Thin-film deposits are useful for a lot of different things. They are needed for everything from the mundane household mirror to the complex lab environments which use thin-film for quantum confinement superlattice structures. Thin films are also used for electronics manufacturing for things like thin-film transistors, resistors and capacitors.
This article is regarding high-voltage sputtering techniques as a means to achieve a thin-film coating. However, there are several other methods of thin-film deposition such as thermal evaporation, pulsed laser deposition and spin coating just to name a few. The method required depends on what you are trying to achieve. Sputtering is a versatile method because it requires a low working temperature and the concepts involved aren't too hard to understand. Implementation however can be pretty expensive depending on how you get your materials.
I originally started researching sputtering because I want to make some electrochromic glass. Hopefully there will soon be a project page on how to make it. There are many factors that made me postpone working on the electrochromic glass. The cost requirements were spiraling out of control as I got deeper into it and started tallying up all the necessary equipment. The chemicals between the glass panes aren't so tough to make or buy, but the really difficult part to acquire is the ITO glass (indium tin oxide) or some similar transparent conducting glass sheet.
Transparent conductive glass is made by applying a thin film of conducting material. Often what is used is a compound called Indium-Tin-Oxide, or ITO. Most of the ITO glass manufacturers are located in China. I was able to get two sheets about 1.5ft by 2.5ft in size from a manufacturer in china. They gave them to me for free as a sample, but it cost me about $150 just to freight ship it from the plant in china. There was no way around that. That's where I started getting into sputtering as a means of creating my own ITO glass.
Basically, to make the the ITO glass: get a the substrate material (ITO), put it in a vacuum and bombard it with argon plasma ions. This will dislodge the ITO atoms which shoot off at the glass and make a film that is conductive. It's do-able at home, but there are some special pieces of equipment that are required:
Without an ultra high vacuum environment you will not be able to create the plasma required for deposition. In order to create the proper vacuum you need to perform two stages. The second stage will create a "high vacuum". This is accomplished by using a common lab vacuum like the ones below. The vacuum will pump air out of the chamber it is attached to. The pump outputs this into the open air of the room it sits in. You can find these for anywhere between $150 to $500. Ebay usually has some good quality used ones.
Next is the first stage which creates the "ultra high vacuum". This is accomplished with a turbo-molecular pump which gets the pressure waaaay down to a usable setting. They are very pricey at around $1400. Below you can see a cross section on the left which shows the multiple blades that compose a vacuum like this. The implementation is described in the diagram in the middle. The "backing pump" here refers to our 2nd stage vacuum pump in the notes above. Without the 2nd stage pump the turbo could not achieve the ultra high vacuum because the pressure differential is too great between the inlet and outlet. On the right you can see this type of vacuum in a typical setting.
If you're planning on creating an ultra-high vacuum then the vacuum chamber you choose and how you construct it will be very important. The vacuum chamber should be made out of stainless steel because of its ability to hold a high vacuum. Stainless steel can hold a high vacuum because it is a low-outgassing material. Every material holds onto some amount of air. More porous metals hold onto a lot more are. The more air trapped inside the material the more outgassing will occur when you are trying to obtain an ultra high vacuum. The air trapped inside the metal is sucked into the vacuum chamber preventing you from achieving the ultra high vacuum pressures necessary.
Here are some various type of chambers available on Ebay at the time of this writing:
For some scientific procedures you have to be very careful about achieving specific vacuum pressures and preventing any contaminants from entering the system. The vacuum required for the deposition process does not have to be perfect. You can follow some of the typical techniques used as shown below, but if the procedures are too cost prohibitive just do your best with what you have. Keep in mind that the more strict your requirements, the more expensive it will be. What we are trying to achieve here is a pressure which is low enough to allow the plasma to form.
To further prevent outgassing problems the following techniques are used:
- Use low-outgassing materials such as glass or stainless steel.
- Create a chamber with the smallest possible amount of surface area exposed to the vacuum.
- Moisture is a big contributor to outgassing. Before trying to achieve high vacuum pressures, the chamber should initially be heated at 200 to 400 degrees C while the vacuum pumps are pulling a vacuum within the chamber. I think the ideal way to do this would be to use an induction heating coil on the inside of the chamber which is connected through the chamber walls using high vacuum voltage feed-through connectors. Ideally, the chamber should go through offgassing any time after it is opened up. Though I'm curious to know if you could get away with not baking it each time.
- For all seals on connection points of the chamber, special all-metal high vacuum seals are used. These are one-time seals - they have knife-edges on the stainless portions which sandwich over a copper ring. The knife-edges bite into the copper creating a seal which is adequate for ultra-high vacuum pressures.
- All surfaces inside the chamber should be as smooth as possible to avoid pits or crevices. These can result from weld joints. Weld joints should be smoothed over as much as possible. Professional chamber manufacturers use an electropolishing procedure which is basically the opposite of electroplating. It removes a thin layer off the surface of the chamber which makes it very smooth.
- Try not to have any bolt heads exposed on the inside of the chamber as this can trap gas between the bolt head and the chamber.
Typically, if you are trying to create argon plasma you need to get the vacuum chamber down to a pressure of about 10-6 Torr. After that is achieved, you allow some argon gas to flow into the chamber until the pressure is raised to about 10-3 Torr.
To allow for all these gas exchanges and airflow you need to have your various apparatus connected through special vacuum tubes and seals. The tubes used in a vacuum chamber is also made of stainless steel. The connection seals at each end of a tube is called a vacuum flange. The typical vacuum flange used in this case is the Conflat, or CF flange. This is seen below to the left. As mentioned before, each face of the two mating ends of the CF flange has a knife edge on it to bite into a soft copper gasket. This creates a seal which is adequate for obtaining ultra high vacuums. Another common type is the KF or QF. See this one on the right. This is the ISO standard for quick-release flanges. The name comes from Klein Flange (KF) or Quick Flange (QF). The KF flange has a rubber seal around a metal ring. Then the clasp fits around the flange and locks everything tight.
