Embedded chip enclosure with solder-free interconnect

Details Embedded chip enclosure with solder-free interconnect Embedded chip enclosure with solder-free interconnect

2002-02-04

2004-10-05

Parekh, Nitin (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Encapsulated

With specified encapsulant

C257S730000, C257S702000, C257S701000, C257S706000, C257S707000, C257S713000, C257S785000, C438S126000

Reexamination Certificate

active

06800949

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to what are known in the semiconductor field as chip (die) enclosures. More specifically the invention relates to an enclosure wherein one or more semiconductor die/dice are embedded within the base of the enclosure rather that on its surface. In greater specificity, the invention relates to an embedded die enclosure wherein electrical connection to the enclosure is performed through solder-free interconnects.
Enclosures for semiconductor die, sometimes referred to as packages when containing a single die and as multi-chip modules when containing plural die, fall largely into two categories. These categories are based upon enclosure “base” material. The common enclosure base is either ceramic or plastic. The way in which a semiconductor die is operably coupled to such an enclosure and the way in which the enclosure is then operably coupled to the outside world (interconnected) is largely contingent upon this enclosure material.
For typical ceramic enclosures, a semiconductor die is “die attached” to a gold film disposed on a ceramic base. In this stage, a gold/silicon eutectic solder is formed at the die/base interface. Electrical leads for the die are typically configured into an electrical lead frame that is embedded in glass disposed at the periphery of the ceramic base. The bonding pads of the semiconductor die are then bonded to the “fingers” of the electrical lead frame, using ultrasonics for aluminum fingers or gold ball bonding techniques for gold wire fingers. The enclosure and die are then sealed in either a belt furnace or other sealer device using glass or a low temp solder such as gold-tin. Electrical connection of the enclosure such as to a printed circuit board is through solder connections.
For typical plastic enclosures, a semiconductor die is die attached to a gold plated pad on an electrical lead frame. A gold/silicon eutectic solder is formed at the interface. Gold wire is then gold ball bonded from the aluminum bonding pads of the semiconductor die to the “fingers” of the electrical lead frame. The structure is then encapsulated with an “epoxy encapsulant”, such as in a traditional hot-press transfer-molding machine. As with ceramic enclosures, electrical connection to the plastic-based enclosure is through solder connections.
Either of the above-two designs have their limitations.
In the ceramic enclosure field, die attach temperatures are typically greater than 400° C. While these temperatures have allowed successful ceramic enclosure operation, lower processing temperatures are desirable to enhance quality and reliability.
The electrical lead frames used in ceramic enclosures are relatively expensive. In addition, the combination of gold plated lead frame fingers and die aluminum bonding pads leads to a potential long term reliability problem in the form of purple plague/intermetallics and Kirkendall voiding. Further, the wire bonding process requires one wire to be bonded at a time. This is a relatively costly endeavor.
As with all enclosures containing a void therein, there is a possibility of conductive particles (contaminants) becoming inadvertently sealed within the enclosure. These particles have the potential of shorting elements of the enclosed electronics. Finally, the sealing process itself is typically done at high temperatures of about 400° C. As with die attach temperatures, lowering the seal processing temperatures is desirable to improve quality and reliability.
As described, the enclosure is typically electrically interconnected to components outside the enclosure by soldering the enclosure's leads to printed circuit boards. If this soldering process is not precisely performed, problems with temperature and solder cracking may result.
Plastic enclosures have drawbacks as well. Data indicates that the epoxy encapsulant materials used with these enclosures contain additives that due to the close proximity to the semiconductor die create degradation of the device during all radiations levels. It is also known that gold ball bonding to aluminum bonding pads presents a potential long term reliability problem in plastic enclosures. This is due to both a bimetallic system and the presence of additive materials of the epoxy encapsulant. These problems embody themselves in the form of purple plague, inter-metallics and Kirkendall voiding.
Plastic enclosed modules also experience what is termed “popcorning,” where internal enclosure delamination occurs due to moisture and soldering temperatures, creating enclosure cracking and electrical problems.
Finally, as with ceramic enclosures, the solder interconnections by which the enclosure is electrically connected to its outside world must be precisely performed. If done imprecisely, temperature and solder cracking problems may result.
To overcome the difficulties and drawbacks of prior art ceramic and plastic enclosure designs, a radically new approach to the enclosure art is needed. The new enclosure design should find a way to dispose of the complexities and temperatures of traditional die attach and sealing methods. The new approach should discard the wire bonding techniques necessary to attach the die to its electrical lead frame. The approach should ideally do away with traditional electrical lead frames altogether and their need for solder interconnection to printed circuit boards.
SUMMARY OF THE INVENTION
An example of the invention begins with a thin fused silica substrate, such as one made from quartz. The substrate is processed to a thickness that allows it to be easily flexed. The substrate is then mettalicized on its major planar surfaces. Through techniques such as those used in the tuning fork industry, an opening is etched in the silica substrate. The substrate is then cleaned of its mettalicized layers.
A die having a patterned topside is then processed to the thickness of the substrate by lapping the opposite side (backside) of the die. Using conventional “pick and place” equipment, the thinned die is positioned within the opening of the substrate. The substrate with die-in-place is mounted on a spinner to enable glass to be spun on both the top and backside surfaces of the die/substrate combination. The low-viscosity spun-on-glass flows not only on these surfaces but also between the die and the substrate.
Traditional processing techniques are then used to remove the spun-on-glass lying over the patterned bonding pads of the die. For enhanced thermal characteristics, at least a portion of the backside spun-on-glass can also be removed where it lies adjacent to the die.
The oxide AL
2
O
3
on the aluminum bonding pads of the die is then removed, such as with an eximer laser or equivalent in a vacuum system. After a sputter etch or equivalent in the pumped down system, aluminum is deposited over the entire topside of the structure, making good ohmic contact to the AL
2
O
3
—free bonding pads.
Using traditional techniques, conductive traces are then constructed to provide electrical connection from the bonding pads of the embedded die to the periphery of the enclosure for external electrical interconnect. Since spacing is not dictated by traditional bonding pad sizes and wire bonding requirements, it is possible to provide a greater number of connectivity lines per chip side. Metal may also be deposited on the backside of the enclosure for enhanced thermal heat dissipation.
The flexural properties of the thin fused silica (or equivalent) permit the enclosure to be made arcuate (non-linear) and thereby inserted into a PCB board without solder, making good ohmic interconnect contact by way of a pressure fit. Multi-sided enclosures and even circular enclosures are envisioned to be able to be mounted in this manner.
As a final processing step, a metal such as copper may be applied to the aluminum traces at the outer edges of the enclosure where the enclosure's leads will be interconnected with components “outside” of the enclosure.
An object of this invention is to provide a die enclosure wherein traditional die attach

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