SCALING OF MICRODISCHARGE DEVICES: PYRAMIDAL STRUCTURES*

Transcription

1 SCALING OF MICRODISCHARGE DEVICES: PYRAMIDAL STRUCTURES* Mark J. Kushner Dept. Electrcal and Computer Engneerng Urbana, IL USA October 2003 * Work supported by the Natonal Scence Foundaton and Electrc Power Research Insttute/ALITE. GEC03_01

2 AGENDA Introducton of pyramdal mcrodscharge devces. Descrpton of model. Fundamental propertes of MDs sustaned n neon. Transton from Townsend to negatve glow. Scalng of MDs Concludng remarks. GEC03_02

3 MICRODISCHARGE PLASMA SOURCES Mcrodscharges are plasma devces whch leverage pd scalng to operate dc atmospherc glows 10s 100s µm n sze. MEMS fabrcaton technques enable nnovatve structures for dsplays and detectors. Although smlar to PDP cells, MDs are dc devces whch largely rely on nonequlbrum beam components of the EED. Electrostatc nonequlbrum results from ther small sze. Debye lengths and cathode falls are commensurate wth sze of devces. L 1/ 2 ( 2V ε / ( qn )) m cathode Fall = c 0 I µ ISPC03_27 1/ 2 T ev λd 750 cm 10µ m, 3 ne( cm )

4 PYRAMIDAL MICRODISCHARGE DEVICES S MDs wth 10s µm pyramdal cavtes dsplay nonequlbrum behavor: Townsend to negatve glow transtons. Small sze also mples electrostatc nonequlbrum. Delectrc Slcon Cathode Plasma Anode S.-J. Park, et al., J. Sel. Topcs Quant. Electron 8, 387 (2002); Appl. Phys. Lett. 78, 419 (2001). ISPC03_28

5 Charged partcle contnuty (fluxes by Sharfetter-Gummel form) Posson s Equaton for Electrc Potental Bulk contnuum electron energy transport and MCS beam. Neutral contnuty and energy transport. ( ) ( ) S N D qn t N + Φ = v v µ ρ V ρ S Φ ε + = ( ) e e e e q j, T 2 5 N n E j t n φ λ εϕ κ ε r r r r = = ( ) g o o P T t ct, S N N DN v t N + = + = κ ρ r 2-D MODELING OF MICRODISCHARGE SOURCES ISPC03_29

6 DESCRIPTION OF MODEL: MCS AND MESHING Transport of energetc secondary electrons s addressed wth a Monte Carlo Smulaton. ISPC03_30 Supermpose Cartesan MCS mesh on unstructured flud mesh. Construct Greens functons for nterpolaton between meshes. Electrons and ther progeny are followed untl slowng nto bulk plasma or leavng MCS volume. Electron energy dstrbuton s computed on MCS mesh. EED produces source functons for electron mpact processes whch are nterpolated to flud mesh.

7 MODEL GEOMETRY: S PYRAMID MICRODISCHARGE Investgatons of a cylndrcally symmetrc S pyramd mcrodscharge were performed. GEC03_03

8 MODEL GEOMETRY: S PYRAMID MICRODISCHARGE Meshng s absolutely crtcal to resolve small structures and dstant boundares. Typcal Mesh: 5,000-10,000 nodes, dynamc range EPRI_0103_26

9 BASE CASE: Ne, 600 Torr, 50 µm DIAMETER Optmum operaton produces large enough charge densty to warp electrc potental nto cathode well. Inspte of large T e, onzaton s domnated by beam electrons Ne, 600 Torr, 50 µm, 200 V, 1 MΩ GEC03_04

10 BASE CASE: Ne, 600 Torr 50 µm DIAMETER There s essentally no regon of quasneutralty or whch s postve column-lke. Monomer and dmer ons are segregated. Excted state denstes > cm -3 rval macroscopc devces Ne, 600 Torr, 50 µm, 200 V, 1 MΩ GEC03_05

11 TRANSITION TO NEGATIVE-GLOW BEHAVIOR Although geometry precludes true hollow cathode behavor, negatve glow behavor sets n a lower pressures. Characterze negatve glow by S[Ne 2+ ] / (S[Ne + ] + S[Ne 2+ ] ) GEC03_06 Ne, 50 µm dameter, 200 V, 1 MΩ

12 SCALING WITH PRESSURE: PASCHEN BEHAVIOR Wth pd=1-10 Torr-cm, these mcrodscharge devces dsplay Paschen behavor. Although senstve to ballastng and current densty lower pressures requrng larger appled voltages also produce large plasma denstes. GEC03_07 Ne, 50 µm dameter, 1 MΩ

13 SCALING WITH PRESSURE: PLASMA PROPERTIES Over a range of pressures that V(appled) and R(ballast) can be constant, confnement at hgher pressures produces hgher peak plasma denstes. [e] x cm Torr [2.1 x cm -3 ] 650 Torr [3.9 x cm -3 ] 750 Torr [5.6 x cm -3 ] GEC03_08 Ne, 50 µm dameter, 200V, 1 MΩ

14 SCALING CONSIDERATIONS: CATHODE FALL THICKNESS In MDs, the cathode fall thckness may be commensurate wth cavty sze. Current densty s therefore crtcal to scalng. Low j (and [e]) may result n cathode fall not beng conformal to cathode V, 1 MΩ [e]= 4.9 x cm V, 1.75 MΩ [e]= 5.3 x cm -3 GEC03_06 Ne, 50 µm dameter, 600 Torr

15 SCALING WITH SIZE: pd, BALLAST = CONSTANT Scalng whle mantanng pd, V(appled) and R(ballast) constant results n a reduced j and [e] n the larger devce. The plasma s not conformal to the cathode. GEC03_10 Ne, -200 V, 1 MΩ

16 SCALING WITH SIZE: pd, j = CONSTANT Scalng whle mantanng pd and j constant produces smlar plasma denstes and conformalty to the cathode. 400 Torr 600 Torr 1000 Torr GEC03_11 Ne, -200 V

17 CONCLUDING REMARKS MDs dffer from macroscopc devces n that plasma scale lengths are commensurate wth devce dmensons. Scalng of MDs wth pressure (tradtonally pd ) lkely also requred λ/l to reman constant or less than a crtcal value. Scalng wth complex shapes must consder all dmensons. The transton from Townsend to negatve glow s largely geometrcally dependent, and can be controlled to some degree by shape. GEC03_12