The Light Emission Spectral Analysis: The Connection between the

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: [email protected]

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: [email protected]

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: [email protected]