Thermal Evaporation Procedure

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THERMAL EVAPORATOR

Document prepared by

Pavithra Prashanth

Equipment technologist

Center for Excellence in Nanoelectronics

Indian Institute of Science

[email protected]
mailto:[email protected]:[email protected]:[email protected]


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Bell jar

Substrate table

Crystal Monitor

Substrate holder andsample

Path of vapor

Clamps connected tohigh current source

Filament boat filledwith evaporant

High vacuum created byDiffusion pump backed byRotary pump


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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 gas-

phase 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 alot of material is lost in the process.

Optimization

Purity of the film depends on the purity of the source materialand the quality of the vacuum.

Thicknesses of the film vary due to the geometry of thechamber.

Collision with the residual gases aggravates no uniformity ofthe thickness.

Advantages

It is simple and cheap. Less substrate surface damage. Excellent purity of films.

Disadvantages

Limited to low melting point metals.


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Its 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 5x10-5mbar. Typical filament currents are 100-200Amps.

Exposes substrate to visible or IR radiation. Maximum deposition thickness that can be achieved is600nm.

Substrate temperature can be increased up to 150oc.


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Heating sources for Thermal Evaporation


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Crystal Monitor

Quartz crystal is used to monitor the thickness of thedeposited 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 thethickness display to zero and open the shutter).

Once the desired thickness is achieved close the shuttermechanically.

Press the stop button.


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Material guide for Thermal Evaporator


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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 5X10-5m bar


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Operating procedure for Vacuum technique Thermal

Evaporator

Precautions

Only the trained and authorized users are permitted to operatethe instrument themselves.

Check if all the valves are CLOSED, before switchingON the system.

Procedure

1. Check if all the valves are closed. Switch ON the system mains.


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2. Switch ON the Rotary pump. Open the Gas ballast. Also switch

on the Pirani gauge (GH 1). Press to on rotary pump Green lightglows if rotary pump is on Press to on diffusion Green light glows

if diffusion pump is on.

Green light glows if

Diffusion pump is onGreen light glows if

rotary pump is onLT

indicatorHT indicator

Press to on rotaryPress to on

diffusion pump


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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).

4. Once the pressure reaches 0.02 mbar, open the coolant watersupply. Switch ON the Diffusion pump. The diffusion pump needs

to be heated for about 20-25 minutes.

Roughing valve

Backing Valve

Pirani gauge Penning gauge


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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.

6. After about 20-25 minutes Diffusion pump will becomeoperational. Close the Backing valve and open the Roughing valve

slowly. Switch on to GH2. Wait till the Pirani gauge reads

0.01mbar.

Air Admittance valve


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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 ofabout 5X10

-5mbar 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


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10. Switch on the LT /HT Mains. Increase the voltage slowly.

During this time the shutter should be closed. The tungstenfilament (or tungsten boat) becomes red hot and the metal starts

melting or wets the filament. Open the shutter now and increase

the voltage.

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.

LT and HT

mains

Hoist

u /down

Radiant heater

switch

LT and HT

switch


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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 thechamber, 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.


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Aluminum film thickness measured using CrystalMonitor 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

Thermal Evaporation Procedure

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