Semiconductor bump-bonded X-ray imaging device

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 cu-pillar bump bonds.

Claims

1. 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 bump bonds connecting said detector pixels and readout pixels, each bump bond comprising a rigid pillar portion in the form of a rigid bump leg (8) and a bump pedestal layer (7) on the bump leg (8), the bump pedestal layer (7) having a width of 19.5 micrometers or less, wherein said detector pixels comprises an under bump metal bulk layer (4).

2. The x-ray imaging device of claim 1, wherein, said under bump metal bulk layer (4) has a diameter (d) in a range from 2.4 micrometers to 24 micrometers.

3. The x-ray imaging device of claim 2, wherein said under bump metal bulk layer (4) has a thickness in a range from 68 nanometers to 510 nanomenters.

4. The x-ray imaging device of claim 3, wherein said under bump metal bulk layer (4) comprises nickel (Ni).

5. The x-ray imaging device of claim 2, wherein said under bump metal bulk layer (4) comprises nickel (Ni).

6. The x-ray imaging device of claim 1, wherein said under bump metal bulk layer (4) has a thickness in a range from 68 nanometers to 510 nanomenters.

7. The x-ray imaging device of claim 6, wherein said under bump metal bulk layer (4) comprises nickel (Ni).

8. The x-ray imaging device of claim 1, wherein said under bump metal bulk layer (4) comprises nickel (Ni).

9. The x-ray imaging device of claim 1, wherein said rigid bump legs (8) have an average height of 5 micrometers or more, and said bump bonds further comprise bump solder hats (6) positioned on top of the rigid bump legs, an average post bonding height of the solder hats (6) being less than 6.5 micrometers.

10. 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 bump bonds connecting said detector pixels and readout pixels, each bump bond comprising a rigid pillar portion in the form of a rigid bump leg (8) and a bump pedestal layer (7) on the bump leg (8), the bump pedestal layer (7) having a width of 19.5 micrometers or less, wherein said detector pixels comprise an under bump metal solderpad (5).

11. The x-ray imaging device of claim 10, wherein said under bump metal solderpad layer (5) has a diameter (c) in a range from 2.6 micrometers to 25.5 micrometers.

12. The x-ray imaging device of claim 11, wherein said under bump metal solderpad (5) has a thickness in a range from 20 nanometers to 150 nanometers.

13. The x-ray imaging device of claim 12, wherein said under bump metal solderpad (5) comprises gold (Au).

14. The x-ray imaging device of claim 13, wherein said rigid bump legs (8) have an average height of 5 micrometers or more, and said bump bonds further comprise bump solder hats (6) positioned on top of the rigid bump legs, an average post bonding height of the solder hats (6) being less than 6.5 micrometers.

15. The x-ray imaging device of claim 11, wherein said under bump metal solderpad (5) comprises gold (Au).

16. The x-ray imaging device of claim 11, wherein said rigid bump legs (8) have an average height of 5 micrometers or more, and said bump bonds further comprise bump solder hats (6) positioned on top of the rigid bump legs, an average post bonding height of the solder hats (6) being less than 6.5 micrometers.

17. The x-ray imaging device of claim 10, wherein said under bump metal solderpad (5) has a thickness in a range from 20 nanometers to 150 nanometers.

18. The x-ray imaging device of claim 17, wherein said rigid bump legs (8) have an average height of 5 micrometers or more, and said bump bonds further comprise bump solder hats (6) positioned on top of the rigid bump legs, an average post bonding height of the solder hats (6) being less than 6.5 micrometers.

19. The x-ray imaging device of claim 10, wherein said under bump metal solderpad (5) comprises gold (Au).

20. The x-ray imaging device of claim 10, wherein said rigid bump legs (8) have an average height of 5 micrometers or more, and said bump bonds further comprise bump solder hats (6) positioned on top of the rigid bump legs, an average post bonding height of the solder hats (6) being less than 6.5 micrometers.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 presents a bump structure on a readout substrate.

