Operating Instructions: wbgeneral

General Purpose Wet Bench

wbgeneral

Operating Instructions

1. SYSTEM OVERVIEW

1.1 Process

The wbgeneral wet bench is for wet, non-solvent processing of standard and non-standard materials. Such materials include wafer pieces or substrates with gold or other non-standard films. Non-standard processing includes KOH etching of silicon and etching of non-standard substrates such as glass. There are no automated wafer processing modules at this station; all processing is done manually in station-dedicated or labuser-provided labware (beakers, dishes, etc.) With appropriate, dedicated quartzware, KOH cleanup can be done to decontaminate wafer for later processing in CMOS-compatible equipment. The station contains one hot plate and a constant temperature circulating bath for heating solutions in beakers. The station also contains a sink for rinsing, and an aspirator and HF drain for disposal of common waste. For additional process info, see section 9.

1.2 Substrates

Substrates may be of most any type, size, shape and any material, subject to the constraints in the Materials Allowed section (1.3). GaAs, however, is not allowed for processing here.

1.3 Materials Allowed

The wbgeneral appears in all three equipment groups (clean, semiclean, and gold) as part of the general-purpose wet benches tool set. Processing may require dedicated labware (see section 4.1.2). Any labware at the wbgeneral station is considered gold-contaminated. For more details, consult the Materials Allowed section of the wbgeneral web page.

No solvents are allowed at this bench (solvent processing must be done at one of the solvent benches.) SNF standard etchants (consult the Materials List on the SNF website for acceptable chemicals) are allowed at this bench, for standard processing only. Consult staff if your work requires non-standard processing conditions (such as elevated temperatures, unusually extended process times, or mixing of chemicals), even if standard SNF etchants and chemicals are to be used.

1.4 Facilities

The wbgeneral station has 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 one heater module and a constant temperature bath, all used for heating solutions in beakers or other containers. These modules have controllers mounted in the upper panel. There is a separate, covered, HF disposal module which drains into the HF waste collection system. The aspirator and the rinse sink both drain into the acid waste neutralization system. The wet bench plenum, over which all the modules sit, collects runoff from the benchtop and drains into the acid waste neutralization system. The wet bench also contains an N2 gun and a DI water hand sprayer.

Because many commonly used etch processes are quite long yet may not require the operator to be present at all times (for example, KOH etching all the way through silicon), the wbgeneral station is set up to allow two users at once and, thus, has two Coral names ( wbgen-hpr, and wbgen-ctb.)

2. SAFETY

2.1 Chemical Hazards

Any of the SNF-standard, non-solvent etchants may be in use at any time at this bench. Any non-SNF-approved etchants must undergo safety and contamination review by the SpecMat team (consult the Materials portion of the SNF website for the specific procedures.) Because this is a shared bench, you must read the Material Safety Data Sheets and be aware of the hazards, not only of the chemicals you are using, but also for chemicals that are commonly used here (see below and section 9) which may be in use by others. Because mixing and pouring of chemicals into open containers are performed at this wet bench, users must be keenly aware of the potential handling hazards of these chemicals. Be aware that wbgeneral is an entirely manual station, without safety features of other wet benches to prevent chemical mixing, spills, or other potential problems. Above all, users must understand and practice safe chemical handling procedures for working at this station.

Some of the chemicals you might find at this bench are: Sulfuric acid, hydrogen peroxide, hydrogen chloride, KOH, and hydrofluoric acid (50:1 HF, 20:1 BOE, 6:1 BOE, 49% HF) and mixtures thereof. 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 some of the chemical mixtures which may be used at this bench are:

For 70% - 90% sulfuric/peroxide - Corrosive, oxidizer, air/water reactive
For 50:1 HF, 10:1 HF, 20:1 BOE and 6:1 BOE - Corrosive, toxic
For PRX-127 or PRS-1000 - Flammable, mildly corrosive
For Hot phosphoric - Corrosive, oxidizer, air/water reactive
30% KOH - Corrosive
20% TMAH, Tetra Methyl Ammonium Hydroxide- Corrosive

Remember: because of fire/explosion risk, flammables should not be mixed with corrosives or oxidizers.

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, this particular one more so than the other because processing is completely manual. Be sure you understand all hazards and proper handling procedures before working at this or any other wet bench.

2.4 Interlocks

The wbgeneral bench has NO safety interlocks to protect personnel from chemical and process hazards described above. So it is absolutely imperative that users must thoroughly understand the chemicals, materials and processes they are working with and their associated hazards.

2.5 Alarms

The sink, aspirator, and 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 wbgaas/wbnonmetal) will flash. When the AWN warning system is flashing, no processing at any of the acid/base wet benches is allowed.

3. QUALIFICATION CHECKLIST

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

Chemical safety knowledge:

Which chemicals may used at this bench.

