Etch thinning techniques for wafer-to-wafer vertical stacks

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the manufacture of microelectronic devices. In particular, the present invention relates to a method of thinning a wafer-to-wafer vertical stack, which prevents edge chipping and/or cracking.

[0003] 2. State of the Art

[0004] Greater packaging density of microelectronic devices is an ongoing goal of the computer industry. One method of increasing the density of microelectronic devices is to stack the individual microelectronic dice within these devices. One method of fabricating a stacked microelectronic device is to fabricate integrated circuitry on active surfaces of individual microelectronic wafers, stack them, then dice the stacked wafers into individual stacked microelectronic devices. FIG. 13 illustrates a first microelectronic wafer 202 having an active surface 204 and an opposing back surface 206. The active surface 204 includes integrated circuitry (illustrated generically as an integrated circuitry layer 208 within the illustrated dash lines) which is formed in the first microelectronic wafer 202 less than about 10 microns deep into the first microelectronic wafer 202. This integrated circuitry layer 208 is the functional area of the first microelectronic wafer 202. As will be understood by those skilled in the art, the microelectronic wafer 202 may be about 700 to 800 microns thick.

[0005] As shown in FIG. 14, an interconnect layer 212 is formed on the first microelectronic wafer active surface 206. The interconnect layer 212 may comprise multiple layers of conductive traces (not shown) separated by dielectric material layers (not shown). The first microelectronic wafer interconnect layer 212 provides routes for electrical communication between integrated circuit components within the integrated circuits and between integrated circuit components and external devices (not shown). As further shown in FIG. 14, the structure includes an exclusion zone 238 around the outer edge 210 of the first microelectronic wafer 202. The exclusion zone 238, usually between about 2 and 3 mm width, is necessary for uniform current distribution into a seed layer (not shown) for electroplating processes during the fabrication. Thus, any material used in during fabrication (e.g., photoresist, etc.), which falls within the exclusion zone 238 is removed (e.g., edge bead removal processes, etc.).

[0006] As shown in FIG. 15, a second microelectronic wafer 216 is provided, which also has an active surface 218, an integrated circuitry layer 222, and an interconnect layer 224 disposed thereon. The first microelectronic wafer interconnect layer 212 is aligned with the second microelectronic wafer interconnect layer 224 and attached using an electrically isolated metal bonding technique, as will be understood by those skilled in the art. The attachment of the first microelectronic wafer interconnect layer 212 and the second microelectronic wafer interconnect layer 224 may electrically interconnect the first microelectronic wafer integrated circuitry layer 208 and the second microelectronic wafer integrated circuitry layer 222.

[0007] Although the 700 to 800 micron thickness of the first microelectronic wafer 202 is required for the fabrication of the first microelectronic wafer integrated circuit layer 208, only the first microelectronic wafer integrated circuitry layer 208 is functional (it is, of course, understood that the second microelectronic wafer integrated circuitry layer 216 is also functional). Thus, after the fabrication of the first microelectronic wafer integrated circuit layer 208 and the first micro-electronic wafer interconnection layer 212, a substantial amount of the first microelectronic wafer 202 may be removed (i.e., “thinned”) without affecting the first microelectronic wafer integrated circuitry layer 208. Thinning a micro-electronic wafer makes it possible to route input-output signals, power, and ground to and from the integrated circuitry layer to the back surface of the microelectronic wafer, as will be discussed.

[0008] As shown in FIG. 16, the first microelectronic wafer 202 is thinned to a thickness of between about 10 and 100 microns forming a thinned back surface 226. A plurality of conductive vias 228 are formed to extend from the first microelectronic wafer thinned back surface 226 to the first microelectronic wafer integrated circuitry layer 208 to make electrical connections therewith, as shown in FIG. 17. A plurality of interconnect devices 232, such as solder balls, are then attached to the plurality of conductive vias 228 at the first microelectronic wafer thinned back surface 226, as shown in FIG. 18, to form a stacked wafer structure 234. The stacked wafer structure 234 may then be diced or singular, such as with a wafer saw or a laser (not shown) to form discrete packages 236, as shown in FIG. 19.

