Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having schottky gate
Reexamination Certificate
2002-10-03
2004-01-06
Dang, Phuc T. (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having schottky gate
C438S222000, C438S931000
Reexamination Certificate
active
06673662
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates microelectronic devices and more particularly to edge termination for silicon carbide Schottky devices.
BACKGROUND OF THE INVENTION
High voltage silicon carbide (SiC) Schottky diodes, which can handle voltages between 600V and 2.5 kV, are expected to compete with silicon PIN diodes fabricated of similar voltage ratings. Such diodes may handle as much as 100 amps of current, depending on their active area. High voltage Schottky diodes have a number of important applications, particularly in the field of power conditioning, distribution and control.
An important characteristic of a SiC Schottky diode in such applications is its switching speed. Silicon-based PIN devices typically exhibit relatively poor switching speeds. A silicon PIN diode may have a maximum switching speed of approximately 20 kHz, depending on its voltage rating. In contrast, silicon carbide-based devices are theoretically capable of much highe in excess of 100 times better than silicon. In addition, silicon carbide devices may be capable of handling a higher current density than silicon devices.
However, reliable fabrication of silicon carbide-based Schottky devices may be difficult. Typical edge termination in SiC Schottky diodes require ion implantation of p-type dopants into the crystal. Such implants may cause substantial damage to the crystal lattice, which may require high temperature annealing to repair such defects. This high-temperature anneal step (>1500° C.) may be undesirable for a number of reasons. For example, high temperature anneals tend to degrade the surface of SiC on which the Schottky contact is to be made, as silicon tends to dissociate from exposed surfaces of the crystal under such a high-temperature anneal. Loss of silicon in this manner may result in a reduced quality Schottky contact between metal and the semiconductor surface. High temperature anneals have other drawbacks as well. Namely, they are typically time-consuming and expensive. Moreover, implantation of p-type (Al) dopants may cause substantial lattice damage, while other species (B) have poor activation rates.
A conventional SiC Schottky diode structure has an n-type SiC substrate on which an n
−
epitaxial layer, which functions as a drift region, is formed. The device typically includes a Schottky contact formed directly on the n
−
layer. Surrounding the Schottky contact is a p-type JTE (junction termination extension) region which is typically formed by ion implantation. The implants may be aluminum, boron, or any other suitable p-type dopant. The purpose of the JTE region is to prevent the electric field crowding at the edges, and to prevent the depletion region from interacting with the surface of the device. Surface effects may cause the depletion region to spread unevenly, which may adversely affect the breakdown voltage of the device. Other termination techniques include guard rings and floating field rings which are more strongly influenced by surface effects. A channel stop region may also be formed by implantation of n-type dopants such as Nitrogen or Phosphorus in order to prevent the depletion region from extending to the edge of the device.
Additional conventional termination of SiC Schottky diodes are described in “Planar Terminations in 4H—SiC Schottky Diodes With Low Leakage And High Yields” by Singh et al., ISPSD '97, pp. 157-160. A p-type epitaxy guard ring termination for a SiC Schottky Barrier Diode is described in “The Guard-Ring Termination for High-Voltage SiC Schottky Barrier Diodes” by Ueno et al., IEEE Electron Device Letters, Vol. 16, No. 7, July, 1995, pp. 331-332. Additionally, other termination techniques are described in published PCT Application No. WO 97/08754 entitled “SiC Semiconductor Device Comprising A PN Junction With A Voltage Absorbing Edge.”
SUMMARY OF THE INVENTION
Embodiments of the present invention may provide a silicon carbide Schottky rectifier having a silicon carbide voltage blocking layer having a predefined surface doping level and a Schottky contact on the silicon carbide voltage blocking layer. A silicon carbide epitaxial region is also provided on the silicon carbide voltage blocking layer and adjacent the Schottky contact. The silicon carbide epitaxial region has a thickness and a doping level designed to provide a selected charge per unit area in the silicon carbide epitaxial region. The charge per unit area in the silicon carbide epitaxial regions, also referred to as the junction termination extension (JTE) charge, is selected based on the surface doping of the blocking layer. In particular embodiments, the JTE charge is greater than 50% of an optimal JTE charge as determined by the surface doping of the blocking layer. Furthermore, it is preferred that the JTE charge is not greater than the optimal charge value.
In further embodiments of the present invention, a silicon carbide Schottky rectifier is provided having a silicon carbide voltage blocking layer and a Schottky contact on the silicon carbide voltage blocking layer. A silicon carbide epitaxial termination region is provided on the voltage blocking layer and adjacent the Schottky contact. The product of the thickness and doping concentration of the silicon carbide epitaxial region is greater than about 50% of
(
ϵ
r
×
ϵ
0
×
E
C
)
q
;
where:
∈
r
is the relative dielectric constant of SiC;
∈
0
so is dielectric constant of air;
E
C
is the critical electric field of SiC; and
q is the electronic charge.
In further embodiments, the product of the thickness and doping concentration are not greater than about 100% of
(
ϵ
r
×
ϵ
0
×
E
C
)
q
.
In still further embodiments, the thickness and doping concentration are not less than about 75% of
(
ϵ
r
×
ϵ
0
×
E
C
)
q
.
In still further embodiments of the present invention, the silicon carbide epitaxial region extends from the Schottky contact from about 1.5 to about 5 times the thickness of the blocking layer. Additionally, a non-ohmic contact may be provided between the silicon carbide epitaxial termination region and the Schottky contact.
In embodiments of the present invention where the silicon carbide epitaxial region has a first conductivity type and the voltage blocking layer has a second conductivity type opposite the first conductivity type, the edge termination may also include a region of first conductivity type silicon carbide in the voltage blocking layer having a carrier concentration higher than that of the voltage blocking layer and adjacent a periphery of the silicon carbide epitaxial region opposite the Schottky contact.
In additional embodiments of the present invention, the Schottky rectifier may also include a first layer of silicon carbide of a first conductivity type the same as a conductivity type of the blocking layer and disposed between the blocking layer and a silicon carbide substrate. The first layer of silicon carbide may have a carrier concentration higher than the blocking layer. A second layer of silicon carbide of the first conductivity type may also be provided on the substrate opposite the first layer of silicon carbide so as to provide a layer of silicon carbide having a carrier concentration higher than a carrier concentration of the substrate. An ohmic contact may be provided on the second layer of silicon carbide. In such embodiments, the second layer may be an implanted layer of first conductivity type silicon carbide. Furthermore, the silicon carbide epitaxial region may be of a second conductivity type opposite that of the first conductivity type. In particular, the first conductivity type may be n-type and the second conductivity type may be p-type.
In other embodiments of the present invention, a Schottky rectifier is provided which includes an n-type silicon carbide substrate, an n-type silicon carbide blocking layer on the silicon carbide substrate, a Schottky contact on the silicon carbide blocking layer, an epitaxial region of p-type
Cree Inc.
Dang Phuc T.
Myers Bigel Sibley & Sajovec P.A.
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