121 International Symposium on Space Terahertz Technology

Characterization of NbN Thin Films produced onQuartz Substrates using MgO Seed Layers forphonon cooled Hot-Electron Bolometer Devices

M. Frommberger, P. Sabon, M. Schicke and K.F. Schuster

Institut de Radioastronomie Millimetrique300, rue de la piscine, Domaine Universitaire

38406 St. Martin d'Ilêres, Francefrommber@iram.fr

INTRODUCTION

NbN phonon-cooled hot-electron bolometer (HEB) mixer devices have turnedout to be very useful for space terahertz applications. High intermediatefrequencies in the range of several GHz are demanded for those applications.The intermediate frequency bandwidth of phonon-cooled HEBs can beincreased by reduced film thickness [1]

Thin NbN films of about 3 nm produced on fused quartz substrates arenot superconducting at liquid helium temperature [2] To improve the filmproperties, one can use crystalline substrates as MgO, silicon or sapphire butthose high-E materials pose particular problems in terms of electromagneticimpedance matching and are fairly expensive.

We present an investigation on NbN thin films deposited on MgO seedlayers on r- fused and crystal quartz substrates. This MgO layer improvedsignificantly the superconducting properties of our NbN films, sputtered atambient temperature. Atomic force microscopy and x-ray diffraction were usedto examine the surface properties and crystallinity of our thin films.

We present preliminary results using small angle x-ray reflectornetry todetermine the thickness of our sputtered NbN/Mg0 bilayers. The results arecompared to ellipsometry and anodization measurements.

Phonon-cooled NbN/Mg0 HEBs have been produced with a newprocess, using a negative resist for the definition of the width of themicrobridge by electron beam lithography.

First results, obtained at a frequency of 800 GHz showed an IF-gainbandwidth of 1.8 GHz and a receiver noise temperature of 670 K (corrected forbeam splitter loss).

• " INbN20 hk141.135 11147.865 20070.016 22084.553 3119.276 222

MgO20 hkl43.167 11150.277 20073.841 22089.565 311

50• • • • • •

42 44 46 4840

NbN 11-1 NbN 002

12(h International Symposium on Space Terahertz Technology

I. CRYSTALLINE PROPERTIES OF THE BILAYER

The influence of an MgO seed layer on fused quartz and crystal quartz on thecrystalline properties of the NbN thin films has been investigated by x-raydiffraction measurements.

NbN on MgO seed layer on fused quartz substrate

40 45 50 55 60 65 70 75 80 85 90

2e

Fig.1.1.: 9/20 scan of an NbN/Mg0 bilayer on a fused quartz substrate

NbN on MgO buffer on crystal quartz substrate

20

Fig.1.2.: 0/20 scan of an NbN/Mg0 bilayer on a crystal quartz substrate.

. sourcedetector

20

sample

161

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The pictures show x-ray diffraction data (Co IC 1.789 A) of an approximately5 nm thick NbN film on a 45 nm MgO seed layer on a fused (fig.1.1) andcrystal quartz substrate (fig:1.2). On the fused quartz substrate the films aregrowing amorphously. The crystalline substrate allows the MgO and the NbNlayer to grow mainly in (002) orientation. From the full width half maximumvalue of the (002) peak of NbN (fig.1.2) we can estimate a NbN grain size of

nm.

omega scan of the (002) NbN and MgO peak

0 r • I i t . • • I • • • i •8 10 12 14 16 18 20 22 . 24 26 28 30 32

Fig.1.3.: Omega scan on the (002) peak of NbN and MgO

If one fixes the 0/20 geometry for a given peak and turns the samplearound the angle co (= 0), one obtains a so called omega scan. The omega scanis a measure of the desorientation of the crystallites in the deposited layers.Figure 1.3 shows a scan, carried out on the NbN and MgO (002) peaks whichconfirms that the NbN growth is following the MgO seed layer.

34 36 38 40

a0

P O W

2

Fig.1.4.: Kiessig fringes of the NbN/Mg0 bilayer on a crystal quartz substrate.

