Bonding structure with pillar and cap

[0001] This application is related to application Ser. No. 09/798,654 filed on Mar. 5, 2001, now U.S. Pat. No. 6,818,545; and related to application Ser. No. 10/935,451 filed on Sep. 7, 2004, now pending; and related to application Ser. No. 09/953,525 filed on Sep. 17, 2001, now U.S. Pat. No. 6,442,136; and related to application Ser. No. 10/638,454 filed on Aug. 11, 2003, now U.S. Pat. No. 6,917,119; and related to application Ser. No. 11/120,234 filed on May 2, 2005, now pending; and related to application Ser. No. 09/837,007 filed on Apr. 18, 2001, now pending; and related to application Ser. No. 10/055,580 filed on Jan. 22, 2002, now pending; and related to application Ser. No. 10/174,357 filed on Jun. 17, 2002, now U.S. Pat. No. 6,784,087; and related to application Ser. No. 10/695,630 filed on Oct. 27, 2003, now pending; and related to application Ser. No. 11/123,328 filed on May 6, 2005, now pending, all assigned to a common assignee.

[0002] 1. Field of Invention

[0003] The present invention relates to a cylindrical bonding structure and its method of manufacture. More particularly, the present invention relates to a cylindrical bonding structure for a flip chip package and a method of fabricating the cylindrical bonding structure.

[0004] 2. Description of Related Art

[0005] In this information-saturated society, working with electronic products has become an integral part of our daily life. Currently, integrated circuit products are used for doing business, educating our children or providing us with games for recreation. As a result of rapid progress in electronic technologies, devices having powerful functions and personalized designs have been developed. Moreover, most electronic products have light and compact design. Nowadays, high-density integrated circuits are frequently housed within compact semiconductor packages such as a flip-chip package and a ball grid array (BGA) package.

[0006] In the flip-chip technique, bumps are formed on the bonding pads of a chip so that the bumps may be attached to corresponding contact points on a substrate after flip over. Compared with conventional wire bonding and tape automatic bonding (TAB) packaging techniques, a flip-chip package has the shortest signal transmission path between the chip and the substrate and hence has superior electrical properties. In addition, a flip-chip package may be designed to have its back exposed so as to increase heat dissipation rate. Due to the above reasons, flip-chip packaging techniques are widely adopted in the semiconductor fabrication industry.

[0007] FIG. 1A is a partially magnified view showing a connection configuration between a bump on a chip and a contact point on a substrate in a conventional flip-chip package. A chip 10 normally has a plurality of bonding pads 112 (only one is shown in FIG. 1A). Each bonding pad 112 has a bump 114. In general, the bump 114 is a solder bump so that the flip-over chip 110 may directly connect with one of the bonding pads 122 (only one is shown in FIG. 1A) on the substrate 120. Since the chip 110 and the substrate 120 each has a different coefficient of thermal expansion (CTE), a standoff distance must be provided between the chip 110 and the substrate 120 so that differential thermal expansion will not accumulate too much shear stress to break the bumps 114 prematurely.

[0008] Thus, to prevent shear stress from damaging the bumps 114, bumps 114 having a great height are often attached to the bonding pads 112 of the chip 110 so as to increase the distance of separation between the chip 110 and the substrate 120 as much as possible. However, increasing the overall height of the bumps 114 must be accompanied by a corresponding increase in outer diameter and volume of the bumps. Moreover, to prevent short-circuiting, pitch between neighboring bumps 114 must be increased. Ultimately, distance between neighboring bonding pads 112 on the chip 110 is hard to reduce.

[0009] In addition, pre-solder material is often applied on the pads 122 of the substrate 120 before the lower end of the bumps 114 are put against the pads 122. In a heat treating operation, the low melting point pre-solder melts and joins the bumps 114 and the pads 122 together. Because an additional step of applying low melting point solder over the pads 122 of the substrate 120 has to be conducted, cost of fabricating the substrate 120 is increased. Furthermore, to increase the distance of separation between the chip 110 and the substrate 120, high lead solder is a principle ingredient of the bumps 114. Since a high temperature treatment of the bump material to form a spherical shape bump often produces oxide material near the surface, the bumps 114 and the pads 122 often have poor adhesion after the solder heat treating process. Poor adhesion often leads to bad electrical connections between the chip and the substrate and a low overall yield of the flip chip package.

