US20150276945A1 - Semiconductor bump-bonded x-ray imaging device - Google Patents

Semiconductor bump-bonded x-ray imaging device Download PDF

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US20150276945A1
US20150276945A1 US14/224,594 US201414224594A US2015276945A1 US 20150276945 A1 US20150276945 A1 US 20150276945A1 US 201414224594 A US201414224594 A US 201414224594A US 2015276945 A1 US2015276945 A1 US 2015276945A1
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bump
um
ray imaging
imaging device
detector
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Konstantinos Spartiotis
Henri Tapio Nykanen
Limin Lin
Tuomas Heikki Elmeri Lahtinen
Pasi Juhani LAUKKA
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Ajat Ltd Oy
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01TMEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16Measuring radiation intensity
    • G01T1/24Measuring radiation intensity with semiconductor detectors
    • G01T1/247Detector read-out circuitry
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144Devices controlled by radiation
    • H01L27/146Imager structures
    • H01L27/14601Structural or functional details thereof
    • H01L27/14634Assemblies, i.e. Hybrid structures
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144Devices controlled by radiation
    • H01L27/146Imager structures
    • H01L27/14601Structural or functional details thereof
    • H01L27/14636Interconnect structures
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144Devices controlled by radiation
    • H01L27/146Imager structures
    • H01L27/14643Photodiode arrays; MOS imagers
    • H01L27/14658X-ray, gamma-ray or corpuscular radiation imagers
    • H01L27/14659Direct radiation imagers structures
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144Devices controlled by radiation
    • H01L27/146Imager structures
    • H01L27/14643Photodiode arrays; MOS imagers
    • H01L27/14658X-ray, gamma-ray or corpuscular radiation imagers
    • H01L27/14661X-ray, gamma-ray or corpuscular radiation imagers of the hybrid type
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L28/00Passive two-terminal components without a potential-jump or surface barrier for integrated circuits; Details thereof; Multistep manufacturing processes therefor
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L31/00Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256Semiconductor devices sensitive to infra-red radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264Inorganic materials
    • H01L31/028Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic System
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
    • A61B6/14Applications or adaptations for dentistry
    • A61B6/145Applications or adaptations for dentistry by intraoral means

Abstract

A high pixel density intraoral x-ray imaging sensor includes a direct conversion, fully depleted silicon detector bump bonded to a readout CMOS substrate by capillary bump bonds.

Description

    BACKGROUND OF THE INVENTION
  • The current invention relates to a direct conversion, semiconductor x-ray imaging device where the detector substrate is bump bonded to the readout substrate. X-rays (or other type of radiation) impinge upon the detector and electron-holes pairs are created inside the detector substrate (thus the term “direct conversion”) in response to the absorbed energy. Under the influence of an electric field applied across the detector these electron(holes) are transferred to charge collection electrodes. The charge collection electrodes are connected to corresponding readout electrodes on a readout substrate, which act as the input to a readout pixel on the readout substrate. The connection is made via bumps and the known flip-chip bonding technique.
  • DESCRIPTION OF THE RELATED ART
  • The technique of bumping and flip-chip bonding is wide spread in the manufacturing of direct conversion x-ray imaging devices. Typically the bumps are grown with electroplating or electroless on the readout substrate side at a wafer scale. Then the wafer is diced and flip-bonded to the detector substrate. The bumps can however be grown on both sides, i.e., on the readout and/or the detector substrate. Typical bump composition found in imaging devices are PbSn, BiPbSn, BiSn, Au, AgSn, In. Each has its advantages. Examples of bump-bonded semiconductor radiation imaging devices can be found in U.S. Pat. No. 5,952,646A and U.S. Pat. No. 6,933,505B2. In NIM A Vol 527 Issue 3, “A CdTe real time X-ray imaging sensor and system”, a detailed embodiment of a CdTe x-ray imaging device is disclosed where the bumps are BiSn, grown on the CMOS. The pixel size is 100 um (one hundred micrometers) and by way of example the bump size is approximately 25 um (twenty five micrometers) while the bump size is roughly spherical. After the bonding the bumps are squashed and the bump is more like an ellipsoid with post bonding height of about 15 um (fifteen micrometers). In NIM A501 2003 “A directly converting high-resolution intra-oral X-ray imaging sensor”, an x-ray imaging sensor for intraoral imaging is disclosed. The readout substrate is again a CMOS and the detector is fully depleted Si. The pixel size in this case is 35 um (thirty five micrometers). For so small pixel size the bumps are expected to be of the order 10 um-15 um and the post bonding height around 10 um.
