U.S. patent application number 15/945803 was filed with the patent office on 2019-05-23 for diamond air bridge for thermal management of high power devices.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Travis J. Anderson, Tatyana I. Feygelson, Karl D. Hobart, Andrew D. Koehler, Francis J. Kub, Lunet E. Luna, Marko J. Tadjer.
Application Number | 20190157181 15/945803 |
Document ID | / |
Family ID | 63792779 |
Filed Date | 2019-05-23 |
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United States Patent
Application |
20190157181 |
Kind Code |
A1 |
Hobart; Karl D. ; et
al. |
May 23, 2019 |
Diamond Air Bridge for Thermal Management of High Power Devices
Abstract
A device structure and method for improving thermal management
in highly scaled, high power electronic and optoelectronic devices
such as GaN FET and AlGaN/GaN HEMT devices by implementing diamond
air bridges into such devices to remove waste heat. The diamond air
bridge can be formed from a polycrystalline diamond material layer
which can be grown on the surface of a dielectric material layer,
on the surface of a III-nitride material, or on the surface of a
diamond polycrystalline nucleation layer, and may be optimized to
have a high thermal conductivity at the growth interface with the
underlying material.
Inventors: |
Hobart; Karl D.;
(Alexandria, VA) ; Koehler; Andrew D.;
(Alexandria, VA) ; Kub; Francis J.; (Arnold,
MD) ; Anderson; Travis J.; (Alexandria, VA) ;
Feygelson; Tatyana I.; (Springfield, VA) ; Tadjer;
Marko J.; (Springfield, VA) ; Luna; Lunet E.;
(Wheaton, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Arlington
VA
|
Family ID: |
63792779 |
Appl. No.: |
15/945803 |
Filed: |
April 5, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62483745 |
Apr 10, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/42356 20130101;
H01L 29/42376 20130101; H01L 29/4232 20130101; H01L 29/1602
20130101; H01L 29/2003 20130101; H01L 29/7787 20130101; H01L
29/7786 20130101; H01L 29/452 20130101; H01L 29/205 20130101; H01L
23/3732 20130101; H01L 21/4803 20130101; H01L 29/475 20130101; H01L
29/66462 20130101; H01L 23/3171 20130101; H01L 29/45 20130101; H01L
29/0649 20130101; H01L 29/0847 20130101; H01L 23/3192 20130101 |
International
Class: |
H01L 23/373 20060101
H01L023/373; H01L 29/20 20060101 H01L029/20; H01L 29/205 20060101
H01L029/205; H01L 29/45 20060101 H01L029/45; H01L 29/47 20060101
H01L029/47; H01L 29/778 20060101 H01L029/778; H01L 29/66 20060101
H01L029/66; H01L 21/48 20060101 H01L021/48 |
Claims
1. An electronic device comprising: a buffer layer disposed on a
substrate; a barrier layer epitaxially grown on an upper surface of
the buffer layer; a source, a gate, and a drain contact disposed on
an upper surface of the barrier layer; a polycrystalline diamond
material layer extending between the source, gate, and drain;
wherein the diamond material layer is separated from the upper
surface of the barrier layer by an air gap so as to form a
suspended diamond air bridge between the source and the gate and
between the gate and the drain.
2. The electronic device according to claim 1, wherein the diamond
material layer extends along an upper surface of the source, gate,
and drain and is patterned to form openings for electronic contact
to the source, gate, and drain.
3. The electronic device according to claim 1, wherein the diamond
material layer is extends along an upper surface of the source and
drain and is patterned to form openings for electronic contact to
the source and the drain, the diamond material layer further being
patterned to form an opening for the gate, the gate protruding
above and extending laterally along an upper surface of the diamond
material layer.
4. The electronic device according to claim 1, wherein the diamond
material layer is a nanocrystalline diamond material layer.
5. The electronic device according to claim 1, further comprising a
passivation layer disposed on the upper surface of the barrier
layer between the source and the gate and between the gate and the
drain.
6. The electronic device according to claim 5, further comprising a
dielectric layer disposed on an upper surface of the passivation
layer, along the sides of the gate, and between the gate and the
barrier layer.
7. The electronic device according to claim 1, further comprising a
dielectric layer disposed on an upper surface of the diamond
material layer.
8. The electronic device according to claim 1, wherein a portion of
the gate electrode extends through the diamond material layer to
the upper surface of the barrier layer, the device further
comprising a dielectric layer disposed on an upper surface of the
diamond material layer and along the sides of the gate so as to
encapsulate the portion of the gate extending though the diamond
material layer to the upper surface of the barrier layer.
9. The electronic device according to claim 1, further comprising a
first dielectric layer disposed on an upper surface of the barrier
layer, on a lower surface of the diamond material layer, and on
sides of the source, gate, and drain electrodes so as to
encapsulate the air cap between the diamond material layer and the
barrier layer.
