Silicide Wet Bench
wbsilicide
Operating Instructions
The wbsilicide wet bench is used for photoresist removal and cleaning wafers prior to processing in silicide furnaces or metal deposition. Only Si, SiGe, or quartz (no GaAs) wafers may be cleaned here. Substrates may contain only the standard Al, Ti, or W silicides (not free metals), deposited at SNF. The station contains (from left to right, at the back of the bench) three quartz hot pots for wafer cleaning (90% sulfuric/peroxide resist strip; 4:1 sulfuric/peroxide clean; 5:1:1 H2O:H2O2:HCl clean, respectively), and two teflon tanks containing 50:1 HF and 6:1 BOE. The front of the bench contains two automatic dump rinsers, a glove wash, DI hand sprayer, and an N2 gun. There is a dedicated spin-rinse dryer just to the right of the main bench. For more process details, see section 9.
Substrates must be 3" or 4" 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.
The wbsilicide bench is in the "semiclean" equipment group (click <here> for Equipment Group Policy) but is subject to additional special constraints. Substrates containing any metal films other than the standard SNF metal silicides are strictly prohibited from being processed here, even if the free metal has been previously removed. Wafers with standard SNF photoresists may be cleaned here only in the left hot pot. Wafers are considered contaminated with standard metals following processing at this station. Under special circumstances, the right hot pot may be used for decontamination of KOH-processed materials (see the Process Info links.) Consult staff members or the Materials Section of the website for more details about which specific materials are acceptable at this station.
wbsilicide 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).
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; three Teflon-lined tanks (for HF etching) which drain into HF waste collection system; and two dump rinsers and a spin-rinse dryer 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.
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 90% sulfuric/peroxide and 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.
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.
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.
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.
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 is already in a chemical bath, finish the process and place the cassette in the nearest dump rinser without starting the rinse. If a dump rinser is in use, stop the rinse while the rinser is filled up with water. Don't start the spin-rinse dryer.
Operators of this station must be able to demonstrate or describe the activities on this list:
Chemical safety knowledge:
Contamination control knowledge:
Process knowledge:
Equipment knowledge:
The control panel modules are, from left to right are:
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):
4.2.3 3150 Process Controller Functions (Hot Pots)
4.2.5 3170A Process Timer Functions (HF Tanks)
4.2.6 Spin/Rinse Dryer
Please leave the dump rinser filled with water. It will automatically flush the water every 90 minutes. This keeps the water lines clean.
5. ROUTINE MAINTENANCE AND CALIBRATION
- N2 gun filters should be changed.
Changing chemicals:
6.1 Standby -- See section 4.6.
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.
Hot pots, tanks, dump rinsers and plenum should be drained and thoroughly rinsed. Power should be shut off to the system.
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.)
For the procedures of the specific processes which are normally performed at this station, check the Process Info link for wbsilicide on the SNF website:
http://snf.stanford.edu/Equipment/wbsilicide/ProcessInfo.html
For processes which may be performed at other wet bench stations, check the Processes section of the SNF website.
9.1 Photoresist Strip/Piranha Clean
This solution consists of 90% concentrated sulfuric acid (H2SO4) and 10% hydrogen peroxide (H2O2) 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.
Wafers with photoresist or if they have just been scribed must undergo a 20 minute clean in the 90% sulfuric before proceeding to pre-diffusion or pre-deposition cleans.
9.2 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.
9.3 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.
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.
9.5 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.
9.5.1 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.
9.5.2 Buffered Oxide Etchants: BOEBOE 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.
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.
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.
|
_______________________________________________________________
Stanford Nanofabrication Facility webmaster@snf.stanford.edu Last Modified 05/09/2007 |
