U.S. patent application number 15/407144 was filed with the patent office on 2017-06-29 for devices and methods related to a sputtered titanium tungsten layer formed over a copper interconnect stack structure.
The applicant listed for this patent is Skyworks Solutions, Inc.. Invention is credited to Kezia Cheng.
Application Number | 20170186694 15/407144 |
Document ID | / |
Family ID | 49001958 |
Filed Date | 2017-06-29 |
United States Patent
Application |
20170186694 |
Kind Code |
A1 |
Cheng; Kezia |
June 29, 2017 |
DEVICES AND METHODS RELATED TO A SPUTTERED TITANIUM TUNGSTEN LAYER
FORMED OVER A COPPER INTERCONNECT STACK STRUCTURE
Abstract
Disclosed are devices and methods related to metallization of
semiconductors. A metalized structure can include a stack disposed
over a compound semiconductor, with the stack including an ohmic
metal layer, a titanium/chromium layer, a metal nitride layer such
as a titanium nitride layer, and a copper/aluminum layer. The
titanium/chromium layer and metal nitride layer can act as a
barrier between the copper/aluminum layer and a substrate.
Inventors: |
Cheng; Kezia; (Lowell,
MA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Skyworks Solutions, Inc. |
Woburn |
MA |
US |
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|
Family ID: |
49001958 |
Appl. No.: |
15/407144 |
Filed: |
January 16, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15224993 |
Aug 1, 2016 |
9576906 |
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15407144 |
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13774988 |
Feb 22, 2013 |
9443803 |
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15224993 |
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61602886 |
Feb 24, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 23/53223 20130101;
H01L 23/485 20130101; H01L 2924/1305 20130101; H01L 2924/1305
20130101; H01L 29/41 20130101; H01L 2224/49171 20130101; H01L
21/76841 20130101; H01L 2924/181 20130101; H01L 2924/12042
20130101; H01L 2224/48463 20130101; H01L 24/48 20130101; H01L
21/76847 20130101; H01L 21/76843 20130101; H01L 2924/12041
20130101; H01L 21/76846 20130101; H01L 29/452 20130101; H01L 21/469
20130101; H01L 2924/13063 20130101; H01L 23/53238 20130101; H01L
2924/12042 20130101; H01L 2924/1306 20130101; H01L 23/5283
20130101; H01L 23/532 20130101; H01L 2224/04042 20130101; H01L
2924/181 20130101; H01L 27/0605 20130101; H01L 21/8252 20130101;
H01L 2224/4911 20130101; H01L 29/475 20130101; H01L 2924/1306
20130101; H01L 24/05 20130101; H01L 21/76895 20130101; H01L
21/76816 20130101; H01L 2924/13063 20130101; H01L 2924/00 20130101;
H01L 2924/00 20130101; H01L 2924/00 20130101; H01L 2924/00
20130101; H01L 2224/45099 20130101; H01L 2924/00 20130101; H01L
2924/00012 20130101; H01L 2223/6677 20130101; H01L 21/76832
20130101; H01L 2224/05666 20130101; H01L 23/66 20130101; H01L
2924/00014 20130101; H01L 23/4855 20130101; H01L 21/76852 20130101;
H01L 21/768 20130101; H01L 2924/00014 20130101; H01L 23/535
20130101; H01L 24/49 20130101; H01L 21/76829 20130101; H01L
2924/12041 20130101 |
International
Class: |
H01L 23/532 20060101
H01L023/532; H01L 21/768 20060101 H01L021/768 |
Claims
1. (canceled)
2. A metalized stack structure for a semiconductor device, the
stack comprising: an ohmic metal layer disposed over a substrate; a
first titanium or chromium layer disposed over the ohmic metal
layer and having a thickness of 1,000 angstroms or more; a first
metal nitride layer disposed over the first titanium or chromium
layer and having a thickness of 500 angstroms or more; and a copper
or aluminum layer disposed over the first metal nitride layer, the
first titanium or chromium layer and the first metal nitride layer
configured to act as a barrier between the copper or aluminum layer
and the substrate.
3. The metalized stack structure of claim 2 further including a
second metal nitride layer disposed over the copper or aluminum
layer.
4. The metalized stack structure of claim 3 further including a
platinum, palladium, or nickel layer disposed over the second metal
nitride layer.
5. The metalized stack structure of claim 4 further including a
gold layer disposed over the platinum, palladium, or nickel
layer.
6. The metalized stack structure of claim 5 further including a
second titanium or chromium layer disposed over the gold layer.
7. The metalized stack structure of claim 5 wherein the gold layer
has a thickness of 1,200 angstroms or more.
8. The metalized stack structure of claim 6 wherein the second
metal nitride layer has a thickness of 500 angstroms or more, the
platinum, palladium, or nickel layer has a thickness of 250
angstroms or more, and the second titanium or chromium layer has a
thickness of 90 angstroms or more.
9. The metalized stack structure of claim 2 wherein the metal
nitride layer is formed by use of an evaporator with an ion source
in conjunction with introduction of nitrogen (N.sub.2) gas during
an evaporation process.
10. The metalized stack structure of claim 2 wherein the first
metal nitride layer is a titanium nitride layer, the first titanium
or chromium layer is a titanium layer, and the copper or aluminum
layer is a copper layer.
11. The metalized stack structure of claim 2 wherein the ohmic
metal layer is an ohmic gold layer.
12. The metalized stack structure of claim 2 wherein the substrate
is gallium arsenide.
13. A semiconductor device comprising: a semiconductor device
formed on a substrate; and a metalized stack structure configured
for use with the semiconductor device and including an ohmic metal
layer disposed over the substrate, a first titanium or chromium
layer disposed over the ohmic metal layer and having a thickness of
1,000 angstroms or more, a first metal nitride layer disposed over
the first titanium or chromium layer and having a thickness of 500
angstroms or more, and a copper or aluminum layer disposed over the
first metal nitride layer, the first titanium or chromium layer and
the first metal nitride layer configured to act as a barrier
between the copper or aluminum layer and the substrate.
14. The semiconductor device of claim 13 further including a second
metal nitride layer disposed over the copper or aluminum layer, a
platinum, palladium, or nickel layer disposed over the second metal
nitride layer, a gold layer disposed over the platinum, palladium,
or nickel layer, and a second titanium or chromium layer disposed
over the gold layer.
15. The semiconductor device of claim 14 wherein the first and
second titanium or chromium layers are titanium layers, the first
and second metal nitride layers are titanium nitride layers, the
copper or aluminum layer is a copper layer, and the platinum,
palladium, or nickel layer is a platinum layer.
16. The semiconductor device of claim 13 wherein the semiconductor
device is an N-ohmic or P-ohmic semiconductor device.
17. A method of forming a semiconductor device, the method
comprising: forming an ohmic metal layer disposed over a substrate;
forming a first titanium or chromium layer disposed over the ohmic
metal layer and having a thickness of 1,000 angstroms or more;
forming a first metal nitride layer disposed over the first
titanium or chromium layer and having a thickness of 500 angstroms
or more; and forming a copper or aluminum layer disposed over the
first metal nitride layer, the first titanium or chromium layer and
the first metal nitride layer configured to act as a barrier
between the copper or aluminum layer and the substrate.
18. The method of claim 17 wherein the first metal nitride layer is
a titanium nitride layer and forming the titanium nitride layer
includes evaporating titanium with an assist from an ion source and
introducing nitrogen gas during the evaporating.
19. The method of claim 17 further including forming a second metal
nitride layer disposed over the copper or aluminum layer, forming a
platinum, palladium, or nickel layer disposed over the second metal
nitride layer, forming a gold layer disposed over the platinum,
palladium, or nickel layer, and forming a second titanium or
chromium layer disposed over the gold layer.
20. The method of claim 19 wherein the first and second titanium or
chromium layers are titanium layers, the first and second metal
nitride layers are titanium nitride layers, the copper or aluminum
layer is a copper layer, and the platinum, palladium, or nickel
layer is a platinum layer.
21. The method of claim 17 wherein the semiconductor device is an
N-ohmic or P-ohmic semiconductor device.
Description
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS
[0001] Any and all applications for which a foreign or domestic
priority claim is identified in the Application Data Sheet as filed
with the present application are hereby incorporated by reference
under 37 CFR 1.57.
BACKGROUND
[0002] Field
[0003] The present disclosure generally relates to devices and
methods related to copper interconnects for compound
semiconductors.
[0004] Description of the Related Art
[0005] Compound semiconductor devices use metals such as gold to
form interconnecting lines such as bus lines. These interconnects
are commonly referred to as Metal 1, Metal 2 and Metal 3. Metal 1
or M1 typically refers to a first layer of metal interconnects that
contacts, for example, ohmic metals and other active and/or passive
components (e.g., resistors, capacitors and inductors) associated
with integrated circuits. Metal 2 or M2 typically refers to a layer
that connects to the M1. Metal 3 or M3 typically refers to, for
example, a heat spreader in devices such as power amplifiers.
[0006] Gold is selected for some or all of the foregoing
applications due to its properties such as low electrical
resistivity, chemical inertness and desirable wire bonding
properties. However, the price of gold is relatively high, thereby
increasing costs associated with fabrication of compound
semiconductor devices.
SUMMARY
[0007] In some implementations, the present disclosure relates to a
metalized structure for a compound semiconductor device. The
structure includes a first titanium (Ti) layer disposed over a
substrate associated with the compound semiconductor device. The
structure further includes a first barrier layer disposed over the
first Ti layer. The structure further includes a second Ti layer
disposed over the first barrier layer. The structure further
includes a copper (Cu) layer disposed over the second Ti layer,
with the second Ti layer being configured to inhibit alloying of
the Cu layer and the barrier layer.
[0008] In some embodiments, the first Ti layer, the first barrier
layer, and the second Ti layer can be configured to yield a barrier
between the Cu layer and an ohmic metal layer formed on the
substrate. In some embodiments, the first barrier layer can include
platinum (Pt), palladium (Pd), or nickel (Ni).
[0009] In some embodiments, the structure can further include a
third Ti layer disposed over the Cu layer and a second barrier
layer disposed over the third Ti layer. The second barrier layer
can include platinum (Pt), palladium (Pd), or nickel (Ni). In some
embodiments, first barrier layer can be in direct contact with the
first Ti layer, the second Ti layer can be in direct contact with
the first barrier layer, the Cu layer can be in direct contact with
the second Ti layer, the third Ti layer can be in direct contact
with the Cu layer, and the second barrier layer can be in direct
contact with the third Ti layer. In some embodiments, thicknesses
of the first barrier layer and the second Ti layer can be selected
to provide sufficient barrier functionality between the Cu layer
and an ohmic metal layer disposed between the first Ti layer and
the substrate. The first Ti layer can have a thickness sufficient
to function as an adhesion layer. As examples, the first Ti layer
can have a thickness of approximately 1,000 angstroms, the first Pt
layer can have a thickness of approximately 500 angstroms, and the
second Ti layer can have a thickness of approximately 1,000
angstroms.
[0010] In some embodiments, the Cu layer can have a thickness
selected to yield a resistivity value similar to that of a gold
layer being replaced by the Cu layer. As an example, the Cu layer
can have a thickness of approximately 25,000 angstroms.
[0011] In some embodiments, the structure can further include a
gold (Au) layer disposed over the second barrier layer. Thicknesses
of the third Ti layer and the second barrier layer can be selected
to provide sufficient passivation functionality between the Cu
layer and the Au layer. As examples, the third Ti layer can have a
thickness of approximately 500 angstroms, and the second barrier
layer can include a platinum (Pt) layer having a thickness of
approximately 500 angstroms.
