# Patent

## Title
High surface area aluminum bond pad for through-wafer connections to an electronic package

## Abstract

A bond pad for effecting through-wafer connections to an integrated circuit or electronic package and method of producing thereof. The bond pad includes a high surface area aluminum bond pad in order to resultingly obtain a highly reliable, low resistance connection between bond pad and electrical leads.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the provision of a bond pad for effecting through-wafer connections to an integrated circuit or electronic package, and which pad comprises a high surface area aluminum bond pad in order to resultingly obtain a highly reliable, low resistance connection between bond pads and electrical leads.
In the current state-of-the-technology, through-wafer connections are frequently employed in the formation of advanced types of electronic packages, for instance, such as, but not limited to, 3D packaging, MEMS packaging, or CMOS imager packaging. In particular, the process which is utilized for these connections is designed to etch a via through the rear side of the wafer and through the bond pads, so as to expose the edges of the respective bond pads. Leads are then formed so as to connect the edges of the bond pads to solder bumps, which are arranged on the rear sides of the electronic package. Thus, for multilevel aluminum (Al) wiring, multiple levels are employed in order to form the connection between the leads and the bond pads so as to be able to obtain low resistance electrical connections. However, when employing (Cu) copper wiring, this particular approach is subject to a poor or relatively low degree of reliability in the use of multiple Cu levels to form connections between the bond pads and leads, as a consequence of oxidation and corrosion of the copper material. A single aluminum pad is frequently employed as a final metal layer in Cu interconnect processes. Nevertheless, the use of such a single aluminum bond pad in effecting a connection to the leads may result in the formation of a high resistance connection, which has an adverse effect on reliability and may generate a high degree of heat shortening the service life of the electronic package in which it is installed.
Various aspects of the applicable technology have been developed, and prior art publications are in existence, which address themselves to the concept of employing vias under bond pads to provide for through-wafer connections or for the protection of underlying dielectrics. However, these constructions require either an extra mask, for instance, such as for through-wafer connections, or alternatively, necessitate that a metal layer be arranged beneath the via for the protection of underlying dielectrics.
2. Discussion of the Prior Art
Chisholm, et al., U.S. Pat. No. 6,586,839 B2 disclose the provision of vias beneath bond pads, which vias are intended to protect low-k dielectrics from sustaining any damage. These vias land on the underlying metal; and are required to employ this structure in order to mechanically strengthen the latter. This, however, requires the provision of arranging metal below the vias, inasmuch as such metal would of necessity be copper and would be subject to being exposed to ambient conditions and thereby susceptible to corrosion. In contrast to this prior art, which is plainly used for mechanical or structural integrity of low-k dielectrics beneath a bond pad, the inventive approach is intended to improve the electrical characteristics and increase cross section area of contacts from pads to leads on the backside of wafer.
Rolfson, U.S. Pat. No. 6,060,378 discloses the formation of a thick bond pad by adding a dielectric and a mask, and forming a thick damascene metal pad on top of a standard bond pad. Although this is somewhat similar to a general aspect of the present invention, in which the latter adds metal to the top of a bond pad using a shadow mask or selective plating, in contrast with the prior art the inventive approach resides in that there is an elimination of any deposit of additional layers over the remainder of the chip, inasmuch as such additional layers would be detrimental to the microlens, or similar MEMS structures.
With regard to the disclosures of Siniaguine, U.S. Pat. No. 6,639,303 B2; Cheng, et al., U.S. Patent Publication No. 2004/0141421 A1; and Pogge, et al., U.S. Patent Publication No. 2004/0097002 A1, these all require the application of an extra mask in order to produce through wafer vias below a bond pad. These vias extend all the way through a silicon wafer, whereas contrastingly, pursuant to the invention the via only extends through the dielectric on top of the silicon (Si), so as to structurally, distinguish over these patent publications.
SUMMARY OF THE INVENTION
To the contrary, in order to provide clear distinctions and advantages over the current state-of-the-art, pursuant to the invention there are employed vias or via bars beneath an aluminum bond pad in order to increase the cross-sectional contact area of metal, such as when there is provided a cut through the bond pad for electronic packaging. Pursuant to the present invention, there are provided advantages over the prior art in that there is eliminated the need for an extra mask in order to form the via or via bar, and whereas moreover, there are eliminated the arranging of metal lines beneath the via bar, this being an undesirable aspect as is presented in the prior art, inasmuch as such metal lines would normally be constituted of copper and would be exposed to an ambient environment during package forming and, consequently, subject to potentially corrosive conditions.
Accordingly, an object of the present invention is directed to utilizing various novel and unique aspects in that there are provided via bars under a bond pad pursuant to the first embodiment, and in an alternative manner, according to another embodiment there is added metal to the top of a bond pad in order to provide an extended or additional contact area to thereby produce a high surface area aluminum bond pad for the formation of a through-wafer connection to an electronic package.



