Abstract:
The radiation detector/imaging substrate arrays in an x-ray and gamma-ray radiation energy imaging device are described which use an electrically conductive adhesive to provide electron charge signaling continuity between the detector and read-out substrates of the device. The present device utilizes a plurality of electrically conductive bonds each discretely connecting a pixel contact in the pixel pattern to a signal contact in the signal contact pattern, the bonds being an electrically conductive adhesive. This bonding technique is especially useful in detection/imaging arrays having detector substrates comprising Cadmium and Tellurium compositions. The present invention is practicable with semiconductor detector and read-out substrates with or without “bumped” electrical contacts. The electrically conductive bonds utilize either isotropically or anisotropically conductive adhesives.

Description:
[0001]    The present application claims the benefit of prior filed U.S. Provisional Application, serial No. 60/368,540, filed Mar. 29, 2002, to which the present application is a non-provisional U.S. Patent Application, and the content of which is incorporated herein by reference.  
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is in the field of semiconductor devices for detecting invisible radiant energy. More specifically, the present invention relates to imaging systems for forming an image of an object and having detection means including a Cadmium Telluride substrate bonded to a specific semiconductor arrangement using a conductive adhesive.  
         BACKGROUND OF THE INVENTION  
         [0003]    In the field of semiconductor radiation imaging devices comprising a semiconductor detector substrate solder bump-bonded to a semiconductor processing/readout substrate, the occurrence of “cold” solder joints can be a problem. See U.S. Pat. No. 5,379,336; U.S. Pat. No. 5,812,191; EP 1162833; and EP 0421869. Cold solder joints, also called cold welds, can occur in a variety of solder compositions, including solder alloys containing Indium, Lead, Tin and other metals.  
           [0004]    This has motivated the field to seek alternative soldering or bump-bonding methods to reduce the occurrence of cold joints, including a low-temperature bump-bonding techniques (see U.S. Pat. No. 5,952,646). However, current soldering or the above mentioned bump-bonding techniques cannot fulfill all the requirements of new high-density, imaging device assembly processes which utilize temperature sensitive semiconductor materials in the detector substrates. These temperature sensitive semiconductor detector substrates include some of the currently most promising inter-metallic materials, such as Cadmium Telluride (CdTe) compositions and Cadmium Zinc Telluride (CdZnTe) compositions.  
           [0005]    When bonding high density semiconductor substrates together with solder, the interconnection volumes are very small in the resulting imaging devices. This small interconnect area coupled with the relatively more brittle character of some of the promising inter-metallic detector substrates can result in a decrease in the mechanical stability of the interconnection and decreased reliability when solder bump-bonding is used with some of these inter-metallic materials. Additionally, the use of metallic Lead (Pb) in solder to bond the semiconductor substrates increases the amount of this toxic metal potentially exposed to the environment.  
           [0006]    Therefore, it would be beneficial to the industry to have a semiconductor substrate bonding technique that does not subject the brittle and temperature sensitive inter-metallic semiconductor materials to excessive heat or pressure. Additionally, it would be beneficial to the industry to have an alternative method for producing semiconductor imaging substrates that is simpler than the current bonding techniques, less susceptible to the problem of cold solder joints or cold welds, and which reduces the amount of metallic lead required to accomplish the bonding process.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is a digital radiation imaging device and method for producing the device to digitally image radiation in excess of 1 keV. The imaging device is relatively robust, reliable and cost effective. The present radiation imaging device comprises a semiconductor substrate which generates charge directly in response to incoming radiation, a readout/processing semiconductor substrate for processing and reading out the generated charge, and an electrically conductive adhesive means for joining or bonding the two semiconductor substrates together.  
           [0008]    The adhesives of the present invention create mechanically solid and electrically conductive contacts of joint between the semiconductor substrates. The cured adhesives are also thermally conductive and protect the electrical contacts from the environment. Most often this is achieved by incorporating electrically conductive or fusible particles into a polymer adhesive.  
           [0009]    The adhesives can be either isotropic or anisotropic depending on their physical characteristics and how they are applied to a semiconductor substrate, i.e., in discrete locations or as a film. Isotropic adhesives conduct electricity substantially equally well in all directions, once they are cured in the imaging device. Anisotropic adhesives preferentially conduct electricity well in one plane direction, once they are cured in the imaging device. The mating charge signaling contacts between the semiconductor substrate can be either bumped or bumpless to connect a readout CMOS substrate to its matching detector substrate. The connections between the mating charge signaling contacts are accomplished using either isotropically conductive or anisotropically conductive adhesives. This is particularly useful with CdTe or CdZnTe detector substrates, but can be used with other radiation imaging detector substrates, including PbI, HgCdTe, GaAs, when the present benefits are desired.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is an illustration of the device of the present invention comprising the two dimensional imaging array of a radiation imaging system.  
