A SENSITIVE DETECTION METHOD FOR LIFETIMES IN
SEMICONDUCTOR WAFERS
P. Bakker, M.J.J. Theunissen, PHILIPS RESEARCH LABS
P.J.T. Bruinsma, B. Heutenik, NIKHEF-K, POB 4395,1009 AJ Amsterdam
Presented at the 3rd symposium on Sensors and Actuators, October 1986, Twente
Univ. of Techn, Enschede, the Netherlands
Abstract
A sensitive method will be described to measure the quantity and nature of impurities and
doping levels in silicon wafers. By means of a pulsed source of light, photo-induced carriers are
generated. Information of the depth can be obtained by variation of the wavelength of the
source of light.
The carriers are detected using a microwave field. The relaxation time of the observed
phenomena provides information on the nature of the doping material and in some cases on the
quantity of the impurities.
Special compensation techniques increase the sensitivity to the order of 1011 atoms/cm3.
Results will be presented and a preliminary explanation will be given.
INTRODUCTION
Semiconductor materials are increasingly employed in the production of transistors,
integrated circuits, radiation detectors and miniature sensors and actuators.
Monocrystalline silicon wafers are frequently used with different doping ions and
various impurity levels and varying depths. An important quantity of semiconductor
materials is the lifetime of the charge carriers. An overdose of carriers will reduce
to an equilibrium value by recombination. A shortage of carriers will reduce by
thermal generation. The generation-recombination process for indirect
semiconductors like silicon takes place at imperfections in the crystal lattice or near
metallic impurities. The measurement of the carrier lifetimes therefore is an
effective tool to evaluate the degree of crystalline perfection of silicon wafers.
A large variety of detection methods have been developed in the semiconductor
technology to measure impurity levels and depth. From simple methods such as Van
der Pauw and Hall-effect measurements, with which the mobility and the
concentration of majority carriers can be obtained, to very complex methods such as
SIMS, RBS, neutron-activation analysis, electron-microscopy and Auger
measurements, with which more information about the structure can be obtained.
With most of these techniques the sample material will be damaged or modified, due
1
to pinning, contamination or special preparation of the samples. Some methods are
time consuming and some present difficulties when oxide layers are present.
A contactless method circumvents most of the mentioned disadvantages.
The principle of this method is based on the excitation of electron-hole pairs by
means of pulsed light. The recombination of these electron-hole pairs can be
detected by means of the interaction -absorption and reflection- with an externally
applied electro-magnetic field. The measurement is complicated due to the low
sensitivity of the method. The number of photo-induced carriers is usually small in
comparison with the carriers in thermal equilibrium, thus providing a large steadystate signal. Several contactless methods have been described in literature using
radio frequency absorption -'Q'- measurements [2,3,5], or measurements of the
reflected r.f. power [1]. A small change in the absorbed r.f. power must be detected,
which is obscured by mechanical instability, electrical noise, etc.
In this paper a sensitive instrument based on opto-microwave techniques is
described. A sharply focussed light pulse can be scanned across the surface of the
silicon substrate. The penetration depends on the wavelength the source of light [4].
The recombination of the charge carriers is detected with sensitive microwave
reflection measurements, based on compensation techniques. The instrument will
produce a time dependent oscilloscope signal. The relaxation time is thought to be
correlated to the lifetime of the photo-induced carriers.
Fig 1 Blockdiagram of the instrument
2
DESCRIPTION OF THE INSTRUMENT
A light emitting diode -LED- illuminates a small section of the silicon wafer to be
measured with a short light pulse. The photo-induced carriers are detected using the
reflected power of an incident microwave field. A combination of various microwave
components with compensation techniques reduces the detection signal in the
absence of the optical excitation. The instrument has been assembled to a large
extend from commercially available components. It can be subdivided into four
systems (fig 1).
Active probe
A special microstrip antenna with a small hole has been fabricated. The probe is
located approx. 1 mm above the surface of the wafer. Through this hole the light is
transmitted onto the surface of the wafer. A strong electric field exists under this
hole which couples the microwave power into and out of the wafer.
Fig 2 The active probe
Microwave system.
The microwave source, a standard signal generator, is coupled onto the probe through
a three-port circulator. A stub-tuner is necessary as matching element to
accommodate various wafers and frequencies. The reflected power can be obtained
from the third port of the circulator and is added to a small fraction of the source
signal in a power-combiner. By means of an attenuator and a phase-shifter the output
of the power-combiner can be varied to compensate for component imperfections and
steady-state sample reflections. This provision increases the sensitivity of the
instrument by two orders of magnitude because imperfections in the various
microwave components may otherwise generate an offset signal of approx. 1% of the
incident microwave power.
