Primary Standards for Humidity

(1)

Primary Standards

for Humidity

Andrea Peruzzi

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Humidity is everywhere

¾ You step out of your morning shower, you look at the mirror of your bathroom and its surface is covered with condensed water.

¾ The moment you step out of your shower, you start drying off by way of evaporation (which makes you feel cold)

¾ The rate of evaporation of the water from your skin depends on the relative humidity: the lower the relative humidity the higher the evaporation rate.

¾ In warm weather, your body is using evaporation of sweat to cool down.

¾When humidity is high, the temperature perceived by your body is higher than the actual temperature (because you cannot cool down by way of evaporating your sweat)

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Hierarchy of Humidity Standards

SI unit

Gravimetric hygrometers

Thermodinamically-Based Humidity generators

kilogram kelvin, pascal

Primary standards

Secondary standards

Working standards Humidity analyzers (Chilled Mirror

Hygrometers, CRDS)

Industry labs,

every day life Traceable humidity measurements

Traceability: relate a measurement to the corresponding SI unit through an unbroken chain of calibrations

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Summary

• Gravimetric hygrometer

• Thermodynamically-based Humidity generators:

¾ Theoretical basis

¾ Operating principles

¾ PTB standard humidity generator (two pressure)

¾ NPL standard humidity generator (two-temperature)

¾NMi VSL standard humidity generator (two-temperature) • Low Frost-Point Generators:

¾ The need

¾ NIST LFPG (ppb level)

¾ NIST dilution system (ppt level)

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Gravimetric Hygrometer

¾ Most fundamental and accurate measure of humidity of a gas

• Given a stream of moist gas

• the water vapour is separated from the gas stream

• the masses of the two components (water and dry gas) are collected and weighted

• r = (mass of water) / (mass of dry gas) (mixing ratio)

¾ Time consuming and

impractical for calibration service

¾Available at NPL (UK),

PTB (Germany), NIST (USA) and NMIJ (Japan)

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Thermodynamically-based Humidity

Generator

Theoretical basis:

¾ An inert gas stream (air or nitrogen)…

¾ … is saturated with water vapour …

¾ … by flowing the gas over a plane surface of isothermal water (or ice) …

¾ … at known temperature T_S and pressure P_S

( ) ( )

_s _s s s w w f T P P T e x = ⋅ , Where:

x_w = mole fraction of water vapour T_S= temperature of the saturator P_S= pressure of the saturator

e_w(T_S) = saturation vapour pressure of the condensate (water or ice) f(T_S, P_S) = enhancement factor

Dry gas Moist gas

Saturator at {T_S, P_S}

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Humidity Generator requirements:

The mole fraction of water x_w can be calculated only if:

¾ Within the saturator, the gas mixture is in thermodynamic equilibrium with the condensate (temperature and pressure of both phases are equal)

If a test instrument is attached downstream of the saturator:

But only if:

¾ No condensation and no evaporation occurs along the pipes

¾ Chilled mirror and sample stream are in local thermodynamic equilibrium

( ) ( )

s s s s w w f T P P T e x = ⋅ ,

( ) ( ) ( ) (

h h

)

h h w s s s s w w f T P P T e P T f P T e x = ⋅ , = ⋅ ,

Dry gas Moist gas in

Saturator {T_s, P_s} Water

Moist gas out Chilled Mirror Hygrometer {T_h, P_h}

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Humidity Generators: operating principles

Two-temperature generator:

¾ The carrier gas is saturated at one temperature and

¾ After saturation, the mixture is raised to another known temperature to produce the desired relative humidity

Two-pressure generator:

¾ The carrier gas is saturated at a known temperature and elevated pressure

¾ After saturation, the carrier gas is expanded to produce the desired level of humidity

¾ Advantage: fast switching from one set point to another (by changing the saturator pressure)

¾ Disadvantage: the expansion leads to additional uncertainties

Two-flow generator:

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PTB Standard Humidity Generator (SHG)

Pre-saturator Pressure controller Dry gas Saturator Hygrometer under test Temperature-controlled baths Pressure gauge Pressure reducing valve

¾ The wet gas is saturated at elevated pressure

¾After saturation, the carrier gas is expanded through the pressure reducing valve

¾ Conservation of water vapour mole fraction:

( ) ( ) ( ) (

_h _h

)

h h w s s s s w w f T P P T e P T f P T e x = , = ,

¾ Two-pressure humidity generator: P_h< P_s T_h< T_s

{T_s, P_s}

{T_h, P_h}

-50°C < T_d< +75°C U(T_h) = 0.05 K

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NPL Standard Humidity Generator (SHG)

¾ Presaturation: the dry gas is preconditioned to humidity virtually identical to the saturator

¾ Recirculation: the loss of gas φ_d is only a fraction of the recirculated gas φ_r

