Thin Solid Films 518 (2010) 3646–3649

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Thin Solid Films

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Effects of Laser in situ annealing on crystal quality of NiSi film grown onSi(001) substrate

Li Wan ⁎, Xuefei Zhang, Bo Tang, Yiming Ren, Xinhong Cheng, Dapeng Xu, Hijun Luo, Yunmi HuangDepartment of Physics, Wenzhou University, People's Republic of China

⁎ Corresponding author.E-mail address: liwan_china@yahoo.com.cn (L. Wan

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.09.084

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 October 2008Received in revised form 11 September 2009Accepted 23 September 2009Available online 1 October 2009

Keywords:NiSiNickel silicideLaser annealingRaman spectroscopyElectron beam evaporation

NiSi thin films grown on Si(001) substrates with a main crystal orientation of NiSi[200]//Si[001] wereprepared and Raman spectroscopy was used to study effects of laser annealing on the NiSi thin films. Resultsshow that the crystal quality of the NiSi films can be improved after laser annealing for 60 min at a laserpower of 140 mW. We have shown that laser annealing can also be used to make structural phase transitionsof NiSi domains under an annealing power of 480 mW for 10 min.

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Silicides are very important materials to be used in complementarymetal oxide semiconductor (CMOS) technology as interconnect andcontact application [1]. To improve the performance speed of the CMOSdevices, suitable silicide materials with the reduction of resistancesshould be found to decrease the RC delay time of circuits. TiSi2 and CoSi2as present silicide materials have been tried for the CMOS fabrication.However, they have certain limitations for the application on thedevices, such as the line-width-dependent sheet resistance and highsilicon consumption. Furthermore, the interfaces between the CoSi2 andthe Si substrate are non-uniform, which results in high diode leakage.NiSi material has been considered a promising candidate material toreplace the TiSi2 and CoSi2 for the Si device fabrication due to the lowformation temperature, line-width-independent resistance, and lowsilicon consumption of theNiSimaterial [1–3]. Intense efforts have beenfocused on the growth of NiSi material.

Various substrates have been used for the growth of the NiSi films,such as Si(001), and Si(111) [4–7]. The obtained NiSi films arepolycrystalline with a variety of crystal orientations. When theannealing temperature for the growth of NiSi thin films is higher than800 °C, some NiSix clusters with x≠1 are formed in the films [4]. Inorder to improve the thermal stability of the NiSi films, interlayermaterials (Pt or Pd metals) have been used to suppress the phasetransition from NiSi to NiSi2 clusters in the films [4]. However, growing

NiSi thin film with a high crystal quality, such as with only one crystalorientation, still remains a big challenge. In this context, we used laserannealing treatment to improve the crystal quality of the NiSi thin film.Raman spectroscopy is a powerful, non-destructive tool for theidentification of silicide phases in the silicide thin films [7–10]. Ramanpeaks of the silicides were used as fingerprints for probing the effects oflaser annealing treatment on the crystal quality of the silicides [11–13].

2. Experimental details

Electron-beam evaporation with a background pressure of 10−6–

10−7Pa in the growth chamberwas used to grow the samples. Nimetalfilmswith thickness of 200 Åwere deposited by evaporation of pureNitarget (99.5%) on Si(001) substrates. Samples were then subjected tofurnace annealing in N2 ambient for silicidation at a temperature of500°C for 30 min. Chemical component and crystal orientation of suchobtained silicide thin films were analyzed by using X-ray diffraction(XRD) in the conventional Bragg–Brentano geometry. The XRDmeasurement was carried out with BEDE D1 diffractometer, equippedwith Cu Kα radiation source. XRD result shows that the silicide filmsare NiSi films, which will be discussed in detail later.

Then the obtained NiSi thin films were laser-annealed. Before thestudy of laser annealing treatment, we had carefully tested heatingeffects of various laser powers. We found that when the laser power isless than 40mW, heating effect of the laser irradiation on the NiSi thinfilms can be neglected. When the laser power is increased from 40 mWto 120 mW, the laser heating introduces only an elastic thermal straininto the NiSi lattices. This makes the NiSi Raman peaks shift, with the

Fig. 2. Raman spectrum of a NiSi thin film annealed at 140 mW for 50 min and measuredunder a laser power at 40 mW.

