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Ch7 Plasma

Introduction to Semiconductor Processing

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What is a Plasma?4th state matterIonized gasIons, electrons, and neutral species

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Plasma Characteristics

0.001%

(non equilibrium)

HDP 1%(100%)

Large number of species

Ar

O

O2 O2+

Ar+

e-O+ nn, Tn

ne, Teni, Ti

ni,~ nenn>>ni,ne

Te>>Ti, Tn

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- RF

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Photo Courtesy: UT Dallas

Wafer location:

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(Ionization)

e + A A+ + 2 e

Threshold energy ~ ionization energy

Electron-Impact Reactions -- Ionization

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7

:

e- + AB A + B + e-

CVD Not important for sputtering (Why?) Eg.: O2 + e

Cl2 + e SiH4 + e

Electron-Impact Reactions -- Dissociation

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- (Excitation Relaxation)

e + A A* + e

A* A + h (Photos)

endpoint

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Endpoint Detection

Forming Cl radicalsCl2 + e 2Cl + e

Al etching:Al + x Cl AlClx (x=1~3)

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CF4

e + CF4 CF3 + F + e

4F + SiO2 SiF4(g) + O2(g)

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PECVD SiH4 NO2 ()e + SiH4 SiH2 + 2H + e

e + N2O N2 + O + e

SiH2 + 3O SiO2 + H2O

PECVD

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Mean Free Path vs. Plasma Stability

?1. (fast electron

vs. slow molecules/atoms)2. (

),

3. (760 torr)(Electric field)(glow-to-arc transition)Calculation of mfp:

=

n: density, : cross section

Ground

d

V

V/d E E/N

d

Low P

High P

VGround

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vs.

1 eV = 11594 K (~2 eV)

v ~ 5.9 *107 cm/sec ~ 1.33 * 107 mph Ar+ (~0.05 eV)

v ~ 3.46 *104 cm/sec ~ 774 mph Room Temperature ( ~0.026 eV)

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:

F = qvB

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, E

f(E)

2 - 3 eV

(Boltzmann Distribution)

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Sheath Formation

*Figure Courtesy: Michael A. Lieberman, Principles of Plasma Discharges and Materials Processing

Why + potential??

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+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-++

++-+-+-+-++

++-+-+-+++

+++-+-+++

+++-++++

+++++++

Vp

Vf

x

(Ion Bombardment)

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Spatial and Temporal Potential Distribution

*Figure Courtesy: Michael A. Lieberman, Principles of Plasma Discharges and Materials Processing

Plasma always remains positive in the bulk. The time-averaged bulk potential depends on the driving electrode voltage amplitude. When the frequency is high, electrons see the change of the field; ions (heavy) only see the averaged field.

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CVD

Vp = 10 20 V

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V 2

A2

A1

V1/V2 =(A2/A1)

V1 = 200 to 1000 V

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V1

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Plasma

Shower head:

ESC ( )

BSC ( )

B-field

Key challenge:

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H2O, O2

OO H

H2O, CO2,

O O OH H

Remote-plasma key features:1.

2.

Major idea:1. Electron decays rapidly

upon formation2. No E-field to heat the

electrons at the downstream

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ICP (Inductively Coupled Plasma)

RF

ICP Key Feature: independent control of

_________________ and ___________________

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RF

ECR

ECR

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Advantages/Uniqueness of Plasma Processes

Existing of reactive radicals under low temperatureUnique reaction pathwayAdjustable ion bombardment energyGap filling capability (e.g. 50 nm trench)Low operating cost (gas vs. wet chemicals)System (chamber) cleaning (gas phase process).

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