Transition Edge Sensor (QET) Device...
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Improvements in Tungsten / Aluminum Quasiparticle-Trap-Assisted Electrothermal Feedback Transition Edge Sensor (QET) Device Fabrication J.M. Kreikebaum †# , J. J. Yen † , B. A. Young *† , B. Cabrera † , P. L. Brink § , M. Cherry § and A. Tomada § †Department of Physics, Stanford University, *Department of Physics, Santa Clara University, §SLAC National Accelerator Laboratory We have developed 1 a wet-etch method that provides significantly improved step-coverage for sputtered 40 nm-thick W films deposited over patterned, energy-absorbing Al films ~500 nm-thick (or greater) in QET-style detectors similar to those used in the Cryogenic Dark Matter Search (CDMS) experiment. While our new method works well and provides improved film-to-film connectivity (as well as smoother sidewalls that should reduce quasiparticle energy loss in real devices), it cannot remove fundamental geometric limitations on Al film thickness process compatible with a given W-TES thickness. To mitigate this limitation, we most recently developed a robust QET device fabrication approach in which the superconducting films are deposited in reversed ("inverted") order (with Al over W- TES) compared to our traditional approach (W-TES over Al). We can now fabricate devices with significantly thicker Al energy collection films than ever before, with no additional loss of TES functionality and up to 100x better physical connection between W and Al films at the edges of each W-TES. The new method also requires fewer hours in the fabrication lab due to the way the W and Al films are wet-etched for our applications. Results from highly successful X-ray detection experiments performed with 'sister' devices made in the "inverted" vs. "non-inverted" film geometries are reported in a separate LTD-16 paper by J.J. Yen, et al. 3 LTD-16 (Grenoble, FRANCE) July 20-24, 2015 # Corresponding author: [email protected] Figure 1. New wet-etch process provides effective step-coverage for 40 nm-thick W patterned over 600 nm-thick Al steps. (a) QET X-ray test device with two W-TESs attached to a central Al energy-collection film. A surrounding ring of QETs is used to veto events that occur in the Si substrate. (b) W-TES overhang after final etching of a finished device. Initially, a horizontal W overhang is created when Al etchant undercuts the W-capped Al base layer of these devices. The overhang gets bent towards the wafer’s surface when the active W-TES film is later deposited. Ultimately, the undercut allows for a tenuous connection between the W-TES and the Al film below. (c-e) Etched 300 nm-thick sputtered Al films using KMG 16:1:1:2 NP Al etchant at room temperature: (c) An uninterrupted 300 sec. etch dip leaves rough sidewalls with pronounced cavities ~200-300 nm in diameter. (d) Results of 17 x 30 sec. etch dips interspersed with DI quench dips. (e) Results from a series of 59 x 10 sec. etch dips with DI quenches show that sidewall roughness is reduced even further. Cavities only ~10 nm in diameter remain. (f) QET device where a 40 nm-thick W film achieves continuous step coverage over patterned 600 nm-thick Al features. In our confocal sputtering system, step coverage quality depends on the step’s orientation and distance from the center of the wafer carrier. a b e d c f References and Further Reading 1. J. M. Kreikebaum, et al., J. Vac. Sci. Technol. B 33, 011203 (2015). 2. A. Jastram, et al., Nucl. Instrum. Meth. A772, 14 (2015). 3. J. J. Yen, et al., LTD-16, JLTP (these proceedings). 4. . B. Shank. et. al. AIP Advances 4, 117106 (2014). 5. J. J. Yen, et. al., App. Phys. Lett. 105, 163504 (2014). Conclusions • A new etch technique was shown to greatly improve step-coverage of patterned, thick Al base-layers by thin W-TESs. • A new "inverted" device geometry allows CDMS-style TES sensors to be used with a wide variety of Al energy collector films; the fabrication process is simpler than before and, once optimized, should provide significantly enhanced energy sensitivity of QETs. Figure 2. New “inverted” (Al over W) fabrication approach yields excellent devices with arbitrarily thick Al energy-collection films coupled to W-TESs. (a) Inverted QET device with 600 nm-think Al film deposited and etched after 40 nm-thick W-TESs were patterned on Si. (b) Residual Al left on the W-TES after patterning and etching the Al film. (c) Residual Al was easily removed with additional etching. (d) For this test the same masks were used as for our traditional “non-inverted” QET device geometry. This caused the observed etch-back of the thick Al side walls (~150% of the thickness of the film) relative to the W-TES edges. We can easily compensate for this in the future by editing one mask. Overall, we showed that our “inverted” QET fabrication method is