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Microelectronic Engineering 83 (2006) 206–212

Fabrication of a low resistivity tantalum nitride thin film

Hong Shen *, Ravi Ramanathan

Skyworks Solutions Inc., 2427 W. Hillcrest Drive, Newbury Park, CA 91320, United States

Received 18 May 2005; received in revised form 26 July 2005; accepted 18 August 2005Available online 22 September 2005

Abstract

Novel bilayer TaN film has been developed using reactive ion DC sputtering. The film has a low resistivity of 80 lX-cm and is morestable compared with high resistivity films with high nitrogen flow rates. Low angle X-ray diffraction results show that the low resistivityTaN film is highly crystallized than that of each individual film deposited in the bilayer design. The crystalline structure and the filmresistivity of the bilayer TaN film remain unchanged even when the thickness of each layer is changed, indicating large process window,that is critical for high volume manufacturing. 2005 Elsevier B.V. All rights reserved.

Keywords: TaN; Thin film resistor; DC sputtering; Barrier layer; Glancing angle XRD

1. Introduction

Tantalum nitride (TaN) has been used effectively insemiconductor industry because of its high melting point,good resistivity uniformity, low temperature coefficient ofresistivity (TCR), low voltage coefficient of resistivity(VCR), good thermal stability, good selectivity to chemicalmechanical polishing (CMP), and good barrier propertiesin Cu interconnects. Although TaN deposition processcan be tuned to achieve wide range of electrical propertiesby varying the film stoichiometry [1–8], as one of the bar-riers for copper back end-of-line (BEOL) processing, highresistivity and narrow process window continue to limitthe usability of these films as thin film resistors in SiliconIC manufacturing.

TaN has good adhesion to GaAs [9,10] and has beenused as thin film resistors in the fabrication of power ampli-fiers [11]. Reported resistivity value of DC-sputtered TaNthin film ranges from 200 to 1000 lX-cm [8,12]. It has longbeen proven that grain boundaries are the most importantdiffusion paths for the polycrystalline TaN films [13–16].Several attempts were made in altering the stoichiometry

0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.mee.2005.08.006

* Corresponding author. Tel.: +1 805 480 4481; fax: +1 805 480 4212.E-mail address: hong.shen@skyworksinc.com (H. Shen).

of TaNx films and change their grain boundary density[17–19]. Changes in the barrier properties will not only af-fect copper diffusion along copper/barrier interface, butalso affect the grain size and crystallographic orientationof the copper, which will also in turn affect the reliabilityof the interconnects. TaNx, with x > 0.5 has the lowestgrain boundary density thus the leakage between transmis-sion lines is the lowest compared to tantalum film, andtantalum-rich TaNx (x < 0.5) films [19]. However, withincreased nitrogen flow in DC sputtering, the nitrogencontent in the TaN film also increases along with rapidincrease in resistivity.

In this paper, a low resistivity TaN film fabricated byDC reactive ion sputtering will be discussed. A novel dou-ble layer (bilayer) TaN thin film scheme is developed forthe first time, and is shown to be highly oriented, crystal-line, large grain size, as well as significantly reduced bulkresistivity.

2. Experimental

Reactive ion sputtering was performed in a BalzerLLS502 sputter system, with DC power, tantalum target,and independently controlled N2 and Ar gas flows. GaAsmechanical wafers were used as substrates. To obtain the

H. Shen, R. Ramanathan / Microelectronic Engineering 83 (2006) 206–212 207

bilayer film, a thin TaN seed layer was deposited on GaAs(first layer), followed by the deposition of a thick TaNlayer (second layer) without breaking the vacuum. The firstlayer was deposited using low DC power, with N2/Ar ratioof 10%. The second layer was deposited with higher DCpower, with N2/Ar ratio of 1.2%. The base pressure forboth processes was kept the same and was controlled bya throttle valve in the system. Film stress was obtainedby measuring the wafer bow, before and after the filmdeposition, by using a laser film stress measurement(FSM) tool. Thickness was measured using both cross-sec-tion Scanning Electron Microscope (SEM) and a profilom-eter. Sheet resistance was measured using 4-point probetechnique. Glancing angle X-ray diffraction (XRD) spectraat a glancing angle of 1, scanning from 20 to 80, werecollected to determine the crystalline orientations of thesingle and bilayer thin films. All the spectra were collectedusing Cu Ka band at 45 kV, 40 mA.

