Logo PTB
symbolic picture: "magazines"

Thermometer for nanocircuits

Magnetic tunnel junctions enable absolute, time-resolved temperature measurements of nanocircuits

PTB-News 1.2018
12.01.2018
Especially interesting for

manufacturers of magnetic sensors

thermal investigations on nanocircuits

PTB has developed a method that can be used to manage the temperature of nanocircuits. The method is based on magnetic tunnel junctions and enables quantitative temperature measurements with a time resolution smaller than a nanosecond. Hereby, the tunnel junction acts as a calibrated thermometer by exploiting the temperature dependence of the tunnel resistance. The principle has been demonstrated at PTB for the time-resolved measurement of laser-induced increases in temperature; this principle can be used in numerous nanocircuits.

Principle of absolute, time-resolved temperature measurement The optically induced increase in temperature is read out with subnanosecond time resolution by means of a magnetic tunnel junction which is located inside a nanostructure.

Given the rate at which technological advances have been progressing over the past few decades, many components containing nanocircuits have been created. Due to ever smaller dimensions and the associated high current densities, it is becoming increasingly important to monitor the temperature evolution in these components. Insufficient heat dissipation can modify or even destroy nanocircuits.

A procedure has been developed at PTB to measure the absolute temperature in nanostructures with a time resolution in the subnanosecond range. Hereby, the nanostructure is integrated into a magnetic tunnel junction. This junction is composed of two magnetic layers that are separated by a thin oxide layer. The tunnel resistance strongly depends on whether these layers are magnetized parallel or antiparallel to each other. A change in the magnetization direction can modify the resistance by more than 100 %. Due to complex physical effects, this change in resistance is temperaturedependent and decreases with increasing temperatures. The tunnel junction can therefore act as a fast thermometer by reading out its electric resistance.

To demonstrate this principle, a tunnel junction was integrated into a series of nanolayers and the temperature-induced change in the tunnel resistance was then calibrated first. For this purpose, a known temperature was adjusted by means of an electric heater. This calibration allowed the average change in the tunnel resistance, which occurred due to the heating of the nanolayers with a short laser pulse, to be converted into a temperature. In a layer lying more than 100 nm below the sample surface, a pulse train from a femtosecond laser with a pulse energy of 5 nJ and a repetition rate of 76 MHz leads to a mean temperature increase of 80 K. In addition, a very fast readout of the tunnel junction allowed the absolute temperature evolution to be determined with time resolution. This showed that in addition to the mean temperature increase mentioned, each laser pulse also caused a temperature peak. Approximately 4 ns after the laser pulse’s impact on the sample surface, this fast temporal temperature increase attains its maximum of 2 K at the tunnel junction.

Such absolute, time-resolved temperature measurements in nanostructures that lie several 100 nm below the surface of a component could, in the future, be used to validate heat transport simulations, which makes them an important method to manage the temperature of nanocircuits.

Contact

Mark Bieler
Department 2.5
Semiconductor Physics and Magnetism
Phone: +49 531 592-2540
E-mail: mark.bieler(at)ptb.de

Scientific publication

H. F. Yang, X. K. Hu, N. Liebing, T. Böhnert, J. D. Costa, M. Tarequzzaman, R. Ferreira, S. Sievers, M. Bieler, H. W. Schumacher: Electrical measurement of absolute temperature and temperature transients in a buried nanostructure under ultrafast optical heating. Appl. Phys. Lett. 110, 232403 (2017)