Tinta Invisible

Titanium dioxide (TiO?) has many interesting phy­ sical properties that make it suitable for a variety of applications. Its high dielectric constant, due to its large lattice polarisability, gives it potential as a capacitor for charge storage insulators in dynamic random access memories (DRAMs), and as a potential replacement material for silicon dioxide (SiOz) as tunnelling leak­ age cunents limit a further downscaling of microelec­ tronic dimensions below approximately 1.5 nm. TiO^ is chemically stable, has excellent optical transparency in the visible and near-infrared regions and a high refractive index, which makes it useful for anti-reflec­ tion coatings in optical devices. Dielectric constant

C o iT e sp o n d in g author. T el.: -1 -4 4 -2 0 7 -4 1 9 -3 1 9 6 ; fax: + 4 4 - 2 0 7 - 3 8 8 - 9 3 2 5 .

E -m a il a d d r e s s ’, n k a liw o h @ e e .u c l.a c .u k (N . K a liw o h ).

values between 30 and 100 have been reported [ 1 ^ ] for a range of such films prepared by methods includ­ ing photo-induced sol-gel processing [5], metal-

organic chemical vapour deposition (MOCVD), [6],

CVD [7], r.f. sputtering [3], photo-induced CVD using

excimer lasers [8] amongst other methods. Photo-CVD

is appeahng for the deposition of TiO^ for microelec­ tronic devices because deposition conditions are con­ trollable. Moreover, excimer lamps can be selectively used to deposit materials over wide areas. Combined with an injection liquid source, which has previously been used to grow other high dielectric constant mate­ rials including TaiOg, precise properties can be achieved at low temperatures that are compatible with microelectronic processing over wide areas [9,10]. In this paper, we report the growth and characterisation of TiO^ thin films on Si substrates and on quartz at temperatures in the range 50-350 °C by photo-CVD using KrCl excimer lamps.

0 1 6 9 - 4 3 3 2 /0 2 / $ - s e e fro n t m atter (g) 2 0 0 2 P u b lish e d b y E ls e v ie r S c ie n c e B .V . PII: 5 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 6 0 0 - 6

N . K a liw o h e t a l . / A p p l i e d S u r fa c e S c ie n c e 1 8 6 ( 2 0 0 2 ) 2 4 1 - 2 4 5 243 J^pplied to the deposition chamber, is also dissociated

l,y the UV, yielding excited oxygen atoms 0 ( 'D ) . fhese species attack the ligands to which the Ti atoms jfe attached, subsequently forming T i-O -T i bonds in itie gas phase and on the substrate surface, leading to [he growth o f Ti0 2 on the substrates. W ith the injector jystem, a precise number of droplets o f identical size ensures accurate control of this process.

Fig. 1 shows the dependence o f thickness and refractive index on deposition time at a constant lemperature o f 350 °C. High refractive index values Iretween 2.2 and 2.54 were obtained. The increase in refractive index is attributed to an increase in film density with the higher values being achieved for the ihickest films. The higher values compare favourably ivith the value o f 2.58 recorded for the bulk material at a wavelength o f 632 nm [13]. The m easured film

thicknesses were between 2 0 and 510 nm, providing

a calculated deposition rate o f approxim ately 50 nm/ min at 350 °C. Sim ilar deposition rates have also been achieved by plasm a-enhanced CVD [14].

Fig. 2 shows an Arrhenius plot o f the deposition rate of films. Deposition time was 600 s for all data

shown. A linear relationship between In

R

leciprocal o f tem perature was observed for tem pera­ tures between 150 and 350 °C, providing an activation energy o f 0.382 eV. Such kinetically controlled deposition is determ ined by surface decomposition of the precursor [15]. At temperatures below 150 °C,

the film growth rate appears to increase with decreas­ ing tem perature due to vapour condensation on the substrate, resulting in only partial decomposition of the precursor. This may be characterised by an activa­ tion energy o f -0 .0 3 9 eV. These com pare favourably with values for TiO? films deposited by laser-induced

chemical vapour deposition (LICVD) [8] and are

lower than the values obtained by MOCVD [6].

Fig. 3 shows the FTIR of films grown at 350

°C

for different time periods. A strong absorption peak generally attributed to TiOi is observed around 433 cm “ * [16]. The inset verifies the absence of OH absorption at 3300 cm ” ’ for these films. However, such

100 S e 60 08 6 0 s 1 0 0 s 2 0 0 s 3 0 0 s 5 0 0 s E e 2 H 800 1000 200 400 600 Wavelength (nm) ! E

s

^8- 3. FTIR o f Ti02 films deposited at 350 °C. Inset shows the W e o f OH bonds. 0.5 0 .4 6 B s § 0.2

•I

Fig. 4. Transmittance (top panel) and extinction (bottom panel) coefficient o f TiOg films as functions o f wavelength.

