U. Glaeser

1.5 Source and Drain
Figure 1.25 shows the changes of S/D (source and drain) formation process and structure. S/D becomes
shallower for every new generation in order to suppress the short-channel effects. Before, the extension
part of the S/D was called as LDD (Lightly Doped Drain) region and low doping concentration was
required in order to suppress electric field at the drain edge and hence to suppress the hot-carrier effect.
Structure of the source side becomes symmetrical as the drain side because of process simplicity. Recently,
major concern of the S/D formation is how to realize ultra-shallow extension with low resistance. Thus,
the doping of the extension should be done as heavily as possible and the activation of the impurity
should be as high as possible. Table 1.4 shows the trends of the junction depth and sheet resistance of
the extension requested by ITRS 2000. As the generation proceeds, junction depth becomes shallower,
but at the same time, the sheet resistance should be reduced. This is extremely difficult. In order to satisfy
this request, various doping and activation methods are being investigated. As the doping method, low
energy implantation at 2–0.5 keV [48] and plasma doping with low energy [49] are thought to be the
most promising at this moment. The problem of the low energy doping is lower retain dose and lower
activation rate of the implanted species [48]. As the activation method, high temperature spike lamp
anneal [48] is the best way at this moment. In order to suppress the diffusion of the dopant, and to keep
the over-saturated activation of the dopant, the spike should be as steep as possible. Laser anneal [50]
can realize very high activation, but very high temperature above the melting point at the silicon surface
is a concern. Usually laser can anneal only the surface of the doping layer, and thus deeper portion may
be necessary to be annealed by the combination of the spike lamp anneal.
TABLE 1.4 Trend of S/D Extension by ITRS
© 2002 by CRC Press LLC
1999 2000 2001 2002 2003 2004 2005 2008 2011 2014
Technology
node (nm)
180 130 100 70 50 35
Gate length (nm) 140 120 100 85 80 70 65 45 32 22
Extension Xj (nm) 42–70 36–60 30–50 25–43 24–40 20–35 20–33 16–26 11–19 8–13
Extension sheet
resistance (Ω/�)
350–800 310–760 280–730 250–700 240–675 220–650 200–625 150–525 120–450 100–400
Gas / Solid phase diffusion P, B Ion Implantation As, P, B
LDD
S Diffused layer D
P, As, B, BF 2
FIGURE 1.25 Source and drain change.
Extension
Low E Ion Imp.
As, BF 2
LDD (Lightly Doped Drain) Extension Pocket
Pocket / Halo
As, BF2 , In