteristic hydrophilic methyl groups. Fig.2 showed that the C-H/Si ratio after hybrid O 2 treatment increased with increasing incidence angles. In contrast, the C-H/Si ratio remained almost constant after radical O 2 treatment a b FTIR Peak Area Ratio Si-CH 3 /Si-O Si-OH/Si-O Hybrid O 2, 15 min FTIR Peak Area Ratio Si-CH 3 /Si-O Si-OH/Si-O Radical O 2, 15 min Incident Angle Incident Angle. Figure. 3. Relation between FTIR peak area ratio of Si-CH3/Si-O and Si-OH/Si-O and the beam incident angles after (a) 400 ev hybrid O 2 and (b) radical O 2. The FTIR peak ratios of Si-CH 3 (1274 cm -1 ) and Si-O (1250~950 cm -1 ), Si-OH (3600~3200 cm -1 ) and Si-O are shown in Fig.3a and 3b. After hybrid O 2 treatment, the Si- CH 3 /Si-O ratio increased while the Si-OH/Si-O decreased with increasing incidence angles.

4 Interestingly, the Si-CH 3 /Si-O and Si-OH/Si-O ratios in the low k film after radical O 2 beam treatments at varying angles did not show notable differences. Measurement of water contact angles and refractive indexes showed a similar trend. The water contact angle provides a measure of the intensity of the surface polar terms. In Fig. 4 (a), the contact angle gradually increased from ~68 to ~90 as the O 2 hybrid beam impinging from the vertical to the parallel direction. For radical O 2 beam, the contact angles remained at around 85Ü, independent of the beam incident angles where the small ±5 variation in the contact angle was probably due to process variations. To examine the extent of the dielectric loss, the refractive index n which can be correlated to the dielectric constant was measured as a function of incidence angle. In Fig. 4 (b), the refractive index at 633 nm was found to decrease with increasing incidence angles after hybrid O 2 beam treatment. In contrast, there was almost no angular dependence of the refractive index with the incidence angle for the radical beam a 1.44 b Water Contact Angle ( 0 ) Hybrid O 2 Radical O 2 Refractive Index Hybrid O 2 Radical O 2 65 Incident Angle ( 0 ) 1.36 Incident Angle ( 0 ) Figure. 4. Relation between (a) water contact angles, (b) refractive indexes and the beam incident angles after 400 ev hybrid O 2 and radical O 2. Table 1. Density, thickness and roughness of the low-κ films after hybrid and radical O 2 treatment at 0 0 and Sample treatment Density (g/cm 3 ) Thickness (nm) Roughness (nm) Hybrid O 2, (top) (Vertical) 1.34 (bottom) 57.1 ~10 Hybrid O 2, (top) (Parallel) 1.25 (bottom) 76.0 ~20 Radical O 2, 0 0 & 1.52 (top) (bottom) 46.1 ~15 The film density, thickness and roughness were extracted from x-ray reflectivity measurements. The results in Table.1 show a large difference in interface roughness between the top and the bottom layers, confirming the carbon density gradient observed by EELS. The highest density of the damaged layer after the hybrid beam treatment was found to be at 0, which can be attributed to the directional ion bombardment of the hybrid beam. At 90, with the sample surface parallel to the beam direction, the hybrid O 2 beam caused only a limited surface densification while the bulk remained largely unchanged. Compared with the 0 vertical hybrid O 2 treatment, the thickness of the densification layer was also reduced. The density gradient after

radical O 2 treatment was similar at 0 and 90. Compared with hybrid O 2, the radical O 2 induced a thicker densification layer although its density was lower.

5 radical O 2 treatment was similar at 0 and 90. Compared with hybrid O 2, the radical O 2 induced a thicker densification layer although its density was lower. The density of the bottom layer after radical O 2 treatment was larger than that after hybrid beam treatment. This indicates that the neutral oxygen can diffuse deep into the bulk of the low-κ film to cause methyl loss while the surface densified layer after hybrid O 2 treatment blocked the penetration of ions and neutrals. Dielectric recovery by silylation Sidewall recovery was attempted with an optimized TMCS vapor silylation and the effect was examined by nano-beam EELS. Considering the bulk is much less damaged and is therefore regarded as a reference point, the signal ratio of EELS carbon K edge at -284 ev on the sidewall to that in the bulk was used to evaluate the carbon recovery on damaged patterned films(fig. 5). The ratio was only 0.56 on the hybrid O 2 damaged trenches, and it rose to 1.52 on radical O 2 damaged trenches. These results showed only partial recovery of surface carbon on the sidewall damaged by hybrid O 2. However, the recovery was more complete on the sidewall damaged by radical O 2. The difference of damage recovery could be attributed to more surface densification by ions that inhibited the rate-limiting mass transport of vapor chemicals. As indicated by Table 1, the average densities of the damaged layer after hybrid O 2 and radical O 2 at 90 were 1.67 g/cm 3 and 1.52 g/cm 3 respectively. The more densified surface after hybrid O 2 damage might inhibit the mass transfer of silylation agent, and thus weakened the dielectric recovery. C-K Hybrid O 2 + TMCS C-K Radical O 2 + TMCS a C s :C B ~0.56:1 b C s :C B ~1.52:1 Fig. 5. Nano-beam EELS analysis of carbon recovery on the trench sidewall damaged by (a) hybrid and (b) radical O 2 plasma respectively. (Beam size=0.7 nm). C S and C B are carbon K edge height at sidewall surface and bulk. CONCLUSIONS Plasma damage to patterned low-κ dielectrics is a complicated phenomenon depending on the plasma species and the pattern structures. The energetic ions can remove methyl groups and induce surface densification but the effect is anisotropic and thus depends on the impingement angles to the low-κ surface. Radicals, with less kinetic energy, are chemically active but are isotropic, which induce relatively uniform damage independent of the impingement angle. The depth of carbon depletion is determined by the diffusion of neutral species into low-κ. Therefore, the radicals can cause deeper damage to the sidewall than ions, particularly for porous low-κ dielectrics. In comparison, dielectric recovery by TMCS vapor silylation was more complete on the sidewall after radical plasma damage because the surface densification by ions can block the mass transport of silylation agents.

6 ACKNOWLEDGMENTS This work was performed in part at the Microelectronics Research Center at UT Austin of National Nanofabrication Infrastructure Network supported by National Science Foundation under award # REFERENCES 1. International Technology Roadmap for Semiconductors, Semiconductor Industry Association, D. Shamiryan, M. R. Baklanov, S. Vanhaelemeersch and K. Maex, J. Vac. Sci. Technol. B 20, 1923 (2002) 3. H. Shi, J. Bao, J. Liu, H. Huang, S. Smith, Q. Zhao, P.S. Ho, M.D. Goodner, M. Moinpour, G.M. Kloster, Proc. of AMC, 64, (2007) 4. T. Abell, J. Lee, M. Moinpour, Mater. Res. Soc. Symp. Proc. 914, F04-02 (2006) 5. J. Bao, H. Shi, J. Liu, H. Huang, P. S. Ho, M. D. Goodner, M. Moinpour and G. M. Kloster, J. Vac. Sci. Technol. B 26, 219 (2008). 6. H. Shi, J. Bao, H. Huang, J. Liu, S. Smith, W. Kim, Y. Sun, P. S. Ho, M. L. McSwiney, M. Moinpour and G. M. Kloster, Mater. Res. Soc. Symp. Proc. 1079, N02-10, (2008) 7. J. Bao, H. Shi, H. Huang, P. S. Ho, M. L. McSwiney, M. D. Goodner, M. Moinpour and G. M. Kloster, AVS 54 th Symp., 2007

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