# Patent

## Title
Bump structure having a single side recess

## Abstract

A bump structure includes a first end, and a second end opposite the first end. The bump structure further includes a first side connected between the first end and the second end. The bump structure further includes a second side opposite the first side. The second side is connected between the first end and the second end, and the second side comprises a recess for a reflowed solder material to fill.

## Background
CROSS REFERENCES TO RELATED APPLICATIONS
The present application is a continuation-in-part of U.S. application Ser. No. 13/192,302, filed Jul. 27, 2011, entitled “BUMP STRUCTURES,” which is related to U.S. application Ser. No. 13/035,586, entitled “EXTENDING METAL TRACES IN BUMP-ON-TRACE STRUCTURES,” filed on Feb. 25, 2011. The present application is also related to U.S. application Ser. No. 13/095,185, entitled “REDUCED-STRESS BUMP-ON-TRACE (BOT) STRUCTURES,” filed on Apr. 27, 2011. All U.S. applications are incorporated herein by reference in their entireties.


BACKGROUND
Bump-on-Trace (BOT) structures have been used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is relatively low. However, there are technical challenges related to BOT structures.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1A and 1B illustrate cross-sectional views of a package structure in accordance with an embodiment.
FIGS. 2A and 2B illustrate top and cross-sectional views of a bump-on-trace (BOT) region, in accordance with some embodiments.
FIGS. 3A and 3B illustrate cross-sectional and top views of a metal bump, in accordance with some embodiments.
FIG. 3C illustrates protruding solder shorting with a neighboring metal trace, in accordance with some embodiments.
FIGS. 4A-4E illustrate various embodiments of metal bumps with recess regions to reduce solder protrusion, in accordance with some embodiments.
FIGS. 5A-51 are top views of a bump structure having a single side recess in accordance with some embodiments.
FIG. 6 is a top view of a bump structure having a single side recess in accordance with some embodiments.
FIG. 7 is a top view of an array of bump structures having a single side recess on a die in accordance with some embodiments.
FIG. 8 is a top view of a portion of an array of bump structures having a single side recess on a die in accordance with some embodiments.
FIG. 9 is a top view of a die having bump structures having a single side recess in accordance with some embodiments.
FIG. 10A is a top view of a of a BOT region in accordance with some embodiments.
FIG. 10B is a cross-sectional view of a BOT region in accordance with some embodiments.
FIG. 11A is a cross-sectional view of a BOT region in accordance with some embodiments.
FIG. 11B is a perspective view of a bump structure having a single side recess in accordance with some embodiments.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative, and do not limit the scope of the disclosure.
Cross-sectional views of a package structure comprising a Bump-on-Trace (BOT) structure 150 is provided in FIGS. 1A and 1B, in accordance with some embodiments. The package structure 150 includes work piece 100 bonded to work piece 200. Work piece 100 may be a device die that includes active devices such as transistors (not shown) therein, although work piece 100 may also be an interposer that does not have active devices therein. In an embodiment wherein work piece 100 is a device die, substrate 102 may be a semiconductor substrate such as a silicon substrate, although it may include other semiconductor materials. Interconnect structure 104, which includes metal lines and vias 106 formed therein and connected to the semiconductor devices, is formed on substrate 102. Metal lines and vias 106 may be formed of copper or copper alloys, and may be formed using damascene processes. Interconnect structure 104 may include a commonly known inter-layer dielectric (ILD, not shown) and inter-metal dielectrics (IMDs) 108. IMDs 108 may comprise low-k dielectric materials, and may have dielectric constants (k values) lower than about 3.0. The low-k dielectric materials may also be extreme low-k dielectric materials having k values lower than about 2.5.
Work piece 100 may further include under-bump metallurgy (UBM) layer 110 and a copper post (or pillar) 112 on UBM layer 110. Throughout the description, the copper post 112 is also referred to as a copper-containing bump or metal bump. Although copper post 112 is used as an example in the description here and below, other types of metal bumps, such as solder bumps, may also be used in place of copper post 112. The UBM layer 110 is disposed on a metal pad 105, which is part of interconnect structure 104. Between the interconnect structure 104 and the UBM layer 110 not contacting the metal pad 105, there is a passivation layer 107. In some embodiments, the passivation layer 107 is made of polyimide.
Work piece 200 may be a package substrate, although it may be other package components such as interposers, for example. Work piece 200 may include metal lines and vias 202 connecting metal features on opposite sides of work piece 200. In an embodiment, metal trace(s) 210 on the topside of work piece 200 is electrically connected to ball grid array (BGA) balls 212 on the bottom side of work pieces 200 through metal lines and vias 202. Metal lines and vias 202 may be formed in dielectric layers 214, although they may also be formed in a semiconductor layer (such as a silicon layer, not shown) and in the dielectric layers that are formed on the semiconductor layer.
Metal trace 210 is formed over a top dielectric layer in dielectric layers 214. Metal trace 210 may be formed of substantially pure copper, aluminum copper, or other metallic materials such as tungsten, nickel, palladium, gold, and/or alloys thereof. FIG. 1A shows that the copper post (or metal bump) 112 has a length of L1, in accordance with some embodiments. Work pieces 100 and 200 are bonded to each other through solder layer 220, which may be formed of a lead-free solder, a eutectic solder, or the like. Solder layer 220 is bonded to, and contacts, the top surfaces of metal trace 210 and copper post 112.
FIG. 1B illustrates a cross-sectional view of the package structure 150 shown in FIG. 1A, wherein the cross-sectional view is obtained from the plane crossing line 2-2 in FIG. 1A. As shown in FIG. 1B, solder layer 220 may also contact the sidewalls of metal trace 210 after solder reflow. The reflowed solder layer 220 may also move along surfaces 113 of copper post 112 and cover portions or all of surfaces 113 (not shown). In some embodiments, there is a capping layer between copper post 112 and solder layer 220. The capping layer could be used to prevent the formation of inter-metallic compound(s) from copper and solder. In some embodiments, the capping layer includes nickel (Ni). Exemplary details of materials and processes used in forming work piece 100 are described in U.S. application Ser. No. 13/095,185, entitled “REDUCED-STRESS BUMP-ON-TRACE (BOT) STRUCTURES,” filed on Apr. 27, 2011, which is incorporated herein by reference in its entirety.
After the bonding of work pieces 100 and 200, a mold underfill (MUF) (not shown) may be filled into the space between work pieces 100 and 200, in accordance with some embodiments. Accordingly, a MUF may also be filled into the space between neighboring metal traces 210. Alternatively, no MUF is filled, while air fills the space between work pieces 100 and 200, and fills the space between neighboring metal traces 210. FIG. 1B shows that the copper post (or metal bump) 112 has a width of W1, in accordance with some embodiments. FIG. 1B also shows that the metal trace 210 has a width W2, in accordance with some embodiments.
