The dielectric measurements (fig. 1) are performed with LCR Hioki impedance analyzer. It enables measurements of capacitance, conductance and impedance at frequencies between 100Hz and 4MHz under applied electric voltage up to 30V. The measurements can be done at room temperature or during heating at temperatures up to 200oC. The measurement system is computerized and operated by a labview program.
Stress in Thin films
The stress in thin films is determined in our lab using a laser scanning system (fig. 2). The system consists of a He-Ne laser beam that scans the sample by a rotating mirror and a position sensitive detector (PSD) that detects the reflected beam from the sample. The radius of curvature of the sample is determined by the ratio between the scanning steps of the beam on the sample to the reflected steps of the beam on the PSD. The stress in the film is determined by measuring the sample radius of curvature prior and after the film deposition and using the Stonney equation:
is the biaxial elastic modulus of the substrate,
is the film thickness,
is the thickness of the substrate,
are the final and initial radius of curvature of the sample, respectively.
Fig. 2. The laser system
The system enables measurements of stress during heating up to about 300oC and cooling to room temperature, under either vacuum or protective gas environment, which provides data on elastic and plastic thermal strains. The system also used for determining the piezoelectric coefficient of thin film. The measurement consists of applying electric field in-vertical to the film plane and measuring the resulted change in the sample radius of curvature due to the piezoelectric effect. The piezoelectric coefficient is determined by the following expression:
– Biaxial elastic modulus of the substrate.
– Biaxial elastic modulus of the film.
– Substrate thickness.
– Radius of curvature
– Applied voltage
Deposition of thin films
Thin films are deposited in our lab in two systems: resistive boat evaporation and sputtering.
The resistive boat system (fig. 4) is capable of evaporation from 3 sources elemental and compound materials. During evaporation the substrate can be heated up to about 700oC and nitrogen can be introduced to the chamber for reactive formation of nitrides.
The sputtering system was mostly used to deposit compound thin films that consist of oxides and nitrides. The films are deposited by reactive rf sputtering of metal targets in either oxygen or nitrogen gas environment. In some cases, the targets are made of the compound material and the gars environment helps in adjusting the stochiometry of the deposited film, like in the case of BaTiO3. Another useful advantage of the system is co-sputtering from two targets to form multi-component thin film. In such a case, one target is sputtered under rf conditions while the other target under dc conditions. This method was used in depositing AlCuOx films from Cu and Al targets under oxygen gas environment. During deposition, the samples can be heated up to about 300oC. The system operation is controlled by a computer and capable of depositing ultra-thin films as thin as few nanometers.
Electrochemical anodization of thin Al films
Thin porous alumina films are prepared in our lab by electrochemical anodization (fig. 5) of pure Al films. The preparation process consists of two main stages. The first stage is polishing the surface to a high degree of smoothness. The second stage is anodization in a dilute phosphoric aqueous solution or an oxalic dilute aqueous solution. The size of the pores and thickness of the porous film depend on various parameters such as the composition and temperature of the liquid solution as well as the amplitude and time of the applied electric field.
Fig. 5. The electrochemical anodization system.
Thermal heat treatments are done in two furnaces capable of heating up to 1100oC in either vacuum (about 10-5Torr) or protective gas environment (Ar, N2). In addition, reactive heat treatments with oxygen up to 1100oC can be done in a third furnace.