FACTORES SOCIALES.

For each deposition, the two cavity tops were placed at the same radial distance from the centre of the work table and on a line perpendicular to the target electrode gap. The cavity parameters were measured using the waveguide reflectometer apparatus shown in figures 8-3 and 8-4 . The use of the Gunn diode source yielded a maximum error in ten Q measurements of _+ 0.2^j and an error of _+ 0.1 MHz for the resonant frequency, the cavity being dismantled for each

1 measurement.

10-4 Results for Permittivity, Toss Tanprent and Denosition Rate

The results obtained for the eermittivity, loss tangent and deposition rate measurements are given in tables 10-2, -3, and -4

respectively. Duplication of the measurements allowed tho commuter programme given in Appendix 2 to be used to assess tho significance of tho effects and interactions. All depositions wore performed with a sourco/substrate distance of 39mm.

10— 4— 1 Permittivity Results (Table 10-?)

The majority of tho permittivity observations exhibit a dielec­ tric constant lower than tho accopted value of 3*8 for bulk silicon dioxide. The only significant effect is that due to tho orecer-ce of

— 2 2

the magnetic field. Applying a magnetic field of 1.25 X10 Wb/m causos a general lowering of tho permittivity. The magnetic field and

sputtering pressure are also shown to interact. Further calculations show that the application of the magnetic field at 10 ^ Torr causes a decrease in the permittivity of 6

¡ t

_2

served at a pressure of 10 Torr. These effects are, however, dependent upon the RiJ input power as is indicated by the second order interaction WMP. The only general conclusion is that tho magnetic field causes a decrease in tho permittivity, the actual decrease ob­ tained being governed by the other parameters. This effect may be explained in terms of the argon content of the film. Schwartz end Jones ^ 5 ) have shown that the argon content of silicon dioxide films is increased b y the presence of a magnetic field. In sputtering, the substrate and growing film will receive bombardment by enorgetic

(

argon ions and electrons. Vossen

v"

156

10-4-2 Loss Tangent Results (Table IQ-i)

All three parameters significantly affect the dielectric loss. Increasing the DC input power increases the loos tangent of the film. This may he attributed to the increased deposition rote (Tablo 10-4) causing an increase in the number of impurities trapped in the film and also trapping of deposited silicon dioxide molecules in non— ideal

positions. The application of a magnetic field is again significant,

in general causing a decrease in the dielectric loss. This may be

explained again in terms of an inrease in the negative bias developed

on the film surface. Argon ion bombardment of the film will result

in some re-sputtering of the film. Impurities and silicon dioxide

molecules situated in non-ideal positions will be preferentially

sputtered owing to their low binding energy within the film. This

will result in improved film quality and lower dielectric loss.

P-etch is often used to evaluate the quality of silicon dioxide films

(^4) # Logan studied the effect of the substrate bias voltago

on the P-etch rate of silicon dioxide films and found that a dramatic decrease in the etch rate (corresponding to a high-purity, low- stressed film) occurred for negative bias voltages in the range

10 to 20 volts. Comparison of treatments x and m shows that the loss tangent for the films deposited for treatment m is higher than for

those deposited for treatment x. This is due to the increase depos­

ition rate caused by the magnetic field (Table 10-4). As was explained for the effect of DC input power, the increased deposition rate will increase the dieleotric loss while the increased bombardment of the film by argon ions caused by the magentio field will tend to

counteract this effect. Pressure has a very significant effect on

_2

. An increase in pressure to 10 Torr can incroase

157

the loss tangont by an ordor of magnitude. Jones ot al havo shown that tho floating potential of a silicon wafer in an HP dis­ charge is groatly roducod for increasing pressures which reduce the

f rrq )

amount of re-sputtering. These workers ' 7 / have also shown that a high re- emission coefficient (i.e. the ratio of the amount of sputtered material re-emitted from the substrate to the amount of incident material) is essential if a high quality film is to bo ob­ tained. It is therefore believed that the observed effect of pressure is caused b y there being only a small amount of re-sputtering of the film (i.e. a low re-emission coefficient); this results in a high impurity concentration and poorlypositioncd silicon dioxide molecules.

Finally, interaction KP indicates that the effect of the magnetic

- 2 - 3

field is far greater at a pressure of 10 Torr than at 10 Torr.

This suggests that the amount of re-sputtering caused by the magnetic

_2

field at 10 Torr has a greater effect on the film properties than

the accompanying increase in the deposition rate. This is the

opposite to that observed for treatments x and m discussed earlier in this section.

10-4-3 Jenosition Rate Results (Table 10-4)

As was shown in Chapter 3, an increase in the energy of the ions bombarding the target consequent upon an increase in the power input to the discharge increases the deposition rate. This is indicated b y main effect

W.