Controlled chemical reduction of surface film

A process is disclosed for applying an electrical contact to the surface of a semiconductor device. A layer of metal selected from metals such as nickel, silver, copper, or alloys of these metals contacts a selected surface region of the device. A metallic contact is then soldered or otherwise joined to the layer of metal. To facilitate the joining, any native oxide present on the surface of the metal layer is first reduced by the low energy implantation of hydrogen ions into the metal surface.

BACKGROUND OF THE INVENTION 
This invention relates generally to the controlled chemical reduction of 
surface films, and more specifically, to a process for removing a native 
film from a metal surface so that another metal can be joined to that 
surface. 
In the process of joining two metallized objects, such as by soldering, 
welding, or the like, a film of metallic oxide on the surface of one of 
the metallized objects can impede the process of joining. The metal oxide 
can totally prevent the joining, make the joining unreliable, or make the 
use of flux or other material or process necessary to effect an acceptable 
joining. For example, when soldering to a nickel surface, a nickel oxide 
layer provides such an impediment. Because nickel oxidizes very readily 
and rapidly upon exposure to air, soldering to nickel is difficult because 
of the almost omnipresent oxide layer. Soldering to nickel is, therefore, 
usually accomplished by using a soldering flux, by high temperature 
reduction of the oxide in a hydrogen ambient, or by other techniques for 
reducing the oxide layer. 
In the fabrication of semiconductor devices, for example, it is often 
necessary to join a metallized semiconductor die to a metallized package 
or to join an electrical lead to a metallized region on the surface of the 
semiconductor device. The joining can be by soldering or by one of the 
bonding techniques such as ultrasonic or thermo-compression bonding. In 
any of these joining techniques the presence of an oxide layer on the 
metal surface impedes the joining. The very nature of semiconductor 
devices, as well as the nature of many other structures, limits the use of 
standard techniques for reducing the oxide prior to joining. For example, 
many semiconductor devices involve very shallow device regions which would 
be adversely affected by high temperature reduction techniques. Large area 
devices such as photovoltaic cells are too large and to fragile to permit 
the use of solder fluxes since the size of the device makes the proper 
cleanup of the device difficult is not impossible. Additionally, the use 
of flux with any device entails additional processing steps, including 
cleanup, which significantly add to the cost of the device. Maintaining a 
controlled reducing atmosphere is expensive, requiring large quantities of 
gases as well as equipment. 
A need therefore existed for a process which would overcome the 
deficiencies of the prior art processes to allow the joining of metallic 
parts. 
It is therefore an object of this invention to provide an improved process 
for applying an electrical contact to a semiconductor device. 
It is another object of this invention to provide an improved process for 
fabricating a photovoltaic cell. 
It is yet another object of this invention to provide an improved process 
for bonding together metal objects. 
It is a further object of this invention to provide an improved process for 
removing an oxide layer or other native film layer from a metallized 
surface prior to soldering. 
BRIEF SUMMARY OF THE INVENTION 
The foregoing and other objects and advantages are achieved in accordance 
with a process which utilizes the ion implantation of reducing ions into 
the surface of a metal layer. In one embodiment of the invention an 
electrical contact is applied to the surface of a semiconductor device by 
forming a layer of metal contacting the surface. Hydrogen ions are 
implanted into the surface of that metal layer to reduce any oxides formed 
thereon, and then a metallic contact is soldered to the layer of metal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustrates, in cross section, a portion of a semiconductor device 
10 during processing of that device. Semiconductor device 10 includes a 
substrate 12 in which device regions 14 of opposite conductivity type are 
formed. Device 10 may be, for example, a discrete transistor, diode, 
photovoltaic cell, or an integrated circuit. In order to operate the 
device, electrical contact must be made to device region 14. To achieve 
the electrical contact, a metal layer 16 is applied to the device surface 
in contact with region 14. Undesired contact to other parts of the device 
structure is prevented by a patterned insulation layer 18. In accordance 
with the invention, metal layer 16 can be, for example, nickel, silver, 
copper, or alloys of these materials. Additionally, metal layer 16 can be 
any other metal or alloy which forms an easily reducible chemical compound 
such as a metal oxide on its surface upon exposure of the metal to the 
ambient. The presence of an oxide or other compound on a metal surface is 
indicated by the layer 20. Depending on the metal used and the structure, 
contact metal 16 may be evaporated, sputtered, plated, or otherwise 
applied in known manner. 
Although layer 20 is referred to as a layer of metal oxide, the invention 
is also applicable to the reduction of other metal compounds. Accordingly, 
as used herein, the terms "oxide" and "metal oxide" will be used to refer 
to any native metal compound film formed on the metal surface. 
To insure good electrical contact to device region 14, it may be necessary 
in certain device structures to form an additional contact layer between 
metal 16 and region 14. This additional contact metal layer is indicated 
by the dash lines 22. The additional contact layer can be, for example, 
platinum silicide, palladium silicide, or the like, which is formed by 
depositing platinum or palladium on a silicon device surface and then 
heating to form the silicide or by direct deposition of the silicide on 
the surface regardless of substrate type. Alternatively, a first contact 
metal or barrier metal can be deposited on the surface of region 14 before 
contact metal 16 is applied. A contact metal such as nickel has a tendency 
to rapidly form a nickel oxide layer 20 on its surface. For example, 
nickel oxide can form on a freshly plated electroless nickel surface in a 
matter of minutes upon exposure to room air. Such a layer of oxide 
interferes with reliable bonding where the bonding involves joining a 
metal wire or other conductor to metal 16 by soldering, thermocompression 
bonding, ultrasonic bonding, welding, or the like. For optimum bonding, 
the two metal surfaces to be joined must be clean and free from oxides or 
other metallic compounds. 
