Method for forming an antifuse element with electrical or optical programming

A programmable antifuse element comprising adjacent bodies of germanium and aluminum or aluminum alloy form forming a low resistance connection of good mechanical and thermal properties when heated to a temperature where alloying of the aluminum and germanium occurs. Heating for the purpose of programming the antifuse element can be done by electrical resistance heating in the germanium, which may be doped to achieve a desired resistance value, or by laser irradiation. Due to the high resistance of intrinsic or lightly doped germanium, a resistance change ratio of greater than 10,000:1 is achieved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to programmable structures for 
forming programmable connections in integrated circuit devices and, more 
particularly, to an antifuse structure for selectively forming connections 
between circuits and circuit elements in integrated circuit devices. 
2. Description of the Prior Art 
As the scale of integration has increased in the manufacture of integrated 
circuits, it has become economically necessary to provide redundant 
circuits in virtually all devices manufactured in order to preserve 
acceptable levels of manufacturing yield. Upon formation of the integrated 
circuit to certain levels of completeness, testing may be done and 
defective circuits disconnected by destroying a fusible link or fuse with 
a high level of current. A similar mechanism is employed in known 
"antifuses" such as that disclosed in U.S. Pat. No. 4,943,538 to Mobsen et 
al. where an insulator is destroyed in order to form a connection rather 
than to interrupt a connection, often accompanied by flow of molten 
conductive material at a high temperature. These practices, while widely 
employed have several significant drawbacks even though a substantial 
economic gain is realized. 
The destruction of known fusible links and insulators in known antifuses 
inherently causes electrical, mechanical and thermal stressing of at least 
a portion of the integrated circuit and may also cause damage beyond the 
capability of current repair techniques. The thermal stressing may also 
initiate processes such as metal migration in other connectors or changes 
in transistor characteristics which compromise integrated circuit 
performance. Therefore, the relatively violent processes associated with 
fuses and, especially, antifuses may cause significant loss of 
manufacturing yield. Further, when the present state of the art provides 
only for the destruction of connections with acceptably low likelihood of 
chip damage, often complex arrangements may be required during integrated 
circuit design in order to insure that redundant structures will be 
connected when the fusible links are destroyed. Alternatively, redundant 
pinouts or address decoder modifications may be required in the same 
fashion that fusible links are destroyed to assure that redundant 
structure can be effectively and functionally substituted for the 
disconnected structure. 
Further, electrical programming of fuses and antifuses may be difficult in 
some cases due to the difficulty of making connections to the integrated 
circuit chip to be modified. Often, different electrical connections must 
be made to the chip for each fuse or antifuse to be programmed. Further, 
to reduce the risk of damage to other circuit elements when the 
programmable element is destroyed, a relatively large "footprint" must be 
dedicated to the fuse in order to allow for temperature reduction through 
thermal conduction and heat dissipation in the vicinity of each fuse. 
Additionally, in programming of fuses and antifuses, the process tolerances 
(e.g. operating margins for programming) are sufficiently small that 
programmable elements may not be completely destroyed when programming is 
done. The fuses also must be fabricated with a significant resistance and 
the difference in resistance between an intact fuse and a "destroyed" fuse 
may only be a few orders of magnitude (e.g. a factor of several hundred). 
Conversely, the delicacy of programmable elements in fuses and antifuses 
often allows programming to occur accidentally during electrical burn-in. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an antifuse 
element which is electrically or optically programmable connection between 
conductors of an integrated circuit device. 
It is another object of the invention to provide an electrically or 
optically programmable connection structure of small lateral dimensions. 
It is a further object of the invention to provide an electrically or 
optically programmable connection structure which reliably exhibits a 
change of resistance, when programmed, differing by a factor of ten 
thousand, or more, in comparison to an unprogrammed connection structure 
in accordance with the invention. 
In order to accomplish these and other objects of the invention, an 
integrated circuit is provided including a connector structure which 
includes a body of germanium adjacent a body containing aluminum. 
In accordance with another aspect of the invention, an antifuse element is 
provided comprising at least one body containing aluminum and a body of 
germanium adjacent said body containing aluminum. 
In accordance with a further aspect of the invention, a method of 
fabricating a programmable connection is provided including the steps of 
depositing a body containing aluminum, depositing a body of germanium 
adjacent the body of aluminum, and heating at least a portion of each of 
the bodies of germanium and aluminum to a temperature of approximately 
450.degree. C. with electro-magnetic radiation. 
