U.S. patent application number 15/439191 was filed with the patent office on 2018-07-05 for method and apparatus to model and monitor time dependent dielectric breakdown in multi-field plate gallium nitride devices.
The applicant listed for this patent is Texas Instruments Incorporated. Invention is credited to Jungwoo Joh, Dong Seup Lee, Sameer Pendharkar.
Application Number | 20180190550 15/439191 |
Document ID | / |
Family ID | 62683629 |
Filed Date | 2018-07-05 |
United States Patent
Application |
20180190550 |
Kind Code |
A1 |
Lee; Dong Seup ; et
al. |
July 5, 2018 |
METHOD AND APPARATUS TO MODEL AND MONITOR TIME DEPENDENT DIELECTRIC
BREAKDOWN IN MULTI-FIELD PLATE GALLIUM NITRIDE DEVICES
Abstract
A first set of test structures for a gallium nitride (GaN)
transistor that includes N field plates is disclosed, where N is an
integer and X is an integer between 0 and N inclusive. A test
structure TS.sub.X of the first set of test structures includes a
GaN substrate, a dielectric material overlying the GaN substrate, a
respective source contact abutting the GaN substrate and a
respective drain contact abutting the GaN substrate. The test
structure TS.sub.X also includes a respective gate overlying the
substrate and lying between the respective source contact and the
respective drain contact and X respective field plates
corresponding to X of the N field plates of the GaN transistor, the
X respective field plates including field plates that are nearest
to the GaN substrate.
Inventors: |
Lee; Dong Seup; (McKinney,
TX) ; Joh; Jungwoo; (Allen, TX) ; Pendharkar;
Sameer; (Allen, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Texas Instruments Incorporated |
Dallas |
TX |
US |
|
|
Family ID: |
62683629 |
Appl. No.: |
15/439191 |
Filed: |
February 22, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62440049 |
Dec 29, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 22/34 20130101;
G01R 31/2621 20130101; H01L 27/088 20130101; H01L 29/2003 20130101;
H01L 29/41725 20130101; H01L 29/0649 20130101; G01R 31/2642
20130101; G01R 31/2884 20130101; H01L 29/404 20130101; H01L 29/7786
20130101; H01L 23/562 20130101; G01R 31/12 20130101 |
International
Class: |
H01L 21/66 20060101
H01L021/66; H01L 29/06 20060101 H01L029/06; H01L 23/544 20060101
H01L023/544; H01L 29/40 20060101 H01L029/40; H01L 29/20 20060101
H01L029/20; H01L 29/417 20060101 H01L029/417; H01L 27/088 20060101
H01L027/088; G01R 31/12 20060101 G01R031/12; G01R 31/28 20060101
G01R031/28 |
Claims
1: A first set of (N+1) test structures for a gallium nitride (GaN)
transistor that comprises N field plates, N being an integer and X
being an integer between 0 and N inclusive, each test structure
TS.sub.X of the first set of test structures comprising: a GaN
substrate; a dielectric material overlying the GaN substrate; a
respective source contact abutting the GaN substrate; a respective
drain contact abutting the GaN substrate; a respective gate
overlying the substrate and lying between the respective source
contact and the respective drain contact; and X respective field
plates corresponding to X of the N field plates of the GaN
transistor, the X field plates comprising field plates that are
nearest to the GaN substrate.
2: The first set of (N+1) test structures as recited in claim 1
test structure TS.sub.X further comprising, a respective
input/output pad coupled to each of the respective source contact,
the respective drain contact and the respective gate of each test
structure.
3: The first set of (N+1) test structures as recited in claim 2
further comprising an epitaxial layer formed in the GaN
substrate.
4: The first set of (N+1) test structures as recited in claim 3
wherein a first field plate of the N field plates is electrically
coupled to the gate and extends laterally beyond the gate towards
the drain contact.
5: A first set of (N+1) test structures for a gallium nitride (GaN)
transistor that comprises N field plates, N being an integer and X
being an integer between 0 and N inclusive, a test structure
TS.sub.X of the first set of test structures comprising: a GaN
substrate; a dielectric material overlying the GaN substrate; a
respective source contact abutting the GaN substrate; a respective
drain contact abutting the GaN substrate; a respective gate
overlying the substrate and lying between the respective source
contact and the respective drain contact; and X respective field
plates corresponding to X of the N field plates of the GaN
transistor, the X field plates comprising field plates that are
nearest to the GaN substrate, wherein a first field plate of the N
field plates is electrically coupled to the gate and extends
laterally beyond the gate towards the drain contact, and wherein a
second field plate of the N field plates is electrically coupled to
a first structure selected from a group consisting of the gate and
the source contact and extends laterally beyond the first field
plate towards the drain contact.