3. Results and Discussions

In reactive ion DC sputtering, Ar ions strike the Ta tar-get and resultant Ta ions will come out into the plasmaphase. In the plasma mixture the tantalum ions will reactwith nitrogen ions to form TaN compounds which will inturn deposit onto the substrate surface to form TaN thinfilm. At low DC power, which transpires to a low Ta ion-ization rate, combined with high N2/Ar ratio in the plasmamixture, the TaN reaction is thermodynamically con-trolled. Fig. 1 shows the XRD spectrum of TaN film thatwas deposited at low DC power and 10% N2/Ar ratio.The thickness of this film is 190 A with a compressive stressof 4000 MPa, and its resistivity is over 480 lX-cm. ThisTaN film is polycrystallized with a 7.18% N content. Thereare many different crystalline orientations, TaN (1 1 1),TaN (2 0 0), TaN (2 2 0), and TaN (3 1 1), as shown in

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

20 30 40

coun

ts/s

TaN(111)

TaN(200)

Fig. 1. Glancing angle XRD pattern of th

Table 1. The average grain size of this film is estimatedto be around 40 A, using Scherrers Formula, given inEq. (1)

d ¼ 0:9 kb cos h

; ð1Þ

where d is the grain size, k is the X-ray wavelength, b is thepeaks full-width at half-maximum (FWHM), and h is thepeak position angle (2h divided by 2). With such smallgrain size the grain boundary density in the film is muchhigher and the chance of copper diffusion along grainboundary is very high.

On the other hand, high DC sputtering power will in-crease the ionization rate of tantalum. At 1.2% N2/Ar ra-tio, the nitrogen is in deficit, the total reaction becomesmore dynamically controlled. Fig. 2 is the XRD spectrumof a TaN film deposited with high DC power and lowN2/Ar ratio. The total thickness of this film is 750 A witha compressive stress of 900 MPa, and its resistivity is154 lX-cm. Instead of TaN polycrystalline structure inthe high resistivity film, this film has a single crystallinephase of TaN0.1, which has 10% nitrogen in the film. An-other identification peak of this crystalline phase showsup at 68.38 (2h), which is the (2 1 1) orientation of thisbody-centered cubic (BCC) structure of TaN0.1. This indi-cates that even at a deficit N2/Ar flow in the sputtering gasthe nitrogen content in the film is actually increased due tothe high sputtering power and increased tantalum ioniza-tion. This film is also highly crystallized along its (1 1 0)plane for the intensity of X-ray diffraction is much highercompared to the previous film. The FWHM at 37.74(2h) is 2.71, which translates to a grain size of 30 A, andis smaller than the high resistivity film. Copper diffusioncan be expected even faster in this film because of moregrain boundaries formed by smaller crystalline. The(1 1 0) peak in Fig. 2 is not a well-defined Gaussian peak.

50 60 70 802θ

GaAs Substrate

TaN(220)TaN(311)

TaN(222)

e high resistivity TaN film on GaAs.

Table 1Peak position, FWHM, and their identifications of different TaN films

Sample ID Position2h ()

Height(CPS)

FWHM2h ()

HKL index

Sample 1 (Fig. 1) 35.12 4525 2.36 TaN(1 1 1)40.42 1921 2.48 TaN(2 0 0)59.58 1365 2.19 TaN(2 2 0)71.52 1072 2.26 TaN(3 1 1)

Sample 2 (Fig. 2) 33.82 4327 0.31 TaN0.1 (1 1 0)Kb37.78 20253 2.71 TaN0.1 (1 1 0)64.68 2905 3.79 Substrate peak ?68.28 3169 2.29 TaN0.1 (2 1 1)

