Preliminary work on microplasmas and APGDs

p.m.bryant@liv.ac.uk

Paul Bryant

Department of Electrical Engineering and Electronics, Brownlow Hill, University of

Liverpool, L69 3GJ.

Recent technological and scientific advances in atmospheric pressure discharges have opened up an exciting area of research with opportunities for novel applications. These can be broadly classified into:

• Large area atmospheric pressure discharges

• AC and RF excited barrier and barrier free discharges e.g. DBDs (Dielectric barrier discharges) and bare electrode discharges. (Textiles treatment)

• Packed bead reactors (de-toxification of car gas exhausts NOx etc)

• Corona and Arc discharges (Textile treatment, environmental detoxification of gas plumes etc)

and …

• Micron - sized discharges (microplasmas)

• PDPs Plasma display panels (micro DBDs)

• Si based microplasma arrays (displays, UV sources)

• Plasma needles (dentistry)

• Micro hollow cathodes

Introduction

1) Si-based Microplasmas (with J. G. Eden and S. J. Park, University of Illinois, USA)

2500 (50 x 50) plasma pixel array based on pSi.

Each pixel is an inverted pyramid 50 m square by 30 m deep.

AC

pSi

Polyimide3 m

Ni

Si3N4

1.8 m

1.7 m

Plasma ball

Distance between pixels approx. 50 m.

Experimental set- up< 1 kVpk-pk, < 20 mA

5, 10, 15 and 20 kHz

AC Pwr

supply

Vacuum chamber and rotary pump

IdVd Active

strain gauge

Ar Ne

SRS Delay Gen.

Computer

Osc. ICCD camera

Plasma on!

Micro-Langmuir probe

Filters: 810nm, 750nm

700 V Ne

-12

-6

0

6

12

0 0.02 0.04 0.06 0.08 0.1

t / ms

I d /

mA

-2.4

-1.2

0

1.2

2.4

Ip /

mA NOPLASMA

PLASMA

diff

5 and 10kHz 500Torr Ar (B)

0

200

400

600

800

1000

0 5 10 15 20 25

Ip-p / mA

Vp

-p /

V 5 kHz

10kHz

Preliminary Results

AC

Polyimide capacitor

Zs Za<<

plasma

array

Id ip

IV independent of pressure, gas etc ! plasma is a parasitic load with Id >> ip

C10kHz = 0.46 nF

Ip per pixel approx 1.20mA / 2500 = 0.48 A!

plasma current peaks (green) increase with Vd and move towards voltage peak.

C5Khz0.57nF

C = 0.4 nF (measured independently)

Imaging results: (with Dr Greg Clarke)

700Torr Ne

7

Matching unit

2) APGD Atmospheric Pressure Glow Discharge

Dielectric Barrier Discharge

d = 1 – 2 mmAC

Filamentary plasmaHomogeneous

plasmaunstable !

Glass, quartz

rf DBD MHz

13.56MHz

Stable! With and modes like in low pressure rf CCP discharges

AC

Barrier free discharge

10 -100 kHz

Flow: 5 slm

He

50Hz, kHz

Ar, He, Ne,

O2

Collaborative Project withIn high pressure plasmas excimers (excited molecular complexes) are readily formed via three body collisions.

The resulting excimer Rg2* is unstable and rapidly disintegrates (within a few ns) typically emitting photons via spontaneous emission.

Table 1: Rare gas excimer peak radiation wavelength in nm

He2* Ne2

* Ar2* Kr2

* Xe2*

74 83 126 146 172

Glass

Stack

TiO2

ZnO2

Ag

ZnO2

ZnSnOx

18-20 nm

5 nm

10-12 nm

3-5 nm

40 nm

Figure 2. Schematic of glass sample stack structure

UV wavelengths good for photon-induced bond-breaking.

E.g. Modifying properties of glass stacks.

Need large area, stable & homogeneous plasma source.

Rg* + Rg + Rg Rg2* + Rg.

Experimental set-upExperimental set-up

1-2 mm

Current probe

High voltage probeHH audio

amp.

Audio Osc.

Oscillo.

Figure 1. Schematic of the existing APGD source set-up.

Gas in

2800 : 54

Active strain gauge

He Ne Ar

Vacuum chamber & Rotary pump

1st stage: Ignite a barrier-free discharge (simpler!)

2nd stage: rfDBD – need RF power feedthrough, ceramic plates

< 5KV, < 2A

Barrier free arrangement with 2 cm diam. Stainless steel electrodes.

Preliminary results:

Ne 10 kHz 700Torr

-300-200-100

0100200300

0 0.02 0.04 0.06 0.08 0.1

t / ms

Va

/ V

-100

-50

0

50

100

Ia /

mA Va

Ia

At ignition (approx 1 kVp-p) observe a discharge column:

Stable Ne, Ar glow

High voltage, low current

No electrode damage

Ne 700 Torr 10kHz

Not an arc !

In rfDBD by decreasing Va discharge column should fill electrode gap with uniform plasma glow …

But in our case it extinguishes!

Possible reason:

Gas flow – reduces breakdown voltage, easier to ignite volume

Summary• Microplasma array:

• plasma array ignited only at several points during the driving voltage cycle, and seem to be related to the plasma current spikes and power maximums. Various other studies are underway (i.e. Paschen curves, micro-Langmuir probe, filtered ICCDs, material treatment) to shed further light on plasmas ignition process.

• APGD:

• Upon ignition a stable discharge column (filamentary discharge?) appears. However, unlike in rfDBDs and other barrier free discharges, reducing the applied voltage does not lead to a uniform glow filling the inter-electrode space. Reason might be due to absence of flow of approx 5slm!

• Is there room for dust in microplasmas / APGDs? Yes!

• See “Observation of individual particles and Coulombic solids in a microdischarge” John, P.C., et al IEEE Trans. Plasma Science, 27, 199.

• APGDs? Reactive gases could produce dust disrupting processes, coating of dust particles, dust crystals at high pressure new phenomena!