An interconnection device for providing electrical connection between two separable conducting elements that requires less applied force than a standard ohmic connection device of the same connection area includes a surface of at least a first conducting element that includes a plurality of atomically sharp projections for creating a strong electric field near the tip of each projection, each projection being disposed within a locally depressed portion of an insulating layer that serves to maintain a space between each tip and a second conducting element that contacts the insulating layer. The strong electric field at each tip induces a variety of conduction modes each contributing to an aggregate current flow from the first conducting element to the second. In an alternate embodiment, a plurality of projections are disposed on the peaks and valleys of a rough surface without an insulating layer, the projections providing a variety of conduction modes.

FIELD OF THE INVENTION 
This invention relates to interconnection devices, and more particularly to 
low-contact-force interconnection devices. 
BACKGROUND OF THE INVENTION 
In known electrical interconnection devices, electrical connection between 
two conducting elements is achieved by bringing them into physical contact 
such that an ohmic connection is formed. Such an ohmic connection is 
characterized by a contact resistance. On a microscopic scale, current is 
transferred between the two conducting elements via randomly distributed 
load-bearing areas, referred to as a-spots, that form between the elements 
when they are in mutual contact. Increasing the mechanical force that 
brings the elements into contact tends to increase the number and size of 
the a-spots. Accordingly, the a-spots contribute a constriction resistance 
to the contact resistance, the constriction resistance being proportional 
to the mechanical force applied between the elements, where the 
constriction resistance is described by: 
EQU R.sub.s =.rho./na (Eq. 1) 
where .rho. is the resistivity of the conducting element, n is the number 
of a-spots created, and a is the average linear dimension of the a-spots. 
Since the dominant conduction mechanism is ohmic, the contact resistance 
is constant over a wide voltage range. 
It is common for an insulating surface film of lubricant, metal oxide, or 
other contaminant to be found on the contacting surface of one or both 
contact elements, thereby contributing an additional resistance referred 
to as an effective resistance. Consequently, the contact resistance is the 
sum of the constriction resistances and the effective resistances of the 
respective surfaces. 
If the surface film is thin, i.e., less than 100 .ANG.ngstroms, some 
conduction will occur due to electron tunneling through the film. Such 
tunneling can occur by several mechanisms. As illustrated in FIG. 1, when 
a voltage is applied across the contacts, the Fermi level 10 of the metal 
constituting the positive contact is lower than the Fermi level 12 of the 
metal of the negative contact. Although electrons at the Fermi level 12 
will not have enough energy to cross over the potential barrier 14 at the 
metal-film interface, there will be some probability that electrons will 
"tunnel" through this barrier, in accordance with Schrodinger's equation 
from elementary quantum mechanics. Thus, a small but measurable current 
will flow, despite the presence of the insulating surface film on the 
contact elements. 
As the applied voltage over the surface of the contact elements is 
increased to, for example, 10.sup.6 V/cm, a second effect, known as field 
emission, takes place. This effect is described by the Fowler-Nordheim 
equation, which is approximately: 
EQU J.apprxeq.AE.sub.2 /.phi. exp[-B.phi..sup.3/2 /E] (Eq. 2) 
where J is the current density, E is the electric field, .phi. is the work 
function of the material, and A and B are constants. 
In present-day connector technology, the total tunneling current is a very 
small fraction, typically 10.sup.-6 to 10.sup.-3 of the total current 
carried by the a-spots. This is because the radius of curvature of the 
a-spots, typically 10.sup.4 .ANG. to 10.sup.5 .ANG., is large enough to 
promote ohmic conduction. 
If the surface film is too thick to allow electron tunneling, another 
phenomena, known as "fritting" occurs. In high applied electric fields, 
such as fields greater than 10.sup.6 V/cm, electrons injected into the 
film due to field emission cause an avalanche breakdown of the film at the 
point of injection. A channel created by the breakdown causes localized 
heating of the contacts, which softens the metal surface and thereby 
causes an a-spot. This a-spot will widen as the current conducted through 
the a-spot increases. Note that fritting occurs at sites similar to those 
that cause tunneling, i.e., at protuberances at the contact surface, the 
protuberances serving to concentrate the electric field. 
The presence of particulate contaminants, such as dust, on the surface of 
the conducting elements further increases the contact resistance. This 
occurs by imposing a barrier to complete contact closure. Referring to 
FIG. 2A, if the metal of the conducting element 16 is softer than the dust 
particle 18 in contact therewith, it will take an amount of force 
proportional to the hardness of the metal to deform the contact enough to 
cause complete contact closure between the element 16 and a complementary 
element 20, as shown in FIG. 2B. For a particle with a cross-sectional 
area A, the mechanical pressure P applied to the contact is: 
EQU P&gt;AH (Eq. 3) 
where H is the hardness of the contact metal. For particles of 2 
.mu.m.sup.2 -200 .mu.m.sup.2 area and H values of 10.sup.9 N/m.sup.2, P 
must be greater than 20 grams of force in order to insure contact. For 
devices with many concurrent contact elements, such as pin-grid arrays, 
such a high P value per contact element results in unacceptably high total 
applied pressure. For example, a 300 pin-grid array socket would require a 
total applied pressure of 6 kg to provide reliable contact. 
