C. H. Bjorkman

Correlation between midgap interface state density and thickness‐averaged oxide stress and strain at Si/SiO2 interfaces formed by thermal oxidation of Si

Correlations between midgap interface state density (Dit) and thickness‐averaged stress in thermally grown SiO2 thin films have been investigated by infrared spectroscopy, an optical beam deflection technique, and capacitance‐voltage measurements. We find no correlations between Dit and either (i) the maximum stress in the Si or SiO2 at the Si/SiO2 interface or (ii) the stress gradient in the SiO2 film. By direct measurements of the strain‐induced bending of the Si wafer, and by calculating the microscopic strain from the SiOSi bond‐stretching vibrational frequency, we have established linear relationships between Dit and the thickness‐averaged stress and strain in the oxide.

Local atomic structure at thermally grown Si/SiO2 interfaces

Abstract This paper presents an experimental study of the properties of SiO 2 and Si, in the immediate vicinity of thermally grown SiO 2 /Si interfaces. We have examined properties of the SiO 2 films, including the center frequency and width of the SiO IR-active bond-stretching vibration, the optical index of refraction at 632.8 nm, and the intrinsic growth stress, as well as the photoreflectance and Raman scattering of the Si substrate at the growth interface. We have interpreted the temperature and thickness dependence of these properties in terms of a model which includes the molar volume mismatch between the SiO 2 and Si at the growth interface, and visco-elastic relaxation of the oxide. We conclude that the thermal history of SiO 2 films is generally non-homogeneous due to differences between the processing times, and the times for visco-elastic relaxation of the interface-generated stress. Even under conditions approaching a homogeneous thermal history, there are still significant stress gradients in the SiO 2 und underlying Si in the immediate vicinity of the SiO 2 /Si interface.

The effect of post-deposition thermal processing on MOS gate oxides formed by remote PECVD

The effects of post-deposition thermal exposure, at temperatures typical of MOS fabrication processes, on gate oxides formed by remote plasma enhanced chemical vapor deposition (RPECVD) is discussed. SiO2 films were prepared by (1) thermal oxidation of silicon at temperatures from 700 to 1150° C, and (2) by RPECVD at a substrate temperature of 350° C. Post deposition thermal processing was achieved by rapid thermal annealing for 100 sec from 850–1200° C. Film properties were studied by infrared spectroscopy (IR), ellipsometry, and by measurements of stress, capacitance voltage characteristics, and dielectric breakdown. Post-formation, thermal processing in the range of 850–1200° C was shown to modify both thermally grown and deposited oxides, but it has been shown that RPECVD films could be stabilized against post-deposition changes by rapid thermal annealing at temperatures of about 900° C for periods of at least 100 sec.

Chemically Modified Second Harmonic Generation at Surfaces on Vicinal Si(111) Wafers

Using surface second harmonic generation (SSHG) at 1053nm, we study the influence of off-axis orientation and surface structure of silicon (111) surfaces. We study wafers cut at angles between 0° and 5° in the [112] direction. The surface structure is varied by thermal oxidation at 850°C, annealing, and thinning the oxide in a HF solution. For comparison, nitride films are also investigated. The characteristic rotational symmetry of the SSHG-signal for (111) flat (non-vicinal) surfaces is enhanced by a Si/SiO 2 interface. The oxide layer also influences the signals due to the steps on vicinal surfaces. The results are discussed in comparison with a microscopic model of the oxidized misoriented surface.

Intrinsic Growth Stress in Thermally Grown and Annealed SiO 2 Thin Films: Control of Stress-Induced Electronically Active Defects at Si/SiO 2 Interfaces

We find that the density of electronically active defect states, formed close to the Si/SiO 2 interface during high temperature oxidation (T ox > 800°C), is proportional to the thickness-averaged stress or strain in the oxide layer. This was established by measuring the midgap interface state density, D it , and correlating it with: i) a direct measurement of the stress, using a beam deflection technique; and ii) a determination of the oxide strain, using infrared (IR) spectroscopy and a model that relates the frequency of the Si-O bond-stretching vibration to the microscopic strain in the oxide.In addition, the elastic stress at the Si/SiO 2 interface has been modified by film deposition onto the SiO 2 surface of Si: 3 N 4 film or a variable thickness of an Al gate metal. The additional elastic stress introduced by these depositions did not influence Dit. These observations lead us to conclude that interface traps are generated by “local plastic deformations” that occur during the oxidation process, and are localized in the immediate vicinity of the Si/SiO 2 interface.

The Influence of Crystal Orientation and Processing Conditions on the Energy Distribution of Traps at the Si-SiO2 Interface

We have studied the local atomic strain as a function of crystal orientation and processing conditions, and its relationship to the energy distribution of interface traps in the Si bandgap. For a given oxidation temperature (up to 1050°C), the strain in SiO2 films grown on (100), (110), and (111) Si was reduced by a rapid thermal anneal, RTA, at 1100°C. This was accompanied by a reduction in the midgap interface state density at midgap. Moreover, for oxides grown on Si(100) surfaces, an increase in oxidation temperature promotes strain relaxation and reduction of interface states at energies above 0.4 eV in the Si bandgap, but not at energies between 0.2 and 0.4 eV.

Effective electron affinities in doped a-Si:H and μc-Si films as determined from studies of MOS capacitors

We have prepared heavily-doped a-Si:H and μc-Si by remote plasma-enhanced chemical-vapor deposition, Remote PECVD; and have used these materials as electrodes in MOS capacitors. Shifts in the flatband voltages of MOS devices with heavily-doped a-Si:H and μc-Si electrodes are used to determine effective electron affinities for these electrode materials. Shifts in V FB for capacitors with lower doping concentrations can not be attributed to differences in the Fermi levels alone.

Second Harmonic Generation on Chemically Treated Surfaces of Vicinal Si(111) Wafers

Surface Second Harmonic Generation (SSHG) is a sensitive probe for surface studies in centrosymmetric materials [1, 2]. Recent investigations of the Si/SiO2 system have shown that optical harmonic generation can be used to study this technologically important material [3, 4, 5, 6].

Integration of wet-chemical processing with low-temperature plasma-assisted processes for the formation of device-quality Si/SiO 2 interfaces on Si(111) surfaces

Abstract An integrated sequence of ex-situ wet-chemistry, and on-line low-temperature remote plasma-assisted techniques, for fabricating device-quality Si/SiO 2 interfaces on Si(111) wafers is reported. Three factors contribute to the device quality interfaces i) the orientation of the Si surface relative to a perfect (111) alignment, ii) the pH of the final HF/NH 4 F rinse used in the pre-deposition wet-chemistry processing, and iii) the specific plasma-assisted oxidation and deposition processes used to form the SiO 2 /Si heterostructure.

Deconvolution of Thickness-Averaged Structural and Optical Properties of Thermally Grown and Rpecvd SiO 2 Films

This study compares the optical characteristics of SiO2 thin films, grown by high temperature dry thermal oxidation of crystalline Si, or deposited by low-temperature remote plasma enhanced chemical vapor deposition (RPECVD), and subjected to rapid thermal annealing (RTA). Infrared (IR) spectrophotometry is used to determine the frequency of the Si-O-Si bond-stretching vibration for films of varying thickness, both before and after RTA. The thickness dependence of the stretching frequency as a function of distance from the SiO2/Si interface is determined from analysis of these measurements. The resulting profiles combined with a previous study of the index of refraction, n, for thermally grown SiO2 films is used to estimate the variation of n as a function of the distance from the SiO2/Si interface.