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Old 28-07-2009, 11:25 PM   #31
Sponge Jockey
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Wax Thickness Test Results

Well here are some results of wax thickness as measured by reflectometry and ellipsometry.

The units given below are in nanometres, abbreviated as nm. For reference, 20 nm is 1/50th of a micron.
One micron is roughly 1/80th of the thickness of a human hair (hair thickness depends on hair colour/ethnic origin etc).

(Some technical details for the scientifically minded:- The methods used are capable of measuring to sub nanometre precision on clean silicon wafers.
However, the accuracy of the methods depend to some extent on the refractive index of the materials being measured.
Here we have used computer fitting to derive the thickness and refractive index dispersion simultaneously.
The value of refractive index obtained was 1.413 (at 632.8 nm) and the value quoted in the literature for pure Carnauba wax is 1.454 (wavelength not quoted).
This gives some confidence that the values of thickness are correct.
The clean silicon wafers were measured beforehand and thicknesses of 2 to 3 nanometres of silicon dioxide were shown to be present (as expected).
Silicon dioxide has a very similar refractive index to the wax and so cannot be distinguished from wax in the final analysis.)

The first tests reported below used Collinite no 915 applied to clean 6 inch diameter silicon wafers using a damp microfibre cloth (as manufacturer's recommendations).
Four silicon wafers were used, with slight variations on the waxing technique as given below.

Wafer 1. Wax was applied thinly with damp cloth, left for 5 minutes to haze over and then buffed off with clean microfibre cloth. No great effort was applied.
Measured Thickness = 22.7 nm (in centre of wafer)

Wafer 2. Wax was applied thickly with damp cloth, left for 15 minutes to haze over and then buffed off with clean microfibre cloth. This layer required a fair degree of effort to remove compared to wafer 1.
Measured Thickness = 20.6 nm (in centre of wafer)

Wafer 3. Wax was applied thinly with damp cloth, left for 15 minutes to haze over and then buffed off with clean microfibre cloth. No great effort was necessary.
Measured Thickness = 15.9 nm (in centre of wafer)

Wafer 4. Wax was applied thinly with damp cloth and buffing with clean microfibre started immediately.
= Measured Thickness = 16.9 nm (in centre of wafer).

The actual wax thicknesses will be 2 to 3 nm less because of the native oxide layer as explained above. (I do have the exact values)

One more thing before I finish for today. Our equipment can also measure the thickness distribution over the 6 inch wafer and a thickness map for wafer 4 is shown in the picture below.
Dark blue in the picture corresponds to thickest and light blue to thinnest.
In case it is not clear, the thinnest value was 15.05 nm and the thickest value was 20.26 nm (standard deviation is 1.25 nm and thickness uniformity is +-14.9%)

My first reactions are that I am surprised how little variation there is between wafers or over the surface of wafer 4. A standard deviation of 1.25 nm for a layer applied by a damp cloth is pretty impressive!
The actual thicknesses of 15 to 20 nm (or 12 to 17 nm minus the oxide layer) is not surprising because the wax layers look extremely smooth and there do not show any optical interference effects even on the ultra smooth silicon wafers
(like you see with oil on water or a soap bubble as it thins down to bursting). This in itself implies that the thicknesses are less than 100 nm.

Tomorrow I will try to add another layer to each wafer!
Attached Images
File Type: jpg wafer4.jpg (86.3 KB, 234 views)
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