SEMICONDUCTOR BUMP-BONDED X-RAY IMAGING DEVICE
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.
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 ARTThe 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:
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- 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 INVENTIONThe 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).
With reference to
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.
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
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
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
Claims
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.
Type: Application
Filed: Mar 25, 2014
Publication Date: Oct 1, 2015
Inventors: Konstantinos Spartiotis (Expoo), Henri Tapio Nykanen (Helsinki), Limin Lin (Espoo), Tuomas Heikki Elmeri Lahtinen (Vantaa), Pasi Juhani Laukka (Espoo)
Application Number: 14/224,594