10. The electronic device according to claim 9, further comprising
a second dielectric material layer disposed on an upper surface of
the diamond material layer and along sides and a top of the gate;
and further comprising a passivation layer deposited on an upper
surface of the second diamond material layer.
11. The electronic device according to claim 1, wherein the
electronic device comprises an AlGaN/GaN high-electron mobility
transistor having a GaN buffer layer and an AlGaN barrier
layer.
12. The electronic device according to claim 1, wherein the source
and drain electrodes comprise titanium, aluminum, platinum, nickel,
gold or layered combinations thereof.
13. The electronic device according to claim 1, wherein the source
and drain comprise an n+ epitaxial layer having an ohmic layer
deposited thereon.
14. The electronic device according to claim 1, wherein the gate
comprises a metal nitride material layer, a refractory metal layer,
an alloy metal layer, or a copper layer in contact with the surface
of the III-nitride material.
Description
CROSS-REFERENCE
[0001] This Application is a Nonprovisional of and claims the
benefit of priority under 35 U.S.C. .sctn. 119 based on U.S.
Provisional Patent Application No. 62/483,745 filed on Apr. 10,
2017. The Provisional Application and all references cited herein
are hereby incorporated by reference into the present disclosure in
their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to High Electron Mobility
Transistors (HEMTs), particularly to HEMTs having a diamond layer
integrated therein to provide thermal management of the device.
BACKGROUND
[0003] The parasitic capacitance between the gate and drain and
between the gate and source can significantly affect the efficiency
of GaN-based High Electron Mobility Transistors (HEMT), especially
at high frequencies. A key determinant of the capacitance of the
gate to the drain, source and body is the thickness and dielectric
constant of passivation materials in the gate to drain and gate to
source regions. A passivation material layer with a lower
dielectric constant will have lower gate to drain and gate to
source capacitances.
[0004] The past several years have seen remarkable improvement in
GaN-based High Electron Mobility Transistor (HEMT) technology. Much
of this is due to improvements in material growth, device design
and device fabrication. Despite significant improvements in power
added efficiencies, it is becoming increasingly clear that GaN
HEMTs designed for RF, microwave, millimeter wave, and power
switching applications are severely limited by the ability to
dissipate heat and thus must run at significantly reduced power
levels, pulse length and duty cycle.
[0005] Recent thermal simulations indicate that the substrate is
not the primary source of the thermal impedance, rather it is the
ability of the III-nitride semiconductor material layers to locally
spread the heat to the surrounding material and substrate due to
the extraordinarily high power dissipation density in the
near-channel device region (estimated at many megawatts) and the
strong reduction in thermal conductivity with increasing
temperature. Integrating the capability to locally spread the
thermal power dissipated near the channel will have a large impact
on overall device performance and allow significant total power
output improvements.
[0006] Past methods to implement thermal management by depositing
diamond on electronic devices resulted in additional adverse
capacitance. For example, as described in U.S. Pat. No. 8,039,301
to Kub et al., "Gate After Diamond Transistor," the insulating
diamond is positioned between the gate and the drain of a AlGaN/GaN
High Electron Mobility Transistor (herein called a HEMT) to extract
heat from the high power density area located adjacent to the gate.
The diamond material has a dielectric constant of approximately 6
which, while low, results in an additional capacitive coupling
between the gate and the drain resulting a loss of gain at high
frequencies. See also U.S. Pat. No. 9,159,641 to Hobart et al.,
"Nanocrystalline diamond three-dimensional films in patterned
semiconductor substrates"; U.S. Pat. No. 9,246,305 to Kub et al.,
"Light-emitting devices with integrated diamond"; U.S. Pat. No.
9,305,858 to Hobart et al., "Nanocrystalline diamond
three-dimensional films in patterned semiconductor substrates";
U.S. Pat. No. 9,331,163 to Koehler et al., "Transistor with Diamond
Gate"; and U.S. Pat. No. 9,466,684 to Koehler et al., "Transistor
with Diamond Gate."
[0007] This problem becomes significantly more acute for very high
frequency devices (transistors) because the gate-to-drain spacing
is scaled down to achieve high transit time. However,
simultaneously, the gate-to-drain capacitive coupling increases.
Inserting a high dielectric constant material between the gate and
drain in these highly scaled devices only increases the capacitive
coupling resulting in loss of gain at high frequencies.
SUMMARY
[0008] This summary is intended to introduce, in simplified form, a
selection of concepts that are further described in the Detailed
Description. This summary is not intended to identify key or
essential features of the claimed subject matter, nor is it
intended to be used as an aid in determining the scope of the
claimed subject matter. Instead, it is merely presented as a brief
overview of the subject matter described and claimed herein.
