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Materials Science and Engineering B 133 (2006) 235–240

Short communication

Laser annealing of BST thin films with reduced cracking at anelevated temperature

Sandip Halder a,∗, Ulrich Boettger a, Theodor Schneller a, Rainer Waser a,Oliver Baldus b, Philipp Jacobs c, Martin Wehner c

a Institut fuer Werkstoffe der Elektrotechnik, RWTH Aachen, Aachen D-52056, Germanyb Institut fuer Materialforschung-I, Forschungszentrum Karlsruhe, Karlsruhe D-76021, Germany

c Fraunhofer Institute for Lasertechnology, Aachen, D-52074 Aachen, Germany

Received 3 November 2005; received in revised form 31 May 2006; accepted 3 June 2006

bstract

An excimer laser (KrF, 248 nm) was used to crystallize barium strontium titanate thin films of thickness 95 nm. Numerical simulation was firsterformed to get an idea about the laser energy suitable for crystallization. Amorphous films deposited by chemical solution deposition wererradiated at different energies with different frequencies (40–l50 Hz), laser fluences (100–150 mJ/cm2) and pulses (400–1000) while maintaininghe substrate at 25 and 250 ◦C. Films were found to crystallize above 100 mJ/cm2 in both cases. Films crystallized with substrate temperatures of50 ◦C were noticed to have less cracks. The microstructure of these films were analyzed with a scanning electron microscope. From the XPSnalysis, it is inferred that the top few nanometers have a different structure compared to normal furnace annealed thin films. This top surface layer

as found to be dependent on the laser energy used for crystallization. The dielectric constant was found to increase from 60 to 180 for the 95 nm

hin films on increasing the laser fluence. Films crystallized with 120 mJ/cm2 while maintaining a substrate temperature of 250 ◦C have leakageurrent densities of 1 × 10 −7 A/cm2 at 1.5 V and a dielectric constant of 200.

2006 Elsevier B.V. All rights reserved.

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eywords: Thin films; Sol–gel; Excimer laser annealing; Barium strontium tita

. Introduction

Laser induced phase transformation of amorphous to per-vskite films at room temperature has recently been of greatnterest. This idea of using an excimer laser, to crystallize per-vskites, originated from the use of excimer lasers for crys-allization of poly-Si based thin film transistors (TFT) [1–3].mong the techniques that have been proposed to fabricate

hese poly-Si based TFT’s, excimer laser based crystallizationf amorphous Si thin films is the most promising. This is mainlyue to the fact that laser crystallization allows for the use of lowemperature inexpensive substrates like glass or plastic, since itnvolves superfast melting and resolidification of the near surface

egion of the sample and, consequently, minimal heating of theubstrate. Furthermore, the scanning area can be freely selectedithout heating other regions. Perovskite films have recently

∗ Corresponding author. Tel.: +49 241 8027817; fax: +49 241 8022300.E-mail address: halder@iwe.rwth-aachen.de (S. Halder).

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921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.06.003

een given a lot of attention due to its probable use in a num-er of microelectronic devices. Lead zirconate titanate (used inerroelectric random access memories) [4,5], barium strontiumitanate (used in phase shifters, decoupling capacitors) [6] areome of the most currently researched topics in microelectron-cs. Thin films of such materials have been deposited by various

ethods like PLD [7], MOCVD [9], sputtering [8] and chemicalolution deposition [10] methods.

A critical problem in integrating these perovskites is oftenheir high processing temperatures (above 600 ◦C). Depositionemperatures of around 450 ◦C at the maximum are allowed after

etallization in order to avoid interface reactions of intercon-ects with the dielectric and formation of intermetallic phasesn the electronic components [11]. Laser crystallization of oxidehin films is now being considered for the crystallization of thesexides. Recently, there has been an effort to crystallize lead zir-

onate titanate (PZT) thin films with different lasers [12–14].or barium strontium titanate (BST) lately there have been onlyfew publications mostly concentrating on numerical simula-

ions [15]. The process of crystallization of BST thin films is

2 and Engineering B 133 (2006) 235–240

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FosofatFwt[ta function of depth after 20 ns. We notice that there is a largedrop in temperature (almost 400 ◦C) across the TiO2 layer. Inorder to reduce the thermal gradient across the TiO2 layer, weperformed the second set of experiments while maintaining a

36 S. Halder et al. / Materials Science

istinctly different from that of lead based compounds whichre much more volatile and easier to crystallize at lower temper-tures. In this report, we start with numerical simulation to getn idea about the laser energy window within which it woulde most suitable for crystallization and then analyze the films inerms of microstructure and electrical properties.

