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Plasma Quest ECR Etcher, pquest

Plasma Quest (pquest) is an ECR plasma etcher primarily used for etching GaAs. It only accepts 4-inch wafers.

Picture

 

Plasma Quest Picture

Plasma Quest ECR Etch Tool

Background

ECR Plasma Etch

In an ECR plasma etcher, the plasma is created by the electron-cyclotron resonance effect. The microwaves are tuned to the cyclotron resonance frequency of electrons in the gas. They excite the atomic electrons to the point where they gain enough kinetic energy to be stripped off of the atoms, ionizing the gas. This allows plasma to be created without an electrical discharge and without increasing the temperature of the ions in the plasma. The resulting plasma can thus have a low temperature and a low density, and also has a high ionic fraction.

The RF generator applies a radio-frequency signal to the chuck that holds the sample. The signal is relative to the walls of the chamber, which are grounded. This creates a depletion region above the sample, effectively biasing the sample relative to the plasma. The ions in the plasma are thus accelerated through the depletion region by this bias, giving them the kinetic energy required for an anisotropic etch.

 

Process Capabilities

Cleanliness Standard

Because pquest is primarily used for etching GaAs substrates, the pquest is operated in the GOLD EQUIPMENT GROUP.  

Due to contamination issues, NO FLUORINE CHEMISTRY is RESTRICTED to specified weeks in a month

- Pquest will undergo a wet/dry clean sequence on the 1st and 3rd Mondays of every month.
- Etches during the following 7 days (inclusive of the 2nd and 4th Mondays) will be restricted to recipes that do not contain Fluorine, and chuck temperatures <=50C.

 (e.g. F is not allowed during the 1st and 3rd weeks of the month)

Compatible Carrier Wafers

Pquest only accept 4" wafers.   When the wafer is loaded in the chamber, it is held place by 3 teflon pins.  A minor flat cannot exist where the pin (180 deg and 45 deg each direction from the major flat) will support the wafer, otherwise the wafer will fall and possibly break.  Prime wafers are recommended since test wafers may be too small (see the image below). 

Furthermore, a clamp holds the wafer in place at the edge so that any thick, sticky layer will likely (such as a thick resist edge bead) cause the wafer to stick to the clamp and/or break. 

 

pquest carrier wafer

Selecting the proper carrier wafer will ensure you do not have problems with wafer loading on this system.  Wafers with flats at 180 or 45 deg either direction from the major flat may fall off the pins. 

100mm wafer stock to use

Recommended prime wafers: 111 p-type, 100 p-type

Avoid: 111 n-type, 100 ntype, test wafers

Also, do not add any material (copper tape, etc) to the 1cm wide crosshatched zone in the diagram. 

Mounting pieces

Since the system only supports 4" wafers, wafer pieces must be mounted to a carrier wafer (see guidelines above).  Pieces may be attached using carbon dots or tape, wet procedure PMMA or resist.  For more details on chip mounting go here.  Only a small piece of tape should be used to mount the sample, and make sure that no tape is exposed to the plasma.  Do not re-use pieces of tape. 

Carrier wafers etch as quick as your sample, and eventually become thin and brittle. To prevent the carrier wafer from breaking inside the chamber, it should not be overused. When a wafer has been etched away about 50 um, the carrier wafer should no longer be used.

Performance of the PQuest

What the pquest CAN do

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What the pquest CANNOT do

  • Pquest cannot be run with fluorine-containing recipes except 4 days before a wet clean which is done twice a month.  Check badger..  This causes contamination problems for other users. 
  • Using may high pressure interfere with the ECR causing the plasma to detach causing a fireball at the microwave window.  Five mT is a good starting pressure.

 

Process Monitoring

 <to be written>

Contact List and How to Become a User

Contact List

Refer to the equipment summary list at:  https://snf.stanford.edu/SNF/equipment

 

Training to Become a Pquest User

Please follow the procedure below to become a qualified user of the Pquest.

  1. Read all material on the SNF website concerning the Pquest.
  2. Contact someone from the pquest "Super-User" to arrange for training.  Watch the training video found here.  In your initial contact, state why you wish to use the pquest etch tool.   Prepare any questions for the super-users. 
  3. Contact and schedule a training session with one of the pquest super-users.  After successful completion of this session, you will be granted status as a full user. 

