Under the plasma conditions used, the etch rate in SF6
gas measured on large patterned areas (100
) is approximately 700 nm/min and etching is isotropic. In the case of etching through the PAA mask, the etch rate was found to be much lower (in the range of 140 to 180 nm/min). This etch rate reduction is expected and is due to the small diameter of the alumina pores (this effect is known as ‘etch lag’).
The addition of O2
is known to result in higher etching anisotropy than with the SF6
gas. This is attributed to a different composition of the fluorine-rich polymer formed on the etched Si sidewalls in the case of SF6
compared to SF6
, which provides better surface passivation of the etched sidewalls. More specifically, a SiOx
layer is formed at the etched Si sidewalls when SF6
is used. By adding O2
to the SF6
gas, the number of fluorine atoms in the above fluoropolymer decreases and the number of oxygen atoms per Si increases, thus leading to a more resistant passivation layer on the etched sidewalls and a better etching anisotropy. In the case of our experiments, better anisotropy was observed with SF6
compared with SF6
; however, the etch rate in both cases was quite similar. This is illustrated in Table which shows the etch rate with the three different gases in the case of a large area pattern (100
) with a resist mask, compared with the PAA mask pattern.
Etch rate of Si through an Al mask compared to a SiO2 mask with large openings
With SF6, the etch rate is drastically reduced through the PAA mask compared with the large area etch rate. However, the addition of oxygen in SF6 does not create any significant difference in the etch rate compared with SF6, as in the case of large area etching. The only effect is a slightly better anisotropy. The significant difference is between these two gases and SF6/CHF3. In this last case, the etch rate is lower, and better anisotropy is achieved compared to the first two cases.
In general, the mixture SF6
gives highly anisotropic Si etching. This is due to the fact that with the addition of CHF3
to the SF6
radicals are produced that form a Cx
blocking layer on the Si sidewalls during etching [22
]. This thin fluorocarbon polymer limits the rate at which fluorine radicals from the plasma reach the Si surface. In addition, it limits the rate of diffusion of volatile SiFy
species into Si and, therefore, slows down the chemical etching. Concerning the etch rate in SF6
, it is lower compared with both SF6
gases. This is due to the fact that the F-atom density is barely higher in this mixture compared to the two other cases, thus retarding Si etching [23
In Table , a comparison is made between the etch rate of a 100
Si area formed using a resist mask and the etch rate of Si through the PAA mask (pore diameter in the range of 35 to 45 nm). The thickness of the PAA mask was 400 nm. Several samples were considered, and the range of given values is an average of all measured values. As described above, the etch rate is similar with SF6
, while it is lower with SF6
. By increasing the PAA mask thickness from 400 to 500 nm, the etch rate in SF6
was reduced from approximately 70 to 50 nm/min.
Table shows the feature etch depth on nanopatterned Si surface for the three different PAA layer thicknesses and the three different etching times. The first PAA layer was 390-nm thick, and no Al annealing was used before PAA formation. The two other layers were 400- and 560-nm thick, respectively, and an annealing step at 500°C for 30 min was applied to the Al film before anodization. We have observed that although the annealing resulted in a better adhesion of the PAA layer on the Si surface (no detachment even after 60 s of etch time), it also created an undulation of the PAA/Si interface, which led to etching inhomogeneities on the Si surface. In these two last cases, the etch depth varied from zero (non-etched areas) to the maximum value indicated in Table . In the case of the non-annealed sample, the etch depth was homogeneous in the whole film. The problem was that for an etching time above 40 s, the lateral etching of the Si film underneath the mask led to mask detachment. The maximum etch depth achieved in that case was around 45 nm.
Feature etch depth using SF6/CHF3