Clean Room & Wafer Cleaning
Clean Room & Wafer Cleaning ESS4810 Lecture Fall 2010 Clean Room • A specially constructed enclosed area that is environmentally controlled with respect to airborne particulates, temperature, air pressure, humidity, vibration, and lighting Clean Room Classification - U.S. Federal Standard 209b • Class 1: the particle count does not exceed 1 particle per cubic foot with particles of a size of > 0.5 μm • Class 100: the particle count does not exceed 100 particles per cubic foot with particles of a size of > 0.5 μm Wafer Cleaning Methods • • • • • • RCA1 and RCA2 Thermal treatment Plasma or glow discharge techniques Ultrasonic agitation Polishing with abrasive compounds Supercritical cleaning RCA Cleaning Procedure • RCA1: Add 1 part of NH3 (25% aqueous solution) to 5 parts of DI water; heat to boiling and add one part of H2O2. Immerse the wafer for ten minutes. This procedure removes organic dirt (resist). • RCA2: Add 1 part of HCl to 6 parts of DI water; heat to boiling and add 1 part H2O2. Immerse the wafer for ten minutes. This procedure removes metal ions. Piranha Clean • Add 1 part of H2O2 to 10 parts of H2SO4; heat to 120ºC. Immerse the wafer for ten minutes. This procedure removes organic residues and complex heavy metal ions. RCA Cleaning Procedure Supercritical Cleaning Thermal Oxidation ESS4810 Lecture Fall 2010 Thermal Oxidation • The formation of the oxide of silicon (SiO2) on a silicon surface is termed oxidation • Although there are several ways to produce SiO2 directly on the Si surface, it is most often accomplished by thermal oxidation, in which the silicon is exposed to an oxidizing ambient (O2, H2O) at elevated temperatures Wet and Dry Oxidation • Wet – H2O as oxidant • Dry – O2 as oxidant Properties • Excellent electrical insulator – Resistivity > 1x1020 ohm-cm – Energy gap ~ 9 eV • High breakdown electric field > 10 MV/cm • Stable and reproducible Si/SiO2 interface • Conformal oxide growth on exposed Si surface Properties • Good diffusion mask for common dopants DSiO << DSi 2 • Very good etching selectivity between Si and SiO2 RSiO << RSi or 2 RSiO >> RSi 2 SiO2 SiO2 Si Si or Si Oxidation Furnace Thickness • Oxidize 1 μm of Si will generate 2.17 μm of SiO2 Kinetics of SiO2 Growth Deal-Grove Model Growth Rate Derivation • Steady state condition: – F1=F2=F3 (2 equations) • 2 unknowns Co and Ci – CS and Co are related by Henry’s Law – CG is a controlled process variable Derivation • Henry’s law • Result Derivation • Define • F1 Derivation • F2=F3 • F1=F2 Derivation • Convert F into growth rate Derivation • Condition Xox • Solution Deal-Grove Model Growth rate slows down with increase thickness Parameters B and B/A Parameters B and B/A Parabolic constant B Linear constant B/A Oxidation Chart based on Xi=0 Thickness Estimation Xox Xi = 4000Å, wet oxidation, 1100ºC, 33 minutes • 4000Å oxide – 1000ºC, 1 hour – 1100ºC, 24 minutes – CVD oxide –… Thickness Estimation Xox Xi = 4000Å, wet oxidation, 1100ºC, 33 minutes • Method 1 – Find B and B/A from charts – Solve Thickness Estimation Xox Xi = 4000Å, wet oxidation, 1100ºC, 33 minutes • Method 2 – Use oxidation chart 24 + 33 = 57 Effect of Xi on Wafer Topography More oxide grown Less oxide grown High Pressure Oxidation CA CA PG PG • When PG increases, both B and B/A will increase. Therefore oxidation rate increases. • Either time or temperature can be reduced if the pressure is increased. Doping Concentration Effect • Highly doped Si has more vacancies • Higher growth rate Orientation Dependence • Difference more obvious for thin oxides • Most IC’s made with (100) Si Orientation Dependence • Density of Si atoms < 7 x 1014/cm2 • Density of Si atoms ~ 8 x 1014/cm2, more bonds are available for reaction Ks(111) > Ks(100) Oxidation with Cl-containing Gas • Introduction of halogen species during oxidation – reduction in metallic contamination – improved SiO2/Si interface properties Effect of HCl on Growth Rate Dependence of B/A and B on Processing Parameters Thickness Characterization • Compare the color of the wafer with the reference color chart • Ellipsometer • Surface Profiler Process Overview ESS4810 Lecture Fall 2010 Process Flow Wet Oxide Si Lab 1: 1. Wafer cleaning 2. Thermal Oxidation PR AZ 5214 (positive) Si Part A Dry etch Part B Wet etch Si Lab 2-1: 1. Lithography (PR AZ 5214, mask #1 for bulk etching window) Lab 2-2: -2: 1. Break wafer into A & B 2. B: BOE wet etching A: RIE dry etching 3. PR strip, wafer cleaning Process Flow Cr/Ni Ni Si Lab 3: 1. A: E-beam evaporation Cr/Ni 0.05/0.15 μm 2. B: TMAH bulk etching Si Lab 4-1: -1: 1. Lithography Ni, by wet etching (mask #2) AZ4620 Si Lab 4-2: 1. Lithography AZ4620 (mask #3) 2. Electro-Plating, Ni Lab 5: 1. PR strip 2. Oxide (sacrificial layer) etching surface micromachining Wafer Layout Surface micromachining Part A Bulk micromachining Part B 本週實驗 ‧本週實驗分兩組上課 –週二組下午六點半開始上課 –週四組下午六點半開始上課 ‧請於六點二十分到工科舊館506教室 或於六點三十分直接到工科新館七樓集合 實驗報告格式 ‧一、封面須註明學號、姓名、實驗名稱與日期。 ‧二、實驗項目與步驟 (以中文簡單描述) ‧三、實驗原理 ‧四、實驗結果(原始數據、照片、分析)與討論 ‧五、回答實驗相關問題 ‧六、結論與心得
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