Sacrificial alignment ring and self-soldering vias for wafer bonding

Abstract

A method of bonding a first substrate to a second substrate, wherein the first substrate includes first electrical contacts on a top surface of the first substrate, and wherein the second substrate includes second electrical contacts on a bottom surface of the second substrate. The method includes forming a block of polyimide on the top surface of the first substrate, wherein the block of polyimide has a rounded upper corner, and vertically moving the top surface of the first substrate and the bottom surface of the second substrate toward each other until the first electrical contacts abut the second electrical contacts, wherein during the moving, the second substrate makes contact with the rounded upper corner of the polyimide causing the first and second substrates to move laterally relative to each other.

Claims

1. A method of bonding a first substrate to a second substrate, wherein the first substrate includes first electrical contacts on a top surface of the first substrate, and wherein the second substrate includes second electrical contacts on a bottom surface of the second substrate, the method comprising: forming a block of polyimide on the top surface of the first substrate, wherein the block of polyimide has a rounded upper corner; and vertically moving the top surface of the first substrate and the bottom surface of the second substrate toward each other until the first electrical contacts abut the second electrical contacts, wherein during the vertical moving, the second substrate makes contact with the rounded upper corner of the polyimide causing the first and second substrates to move laterally relative to each other.

2. The method of claim 1, wherein the polyimide block has a ring shape that encircles the first electrical contacts.

3. The method of claim 1, further comprising: forming a layer of inorganic material disposed between the polyimide block and the first substrate.

4. The method of claim 3, wherein the inorganic material is one of oxide and nitride.

5. The method of claim 1, wherein each of the first electrical contacts includes SnCu material.

6. The method of claim 5, wherein the SnCu material includes between 0.5% to 5% Cu as a percentage of overall composition.

7. The method of claim 5, wherein each of the first electrical contacts further includes a metal block in contact with the SnCu material.

8. The method of claim 5, further comprising: applying heat to the first and second electrical contacts so that a solder connection is formed between each of the first electrical contacts and one of the second electrical contacts.

9. The method of claim 1, further comprising: removing the polyimide block after the moving.

10. The method of claim 1, wherein the first substrate includes a third electrical contact on the top surface, the method further comprising: forming an aluminum pad on the third electrical contact, wherein a portion of the polyimide block is directly on the aluminum pad; and connecting a wire to the aluminum pad.

11. The method of claim 1, wherein the forming of the polyimide block comprises: forming a polyimide layer over the top surface of the first substrate; exposing portions of the polyimide layer to light; and removing the portions of the polyimide layer that were exposed to light.

12. A method of bonding a first substrate to a second substrate, wherein the first substrate includes first electrical contacts on a top surface of the first substrate, and wherein the second substrate includes second electrical contacts on a bottom surface of the second substrate, the method comprising: forming a first material over the top surface of the first substrate and over the first electrical contacts; forming vias extending through the first material to expose the first electrical contacts; forming SnCu material in the vias; forming a layer of polyimide over the top surface of the first substrate; selectively removing one or more portions of the layer of polyimide, leaving a block of the polyimide over the top surface of the first substrate, wherein the block of polyimide has a rounded upper corner; and vertically moving the top surface of the first substrate and the bottom surface of the second substrate toward each other until the SnCu material abuts the second electrical contacts, wherein during the vertical moving, the second substrate makes contact with the rounded upper corner of the polyimide causing the first and second substrates to move laterally relative to each other.

13. The method of claim 12, wherein the polyimide block has a ring shape that encircles the first electrical contacts.

14. The method of claim 12, further comprising: forming a layer of inorganic material between the polyimide block and the first substrate.

15. The method of claim 14, wherein the inorganic material is one of oxide and nitride.

16. The method of claim 12, wherein the SnCu material includes between 0.5% to 5% Cu as a percentage of overall composition.

17. The method of claim 12, further comprising: applying heat to the SnCu material so that a solder connection is formed between the SnCu material and the second electrical contacts.

18. The method of claim 12, wherein the forming of the SnCu material comprises: forming discrete, alternating layers of Sn material and Cu material; and annealing the alternating layers so that the Sn material layers alloys with the Cu material layers.

19. The method of claim 12, wherein the forming of the SnCu material comprises: forming a layer of SnCu alloy over the first material and in the vias; and removing the layer of SnCu alloy over the first material while leaving the SnCu alloy in the vias.

