Notes on Aspect Ratio
rates have been observed to depend on aspect ratio
~
depth/width
!
rather than the absolute feature size.
1
Several mechanisms have been invoked to explain the ‘‘rule’’ of aspect-ratio-dependent etching
~
ARDE
!
, but no general theory has emerged that captures the variety of seemingly conflicting experimental observations reported in the literature. 1,2
For example, while an ion-neutral synergy model with pure neutral flux shadowing appears to be con- sistent with a wealth of ARDE measurements in semiconductors, 2 it does not hold for the etching of insula- tors. Indeed, Doemling et al.
3
have reported inverse ARDE of trenches and holes in SiO
2
in a high-density CHF
3
plasma at 20 mTorr. Remarkably, they also reported aspect ratio independent etching
~
ARIE
!
when the pressure was lowered to 6.7 mTorr; for fixed etching time, the etch depth was the same for a variety of trench widths and hole diameters
~
as low as 0.25 m m, corresponding to an aspect ratio of 8.5:1
!
.
These authors convincingly argued that the strong influence of feature geometry on neutral flux of an etch inhibitor, pro- duced in the CHF
3
plasma, is responsible for the inverse
ARDE at the higher pressure. The low pressure results were explained by hypothesizing that the neutral density at the bottom of the trench or hole, while ‘‘too low to cause inverse
ARDE, it was still sufficient to suppress regular ARDE.’’
This hypothesis must be valid for all trenches and holes etched at the low pressure, which spanned the regime of aspect ratios between 2.1:1 and 8.5:1. Bailey and Gottscho
4
examined in detail the possibility of ARIE by exploiting the ability of etch inhibitors to slow down the etch rate in smaller aspect ratio trenches and concluded that this method
‘‘may be useful in minimizing ARDE but only over a limited range of aspect ratios and only with peculiar inhibitor fluxes.’’ Even in the best of cases, with an ad hoc
~
depth/width
!
rather than the absolute feature size.
1
Several mechanisms have been invoked to explain the ‘‘rule’’ of aspect-ratio-dependent etching
~
ARDE
!
, but no general theory has emerged that captures the variety of seemingly conflicting experimental observations reported in the literature. 1,2
For example, while an ion-neutral synergy model with pure neutral flux shadowing appears to be con- sistent with a wealth of ARDE measurements in semiconductors, 2 it does not hold for the etching of insula- tors. Indeed, Doemling et al.
3
have reported inverse ARDE of trenches and holes in SiO
2
in a high-density CHF
3
plasma at 20 mTorr. Remarkably, they also reported aspect ratio independent etching
~
ARIE
!
when the pressure was lowered to 6.7 mTorr; for fixed etching time, the etch depth was the same for a variety of trench widths and hole diameters
~
as low as 0.25 m m, corresponding to an aspect ratio of 8.5:1
!
.
These authors convincingly argued that the strong influence of feature geometry on neutral flux of an etch inhibitor, pro- duced in the CHF
3
plasma, is responsible for the inverse
ARDE at the higher pressure. The low pressure results were explained by hypothesizing that the neutral density at the bottom of the trench or hole, while ‘‘too low to cause inverse
ARDE, it was still sufficient to suppress regular ARDE.’’
This hypothesis must be valid for all trenches and holes etched at the low pressure, which spanned the regime of aspect ratios between 2.1:1 and 8.5:1. Bailey and Gottscho
4
examined in detail the possibility of ARIE by exploiting the ability of etch inhibitors to slow down the etch rate in smaller aspect ratio trenches and concluded that this method
‘‘may be useful in minimizing ARDE but only over a limited range of aspect ratios and only with peculiar inhibitor fluxes.’’ Even in the best of cases, with an ad hoc