Chapter 6.15

ULTEK E-Beam Evaporator

(ultek)

1.   Introduction

The ultek is an e-beam evaporation system. This system is cryo pumped and can attain a base pressure of 2e-7 Torr. The ultek is set up to hold one 4" wafer.  Users may attach smaller samples to the existing 4" sample holder, which has screws and washers, or they can design their own sample holder. An Inficon, thin-film crystal monitor measures deposition rate and accumulated thickness of the deposited films. Evaporation at hi vac is a line of sight process. The material travels from the source in a straight line and stops on the first surface it hits. Sidewalls of structures or topography do not get coated.

E-beam evaporation uses an electron beam to heat a source of material in a crucible (hearth liner).  Electrons are emitted from a heated filament and are accelerated to a high velocity by several kV. Electron current is typically small, around 100 mA. A permanent, horseshoe magnet bends and guides the e-beam in a circular path from the filament to the source.

Material selection of the crucible should be appropriate for the material being evaporated.  Crucibles made of Cu or graphite are stocked in the Microlab Office, Room 406.  Users should purchase their own crucibles, and their own source material.

The most common error is to have too much or too little material in the crucible. The crucible should be at least 1/3 full, but not more than ¾ full. If there is not enough material, it is possible to burn a hole through the bottom of the crucible and supporting copper hearth. If there is too much material it can escape from the crucible and contaminate the system. 

Material can be added to the crucible as needed. Typically the material is in the form of 1/8 inch or ¼ inch pellets (not powder, which is bad for contaminating the system).

For the cleanest deposition, the beam current should be adjusted to just melt a spot in the middle of the material. If the power is too high, all the material will melt, and then the crucible material can diffuse into it (may be ok, depending on what you are doing). As power is increased and more material is melted, the deposition rate increases, but also the amount of spitting of droplets of material increases. You need to look through the glass to see if this is happening, and then find a compromise setting that minimizes spitting, but still gives a reasonable dep rate. Typical dep rates are 1 to 5 Angstroms / second, but can be higher for some materials. Some materials have a high enough vapor pressure that a spot does not need to be melted, and it can sublimate directly from the solid. In order to look at the hot material, you need to used the welding glasses. The darker ones (rated”12”) are needed for the high melting point materials (e.g., Ta), while the one rated “5” can be used for lower melting materials (e.g., Au, Si).

After buying high purity pellets of material for evaporation, you do not want it to get contaminated. Therefore you need to check to see if the chamber is clean. If you need to clean it, then first cover the pump port with a clean piece of aluminum foil so particles don’t fall down in it since they can prevent the hi vac valve or roughing valve from sealing. Check the bottom of the shutter for built up deposits that may be flaking off and may fall into your crucible. Check aluminum foil that is wrapped around the components that need protection. If the deposits look like they may flake off, replace with new foil. If there is material on the copper hearth, you can carefully remove it with a clean room cloth, or if it is stuck on, very carefully use a clean razor blade. Finally, vacuum out all loose debris with the vacuum cleaner. Put in new clean microscope slides to protect the viewing areas of the glass bell jar from being covered over by deposits. There are two 2 in x 3 in slides mounted vertically on the side, and one 1x3 inch slide mounted at 45 degrees at the top of the chamber. Remove the Al foil that you put over the pump port.

You need to have clean gloves. Gloves quickly get dirty since people unconsciously touch their noses, cell phones, etc, and then transfer the oily, sodium rich goop to all the door handles, deck hoses, keyboards, and equipment knobs in the lab.  Therefore, put on new gloves, and then do not touch anything with your finger tips. Reserve the finger tips for handling things in the chamber (and use tools for handling things when possible).

Another common source of contamination is kapton tape. This will outgas under the radiant heating from the evaporating material source. You should only use metal clips, springs, and screws to mount your samples (no tape, or rubber bands).

Correct heat flow is needed for consistent results. The most consistent setup is with the thermal contact only between the copper hearth and the bottom of the crucible (not the sides). This assumes that you have cleaned out the bottom of the hearth with a cotton swab and vacuum cleaner, so no particles are there to hold the crucible out of contact with the hearth.  If your carbon crucible cracks, you probably had nonsymmetric heating and contact stress on the sidewall.

2.   Stand-by Condition

When the ultek is not being used, it should be left under hi vac, in the following standby mode:

AUTO button on

VENT button off

Ion gauge off

Cooling water on

e-beam power supply off

Inficon  off

3    Operating Procedure

(1)   Verify that the cooling water is on: the water wheel is turning.

(2)   Check to see that the crystal monitor is functioning.

Turn on the crystal monitor.  Press the XTAL button. The first readout will tell you the percentage of the crystal life that has been used up. Readings are unreliable when crystal use is above 90%.  Replace the crystal if needed. Spare crystals are kept between the nrc and v401 or may be procured in the Microlab office.  They are not recharged. Lab members are responsible for crystal changes and should make sure a spare is also available for the next user in case the office is closed when they need it.