In accordance with yet another aspect of the invention, a method of 
fabricating a programmable connection is provided including the steps of 
depositing a body containing aluminum, depositing a body of germanium 
adjacent the body of aluminum, doping the body of germanium to reach a 
predetermined resistance value range, and alloying at least a portion of 
the body of germanium with at least a portion of the body containing 
aluminum by electrical resistance heating of the body of germanium.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, an early 
stage of the fabrication of the antifuse 100 (FIG. 3) is shown. Layer 10 
represents a plurality of layers which form an integrated circuit 
comprising at least two transistors and is referred to hereinafter as a 
"masterslice". The assumption of at least two transistors is intended to 
indicate that the present invention is applicable to integrated circuits 
of any scale of integration or integration density in which redundant 
circuits exist and which are to be selectively connected to contacts or 
other circuits to improve manufacturing yield of the overall manufacturing 
process if one or more circuits are found, upon testing to be of 
questionable operability or stability. In practice, the formation of such 
redundant circuits is generally employed only with large scale integrated 
circuits, but, in principle, the present invention could be applied to any 
device in which selective connections for programming were to be carried 
out. Therefore, the invention should not be considered as limited to large 
scale integrated circuits but is also applicable to transistors and such 
other circuits as read-only memories and decoders. 
On the surface of the masterslice 10, it is also assumed that an oxide 
insulating layer 20 and a metal conductor 30 are formed in sequence. For 
improved adhesion between layers metal nitride layers 31, 32 are also 
shown but are not particularly important to the practice of the invention. 
Preferred materials for conductor 30 are aluminum and aluminum - copper 
alloys which may be further alloyed with titanium, nickel or cobalt, such 
as Al-Cu/Ti or Al-Cu-Ge/Ti. Alternatively, aluminum - silicon alloys can 
be employed in the practice of the invention. The principal requirement is 
that the conductor or other body of material adjacent to a body of 
germanium contain and be able to serve as a source of aluminum for 
alloying. 
The conductor 30 is preferably formed by a blanket deposition followed by 
patterning by etching. However, the technique of conductor formation is 
not critical to the invention. The conductor layer will be connected to 
some point (e.g. connection pad) of the masterslice 10 in order to form a 
connection to some portion of the integrated circuit therein. The 
construction of this connection 30 to masterslice 10 through insulator 20 
is not important to the practice of the invention. However, a stud 40 of 
known construction would preferably be used for the purpose. It should be 
noted, however, that the invention can be applied to any stud layer and 
could, for example, be carried out at location 40 in the same manner as 
will now be described for location 50. 
The basic object of any programmable connection structure, such as the 
fuses known in the prior art, is to form a connection structure extending 
at least to a point where it is accessible by another conductor in the 
same or a different layer and which, after formation allows change of the 
conductivity characteristics of the structure by a later operation. In 
accordance with the invention, a further insulative oxide layer 60 is 
applied over the termination of conductor 30 and planarized to accept 
deposition of another conductor. The illustrated termination of connection 
30 is to be regarded as symbolic of a terminal node at which a connection 
can be made to a portion of an integrated circuit and actual termination, 
as shown, is not necessary. 
The oxide is then patterned, preferably with a mask, not shown, and a via 
70 is etched through the oxide and preferably, if economically possible, 
to a significant fraction of the thickness of conductor 30. Then, as shown 
in FIG. 3 the via is partially or completely filled with intrinsic (e.g. 
undoped) germanium, preferably by selective germanium growth within the 
vias with nucleation provided by aluminum on the bottoms of the vias. 
Chemical vapor deposition using germane (GeH.sub.4) or di-germane 
(Ge.sub.2 H.sub.6) or some similar gas at a temperature of about 
350.degree. C. and other techniques such as collimated sputtering and 
electron cyclotron resonant (ECR) sputtering are also suitable. The 
antifuse in accordance with the invention could be considered to be 
complete at this point and, as will be discussed below, could be 
programmed at this point in the process if programming is to be done by 
optical techniques (that is, before electrical connections 110, allowing 
electrical programming, are formed on the upper side). However, it should 
be noted that later programming by optical means is still available even 
after a the formation of a tungsten cap or a reflective aluminum 
connection since it is not necessary that optical energy reach the 
antifuse structure but only that heat be conducted thereto. Therefore, 
even if a reflective aluminum connection is applied over the antifuse 
structure, a dark, light absorptive layer of, say, a refractory material 
could be applied thereover at the antifuse location. It should also be 
noted in this regard that melting of the germanium and/or aluminum is not 
necessary but only the heating to a temperature where a solid state 
diffusion reaction will occur fairly rapidly. 
