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R. B. Darling / EE-527 / Winter 2013 EE-527: MicroFabrication Compressed Gas Systems

Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

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Page 1: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

EE-527: MicroFabrication

Compressed Gas Systems

Page 2: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Compressed Gases in Microfabrication

• Gases offer extremely high purity chemicals for microfabrication processes, if handled properly.

• Carrier gases: N2, He, Ar• Silicon sources: SiH4, SiCl4, SiF4, Si2H6, SiH2Cl2, TEOS• Oxidizers: O2, H2O, N2O• Reducers: H2, NH3, FG• Dopant gases: AsH3, PH3, B2H6

• Etch gases: CF4, CHF3, SF6, Cl2, F2, many others …

Page 3: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Compressed Gas Hazards• Mechanical

– Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored in a cylinder, even

without consideration of the chemical energy of the contents. – Small parts can be propelled at bullet-like velocities if fittings are

disconnected while still pressurized.

• Chemical– Many process gases are (nearly) inert, but some are extremely flammable

and reactive when mixed with air.

• Health– Any compressed cylinder of gas, if discharged into a confined space, can

displace air and/or oxygen and become a simple asphyxiant. – Many microfabrication gases are extremely toxic even at low levels.

Page 4: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Gas Cylinder Handling and Storage• Cylinders are steel; valves are brass.

– Biggest hazard is having a cylinder fall and have its valve broken off. – The high pressure gas inside will turn the cylinder into a torpedo which

can easily penetrate building walls.

• Precautions to be taken to avoid cylinder falls: – Standing cylinders are always chained up to their rack.

• NEVER, EVER leave a cylinder standing unattended in the laboratory!• Place safety chains about 2/3 of the way up for best protection. • Chains should have minimum slack, but do not need to be tensioned.

– Cylinder caps must be replaced when the cylinder is disconnected. – Cylinders can be heavy – always use a proper cylinder cart which has a

cylinder cradle for transporting. • Common hand trucks are not designed for cylinders and are accident prone.

– If no cylinder rack and chain are available, the best practice is to gently lay the cylinder down horizontally on the floor.

Page 5: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Standard High Pressure Gas Cylinder Sizes

Size DOT Specification Dimensions Tare Weight Internal Volume

A 3AA2400 9” dia. x 55” tall 137 lbs. = 62 kg 1.76 ft3 = 49.8 L

B 3AA2265 9” dia. x 51” tall 119 lbs. = 64 kg 1.55 ft3 = 43.9 L

C 3A2015 7” dia. x 33” tall 57 lbs. = 26 kg 0.56 ft3 = 15.9 L

D 3AA2015 4” dia. x 17” tall 9 lbs. = 4 kg 0.10 ft3 = 2.8 L

LB 3E1800 2” dia. x 12” tall 2 lbs. = 0.7 kg 0.015 ft3 = 0.43 L

ME 3AA2015 4” dia. x 26” tall 14 lbs. = 6 kg 0.16 ft3 = 4.5 L

BX 3AA6000 10” dia. x 51” tall 300 lbs. = 136 kg 1.49 ft3 = 42.2 L

BY 3AA3500 9” dia. x 51” tall 187 lbs. = 85 kg 1.53 ft3 = 43.3 L

Size refers to cylinder designation used by Air Products and Chemicals, Inc.

LB = Lecture Bottle; ME = Medical size E; BX, BY = very high pressure cylinders

Note: First digit and letters of DOT specification give the construction, e.g. 3AA = steel.

Note: Last 4 digits of DOT specification give the service or working pressure in psi.

Page 6: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

The Most Common Gas Cylinder Sizes for Microfab

Cylinder size designations are those used by Air Products; other gas vendors use different designations for the same cylinder sizes.

Size A cylinder9 in dia x 55 in tall

Size B cylinder9 in dia x 51 in tall

Size C cylinder7 in dia x 33 in tall

Lecture Bottle2 in dia x 12 in tall

Some vendors refer to this as a ‘K’ size cylinder.

Page 7: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Cylinder Valves

• Have Compressed Gas Association (CGA) standard outlet ports. • Cylinder valve is an ON/OFF switch: open or close completely.

– The cylinder valve is NOT used to regulate gas flow or pressure. – When opened completely, the valve seals off its own packing box.

