U. Glaeser

TABLE 1.5 Physical Properties of Silicides
In order to further reduce the sheet resistance, elevated S/D structure of the extension is necessary, as
shown in Fig. 1.26 [6]. Elevated S/D will be introduced at the latest from the generation of sub-30 nm
gate length generation, because sheet resistance of S/D will be the major limiting factor of the device
performance in that generation.
Salicide is a very important technique to reduce the resistance of the extrinsic part of S/D—resistance
of deep S/D part and contact resistance between S/D and metal. Table 1.5 shows the changes
of the salicide/silicide materials. Now CoSi 2 is the material used for the salicide. In future, NiSi is
regarded as promising because of its superior nature of smaller silicon consumption at the silicidation
reaction [10].
1.6 Channel Doping
Channel doping is an important technique not only for adjusting the threshold voltage of MOSFETs
but also for suppressing the short-channel effects. As described in the explanation of the scaling method,
the doping of the substrate or the doping of the channel region should be increased with the downsizing
of the device dimensions; however, too heavily doping into the entire substrate causes several problems,
such as too high threshold voltage and too low breakdown voltage of the S/D junctions. Thus, the
heavily doping portion should be limited to the place where the suppression of the depletion layer is
necessary, as shown in Fig. 1.27. Thus, retrograde doping profile in which only some deep portion is
heavily doped is requested. To realize the extremely sharp retrograde profile, undoped-epitaxial-silicon
growth on the heavily doped channel region is the most suitable method, as shown in the figure [7–9].
This is called as epitaxial channel technique. The epitaxial channel will be necessary from sub-50 nm
gate length generations.
© 2002 by CRC Press LLC
MoSi 2 WSi 2 C54–TiSi 2 CoSi 2 NiSi
Resistivity (µΩ cm) 100 70 10~15 18~25 30~40
Forming temperature (°C) 1000 950 750~900 550~900 400
Diffusion species Si Si Si Co ∗
Ni
∗ Si(CoSi), Co(Co2Si).
Series resistance [Ω− µ
m]
FIGURE 1.26 Elevated source and drain.
500
400
300
200
100
0
0 −1.5 -2.0
S [mV/decade]
200
V g [V]
150
SPDD
100
50
S 4 D
LDD
LDD
SPDD
S 4 D
V d = −1.5 V
0.1 1.0
Lg [ µ m]