Elsevier

Applied Surface Science

Volume 252, Issue 18, 15 July 2006, Pages 6323-6331
Applied Surface Science

Investigation on hexamethyldisilazane vapor treatment of plasma-damaged nanoporous organosilicate films

https://doi.org/10.1016/j.apsusc.2005.08.060Get rights and content

Abstract

Hexamethyldisilazane (HMDS) vapor treatment of plasma-damaged nanoporous organosilicate thin films has been studied as a function of treatment temperature in this work. Although, the HMDS vapor treatment facilitated incorporation of methyl (CH3) groups subsequent to the removal of free hydroxyl (OH) groups in the damaged films at treatment temperature as low as 55 °C, the bonded OH groups were not removed. More significantly, detailed analysis of the results reveals that HMDS vapor modified only the surface of the plasma-damaged samples and not the entire film as expected. This is attributed to the formation of a thin solid layer on the surface, which effectively prevents penetration of HMDS vapors into the bulk. The Fourier transform-infrared (FT-IR) absorption and dielectric constant measurements confirm that the vapor treatment assists only partial curing of the plasma-damaged films. Alternative processes of curing the films with HMDS dissolved in supercritical carbon dioxide (SCCO2) as a medium of reaction in static and pulsed modes were also attempted and the results are presented in this paper.

Introduction

High on-chip speeds require the use of low dielectric constant (k) materials for insulation. Lowering the k-value reduces the cross-talk noise and minimizes the dissipated power in the circuit by lowering the resistance–capacitance (RC) constant of the device [1], [2], [3]. Many research groups are involved in the development of a suitable low-k material. A wide range of spin-on and plasma enhanced chemical vapor deposited films are being explored as potential candidates. Materials with k-value less than 2 have been reported [4], [5], [6], [7], [8]. Nanoporous silica-based films have attracted special attention in recent years as their k-value films can be tailored by tuning their porosity. However, there are serious integration issues concerned with the implementation of these films. For example, plasma ashing, etching and cleaning are important process steps involved in the production of Si integrated circuits and these integration steps are found to destroy the structural and electrical properties of the nanoporous films.

Recent studies signify that the hydrophobic CH3 groups are removed during photoresist ashing and replaced with the hydrophilic silanol (Sisingle bondOH) groups [9], [10], [11]. Highly polar SiOH attracts more moisture and this increases the k-value of the porous material considerably. The removal of the hydrophobic CH3 groups and the subsequent increase in the k-value of the nanoporous films is referred to as the plasma-damage. Hence, it is vital to cure plasma-damaged films and restore their original structural and electrical characteristics. Researchers have reported a few approaches during the recent years for minimizing or curing the plasma-damage caused during photoresist stripping. One of the approaches is the use of He or NH3 pretreatment of the as-deposited or as-spun films for a few minutes before the removal of photoresist with O2 ashing and chemical wet stripper process [12], [13]. For example, it is reported that the NH3 plasma pre-treatment transformed the low-k hybrid-organic-siloxane-polymers (HOSP) surfaces into a thin nitrogen-containing layer, which acts as a good barrier against O2 plasma-damage and high-alkalinity wet stripper solution attack [13]. Alternatively, treatment with hexamethyldisilazane (HMDS) or trimethylchlorosilane (TMCS) is found to be as a potential method to cure the damaged films [9], [10], [11]. Previous works involved spin coating of HMDS on the surface of damaged porous films and then subjecting the films to hot plate baking at different temperatures [11] or by flowing the HMDS vapor in the treatment chamber [10]. However, the extent of curing by HMDS vapor treatment has not been clearly addressed. The work reported in this paper attempts to bring further clarity on the effectiveness of curing with HMDS vapors. In the present work, HMDS vapor treatment of plasma-damaged nanoporous methylsilsesquioxane (MSQ) films at different temperatures was studied in detail. The effects of treatment temperature on the structural and electrical properties of these films have been comprehensively investigated in this work. Fourier transform infrared (FT-IR) spectroscopy was used extensively to monitor the structural changes in the films after each treatment. Dielectric constant measurements were also carried out and correlated with the FT-IR observations recorded before and after repair of the plasma-damaged films.

More recently, the use of supercritical carbon dioxide (SCCO2) as a medium of reaction with silylating agents (HMDS/TMCS) as a good alternative has been reported [14], [15], [16]. Carbon dioxide (CO2) becomes supercritical above a critical temperature of 31 °C and a critical pressure of 1070 psi. SCCO2 has a gas-like diffusivity, a liquid-like density and it does not suffer from surface tension and capillary forces [17]. Because of these attractive properties; SCCO2 may facilitate the removal of OH groups from the damaged films. Keeping this in mind, we have also conducted a few sets of experiments with SCCO2 as a medium of reaction and HMDS in static and pulsed modes. These results are also presented and discussed in this paper. However, the main focus of this study is to investigate the efficiency of HMDS vapor on the repair of plasma-damaged films. The present study is also intended to determine whether this curing process is complete or partial.

Section snippets

Experimental

In this work, nanoporous MSQ type films supplied by Tokyo Electron America (TEL) were used. CF4 plasma was used to etch the top 1500 Å and then one set of etched films was subjected to N2/H2 ashing while another set was subjected to Ar/O2 ashing. For convenience, these two sets, namely N2/H2 and Ar/O2 plasma ashed films, are referred to as MSQ1 and MSQ2, respectively and the undamaged sample (no plasma etching/ashing/cleaning) is referred to as MSQ3 throughout the text. The thickness of the MSQ3

Results and discussion

The FT-IR absorbance spectrum of MSQ3 film (undamaged) is shown in Fig. 1a. This spectrum reveals a strong peak at 1060 cm−1 with a shoulder at 1140 cm−1 corresponding to the Sisingle bondOsingle bondSi network and Sisingle bondOsingle bondSi cage structure vibrations, respectively [18]. The absorption band observed at 1250 and 840 cm−1 are attributed to the CH3 symmetric deformation mode in (Sisingle bondCH3)n group and CH3 rocking vibration in Sisingle bondCH3, respectively [18]. A peak at 2970 cm−1 corresponding to the CH3 asymmetric stretching vibrational

Conclusion

Curing of plasma-damaged nanoporous organosilicate films by HMDS vapor treatment has been thoroughly studied in this work. It is significant that CH3 groups have been incorporated even at a low processing temperature as low as 55 °C. However, detailed analysis of OH region of the FT-IR absorbance spectra tells us that only free water is removed and not bonded silanol groups.

The remarkable increase in the CH3 concentration without significant decrease in the concentration of bonded OH groups

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