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Home > Knowledge > 5.Silicon Carbide Technology > 5.5.2 SiC-Selective Doping: Ion Implantation
5.5.2 SiC-Selective Doping: Ion Implantation
5.5 SiC Device Fundamentals
 
5.5.2 SiC-Selective Doping: Ion Implantation
 
The fact that diffusion coefficients of most SiC dopants are negligibly small (at 1800°C) is excellent for
maintaining device junction stability, because dopants do not undesirably diffuse as the device is operated
long term at high temperatures. Unfortunately, this characteristic also largely (except for B at extreme
temperatures ) precludes the use of conventional dopant diffusion, a highly useful technique widely
employed in silicon microelectronics manufacturing, for patterned doping of SiC.
Laterally patterned doping of SiC is carried out by ion implantation. This somewhat restricts the depth
that most dopants can be conventionally implanted to <1 μm using conventional dopants and implantation
equipment. Compared to silicon processes, SiC ion implantation requires a much higher thermal budget
to achieve acceptable dopant implant electrical activation. Summaries of ion implantation processes
for various dopants can be found in . Most of these processes are based on carrying out implantation at
temperatures ranging from room temperature to 800°C using a patterned (sometimes high-temperature)
masking material. The elevated temperature during implantation promotes some lattice self-healing during
the implant, so that damage and segregation of displaced silicon and carbon atoms does not become
excessive, especially in high-dose implants often employed for ohmic contact formation. Co-implantation
of carbon with dopants has been investigated as a means to improve the electrical conductivity of the more
heavily doped implanted layers .
Following implantation, the patterning mask is stripped and a higher temperature (~1200 to 1800°C)
anneal is carried out to achieve maximum electrical activation of dopant ions. The final annealing
conditions are crucial to obtaining desired electrical properties from ion-implanted layers. At higher
implant anneal temperature, the SiC surface morphology can seriously degrade . Because sublimation
etching is driven primarily by loss of silicon from the crystal surface, annealing in silicon overpressures
can be used to reduce surface degradation during high-temperature anneals . Such overpressure can
be achieved by close-proximity solid sources such as using an enclosed SiC crucible with SiC lid and/or
SiC powder near the wafer, or by annealing in a silane-containing atmosphere. Similarly, robust deposited
capping layers such as AlN and graphite, have also proven effective at better preserving SiC surface
morphology during high-temperature ion implantation annealing .
As evidenced by a number of works, the electrical properties and defect structure of 4H-SiC doped
by ion implantation and annealing are generally inferior to SiC doped in-situ during epitaxial
growth . Naturally, the damage imposed on the SiC lattice roughly scales with implantation dose. Even
though reasonable electrical dopant activations have been achieved, thermal annealing processes
developed to date for SiC have not been able to thoroughly repair all damage imposed on the
crystal lattice by higher-dose ion implantations (such as those often used to form heavily doped layers
in preparation of ohmic contact formation, Section 5.5.3). The degraded crystal quality of highly
implanted SiC layers has been observed to degrade carrier mobilities and minority carrier lifetimes,
thereby causing significant degradation to the electrical performance of some devices . Until
large further improvements to ion-implanted doping of SiC are developed, SiC device designs will have
to account for nonideal behavior associated with SiC-implanted layers.