6: The first set of (N+1) test structures as recited in claim 5
wherein a third field plate of the N field plates is electrically
coupled to a second structure selected from a group consisting of
the gate and the source contact and extends laterally beyond the
second field plate towards the drain contact.
7: The first set of (N+1) test structures as recited in claim 4
further comprising respective isolation structures separating each
respective pair of adjacent test structures.
8: The first set of (N+1) test structures as recited in claim 7
wherein the respective isolation structures are trench
isolation.
9: The first set of (N+1) test structures as recited in claim 7
wherein the respective isolation structures are isolation
implantations.
10: The first set of (N+1) test structures as recited in claim 7
wherein the respective gate further overlies at least a portion of
the dielectric material.
11: The first set of (N+1) test structures as recited in claim 1
further comprising a second set of (N-1) test structures with Y
being an integer between 1 and (N-1) inclusive, a test structure
TS.sub.Y of the second set of (N-1) test structures comprising: a
respective source contact abutting the GaN substrate; a respective
drain contact abutting the GaN substrate; and a respective field
plate corresponding to the Y.sup.th field plate of the GaN
transistor.
12: A test method comprising: providing a first set of test
structures TS.sub.0 through TS.sub.N for a gallium nitride (GaN)
transistor that comprises N field plates, N being an integer and X
being an integer between 0 and N inclusive, a test structure
TS.sub.X of the first set of test structures comprising: a GaN
substrate, a dielectric material overlying the GaN substrate, a
respective source contact abutting the GaN substrate, a respective
drain contact abutting the GaN substrate, a respective gate
overlying the substrate and lying between the source contact and
the drain contact, X respective field plates corresponding to X
field plates of the N field plates of the GaN transistor that are
nearest to the GaN substrate, and a respective input/output pad
coupled to each of the respective source contact, the respective
drain contact and the respective gate; for each test structure
TS.sub.X, the test method comprising: applying a stress voltage to
the drain contact of test structure TS.sub.X until a dielectric
breakdown condition is detected; and recording the time-to-failure
of test structure TS.sub.X at the stress voltage.
13: The test method as recited in claim 12 wherein the first set of
test structures are provided in a scribe structure and the test
method further comprises determining whether there is in a
determined lifetime of the GaN transistor at the stress
voltage.
14: The test method as recited in claim 12 further comprising:
incrementing the stress voltage; performing the applying, recording
and incrementing a selected number of times, each of the selected
number of times being performed on a new first set of test
structures; and using a plurality of the recorded times-to-failure
of the test structure to extrapolate a mean time-to-failure at
normal operating voltages.
15: The test method as recited in claim 14 further comprising using
the plurality of the recorded times-to-failure of the test
structure to determine a time-to-failure distribution.
16: The test method as recited in claim 12 further comprising:
incrementing the stress voltage; performing the applying, recording
and incrementing a selected number of times, each of the selected
number of times being performed on a new first set of test
structures; and determining whether there has been a drift in a
voltage acceleration factor.
17: The test method as recited in claim 12 further comprising
holding the gate at a voltage that is lower than the threshold
voltage of the GaN transistor.
18: The test method as recited in claim 12 further comprising
holding the source contact at a lower rail.
19: The test method as recited in claim 12 wherein the stress
voltage is in the range of approximately three to six times the
normal operating voltage.
20: The test method as recited in claim 10 further comprising:
providing a second set of (N-1) test structures with Y being an
integer between 1 and (N-1) inclusive, a test structure TS.sub.Y of
the second set of (N-1) test structures comprising: a respective
source contact abutting the GaN substrate; a respective drain
contact abutting the GaN substrate; and a respective field plate
corresponding to the Y.sup.th field plate of the GaN transistor;
for each test structure TS.sub.Y, the test method comprising:
applying a stress voltage to the drain contact of TS.sub.Y until a
dielectric breakdown condition is detected; and recording the
time-to-failure of TS.sub.Y at the stress voltage.
21-22: (canceled)
Description
FIELD OF THE DISCLOSURE
[0001] Disclosed embodiments relate generally to the field of
testing integrated circuit devices. More particularly, and not by
way of any limitation, the present disclosure is directed to a
method and apparatus to model and monitor time dependent dielectric
breakdown in multi-field plate gallium nitride (GaN) devices.
BACKGROUND
[0002] Unlike semiconductor processing on silicon-based wafers,
which has been studied and refined for more than half a century,
gallium nitride processing is a relatively new technology, for
which standardized testing is still being devised. In particular,
in GaN high voltage technology, no standard qualification criteria
have yet been determined. Processes are needed that can validate
GaN and similar devices for long term use in a relatively short
amount of time.
