High Vacuum, Thermal PVD System
High Vacuum, Thermal PVD System
High Vacuum, Thermal PVD System
Creating a configurable, water cooled, metal-melting, physical vapor deposition system in my bedroom.
Creating a configurable, water cooled, metal-melting, physical vapor deposition system in my bedroom.
Background
Background
Background
Industrially, physical vapor deposition systems can be accomplished with many different techniques and phenomenon but they all operate on the same fundamentals. When a metal is brought into a free flight state (either by ion bombardment or vaporization) inside a vacuum, that metal will fly unimpeded until it strikes a surface. This metal is then deposited onto the surface with fairly variable bonding strengths depending on many different factors. This technique allows you to deposit very thin layers of metals (or some non-metals) onto a different part or surface, and is critical to processes such as computer chip making and optics coatings.
This project was initially inspired by this phenomenon occurring my graduate research's laser welding vacuum chamber. Metal melted and vaporized by the laser would be immediately deposited onto all visible surfaces and proved to be a huge issue when trying to maintain clear optics for a >1kW laser. As annoying as it was, I found the idea fascinating.
Two designs were considered for this project: a magnetron sputterer or a thermal PVD system. Magnetron sputtering systems use high voltage to ionize a gas which is then constrained by magnetic fields to the donor surface. The highly energetic ionized gas will "knock off" atoms of the donor surface (often a metal) which will fly and deposit onto visible surfaces. This is highly controllable by gas flow, voltage, current, etc but is somewhat complicated to build.
Instead, thermal PVD systems simply melt and vaporize metals to re-deposit on visible surfaces. This is theoretically simple to build but requires high currents and creates a lot of excess heat. This type of PVD system was chosen for this project due to the simplicity, though I plan to build a magnetron sputterer too.
Industrially, physical vapor deposition systems can be accomplished with many different techniques and phenomenon but they all operate on the same fundamentals. When a metal is brought into a free flight state (either by ion bombardment or vaporization) inside a vacuum, that metal will fly unimpeded until it strikes a surface. This metal is then deposited onto the surface with fairly variable bonding strengths depending on many different factors. This technique allows you to deposit very thin layers of metals (or some non-metals) onto a different part or surface, and is critical to processes such as computer chip making and optics coatings.
This project was initially inspired by this phenomenon occurring my graduate research's laser welding vacuum chamber. Metal melted and vaporized by the laser would be immediately deposited onto all visible surfaces and proved to be a huge issue when trying to maintain clear optics for a >1kW laser. As annoying as it was, I found the idea fascinating.
Two designs were considered for this project: a magnetron sputterer or a thermal PVD system. Magnetron sputtering systems use high voltage to ionize a gas which is then constrained by magnetic fields to the donor surface. The highly energetic ionized gas will "knock off" atoms of the donor surface (often a metal) which will fly and deposit onto visible surfaces. This is highly controllable by gas flow, voltage, current, etc but is somewhat complicated to build.
Instead, thermal PVD systems simply melt and vaporize metals to re-deposit on visible surfaces. This is theoretically simple to build but requires high currents and creates a lot of excess heat. This type of PVD system was chosen for this project due to the simplicity, though I plan to build a magnetron sputterer too.
Magnetron Sputtering
Image: Denton Vacuum
Thermal Evaporation
Image: SAM Sputter Targets
Design Choices
Design Choices
Design Choices
With the theory in mind, I could start coming up with constraints to guide the design.
The first consideration would be how to actually melt metal inside the chamber. Industrial systems often use a tungsten boat as these allow you to melt nearly any metal without damaging the tungsten itself. After some searching, I found a supplier for cheap 14mm x 60mm tungsten boats to use in the system.
In order to pass current through this boat, I would need a fixture that could withstand the extreme temperatures. I did not want to risk melting my electrodes or be limited by run time so early on decided that I would build a water cooling system for the jaws. This would be quite the challenge as I had to get pressurized water into and out of the chamber without leaking into the sensitive vacuum system.
With these constraints, I completed the design of the system. The full system was built as a unit mounted to the top ISO160 flange such that it could simply be slid in and out of the vacuum chamber body. This allowed me to easily make modifications to the PVD system without modifying the rest of the chamber, as well as easily swapping out the chamber's functionality.
The flange started as an ISO160 to KF-16 adapter but had some major changes made to it. Two additional KF-16 ports were welded into the flange to fit the two high current feedthroughs. Additionally, stainless hose barbs were welded through the flange to allow coolant in and out. The central port is currently used for the vacuum gauge, but is planned to eventually be used as a port for custom rotary feedthrough. This will allow me to "shutter" the sputtering and thus have much greater control over the process.
The electrode assembly is the most complex assembly of the project and is discussed below.
With the theory in mind, I could start coming up with constraints to guide the design.
The first consideration would be how to actually melt metal inside the chamber. Industrial systems often use a tungsten boat as these allow you to melt nearly any metal without damaging the tungsten itself. After some searching, I found a supplier for cheap 14mm x 60mm tungsten boats to use in the system.
In order to pass current through this boat, I would need a fixture that could withstand the extreme temperatures. I did not want to risk melting my electrodes or be limited by run time so early on decided that I would build a water cooling system for the jaws. This would be quite the challenge as I had to get pressurized water into and out of the chamber without leaking into the sensitive vacuum system.
With these constraints, I completed the design of the system. The full system was built as a unit mounted to the top ISO160 flange such that it could simply be slid in and out of the vacuum chamber body. This allowed me to easily make modifications to the PVD system without modifying the rest of the chamber, as well as easily swapping out the chamber's functionality.
