Optical Thin Films on Complex Substrate Geometries
- Friday, 01 March 2013
Anext-generation low pressure chemical vapor deposition (LPCVD) optical thin film coating process is permitting the manufacture of interference filter coatings, such as single wavelength, dual band and broadband AR, cold mirror, dichroic, and conductive. The enhanced thin film IsoDyn™ process, designed by Deposition Sciences, Inc. (DSI) is now being used to produce conformal coatings on complex substrate geometries. This capability allows for a myriad of new applications that may require uniform, multilayer coatings on complex shapes, ranging from simple ball lenses to almost any imaginable optical shape.
With broad wavelength coverage from 300nm to 5μm, the new LPCVD thin film technology opens the door for novel optic designs. Such designs may not have been considered in the past due to the limitations of more common deposition methods such as evaporation or sputtering. While excellent for some applications, these deposition methods cannot match the conformal coverage and coating uniformity that LPCVD offers for non-planar and asymmetrical optical components (Figure 1).
The IsoDyn low pressure chemical vapor deposition process is similar to technology commonly used in the semiconductor industry. It has been optimized to produce pinhole-free, low particulate, high-quality optical coatings with excellent surface quality. Scratch/ dig quality of a substrate surface is not degraded by deposition and films of low surface roughness (i.e. < 5nm) can be obtained.
LPCVD is essentially a thermal process used to deposit thin films from gas-phase precursors at subatmospheric pressures. Deposition occurs by diffusion of reactants onto a heated substrate surface, where an irreversible surface reaction takes place. The chemical reaction at the surface could be one of a number of possible mechanisms including thermal decomposition (pyrolysis), reduction, hydrolysis, oxidation, carburization, and nitridation. The hot substrate, commonly in excess of 400°C, provides the energy for the reaction to occur.
LPCVD differs from other deposition processes like evaporation, sputtering, and even atmospheric chemical vapor deposition (CVD) in a number of important and advantageous ways. Physical vapor deposition (PVD) techniques, such as evaporation and sputtering, are limited to line-of-sight geometries, and cannot be used to coat deeply recessed shapes. LPCVD, on the other hand, can easily provide uniform coatings on all substrate shapes including deeply recessed shapes and even tubes, due to its small mean free path. The mean free path, the average distance between molecular collisions, is many orders of magnitude smaller for LPCVD than for PVD. This means that there are many more collisions between atoms and molecules in the gas phase prior to encountering the substrate. While a “billiard ball” model is often used to describe PVD processes, CVD is more comparable to fluid flowing through a pipe. Put simply, with LPCVD all exposed surfaces are going to get “wet”. Furthermore, LPCVD does not require the high vacuum (very low pressures) that is needed for PVD.
When compared with atmospheric CVD, LPCVD enables more uniform conformal coatings. Due to the reduced pressure and elevated deposition temperatures used in LPCVD, thermal diffusivity is large, thus facilitating an even distribution of reactants within a given cross-section of the deposition chamber. Proper consideration of the flow conditions is one of the keys to the successful development of CVD processes (Figure 2). LPCVD is characterized by continuum flow conditions operating within the laminar regime. Reactor geometry is a critical factor to be considered in LPCVD process setup and optimization.
These fundamental properties of LPCVD enable deposition processes to be developed that provide uniform coverage on all surfaces of the substrate. This attribute has led to the wide use of LPCVD within the semiconductor industry since excellent step coverage of micron and submicron features can be similarly obtained. In contrast, the large mean free path and molecular gas behavior that characterize PVD processing provide for mainly line-of-sight deposition.