Vacuum Technology-2
Concepts and Key Points
Jim Jr-Min Lin 林志民
Institute of Atomic and Molecular Sciences
Academia Sinica, Taipei, Taiwan 106
原分所
Department of Applied Chemistry
National Chiao Tung University, Hsinchu, Taiwan 300
交大應化系
1
Mean Free Path of gas molecules: Viscose flow Vs. Molecular flow�Gas flow: Throughput, Conductance, Pumping speed
Pumps: Mechanical, Roots, Turbo, Diffusion, Dry, Ion, Cryo,
Gauges: Mechanical, Thermal conductance, Ionization,
Chambers: Joints (metal, elastomer), parts
2
Practical concerns:
Surface
Material: SUS, Al alloy, ceramic, plastic,
Baking
Virtual leak
Leak test
UHV
Outlines:
Surface Outgas Concern
example: 10-3 mol H2O, 18 mg, liquid 1.8x10-2 cm3; gas 2.4x10-2 ℓ at 1 atm
If 10-3 mol H2O in chamber surface and if the outgas pressure is 10-7 torr, how long it takes to pump down to 10-8 torr when the pumping speed is 1000 ℓ/s?
Initial pumping throughput is
10-7 torr * 1000 ℓ/s = 10-4 torr ℓ/s
Total amount of water is
1 atm * 2.4x10-2 ℓ = 760 torr * 2.4x10-2 ℓ = 1.8 x 101 torr ℓ
Assume: outgas rate ∝ water amount ∝ P,
i.e. ∝ pump out throughput Q=PS
i.e. first order rate equation
example of exponential decay
same area under both curves
2.3 τ = 115 hr to reduce 10x pressure
Often for a chamber with big surface area
Pump out time constant = τ
Big chamber ≠ big surface area
How to minimize surface area
Remove thick surface oxide:
electro polish SUS chamber and parts
basic wash (NaOH solution) Al alloy
acid wash copper/brass parts
sand blast
Oxide could be porous
Dirty surface is thicker
Strong detergent is much more efficient than solvent
Cleaning
Estimate the effect of Baking
When temperature rises by 100 oC, outgas rate rises by roughly two orders of magnitude, i.e., 10-5 torr instead of 10-7 torr
Initial pumping throughput is 10-5 torr * 1000 ℓ/s = 10-2 torr ℓ/s
to P = 10-10 torr, P0/P = 105
Practically, it takes a little bit longer (≲100 hr)
∵ Single exponential delay is only an approximation
Deeper water has smaller outgas rate, thus longer τ
bake uniformly is important to avoid distortion
Don’t bake oily surface. oil → tar
Aluminum foil on SUS chamber, heating tape on the aluminum foil, another layer of aluminum foil to reduce heat loss
Degas ion gauge during baking
Clean ion gauge and its surrounding by excess heating
Estimate the effect of using plastic parts
plastic may absorb H2O to 1~2 % w/w
Assume 100 g plastic can absorb ~1.8 g H2O = 0.1 mol
If the initial outgas pressure is 10-7 torr, τ = 5000 hr
If the initial outgas pressure is 10-6 torr, τ = 500 hr
more troublesome is that most plastics cannot be baked
Use only
Inert material: Teflon, PE, PP, Kel-F, Viton,
Teflon insulated wire
High temperature material : polyimide (Vespel, Kapton), Kalrez (O-ring)
Less absorption
Bakable to
200oC
less inert than Teflon
Material outgas (volume outgas)
SUS: H2 & CO.
SUS316L can be vacuum firing at 1000 oC to remove deeper contaminants
Al alloy: less H2 & CO. Bakable to 120 oC
Zn & Cd alloy have high vapor pressure
High temperature increase outgas @ bake out
Cooling can reduce outgas @ use
It lasts forever!
More than your life!
Metal seal: copper gasket & ConFlat flange are preferred
Sealing Concern:
100% seal
low outgas
bakable
O-ring seal: Viton O-ring bakable to 100 oC
15 ~ 18 % compression to seal
volume compression is not allowed
sealing surface polish is important
small leak is possible
(Hard to find small leaks)
convenient
non-consuming
Careful to use viton gasket on conflat flanges
very easy to leak for size larger than 4.5”
two surfaces may fuse together
use silver plated screws in SUS taps
not cheap
Dynamic Seal
a virtual leak
without differential pumping
760 torr * L = 10-7 torr * 1000 ℓ/s
L = 1.3 x10-7 ℓ/s
760 torr
10-7 torr
1000 ℓ/s
760 torr
760 torr
P2
0.1 ℓ/s
P3
with differential pumping
760 torr * L = P2 * 0.1 ℓ/s
P2 = 1 x10-3 torr
P2*L=P3*1000 ℓ/s
P3= 1.3x10-13 torr
O-ring: 15 ± 2 % compression + Grease
polished surfaces to have the above leak rate
careless work makes 1~2 orders worse
O-ring may trap gas & water
⇒ Virtual leak
rotatable
or
translational
Virtual Leak: Gas trapped in a vacuum system, can’t be found from outside
1 cc at 1 atm = 760 x 107 cc at 10-7 torr = 7.6 x 106 ℓ at 10-7 torr
10-1 pressure drop takes 2.3τ
Estimate pump down time constant if leak out rate is 1.32x10-7 ℓ/s and the chamber is pumped at 1000 ℓ/s, that is, 760 torr x 1.32x10-7 ℓ/s = 10-7 torr x 1000 ℓ/s
O-ring can trap small bubbles and release them when being moved.
leak: 1.32x10-7 ℓ/s
1 cc
at 1 atm
pumped at 1000 ℓ/s,
1x10-7 torr
Leak Check
Spread CH3OH or C2H5OH on a possible leak to see if pressure rises
Acetone is OK for metal, bad for O-ring, bad for health
Response rise time ~ few seconds, Don’t move too fast.
