U.S. patent application number 14/601629 was filed with the patent office on 2015-11-12 for device package and methods for the fabrication thereof.
The applicant listed for this patent is Nuvotronics, LLC. Invention is credited to James D. MacDonald, David W. Sherrer.
Application Number | 20150327410 14/601629 |
Document ID | / |
Family ID | 45816804 |
Filed Date | 2015-11-12 |
United States Patent
Application |
20150327410 |
Kind Code |
A1 |
Sherrer; David W. ; et
al. |
November 12, 2015 |
DEVICE PACKAGE AND METHODS FOR THE FABRICATION THEREOF
Abstract
A microwave assembly having a substrate comprising a microwave
device; said device having a die, a first layer having a dielectric
constant between about 1.00 and about 1.45 and a thickness between
about 0.05 and about 2 mm along with one or more layers chosen from
an absorbing layer, an EMI blocking layer, a layer comprising
conductive material or a metal cover.
Inventors: |
Sherrer; David W.; (Cary,
NC) ; MacDonald; James D.; (Chandler, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nuvotronics, LLC |
Radford |
VA |
US |
|
|
Family ID: |
45816804 |
Appl. No.: |
14/601629 |
Filed: |
January 21, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13237931 |
Sep 20, 2011 |
8969132 |
|
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14601629 |
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61521742 |
Aug 9, 2011 |
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61421854 |
Dec 10, 2010 |
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61384374 |
Sep 20, 2010 |
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Current U.S.
Class: |
361/818 ;
427/123; 427/125; 427/58 |
Current CPC
Class: |
H01Q 23/00 20130101;
B05D 5/08 20130101; C23C 28/42 20130101; H01L 2224/48227 20130101;
H01L 2224/48091 20130101; B05D 3/007 20130101; C23C 24/082
20130101; C23C 16/06 20130101; B05D 3/0272 20130101; H01Q 17/00
20130101; H01L 23/564 20130101; H01L 23/047 20130101; H01L 23/24
20130101; C23C 18/42 20130101; C23C 16/45555 20130101; H05K 9/0024
20130101; C23C 14/14 20130101; C23C 16/0227 20130101; C23C 28/042
20130101; H01L 2224/48091 20130101; H01L 2224/49109 20130101; C23C
16/0272 20130101; H01L 23/06 20130101; C23C 18/38 20130101; B05D
1/005 20130101; H01L 2924/00014 20130101; H01L 2924/09701
20130101 |
International
Class: |
H05K 9/00 20060101
H05K009/00; C23C 16/06 20060101 C23C016/06; B05D 1/00 20060101
B05D001/00; B05D 5/08 20060101 B05D005/08; B05D 3/02 20060101
B05D003/02; C23C 18/38 20060101 C23C018/38; C23C 18/42 20060101
C23C018/42; C23C 16/02 20060101 C23C016/02; B05D 3/00 20060101
B05D003/00; C23C 16/455 20060101 C23C016/455; C23C 14/14 20060101
C23C014/14 |
Goverment Interests
[0002] The subject matter of the present application was made with
Government support from the Naval Surface Warfare Center under
contract number N65538-10-M-0114. The Government may have rights to
the subject matter of the present application.
Claims
1. A method of forming a microwave assembly, comprising: a.
providing a microwave circuit comprising at least one microwave
transmission line. b. forming a dielectric layer having a
dielectric constant between about 1.01 and about 2.00 over the
circuit; c. forming an ionic barrier layer comprising a silicone
over the dielectric layer; d. forming an electromagnetic
interference blocking layer over the ionic barrier layer; and e.
forming a second ionic barrier layer comprising a silicone over
said electromagnetic interference blocking layer.
2. A method of forming a microwave assembly comprising: a.
providing a microwave circuit comprising at least one microwave
transmission line; b. forming at least one dielectric layer having
a dielectric constant between about 1.01 and about 2.00 over the
circuit; c. forming a least one electromagnetic interference
blocking layer over the at least one dielectric layer; and d.
forming at least one ionic barrier layer over the at least one
electromagnetic interference blocking layer or the at least one
dielectric layer.
