RADIATION DETECTOR AND METHOD FOR MANUFACTURING A RADIATION DETECTOR

Abstract

A radiation detector includes a substrate and a membrane suspended above the substrate by spacers, wherein the spacers electrically contact a radiation sensor formed in the membrane and thermally insulate the membrane from the substrate.

Claims

1. A radiation detector comprising a substrate and a membrane suspended above the substrate by spacers, wherein the spacers electrically contact a radiation sensor formed in the membrane and predominately thermally insulate the membrane from the substrate.

2. The radiation detector according to claim 1, wherein the membrane is suspended on the spacers without ridges or wherein the membrane is suspended on at least one of the spacers by a ridge, the thermal insulation at the at least one spacer being favored by the spacers.

3. The radiation detector according to claim 1, wherein a reflector is disposed between the substrate and the membrane.

4. The radiation detector according to claim 3, wherein the reflector is disposed between the substrate and the membrane by further spacers.

5. The radiation detector according to claim 3, wherein the reflector comprises a metal layer.

6. The radiation detector according to claim 3, wherein the distance between the reflector and the membrane is an odd, integral multiple of a quarter of a main wavelength to be detected.

7. The radiation detector according to claim 1, wherein the spacers are hollow on the inside.

8. The radiation detector according to claim 1, wherein the spacers are manufactured by using ALD and a sacrificial layer method.

9. The radiation detector according to claim 1, wherein a wall of the spacers comprises a plurality of layers of different materials.

10. The radiation detector according to claim 9, wherein the plurality of layers comprise at least one layer of TiN, Ti, Cu, W, Sn, Ni, Au, Al or a combination thereof, surrounded by an oxide layer.

11. The radiation detector according to claim 1, wherein a cross-sectional area of the spacers is smaller than or equal to 7 μm.sup.2 or smaller than or equal to 3 μm.sup.2 or smaller than or equal to 0.8 μm.sup.2 across a total length.

12. The radiation detector according to claim 1, wherein a wall of the spacers comprises one layer of a single material.

13. The radiation detector according to claim 1, the radiation detector comprising one layer of a single material.

14. The radiation detector according to claim 1, wherein a length of the spacers between the membrane and the substrate is smaller than or equal to 100 μm.

15. The radiation detector according to claim 1, wherein a length of the spacers between membrane and substrate is larger than or equal to 0.4 μm.

16. The radiation detector according to claim 1, wherein a wall of the spacer comprises at least one layer of a material, wherein the maximum layer thickness is smaller than or equal to 1 μm.

17. The radiation detector according to claim 1, wherein the ratio of a length to a width of the spacer is larger than or equal to 10 or larger than or equal to 5 or larger than or equal to 1.

18. The radiation detector according to claim 16, wherein the layer thickness varies across a total length of the spacers between the substrate and the membrane by less than 50 nm or less than 25 nm or less than 5 nm.

19. The radiation detector according to claim 1, wherein the radiation sensor is a temperature-dependent electrical resistor or a temperature-dependent capacitance or a temperature-dependent inductivity or a diode.

20. The radiation detector according to claim 1, wherein the substrate comprises a temperature evaluation circuit.

21. The radiation detector according to claim 1, the radiation detector further comprising laterally extending ridges by which the membrane is suspended on the suspensions, which increase a thermal insulation of the membrane from the substrate by the spacers and which are electrically contacted to the radiation sensor.

22. The radiation detector according to claim 1, the radiation detector being a bolometer.

23. An array of radiation detectors according to claim 1.

24. A method for manufacturing a radiation detector comprising a substrate and a membrane suspended above the substrate by spacers, wherein the spacers are manufactured by using ALD and a sacrificial layer method so that the spacers electrically contact a radiation sensor formed in the membrane and predominantly thermally insulate the membrane from the substrate.

25. The method according to claim 24, wherein an opening in a sacrificial layer, into which one of the spacers is deposited, is etched by using a DRIE process.