[0003] Due to the high critical electric field of GaN, Time
Dependent Dielectric Breakdown (TDDB) can be one of the main
factors that limit the device lifetime. Most GaN high voltage
devices utilize a structure that includes multiple field plates to
shield underlying structures from the high voltages present.
However, the presence of multiple field plates increases the
challenge in determining the TDDB lifetime of the inner field plate
region due to the electric field shielding from the outer field
plates.
SUMMARY
[0004] Disclosed embodiments provide a first and a second set of
test structures and a method of using these test structures to test
and monitor variations in design and processing that can affect the
TDDB lifetime of a GaN device or other device utilizing multiple
field plates. The first set of test structures includes a separate
test structure for each level of field plate and another separate
structure for the gate. The test structure designed to test a
particular field plate or gate eliminates the overlying field
plate(s) that can shield the tested structure. For example, a test
structure for the gate includes only the source contact, the drain
contact and the gate, with no field plates, allowing the gate to be
separately qualified; a test structure for the first field plate
(which for the purposes of this application are numbered
sequentially from the field plate closest to the substrate outward)
includes the source contact, the drain contact, the gate and the
first field plate; a test structure for the second field plate
includes the source contact, the drain contact, the gate, the first
field plate and the second field plate; etc. The second set of test
structures focuses on the inner field plates. In this second set of
test structures, only the source and drain contacts and the inner
field plate under test are provided in the respective test
structure.
[0005] Testing includes subjecting each of the first test
structures to high voltages on the drain contact, i.e., higher than
the normal operating voltages to which the specific
structure-under-test will be exposed, and determining the mean time
to failure (MTTF) of the device under each of the subject voltages.
Testing can also include determining a time to failure distribution
in order to build a reliability model. The values determined for
the MTTF can then be projected for normal operating conditions. If
the projected MTTF is acceptable, testing is completed; otherwise
changes can be proposed for the device structure and retested. Once
testing has been completed and the design and processing are
finalized, the disclosed test structures can be incorporated into
scribe structures on the production chip. This provides a mechanism
by which production can be monitored on a periodic basis, by
probing the scribe structures to ensure that processing has not
drifted away from specifications.
[0006] The second set of test structures is useful when one or more
of the inner field plates fails earlier than the outer field plates
for any reason. This set provides the simplest structure that can
be used to test a particular field plate without having to worry
about the reliability of other field plates. Testing on the second
set of test structures can take two forms. In a three-terminal
mode, a voltage is applied to the field plate that is less than the
threshold voltage for the field plate to pinch off the channel. At
the same time, the source is held at the lower rail and a test
voltage is applied to the drain until failure. In a two-terminal
mode, the source contact is either floating or not connected, while
the test voltage is applied to the drain contact until the field
plate fails.
[0007] In one aspect, an embodiment of a first set of (N+1) test
structures for a gallium nitride (GaN) transistor that comprises N
field plates is disclosed, N being an integer and X being an
integer between 0 and N inclusive. A test structure TS.sub.X
includes a GaN substrate; a dielectric material overlying the GaN
substrate; a respective source contact abutting the GaN substrate;
a respective drain contact abutting the GaN substrate; a respective
gate overlying the substrate and lying between the respective
source contact and the respective drain contact; and X respective
field plates corresponding to X of the N field plates of the GaN
transistor, the X field plates comprising field plates that are
nearest to the GaN substrate.
[0008] In another aspect, an embodiment of a test method is
disclosed. The test method includes providing a first set of test
structures TS.sub.0 through TS.sub.N for a gallium nitride (GaN)
transistor that comprises N field plates, N being an integer and X
being an integer between 0 and N inclusive, a test structure
TS.sub.X of the first set of test structures comprising: a GaN
substrate, a dielectric material overlying the GaN substrate, a
respective source contact abutting the GaN substrate, a respective
drain contact abutting the GaN substrate, a respective gate
overlying the substrate and lying between the respective source
contact and the respective drain contact, X field plates
corresponding to X field plates of the N field plates of the GaN
transistor that are nearest to the GaN substrate, and a respective
input/output pad coupled to each of the respective source contact,
the respective drain contact and the respective gate; for each test
structure TS.sub.X, the test method comprising: applying a stress
voltage to the respective drain contact of TS.sub.X until a
dielectric breakdown condition is detected; and recording the
time-to-failure of TS.sub.X at the stress voltage.
