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Wet Bench Operating Instructions

Wet bench operating instructions for wbnonmetal, wbnitride, and wbmetal.

Safety

Chemical Hazards

You must read the Material Safety Data Sheets for these chemicals and understand safe chemical handling procedures.

We are required by the Stanford University - Utilities Services (Environmental Quality and Water Efficiency) to distribute the Best Management Practices (BMP) fact sheet, you'll find the link here in the Stanford University - Utilities Services folder.
Please read the fact sheet and reduce the amount of pollutant for example by using smaller quantities of chemicals.

HF-based chemicals may cause serious, delayed tissue damage upon skin contact. The mechanism of action is the depletion of calcium from muscle and bone by free fluoride, which can readily pass through skin. If skin contact with HF-based chemicals is suspected, rinse the affected area thoroughly, and then immediately apply calcium gluconate gel to the affected area, following the SNF guidelines for first aid for HF exposure in the SNF Lab Manual, Part II. HF-kits containing tubes of gel (along with instructions for use) are located in plastic bags at each wet station.

Equipment Hazards

Input power is 208 volts AC, three phase at 30 amps. Only qualified personnel are allowed to work on the electrical parts of this system. If any electrical problems are seen with this system, press the EMERGENCY POWER OFF button, shut the system down, and contact Maintenance staff.

Process Hazards

General process hazards involve handling of chemicals and materials which come into contact with chemical used at this station. Wet benches are potentially the most dangerous operations in the lab. Be sure you understand all hazards and proper handling procedures before working at any wet bench.

Interlocks

The wet benches are equipped with several interlocks. It is important to bear in mind, however, that no system is fool-proof; there is absolutely no substitute for a thorough understanding of the operating procedures, maintenance procedures, potential hazards, and safety issues associated with the wet benches.

For the hot pots:

    • Heaters cannot be turned on if the bench is not enabled.
    • If the level sensor detects that the acid level in the hot pot is too low, the heater will shut off and cannot be turned on.
    • When DRAIN/ASP is on (indicator lit), the HEATER ON/OFF button is disabled.
    • A thermal sensor prevents the hot pots from being aspirated if the temperature is 120 degrees C or above. Any attempt to do so will result in an alarm. As a failsafe, in case the temperature sensor itself is damaged, the signal from the sensor is continually monitored for electrical continuity. If damage is suspected, the heater will be disabled and all four digits on the temperature display will flash. If this happens, call Maintenance.
    • The heater cannot be turned off unless the timer is reset. This prevents someone from turning off the heater during a clean cycle.
    • The temperature set point, undertemp limit, overtemp limit, process time, and prewarn time are all programmed into the controller. Only staff has access to modify these setpoints.

 

Alarms

The hot pots, the dump rinsers, the spin-rinse dryer, and the plenum all drain into the general lab acid waste neutralization (AWN) system. These modules may be used only when the AWN is functioning. When there is a problem with the AWN, the yellow light (above wbnonmetal and wbflexcorr-1) will flash.
When the AWN warning system is flashing, no processing at any of the acid wet benches is allowed. If a cassette sits in a chemical bath, finish the process, place the cassette in a dump rinser but don’t start the water rinse. If a cassette sits in a dump rinser, stop the water rinse while filled up. If you are draining chemicals from a hot pot, stop the drain. Don’t start the spin-rinse dryers.

Preparation 

Gloves

When working with acids, yellow chemical resistant Nitrile cleanroom gloves should be worn over latex gloves; vinyl gloves should then be worn over the yellow Nitrile cleanroom gloves (for a total of three pairs of gloves). The yellow Nitrile cleanroom gloves provide chemical protection. The vinyl gloves protect the wet bench from potential contamination from the yellow Nitrile cleanroom gloves. Gloves must be worn in this way when transferring cassettes in and out of acid baths. In addition to full acid gear, gloves should be worn like this for pouring chemicals. After handling chemicals, vinyl gloves must be changed before handling cassettes.

