Frequently asked questions: Parameters we can measure
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Which parameters can be measured using LayTec systems?
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How accurate is the temperature measurement?
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In the temperature range between 450-1300 or 500-1400°C the accuracy is better +/-1K.
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Can all wafers in a multi-wafer system be measured separately?
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Yes, LayTec systems offer wafer selective measurements.
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How do optical measurements work?
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Advantages of emissivity corrected pyrometry compared with band-edge detection
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What spectral range (EpiRAS, EpiR M or EpiR DA) is available?
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For reflectance measurements we offer three different spectral ranges: 280-750nm, 500-1000 or 400-1600nm. For RAS measurements we offer 280-800nm.
Lightsource: XBO for UV and halogen for IR spectral range. -
What detecting wavelength are available for EpiTT / EpiTwin TT?
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For reflectance measurements one of 405, 488, 633 or 950nm sensing wavelength can be selected. The pyrometer detector senses 950 nm always in parallel. The lightsource is LED.
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Advantages of spectroscopic measurement compared with single wavelength reflectance
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For research or process development the spectroscopic systems like EpiR offer much more information and are advantageous over single wavelength systems like EpiTT. During just one measurement different properties like growth rate, roughness etc. can be analyzed at the same time.
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How does composition monitoring work? How precise can composition be monitored?
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Composition can be measured quantitatively through the refractive index n. n changes with x and with T. Therefore, TT-option is needed for an accurate composition measurement. Composition measurement is in principle possible for all ternary materials. For quaternaries (x;y) it is essential, that at least one parameter (x or y) is known. RAS measurement is very sensitive to surface stoichiometry changes, but currently applied only qualitative to observe deviations in the composition.
The growth rate fitting tool (included in all LayTec sensors) determine the composition of selected ternary materials (AlGaAs with an accuracy around 1 percentage point) from reflectance transients via the refractive index.
AlGaN and InGaN composition can be obtained by EpiCurve quantitatively analysis of the curvature slope. -
Can the doping concentration be determined automatically?
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LayTec's EpiRAS in-situ tool is sensitive to composition in conventional III-V's. Reflectance signal is at appropriate wavelengths capable of sensing qualitatively doping concentration. LayTec R&D is currently working on guidelines for doping measurement.
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Does in-situ monitoring work for patterned growth?
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Yes! LayTec's in-situ tools are capable to measure at smooth overgrown patterned substrates and can be used for reproducibility measurements during covalecence.
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How thick needs the layer to be for a precise growth rate fit?
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It depends on the wavelength of the measurements performed. In general at least one Maximum and one Minimum of the Fabry-Perot-Oscillation should be monitored for a precise fit. This relates to an optical thickness of l/2.
However, it is possible to determine the thickness of even thinner layers, as shown by the following example for a DBR mirror (1000 nm centre wavelength):
consisting of layers with the optical thickness of l/4
choosing a measurement wavelength of e.g. 500nm,
assuming a refractive index of n = 4 an optical thickness of l/2 relates to a geometrically thickness of 62.5 nm
If n and k are known and the temperature is measured precisely even thinner layers can be fitted. -
How does the fitting of growth rates work?
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LayTec's analys is based on the virtual layer approach. For this very precise fitting routine, the reflectance is always normalized to the defined substrate reflectance (also automatically correcting for slight effects from window coverage). Whatever material is grown later in the growth run, the growth rate can be determined just by setting the range where the fit should be made.