3. The method claim 2, further comprising a rework in which at
least one of the dielectric coating, the conductive coating or the
encapsulating coating is removed and replaced.
4. The method according to claim 2, wherein the at least one
electromagnetic interference blocking layer comprises a conductive
coating, a metal coating, a metallized plastic, a metal lid or
combinations thereof.
5. The method according to claim 2, wherein the at least one
electromagnetic interference blocking layer is a conductive coating
comprising gold, silver, copper, palladium, or platinum.
6. A method of forming a microwave assembly, comprising: a.
providing a microwave circuit comprising at least one microwave
transmission line; b. forming at least one dielectric layer having
a dielectric constant between about 1.01 and about 2.00 over the
microwave circuit; and c. forming at least one ionic barrier layer
over the at least one dielectric layer.
7. The method according to claim 6, wherein the thickness of the
dielectric layer is at least 25 microns.
8. The method according to claim 6, further comprising forming at
least one electromagnetic interference blocking layer over the at
least one dielectric layer.
9. The method according to claim 6, further comprising forming an
electromagnetic interference absorbing layer.
10. The method according to claim 6, wherein the electromagnetic
interference absorbing layer comprises at least one of one or more
electrically lossy materials, one or more magnetically lossy
materials, or a combination thereof.
11. The method according to claim 6, wherein the at least one
dielectric coating comprises at least one of a syntactic foam, an
expanded foam, an aerogel, or a highly porous material formed from
a composite comprising a porogen.
12. The method according to claim 6, wherein the at least one ionic
barrier layer comprises at least one of a silicone, a parylene, a
polyimide, or a curable BCB resin.
13. The method according to claim 6, wherein the at least one ionic
barrier comprises an electronics grade silicone.
14. The method according claim 6, further comprising forming a
dielectric layer on the microwave circuit by aerosol, atomic layer
deposition, or vapor deposition.
15. A non-hermetic package for protecting a microwave assembly
comprising: a. a microwave circuit comprising at least one
microwave transmission line; b. at least one dielectric layer
having a dielectric constant between about 1.01 and about 2.00 over
the microwave circuit; c. at least one electromagnetic interference
blocking layer over the at least one dielectric layer; and d. at
least one ionic barrier layer over the at least one electromagnetic
interference blocking layer or the at least one dielectric
layer.
16. The non-hermetic package of claim 15, wherein the at least one
electromagnetic interference layer absorbs microwave radiation.
17. The non-hermetic package of claim 15, wherein the at least one
electromagnetic interference blocking layer comprises a conductive
coating, a metal coating, a metallized plastic, a metal lid or
combinations thereof.
18. The non-hermetic package of claim 15, wherein the at least one
electromagnetic interference blocking layer comprises a conductive
coating comprising at least one metal chosen from gold, silver,
copper, palladium, platinum or combinations thereof.
19. A non-hermetic package for protecting a microwave assembly
comprising: a. a microwave circuit comprising at least one
microwave transmission line; b. at least one dielectric layer
having a dielectric constant between about 1.01 and about 2.00 over
the microwave circuit; and c. at least one ionic barrier layer over
the at least one dielectric layer.
20. The non-hermetic package of claim 19, wherein the thickness of
the dielectric layer is at least 25 microns.
Description
REFERENCE TO PRIOR FILED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/237,931, filed Sep. 20, 2011, which claims
the benefit of priority of Provisional Application No. 61/521,742,
filed Aug. 9, 2011. U.S. patent application Ser. No. 13/237,931,
also claims the benefit of priority to Provisional Application No.
61/421,854, filed Dec. 10, 2010. Additionally, U.S. patent
application Ser. No. 13/237,931 claims the benefit of priority of
Provisional Application No. 61/384,374, filed Sep. 20, 2010. The
entire contents of all recited U.S. Patents and patent applications
are herein incorporated by reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to device packages,
and in particular to non-hermetic packages for microwave devices
and assemblies. Disclosed herein are protected microwave device
assemblies having a layer or layers; at least one of which has a
low dielectric constant.
