Modification of Nanoporous Silica Structures by Fluorocarbon Plasma Treatment
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Modification of Nanoporous Silica Structures by Fluorocarbon Plasma Treatment Woojin Cho, Ravi Saxena, Oscar Rodriguez, Ravi Achanta, Manas Ojha, Joel L. Plawsky, and William. N. Gill Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA Mikhail R. Baklanov IMEC, Leuven, Belgium ABSTRACT Polymerization occurring during fluorocarbon plasma treatment as a potential method for pore sealing was investigated. CHF3 was used as a reactant gas to expedite the rate of polymerization due to the presence of hydrogen and the low C/F ratio. The reactor pressure was varied from 30mTorr to 90mTorr to change the number of neutrals that act as the polymerizing species. The films were exposed to the plasma for times of 1min, 3min, and 5 min to observe the penetration depth of neutrals and the thickness of modified layer as a function of time. Dielectric constants were measured before and after plasma treatment. The film morphology was investigated by scanning electron microscopy before and after plasma treatment and a featureless surface morphology was observed at 90mTorr on a 56% porosity film. After plasma treatment, the average pore neck size decreases which may help reduce metal precursor penetration during metallization. INTRODUCTION Future integrated circuits require a low dielectric constant (k) material as an interlayer dielectric (ILD) [1, 2]. The incorporation of porosity is the most plausible way of reducing the dielectric constant of a material below 2. Nanoporous silica or silica xerogels are one promising alternative dielectric material as they can be made hydrophobic and the porosity and thickness can be tailored to desired values [2-4]. However, porous materials have several shortcomings such as metal precursor penetration into the pores during CVD, ALD and/or PVD processes [5, 6], poor mechanical strength [7], and low thermal conductivity [8]. The sealing of surface pores has been proposed as one way of preventing metal precursor penetration during metallization, without affecting (or even improving) the dielectric properties of the porous material. In this work, we study the formation and use of the polymer layer that forms during exposure to a fluorocarbon plasma for pore sealing. EXPERIMENTAL DETAILS Nanoporous silica films were prepared by a spin-on, binary solvent technique and the film porosity was controlled by changing the amount of glycol as the pore former [9]. The effect of plasma treatment in the nanoporous silica films was studied in a capacitively coupled plasma reactor (Plasmatherm 73, RPI). The plasma power was fixed at 247 W, the reactant gas (CHF3) flow rate was 50 sccm, and the RF was 13.56 MHz. The films were
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exposed to plasma for 1 minute at three different pressures: 30, 60 and 90 mTorr. The films were also exposed to plasma at different times under 90 mTorr. The refractive index and thickness of the films before and after plasma treatment were measured by ellipsometry. The wavelength of the light was 632.8 nm and the ang
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