Ion-beam Assisted Sputtering of Titanium Nitride Thin Films Timothy Draher12 Tomas Polakovic3 Juliang Li4 Yi Li1 Ulrich Welp1 Jidong Samuel

2025-05-03 0 0 923.2KB 10 页 10玖币

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Ion-beam Assisted Sputtering of Titanium Nitride

Thin Films

Timothy Draher1,2, Tomas Polakovic3, Juliang Li4, Yi Li1, Ulrich Welp1, Jidong Samuel

Jiang1, John Pearson1, Whitney Armstrong3, Zein-Eddine Meziani3, Clarence Chang4,

Wai-Kwong Kwok1, Zhili Xiao1,2, and Valentine Novosad1,*

1Argonne National Laboratory, Materials Science Division, Lemont Illinois, 60439, USA

2Northern Illinois University, Department of Physics, Dekalb Illinois, 60115, USA

3Argonne National Laboratory, Physics Division, Lemont Illinois, 60439, USA

4Argonne National Laboratory, High Energy Physics Division, Lemont Illinois, 60439, USA

*novosad@anl.gov

ABSTRACT

Titanium nitride is a material of interest for many superconducting devices such as nanowire microwave resonators and photon

detectors. Thus, controlling the growth of TiN thin ﬁlms with desirable properties is of high importance. This work aims to

explore effects in ion beam-assisted sputtering (IBAS), were an observed increase in nominal critical temperature and upper

critical ﬁelds are in tandem with previous work on Niobium nitride (NbN). We grow thin ﬁlms of titanium nitride by both, the

conventional method of DC reactive magnetron sputtering and the IBAS method, to compare their superconducting critical

temperatures

as functions of thickness, sheet resistance, and nitrogen ﬂow rate. We perform electrical and structural

characterizations by electric transport and x-ray diffraction measurements. Compared to the conventional method of reactive

sputtering, the IBAS technique has demonstrated a 10% increase in nominal critical temperature without noticeable variation in

the lattice structure. Additionally, we explore the behavior of superconducting

in ultra-thin ﬁlms. Trends in ﬁlms grown at high

nitrogen concentrations follow predictions of mean-ﬁeld theory in disordered ﬁlms and show suppression of superconducting

due to geometric effects, while nitride ﬁlms grown at low nitrogen concentrations strongly deviate from the theoretical models.

Introduction

TiN has been extensively studied for its many useful mechanical, electrical, and optical properties. When fabricated into

superconducting devices such as nanowire microwave resonators and photon detectors, TiN serves as an important material for

fundamental structures in quantum electrical circuits, such as resonators used to multiplex large arrays of qubits

. TiN has

been shown to meet the criteria desired for quantum computations and photon detection such as low RF losses at both high

and low driving powers, high kinetic inductance, and tunable

Tc1–8

. In addition, as a superconducting nitride, TiN has a high

superconducting Tc, relative to elemental Ti and Ti2N, for highly stoichiometric phases. It is a hard, mechanically robust, and

stable material

9–12

. The composition of deposited TiN

compounds can be varied by changing the ﬂux of reactive nitrogen gas

present during fabrication, where varying the nitrogen concentration not only tunes the superconducting

, but also alters the

ﬁlm’s crystal structure and kinetic inductance12,13.

For the lowest nitrogen concentrations, an

-Ti phase initially forms where nitrogen is incorporated interstitially. With

little increase in nitrogen, there is an atomic fraction of nitrogen that forms the Ti

N phase which is known to suppress

Ti-N compounds

. Next, in the higher nitrogen ﬂow regime, TiN becomes the most predominant and stable compound

A mix of the TiN (111) and TiN (002) phases can form. TiN (002) is the orientation with lower surface energy and forms

more elastic grains comparatively to TiN (111), however, many deposition parameters can drive the preferred growth of either

orientation such as the deposition pressure, substrate bias/temperature, ion ﬂux, and gas composition

14,16,17

. Growth of TiN can

be conducted using a variety of physical vapor deposition (PVD) techniques including sputtering, evaporation, and molecular

beam epitaxy (MBE).

MBE allows for highly stoichiometric and ordered growth of multi-component ﬁlms like TiN at low temperatures inside

an ultra-high vacuum environment

, while the use of reactive sputtering or evaporation promotes a more polycrystalline and

amorphous lattice structure. The latter techniques offer faster growth and higher throughput at the cost of less control over

crystal structure during deposition. However, sputtering and evaporation still offers the ability to grow ﬁlms of high quality

with desirable characteristics by tailoring the deposition parameters9.

In reactive DC magnetron sputtering, the target material is connected to a high power DC source that creates a plasma out

of a mixture of inert gas (usually argon) and a reactive gas (in this case nitrogen) which is then conﬁned by magnetic ﬁelds

arXiv:2210.15065v3 [cond-mat.supr-con] 12 Apr 2023

local to the source target. The gas particles are ionized by the strong electric ﬁelds and are accelerated towards the target, which

knocks loose the desired sputtering atoms that then recombine with the reactive gas to form the thin ﬁlm. Ion-beam assisted

sputtering (IBAS) utilizes the enhanced kinematic effects of an additional ion source to bombard the sample surface during the

reactive sputtering process. This effectively anneals the ﬁlm surface, and promotes better adhesion

19,20

. In the case of reactive

IBAS, the ion-beam source also functions as the supply of the reactive gas.

