EE143 S06 Defects Tutorial Defects and Damage Point Defects Point Defects clusters Line Defects Extended Defects Ion Implantation Defects Amorphization Secondary Defects end of range loops Effect of defects on Electrical resistivity PN junction leakage current Diffusion Mechanical stress Professor N Cheung U C Berkeley 1 EE143 S06 Defects Tutorial Simple Point Defects Elemental crystal Professor N Cheung U C Berkeley 2 EE143 S06 Defects Tutorial More complicated Point Defects AB compound Professor N Cheung U C Berkeley 3 EE143 S06 Defects Tutorial For reference only Si Native Point Defects Si vacancy 1 Thermal equilibrium values of Si neutral interstitials and vacancies at diffusion temperatures doping concentration of interest 1015 1020 cm3 Si interstitial At 1000oC CIo 1012 cm3 CVo 1013 cm3 2 Diffusivity of Si interstitials and Si vacancies diffusivity of dopants Professor N Cheung U C Berkeley 4 EE143 S06 Defects Tutorial Neutral and Charged Point Defects Professor N Cheung U C Berkeley 5 EE143 S06 Defects Tutorial Line Defects dislocations A dislocation line can Create mechanical stress Getter Impurities Professor N Cheung U C Berkeley 6 EE143 S06 Defects Tutorial Movement of Dislocation can create slippage Professor N Cheung U C Berkeley 7 EE143 S06 Defects Tutorial Misfit Dislocation of Epitxial layers Professor N Cheung U C Berkeley 8 EE143 S06 Defects Tutorial Two Dimensional Defects Twins Boundary Grain Boundary Professor N Cheung U C Berkeley 9 EE143 S06 Polycrystalline Solids Defects Tutorial Amorphous Solids Silicate Glass Polymers Professor N Cheung U C Berkeley 10 EE143 S06 Defects Tutorial Implantation Damage Substrate Interstitials and Vacancies are created by momentum transfer collision process Professor N Cheung U C Berkeley 11 EE143 S06 Defects Tutorial End of Range EOR Dislocation Loops Recoiled Si More Vacancies SiV More Interstitials SiI annealing extra plane of Si atoms Extrinsic dislocation loops This is called secondary defect because it is caused by the primary point defects created by ion implantation Plummer et al Si VLSI Technology Professor N Cheung U C Berkeley 12 EE143 S06 Defects Tutorial Dopant Activation Boron implant 30min for each anneal Professor N Cheung U C Berkeley From 450 to 550C Si interstital compete with B for Si substuitional sites or Si interstials pairs with B to form inactive complex 13 EE143 S06 Defects Tutorial Example Poly Si Average grain size depends on deposition doping conc annealing conditions Professor N Cheung U C Berkeley 14 EE143 S06 Defects Tutorial Electrical Resistivity of Polycrystalline Materials For metals GB has negligible effect For doped semiconductors poly material has higher resistivity than single crystal material Trapped charges at GB create energy barriers for mobile carriers Professor N Cheung U C Berkeley 15 EE143 S06 Defects Tutorial Defects and PN Juncion Leakage current Native Si points defects or some special impurities trapped by point line extended defects can create additional electronic states inside the energy gap The inter gap states will increase reverse biased current of a pn junction Id Edefect Vd Excessive leakage current Professor N Cheung U C Berkeley 16 EE143 S06 Defects Tutorial How processing steps affect point defect concentrations Neutral interstitial and vacancy point defects present at thermal equilibrium At 1000oC CIo 1012 cm3 CVo 1013 cm3 Charged Point Defects enhanced by heavy doping total point defect concentrations enhanced by 10x I Io IV Vo V V Point defects Injected by interfaces during oxidation total point concentrations enhanced by 10x Implantation collisions total point defect concentration enhanced by 1000X Professor N Cheung U C Berkeley 17 EE143 S06 Defects Tutorial Diffusion Mechanisms in Si A No Si Native Point Defect Required Example Cu Fe Li H a Interstitial Diffusion Fast Diffusion Cu 10 6 cm2 sec Professor N Cheung U C Berkeley Au 18 EE143 S06 Defects Tutorial Diffusion Mechanisms in Si B Si Native Point Defects Required Si vacancy and Si interstitials Example Dopants in Si e g B P As Sb a Substitutional Diffusion Professor N Cheung U C Berkeley b Interstitialcy Diffusion 19 EE143 S06 Defects Tutorial Diffusivity Comparison Dopants Si interstitial and interstitial diffusers For reference only 108 times higher Professor N Cheung U C Berkeley 20 EE143 S06 Defects Tutorial Diffusivity along defect paths Surface diffusion GB diffusion Bulk diffusion Professor N Cheung U C Berkeley 21 EE143 S06 Defects Tutorial Defects and Thin film Stress Growth morphology Lattice misfit Phase transformation Defects can create intrinsic thin film stress which is of big concern for MEMS fabrication and small feature IC devices Professor N Cheung U C Berkeley 22 EE143 S06 Defects Tutorial Relevance of Defects to Microfabrication Deposited thin films are typically polycrystalline or amorphous One can obtain single crystal film only with special Epitaxial Growth conditions monocrystalline substrate ultras clean surface and high deposition temperature Monocrystalline semiconductor is need for active regions of highperformance devices such as integrated circuits If Polycrystalline or amorphous semiconductors is used performance will be compromised e g Poly Si thin film transistors Heavily doped Poly Si can be used as a metallic conductor e g the gate material of a MOSFET which is not part of the active device region Point defect concentration and their distribution controls the diffusivity of dopants Defects and the impurities they trap will give excess leakage current in active device regions e g pn junctions Defects alter the mechanical properties of thin films Professor N Cheung U C Berkeley 23
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