 

Operating Procedures

 

Pquest Power Supplies

 

Initial system checks

  1. Turbo pump is spinning and normal.  The following green lights on the Osaka Vacuum power supply should be on, Magnetic Suspension: Power, Levitating, Battery Check, and Driver: Normal and Rotating.  If not, or there is any red light illuminated, DO NOT USE and SHUT DOWN the system on Coral. 
  2. Microwave power supply, RF power supply, and RF tuning systems are switched off.
  3. The chiller for the chuck cooling system is switched ON, and set to "MIN".  The bath should be set and stabilized to 20 deg C.  The light on the water filter should be a solid green, with water visibly flowing through the lines.  If the light is red (or flickering red), report the problem on Coral.  Make sure there is enough fluid in the chiller -- the water level should be between the high and low lines.  If the level is low, add DI water and comment it on Coral. 
  4. The temperature controller is set to 20C.
  5. The computer screen saver is showing.  Open the keyboard and press the spacebar to wake the computer.  The system should be in idle mode, indicated by a window displaying the Idle pressure and Ar flow.  In the idle mode, the chamber is purged with 50 sccm Ar and the load lock with N2. The purge gases prevent back streaming of oil from mechanical pumps.

  6. For higher temp etching do not set the heat exchanger more that 80C.  Gas cooling may be used from 50-200C. Contact responsible people about higher temp etching.

(If the system was not left in the correct standby mode as listed by steps 2-5, gently remind the previous user about the shutdown procedure)

 

Startup

  1. Check Coral for reservations, problems, and to see if it is already enabled by another user.

  2. If the system is available and the initial system and status checks are good, you may enable the system on Coral.

  3. Turn on the tuner, RF power supply, and the Microwave power supply. Press the "Ready" button to put the microwave power supply in "Remote" mode.

  4. Set the chiller and the chuck temperature controller to the desired temperature and wait for it to stabilize.  Chuck temperatures up to 50C can be used with water cooling.  It is recommended to set the chiller about 10C lower than the desired chuck temperature.  Chuck temperatures up to 200C are possible using N2 gas cooling, however this is an advanced procedure and requires special training. 

  5. Use the light pen to click on "Run Recipe". Enter password to run recipe mode.

  6. Visually inspect to make sure there is no wafer in the process chamber. If there is, unload the wafer.

Creating and Modifying Recipes

  • You should use your own recipes as much as possible. Shared recipes can be used as guidelines. Do not make any changes to shared recipes.

  • To choose an existing recipe, click on the "Select Recipe" button.

  • To modify the selected recipe, click on the "Edit Recipe" button.

  • To modify values in the recipe, click the button with the parameter you want to modify, and enter the new parameter value. To select the recipe step to modify, use the "<<<" and ">>>" buttons at the bottom.

  • Most recipes will have the following steps:

    1. Set the backside helium pressure to the desired value (usually 10 Torr). Allow about 30 sec. to stabilize.

    2. Set the gas flow rates and the process pressures to the desired values. Allow 45 sec. to stabilize.

    3. Set the ECR power and the RF power to the desired values. This step will constitute the actual etching.

  • Tolerance delay times should be set to 15 – 30 sec. In some cases, large offsets will require tolerance checking to be turned off; however, this is not a good idea as a general policy.

  • Choose your preferred settings for wafer loading and unloading. In general, it is highly recommended to chose  'Prompt' for wafer loading, wafer unloading, and load-lock venting.  If you ask for a load and there is already a wafer in the chamber the system will break the wafer in the chamber!  The tool will be unusable.

Loading Wafers

Note: BE VERY CAREFUL when loading and unloading wafers as pquest can break wafers if not handled properly.  If it looks like wafer destruction is imminent, PRESS ESC on the keyboard to stop all mechanical motion.  System Reset should will recover the system.  You may have to try twice.   Carefully read and consider the prompts- they may change depending on system state.

Do not use the Escape Button unless it is a health and/or safety situation.

  1. Press "LL->ATM" to vent the load lock to atmosphere.

  2. Place the dummy wafer on the load arm. Make sure the major flat lines up on the slot on the load arm.

  3. Verify the is no wafer in the chamber.  Press "Load" to load the wafer into the chamber. The load lock will be pumped down.

  4. WATCH as the transfer arm moves into the chamber and the wafer is lifted up onto the pins. If there is any problem such as the wafer falling off the pins, hit "Esc." to abort the loading, perform a machine shutdown, and notify maintenance.