[0009] As previously discussed, the first and second microelectronic wafers 202, 216 each include exclusion zones 238 and 240, respectively, resulting in a portion 250 of the first microelectronic wafer 202 being unsupported. Thus, when the first microelectronic wafer 202 is thinned, the unsupported portion 250 is susceptible to chipping and cracking. As shown in FIG. 20, if the unsupported portion 250 (see FIGS. 16 and 17) chips off during thinning, a chip 242 can extend into the integrated circuitry layer 208, which can damage or destroy the functionality thereof. As shown in FIG. 21, if the unsupported portion 250 (see FIGS. 16 and 17) flexes during thinning, cracks 244 can propagate and extend into the integrated circuitry layer 208 also damaging or destroying the functionality thereof. Furthermore, chips and cracks may also facilitate contaminant incursion during subsequent processing, which may also damage or destroy the functionality of the integrated circuitry layer 208.

[0010] Therefore, it would be advantageous to develop methods for fabricating stacked microelectronic device, which reduces or substantially chipping and/or cracking of the microelectronic wafers during a thinning process.

[0027] In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein, in connection with one embodiment, may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.

[0028] The present invention includes methods for thinning a wafer-to-wafer vertical stack, also known as a stacked wafer structure. In particular, the present invention including removing unsupported portion of at least one wafer in the vertical stack by chemical etching. The removal of the unsupported portion eliminates potential cracking and chipping, which can occur during the thinning process when the unsupported portion exists.

[0029] FIGS. 1-12 illustrate a method of fabricating a stacked microelectronic device. FIG. 1 illustrates a first microelectronic wafer 102 having an active surface 104, a back surface 106 (opposing said active surface 104), and at least one edge 108. The first microelectronic wafer active surface 104 includes integrated circuitry components (illustrated generically as an integrated circuitry layer 112 within the illustrated dash lines), which may be formed in the first microelectronic wafer 102 less than about 10 microns deep from the first microelectronic wafer active surface 104 into the first microelectronic wafer 102. This first microelectronic wafer integrated circuitry layer 112 is the functional area of the first microelectronic wafer 102. As will be understood by those skilled in the art, the first microelectronic wafer 102 may have an initial thickness 116 between about 700 and 800 microns thick and may include any substrate capable of having integrated circuitry formed therein, including but not limited to, silicon, silicon-on-insulator, germanium, indium antimonide, lead telluride, indium arsenide, indium phosphide, gallium arsenide, or gallium antimonide. It is, of course, understood the integrated circuitry of the first microelectronic wafer integrated circuitry layer 112 may be any circuitry, including but not limited to, circuitry used in central processing units (CPUs), chipsets, memory devices, ASICs, and the like.

[0030] As shown in FIG. 2, an interconnect layer 114 is formed on the first microelectronic wafer active surface 104 by techniques known in the art. The first microelectronic wafer interconnect layer 114 may comprise multiple conductive traces (not shown) separated by a plurality of dielectric material layers (not shown). The first microelectronic wafer interconnect layer 114 provides routes for electrical communication between integrated circuit components within the integrated circuits and between integrated circuit components and external devices (not shown). As further shown in FIG. 2, the structure includes an exclusion zone 110 around the outer edge 108 of the first microelectronic wafer 102. The exclusion zone 110, usually between about 2 and 3 mm width, is necessary for uniform current distribution into a seed layer (not shown) for electroplating processes during the fabrication. Thus, any material used in during fabrication (e.g., photoresist, etc.), which falls within the exclusion zone 110 is removed (e.g., edge bead removal processes, etc.).

[0031] As shown in FIG. 3, a second microelectronic wafer 122 is provided, which also has an active surface 124, a back surface 126 (opposing said active surface 124), at least one edge 128, an integrated circuitry layer 132, an exclusion zone 130, and an interconnect layer 134 disposed thereon. The second microelectronic wafer 122 may also have an initial thickness 136 of between about 700 and 800 microns thick. The first microelectronic wafer interconnect layer 114 is aligned with the second microelectronic wafer interconnect layer 134 and attached thereto. The attachment is preferably achieved using an electrically isolated metal bonding technique, as will be understood by those skilled in the art. The attachment of the first microelectronic wafer interconnect layer 114 and the second microelectronic wafer interconnect layer 134 may electrically interconnect the first microelectronic wafer integrated circuitry layer 112 and the second microelectronic wafer integrated circuitry layer 132. It is, of course, understood the integrated circuitry of the second microelectronic wafer integrated circuitry layer 132 may be any circuitry, including but not limited to, circuitry used in central processing units (CPUs), chipsets, memory devices, ASICs, and the like.