[0012] In some embodiments, the structure can further include a
fourth Ti layer disposed over the Au layer. As examples, the Au
layer can have a thickness of approximately 1,200 angstroms, and
the fourth Ti layer can have a thickness of approximately 90
angstroms. In some embodiments, each of the first Ti layer, the
first barrier layer, the second Ti layer, the Cu layer, the third
Ti layer, the second barrier layer, the Au layer, and the fourth Ti
layer can be formed by evaporation.
[0013] In accordance with a number of implementations, the present
disclosure relates to a method for forming a metalized structure
for a compound semiconductor device. The method includes forming a
first titanium (Ti) layer over a substrate associated with the
compound semiconductor device, The method further includes forming
a first barrier layer over the first Ti layer. The method further
includes forming a second Ti layer over the first barrier layer.
The method further includes forming a copper (Cu) layer over the
second Ti layer.
[0014] In some embodiments, the method can further include forming
a third Ti layer over the Cu layer, and forming a second barrier
layer over the third Ti layer. In some embodiments, the method can
further include forming a gold (Au) layer over the second Pt layer.
In some embodiments, the method can further include forming a
fourth Ti layer over the Au layer.
[0015] In some embodiments, each of the first Ti layer, the first
barrier layer, the second Ti layer, the Cu layer, the third Ti
layer, the second barrier layer, the Au layer, and the fourth Ti
layer can be formed by evaporation. In some embodiments, each of
the first barrier layer and the second barrier layer can include
platinum (Pt), palladium (Pd), or nickel (Ni).
[0016] According to a number of implementations, the present
disclosure relates to a compound semiconductor die that includes a
semiconductor device formed on a compound semiconductor substrate.
The die further includes an interconnect metalized stack for the
semiconductor device. The metalized stack includes a first titanium
(Ti) layer disposed over a substrate associated with the
semiconductor device, a first barrier layer disposed over the first
Ti layer, a second Ti layer disposed over the first barrier layer,
and a copper (Cu) layer disposed over the second Ti layer.
[0017] In some embodiments, the interconnect metalized stack can
further include a third Ti layer disposed over the Cu layer, and a
second barrier layer disposed over the third Ti layer. In some
embodiments, the interconnect metalized stack can further include a
gold (Au) layer disposed over the second barrier layer, and a
fourth Ti layer disposed over the Au layer. In some embodiments,
each of the first barrier layer and the second barrier layer can
include platinum (Pt), palladium (Pd), or nickel (Ni).
[0018] In some embodiments, the compound semiconductor die can be a
gallium arsenide GaAs die. In some embodiments, the semiconductor
device can include a pseudomorphic high electron mobility
transistor (pHEMT), a metal semiconductor field effect transistor
(MESFET), a heterojunction bipolar transistor (HBT), a Mach-Zehnder
(MZ) modulator, a photovoltaic device, a light-emitting diode
(LED), a bipolar FET (BiFET), a bipolar HEMT (BiHEMT), a laser
diode such as a vertical cavity laser (VCSEL) diode, or a surface
acoustic wave (SAW) device such as a SAW filter or resonator.
[0019] In a number of implementations, the present disclosure
relates to a radio-frequency (RF) module that includes a packaging
substrate configured to receive a plurality of components. The
module further includes a die mounted on the packaging substrate
and having a semiconductor device formed on a compound
semiconductor substrate. The die further includes an interconnect
metalized stack for the semiconductor device. The metalized stack
includes a first titanium (Ti) layer disposed over a substrate
associated with the semiconductor device, a first barrier layer
disposed over the first Ti layer, a second Ti layer disposed over
the first barrier layer, and a copper (Cu) layer disposed over the
second Ti layer.
[0020] According to some implementations, the present disclosure
relates to a radio-frequency (RF) device that includes an antenna,
and an RF circuit in communication with the antenna and configured
to provide transmit and/or receive functionality. The RF device
further includes a module configured to facilitate the operation of
the antenna and the RF circuit. The module includes a semiconductor
device formed on a compound semiconductor substrate. The module
further includes an interconnect metalized stack for the
semiconductor device. The metalized stack includes a first titanium
(Ti) layer disposed over a substrate associated with the
semiconductor device, a first barrier layer disposed over the first
Ti layer, a second Ti layer disposed over the first barrier layer,
and a copper (Cu) layer disposed over the second Ti layer.
[0021] In some implementations, the present disclosure relates to a
metalized structure for a compound semiconductor device. The
structure includes a first titanium (Ti) layer disposed over a
substrate associated with the compound semiconductor device. The
structure further includes a first titanium nitride (TiN) layer
disposed over the first Ti layer. The structure further includes a
copper (Cu) layer disposed over the first TiN layer.
[0022] In some embodiments, the first Ti layer and the first TiN
layer can be configured to yield a barrier between the Cu layer and
the substrate. In some embodiments, the structure can further
include a second TiN layer disposed over the Cu layer and a first
platinum (Pt) layer disposed over the second TiN layer. The first
TiN layer can be in direct contact with the first Ti layer, the Cu
layer can be in direct contact with the first TiN layer, the second
TiN layer can be in direct contact with the Cu layer, and the first
Pt layer can be in direct contact with the second TiN layer.
[0023] In some embodiments, a thickness of the first TiN layer can
selected to provide sufficient barrier functionality between the Cu
layer and an ohmic metal layer disposed between the first Ti layer
and the substrate. The first Ti layer can have a thickness
sufficient to function as an adhesion layer. As examples, the first
Ti layer can have a thickness of approximately 1,000 angstroms, and
the first TiN layer can have a thickness of approximately 500
angstroms.
[0024] In some embodiments, the Cu layer can have a thickness
selected to yield a resistivity value similar to that of a gold
layer being replaced by the Cu layer. As an example, the Cu layer
can have a thickness of approximately 25,000 angstroms.
[0025] In some embodiments, the structure can further include a
gold (Au) layer disposed over the first Pt layer. In some
embodiments, the structure can further include a second Ti layer
disposed over the Au layer. As examples, the Au layer can have a
thickness of approximately 1,200 angstroms, and the second Ti layer
can have a thickness of approximately 90 angstroms. In some
embodiments, each of the first Ti layer, the first TiN layer, the
Cu layer, the second TiN layer, the first Pt layer, the Au layer,
and the second Ti layer can be formed by evaporation.
[0026] In accordance with some implementations, the present
disclosure relates to a method for forming a metalized structure
for a compound semiconductor device. The method includes forming a
first titanium (Ti) layer over a substrate associated with the
compound semiconductor device. The method further includes forming
a first titanium nitride (TiN) layer over the first Ti layer. The
method further includes forming a copper (Cu) layer over the first
TiN layer.
[0027] In some embodiments, the method can further include forming
a second TiN layer over the Cu layer, and forming a first Pt layer
over the second TiN layer. In some embodiments, the method can
further include forming a gold (Au) layer over the first Pt layer.
In some embodiments, the method can further include forming a
second Ti layer over the Au layer.
[0028] In some embodiments, each of the first Ti layer, the first
TiN layer, the Cu layer, the second TiN layer, the first Pt layer,
the Au layer, and the second Ti layer can be formed by evaporation.
The evaporation of each of the first and second TiN layers can
include evaporating Ti with an assist from an ion source, and
introducing nitrogen gas during at least a portion of the Ti
evaporation from the ion source to form the TiN layer.
[0029] In accordance with a number of implementations, the present
disclosure relates to a compound semiconductor die that includes a
semiconductor device formed on a compound semiconductor substrate.
The die further includes an interconnect metalized stack for the
semiconductor device. The metalized stack includes a first titanium
(Ti) layer disposed over a substrate associated with the
semiconductor device, a first titanium nitride (TiN) layer disposed
over the first Ti layer, and a copper (Cu) layer disposed over the
first TiN layer.
[0030] In some embodiments, the interconnect metalized stack can
further include a second TiN layer disposed over the Cu layer, and
a platinum (Pt) layer disposed over the second TiN layer. In some
embodiments, the interconnect metalized stack can further include a
gold (Au) layer disposed over the Pt layer, and a second Ti layer
disposed over the Au layer.
[0031] In some embodiments, the compound semiconductor die can be a
gallium arsenide GaAs die. In some embodiments, the semiconductor
device can include a pseudomorphic high electron mobility
transistor (pHEMT), a metal semiconductor field effect transistor
(MESFET), a heterojunction bipolar transistor (HBT), a Mach-Zehnder
(MZ) modulator, a photovoltaic device, a light-emitting diode
(LED), a bipolar FET (BiFET), a bipolar HEMT (BiHEMT), a laser
diode such as a vertical cavity laser (VCSEL) diode, or a surface
acoustic wave (SAW) device such as a SAW filter or resonator.
[0032] In a number of implementations, the present disclosure
relates to a radio-frequency (RF) module that includes a packaging
substrate configured to receive a plurality of components. The
module further includes a die mounted on the packaging substrate
and including a semiconductor device formed on a compound
semiconductor substrate. The die further includes an interconnect
metalized stack for the semiconductor device. The metalized stack
includes a first titanium (Ti) layer disposed over a substrate
associated with the semiconductor device, a first titanium nitride
(TiN) layer disposed over the first Ti layer, and a copper (Cu)
layer disposed over the first TiN layer.
[0033] In some implementations, the present disclosure relates to a
radio-frequency (RF) device that includes an antenna, and an RF
circuit in communication with the antenna and configured to provide
transmit and/or receive functionality. The RF device further
includes a module configured to facilitate the operation of the
antenna and the RF circuit. The module includes a semiconductor
device formed on a compound semiconductor substrate. The module
further includes an interconnect metalized stack for the
semiconductor device. The metalized stack includes a first titanium
(Ti) layer disposed over a substrate associated with the
semiconductor device, a first titanium nitride (TiN) layer disposed
over the first Ti layer, and a copper (Cu) layer disposed over the
first TiN layer.
[0034] In a number of implementations, the present disclosure
relates to a metalized structure for a compound semiconductor
device. The structure includes a stack disposed over a substrate
associated with the compound semiconductor device. The stack
includes a barrier and a copper (Cu) layer disposed over the
barrier. The structure further includes a sputtered titanium
tungsten (TiW) layer disposed over the stack.
[0035] In some embodiments, the barrier can include a titanium
nitride (TiN) layer disposed over a Ti layer. In some embodiments,
the barrier can include a first titanium layer, a barrier layer
disposed over the first Ti layer, and a second Ti layer disposed
over the barrier layer. The barrier layer can include platinum
(Pt), palladium (Pd), or nickel (Ni).
[0036] In some embodiments, the stack can further include a
titanium (Ti) layer disposed between the Cu layer and the sputtered
TiW layer. In some embodiments, the stack can further include a
gold (Au) layer disposed over the sputtered TiW layer. The
sputtered TiW layer can have a thickness sufficient to inhibit
interaction between the Cu layer and the Au layer. In some
embodiments, the stack can further include a Ti layer disposed over
the Au layer. In some embodiments, each layer of the stack can be
formed by evaporation. In some embodiments, each of the Au layer
and the Ti layer above the Au layer can be formed by evaporation.
In some embodiments, the sputtered TiW can be substantially free of
wing or stringer features.
[0037] In accordance with a number of implementations, the present
disclosure relates to a method for forming a metalized structure
for a compound semiconductor device. The method includes forming a
resist mask that defines an opening over a substrate associated
with the compound semiconductor device. The method further includes
forming a stack within the opening and over the substrate. The
stack includes a barrier and a copper (Cu) layer formed over the
barrier. The method further includes sputtering a titanium tungsten
(TiW) layer over the stack in a manner that allows the resist mask
to be removed by a liftoff process.
