Introduction
In physical vapor deposition system, material is deposited on to the desired substrate. There are many ways by which one can do this, most used one are thermal vapor deposition or sputtering. Here I am going to discuss thermal vapor deposition.
At certain temperature every material changes into vapor state. These vapor can then be deposited into any substrate of choice. For example aluminum (Al) melts at 933K and evaporates at ~2800K. These vapors can then be deposited into substrate which is kept at much lower temperature. For heating the metal usually Tungsten heaters are used. When very high current is allowed to pass through Tungsten it heats up. Tungsten is used because of its low electrical resistance (52nohm.m), high melting point (~3700K). These Tungsten heaters are in the form of boats to hold the melted metals. In my system, I use a 5V power supply capable of providing very high current. My system is described below
Statement of Operation of the my tool is at the end of this document and can be useful to understand how these systems work.
Depositing Metals
1) Lead (Pb)
Its fairly easy to deposit lead as it melts at 600K and evaporates at 2022K. Tungsten (W) boat can be used for evaporating lead as it does not form ally with Pb
For measuring thickness, the system use a quartz crystal who's resonant frequency changes as metal deposit on it. This change in resonance frequency is used to calculate the thickness of deposited metal. Since each metal has different frequency, one need to specify this into the thickness meter to correctly calculate the thickness of corresponding metal. The parameters for all the metals can be found out in the "Thickness monitor manual". For Pb these parameters are -
Density - 11.3
z - factor (acoustic impedance) - 7.81
Tooling factor - 1.0
(this should be determined by first actually calculating deposited thickness and then back calculating tooling factor. For now, I am just using 1.0)
In thickness monitor system, I have set layer 8 for lead, were all this parameters are already entered.
2) Aluminum (Al)
Aluminum wets the tungsten and make alloy so usually alumina coated tungsten boast is used for Al deposition. However, because alumina causes thermal resistance usually 20%-50% more power is required to melt Al than using only tungsten boat.
First, I tried alumina coated tungsten boat and only when I use the boat for the first time I could manage to get 0.1nm/sec deposition rate at 13A, however increasing the current to 14A will cause the circuit break then power supply will shut off. Using the second boat for second time did not even gave 0.1nm deposition rate at 13A and it used to get circuit break if I try to increase the current to 14A. So I concluded that my system simply can not provide enough power to deposit aluminum using alumina coated boat.
Next, I tried tungsten boat. At around 10A the aluminum melted. The moment aluminum melts it wets the boat decreases the resistance of the boat and which cause sudden decrease in the boat temperature (I could see the glow stops when aluminum melts and the recover in 2-3sec). The melted aluminum keep on flowing over the boat covering entire boat. The deposition rate was first 0.5nm/sec which decreased to 0.2nm/sec possibly because all the alumina wetted the boat and there was nothing to evaporate.I further increased the current to 12A and could see increase in the deposition rate. I used 2 pellets and was able to deposit 88nm thick aluminum. I hoping since my boat is already wet with aluminum now, next time when I will use the same boat I will be able to deposit more aluminum.
Shadow Mask
I am making shadow mask for myself so that I can make many devices per lot without manyally sticking all sample substrate with the sample holder. For this I am using 0.050" thick stainless steel sheets. I will water jet cut windows in this laser cut sheet for mask. I am buying the sheet from McMaster product number - 1773T15. For mask I will order a laser cut or etch mask which will be sandwiched between these to stainless steel sheets. Sample size is (12.3,13.16).
Size of center hole - 6.5mm
Side hole size -4.0mm
Thickness of mask sheet - 0.006" stainless steel
8. Pump maintenance
In physical vapor deposition system, material is deposited on to the desired substrate. There are many ways by which one can do this, most used one are thermal vapor deposition or sputtering. Here I am going to discuss thermal vapor deposition.
At certain temperature every material changes into vapor state. These vapor can then be deposited into any substrate of choice. For example aluminum (Al) melts at 933K and evaporates at ~2800K. These vapors can then be deposited into substrate which is kept at much lower temperature. For heating the metal usually Tungsten heaters are used. When very high current is allowed to pass through Tungsten it heats up. Tungsten is used because of its low electrical resistance (52nohm.m), high melting point (~3700K). These Tungsten heaters are in the form of boats to hold the melted metals. In my system, I use a 5V power supply capable of providing very high current. My system is described below
Statement of Operation of the my tool is at the end of this document and can be useful to understand how these systems work.
