Fig. 5.24 shows the input structure for the ion implantation process step. It is one half of an NMOS-Transistor which is cut through the gate. The crystalline silicon substrate is partly covered with a thick silicon dioxide layer forming the isolation and a thin silicon dioxide layer serving as a scattering oxide. The isolation is formed by a local oxidation of silicon (LOCOS) process.
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The implantation is performed with BF ions with an energy of 50 keV and a dose
of
cm
. The ion beam was tilted by 7
and rotated by
0
. By this implantation a boron concentration up to
cm
is introduced in the silicon substrate, which has a positive
background doping of
cm
, below the gate region (narrow region below
the scattering oxide) and also below the source/drain region (large rectangular
region below the scattering oxide) as shown in the top figure of
Fig. 5.25.
Just the doping below the gate has a significant influence on the final device
behavior, while the source/drain region is additionally doped by a succeeding
ion implantation steps with a higher implantation dose. The effect of this
implantation is to adjust the threshold voltage of the
transistor. The threshold voltage is defined as the gate voltage above which the
transistor becomes conductive due to an inversion of a thin layer below the
gate. The voltage which is necessary to create an inversion layer strongly
depends on the original doping concentration, which is adjusted by this
implantation. Threshold voltage adjust implantations are always performed with
low doses, because just slight modification of the gate concentration are
sufficient for the adjustment. Typical doses are
cm
to
cm
([91]).
The simulation was performed using the molecular method (Sec. 4.5.2). Therefore, the impurity distributions of the boron atoms and of the fluorine atoms have been calculated. Fig. 5.25 shows the distribution of the boron atoms (top) and of the fluorine atoms (bottom) in the transistor. The doping profiles are visualized within three cuts through the active area of the transistor in common with an outline of the transistor structure. 2300000 distinct ions where simulated on a DEC-600 workstation with 333 MHz CPU clock frequency. The simulation took approximately 5 hours of CPU time.
As expected there is not much difference between the boron and the fluorine
distribution, except that the fluorine concentration is twice the boron
concentration due to the larger number of fluorine atoms in the BF
molecule. Therefore an ion implantation with BF
ions can also be simulated quite
accurately with the simplified molecular method as already mentioned in
Sec. 4.5.2. If the same simulation is performed with the simplified
molecular method (Sec. 4.5.2) the simulation time reduces to 3 hours.
Alternatively this simulation can be performed with the analytical method (Sec. 3.1) or if a higher accuracy is required with the point response interface method (Sec. 3.2). The analytical method requires a simulation time of approximately 30 minutes while the point response interface method requires a simulation time of approximately 2 hours. By using the point response interface method the simulation time is mainly determined by the time required for the calculation of the point response function.
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