Diffusion Wet Bench

wbdiff

Operating Instructions

1. SYSTEM OVERVIEW

1.1 Process

The wbdiff wet bench is used for cleaning wafers prior to processing in diffusion furnaces, LPCVD or metal deposition. Only 3", 4",and 6" silicon or similarly contamination-free substrates may be cleaned here (no GaAs wafers allowed). Wafers must not have (nor ever have had) any metals or silicides on them. Wafers must not have any photoresist. The cleans performed here are high quality, final cleans which precede high temperature processing. This station contains (from left to right, at the back of the bench) two quartz hot pots (the left with 4:1 sulfuric/peroxide; the right with 5:1:1 H2O:H2O2:HCl), and two teflon tanks containing 50:1 HF and 6:1 BOE. The front of the bench contains three automatic dump rinsers, a glove wash, DI hand sprayer, and an N2 gun. There are two dedicated spin-rinse dryers just to the right of the main bench, the bottom one is for 3" or 4" wafer and the top one is for 6" wafers. For more process details, see section 9.

1.2 Substrates

Substrates must be 3", 4", or 6" rounds, which can be handled by the dedicated cassettes at this station. Only silicon, silicon germanium, and quartz substrates may be process at this station, subject to the constraints in the Materials Allowed section (section 1.3). Odd-sized substrates must be processed at wbgeneral.

1.3 Materials Allowed

The wbdiff bench is in the "clean" equipment group (click <here> for Equipment Group Policy). Wafers that at any point have received processing on any equipment outside of the "clean" equipment group cannot be processed here. Substrates containing absolutely any metals or metal films are strictly prohibited from being processed here, even if the metal has been previously removed. Wafers must not contain any photoresist (they must be cleaned at wbnonmetal first). If wafers have previously come into contact with KOH, decontamination must be performed (see Process section of the SNF website). Wafers processed outside SNF may not be processed at this station without first consulting with staff. Consult staff members or the Materials Section of the website for more details about which specific materials are acceptable at this station.

1.4 Facilities

Wbdiff contains the following 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).

1.5 Equipment Description

This is a Santa Clara Plastics brand wet bench. It is equipped with: two quartz-lined, temperature controlled hot pots (for sulfuric and HCl cleans) which aspirate into the acid waste neutralization system; two Teflon-lined tanks (for HF etching) which drain into HF waste collection system; and three dump rinsers and two spin-rinse dryers which drain into the acid waste neutralization system. Each of these modules has its own microcontroller unit, mounted in the overhead control panel. The wet bench also contains a glove wash, an N2 gun, and a DI water hand sprayer. The wet bench plenum, over which all the modules sit, collects runoff and overflow, and drains into the acid waste neutralization system.

2. SAFETY

2.1 Chemical Hazards

Sulfuric acid, hydrogen peroxide, hydrofluoric acid, and hydrochloric acid are used at this wet bench. You must read the Material Safety Data Sheets for these chemicals and understand safe chemical handling procedures before working at this station.

The primary hazard classifications for the chemical mixtures used at this bench are:

For 4:1 sulfuric:peroxide - Corrosive, oxidizer, air/water reactive
For 50:1 HF and 6:1 BOE - Corrosive, toxic
For 5:1:1 H2O:H2O2:HCl - Corrosive, oxidizer

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. HF-kits containing tubes of gel (along with instructions for use) are located in plastic bags at each wet station.

2.2 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 on Coral, and contact Maintenance staff.

2.3 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.

2.4 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.

2.4.1 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. This prevents the potentially dangerous situation of heating an empty pot.
• 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.

2.4.2 General wet bench functions:

• The dump rinsers will rinse under a dump/rinse cycle automatically every 90 minutes. This prevents water from stagnating in the lines and in the tank. The lid must be closed and the dump rinser filled up with water for this to occur.
• The drain/aspirators will automatically time out after 10 minutes, unless the DRAIN/ASP button is pressed again.

2.5 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, wbgaas 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. Don’t start the spin-rinse dryers.

