Download Proton implanted silicon wafers investigated by electron beam

Proton implanted silicon wafers investigated by
electron beam induced current measurements
S. Kirnstötter 1,2, P. Hadley1, W. Schustereder², J. Laven²,³ H. J. Schulze³
1
Institute of Solid State Physics, Technical University Graz, Austria
² Infineon Technologies Austria AG, Siemensstraße 2, 9500 Villach, Austria
3 Chair of Electronic Devices, University Erlangen-Nürnberg, Cauerstraße 6, 91058 Erlangen, Germany
Micromanipulator Probe
Station
Abstract
Electron Beam Induced Current (EBIC) is an analysis method used in a Scanning Electron
Microscope (SEM) to investigate buried junctions or defects in semiconductors. [1-3] During an EBIC
measurement, the electron beam enters a semiconductor and generates electron-hole pairs. If the
charge carriers diffuse into a region where there is a built-in electric field, such as a pn junction or a
Schottky contact, charge separation will occur and a current will flow.
We have used EBIC to investigate proton implanted silicon wafers with implantation doses from
1×1013 p+/cm² to 1×1015 p+/cm² and with implantation energies from 500 keV to 5 MeV. [4] The
implantation introduces vacancies, silicon interstitials, and hydrogen into the crystal. The sample is
then annealed in the temperature range from 350-550°C and defect complexes form. The microscopic
structure of these defect complexes is not completely understood. There is a class of oxygen-vacancy
defect complexes called thermal donors that are known to act as donors in silicon. Besides EBIC we
used voltage contrast imaging, Schottky contacts of tungsten tips, and spreading resistance profiling
(SRP) to investigate the properties of these wafers.
Voltage Contrast Imaging
Voltage contrast imaging in a scanning electron microscope (SEM) is a technique for
studying potentials and potential distributions on a sample. The principle of voltage
contrast is that fewer secondary electrons at a conducting surface escape when the metal is
positively biased. Here we used it for visualizing a pn junction across the surface of a doped
silicon wafer. We put the wafer in a sample holder, where each side of the pn junction was
contacted to one of the electrodes, and applied a voltage of +10 V to -60 V across the sample.
Fig. 2: Voltage contrast images, with (a) +10V, (b) 0V, (c) -10V, (d) -20V, (e) -60V applied over
the sample
Fig. 1: Micromanipulator installed in a FEI Quanta 200 (photograph taken by
Kleindiek)
1D- and 2D-EBIC
a)
For the EBIC measurements, silicon wafers that have
been implanted with hydrogen are diced and placed so
that the edge of the die is visible in the SEM. The
electron beam is then scanned from the front-side to
the back-side of the wafer and the current is recorded.
Typical one and two dimensional EBIC measurements
of the wafer are shown in Fig. 4 (a) are shown in Fig. 4
(b) and (c). The currents collected at the front and the b)
back of the sample have equal magnitudes but opposite
signs. The magnitude of the EBIC current can be much
larger than the beam current since the high energy
electrons in the beam can each generate many electronhole pairs. A peak in the EBIC signal indicates the
position of a pn-junction.
c)
Fig. 4: (a) SEM image, (b) 1dimensional and (c) 2
dimensional EBIC image of the cross section of proton
implanted silicon wafers arranged under each other
Schottky contacts
Fig. 3: Voltage drop measurement with a bias voltage of 20 V
Spreading Resistance Profiling
To better visualize the defects caused by the proton implantation, a tungsten tip was placed on the silicon to
form a Schottky contact. The EBIC signal was then measured as the electron beam was scanned in a two
dimensional region around the Schottky contact. The Schottky contact separates electron-hole pairs that
diffuse to it from the surrounding silicon. Since electron-hole pairs recombine at defects, the EBIC signal is
decreased if the charge carriers have to diffuse past defects on their way to the Schottky contact.
Determination of the position of the pn junction
SRP is a technique used to analyze resistivity vs. depth in semiconductors. The carrier
concentration can be inferred from the resistivity profile provided by SRP. In general two
tips are placed about 20 microns apart on a bevelled surface. The tips are pressed to the
surface and so when a voltage is applied the contact from the tip to the silicon is nearly
ohmic, and so the local resistivity can be determined.
Fig. 5: (left) EBIC measurement of a proton implanted wafer from the front side of the wafer. (right)
The spreading resistance profile measured in the same region.
[1] H. J. Leamy, J. Appl. Phys. 53, R51–R80, (1982)
[2] R. F. Egerton, in Physical principles of electron microscopy (Springer, Berlin, 2005)
[3] O. Breitenstein, J. Bauer, M. Kittler, T. Arguirov, W. Seifert, EBIC and luminescence
studies of defects in solarcells, Scanning 30, 331 (2008)
[4] J. Laven, Phys. Status Solidi 8, 697-700 (2011)
[5] P. Pichler, in Intrinsic Point Defects, Impurities, and Their Diffusion in Silicon
(Springer, Wien, 2004)
Fig. 6: A series of two dimensional EBIC images of a tungsten tip on different positions on the cross section of
the silicon wafer.
Stefan Kirnstötter [email protected]
www.if.tugraz.at