B. Balland

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

Abstract This paper reviews the present knowledge on tantalum pentoxide (Ta 2 O 5 ) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta 2 O 5 in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta 2 O 5 and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta 2 O 5 has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.

Fabrication and characterization of Si-MOSFET's with PECVD amorphous Ta 2 O 5 gate insulator

Silicon MOS transistors having amorphous Ta/sub 2/O/sub 5/ insulator gates have been fabricated. The Ta/sub 2/O/sub 5/ films were deposited using a low pressure (a few mtorr) plasma-enhanced CVD process in a microwave (2.45 GHz) excited electron cyclotron resonance reactor. The source gas was TaF/sub 5/. Electrical characteristics of p-channel Al gate transistors are presented.

Use of carbon-free Ta2O5 thin-films as a gate insulator

Abstract A novel electron cyclotron resonance plasma-enhanced chemical vapor deposition process using an alternative carbon-free source, namely TaF 5 , is proposed to obtain high quality amorphous Ta 2 O 5 films. The excellent physical and electrical properties suggest that this material is clearly compatible with the requirements of high density CMOS operation, as demonstrated by the fabrication of p-channel MOS transistors with a Ta 2 O 5 gate insulator.

Nitridation of SiO2 thin films at low ammonia pressures: Composition and electrical properties

Abstract Thin thermal SiO 2 films on crystalline silicon substrates were nitrided at low ammonia pressures (10 −6 mbar ⩽ P NH 3 ⩽ 10 −1 mbar) for times varying from 1 to 15h held at short intervals (5–30 min) by means of two techniques: (i) surface nitridation has been achieved by thermal activation at high temperature (HT), in the range 800–1200 °C; (ii) a new process at low temperature (LT) was carried out at T ≈ 30 °C, under electron beam irradiation. The nitridation reaction rate, the nitrogen distribution in the film and the chemical composition were found to be a function of pressure, temperature and time for the HT process, and a function of electron flux and energy, pressure and time for the LT process. The electrical properties of nitrided films were compared with those of thin SiO 2 films. Conduction, electron trapping, fixed charge, interface trapped charge densities and the breakdown behaviour of nitrided oxide films depend on the amount of nitrogen incorporated into the bulk of the films and/or at the SiO 2 Si interface. We have shown that metal-insulator-semiconductor structures with nitrided oxide as the gate insulator exhibit two attractive electrical properties: a sufficiently low density of interfacial fast states can be achieved by optimization of nitridation parameters and a high density of shallow and/or deep traps exists; however, there is a balance between the trapping of injected electrons and detrapping by tunnel emission (saturation of the flat band voltage shift under bias stress).

Spatial distribution of surface states in MOS transistors

Abstract In this paper, we show that the interface state density in the middle of the channel obtained by charge pumping measurements on small size MOS transistors are always over-estimated, because of the actual non uniform spatial distribution along the Si-SiO2 interface. It appears to be necessary to take into account the local densities of surface states before degradation, when the charge pumping technique is used to analyse the rate of creation of new interface states in MOS transistors subjected to aging stresses. A new calculation method has been proposed which allowd the determination of surface state density in the vicinity of the drain after channel hot carrier injection.

Three-level charge pumping study of radiation-induced defects at SiSiO2 interface in submicrometer MOS transistors

Abstract The charge pumping response of interface traps and near-interfacial oxide traps (border traps) induced by Co60 gamma rays in submicrometer (0.5 μm channel length) metal-oxide-semiconductor transistors has been studied. Using an improved three-level charge pumping technique, the energy distribution of interface trap parameters (emission times, capture cross-sections and interface state density) has been determined after irradiation in both the upper and lower parts of the silicon band gap on n-channel devices. The influence of border traps on three-level charge pumping measurements is demonstrated for the first time. Good agreement has been found between standard charge pumping and three-level charge pumping characteristics in terms of ‘breakpoint frequency’ at which the charge recombined per cycle deviates from the fast interface state response. The distance of border traps from the interface has been estimated to be ∼ 15–20 A from a trap-to-trap tunneling model. In addition, a new technique is presented based on three-level charge pumping measurements to determine a border trap distribution in the silicon band gap.