12 International Symposium on Space Terahertz Technology

x-ray reflectornetry, NbN/Mg0 bilayer on crystalline quartz substrate

As the equipment at DRFMC (Department of fundamental research oncondensed matter) allowed also small angle measurements, we observed theKiessig fringes of an NbN/114g0 bilayer on crystal quartz (fig.1.4). The smalloscillations correspond to the thicker MgO film and give a thickness of about46 nm. The large oscillation is due to the ultrathin NbN layer and results in athickness of 4 nm ±1. Surface, interface and substrate roughnesses determinedfrom the x-ray small angle measurements could be estimated as 0.5 nm. This isabout twice The RMS values obtained by AFM measurements carried outindividually on a crystal sivartz substrate, an MgO layer on crystal quartz andan NbN/Mg0 bilayer on crystal quartz.

II. THICKNESS CONTROL BY ELLIPSOMETRY AND ANODIZATION

The MgO layer thickness obtained by the reflectivity measurement wasverified by ellipsometry of a film sputtered on a 2" Si wafer with the samedeposition parameters. We obtained an average seed layer thickness of 42 nmin good agreement with the x-ray results (fig.2.1).

sometry. Mapping of the hole 2".Fig. 2. MgSi wafer.

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Etliptameter 310owe

0 0 lo 0 20 0 30 00 I

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12th International Symposium on Space Terahertz Technology

The 'thickness of our 'NbN layers is always measured directly afterdeposition by anodization :[4].!. The graph in figure 2.2 shows an .anodizationcurve of a 5 mn ±1 NbN :film. The thickness of the superconducting layerdetermines the resistivity of the film in the normal conducting *ate and so thelength and the width of the final NbN :tnicrobriiige of' a HEEL.

VOL IAGL,. stif)

NbN film thickness control by anodization. The plot shows the curveof a —.5 nm MN thin

163

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450 -

400-

c9 350

›, 300.> -IS 250 -17)

200 -

15.0 -

NbN on fused quartz

•

NbN/Mg0 on fused quartzA

aa Nb1\18/1g0 on crystal quartz -

a

12 International Symposium on Space Terahertz Technology

III. SUPERCONDUCTING PROPERTIES OF NBN/MGO BILAYERS

To examine the superconducting properties of our NbN thin films, sampleshave been fabricated on quartz substrates. We determined the electrical proper-ties by four point measurements on 1 mm x 10 mm samples which werecarried out in a dip-stick setup. The film thicknesses were controlled byanodization. Film resistivity, the critical temperature Tc and their dependenceon film thickness were investigated.

The MgO seed layer is substantially increasing the quality of our thinNbN films [31 The resistivity (shown in figure 3.1) is much lower ascompared to NbN films on fused quartz and shows only a weak dependence onthe film thickness. This allows easier impedance matching of the HEB to theplanar antenna.

resistivity of NbN thin films on different substrates

2 3 4 5 6

thickness (nm)

Fig.3.1.: Resistivity vs. 1VbN film thickness of NbN/Mg0 bilayers compared toNbN on fused quartz substrates.

We observed a strong increase of the critical temperature T ascompared to NbN on fused quartz substrates (figure 3.2). The increased Tcallows working with thinner NbN films, which should result in a larger IF-gainbandwidth.

100

I . 1 ' I • 1 1 r I • I' 1 • • • I • • • I •

• , A ...... A

s'

WT

15-

14:

EZ 13-

a) 12

E°

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Ta• 8 -

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6 -

5 -

4

.•.

*-

-a - NbN/Mg0 on crystal quartz- - - NbN/1\11g0 on fused quartz -

- - -a- - - NbN on fused quartz [1]

12th International Symposium on Space Terahertz Technology

critical temperature Tc of NbN on different substrates

Fig.3.2.: Critical temperature vs. film thickness of NblV/Mg0 bilayerscompared to NbN on fused quartz substrates.

The thickness of the MgO seed layer was 42 nrn for the tested films,except for the two encircled points shown in fig.3.2. The latter weremeasurements on NbN films using a 15 nm thick MgO seed layer, showingthat this thickness already provides a better film quality of NbN [5] Thespread in the data is mostly due to the uncertainty of the NbN layer thickness[4].

Iv. DEVICE FABRICATION

In order to investigate the influence of an MgO seed layer on the properties ofa phonon-cooled NbN HEB we first produced devices on fused quartzsubstrates. The HEBs have been produced with a new process, using anegative resist for the definition of the width of the microbridge by electronbeam lithography.

