Method of fabricating a microelectronic device utilizing unfilled epoxy adhesive

A method of bonding microelectronic components (10, 23, 24) is disclosed. A light emitting diode (10) is bonded to a conductive lead (24) and/or a portion of a lead frame (23) using an adhesive having no metallic particles therein. The diode (10) is clamped to the conductive lead (24) and/or the lead frame (23) as the epoxy is cured. Such bonds have been found to exhibit low contact resistance in addition to long life and reliability.

TECHNICAL FIELD 
This invention relates to a method of fabricating microelectronic devices. 
More particularly, the method is directed to electrically connecting 
components of the microelectronic devices using adhesive bonding 
techniques. 
BACKGROUND OF THE INVENTION 
A well-known solution in the prior art to the problem of bonding 
microelectronic components together is the use of an electrically 
conductive adhesive, such as an epoxy adhesive, typically comprising an 
epoxy material "filled" with metallic particles, e.g., silver, gold, 
copper, etc. dispersed therethrough, as described in an article titled 
"Where Epoxy Die Bonding for Microelectronics Stands Today" by F. W. 
Kulesza in Insulation/Circuits, November 1974, pages 31 to 33. That 
article describes the bonding of electrically conductive leads to 
terminals and electrical devices using the filled epoxy adhesive. Such a 
process has been found to be most effective and economically beneficial. 
However, when the devices to be bonded are certain semiconductor devices, 
such as light emitting diodes (LED's) having exposed junctions, it has 
been found that metallic ions associated with the particles in the 
adhesive migrate to the junction region and deleteriously affect the 
operation thereof. 
SUMMARY OF THE INVENTION 
The foregoing problem has been overcome by the instant method for providing 
an electrically conductive bond between first and second microelectronic 
components. The method is accomplished by coating at least a portion of 
the first and second components with a noble metal; applying an unfilled 
adhesive coating on the coated surface of at least one of the components 
and urging the components into intimate contact at the gold coated 
surfaces with the unfilled adhesive therebetween; and applying a clamping 
force to the components while curing the adhesive. 
Surprisingly, the bond between the microelectronic components results in a 
low resistance connection having high mechanical strength and of a quality 
at least as acceptable as the bond formed using a filled adhesive. 
Advantageously, the use of such unfilled adhesive obviates the 
aforementioned migration problem associated with the prior art technique. 
Furthermore, the use of such unfilled adhesive precludes the need for 
precise adhesive deposition techniques, since the presence of the unfilled 
material in areas other than between the components being connected does 
not have the deleterious effects associated with the use of conductive 
fillers.

