Curley, Garrett A. (2008) The dynamics of the charged particles in a dual frequency capacitively coupled dielectric etch reactor. PhD thesis Physique des plasmas, Laboratoire de Physique et Technologie des Plasmas, EP/X p.193.

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Abstract

Dual frequency capacitively coupled plasmas are used for the etching of dielectric materials as part of the microelectronics fabrication process. The use of two frequencies is intended to allow for the independent control of the ion flux and the ion energy. Fluorocarbon gases play a key role in producing the precursor species that eventually etch the nano-scale patterns. These fluorocarbon-containing plasmas are complex in nature, forming many types of neutral radicals, positively charged ions and negatively charged ions. We have studied a customized industrial etch reactor, running in Ar/O2/C4F8 and Ar/O2/CF4 gas mixtures at pressures in the region of 50 mTorr (6.6 Pa) and driven by 2 and 27 MHz RF power. The measurement of negative ion densities and their effect on the plasma's electrical properties are the main focus of this thesis. Several diagnostic techniques have been implemented to characterise the densities and fluxes of the various charged species. An RF biased ion flux probe, installed in the upper electrode, is used to measure the ion flux. The electron density in the centre of the plasma was measured using a microwave resonator probe, known as a hairpin probe. Cavity ring-down spectroscopy (CRDS) is applied to the measurement of the negative fluorine ion density by the absorption in its broadband photodetachment continuum. The negative ion fraction was deduced from the probe measurements by comparing the ratio of the ion flux to the electron density to the theoretical ratios obtained by an electronegative plasma fluid model. It was found that the temperature of the negative ions must be very high (about 1 eV) if the experimental and theoretical results are to agree.

Table of content

1 Introduction 1

1.1 The context and objectives of this study - 1

1.2 Low-pressure plasmas for surface processing - 5

1.2.1 Basic plasma equations - 8

1.2.2 Case of a bounded plasma - 10

1.2.3 Edge-to-centre density ratio - 12

1.2.4 Radio frequency plasma sources - 13

1.3 Dual-frequency capacitively-coupled plasmas - 18

1.3.1 History - 20

1.3.2 Principle of operation of dual freqency capacitively coupled plasma

sources - 21

1.3.3 Triple and multiple frequency plasma excitation - 23

1.4 Dielectric etching - 24

1.4.1 Fluorocarbon containing plasmas - 24

1.5 Electronegative Plasmas - 28

1.5.1 Negative ions in fluorocarbon plasmas: production and loss - 29

1.5.2 Plasma theory in the presence of negative ions - 32

1.5.3 The role of negative ions in industrial plasmas - 39

1.5.4 Measurement of negative ion densities - 41

2 Experimental setup and diagnostic techniques 49

2.1 The dual-frequency plasma chamber - 49

2.1.1 The plasma chamber - 50

2.1.2 The vacuum/pumping system - 51

2.1.3 The rf power generators - 53

2.1.4 The electrostatic clamping (chuck) unit - 54

2.1.5 Cooling - 56

2.2 Electrical diagnostics - 56

2.2.1 The ion flux probe - 56

2.2.2 The microwave resonator probe - "Hairpin probe" - 67

2.2.3 Calculation of the hairpin sheath size - 77

2.2.4 The Floating Hairpin - 79

2.2.5 Design and construction of the hairpin probe - 80

2.2.6 Operation of the hairpin probe - 82

2.2.7 Comparison between two hairpin probe designs - 84

2.3 Cavity Ring-Down Spectroscopy - 87

2.3.1 Principle of absorption spectroscopy - 87

2.3.2 Absorption of the negative fluorine ion - 90

2.3.3 Cavity ring-down spectroscopy - 92

3 Results and analysis 101

3.1 Introduction - 102

3.2 Fluorine negative ion densities - 103

3.2.1 Ar/O2/c-C4F8 - 103

3.2.2 Ar/O2/CF4 - 105

3.3 Determining the negative ion fraction - 107

3.3.1 Negative ion fractions from CRDS and the hairpin probe - 108

3.3.2 Two-probe technique: simple model - 109

3.3.3 Two-probe technique: fluid model - 115

3.3.4 Comparison of the various estimations of the negative ion fractions 118

3.3.5 Introducing variable mobility to the fluid model - 120

3.4 Charged particle dynamics - Effect of gas composition - 126

3.4.1 Ar/O2/c-C4F8 – varying the c-C4F8 gas flow - 126

3.4.2 Ar/O2/c-C4F8 – varying the O2 and c-C4F8 gas flows - 130

3.4.3 Ar/c-C4F8 – varying the c-C4F8 gas flow - 133

3.4.4 Ar/O2/CF4 – varying the O2 and CF4 gas flows - 135

3.4.5 Ar/O2/CF4 – varying the O2 gas flow - 136

3.4.6 Ar/O2/SF6 – varying the O2 and SF6 gas flows - 138

3.4.7 CF4 plasma – effects due to SF6 addition - 139

3.5 Charged particle dynamics – Effect of pressure - 140

3.5.1 Ar/O2/c-C4F8 – varying the pressure: 25 – 100 mTorr - 141

3.5.2 Pressure effects on the ion flux probe I–V curves - 141

3.6 Charged particle dynamics – Effect of applied power - 143

3.6.1 Ar/O2/c-C4F8 : 160/8/16 sccm - 144

3.6.2 Ar/c-C4F8 : 160/24 sccm - 150

3.6.3 Ar/O2 : 160/24 sccm - 151

4 Conclusions and perspectives 155

4.1 Diagnostics in DF-CCP industrial chambers - 155

4.1.1 Ion flux probe - 155

4.1.2 Hairpin probe - 156

4.1.3 Cavity ring-down spectroscopy - 156

4.2 Validity of the two-probe technique for measuring ® - 156

4.3 The relationship between ne and ¡i - 157

4.4 The role of dual frequency RF power coupling - 158

4.5 Processing plasmas and the role of electronegativity - 158

4.6 Perspectives - 159

Appendix A - Hairpin Sheath Correction 161

Appendix B - Comparison of Stenzel’s hairpin with Kim’s hairpin 163

Appendix C - Normalisation of ion fluid equations - variable mobility 165

Bibliography 168

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