[0010] FIG. 1B is a partially magnified view showing an alternative connective configuration between a bump on a chip and a contact point on a substrate in a conventional flip-chip package. A solder mask 124 is formed over the substrate 120 to pattern out contact area around the pads 122. In fact, there are two major patterning techniques that employ the solder mask 124. The first one is called a ‘solder mask define’ (SMD) and the other one is called a ‘no solder mask define’ (NSMD). In FIG. 1A, a ‘solder mask define’ (SMD) technique is used. An opening 126 in the solder mask 124 exposes a portion of the pad 122 and a bump on the chip 110 can be bonded onto a portion of the pad 122 on the substrate 120. In FIG. 1B, a ‘no solder mask define’ (NSMD) technique is used. An opening 126 in the solder mask 124 completely exposes a pad 122 and a bump is completely connected to the pad 122 on the substrate 120. The most commonly used material for forming the solder mask 124 is, for example, green lacquer.

[0011] To shorten pitch between neighboring pads 122, SMD technique such as the one shown in FIG. 1A is often employed. Only a portion of the pad 122 is exposed through the solder mask 124 for contact with the lower edge of a bump 114 (shown in profile by dash lines 114a). However, because actual dimension of a bump 114 may vary from the standard dimension by ±10%, variation in positional accuracy between the bump 114 and the pad 122 of up to 10 μm is possible. Furthermore, the opening 126 in the solder mask layer 124 may have an intrinsic diametrical variation of about 15 μm. Hence, when the bump 114 and the pad 122 are laid on top of each other, the lower edge of the bump 114 may not come into direct contact with the surface of the pad 122. In extreme cases, part of the outer edge of the bump 114 may lean upon the upper corner of the opening 126 of the solder mask layer 124 shown by the dash line 114b in FIG. 1A. Hence, after a solder heat treating operation, the bump 114 may not be properly bonded with the pad 122 to form a good electrical connection. To ensure proper bonding between the lower edge of the bump 114 with the pad 122, diameter of the opening 126 of a conventional solder mask 124 is generally larger than the external diameter of the bump 114. Since distance between neighboring pads 122 must be increased to accommodate the extension, ultimate level of integration is greatly reduced.

[0012] Accordingly, one object of the present invention is to provide a cylindrical bonding structure and its method of manufacture capable of reducing the separation between neighboring bonding pads on a chip while increasing distance of separation between the chip and a substrate. Ultimately, reliability of the junctions connecting the chip and the substrate is improved and post-packaging life of the chip is extended.

[0013] A second object of this invention is to provide a cylindrical bonding structure and its method of manufacture capable of reducing the diameter of openings on a solder mask for exposing a pad so that distance of separation between neighboring pads on the substrate is reduced. Consequently, the distance of separation between neighboring bonding pads (bumps) on the chip is also reduced.

[0014] A third object of this invention is to provide a cylindrical bonding structure and its method of manufacture that requires no application of low melting point solder material on the pads of a substrate or the surface of bumps before conducting a heat treating process. Thus, production cost of a flip-chip package is reduced.

[0015] To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a cylindrical bonding structure and its method of manufacture. A ball contact metallic layer is formed over the entire surface of a silicon wafer. A patterned mask layer is formed over the ball contact metallic layer. The mask layer has openings that correspond in position to bonding pads on the wafer and expose a portion of the underlying ball contact metallic layer. By conducting an electroplating process, for example, conductive material is deposited into the openings of the mask layer to form conductive pillars. Through electroplating or printing, solder material is deposited into the openings of the mask layer to form a cylindrical solder cap on the upper surface of the conductive pillars. The mask layer and the ball contact metallic layer outside the conductive pillar are removed. The residual ball contact metallic layer, the conductive pillar and the solder cap together form a cylindrical bonding structure.