SUMMARY OF THE INVENTION 
An interconnection device is disclosed for providing electrical connection 
between two conducting elements that requires less applied force than a 
standard ohmic connection device of the same connection area. A surface of 
at least a first conducting element includes a plurality of atomically 
sharp projections for creating a strong electric field near the tip of 
each projection, each projection being disposed within a locally depressed 
portion of an insulating layer that serves to maintain a space between 
each tip and a second conducting element that contacts the insulating 
layer. The strong electric field near each tip induces a variety of 
conduction modes each contributing to an aggregate current flow from the 
first conducting element to the second. The projections are distributed 
with an area density sufficient to provide a current density per unit area 
at least as great as a standard ohmic connection of the same area, yet 
with significantly lower contact force, or a standard ohmic connection of 
significantly less area with the same contact force. To further increase 
the area density of projections, thereby increasing the number of 
conduction sites, an alternate embodiment includes projections on both the 
first and second conducting elements. In a further alternate embodiment, a 
metal layer resides upon the insulating layer of at least the first 
conducting element. Each sharp projection is disposed under a window in 
the metal layer, the edge of each window serving as a gate electrode 
cooperative with the sharp projection and the second conducting element to 
provide a switching or filter action. In a yet further embodiment, a 
plurality of projections are disposed on the peaks and valleys of a rough 
surface without an insulating layer, the projections providing a variety 
of conduction modes. In particular, the projections disposed on the peaks 
form a-spots, and tunneling and fritting sites, and the projections 
disposed in the valleys form field emission sites. 
The sharp projections of the interconnection device of the invention serve 
as conduction sites, such as a-spots, tunneling sites, field emission 
sites, and fritting sites. Thus, conduction takes place without the need 
to apply significant mechanical pressure between the first and second 
conducting elements to create or increase the likelihood of these 
conduction sites. The sharp projections can be sharper than any randomly 
created conduction site, and they can be fabricated at higher area 
densities than the typical distribution of conduction sites as well. Thus, 
conduction can occur at lower applied voltages, higher surface contaminant 
densities, thicker insulating film thicknesses, and lower applied 
mechanical pressures. Furthermore, since it is no longer necessary for one 
of the contact elements to deform to create conduction sites, it is 
permissible to use harder materials, such as tungsten, titanium nitride, 
or silicon, thereby eliminating the need for more expensive and less 
durable soft metals, such as gold. 
The invention is especially useful in high density, small signal 
applications where the excessive force required to produce low resistance 
contacts would be detrimental to standard connection devices, and in 
situations where the contact must be made and broken thousands of times 
over the life of the connecting device. The invention is also of great 
utility for making contact to the planar leads employed in surface mounted 
device technology. The alternate embodiment is particularly advantageous 
in applications that require a noise filter.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 3, the interconnection device of the invention 
provides electrical interconnection between a first conducting element 22 
and a second conducting element 24 such that less applied force is 
required than a standard ohmic connection device of the same connection 
area. The first conducting element 22 includes a metal substrate 26 with a 
plurality of integral sharp projections 28. Each sharp projection 28 is 
disposed at the bottom of a depression 30 in an insulating layer 32, and 
extends upward from the metal substrate 26 no further than the maximum 
thickness of the layer 32. Thus, there is a space between the tip 34 and 
the second conducting element 24. The tip of each projection 28 is 
atomically sharp, with a tip radius of less than, for example, 50 
.ANG.ngstrom units, so that a high intensity electric field can be 
produced in the vicinity of the tip 34 of the projection 28. When the 
second conducting element 24 is brought into contact with the first 
conducting element 22, the distance from the second conducting element to 
the tip 34 of the projection 28 is preferably less than 1 .mu.m. 
With reference to FIG. 4, the sharp projections 28 and their associated 
depressions 30 are randomly distributed with a preferred average density 
of about, for example, 10.sup.7 -10.sup.8 projections/cm.sup.2, so as to 
achieve current densities at least as great as those produced between two 
standard ohmic conducting elements. 
