.

In various embodiments, the second substrate 140 is similar to the substrate 110 in that it may also have one or more thermally conductive layers formed thereon. In one embodiment, the bonding surface 113 of the second substrate 140 is aluminum oxide. In another embodiment, the bonding surface 113 of the second substrate 140 is aluminum nitride. In other embodiments, the bonding surface 113 of the second substrate 140 is a silicon-containing surface, such as silicon (Si) or silicon oxide (SiO₂). However, the bonding surface 113 may also be a silicon-containing surface that includes other silicon-containing materials, such as silicon nitride (Si₃N₄), SiC, silicon carbonitride (SiCN), and others.

FIG. 2 illustrates a cross-sectional view of an example substrate that may be used to form bonded substrate structures described herein, the substrate having a thermally conductive layer of aluminum nitride underlying a thermally conductive layer of aluminum oxide in accordance with embodiments of the invention.

Referring to FIG. 2, a substrate 210 includes an aluminum nitride layer 221 (a specific example of a thermally conductive layer, such as the first thermally conductive layer 120 of FIG. 1) disposed thereon. An aluminum oxide bonding layer 231 is formed (e.g., deposited using a suitable technique, such as ALD, CVD, PE-CVD, etc.). The aluminum oxide bonding layer 231 is a specific example of a thermally conductive layer, such as the second thermally conductive layer 130 of FIG. 1, for example. It should be noted that here and in the following a convention has been adopted for brevity and clarity wherein elements adhering to the pattern [x10] where ‘x’ is the figure number may be related implementations of a substrate in various embodiments. For example, the substrate 210 may be similar to the substrate 110 except as otherwise stated. An analogous convention has also been adopted for other elements as made clear by the use of similar terms in conjunction with the aforementioned numbering system.

The aluminum nitride layer 221 (e.g., an aluminum nitride film) may have a certain structure that contributes to properties such as thermal conductivity and surface roughness. For example, the aluminum nitride layer 221 may have a crystalline structure or multiple regions with a crystalline structure. In some embodiments, the aluminum nitride layer 221 has a columnar polycrystalline structure (as shown), which may contribute to its high thermal conductivity properties. However, the crystalline and polycrystalline structures (such as the columnar polycrystalline structure) may also contribute to increased surface roughness.

The aluminum oxide bonding layer 231 reduces the surface roughness of the aluminum nitride layer 221, which may have the advantage of increasing the bonding capability of the substrate 210 to another substrate. Although aluminum nitride is provided as an example here, it is worth noting that the aluminum oxide bonding layer 231 may also decrease the surface roughness of other thermally conductive materials that could be used instead of the aluminum nitride layer 221, such as diamond, for example.

Additionally, the aluminum oxide bonding layer 231 already includes oxygen, which may advantageously avoid the use of oxygen plasma during a surface activation step. For example, in the absence of the aluminum oxide bonding layer 231, an oxygen plasma activation step may be performed to try and incorporate oxygen at the surface along with the aluminum and nitrogen. However, by using the aluminum oxide bonding layer 231, oxygen is already present. When included an activation step may use nitrogen plasma to create dangling bonds, which may then be followed with a rinse step, (e.g., with deionized (DI) water) to create hydrophilic hydroxyl groups, for example.

FIG. 3 illustrates a cross-sectional view of an example bonded substrate structure with a first substrate bonded to a second substrate, where the first substrate has a first thermally conductive layer underlying a second thermally conductive layer and the second substrate has a third thermally conductive layer underlying a fourth thermally conductive layer in accordance with embodiments of the invention. The bonded substrate structure of FIG. 3 may be a specific example of other bonded substrate structures described herein, such as the bonded substrate structure of FIG. 1, for example. Similarly labeled elements may be as previously described.

Referring to FIG. 3, a bonded substrate structure 300 includes a substrate 310 (i.e., a first substrate) that is bonded to a second substrate 340 using a bonding region 318 that includes a first thermally conductive layer 320 and a second thermally conductive layer 330. The bonded substrate structure 300 is a specific example of the bonded substrate structure 100 of FIG. 1 in which the second substrate 340 also includes thermally conductive layers. Specifically, the second substrate 340 includes a third thermally conductive layer 350 and a fourth thermally conductive layer 360 that acts as a bonding layer of the second substrate 340. In this specific example, the fourth thermally conductive layer 360 forms a bonding surface 313 that is brought in contact with the second thermally conductive layer 330 at a bonding interface 312.

