BRIEF DESCRIPTION OF THE DRAWINGS
An example of a solid state detector and method according to the present invention is now described with reference to the accompanying drawings, in which:
FIG. 1 shows a prior art grooved detector, partly in section;
FIG. 2 shows a prior art “top hat” detector, partly in section, with a small rear electrode;
FIG. 3 shows a prior art “grooved” detector, partly in section, with a small rear electrode;
FIG. 4 shows a prior art detector, partly in section, having a front electrode extended around the sidewalls;
FIG. 5 shows an example detector, partly in section, according to the invention; and,
FIGS. 6(
a), 6(b) and 6(c) show major steps in an example method of manufacturing a detector according to the invention.
DETAILED DESCRIPTION
An example of a detector according to the invention is shown in FIG. 5. FIG. 5 shows, partly in section, a “grooved” Si(Li) detector having an absorber in the form of a detector crystal 10. This is a circular disk a few mm in thickness. The crystal has front and rear planar opposed faces. In a similar manner to known detectors a front electrode 20 is formed from a thin electrically conductive coating applied across the front face of the absorber crystal.
A rear electrode 30 is formed by diffusing lithium into the silicon wafer prior to cutting out the shape of the crystal absorber. The application of a potential difference between the front and rear electrodes produces a semiconductor depletion zone 40 which extends to the front of the detector and throughout the Li-compensated silicon. The region of intersection of the depleted region with the front surface approximately defines the extent of the active region of the front surface. If a collimator is present, as is the case in FIG. 5 at 70, then the active region is that part of the front face upon which ionising radiation is incident when in use.
At the periphery of the compensated region, and beyond the active region at the front face, an effectively conductive region of undepleted silicon 50 is located. This is in electrical contact with the front electrode. Within the depletion zone, there is a strong field, shown by arrows and equipotential field lines. X-rays which are incident upon the detector enter the detector in the direction indicated at 60 and liberate charge upon interaction with the depleted silicon in the known manner.
The object of the invention, namely improvement in detector performance, is achieved with a detector crystal having smooth tapered sidewalls (shown at 80). The rear electrode 30, as a readout anode, covers the whole rear face. Each of the rear face and the rear electrode therefore have a substantially smaller area than the active region of the “entrance” front electrode 20. All the material that is not adjacent to the back contact has been removed and there are smooth tapered sidewalls that extend towards the larger diameter front of the detector. As with prior art Si(Li) detectors, the sidewalls are passivated to render them non-conductive so that they can sustain a high field near to the surface. The effect of the modified shape is to strengthen the field significantly along the surface near the periphery of the rear electrode, as shown by “S” in FIG. 5. This resolves the charge collection problems of prior art designs such as FIG. 3. The capacitance is 0.49 pF which is less than that of the prior art example of FIG. 3. This value is much less than the 0.94 pF of the device in FIG. 1, which has essentially the same front active area. Thus the goal of reducing capacitance while maintaining large active area and good charge collection is achieved. The structure of FIG. 5 can also be achieved with a simpler processing sequence than described by Rossington et al (discussed above in relation to FIG. 4) and the final device has the advantage that the undepleted silicon at the periphery can be used to grasp the device without contacting the critical surfaces.
A variant of this shape, to which the invention can be applied, is the “top-hat” structure shown in Goulding et al (or Tikkanen et al as shown in FIG. 2) where the outer regions beyond the groove are not present but there is still a peripheral ring of undepleted Si around the front contact.
Thus the invention provides an improvement to the design and manufacture of radiation detectors which offers increased active area with no significant degradation of noise and charge collection performance.
A method of manufacturing an ionising radiation detector according to the invention is shown in FIG. 6.
A conventional grooved Si(Li) crystal is made with a Li-diffused back contact (FIG. 6(a)). A special drill bit is then used to cut a conical surface that reduces the size of the rear face and produces a smooth surface extending away from this towards the front face (FIG. 6(b)). This produces the sloping sidewalls. Alternatively, these could be created with chemical etching or deep reactive ion etching. The sloping side walls are then etched and passivated (shown at 90) using the same process that would be used for the side walls for a conventional grooved Si(Li) structure. A conventional front contact electrode for use with a conventional grooved Si(Li) detector is then applied to the front face to produce the structure shown in FIG. 6(c). This has essentially the same front face area as the unmodified grooved Si(Li) detector but has significantly reduced capacitance. The complete detector is then finished using conventional processing steps, these including connecting electrical wiring to the electrodes and housing the structure.
The process steps described above are simpler than those described by Rossington et al (in relation to FIG. 4). As a further advantage, the final crystal is compatible with the same handling and mounting arrangements as would be used for a prior art Si(Li) crystal.
Different geometries of taper can be used on the side walls. A shallow taper only produces a slight reduction in capacitance. The taper does not have to reach the “bottom” of the groove. However, the preferred embodiment uses a taper that almost reaches the bottom of the groove so that when the normal etching processes are used to prepare the side walls for passivation, the cross section is similar to that shown in FIG. 5 where the distance from the front surface to the rear contact electrode is about 3 mm. In this case, the crystal can have an active front face area of 30 mm2 but with a capacitance as small as that of a conventional grooved Si(Li) detector with only 10 mm2 active area. Thus, for example a 30 mm2 active area detector can be manufactured that has essentially the same electronic noise and energy resolution as a prior art 10 mm2 detector. This improves sensitivity by a factor of three with no penalty in spectral quality.
It should be noted that because material has been removed from the back of the detector, high energy x-rays entering the front face may now penetrate through the side wall and go undetected. However, only about 2% of 10 keV x-rays will penetrate 0.5 mm and this is not enough to reach the tapering wall of FIG. 5. As a result, such a detector will not show any loss of efficiency for x-rays below 10 keV in energy. Therefore, the invention is particularly useful for low energy x-ray analysis.
In FIG. 6, the taper is applied after the Si crystal has been Li-compensated. In an alternative embodiment, the taper is applied to the crystal before it is subjected to the conventional Li-compensation process.
The same tapering principle can be used with other detector materials such as high purity silicon. In this case, the process is similar to that in FIG. 6(a) but the starting crystal is now purified silicon rather than Li-compensated silicon and the back contact can be either diffused Li or could be an implanted contact such as that described in U.S. Pat. No. 6,153,883. Similarly, high purity germanium or other materials such as CdTe, CdHgTe, CdZnTe could be used. The front contact could also be ion implanted.
Whereas FIG. 6(a) shows a grooved structure, the starting point can alternatively be a “top hat” structure as shown in Goulding et al for example and the final crystal still has peripheral regions around the rim of the front face that can be used for handling.
The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
Patent Application
20070290141