(2) FIG. 2 shows an imaging device with the detector pixel bump bonded to the readout pixel, exemplifying the possible short-circuit issues with current bump-bonding technology.

(3) FIG. 3 shows a pillar with a solder hat on a readout pixel in accordance with an embodiment of the current invention and bonded to a detector substrate

(4) FIG. 4 shows an imaging device in accordance with an embodiment of the current invention.

(5) FIG. 5 shows an imaging device in accordance with an embodiment of the current invention, in which embodiment the post bonding height is minimum but still in excess of the pillar height (and no short circuits).

MODES FOR CARRYING OUT THE INVENTION

(6) With reference to FIG. 1, an imaging device, which can be made using previously known technology, is shown where a CMOS pixel 101 is bump-bonded to the corresponding detector pixel 102 via bump 406. A bump 406 is seen on a semiconductor readout pixel 101 (e.g., a CMOS). The bump 406 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. Such a bump-bonded imaging device 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:

(7) TABLE-US-00001 TABLE 1 Thickness (μm) Name Number Material (example average) Detector pad 401 Pt (Platinum) 0,050 UBM 1 402 Au (Gold) 0,030 UBM 2 403 Ni (Nickel) 0,050 UBM 3 404 Au (Gold) 0,080 Detector passivation 405 AlN 0,150 (Aluminum Nitride) Bump solder 406 SnBi 10,000 (Tin Bismuth) Bump pedestal 407 Ni (Nickel) 1,600 Bump seed bulk 408 Cu (Copper) 0,500 Bump seed adhesion 409 TiW 0,040 (Titanium tangsten) CMOS passivation 410 SiO2 (Silicon 0,800 oxide) CMOS pad 411 Al 1,200 (Aluminum)

(8) The bump 406 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 406 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 μm to 400 μm and most often the pixel size is in the range of 75 μm to 120 μm. The bumps in the prior art of bump bonded x-ray imaging devices are approximately of spherical shape or ellipsoid shape and with sizes typically in the range from 20 μm (in diameter) to 50 μm (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 μm and 50 μm.

(9) FIG. 2 shows an end result of the bonding process with the bump of FIG. 1 in a problematic case. The bumps 406 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 μm the post bonding height, i.e., the distance between the CMOS substrate 101 and the detector substrate 102 is typically 10 μm to about 15 μm. Simultaneously the bumps 406 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 short 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 μm 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 μm (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 μm, there is not enough or hardly enough separation between the bumps, post bonding. As an example, consider a 25 μm 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 μm. This would mean that the bump to bump separation 200 would be around 10 μm. After the bonding, the post bonding height 210 between the detector 102 and CMOS readout 101 would be 5 μm to no more than 10 μm, 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 shorted bump connections. This results to loss of resolution and reliability issues.

(10) 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 cm or 2 cm×3 cm, which is typical in x-ray imaging intraoral sensors, the post bonding height will vary between 5 μm and 10 μm 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.

(11) Finally, trying to control the post bonding height 210 within the range of 5 μm to 10 μm, brings manufacturing close to the limits (the accuracy) of available bonding equipment. With reference to FIG. 3, an embodiment of the invention 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 cu-pillary bump comprising a rigid bump leg 8 and a bump solder hat 6 positioned on top of the bump leg 8, preferably grown on the CMOS wafer prior to dicing. Thus, the bump legs 8 form the rigid portions of the bump bond in these embodiments. 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 SiO.sub.2 11. The passivation 11, has openings to the Al pad 12, said opening having a diameter “g”. In an embodiment, then the following seed metals are deposited 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, a Cu pillar (Bump Leg) 8 is grown using electroplating in the embodiment of FIG. 3. This can be made for example such that the pillars are grown in openings in a photoresist layer provided on the surface of the wafer, as is known in the art. Following the Cu pillar 8, the embodiment 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.

(12) 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 μm or even smaller, e.g., 5 μm to 20 μm pixel size, such as 10 μm to 15 μm.

(13) 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 SiO.sub.2 and with an opening “f”. In accordance with the embodiment, 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”.

