Raman Thermometry Technique on Hi-Rel EEE Parts

As typical length in integrated circuits decreases thermal management is becoming a key factor determining the reliability and lifetime of such systems. Furthermore, micrometric and submicrometric objects in integrated circuits modify local heat transport and may lead to the formation of unintentional hot-spots. Thus, the accurate measurement of the local temperature with micrometric spatial resolution is crucial to confirm the suitable operation of highly integrated systems.

Diagram of Raman Scattering Processes

In their spectroscopic facet, Raman techniques are extensively used for the identification of chemical compounds as well as the characterization of the bond structure and solid-state structure of the investigated materials. On the other hand, Raman microscopy has proven to be a useful tool for determining the local temperature in heterogeneous systems. In the most common approach for this application, both the Stokes and Anti-Stokes peaks are registered. Their relative intensities (I_S and I_AS) are correlated to the local temperature of the illumination spot according to the following equation in the case of solid-state semiconductors, where the and are the frequencies of the Stokes and Anti-Stokes modes and is the frequency of the generated/annihilated phonon.

Practical Applications

Raman thermal imaging of integrated circuits or other microelectronic devices provides useful information about temperature gradients. The images below show the thermal maps acquired by Raman thermometry in the case of AlGaN/GaN HFET when applying 40V (V_SD) and I_SD = 25 mA. The Raman thermal maps are registered at different depths; in the GaN layer (boom left) and in the SiC layer (bottom right). The figure illustrates the formation of a hot spot under the region between the gain and the drain that affects in a lesser extent to the temperature distribution within the SiC underlayer.

IEEE TRANSACTION ON ELECTRON DEVICES 0018-9383

 Raman temperature map of GaN and SiC layers in an operative HFET systems

Although the technique is generally applied to steady-state temperature mapping, it is also eventually used to characterize the transient-state temperatures during pulse operation. Time-resolved Raman thermography can be used to measure temperature transients with nanosecond temporal resolution.

The figure shows the transient thermal response of a GaN-on-SiC HEMT measured using time-resolved Raman thermography. It is shown that there is a high-temperature peak between the gate an the drain which leads to the formation of steady state hot spot.

How it works

Raman thermometry is based in the inelastic scattering that a minority of IR phonons experience when they impinge onto a material with the suitable energy to interact with the lattice (phonons) quantized vibration levels. As it is illustrated in the figure during this an unstable “virtual state” is generated and a photon is immediately elastically or inelastically (Raman) scattered. In turns, within Raman scattering we can distinguish between two phenomena, i) Stokes Raman scattering when the emitted photon has lower energy than the incident beam and (phonon generation) ii) Anti-Stokes Raman scattering in which the generated photon absorbs energy from the system (phonon annihilation).

IEEE TRANSACTION ON ELECTRON DEVICES 1530-4388

Time-resolved Raman thermography on 125 micron HEMT located area