EE143 F05 Lecture 8 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 F05 Lecture 8 Kinetic Energy of Multiple Charged Ions accelerating voltage x kV Singly charged Doubly charged Triply charged Professor N Cheung U C Berkeley B P As Kinetic Energy x keV B Kinetic Energy 2x keV B Kinetic Energy 3x keV 2 EE143 F05 Lecture 8 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 v B 2 1 2 K E of F mF vB 2 K E of B 11 20 11 19 19 K E of BF2 K E of B 3 EE143 F05 Lecture 8 All Atomic Components have same Velocity F B after B Binding energy of molecule several eV is negligible compared with implantation 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 F05 Lecture 8 Molecular Implantation for Shallow Junctions For conventional beamline implanters Beam current I as accelerator voltage B I B as voltage 5kV BF2 I BF2 can still be high with 25kV accelerating voltage but the effective B implantation energy is 5 keV For ultra shallow junction which needs 1keV B energy B10H14 at 10keV is proposed Professor N Cheung U C Berkeley 5 EE143 F05 Lecture 8 Implantation Damage After implantation we need an annealing step A typical 900oC 30min will 1 Restore Si crystallinity 2 Put dopants into Si substitutional sites for electrical activation Professor N Cheung U C Berkeley 6 EE143 F05 Lecture 8 Implantation Damage Professor N Cheung U C Berkeley 7 EE143 F05 Lecture 8 Amount and type of Crystalline Damage Professor N Cheung U C Berkeley 8 EE143 F05 Lecture 8 Solid Epitaxial Growth through the Implant Damaged Region Professor N Cheung U C Berkeley 9 EE143 F05 Lecture 8 Solid Epitaxial Growth through the Implant Damaged Region cont Professor N Cheung U C Berkeley 10 EE143 F05 Lecture 8 Dopant Activation Versus Annealing Temp InterstitialVacancy recombination Secondary defects Annealed out Agglomeration of secondary defects Professor N Cheung U C Berkeley 11 EE143 F05 Lecture 8 Dopant Activation Sheet Resistance is limited by dopant solid solubility Shallower junctions will have higher RS Professor N Cheung U C Berkeley 12 EE143 F05 Lecture 8 Ion Channeling Si Crystal deeper penetration C x Random component channeled component random scattering path Random Planar Channeling x Axial Channeling o To Tominimize minimizechanneling channeling we wetilt tiltwafer waferby by77owith withrespect respectto toion ionbeam beam Professor N Cheung U C Berkeley 13 EE143 F05 Lecture 8 Lucky Ions 7o C x Some Somescattered scatteredions ionsfall fallinto into other otherchanneling channelingdirections directions causing causingdeeper deeperpenetration penetration No Channeling With channeling tail CB xj Professor N Cheung U C Berkeley x xj 14 EE143 F05 Lecture 8 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 Disadvantage Needs Needsan anadditional additionalhigh dose high doseimplantation implantationstep step Professor N Cheung U C Berkeley 15 EE143 F05 Lecture 8 Transverse or Lateral Straggle Rt or R Rt Rp 1 Rt Rp Rt Professor N Cheung U C Berkeley 16 EE143 F05 Lecture 8 Lateral Scattering Causes Feature Enlargement y Mask x x Rp Lower concentration Higher concentration Implanted Implantedspecie specie has haslateral lateraldistribution distribution larger largerthan thanmask maskopening opening C y at x Rp y Professor N Cheung U C Berkeley 17 EE143 F05 A 2 D formulation of Implantation Profile Lecture 8 mask y a y a x C x y K C x a a y y 2 e 2 Rt y a K C x erfc 2 Rt Professor N Cheung U C Berkeley 2 dy y a erfc 2 R t 18 EE143 F05 Lecture 8 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 F05 Lecture 8 Semi log Plots of Gaussian and erfc functions details of erfc function also covered in Diffusion section of EE143 Reader Professor N Cheung U C Berkeley Curves also given in Jaeger 20 EE143 F05 Lecture 8 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 F05 Lecture 8 Transmitted Fraction T 0 C x dx x dx d C 0 Rp Rp d Rp 1 erfc 2 2 Rp erfc x 1 2 x y2 e dy 0 are values of for ions into the masking material Rule of thumb Good masking thickness d Rp 4 3 Rp Professor N Cheung U C Berkeley C x d 4 10 C x Rp 22
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