Nuclear Instruments and Methods in PhyGc\ Research A 370 (I9961 3X-10 NUCLEAR

INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH

Secrlon A

X-ray photon detection with multilayered Josephson junctions

C. Thomas*, S.R. Maglic, S.N. Song, M.P. Ulmer, J.B. Ketterson

Abstract

Superconducting tunnel junctions can be configured as high resolution X-ray spectrometers however, because they are thin, STJs suffer from inherent photoabsorption inefficiency. A common approach to enhance quantum efficiency is to

couple the junctions to superconducting absorbers and employ quasiparticle traps. An alternative approach is to use vertical

stacks of tunnel junctions as detectors. These multilayered superconducting tunnel junctions (MSTJs) need not be thin and may have additional advantages including increased signal to noise ratio. We report on the fabrication of multilayered tunnel

junctions and successful X-ray detection by an MSTJ.

1. Introduction

Superconducting tunnel junctions ( STJs) represent an alternative to present state of the art X-ray detectors. In

short, they potentially offer position resolutions and de- tection rates comparable to conventional semiconductor

based detector and energy resolutions that are competitive

with bolometric delectors. Several reports contirm that

significant progress toward realizing the potential of these detectors has been accomplished [l-3]. Some efforts focus upon overcoming the problem of inherent thinness (and specifically the resulting low X-ray absorption efficiency) of STJs. A good X-ray absorber which also acts as a quasiparticle trap such as tantalum is generally used to

increase the X-ray absorption and reduce quasiparticle escape in the detectors but even with this feature incorpo-

rated into an STJ, a poor quantum efficiencies of less than 15% for 6 keV photons still results [3].

Largely unexplored experimentally is the method of

increasing detector efficiency by vertically stacking the detecting junctions. Proposed as a detector as early as

1981, multilayered superconducting tunnel junctions utilize the same detection mechanism as their single layer coun-

terparts and possess geometrical advantages which directly affect the energy resolution such as reduced quasiparticle and phonon escape areas [4]. Another advantage of multilayers is the possibility of increased signal to noise ratio. In a single layer detector, those quasiparticles that live after tunneling are available for further tunneling resulting in an electrical gain and an increase in signal to noise ratio [5]. Recently it has been shown that this type of

*Corresponding author. Tel + I 708 491 8617, fax + I 708 491

9982, e-mail cdt@casbah.acns.nwu.edu.

multiple tunneling leads to additional statistical noise and therefore loss of energy resolution [6,7]. Kurakado has demonstrated the signal to noise ratio for a multilayered

detector coupled to an FET based preamp can be written as:

(SIN) = n”‘(SIN), ,

where (SIN), refers to the signal to noise ratio of a single layer junction and n is the number of layers in the stack. a

result that should not be surprising in light of the inherent- ly larger subgap resistance and smaller capacitance of a vertical stack 141. This scheme is particularly attractive because the resulting higher signal to noise ratio does not necessarily rely upon multiple tunneling. It should be noted that this claimed increase is probably not obtainable when using a SQUID based preamp such as those pre-

sented elsewhere at this conference. We discuss below one

technique for fabricating MSTJs. the progress made toward adapting them as X-ray detectors, and experimental dif- ficulties both expected and encountered.

2. Device fabrication

Vertical stack fabrication presents a challenge because of the potentially large thickness of the junctions. If one chooses to use reactive ion etching, an etch stop superior to photoresist may be required which will survive the pro- longed etching times. An alternative to etching, which we chose, is found in an all lift off technique. The lift off technique described below involves immersing the sub- strate in a photoresist solvent such as acetone; unwanted material is removed along with the photoresist.

Referring to Fig. I, the fabrication process begins with

Elsevier Science B.V.