3. QUALIFICATION CHECKLIST

Operators of this station must be able to demonstrate or describe the activities on this list:

Chemical safety knowledge:

• Which chemicals are used at this bench.
• Hazards of each chemical.
• Where to obtain chemicals and how to properly transport them.
• How to pour and mix the chemicals.
• Necessary protective gear for pouring chemicals or wafer processing.
• How to deal with drips on the wet bench deck.
• How to deal with a spill.
• Point out the nearest safety shower and eye wash.
• Know where the HF kits are and how to use.
• How to report spills, safety problems, mistakes, hazard violations to staff.

Contamination control knowledge:

• Contamination level at this bench (i.e., what the acceptable materials are.)
• Dedicated cassettes.
• Wafer transfer procedures to dedicated cassettes.
• Glove cleanliness precautions.
• Proper use of tweezers.

Process knowledge:

• When to change chemicals.
• When and how to fill out the chemicals change log sheet.
• Why and how each chemical is used at this bench.
• The various cleans used, their sequence, and why.
• Recleaning wafers.

Equipment knowledge:

• Identify the modules, what chemicals they may contain, and where they drain.
• List the functions of the main interlocks (low level, temperature and DRAIN/ASP valve.)
• List the functions of all the buttons on each of the controller panels.
• List the startup check procedures.
• Point out the location of the AWN alarm.

4. OPERATION

4.1 Preparation

4.1.1 Gloves

o        When working with acids, yellow 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.

o        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 cassetted, 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.

4.1.2 Cassettes

o        There are dedicated cassettes and handles for all wet benches. This bench has Teflon cassettes and handles with a circular cutout, no colored button. Use only these designated cassettes and handles!

o        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. Follow the special procedures in section 4.5.

4.1.3 Tweezers

o        Tweezers used for loading wafers into wbdiff cassettes should be teflon-coated stainless steel. They should be decontaminated according to the standard lab procedures for tweezer 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.

4.1.4 Substrates

o        Wafers must not have any photoresist or scribe dust on them. Photoresist clean must be done at wbnonmetal prior to cleaning at wbdiff.

o        After wafer cleaning at wbdiff, wafers must be loaded into the furnace within one hour, otherwise they must be recleaned. Wafers going between two furnace steps must also be recleaned if there is more than a one hour gap in between.

4.1.5 Cleanroom Wipes

o        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.

4.2 Equipment Setup

4.2.1 Control Panel

The control panel modules are, from left to right are:

1.      3130 EPO. This controls power to the control panel. It contains the Emergency Off button (top) and the main power On/Off switch. When in use or under standby conditions, the main power should always be left on.

2.      Plenum Flush. This is for flushing the plenum. To be used for maintenance purposes by staff.

3.      3150 Process Controller. This controls heating, the aspirator, and the timer for the left hot pot (4:1 sulfuric/peroxide). For more details about the microcontroller functions, see section 4.2.3.

4.      ISE Dump Rinser DRCa. This controls the left dump rinser. See section 4.2.4 for more details.

5.      3150 Process Controller. This controls heating, the aspirator, and the timer for the right hot pot (5:1:1 H2O:H2O2:HCl). For more details about the microcontroller functions, see section 4.2.4.

6.      ISE  Resistivity Monitor CRM-120. This measures the water resistivity of the center dump rinser.

7.      ISE Dump Rinser DRCa. This controls the center dump rinser. See section 4.2.4 for more details.

8.      Enable/Disable LED. This shows the Coral enable/disable status of wbdiff.

9.      3170A Process Timer. This is the timer for the left HF tank (50:1 HF). For more details, see section 4.2.5.

10. Drain Switches. This module controls the drain valves for both HF tanks.

11. 3170A Process Timer. This is the timer for the right HF tank (6:1 BOE). For more details, see section 4.2.5.

