Fundamentals of Vacuum Technology (1248463), страница 23
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Depending on the design of the baffle, 30 to 50percent of all N2 molecules are reflected.H2 arrives at the cryosorption panels after further collisions and thus coolingof the gas. In the case of optimally designed cryopanels and a good contactwith the active charcoal up to 50 percent of the H2 which has overcome thebaffle can be bonded. Due to the restrictions regarding access to thepumping surfaces and cooling of the gas by collisions with the walls insidethe pump before the gas reaches the pumping surface, the measuredpumping speed for these two gases amounts only to a fraction of thetheoretical pumping speed.
The part which is not pumped is reflectedchiefly by the baffle. Moreover, the adsorption probability for H2 differsbetween the various adsorbents and is < 1, whereas the probabilities forthe condensation of water vapor and N2 Å 1.Three differing capacities of a pump for the gases which can be pumpedresult from the size of the three surfaces (baffle, condensation surface atthe outside of the second stage and sorption surface at the inside of thesecond stage). In the design of a cryopump, a mean gas composition (air)is assumed which naturally does not apply to all vacuum processes(sputtering processes, for example.
See 2.1.9.6 ÒPartial RegenerationÓ).2.1.9.6Characteristic quantities of a cryopumpThe characteristic quantities of a cryopump are as follows (in no particularorder):••••••••••Cooldown timeCrossover valueUltimate pressureCapacityRefrigerating power and net refrigerating powerRegeneration timeThroughput and maximum pV flowPumping speedService life / duration of operationStarting pressure57HomeVacuum generationCooldown time: The cooldown time of cryopumps is the time span fromstart-up until the pumping effect sets in. In the case of refrigeratorcryopumps the cooldown time is stated as the time it takes for the secondstage of the cold head to cool down from 293 K to 20 K.Crossover value: The crossover value is a characteristic quantity of analready cold refrigerator cryopump. It is of significance when the pump isconnected to a vacuum chamber via an HV / UHV valve.
The crossovervalue is that quantity of gas with respect to Tn=293 K which the vacuumchamber may maximally contain so that the temperature of the cryopanelsdoes not increase above 20 K due to the gas burst when opening thevalve. The crossover value is usually stated as a pV value in in mbar · l.The crossover value and the chamber volume V result in the crossoverpressure pc to which the vacuum chamber must be evacuated first beforeopening the valve leading to the cryopump. The following may serve as aguide:pc ≤35 ·· Q 2 (20 K ) mbarV(2.27)V = Volume of the vacuum chamber (l),.Q.2(20K) = Net refrigerating capacity in Watts, available at the second stageof the cold head at 20 K.Ultimate pressure pend: For the case of cryocondensation (see Section2.1.9.4) the ultimate pressure can be calculated by:pend = ps( TK ) ·TGTK(2.28)pS is the saturation vapor pressure of the gas or gases which are to bepumped at the temperature TK of the cryopanel and TG is the gastemperature (wall temperature in the vicinity of the cryopanel).Example: With the aid of the vapor pressure curves in Fig.
9.15 for H2 andN2 the ultimate pressures summarized in Table 2.6 at TG = 300 K result.The Table shows that for hydrogen at temperatures T < 3 K at a gasTK (K)Ultimate pressureUlt. press. (mbar)(according to equ. 2.28)H2Ult. press.(mbar)N210.95 á ps3.28 á 10Ð14immeasurably low4.28.66 á ps4.33 á10Ð9immeasurably low203.87 á ps3.87 á 10+32.53.87 á 10Ð11Table 2.6 Ultimate temperatures at a wall temperature of 300 Ktemperature of TG= 300 K (i.e. when the cryopanel is exposed to thethermal radiation of the wall) sufficiently low ultimate pressures can beattained.
Due to a number of interfering factors like desorption from the walland leaks, the theoretical ultimate pressures are not attained in practice.Capacity C (mbar á l): The capacity of a cryopump for a certain gas is thatquantity of gas (pV value at Tn = 293 K) which can be bonded by thecryopanels before the pumping speed for this type of gas G drops to below50 % of its initial value.The capacity for gases which are pumped by means of cryosorptiondepends on the quantity and properties of the sorption agent; it is pressuredependent and generally by several orders of magnitude lower comparedto the pressure independent capacity for gases which are pumped bymeans of cryocondensation..Refrigerating power Q (W): The refrigerating power of a refrigerationsource at a temperature T gives the amount of heat that can be extractedby the refrigerating source whilst still maintaining this temperature.
In thecase of refrigerators it has been agreed to state for single-stage cold headsthe refrigerating power at 80 K and for two-stage cold heads therefrigerating power for the first stage at 80 K and for the second stage at 20K when simultaneously loading both stages thermally. During themeasurement of refrigerating power the thermal load is generated byelectric heaters.
