L/O/G/O

Chapter 2 Vacuum Technology for

Applied Surface Science

講義下載：http://www.che.ncku.edu.tw/FacultyWeb/LeeYL

Kinetic theory of gases:

Explain the macroscopic properties of gases (P, T, V..) by microscopic

behavior of the molecules.

ρ: gas density , n=number density of particles, (m-3),

:mean square velocity

m: mass of a molecule

k: Boltzmann constant, (JK-1).

T: Absolute temperature,(K).

2C

2C3

1p 2

C3

1nmp or (1)

(2) m

kT3C

2

Collision rate (Z):

the average number of collisions per second between particles.

λ: mean free path -

the mean path which each particle makes between collision.

The quantity is called the collision cross-section of the molecule and is

often denoted by the symbol .

(5)

(4) (3)

cz

pm

kTc

8

cm

torrP

105

22

1 3

2

rn

for air

22r

,

Vacuum(真空):

A defined space where the pressure is significantly smaller than the atmosphere.

concept：

宇宙內沒有絕對的真空(無任何物質存在)亦無任何人為的方式可造成絕對的真空

※壓力之單位：

1 atm = 76 cm-Hg

= 76 x 13.6 gw / cm2 = 1033.6 gw/cm2

= 76 x 13.6 x 980 dyne / cm2 = 1.01325 x 106 dyne / cm2

= 1.01325 x 105 (N / m2 ) = 1.01325 x 105 (Pa)

Chapter 2 Vacuum Technology

dyne/cm2 磅/平方英吋 mmHg atm Pa

lb/in2 (psi) Torr 1 N/m2

1013250 14.7 760 1 101325

1 bar (巴) = 106 dyne / cm2

∴ (1 atm = 1.01325 巴 ； 1 巴 = 0.98692 atm )

絕對單位 N / m2 Pascal ( Pa )

1 bar = 106 dyne / cm2

= 106 x (10-5 N / 10-4 m2 ) = 105 N / m2

∴ 1 bar = 105 Pa = 0.987 atm = 750 torr

1 torr =133 Pa (N / m2 )

1 mbar = 0.75 torr

1 atm = 760 torr ≈ 105 Pa

(一) Divisions of vacuum

Rough and mediate vacuum：1 atm ~ 10-3 torr

High Vacuum (高真空) ：10-3 torr ~ 10-7 torr

Ultra-high Vacuum (超高真空) ： P < 10-7 torr

These divisions are somewhat arbitrary.

Gas in each region would have similar characteristic

(二) Density of Gas

1 mole 6.02 x 1023 molecules

Density at a pressure of 1 torr：

PV=NRT N / V = P / RT = ( 1 / 760 ) / ( 0.082 x 298 )

分子數：molecale / cm3 = [( 1 / 760 ) / ( 0.082 x 298 ) ] x

(6.02 x 1023 / 1000 ) ≈ 3 x 1016 molecule / cm3

At UHV, the gas density is ca. 109 molecule / cm3

The Pressure Regions of Vacuum and their

Characteristics

(三) mean free path (氣體的平均自由徑) λ ：

The average traveling distance of gas molecules between

successive collision. (一個粒子在碰撞其他粒子前所走的平均距離)。

at 1 atm ( 760 torr ) λ ≈ 6.5 x 10-6 cm

10-3 torr λ ≈ 5.1 cm

10-6 torr λ ≈ 5,000 cm

For air at room temp λ ( cm ) = ( 5 x 10-3 ) / P ( torr )

高真空時氣體要走很長距離才會碰撞另一分子，若考慮容器因素，通常在未碰撞其分子前已碰撞器壁。

高真空時，氣體分子間之互相作用可忽略。

cm

torrP

105

22

1 3

2

rn

Flow behavior of gas in the vacuum

λ ：mean free path of the particle

d ：diameter of the conducting tube

Viscous flow：λ << d, (λ / d < 0.01) ---- rough vacuum

Successive intermolecular collisions are much higher than the collisions

with the wall, The mutual interaction of the particles with one another determined

the character of the flow. (D ≈ u λ/3)

Free molecular flow： λ > d, (λ / d > 1) --- high and ultra-high vacuum

Particle move freely without any mutual hindrance. Gas particle can

have any random direction of moving. Gas flow is independent on the pressure

difference but is related to the molecular weight of gas,

temperature, and the geometry of pumping system. The conductance of a pumping

system is controlled totally by the geometry of the system.