26. The method according to claim 25, wherein the Bosch process or the cryogenic process is used as the DRIE process.

27. The method according to claim 24, wherein a wall of the spacers comprises a layer manufactured by using ALD.

28. The method according to claim 24, wherein the method further comprises forming a reflector between the substrate and the membrane suspended on further spacers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which:

[0020] FIG. 1 shows a schematic illustration of a radiation detector, wherein a membrane is suspended on a substrate by spacers;

[0021] FIG. 2a shows a schematic illustration of a spacer consisting of two layers of different materials and is hollow on the inside;

[0022] FIG. 2b shows a schematic illustration of a spacer consisting of two layers of differing materials and is hollow on the inside;

[0023] FIG. 3 shows a cross-section of a radiation detector, wherein a reflector is disposed on the substrate, and the contacting and the layers of the membrane are shown;

[0024] FIG. 4 shows a schematic illustration of a radiation detector, wherein the reflector is suspended above the substrate on further spacers;

[0025] FIG. 5 shows a cross-section of a radiation detector, wherein the spacers as well as the membrane consist of one layer of a material;

[0026] FIG. 6 shows a schematic illustration of a radiation detector, wherein the membrane is suspended on the spacers by ridges;

[0027] FIG. 7a shows a schematic illustration of an array of radiation detectors;

[0028] FIG. 7b shows a schematic illustration of a top view of the spacers;

[0029] FIG. 7c shows a schematic illustration of a side view of the spacers;

[0030] FIG. 8a-h show schematic illustrations of method steps of the manufacturing method;

[0031] FIG. 9 shows a schematic illustration of a radiation detector, wherein a membrane is suspended on a substrate by spacers and the spacers are stabilized by an oxide layer;

[0032] FIG. 10 shows a section around a contact of the substrate;

[0033] FIG. 11 shows a schematic illustration of a known microbolometer;

[0034] FIG. 12 shows a schematic illustration of a top view of a known membrane, wherein the membrane is suspended on metal contacts by ridges.

DETAILED DESCRIPTION OF THE INVENTION

[0035] FIG. 1 shows a radiation detector with a substrate 20 and a membrane 10 suspended above the substrate 20 by spacers 45. A radiation sensor 50 is formed in the membrane 10 which may be electrically contacted to the substrate 20 and thermally insulated from the substrate 20 by the spacers 45. In the following, the spacers 45, the membrane 10 and the substrate 20 will be described in detail.

[0036] The spacers 45 may realize the thermal insulation of the membrane 10 from the substrate 20, while the electromagnetic radiation detectors are simultaneously electrically contacted, wherein the latter may be embodied, for example, as sufficiently long and thinly layered hollow tubes which, for example, may be manufactured by using technologies and methods of microsystem technology. As described above, due to the thick metal coating, the respective thermal conductance of the contacts, more specifically of the spacers, is thus far very large as compared to the ridges and does therefore not contribute to thermal insulation. The proportion in the thermal insulation amounts to less than 2%. However, if the walls of the, e. g., round contacts are coated sufficiently thinly with a suitable metal layer, a corresponding thermal conductance may result therefrom, which may be comparable to that of the ridges 22 or may even be significantly smaller. The thermal conductance of the spacers 45 may be calculated analogously to Eq. 4, wherein in this example, as shown in FIG. 2, circular rings form the respective cross-sectional areas.

[00004]$\begin{array}{cc}g=\frac{\pi }{2.Math..Math.l}.Math..Math..Math.{\lambda }_i\left(r_{i,2}^2-r_{i,1}^2\right)& \left(\mathrm{Eq}..Math.4\right)\end{array}$