         [0011]    [0011]FIG. 2A is a partial cross-sectional side view of an embodiment of the present invention wherein a semiconductor flip-chip substrate is adhesively bonded to a bumped readout CMOS semiconductor substrate using discrete application of an isotropically conductive adhesive.  
         [0012]    [0012]FIG. 2B is an exploded side view of the device of FIG. 2A showing a pixel cell before conductive adhesive bonding of the flip-chip substrate to the bumped CMOS substrate.  
         [0013]    [0013]FIG. 2C is an exploded side view of the device of FIG. 2A showing a pixel cell after conductive adhesive bonding of the flip-chip substrate to the bumped CMOS substrate.  
         [0014]    [0014]FIG. 3A is a partial cross-sectional side view of an embodiment of the present invention wherein a semiconductor flip-chip substrate is adhesively bonded to a bumpless readout CMOS semiconductor substrate using discrete application of an isotropically conductive adhesive.  
         [0015]    [0015]FIG. 3B is an exploded side view of the device of FIG. 3A showing a pixel cell before conductive adhesive bonding of the flip-chip substrate to the bumpless CMOS substrate.  
         [0016]    [0016]FIG. 3C is an exploded side view of the device of FIG. 3A showing a pixel cell after conductive adhesive bonding of the flip-chip substrate to the bumpless CMOS substrate.  
         [0017]    [0017]FIG. 4A is a partial cross-sectional side view of an embodiment of the present invention wherein a semiconductor flip-chip substrate is adhesively bonded to a bumped readout CMOS semiconductor substrate using a layer of an anisotropically conductive adhesive, and the directional nature of its conductivity.  
         [0018]    [0018]FIG. 4B is an exploded side view of the device of FIG. 4A showing a pixel cell before conductive adhesive bonding of the flip-chip substrate to the bumped CMOS substrate.  
         [0019]    [0019]FIG. 4C is an exploded side view of the device of FIG. 4A showing a pixel cell after conductive adhesive bonding of the flip-chip substrate to the bumped CMOS substrate.  
         [0020]    [0020]FIG. 5A is a partial cross-sectional side view of an embodiment of the present invention wherein a semiconductor flip-chip substrate is adhesively bonded to a bumpless readout CMOS semiconductor substrate using a layer of an anisotropically conductive adhesive, and the directional nature of its conductivity.  
         [0021]    [0021]FIG. 5B is an exploded side view of the device of FIG. 5A showing a pixel cell before conductive adhesive bonding of the flip-chip substrate to the bumpless CMOS substrate.  
         [0022]    [0022]FIG. 5C is an exploded side view of the device of FIG. 5A showing a pixel cell after conductive adhesive bonding of the flip-chip substrate to the bumpless CMOS substrate.  
     
    
       [0023]    Figure Reference Numerals:  
         [0024]    imaging system  10   
         [0025]    imaging device  11   
         [0026]    image detector  20   
         [0027]    readout CMOS  21   
         [0028]    pixel contact (electrode)  22   
         [0029]    pixel cell  24   
         [0030]    bump  25   
         [0031]    additional metal layer  26   
         [0032]    isotropically conductive adhesive (ICA)  27   
         [0033]    anisotropically conductive adhesive (ACA)  28   
         [0034]    continuous electrode  31   
         [0035]    semiconductor detector substrate  32   
         [0036]    detector insulator  33   
         [0037]    under bump metal layers (UBM)  34   
         [0038]    CMOS insulator  35   
         [0039]    CMOS pixel signal circuit  36   
         [0040]    conductor  37   
         [0041]    adhesive matrix  38   
         [0042]    filler particle  39   
         [0043]    detector openings  42   
         [0044]    pixel cell pitch P  
         [0045]    contact-to-contact gap G  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0046]    In accordance with a preferred embodiment of the present invention the radiation imaging device comprises a semiconductor substrate for generating charge directly in response to incoming radiation hits, a readout/processing semiconductor substrate for processing and reading out the generated charge and an isotropically conductive adhesive (ICA) for bonding the two substrates together. Such ICAs have a conductive filler with a high filler loading rate, and they are conductive substantially in all directions equally well. Only a minimum bonding pressure is required when bonding with ICAs. Hence, the brittle CdTe or CdZnTe detector substrates are not subjected to excessive pressure and the risk of damage to the detector substrate is minimized.  
