13th IEE Intemational Conference on Advanced Thermal Processing of Semiconductors - RTP 2005

Ni2Si AND NiSi FORMATION BY LOW TEMPERATURE SOAK AND SPIKE RTPs

Eun-Ha Kim', Hali Forstner2, Majeed Foad2, , Norman Tam2, Sundar Ramamurthy2, Peter B. Griffin' andJames D. Plummer'

'Center for Integrated Systems, Stanford University, Stanford, CA 943052Applied Materials Inc., Sunnyvale, CA 94086

As the size of metal oxide semiconductor devices continues to be scaled down to sub-90 nm, novel materials must be integrated successfully in order to meet the technicaldemands. Nickel silicide (NiSi) is being considered as an alternative material to cobaltsilicide (CoSi2) for the self-aligned silicide process, because it forms at lowertemperatures with less silicon consumption and is compatible with SiGe. In order toprevent excessive silicidation in narrow gate lines and at the edges of source/drainregions, NiSi integration requires limiting silicidation kinetics via reduced thermalbudgets followed by forming the low resistance phase. This paper focuses on the lowtemperature regime of the Ni-Si reaction through the use of soak RTP at 300 °C andspike RTP at 300 - 400 'C. In order to study the formation of Ni2Si and NiSi and thetransformation from Ni2Si to NiSi, the silicide films are characterized by Rs sheetresistance measurements, XRD for phase identification, and TEM for microstructure. Theintermediate phase of Ni2Si is formed at 270 'C and its growth is observed withincreasing anneal time. At temperatures above 300 'C, the NiSi phase is found inaddition to the Ni2Si phase, and the transformation from Ni2Si to NiSi is observed. Thesequence of the Ni2Si-NiSi transformation involves the initial formation of NiSi and thechange in the alignment of the crystal planes as the low resistance phase of NiSi forms.Two RTP schemes, soak RTP and spike RTP, follow parallel trends in the sequence ofthe Ni2Si-NiSi transformation with marked differences in the reaction kinetics.

INTRODUCTION

Self-aligned silicide has been used to reduce thesheet resistance of poly-silicon gates and the contactresistance to silicon source/drain regions. For deviceswith a design rule of sub-90 nm, source/drain regionsrequire shallow junctions with high dopingconcentration to minimize the short channel effectand the diffusion sheet resistance. Nickel silicide(NiSi) emerges as a promising self-aligned silicidematerial for sub-9Onm devices, because NiSi forms atlower temperatures with smaller silicon consumptionthan other common silicides (TiSi2 and CoSi2) [1].NiSi also has superior compatibility with SiGe that is

often used in the source/drain region of the advanceddevice structure to induce strain in the silicon channel.The low resistance phase of NiSi formation

involves the nickel film deposition and the rapidthermal processing (RTP) anneal for the silicidationreaction between nickel and silicon. During thereaction, as nickel is the dominant diffusing species,its fast diffusivity causes the excessive NiSiformation in the narrow gates as well as at the edgesof source/drain regions [2]. The thicker silicide layerscan result in poor leakage behavior in junctions anduncontrollable fully-silicided gates [3] that can alterdevice characteristics. To prevent the excessivesilicidation, a two-step RTP has been suggested. Theprocess includes the first RTP (RTPI) at

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temperatures lower than 300 °C to create anintermediate Ni-rich phase followed by unreacted Niremoval and the second RTP (RTP2) at 400-600 °Cto form the low resistance phase of NiSi [2].Introducing RTPI step at a low temperature iseffective in limiting the Ni diffusion, and thusalleviating excessive silicidation. Low temperaturespike RTP is also being investigated for the RTP1step, since it can further control the thermal budgetwith fast temperature ramping and negligible annealtimes at peak temperatures.

In this work, we investigate the low temperatureregime of the Ni-Si reaction through the use of soakRTP and spike RTP at temperatures below 400 °C,including the formation of Ni2Si and its evolution toNiSi. Furthermore, by comparing soak RTP withspike RTP, we explore the effect of temperatures andreducing anneal times on the silicidation reaction.