BACKGROUND
[0004] Many high end microwave assemblies are still hermetically
packaged, such as transmit and receive modules for phased arrays,
components for defense applications, power amplifiers, assemblies
requiring chip and wire construction, high performance devices and
circuit operating in the upper microwave and mm-wave bands, and so
on. The reasons include ensuring reliability under environmental
variations and a lack of organic or polymer protective layers that
do not reduce or interfere with device performance due to factors
such as dielectric loss or attenuation, or due to changing the
electrical impedance of transmission lines contained in or on the
device. Hermetic packaging has substantial drawbacks. The
substantial cost and yield impact may be attributed to the
specialized nature of the parts used in hermetic packaging, such as
metal or ceramic housings, solderable or weldable lids, hermetic
seals such as glass-metal seals for connectors, and the manual
labor usually required for assembly and test and rework. Meanwhile,
most consumer electronics traditionally operating at lower
frequencies have been able to move to lower cost non-hermetic
packaging through the use of protective coatings, underfill
polymers, encapsulants and the like. Such approaches enable more
automated batch production on large area circuit boards.
Non-hermetic cavity packaging has been done in some cases; however,
in environments where there is high humidity and fluctuating
temperatures, condensation of water can occur inside the package.
In addition, ionic contaminates such as sodium, potassium, and
calcium can come from environmental sources including fingerprints,
the air or salt water can penetrate many non-filled cavity
structures and produce electrical reliability problems such as
conductivity between circuits and/or corrosion. The problems from
such condensation and ionic contaminates can be eliminated by
employing polymer or silicone encapsulations over the electronics
components.
[0005] Traditional approaches used to package devices for consumer
electronics do not work well on microwave devices and the circuit
boards on which they are mounted because of the field interaction
from the transmission lines in the circuits extend into the
surrounding mediums and often extend into the encapsulants or
coatings producing problems such as attenuation, changing
transmission line impedance, and otherwise interfering with the
function of the circuit. For example, a microstrip transmission
line with a coupled-line filter can have substantial field lines
that interact on or around the signal line upper surfaces. The same
is also true of a coplanar waveguide (CPW) transmission line.
Traditional materials such as those based on silicones, acrylates,
and even high performance vapor deposited coatings such as parylene
have substantially higher than air dielectric constants changing
the transmission line impedance or function. Moreover, the loss
tangents of these materials can substantially alter circuit
performance in ways that cause it to deviate from design targets,
particularly at high microwave frequencies. While such materials
can be valuable in non-hermetic packaging, their use in microwave
devices and modules without some means of separating them from the
microwave devices will usually result in detrimental interactions.
To address these problems, some microwave devices such as
monolithic microwave integrated circuits (MMICs) and circuit boards
have been designed with buried transmission lines to minimize field
interactions with the surrounding environment, specifically with,
for example, underfill materials or encapsulants. While this
addresses the problem of the surrounding environment substantially
changing their function, it does not enable optimal performance as
the losses in many semiconductors and circuit board materials are
high and typically increase with frequency. As frequency moves up
from 2 to 10 to 40 GHz or above the problem of losses in packaging
materials becomes increasingly demanding. Accordingly, there
remains a need to have a low cost non-hermetic packaging technology
for devices containing microwave circuitry with transmission lines
for example such as microstrip or CPW or any primarily
air-dielectric or suspended transmission lines, including
waveguides and air-dielectric coaxial transmission lines, that
allows RF, microwave, and mm-wave components, devices, and
assemblies containing such transmission lines to operate with
minimal interaction with the dielectrics surrounding them while
protecting them from the environment.
[0006] A possible solution to this problem, as further disclosed
herein, is the use of a very low-k layer of material, such as a
foam, that does not substantially interfere with the operation of a
circuit designed for operation in air or in a vacuum environment.
