The properties of plasma-grown SiO2 films

SURFACE

SCIENCE 7 (1967) 490-495 0 North-Holland Publishing Co., Amsterdam

THE

PROPERTIES

OF PLASMA-GROWN

SiO,

FILMS

Received 1 March 1967 A range of SiO, films has been grown using a low-temperature plasma technique originally described by Ligenzal), in which a field is applied between two silicon electrodes immersed in a microwave oxygen plasma. The experimental procedure, shown in fig. 1, has been extended to incorporate water-cooled interchangeable electrodes, spectroscopically pure oxygen, and either ac or dc fields can be applied to the electrodes. RF bombardment can also be used to clean the electrodes in situ prior to oxidation. With a dc field applied to the electrodes, oxide growth occurs mainly at the anode, whilst an ac applied field results in growth at both electrodes. initially, preferential growth occurs at the centre of the substrate, giving rise to a radial oxide thickness variation. The growth rate decreases with increasing oxide thickness, and the initial parabolic rate constant is of the order 10’ A2/min for oxygen pressure 0.5 Torr, 50-70 V dc or ac, current densities l-10 mA/cm2. For thicknesses greater than 3000 A, deviations from parabolic growth have been observed with a reduced rate constant, fig. 2. For pedestal temperatures of 100°C and 300°C no difference in growth characteristics was observed. The deliberate introduction of partial pressures of water vapour resulted in an appreciable change in growth characteristics. Partial pressures of water vapour between 0.1-0.4 Torr in a total pressure of 0.5 Torr resulted in oxides of approximately 2200 f% compared with 5600 .J%for dry oxygen. These results are unexplained at the present time. The physical properties of plasma oxides were investigated and compared with conventional thermal oxides by studies of etch rate, infra-red vibrational spectra, and refractive index. Etch rates for steam (12OO”C), dry oxygen (1200°C) and plasma oxides, measured using Pliskin and Lehman 2, P etch at 25”C, gave the values 2.50, 2.34 and 2.58 A per set respectively. Infra-red analysis of all three oxides gave no detectable shift in the position of the 9 and 12 micron vibrational bands. The refractive index was measured using an immersion technique with which accuracies to 0.1% can be achieved. The results were in agreement with Ligenzal), and are shown in table 1.

FLAS~A-BROWN

SiOz

491

FILMS

The close similarity in these three sets of results indicates that there appears to be little or no difference in stoichiometry and density of the films and that the composition probably lies close to SiOz. The electrical properties of plasma oxides have been investigated by C-V measurements on MOS structures, using gold as electrode material and both n and p type silicon substrates. The initial flat-band electrode charge density is typically in the range 10”-10’2 per cm2, and the instability observed as a result of thermal-bias treatments is typical of the so-called sodium contamination. Radiochemical neutron activation analysis has revealed sodium contamination, the sodium being piled up at the outer oxide surface. An analysis of C-V characteristics after Lindners) has not revealed the presence of any particular high density of surface states near to the middle of the gap.

DEMOUNTABLE ELECTRODE

SILICON

WATER

COOLED

SUBSTRATE

~

TUNABLE TO 2450

CAVITY COUPLED t&/SEC, 5 kV

POWER

SUPPLY

ILICA

DISCHARGE

SILICON

TO AND

SUBSTRATE

PUMPING SYSTEM SPECPURE OXYGEN

EMOUNTAELE

Fig. 1.

TUBE

WATER

Plasma oxidation tube.

COOLED

492

E. R. SKELT

AND

G. M. HOWELLS

(THICKNESS G 0.15

.

8OV

DC

2-4

0

7ov

DC

I-

x

60V

AC

-

RATE

3

o,

I

mA ,

0.5

!

CONSTANT

7

TIME

Fig. 2.

0,

0.5’f f

o,-

I IO’

,f?/MlN

I

I

I

0.5-f

I

I INITIAL

mA 4 mA

7

HOUR! 5

Growth rate characteristics

Using the method of Gray and Brown*), plasma oxides have shown rather broad surface-state bands located near to the energy gap edges, with typical surface state densities of the order 10”-few x 1013 states/cm2-eV. This is in contrast to the more sharply defined peaks which have been reported with thermal oxides. A higher incidence of pinholes in plasma films compared with thermal films has been observed, defects being demonstrated by high electrical leakages across the oxide layers and by electrically pulsing the films, which

TABLE

1

Refractive indices Plasma oxide I) Bulk vitreous silica’) Plasma oxide Steam oxide Dry oxygen oxide

1.47 1.458 1.470 1.466 1.463

_ at at at at at

5460 5460 5890 5890 5890

A 8, 8, 8, 8,

PLASMA-GROWN

sio2

FILMS

493

Fig. 3.

Electron shadow-microscope

showing type “a” features (X lo4 magnificatic 3n).

Fig. 4.

Electron shadow-microscope

showing type “b” features ( x lo4 magnification).

494

Fig. 5.

Fig. 6.

E. R. SKELT

ztron

AND

0.

hf. HOWELLS

shadow-microscope showing two features (x 104 magnification).

associated

wit:h P

rles

Electron shadow-rni~rosc~~ showing severe etching of type “a” features res .aing in holes (4 x 103 magni~cation).

PLASMA-GROWN

SiOz FILMS

495

burns away the thin gold electrode in the region of the defect. A scanning electron microanalyser has shown that the defects do not appear to be associated with a localized gross impurity. A small number of oxides appear to have visibly rough areas which have been identified with high pinhole densities. These areas have been studied in some detail using replicate electron microscopy to characterize the surface. Certain feature have been observed repeatedly: they are: (a) fiat bottomed crater-like depressions _t-2 pm across, fig. 3 ; (b) shallow saucer-shaped depressions and hills i-1 pm across, fig. 4; (c) small pits 0.1 j.fm or less a&oss. Prior to etching, no pinholes can be observed by this technique, but after a catechol etch treatment, features associated with pinholes can be observed, fig. 5, and the bottoms of many craters become smoother. Further etching causes pinholes to appear in the craters, fig. 6, probably caused by the oxide etching effect. Removal of the oxide shows that the craters are associated with upstanding pedestals in the Si and hence this defect is believed to be due to a mechanism which inhibits or delays oxide growth. Since an oxide grows to approximately twice the thickness of the silicon used, a mechanism inhibiting oxide growth would cause craters in the oxide surface and upstands in the silicon surface. This we believe to be due to contamination during growth. The cause of pinholes may thus lie with either the small pits or with the structure seen in many of the hills, To summarize our work, plasma oxides can be grown which are physically and electrically similar to thermal oxides, but higher defect densities have often been observed. We are pleased to acknowledge the experimental assistance of G. Moule throughout

this work.

Central Research Laboratory, AEI Ltd., Rugby, Warwick&ire, England

E. R. SKELTand G, M. HOWELLS

References 1) J. R. Ligenza, J. Appl. Phys. 36 (1965)2703. 2) W. A. Pliskin and H. S. Lehman, J. Electrochem. Sot. 112 (1965) 1013. 3) R. Lindner, 3. S. T. J. 41 (1962) 803. 4) P. V. Gray and D. M. Brown, Appl. Phys. Letters 8 (1966) 31.