Electrooptic assembly

[0001] The present invention relates to apparatus and methods for packaging a microelectronic component with one or more optical components in a compact yet functional manner. In particular, the present invention relates to a packaging technology that aligns and secures an electrooptic element between a microelectronic package and a substrate with a waveguide.

[0002] Higher bandwidth, improved performance, lower cost, and increased miniaturization are ongoing goals of the computer industry to enhance communication between chips within a computer. It is known that integrated circuit components have undergone increased miniaturization and, as this goal is achieved, microelectronic dies become still smaller. Similar miniaturization of the microelectronic package is desirable as it would enable reductions in cost and component external dimensions (form factor).

[0003] As for achieving higher bandwidth, e.g., on the order of 10 Gb/s or more, optical interchip communications face many challenges. Among these challenges are high bandwidth, low latency communication between the microprocessor and the optical emitter/detector chips, alignment and coupling between the optical emitter/detector chips and a waveguide, and maintaining acceptable cost. Until now, the difficulty of meeting all of these challenges has meant that interchip communications are typically achieved in an electronic manner.

[0004] Meeting the challenges facing optical interchip communications will provide significant benefits, including the higher bandwidth that optical interchip communication provides as well as reduced noise from sources such as electromagnetic interference (EMI) and crosstalk. However, it will be necessary for electrooptic assemblies to provide good alignment to waveguide structures that may be located on the printed circuit board, low electrical parasitics between the microprocessor and optical emitter/detector chips, and relatively low cost fabrication.

[0005] Therefore, it would be advantageous to develop new apparatus and methods for securing a microelectronic package to a substrate such as a printed circuit board with an electrooptic element in alignment with a coupler and waveguide integrated into the printed circuit board (electrooptic PCB) to provide a suitable electrooptic assembly.

[0019] Although FIGS. 1–12 illustrate various views of the present invention, these figures are not meant to portray microelectronic and optical assemblies in precise detail. Rather, these figures illustrate microelectronic and optical assemblies in a manner to more clearly convey the concepts of the present invention. Additionally, it should be noted that elements common between the various figures retain the same numeric designation throughout the various views.

[0020] The present invention includes a packaging technology that secures a microelectronic package to an electrooptic PCB in a manner that secures an electrooptic element in alignment with a coupler and waveguide in the electrooptic PCB. The electrooptic element is disposed between the microelectronic package and the electrooptic PCB to convert an electrical signal from a microelectronic die in the microelectronic package to an optical signal that is transmitted to the coupler and waveguide. Alternatively, the electrooptic element can convert an optical signal from the coupler and waveguide in the electrooptic PCB to an electrical signal that is transmitted to the microelectronic package.

[0021] FIG. 1 illustrates a microelectronic package core 20 which comprises a substantially planar material used to fabricate a microelectronic package having an opening 22 formed therein. The opening 22 extends completely through the microelectronic package core 20 from a first, active surface 24 to an opposing, second surface 26. As will be appreciated, the opening 22 may be any shape and size with the only limitation being that it be appropriately sized and shaped to house a corresponding microelectronic die therein, as will be discussed below.

[0022] FIG. 1 illustrates the microelectronic package core second surface 26 abutting a protective film 28. A microelectronic die 30 having an active surface 32 and a back surface 34 is placed in the opening 22 of the microelectronic package core 20, as shown in FIG. 2. The microelectronic die 30 is placed such that its active surface 32 abuts the protective film 28 which may have an adhesive, such as silicone, thereon. The protective film 28 attaches to the first, or active, surface 24 of the microelectronic package core 20 and the active surface 32 of the microelectronic die 30, as shown in FIG. 4. Alternatively, the protective film 28 may be a non-adhesive film depending upon the encapsulation process.

[0023] In particular, an adhesive-type protective film 28 may be applied prior to placing the microelectronic die 30 and microelectronic package core 20 in a mold or other piece of equipment which can be used for an encapsulation process. The protective film 28 may also be a non-adhesive film, such as a EPFE (ethylene-petrafluoroethylene) or Teflon® film, which is held on the microelectronic die active surface 32 and the microelectronic package core first, or active surface 24 by an inner surface of a mold or other piece of equipment which can be used for an encapsulation process. In either case, the protective film 28 will be removed from the active surface 32 of the microelectronic die 30 and the first, or active surface 24 of the microelectronic package core 20, following an encapsulation process.

[0024] The microelectronic die 30 is then encapsulated using an encapsulation material 36 to fill the portions of the opening 22 in the microelectronic package core 20 which are not occupied by the microelectronic die 30, as shown in FIG. 5. The encapsulation of the microelectronic die 30 may be achieved by any known process, including but not limited to transfer and compression molding, and dispensing. It is also possible to perform such an encapsulation process without use of a protective film. The encapsulation material 36 serves to secure the microelectronic die 30 within the microelectronic package core 20 and provides mechanical rigidity for the resulting structure together with surface area for the subsequent build-up of conductive trace layers.

[0025] After encapsulation, the protective film 28 is removed, as shown in FIG. 4, to expose the microelectronic die active surface 32 with the encapsulation material 36 serving as a filler for the space between the microelectronic package core 20 and the microelectronic die 30. The result is at least one surface 38 that is substantially coplanar with the microelectronic die active surface 32 and the microelectronic package core first, or active surface 24. FIG. 5 illustrates a microelectronic die 30 encapsulated with encapsulation material 36 within the microelectronic package core 20. The microelectronic die 30 includes a plurality of electrical contacts 40 located on the microelectronic die active surface 32. The electrical contacts 40 are electrically connected to circuitry (not shown) within the microelectronic die 30 but, for sake of simplicity and clarity, it will be appreciated that only four electrical contacts 40 have been shown in FIG. 5.