[0009] In yet another aspect, an embodiment of an integrated
circuit (IC) chip is disclosed. The IC chip includes a substrate
comprising gallium nitride (GaN); a dielectric material overlying
the substrate; a transistor formed in an active circuitry region of
the IC chip, the transistor comprising a first gate overlying the
substrate, a first source contact abutting the substrate, a first
drain contact abutting the substrate and N field plates overlying
the gate, N being an integer; and a scribe structure formed outside
the active circuitry region of the IC chip, the scribe structure
comprising a set of (N+1) first test structures for the transistor,
a test structure TS.sub.X of the first set of test structures
comprising: a respective source contact abutting the GaN substrate,
a respective drain contact abutting the GaN substrate, a respective
gate overlying the substrate and lying between the respective
source contact and the respective drain contact, X field plates, X
being an integer between 0 and N inclusive, the X field plates
corresponding to X field plates of the N field plates of the
transistor that are nearest to the substrate, and a respective
isolation barrier separating test structure TS.sub.X from adjacent
test structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Embodiments of the present disclosure are illustrated by way
of example, and not by way of limitation, in the figures of the
accompanying drawings in which like references indicate similar
elements. It should be noted that different references to "an" or
"one" embodiment in this disclosure are not necessarily to the same
embodiment, and such references may mean at least one. Further,
when a particular feature, structure, or characteristic is
described in connection with an embodiment, it is submitted that it
is within the knowledge of one skilled in the art to effect such
feature, structure, or characteristic in connection with other
embodiments whether or not explicitly described. As used herein,
the term "couple" or "couples" is intended to mean either an
indirect or direct electrical connection unless qualified as in
"communicably coupled" which may include wireless connections.
Thus, if a first device couples to a second device, that connection
may be through a direct electrical connection, or through an
indirect electrical connection via other devices and
connections.
[0011] The accompanying drawings are incorporated into and form a
part of the specification to illustrate one or more exemplary
embodiments of the present disclosure. Various advantages and
features of the disclosure will be understood from the following
Detailed Description taken in connection with the appended claims
and with reference to the attached drawing figures in which:
[0012] FIGS. 1A-1F depict individual test structures of a set of
test structures for a GaN device having multiple field plates
according to an embodiment of the disclosure;
[0013] FIG. 2 depicts test data collected to determine mean time to
failure for the innermost field plate using two different process
conditions according to an embodiment of the disclosure;
[0014] FIG. 3A depicts an overhead view of an example semiconductor
wafer that includes a number of dies on which are formed GaN
circuits according to an embodiment of the disclosure;
[0015] FIG. 3B depicts an enlargement of a portion of the wafer of
FIG. 3A;
[0016] FIGS. 3C and 3D depict cross sections through an example
scribe structures that includes the disclosed set of test
structures according to an embodiment of the disclosure;
[0017] FIGS. 4A and 4B depict a method of testing a GaN or
semiconductor device having multiple field plates according to an
embodiment of the disclosure; and
[0018] FIGS. 4C and 4D depict a further method of testing specific
field plates of a GaN or semiconductor device having multiple field
plates according to an embodiment of the disclosure.
DETAILED DESCRIPTION OF THE DRAWINGS
[0019] Specific embodiments of the invention will now be described
in detail with reference to the accompanying figures. In the
following detailed description of embodiments of the invention,
numerous specific details are set forth in order to provide a more
thorough understanding of the invention. However, it will be
apparent to one of ordinary skill in the art that the invention may
be practiced without these specific details. In other instances,
well-known features have not been described in detail to avoid
unnecessarily complicating the description.
[0020] A typical GaN transistor includes a thin layer of aluminum
gallium nitride (AlGaN) located above a GaN layer. Because the
AlGaN and GaN layers exhibit different bandgaps, they are said to
meet at a heterojunction. Under proper conditions, a
two-dimensional electron gas (2-DEG) is formed at this
heterojunction interface of the GaN and AlGaN layers. In the 2-DEG,
some electrons are unbound to atoms and free to move, providing
higher mobility as compared with other types of transistors.
Accordingly, little or no doping of the substrate is required for
operation of the GaN transistor.
[0021] FIG. 1A represents a cross-section of both an example GaN
device and a first test structure 100A for the GaN device according
to an embodiment of the disclosure. The GaN device in this example
embodiment is a power transistor designed for high voltages, e.g.,
600 volts. However, the teachings presented herein are equally
applicable to other semiconductor devices and is not limited to the
particular devices described below. Test structure 100A includes a
substrate 102 and an epitaxial layer 104 formed on the upper
surface of substrate 102. In one embodiment, substrate 102
comprises gallium nitride and epitaxial layer 104 comprises
aluminum gallium nitride. In other embodiments, two active layers,
such as a GaN layer and an AlGaN layer, are formed on a substrate
comprising, e.g., silicon, silicon carbide, sapphire, or other
semiconductor material.