         [0047]    In another preferred embodiment, the imaging device of the present invention comprises a semiconductor substrate for generating charge directly in response to incoming radiation hits , a readout/processing semiconductor substrate for processing and reading out the generated charge and an anisotropically conductive adhesive (ACA) material for bonding the two substrates together. ACAs also have a conductive filler, but achieve electrical conduction by using conductive filler loading rate which provide conduction substantially across a single plane (e.g., z-plane). In anisotropic adhesives, the loading rates are low enough to ensure that electrical insulation is maintained in the other two physical planes (e.g., the x-y-planes) of the bond interface.  
         [0048]    The present electrically conductive adhesives comprise a polymer resin, a curing agent, other additives and conductive filler particles. The polymer resin provides a matrix in which the conductive filler particles are suspended. The adhesive matrix can be either thermosetting or thermoplastic. When thermosetting polymer resins are cured, a chemical reaction takes place and a three-dimensional network of cross-linked polymers is formed. As result, thermosetting adhesives are relatively temperature stable and do not melt under heating. They are also essentially infusible, insoluble and show good creep resistance. Epoxy resin is the most frequently used thermosetting adhesive matrix.  
         [0049]    On the other hand, thermoplastic adhesives are based on a polymer dispersed in a solvent. Curing of such thermoplastic resins is a drying process, where heating evaporates the solvent content of the resin, thus no chemical reaction takes place. Therefore, the adhesive matrix can be re-melted when subjected to temperatures above a specific melting point or melting range. Hence, thermoplastic adhesives can be used in so called polymer bumps where re-melting is needed. Examples of solvent labile polymers useful in the present invention include polyamides and polyesters.  
         [0050]    Conduction mechanisms for providing electrical communication between the semiconductor substrates are highly dependent upon the type of conductive particle that is formulated into the adhesive matrix. The adhesive matrix may be filled with metal, metal alloy or metal coated particles. Examples of metal particles practicable in the practice of the present invention include nickel (Ni), gold (Au) or silver (Ag). In the case of ACAs, the conduction mechanism is based upon applying sufficient pressure during assembly in a such way that the metal particles penetrate through the oxide layer into of the semiconductor substrate, and plastically deform the electrical contact metallizations. On the other hand, in the case of ICAs, the electrical connection between the detector substrate and readout CMOS substrate is created by numerous bridged particles, not by single particles (as in ACAs).  
         [0051]    Metal coated particles may, for example, comprise polymer spheres which are coated with Ni/Au or Au. The conduction mechanism in this case is based upon applying sufficient pressure during assembly in a such way that the particles themselves plastically deform and result in overall contact area larger the initial contact area. A benefit of using coated polymer particles filler is that the compressible nature of a single polymer particle (or a cluster of particles) will accommodate some horizontal or vertical movement of the joint area, and hence the electrical connection will remain stable during thermal expansion or other movement. Also, compressibility the polymer-cores of coated polymer particles fillers can provide the advantage of compensating small non-coplanarity discrepancies between the planes of the readout CMOS or its bump and pixel contact of the CdTe or CdZnTe detector substrate.  
         [0052]    Traditional solder materials, such as tin-lead (SnPb), or low melting point alloys, such as tin-bismuth (SnBi), tin-lead-bismuth (SnPbBi), tin-indium (SnIn), are examples of metal alloy particles utilized in electrically conductive adhesives.  
         [0053]    An adhesive filled with these type of particles creates small, local solder joints or connections between the readout CMOs&#39; contacts and pixel electrode contacts, while the adhesive matrix simultaneously provides an underfilling. The metallurgical nature of these joints mean that any relaxation of the adhesive matrix, which may occur during the operational lifetime of the particular device, should not affect the electrical continuity of the joints. Also, when the metal filler is metallurgically compatible with the contact metal, a local solder joint is formed even though the melting point of the filer metal or metal alloy is not reached. Bismuth filler with tin contact metal is an example of this technique.  