  5. Watch as the transfer arm moves out of the chamber and the clamp lowers the wafer down onto the chuck.

Running Recipes

  1. If the machine has recently been opened for maintenance, it will need to be cleaned and conditioned.

  2. Run oxygen plasma "oxygen3" for 10 min. with a dummy wafer on the chuck.

  3. Run your etch for 10 min. with the same dummy wafer on the chuck to condition the chamber.

  4. Remove the dummy wafer and continue with the normal procedure.

  5. Load your wafer into the chamber.

  6. Set the microwave tuning stubs to values obtained from the log sheets for previous runs.

  7. Press "Start" to run the recipe.

  8. Monitor the system as the recipe runs through its steps. Record the measured process parameters on the growth sheet.

  9. If there are any major mechanical problems during the run, press "Esc." To abort a recipe, perform a machine shutdown, and notify maintenance. It is also possible to quit the recipe by pressing the "Stop" button, or to stop a particular step in the recipe by pressing the "End Step" button.

  10. When the microwave power is turned on, the plasma should ignite. There may be a delay of a few seconds; if it is much longer than this, there is a problem. Once the plasma has ignited, tune the microwave stubs to minimize the reflected power. For a good etch, the reflected power should be less than 10 W. Note the stub set points in the logbook.

  11. After the recipe has finished, the machine will run a 60-second purge of argon to clean the chamber.

  12. Unload the wafer by pressing the "Unload" button or by answering, "Yes" to the prompt.

  13. Watch as the wafer is lifted up by the pins and lowered onto the transfer arm. Abort immediately by pressing "Esc." if there are any problems.

  14. Vent the load lock to atmosphere by pressing "LL->ATM" or by answering, "Yes" to the prompt. Select "Yes" if you are asked whether you want to leave the nitrogen purge on.

  15. Take out the dummy wafer and put on the transfer arm the carrier wafer with the sample you want to etch. Press "Load" to load the sample into the chamber. Follow the same steps as above, being sure to watch as the sample is lifted and lowered by the pins.

  16. Press, "Start" to run the recipe. Retune the microwave stubs if necessary.

  17. Unload the wafer as above. Remove the wafer from the load lock.

  18. If you have additional samples to etch, load them and run the recipes as above.

  19. When finished, press "System Reset" to pump down the load lock and return the machine to its default values. Make sure the log sheet is complete.  Do not disable the tool on badger until the load lock pump down is complete.  If disabled prematurely the LL will continue to pump.

 

Shutdown Procedure

  1. If you have changed the temperature of the chiller, return it to 20C. Return power to "Min." Leave the chiller power on.  

  2. Set the chuck temperature controller to 20C.

  3. Press the "Close" button to exit Run Recipe mode.

  4. Press the "Idle Mode" button. Press "Go Idle". This will run a constant flow of argon through the machine to keep it clean. The machine should always be left in idle mode.

  5. Press "Sleep" to turn on the screen saver.

  6. Turn off the RF tuner, the RF power supply, and the Microwave power supply.

  7. Make sure the system is in the standby condition.   Cleanup the area, and make any notes in the logbook regarding the performance of the tool. 

  8. Disable "PQuest" on badger.

 

Process Monitoring and Machine Qualification

Pquest Qualification Run

To ensure continued machine performance the pquest is regularly put through a qualification procedure meant to capture basic functionality and potential errors in the system.

 

Frequency

 

 

Procedure

 TBD

Responsibility

Performance of the qualification procedure is primarily the responsibility of the process members of the tool’s quality circle. The super-users are available to assist in times of need with running, testing, and reporting of the qualification procedure.

 

Recommendation to users with critical processes

If you have a critical etch it may be in a user’s best interest to perform their own run of the qualification procedure before entrusting their precious process wafers to the pquest.  Remember, etching is irreversible!

 

Machine Status States

Red: the Pquest is in red-light condition and should not be used. It can be in this state for any of the following reasons:

  1. The machine failed to pass the qualification procedure within spec.
  2. The machine is currently under repair.
  3. There are broken / shattered wafers in the system, or the previous user could not unload his/her waferN2
  4. The software has "locked-up"
  5. The turbo pump is OFF, or the machine cannot hold requested chamber pressure. 

 

Yellow: the Pquest is in yellow-light condition and should only be used after careful consideration of posted comments and evaluation of likely effect to your process. NOTE: if the machine is in Yellow or Green it has successfully passed the qualification procedure in the most recent test.

  1. Plasma striking problems.
  2. RF / Microwave parameters suspect (DC bias, reverse power, auto-tuning problems etc.)
  3. User reported problems or issues.

 

Green: the Pquest is in green-light condition and is free of any known problems. NOTE: this does not mean that any specific ultra-critical process will work immediately without flaw, and users should take appropriate care with their wafers.

 

Process Monitoring Results

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