For general use when not working directly with acids, vinyl gloves can be worn directly over latex gloves: namely, when unloading cassettes from the dump rinser or loading and unloading wafers into the spin/rinse dryer. The station-dedicated cassettes, handles, the inside of the wafer boxes and their lids, are all to be kept free of contamination. A glove touching anything else is considered contaminated. Remember, the benchtop and the control panels are considered contaminated. Change gloves frequently and especially whenever any contamination might be suspected. New gloves are much less costly than your time and your experiment. Always follow the guidelines in the SNF Contamination Control and Wafer Handling Procedures.

Additional Chemical Protection Gear:  face shield and chemicals apron

Always wear full safety protection (safety glasses, face shield, yellow chemical resistant Nitril gloves, chemicals apron) when working with chemicals, which includes transferring chemicals from the storage area to the wet bench and back,  pouring chemicals, placing a sample in and out of chemical baths, start rinsing the sample in Di water, draining the etchant, and cleaning the bench.

 

Tweezers

Tweezers used for loading wafers into wbclean1and2 cassettes should be Teflon-coated stainless steel. They should be decontaminated according to the standard lab procedures for tweezers cleaning and their dedicated use should be for "dirty non-metal" applications. Wafers should then be unloaded directly from the station-dedicated cassettes into the furnace boats using dedicated vacuum wands at the furnace station or "clean non-metal" tweezers.

Cleanroom Wipes

Cleanroom wipes are to be kept off the benchtop, because they can get sucked up into the back of the bench, into the acids and into the exhaust system. This poses a fire hazard and may damage the exhaust system. When using wipes, keep them off to the side or on top of the bench shield.

 

Equipment Setup

Hot Pot 3150 Process Controller Functions

 

Hot pot 3150 controller

 

The process timer is in the upper left portion of this controller. The time is set by default to 20 minutes, which is the normal time for a sulfuric clean process. START begins the timer countdown. STOP pauses the timer. RESET resets the timer to 20 minutes. Because of the heater interlocks, press RESET when turning the heater off.

The heater and aspirator controls are in the lower left portion of this controller. HEATER ON/OFF controls the heater switch. DRAIN/ASP controls the aspirator. The three indicator lights are as follows: HEATER ON, LOW LEVEL, and DRAIN/ASPIRATOR. When any of these indicators is lit, the corresponding function or interlock is active. During normal operation and heating of the hot pot, only the HEATER ON indicator should be lit.

The process temperature readout is in the upper right portion of this controller. For normal operation, the PROCESS TEMPERATURE NORMAL indicator (green) should be lit. When the process temperature is outside of pre-programmed operating range, the PROCESS TEMPERATURE RANGE OUT indicator (in red) will be lit.

The process temperature setpoint controls are located in the lower right portion of this controller. Temperature setpoints are to be adjusted only by staff members.

If the power is turned off to the station, the memory will be stored for ~10,000 hours.

Timers

The timers for the hot pots have been preset at 10 or 20 minutes. Press the start button, the alarm will begin sounding one minute (the prewarn time) before the timer times out. To silence the alarm, press STOP. Press RESET to reset the timer.

There is a user-set process timer for each of the HF tanks, the timers are left at 30 seconds by default, but can be set by the operator. Each of the three digits on the LED is set by the SET and RESET buttons beneath it.
Timer 30 seconds

Startup

  • Users are not allowed to make a reservation for wbnonmetal, first come first serve.
  • Ensure that the main power to the bench is on (upper left on the control panel).
  • Check the exhaust gauge (magnehelic) for sufficient exhaust, it should read greater than 0.50.
    Exhaust Gauge
    (located near the floor, on the right side of the bench). 
  • Check that the acid waste neutralization (AWN) system is functioning. When there is a problem with this system, the yellow warning light (there is one over wbnonmetal) will flash. If this happens, none of the wet benches may be used, as the waste will not be properly treated.
    Before you are draining a hot pot, ensure that the neutralization system alarm is not flashing (yellow light above wbnonmetal.) If it is flashing, the neutralization waste treatment system is not working, so the hot pots cannot be drained. This does not affect the HF tanks, which drain into a separate HF waste collection system. (However, since the dump/rinsers, spin/rinse dryer and plenum all drain into the acid waste neutralization system as well, they cannot be used either.)