Hazards of each chemical.

Where to obtain chemicals.

How to pour and mix the chemicals.

How to fill out the "In-use Hazardous Materials Sheet" ("blue card").

List the three methods for disposing of used chemicals.

Describe additional precautions required for disposing of heated 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.

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.)

Which cassettes may be used at this bench.

Glove cleanliness precautions.

Proper use of tweezers.

Process knowledge:

List the processes most commonly performed at this station and the chemicals used.

Which containers are compatible with commonly used etchants.

How to fill out the chemicals waste log sheet.

Equipment knowledge:

Identify each of the modules, their purpose, and where they drain.

How to enable each of the modules on Coral.

List the startup check procedures.

4. OPERATION

4.1 Preparation

4.1.1 Gloves

When working with chemicals, yellow chemical resistant Nitril gloves should be worn over latex gloves; vinyl gloves should then be worn over the yellow chemical resistant Nitril gloves (for a total of three pairs of gloves). The yellow chemical resistant Nitril gloves provide chemical protection. The vinyl gloves protect the wet bench from potential contamination from the yellow chemical resistant Nitril gloves. Gloves must be worn in this way when transferring cassettes in and out of chemical baths. In addition to full chemical protection 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 chemicals, vinyl gloves can be worn directly over latex gloves. 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.

4.1.2 Cassettes and other Labware

Cassettes at wbgeneral do not have any markings; some have been cut in half for ease of use in beakers. Any cassettes with buttons or circular cutouts belong to other wet bench stations and so must not be used at wbgeneral because they will become contaminated. (Another note: please handle Teflon cassettes with care -- they cost about $250 each to replace.)

Labware found at wbgeneral is considered gold-contaminated; any substrates processed in this labware is considered gold-contaminated thereafter. You may use your own personally labeled, dedicated labware to prevent contamination of substrates. In order to be "clean" compatible, the labware must never have been in contact with solutions or materials containing alkali (Na, K, Li) or any metals. To be "semiclean", labware must never come into contact with alkali nor any non-standard metals (standard metals are acceptable.) Personal labware must be stored in personal storage space or will be considered contaminated. Dedicated "clean" (uncontaminated) labware must be made of either fused silica ("quartz") or electronics-grade Teflon. Standard Pyrex-brand glassware, which contains up to 5% sodium, is unacceptable as "clean" labware.

Labware must be compatible with the etchant to be used. Pyrex is not acceptable for HF or BOE etchants, although it is acceptable for sulfuric/peroxide ("piranha") cleans. Likewise, for quartz. Teflon is acceptable for any of the commonly used etchants, although should not be heated (heat is transferred poorly and Teflon will absorb etchant). Polypropylene is not acceptable for piranha (as it melts) although is acceptable for room-temperature HF or BOE etchants. In summary:

Labware material	Piranha	HF or BOE	KOH/TMAH
*Pyrex*	OK	Will etch	OK
Quartz	OK	Will etch	OK
Teflon	OK	OK	Chemically OK Can't heat
Polypropylene	Chemically OK but may melt	OK	Chemically OK but may melt

For other etchants and material combinations, you may wish to consult SNF staff.

4.2 Equipment Setup

4.2.1 The Control Panel

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

4.2.1.1 3130 EPO. This controls power to the control panel, 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 (the Coral system will automatically turn the power off, when completely disabled.)

4.2.1.2 Hood Light.

4.2.1.3 Gooseneck/Aspirator. This controls water flow at the bench. The "Gooseneck" switch turns the tap in the rinse sink on and off. The "Aspirator" switch turns on the aspirator system, allowing the user to drain chemicals (see section 4.5.3 for procedures.) After three minutes, the aspirator will turn itself off; press the switch again to restart.

4.2.1.5 Hot Plate Controller. This controls the heating element on the hot plate.

4.2.1.6 Constant Temperature Bath. This controls heating of the constant temperature bath.

4.3 Startup

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 wbgaas or wbnonmetal) will flash. If this happens, none of the wet benches may be used, as the waste will not be properly treated.

Switch the main power to the bench 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.

Enable a module ( wbgen-hpr, or wbgen-ctb) on Coral. If your process requires the complete bench top or if you do not otherwise want to share the space with other users, make sure to reserve both modules on Coral for the time period you require. Even if you do not use a module, you must enable at least one of them.

4.4 Test Run - N/A

4.5 Processing

4.5.1 Labeling Containers with Blue "In-use Hazardous Chemical" Tags:

Every beaker or open container containing any chemical must have a "In-use Hazardous Chemical" tag with it. This rule includes containers of DI water. The purpose of the tag is to identify the contents of the container as well as the labmember who is using the chemical. Fill the tag out before you pour chemicals in a beaker.

Compositions of the solutions (including water) must be listed on the tag.

