Plasma vacuum ultraviolet emission in an electron cyclotron resonance etcher

Abstract

This work investigates the vacuum ultraviolet ͑VUV͒ emission from various feed gases producing plasmas in an electron cyclotron resonance etcher. Absolute measurements of plasma VUV emission at typical pressures for processing between 0.5 and 5 mTorr, and microwave powers between 700 and 1300 W, show levels of irradiance at the wafer position of the order of tenths of mW/cm 2 and integrated photon fluxes in the 10 14 photons/cm 2 s range. The reported level of VUV emission is sufficient to induce radiation damage in typical metal-oxide-semiconductor devices in the form of flatband voltage shift and inversion of lightly doped substrates. © 1999 American Institute of Physics. ͓S0003-6951͑99͒04718-X͔ During the last decade, the problem of gate oxide damage from plasma processing of metal-oxide-semiconductor ͑MOS͒ devices has been widely studied, because it is increasingly perceived as a threat for the production of advanced integrated circuits. As gate oxide thicknesses approach 20 Å for advanced MOS processes, sources of damage considered minor in the past now hold the potential to degrade device reliability and performance, and the relative impact of different damage mechanisms may also change.Although charging of floating gates is considered to be the most important MOS damage mechanism, gate oxides may also be damaged during plasma processing by x-ray, vacuum ultraviolet ͑VUV͒, and ultraviolet irradiation. 1-3High-energy photons ͑hϾ9 eV͒ can be generated from recombination and relaxation processes in the plasma. Depending on the energy of the incident photons, two cases that may introduce damage can occur: ͑1͒ photons with energies higher than the band gap of SiO 2 ͑ϳ9 eV͒ generate electronhole pairs in the oxide, 4 and ͑2͒ photons with energies lower than the SiO 2 energy gap but greater than 4.2 eV ͑the height of the minimum energy barrier between the Si substrate valence band and the oxide conduction band͒ cause electron injection from the silicon surface into the oxide through the photoelectric effect. In case ͑1͒, it has been established that electron-hole-pair generation increases the bulk and interface trapped-charge density, which will affect device reliability accordingly. 5,6 In case ͑2͒, there is a controversy: while some studies show a similar phenomenon to the one proposed, 7,8 others report that photocarrier injection from the substrate is beneficial, proposing that it anneals the positive interface charge 9 through a recombination process. Consequently, it is apparent that characterizing the plasma VUV irradiation impinging on the wafer surface during processing both qualitatively and quantitatively is essential for understanding radiation damage in MOS devices. This is the key goal of this work.The electron cyclotron resonance ͑ECR͒ plasma etching system employed in this study incorporates a 1.5 kW microwave plasma source and a pair of magnets arranged in a vertical magnetic-mirror configuration. 10 The wafer stage is located 19 cm below the resonance region, and is provided with a radio frequen...

Surface potential measurements of vacuum ultraviolet irradiated Al/sub 2/O/sub 3/, Si/sub 3/N/sub 4/, and SiO/sub 2/

“…Charge separation causes the dielectric to become polarized, which also contributes to the surface potential of the dielectric. To separate the damage effects, plasma-generated VUV radiation from charged-particle bombardment, we used monochromatic synchrotron radiation in the same photon energy range that is typically found in most processing plasmas [1]- [3]. The synchrotron source at the University of Wisconsin, Madison was used as a source of VUV photons to expose unpatterned dielectric-coated wafers.…”

In situ electrical characterization of dielectric thin films directly exposed to plasma vacuum-ultraviolet radiation

“…In contrast, argon plasma has most of its VUV emission at higher energies ͑ϳ12 eV͒. 5 The lower energy photons emitted by the oxygen plasma will penetrate deeper in the SiO 2 layer, 15 inducing the generation of electron-hole pairs throughout the oxide, while the higher energy photons emitted by the argon plasma will penetrate only a few hundreds of angstroms and will generate electron-hole pairs in this outermost layer only. Therefore, it is expected that the conductivity of oxide layers of more than a few hundreds of angstroms thick will be higher under oxygen plasma exposure than under argon plasma exposure.…”

“…This model is defined by the following expression: where is the ionizing radiation flux with energies higher than the energy bandgap of SiO 2 ͑approximately 9 eV͒, E ox is the electric field across the oxide layer (E ox ϭV ox /t ox ), t ox is the oxide thickness, and C is a fitting parameter. The radiation flux was measured previously to be 3.6 ϫ10 14 photons/cm 2 s for argon and 5.1ϫ10 13 photons/cm 2 s for oxygen, 5 in the ECR etcher, at 2 mTorr neutral pressure and 1000 W microwave power. For the oxide thicknesses used in this work, the high electric field, in connection with the density of impurities and imperfections in the oxide layer, results in an exponential trend of the current density with the applied electric field.…”

“…However, these measurements were not performed in situ and, in addition, only the effects of plasma VUV radiation on the electrical characteristics of SiO 2 for photons between 15 and 70 eV were determined, while most of the plasma VUV emission is between 9 and 15 eV. 5 In addition, any temporary effects that could result in in situ changes in the electrical characteristics of the dielectric layers were not measured.…”

“…1 During plasma processing, the electrical characteristics ͑e.g., conductivity͒ of dielectrics that are directly exposed to the plasma are influenced by plasma-emitted vacuum-ultraviolet ͑VUV͒ radiation. [2][3][4] It was determined that most high-density plasmas emit radiation in the VUV energy band between 4 and 30 eV, with most of the radiation above 9 eV, 5 the latter of which is approximately the energy band gap of SiO 2 . This radiation can be absorbed in exposed dielectric layers, and it results in the generation of electron-hole pairs.…”