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Alternative technology concepts for low-cost and high-speed 2D and 3D interconnect manufacturing
Keywords: TSV, 3D Interconnect, DRIE
We report on alternative, disruptive technology concepts with potential for cost-effective and high-speed interconnect manufacturing of advanced interconnects like Through-Silicon Vias (TSVs) and of redistribution layers. Options studied are 1) plasma etching by Spatially-divided Deep Reactive Ion Etching, S-DRIE, with intermittent ALD-oxide passivation steps and 2) Laser-Induced Forward Transfer (LIFT) deposition of metals like copper. ----------------- S-DRIE. Conventional DRIE, known as the 'Bosch'-process', concerns low-pressure (~100 mTorr), near-room temperature etching with alternating half-cycles of 1) Si-etching with SF6 forming volatile SiFx, and 2) passivation with C4F8, forming a protecting polymer on the feature sidewalls and bottom. The etch half-cycles are isotropic, and proceed - if non-interrupted - mainly by non-directional F-radicals. To minimize lateral etching each etch step is interrupted by C4F8-passivation. During each etch cycle the substrate is voltage-biased which generates a directional physical ion bombardment from the plasma onto the substrate. This sputters the polymer off the feature bottom part, leaving the sidewall passivation intact and sustaining deep etching. Half-cycles last ~1-10 s with 0.1-1 um etched per cycle. The process enables deep micro-feature etching at relatively high etch rates and selectivities against a hard mask (usually SiO2). ------------------- An accelerated alternative replaces time-separated by spatially separated processing, which may lead to unprecedented etch rates and anisotropy control. Spatial separation can be accomplished by N2-gas bearing 'curtains' of heights down to ~20 um, that confine the reactive gases to individual injection zones constructed in a gas injector head. By horizontally moving the substrate back and forth under the injector head one creates the alternate exposures to the overall cycle. Optimum pressures in the injection slots are obtained by balancing the gas flows injected into and exhausted from the slots and by properly designing the distance between the slots and the gas bearing gap height; see ref. 1. -------------------- Another improvement is the replacement of CVD-based C4F8 passivation by ALD-based deposition cycles of SiO2, or others like Al2O3. Unlike the C4F8 case the ALD-based protection layer is self-limiting, chemisorptive and easier in thickness control and thus anisotropy control of the total DRIE process. Both Al2O3-layers deposited by atmospheric thermal, spatial ALD over high aspect ratio (30:1) trenches and SiO2-layers deposited in (130:1) trenches using near room-temperature O2-plasma-assisted temporal ALD with H2Si[N(C2H5)2]2-precursor dosing times down to ~20 ms, gave good conformality. This indicates that recombination of O-radicals in such trenches plays no dominant role [ref. 2]. -------------------------- LIFT is a maskless, 'solvent'-free deposition method, using pulsed lasers to deposit thin material layers with um-range resolution from a transparent carrier (ribbon) onto a close-by acceptor substrate [ref. 3]. This single-step, dry room-temperature process in air, is suitable to fabricate different types of interconnect (redistribution layers, TSV fills), without use of wet chemistry. We demonstrated the feasibility of this technique to create 2D and 3D copper-based interconnects (60x200 um TSVs). We investigated the electrical resistivity of the deposited copper as well as the geometric capabilities. The quality of thin LIFT layers can often be superior to that of other deposition techniques. Smaller features (20x100 µm TSVs) are now under investigation.
Fred Roozeboom, Professor
Eindhoven Univ. of Technology
Eindhoven, MB

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