EE143 F2010 Lecture 9 1 Implant Profile depends only on incident ion momentum NOT on charge state A A A Same momentum Same implant profile Note Kinetic Energy momentum 2 2M 2 Charge carried by ions will be neutralized by charges in the substrate after implantation 3 n p Nd Na charges in semiconductors are caused by the chemistry of the implanted dopants and are NOT related to charges carried by the ions Professor N Cheung U C Berkeley 1 EE143 F2010 Lecture 9 Kinetic Energy of Multiply Charged Ions With Accelerating Voltage x kV Singly charged Doubly charged Triply charged B P As Kinetic Energy x keV B Kinetic Energy 2x keV B Kinetic Energy 3x keV Note Kinetic energy is expressed in eV An electronic charge q experiencing a voltage drop of 1 Volt will gain a kinetic energy of 1 eV Professor N Cheung U C Berkeley 2 EE143 F2010 Lecture 9 Molecular Ion Implantation Kinetic Energy x keV B F F BF2 accelerating voltage x kV B has 11 amu F has 19 amu Professor N Cheung U C Berkeley Molecular ion will dissociate immediately into atomic components after entering a solid All atomic components will have same velocity after dissociation Velocity Solid Surface vB vF vF 1 2 mB vB 2 1 2 K E of F m F v B 2 K E of B 11 20 11 19 19 K E of BF2 K E of B 3 EE143 F2010 Lecture 9 Proof Dissociated atomic components having same velocity F B after B Binding energy of molecule several eV is negligible compared with ion energy many keV F F F V V B F Same velocity V V B F move together as a molecule 1 1 1 1 2 2 2 mBvB 2 mFvF mBv B 2 mFv F2 2 2 2 2 mB 2mF vB mBv B 2mFv F 1 2 The only way to satisfy both 1 and 2 is v B v F vB vF Professor N Cheung U C Berkeley 4 EE143 F2010 Lecture 9 Molecular Implantation for Shallow Junctions For conventional beamline implanters Beam current as accelerator voltage B Beam current B as accelerating voltage 1kV BF2 Beam current BF2 can still be maintained high with 5kV accelerating voltage but the effective B implantation energy is only 1 keV Professor N Cheung U C Berkeley 5 EE143 F2010 Lecture 9 Implantation Damage Professor N Cheung U C Berkeley 6 EE143 F2010 Lecture 9 Amount and type of Crystalline Damage Professor N Cheung U C Berkeley 7 EE143 F2010 Lecture 9 Solid Epitaxial Growth through the Implant Damaged Region Professor N Cheung U C Berkeley 8 EE143 F2010 Lecture 9 Solid Epitaxial Growth through the Implant Damaged Region cont Professor N Cheung U C Berkeley 9 EE143 F2010 Lecture 9 Dopant Activation Versus Annealing Temp InterstitialVacancy recombination Secondary defects Annealed out Agglomeration of secondary defects Professor N Cheung U C Berkeley 10 EE143 F2010 Dopant Activation Lecture 9 Sheet Resistance is limited by dopant solid solubility Shallower junctions will have higher RS Professor N Cheung U C Berkeley 11 EE143 F2010 Lecture 9 Post Implantation Annealing Summary After implantation we need an annealing step A typical 900oC 30min will 1 Restore Si crystallinity 2 Place dopants into Si substitutional sites for electrical activation Professor N Cheung U C Berkeley 12 EE143 F2010 Lecture 9 Ion Channeling Si Crystal deeper penetration C x Random component channeled component random scattering path Random Planar Channeling x Axial Channeling To minimize channeling we tilt wafer by 7o with respect to ion beam Professor N Cheung U C Berkeley 13 EE143 F2010 Lecture 9 Chaneling with Lucky Ions Ion beam already tilted 7o away from normal of surface Some scattered ions can still fall into other crystallographic channeling directions Si substrate 7o C x No Channeling with channeling tail of lucky ions CB Professor N Cheung U C Berkeley xj xj x 14 EE143 F2010 Lecture 9 Prevention of Channeling by Pre amorphization Step 1 Si High dose Si implantation to covert surface layer into 2 1 E15 cm amorphous Si Step 2 Implantation of desired dopant into amorphous surface layer Si crystal Amorphous Si Si crystal B Disadvantage Needs an additional high dose implantation step Professor N Cheung U C Berkeley 15 EE143 F2010 Lecture 9 Transverse or Lateral Straggle Rt or R Rt Rp 1 Rt Rp Rt Professor N Cheung U C Berkeley 16 EE143 F2010 Lecture 9 Lateral Ion Scattering Causes Feature Enlargement y Mask x x Rp Lower concentration Higher concentration Implanted specie has lateral distribution larger than mask opening C y at x Rp y Professor N Cheung U C Berkeley 17 EE143 F2010 A 2 D formulation of Implantation Profile Lecture 9 mask y a y a x C x y K C x a a K C x erfc Professor N Cheung U C Berkeley y y 2 e 2 Rt y a 2 R t 2 dy y a erfc 2 R t 18 EE143 F2010 Lecture 9 A 2 D formulation of Implantaion Profile cont For a i e no mask C x y C x C x y C x K erfc erfc C x C x K 2 K 1 2 Professor N Cheung U C Berkeley 19 EE143 F2010 details of erfc function also covered in Diffusion section of EE143 Reader Professor N Cheung U C Berkeley Lecture 9 Semi log Plots of Gaussian and erfc functions Curves also given in Jaeger 20 EE143 F2010 Lecture 9 Transmission Factor of Implantation Mask Mask material e g photoresist C x What fraction of dose gets into Si substrate Si substrate x 0 C x x d Mask material with d x 0 Professor N Cheung U C Berkeley x d 21 EE143 F2010 Lecture 9 Transmitted Fraction T 0 C x dx d C 0 d Rp 1 erfc 2 2 Rp 2 x erfc x 1 0 e x dx Rp Rp y2 dy are values of for ions into the masking material Rule of thumb Good masking thickness d Professor N Cheung U C Berkeley Rp 4 3 Rp Cx d 10 C x Rp 4 22 EE143 F2010 Example Application SIMOX Separation by IMplantation of OXygen Lecture 9 SOI Wafer SIMOX depends on a peaked implant profile The sharp interfaces are formed during high temperature annealing 4000 of buried oxide requires a high oxygen dose of 2E18 cm 2 Slow process using beam line implanters Professor N Cheung U C Berkeley 23 EE143 F2010 Lecture 9 Example Applications Ion Cut Hydrogen Implantation 1 H Ion Implantation 6E16 dose H Hydrogen peak 2 Direct Wafer Bonding SiO2 Si donor Bonding interface 3 Donor wafer cleavage with heat treatment or mechanical cleavage Handle wafer Si donor Handle wafer Si donor Transferred Si overlayer Tong and Gosele Advanced Materials p 1409 1999 Professor N Cheung U C Berkeley Handle wafer Thermal or mechanical cleavage SOI Wafer 24 EE143 F2010 Lecture 9 Dose Energy Phase Space for Ion Implantation Applications Professor N Cheung U C Berkeley 25 EE143 F2010 Lecture 9 Summary of Implantation Module Ion Implanter Ion source mass selection accelerator …
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