20. The method of claim 12, wherein the first substrate includes a third electrical contact on the top surface, the method further comprising: forming an aluminum pad on the third electrical contact, wherein a portion of the polyimide block is directly on the aluminum pad; and connecting a wire to the aluminum pad.

21. The method of claim 12, further comprising: removing the polyimide block after the moving.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1-9 are cross sectional side views illustrating the steps in forming the polyimide alignment structure.

(2) FIGS. 10-15 are side cross sectional side views illustrating the steps of aligning and bonding the die to the wafer.

DETAILED DESCRIPTION OF THE INVENTION

(3) The present invention is an alignment and electrical connection technique and alignment structure for bonding the bottom surface of a die to a top surface of a wafer. The wafer can include a substrate 10 on which circuitry and other conductive elements are formed and is shown in FIG. 1 (without showing the circuitry formed thereon), and includes vertically extending metal contacts 12 at the substrate's top surface. To facilitate bonding and the electrical connections between the metal contacts 12 and a die, a layer of insulation material 14 (e.g., inter-layer dielectric IMD) is formed over the structure and planarized, as shown in FIG. 2. Vias 16 are formed in the insulation 14, with each via 16 extending down to and exposing one of the metal contacts 12, as shown in FIG. 3. The vias 16 can be formed using a photolithography process, where photo resist is formed over the insulation 14 and selectively exposed and developed using a mask. Selective portions of the photo resist are then removed, exposing the insulation 14 above each metal contact. Then an etch is performed on the exposed portions of insulation 14 to create the vias 16 therein.

(4) A layer of SnCu alloy is deposited over the structure, filling the vias 16. The SnCu alloy is then dry etched or polished back using a chemical mechanical polish (CMP) so that the SnCu alloy is removed from the top surface of the insulation 14, but leaves the vias filled with SnCu contacts 18, as shown in FIG. 4. A passivation layer 20 (of inorganic material such as oxide or nitride) is formed over the structure. Aluminum pads 22 can be formed over some of the SnCu contacts 18, by selectively etching through the passivation layer 20, covering the structure with aluminum, and performing an aluminum etch to remove the aluminum except where the passivation layer was etched, as shown in FIG. 5.

(5) A second passivation layer 24 is formed over the structure, as shown in FIG. 6. This second passivation layer is formed of polyimide. Selective portions 24a of the polyimide 24 are exposed to photons in a photolithography process, as shown in FIG. 7. Alternately, a whole wafer contact mask could be used to do this patterning. The exposed portions 24a of the polyimide 24 are removed, leaving a ring 24b of the polyimide surrounding the SnCu contacts 18 which will bonded to the die, as shown in FIG. 8. The ring of polyimide 24b is cured, rounding its edges so that its upper corners 24c are tapered. The passivation layer 20 inside the ring is removed through an etch, exposing the SnCu contacts 18, as shown in FIG. 9. The resulting alignment structure 26 surrounding the SnCu contacts includes a ring of polyimide 24b over a ring of the passivation material 20, which together have a total height of H relative to the SNCu contacts 18. In a non-limiting example, the total height H of the alignment structure can be 15-20 m.

(6) Using mechanical-robot assisted rough alignment, a die 30 (e.g., a 300 mm die with bottom surface electrical contacts 32, preferably made of copper) is placed over and aligned as best as possible to a wafer for bonding. As shown in FIG. 10, there may be some initial lateral misalignment. As shown in FIGS. 11-13, as the die 30 is lowered in a misaligned state, it makes contact with the tapered corner 24c of the polyimide 24b of the alignment structure 26, where the polyimide absorbs the impact (FIG. 11) and the sloped profile of the tapered corner 24c of the polyimide deflects the die laterally (FIG. 12) guiding it toward its proper alignment as it reaches wafer (FIG. 13). After final placement, the SnCu contacts 18 of the wafer are in electrical contact with corresponding contacts 32 on the die 30. A certain amount of force is preferably applied, pressing the die 30 against the wafer, and heat is applied until the SnCu contacts 18 of the wafer auto-solder to the copper contacts 32 of the die 30 (i.e., by creating solder bonds 34 between contacts 18 and 32 as shown in FIG. 14). After cooling, the bonding is complete, with solder bonds 34 connecting the wafer contacts 18 and die contacts 32 together. A wire 36 can be connected to the aluminum contact 22 after the die 30 is bonded in place, as shown in FIG. 15.