After an overall deposition of a 42 nrn MgO seed layer, the NbN film isdeposited in situ. Antenna and filter structures are defined byphotolithography. To define the lateral dimensions of the microbridge we use

0.6 0.7 0.8 0.9 1 3 4

direct injection measurement

M40.1.78 GI-1z

12 International Symposium on Space Terahertz Technology

Ebearn lithography. In a first step we fix the microbridge length by definingthe Au contact pads by a single INVINIA layer lift-off process.

In a second Ebeam step we generate a thin resist line using a negativeEbeam resist (Microsresist ma-N 2405) {6,71 This resist line is used as an etchmask. We remove the surrounding NbN by reactive ion etching (CF 4, 02). Theresist etch mask is removed in aceton. Finally the substrate is diced and theindividual devices are characterized by dc-measurements.

V. CHARACTERIZATION OF AN NBN/MGO HEBON A FUSED QUARTZ SUBSTRATE

We determined the noise temperature of our NbN/114g0 HEB by a Y-factormeasurement. The improved film quality (as compared to NbN on fused quartz[2]) increased the critical current of our standard 5 run NbN thin film andmade higher pumping power necessary. With a beam splitter of 60%transmission at 300 K the best receiver noise temperatures were of the order of670 K (corrected for beamsplitter loss) at 798 Gliz and 1 GHz T.

The IF-gain bandwidth was measured in a heterodyne mixingexperiment with two local oscillators (L0), using 1,01 to pump the device andL02 as a signal. The variation of the IF was obtained by changing thefrequency of L02 and observing the output signal with a frequency analyzer.The NbN/Mg0 HEB showed a -3 dB IF-gain bandwidth of 1.8 GHz, as shownin figure 5.1.

frequency pHz)

Fig.5.1.: Pumped 1(V)-curve and conversion curves of an NbN/Mg0 HEB; IFbandwidth determined by double local oscillator injection.

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VI. CONCLUSION AND OUTLOOK

An MgO seed layer on fused quartz, or crystal quartz substrates, provides abetter crystal growth of our NbN thin films, resulting in lower normal stateresistivity and higher critical temperature.

High frequency measurements on phonon cooled NbN HEBs fabricatedon fused quartz substrates using an MgO seed layer showed a better noisetemperature and a higher IF-gain bandwidth than comparable devices on fusedquartz [21

We are currently investigating the influence of the MgO seed layer onthe acoustic matching between the bolometer and the substrate.

NbN/Mg0 HEBs were produced on crystal quartz substrates showingcritical currents of about 150 AA and a 're of 10 K. The high frequencyproperties of such devices are currently investigated.

ACKNOWLEDGEMENT

The authors like to thank V. Fasquelle (LEPMI, INPG Grenoble) for the AFMobservations and I. Schuster (DRFMC, CEA Grenoble) for helpful discussionsconcerning the x-ray measurements.

REFERENCES

[1] A. V. Sergeev, M. Yu. Reizer, Photoresponse mechanisms of thin superconducting filmsand superconducting detectors, Int. J. of Mod. Phys. B 10 (6), 1996[2] C. Misch, Herstellung und Charakterisierung supraleitender Hot-Electron-Bolometer alsFrequenzmisch�r; für Submillimeterwellen, Ph.D. thesis, IRAM, University of Cologne,January 2000[3] M. Frommberger, P. Sabon, M. Schicke, F. Mattiocco and K. F. Schuster, Fabrication andCharacterization of Hot-Electron Bolometers for THz Applications, IEEE Trans. Appl.Supercond., Proceedings of the ASC2000, Virginia Beach, September 2000[41 M. Schicke, Superconducting Mixer Elements for terahertz Frequencies, Ph.D. thesis,Forschungs-Report, VDE Verlag, 1999[5] V. Laffey, Etude et realisation de jonctions SIS a base de nitrure de niobium et d'unebarriere tunnel adapt& pertnettant la monthe aux frequences THz des instrumentsheterodynes, Ph.D. thesis, CEA, Grenoble, 1998[6] I. POron, P. Pasturel and K. F. Schuster, Fabrication of SIS junctions for space bornesubmillimeter wave mixers using negative resist e-beam lithography, IEEE Trans. Appl.Supercond., Proceedings of the ASC'2000, Virginia Beach, September 2000[7]Microresist technology, Berlin, Germany