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. ((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].))
2 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]. 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).
3 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 [ ]. 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].
4 [ ]. 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 . 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=.
5 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. 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. The anisotropy can also be restored by operating at low pressure. pressure 21. O tli Outline 1 Introductory Concepts 1. 2. PLASMA FUNDAMENTALS 3.
6 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. 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 .
7 24. 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. Gas Solid Systems Gas-Solid Solid Etch Gas Etch Product Silicon CF4,Cl2, SF6 SiF4, SiCl4, SiCl2. SiO2, SiNx CF4, C4F8, SiF4, CO, O2, N2, CHF3, SF6 FCN. Al BCl3/Cl2 Al2Cl6, AlCl3. Ti, TiN Cl2, CF4 TiCl4, TiF4. Organic Solids O2, O2/CF4 CO, CO2, GaAs & III-V Cl2/Ar, BCl3 Ga2Cl6, AsCl3.
8 Cr Cl2/O2 CrO2Cl2. Difficult materials to etch: Fe, Ni, Co, Au, Ag, Pt halides not volatile Cu Cu3Cl3 is volatile above 200C. 27. Halogen Si Sizee Effect [Handbook of Advanced PLASMA Processing Techniques by Pearton]. 28. O tli Outline 1 Introductory Concepts 1. 2. PLASMA FUNDAMENTALS 3. The Physics and Chemistry of Plasmas 4 Anisotropy Mechanisms 4. 5. The ETCHING of Si and its Compounds 6. The ETCHING of Other Materials 29. D fi iti Definition Etch rate Mask G ld Gold Mask (Photoresist, Metal, SiO2, ). Substrate Selectivity L. Anisotropy degree H. L. Af 1 . H. 30. Reactive ETCHING R. Reactive ti ETCHING t hi is i process! i an isotropic i t Has very high selectivity! Si + 4F SiF4 (gas). Mask SiF4. F. Substrate Isotropic etch 31. Ion ETCHING Ion ETCHING or mechanical ETCHING is an anisotropic process! Has lower selectivity y and etch rate!
9 Si + Ar+. S Si + Ar+. S. Si Mask Ar+. Substrate Directional etch Vertical etch 32. Reactive ion ETCHING Reactive ion ETCHING is an anisotropic process! Has better selectivity and much higher etch rate! Effect of Ions: Breaks bonds, raises temperature locally on the surface and provides activation energy [J. Appl. Phys. 50, 3189 (1979)]. 33. Side all Passivation Sidewall Passi ation Deposition of carbon polymer material on the sidewalls where: (a) Either the carbon is provided by the feed gas through the chamber such as CHF3, C4F8. (b) Or the carbon is provided by the erosion of the photoresist etch mask. 34. Side all Passivation Sidewall Passi ation Oxidation of the sidewall by adding O2 gas. 35. B hP. Bosch Process Switching SF6 and C4F8 500um Silicon Etched by T. Maleki using STS ASE. (8um/min etch rate). Th sidewall The id ll fil film thickness hi k d depends d to the h ddeposition i i or passivation i i time.
10 I 36. T. Temperature t effects ff t ini PLASMA l ETCHING t hi Wafer surface temp. depends on: - Chuck temperature - Ion's I ' energy anddd density it Reaction probabilityy of radicals depends on substrate temperature. Helium backside cooling helps anisotropic etch by preventing or reducing reaction of F and Cl species with sidewalls. 37. Notch Effect Notching effect due to charging oxide by ions Can be reduced by using low frequency (LF) 380kHz bias generator in pulsed mode e- +. +++++++. RF bias RF bias on off [J. Vac. Sci. Technol. B , Sep/Oct 2001]. STS ASE has LF pulsed generator!! 38. Other Effects Grass or micromasking g issue mainly y because of metal mask sputtered on the wafer Aspectt Ratio A R ti Dependent D d t Eff Effectt (ARDE). Typically large open areas etch faster than smaller features! 39. Other Effects Microloading effect: Feature of the same size etch more slowly in dense patterns Than in wide open areas!