Experimental detection of time dependent temperature maps in power bipolar transistors G. Breglio * , P. Spirito Dipartimento di Ingegneria Elettronica e delle Telecomunicazioni, Universita ` di Napoli “Federico II”, Via Claudio 21, 80127 Napoli, Italy Abstract In this paper, we present a completely automated experimental set-up for transient temperature distribution measurements on the surface of the semiconductor power devices. Based on direct radiometric measurements, the proposed system is able to acquire, with high spatial (less than 10 mm) and quite good time (less than 10 ms) resolution, the temperature maps of devices biased in critical working conditions without damaging them. As an example of the measurement set-up capability, we show some measurements on the onset of the hot spot in two different kind of cellular BJTs. The obtained results confirm that the electro-thermal instability is mainly dependent on the lay-out of the device under test. Moreover, hot-spot growth is much faster than the heating time constant of the chip. Finally, we obtained the result that the hot-spot size is almost constant, independently from the chip area.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Transient temperature distribution; Time dependent temperature maps; Power bipolar transistors 1. Introduction Our experimental activity has been oriented to the study of the physical phenomena that limit the Safe Operating Area of power bipolar transistors in Forward Bias (FBSOA), with particular regard to the electro-thermal instabilities, named hot spots. Power transistor FBSOA is generally limited by thermal instabilities due to the positive temperature coefficient of the current gain, that could cause large non-homogenous current density over the device area [1]. The consequence is that the power rating decreases with respect to limits set by uniform thermal dissipation and, eventually, the device fails if such phenomenon is unstable [2]. To investigate the non-equilibrium distribution of the temperature on the chip surface and to detect the onset and the position of thermal instabilities, it is necessary to use an experimental system which is able to perform dynamic temperature mapping. Generally, the techniques by which electro-thermal instabilities in power devices have been measured present some limits: the measurements of variations of electrical parameters sensitive to temperature [3–5] give only global information over the chip; laser based systems that are able to measure the silicon refractive index, temperature depen- dent, [6] need devices without back metallization and complex elaboration of the acquired signal; emissivity mapping systems based on camera sensor arrays [7,8] have fixed number of acquired points and can only give static map measurements (frame rate 1/30 s). In order to overcome the previously explained limita- tions, we have proposed the direct non-contact thermal mapping described in the next section. The advantage of using an open experimental set-up and a self-made software which controls the different measurement steps and elabo- rates the acquired data is the greater flexibility. As a matter of fact, we are able to overcome a lot of errors that trouble these kind of measurements, such as: narcissus effect, back- ground correction, mechanical error position between the calibration and the measurement map acquisitions, increase of the radiometric S=N ratio. 2. Measurement set-up The system used to detect dynamic temperature maps is schematically described in Fig. 1; we use an radiometric microscope with a cooled InSb sensor. The microscope is equipped with an objective which gives at the focal plane (2.5 mm of working distance) an equivalent spot size of about 8 mm. The 2D scanning of the measured surface is achieved by means of an x – y motorized stage controlled via Microelectronics Journal 31 (2000) 735–739 Microelectronics Journal 0026-2692/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0026-2692(00)00052-5 www.elsevier.com/locate/mejo * Corresponding author. Tel.: +39-081-768-3128; fax: +39-081-593- 4448. E-mail address: breglio@unina.it (G. Breglio).