PHTN1432: Vacuum Systems and Thin Film Technology
Basic Thin-Film Techniques - Thermal Evaporation (2015W)

Introduction

Monitoring Thickness During Deposition
A lab group monitoring deposition progress during an aluminum deposition. Deposition rate as well as overall thickness must be controlled carefully during the process. In this updated lab a basic thermal evaporator using a diffusion pump is used to deposit thin films. During depostion film thickness is monitored via a quartz crystal thickness monitor.

Purposes

Pre-Lab

Bring safety glasses Failure to bring glasses to the lab will result in being denied entry and hence being marked as absent.

Beginning The Lab

The deposition will be done in the class-1000 cleanroom hence all persons entering the room MUST wear proper cleanroom attire, PERIOD. A clean pen (not pencil – it creates dust) will be available in the cleanroom as cleanroom paper for observations (see the end of this lab for a copy of the worksheet). The substrate will be supplied in the cleanroom and will be taken out in a zip-lock bag.

Gown in the outer airlock before entering the cleanroom (don’t even ASK if these are "all required"):
Cleanroom coveralls (‘bunny suit’)
Safety Glasses
Shoe covering (‘booties’)
Hair covering (bouffant)
Cleanroom gloves

Cleanroom entry protocol:

A lab group in the cleanroom
A lab group in the cleanroom using the older Edwards system. Note the cleanroom wear required to maintain a high purity environment. Depending on other uses of the lab during the term, face masks may also be required while in the cleanroom.

The room is class-1000, lets try to keep it that way! While in the cleanroom avoid the wet-bench. That apparatus contains hydrofluoric acid and is extremely dangerous.

Before We Begin … A Few Words To The Wise

IF YOU HAVE ANY QUESTIONS DURING THE LAB, ASK … THIS IS A FIRST LAB IN THE CLEANROOM AND SHOULD BE A LEARNING EXPERIENCE, NOT A DESTRUCTION SESSION. THE PROFESSOR WILL BE PRESENT SO NOW IS A GOOD TIME TO CLARIFY ANY DETAILS YOU STILL FIND ‘SKETCKY’.

Preparing the Vacuum System

The diffusion and mechanical pumps will be running since these take some time to ‘warm up’. The professor will switch ON the pumps at least an hour before the lab since warm-up takes at least 45 minutes.

For future labs, we will leave the chamber under vacuum as much as possible. For this lab, I’d rather you take the time to study the system and load the source carefully!

Preparing the Substrate

Clean the substrate with a wipe of methanol. Lay the substrates on a cleanroom wiper, apply a few drops of methanol to each, and wipe across the top using a second wipe. Load the substrates into the holders above the source deck - they will fit into the larger recessed holes (the holders are designed to accomodate both square and round substrates). AVOID TOUCHING THE SIDE TO BE METALLIZED, EVEN WITH GLOVES. Carry the substrate to the evaporation chamber and mount it face-down into the open circles in the holder.

Loading Substrates into the chamber
A photo of the substrates being loaded into the chamber. Substrates are loaded into the holder located above the source deck, filaments on the source deck are loaded with evaporants.

Loading the Evaporant (Source)

The filament will already be installed into the holder. We will use small, highly purified aluminum ‘horseshoes’ (each 29.9mg) as sources. Wearing gloves and using tweezers, carefully place the aluminum pieces onto the filament. Don’t overload the filament too much since excess aluminum will be wasted as it falls to the plate below - use the nomograph method explained in class to determine the number of pieces to add..

If a second aluminum filament is installed, ensure it, too, is loaded (this is a backup filament in the event the first fails).

WARNING: Do not EVER allow objects, dirt, etc. to fall onto the high-vacuum valve under the source deck … that is an easy way to contaminate a diffusion pump.