[0009] The present invention provides a device structure and method
for improving thermal management in highly scaled, high power
electronic and optoelectronic devices such as GaN Field Effect
Transistor (FET) and more specifically, AlGaN/GaN HEMT (High
Electron Mobility Transistor) devices, by implementing diamond air
bridges into such devices to remove waste heat.
[0010] In accordance with the present invention, a diamond material
layer is deposited on a sacrificial layer in a semiconductor
heterostructure. The sacrificial layer is then removed, leaving the
diamond material layer suspended as a diamond air bridge.
[0011] In many embodiments, the diamond material layer will be a
nanocrystalline diamond material layer. The diamond material layer
can be grown on the surface of a dielectric material layer, on the
surface of a III-nitride material, or on the surface of a diamond
nanocrystalline nucleation layer that is formed on the surface of
the dielectric material layer or the III-nitride material surface,
and may be optimized to have a high thermal conductivity at the
growth interface with the underlying material.
[0012] In some embodiments, the diamond material layer is formed
before the gate electrode is formed, while in other embodiments,
the diamond material layer is formed afterwards.
[0013] In some embodiments, a passivation layer is deposited on the
surface of the III-nitride material layer and the sacrificial layer
is deposited on the passivation layer.
[0014] In some embodiments, a dielectric material layer is
deposited on the passivation layer.
[0015] In some embodiments, a dielectric material layer is
deposited on the diamond layer.
[0016] In some embodiments, a passivation layer is deposited on the
dielectric material layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1A-1F are block schematics illustrating aspects of an
exemplary embodiment of a HEMT having a diamond air bridge in
accordance with the present disclosure.
[0018] FIGS. 2A-2H are block schematics illustrating aspects of
another exemplary embodiment of a HEMT having a diamond air bridge
in accordance with the present disclosure.
[0019] FIGS. 3A and 3B are block schematics illustrating additional
aspects of the HEMTs depicted in FIGS. 1A-1F and FIGS. 2A-2H.
[0020] FIGS. 4A-4I are block schematics illustrating aspects of
another exemplary embodiment of a HEMT having a diamond air bridge
in accordance with the present disclosure.
[0021] FIGS. 5A-5F are block schematics illustrating aspects of
another exemplary embodiment of a HEMT having a diamond air bridge
in accordance with the present disclosure.
[0022] FIGS. 6A-6H are block schematics illustrating aspects of
another exemplary embodiment of a HEMT having a diamond air bridge
in accordance with the present disclosure.
[0023] FIGS. 7A-7G are block schematics illustrating aspects of
another exemplary embodiment of a HEMT having a diamond air bridge
in accordance with the present disclosure.
[0024] FIGS. 8A-8D are block schematics illustrating aspects of
another exemplary embodiment of a HEMT having a diamond air bridge
in accordance with the present disclosure.
[0025] FIGS. 9A and 9B are SEM images showing aspects of a
reduction to practice of the method of forming a diamond air bridge
in accordance with the present invention.
DETAILED DESCRIPTION
[0026] The aspects and features of the present invention summarized
above can be embodied in various forms. The following description
shows, by way of illustration, combinations and configurations in
which the aspects and features can be put into practice. It is
understood that the described aspects, features, and/or embodiments
are merely examples, and that one skilled in the art may utilize
other aspects, features, and/or embodiments or make structural and
functional modifications without departing from the scope of the
present disclosure.
[0027] The present invention provides a device structure and method
for improving thermal management in highly scaled, high power
electronic and optoelectronic devices such as GaN Field Effect
Transistor (FET) and more specifically, AlGaN/GaN HEMT (High
Electron Mobility Transistor) devices, by implementing diamond air
bridges into such devices to remove waste heat.
[0028] In many embodiments, the diamond material layer will be a
nanocrystalline diamond material layer. In some embodiments, the
diamond material layer may have a thermal conductivity higher than
about 300 W/mK, while in other embodiments, the diamond material
layer may have a thermal conductivity higher than about 800 W/mK,
higher than about 1200 W/mK, higher than about 1600 W/mK, higher
than about 2000 W/mK, or higher than about 2300 W/mK.
[0029] Diamond has a dielectric constant of about 6, and so the
diamond material layer not only has a high thermal conductivity, it
may also be electrically insulating.
[0030] The diamond air bridge concept in accordance with the
present invention significantly reduces capacitive coupling between
the drain and the gate by suspending the diamond above the
transistor. This results in low dielectric constant air (dielectric
constant of 1) between the gate and the drain.
[0031] The implementation of the diamond air bridge in a high
performance RF, microwave, or power switching transistor structures
has many variations but primarily involves the deposition and
patterning of a sacrificial layer (often herein referred to as
"SL") prior to the deposition of the diamond film. The diamond
material layer can be grown on the surface of a dielectric material
layer, the it material surface, or on the surface of a diamond
nanocrystalline nucleation layer that is formed on the surface of
the first dielectric material layer or the III-nitride material
surface, and may be optimized to have a high thermal conductivity
at the growth interface with the underlying material.