[0016] In addition, the cylindrical solder cap may undergo a heat treating treatment to transform the cylindrical solder cap into a solder cap attached to the upper surface of the conductive pillar. Alternatively, the deposition of solder material into the openings may be deleted. After the formation of the conductive pillars, the mask layer and the ball contact metallic layer outside the conductive pillars are removed. Thereafter, a ball implant process is conducted to attach a solder ball directly onto the exposed surface of each conductive pillar. The residual ball contact metallic contact, the conductive pillar and the solder ball together form a pillar bonding structure.

[0017] This invention also provides an alternative cylindrical bonding structure and its method of manufacture. A ball contact metallic layer is formed over the entire surface of a silicon wafer. A patterned first mask layer is formed over the ball contact metallic layer. The first mask layer has openings that correspond in position to bonding pads on the wafer and expose a portion of the underlying ball contact metallic layer. By conducting an electroplating process, for example, a conductive material is deposited into the openings of the mask layer to form a conductive pillar. A patterned second mask layer is formed over the first mask layer. The second mask layer has openings that expose the upper surface of the conductive pillars. Similarly, by conducting another electroplating operation, solder material is deposited into the openings of the mask layer to form cylindrical solder caps on the upper surface of all conductive pillars. The first mask layer, the second mask layer, and the ball contact metallic layer outside the conductive pillar are removed. The residual ball contact metallic layer, the conductive pillar and the cylindrical solder cap together form a cylindrical bonding structure. In addition, the cylindrical solder cap may be designed to have an outer diameter smaller than the diameter of the opening in the solder mask. Hence, the cylindrical solder cap may pass through the solder mask opening to contact the pad on the substrate when the chip is flipped over the substrate.

[0018] It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

[0029] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

[0030] FIGS. 2A˜2F are schematic cross-sectional views showing the progression of steps for producing a cylindrical bonding structure according to a first embodiment of this invention. As shown in FIG. 2A, a silicon wafer 210 is provided. Since each chip (not shown) is formed by cutting up the wafer 210 and any redistribution layer or bumps are formed before cutting, the wafer 210 actually represents a congregation of uncut chips. Furthermore, the active surface 212 of the wafer 210 has a passivation layer 214 and a plurality of bonding pads 216 (only one bonding pad is shown in FIG. 2A). The active surface 212 of the wafer 210 refers to the side of the wafer 210 where active devices are formed. The patterned passivation layer 214 exposes the bonding pads 216. Note that bonding pads 216 includes those original pads on the chips or the bonding pads of any redistribution circuit layer on the chips. The purpose of having a redistribution layer is to relocate the original bonding pads on the chip to some other places on the chip.

[0031] As shown in FIG. 2A, a ball contact metallic layer 220 is formed over the entire active surface 212 (that is, the passivation layer 214) of the wafer 210 by conducting, for example, an electroplating operation, an evaporation plating operation or sputtering. The ball contact metallic layer 220 covers the bonding pads 216 and serves as an interface between a subsequently formed conductive pillar 240 (as shown in FIG. 2C) and the bonding pad 216. Hence, the ball contact metallic layer must be a material that produces as little stress as possible, has a high adhesive strength, resists corrosion and sticks to surface quite easily. In general, the ball contact metallic layer is a composite layer comprising two or more metallic layers, for example, including an adhesive layer and a wetting layer. Common metallic material for fabricating the ball contact metallic layer includes titanium (Ti), tungsten (W), chromium (Cr), copper (Cu), nickel (Ni), cobalt (Co), silver (Ag), gold (Au), tin (Sn), vanadium (V), palladium (Pd) or an alloy of some of the aforementioned metals.

[0032] As shown in FIG. 2B, a patterned mask layer 230 is formed over the ball contact metallic layer 220. The mask layer 230 has a plurality of openings 232 (only one is shown in FIG. 2B) that corresponds in position to the bonding pads 216 and exposes a portion of the ball contact metallic layer 220. The material of the metallic layer 220 comprises titanium, titanium-tungsten alloy, chromium, chromium-copper alloy, tantalum, or tantalum nitride. The patterned mask layer 230 is formed, for example, by forming a photoresist layer over the ball contact metallic layer 220, conducting a photographic exposure and developing the photoresist layer. Aside from depositing photoresist and conducting photographic procedures, the mask layer 230 with openings 232 thereon may also be formed by a corresponding method using some other materials.