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Raman peaks shifting back to the previous positions of unheated NiSifilmswhen the laser power isdecreasedback to 40 mW.However,whenthe laser power is increased to more than 120 mW, inelastic changesoccur to the NiSi Raman peaks. Furthermore, when the laser powerexceeds 500 mW, other phases of NiSi compounds such asNiSi2 clustersare formed in the thin film. Thus, we limited the laser power rangingfrom 120 mW to less than 500 mW for the laser annealing study. Aftereach annealing cycle, the laser power was decreased to 40 mW for theRaman measurement to keep the same measurement condition. Wefirst verified the cumulative effect of up to six annealing cyclesperformed at 140 mW for 10 min. We then tested the effect of theannealing power by ranging the power from 140 to 480 mW using aconstant annealing time of 10 min.

Micro-Raman spectroscopy was performed with the Horiba JobinYvon T64000 at room temperature using the 514.5 nm line of an argonion laser. The scattered light was dispersed through a triplemonochromator system attached to an air-cooled CCD detector. Thescattered light from the sample was detected in the backscatteringgeometry with the wave vector of the incident laser beam parallel tothe scattered light, which was normal to the film.

3. Results and discussion

3.1. XRD results

Result of a symmetric XRD scan performed on an as-deposited NiSisample is shown in Fig. 1. It is seen that the NiSi film develops atexture with the main crystal orientation of NiSi[200]//Si[001]. Someother peaks, such as NiSi(112) and NiSi(202) and NiSi(013), have alsobeen observed, indicating that there exist some NiSi domains withdifferent crystal orientations from the NiSi[200]//Si[001] in the NiSimatrix.

3.2. Raman measurement

According to the group theory, NiSi material with an orthorhombiccrystal structure (space group Pnma, D2h

16) has 12 Raman active phononmodes in the Brillouin zone center: ΓRaman=4Ag+2B1g+4B2g+2B3g[14]. However, for the NiSi films, fewer phononmodes can be detecteddue to the Raman selection rules with respect to the uniform crystalorientation. A Raman spectrum of a NiSi thin film laser-annealed at140 mW for 50 min is presented in Fig. 2 for overview. For thespectrum measurement, the laser power was reduced to 40 mW. InFig. 2, six peaks at 196 cm−1, 213 cm−1, 256 cm−1, 293 cm−1,363 cm−1, and 398 cm−1 are detected. Compared to the Raman

Fig. 1. XRD result of the as-deposited NiSi thin film grown on Si(001) substrate.

peaks from NiSi powder, the NiSi film peak positions are shifted byseveral wave numbers [5]. The peak shifts had been attributed to theexistence ofmicrostructures in the NiSi film, such as film strain, to filmcompositional variation, and differences in the percentage of crystal-linity [5]. The two peaks of 196 cm−1 and 213 cm−1 are used asfingerprint of NiSi material. The intensity of 213 cm−1 peak is higherthan that of 196 cm−1 peak due to a larger Raman scattering crosssection of the 213 cm−1 peak, which can be revealed by the Ramanspectroscopy of NiSi powder [5].

In Fig. 1, the XRD NiSi(200) peak has a very broad peak width,indicating the existence of a large number of defects in the NiSi film.Those defects influence the Raman scattering processes and change thescattering intensity of each Raman peak. We first studied the effect ofcumulative annealing time using a laser power of 140 mW. The Ramanspectra of annealed samples are presented in Fig. 3. In the as-depositedsamples, the intensities of the two peaks located at 196 cm−1 and213 cm−1 are very low, and the other Raman peaks are undetectable.However, with the increase of the number of 10-min annealing cycles,the peak intensities of 196 cm−1 and 213 cm−1 are enhanced. And afterannealing the sample three cycles of 30 min, Raman peaks at 256 cm−1,293 cm−1, 363 cm−1, and 398 cm−1 can be detected. Integrated

Fig. 3. Raman spectra obtained under a laser power of 40 mW for samples laser-annealed for different numbers of 10 min annealing cycle at a power of 140 mW. Curvesare shifted for clarity.

