Positive Charge Buildup Research Articles

Fowler–Nordheim stressing of polycrystalline Si oxide Si structures: Observation of stress induced defects in the oxide, oxide/Si interface, and in bulk silicon

Deep level transient spectroscopy and capacitance voltage measurements were used to study the Fowler–Nordheim (FN) stress induced defects in n+-polycrystalline-Si/90-Å-thick-SiO2/p-Si substrate capacitors. These capacitors were fabricated using a 0.5 μm complementary metal–oxide–silicon process flow. The capacitors were subjected to constant voltage FN stress at temperatures between 300 and 50 K. At all stress temperatures positive charge buildup was observed to take place in the gate oxide, whereas at stress temperatures ⩾150 K a band of hole traps centered at 0.55 eV above the top of the valence band and ∼0.2 eV wide was seen to be induced by the stress. However, FN stress below 150 K was observed to induce two bulk silicon defects 600 to 1000 Å from the SiO2/Si interface. One of these defects is configurationally bistable with electronic states at 0.35 and 0.30 eV below the conduction band edge Ec. The second defect gives rise to an electron trap located at Ec−0.37 eV.

Fowler-Nordheim stress degradation in gate oxide: results from gate-to-drain capacitance and charge pumping current

The buildup of positive oxide charge and interface trap charge, due to Fowler-Nordheim stress, is observed in the gate-drain overlap region of the MOSFET. Results from gate-to-drain capacitance and charge pumping current show a steady increase in positive charge near the anode interface. Interface trap generation becomes significant when injected electron fluence exceeds /spl sim/10/sup 14/ cm/sup -2/, and dominates net charge creation at higher fluence.

Image acceleration of highly charged ions on metal, semiconductor, and insulator surfaces

Very slow, highly charged ions impinging on metal surfaces are known to be accelerated by their image and to drop irremediably on the surfaces which they touch or slightly penetrate. We present experiments which demonstrate that above insulators or semiconductors, at normal incidence, the ions are backscattered at a certain distance from the surface and do not touch it. This finding is explained by the transient buildup of positive charges due to the removal of valence electrons, which overcome the acceleration of the ion by its own image. This effect is found to cancel out at grazing incidence.

Sputtering investigation of boron nitride with secondary ion and secondary neutral mass spectrometry

Sputtering of boron nitride with xenon ions was investigated using secondary ion (SIMS) and secondary neutral (SNMS) mass spectrometry. The ions generated from the ion gun were incident on the target at an angle of 50° with respect to the surface normal. The energy of ions ranged from 100 eV to 3 keV. A flood electron gun was used to neutralize the positive charge buildup on the target surface. The intensities of sputtered neutral and charged particles, including single atoms, molecules, and clusters, were measured as a function of ion energy. Positive SIMS spectra were dominated by the two boron isotopes whereas BN− and B− were the two major constituents of the negative SIMS spectra. Nitrogen could be detected only in the SNMS spectra. The intensity–energy curves of the sputtered particles were similar in shape. The knees in p-SIMS and SNMS intensity–energy curves appear at around 1 keV, which is significantly higher than the 100–300 eV energy range at which knees appear in the sputtering of medium and heavy elements by ions of argon and xenon. This difference in the position of the sputter yield knee between boron nitride and heavier targets is due to the reduced ion energy differences.

Observation of DC currents induced in MOS capacitors during NBTS aging

A current which is directly related to build-up of positive charge in thermal oxide due to negative bias temperature stress (NBTS) aging in the temperature range from 25°C to 350°C has been observed in MOS capacitors. The current was very noticeable in the MOS capacitors which had been exposed to plasma in a magnetron sputtering system or in a reactive ion etching system. Two extreme situations were observed. One was that most of the holes injected in oxide move a short distance before being captured. Another was that most of the injected holes transport through the entire oxide, arriving at the metal interface and recombining with electrons there. It was found that, during negative weak field thermal stimulation current (TSC) measurement performed on a heavily charged capacitor, the observed current in the external circuit flows in the direction opposite to the external bias field. A mechanism which is responsible for the slow-trapping related current is proposed.

Nitration of α,β-unsaturated esters. Evidence for positive charge build-up adjacent to carbonyl carbon

Reactive intermediates formed in the nitration of certain α,β-unsaturated esters with nitronium tetrafluoroborate exhibit behaviour expected of highly reactive α-carbonyl cations. Three diagnostic reaction types have been observed which indicate the presence of these destabilised cations: (i) trapping in a Ritter reaction, (ii) cyclopropane formation from propyl cations, (iii) Wagner–Meerwein migration of alkyl groups. Semi-empirical calculations of the relative gas-phase stabilities of the proposed intermediate cations are useful in rationalising the observed chemistry.

