ECE6450L5-Ion Implantation

ECE6450L5-Ion Implantation - Lecture 5 Ion Implantation...

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ECE 6450 - Dr. Alan Doolittle Georgia Tech Lecture 5 Ion Implantation Reading: Chapter 5
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ECE 6450 - Dr. Alan Doolittle Georgia Tech Shockley patented the concept of Ion Implantation for semiconductor doping in 1956 (2 years after Pfann patented the diffusion concept). First commercial implanters were introduced in 1973. Modern implanters are multi-million dollar machines! Concept: Ions (charged atoms or molecules) are created via an enormous electric field stripping away an electron. These ions are filtered and accelerated toward a target wafer, where they are buried in the wafer. The depth of the implantation depends on the acceleration energy (voltage). The dose is very carefully controlled by integrating the measured ion current. This integration process tends to minimize noise in the measurement of the ion current, resulting in several decimal places of accuracy in the dose Ion Implantation = dt I Area q Q T 1
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ECE 6450 - Dr. Alan Doolittle Georgia Tech Advantages of Ion Implantation: 1.) Very precise control of the dose 2.) Independent control of impurity depth and dose 3.) Very fast (1 6" wafer can take as little as 6 seconds for a moderate dose) 4.) Can perform retrograde profiles that peak at points inside the wafer (as opposed to the wafer surface). Draw an example 5.) Complex profiles can be achieved by multi-energy implants. Disadvantages of Ion Implantation: 1.) Very deep and very shallow profiles are difficult 2.) Not all the damage can be corrected by annealing. 3.) Typically has higher impurity content than does diffusion. 4.) Often uses extremely toxic gas sources such as arsine (AsH 3 ), and phosphine (PH 3 ). 5.) Expensive Applications: Doping, SIMOX, H and He isolation in GaAs, and Smart cut technologies Ion Implantation
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ECE 6450 - Dr. Alan Doolittle Georgia Tech 1.) Nuclear collisions: ions collide with atoms. The positively charged ions are coulombically repealed by the positive cores of the wafers lattice atoms. This coulombic repulsion is “screened” by the cloud of electrons surrounding each atom. 2.) Electronic stopping: If ions graze the lattice atoms, they do not interact with the lattice atom’s electrons and not the positive core. This interaction slows the ions by “viscous friction” similar to a rock thrown into water. Note: many electrons are freed from the lattice atoms creating an “ocean of electrons” that the ions must pass through. The stopping power, S = energy loss per unit length of the ion path is , e n electronic nuclear S S dx dE dx dE S + = + = Ions are imbedded into the wafer and are scattered at random angles. The ions loose kinetic energy, thus, slowing to a stop, by 2 mechanisms: Ion Implantation Electronic: Electric field “drag” created by positive ion moving in a flood of electrons Nuclear: Impact with cores of atoms causes damage.
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ECE 6450 - Dr. Alan Doolittle Georgia Tech Ion Implantation
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ECE 6450 - Dr. Alan Doolittle
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