  • In other prior art examples, the bumps are grown on both the readout substrate (CMOS) and the detector substrate (Si, CdTe, CdZnTe etc). In such examples in prior art one finds In bumps and/or Au studs.
  • The prior art techniques in bump-bonded semiconductor imaging devices work and are efficient due to the relatively large pixel size. By large pixel size is meant pixel pitch of thirty five micrometers (35 um) to two hundred micrometers (200 um). At the low end (close to 35 um) the above described bump bonding techniques suffer from sever limitations:
      • During bonding the spherical shape of the bump becomes ellipsoid and the bump is squashed and extends laterally. There is a high risk of sorting a bump with its neighboring bump(s).
      • The surface (active area) of the detector and the CMOS (readout substrate) can be several square cm, and the uniformity of the spherical bumps becomes critical. A non-uniformity of the spherical bump shape of ±3 um becomes critical in a substrate size of 2 cm×1 cm or larger. The manufacturing ability gets even more compromised for small pixel sizes, i.e., for pixels of 35 um or less. For such small pixels the spherical bumps need to be 15 um or smaller and such bumps become increasingly difficult to manufacture over large areas with sufficient uniformity (±3 um) using conventional electroplating or electroless technique.
      • For pixel sizes less than 35 um, the spherical bumps need be of the order of 5 um-15 um and as mentioned above making such PbSn, BiSn, AgSn, In (etc) spherical (or almost spherical) bumps of such small size, becomes increasingly difficult, especially given the large area and uniformity constraints.
      • The current bumps and interconnect technologies in semiconductor direct conversion radiation imaging devices have a deforming structure. This means that the whole bump or bonding element (which may have some other general shape as well) is deformed during the bonding process. As a result there is no “guaranteed” minimum post bonding height. The post bonding height depends on the how much the bump (or bonding element) will be deformed, i.e., it depends on the bonding process, the bump size and bump uniformity across the readout substrate.
  • It is therefore no coincidence that the breakthrough intraoral sensor described in NIM A501 2003 “A directly converting high-resolution intra-oral X-ray imaging sensor”, never came to the market despite the efforts of several sensor manufacturers trying to employ the above mentioned conventional bump bonding techniques. The yield was too low and the manufacturing cost too high.
  • Furthermore, there are no known direct conversion, bump-bonded, high pixel density x-ray (or gamma ray, beta ray or other form of radiation) imaging devices, at least none produced regularly and with high yield. High pixel density means a readout pixel with size of less than sixty micrometers (<60 um) and preferably less than thirty five micrometers (<35 um) bump bonded to a detector pixel with size of less than thirty five micrometers (<35 um).
  • SUMMARY OF THE INVENTION
  • The current invention provides a direct conversion radiation imaging device that overcomes the limitations of prior art. Specifically, in accordance with the current invention, the direct conversion x-ray comprises a semiconductor detector substrate, a readout substrate and the two are bump bonded together with each detector pixel bonded to one or more readout pixels by means of capillary bump bonds.
  • A capillary bump has essentially a substantially rigid portion, usually of the element copper (Cu) or other metals such as Nickel (Ni), Aluminum (Al) etc., with high melting point and a bump solder “hat” grown on top of the rigid element. The bump solder hat has initially, during the manufacturing process, a semi spherical shape and is usual made from one of: tin (Sn), lead-tin (PbSn), bismuth-tin (BiSn), silver-tin (AgSn) etc. In the manufactured imaging device, the bump solder hat has a final cross-section shape of a compressed spherical shape, with upper and lower surfaces that are generally flat and parallel, and arcuate end surfaces connecting the upper and lower surfaces. The final shape of the bump solder hat may be a compressed ellipsoidal-like structure compressed along its minor axis, with upper and lower surfaces that are generally flat and parallel, and arcuate end surfaces connecting the upper and lower surfaces.