In some other embodiments, the ratio of L1/W1 is greater than 1. In some embodiments, the ratio of L1/W1 is equal to or greater than about 1.2. In some embodiments, the L1 is in a range from about 10 μm to about 1000 μm. In some embodiments, W1 is in a range from about 10 μm to about 700 μm. In some embodiments, W2 is in a range from about 10 μm to about 500 μm. The structure as shown in FIGS. 1A and 1B is referred to as being a BOT structure, because solder layer 220 is formed directly on the top surface and sidewalls of metal trace 210, and not on a metal pad that has a width significantly greater than width W2 of metal trace 210. In some embodiments, the ratio of W1/W2 is in a range from about 0.25 to about 1.
FIG. 2A shows a top view of a BOT region 200, in accordance with some embodiments. FIG. 2A shows a number of metal bumps 201-208 over metal traces 211-218. The metal traces provide the function of interconnection and connects metal bumps to one another. For example, metal trace 211 connects metal bump 201 and metal bump 202, in accordance with some embodiments. Metal bumps 201-208 include copper post 112, UBM layer 110 and solder layer 220 described above.
FIG. 2B shows a cross-sectional view of the BOT region 200 cut along A-A line, in accordance with some embodiments. FIG. 2B shows that metal bumps 201, 203, and 205 are placed on metal traces 211, 213, and 215. FIG. 2B also shows cross-sections of metal traces 212 and 214. Cross sections of metal bumps 201, 203, and 205 show a UBM layer 110, copper posts 112 with solder layer 220. The solder layer 220 wrap around the exposed surfaces of metal traces 211, 213 and 215 after reflow. FIG. 2B also shows that the solder layer 220 between copper post 112 of metal bump 201 protrudes beyond surface 231 of copper post 112 with a distance “D1”. As mentioned above, the reflowed solder of the solder layer 220 may also move along surfaces 113 of copper post 112 and cover portions or all of surfaces 113. Due to the pressure exerted by the work piece 100 on work piece 200, the surface 221 of solder layer 220 extends beyond the surface 113 of the copper post with a maximum distance of “D1.” Larger D1 reduces the distance D2 between the surface 113 and the neighboring metal trace surface 232 and increases the risk of shorting between metal traces 211 and 212. In addition, D2 may be shortened by mis-alignment or by alignment error. With shrinking feature sizes and pitches, minimizing D1 is important to improve yield. In some embodiments, D2, the minimum distance between a bump and a neighboring metal trace, is specified to be equal to or greater than about 0.1 μm to avoid shorting. In some embodiments, the distance between the edge of copper post 112 to a closest edge of a metal trace 212, or D1+D2, is in a range is equal to or greater than about 1 μm. In some other embodiments, the distance is equal to or greater than about 5 μm.
FIG. 3A shows a cross-sectional view of metal bump 201 before it is coupled to metal trace 211 and before solder reflow, in accordance with some embodiments. FIG. 3A shows that metal bump 201 includes a copper post 112 and a solder layer 220. There is an optional capping layer 126 between the copper post 112 and the solder layer 220. The cap layer 126 could act as a barrier layer to prevent copper in the Cu pillar 125 from diffusing into a bonding material, such as solder alloy, that is used to bond the substrate 101 to external features.
The solder layer 220 and copper post 112 may be formed by plating over the UBM layer 110, in accordance with some embodiments. Prior to the solder reflow, the solder layer 220 and copper post 112 share the same surfaces 113. FIG. 3B shows a top view of metal bump 201 of FIGS. 2A and 2B before bonding and reflow, in accordance with some embodiments. FIG. 3B shows the outline 310 of copper post 112. Outline 310 is also the outline for UBM layer 110. The copper post 112 shown in FIG. 3B is in the shape of a race track with two hemispheres (M sections) coupled to a rectangle (N section). The diameter of the two hemispheres W1 is the same as the width of the rectangle, which is also shown in FIG. 1B. The total length of the metal bump 201 is L1, as shown in FIG. 1A, and the length of the rectangle is LR. Although the example in FIG. 3B is in the shape of a race track, other elongated shapes, such as oval shape, etc, are also applicable.
After reflow and under the pressure of being pressed against metal trace 211, solder layer 220 tends to protrude the most near the center region of the rectangle (N section). This could be due to less surface tension on the side walls of the center region (N section), in comparison to the edge sections (M sections).
As described above, the protrusion of the solder layer 220 (with a maximum protruding distance D1) increases the risk of shorting. FIG. 3C shows a cross-sectional view of a BOT structure, in accordance with some embodiments. FIG. 3C shows that the solder layer 220 of a metal bump 201* making contact with a neighboring metal trace 212* due to solder extrusion and some alignment error, which is expected due to process variation. As a result, it is desirable to reduce the protruding distance D1 to reduce of risk of shorting between metal bump 201, metal trace 211, and metal trace 212.
FIG. 4A shows a top view of a metal bump 400, in accordance with some embodiments. The metal bump is similar to metal bumps 201 described above. FIG. 4A shows an outline 401 of outer boundary of the UBM layer 100, which also significantly match the outer boundaries of copper post 112 and solder layer 220 before solder reflow. FIG. 4A shows that in order to reduce solder protrusion to reduce the shortest distance between the metal bump, such as metal bump 201, and a neighboring metal trace, such as metal trace 212, the width of the rectangular section (N section) of the metal bump is reduced from W1 to W4. Each side of the rectangular section is reduced by a width of W3. FIG. 4B shows a side view of metal bump 400, in accordance with some embodiments. Metal bump 400 includes a solder layer 220, a copper post 112 and an UBM layer 110, in accordance with some embodiments. Alternatively, metal bump 400 could refer only to the copper post 112 or the copper post 112 with the UBM layer 110. The height of the solder layer 220 is H1 and the height of the copper post is H2. The height of the UBM layer is H3. In some embodiments, H1 is in a range from about 10 μm to about 50 μm. In some embodiments, H2 is in a range from about 10 μm to about 70 μm. In some embodiments, H3 is in a range from about 3 μm to about 15 μm.
As mentioned above, solder layer 220 tends to protrude in the middle section (or N section). By reducing the width of the middle section, the reflowed solder will fill the recess space created by the reduced width of the middle section (or rectangular section N). As a result, the risk of shorting due to protruding solder material can be reduced and yield can be improved. Such reduction to reduce shorting and to improve yield is important for advanced packaging. In some embodiments, the recess region is region R, which is shown in FIG. 4C. FIG. 4A shows that there are two recess regions R for metal bump 400. Recess region R includes recess region A of the solder layer 220, recess region B of copper post 112, and recess region C of the UBM layer 110, as shown in FIG. 3C in accordance with some embodiments. Equation (1) shows the volume of region R, in accordance with some embodiments.

