Introduction to Plasma Immersion Ion Implantation Technologies

1. Introduction to Plasma Immersion Ion Implantation Technologies Emmanuel Wirth

2. Plan of the presentation • Ion beam / plasma ion immersion • Generation of ions • Reminder of basic plasma parameters • The plasma immersion device • Zoom into the substrate zone

3. Ion implantation • Ion beam: Conventional ion implantation setups extract a stream of ions from an ion source and focuses them into a beam • Expensive • Plasma Immersion: the target is placed inside the ion source itself • Much cheaper

4. Ion e - Atom e - Ee > Ei e - Ar Ar+ Generation of a ionelectron ionisation • An incident electron collides one neutral atom • If the incident electron has enough energyEe > Ei one electron of the valence layer can be ejected

5. Collision S Definition: PlasmaMain free path • Plasma: partially ionised gas which contains no net space charges • The mean free path is the average distance between collisions in a gas S= cross section N= number of particles per unit volume

6. Ti=Te: balanced plasma Ti≠Te: unbalanced plasma Basic parameters of the plasma • 3 species: neutral, ions, electrons • Temperature • Ti,Te,Tn • Density • ne,ni,nn • Debye length • Screening distance over which electric field are effectively excluded

7. Cathode Sheath Cathode Plasma Anode Generation of Ion: Use of Plasma • 3 Processes occur: • Self sustained regime • Thermoelectronic emission • Field emission • The e- are accelerated in the cathode sheath Intensive ionization V I-V characteristic of gas discharge

8. Generation of Ion: Use of Plasma • 3 Processes occur: • Self sustained regime • Thermoelectronic emission • Field emission α : Number of ionisation per 1 e- for 1 m along E γ : Number of ions necessary to create one 2nd e- at the cathode Cathode surface I-V characteristic of gas discharge

9. Avalanche + + - - Thermoelectronic emission + + - - + + - - + - - + + - - + + - - + + + - - Generation of Ion: Use of Plasma • 3 Processes occur: • Self sustained regime • Thermoelectronic emission Electrons of surface receive sufficient kinetic energy to pass the potential barrierand are emitted from cathode • Field emission Intensive Ion bombardment T  E kin of e-  Cathode surface I-V characteristic of gas discharge

10. Avalanche Field emission + + - - + + - - + + - - + - - + + - - + + - - Cathode Sheath + + - - C A Plasma Generation of Ion: Use of Plasma • 3 Processes occur: • Self sustained regime • Thermoelectronic emission • Field emission I increase  extra ionisation  space charge approachesthe cathode  E  near cathode  Channelling effect Cathode surface I-V characteristic of gas discharge

11. Plasma Plasma as a source of ion • Generation of a plasma by auxiliary device • DC • Magnetron • Immersion of the substrate in the plasma A field to compensate the negative bias take place: a sheath is created Cathode U < 0 Anode

12. + + + δ + + + Zoom in the sheath zone • Surface of plasma = source of ions • Positive ions are attracted from the plasma • Within the sheath ρ ≠ 0 + + + + + - - - - - + + + + + + - - - - U < 0

13. If δ > λ Ions perform collisions 2 cases for the sheath • If δ < λ • Ions pass the sheath like in vacuum Ions go directly to substrate You have scattering of ions • It is very important to have an estimation of the thickness of the sheath δ

14. If δ < λ: expression of δ • If gap can be considered as vacuum, you can calculate the thickness of the sheath layer • Start from the Poisson equation • You can estimate x= δ You measure V, jimi: mass of the ion qi: charge of the ion

15. Conclusion • Plasma is a cheaper way to implant ions • Plasma is formed by a gas discharge • Negative bias voltage on substrate create a sheath • The vacuum chamber should be larger than δ!