Gas flow in various vacuum regions

Transition region：(0.01 < λ / d < 1 )

– Also called Kundsen flow. Collision between molecules and with the

wall are both important. The diffusivity is less than the normal value.

The diffusion process is call Knudsen diffusion and the diffusivity for

a cylindrical pore is

Dk = 9700 r (T/M)1/2

Dk: Knudsen diffusivity

M : molecular weight

r : pore radius

1/Dpore = 1/DAB + 1/Dk

(四) Gas flow in various vacuum regions

Viscous flow region (黏滯流) λ / d < 0.01

The mutual interaction of the particles determines the character of the flow.

That is, the viscosity or the inner friction of the fluid plays the dominant

role.

氣體分子由一端走向另一端必須遭受多次碰撞，故受分子間的阻

力 ， 表現出顯著的黏滯性

Free molecular flow (自由分子氣流) , λ / d > 1 (high vacuum and UHV)

The particles move freely without mutual interaction.

• 氣體分子由器壁一端至另一端不遭受任何碰撞

• gas flow 與壓力無關

Gas impingement on surfaces.

Impingement rate---the number of molecules that strike upon unit

surface area per unit time (ZA):

ZA = n〈C〉/4

=3.513 x 1022 x P/(MT)0.5 (molecules/cm2-sec) -- for ideal gas

Monolayer time (τ) ─ time required to form a monolayer of

adsorption gas by the impingement of gas. If the number of free places (sites)

for the adsorption of gas per unit area is a:

τ = a /ZA , if a =1015 atom/cm2 and ideal gas

τ = 2.85 x 10-8 (MT)1/2 / P (sec), (P in torr)

at 1 atm, τ =3.49 x 10-9 sec, 10-5 torr, τ = 1 sec

at 10-10 torr, τ = 7.3 hours.

λ / d > 1

cm

torrP

105

22

1 3

2

rn

The division of vacuum

Rough vacuum 1000-1 mbar

Medium vacuum 1-10-3 mbar

High vacuum 10-3- 10-7 mbar

Ultra-high vacuum 10-7 and below

In high and ultra-high vacuum, there are more gas molecules on

the surfaces of the vacuum chamber than there are in the gas space

(1 liter volume). Assume a monolayer is adsorbed on the inner wall,

the value of n(adsorbed)/n(in space) is:

1 mbar 10-2

10 -6 mbar 104

10-11 mbar 109

Performance of a pump is usually expressed by the pumping speed,

S, which is defined as the volume flow rate of gas through the inlet

aperture of the pump.

S=dV/dt ----- does not indicate the number of particles flowing per

unit time. More reasonable expressed by the gas mass

flow rate (qm)

G=PV(M/RT) ----gas mass

qm= G/Δt----------mass flow rate

It can also be expressed as the throughput, qPV,

qPV=P(ΔV/Δt) = PS ----varies with P and V

After pumpdown, p and ΔV are constant, the throughput of a pump is constant.

qPV=PS

Vacuum Pumps

The ability of a pump to remove the gas :

(1) Throughput of the pump,

(2) elements in the pumping system to transmit the gas, terms as the

conductance of the element, C, which is determined by

geometrical nature of the elements.

qPV=C(P1-P2)

If several elements, A, B, C, etc. are connected in series:

If connected in parallel:

CBAtot CCCC

1111

CBAtot CCCC

沒有一種 pump 可以將真空度由 1 atm 抽至超高真空。故高真空的

達到必須由幾種不同 pump搭配使用才可達到。

幫浦分類

(A) 粗略真空或中度真空 pump ( 1 atm ~ 10-3 torr )

Mechanical pump (機械幫浦): rough pump，fore-way pump

• rotary oil-sealed pump (迴轉油墊pump )

• rotary blower pump (迴轉吹送 pump )

(B)高真空，或超高真空 pump ( 10-3 ~ 10-7 torr 或以下 )

• diffusion pump and ejection pump ( 蒸氣噴流 pump )

• turbo – molecular pump (渦輪分子 pump )

• Sorption pump ( 吸附 pump )，Cryopump ( 冷凍 pump )