CGA outlet fittings

packing box

valve seal

attached to gas cylinder

safetyoverpressurerelease

to regulator

Page 8: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

CGA Outlet Fittings

(from Coyne, 1992)

C3H8 (propane)

O2

N2, Ar, He, CF4

SF6

Cl2, CHF3

Note: flammable gases usually have LH threads, indicated by notches on their nuts.

CGA fittings are designed NOT to interchange by using different LH/RH, male/female, TPI, and diameters.

Page 9: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Pressure Regulators• Adjustment knob is used to apply

pressure to the ambient side of the diaphragm.

• Spring pressure opens up the valve seat, letting high pressure gas from the supply side fill the backside of the diaphragm cavity.

• When the pressure in the backside cavity equals the spring pressure, the valve seat will close.

• As gas is drawn from the delivery side, an equilibrium is set up between the spring pressure and the backside diaphragm pressure.

• Fspring = Pdelivery Adiaphragm

• Must “unscrew” the adjusting knob to reduce the pressure.

adjustmentknob

pressurespring

diaphragm

valve seat

valvebody

Page 10: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Pressure Regulator Types• Low delivery pressures require large diaphragms. • One solution is to use a 2-stage regulator.

single-stageregulator

two-stageregulator

The first stage is preset.

The second stage is adjustable.

Page 11: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Pressure Gauges

• The most common mechanical pressure gauge is the Bourdon tube, used for both gases and liquids.

• Common pressure units: – 1 atmosphere =

• = 760 mmHg = 760 Torr• = 29.9213 inHg• = 407.189 inH2O• = 101,325 Pa (N/m2)• = 1.01325 bar• = 14.6959 psi

• Pressure measurements: – Gauge pressure: P1 – 1 atm (ambient!)– Absolute pressure: P1

– Differential pressure: P1 – P2

psi

Page 12: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Pressure Regulator and Gauge Combination

psipsi

highpressure

side

lowpressure

side

Typical:0 – 100 psi

Typical:0 – 3000 psi

Page 13: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Cylinder & Pressure Regulator Connection

psipsi

deliverypoppetvalve pig tail

pressureregulator

gas cylinder

cylindervalve

deliverypressuregauge

cylinderpressuregauge

CGA fitting

Page 14: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Procedure to Connect a Cylinder to a Regulator • Move the cylinder into position and secure it with a safety chain. • Remove the cylinder cap and clean the CGA fitting. • Attach either a regulator or a pigtail to the CGA fitting.

– Run the threads down all the way first by hand. – Finally, tighten the connection with a wrench, but only snug is enough.

• Insure that the delivery poppet valve is closed and the regulator adjustment knob is backed out and loose.

• Slowly open the cylinder valve. – The cylinder pressure gauge should rise and show the cylinder pressure. – Finally, open the cylinder valve all the way.

• Tighten the regulator adjustment knob to achieve the desired delivery pressure.

• Open the delivery poppet valve to supply the gas to the system.

Start at the cylinder and work towards the delivery point

Page 15: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Procedure to Disconnect a Cylinder from a Regulator

• Close the cylinder valve completely. • Release the pressure in the lines throughout the system.

– The system may have a special vent for this. – Insure that BOTH the delivery and cylinder pressure gauges read ZERO. – NEVER attempt to break any line which is still pressurized!

• Close the delivery poppet valve. • Fully loosen the regulator adjustment knob. • Break the line at the CGA fitting.

– Loosen the nut with a wrench and then spin the nut off by hand. – If regulator is directly connected to the cylinder, support it to keep it from

falling. • Replace the cylinder cap. • Remove the safety chain and move cylinder onto a cart or into a

storage rack.

Page 16: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Poppet Valves• Used for ON/OFF gas flow control. • Cannot be used to adjust either pressure or flow. • Most are actuated by 1/4 turn of their handle. • Orientation of the handle relative to the pipe usually

indicates the open or closed state. (But not always!)• Note: ball valves, commonly used for fluids, are not used

for high pressure gases.

Page 17: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Needle Valves

• Used to control gas flow, not pressure, by means of an adjustable orifice.

• These use a narrow, tapered needle within an orifice to adjust the flow with fine control.

• These should never be used as an ON/OFF valve.

• Tightening the valve all the way closed will damage the orifice and the needle!

Note: One can always tell a flow-regulation needle valve from an on-off poppet valve by the presence of graduations around its knob.

Page 18: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Flow Meters

• Also known as “rotameters.” • These use a tapered tube and a

ball which floats in the upward flowing gas stream.