[0037] In FIG. 2a, the radiuses r.sub.1,1, r.sub.1,2 and r.sub.2,1, r.sub.2,2, respectively, are exemplarily illustrated, however the spacers may also consist of one or a plurality of layers. The term r.sub.1,2−r.sub.1,1=d.sub.i is equivalent to the thickness of the individual materials/layers inside of the contacts, l denotes a length of the spacers and λ.sub.i denotes the thermal conductance of the layer i. i is therefore the index of a layer as well as the running index of the sum and has the value range 1≦i≦N, wherein N is the number of the layers. N may be any natural number. i, for example, increases from the inside to the outside, that is, i corresponds to the rank of the layer from the inside to the outside. The second index of the radiuses describes the inner radius (1) and the outer radius (2) of a layer. Further, r.sub.1,2=r.sub.1+1,1 applies when the layers rest on one other without a gap. If the spacer 45 is configured of only one layer (N=1), the sum may be formed of one element only and is therefore to be omitted. Analogously, instead of the circular design, the spacers may comprise any other shape. Likewise, the spacers 45 may be filled, for example, with a thermally insulating material instead of being hollow on the inside. In this case, for example r.sub.1,1=0 would apply.

[0038] Similar to the ridges, the spacers 45 should also be as long as possible and consist of very thin materials/layers comprising a low thermal conductivity. This emerges, for example, from Eq. 4. In other words, it is desirable that, when the overall diameter D of the contacts, that is, r.sub.N,2 (in FIG. 2 denoted as r.sub.2,2), is selected to be as small as possible, for example such that a cross-sectional area of the spacer 45 is smaller than or equal to 7 μm.sup.2, in another embodiment smaller than or equal to 3 μm.sup.2 and in an embodiment smaller than and equal to or smaller than 0.8 μm.sup.2. Otherwise, many thin layers or few thick layers may form a large diameter, in both cases resulting in a larger conductance. Hence, for a lower conductance, the aspect ratio of the length I to the diameter D also needs to be taken into account. For example, it is equal to or larger than 10, however, in another embodiment it may also be equal to or larger than 5 or in an alternative embodiment it may be equal to or larger than 1. These principles also apply to any other spacers apart from the round spacers described herein; however, Eq. 4 needs to be adapted accordingly for this. The cross-section of the spacers could, for example, also be square or rectangular or any other shape. Generally, for a layer i, the pair r.sub.i,2 and r.sub.i,1 could be selected so that r.sub.i,2.sup.2 corresponds to the area bound by the outside of the cross-section of the layer i and r.sub.i,2.sup.2-r.sub.i,1.sup.2 corresponds to the area of the cross-section of the layer i. r.sub.N,2 would then correspond to a “width”. FIG. 2b shows another dimensioning of the spacers.

[0039] Beside the thermal insulation, the second function of the spacers may consist of electrically contacting this element. Generally, the spacers consist of one electrically conducting layer. To protect the electrically conducting layer during the etching process of the sacrificial layer, other protection layers may optionally also be deposited with the atomic layer coating. The protection layers should be electrically and thermally insulating and stable with respect to the etching medium of the sacrificial layer. The thicknesses of all layers of the spacers are within a range of 0.1 nm-1 μm, but may also be thinner or thicker.

[0040] As described above, the membrane 10 supported by the spacers 45 consists of an element changing its electrical characteristics when fed with heat, and an absorber layer. The temperature sensitive element consists of a temperature-dependent electrical resistor, a temperature-dependent capacitance, a temperature-dependent inductivity or a diode (pn junction). The absorber layer may be selected such that its reflection coefficient is similar to that of air so that a smallest possible part of the incident radiation is reflected.

[0041] The substrate may, for example, be made of a semiconductor or a different material. Beside the base area for the spacers, a temperature evaluation circuit may be integrated in the substrate. For example, the same is disposed directly below the membrane; however, it may be disposed at a different location.

[0042] FIG. 3 shows an application example of the realization of the thermal insulation and contacting of electromagnetic radiation detectors by using the suspension. Exemplary, it is the cross-section of a resistance microbolometer. In this embodiment, a reflector 35 reflecting back the radiation that is transmitted by the membrane 10 onto the radiation sensor 45 is disposed on the substrate 20.

[0043] The membrane may consist of a sensor layer 30 and an absorber layer 25, for example, which may consist of about 300 nm a-SI and about 7 nm TiN. However, other materials and layer thicknesses are possible. Below the sensor layer 30 and above the absorber layer 25, respectively, an aluminum oxide (Al.sub.2O.sub.3) layer 70 with a thickness of 10 nm may be located which serves as protection during etching of the sacrificial layer. Again, other materials and/or differing layer thicknesses are possible as an alternative.

