Rough (low) vacuum : Medium vacuum : High vacuum (HV) : Ultrahigh vacuum (UHV) :

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1 4. UHV & Effects of Gas Pressure 4.1 What is Ultra-high Vacuum (UHV)? 4.2 Why is UHV required for Surface Studies? 4.3 Exercises - Effects of Gas Pressure Vacuum technology has advanced considerably over the last 25 years and very low pressures are now routinely obtainable. Firstly, let s remind about of the units of pressure : The SI unit of pressure is the Pascal ( 1 Pa = 1 N m -2 ) Normal atmospheric pressure ( 1 atm.) is Pa or 1013 mbar (1 bar = 10 5 Pa). An older unit of pressure is the Torr ( 1 Torr = 1 mmhg ). One atmosphere is ca. 760 Torr ( i.e. 1 Torr = Pa ). Whilst the mbar is often used as a unit of pressure for describing the level of vacuum, the most commonly employed unit is still the Torr. (The SI unit, the Pa, is almost never used! ) Classification of the degree of vacuum is hardly an exact science - it very much depends upon who you are talking to - but as a rough guideline : Rough (low) vacuum : Medium vacuum : High vacuum (HV) : Ultrahigh vacuum (UHV) : Torr Torr Torr < 10-9 Torr

2 4.2 Why is UHV required for surface studies? Ultra high vacuum is required for most surface science experiments for two reasons : To enable the preparation of atomically clean surfaces and to maintain them free of contaminants for the duration of the experiment. To permit the use of low energy electron and ion-based experimental techniques without undue interference from gas phase scattering. 1. Gas Density The gas density is easily estimated from the ideal gas law : n = ( N / V ) = P / (k T) [ molecules m -3 ] where :P - pressure [ N m -2 ] k - Boltzmann constant ( = 1.38 x J K -1 ) T - temperature [ K ] 2. Mean Free Path of Particles in the Gas Phase The average distance that a particle (atom, electron, molecule..) travels in the gas phase is given by : where : P - pressure [ N m -2 ] ; k - Boltzmann constant ( = 1.38 x J K -1 ) T - temperature [ K ] ; σ - collision cross section [ m 2 ]

3 3. Incident Molecular Flux on Surfaces One of the crucial factors in determining how long a surface can be maintained clean (or, alternatively, how long it takes to build-up a certain surface concentration of adsorbed species) is the number of gas molecules impacting on the surface from the gas phase. The incident flux is the number of incident molecules per unit time per unit area of surface. Step 1 : it can be readily shown that the incident flux, F, is related to the gas density above the surface by [ molecules m -2 s -1 ] where : n - molecular gas density [ molecules m -3 ] c - average molecular speed [ m s -1 ] Step 2 : the molecular gas density is given by the ideal gas equation, namely n = N / V = P / (kt) [ molecules m -3 ] Step 3 : the mean molecular speed is obtained from the Maxwell-Boltzmann distribution of gas velocities by integration, yielding [ m s -1 ] where : m - molecular mass [ kg ] ; k - Boltzmann constant ( = 1.38 x J K -1 ) T - temperature [ K ] ; π =

4 Step 4 : combining the equations from the first three steps gives the Hertz-Knudsen formula for the incident flux [ molecules m -2 s -1 ] Note: all quantities in the above equation need to be expressed in SI units the molecular flux is directly proportional to the pressure 4. Gas Exposure - the "Langmuir The gas exposure is a measure of the amount of gas which a surface has been subjected to. It is numerically quantified by taking the product of the pressure of the gas above the surface and the time of exposure (if the pressure is constant, or more generally by calculating the integral of pressure over the period of time of concern). Although the exposure may be given in the SI units of Pa s (Pascal seconds), the normal and far more convenient unit for exposure is the Langmuir, where 1 L = 10-6 Torr s. i.e. (Exposure/L) = (Pressure/Torr) x (Time/s) x 10 6

5 S = f ( surface coverage, temperature, crystal face... ) The surface coverage of an adsorbed species may itself, however, be specified in a number of ways : 1.as the number of adsorbed species per unit area of surface (e.g. in molecules cm -2 ). 2.as a fraction of the maximum attainable surface coverage i.e. - in which case θ lies in the range 0 to 1. 3.relative to the atom density in the topmost atomic layer of the substrate i.e. - in which case θ max is usually less than unity, but for an adsorbate such as H can exceed unity. How long will it take for a clean surface to become covered with a complete monolayer of adsorbate? assuming a unit sticking probability (i.e. S = 1) and noting that monolayer coverages are generally of the order of per cm 2 or per m 2. Then Time / ML ~ ( / F ) [ s ]