In a previous work with niobium nitride, IBAS was shown to decrease the sensitivity of nitrogen to forming ideal

superconducting stoichiometric ﬁlms and increase

Tc21

. In this study, we aim to compare the IBAS method with conventional

reactive magnetron sputtering of TiN and explore its effects on superconducting Tc, structure, and electrical properties.

Methods

TiN ﬁlms were deposited on 2-inch high resistance

(ρ>10

Ω

)

Si (100) wafers with a thin layer of native oxide inside

a commercial ultra-high vacuum sputtering system from Angstrom Engineering

. Two separate growth techniques were

utilized at room temperature. The ﬁrst being conventional DC reactive magnetron sputtering and the second with the added

bombardment of nitrogen ions from a diffusive ion-beam source, adapting the IBAS method. Before deposition, the chamber

vacuum was pumped down to

5×10−9

Torr and the substrate surface was etched of water or organic contamination using

a low energy argon ion beam. Moreover, the substrate was continuously rotated during deposition to assure uniform ﬁlm

growth. Samples were not heated or annealed during deposition and the temperature did not exceed 30

C. Sputtering rates

were determined by use of x-ray reﬂectometry and proﬁlometer measurements on a masked twin sample.

For both methods, the chamber pressure was held ﬁxed at 3 mTorr with a continuous mass ﬂow of 99.9999% argon at

30 sccm. While the reactive ultrahigh purity (99.9997%) nitrogen gas concentration was varied from 0 up to 10 sccm. A

99.995% titanium target was sputtered from a 3-inch diameter magnetron sputtering gun powered by a DC power source with

≈

11.6 W

−2

. The substrate-to-target distance sits at 5-inches with a 33

angle relative to the substrate surface normal.

The ion-beam source was an end-Hall ion gun with attached hollow cathode for thermionic emission of electrons to neutralize

the beam plasma23. It rests at a 40° angle from the substrate and 20° azimuthal from the Ti gun. During IBAS deposition, the

nitrogen ﬂow is supplied only from the ion source rather than uniformly around the substrate during conventional sputtering.

Ion energies of N

were kept low at 100 eV to minimize any structural damage to the ﬁlms and reducing the formation of

microcracks that lead to pores along the surface

24,25

. While maintaining a 0.5 A ion current, this is equivalent to an ion power

density of 70 mW·cm−2. Table 1summarizes the general deposition parameters used.

The superconducting

of the TiN ﬁlms was measured via a standard four-wire probe method in an ICEoxford Dry Ice

cryostat and Bluefors dilution refrigerator. In addition, x-ray diffraction (XRD) analysis was conducted on ﬁlms grown from

both methods to determine the phase of TiN. Sheet resistance measurements followed via a four probe on a circular sample, to

correct for geometric factors26.

Parameter Value

Base Pressure (Torr) 5×10−9

Working Pressure (mTorr) 3

Nitrogen Flow (sccm) 0 - 10

Argon Flow (sccm) 30

Target/Substrate Distance (inch) 5

Deposition Temperature (°C) ≤30

Ion Energy (eV) 100

Ion Current (A) 0.5

Table 1. TiN sputtering chamber deposition parameters.

Results

The advantages of the IBAS method for TiN are best demonstrated by direct comparison of superconducting critical temperatures

of thin ﬁlms grown by conventional reactive sputtering under identical chemical conditions as the IBAS ﬁlms.

The nitrogen ﬂow dependence on superconducting

for 300 nm ﬁlms grown with both techniques is shown by Fig. 1with

resistive transitions inset for IBAS grown ﬁlms. While the superconducting

changes little with higher nitrogen ﬂow rates,

there is large variation near the ﬂow range of 0.5-2 sccm, where the

increases sharply from 0.5 K to 4-4.5 K. The IBAS

grown ﬁlms show a 10% increase in nominal superconducting

. The sharp increase in the superconducting

is due to the

formation of stoichiometric TiN as the nitrogen content increases12.

2/10

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Ion-beamAssistedSputteringofTitaniumNitrideThinFilmsTimothyDraher1,2,TomasPolakovic3,JuliangLi4,YiLi1,UlrichWelp1,JidongSamuelJiang1,JohnPearson1,WhitneyArmstrong3,Zein-EddineMeziani3,ClarenceChang4,Wai-KwongKwok1,ZhiliXiao1,2,andValentineNovosad1,*1ArgonneNationalLaboratory,MaterialsScienceDivision...

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Ion-beam Assisted Sputtering of Titanium Nitride Thin Films Timothy Draher12 Tomas Polakovic3 Juliang Li4 Yi Li1 Ulrich Welp1 Jidong Samuel

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