Antifuse development using .alpha.-c:h,n,f thin films

The present application discloses a metal-to-metal antifuse with an amorphous carbon dielectric which provides a very high resistance off state and can be programmed at voltages compatible with deep submicron devices. Furthermore, the programmed filament achieves low resistance with low programming current while maintaining a high level of stability.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to integrated circuit antifuse structures. 
The first commercial antifuse based Field Programmable Gate Array (FPGA) 
employed an oxide/nitride/oxide (ONO) dielectric sandwiched between 
heavily doped polysilicon and a heavily doped diffusion in single crystal 
silicon. Because the antifuse is constructed on substrate next to the 
logic modules, this type of antifuse takes up valuable silicon area. In 
addition, because of reliability constraints, the programming voltage of 
this dielectric could not be scaled to less than about 14 volts. This 
causes an increasing problem as supply voltages are scaled. 
The next commercial antifuse based FPGA employed an amorphous silicon 
antifuse dielectric sandwiched between two of the interconnect layers 
overlying the logic modules. This significantly reduced the size of the 
antifuse chip because antifuses could be constructed above rather than 
alongside the logic modules. In addition, the programming voltage was 
reduced to about 10 volts which helps with scaling supply voltages. The 
limitations of this dielectric are that (1) the programming voltage cannot 
be scaled below about 8 volts because of a rapid increase in off state 
leakage as the amorphous silicon thickness is reduced, and (2) a 
programming current of about 20 mA is required to stabilize the conductive 
filament. With amorphous silicon, insufficient programming current can 
result in a filament that will switch to a high resistance state during 
use. The amorphous carbon and nitrided amorphous carbon dielectrics 
described herein overcome many of these limitations. The programming 
voltage of the amorphous carbon dielectric antifuses can be scaled to less 
than 5 volts while still maintaining picoamp leakage current in the off 
state. The resistance of the programmed filament can be reduced to less 
than 100 ohms with 5 mA of programming current. Furthermore, the filament 
exhibits none of the resistance instability problems seen with amorphous 
silicon. 
The present application discloses a metal-to-metal antifuse with an 
amorphous carbon dielectric which provides a very high resistance off 
state and can be programmed at voltages compatible with deep submicron 
devices. Furthermore, the programmed filament achieves low resistance with 
low programming current while maintaining a high level of stability. 
One limitation of the hydrogenated amorphous carbon films is that they 
begin to evolve hydrogen and convert to low resistance graphite at 
temperatures above 250.degree. C. This imposes a constraint on the maximum 
temperature that can be used in post antifuse processing of the integrated 
circuits. Either nitrogen and/or fluorine doping of hydrogenated films or 
the deposition of low hydrogen content amorphous carbon films stabilizes 
the films to temperatures exceeding 400.degree. C. These temperatures are 
commonly used in back-end semiconductor processing. 
The antifuse structure can be constructed between two of the interconnect 
layers overlying the logic modules in an FPGA, permitting a very compact 
layout. The antifuse structure consists of the bottom interconnect metal, 
the amorphous carbon dielectric, and the top interconnect metal. In 
between the interconnect metal layers and the amorphous carbon dielectric 
a layer of barrier metal such as TiW, Ti, or TiN, may be used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The numerous innovative teachings of the present application will be 
described with particular reference to the presently preferred embodiment 
(by way of example, and not of limitation), in which: 
The antifuse structure is formed between interconnect layers in an 
integrated circuit as shown in FIGS. 1a and 1b. In FIGS. 1a and 1B, an 
amorphous carbon film 2 is deposited into a via opening that has been 
formed in the intermetal dielectric 3 between top and bottom interconnect 
layers 1. FIG. 1A shows the addition of barrier material 4 which is 
sometimes used in integrated circuits to enhance electromigration 
resistance of the aluminum interconnect layers. 
The amorphous carbon dielectric can be deposited using sputtering or 
Electron Cyclotron Resonance (ECR) or Plasma Enhanced Chemical Vapor 
Deposition (PECVD) equipment commonly used for thinfilm deposition in the 
semiconductor industry. Depending upon deposition conditions, film 
properties can range from a fine sooty lampblack-like material to a 
diamond like film. The preferred deposition conditions for the antifuse 
dielectric produces a more diamond-like film. The preferred deposition gas 
for PECVD or ECR is CH.sub.4 or a mixture of CH.sub.4 and NF.sub.3, but 
other carbon and nitrogen gases could be used as well. The preferred 
process for sputtering utilizes a graphite electrode and butane gas. 
The preferred deposition conditions of the diamond-like amorphous carbon 
film (DLC) using PECVD plasma are 
CH.sub.4 =200 sccm, 
Pressure=2 Torr, 
Substrate temperature=100.degree. C. and 
Power=100 Watts. The optical band gap of the resulting material is 2.7 to 3 
eV and the hydrogen content is at 50% to at 60%. 
Leakage and breakdown characteristics of this film are shown in FIG. 2. 
Leakage on this graph is the total leakage through the 0.2 mm.sup.2 dot, 
and leakage is in the picoamp/.mu.m.sup.2 range up to a programming 
voltage of about 3 volts. This voltage can be adjusted by changing the 
amorphous carbon film thickness. The breakdown causes a low resistance 
filament to form. Filament resistance as a function of the programming 
current is shown in FIG. 3. A programming current of less than 2 mA is 
required to reduce the filament resistance to below 100 ohms. This 
filament is believed to be graphite-like in composition having a high 
level of stability. Subjecting this filament to prolonged current stress 
shows no resistance instability and none of the switching behavior 
characteristics of amorphous silicon films. 