EXPERIMENTAL

P-type (100) silicon wafers with a resistivity of 10-20 Q-cm were used. After removal of native oxide indilute HF, 10 nm-thiclNi films were deposited in anEndura ALPS Ni PVD chamber of Applied Materials.10 nm-thick TiN cap layers were then sputtered ontop ofNi films. Using the Centura RadiancePlus RTP,the silicidation was performed in the temperaturerange of 270 - 300 °C varying soak times from 1 to200 seconds. In addition to soak anneals, lowtemperature spike anneals, which use a fast ramp tothe peak temperature with a negligible soak, wereperformed at higher temperatures from 300 to 400 'C.TiN caps and unreacted nickel films were removedselectively using H2SO4+H202 prior to electrical andmaterial characterizations. Sheet resistance (Rs) wasmeasured to determine the electrical conductivity ofthe silicide films. X-ray diffraction (XRD, PhilipsX'Pert) was employed to identify the phase of thesilicide films. With an X-ray generated at 45 KeV, 30A, the diffractometer was configured for the theta-2theta scan to detect the planes parallel to the surfaceand for the glancing incidence XRD (GIXRD) scan ata fixed incidence of 0.5 °. Further analysis wasperformed on the selected samples by transmissionelectron microscopy (TEM, Philips CM20). TEM

specimens were prepared by thinning and ion-millingmethods, where every procedure was performed attemperatures below 100 °C in order to prevent anysubsequent annealing effect on the silicide films.

RESULTS AND DISCUSSION

1. Soak RTP annealsThe Rs data measured on the silicide films after

RTP at 270 and 300 °C is shown in Fig. 1. Withincreasing soak times at 270 °C, we observe that Rsdecreases from 120 to 40 ohms/sq in the short timerange from 5 to 45 sec, then plateaus at 40 ohms/squntil 200 sec. Raising the RTP temperature to 300 °Cyields a different Rs trend that shows an initialplateau at 40 ohms/sq until 45 sec and a gradualdecrease after 60 sec. This Rs behavior can beassociated with the progress in the silicide formationreaction, which involves the Ni-film consumptionand the Ni2Si film growth at 270 °C and thetransformation into the low resistance phase of NiSiat 300 °C.

120

t 100C)Eo 800

rs 60.UM 40

, 20to

0 50 100 150

Soak Time (Sec)200

Fig. 1. Sheet resistance of films after soak RTP at270 and 300 °C for 1 - 200 sec.XRD scans are displayed in Fig. 2 for the silicide

films formed by RTP at 270 IC. Ni2Si is found as themain phase from all the samples for various soaktimes. With increasing the soak time from 5 to 45 sec,the intensities of the peaks increase, which is anindication of the growth of the Ni2Si films. For thesamples annealed longer than 45 sec, the intensitiesare more or less the same until 120 sec. This resultsuggests that the silicidation reaction, mainly Ni2Si

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growth at this temperature, is completed after 45 sec,with full consumption of the deposited Ni-films. Thephase identification from XRD measurements isdirectly related to the Rs behavior that shows theinitial decrease followed by saturation after 45 sec.

-iD0.Z

30 35 40 45 50

2 theta (degree)55 60

Fig. 2. XRD spectra of the silicide films after soakRTP at 270 'C.In Fig. 3 (a), XRD data for the 300 'C samples

show the formation of NiSi in addition to Ni2Si evenafter the short time anneal of 5 sec. As the annealtime increases from 5 to 45 sec, a decrease in Ni2Sipeaks and an increase in NiSi peaks are observed,which indicates that the initial phase of Ni2Siconverts to NiSi progressively with time at thetemperature of 300 'C. Comparing with the Rs datafrom these samples from Fig.1, different fractions ofNi2Si and NiSi do not seem to affect the Rs of films.For the 80-150 sec samples, which show the gradual

(a _ (b)r0 0 0

30 15 5 e0 15 e

lOOsec lOO~~~~~10sec-80Bsec8Oe

45sec 4.Ssec3Osec 30 ec

lOsec lOsec

5sec 5ec

30 35 40 45 55 55 6030 35 40 45 50 55 602 theta (degree) 2 theta (degree)

Fig. 3. (a) XRD and (b) GIXRD spectra from the silicidefilms after soak RTP at 300 'C.

decrease in Rs, XRD spectra reveal that while theNi2Si peaks are steady with the minimum height, theNiSi (121) peaks diminish from the 80 - 150 secsample. In order to confirm the formation of NiSi inthese samples, glancing incidence XRD (GIXRD)spectra are displayed in Fig. 3 (b). As can be seen,the 80 - 150 sec samples show a number of NiSipeaks, which are not seen from the XRDmeasurement in a co-20 scan mode.

