Fluxless ion beam soldering process

A method for fluxlessly joining members having relatively low melting materials is provided. The members to be joined are exposed to ion beam radiation of sufficient intensity and a time sufficient to cause cleaning of the low melting materials after cooling. The members are then placed into juxtaposition with each other and again exposed to ion beam radiation of an intensity and for a time sufficient to cause reflow of the low melting materials which upon cooling joins said members.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a fluxless joining process utilizing ion beam 
milling and reflow techniques. 
2. Prior Art 
In the preparation of present day large scale integration technology, both 
for semiconductor and Josephson type devices, it is necessary to 
interconnect circuit chips to a substrate. Normally, solder joints are 
used in order to make these interconnections. The present day procedure 
entails the placement of solder pads on the chip which are to be joined to 
metal or solder pads which have been established in the substrate. The 
solder pads are treated with a wet chemical flux in order to dissolve 
surface oxide which forms thereon. The flux treatment serves to permit 
solder flow upon heating. 
This prior art method has a severe drawback in that the flux is most 
difficult to remove and remanents thereof remain as a corrosive 
contaminant. Additionally, to join the flux treated solder pads it is 
necessary to heat the entire chip assembly. 
More recently, it has been recognized that surface oxide can be removed 
from solder pads by means other than the use of chemical fluxes. For 
example, in the publication to R. J. Herdizik et al., IBM Technical 
Disclosure Bulletin, Vol. 23, #11, April 1981, it is recognized that ion 
milling can be used to effect solder milling i.e., removal of the oxide 
film. The so treated solder is then heated on a heating stage to effect 
solder reflow. 
In U.S. Pat. No. 3,294,951 to K. O. Olson, there is described a method for 
micro-soldering using a finely focused electron beam. This method cannot 
be used for large scale integration as contemplated by the present 
invention. This method cannot separately remove oxides and cause solder 
reflow, as well as it cannot be used to cover large areas simultaneously. 
Additionally, because electron beams are highly energized, they can be 
damaging to sensitive components. 
SUMMARY OF THE INVENTION 
What has been discovered here is that a method for fluxless joining on a 
large scale is obtained through the use of ion beam techniques. The 
joining of solder or metal pads can be effected in a single step or in a 
two step process depending upon the control of the ion beam radiation. 
In summary the process can be defined as follows: 
(a) providing chips and a substrate to which said chips are to be joined 
with joining pads; 
(b) abutting said chips and substrates so that said joining pads to be 
joined are aligned, and 
(c) exposing said joining pads to ion beam radiation whereby metal flow is 
effected to join said joining pads. 
In an alternative embodiment of the invention the method can be 
characterized as; 
(a) providing chips and substrates, to which said chips are to be joined 
with joining pads; 
(b) exposing said joining pads to ion beam radiation to remove oxide films 
therefrom; 
(c) removing said joining pads from said ion beam radiation; 
(d) positioning said chips onto said substrate to match said joining pads 
to be joined; and 
(e) exposing said chips to ion beam radiation of sufficient energy to cause 
metal reflow whereby said joining pads are joined. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide an improved method for fluxless 
joining. 
It is another object of the invention to provide an improved method for 
fluxless joining utilizing ion beam radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, there is shown the ion beam system which can be used to perform 
the present invention. The system is shown to be comprised of a vacuum 
chamber generally designated 10. The chamber 10 has an opening through 
which a gas conduit 12 is permitted to enter the system. It also has an 
opening 14 through which a vacuum can be created by means of a vacuum pump 
not shown. Situated within chamber 10 is the ion beam source comprising an 
ion generation chamber 16. Chamber 16 is formed by source body 18 and 
extraction grids 20 and 22. Within chamber 16 are the electrodes, cathode 
24 and anode 26. In chamber 16, electrons from the hot filament (cathode 
24) collide with gas atoms which are admitted into the chamber 16 via 
conduit 12. These collisions create ions. These ions are extracted and 
accelerated as they pass through grids 20 and 22 to form ion beam 28. The 
beam 28 is directed and impinged upon grounded specimen or chip 30. The 
charge of the ion beam 28 is neutralized by electrons emitted by a hot 
wire neutralizer 25 thereby allowing insulating specimens to be bombarded. 
Referring to FIGS. 2.1 through 2.4, there is shown the progressive effects 
of exposure to ion beams on to solder pads on a silicon chip. 
FIG. 2.1 shows a Si chip 30 having evaporated pyramidal solder pads 32 
thereon. These pads are initially coated with a film 34 of oxide which 
grows by exposure to air. During exposure to ion beam 28 (FIG. 1), the 
oxide film 34 is removed (FIG. 2.2). This is hereafter referred to as a 
cleaning step. Upon further exposure solder 32 forms a hemispherical melt 
(FIG. 2.3). Just prior to joining Si chip 30 onto a substrate 36 having 
either solder pads or metal pads 38, which has been similarly exposed to 
ion beam 28, the beam 28 is turned off to permit solder 32 to form a solid 
as illustrated in FIG. 2.4. 