(14) The CMOS readout pixel array 101 carries the cu-pillary 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 μm. In other words the values in Table 2 have been optimized to achieve a pixel size of 25 μm in an intraoral Si x-ray imaging sensor. However, other parameter ranges can also be used in accordance with the embodiments. Further, the optimal ranges may differ from application to application and when the pixel size is different.

(15) TABLE-US-00002 TABLE 2 Thickness (μm) EXAMPLE PREFERED Name Number Material EMBODIMENT Detector pad 1 Al 1,200 Detector passivation 2 SiO.sub.2 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 (from 4 to 7) Bump pedestal 7 Ni 1,600 (from 1 to 2) Bump leg 8 Cu  8,000 (from 5 to 12) Bump seed bulk 9 Cu 0,300 Bump seed adhesion 10 TiW 0,015 ASIC/CMOS passivation 11 SiO.sub.2 0,800 ASIC/CMOS pad 12 Al 1,200 Width (μm) EXAMPLE PREFERED Name Letter Material EMBODIMENT Detector pad a Al 15,000 (from 13 to 17) Bump pedestal b Ni 13,000 (from 12 to 16) UBM solderpad c Au 14,000 (from 13 to 17) UBM bulk d Ni 13,000 (from 12 to 16) UBM adhesion e TiW 12,000 (from 10 to 14) UBM opening f — 6,000 (from 4 to 8)  ASIC/CMOS Opening g — 6,000 (from 4 to 8)  Bump seed adhesion h TiW 10,000 (from 7 to 13)  Bump leg i Cu 10,400 (from 7 to 13)  ASIC/CMOS pad j Al 15,000 (from 12 to 18) k — — l — —

(16) According to another embodiment, the dimensions are within 20% to 150% of the nominal values indicated in Table 2. In another embodiment, the dimensions are within 50% to 125% of the values indicated in Table 2. In a further embodiment, the dimensions are within 75% to 110% of the values indicated in Table 2. In even further embodiments, the above-mentioned parameter ranges or their combinations apply otherwise but the thickness of the bump leg 8 layer is at least 4 μm, at least 5 μm or at least 6 μm.

(17) In an embodiment, the cu-pillary bump comprises a bump leg 8 having a thickness (height) of 5-12 μm and a width i of 7-13 μm, and a plurality of other layers with their total thickness of at least 2 μm, such as 3-7 μm.

(18) In an embodiment, the total height of the bump bond is greater than the general width (i) of the rigid portion of the bump bond.

(19) FIG. 4 schematically shows a Si intraoral sensor cross section of two CMOS readout pixels 101 bump bonded by means of the disclosed cu-pillary bump bonds to their corresponding Si detector pixels 102. The center to center distance 300 of the cu-pillary bumps is 25 μm, 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 μm, reduced by the amount the solder hats 6 have been squeezed. Therefore in practice the post bonding height 310 is between 10 μm to 15 μm. This distance is sufficient to keep the input node capacitance reasonably low. The current embodiments implement cu-pillary bump bonds in x-ray imaging devices and this is particularly beneficial when the center to center distance 300 between the bonds is 75 μm or less, while simultaneously the post bonding height 310 remains 5 μm or more. Preferably the center to center distance 300 between the bonds is 55 μm or less, while simultaneously the post bonding height 310 remains 8 μm or more. Even more preferably the center to center distance 300 between the bonds is 25 μm or less, while simultaneously the post bonding height 310 remains 8 μm or more.

(20) FIG. 5 shows, in a schematic way, the event of the effect of extreme pressure that may be 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 a sufficient clearance 330 between them. In other words, the short 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 these embodiments 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 μm in this example. This feature of a “guaranteed” post bonding height is beneficial for bump-bonding Si intraoral sensors because the detector substrate 102 and the readout substrate 101 are very large in area. Specifically, with the described embodiments, 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 μm or smaller, 35 μm or smaller, 25 μm or smaller and even as small as 15 μm. The intraoral fully depleted Si x-ray imaging device is the preferred embodiment in this exemplified description.