1. Microelectronics Processing
Ion Implantation

2. Issues in Ion Implantation
Equipment
Dose, Range, Straggle
Implantation Profile
Junction Depth
Channeling
Energy Loss Mechanisms
Damage - Anneal

3. Ion Implantation
A process in which energetic, charged atoms or molecules are directly introduced into a substrate.
Acceleration energies range between KeV. (Today also up to several MeV)
Primarily used to add dopant ions into the surface of silicon wafers.
Goal : to introduce a desired atomic species, with a specified quantity (dose), into the required depth, with lateral selectivity.

4. Advantages of ion implantation

5. Ion implantation- Equipment - I
An ion implanter is a high voltage particle accelerator producing a high-velocity beam of ions which can penetrate the surface of silicon target wafers.
Components:
•Ions generated in a source
(from feed gas, e.g. BF3, AsH3, PH3 ...
or heated solid source, then ionized in
arc chamber by electrons from hot filament)
•Accelerate for mass spectroscopy
•Select desired species by q/m, using
a magnet (mass spectrometer),
•Accelerate by an E-field and focus
using electrostatic lenses
impact substrate in raster pattern.
Ion source
Mass spectrometer
High V accelerator
Scanning system
Target chamber

7. State of the art semiconductor ion implantation system
Axcelis Optima MD ion implantation tool for semiconductor production. (Used with permission from Axcelis Technologies)

8. Ion implantation- Equipment - II
Schematic diagram of an ion implantation system.

9. An Ion Source

10. Ion Implantation: A random Process
Each ion follows a random trajectory,
scattering off silicon atoms, losing energy
and coming to rest.
Since a large number of ions is implanted,
the average range Rp and their straggle Rp
can be precisely predicted

11. Ion implantation-Range and Straggle
Schematic diagram to show the range, the projected range RP , the projected (ΔRP ) and lateral ΔR┴ straggle in ion implantation. y

12. Distribution of Ions in Silicon Implanted at 200 keV

13. Playing billiards with atoms and electrons…
Ion impact leads to cascades of recoil atoms and electrons
Atomic cascades act as a nano-blender changing crystal structure and mixing atoms
Electron cascades cause chemical changes (radiolysis)
Foreign ion comes to rest under surface of material – ion implantation doping. Changes chemical and electronic behaviour

14. Monte Carlo simulation of 50keV Boron implanted into Si
TRIM

15. Mathematical model for ion implantation
The distribution can be described statistically and is modeled to a first order by a symmetric Gaussian distribution:
The total number of ions
implanted is defined as the dose:
(C N in some of the figures)

16. Factors in implantation
Range and profile shape depend on the ion energy (for a particular ion/substrate combination.
Height (i.e. concentration) of profile depends on the implantation dose.
Mask layer thickness can block ion penetration.
C(x) in #/cm3
C(x)- concentration = # of atoms/cm3
Q- dose = # of atoms/cm2

17. Meaning of dose and concentration

18. Simplified description of Ion implantation
profile of implanted impurities, C(x):
General profile of ion implanted impurities with a peak concentration Cpeak at depth Rp, and symmetrical distributions on either side of ΔRP .

19. Ion implantation-Projected range and Straggle
Projected range of B, P and As in Si and SiO2 vs ion energy;
projected and lateral straggle of B, P and As ions in Si.
(a)
(b)

20. Distribution of Ions in Silicon Implanted at 200 keV
Profile
not just
Gaussian!

21. Rp

24. Ion Implantation energy loss mechanism

25. LSS Theory of Ion Stopping

32. Nuclear Stopping Power

33. Electronic Stopping Power
Se(E)  kSi E1/2

34. Energy dependence of dE/dx

35. Ion distribution: perpendicular and lateral range
Implanted species
Contours of implanted equal ion concentration into Si

36. Junction depth
The point at which the diffused impurity profile intersects the background concentration, CB, is the metallurgical junction depth, xj. The net impurity concentration at xj is zero. Setting C(xj)=CB we find:
Both roots may be meaningful, as indicated in the figure.

37. Sheet resistance The resistance of a rectangular block is:
R = ρL/A = (ρ/t)(L/W) ≡ Rs(L/W)
Rs is called the sheet resistance. Its units are termed Ω/ .
L/W is the number of unit squares of material in the resistor.

38. Sheet resistance

39. Irving’s curves: Motivation to generate them

40. Figure illustrating the relationship of No, NB, xj, and Rs

41. Irving’s curves

42. Channeling The Si lattice viewed along the <110> axis.
The previous, LSS results, are based on the assumption that the target material is amorphous, having a completely random order.
The Si lattice viewed along the <110> axis.

44. Atom strings and planes in a crystal

45. Channelling
Enhanced scattering with atomic rows
 Critical axial channeling angle
Minimum yield from scattering by surface atoms on the end of rows
480 keV H+ →(100) W angular scan (After J.U. Andersen Mat. Fys. Medd, Dan. Vid. Selsk. 36,No 7(1967))

46. Ion Channeling

47. “Lucky Ions”

49. Tilting
Channeling can be reduced by tilting the <100> silicon by approximately 7o relative to the ion beam.
Tilted implant can produce a doping profile with a junction depth that is closer to the theoretical calculations.

50. Implantation damage

51. Implantation damage - Amorphization
A plot of the dose required to form an amorphous layer on silicon versus reciprocal target temperature.

52. Post Implant Anneal Electrical activation of implanted Impurities
Annealing of primary crystalline defect damage
Annealing of amorphous layers
Dynamic annealing effects
Diffusion of implanted impurities

53. Heat Treatment - Additional
Annealing to restore the crystal structure after the implantation of dopant atoms (1000 C).
Alloying to ensure good electrical conduction between metal layers and the wafer surface (450 C).
No material is added in this process.

54. Diffusion of Gaussian implantation profile upon annealingQ
Note: Q is the implantation dose.

55. Diffusion of Gaussian implantation profile (arbitrary Rp)

56. Multiple implants for uniform profile
Construction of a composite doping profile using multiple implants at different energies.

57. Additional features Wafer annealing:
Damage removed by annealing the wafer at high temperature
for a short period; implanted impurity atoms are "activated".
Proton isolation
Hydrogen ions are used to deliberately change the crystal and
convert it into electrically insulating material.
Predeposition by ion implantation
Ion implantation predeposition of a fixed number of impurities
into the semiconductor in preference to predeposition by
thermal diffusion
Threshold voltage adjustment:
By shallow ion implants through the SiO2 gate oxide layer