Electrical and Physical Characterization of Tetraethylorlthosilicate‐ O 3 Borophosphosilicate Glass

The reflow properties of tetraethylorthosilicate‐ (TEOS‐) borophosphosilicate glass (BPSG) under furnace and rapid thermal processing conditions that we have studied show good void‐free, subquarter micron gapfill, and planarization. Thick BPSG films on reflow exhibit positive charge buildup at the oxide/Si interface that cause a shift in the flatband voltage. The amount of phosphorous in the BPSG film, the BPSG film thickness, and the BPSG deposition temperature determine the extent of this flatband shift. We observed a linear correlation of the flatband shift and the amount of carbon migration and pileup at the oxide/Si interface during reflow. Using current‐voltage, triangular voltage sweep, and secondary ion mass spectroscopy and field threshold voltage measurements, we demonstrate that under appropriate deposition conditions the flatband shift can be brought to negligible levels.

Oxynitride gate dielectric grown in nitric oxide (NO): the effect of reoxidation on dielectric reliability of the active edge

Reoxidation of an oxynitride gate dielectric grown by NO anneal of thermal oxide has been studied. This process has demonstrated /spl sim/3-5X improvement of Q/sub BD/ of active edge intensive capacitors in comparison to thermal oxide, N/sub 2/O and NO oxynitride. This improvement is believed to be due to the reduction of local thinning of the gate dielectric at the field oxide edge which also reduces local build-up of positive charge near the gate electrode at the isolation edges.

Use of MOS structures for the investigation of low-dose-rate effects in bipolar transistors

A possible physical mechanism for bipolar transistor low-dose-rate irradiation response is discussed. This mechanism is described in terms of shallow electron traps in oxide. The experimental results on positive charge build-up at low dose-rates and small electric field in oxide are presented. The use of an MOS transistor in bipolar mode for investigation of surface peripheral recombination current in bipolar transistors and extraction of MOS structure physical parameters is described.

Sensitivity of a modified gateless FET structure to ethanol vapour

Evidence for the sensitivity to ethanol vapour of a modified n-channel FET structure, based on Si, was found. The detection is based on the increase in the drain current (at constant drain voltage) in the presence of ethanol vapour. Good ethanol sensitivity can be obtained using thin and ultrathin Si0 2 as well as a plasma nitrided thin thermal SiO 2 layer as an active dielectric. The response time is within seconds and the response range covers more than five decades for 100% ethanol vapour concentration in the working cell. The structure may operate as an alcohol sensor for about 150 measurement cycles without loss of detection speed or accuracy. The experimental results provide evidence that the rapid increase of the drain current is closely related to positive charge build-up in the structure during ethanol vapour exposure. Some concepts are proposed for the interpretation of the results obtained, including arguments used to isolate the contribution of the surface oxide charge on one hand, and the charge in the oxide and in the interface states on the other, to the total positive charge generated.

Kinetics and mechanism of addition of tertiary phosphines and phosphites to the dicarbonyl(η5-cyclopentadienyl)-(η-ethene)iron cation

Kinetic studies of the addition of a range of tertiary phosphine and phosphite nucleophiles PR3 to the cation [Fe(cp)(CO)2(η-C2H4)]+1(cp =η5-C5H5)[equation (i)] revealed the general rate law, Rate =k1[Fe][PR3]. The second-order rate constants k1 decrease markedly down the order P(C6H4OMe-2)3 > PBun3 > P(C6H4OMe-4)3 > P(C6H4Me-4)3 > P(C6H4Me-4)Ph2 > PPh3 > P(C2H4CN-2)Ph2 > P(C2H4CN-2)3 > P(C6H4Cl-4)3 > P(OBun)3. This reactivity order parallels that of decreasing electron availability at the phosphorus centre, as shown quantitatively by the good correlation between log k1 and the Tolman Σχ values. An excellent fit to the Hammett and Bronsted equations is also observed for reaction (i) with the nucleophiles P(C6H4X-4)3. The moderate Bronsted slope α of [Fe(cp)(CO)2(η-C2H4)]++ PR3→[Fe(cp)(CO)2(C2H4PR3)]+(i) 0.46 establishes the importance of phosphine basicity in determining nucleophilicity towards the ethene ligand in cation 1. These results, together with the large negative entropy of activation with PPh3(ΔS‡1=–103 J K–1 mol–1), are interpreted in terms of direct addition (k1) of the phosphorus nucleophiles to the ethene ligand in 1 and suggest a transition state in which there is build-up of positive charge on the phosphorus centre and considerable phosphorus–carbon bond formation.

