Fully reticulated detectors for curved focal plane arrays.

Abstract

To provide a focal plane array (FPA) formed from an array of fully reticulated detectors.SOLUTION: A curved FPA comprises an array of detectors 12. Metallization 26 connects the detectors 12 electrically and thereby provides a common ground between them. Strain induced by bending the FPA into a curved shape is across the metallization and backfill epoxy 22, rather than across the detectors. Indium bumps 18 are evaporated onto respective detectors for connection to a readout integrated circuit (ROIC) 20. The ROIC 20 coupled to the detectors is thinned.SELECTED DRAWING: Figure 1B

Description

[Technical Field]

[0001] Related Applications

[0001] This application claims the benefit of Provisional Patent Application No. 62 / 923,189, filed October 18, 2019, to Majid Zandian.

[0002] This invention relates generally to focal plane arrays (FPAs), and more particularly to structures and methods for fabricating curved FPAs. [Background technology]

[0003]

[0003] It is difficult to focus light on all areas of a flat, planar surface such as a conventional FPA. It is much easier to focus light on a spherical surface, especially over a large field of view. Therefore, it makes the optical system easier and provides better performance when the FPA can be a spherical surface. A curved image surface also provides a way to reduce the number of optical elements, reduce aberrations including astigmatism and coma, and increase off-axis brightness and sharpness.

[0004] However, preparing a curved FPA is difficult: One approach is to simply bend the FPA into a curved shape, but the bending can induce distortions in the detectors that make up the array, which can degrade the performance of those detectors and / or damage the array. Summary of the Invention

[0005]

[0005] Presented herein is an FPA formed from an array of fully reticulated detectors, which enables the fabrication of curved FPAs by reducing the distortion induced on the detectors due to FPA bending.

[0006] The present curved FPA includes an array of detectors, with mesas etched between the detectors so that the detectors are electrically and physically isolated from one another. Metallization deposited on the underside of the mesas electrically reconnects the detectors, thereby providing a common ground between the detectors. When so configured, strain induced by bending the FPA into a curved shape is not across the detectors, but across the metallization and any backfill epoxy. The array of detectors is preferably fabricated with a common photosensitive base layer and a common buffer layer, and the mesas are etched through the base layer and buffer layer.

[0007]

[0007] Indium bumps will typically be deposited on each detector for connection to a readout integrated circuit (ROIC). The ROIC bonded to the detector is preferably thinned, and the backside of the ROIC may even include a mesa so that the ROIC is reticulated.

[0008] These and other features, aspects, and advantages of the present invention will become better understood with reference to the drawings, description, and claims that follow. [Brief explanation of the drawings]

[0009] [Figure 1A]

[0009] FIG. 1 is a cross-sectional view of an FPA prior to reticulation. [Figure 1B]

[0010] FIG. 2 is a cross-sectional view of an FPA after reticulation according to the present invention. [Figure 2A]

[0011] FIG. 1 is a cross-sectional view of an FPA prior to reticulation. [Figure 2B]

[0012] FIG. 2 is a cross-sectional view of an FPA after reticulation according to the present invention. [Figure 3A]

[0013] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3B] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3C] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3D] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3E] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3F] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3G] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3H] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3I] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3J] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3K] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3L] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3M] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3N] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3O] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3P] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3Q] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. [Figure 3R] 1A-1D are cross-sectional views of process steps that may be used to fabricate a curved FPA in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010]

[0014] A curved focal plane array (FPA) according to the present invention includes an array of detectors, mesas etched between the detectors so that the detectors are electrically and physically isolated from one another, and metallization at the bottom of the mesas to electrically reconnect the detectors, thereby providing a common ground between the detectors.

[0011]

[0015] The basic structure is illustrated in Figures 1A and 1B. Figure 1A depicts the FPA prior to mesa etching. The FPA structure 10 includes three detectors 12 in this exemplary embodiment, preferably made from a common buffer layer 14 fabricated on a substrate (removed in Figure 1A) and a common photosensitive base layer 16 on the buffer layer, on which pn junctions 12 are formed.

[0012]

[0016] A typical FPA will further include indium bumps 18 deposited on each detector 12 for connection to a readout integrated circuit (ROIC), and an ROIC 20 bonded to the detector. An epoxy backfill 22 will typically be aspirated into the area between the detector and the ROIC and cured.

[0013]

[0017] 1B, mesas 24 are etched between the detectors so that they are electrically and physically isolated from one another. Metallization 26, located at the bottom of the mesas, electrically reconnects the detectors, thereby providing a common ground between the detectors. By doing so, when the FPA is bent as it curves, the resulting strain is across the metallization 26 and the backfill epoxy 22.

[0014]

[0018] Fabrication of a structure such as that described above will typically be performed with the substrate in place. After the epoxy has cured, the substrate will typically be removed. Once the substrate is removed, an anti-reflective (AR) coating 28 can be applied to the backside of the FPA. A coating thickness of 3000-5000 Å is preferred.