Sample 3 (Fig. 3) 37.92 65865 0.83 TaN0.1 (1 1 0)54.62 3484 1.78 TaN0.1 (2 0 0)68.58 11530 1.28 TaN0.1 (2 1 1)

Sample 4 (Fig. 4) 37.78 53232 0.92 TaN0.1 (1 1 0)53.12 4699 0.14 ?54.62 3484 1.84 TaN0.1 (2 0 0)68.32 10559 1.45 TaN0.1 (2 1 1)

Sample 5 (Fig. 4) 37.82 57667 0.88 TaN0.1 (1 1 0)53.12 4752 0.16 ?54.62 3582 1.68 TaN0.1 (2 0 0)68.48 10378 1.35 TaN0.1 (2 1 1)

Sample 6 (Fig. 4) 37.82 57763 0.89 TaN0.1 (1 1 0)53.12 4779 0.21 ?54.58 3375 1.66 TaN0.1 (2 0 0)68.48 9630 1.34 TaN0.1 (2 1 1)

Sample 7 (Fig. 6) 35.48 5785 2.84 TaN(1 1 1)37.62 5275 3.47 TaN0.1 (1 1 0)39.72 3848 3.50 TaN(2 0 0)59.92 1544 2.27 TaN(2 2 0)71.42 1347 2.18 TaN(3 1 1)

Sample 8 (Fig. 7) 37.92 65865 0.88 TaN0.1 (1 1 0)53.12 4665 0.16 ?54.62 3484 1.68 TaN0.1 (2 0 0)68.58 11930 1.35 TaN0.1 (2 1 1)

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15000

20000

25000

20 30 40

cou

nts

/s

TaN0.1(110)

GaA

Fig. 2. Glancing angle XRD pattern of t

208 H. Shen, R. Ramanathan / Microelectronic Engineering 83 (2006) 206–212

There might be several peaks embedded in the same (1 1 0)peak. Possible candidates include diffraction peak of hex-agonal Ta2N phase at 38.73 [11], which has 30% nitrogenand is amorphous. However, compared to the intensity ofthe (1 1 0) peak, one can determine that the Ta2N contentin the film is rather small.

When these two films are put together, such that the lowresistivity film is deposited on top of the high resistivityone, the film properties change dramatically. Fig. 3 showsthe XRD spectrum of such a bilayer film, which containsa 40 A thick high resistivity TaN similar to that depictedin Fig. 1, and a 750 A thick low resistivity TaN as depictedin Fig. 2, without breaking the vacuum. All the other TaNcrystalline phases disappeared, with only TaN0.1 left, indi-cating the domination of the second layer. The bilayer filmis much better oriented than the second film alone, with apeak intensity ratio almost 3 times higher than that ofthe low resistivity film. Regardless of different TaN film,for a fixed film thickness, the GaAs substrate XRD peakshould not change its intensity and should provide a com-mon reference point for the TaN film crystallization evalu-ation. In Fig. 2, the peak intensity ratio of TaN0.1(1 1 0)/GaAs is 1.4. In Fig. 3, in the bilayer design, this ratio is4.2. Apparently, the TaN0.1(2 1 1) content in the film alsoincreased, with peak intensity ratio TaN0.1(2 1 1)/GaAschanging from 0.21 in Fig. 2 to 0.73 in Fig. 3. The resistiv-ity of this bilayer film is 79.6 lX-cm, with a compressivestress of 400 MPa. By convention, the combined filmresistivity can be calculated by treating the two films astwo resistors connected in parallel. The theoretical valueof the final film by this method should be close to the toplow resistivity layer, which is about 149.8 lX-cm. The mea-sured resistivity of 79.6 lX-cm for the bilayer film is muchlower than its theoretical value, indicating that these twofilms have much greater interaction between them and

50 60 70 80

s Substrate

TaN0.1(200)

TaN0.1(211)

he low resistivity TaN film on GaAs.

