Resonant Auger electron spectroscopy for analysis of the chemical state of phosphorus
segregated at SiO2/Si interfaces.
M. Oshima, Y.Yoshimura, K.Ono, H.Fujioka, Y.Sato, Y. Baba, K. Yoshii, T.A. Sasaki
Introduction.
On this paper they are interested on identify the matrix of the segregated phosphorus atoms at
the SiO2/Si interface. Since p-Mos transistors utilize phosphorus implantation into p-type silicon
substrates to make an n-type well, it is very important to identify the structure and the electrical
influence of segregated phosphorus. If phosphorous exist in the silicon side, it is very important
to distinguish the substitutional site from the interstitial site because phosphorus in the
substitutional site is electrically active, causing a serious detrimental effect on the inversion
layer of p-Mos transistors.
Different methods have been used to investigate diffusion processes, chemical states and
position of the segregated phosphorus atoms at the interface. XPS (X-ray photoelectron
spectroscopy) was used to determine the chemical state of phosphorus implanted in SiO2 and
showed the post-annealing chemical shift. However, it’s difficult to determine if the phosphorus
exist in the silica side or in the silicon side of the interface. That is why they used another
method to analyze the matrix of the segregated phosphorus atoms at the SiO2/Si interface
“resonant Auger electron spectroscopy”.
Experimental.
Samples were p-type Si(100) wafers. First, the wafers were cleaned by dipping in a piranha
solution and in HF solution, repeatedly, and then phosphorus atoms were implanted. After the
growth of thin chemical oxide films in a piranha solution, the sample was annealed in a rapid
thermal annealing chamber, resulting in phosphorus piling up at the SiO2/Si interface. They use
phosphosilicate glass (PSG) film and a phosphorus-doped poly-Si film. Here they used the
phosphorus-doped poly-Si sample as a reference for phosphorus atoms at the substitutional
sites.
Results.
This figure shows P1s photoelectron spectra from SiO2/Si sample with segregated phosphorus,
the PSG film and the phosphorus-doped poly-Si film. The P 1s peak for the phosphorussegregated SiO2/Si has lower energy that that for the PSG sample. This chemical shift indicates
that the phosphorus atoms are not in the oxidized state. However in this experiment, because of
the inadequate energy resolution and the small chemical shift of P 1s, it is difficult to give a
conclusion about the position of the phosphorus.
There is no method to distinguish phosphorus in silicon from unoxidized phosphorus in silica,
because both are in almost identical chemical state. So in this paper they apply resonant Auger
electron spectroscopy to do matrix analysis of the segregated phosphorus.
In the case of normal Auger process, the P 1s electron is excited to the free electron state and
the remaining hole is immediately filled by an electron from the P2p level, then ejecting a normal
Auger electron. In case of resonant auger process, if the P 1s electron is excited just to the
conduction band of pπ orbital, the successive decay process is strongly influenced by this
excited electron. So here the outgoing electron gets energy from the excited electron which
loses that energy and is de-excited to the bottom of the conduction band. Therefore, the kinetic
energy of the resonant Auger electron depends on the photon energy.
Figure 2 shows Auger decay spectra of the PSG sample exited by various photon
energies. When the photon energy is below the threshold, no Auger peak appeared. When the
photon energy is above the threshold a normal Auger peak appeared together with the resonant
Auger peak. In Fig 3 we can observed the kinetic energy of the resonant Auger electrons
increases with increasing photon energy, while kinetic energies of normal Auger peaks remain
unchanged. the slope on resonant Auger peaks indicates the excess energy is given to the
emitted Auger electron.
This figure shows Auger decay spectra of the SiO2/Si sample with segregated phosphorus
around the P 1s edge excited by various photon energies. No Auger peaks are observed below
2142 eV. A large Auger peak is observed at about 1852 eV of KE when P 1s is excited by 2144
eV photons. Even when the photon energy increased the Auger peak position stay in the same
place, this indicates this peak is caused by a normal Auger decay process. There are no
resonant Auger peaks for the SiO2/Si sample which means P 1s core hole are immediately
screened by conduction electrons in silicon and that the potential of the P 1s core hole does not
affect the excited electron, resulting in delocalization of excited electrons. Thus, they can
exclude the possibility that the piled up phosphorus atoms exist in the silica side of the interface.
Conclusion.
Resonant Auger electron spectroscopy was used to investigate the phosphorus atoms pile up at
the SiO2/Si interface, and determined the chemical state and matrix of phosphorus atoms
segregated at the SiO2/Si interface.
Since no resonant Auger spectra was observed from the segregated phosphorus, it is
concluded that the phosphorus atoms exist in the silicon side of the interface, probably at the
interstitial sites.