Porous SiOCH Modifications by O2, NH3 and CH4 Plasmas Used as Ashing and Pore Sealing Steps M.Darnon1, T.Chevolleau1, N.Posseme2, T.David2, C.Licitra2, J.Torrès3, O.Joubert1 1 2
CNRS/LTM, (CEA/LETI-Minatec), 17 rue des martyrs, F38054 Grenoble cedex 09 CEA/LETI-Minatec, 17 rue des martyrs, F38054 Grenoble cedex 09 3 STMicroelectronics, Central R&D, 850 rue J. Monnet, F38926, Crolles cedex Email:
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The effect of oxidizing and reducing plasmas (O2, NH3, CH4) on porous SiOCH material modification has been investigated. We have used surface and volume characterization techniques such as X-ray Photoelectron Spectroscopy, X-Ray Reflectometry, Fourier Transform InfraRed spectroscopy and Ellipsometric Porosimetry to monitor the porous SiOCH modification induced by such plasmas used as ashing and pore sealing techniques. After plasma exposure, a densified over layer is formed and its composition is SiO2 like, SiON and SiOC carbon rich with O2, NH3 and CH4 plasma treatments, respectively. With an O2 or NH3 based plasma, this overlayer is depleted in carbon, and hydrophilic. The underlying material is hardly modified with a slight moisture uptake, which leads to k increase. With a CH4 based plasma the surface remains hydrophobic, and the underlying material is carbon depleted, but does not present moisture uptake, and thus k is hardly increased. Whatever the plasma treatments, toluene can diffuse through the surface. Nevertheless, RBS studies after TiN coating reveal that the diffusion of barrier precursors is strongly reduced. The whole results highlight that moisture uptake is worse than carbon depletion in terms of k increase, and that plasma treatment can be efficient to prevent barrier precursor diffusion even if a complete pore sealing is not achieved.
Introduction To reduce the RC delay in microelectronics circuits, low dielectric constant materials such as porous SiOCH have to be integrated for future technological nodes. Porous materials introduction leads to severe issues because species can diffuse through their pore network. In particular, ashing plasma species such as oxygen radicals can diffuse into the material and degrades its characteristics [1]. Metallic barrier precursors can also diffuse and alter the material properties. To prevent those degradations, low damage ashing treatments and pore sealing treatments are under development [2]. In this paper, we will compare three different oxidizing or reducing plasmas proposed to remove the photoresist and prevent the metallic barrier diffusion at once. We will study the film modifications of a porous SiOCH low-k material induced by O2 and NH3 plasmas in a capacitive discharge (RIE), and by a CH4 plasma in an inductively coupled discharge (ICP). To understand the mechanisms of plasma induced material degradation, modifications of the remaining film have been investigated using different surface analysis techniques such as X-Ray Photoelectron Spectroscopy (XPS), Infrared Spectroscopy (FTIR), X-Ray Reflectometry
(XRR) and mercury probe capacitance measurements. Finally, we have verified the pore sealing efficiency by Ellipsometric Porsimetry (EP) and RBS measurement on plasma treated films, coated by a CVD TiN.
Experimental The experimental work has been performed on a 200 mm etch platform. The etch experiments have been carried out in a metal etcher (DPS+ from AMAT) and an oxide etcher (eMax from AMAT). An XPS chamber is connected to the etch platform via a pumped transfer chamber allowing to perform quasi in-situ analysis. Wafers used in this study are 200 mm diameter silicon wafers coated by spin on deposition with 300 nm porous SiOCH (LKD5109 from JSR). This material has a k value of 2.2 and an interconnected microporosity of 45%. The plasma treatments investigated in this work are the following: 1) a O2 RIE plasma performed at 15°C, a pressure of 75 mTorr and a plasma power of 200 W 2) a NH3 RIE plasma performed at 15°C, a pressure of 80 mTorr and a plasma power of 350 W 3) a CH4 ICP plasma performed at 250°C, a pressure of 5 mTorr, a source power of 800 W, and no bias power.
Results and discussion
CH4 NH3 O2 pristine
Intensity (au)
RBS measurements after TiN coating of wafers show [3,4] that the damaged layer formed with NH3 and O2 plasma and that the carbon rich overlayer with CH4 plasma strongly reduce the diffusion of the TiN barrier precursor. However, EP measurement using toluene (figure 2) show that those plasma treatments do not make an efficient pore sealing, since the toluene can diffuse into the porous material. 50
% of Toluene
After plasma exposure, the chemical composition of the top surface of the dielectric material has been determined by XPS. With O2 and NH3 plasma, the top surface is SiO2 like and SiON, respectively, while with CH4 plasma, a carbon rich top layer is formed. FTIR spectra (figure 1) show a decrease of the carbon content into the dielectric material whatever the plasma investigated. The carbon depletion has been estimated by calculating the area ratio of the Si-CH3 peak at 1270 cm-1 over the SiOSi peak between 950 and 1270 cm-1. With O2 and NH3 plasma the FTIR spectra show moisture uptake and Si-OH bonds. With CH4 plasma, no moisture uptake is clearly evidenced but a new peak is observed at 800 cm-1 which is assigned to Si-H bonds. After a HF dip, FTIR spectra almost go back to the pristine material (only a slight moisture uptake is still detected) with O2 and NH3 plasma while no change of the FTIR spectrum is observed with CH4 plasma.
formed over an almost unchanged material (only slightly hydrophilic) with O2 and NH3 plasma exposure. With CH4 plasma, a carbon rich layer is formed over a highly carbon depleted material. The whole material remains hydrophobic. Based on the k value measurements and FTIR analyses, the results demonstrate that the k increase is mainly related to moisture uptake and not to carbon depletion.
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Figure 1: FTIR spectra of pSiOCH after the different plasmas exposure
The density evolution of the top surface can be monitored by XRR. XRR spectra show an increase of the critical angle with all plasma investigated, indicating an increase of the top surface density. After HF dip, the critical angle goes back to its pristine value with NH3 and O2 plasma whereas the XRR spectrum hardly changes with CH4 plasma. Thanks to XRR measurements, we can estimate that HF dip removes a material thickness of 37 nm and 62 nm with O2 and NH3 plasmas, respectively, which corresponds to the modified layer thickness. We have measured an increase of the k value of the remaining porous SiOCH film with all the plasma investigated. The k value increases from 2.2 (pristine material) up to 2.5, 2.8 and 2.3 with O2, NH3 and CH4 plasma, respectively. The whole results highlight that a damaged layer of several tens nanometers is
Figure 2: Ellipsometric Porosimetry spectra of treated pSiOCH. The solvent is toluene
Conclusions As a conclusion, an ashing and pore sealing plasma treatment has to preserve the hydrophobic nature of the low-k material to prevent k increase, and must limit the diffusion of metallic barrier precursors. This can be achieved even with strong carbon depletion and even without a complete pore sealing.
References [1] K.Maex, M.R.Baklanov, D.Shamiryan, F.Iacopi, S.H. Brongersma, Z.S. Yanovitskaya, J. Appl. Phys. 93, 8793 (2003) [2] N.Posseme, T.David, T.Chevolleau, O.Joubert, Electrochem. Solid-State Lett., 8, G112 (2005) [3] T.David, N.Posseme, T.Chevolleau, M.Darnon, O.Louveau, G.Passemard, O.Joubert, Advanced Metallization Conference 2005 (AMC 2005), 405 (2005) [4] N.Posseme, T.David, T.Chevolleau, P.Meininger, O.Louveau, M.Fayolle and O.Joubert, Presented at the Int. Dry Process Symp., Tokyo, Japan, 2004, Nov.30 Dec.1
