Rotating cruicible electron beam evaporation technique for mixed oxide films

Abstract

Recent developments in thin film deposition techniques for preparing a variety of devices such as superconducting films, optical interference coatings, solar selective coatings, etc., have shown the necessity of preparing stoichiometric multicomponent oxide films.

It has also been realized that most of the available Physical Vapour Deposition techniques are not suitable to prepare these films with the required stoichiometry. This has posed a big challenge to the thin film technologists and motivated them not only to modify the existing deposition techniques but also to design new techniques to meet the stringent requirements. All possible innovations have been tried to achieve this goal.

The objective of the present work is to try one such innovation, i.e., to modify the existing electron beam evaporation technique and make it simple and more suitable for preparing mixed oxide films with the required stoichiometry. The outcome of this work is the design and fabrication of a rotating crucible electron beam evaporation (ROCEE) technique and its performance study in preparing homogeneous and graded multicomponent oxide films with a wide range of refractive indices.

The use of multiple electron beam guns has been avoided by replacing the conventional crucible with a grooved crucible and filling the different materials in the groove with partitions. The crucible is mounted in an electron beam gun assembly and is continuously rotated using a stepper motor operated from outside the vacuum system. If two materials are to be deposited, they are loaded into two segments of the crucible and the crucible is rotated. A position sensor is used to enable the electron beam to sense the initial position of the crucible during rotation. The necessary hardware and software automatically adjust the electron beam emission current and rotation speed of the crucible to achieve the required deposition rate for that material. These values are retained till the crucible crosses the first evaporating material.

When the electron beam sees the second evaporating material, the emission current and crucible rotation speed are automatically set to the other predetermined values, enabling the deposition of the second material. This process is continued till the film of required thickness is deposited. The entire operation is controlled using an INTEL 8085 microcomputer. The design details on electronic hardware and software are described.

Some of the novel ideas used to solve many problems that came up in the fabrication of this system are:

Control of emission currents at the boundaries of the materials to avoid contamination from the crucible.

Providing the necessary software to achieve required emission currents and rotation speeds for the different constituents.

Design of a novel substrate shutter mechanism which enables deposition of different constituents on different substrates so that their thickness and optical constants could be independently evaluated.

Optimization of deposition conditions to achieve high packing densities for the films.

Generation of necessary hardware and software to prepare not only homogeneous films but also films with the required gradation in composition.

Automatic process termination by opening and closing the evaporation shutter when the concentration of each material reaches the preset value.

Automatic degassing of the evaporants in a prescribed time.

To evaluate the performance of this system and to test its suitability for depositing mixed oxide films, films of single layers and mixed films of CeO? and SiO? materials were prepared. Thin films were prepared at ambient and at 200°C, 300°C, and 400°C substrate temperatures. The optical properties (refractive index, extinction coefficient, packing density, optical inhomogeneity, and optical band gap) were studied using a spectrophotometer and ellipsometer. The structural properties were studied using X-ray diffractometer and transmission electron microscope (TEM). The composition was studied using Auger electron spectroscopy, energy dispersive X-ray analysis, and Rutherford backscattering spectroscopy.

The technique enabled preparation of mixed oxide films with the required composition and refractive index over a wide range of values. The results of single layer films with CeO? and SiO? materials were found to be in good agreement with reported values. Mixed oxide films were found to have good homogeneity and were absorption-free. They were found to obey the Lorentz-Lorenz relation.

The graded index films of CeO? and SiO? materials were also prepared at ambient and at 300°C substrate temperature by varying the emission current of individual materials in a systematic manner. The gradation of the film with respect to its thickness along the depth was analyzed using Auger depth profile and was found to be in good agreement with the estimated composition.

The technique developed is simple and can be easily arranged in all deposition systems. This technique is also suitable for the preparation of other multicomponent films such as alloys, cermets, and composites.