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Microelectronic Engineering 84 (2007) 678–683

Effect of a thin W, Pt, Mo, and Zr interlayer on the thermal stabilityand electrical characteristics of NiSi

Wei Huang *, Lichun Zhang, Yuzhi Gao, Haiyan Jin

Institute of Microelectronics, Peking University, Beijing 100871, PR China

Received 21 March 2006; received in revised form 21 November 2006; accepted 21 November 2006Available online 20 December 2006

Abstract

It is reported that the thermal stability of NiSi is improved by employing respectively the addition of a thin interlayer metal(W,Pt,Mo,Zr) within the nickel film. The results show that after rapid thermal annealing (RTA) at temperatures ranging from650 �C to 800 �C, the sheet resistance of formed ternary silicide Ni(M)Si was less than 3 X/h, and its value is also lower than that ofpure nickel monosilicide. X-ray diffraction (XRD) and raman spectra results both reveal that only the Ni(M)Si phase exists in these sam-ples, but the high resistance NiSi2 phase does not. Fabricated Ni(M)Si/Si Schottky barrier devices displayed good I–V electrical char-acteristics, with the barrier height being located generally between 0.65 eV and 0.71 eV, and the reverse breakdown voltage exceedingto 40 V. It shows that four kinds of Ni(M)Si film can be considered as the satisfactory local connection and contact material.� 2007 Elsevier B.V. All rights reserved.

Keywords: NiSi; XRD (X-ray diffraction); Raman spectral analysis; RBS (Rutherford backscattering spectrometry); SBD (Schottky barrier diode)

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6782. Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6793. Experiment results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679

0167-9doi:10.

* CoE-m

3.1. The sheet resistance of NiSi, Ni(W)Si, Ni(Pt)Si, Ni(Mo)Si, and Ni(Zr)Si films . . . . . . . . . . . . . . . . . . . . . . . . . . . 6793.2. Physical phase analysis of several NiSi, Ni(W)Si, Ni(Pt)Si, Ni(Mo)Si, and Ni(Zr)Si films. . . . . . . . . . . . . . . . . . . . 6803.3. Electrical characteristics of NiSi/Si, Ni(W)Si/Si, Ni(Pt)Si/Si, Ni(Mo)Si/Si, and Ni(Zr)Si/Si Schottky diodes . . . . . . 682

4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683

1. Introduction

With the feature size of the devices down to submicronin the very-large and ultra-large scale integration semicon-

317/$ - see front matter � 2007 Elsevier B.V. All rights reserved.1016/j.mee.2006.11.006

rresponding author. Tel.: +86 10 62752579; fax: +86 10 62751789.ail address: huangw@pku.org.cn (W. Huang).

ductor fabrication recent years, NiSi gradually substitutesTiSi2 and CoSi2, and becomes an attractive silicide materialbecause of its lower silicon consumption, low resistivity,low stress, and no bridge effect. However, a critical prob-lem comes from the occurrence of agglomeration at tem-peratures in excess of 650 �C and also the phasetransition from the low resistivity NiSi to the high resistiv-ity NiSi2 at temperatures greater than 750 �C [1,2].

650 700 750 800 850 900 9501

2

3

4

5

6

She

et r

esis

tivity

/Ω/

Annealing temperature / c

Cap 30nm Ti/30nm Ni/Si 15nm Ni/2nm Pt/15nm Ni/Si 15nm Ni/2nm Mo/15nm Ni/Si 15nm Ni/2nm W/15nm Ni/Si 15nm Ni/2nm Zr/30nm Ni/Si

Fig. 1. The dependence of sheet resistance of several kinds of these nickelfilms on annealing temperatures.