(3)   Check System base pressure  (Granville  Phillips Gauge controller)

The DEGAS function is intended to improve the accuracy of readings by desorbing absorbed gases from the ion gauge anode. Do not leave the ultek unattended while the ion gauge controller is in DEGAS mode. Excessive DEGAS will eventually sag the ion gauge, requiring expensive replacement.  First turn off the filament.  Next, flip up the switch labeled DEGAS. This will begin heating the ion gauge tube's anode. Turn degas off after 5-10 minutes.  When the filament is turned back on the pressure should be 2 - 4 × 10E-7 Torr. Ion gauges should not be used to measure pressures greater than 10E-4 Torr. Ion gauge controllers are equipped with circuits that automatically turn off above this pressure. High system base pressure may mean the cryopump needs regeneration.

(4)   Turn off the ion gauge filament current.

(5)   Close the gate valve to the cryo pump.    

1.      Push the lighted AUTO button. The AUTO button will go dark and the hi vac valve will close. Listen for the hi vac valve to close.

2.      Push the VENT button. The vent button lights up. N2 enters the chamber. Venting is finished when the bell jar is loose and can be moved.

(6)   Raise the bell jar manually.

Move slowly, be careful not to bang the bell jar against the other components. Once the bottom of the bell jar is higher than the structures in the chamber you can rotate the column (pull the red handle on the column) so the bell jar goes to the back, out of the way. (Safety tip: before opening the bell jar, look at the high voltage power supply to make sure the main power switch is off. Although there is an interlock to prevent it, you just want it to be impossible for the high voltage to be on when your hands are in the chamber).

(7)   Put on new gloves.

Keep the finger tips clean as described above

(8)   Lift up the stainless steel shroud.

 Again, move slowly and guide it gently over the structures so you don’t apply high forces to anything. Set the shroud on clean aluminum foil.

(9)   Clean the chamber.

As described above.

(10)     Load your material source(s).

As described above. Use clean tweezers.  Turn the red handle of the hearth positioning screw to align the center of the crucible to be adjacent to the cathode of the e-gun. If you will be evaporating from more than one crucible, make sure you know where the marks on the leadscrew (positioning screw) are when the different crucibles are lined up with the e-gun. Make sure the shutter is closed.

(11)     Protecting the bell jar.

If your target substrate (e.g., wafer or die) has any apertures that would let evaporated material travel to the bell jar, then you need to cover it with aluminum foil. Otherwise deposits will build up on the bell jar. Make sure two new 2" x 3" glass slides are in the window, and a new 1” x 3” slide is in the 45 degree holder on top. These slides allow viewing the sources and must be replaced after each evaporation.

Failure to protect the bell jar will make it increasingly difficult to see into the system.

(12)     Replace the stainless steel shroud.

      Slowly, carefully, guide the shroud over the chamber structures so you don’t damage anything.

(13)     Make sure the gasket on the bottom of the bell jar and the mating surface on the baseplate are clean.

(Wipe with clean glove finger) and then lower the bell jar. The N2 vent pressure holds the bell jar up a little bit above the base plate, so push down on it with your left hand, so you can operate the valve control buttons with your right hand.

(14)     Pump down.

Push the vent button (lighted). The vent button will go dark.

Push the AUTO button. The AUTO button will light up.

The system will now pump down automatically, but you have to stay there to make sure it works: the roughing valve will open. When the convectron gauge reaches 200 mT, the roughing valve will close. The system will wait about 1 minute, then open the HI VAC valve to the cryo. After that happens you can turn on the ion gauge, and wait for base pressure to be reached (or, depending on what you are doing, it may be ok for you to do the evaporation when it reaches 3x10-6 T).

(15)     Program the crystal monitor.

(See the Inficon manual which is kept on top of the power supply cabinet.) Push the PROGRAM button, then step through the variables using the E  (enter) button. The Z-ratio and densities of different metals are listed in the Inficon manual and are posted on the right side of the NRC. The tooling ratio for the ultek has been estimated at 145%. All other parameters can be left at the default value. Press the PROGRAM button again to exit program mode.

(16)     Turn on the e-beam power supply.

 Flip the circuit breaker switch labeled “main power” to the on position.  Set the beam current control variac to 0. Turn the variac on.  Turn on the high voltage (push the high voltage “ON” button). The red light will turn on (may take a minute). Slowly raise the e-beam current by turning the knob on the variac clockwise. Watch all of the following: the milliammeter on the high voltage front panel, the brightness of the e-gun cathode, and the location of the brightest spot on the evaporation source (where the e-beam is hitting, will be hottest and looks brighter than the surrounding area). Put the beam on the center of the material by adjusting the leadscrew, and by flipping the toggle switch labeled “toward filament”/ “away” (whichever you want) and then turning the knob on the adjacent front panel variac to move the beam in the direction perpendicular to the leadscrew (If the voltage adjustment for the this direction does not work, press the white buttons labeled 3 next to the voltage knob. This resets the circuit breakers. If it still doesn’t work, report it on FAULTS).The main thing is that the beam should not be hitting the crucible, but only the source material. Increase the beam current slowly enough so the material has time to heat up and not lag too far below equilibrium temperature for a given beam current setting. Continue increasing the beam current until you get the desired amount of material melted, and a dep rate reading on the crystal monitor. Wait 5-10 minutes, allowing the source to outgas. Open the shutter and adjust the beam current so that deposition rate is about 1 to 5 angstroms per second (can be higher for some materials).

