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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

LPCVD Optical Coatings for Complex Shapes and Uneven Surfaces

Process is effective for spherical, hemispherical, hyperhemispherical, and dome-shaped optics
Figure 1
© Deposition Sciences
Figure 1: The LPCVD Reactor Setup. Chemicals (precursors) are vaporized in a high-temperature manifold outside the reaction chamber and flow by diffusion into the reaction chamber to the substrate to be coated.
Figure 2
© Deposition Sciences
Figure 2: Physical Vapor Deposition (PVD) coating equipment involves creating optical films using physical processes rather than chemical reactions at the surface of the substrate. The material used for coating is either heated in a vacuum to a great enough temperature to produce a high vapor pressure, or the atoms of the material are knocked off by interactions with a plasma.
Figure 3
© Deposition Sciences
Figure 3: In the manufacture of ball lenses and complex shapes, a uniform coating is critical; without uniformity, the net optical characteristics of the lens and coating would depend entirely on the orientation of the lens.
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By Jay Kane, Ph.D. and Bob J. Crase

Applying optical thin film coatings evenly to various substrates, shapes and sizes is becoming increasingly critical to meeting higher performance parameters and durability requirements. From telecommunications to aerospace and defense, to commercial lighting, information displays, solar simulation and material aging, an array of highly specialized optical coating solutions are enabling precision spectral performance for many diverse applications.

LPCVD for Uniform Coatings on Uneven Surfaces

Low pressure chemical vapor deposition (LPCVD) is one process by which thin film coatings are applied to surfaces to produce various optical effects. These coatings are designed to produce specific reflectance and transmittance properties for products in many different fields, such as the telecom industry, the lighting industry, defense, medical and scientific research. While there are other processes that are used to produce thin-film coatings, LPCVD offers some unique and valuable benefits. One of the most common methods for applying thin-film coatings is physical vapor deposition (PVD). This process, however, has a limitation that arises when a surface with a non-flat topology must be uniformly coated. LPCVD processes have the ability to apply uniform coatings on these uneven surfaces and complex shapes. This article will explore both PVD and LPCVD methods of optical thin-film-coating for different tasks.

How it works

The basic LPCVD process begins with two chemicals that will be used to produce an alternating stack of layers with different indices of refraction. By adjusting the thicknesses of each layer in the stack the desired reflecting and transmitting properties of the coating can be obtained. The chemicals used are typically organometallic chemicals called precursors. These precursors are vaporized in a high-temperature manifold outside the reaction chamber and flow by diffusion into the reaction chamber to the substrate to be coated (Figure 1).

At the surface of the substrate the precursor adsorbs and undergoes a chemical reaction that produces the desired optical coating material and a gaseous byproduct. By adjusting precursor flows to the reaction chamber, the thickness of the film formed at the surface of the substrates can be controlled. By alternating different precursors of different refractive indices, the desired stack of layers is produced.

At Deposition Sciences, the precursors used in the chemical vapor deposition system are tantalum ethoxide (TaO­5C10H25) to produce Ta2O5, titanium ethoxide (TiC8H20O4) to produce TiO2, and silicon di-t-butoxide acetate (SiC12H24O6) to produce SiO2. The precursors begin in liquid form and are passed through a flow meter to control the rate of flow. After this they pass to a high temperature manifold where they are vaporized. From here the gas flows into the reaction chamber at a pressure in the Torr range (1/760 of an atmosphere).

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