Patent Publication Number: US-2021193848-A1

Title: Electronic imaging detector with thermal conduction layer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119, based on U.S. Provisional Application No. 62/952,852, filed Dec. 23, 2019, the disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     Electron imaging detectors based on thinned transmission silicon active pixel sensors have revolutionized cryo-electron microscopy. These devices have enabled scientists to investigate the structure of large biological macromolecules at levels not achieved previously. Because these are transmission devices, electrons which have interacted with the detection layer ideally exit the sensor without being scattered back into the detection layer. The use of silicon as the sensor substrate minimizes the scatter of electrons within the sensor back into the detection layer. Active sensors maximize the signal to noise ratio for detecting positions of individual incoming electrons through amplification of electron signals. 
     As shown in  FIG. 1 , an active pixel sensor  10  is built up in three layers: a substrate (SUB) layer  12 , an epitaxially grown detection (EPI) layer  14 , and a back-end-of-line (BEOL) layer  16 . SUB layer  12  is inert from the standpoint of detecting electrons and is used as the base on which to form the detection layers. EPI layer  14  includes a material that generates sensor electrons in response to incoming electron microscope electrons. These sensor electrons are collected. BEOL layer  16  is provided above EPI layer  14  and contains metal interconnects and oxide insulation for reading out the sensor. 
     In some known electron imaging detectors, SUB layer  12  is thick enough that no electrons pass all the way through it. Two example electron paths are shown in  FIG. 1 . Electrons paths A and B are scattered by the substrate back into the EPI detection layer  14  and are detected a second time at locations  20 ,  22 , which are less correlated with the actual entry points of the backscattered electrons into EPI layer  14 . This extra detection reduces imaging quality. Thinning SUB layer  12  from the back can reduce backscatter and the secondary detections, as shown in  FIG. 2 , where electron A does not backscatter up into the detection layer  14 . 
     In the example image using the same electron paths, in  FIG. 2 , electron A, which penetrated the substrate layer to greater depth, is no longer scattered back to the detection layer. Electron B, which is scattered at a shallower depth, still returns to the detection layer. Backscatter can be minimized by minimizing the amount of substrate in this front-illuminated configuration. 
     Thinning the substrate reduces the available material for removal of heat from the device. As the number of pixels per a given area is increased by advances in process technology, heat removal becomes an ever greater problem. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of a prior art silicon imaging detector; 
         FIG. 2  is cross section of another prior art silicon imaging detector; 
         FIG. 3  is a cross section of an exemplary silicon imaging detector according to implementations described herein; 
         FIG. 4A  is a graph showing detective quantum efficiency (DQE) for ideal, front, and back illuminated imaging sensors; 
         FIG. 4B  is a graph comparing DQE for front versus back illuminated imaging sensors to the ideal sensor DQE curve in  FIG. 4A ; 
         FIGS. 5A-C  are cross sections of processing stages for an exemplary silicon imaging detector according to embodiments described herein; and 
         FIGS. 6A-C  are cross sections of processing stages for an exemplary silicon imaging detector according to embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Those skilled in the art will recognize other detailed designs and methods that can be developed employing the teachings of the present invention. The examples provided here are illustrative and do not limit the scope of the invention, which is defined by the attached claims. The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. 
     In accordance with embodiments described herein, as shown in  FIG. 3 , an active pixel silicon imaging detector  300  comprises a SUB layer  310 , an EPI layer  312 , a BEOL layer  314 , and a thermal conduction layer (“TCL”)  316 . TCL  316  is applied to the BEOL layer  314  after manufacture of the BEOL layer. Consistent with embodiments described herein, detector  300  is exposed to incoming electrons initially 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, in a front-illuminated configuration, lies in front of the detection EPI layer. While pre-detection scatter by the BEOL has little or no effect on the number of detections per incoming electron, the scatter may reduce the spatial correlation between entry point and detection point and may therefore limit the ultimate resolution of the detector. The BEOL cannot be removed as it performs the necessary readout function of the detector. However, back-illumination places the BEOL after the EPI detection layer and eliminates the pre-detection scatter caused by the BEOL. 