[0022] Although the discussion that follows is directed primarily
to embodiments based on GaN, it will be understood that the
disclosed apparatus and method are not so limited. In one
embodiment, the test structures contain nitride compounds of
elements from Group III of the Periodic Table of Elements. In one
embodiment, the active layers have the formula
B.sub.wAl.sub.xIn.sub.yGa.sub.zN, in which w, x, y and z each has a
suitable value between zero and one (inclusive). The reference
herein to B.sub.wAl.sub.xIn.sub.yGa.sub.zN or a
B.sub.wAl.sub.xIn.sub.yGa.sub.zN material refers to a semiconductor
material having nitride and one or more of boron, aluminum, indium
and gallium. Examples of B.sub.wAl.sub.xIn.sub.yGa.sub.zN materials
include GaN, AlN, AlGaN, AlInGaN, InGaN, and BAlInGaN, by way of
illustration. A B.sub.wAl.sub.xIn.sub.yGa.sub.zN material may
include other materials besides nitride, boron, aluminum, indium
and/or gallium. For example, a B.sub.wAl.sub.xIn.sub.yGa.sub.zN
material may be doped with a suitable dopant such as silicon and
germanium.
[0023] A source contact 106, drain contact 108 and gate 110, which
can be formed of conductive material such as titanium, nickel,
aluminum, gold and ohmic metals, complete the basic elements of the
transistor. Source contact 106 and drain contact 108 both abut the
substrate. In one embodiment (not specifically shown), gate 110
also abuts the substrate, e.g. with a p-type GaN substrate in an
enhancement mode transistor. In one embodiment shown in FIG. 1A, a
gate dielectric separates gate 110 from substrate 102. Given the
need to place gate 110 near or touching epitaxial layer 104, which
causes the dielectric layer under gate 110 to be relatively thin,
gate 110 is not able to withstand the electric field generated by
the large gate/drain voltages that exist during operation of the
GaN transistor 100A. To protect gate 110 from these high voltages,
one or more field plates 112, 114, 116, which are also formed of a
conductive material, are used to reduce the electric field produced
by the high voltage. As seen in this embodiment, field plate 112 is
coupled to gate 110, so that these two structures share the same
voltage, but field plate 112 extends laterally closer to drain
contact 108 that does gate 110. Field plate 114 is coupled to
source contact 106, lies above both gate 110 and field plate 112
and extends laterally closer to drain contact 108 than the two
lower elements. Field plate 116 is also coupled to source contact
106, lies above field plate 114 and extends laterally closer to
drain contact 108 than field plate 114. In one embodiment, not
specifically shown, each of field plates 112, 114, 116 are coupled
to gate 110. In one embodiment, also not specifically shown, field
plates 112, 114 are coupled to gate 110 and field plate 116 is
coupled to the source. In one example, for a GaN device that is
subjected to 600 volts at the gate, field plate 116 shields gate
110 and field plates 112, 114 from approximately 200 volts; field
plate 114 shields both gate 110 and field plate 112 from an
additional 200 volts and field plate 112 shields gate 110 from a
final 200 volts. A dielectric material 118, which can be silicon
oxide, silicon nitride, or other dielectric material, fills the
spaces between the conductive elements and insulates the various
elements. It will be understood that although dielectric material
118 is shown as a monolithic material, dielectric material 118 is
generally laid down in various layers as different metallization
layers are formed. It will further be understood that the specific
materials used to form the GaN device are not relevant to the
disclosed test structures and method of testing, but are given
solely as examples. Additionally, although three field plates are
shown in FIG. 1A, a GaN device can have either more or fewer field
plates, depending on the voltages used, the materials, and the
specific needs of the device.
[0024] As noted previously, one of the most common failures seen in
testing device 100A is time dependent dielectric breakdown. One of
the specifications that the Joint Electronic Device Engineering
Council (JEDEC) has provided for silicon is operation for 1000
hours at 150.degree. C. However, in testing the longevity of GaN
device 100A, examples of TDDB have occurred at 2000 hours, e.g., at
a corner of the gate 110 or field plate 112. Such failures
illustrate the need for a testing methodology for the GaN device
that can be performed quickly, e.g., in less than an hour, to
certify that all levels of the GaN device can provide a desired
lifetime, typically ten years. However, given the shielding action
of the field plates 112, 114, 116, testing the reliability of
device 100A as a whole will generally indicate only the reliability
of outer field plate 116 and will not test the reliability of the
underlying structures separately.
