Presentation Information
[ED6-04]FIB-Induced TC Modulation on Small Superconducting Iridium Thin Films
*M. Amin Choghadi1, Yuki Mitsuya1, Hiroyuki Takahashi1 (1. The University of Tokyo (Japan))
Keywords:
Critical Temperature,Thin Film Superconductor,Transition Edge Sensor,Focused Ion Beam,Iridium
Thin film superconductors have been used in various fields of applied superconductivity for several decades due to their superior characteristics and improved electronic properties over the bulk superconductors. In addition, the critical temperature (TC) of thin films depends highly on the thickness of the film as well as the stress between the film and the substrate. Some superconducting thin films exhibit sharp transition in ultra-low temperatures where overall thermal noise could be neglected, making them ultra-sensitive to any thermal fluctuation caused by even a single light photon. This property makes them suitable for fabricating microcalorimeters, particularly transition edge sensors (TES).
In this work we studied the effect of focused ion beams (FIB) on the TC of small iridium thin films designed and fabricated to be used as TES. The film dimensions were 10x10 mm2 with 24 nm thickness. First, iridium was sputtered on a 20x20 mm2 silicon substrate, then photoresist JSR7790G was coated on top, and by direct laser writing, a pattern consisting of 6 squares of 10x10 mm2 was formed, and eventually after alkali development, the excess iridium outside the squares was removed by reactive ion etching (RIE). Later, niobium lines were also patterned with photolithography lift-off method to form electrodes and pads for wire-bonding to the measurement devices. At the end, they were separated from the chip by deep RIE (DRIE). This structure with a thin iridium film and niobium electrodes could be simply used as a TES when combined with SQUIDs or any other electronic readout circuit.
One of the samples was used as a reference, the other 5 were inserted in the FIB-SEM chamber. Eucentric height adjustment was done by FIB (80 pA) 100 mm away from each film. Acceleration voltage was kept at 30 kV in all processes. There was no direct exposure on the 2nd sample. The 3rd sample was scanned few times by a 10-pA beam. The 4th sample was scanned once and cut by the same beam to form two parallel strips of Ir films. Finally, the 5th and 6th samples each were scanned once with the same beam and then cut by an 80-pA beam to make two strips and three strips, respectively.
The TC of the reference sample was 324 mK. For the 2nd sample, despite no direct exposure, TC was 25 mK lower (299 mK). The 3rd sample, that was scanned few times, showed TC at 284 mk, and about 20 mk further shift down was observed for the 5th and 6th samples which were cut in 5 and 8 seconds, respectively (TC at 266 and 264 mK). However, the 4th sample, that was cut by the weaker beam but longer processing time of 60 seconds, showed no transition at all. This means the exposure time plays a significant role in this process. Further quantitative processes to control the FIB exposure and dosage is being carried out to analyze TC modulation on Ir films more precisely.
Acknowledgement
This work was partially supported by JST Moonshot R&D Grant Number JPMJMS2064-2.
In this work we studied the effect of focused ion beams (FIB) on the TC of small iridium thin films designed and fabricated to be used as TES. The film dimensions were 10x10 mm2 with 24 nm thickness. First, iridium was sputtered on a 20x20 mm2 silicon substrate, then photoresist JSR7790G was coated on top, and by direct laser writing, a pattern consisting of 6 squares of 10x10 mm2 was formed, and eventually after alkali development, the excess iridium outside the squares was removed by reactive ion etching (RIE). Later, niobium lines were also patterned with photolithography lift-off method to form electrodes and pads for wire-bonding to the measurement devices. At the end, they were separated from the chip by deep RIE (DRIE). This structure with a thin iridium film and niobium electrodes could be simply used as a TES when combined with SQUIDs or any other electronic readout circuit.
One of the samples was used as a reference, the other 5 were inserted in the FIB-SEM chamber. Eucentric height adjustment was done by FIB (80 pA) 100 mm away from each film. Acceleration voltage was kept at 30 kV in all processes. There was no direct exposure on the 2nd sample. The 3rd sample was scanned few times by a 10-pA beam. The 4th sample was scanned once and cut by the same beam to form two parallel strips of Ir films. Finally, the 5th and 6th samples each were scanned once with the same beam and then cut by an 80-pA beam to make two strips and three strips, respectively.
The TC of the reference sample was 324 mK. For the 2nd sample, despite no direct exposure, TC was 25 mK lower (299 mK). The 3rd sample, that was scanned few times, showed TC at 284 mk, and about 20 mk further shift down was observed for the 5th and 6th samples which were cut in 5 and 8 seconds, respectively (TC at 266 and 264 mK). However, the 4th sample, that was cut by the weaker beam but longer processing time of 60 seconds, showed no transition at all. This means the exposure time plays a significant role in this process. Further quantitative processes to control the FIB exposure and dosage is being carried out to analyze TC modulation on Ir films more precisely.
Acknowledgement
This work was partially supported by JST Moonshot R&D Grant Number JPMJMS2064-2.