The flange started as an ISO160 to KF-16 adapter but had some major changes made to it. Two additional KF-16 ports were welded into the flange to fit the two high current feedthroughs. Additionally, stainless hose barbs were welded through the flange to allow coolant in and out. The central port is currently used for the vacuum gauge, but is planned to eventually be used as a port for custom rotary feedthrough. This will allow me to "shutter" the sputtering and thus have much greater control over the process.
The electrode assembly is the most complex assembly of the project and is discussed below.
Electrode Assembly
Electrode Assembly
Electrode Assembly
A few important constraints guided the design of the electrode assembly:
The jaws needed to firmly clamp onto the tungsten boat.
There should be a short thermal path through solid metal from the tungsten boat to the water blocks.
The two jaws needed to be electrically isolated but still rigidly mounted to a connection plate that will not deteriorate or melt under the extreme light irradiation.
Ideally, the jaws should be flexible so that different sizes of boat could be used.
The result of these constraints is the below design. The two electrodes are identical and mirrored across the centerline. Each electrode has a primary body that attaches the tungsten boat via a clamp plate on the top, the water block via a clamp plate on the back, and the mounting plate on the bottom. All parts were made out of (scrap) copper where possible for a high thermal mass and conductivity.
To achieve electrical isolation while bolting the assembly together, two different insulators were needed. A PTFE sheet is used to prevent the body from directly touching the electrode mounting plate. To prevent the bolt from shorting the two together, custom PTFE bushings are used to isolate the bolt from the mounting plate.
The heating is powered by a salvaged microwave oven transformer with a secondary rewired for low voltage, but very high currents.
A few important constraints guided the design of the electrode assembly:
The jaws needed to firmly clamp onto the tungsten boat.
There should be a short thermal path through solid metal from the tungsten boat to the water blocks.
The two jaws needed to be electrically isolated but still rigidly mounted to a connection plate that will not deteriorate or melt under the extreme light irradiation.
Ideally, the jaws should be flexible so that different sizes of boat could be used.
The result of these constraints is the below design. The two electrodes are identical and mirrored across the centerline. Each electrode has a primary body that attaches the tungsten boat via a clamp plate on the top, the water block via a clamp plate on the back, and the mounting plate on the bottom. All parts were made out of (scrap) copper where possible for a high thermal mass and conductivity.
To achieve electrical isolation while bolting the assembly together, two different insulators were needed. A PTFE sheet is used to prevent the body from directly touching the electrode mounting plate. To prevent the bolt from shorting the two together, custom PTFE bushings are used to isolate the bolt from the mounting plate.
The heating is powered by a salvaged microwave oven transformer with a secondary rewired for low voltage, but very high currents.
Using the System
Using the System
Using the System
Initial tests proved to be too effective.
A small amount of copper was added to the boat which was very quickly melted. Unfortunately, it wetted well enough that attempts to remove the ball broke the tungsten boat.
Initial tests proved to be too effective.
A small amount of copper was added to the boat which was very quickly melted. Unfortunately, it wetted well enough that attempts to remove the ball broke the tungsten boat.
First melted metal with PVD system
Thermal Evaporation
Image: SAM Sputter Targets
The next test greatly increased the amount of copper in the boat. This copper was very quickly melted and allowed to sit at above it's melting point for <30 seconds. However, too much had been added. An oscillatory motion formed due to the size of the pool creating a path with a much lower resistance than that of the boat. This would then create cold spots causing the copper to then move to that region, repeating continuously. Eventually the pool of copper made its way around the side of the boat and onto the bottom of the boat. In this position, with such close proximity to solid surfaces, it very rapidly formed a thick deposition layer that flaked off into extremely thin copper foil. The result of this test is below.
The next test greatly increased the amount of copper in the boat. This copper was very quickly melted and allowed to sit at above it's melting point for <30 seconds. However, too much had been added. An oscillatory motion formed due to the size of the pool creating a path with a much lower resistance than that of the boat. This would then create cold spots causing the copper to then move to that region, repeating continuously. Eventually the pool of copper made its way around the side of the boat and onto the bottom of the boat. In this position, with such close proximity to solid surfaces, it very rapidly formed a thick deposition layer that flaked off into extremely thin copper foil. The result of this test is below.
Extreme deposition on the copper jaws.
Deposited foil. Thin enough to float away at the smallest breeze.
Unintentional deposition on an internal camera and light.
Very consistent layer left on viewing window glass.
With the lesson learned to minimize melt pool volume, the next test used a small piece of commerical pure nickel (melting point ~1450 deg C). This also effectively deposited on multiple surfaces in the chamber.
With the lesson learned to minimize melt pool volume, the next test used a small piece of commerical pure nickel (melting point ~1450 deg C). This also effectively deposited on multiple surfaces in the chamber.
Nickel on copper.
Very consistent layer left on viewing window glass.
End Result
End Result
End Result
As seen above, this PVD system works extremely well for the rapid melting and depositing of metals. Copper and Nickel have so far been tested but with those temperatures proven, most common metals could be deposited with this system.
Future work involves a shutter system to control deposition more accurately, a method of moving samples to more consistently coat them, and a variac to accurately tune the boat temperature.
As seen above, this PVD system works extremely well for the rapid melting and depositing of metals. Copper and Nickel have so far been tested but with those temperatures proven, most common metals could be deposited with this system.
Future work involves a shutter system to control deposition more accurately, a method of moving samples to more consistently coat them, and a variac to accurately tune the boat temperature.