Electronic imaging detector with thermally conductive layer

Abstract

A solid state active pixel image sensor for backside illumination by an electron beam is described. The image sensor includes a thermally conductive layer for heat removal. The image sensor may also include a thinned silicon substrate on which an epitaxial layer is formed. The substrate may also be completely removed before or after the thermally conductive layer is applied. The thermally conductive layer may include a metal, a metal compound, silicon, diamond, or graphite.

Description

[0001] This application is a divisional application of the invention patent application with application number 202011524890.8 and the application date of December 22, 2020, entitled “Electronic imaging detector with thermal conductive layer”. Background Art

[0002] Electron imaging detectors based on thinned transmission silicon active pixel sensors have revolutionized cryogenic electron microscopy. These devices enable scientists to study the structure of large biomacromolecules at a level not previously achieved. Because these are transmission devices, electrons that interact with the detection layer ideally leave the sensor without scattering back into the detection layer. Using silicon as the sensor substrate minimizes the scattering of electrons within the sensor back into the detection layer. Active sensors maximize the signal-to-noise ratio for detecting the position of a single incident electron by amplifying the electronic signal.

[0003] like Figure 1 As shown, the active pixel sensor 10 is composed of three layers: a substrate (SUB) layer 12, an epitaxially grown detection (EPI) layer 14, and a back-end-of-line (BEOL) layer 16. The SUB layer 12 is inert from the point of view of detecting electrons and serves as a basis for forming the detection layer. The EPI layer 14 includes materials that generate sensor electrons in response to incident electron microscope electrons. These sensor electrons are collected. The BEOL layer 16 is disposed above the EPI layer 14 and includes metal interconnects and oxide insulation for reading the sensor.

[0004] In some known electron imaging detectors, the SUB layer 12 is thick enough that no electrons pass completely through it. Figure 1 Two example electron paths are shown in FIG. Electron paths A and B are scattered by the substrate back into the EPI detection layer 14 and detected a second time at locations 20, 22, which are less relevant to the actual entry point of the backscattered electrons into the EPI layer 14. This additional detection degrades the imaging quality. Figure 2 As shown, the SUB layer 12 thinned from the back side can reduce backscattering and secondary detection, wherein electrons A will not be backscattered into the detection layer 14 .

[0005] exist Figure 2 In the example image using the same electron path, electron A that penetrates the substrate layer to a greater depth is no longer scattered back to the detection layer. Electron B scattered at a shallower depth still returns to the detection layer. In this front-illuminated configuration, backscattering can be minimized by minimizing the amount of substrate.

[0006] Thinning the substrate reduces the material available to remove heat from the device. As process technology advances and the number of pixels per given area increases, heat removal becomes an increasing problem. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a cross section of a prior art silicon imaging detector;

[0008] Figure 2 is a cross section of another prior art silicon imaging detector;

[0009] Figure 3 is a cross-section of an exemplary silicon imaging detector according to embodiments described herein;

[0010] Figure 4A is a graph showing the detection quantum efficiency (DQE) of an ideal, front-illuminated and back-illuminated imaging sensor;

[0011] Figure 4B The DQE of front-illuminated vs. back-illuminated imaging sensors is Figure 4A A diagram comparing the ideal sensor DQE curve in ;

[0012] Figure 5A - C is a cross-section of a processing stage of an exemplary silicon imaging detector according to embodiments described herein; and

[0013] Fig. 6A -C is a cross-section of a processing stage of an exemplary silicon imaging detector according to embodiments described herein. DETAILED DESCRIPTION

[0014] Those skilled in the art will recognize other detailed designs and methods that can be developed using the teachings of the present invention. The embodiments provided herein are illustrative and do not limit the scope of the present invention, which is defined by the appended claims. The following detailed description refers to the accompanying drawings. The same reference numerals in different drawings may identify the same or similar elements.

[0015] According to the embodiments described herein, Figure 3As shown, the active pixel silicon imaging detector 300 includes a SUB layer 310, an EPI layer 312, a BEOL layer 314, and a thermally conductive layer ("TCL") 316. The TCL 316 is applied to the BEOL layer 314 after the BEOL layer is fabricated. Consistent with the embodiments described herein, the detector 300 is exposed to electrons initially incident from the substrate side 310, i.e., the sensor is "back-illuminated." The sensor is back-illuminated because some scattering also occurs in the BEOL, which is located in front of the detection EPI layer in a front-illuminated configuration. The pre-detection scattering caused by the BEOL has little or no effect on the number of detections per incident electron, but the scattering may reduce the spatial correlation between the entry point and the detection point, and therefore may limit the ultimate resolution of the detector. The BEOL cannot be removed because it performs the necessary readout function of the detector. But back-illumination places the BEOL after the EPI detection layer and eliminates the pre-detection scattering caused by the BEOL.