244 _{N. K a lin o li et { il./A p p lie d S u /fa c e S cience 186 (2(K)2j 2 4 1 -2 4 5} a) 200T 20 25 50 35 40 45 2 9 (d e (> r e e s ) 50 55 60 b) 350T 20 25 30 35 40 45 50 55 60 2 0 ( d e g r e e s )

l'ig. 5. XRI3 o f TiO ] films deposited al 200 °C (a) and 350 °C (h).

tibs(M‘ption w a s fou n d to be present in layers d e p o site d at tem p eratures b e lo w 2 5 0 "C.

i-'ig. 4 s h o w s the transm ittance (top ) and ex tin ctio n (b o tto m ) c o e fh c ie n t diagram s for film s grow n at 3 5 0 C. T ransm ittance valu es for the thinner film s ( ~ 6 0 0 A ) w a s m ore than 90% a b o v e 5 0 0 nm . T he average e x tin ctio n c o e ffic ie n t, c a lc u la te d for all film s on their r e s p e ctiv e th ic k n e sse s w a s foun d to be 3 .5 X 10 - K at 5 0 0 nm . w h ich is slig h tly higher than that o b ta in ed by r.f. m agnetron sp u tterin g [17].

X K D sh o w e d that film s d e p o site d b e lo w 2 0 0 °C w ere am orp h ou s, w h ereas th ose d ep o s ite d at high er tem p eratures w ere anatase. b ig. 5a and b sh o w s the

X R D o f T iO ] film s d ep o sited at 2 0 0 and 3 5 0 °C . resp ectiv ely . S everal p eaks fou n d in the a s-d ep o site d film s w ere id en tified as cry sta llin e an atase p eak s. F ilm s d ep o sited at 2(X) had the (1 0 1). (2 0 0 ) and (2 11) orien tation s as film s d e p o site d at 3 5 0 ‘'C had sim ila r p eak s and an ad d ition al (1 1 2) peak.

4. Conclusions

T h e lo w -tem p eratu re d ep o sitio n o f thin T i0 2 film s on cry sta llin e Si and quartz by p h o to -in d u ce d C V D u sin g U V e m p lo y in g a d ie le c tr ic barrier d isch a r g e

N. K a liw o h et a l./A p p li e d S u rfa ce S c ie n ce 1 8 6 (2 0 0 2 ) 2 4 1 -2 4 5 245 e x c im e r la m p s at X = 2 2 2 nm h a s b e e n d e m o n stra te d .

T h e g ro w th o f film s fr o m 2 0 to 5 1 0 n m in th ic k n e ss w ith r e fr a c tiv e in d ic e s fr o m 2 .2 0 to 2 .5 4 h a s b e e n a c h ie v e d at te m p e r a tu r e s b e t w e e n 5 0 and 3 5 0 °C . R e fr a c tiv e in d e x v a lu e s in d ic a te th e film s d e p o s ite d b e tw e e n 2 5 0 and 3 5 0 C w e re o f th e b e s t q u a lity . F ilm s g r o w n at te m p era tu re s o f 2 0 0 °C and a b o v e w e r e c r y s ta llin e . T h e m e a su r ed d e p o s itio n rate o f arou n d 5 0 n m /m in at 3 5 0 ' C is h ig h e r th an v a lu e s a c h ie v e d b y m o s t m e th o d s .

References

[1 ] B . H e . N . H o ilie n . R . S m ith . T. M a , C . T a y lo r, I.St. O m er, S .A . C a m p b e ll. W .L . G la d fe lter , M . G r ib ely u k , D . B u ch a n a n , in; P r o c e e d in g s o f th e I E E E U n iv e r s ity /G o v e r n m e n t/I n d u s tr y M ic r o e le c ti'o n ic s S y m p o s iu m , 1 9 9 9 , p. 3 3 .

[2] J. Z h a n g . J .S . Y uan . Y. M a . A .S . O a te s, S o lid S tate E le c tr o n . 4 4 ( 2 0 0 0 ) 2 1 6 5 .

[3 ] M .D . S ta m a te . T h in S o lid F ilm s 3 7 2 ( 2 0 0 0 ) 2 4 6 . [4] A . S h ib a ta . Jpn. J. A p p l. P h y s. 3 0 ( 1 9 9 1 ) L 6 5 0 .

[5 ] N . K a liw o h , J.-Y . Z h a n g , I.W . B o y d , Surf. C o a t. T e c h n o l. 1 2 5 ( 2 0 0 0 ) 4 2 4 . [6 ] B . K a n g , S . L e e , J. B o o , S u rf. C o a t. T e c h n o l. 13 1 ( 2 0 0 0 ) 8 8 . [7 ] A . T u an , M . Y o o n , V. M e d v e d e v , Y. O n o , Y. M a , J.W . R o g e r s , T h in S o lid F ilm s 3 7 7 - 3 7 8 ( 2 0 0 0 ) 7 6 6 . [8 ] E . H alary, E . H a r o -P o n ia to w sk i, G . B e n v e n u ti, P. H o ffm a n , A p p l. Surf. S c i. 1 6 8 ( 2 0 0 0 ) 6 1 .

[9 ] F. F e lte n , J.P. S én a teu r, M . L a b ea u , K . Y u -Z h a n g , A . A b r u tis, T h in S o lid F ilm s 2 9 6 ( 1 9 9 7 ) 7 9 .

