detectors technology
project was Royal Observatory of Belgium, based in Brussels, working on system specifications. The project’s primary goal is fabrication of a EUV imaging instrument that can be placed onboard the Solar Orbiter spacecraft and used to study the Sun’s atmosphere. Technical specifications for the imager are very challenging: It should provide EUV images not only with a 10 µm pixel-to-pixel pitch, but also with a rejection ratio of visible radiation of several orders of magnitude. To meet these goals, we and our project partners have agreed on a design that involves an AlGaN-on-silicon active layer on which the 256x256 pixel focal plane array (FPA) is fabricated (see Figures 2 and 3). A flip-chip bonding technique integrates the AlGaN 2D pixel array with the custom-design readout, manufactured in a commercial CMOS technology.
Indium bumps are used as high-density interconnects. These must have good uniformity to ensure reliable connection. Meeting these criteria is tough, because the distance between two adjacent bumps must be less than half-pitch, which equates to 5 µm, and that the height of the bump must be at least 3 µm.
Fig. 3. AlGaN Focal Plane Array with 10 µm pitch: a) schematic showing consequent processing steps together; b) array fragment after MESA etching and deposition of the ohmic and Schottky contacts; c) array after indium bumps deposition, with one bump per one Schottky diode pixel; and d) the same array seen from the backside after silicon substrate removal (through the optically transparent AlGaN layer), showing all levels from a) from the other side of the wafer
A ‘face-down’ approach also has its downsides. First and foremost, you have to work in the backside illumination configuration.
Several groups, who all have faced challenges associated with substrate absorption, have used this geometry. Building a detector of low-energy ultraviolet radiation is relatively easy, because sapphire is transparent in this spectral range. But this material is opaque in the EUV regime.
If silicon is used for AlGaN epitaxy, backside illumination is impossible, so the substrate has to be completely removed. Do this, and you are left holding an epitaxial stack that is less than a micron thick. Handling this without damaging it is tricky, but even so, this is still the most promising way to make AlGaN detectors.
At Imec, which is based in Leuven, Belgium, we have adopted this approach for an EUV imager that has been developed in the framework of the Blind to Optical Light Detectors (BOLD) project from the European Space Agency (ESA). The concept was established together with CRHEA-CNRS, based in Valbonne, France, responsible for the AlGaN epitaxy. Another partner in the
Another challenge is removing the silicon substrate. This is done using an SF6
-based, inductively coupled plasma
reactive-ion etching, which is highly selective to the AlN at the interface of the epitaxial layer and the substrate. A submicron membrane of the active material that is supported by an array of indium bumps is left after this
Fig. 4. A 100x25 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 13 nm is demonstrated with the synchrotron radiation
January / February 2011
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