Ion Implantation M H Nemati Sabanci University 1

Ion Implantation M. H. Nemati Sabanci University 1

Ion Implantation • • • Introduction Safety Hardware Processes Summary 2

Introduction • Dope semiconductor • Two way to dope – Diffusion – Ion implantation • Other application of ion implantation 3

Dope Semiconductor: Diffusion • Isotropic process • Can’t independently control dopant profile and dopant concentration • Replaced by ion implantation after its introduction in mid-1970 s. 4

Dope Semiconductor: Ion Implantation • Used for atomic and nuclear research • Early idea introduced in 1950’s • Introduced to semiconductor manufacturing in mid-1970 s. 5

Dope Semiconductor: Ion Implantation • Independently control dopant profile (ion energy) and dopant concentration (ion current times implantation time) • Anisotropic dopant profile • Easy to achieve high concentration dope of heavy dopant atom such as phosphorus and arsenic. 6

Ion Implantation, Phosphorus Si. O 2 Poly Si n+ P+ n+ P-type Silicon 7

Ion Implantation Control • Beam current and implantation time control dopant concentration • Ion energy controls junction depth • Dopant profile is anisotropic 8

Stopping Mechanism • • Ions penetrate into substrate Collide with lattice atoms Gradually lose their energy and stop Two stop mechanisms 9

Two Stopping Mechanism • Nuclear stopping – Collision with nuclei of the lattice atoms – Scattered significantly – Causes crystal structure damage. • electronic stopping – Collision with electrons of the lattice atoms – Incident ion path is almost unchanged – Energy transfer is very small – Crystal structure damage is negligible 10

Implantation Processes: Channeling • If the incident angle is right, ion can travel long distance without collision with lattice atoms • It causes uncontrollable dopant profile Lots of collisions Very few collisions 11

Channeling Effect Lattice Atoms Channeling Ion Collisional Ion q Wafer Surface 12

Implantation Processes: Channeling • Ways to avoid channeling effect – Tilt wafer, 7° is most commonly used – Screen oxide – Pre-amorphous implantation, Germanium • Shadowing effect – Ion blocked by structures • Rotate wafer and post-implantation diffusion 13

Implantation Processes: Damage • Ion collides with lattice atoms and knock them out of lattice grid • Implant area on substrate becomes amorphous structure Before Implantation After Implantation 14

Implantation Processes: Anneal • Dopant atom must in single crystal structure and bond with four silicon atoms to be activated as donor (N-type) or acceptor (P-type) • Thermal energy from high temperature helps amorphous atoms to recover single crystal structure. 15

Thermal Annealing Lattice Atoms Dopant Atom 16

Thermal Annealing Lattice Atoms Dopant Atom 17

Thermal Annealing Lattice Atoms Dopant Atom 18

Thermal Annealing Lattice Atoms Dopant Atom 19

Thermal Annealing Lattice Atoms Dopant Atom 20

Thermal Annealing Lattice Atoms Dopant Atom 21

Thermal Annealing Lattice Atoms Dopant Atom 22

Thermal Annealing Lattice Atoms Dopant Atoms 23

Implantation Processes: Annealing Before Annealing After Annealing 24

Ion Implantation: Hardware • • Gas system Electrical system Vacuum system Ion beamline 25

Implantation Process Gases and Vapors: P, B, BF 3, PH 3, and As. H 3 Implanter Select Ion: B, P, As Select Ion Energy Next Step Select Beam Current 26

Ion Implanter Gas Cabin Ion Source Electrical System Analyzer Magnet Vacuum Pump Beam Line Electrical System Vacuum Pump Plasma Flooding System Wafers End Analyzer 27

Ion Implantation: Gas System • Special gas deliver system to handle hazardous gases • Special training needed to change gases bottles • Argon is used for purge and beam calibration 28

Ion Implantation: Electrical System • High voltage system – Determine ion energy that controls junction depth • RF system – Some ion sources use RF to generate ions 29

Ion Implantation: Vacuum System • Need high vacuum to accelerate ions and reduce collision • MFP >> beamline length • 10 -5 to 10 -7 Torr • Turbo pump and Cryo pump • Exhaust system 30

Ion Implantation: Control System • • Ion energy, beam current, and ion species. Mechanical parts for loading and unloading Wafer movement to get uniform beam scan CPU board control boards – Control boards collect data from the systems, send it to CPU board to process, – CPU sends instructions back to the systems through the control board. 31

Ion Implantation: Beamline • • • Ion source Extraction electrode Analyzer magnet Post acceleration Plasma flooding system End analyzer 32

Ion Beam Line Suppression Electrode Vacuum Pump Ion Source Extraction Electrode Post Acceleration Electrode Plasma Flooding System Analyzer Magnet Beam Line Vacuum Pump Wafers End Analyzer 33

Ion implanter: Ion Source • Hot tungsten filament emits thermal electron • Electrons collide with source gas molecules to dissociate and ionize • Ions are extracted out of source chamber and accelerated to the beamline • RF and microwave power can also be used to ionize source gas 34

Ion Implantation: Extraction • Extraction electrode accelerates ions up to 50 ke. V • High energy is required for analyzer magnet to select right ion species. 35

Ion Implantation: Analyzer Magnet • Gyro radius of charge particle in magnetic field relate with B-field and mass/charge ratio • Used for isotope separation to get enriched U 235 • Only ions with right mass/charge ratio can go through the slit • Purified the implanting ion beam 36

Analyzer Magnetic Field (Point Outward) Ion Beam Larger m/q Ratio Flight Tube Smaller m/q Ratio Right m/q Ratio 37

Ion Implantation: The Process • CMOS applications • CMOS ion implantation requirements • Implantation process evaluations 38

Implantation Process: Well Implantation • High energy (to Me. V), low current (1013/cm 2) P+ Photoresist N-Well P-Epi P-Wafer 39

Implantation Process: VT Adjust Implantation Low Energy , Low Current B+ Photoresist USG STI P-Well P-Epi N-Well P-Wafer 40

Lightly Doped Drain (LDD) Implantation • Low energy (10 ke. V), low current (1013/cm 2) P+ Photoresist USG STI P-Well P-Epi N-Well P-Wafer 41

Implantation Process: S/D Implantation • Low energy (20 ke. V), high current (>1015/cm 2) P+ Photoresist STI n+ n+ USG P-Well N-Well P-Epi P-Wafer 42

Process Issues • • Wafer charging Particle contamination Elemental contamination Process evaluation 43

Ion Implantation: Safety • One of most hazardous process tools in semiconductor industry • Chemical • Electro-magnetic • Mechanical 44

Summary of Ion Implantation • Dope semiconductor • Better doping method than diffusion • Easy to control junction depth (by ion energy) and dopant concentration ( by ion current and implantation time). • Anisotropic dopant profile. 45

Summary of Ion Implantation • • • Ion source Extraction Analyzer magnets Post acceleration Charge neutralization system Beam stop 46

Summary of Ion Implantation • • Well Source/Drain Vt Adjust LDD High energy, low current Low energy, high current Low energy, low current 47