W. Huang et al. / Microelectronic Engineering 84 (2007) 678–683 679

Recently the insertion of a thin Pt interlayer within thenickel film could improve the thermal stability of NiSi[3–5], but its application is limited because of high pricein unit-step. We deeply considered that other relativelycheap metal (M) is added into the nickel films to improvethe thermal stability of NiSi. This kind of the interlayeredmetal should be of two properties below. At first, eventhough this metal makes silicidation at high annealing tem-perature, the formed silicide’s sheet resistance is approxi-mately equivalent to that of NiSi film. Secondly, thesilicide is of steadier property than NiSi, and the low resis-tivity phase transformation of the silicide film happensmore difficultly. Hence the appearance of NiSi2 peak isdelayed and the thermal stability of the NiSi film can beimproved. Based on the analysis above, the dependenceof the sheet resistance on rapid annealing temperature ofNi(M)Si was investigated at the beginning After the com-pletion of last step, both XRD and raman spectra analysiswere used to microcosmically understand the phase trans-formation of the formed ternary silicide at different anneal-ing temperatures, and RBS was also employed to measurethe atomic ratio of the interlayered metal in this formed sil-icide. Finally, the Schottky barrier diode with the guardring structure was fabricated to further investigate the com-patibility with the NiSi film and the integrated circuit fab-rication. According to the method, adding each of a thinW, Pt, Mo, Zr interlayer within the nickel film is investi-gated in the paper to improve the thermal stability of NiSi.

2. Experiment

After cleaned by traditional, and washed by a dilute HFsolution, the wafers with an epitaxially grown layer of thephosphorus doping concentration about the order of5 · 1015 cm�3, were immediately loaded into an S-gun mag-netron sputtering system with a base pressure of4.6 · 10�5 Pa. While the working pressure kept at 10�3 Pain the Ar ambient, 15 nm Ni/2 nm M/15 nm Ni were sput-tered sequentially, which M stand for W, Pt, Mo, and Zrelement in turns . The silicidation process was carried outwith the first RTA of 650 �C for 50 s in an ambient of aflowing pure N2, and then unreacted metal was selectivelyetched with a mixture of H2SO4 and H2O2. Annealing tem-peratures for the second RTA were varied from 650 �C to950 �C for 40 s.

The sheet resistance of these formed samples was mea-sured by four-point probe (FPP). Nickel silicide phaseswere identified by XRD and raman spectra. The XRDmeasurement instrument was Rigaku-RA made in USA,and the raman spectra measurement instrument was RM-1000 made in British. A RBS measurement was used toestimate the atomic ratio of the M element in the Ni(M)Sifilm and the thickness of the ternary silicide. The RBS mea-surement instrument was Pandem accelerator. A Hewlett–Packard4156B semiconductor parameter analyzer wasemployed to measure I–V characteristics of these fabri-cated Schottky barrier diodes with the guard ring structure.

3. Experiment results and discussion

3.1. The sheet resistance of NiSi, Ni(W)Si, Ni(Pt)Si,

Ni(Mo)Si, and Ni(Zr)Si films

Fig. 1 shows the dependence of the sheet resistance onrapid annealing temperature of Ni/Si and Ni/W/Ni/Sifilms, also including three other Ni/Pt/Ni/Si, Ni/Mo/Ni/Si, and Ni/Zr/Ni/Si films. For 30 nm Ni film on n-Si, thisfilm’s sheet resistance was nearly 3.1 X/h after beingannealed at temperatures from 650 �C to 700 �C. At thattime, the low resistivity NiSi was the dominant phase andthe grain size of NiSi seemed minute. However, as anneal-ing temperature increased to 750 �C, the sheet resistancerose to 4.5 X/h and the high resistivity phase, NiSi2appeared. At that time the surface of the film becamedegraded and many dark blobs grew on it. Previous resultswere reported by Zhang et al. [8].