[0016] like Figure 3 As shown, electrons A and B are detected in the EPI layer 312 and then pass through the BEOL layer 314. The thickness and material composition of the thermally conductive layer 316 are designed to have a high thermal conductivity, but a low tendency to reflect electrons back into the EPI detection layer 312. Prior art substrate thicknesses have ranged from 10 μm to 40 μm, producing backscatter fractions ranging from less than 2% to greater than 10% for electrons at 200 kV and 300 kV. Advantageously, this backscatter fraction can be reduced by a reduced Z (atomic number) of the thermally conductive substrate or by reducing the thickness, which is possible through higher thermal conductivity. In practice, the material is varied simultaneously through these parameters. As Figure 3 As shown, electron A exits TCL layer 316 without being reflected back into the sensor. Electron B is reflected by the TCL layer and detected a second time in detection layer 312. The thickness of the TCL layer and its composition determine the probability of an electron passing through the TCL layer or being reflected back into sensor 300.

[0017] Figure 4A The simulated linear detected quantum efficiency (DQE) is shown for three configurations: the solid line is the idealized epilayer only (“EPI only”), the middle curve (large dashed line) is a back-illuminated device with BEOL and epilayers, where the epilayers are directly exposed to electrons, and the lower curve (short dashed line) is a front-illuminated device with BEOL and epilayers (i.e., illuminated by the BEOL). Figure 4B Shown with Figure 4A The idealized epitaxial layer-only sensor curve is compared to that with BEOL Figure 4A The simulated ratio of the DQE of the two sensors.

[0018] Two pictures ( Figure 4A, 4B) show the performance quantified by linear DQE vs. spatial frequency. The zero spatial frequency at the origin on the left side of each figure corresponds to the performance of the detector as a whole. The spatial frequency of one times the Nyquist frequency on the right side of each figure shows the image signal modulation performance that goes up and down every other pixel. This is the highest spatial modulation frequency that a pixelated detector can achieve without aliasing errors.

[0019] Figure 4A and 4B It is shown that while the materials of both the entrance and exit surfaces of the detector increase scattering at elevated levels of spatial frequency, the scattering from materials mounted on the exit surface that occurs after detection (large dashed curve in both figures) is less detrimental to high-resolution signal quality. The simulation results also suggest that any material in front of the EPI detection layer should be minimized. Figure 3 As shown, the sensor is exposed from the back side, leaving the substrate in front of the epitaxial layer. Figure 4A and 4B The conclusion is that anything in front of the EPI detection layer degrades the DQE, which means that all or most of the substrate now at the incident face should be removed (leaving no more than 5 microns). This has been impractical so far due to the role the substrate plays in providing mechanical support and thermal conductivity for the sensor. Consistent with the embodiments herein, a thermally conductive layer can support the EPI and BEOL layers from the BEOL layer side (i.e., the exit side in a back-illuminated configuration). Using a layer next to the BEOL layer to support the device, the original substrate layer can be completely removed. This in turn allows direct exposure of the EPI layer, which has not been possible so far.

[0020] Because the thermally conductive layer is not part of the original sensor fabrication (i.e., not part of the epitaxial layer growth or BEOL layer deposition), there is an opportunity to find a better thermally conductive layer material than silicon (which comprises the original substrate and most of the heat removal in prior art devices) for this purpose. The thermally conductive layer material can be superior to the original silicon substrate in two ways. It can have better thermal conductivity, and it can have a lower tendency to scatter electrons. The benefit of the combined effect is proportional to the thermal conductivity κ and inversely proportional to the square of the atomic number Z. For the purpose of an electron transmissive thermally conductive layer, a heuristic quality factor can be defined as Q = κ / Z 2 The following table summarizes estimated values ​​of κ, Z, and Q for some representative materials. The estimated values ​​of κ, and therefore Q, are subject to manufacturing methods, binder fraction, etc. This table is not exhaustive, but illustrates the principles used to identify materials as more or less suitable for electron transmissive thermal conductive layers than silicon.

[0021] Table 1

[0022]

[0023] Due to the fragility of the thin EPI detector layer, especially after all substrate material has been removed, the method used to attach the thermally conductive layer to the thin device must be either very low force / pressure or a strong temporary support for the thin layer that must be provided as part of any higher pressure process steps. The method used to adhere the thermally conductive layer to the BEOL of a back-illuminated sensor differs depending on the conductive material.