[0032] This basic SL and diamond air bridge formation process can
be implemented at any one of several points in any device
fabrication sequence.
[0033] In some embodiments, the diamond material layer is formed
before the gate electrode is formed, while in other embodiments,
the diamond material layer is formed afterwards. In some
embodiments, an optional diamond material layer is also formed
after formation of the gate electrode.
[0034] In some embodiments, a dielectric material layer can be
placed between the gate metal electrode and the AlGaN surface of an
AlGaN/GaN EMT. Such a dielectric layer should be optimized for the
capability to have a low surface state density, low hot electron
carrier trapping, high dielectric field strength, low transistor
current collapse characteristics, low transistor dispersion
characteristics. The dielectric layer should be sufficiently thin
so that the thermal impedance from the semiconductor surface to the
diamond heat-spreading layer is low.
[0035] The dielectric layer can be a composite dielectric layer
consisting of one or more dielectric layers on the AlGaN or GaN
surface grown using either in-situ or ex-situ growth
techniques.
[0036] Whether in-situ or ex-situ, the dielectric layers will
typically be from the group of silicon nitride, silicon oxide,
aluminum nitride, or metal oxide with typical metal oxide layers
being such thin film layers as aluminum oxide, hafnium oxide,
lanthanium oxide, gadolinium oxide, or strontium oxide, or a
composite dielectric consisting of combination of silicon nitride,
silicon oxide and/or metal oxide. In addition, it will often be
desirable that the dielectric be deposited using plasma techniques
that have low ion energies so as to not damage the III-nitride
surface to facilitate low current collapse and low dispersion in
AlGaN/GaN HEMTs.
[0037] In-situ grown dielectric layers will typically be grown by a
chemical vapor deposition (CVD), metal organic chemical vapor
deposition (MOCVD), molecular beam Epitaxy (MBE), atomic layer
deposition, atomic layer epitaxy, or plasma deposition
technique.
[0038] Ex-situ dielectric layer deposition approaches include all
of the commonly known thin film growth techniques but can also
include new growth techniques such as atomic layer deposition,
atomic layer epitaxy, plasma enhanced chemical vapor deposition,
and chemical vapor deposition. An optional anneal at approximately
600.degree. C. can be performed to increase the dielectric strength
of the ex-situ deposited dielectric layer.
[0039] An example of a candidate first dielectric layer would be a
1 nm thick in-situ grown silicon nitride layer followed by the
deposition of an ex-situ 3 nm thick metal oxide layer or aluminum
nitride layer formed by an Atomic Layer Deposition (ALD) process.
In the case that an in-situ silicon nitride layer is not used, it
can be desirable to expose the AlGaN or GaN surface to a nitrogen,
ammonia, or plasma nitrogen step to react nitrogen with the AlGaN
or GaN surface to minimize nitrogen vacancies at the AlGaN or GaN
surface.
[0040] The dielectric layer can also act as an insulating gate
dielectric layer beneath a metal gate. In addition, it can protect
the GaN or AlGaN surface from damage during diamond deposition
process or from chemical decomposition during the growth of the
diamond film. The growth environment for diamond typically includes
a CH/H.sub.2 gas mixture; these gases can decompose the GaN surface
at typical diamond growth temperatures, and so a thin layer of
silicon nitride or other dielectric material can protect the GaN
surface during diamond growth.
[0041] In some embodiments, a diamond material layer may be
deposited on the surface of the first dielectric layer, where the
diamond layer may be in the form of a nanocrystalline diamond
nucleation layer. The diamond deposition process typically uses the
ultrasonic deposition of nanocrystalline diamond particles that act
as a nucleation layer for diamond growth. The nanocrystalline
diamond particles accelerated by ultrasonic energy can collide with
the GaN surface causing mechanical damage if the first dielectric
layer is not present.
[0042] Surface passivation of the device can be implemented before
or after the formation of the gate electrode but in all cases
passivation is necessary to provide device stability against
current collapse due to trapping of electrons near the surface.
Surface passivation materials include Si.sub.3N.sub.4 and AlN. The
best surface passivation of GaN is typically obtained for silicon
nitride layer deposited directly on GaN or AlGaN or AlN
surface.
[0043] Various embodiments of a device structure incorporating a
diamond air bridge and methods for fabricating the same will now be
described in the context of the FIGURES submitted with and forming
a part of the present disclosure. It will be noted that in the
FIGURES and in the description below, like structural elements are
numbered alike, with only the first numeral being changed to
reflect the FIGURE in which the element is depicted. For example,
the diamond layer forming the diamond air bridge is referred to as
element 108 in the discussion of the embodiment illustrated in
FIGS. 1A-1F, element 208 in the discussion of the embodiment
illustrated in FIGS. 2A-2H, element 308 in the discussion of the
embodiment illustrated in FIGS. 4A-4H, etc. In addition, for the
sake of brevity, elements introduced and described with respect to
one FIGURE are not described in detail when they appear in
subsequent FIGURES unless such description is needed for clarity.