     As shown in  FIG. 3 , electrons A and B are detected in the EPI layer  312  and then pass through the BEOL layer  314 . The thermal conduction layer  316  is designed both in thickness and material composition to have a high thermal conductivity but a low propensity to reflect electrons back into the EPI detection layer  312 . Prior art substrate thickness has been in the range of 10 um to 40 um yielding backscatter fractions from less than 2% to more than 10% for 200 kV and 300 kV electrons. It would be advantageous to reduce that backscatter fraction through reduced Z (atomic number) of the thermal conduction substrate or by reducing the thickness which would be possible with a higher thermal conductivity. In practice materials vary by both these parameters simultaneously. As shown in  FIG. 3 , Electron A, passes out of the TCL layer  316  without being reflected back into the sensor. Electron B is reflected by the TCL layer and detected a second time in the detection layer  312 . The thickness of the TCL layer and its composition determine the likelihood of electrons passing through the TCL layer or being reflected back into the sensor  300 . 
       FIG. 4A  shows simulated linear detective quantum efficiency (DQE) for three configurations: the solid curve is for an idealized epitaxial layer only (“just EPI”), the middle curve (large dashes) is for a back side illuminated device having a BEOL and epitaxial layer wherein the epitaxial layer is exposed to the electrons directly and the lower curve (short dashes) is for a front side illuminated device having a BEOL and an epitaxial layer (i.e., illuminated through the BEOL).  FIG. 4B  show simulated ratios of the DQE for the two sensors of  FIG. 4A  having a BEOL, as compared to the idealized epitaxial layer only sensor curve of  FIG. 4A . 
     Both graphs ( FIGS. 4A, 4B ) show performance as quantified by linear DQE vs. spatial frequency. Zero spatial frequency at the origin on the left of each graph corresponds to performance of the detector functioning as a whole. Spatial frequency of one times the Nyquist frequency at the right of each graph shows performance for modulations of image signal which go high and low every other pixel. This is the highest spatial modulation frequency that can be acquired by a pixelated detector without aliasing errors. 
       FIGS. 4A and 4B  show that while material on both the entrance and exit faces of the detector increases scatter at increasing levels of spatial frequency, the scatter that occurs post-detection from material mounted on the exit face (large dash curve in both graphs) is less damaging to high-resolution signal quality. The simulation results also imply that any material in front of the EPI detection layer should be minimized. Exposing the sensor from the back side as in  FIG. 3  leaves substrate in front of the epitaxial layer. The conclusion from  FIGS. 4A and 4B  is that anything in front of the EPI detection layer reduces DQE, which means that all or most of the substrate (leaving no more than 5 microns), now at the entrance surface, should be removed. Up to now this has been impractical due to the role played by the substrate in providing mechanical support and heat conduction for the sensor. Consistent with embodiments herein, a thermal conduction layer can support the EPI and BEOL layers from the BEOL layer side (i.e. the exit side in a back-side illuminated configuration). Supporting the device with a layer next to the BEOL layer enables complete removal of the original substrate layer. This in turn allows for direct exposure of the EPI layer, which heretofore has not been possible. 
     Because the thermal conduction layer is not part of the original sensor fabrication (i.e. not part of the growth of the epitaxial layer or the deposition of the BEOL layer), there is an opportunity to find a thermal layer material better than silicon (which comprises the original substrate and most of the heat removal in prior art devices) for the purpose. The thermal conductivity layer material can be better than the original silicon substrate in two ways. It can have a better thermal conductivity and it can have a lower tendency to scatter electrons. The benefit of the combined effects is proportional to thermal conductivity κ and inversely proportional to the square of atomic number Z. A heuristic quality factor for the purpose of an electron-transmissive thermal conduction layer can be defined as Q=κ/Z 2 . The following table summarizes estimates of κ, Z and Q for some representative materials. The estimates of κ and therefore also of Q are subject to manufacturing method, binder fraction, etc. The table is not meant to be exhaustive but illustrative of the principal used for identifying materials as more or less suited for an electron-transmissive thermal conductive layer as/than silicon. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 min thermal 
                 stoichiometric 
                   