[0025] In order to test each element of transistor 100A, a first
suite or set of additional test structures is provided, as shown in
FIGS. 1B-1D. For example, because test structure 100B in FIG. 1B
does not include field plate 116, test structure 100B can be
utilized to test the reliability of field plate 114. Similarly,
test structure 100C in FIG. 1C, which does not include either of
field plates 114, 116, can be utilized to test the reliability of
field plate 112; and test structure 100D in FIG. 1D, which does not
include any field plates 112, 114, 116, can be utilized to test the
reliability of gate 110. A second set of test structures can also
be provided to test the inner field plates in an even more
simplified structure, as shown in FIGS. 1E-1F. For the example
transistor having three field plates 112, 114, 116, a further test
structure for each of the inner field plates includes only the
field plate under test, the source contact and the drain contact.
Test structure 100E in FIG. 1E includes only source contact 106,
drain contact 108 and field plate 114, with field plate 114 acting
as a gate. Test structure 100F in FIG. 1F includes only source
contact 106, drain contact 108 and field plate 112, with field
plate 112 acting as a gate. When these test structures are
subjected to high voltages, the remaining field plate acts as the
gate in the structure.
[0026] Once all of the test structures, e.g., test structures
100A-100F, are fabricated using desired process conditions, testing
can be performed. The testing is designed to accelerate failure of
the element being tested, either a field plate or gate. In one
embodiment, during the testing, source contact 106 is grounded and
the voltage on gate 110 is held below the threshold voltage
V.sub.TH to ensure that the transistor is turned off. A high drain
voltage is then applied to accelerate the electron-field between
gate 110 and drain contact 108. The specific voltage applied during
testing is dependent on the specific test structures and the
voltage acceleration factor. In one embodiment, the applied voltage
is three to six times the normal operating voltage to which the
structure-under-test is usually subjected. This bias condition is
held, while the gate-to-drain leakage is monitored. When the device
fails due to dielectric breakdown, the leakage current increases
abruptly; the time-to-failure is then noted as the time under
stress up to the point of failure. FIG. 2 discloses a graph that
plots the square root of the gate/drain voltage (V.sub.DG) against
a logarithmic scale of time in seconds. When plotted in this
manner, the mean time to failure, as determined at various
voltages, tends to respond linearly and can be projected to lower
voltages to provide an estimated lifetime at normal operating
voltages. As seen in graph 200, a first process condition can be
predicted to provide a lifetime of 1.times.10.sup.9 seconds under
normal conditions or a bit over thirty years. A second process
condition can be predicted to provide a lifetime greater than
1.times.10.sup.11 seconds, or over a hundred percent increase in
predicted lifetime. This test is, of course, only testing one level
of the device; however, if all levels test as having a lifetime at
or above the desired range, the device as a whole can be verified
for the desired lifetime.
[0027] The disclosed structures and process provide valuable
information to assist in proper design of a GaN device. However,
both the structures and the process can also be utilized to ensure
that processing of the wafers during production remain within
specification. In order to do so, the test structures can be
incorporated into the scribe structures adjacent the scribe lines
for the wafer. FIG. 3A depicts an overhead view of a semiconductor
wafer 300A that includes multiple dies 304 on which are formed a
GaN device having multiple field plates. In one embodiment, the GaN
device is a power transistor. The number of dies 304 can vary
greatly depending upon the device geometry, the size of the wafer
and other factors; the layout shown is only one possible example.
The dies 304 may be virtually any type of integrated circuits, such
as microprocessors, graphics processors, combined microprocessors
and graphics processors, memory devices, application specific
integrated circuits or virtually any other type of semiconductor
based circuits. If implemented as processors of one sort or
another, the dies 304 may be single or multicore. Four of the dies
304 are located in region 302, which is shown encircled by a dotted
line. FIG. 3B depicts an enlargement of region 302 and the four
included dies 304.
[0028] FIGS. 3A and 3B depict the semiconductor wafer 300A from the
circuitry side of the dies 304 and at a stage in processing just
prior to die singulation. Thus, the surfaces that are visible in
FIGS. 3A and 3B consist of inter-level dielectric materials and
scribe lines as described more fully below. The portions of the
wafer 300A that consist of semiconductor materials are generally
beneath those portions that are visible in FIGS. 3A and 3B. In
enlargement 300B, dies 304 are shown to be separated by dicing
alleys 306 that run between the separate dies 304; dies 304 will be
singulated from the wafer 300A generally along the dicing alleys
306. The singulation may be carried out by a variety of techniques,
such as sawing, laser cutting, combinations of these with or
without breaking or the like. Whether by saw, laser or the like,
the singulation process places great stresses on the perimeter of
the die.
