MOCVD of Very Thin Films of Lead Lanthanum Titanate
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Mat. Res. Soc. Symp. Proc. Vol. 415 0 1996 Materials Research Society
a leakage current less than 1 x 10-8 A/cm 2 at 1 volt. One figure of merit for an insulator in DRAM applications which takes into account both film thickness and dielectric constant is the capacitance per unit area, expressed in fF/ymm2 . The requirement for 1 gigabit Capacitor DRAM is likely to be in excess of 100 fF/Mm 2 , while the Dielectric best results obtained for PLT in our sol-gel work are in the range of 50 fF/Mm 2 . Therefore, it is likely that the area available for the deposition of the capacitor will need to be l Bit rincreased by at least a factor of 2. One method currently used for increasing capacitor area is the use of a "stack" on top of the transistor of the memory cell. This is depicted Figure 1. Stack capacitor in Figure 1. To construct such a structure, it will be structure. necessary to conformally coat the stack structure. This is best accomplished using a CVD process. This paper reports preliminary studies on MOCVD of PLT. The goals of this work are to deposit PLT in the thickness region of 600 to 1000 A, and to minimize the degradation of the electrode by depositing at low temperatures and using rapid thermal processing. EXPERIMENTAL The reactor used in this study is depicted in Fig. 2. Precursors, dissolved in solvent, were metered using an Eldex Model A-120-S liquid metering pump designed for use in liquid chromatography. For the metering pump to work correctly, it was necessary to insert a back-pressure regulator which consisted of a valve held in its seat by a spring. A pressure of -30 psig was required to open the valve. Oxygen was allowed to flow around the tube carrying the solution to prevent sublimation
Back-pressure 225 C Regulator r"2
i0
500 C 0 0 0 0 0 0 0 0
0\
Pump -Mc
M~ha,,:,i ýPic~ate Throttle
So ution Reservoir
Pump
Trap
Valve
Figure 2. Hot-wall MOCVD system with liquid precursor delivery system. of precursor in the upstream direction. The metered solution entered the evaporator which was constructed of 1 inch diameter quartz tubing with several baffles to prevent transport of droplets into the main reactor. The evaporator was heated externally by a tube furnace held at about 225 'C. This
226
temperature was as low as possible to prevent premature solvent evaporation or precursor decomposition. The main reactor consisted of a 3 inch diameter quartz tube heated by a three-zone electric tube furnace. A throttle valve in the pumping line allowed a constant reactor pressure independent of the flow settings. The lead precursor used in this study was lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate), Pb(THD) 3 , (StremChemicals) and was purified by sublimation at 145 'C at 10-1 torr. The lanthanum precursor was lanthanum tris(2,2,6,6-tetramethyl-3,5-heptanedionate), La(THD)3 , (StremChemicals) which was also purified by sublimation at 210 0 C at 10-1 torr. The titanium precursor was titanium bis(isopropoxide)-bis(2,2,6,6-tetramethyl-3,5-heptanedionate), Ti(iOPr) 2(THID) 2 , which was synthesize
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