Theoretical and experimental study of the conduction mechanism in Al/Ta2O5/SiO2/Si and Al/Ta2O5/Si3N4/Si structures

Abstract The conduction mechanisms in Al/Ta 2 O 5 /SiO 2 /n-Si and Al/Ta 2 O 5 /Si 3 N 4 /n-Si capacitors were studied from both the experimental and theoretical points of view. Amorphous Ta 2 O 5 thin films were deposited by plasma enhanced chemical vapour deposition from TaF 5 on thermally grown SiO 2 and low pressure chemical vapour deposited Si 3 N 4 . To identify the conduction processes, the experimental current–voltage traces were compared to theoretical curves calculated in steady-state regime by using the current continuity equation and the Kirchhoff voltage law. In Ta 2 O 5 /SiO 2 capacitors, an electronic hopping conduction process and a field emission were identified in SiO 2 , associated with a Poole–Frenkel effect in Ta 2 O 5 . In Ta 2 O 5 /Si 3 N 4 structures, an electronic hopping conduction process and a Fowler–Nordheim tunneling were observed in Si 3 N 4 with an electronic conduction and a Poole–Frenkel effect in Ta 2 O 5 . The electric field is localised in the nitride layer, and the leakage current densities of the Ta 2 O 5 /Si 3 N 4 capacitors are reduced by the increase of the Si 3 N 4 thickness.

Three-level charge pumping on submicronic MOS transistors

Abstract A modified three-level charge pumping technique on submicronic MOS transistors is used to determine the energetic distribution of capture cross sections of electron Si/SiO 2 interface states and their density on an energy scale in the silicon bandgap including the midgap. The results are compared with values obtained by standard charge pumping technique.

Dynamic stressing of thin tunnel oxides : a way to emulate a single EEPROM cell programming function

Abstract An experimental set-up was implemented by which metal-oxide-semiconductor (MOS) capacitors are subjected to bipolar high voltage (up to 20 V) pulses similar to those used in programming electrically erasable programmable read-only memory (EEPROM) devices. Thin (9 nm) tunnel SiO2 oxides MOS capacitors were used. During stress, by means of capacitive coupling, the capacitors gate node was kept floating so its potential was equivalent to that of the isolated floating gate of a memory cell. The written and erased operations of memory cells which are based on Fowler–Nordheim (FN) tunneling injection mechanisms, were then reproduced on simple MOS capacitors. Via a high input impedance electronic circuit, the floating gate potential was monitoring. A model based on a simple equivalent electrical circuit was used to simulate the transient regime of the FN current and the resulting floating gate charge and potential during dynamic stressing. It was shown that the floating gate accumulated charge is proportional to the maximum control gate voltage but is independent of the control gate pulse rise time.

Analysis of low energy boron implants in silicon through SiO2 films: implantation damage and anomalous diffusion

Abstract The experimental investigation reported in this paper focuses on the effect of induced implantation damage on the boron diffusion process. Boron is implanted at various fluences and energies in Cz-(100) silicon through different oxide layer thicknesses. Rapid thermal Annealing (RTA) is used to activate shallow p+ layers (0·1–0·15 μm) following boron implantations Concentrations versus depth boron profiles are measured using a secondary ion mass spectrometry (SIMS) analyser. An enhanced boron diffusion is detected in the tail region when the oxide thickness decreases. If the concentration peak is located at the oxide/silicon interface or in the substrate, further implantation damage is generated. This observed enhanced boron diffusion is thus attributed to the implantation-induced damage. The point defects, which act as a driving force for this enhanced boron diffusion at different annealing stages, are detected by the deep level transient spectroscopy (DLTS) technique. In particular, the effect of knocked-on oxygen during the implantation step on the generation of deep centres and defects is investigated. Finally, all the results reveal that DLTS coupled to SIMS analysis provides an efficient method with which to identify the origin and the nature of implanted and RTA related defects.