Shigeto Fukatsu

Self-diffusion of Si in thermally grown SiO2 under equilibrium conditions

Self-diffusion coefficients of Si in thermally grown SiO2 on a semiconductor-grade silicon wafer have been determined at temperatures between 1150 and 1300u2009°C under equilibrium conditions using isotope heterostructures (natSiO2/28SiO2). Si self-diffusion was induced by appropriate heat treatments, and the diffusion depth profiles of 30Si isotope from natSiO2 to 28SiO2 layers were determined by secondary ion mass spectrometry (SIMS). The diffusion coefficients found in the present study for 1150–1300u2009°C are more than two orders of magnitude smaller than the values measured with semiconductor-grade SiO2 in the presence of excess silicon, i.e., in nonequilibrium conditions, and agree very well with previously reported values of Si self-diffusion in fused silica under equilibrium conditions.

Effect of the Si/SiO2 interface on self-diffusion of Si in semiconductor-grade SiO2

Self-diffusion of ion-implanted 30Si in SiO2 formed directly on Si substrates by thermal oxidation was studied as a function of the temperature and SiO2 thickness (200, 300, and 650 nm). The diffusion coefficient increases by about an order of magnitude with decreasing SiO2 thickness from 650 to 200 nm when silicon–nitride capping layers are placed on top of the SiO2, i.e., the distance between the 30Si diffusers and Si/SiO2 interface has a strong influence. Because the stress on SiO2 by nitride estimated for such a change in diffusivity is unrealistically large, Si species, most likely SiO, generated at the Si/SiO2 interface and diffusing into SiO2 must be affecting the self-diffusion of Si in SiO2.

Modeling of Si self-diffusion in SiO2: Effect of the Si/SiO2 interface including time-dependent diffusivity

Self-diffusion of Si in thermally grown SiO2 is modeled taking into account the effect of SiO molecules generated at the Si/SiO2 interface and diffusing into SiO2 to enhance the self-diffusion. Based on the model, a recent self-diffusion experiment of ion-implanted 30Si in SiO2, which showed increasing self-diffusivity with decreasing distance between the 30Si diffusers and Si/SiO2 interface [Fukatsu et al., Appl. Phys. Lett. 83, 3897 (2003)], was simulated, and the simulated results fit the experimental profiles. Furthermore, the simulation predicts that the self-diffusivity would increase for a longer annealing time because more SiO molecules should be arriving from the interface. Such time-dependent diffusivity was indeed found in our follow-up experiments, and the profiles were also fitted by the simulation using a single set of parameters.

Correlated diffusion of silicon and boron in thermally grown SiO2

Si self-diffusion and B diffusion in SiO2 were simultaneously investigated in thermally grown Si28O2 co-implanted with Si30 and B. The B diffusivity increases with decreasing distance between the implanted B and Si∕SiO2 interface, in the same way as Si self-diffusivity. This result together with a numerical simulation shows that SiO molecules, which are generated at the Si∕SiO2 interface and diffusing into SiO2, enhance not only Si self-diffusion, but also B diffusion. In addition, we found that the diffusivities of both Si and B increase with higher B concentration in SiO2. The experimental results can be quantitatively explained by a numerical simulation assuming that the diffusivity of SiO, which enhances the diffusivities of Si and B, increases with higher B concentration.

Effect of Si/SiO2 Interface on Silicon and Boron Diffusion in Thermally Grown SiO2

Silicon self-diffusion and boron diffusion in SiO2 were investigated as functions of the distance of diffusing silicon from the Si/SiO2 interface at various temperatures in the range of 1150–1250°C using natSiO2/28SiO2 isotope heterostructures and 30Si- and B-implanted 28SiO2 without and with a 30-nm-thick silicon nitride layer on the surface of each sample. The self-diffusivity of Si in SiO2 did not depend on the oxygen concentration in the annealing ambient without the silicon nitride layer. The diffusion profiles of Si and B in the sample capped with the silicon nitride layer became broader as the distance from the Si/SiO2 interface decreased. This dependence on the distance from the interface was caused by SiO molecules, which are generated at the interface and diffuse into SiO2. The simulated results, taking into account the role of SiO molecules, showed good agreement with each experimental profile of 30Si and B.

Simulation of correlated diffusion of Si and B in thermally grown SiO2

Simultaneous diffusion of Si and B in thermally grown SiO2 is modeled taking into account the effect of SiO molecules generated at the Si∕SiO2 interface and diffusing into SiO2 to enhance both Si and B diffusion. Based on the model, we simulated experimental profiles of coimplanted Si30 and B in SiO228, which showed increasing diffusivities with decreasing distance from the interface. The simulation results show that the SiO diffusion is so slow that the SiO concentration at the near-surface region critically depends on the distance from the interface. In addition, the simulation explains that the diffusivities of both Si and B increase with longer annealing times because more SiO molecules arrive from the interface. Furthermore, we examined the effect of high-concentration B on the diffusivities of Si and B in SiO2, both of which increase with higher B concentration. The experimental results were simulated assuming that the diffusivity of SiO, which enhances the diffusivities of Si and B, increases with ...

The Effect of Partial Pressure of Oxygen on Self-Diffusion of Si in SiO2

The self-diffusion coefficient of Si in thermal oxides (SiO2) formed on semiconductor silicon wafers has been determined with isotope heterostructures, natSiO2/28SiO2, as a function of the partial pressure of oxygen mixed into argon annealing ambient. The natSiO2 layers contain 3.1% of 30Si stable isotopes while the 28SiO2 layers are depleted of 30Si stable isotopes down to 0.003%, and the diffusion depth profiles of 30Si isotopes from the natSiO2 to 28SiO2 layers after thermal annealing have been determined by secondary ion mass spectrometry (SIMS). The Si self-diffusivity is found not to depend on the partial pressure of oxygen within our experimental error of about ±33%.

Method of Decoupling the Bias Temperature Instability Component from Hot Carrier Degradation in Ultrathin High-k Metal–Oxide–Semiconductor Field-Effect Transistors

Hot-carrier (HC) degradation becomes more critical as the channel length is reduced. Furthermore, both positive and negative bias temperature instabilities (PBTI and NBTI, respectively) are significant in high-k devices. Under HC stress, BTI caused by the vertical electric field is unavoidable. The decoupling of the BTI component from HC degradation is necessary to predict device lifetime more accurately. In this study, a new decoupling method of HC degradation is proposed. By using the relation between ΔVth and Jg, the BTI component can be decoupled from the degradation under Vg=Vd stress. The application of our method to HfSiON metal–oxide–semiconductor field-effect transistors (MOSFETs) is demonstrated. The channel length dependence of each component shows the improvement resulting from the decoupling method. Furthermore, the activation energy of the NBTI component in HC degradation coincides with that of NBTI.