BRIEF DESCRIPTION OF THE DRAWINGS
Reference may now be made to the following detailed description of the preferred embodiments of the invention, taken in conjunction with the accompanying drawings; in which:
FIGS. 1A through 1C describe sequential steps in the formation of the etching of vias in the back sides of an electronic package, and then forming the leads to connect the edges of bond pads to solder bumps in accordance with the prior art;
FIGS. 2A and 2B illustrate, respectively, perspective and enlarged views of a section of the bond pads and leads after singulation pursuant to the prior art;
FIG. 3 illustrates an enlarged diagrammatic detail of a portion of the bond pads and lead connection pursuant to the prior art;
FIGS. 4A through 4C illustrate sequential steps in the formation of a first embodiment of a structure utilizing via bars underneath bond pads pursuant to the invention;
FIGS. 5A and 5B illustrate a further embodiment showing sequential steps of adding metal to the top of a bond pad, respectively, before and after package via etch pursuant to the invention; and
FIGS. 6A and 6B illustrate, diagrammatically, plan and end views of typical corrugated aluminum pads with slotted vias therebeneath.



DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGS. 1A through 1C of the drawings, setting forth a prior art example of producing an electronic package, this is directed to a process for forming through-wafer connections by etching a via through the backside of a wafer and through bond pads, so as to expose the edges of the pads.
Hereby, as shown in FIG. 1A of the drawings, a front glass plate 10 has a plurality of spacer rims 12 attached thereto, and a die 14 is located to form a cavity 16 between the backside 18 of the front glass plate 10 and a silicon pad 20, etches 22 are implemented from the backside of the die so as to form I/Os (inputs/outputs) to the die. Thereafter, as shown in FIG. 1B of the drawings, a layer of glue 24 is employed to attach a back glass plate 26, and notching at angles to a normal is implemented for forming the initial structure 30 prior to singulation (by dicing).
Thereafter, as shown in FIG. 1C of the drawings, BGA solder bumps 32 are suitably fastened to the back plate and leads 34, which are connected extending along the notched surface 36 so as to connect to bond pads 38 formed beneath the spacer rim portions 12. Thereafter, final dicing along lines A is implemented to provide for singulation and to form discrete structures, as shown in FIGS. 2A and 2B of the drawings. In this instance, as illustrated, epoxy 40 attaches the silicon 20 to the back glass plate 26 and attachment surfaces are also provided for the BGA solder bumps 32 and the back surface of the back glass plate.
As set forth hereinabove, the spacer rims area comprise the bond pads 38, which are connected to the BGA solder bumps 32 by means of leads 34 extending along the angularly notched sides of the structure. A cavity 44 is formed below the back surface of the front glass plate 10 and the cavity provides an optically or mechanically sensitive area, as is known in the technology.
As shown in the enlarged detail of FIG. 3, this structure 50 discloses prior art multi-level aluminum-tungsten vias 52 with multi-level bond pads 54 to form connections between the leads 56 and the bond pads 54. This essentially provides a low-resistance connection to the aluminum and tungsten via structures; however, due to the use of copper wiring, normally employing multiple-level through connections between the bond pads and leads, poor reliability may be encountered due to oxidation and corrosion of the copper. Moreover, a single aluminum pad is frequently employed as the final metal layer in the copper interconnect processes and, if connected to the leads, may produce a high-resistance connection, with the attendant drawbacks.
The present invention may use a single aluminum pad, but increases the surface area of this pad by forming via bars of either W (via bar width <2× the thickness of W) or W+Al (via bar width >2× the thickness of W) underneath the pad, or alternatively by adding metal on top of the pad.
Via bars can be formed using the standard via mask or by using an additional mask. The depth of the via bar can be enhanced in two ways: (1) use RIE (reactive ion etch) lag to obtain a much greater etch depth for the via bar compared to the via, or (2) use an additional mask to form the via bars before the vias. Hereby, the surface area can also be increased by using an isotropic etch of the dielectric which is selective with regard to the metal (dilute HF for SiO2 dielectric, O2 plasma for organic dielectric), so as to recess the edge of the package via with respect to the metal.
A first one of these inventive embodiments (W+Al via bar, no additional masks) is shown in FIGS. 4A through 4C of the drawings.