robust, extendable to the use with arbitrarily thick Al energy-collection films, and yields excellent devices with W-TES T c unaffected by the processing changes compared to our well-established “non- inverted” devices. b a c d Figure 4. Comparing quasiparticle energy collection signals in opposing TESs of an "inverted" (Al over W) vs. "non-inverted" (W over Al) QET test device. Both devices used 250 μm x 250μm x 40 nm- thick W-TESs coupled to a central 500 μm-long x 250 μm-wide x 300 nm-thick Al film, a style similar to that in Fig. 1(a). Color in the 2-D energy plots indicates fraction of energy collected in the "guard-ring" used to veto substrate events. Deep blue points correspond to direct hits in the Al or W film and little energy loss to substrate. [Insets: Inverted (left) and non-inverted (right) film geometries; generic TES pulse for Al "direct-hit" events. Pulse shape differences (e.g. fall time) in these 'proof-of-principle' devices are largely due to the difference in W-T c of the two devices tested (65 mK, 94 mK). Results with significantly thicker Al films in the "inverted" geometry and further analysis of device performance as a function of Al quasiparticle collector film geometry can be found in Ref. 3. Figure 3. Set-up for characterizing "inverted" and "non-inverted" W/Al QET test devices operated at ~ 35 mK. An 55 Fe X-ray source strikes a NaCl reflector, producing 2.6 keV Cl, etc., X-rays that strike our test device after passing through a 1.6 mm diameter collimator hole covered with 12.5 μm of Kapton (to absorb Auger electrons).
Transcript of Transition Edge Sensor (QET) Device...
Improvements in Tungsten / Aluminum Quasiparticle-Trap-Assisted Electrothermal Feedback Transition Edge Sensor (QET) Device Fabrication
J.M. Kreikebaum†#, J. J. Yen†, B. A. Young*†, B. Cabrera†, P. L. Brink§, M. Cherry§ and A. Tomada§
†Department of Physics, Stanford University, *Department of Physics, Santa Clara University, §SLAC National Accelerator Laboratory
We have developed1 a wet-etch method that provides significantly improved step-coverage for sputtered 40 nm-thick W films deposited over patterned,energy-absorbing Al films ~500 nm-thick (or greater) in QET-style detectors similar to those used in the Cryogenic Dark Matter Search (CDMS) experiment. Whileour new method works well and provides improved film-to-film connectivity (as well as smoother sidewalls that should reduce quasiparticle energy loss in realdevices), it cannot remove fundamental geometric limitations on Al film thickness process compatible with a given W-TES thickness. To mitigate this limitation, wemost recently developed a robust QET device fabrication approach in which the superconducting films are deposited in reversed ("inverted") order (with Al over W-TES) compared to our traditional approach (W-TES over Al). We can now fabricate devices with significantly thicker Al energy collection films than ever before, withno additional loss of TES functionality and up to 100x better physical connection between W and Al films at the edges of each W-TES. The new method alsorequires fewer hours in the fabrication lab due to the way the W and Al films are wet-etched for our applications. Results from highly successful X-ray detectionexperiments performed with 'sister' devices made in the "inverted" vs. "non-inverted" film geometries are reported in a separate LTD-16 paper by J.J. Yen, et al.3
LTD-16 (Grenoble, FRANCE) July 20-24, 2015 # Corresponding author: [email protected]
Figure 1. New wet-etch process provides effective step-coverage for 40 nm-thick W patterned over 600 nm-thick Al steps. (a) QET X-ray test device with two W-TESs attached to a central Al energy-collection film. A surrounding ring of QETs is used to veto events that occur in the Si substrate. (b) W-TES overhang after final etching of a finished device. Initially, a horizontal W overhang is created when Al etchant undercuts the W-capped Al base layer of these devices. The overhang gets bent towards the wafer’s surface when the active W-TES film is later deposited. Ultimately, the undercut allows for a tenuous connection between the W-TES and the Al film below. (c-e) Etched 300 nm-thick sputtered Al films using KMG 16:1:1:2 NP Al etchant at room temperature: (c) An uninterrupted 300 sec. etch dip leaves rough sidewalls with pronounced cavities ~200-300 nm in diameter. (d) Results of 17 x 30 sec. etch dips interspersed with DI quench dips. (e) Results from a series of 59 x 10 sec. etch dips with DI quenches show that sidewall roughness is reduced even further. Cavities only ~10 nm in diameter remain. (f) QET device where a 40 nm-thick W film achieves continuous step coverage over patterned 600 nm-thick Al features. In our confocal sputtering system, step coverage quality depends on the step’s orientation and distance from the center of the wafer carrier.
