1. 24/10/02YACOV SHNEIDER WAFER CLEANING PROCESSING Yacov Shneider Processing Engineer Microelectronics Center Dept. of Electrical Eng. TECHNION

2. 24/10/02YACOV SHNEIDER Wafer 300 mm - Infineon

3. 24/10/02YACOV SHNEIDER 300 mm wafer - Infineon

4. 24/10/02YACOV SHNEIDER IBM 300 mm FAB

5. 24/10/02YACOV SHNEIDER IBM 300 mm FAB wet clean

6. 24/10/02YACOV SHNEIDER Copper integration clean room

7. 24/10/02YACOV SHNEIDER IBM 300 mm FAB wafer check

8. 24/10/02YACOV SHNEIDER IBM 300 mm FAB mask clean

9. 24/10/02YACOV SHNEIDER IBM 300 mm FAB engineers

10. 24/10/02YACOV SHNEIDER Crystal silicon Ingots

11. 24/10/02YACOV SHNEIDER VIAs before cleaning, VERTEQ

12. 24/10/02YACOV SHNEIDER VIAs after cleaning, VERTEQ

13. 24/10/02YACOV SHNEIDER CMP contaminations

14. 24/10/02YACOV SHNEIDER CMP contaminations

15. 24/10/02YACOV SHNEIDER Six layer metallization design

16. 24/10/02YACOV SHNEIDER CLEANING CONCEPTION Define the nature of the contamination. Analytical procedures (methods) for detecting contamination. Cleaning procedures (techniques) to remove contaminations. TERMINOLOGY CLEANLINESS – The ability to control the surface atomic homogeneity. The surface is naturally covered by layers of elements (O2) or compounds (H2O). REQUIREMENTS for “clean” surface Uniform on a molecular or atomic level. Volatile. Non-interfering with diffusion or oxidation processes. Unable to affect the electrical properties of a finished device.

17. 24/10/02YACOV SHNEIDER CONTAMINATION KINDS PARTICULATES – CHUNKS OF GRANULAR MATTER Diffusion locally providing unde- sirable masking effects. Unwanted impurities, defects. Scratches in photomasks during contact printing. FILMS – ATOMIC, IONIC or POLYMERIC Gives uniform undesirable effects In oxidation and evaporation processes.

18. 24/10/02YACOV SHNEIDER ANALYTICAL METHODS FOR DETECTING CONTAMINATIONS PARTICULATES OPTICAL MICROSCOPY Brightfield / darkfield; Nomarsky interference contrast; Phase contrast. SEM; LASER SURFSCAN; COLLIMATED light BEAM; AUGER chemical analysis; TEM; SIMS. FILMS ELLIPSOMETRY; AUGER analysis; TEM; SIMS; FLUORESCENCE microscopy; TECHOLOGICAL TESTS Water break test (hydrophobic); Breath tests (cold chuck); C-V technique for MOS; Lifetime measurements.

19. 24/10/02YACOV SHNEIDER CONTAMINATION NATURE PARTICULATES - CHUNKS OF GRANULAR MATTER DUST from abrasion grinding and handling. INORGANIC “GRIT”-abrasive particles, sand, clay (from air- borne or chemicals). LINT from clothing, skin, hair -organic in nature, bacteria and etc. FILMS - ATOMIC, IONIC OR POLYMERIC ORGANIC INORGANIC Resist residues Metal layers left by evapora- Ions from resist tion of solvents and reagents Oil from water Residues from and handling. reagents, and handling

20. 24/10/02YACOV SHNEIDER Introduction The device performance, reliability, and product yield of Integrated Circuit are critically affected by the presence of chemical contaminants and particulate. ULSI technology needs stringent and reliable means: To control the surface smoothness and; To remove metallic and organic residues. ITRS 2003 requirements: Metallic impurities less 10 9 atoms/cm 2, particles less 0.1/cm 2, (for particle size greater 0.1mkm). Process Standard and modified RCA Wet Cleaning. Dry Cleaning that meets cluster-processing system.

21. 24/10/02YACOV SHNEIDER CLEAN wafer: free from particles, organic contamination, metal ions contamination, low surface micro roughness and native oxide films. Basic Concepts of Cleaning RCA Cleaning by Kern and Puotinen in 1970. SC-1 : NH 4 OH:H 2 O 2 :H 2 O = 1:1:5 to 1:2:7 at 70-90 o C. Remove organic contamination and particles by oxidation. SC-2 : HCL:H 2 O 2 :H 2 O =1:1:6 to 1:2:8 at 70-90 o C. Remove metal contamination by forming a soluble complex. Particles : General Guideline (smaller than 1/10 of feature size). Particle adhesion occurs during the process from the equipment, ambient, gas, chemicals and DI water.

