EE-527: MicroFabrication

Compressed Gas Systems

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 …

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.

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. 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.

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.

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

Size A cylinder Size B cylinder Size C cylinder Lecture Bottle 9 in dia x 55 in tall 9 in dia x 51 in tall 7 in dia x 33 in tall 2 in dia x 12 in tall 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 to regulator

safety overpressure attached to gas cylinder release

R. B. Darling / EE-527 / Winter 2013 CGA Outlet Fittings

C3H8 (propane)

O 2 Note: flammable gases usually have LH threads, indicated by N2, Ar, notches on their nuts. He, CF4 CGA fittings are designed NOT to SF6 interchange by using different LH/RH, Cl2, CHF3 male/female, TPI, and diameters.

(from Coyne, 1992) R. B. Darling / EE-527 / Winter 2013 Pressure Regulators • Adjustment knob is used to apply pressure to the ambient side of the adjustment diaphragm. knob • Spring pressure opens up the valve seat, letting high pressure gas from the supply side fill the backside of pressure the diaphragm cavity. spring • When the pressure in the backside cavity equals the spring pressure, the valve seat will close. diaphragm • As gas is drawn from the delivery valve seat side, an equilibrium is set up between the spring pressure and the backside diaphragm pressure. valve body •Fspring = Pdelivery Adiaphragm • Must “unscrew” the adjusting R. B. Darling / EE-527 / Winter 2013 knob to reduce the pressure. Pressure Regulator Types • Low delivery pressures require large diaphragms. • One solution is to use a 2-stage regulator.

The second stage is adjustable.

two-stage regulator

single-stage The first stage regulator is preset.

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 psi • Pressure measurements:

– Gauge pressure: P1 – 1 atm (ambient!)

– Absolute pressure: P1

– Differential pressure: P1 –P2 R. B. Darling / EE-527 / Winter 2013 Pressure Regulator and Gauge Combination

Typical: Typical: 0 – 100 psi 0 – 3000 psi

low high pressure pressure side side i p ps si

R. B. Darling / EE-527 / Winter 2013 Cylinder & Pressure Regulator Connection

delivery cylinder pressure pressure gauge gauge CGA fitting

i p ps si cylinder valve delivery poppet pressure regulator valve pig tail

gas cylinder

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 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.

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.

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. Note: One can always tell a flow- • Tightening the valve all the regulation needle valve from an on- off poppet valve by the presence of way closed will damage the graduations around its knob. orifice and the needle!

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! R. B. Darling / EE-527 / Winter 2013 Flow Meter Calibration

Gas Correction Factor • Flow meters are calibrated Acetylene 1.04 for air. Ammonia 1.30 • For other gases, multiply Argon 0.85 the flow meter reading by Carbon Dioxide 0.81 a correction factor to Helium 2.69 obtain the actual flow rate Hydrogen 3.81 for that gas. Methane 1.35 • Less viscous gases have a Nitrogen 1.01 higher correction factor. Oxygen 0.95 • Gas flow rate units: Propane 0.80 SCFM = Standard Cubic Feet per Minute SLPM = Standard Liters Per Minute 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. R. B. Darling / EE-527 / Winter 2013 Gas Cylinder Capacity and Consumption

• Sample calculation: How long will a size A cylinder of N2 provide 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

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.

electronic control & interface solenoid

differential metering pressure valve sensor

FLOW

inlet flow exhaust restrictor R. B. Darling / EE-527 / Winter 2013 3-Valve Inert Gas Purge Panel

gas purge • These are used to keep from supply exhaust contaminating a feed gas line with air or oxygen during a purge valve cylinder changeover. low high pressure pressure i p • After new cylinder is attached side ps si side to the pig tail, cylinder valve is

opened, and pig tail is purged. delivery regulator valve pressure • The regulator tap valve is then regulator tap valve opened to supply gas to pig tail regulator. cylinder valve • Regulator tap valve is closed before disconnecting cylinder. • Purge valve is then used to depressurize the pig tail after closing the cylinder valve. gas cylinder 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.

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

R. B. Darling / EE-527 / Winter 2013 Swagelok® Fittings

• These utilize a pair of nesting ferrules to create a swaged on tubing sealing surface to the tubing. • The first assembly permanently swages the ferrules on to the nut tubing: – Finger tight plus 1 & 1/4 turns small ferrule – (3/4 turn for 1/16 to 3/16 inch) large ferrule • 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 base – Same tubing and ferrules • Example: for connecting plastic tubing to a brass base: use plastic ferrules and a brass nut. R. B. Darling / EE-527 / Winter 2013 Cajon® VCR Fittings • These utilize a flat compressible tubing metal gasket between a pair of hemicircular beads on each sealing gland. nut • Gasket materials: – Nickel – Stainless steel – Copper welded • These can be reassembled many tubing gland times, but a new gasket is required each time. gasket • Finger tight plus 1/4 turn for Cu gaskets; finger tight plus 1/8 base gland turn for other gasket materials. • The nut usually has sniffer

holes for leak testing. R. B. Darling / EE-527 / Winter 2013 Liquid Nitrogen (LN2) Cylinders - 1

The most common LN2 cylinder size is the LS160, which holds

160 liters of LN2. Most of these cylinders can supply both liquid and

gaseous N2.

from Coyne, p. 269 R. B. Darling / EE-527 / Winter 2013 Liquid Nitrogen (LN2) Cylinders - 2

from Coyne, p. 270 R. B. Darling / EE-527 / Winter 2013 Liquid Nitrogen (LN2) Cylinders - 3

from Coyne, p. 272 R. B. Darling / EE-527 / Winter 2013 Liquid Nitrogen (LN2) Cylinders - 4

from Coyne, p. 273 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.

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 = −196C. –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.

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