A carbon-restoring silylation process for low-k dielectric repair
07/01/2007
Organo-silicate glass (OSG) based low-k dielectric materials are widely used to minimize resistance-times-capacitance (RC) delays in the BEOL interconnect for <90nm node logic devices. Further reductions in RC delay at the 45nm node and beyond require the use of ultra low-k materials, such as porous OSG dielectrics. As the carbon content and porosity of these materials increases to achieve lower k values, they become more susceptible to damage from plasma processes for etch and strip as well as from exposure to wet clean/strip chemicals. The damage is a result of carbon depletion in the film, which results in the degradation of the film’s dielectric properties as well as other issues such as CD loss due to low wet-etch resistance, poor barrier integrity, and moisture absorption in the film causing reliability failures.
Attempts to resolve the plasma damage issue have included improving the low-k film’s plasma resistance and the use of interconnect architectures that minimize damage [1], reducing damage during etch, strip, and other processes [2, 3], and repairing the low-k damage by post-treatments [4-8]. It is likely that one or more of these approaches may be used to overcome plasma damage to porous OSG materials. Repairing the damage by carbon restoration could potentially increase the process window for other steps.
A promising method for repairing carbon damaged low-k dielectric materials is through the use of a silylation reaction. A silylating agent typically has two distinct components joined by a Si atom: an organic substituent and a hydrolysable substituent. The reaction can proceed through an intermediate hydrolysis in hydrated media or as a direct reaction with SiOH bonds in the film in the absence of moisture. Nitta et al [6] have reported a systematic study of key aspects of silylating agents. They showed that difunctional silylating agents (those with two hydrolysable groups) were most effective at restoration. Monofunctional silylating agents such as HMDS (hexamethyldisilazane) and TMCS (trimethylchlorosilane), which were the focus of earlier studies, were not as efficient in eliminating SiOH groups in the film. Trifunctional silylating agents suffered from a tendency to undergo self-condensation reactions leading to oligomer formation, which can induce defect formation. Difunctional agents may also be susceptible to oligomer formation, albeit to a lesser extent than trifunctional agents. Most silylation processes require a significant thermal treatment after exposure of the silylating agent to the damaged film to eliminate hydrogen bonded SiOH.
The key criteria for an effective and manufacturable silylation process are: high level of k and carbon restoration, no addition of residues/defects, low thermal budget, and high throughput. We evaluated dielectric restoration using a material developed by Honeywell called Toughening Agent (TA). The application of TA to hybrid dual-damascene structures has been previously reported [1, 7, 8]. TA is designed to achieve high level of restoration with a low thermal budget, while preventing formation of non-volatile residues through self-condensation. TA can be applied in liquid or vapor state and the processing is done using standard equipment, such as spin coaters, wet clean tools, and vacuum processing chambers. This paper presents the results from the use of TA in a spin coater with integrated hot plate bake modules.
Characterizing low-k films
The initial evaluations of TA were carried out at Honeywell using spin-on and CVD deposited porous low-k films. The FTIR spectra of Nanoglass-a porous spin-on low-k dielectric film (k = 2.2 after thermal cure @ 425°C, 1hr)-were observed after plasma damage, and after silylation. To simulate plasma damage, the following conditions were used on a blanket film: etch process in TEL DRM-85 (C4F8/CO/Ar/N2, 40mT, 1000W), followed by a photoresist strip process in the same chamber (O2, 45mT, 400W).
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Silylation was performed using either HMDS or TA by spin coating the silylation agent on the wafer followed by a hotplate bake sequence (125˚C, 200˚C, 350˚C, 1 min each). Damage during etch and strip processes lead to severe reduction in CH and Si-CH3 peaks and an increase in SiOH compared to the post-cure results (Table 1). Both silylation treatments increased the carbon containing peaks and reduced SiOH; however TA was more effective and eliminated moisture. Dielectric constant reduction is superior with TA treatment compared to HMDS (Table 2).
It is important to manage the trade-off between achieving a high level of restoration at low thermal budget and avoiding excessive oligomer formation that may lead to residues and defects. For the non-optimized silylating agent, defect density on bare silicon wafers increased dramatically with increasing concentration of the silylation agent, due to formation of residues on the wafer surface. The “optimized chemistry” of TA is designed to prevent such residue formation regardless of the concentration used, and shows no detectable defect addition.