As previously discussed, the substrate 310 has a thermally conductive layer thickness that may be thicker than a bonding layer thickness. In this specific example, the second substrate 340 also has a thermally conductive layer thickness that may be thicker than a bonding layer thickness (whether the same or different than those of the substrate 310). The thicknesses of the second thermally conductive layer 330 and the fourth thermally conductive layer 360 combine to form a total bonding layer thickness 336. In various embodiments, the total bonding layer thickness 336 is less than about 10 nm and is less than about 5 nm in some embodiments.

Similarly, the first thermally conductive layer 320 and the third thermally conductive layer 350 combine to form a total thermally conductive layer thickness 326. In various embodiments, the total thermally conductive layer thickness 326 is greater than about 50 nm, and is greater than about 200 nm in some embodiments. The total thermally conductive layer thickness 326 may also be even higher such as greater than about 500 nm, greater than about 1000 nm, and higher.

Embodiments of this application may be applied to wafer to wafer (W2 W) bonding or die to wafer (D2 W) bonding techniques using direct bonding. Certain aspects of the process flow will be described using FIGS. 4A-4B, 5A-5B, and 6 in which a substrate is illustrated during various stages of a bonding process.

FIGS. 4A and 4B illustrate a substrate during various stages of an example bonding process that may be used to form bonded structures described herein where FIG. 4A shows a substrate with a thermally conductive layer of aluminum nitride formed thereon and FIG. 4B shows the substrate after a layer formation step during which a thermally conductive layer of aluminum oxide is formed over the first thermally conductive layer as a bonding layer in accordance with embodiments of the invention.

Referring to FIG. 4A, a direct bonding process 400 begins with a substrate 410 that has an aluminum nitride layer 421 disposed thereon. For example, the aluminum nitride layer 421 may be formed during a layer formation step 401 as part of the direct bonding process 400. Alternatively, the aluminum nitride layer 421 may already exist on the substrate 410 at the start of the direct bonding process 400. At this stage, the substrate 410 may include various device regions, metallization layers, passivation layers, and/or protection layers (not shown) that have been formed using standard semiconductor fabrication processes prior to the formation of the aluminum nitride layer 421.

The aluminum nitride layer 421 may be deposited using various deposition techniques. In one embodiment, the aluminum nitride layer 421 is deposited using a PVD technique (e.g., a medium temperatures, such as about 200° C.), such as a sputtering process. In some embodiments, the deposition may be performed at low temperatures, for example, at about 25° C., or between about 25° C. and about 300° C. in various embodiments. Various additional steps may be performed to improve the thermal conductivity of the aluminum nitride layer 421, such as a thermal anneal (e.g., at between about 200° C. to about 400° C.) after the deposition of the aluminum nitride layer 421. Other techniques may also be used to form the aluminum nitride layer 421, such as CVD, PE-CVD, etc.

At this stage of the process, the surface of the aluminum nitride layer 421 may have a certain degree of roughness. The surface roughness of the aluminum nitride layer 421 may increase as its thickness increases. This relationship between thickness and roughness may present difficulty for subsequent bonding steps, particularly when thicker layers are desired for their enhanced thermal conductivity properties. For example, the root mean square (RMS) surface roughness may increase from about 1-2 Rq for thinner layers to about 3-4 Rq or more for thicker layers. The inventors found that performing a chemical mechanical polishing on an aluminum nitride layer resulted in increased surface roughness.

Referring now to FIG. 4B, an aluminum oxide bonding layer 431 is formed on the aluminum nitride layer 421 during a layer formation step 402. The formation of the aluminum oxide bonding layer 431 may be performed using various techniques. In one embodiment, the layer formation step 402 uses an ALD process to deposit the aluminum nitride layer 421. An ALD process may allow for precise control of the layer thickness and uniformity. In another embodiment, the layer formation step 402 uses a CVD process. In various embodiments, the thickness of the aluminum oxide bonding layer 431 is thinner than the aluminum nitride layer 421, which may be consistent with a desired balance between thermal conductivity and achievable bond strength.