Use complete chemical names on the tag (no chemical formulas, no abbreviations, and no jargon such as "piranha").

Extra tags are available in the gowning room.

4.5.2 Pouring Chemicals:

No solvents may be used at this bench (use wbsolvent for processes with solvents.) Only acids and bases or other standard SNF, non-solvent etchants may be used at wbgeneral.

In obtaining chemicals, use proper protection (safety glasses, face shield, yellow chemical resistant Nitril 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 (plastic) 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 chemicals) 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.

Before pouring chemicals, plan how you will dispose of used chemicals (section 4.5.3), especially if you are etching metals. Organize the work bench accordingly.

Make sure to label chemicals in beakers, even water, with the "In-use Hazardous Materials" card.

It is advisable to have a beaker of water for rinsing your substrates.

Now, to pour the chemical. Place the chemical 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 chemical into the beaker or other container. Pour slowly, to avoid any splashing.

For sulfuric/peroxide "piranha" clean, choose a container that is roughly twice the volume of what you plan to pour (boiling piranha tends to "spit".)

First pour the concentrated sulfuric acid.

Because sulfuric is heavy and viscous while peroxide is thin, they do not always mix well. Mix them by taking an empty cassette or appropriate handle and gently place it in the acid and bring it out several times (don't stir!)

After pouring the chemical, replace the cap on the bottle. If any chemical 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!)

If you are using the hot plate, the wbgen-hpr module should be enabled. DO NOT turn the heater. Place your glass or quartz beaker on the hot plate, making sure it is filled with your chemical. Do not use beakers made of materials other than glass or quartz which may melt. Do not heat an empty beaker. Do not walk away from the station when heating. Place a thermometer in the solution (make sure it is compatible with your chemical). Start with the heater setting at "2"; this will heat up approximately to 60 C, depending on the volume and beaker. If required for your process, ramp the temperature setting up SLOWLY. If unattended, the hot plate will shut off automatically after two hours.

4.5.3 Chemical Handling:

Keep all open containers at least 6 inches from the front of the bench to ensure proper exhaust. The red tape line indicates 6 inches.

Chemicals must remain at this bench. Open containers with chemicals may not be transferred to or from this bench.

If you wish to re-use the chemical later, obtain a clean, chemically compatible container with the screw cap lid. Transfer your chemical to that container, using a funnel. Obtain and fill out a permanent yellow personal chemical identifier label for this container. (See the safety section of the website.)

4.5.4 Etching Substrates:

Prepare your substrates for etching. Various holders for wafers and wafer pieces may be found at the wbgeneral station. Ensure that whatever you choose to use is compatible with the etchant (section 4.1.2) and that there is no risk of substrates or holders of slipping or being dropped.

Set your timer or stopwatch (timers are not built into this bench).

Etch your substrates for the appropriate time.

Remove the substrates and place in a rinse beaker.

With the rinse beaker in the rinse sink, turn on the gooseneck and allow rinse water to overflow in the rinse beaker for at least five minutes. Alternatively, you may use the DI hand sprayer to rinse a single piece. For very small pieces, you may rinse in a beaker.

To dry substrates, you may use the N2 gun.

4.5.5 Disposing of Used Chemicals

HF-based acids such as BOE and acid mixtures containing HF must be poured down the HF drain. (These acids must NOT be aspirated or poured down the sink drain). First, carefully lift the HF drain lid. Then, using both hands gently pour the acid waste down the drain, being careful not to splash. Log the approximate amount and composition of acid drained on the station log sheet, located on the left-side of the bench.

All standard, non-HF-based etchants, other than metal etchants must be aspirated. For safety reasons, chemicals must be not be very hot (less than about 120 C) before aspirating. Do not pour these chemicals down the sink drain, as the sink is to be used only for rinsing. The sink is not completely within the exhausted area of the bench (6" from the front edge), so fumes from the chemicals poured here will escape into the lab.

Wearing appropriate chemical protection gear. Remove the aspirator nozzle from its storage space and place the nozzle into the chemical container. Press the "Aspirator" button

Once the container is empty, use the DI hand sprayer to rinse the inside. The aspirator will drain the rinse water. Do this three times. If the aspirator times out before you are done, press the "aspirator" switch again.

Lift the aspirator nozzle off the bottom of the container. Hold it either over the container or over the rinse sink. Thoroughly rinse the end of the nozzle with the DI hand sprayer. Continue to hold the aspirator until the nozzle end stops dripping back. Dry with a cleanroom wipe and replace the aspirator nozzle in its storage space.

Rinse the container again in the rinse sink.

Log the approximate amount and composition of chemicals drained on the station log sheet, located on the left side of the bench.

