EE143 F2010 Lecture 2 Microstructure of Electronic Materials Amorphous materials Professor N Cheung U C Berkeley Single Crystal Material 1 EE143 F2010 Lecture 2 The Si Atom The Si Crystal diamond structure High performance semiconductor devices require defect free crystals Professor N Cheung U C Berkeley 2 EE143 F2010 Lecture 2 Crystallographic Planes Unit cell Si lattice constant 5 431 5 x 1022 atoms cm3 View in 100 direction Professor N Cheung U C Berkeley View in 110 direction View in 111 direction 3 EE143 F2010 Lecture 2 Crystallographic Notation Miller Indices Notation hkl Interpretation hkl equivalent planes hkl crystal direction hkl equivalent directions crystal plane h inverse x intercept k inverse y intercept l inverse z intercept Intercept values are in multiples of the lattice constant h k and l are reduced to 3 integers having the same ratio Professor N Cheung U C Berkeley 4 EE143 F2010 Lecture 2 Carrier Concentrations of Intrinsic undoped Si electron Bottom of conduction band Energy gap 1 12 eV hole Top of valence band n electron conc p hole conc ni Professor N Cheung U C Berkeley 5 EE143 F2010 Dopants in Si Lecture 2 By substituting a Si atom with a special impurity atom Column V or Column III element a conduction electron or hole is created Donors P As Sb Professor N Cheung U C Berkeley Acceptors B Al Ga In 6 EE143 F2010 Lecture 2 n type Semiconductor If ND NA such that ND NA 10 ni ND cm3 n type NA cm3 2 n ND N A and ni p ND N A Note n p Professor N Cheung U C Berkeley 7 EE143 F2010 Lecture 2 p type Semiconductor If NA ND such that NA ND 10 ni ND cm3 NA cm3 p type 2 p N A ND and ni n N A ND Note p n Professor N Cheung U C Berkeley 8 EE143 F2010 Adding parts billion to parts thousand of dopants to pure Si can change resistivity by 8 orders of magnitude 1 Note m 100 Resistivity Range of Materials Lecture 2 Si with dopants SiO2 Si3N4 cm Professor N Cheung U C Berkeley 9 EE143 F2010 Lecture 2 Principle of Monolithic Process Integration A sequence of Additive and Subtractive steps with lateral patterning Example CMOS Integrated Circuit Processing Steps Si wafer Professor N Cheung U C Berkeley EE143 F2010 Lecture 2 Czochralski Crystal Growth Crystal Pulling Shaping and Polishing Professor N Cheung U C Berkeley Crystal Ingots 300 mm wafer 11 EE143 F2010 Professor N Cheung U C Berkeley Lecture 2 12 EE143 F2010 Professor N Cheung U C Berkeley Lecture 2 13 EE143 F2010 Lecture 2 Purity of Starting IC Si Wafer 99 999999999 so so called eleven nines Maximum impurity of starting Si wafer is equivalent to 1 mg of sugar dissolved in an Olympic size swimming pool Professor N Cheung U C Berkeley EE143 F2010 Lecture 2 Solar Cell Grade Silicon For reference only Metallurgic Grade Silicon MG Si 90 99 US 1 2 5 kg Solar Grade Silicon SG Si 99 99 99 999 US 30 40 kg Electronic Grade Silicon EG Si 99 9999 over US 60 kg Professor N Cheung U C Berkeley EE143 F2010 Lecture 2 Photolithography glass plate chromium Processing Temperature Ambient Positive Resist Region exposed to light will be dissolved in development solution Professor N Cheung U C Berkeley 16 EE143 F2010 Example Deep UV Photolithography Sequence 1 Surface Prime 2 Coat 3 Prebake 4 Expose 5 Post Exposure bake 6 Develop 7 Hard Bake Professor N Cheung U C Berkeley Lecture 2 EE143 F2010 Lecture 2 Example Deep UV Photolithography continued Professor N Cheung U C Berkeley EE143 F2010 Example Deep UV Photolithography continued Professor N Cheung U C Berkeley Lecture 2 All baking sub steps are similar but with different temperature and time EE143 F2010 Etching Lecture 2 Pattern resist mask Etching thin film Etching completed Remove resist mask Anisotropic Processing Temperature e g Reactive Ion Etching RIE Ambient Professor N Cheung U C Berkeley Isotropic e g Wet Etching 20 EE143 F2010 Lecture 2 Etching Selectivity Example HF solution etches SiO2 but not Si HF SiO2 Si solution Si A high etching selectivity is usually desired Professor N Cheung U C Berkeley 21 EE143 F2010 Anisotropic Wet Etching of Si Crystals Lecture 2 Etchants KOH or EDP Ethylene Diamine Pyrocatechol Cross section Top view 100 Si substrate Etching stops Etching continues Professor N Cheung U C Berkeley Effect of different mask opening 22 Thermal Oxidation EE143 F2010 Lecture 2 Processing Temperature 900 1100 oC Si O2 SiO2 Si 2 H2O SiO2 2H2 Oxide Xox thickness t t O2 or H2O diffuses through SiO2 and reacts with Si at the interface to form more SiO2 1 m of SiO2 formed consumes 0 44 m of Si substrate Thin oxide growth e g gate oxide use O2 Dry oxidation Thick oxide growth e g field oxide use H2O Wet oxidation Oxidation time t Professor N Cheung U C Berkeley 23 EE143 F2010 Lecture 2 Uneven surface topography with window oxidation 1st oxidation SiO2 Si Si Realistic topography with 2 dimensional effect SiO2 Si Pattern oxide window by litho and etch 2nd oxidation SiO2 SiO2 Si Si Note uneven Si surface after window oxidation Professor N Cheung U C Berkeley 24 EE143 F2010 Lecture 2 Local Oxidation silicon nitride as oxidation mask O2 Si3N4 pad oxide 100 A Si Thermal Oxidization LOCOS Process nitride SiO2 Si Professor N Cheung U C Berkeley 25 EE143 F2010 Ion Implantation Lecture 2 typically used to introduce dopants into semiconductors Ion Energy 1 keV to 200 keV Processing Temperature Room temp during implantation After implantation a 900oC 1000oC anneal step is needed to 1 activate dopants 2 restore Si crystallinity Professor N Cheung U C Berkeley 26 Diffusion EE143 F2010 Lecture 2 To introduce dopants into semiconductors Predeposition To spread out the dopant profile Drive in Processing Temperature 850 1150 oC D D0 e Q kT D Diffusion Constant Q Activation Energy T Temp in K D as T Professor N Cheung U C Berkeley 27 EE143 F2010 Lecture 2 Predeposition Si surface concentration maintained at constant Cs solid solubility during predep Dose of dopant incorporation in cm2 C s 2 Dt Professor N Cheung U C Berkeley 28 EE143 F2010 Lecture 2 Predeposition and Drive in Drive in means removing dopant supply after Predep step and anneal at high temperature Half gaussian depth profile after long drive in Predep only Predep Drive in Dopant dose conserved during drive in Diffusion distance Dt Professor N Cheung U C Berkeley Concentration in cm3 versus Depth 29 EE143 F2010 Lecture 2 Physical Vapor Deposition 1 Evaporation Deposition Si Substrate Substrate at room temp Deposited Al film polycrystalline evaporation Al charge Tsource Tboiling of Al 700OC Professor N Cheung U C Berkeley 30 EE143
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