[0024] Solid refractory dielectrics, including but not limited to boron nitride, beryllium oxide, and diamond, can be contact bonded to the sensor BEOL with or without elevated temperature and with or without applied pressure, and then excess thermally conductive layer material is removed, and the silicon substrate layer is removed. Contact bonding is described in U.S. Pat. Nos. 5,852,622 and 5,441,803, which are incorporated herein by reference. The substrate is first left intact to allow application of pressure to contact bond the thermally conductive layer to the BEOL.

[0025] Figures 5A-5C A cross section of a production process for an imaging sensor according to one aspect of the present invention is shown. Figure 5A A sensor 510 including a silicon substrate 512 , an epitaxial layer 514 , and a BEOL 516 is shown. Figure 5A Also shown is a thermally conductive layer 520 supported by a thermally conductive layer substrate 522. The TCL substrate 522 may be the same or a different material than the TCL itself. Figure 5B The bonding of the BEOL 516 of the sensor 510 to the thermally conductive layer 520 is shown. At this stage, both the imaging sensor 510 and the thermally conductive layer are supported by substrates 512, 522 to allow pressure to be applied during the contact bonding process. Note that in some embodiments, contact bonding can be performed with or without pressure and with or without elevated temperature. Figure 5C A completed imaging sensor 510 is shown with the original sensor substrate 512 removed and the thermally conductive layer substrate 522 removed. Removal of substrate material or excess thermally conductive material can be accomplished by a variety of means including, but not limited to, polishing, chemical mechanical polishing, ion milling, and laser ablation. Figure 5C The resulting sensor shown in has an unobstructed epitaxial sensing layer 514 that can be directly exposed to electrons without any initial scattering, and also has a thermally conductive layer 520 to remove heat from the device and physically support the device.

[0026] The thermally conductive layer to be contact bonded to the sensor can be derived from a variety of initial starting forms and can be processed in a variety of ways. Consistent with the embodiments described herein, the TCL / SUB layer 502 / 522 bonded to the BEOL can be made of a single material that is thicker than the final TCL 520. The additional thickness is necessary for strength during the bonding process. This layer is contact bonded to the BEOL layer of the sensor and the excess material is polished off. In this case, the final thickness of the thermally conductive layer is determined by closely monitoring the thickness during the polishing process.

[0027] In another embodiment, a single material thicker than the final thermally conductive layer is used as the starting material, but the starting material is ion implanted to a predetermined depth on the side to be bonded. The ion implantation changes the etch rate of the chemical or chemical mechanical thinning process without changing the thermal or scattering properties of the thermally conductive layer material. Once bonded to the BEOL layer, the thermally conductive material is thinned. The etch stop created by the ion implantation increases the reliability of obtaining a thermally conductive layer of a given thickness.

[0028] In another embodiment, the thermally conductive layer is epitaxially grown or chemically vapor deposited onto a substrate of a different material than the layer being grown or deposited. The substrate serves as a transfer layer. The combined layer is contact-bonded to the BEOL layer as described above. Once bonded to the BEOL layer, the substrate material is removed by one or more of the methods described above.

[0029] In another embodiment, a thermally conductive powder layer is sintered onto a substrate having a similar melting point and thermal expansion coefficient as the powder. The substrate acts as a transfer layer. The combined layer is contact-bonded to the BEOL layer and then the substrate material is completely removed after contact bonding.

[0030] Metal layers may be sputtered onto the BEOL. Fig. 6A -C shows a cross section of an exemplary sputter deposition of a thermally conductive layer 616 onto the BEOL 614 . Fig. 6A A sensor 610 is shown including epitaxial layers 612 and BEOL 614. The original substrate on which the sensor was formed has been removed in this embodiment. Figure 6B Sputtering of material onto the BEOL to create a thermally conductive layer 616 is shown. Figure 6C The completed sensor is shown with the thermally conductive layer 616 fully deposited.

[0031] Powder-based materials can be applied by sedimentation or slurry coating processes in a solvent-diluted binder, such as for powder phosphorescent scintillators. The powder-binder mixture is applied to the BEOL layers by means including silk screen printing or gravity sedimentation. Liquid powder suspensions can also be spin-coated onto the BEOL layers. Exemplary binders include, but are not limited to, sodium silicate, ethyl cellulose and ammonium polymethacrylate, and nitrocellulose. Although the binder sacrifices some thermal conductivity, the net conductivity still becomes an improvement over silicon given the extremely high conductivity of graphite or boron nitride powders and a sufficiently low binder fraction.

[0032] Powder-based thermal conductors can also be deposited directly on BEOL layers by spin coating a liquid powder suspension.