[0038] In some embodiments, the resist mask can have a re-entrant
profile at the opening. In some embodiments, forming of the stack
can further include forming a titanium (Ti) layer between the Cu
layer and the TiW layer. In some embodiments, the method can
further include forming a gold (Au) layer over the TiW layer. In
some embodiments, the method can further include forming a Ti layer
over the Au layer. In some embodiments, the method can further
include lifting off the resist mask to thereby yield a trapezoid
shaped stack.
[0039] According to some implementations, the present disclosure
relates to a compound semiconductor die that includes a
semiconductor device formed on a compound semiconductor substrate.
The die further includes an interconnect metalized stack for the
semiconductor device. The metalized stack includes a copper (Cu)
layer disposed over a barrier. The metalized stack further includes
a sputtered titanium tungsten (TiW) layer disposed over the Cu
layer.
[0040] In some embodiments, the metalized stack can further include
a titanium (Ti) layer disposed between the Cu layer and the TiW
layer.
[0041] In a number of implementations, the present disclosure
relates to a radio-frequency (RF) module that includes a packaging
substrate configured to receive a plurality of components. The
module further includes a die mounted on the packaging substrate
and including a semiconductor device formed on a compound
semiconductor substrate. The die further includes an interconnect
metalized stack for the semiconductor device. The metalized stack
includes a copper (Cu) layer disposed over a barrier. The metalized
stack further includes a sputtered titanium tungsten (TiW) layer
disposed over the Cu layer.
[0042] In some implementations, the present disclosure relates to a
radio-frequency (RF) device that includes an antenna and an RF
circuit in communication with the antenna and configured to provide
transmit and/or receive functionality. The RF device further
includes a module configured to facilitate the operation of the
antenna and the RF circuit. The module includes a semiconductor
device formed on a compound semiconductor substrate. The module
further includes an interconnect metalized stack for the
semiconductor device. The metalized stack includes a copper (Cu)
layer disposed over a barrier. The metalized stack further includes
a sputtered titanium tungsten (TiW) layer disposed over the Cu
layer.
[0043] In accordance with a number of implementations, the present
disclosure relates to a metalized structure for a compound
semiconductor. The structure includes an adhesion layer disposed
over a surface of the compound semiconductor. The structure further
includes a barrier layer disposed over the adhesion layer. The
structure further includes a copper (Cu) layer disposed over the
barrier layer. The barrier layer is configured to substantially
survive a high temperature operating life (HTOL) stress at
approximately 200.degree. C. for at least 100 hours.
[0044] In some embodiments, the structure can further include a
passivation layer disposed over the Cu layer. In some embodiments,
the adhesion layer can include a first titanium (Ti) layer.
[0045] In some embodiments, the barrier layer can include a second
Ti layer and a first platinum (Pt) layer, with the second Ti layer
being disposed over the first Pt layer. The passivation layer can
include a second Pt layer and a third Ti layer, with the second Pt
layer being disposed over the third Ti layer. In some embodiments,
the structure can further include a gold layer disposed over the
second Pt layer.
[0046] In some embodiments, the barrier layer can include a first
titanium nitride (TiN) layer. The passivation layer can include a
second Pt layer and a second TiN layer, with the second Pt layer
being disposed over the second TiN layer. In some embodiments, the
structure can further include a gold layer disposed over the second
Pt layer.
[0047] In some embodiments, each of the adhesion, barrier, Cu, and
passivation layers can be formed by evaporation deposition. In some
embodiments, at least some of the passivation layer can include a
sputtered layer. In some embodiments, the passivation layer can
includes a sputtered titanium tungsten (TiW) layer and an
evaporated titanium (Ti) layer, with the TiW layer being disposed
over the Ti layer.
[0048] In some embodiments, the adhesion, barrier, Cu, and Ti
layers can have a side profile corresponding to a re-entrant resist
profile. The Cu layer can have a sufficient thickness such that
copper deposited on the re-entrant resist profile yields a modified
resist profile dimensioned to allow formation of the sputtered TiW
layer substantially without wing or stringer features to thereby
facilitate a liftoff process.
[0049] In some embodiments, the structure can further include a
gold layer disposed over the second TiW layer. In some embodiments,
the compound semiconductor can include gallium arsenide (GaAs).
[0050] According to some implementations, the present disclosure
relates to a semiconductor die that includes an integrated circuit
(IC) having at least one transistor formed on a compound
semiconductor substrate. The die further includes a metalized
structure configured to facilitate an electrical connection for the
transistor. The metalized structure includes an adhesion layer
disposed over a surface of the compound semiconductor, a barrier
layer disposed over the adhesion layer, and a copper (Cu) layer
disposed over the barrier layer, with the barrier layer being
configured to substantially survive a high temperature operating
life (HTOL) test at a temperature between 270.degree. C. and
273.degree. C. for at least 100 hours or 500 hours.
[0051] In some embodiments, the compound semiconductor substrate
can include gallium arsenide (GaAs). In some embodiments, the at
least one transistor can include a heterojunction bipolar
transistor (HBT) or a pseudomorphic high electron mobility
transistor (pHEMT).
[0052] In a number of implementations, the present disclosure
relates to a packaged electronic module that includes a packaging
substrate configured to receive a plurality of components. The
module further includes a semiconductor die mounted on the
packaging substrate. The die includes an integrated circuit having
at least one transistor formed on a compound semiconductor
substrate. The die further includes a metalized structure
configured to facilitate an electrical connection for the
transistor. The metalized structure includes an adhesion layer
disposed over a surface of the compound semiconductor, a barrier
layer disposed over the adhesion layer, and a copper (Cu) layer
disposed over the barrier layer. The barrier layer is configured to
substantially survive a high temperature operating life (HTOL) test
at a temperature between 270.degree. C. and 273.degree. C. for at
least 100 hours or 500 hours. The module further includes a contact
pad disposed on the packaging substrate and electrically connected
to the metalized structure.
[0053] In some implementations, the present disclosure relates to a
radio-frequency (RF) device that includes an RF circuit configured
to generate a transmit signal and/or process a received signal. The
RF device further includes a module configured to facilitate
transmission of the transmit signal and/or processing of the
received signal. The module includes a semiconductor die having at
least one transistor formed on a compound semiconductor substrate.
The die further includes a metalized structure configured to
facilitate an electrical connection for the transistor. The
metalized structure includes an adhesion layer disposed over a
surface of the compound semiconductor, a barrier layer disposed
over the adhesion layer, and a copper (Cu) layer disposed over the
barrier layer. The barrier layer is configured to substantially
survive a high temperature operating life (HTOL) test at a
temperature between 270.degree. C. and 273.degree. C. for at least
100 hours or 500 hours.
[0054] In some embodiments, the RF device can include a wireless
device. In some embodiments, the wireless device can include a
cellular phone.
[0055] In some implementations, the present disclosure relates to a
method for forming a metalized structure. The method includes
providing a compound semiconductor substrate. The method further
includes forming a photoresist layer over the substrate. The method
further includes patterning the photoresist layer to define an
opening. The method further includes evaporating copper stack over
the patterned photoresist layer. The copper stack includes an
adhesion layer, a barrier layer, and a copper layer. The method
further includes forming a passivation layer over the copper stack.
The method further includes performing a liftoff process to remove
a portion of the copper stack that is over the photoresist layer to
yield the metalized structure at a location of the opening.
[0056] For purposes of summarizing the disclosure, certain aspects,
advantages and novel features of the inventions have been described
herein. It is to be understood that not necessarily all such
advantages may be achieved in accordance with any particular
embodiment of the invention. Thus, the invention may be embodied or
carried out in a manner that achieves or optimizes one advantage or
group of advantages as taught herein without necessarily achieving
other advantages as may be taught or suggested herein.
[0057] The present disclosure relates to U.S. patent application
Ser. No. 13/774,228, titled "COPPER INTERCONNECTS HAVING A
TITANIUM-BARRIER-TITANIUM ASSEMBLY BETWEEN COPPER AND COMPOUND
SEMICONDUCTOR," and U.S. Pat. No. 8,878,362, titled "COPPER
INTERCONNECTS HAVING A TITANIUM-TITANIUM NITRIDE ASSEMBLY BETWEEN
COPPER AND COMPOUND SEMICONDUCTOR," each filed on even date
herewith and each hereby incorporated by reference herein in its
entirety.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] FIG. 1 schematically shows that in some implementations, a
device having one or more features as described herein can be
formed on a compound semiconductor substrate such as a wafer.
[0059] FIGS. 2A and 2B show examples of the device of FIG. 1, where
the device can include a copper-based metalized structure.
[0060] FIG. 3 shows an example of the copper-based metal stack
structure.
[0061] FIG. 4 schematically shows a first example configuration of
the copper-based metal stack structure.
[0062] FIG. 5A shows a photograph of a sectional view of the
example metalized stack of FIG. 4.
[0063] FIG. 5B shows a closer view of the example metalized stack
of FIG. 5A.
[0064] FIG. 6A shows an example of how a barrier can fail without
one or more features of the example metalized stack of FIGS. 4 and
5.
[0065] FIG. 6B shows an example of how M1 can become discolored and
have reduced wire bonding performance in a configuration without
one or more features of the example metalized stack of FIGS. 4 and
5.
[0066] FIG. 7 schematically shows a second example configuration of
the copper-based metal stack structure.
[0067] FIG. 8A shows a photograph of a sectional view of the
example metalized stack of FIG. 7.
[0068] FIG. 8B shows a closer view of the example metalized stack
of FIG. 8A.
[0069] FIG. 9A shows an example of how significant voids can form
in a copper layer in a configuration without one or more features
of the example metalized stack of FIGS. 7 and 8.
[0070] FIG. 9B shows an example of how M1 can become discolored and
have reduced wire bonding performance in a configuration without
one or more features of the example metalized stack of FIGS. 7 and
8.
[0071] FIG. 10 schematically shows a third example configuration of
the copper-based metal stack structure.
[0072] FIG. 11 shows a pre-liftoff stage that can be implemented to
fabricate the example metal stack structure of FIG. 10.
[0073] FIG. 12 shows an underside of a metal layer that has
undergone a successful liftoff process.
[0074] FIG. 13 shows a process that can be implemented to fabricate
the first example metal stack of FIGS. 4 and 5.
[0075] FIG. 14 shows various stages of the example fabrication
process of FIG. 13.
[0076] FIG. 15 shows a process that can be implemented to fabricate
the first example metal stack of FIGS. 7 and 8.
[0077] FIG. 16 shows various stages of the example fabrication
process of FIG. 15.
[0078] FIGS. 17A and 17B show a process that can be implemented to
fabricate the first example metal stack of FIGS. 10 and 11.
[0079] FIGS. 18A and 18B show various stages of the example
fabrication process of FIGS. 17A and 17B.
[0080] FIG. 19 shows an example of high temperature operating life
(HTOL) test performance for samples of the metallization structure
of FIGS. 4 and 5.
[0081] FIG. 20 shows an example of HTOL test failure for samples
that do not have the barrier functionalities of the metallization
structure of FIGS. 4 and 5.
[0082] FIG. 21 shows an optical image of the metallization
structure on one of the samples of FIG. 19, after 1,400 hours of
the HTOL test.
[0083] FIG. 22 shows an optical image of the metallization
structure on one of the samples of FIG. 20, after failing the HTOL
test.
[0084] FIG. 23 shows a process that can be implemented to fabricate
a die having one or more metalized structures as described
herein.
[0085] FIG. 24 schematically depicts a die that can result from the
fabrication process of FIG. 23.
[0086] FIGS. 25A and 25B schematically depict a module having one
or more dies of FIG. 24.