[0044] In this case, the spacers consist of three layers 75, 80, 85 deposited, for example, by using the atomic layer deposition ALD, as described above. The method for manufacturing a radiation detector will be described in detail below. The inner and outer layers 75 and 85 may also be, for example, aluminum oxide or another material. The inner layer 75 comprises a typical total thickness of, for example, 20 nm and the outer layer 85 comprises a typical thickness of, for example, 10 nm. These oxide layers are also to protect the middle metal layer during etching of the sacrificial layer. Moreover, the inner Al.sub.2O.sub.3 layer may serve as an insulator between the metal electrodes 90 on the sensor layer and the absorber layer 25. The middle metal layer 80, which may serve as contact between readout circuit and sensor layer, consists of, for example, TiN or another electrically conductive but thermally insulating material and comprises, for example, a typical thickness of 5 nm. Each of the layers comprises, for example, a thickness of 1 μm or less. The layer may also be thicker than 1 μm. The diameter of the spacers may be, for example, 400 nm. Generally, for example even in cases of non-round spacers, the cross-sectional area in a first embodiment is smaller than 7 μm.sup.2, in a second embodiment smaller than 3 μm.sup.2 or in a third embodiment smaller than 0.8 μm.sup.2. However, it may also be larger. The length depends on the resonator condition, for example, about 6 μm is typical. As described above, the resonator condition should be met for a maximum absorption of the incident infrared radiation. For this reason, the length of the spacers 45 may not be selected arbitrarily, but should be adapted according to Eq. 1 and/or Eq. 2. Generally, the length of the spacers 45 between substrate 20 and membrane 10 is at least 3 μm and at the most 100 μm. However, it may also exceed this range.

[0045] FIG. 4 shows an embodiment wherein the restriction due to the resonator condition is omitted by realizing the reflector in terms of process in such a manner that the optical path between itself and the absorber layer 25 is exactly a quarter or an integral multiple of a quarter of a main wavelength to be detected, regardless of the length of the spacers 45. This may be made possible by suspending and therefore elevating the reflector by means of additional spacers 95. The spacers 95 may then be selected such that the resonator condition may be met.

[0046] FIG. 5 shows a further embodiment wherein the spacers 45 as well as the sensor layer 30 and the absorber layer 25 consist of only one layer 80 of a material. For example, the spacers may be embodied very thinly and may therefore thermally insulate whereas a thicker layer of the same material may represent a thermal sensor. The cross-sectional area may match the cross-sectional areas of multi-layer spacers mentioned above; however, in the case of a single-layer spacer, the cavity inside the spacers comprises a larger cross-sectional area. Alternatively, the cross-section of the single-layer spacer may also be selected to be smaller. However, they may also comprise a larger cross-section. Optionally, this material may be protected by further layers. By this design, the manufacturing process is simplified significantly.

[0047] FIG. 6 shows a further case of application. In addition to the thermal insulation of the membrane from the substrate by the spacers described above, the membrane may be suspended on the spacers by ridges 22. In this case, the advantage consists of a further improvement of the thermal insulation, however, at the expense of the effective absorber area. However, numerous combination possibilities arise between spacer design and ridge design, whereby the sensor may be manufactured flexibly. This arrangement is shown in FIGS. 7a-c on the basis of a schematic illustration of a QVGA microbolometer testing structure array. The pixel pitch could be, for example, 17 μm. In this case, the thermal insulation is realized by the spacers and conventional ridges described herein. The ridges 22 may be in a plane with the membrane 10 and be formed of one or multiple layers forming the membrane 10. The ridges 22 may be separated from the membrane 10 by slots 98, except for suspension interfaces between ridges and membrane. The ridges 22 contact the spacers 45 at one end and the membrane 10 at another end. Due to the insulation of the spacers 45, the ridges 22 may be configured shorter and/or narrower compared to the same serving in regards to the electrical contacting only. Further, it is possible to contact only a first spacer 45 by a ridge 22 and to contact a second spacer directly with the membrane 10. In the cases specified, the thermal insulation is predominantly accomplished by the spacers 45, that is, the thermal conductance of the ridges 22 is, for example, larger than the thermal conductance of the spacer 45 connected to the membrane by the ridge 22. However, it may also occur that the spacers 45 comprise a larger thermal conductance.

