Vertex System A retarding field energy analyser with ion angular discrimination “

Talk Outline Angle Theory Overview Variable aspect ratio Combining angle and energy discrimination V angle Example: Ion energy distribution as a function of angle Summary

The incoming ions have an energy component in the X direction perpendicular to the sampling aperture E I and an energy component in the Y direction E II parallel to the sampling aperture The Idea Incoming ion with angle α E II EIEI α G1 G2 G3 C

The aspect ratio determines if the E II energy component is such that the ion gets collected at the aperture electrodes or passes through the apertures for detection at the collector of the RFEA. Variable Aspect Ratio Ion not collected L L Ion collected

The electric field applied in the E I direction determines if the E II energy component is such that the ion gets collected at the aperture electrodes or passes through the apertures for detection at the collector of the RFEA. Variable Aspect Ratio Using a Variable Bias V angle E L Ion collectedIon not collected L

Incoming ion with angle α E II EIEI α G1 G2 G3 C V angle Aperture Between Grid 2 and Grid 3

Theory The RFEA is designed to have electric fields in the X direction only, it is a planar system with all grids parallel to each other. The discriminator potential at G2 is used to separate ions with different energy and acts only on the E I component of the incoming ion. The E II component of the ion energy is unaffected by the electric fields inside the sensor. At any location inside the sensor the E II component of the ion energy is identical to the E II energy component of the ion as it entered the sensor through the sampling aperture. By varying the potential difference between G2 and G3, V angle the acceptance angle for which ions can enter the sensor is varied.

Discussion The incoming ion arrives at the surface of G1 at an angle α Based on positive voltage bias at G2 (w.r.t. G1) the ion decelerates and loses perpendicular energy component E I and reaches G2 at a different angle β Due to negative voltage bias at G3 (w.r.t. G2) the ion accelerates and gains perpendicular energy componenet E I and reaches G3 with different angle ɤ. G2 G3 C Ion with angle β Ion with angle ɤ Deceleration Acceleration G1 Incoming ion with angle α α E II EIEI β EIEI ɤ EIEI

Concept Tan( ɤ ) = E II / E I The ion will be collected at C only if Tan( ɤ ) < 2a / L No ion will be collected at C if Tan( ɤ ) > 2a / L G2 G3 C L 2a ɤ E II EIEI

Concept Tan ( ɤ ) = 1/r x Tan (β) r = ratio of potential difference b/w G2 & G3 and G2 potential By varying r, acceptance angle ɤ can be varied

Results E II EIEI ɤ1ɤ1 ɤ2ɤ2 ɤ4ɤ4 ɤ3ɤ3 ɤ5ɤ5 ɤ6ɤ6 ɤ 1 > ɤ 2 > ɤ 3 > ɤ 4 > ɤ 5 > ɤ 6 Current Angle Derivative ɤ1ɤ1 ɤ2ɤ2 ɤ3ɤ3 ɤ4ɤ4 ɤ5ɤ5 ɤ6ɤ6

Ion Angle Distribution The discriminator grid G2 selects the energy window for ion detection. At each energy selected the potential difference b/w G2 and G3 is swept through the various ion angles. Current is collected at each set of angles The current derivative shows the ion angle distribution

Ion Angle Distribution -5.00E E E E E E E-07 0 eV10 eV 20 eV30 eV40 eV50 eV Ion Angle Measurement in CCP plasma 0 degrees 3 degrees 6 degrees

Ion Angle Distribution 0.00E E E E E E E E E-08 0 degrees6 degrees 9 degrees 12 degrees average over 18 to 30eV 15 degrees18 degrees

Summary Angle is determined by ratio of E I and E II Collected current is reduced when high angle ions cross the aperture Varying aperture aspect ratio will vary collected ion current. Effective aspect ratio can be changed by applying V angle to aperture. Collected current becomes a function of V angle depending on ion angular distribution entering Semion. Derivative of ion current as a function of V angle gives Ion Anglular Distribtuion function. Angle resolution 3 degrees over range 0 to 45 degrees