6 Variation of Parameters with Pressure All values given below are approximate and are generally dependent on factors such as temperature and molecular mass. Vacuum level Pressure (Torr) Gas Density (molecules m -3 ) Mean Free Path (m) Time / ML (s) Atmospheric x x Low 1 3 x x Medium x x High x UltraHigh x x We can therefore conclude that the following requirements exist for : Collision Free Conditions Maintenance of a Clean Surface P < 10-4 Torr P < 10-9 Torr

7 What is VACUUM good for? Main use of vacuum (from science to everydays life) reduce the concentration of a given gas below some critical layer (e.g. O 2 in light bulbs). Avoid chemical processes caused by the action of atmospheric gases (casting of reactive metals like Mo, W, Ta, mantainment of controlled conditions in gas-surface experiments). Thermal insulation (thermos, dewars for cryogenic liquids). Elimination of dissolved gases or contaminants from a material (degassing of oils e liofilization). Simulate particular physical conditions (chambers to simulate the space conditions for the testing of satellites and space ships). Increase the mean free path of particles (molecules, electrons, ions) up to macroscopic distances (cathodic tube, thermoionic tubes, electron spectroscopies, particle accelerators).

8 Flux and pumping speed volumetric flow: S=vA or S=V/t Mass flow: G=ρS=ρvA=ρV/t Under vacuum conditions one always assumes a constant gas temperature. G can then be expressed as feed through : Q=pS (in torr L/s) Note: torr L/s= (g cm 2 )cm 3 /s = g cm/s = J/s =W 1 W = 7.5 torr L/s The mass flowhas therefore an associated energy. Pumps limited in power will also have a limited flux.

9 Conductance C = Q P P 1 2 = pva p in L/s; or better, torr L/s for each torr in the pressure gradient The conductance limits the pressure drop in a tube. For molecular flux C is independent of pressure and depends only on the geometry of the conductor. Fig pag 53 The molecular flux through a conductor of uniform area is: 4A 2 v C= 3 LF A area v velocity of the molecules L length of the conductor F perimeter of the conductor which becomes Combination of conductances in parallel: C = 3 2πR v For a conductor with circular shape 3L C = C + C C n Combination of conductances in series: 1 C 1 = C C C n

10 UHV Chambers UHV chambers in inox have the possiiblity to installe equipment through flanges: - pumping systems; -Pressure gauges; - Manipolators to handle the samples; - Preparation and analysis facilities

11 How to make a UHV chamber tight? Different parts need to be connected in irreversible or detachable arrangements. Permanent connections -TIG welding performed in inert gas atmosphere to avoid oxidation -- Glas or Ceramic metal weldings Removable connections Low Vacuum : ISO-KF standard with viton or teflon O-rings COPPER GASKET High and Ultrahigh vacuum CONFLAT flanges with oxygen free Cu gaskets

12 Pumping systems

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14 Low vacuum : rotative pump Single /double stage Typical performances: Pumping speed: 3-5 liters/s Limit pressure: 10-3 / 10-4 mbar Work principle: The gas enters from the inlet; It is compressed by the rotor It is released in the atmosphere Q e, Q i S P d k P Limit pressure: Q S Q + S e i ult = + P k external, internal charge pumping velocity out let pressure compression rate d For P close to UHV conditions the ultimate pressure P ult is the sum of the three contributions for each pumped gas Determination of P ult is complicated by - Internal degassassing (oil); - Affidability of pressure reading (vapor condensation) - Different efficiency for the different gases (Helium)

15 Low Vacuum : rotative pump Necessity to use special oils : -Low vapour tension; -Chemically inert with respect to the pumped gases. FUNCTIONS: - Make rotor and blade tight. - Lubrificate the pump. - Dissipate the heat maintaining the pump at an acceptable temperature. DISADVANTAGES: Back-flow into the vacuum chamber ZEOLITE TRAPS Zeolites: minerals with high surface area with pores of molecular dimensions Use of oil free pumps -E.g. Scroll pumps; The trap, inserted between vacuum chamber and the pump, significantly reduces the back flow. If properly designed the trap riduces the pumping velocity by not more than ~10%.

16 UHV: turbomolecular pump Working mechanism the axial flux turbine, maximizes the volumetric efficientcy for a given diameter and volume. It consists of rotors (13) and statora (12). The external forces are simmettric on the borders elevated balance allows for high angular velocity. The pumping takes place essentially by transfering kinetic momentum from the rotating blades to the gas. There is no surface exposed sequentially to high and low pressure. Degassing effects are limited. Lubrification may take place with oil or grease and the pump may be cooled by air or water flow. Alternatively the pump may be magnetically suspended The inclination of the blades maximises the probability that the molecules are pushed in the direction of the flux and minimizes backstreaming. Inclination of the blades: Small inclination to maximize the pumping speed. Large inclination to maximize the compression factor. In practice we have to choose a compromize and different inclinations for the different stator / rotor couples.