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70000

202θ

cou

nts

/s

TaN0.1(110)

GaAs Substrate

TaN0.1(200)

TaN0.1(211)

30 40 50 60 70 80

Fig. 3. Glancing angle XRD pattern of the ultra low resistivity bilayer TaN film on GaAs.

H. Shen, R. Ramanathan / Microelectronic Engineering 83 (2006) 206–212 209

the bilayer film undergoes some major structural changes,compared to each of their individual constituents. In addi-tion, the grain size in the bilayer film is 100 A, much largerthan their individual counterpart, 40 A for high resistivityfilm and 30 A for low resistivity film, respectively, also indi-cating a major structural change in the combined film. Thisis proven by the analysis of the XRD pattern in Fig. 3. TheFWHM is much smaller for the (1 1 0) peak and the calcu-lated grain size from the FWHM is 100 A. The grain sizemight not be large enough to block the copper diffusionalong grain boundaries but compared with the low resistiv-ity film the bilayer film is a promising candidate as barrierfor Cu interconnects.

One possible explanation, for the dominance of theTaN0.1 phase, is that the thickness of the first layer is40 A and its grain size is also close to 40 A, so that the high

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cou

nts

/s

TaN0.1(110)

G

Fig. 4. Glancing angle XRD pattern of the ultra low resistivity bilayer TaN

resistivity film is essentially masked by the low resistivityfilm on top. In order to examine the influence of first layerthickness on the bilayer film properties, a second set of sam-ples were prepared with different first layer thicknesses.Fig. 4 shows three films with the bilayer design, with the firstlayer thickness changing from 20 to 40 and 80 A and the to-tal film thickness was kept the same at 800 A. The resistivityand stress were identical for all three bilayer films. When thefirst layer thickness increases the intensity of the (1 1 0) peakis also increased slightly, indicating more crystallization.Meanwhile the FWHM decreases as the film thickness in-creases, indicating slightly larger grain size. Fig. 5 summa-rizes the impact of first layer thickness on the TaN filmresistivity. By adding a high resistivity TaN as a seed layer,the overall film is better crystallized and oriented than eachone of the films deposited, and the overall film resistivity

50 60 70 802θ

TaN0.1(200)

TaN0.1(211)

aAs Substrate

film on GaAs. First layer thickness is changed from 20 to 40 and 80 A.

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100

1000

0 100 200 300 400 500 600 700 800 900

First Layer Thickness, A

Res

isti

vity

,µΩ

cm

Fig. 5. Impact of first layer thickness on resistivity of bilayer TaN film.

210 H. Shen, R. Ramanathan / Microelectronic Engineering 83 (2006) 206–212

also dropped well below each of the two constituent films.These results indicate that the first layer film in the bilayer,though exhibits much higher resistivity, acts as a seed layerto force the orientation of the low resistivity film on the topthat leads to highly crystallized and lower overall resistivitythan each of the two films. As the thickness of the first layerfilm increases, the combined film eventually loses its bilayercharacteristics and behaves like a conventional resistors-in-parallel configuration.

When the order of the layers in the bilayer design is re-versed, the final film starts to show both the characteristicsof two films. Fig. 6 shows the XRD spectrum for a filmwith a 240 A low resistivity film as the first layer and a

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cou

nts

/s

TaN(111)TaN0.1(110)

TaN(200)

Fig. 6. Glancing angle XRD pattern of the ultra low re

190 A high resistivity film on the top. The resistivity ofthe combined film is over 300 lX-cm and its stress is1400 MPa. This film consists of TaN(1 1 1), TaN(2 0 0),TaN(2 2 0), and TaN(3 1 1), which belong to the high resis-tivity film, and TaN0.1(1 1 0), which belongs to the lowresistivity film, and possibly some amorphous Ta2N aswell. None of the individual films appear to be dominatingin the crystalline structure. Grain size is difficult to calcu-late for this film as all the peaks are overlapping and it isdifficult to measure the FWHM. However, these peaksare rather broad, indicating small grains in the film. In thiscase, the two films behave exactly like two resistors inparallel and there is no physical or chemical interaction

50 60 70 80

GaAs Substrate Peak

TaN(220) TaN(311)

2θ

sistivity bilayer TaN film in reverse order on GaAs.

