Ion implantation

Ion implantation is used in semiconductor device fabrication and in metal finishing, as well as in material science research.
it is a low temperature process that includes the acceleration of ions of a particular element towards a target, altering the chemical and physical properties of the target.

1. ION IMPLANTATION PRESENTED BY: SWATHI P 1 ST YEAR M.TECH- ECE PONDICHERRY UNIVERSITY

2. C ontents Ion implantation Impurity distribution of implanted ions Stopping power Channeling Advantages Drawbacks Applications

3. ION IMPLANTATION Ion Implantation is the alternative method of diffusion and it is the most common method and is usually done at low temperature. It is a method to implant impurities on a wafer. The impurities used here are in the form of ions. These ions are first generated usually by ionization of gaseous chemical compound.

4. From these ions necessary ions are separated by the appropriate application of magnetic fields . This ions are then focused using electric field to form an ion-beam and then made to target on wafer. If energy of ion beam is adequate the ions can penetrate the target wafer and then collide with Si lattice atom and with surrounding electrons and eventually comes to rest with in the wafer.

5. In ion implantation the impurity atom which are ionized is accelerated through an electric field strikes the wafer surface. The dose of impurity can be tightly controlled by measuring the ion current. The penetration depth of impurity ion can be controlled by electrostatic field. The depth of penetration of any particular type of ion will increase with increasing accelerating voltage. The penetration depth will be in the range of 0.1 to 1.0  

6. Impurity distribution of implanted ions The distribution of implanted ions as a function of distance x from the silicon surface will be a Gaussian distribution given by [ ] Peak concentration of implanted ion = Projected range = Distance into substrate from surface = standard deviation of projected range  

7. An ion implantation impurity profile is shown

8. The peak implanted ion concentration is related to implantation dosage by The implantation dosage is the number of implanted ion per unit surface area. The ion density drops off rapidly from the peak value with distance as measured from in either direction.  

9. Stopping power When an energetic ion enters a solid, it losses energy. The energy loss in the target mainly due to two mechanisms. Ion-electron interaction [Electronic stopping power] Ion-nucleus interaction [Nuclear stopping power] Le t = Energy of bombarding ion  

10. = Number of silicon atoms/unit volume of target lattice Nuclear stopping power Electronic stopping power  

11. C hanneling The target in ion implantation (i.e., wafers to which ions are implanted) will be having crystalline regular arrangement of atom. The atoms arrangement must be such that there must be open space between them through which the ion entering can travel without significant scattering. Ions must be steered down the channels by glancing collision (collisions in smaller angle) with the atom row or plane, extending the ion distribution deeper into the target.

12. The below figure shows the channeling of ion into a simple cubic lattice structure (i.e., our target).

13. Ion A is well aligned with a channel and so suffer only glancing collisions with the walls as it travels far into the lattice. Ion B is scattered into a channel after a short distance, perhaps because of a lattice imperfection. Ion C is not properly channeled thus has a random collision with lattice atoms. Channeling is characterized by a critical angle ; which is the maximum angle between ion and channel for a glancing collision to occur .  

14. Critical angle = Incident ion atomic number = Target atom atomic number = Atomic spacing along the ion direction. = Ion energy in  

15. Channeling effect occur when ion velocity is parallel to a major crystal orientation . Some ions may travel considerable distances with little energy loss.

16. Advantages Much more precise control over density of dopants ( ) deposited into wafer. Done at relatively low temperature. It reduces the threshold voltage of the MOSFET. Less sensitive to surface cleaning procedures. Complex profile can be achieved by multi-energy implants.  

17. Drawbacks Incident ion may damage the semiconductor lattice. Very shallow and deep profiles are difficult or impossible. Equipment is very expensive. At high dose values, throughputs is less than diffusion. Masking materials can be “knocked” into the wafer creating unwanted impurities, or even destroying the quality of the interface.

18. Applications Nitrogen or other ions can be implanted into a tool steel target (example: drill bits). Metal parts on heart valves are ion implanted by carbon to make them biocompatible. Radioisotopes are implanted in prosthesis for localized radiotherapy. Ion beam mixing, i.e., mixing up atoms of different elements at an interface.

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