12. ISE Dump Rinser DRCa. This controls the right dump rinser. See section 4.2.4 for more details.

4.2.2 Lower Front Panel Controls

To allow the user to avoid reaching over wafers and acid, many of the module functions can be actuated from the lower front panel. The front panel functions available are as follows (from left to right):

1.      Process timer for the left hot pot (4:1 sulfuric:peroxide).

2.      Process timer for right hot pot (5:1:1 H2O:H2O2:HCl).

3.      Process timer for left HF tank (50:1 HF).

4.      Process timer for right HF tank (6:1 BOE).

4.2.3 3150 Process Controller Functions (Hot Pots)

o        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, it is important to press RESET when turning the heater off (see section 2.4).

o        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. See section 2.4 for explanation of interlocks. During normal operation and heating of the hot pot, only the HEATER ON indicator should be lit.

o        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 preprogrammed operating range, the PROCESS TEMPERATURE RANGE OUT indicator (in red) will be lit.

o        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.

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

4.2.4 ISE Dump Rinser DRCa Functions

o        Under normal operation, 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.

o        START begins the dump/rinse cycling.

o        STOP/RESET pauses the dump/rinse cycling.

o        OPEN drains the tank (see section 2.4).

4.2.5 3170A Process Timer Functions (HF Tanks)

o        There is a user-set process timer for each of the HF tanks.

o        Each of the three digits on the LED is set by the SET and RESET buttons beneath it.

4.2.6 Spin/Rinse Dryer

Do Not run the spin/rinse dryer without a cassette. The settings on the spin/rinse dryer are:

·  Auto

·  Rinse time 180 seconds (400 rpm). If the resistivity does not reach 16.0 megaohms, rinsing will continue beyond this time.

·  Dry time at 240 seconds (1800 rpm)

4.3 Startup

• Enable "wbdiff" on Coral.
• Ensure that the main power to the bench is on (upper left on the control panel.)
• Check the exhaust gage (magnehelic) (located near the floor, on the right side of the bench). For sufficient exhaust, it should read greater than 0.50.
• Check that the acid waste neutralization 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.
• 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 section 5.4.
• To heat 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.
• 90 degrees C +/- 10 for the 4:1 sulfuric:peroxide hot pot
• 70 degrees C +/- 10 for the 5:1:1 H2O:H2O2:HCl hot pot

4.4 Test Run - N/A

4.5 Processing

• For detailed process procedures, consult the Process Info links for the wbdiff station on the SNF website. In brief, the standard processes are:
• Standard Prediffusion Clean consists of the following steps:
• 1. 4:1 Sulfuric:H2O2 (10 minutes), followed by dump/rinse
• 2. 50:1 HF (30 seconds), followed by dump/rinse
• 3. 5:1:1 H2O:H2O2:HCl (10 minutes), followed by dump/rinse
• 4. Spin dry
• Standard Pre-LPCVD or Pre-Metal clean consists of similar steps, but in different order:
• 1. 4:1 Sulfuric:H2O2 (10 minutes), followed by dump/rinse
• 2. 5:1:1 H2O:H2O2:HCl (10 minutes) followed by dump/rinse
• 3. 50:1 HF (30 seconds), followed by dump/rinse
• 4. Spin dry
• Transfer your wafers to the designated Teflon cassettes (section 4.1.2.)
• Remember to use clean gloves.
• Place the station-dedicated cassette box on the bench top. Remove the cassette box lid and place it, inside facing up, on the bench top.
• The station cassette may be placed on the top of the box lid, which is considered "clean". Do not place the cassette on the bench top or on the splash shield, as these are not considered "clean". Cleanroom wipes, for the purposes of this bench, are not considered "clean".
• Transfer your wafers. Tweezers must be decontaminated and "dirty non-metal" for loading wafers into wet bench cassettes.
• Check the timer for the module you will be using. For the hot pots, the timers have been preset at 10 minutes. The timers for the HF tanks are left at 30 seconds by default, but can be set by the operator.
• Slip the handle on to the cassette. Make sure the handle is firmly against the two back "stops" on the cassette. Lower the cassette 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.
• Lift the lid of the nearest dump rinser.
• 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. (Remember, clean gloves.)
• After each chemical dip, 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.

1.       Push START to start the multi-cycle rinse.

2.       When rinse is done push STOP/RESET to stop the alarm.

3.       Push OPEN to drain the water and remove handle and cassette.

4.       Push STOP/RESET to close the drain valve.

5.       Push START to fill the bath with water.

6.       Once the bath is filled with water push STOP/RESET.  The water will continue to over flow.

7.       Push STOP/RESET to stop the water from flowing.

Please leave the dump rinser filled with water.  It will automatically flush the water every 90 minutes.  This keeps the water lines clean.
 