    AWN alarm
  • Check the bench for any damage or any other obvious problems. Make a note on Badger and notify Maintenance if any problems are observed.
  • The benchtop should be clean and dry before you work on it. Thoroughly rinse off any drips (assume they are acid) with the DI hand sprayer, gently dry with the N2 gun and, if necessary, remove residual water with cleanroom wipes. Avoid using cleanroom wipes to clean up unknown drips; these are then considered hazardous waste and must then be handled and disposed of accordingly. Do not leave cleanroom wipes on the bench top.
  • Check the logsheet for the last acid change in the hot pot or tank you will be using. If you need to change the acid, carefully follow the standard safety practices and the procedures outlined in Changing Chemicals.
  • Enable wbnonmetal, wbnitride, or wbmetal.

    Changing Chemicals

    • To remove old acid from the hot pot or tank, press the DRAIN/ASP button for the appropriate module.
      The hot pots must be below 120 degrees C to be aspirated (safety interlocked to prevent aspirating hot acid.). The DRAIN/ASP indicator will light up when the aspirator is on (hot pots) or the drain valve is open (HF tanks).
      Cooling a heated hot pot takes hours. Absolutely do not try to speed up the process by adding water.
    • Once acid is removed from the hot pot or tank, use the DI sprayer to rinse the hot pot or tank well. Rinse down the sides from top to bottom, but do not splash water outside the hot pot or tank. Be careful not to over-rinse the HF tanks, because the HF waste collection tank has limited capacity.
    • Press DRAIN/ASP to close off the aspirator (hot pots) or the drain valve (HF tanks). The indicator light will be off.
      The aspirators for the hot pots will automatically time out after 10 minutes, unless the DRAIN/ASP button is pressed again.

Transport Chemicals

 

  • If the chemicals need to be changed, use proper protection (safety glasses, face shield, yellow Nitrile cleanroom gloves/vinyl gloves, chemicals apron) to transfer chemicals from the storage area to the wet bench. Always double check the labels to ensure you have the correct chemicals!
  • Always use transfer carts to transport chemicals; chemicals must never be hand-carried in the lab!
    Follow the instructions for transporting chemicals, which are posted on the chemicals passthrough door.

    How to fill the tanks

    Place the acid bottle far enough under the hood to prevent fumes from escaping into the lab. Remove the bottle cap and place it topside down on the benchtop, to prevent contamination. Avoid contamination of the bottle: do not touch the inside of the bottle cap or the mouth of the bottle with your gloves. If the cap is inadvertently contaminated, rinse thoroughly before replacing it on the bottle.
    Using one hand around the neck and the other around the base of the bottle, gently pour the acid into the hot pot or tank. Pour slowly, to avoid any splashing.

    • To fill the 9:1 piranha hot pot:
      Pour sulfuric acid to the bottom of the Teflon dip-stick (you need more than one bottle) and add hydrogen peroxide to the mark on the dip-stick, about 1/3 gallon of peroxide. Be very careful: sulfuric acid and hydrogen peroxide give off heat when mixed.
      Because sulfuric is heavy and viscous while peroxide is thin, they do not always mix well. To help in mixing, place an empty cassette (use station-dedicated handles and cassettes!) into the acid mixture and lift it out. This should be done with the heat off. Rinse the cassette/handle in the dump rinser and dry in the spin-rinse dryer.
    • To fill the HCL hot pot:
      Fill DI water to the bottom of the Teflon dip-stick. Add hydrogen peroxide to the mark on the dip-stick and then add the hydrochloric acid to the last mark on the dip-stick.
    • To fill the NH4OH hot pot:
      Fill DI water to the bottom of the Teflon dip-stick. Add hydrogen peroxide to the mark on the dip-stick and then add the ammonium hydroxide to the last mark on the dip-stick.