 Figure 1. Effect of temperature and time bake conditions on toughening agent (TA) used to restore the dielectric constant of plasma-damaged porous OSG low-k films. |
Figure 1 shows the effect of bake conditions used during TA treatment on the dielectric constant of the treated film. For a total baking time of 1 minute (desirable for higher throughput), a dramatic reduction in k occurs as the bake temperature is increased from 150˚C to 225˚C. Beyond 225˚C, further increases in temperature still result in k decreases, but at a slower rate. Additionally, at a given temperature (200˚C), increasing the bake time leads to lower k. A relatively low temperature (350˚C) and short processing time (less than 5 min) is adequate for achieving nearly complete restoration of the dielectric material in terms of its k value.
Figure 2 shows Time-of-Flight SIMS (TOF-SIMS) profiles for three different samples which illustrate the effectiveness of silylation using TA. The first one is of a pristine, k = 2.1 Nanoglass film; the second is of a Nanoglass film that was subjected to an oxidizing plasma process; and the last is similar to the second sample but was subsequently treated with TA. All the treatments and depositions were performed at IBM.
Figure 2b shows that very little carbon (C) remains through the depth of the porous OSG film after exposure to an oxidizing plasma process. Figure 2c shows that silylation successfully increases the C content through the depth of the film suggesting that the silylation agent penetrated and replaced silanols all the way through the depth of the damaged region. It can also be observed that the surface and subsurface regions have the greatest amount of carbon depletion and the least amount of C recovery. We hypothesize that this is due to a densified skin like layer that is formed at the surface and subsurface regions. However, it is interesting to note that the formation of this region does not affect the penetration of the TA to damaged regions beyond this layer.
Restoring interconnect structures
Figure 3 shows SEM cross sections of a single-damascene line. For this type of build, wafers are sent through an aggressive wet etch immediately after the formation of the damascene line (Fig. 3a). In this experiment, the interconnect structure shown in Fig. 3b was silylated immediately after etch, prior to the aggressive wet etch. A comparison of the line dimensions of these two structures shows that the silylated wafer has a straighter profile with a smaller line CD compared to that of the unsilylated wafer (Table 3). This clearly demonstrates CD control which we attribute to the increased hydrophobicity of the ILD after silylation, and, hence more resistance to further etch by the aggressive wet clean.
Figure 4 shows that silylation reduces at-level capacitance of a highly porous, k = 2.3 OSG film integrated in a dual-damascene interconnect structure using a via-first scheme at IBM. A somewhat similar trend can be observed in the leakage values.
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Conclusion
Carbon restoration to a damaged low-k dielectric film is shown to be an effective and manufacturable method to restore the hydrophobicity of porous OSG films used in BEOL interconnects. Through proper selection of a silylating agent and optimization of its composition and process conditions, it is possible to achieve close to complete k restoration of most highly porous dielectric materials with no added defects. Other benefits of carbon restoration include improved resistance to CD loss during aggressive wet cleaning, and improved leakage and capacitance of interconnect structures containing highly porous OSG films as ILDs.
Acknowledgments
Thanks to R. Leung, T. Nguyen, and Y. Negga of Honeywell International Inc., Electronic Materials business unit, for valuable work and discussion. Toughening Agent and Nanoglass are trademarks of Honeywell International Inc.
References
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Anil Bhanap is a project manager at Honeywell Electronic Materials, Dielectrics R&D, 1349 Moffett Park Drive, Sunnyvale, CA 94089; ph 408/962-2090, e-mail [email protected].
Boris Korolev is a senior scientist at Honeywell Electronic Materials and joined Honeywell in 1995.
Satya Nitta is a research staff member in the Advanced Interconnect Technology group at the IBM T.J. Watson Laboratory, Yorktown Heights, NY.
Griselda Bonilla is an advisory engineer in the BEOL Technology Strategy group at IBM.
Sampath Purushothaman manages the Advanced Interconnect Technology group at the IBM T.J. Watson Laboratory, Yorktown Heights, NY.
E. Todd Ryan is a senior member of the technical staff atAdvanced Micro Devices, Sunnyvale, CA.