It takes very long to dry out the solvent. Very long fall time
From lower spots to higher spots.
Helium leak check:
Spread He to see if PHe rises
MASS is required. RGA or He leak detector
fast ≲ 1 sec ∵ light mass ∴ fast speed
He is fast to escape, fast to pump down
low background
inert
from higher spots to lower spots
He is easy to reach a nearby spot. ⇒ isolation
high m28/m32 (4:1) indicate air leak
daughter ion is useful. CO+ C+ O+ Vs. N2+ N+
Don’t make vacuum chamber wet, especially at a rainy day
H2O is very common
RGA provide very important information
Good vacuum practices
No leak
Clean: traps for oil pumps: molecular sieve, LN2
Metal & non-porous ceramic is excellent
Plastic and grease: as less as possible
Confident sealing. Finding a leak is labor consuming.
Bakable for 10-10 torr or better
Good local conductance for pumping speed
Gas composition (partial pressure) is often more important than the total pressure, as most vacuum parameters are species dependent. e.g. surface laser burn, background masses
RGA is very nice to have
Example of utilizing a sorption pump to recycle isotope gases, see §3.4.3
pump 2
turbo pump 1
2000 ℓ/s
P1= 1x10-5 torr
O2 (18O, 97%, > NT$10000/ℓ)
ℓ: std liter (1 ℓ at standard condition, i.e. 298K, 760 torr)
If pump 2 = mechanical pump, O2 will be mixed with air and oil.
If pump 2 = molecular sieve sorption pump, O2 can be absorbed at liquid nitrogen temperature and retrieved at room temperature.
molecular sieve
gas inlet
Question:
How much O2 (in atm ℓ) is used per hour?
φ 25mm
φ 6mm
Quiz: Write answer on a page of A4 paper
A mixture of 10% C4H10 in He flows into a vacuum chamber pumped by a turbo pump. The Ion Gauge reading is 5x10-6 torr.
(a) What is the true pressure?
(b) What is the pressure reading for a 20% mixture at the same flow rate?
Note: Given that the relative electron impact cross section:
σ(N2)=1, σ(He)=0.15, σ(C4H10)=5
Untra High Vacuum < 1x10-10 torr
Example of 1 x 10-12 torr
Practically
clean chamber, turbo pump, not baked, 10-9 torr
clean chamber, 2 serial turbo pumps, baked, 10-11 torr
(compression ratio for H2)
LN2 78K
<10K
50K
He cold head
(remove displacer to bake)
room temperature
5x10-11 torr
LN2 3x10-11 torr
(much cleaner)
cold head
1x10-12 torr
How to make Ultra High Vacuum (UHV): Outgas rate = pumping speed x pressure
100 ℓ/s x 10-11 torr
Ultimate
pressure
of pumps
Outgas
rate
Effective
pumping speed
2 serial Turbo Pumps: 10-11 torr, H2 compression ratio
Ion Pumps : 10-11 torr, small pumping speed, memory effect
Getter Pumps: Titanium Sublimation pump, Non-evaporative getter
Cryopump: 10-10 torr, not-bakable, memory effect
Materials: Stainless Steel, Al alloy, OFHC copper, ceramic, teflon, kapton
S-1 = S1-1 + S2-1
Bake & Contamination: Firing, porous oxides, oil→tar→charcoal
Virtual Leaks
Low temperature: LN2, cryohead
Surfaces: mechanical polish (glue on sand paper!), electro polish, acid, base
Turbo pumps: 400 ℓ/s, 600 ℓ/s
compression ratio: 104 for H2, 106 for He,109 for N2
150oC
Oil, grease, or magnetic bearing and insulated wires
Fore line back stream when Electric shutdown
Getter pumps: Ion, Ti, NEG
No foreline needed, no continuous electricity needed
250oC, 400oC
Not for every gas, memory effect
Low maintenances for low load systems
Cryopumps: >1500 ℓ/s,
bake to 70oC,
Not for every gas, memory effect
Outgas due to activated carbon absorber
Electric shutdown
Cryopump + turbo pump: Very high pumping speed even for H2 at 10-9 torr
Bakable Cryohead without absorber: high pumping speed for H2 at <10-11 torr
Low outgas ⇒ 10-12 torr
SUS304, SUS304L: Cr 18%, Ni 10%, Fe, C<0.2 % or Low carbon < 0.08%
SUS316, SUS316L: Cr 18%, Ni 10%, Mo 2-3%, std and Low carbon
Sand ballasting, basic detergent, Acid dip, Electro polish, DI water
Easy to be welded,
Bake to 250oC, SUS316L: Fire at 1000oC at 10-8 torr
Major outgas: H2, CO
Al alloy 6061-T6, and others
Low H2, CO outgas
Welding at outside, must clean before welding
NaOH(aq), HNO3(aq),
Al2O3 is porous. Mirror finish parts is available (Japan)
120oC, high temperature will change tempering condition
Plastic: gas/water permittivity is high
Teflon absorb water 10 times less than usual plastics, but still too much for UHV
Teflon, PE, PP, might be OK for 10-8 torr, others are only good to 10-6 torr.
polyimide (Kapton) is bakable, 10-10 torr
Oxygen Free High Conductance copper, Beryllium copper
Brass and bronze could be dirty (zinc, phosphor)
Special (strong) acid brightening
Ceramic could be porous
Al2O3 (alumina) (thermal conductance better than SUS)
Vacuum firing