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Wet Etching and Cleaning: Surface Considerations and Process Issues

Wet Etching and Cleaning: Surface Considerations and Process Issues. Dr. Srini Raghavan Dept. of Chemical and Environmental Engineering University of Arizona  1999 Arizona Board of Regents for The University of Arizona. Outline. Etching and cleaning solutions/processes

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Wet Etching and Cleaning: Surface Considerations and Process Issues

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  1. Wet Etching and Cleaning: Surface Considerations and Process Issues Dr. Srini Raghavan Dept. of Chemical and Environmental Engineering University of Arizona  1999 Arizona Board of Regents for The University of Arizona

  2. Outline • Etching and cleaning solutions/processes • Particle adhesion theory • Surface charge and chemistry • Contamination

  3. Etching and Cleaning Solutions • HF Solutions • Dilute HF (DHF) solutions - prepared by diluting 49% HF with dionized water • Buffered HF solutions - prepared by mixing 49% HF and 40% NH4F in various proportions • example: Buffered Oxide Etch (BOE) - patented form of buffered HF solution • May contain surfactants for improving wettability of silicon and penetration of trenches containing hydrophobic base • nonionic or anionic • hydrocarbon or fluorocarbon

  4. Etch Rate of SiO2 Etch Rate (Å/min) at constant temp. Etch Rate (Å/min) Temperature More NH4F Less NH4F 100 0 Weight % HF NH4F/HF Ratios Etch rate of SiO2 increases with increasing weight % of HF in the etch solution, as well as higher ratios of NH4F buffer in BHF solutions. Etch rate also directly increases with increasing temperature.

  5. Etching and Cleaning Solutions (cont’d) • Piranha • H2SO4 (98%) and H2O2 (30%) in different ratios • Used for removing organic contaminants and stripping photoresists • Phosphoric acid (80%) • Silicon nitride etch • Nitric acid and HF • Silicon etch

  6. Etching and Cleaning Solutions (cont’d) • SC-2 (Standard Clean 2) • HCl (73%), H2O2 (30%), dionized water • Originally developed at a ratio of 1:1:5 • Used for removing metallic contaminants • Dilute chemistries (compositions with less HCl and H2O2) are being actively considered

  7. Alkaline Cleaning Solutions • SC-1 (Standard Clean 1) • NH4OH (28%), H2O2 (30%) and dionized water • Classic formulation is 1:1:5 • Typically used at 70 C • Dilute formulations are becoming more popular • Tetramethyl Ammonium Hydroxide (TMAH) • Example: Baker Clean • TMAH (<10%), nonionic surfactant (<2%), pH regulators for a range of 8-10, and chelating/complexing agents • Could possibly be used with H2O2 to replace SC1 and SC2 sequence

  8. Surfactants • Alkyl phenoxy polyethylene oxide alcohol • Nonionic compounds • Alkyl group: 8 - 9 carbons • 9 - 10 ethylene oxide groups • Examples: NCW 601A (Wako Chemicals), Triton X-100 (Union Carbide) • Alkyl phenoxy polyglycidols • Nonionic surfactants • Example: Olin Hunt Surfactant (OHSR) • Fluorinated alkyl sulfonates • Anionic surfactants • Typically 8 carbon chain • Example: Fluorad FC-93 (3M)

  9. Surfactants (cont’d) • Acetylenic alcohols • Unsaturated triple bond in the structure • Nonionic • Example: Surfynol 61 (APCI) • Betaines • Zwitterionic in nature • Used mostly in alkaline clean • Example: Cocoamidopropyl betaine

  10. RCA Cleaning • Two-step wet cleaning process involving SC-1 and SC-2: • 1) 1:1:5 NH4OH-H2O2-H2O at ~70 C • Oxidizing ammoniacal solution • Ammonia complexes many multivalent metal ions (e.g. CU++) • Treatment leaves a thin “chemical” oxide • Without H2O2, Si will suffer strong attach by NH4OH • 2) 1:1:5 HCl-H2O2-H2O at ~70 C • HCl removes alkali and transition metals (e.g. Fe)

  11. Problems with SC1 Clean • Some metals (e.g. Al) are insoluble in this oxidizing, highly basic solution and tend to precipitate on the surface of Si wafers • High Fe contamination of the wafer surface after a SC1 clean • Rough surface after cleaning • SC1 solutions with lower ammonia content (X:1:5, X<1) are being actively investigated

  12. Particle Removal During SC1 Clean • H2O2 promotes the formation of an oxide • NH4OH slowly etches the oxide • In a 1:1:5 SC1, the oxide etch rate is ~0.3 nm/min at 70 ºC. At the alkaline pH value of SC1 solution, most surfaces are negatively charged. Hence, electrostatic repulsion between the removed particle and the oxide surface will prevent particle redeposition.