         [0054]    In an alternative embodiment, the filler particles may have a very thin, nonconductive outer surface coating. This very thin insulating coating prevents the particles from creating electrically conductive bridges across the thickness of the adhesive. However, for those filler particles compressed between the bump and pixel electrode contact during bonding of the two substrates together, the nonconductive coating is broken and an electrically conductive pathway is created. The non-conductive coating may be chosen so that breakage of the coating is accomplished or facilitated by heating. Hence, a high particle-adhesive matrix ratio can be used without the risk of short circuiting.  
         [0055]    It is not only the choice of filler material, but also the size and shape of the filler particles that influence the properties of the cured adhesive bonds. Some of the properties of the cured adhesive bonds that can be influenced by particle size and shape are: electrical conductivity, thermal conductivity, tensile strength, viscosity, weight loss and rheology. The filler particles may be different configurations as selectable by one of ordinary skill in the art. For example, the filler particles may be flakes, spheres or other shapes, and may have a size ranging from nanometers to several micrometers.  
         [0056]    Due to their nature different adhesive types have to be applied to the contact areas accordingly. ICAs have to be applied with high accuracy. For example, screen printing, dispensing or pin-transfer methods can be used. Also, if either the readout CMOS substrate or the detector substrate has bumps, the adhesive can be applied with dipping the CMOS or detector bumps into the ICA. Furthermore, if the adhesive matrix is composed of thermoplastic polymers, the ICA can be cured before actual bonding and a so called polymer bump is formed. This is followed by a normal bump-bonding where the polymer bump re-melts during bonding and an electrical connection is formed. These polymer bumps can also be formed with a masking process, and hence no under bump metals (UMP) or bumps have to be initially deposited on the readout CMOS substrate or the detector substrate. However, additional metal layers may be deposited on top of the conductor in order to improve conductivity and material compatibility (and hence improve reliability). Furthermore, a thermoplastic photoresist mask may be used which, if not removed, can work as an underfill. On the other hand, ACAs can be applied to the whole joining area, not only on each contact or joint area, as ACAs achieve electrical conductivity only in a single plane direction. This greatly reduces the accuracy requirements on whatever method is used to apply the conductive adhesive. Additionally, the conductive adhesive may also be provided as a film which has either randomly or uniformly distributed filler particles in it. This can further decrease the risk of short circuiting due to bridged particles and therefore are often used in very fine pitch applications.  
         [0057]    Referring now to the drawings, the details of preferred embodiments of the present invention are graphically and schematically illustrated. Any like elements in the drawings are represented by like numbers, and any similar elements are represented by like numbers with a different lower case letter suffix.  
         [0058]    [0058]FIG. 1 schematically illustrates an imaging system  10  which consists of one or more imaging devices  11 . The imaging device  11  comprises of a readout CMOS substrate  21  and image detector substrate  20  which are bonded together with an electrically conductive adhesive  25 . In the preferred embodiment shown, the imaging device II is composed of a plurality of individual pixel cells  24 . These pixel cells  24  detect high energy radiation (indicated by the large arrows in figures) and pass the collected information through the readout CMOS  21  to the control electronics for further processing and analysing.  
         [0059]    [0059]FIG. 2A is a schematic cross section of an adhesive bonded imaging device  11 . In this example the image detector substrate  20  is bonded to a bumped readout CMOS  21  with a conductive adhesive  38 . In the case illustrated either an isotropically conductive adhesive (ICA)  27  or an anisotopically conductive adhesive (ACA)  28  could have been used as the conductive adhesive. FIGS. 2B and 2C are more detailed schematic illustrations of a single pixel cell  24  before ( 2 B) and after bonding ( 2 C). The image detector  20  side of a single pixel cell  24  comprises a semiconductor detector substrate  32  which has a continuous electrode  31  on the back side and pixel contacts (electrodes)  22  on the front side of the detector substrate  32 . The pixel contacts  22  are protected with a detector insulation (passivation) layer  33  having small detector openings  42  for electrical connection pathways.  
         [0060]    On the readout CMOS substrate side of the pixel cell  24  is a corresponding CMOS pixel signal circuit  36 . This CMOS signal circuit  36  of the pixel cell  24  has a signal contact  37  conductor which is connected (see FIG. 2C) to the pixel contact  22  of the semiconductor detector substrate  32  by means of a conductive  38 . The readout substrate  21  in this example has bumps  25  comprised of, for example, nickel-gold (Ni/Au), nickel-palladium (Ni/Pd), normal solder or a low temperature solder material. Although shown in FIG. 2B as being disposed the CMOS readout substrate  21 , the bump  25  alternatively may be disposed on the pixel contact  22  of the detector substrate  20  or on the contacts  22  &amp;  37  of both substrates  20  &amp;  21 .  