(17)     Shutdown.

When the desired thickness has been deposited, rotate the shutter over the crucible. Turn beam current control variac to 0. Turn the variac power off. Push the high voltage “off” button. Switch the high voltage power supply “main power” circuit breakers off.  Let cool 20 minutes. Vent the chamber. Raise the bell jar. Remove your wafer. Lift out the stainless steel shroud. Remove your crucible(s). Clean the chamber (as described earlier). Replace the 3 microscope slides. Put the stainless steel shroud back in. Pump down. Turn off the Inficon. Turn off the ion gauge. Leave the water on (it is on the lab recirc, so it isn’t just going down the drain). Make sure you leave the area clean. Some users think that it is ok for them to leave their used glass slides lying around, or even leave kapton tape with evaporated films on them stuck on the bench. These are not civilized people, and have no right to be in the lab.

5.      Regeneration

Cryopumps are captive pumps and require periodic regeneration.  In addition to normal regeneration cycles, mistakes and mechanical failures may necessitate regeneration of the cryopump.  An indication of a need for regeneration is a higher than normal base pressure or excessive cropump temperature. A cryopump temperature monitor is located in the instrument. The cryopump should be less than 20 Kelvin for proper operation. If the cryopump needs regeneration file a FAULT report via the WAND.

Refer to the comments section of the wand for additional suggestions and lab member entries.

6.      Checklist

1.      Verify water flow (waterwheel turning)

2.      Verify crystal function

3.      Verify system base pressure

4.      To vent:

1.      Turn off ion gauge.

2.      Push AUTO.

3.      Wait for valve to close.

4.      Push VENT.

5.      Manually raise the bell jar.

6.      Lift out stainless steel shroud.

7.      Clean the chamber (should already be clean if last user is a good citizen).

8.      Verify clean glass microscope slides are in place.

9.      Put crucible(s) of material in hearth.

10.  Align crucible with cathode.

11.  Position shutter over crucible.

12.  Put stainless steel shroud back in.

13.  To pump down:

1.      Hold bell jar down.

2.      Push VENT to turn it off.

3.      Wait for valve to close.

4.      Push AUTO.

14.  Program Inficon

1.      Enter density.

2.      Enter z-factor.

15.  Evaporate

1.      Verify variac set to 0, turn it on.

2.      Turn on power supply.

3.      Push high voltage ON button, then wait for the red light to go on.

4.      Slowly turn up variac to melt some of the material.

5.      Look through glass and adjust position of hot spot as needed.

6.      Adjust variac to get desired dep rate (look to make sure material is not spitting excessively, use welding glasses).

7.      Open shutter.

16.  Shutdown

1.      Turn variac to 0, turn off.

2.      Push high voltage “off” button.

3.      Turn off main power switch.

4.      Let cool 20 minutes.

5.      Turn off ion gauge.

6.      Vent.

7.      Remove wafer.

8.      Remove stainless steel shroud.

9.      Remove crucible(s).

10.   Clean chamber.

11.   Replace the 2 x 3-inch glass slides (2) and the 1 x 3-inch slide.

12.   Replace the stainless steel shroud.

13.   Pump down.

14.   Leave water running.

15.   Leave the whole work area clean.

7.      Crucibles

The following recommendations  (from Poco Graphite, a specialty material supplier) address several common crucible problems.

7.1        Melt Levels

The most common cause of crucible failures is overfilling. Overfilling can cause the melt to spill over the edge of the crucible. When a spillover occurs, a thermal hort is created between the crucible and the hearth. The resultant thermal stress causes the crucible to crack. For this reason a maximum melt level of 80% of the crucible capacity and a minimum melt level of 30% of the crucible capacity are recommended.

7.2        Crucible Contact

Another significant cause of crucible failures is cracking due to the improper seating of the crucible in the hearth. Out of round or chiseled hearths often create nonuniform mechanical stresses on the crucible walls. For the longest crucible life and for the most reproducible evaporation results, contact between the graphite crucible and the copper hearth should be restricted to the bottom of the crucible and the bottom on the hearth cavity. A circular graphite or copper shim is frequently used to achieve proper contact.

7.3       Handling

Improper crucible handling and storage also can be the source of crucible life problems. Crucibles should be handled with tongs, gloves or finger cots; never with bare hands or fingers. Used crucibles available for reuse should be stored in a dry, oxygen-free environment.

7.4        Aluminum Melts

Aluminum carbide formation affects the life of crucibles used for aluminum evaporation. The aluminum carbide forms a transparent, yellowish film on the surface of the aluminum. When the film covers the entire surface of the aluminum, the evaporation rate is reduced to near zero. The presence of this phenomenon is indicative of excessive crucible temperature. The beam power should be reduced to minimize the formation of aluminum carbide.

Rev. 00 – 1/07, C. Keller – Rewrite of the chapter.

Rev. 01 – 1/07, W. Flounders –  Added four informational paragraphs from Poco Graphite as Section 7 – Crucibles.