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Hybrid titaniumaluminum oxide layer as alternative high-k gate dielectricfor the next generation of complementary metaloxidesemiconductordevicesO. Auciello, W. Fan, B. Kabius, S. Saha, J. A. Carlisle et al.Citation:Appl. Phys. Lett. 86, 042904 (2005); doi: 10.1063/1.1856137View online: http://dx.doi.org/10.1063/1.1856137View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v86/i4Published by theAIP Publishing LLC.Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/Journal Information: http://apl.aip.org/about/about_the_journal

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Hybrid titaniumaluminum oxide layer as alternative high-k gate dielectricfor the next generation of complementary metaloxidesemiconductordevices

O. Auciello,a W. Fan, B. Kabius, S. Saha, and J. A. CarlisleMaterials Science Division, Argonne National Laboratory, Argonne, Illinois 60439

R. P. H. Chang

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208

C. Lopez and E. A. IreneDepartment of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599

R. A. BaragiolaDepartment of Materials Science, University of Virginia, Charlottesville, Virginia 22904

Received 2 September 2004; accepted 30 November 2004; published online 20 January 2005

Research is focused on finding reliable high-dielectric constant k oxides with high capacitance and

all critical properties required for the next generation of complementary metaloxide

semiconductor CMOS gates. A trade-off between dielectric constant and band-offset height is

generally observed on gate oxides. Combining TiO2 and Al2O3, with the two extremes of high

permittivity k and high band offset, we produced a TixAl1xOy TAO oxide layer with k= 30

and low dielectric leakage for a next generation of high-k dielectric gates. We developed a low

temperature oxidation process, following room temperature sputter-deposition of TiAl layers, to

produce ultrathin TAO layers on Si with subatomic or no SiO2 or silicide interface formation. We

demonstrated TAO layers with 0.5 nm equivalent oxide thickness on Si and thermal stability

under rapid thermal annealing up to about 950 C. The data presented here provide insights into

fundamental physics and materials science of the TAO layer and its potential application as gate

dielectric for the next generation of CMOS devices. 2005 American Institute of Physics.

DOI: 10.1063/1.1856137

The technology roadmap1

for the next generation of

complementary metaloxidesemiconductor CMOS de-vices requires an equivalent oxide thickness EOT of SiO2of less than 1.5 nm to maintain a suitable capacitance for

sub-100 nm gate. Materials with intermediate dielectric con-

stants 12k201,2

trading-off between high k and offset

barrier height are investigated as gate layers. Thin layers of

HfO2 and ZrO2 with EOT less than 1.5 nm and low leakage

have been reported.1,2

However, higher dielectric constant

might be desirable for future CMOS requirements TiO2 at-

tracted attention because of its high k50, arising from soft

phonons involving Ti ions. However, TiO2 exhibit high leak-

age current, due to a near zero offset barrier to Si,3

and

undesirable crystallization at low temperature 400 C .

Alternatively, Al2O3 exhibits the largest band gap Eg=8.8 eV next to SiO2 with a conduction band offset of

2.8 eV with respect to Si, 4.9 eV valence band offset, andthermodynamic stability against Si, but has relatively low k

=810. The idea of the TixAl1xOy TAO high-k layer pre-sented in this letter evolved from our prior work developing

a Ti0.5Al0.5 alloy layer as oxygen diffusion barrier for inte-

gration of complex oxide layers with Cu-based electrodes.4

Our studies showed that the TiAl oxygen barrier resulted

from formation of an amorphous TAO thin 46 nm thicksurface layer, including TiO2 and Al2O3 the two extremes of

high permittivity and high band gap, respectively . Thus,

TAO with Ti:Al=75:25at. % could be a dielectric material

with combined high permittivity, low leakage current, and

high thermal stability.

A limitation to achieve a sub-1 nm EOT dielectric layer

is the formation of an interfacial low-k SiO2

layer, formed

during high temperature growth of the high-k film, which

lowers the MOS capacitance when in series with a high-k

layer. Therefore, a low partial pressure of oxygen/growth

temperature processes is desirable to inhibit or minimize Si

oxidation.

The initial studies of the TAO layer discussed here were

performed using a sputter-deposition system integrated with

time-of-flight mass spectroscopy of recoil ions TOF-MSRI

and x-ray photoelectron spectroscopy XPS5

that provides

controlled oxide growth and enables in situ analysis of the

layers without exposure to air. We are also investigating mo-

lecular beam epitaxial and atomic layer deposition methods.

