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KOH etching of Silicon

KOH ETCHING AND DECONTAMINATION PROCEDURES


OVERVIEW

Heated KOH solutions can be used for preferential etching of silicon along crystal planes. The etch rate will depend on the doping and crystal orientation of the silicon and the type of KOH solution used, but is typically on the order of about a micron per minute. Potassium (K+) is an extremely fast-diffusing alkali metal ion, and a lifetime killer for MOS devices. Thus, KOH etching is limited to wbflexcorr1-2 or 3-4. Labmembers using KOH absolutely must observe proper procedures to avoid contaminating any metal-ion sensitive processes and equipment elsewhere in the lab. KOH-etched substrates, however, may be later processed in "Clean" equipment, but only providing that the procedures for decontamination described here are strictly followed.

SAFETY

Chemical Hazards

KOH solutions are caustic. The primary hazard classifications for KOH solutions are: Corrosive, air/water reactive. If you are using Isopropyl Alcohol in your KOH solution, remember that it is a solvent and that it is flammable.

The 5:1:1 H2O:H2O2:HCl solution is used for decontamination of wafers and labware following KOH etching. The primary hazard classifications for these solutions are: Corrosive, oxidizer, air/water reactive.

Find more information about primary hazard categories in the SNF Lab Manual, Part II.

Process Hazards

General process hazards involve handling of chemicals, and materials which come into contact with chemicals, 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. Be aware that KOH etching solutions are heated and not only present thermal hazards, but also chemical hazards that are more severe than what may be listed in the typical MSDS.

 

OPERATION/PROCESS

KOH Etching of Silicon at wbflexcorr-4

KOH etching is done in the constant temperature bath at the wbflexcorr-4 station.
KOH decontamination may be performed at wbflexcorr (with appropriately clean quartz containers) or at wbdecon in the designated hot pot.

Enable wbflexcorr-4, make sure the on/off toggle is OFF!
Unfortunately, there is no level sensor in the tank.

  1. Fill the constant temperature bath.
    Fill the bath with water, leaving a couple of inches at the top so that it does not overflow when the beaker is placed inside. There are plastic balls in the tank, they slow down the water from evaporating.

  2. Select the appropriate labware.
    KOH etching requires an etch beaker with a lid, a rinse beaker, and teflon wafer holder (either a boat & handle or single wafer holder). There is a designated set "Clean" labware consisting of a quartz etch beaker with a Pyrex condensor lid, a Pyrex rinse beaker, and various wafer holders/boats. The "Clean" labware is specifically for processing material that will be KOH-decontaminated for subsequent processing in "Clean" equipment. These are stored in the cabinet at wbflexcorr and may not be used for any materials that have (or have ever had) any metals. There is also a designated set of "Metal- and Gold-Compatible" labware (beaker, beaker lid, and wafer handling tools) for KOH etching of all other materials. These are located on the shelf at wbflexcorr.

  3. Mix up your KOH solution in the beaker and place it in the bath.
    Pour the desired amount of KOH into the beaker. The KOH solution supplied in the lab is 45%. Depending on your process, you may want to dilute with water or water and isopropanol (although processing of materials using solvents is not allowed at this bench, KOH etching is an exception). A ruler placed against the side of the beaker will help with determining the correct proportions. People tend to use 30% KOH in water, which etches at ~60 microns per hour at 80 C. Carefully place the KOH beaker into the constant temperature bath.

  4. Assemble the condensor.
    Place the condensing unit on top of the beaker with the lower hose connected to the industrial water connection and the upper hose going down the drain. Turn on the water to obtain a slow, steady flow. The actual temperature of the bath can be monitored by placing a thermometer in the constant temperature bath. The thermal capacity of the bath is much higher than that of the beaker and its contents, so you may assume that the etching solution is at the same temperature as the surrounding bath.

  5. Heat the bath.
    Using the on/off toggle, turn on the heater on the constant temperature bath and set the temperature desired. The actual temperature in the bath will be approximately 15 C lower than the set temperature! The temperature setting should not be set above 90 C (water boils at 100 C). Set the Limit Adjust knob at about 130C (if it is too low it will trip the breaker and cool down the bath.) It takes about 45 minutes to heat the bath to 80 C. Check the water level and add water to the bath as needed.

  6. Etch your wafers.
    Place your wafers in the designated teflon wafer boat or holder. When the bath/beaker have come up to temperature, carefully remove the condensing unit from the beaker. Place your wafers into the solution. Replace the condensing unit and proceed with etching.

  7. Check the water level frequently.
    Add water to the bath as needed. The temperature will be more stable if small amounts of water are added frequently (rather than large amounts, less often.) Do not leave the bath heating and unattended for more than four hours (and certainly not overnight) as it will boil dry. There is no level senor, without water in the tank, the heating coil will melt the plastic balls!

  8. Finish etching and clean up.
    Remove your wafers and thoroughly rinse in the appropriate rinse beaker. Turn off the heater to the bath. Disassemble, remove, and rinse the condensor lid. Pour the KOH from the quartz beaker into a "gold contaminated" beaker and aspirate the KOH from this beaker. The tip of the aspirator is dirty and should not contaminate the quartz beaker. Thoroughly rinse the beaker. Clean up labware. Make sure the bench is left clean and dry.

  9. Log the KOH into the station checklist.

DECONTAMINATING KOH-ETCHED SUBSTRATES

Decontamination is required only if you plan on processing your KOH-etched materials in any "Clean" equipment. KOH-cleanup/decontamination may be performed at wbflexcorr1-2, 3-4 or at wbdecon.

