Substrates with Ultra Fine Pitch Flip Chip Bumps

Abstract

A method of attaching a chip to the substrate with an outer layer comprising via pillars embedded in a dielectric such as solder mask, with ends of the via pillars flush with said dielectric, the method comprising the steps of: (o) optionally removing organic varnish, (p) positioning a chip having legs terminated with solder bumps in contact with exposed ends of the via pillars, and (q) applying heat to melt the solder bumps and to wet the ends of the vias with solder.

Claims

1. A method of terminating a side of a multilayer composite structure having an outer layer of via posts embedded in a dielectric, comprising the steps of: (i) thinning away the outer layer to expose the copper vias; (ii) sputtering a layer of copper over the thinned surface; (iii) applying, exposing and developing a penultimate pattern of photoresist; (iv) electroplating an external feature layer into the pattern; (v) stripping away the penultimate pattern of photoresist; (vi) applying, exposing and developing an ultimate pattern of photoresist corresponding to the desired pattern of micro bumps; (vii) pattern plating copper via posts into the ultimate pattern of photoresist; (viii) pattern plating solderable metal over the copper via posts; (ix) stripping away the ultimate pattern of photoresist; (x) etching away the seed layer; (xi) laminating a dielectric outer layer; (xiv) plasma etching the dielectric outer layer to expose the solderable cap of the via post, and (xv) applying a finishing treatment of the solderable cap.

2. The method of claim 1 wherein the dielectric outer layer is selected from the group consisting of a film dielectric and a dry film solder mask.

3. The method of claim 1 where step (xv) comprises applying pressure to the solder cap along axis of the via post resulting in a flat coined solderable cap.

4. The method of claim 1 where step (xv) comprises applying pressure along axis of the via post together with heat to cause reflow under pressure, resulting in a flat coined solderable cap.

5. The method of claim 1 where step (xv) comprises applying heat to cause reflow without applying pressure such that the solderable cap assumes a dome shape due to surface tension.

6. The method of claim 1 where step (xiv) of plasma etching comprises exposing to ion bombardment in a low pressure atmosphere comprising ionizing at least one of the gases selected from the group consisting of oxygen, tetrafluoride carbon and fluorine.

7. The method of claim 1 further comprising step (xiii) of applying terminations on other side of the substrate.

8. The method of claim 7, wherein applying terminations comprises: (a) thinning the other side to expose the ends of copper vias; (b) sputtering a copper seed layer; (c) applying, exposing and developing a layer of photoresist; (d) electroplating copper pads into the photoresist; (e) removing the photoresist, and (f) depositing solder mask over substrate between and overlapping the copper pads.

9. A method of applying solderable bumps to ends of via posts comprising electroplating the via posts into a patterned photoresist; plating a solderable material over the via posts, and removing the patterned photoresist.

10. The method of claim 9 further comprising coining the solderable material.

11. The method of claim 9 further comprising reflowing the solderable material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings.

[0082] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:

[0083] FIG. 1 is a flowchart illustrating the steps of a process for manufacturing very fine pitch ball grid array terminations on a multilayer composite electronic structure for connecting an IC thereto, using flip chip technology;

[0084] FIG. 2 is a schematic illustration of a multilayer composite electronic structure;

[0085] FIG. 3 is a schematic illustration of the multilayer composite electronic structure of FIG. 2 having a first side thinned to expose the ends of embedded pillars;

[0086] FIG. 4 is a schematic illustration of the multilayer composite electronic structure of FIG. 3 with a copper seed layer sputtered onto the thinned surface;

[0087] FIG. 5 is a schematic illustration of the multilayer composite electronic structure of FIG. 4 after application, exposure and developing of a photoresist to provide a pattern of pads;

[0088] FIG. 6 is a schematic illustration of the multilayer composite electronic structure of FIG. 5 after plating copper into the photoresist;

[0089] FIG. 7 is a schematic illustration of the multilayer composite electronic structure with upstanding copper pads after stripping away the photoresist;

[0090] FIG. 8 is a schematic illustration of the multilayer composite electronic structure after application, exposure and developing of a photoresist to provide a pattern of termination pegs;

[0091] FIG. 9 is a schematic illustration of the multilayer composite electronic structure after plating copper into the patterned photoresist;

[0092] FIG. 10 is a schematic illustration of the multilayer composite electronic structure after plating a solderable metal or alloy over the copper into the patterned photoresist

