7<\/td> Others<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nAs you see in Fig.1, the vapor pressure of water vapor becomes below 10-8<\/sup>8 Pa at temperature of below 130K, and thus water vapor is pumped as a result of condensation on the 80K baffle and 80K shield. The next group of gases, nitrogen, oxygen, argon, and other gases of similar molecular weight are pumped at the exposed surface of 15K cryopanel(1) at the second stage, which is kept below 20K. The third group, mainly hydrogen, helium, and neon will not condense at 20K and will not be pumped efficiently by the metal surface because the equilibrium vapor pressure for cryosorption will be too high. To improve this situation, cryopumps have adsorbent of porous materials such as charcoal on the 2nd stage cryoarrays. The adsorbent is bonded to the inner surface of the 15K cryopanel(1) to prevent it from being covered with condensable gases.In order to monitor those temperature, K(CA) thermocouple for 80K shield, and hydrogen vapor pressure gauge(H2VP) or CRYO METER MBS for 15K cryopanel are installed to the cryopumps. (The standard for the electromotive force at 130K of K(CA) thermocouple is -5.5mV.)<\/p>\n\n\n\n
3.Regeneration and Pressure Relief Valve<\/h3>\n\n\n\n Cryopumps are not continuous throughout pumps such as oil diffusion pumps and turbo molecular pumps. As a cryopump keeps gases inside on 15K cryopanels by condensation and adsorption, it needs to be degassed and regenerated on a regular basis. During this regeneration process, the cryopump is warmed up, and condensed or adsorbed gases are turned into gas again. If large amount of gases are pumped, there is a risk of explosion. In order to prevent the explosion danger, all cryopumps feature a pressure relief valve.
4.Cryopump System<\/h3>\n\n\n\n Cryopump system consists of<\/p>\n\n\n\n
\u300a1\u300bCryopump Unit\uff08incl. Cold Head\uff09Figure 3.<\/strong> In addition, rough pump (customer-supplied) is necessary to operate and regenerate cryopumps. (Cryopumps cannot start from the atmospheric pressure.)<\/p>\n\n\n\n
Performance Characteristics<\/h2>\n\n\n\n The essential performance characteristics for cryopump operation are: \u2460Cooldown, \u2461Pumping Speed, \u2462Pumping Capacity, \u2463Maximum Throughput, \u2464Crossover Pressure, \u2465Ultimate Pressure, and \u2466Thermal Load Capacity. For details, see each section below.<\/p>\n\n\n\n
1.Cooldown Characteristics<\/h3>\n\n\n\n The vacuum chamber must be rough pumped before cryopumping since cryopumps cannot start its operation from atmospheric pressure. When rough pumping a CRYO-U\u00ae cryopump with a rotary pump, roughing pressure of 40Pa is sufficient since backflow of oil vapor will not occur. The residual gas will be adsorbed to adsorbent inside cryopump.Cooldown time differs depending on conditions as shown in table below.<\/p>\n\n\n\n
Table 1.Factors which affects Cooldown Time<\/p>\n\n\n\nFactor<\/th> Cooldown Time<\/th><\/tr> 1.Rough Pumping Pressure<\/td> HIGH<\/td> Extended<\/td><\/tr> 2.Pump Temperature<\/td> HIGH<\/td> Extended<\/td><\/tr> 3.Residual Gas Composition After Rough Pumping<\/td> DRY\uff08Dry inside cryopump\uff09<\/td> Extended<\/td><\/tr> WATERY<\/td> Shortened<\/td><\/tr> 4.Contamination of Cryopump<\/td> CONTAMINATED<\/td> Extended<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nCooldown time is affected by regeneration method. Use of N2 purge or Band Heater may extend the cooldown time causing temperature rise, or dry condition which hardly achieves vacuum insulation. Also, slight leakage may cause extended cooldown time or cooldown failure.(Especially due to leakage from pressure relief valve.) Cooldown time may be reduced by 10 to 15% at 60Hz compared to that at 50Hz. Generally, cooldown time is defined as the amount of time for 15K cryopanel temperature to drop to 20K or below. See Table 2 for details.<\/p>\n\n\n\n