[0033] In another embodiment, the starting material for the thermal conductor layer is a flexible sheet containing a flexible polymer embedding medium for a powdered thermally conductive material. The flexible sheet can be contact bonded in a vacuum with or without elevated temperature to prevent gas entrainment. After all high temperature processing is completed on the sensor itself, the material should be pasted to the BEOL layer. In this case, the connection (attachment) is performed on the fragile and thin final sensor, retaining only the BEOL and epitaxial layers. Therefore, the connection needs to use only a small force / pressure. Possible small force connection methods include, but are not limited to: guiding a gas jet to the outer surface of the flexible layer to pressurize and smooth the layer with low temperature heating to maximize the flexibility and adhesion of the flexible layer. The warm gas jet combines the two effects of smoothing and heating. In another embodiment, centrifugal force is used to flatten the flexible sheet during adhesion to the BEOL layer of the sensor. In another embodiment, electrostatic-dielectric attraction should be used with caution to prevent electrostatic damage to the sensor. Some types of flexible sheets are electrically conductive (eg, graphite-containing), while others are electrically insulating (eg, boron nitride) and may require different treatment during electrostatic methods of attachment.

[0034] Powder-based thermal conductors can be deposited directly on BEOL layers by precipitation or slurry coating in a solvent-diluted binder, such as for powder phosphorescent scintillators. Exemplary binders include, but are not limited to, sodium silicate, ethyl cellulose, and ammonium polymethacrylate, as well as nitrocellulose. Although the binder sacrifices some thermal conductivity, the net conductivity still becomes an improvement over silicon given the extremely high conductivity of graphite or boron nitride powders and a sufficiently low binder fraction.

[0035] Powder-based thermal conductors can also be deposited directly on BEOL layers by spin coating a liquid powder suspension.

[0036] Although the present invention has been described in detail above, it should be clearly understood that it is clear to those skilled in the relevant art that the present invention can be modified without departing from the spirit of the present invention. Various changes in form, design or arrangement can be made to the present invention without departing from the spirit and scope of the present invention. Therefore, the above description should be considered as exemplary, rather than restrictive, and the true scope of the present invention is defined by the appended claims.

[0037] Unless explicitly described, any element, action or instruction used in the description of this application should not be interpreted as being critical or essential to the present invention. Similarly, the article "a" as used herein is intended to include one or more items. In addition, the phrase "based on" is intended to mean "based at least in part on", unless otherwise explicitly stated.

Claims

1. A solid-state active-pixel image sensor for direct back-illuminated exposure to an electron beam, comprising: an epitaxial layer formed on a substrate comprising silicon; A substrate comprising silicon having a thickness no greater than 5 micrometers; a back-end-of-the-line (BEOL) layer formed on the epitaxial layer; and A thermally conductive layer directly adhered or formed on the back-end process layer by contact bonding, The heat conducting layer comprises metal, boron nitride, diamond or graphite.

2. A solid-state active-pixel image sensor according to claim 1, wherein the thermally conductive layer is applied to the back-end processing layer by slurry coating or deposition or spin coating implemented as a powder in a solvent-diluted binder, or by sputtering, contact bonding or electrostatic bonding.

3. The solid-state active-pixel image sensor according to claim 1 or 2, wherein the thermally conductive layer applied to the back-end-of-line process layer is thinned by polishing, chemical mechanical polishing, ion milling or laser ablation.

4. A method for producing a solid-state active-pixel image sensor comprising a thermally conductive layer, comprising: forming an epitaxial image sensing layer on a silicon substrate; forming a back-end-of-line layer including interconnects on top of the epitaxial image sensing layer; and A heat conducting layer is directly applied on the back-end process layer by contact bonding, or a heat conducting layer is formed on the back-end process layer, wherein the heat conducting layer comprises metal, boron nitride, diamond or graphite. 5 . The method of claim 4 , further comprising thinning the thermally conductive layer by polishing, chemical mechanical polishing, ion milling, or laser ablation. 6 . The method according to claim 4 , further comprising contact bonding the thermal conductive layer to the back-end-of-line layer, and then removing some or all of the silicon substrate and a predetermined amount of the thermal conductive layer.

7. The method of claim 4, further comprising removing some or all of the silicon substrate and forming the thermally conductive layer of a first material, doping the thermally conductive layer to a predetermined depth to produce a doped portion and an undoped portion, and removing the undoped portion after the applying.

8. The method of claim 4, further comprising removing some of the silicon substrate to a thickness of no greater than 5 microns, or removing all of the silicon substrate, by polishing, chemical mechanical polishing, ion milling, or laser ablation.

9. The method of claim 4, further comprising applying the thermally conductive layer to the back-end processing layer by slurry coating or deposition or spin coating as a powder in a solvent-diluted binder, or by sputtering, contact bonding or electrostatic bonding.

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