[0093] FIG. 11 is a schematic illustration of the multilayer composite electronic structure with an array of upstanding copper and solder bumps after stripping away the photoresist;

[0094] FIG. 12 is a schematic illustration of the multilayer composite electronic structure with an array of upstanding copper and solder bumps after etching away the copper seed layer;

[0095] FIG. 13 is a schematic illustration of the multilayer composite electronic structure with a film dielectric or dry film solder mask laminated over the solder bumps array;

[0096] FIG. 14 is a schematic illustration of the multilayer composite electronic structure after an optional stage of planarizing the film dielectric or dry film solder mask laminated over the solder bumps array, typically using chemical mechanical polishing (CMP);

[0097] FIG. 15 a shows the other side of the multilayer composite electronic structure ground down to expose the ends of the copper vias;

[0098] FIG. 16 shows the other side of the multilayer composite electronic structure with a copper seed layer sputtered thereon;

[0099] FIG. 17 shows the other side of the multilayer composite electronic structure with a pattern of photoresist after application, exposure and development;

[0100] FIG. 18 shows the other side of the multilayer composite electronic structure with a copper layer electroplated into the pattern of photoresist;

[0101] FIG. 19 shows the other side of the multilayer composite electronic structure after stripping away the photoresist;

[0102] FIG. 20 shows the other side of the multilayer composite electronic structure after etching away the seed layer;

[0103] FIG. 21 shows the other side of the multilayer composite electronic structure after depositing a patterned solder mask;

[0104] FIG. 22 shows the first side after thinning the dielectric film to expose the solderable cap over the copper via post;

[0105] FIG. 23 shows the first side after densifying under pressure;

[0106] FIG. 24 shows the first side after densifying by reflow;

[0107] FIG. 25 is a flowchart illustrating the process for terminating the other side of the substrate with a ball grid array;

[0108] FIG. 26 is a schematic illustration of an in-line plasma etching station;

[0109] FIG. 27 is a scanning electron micro photograph (SEM micrograph) showing copper pads separated with dielectric on the surface of a substrate and showing upstanding copper via posts thereupon from above, i.e. from an angle of 0°;

[0110] FIG. 28 is a scanning electron micrograph showing copper pads separated with dielectric on the surface of the substrate and having upstanding copper via posts thereupon from above and from an angle of 45°, at a magnifications such that the scale bar is 100 microns;

[0111] FIG. 29 is a scanning electron micrograph showing copper pads separated with dielectric on the surface of the substrate and having upstanding copper via posts thereupon from above and from an angle of 45°, at a magnifications such that the scale bar is 20 microns, and the copper via post and tin layer electroplated thereover are both clearly visible;

[0112] FIG. 30, is a scanning electron micrograph at the magnification and tilt of FIG. 4c, showing the tin layer 410 as a dome, after reflow;

[0113] FIG. 31 is a scanning electron micrograph at very high magnification wherein the scale bar is 10 microns. An upstanding copper via 405 with a tin cap 407 electroplated thereover using the same patterned photoresist to achieve perfect alignment is shown;

[0114] FIG. 32 is a scanning electron micrograph at very high magnification wherein the scale bar is 10 microns. An upstanding copper via 405 with a tin cap 407 electroplated thereover as per FIG. 30, but after subjecting to reflow;

[0115] FIG. 33 is an intermediate magnification scanning electron micrograph of solderable caps that have been subjected to pressure in the direction of the axis of the via posts;

[0116] FIG. 34 is a higher magnification scanning electron micrograph of a solderable cap that that has been pressed in the direction of the axis of the via posts;

[0117] FIG. 35 is an intermediate magnification scanning electron micrograph of solderable caps that have been subjected to pressure in the direction of the axis of the via posts;

[0118] FIG. 36 is a higher magnification scanning electron micrograph of a solderable cap that that has been subjected to pressure in the direction of the axis of the via posts by inserting into a press, and heated to cause reflow.

[0119] Like reference numbers and designations in the various drawings indicated like elements.

DETAILED DESCRIPTION

[0120] In the description hereinbelow, support structures consisting of metal vias in a dielectric matrix, particularly, copper via posts in a polymer matrix, such as polyimide, epoxy or BT (Bismaleimide/Triazine) or their blends, reinforced with glass fibers are considered.

[0121] It is a feature of Access' photo-resist and pattern or panel plating and laminating technology, as described in U.S. Pat. No. 7,682,972, U.S. Pat. No. 7,669,320 and U.S. Pat. No. 7,635,641 to Hurwitz et al., incorporated herein by reference, that large panels comprising very large arrays of substrates with very many via posts may be fabricated. Such panels are substantially flat and substantially smooth.