2.Pumping Speed Characteristic<\/h3>\n\n\n\n2-1.Pumping Speed for Water<\/h4>\n\n\n\n It is usually assumed that the water condensation probability on cryosurface is approx. one, provided that the cryosurface temperature is below 150K. In normal operation, temperature of 80K shield and baffle is below 130K(80K or below in normal use). Therefore the pumping speed for water is equivalent to ideal pumping speed of cryopump which has a diameter of 80K shield. The ideal pumping speed (s) per unit area for molecular weight M is given by:1 \/ 2<\/sup>(L\/s\/cm2<\/sup>)(20\u2103)2<\/sup>).2<\/sup>). For 8-inch cryopump, the area of 80K shield inlet is approx. 275cm2<\/sup>, thus, the pumping speed for water is 4000L\/s. The same calculation is available for gases condensed and pumped on 80K baffle such as CO2<\/sub> and NH4<\/sub> etc.\u3000\u3000<\/p>\n\n\n\nCRYO-U8H pumping speed for water is 4000L\/s. Since molecular weight of CO2<\/sub> is 44, CRYO-U8H pumping speed for CO2<\/sub> is given by:\uff12<\/sub>\uff1d\uff33\uff28\uff12<\/sub>\uff2f\u00d7 (\uff11\uff18\/\uff14\uff14)\uff11\/\uff12\uff1d\uff12\uff15\uff16\uff10\uff2c\/\uff53<\/p>\n\n\n\nAs you see, the capture probability(sticking coefficient) of condensing array for condensable gases is one, provided the temperature of cryosurface is below 20K. Also, cryopump pumping speed at molecular flow region is constant since the conductance from inlet to cryopanel is constant at molecular flow region.\uff12<\/sub>\u00d7\uff08\uff12\uff18\/\uff2d\uff09\uff12\/\uff11<\/sup>\uff08\uff2c\/\uff53\uff09\u3002<\/p>\n\n\n\nTable 2.Pumping Speed of Various Cryopump for Nitrogen(Value from Brochure)<\/p>\n\n\n\nDiameter<\/th> Model<\/th> Pumping Speed\uff08L\/s)<\/th><\/tr> 6<\/td> U6H<\/td> 2100<\/td><\/tr> 8<\/td> U8H,U8H-U,U8HSP<\/td> 4000<\/td><\/tr> 10<\/td> U10PU<\/td> 6900<\/td><\/tr> 12<\/td> U12H,U12H-K2,U12HSP<\/td> 9500<\/td><\/tr> 16<\/td> U16,U16P<\/td> 16000<\/td><\/tr> 20<\/td> U20P<\/td> 29000<\/td><\/tr> 22<\/td> U22H<\/td> 39000<\/td><\/tr> 30<\/td> U30H<\/td> 70000<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nPumping Characteristic for Argon and Nitrogen (Condensable Gas)<\/h4>\n\n\n\n A group of gases such as N2<\/sub>\u3001Ar\u3001CO\u3001O2<\/sub> and other gases of similar molecular weight are condensed and pumped at 20K or below, therefore they are not condensed on 80K baffle and shield. In general, the limit temperature that gases can be pumped by condensation is 20K. The temperature of condensing array when condensation probability of gases becomes below one at 300K is:2<\/sub>\u00d7(28\uff0fM)1\/ 2<\/sup>\uff08L\/s\uff09\u30fb\u30fb\u30fb\u30fb\u30fb\u30fb\u30fb(1)2<\/sub>\uff1aPumping Speed for Nitrogen\uff08L\/s\uff09<\/p>\n\n\n\nFor example, CRYO-U8H pumping speed for argon is (See Table 2):2<\/sub>\uff1d1700\uff08L\/s\uff09and also molecular weight of argon is 40, thus, the calculation is:1 \/ 2<\/sup>=1400L\/s<\/p>\n\n\n\nModel<\/th> Pumping Speed\uff08L\/s\uff09\uff08L\/s\uff09<\/th><\/tr> U6H<\/td> 750<\/td><\/tr> U8H\/U8H-U\/U8HSP<\/td> 1700<\/td><\/tr> U10P<\/td> 2300<\/td><\/tr> U12H<\/td> 4000<\/td><\/tr> U12HSP<\/td> 4100<\/td><\/tr> U16\/U16P<\/td> 5000<\/td><\/tr> U20P<\/td> 10000<\/td><\/tr> U22H<\/td> 17000<\/td><\/tr> U30H<\/td> 28000<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nTable 3.Pumping Speed of Various Cryopump for Nitrogen(Value from Brochure)<\/p>\n\n\n\n