. Experimental

Films were deposited by a carboxylate route, the detailsf which are published elsewhere [18]. The solutions wereeposited on Pt coated Si substrates (Si/SiO2/TiO2/Pt) of 4 in.iameter. After deposition of the solution the films were spunt 500 rpm for 5 s followed by 30 s at 4000 rpm. Solutions usedere of 0.1 M strength. The coated wafer was pyrolysed each

ime at 450 ◦C. This coating and pyrolysis was performed 11imes to reach the desired thickness of approximately 95 nm.he 4-in. wafers were then diced into 1 cm2 pieces. In the firstet of experiments these 1 cm2 pieces were laser annealed at dif-erent fluences and different frequencies while in the second sethe films were laser annealed while maintaining a substrate tem-erature of 250 ◦C. Top electrodes of platinum were depositedy a photolithographic lift-off process after an initial SEM exam-nation.

. Results and discussion

.1. Numerical simulation

To understand the key parameters during a laser anneal-ng process, thermal simulation is essential. One of the mostommon equations used to study the problem of heat diffu-ion on irradiating a complex heterostructure containing themorphous PZT film is the simplified one-dimensional heatquation [19]. We use a three-dimensional axially symmetricodel for the thermalization of optical energy and heating ofmultilayer structure [20]. In this model, crystallization and

tress development due to inhomogeneous temperature distri-ution, mismatch of coefficients of thermal expansion betweenifferent layers and material contraction during amorphous torystalline phase transformation was considered. The governingquations are integrated by finite element method (FEM) usingwo-dimensional second order elements of the of the Serendipitylass with eight nodes per element [16]. On laser irradiation partf the incident radiation is reflected and part of it is absorbedccording to the Lambert Law. The absorbed optical energy isonverted to thermal energy, including a term that incorporateshe heat of fusion (Stefan problem) [23]. The following equationescribes the non-stationary heat flow.

∂(ρCpT )

∂t= ∇(λ∇T ) + qL (1)

here λ is thermal conductivity, ρ the density of the film, Cp the

eat capacity of the material and qL is the heat of fusion. Theeft side specifies the heat energy accumulated per time intervalhile the right side gives the difference of the inflowing andutflowing heat. The increasing temperature initiates the trans-

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ig. 1. Time dependence of temperature from the surface at different energyensities for a single KrF pulse.

ormation from an amorphous to a crystalline phase. Using theohnson–Mehl–Avrami (JMA) equation (Eq. (2)), the fractionf the transformed phase X at time t is computed.

(t) = 1 − exp[−(kt)n], k = νo exp

(− Ea

RT

)(2)

urther details of the numerical simulation method with valuesf the different constant can be found elsewhere [20–22]. Fig. 1hows the crystallization kinetics of the BST film as a functionf the distance from the surface at different energy densitiesor a single KrF pulse. In Fig. 2, the top surface temperaturend the bottom interface temperature is shown for the differenthickness films as a function of time for a single 100 mJ/cm2.rom the simulation it was decided that films around 100 nmould have an interface temperature of about 600 ◦C which is

he minimum temperature required for the crystallization of BST17] by chemical solution deposition methods. In Fig. 3, theemperature distribution within the heterostructure is shown as

ig. 2. Time dependence of temperature for different film thicknesses (30, 65,5, 130 and 200 nm) for a single 100 mJ/cm2 laser pulse.

S. Halder et al. / Materials Science and Engineering B 133 (2006) 235–240 237

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ig. 3. Temperature distribution within the heterostructure is shown as a functionf depth after 20 ns for different laser fiuences between 40 and 220 mJ/cm2 inteps of 20 mJ/cm2.

ubstrate temperature of 250 ◦C. Based on these simulation datahe starting laser energy was decided upon to be 100 mJ/cm2 andncreased till 150 mJ/cm2.

