Moisture Induced Degradation of Porous Low-k Materials
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INTRODUCTION One of the most difficult challenges during the integration of porous low-k materials in ULSI technology is related to their degradation during different technological processes such as thermal annealing, plasma etch and resist strip, cleaning and chemical mechanical polishing (CMP) etc. Removal of carbon containing hydrophobic groups and further adsorption of water and other polar molecules significantly increases the effective dielectric constant of low-k films [1]. Degree and depth of such damages increase with pore size and their interconnectivity. Therefore, ultra low-k materials, which have high porosity and relatively large pore size, suffer much more than microporous materials [2, 3]. The depth and profile of the carbon depletion are normally evaluated using various analytical techniques such as Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ions Mass spectroscopy (TOF-SIMS), Energy Filtered Transmission Electron Microscope (EFTEM) etc. [1]. It is assumed that the carbon depletion is directly responsible for the hydrophilicity and moisture adsorption, which finally defines effective dielectric constant of the film. The Si-O-Si backbone of hybrid low-k materials renders the matrix mechanically fragile and susceptible to stress-corrosion cracking in moist environments [4]. Increasing hydrophilicity caused by plasma processing makes this problem even more challenging. In this paper, we present the basic ideas and fundamentals of recently developed method for evaluation of hydrophilic properties of low-k materials [5]. Typical experimental results related
to evaluation of thermal and plasma damage are also presented. Possibility of application of this method (water EP) for quantitative evaluation of thermal and plasma damage has already been demonstrated. Comparative evaluation showed that degree of hydrophilisation and depth of thermal and plasma damage have qualitative agreement with the carbon depletion determined by TOF SIMS and HF dip test [5]. Most of data reported in this paper were obtained with Spin-on-Glass (SOG) low-k films damaged by thermal annealing and etch/strip plasma. EXPERIMENTAL DETAILS The fundamental ideas of “water EP” are similar to Ellipsometric Porosimetry (EP) [6, 7]. EP set-up with a water source allows changing of water vapor pressure from 10-3 to saturated ones (P0 ≈ 20 torr at room temperature). The volume concentration of the adsorbed water (Cwat) is calculated from change of refractive indices occurring during the water adsorption by using the Lorenz-Lorentz equation [6]:
Cwat (%) = 100 ⋅
2 ⎡⎛ n 2 − 1 ⎞ ⎛ nmin Vwater − 1 ⎞⎤ ⎟⎟⎥ = 100 ⋅ ⎢⎜ 2 ⎟ − ⎜⎜ 2 V film n n 2 2 + + ⎝ ⎠ min ⎠⎦ ⎝ ⎣
2 ⎛ nwat −1 ⎞ ⎟⎟ ⎜⎜ 2 ⎝ nwat + 2 ⎠
(1)
where Vwater and Vfilm are volumes of adsorbed water and low-k film, respectively. n is refractive index of the film with adsorbed water, nmin and nwat are refractive indices of the film with empty pores and bulk liquid water, respectively. Amount of the adsorbed water in prist
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