The aluminum oxide bonding layer 431 decreases the surface roughness (i.e., the surface of the aluminum oxide bonding layer 431 is smoother than that of the aluminum nitride layer 421 and is more suitable for subsequent bonding steps). In some embodiments, additional steps (such as a planarization) may be performed on the aluminum oxide bonding layer 431 to achieve the smoother surface. In other embodiments, the deposition process itself results in a smoother surface. The aluminum oxide bonding layer 431 may have an RMS surface roughness that is suitable for direct bonding processes, for example, less than about 0.5 nm, or between about 0.01 nm and about 0.7 nm. The surface roughness of the aluminum oxide bonding layer 431 may be sufficiently low to advantageously enable strong and void-free bonding in subsequent steps.

FIGS. 5A and 5B illustrate a substrate during various stages of an example bonding process that may be used to form bonded structures described herein where FIG. 5A shows a substrate after a layer formation step during which a thicker aluminum oxide layer is formed and FIG. 5B shows the substrate after a planarization step that planarizes the thicker aluminum oxide layer to form an aluminum oxide bonding layer in accordance with embodiments of the invention.

Referring to FIG. 5A, a direct bonding process 500 may include a layer formation step 502 during which a thick aluminum oxide layer 535 is formed (e.g., deposited, such as with CVD or ALD) on an aluminum nitride layer 521 that is disposed on a substrate 510. For example, the substrate 510 prior to layer formation step 502 may be similar to the substrate 410 at the beginning of the direct bonding process 400. While the thick aluminum oxide layer 535 may be smoother than the aluminum nitride layer 521, it may also substantially retain the topography of the aluminum nitride layer 521 (as shown). However, the thick aluminum oxide layer 535 may have the advantage of being more amenable to a planarization process than the aluminum nitride layer 521.

Turning now to FIG. 5B, the thick aluminum oxide layer 535 is smoothed during a planarization step 503 (which also may result in decreasing its thickness, as shown) to form an aluminum oxide bonding layer 531. In one embodiment, the planarization step 503 includes a CMP process. However, other planarization techniques may be used instead of or in addition to a CMP process. In one embodiment, the planarization step 503 includes an ion milling process. In one embodiment, the planarization step 503 includes a GCIB process. The planarization step 503 may result smoother bonding layer. For example, the aluminum oxide bonding layer 531 may be similar to the aluminum oxide bonding layer 431 of the direct bonding process 400.

FIG. 6 illustrates a substrate after a direct bonding step of an example bonding process that may be used to form bonded structures described herein where an aluminum oxide bonding layer of the substrate is bonded to an additional aluminum oxide bonding layer of a second substrate during the bonding step in accordance with embodiments of the invention.

Referring to FIG. 6, a direct bonding process 600 ends with a direct bonding step 604 during which a substrate 610 is bonded to a second substrate 640 using a bonding region 618. In this specific example, both the substrate 610 and the second substrate 640 may have been formed using the process steps described in FIGS. 4A and 4B and/or in FIGS. 5A and 5B. Specifically, the substrate 610 includes an aluminum nitride layer 621 with an aluminum oxide bonding layer 631 formed thereon. Similarly, an additional aluminum oxide bonding layer 661 is formed on an additional aluminum nitride layer 651 disposed on the second substrate 640.

The aluminum oxide bonding layer 631 and the additional aluminum oxide bonding layer 661 are brought into contact at a bonding interface 612 to bond the substrate 610 to the second substrate 640 and form a bonded substrate structure. In this specific example, the bonding region 618 includes an aluminum oxide region (formed by bonding the aluminum oxide bonding layer 631 and the additional aluminum oxide bonding layer 661 together, which may or may not leave a discernible seam or bonding region) that is between two aluminum nitride regions (from the aluminum nitride layer 621 and the additional aluminum nitride layer 651). However, as will be explained in more detail in FIGS. 7-10, the material composition may vary depending the specific details of a given application.

Direct bonding processes, such as the direct bonding process 600, may include a number of steps that are performed to attain a bonded substrate structure (e.g., a bonded wafer structure as in W2 W processes or a hybrid bonded structure as in D2 W processes). Prior to bonding, the surfaces of the aluminum oxide bonding layer 631 and the additional aluminum oxide bonding layer 661 may be subjected to a surface activation plasma treatment (which may be a nitrogen plasma when aluminum oxide is used, as here) that is followed by rinse (e.g., with DI water) form hydrophilic groups on the exposed surfaces.