• Load the cassette into the spin-rinse dryer.
• 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.
• 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 11.9 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, gently remove the cassette (remember, clean gloves!)
• Place the cassette into the next chemical bath, or:
• Store your wafers for transfer to the next furnace or deposition step.
• Remember to use clean gloves.
• Place the station-dedicated cassette box on the bench top. Remove the cassette box lid and place it, inside facing up, on the bench top.
• The station cassette may be placed on the top of the box lid, which is considered "clean". Do not place the cassette on the bench top or on the splash shield, as these are not considered "clean". Cleanroom wipes, for the purposes of this bench, are not considered "clean".
• Place the station cassette into the cassette box. Transfer the cassette box to the next station.

4.6 Standby

• Clean up your workspace. Ensure the benchtop is clean and dry.
• Turn off the heaters to the hot pots.
• Leave the main power on.
• Please leave the dump rinsers filled with water and the lids closed.
• Be sure you are leaving the station in neat condition (see section 5.1).
• Disable wbdiff on Coral.

5. ROUTINE MAINTENANCE AND CALIBRATION

5.1 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 Coral and notify Maintenance if any problems are observed.

5.2 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.

5.3 Monthly -- N/A

5.4 Other

Changing chemicals:

• Frequency:
• The 4:1 sulfuric/peroxide clean should be changed once every five days.
• The 5:1:1 H2O:H2O2:HCl hot pot should be changed every four hours.
• The 50:1 HF tank should be changed once/day.
• The 6:1 BOE is changed once per month or on an as-needed basis.
• 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.

• 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.

• If 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 sytem as well, they cannot be used either.)
• 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 aspirator will also shut itself off after 10 minutes from the last time the DRAIN/ASP button is pressed.
• Now, to pour the acid. 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 4:1 piranha hot pot:
• Pour 4 parts sulfuric acid and one part hydrogen peroxide. It takes about 1 1/3 gallons of sulfuric and about 1/3 gallon of peroxide to fill the pot. The proportion of sulfuric to peroxide does not need to be exact. Be very careful: sulfuric acid and hydrogen peroxide give off heat when mixed.
• Do not overfill this hot pot. The acid should be a couple of inches below the top of the pot (this allows for volume displacement by a full cassette and some extra room for additional peroxide, as the piranha solution ages.)
• 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 HCl to the last mark on the dip-stick.
• 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!)

6. SHUTDOWNS

6.1 Standby -- See section 4.6.

6.2 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. This will shut off all power to the bench.

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

6.3 Extended Shutdown

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

7. TROUBLESHOOTING

For process troubleshooting, see section 9.

If you are having any problems with equipment, contact Maintenance (because of potential safety and contamination issues, only qualified personnel are allowed to work on the wet benches.)

8. BACKUP EQUIPMENT - N/A

9. ADDITIONAL PROCESS INFO

For the procedures of the specific processes which are normally performed at this station, check the Process Info link for wbdiff on the SNF website:

http://snf.stanford.edu/Equipment/wbdiff/ProcessInfo.html

For processes which may be performed at other wet bench stations, check the Processes section of the SNF website.

9.1 About the "RCA" clean (from the MIT MTL website)

"Contaminants present on the surface of silicon wafers at the start of processing, or accumulated during processing, have to be removed at specific processing steps in order to obtain high performance and high reliability semiconductor devices, and to prevent contamination of process equipment, especially the high temperature oxidation, diffusion, and deposition tubes. In 1970, the RCA Laboratories developed a cleaning procedure for silicon semiconductor device fabrication technology, which has become the industry standard; it uses several reagents containing hydrogen peroxide.

"The RCA cleaning procedure has three major steps used sequentially:

I. Removal of insoluble organic contaminants with a 5:1:1 H2O:H2O2:NH4OH solution (SC-1).

II. Removal of a thin silicon dioxide layer where metallic contaminants may accumulated as a result of (I), using a diluted 50:1 H2O:HF solution.