[0048] FIG. 7a shows an array of radiation sensors on a substrate 20. The array of radiation sensors may, for example, form an image sensor for thermal radiation. The membrane is, for example, suspended on the spacers 45 by ridges 22. The spacers electrically contact the substrate and thermally insulate both from each other. FIG. 7b shows a close-up view of the spacers 45 as a plane view and FIG. 7c as a side view.

[0049] As already shown in FIG. 7a, forming an array of radiation detectors is also possible, for example, to detect the intensity of multiple radiation sources or to create a two-dimensional image. For this, several membranes may be contacted and thermally insulated with one or several substrates by a plurality of spacers. The temperature evaluation circuits, which, for example, may be disposed in the substrate and may be located, for example, below or next to the membrane, may be, for example, electrically connected to each other, for example, to allow a serial readout, or electrically insulated from each other, for example, to simultaneously read out the information of the temperature evaluation circuits.

[0050] FIGS. 8a-h show an exemplary sequence of the method for manufacturing radiation detectors with a reflector suspended on spacers 95 by using the spacers 45 described above for the suspension of the membrane. A possible process flow is shown, outlining the realization of the microbolometer described above including the elevated reflector. For example, the reflector may consist of 200 nm TiN. The inner material of the spacers suspending the reflector may consist of, for example, 50 nm Al.sub.2O.sub.3. However, other materials may be used and other layer thicknesses may be used.

[0051] FIG. 8a shows the substrate 20 as well as its contacts 90 embodied on the surface. In the following step (FIG. 8b), a sacrificial layer 100a, for example consisting of a-SI, as well as the reflector material 35, for example TiN, are applied, and in the case of the reflector material, for example a metal, already structured, that is, for example, brought to the predetermined size. Subsequently (FIG. 8c), the sacrificial layer and the reflector are being structured. This may consist of, for example, etching of holes, for example, coated with Al.sub.2O.sub.3, and therefore, after removing the sacrificial layer, form the spacers 95 on which the reflector is suspended. In the following (FIG. 8d), a further sacrificial layer 100b may be applied onto the existing sacrificial layer 100a as well as onto the reflector. For example, the sensor layer 30 of the membrane, for example an a-SI layer, may be applied onto this sacrificial layer 100b, protected by a protection layer 105, for example, consisting of Al.sub.2O.sub.3. FIG. 8e describes the step of structuring the sacrificial layers 100a and 100b and of the protection layer 105 of the sensor layer 30 of the membrane 10. This consists of, for example, the etching of the sacrificial layer as well of the protection layer 105, for example, by using the Bosch process, as well as the deposition of a further protection layer 75 onto the inner edges of the sacrificial layer. Subsequently (FIG. 8f), the contact layer 80, for example a layer of TiN, is deposited onto the protection layer 75 in the openings as well as the contacts of the membrane and the substrate, followed by a further protection layer 85. After applying the absorber layer 35 of the membrane 10, this protection layer 85 may be broadened and extended over the absorber layer 35 (FIG. 8g). Finally, the sacrificial layers 100a and 100b may be removed (FIG. 8h).

[0052] FIG. 9 shows a further embodiment similar to the embodiment of FIG. 1. For stabilizing the spacers, an oxide layer 110 is applied onto the substrate before of the sacrificial layer, being very selective towards the sacrificial material during the final etching. When the sacrificial layer is removed, the oxide layer 110 remains and further stabilizes the spacers 45.

[0053] FIG. 10 shows a section around a contact of the substrate in a cross-sectional view. In this case, the spacer 45 illustrated comprises only one layer 80, but may comprise a plurality of layers, as described above. The spacer 45 is disposed on the contact 90 of the substrate 20.