[0087] FIG. 26 schematically depicts a radio-frequency (RF) device
that includes one or more dies and/or one or more modules having
one or more features as described herein.
[0088] FIG. 27 shows a more specific example of the RF device of
FIG. 26 implemented as a wireless device.
DETAILED DESCRIPTION OF SOME EMBODIMENTS
[0089] The headings provided herein, if any, are for convenience
only and do not necessarily affect the scope or meaning of the
claimed invention.
[0090] Metalized features formed on semiconductors such as compound
semiconductors can be configured to provide, for example,
interconnect functionalities. Such interconnects are commonly
referred to as Metal 1, Metal 2 and Metal 3. Metal 1 or M1 commonly
refers to a layer of metal interconnect that contacts ohmic metals
and/or other active or passive components (e.g., resistors,
capacitors, inductors) in circuits. Metal 2 or M2 commonly refers
to a metal layer that connects to an M1 layer. Metal 3 or M3
commonly refers to a heat spreader (e.g., in a power
amplifier).
[0091] In some implementations, gold can be chosen for use as M1
interconnects due to its properties such as low electrical
resistivity, chemical inertness and desirable wire bonding
properties. However, gold is relatively expensive, and its price
has been rising steadily. Consequently, costs associated with
wafers and resulting devices using gold as M1 interconnects can
also increase.
[0092] Accordingly, a less expensive alternative having one or more
desirable properties of gold is desirable. Copper can be such an
alternative. By way of examples, copper has about 20% lower
electrical resistivity than gold (approximately 1.7 .mu..OMEGA.-cm
for copper, and approximately 2.2 .mu..OMEGA.-cm for gold). Copper
also has higher thermally conductivity than gold (approximately 385
W m.sup.-1 K.sup.-1 for copper, and approximately 314 W m.sup.-1
K.sup.-1 for gold), making it a desirable replacement for gold for
M3 heat spreader application.
[0093] Use of copper, however, can include challenges when
implemented on semiconductor substrates. For example, copper is a
relatively fast diffuser; so a barrier layer is typically needed to
prevent the metal from diffusing into a semiconductor substrate.
Copper is also chemically reactive and oxidizes readily in air; and
therefore it typically needs to be passivated so as to be protected
from process chemicals and/or fabrication environment between
steps.
[0094] Described herein are structures, devices and methods related
to metal structures (such as M1 interconnects) based on copper.
Various examples of barriers and passivation configurations that
provide desirable functionalities that facilitate use of copper as
an interconnect conductor are described. Although described in the
context of M1 interconnects, it will be understood that one or more
features of the present disclosure can also be implemented in other
types of copper-based metal structures.
[0095] FIG. 1 shows that in some implementations, a device 12
formed on a compound semiconductor substrate (such as a wafer 10)
can include one or more copper-based interconnect structures having
one or more features as described herein. Various examples
described herein are in the context of gallium arsenide (GaAs)
compound semiconductor on which such copper-based interconnect
structures can be implemented. However, it will be understood that
such copper-based interconnect features can also be implemented on
other compound semiconductors. For example, compound semiconductors
such as InP, GaN, InGaP, and InGaAs can be utilized as a substrate
on which copper-based interconnects having one or more features as
described herein are formed.
[0096] It will also be understood that although various examples
herein are described in the context of compound semiconductors, one
or more features of the present disclosure can also be implemented
on elemental semiconductors. For example, elemental semiconductors
such as silicon and/or germanium can be utilized as a substrate on
which copper-based interconnects having one or more features as
described herein are formed.
[0097] FIGS. 2A and 2B show examples of devices 12 for which
copper-based interconnect structures can be implemented. In FIG.
2A, a heterojunction bipolar transistor (HBT) 20 is shown to
include a number of copper-based structures 100 formed as various
contacts. For example, copper-based contacts 100 are shown to be
formed over a sub-collector layer 24 (which is formed over a
semi-insulating compound semiconductor substrate 22 such as GaAs).
In another example, copper-based contacts 100 are shown to be
formed over a base layer 28 (which is formed over a collector layer
26). In yet another example, a copper-based contact 100 is shown to
be formed over an emitter stack 30 (which is formed over the base
layer 28). In the example configuration 20 of FIG. 2A, ohmic metal
layers 32 are shown to be disposed between the copper-based
contacts 100 and their respective semiconductor substrates.
[0098] In FIG. 2B, a high electron mobility transistor (HEMT) such
as a pseudomorphic HEMT (pHEMT) 40 is shown to include a number of
copper-based structures 100 formed as various contacts. The pHEMT
40 is shown to include a compound semiconductor substrate 42 (such
as GaAs) over which is formed a buffer layer 44. A superlattice
structure 46 is shown to be formed over the buffer layer 44, and a
channel layer 48 is shown to be formed over the superlattice
structure 46. A barrier layer 50 is shown to be formed over the
channel layer 48, and capping layers 52 are shown to be formed over
portions of the barrier layer 50. Copper-based contacts 100 are
shown to be formed over the capping layers 52 so as to form source
and drain contacts. A copper-based contact 100 is also shown to be
formed over the barrier layer 50 so as to form a gate contact. In
the example configuration 40 of FIG. 2B, ohmic metal layers 54 are
shown to be disposed between the copper-based contacts 100 and
their respective semiconductor substrates.
[0099] In some embodiments, copper-based structures 100 can also be
implemented in other semiconductor devices. Examples of such
semiconductor devices are described herein in greater detail.
[0100] FIG. 3 shows that a copper-based metalized structure 100
such as an M1 structure can include a copper (Cu) layer 110
disposed over a barrier 106. The barrier 106 is shown to be formed
over an ohmic metal layer 104 which is formed over a compound
semiconductor substrate 102. As further shown in FIG. 3, the
metalized structure 100 can also include a passivation structure
112 formed over the copper layer 110. Various non-limiting examples
of the barrier 106 and the passivation structure 112 and how they
can be formed are described herein.
EXAMPLE 1
[0101] FIG. 4 schematically shows a first example configuration 120
of a copper-based metal stack structure 100. An ohmic metal layer
104 can be formed over a compound semiconductor substrate 102 such
as GaAs. A first titanium (Ti) layer 122 (thickness d1) can be
formed over the ohmic metal layer 104. A first platinum (Pt) layer
124 (thickness d2) can be formed over the first Ti layer 122. A
second Ti layer 126 (thickness d3) can be formed over the first Pt
layer 124. A copper (Cu) layer 110 (thickness d4) can be formed
over the second Ti layer 126. Examples of desirable functionalities
that can be provided by some or all of the foregoing Ti--Pt--Ti
combination, as well as some possible alternate configurations, are
described herein in greater detail.
[0102] In some embodiments, a third Ti layer 130 (thickness d5) can
be formed over the Cu layer 110 A second Pt layer 132 (thickness
d6) can be formed over the third Ti layer 130 A gold (Au) layer 134
(thickness d7) can be formed over the second Pt layer 132 A fourth
Ti layer 136 (thickness d8) can be formed over the Au layer 134.
Examples of desirable functionalities that can be provided by some
or all of the foregoing combination, as well as some possible
alternate configurations, are described herein in greater
detail.
[0103] Table 1 lists example thicknesses of the various layers
shown in FIG. 4.
TABLE-US-00001 TABLE 1 Layer Dimension Approximate value (Angstrom)
First Ti d1 1,000 First Pt d2 500 Second Ti d3 1,000 Cu d4 25,000
Third Ti d5 500 Second Pt d6 500 Au d7 1,200 Fourth Ti d8 90
[0104] FIG. 5A shows a sectional view of a photograph of a
metalized stack 120 having the example configuration described in
reference to FIG. 4. FIG. 5B shows a closer view of the metalized
stack 120 of FIG. 5A. In both photographs, one can see that the
example assembly of layers Ti--Pt--Ti (122, 124, 126) can be formed
in an opening formed in a passivation layer 150 that has been
formed over the ohmic metal layer 104 and the GaAs substrate 102.
Further, FIGS. 5A and 5B show that a passivation layer 152 can be
formed so as to cover some or all of the side and upper surfaces
that would otherwise be exposed.
[0105] In some implementations, at least some of the layers of the
example metalized stack 120 described in reference to FIGS. 4 and 5
can be formed by electron-beam evaporation deposition. Applicant
has found that Ti can be an adequate barrier for Cu, and Pt can be
a good barrier for Cu for preventing or reducing diffusion. In the
metalized stack 120, the Cu layer is shown to be clad between two
Ti layers. Applicant's reliability tests have shown that without
the Ti layers, Pt would alloy with the Cu causing the barrier to
fail (FIG. 6A). When this happens, the resistivity of the M1 stack
can increase drastically, thereby resulting in failure of the
device associated with the M1 stack. The Cu would also reach the
top 1200A of Au causing the M1 to discolor and also resulting in
wire bonding issues (FIG. 6B).
[0106] FIG. 6A shows an example configuration 140 where Ti layers
are absent on both sides of the Cu layer 110. Thus, the example
stack 140 includes layers Ti (122), Pt (124), Cu (110), Pt (132 in
FIG. 4), Au (134 in FIG. 4) and Ti (136 in FIG. 4) formed over an
ohmic metal layer 104. FIG. 6A shows that without a Ti layer (126
in FIG. 4) between the Cu layer 110 and the Pt layer 124, the Pt
layer 124 becomes degraded, and thus is not a reliable barrier for
Cu 110. It is believed that such a degradation is due to copper and
platinum forming an alloy at relatively low temperatures.
[0107] Referring to FIG. 6B, with the absence of a Ti layer (130 in
FIG. 4) between the Cu layer 110 and the upper Pt layer (132 in
FIG. 4), copper is also able to reach the gold layer (134 in FIG.
4), thereby resulting in discoloration (e.g., non-uniform patterns
on the surfaces) of the M1 structure 140 and undesirable features
such as wire bonding issues. Again, it is believed that such a
degradation of the upper Pt layer is due to copper and platinum
forming an alloy at relatively low temperatures.
[0108] For the example metalized stack 120 of FIGS. 4 and 5, the
first Ti layer 122 can be configured to provide functionalities
such as adhesion and covering of surface irregularities due to, for
example, roughness of the ohmic metal film 104. An example of how
the first Ti layer 122 can be formed is described herein in greater
detail. It will be understood that the thickness of the first Ti
layer 122 can be less than or greater than the example thickness of
approximately 1,000 angstroms. In some implementations, the
thickness of the first Ti layer 122 can be selected to be
sufficient to provide one or more of the foregoing
functionalities.
[0109] For the example metalized stack 120 of FIGS. 4 and 5, the
first Pt layer 124 can be configured to act as, for example, a
barrier that reduces or inhibits migration of gold, copper and/or
other atoms between the ohmic metal layer 104 (through the first Ti
layer 122) and the Cu layer 110. An example of how the first Pt
layer 124 can be formed is described herein in greater detail. It
will be understood that the thickness of the first Pt layer 124 can
be less than or greater than the example thickness of approximately
500 angstroms. In some implementations, the first Pt layer 124 can
be formed so as to have a thickness d2 that is at least
approximately 250 angstroms. In some embodiments, the thickness d2
can be significantly larger than the example thickness of 250
angstroms. In some implementations, the thickness of the first Pt
layer 124 can be selected to be sufficient to provide one or more
of the features as described herein.
[0110] For the example metalized stack 120 of FIGS. 4 and 5, the
second Ti layer 126 can be configured to, for example, reduce or
inhibit the Cu layer 110 from alloying with the first Pt layer 124.