  13. Particle Removal Efficiency vs. Immersion TimeSC1 solutions w/ varying NH4OH concentration 1.0 1:1:5 NH4OH:H2O2:H2O The efficiency curve is steeper with a higher concentration of NH4OH in the SC1 solution. Particle Removal Efficiency 0 Immersion Time

  14. Standard Clean for Silicon • Step 1 - Piranha/SPM • 4:1 H2SO4 (40%):H2O2 (30%) @ 90 C for 15 min • Removes organic contaminants • Step 2 - DI water rinse • Step 3 - DHF • HF (2%) for 30 sec • Step 4 - DI water rinse • Step 5 (SC-1/APM) • 1:1:5 NH4OH (29%):H2O2 (30%) H2O at 70 C for 10 min • removes particulate contaminants • desorbs trace metals (Au, Ag, Cu, Ni, etc.)

  15. Standard Clean for Silicon (cont’d) • Step 6 - DI water rinse • Step 7 - SC-2 • 1:1:5 HCl (30%):H2O2 (30%):H2O at 70 C for 10 min • dissolves alkali ions and hydroxides of Al3+, Fe3+, Mg3+ • desorbs by complexing residual metals • Step 8 - DI water rinse • Step 9 - Spin rinse dry

  16. Adhesion of Particles to Surfaces • Attractive Forces (AF) • van der Waals forces (short range) • Electrostatic (if the charge on the particles is opposite to the charge on the surface (typically longer range) • Repulsive Forces (RF) • Electrostatic (charge on the particle has the same sign as that on the surface) • Steric forces (due to absorbed polymer layers on the surface of the particles and wafer) (short range) • When AF > RF, particle deposition is favorable

  17. Particle Deposition Model • Parameters controlling deposition • zeta potential of wafers • size and zeta potential of particles • ionic strength and temperature of solution • Transport of particles towards the wafer requires diffusion through a surface boundary layer (particles move along the flow in the solution and deposit by diffusion). Substrate Along the flow Diffusion layer

  18. Surface Charge and Surface Electricity • Development of surface charge • Adsorption of H+ and OH- ions (oxides) • Selective adsorption of positive or negative ions (hydrophobic materials) • Ionization of surface groups (polymers such as nylon) • Fixed charges in the matrix structure exposed due to counter ion release • example: positively charged modified filters used in DI water purification

  19. Surface Charge Development on SiO2Immersed in Aqueous Solutions -O-Si... H+ Aqueous Solution Bulk SiO2 -Si-O... OH- -O-Si... -Si-O... Acidic Solutions (low pH) Basic Solutions (high pH) H+ OH- -O-Si-OH2+ -O-Si-O- Bulk Solid Bulk Solid -O-Si-OH2+ Solution -O-Si-O- Solution -O-Si-OH -O-Si-OH

  20. Point of Zero Charge (PZC) of Materials 20 • PZC = the solution pH value at which the surface bears no net charge; i.e. surf = 0 Material pHPZC SiO2 2-2.5 TiO2 5.5-6 Al2O3 ~9 Si ~4 Ny lon ~6 PZC surf (microcoulombs/cm2) 0 pH -20 • Development of + or - charge at a given pH depends on the nature of the metal-oxygen bond and the acid/base character of the surface MOH groups. Acidic oxides have a lower PZC than basic oxides.

  21. Surface Potential (o) and Zeta Potential () + - - + + - + - + - - - Solid + - Liquid o  • Zeta Potential ( ): • Potential in the double layer at a short distance (typically the diameter of a hydrated counter ion) from the solid surface • Experimentally measurable through electrokinetic techniques • Decreases (more negative) with increasing pH 0 • Surface Potential (o ): • Not experimentally measurable • Oxides immersed in aqueous soln’s, o = 0.059 (PZC-pH) volts

  22. Zeta Potential Electrophoretic Method E  •  = dielectric constant of liquid •  = viscosity of liquid • K = constant dependent on particle size >> 1/ or << 1/  • (1/  is the electrical double layer thickness) • Technique useful for particles suspended in aqueous or non-aqueous media

  23. Zeta Potential from Streaming Potential LIQUID IN P LIQUID OUT (+) and (-) charges V • Generation of an electrical potential due to the flow of liquid past a charged surface • Potential generated = streaming potential (Estr), which is related to zeta potential , , and k are viscosity, dielectric constant, and conductivity of solution; Es/P is the slope of the streaming potential vs. pressure drop.

  24. Streaming Potential CellSchematic Sketch - 6” wafers LIQ IN LIQ OUT Electrode Electrode Cell Block Channel LIQ IN LIQ OUT

  25. Zeta Potential vs. pHOxide Wafer - Activation Etch 0 (-) Zeta Potential, mV pH

  26. Contamination Mechanisms • Liquid film draining (liquid/air interface) • Bulk deposition from liquids • Contaminant pick-up from air A A (OR) Hydrophobic Hydrophilic L L

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