LayTec is expanding the application of its sensors to new
fields, one of which is growth of oxides on silicon. In collaboration
with Dr. Lutz Geelhaar from Namlab in Dresden (Germany)
– a joint venture of Qimonda and TU Dresden – first
promising in-situ measurement results have been gained.
Dr. Steffen Uredat from LayTec will present the method at the
SVC Conference in Chicago (USA) in April.
In this work LayTec will demonstrate how reflectivity measurements
at optimised UV wavelength can be used to monitor
the deposition of extremely thin ZrO2 and Al2O3 layers
(down to 30 Å) on silicon in MBE. The accuracy of this method
was shown to be up to ±2.5 Å for 50 Å ZrO2 layers.
Fig. 1 shows the reflectance signals at wavelengths of 300
and 405 nm as measured during the deposition of 115 Å ZrO2 on Si, which had previously been coated by 46 Å TiN. The reflectance
decreases with increasing thickness of the oxide layer.
For ultra-thin layers, the 300 nm signal is most sensitive
to changes in layer thickness.
However, when thicker layers are grown, the 300 nm signal
passes a Fabry-Perot minimum. In this case LayTec‘s doublewavelength
set-up provides the second wavelength to close
the gap. In the case of oxides, additional 405 nm reflectance
allows to expand the effective range of measurements to all
typical thicknesses between 30 Å and 500 Å.
If the refractive index and the absorption coefficient of the
oxide layer are well known, the layer thickness can be calculated
directly from the measured reflectance. Fig. 2 shows the
change of the thickness of a 115 Å layer of ZrO2 as obtained
from the 300 nm reflectance measurement, revealing a fairly
linear growth with a growth rate of 0.085 Å/s, as indicated by
the straight red line in the figure.
Beyond growth rate studies, reflectance measurements at
longer wavelengths were used to determine the wafer temperature
based on a double wavelength band-edge approach.
It was found out that during oxide growth the wafers experience
a temperature offset of 20°C between process temperature
and the wafer surface temperature, which is attributed
to heating effects from the hot MBE sources.
This work opens a new field of application for LayTec tools
not only in MBE but also in sputter and other thin-film techniques.