TAO layers were produced via room temperature RTsputter-deposition of thin TiAl alloy films in the ultrahighvacuum system 109 Torr base pressure described earlier,followed by in situ oxidation. Amorphous TiAl

75/25 at. % layers were deposited on hydrofluoric acid

finished n Si 100 = 4 6 cm and p Si 100

=1216 cm substrates using a broad beam of 500 eV Xe

ions. TOF-MSRI, used to monitor the TiAl growth on Si,

indicated that a pinhole free 0.9 nm TiAl layer covers the Si

substrate. Atomic oxygen delivered by an Oxford source at104 Torr was used to convert the TiAl metallic into fully

oxidized TAO via postdeposition oxidation at 25700 C.

In situ XPS analysis of the TiAl layers after oxidationa Electronic mail: [email protected]

APPLIED PHYSICS LETTERS 86, 042904 2005

0003-6951/2005/86 4 /042904/3/$22.50 2005 American Institute of Physics86, 042904-1

http://dx.doi.org/10.1063/1.1856137


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revealed complete transitions of Ti0 to Ti4+ and Al0 to Al3+

within the 25 700 C range Figs. 1 a and 1 b , indicating

that stoichiometric alloy oxides with identical chemicalbonding are produced by oxidation with atomic oxygen at all

temperatures, unlike molecular oxygen O2 . The XPS re-

sults were confirmed by near-edge x-ray absorption fine

structures NEXAFS studies of the O K-edge and Ti L-edge

of TAO thin layers on Si. NEXAFS is sensitive to the local

bonding environment, providing information on crystal

structures and forms of titanium oxides. In Fig. 2 a , the

doublet shown at the O K-edge, corresponding to an electron

transfer from the O 1s orbit to a covalently mixed state from

the O 2p and Ti 3d states, agrees with published NEXAFS

spectra for TiO2.6

The broad peak at 550 eV correspondsto the superposition of O p states hybridized with Al 3sp and

Ti 2t2g and 3eg, and its shape is similar to the O K-edge froma 34-ML-thick TiO2 film grown on Al2O3.7

The two

Ti L-edge-peaks in Fig. 2 b arise from the spinorbit inter-

action of the Ti 2p core level, and could not be confused

with Ti3+ and Ti2+ oxidation states from analysis of Ti2O3and TiO.

8The similarity of the O K- and Ti L-edge spectra

from TAO layers grown at 500 C and RT indicates that low

temperature oxidation produces stoichiometric TAO.

Thermodynamically, Ti and Al oxidize favorably over

Si, However, SiO2 formation is determined by kinetics when

nonequilibrium or excess amount of oxygen is present. In

situ XPS studies of the TAO/Si interface were performed on

3-nm-thick TiAl layers deposited on Si Fig. 1 c to allowdetection of photoelectrons from the interface escaping

through the TAO layer. The formation of a SiO2 interface

layer is revealed by the Si3+ peak at 102.5 eV, but the lowintensity relative to the Si0 Fig. 1 d is indicative of a largesuppression of SiO2 formation at low oxidation temperature,

due to the excellent barrier property of the amorphous TAO

surface layer.4

The low-temperature 50 C oxidation

used in this study practically eliminates the possibility of

undesirable silicide formation.

Cross-sectional transmission electron microscopy TEM

studies of the RT oxidized TAO thin layer were performed

with a microscope equipped with Cs corrector for the objec-

tive lens, to avoid contrast delocalization.9

Two amorphous

layers can be distinguished in Fig. 3 a , i.e., a darker top

layer L2 3 4 nm thick and a brighter intermediate

layer L1 2 nm thick . The line scan across the imagewith a linewidth of 3 nm shows a sharp boundary between

the crystalline Si substrate and the amorphous TAO layer. A

Fourier transformation of the line scan at the L1 and L2

layers location did not reveal any periodicities. We used en-

ergy filtering TEM to obtain a qualitative mapping of the

elements Si, O, Ti, and Al in a 200 kV TEM TECNAIF20ST . For energy width of 10 eV, a resolution of better

than 0.6 nm can be expected from the dampling envelopes of

spatial and temporal coherence.10

Elemental maps using the

Al L-, Si L-, Ti L-, and O K-edges were obtained using the

three-window method11 Fig. 3 b . The TAO layer L2 and

a layer containing mostly Al and oxygen L1 , which pro-

vides a much more stable interface than having direct contactbetween Ti and Si, are recognizable in the high-resolution

FIG. 1. In situ XPS analysis of TiAl thin layer on Si before and after

oxidation with atomic oxygen. The Al 2p a and Ti 2p b spectra showcomplete transitions to Al3+ and Ti4+ for both oxidation processes. The Si 2p

spectra c and the Sin+/Si0 intensity ratio vs oxidation temperature d re-

veal inhibition of SiO2 interfacial layer formation.

FIG. 2. NEXAFS analysis of TAO thin layers produced with atomic oxygen

oxidation at RT and at 500 C for 30 min. The O K-edge a and Ti L-edge

b spectra show identical O and Ti local binding environments for bothoxidation conditions, consistent with XPS analysis.