• titantium sublimation pump (鈦昇華 pump )

• sputtering ion pump ( 濺射離子 pump )

Vacuum Pumps

Rotary-oil-seals pump

Mechanical pump

Rotary-oil-sealed pump

Function of oil ：

(1) lubrication and cooling

(2) sealing medium

抽空氣效率高。但若有易凝結

氣體存在(ex：H2O、ethanol)，

蒸氣將凝結於器壁或混入幫浦

油中，系統可達的 最低 壓力：

凝結液體之飽和蒸氣壓 (water at 70 oC, pmin=312 mbar)

處理方式

(1)更換 pump oil, (2) 空氣混抽法 (gas ballast)

gas in gas out

Without gas

ballast With

gas ballast

Rotary blower pump (Roots Pump)

• 無 pump oil ，所以轉子與轉子間的間隙必須十分精密(不大於 1 / 10 mm )

• 利用轉子迅速轉動將氣體由低壓送往高壓

• 壓力範圍 10 ~ 0.01 ( 0.0001 ) torr

• 無油氣回流真空室之問題，

常用於真空冶煉、真空熱處理、

真空乾燥(LPCVD， P=1 torr )

• 不可有固體微粒進入，否則會磨損

機件，產生漏氣刻痕(進氣口加濾網)

Mechanical pump

Rotary blower pump

High Vacuum Pump

Mechanical molecular pump (機械分子 pump )

利用分子碰撞高速運動的機件而獲得動能，向機件運動的方向流動(由低壓→高壓) 再由前段 pump接受至系統之外。

渦輪分子 pump ( turbo-molecular pump ) 10-9~10-12 torr

• 旋轉速度 20,000~30,000 rpm

• 質量大的分子所受離心力大 效率較高

• 軸承處有潤滑油，故有時需冷卻

• 缺點：價格高

High Vacuum Pump

Pumping speed for turbomolecular pump

High Vacuum Pump

蒸氣噴流幫浦(vapor steam pump )、擴散幫浦( diffusion

pump ) 及噴射幫浦( steam ejector pump )

原理：液體蒸發為氣體後衝向真空系統的抽氣口，將真空系統中的氣體分子帶走，而後由前段 pump 抽送至大氣中。

• vapor 之 mean free path > d 因碰撞而散射回真空系統之

vapor 或 gas molecule 少

• 蒸氣噴流速度越大則 pump 效率越高 (故需用高分子量之液體作為 pump oil )

• pump 級數增加，所抽真空度亦越高

• pump fluid 之 vapor 需冷卻水

• 需由前段 pump 抽至中度真空 ( 0.01 torr ) 才有用。與之串聯

使用

• 最終壓力受 pump oil 氣壓之限制

Diffusion pump

Pumping speed of a DP

Rotary pump Diffusion pump

Sorption Pump (吸附幫浦) (Cryo pump , 冷凍pump)

– high vacuum pump : 10-3~10-10 torr

Rely on the condensation of vapor molecules on surfaces cooled

below 120 K (by Liquid N2)

Working media of the common sorption pump

—molecular sieve materials with microporous surface of very large

area (Zeolite, Activated charcoal)

Binding of the adsorbed gas molecules:

van der Waals forces (depend on T), no chemical bonding, reversible

Require an initial fore-pressure of about 10-3 torr

一般用 LN2(-195.8 oC, 77K)

H2O, CO2, organic vapor → condense to be solid phase

But H2, He, N2, → still in gas phase

• The panel (with activated charcoal) is cooled to below 20 K(by

subcooled He gas, or L He, 4.2 K)

可自 1 atm 開始抽氣，但若先以機械 pump 或 diffusion pump

抽氣，起始壓力較低，所能達到的真空度越高。

Crro pump (冷凍幫浦)

High vacuum flange 77K shield

Fore-vacuum port

20 K condenser

Chemisorption pump - The gases are chemically absorbed by

the surface or inner part of a solid material (Ti, Zr, Ta….)

called as: getting pump

Titanium sublimation pump (鈦昇華幫浦)

The getter material (Ti) is evaporated and deposited on the inner wall

of the vacuum chamber as a getter film. Particles from the gas which

impinges on the getter film become bound to it by chemisorption.

