Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2003-05-15
2004-07-06
Le, Dung A. (Department: 2818)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S637000, C438S618000
Reexamination Certificate
active
06759331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface zener diodes and, more particularly, to an apparatus and method for reducing surface zener drift in a zener diode.
2. Description of the Related Art
A zener diode is a pn junction that has a reverse breakdown voltage that defines two distinctly different regions of reverse-bias operation. When the pn junction is reverse biased, but the reverse-biased voltage is less than the reverse breakdown voltage, only a small leakage current flows through the junction.
On the other hand, when the reverse-biased voltage is increased to exceed the reverse breakdown voltage, a large breakdown current flows through the junction. Zener diodes are commonly used to provide a stable reference voltage by permanently biasing the diode to have a reverse-biased voltage that is greater than the reverse breakdown voltage.
FIG. 1
shows a cross-sectional view that illustrates a conventional surface zener diode
100
. As shown in
FIG. 1
, diode
100
, which is formed in a n− semiconductor material
108
, includes a n+ region
110
and an adjacent and overlapping p region
112
that are formed in n− material
108
.
In operation, when a first voltage is placed on n+ region
110
and a lower second voltage is placed on p region
112
such that the reverse biased voltage is less than the reverse breakdown voltage, only a small leakage current flows through the junction.
However, when the voltage on n+ region
110
is increased to exceed the reverse breakdown voltage of diode
100
, a large breakdown current flows through the junction. When the breakdown current flow is primarily a lateral flow across the junction at the surface of p region
112
, diode
100
is often referred to as a lateral diode or a surface zener diode.
One surface zener diode problem, known as surface zener drift, occurs when the reverse breakdown voltage of the diode drifts over time. Since the active junction formed between n+ region
110
and p region
112
is primarily a surface junction, it is more susceptible to the presence of hydrogen. The presence of hydrogen can significantly increase the reverse breakdown voltage characteristics of the diode, resulting in an observed voltage drift over time. When the diode is used as a stable reference voltage (the diode is permanently biased to have a reverse-biased voltage that is greater than the reverse breakdown voltage), the drift can lead to degraded circuit operation and potential device failure.
Thus, there is a need for a method of forming a surface zener diode that minimizes drift in the reverse breakdown voltage over time.
SUMMARY OF THE INVENTION
The present invention provides a method of forming a surface zener diode that substantially reduces drift in the reverse breakdown voltage of the diode. A semiconductor structure formed in accordance with the present invention is formed in a semiconductor material of a first conductivity type. The semiconductor structure includes a first region of the first conductivity type that is formed in the semiconductor material. The first region has a dopant concentration that is greater than the dopant concentration of the semiconductor material.
The semiconductor structure also includes a second region of a second conductivity type that is formed in the semiconductor material to adjoin the first region, and a layer of isolation material that is formed on the semiconductor material.
The semiconductor structure also includes a conductive contact that is formed through the layer of isolation material to make an electrical contact with the first region. In addition, a first metal trace is formed over the layer of isolation material and the conductive contact.
The semiconductor structure additionally includes a layer of insulation material that is formed on the first metal trace, and a conductive via that is formed through the layer of insulation material to make an electrical contact with the first metal trace. Further, the structure includes a second metal trace that is formed on the layer of insulation material and the conductive via to make an electrical contact with the conductive via. In addition, a layer of passivation material is formed over the second metal trace. The layer of passivation material, in turn, includes nitride.
In accordance with the present invention, the semiconductor structure also includes a titanium protection layer that is formed over the layer of isolation material and the conductive contact, and below the layer of passivation material. The titanium protection layer can be formed on the isolation layer and the conductive contact under the first metal trace, or on and over the second metal trace. Alternately, the titanium protection layer can be formed on the insulation layer and the conductive via under the second metal trace, or on and over the second metal trace.
The present invention also includes a method for forming a semiconductor structure in a semiconductor material of a first conductivity type. The semiconductor structure includes a first region of the first conductivity type that is formed in the semiconductor material. The first region has a dopant concentration that is greater than the dopant concentration of the semiconductor material.
The semiconductor structure also includes a second region of a second conductivity type that is formed in the semiconductor material to adjoin the first region, and a layer of isolation material that is formed on the semiconductor material. The semiconductor structure also includes a conductive contact that is formed through the layer of isolation material to make an electrical contact with the first region.
The method of the present invention includes the steps of forming a first metal trace over the layer of isolation material and the conductive contact, and forming a layer of insulation material on the first metal trace. The method also includes the step of forming a conductive via through the layer of insulation material to make an electrical contact with the first metal trace.
The method further includes the step of forming a second metal trace on the layer of insulation material and the conductive via to make an electrical contact with the conductive via. The method additionally includes the steps of forming a layer of passivation material over the second metal trace, and forming a titanium protection layer over the layer of isolation material and the conductive contact, and below the layer of passivation material. The layer of passivation material includes nitride.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.
REFERENCES:
patent: 5117276 (1992-05-01), Thomas et al.
James Dunkley et al., “Hot Electron Induced Hydrogen Compensation of Boron Doped Silicon Resulting From Emitter-Base Breakdown”, IEEE, (1992) pp. 31.4.1-31.4.4.
M. Soltani-Farshi et al., “Thermal Behaviour of Implanted Nitrogen and Accumulated Hydrogen in Titanium”, (1998), Mat. Res. Soc. Symp. Proc. vol. 504, Materials Research Society, pp. 231-237.
O.N. Senkov and J.J. Jonas, “Dynamic Strain Aging and Hydrogen-Induced Softening in Alpha Titanium”, Metallurgical and Materials Transactions A. vol. 27A, Jul. 1996, pp. 1877-1887.
Stanley Wolf Ph.D., Silicon Processing For the VLSI Era, vol. 2: Process Integration, Lattice Press, (1990), pp. 191 and 124-125.
Kurihara Steven
Min Heikyung
Spence Robert
Le Dung A.
National Semiconductor Corporation
Pickering Mark C.
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