For 15 nm Ni/2 nm M/15 nm Ni metal film on n-Si, thesheet resistance of Ni(M)Si formed at annealing tempera-tures ranging from 650 �C to 800 �C is less than 2.5 X/h.At that time, the surfaces of the silicided film exhibitedsmall and bright silicide particles that were distributedevenly. However, at higher silicidation temperature above850 �C, there was a slight increment in the sheet resistanceof the formed silicide. Due to agglomeration, the silicideinterface became rough with dark blobs, and the grainsof Ni(M)Si were clearly observed to expand, apparentlyaccompanied by severe degradation in the silicide surfaces.Therefore, the insertion of a thin W, Pt, Mo, and Zr inter-layer into the nickel film can effectively reduce the degrada-tion of the NiSi film. Moreover, the upper boundary of thesilicidation window is successfully extended by at least100 �C until the increase of the sheet resistance is achievedat 800 �C.

20 30 40 50 60 70 80 90101

102

103

104

105

106

107

108

109

1010

1011

1012

1013

1014

1015a

Inte

nsity

/ C

PS

2θ/ deg.

2θ/ deg.N

iSi 2(

311)

NiS

i(12

1)N

iSi(

121)

NiS

i(20

3)

NiS

i(30

1)

NiS

i(11

2)

NiS

i(10

1)

Si(1

00)

Si(1

00)

NiS

i 2(42

2)

NiS

i(12

1)N

iSi(

203)

NiS

i(11

2)

Si(1

00)

NiS

i 2(42

2)

NiS

i 2(31

1)

Si(4

00)

Si(1

00)

800˚C

750˚C

700˚C

650˚C

20 30 40 50 60 70 80 90100

103

106

109

1012

1015

1018

1021

1024

1027

1030

b

Ni/Zr/Ni/Si <111>

Ni/Mo/Ni/Si <100>

Ni/W/Ni/Si <100>

Ni/Pt/Ni/Si <100>

800˚C

850˚C

800˚C

800˚C

850˚C

850˚C

800˚C

NiS

i(20

3)

NiS

i(112

)Si(1

11)

NiS

i 2(4

22)

NiS

i 2(1

13)

NiS

i(222

)

Si(4

00)

NiS

i(112

)

NiS

i(102

)

Si(1

00)

NiS

i(103

)

NiS

i(112

)

NiS

i(002

)

Si(1

00)

NiS

i 2(4

22)

NiS

i 2(3

11)

NiS

i 2(2

02)

Si(1

00)

NiS

i(222

)

NiS

i(400

)

NiS

i(013

)

Si(1

00)

NiS

i(101

)

NiS

i 2(4

22)

NiS

i 2(3

31)

NiS

i 2(31

1)

Si(1

00)

Inte

nsity

/ C

PS

Fig. 2. XRD spectra of (a) Ni/Sih100i and (b) Ni/M/Ni/Si films annealed at different temperatures.

680 W. Huang et al. / Microelectronic Engineering 84 (2007) 678–683

3.2. Physical phase analysis of several NiSi, Ni(W)Si,

Ni(Pt)Si, Ni(Mo)Si, and Ni(Zr)Si films

XRD spectra of Ni/Si and Ni/M/Ni/Si films for variousannealing temperatures is shown in Fig. 2a and b. Forannealed Ni/Sih10 0i samples that corresponds to Fig. 2a,some diffraction peaks corresponding to NiSi wereobserved only in the sample formed at 650 �C and700 �C, such as (112) NiSi and (203) NiSi. However, forthe sample annealed at 750 �C, some high resistivity peaksbegan to appear, corresponding to (311) NiSi2 and (422)NiSi2, and at that time the resistance of the NiSi film beganto rise. This result shows that the temperature above700 �C seriously limit the application of NiSi. On the other

hand, XRD spectra of four kinds of samples, Ni/Pt/Ni/Sih10 0i, Ni/W/Ni/Sih100i, Ni/Mo/Ni/Sih100i, and Ni/Zr/Ni/Sih111i annealed by 800 �C and 850 �C is com-pared. For these samples Ni/M/Ni/Si annealed at 800 �C,the XRD spectra exhibited only nickel monosilicide diffrac-tion peaks, such as (112) NiSi and (222) NiSi, and noNiSi2 phases were detected. For the Ni/M/Ni/Si sampleat the RTA temperature of 850 �C, there occurred severalhigh resistance peaks scanned by X-ray, such as (311)NiSi2 and (42 2) NiSi2.