Charge build-up and oxide wear-out during Fowler-Nordheim electron injection in irradiated MOS structures

The degradation phenomena and charge build-up during high-field Fowler-Nordheim stress in silicon dioxide layers of MOS structures, pre-degraded by ionizing radiation, has been studied from the point of view of dielectric strength wear-out and long-term reliability. External bias of both polarities was applied to the structures during irradiation to separate generated carriers, force the current flow, and to enhance or inhibit radiation-induced degradation. It has been found that breakdown and wear-out properties of the oxides remain unchanged after the irradiation regardless of irradiation conditions. On the other hand, while the rate of electron trap generation during the prolonged stress has not been found sensitive to irradiation, we have found that the initial voltage transients during constant-current stress may be strongly affected by the changes in the radiation-induced hole trap occupancy. The character of the initial transient may indicate both net positive or negative charge build-up, depending on the hole trap occupancy prior to the high-field degradation.

Radiation Properties of Silicon Metal-Oxide-Semiconductor Structures with Zinc-Doped Oxides

In this paper we present the results of investigation of radiation-induced effects in metal-oxide-semiconductor (MOS) structures with oxides doped with moderate amounts of zinc during the oxide growth. It is shown through secondary ion mass spectroscopy (SIMS) analysis that zinc, when present in the oxidation ambient, is effectively incorporated into the oxide matrix. Electrophysical parameters of the modified MOS structures are similar to those of the reference ones. On the other hand, radiation sensitivity of zinc-doped structures is significantly affected. While observing an enhanced radiation-induced positive charge buildup, we have found that the Si-SiO2 interface and the near-surface silicon region were significantly hardened in these structures. The observed effects strongly depend on the amount of zinc incorporated in the oxide. It is also shown that postirradiation low-temperature annealing can result in the improvement of the oxide and the Si-SiO2 interface characteristics. The main parameters characterizing the system, namely, the oxide charge, the density of interface states, and the minority carrier generation lifetime, are improved after the irradiation-then-annealing treatment compared to those of simply postmetallization-annealed MOS structures. A simple model to explain the observed radiation-induced ordering effect is also presented.

Theory of high-field electron transport and impact ionization in silicon dioxide.

A detailed theoretical study of impact-ionization-related transport phenomena in ${\mathrm{SiO}}_{2}$ thin films is presented. The Boltzmann transport equation is integrated by the Monte Carlo method using acoustic-phonon-scattering rates derived from photoinduced electron transmission experiments. It is shown that these empirical scattering rates necessitate the inclusion of impact ionization at fields F>${\mathit{F}}_{\mathrm{th}}^{\mathrm{ii}}$=7 MV/cm because phonon scattering alone can no longer stabilize the electron energy distribution below the ionization energy of 9 eV. However, even above ${\mathit{F}}_{\mathrm{th}}^{\mathrm{ii}}$, acoustic-phonon scattering is found to considerably delay the heating of electrons, leading to a wide dark space in which impact ionization cannot take place or is strongly reduced. Therefore, the electron multiplication factors m(F,${\mathit{t}}_{\mathrm{ox}}$) decrease rapidly with decreasing oxide thickness, ${\mathit{t}}_{\mathrm{ox}}$, for ${\mathit{t}}_{\mathrm{ox}}$30 nm. These predictions are shown to be consistent with results of several high-field transport experiments in silicon--silicon-dioxide device structures. The calculated electron energy distributions develop high-energy tails which extend beyond the band-gap energy at fields larger than ${\mathit{F}}_{\mathrm{th}}^{\mathrm{ii}}$, as observed by vacuum emission experiments. The calculated impact-ionization coefficients are found to be in good agreement with values derived from experiments.The hole generation factors m-1 quantitatively agree with substrate hole-current to channel-current ratios measured by the carrier separation technique in n-channel field effect transistors with gate oxide thicknesses ${\mathit{t}}_{\mathrm{ox}}$\ensuremath{\ge}25 nm. The field and thickness dependence of the measured positive charge buildup (hole trapping) near the Si/${\mathrm{SiO}}_{2}$ interface can be quantified in terms of impact ionization in the oxide film. The calculated carrier multiplication, however, cannot fully account for the substrate hole currents and the hole trapping measured in thinner oxides, ${\mathit{t}}_{\mathrm{ox}}$\ensuremath{\le}20 nm, indicating that another mechanism, likely related to hole injection from the anode, becomes the dominant source for hole currents in thin oxides. Dielectric breakdown of thin ${\mathrm{SiO}}_{2}$ films on silicon is reevaluated on the bases of all of these findings. It is proposed that time-dependent breakdown is the result of cumulative degradation of the oxide near its interfaces caused by impact ionization and by hot-electron-induced hydrogen release together. This ansatz is shown to yield a good understanding for the oxide field and thickness dependence of the interface-state generation and of the charge to breakdown. Since impact ionization is strongly suppressed in thin films, ${\mathit{t}}_{\mathrm{ox}}$25 nm, degradation and time-dependent breakdown appear to be largely caused by hydrogen release and its subsequent secondary reactions in these thin films.

