LPCVD Optical Coatings for Complex Shapes and Uneven Surfaces Allston MA

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.

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).

The temperature inside the chamber must be optimized for the operating pressure and the coating materials used. DSI's chamber temperature is nominally 500 degrees C. In the coating chamber, the precursors adsorb onto the surface of the substrate and react forming either Ta₂O₅, TiO₂ or SiO₂, which constitute the materials that will comprise the layer stack. During this process byproducts are formed and desorb from the surface. These waste products must then be pumped out of the chamber to avoid cross contamination between the two different precursors. After they are pumped out of the chamber they pass through cryo traps where they condense and are collected. With this process, coatings are applied at the rate of a few angstroms per second.

PVD VS. LPCVD

Another important process by which thin films can be applied to a surface is physical vapor deposition. PVD 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. The latter is known as sputtering; this is when an inert gas (usually argon) is brought to a plasma state and the ions strike the target material and release the atoms, which in turn will deposit on the substrate.

As mentioned earlier, LPCVD is ideal for coating complex or uneven surfaces, which can be difficult to do in sputtering processes. In sputtering, ions from the plasma strike the material to be used for coating (the target), and knock the atoms off in different directions. These atoms then deposit onto the surface of the substrate to form the film. The rate of this deposition will depend on the distance from the target to the substrate. For a non-flat substrate surface, the portion that is closer to the target will therefore receive a higher deposition rate and the coating will be thicker in that region, and therefore uneven overall. Another problem arises when coating a surface with a convex shape in relation to the target. Consider a spherical substrate. The point closest to the target will receive a coating at a certain rate, but as the azimuthal angle increases with respect to that point, the coating will be thinner as there is less of the surface area's perpendicular component that is exposed to the target. This azimuthal anisotropy (in this particular example) can be extremely problematic in many applications, as we will discuss further. In addition, in the absence of specialized tooling, the same face of the substrate will be oriented toward the target at all times, and thus a portion of the substrate will be completely uncoated.

LPCVD IN REAL-WORLD APPLICATIONS

Since LPCVD deposition is not directional in nature, these uniformity issues are avoided. The coating process in LPCVD relies on the precursor gas diffusing in a low pressure environment, and all exposed surfaces can be coated uniformly. There are many situations where this feature can be effectively used. Sputtering can coat concave surfaces, provided the aspect ratio is not too great, but the coating will be non-uniform. LPCVD processes can evenly coat these concave shapes. Substrates with these shapes are common in the lighting industry as reflectors and light collectors. Deposition Sciences, with its proprietary technology, routinely applies high reflective coatings to substrates with aspect ratios of 3:1 (reflector opening to reflector depth). DSI has developed techniques to uniformly apply coatings to the inside of completely cylindrical objects, such as glass tubes, which would not be possible with conventional physical vapor deposition processes.

There are many common applications that lend themselves to the LPCVD process. These are the coatings of spherical, hemispherical, hyperhemispherical, and dome-shaped optics. One example of a spherical optic is the ball lenses used in the telecom industry. The LPCVD process is able to provide a uniform coating over the entire surface of the sphere. If these lenses were coated in PVD, the coating would be non-uniform over the surface, and only a portion of the lens would be coated correctly. In the manufacture of ball lenses, 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.

This tried and true method of coating spheres has been developed and tested at DSI's northern California facility. Its proprietary LPCVD process routinely delivers consistent, highly durable spherical coatings. The same is true for hemispherical and hyperhemispherical optics used in the projection display and medical device markets. The LPCVD process is able to coat these optics uniformly over the entire surface or both surfaces in the case of a hollow optic.

More LPCVD applications that require uniform, highly durable, thin film coatings are for the optically transmissive domes used to cover sensors with a wide field of view. The domes for seeker heads for smart ordnance and covers for security cameras or machine vision sensors are applications whereby the devices are required to have large fields of view. The dome shape of the optic allows the light rays to reach the sensor at a near normal angle of incidence. With conventional PVD coating technology intricate masking needs to be developed to deposit the coating uniformly over the curved surface. Even after developing this intricate and expensive masking, the coating will be compromised near the edges of the dome because of the high angle at which the coating is deposited. However a coating deposited with the LPCVD process will be of uniform quality out to the extreme edges of the dome.

The LPCVD process is able to produce a wide range of coatings on many different types of surfaces. Although there are some surfaces and situations where LPCVD is not the ideal coating method, LPCVD is uniquely suited to coating substrates with complex shapes, spherical or non-flat surface topologies. The benefits of the LPCVD process are valuable to many specialized optical products.

Jay Kane, Ph.D., is Process Engineer and Bob J. Crase is Program Manager for Commercial Products at Deposition Sciences, Inc., Santa Rosa, Calif. For more information, visit www.depsci.com.