Raman spectra was also employed to further comparethe physical orientation of NiSi with that of Ni(M)Si.The raman spectral experiment was performed on the con-ditions of spectral line Renishaw1000 and 633 nm laser

150 200 250 300 350 400 450 500

100

200

300

400

500

600

700

800

900

Ram

an in

tens

ity /

a.u.

800

750

650

Raman shift / cm-1

150 200 250 300 350 400 450 500

600

800

1000

1200

1400

1600

1800

Ni/W/Ni/Si

Ni/Zr/Ni/Si

850

800

800

Raman shift / cm-1

Ram

an in

tens

ity /

a.u.

C

C

C

C

850 C

C

C

a

b

Fig. 3. Raman spectra of (a) Ni/Si, (b) Ni/W/Ni/Si, and Ni/Zr/Ni/Si films annealed at different temperatures.

W. Huang et al. / Microelectronic Engineering 84 (2007) 678–683 681

wavelength, and scanning time was set on 30 s. Ramanspectra of Ni/Si and Ni/M/Ni/Si film for various annealingtemperatures is shown in Fig. 3a and b. For Ni/Si samples,all experimental results are shown in Fig. 3a. There existedtwo strong peaks of raman spectra for Ni/Sih100i sampleannealed at 650 �C, which were at 195 cm�1 and215 cm�1 respectively. The peak locating at 195 cm�1 stan-ded for NiSi peak, so did another peak locating at215 cm�1 [9,10]. However, with annealing temperature of750 �C, besides two NiSi peaks at 195 cm�1 and215 cm�1, there were other strong peaks in correspondencewith NiSi2 at 297 cm�1 and 402 cm�1. For Ni/Si samplesannealed at 800 �C, two NiSi peaks at 195 cm�1 and215 cm�1 disappeared, but two NiSi2 peaks became strongthan the sample annealed at 750 �C. This effect shows thatNiSi has begun to transform into NiSi2 at annealing tem-perature of about 750 �C [11]. For two samples, Ni/Zr/

Ni/Sih111i, Ni/W/Ni/Sih100i samples annealed at800 �C in Fig. 3b, two peaks of NiSi from these two sam-ples were clearly observed at 195 cm�1 and 215 cm�1

respectively. However, for these Ni/Zr/Ni/Si, Ni/W/Ni/Sisamples annealed at 850 �C, the raman spectra instrumentdid not detect the existence of the NiSi peak at 195 cm�1

and 215 cm�1, instead several high resistivity NiSi2 peaksoccurred at five places, 232 cm�1, 297 cm�1, 320 cm�1,380 cm�1, and 402 cm�1.

From the result of physical phase analysis for these sam-ples, it is apparent that the addition of a thin W, Pt, Mo,and Zr interlayer can play an important role in delayingthe formation of NiSi2, and the transformation tempera-ture from NiSi to NiSi2 is raised at least 800 �C, becausethe formation of the high resistance peak is delayed.

RBS experimental was carried out for the silicidationsample of 15 nm Ni/2 nm M/15 nm Ni film at 800 �C. It

Table 1The barrier height and the ideality factor of NiSi/Si, Ni(M)Si/Si Schottkydiodes

Barrier height (UB/eV) Ideality factor (n)

650 �C NiSi/Si 0.63 1.04850 �C Ni(Pt)Si/Si 0.71 1.01800 �C Ni(Mo)Si/Si 0.66 1.07800 �C Ni(W)Si/Si 0.66 1.07800 �C Ni(Zr)Si/Si 0.65 1.01

682 W. Huang et al. / Microelectronic Engineering 84 (2007) 678–683

is found that the M concentration in the ternary Ni(M)Sisilicide was 5 at.% or so, and that the thickness of thisformed silicide was about 80 nm.