¾ Two-temperature generator: near atmospheric pressure, T_s¡ T_hand P_s¡ P_h Pre-saturator Pressure controller Dry gas 105 KPa Saturator Pump Hygrometer under test Temperature-controlled baths Flow meter φr d φ d r φ φ + r φ d φ {T_s, P_s} {T_h, P_h} -90°C < T_d< +90°C U(T_h) = 0.03 – 0.10°C 1 min 14 . 0 l d r d = = φ φ φ

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NMi VSL Standard Humidity Generator

Temperature-controlled bath Pre-saturator Mass Flow Controller Dry gas HT Saturator Pump Hygrometer under test LT Saturator Temperature-controlled bath

¾ LTS (Low Temperature Saturator): -60°C < T_d < 20°C

¾ HTS (High Temperature Saturator): 5°C < T_d < 70°C Two-temperature generator

-60°C < T_d< +70°C U(T_h) = 0.04 – 0.07°C

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Trace moisture: ppm and ppb range

( ) ( )

s s s s w w f T P P T e x = ⋅ , 1 ≈ Dew Point Temperature /°C Saturation vapour pressure of ice /Pa

Water vapour molar fraction / mol/mol 0 611 6·10-3 -10 260 2.6·10-3 -20 103 1·10-3 -30 38 3.7 ·10-4 -40 13 1.3 ·10-4 -50 4 3.9·10-5 -60 1 1·10-5 -70 0.26 2.6 ·10-6 -80 0.054 5.3 ·10-7 -90 0.0097 9.6·10-8 -100 0.0014 14 ·10-9 -110 0.00016 1.6 ·10-9 ppm range ppb range

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Semiconductor manufacturing: the

driving force of trace moisture metrology

300 mm silicon wafer

Techniques for trace moisture measurement have progressed by orders of magnitude over the last decade

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The needs of semiconductor industry

¾ Yield = (number of silicon wafers batches that can be sold)/ (number of silicon wafers batches that can be made)

¾ In semiconductor manufacturing, very small quantities (100 ppb) of water vapour can adversely affect the performance and the yield of silicon-based semiconductors

¾ Semiconductor manufacturing requires ultra-high-purity conditions in: • sample preparation, in which high-purity purge gases are used to

sweep atmospheric constituents or residual compounds from prior processes

• reactive flows, in which high purity gases are used for chemical incorporation in film growth

¾Humidity-generation devices and humidity analyzers must be

available for specialty gas suppliers and end-users with the required sensitivity and stability.

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The NIST Low Frost-Point Generator

(LFPG): ppb level

Mass Flow Controller Dry gas Hygrometer Under test Heater 2 l/min Back pressure regulator Vacuum chamber

¾ The back pressure regulator maintains the pressure above the minimum required by the hygrometer under test for proper operation

¾ -100°C < T_f< -5°C (14 nmol/mol < x_w < 3 mmol/mol)

¾ Relative uncertainty in x_w(k = 2): 0.8% > u > 0.2%

Refrigeration

(Liquefied ethylene) Temperature

control Saturator

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The NIST Low Frost-Point Generator

(LFPG): ppb level

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The NIST dilution system: ppt-level

Mass Flow Controller Laminar Flow Element Low Frost-Point Generator Purifier Mass Flow Controller Laminar Flow Element CRDS Hygrometer Back pressure regulator

{

x_wet,φ_wet

}

{

xdry,φdry

}

tot x

¾ The LFPG operating range (ppb) is extended to ppt levels by diluting the water vapour/gas mixture produced by the LFPG with ultra-dry gas

¾ Identical nominal output concentration are produced by varying both the output of the LFPG and the level of flow dilution

¾ The calculated x_tot is compared to the reading of a CRDS hygrometer

(

)

(

)

(

1

)

1 1 ) 1 ( 1 + − − − − + − = dry wet wet dry dry wet tot x x x x x x x φ φ φ φ dry wet wet φ φ φ φ + =

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The NMi VSL dilution system (ppb range)

Pressure Controller Standard Humidity Generator Mass Flow Controller 2 nm Filter Purifier Test instrument Back pressure regulator

{

x_wet,φ_wet

}

{

xdry,φdry

}

tot x N₂ N₂ MFC1 MFC2 MFC3

(

)

(

)

(

1

)

1 1 ) 1 ( 1 + − − − − + − = dry wet wet dry dry wet tot x x x x x x x φ φ φ φ dry wet wet φ φ φ φ + = Minimum frostpoint: -85°C Uncertainty: ±1°C

¾ The LTS operating range is extended to ppb levels by diluting the water vapour/gas mixture produced by the LTS with ultra-dry gas

¾ Identical nominal output concentration are produced by varying both the output of the LTS and the level of flow dilution

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Portable Humidity-Generation Transfer

Standards

The need:

• On-site calibration of humidity analyzers

• Testing analyzers response

The answer:

• Permeation tube humidity generators (PTGs) PTG:

• Simple device based on the controlled dilution of water vapour by a purified carrier-gas stream

• A tube filled with liquid water … • water evaporates …

• the water vapour diffuses through the walls of the tube …

• into a temperature-controlled chamber. • A stream of purified carrier-gas dilutes the water vapour