[0033] As shown in FIG. 2C, an electroplating operation is conducted to deposit conductive material into the opening 232 using the ball contact metallic layer 220 as a seed layer. The opening 232 is partially filled to form a conductive pillar 240 over the ball contact metallic layer 220. The conductive material 240 deposited into the opening 232 is a high melting point metal or alloy such as tin (Sn), lead (Pb), copper (Cu), gold (Au), silver (Ag), zinc (Zn), bismuth (Bi), magnesium (Mg), antimony (Sb), indium (In) or an alloy containing various combination of the aforementioned metals. For example, the conductive material of the conductive pillar 240 comprises copper or tin-lead alloy with high lead.

[0034] As shown in FIG. 2D, an electroplating operation or a printing operation is conducted to fill the remaining space of the opening 232 with solder material using the conductive pillar 240 as a seed layer. The solder material forms a cylindrical solder cap 250 over upper surface of the conductive pillar 240. Note that the solder material is a metal or alloy having a melting point lower than that of the conductive pillar. Suitable solder material includes tin (Sn), lead (Pb), copper (Cu) gold (Au), zinc (Zn), bismuth (Bi), magnesium (Mg), antimony (Sb), indium (In) or an alloy containing various combinations of the aforementioned metals. For example, the material of the solder cap 250 comprises tin-lead alloy, tin-silver-copper alloy or other lead free alloy.

[0035] As shown in FIG. 2E, the mask layer 230 and the ball contact metallic layer 230 outside the conductive pillar 240 are removed. The reserved ball contact metallic layer 220, the conductive pillar 240 and the cylindrical solder cap together constitute a cylindrical bonding structure 260. As an example, the conductive pillar 240 may contain tin and lead in the ratio 5:95 (5 Sn/95 Pb) or 10:90 (10 Sn/90 Pb) and the cylindrical solder cap 250 may contain tin and lead in the ratio 63:37 (63 Sn/37 Pb) or 60:40 (60 Sn/40 Pb). The conductive pillar 240 can also be a copper rod while the cylindrical solder cap 250 can be a tin cap. Alternatively, the conductive pillar 240 can be a rod made from a high melting point lead-free alloy such as a tin-silver-copper (Sn/Ag/Cu) alloy and the cylindrical solder cap 250 can be a cap made from a low melting point lead-free alloy such as tin-bismuth (Sn/Bi) alloy.

[0036] As shown in FIG. 2F, a heat treating treatment is conducted after the cylindrical bonding structure 260 is exposed. In the heat treating process, the cylindrical solder cap 250 is partially melted to form a solder cap 250a having a hemispherical profile over the upper surface of the conductive pillar 240.

[0037] As shown in FIGS. 2E and 2F, the cylindrical bonding structure 260 of the first embodiment mainly comprises the conductive pillar 240 and the solder cap 250a. The solder cap may have a cylindrical shape (the cylindrical solder cap 250 shown in FIG. 2E) or a hemispherical shape (shown in FIG. 2F). The cylindrical bonding structure 260 serves a similar function as the bump 114 in FIGS. 1A and 1B. When the solder cap 250a melts, the conductive pillar 240 and the pad 122 are joined together. Hence, the cylindrical bonding structure 260 not only serves as a medium for connecting the chip 110 and the substrate 120 together electrically, the conductive pillar 240 also serves as an cushioning pad from the chip 110 that pushes the solder cap further towards the substrate 120. Note that outer diameter of the conductive pillar 240 is fixed even when height of the conductive pillar 240 is increased. Hence, distance of separation between neighboring cylindrical bonding structures 260 and hence neighboring bonding pads 114 (or bonding pads 216) on the chip 110 is shortened. Additionally, as shown in FIG. 2G, there are multiple cylindrical bonding structures 260 formed on the bonding pads 216 of the wafer 210, respectively. Besides, the wafer 210 includes a redistribution circuit layer 290 and the cylindrical bonding structures 260 can be formed on the bonding pads 292 of the redistribution circuit layer 290, as shown in FIG. 2H. The redistribution circuit layer 290 is formed to relocate the original bonding pads 216 to other places over the wafer 210, for example, the relocated bonding pads 292.