      NOTE:
      the H2O2 in the NH4OH, HCl, and piranha hot pots will 'boil' off in time and need to be replenished.  This maintains the correct concentration of the mixture.  Simply add H2O2 back to the correct level on the dip-stick.

    • To fill the HF tanks:
      The HF tanks have no level indicator. One bottle of HF should be enough to cover a 4 inch cassette.
    After pouring acid, replace the cap on the bottle. If any acid drips onto the outside of the bottle, thoroughly rinse and dry the bottle before returning to the transfer cart. Return unused chemicals or empty bottles to the pass-through as soon as you are done pouring; never leave chemicals at the wet bench or on the transfer cart. Make sure the chemicals are returned to the appropriate labeled shelf (double-check!)

    Heating the hot pots
  • Turn on the heater by pressing the heater ON/OFF button for the appropriate pot. The temperatures are pre-programmed in the controller. Hot pots will heat to process temperature in about 10 minutes.
    wbnonmetal: 120 degrees C +/- 10 for the 9:1 sulfuric:peroxide hot pots

   

 

    Transfer wafers to the designated Teflon cassette, make sure wafers are not "cross slotted".

    There are dedicated cassettes and handles for all wet benches.
     
    The wbnonmetal and wbnitride share Teflon cassettes and handles with a yellow button.

    The wbmetal has Teflon cassettes and handles with a blue button.

    Use only these designated cassettes and handles!

    There are separate, dedicated cassettes and handles for 3", 4", and 6" wafers. A special stainless steel adapter is required to run 3"cassettes in the spin-rinse dryer.
      • Place the station-dedicated cassette box on the splash shield. Remove the cassette box lid and place it, inside facing up, on the shield. Place your wafer box and your tweezers box on the shield too and open both.
      • Wear clean vinyl gloves over latex gloves.
      • Place the station cassette, which is considered "clean", on the top of the box lid, which is considered "clean" too. Do not place the cassette on the bench top, the splash shield, or cleanroom wipes as these are not considered "clean".
      • Use your own decontaminated dirty non-metal "dirty non-metal" tweezers for loading wafers into wet bench cassette.
      • Slip the station-dedicated handle, which is considered "clean" on to the cassette. Make sure the handle is firmly against the two back "stops" on the cassette.

        Processing in hot pot or HF tank

    • Use proper protection: safety glasses, face shield, clean vinyl gloves over yellow Nitrile cleanroom gloves, and chemicals apron.
    • Lower the cassette (wear clean vinyl gloves) carefully into the appropriate hot pot or tank.
    • Press the start button on the appropriate timer.
    • The alarm will begin sounding one minute (the prewarn time) before the timer times out. To silence the alarm, press STOP. Press RESET to reset the timer.

      Dump Rinser

      After each chemical dip, rinse the cassette in one of the dump rinsers. 

    • Open the lid of the nearest dump rinser.
      • Change vinyl gloves!
      • Use the handle to gently lift the cassette out of the hot pot or tank. Hold the cassette over the hot pot or tank for several seconds to allow excess acid to drip off.
      • Place the cassette into the dump rinser.
      • Remove the handle and place it next to the cassette. This will ensure that the handle is completely rinsed. If the etch time is critical for your process, you should allow for the time required to move wafers from the etch tank to the dump rinser.
      • Close the lid.
      • Push START to start the multi-cycle rinse. This rinse tank fills and drains five times. The number of dump/rinse cycles is indicated by the LED at the top of this module.
      • When rinse is done push STOP/RESET to stop the alarm.
      • Push OPEN to drain the water.
      • Open the lid.
      • Use clean vinyl gloves and remove handle and cassette. Place the cassette into the next chemical bath or place them into the spin/rinse dryer.
      • Close the lid and press the up arrow key to fill up the tank with water. Please leave the dump rinser filled with water. If not in use, the dump rinsers will rinse under a dump/rinse cycle automatically every 90 minutes. This keeps the water lines clean.
      • When the AWN warning system is flashing during the dump-rinse cycle, make sure the cassette is covered with water, then press STOP/RESET button to pause the dump/rinse cycling.