In addition, The Gibbs free energy theory was used toexplain the underlying reason why the addition of thesefour kinds of these metal elements can play a key role inenhancing the thermal stability of nickel monosilicide [5–7].

3.3. Electrical characteristics of NiSi/Si, Ni(W)Si/Si,

Ni(Pt)Si/Si, Ni(Mo)Si/Si, and Ni(Zr)Si/Si Schottky

diodes

Besides the thermal stability of the thin Ni(M)Si film,the contact characteristics of formed Ni(M)Si film with Sisubstrate is further investigated. Wafers with epitaxial lay-ers were used as substrates. This kind of wafer consisted ofan epitaxially grown layer of about 9–10 lm thick with thephosphorus concentration of about 5 · 1015 cm�3 andarsenic-doped substrate h111i with the concentration of1 · 1019 cm�3. The Ni(M)Si/Si Schottky barrier diode witha guard ring structure was fabricated on the substrate. It isworth noting that the self-alignment formation of theNi(M)Si/Si Schottky barrier contact is important. Afterthe 15 nm Ni/2 nm M/15 nm Ni structure was depositedin the sputtering system, the first silicidation reaction wascarried out in the ambient of pure N2. The annealing tem-perature was 650 �C for 50 s. After the selective removalstep to remove unreacted metal, the conditions of the sec-ond RTA were in turn set at 650–800 �C for 40 s. The for-ward and reverse current–voltage (I–V) characteristics ofthese SBD diodes were tested with a HP4156B semiconduc-tor parameter analyzer.

Fig. 4 shows the forward I–V measurements of a Scho-ttky junction, Ni(W)Si/Si, Ni(Mo)Si/Si, Ni(Zr)Si/Si at800 �C, Ni(Pt)Si/Si at 850 �C, and NiSi/Si at 650 �C. Com-pared with NiSi/Si Schottky barrier diode formed at650 �C. Four kinds of 15 nm Ni/2 nm M/15 nm Ni/Si

0.0 0.2 0.4 0.6 0.8 1.0

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

650 /850 / 40s Ni ( Pt ) Si / Si

40s Ni ( Pt ) Si / Si

800 / 40s Ni ( Mo ) Si / Si800 / 40s Ni ( W ) S i / S i800 / 40s Ni ( Zr ) Si / SiF

orw

ard

volta

ge/A

Forward Voltage/V

CC

CCC

Fig. 4. Forward I–V characteristics of the NiSi/Si, Ni(M)Si/Si Schottkydiodes annealed at different temperatures.

Schottky barrier diode almost show the same rectification.These results imply that because a thin interlayer, W, Pt,Mo, Zr metal element is inserted into the nickel film, thevariation of annealing temperatures arranging from100 �C to 150 �C hardly has any influence on the rectifyingcharacteristic of these silicided diodes.

Furthermore, the barrier height (UB) and the idealityfactor (n) of the Schottky diodes were derived from the cur-rent function of forward biased voltage as follows:

I ¼ I s expqV

nKT

� �; ð1Þ

I s ¼ ST 2A� exp � UB

KT

� �; ð2Þ

where A* is the Richardson constant (110 A/cm2 K2 for n-Si), S is the area of the diode (4 · 10�4 cm2), T is the Kelvintemperature (300 K), I is the forward current, Is is the sat-uration current, uB is the barrier height, and n is the ideal-ity factor. The Schottky barrier height and the idealityfactor of Ni(M)Si/Si diodes were deduced with expressions(1) and (2) above and they are displayed in Table 1. It isseen that the range for the barrier height of the diodeswas from 0.65 eV to 0.71 eV, and that the ideality factorof the diodes was close to 1.

