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G12.9.1

High-resolution thermoreflectance microscopy S.A. Thorne, S.B. Ippolito, M.S. Ünlü, and B.B. Goldberg Departments of Physics and Electrical and Computer Engineering and Photonics Center, Boston University, Boston, MA 02215, U.S.A. ABSTRACT We present very high-resolution thermal microscopy using the technique of thermoreflectance, a non-contact measurement of the temperature in and around active semiconductor devices. By measuring the local change in reflectivity and comparing to the optical index versus temperature for the interface materials, thermoreflectance can determine the local temperature distribution. Thermoreflectance allows us to work at wavelengths much smaller than those used in typical blackbody imaging, and thus the spatial resolution is significantly improved over that of traditional thermal microscopy. In our experimental setup, we have a confocal scanning optical microscope with a tunable laser, where reflected light is detected by a silicon photodiode in a heterodyne scheme. The sample consists of a 600 nm wide poly-silicon wire embedded in silicon dioxide on top of a silicon substrate. Varying the amount and temporal shape of the current through the poly-silicon wire, we generate a controlled thermal profile to test the imaging capability. Our preliminary results indicate sub-micron thermal resolution. INTRODUCTION Thermal microscopy is used widely in the semiconductor industry to determine the temperature in and around active devices. The traditional method used in thermal microscopy is blackbody imaging at infrared wavelengths. However, because wavelengths in the blackbody thermal regime are typically several microns [1], diffraction limited microscopy provides resolution about ten times the size of today’s devices (which can be as small as 0.13 µm). The obvious solution to this problem is to decrease the wavelength to the order of magnitude of the structures one wishes to resolve. By heating the sample significantly above the ambient temperature, blackbody imaging can be conducted at reduced wavelengths. However, if one wishes to test the device under normal operating conditions, other methods must be used. A promising solution is offered by the technique of thermoreflectance. Thermoreflectance is based on the change of refractive index of materials as a function of temperature. Because of this, the intensity of reflected light, R, can be related to temperature, T, in the following manner [2]:

∆R(T (t ))  1 ∂R  = ∆T (t ) = Cth ∆T R  R ∂T 

(1)

Thus, relative temperature can be found by determining the change in reflectance. The advantage is that thermoreflectance allows one to work with light at wavelengths much smaller than that of blackbody imaging, so both spatial and thermal resolution can be

G12.9.2

significantly improved. Additionally, absolute temperature readings can be determined by finding the initial temperature using a thermocouple [3]. The thermoreflectance constant, Cth, is dependent on the material. For silicon, Cth is approximately 1.5x10-4, and for most me