[0122] It is a further feature of Access' technology that vias fabricated by electroplating using photoresists and may be narrower than vias created by drill & fill. At present, the narrowest drill & fill vias are about 60 microns. By electroplating using photoresists, a resolution of under 50 microns, or even as little as 30 microns is achievable. Coupling ICs to such substrates is challenging. One approach for flip chip coupling is to provide solder on pads (SoP) terminations, where solder bumps are applied to the support structure to terminate copper vias. This is difficult to achieve because of the fine pitch and small scale.

[0123] Embodiments of the present invention address this issue by providing solder bumps at the end of the copper vias of the support structure.

[0124] One embodiment consists of Cu pillars with a tin tip.

[0125] With reference to FIG. 1 and to FIGS. 2 to 15, a process for manufacturing very fine pitch ball grid array terminations on a multilayer composite electronic structure for connecting an IC thereto, using flipchip technology is described.

[0126] Firstly, a multilayer composite support structure of the prior art is obtained—step 1(i). As shown in FIG. 2 the multilayer support structures 100 includes functional layers 102, 104, 106 of components or features 108 separated by layers of dielectric 110, 112, 114, 116, which insulate the individual layers. Vias 118 through the dielectric layer provide electrical connection between features 108 in the adjacent functional or feature layers 102, 104, 106. Thus the feature layers 102, 104, 106 include features 108 generally laid out within the layer, in the X-Y plane, and vias 118 that conduct current across the dielectric layers 110, 112, 114, 116. Vias 118 are generally designed to have minimal inductance and are sufficiently separated to have minimum capacitances therebetween.

[0127] The vias could be fabricated by drill & fill, but to provide greater flexibility in fabrication, higher precision and more efficient processing by enabling large numbers of vias to be fabricated simultaneously, preferably the vias are fabricated by electroplating using the technology described in U.S. Pat. No. 7,682,972, U.S. Pat. No. 7,669,320 and U.S. Pat. No. 7,635,641 to Hurwitz et al. The via post technology allows different diameter vias, non circular vias, faraday cages, embedded passive components and other features. It will be appreciated that FIG. 2 is a schematic illustration for purposes of explanation. Real substrates may have more or less feature layers and more or less vias. Typically, substrates 100 comprise very large numbers of vias. The relative dimensions of vias, feature layers and dielectric, and, in subsequent schematics, of additional elements, are illustrative only, and are not to scale.

[0128] The side of the multilayer composite electronic structure 100 to which a chip is to be coupled by flip chip bonding is first thinned—step (ii) to expose the ends of the copper vias 110, see FIG. 3. Chemical, mechanical, or preferably, Chemical Mechanical Polishing CMP may be used. Next, a seed layer of copper 120 is sputtered over the thinned surface—step (iii). The resulting structure is schematic illustrated in FIG. 4.

[0129] With reference to FIG. 5, a layer of photoresist 122 is applied, exposed and developed to provide a pattern of pads—step (iv). As shown in FIG. 6, copper pads 124 are then plated into the photoresist—step (v), the copper seed layer 120 serving as an anode.

[0130] Now, the photoresist 122 FIG. 7 is stripped away—step (vi), exposing the upstanding copper pads 124 and the seed layer 120 therebetween.

[0131] With reference to FIG. 8 a second layer of photoresist 126 is applied, exposed and developed to provide a pattern of termination pegs—step (vii).

[0132] Copper is now plated into the patterned photoresist 126—Step (viii) to provide the structure schematically shown in FIG. 9.

[0133] A solderable metal or alloy 130, typically tin (Sn) is electroplated over the copper 128 into the patterned photoresist 126—step (ix), providing the structure illustrated schematically in FIG. 10.

