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 Generation of a ion electron ionisation An incident electron collides one neutral atom If the incident electron has enough energy E e > E i  one electron of the valence layer can be ejected e - E e > E i Atom e -Ion e - Ar Ar +

5 Definition: Plasma Main 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 Collision S= cross section N= number of particles per unit volume

6 Basic parameters of the plasma 3 species: neutral, ions, electrons Temperature T i, T e,T n Density n e, n i, n n Debye length Screening distance over which electric field are effectively excluded Ti=Te: balanced plasma Ti≠Te: unbalanced plasma

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

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 2 nd e- at the cathode I-V characteristic of gas discharge Cathode surface

9 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 barrier and are emitted from cathode Field emission I-V characteristic of gas discharge Cathode surface Avalanche - Thermoelectronic emission + Intensive Ion bombardment T   E kin of e- 

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

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

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

13 2 cases for the sheath If δ < λ Ions pass the sheath like in vacuum If δ > λ Ions perform collisions You have scattering of ions Ions go directly to substrate 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, j i m i : mass of the ion q i : 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!