X-ray reflectometry allows the determination of the thickness, density, absorption and rms roughness of a stack of thin layers on a substrate from several nanometres to some hundred nanometres. Inversion of the experimental reflectivity data is usually realized by a trial-and-error method based on the theoretical computation of the reflectivity curve after having extracted initial values of the layer thicknesses from the result of a classical FFT of the reflectivity data. However, the order information of the layers is lost during classical FFT. The order of the layers then has to be known a priori. Besides, the classical Fourier transform does not reveal anything about the stack parameters (density, absorption and the rms roughness). As this trial-and-error method is efficient provided that one has a good idea of the stack parameter values, it is important to extract some valuable information directly from the experimental reflectivity. In this paper, it will be shown that the order of the layers can be obtained by the so-called joint time-frequency representations. Furthermore, the continuous wavelet transform allows qualitative determination of the stack parameters and helps in the determination of an appropriate starting model for the trial-and-error method. The points of interest of this method are illustrated by experimental examples.

Low-temperature deposition (room temperature, RT-250 °C) of high-resistivity SiO2 layers has been successfully developed on InP substrates. Complete electrical characteristics of metal-insulator-semiconductor (MIS) diodes show promising characteristics in terms of barrier height at the SiO2-InP interface and in terms of interface state distribution (NSS). Leakage current is essentially bulk limited (ρ=4×1015 Ω cm) until a high electrical field in the range 4–6 MV cm−1 and a minimum value of NSS of 2×1011 cm−2 eV−1 range is achieved, without particular surface treatment. These results show that the technique is well adapted to n-type depletion-mode MIS field-effect transistor processing.

Silicon dioxide thin films have been deposited at low substrate temperatures (Ts < 120~ using a microwave plasma. A new type of microwave excitation, the distributed electron cyclotron resonance (DECR), which provides high density plasma (~ 1011 cm -3) of low-energy ions, has been used. Pure N20 and Sill4 are mixed in the discharge. At constant pressure (0.1 pa), the ratio of N20 flow to Sill4 flow (Ro) was varied from 1-9. We have studied the effects of the gas phase composition at two different microwave powers (800 and 1200 W) on the refractive index, atomic composition, infrared absorption bands, etch rate, and electrical properties of the films. For ratio Ro larger than 4, near-stoichiometric films are obtained, with N and H atomic contaminations below 2 and 5 atomic percent (a/o), respectively, even at low temperature and without post deposition annealing. For Ro = 9, the films have physical and chemical properties similar to those obtained by RF plasma enhanced chemical vapor deposition at 350~ in terms of refractive index in the range of 1.47-1.48, etch rate in "P-etch" (6/~/s), and Si-O-Si stretching mode vibration (1058 cm-1). These films exhibit, at Ro = 9 and 1200 W, electrical resistivity of 10 ~ 1] cm and critical field larger than 3.5 MV cm -~, without post-deposition annealing.