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Permeation Tubes Generators (PTGs)

Temperature-controlled chamber

PTGs widely used in semiconductor industry:

• Produce reference-gas streams in the trace-moisture region (ppb)

• Specialty gas suppliers use PTGs for calibrating a wide range of trace-humidity analyzers (e.g. point-of-use analyzers that monitor water contamination in silicon wafer processing)

• Analyzer manufacturers rely on PTGs for their in-house metrology programs and even install PTGs inside humidity analyzer instruments as in situ reference gas sreams with a fixed water vapor mole fraction But:

•They must be calibrated because the permeation rates of PTGs cannot be predicted reliably from the first principles

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The NMIJ permeation tube calibration

facility

Dry N₂ Mass Flow Controller 1-20 l/min Mass Flow Controller 0.1 l/min CRDS Hygrometer Magnetic Suspension Balance PTG Temperature-controlled chamber

¾ Evaporation rate of the PTG is measured by the mass-change rate of the diffusion cell using the MSB

¾ Observed and calculated evaporation rates are compared

( ) ( ) ( )( )( )α T T P P P T D P T D lRT Se P T D n_w _w 0 0 0 0, , , = =

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The NIST permeation tube calibration

facility

Purified Nitrogen Supply Laminar Flow Element Mass Flow Controllers Permeation Tube Commercial Analyzer NIST LFPG

PTG subsystem in temperature-controlled oven

UHP Diaphragm valve Back

Pressure Controllers

UHP Diaphragm valve

¾ The flow streams of the PTG and LFPG are alternatively sampled by the analyzer

¾ The LFPG is adjusted to produce an analyzer response that is nearly indistinguishable from the analyzer response to the PTG system

¾ Using a commercial QCM as comparative analyzer brings a

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The NIST permeation tube calibration

facility

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Recirculation flow rate

How does the recirculation flow rate affect the efficiency of the saturator? Is it better to have a low or high recirculation flow rate?

• For a slow recirculation rate, the gas passes over the water (or ice) surface at low velocity and is in contact with the surface for a longer time per circulation but will circulate fewer times before being drown off to the instrument under test

• For a high recirculation rate, the gas passes over the surface at higher velocity, is in contact with the surface for a shorter time per circulation, but will circulate more times before being drown off

• A fast flow rate increases the mixing of the new incoming gas with the

recirculating gas, but decreases the efficiency of the temperature preconditioning

• A slow flow rate decrerases the mixing but increase the efficiency of the temperature conditioning

•A fast flow reate reduces the effects of leaks and desorption between the

saturator and the instrument under test (leakks elsewhere in the pipework are less importand as the gas always passes through the saturator after)

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Desorption and adsorption

Even with the best electropolished surface, water molecules are always present in an adsorbed layer at the surface.

Desorption: water molecules migrate between the surface layer and the test gas, tending to bring the local humidity towards equilibrium with the adsorbed water layer. (This is a problem at low humidity)

Adsorption: water molecules migrate from the humid gas to the surface. This is less important because this happens when at high dewpoints (high humidity), when the loss of a small amount of water from the humid gas is less significant.

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Humidity is everywhere

¾ You step out of your morning shower, you look at the mirror of your bathroom and its surface is covered with condensed water.

¾ The moment you step out of your shower, you start drying off by way of evaporation (which makes you feel cold)

¾ The rate of evaporation of the water from my skin depends on the relative humidity: the lower the relative humidity the higher the evaporation rate.

¾ In warm weather, my body is using evaporation of sweat to cool down.

¾When humidity is high, the temperature perceived by my body is higher than the actual temperature (because I cannot cool down by way of evaporating my sweat)

(27)

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I watch out of my window in the early morning and I see droplets of water trapped in a spider web.

Where is the water coming from?

Water vapour contained in the air condensed into water.

Why does it happen?

Because the amount of water that air can retain depends on the temperature:

during the night the temperature goes down and it reaches a value at which air can not hold all the water vapour.

Why does the water take the form of droplets?

Surface tension: Liquids adjust their shape in order to expose the smallest possible surface area.

The film that initially settles on the threads is unstable and breaks up spontaneously into droplets (Plateau-Rayleigh instability).

Why are some droplets bigger than the other?

Secondary instability of the liquid film when two adjacent droplets are in the process of forming.

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Yield Enhancement in semiconductor

industry

¾Yield = (number of silicon wafers batches that can be sold)/ (number of silicon wafers batches that can be made)

¾ Every time there is a wafer size transition, the success of a

semiconductor industry in the market is measured by its yield ramp: how rapid is ramping Y=Y(t)?

¾ Yield enhancement: the process of improving the starting yield from R&D yield to mature yield

¾ It is a learning process involving in-line detection and reduction of defects, isolation of failure techniques, identification of killer defects

¾ The optimum balance between amount of inspection and costs

must be found

¾ Statistically optimized sampling algorithms are needed to maximize the yield learning