.2. Structural characterization

.2.1. XRD analysisGlancing angle X-ray diffraction was performed on the dif-

erent films in order to check the phase formation. Films whichere crystallized with 100, 110, 120 and 150 mJ/cm2 are only

eported here. Lower energy films were also checked but noteported here since most of them were non-crystalline. Whilelms which were irradiated with a higher energy were ofteneen to suffer from larger cracks and ablation. In Fig. 4, we seehat more number of pulses or a higher energy usually lead toetter crystallization. But various other disadvantages crop upn using higher number of pulses or higher energy, mainly lead-

ng to cracks. We notice that for frequencies above 80 Hz theres not much effect in crystallization (Fig. 4b). Therefore, mostf the data are based on films with different energies and shotsut a fixed frequency of 80 Hz.

ig. 4. XRD of the films crystallized at different fiuences and frequencies. The-axis shows the 2θ values while Y-axis shows the relative intensity.

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ig. 5. Films crystallized with different energies (a) 110 mJ, (b) 120 mJ, (c)50 mJ and (d) 110 mJ while maintaining a substrate temperature of 250 ◦C.

.2.2. SEM analysisIn order to study the microstructure scanning electron

icroscopy is one of the most common methods. We per-ormed a surface SEM of almost all the films we depositedn order to study the microstructure development with laserrradiation (Fig. 5(a–d)). Films crystallized with 110 mJ/cm2

rystallized with less cracks (Fig. 5a). In Fig. 5b, the topurface of the 120 mJ/cm2 is shown. When the amorphous films irradiated with 110 mJ/cm2 while maintaining a substrateemperature of 250 ◦C the cracks are minimized furtherFig. 5(d)). Films irradiated with 150 mJ/cm2 were found toave large cracks and ablation (Fig. 5c). From the SEM’s and,ogether with numerical simulation studies it was estimatedhat films treated with 110 and 120 mJ/cm2 while maintain-ng a substrate temperature of 250 ◦C would be ideal forrystallization.

.2.3. XPS analysisAlthough photon penetration depth is relatively large in solid

aterials (a few micrometers for keV X-rays), the analyzed pho-oelectrons come from the superficial layers only since electronshoto-emitted from deeper layers suffer inelastic collisions inhe material. Electrons detected in the photoelectron peaks comerom an average depth of a few tens of angstroms below the sur-ace. Therefore, photoelectron spectroscopy is only sensitive tohe surface of the material. In our case, this analysis plays anmportant part because the interface always plays a crucial partn the final device characteristics. A low dielectric constant layer

t the interface would drastically reduce capacitance values ofhe capacitor. Similar phases at the top surface were reported byingh and Craciun [26] during UV pulsed laser deposition ofST thin films.

238 S. Halder et al. / Materials Science and Engineering B 133 (2006) 235–240

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Fig. 6. XPS spectra of C ls.

Each of the Figs. 6–10 gives the C ls, O ls, Ti 2p, Ba 3d and Srd photoelectron peaks for the (a) amorphous films; (b) for theaser crystallized films with 400 shots at 100 mJ/cm2; (c) laserrystallized films with 400 shots at 150 mJ/cm2; (d) normal oxy-en crystallized films at 750 ◦C. The C ls photoelectron peaksFig. 6) for the four differently crystallized films mainly showhree contributions. Peaks at 288.9 and 284.8 eV were assigned

o CO3

2− and C–H/C–C, respectively. Smaller peaks for C–Ot 286.6 are noticed after deconvolution of the spectrum. Sincehese films are derived from a solution with butanol its presence

Fig. 7. XPS spectra of Ti 2p.

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Fig. 8. XPS spectra of O 1s.

s expected and is strongest for the amorphous films. It is interest-ng to note that the CO3

2− peak almost does not exist before theaser irradiation but can be seen after an initial irradiation of with00 pulses at 100 mJ/cm2. On increasing the energy it diminishesndicating a decrease of the carbonates. This can be explainedn the basis of an intermediate oxo-carbonate phase which usu-lly forms when carboxylate based precursors are used. This

ntermediate oxo-carbonate phase usually decomposes at higheremperature and so with higher pulse energy it tends to decrease.he Ti 2p peaks at 458.4 and 464.1 are for the Ti 2p3/2 and Ti

Fig. 9. XPS spectra of Ba 3d5/2.