[0134] There are various solderable alloys that may be electroplated. The most common of these is the tin-lead eutectic mixture Sn.sub.63Pb.sub.37 having a melting point of 183° C. Other solder materials include pure lead. However, in the drive to limit usage of lead, various lead free solders have been developed. These include pure tin, tin-silver Sn.sub.96.5Ag.sub.3.5 having a melting point of 221° C., and various tin silver copper alloys such as Sn.sub.96.5Ag.sub.3.0Cu.sub.0.5 with a melting point of 218-219° C., Sn.sub.95.8Ag.sub.3.5Cu.sub.0.7 with a melting point of 217-219° C., Sn.sub.95.5Ag.sub.3.8Cu.sub.0.7 with a melting point of 217-219° C., Sn.sub.95.2Ag.sub.3.8Cu.sub.1 with a melting point of 217° C. and Sn.sub.95.5Ag.sub.4Cu.sub.0.5 with a melting point of 217-219° C. There are also some silver free compositions such as Sn.sub.99.3Cu.sub.0.7 with a melting point of 227° C. and Sn.sub.99.3Cu.sub.0.7+Ni with a melting point of 227° C. All of these electroplate well onto the shorted copper via posts within the photoresist. Another candidate material is pure tin. DOW Chemicals provides a sulfonic acid based tin plating solution Solderon ECT Matte Tin which has been found to perform very well.

[0135] It will be appreciated that aligning solder bumps with drill fill vias is extremely difficult and increasingly so as via diameters decrease and the number of vias per unit area increase. This lowers yields and reliability. In the present method described herein, the same pattern is used to electroplate via posts and the solder bumps thereon. This manufacturing technique totally overcomes these problems, ensuring good alignment of solder bumps with the underlying copper via posts.

[0136] The photoresist 126 is now stripped away—step x, providing the structure illustrated in FIG. 11 which shows the multilayer composite electronic structure with an array of upstanding copper and solder bumps.

[0137] The copper seed layer 120 is now etched away—step (xi). Providing the structure shown in FIG. 12.

[0138] A film dielectric or dry film solder mask 132 is laminated—step (xii) over the array of solder bumps 130. A schematic illustration of the multilayer composite electronic structure 100 with the film dielectric or dry film solder mask 132 laminated over the array of solder bumps 130 is shown in FIG. 13.

[0139] Although not shown, it will be appreciated that refluxing whilst the solder caps 130 on the underlying copper via posts 128 are isolated from each other, is one way to prevent solder flow from shorting adjacent bumps.

[0140] Often, surface of the film dielectric/dry film solder mask 132 is rather bumpy, and optionally, the film dielectric/dry film solder mask 132 is planarized—step (xiii), see FIG. 14, typically using chemical mechanical polishing (CMP).

[0141] At this stage, it is convenient to terminate the other side of the substrate 100 with a ball grid array. The process for so doing is shown in FIG. 25, and the various structures are illustrated in FIG. 15 to FIG. 24.

[0142] Thus, with reference to FIGS. 15 to FIG. 24 and to FIG. 25, to terminate the other side of the multilayer composite electronic structure 100, the other side is ground down—step a, to expose the ends of the copper vias 116, as schematically shown in FIG. 15. Copper is then sputtered—step b—over the ground surface to form a copper seed layer 134 as schematically shown in FIG. 16. Referring to FIG. 17 photoresist 136 is now applied, exposed and developed—step c. As shown in FIG. 18, a copper layer 138 is now electroplated—step d—into the pattern of photoresist 136. The photoresist 136 is now stripped away—step e, providing the structure as illustrated in FIG. 19. The seed layer 134 is now etched away—step f, providing the structure illustrated in FIG. 20, and then a patterned solder mask 140 is applied—step g—around and overlapping the copper pads 138. forming the structure shown in FIG. 21.

[0143] Solder balls may then be applied onto the copper pads 138 to create a ball grid array (BGA) interconnect of the finished package (after die assembly).

[0144] With reference to FIG. 26, an in-line plasma etching station 300 is schematically shown. This consists of a vacuum chamber 302 within which a carrier 304 supports a substrate 306. Gases to be ionized for the plasma etching process, such as Oxygen, Tetrafluoro-carbon (CF.sub.4) and Argon, for example, may be introduced through inlet 312 into the vacuum chamber 302. By maintaining a potential difference between the substrate 306 and an upper electrode 308, a plasma zone 314 is created. Optical emission spectrometer analyzers 310 detect the end point when the Sn is exposed and the copper is just covered in real time, allowing accurate computer control.

[0145] By an ion assisted plasma etching process using the equipment 300 schematically shown in FIG. 3, the dielectric film 132 may be removed to expose the solderable cap 130, typically of tin or a tin alloy—step (xv), see FIG. 22.

[0146] After electroplating, solderable alloys may include high surface roughness that without the usage of the right flux material during die assembly—may create voids between the substrate bumps to the die bumps during the die assembly process. As a result, it is often required to apply a finishing treatment such as to “smooth” or “coin” the top surface of the electro-plated bump on the substrate in order to further ease and assist with the flip chip assembly process—step (xvi)a. Different surface treatment techniques may be used.