Detector
The output from the power-combiner now is the reflected power originating from the
carriers generated in the sample volume under the probe. After amplification and
3
linear detection, the signal can be displayed using a standard oscilloscope. The scope
picture now displays the number of carriers as a function of time.
Manipulator
A standard X-Y table can be used. Provisions should be made to shield external light
from the wafer.
EXPERIMENTAL RESULTS
A number of measurements have been made, mainly using P- and N-type mono and
polycrystalline silicon. The wafers have been implanted with P+, B+, Au+, As+ under
various thermal treatments (annealing) and with or without oxide isolation layers. The
implanted dose varied between 5.1011 to 1015 ions/cm2 with implantation depths of
0.05 to 1 µm (implantation energy from 20 to 400 keV) [6]. The wavelength of the
lightsource could be selected from 480 - 950 nm with intensities between 0.6-12
mW/sr, by using different types of LED's. In fig 3 the relation between wavelength
and penetration depth is given.
Fig 3. Penetration depth of the light as a function of wavelength
The microwave frequency was 2.5 GHz and the power varied between 10 and 100 mW.
The penetration depth of the microwave field is 3-4 mm for resistivities between 1020 Ohm.cm.
In fig 4 the decay curves of different doping atoms in N-type annealed silicon under
the same conditions are displayed. The observed difference in lifetime is large, as is
expected from the theory. The recombination process is strongly dependent on the
energy band structure resulting from the doping material.
4
Fig 4. The decay curves for different doping ions in annealed wafers.
Fig 5. The decay curves for Boron implanted in P- and N- type silicon.
The effect of the implanted level using P- and N-type silicon is displayed in figure 5.
A tentative explanation of this may be:
- P-type silicon: Normally the lifetime is determined by the recombination at surface
states and bulk states. With an increasing dose of P+ in P-type crystal the minority
carriers, the electrons, are pushed away from the surface by the potential
difference created by the higher boron concentration. This will result in the
elimination of the surface recombination, therefore only the bulk recombination
remains.
- N-type silicon: The recombination process in boron doped silicon takes place in the
depletion area of the P+N junction and is not influenced much by the increased boron
dose.
5
CONCLUSIONS
A sensitive method has been developed to measure the photo-induced carriers in
silicon wafers, with the following tentative conclusions and features:
- The instrument is simple and can easily be adapted to monitor the various stages
during the production process of semiconductor components.
- Lifetimes corresponding to a concentration of 1011 gold atoms/cm3 can be detected.
- Information on the substrate, the doping material, the depth of penetration, and
annealing of various wafers can be obtained.
- By eliminating the surface recombination by high implanted doses, pure bulk
recombination can be measured.
- For wafers with similar surface passivation there exists a direct correlation
between the lifetime of the photocarriers and the size of the depletion free zone DFZ- [7].
- Information can be obtained without special preparation such as making contacts,
which can change the properties of the material.
However more research is necessary in order to fully understand the phenomena.
REFERENCES
[1] Tatsuya Yamazaki, Yoh-ichiro Ogita, et al.,'Contactless measurement of short
carrier lifetime in heat-treated N-type silicon', Jap. Journ. of Appl. Physics, Vol 23,
no 3. March 1984, pp 322-325.
[2] Verkuil R.L. et al., 'A contactless method for high sensitivity measurement of PN
junction leakage', IBM Res. Dev. Vol. 24, no3, 1980.
[3] Tiedje T. et al., 'An RF bridge technique for contactless measurements of the
carrier lifetime in silicon wafers', EXXON, J.Appl. Phys (5) May 1983.
[4] Regtien P.P.L., Wolffenbuttel R.F., 'A novel solid-state color sensor suitable for
robotic applications', Proc. 5th Int. Conf. on Robotic Vision and Sensory Controls,
Amsterdam, Oct 1985, p.259-266.
[5] Gregorkiewicz T., 'Microwave contactless method of conductivity measurement in
the studies of ion implantation effect', Radiation Effects 1980 vol. 52.
[6] Hofker W.K., 'Implantation of Boron in Silicon', Thesis, Univ. of Amsterdam, 1975
[7] Borrego J.M. et al, 'Characterization of the DFZ in silicon wafers by a nondestructive technique', MRS Spring meeting 1986, Palo Alto.
6