In FIG. 3, there is shown the Si chip 30 joined to substrate 36. The Si 
chip 30 with its solid hemispherical solder 32 is flipped over onto 
substrate 36 with its matching solder or metal pads 38. The flipped over 
chip 30 is again exposed to ion beam 28, a time sufficient to effect 
joining of the chip to the substrate. (Heat may also be applied by 
auxiliary heating stage). 
In operation, a silicon chip having a joining material thereon is 
conventionally prepared. Typically, joining materials contemplated by this 
invention includes those materials having softening and/or melting points 
below 1,000.degree. C. These materials are capable of flowing and are 
thereby capable of joining with each other. Among the preferred materials 
are metals having relatively low melting point temperatures. Included 
among these materials are solders. Typically, the chips are prepared by 
evaporation solders prepared from lead tin, lead indium and lead bismuth 
alloys thereon. 
For example, an alloy containing about 95% Pb and 5% Sn is found suitable 
for the purposes of the present invention. Similarly a substrate such as a 
ceramic substrate is prepared having corresponding metal lands or pads 
fashioned thereon. The so prepared chip and substrate are then placed in 
the ion beam generating system of FIG. 1. They constitute the target upon 
which the ion beam is trained. The system is then evacuated to about 
10.sup.-6 Torr. It is then backfilled with a suitable gas. For example, 
the following gases may be used; argon, krypton, neon, xenon, mixtures of 
hydrogen and argon or forming gas in a mixture of hydrogen plug nitrogen. 
The system is backfilled to a pressure in the range of about 10.sup.-5 to 
about 10.sup.-4 Torr. 
Typical operation parameters are anode 26, maintained at about 1000 V while 
cathode 24 is maintained at about 950 V. Screen grid 20 is kept at the 
same potential as cathode 24 while accelerator grid 22 is maintained at a 
potential of -150 V. Cathode 24 is heated to a temperature sufficient to 
effect electron emission. Typically, the cathode is fashioned from a 0.010 
inch tungsten wire shaped in a circle 10 cm in diameter. A current of 
about 10 amperes to about 15 amperes is passed through the cathode to 
obtain the temperature needed to effect electron emission. 
Ions are created by applying a voltage of from about 50 V to about 100 V 
across chamber 10 between anode 24 and cathode 24 (FIG. 1). Cathode 24 is 
heated to a temperature high enough for electron emission. (Typically 
10-15 amp. for 0.010" tungsten wire in 10 cm diam. ion source). 
The process can be performed stepwise i.e., the solder pads can be cleaned 
melted, solidified and subsequently joined to the matching metal or solder 
pads of the substrate. For example, in the cleaning step of the solder 
pads are exposed to ion beams of a density of typically 1000eV, 
1.0mA/cm.sup.2. They are melted within approximately 5 secs. to 20 secs. 
at an ion beam density of about 1.0 mA/cm.sup.2. Solidification occurs by 
simply turning off the ion beam and permitting the irradiated solder pads 
to cool. This requires approximately 10 secs. The joining step requires 
that the Si chip be mechanically flipped onto the substrate so that the 
solder pads of the chip are in contact with matching pads on the 
substrate. The chip is then irradiated in an ion beam of about 1.0 
mA/cm.sup.2 to 10 mA/cm.sup.2 density from about 20 secs. to about 200 
secs. 
In an alternate embodiment of the invention joining can be effective in a 
one step process. As shown in FIG. 4.1, two chips or a chip 30 and a 
substrate 36 are abutted so that solder pads 32 on each are matched and 
are in contact with each other. The abutting members are then exposed to 
ion beam radiation and joined as in FIG. 4.2. The intensity of the 
radiation is such that cleaning and melting is accomplished in a single 
exposure. For example, the density of the radiation is maintained from 
about 1 mA/cm.sup.2 to about 10 mA/cm.sup.2 at 1000eV and for a time of 
from about 5 secs to about 30 secs. 
The above described methods have the advantages and capability of joining 
solder pads without flux and without significant electrical damage to 
components since heating is both local and rapid. The process can be 
stepwise because it uses a charged particle beam, which allows selective 
bombardment of conducting solder pads and minimal bombardment of 
insulating surfaces. This highly localizes the cleaning and heating 
process. This selective bombardment is accomplished by using low energy 
ions and applying a potential to pad lines, thereby precisely controlling 
ion bombardment by sequential biasing of the pads.