gSrO.2 Ti03 thin films have been deposited on silicon Si/Si02fTifriNIPt substrate. ... constant ofBST film in Si/Si02fTifriNIPtIBST/AI thin film capacitor is found.
Indian Journal of Pure & Applied Physics Vol. 37, April 1999, pp. 318-320
Thickness dependence of dielectric constant in barium strontium titanate thin films B Panda, G D Nigam & S K Ray Department of Physics & Meteorology, Indian Institute of Technology. Kharagpur 721 302 Received 3 February 1999 Radio frequency magnetron sputtered Bao.gSrO.2 Ti0 3 thin film s have been deposited on silicon Si/Si0 2fTifriNIPt substrate. The structural and electrical properties have been investigated using X-ray diffraction and capacitance-voltage characteristi cs of fabricated capacitors. The growth and orientation of the film have been found to depend upon the type of substrates and deposition temperatures. The effective dielectric constant ofBST film in Si/Si0 2fTifriNIPtIBST/AI thin film capacitor is found to increase with the increasing thickness of the film . A space charge layer with low dielectric constant is found to form at the film electrode interface.
1 Introduction The search for a high permittivity material, that could increase the storage capabilities of next generation dynamic random access memory (DRAM) cell is underway! . Ferroelectric perovskite oxide films are very much attractive due to their high dielectric constant, and 2 are being exploited in the VLS IIULSI applications -4. The potential uti lization of high diel ectric constant film s can be made for fab ricating new generation of memory devices by studying the synthesis, characterization and determin in g the process composition microstructureproperty relationship of ferroelectric thin film s. Although. lot of studies have been undertaken to make a good progress in achieving the above goals, there exists a considerable scope for further research in this challenging field. in recent years, bar iu m stronti um titanate (BST) thin films are found to be very attractive because oftheir very high die lectric constan t. The present illYestigation is concerned with the studies on the structural and thickness dependence of dielectric constant of nonlead based bariu m strontium titanate (B ST) thin film s 5 prepared by rf magnetron sputtering .
2 Experimental Details A 13.56 MHz, 0-500 W, rf magnetron sputteri ng system (EDWARDS ESM 100) with 7.5 cm diameter cathode was used fo r deposition ofBao sSro2 Ti0 3 (BST) films. These films were deposited at differen t chamber pressures maintai ni ng an argon to oxygen ratio 90: lO on silicon and Si/Si0 2/Ti/TiN /Pt substrates. T iN(2000A) on Si 0 2/Si substrate was used as an intermediate barrier
layer. Pt(300A) e lectrode wa s deposited on
Si/SiOiTiffiN substrates bye-beam evaporation at a substrate temperature of 450°C. Here T i layer is used as an adhesive layer6. Sputter deposit ion of BST film was carried out at substrate temperatures of 480°, 530° and 580°C respectively. The influence of bottom electrode, substrate temperature and deposition temperature on film orientation and phase fo rmat ion was investigated using X-ray diffraction ana lysis. Dielectric properties of BST films were eval uated from capacitance voltage (C-V) characte ri stics of fabricated metal-insulatorsemiconductor (MIS) and metal-insulator-metal (MlM) capacitors. All the measurements were made us ing HP 406J-A computerized semiconductor test system consisting of multi-frequency LCR meter (HP-4275-A), a pico-ammeter/dc voltage source (HP 4140-B) and a switchin g subsystem.
3 Results an d Discussion Structura l characteristics of BST film s were studied fro m X-ray diffraction (X RD) pattern of the deposited fil ms using CuK" radiation . X-ray diffraction spectra of as grown BST fi lms deposited on si licon (100) face at various substrate temperatures are shovv'I1 in Fig. 1. As can be seen from the figures, th e orientati on and crystallinity of the film s are strongly dependent on the . deposition tem perature. Since sputtering from a multicomponent target like Bao.SSrO.2Ti0 3 causes fragmentation into BaO, SrO an d Ti0 2 com ponents , we investigated the lowest temperature required fo r the formation of BST film during in situ deposition. It was
319
PANDA et a/: B3O.8 Sro.2Ti03 THIN FILMS
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observed from X-ray diffraction that a perovskite phase of SST was formed at a temperature greater than 480°C. In situ deposition and heating at 580°C resulted in the growth of oriented films. The crystallinity of the film s increased with increasing temperature. Higher degree of preferred orientation of the SST film is observed on Si/Si0 2/SiNIPt structure (F ig. 2) as compared to Si su bstrate. It is clear fro m Figs I and 2 that the film deposited on sil icon is polycrystalline at a deposition temperature around 500°C and gradually orients itself a long di rection at a higher substrate temperatu re (5 80°C), while in the case of platinum the orientation takes place ata lower temperature (530°C). Lattice mismatch is very large between si licon (a= 5.43 A) and SST (a= 3.93A), while 1he lattice constant of plati num ( a = 3.93 A) is very close to that of SST. The films at higher substrate temperatures preferentially orient along direction which is considered to be related to the surface energ/ and the structure factor of the perovskite phase fo rmation. The effecti ve die lectric constant of S ST film used in the mul t il ayered capac itor, S i/S i0 2/Ti/TiN I PtlBST/AI, increases with the increase of S ST fil m thickness and ul ti mately tends to a saturation (Fig. 3) . The local fie ld in the film origi nati ng from the space charge layers at the interfaces, located at both the top and the bottom ends of the fi lm and the e lectrode, is the 8 route cause of thi s thickness dependence . If C BsT be the capacitance due to S ST fil m and C; be the interfac ial capacitance, the effecti ve capacitance is given by : . . .( I) C = (CBSTC\) I (CBST + C i ) If CasT « C; , i.e., SST film thickness is very high, the effective capacitance will be due to the capacitance
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Fig. 2 - XRD spectra of BST film s deposited on Si/Si02ITiITiN/Pt substrate at substrate temperatures (a) 480°C (b) 530°C
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Fig. 3 - Dielectric constant as a function of fi lm thickness fo r BST thin films Si/ Si0 2 1TiITiJ~J/PtJBST (80/20) ; frequency = I MHz . substrate temperature = 530 0 C
of the SST film on ly. So, at higher thickness the effective capaci tance i.e. the effective d ie lectric constant will te nd towa rd s the actua l di e lectri c co nstant of the SST fil m . Fig. 4 shows the EoA/C as the function of film thickness d, where A is the electrode area, C is the measured capacitance and Eo is th e di e lectric constant of the free space. The pos iti ve va lue of EoAIC at zero film thickn ess indicates the ass umpt ion of the formation of an interfac ial space charge layer. 4 Conclusions
SST
film s
ha ve b ee n d e p os it ed on Si/SiOi Ti/TiN /PtiSST (80/20) substrate by rf magnetron sputtering. The crystallinity of SST films has been
INDIAN 1 PURE & APPL PHYS, VOL 37, APRIL 1999
320
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compared to that on silicon. The effective dielectric constant of the film is found to increase with the increasing thickness of film. The formation of an interfacial space charge layer is assumed to be the cause behind the thickness dependent dielectric constant of the film . References I
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02 0., ().6 G8 1·0 FILM THiCkNESS(JJm)
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found to increase with substrate temperature. Films deposited on platinum coated ,substrafes orient preferentially along direction at a lower temperature
Sedlar M, Sayer M & Amm D T , J Appl Phys, 80 ( 1996)
367. 3
Fig. 4 - EQAlC as a function of film thickness for BST film Si/Si02(fiffiNIPtIBST (80/20); frequency = I M~z, substrate temperature = 530 0 C
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