17 UHV: turbomolecular pump Typical performances S = L/S Limit pressure < 2 x mbar S = S(P,GAS) 10-2 mbar TURBO STAGES DRAG STAGES To increase the operating pressure in the outgas above 20 mbar a molecular drag stage has been added to the turbomolecular stage. It consists of a rotating cylinder and of a wall with a path carved in it.

18 Vacuum Measurements Pressure gauges 15 orders of magnitude ( mbar) No single instrument can cover sucha wide pressure range. In the vacuum chamber at least two gauges are needed: for thte low and for the ultra high vacuum. The smaller the pressure the more difficult it is to perform accurate measurements. However under ultra high vacuum the interesting quantity is non so much the pressure but the density of the gas which can be related to the pressure by the perfect gas law. These gauges measure the total pressure 4 work principles : - Force - Transferred Momentum - Dissipated Heat - Ionization Gas Analyzers Measure the partiual pressure of the different gases within a well defined volume.

19 Pressure gauges.the type of transducer depends on the pressure range of interest Direct measurements The pressure is determined from the force of the flux of particles on a surface Examples: DIAPHRAGM BOURDON CAPACITANCE SPINNING ROTOR Indirect measurements The pressure is determined by measuring a property of hte gas which depends on its density. Examples: ION GAUGE,TC, PIRANI

20 Heat transfer gauges: TC e PIRANI Both gauges are based on the variation of the thermal conductivity of the gas. The heat transferred though the gas from a heat source to the room temperature walls is measured. Working condition: λ d (distance source cage at RT) Low Vacuum PIRANI: ( or 0.1 mtorr) The heat loss of a filament is determined under vacuum conditions by a Wheatstone bridge. The circuit heats a filament and measures its resistence. The gauge and the compensation element have to be as similar as possible. The former is mounted in a hole in contact with the vacuum. The other in a tight hole with known press (P<1 mtorr). The voltage is kept constant V. A variation of p in the open hole causes a variation of the temeprature of the gauge and an unbalance in the electric bridge circuit

21 Heat transfer gauges: TC e PIRANI Both gauges are based on the variation of the thermal conductivity of the gas. The heat transferred though the gas from a heat source to the room temperature walls is measured. Working condition: λ d (distance source cage at RT) Low Vacuum Thermocouple gauge: ( mtorr) Similar to the Pirani gauge, but the temperature of the resistance is now measured by a thermocouple. For p<10 mtorr the accuracy is limited by: - variation of the gas composition; - aging; - contamination; - variation of the external temperature

22 Heat transfer gauges: TC e PIRANI Both gauges are based on the variation of the thermal conductivity of the gas. The heat transferred though the gas from a heat source to the room temperature walls is measured. Working condition: λ d (distance source cage at RT) Low Vacuum λ d heat transfer proportional to the concentration of molecules. λ < d non linearity because of the collisions between the gas phase molecules.

23 Ionization gauges For high precision measurements of high and ultra high vacuum. Cold cathode Hot cathode COLD CATHODE IONIZATION GAUGES torr Typically one has to apply some 4kV which accelerate molecules ionized by cosmic rays. Such molecules create then a cascade which gives a measurable total current. Advantages: - Absence of a hot filament; - high sensitivity. -Disadvantages: - Discontinuity in the calibration; - Induction period at low pressure (works only after one molecule has been ionized buy chance by a cosmic ray.

24 Ionization gauges HOT CATHODE IONIZATION GAUGE TRIOD DESIGN: - Electrons are emitted by a hot filament. They are accelerated towards a grid and hit against the gas phase molecules causing their ionization. The ions a re collected by a collector and the pressure is evaluated from the measured current. - Range: torr. - Limited sensitivity because the energetic incident electrons cause X ray emission which may hit the collector and cause photoemission. Bayard-Alpert gauge: - It s a modified version of the triod gauge which minimizes the photoelectron current -The collector placed inside the grid 3 main advantages : 1) The limited dimension of the collector wire make so that only a small part of the produced X Rays hit against it. 2) the potential difference between grid and collector make so that all the gas in the volume of the grid is uniformly ionized. 3) The efficiency of the collector is improved by its central position within the grid Filament Collector Grid B-A gauge with spiral grid P torr B-A gauge UHV 24 Closed grid and very small collector =I coll /(I el xp)=24 A/(Axtorr) P torr