Previously, Detavemier et al. [4] reported that theNiSi films on the Si substrates prefer a planaralignment across the interface. Noting that theinterplanar spacings for NiSi (121) and (220) arealmost identical to that for Si (220), the alignment ofthese NiSi planes results in a fiber-like texture that isnot parallel to the surface. Looking back at our XRDdata, it describes not only the transformation fromNi2Si to NiSi but also the change in the alignment ofthe NiSi (121) planes. For the samples annealed for 5- 80 sec, the NiSi (121) peak is found as the first signof the NiSi phase formation and its intensityincreases as the anneal continues. During theextended anneal from 80 to 150 sec, the NiSi (121)peak reduces and disappears in the w-20 scans, whichcan be described as the preferred texture. NiSi (121)planes are no longer aligned parallel to the Si (001),but possibly parallel to the Si (220), consistent withprevious studies. This texture occurrence in the NiSifilms appears to be directly related to the decrease inRs.

2. Spike RTPfor the NiSi FormationSpike RTP was performed in order to investigate

the effect of the peak temperature on the silicide filmformation and to compare with the results obtainedfrom the soak RTP samples. During each anneal,across-wafer temperatures were measured usingmultiple pyrometers. Examples of such temperaturetraces for spike RTP at 340 °C and for soak RTP at300 °C, for a comparison, are shown in Fig. 4 (a), (b)respectively. As can be seen in the temperature traces,the spike RTP heats and cools the wafer faster with arate of 10 °C/sec than the soak RTP (- 5 °C/sec).

Fig. 5 presents the Rs data measured on thesamples that received the spike RTPs as a function ofthe peak temperature. Spike anneals at temperaturesof 300 350 °C create silicide films with similar Rs of

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360

- 340

- 320cm

a 300-

; 280

5 260QE 240

220

200

360

340

- 320co

23 300-

' 280

- 260QE 240

1- 220-

0 10 20 30 40 50

Time (Sec)60

200 . .,0 10 20 30 40 50

Time (Sec)

70 80

60 70 80

Fig. 4. Temperature profiles of (a) spike RTP at 340°C and (b) soak RTP at 300°C for 30 sec.

40 Q/sq. With an increase in the temperature from350 to 370 °C, it is observed that Rs drops from 40to 7 Q/sq, which corresponds to the Ni2Si-NiSitransformation.XRD and GIXRD spectra are shown in Fig. 6 (a)

and (b) respectively. In the co-20 scans, as the spiketemperature increases from 300 to 350 °C, the Ni2Sipeaks decrease and the NiSi peaks increase. Thestrong NiSi (121) peak is found in the 340 °C sample,then it disappears in the 370 and 400 °C samples,while a small peak of NiSi (301) is observed. In theGIXRD scans, a number ofNiSi peaks are seen in thesamples annealed at 370 and 400 'C. Together withthe Rs plot, these XRD results illustrate the evolution

120

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A 80

00,x 60

40

<, 20 \0

cl) n

280 300 320 340 360 380 400 420

Peak Temperature (degree)Fig. 5. Sheet resistance of films after spike RTPsat 300 -400 'C.

of the Ni2Si-NiSi transformation. At the early stage

of the transformation into NiSi (300-350 °C), whilethe XRD patterns show increasing NiSi (121) peaksand decreasing Ni2Si peaks, Rs does not reflect theincrease in the fraction of the NiSi phase. Asubstantial drop in Rs is measured when the NiSi(121) peak is no longer detected from the o-20 scans,

indicating that the low resistance NiSi bears theparticular texture where the NiSi (121) planes are not

aligned parallel to the Si (001) planes but possiblyparallel to the Si (220) planes, as discussed earlier.