22. 24/10/02YACOV SHNEIDER Metallic impurity Concentration (1X10 10 atoms/cm 2 ) Calcium <5.0 Cobalt <0.3 Copper <0.4 Chromium <0.5 Iron 0.8 Manganese <1.0 Nickel <0.3 Titanium <0.9 Zinc <0.3 Typical concentrations of trace metallic impurities on wafer surfaces

23. 24/10/02YACOV SHNEIDER Sulfuric Acid has the highest number of particles and HF the lowest. Adhesion of Particles: 1. Van der Waals Forces. 2. Forces due to the formation of an electrical double layer. 3. Forces due to capillary action around particle. 4. Chemical bond between the particle and the surface. Particle removal mechanisms 1. Dissolution. 2. Oxidizing degradation and dissolution. 3. Lift-off by slight etching of the wafer surface. 4. Electric repulsion between particles. H 2 O 2 can oxidize the silicon surface and OH - group (from NH 4 OH) provide negative charge on silicon. The deposition of particles is a strong function of pH values of the solution. With increasing pH value above 10 results in low particle deposition (SC-1 have the highest removal efficiency).

24. 24/10/02YACOV SHNEIDER Megasonic cleaning removes organic and inorganic particles from the surface at a temperature of less than 40 o C. The high power and high frequency sonic pressure waves wet the particles first and the solvent diffuses into the interface, and then, the particle is removed from the surface. Metal Ions Contamination Source : Chemical solutions, Ion implantations, Plasma processes. Effect on Device : 1. Structural defects at the interface, after high temperature anneal. 2. Stacking faults during later Oxidation or Epi-process. 3. Increased leakage current of P-N junctions. 4. Reduced minority carrier lifetime.

25. 24/10/02YACOV SHNEIDER

26. 24/10/02YACOV SHNEIDER TWO mechanism for precipitation of metal impurities on Si wafer. 1. Direct bonding to the Si by charge exchange between a metallic ion and hydrogen atoms that terminated on the Si substrate: For examples Noble metal such as Gold which has higher electronegativity. 2. Metals such as Al, Cr, and Fe tend to oxidize when the silicon surface is oxidized and are included in the oxide films. These oxide can be removed by HF etching. Method for removing Metal Contamination Currently the wet cleaning process is the most effective method for removing metallic contamination : HF: 0.5%, H 2 O 2 : 10% clean. Both SC-1, SC-2 have the capability of removing metallic impurity on Si wafer due to high oxidizing mechanism of H 2 O 2. Ca contamination causes rough surface and defect density in oxide: The threshold value for Ca contamination is 10 9 atoms/cm2.

27. 24/10/02YACOV SHNEIDER Fe contamination: for thin oxide (<100 Ǻ) threshold concentration is as low as 1x 10 9 atoms/cm 3. Method for removing Organic Contamination Source: organic vapor in the ambient, storage container, residue of photo-resist (main source). Take Effect to the yield: Incomplete cleaning of the surface, leaving contaminants such as the native oxide or metal impurities which cause micro-masking effect in the Plasma process. Current stripping technology: Photoresist removal by Plasma and Wet cleaning (H 2 SO 4 :H 2 O 2 = 3:1 to 4:1 at 120-130 o C). Depletion of H 2 O due to high wafer temperature (120 o C) cause unstable process control > Alternative: add ozone in water which can be used as strong oxidizing agent that decomposes organic impurities. The oxide thickness increases as the immersion time increases and with the concentration of ozone.

28. 24/10/02YACOV SHNEIDER Based on XPS analysis, the addition of ozone can reduce residual oxide after HF dip. Ozone – unstable, toxic gas (Threshold LV 100 ppb). Surface Microroughness problem ULSI device need <40 Ǻ oxide: surface should be atomic flatness. RCA SC-1 solution cause microroughness: NH 4 OH acts as an etchant of the oxide while H 2 O 2 acts as the oxidant. Etching and Oxidation simultaneously in SC-1 solution. Methods to reduce surface microroughness 1. Reduce the proportion of NH 4 OH. 2. Reduce the temperature of the bath. 3. Reduce cleaning time.