[0038] According to the first embodiment, the steps involved in fabricating the cylindrical bonding structure include forming a ball contact metallic layer globally over a wafer and then forming a patterned mask layer over the ball contact metallic layer. The mask layer has an opening that surrounds a bonding pad and exposes a portion of the ball contact metallic layer. An electroplating operation is conducted to partially fill the mask opening with conductive material, thereby forming a conductive pillar. Another electroplating operation or printing operation is conducted to fill up the remaining space of the opening, thereby forming a cylindrical solder cap on the upper surface of the conductive pillar. Finally, the mask layer and the ball contact metallic layer outside the conductive pillar are removed to form the cylindrical bonding structure. Furthermore, a heat treating operation may also be conducted to transform the cylindrical solder cap into a solder cap having a hemispherical shape attached to the upper surface of the conductive pillar.

[0039] In summary, the method of fabricating the cylindrical bonding structure according to the first embodiment of this invention mainly involves forming a conductive pillar over the bonding pad of a chip. The conductive pillar serves as a conductive medium as well as a pad for cushioning up the distance between the chip and the substrate. In addition, by attaching a solder cap on the upper end of the conductive pillar, the conductive pillar and the pad on the substrate are bonded together after the solder cap material is partially melted in a heat treating operation. Hence, at the same height level, the conductive pillar can be designed to have an outer diameter smaller than the outer diameter of a spherical bump in a conventional design. Ultimately, the distance of separation between neighboring cylindrical bonding structures and hence the corresponding distance of separation between neighboring bonding pads on the chip can be reduced.

[0040] FIGS. 3A˜3E are schematic cross-sectional views showing the progression of steps for producing a cylindrical bonding structure according to a second embodiment of this invention. The second embodiment differs from the first embodiment in that a solder ball is planted onto the upper surface of the conductive pillar instead of forming the solder cap (or the cylindrical solder cap). Since the initial steps as shown in FIGS. 3A˜3C for forming the cylindrical bonding structure are identical to the ones shown in FIGS. 2A˜2C, detailed description is omitted.

[0041] As shown in FIG. 3D, the mask layer 330 and the ball contact metallic layer 320 outside the coverage of the conductive pillar 340 are removed. As shown in FIG. 3E, a preformed solder cap 350, such as ball-like shaped, is provided and then a ball placement operation is conducted to attach a solder cap 350 on the upper surface of the conductive pillar 340. A cylindrical bonding structure 360 is formed on the bonding pad 316 provided on the active surface 312 of the wafer 310.

[0042] In summary, one major aspect of both the first and the second embodiment of this invention is to form a block of solder material on top of a conductive pillar for joining the conductive pillar with a pad on the substrate. The block of solder material may be shaped into a variety of forms including cylindrical, spherical or hemispherical. The solder cap is formed over the conductive pillar by depositing solder material into the same opening for forming the conductive pillar through conducting an electroplating operation or printing operation as in the first embodiment. Alternatively, a solder ball is planted on top of the conductive pillar as in the second embodiment.

[0043] FIGS. 4A˜4F are schematic cross-sectional views showing the progression of steps for producing a cylindrical bonding structure according to a third embodiment of this invention. One major aspect in the third embodiment that differs from the first and the second embodiment of this invention is the control of outer diameter and length of the solder cap (or cylindrical solder cap) so that pitch between neighboring bonding pads on a chip can be further reduced.

[0044] As shown in FIG. 4A, a wafer 410 having an active surface 412 is provided. The active surface 412 has a plurality of bonding pads 416 thereon. A passivation layer covers the active surface 412 but exposes the bonding pads 416. A ball contact metallic layer 420 is formed over the entire active surface 412 (the passivation layer 414) of the wafer 410 including the bonding pads 416 by conducting an electroplating operation, evaporation plating operation or sputtering, for example.

[0045] As shown in FIG. 4B, a first patterned mask layer 430 is formed over the ball contact metallic layer 420. The first mask layer 430 has a plurality of openings 432 that corresponds in position to the bonding pads 416 and exposes a portion of the ball contact metallic layer 420. Since the patterned first mask layer 430 is formed in a manner similar to the mask layer 230 in the first embodiment, description is not repeated here.

