Science of plasma arc machining

PRESENTED BY -VINIT SINGH2016PPE5370

Science of Plasma Arc Machining

SUBMITTED TO-Dr. HARLAL SINGH MALIDEPT. OF MECHANICAL ENGINEERING

CONTENTS

•WHAT IS PLASMA• PLASMA CATEGORY• PLASMA PROPERTIES•USE OF PLASMA IN MANUFACTURING• PLASMA FORMATION• ETCHING• PLASMA ETCH REACTORS•ADVANTAGES OF RF PLASMA• REGIONS FORMED DURING PLASMA• PLASMA NOZZLE• REFRNECES

WHAT IS PLASMA?

- Can be defined as “SUPERHEATED ELECTRICALLY IONIZED GAS”.- Creation of plasma is possible by heating the gas and subjecting through electromagnetic field.

PLASMA 4th STATE OF MATTER

- Plasma consist of a collection of free moving electrons and ions- atom that have lost electrons.

- Energy is needed to strip electrons from atoms to make plasma .

PLASMA CATEGORY According to the temperature

PLASMA

HIGH TEMPERATURE PLASMA (10^8-10^9 K)

LOW TEMPERATURE PLASMA

HOT PLASMA

COLDPLASMA

PLASMA PROPERTIES (1) PLASMA DENSITY - NO. of free electron per unit volume. (2) PLASMA TEMPERATURE - Temperature controls the degree of plasma ionization.

(3) DEBYE LENGTH - The Debye length is the distance over which significant charge separation can occur. (4) PLASMA FREQUANCY - Plasma Frequency are rapid oscillations of the electron density in conducting media.

USE OF PLASMA IN MANUFACTURING

- Plasma Arc Welding HOT PLASMA �- Plasma Cutting �- Plasma Spraying�- Plasma Medicine COLD�- Food processing (non PLASMA thermal plasma, "cold plasma")- Plasma arc waste disposal (convert waste into reusable material with plasma).

Plasma Formation1.By electric field2.By shock front

Etching

• Electrons gain energy from RF (radio frequency) or µw (microwave)fields.

• Electrons impact with feed gas to generate ions, reactiveneutrals and more electrons.

• Ions and reactive neutrals diffuse and drift to wafersurface to remove the material.

Plasma Etch Reactors

1. Capacitive Coupled Plasma CCP Type

2. Inductive Coupled Plasma (ICP) Type

Capacitive Coupled Plasma (CCP)

• To start plasma formation initial voltage must exceed breakdownvoltage. It depends on gas pressure and spacing. Its value is nearly equalto 300 to 600 V

• RF current through gas maintains steady state discharge by heatingelectrons.

CCP Characteristics

•The plasma is generated by RF current flow between electrodes •Plasma density (ne) tends to increase linearly with RF power •RF current across a sheath generates a dc voltage • Ions gain energy from the dc sheath voltage •ne and Eion tend to be coupled and increase together

Inductive Coupled Plasma (ICP) Type

• Current in coil induces current loop in plasma in glass tube • In ICP, power is transferred to plasma by the oscillating B field.

Advantages of RF Plasma

•RF current through wafer causes no damage •Easy to get induced self or DC bias •No charging damage if plasma is uniform

Regions formed during plasma

1.Plasma Region • Ions are generated in this region. These are important for

etchingprocess. • Electrons carry the RF current in this region.

2.Sheath Region •Electron depletion region forms at all surfaces to keep

electrons inplasma region. • In this region, number of positive ions are much

higher than number ofnegative ions. •Due to high electric field in this region, only few

electrons will be ableto overcome this region • Ions gain energy and directionality •RF current is carried by displacement (capacitor)

current

Plasma Nozzle•Gas Swirling :

Swirling the gas assists cutting in several ways. Swirling increases cooling. The un-ionized gas atoms are heavier/cooler and are thrown to the outside of the spinning gas stream. This cool barrier provides protection for the copper nozzle. As amperage is increased, the amount of ionization increases (changing the 30/70% ratio) and cooling decreases, shortening the life of the nozzle. Nozzles are designed to operate within a specific current (amp) range.

REFERANCES

(1)Plasma fundamentals part: http://www.prism.gatech.edu/~gtg928x/NNIN_reading_material/Fundamentals%20Part%201.pdf

(2) Plasma discharge fundamentals http://people.physics.anu.edu.au/~jnh112/AIIM/c17/Plasma_discharge_fundamentals.pdf

(3) Plasma physics Wikipedia

THANK YOU