Protective coating useful as passivation layer for semiconductor devices

A protective coating useful as a passivation layer for semiconductor devices incorporates a thin film of an amorphous diamond-like carbon. In one implementation, a thin film of amorphous silicon is deposited over the carbon material. The semiconductive passivation coating eliminates electrical shorts, dissipates charge build-up and protects against chemical contamination.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a protective coating for electrical 
devices and in particular to a thin film assembly useful as a passivating 
coating for a semiconductor device. 
2. Description of the Prior Art 
In present day semiconductor technology, passivation layers are used to 
provide several functions for protection of the semiconductor device 
structure against environmental influences that arise in the manufacture 
and application of semiconductor devices. Passivation layers protect the 
semiconductor devices from the effects of moisture and contaminants which 
may occur during oxidation or other steps of the manufacturing process 
such as during assembly of circuits using the devices, or during the 
practical operation of the devices in circuit environments. By using 
passivation layers, production yield is increased and deleterious effects 
are minimized when the semiconductor device is operating in the field. 
Especially desirable are passivation layers that effectively passivate 
surface states, which are electron energy levels at the surface of the 
semiconductor substrate, characterized by electrical charge and discharge 
having variable time constants. This phenomenon causes electrical drift, 
which may be short term or long term and which undesirably changes the 
characteristics of a field effect transistor. Surface states are 
"passivated" when a layer overlying the semiconductor surface interacts 
with atoms at the surface in such a way as to reduce the time constants 
characterizing the electrical charge and discharge of the surface states 
to values small enough to eliminate electrical drift problems. Surface 
states on a silicon crystal can be passivated by a silicon dioxide layer 
produced by thermal oxidation, for example. Surface states on gallium 
arsenide can be passivated by a layer of semiconducting material other 
than gallium arsenide, for example, providing the interface between the 
gallium arsenide surface and the semiconducting layer is appropriately 
controlled. The resulting junction between the semiconductor surface and 
the semiconducting layer is called a "heterojunction". 
Passivation layers act as insulators and protect against electrical 
shorting and low breakdown voltages. Passivation layers also act as 
potting materials that protect against surface scratches and thus prevent 
electrical shorting. 
During some processes of semiconductor device handling vacuum wands or 
other tools are used to move wafers or chips from one position to another. 
In such cases, the tool may cause a displacement or abrasion of exposed 
metal conductors that are formed on the wafers. Passivation layers help to 
eliminate this problem. Another problem that is encountered is found with 
semiconductor devices that incorporate an air bridge, which is a metal 
connection to a metal conductor that skips over an adjacent conductor, so 
that capacitive coupling is not added between the two conductors. The air 
disposed between the air bridge and the skipped over conductor has a low 
dielectric constant of nearly unity. However, if the metal air bridge is 
subjected to mechanical pressure causing it to contact the skipped over 
metal conductor, an electrical short would result. A passivation layer 
overlying the skipped over metal conductor can act as an electrical 
insulator to insure against such shorting. 
Passivation layers generally are composed of silicon dioxide or silicon 
nitride, for example. Passivation layers using such materials are usually 
relatively thick, about 2000.ANG. or more, and require long deposition 
time, which adds to the cost of the semiconductor devices. With dielectric 
constants more than three times that of air these thick layers increase 
the capacitances between various parts of a semiconductor device, thereby 
degrading the device's high-frequency performance. Also, silicon dioxide 
and silicon nitride do not adhere very well to gold which is used for 
electrodes or conductors and actually are known to separate from gold 
conductors so that circuit problems are caused. Thick layers of insulators 
or passivation material using silicon dioxide or silicon nitride are 
subject to strain, and as they are relatively brittle in nature, can 
experience cracking and do not seal well. Furthermore, insulators such as 
silicon dioxide or silicon nitride do little to passivate surface states 
on some semiconductors such as gallium arsenide and can themselves act as 
charge traps and produce drift problems. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a novel coating that is effective 
to protect electrical devices from electrical shorting and failure. 
Another object of this invention is to provide passivation material for a 
semiconductor device that realizes an improvement in production and 
handling yields and enhances performance and reliability of the operating 
device. 
Another object of this invention is to provide a semiconducting passivation 
material for a semiconductor device that partially passivates surface 
states and reduces drift problems in the operating device. 
In accordance with this invention, a protective coating useful for a 
semiconductor device is formed with a carbon material that has 
diamond-like characteristics and is semiconducting. To obtain the desired 
carbon coating, a ring of graphite material is used as a target in a 
sputtering system to deposit a thin film of carbon to a specified 
thickness. The passivation layer covers exposed portions of the substrate, 
electrodes and metal conductors that are exposed. In one embodiment, a 
very thin film of amorphous silicon is deposited over the carbon layer. 
The silicon thin film protects the carbon from removal during plasma 
cleaning that occurs during the semiconductor manufacturing process and 
provides additional sealing and protection. In another implementation, 
wherein an air bridge connection to conductive elements on the substrate 
is formed, the passivation layer serves to insulate the conductive air 
bridge from adjacent exposed conductors where an electrical shorting 
problem could exist.

