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Title:Laser-enhanced plasma etching of semiconductor materials
Author(s):Peck, Jason A
Director of Research:Ruzic, David N.
Doctoral Committee Chair(s):Ruzic, David N.
Doctoral Committee Member(s):Eden, James G.; Brooks, Caleb S.; Jurczyk, Brian E.
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:In this work, laser exposure was coupled with plasma etch processes for local etch rate enhancement, and under some conditions, etch activation. Materials were tested which are most-frequently used in semiconductor devices – namely Si, SiO2, and Cu. A 100 Hz, 7 ns pulse width Q-switched Nd:YAG laser was applied at its 1064, 532, and 266 nm modes. Using the 532 nm line on Si (40 mJ/cm²/pulse) with a radiofrequency inductively-coupled plasma (RF-ICP) source placed upstream, laser etch enhancement effect is 4 Å/s in 50:4 sccm Ar/SF6 and 3 Å/s etch enhancement at 50:8:2 sccm Ar/C4F8/O2. With no O2 flow in a 50:8 sccm Ar/C4F8 chemistry in an RF capacitively-coupled plasma (RF-CCP) source with a measured self-bias of -140 V, etch activation occurs at 0.62±0.07 W/cm² (6.2±0.7 mJ/cm²), with etch rates linearly increasing with laser intensity. The 266 nm line sees etch activation at roughly the same intensity, though etch rate scaling with laser intensity is roughly 6 times higher than at 532, corresponding to the drastically-larger absorption depth of 266 nm in Si. No etch enhancement occurs in either chemistry for SiO2 due to its transparency across the UV-VIS-NIR spectrum down to 200 nm. CFx polymer thinning occurs on both Si and SiO2 at 266 nm but only on Si at 532 nm, indicating a thermally-driven desorption mechanism which relies on heating the material beneath. Continuous wave (CW) laser sources of 405, 455, and 520 nm fail to stimulate etch enhancement even up to intensities of 200 W/cm², demonstrating the necessity of rapid heating of the Q-switched Nd:YAG source (~10s of MW/cm² over 7 ns) to temporarily but drastically increase wafer surface temperature. COMSOL simulations showed that a Si surface over the duration of a 532 nm laser pulse would increase temperature by 2.7°C per mJ/cm² – a reliably linear rate even at high intensity. Testing of highly-doped Si wafers revealed a substantial increase in etch enhancement – 1e19 and 1e21 cm⁻³ P-doped wafers show 1.7× and 3.7× higher etch rates over intrinsic Si, respectively. The increased absorption coefficient in these doped wafers confirmed that the etch enhancement mechanism at 532 nm is due to desorption of etch products through thermal heating, rather than through photolytic bond breaking. Pulse rate variation with fixed pulse energy at low pressure showed that material removal per pulse saturating at 0.037±0.003 Å/pulse when the time between laser pulses exceeds the predicted F monolayer formation time in SF6 etch chemistry. This indicates that the instantaneous thermal heating of the pulsed laser promotes removal of involatile etch products on the saturated surface. This effect was further verified in glancing-angle XPS characterization of surface F content with increasing laser intensity. A similar saturation effect occurs in a polymer-rich C4F8/O2 etch recipe, with peak removal rates depending on pressure. However, the presence of fluorocarbon radicals necessitates a minimum pulse rate to actively combat polymer growth, resulting in an intermediate optimized pulse rate for material removal per pulse: 0.031 Å/pulse at 100 Hz for 1.0 mTorr, 0.021 Å/pulse at 50 Hz for 0.2 mTorr, and 0.015 Å/pulse at 25 Hz for 0.1 mTorr (all using the same laser intensity of 40 mJ/cm²/pulse). High resolution SIMS depth profiling and XPS surface analysis showed that laser stimulation as a material removal mechanism drastically mitigates the penetration of residuals/contaminants into the etched wafer compared to ion bombardment, with the atomic fidelity of the Si surface being preserved. Damaged layers for 40 mJ/cm² laser exposure stayed below 1.5 nm thickness, while ion bombardment under -140 VDC self-bias causes noticeable mixing of etch residue into the Si and a damaged layer of >4.0 nm. Finally, etch tests of 100 nm full-pitch, 100 nm deep trenches showed the ability to tailor etch profile based on wafer orientation. Polarization parallel to the trench line enhances etching at the top of the features, while perpendicular to the trench line increases trench bottom etch rate.
Issue Date:2017-06-05
Rights Information:Copyright 2017 Jason A. Peck
Date Available in IDEALS:2019-08-23
Date Deposited:2017-08

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