Chapter 14 Getter and Ion Pumps (Capture pumps)

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1 Chapter 14 Getter and Ion Pumps (Capture pumps) ** Getter pump (titanium sublimation pump, TSP; non-evaporable getter pump, NEG) and ion pumps are capture pumps. -- Chemical method of pumping Getter pump and ion pump (vs Physical method of pumping cryopump, sorption pump, LN 2.) -- All gases are not pumped equally well. Two or more capture processes are usually combined to pump effectively a wide range of active and noble gases. -- Clean pump with no backstream of heavy organic molecules. -- Compliations: C in some metals + H 2 O CH 4 + CO 2 Displacing other adsorbed gases. Desorbing some adsorbed H 2, inert gases, and particles Getter Pumps ** Reactive metals rapidly pump large quantities of active gasses by gettering. ** Titanium sublimation pump (TSP) A surface getter pump. Gases reacts to form a surface compound (TiO) or diffuse (similar to H 2 ) into the bulk of the getter. cooled to enhance the sticking coefficient.

2 Titanium Sublimation Pump (TSP) ** Clean surface of reactive metals such as titanium, molybodenum, niobium, tantalum, zirconium, or aluminum are surface getters for active gases, and will pump N 2, O 2, CO 2, H 2 O, and CO by chemisorption. ** TSP is typically used because it can sublime at much lower temperatures, and it is inexpensive, and pumps a large number of gases. -- Ti filament + heat titanium sublimation and deposition on adjacent walls. -- Active gases are captured on the fresh titanium surface, which is cooled with water or LN Heating can not desorb the pumped gases. Periodic deposition of a fresh titanium layer to ensure continuous pumping.

3 Titanium Sublimation Pump (TSP) At room temp ** Active gases (CO, O 2, H 2 O vapor, C 2 H 2 ) are pumped with high sticking coefficients in the 77 to 300 K range. -- H 2 O dissociates into O 2 and H 2, which are then pumped separately. ** The sticking coefficients for intermediate gases (H 2 and N 2 ) are low at room temperature but increase at 77 K. -- After adsorbing, hydrogen will diffuse into the underlying film. ** Chemically inert gases (He, Ar, CH 4 ) are not pumped at all at room temperature. -- Methane is only slightly adsorbed on titanium at 77 K.

4 Titanium Sublimation Pump (TSP) ** Memory effect The replacement of one previously adsorbed gas by another gas. Sticking coefficients depend on the nature of the underlying adsorbed gas. ** TSP operates at pressure < 10-1 Pa.-- P < 10-4 Pa: Constant pumping speed determined by the surface area. -- P > 10-4 Pa: Titanium gas collisions no sublimation Pumping speed Titanium deposition on the surface Q Titanium sublimation rate, S 1 / P (PS = constant)

5 Titanium Sublimation Pump (TSP) ** Pumping speed at the low-pressure region: 1/S = 1/S i + 1/C a, S i : Intrinsic speed of the surface. S i (L/s) = 1000 A (v avg / 4)s, A, m 2 ; v avg, m/s; s, sticking coeff. C a : Conductance of the aperture at the end of the cylindrical surface on which the titanium is deposited. ** Pumping speed at high pressures (titanium is fully reacted with gas): -- Throughput: Q(Pa-L/s) = 10 8 V 0 TSR/ nn A = 1.25x10-2 TSR μg Ti/s /n TSR: Titanium sublimation rate in atoms/s. V 0 : Normal specific volume of a gas ( L/mol for an ideal gas). N A : Avogadro s number. n: Number of titanium atoms that react with each molecule of gas. n = 1 for CO; n = 2 for N 2, H 2, O 2, and CO Pumping speed: S = Q / P

6 Titanium Sublimation Pump (TSP) ** Typical TSP operation, in combination with other pumps that will pump inert gases and methane. 1. A sorption pump + small TSP for a short time + ion pump. Pressure crossover 2. Large TSP + small ion pumps for long-term and high-throughput pumping. 3. Intermeittent TSP is used for long periods at low pressure to provide high-speed pumping for reactive gases. 4. At low pressure, titanium is sublimed only periodically to retain the pumping speed. TSP is turned off until the film saturated.

7 14.2 Ion Pumps Penning cell Early forms of the diode sputter - ion pump ** Pumping gases by ions (considered detrimental to vacuum gauge operation). ** Two cathodes and the axial magnetic field forces the electrons into circular orbit that prevent them from reaching the anode. The electrons travel long distance before colliding with the anode. High probability of ionizing collision with gas. ** Steady state density of electrons ~ electrons / cm 3. Starting time 10-9 s at 10-1 Pa, 500 s at 10-9 Pa.

8 14.2 Ion Pumps Ionizing collision with gas: ** Ions collide, sputter away the cathode, and release secondary electrons that in turn are accelerated by the field. -- Additional processes: Low-energy neutrals by molecular dissociation. High-energy neutrals from energetic ions by charge neutralization as they approach the cathode, collide, and recoil elastically. The mechanism of pumping in an ion pump: (1) Precipitation or adsorption following molecular dissociation. (2) Gettering by freshly sputtered cathode material (titanium). (3) Surface burial under sputtered cathode materials. (4) Ion burial following ionization in the discharge. (5) Fast neutral atom burial. Pump Steady state (reemission rate = pumping rate) memory effect Magnets: T Cathode voltage: ~ 5 kv

9 14.2 Ion Pumps Inert gas ionized before burial.(δ) Inert gases buried as neutral particles. ( ) Chemically active gases ionized before burial.( ) ** Organic gases are pumped by adsorption and precipitation after dissociation by electron bombardment. ** Active gases (O 2, CO, N 2 ) are pumped by reaction with titanium, which is sputtered on the anode surface, and by ion burial in the cathode. ** Hydrogen is initially pumped by ion burial and neutral adsorption, and diffuses into the bulk to form a hydride. A surface covered with compounds that prevent H 2 diffusion will limits the pumping speed for H 2. Chemically active gases buried as neutral particles.( ) (P. 257)

10 14.2 Ion Pumps Diode vs Triode ** Noble gases are not pumped so efficiently as active gases in a diode pump. (memory effect) Triode pump Three major ways differ the triode from the diode: (1) Each titanium plate is insulated from the pump wall. (2) A high negative potential is imposed on the plate; and the plate has multiple slots through it. (3) The triode's slotted plates sputtered titanium to deposit on the pump walls behind the plate. Much less likely to suffer ion bombarded even at high pressures with high bombardment rates. -- Any inert gas physically buried less likely to be released.

11 14.2 Ion Pumps ** Ion pumps cover a very wide pressure range in 10-2 to 10-8 Pa. ** Other than starting, operation at high pressure is NOT recommended because it shortens the pump life. ~ 5000 h at 10-3 Pa, or h at 10-4 Pa. Tridode has a lifetime of less than half the diode. ** The ultimate pressure after bakeout is generally in the region of 2 x 10-9 Pa. ** At high pressure side of operation, discharge intensity is proportional to pressure and thus the pump serves as its own pressure gauge. ** Ion pumps require no cooling water or backing pump, and introduce no hydrocarbon or other contamination to the vacuum system. Ion pumps does suffer from the re-emission of previously pumped gas (memory effect), especially H 2, CH 4, and noble gases. ** Ion pumps continue pumping by getter action even if the power fails. ** Stray electric and magnetic fields are unacceptable.