 KOH decontamination at wbflexcorr1-2 or 3-4:

  1. "Clean KOH" labware should have been used for KOH etching.
    You should have performed your KOH etching in the appropriate, designated "Clean KOH" labware. Note that in this case "Clean KOH" is not the same as the lab definition of "Clean" -- because the labware is used for KOH etching, it is contaminated with potassium, an alkali metal which is a lifetime killer in devices.

  2. Obtain metal-free labware (not provided).
    Labware for decontamination must be quartz or Teflon. Pyrex is not acceptable, as it contains ~5% sodium. Polypropylene is not acceptable, because it has a low melting temperature. The quartz and Teflon labware used for KOH etching is not acceptable. If you wish to use a separate beaker for rinsing your substrates, it must also be quartz or Teflon. Place your wafers in the Teflon holders.


  3. Decontaminate wafers (and labware).
    Twenty minutes of immersion in a solution of 5:1:1 H2O:H2O2:HCl is used to decontaminate wafers and labware. Mix up the solution by pouring H2O, then 30% H2O2, and then finally adding the concentrated HCl. Add the HCl slowly. Be careful, as this mixture self-heats. If you are using a rinse beaker, you must also decontaminate it with this solution, and then rinse thoroughly. Place the wafers into the HCl acid solution. Leave the wafers in solution for 20 minutes. Remove the wafers and rinse thoroughly.

  4. Clean up.
    Aspirate the H2O:H2O2:HCl solution. Rinse beakers thoroughly. Log acid on the logsheet. Leave the bench clean and dry.

 

 

KOH decontamination at wbdecon (former wbsilicide)

KOH-wafer cleanup/decontamination may be done only in the left HCl hotpot at the wbdecon station.  Reserve wbdecon in advance. 

  1. Pour fresh 5:1:1 H2O:H2O2:HCl solution.
    The tank should be empty, if not, drain the old solution, and pour fresh solution, as described in the wbdecon operating instructions. Heat the solution to 70 C.


  2. Load your wafers into a wbdecon cassette.
    Observe proper use of designated tweezers. Use KOH-contaminated tweezers to load wafers into the wbdecon cassette. Do not use these tweezers after this point. The cassette with KOH-etched wafers is now considered KOH-contaminated. Do not put this cassette into the dump rinser, spin/rinse dryer, or any other hot pot, at this point.


  3. Place the cassette with wafers into the HCl hotpot.
    Use a dedicated wbdecon cassette handle to place the cassette in the hotpot. Leave the wafers in the heated HCl solution for 20 minutes.

  4. Rinse wafers in the dump rinser, then in the spin/rinse dryer.
    After the HCl hotpot, the wafers and cassette are no longer considered KOH-contaminated.

  5. Remove wafers and store.
    Use "Non-metal/photoresist" tweezers to remove wafers from the silicide cassette. Store the wafers in a non-metal, non-KOH storage box.

  6. Leave bath empty.
    Drain the KOH-contaminated HCl solution.
    Rinse the hot pot thoroughly by completely filling and aspirating at least once with DI water. Leave the hot pot empty.
    In the chemical change log, write "KOH decon." and indicate if the hot pot was left empty.

  7. If you want to go into clean equipment.
    If you wish to process these wafers in one of the "Clean" tools, the wafers must be cleaned first in 9:1 sulfuric acid/ hydrogen peroxide solution at wbnonmetal, and then through diffusion clean at wbclean-1 or -2.

 

ADDITIONAL PROCESS INFO, Etch Rates

 KOH, Potassium Hydroxide: Etching of Silicon

45% KOH can be found in the Chemicals passthrough and can be diluted to provide a 30% solution. Heated 30% KOH in water is used to etch silicon. Detailed procedures for KOH etching can be found in the processes link. KOH is a strong, corrosive base and should be handled with extreme care. Potassium is an extremely fast-diffusing alkali metal which will have disastrous effects on the performance of any electronic device should it come into contact with any wafers, wafer handling tools, or processing equipment in the "clean" or "semiclean" equipment groups. Be extremely careful about handling KOH and all the labware and wafers which come into contact with it. Be extremely conscientious about decontaminating your substrates if they are to be returned to "clean" or "semiclean" equipment.

Hot, concentrated solutions of KOH (and other alkali metal hydroxides) will etch along the (100) crystal plane several hundred times faster than along the (111) plane. KOH etching through mask openings >1mm will result in a V-shaped pit that can go all the way through a (100) wafer of standard thickness. MEMS engineers frequently design structures which exploit this etch preference, thus making KOH the most common method of machining silicon.

Photoresist will not hold up to KOH etching. Silicon oxide can serve as a mask, although it still etches somewhere on the order of about 1 nm/min (oxide to silicon selectivity is a bit better in TMAH solutions). Silicon nitride is the preferred mask material (as little as 250 A is sufficient for masking (100) etch all the way through a wafer). For detailed process info, including references and recipes, consult Greg Kovacs' indispensable book, "Micromachined Transducers Sourcebook."

Because potassium (K+) is a lifetime killer for MOS devices, extensive decontamination procedures are defined here, for those applications which require later processing in "Clean" equipment. If you wish to avoid the decontamination process, you may use the TMAH etch process instead. TMAH etching must be performed entirely in quartz beakers; Pyrex is unacceptable because it contains about 5% sodium, another fast-diffusing, MOS lifetime killer. If you use TMAH, all quartzware and Teflon wafer ware must be decontaminated before processing your wafers. To decontaminate, use the 5:1:1 H2O:H2O2:HCl procedures.

 

 

Last updated Feb 14, 2014

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