  • During the bonding process the temperature used is from 70 C to 250 C and the solder hat is in a reflow state or almost reflow state and is squeezed, just as an ordinary bump, found in the prior art, would be squeezed. However, the capillary element (in the form typically of cylindrical or other type/shape of pillar) stays rigid and acts as a pillar that will not allow the two substrates, i.e., the detector and readout, to come closer than the height of the pillar. In this way the semiconductor direct conversion imaging device has a well-defined post bonding height, the solder hats are not sorted with each other or with the readout pixels and can be reliably manufactured even for the smallest pixel sizes, i.e., for pixels less than sixty micrometers (<60 um), even less than thirty five micrometers (<35 um) and even less than or equal to twenty five micrometers 25 um).
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIG. 1 presents a bump structure on a readout substrate as found in the prior art.
  • FIG. 2 shows an imaging device as found in prior art with the detector pixel bump bonded to the readout pixel, exemplifying the possible sort-circuit issues with current bump-bonding technology.
  • FIG. 3 shows a pillar with a solder hat on a readout pixel in accordance with the current invention and bonded to a detector substrate
  • FIG. 4 shows an imaging device in accordance with the current invention.
  • FIG. 5 shows an imaging device in accordance with the current invention where the post bonding height is minimum but still in excess of the pillar height (and no sort circuits).
  • DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • With reference to FIG. 1, an imaging device in accordance with prior art is shown where a CMOS pixel 101 is bump-bonded to the corresponding detector pixel 102 via bump 6. A bump 6 of the prior art is seen on a semiconductor readout pixel 101 (e.g., a CMOS). The bump 6 is of spherical shape. Under the bump one or more seed metal layers have been deposited. Typically, the seed layers are grown on the CMOS readout wafer 101 via sputtering or evaporation technique. The bump-bonded imaging device of the prior art is shown in FIG. 1 and Table 1 all the elements are described with like numbers in the table below, indicating also the average thickness:
  • TABLE 1 Thickness (um) (example Name Number Material average) Detector pad 1 Pt (Platinum) 0.050 UBM 1 2 Au (Gold) 0.030 UBM 2 3 Ni (Nickel) 0.050 UBM 3 4 Au (Gold) 0.080 Detector passivation 5 AlN 0.150 (Aluminum Nitride) Bump solder 6 SnBi 10.000 (Tin Bismuth) Bump pedestal 7 Ni (Nickel) 1.600 Bump seed bulk 8 Cu (Copper) 0.500 Bump seed adhesion 9 TiW (Titanium 0.040 tangsten) CMOS passivation 10 SiO2 (Silicon 0.800 oxide) CMOS pad 11 Al (Aluminum) 1.200
  • The bump 6 need not be just SnBi, but can be composed by other types of solders like: PbSn, BiPbSn, AgSn, In, or other types of solder. The composition of the bump 6 is important in view of the bonding process. During the bonding process the CMOS readout substrate and the detector substrates are heated, then flipped and bonded together in accordance with a thermal-compression profile which defines the temperature ramp and pressure as a function of time. In some cases the bump is in a reflow state during bonding and in some other cases the bump is merely softened and compressed (for example with In). In radiation imaging the pixel sizes are typically in the range from few micrometers and up to one millimeter. The x-ray imaging devices pixel size where the flip-chip bonding technique is applied is in most cases in the range of 60 um to 400 um and most often the pixel size is in the range of 75 um to 120 um. The bump sizes in the prior art are typically in the range from 20 um (in diameter) to 50 um (diameter). Therefore the pre-bonding distance between the CMOS readout substrate and the detector is of the order of the size of the bump, i.e., between 20 um and 50 um.
