Vacuum 65 (2002) 133–136

Formation of ultra-thin PtSi film by vacuum annealing

Liu Shuanga,*, Zhong Zhiyonga, Ning Yonggonga, Chen Aia, Zhang Huaiwua,Yang Jiadeb

a Institute of Information Material Engineering, University of Electronic Science and Technology of China, Chengdu 610054,

People’s Republic of ChinabChongqing Optoelectronics Research Institute, Chongqing 400060, People’s Republic of China

Received 7 August 2001; accepted 26 September 2001

Abstract

The quantum efficiency of PtSi infrared detector is restricted by PtSi film thickness. A new approach to the formation

of ultra-thin PtSi film is introduced in this article. Based on solid phase reaction theory, an ultra-thin PtSi film of

thickness 4 nm is obtained using vacuum annealing and wet etching. The analysis results of X-ray diffractometer, X-ray

photoelectron spectroscopy and transmission electron microscopy are also given. It is shown that the approach has

advantages in terms of low reaction temperature and short reaction time, and a uniform, continuous and smooth PtSi

film of thickness 4 nm is formed by the new approach. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: PtSi thin film; Vacuum annealing; XPS; XRD; TEM

1. Introduction

Thin silicide films are becoming more importantwith increasing packing density of integratedcircuits [1,2]. Platinum silicide is a candidate forapplications in making both ohmic and Schottky-barrier contacts to MOS [2] and bipolar devices.Now, the super conducting transition has beenobserved on the thin PtSi films down to d ¼ 4 nm[3]. Since Platinum silicide has a relatively highwork function, the most attractive application is theformation of Schottky-barrier junction which isimportant for 3–5mm infrared charge coupled device(IRCCD) imager arrays with P-type silicon [4].

Previous works show that the quantum effi-ciency of PtSi infrared detector is restricted by PtSifilm thickness, one important technique for fabri-cating high performance Schottky-barrier detec-tors is the formation of very thin films [5]. Verythin PtSi films have a larger coefficient ofabsorption than thick films and also in the rangeof longer wavelength [6]. At a thickness of 9 nm,the quantum efficiency is about 10 times greaterthan at a thickness of 78 nm [5], The Schottky-barrier of a PtSi film of 10 nm or less shows thehighest infrared reflectivity. Therefore tight con-trol of the PtSi layer uniformity and thickness hasbecome very important for the optimal detectorbehavior.

Most work on the formation of ultra-thin PtSifilm has been performed over the last few decades.The common techniques for the formation of the

*Corresponding author. Tel.: +86-28-3202563; fax: +86-28-

3254131.

E-mail address: cdliushuang@263.net (L. Shuang).

0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 1 9 - 5

thin PtSi films are electron beam deposition orsputter deposition of a Pt layer in clean vacuumconditions. Then annealing is performed to obtainthe metal silicides under protective gases. Silicida-tion by means of conventional furnace annealingprocesses (RTP) has been reported [4–7].

This paper, reports a new technique for formingultra-thin (4 nm) PtSi films using sputtering formetal Pt deposition under 10�5 Pa; vacuumannealing, wet etching rich Pt. X-ray diffract-ometer (XRD) and X-ray photoelectron spectro-scopy (XPS) are used for characterizing the phaseof PtSi films and transmission electron microscopy(TEM) is used for determining the continuity,smooth, uniformity and thickness of the PtSi films.

2. Experimental procedures

The samples are fabricated on P-type Si(1 0 0)wafers with a resistivity of 1271O cm. Thesubstrates are chemically cleaned and etched indiluted HF for 2–3min to remove the native oxide.The wafers are immediately loaded into thevacuum chamber for deposition prior to sputterat a base pressure of 10�5 Pa, the substrate isheated to 2001C for 2min and kept at thistemperature during the Pt sputter deposition andthe working pressure is 10�3 Pa. Sputter powervalue is 0.8–1 kW, the target voltage is 400–450V,and sputter time is about 2min. The sputter Ptlayer is of about 20 nm thickness. On taking outthe sample and removing the un-reacted Pt by wetetching in H2O+HCl+HNO3 at 751C, etchingtime is 3min. After the samples are cleaned, theyare loaded into a vacuum chamber for annealing.

Thermal annealing is performed at temperaturesbetween 2501C and 3501C and a pressure of about10�4 Pa. The ultra-thin PtSi films are obtained atan annealing time of 5min. XRD, XPS and highresolution TEM are used to characterize theformed layers.