Moreover, although the devices and fabrication methods thereof are
described below in the context of HEMT devices, particularly
AlGaN/GaN HEMT devices, one skilled in the art will readily
understand that the features and fabrication methods described
herein may be easily modified to apply to other electronic devices,
and application of the principles and aspects of diamond bridge
formation to all such additional devices and fabrication methods is
deemed to be within the scope of the present disclosure.
[0044] FIGS. 1A-1F illustrate aspects of a first embodiment of a
semiconductor device structure having a diamond air bridge
incorporated therein and a method for fabricating the same, where
the device is an AlGaN/GaN HEMT in which the gate is formed prior
to the diamond air bridge fabrication process.
[0045] The fabrication process starts at Step 1 shown in FIG. 1A,
where an AlGaN/GaN HEMT comprising GaN buffer layer 101 and AlGaN
barrier layer 102 is formed on a substrate (not shown). In the
embodiment illustrated in FIGS. 1A-1F, passivation layer 106 is
formed on an upper surface of AlGaN barrier layer 102, with source,
gate, and drain electrodes 103, 104, and 105 being formed in
patterned openings in the passivation layer. The source and drain
electrodes can be formed from any suitable material such as
titanium, aluminum, platinum, nickel, gold or layered combinations
of those metals, while the gate electrode can comprise a metal
nitride material layer, a refractory metal layer, an alloy metal
layer, nickel, gold or a copper layer in contact with the surface
of the III-nitride material in the case of AlGaN/GaN HEMTs.
[0046] In Step 2 shown in FIG. 1B, a sacrificial layer (often
referred to as simply "SL" as shown in the FIGS. 107 is deposited
on the passivation layer and source, gate, and drain electrodes. SL
106 can comprise any suitable material that can be wet or dry
etched with high selectivity to the other materials. Candidate
materials for the sacrificial layer are polycrystalline Si and
amorphous SiO.sub.2. SIn Step 3 shown in FIG. 3, SL 107 is
selectively etched so that the source, gate, and drain materials
remain unaffected by the etching, with the result that SL 107
extends only between the source, gate, and drain electrodes, while
upper surfaces of the source, gate, and drain are exposed. Such
selective etching of SL 107 can be accomplished, e.g., using
XeF.sub.2 (for a polycrystalline sacrificial layer) or by vapor or
anhydrous HF (for an amorphous SiO.sub.2 sacrificial layer)
[0047] At Step 4 shown in FIG. 1D, diamond film 108 is deposited on
the upper surface of the sacrificial layer and on the exposed upper
surfaces of the source, gate, and drain. Diamond film 108 can be in
the form of ultra-nanocrystalline diamond, nanocrystalline diamond,
and polycrystalline diamond; can be grown by microwave assisted
chemical vapor deposition or hot filament chemical vapor deposition
from precursors including methane CH.sub.4), hydrogen, nitrogen,
argon, and diborane for p-type doping; and can be deposited at
temperatures between 300 C and 900 C. Diamond films. In some cases,
a diamond seed layer can be deposited prior to deposition of
diamond film 108 to initiate growth of the diamond material.
[0048] At Step 5 shown in FIG. 1E, diamond layer 108 is patterned,
e.g., by etching, to expose portions of the source, gate, and drain
electrodes so that electrical contact can be made to the
device.
[0049] Finally, at Step 6 shown in FIG. 1F, SL 107 is removed,
e.g., by etching, with diamond layer 108 remaining suspended above
the passivation layer so as to form a diamond air bridge between
the source and the gate and between the gate and the drain in the
HEMT.
[0050] The block schematics in FIGS. 2A-2H depict aspects of
another embodiment for fabricating a semiconductor device with a
diamond air bridge in accordance with the present invention. In
this embodiment, the same basic processing steps described above
with respect to FIGS. 1A-1F are followed, except that the gate is
formed after the sacrificial layer and the diamond layer are
deposited but prior to the removal of the sacrificial layer. This
implementation is useful when the gate electrode cannot tolerate
the conditions present during the sacrificial layer or diamond
deposition processes.
[0051] Fabrication of such a device starts with the formation of a
basic device structure in Step 1, where the structure comprises an
AlGaN/GaN HEMT having GaN buffer layer 201 and AlGaN barrier layer
202 and source and drain electrodes 204 and 205 formed on an upper
surface of the AlGaN barrier layer. In addition, in this
embodiment, passivation layer 206 is formed on an upper surface of
the AlGaN barrier layer between the source and the drain. At Step 2
shown in FIG. 2B, SL 207 is formed on the passivation layer, and at
Step 3, shown in FIG. 2C, diamond layer 208 is formed on an upper
surface of the sacrificial layer, with diamond layer 208 extending
laterally to cover the upper surface of the source and drain
structures as well.
