High temperature refractory metal contact in silicon integrated circuits

A contact and interconnect structure for a semiconductor integrated circuit includes a thin layer of refractory metal on a contact surface of the substrate through an opening in an overlying insulation layer with boron ions implanted into the substrate through the layer of refractory metal and the contact surface to ensure a uniform ohmic contact. An interconnect structure is then formed on the insulation layer and on the thin layer of refractory metal including a first layer of a refractory metal nitride on the insulation layer, a second layer of refractory metal on the first layer of refractory metal nitride, and a second layer of refractory metal nitride on the second layer of refractory metal.

BACKGROUND OF THE INVENTION 
This invention relates generally to integrated circuits fabricated in 
silicon substrates, and more particularly, the invention relates to 
electrical interconnections and contacts in such integrated circuits. 
It is well known that a multilayer integrated circuit interconnection 
structure is desirable to improve circuit performance as well as to 
increase circuit density. Prior art structures include etched contact 
holes formed in an oxide coated wafer surface, a layer of aluminum formed 
on the oxide surface and in the contact holes to form contact with 
selected regions of the wafer. The aluminum layer is patterned to 
interconnect specific portions of the integrated circuit. This contact and 
interconnection structure has not been especially satisfactory for many 
applications because the highly reactive aluminum pits the silicon and 
silicon oxide during high temperature processing reducing the structure's 
breakdown voltage. Subsequent processing steps such as passivation and 
multilayer interconnection increases the reaction between the aluminum and 
the silicon oxide. Elevated temperature required for some processing 
steps, on the order of 900.degree. C., further promotes aluminum and oxide 
reaction. The aluminum may recrystallize in formations which crack the 
insulating layer separating multiple metal layers. Thus, the use of 
aluminum contacts and interconnections severely restricts subsequent 
processing steps at elevated temperatures. 
U.S. Pat. No. 4,042,953 issued to John H. Hall, applicant herein, for HIGH 
TEMPERATURE REFRACTORY METAL CONTACT ASSEMBLY AND MULTIPLE LAYER 
INTERCONNECT STRUCTURE discloses an ohmic contact and interconnect 
structure including a layer of refractory metal such as molybdenum between 
two layers of silicon. The bottom silicon layer provides an adherent 
interface with an underlying insulating layer such as silicon oxide or to 
contact surfaces of the substrate, while the top silicon layer is provided 
so that ohmic contact can be easily made to the refractory metal. While 
the multilayer structure is relatively inert and withstands subsequent 
high temperature processing, the contact structure can develop a high 
resistance after being exposed to higher temperatures. This is due to the 
fact that silicon in the contact holes is purified by reaction with the 
molybdenum to raise its resistance. There is also a problem with residual 
silicon dioxide on the surface of the silicon substrate in the contacting 
areas which causes erratic contact resistance. 
The present invention is directed to an integrated circuit having improved 
high temperature contacts and interconnect structures which overcome 
problems associated with prior art structures. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a multilayer refractory metal and 
refractory metal nitride interconnect and contact structure is provided. 
The refractory metal nitride structure is electrically conductive and has 
the same work function as the refractory metal thereby facilitating high 
temperature processing. The resulting structure can withstand processing 
temperatures in excess of 1000.degree. C., which will be encountered in 
next generation semiconductor device fabrication. 
In forming circuit contacts to surfaces of the silicon substrate, a thin 
layer of the refractory metal and a second refractor metal nitride layer 
is first deposited on the substrate surface in contact holes through an 
overlying insulating layer. Reaction of the refractory metal layer with 
silicon is minimized by limiting the thickness of the refractory metal in 
the sandwich structure. Boron ions are then implanted into the substrate 
through the refractory metal layer. The ion implantation drives the 
refractory metal atoms through any silicon oxide on the contact surface 
and into the substrate, thereby allowing a consistent ohmic contact to be 
formed. 
The invention and objects and features thereof will be more readily 
apparent from the following detailed description and appended claims when 
taken with the drawing.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT 
Referring now to the drawing, FIG. 1 is a section view of a portion of a 
silicon substrate 10 in which an integrated circuit is formed and an 
overlying insulation layer 12 of silicon oxide. The layer 12 is formed on 
the surface of the silicon substrate 10 by thermal oxidation of the 
surface in a steam atmosphere at 1000.degree. C. Thereafter, as 
illustrated in FIG. 2, the silicon oxide insulation layer 12 is 
selectively masked and etched to form a contact opening therethrough and 
exposing a contact surface on substrate 10. 
In FIG. 3, a thin layer 14 of a refractory metal, molybdenum, for example, 
is deposited by sputtering on the contact surface of substrate 10 through 
the contact opening in the silicon oxide layer 12, and then a boron 
implant of approximately 1E15 at 60 kv is made through the layer of 
molybdenum into the underlying substrate, thereby driving the refractory 
metal atoms through any residual silicon oxide formed on the surface of 
the substrate, and allowing a consistent ohmic contact to be formed. Boron 
is selected for this implant since it enhances contact to P type material 
and does not interfere with the contact to high concentration N type 
material, which is usually found in source, drain, or emitter diffusions 
in integrated circuits. To limit reaction of the refractory metal and the 
substrate, the thickness of the layer 14 is limited to approximately 200 
.ANG., and a second layer of metal nitride of 300 .ANG. is formed to act 
as a passivating layer. 
After formation of the refractory metal layer and ion implantation, a 
multilayer contact and interconnect structure is formed on the surface of 
the integrated circuit over the insulation layer 12 and on the molybdenum 
layer 14. As illustrated in FIG. 4, the multilayer structure includes a 
molybdenum nitride layer 16, a layer 18 of molybdenum, and a top layer 20 
of molybdenum nitride. 
As described in copending application 14982-6, the multilayer interconnect 
structure is formed by placing substrate 10 in a vacuum sputtering 
apparatus along with a target of molybdenum. The first layer 16 of 
molybdenum nitride is formed by sputtering the refractory metal in a 
plasma atmosphere including nitrogen. Thereafter, the nitrogen gas is 
purged from the atmosphere, and the refractory metal is sputtered in an 
inert argon plasma to form the layer 18 of pure molybdenum. Thereafter, 
nitrogen is again introduced into the atmosphere and the second layer 20 
of molybdenum nitride is deposited on the surface of the refractory metal, 
thereby completing formation of the multilayer structure. The structure 
can then be selectively etched to define a desired interconnect pattern. 
While the illustrative embodiment is described with molybdenum refractory 
metal, other refractory metals such as tungsten, and tantalum can be 
employed in practicing the invention. Further the metal and metal nitrides 
can be deposited by chemical vapor deposition. The implanted ions can be 
gallium or indium as well as boron. Thus, while the invention has been 
described with reference to a specific embodiment, the description is 
illustrative of the invention and is not to construed as limiting the 
invention. Various modifications and applications may occur to those 
skilled in the art without departing from the true spirit and scope of the 
invention as defined by the appended claims.