UV Tool Maps Nitride Temperatures
Profitable chip making demands low development costs and high production yields. To cut expense, growth recipes should be established using minimal runs, because this optimization process can consume a large proportion of the development budget. To do this, process engineers must know as much as possible about the reactor s local environment, including wafer temperature – a primary driver of epilayer growth rates and compositions.
Pyrometry is the standard method for measuring the wafer s temperatures within a reactor. The technique involves measuring the intensity of thermal radiation emitted by the wafers over a narrow wavelength band using a photodetector, and then correlating this intensity to a temperature. The temperature of wafers based on InP and GaAs material systems can be measured with a pyrometer operating at 950 nm. However, this spectral region is useless for nitrides, because they do not produce any radiation at this wavelength.
To address this deficiency, pyrometers that operate at 400 nm have been built for nitride growth. The first of these was constructed by J Randall Creighton and co-workers from Sandia National Laboratories, NM, and last year in situ monitoring specialist LayTec introduced a commercial version of this tool, the Pyro 400, at the MRS fall meeting in Boston, MA. This instrument is primarily designed for Aixtron multi-wafer reactors, but could be adapted for Thomas Swan tools.
Developing a pyrometer that operates in the near ultraviolet is not easy, according to LayTec s president Thomas Zettler: "In a reactor at 1000 °C there is very intense radiation at all wavelengths, and it s a real challenge to measure 400 nm emission accurately." Emission at 400 nm is many orders of magnitude weaker than infrared radiation, and the careful selection of several filters is required to block out this unwanted radiation. Operating in this wavelength range enables LayTec s instrument, which consists of collection optics, filters and a detector, to measure the surface temperature of multiple wafers with a precision of ±0.1 °C or less. Such precision exceeds that of all other commercial tools, says Zettler.
Although the Pyro 400 can measure temperatures with very high precision, it does not feature emissivity correction. This means that it ignores the variations in emissivity between different objects, and that the growth of an anti-reflection film would produce a change in the value of the recorded temperature. However, Zettler believes that this weakness is not a big deal. That s because the tool is primarily designed to check bare wafer surface temperature, and there is also only a small difference between the emissitivities of GaN and AlN.
To drive sales, Laytec will have to convince its customers to add the Pyro 400 to other in situ instruments already installed in reactors, such as laser deflection and reflectance tools that monitor surface bowing and susceptor surface temperature. These monitors can ensure wafer flatness, but they cannot reveal whether temperature variations occurred across the substrate during the growth. These differences can exist even before growth begins, says Zettler, as they can come from slight variations in the susceptor s geometry.
Zettler believes that the Pyro 400 can deliver the greatest benefit as a complementary tool to LayTec s Epicurve TT sensors during the development of growth recipes for future products. These recipes must produce wafers with uniform, high-quality active regions and a low degree of bowing. The number of runs required to fine-tune this growth can be reduced with the pyrometer, says Zettler, adding that the in-house expertise generated from developing the instrument has also enabled the company to show customers how to get more out of its wafer-bowing equipment.
At the MRS meeting, Zettler s colleague Elizabeth Steimetz presented a poster detailing the capabilities of the Pryo 400, which was co-authored by researchers from the Ferdinand Braun Institute for High Frequency Technology (FBH) in Berlin. The pyrometer was fitted to an Aixtron AIX2600HT planetary reactor equipped with LayTec s EpiCurve sensor, and mapped the temperature of a platter containing 11 different 2 inch substrates and epiwafers with a precision of ±0.1 °C and a ±3 mm spatial resolution.
These trials – which were conducted at 1100 °C and a platter rotation speed of 6 rpm – revealed that a 10 μm bow caused by depositing GaN on sapphire produces a 4 °C temperature variation across the wafer. Switching to a SiC platform replaces a convex bow with a concave one, and variations in wafer temperature decrease to 1.2 °C per 10 μm of bow. According to the researchers, lowering the reactor s temperature to 800 °C and tuning the GaN nucleation layer growth with the Pyro 400 and a curvature sensor dramatically reduced the bowing of both of these epiwafers, leading to growth of quantum wells with excellent uniformity.