An example of how the second Ti layer 126 can be formed is
described herein in greater detail. It will be understood that the
thickness of the second Ti layer 126 can be less than or greater
than the example thickness of approximately 1,000 angstroms. In
some implementations, the thickness of the second Ti layer 126 can
be selected to be sufficient to provide one or more of the features
as described herein.
[0111] For the example metalized stack 120 of FIGS. 4 and 5, the Cu
layer 110 can be configured to, for example, yield a desired
resistivity. Because the Cu layer 110 can be configured as a main
conductive layer of the stack 120, the Cu layer's sectional
dimensions (e.g., thickness and/or width) can be selected to
provide a desired conductive property of the stack 120. In the
example stack 120 of FIGS. 4 and 5, the approximately 25,000
angstrom Cu layer 110 was selected to provide a roughly same sheet
resistance associated with an approximately 30,000 angstrom gold
layer which the Cu layer 110 is replacing. An example of how the Cu
layer 110 can be formed is described herein in greater detail. It
will be understood that the thickness of the Cu layer 110 can be
less than or greater than the example thickness of approximately
25,000 angstroms. In some implementations, the thickness of the Cu
layer 110 can be selected to be sufficient to provide one or more
of the features as described herein.
[0112] For the example metalized stack 120 of FIGS. 4 and 5, the
third Ti layer 130 can be configured to, for example, reduce or
inhibit the Cu layer 110 from alloying with the second Pt layer
132. As described in reference to FIGS. 6A and 6B, such an alloying
of the copper layer and the platinum layer can result in some of
the copper reaching the gold layer and yielding discoloration and
wire bonding issues associated with the metalized structure. An
example of how the third Ti layer 130 can be formed is described
herein in greater detail. It will be understood that the thickness
of the third Ti layer 130 can be less than or greater than the
example thickness of approximately 500 angstroms. In some
implementations, the thickness of the third Ti layer 130 can be
selected to be sufficient to provide one or more of the features as
described herein.
[0113] For the example metalized stack 120 of FIGS. 4 and 5, the
second Pt layer 132 can be configured to act as, for example, a
barrier that reduces or inhibits migration of gold, copper and/or
other atoms between the Au layer 134 (through the third Ti layer
130) and the Cu layer 110. An example of how the second Pt layer
132 can be formed is described herein in greater detail. It will be
understood that the thickness of the second Pt layer 132 can be
less than or greater than the example thickness of approximately
500 angstroms. In some implementations, the second Pt layer 132 can
be formed so as to have a thickness d6 that is at least
approximately 250 angstroms. In some embodiments, the thickness d6
can be significantly larger than the example thickness of 250
angstroms. In some implementations, the thickness of the second Pt
layer 132 can be selected to be sufficient to provide one or more
of the features as described herein.
[0114] For the example metalized stack 120 of FIGS. 4 and 5, the Au
layer 134 can be configured to, for example, allow wire bonding of
the metalized stack 120 and/or to protect the Cu layer 110 from
oxidation. An example of how the Au layer 134 can be formed is
described herein in greater detail. It will be understood that the
thickness of the Au layer 134 can be less than or greater than the
example thickness of approximately 1,200 angstroms. In some
implementations, the thickness of the Au layer 134 can be selected
to be sufficient to provide one or more of the features as
described herein.
[0115] For the example metalized stack 120 of FIGS. 4 and 5, the
fourth Ti layer 136 can be configured to, for example, provide a
passivation layer for the Au layer 134, and/or to act as an
adhesion layer for an additional passivation layer (e.g., a nitride
layer) (e.g., 152 in FIG. 5A). An example of how the fourth Ti
layer 136 can be formed is described herein in greater detail. It
will be understood that the thickness of the fourth Ti layer 136
can be less than or greater than the example thickness of
approximately 90 angstroms. In some implementations, the thickness
of the fourth Ti layer 136 can be selected to be sufficient to
provide one or more of the features as described herein.
[0116] In some implementations, a combination of layers that
includes the first Ti layer 122, the first Pt layer 124, and the
second Ti layer 126 can be configured to sufficiently reduce or
prevent gold in the ohmic metal (104) from migrating to the Cu
layer 110, and copper in the Cu layer 110 from diffusing into the
ohmic metal (104). While it is not desired or intended to be bound
by any particular theory, the Ti/Pt/Ti structure can be modeled or
approximated electrically as two diodes back to back, to thereby
inhibit or reduce Au in the ohmic layer (104) from moving towards
the Cu layer 110, and to inhibit or reduce Cu (of the Cu layer 110)
from moving into the Pt layer 124.
[0117] For a combination of layers that includes the third Ti layer
130, the second Pt layer 132, and the Au layer 134, such a
combination can be configured to sufficiently reduce or prevent
migration and/or diffusion between the Cu layer 110 and the Au
layer 134. Such a combination can also be configured to be
sufficiently robust mechanically to be able to withstand stress of
wire bonding and/or probing.
[0118] In the example configuration 120 of FIGS. 4 and 5,
dimensions (e.g., thickness) of the various layers can be different
than their respective example values listed in Table 1. For
example, the first Ti layer (122) can have a thickness that is in a
range of 50 to 5,000 angstroms, in a range of 100 to 4,000
angstroms, 200 to 3,000 angstroms, 500 to 2,000 angstroms, or 750
to 1,250 angstroms. In another example, the first Pt layer (124)
can have a thickness that is in a range of 50 to 5,000 angstroms,
in a range of 100 to 2,500 angstroms, 200 to 1,000 angstroms, 300
to 700 angstroms, or 400 to 600 angstroms. In yet another example,
the second Ti layer (126) can have a thickness that is in a range
of 50 to 5,000 angstroms, in a range of 100 to 4,000 angstroms, 200
to 3,000 angstroms, 500 to 2,000 angstroms, or 750 to 1,250
angstroms. In yet another example, the Cu layer (110) can have a
thickness that is in a range of 500 to 50,000 angstroms, in a range
of 5,000 to 40,000 angstroms, 10,000 to 35,000 angstroms, or 20,000
to 30,000 angstroms. In yet another example, the third Ti layer
(130) can have a thickness that is in a range of 50 to 5,000
angstroms, in a range of 100 to 2,500 angstroms, 200 to 1,000
angstroms, 300 to 700 angstroms, or 400 to 600 angstroms. In yet
another example, the second Pt layer (132) can have a thickness
that is in a range of 50 to 5,000 angstroms, in a range of 100 to
2,500 angstroms, 200 to 1,000 angstroms, 300 to 700 angstroms, or
400 to 600 angstroms. In yet another example, the Au layer (134)
can have a thickness that is in a range of 100 to 5,000 angstroms,
in a range of 200 to 4,000 angstroms, 300 to 3,000 angstroms, 500
to 2,000 angstroms, or 900 to 1,500 angstroms. In yet another
example, the fourth Ti layer (136) can have a thickness that is in
a range of 20 to 500 angstroms, in a range of 40 to 300 angstroms,
60 to 200 angstroms, or 70 to 110 angstroms.
[0119] In some embodiments, some or all of the layers in the
example configuration 120 of FIGS. 4 and 5 can be formed from
different materials to provide similar functionalities. For
example, the first Ti layer (122) can be substituted with materials
such as chromium (Cr). In another example, the first Pt layer (124)
can be substituted with materials such as palladium (Pd) or nickel
(Ni). In yet another example, the second Ti layer (126) can be
substituted with materials such as chromium (Cr). In yet another
example, the Cu layer (110) can be substituted with materials such
as aluminum (Al). In yet another example, the third Ti layer (130)
can be substituted with materials such as chromium (Cr). In yet
another example, the second Pt layer (132) can be substituted with
materials such as palladium (Pd) or nickel (Ni). In yet another
example, the fourth Ti layer (136) can be substituted with
materials such as chromium (Cr).
[0120] In the example configuration 120 of FIGS. 4 and 5, the ohmic
layer 104 is described as being formed from gold. It will be
understood that other types of ohmic layer can also be implemented.
For devices such as MESFET and pHEMT, the ohmic layer can be an
N-ohmic layer. For N-type semiconductors, the ohmic layer can
include nickel, germanium (or gold-germanium AuGe) and/or gold.
Different variations of such materials can be utilized, including
for example, Ni/Ge/Au, Ni/Au/Ge/Au, Ni/AuGe/Au, Ni/Au/Ge/Ni/Au,
Ge/Ni/Au, and AuGe/Ni/Au. For devices such as HBT, BiFET, BiHEMT,
the ohmic layer can be N-ohmic or P-ohmic. The P-ohmic type can
have Au in the layer with other metals such as Pt and Pd. Other
configurations are also possible.
EXAMPLE 2
[0121] FIG. 7 schematically shows a second example configuration
160 of a copper-based metal stack structure 100. An ohmic metal
layer 104 can be formed over a compound semiconductor substrate 102
such as GaAs. A first titanium (Ti) layer 162 (thickness d1) can be
formed over the ohmic metal layer 104. A first titanium nitride
(TiN) layer 164 (thickness d2) can be formed over the first Ti
layer 162. A copper (Cu) layer 110 (thickness d3) can be formed
over the first TiN layer 164. A second TiN layer 170 (thickness d4)
can be formed over the Cu layer 110. A Pt layer 172 (thickness d5)
can be formed over the second TiN layer 170. A gold (Au) layer 174
(thickness d6) can be formed over the Pt layer 172. A second Ti
layer 176 (thickness d7) can be formed over the Au layer 174.
Examples of desirable functionalities that can be provided by some
or all of the foregoing combination, as well as some possible
alternate configurations, are described herein in greater
detail.
[0122] Table 2 lists example thicknesses of the various layers
shown in FIG. 7.
TABLE-US-00002 TABLE 2 Layer Dimension Approximate value (Angstrom)
First Ti d1 1,000 First TiN d2 500 Cu d3 25,000 Second TiN d4 500
Pt d5 500 Au d6 1,200 Second Ti d7 90
[0123] FIG. 8A shows a sectional view of a photograph of a
metalized stack 160 having the example configuration described in
reference to FIG. 7. FIG. 8B shows a closer view of the metalized
stack 160 of FIG. 8A. In both photographs, one can see that the
example assembly of layers Ti--TiN (162, 164) can be formed in an
opening formed in a passivation layer 190 that has been formed over
the ohmic metal layer 104 and the GaAs substrate 102. Further,
FIGS. 8A and 8B show that a passivation layer 192 can be formed so
as to cover some or all of the side and upper surfaces that would
otherwise be exposed.
[0124] In some implementations, at least some of the layers other
than the TiN layers 164, 170 of the example metalized stack 160
described in reference to FIGS. 7 and 8 can be formed by
electron-beam evaporation deposition. The TiN layers 164, 170 can
be formed by use of an evaporator with such an ion source, in
conjunction with introduction of N2 gas during such an evaporation
process. In the metalized stack 160, the Cu layer is shown to be
clad between two TiN layers 164, 170; and such TiN layers can
reduce or inhibit interactions between the Cu layer 110 and the
ohmic metal layer 104 (via the first TiN layer 164) and the Au
layer 174 (via the second TiN layer 170).
[0125] Applicant's reliability tests have shown that without the
TiN layers (e.g., a stack 180 having Ti/Cu/Ti/Au/Ti layers in FIG.
9A), Ti alone is not sufficient to act as a barrier. A failure of
such a metalized structure typically begins at, for example, a step
where the film is porous due to the nature of a liftoff evaporation
process. It has been observed that a wafer with relatively greater
amount of topography tends to fail more quickly. Focused ion
beam/scanning electron microscopic analysis shows that significant
voids 184 can form in the Cu layer 110 (FIG. 9A) thereby
contributing to the failure. As with the first example, the Cu can
also reach the top 1200A of Au causing the M1 180 to discolor and
also resulting in wire bonding issues (FIG. 9B).
