Light Emission Spectral Analysis: The Connection Between the Electric Field
and the Spectrum
Daniel L. Barton*, Paiboon Tangyunyong, Jerry M. Soden, Edward I. Cole, Jr., and Christopher
L. Henderson, Sandia National Laboratories, Albuquerque, NM, USA
Rainer Danz, Peter Schaeffer, Carl Zeiss, Jena, Germany
Zbigniew Iwinski, Carl Zeiss Inc. Thornwood, NY, USA
The relationship between the electric fields associated with the various defect conditions
encountered on microelectronic devices and the associated light emission spectrum has interested
the failure analysis community for years. The intent is to use the light emission spectrum to help
identify the type of failure and limit the need to de-process the sample to identify the failure
mechanism, saving both time and money. The problem has been the availability of
spectrometers that are capable of measuring the spectral content of the very weak light emission
from semiconductor device defects.
In recent years, there have been several approaches published with sufficient sensitivity for
this application [1,2]. In this work, the spectrometer uses light emitted from a faulty region on
the device-under-test (DUT) that is collected by an objective lens, collimated, and passed
through a transmission grating as shown in Fig. 1. The grating splits the diffracted light into its
zero order (emission spot) and higher diffracted order (“plus-minus“) components. The
diffraction pattern is subsequently focused onto a cooled, CCD camera. Since the spectrometer
uses a blaze transmission grating, most of the emission light intensity is concentrated within the
(“plus“) first order. The spectrometer is calibrated by collecting a spectrum from a point-like,
halogen light source at a fixed color temperature (3200K) and correcting the spectrum to match
the appropriate blackbody spectrum. In its present state, the spectrometer has the same spectral
response of the Si CCD camera used for image collection (400 – 1100 nm). Future versions will
be available with extended response into the near-infrared (to 1400 nm).
In our previous work [3], we reviewed the basics of light emission to stress the need to
move to near-infrared detector technologies to better match the camera response to the emission
from silicon-based devices. Fig. 2 shows the emission spectra from both forward and reverse
biased pn junctions [4]. From these data, we argued that all spectra collected from silicon
devices would follow these two curves and would depend on the electric field governing the
emission process.
To demonstrate this relationship, we have included data collected from an n-channel
MOSFET under various bias conditions as an example. The spectra (Fig. 3) show that the short
wavelength emission (< 800 nm) is controlled by the length of the pinchoff region in the
transistor and the voltage across it. From basic device physics, we know that the drain to source
current in a MOSFET in saturation is related to the gate to source voltage by:
 W 
V  VTH  2
I DS  C ox 
 L  GS
 eff 
The current is controlled by VGS by modulating the width of the pinchoff region and, thus, the
electric field responsible for transporting carriers from the channel to the drain. Since the
channel will be pinched off at the point where the voltage between the source and drain is less
than VGS – VTH, the electric field becomes:
V  VGS  VTH 
E  DS
Leff  L'
*Daniel L. Barton, P.O. Box 5800, MS 1081, Sandia National Laboratories, Albuquerque, NM, 87185-1081
Tel: (505)844-7085, Fax: (505)844-2991, email: bartondl@sandia.gov
Where L’ is the length of the pinched-off channel and Leff is the effective length between the
source and drain diffusions. This relationship demonstrates that as VGS decreases, the voltage
across the pinchoff region increases while the length of the pinchoff region decreases; both
effects act to increase the electric field between the source and drain. This effect is clearly
demonstrated in Fig. 3 where the fraction of the light emitted at short wavelengths increases as
VGS reduces toward the threshold voltage.
In the full paper, we will focus on the device physics necessary to gain theoretical insight
into the relationship between the bias conditions and the associated electric field for various
defect-like conditions (forward and reverse biased junctions, MOSFET saturation, latchup, gate
oxide breakdown, etc.). The relationships will be verified by spectra collected from test samples
under various bias conditions. The paper will conclude with a series of examples that
demonstrate the utility of spectral analysis techniques for defect identification and the associated,
non-electric field related information contained in the spectra.
Key Words: light emission, spectral analysis and electric field.
1. M. Rasras, I. De Wolf, G. Groeseneken, H. E. Maes, S. Vanhaeverbeke, P. De Pauw, ISTFA 97, pp. 153 – 157.
2. J. M. Tao, W. K. Chim, D. S. H. Chan, J. C. H. Phang, and Y. Y. Liu, IRPS 96, pp. 360 – 365.
3. D. L. Barton, P. Tangyunyong, J. M. Soden, A. Y. Liang, F. J. Low, A. N. Zaplatin, K. Shivanandan, and G.
Donohoe, ISTFA 96, pp. 9 – 17.
4. A. G. Chynoweth and K. G. McKay, Phys. Rev., 102 (2), 369-376 (1956).

g
=ß
Carrier plate
Grating (plastics, refr. index n)
: prism angle (= blaze angle)
: deflection angle (= angle ß of the first diffraction order)
g: grating (grism) constant
Fig. 1. Principle of operation of a blaze transmission grating
*Daniel L. Barton, P.O. Box 5800, MS 1081, Sandia National Laboratories, Albuquerque, NM, 87185-1081
Tel: (505)844-7085, Fax: (505)844-2991, email: bartondl@sandia.gov
Intensity (arbitrary units)
50
Forward Bias
Reverse Bias
40
30
20
10
0
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
Wavelength (microns)
Fig. 2. Measured spectra from forward and reverse biased Si, pn junctions [4]
V GS = 1.5 V
Norm alized Intensity (arb. units)
100
1.75 V
2.0 V
2.5 V
80
3.125 V
3.75 V
60
4.25 V
5.0 V
40
1.2 x 30  m n-M OSFET
20
V DS = 5.0 V
V SB = 0 V
0
400
500
600
700
800
900
1000
W avelength (nm)
Fig. 3. Light emission spectra from an n-channel MOSFET at various gate voltages with a constant drain
voltage.
*Daniel L. Barton, P.O. Box 5800, MS 1081, Sandia National Laboratories, Albuquerque, NM, 87185-1081
Tel: (505)844-7085, Fax: (505)844-2991, email: bartondl@sandia.gov