Rvolume=W3×LR×(H1+H2+H3)  (1)

Although reducing W1 could reduce the risk of shorting, W1 cannot be reduced too much to prevent insufficient coverage of solder on the metal trace underneath. In addition, small W1 would lead to small UBM area, which could increase the stress at the interface 118 (as shown in FIGS. 1A and 1B) next to IMDs 108 and could result in interfacial delamination. Such interfacial delamination is a reliability concern and can affect yield. In some embodiments, the maximum width of the recess region W3 is in a range from about 1 μm to about 30 μm. In some embodiments, the ratio of W3 to W1 is in a range from about 0.02 to about 0.5. In some embodiments, the volume ratio of recess regions R to the solder layer 220 of the metal bump 400 is greater than or equal to about 0.01, which means that the recess regions R for metal bump 400 is equal to or greater than about 1% of the volume of the solder layer 220. In some other embodiments, the volume ratio of recess regions R to the solder layer 220 of the metal bump 400 is equal to or less than about 0.1. In some embodiments, a ratio of the surface areas (or cross-sectional areas) of recess regions R, as viewed in FIG. 4A, to the surface area (or cross-sectional area) of bump 400 is equal to or greater than about 0.01. In some other embodiments, a ratio the surface areas (or cross-sectional areas) of recess regions R, as viewed in FIG. 4A, to the surface area (or cross-sectional area) of bump 400 is equal to or less than about 0.1.
Other shapes of recess regions combined with various profiles of metal bumps may also be used to reduce solder metal protrusion. FIG. 4D shows a top view of a metal bump 400′, in accordance with some embodiments. Metal bump 400′ is similar to metal bump 400. The corners 402 of regions R of FIGS. 4A, 4B, and 4C are straight (or 90°). The corners 403 of the recess region R′ of FIG. 4D is has an angle α. Angle α may be equal to or greater than 90°. A greater than 90° corner angle, α, may have less stress than a straight corner. However, angle a may be designed with an angle less than 90°, in accordance with some embodiments. In some embodiments, the M sections (or end sections) of metal bump 400 do not need to be in hemispherical shape. Other shapes are also possible. Further, the recess regions do not need to be formed by straight walls. FIG. 4E shows a metal bump 400* with the recess regions having curved side walls, in accordance with some embodiments. Other shapes and curvatures of sidewalls of recess regions are also possible.
The metal bumps described above without the recess regions have a cross section in the shape of a race track. Bumps with other shapes of cross sections may also be used. For example, the shape of a cross section may be an oval shape. The top views of metal bumps 400 may be in any elongated shapes, including a rectangle with rounded corners. Recess region(s) may be formed in such bumps to allow solder layers to fill (fully or partially) in the recess region(s) after reflow to reduce the risk of shorting.
The embodiments of bump and bump-on-trace (BOT) structures provide bumps with recess regions for reflowed solder to fill. The recess regions are placed in areas of the bumps where reflow solder is most likely to protrude. The recess regions reduce the risk of bump to trace shorting. As a result, yield can be improved.
FIG. 5A is a top view of a bump structure 500 having a single side recess in accordance with some embodiments. Bump structure 500 is similar to bump structure 400 (FIG. 4A) except that bump structure 500 includes a recess on a single side of the bump structure. Bump structure 500 has a general race track shape. Bump structure 500 includes a recess 510 in one side. A side 520 of bump structure 500 opposite recess 510 is substantially straight. Ends 530 of bump structure 500 connect side 520 to the side having recess 510. Bump structure 500 has an overall width “a” and an overall length “b”. Recess 510 has a first length “c” closest to an outer surface of bump structure 500. Recess 510 has a second length “d” closest to side 520. A depth “e” of recess 510 is a distance between the outer surface of bump structure 500 and a point of the recess closest to side 520.
Bump structure 500 is usable for connecting one die to another in a package. Bump structure 500 includes a conductive material. In some embodiments, the conductive material is copper, aluminum, tungsten, or another suitable conductive material. Bump structure 500 connects one die to another using a reflowed solder layer or a reflowed solder ball. During a reflow process, liquefied solder flows into recess 510. In comparison with bump structures which do not include recess 510, bump structure 500 is able to achieve a smaller pitch between adjacent bump structures with reduced risk of bridging between solder material of the adjacent bump structures. In some embodiments, bump structure 500 is part of a bump on trace (BOT) structure. In some embodiments, bump structure 500 is configured to connect to another bump structure. In some embodiments, the other bump structure includes at least one recessed side. In some embodiments, the other bump structure includes no recessed sides.
Ends 530 of bump structure 500 are continuous curves. In some embodiments, ends 530 are straight edges with rounded corners, such that a general shape of bump structure 500 is a rectangle having rounded corners as seen in FIG. 5B. In some embodiments, ends 530 have a different shape, such as a triangular (FIG. 5C), a polygon (FIG. 5D), a discontinuous curve (FIG. 5E) or another suitable shape. Ends 530 have a same shape. In some embodiments, one end 530 has a different shape from the other end 530 (FIG. 5F).
Bump structure 500 is usable for a 16 nanometer (nm) technology chip. In some embodiments, bump structure 500 is usable for a 28 nm technology chip. In some embodiments, bump structure 500 is usable for a 20 nm technology chip. In some embodiments, bump structure 500 is sized for a technology node other than 16 nm, 20 nm or 28 nm. In some embodiments, overall width “a” ranges from about 10 microns (μm) to about 200 μm. In some embodiments, overall width “a” ranges from about 25 μm to about 50 μm. If the overall width of bump structure 500 is too small, an electrical resistance of bump structure 500 increases and negatively impacts performance of a die connected to the bump structure; or a risk of the bump structure breaking during a packaging process increases. In addition, a risk of an open connection during a packaging operation due to misalignment increases if the overall width is too small. If the overall width of bump structure 500 is too great, a risk of bridging of solder materials of adjacent bump structures is increased. In some embodiments, overall length “b” ranges from about 20 μm to about 400 μm. In some embodiments, overall length “b” ranges from about 50 μm to about 80 μm. If the overall length of bump structure 500 is too small, the electrical resistance of the bump structure increases and adversely impacts performance of a device connected to the bump structure, in some instances. In addition, a mechanical strength of bump structure 500 is reduced and a risk of breaking during a packaging process increases if the overall length of the bump structure is too small. A risk of misalignment causing an open connection also increases if the overall length of bump structure 500 is too small. If the overall length of bump structure 500 is too large, a risk of bridging between solder materials of adjacent bump structures increases. In some embodiments, a ratio of overall width “a” to overall length “b” ranges from about 0.5 to about 1.0. If the ratio of overall width “a” to overall length “b” is too small, a mechanical strength of bump structure 500 is adversely impacted, in some instances. If the ratio of overall width “a” to overall length “b” is too great, a size of bump structure 500 is increased without a significant increase in performance and functionality, in some instances.
