N. S. Saks

Radiation Effects in MOS Capacitors with Very Thin Oxides at 80°K

Radiation induced flatband voltage shifts are measured at 80°K in MOS capacitors with oxides 6.0-50 nm thick. Previous studies have found that for relatively thick oxides (greater than 20 nm) the flatband voltage changes with radiation dose as the square of the oxide thickness suggesting that the holes created by the ionizing radiation in the oxide are uniformly created and trapped. For the thinner oxides examined in the present work, significantly smaller shifts than predicted by the oxide thickness squared dependence were observed indicating that many of the generated holes are escaping the thin oxide. Physical mechanisms to explain this effect, of which recombination of trapped holes by carrier tunneling appears the most important, are discussed.

Lateral distribution of hot-carrier-induced interface traps in MOSFETs

The spatial profiles of hot-carrier-induced interface traps in MOSFETs with abrupt arsenic junctions and oxide thickness of 10-38 nm are determined using charge pumping both in the conventional manner and with a modified constant-field approach. For the thinnest oxides the damage is highly localized in a very sharp peak that is located inside the drain at the point of maximum lateral electric field. In thicker oxides, the damage peak is broader and is shifted toward the edge of the drain junction. Two-dimensional device simulations using the measured profiles are in qualitative agreement with measured I-V characteristics after degradation. However, the magnitude of the predicted degradation is underestimated, suggesting that significant electron trapping occurs also. >

Interface trap formation via the two-stage H/sup +/ process

The time dependence and oxide-field dependence of interface trap (N/sub it/) formation in MOSFETs have been studied following pulsed ionizing radiation. Results are compared with the so-called two-stage model for N/sub it/ formation involving slow drift of radiation-induced H/sup +/ ions in the SiO/sub 2/. Detailed data on the gate-oxide-field dependence during each individual stage are presented and discussed. A model is developed for the production of H/sup +/ throughout the oxide. Calculations based on this model correctly predict the complete time-dependent N/sub it/ formation curves. It is also shown that N/sub it/ formation is at a maximum near zero first-stage gate bias. This unexpected behavior apparently arises from the oxide-field dependence of the H/sup +/ production during the first stage. >

Time dependence of radiation-induced interface trap formation in metal-oxide-semiconductor devices as a function of oxide thickness and applied field

This work is a study of the formation mechanisms of interface traps (Nit) in metal‐oxide‐semiconductor devices. The time‐dependence of the Nit formation has been measured as a function of oxide thickness following a short radiation pulse. The Nit formation time is found to increase as t2.6ox when the gate bias is negative during irradiation and positive afterward. This result is in excellent agreement with predictions of a hydrogen transport model where drift of hydrogen ions (H+) is the rate‐limiting step. When the gate bias during irradiation is positive, interpretation of the correlation between data and model suggests that the hydrogen ions are preferentially created near the Si‐SiO2 interface. Finally, the Nit formation time is found to decrease with increasing oxide field as E−1.73ox. This result is compatible with the hydrogen transport model if the average displacement per hop is assumed to be proportional to Em.

Oxide, interface, and border traps in thermal, N2O, and N2O‐nitrided oxides

We have combined thermally stimulated‐current (TSC) and capacitance–voltage (C–V) measurements to estimate oxide, interface, and effective border trap densities in 6–23 nm thermal, N2O, and N2O‐nitrided oxides exposed to ionizing radiation or high‐field electron injection. Defect densities depend strongly on oxide processing, but radiation exposure and moderate high‐field stress lead to similar trapped hole peak thermal energy distributions (between ∼1.7 and ∼2.0 eV) for all processes. This suggests that similar defects dominate the oxide charge trapping properties in these devices. Radiation‐induced hole and interface trap generation efficiencies (0.1%–1%) in the best N2O and N2O‐nitrided oxides are comparable to the best radiation hardened oxides in the literature. After ∼10 Mrad(SiO2) x‐ray irradiation or ∼10 mC/cm2 constant current Fowler–Nordheim injection, effective border trap densities as high as ∼5×1011 cm−2 are inferred from C–V hysteresis. These measurements suggest irradiation and high‐field s...

Generation of Interface States by Ionizing Radiation in Very Thin MOS Oxides

The creation of interface states Dit by ionizing radiation is investigated in MOS capacitors as a function of oxide thickness in the range 6-50 nm. A comparison of the thickness dependence in etchback and asgrown oxides supports the idea that the number of defects at the Si-SiO2 interface increases with oxidation time. For relatively thin oxides (tox<12 nm), the rate of increase in Dit is significantly smaller than would be extrapolated from the behavior of thicker oxides for both oxide types. This effect is probably caused by tunneling of trapped holes near the oxide interfaces.

Time dependence of interface trap formation in MOSFETs following pulsed irradiation

The time dependence of interference trap (N/sub it/) formation in MOSFETs was studied as a function of gate oxide thickness, oxide growth type, substrate orientation, temperature, and gate bias. Two different N/sub it/ formation mechanisms are observed. Most (typically 90%) of the formation, called the late process, occurs slowly at long times (1-10000 s) after the radiation pulse. From a variety of experimental data, it is concluded that the rate of the late process is limited by drift of a radiation-induced positive ion, probably H/sup +/, through the gate oxide to the Si-SiO/sub 2/ interface where the N/sub it/ are formed. A relatively fast, or early, process is responsible for a small percentage of the total N/sub it/ formation. The time constant for this process appears to be consistent with hole drift through the oxide. >

Nitrogen depletion during oxidation in N2O

The incorporation of nitrogen in oxides has been studied after furnace oxidation in N2O at 900 °C. We observe that nitrogen is removed from the oxide bulk during oxidation in N2O, while simultaneously nitrogen is incorporated at the growing Si–SiO2 interface. This results suggests that nitrogen incorporation involves a dynamic equilibrium between competing processes which causes both nitrogen incorporation and depletion. A chemical model for nitrogen removal is proposed based on a reaction with NO. Reaction energies, estimated from semiempirical quantum‐mechanical calculations, support the proposed model.

Observation of H/sup +/ motion during interface trap formation

The time dependence of changes in the oxide trapped charge during interface trap formation is investigated. Changes in MOSFET threshold voltage V/sub th/ and number of interface traps N/sub it/ are measured in the same sample as a function of time following pulsed irradiation. When the gate bias during irradiation V/sub gl/ is positive, the initial mod Delta V/sub th/ mod is large due to trapping of radiation-induced holes at the Si-SiO/sub 2/ interface and the postirradiation time dependence of Delta V/sub th/ is dominated by hole detrapping, as expected. When V/sub gl/ is negative, interfacial hole trapping is minimized. In this case, an unusual peak in the Delta V/sub th/ vs. time curve provides evidence of the involvement of H/sup +/ ions in the N/sub it/ formation process. >

Formation of interface traps in MOSFETs during annealing following low temperature irradiation

The formation of interface traps, N/sub it/, was studied in MOSFETs during isochronal annealing up to 350 K, following exposure to ionizing radiation at 78 K. Two distinct N/sub it/ formation processes are observed. A small (1-10% of total) process occurs at 100-150 K, which is caused by neutral atomic hydrogen, and a second, higher-temperature (200-300 K) process that accounts for most ( >90%) of the N/sub it/ formation also occurs. The characteristics of the high-temperature process support the proton (H/sup +/) model of N/sub it/ formation and are not in agreement with several other common models. Charge pumping and inversion layer mobility techniques for determining N/sub it/ are compared. It is found that the mobility cannot be used to determine N/sub it/ at 78 K (in contrast to its successful use at 295 K), probably because of lateral uniformities in the large radiation-induced fixed oxide charge. >