3. Results and discussion

Fig. 1 shows XRD spectra of the sample, peak 3is wider, its 2y is 431–451. When I=I0 ¼ 100; 2y of

PtSi and Pt2Si are 43.581 and 44.681, respectively,peak 3 shows that the sample phase is a mixture ofPtSi and Pt2Si, and when peak 3 is very small andweak, it shows that the film is very thin.

Because the film is very thin, the phase of PtSifilm was characterized using XPS for chemicalanalysis. Fig. 2 shows XPS spectra of Pt4f and thecurve fitting of the spectra [branching ratio is 3:4,spin orbit spitting line width and line position arekept constant]. The spectra are taken using MgKa

as the source. The pass energy of the hemisphereexperiment is conducted in VG MICROLAB MKII.XPS analyzer, which is set at 20 eV. The bindingenergy of XPS core-level is calibrated by C1s

ðEB ¼ 284:6 eVÞ: Because the Pt4f7/2 line positionsof PtSi and Pt2Si are 72.5 and 71.9 eV, respectively[8], they prove the phase result of XRD analysis,and the fact that the phases of the film are PtSi andPt2Si.

Figs. 3 and 4 show TEM crystal lattice images ofsample. The sample is first cut into two pieces,

Fig. 1. XRD spectra of the sample.

Fig. 2. XPS Pt4f spectra and the curve fitting of the spectra of

the sample.

L. Shuang et al. / Vacuum 65 (2002) 133–136134

which are then glued together with the grownlayers face to face. After that, the cross-section ismechanically thinned to a thickness of about80 mm, followed by dimpling to a thickness of

about 10 mm. The dimpled sample is finally ionmilled to electron transparent, and then observedin a field emission 300 kV Philips CM 300 TEM.To know more about the continuity, smoothnessand uniformity of the films, the larger area TEMcrystal lattice images of sample were developed. Asin Fig. 3, it is seen that the film is very continuous,smooth and uniform. Fig. 4 shows smaller areahigh resolution, TEM crystal lattice images ofsample, which shows that films thickness is about4 nm.

When a thin layer of platinum on excess siliconis annealed (in vacuum or inert atmosphere) thesequential appearance of two phases is observed[9]. Initially, the reaction between the platinummetal and silicon produces the first phase, Pt2Si,the Pt2Si subsequently reacts further with thesilicon to form the second phase PtSi, until all thePt2Si has been consumed: Pt+Si-Pt2Si+Si-PtSi. The growth kinetics of both phases exhibitt1=2 time dependence [10], indicating diffusion-limited reactions. It is observed that the twophases each grow in a well-defined laterallyuniform manner and the temperature dependenceof the reaction rate each follows an Arrbenius law,with activation energies of 1.370.2 and1.570.2 eVfor Pt2Si and PtSi, respectively [10], over thetemperature range 200–3251C.

Because the activation energies for Pt2Si arelower than for PtSi, the annealing at shorter time(2min) and lower temperature (B2001C) can leadto make the sputtered Pt film forming an ultra-thincontinuous Pt2Si layer at first, and when theworking pressure is not high, oxygen from theambient diffuses into the Pt layer and reacts withthe front of Pt2Si phase to form an oxide layer.The oxide layer thus formed hinders furthergrowth of the Pt2Si layer [11], therefore, the Pt2Silayer cannot become thick. A thinner sputtered Ptlayer (20 nm) can ensure film continuity, and canobtain a continuous Pt2Si film. On wet etching thePt metal, only Pt2Si layer is left on the substrates.The Pt2Si forms PtSi by annealing. The annealingin vacuum is better than in gases such as N2, Ar; itcan reduce the affect of oxygen and makePt2Si+Si-PtSi reaction complete. The continuityof this film was improved and its thicknessremained ultra-thin.

Fig. 3. TEM crystal lattice image of sample (larger area).

Fig. 4. TEM crystal lattice image of sample (smaller area).

L. Shuang et al. / Vacuum 65 (2002) 133–136 135

4. Conclusion

Taking advantage of the fact that activationenergy for Pt2Si is lower than for PtSi, and oxygenfrom the ambient can hinder further growth of thePt2Si layer, the ultra-thin PtSi film can be obtainedby two-step vacuum annealing and wet etchingunreacted Pt. The ultra-thin PtSi film thickness isabout 4 nm. The approach has advantages of lowreaction temperature and short reaction time, andthe formed PtSi ultra-thin films have gooduniformity, continuity and smooth.

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