The substrates are then aligned face to face and bonded together using van der Waals forces. This process involves bringing the two substrates together and initiating a bond front by striking a region of the upper chuck holding the upper substrate with a striker. The forces of the strike causes the upper substrate to bond with the lower substrate locally, and then the bond front propagates across the substrates. This process may be performed in an ambient environment, or may be performed in vacuum to avoid back pressure from stopping the bonding and introducing voids.

If the surfaces of the substrates being bonded are too rough, the bonding results in the formation of voids (even in vacuum), which causes in poor bond quality (low bond strength). In other words, the bond energy of the bonded assembly may be sufficiently low to allow undesirable separation of the substrates during subsequent processing or during the product lifetime. Because of the smoother surface of the aluminum oxide bonding layer 631 and the additional aluminum oxide bonding layer 661, a void free bond interface may be formed. Embodiments with bonding energy greater than 2.5 J/m²may be formed using embodiments discussed in this application.

Various post-bonding processes may also be performed, such as involving flipping the bonded substrate structure (e.g., the bonded wafers) and thinning and etching the surfaces of the bonded assembly. Other steps may also be performed, such as via formation, wafer dicing, and so on.

FIG. 7 illustrates a cross-sectional view of an example bonded substrate structure with a bonding region that includes an aluminum oxide region between two aluminum nitride regions in accordance with embodiments of the invention. The bonded substrate structure of FIG. 7 may be a specific example of other bonded substrate structures described herein, such as the bonded substrate structure of FIG. 3, for example. Similarly labeled elements may be as previously described.

Referring to FIG. 7, a bonded substrate structure 700 includes a silicon substrate 711 (i.e., a first substrate) that is bonded to a second silicon substrate 741 using a bonding region 718 that includes an aluminum oxide region between two aluminum nitride regions. Specifically, the silicon substrate 711 includes an aluminum nitride layer 721 and an aluminum oxide bonding layer 731 while the second silicon substrate 741 also includes an additional aluminum nitride layer 751 and an additional aluminum oxide bonding layer 761. The aluminum oxide bonding layer 731 and the additional aluminum oxide bonding layer 761 are brought together at a bonding interface 712 to form the bonded substrate structure 700. In this case, the bonding surface of the second silicon substrate 741 is an aluminum oxide surface 715. The bonded substrate structure 700 is a specific example of the bonded substrate structure 300 of FIG. 3.

FIG. 8 illustrates a cross-sectional view of an example bonded substrate structure with a bonding region that includes an aluminum oxide region between a silicon-containing region and an aluminum nitride region in accordance with embodiments of the invention. The bonded substrate structure of FIG. 8 may be a specific example of other bonded substrate structures described herein, such as the bonded substrate structure of FIG. 1, for example. Similarly labeled elements may be as previously described.

Referring to FIG. 8, a bonded substrate structure 800 includes a silicon substrate 811 (i.e., a first substrate) that is bonded to a second silicon substrate 841 using a bonding region 818 that includes an aluminum oxide region between a silicon-containing region and an aluminum nitride region. Specifically, the silicon substrate 811 includes an aluminum nitride layer 821 and an aluminum oxide bonding layer 831 while the second silicon substrate 841 includes a silicon-containing surface 814. The aluminum oxide bonding layer 831 is brought into contact with the silicon-containing surface 814 at a bonding interface 812 to form the bonded substrate structure 800. The bonded substrate structure 800 is a specific example of the bonded substrate structure 100 of FIG. 1.

The silicon-containing surface 814 may be various silicon-containing materials. In various embodiments, the silicon-containing surface 814 is silicon and the second silicon substrate 841 includes a backside power distribution network (BSPDN) in one embodiment. In another embodiment, the silicon-containing surface 814 is silicon oxide. In still another embodiment, the silicon-containing surface 814 is silicon nitride. In yet another embodiment, the silicon-containing surface 814 is silicon carbide. In still yet another embodiment, the silicon-containing surface 814 is silicon carbonitride.

FIG. 9 illustrates a cross-sectional view of an example bonded substrate structure with a bonding region that includes an aluminum oxide region between a silicon oxide region and an aluminum nitride region in accordance with embodiments of the invention. The bonded substrate structure of FIG. 9 may be a specific example of other bonded substrate structures described herein, such as the bonded substrate structure of FIG. 1, for example. Similarly labeled elements may be as previously described.

Referring to FIG. 9, a bonded substrate structure