While the chamber is still open, turn on the Inficon thickness monitor ensure the system is operating properly. If using the XTM deposition monitor ensure the XTAL lamp is not blinking and check the crystal lifetime using the TC/CHK button. If using the XTM/2 deposition monitor ensure there are no 'XTAL FAIL' warning on the display. Change the crystal if required.

Vacuum System Operation

Close the chamber and pump-down as per standard procedure. Remember to ROUGH PUMP the chamber below 1*10-3 torr before switching to high-vacuum/diffusion pump mode. The chamber gauge will 'bottom out' when proper switchover pressure is reached - this will reduce the chance of backstreaming. If this is not possible (for example due to the need to frequently 'back' the diffusion pump due to the evolution of gases as seen on the foreline gauge), the high-vac valve may be opened extremely slowly to reduce the gas load before the high-vac valve is completely opened.

The cryotrap should also be filled with liquid nitrogen now just before the high-vacuum valve is opened. Close the roughing valve, open the foreline valve, and open the HIGH-VAC VERY SLOWLY while watching the foreline TC gauge. If the foreline pressure rises too fast, close the HIGH-VAC valve a little. Keep the foreline pressure at or below 100mTorr during this procedure. It may take several minutes to fully open the valve. When the foreline pressure no longer rises as the valve is opened it may be opened fully. Pump the chamber for at least ten minutes to reach a suitable pressure (below 5*10-6 torr) and we are ready to deposit the film.

You may safely switch on the ION GAUGE at any time when the high vacuum gate valve is FULLY OPENED since the chamber pressure at that point will certainly be below 1*10-4 torr.

Deposition

Prepare the XTM/2 deposition monitor by selecting the proper material. Press PROG and press the E key several times to highlight the FILM function. Press 2 to select film #2 then PROG again to exit program/configuration mode. Press ZERO/3 to zero the film thickness before the deposit.

Set the shutter to cover the aluminum filament as seen through the window. Set the filament selector switch to filament #2 (Aluminum). Turn the filament on and raise the power until the filament glows dull-red (about 50A for a short wire filament to begin melting the aluminum) but be sure to increase the current SLOWLY to avoid thermal stress on the filament. As you do this the chamber pressure will rise due to outgassing and oxygen will be liberated from the filament. Wait a moment for the pressure to lower then OPEN the shutter to deposit the film. Vary the filament current to maintain a deposition rate of about 5 Angstroms/sec (rate control is not as critical with aluminum as it is with other materials we will use in subsequent labs). When close to the final thickness of 500 Angstroms slow the deposition rate and finally CLOSE the shutter when the desired thickness is complete. Turn off the filament power.

Aluminum Filament during deposition
A photo of the aluminum filament during deposition as seen through the chamber window. The filament should glow an orange-pink colour during deposition. Rate must be constantly monitored and adjusted - the rate must never exceed 50 Angstroms/sec for aluminum.

Get a final reading of the actual thickness deposited from the XTM monitor, close the HIGH-VAC valve, and wait two minutes for the sources to cool before opening the VENT valve to admit air into the chamber. WATCH THE FORELINE TC GAUGE as the chamber is vented - if the high-vac valve is not tight enough the foreline will rise in which case the HIGH-VAC valve must be tightened - QUICKLY. When atmospheric pressure is reached, RAISE the chamber and remove the substrates. If you are the last group to use the system switch the diffusion pump OFF to allow it to cool but leave the mechanical pump on to back the foreline during cooling and leave the water supply on as well - the pump must cool for twenty minutes minimum. Close the chamber and ROUGH PUMP it again to a pressure below 100 millitorr - NEVER leave a chamber at atmospheric pressure, not even for an hour!

Lab Report

There is no lab report associated with this lab procedure - the procedure will be part of that followed in the subsequent lab. Lack of attendance at this preparatory lab will result in a penalty applied to Lab 5 however so attendance is mandatory.

LAB Worksheet (PDF Document) Since paper is not allowed in the cleanroom, print this worksheet and tape it to the outer door of the cleanroom (facing towards the cleanroom) in V14.