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References and Further Reading
1. J. M. Kreikebaum, et al., J. Vac. Sci. Technol. B 33, 011203 (2015).2. A. Jastram, et al., Nucl. Instrum. Meth. A772, 14 (2015).3. J. J. Yen, et al., LTD-16, JLTP (these proceedings).4. . B. Shank. et. al. AIP Advances 4, 117106 (2014).5. J. J. Yen, et. al., App. Phys. Lett. 105, 163504 (2014).
Conclusions
• A new etch technique was shown to greatly improve step-coverage of patterned, thick Al base-layers by thin W-TESs.
• A new "inverted" device geometry allows CDMS-style TES sensors to be used with a wide variety of Al energy collector films; the fabrication process is simpler than before and, once optimized, should provide significantly enhanced energy sensitivity of QETs.
Figure 2. New “inverted” (Al over W) fabrication approach yields excellent devices with arbitrarily thick Al energy-collection films coupled to W-TESs. (a) Inverted QET device with 600 nm-think Al film deposited and etched after 40 nm-thick W-TESs were patterned on Si. (b) Residual Al left on the W-TES after patterning and etching the Al film. (c) Residual Al was easily removed with additional etching. (d) For this test the same masks were used as for our traditional “non-inverted” QET device geometry. This caused the observed etch-back of the thick Al side walls (~150% of the thickness of the film) relative to the W-TES edges. We can easily compensate for this in the future by editing one mask. Overall, we showed that our “inverted” QET fabrication method is robust, extendable to the use with arbitrarily thick Al energy-collection films, and yields excellent devices with W-TES Tc unaffected by the processing changes compared to our well-established “non-inverted” devices.
ba
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Figure 4. Comparing quasiparticle energy collection signals in opposing TESs of an "inverted" (Al over W) vs. "non-inverted" (W over Al) QET test device. Both devices used 250 µm x 250µm x 40 nm-thick W-TESs coupled to a central 500 µm-long x 250 µm-wide x 300 nm-thick Al film, a style similar to that in Fig. 1(a). Color in the 2-D energy plots indicates fraction of energy collected in the "guard-ring" used to veto substrate events. Deep blue points correspond to direct hits in the Al or W film and little energy loss to substrate. [Insets: Inverted (left) and non-inverted (right) film geometries; generic TES pulse for Al "direct-hit" events. Pulse shape differences (e.g. fall time) in these 'proof-of-principle' devices are largely due to the difference in W-Tc of the two devices tested (65 mK, 94 mK). Results with significantly thicker Al films in the "inverted" geometry and further analysis of device performance as a function of Al quasiparticle collector film geometry can be found in Ref. 3.
Figure 3. Set-up for characterizing"inverted" and "non-inverted" W/AlQET test devices operated at ~ 35 mK.An 55Fe X-ray source strikes a NaClreflector, producing 2.6 keV Cl, etc.,X-rays that strike our test device afterpassing through a 1.6 mm diametercollimator hole covered with 12.5 μmof Kapton (to absorb Auger electrons).