The refrigerating power is greatest at room temperatureand is lowest (Zero) at ultimate temperature..Net refrigerating power Q (W): In the case of refrigerator cryopumps thenet refrigerating power available at the usual operating temperatures(T1 < 80 K, T2 < 20 K) substantially defines the throughput and thecrossover value. The net. refrigerating power is Ð depending on theconfiguration of the pump Ð much lower than the refrigerating power of thecold head used without the pump.pV flow see 1.1Regeneration time: As a gas trapping device, the cryopump must beregenerated after a certain period of operation. Regeneration involves theremoval of condensed and adsorbed gases from the cryopanels by heating.The regeneration can be run fully or only partially and mainly differs by theway in which the cryopanels are heated.In the case of total regeneration a difference is made between:1.
Natural warming: after switching off the compressor, the cryopanels atfirst warm up only very slowly by thermal conduction and then inaddition through the released gases.2. Purge gas method: the cryopump is warmed up by admitting warmpurging gas.3. Electric heaters: the cryopanels of the cryopump are warmed up byheaters at the first and second stages. The released gases aredischarged either through an overpressure valve (purge gas method) orby mechanical backing pumps.
Depending on the size of the pump onewill have to expect a regeneration time of several hours.Partial regeneration: Since the limitation in the service life of a cryopumpdepends in most applications on the capacity limit for the gases nitrogen,argon and hydrogen pumped by the second stage, it will often be requiredto regenerate only this stage. Water vapor is retained during partialregeneration by the baffle.
For this, the temperature of the first stage mustbe maintained below 140 K or otherwise the partial pressure of the watervapor would become so high that water molecules would contaminate theadsorbent on the second stage.In 1992, LEYBOLD was the first manufacturer of cryopumps to develop amethod permitting such a partial regeneration. This fast regenerationprocess is microprocessor controlled and permits a partial regeneration ofthe cryopump in about 40 minutes compared to 6 hours needed for a totalregeneration based on the purge gas method. A comparison between thetypical cycles for total and partial regeneration is shown in Fig. 2.70. Thetime saved by the Fast Regeneration System is apparent.
In a productionenvironment for typical sputtering processes one will have to expect one58HomeVacuum generationhigher pV flow is permissible (see crossover value).Pumping speed Sth: The following applies to the (theoretical) pumpingspeed of a cryopump:Temperature (K)S th = A K · SA · α · 1 −2(1)(2)40 minutesTimeabout 6 hoursαpendpSize of the cryopanelsSurface area related pumping speed (area related impact rateaccording to equations 1.17 and 1.20, proportional to the meanvelocity of the gas molecules in the direction of the cryopanel)Probability of condensation (pumping)Ultimate pressure (see above)Pressure in the vacuum chamberComparison between total (1) and partial (2) regenerationtotal regeneration after 24 partial regenerations.Throughput and maximum pV flow: (mbar l/s): the throughput of acryopump for a certain gas depends on the pV flow of the gas G throughthe intake opening of the pump:QG = qpV,G;the following equation appliesQG = pG · SGwithSth = AK · SAGasHydrogen(2.29a)withThe maximum pV flow at which the cryopanels are warmed up to T Å 20 Kin the case of continuous operation, depends on the net refrigeratingpower of the pump at this temperature and the type of gas.
For refrigeratorcryopumps and condensable gases the following may be taken as a guide:.Qmax = 2.3 Q 2 (20 K) mbar · l/s.Q 2 (20 K) is the net refrigerating power in Watts available at the secondstage of the cold heat at 20 K. In the case of intermittent operation, aSymbolThe equation (2.29) applies to a cryopanel built into the vacuum chamber,the surface area of which is small compared to the surface of the vacuumchamber. At sufficiently low temperatures α = 1 for all gases.
The equation(2.29) shows that for p >> pend the expression in brackets approaches 1 sothat in the oversaturated casep >> pend > Ps so that:pG = intake pressure,SG = pumping capacity for the gas GH2(2.29)1AKSAFig. 2.70pendp R · TGTGSA = c == 365.`/ s · cm242· π · MMTGMGas temperature in KMolar massGiven in Table 2.7 is the surface arearelated pumping speed SA inl · s-1 · cm-2 for some gases at two different gas temperatures TG in Kdetermined according to equation 2.29a.
The values stated in the Table arelimit values. In practice the condition of an almost undisturbed equilibriumMMolarmassg/molSAat 293 Kgas temp.l/s · cm2SAat 80 Kgas temp.l/s · cm2TSBoiling point1013 mbarKTriple point(= melting point)TtKptmbar2.01643.8822.9320.2713.8070.4HeHelium4.00331.1416.274.2222.17350.52CH4Methane4.00315.568.13111.6790.67116.7H2OWater18.01514.68Ð373.15273.166.09NeNeon20.18313.877.2527.10224.559433.0COCarbon monoxide28.00011.776.1581.6768.09153.7N2Nitrogen28.01311.776.1577.34863.148126.1Air28.9611.586.05Å 80.5Å 58.5ÐO2Oxygen31.99911.015.7690.18854.3611.52ArArgon39.9489.865.1587.2683.82687.5KrKrypton83.806.813.56119.4115.94713.9XeXenon131.35.442.84165.2161.4Table 2.7Surface-related pumping speeds for some gases59HomeVacuum generation(small cryopanels compared to a large wall surface) is often not true,because large cryopanels are required to attain short pumpdown times anda good end vacuum.
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