A set of experiments were undertaken to examine the effect of pressure, RF power on the deposition rate and film quality of silica, primarily to characterise and test the system. The silica films were grown on single crystal [100] silicon substrates. The as-grown films were characterised by general appearance, film thickness and uniformity, refractive index and microscopic inspection.
4.5.0 Gas flow ratio of S1H4 to N2O
Since the silane source used was a 5% mix in argon, this implied that the SiH4iAr
mix had to be sufficiently Si-rich to produce the desired initial gas ratio. It could be argued that a flow rate of lOOsccm 5%-SiH4/Ar meant an equivalent flow rate
of 5 seem SiH4. To produce SiOi requires two oxygens for every one silicon atom
in an ideal building-block synthesis. Therefore, to achieve a 10:1 ratio this would require a flow rate (for example) of 1 0 0 seem of 5%-SiH4/Ar and 1 0 0 seem N2O.
For the silicon-rich silica fabrication, ratios were used that substantially increased the silane content of the gas mix.
For this initial study; the ratio of around 20:1 was chosen which suggested that the reaction schemes would take place in an oxygen rich environment. The refractive index of the samples measured by ellipsometry was found to be around 1.457 or lower indicating that some of the samples were probably oxygen rich. Eighteen films were grown and the deposition rate was plotted as a function of RF power and operating pressure. The uniformity of the films was also studied by examining the optical fringes produced under fluorescent lamp.
Chapter Four: Plasma enhanced chemical vapour deposition 1400 1200 1000 c S >< % 800 2
I
600 "cAa
Q 40010 w
200
300 350 150200
250 100Operating pressure (mtorr)
Figure 4.3 Deposition rate as a function of operating pressure and RF power.
500 I 400
1
B 2 300 1) s .2 200 O a, Q 100 - I --- 1— I--- 1--- 1— I- --- 1- --- 1--- 1— I--- 1--- 1--- 1--- 1--- 1--- 1--- 1--- 1--- 1--- 1--- r J__ , O p era tin g i p r e s s u r e o 124 m to rr L IN ■ L _ J I I I L 10 15 RF power (W) 20 25Figure 4.4 Deposition ra te as a function of RF power at a fixed pressure
C h apter Four: Plasm a enhanced ch em ical vapour deposition
4.5.1 Discussion
Figure 4.3 and 4.4 show the deposition rate as a function of RF power and operating pressure. The highest powers give the fastest deposition rate at around 1200 Â/min. The deposition rates for the lowest operating pressure are similar with the exception of the 5W sample. However, as the operating pressure is increased, the deposition rate also increases, this being more marked at higher RF powers. Fig. 4.4 shows a reasonably linear increase in deposition rate as the RF power increases (for a chamber pressure of 124mT) over the power range of 5- 20W. When the refractive index and optical fringing was examined the best films were those grown at 5-lOW in terms of correct index (compared to thermally grown amorphous silica on silicon) and good uniformity across the film.
For the powers of 5,10, and 20W (Fig. 4.3) there is an apparent threshold pressure around 160mT where the deposition increases more rapidly then reaches a maximum and then decreases as the pressure increases. From this, three regimes can be identified: 1) low pressure: where the deposition rate is relatively low and
stable, 2) the transition region: where the deposition rate increases suddenly, and
3) high pressure: where the deposition rate gradient decreases for lower RF powers but carries on increasing for the higher RF power of 20W.
This can be explained as follows: When the system pressure is low (typically <100mT) the mean free paths of the electrons become large and the collision probability decreases hence lowering the dissociation and ionisation of the gas molecules. This results in lower deposition rates and plasma instability. If the pressure is raised, molecular collisions become too frequent and the onset of electron impact ionisation and dissociation of the SiH4 and N2O occurs in the
sheaths very close to the electrodes and substrate; this significantly increases the deposition rate. In this somewhat unstable pressure region, the result on the growing film is an increase in homogeneous nucléation that decreases the uniformity of both the thickness and the composition o f the film.
In the high pressure regime, the subsequent decrease in the deposition rate after reaching a maximum is caused by the decrease in electron temperature, and the deposition rate becomes limited by gas phase polymerisation that causes the powder formation in the plasma region.
C h apter Four: P lasm a enhanced chem ical vapou r deposition
Increasing the RF power at constant pressure leads to an enhancement of both energy and the flux of ions to the substrate. Increasing the pressure at constant RF power results in a decrease in the ion flux and overall energy by the increase number of collisions within the plasma.
Higher RF powers increases the ion bombardment on the film and also increases the energy of the ions striking the surface. This can have two consequences on the growing film: the energetic ions can enhance the surface mobility of the adsorbed species and remove loosely bound species that are often undesirable contaminants. On the other hand, the ions can damage the growing film creating defects in the material.