[0046] As shown in FIG. 4C, an electroplating operation is conducted to deposit conductive material into the openings 432 using the ball contact metallic layer 420 as a seed layer. Hence, conductive pillars 440 are formed over the ball contact metallic layer 420. Note that the conductive material is a high melting point metal or alloy.

[0047] As shown in FIG. 4D, a second patterned mask layer 434 is formed over the first mask layer 430. The second mask layer 434 has a plurality of openings 436 and exposes the central region of the conductive pillar 440. The openings 436 have transverse cross-sectional areas smaller than those of the corresponding openings 432, respectively. Since the patterned second mask layer 434 is formed in a manner similar to the mask layer 230 in the first embodiment, description is not repeated here.

[0048] As shown in FIG. 4E, another electroplating operation is conducted to deposit conductive material into the openings 436 using the conductive pillar 440 as a seeding layer. Hence, a cylindrical solder cap 450 is formed on the upper surface of each conductive pillar 440. Note that the conductive material deposited into the openings 436 is a low melting point metal or alloy so that the cylindrical solder cap 450 has a melting point lower than the conductive pillar 440. The solder cap 450 has transverse cross-sectional area smaller than that of the conductive pillar 440.

[0049] As shown in FIG. 4F, the first mask layer 430, the second mask layer 434 and the ball contact metallic layer 420 outside the conductive pillar 440 are removed. The remaining ball contact metallic layer, the conductive pillar 440 and the cylindrical solder cap 450 together form a cylindrical bonding structure 460.

[0050] FIGS. 5A˜5C are schematic cross-sectional views showing an application of the third cylindrical bonding structure according to this invention to the fabrication of a flip-chip package. As shown in FIG. 5A, a cylindrical bonding structure 514 according to the third embodiment of this invention is formed on the bonding pad 512 of a chip 510. The cylindrical bonding structure 514 comprises a ball contact metallic layer 514a, a conductive pillar 514b and a cylindrical solder cap 514c. In addition, a substrate 520 having a solder mask layer 524 and a pad 522 thereon is also provided. The solder mask 524 has a plurality of openings 526 that exposes the pads 522. In one aspect, the solder cap 514c has a transverse length w1 smaller than the corresponding transverse length w2 of the opening 526 in the solder mask layer 524 of the substrate 520. In one aspect, the solder cap 514c has a length D1 greater than the corresponding depth D2 of the opening 526 in the solder mask layer 524 of the substrate 520.

[0051] As shown in FIG. 5B, the cylindrical solder cap 514c has an outer diameter smaller than the diameter of the opening 526 on the solder mask 524. Hence, tolerance between the cylindrical solder cap 514c on the cylindrical bonding structure 514 and the pad 522 on the substrate 520 is greatly increased. Furthermore, if the conductive pillar 514b has an outer diameter greater than the diameter of the opening 526, the cylindrical solder cap 514c must be designed to have a length greater than the depth of the opening 526. Hence, when the cylindrical solder cap 514c is lowered into the opening 526, the upper end of the cylindrical solder cap 514c is able to contact the pad 522.

[0052] As shown in FIG. 5C, a heat treating process may be conducted after the upper surface of the cylindrical solder cap 514c is positioned to contact the pad 522. In the heat treating process, the cylindrical solder cap 514c partially melts and joins together the conductive pillar 514b and the pad 522. Moreover, an underfill material may be injected into the space between the chip 510 and the substrate 520 to protect the cylindrical bonding structure 514 and serve as a vibration damper.

[0053] The method of fabricating the cylindrical bonding structure according to the third embodiment includes forming a ball contact metallic layer over the surface of a wafer surface and forming a patterned first mask layer over the ball contact metallic layer. The first mask layer has openings that correspond in position to various bonding pads on the wafer and exposes a portion of the ball contact metallic layer. An electroplating operation is conducted to deposit conductive material into the openings of the first mask layer to form conductive pillars. A patterned second mask layer is formed over the first mask layer. The second mask layer has openings that expose a portion of the upper surface of the conductive pillars. Similarly, solder material is deposited into the openings of the second mask by conducting an electroplating operation to form cylindrical solder caps over the conductive pillars. The first mask layer, the second mask layer and the ball contact metallic layer outside the conductive pillar are removed so that the remaining ball contact metallic layer, the conductive pillar and the cylindrical solder cap together form a cylindrical bonding structure on the chip.