               
               
                   
                 conductivity 
                 average atomic 
                 quality factor 
               
               
                 Material 
                 (W/mK) 
                 number (Z) 
                 Q = κ/Z 2   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 aluminum 
                 170 
                 13 
                 1.01 
               
               
                 aluminum nitride 
                 180 
                 10 
                 1.80 
               
               
                 diamond 
                 1000 
                 6 
                 27.78 
               
               
                 pyrolized graphite 
                 700 
                 6 
                 19.44 
               
               
                 beryllium 
                 200 
                 4 
                 12.50 
               
               
                 beryllium oxide 
                 265 
                 6 
                 7.36 
               
               
                 boron nitride 
                 1700 
                 6 
                 47.22 
               
               
                 silicon 
                 190 
                 14 
                 0.97 
               
               
                   
               
            
           
         
       
     
     Due to the fragility of the thin EPI detector layer, especially after removal of all substrate material, methods for attachment of a thermally conductive layer to the thin device must either be very low force/pressure or a robust temporary support for the thin layers must be provided as part of any higher pressure process step. Methods for affixing a thermal conduction layer to the BEOL of the back-illuminated sensor vary depending on the conduction material. 
     Solid refractory dielectrics, including but not limited to boron nitride, beryllium oxide and diamond can be contact bonded onto the sensor BEOL either with or without temperature elevation and with or without applied pressure, followed by removal of excess thermal conduction layer material and also removal of the silicon substrate layer. Contact bonding is described in U.S. Pat. Nos. 5,852,622 and 5,441,803, which are incorporated by reference herein. The substrate is left intact at first, to allow pressure applied for contact bonding of the thermal layer to the BEOL. 
       FIGS. 5A-5C  show cross sections of the manufacturing process for an imaging sensor according to an aspect of the invention.  FIG. 5A  shows a sensor  510  comprising a silicon substrate  512 , an epitaxial layer  514  and a BEOL  516 .  FIG. 5A  also shows a thermal conduction layer  520  supported by a thermal conduction layer substrate  522 . The TCL substrate  522  can be the same or different material as the TCL itself.  FIG. 5B  shows the bonding of the BEOL  516  of the sensor  510  to the thermal conduction layer  520 . At this stage, both the imaging sensor  510  and the thermal conduction layer are supported by substrates  512 ,  522  to allow pressure to be applied in a contact bonding process. Note that contact bonding may be performed in some embodiments with or without pressure and with or without temperature elevation.  FIG. 5C  shows the completed imaging sensor  510  with the original sensor substrate  512  removed and the thermal conduction layer substrate  522  removed. Removal of substrate material or excess thermal conduction material can be accomplished by a variety of means including, but not limited to, polishing, chemo-mechanical polishing, ion milling, and laser ablation. The resulting sensor shown in  FIG. 5C  has an unhindered epitaxial sensing layer  514  that can be directly exposed to electrons without any initial scattering, plus a thermal conduction layer  520  to remove heat from the device as well as physically support the device. 
     The thermal conduction layer to be contact bonded to the sensor can be derived from a variety of initial starting forms and processed in a variety of ways. Consistent with embodiments described herein, the TCL/SUB layer  502 / 522  bonded to the BEOL can be made of a single material which is thicker than the final TCL  520 . The extra thickness is necessary for strength in the bonding process. The layer is contact bonded onto the BEOL layer of the sensor and the extra material is polished off. In this case the ultimate thickness of the thermal conduction layer is determined by close monitoring of the thickness during polishing. 
     In a further embodiment, a single material thicker than the final thermal conduction layer is used as a starting material, but the starting material is ion-implanted on the side to be bonded to a predetermined depth. This ion implantation changes the etch rate for a chemical or chemo-mechanical thinning process without changing the thermal or scattering properties of the thermal conduction layer material. Once bonded to the BEOL layer, the thermal conduction material is thinned. The etch stop created by the ion implantation increases the reliability of achieving a given thickness thermal conduction layer. 