A surface of recess 510 closest to side 520 is parallel to side 520. In some embodiments, the surface of recess 510 closest to side 520 is angled with respect to side 520 (FIG. 5G). In some embodiments, the surface of recess 510 closest to side 520 is curved (FIG. 5H). In some embodiments, the curve is convex. In some embodiments, the curve is concave. In some embodiments, the second length “d” is less than or equal to about 30 μm. In some embodiments, the second length “d” is less than or equal to about 15 μm. In some embodiments, second length “d” is equal to 0 (FIGS. 5G and 5H). In some embodiments, a ratio of the second length “d” to the overall length “b” is less than or equal to about 0.3. In some embodiments, the ratio of the second length “d” to the overall length “b” is less than or equal to about 0.15. The second length is substantially zero when the surface closest to side 520 is curved or sidewalls of recess 510 intersect. If second length “d” or the ratio between the second length and the overall length “b” is too large, the electrical resistance of bump structure 500 increases and negatively impacts performance of a die connected to the bump structure; or a risk of the bump structure breaking during a packaging process increases, in some instances. In addition, a risk of an open connection during a packaging operation due to misalignment increases if second length “d” or the ratio between the second length and the overall length “b” is too large.
First length “c” of recess 510 provides an opening for solder material to flow into the recess during a reflow process. In some embodiments, first length “c” ranges from about 5 μm to about 50 μm. In some embodiments, first length “c” ranges from about 8 μm to about 15 μm. In some embodiments, first length “c” is substantially equal to second length “d” (FIG. 5I). In some embodiments, a ratio between first length “c” and overall length “b” ranges from about 0.3 to about 0.5. If first length “c” is too large or the ratio between first length “c” and overall length “b” is too great, the electrical resistance of bump structure 500 negatively impacts performance of a die connected to the bump structure, in some instances. Additionally, a risk of bump structure 500 breaking during a packaging process increases or an open connection during a packaging operation due to misalignment increases if first length “c” is too large or the ratio between first length “c” and overall length “b” is too great. If first length “c” is too small or the ratio between first length “c” and overall length “b” is too small, a size of recess 510 is not sufficient to reduce the risk of bridging between adjacent bump structures, in some instances.
Depth “e” of recess 510 provides a volume to receive solder material during a reflow process. In some embodiments, depth “e” ranges from about 0.5 μm to about 15 μm. In some embodiments, depth “e” ranges from about 1 μm to about 15 μm. In some embodiments, a ratio of depth “e” to overall width a ranges from about 0.05 to about 0.2. If depth “e” is too large or the ratio between depth “e” and overall width a is too great, the electrical resistance of bump structure 500 negatively impacts performance of a die connected to the bump structure, in some instances. Additionally, a risk of bump structure 500 breaking during a packaging process increases or an open connection during a packaging operation due to misalignment increases if depth “e” is too large or the ratio between depth “e” and overall width “a” is too great. If depth “e” is too small or the ratio between depth “e” and overall width “a” is too small, the size of recess 510 is not sufficient to reduce the risk of bridging between adjacent bump structures, in some instances.
As a pitch between bump structures decreases, an overall size of the bump structures is reduced. For example, a 28 nm technology node chip includes a bump pitch of about 100 μm to about 160 μm, in some instances. For a chip which is about 10 mm×10 mm, the number of bump structures is approximately 1000 bump structures to connect the chip to another structure. In contrast, a 16 nm technology node chip includes a bump pitch of about 80 μm to about 120 μm, in some embodiments. The 16 nm technology node chip will have 3-4 times the number of bump structures for a 10 mm×10 mm chip in comparison with the 28 nm technology node chip. In some embodiments, a 10 nm technology chip includes a bump pitch of about 40 μm to about 100 μm. The 10 nm technology chip will have even more bump structures for a 10 mm×10 mm chip than a 16 nm technology node chip. Due to the increased number of bump structures on a chip as technology nodes decrease, recessing both sides of the bump structures has a greater overall impact in an ability of a chip including the recessed bump structures to maintain functionality when packaged with another structure due to increased resistance. In addition, mechanical strength of a connection point between a bump structure having a single recess side (FIG. 5A) and a die is greater than a connection point between a bump structure having recesses on both sides (FIG. 4A) and a die. By recessing a single side of bump structures 500, the increased resistance and reduced mechanical strength of the bump structure resulting from the recess is reduced in comparison with a bump structure which is recessed on both sides of the bump structure.
FIG. 6 is a top view of a bump structure 600 having a single side recess in accordance with some embodiments. Bump structure 600 includes a recess 610 offset from a center of a length of the bump structure. In some embodiments, an offset distance z from a center of recess 610 to the center of the length of bump structure 600 is less than or equal to about 7 μm. In some embodiments, a ratio of offset z to an overall length of bump structure 600 is less than or equal to about 0.15. If offset z is too large or the ratio between offset z and the overall length of bump structure 600 is too great, a risk of an open connection during a packaging operation due to misalignment increases, in some instances.
