General Introduction

-- Optical thin film layers are common in the natural world. Their effects produce colors seen in soap bubbles and oil slicks, as well as in some animals. For example, the light collecting tapetum lucidum of many nocturnal species and the photophores of bioluminescent squid (e.g. the Bobtail squid). In many cases, iridescent colors that were once thought to result from planar layers, such as in opals, peacocks, and the Blue Morpho butterfly, turn out to result from more complex periodic photonic-crystal structures. (Courtesy from Wikipedia)

Optical Thin Film is related to two fields. optical thin film Optics and optical thin film Deposition (coating) Technology.

Thin film optics

Thin film is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin film optics, the thickness of the layers of material must be on the order of the wavelengths of visible light (about 500 nm). Layers at this scale can have remarkable reflective properties due to light wave interference and the difference in refractive index between the layers, the air, and the substrate.

...optical thin film optics is based on the interference phenomenon when light passing through two different (or more) material layers. When light pass through these optical thin film(s), it will alter in many characteristic. Including transmittance, reflection, absorption, scattering, polarization, phase change....ect. Its basic principles can be interpreted by physic electromagnetic wave theory. Via Maxwell equation, we could understand the haracteristic of light interact with the different material interface. The classic interpretation is well developed from electromagnetic wave theory. The key ponint of this discipline is how to applicate it.

...Design the optical thin film layers to meet your requirement became the first target of this discipline. Two popular methods are widely used to design the optical thin film layers: Novel Monitoring Method and Vector Method. Before the computer age, the Novel Monitoring Method is more suitable for human to design the optical thin films. It can be done by hand drawing and calculation. Vector Method need tremendous calculation and is not a practical at that time. After the computer entering the human life, most software is base on the Vector Methods which can provide more precisely result.

...After WWII, optical thin film optics applied to many optoelectronic devices more widely than before. Such as interferometer, camera, telecope, binoculars, microscope, optical engine, monitor, projector, optical communication device, artificial satellite, optical sensor, missile, illumination, decoration, .....The optical thin film optics became a very important role in optics discipline. You can easily find a good textbook to detailed understand relative knowledge.

Optical Thin Film Deposition Technology.

...Please note that, optical thin film coating is a part of optical thin film deposition technology. There are many other optical thin film types applied to many aspects other than optics. Some film applied to electrical characteristics. Some films applied to mechanic characteristics. Some of them can change the chemical corrosiveness. Most important of all maybe applied in semiconductor industry. Such as Nucleation, Grain Growth, Coalescence, Film Growth, Filling of Channels,.. These semiconductor process widely use optical thin film deposition technology toA achieve. The application of optical thin film is so widely and can't be listed here. Only optical optical thin film deposition technology will be discuss here.

...The optical optical thin film existing in nature world is hard to adapt and re-assemble to the device we desired. It is so thin ( its thickness scale measured in nanometer, nm ) make it impossible to adapt it form nature world. All usable optical thin film must be made by artifical means.

There are two classification to "creat" an optical optical thin film: Liquid Forming Method and Vepor Depositon Method.



Liquid Forming Method: It is on the basis of merging the substrate in liquid. Via chemical or electrochemistrial reaction to form a optical thin film on the substrate.

Erosion: Let substrate merged in the hydrochloric acid or nitric acid solution. There will be a thin silicic acid layer gererated on the surface. Bake and dry the substrate, the layer is an Anti-Reflection optical thin film. Solution Deposition: We can separate the the metal-ion from the metal alloy merging in proper electrolyte solution. And by reducing agent (reductant) effect, we can reduct / deposite a metal optical thin film upon the substrate. Such as Cu, Au, Ni, Co optical thin film.

Electroplating:The target metal connect to positive electrode, the substrate connect to negative electrode. Merging with a proper electrolyte solution. When current pass through, the traget will be separated to atomic state and deposite on negative electrode (substrate). Such as metel reflection layer (mirror) could be made.

Sol-Gel: The sol-gel method is a chemical solution deposition technique to produce a metal oxide optical thin film. It usually start from a chemical solution (as "Sol" named here) or colloidal particles to generate an integrated gel. The precursors are usually metal alkoxides and metal chlorides. After undergoing hydrolysis and polycondensation reactions to form a colloid gel. The sol is in a formation of an inorganic network containing a liquid phase. Using dip or spin coating method, the gel can uniformly spread over the substrate. To dry the substrate and the film material will make the product you desired. Such as AnitReflection, beamsplitter, ITO , Protective optical thin film.

Langmuir - Blodgett :A Langmuir-Blodgett film contains one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing (or emersing) the solid substrate into (or from) the liquid. A monolayer is added with each immersion or emersion step, thus films with very accurate thickness can be formed. The monolayers are usually composed of amphiphilic molecules (see Chemical polarity) with a hydrophilic head and a hydrophobic tail (example: fatty acids). (courtesy from Wikipedia) It is applied in solar energy device, light emitting device....

Liquid Phase Epitaxy: Heating the solute in the solvent. Make it a homogenized solution.Spread on the surface of a crystalline substrate and cool down. it will generate a epitaxy optical thin film. It is applied in seconductor laser, optoelectro material (GaAs, InP) optical thin film.

Vepor Depositon Method.
In "vapor deposition", there also be classified into two fields of PVD(Physical Vapor Deposition) and CVD (Chemical Vapor Deposition), depending on the different principles causing film deposition. A PVD method evaporates or sputters (physical method) a material, which could be added by a gaseous plume or beam to deposit films on the glass or substrate. A CVD method uses reactive, chemical compounds (chemical method) that decompose on a heated substrate. The materials are often organo- or hydrido-compounds that pyrolyse at relatively low temperatures into a non-volatile (film) component and a pumpable vapor/gas. Both methods sub-divide into a variety of techniques with auxiliary mechanisms to deposite the films.

Chemical Vapor Deposition (CVD) is a chemical process used to produce high-purity optical thin film. In a typical CVD process, substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. It is widely used in semiconductor industry. It is also used to deposite optical optical thin film in optics industry.

For the technology today, many added high technology methods is in use to speed reaction or deposition steps. Like Plasma, Laser, Electron Cyclotron, Synchrotron Radiation, Plasma Impulse. It make this method produce a high purity and high strength optical thin film.

MOCVD

CVD method is not the major technique adapted by ETAFILM. No further introduction will be described. You can easily find more information in Internet.


Physical Vapor Deposition (PVD) is a physical process to evaporate or sputter target material to deposite on substrate surface.
It has three basic steps to depostie a optical thin film.
I. Sublimate solid state of target material to vapor state.
II. Atom or molecule or ion in vapor state passing through the vaccum space to the surface of substrate.
III. Deposite target material on the surface of substrate to form a optical thin film.

Different physical techniques are used in PVD method. Each technique can be divided into more sub-techniques which apply different mechanisms to achieve its goal.
Such as
A. Thermal Evaporation Deposition
.....- Resistive Heating
.....- Laser Deposition
.....- Arc Deposition
.....- Electron Beam Gun Evaporation
.....- RF- Heating
.....- Molecular Beam Epitacy, MBE

B. Plasma Sputtering Deposition.
....- Planar Diode Sputtering Deposition (DC Sputtering Deposition)
....- RF Sputtering Deposition
....- Dual Cathodes Sputtering Deposition
....- Triode Sputtering Deposition
....- Magnetron Sputtering Deposition

C. Ion Beam Sputtering Deposition, IBSD.

The detailed PVD Technology introduction will be described in following webpages.