SSDI 016%9002(95)01040-8

in P1gx Rex. A -370 (IY96) .3X-40 39

Photoresist Multilayered Junction

(e)

Fig. I. Multilayer junction processing diagram showing (a). (b)

htlse electrode definition; (b). (cl MSTJ definition. id) quartz

insulation layer, and (e) top wirin g layer. Note: the drmmgs are

not to scale.

lithographic definition of the base electrode in photoresist on a 0.50 inch square silicon (100) substrate. Niobium is then deposited over the photoresist and lifted off thus leaving the base electrode. Next, more photoresist is laid

down over the base electrode and the multilayered junction material is deposited. These layers are lifted off to form the

junctions. To reduce possible conduction paths along the edges of the junctions, the substrate is anodized placing about 100 A of oxide along the side of the stack while the

remainder of the substrate is protected with photoresist.

The lift off process continues through the next two steps: deposition of an insulating quartz layer, and finally defini- tion of the wiring layer.

3. Results and discussion

The most significant problem encountered in the fabrica- tion of multilayered tunnel junction is the requirement of

layer consistency, a necessity if each layer is to be properly biased and if each layer is expected to give a similar response to irradiation. The argument supporting increased

signal to noise ratio assumed a collective response of the entire stack but if the junction layers are inconsistent. then it can be expected that the response to non-equilibrium conditions will be determined by a particular layer rather than the stack. While reducing quasiparticle and phonon escape channels will have a beneficial effect on the energy resolution. these benefits may be offset by the loss of resolution due to the responses of the various layers.

The sample used in this study was I IO pm X I IO pm two layer Nb/AIOY junction with overall thickness of e approximately 2500 A fabricated at the Universiti de Salerno with the liftoff technique described above. The current-voltage relationship of this junction illustrates another problem encountered in attempting to detect X-

rays with a multilayered Josephson junction: The de-

termination of a proper bias point. Fig. 2 shows the I-V characteristic of the junction taken at 330 mK with an

applied field of I24 G. Note the presence of numerous Fiske steps. The steps represent bias points at which a

spontaneous DC current flows in the subgap region and hence are obstacles for X-ray detection. For the case of

single layer junctions one expects Fiske steps, or self resonant modes, to occur at evenly spaced voltages given

by:

for integer II. where L is the linear size of the junction and

c,] is the electromagnetic wave velocity in the junction.

Many more resonant modes exist in multilayers. For instance, in the case of a two layer junction both symmet-

ric and anti-symmetric resonant modes are present leading

to twice as many current steps [S]. The two layer junction was used to successfully detect

5.89 keV X-rays at the U.S. Naval Research Laboratory.

Fig. 2. Current-voltage relationship for 110 km X I10 pm two-

layer junctions at 330 mK with 124 G field. The lower curve shows details of the sub-gap region with a current scale of 100 nA/divisibn and voltage scale at 0.1 mV/division.

III. TUNNEL JUNCTIONS

Reapmee to a Single -Fe X-ray

2*oI.,,.,....

10 Tme20j&sec) 30 40

Fig. 3. Detected X-ray pulse from I 10 pm X 1 IO pm two-layer junction.

Fig. 3 shows a typical X-ray pulse. Qualitatively, the junction was measured to have unspectacular characteris-

tics, again Fig. 2 shows the current-voltage relationship. The ratio RdIR, was less than 400. The subgap leakage

current was greater than I FA and independent of tempera-

ture below I K. Because of the well established measure-

ment facilities that were used, low junction quality proba- bly accounts for a poor measured energy resolution

( 1.5 keV at 6 keV). It should be noted that this junction was not customized for X-ray detection. For instance the connecting leads were as wide as the junction allowing for quasiparticle diffusion away from the barrier, further

degrading the energy resolution. In summary, we have demonstrated that multilayered

superconducting tunnel junctions are capable of detecting X-rays. Unfortunately. due to the poor quality of the tested junction, it is very difficult to draw conclusions as to whether these advantages can be realized or not and admittedly, the predicted advantages of MSTJs have not

been demonstrated yet.

Acknowledgements

We would like to thank F. Scott Porter and Deborah Van Vechten at NRL for the use of their detecting electronics

and their helpful expertise. We also thank R. Monaco for

the donation two layer and other junctions. research is supported by NASA Graduate Student Researchers Pro-

gram Grant Number NGT 5 I30 1.

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