Other things that are required for the chamber are instruments and windows. Instruments and voltage probes are attached to the chamber using vacuum feedthrough accessories. These can have many different shapes and sizes according to their purpose. Some of the twist and turn or slide in and out while some of them are stationary and only used for voltage. Here are a couple of high voltage feedthrough items:
Windows are called viewports in the vacuum world. They are just the same as a flange except instead of an extending tube or a cap, the viewport is made of glass on its face. The mating surfaces are the same as any other flange.
The target is made of any material that you want to deposit onto the substrate. In the case of ITO glass a substrate of ITO is used. These targets come in discs about 2 inches to 3 inches in diameter and cost about $300.
To hold the substrate in place you need what is called a source gun. The source gun (magnetron) also has a magnetic field which holds the excited argon atoms away from the surface of the substrate. That keeps the substrate from burning up.
Buying it online (eBay) you could get it for about $1500. You can also make one yourself using strong magnets in the correct pattern. Here are a couple examples of what a magnetron sputtering source gun looks like:
The source gun has to be water cooled as the plasma will warm it significantly. To keep it water cooled you need a vacuum feedthrough that has a pipe which can accommodate the size of the tubing which leads up to the source gun. If you buy a source gun then the feedthrough tube is usually included with the purchase. A $50 water pump can accomplish the task of supplying a steady flow of cooling water to the head of the source gun.
The substrate is what you are depositing the thin-film onto. This can be a silicon wafer, glass, metal...etc. In manufacturing of potato chip bags, sputtering is used on plastic bags to coat the insides with a metal film. In the case of ITO the substrate is usually glass, but there are certain plastics that can be used for flexible transparent conducting elements. The substrate is attached to a holder at the top of the chamber. It is held in place by a vacuum or sometimes with adhesives.
To develop a uniform film the substrate holder is sometimes rotated using a rotary feedthrough attached to a stepper motor on the outside of the chamber.
To excite all the atoms of argon you will need to set up a high voltage electric field through the source gun. That creates the plasma out of the high-vacuum argon environment. This should be about 600 volts (not too high) at 1 to 2 amps. You can find voltage supplies like this on eBay or straight from a company specializing in them.
There are two types of voltage supplies you can use for sputtering: DC, or direct current, and RF, alternating current at high frequency. The DC method is effective, but the deposition rate is slow, and because the bombardment of the target is continuous, the target can overheat and cause structural damage which reduces the life span of the target material. RF is much more effective because it uses the target more efficiently. High powered magnets hold the free electrons close to the surface of the target. Holding all the free electrons in one place increases the likelihood of ionizing the argon gas molecules. As a result of the high concentration of ions, there is more bombardment of the target material. This allows the target material to be deposited more quickly on to the surface of the substrate.
You can get a DC power supply in this range for about $1200. You can get an RF power supply for about $2000.
The Argon can be bought from a welding supply store. The tank costs between $75 and $150. Once you buy the tank you can refill it for about $40. You will also need a high-vacuum valve to connect the argon tank to the chamber. You can get these types of valves for $150 to $300.
Other How-To articles:
- ITO deposition and contact fabrication: http://www.rit.edu/kgcoe/ue/thesisguidelinespages/2006_Jianming_Zhou.pdf
- ITO deposition on Plastic substrate: http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2729&DID=155341&action=detail
- ITO deposition on color filters: http://www.ece.ust.hk/~eekwok/publications/2004/JSID.pdf
- Run – to –Run Control of ITO Deposition Process: http://www.eecs.umich.edu/~impact/Publications/SIDpaper.pdf
- Study of ITO for Novel Optoelectronic Devices: http://www.betelco.com/sb/phd/index.html
- Method of preparing an ITO target: http://www.patentstorm.us/patents/6187253/description.html
- Forum entry on creating your own gold target: http://www.biotech.ufl.edu/EM/data/poundit.html
- Vacuum Pumps (Wikipedia): http://en.wikipedia.org/wiki/Vacuum_pump
- Ultra High Vacuum (Wikipedia): http://en.wikipedia.org/wiki/Ultra_high_vacuum
- Vacuum Flange (Wikipedia): http://en.wikipedia.org/wiki/Vacuum_flange
- Ion Gauge for Vacuum pressure sensing (Wikipedia): http://en.wikipedia.org/wiki/Ion_gauge
- Electropolishing - creates a smooth surface on the chamber walls (Wikipedia): http://en.wikipedia.org/wiki/Electropolishing
- Turbomolecular Pumps (Wikipedia): http://en.wikipedia.org/wiki/Turbomolecular_pump
Where to buy equipment:
- Vacuum pressure sensor - Ion Gauge search (eBay): http://shop.ebay.com/i.html?_nkw=ion+gauge&_sacat=0&_trksid=p3286.m270.l1313&_odkw=ion+gauge+cable&_osacat=0&bkBtn=1
- Huntington - Vacuum chambers and accessories: http://www.huntvac.com/
- Denton Vacuum - Targets, chambers...etc: http://store.dentonvacuum.com/
- Books related to sputter deposition: http://astore.amazon.com/guad-20?_encoding=UTF8&node=10
- ITO powder material info: http://www.reade.com/Products/Oxides/indium_tin_oxide.html
- Pressure conversion chart: http://wiki.xtronics.com/index.php/Pressure_Conversion_Table