Reverting in greater particularity to the drawings, and especially the first embodiment as exemplified in FIGS. 4A through 4C, there is diagrammatically shown in FIG. 4A a side view of an electronic package 60 a notch has been formed to expose the bond pads Provided is a glass plate 62, the back surface thereof includes a polymer layer 64, and along the edges at one side consisting of silicon dioxide or silicon nitrate, and the rear surface of which is contacted an Al or aluminum bond pad 66. A via 68 is provided beneath the bond pad 66. Tungsten (W) via bars 68 are provided at one end of bond pad 66, to provide connection to the circuits. Tungsten (W) via bar 70 is provided beneath the bond pads 66, and therebeneath a further glass plate is bonded thereto by means of epoxy and which forms the rear surface for the attachment thereto of BGA solder bumps (not shown). As shown in FIG. 4B of the drawings, which is a side view after the via etch has been implemented to the electronic package 60, there is indicated that in the structure thereof an optional isotropic etch 76 may be provided extending diagonally across one edge so as to increase the surface area of the bond pad 66. The via bars can also be formed from metals, such as tungsten, a combination of tungsten and aluminum, Cr, Au, Ni, NiMoP, Co, CoWP or CoWB.
As shown in an edge view of the electronic package in the direction of the arrow A in FIG. 4B, in FIG. 4C there are shown the various tungsten and aluminum via bars 80 beneath the aluminum bond pad 66, which provide for an increased surface area for a low resistance electrical contact.
As previously noted, the edge 82 of the electronic package 60 may be cut at an angle relative to the normal for positioning of the leads interconnecting the bond pads. This eliminates the need for applying of the additional masks, as is provided for in the prior art.
As indicated, an RIE lag can be used so that via bars 70 are at a greater depth than vias 68, and the isotropic etch, which is optional of the dielectric may be provided in connection with the type of metal employed where dilute HF is employed for SiO2 dielectric, and O2 plasma for an organic dielectric, for recessing or angling the edge of the package with regard to the metal. This also eliminates the requirement for an additional mask in comparison with the prior art, as shown for the invention in FIGS. 4A through 4C.
As shown in the embodiment of FIGS. 5A and 5B, in which the same reference numerals are used as in the previous embodiment for identical or similar elements, in this instance, an additional metal layer, such as aluminum, gold, silver, silicon based solder, lead based solder, palladium, platinum, chromium, nickel, copper or alloys thereof can be added on top of the bond pad after forming a terminal via. The additional metal is formed, as shown in FIG. 5A, by using a shadow mask to deposit the metal, wherein the aluminum is evaporated, and, if required as well, a barrier layer, such as of titanium, tantalum, tungsten, tantalum nitrate, tungsten nitrate, or titanium tungsten. These materials are not shown in the drawings.
Suitable electroplating or electroless plating may be employed to add the metal layer, whereby electroless plating requires no additional masks. On the other hand, electroplating will require one or two additional masks, which define a seed layer used to electroplate metal, as is known in the art.
Thereafter, an optional isotropic etch may be applied, as in the previous embodiment, as shown in the side view of FIG. 5B, after an electronic package etch, so as to increase the bond pad surface at an essentially angular relationship relative to the normal.
From the foregoing, it is also shown in FIGS. 6A and 6B, that each aluminum pad 80 may have slotted vias 82 provided under the respective bond pad into which the tungsten via bars 84 can be inserted and connected by means of leads 86 to the electronic package structure 88, as shown in FIG. 6A and sectional view of FIG. 6B.
From the foregoing, it becomes apparent by the invention that a highly reliable connection is produced, which avoids the use of copper for the connection between bond pads and leads, thereby reducing any possibility of corrosion; and there is also provided a low-resistance connection between the bond pads and the leads due to the high cross-sectional surface area, and also the possibility of utilizing single layer aluminum bond pads.
A particular use of the invention may also be ascertained in that it can be utilized in chip scale packaging (CSP) resulting in smaller integrated circuit package sizes. Further, in another application of the invention, there may be utilized image sensors, which benefit from the resultingly smaller packages, since a reduction in the size of an image sensor package enables the installation and utilization thereof in cell phone cameras. Pursuant to another application of the invention, there may be utilized MEMS sensors, which benefit from the resultantly smaller packages, since a reduction in the size of a MEMS sensor package enables the installation and utilization thereof in consumer portable electronics (GPS devices, and the like).
While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the spirit and scope of the appended claims.

