THERMAL EVAPORATOR Document prepared by Pavithra Prashanth Equipment technologist Center for Excellence in Nanoelectronics Indian Institute of Science mspb12@gmail.com
Bell jar Substrate table Crystal Monitor Substrate holder and sample Path of vapor Clamps connected to high current source High vacuum created by Diffusion pump backed by Rotary pump Filament boat filled with evaporant
A thermal evaporator uses an electric resistance heater to melt the material and raise its vapor pressure to a useful range. This is done in a high vacuum, both to allow the vapor to reach the substrate without reacting with or scattering against other gasphase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber. Thermal evaporation is the simplest way of depositing material onto a substrate. One major disadvantage of this is that a lot of material is lost in the process. Optimization Purity of the film depends on the purity of the source material and the quality of the vacuum. Thicknesses of the film vary due to the geometry of the chamber. Collision with the residual gases aggravates no uniformity of the thickness. Advantages It is simple and cheap. Less substrate surface damage. Excellent purity of films. Disadvantages Limited to low melting point metals.
It s not possible to evaporate the Dielectric materials. Filament limits the amount of material that can be deposited. Density is poor. Adhesion is poor. Step coverage is more difficult to improve. Specifications of the system Uses tungsten or Molybdenum filaments to heat evaporants. Ultimate chamber pressure 5x10-5 mbar. Typical filament currents are 100-200Amps. Exposes substrate to visible or IR radiation. Maximum deposition thickness that can be achieved is 600nm. Substrate temperature can be increased up to 150 o c.
Heating sources for Thermal Evaporation
Crystal Monitor Quartz crystal is used to monitor the thickness of the deposited film and also to control the rate of evaporation. Crystal has to be cleaned or changed periodically. Steps to use Crystal Monitor Switch ON the crystal monitor. Select the film number depending on the material you are going to deposit. Enter the density of the material. Acoustic impedance Tooling factor When you start the evaporation press the Start button(set the thickness display to zero and open the shutter). Once the desired thickness is achieved close the shutter mechanically. Press the stop button.
Material guide for Thermal Evaporator
Vacuum Techniques Thermal Evaporator Thermal Evaporator make Vacuum techniques Crystal monitor make Model DTM -10 Heating filaments/boats Tungsten, molybdenum Materials evaporated Low melting point metals (Al, Cu, In, Au, Ag..) Pressure in System 5X10-5 m bar
Operating procedure for Vacuum technique Thermal Evaporator Precautions Only the trained and authorized users are permitted to operate the instrument themselves. Check if all the valves are CLOSED, before switching ON the system. Procedure 1. Check if all the valves are closed. Switch ON the system mains.
2. Switch ON the Rotary pump. Open the Gas ballast. Also switch on the Pirani gauge (GH 1). Press to on rotary pump Green light glows if rotary pump is on Press to on diffusion Green light glows if diffusion pump is on. Green light glows if rotary pump is on Green light glows if Diffusion pump is on LT indicator HT indicator Press to on rotary Press to on diffusion pump
3. Close the Gas ballast after 5 minutes. When the pressure drops below 0.02 mbar, open the Backing valve slowly. (The pressure increases and again starts decreasing). Roughing valve Backing Valve 4. Once the pressure reaches 0.02 mbar, open the coolant water supply. Switch ON the Diffusion pump. The diffusion pump needs to be heated for about 20-25 minutes. Pirani gauge Penning gauge
5. Meantime, open the air admittance valve slowly, hoist up the chamber. Clean the chamber and the boat/filament contacts, replace the glass near the window, check the contacts, load the metal (Al, Cu, etc.,) into tungsten boat/filament, load the samples, hoist down the chamber and close the air admittance valve. Air Admittance valve 6. After about 20-25 minutes Diffusion pump will become operational. Close the Backing valve and open the Roughing valve slowly. Switch on to GH2. Wait till the Pirani gauge reads 0.01mbar.
7. Once this vacuum is achieved, close the roughing valve and open the Backing valve slowly. Then open the Baffle (main) valve slowly. On the Penning gauge (Ring 1). 8. Then pour about 5 liters of Liquid Nitrogen, Wait pressure of about 5X10-5 mbar is reached on penning gauge (Ring 2). Note: The penning gauge should not be kept on continuously. 9. If the evaporation of the metal is to be done at a certain temperature, the Radiant Heater switch needs to be turned ON after the above told pressure is reached and the temperature needs to be set. The set temperature can be seen on the temperature controller. 9. Once the required pressure is reached switch on the crystal monitor and enter the proper values, depending upon the material to be deposited. Crystal monitor
10. Switch on the LT /HT Mains. Increase the voltage slowly. During this time the shutter should be closed. The tungsten filament (or tungsten boat) becomes red hot and the metal starts melting or wets the filament. Open the shutter now and increase the voltage. Hoist up/down Radiant heater switch LT and HT switch LT and HT mains 11. Once the metal gets completely evaporated, decrease the voltage to ZERO, switch off the LT Mains. After 5 minutes, close the baffle valve and switch off the diffusion pump. Switch off the Penning gauge.
12. Allow the substrate to cool down to room temperature. 13. Close the Backing valve. Open the air admittance valve slowly, hoist up the chamber and remove the samples, hoist down the chamber, close the air admittance valve. 14. Open the roughing valve for about 15 to 20 minutes, to get a pressure of 0.01 mbar which can be seen on the pirani gauge (GH 2). This is done to keep the system always in vacuum. 15. Close the Roughing valve. Open the Backing valve. 16. After 10 minutes, close the Backing valve and open the Gas ballast for 5 minutes. 17. Close the Gas ballast and switch of the Rotary pump. 18. Switch off the mains.
Aluminum film thickness measured using Crystal Monitor and Dektek profiler Crystal monitor Thickness in nm Dektek Profiler Thickness in nm 52 53 83 82 91 94 93 98 108 112 120 113 193 189 237 235