1. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 2 Outline Etching and cleaning solutions/processes Particle adhesion theory Surface charge and chemistry Contamination

3. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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% NH 4 F 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 4 Etch Rate of SiO 2 Etch Rate (Å/min) at constant temp. Weight % HF0 100 Etch Rate (Å/min) NH 4 F/HF Ratios Temperature Etch rate of SiO 2 increases with increasing weight % of HF in the etch solution, as well as higher ratios of NH 4 F buffer in BHF solutions. Etch rate also directly increases with increasing temperature. More NH 4 FLess NH 4 F

5. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 13 Particle Removal Efficiency vs. Immersion Time SC1 solutions w/ varying NH 4 OH concentration Particle Removal Efficiency0 1.0 Immersion Time The efficiency curve is steeper with a higher concentration of NH 4 OH in the SC1 solution. 1:1:5 NH 4 OH:H 2 O 2 :H 2 O

14. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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). Along the flow Diffusion layer Substrate

18. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 19 Surface Charge Development on SiO 2 Immersed in Aqueous Solutions -O-Si... -Si-O... -O-Si... -O-Si-OH 2 + -O-Si-OH -O-Si-O - -O-Si-OH Bulk Solid Solution Bulk Solid Solution H+H+ OH - Bulk SiO 2 Aqueous Solution Acidic Solutions (low pH) Basic Solutions (high pH) H+H+ OH -

20. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 20 Point of Zero Charge (PZC) of Materials PZC = the solution pH value at which the surface bears no net charge; i.e.  surf = 0 0  surf (microcoulombs/cm 2 ) pH 20 -20 PZC MaterialpH PZC SiO22-2.5 TiO25.5-6 Al2O3~9 Si~4 Ny lon~6 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. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 21 Surface Potential (  o ) and Zeta Potential (  ) SolidLiquid + - - + + - + - - - + - 0 oo  Surface Potential (  o ): Not experimentally measurable Oxides immersed in aqueous soln’s,  o = 0.059 (PZC-pH) volts 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

22. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 22 Zeta Potential Electrophoretic Method  = 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 E 

23. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 23 Zeta Potential from Streaming Potential V (+) and (-) charges LIQUID IN  P LIQUID OUT Generation of an electrical potential due to the flow of liquid past a charged surface Potential generated = streaming potential (E str ), which is related to zeta potential , , and k are viscosity, dielectric constant, and conductivity of solution;  E s /  P is the slope of the streaming potential vs. pressure drop.

24. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 24 Streaming Potential Cell Schematic Sketch - 6” wafers Block Cell LIQ OUTLIQ IN Channel Electrode LIQ INLIQ OUT Electrode

25. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 25 Zeta Potential vs. pH Oxide Wafer - Activation Etch 0 (-) Zeta Potential, mV pH

26. NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Raghavan 26 Contamination Mechanisms Liquid film draining (liquid/air interface) Bulk deposition from liquids Contaminant pick-up from air A L Hydrophilic Hydrophobic A L (OR)