    Spin-Rinse Dryer (SRD)

     

    WARNING: WAFERS MIGHT BREAK DUE TO:
    • High RPM,

      Thin wafers, chipped wafers, wafers that are scribed deeply,

      Stressed wafers, etched through wafers, wafers with jagged edges,…

      Make sure wafers are not “cross slotted” in the cassette!

       

      Wafers might stick to the Teflon rod after the spin-dry!

      Use extreme caution when removing the cassette!

    • Do not run the spin/rinse dryer without a cassette.
    • Rinse time is 180 seconds (400 rpm). If the resistivity does not reach setpoint, rinsing will continue beyond this time.
      Dry time at 240 seconds (1800 rpm).

      Do not change the rinse or dry times. (NOTE:  pure DI water is 18.2 Mohms resistance.)

      • Open the door to the lower spin-rinse dryer if you use a 4" cassette and carefully load the cassette. If you have a 3" cassette, you must insert the special stainless steel adapter into the spinner assembly first, then load your cassette. Use clean gloves to handle the adapter. If you have a 6" cassette use the upper spin-rinse dryer.
      • Close the door.
      • Press the green START button to start the cycle. In the Auto mode, this module will continue in the slow spin/rinse step of the cycle until the resistivity of the rinse water reaches 16.0 mega-ohms. This ensures that there is no residual acid is left on the wafers. When resistivity is reached, the fast spin/dry step will start. When the cycle is done, the rotor will stop spinning and the door interlock will be released. Open the door, and making sure that the cassette is upright. Wafers might stick to the Teflon rod after the spin-dry! Use extreme caution when removing the cassette (remember, clean gloves!)
      • Place the cassette into the station dedicated box and transfer the cassette box to the next furnace or deposition step.
      • After wafer cleaning at wbdiff, wafers must be loaded into the furnace within one hour, otherwise they must be re-cleaned. Wafers going between two furnace steps must also be re-cleaned if there is more than a one hour gap in between.
      • Return diffusion cassette and box to the diffusion wet bench after loading the wafers into the next tool. Do not store your wafers in the station dedicated Teflon cassette!

    Shutdown

      • Clean up your workspace. Ensure the benchtop is clean and dry.
      • Turn off the heaters to the hot pots.
      • Fill out the log sheet.
      • Please leave the dump rinsers filled with water and the lids closed.
      • Leave the main power on.
      • Be sure you are leaving the station in neat condition.
      • Disable wbnonmetal, wbnitride, or wbmetal.

     

    ROUTINE MAINTENANCE AND CALIBRATION

    Daily

      • Keep the benchtop clean. Thoroughly rinse off any drips (assume they are acid) with the DI hand sprayer, gently dry with the N2 gun and, if necessary, remove residual water with cleanroom wipes. Avoid using cleanroom wipes to clean up unknown drips; these are then considered hazardous waste and must then be handled and disposed of accordingly. Do not leave cleanroom wipes on the bench top.
      • Once a day, a clean station cassette should be run through the spin-rinse dryers. This will purge the lines of any standing water and serve as a system check. (Do not run the spin-rinses dryer without a cassette.)
      • Check the bench for any damage or any other obvious problems. Make a note on Badger and notify Maintenance if any problems are observed.

    Weekly (Maintenance)

      • N2 gun filters should be changed.
      • Chemicals aprons and face shields should be checked and changed if any staining or other damage is observed.

    Emergency Shutdown

     

    In the event of an emergency, the wet bench should be shut down by pressing the EMERGENCY POWER OFF button located on the upper left corner of the control panel. The hot pots will stop heating.

    If the exhaust shuts down while there are chemicals in the hot pots and tanks, EVACUATE THE LAB.

    Extended Shutdown

    Hot pots, tanks, dump rinsers and plenum should be drained and thoroughly rinsed. Power should be shut off to the system.

     

    Facilities

    HF acid waste drain (awhf-f13), acid waste neutralization drain (awn-f15), deionized water (dih2o-f9), scrubber exhaust (exhsc-f16), house nitrogen (hn2-f1) and industrial water (iw-f12).

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