[0026] As shown in FIG. 6, a dielectric layer 42, such as epoxy resin, polyimide, bisbenzocyclobutene, and the like, is disposed over the microelectronic die active surface 32. The dielectric layer 42 covers the electrical contacts 40 as well as the microelectronic package core first, or active, surface 24 and the encapsulant material surface 38. The formation of the dielectric layer 42 may be achieved by any known process, including but not limited to lamination, spin coating, roll coating, and spray-on deposition.

[0027] As shown in FIG. 7, a plurality of vias 44 is then formed through the dielectric layer 42 by any method known in the art, including but not limited to laser drilling, photolithography, etc. If the dielectric layer 42 is photoactive, the plurality of vias 44 may be formed in the same manner that a photoresist mask is made in a photolithographic process, as known in the art.

[0028] A plurality of conductive traces 46 is formed on the dielectric layer 42, as shown in FIG. 8. A portion of each of the plurality of conductive traces 46 is formed so as to extend into at least one of the plurality of vias 44 (see FIG. 7) in order to make electrical contact with the contacts 40 on the microelectronic die 30. The plurality of conductive traces 46 may be formed of any suitable conductive material, such as copper, aluminum, alloys thereof, and conductive polymer resins.

[0029] The plurality of conductive traces 46 may be formed by any known technique, including but not limited to semi-additive plating and photolithographic techniques. An exemplary semi-additive plating technique can involve depositing a seed layer, such as a sputter-deposited or electroless-deposited metal on the dielectric layer 42. A resist layer can then be deposited and then patterned on the seed layer. A layer of metal, such as copper, can then be applied by electrolytic plating on the seed layer exposed by open areas in the patterned resist layer. The patterned resist layer can then be stripped and portions of the seed layer not having the layer of metal plated thereon can be etched away to complete the formation of the conductive traces 46. While the foregoing sets forth one known technique for forming conductive traces, other methods of forming the plurality of conductive traces 46 will be apparent to those skilled in the art.

[0030] The sequence of process steps used to deposit the dielectric material, form vias, and form traces can be repeated as needed to dispose the traces in such positions as may be required to achieve a suitable interconnection or to meet electrical or other performance requirements. Once the conductive traces 46 are formed, they can be used in the formation of conductive interconnects with solder bumps, solder balls, pins, and the like for communication with external components, such as an optical substrate. For example, a soldermask material 48 can be disposed over the dielectric layer 42 and the conductive traces 46 with soldermask openings 51, as shown in FIG. 9, and a plurality of vias 50 can then be formed in the solder mask material 48 to expose at least a portion of each of the conductive traces 46. A plurality of conductive bumps 52 such as a ball grid array (BGA) can be placed in contact with the exposed portion of the conductive traces 46 and secured thereto by a reflow process, as shown in FIG. 10.

[0031] FIG. 11 illustrates an electrooptic element 54 such as a vertical cavity surface emitting laser (VCSEL) or photodetector in contact with at least one conductive trace 46 to convert an electrical signal from the microelectronic die 30 to an optical signal or vice versa. The electrooptic element 54, as shown in FIG. 11, is in contact with a pair of conductive traces 46 through solder balls 56 that by means of a reflow process secure the electrooptic element 54 to the microelectronic package 58. As will be appreciated, the microelectronic package 58 is defined by the microelectronic package core 20, the microelectronic die 30, the encapsulant material 36, and the build-up layer or layers 60 defined by the dielectric layer 42, the conductive traces 46, and the solder mask material 48.

[0032] As shown in FIG. 12, an electrooptic PCB 62 secured to the microelectronic package 58 with the BGA 52 has a first surface 66 and a second surface 68 opposite the first surface 66. FIG. 12 illustrates that a coupler 70 and waveguide 72 is provided in or on one of the first and second surfaces, such as the first surface 66, or at an intermediate position, and they are positioned relative to the active side of the microelectronic package 58 to be spaced from but facing the microelectronic die active surface 32. FIG. 12 also illustrates that the active side of the microelectronic package 58 is defined at least in part by the active surface 32 of the microelectronic die 30 and the corresponding active surfaces 24 and 38 of the microelectronic package core 20 and the encapsulation material 36, respectively, all of which are coplanar. FIG. 12 also further illustrates that the build-up layer or layers 60 on the active side of the microelectronic package 58 is provided on at least one, and preferably all, of the microelectronic die active surface 42 and corresponding active surfaces 24 and 38 of the microelectronic package core 20 and the encapsulation material 36, respectively. The positions of the BGA joints illustrated in FIG. 12 are determined by the positions of the soldermask openings 51. The soldermask openings for the BGA balls which secure the microelectronic package 58 to the electrooptic printed circuit board 64 and the BGA balls which secure the electrooptic element 54 would preferably be made as part of the same patterning process, or with a single exposure of the same mask, if the patterning process is photo-lithographically based.

[0033] Alignment between the electrooptic element and the coupler may be achieved by solder self-alignment. Such self-alignment may be achieved, for example, by BGA reflow. Successful alignment by this method requires accurate placement of the coupler with respect to the BGA solder mask openings of the optical substrate.

[0034] Still referring to FIG. 12, the electrooptic element 54 may include an integrated lens 76 to focus and transmit the optical signal to or from the coupler 70 and waveguide 72. A refractive-index matched material as at 78 may be provided on a surface of the integrated lens 76 generally adjacent the coupler 70 and waveguide 72. FIG. 12 also illustrates that the optical substrate 62 may comprise a printed circuit board having the coupler 70 and waveguide 72 therein, for example, on the side facing the microelectronic package 58.

[0035] Having thus described in detail embodiments of the present invention, it is understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.