FIG. 7 is a top view of an array 700 of bump structures 750 having a single side recess on a die in accordance with some embodiments. Array 700 includes a pitch P between adjacent bump structure 750. Center lines 710a and 710b divide array 700 into four substantially equal quadrants. Each bump structure 750 of array 700 includes a single recess, similar to bump structure 500 (FIG. 5). A line 710c extends from a center of array 700 to a corner of the array. In some embodiments, bump structures 750 which are in a core region or a corner region of array 700 are angled so that a longitudinal axis of the bump structure is substantially parallel to line 710c. 
A side of bump structures 750 including a recess is oriented toward a closest center line of center line 710a and center line 710b. That is, a bump structure, such as bump structure 750a, which is located closer to center line 710a than center line 710b includes a recess in the side of the bump structure facing center line 710a. Conversely, a bump structure, such as bump structure 750b, located closer to center line 710b than center line 710a includes a recess in the side of the bump structure facing center line 710b. In some embodiments, a bump structure, such as bump structure 750c, which is equidistant from center line 710a and center line 710b includes a recess in the side of the bump structure facing center line 710b. In some embodiments, a bump structure 750 which is equidistant from center line 710a and center line 710b includes a recess in the side of the bump structure facing center line 710a. In some embodiments, a first bump structure 750 which is equidistant from center line 710a and center line 710b has a recess in the side of the first bump structure facing center line 710a and a second bump structure 750 which is equidistant from center line 710a and center line 710b has a recess in the side of the second bump structure facing center line 710b. In some embodiments, pitch P between adjacent bump structures 750 ranges from about 40 μm to about 200 μm.
Bump structures 750 are bonded to another die using a solder reflow process. In some embodiments, bump structures 750 are part of an active die, a passive die, an interposer, or another suitable connection structure. The recess of bump structures 750 is oriented toward the closest center line 710a or 710b in order to capture reflowed solder material during a packaging process. During the solder reflow process, a die being bonded to bump structures 750 is heated. In some embodiments, the die is an active die, a passive die, an interposer or another suitable connection structure. When the die cools the die shrinks. A magnitude of the shrinkage of the die is based on a coefficient of thermal expansion of a material of the die and an overall size of the die. This shrinking causes edges of the bonded die to move inwardly in a plane parallel to array 700 toward center line 710a and center line 710b. The movement of the edges of the die pulls solder material, which is still cooling from the reflow process toward, center line 710a and center line 710b. For example, referring to FIG. 3C, a center line of a die having trace 211* and trace 212* is located to a right side of trace 212*. As the die shrinks toward a center of the die following a reflow process solder 220 is pulled toward the center of the die. In structures which do not include the recess of bump structures 750, the pulling of the solder material during shrinkage of the die is most likely to cause bridging in a direction toward the center line 710a and center line 710b. By including the recesses of bump structures 750 oriented toward a closest center line of center line 710a or center line 710b, the risk of bridging in the direction of shrinkage is reduced in comparison with structures that do not include the recess. In some embodiments, a depth, e.g., depth “e” (FIG. 5A), of the recess of bump structures 750 is determined based on the coefficient of thermal expansion of the die bonded to bump structures 750 or a size of the die boned to bump structures 750. In some embodiments, as the coefficient of thermal expansion of the die bonded to bump structures 750 increases the depth of the recesses of bump structures 750 increases. In some embodiments, as the size of the die bonded to bump structures 750 increases, the depth of the recesses of bump structures 750 increases.
FIG. 8 is a top view of a portion of an array 800 of bump structures having a single side recess on a die in accordance with some embodiments. FIG. 8 includes half of array 800, i.e., the half of array 800 below a center line 810a. In comparison with array 700 (FIG. 7), array 800 includes at least one bump structure which does not include a recess. Array 800 includes a core region 820 having bump structures. Bump structures in core region 820 include non-recessed sides. In some embodiments, at least one bump structure in core region 820 includes a recess. The bump structures of core region 820 have a longitudinal axis at an angle with respect to center line 810a and center line 810b. Array 800 also includes a corner region 830 having bump structures. Bump structures in corner region 830 include non-recessed sides, similar to bump structures in core region 820. In some embodiments, at least one bump structure in corner region 830 includes a recess. The bump structures in corner region 830 have a longitudinal axis oriented at an angle to center line 810a and center line 810b. A central peripheral region 840 of array 800 also includes bump structures. Bump structures of central peripheral region 840 include non-recessed sides. In some embodiments, at least one bump structure in central peripheral region 840 includes a recess. The bump structures in central peripheral region 840 have a longitudinal axis substantially parallel to a closest center line of center line 810a or center line 810b. In some embodiments, an arrangement of bump structures in corner region 830 and core region 820 has a layout based on the bump arrangement described in U.S. Pat. No. 8,598,691, which is incorporated herein by reference in its entirety. Array 800 also includes a recess peripheral region 850 including bump structures. Each bump structure in recess peripheral region 850 includes a recess oriented toward a closest center line of center line 810a or center line 810b. Recess peripheral region 850 is located at an edge of array 800 between central peripheral region 840 and corner region 830. The bump structures of recess peripheral region 850 have a longitudinal axis substantially parallel to the closest center line of center line 810a or center line 810b. 
As discussed above, recessed bump structures have increased electrical resistance in comparison with a bump structure having non-recessed sides. The recessed bump structure also has a lower mechanical strength than non-recess bump structures. By concentrating recessed bump structures in regions of array 800 where a bridging risk is highest, the above drawbacks of recessing the sides of the bump structures are mitigated in other portions of array 800. Recess peripheral region 850 is a location of highest bridging risk of array 800 because the edge of the die bonded to the bump structures of array 800 experiences a largest magnitude of shrinkage following a reflow process.