When gas flow changes from molecular flow to intermediate flow(transition flow), pumping speed increases since the conductance increases proportional to the pressure. However, since incoming heat increases as pressure increases, cryopump reaches its limit of pumping ability when the thermal load exceeds cold head refrigerating capacity. ULVAC CRYOGENICS defines the flow rate when the cryopanel reaches 20K by the thermal load as the maximum throughput.\uff08The point marked “\u25cb” in Figure 2.\uff09 The maximum throughput may be increased with large refrigerating capacity, but it may generate temperature gradient in thickness direction since the thermal conductivity for cryoarray is limited even with a large refrigerating capacity. If the temperature of condensation array becomes too high exceeding its limit, gases will not be condensed, and then pumping capacity becomes zero. That is to say a physical limit of pumping.<\/p>\n\n\n\n
2-3.Pumping Speed for Hydrogen, Helium, and Neon\uff08Non-condensable Gas\uff09<\/h4>\n\n\n\n The next group of gases such as hydrogen, helium, and neon will not condense at 20K and will not be pumped efficiently by metal surface because the equilibrium vapor pressure for cryosorption will be too high. These gases cannot be pumped through condensation, therefore adsorbent cooled below 20K is provided to adsorb these gases. The pumping speed will decrease as the surface coverage increases. When the pumping speed is down to 80% of its starting value, the total amount of pumped gases is defined as the pumping capacity. (see below.) Pumping speed for non-condensable gases depends on the following:Model Pumping Speed Maximum Throughput Pumping Capacity -U6H<\/td> 1100<\/td> 1.1\u00d7102<\/sup><\/td>3.1\u00d7105<\/sup><\/td><\/tr>-U8H<\/td> 2700<\/td> 2.4\u00d7102<\/sup><\/td>1.0\u00d7106<\/sup><\/td><\/tr>-U8HSP<\/td> 3200<\/td> 2.4\u00d7102<\/sup><\/td>1.0\u00d7106<\/sup><\/td><\/tr>-U10PU<\/td> 3600<\/td> 1.5\u00d7102<\/sup><\/td>6.7\u00d7105<\/sup><\/td><\/tr>-U12H<\/td> 6000<\/td> 4.1\u00d7102<\/sup><\/td>9.8\u00d7105<\/sup><\/td><\/tr>-U12HSP<\/td> 6000<\/td> 4.1\u00d7102<\/sup><\/td>1.6\u00d7106<\/sup><\/td><\/tr>-U16<\/td> 10000<\/td> 4.1\u00d7102<\/sup><\/td>2.4\u00d7106<\/sup><\/td><\/tr>-U16P<\/td> 10000<\/td> 4.5\u00d7102<\/sup><\/td>2.4\u00d7106<\/sup><\/td><\/tr>-U20P<\/td> 18000<\/td> 5.0\u00d7102<\/sup><\/td>4.6\u00d7106<\/sup><\/td><\/tr>-U22H<\/td> 25000<\/td> 1.3\u00d7103<\/sup><\/td>8.5\u00d7106<\/sup><\/td><\/tr>-U30H<\/td> 43000<\/td> 7.4\u00d7102<\/sup><\/td>1.5\u00d7107<\/sup><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nTable 4.CRYO-U Pumping Performance for Hydrogen<\/p>\n\n\n\n
3.Pumping Capacity of Cryopumps<\/h3>\n\n\n\n Since a cryopump does not have a continuously functioning exhaust port, it has a limiting value of pumping. That is called the pumping capacity. When a cryopump becomes saturated, it is required to be regenerated because the performance degradation such as ultimate pressure rise and lowered pumping speed will occur. There is a big difference of cryopump pumping capacity between non-condensable gases and condensable gases. For instance, the pumping capacity for hydrogen is about 1% of that for nitrogen and argon.<\/p>\n\n\n\n