FIG. 3. Color HRTEM and EELS analyses of room-temperature oxidized

TAO layer grown on Si 100 . a HRTEM image, b elemental maps, and c integrated line scans for Al, Si, O and Ti, respectively.

042904-2 Auciello et al. Appl. Phys. Lett. 86, 042904 2005



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TEM image. An intermediate layer I consisting mostly ofSi and O is evident between the substrate and L1, in agree-

ment with the XPS analysis showing a minimal Si3+ intensity

on the TAO layer oxidized at low temperature Fig. 3 .The C V analysis of capacitors defined with 100-

to 500-m-diam top Pt electrodes, with various TAO physi-

cal thicknesses, demonstrated a permittivity of 30 for the

dielectric layer. A significant increase for the accumulation

capacitance density was observed for the TAO dielectric

layer produced at temperatures 200 C with atomic oxy-

gen, due to a large suppression of the interfacial SiO2 forma-

tion. Figure 4 shows the C V characteristics for 4-nm-thick

TAO layers produced on n Si and n+ Si by RT oxidation.

The dual frequency technique12

was used to extract the C V

behavior of TAO layers grown on n Si, to eliminate the

parasitic resistance influence from the substrate. High capaci-

tance densities of 7.7 8.3 F/cm2

EOT=0.420.48 nm

were obtained on all n and p Si substrates, consistent withthose measured from the same TAO layers grown on heavily

doped Si =0.004 cm . The leakage current across the

TAO layers, at 1 V, was about 5 A/cm2 on n and p sub-

strates i.e., 45 decades lower than that for SiO2 with asimilar EOT . Preliminary band gap measurements via spec-

troscopic ellipsometry and EEELS revealed a band gap of

4 eV for the TAO composition reported here. Initial mea-surements of the trapping density of states revealed values of

81011 to 1012. It is encouraging that by adding a very

small amount of Al2O3 into a TiO2 matrix, the band gap of

the latter is increased by 1 eV, and the density of trapping

states is within the same order of magnitude as for frontrun-

ners amorphous oxide candidate materials. Atomic force mi-

croscopy analysis demonstrated that the surface morphology

of the thin TAO layer remained practically the same as that

of the original atomically smooth Si surface after 30 s rapid

thermal annealing up to 950 C, indicating that no crystalli-

zation or interfacial reaction has taken place. There is sub-stantial room for compositional improvement in the TAO

layer to optimize its performance as a high-k dielectric.

In summary, a room temperature oxidation process via

atomic oxygen exposure of room temperature sputter-

deposited metallic alloy films was developed to produce

TAO high-k thin layer with highly suppressed or no SiO2interface and free of silicide. The resulting amorphous TAO

layer on Si exhibits high capacitance density with exception-

ally low leakage. A TAO layer with EOT0.5 nm was pro-

duced on Si, satisfying capacitance density and leakage re-

quirement of next generation CMOSs.1

The authors acknowledge support from the Departmentof Energy-Office of Science-Basic Energy Science under

Contract No. W-31-109-ENG-38. The authors thank S.

Lentzen and K. Urban from the Research Center in Jlich,

Germany, for experimental time on the Cs corrected TEM.

1International Technology Roadmap for Semiconductors, 2001 edition,

ITRS Home Page, http://public.itrs.net 2001 .2Alternative Gate Dielectrics f or Microelectronics, edited by R. M. Wallace

and G. Wilk, MRS Bull. 27, 3 2002 .3J. Robertson, J. Vac. Sci. Technol. B 8, 1785 2000 .

4W. Fan, B. Kabius, J. M. Hiller, S. Saha, J. A. Carlisle, O. Auciello, R. P.

H. Chang, and R. Ramesh, J. Appl. Phys. 94, 6192 2003 .5O. Auciello, A. R. Krauss, J. Im, and J. A. Schultz, Annu. Rev. Mater. Sci.

28, 375 1998 .6F. M. F. de Groot, J. C. Fuggle, B. T. Thole, G. A. Sawatzky, and H.

Peterson, Phys. Rev. B 40, 5715 1989 .7M. Snchez-Agudo, L. Soriano, C. Quirs, J. Avila, and J. M. Sanz, Surf.

Sci. 482485, 470 2001 .8V. S. Lusvardi, M. A. Barteau, J. G. Chen, J. Eng Jr., and A. V. Teplyakov,

Surf. Sci. 397, 237 1998 .9M. Haider, S. Uhlemann, E. Schwan, H. Rose, B. Kabius, and K. Urban,

Nature London 392, 768 1998 .10

K. Ishizuka, Ultramicroscopy 5, 55 1980 .11

R. F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Micro-

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FIG. 4. C V characteristics of the TAO-based MOS capacitors on n and

n+ Si with Pt top electrodes, measured at different frequencies.

042904-3 Auciello et al. Appl. Phys. Lett. 86, 042904 2005

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