DETAILED DESCRIPTION OF THE INVENTION 
Although the description is directed to the use of a passivation material 
for a semiconductor device, it should be understood that the protective 
coating disclosed herein is also applicable to other electrical assemblies 
such as thin film circuits, microwave monolithic integrated circuits and 
for use as capacitor dielectrics, by way of example. 
With reference to the drawing, a semiconductor device includes a substrate 
10 which may be made of gallium arsenide or silicon by way of example. 
During the manufacture of the semiconductor device, an ohmic contact layer 
12 is deposited on a wafer or substrate, followed by the deposition of a 
metallization layer 14 and the formation of a gate electrode 15. The 
metallization and gate may be formed of gold or another conductive metal, 
as is well known in the art. The wafer is then positioned in a vacuum 
chamber fitted with a Sloan Model S-310 sputtergun. 
In accordance with this invention, a graphite ring target is placed in the 
sputtergun at a specified working distance of about 1.7 inches from the 
wafer. The sputtering system is operated at room temperature, and an argon 
environment at a pressure of about 8 microns is provided within the 
chamber enclosing the target and the wafer. A current of about 0.2 Amps DC 
is applied to the sputtergun so that the deposition rate of the carbon 
onto the wafer is about 50 .ANG./min. The sputtering deposition serves to 
deposit one atom at a time on the surface of the wafer so that a thin 
film, having a thickness in the range of approximately 100-20,000.ANG., 
and preferably of about 300.ANG., is deposited on the exposed surface 
portions of the wafer facing the target. The deposited carbon 16 is much 
harder than graphite and has diamond-like carbon characteristics. The 
carbon adheres well to the exposed areas of the substrate and to the 
metallization which is deposited on the substrate. 
In one embodiment, a thin film of almost pure amorphous silicon 18 is 
deposited on the carbon layer by plasma enhanced chemical vapor 
deposition. The silicon deposition is accomplished at room temperature, 
using silane gas at a low pressure, which is 10 microns or less of 
mercury, by way of example. R.F. power excitation is applied so that a 
thin film of about 200-300.ANG. of silicon is deposited over the carbon. 
The silicon conforms to the underlying carbon and effectively seals pin 
holes and protects the thin carbon material from removal during further 
processing. Additionally, the silicon film further protects the substrate 
surface from degrading effects. 
After the deposition of the silicon, the semiconductor device is then 
processed to provide a third metallization layer 20 of gold that makes 
contact with the metallization layer 14 below the passivation material via 
through holes. The semiconductor structure is thus provided with 
conductive lines and bonding pads to enable electrical connection to 
external circuitry, in a well known manner. 
In another implementation, the semiconductor device is fabricated with an 
air bridge 22, which may be made of gold, that leads from an electrode 14A 
to the electrode of an adjacent device structure (not shown). The air 
bridge in effect skips over the conductor 14B to make connection to the 
adjacent device. If pressure is inadvertently applied to the air bridge so 
that it is displaced towards the substrate surface, there is a chance that 
the air bridge will contact the metallic conductor 14B that normally is 
separated by air from the air bridge. By providing a passivation layer 
made of diamond-like carbon, or diamond-like carbon in combination with a 
silicon thin film, between the air bridge 22 and the opposing conductor 
14B, the electrical shorting condition that would occur upon contact 
between the air bridge and the skipped over conductor 14B is prevented. 
In an alternative implementation, the passivation assembly is formed with a 
sputtered carbon material covered by a second harder diamond-like carbon 
material deposited by plasma enhanced chemical vapor deposition. A silicon 
thin film can be deposited over the second layer of carbon. 
It should be understood that the invention is not limited to the specific 
arrangements and parameters set forth above. For example, the passivation 
material may be amorphous carbon, diamond-like carbon, polycrystalline 
diamond and/or monocrystalline diamond. The passivation material may be 
deposited over a third metallization layer of the semiconductor device. 
The semiconductor device may be bipolar as well as a field effect 
transistor. The magnitudes of the current, temperature, pressure, the 
working distance between the target and wafer, and the chemical makeup of 
the gas may be varied within the scope of this invention. Also, methods 
other than sputter deposition or plasma enhanced chemical vapor deposition 
may be employed to create the carbon or silicon layers. 
The passivation structure affords a very thin film that increases 
interelectrode capacitances very little and lends itself to enhanced 
integrated circuit performance. This is in contrast to the thick 
passivation layers used in prior art devices that tend to degrade 
performance significantly, as much as 0.5-1 dB in high-frequency gain, for 
instance. Also, even though the passivation film is relatively thin, it 
still provides sufficient protection against chemical penetration. The 
semiconductive passivation material having diamond-like carbon 
characteristics dissipates charge build-up very quickly so that drift 
problems are minimized. The diamond-like carbon, being itself a 
semiconductor, can form a heterojunction with the semiconductor surface 
and thereby partially passivate the surface states.