3-1.Pumping Capacity for Condensable Gases<\/h4>\n\n\n\n The gases pumped by condensation are \u2460gases pumped at 80K shield and baffle (mainly H2O) and \u2461gases pumped at 15K cryopanel (nitrogen, argon, oxygen, etc.)<\/p>\n\n\n\n
(1) Pumping Capacity for WaterModel<\/th> Pumping Capacity \uff08\uff47\uff09<\/th><\/tr> CRYO-U6H<\/td> 40<\/td><\/tr> CRYO-U8H,U8H-U<\/td> 90<\/td><\/tr> CRYO-U10PU<\/td> 170<\/td><\/tr> CRYO-U12H<\/td> 260<\/td><\/tr> CRYO-U16,U16P<\/td> 500<\/td><\/tr> CRYO-U20P<\/td> 1000<\/td><\/tr> CRYO-U22H<\/td> 1400<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nTable 5.Pumping Capacity for Water(as measure)<\/p>\n\n\n\n(1) Materials which release much water during processing are<\/th><\/tr> Plastic<\/td><\/tr> Glass<\/td><\/tr> Ceramic<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n(2) Cautions for regeneration at atmosphere with much water are<\/th><\/tr> Melt ice completely when warming up.<\/td><\/tr> Do not freeze water when rough pumping.<\/td><\/tr> Remove water from inside cryopump completely.<\/td><\/tr> Confirm the operation of rotary pump.\uff08Avoid emulsifying of oil.\uff09<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n(2)Pumping Capacity for Argon<\/p>\n\n\n\n
In sputtering process, there is a problem concerning the pumping capacity for argon condensed at 15K cryopanel. If the accumulated argon frost on the external surface of 15K cryopanel becomes thick, it may reach 80K baffle and shield which are both high temperature parts compared to 15K cryopanel. In addition, there is a possibility of insufficient pumping for argon since the temperature of argon frost reached to 80K baffle and shield may become too high to condense argon more than that. The amount of argon, which is pumped previously until then, is called the pumping capacity for argon. Pumping capacity for argon is defined as the amount of previously pumped argon when introduction of argon has stopped, and the pressure after 5 minutes with closing main valve does not drop below 1.3X10-4<\/sup>Pa. Even if the introduction of argon is stopped, the pumping ability will be lost if the pumping capacity for argon has reached its capacity. Figure 3 shows the pressure build up of CRYO-U12HSP. In this figure, argon has been introduced continuously at 200SCCM, and then the pressure 5 minutes after the introduction of argon has stopped was recorded. As you see from the figure, the pressure buildup worsens significantly at 4.3X108<\/sup> Pa\uff65L, and thus it is assumed that the pumping capacity is 4.3X108<\/sup> Pa\uff65L. Various cryopump pumping capacity for argon is also shown in Table 6.<\/p>\n\n\n\nFigure 3.Pressure Buildup of CRYO-U\u00ae12HSP\uff08Example\uff09ModelCRYO-<\/th> Pumping Capacity(Pa\u30fbL)<\/th><\/tr> -U6H<\/td> 5.6\u00d7107<\/sup><\/td><\/tr>-U8H,U8H-U<\/td> 1.0\u00d7108<\/sup><\/td><\/tr>-U8HSP<\/td> 2.5\u00d7108<\/sup><\/td><\/tr>-U10PU<\/td> 1.0\u00d7108<\/sup><\/td><\/tr>-U12H<\/td> 2.1\u00d7108<\/sup><\/td><\/tr>-U12HSP<\/td> 4.3\u00d7108<\/sup><\/td><\/tr>-U16,U16P<\/td> 4.3\u00d7108<\/sup><\/td><\/tr>-U20P<\/td> 5.8\u00d7108<\/sup><\/td><\/tr>-U22H<\/td> 8.1\u00d7108<\/sup><\/td><\/tr>-U30H<\/td> 