[0054] One major difference between the cylindrical bonding structure according to the third embodiment and the first two embodiments is that the cylindrical solder cap is designed to have an outer diameter smaller than opening diameter on the solder mask. Hence, the cylindrical solder cap may easily lower into the opening to contact the pad on the substrate. This increases the yield of fabricating a flip-chip package and reduces the diameter of the opening. Ultimately, distance of separation between neighboring pads on a substrate and distance of separation between neighboring bonding pads on a chip may both be reduced.

[0055] FIGS. 6A˜6E are cross-sectional views showing cylindrical bonding structures fabricated according to this invention with each cylindrical bonding structure having an additional transition layer between the conductive pillar and the solder cap. As shown in FIGS. 6A˜6E, a transition layer 670 is inserted between the conductive pillar 640 and the solder cap 650 in each case. The transition layer 670 may provide different functions according to the constituent materials. Furthermore, the transition layer 670 can be a single layer or a multiple of layers. In FIG. 6A, the transition layer 670 provides a function very similar to the ball contact metallic layer 620 between the bonding pad 616 and the conductive pillar 640. The transition layer 670 may contain one or a more layers. The transition layer 670 mainly boosts the connectivity between the conductive pillar 640 and the solder cap 650 or prevents the collapse of solder cap 650 material onto the peripheral section of the conductive pillar 640 after conducting a heat treating operation leading to a short-circuit between neighboring conductive pillars. The material of the transition layer 670 is nickel, for example.

[0056] The transition layer 670 is fabricated after forming the conductive pillar 640. The transition layer 670 is formed over the upper surface of the conductive pillar 640. Thereafter, a cylindrical solder cap 650 is formed over the transition layer 670 in FIG. 6A, while a solder cap having a hemispherical shape is formed over the transition layer 670 in FIG. 6B. In FIG. 6C, the transition layer 670 is also fabricated on the upper surface of the conductive pillar 640 after forming the conductive pillar 640. However, a solder ball 650 is attached to the transition layer 670 instead of a solder cap. Similarly, in FIGS. 6D and 6E, the transition layer 670 is fabricated on the upper surface of the conductive pillar before forming a solder cap over the transition layer 670. One major difference is that the transition layer 670 in FIG. 6D is formed inside the opening of the patterned first mask layer 430 (in FIG. 4C) while the transition layer 670 in FIG. 6E is formed inside the opening of the patterned second mask layer 434 (in FIG. 4D).

[0057] In conclusion, the cylindrical bonding structure according to this invention is formed by constructing a conductive pillar over the bonding pad of a chip and using the conductive pillar to cushion up the distance of separation between the chip and a substrate. The solder cap on the tip of the conductive pillar is also used to join the conductive pillar to a pad on the substrate. Compared with a conventional design using spherical bumps, the cylindrical bonding structure can provide a smaller contact separation. In addition, the solder cap may have a variety of profiles including cylindrical, spherical or hemispherical shape. Note that when the solder cap has a cylindrical shape, the length and outer diameter of the pillar may be adjusted to fit into the opening leading to the pad. Consequently, outer diameter of the opening may be reduced and separation between neighboring pads may be reduced. In other words, separation of neighboring bonding pads on a chip may be reduced.

[0058] Because the conductive pillar and the pad are connected by partially melting the solder cap in a heat treating process, the step of applying a low melting point solder material on the pads of the substrate or the surface of bumps in a conventional design can be eliminated. Hence, production cost of the flip-chip package is reduced.

[0059] Furthermore, the conventional high-temperature heat treating process for shaping the bumps into a spherical shape may result in the formation of excessive oxide material on bump surface and may lead to poor bonding between the bump and the pad. In this invention, however, the solder cap is formed on the upper surface of the conductive pillar. A high-temperature heat treating process for shaping the solder cap into a spherical form is not absolutely required. Even if a spherical shape is demanded, the solder cap is shaped using a low-temperature heat treating process. Hence, not much oxidation occurs at the surface of the solder cap material. Ultimately, a better junction structure is formed linking up the conductive pillar and the pad on the substrate.

[0060] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.