  • FIG. 2, shows the end result of the bonding process, in a problematic case in which an imaging device in accordance with prior art is shown. The bumps 6 have been deformed and the distance between the readout substrate and the detector substrate (post bonding height 210) is smaller or much smaller than the original bump height. For example for a bump height of 25 um the post bonding height, i.e., the distance between the CMOS substrate 101 and the detector substrate 102 is typically 10 um to about 15 um. Simultaneously the bumps 6 have acquired an ellipsoid or asymmetrical shape and extend laterally approaching or exceeding the border of the bump in the next pixel. Even worse, “overwetting” may occur during the bonding reflow and the bumps on the CMOS side 101, flow towards one another and create sort circuits as shown in FIG. 2. This may not really happen or be a problem when the pixel size (or pitch between the pixels) is of the order of 100 um or larger (center to center distance 200 between the bumps), because there is enough distance to separate the bumps, even after they get squeezed. However, for small pixel sizes, i.e., for pixels less than 60 um (i.e., it effectively means that the center to center distance 200 between the bumps) or even worse for pixel sizes of less than 35 um, there is not enough or hardly enough separation between the bumps, post bonding. As an example, consider a 25 um pixel size which is typical in intraoral x-ray imaging sensors. With such pixel size, the bump diameter would necessarily have to be approximately 15 um. This would mean that the bump to bump separation 200 would be around 10 um. After the bonding, the post bonding height 210 between the detector 102 and CMOS readout 101 would be 5 um to no more than 10 um, while the squeezed bumps would extend laterally and in many cases the separation 200 is eliminated at the CMOS side (as shown in FIG. 2) or on the detector side (not shown) and the imaging device has sorted bump connections. This results to loss of resolution and reliability issues.
  • Another important consideration is that the post bonding height 210 relates to the input node capacitance of the readout CMOS pixels. A bigger separation 210 between the detector and readout is desirable because it reduces the input node capacitance which means a better signal. The input node capacitance and the gain are related as is well known “V=Q/C”, where (V) is the gain amplitude for a charge (Q) generated inside the detector substrate in response to incident radiation, with input node capacitance (C). With the traditional bump and bonding techniques the post bonding height is not controlled and can actually be quite small for small pixel sizes. Especially in an area of 3 cm×4 mm or 2 cm×3 cm, which is typical in x-ray imaging intraoral sensors, the post bonding height will vary between 5 um and 10 um as a result of parallelism inaccuracies between the two substrates. Therefore the input node capacitance will vary across the imaging device which is another down-side in addition to the risk of pixels been shorted with one another.
  • Finally, trying to control the post bonding height 210 within the range of 5 um to 10 um, brings manufacturing close to the limits (the accuracy) of available bonding equipment.
  • With reference to FIG. 3, a preferred embodiment is illustrated. A semiconductor Application Specific Integrated Circuit (ASIC) readout substrate, preferably a CMOS 101, is bump bonded to a direct conversion detector substrate 102 by means of a capillary bump comprised of a rigid bump leg 8 and a bump solder hat 6 positioned on top of the bum leg 8, preferably grown on the CMOS wafer prior to dicing. Ordinarily the CMOS wafers 101 are manufactured with a circular Al pad 12 (with diameter “j”) at the input of the readout pixel. On the top of the Al pad 12, the CMOS manufacturer will ordinarily have deposited passivation layer SiO2 11. The passivation 11, has openings to the Al pad 12, said opening having a diameter “g”. The invention then deposits the following seed metals on top of the Al pad 12: TiW Bump Seed Adhesion layer 10 with inner diameter “h” and outer diameter “i”, Cu Bump Seed Bulk metal 9 with diameter “i”. These seed metals are typically deposited using sputtering technique. Following these seed metals, the invention grows a Cu pillar (Bump Leg) 8 using electroplating. Following the Cu pillar 8, the invention deposits a Ni Bump Pedestal layer 7 with diameter “b”, also with electroplating. Following the Ni layer 7, the Bump Solder hat 6 is deposited. The Bump Solder hat 6 is preferably Sn, but can also be BiSn, PbSn, BiPbSn, AgSn or other solder types. As shown in FIGS. 3-4, the bump solder hat has a final cross-section shape of a compressed spherical shape, with upper and lower surfaces that are generally flat and parallel, and arcuate end surfaces connecting the upper and lower surfaces.
