Interferences of Peltier thermal waves produced in ohmic contacts upon integrated circuits

We have experimentally detected a Peltier effect produced by the current flow 111 ohmic contacts upon integrated circuits. These contacts are small size heat sources which are very useful for investigating integrated circuits. We have characterised their thermal properties with a high resolution interferometric laser probe by measuring absolute thermal expansions. Subpicometric resolution has allowed original imaging of semiconductor components : we have mapped the thermal waves created at ohmic contacts under normal operating conditions. Thermoelectrical effects, like Joule, Peltier, Thomson and Seebeck occur in semiconductor devices. These effects have been studied experimentally before [I, 21. We have developed an appropriate instrumentation for the thermal characterisation of semiconductor devices [3, 41 at a microelectronic scale and have undertaken systematic studies of thermoelectric effects in integrated circuits. In the present work we show metal semiconductor contacts to be the source of a Peltier thermal wave. Such original heat sources can be very useful for thermal probing inside semiconductor devices. In integrated circuits, links between the power supply and semiconductor integrated components are realised through ohmic contacts. An ohmic contact is defined as a metal-semiconductor connection that has negligible resistance relative to the bulk or spreading resistance of the semiconductor. We have investigated the thermal effects of current in Al-Si contacts and have observed, besides the Joule heating (I2 dependent), an important Peltier heating process (I dependent) located at the metal-semiconductor interface. The Peltier heat source in ohmic contacts is a very interesting one in terms of thermal wave probing : it is of very small size (located within a pm3 volume), it is very easy to modulate and it is part of all components in integrated circuits. It is worth to note that for sine wave current excitation, there is no average temperature change due to the Peltier effect. Furthermore, there are no electron-hole pairs formed like in conventional photothermal analysis of semiconductors. We have followed the propagation amplitude and phase of low frequency surface waves in a doped semiconductor, resulting from thermal wave propagation. We have also studied interferences from two thermal waves originated at ohmic contacts linked together by a semiconductor resistance where a sine wave current is flown. The Peltier heat sources at the two ohmic contacts, have opposite phases and the associated thermal waves will interfere destructively at mid distance between the sources. We also show an example of Peltier thermal wave imaging of a semiconductor component. The image is obtained by mapping the thermal wave amplitude propagating from the ohmic contacts of the functioning component. This imaging technique shows to be interesting for semiconductor homogeneity and reliability testing. We have studied these surface waves with a high resolution stabilised Michelson interferometer we have built [3]. We have observed with this probe, absolute surface normal displacements amplitudes Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994747 C7-196 JOURNAL DE PHYSIQUE IV of a few 10-15 m upon silicon circuits for sine wave current excitation. In our approach, the component has its normal electrical excitation and the resulting thermal behaviour is detected in synchronism with the electrical source. 2.HIGH RESOLUTION INTERFEROMETRIC LASER PROBE We have built and developed a Michelson type interferometer designed for the analysis of semiconductor components [I]. Three main requirements have to be met: to visualise the point under test upon the device with micrometric resolution, to cope with the different reflectances of the materials involved in electronic components (Al, Si, ...) and finally to detect the interferometer response in synchronism with the circuit. Figure 1 shows a schematic view of the interferometric laser probe, a homodyne stabilised Michelson interferometer. The laser is a stabilised polarised HeNe laser . The beam splitting element of the interferometer is a polarising prism. By rotating a half wave plate (V2) it is possible to partition the intensity of the reference arm to that of the probe arm in order to equalise the reflected intensities and to obtain high contrast interference fringes. In the reference and probe arm a quarter wave plate (U4) is inserted. The linear polarisation of the incoming light is rotated by 90" when coming out, as it has passed twice the plate. This allows to reflect all the intensity of the returning beams to the photodetector and the polarising prism acts as an optical insulator. The two beams have orthogonal linear polarisations. To obtain interferences, both polarisations are projected at 45' upon a same axis by passing through a polarising beam splitter (prism) before the photodetector. A microscope objective focuses the probe beam upon the surface of the component under test. The phase of the reflected beam is modulated by the surface normal displacements. The sample is mounted on a 3-D micrometric translation stage. The laser impact upon the sample can be viewed on a CCD camera by moving the mirror in front of the reflected probe beam and by reducing the laser beam intensity with an attenuator. The lateral am resolution is 1 pm. For small surface displacements (< h/20, h = 6328 A ) , the sample interferometer is actively stabilised. FYL The signal is measured in synchronism with the anslation excitation signal and recorded by a digital oscilloscope or a lock-in amplifier. Absolute values of the surface displacement are : &; ; obtained from comparison of the photodetected signal amplitude with the fringe amplitude obtained by moving the piezomirror i n the jigure 1: the inter3cerometer reference arm.