This work focuses on the impact of oxidizing and reducing ash chemistries on the modifications of two porous SiOCH films with varied porosities (8% [low porosity (lp)-SiOCH] and 45% [high porosity (hp)-SiOCH]). The ash processes have been performed on SiOCH blanket wafers in either reactive ion etching (RIE) or downstream (DS) reactors. The modifications of the remaining film after plasma exposures have been investigated using different analysis techniques such as x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy (FTIR), x-ray reflectometry, mercury probe capacitance measurement (C-V), and spectroscopic ellipsometry (SE). FTIR analyses show that the lp-SiOCH film is not significantly altered by any of the ash processes investigated (DS-H2∕He, RIE-O2, and RIE-NH3), except by downstream oxidizing plasmas (DS-O2 or DS-N2∕O2) which induce some carbon depletion and moisture uptake, resulting in a slight increase of the k value. The porosity amplifies the sensitivity of the material to plasma treatments. Indeed, hp-SiOCH is fully modified (moisture uptake and carbon depletion) under oxidizing downstream plasma exposures (DS-O2 and DS-N2∕O2), while it is partially altered with the formation of a denser and modified layer (40–60nm thick), which is carbon depleted, hydrophilic, and composed of SiOxNyHz with RIE-NH3 and DS-N2∕H2 plasmas and SiOxHy with RIE-O2 plasma. In all the cases, the k value increase is mainly attributed to the moisture uptake rather than methyl group consumption. hp-SiOCH material is not altered using reducing DS chemistries (H2∕He and H2∕Ar). The porous SiOCH film degradation is presented and discussed with respect to chemistry, plasma parameters, and plasma mode in terms of film modification mechanism.
For the next technological generations of integrated circuits, the traditional challenges faced by etch plasmas (profile control, selectivity, critical dimensions, uniformity, defects, ...) become more and more difficult, intensified by the use of new materials, the limitations of lithography, and the recent introduction of new device structures and integration schemes. Particularly in the field of the interconnect fabrication, where dual-damascene patterning is performed by etching trenches and vias in porous low-k dielectrics, the main challenges are in controlling the profile of the etched structures, minimizing plasma-induced damage, and controlling the impact of various types of etch stops and hard mask materials. Metallic hard masks can help thanks to their high selectivity toward low-k materials, and by avoiding low-k exposure to potentially degrading ashing plasmas. In this paper, we will present some key issues related to the patterning of narrow porous SiOCH trenches with a metallic (TiN) hard mask. Narrow trenches (down to 40 nm width) can be opened into TiN with a critical dimensions bias (around 10 nm) attributed to carbon and silicon containing deposits on the photoresist and TiN sidewalls during the etching. Porous SiOCH etching using a TiN hard mask instead of the conventional SiO 2 hard mask may lead to severe profile distortions, attributed to TiF x compounds which settle on the trenches sidewalls. A chuck temperature of 60°C and fluorine-rich plasmas are required to minimize those distortions. An etching process leading to almost straight porous SiOCH profiles presenting a slight bow has been developed. However a wiggling phenomenon has been evidenced for the etching of narrow and deep trenches. This phenomenon is attributed to the highly compressive residual stress in the TiN hard mask, which is released when the dielectric is not mechanically strong enough to withstand it.
International audiencePorous SiCOH materials integration for integrated circuits faces serious challenges such as roughening during the etch process. In this study, atomic force microscopy is used to investigate the kinetics of SiCOH materials roughening when they are etched in fluorocarbon plasmas. We show that the root mean square roughness and the correlation length linearly increase with the etched depth, after an initiation period. We propose that: (1) during the first few seconds of the etch process, the surface of porous SiCOH materials gets denser. (2) Cracks are formed, leading to the formation of deep and narrow pits. (3) Plasma radicals diffuse through those pits and the pore network and modify the porous material at the bottom of the pits. (4) The difference in material density and composition between the surface and the bottom of the pits leads to a difference in etch rate and an amplification of the roughness. In addition to this intrinsic roughening mechanism, the presence of a metallic mask (titanium nitride) can lead to an extrinsic roughening mechanism, such as micromasking caused by metallic particles originating form the titanium nitride mask
Skeletal silica characterization in porous-silica low-dielectric-constant films by infrared spectroscopic ellipsometry
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This work focuses on the impact of oxidizing (O2) and reducing plasma ashing chemistries (NH3, CH4) on the modifications of dielectric materials in a porous or an hybrid state (SiOCH matrix+porogen). The plasma ashing processes have been performed on blanket wafers using O2, NH3, and CH4 based plasmas. The modifications of the remaining film after plasma exposures have been investigated using different analysis techniques such as x-ray photoelectron spectroscopy, infrared spectroscopy, x-ray reflectometry, and porosimetric ellipsometry. For the porous material the authors have shown that NH3 and O2 plasmas induce carbon depletion and moisture uptake while the CH4 plasma only leads to important carbon depletion without moisture uptake and to the formation of a thin carbon layer on the surface. For the hybrid material, no significant material modification is evidenced with the O2 plasma while an important methyl depletion and porogen degradation are observed with reducing chemistries such as CH4 and NH3 plasmas. The impact of the porogen on the film modification and the value of the dielectric constant will be presented and discussed.
The etching of sub-100-nm porous dielectric trenches has been investigated using an organic mask. The etching process that is performed in an oxide etcher is composed of three steps: a thin dielectric antireflective coating (DARC) layer (silicon containing layer) is etched in the first step, the organic mask [carbon-based layer (CL)] is opened in the second step, and the dielectric layer is etched in the last step. The DARC layer is open in a fluorocarbon-based plasma (CF4∕Ar∕CH2F2) and the main critical dimension issue is the critical dimension control of the trench, which can be adjusted by controlling the amount of polymer generated by the etching chemistry (% of CH2F2). The CL is etched using NH3 based plasmas, leading to straight trench profiles. For dielectric patterning, the etch process results from a delicate trade-off between passivation layer thickness and mask faceting. This is driven by the polymerizing rate of the plasma (% of CH2F2) which controls the trench width. Using an optimized etching process (CF4∕Ar∕2%CH2F2), p-SiOCH trenches can be patterned with straight etch profiles down to 75nm trench width. In this article, the authors have also compared the organic mask and TiN metal hard mask strategies in terms of patterning performances (profile control, porous SiOCH modification, and reactor wall cleaning processes).
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