[0126] FIG. 9A shows an example configuration 180 where TiN layers
are absent on both sides of the Cu layer 110. Thus, the example
stack 180 includes layers Ti (162), Cu (110), Ti (182), Au (174)
and Ti (176) formed over an ohmic metal layer 104. FIG. 9A shows
that without a TiN layer between the Cu layer 110 and the Ti layer
162, the Cu layer can form significant voids 184 that contribute to
the degradation of the stack 180.
[0127] Referring to FIG. 9B, with the absence of a TiN layer
between the Cu layer 110 and the upper Ti layer 182, copper from
the Cu layer 110 is also able to reach the Au layer 174. Such an
effect can result in discoloration (e.g., non-uniform patterns on
the surfaces) of the M1 structure 180 and undesirable features such
as wire bonding issues.
[0128] For the example metalized stack 160 of FIGS. 7 and 8, the
first Ti layer 162 can be configured to provide functionalities
such as adhesion and covering of surface irregularities due to, for
example, roughness of the ohmic metal film 104. An example of how
the first Ti layer 162 can be formed is described herein in greater
detail. It will be understood that the thickness of the first Ti
layer 162 can be less than or greater than the example thickness of
approximately 1,000 angstroms. In some implementations, the
thickness of the first Ti layer 162 can be selected to be
sufficient to provide one or more of the foregoing
functionalities.
[0129] For the example metalized stack 160 of FIGS. 7 and 8, the
first TiN layer 164 can be configured to act as, for example, a
barrier that reduces or inhibits migration of gold, copper and/or
other atoms between the ohmic metal layer 104 and the Cu layer 110.
An example of how the first TiN layer 164 can be formed is
described herein in greater detail. It will be understood that the
thickness of the first TiN layer 164 can be less than or greater
than the example thickness of approximately 500 angstroms. In some
implementations, the thickness of the first TiN layer 164 can be
selected to be sufficient to provide one or more of the features as
described herein.
[0130] For the example metalized stack 160 of FIGS. 7 and 8, the Cu
layer 110 can be configured to, for example, yield a desired
resistivity. Because the Cu layer 110 can be configured as a main
conductive layer of the stack 160, the Cu layer's sectional
dimensions (e.g., thickness and/or width) can be selected to
provide a desired conductive property of the stack 160. In the
example stack 160 of FIGS. 7 and 8, the approximately 25,000
angstrom Cu layer 110 was selected to provide a roughly same sheet
resistance associated with an approximately 30,000 angstrom gold
layer which the Cu layer 110 is replacing. An example of how the Cu
layer 110 can be formed is described herein in greater detail. It
will be understood that the thickness of the Cu layer 110 can be
less than or greater than the example thickness of approximately
25,000 angstroms. In some implementations, the thickness of the Cu
layer 110 can be selected to be sufficient to provide one or more
of the features as described herein.
[0131] For the example metalized stack 160 of FIGS. 7 and 8, the
second TiN layer 170 can be configured to, for example, reduce or
inhibit some of the copper from the Cu layer 110 from reaching the
Au layer 174 and yielding discoloration and wire bonding issues
associated with the metalized structure. An example of how the
second TiN layer 170 can be formed is described herein in greater
detail. It will be understood that the thickness of the second TiN
layer 170 can be less than or greater than the example thickness of
approximately 500 angstroms. In some implementations, the thickness
of the second TiN layer 170 can be selected to be sufficient to
provide one or more of the features as described herein.
[0132] For the example metalized stack 160 of FIGS. 7 and 8, the Pt
layer 172 can be configured to act as, for example, a barrier that
reduces or inhibits migration of gold, copper and/or other atoms
between the Au layer 174 and the Cu layer 110. An example of how
the Pt layer 172 can be formed is described herein in greater
detail. It will be understood that the thickness of the Pt layer
172 can be less than or greater than the example thickness of
approximately 500 angstroms. In some implementations, the Pt layer
172 can be formed so as to have a thickness d5 that is at least
approximately 250 angstroms. In some embodiments, the thickness d5
can be significantly larger than the example thickness of 250
angstroms. In some implementations, the thickness of the Pt layer
172 can be selected to be sufficient to provide one or more of the
features as described herein.
[0133] For the example metalized stack 160 of FIGS. 7 and 8, the Au
layer 174 can be configured to, for example, allow wire bonding of
the metalized stack 160 and/or to protect the Cu layer 110 from
oxidation. An example of how the Au layer 174 can be formed is
described herein in greater detail. It will be understood that the
thickness of the Au layer 174 can be less than or greater than the
example thickness of approximately 1,200 angstroms. In some
implementations, the thickness of the Au layer 174 can be selected
to be sufficient to provide one or more of the features as
described herein.
[0134] For the example metalized stack 160 of FIGS. 7 and 8, the
second Ti layer 176 can be configured to, for example, provide a
passivation layer for the Au layer 174, and/or to act as an
adhesion layer for an additional passivation layer (e.g., a nitride
layer) (e.g., 192 in FIG. 8A). An example of how the second Ti
layer 176 can be formed is described herein in greater detail. It
will be understood that the thickness of the second Ti layer 176
can be less than or greater than the example thickness of
approximately 90 angstroms. In some implementations, the thickness
of the second Ti layer 176 can be selected to be sufficient to
provide one or more of the features as described herein.
[0135] In some implementations, a combination of layers that
includes the first Ti layer 162 and the first TiN layer 164 can be
configured to sufficiently reduce or prevent gold in the ohmic
metal (104) from migrating to the Cu layer 110, and copper in the
Cu layer 110 from diffusing into the ohmic metal (104). For a
combination of layers that includes the second TiN layer 170, the
Pt layer 172, and the Au layer 174, such a combination can be
configured to sufficiently reduce or prevent migration and/or
diffusion between the Cu layer 110 and the Au layer 174. Such a
combination can also be configured to be sufficiently robust
mechanically to be able to withstand stress of wire bonding and/or
probing.
[0136] In the example configuration 160 of FIGS. 7 and 8,
dimensions (e.g., thickness) of the various layers can be different
than their respective example values listed in Table 2. For
example, the first Ti layer (162) can have a thickness that is in a
range of 50 to 5,000 angstroms, in a range of 100 to 4,000
angstroms, 200 to 3,000 angstroms, 500 to 2,000 angstroms, or 750
to 1,250 angstroms. In another example, the first TiN layer (164)
can have a thickness that is in a range of 50 to 5,000 angstroms,
in a range of 100 to 2,500 angstroms, 200 to 1,000 angstroms, 300
to 700 angstroms, or 400 to 600 angstroms. In yet another example,
the Cu layer (110) can have a thickness that is in a range of 500
to 50,000 angstroms, in a range of 5,000 to 40,000 angstroms,
10,000 to 35,000 angstroms, or 20,000 to 30,000 angstroms. In yet
another example, the second TiN layer (170) can have a thickness
that is in a range of 50 to 5,000 angstroms, in a range of 100 to
2,500 angstroms, 200 to 1,000 angstroms, 300 to 700 angstroms, or
400 to 600 angstroms. In yet another example, the Pt layer (172)
can have a thickness that is in a range of 50 to 5,000 angstroms,
in a range of 100 to 2,500 angstroms, 200 to 1,000 angstroms, 300
to 700 angstroms, or 400 to 600 angstroms. In yet another example,
the Au layer (174) can have a thickness that is in a range of 100
to 5,000 angstroms, in a range of 200 to 4,000 angstroms, 300 to
3,000 angstroms, 500 to 2,000 angstroms, or 900 to 1,500 angstroms.
In yet another example, the second Ti layer (176) can have a
thickness that is in a range of 20 to 500 angstroms, in a range of
40 to 300 angstroms, 60 to 200 angstroms, or 70 to 110
angstroms.
[0137] In some embodiments, some or all of the layers in the
example configuration 160 of FIGS. 7 and 8 can be formed from
different materials to provide similar functionalities. For
example, the first Ti layer (162) can be substituted with materials
such as chromium (Cr). In another example, the first TiN layer
(164) can be substituted with materials such as any metal nitride
that can be evaporated with ion assist. In yet another example, the
Cu layer (110) can be substituted with materials such as aluminum
(Al). In yet another example, the second TiN layer (170) can be
substituted with materials such as any metal nitride that can be
evaporated with ion assist. In yet another example, the Pt layer
(172) can be substituted with materials such as palladium (Pd) or
nickel (Ni). In yet another example, the second Ti layer (176) can
be substituted with materials such as chromium (Cr).
[0138] In the example configuration 160 of FIGS. 7 and 8, the ohmic
layer 104 is described as being formed from gold. It will be
understood that other types of ohmic layer can also be implemented.
For devices such as MESFET and pHEMT, the ohmic layer can be an
N-ohmic layer. For N-type semiconductors, the ohmic layer can
include nickel, germanium (or gold-germanium AuGe) and/or gold.
Different variations of such materials can be utilized, including
for example, Ni/Ge/Au, Ni/Au/Ge/Au, Ni/AuGe/Au, Ni/Au/Ge/Ni/Au,
Ge/Ni/Au, and AuGe/Ni/Au. For devices such as HBT, BiFET, BiHEMT,
the ohmic layer can be N-ohmic or P-ohmic. The P-ohmic type can
have Au in the layer with other metals such as Pt and Pd. Other
configurations are also possible.
EXAMPLE 3
[0139] FIG. 10 schematically shows a third example configuration
200 of a copper-based metal stack structure 100. This particular
example includes methods related to deposition of various layers,
including the upper portion of the stack structure 200.
[0140] One of the challenges of a Cu M1 scheme is to prevent the
top layers from failing. In the context of wire bonding based
devices, the upper portion of an M1 stack generally needs to be
able to handle the stress associated with wire bonding. In some
situations, mechanical and/or ultrasonic forces associated with
ball bonding can weaken a top barrier such as a Ti/Pt/Au
combination. Film failure can originate from such a weakened site
and propagate.
[0141] A sputtered refractory metal like TiW can function as a good
barrier. However, a sputtered film typically cannot be lifted off
successfully due to its conformal coating property. In some
implementations, such a challenge associated with sputtered film
and subsequent liftoff can be addressed by use of some types of
resists, including those having a re-entrant profile. As described
herein, a deposition of a relatively thick Cu layer (e.g., 25,000
angstroms) using a re-entrant profile can yield a change in a
resist profile to make it possible to sputter a barrier metal such
as TiW with little or no wings or stringers, to thereby facilitate
an effective liftoff process.
[0142] By way of an example, FIG. 10 shows that the metal stack
structure 200 that can result from the foregoing sputtering of a
barrier (such as TiW) and a liftoff process thereafter. The metal
stack structure 200 can include a copper stack 202 (e.g., including
Ti/barrier/Cu/Ti) formed over an ohmic metal layer 104 which is
formed over a substrate 102 such as GaAs. The metal stack structure
200 is shown to further includes a sputtered barrier layer 204
(such as TiW) formed over the copper stack 202. A gold layer 206 is
shown to be formed over the sputtered barrier layer 204. Examples
of how the copper stack 202, sputtered barrier layer 204, and the
gold layer 206 can be formed are described herein in greater
detail.
[0143] FIG. 11 shows a pre-liftoff stage where the barrier layer
(e.g. TiW) 204 has been sputtered on over the copper stack 202, and
the gold layer 206 has been deposited over the barrier layer 204 by
evaporation. A photoresist 212 is shown to have a re-entrant
profile so as to define an opening for the formation of the copper
stack 202. The photoresist 212 is shown to be formed over an ohmic
metal layer 104 which is formed over a compound semiconductor
substrate such as a GaAs wafer 210.