In accordance with the invention, the oxide layer is removed by implanting 
the surface of the metal, including the oxide layer, with hydrogen ions as 
indicated schematically by arrows 23. The hydrogen ions act to reduce the 
oxide layer and are effective with oxide layers on nickel, silver, copper, 
and alloys thereof, as well as other materials having easily reducible 
oxides. The technique in accordance with the invention, however, has not 
been found applicable to oxides of either aluminum or silicon, since these 
oxides are apparently difficult to reduce. 
As illustrated in FIG. 2, after ion implanting metal 16 with hydrogen ions 
to reduce any oxide layer formed on the surface of the metal, a conductor 
24 can be soldered to metal 16 using a conventional solder 26. The 
soldering can be carried out at low temperatures without a reducing 
ambient and without using a flux. Conventional lead-tin solder such as 
60:40, for example, can be used to solder a nickel clad lead 24 to nickel 
metallization 16. 
The reduction process, in accordance with the invention, requires a high 
dose of reducing ions, and preferably hydrogen ions. In the case of 
reducing a metal oxide, the dose must be sufficiently high to supply about 
2 hydrogen ions for each oxygen atom associated with the metal oxide 
molecule. A high flux of hydrogen ions, of the order of 10.sup.17 per 
square centimeter, can easily be supplied by low energy, unanalyzed 
hydrogen ion beams. The ion implant voltage should be about 5 Kv or less 
and is preferably about 1-2 Kv. The relatively low energy ions are optimum 
for the reaction since the oxide to be reduced is a surface oxide and the 
low energy ions have a shallow range and are stopped near the surface. 
In accordance with the invention, a photovoltaic cell was fabricated by 
forming a shallow N-type region near the surface of a P-type silicon wafer 
to form a PN junction. An antireflective coating was formed on the surface 
of the N-type region and patterned to allow the N-type region to be 
selectively contacted. Contact to the N-type region was made by plating 
palladium onto portions of the N-type region exposed through openings in 
the antireflective coating. The palladium was sintered to form palladium 
silicide contacting the N-type region. Electroless nickel was then plated 
onto the palladium silicide to a thickness of about 150 nanometers. In 
accordance with the invention, selected photovoltaic cells were ion 
implanted with hydrogen at about 1 Kv and a dose of about 
5.times.10.sup.17 per square centimeter. No heating of the photovoltaic 
cell was observed during the ion implantation. Solder coated leads were 
then soldered to the electroless nickel by heating in an inert ambient to 
a temperature of about 225.degree. C. It was observed that the soldering 
operation could be carried out as much as 24 hours after the ion 
implantation without any impediments to the soldering. Other photovoltaic 
cells which did not receive the ion implantation could not be soldered 
even minutes after the electroless nickel plating under the low 
temperature soldering conditions above. Only by heating the cells to a 
temperature greater than or equal to 245.degree. C. in a hydrogen ambient 
for more than 15 minutes could the soldering be properly carried out. The 
extended high temperature hydrogen treatment, however, is sufficient to 
form a nickel silicide and to diffuse nickel into the silicon. Because of 
the shallow junctions utilized in photovoltaic cells, the nickel diffusion 
adversely affects the PN junction and thus the performance of the 
photovoltaic cell. 
Silver plated substrates were prepared having a bright, metallic-looking 
surface. After exposure to room ambient, the silver surface tarnished and 
assumed a grayish color. The substrates were ion implanted with hydrogen 
at 2 Kv to a dose of about 2.times.10.sup.17 cm.sup.-2. After the 
implantation the surfaces were again characterized by the original bright, 
metallic-looking appearance. Soldering to the surface was achieved in an 
inert ambient without the use of a soldering flux. 
Thus it is apparent that there has been provided, in accordance with the 
invention, an improved process for reducing oxides on the surface of 
metals, for joining metals together, and for forming semiconductor devices 
which fully meets the objects and advantages set forth above. Although the 
invention has been described and illustrated with reference to specific 
embodiments thereof such as the fabrication of specific semiconductor 
devices, it is not intended that the invention be limited to these 
illustrative embodiments. In the fabrication of semiconductor devices, for 
example, nickel plated packages or lead frames are often employed. The 
invention can be used to reduce oxides on these packages or lead frames 
before attaching a semiconductor die thereto. Additionally, silver or 
copper is often used as a metallization on the back of semiconductor die 
for use in attaching that die to a metallic package member. The invention 
may be employed to prepare those back metal surfaces prior to die 
attachment. Additionally, the invention applies to other industries than 
the semiconductor industry. Further, it is contemplated that the low 
voltage ion implantation can be implemented by the use of high pressure 
plasma equipment such as that described in U.S. Pat. No. 4,343,830. 
Hydrogen can be supplied to the reaction as pure hydrogen, as a mixture 
including hydrogen or as a gaseous hydrogen compound. Accordingly, it is 
intended to include within the invention all such variations which fall 
within the scope of the appended claims.