S. Halder et al. / Materials Science and Engineering B 133 (2006) 235–240 239

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Fig. 11. Average dielectric constant of the different films at different fluencesww

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teticibssseifound to suffer from cracks even when the substrates were main-tained at 250 ◦C.

Fig. 10. XPS spectra of Sr 3d.

p1/2, respectively (Fig. 7). On laser annealing the Ti 2p3/2 peaks found to red shift slightly which could be due to the changen the microenvironments as the process of crystallization takeslace. Gust et al. [24] have already shown the initial formationf TiO2 nuclei prior to the formation of BaTiO3 by TEM. The Os signal (Fig. 8) also shows three components. Peaks at around29.6 eV are those of oxides. At 530.8 eV the peaks are due toO3

2−, which reduce on laser crystallization with higher laserulse energy. Presence of H2O or contamination leads to peakst 531.7 and 532.8 [25]. Photopeaks for Ba 3d may be resolvednto two components (Fig. 9). The peaks around 779.0 eV [27]re for Ba atoms associated in the perovskite structure while theeaks at 780.0 eV could be due to Ba atoms within a decomposedarium carbonate/oxocarbonate phase [28]. Fig. 10 shows the Srd5/2 and Sr 3d3/2 photo peaks. Hence at the surface there existslayer which is quite different from the bulk crystalline BST and

his layer was dependent on the laser energy used for crystalliza-ion. This surface layer achieved an almost similar structure tohe normal oxygen furnace processed films as the laser energyas increased.

.3. Electrical characterization

The capacitance versus voltage was measured for the filmsrystallized with different energy densities keeping the fre-uency (80 Hz) and number of shots (400) fixed (Fig. 11). Thenset shows the dielectric constant versus voltage of the filmnnealed with 120 mJ/cm2. The dielectric constant is found to

ncrease from 60 to 180 for the films when treated by an increas-ng laser energy, which is quite good considering the thicknessf the films to be 95 nm. The reason for the low tunability inhe films could be due to the different surface layer of the films.

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ith 400 shots. Inset shows the dielectric constant vs. voltage of the film annealedith 120 mJ/cm2.

ften a low dielectric constant layer at the top electrode inter-ace can lead to low dielectric constant in the total capacitancef the capacitor.

An important property of perovskite type titanate thin films ishe charge transport mechanism. The dc leakage currents werevaluated using a step voltage technique. For the step voltageechnique a voltage is applied to the sample and current flows measured as a function of time, at times lesser than somerossover time this measurement is dominated by some polar-zation current, while at times longer than this it is dominatedy the true leakage current. But care should be taken so that thetep time period does not reach the degradation regime of theample. The leakage current density (JC) of the films was mea-ured and it was found that when irradiated with higher lasernergy (120 mJ/cm2) the leakage current density reduced. Thiss shown in Fig. 12. However, beyond 120 mJ/cm2 films were

ig. 12. Leakage current density (JC) of the films crystallized with differentuences. Films beyond 120 mJ/cm2 had more microcracks and so were difficult

o characterize.

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. Conclusion

In conclusion, thin films of 95 nm thickness were crystallizedy KrF excimer laser. Numerical simulation was first performedo get an idea about the range of laser energy suitable for therystallization to take place. Films were crystallized above annergy density of 100 mJ/cm2 but above 120 mJ/cm2 there wasome amount of ablation and cracking. From SEM studies, itas seen that films which were annealed while maintaining a

ubstrate temperature of 250 ◦C and an energy density lowerhan 120 mJ/cm2 had less cracks. Films which were crystallizedith 120 mJ/cm2 had the best electrical properties. From an XPS

tudy, it is inferred that with 400 pulses at 110 mJ/cm2 the sur-ace has a structure different from the bulk but on increasinghe energy to 120 mJ/cm2 this top layer achieved a structurelmost similar to that present in normal furnace annealed films.ielectric constant of the films increased from 60 to 180 on

aser annealing with higher fluences which is comparable to theeported values of the normal furnace processed films. Leakagef the films decreased when higher fluences were used for therystallization.

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