Array 800 includes recess peripheral region 850 having a single row of bump structures. In some embodiments, recess peripheral region 850 includes multiple rows of bump structures. In some embodiments, array 800 includes a single recess peripheral region 850. In some embodiments, the single recess peripheral region 850 includes a single recessed bump structure. In some embodiments, the single recessed bump structure is adjacent to a corner region 830 of array 800.
In some embodiments, as the coefficient of thermal expansion of the die bonded to array 800 increases, a size of recess peripheral region 850 increases. In some embodiments, as the size of the die bonded to array 800 increases, the size of recess peripheral region 850 increases. In some embodiments, the size of recess peripheral region 850 is determined based on empirical evidence from previous package structures.
FIG. 9 is a top view of a die 900 having bump structures having a single side recess in accordance with some embodiments. Die 900 includes core region 920 similar to core region 820 (FIG. 8). Die 900 further includes corner region 930 similar to corner region 830. Die 900 further includes recess peripheral region 950 similar to recess peripheral region 850. In some embodiments, die 900 also includes a central peripheral region similar to central peripheral region 840.
A first corner region extending parallel to center line 910a has a length x1 parallel to center line 910a. In some embodiment, a ratio of length x1 to an overall length L of die 900 parallel to center line 910a ranges from about 0.02 to about 0.1. If the ratio of length x1 to the overall length of die 900 is too great, a risk of bridging of bump structures within the first corner region increases, in some instances. If the ratio of length x1 to the overall length of die 900 is too small, a mechanical structure of the bump structures in the first corner region is needlessly reduced or the electrical resistance of the bump structures in the first corner region is needlessly increased, in some instances.
A first recess peripheral region extending parallel to center line 910a has a length y1 parallel to center line 910a. In some embodiment, a ratio of length y1 to an overall length L of die 900 parallel to center line 910a ranges from about 0.2 to about 0.3. If the ratio of length y1 to the overall length of die 900 is too great, a mechanical structure of the bump structures in the first recess peripheral region is needlessly reduced or the electrical resistance of the bump structures in the first recess peripheral region is needlessly increased, in some instances. If the ratio of length y1 to the overall length of die 900 is too small, a risk of bridging of bump structures within the first corner region increases, in some instances.
A second corner region extending parallel to center line 910a has a length x2 parallel to center line 910a. In some embodiment, a ratio of length x2 to an overall length of die 900 parallel to center line 910a ranges from about 0.02 to about 0.1. If the ratio of length x2 to the overall length L of die 900 is too great, a risk of bridging of bump structures within the first corner region increases, in some instances. If the ratio of length x2 to the overall length of die 900 is too small, a mechanical structure of the bump structures in the first corner region is needlessly reduced or the electrical resistance of the bump structures in the first corner region is needlessly increased, in some instances. In some embodiments, length x2 is equal to length x1. In some embodiments, length x2 is different from length x1. In some embodiments, a magnitude of length x1 or length x2 is determined based on empirical information, a coefficient of thermal expansion of a die bonded to die 900, or a size of the die bonded to die 900.
A second recess peripheral region extending parallel to center line 910a has a length y2 parallel to center line 910a. In some embodiment, a ratio of length y2 to an overall length L of die 900 parallel to center line 910a ranges from about 0.2 to about 0.3. If the ratio of length y2 to the overall length of die 900 is too great, a mechanical structure of the bump structures in the first recess peripheral region is needlessly reduced or the electrical resistance of the bump structures in the first recess peripheral region is needlessly increased, in some instances. If the ratio of length y2 to the overall length of die 900 is too small, a risk of bridging of bump structures within the first corner region increases, in some instances. In some embodiments, length y2 is equal to length y1. In some embodiments, length y2 is different from length y1. In some embodiments, a magnitude of length y1 or length y2 is determined based on empirical information, a coefficient of thermal expansion of a die bonded to die 900, or a size of the die bonded to die 900.
A third corner region extending parallel to center line 910b has a length i1 parallel to center line 910b. In some embodiment, a ratio of length i1 to an overall length K of die 900 parallel to center line 910b ranges from about 0.02 to about 0.1. If the ratio of length i1 to the overall length of die 900 is too great, a risk of bridging of bump structures within the first corner region increases, in some instances. If the ratio of length i1 to the overall length of die 900 is too small, a mechanical structure of the bump structures in the first corner region is needlessly reduced or the electrical resistance of the bump structures in the first corner region is needlessly increased, in some instances.
A third recess peripheral region extending parallel to center line 910b has a length j1 parallel to center line 910b. In some embodiment, a ratio of length j1 to an overall length K of die 900 parallel to center line 910b ranges from about 0.2 to about 0.3. If the ratio of length j1 to the overall length of die 900 is too great, a mechanical structure of the bump structures in the first recess peripheral region is needlessly reduced or the electrical resistance of the bump structures in the first recess peripheral region is needlessly increased, in some instances. If the ratio of length j1 to the overall length of die 900 is too small, a risk of bridging of bump structures within the first corner region increases, in some instances.
A fourth corner region extending parallel to center line 910b has a length i2 parallel to center line 910b. In some embodiment, a ratio of length i2 to an overall length K of die 900 parallel to center line 910b ranges from about 0.02 to about 0.1. If the ratio of length i2 to the overall length of die 900 is too great, a risk of bridging of bump structures within the first corner region increases, in some instances. If the ratio of length i2 to the overall length of die 900 is too small, a mechanical structure of the bump structures in the first corner region is needlessly reduced or the electrical resistance of the bump structures in the first corner region is needlessly increased, in some instances. In some embodiments, length i2 is equal to at least one of length x1, length x2 or length i1. In some embodiments, length x2 is different from at least one of length x1, length x2 or length i1. In some embodiments, a magnitude of length i1 or length i2 is determined based on empirical information, a coefficient of thermal expansion of a die bonded to die 900, or a size of the die bonded to die 900.