     In a further embodiment, the thermal conduction layer is epitaxially grown or chemical vapor deposited onto a substrate of a different material than the grown or deposited layer. The substrate is used as a transfer layer. The combined layers are contact bonded to the BEOL layer as above. Once bonded to the BEOL layer the substrate material is removed by one or more of the processes described above. 
     In a further embodiment, a thermally conductive powder layer is sintered onto a substrate of similar melting point and coefficient of thermal expansion as the powder. The substrate acting as a transfer layer. The combined layers are contact bonded to the BEOL layer, followed by complete removal of the substrate material after contact bonding. 
     Metal layers can be sputtered onto the BEOL.  FIGS. 6A-C  show cross sections of an exemplary sputtering deposition of a thermal conduction layer  616  onto a BEOL  614 .  FIG. 6A  shows a sensor  610  comprising an epitaxial layer  612  and a BEOL  614 . The original substrate on which the sensor was developed has already been removed in this embodiment.  FIG. 6B  shows the sputtering of material onto the BEOL to create a thermal conduction layer  616 .  FIG. 6C  shows a completed sensor with a fully deposited thermal conduction layer  616 , 
     Powder-based materials can be applied by settling or slurry coating processes in a solvent-thinned binder such as is done for powder phosphor scintillators. The powder-binder mixture is applied to the BEOL layer by means including silk screening or gravitational settling. A liquid powder suspension may also be spin coated onto the BEOL layer. Exemplary binders include but are not limited to sodium silicate, ethyl cellulose, and ammonium polymethacrylate, and cellulose nitrate. While a binder sacrifices some of the thermal conductivity, given the extremely high conductivity of a graphite or boron nitride powder and given sufficiently low binder fraction, the net conductivity will still constitute an improvement over silicon. 
     Powder-based thermal conductors can also be deposited directly onto the BEOL layer by spin-coating of a liquid powder suspension. 
     In a further embodiment, the starting material for a thermal conductor layer is a flexible sheet comprising a flexible polymer embedding medium for powder heat conduction material. The flexible sheet material can be contact bonded in vacuum to prevent gas entrapment, with or without temperature elevation. The material should be affixed to the BEOL layer after all high-temperature processes are complete on the sensor itself. In this case attachment is performed on a fragile, thin final sensor with only BEOL and epitaxial layers remaining. Thus attachment will need to use only low forces/pressures. Possible low force attachment methods include but are not limited to: directing a gas jet at outer surface of the flexible layer to press and smooth the layer along with low temperature heating to maximize the flexibility and adhesion of the flexible layer. A warm gas jet combines the two effects of smoothing and heating. In a further embodiment, centrifugal force is used to flatten the flexible sheet during adhesion to the BEOL layer of the sensor. In a further embodiment electrostatic-dielectric attraction is used with due care to prevent electrostatic damage to the sensor. Some types of flexible sheets are conductive (graphite-containing for instance) and others are electrically insulating (boron-nitride for instance) and may require different handling in an electrostatic method of attachment. 
     Powder-based thermal conductors can be deposited directly onto the BEOL layer by settling or slurry coating in a solvent-thinned binder as is done for powder phosphor scintillators. Exemplary binders include but are not limited to sodium silicate, ethyl cellulose, and ammonium polymethacrylate, and cellulose nitrate. While a binder sacrifices some of the thermal conductivity, given the extremely high conductivity of a graphite or boron nitride powder and given sufficiently low binder fraction, the net conductivity will still constitute an improvement over silicon. 
     Powder-based thermal conductors can also be deposited directly onto the BEOL layer by spin-coating of a liquid powder suspension. 
     Although the invention has been described in detail above, it is expressly understood that it will be apparent to persons skilled in the relevant art that the invention may be modified without departing from the spirit of the invention. Various changes of form, design, or arrangement may be made to the invention without departing from the spirit and scope of the invention. Therefore, the above-mentioned description is to be considered exemplary, rather than limiting, and the true scope of the invention is that defined in the following claims. 
     No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.