[0052] At Steps 4, 5, and 6 (FIGS. 2D, 2E, and 2F), the diamond
layer, sacrificial layer, and passivation layers are patterned to
form an opening for the formation of gate 204 in Step 7, which
extends down to and contacts the AlGaN layer and extends laterally
over the upper surface of the diamond layer, as shown in FIG. 2G.
Finally, at Step 8 shown in FIG. 2H, the sacrificial layer is
removed, e.g., by etching, with diamond layer 208 remaining
suspended above the passivation layer so as to form a diamond air
bridge between the source and the gate and between the gate and the
drain in the HEMT.
[0053] In both cases, either the "gate first" process or the "gate
last" process, in some embodiments, a gate insulator dielectric
layer can be deposited prior to the formation of the gate as shown
in FIG. 3A ("gate first"' or FIG. 3B ("gate last") to insulate the
gate and reduce current leakage in the device. Any suitable
insulating dielectric can be used, with Al.sub.2O.sub.3, ZrO.sub.2,
SiO.sub.2, Si.sub.3N.sub.4, and HfO.sub.2 being particularly
suitable.
[0054] In another embodiment of a HEMT device having a diamond air
bridge incorporated therein, the passivation, SL, and diamond
layers are deposited first, followed by formation of the ohmic
contact (source and drain) and gate electrodes.
[0055] The block schematics in FIGS. 4A-4H illustrate aspects of
such a device and the fabrication process thereof.
[0056] As shown in FIG. 4A, in Step 1, a basic HEMT structure is
formed, comprising GaN buffer layer 401 and AlGaN barrier layer
402, with passivation layer 406 being formed on an upper surface of
the AlGaN layer. At Step 2 shown in FIG. 4B, SL 407 is formed on an
upper surface of the passivation layer and in Step 3 shown in FIG.
4C, diamond layer 408 is deposited on an upper surface of the
sacrificial layer.
[0057] In Steps 4, 5, and 6 shown in FIGS. 4D, 4E, and 4F, the
diamond layer, sacrificial layer, and passivation layer are etched
to provide openings for the formation of the source and drain
contacts 403/405 in Step 7 as shown in FIG. 4G and for formation of
gate electrode 404 in Step 8 shown in FIG. 4H. Finally, in Step 9
shown in FIG. 4I, the sacrificial layer is removed, leaving
suspended diamond air bridge 408 remaining above the passivation
layer.
[0058] The block schematics shown in FIGS. 5A-5F illustrate a
process where further reduction of the gate-to-drain capacitance is
achieved by increasing the thickness of the SL to provide greater
separation of the diamond layer from both the gate and the drain.
Such a structure in accordance with this aspect of the present
invention reduces the capacitive coupling between the gate and the
drain, which improves device performance and efficiency.
[0059] Formation of a HEMT with a diamond air bridge in accordance
with this embodiment of the present invention begins at Step 1
shown in FIG. 5A, where, as described above, an AlGaN/GaN HEMT
comprising GaN buffer layer 501 and AlGaN barrier layer 502 is
formed, with source, gate, and drain 503, 504, and 505 and
passivation layer 506 formed on an upper surface of the AlGaN
layer, where passivation layer 506 extends between the source and
the gate and between the gate and the drain.
[0060] At Steps 2 and 3 shown in FIGS. 5B and 5C, SL 507 is
deposited on an upper surface of the structure. In this embodiment,
the sacrificial layer is deposited so as to cover the passivation
layer between the source and the gate and between the gate and the
drain, and further so as to cover the upper surfaces of the source,
gate, and drain, with all of the passivation layer, the source,
gate, and drain being covered with a thick sacrificial layer as
shown in FIG. 5C. At step 4 shown in FIG. 5D, SL 507 is etched to
provide exposed areas for electrical contact with the source, gate,
and drain, leaving the remaining thick sacrificial layer in the
unetched areas.
[0061] At Step 5 shown in FIG. 5E, diamond layer 508 is grown over
all of the exposed surfaces in the device, including on the upper
surfaces and sides of the source, gate, and drain and the upper
surfaces of the thick SL.
[0062] Finally, at Step 6 shown in FIG. 5F, the sacrificial layer
is etched away to leave the suspended diamond air bridge above the
passivation layer and portions of the source, gate, and drain, with
other portions of the source, gate, and drain having the diamond
layer in contact therewith to provide thermal management in those
layers. As noted above, because of the thickness of the SL, the
diamond air bridge in this embodiment of the present invention is
suspended at a greater distance above the structure, particularly
above the gate and the drain, which reduces the capacitive coupling
between the gate and the drain in the device.