[0144] As shown in FIG. 11, deposition of the copper stack 202 by,
for example, evaporation results in a significant increase in
resist liftoff height by "t" which is approximately the height of
the copper stack 202' formed over the photoresist 212. For the
first and second examples (120, 160) described herein, the added
thickness "t" can be at least 25,000 angstroms. As further shown in
FIG. 11, the evaporated copper stack 202' over the photoresist 212
can generally continue the re-entrant profile of the photoresist
212, thereby reducing the critical dimension of the opening through
which the upper layers (e.g., 204, 206) will be formed. Such a
reduction in the opening's critical dimension can reduce or
substantially eliminate formation of wings during the sputtering of
the TiW barrier layer 204. Such a sputtered TiW barrier 204 can
further allow removal of the metal layer 202' by a liftoff process.
FIG. 12 shows an underside of a metal layer 220 (such as the metal
layer 202') that has undergone a successful liftoff process.
[0145] In some implementations, the copper stack 202 of FIGS. 10
and 11 can include the copper layer 110 of FIGS. 4 and 7, and any
combination of below-copper layers and above-copper layers of the
first and second example configurations 120, 160 of FIGS. 4 and 7.
Dimensions and materials for such implementations can be similar to
those described herein. In some implementations, one or more layers
above the copper layer and/or one or more layers below the copper
layer can be different from the examples described in reference to
FIGS. 4 and 7.
[0146] In the example configuration 200 of FIGS. 10 and 11, the
sputtered TiW layer (204) can have a thickness selected to provide
desired barrier functionality and to facilitate removal of the
metal layer 202' by a liftoff process.
Example Method for Fabrication of the First Example Metal Stack
[0147] FIG. 13 shows a process 300 that can be implemented to
fabricate the example metal stack 120 of FIGS. 4 and 5. FIG. 14
shows various stages of fabrication of the metal stack 120. In
block 302, a compound semiconductor substrate (102) such as GaAs
can be provided. In block 304, an ohmic contact (104) can be formed
over the substrate (102). In some implementations, the ohmic
contact layer (104) can be formed in a known manner.
[0148] In block 306, a first Ti layer (122) can be formed over the
ohmic contact layer (104). In some implementations, the first Ti
layer (122) can be formed by an electron-beam evaporation
deposition process. In some implementations, such a deposition can
start at a vacuum of about 7.5.times.10.sup.-6 Torr; and deposition
rates can be between about 5 to 15 angstroms/sec. A circular beam
sweep pattern of about 2 revolutions per second, as well as a
static beam configuration can be used. This deposition step can be
performed without a crucible. Other deposition methods and
configurations can also be utilized.
[0149] In block 308, a first Pt layer (124) can be formed over the
first Ti layer (122). In some implementations, the first Pt layer
(124) can be formed by an electron-beam evaporation deposition
process. In some implementations, such a deposition can be
performed so as to yield an evaporation rate of about 3
angstroms/sec without a beam sweep. Other deposition methods and
configurations can also be utilized.
[0150] In block 310, a second Ti layer (126) can be formed over the
first Pt layer (124). In some implementations, the second Ti layer
(126) can be formed by an electron-beam evaporation deposition
process similar to that associated with the first Ti layer (122).
Other deposition methods and configurations can also be
utilized.
[0151] In block 312, a Cu layer (110) can be formed over the second
Ti layer (126). In some implementations, the Cu layer (110) can be
formed by an electron-beam evaporation deposition process. In some
implementations, a relatively long cool down time can be provided
to ensure the source material is sufficiently cool before venting
the chamber for service or to add metal. A tungsten (W) or
molybdenum (Mo) crucible can be used for the evaporation of Cu. Use
of such a liner can provide advantageous features such as reducing
thermal contact to the hearth (which is water cooled in some
situations), thereby allowing a higher deposition rate for a given
beam power. In some implementations, a deposition rate of about 8
to 20 angstroms/sec can be achieved with little or no signs of
spitting which is a problem commonly associated with evaporated
gold. Other deposition methods and configurations can also be
utilized.
[0152] In block 314, a third Ti layer (130) can be formed over the
Cu layer (110). In some implementations, the third Ti layer (130)
can be formed by an electron-beam evaporation deposition process
similar to that associated with the first Ti layer (122). Other
deposition methods and configurations can also be utilized.
[0153] In block 316, a second Pt layer (132) can be formed over the
third Ti layer (130). In some implementations, the second Pt layer
(132) can be formed by an electron-beam evaporation deposition
process similar to that associated with the first Pt layer (124).
Other deposition methods and configurations can also be
utilized.
[0154] In block 318, an Au layer (134) can be formed over the
second Pt layer (132). In some implementations, the Au layer (134)
can be formed by an electron-beam evaporation deposition process.
Other deposition methods and configurations can also be
utilized.
[0155] In block 320, a fourth Ti layer (136) can be formed over the
Au layer (134). In some implementations, the fourth Ti layer (136)
can be formed by an electron-beam evaporation deposition process
similar to that associated with the first Ti layer (122). Other
deposition methods and configurations can also be utilized.
[0156] In some implementations, a portion of the third Ti layer
(130) can be formed (e.g., about 100 angstroms) so as to cap the Cu
layer (110) of a wafer. Thereafter, the wafer can be processed
further in a different evaporator for deposition of the remaining
layers (e.g., about 400 angstroms of the third Ti layer, about 500
angstroms of the second Pt layer, about 1,200 angstroms of the Au
layer, and about 90 angstroms of the fourth Ti layer).
Example Method for Fabrication of the Second Example Metal
Stack
[0157] FIG. 15 shows a process 330 that can be implemented to
fabricate the example metal stack 160 of FIGS. 7 and 8. FIG. 16
shows various stages of fabrication of the metal stack 160. In
block 332, a compound semiconductor substrate (102) such as GaAs
can be provided. In block 334, an ohmic contact (104) can be formed
over the substrate (102). In some implementations, the ohmic
contact layer (104) can be formed in a known manner.
[0158] In block 336, a first Ti layer (162) can be formed over the
ohmic contact layer (104). In some implementations, the first Ti
layer (162) can be formed by an electron-beam evaporation
deposition process. In some implementations, such a deposition can
start at a vacuum of about 7.5.times.10.sup.-6 Torr; and deposition
rates can be between about 5 to 15 angstroms/sec. A circular beam
sweep pattern of about 2 revolutions per second, as well as a
static beam configuration can be used. This deposition step can be
performed without a crucible. Other deposition methods and
configurations can also be utilized.
[0159] In block 338, a first TiN layer (164) can be formed over the
first Ti layer (162). In some implementations, an ion assist
technique can be utilized to form a TiN.sub.x layer. Such a
technique of using ion assist evaporation can yield a
Ti.sub.xN.sub.y film (which is typically sputtered reactively). To
achieve such an ion assisted evaporation process, the following
example can be implemented. A chamber vacuum of about
7.5.times.10.sup.-7 Torr can be provided. Nitrogen (N2) can be bled
into the chamber until the chamber pressure reaches about
5.times.10.sup.-5 Torr. Deposition rate of about 2.5 angstroms/sec
can be achieved using an ion source power of about 1,000 V at about
20 mA. Higher ion gun power and other deposition techniques can
also be utilized. In some embodiments, ion assisted evaporation of
TiN can yield a densified barrier film that is more resistant to
failures. Other deposition methods and configurations can also be
utilized.
[0160] In block 340, a Cu layer (110) can be formed over the first
TiN layer (164). In some implementations, the Cu layer (110) can be
formed by an electron-beam evaporation deposition process. In some
implementations, a relatively long cool down time can be provided
to ensure the source material is sufficiently cool before venting
the chamber for service or to add metal. A tungsten (W) or
molybdenum (Mo) crucible can be used for the evaporation of Cu. Use
of such a liner can provide advantageous features such as reducing
thermal contact to the hearth (which is water cooled in some
situations), thereby allowing a higher deposition rate for a given
beam power. In some implementations, a deposition rate of about 8
to 20 angstroms/sec can be achieved with little or no signs of
spitting which is a problem commonly associated with evaporated
gold. Other deposition methods and configurations can also be
utilized.
[0161] In block 342, a second TiN layer (170) can be formed over
the Cu layer. In some implementations, the second TiN layer (170)
can be formed in a manner similar to that of the first TiN layer
(164). Other deposition methods and configurations can also be
utilized.
[0162] In block 344, a Pt layer (172) can be formed over the second
TiN layer (170). In some implementations, the Pt layer (172) can be
formed by an electron-beam evaporation deposition process. In some
implementations, the Pt layer (172) can be formed in a manner
similar to the example described in reference to block 308 of FIG.
13. Other deposition methods and configurations can also be
utilized.
[0163] In block 346, an Au layer (174) can be formed over the Pt
layer (172). In some implementations, the Au layer (174) can be
formed by an electron-beam evaporation deposition process. In some
implementations, the Au layer (174) can be formed in a manner
similar to the example described in reference to block 318 of FIG.
13. Other deposition methods and configurations can also be
utilized.
[0164] In block 348, a second Ti layer (176) can be formed over the
Au layer (174). In some implementations, the second Ti layer (176)
can be formed by an electron-beam evaporation deposition process
similar to the example associated with the first Ti layer (162).
Other deposition methods and configurations can also be
utilized.
Example Method for Fabrication of the Third Example Metal Stack
[0165] FIGS. 17A and 17B show a process 360 that can be implemented
to fabricate the example metal stack 200 of FIGS. 10 and 11. FIG.
18 shows various stages of fabrication of the metal stack 200. In
block 362, a compound semiconductor substrate (102) such as GaAs
can be provided. In block 364, an ohmic contact (104) can be formed
over the substrate (102). In some implementations, the ohmic
contact layer (104) can be formed in a known manner.
[0166] In block 366, a liftoff resist (212) having a re-entrant
profile can be formed over the substrate (102) and a portion of the
ohmic contact layer (104). In some implementations, such a liftoff
resist can be formed utilizing a known technique.
[0167] In block 368, a copper stack having at least a lower barrier
can be formed. Formation of such a copper stack can yield a
trapezoid shaped stack 202 inside the re-entrant profile opening,
and a stack 202' above the resist layer 212. The copper stack 202'
above the resist layer 212 can yield an increased liftoff height,
as well as a reduced critical dimension of the re-entrant opening.
In some implementations, the copper stack (202, 202') can be based
on one or any combination of the examples described herein. In some
implementations, the copper stack (202, 202') can include a
combination of layers having Ti, barrier, Cu, and Ti layers; and
such a stack can be formed by evaporation. Other deposition methods
and configurations can also be utilized.
[0168] In block 370, a TiW layer can be formed over the copper
stack (202, 202') by sputter deposition. Such a sputter deposition
can yield a TiW layer 204 over the copper stack 202. In some
implementations, the reduced critical dimension of the re-entrant
profile opening can allow the sputtered TiW layer 204 to form with
little or no wings or stringers, to thereby allow liftoff of the
metal stack 202'. Other deposition methods and configurations can
also be utilized.
[0169] In block 372, an Au layer (206) can be formed over the TiW
layer (204). In some implementations, the Au layer (206) can be
formed by an electron-beam evaporation deposition process. In some
implementations, the Au layer (206) can be formed in a manner
similar to the example described in reference to block 318 of FIG.
13. Other deposition methods and configurations can also be
utilized.
[0170] In block 374, the metal stack (202') above the resist layer
(212) can be removed by a liftoff process. In some implementations,
such a liftoff process can be achieved in a known manner.
