PLASMA RIE ETCHING FUNDAMENTALS AND ...

1 PLASMA RIE Birck ETCHING . Nanotechnology Center FUNDAMENTALS AND APPLICATIONS. 1. O tli Outline 1 Introductory Concepts 1. 2. PLASMA FUNDAMENTALS 3. The Physics and Chemistry of Plasmas 4 Anisotropy 4. A i t Mechanisms M h i g of Si and its Compounds 5. The ETCHING p 6. The ETCHING of Other Materials 2. DEFINITIONS. Electron ((e-). Positive ion (Ar+, Cl+, SiF4+, CF3+). Positive ion mass in RIEs >>mass of electron Radical (F, Cl, O, CF3). Uncharged atoms with unsatisfied chemical bonding 3. DEFINITIONS (continued). ( ti d). Mean free ppath average g distance a pparticle travels before collisions 5.)

2 ((cm cm) ((Dependent p on the species). p ). P (mT ). Pressure 1atmosphere= 760 Torr = 1*105 Pascals Pumping speed (S) [liters/sec]. Gas flow rate (Q) [Torr-liters/sec] or [sccm]. 4. Pl PLASMA Vacuum V S. System t Gas lines Mixed gas [Q]. valve MFC box APC & gate Chamber valve Vacuum pump [S]. VRF. Matching network He backside cooling ( ) (Q - S P(t)). dP(t) ( )). = V= chamber volume dt V. In Steady state : Q = S P. 5. M h i l Pumps Mechanical P. Wet and Dry Pumps Pumping speed: 20-500 m3/h Ultimate pressure: 1-10 mTorr [BOC Edwards Dry Pump]. [Kurt J. Lesker].

3 6. T b Pumps Turbo P. Rotation speed= 20000-90000. 20000 90000 rpm Pumping speed: 50-3000 l/s Ultimate pressure: 10-5-10-8 Torr [Wiki]. [BOC Edwards Turbo Pump]. [TP controller]. 7. M Flow Mass Fl Controller C t ll (MFC). Thermal-based flow meter Heater Q [sccm]. T1 T2. T2 T1= Cp Q. Cp is specific heat. [HORIBASTEC]. 8. MFC or G. Gas B. Box Mixed gas line Gas G. lines Panasonic MFC Box 9. A t Automatic ti Pressure P Controller C t ll (APC). & Gate Valve Pendulum valve Butterfly valve Q = S P. 10 [VAT]. Cl Clamp or Electrostatic El t t ti Chuck Ch k Clamp Electrostatic Chuck (ESC).

4 Si wafer Dielectric ------- +++++. +++++ ------- Base Plate +V + - -V. He 11. RF G. Generator t & Matching M t hi Network N t k Matching Chamber ZL. RF Generator PF PL. Network ZIn PR. ZS= 50 . PL=PF- PR. In general: ZL ZS. Purpose of Matching Network: Zin= ZS to maximize power delivery from source. ZS L C1. Manual or Automatic ZL. Matching Network C2. [Gambetti]. ZIn 12. O tli Outline 1 Introductory Concepts 1. 2. PLASMA FUNDAMENTALS 3. The Physics and Chemistry of Plasmas 4 Anisotropy 4. Anisotrop Mechanisms 5. The ETCHING of Si and its Compounds 6. The ETCHING of Other Materials 13 [ ].

5 Wh t is What i PLASMA ? Pl ? PLASMA is the fourth state of matter. It is an ionized ggas, a ggas into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons to coexist. electrons, coexist [Plasmas org]. [ ]. 14. H tto Make How M k Pl PLASMA ?? Capacitive p RIE PLASMA - Low density PLASMA ne 109 [electron/cm3]. Ionization efficiency 10-7. Inductive RIE Magnetic field - High density PLASMA ne 1013 [electron/cm3] Wafer Ionization efficiency 10-3. [Oxford Instruments]. 15. DC Glow Discharge Only used for sputtering system not for ETCHING .

6 ETCHING Vc=0 Vc= -100. n Red: ni X. Black: ne V( ). V(x) V(x). Vp Vp X X. -100v Vp= a few volts 16. RF Glow Discharge Used for any materials (insulating and conductive) . VRF(t). VRF ZG. Vp (t) = VRF (t). ZG + ZE. Vp(t) AE AG ZG ZE. Vp (t) VRF (t). VRF(t). ZE ZG. V(x). VRF. X. 17. RF Glow Discharge ZG. Actual RIE Vp (t) = VRF (t). ZG + ZE. AA E AG Z. <<A Z ZE. G >>Z. E G E G. VRF(t) VpV(t) VRF0(t). p(t). Ion transit time (Tion) is the time it takes the ion to traverse the sheath. q << Tion ! 1/Freq Freq=. q MHz 18. P h ' L. Paschen's Law Describes how the breakdown voltage depends on electrode separation and the pressure based on ideal gas law.

7 A (p ( )) V. V=. l n( ) + b 800. 600. V: Voltage p: Pressure d: gap distance 400. a & b: constants 10-1 100 101 Pd [Torr cm]. 19. I d ti Coupled Inductive C l d PLASMA Pl RIE. STS ASE and AOE systems 20. Wh High Densit Why Density Plasmas? Lower ion bombardment energies improve selectivity and reduce ion- bombardment-induced physical damage of the wafer surface. Lower ion energies, g , however,, result in the lower etch rates and reduced anisotropy! However However, the etch rate can be increased by using much higher ion fluxes due to high density plasmas.

8 The anisotropy can also be restored by operating at low pressure. pressure 21. O tli Outline 1 Introductory Concepts 1. 2. PLASMA FUNDAMENTALS 3. The Physics and Chemistry of Plasmas 4 Anisotropy 4. Anisotrop Mechanisms 5. The ETCHING of Si and its Compounds 6. The ETCHING of Other Materials 22. Electron Molec le Collisions Electron-Molecule An energetic electron colliding with a neutral etch gas molecule can create any of the following processes: 9 Dissociation AB + e- A+B + e- CF4 + e- CF3+F+e- 9 Ionization AB + e- AB+ + 2e- Ar + e- Ar+ +2e- Often dissociation and ionization Occur in one collision: CF4 + e- CF3++ F+ 2e- 23.

9 R di l andd Ions Radicals I in i Plasmas Pl Positive ions are veryy important p for ETCHING g pprocesses. Radicals are more numerous than ions in gas glow discharges because: 1. The electron energy required in order to break chemical bonds in the molecules is usually less than the energy required to ionize these molecules. 2. Radicals have a longer lifetime in the PLASMA compared to ions because an ion is almost always neutralized during a collision with a surface while radicals often do not react with a surface and are reflected back into the PLASMA . 24.

10 Wh t iis Pl What PLASMA ETCHING ? Et hi ? CF4. CF4 + e- CF3++ F+ 2e- valve Chamber Si + 4F SiF4 (gas). Gate valve Vacuum pump +, CF3 F. SiF4. Si Wafers 1- Need an ETCHING gas 2- Establish a glow discharge 3- Choose chemistry so that the reactive species react with the substrate to form a volatile by-product 4- Pump away the volatile by-product 25. Wh Pl Why PLASMA Et ETCHING ? hi ? Clean process Compatible with automation Anisotropic ETCHING P i pattern Precise tt transfer t f especially i ll for f Nano-scale N l features f t Mask Substrate Isotropic etch Directional etch Vertical etch 26.