[0029] Between dicing alleys 306, each die 304 includes an active
circuitry region 308 that is surrounded by a scribe structure 310.
The active circuitry region 308 performs the work for which the die
was manufactured and includes a number of input/output (I/O) pads
312 that may be bump pads, wire bond pads or other type of I/O pads
as desired. Beneath the I/O pads 312 but not visible in FIG. 3B, is
a series of stacked interconnect layers leading down to device
circuitry, such as the source contact, drain contact and gate of a
transistor. While only a few I/O pads 312 are depicted for
simplicity of illustration, the skilled artisan will appreciate
that there may be thousands of such pads. The I/O pads 312 are
surrounded laterally by inter-level dielectric material 314. The
scribe structure 310 is designed to act as both a crack stop and
also as a protective structure to protect the active circuitry
region 308 during subsequent singulation, as well as providing a
region where test structures can be incorporated for ease of
testing after a production run.
[0030] FIG. 3C depicts a cross-section of a scribe structure taken
along the line A-A' according to an embodiment of the disclosure.
Scribe structure 300C includes test structures TS.sub.0 through
TS.sub.3, such that a test structure is available for testing gate
320 and for testing each of field plates 322, 324, 326. Test
structure TS.sub.3 includes source contact 316, drain contact 318,
gate 320, and field plates 322-326. Isolation structures 330
separate the various test structures from each other to prevent
electrical interference between the test structures. In the
embodiment shown, isolation structures 330 are trench isolation,
although isolation implantation can also be used for isolation.
Test structure TS.sub.0 can be utilized to test gate 320; test
structure TS.sub.1 can be utilized to test field plate 322; test
structure TS.sub.2 can be utilized to test field plate 324; and
test structure TS.sub.3 can be utilized to test field plate 326.
Although not specifically illustrated in FIG. 3C, source contact
316, gate 320 and drain contact 318 of each of test structures
TS.sub.0, TS.sub.1, TS.sub.2, TS.sub.3 are coupled to a respective
I/O pad 312 as shown in FIG. 3B. Accordingly, appropriate voltages
can be applied to the I/O pads 312 that correspond to source
contact 316, gate 320 and drain contact 318 of each of test
structures TS.sub.0, TS.sub.1, TS.sub.2, TS.sub.3, TS2.sub.1,
TS2.sub.2.
[0031] Similarly, FIG. 3D depicts a cross-section of another scribe
structure taken along the line A-A' according to an embodiment of
the disclosure. Scribe structure 300D includes test structures
TS2.sub.1 through TS2.sub.2, such that a test structure is
available for testing each of inner field plates 322, 324. Test
structure TS2.sub.2 includes source contact 316, drain contact 318,
and field plate 324 and test structure TS2.sub.1 includes source
contact 316, drain contact 318, and field plate 322. Isolation
structures 330 separate the various test structures from each other
to prevent electrical interference between the test structures.
Test structure TS2.sub.1 can be utilized to test field plate 322
and test structure TS2.sub.2 can be utilized to test field plate
324. Using the method described below in FIG. 4, each of test
structures TS.sub.0, TS.sub.1, TS.sub.2, TS.sub.3, TS2.sub.1,
TS2.sub.2 can be tested at the wafer level prior to singulation
and/or after singulation.
[0032] FIGS. 4A-D depict an overall test method for testing a
gallium nitride (GaN) transistor or other semiconductor device that
comprises N field plates, N being an integer, X being an integer
between 0 and N inclusive, and Y being an integer between 1 and
(N-1) inclusive. Test method 400A includes providing (405) a set of
test structures TS.sub.0 through TS.sub.N. A test structure
TS.sub.X of the set of test structures includes a GaN substrate, a
dielectric material overlying the GaN substrate, a source contact
abutting the GaN substrate, a drain contact abutting the GaN
substrate, a gate overlying the substrate and lying between the
source contact and the drain contact, and X field plates
corresponding to X of the N field plates of the GaN transistor that
are nearest to the GaN substrate.
[0033] The test method includes setting (410) X to zero and setting
a stress voltage to a lowest test value. The stress voltage is
applied (415) to the drain contact of TS.sub.X until a dielectric
breakdown condition is detected, i.e., by determining that the
gate/drain leakage has abruptly increased. In at least one
embodiment, the stress voltage is applied while holding the source
contact at a lower rail and holding the gate voltage below the
threshold voltage. The time-to-failure of TS.sub.X at the stress
voltage is recorded (420). A determination is then made (425)
whether all test structures have been tested, i.e., whether X is
equal to N. If all of the test structures have not been tested,
then X is incremented (430) and the method returns to point A to
continue the testing by applying the stress voltage to the next
test structure. If all of the test structures have been tested and
the test structures were formed in the scribe structure of a
production chip, testing at a single voltage can indicate whether
any drift in the lifetime of the structure at the given voltage has
occurred. However, if the testing is being performed in order to
verify the design and extrapolate the lifetime of the device, the
testing continues at point B to test at other voltages. Testing of
multiple voltages on a production chip can be used to determine
whether there is drift in the voltage acceleration factor.