7.8\u00d7108<\/sup><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n3-2.Pumping Capacity for Non-condensable Gases<\/h4>\n\n\n\n Non-condensable gases such as hydrogen, helium, and neon are pumped by porous materials attached to inner surface of 15K cryopanel since they will not condense at 10K and will not pumped by the metal surface. Therefore, not only will the capacity be limited to fractions of a monolayer, but the pumping speed will decrease as the surface coverage increases, and the cryopump will lose its pumping ability at last. When the pumping speed for hydrogen drops to 80% of its starting value, ULVAC CRYOGENICS defined the quantity of hydrogen pumped as the pumping capacity for hydrogen. The adsorbent should be kept clean to attain its adsorption ability. Contamination of adsorbent is caused by:
Figure4.Pumping Speed and Pumping Capacity for Hydrogen
4.Thermal Load and Maximum Throughput<\/h3>\n\n\n\n The thermal load to cryopumps is generated not only by heat radiation from the equipment, but also heat from gases such as thermal conductance or heat transfer with condensation. They are given by the following equations:\u03c3<\/th> \uff1aBoltzmann constant\u30005.67\u00d710-12<\/sup>W\/cm2\/K4<\/td><\/tr>\u03b5AV<\/th> \uff1aMean Radiation Rate<\/td><\/tr> T1<\/th> \uff1aLow Temperature Surface\uff08K\uff09<\/td><\/tr> T2<\/th> \uff1aHigh Temperature Surface\uff08K)<\/td><\/tr> A<\/th> \uff1aHeat Receiving Area\uff08cm2<\/sup>)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n\u03b3<\/th> \uff1aSpecific Heat Ratio<\/td><\/tr> \uff410<\/th> \uff1aMean Thermal Accommodation Coefficient<\/td><\/tr> \uff30<\/th> \uff1aPressure\uff08Pa\uff09<\/td><\/tr> M<\/th> \uff1aMolecular Weight<\/td><\/tr> T1<\/th> \uff1aLow Temperature Surface\uff08K\uff09<\/td><\/tr> T2<\/th> \uff1aHigh Temperature Surface\uff08K\uff09<\/td><\/tr> A<\/td> \uff1aHeat Receiving Area\uff08cm2\uff09<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nMean Thermal Accommodation Coefficient \uff410\uff08A1\uff1cA2\uff09\u03b3<\/th> \uff1aCondensation Heat(Adsorption Heat for H2<\/sub>, He, and Ne) (W\/Pa\u30fbL\/s\uff09<\/td><\/tr>Tc<\/th> \uff1aTemperature Surface\uff08K\uff09<\/td><\/tr> Tg<\/th> \uff1aGas Temperature\uff08K\uff09<\/td><\/tr> S<\/th> \uff1aCryopump Pumping Speed\uff08L\/s)\u3000\u3000SP\uff1a\uff08Pa\u30fbL\/\uff53\uff09<\/td><\/tr> P<\/th> \uff1aPressure\uff08Torr\uff09<\/td><\/tr> Cp<\/th> \uff1aMean Specific Heat of Gas(W\/\uff08Pa\u30fbL\/s\uff09\/K\uff09<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nThermal load to 1st stage of cold head is from radiation heat and heat conduction of gases, but mainly from radiation heat in normal use except for continuous operation at the same level of 10-1<\/sup>Pa. Thermal load to 2nd stage of cold head is from gas condensation heat, and the maximum throughput is depending on it. The maximum throughput may be reduced as the thermal load to 1st stage increases since the thermal load to 1st stage affects the refrigerating capacity of 2nd stage.-1<\/sup>Pa, intermediate flow.Cryopump Maximum Throughput<\/p>\n\n\n\n<\/th> Argon Hydrogen CRYO-U6H<\/th> 1.1\u00d7103<\/sup><\/td>1.1\u00d7102<\/sup><\/td><\/tr>CRYO-U8H,U8H-U,U8HSP<\/th> 1.2\u00d7103<\/sup><\/td>2.4\u00d7102<\/sup><\/td><\/tr>CRYO-U10PU<\/th> 8.0\u00d7102<\/sup><\/td>1.5\u00d7102<\/sup><\/td><\/tr>CRYO-U12H,U12HSP<\/th> 2.0\u00d7103<\/sup><\/td>4.1\u00d710
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