[1 0 ] J.-Y . Z h a n g , I.W . B o y d , A p p l. P h y s . L e tt. 7 7 ( 2 0 0 0 ) 3 5 7 4 . [1 1 ] H . K o m iy a m a , H . In o u e, K a g a tu K o g a k u R o n b u s h u 11 ( 1 9 8 5 )

3 1 7 .

[1 2 ] K . T o k ita , F. O k a d a , J. A p p l. P h y s. 8 0 ( 1 9 9 6 ) 7 0 7 3 . [1 3 ] E .D . P a lik , H a n d b o o k o f O p tic a l C o n sta n ts o f S o lid s ,

A c a d e m ic P r ess, O r la n d o , F L , 1 9 8 5 , p. 8 0 0 . [1 4 ] M . N a k a m u ra , D . K o r z e c , T. A o k i, J. E n g e m a n n , Y. H a ta n a k a , A p p l. Surf. S c i. 1 7 5 ( 2 0 0 1 ) 6 9 7 . [1 5 ] A C . J o n es, T.J. L e e d h a m , P.J. W rig h t, M .J . C r o s b ie , K .A . F le e t in g , D .J . O tw a y , P. O ’B r ie n , M E . P e m b le , J. M a te r . C h e m . 8 ( 1 9 9 8 ) 1 7 7 3 . [1 6 ] H . J o e, A .K . V a su d ev a n , G . A ru ld h a s, A .D . D a m o d a r a n , K .G .K . W arrier, J. S o lid S ta te C h e m . 131 ( 1 9 9 7 ) 181. [1 7 ] S . B e n A m o r , G . B a u d , J.P. B e s s e , M . J a c q u et, M ater. S c i. E n g . B 4 7 ( 1 9 9 7 ) 1 1 0 .

E L S E V IE R A p p lie d S u r fa ce S c ie n c e 1 8 6 ( 2 0 0 2 ) 2 4 6 - 2 5 0

applied surface science w w w .e ls e v ie r .c o m /lo c a t e /a p s u s c

(Ta205)j_^(Ti0 2)^ deposited by photo-induced CVD

using 222 nm excimer lamps

N ever K aliw oh*, Jun-Y ing Z hang, Ian W. B oyd

E lectro n ic a n d E le ctric a l E ngineering, U n iversity C olleg e London, Torrington P la ce, London W C IE 7JE, U K

Abstract

We report the deposition o f thin (Ta2 0 5),_ ,.(T i0 2)^ films on quartz and crystalline p-type Si (1 0 0) by photo-induced C V D using 222 nm excim er lam ps at temperatures from 50 to 350 °C. The alkoxide precursors titanium isopropoxide and tantalum tetraethoxy dim ethylam enoethoxide w ere m ixed in various ratios, vaporised, and then driven into the reaction chamber by an Ar carrier gas, where they w ere exposed to the U V radiation. Film s greater than 500 nm in thickness were grown at a pressure o f several millibars and at a deposition rate o f approximately 60 nm/min at 350 °C. The chem ical bonding o f the films has been analysed by Fourier transform infrared spectroscopy, and indicated that both Ta205 and TiÛ2 stretching absorption peaks were present. The optical properties o f the layers were analysed by U V -V is spectroscopy betw een w avelengths o f 1 9 5 - 1100 nm. Band gap energies ranging between 3.2 and 4.2 eV were obtained for films o f various com positions. © 2002 Published by E lsevier Scien ce B.V.

K eyw ords: D ie le c tr ic ; E x c im e r la m p ; P h o to -C V D ; T a^ O ^ -T iO i

1. Introduction

The Semiconductor Industry A ssociation roadmap predicts that SiO] thickness w ill reach atomic dimen­ sions by 2012 [1]. B elow thicknesses o f 2 nm, direct tunnelling currents w ill lim it the functionality o f devices that incorporate such layers o f S i0 2- Materials

with high dielectric constant, low leakage current and higher dielectric strengths w ill, therefore, be required to produce the same devices possessing the same effective capacitance but using thicker layers. High dielectric constant materials have potential applica­ tions as charge storage capacitors for memory cells in ULSI D R A M s [2] and low inductance decoupling capacitors for the control o f simultaneous switching

C o r r e sp o n d in g author. T el.: 4 - 4 4 - 2 0 7 - 4 1 9 - 3 1 9 6 ; fa x : + 4 4 - 2 0 7 - 3 8 8 - 9 3 2 5 .

E -m ail address', n k a liw o h @ e e .u c l.a c .u k (N . K a liw o h ).

noise in high speed switching ULSI chips [3]. Because o f its thermal and chem ical stability Ta2 0 5 appears to

be one o f several promising materials for such appli­ cations especially because o f its compatibility with ULSI processing [4]. A significant increase in d ielec­ tric constant has been observed when Ta2 0s has been

doped with other oxides including AI2O3 [5], Zr0 2 [6]

and Ti0 2 [7]. O f particular interest is the doping o f