DETAILED DESCRIPTION 
The present invention is described primarily in terms of bonding a light 
emitting diode (LED) to an electrically conductive lead and/or a lead 
frame. However, it will be understood that such description is exemplary 
only and is for purposes of exposition and not for purposes of limitation. 
It will be readily appreciated that the inventive concept described is 
equally applicable to bonding any suitable microelectronic components 
together. It should be further appreciated that the inventive concept 
described is equally applicable to bonding a single assembly or 
simultaneously bonding a multitude of components. 
FIG. 1 is an isometric view of an LED 10 which may be of any well-known 
construction and could include GaAs, GaP, GaPAs, GaAlAs compound 
semiconductor chips having adjacent P-type and N-type semiconductor region 
with a P-N junction 11 therebetween. The N-type and P-type regions are 
formed by well-known techniques such as diffusion, epitaxy or the like. 
The LED 10 exhibits electroluminescence in the vicinity of the P-N 
junction 11 when charge carriers of one type are injected into a region 
where the predominant charge carriers are of the opposite type. Radiation 
is emitted in conjunction with the recombination of pairs of oppositely 
charged carriers. In an exemplary embodiment, each LED 10 was a cube 
wherein each side was approximately 0.015 inch in length. 
As hereinbefore indicated it is well known to form an electrical bond 
between microelectronic components using a filled epoxy (i.e., an epoxy 
having conductive metallic particles therein). However, when devices such 
as the LED 10 are bonded using such a filled epoxy, ions associated with 
the metallic particles therein migrate to the P-N junction 11 which 
seriously affects the operation of the device. The present invention 
overcomes this problem by using an unfilled adhesive (i.e., an epoxy 
having no metallic particles therein) to bond the microelectronic 
components together. From a mechanical standpoint, the bond so formed is 
one having high strength, thermal stability, structural integrity, and 
otherwise provides a connection that is at least equivalent in all 
respects to bonds formed using electrically conductive adhesives. 
Surprisingly, from an electrical standpoint, the bond also exhibits a low 
contact resistance. Although the mechanism is not fully understood, it 
appears that the clamping force used to bond the components causes the 
non-conductive adhesive to be moved into the interstices between high 
points on the surfaces of both components permitting the high points to 
touch or to be in close enough proximity to provide a low resistance, 
electrically conductive path between the components. A current of 
approximately 10 milliamps passes through the bonded LED 10. 
It shold be emphasized that the surface of the components to be joined with 
the unfilled epoxy must be substantially free of insulating films, oxides 
or the like. Such contaminant-free surfaces are obtained and preserved for 
the lifetime of the assembly in the present invention by coating the 
surfaces with a thin layer of gold. Although gold has been found to be 
most effective, any other noble metal may be used. Additionally, other 
techniques, such as cleaning the components, bonding and use of the 
assembly in an environment where deleterious films, oxides or the like 
would be prevented or placing additives in the non-conducting adhesive to 
remove and prevent the formation of the undesirable films at the bond site 
may be used. 
FIG. 2 depicts a lead frame, generally referred to by the numeral 20, 
having a plurality of first electrically conductive posts 21--21 and a 
second plurality of electrically conductive posts 22--22 terminating in 
conductive reflector cups 23--23. A plurality of interconnecting leads 
24--24 having first and second ends 26 and 27, respectively, are shown in 
alignment with and spaced from the posts 21 and 22. In an exemplary 
embodiment, the leads 24--24 were 70 mils long, 5 mils wide and 1.4 mils 
thick with a 0.1 mil coating of gold thereon. 
An LED 10 is shown interposed between the first ends 26--26 of the leads 
24--24 and the reflector cups 23--23, the top surface 29 of the LED having 
a thin coating of gold thereon. In applicants' specific embodiment, the 
surface 29 was not fully coated with gold but a number of islands 30--30 
of thin gold were placed thereon (see FIG. 1) to permit light generated 
within the LED 10 to pass through the surface 29. The bottom surface of 
the LED 10 is also coated with gold, or other noble metal. 
In operation, an epoxy adhesive 31 is deposited in the bottoms of the cups 
23--23 and the LED's 10--10 placed thereon. The epoxy adhesive 31 is also 
deposited on the first and second ends 26 and 27 of the interconnecting 
leads 24--24. The first ends 26--26 of the leads 24--24 are brought into 
contact with the LED's 10--10 and the second ends 27--27 of the leads 
24--24 are brought into contact with the posts 21--21 as shown in FIG. 3. 
A force is then applied to the first and second ends 26 and 27, as 
indicated by arrows and the heat applied for a predetermined period of 
time to cure the epoxy adhesive 31. It should be noted that the invention 
is not limited to high temperature curing, epoxies that cure at room 
temperature have also been used to implement the instant bonding 
technique. 
It should be noted that the epoxy adhesive 31 could be used at all the 
bonding sites, however, unfilled epoxy need not only be placed on the 
first end 26 of the terminal 24 for it is at this location that a filled 
epoxy can deleteriously affect the P-N junction 11 as hereinbefore 
described. Thus, it would only be necessary to apply a force at that 
location in order to obtain the desired connection. At locations where 
metal filled epoxies are used, it is not necessary to apply a force. The 
clamping force used in the exemplary embodiment at the site of the 
unfilled epoxy bond was 200 grams and the epoxy was subjected to a 
temperature between 175.degree. C. and 185.degree. C. for a period of 
about 18 minutes. 
Once the epoxy adhesive 31 has been cured, the bonded articles are then 
subjected to a conventional encapsulation process followed by a 
conventional cutting operation, to remove unwanted portions of the lead 
frame 20. Each individual LED assembly 50 is partially encapsulated in a 
transparent or translucent envelope 51, e.g., an epoxy envelope having 
posts 21 and 22 extending therefrom as shown in FIG. 4. 
It is to be understood that the specific sequence of steps in bonding a 
multi-element assembly 50 such as the above-described LED assembly 50 
forms no part of the instant invention. Any sequence of operation may be 
used as long as an unfilled adhesive is used to electrically bond a 
portion of the microelectronic components together. Clearly, the bonding 
together of two elements (e.g., wire-wire, wire-terminal, etc.) using 
unfilled adhesive fall within the purview of the instant invention.