Depositing Metals
1) Lead (Pb)
Its fairly easy to deposit lead as it melts at 600K and evaporates at 2022K. Tungsten (W) boat can be used for evaporating lead as it does not form ally with Pb
For measuring thickness, the system use a quartz crystal who's resonant frequency changes as metal deposit on it. This change in resonance frequency is used to calculate the thickness of deposited metal. Since each metal has different frequency, one need to specify this into the thickness meter to correctly calculate the thickness of corresponding metal. The parameters for all the metals can be found out in the "Thickness monitor manual". For Pb these parameters are -
Density - 11.3
z - factor (acoustic impedance) - 7.81
Tooling factor - 1.0
(this should be determined by first actually calculating deposited thickness and then back calculating tooling factor. For now, I am just using 1.0)
In thickness monitor system, I have set layer 8 for lead, were all this parameters are already entered.
2) Aluminum (Al)
Aluminum wets the tungsten and make alloy so usually alumina coated tungsten boast is used for Al deposition. However, because alumina causes thermal resistance usually 20%-50% more power is required to melt Al than using only tungsten boat.
First, I tried alumina coated tungsten boat and only when I use the boat for the first time I could manage to get 0.1nm/sec deposition rate at 13A, however increasing the current to 14A will cause the circuit break then power supply will shut off. Using the second boat for second time did not even gave 0.1nm deposition rate at 13A and it used to get circuit break if I try to increase the current to 14A. So I concluded that my system simply can not provide enough power to deposit aluminum using alumina coated boat.
Next, I tried tungsten boat. At around 10A the aluminum melted. The moment aluminum melts it wets the boat decreases the resistance of the boat and which cause sudden decrease in the boat temperature (I could see the glow stops when aluminum melts and the recover in 2-3sec). The melted aluminum keep on flowing over the boat covering entire boat. The deposition rate was first 0.5nm/sec which decreased to 0.2nm/sec possibly because all the alumina wetted the boat and there was nothing to evaporate.I further increased the current to 12A and could see increase in the deposition rate. I used 2 pellets and was able to deposit 88nm thick aluminum. I hoping since my boat is already wet with aluminum now, next time when I will use the same boat I will be able to deposit more aluminum.
Shadow Mask
I am making shadow mask for myself so that I can make many devices per lot without manyally sticking all sample substrate with the sample holder. For this I am using 0.050" thick stainless steel sheets. I will water jet cut windows in this laser cut sheet for mask. I am buying the sheet from McMaster product number - 1773T15. For mask I will order a laser cut or etch mask which will be sandwiched between these to stainless steel sheets. Sample size is (12.3,13.16).
Size of center hole - 6.5mm
Side hole size -4.0mm
Thickness of mask sheet - 0.006" stainless steel
Metal Evaporator Operation Manual
Last update: March 19, 2013 by Prashant Patil
You should be trained before you use this machine. Please email prashant.patil@cba.mit.edu for training informationMachine Overview
This machine can
evaporate metal under high vacuum while measuring the
thickness by quartz crystal microbalance. Up to four different
materials can be loaded in one batch. Since its thermal
nature, metals with relatively low melting temperatures are
more reliable. Following is verified list.
Reliable deposition:
gold, silver, aluminum, copper
Marginal deposition: chrome
Very tough control: titanium
Unsuccessful: platinum
Marginal deposition: chrome
Very tough control: titanium
Unsuccessful: platinum
All the metals are
stored in the nitrogen box beside the refrigerator.
Procedure
Let’s assume the
machine is in vacuum status.
1. Check nitrogen
cylinder open. Check source control ‘off’. Check current dial
‘zero’. Then, press ‘vent’ button in the control panel.
You can hear hissing sound. Venting takes less than 2~3 minutes.
You can hear hissing sound. Venting takes less than 2~3 minutes.
2. Open the chamber
door using handle. If venting is completed, the door opens
without any resistance.
After open, press ‘seal’ button in the control panel. This stops nitrogen supply.
After open, press ‘seal’ button in the control panel. This stops nitrogen supply.
3. Relocate the
venting port plug.