[0015]

[0019] The backside of ROIC 20 is preferably thinned, which makes the hybridized structure more flexible. A mesa (not shown) may be formed on the backside of the ROIC to further reticulate ROIC 20, thereby making the ROIC even more flexible. The bendable FPA can be bent into a semicircle, and if the ROIC is thinned sufficiently, e.g., to 25 microns or less, the FPA can be bent to form a full circle or spherical surface.

[0016]

[0020] For conventional FPAs, when the pixels are connected to the ROIC through indium bumps, the entire array of detectors must match in expansion and contraction to the ROIC. If there is a thermal coefficient mismatch between the detector array and the ROIC, stress is applied to the detectors, which can cause damage upon cooling or heating. In contrast, for the present FPA, reticulating the pixels as described above allows the pixels to expand and contract as needed without being stressed, which serves to reduce or eliminate damage to the FPA due to thermal coefficient mismatch between the detectors and the ROIC.

[0017]

[0021] 1A and 1B illustrate a backside process format for an FPA according to the present invention. Figures 2A and 2B illustrate a frontside process format. As described above, a buffer layer 14 and a common photosensitive base layer 16 are fabricated on a substrate 30, shown in FIG. 2A. A mesa 24 is etched through the base layer 16 and buffer layer 14 down to the substrate 30, and then metallization 26 is provided at the bottom of the mesa to electrically reconnect the detectors, thereby providing a common ground between them. In FIG. 2B, the substrate 30 has been removed and an AR coating 28 has been applied.

[0018]

[0022] The backfill epoxy 22 is preferably soft and flexible to allow the FPA to be easily bent. As mentioned above, the mesa is etched through the base layer 16 and buffer layer 14 down to the substrate 30. The mesa depth, therefore, depends on the wavelength range provided by the FPA. For example, for FPAs sensitive to SWIR wavelengths, the material forming the detector is typically about 6 microns thick, including the base layer, buffer layer, and any capping layers. Thus, a mesa depth of 6 microns is required to reach the substrate. Similarly, for LWIR FPAs, the detector material thickness is typically about 12 microns thick, requiring a mesa depth of 12 microns.

[0019]

[0023] The curved FPA structures and methods described herein can be used for a variety of FPA types, for example, FPAs made of HgCdTe, InGaAs, HyViSi (trademark) They can be grown and processed in silicon, strained-layer superlattice (SLS), and other material systems. The described FPAs largely eliminate the thermal coefficient mismatch between the detector and the ROIC, thereby eliminating the need for a balanced composite structure (BCS). The FPAs can be mounted on many different materials for packaging.

[0020]

[0024] One possible process sequence for fabricating a curved FPA as described herein is shown in Figures 3A-3R. In Figure 3A, a buffer layer (not shown) and a photosensitive base layer 40 are provided on a substrate 42, such as CdZnTe. Base layer 40 may comprise a grown material or a bulk material.

[0021]

[0025] 3B, the surface of the base layer 40 has been processed, typically using photolithography, to form an array of detectors 44. Mesas 46 are etched through the base layer and buffer layer down to the substrate 42 so that the detectors are electrically and physically isolated from one another. Either wet or dry etching can be used; dry etching provides higher aspect ratio sidewalls but is more difficult to passivate and can create surface damage.

[0022]

[0026] 3C, metallization 48 is deposited on the bottom of mesa 46 to electrically reconnect detectors 44, thereby providing a common ground between the detectors. Metallization 48 is preferably evaporated and soft, with indium being preferred.

[0023]

[0027] 3D, metallization 50 is provided on each detector to enable connection of the detector to the ROIC. Metallization 50 preferably includes indium bumps, which are preferably evaporated onto each detector.

[0024]

[0028] 3E and 3F, ROIC 52 is hybridized to detector 44 via metallization 50. Then, as shown in FIG. 3H, epoxy 54 is backfilled into the space between ROIC 52 and mesa 46 (note that the hybridized FPA of FIG. 3F is reproduced in FIG. 3G). When so arranged, when the FPA is bent, the resulting strain is across metallization 48 and backfill epoxy 54.

[0025]

[0029] ROIC 52 is preferably thinned. This process is illustrated in Figures 3I-3L. In Figure 3I, the FPA of Figure 3H is reproduced, except that the FPA is now shown upside down. In Figure 3J (reproduced in Figure 3K), substrate 42 is attached to support 60, preferably using a wax or photoresist (PR) layer 62. Then, in Figure 3L, ROIC 52 is thinned, preferably by polishing, to a thickness of suitably 50 microns. Note that ROIC 52 can alternatively be thinned independently of the rest of the FPA structure before it is contacted with metallization bumps 50 (shown in Figure 3F).

[0026]

[0030] Substrate 42 is removed in Figures 3M-3P (note that Figure 3M is a reproduction of Figure 3L, and Figure 3O is a reproduction of Figure 3N). In Figure 3N, substrate 42 has been removed, the FPA structure has been flipped over, and ROIC 52 is attached to support 64, preferably using a wax or PR layer 66. Then, in Figure 3P, substrate 42 has been removed, preferably by polishing.

