Simple composition control of ferroelectric thin films grown by sputtering with an original oxides target design

Simple composition control of ferroelectric thin films grown by sputtering with an original oxides target design

MICROELECTRONIC ENGINEERING ELSEVIER Microelectronic Engineering 29 (1995) 213-216 Simple composition control of ferroelectric thin films grown by ...

264KB Sizes 0 Downloads 26 Views

MICROELECTRONIC ENGINEERING

ELSEVIER

Microelectronic Engineering 29 (1995) 213-216

Simple composition control of ferroelectric thin films grown by sputtering with an original oxides target design D. Remiens, M. Descamps, B. Jaber, B. Thierry Laboratoire des Mat6riaux Avanc6s C6ramiques (LAMAC) / CRITT "C6ramiques Fines" Universit6 de Valenciennes et du Hainaut-Cambr6sis Z.I. Champ de l'Abbesse - 59600 MAUBEUGE - FRANCE

We propose a new type of target which permits an easy control of composition for complex oxide films grown by sputtering. The targets consists of multilayers of PbO and TiO 2for PbTiO 3films and multilayers of PbO, TiO2, ZrO 2 for PbZrTiO s films obtained by tape-casting and cold pressing. The composition of the film is controlled by the thicknesses of the different oxide layers and the erosion area of the target (maguetron sputtering). With this new target design we have sucessefully realized PbTiO 3 and PbZrTiO 3 thin films. I. INTRODUCTION

PbZrTiO 3 thin films.

Ferroelectric materials such as PbTiO 3, Pb(Zr Ti(1..~)O3 and La doped Pb(Zr Ti(1..))Oa have been applied to many useful electronic and opto-electronic devices since they present excellent dielectric, piezoelectric and optical properties. High quality ferroelectric film growth techniques which offer a good control of the composition (homogeneity and reproductibility) are required for device fabrication. Sputter deposition and sol-gel processes are the favoured techniques. In many works, sputtering target were made by hot or cold pressing of mixed oxides powders or synthesized powders. However, experience with such targets showed a number of disadvantages; the most important problem is the control of the film composition [1]. Another approach based on reactive sputtering of metal targets is also used. The major inconvenience with these targets is the dependence of the sputtering rate upon oxygen gas pressure [2].

H. EXPERIMENTS

This clearly suggests the need for new target materials, or design for improvement in the sputtering techniques. So, we propose a new type of oxide target which permits an easy control of the ferroelectric film composition. These targets are applied to realize PbTiO 3and

A radio-frequency maguetron sputtering system, described previously [3], was used to grow PbTiO 3 and PbZrTiO 3 thin films. Targets are obtained bythe super position and the rolling up of ceramic tapes constituted of different oxides (PbO, TiO2, ZrO 2) (figure 1).

TiO2 Figure 1. Schematic of principle for the production of multilayer oxide targets.

The tape-casting process is used for the formation of these tapes. This process of ceramic shaping requires on one hand the formation of a casting slurry and on the other hand the

0167-9317/95/$09.50 c~ 1995 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317 (95)00148-4

214

D. Remiens et al. /Microe&ctronic Engineeling 29 (1995) 213-216

casting of this suspension on a bench which allows to obtain ceramic tapes of a significant length and a well-defined thickness. The slurry dispersion is carried out in a b i n a r y solvent m i x t u r e of 72-28 vol % t r i c h l o r o e t h y l e n e - e t h a n o l . The selected dispersing agent used is a phosphate ester. This organic compound allowed a electrosterical stabilization of oxide suspension. Indeed, electrostatic repulsion effects due to particle charge is combined with a steric repulsion ensured by the polymeric chains of the dispersant. Suspensions which exhibit very low viscosities and a high amount of dry matter are obtained by using this dispersing agent. After the dispersion step, other organic constituents (binder and plasticizer) are added to the slurry. These compounds provide the green tape mechanical properties which are required for post-casting operations (handling, c u t t i n g , storing). The binder used is epolyvinylbutyral resin (PVB) which allows a linkage between the powder particles and confers the strength to the green tape. The plasticizer used is the dibutyl phtalate (DBP), it ensures the flexibility to the ceramic tape. Slurry compositions used to form PbO, TiO2,ZrO2 tapes are showed in table 1. Table 1 Slurry compositions for PbO, TiO2, ZrO2 tapes PbO TiO2 ZrO2 (g) (g) (g) Powder 72 Dispersing agent 0.36 (phosphate ester) Binder (PVB) 3.24 Plasticizer (DBP) 3.24 Solvents 21.16 (Trichloroethylene/ethanol)

62 0.5

68 0.54

2.79 3.06 4.18 4.59 30.53 23.81

After the milling and mixing ofthe organic and mineral compounds is carried out with a planetary ball milling, the slurry is poured into the bench casting reservoir. The casting is

accomplished by a relative movement of reservoir with respect to the carrier surface (doctor blade process). The tape thickness obtained with this process can vary between 30 ~tm to 1000 pm. The tape drying is carried out by the evaporation of organic solvents. When tapes are dry, they are stripped from the carrier, then stacked and wound in order to obtain a multilayer material. The organic constituents are eliminated by a thermal treatment (400°C for 30 min). After this debinding, the target undergoes an uniaxial compression (200 MPa), in order to improve the target tape cohesion. Disk targets 25.4 mm in diameter and 2 mm thick with variable thickness ratios of PbO! TiO 2 and PbO/TiO2/ZrO 2 are then utilized for the formation of PbTiO 3 and Pb(Zrx Til.x)O3 films. The thickness of each layer can varied between 30 ~tm and 1 mm. The target dimension can be modified. Targets of diameter 76.2 mm are now in study. The sputtering conditions used in this experiment are summarized in table 2. Identical process parameters have been used to grow stoichiometric film (Pb/Ti = 1) with conventional oxide target [4]. Since the sputtering p a r a m e t e r s and in p a r t i c u l a r the s p u t t e r i n g p r e s s u r e are maintained constant, the target erosion area, for all the targets used in this experiment, is unchanged. The substrates used are Si (100) n-type covered with thermal SiO2 (5000/~) ; before the growth, the substrates are cleaned in different organic solvents. Table 2 Sputtering conditions for the preparation of PbTiO 3 and PbZrTiO 3 films R F power density Target diameter Target thickness Gas pressure Inter-electrode distance