[0034] At point B in method 400B, a determination is made (435)
whether the time-to-failure has been determined at all stress
voltages. If this has not occurred, X is set (440) to zero again
and the stress voltage is set to a next test value, then the method
returns to point A to apply the new test voltage. It will be
understood that since the device is tested to failure, each new
test voltage will be applied to a new set of test structures. If
the time-to-failure has been determined for all test voltages, the
entire testing operation, i.e., operations 410 through 440 can be
repeated (445) a selected number of times in order to obtain
average values of the time-to-failure. The number of times the
entire operation is performed can be part of the design of the
testing protocol. Once all testing has been completed, the lifetime
of the device at normal operating conditions can be extrapolated
(450) from the test results. Additionally, the obtained test
results can also be used to determine (455) the time-to-failure
distributions, which can provide additional information regarding
reliability.
[0035] Test methods 400C and 400D perform the same operations on
the second set of test structures. Method 400C includes providing
(460) a set of test structures TS2.sub.1 through TS2.sub.(N-1). A
test structure TS2.sub.Y of the second set of test structures
includes a GaN substrate, a dielectric material overlying the GaN
substrate, a source contact abutting the GaN substrate, a drain
contact abutting the GaN substrate, and a Y.sup.th field plate
corresponding to the Y.sup.th of the N field plates of the GaN
transistor. As noted previously, the Y.sup.th field plate will act
as the gate for the TS2.sub.Y structure.
[0036] The test method includes setting (465) Y to one and setting
a stress voltage to a lowest test value. The stress voltage is
applied (470) to the drain contact of TS2.sub.Y until a dielectric
breakdown condition is detected. In at least one embodiment, the
stress voltage is applied while holding the source contact at a
lower rail and holding the gate voltage below the threshold
voltage. In one embodiment, the stress voltage is applied while
allowing the source contact to float or be disconnected. The
time-to-failure of TS2.sub.Y at the stress voltage is recorded
(475). A determination is then made (480) whether all test
structures have been tested, i.e., whether Y is equal to (N-1). If
all of the test structures have not been tested, then Y is
incremented (485) and the method returns to point C to continue the
testing by applying the stress voltage to the next test structure.
Otherwise, the testing continues at point D in method 400D to test
at other voltages.
[0037] At point D, a determination is made (490) whether the
time-to-failure has been determined at all stress voltages. If this
has not occurred, Y is set (492) to one again and the stress
voltage is set to a next test value, then the method returns to
point C to apply the new test voltage. Again, each new test voltage
will be applied to a new set of test structures. If the
time-to-failure has been determined for all test voltages, the
entire testing operation, i.e., operations 465 through 492 can be
repeated (494) a selected number of times in order to obtain
average values of the time-to-failure, which can be part of the
design of the testing protocol. Once all testing has been
completed, the lifetime of the specific field plates at normal
operating conditions can be extrapolated (4496) from the test
results. Additionally, the obtained test results can also be used
to determine (498) the time-to-failure distributions, which can
provide additional information regarding reliability.
[0038] Applicants have disclosed a set of test structures and a
method of utilizing the set of test structures to validate an
expected lifetime of a GaN device that uses a plurality of field
plates. Successive test structures in the set of test structures
remove one or more of the outermost field plates in order to test
the remaining structures. The disclosed structures and methods can
be utilized both in testing and validating a new design and in
monitoring a production process to ensure that the quality of the
processing and the GaN device remains at the design level.
[0039] Although various embodiments have been shown and described
in detail, the claims are not limited to any particular embodiment
or example. None of the above Detailed Description should be read
as implying that any particular component, element, step, act, or
function is essential such that it must be included in the scope of
the claims. Reference to an element in the singular is not intended
to mean "one and only one" unless explicitly so stated, but rather
"one or more." All structural and functional equivalents to the
elements of the above-described embodiments that are known to those
of ordinary skill in the art are expressly incorporated herein by
reference and are intended to be encompassed by the present claims.
Accordingly, those skilled in the art will recognize that the
exemplary embodiments described herein can be practiced with
various modifications and alterations within the spirit and scope
of the claims appended below.
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