Performance of the Example Metal Stacks
[0171] Each of the three example configurations described herein
(120 in FIGS. 4 and 5, 160 in FIGS. 7 and 8, and 200 in FIGS. 10
and 11) passed a 3-week bake test. More specifically, wafers having
Cu M1 structures according to the foregoing configurations were
kept in a convection oven at about 225.degree. C. for three weeks,
and the resulting Cu M1 structures showed no noticeable
discoloration.
[0172] The first example configuration (120 in FIGS. 4 and 5) was
further subjected to a high temperature operating life (HTOL) test
which is typically a standard reliability test, and is much more
demanding than the foregoing bake test.
[0173] Wafers having Cu M1 structures according to the first
example configuration (120 in FIGS. 4 and 5) were subjected to a
protracted duration under an elevated temperature. Such Cu M1
structures were associated with 2.times.230 .mu.m single-gate pHEMT
devices, and such devices were placed on a hot chuck (temperature
of approximately 200.degree. C.) and biased. The resulting bias
yielded a device temperature of approximately 273.degree. C. More
particularly, Vds of about 5.0 V and Ids of about 35 mA (stressed
as 1.times.230 .mu.m gate) resulted in Tchannel=273.degree. C.
[0174] In some situations, a zero failure of devices at the
foregoing stress conditions at 500 hours implies a reliable
operation (failure rate of 0.1% or less) for at least 10.sup.4
hours of use when operated at Tchannel=125.degree. C.
[0175] FIG. 19 shows plots of parameter (saturation current Idss)
change (in %) for nine different samples having the first example
configuration (120 in FIGS. 4 and 5), where Idss was measured for
each sample over time. As shown, none of the nine samples showed
any significant degradation in Idss for at least 1,400 hours which
is far longer than the foregoing standard HTOL duration of 500
hours.
[0176] Based at least on the foregoing, copper-based metalized
structures having one or more features as described herein can be
configured to substantially survive a high temperature operating
life (HTOL) test at approximately 200.degree. C. for at least 100
hours, 200 hours, 500 hours, 1,000 hours, or 1,400 hours.
[0177] FIG. 20 shows plots of the same parameter (saturation
current Idss) change (in %) for a number of samples (e.g., FIGS. 6A
and 6B) that do not have the barrier functionalities of the first
example configuration (120 in FIGS. 4 and 5). Such barrier-less
samples are shown to fail before a first read point, usually after
about 48 hours of stress.
[0178] FIG. 21 shows an optical image (plan view) of the M1 Cu
structures on one of the samples of FIG. 19, after 1,400 hours of
the HTOL stress. One can see that there is no noticeable signs of
diffusion on the M1 Cu structures.
[0179] In contrast, FIG. 22 shows an optical image (plan view) that
is similar to the image of FIG. 6B, of the copper structures on one
of the samples of FIG. 20, after failing the HTOL stress test. As
in FIG. 6B, the copper structures show significant discoloration
resulting from, for example, diffusion.
[0180] The foregoing failed devices of FIGS. 20 and 22 also include
significant voids in the copper layer when viewed with, for
example, a focused ion beam (FIB) imager. Such voids and related
failures can be similar to those described in reference to FIG.
9A.
[0181] In some embodiments, metalized structures having one or more
features as described herein can be implemented in a compound
semiconductor wafer, which in turn can yield a plurality of dies.
Each of such dies can include an integrated circuit configured to
perform a number of functions. FIG. 23 shows a process 400 that can
be implemented to fabricate a die (410 in FIG. 24) having one or
more metalized structures 100 as described herein. In block 402,
one or more semiconductor structures (412 in FIG. 24) can be formed
on a compound semiconductor substrate such as GaAs. In block 404,
one or more M1 stacks having copper can be formed for such
structures. In block 406, a die having an integrated circuit that
includes the M1 metalized semiconductor structures can be formed.
Although described in the context of metalizing semiconductor
structures (412), it will be understood that copper-based metal
stacks having one or more features as described herein can also be
implemented in other portions (such as a passive device 414) of the
die 410.
[0182] In some embodiments, a die having one or more copper-based
metalized stacks as described herein can be implemented in a
module. FIG. 25A shows an example of such a module 420, and FIG.
25B shows a block diagram representation of the same.
[0183] The module 420 can include a die 410 such as the die
described in reference to FIGS. 23 and 24. Such a die can be
mounted on a packaging substrate 422, and can be protected by an
overmold structure 425. Electrical connections to and from the die
410 can be facilitated by connections 426 such as wirebonds. Such
wirebonds can be interconnected to connection pads 428 formed on
the module so as to facilitate connectivity to other modules and/or
external components.
[0184] In some embodiments, the module 420 can also include one or
more surface-mount devices (SMD) mounted on the packaging substrate
422 and configured to facilitate and/or complement the
functionality of the integrated circuits in the die 410. In some
embodiments, the module 420 can also include one or more packaging
structures to, for example, provide protection and facilitate
easier handling of the module 420. Such a packaging structure can
include an overmold formed over the packaging substrate 422 and
dimensioned to substantially encapsulate the various circuits and
components thereon.
[0185] It will be understood that although the module 420 is
described in the context of wirebond-based electrical connections,
one or more features of the present disclosure can also be
implemented in other packaging configurations, including flip-chip
configurations.
[0186] In some embodiments, a module or a die having one or more
copper-based metalized stacks as described herein can be
implemented in a radio-frequency (RF) device. FIG. 26 shows an
example of such an RF device 430.
[0187] The RF device 430 can include a module 420 such as the
module described in reference to FIGS. 25A and 25B, and/or an
unpackaged die such as the die 410 described in reference to FIG.
24. In some implementations, such a module can facilitate operation
of a transmit/receive circuit 432 and an antenna 434. In some
embodiments, the module 420 can be configured to provide, for
example, amplification of RF signals, switching of RF signals,
and/or other RF related functions.
[0188] In some implementations, a device and/or a circuit having
one or more features described herein can be included in an RF
device such as a wireless device. Such a device and/or a circuit
can be implemented directly in the wireless device, in a modular
form as described herein, or in some combination thereof. In some
embodiments, such a wireless device can include, for example, a
cellular phone, a smart-phone, a hand-held wireless device with or
without phone functionality, a wireless tablet, etc.
[0189] FIG. 27 schematically depicts an example wireless device 500
having one or more advantageous features described herein. In the
example wireless device 500, one or more low-noise amplifiers
(LNAs) 526 are shown to receive RF signals from an antenna 516
through a switch 514 and their respective duplexers 512. Such LNAs
and related channels can facilitate, for example, multi-band
operation of the wireless device 500.
[0190] The LNAs 526 are shown to pass their amplified signals to a
transceiver 510 for further processing by a receiver circuit (not
shown). The transceiver 510 can also generate RF signals for
transmission and pass such signals to power amplifiers (PAs) 511.
Outputs of the PAs 511 are shown to be matched (via match circuits
520) and routed to the antenna 516 via their respective duplexers
512a-512d and the band-selection switch 514. The band-selection
switch 514 can include, for example, a single-pole-multiple-throw
(e.g., SP4T) switch to allow selection of an operating band (e.g.,
Band 2). In some embodiments, each duplexer 512 can allow transmit
and receive operations to be performed simultaneously using a
common antenna (e.g., 516).
[0191] The transceiver 510 is also shown to interact with a
baseband sub-system 508 that is configured to provide conversion
between data and/or voice signals suitable for a user and RF
signals suitable for the transceiver 510. The transceiver 510 is
also shown to be connected to a power management component 506 that
is configured to manage power for the operation of the wireless
device. Such power management can also control operations of the
baseband sub-system 508 and other components.
[0192] The baseband sub-system 508 is shown to be connected to a
user interface 502 to facilitate various input and output of voice
and/or data provided to and received from the user. The baseband
sub-system 508 can also be connected to a memory 504 that is
configured to store data and/or instructions to facilitate the
operation of the wireless device, and/or to provide storage of
information for the user.
[0193] In some implementations, M1 Cu contacts or interconnects
having one or more features as described herein can be implements
in a number of components in the example wireless device 500. For
example, the power amplifiers 511 and the switch 514 are
non-limiting examples of components that can include one or more M1
Cu structures. In another example, a component that includes any
device that has or be connected with an M1 gold contact can benefit
by having the gold replaced by copper as described herein.
[0194] A number of other wireless device configurations can utilize
one or more features described herein. For example, a wireless
device does not need to be a multi-band device. In another example,
a wireless device can include additional antennas such as diversity
antenna, and additional connectivity features such as Wi-Fi,
Bluetooth, and GPS.
[0195] In the context of wireless devices, pseudomorphic high
electron mobility transistor (pHEMT), metal semiconductor field
effect transistor (MESFET), heterojunction bipolar transistor
(HBT), bipolar FET (BiFET), bipolar HEMT (BiHEMT), and surface
acoustic wave (SAW) device (e.g., SAW filter or resonator) are
examples of semiconductor devices that can include or be connected
with Cu M1 structures having one or more features as described
herein. It will be understood that other semiconductor devices
utilized in wireless devices can also benefit from such Cu M1
structures.
[0196] It will also be understood that semiconductor devices
associated with other types of electronic devices can also benefit
from Cu M1 structures having one or more features as described
herein. By way of non-limiting examples, electrical contacts and/or
interconnects associated with devices such as Mach-Zehnder (MZ)
modulator, photovoltaic device, light-emitting diode (LED), laser
diode such as vertical cavity laser (VCSEL) diode, etc. can utilize
one or more Cu M1 structures as a substitute for more costly
structures such as gold M1 structures.
[0197] Unless the context clearly requires otherwise, throughout
the description and the claims, the words "comprise," "comprising,"
and the like are to be construed in an inclusive sense, as opposed
to an exclusive or exhaustive sense; that is to say, in the sense
of "including, but not limited to." The word "coupled", as
generally used herein, refers to two or more elements that may be
either directly connected, or connected by way of one or more
intermediate elements. Additionally, the words "herein," "above,"
"below," and words of similar import, when used in this
application, shall refer to this application as a whole and not to
any particular portions of this application. Where the context
permits, words in the above Detailed Description using the singular
or plural number may also include the plural or singular number
respectively. The word "or" in reference to a list of two or more
items, that word covers all of the following interpretations of the
word: any of the items in the list, all of the items in the list,
and any combination of the items in the list.
[0198] The above detailed description of embodiments of the
invention is not intended to be exhaustive or to limit the
invention to the precise form disclosed above. While specific
embodiments of, and examples for, the invention are described above
for illustrative purposes, various equivalent modifications are
possible within the scope of the invention, as those skilled in the
relevant art will recognize. For example, while processes or blocks
are presented in a given order, alternative embodiments may perform
routines having steps, or employ systems having blocks, in a
different order, and some processes or blocks may be deleted,
moved, added, subdivided, combined, and/or modified. Each of these
processes or blocks may be implemented in a variety of different
ways. Also, while processes or blocks are at times shown as being
performed in series, these processes or blocks may instead be
performed in parallel, or may be performed at different times.
[0199] The teachings of the invention provided herein can be
applied to other systems, not necessarily the system described
above. The elements and acts of the various embodiments described
above can be combined to provide further embodiments.
[0200] While some embodiments of the inventions have been
described, these embodiments have been presented by way of example
only, and are not intended to limit the scope of the disclosure.
Indeed, the novel methods and systems described herein may be
embodied in a variety of other forms; furthermore, various
omissions, substitutions and changes in the form of the methods and
systems described herein may be made without departing from the
spirit of the disclosure. The accompanying claims and their
equivalents are intended to cover such forms or modifications as
would fall within the scope and spirit of the disclosure.
* * * * *