  • The detector material 102 for converting directly incoming x-ray radiation to electron-hole pairs is preferably fully depleted Si of thickness 0.5 mm to 2 mm. Alternatively, the detector material maybe CdTe or CdZnTe or GaAs. In the preferred embodiment of the current invention the detector is as mentioned Si, in single crystal form. Single crystal Si, fully depleted detector has the benefit of extreme uniformity and planarity and can be manufactured using conventional semiconductor industry's wafer level equipment. As a result very small pixel sizes can be achieved. For example in the preferred embodiment of the current invention an intraoral x-ray imaging sensor comprises Si fully depleted detector of thickness 0.5 mm to 2.0 mm with pixel size 25 um or even smaller, i.e., 10 um to 25 um pixel size.
  • Always with reference to FIG. 3, the Si detector pixel 102 arrives from the factory with an Al Detector Pad 1 of diameter “a”. Through this pad the signal from the direct conversion of x-ray to electron-hole pairs is collected (under the influence of an electric field). On top there is a Detector Passivation layer 2 which is most often SiO2 and with an opening “f”. In accordance with the invention, an Under Bump Metalization (UBM) adhesion layer 3 is next deposited, which is preferably TiW with outer diameter “e” and inner diameter “f”. Then the UBM bulk layer 4 with diameter “d” is deposited, which is preferably Ni and then UBM solder pad which is preferably Au with diameter “c”.
  • The CMOS readout pixel array 101 carries the capillary bumps described above and is then flipped and bonded to the Si detector array with a corresponding number of detector pixels 102, as shown in FIG. 3. Table 2 below specifies example values of the dimensions and aspect ratios described above that the inventors have found are optimal to achieve a pixel to pixel distance of 25 um. In other words the values in Table 2 have been optimized to achieve a pixel size of 25 um in an intraoral Si x-ray imaging sensor.
  • TABLE 2 Thickness (um) EXAMPLE Name Number Material PREFERED EMBODIMENT Detector pad 1 Al 1.200 Detector passivation 2 SiO2 0.800 UBM adhesion 3 TiW 0.040 UBM bulk 4 Ni 0.340 UBM solderpad 5 Au 0.100 Bump solder 6 Sn 6.500 Bump pedestal 7 Ni 1.600 Bump leg 8 Cu 8.000 Bump seed bulk 9 Cu 0.300 Bump seed adhesion 10 TiW 0.015 ASIC/CMOS 11 SiO2 0.800 passivation ASIC/CMOS pad 12 Al 1.200 Width (um) EXAMPLE Name Letter Material PREFERED EMBODIMENT Detector pad a Al 15.000 Bump pedestal b Ni 13.000 UBM solderpad c Au 14.000 UBM bulk d Ni 13.000 UBM adhesion e TiW 12.000 UBM opening f 6.000 ASIC/CMOS Opening g 6.000 Bump seed adhesion h TiW 10.000 Bump leg i Cu 10.400 ASIC/CMOS pad j Al 15.000 k l
  • FIG. 4 schematically shows a Si intraoral sensor cross section of two CMOS readout pixels 101 bump bonded by means of the disclosed capillary bump bonds to their corresponding Si detector pixels 102. The center to center distance 300 of the capillary bumps is 25 um, which defines the pitch or pixel size. This is essentially the resolution of the final image to be displayed. The distance between the pillars 320 is also shown as well as the distance 330 between the solder hats 6. The post bonding height 310 in FIG. 4 is essentially the sum of the pillar leg 8 plus the solder hat 6 plus the bump pedestal 7, i.e., in the exemplified embodiment and with reference to Table 2 the post bonding height is of the order 8+6.5+1.6=16.1 um, reduced by the amount the solder hats 6 have been squeezed. Therefore in practice the post bonding height 310 is between 10 um to 15 um. This distance is sufficient to keep the input node capacitance reasonably low. The current invention implements capillary bump bonds in x-ray imaging devices and this is particularly beneficial when the center to center distance 300 between the bonds is 75 um or less, while simultaneously the post bonding height 310 remains 5 um or more. Preferably the center to center distance 300 between the bonds is 55 um or less, while simultaneously the post bonding height 310 remains 8 um or more. Even more preferably the center to center distance 300 between the bonds is 25 um or less, while simultaneously the post bonding height 310 remains 8 um or more.