Table 2Summary of TaN bilayer film designs

Final grain size 100 A SmallFinal resistivity 79.6 lX-cm >300 lX-cm

2nd Film Thickness 750 A 190 AGrain size 30 A 40 AResistivity 154 lX-cm 480 lX-cm

1st Film Thickness 40 A 240 AGrain size 40 A 30 AResistivity 480 lX-cm 154 lX-cm

Substrate GaAs thickness 650 lX

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20

cou

nts

/s

TaN 0.1 (110)

30 40

Fig. 7. Glancing angle XRD pattern of the TaN

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cou

nts

/s

TaN0.1(110)

G

Fig. 8. Glancing angle XRD pattern of the ultra low resistivity

H. Shen, R. Ramanathan / Microelectronic Engineering 83 (2006) 206–212 211

between these two films. Details of the layer sequences andthe film properties are summarized in Table 2.

To investigate the influence of the starting substratematerial on the quality of the bi-layer TaN thin film, thebilayer film was deposited on Plasma Enhanced ChemicalVapor Deposition (PECVD) Silicon Nitride on Si sub-strate. The glancing angle XRD pattern, shown in Fig. 7,indicates strong (1 1 0) and (2 1 1) peaks, similar to thatdeposited on GaAs substrate. The sheet resistance of thebi-layer film on SiNx is also identical. These results suggestthat the crystalline structure of the bilayer TaN, reported in

2θ

TaN 0.1 (200)

TaN 0.1 (211)

50 60 70 80

bilayer film on PECVD SixNy/Si substrate.

50 60 70 802θ

Pre-Heat Post-Heat

aAs Substrate

TaN0.1(200)

TaN0.1(211)

bilayer TaN film on GaAs, before and after heat treatment.

212 H. Shen, R. Ramanathan / Microelectronic Engineering 83 (2006) 206–212

this study, primarily depends on the seed TaN layer. Thelow value TiN or TaN thin film resistors, in mature Si tech-nology, have to be fabricated after the pre-metal dielectric(PMD) process step so that the thin film resistor can be di-rectly wired to the Al or Cu metal interconnect. It is notdesirable to place the low value resistor below PMD layerin order to avoid high contact resistance arising fromTungsten plugs and also to avoid subjecting the thin filmresistors to high temperature annealing required for salici-dation and PMD densification process steps. In general,the PMD oxide (Boron and Phosphorus doped) is oftencapped with PECVD or HDP SixNy and hence suitablefor the bilayer TaN film.

The stability of the bilayer film was also confirmed bysubjecting the film to an isothermal annealing in nitrogenambient at 300 C for an hour. XRD spectra are taken be-fore and after the heat treatment. From Fig. 8, it is evidentthat the crystalline phases and their orientations remainunchanged even after the heat treatment, indicating ade-quate stability of the bilayer TaN film. The FWHM dataalso show that after the heat treatment the grain size didnot change either.

4. Conclusions

By carefully designing and stacking TaN films with dif-ferent properties, the resistivity of the combined bilayerfilm can be decreased much lower than either of the indi-vidual films with larger grain size. By using a high resistiveTaN layer as a seed layer, a second film deposited at highDC sputtering power and carefully tuned N2/Ar ratio, isshown to exhibit high crystalline orientation, larger grainsize and reduced grain boundary density, leading to thelowest resistivity TaN films. Influence of seed layer thick-ness, starting substrate material and the order of the layerstacking on the crystalline orientation and resistivity hasbeen extensively studied. The stability of the bilayer TaNfilm is demonstrated by subjecting the film to isothermalanneal at 300 C. The bilayer TaN film, with its unique

properties and wide process window can be utilized aslow value thin film resistors in both GaAs and Si processtechnologies.

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