Radiation tolerance of the FOXFET biasing scheme for AC-coupled Si microstrip detectors

The radiation response of FOXFETs (field oxide FETs) has been studied for proton, gamma, and neutron exposures. It is shown that the punch-through (PT) conduction mechanism is much less sensitive to radiation damage than the usual surface conduction in FOXFETs. The threshold voltage is strongly affected by the huge buildup of positive charge in the thick gate oxide. This effect can be reduced by decreasing the oxide thickness, but this modification seems to be unnecessary for detector applications. In fact, variations of a few volts of the PT voltage after irradiations in the Mrad range will result in a corresponding similar increase of the strip self-bias in detectors. Such modifications are still acceptable, and can be properly compensated by a suitable increase of the detector backside bias in order to ensure full depletion operation. It is also shown that the radiation-induced variations of the PT voltage can be tailored by the proper choice of the gate bias. Computer simulations have shown that the main radiation effects affecting the PT mechanism are the charge accumulation in the oxide and substrate-type inversion. >

Net positive-charge buildup in various MOS insulators due to high-field stressing

The buildup of net positive charge with field stressing has been observed in all thermally grown oxides, viz. dry, pyrogenic, reoxidized nitrided oxides (RNO), and reoxidized nitrided pyrogenic oxides (RNPO). The authors observed a faster rate of growth of net positive charge for dry oxides given a postmetalization anneal (PMA) in hydrogen than for those given a PMA in nitrogen; the fastest growth of net positive charge, however, was observed in pyrogenic oxides. It has been observed that in dry and pyrogenic oxides the positive-charge growth as a function of time obeys a power law with time under the stress of constant current or voltage. On the other hand, growth of positive charge in RNO and RNPO shows a two-piece linear growth of positive charge. These results suggest that positive-charge growth at high fields is related to both the hydrogen concentration and its drift in the oxide. >

Radiation damage test of position sensitive silicon detectors

A study of radiation damage of position sensitive silicon detectors (PSDs), for use as a position monitor of high energy heavy ion beams, was carried out. It is revealed that the position linearity of the standard PSD is strongly affected even by an absorbed dose around 100 Gy. The reason of the distortion of position linearity was considered to be a positive charge build-up in the SiO 2 layer covering the PSD. To overcome the problem, new types of PSDs have been developed raising impurity density in the resistive layer and removing the SiO 2 layer. A PSD with a distortion of less than 100 μm over an effective area of 13 mm × 13 mm is obtained which is usable as a heavy ion beam monitor.

Radiation hardness of MOSFETs with N/sub 2/O-nitrided gate oxides

Radiation hardness of furnace N/sub 2/O-nitrided gate oxides was investigated for both n- and p-channel MOSFETs by exposing devices in an X-ray radiation system. An enhanced degradation was observed in both control and N/sub 2/O-nitrided MOSFETs with reduction in the channel length. Compared to MOSFETs with control oxides, N/sub 2/O-nitrided MOSFETs show an enhanced radiation hardness against positive charge buildup and interface state generation. The effects of channel hot carriers on the irradiated devices with subsequent low-temperature forming gas annealing were also studied. The results show that N/sub 2/O-nitrided oxides have a greatly enhanced resistance against radiation-induced neutral electron trap generation. >

The effect of oxide charges at LOCOS isolation edges on oxide breakdown

The degradation of time dependent dielectric breakdown (TDDB) characteristics at LOCOS isolation edges has been studied using MOS capacitors with and without field oxide edges under gate electrodes. The wear-out mode was shifted toward shorter breakdown time by field oxide etching right after the conventional LOCOS process. An increase in leakage current was observed at the isolation edge, so that the current enhancement is regarded as one of the main causes of the degradation. The current was reduced by constant current stress, suggesting the neutralization of positive charges due to electron trapping. The authors attribute the current enhancement causing the degradation to the buildup of positive charges at the isolation edges. Carrier injection before TDDB tests was found to improve the degradation of the wear-out mode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the different time dependence of interface trap generation and charge trapping during hot carrier degradation in CMOS

In this paper, both I–V and charge pumping (CP) measurements are used to study the time dependence of the hot carrier degradation of n- and pMOSFET's under various stress conditions. It will be shown that for the interface trap generation, in all cases a power law relationship is obtained, while the build-up of positive and negative charge behaves logarithmically with time. The time dependence of the degradation monitored with I–V will of course be determined by the dominant degredation mechanism. In this way, the duality between the degradation of n- and pMOSFET's is confirmed. Also the relevance of these time dependencies for dynamic degradation is treated.