Diffusion in silicon of elements from columns III and V of the Periodic Table is reviewed in theory and experiment. The emphasis is on the interactions of these substitutional dopants with point defects (vacancies and interstitials) as part of their diffusion mechanisms. The goal of this paper is to unify available experimental observations within the framework of a set of physical models that can be utilized in computer simulations to predict diffusion processes in silicon. The authors assess the present state of experimental data for basic parameters such as point-defect diffusivities and equilibrium concentrations and address a number of questions regarding the mechanisms of dopant diffusion. They offer illustrative examples of ways that diffusion may be modeled in one and two dimensions by solving continuity equations for point defects and dopants. Outstanding questions and inadequacies in existing formulations are identified by comparing computer simulations with experimental results. A summary of the progress made in this field in recent years and of directions future research may take is presented.

Point defects and dopant diffusion in silicon

Bulk silicon etching techniques, used to selectively remove silicon from substrates, have been broadly applied in the fabrication of micromachined sensors, actuators, and structures. Despite the more recent emergence of higher resolution, surface-micromachining approaches, the majority of currently shipping silicon sensors are made using bulk etching. Particularly in light of newly introduced dry etching methods compatible with complementary metal-oxide-semiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future. The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. For each category, the available processes are reviewed and compared in terms of etch results, cost, complexity, process compatibility, and a number of other factors. In addition, several example micromachined structures are presented.

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Bulk micromachining of silicon

The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices...

Ultrathin (<4 nm) SiO2 and Si-O-N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits

Core-level binding-energy shifts at surfaces and in solids

An empirical model of radiation-induced interface states at the SiO2/Si interface in SiO2 MOS capacitors is developed. The formulation explicitly addresses the time-dependent two stage nature of the buildup process, and it gives the mathematical dependencies of the experimentally observed buildup on time, field, temperature, and dose. The model is applied to both wet and dry grown oxides. The implications of the empirical model for microscopic mechanisms involved in the buildup are discussed. In particular, it is argued that the experimental observations can best be understood in terms of a positive ion (probably H+) release in the SiO2 bulk and the subsequent transport of the liberated ions to the SiO2/Si interface (for positive gate bias). The induced interface states result from an interaction of the ions at the interface.

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A Framework for Understanding Radiation-Induced Interface States in SiO2 MOS Structures

The chemical structure of thin Si${\mathrm{O}}_{2}$ films and Si${\mathrm{O}}_{2}$-Si interfaces has been investigated using high-resolution x-ray photoelectron spectroscopy. The data are consistent with a continuous random network of four-, six-, seven-, and eight-member rings of Si${\mathrm{O}}_{4}$ tetrahedra joined together by bridging oxygens. This distribution changes substantially within 30 \AA{} of the Si${\mathrm{O}}_{2}$-Si interface. The near-interface region is comprised of ${\mathrm{Si}}_{2}$${\mathrm{O}}_{3}$, SiO, and ${\mathrm{Si}}_{2}$O. This structure is interpreted by means of a structure-induced-charge-transfer model.

High-Resolution X-Ray Photoelectron Spectroscopy as a Probe of Local Atomic Structure: Application to Amorphous Si O 2 and the Si-Si O 2 Interface

The chemical structures of thin SiO2 films, thin native oxides of GaAs (20-30 A), and the respective oxide-semiconductor interfaces, have been investigated using high-resolution X-ray photoelectron spectroscopy. Depth profiles of these structures have been obtained using argon ion bombardment and wet chemical etching techniques. The chemical destruction induced by the ion profiling method is shown by direct comparison of these methods for identical samples. Fourier transform data-reduction methods based on linear prediction with maximum entropy constraints are used to analyze the discrete structure in oxides and substrates. This discrete structure is interpreted by means of a structure-induced charge-transfer model.

Local atomic and electronic structure of oxide/GaAs and SiO2/Si interfaces using high-resolution XPS

A correlation between the nitridation condition dependent nitrogen distribution and the magnitude of the flatband voltage shift in SiOxNy/Si structures is identified and an explanation proposed in terms of a strain‐dependent kinetics of formation of defects near the interface. The proposed explanation derives from, and is consistent with, the recent demonstration of the influence of the interfacial strain on the nitridation kinetics of thermally nitrided SiOxNy/Si films.

Strain‐dependent defect formation kinetics and a correlation between flatband voltage and nitrogen distribution in thermally nitrided SiOxNy/Si structures

We report on the first nondestructive measurement of the chemical and physical characteristics of the interface between bulk SiO2 and thick aluminum films. Both x‐ray photoelectron spectroscopy (XPS) and electrical measurements of unannealed, resistively evaporated Al films on thermal SiO2 indicate an atomically abrupt interface. Post metallization annealing (PMA) at 450 °C induces reduction of the SiO2 by the aluminum, at a rate consistent with the bulk reaction rate. The XPS measurement is performed from the SiO2 side after the removal of the Si substrate with XeF2 gas and thinning of the SiO2 layer with HF:ETOH. This represents a powerful new approach to the study of metal‐insulator and related interfaces.

A novel X-ray photoelectron spectroscopy study of the Al/SiO2 interface

X-ray photoelectron spectroscopy has been used to study the composition of 100-A thermally grown SiO2 films that have been thermally nitrided in ammonia. The SiO(x)N(y)/Si interface was studied both by chemical depth profiling of the oxynitride and by removal of the Si substrate with XeF2. It is found that N is distributed throughout the film, but with the concentration higher at the surface and in a region centered 25 A from the film/substrate interface. The interface region itself is found to be oxygen-rich relative to the rest of the film. Possible models which can explain these results are discussed.

R. P. Vasquez

Papers

High-Resolution X-Ray Photoelectron Spectroscopy as a Probe of Local Atomic Structure: Application to Amorphous Si O 2 and the Si-Si O 2 Interface

Local atomic and electronic structure of oxide/GaAs and SiO2/Si interfaces using high-resolution XPS

Strain‐dependent defect formation kinetics and a correlation between flatband voltage and nitrogen distribution in thermally nitrided SiOxNy/Si structures

A novel X-ray photoelectron spectroscopy study of the Al/SiO2 interface

X‐ray photoelectron spectroscopy study of the chemical structure of thermally nitrided SiO2