  • FIG. 5 shows, in a schematic way, the event of the effect of extreme pressure that maybe applied during bump-bonding. The final shape of the bump solder hat is a compressed ellipsoidal-like structure compressed along its minor axis, with upper and lower surfaces that are generally flat and parallel, and arcuate end surfaces connecting the upper and lower surfaces. As can been seen the bump solder hats 6 have been severely deformed but still there is an sufficient clearance 330 between them. In other words, the sort circuit of the pixels has been avoided unlike the situation of conventional bump bonded x-ray imaging device shown in FIG. 2. Also it can be seen that the pillar 8 and the solder bump pedestal 7 remaining essentially intact (rigid), regardless the fact that the solder hat has suffered severe deformation as a result of the bump-bonding. In other words with this invention one is able to control reliably the post bonding height 310. The lower limit of the post bonding height is the height of the pillar leg 8 plus the bump pedestal 7, i.e., 8+1.6=9.6 um in this example. This feature of a “guaranteed” post bonding height is essential for bump-bonding Si intraoral sensors because the detector substrate 102 and the readout substrate 101 are very large in area. Specifically, with the current invention, substrates of 1 cm×2 cm and up to 5 cm×5 cm can be reliably bonded with pixel sizes (interpixel pitch) that are 55 um or smaller, 35 um or smaller, 25 um or smaller and even as small as 15 um. The intraoral fully depleted Si x-ray imaging device is the preferred embodiment in this exemplified description.

Claims (20)

1. An intraoral x-ray imaging sensor, comprising:
a silicon (Si) detector substrate with detector pixels thereon, said Si detector substrate for converting incident radiation directly to an electronic signal;
a readout substrate with readout pixel circuits thereon for receiving, storing and reading said electronic signals;
rigid bump bonds that interconnect said Si detector substrate and readout substrate to one another.
2. An x-ray imaging device comprising:
a direct conversion detector substrate having detector pixels for collecting electronic signals generated in response to incident radiation; a readout substrate having readout pixels for receiving said electronic signals; and
capillary bump bonds connecting said detector pixels and readout pixels.
3. An x-ray imaging device according to claim 2, wherein a center to center distance (300) between the capillary bump bonds is less than or equal to 75 um and a post bonding height (310) is more than or equal to 5 um.
4. An x-ray imaging device according to claim 2, wherein a center to center distance (300) between the capillary bump bonds is less than or equal to 25 um and a post bonding height (310) is more than or equal to 8 um.
5. An x-ray imaging device according to claim 2, wherein the capillary bump bonds comprise a rigid bump leg (8) and a bump solder hat (6) positioned on top of the bump leg.
6. An x-ray imaging device according to claim 5, wherein a height of the bump leg (8) is 5 um or more.
7. An x-ray imaging device according to claim 5, wherein the bump leg comprises copper (Cu).
8. An x-ray imaging device according to claim 5, wherein the bump solder hat (6) comprises Tin (Sn), Bismuth Tin (BiSn), Lead Tin (PbSn), or Silver Tin (AgSn).
9. An x-ray imaging device according to claim 1, wherein,
the rigid bonds comprise capillary bonds, said capillary bonds being comprised of i) a rigid bump leg (8) and ii) a bump solder hat (6) positioned on top of the bump leg, and
the bump leg being sufficiently rigid that during a bonding process at a temperature from 70 C to 250 C with the bump solder hat being squeezed, the bump leg stays rigid and maintains an initial height of the bump leg.