The preferred deposition conditions of the nitrogen doped, diamond-like, 
amorphous carbon film (N-DLC) using PECVD plasma are CH.sub.4 =200 SCCM, 
NF.sub.3 =20 SCCM, Pressure=2 Torr, Substrate temperature=100.degree. C., 
and Power=100 Watts. Antifuse structures were formed by depositing an 
N-DLC film, having a thickness half that of the previous embodiment, onto 
a TiW substrate and then evaporating a 0.2 mm.sup.2 aluminum top electrode 
through a shadow mask. Leakage and breakdown characteristics of this film 
are shown in FIG. 2. Leakage on this graph is the total leakage through 
the 0.2 mm.sup.2 dot, and leakage is in the picoamp/.mu.m.sup.2 range up 
to a programming voltage of about 6 volts. This voltage can be adjusted by 
changing the N-DLC film thickness. 
The breakdown causes a low resistance filament to form. Filament resistance 
as a function of the programming current is shown in FIG. 4. A programming 
current of less than 2 mA is required to reduce the filament resistance to 
below 100 ohms. This filament, like the DLC film, is very stable and shows 
no tendency to change resistance under prolonged current stress. 
The DLC film shows a decrease in resistivity starting at about 200.degree. 
C. The DLC film begins to evolve hydrogen at these temperatures and 
convert to a more graphite-like, conductive film. Nitrogen and fluorine 
doping increases the temperature stability of the film making it easier to 
integrate into the back-end interconnect part of semiconductor flows where 
temperatures are limited by the aluminum leads to 450.degree. C. or less. 
The temperature stability of DLC films is increased also by reducing the 
hydrogen content of the films. Reduction in the hydrogen content of the 
film by at 30% (to about at 35%) resulted in an increase in the breakdown 
voltage by a factor of two, and an increase in thermal stability up to 
400.degree. C. or more. The preferred method of depositing the reduced 
hydrogen amorphous carbon films is by reactive sputtering. The preferred 
deposition conditions for sputtering are: graphite electrode, butane gas, 
pressure=40 mtorr, cathode power=40 watts, substrate 
temperature=100.degree. C., and anode bias of -60 Volts DC. The optical 
bandgap is 1.9 eV and the hydrogen content is approximately 35% at. 
Antifuse structures were formed by depositing approximately 100 nm reduced 
hydrogen amorphous carbon films on top of a TiW substrate and then 
evaporating a 0.4 mm.sup.2 aluminum top electrode through a shadow mask. 
The leakage current is about 100 picoamp for a 0.4 mm.sup.2 antifuse 
capacitor. The device breaks down at about 14 Volts and filament 
resistance is 200 ohms at 0.3 mA programming current. No change in 
resistivity of the unprogrammed device occurs up to 400.degree. C. anneal. 
The programming voltage of the reactively sputtered film can be reduced by 
reducing the film thickness. 
The films described in the above application were deposited using PECVD 
ECR, or reactive sputtering but, as is known to those skilled in the art, 
other techniques can be used equally well to deposit these films. The 
gases of choice are CH.sub.4 and NF.sub.3, but other carbon and nitrogen 
containing gases could be used to deposit like films by those skilled in 
the art. The resistivity, breakdown voltage, and thermal stability of the 
amorphous carbon films can be controlled by varying the plasma parameters 
such as rf power, anode bias, gas pressure, gas composition, substrate 
temperature, and thickness of the film. 
According to a disclosed class of innovative embodiments, there is provided 
an integrated circuit, comprising: an antifuse structure having first and 
second interconnect layers being disposed over logic modules, optional 
first and second barrier layers being disposed between said first and 
second interconnect layers, and an amorphous carbon dielectric layer being 
disposed between said first and second interconnect layers; whereby said 
dielectric layer can be programmed at voltages compatible with deep 
submicron devices. 
According to another disclosed class of innovative embodiments, there is 
provided an antifuse structure, comprising: first and second interconnect 
layers being disposed over logic modules; first and second barrier layers 
being disposed between said first and second interconnect layers; and an 
amorphous carbon dielectric layer being disposed between said first and 
second barrier layers. 
According to another disclosed class of innovative embodiments, there is 
provided a method of fabricating an antifuse structure, comprising the 
steps of: providing a substrate composed of a conductive material; 
depositing a dielectric film over said conductive material, patterning and 
etching an opening through the dielectric to said conductive layer, 
depositing an amorphous carbon film into the opening and contacting the 
lower said conductive film, and depositing a top layer of conductive film. 
MODIFICATIONS AND VARIATIONS 
As will be recognized by those skilled in the art, the innovative concepts 
described in the present application can be modified and varied over a 
tremendous range of applications, and accordingly the scope of patented 
subject matter is not limited by any of the specific exemplary teachings 
given. 
Of course, the specific etch chemistries, layer compositions, and layer 
thicknesses given are merely illustrative, and do not necessarily delimit 
the scope of the claimed inventions. 
In particular, the disclosed antifuse structure can also be used with other 
materials for top or bottom contact. The use of metal layers as described 
is very convenient, but various other conductive materials can be used 
instead.