During venting, port
plug pops out as shown in the picture. If contaminant goes
into the venting hole, that may interfere the valve operation.
Therefore, relocate the port plug right after the chamber
opening.
4. Attach your sample to the sample holding
disk.
Kapton or carbon tape is convenient to use.
However, if it is exposed to the high temperature source for
a long time, the sticky adhesives remain residue. In such a
case, use screws and clips instead.
5. Plug the rod of sample holding disk into
the slot. Then, the sample is facing down toward the source
as shown in the picture.
6. Install the boat and put metal pellets.
6. Install the boat and put metal pellets.
Loosen the
knuckles and install the boat. Put 1~2 pellets.
Using source selector, adjust the boat position.
Adjust electrode arm position. It must make a intimate contact to the knuckle body.
Using source selector, adjust the boat position.
Adjust electrode arm position. It must make a intimate contact to the knuckle body.
7. Close the chamber, and press ‘cycle’ to
vacuum the chamber.
Typical
operation vacuum is <2 1="" about="" e-6="" hour.="" it="" mbar.="" span="" takes="">
8. Pressing ‘data’ button changes the
indication mode from rate to layer, density…… Go to ‘layer’.
9. Choose the layer(preset of material properties) according to your metal source using arrow buttons.
9. Choose the layer(preset of material properties) according to your metal source using arrow buttons.
Currently, 1-Au, 2-Ti, 3-Cr, 4-Al, 5-Pt,
6-Cu, 7-Ag.
10. After choosing layer, press ‘run’ button
twice.
Whenever
pressing ‘run’ button, the status changes between ‘close’
and ‘open’. By pressing twice, it will switch
‘close’à’open’à’close’. This resets the
thickness monitor to zero.
The ‘open’ and ‘close’ only mean the thickness monitor(QCM) operation. It is not synchronized with physical shutter.
The ‘open’ and ‘close’ only mean the thickness monitor(QCM) operation. It is not synchronized with physical shutter.
11. Once pressure reaches operation
condition, put source control ‘on’ (Man - manual)
12. Gradually increase the current using current dial while seeing the source boat through the window.
12. Gradually increase the current using current dial while seeing the source boat through the window.
The number
in dial is fake. Only read the current meter.
Typically, you can see the boat glowing at around 8 in the current meter. Up until this point, you can increase the current quite fast.( in about 10~20 seconds)
From 8 point, be patient to watch out the status of the metal. Low melting point metal start to melt around this point. If properly melted, the liquid swirls in the boat. If it does not melt, increase the current slightly(about 1~2 each time) and watch.
It is good to memorize typical operation point of the metal. In my observation, gold and aluminum melt around 10~12.
Typically, you can see the boat glowing at around 8 in the current meter. Up until this point, you can increase the current quite fast.( in about 10~20 seconds)
From 8 point, be patient to watch out the status of the metal. Low melting point metal start to melt around this point. If properly melted, the liquid swirls in the boat. If it does not melt, increase the current slightly(about 1~2 each time) and watch.
It is good to memorize typical operation point of the metal. In my observation, gold and aluminum melt around 10~12.
13. After seeing stable swirling of the
metal source, open the shutter by shutter handle and press
‘run’ button in the thickness monitor simultaneously.
14. See the deposition rate in the thickness monitor. Adjust current dial to desired deposition rate.
15. When desired thickness reaches, close the physical shutter first, then press ‘run’ button to close the QCM.
16. Decrease the current using dial.
14. See the deposition rate in the thickness monitor. Adjust current dial to desired deposition rate.
15. When desired thickness reaches, close the physical shutter first, then press ‘run’ button to close the QCM.
16. Decrease the current using dial.
Decrease
into half, wait about 30 seconds. Repeat these cycle 2~3,
then put the current zero.
17. Turn the source control ‘off’.
18. Cool the substrate at least 15 minutes.
18. Cool the substrate at least 15 minutes.
If opened
immediately, metal film may delaminate.
19. Vent the chamber and remove your sample
and metal source.
20. Vacuum the chamber.
20. Vacuum the chamber.
Metal Pellets stock
Al, Sn, Au, Ni, Cr, In, Ti, Ag, Zn,Pb
Boat Stock-
In
Notes
1. Aluminum foil cover
All inside
wall of the chamber and shutter is covered by aluminum foil.