The reverse I–V measurement results of the NiSi/Si SBDformed at 650 �C and the Ni(M)Si/Si SBD formed at800 �C, 850 �C, are compared in Fig. 5. For the NiSi/Si

0 -10 -20 -30 -40 -500.0

-2.0x10-5

-4.0x10-5

-6.0x10-5

-8.0x10-5

650 / 40s NiH Si /Si (Cap Ti) 850 / 40s Ni ( Pt ) Si / Si 800

/ 40s Ni ( W ) Si / Si 800/ 40s Ni ( Mo ) Si / Si

800 / 40s Ni ( Zr ) Si / Si

Rev

erse

cur

rent

/ A

Reverse Voltage / V

CCCCC

Fig. 5. Reverse I–V characteristics of the NiSi/Si, Ni(M)Si/Si Schottkydiodes annealed at different temperatures.

W. Huang et al. / Microelectronic Engineering 84 (2007) 678–683 683

diode annealed at 650 �C, breakdown appeared abruptly atthe reverse voltage of 45–49 V, and maximum leakage cur-rent barely approached 1 lA. For the Ni(M)Si/Si SBDformed at 800 �C, 850 �C, these diodes’ maximum leakagecurrent, which was a little more than that of NiSi/Si diodeabove, was only 20 lA at the breakdown voltage point of49 V. Therefore, it can be seen that due to the W, Pt,Mo, Zr interlayer within the nickel film, formed Ni(M)Sifilm posses satisfactory thermal stability, as well as formedNi(M)Si/Si Schottky diodes being good electrical quality.

4. Conclusion

It was first reported that inserting a thin W, Pt, Mo, Zrinterlayer within the nickel film can improve the thermalstability of NiSi film. After the Ni/M/Ni/Si sandwich filmwas silicided at annealing temperatures from 650 �C to800 �C, the sheet resistance of the formed Ni(M)Si silicidewas always about 2–3 X/h, which was lower than that ofNiSi film. As was revealed by X-ray diffraction and ramanspectral analysis, only the NiSi phase, instead of the highresistance NiSi2 phase, was detected in these silicide sam-ples. This proves that the incorporation of a thin W, Pt,Mo, Zr interlayer into the nickel film on Si can delay theappearance of the NiSi2 phase and widen the upper bound-ary of the silicidation window lay about 100 �C. Further-more, the I–V characteristics of fabricated Ni(M)Si/SiSchottky devices were measured, from which the deducedbarrier height was generally lay between 0.65 eV and

0.71 eV, and the reverse breakdown voltage exceeding to40 V.

Acknowledgments

The authors thank all workers at the microelectronicengineering lab at Peking University for device fabrication.

References

[1] J.P. Gambino, B. Cunningham, Materials Chemistry and Physics 52(1997) 99–146.

[2] S.H. Zhang, O. Mikael, Critical Reviews in Solid State and MaterialScience 28 (2003) 1–129.

[3] Y.Z. Han, X.P. Qu, Y.L. Jiang, Ni(Pt)Si thin film formation and itselectrical characteristics with Si substrate, in: Solid-State andIntegrated-Circuit Technology Proceedings, 6th International Con-ference, 1, 2001, pp. 513–516.

[4] J.F. Liu, H.B. Chen, J.Y. Feng, Journal of Crystal Growth 220(2000) 488–493.

[5] W. Huang, L.C. Zhang, Y.Z. Gao, H.Y. Jin, Research and Progressof Solid State Electronics in China 25 (2005) 411–415.

[6] W. Huang, L.C. Zhang, Y.Z. Gao, H.Y. Jin, Acta Physica Sinica 54(2005) 2252–2255.

[7] W. Huang, L.C. Zhang, Y.Z. Gao, H.Y. Jin, MicroelectronicsEngineering 83 (2006) 345–350.

[8] H. Zhang, L.C. Zhang, Y.Z. Gao, Research and Progress of SolidState Electronics in China 25 (2005) 167–173.

[9] P.S. Lee, K.L. Pey, D. Mangelinck, IEEE Electron Devices Letters 22(2001) 568–570.

[10] P.S. Lee, K.L. Pey, D. Mangelinck, IEEE Electron Devices Letters 21(2000) 566–568.

[11] F.F. Zhao, J.Z. Zheng, Z.X. Shen, Microelectronics Engineering 71(2004) 104–111.