Metal etchants (including gold and chrome etchants) must absolutely not be aspirated or poured down the rinse drain; they must be collected in waste containers and sent out for hazardous waste disposal. Any waste containing toxics and heavy metals must be collected locally. Toxics and heavy metals include, but are not limited to, the following:

Antimony

Arsenic

Barium

Beryllium

Boron

Cobalt

Manganese

Molybdenum

Selenium

Thallium

Vanadium

Cyanide

Formaldehyde

Gold

Phenols

Any chemicals on the Federal List of Acutely Hazardous Chemicals or the California List of Extremely Hazardous Chemicals must also be collected locally. When in doubt, check with the SNF staff.

Clean, empty waste containers are stored next to the Flammables cabinet, in the semiclean area behind the furnaces. These containers might be previously used for handling chemicals; however, they have been chemically decontaminated and their labels should be defaced (do not use the containers if the labels are intact.) Blank hazardous waste disposal labels are also available next to these designated containers. Do not use empty chemical containers from the chemicals pass-through as they are not decontaminated and pose a chemicals hazard.

Fill out the pertinent information on the hazardous waste disposal label. Attach the label to the waste container. If you are working with multiple chemical mixtures, make sure to use separate containers for collecting different types of waste (acid versus base, for example.)

Use a funnel (make sure it is chemically compatible with the etchant) to pour the used etchant into the appropriately labeled waste container. Rinse the etchant container with water three times, pouring the rinse water each time into the waste container.

Pour the water used to rinse the substrates into the waste container.

Cap the container, making sure that the outside of the container is clean and dry. Make sure the hazardous waste label is securely attached. Transport the container using the transfer cart. Place the container on the appropriate shelf in the chemicals pass-through.

4.6 Standby

Clean up your workspace. Ensure the benchtop is clean and dry. Turn off the heater/stirrers or constant temperature bath if you have used them; otherwise, leave them on (if someone else is using them). Leave the main power on. Disable the module you used (wbgen-hpr, or wbgen-ctb) on Coral. Be sure you are leaving the station in neat condition (see section 5.1).

5. ROUTINE MAINTENANCE AND CALIBRATION

5.1 Daily

Keep the benchtop clean. Thoroughly rinse off any drips (assume they are chemicals) 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.

Remove any cleanroom wipes that may be lying on the benchtop. These have a tendency to get sucked up towards the back of the bench by the exhaust, and into the chemical containers which not only contaminates them, but presents a serious fire hazard.

Check the bench for any damage or any other obvious problems. Make a note on Coral and notify Maintenance if any problems are observed.

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

5.2 Weekly (Maintenance)

N2 gun filters should be changed

5.3 Monthly -- N/A

5.4 Other -- N/A

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 any beakers or other open containers at this station, EVACUATE THE LAB.

6.3 Extended Shutdown

Bench top and plenum should be thoroughly rinsed and drained. Main 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

The wbgaas wet bench may be used in much the same way as wbgeneral. However, priority at wbgaas is given to researchers with GaAs processes to run.

9. ADDITIONAL PROCESS INFO

Background information on some of the processes which are commonly run at wbgeneral is given here. Detailed information and procedures for these and other processes commonly performed at wbgeneral are listed on the website at:

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

9.1 90% Sulfuric/Peroxide Photoresist Strip Piranha Clean

This solution consists of 90% concentrated sulfuric acid and 10% hydrogen peroxide and is heated to 120 +/- 10 degrees C. Sulfuric acid is about 95%-98% pure; the hydrogen peroxide in the lab is 30% in water. This combination is excellent for removing organics. The sulfuric acid converts organic compounds to elemental carbon (which is why the solution may darken temporarily when loaded with photoresist). The peroxide then oxidizes the carbon to carbon dioxide and water (which is why the solution boils and fumes, and eventually clears again.) When the piranha mixture has been around for a while or has been used extensively, the hydrogen peroxide all turns to water, which is a lousy oxidant in this system. So, additional hydrogen peroxide can be added on an as-needed basis to increase the active lifetime of the piranha. However, for the temperatures that we run, additional peroxide can help only so much before the acid becomes diluted and then needs to be changed. For regular usage levels in our lab, a change frequency of about once/week is generally sufficient.

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 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.

9.3.1 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 an eye-catching magenta-colored ty-rap around the neck of the bottle.

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.

9.3.2 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.

9.3.2.1 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.

9.3.2.2 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.

9.4 KOH

Heated 30% KOH in water is used to etch silicon. 45% KOH can be found in the Chemicals passthrough and can be diluted to provide a 30% solution. 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.

_______________________________________________________________
Stanford Nanofabrication Facility
webmaster@snf.stanford.edu
Last Modified 06/09/2008

Antimony	Arsenic	Barium	Beryllium
Boron	Cobalt	Manganese	Molybdenum
Selenium	Thallium	Vanadium	Cyanide
Formaldehyde	Gold	Phenols