A molecular dynamics model is used to understand the layer-by-layer etching of Si and SiO2 using fluorocarbon and Ar+ ions. In these two-step etch processes, a nanometer-scale fluorocarbon passivation layer is grown on the material’s surface using low energy CFx+ ions or radicals. The top layers of the material are then reactive ion etched by Ar+ ions utilizing the fluorocarbon already present on the material surface. By repeating these two steps, Si or SiO2 can be etched with nanometer-scale precision and the etch rate is considerably faster than what traditional atomic layer etching techniques provide. The modeling results show that fluorocarbon passivation films can be grown in a self-limiting manner on both Si and SiO2 using low energy CF2+ and CF3+ ions. The fluorocarbon passivation layer is a few angstroms thick, and its thickness increases with the fluorocarbon ion’s energy. Increasing the ion energy, however, amorphizes the top atomic layers of the material. In addition, the fluorocarbon film becomes F rich with increasing ion energy. Simulations of fluorocarbon passivated SiO2 surface show that Ar+ ions with energy below 50eV etch Si (within SiO2) in a self-limiting manner. Si etching stops once F in the fluorocarbon passivation layer is exhausted or is pushed too deep into the substrate. Oxygen within SiO2 is more easily sputtered from the material surface than Si, and the top layers of SiO2 are expected to become O deficient during Ar+ ion bombardment. Ar+ ion etching of fluorocarbon passivated Si also appears to be self-limiting below 30eV ion energy, and etching stops once F on the material surface is either consumed or becomes inaccessible.
Six unsaturated fluorocarbon (UFC) gases as well as a fluorinated ether were examined for dielectric etch and global warming emissions performance and compared to three perfluorocompound (PFC) gases. All of the gases were capable of etch performance comparable to that of a typical C3F8 process, while exhibiting superior global warming emissions performance compared to the PFCs. A low-flow hexafluoro-2-butyne process was found to have a significant emissions benefit, showing a normalized emissions reduction of 88.2% compared to the C3F8 process. Two other C4F6 isomers (hexafluoro-1,3-butadiene and hexafluorocyclobutene) also exhibited reductions greater than 80%, while hexafluoropropene and octafluorocyclopentene exhibited emissions reductions greater than 70% compared to the typical C3F8 process. For the C4F6 isomers, a large portion of the emissions were a result of CHF3 formation with photoresist as the sole source of the hydrogen. An extended 4 min etch with hexafluoro-1,3-butadiene resulted in a deep via with an aspect ratio of 5:1, very high selectivity to photoresist, and no evidence of etch stopping. © 2002 The Electrochemical Society. All rights reserved.
IntroductionA high performance 0.20pm logic technology has been developed with six levels of planarized copper interconnects. 0.15pm transistors (Lg,,,=0.15+0.04pm) are optimized for 1.8V operation to provide high performance with low power-delay products and excellent reliability. Copper has been integrated into the back-end to provide low resistance interconnects. Critical layer pitches for the technology are summarized in Table 1 and enable fabrication of 7.6pm2 6T SRAM cells.Isolation and Transistors CMP planarized shallow trenches with good electrical isolation down to n+/p+ spacings of 0.5pm were fabricated (Fig. 1). Dual gate 0.15pm transistors with 35A physical gate oxides (accumulation t,,=39A measured at Vg=+l .SV) were formed using super steep retrograde channels, shallow extensions and halos, relatively deep source/drain regions and 1 OOnm nitride spacers. CoSi, was selectively formed on the polysilicon gates and source/drain regions with a nominal sheet resistance of 9Wsq. Rapid thermal processing was utilized as much as possible throughout the flow to minimize transient enhanced dopant diffusion.Fig. 2 shows a typical SEM cross-section of a NMOS transistor with a gate length of 0.15pm. Well delineated shallow S/D extensions and the deeper S/D junctions are clearly observed. The saturation drive currents for nominal gate length NMOS and PMOS devices are shown in Fig. 3 . The nominal drive currents are 630pNpm for NMOS and 230pA/ym for PMOS at 1.8V. The off-state leakage currents of these devices are well below the worst case leakage specification of 2nA/pm. The drain induced barrier lowering (DIBL) measured on NMOS and PMOS devices is plotted as a function of Leff in Fig. 4. Good short channel characteristics are maintained down to effective channel lengths of O.1ym. The Vt roll-off for N and P devices in the linear and saturation regions are shown in Fig. 5. The Vt's are 0.44V and -0.46V for Nch and Pch respectively, at a gate length of 0.15pm and the associated subthreshold slopes are less than 90mv/dec. The use of nitrided gate oxides was investigated due to their superior hot carrier reliability. Fig. 6 compares the degradation under hot carrier stress of nitrided oxides to thermal oxides and highlights the improved reliability of NO-annealed oxides. Peak Gms comparable to those from thermal oxides were obtained (Fig. 7). A further advantage afforded by nitrided gate dielectrics is its superior boron blocking properties, Increasing the poly silicon doping in the P+ gate to reduce poly depletion resulted in only a 88mV Vt shift in nitrided oxides (Fig. 8) compared to a 300mV Vt shift in thermal oxides. A significant reduction in the inversion to, is achieved with the higher gate doping, resulting in improved device characteristics. NMOS transistor design focused on minimizing defect enhanced dopant re-distribution such as TED. To this end, the effect of different source/drain implant energies on NMOS transistor performance is shown in Fig. 9. The lower energy implant results in a significantl...
The goal of the work presented in this article was to provide a preliminary screening for a novel fluorinated compound, oxalyl fluoride, C 2 O 2 F 2 (F-͑CϭO)-(CϭO͒-F), as a potential replacement for perfluorocompounds in dielectric etch applications. Both process and emissions data were collected and the results were compared to those provided by a process utilizing a standard perfluorinated etch chemistry (C 2 F 6 ). In this evaluation, oxalyl fluoride produced very low quantities of global warming compounds under the conditions in which it was tested, as compared to the C 2 F 6 process. A preliminary evaluation of the compound's process performance was also carried out. Patterned tetraethoxysilane-deposited silicon oxide masked with deep UV photoresist having 0.6, 0.45, and 0.35 m via hole features was used as the test vehicle. Although C 2 O 2 F 2 was capable of etching silicon dioxide, low oxide etch rate and poor selectivity to the mask layer were observed. Finally, in addition to the experimental work performed, a set of ab initio quantum chemical calculations was undertaken to obtain enthalpies of dissociation for each of the bonds in the oxalyl fluoride molecule in order to better understand its dissociation pathways in plasma environments.
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