[0063] In another embodiment of a HEMT having a diamond air bridge
in accordance with the present invention, aspects of which are
shown by the block schematics in FIGS. 6A-6F, high regrown n+
epitaxial contacts are used to implement the source and drain in
order to reduce the contact resistance.
[0064] Fabrication of such a device starts with Step 1 shown in
FIG. 6A, in which an n+ epitaxial structure ("n+ epi") 611 is
formed on an upper surface at each end of the AlGaN/GaN HEMT. In
some embodiments, the n+ epi can be formed as a blanket layer and
then be selectively etched away. In other embodiments, the surface
of the transistor can be protected with a dielectric layer, with
the n+ epi being selectively deposited in patterned openings in the
dielectric layer. In either case, ohmic contacts 612 can be formed
on the upper surface of each of the n+ epi structures. SL 607 is
then deposited on the upper surface of the AlGaN layer and along
the sides of the n+ epi and ohmic layers at the opposite ends of
the AlGaN layer.
[0065] In Step 2 shown in FIG. 6B, diamond layer 608 is deposited
on the upper surfaces of the ohmic layers and the sacrificial layer
so as to cover all exposed upper surfaces of the device. At Steps 3
and 4 shown in FIGS. 6C and 6D, the diamond layer (FIG. 6C) and the
sacrificial layer (FIG. 6D) are etched down to the AlGaN layer
surface to provide a trench for formation of the gate.
[0066] In step 5 shown in FIG. 6E, a dielectric layer 609 is
deposited on the upper surface of the diamond layer, on the diamond
and sacrificial layers along the sides of the trench, and on the
upper surface of the AlGaN layer at the bottom of the trench so as
to insulate gate 604 formed in Step 6 (FIG. 6F) from the AlGaN
layer.
[0067] In Step 7 shown in FIG. 6G, the sacrificial layer is
removed, leaving the diamond layer suspended between the ohmic
layers and the trench on either side of the gate. Finally, in Step
8 shown in FIG. 6H, the device is passivated by deposition of
passivation layer 606a on the upper surface of the dielectric layer
and on the sides and upper surface of the gate and of passivation
layer 606b on all exposed surfaces surrounding the air pockets
produced by the removal of the SL. The passivation layer is needed
to passivate the surface and specifically surface traps. The
passivation layer can be deposited by any suitable means, such as
atomic layer deposition (ALD) and can cover some or all of the
surfaces in the device as appropriate. Thus, in the exemplary
embodiment illustrated in FIG. 6H, passivation layer 606b is
deposited by an ALD process which deposits the passivation layer on
all surfaces of the device. It should be noted that in other cases,
such as those illustrated in FIGS. 1A-1F, 2A-2H, 3A-3B, 4A-4I, and
5A-4F, the passivation layer can also be deposited by means of
PECVD.
[0068] In another embodiment, aspects of which are shown in FIGS.
7A-7G, the diamond encases the gate, thereby providing improved
thermal transport away from the gate.
[0069] The fabrication process of a device in accordance with this
embodiment of the present invention includes Step 1 shown in FIG.
7A, wherein SL 707 is deposited on an AlGaN/GaN HEMT having an n+
epi layer 710 with ohmic layer. As shown in FIG. 7A, in this
embodiment SL 707 is deposited at the ends of the AlGaN layer and
on the sides of the epi and ohmic layers, leaving a gap in the
middle where the AlGaN layer is exposed.
[0070] In Step 2 shown in FIG. 7B, diamond layer 708 is then
deposited so as to cover the top of the ohmic contacts, the top and
side surfaces of the sacrificial layer, and the exposed area of the
AlGaN surface, with the diamond forming a trench along the sides of
the sacrificial layer and the AlGaN surface, and at Step 3 shown in
FIG. 7C, the diamond is removed from the bottom of this trench. In
Step 4 shown in FIG. 7D, dielectric layer 709 is deposited on top
of the diamond layer and along the sides and bottom of the trench,
including on the upper surface of the exposed AlGaN layer. In Step
5 shown in FIG. 7E, gate 704 is formed in the trench so as to be
both within the trench and extending above the upper surface of the
dielectric layer, with the diamond and dielectric film encasing the
lower portion of the gate.
[0071] At Step 6 shown in FIG. 7F, the sacrificial layer is
removed, leaving the diamond on the sides of the trench
encapsulating the gate as well as being suspended over the AlGaN,
with the encapsulated gate improving thermal coupling to the
hottest portion of the device. Finally, at Step 7 shown in FIG. 7G,
the device can be passivated by deposition of passivation layer
706a on top of dielectric layer 709 and passivation layer 706b on
the inner surfaces of the diamond layer, AlGaN layer the n+ epi
layers and the ohmic layers exposed after removal of the
sacrificial layer around the encapsulated gate. This structure has
the highest thermal coupling and also a narrow gate foot for higher
frequency of operation.