Each tip 34 serves as a conduction site, where a-spots, tunneling sites, 
field emission sites, and fritting sites can occur. When a bias voltage 36 
is applied across the first and second conducting elements 22, 24, 
extremely high electric fields are produced at each tip 34. For example, 
with an applied voltage of 10 volts, a sharp tip 34 with a radius of 50 
.ANG.ngstrom units would produce an electric field of 5.times.10.sup.7 
V/cm. At this field strength, a second conduction element made of a metal 
with a work function of 4.5 eV, such as tungsten, would exhibit field 
emission. The emission current would be approximately 3 .mu.A per tip. If 
the tips were fabricated at a density of 10.sup.7 -1O.sup.8 tips/cm.sup.2, 
a current density of 30 A/cm.sup.2 would result. 
In an alternate embodiment, shown in FIG. 5, a metal layer 38 is included 
that has a plurality of windows 40, each disposed in generally concentric 
relationship with a sharp tip 34. The width of the window 40 is less than 
the width of the depression at the surface of the insulating layer 32, and 
is approximately the same width as the base of the projection 28. When the 
metal layer 38 is biased with respect to the conducting substrate 26 by 
the voltage source 36, the portion of the metal layer 38 surrounding each 
tip 34 serves as a gate electrode that provides an electric field at each 
tip 34 to generate current by field emission at low voltages. Thus, the 
conducting element 22 can function as a switch or noise filter by biasing 
the metal layer 38 at a specific threshold voltage. For example, a 
connector for a multiconductor cable could have this threshold set to 
eliminate electrical interference transferred to the cable when it is 
exposed to potentially harmful fields. In particular, the effects of a 
large electromagnetic pulse (EMP) on sensitive equipment could possibly be 
reduced or eliminated by including an integrated sensor in the connector 
package to shut off the gate during an EMP event. 
Thus, it is clear that the invention provides more conduction sites, and at 
a greater area density, where both the number and density of the sites is 
controllable during manufacturing. Furthermore, typical spontaneously 
formed a-spots have a radius of curvature on the order of 10.sup.4 
-10.sup.5 .ANG.ngstroms, whereas the sharp tips of the invention have a 
radius of curvature of certainly no more than 10.sup.2 .ANG.ngstroms. Such 
small radii of curvature allow the invention to provide electric field 
strengths high enough to induce field emission, tunneling, and fritting 
using voltages applied to the conducting elements that are well within 
voltage ranges commonly found in contemporary electronic devices. 
The structure of the first conducting element in this embodiment bears some 
similarity to a structure disclosed in Spindt et al., U.S. Pat. Nos. 
3,789,471 and 4,857,799, and included in vacuum tubes and flat panel 
displays for energizing cathodoluminescent areas on a phosphor-coated face 
plate. Both references are silent on using arrays of sharp projections for 
increasing current density in an interconnection device, and furthermore, 
the arrays disclosed by Spindt are of insufficient area density to provide 
useful current densities. Moreover, the methods disclosed for fabricating 
the structures of Spindt are clearly distinguishable, the method of the 
instant invention being inexpensive and highly suitable for 
mass-production. 
The interconnection device of the invention can be used in high density, 
small signal applications where the excessive force required to produce 
low resistance contacts would be detrimental to connection devices. Modern 
trends in semiconductor electronics are progressing towards ever 
increasing numbers of connections required from the electronic devices to 
the outside world. Most integrated circuits (ICs) are soldered into place 
on printed circuit boards. However, many ICs are placed into sockets that 
have been themselves soldered onto a printed circuit board. Chip sockets 
are used when a chip is very expensive and would be vulnerable to damage 
in a soldering operation. Chip sockets are also used when a chip must be 
replaced frequently, as in upgrades or testing prior to shipment from a 
manufacturer. Also, the invention is particularly useful for use with 
surface mounted devices, which are increasingly common on modern printed 
circuit boards. The connection mechanism of the invention is ideally 
suited for making contact to the planar leads found in surface mounted 
device technology. 
Also, the invention provides improvement in wear resistance due to the 
reduced requirement for employing soft metals, such as gold, as contact 
materials. Improved wear resistance is useful in applications where 
contact must be made and broken thousands of times over the life of a 
connector, such as with "smart cards", the credit card-sized computers 
used sometimes in banking applications. Furthermore, electronic locks, and 
ROM modules for computers and peripherals could benefit greatly from the 
reduced insertion forces and decreased wear provided by use of the 
invention. 
To fabricate the array of projections and their associated depressions in 
the insulating layer, the following fabrication sequence can be used. 
Reference numbers in parentheses refer to process steps shown in FIG. 7. 
Referring first to FIG. 6A, a silicon substrate 42 is cleaned and oxidized 
(42) to a desired thickness to form an oxide layer 44. The oxide layer 44 
must be thick enough to prevent significant erosion during a subsequent 
silicon etch step, and to decrease the amount of oxide grown vertically 
during a subsequent tip formation step. Silicon has been chosen due to its 
well-known fabrication characteristics and low cost, although other 
similar materials could be used. 