Regularly, replace this aluminum foil. Otherwise, detached
metal flakes may contaminate the chamber.
2. Cleaning
Regularly,
clean the chamber with cleanroom vacuum cleaner. Especially,
remove metal flakes thoroughly. They frequently stick to
valves/sealing degrading vacuum quality.
3. Windows shield
Window is shielded by a couple sheets of
slide glass. Regularly replace these slide glasses.
Otherwise, it is very hard to see the inside through the
window.
4. Door rubber sealing
Regularly
clean the rubber sealing of the chamber door. First, swipe
the sealing with IPA soaked wiper. Clean and dry thoroughly.
Then, spread a small amount of high vacuum grease(Dow
Corning).
5. QCM replacement
Using ‘data’
button in the thickness monitor, go to ‘usage’. This
indicates the usage of QCM. When new QCM is installed, this
value is around 20. Do not exceed 100.
If QCM needs
replacement, pop out the holder.(Then, thickness monitor
automatically turns off.) Remove old QCM and install new one
as shown in the picture.(electrode on top) Plug in, then you
can see the thickness monitor on again.
QCM is sold
in many venders including Fil-Tech and Maxtek.(ex. Maxtek
P/N 103200-2)
6. Metal boat
Typically,
tungsten boat is used. Order them to Kurt J. Lesker.
Tungsten Boat - P/N-EVS2B015W
Tungsten Boat - P/N-EVS2B015W
Alumina Coated
Tungsten Boat - P/N-EVS2BAOW
For the following
materials, use alumina coated tungsten boat instead.
Aluminum: It easily wets tungsten boat.
Chrome: It forms alloy with tungsten.
Aluminum: It easily wets tungsten boat.
Chrome: It forms alloy with tungsten.
7. Circuit breaker
Circuit
breaker locates just above the turbo pump control unit. The
breaker trips when too large current is drawn, or the circuit
is shortened. In such a case, press the white button. It is
unclear the roles of other two buttons.
8. Pump maintenance
In case of
roughing pump, change the oil regularly. Typically once a
year.
Turbo pump needs cooling. Regularly check the coolant/chiller. However, even if the chiller fails, turbo can be operational under proper air cooling. Install a fan and leave the cabinet door open.
Turbo pump needs cooling. Regularly check the coolant/chiller. However, even if the chiller fails, turbo can be operational under proper air cooling. Install a fan and leave the cabinet door open.
9. Manual control
The machine
parameters can be manually controlled by hidden menu. Press
‘no’ button in control panel. Then, you can see several
submenus by arrow button. Pressing ‘yes’ selects current
indication.
-Sequence run
=> default screen
-Set point
-Manual mode
-Adjust mode
-System ID
-Set point
-Manual mode
-Adjust mode
-System ID
Using Manual
mode, each valves/relays can be operated.
Chiller maintenance
Note:
The maintenance
consists of two parts.
- Circulation water refill
- Recovery from dried-out status
- Emergency operation during chiller broken-down
- Circulation water refill
- Recovery from dried-out status
- Emergency operation during chiller broken-down
1) Circulation water refill
Interval:
every week
Purpose:
Metal evaporator
chiller is not well sealed. Therefore, the water loss is fast,
which means weekly checkup required. The compressor is
air-cooled. No need of external coolant.
Procedure:
1. Open the lid of
the reservoir.
2. If water level is low, refill potable mineral water.
2. If water level is low, refill potable mineral water.
2) Recovery from dried-out status
Interval:
Purpose:
If the water
reservoir remained below minimum level for a long time, the
chiller is automatically turned off and put into standby. It
must be recovered by resetting.
Procedure:
1. Refill the water
first.
2. Push the red reset button on the right side of the unit.
2. Push the red reset button on the right side of the unit.
3) Emergency operation during chiller broken-down
Interval:
Purpose:
This chiller
frequently experiences compressor failure. During fixture,
there is no circulating water. However, fortunately,
turbomolecular pump of metal evaporator had run without
chilling water making no problems. But, as a precaution, set
up an air-cooling platform.
Procedure:
1. Open metal
evaporator cabinet door.
2. Place a fan, and blow the air continuously until chilling water is recovered.
2. Place a fan, and blow the air continuously until chilling water is recovered.
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