         [0061]    As can be seen from FIGS.  2 A-C, the conductive adhesive  38  is discretely applied directly on the bump  25 . For a conductive adhesive (e.g., an ICA) which is conductive substantially equally in all directions, discrete application directly on a bump  25  or a contact  22  &amp;  37  does not create a signal loss problem from shorting. Application of the conductive adhesive  38  can be done with screen printing, dispensing, pin-transfer methods or other methods known to one of skill in the art. The conductive adhesive  38  itself is a matrix composed of an adhesive or cement (e.g. epoxy adhesive) which is filled with electrically conductive or fusible particles  39  (in FIGS.  4 A- 4 C and FIGS.  5 A- 5 C). These particles  39  can be, for example, metal (e.g. Ag), metal alloy (e.g. Ni/Au) or metal coated particles (e.g. polymer sphere coated with Ni/Au). ICAs consist, in general, of about 35-85 vol-% randomly dispersed filler particles  39 , but the filler material, size and amount is determined by the application, the contact materials used, and the pitch P of the pixels and the gap G between the image detector  20  and readout CMOS  21 .  
         [0062]    Outside the connection area, the CMOS pixel circuit  36  is protected from the environment by the CMOS insulation or passivation  35 . In order to improve the compatibility of the materials (in connection with each other) and also to improve adhesion, mechanical and/or electrical conductivity, one or multiple layers of under bump metals (UBM)  34  can be applied between the bump  25  and a pixel circuit signal contact  37 . Furthermore, an imaging device bonded with a conductive adhesive  38  can be underfilled if increased reliability is needed.  
         [0063]    [0063]FIG. 3A shows a detector substrate  20  which is bonded to a readout CMOS  21  with a conductive adhesive  38 , as in FIGS.  2 A- 2 C. However, unlike in FIGS.  2 A- 2 C, in this example the readout CMOS  21   a  is bumpless. The bump  25  and UMB  34  may be replaced with additional metal layer  26  in order to improve conductivity and material compatibility (and hence improve reliability), but still no bumping is needed. As FIGS. 3B and 3C illustrate, the bonding process with a bumpless readout CMOS substrate  21   a  is very similar as it is with a bumped readout CMOS substrate  21 .  
         [0064]    Generally, the conductive adhesive  38  is not cured before bonding. However, if the adhesive matrix of the conductive adhesive  38  is comprised of thermoplastic polymers, the adhesive may be cured to form a “polymer bump before actual bonding of the two semiconductor substrates  20  &amp;  21 . This is followed by a normal bump-bonding technique where the polymer bump is remelted during bonding and forms an electrical connection. Polymer bumps can be formed, for example, with a masking process. Alternatively, a photoresist mask may be used as a stencil and the conductive adhesive  38  screen printed onto the signal contacts  37  or pixel contacts  22 . This process step may also take place prior to dicing (e.g., of the CMOS chips from the wafer). Furthermore, no additional underfill is needed if the photoresist is also composed of thermoplastic polymers and is not removed from the readout CMOS substrate  21  or the detector substrate  20 .  
         [0065]    FIGS.  4 A- 4 C and FIGS.  5 A- 5 C are respectively schematic presentations of a bumped and a bumpless readout CMOS substrate  21 , which are bonded to a detector substrate  20  with a conductive adhesive  38 . As the figures demonstrate, an ACA  28  (FIG. 5B) contains fewer filler particles than an equivalent volume of an ICA  27  (FIG. 4B), i.e., an ACA  28  has a lower filler loading rate than an ICA  27 . An ACA  28  conducts electrical charge only across a single plane (e.g., z-axis) when the filler particles  39  are compressed between, for example, the pixel contact  22  and the bump  25 . Moreover, ACAs  28  may be applied to the whole joining surface areas, not only the contact areas. Additionally, an ACA  28  may be configured as a film layer which is placed between the readout CMOS substrate  21  and detector substrate  20 . The adhesive film may consist of one or multiple layers which all may have different properties according to the application. Furthermore, the conductive filler particles  39  may be either randomly or uniformly distributed in the film.  
         [0066]    While the above description contains many specifics, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of one or another preferred embodiment thereof Many other variations are possible, which would be obvious to one skilled in the art. Accordingly, the scope of the invention should be determined by the scope of the appended claims and their equivalents, and not just by the embodiments.