A fourth recess peripheral region extending parallel to center line 910b has a length j2 parallel to center line 910b. In some embodiment, a ratio of length j2 to an overall length K of die 900 parallel to center line 910b ranges from about 0.2 to about 0.3. If the ratio of length j2 to the overall length of die 900 is too great, a mechanical structure of the bump structures in the first recess peripheral region is needlessly reduced or the electrical resistance of the bump structures in the first recess peripheral region is needlessly increased, in some instances. If the ratio of length j2 to the overall length of die 900 is too small, a risk of bridging of bump structures within the first corner region increases, in some instances. In some embodiments, length j2 is equal to at least one of length y1, length y2 or length j1. In some embodiments, length j2 is different from at least one of length y1, length y2 or length j1. In some embodiments, a magnitude of length j1 or length j2 is determined based on empirical information, a coefficient of thermal expansion of a die bonded to die 900, or a size of the die bonded to die 900.
FIG. 10A is a top view of a BOT region 1000 in accordance with some embodiments. BOT region 1000 is similar to workpiece 200 and similar element have a same reference number increased by 200. In comparison with workpiece 200 BOT region 1000 includes bump structures 1001-1008 including a recess 1050. Each recess 1050 is facing toward a center of a die including BOT region 1000, i.e., to a right side of FIG. 10A. Recess 1050 is configured to receive solder which is reflowed during a bonding of bmp structures 1001-1008 to corresponding traces 1011-1018. Bump structures 1001-1008 are wider than traces 1011-1018. FIG. 10A includes each trace 1011-1018 fully landed on by a corresponding bump structure 1001-1008. In some embodiments, at least one trace 1011-1018 is only partially landed on by a corresponding bump structure 1001-1008. Traces 1011-1018 are conductive lines. In some embodiments, traces 1011-1018 include copper, aluminum, tungsten, or another suitable conductive material.
FIG. 10B is a cross-sectional view of BOT region 1000 taken along line B-B in accordance with some embodiments. BOT region 1000 includes a first work piece 100′ bonded to a second work piece 200′. Second work piece 100′ includes traces 1011-1018. Traces 1011-1018 are electrically connected to active elements or passive elements within second work piece 200′. First work piece 100′ includes bump structures 1001-1008. Bump structures 1001-1008 are electrically connected to active elements or passive elements within the first work piece 100′.
Bump structure 1001-1008 includes a conductive post 1022, a solder material 1024 and a UBM layer 1026. During a bonding process, solder material 1024 is reflowed in order to bond with a corresponding trace 1011-1018. When solder material 1024 is reflowed a portion of the reflowed solder material flows into recess 1050. The portion of solder material 1024 which flows into recess 1050 reduces a total width of the bonded bump structure 1001-1008 in comparison with a bump structure which does not include recess 1050. The reduced width of bump structure 1001-1008 reduces facilitates a reduced pitch between bump structures by reducing a risk of bridging between adjacent bump structures in an array having the reduced pitch. Reducing a pitch between bump structures 1001-1008 facilitates an increased density of connections between first work piece 100′ and second work piece 200′.
FIG. 11A is a cross-sectional view of a BOT region 1100 in accordance with some embodiments. BOT region 1110 is similar to BOT region 1000 (FIG. 10B). Same elements have a same reference number increased by 100. In comparison with BOT region 1000, BOT region 1100 does not include firs work piece 100′. Conductive post 1122 is able to be bonded to an additional work piece. In some embodiments, the additional work piece includes a work piece including active circuitry, a work piece including passive circuitry, an interposer or another suitable work piece. Recess 1150 is indicated as a dotted line because the recess is located in a plane other than the cross-sectional view of BOT region 1100.
FIG. 11B is a perspective view of a bump structure 1100′ in accordance with some embodiments. An overall shape of bump structure 1100′ is rectangular and includes flat end faces and rounded corners. Bump structure 1100′ also includes a trapezoidal recess. In some embodiments, the overall shape of bump structure 1100′ is different from a rectangular shape, as indicated in FIGS. 5A-51. In some embodiments, the recess of bump structure 1100′ is different form a trapezoidal shape, as indicated in FIGS. 5A-51.
One aspect of this description relates to a bump structure including a first end, and a second end opposite the first end. The bump structure further includes a first side connected between the first end and the second end. The bump structure further includes a second side opposite the first side. The second side is connected between the first end and the second end, and the second side comprises a recess for a reflowed solder material to fill.
Another aspect of this description relates to a semiconductor structure. The semiconductor structure includes an array of bump structures configured to electrically connect to a separate die. At least one bump structure of the array of bump structures includes a first end, and a second end opposite the first end. The at least one bump structure further includes a first side connected between the first end and the second end. The at least one bump structure further includes a second side opposite the first side, wherein the second side is connected between the first end and the second end, and the second side comprises a recess for a reflowed solder material to fill.
Still another aspect of this description relates to an array of bump structures of a semiconductor die. The array of bump structures includes a core region having at least one first non-recessed bump structure, wherein the core region is located in a central portion of the semiconductor die. The array of bump structures further includes a corner region having at least one second non-recessed bump structure, wherein the corner region is located at a corner of the semiconductor die. The array of bump structures further includes a recess peripheral region having at least one recessed bump structure, wherein the recess peripheral region is located at an edge of the semiconductor die adjacent to the corner region. The at least one recessed bump structure includes a side having a recess for a reflowed solder material to fill.
Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure.