It is somewhat preferable to only partially fill via 70 to reduce the 
amount of energy (e.g. heat) which must be applied during programming of 
the antifuse. However, filling must be carried out to a significant height 
above the surface of conductor 30 and into the thickness of oxide 60 in 
order to achieve a relatively large resistance (e.g. generally in the 
range of 1K to 10K Ohms and preferably about 5K Ohms). Some light doping 
of the germanium may be necessary to facilitate electrical heating and to 
achieve a resistance near 5K Ohms if filling is done to a greater degree. 
For example, a length of Germanium in the range of 1000 to 3000 Angstroms 
is preferred to minimize and resistance change due to the heat budget of 
other process steps which may cause slight diffusion of aluminum. This 
length would require a dopant concentration in the range of 10.sup.14 to 
10.sup.16 atoms per cubic centimeter to achieve an appropriate resistance. 
In comparison, if intrinsic germanium having a high specific resistance of 
50 Ohm cm. at room temperature were to be used, a length limit of about 
100 Angstroms would be imposed since greater lengths and resistance would 
preclude drawing enough current for electrical resistance heating to occur 
with acceptable differentiation from other elements. At the same time, the 
doping levels of the germanium should be small enough and the resulting 
resistance high enough (e.g. also near 5K Ohms) to prevent programming 
temperatures from being reached during application of burn-in voltages to 
the integrated circuit. 
The remainder of the via 90 is preferably filled with tungsten which can be 
deposited at the same temperature as the germanium. Therefore the two 
depositions can be achieved by a single tool. Alternatively, the entire 
via can be filled with intrinsic germanium if optical programming is 
intended or with impurity doped germanium if suitably doped to allow 
significant differential resistance heating during electrical programming 
at voltages that can be withstood by other portions of the integrated 
circuit. However, extended lengths of germanium may cause difficulty (e.g. 
extended heating time) in achieving sufficient diffusion, particularly if 
intrinsic germanium is used. 
Other via structures can achieve the same advantages for optical 
programming, however, with reduced heat requirements compared with a 
germanium filled via and while retaining the ability to be electrically 
programmed. For example, for optical programming, it would be preferable 
to fill volume 80 with aluminum and complete filling of the via by 
deposition of germanium in volume 90. In this case, the thickness of the 
germanium should also be chosen to achieve a resistance near 5K Ohms. With 
this construction, the germanium would remain at the surface in order to 
be "visible" to laser light used for programming. This type of structure 
including a body of aluminum or aluminum-containing material independent 
of the conductor but adjacent the germanium body would also be useful 
where a higher melting point metal such as copper or a metal that alloys 
less readily with germanium were to be used for conductor 30. In this 
latter case, a layer of tungsten can be applied after the germanium, if 
desired. 
After the via structure is completed by one of the above alternative 
techniques in accordance with the invention, a further connection 110 is 
preferably added to the antifuse and a passivation layer applied. For 
optical programming, an aperture 120 may also be provided in the 
connnection 110, coincident with a portion of the stud location. However, 
it may mot be feasible to form an optically effective hole which does not 
reduce connection cross-sectional area of the conductor and thus provision 
of a material to increase absorption of light energy, such as a black or 
dark refractive material, referred to above, and reliance on heat 
conduction through the connection 110 is preferred. The addition of a 
quartz/silicon passivation layer does not seriously affect later optical 
programming since electromagnetic radiation, preferably infra-red light 
from a carbon dioxide laser, will penetrate insulating layers of quartz or 
polyimide for depths likely to be encountered. Alternatively, if the 
entire chip is heated to about 350.degree.-380.degree. C., the amount of 
additional laser heating required for programming is relatively small and 
optical access is less critical and possibly unnecessary. 
With brief reference now to FIG. 5, showing a phase diagram for alloys of 
germanium and aluminum it is seen that at a temperature of only slightly 
above 420.degree. C., stable alloys of aluminum and germanium are formed 
by dissolution of germanium in aluminum, all of which are of low 
resistance. A specific resistance for eutectic aluminum - germanium alloys 
as low as 6 .mu.Ohms-cm. has been reported in the literature. A typical 
resistance of 0.2 Ohms for a 0.5.times.0.5 micron stud yields a very large 
resistance change ratio (e.g. 25,000:1) relative to an as-formed 
resistance of 5K Ohms. (For this reason, it is preferred but not at all 
critical to adjust the relative volumes of 80 and 90 and/or adjust the 
rate of electrical or optical heating relative to heat conduction in an 
aluminum-containing conductor adjacent the germanium to approximate the 
proportions of aluminum and germanium in a eutectic alloy since eutectic 
alloys will exhibit maximum thermal stability.) Therefore, a connection 
can selectively be made through the antifuse by selective heating thereof 
to form an alloy plug such as at 200, as shown in FIG. 4 (or throughout 
volume 90, if germanium is initially deposited in that volume rather than 
in volume 80). The alloy plug will be principally composed of eutectic 
alloy but the specific alloy is not critical since all are highly 
conductive. All of these alloys are believed to be highly stable 
mechanically by extrapolation from the fact that alloying of aluminum with 
silicon is a known technique to prevent "spiking" of junctions (e.g. 
diffusion of spikes of aluminum into silicon when aluminum contacts are 
applied) in transistors. 