2.3 W / c m 2 25.4 m m 2 mm 100 m T 35 m m

Gas process Substrate temperature Substrate

Ar Ambient S i / S i O 2 (thermal)

D. Remiens et al. / Miclvelectronic Engineetqng 29 (1995) 213-216

215

HI. RESULTS AND DISCUSSION In the first step, we determined the PbO and TiO 2layers thicknesses in order to produce PbTiO a stoichiometric films realized with different t a r g e t s b y v a r y i n g the PbO/TiO 2 thicknesses. Figure 2 shows the variation of the film composition (ratio Pbfri) with the thickness ratio P b O f r i o r 5

....

] ....

I ....

I ....

] ....

u_

o

3

"

2

I Figure 3. SEM micrograph of the erosion area.

<

~ o 0

..~_,,._ 0

%,.H.I

H''',''"

0.2 0.4 0.6 0.8 THICKNESS RATIO OF PbO / TiO 2 TAPES

F i g u r e 2. ( P b / T i ) f i l m v e r s u s t h i c k n e s s layers ratio (PbOfriO2).

This experimental result confirms that the sputtering yield of PbO is higher than that ofTiO 2 ;the stoichiometricfilm is obtained when the thickness ratio (PbO/TiO 2)is equal to 0.394. The targets used in this experiment have been investigated by scanning electron microscopy (SEM) in order to examine the erosion area. An example is shown figure 3, we recognize the well known s p u t t e r erosion [5]. In all films obtained with the different m u l t i l a y e r s targets, we h a v e systematically m e a s u r e d the growth rate. It is in the order of 20 A/rain for stoichiometric films which is higher t h a n t h a t obtained with conventional targets (10 A/rain). In multicomponent oxide targets, the growth r a t e is limited by the element which h a s the smallest s p u t t e r i n g yield, i.e. Ti in our case. In the multilayered oxide targets, this p h e n o m e n a is not observed since each element is i n d e p e n d e n t of the others.

W h e n t h e P b O l a y e r t h i c k n e s s is decreased to zero, we find t h a t the growth r a t e corresponds to the growth r a t e of TiO 2 alone ( - 7/~Jmin m e a s u r e d u n d e r similar s p u t t e r i n g conditions with a TiO~ target). Inversely, w h e n the TiO 2 layer thickness tends to zero we find the growth r a t e of PbO (380 ./~Jmin). Stoichiometric PbTiO 3 films could be p r e p a r e d f r o m t a r g e t s c o m p o s e d of P b O a n d TiO 2 multilayers with the ratio PbO/TiO 2 = 0.4, eg with PbO and TiO 2 thicknesses of 80 ~tm and 200 ~tm respectively. On t h e b a s i s of t h e knowledge obtained for PbTiOs, we have realized a PZT m a t e r i a l t a r g e t also. The PZT t a r g e t design consists of : - PbO layer : the thickness is m a i n t a i n e d const a n t as in PbTiO s target due to the independence of each element. - TiO 2layer : the thickness is divided by a factor 2 in comparison with PbTiO 3 target. - ZrO 2 layer : the thickness is defined such t h a t the thickness of TiO 2 + ZrO 2 is equal to the thickness of TiO 2 in the case of PbTiO 3 target.

216

D. Renliens et nl. / Microelectwnic Engineering 29 (1995) 213-216

Since the sputtering yield of Zr is somewhat higher than that of Ti (0.75 atoms/ ion and 0.58 atoms/ion at 600 eV, respectively) while the sputtering conditions are maintained identical one expects the f&n compositionto be enriched in Zr compared to the 50150 ZrfTi target composition. An analysis of the film by EDS indicatedthe compositionPb(Zr,,5,Ti&O, i.e. a little excess in Zr. No variations in the lead concentration in the flhn were observed by comparison with PbTiO, targets, this confirms the interdependence between the elements. Observations carried out by SEM shows in agreement with this a preferential etching of ZrO, tapes compared to TiO, tapes. IV. CONCLUSION PbTiO, and PbZrTiO, (53/47) thin films have been prepared by sputtering using a new type of oxide target. It consists of multilayersof PbO, TiO, and ZrO, ; the film composition is controlled by the adjustmentof each oxide layer thickness in relation to the oxide sputtering yield and the erosion area. This new process allows the fabrication of sputtering targets for more complex ferroelectric films such as PLZT or doped PZT (with Nb, Nd, . . J. REFERENCES 1. S.B. Krupanidhi, N. Maffei, M. Sayer, J. Appl. Phys. a, p 6601(1983). 2. K. Sreenivas, M. Sayer, J. Appl. Phys. && p 1484 (1988). 3. D. Remiens, J.F. Tirlet, B. Jaber, H. Joire, B. Thierry, Cl. Moriamez, J. Europ. Ceram. Sot. U, p 493 (1994). 4. D. Remiens, J.F. Tirlet, B. Jaber, H. Joire, B. Thierry, Cl. Moriamez, Silicates Industr. IXl-21, p 15 (1995). 5. A.R. Nyaiesh, Vacuum a(S), p 307 (1986).