technology detectors
Our imagers were characterized with the radiation from a synchrotron located at PTB/BESSY II in Berlin, Germany. Using a dedicated readout system with vacuum feedthroughs, the imagers were placed in the beamline and measured after aligning the beam to the active area of the detector chip. Images from a 100x25 pixels region were analyzed as a representative part of the 256x256 array (see figures 4 and 5). Both these samples had a silicon substrate grid patterned on top of the entire AlGaN active layer, apart from the 1-mm-wide frame around it. Thanks to this approach, it is possible to have a fixed pattern in the image and to explore different post- processing schemes.
Fig. 5 A 50x25 pixel fragment of the array after integration and substrate removal, showing the AlGaN layer (yellow) exposed to radiation with the silicon substrate frame around (brown). The response pattern under illumination with wavelengths below the cut-off wavelength (280 nm) corresponds to the shape of the substrate opening. Sensitivity down to the wavelength of 1 nm is demonstrated with the synchrotron radiation. At such a low wavelength silicon becomes transparent, which is visible as more pixels are activated at the edges of the opening, where the Si substrate is thinner
step has been completed. There are several critical requirements for this AlGaN layer: It must be thin enough for optimum performance in the EUV range, where the penetration depth is very small; it must be completely free from cracks; and it must contain as few defects as possible, to minimize the recombination losses after carrier generation. To avoid curling of the AlGaN layer due to relaxation and the resulting debonding of the detector chip, a 1-mm-wide frame of silicon is left around the active area, which is supported by an array of dummy interconnects (see the shape of the silicon substrate in Fig. 2). Integrated chips are then encapsulated in a package, wire-bonded and characterized in any facility equipped with the EUV radiation source.
Spanning the UV
Relatively simple systems can be used to perform measurements under ultraviolet illumination – commercially available lamps in combination with filter wheels or with a monochromator serve the purpose perfectly. However, characterization below 200 nm requires a much more complicated, vacuum configuration, because high-energy radiation is strongly absorbed in air.
Samples had a cut-off wavelength of 280 nm, due to the deployment of an Al0.4
Ga0.6 N active layer. At longer
wavelengths no response was registered, and at lower wavelengths the pixels produced a response corresponding to the area in the substrate opening. Since the substrate is not transparent, it acts as a shadow mask, allowing better distinction of the photogenerated signal. Reducing the wavelength even further, it’s possible to obtain images at the Lyman-α line (121.6 nm) and in the EUV range – the target spectral span for our device.
Excitation of our devices with radiation as short as 13 nm produces a response, which is very promising for applications in the EUV lithography. It is worth noting that at this excitation wavelength more pixels at the edge of the opening show response. This is because at the edge the silicon substrate is thin enough to allow penetration of the high-energy photons. Even though not all the pixels in these examples are operational, this experimental data provides a proof-of-concept for this imager: It is possible to demonstrate a solar-blind response in a two- dimensional array with 10 µm pixel-to-pixel pitch.
Process investigation and optimization is ongoing, and the next goals are to obtain a uniform response and good fabrication repeatability. The results from the first batches of demonstrators reveal many possible improvements. Concerning applications, these AlGaN imagers promise to serve many areas outside the original EUV solar observation. Not only could they have an impact in EUV lithography; they may also serve other scientific applications requiring long-term stability and intrinsic blindness to visible and infrared light.
© 2011 Angel Business Communications. Permission required.
Further reading P.E. Malinowski et al. IEEE Electr. Device Lett. 30 1308 (2009).
F. Barkusky et al. Rev. Sci. Instrum. 80 093102 (2009). P.E. Malinowski et al. IEDM 14.5 (2010).
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