10. An x-ray imaging device according to claim 9, wherein, the bump solder hat has a cross-section shape of a compressed spherical shape, with upper and lower surfaces that are generally flat and parallel, and arcuate end surfaces connecting the upper and lower surfaces.
11. An x-ray imaging device according to claim 9, wherein, the bump solder hat has a cross-section shape of a cross-section shape of a compressed ellipsoidal structure compressed along a minor axis, with upper and lower surfaces that are generally flat and parallel, and arcuate end surfaces connecting the upper and lower surfaces.
12. An x-ray imaging device according to claim 9, wherein,
each rigid bond further comprises a pad (12), a passivation layer (11) on the pad (12), the passivation layer having openings to the pad (12), said opening having a first diameter (g), a bump seed adhesion layer (10) on the passivation layer, the bump seed adhesion layer (10) having with an inner second diameter (h) and an outer third diameter (i), a bump seed bulk metal (9) with a fourth diameter (i), wherein the rigid bump leg (8) is mounted in contact on the bump seed bulk metal (9), a bump pedestal layer (7) with a fifth diameter (b) in contact on the rigid bump leg, and the bump solder hat in contact on the bump pedestal layer (7),
a width of the pad (12) and a width of the passivation layer (11) are the same, and
and a width of the rigid bump leg (8) is less than the width of the pad (12) and the width of the passivation layer (11).
13. An x-ray imaging device according to claim 12, wherein,
said Si detector substrate comprises, for each said rigid bond, a detector pad (1), through the detector pad the signal from the direct conversion of x-ray to electron-hole pairs is collected, a detector passivation layer (2) in contact on the detector pad (1), an under bump metalization (UBM) adhesion layer (3) on the detector passivation layer (2), a bulk under bump metalization layer (4) against the under bump metalization (UBM) adhesion layer (3), and a solder pad (5) contacting the bulk under bump metalization layer (4) and the bump solder hat,
the solder pad (5) having a sixth diameter (c), the sixth diameter (c) being greater than the fifth diameter (b).
14. An x-ray imaging device according to claim 13, wherein,
a center to center distance (300) between most-adjacent capillary bump bonds is less than or equal to 75 um,
and a post bonding height (310) between opposite surface of the silicon detector substate and the readout substrate is within a range of 5 um to 8 um.
15. An x-ray imaging device according to claim 13, wherein,
a center to center distance (300) between most-adjacent capillary bump bonds is less than or equal to 25 um, and
a post bonding height (310) between opposite main surfaces of the silicon detector substate and the readout substrate is more than or equal to 5 um.
16. An x-ray imaging device according to claim 15, wherein,
the post bonding height of the bump leg (8) is 8 um,
the post bonding height of the solder hat (6) is less than 6.5 um, and
the post bonding height of the bump pedestal (7) is 1.6 um.
17. An x-ray imaging device according to claim 15, wherein the post bonding height of the bump leg (8) is 5 um.
18. An x-ray imaging device according to claim 10, wherein,
a center to center distance (300) between most-adjacent capillary bump bonds is less than or equal to 25 um, and
a post bonding height (310) between opposite main surfaces of the silicon detector substate and the readout substrate is between 10 um and 15 um, and
the post bonding height of the bump leg (8) is between 5 um and 8 um.
19. An x-ray imaging device according to claim 11, wherein,
a center to center distance (300) between most-adjacent capillary bump bonds is less than or equal to 25 um,
a post bonding height (310) between opposite main surfaces of the silicon detector substate and the readout substrate is between 10 um and 15 um, and
the post bonding height of the bump leg (8) is between 5 um and 8 um.
20. An x-ray imaging device according to claim 9, wherein,
a center to center distance (300) between most-adjacent capillary bump bonds is less than or equal to 25 um, and
a post bonding height (310) between opposite main surfaces of the silicon detector substate and the readout substrate is 10 um to 15 um.
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