To form a random distribution of contact sites, a solution of latex 
microspheres suspended in isopropanol is applied (44) using a photo resist 
spinner to distribute the spheres over the wafer. The microspheres are 
then softened (46) by heat to produce hemispheres 46 which act as masks 
for a subsequent oxide etch (48) in an etchant solution, such as 
HF/NH.sub.4 F solution, to remove the oxide layer everywhere except under 
the hemispheres 46. The resulting structure is shown in FIG. 6B. 
The mask of latex hemispheres 46 is stripped (50) in acetone, the wafer is 
cleaned, and the silicon is partially etched (52) to form a pedestal with 
oxide on top, as shown in FIG. 6C. The etching step (52) is critical, 
since it determines the size of the tip formed after a subsequent 
oxidation step. 
Silicon dioxide is then deposited (54) on the wafer by a method such as 
electron-beam evaporation, to provide a layer of deposited SiO.sub.2 50 
shown in the profile shown in FIG. 6D. Evaporated films are of poor 
quality with respect to films formed by chemical vapor deposition and 
thermally grown films, since oxygen is liberated from the SiO.sub.2 during 
deposition, resulting in films that are a mixture of Si, SiO, and 
SiO.sub.2. A heat treatment step (56) is applied to densify the film and 
to restore stoichiometry. The thickness of the film determines the 
distance from the tip to the second conducting element, and so must be 
well controlled. 
The wafer is then thermally oxidized (58) to form the structure in FIG. 6E. 
The thermal oxidation step (58) serves to densify and oxidize, thereby 
oxidizing the silicon pedestal 48 to form a silicon tip 52 underneath the 
oxide layer 54. Tips sharpened in this manner have a radius of less than 
200 .ANG.ngstroms, which are ideal for this application. 
In the next step, a gate metal layer 56 is deposited (60), to produce the 
structure shown in FIG. 6F. This metal layer serves to selectively protect 
the insulating oxide layer 54 during a subsequent tip definition etch step 
(62), which results in the profile of FIG. 6G, to provide the alternate 
embodiment that includes a metal gate surrounding each sharp tip of each 
conducting site. In this step, the insulating layer 54 is undercut under 
the metal layer 56. The extent of undercut depends on the etch rate of the 
insulating layer 54, which is a function of the extent of densification of 
the layer 54. 
The metal layer 56 can optionally be stripped away (64) to produce the 
embodiment that includes solely the sharp tips 52 disposed within 
depressions in an insulating layer 54, as described above, and shown in 
FIG. 6H. The metal 56 can optionally be further patterned to provide a 
variety of metal gate geometries. It may also be useful to etch the oxide 
54 to reduce the tip-to-collector distance, or to planarize the structure 
with a spin-on glass or polyimide. The silicon tips 52 could also be 
coated with a very thin layer of another conductor with a lower work 
function so as to improve the emission characteristics of the sharp tips 
52. 
With reference to FIG. 8, a further interconnection device structure 68 is 
shown. A standard conducting element 70, such as one made from brass or 
beryllium copper, includes a film 72 of a hard, highly wear-resistant 
material, such as tungsten or titanium nitride, for example, which has 
been etched to produce a plurality of sharp projections 74, 76. The 
conducting element 70 has a surface roughness such that a plurality of 
sharp projections can reside on each peak and valley of the surface. Sharp 
projections 74 are disposed on the peaks of the rough surface of the 
conducting element 70, and sharp projections 76 are disposed in the 
valleys thereof. The projections provide a variety of conduction modes; 
the projections 74 disposed on the peaks form a-spots, and tunneling and 
fritting sites, and the projections 76 disposed in the valleys form field 
emission sites. Unlike the previously described embodiments, a protective 
insulating layer for is not formed. Instead, some projections 74 directly 
contact a cooperative conductor, and other projections 76 are disposed in 
close proximity and therefore act as field emitters. 
With reference to FIG. 10, to form the structure 68, a film of a hard 
material 72, such as tungsten or titanium nitride, is deposited (80) upon 
the surface of the conducting element 70. Then, a plurality of latex 
microspheres 78 is applied (82) by either immersing or spraying the coated 
conducting element with a solution of latex microspheres. Last, the film 
72 is etched (84) by either a wet method, such as wet chemical etching, or 
a dry method, such as reactive ion etching, to form a plurality of sharp 
projections 74, 76. 
Other modifications and implementations will occur to those skilled in the 
art without departing from the spirit and the scope of the invention as 
claimed. Accordingly, the above description is not intended to limit the 
invention except as indicated in the following claims.