Trench structure for semiconductor devices

Details Trench structure for semiconductor devices Trench structure for semiconductor devices

2001-07-03

2004-01-27

Flynn, Nathan J. (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

With means to increase breakdown voltage threshold

With physical configuration of semiconductor surface to...

C257S487000, C257S491000, C257S328000, C257S355000

Reexamination Certificate

active

06683363

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates in general to semiconductor devices. More specifically, the present invention relates to trench structures, which can be used to enhance the performance of semiconductor devices.
There exist a variety of semiconductor devices commonly used in power applications. One such device is the Schottky barrier. A Schottky barrier comprises a metal-semiconductor interface, which functions as rectifier for controlling current transport.
A figure of merit, which is used to measure the blocking capability of a Schottky barrier rectifier is its breakdown voltage. A breakdown voltage in this context refers to the maximum reverse voltage, which can be supported across the device, while still being able to provide a blocking function. Breakdown in a Schottky barrier rectifier is normally an “avalanche” type breakdown, which is predominantly attributable to a phenomenon known as “impact ionization”.
FIG. 1
shows a cross section of a basic Schottky barrier rectifier
10
. A first metal layer
100
is formed on a semiconductor layer
102
. Typically, the semiconductor layer
102
is comprised of an epitaxial layer
104
, which lends itself as a drift region, and a more heavily doped substrate
106
. Heavily doped substrate
106
and a second metal layer
108
provide an ohmic contact for the device.
Applying a reverse bias voltage VREV across the Schottky barrier rectifier
10
creates a depletion region
110
, across which a majority of the applied voltage is dropped. As the reverse voltage is increased, electric fields in the depletion region
110
become greater. These increasing electric fields cause the charge carriers to accelerate and, if sufficiently accelerated, can cause the creation of electron-hole pairs by collision with dopant atoms. The more carriers that are generated, the more carriers having sufficient energy to cause impact ionization there become. Hence, impact ionization is a snowball effect, whereby a cascade of electron-hole pairs are created by a succession and multiplication of collisions. A point is eventually reached where the rate of impact ionization is so great that the device cannot support any further reverse bias applied across it. This voltage limit is commonly referred to in the art as the “avalanche breakdown voltage”.
The basic Schottky barrier rectifier
10
shown in
FIG. 1
is limited by its reverse blocking capability, since electric fields tend to converge at the edges of the metal layer
100
. Because of this, techniques for terminating the Schottky barrier rectifier have been sought. Two commonly used techniques, which reduce the edge effects are a local oxidation of silicon (LOCOS) structure and the diffused field ring structure described in “Modern Power Devices” by B. J. Baglia, 1987, Reprinted Edition, pp. 437-438. These two approaches are shown here in
FIGS. 2 and 3
. Each of these prior art techniques has the effect of reducing electric field crowding at the metal edges and, consequently, a higher breakdown voltage is achieved.
A technique proposed to achieve even better reverse blocking capabilities in a Schottky barrier rectifier is described in Wilamowski, B. M., “Schottky Diodes with High Breakdown Voltages,”
Solid State Electron.,
26, 491-493 (1983). A cross section of the structure proposed in this article, referred to as a Junction Barrier Controlled Schottky Rectifier (i.e. “JBS rectifier”), is shown here in
FIG. 4. A
series of p-type regions
400
are formed in and at the surface of the drift region
402
of the device. These p-type regions
400
act as screens to lower the electric field near the surface. Since electric fields at the surface are what determine the breakdown voltage of the device, introduction of the p-regions
400
results in a higher breakdown voltage.
An undesirable characteristic of the JBS rectifier relates to the p-n junctions, which are formed between the p regions
400
and the drift region
402
. For silicon devices having a high reverse breakdown voltage, a forward bias exceeding 0.7 volts is required before a reasonable forward conduction current of the Schottky barrier can be realized. Unfortunately, voltages higher than 0.7 volts have the effect of turning on the p-n junctions. When on, minority carriers are introduced, which slow the switching speed of the device. A reduction in switching speed is undesirable, particularly if the Schottky barrier rectifier is to be used in switching applications such as, for example, switch-mode power supplies.
To overcome the forward bias limitations associated with the JBS rectifier, an alternative device structure has been proposed, which utilizes a series of parallel metal oxide semiconductor (MOS) trenches in replace of the p-type regions. This MOS Barrier Schottky Rectifier (i.e. “MBS rectifier”) is proposed in B. J. Baliga, “New Concepts in Power Rectifiers,”
Proceedings of the Third International Workshop on the Physics of Semiconductor Devices,
November 24-28, World Scientific Publ. Singapore, 1985. A cross-section of an MBS rectifier
50
is shown in FIG.
5
A. It comprises a first metal layer
508
, over which a semiconductor layer
502
is formed. Typically, the semiconductor layer
502
is comprised of an epitaxial layer
504
, which lends itself as a drift region, and a more heavily doped substrate
506
. Heavily doped substrate
506
and first metal layer
508
provide an ohmic contact for the device. MBS rectifier
50
also includes a number of parallel trenches
512
formed in epitaxial layer
504
, each of which has an end that terminates (or “merges”) with a termination trench
514
, which includes a segment that runs essentially perpendicular to the parallel trenches
512
. Termination trench
514
and parallel trenches
512
are lined with a dielectric
516
, e.g. silicon dioxide, and are filled with a conductive material
518
, e.g. metal (as shown in FIG. SA) or doped polysilicon. A second metal layer
520
is formed over the entire surface of the structure. Note that in
FIG. 5A
, metal layer
520
is shown as only partially covering the surface. However, this is done so that underlying elements of the rectifier
50
, which would otherwise be covered by metal layer
520
, can be seen. The metal/semiconductor barrier of MBS rectifier
50
is formed at the junction between second metal layer
520
and upper surfaces of mesas
522
formed between parallel trenches
512
.
In many respects, the MBS rectifier is superior to the JBS rectifier. However, it too has limits on its reverse blocking capabilities. These limits can be illustrated by reference to
FIG. 5B
, which shows a top or “layout” view of the MBS rectifier in FIG.
5
A. The arrows, at the ends of mesas
522
, which point at labels “E” (“E” electric field), are present to show how under reverse bias conditions, electric fields tend to crowd toward the ends of mesas
522
. This electric field crowding phenomenon is due to faster depletion in these regions, compared to other regions in semiconductor layer
502
. Accordingly, the breakdown voltage of the MBS rectifier shown in
FIG. 5A
is determined and, therefore, limited by the trench structure geometry illustrated in FIG.
5
B.
SUMMARY OF THE INVENTION
Generally, a broken trench structure enhances the breakdown characteristics of semiconductor devices, according to various aspects of the present invention. For example, as explained in more detail below, a Schottky barrier rectifier, when integrated with the broken trench aspect of the present invention, shows enhanced reverse blocking capabilities, compared to that achievable in prior art structures.
According to a first aspect of the invention, a MOS trench structure integrated with a semiconductor device for enhancing the breakdown characteristics of the semiconductor device comprises a semiconductor substrate; a plurality of parallel trenches formed in the semiconductor substrate, each parallel trench defined by end walls, sidewalls and a bottom and each two adjacent parallel trenches separated by mesas containing the semicon

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