## Claims

1. A bump structure comprising:
a first end;
a second end opposite the first end;
a first side connected between the first end and the second end; and
a second side opposite the first side, wherein the second side is connected between the first end and the second end, and the second side comprises a recess for a reflowed solder material to fill.

2. The bump structure of claim 1, wherein the first side is a non-recessed side.

3. The bump structure of claim 1, wherein a ratio of a depth of the recess to an overall width of the bump structure ranges from about 0.05 to about 0.2.

4. The bump structure of claim 1, wherein the recess comprises a length adjacent to an outer edge of the bump structure, and a ratio of the length to an overall length of the bump structure ranges from about 0.5 to about 0.3.

5. The bump structure of claim 1, wherein the recess comprises a length closest to the first side, and a ratio of the length to an overall length of the bump structure is less than or equal to about 0.3.

6. The bump structure of claim 1, wherein a center of the recess is aligned with a center of the bump structure.

7. The bump structure of claim 1, wherein a center of the recess is offset by a distance from a center of the bump structure.

8. The bump structure of claim 7, wherein a ratio between the distance and an overall length of the bump structure is less than or equal to about 0.15.

9. The bump structure of claim 1, wherein the recess is curved or polygonal.

10. A semiconductor structure comprising:
an array of bump structures configured to electrically connect to a separate die, wherein at least one bump structure of the array of bump structures comprises:
a first end;
a second end opposite the first end;
a first side connected between the first end and the second end; and
a second side opposite the first side, wherein the second side is connected between the first end and the second end, and the second side comprises a recess for a reflowed solder material to fill.

11. The semiconductor structure of claim 10, wherein each bump structure of the array of bump structures comprises a recess for a reflowed solder material to fill.

12. The semiconductor structure of claim 10, further comprising:
a first center line extending parallel to a top surface of the semiconductor structure; and
a second center line extending parallel to the top surface of the semiconductor structure, wherein the second center line is perpendicular to the first center line, and the recess of the at least one bump structure is oriented toward a closest center line of the first center liner or the second center line.

13. The semiconductor structure of claim 12, wherein the at least one bump structure has a longitudinal axis parallel to the first center line or the second center line.

14. The semiconductor structure of claim 10, wherein the at least one bump structure is located at an edge of the semiconductor structure adjacent a corner region of the semiconductor structure, wherein the corner region comprises at least one non-recessed bump structure of the array of bump structures.

15. An array of bump structures of a semiconductor die comprising:
a core region having at least one first non-recessed bump structure, wherein the core region is located in a central portion of the semiconductor die;
a corner region having at least one second non-recessed bump structure, wherein the corner region is located at a corner of the semiconductor die; and
a recess peripheral region having at least one recessed bump structure, wherein the recess peripheral region is located at an edge of the semiconductor die adjacent to the corner region, and the at least one recessed bump structure comprises:
a side having a recess for a reflowed solder material to fill.

16. The array of bump structures of claim 15, further comprising:
a first center line extending parallel to a top surface of the semiconductor die; and
a second center line extending parallel to the top surface of the semiconductor die, wherein the second center line is perpendicular to the first center line, and the recess of the at least one recessed bump structure is oriented toward a closest center line of the first center liner or the second center line.

17. The array of bump structures of claim 16, wherein the at least one recessed bump structure has a longitudinal axis parallel to the first center line or the second center line.

18. The array of bump structures of claim 16, wherein the at least one second non-recessed bump structure has a longitudinal axis angled with respect to the first center line and the second center line.

19. The array of bump structures of claim 16, further comprising a central peripheral region having at least one third non-recessed bump structure, wherein the central peripheral region is located at the edge of the semiconductor die, and the recess peripheral region is between the central peripheral region and the corner region.

20. The array of bump structures of claim 19, wherein the at least one third non-recess bump structure has a longitudinal axis parallel to the first center line or the second center line.