From Proposal to Product: Scaling Up the Chemical Synthesis of MOCVD Oxide Precursors
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Ta Nb Ta Mo W Cr Fe Ga Mn Ni Zn Ba Bi La Pb Mg Nb Ru Sr Ta Ti Zr Al Hf Zr Er Ge Ba Co Ge Fe Ln Sr Ca Y Zr Ce Gd Ba Bi Ca Cu Sr TI Y Sb In Sn
[1] [2] [3] [4] [5] [6] [7] [8]
[91 [10] [11] [12]
79 Mat. Res. Soc. Symp. Proc. Vol. 415 01996 Materials Research Society
TABLE 2 COMMON MOCVD OXIDE PRECURSORS Aluminium
Antimony Barium Bismuth Calcium Cerium Chromium Cobalt Copper Erbium Germanium Hafnium Indium Iron Lanthanum Lead Magnesium Manganese Nickel Niobium Strontium Tantalum Tellurium Thallium Tin Titanium Vanadium Yttrium Zinc Zirconium
sec-butoxide liquid hfac mp 70' triethyl liquid triethoxide liquid thd mp 2200 tdfnd tetraglyme mp 720 triphenyl mp 770 hfod mp 1300 (IV) thd mp 2500 (1II) acac mp 2160 (1II) acac mp 2130 t-butoxide dec 1700 thd mp 1980 thd water adduct mp 1820 ethoxide liquid t-butoxide liquid thd mp 300' thd mp 1730 triethyl liquid acac mp 1840 hfac mp 140' (11)thd mp 123' tetraethyl liquid thd mp 1770 (11)acac dec 2300 acac mp 2050 ethoxide liquid thd mp 1250 ethoxide mp 21' ethoxide liquid ethoxide liquid hfac mp 1260 (IV) t-butoxide mp 450 dimethyltin dichloride mp 1070 isopropoxide mp 190 titanyl bis-thd mp 1680 vanadyl isopropoxide liquid thd tbpo mp 970 acac mp 1240 t-butoxide liquid hfac mp41° thd mp 308'
1 mm 0.3mm 1 mm 5 mm 0.05 mm 1 mm 0.2 mm 0.3 mm 0.05 mm 0.2 mm 1 mm I mm 1 mm 0.1 mm 1 mm 1 mm 0.1 mm 1 mm 0.7 mm 2 mm 0.3 mm 0.3 mm 1 mm 0.1 mm 0.1 mm 1 mm 1 mm 0.3 mm 1 mm 1 mm 2 mm 0.1 mm 1 mm 5 mm 1 mm 1 mm 1 mm 1 mm 0.1 mm 1 mm 2 mm 0.1 mm
* precursor is liquid at 1000 with vapour pressure above mrnm
80
1000 700 630 850 2000 184c 1O0° 1000
1700 1400 1000 1500 1700 1200 2000 550 650 1800 2920 250 1100 1500 1230
40•,
*
2200
1500 1900 1600
2300 1600 505 50'
* * *
1400
800
1070
800 2400 950 1720 1100 65° 1000 1800
* * *
8*
TABLE III CANDIDATE MOLECULES FOR TANTALUM OXIDE PRECURSOR Homologous series Ta 2 (OR) 1 0 [13] mp 520 R - methoxide R = ethoxide mp 210 R = isopropoxide mp 920 R = n-butoxide liquid R = t-butoxide liquid Tetraethoxide derivatives Ta(OEt) 4 L [14] L = acetylacetonate mp 450 L = dimethylaminoethoxide liquid
vp vp vp vp vp
lmm at 1mm at 1mm at 1mm at 1mm at
1600 1600 1600 1900 1600
vp 1mm at 1150 vp 1mm at 1100
in conventional bubblers, having melting points below 1000 and vapour pressures above 1mm at 100'. Eleven of these are alkoxides, four are alkyls and two are diketonates, underlining the importance of alkoxide chemistry to this branch of materials science. Many more candidate molecules have been synthesised worldwide and a comprehensive list would run to several hundred entries. For example table III illustrates seven possible precursors for MOCVD of tantalum oxide. This poses a selection problem for the production chemist and for the deposition scientist. Is the chosen precursor sufficiently well characterised to deposit reproducable thin films ? Our early experience indicated that the respective importance of synthesis criteria changed when scaling by one, two or three orders of magnitude and so we decided to investigate how this a
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