III. Removal of ionic and heavy metal atomic contaminants using a solution of 6:1:1 H2O:H2O2:HCL (SC-2).

"The RCA cleaning technique does not attack silicon, and only a very thin layer of silicon dioxide is removed (in II) in the process. The procedure was also designed to prevent replating of metal contaminants from solution back to the wafer's surface."

At SNF, the procedure has been modified slightly. Instead of SC-1, we use a low temperature, highly oxidizing 4:1 sulfuric: peroxide clean which serves the same purpose in removing trace organics. For ease of use, we also use a slightly different formulation of SC-2 (5:1:1). And for cleans which precede film deposition, the order is switched so that the last dip is the 50:1 HF, to minimize any native oxide that might form.

9.2 4:1 Sulfuric/Peroxide Piranha Clean

This solution consists of 80% concentrated sulfuric acid and 20% hydrogen peroxide. The 4:1 piranha clean is run at 90 degrees C, which is lower than the standard process temperature for 90% clean for photoresist removal. At this station, this dip serves the same purpose as the first SC-1 step of the RCA standard clean process, in removing any trace organics.

9.3 5:1:1 H2O:H2O2:HCl

This solution is heated to 70 degrees C. This HCl clean is a variant of the SC-2 step of the standard RCA clean procedure (the standard SC-2 uses slightly different proportions). This formulation is excellent for removing trace metal cations from silicon surfaces.

9.4 HF-based Etchants There are several varieties of HF-based etchants, different in acid strength and in composition. HF-based etchants include the BOE etchants, Pad etch, and ammonium fluoride (NH4F). Before working with any HF-based etchant, read the information and the links provided in the Safety section about HF.

9.4.1 HF/Water Mixtures: 50:1 HF and 49% HF

SNF stocks two "straight HF" mixtures (which contain only HF and water): 50:1 HF and 49% HF.

Concentrated hydrofluoric acid is approximately 49% HF and 51% water. 50:1 HF is approximately 2% HF in water, about 25-fold less than 49%. Do not confuse these two (yes, this mistake has been made). The 49% HF bottles should have ‘HF’ handwritten on the cap.

The etch rate of thermal oxide in 50:1 HF is generally nominal (about 50 angstroms/minute.) Because this acid is not buffered, the etch rate may vary with acid lifetime or usage. More detailed etch rate info is available on the Processes section of the SNF website.

9.4.2 Buffered Oxide Etchants: BOE

BOE is the acronym for "Buffered Oxide Etch", which is a mixture of ammonium fluoride, HF, and water. Ammonium fluoride is normally a solid with a low temperature of sublimation, but is very soluble in water (concentrated ammonium fluoride is approximately 40% by weight in water.) In the BOE etchants, the ammonium fluoride acts as a buffer, maintaining the pH of the solution which keeps the etch rate stable/constant over time. High concentrations of ammonium fluoride are used in BOE; in fact, the total fluoride ion content is nearly that of concentrated 49% HF, and so BOE etchants are considered to pose the same toxic hazards as 49% HF.

The temperature of BOE etchants is not controlled in SNF. Depending on the manufacturer, BOE acids may also have an added surfactant to help circumvent surface tension, which can prevent etching in small geometries or in areas with high aspect ratios. The exact formulations of BOE's are generally proprietary, but here is a general summary is below.

9.4.2.1 20:1 BOE

20:1 BOE is approximately 20 parts of 40% ammonium fluoride and 1 part of 49% HF. Thus, 20:1 BOE is approximately 38% NH4F, 2.5% HF, and 60% water. The etch rate of thermal oxide is approximately 300 angstroms/minute. More detailed etch rate info is available on the SNF website.

9.4.2.2 6:1 BOE

6:1 BOE is approximately 6 parts of 40% ammonium fluoride and 1 part of 49% HF. Thus, 6:1 BOE is approximately 34% NH4F, 7% HF, and 59% water. The etch rate of thermal oxide is approximately 900 angstroms/minute. More detailed etch rate info is on the SNF website.