• The height of the ball depends upon its size, weight, gas viscosity, tube taper, and the flow rate of the gas.

• A needle valve is often integrated with the flow meter.

• These must be used and installed in a vertical orientation!

Page 19: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Flow Meter Calibration

• Flow meters are calibrated for air.

• For other gases, multiply the flow meter reading by a correction factor to obtain the actual flow rate for that gas.

• Less viscous gases have a higher correction factor.

• Gas flow rate units:

Gas Correction FactorAcetylene 1.04Ammonia 1.30Argon 0.85Carbon Dioxide 0.81Helium 2.69Hydrogen 3.81Methane 1.35Nitrogen 1.01Oxygen 0.95Propane 0.80

SCFM = Standard Cubic Feet per Minute

SLPM = Standard Liters Per Minute

Page 20: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Gas Flow Rates• Gas flow is measured as a volumetric flow rate under

standard temperature and pressure (STP) conditions. • STP depends upon the organization involved:

– International Union of Pure and Applied Chemists (IUPAC): • Pressure = 1.000 bar = 100 kPa = 14.504 psi = 0.986 atm. • Temperature = 0°C = 273.15 K.

– National Institute of Standards and Technology (NIST): • Pressure = 1.01325 bar = 101.325 kPa = 14.696 psi = 1.000 atm. • Temperature = 20°C = 293.15 K.

– Standard Ambient Temperature and Pressure (SATP): • Pressure = 1.01325 bar = 101.325 kPa = 14.696 psi = 1.000 atm. • Temperature = 25°C = 298.15 K.

• Gas flow rates are in SCFM or SLPM, but they don’t say which standard… Use NIST unless specified otherwise.

Page 21: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Gas Cylinder Capacity and Consumption

• Sample calculation: How long will a size A cylinder of N2provide purge for a furnace tube running at 5 SLPM? – A size A cylinder has an internal capacity of 49.8 L ≈ 50 L. – A new, full size A cylinder should arrive with 2200 psig. – The cylinder compression ratio is 2200/14.7 = 149.67 ≈ 150X.

• Using Boyle’s Law, ignoring any temperature change resulting from expansion of the gas.

– The expanded volume of the N2 is 150*50 L = 7500 Liters. – Running at 5 SLPM, this cylinder would last for 1500 min. = 25 hr.

1 SLPM = 0.035315 SCFM = 2.11888 SCFH

Page 22: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Mass Flow Controllers (MFCs)• The flow is measured by differential pressure across a flow restriction. • The flow is metered by an electronically controlled solenoid valve. • Electronic feedback control keeps the flow rate at a desired set point value. • They are self-contained analog control systems. • Host controller sends set point value to MFC; MFC reads back flow rate.

FLOW

solenoid

inlet exhaustflowrestrictor

meteringvalve

differentialpressuresensor

electroniccontrol & interface

Page 23: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

3-Valve Inert Gas Purge Panel

• These are used to keep from contaminating a feed gas line with air or oxygen during a cylinder changeover.

• After new cylinder is attached to the pig tail, cylinder valve is opened, and pig tail is purged.

• The regulator tap valve is then opened to supply gas to regulator.

• Regulator tap valve is closed before disconnecting cylinder.

• Purge valve is then used to depressurize the pig tail after closing the cylinder valve.

psipsi

gas cylinder

cylinder valve

pig tail

pressureregulator

highpressure

side

lowpressure

side

purgeexhaust

purgevalve

deliveryvalve regulator

tapvalve

gassupply

Page 24: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Gas Piping Materials• Metal

– Aluminum: Low pressure only! Not for reactive gases! – Brass and copper: Used only for compressed air pneumatics– Stainless Steel: Usually the best all-round choice for microfab lab

gases and fittings. Type 316 stainless steel is the most common.

• Plastic– Teflon: About the only plastic suitable for process gases, but cold

flow characteristics require special consideration for fittings.

– Inexpensive metal tubing is usually rolled and welded, leading to grease and dirt on the interior. (NOT APPROPRIATE)

– Proper gas tubing is seamless and “cleaned and capped” with the interior bore brushed, degreased, and electropolished.