(7) The use of polyimide to guide the die in place (with the proper mechanical alignment) has many advantages. It allows for reliably bonding the die to the wafer with properly formed electrical connections even with smaller device geometries. The polyimide is photosensitive-light developable in tall and non-brittle alignment structures such as rings. The photosensitive polyimide develops away and may be used without an extra etch. The polyimide further serves as a mask layer to etch the passivation layer to expose the SnCu contacts. The alignment structure 26 includes both an inorganic base (i.e., passivation layer 20) plus an organic upper portion (i.e., a polyimide top portion 24b as the elastic material to make contact with the die, absorb some of the shock of the initial contact, and provide the alignment correcting lateral force). The tapered sidewall 24c of the polyimide 24b effectively guides the die 30 while minimizing damage to either structure. The alignment tolerance of the via to via connection is greater than the variation in the opening and alignment ring critical dimension limits. In some cases, there may be some damage to the ring and the edge vias, which is why the polyimide 24b is preferably sacrificial in the sense that it is preferably removed in its entirety after bonding. Moreover, it may be desirable in some applications for one or more of the electrical contacts adjacent to the polyimide ring to be dummy contacts and not actually used for electrical signals (i.e., no electrical connections).

(8) The use of SnCu alloy contacts for auto-soldering has many advantages as well. It reliably provides electrical connection formation for high density bonding (e.g. thousands of bonds per die), and is compatible with the polyimide alignment structures. The SnCu contacts form solder connections to the counterpart copper contacts of the die simply by applying heat (and optionally some compressive force). The SnCu material has a melting point low enough to allow self-soldering between the wafer and the die, without requiring higher temperatures that could damage the wafer or the die. The relative percentage of Sn to Cu can vary. Too much Sn as a percentage will make CMP difficult, and too much Cu as a percentage will make the etch difficult. It has been determined that 0.5-5% Cu and 95-99.5% Sn as percentage of overall composition ranges strike an ideal balance of percentage of overall composition between CMP processing, etch processing, and effective self-solder formation at sufficiently low enough temperatures. While forming contacts 18 using a homogenous deposited SnCu alloy material is preferable, it is also possible to form contact 18 by depositing alternating and repeating discrete layers of Sn and Cu. Afterward, an anneal would be performed so the Sn is alloyed with the Cu.

(9) It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of any claims. For example, references to the present invention herein are not intended to limit the scope of any claim or claim term, but instead merely make reference to one or more features that may be covered by one or more of the claims. Materials, processes and numerical examples described above are exemplary only, and should not be deemed to limit the claims. Further, while the polyimide alignment structure may be a continuous ring around the location at which the die will be placed, it need not be ring shaped (e.g., could be square or any other shape matching or compatible with that of the die), and it need not be continuous (e.g., it could be one or more individual separate blocks of polyimide alignment structures having a partial ring shape, having multiple blocks of polyimide on opposite sides of the contacts, etc.). The self-soldering solution using SnCu can be implemented without implementing the polyimide alignment structure, and vice versa, however together they provide significant advantages over prior art techniques of die/wafer bonding. Lowering the die onto the wafer includes vertically moving the die bottom surface toward the wafer top surface. However, placing these surfaces in contact can broadly be accomplished by vertically moving the two surfaces toward each other, which can be accomplished by moving the die toward a stationary wafer, moving the wafer toward a stationary die, or moving both the die and wafer toward each other at the same time. Finally, the polyimide alignment structure could be implemented without the underlying passivation layer 22.

(10) It should be noted that, as used herein, the terms over and on both inclusively include directly on (no intermediate materials, elements or space disposed there between) and indirectly on (intermediate materials, elements or space disposed there between). Likewise, the term adjacent includes directly adjacent (no intermediate materials, elements or space disposed there between) and indirectly adjacent (intermediate materials, elements or space disposed there between), mounted to includes directly mounted to (no intermediate materials, elements or space disposed there between) and indirectly mounted to (intermediate materials, elements or spaced disposed there between), and electrically coupled includes directly electrically coupled to (no intermediate materials or elements there between that electrically connect the elements together) and indirectly electrically coupled to (intermediate materials or elements there between that electrically connect the elements together). For example, forming an element over a substrate can include forming the element directly on the substrate with no intermediate materials/elements there between, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements there between.