[0147] For example, with reference to FIG. 23, by applying pressure along the axis of the via posts in a press, for example, the solderable caps may be coined. To aid this process, heat may also be applied, to cause reflow of the substrate bumps. Having an array of flat solderable caps with a fine, smooth surface 130a aids attachment of a bump array of a flip chip and prevents voids at the interface of the die to substrate bumps.

[0148] Alternatively, and usefully for attachment of low I/O count die(s) that do not contain bumps, the solderable caps on the substrate may be exposed to sufficient heat to cause reflow, which, in the absence of a compressing force to generate coining, results in the solderable material melting and forming dome shaped caps 130b due to surface tension of the solder meniscus—FIG. 24. In this case the non-coined bumps on the substrate may be directly attached to non bumped die—directly on its flat pads that may contain Ni/Au or other final metal finishes.

[0149] It will be appreciated that compaction, with or without reflow ensures that the solderable caps 130 are isolated from each other, which helps prevent solder flow shorting adjacent bumps.

[0150] With reference to FIG. 27, there is shown a scanning electron microphotograph (SEM micrograph) showing copper pads 402 separated with dielectric 404 on the surface of a substrate and showing upstanding copper via posts 406 thereupon from above, i.e. from an angle of 0°. The scale bar is 100 microns, and shows that the via posts are approx. 50 microns in diameter.

[0151] Referring to FIG. 28 there is shown a scanning electron micrograph showing copper pads separated with dielectric on the surface of the substrate and having upstanding copper via posts thereupon from above and from an angle of 45°, at a magnifications such that the scale bar is 100 microns.

[0152] With reference to FIG. 29 a scanning electron micrograph is shown, illustrating copper pads 402 separated with dielectric 404 on the surface of the substrate and having upstanding copper via posts thereupon from above and from an angle of 45°, at a magnifications such that the scale bar 409 is 20 microns, and the copper via post 405 and tin layer 407 electroplated thereover are both clearly visible, the denser tin 407 is lighter than the copper 405.

[0153] With reference to FIG. 30, there is shown a scanning electron micrograph at the magnification and tilt of FIG. 29, showing the tin layer 410 as a dome, after reflow. This is the type of finish obtained by the process step xvi, variation b.

[0154] Referring to FIG. 31, a scanning electron micrograph at very high magnification is shown, wherein the scale bar 411 shows 10 microns. This shows an upstanding copper via 405 with a tin cap 407 electroplated thereover using the same patterned photoresist to achieve perfect alignment.

[0155] In FIG. 32, a scanning electron micrograph at the very high magnification of FIG. 4e is shown, wherein the scale bar shows 10 microns. Here, the tin cap 410 has been subject to heat and, due to reflow, has assumed a dome shape 410. This is the type of finish obtained by the process step xvi, variation b.

[0156] Referring to FIG. 33, an electron micrograph wherein a couple of solderable caps 420 that have been subjected to a pressing force without reflow are shown. In FIG. 34, a single solderable cap that have been subjected to a pressing force without reflow is shown. Applying pressure compresses the solderable caps and densifies them, providing a surface to which the bumps of a flip chip IC may be attached.

[0157] Referring to FIG. 35 and FIG. 36, copper vias 426 with compressed reflowed solderable caps 425 that have been subjected to pressure and reflow at the same time are shown. By supplying pressure and heat, flat, dense solderable caps are obtained which are dense and well adhered to the copper vias.

[0158] Ideally the substrate bump has a similar diameter to the solder bumps on the chips. There are typically 60 μm to 110 μm. The technology described hereinabove allows bump diameters of as little as 35 μm. These may be separated by a spacing of about 20 μm, providing a pitch of 55 μm. Indeed, micro bumps of 15 micron diameter separated by 15 micron spaces are also possible.

[0159] There are a number of polymer dielectric films that are commercially available that have been found appropriate for laminating the very high pitch substrate arrays of the outer layers. These include NX04H available from Sekisui, HBI-800TR67680 available from Taiyo and GX-13 available from Ajinomoto.

[0160] The above description is provided by way of explanation only. It will be appreciated that the present invention is capable of many variations.

[0161] Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

[0162] Thus persons skilled in the art will appreciate that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.

[0163] In the claims, the word “comprise”, and variations thereof such as “comprises”, “comprising” and the like indicate that the components listed are included, but not generally to the exclusion of other components.