[0054] The contact 90 may be embodied, for example, of aluminum, tungsten or another electrically conductive material. Due to the large aspect ratio of the spacers 45 and an appropriate method for removing unnecessary material of the spacer on the sacrificial layer, for example an etching process, a material layer of the spacer may remain on the base and/or the bottom of the spacer. The same may increase the contact area with the contact 90 and, therefore, contribute to a better and more secure contacting as well as a secure stand of the spacer. As already shown in FIG. 9, a further material layer, such as that of an oxide, may be applied between the substrate and the sacrificial layer 110 such that, after the removal of the sacrificial layer, for example the etching, an edge or a collar remains around the base and/or the bottom of the spacer and, thus, stabilizes the spacer.

[0055] In other words, the spacers may be manufactured by means of a sacrificial layer process. Therein, a hole is etched into the sacrificial layer initially and coated subsequently. For example, for the etching of the hole, the so-called Bosch process may be used, since with this, it is possible to create steep edge angles at large aspect ratios. The layers are deposited by using atomic layer coating ALD so that, even for the steep edge angles mentioned, the etched holes may be covered, that is, the whole area of the hole etched into the sacrificial layer may be evenly thinly coated. In this case, the tolerance should be less than 50 nm. Alternatively, the tolerance may also be smaller than 25 nm or smaller than 5 nm. Further, the deposited layers may be structured according to the manufacturing process. Finally, the removal of the sacrificial layer follows so that the manufactured spacers are self-supporting. Contrary to the embodiments with two spacers exclusively shown so far, it is also possible to provide this and further embodiments with a plurality of spacers. This applies to both spacers 45 and 95 mentioned. Contrary to the spacers 45 and 95 in the corners of the membrane and the reflector shown so far, the same may also be disposed at other locations, for example, in the middle between two corners. Further, it is also possible to realize spacers, for example, from a thermally insulating material, for example an oxide such as aluminum oxide, as an additional support for mechanically stabilizing without comprising an electrical functionality.

[0056] Beside the method described and the materials and processes used, further solutions are possible. For example, if the reflector is disposed on the substrate, only one sacrificial layer may be necessitated, onto which the membrane is applied and into which the spacers are deposited. It is also possible to use different materials for the protection layers 75 and 85 or the contact layer 80 or to provide only one or no protection layer. In this case, the contact layer should consist of a material, which is not attacked during removal of the sacrificial layer, for example the etching. Further, the membrane and the contact layer may consist of the same material and one layer, which may also be surrounded by a protection layer. Further, by using the method, it is possible to manufacture different components apart from the radiation detectors described.

[0057] As a result of the invention described herein, it is possible to maximize the relative area proportion of the absorber at a given pixel pitch, since lateral ridges, which were so far a main part of the thermal insulation, may be omitted entirely and/or significantly reduced. This saving of space may be used to, for example, expand the effective absorber area. The thermal insulation may be flexibly adapted as per Eq. 4 by the respective layer thicknesses, base radius 65 and length of the spacers 45. The respective maximum is limited by the stability of the spacers 45 and the resonator condition. Therefore, it is even possible to achieve a significantly smaller corresponding thermal conductance than so far possible by using the ridges. Due to both of these factors, the performance of radiation detectors, for existing microbolometers, may be raised significantly. The advantages of a very large absorber area and, at the same time, a good thermal insulation may also result in an enormous benefit, especially when scaling the pixel pitch to smaller values. Due to the smaller dimensions, the spacers may also be denoted as nanotubes. They allow for an enlargement of the available absorber area at a same thermal conductance due to a reduction of the ridge area, the thermal conductance being independently adaptable at the same time.

[0058] Even though some aspects have been described within the context of a device, it is understood that said aspects also represent a description of the corresponding method, so that a block or a structural component of a device is also to be understood as a corresponding method step or as a feature of a method step. By analogy therewith, aspects that have been described within the context of or as a method step also represent a description of a corresponding block or detail or feature of a corresponding device.

[0059] While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.