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intensities and peak heights of the Raman peaks can be fitted usingLorentzian peaks. Ratio of the integrated intensities and of the peakheights of the 213 cm−1 and 196 cm−1 peaks have been calculated andare shown in Fig. 5(a). The ratio of the integrated intensities remainsnearly the same during the annealing treatment, while the ratio of thepeak heights increaseswith the increase of the annealing time. Phononsclose to the Raman peak center represent the Raman scattering at theBrillioun zone center while the phonons away from the peak center arescattered by the microstructures in the crystal through breaking themomentum selection rule. The integrated intensity under one Ramanpeak is contributed from the type of phonons Raman-scattered in thewhole Brillouin zone. In a perfect NiSi crystal, Raman scattering crosssection of 213 cm−1 peak is larger than that of 196 cm−1 peak, whichhas been confirmedby theRamanspectroscopyofNiSi powder [5]. Thus,enhancement of the ratio of the peak heights in Fig. 5(a) reflects that thecrystal quality of the NiSi film laser-annealed is improved approachingto a perfect one. The ratio of the integrated intensities of the twopeaks isthen dependent on the Raman scattering geometries. Raman selectionrules of NiSi films with various scattering geometries have beencalculated in [15]. In our NiSi films with the crystal orientation of NiSi[200]//Si[001], Raman scattering of the two peaks of 196 cm−1 and213 cm−1 is allowed and both peaks should appear in the Ramanspectra. The same value of the ratio of the integrated intensities in Fig. 5(a) indicates that the crystal orientation of the NiSi domains is kept thesameafter the laser anneal at the laser power of 140 mWand there is nostructural phase transitions occurring to the NiSi films laser-annealed.

It was revealed in Fig. 1 that in the as-deposited NiSi film there existsome NiSi domains with different crystal orientations from the NiSi[200]//Si[001] of NiSi matrix, such as NiSi(112)//Si(001), NiSi(202)//Si(001) and NiSi(013)//Si(001). Raman spectra of those domains aredifferent to the spectra of NiSi matrix, which have been studied in [15].We had found those domains by scanning the Raman spectroscopy onthe samples,where Raman scattering of 213 cm−1 peak in the spectra issuppressed.Wehadperformed the laser anneal on those domains at thelaser power of 140 mW. However, we found that the 213 cm−1 peakwas still absent in the Raman spectra of the samples even with aprolonged annealing time.We then tested the effect of annealing poweron the domains by ranging the laser power from 140 to 480 mW usingconstant annealing time of 10 min. Raman spectroscopy was obtainedby decreasing the laser power to 40 mW after each annealing cycle.Results are shown in Fig. 4. It indicates that the 213 cm−1 peak appearswhen the annealing power of 320 mW is applied for 10 min. Andwhen

Fig. 4. Raman spectra obtained under a laser power of 40 mW for samples laser-annealedfor 10 min with laser powers varying from 140 to 480 mW. The curves are shifted forclarity.

the annealing power is increased up to 480 mW, the intensity of the213 cm−1 peak is higher than that of the 196 cm−1 peak. According tothe selection rules calculated in [15], the 213 cm−1 peak previouslysuppressed is active only after the NiSi domains changed theircrystallographic textures to meet the requirement of Raman selectionrules. Thus, results in Fig. 4 confirm that NiSi domains have experiencedstructural phase transitions after the laser annealing at the annealingpowermore than 320 mWfor 10 min. Ratio of the integrated intensitiesand of the peak heights of the two peaks in Fig. 4 have also beencalculated and shown in Fig. 5(b). In the figure, the two ratios are bothenhanced, while the value of the ratio of the peak heights is kept thesame in Fig. 5(a). Thus, we suggest that high annealing power that is notmore than 500 mW can be used to anneal the NiSi domains for thestructural phase transitions.

4. Conclusion

NiSi thinfilms have beengrownon Si(001)with a crystal orientationof NiSi[200]//Si[001]. Laser annealing under the annealing power of140 mW can be used to improve the crystal quality of the NiSi film byprolonging the annealing time. Annealing power of 480 mW used forthe laser annealing by 10 min canmake the structural phase transitionsof NiSi domains.

Fig. 5. Ratio of integrated intensities and of the peak heights of the 213 cm−1 and196 cm−1 Raman peaks for samples (a) annealed at 140 mW for 0 to 60 min and(b) annealed for 10 min at laser powers varying from 140 to 480 mW.

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Acknowledgement

We thank the National Natural Science Foundation of China for thefinancial support (no. 60807002).

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