— not use continuously, but are switched on in short bursts

of a few minutes

— getter pump can not be applied to inert gas, which can be

improved by gettering ion pump

Chemical pump

Heating the element using W wire.

(1) 使 Ti蒸發 – 冷凝於pump wall

(2) 放出電子，經柵極加速，使氣體

(He, Ar) 離子化 → (He+, Ar+…)

打擊至pump wall, 而被其上的

Ti 所吸附， 進而覆蓋。

initiated pressure < 10-3 torr

Sputtering ion-pump(濺射離子幫浦)

i) Ions impinge on the cathode surface of a cold cathode discharge system and

sputter material off it (the material of the cathode is again titanium). The

titanium deposits on surface in the pump and acts as a getter film as before.

ii) The energy of the ions is high enough on sputtering incidence for them to

become deeply embedded in the cathode where they are in essence adsorbed

by ion implantation. The process can pump all types of ions including rare

gases.

Sputtering ion pump

排氣機構

真空壓力計 真空計的分類 : Direct or indirect methods

流體靜態壓力計:

利用液體(水銀、油)之液位高來代表系統內外壓力表。(最低至0.1 torr)。

— 不須複雜電子儀器。

— 實驗室、工業上普通真空常用。

薄膜壓力計(diaphragm gauge)

atm ~ few mbar (torr)

Accuracy 10 torr

Modern gauge:

movement of diaphragm is linked to

an electrical transducer, the pressure

can be displayed digitally.

最低 10-2 torr 、因受齒輪轉動影響，

其靈敏度有限。

Geissler Tube

2. 熱傳導真空計(thermal conductivity gauge), (Pirani guage)

Heat transfer in a headed filament

1. 輻射

2. 由導線傳導

3. 由氣體分子撞擊 ← 利用此種的傳熱方式來測定殘餘氣體。

P : 1~10-4 torr (10~10-3 mbar)

λ diameter of filament

Thermal Gauges

The effects of pressure on the heat transfer of a heated filament

熱電偶(Thermal)真空計

派藍尼 (Pirani)真空計

Variable resistance: 10-10-3 mbar

Constant resistance: 103 – 10-3 mbar

High Vacuum Gauges

一、離子化真空計 (ionization gauge)

亦稱: 三級離子化真空計

1. 燈絲: 通電流加熱，放出電子，在電場中被加速，並碰撞剩餘氣體分子

使之離子化(帶正電)，此正離子被吸往負極 → 離子電流 ( gas pressure)

2. 屏極(plate)

3. 柵極(grid)

收集電子或離子

-25 V

1. 熱離子真空計測定範圍: 10-3~10-7 torr

2. 壓力太大，燈絲易燒斷。

3. 若P<10-7 torr則傳統壓力計指示無法再下降，最初被認為是

壓力的下降極限，後發現原因 :

高速電子撞擊柵極產生柔和X-ray，此X-ray與屏極作用，起光電效應

放出電子， 增加屏極的正電荷 →真空計之壓力大於真空系統內之壓力。

改良之 ion gauge

(A) Bayard-Alpet ionization gauge(B. A ion gauge)

利用反位電極安排，燈絲在柵極外，離子收集極改成細絲

在中心。(降低X光能射到離子收集極之機會)

> 真空度高達5-6 × 10-11 torr

> 10-5 torr 使用壽命較長

> 不宜長期使用

改良之 ion gauge

(B) 李氏真空計:

利用雙離子收集極

電極 A: (0 V) → only X光電子電流。

電極 B: (-60 V) → 收集正離子+X光光電子電流。

由差異電流計，可將X-ray影響消去。

Pressure 達 10-12 torr。

彭寧冷陰極真空計: 亦稱 Philips gauge

原理: 由高電壓 真空放電產生電子。(與 ion gauge 由燈絲加熱方式不同)

利用磁場以螺旋狀途徑運動，增加碰撞氣體分子的機會。

A: 陽極 電壓2000~5000 V(高壓電放壓)

C: 陰極 M: 磁場

> 測定壓力: 10-5~10-2 torr。

> 因不用燈絲，故壽命較長

(不怕突然漏氣或壓力驟增)。

Penning cold – cathode gauge

L/O/G/O

Thank You!

化學工程系 李玉郎