Page 25: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Gas Piping Connections

• Not suitable for clean, leak-free piping: – Standard pipe threads– Flared tubing connectors– O-ring joints

• Suitable for clean, leak-free piping: – Swagelok® fittings– Cajon ® VCR fittings– Orbital TIG welded connections

Page 26: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Swagelok® Fittings

• These utilize a pair of nesting ferrules to create a swaged on sealing surface to the tubing.

• The first assembly permanently swages the ferrules on to the tubing: – Finger tight plus 1 & 1/4 turns– (3/4 turn for 1/16 to 3/16 inch)

• The same fitting parts can be reassembled many times: – Finger tight plus 1/8 to 1/4 turn

• If mixing materials, use: – Same nut and base– Same tubing and ferrules

• Example: for connecting plastic tubing to a brass base: use plastic ferrules and a brass nut.

nut

tubing

base

small ferrule

large ferrule

Page 27: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Cajon® VCR Fittings• These utilize a flat compressible

metal gasket between a pair of hemicircular beads on each sealing gland.

• Gasket materials: – Nickel– Stainless steel– Copper

• These can be reassembled many times, but a new gasket is required each time.

• Finger tight plus 1/4 turn for Cu gaskets; finger tight plus 1/8 turn for other gasket materials.

• The nut usually has sniffer holes for leak testing.

nut

tubing

weldedtubing gland

gasket

base gland

Page 28: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Liquid Nitrogen (LN2) Cylinders - 1

from Coyne, p. 269

The most common LN2cylinder size is the LS160, which holds 160 liters of LN2. Most of these cylinders can supply both liquid and gaseous N2.

Page 29: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Liquid Nitrogen (LN2) Cylinders - 2

from Coyne, p. 270

Page 30: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Liquid Nitrogen (LN2) Cylinders - 3

from Coyne, p. 272

Page 31: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Liquid Nitrogen (LN2) Cylinders - 4

from Coyne, p. 273

Page 32: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Using Liquid Nitrogen (LN2)

• Common Hazards: – Tipping: cylinders are quite heavy when full; > 200 lbs. – Direct LN2 burns: freezes tissue rapidly; same damage to cells as

if they were exposed to a high temperature source. – Indirect cryogenic burns: LN2 may sufficiently chill metal objects

so that these objects can produce low temperature burns, e.g. valves and delivery tubes.

– Asphyxiation: the contents, if vaporized all at once, can displace enough air and oxygen to produce an asphyxiation hazard.

– Frosting: cold surfaces will condense water vapor into frost, which can interfere with equipment operation.

Note: The “fog” produced by evaporating liquid nitrogen is actually the condensation of water vapor in the air which has been chilled, just like in a cloud. Nitrogen is itself colorless and odorless.

Page 33: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Uses for LN2 in Microfabrication

• Cryogenic cooling: – Components containing LN2 can be brought down to its liquefaction

temperature of 77 K = −196C. – Examples:

• LN2 traps on vacuum pumping stacks. • Cryogenically cooled wafer chucks from some processes.

• Source of dry N2: – Evaporated LN2 provides an extremely pure source of dry N2. – Liquid sources can also supply an extremely large amount of dry N2.

• Can figure approx. 1000X expansion from liquid to vapor at STP. – After being filled, LN2 cylinders are constantly warming up, and

slowly loose their contents to evaporation. A standard LS160 will normally last about 1 month, lacking any use of the LN2.

Page 34: Compressed Gas SystemsCompressed Gas Hazards • Mechanical – Compressed gas cylinders are typically 2000 psi or greater when full. – Enormous mechanical potential energy is stored

R. B. Darling / EE-527 / Winter 2013

Transporting Liquid Nitrogen (LN2)• Small dewars (vacuum vessels):

– Glass thermos bottles are quite good, as long as their caps are not screwed down. (Expansion upon warming will explode them.)

– Small commercial dewars are common, typically in sizes from 5 to 25 liters. These usually have a styrofoam cap that does not restrict pressure build-up.

• Pouring and filling LN2: – Gloves: a personal choice, although often required by local protocols.

• Special cryogenic gloves are available to provide thermal insulation and protection against spills.

• Clean room gloves do not provide any protection. • Small splashes from LN2 will often just bounce off, whereas a glove can sometimes

trap the LN2 against the skin. – Safety glasses are always required!

• Splashes of LN2 to the eye are particularly dangerous because of the instant freezing of the water in the eye.

– Beware of residual water in the vessel being filled! • This will instantly be turned into ice by the LN2, and the ice can potentially crack the

vessel when it freezes.