16-08-2011, 02:53 PM
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ION IMPLANTATION
Ion Implantation is an alternative to a deposition diffusion and is used to produce a shallow surface region of dopant atoms deposited into a silicon wafer. This technology has made significant roads into diffusion technology in several areas. In this process a beam of impurity ions is accelerated to kinetic energies in the range of several tens of kV and is directed to the surface of the silicon. As the impurity atoms enter the crystal, they give up their energy to the lattice in collisions and finally come to rest at some average penetration depth, called the projected range expressed in micro meters. Depending on the impurity and its implantation energy, the range in a given semiconductor may vary from a few hundred angstroms to about 1micro meter. Typical distribution of impurity along the projected range is approximately Gaussian. By performing several implantations at different energies, it is possible to synthesize a desired impurity distribution, for example a uniformly doped region.
Ion Implantation System
A typical ion-implantation system is shown in the figure below.
Ion Implantation System
A gas containing the desired impurity is ionized within the ion source. The ions are generated and repelled from their source in a diverging beam that is focussed before if passes through a mass separator that directs only the ions of the desired species through a narrow aperture. A second lens focuses this resolved beam which then passes through an accelerator that brings the ions to their required energy before they strike the target and become implanted in the exposed areas of the silicon wafers. The accelerating voltages may be from 20 kV to as much as 250 kV. In some ion implanters, the mass separation occurs after the ions are accelerated to high energy. Because the ion beam is small, means are provided for scanning it uniformly across the wafers. For this purpose the focussed ion beam is scanned electrostatically over the surface of the wafer in the target chamber.
Repetitive scanning in a raster pattern provides exceptionally uniform doping of the wafer surface. The target chamber commonly includes automatic wafer handling facilities to speed up the process of implanting many wafers per hour.
Properties of Ion Implantation
The depth of penetration of any particular type of ion will increase with increasing accelerating voltage. The penetration depth will generally be in the range of 0.1 to 1.0 micro meters.
Annealing after Implantation
After the ions have been implanted they are lodged principally in interstitial positions in the silicon crystal structure, and the surface region into which the implantation has taken place will be heavily damaged by the impact of the high-energy ions. The disarray of silicon atoms in the surface region is often to the extent that this region is no longer crystalline in structure, but rather amorphous. To restore this surface region back to a well-ordered crystalline state and to allow the implanted ions to go into substitutional sites in the crystal structure, the wafer must be subjected to an annealing process. The annealing process usually involves the heating of the wafers to some elevated temperature often in the range of 1000°C for a suitable length of time such as 30 minutes.
Laser beam and electron-beam annealing are also employed. In such annealing techniques only the surface region of the wafer is heated and re-crystallized. An ion implantation process is often followed by a conventional-type drive-in diffusion, in which case the annealing process will occur as part of the drive-in diffusion.
Ion implantation is a substantially more expensive process than conventional deposition diffusion, both in terms of the cost of the equipment and the throughput, it does, however, offer following advantages.
ION IMPLANTATION
Ion Implantation is an alternative to a deposition diffusion and is used to produce a shallow surface region of dopant atoms deposited into a silicon wafer. This technology has made significant roads into diffusion technology in several areas. In this process a beam of impurity ions is accelerated to kinetic energies in the range of several tens of kV and is directed to the surface of the silicon. As the impurity atoms enter the crystal, they give up their energy to the lattice in collisions and finally come to rest at some average penetration depth, called the projected range expressed in micro meters. Depending on the impurity and its implantation energy, the range in a given semiconductor may vary from a few hundred angstroms to about 1micro meter. Typical distribution of impurity along the projected range is approximately Gaussian. By performing several implantations at different energies, it is possible to synthesize a desired impurity distribution, for example a uniformly doped region.
Ion Implantation System
A typical ion-implantation system is shown in the figure below.
Ion Implantation System
A gas containing the desired impurity is ionized within the ion source. The ions are generated and repelled from their source in a diverging beam that is focussed before if passes through a mass separator that directs only the ions of the desired species through a narrow aperture. A second lens focuses this resolved beam which then passes through an accelerator that brings the ions to their required energy before they strike the target and become implanted in the exposed areas of the silicon wafers. The accelerating voltages may be from 20 kV to as much as 250 kV. In some ion implanters, the mass separation occurs after the ions are accelerated to high energy. Because the ion beam is small, means are provided for scanning it uniformly across the wafers. For this purpose the focussed ion beam is scanned electrostatically over the surface of the wafer in the target chamber.
Repetitive scanning in a raster pattern provides exceptionally uniform doping of the wafer surface. The target chamber commonly includes automatic wafer handling facilities to speed up the process of implanting many wafers per hour.
Properties of Ion Implantation
The depth of penetration of any particular type of ion will increase with increasing accelerating voltage. The penetration depth will generally be in the range of 0.1 to 1.0 micro meters.
Annealing after Implantation
After the ions have been implanted they are lodged principally in interstitial positions in the silicon crystal structure, and the surface region into which the implantation has taken place will be heavily damaged by the impact of the high-energy ions. The disarray of silicon atoms in the surface region is often to the extent that this region is no longer crystalline in structure, but rather amorphous. To restore this surface region back to a well-ordered crystalline state and to allow the implanted ions to go into substitutional sites in the crystal structure, the wafer must be subjected to an annealing process. The annealing process usually involves the heating of the wafers to some elevated temperature often in the range of 1000°C for a suitable length of time such as 30 minutes.
Laser beam and electron-beam annealing are also employed. In such annealing techniques only the surface region of the wafer is heated and re-crystallized. An ion implantation process is often followed by a conventional-type drive-in diffusion, in which case the annealing process will occur as part of the drive-in diffusion.
Ion implantation is a substantially more expensive process than conventional deposition diffusion, both in terms of the cost of the equipment and the throughput, it does, however, offer following advantages.