[0072] FIGS. 8A-8D illustrate aspects of an additional embodiment
of an AlGaN/GaN having a suspended diamond air bridge incorporated
therein. Fabrication of such a device starts with Step 1 shown in
FIG. 8A which depicts an AlGaN/GaN HEMT with n+ epi and ohmic
contacts as described above with respect to FIG. 6A. In addition,
the device structure formed in this Step 1 includes a passivation
layer 806 formed on an upper surface of the AlGaN layer, the
passivation layer being patterned to form a trench therein;
dielectric layer 809 formed on an upper surface of the passivation
layer and along the sides and bottom of the trench; and gate 804
formed as a t-gate in the trench and on an upper surface of the
dielectric layer, where the gate is insulated from the AlGaN layer
by the dielectric layer in the trench.
[0073] In Step 2 shown in FIG. 8B, SL 807 is deposited on an upper
surface of the dielectric layer so as to extend along the sides of
the n+ epi layers on either end of the device and along the sides
of the gate. At Step 3 shown in FIG. 8C, diamond layer 808 is
formed on all exposed surfaces of the device, covering the exposed
upper surfaces and sides of the ohmic contacts, the n+ epi layer,
the sacrificial layer, and the gate to encapsulate the entire upper
surface of the device. Finally, at Step 4 shown in FIG. 8D, the
sacrificial layer is removed, leaving the diamond deposited over
the sacrificial layer suspended as an air bridge in the device.
[0074] The concept of a diamond air bridge was reduced to practice
as shown in FIGS. 9A and 9B. FIG. 9A shows a cross-sectional
scanning electron microscope (SEM) image of a patterned polysilicon
layer that has been coated with poly- or nano-crystalline diamond.
The sample was sectioned prior to imaging. FIG. 9B shows the same
sample after exposure to XeF.sub.2, where the polysilicon is
observed to have been etched leaving a suspended diamond air
bridge.
[0075] Alternatives
[0076] In some alternative embodiments, the metal gate electrode
can be replaced with diamond in any of the device structures
described above.
[0077] The diamond air bridge of the present invention is not
limited to implementation in an AlGaN/GaN HEMT. Other RF,
microwave, or power switching transistors that can benefit from
formation of a diamond air bridge in accordance with the present
invention can include AlGaAs/GaAs HEMTs, InP based heterojunction
bipolar transistors, Si/SiGe bipolar transistors, and laser
diodes.
[0078] In some embodiments, the diamond material layer may be
deposited at a temperature of about 400 C. In some embodiments, the
diamond material layer may be deposited at temperatures of about
750 C. In some embodiments, the diamond material layer may be
deposited at a temperature of about 1000 C.
[0079] In some embodiments, a high-temperature (>500.degree. C.)
diamond layer can deposited after formation of a Schottky gate,
where the Schottky gate is formed from, e.g., a metal nitride,
refractory metal, a metal alloy, titanium nitride, iridium, or
copper material layer in contact with the III-nitride material or a
first dielectric layer.
[0080] Advantages and New Features
[0081] The present invention provides a low capacitance, high
thermal conductivity diamond air bridge that will enable high
performance RF, microwave and power switching transistors. The
advantage of the new approach is that by elevating the diamond
thermal transport layer above the surface of the device, the
dielectric constant that is responsible for capacitive coupling
between the gate and the drain is reduced by a factor of
approximately 6.
[0082] The diamond thin film layer produced in accordance with the
present invention may improve lateral heat spreading locally in and
near the active region of power dense microwave and millimeter wave
devices. The diamond thin film layer will act to improve lateral
heat spreading locally in and near the active region of power dense
microwave and millimeter wave devices. The lateral heat spreading
will increase the area for vertical heat transfer to the substrate.
In addition, the lateral heat spreading can transfer the heat to
thermal shunts built into the substrate. Because of the lateral
heat spreading enable by the diamond thin film, there is reduced
peak channel temperature and improved transistor reliability.
[0083] The embodiments described in this disclosure device and
process provides for improved AlGaN/GaN HEMT pulse length and duty
cycle as well as increased power switching capability and total RF,
microwave and millimeter wave transistor output power and gain. The
embodiments in this proposal also provide for reliable high
temperature operation.
[0084] Silicon nitride has a dielectric constant of about 6.0 and
diamond has a dielectric constant of about 5.5. The low dielectric
constant of diamond material layers will reduce the gate-drain
capacitance and the gate-to-source capacitance.
[0085] Although particular embodiments, aspects, and features have
been described and illustrated, one skilled in the art would
readily appreciate that the invention described herein is not
limited to only those embodiments, aspects, and features but also
contemplates any and all modifications and alternative embodiments
that are within the spirit and scope of the underlying invention
described and claimed herein. The present application contemplates
any and all modifications within the spirit and scope of the
underlying invention described and claimed herein, and all such
modifications and alternative embodiments are deemed to be within
the scope and spirit of the present disclosure.
* * * * *