29. 24/10/02YACOV SHNEIDER Method for removing Native Oxide Uncontrollable thin oxide growth, high contact resistance, and inhibition of selective chemical vapor deposition or epitaxy. If this native oxide is not removed, it serves as the source of metallic impurities which diffuse into the silicon or precipitates at the interface of SiO 2 -Si, resulting in defects. Native oxide free surface is a key factor in obtaining high performance and reliability (after HF-dip: H-passivated surface). Wet Cleaning Technology Standard Method 1. H 2 SO 4 :H 2 O 2 (2:1 to 4:1 at 120-130 o C) : Remove greasy impurities which may be from the cassette or residues from the photoresist. 2. SC-1 : NH 4 OH:H 2 O 2 :H 2 O = 1:1:5, 70-80 o C for 10 min : Remove organic films, desorption of trace metals.

30. 24/10/02YACOV SHNEIDER 3. 1%HF-H 2 O, for 10-20 sec: remove oxide and trace metals in oxide. 4. SC-2 : HCl:H 2 O 2 :H 2 :O = 1:1:6, 70-80 o C for 10 min: dissolve alkali ions and hydroxides of Al +3, Fe +3, Mg +2. 5. DI water rinse (resistivity >17 Mohm-cm). Equipment 1. Immersion technique: Quartz bath to prevent leaching of Al, B, and alkalis which can results if Pyrex glass is used. 2. Megasonic cleaning: SC-1 solution at 35-42 o C. 3. Megasonic cleaning: SC-1, SC-2, DI-water are fed directly onto the spinning wafer (Verteq, Semitool, SEZ). Advanced Wet Cleaning from Ohmi (Tohoku University) 1. H 2 O + O 3 Organic Contamination 2. NH 4 OH:H 2 O 2 :H 2 O = 0.05:1:5 Particle, Organic and Metallic impurity. Effects on micro roughness.

31. 24/10/02YACOV SHNEIDER HF(0.5%) + H 2 O 2 (10%) Native oxide, metallic impurity Ultra pure water Rinsing Omit RCA SC-2 if high purity HF solution is used. Dry Cleaning Technology (25 years efforts) Problems of wet cleaning: particle generation, drying difficulty, cost, chemical waste, incompatibility with advanced cluster process, inflexibility. The dry cleaning process can solve problems: so far not completely successful. The dry cleaning process: require excitation energy to enhance gas- phase chemical reaction at low temperature such as plasma, particle beam, radiation, thermal heating. Additional energy enhance reaction but we have minimize damage on the wafer.

32. 24/10/02YACOV SHNEIDER UV-Ozone Clean Effective way to remove hydrocarbon, although the surface is oxide passivated. Surface excitation process Absorbed impurity + hν (2000-3000 Ǻ UV) > Excited impurity Gas-phase excitation process O 2 + hν (1849 Ǻ UV) > 2O O + O 2 > O 3 O 3 + hν (2537 Ǻ UV) > O + O 2 Excited impurity + (O + O 3 ) > Volatile compound The subsequent HF/H 2 O vapor or Ar/H 2 plasma cleaning can remove surface oxide. HF/H 2 O vapor clean HF dip promotes a hydrogen-passivated surface HF/H 2 O vapor clean induces a fluorine-terminated surface

33. 24/10/02YACOV SHNEIDER Hydrophobic surface (high contact angle) vs. Hydrophilic surface (low contact angle). With oxide layer on silicon: hydrophilic surface. Ar/H 2 Plasma Cleaning The gas molecules are excited or ionized by remote plasma to reduce bombardment damage on silicon wafer. Excited Ar ions physically sputter the surface impurity away. Excited Hydrogen ions chemically etch the surface. By proper adjustment of two process, an optimum cleaning can be obtained. Thermal Cleaning 1. The native oxide can be removed by heating the wafer to 800 o C or above in UHV (10 -10 Torr) to vaporize the oxide.

34. 24/10/02YACOV SHNEIDER 2. High temperature cleaning should be carefully examined due to etching of Si surface: - Si + SiO 2 = 2SiO at high temperature (>800 o C) and low O 2 partial pressure; - SiO is volatile at temperatures above 750 o C and oxide film is removed; - 2Si + O 2 = 2SiO Etching of the surface (micro roughness); At low temperatures and high oxygen partial pressure: smooth surface with thin oxide; At high temperatures and low oxygen partial pressure: clean surface with roughness.