[0032] As previously discussed, the first microelectronic wafer 102 and second microelectronic wafer 122 each include exclusion zones 110 and 130, respectively, resulting in a portion 140 of the first microelectronic wafer 102 being unsupported. Thus, to help prevent potential chipping and cracking during a subsequent thinning step due to the unsupported portion 140, as discussed previously, the first microelectronic wafer unsupported portion 140 is removed.

[0033] First, as shown in FIG. 4, the first microelectronic wafer 102 is preferably thinned to a thickness 116′ forming a first thinned back surface 106′. The first thinning may be achieved by any known process in the art, including but not limited to, grinding, spin etching, and/or chemical mechanical polishing. This first thinning should not be sufficient to induce chipping or cracking of the first microelectronic wafer 102 proximate the first microelectronic wafer unsupported portion 140. As shown in FIG. 5, the first microelectronic wafer first thinned back surface 106′ is placed against a wafer spin processor 142, which has a baffle 144 substantially surrounding the wafer spin processor 142.

[0034] As shown in FIG. 6, the first microelectronic wafer 102 and the second microelectronic wafer 122 are spun by the wafer spin processor 142 (illustrated by rotational arrow 154) as an etchant dispensing apparatus 148 dispenses a etchant 146, preferably a liquid, onto the second microelectronic wafer back surface 126. The etchant 146 may be any material capable of etching the first microelectronic wafer 102 (and may also be suitable to etch the second microelectronic wafer 122, if so desired). For example, if the first microelectronic wafer 102 contains silicon the etchant 146 may include, but is not limited to, TMAH, a mixture of HF, HNO₃ and H₂PO₄, or any chemistry suitable for etching silicon. The etchant 146 flows across the second microelectronic wafer back surface 126, down the second microelectronic wafer edge 128, down the first microelectronic wafer edge 108, and adjacent the wafer spin processor 142. Due to the rotation of the wafer spin processor 142 and the flow of the etchant 146, the baffle 144 directs the etchant 146 substantially radially from the wafer spin processor 142 (illustrated generally by directional arrows 156 and 156′). The etchant dispensing apparatus 148 may be moved during the dispensing of the etchant 146 in order to achieve a desired distribution. The movement of the etchant dispensing apparatus 148 is illustrated generally by directional arrows 152 and 152′.

[0035] As shown in FIG. 7, the first microelectronic wafer edge 108 is etched away, which removes the unsupported portion 140 (shown in FIGS. 2-4) and forms an etched edge 108′, which is preferably proximate the first microelectronic wafer interconnect layer 114. Preferably, the first microelectronic wafer first thinned back surface 106′ (see FIG. 6) is also etched during the process to form a second thinned back surface 106″. Most preferably, the forming of the first microelectronic wafer second thinned back surface 106′ also achieves a desired thickness 116″, preferably to a thickness of between about 10 and 100 microns. However, it is, of course, understood that the first microelectronic wafer 102 can be thinned to a desired thickness after the removal of the unsupported portion 140. It is further understood that the second microelectronic wafer back surface 126 (see FIG. 6) and the second microelectronic wafer edge 128 may be etched during the etching process forming an etched second microelectronic wafer back surface 126′ to an etched thickness 136′, and an etched second microelectronic wafer edge 128′.

[0036] As shown in FIG. 8, when the desired etching is achieved, the dispensing of the etchant 146 (see FIG. 7) and the spinning of the wafer spin processor 142 (see FIG. 7) is discontinued. As shown in FIG. 9, the resulting thinned wafer stack 160 is removed from the wafer spin processor 142. A plurality of conductive vias 162 are then formed to extend from the first microelectronic wafer second thinned back surface 106″ to the first microelectronic wafer integrated circuitry layer 112 to make electrical connections therewith, as shown in FIG. 10, by any known technique. A plurality of interconnect devices 164, such as solder balls, are then attached to the plurality of conductive vias 162 at the first microelectronic wafer second thinned back surface 106″, as shown in FIG. 11, to form a stacked wafer structure 166. The stacked wafer structure 166 may then be diced or singulated, such as with a wafer saw or a laser (not shown) to form a discrete microelectronic device 170, as shown in FIG. 12.

[0037] It is, of course, understood that rather than the plurality of interconnect devices 164 shown in FIG. 11, other structures could be formed, such as build-up layers (i.e., trace networks) or other such structures for the attachment of external devices.

[0038] 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.