[0027]

[0031] Another optional step is to apply an AR coating to the FPA, which is illustrated in Figures 3Q and 3R. In Figure 3Q, an AR coating 68 is applied, preferably by vapor deposition, to the exposed side of the photosensitive base layer 40. Then, in Figure 3R, the process is completed by removing the flexible FPA from the support 64.

[0028]

[0032] Although not shown, an FPA as described herein can be made even more flexible by forming a mesa on the backside of the ROIC 52 such that the ROIC is reticulated.

[0029]

[0033] The present curved FPA has many possible uses, including astronomy, military, and commercial applications. Such an FPA could enable lower cost and higher performance IR cameras and could even be used in visible imaging.

[0030]

[0034] The embodiments of the invention described herein are illustrative and numerous modifications, variations, and rearrangements may be readily devised to achieve substantially equivalent results, all of which modifications, variations, and rearrangements are intended to be within the spirit and scope of the invention as defined in the appended claims. [Explanation of symbols]

[0031] 10 FPA structure 12 Detector, pn junction 14 Common buffer layer, buffer layer 16 Common photosensitive substrate, substrate 18 Indium bump 20 ROIC 22 Epoxy Backfill, Backfill Epoxy 24 Mesa 26 Metallization 28 Anti-reflection (AR) coating, AR coating 30 boards 40 Photosensitive base layer, base layer 42 PCB 44 detectors 46 Mesa 48 Metallization 50 Metallization, Metallization Bump 52 ROIC 54 Epoxy, Backfill Epoxy 60 Support 62 wax or photoresist (PR) layer 64 Support 66 Wax or PR layer 68 AR coating

Claims

1. an array of detectors; mesas etched between the detectors, the mesas electrically and physically isolating the detectors from one another; a metallization disposed under the mesa, the metallization electrically connecting the detectors and providing a common ground between the detectors; A curved focal plane array (FPA), including:

2. A curved FPA as described in claim 1, wherein the array of detectors is supported by a common photosensitive base layer and a common buffer layer, and the mesa is etched through the photosensitive base layer and the buffer layer.

3. The curved FPA of claim 2 further comprising an anti-reflective (AR) coating on the buffer layer.

4. 10. The curved FPA of claim 1 further comprising an indium bump deposited on each said detector for connection to a readout integrated circuit (ROIC).

5. The curved FPA of claim 1 further comprising a ROIC coupled to the detector.

6. The curved FPA of claim 5 , wherein the ROIC has a thickness of 25 micrometers or less.

7. The curved FPA of claim 5 , wherein a backside of the ROIC includes the mesas such that the ROIC is reticulated.

8. The curved FPA of claim 1 , wherein the FPA is a HgCdTe FPA, an InGaAs FPA, a hybrid visible silicon FPA, or an SLS FPA.

9. The curved FPA of claim 1 , further comprising a backfill epoxy such that when the FPA is curved, the resulting strain is across the metallization and the backfill epoxy.

10. 1. A method for fabricating a curved focal plane array (FPA), comprising: forming an array of detectors on a substrate; etching mesas between the detectors such that the detectors are electrically and physically isolated from one another; depositing metallization at the bottom of the mesas to electrically reconnect the detectors, thereby providing a common ground between the detectors; removing the substrate; A method comprising:

11. The method of claim 10 , wherein forming the array of detectors on a substrate includes fabricating a common photosensitive base layer and a common buffer layer.

12. The method of claim 11 , wherein the step of etching the mesa between the detectors comprises etching the mesa through the photosensitive base layer and the buffer layer.

13. 12. The method of claim 11, further comprising depositing an anti-reflective (AR) coating on the buffer layer, wherein the step of depositing the AR coating on the buffer layer is performed after the step of removing the substrate.

14. The method of claim 11 , wherein the photosensitive underlayer comprises a grown material or a bulk material.

15. 11. The method of claim 10, further comprising depositing an indium bump onto each of the detectors for connection to a readout integrated circuit (ROIC).

16. The method of claim 10 , further comprising the step of coupling a ROIC to the detector, wherein the step of removing the substrate is performed after the ROIC is coupled to the detector.

17. The method of claim 16 further comprising thinning the ROIC.

18. 17. The method of claim 16, further comprising forming the mesa on a backside of the ROIC such that the ROIC is reticulated.

19. The method of claim 10 , wherein the mesas between the detectors are etched down to the substrate.

20. The method of claim 10 , wherein the FPA is a HgCdTe FPA, an InGaAs FPA, a hybrid visible silicon FPA, or an SLS FPA.

21. The method of claim 10 , further comprising applying a backfill epoxy such that when the FPA is bent, the resulting strain is across the metallization and the backfill epoxy.

22. The method of claim 10, wherein the step of depositing the metallization at the bottom of the mesa includes a step of vapor-depositing the metallization.

23. The method of claim 10 , wherein the metallization comprises indium.

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