## Claims

1. A method of producing low-electrical resistance connections to a bond pad, comprising providing an aluminum bond pad for effectuating a through-wafer connection to an integrated circuit chip or electronic package, and imparting a high-surface contact area to said bond pad to form said low-electrical resistance connection between said bond pad and electrical leads of said integrated circuit or electronic package, wherein imparting said high surface contact area comprises forming via bars at a bottom surface of said bond pad for increasing the contact area of said bond pad, and implementing an isotropic etch at an angular relationship to said bond pad so as to increase an electrical side contact surface of said bond pad.

2. A method as claimed in claim 1, wherein said via bars are constituted of materials which are resistant to corrosion from applicable ambient conditions, and which are selected from the group of metals consisting of tungsten, a combination of tungsten and aluminum, Cr, Au, Ni, NiMoP, Co, CoWP or CoWB.

3. A method as claimed in claim 1, wherein a plurality of said bond pads are each connected to electrical leads, a plurality of slotted said vias being arranged below each of said bond pads, and said via bars are constituted of materials which are resistant to corrosion from applicable ambient conditions, and which are selected from the group of metals consisting of tungsten, a combination of tungsten and aluminum, Cr, Au, Ni, NiMoP, Co, CoWP or CoWB, and are located in each of said vias for increasing the contact areas of said bond pads.

4. A method as claimed in claim 1, wherein a metal mask-forming layer is provided on the surface of said bond pad, and the surface area of said bond pad is increased.

5. A method as claimed in claim 4, wherein a shadow mask provides said metal mask-forming layer of evaporated metal to be deposited on the surface of said bond pad, or said metal is deposited by a selective plating process, such as electroplating or electroless plating.

6. A method as claimed in claim 4, wherein said mask-forming layer is constituted of a metal selected from the group of materials consisting of aluminum, gold, silver, silicon-based solder, lead-based solder, palladium, platinum, chromium, nickel, copper or alloys of said materials.