U. Glaeser

FIGURE 1.21 Recently reported (a) high-k materials and (b) deposition methods.
Among the candidates, ZrO 2 [29–31,34–37] and HfO 2 [28,32,34,36,38–40] become popular because
their dielectric constant is relatively high and because ZrO 2 and HfO 2 were believed to be stable at the Si
interface. However, in reality, formation and growth of interfacial layer made of silicate (ZrSi xO y, HfSi xO y)
or SiO 2 at the Si interface during the MOSFET fabrication process has been a serious problem. This
interfacial layer acts to reduce the total capacitance and is thought to be undesirable for obtaining high
performance of MOSFETs. Ultrathin nitride barrier layer seems to be effective to suppress the interfacial
layer formation [37]. There is a report that mobility of MOSFETs with ZrO 2 even with these interfacial
layers were significantly degraded by several tens of percent, while with entire Zr silicate gate dielectrics
is the same as that of SiO 2 gate film [31]. Thus, there is an argument that the thicker interfacial silicate
layer would help the mobility improvement as well as the gate leakage current suppression; however,
in other experiment, there is a report that HfO 2 gate oxide MOSFETs mobility was not degraded [38].
For another problem, it was reported that ZrO 2 and HfO 2, easily form micro-crystals during the heat
process [31,33].
Comparing with the cases of ZrO 2 and HfO 2, La 2O 3 film was reported to have better characteristics at
this moment [33]. There was no interfacial silicate layer formed, and mobility was not degraded at all.
The dielectric constant was 20–30. Another merit of the La 2O 3 insulator is that no micro-crystal formation
was found in high temperature process of MOSFET fabrication [33]. There is a strong concern for
its hygroscopic property, although it was reported that the property was not observed in the paper [33].
However, there is a different paper published [34], in which La 2O 3 film is reported to very easily form a
silicate during the thermal process. Thus, we have to watch the next report of the La 2O 3 experiments.
Crystal Pr 2O 3 film grown on silicon substrate with epitaxy is reported to have small leakage current [42].
However, it was shown that significant film volume expansion by absorbing the moisture of the air was
observed. La and Pr are just two of the 15 elements in lanthanoids series. There might be a possibility
that any other lanthanoid oxide has even better characteristics for the gate insulator. Fortunately, the
atomic content of the lanthanoids, Zr, and Hf in the earth’s crust is much larger than that of Ir, Bi, Sb,
In, Hg, Ag, Se, Pt, Te, Ru, Au, as shown in Fig. 1.22.
Al 2O 3 [41,43] is another candidate, though dielectric constant is around 10. The biggest problem for
the Al 2O 3 is that film thickness dependence of the flatband shift due to the fixed charge is so strong that
controllability of the flatband voltage is very difficult. This problem should be solved before it is used
for the production. There is a possibility that Zr, Hf, La, and Pr silicates are used for the next generation
gate insulator with the sacrifice of the dielectric constant to around 10 [31,35,37]. It was reported that
the silicate prevent from the formation of micro-crystals and from the degradation in mobility as
described before. Furthermore, there is a possibility that stacked Si 3N 4 and SiO 2 layers are used for mobile
device application. Si 3N 4 material could be introduced soon even though its dielectric constant is not
very high [44–46], because it is relatively mature for use for silicon LSIs.
© 2002 by CRC Press LLC
Zr alminate
Zr-Al silicate
Pr2O3 Y CeO
2 O3 2
Y silicate
BST
Tix TayO TaOx Ny TiO 2
Ti silicate
La 2 O 3
La silicate
Ta 2 O 5
Ta silicate
Al 2 O 3
Al silicate
HfO 2
Hf silicate
LaAlO3 Gd2O3 ZrO 2
Zr silicate
MBE
MBE
(amorphous)
Sputtering
PLD
(a) (b)
LPCVD
ALCVD
MOCVD
RPECVD
RTCVD