(a (b)

spike~~~~00P at 00C 00°C

fim as the pek epeaur iceae, E

OC~~C 4CO

370C 3 07

mirorahsofhe310,340C and370°Csample30C

3100c 310C

300C

30 35 40 45 50 55 60 30 35 40 45 50 55 t0

2 theta (degree) 2 theta (degree)

Fig. 6. (a) XRD and (b) GIXRD spectra of films after

spike RTPs at 300 -400 'C.

In order to illuminate the evolution of the silicide

films as the peak temperature increases, TEM

micrographs of the 310, 340, and 370 'C samples are

shown in Fig. 7 (a), (b), and (c), respectively. The310 'C sample primarily consists of a single layer ofthe Ni2Si grains typically 20 nm across. A slightcontrast is observed at the interface, suggesting an

interfacial reaction between the Ni2Si and the Sisubstrate to form the NiSi phase. At 340 'C,distinctive two layers are formed. The upper layer ofNi2Si displays the grain structure that is similar to the310 'C sample, and the NiSi layer near the Si showslarge grains and a smooth interface with the Si. Theboundary between the Ni2Si and the NiSi is notabrupt, owing to the development of thetransformation from Ni2Si to NiSi. Thetransformation into NiSi is mostly completed by thespike RTP at 370 'C as shown in the Fig. 7 (c). The370 'C sample contains the NiSi layer with largegrains and the thin top layer of the Ni-rich phase,Ni2Si, which has not tranformed to the NiSi phase.

180

(a)

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(b),....I

_I _ - --II- t.

III_II

! L I LI-__I-IJ I----1____<

I~

l

1awy l l

Li -

I

.

Fig. 7. TEM micrographs of the films received spike RTPs at (a) 310 °C, (b) 340 °C, and (c) 370 'C.

Compared with the isothermally annealed samplesat 300 °C, we notice that the two schemes of soakand spike RTP anneals lead to similarities in thesilicide films and the evolution of the Ni2Si-NiSitransformation. In terms of Rs and XRD pattern ofthe silicide films, the spike RTP at 340 °C appears tobe equivalent to the soak RTP at 300 °C for 30 sec.However, two RTP conditions can result indifferences in the Ni diffusion in Si. The significantlyshorter time with spike anneals limits the amount ofNi diffusion. Additionally, due to the smalleractivation energy for Ni diffusion (Ea=0.47 - 0.76 eV)[5], increasing the anneal temperature does notenhance as much as the Ni2Si formation (Ea=1.55 eV)[1] for the same temperature increase, Thus, the Nidiffusion can be effectively reduced when the spikeRTP is performed at a higher temperature to yield asimilar silicide film. Since Ni diffusion can be asource for the leakage current in silicided junctions,spike RTP can improve the junction characteristicswhile maintaining the low resistance in thesource/drain region.

SUMMARY

We have studied the reaction between deposited 10nm-Ni films and Si substrates by using soak RTPs aswell as spike RTPs. Through Rs and XRDmeasurements, we observe the time-evolving Ni2Sigrowth at 270 °C and the Ni2Si-NiSi transformationat 300 °C and higher temperatures. The sequence ofthe Ni2Si-NiSi transformation involves the initialformation of NiSi and the change in the alignment ofthe crystal planes with respect to the Si substrates.The low resistance phase of NiSi displays a preferredtexture so that the (121) NiSi planes are not aligned

parallel to the (001) Si planes, rather possibly parallelto the (220) planes due to the small mismatch in theinterplanar spacings for the NiSi (121) and the Si(220).The spike and soak RTP anneals lead to similarities

in the silicide films and the sequence of the Ni2Si-NiSi transformation. Since Ni diffusion in silicon hasa smaller activation energy than the silicide formation,a low temperature spike anneal to form to lowresitance silicide films will minimize the Ni diffusioninto the silicon.

ACKNOWLEDGEMENTThe authors would like to thank the technical staff

at Applied Materials, especially Bertha Chang andJulieta Ruiz for Ni-film deposition and Toni Wiscoand Steve Nagy for RTP.

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