Patent Application
20060166402
(1) Field of the Invention
The present invention relates to the fabrication of integrated circuit devices on semiconductor chips, and more particularly relates to making improved elevated bond-pad structures for bonding chips on and to a substrate using flip-chip technology. These elevated bond-pad structures are particularly useful for increasing the density of the input/output (I/O) bond pads on a chip while reducing electrical shorts between closely spaced (adjacent) solder balls (bumps) on the array of pads. The elevated bond pads increase the spacing between chip and substrate during bonding for improved under-fill flow rates, and also reduce Alpha (α) particle emission from Pb/Sn solder balls or bumps into the semiconductor chip.
(2) Description of the Prior Art
In recent years there has been a renewed interest in replacing the conventional wire bond techniques with flip-chip bonding techniques to increase circuit performance and reduce package size. In the flip-chip method lead/tin (Pb/Sn) solder balls (or bumps) are formed on an array of bonding pads on a chip, and the chip is mounted (soldered) upside-down to a substrate, such as a circuit board, including ceramic substrates, and the like. In recent years advances in the semiconductor process technologies have dramatically decreased the semiconductor device feature sizes and increased the circuit density of the integrated circuits on the chip. As a consequence this increase in circuit density has resulted in increased density of the array of I/O pads on the chip, with reduced spacing between adjacent pads and reduced pad areas. Because of surface tension, the volume-to-surface area of the solder is maximized and the solder forms a bead or ball (bump). When the area of the bond pad is reduced and hence the wetting surface is reduced in size, the lead balls are also smaller (e.g., <100 um). When the chip is bonded to the substrate, the reduced spacing between the chip and the substrate makes it more difficult to under-fill between the chip and the substrate (circuit board) with Epoxy+filler to strengthen the solder joints and seal the chip on the “circuit board.”
Numerous methods for making bonding pads for both wire bond and flip-chip bonding have been reported in the literature. For example, one method for wire bonding is described in U.S. Pat. No. 6,376,353 B1 to Zhou et al. in which an Al—Cu alloy bond pad is used to improve the adhesion of the wire-bond solder to the underlying Cu metallurgy. U.S. Pat. No. 6,544,880 B1 to Akram shows a method in which one or more metal barrier layers are deposited on the underlying copper to improve adhesion. In U.S. Pat. No. 5,523,920 to Machuga et al. a method is described for elevating the bonding pads above a polymeric coating on a circuit board to facilitate soldering operations. Methods relating to flip-chip bonding include U.S. Pat. No. 5,891,756 to Erickson in which a wire bond pad is converted to a flip-chip solder bump by electroless plating nickel (Ni) on the underlying Al pad to prevent oxidation. A solder bump pad is then formed on the nickel.
In U.S. Pat. No. 6,578,754 B1 to Tung an elongated pillar structure is described for flip-chip bonding. The lower portion of the pillar is copper to reduce alpha particles, and the upper portion is PbSn for bonding. U.S. Pat. No. 6,692,629 B1 to Chen et al. uses a plating bus over and along the cutting lines (kerf areas) to each bond pad for plating the bump pads prior to separating the chips by cutting (dicing). In U.S. Pat. No. 6,770,547 B1 to Inoue et al., a method is described for making underfill-less flip-chip bonding that allows defective chips to be replaced on the circuit board, and also avoids alpha particle thereby preventing soft errors in the semiconductor circuit. Several Patent Application Publications have been identified that address the flip-chip technology. In Pub. No. U.S. 2002/0121692 A1 to Lee et al., a method is described to form closely spaced (fine pitch) pillar solder bump pads on a chip for flip-chip bonding. Pub. No. U.S. 2004/0157450 A1 to Bojkov et al. describes a method for directly bonding solder bumps to copper studs.
However, there is still a strong need in the semiconductor industry to improve the bonding pad structure for flip-chip (lead bump) technology for high-density integrated circuits without significantly increasing manufacturing process complexity.
A principal object of this invention is to make an array of improved bond-pad structures on chips for increased density when flip-chip bonding (soldering) a chip to a substrate, such as on a circuit board or a ceramic substrate.
A second object of this invention is to make an elevated bond-pad structure to improve the under-fill (Epoxy+filler) and concurrently to reduce alpha particle radiation from the lead/tin balls into the silicon chip
A third object of this invention is to make sidewall spacers on the elevated bond-pad structures to restrict the wetting area for the Pb/Sn ball (bump) to the top area of the elevated bond pads, thereby reducing the electrical shorting between adjacent bond pads on the chip.
Another objective of this invention, by a second embodiment, is to further reduce the top area of the lead wetting layer on the bond pad by patterning by partial etching which forms a second sidewall on which sidewall spacers are formed to further increase the pad density while eliminating electrical shorts between adjacent bond pads during soldering.
In accordance with the objects of the present invention a method for fabricating elevated bond pads with sidewall spacers to improve bond-pad density is achieved. Typically, integrated circuits are made on an array of chips on a semiconductor substrate (wafer) up to and including a top metal to provide areas for wire bonding or flip-chip bonding to a circuit board or ceramic substrate. In the flip-chip method the bond pads and lead/tin (Pd/Sn) bumps are formed prior to dicing the semiconductor substrate to separate the individual chips (die).
In accordance with the objectives of this invention the method for forming these elevated bond-pad structures by a first embodiment begins by providing a semiconductor substrate having an array of semiconductor chips, each chip has an array of top metal pads that are formed in recesses and are planar with a first insulating layer on the substrate. A second insulating layer is formed over the top metal pads with openings to top surface of the top metal pads. A key feature of this invention is to form elevated bond pads in the openings. The elevated bond pads extend above the surface of the second insulating layer to provide exposed sidewalls on the elevated bond pads to increase the height of the chip over the substrate (circuit board) during flip-chip bonding. Another key feature is to form sidewall spacers on the sidewalls of the elevated bond pads to reduce electrical shorts between adjacent bond pads during soldering. A further advantage of the sidewall spacers is to passivate the copper from oxidation during storage. An under-bump metallurgy layer is deposited and patterned on the top surface of the elevated bond pads. The under-bump metallurgy layer is a multilayer that serves as an adhesion layer, a diffusion barrier layer, and a solder-wetting layer.
The method by a second embodiment is similar to the first embodiment up to and including the formation of the second openings in the second insulating layer. Instead of electroless plating copper, as in the first embodiment, a conformal Ti/TiN barrier layer is deposited, and an etchable metal such as aluminum or aluminum-alloy layer is deposited sufficiently thick to form elevated bond pads over the second openings in the second insulating layer. Using a photoresist mask and plasma etching the aluminum is then patterned to form the elevated bond pads. A key feature of this second embodiment is to use a second photoresist mask and partial anisotropic etching to further reduce the top surface area of the elevated bond pads that result in second sidewalls on the elevated bond pads. Sidewall spacers are formed on the first and second sidewalls by depositing a conformal insulating layer, such as SiO2 or Si3N4, and anisotropically etching back the insulating layer to the top surface of the Al bond pads. By reducing the top surface area of the elevated bond pads, the array of solder bumps can be formed closer together. As in the first embodiment, an under-bump material multilayer is deposited to provide an adhesion layer, a diffusion barrier layer, and a solder-wetting layer.
The method for making an array of elevated bond pads is described in detail for a first embodiment. Although the method is described for making an array of elevated bond pads on a chip, only a portion of a substrate having a single bond pad is depicted to simplify the drawings. The method for fabricating elevated bond pads with sidewall spacers to improve bond-pad density is now described.
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While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.