This type of programmable structure has numerous advantages which may not 
be immediately evident. First, the alloy plug is highly stable and can 
thus be used in close proximity to active devices. This is also an 
advantage of the relatively low temperatures at which alloying may be made 
to occur. Second, since no fuse is "blown" and the programming process is 
done at a relatively low temperature, there is little or no risk of damage 
to adjacent structures. Third, in assembly of the chip with other 
structures, bonding temperatures for solder reflow are limited to 
temperature of 400.degree. C. and below and the antifuse is not 
significantly affected thereby. For example, in an extreme case a heat 
budget of, say, 10 hours at 400.degree. C. would result in diffusion 
distance of Aluminum into Germanium of less than 300 Angstroms or about 
10% of the preferred antifuse length. For electrical stressing known as 
burn in, voltages and currents are typically adjusted to limit chip 
temperatures to about 150.degree. C., at which temperature diffusion is 
much more slow. Therefore, while some diffusion of aluminum into germanium 
may occur in an unprogrammed fuse, the large resistance change upon 
alloying of a plug limits the effect on the circuit since diffusion will 
be relatively shallow and will not greatly affect the bulk resistance of 
the germanium. Thus, if the resistance of the germanium is suitably 
controlled by dopant concentration to require a voltage above the burn-in 
voltage, accidental programming is extremely unlikely during either 
further processing steps or burn-in. Fourth, by allowing a connection to 
be made optically, alternative techniques for programming are available 
and electrical isolation for programming and design restrictions and 
modification to avoid damage to other elements of the integrated circuit 
from increased voltages are unnecessary. Fifth, the antifuse in accordance 
with the invention produces a resistance change ratio well in excess of 
10,000:1 which results in much increased process latitude and tolerance 
during programming as well as increased reliability and repeatability. 
Sixth, if programming is to be done optically, doping of the germanium is 
unnecessary and the full specific resistance of intrinsic germanium may be 
exploited to achieve even higher resistance change ratios and even higher 
immunity to significant resistance changes due to diffusion of aluminum 
into germanium. 
In the interest of completeness, electrical programming is preferably done 
with a pulse of about 5 volts (compared to an exemplary burn-in voltage of 
4-4.5 volts) which produces approximately 5 mW of power dissipation. The 
volume of the stud and the length of the pulse should be chosen in 
consideration of the chip temperature (e.g. if preheated) to achieve a 
local temperature of about 450.degree. C. but without significantly 
heating the surrounding structure. Usually, a pulse duration of a small 
fraction of a millisecond is appropriate. If desired, the change of 
resistance can be monitored and controlled in near real time by a 
measurement technique disclosed in U. S. patent application S. N. 
07996,766 (attorney's Docket Number FI9-92-076), filed concurrently 
herewith. 
In view of the foregoing, it is seen that the essence of the antifuse 
structure in accordance with the invention is a body of intrinsic or 
lightly doped germanium adjacent a body of aluminum so that solid state 
diffusion may take place when heat is applied. While doping may be 
required to achieve desired levels of power dissipation for programming by 
electrical resistance heating, it is not necessary to the underlying 
concept of the invention. Further, due to the simplicity of the structure, 
many other structural configurations could be employed in the practice of 
the invention. For example, the invention is not limited to a stud 
structure but could be practiced in a single layer of a device by 
depositing germanium between two deposits of aluminum or 
aluminum-containing alloy which either constitute or are bonded to 
conductors. Upon raising the temperature to 450.degree. C., as above, a 
germanium-aluminum alloy connection will be formed in the same manner as 
in the stud structure. However, it is preferred to form anti-fuses at the 
stud level since the antifuse would have a minimized footprint on the 
chip, separation from other devices for heat dissipation is more easily 
provided and possibly more conductor length may be available for heat 
sinking. 
Therefore, it is seen that the above-described invention provides an 
antifuse element which is programmable electrically, optically or a 
combination thereof. The antifuse element can also be of small lateral 
dimension and provides a high ratio of resistance change with little or no 
risk of damage to surrounding structures during programming or accidental 
programming by other processes. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modification within the spirit and scope of the appended 
claims.