Electron Gun Beam Coater with IAD

As said in prevous pages, the thermal evaporation and sputtering are classified in Physical Method (PVD). Evaporation use thermal and sputtering as tools. The detailed mechanism is described in related chapters.

In this page, discussion will narrow down to a Thermal E-Gun IAD coater. Although, no obvious cue indicated which year and who invented the thermal evaporation method to deposite a thin film. It is believed to be beginning around 1840~50's. In 1890's, two important progress lead optical thermal evaporation into a more commerical practice. One is Nahrwold & Kundt deposited a thin film in a vacuum environment. The other is Toylor founded to coat a thin film in optical glass.

Vacuum
Why vacuum? Since the target material is sublimated into atom or molecule particles, this small particles must pass a distance to condense on the substrate surface. This particle is so light and tiny. If the space is full with gas, the gas molecular will be the obstacle to stop the target material particles. The target material can't attach to the surface firmly. The film will also content impure gas moleculars to decrease the function of desired. A cacuum system consisted by two parts a vacuum chamber and the pump. Chamber is contented space and must be sealed when pump suck out the gas. The pump must be very powerful to such most of gas in chamber. Before 1920's, the vacuum machine is not mature. Basically only laboratory will make vacuum enviroment. Its quantity and quality are not good. High cost was also the problem.

After the invention of rotary and diffusion pump, the high power and efficiency pump machine make the vacuum more easily to get. The capacity of Duffussion pump (with series installation of Rotary pump) now can pump the vacuum to 10^-7 to 10^-8 torr. (The vacuum in universe space is 10^-12 torr) A thin film coater commonly use 10^-4torr as working enviroment. It depends on your target material type and the quality you desired.

In tradition, the vacuum level was divided into 4 classification. Rough vacuum is the range of 760~1 torr. Medium vacuum is the range of 1~10^-3 torr. High vacuum is the range of 10^-3 ~10^-7 torr . Ultra high vacuum is the range of 10^-7 torr~.

Advanced Cryo pump work with Rotary pump can pump the vacuum to level of 10^-9 torr. But this pump is rarely used in thin film coater. For processing a thin film, some gas is needed to react with electron beam gun or ion gun. Usually 10^-3 ~10^-5 torr vacuum is enough for thin film deposition.

A pump system usually consists with two types of pump. The preliminary pump usually Rotary pump. It will pump vacuum to 10^-3 level. A secondary pump (usually Diffussion pump, or Cryo pump) continuous pump the vacuum to 10^-7 level. The leakage is the most difficult problem with vacuum system. The chamber, pipe, connection, pump, oil ring all possible have leakage risk. To find the leakage is difficult and need high technology leak hunting instrument.

Electron Beam Gun
From the history progress, the Thermal evaporation coating use electron beam gun from 1940. Only 10 years to 1950's, the Electron Beam Gun became a prevalence method in high class optical coater.

There are some advantages make this method prevalence. The electron beam can control the evaporation spot size and location. Get a good target material consumption and homogenous thin film thickenss. With high kinetic energy, the target particle will get high momentum and attach to substrate surface firmly. In 1968, Hanks developed 270° Electron Beam Gun. This progress improved the contamination problem. This is most suitable coating method in high class optical thin film.

For optical thin film, two dielectric materials is usually required to compose a multi-layers thin films. Dielectric material need very high thermal temperature to sublimate. Amorphous requirement is also needed. These characteristics make the sputtering system not suitable to make high class optical thin film until 1990's. After 50 years development, Electron Beam Evaporation is comparely mature and low maintance and operation fee for high class products.

Until 1990's, A company developed Ion Beam Sputtering Depostition, IBSD system. This is the first sputtering system commerically produce the high end optical thin film with dielectric target material. However, high machine cost make the product very expensive. Only very high end product (more than 100 layers deposition), like DWDM, Laser Mirror in ingyro scopei, will use this sytem to produce.

After IBSD machine selled in market, many companies developed new coater base on Electron Beam Gun one year later also can produce the these DWDM, Laser products. The price of new E-Gun coater only the 1/2 to IBSD coater. The position of E-Gun evaporation did not be harmed.

Ion - beam Assisted Deposition, IAD
As E-Gun became the domination method in evaporation deposition. Focus turned to the additional or added method to help E-Gun evaporation film better. Such as ARE, BRE, Plasma Assistance, IAD. (Some of these methods can be used in other PVD method. )

Among these methods, the IAD is widely adapted by coater producers for high level coaters. Two important factors. 1. The IAD is added an independent ion source. Easy install and maintance. 2. The ion with high mass (compare to target material atom) and high kinetic energy, can easily knock the target atom into the surface of substrate firmly. The film quality and adhesive force is good.

The ion source use intense magnetic fields and substantial electron currents, with low maintenance, gridless ion sources produce a very high current density at lower energies than is practical with gridded sources.

Monitor system
Monitor system is important to control the thickness of the thin film layers. A thin film layer (no matter high index layer or low index layer) thickness is in nanometer scale. How is the machine stop the layer deposition and start another target material layer?

Two factors is crucial to this problem. First is how to measure correct thickness of thin film layer in nanometer. Second is how sensitive of the coater to stop the film deposition once the thickness is reached.

Remember that, all scale in nanometer. It is a very small dimension for human to detect and control. Three methods is adapted in measurement of thin film thickness. Optical Monitor, Quartz Monitor, and Time Monitor.

Optical monitor use a spectroptometer installed in coater and monitor the thin film deposition on dummy glass. If some optical character meet the previous setting parameter of dummy glass, it means the thickness is reached. The coater can stop the E-gun and Ion source working and make the deposition stop to this layer. Another layer of deposition need a new dummy(or re-calculation new parameters). How many dummy glass installed in the coater basically decided the layers number of the coater can make.

Quartz monitor is also widely used. When thin film depositing on quartz, the weight will change. With a current passing through, the frequence will change in direct proportion relation. It can detect the thickness of deposited thin film precisely.

Time monitor is used in non-optics system, like MBE, MOCVD. In evaporation system, the evaporation amount in chamber of target is unstable. The evaporation velocity is also unstable. Time control only used in some low level optical coater like AR or metallic reflection mirror.

DOME:
Dome is to support the glass substates hanging on chamber. Dome will rotate when the thin film evaporated form crucible. It will increase the uniformity to the dome products if dome can rotate. Uniformity of the film thickness is most important to whole processing. Althought a bigger size dome can make more batch quantity, but the big size also make the bad uniformity which will fail the products. Its size is base on the chamber and uniformity you want. It also decides the quantity of products. Uniformity direct relates the yeild rate of the products. The balance between quaitity and uniformity is an important consideration when choicing chamber size (vacuum requirement is important too)

There are many types of dome and dome rotation model. No best type in practice. It must depand on your products, your chamber, your mechanism allocating. The revolution, revolution-rotation type is commonly used in thin film coater.

Crucible system:
When E-gun evaporate the target material, the crucible which content the target material can not be melted. Crucibles are made by high melting temperature material, like tantalum (Ta), molybdenum (Mo). An dielectric thin film filter usually use two target materials to compose high and low index layers. Sometimes multi-layer (20-80 layers) deposition is also required. Several crucilbes to form a rotating system is necessary. So one empty can rotate to another one. There are many types of crucible arrangement. How to let E-gun consum the target material efficient and homogenous and without contamination will be an important consideration for crucible system.