Such a layer can be applied thick enough to minimize field
interactions, for example 0.5 to 2 mm or more thick, and can be
used as a "spacer layer" to an outer protective set of layers.
Exceptionally low-k materials, such as expanded urethane foams,
sol-gels, aerogels, porogen filled polymers, and syntactic foams
with low dielectric constants, for example, below approximately
two, can serve as a spacer layer without adversely affecting all
but the most sensitive devices (such as, for example, high Q or
narrow band pass filters). Unfortunately, such low-k foams as
currently exist are usually also porous and permeable to moisture
and ionic contaminant penetration. A solution to this is to seal
the low-k material using one or more sealing layers. U.S. Pat. No.
6,713,867 B2 to Mannak et al. discloses a syntactic foam protected
by a "moisture proof top layer" identified as a "polymer lacquer".
U.S. Pat. No. 6,713,867 B2 does not, however, identify any
candidate materials to satisfy this requirement and does not
identify the need for an ionic barrier or more than one sealing
layer. It also does not identify the importance of choosing low
ionic contaminate materials or materials based their ability to
resist contaminants such as sodium, potassium, and other ionic
conductors with high mobility. There are many polymer lacquers that
would not work well and many polymer lacquers that would allow
moisture to penetrate. For example, RTV silicone contains ionic
contaminants. Polyacrylic acid, polyvinyl alcohol, polyvinyl
pyrolidone, phenolic resins such as novalaks or anything with a
hydroxy group would be poor choices due to their ionic
mobility.
[0007] In addition, the reference does not identify various
improvements to the art to enable its practical use. Such
improvements include, for example, the use of adhesion layers
between the silver paints and ionic barriers such as, for example,
electronic grade silicones; the need to protect conductive paint,
for example silver-filled conductive paints which can be used for
EMI shielding, from corrosion; the use of EMI absorbing layers such
as, for example, graphite filled silicones; or the use of secondary
protection layers such as ALD coatings on circuits. Thus, despite
the fact that syntactic foams have been available since the 1960's
and other low-k materials, such as aerogels, have been available to
the packaging industry for over a decade, these materials have
still not found use for microwave device packaging with the
exception of spacer layers in antenna construction. Thus, many high
performance microwave devices for critical applications in airborne
and marine environments, such as T/R modules, radar modules, chip
and wire assemblies, still require expensive hermetic packages.
[0008] U.S. Pat. No. 6,423,566 B1 to Feger et al. discloses a
method to protect the interconnect layers of a semiconductor chip
or wafer where the electrical interconnect layers of the device are
contained in the dielectric, said dielectric including materials
that include a low-k dielectric. Disclosed are a number of possible
thin polymeric barriers that can be applied to the wafer or chip
after manufacturing it to protect the exposed portions of
dielectric materials in the interconnect layers from ionic
contamination and moisture ingress. While U.S. Pat. No. 6,423,566
B1 discloses a polymeric barrier to protect a dielectric disposed
on an interconnect structure that may include a low-k material, it
does not teach a packaging method or technology or structure for
circuits or assemblies or the use or addition of low-k materials
applied to a device at a thickness, for example typically 500 to
700 microns, sufficient to prevent deleterious field interactions
at a distance from the device's RF or microwave transmission lines
to prevent subsequent layers from interfering with the device
performance. In the reference, the low-k material was formed as
part of an electrical interconnect structure that totals on the
order of 5-15 microns in thickness and is instead formed as part of
the integrated circuit manufacturing process and is therefore not
applied as a component of a device packaging technology.
Furthermore, it does not teach the application to microwave devices
or microwave device packaging. It does not teach the use of
multiple layers for EMI blocking or attenuation. In addition, the
ionic and moisture barrier layers taught in the reference are less
than one micron thick whereas the ionic and moisture barrier layers
required for RF packaging are typically on the order of 100-500
times thicker. Finally, while a RF or microwave device is not
taught, if the semiconductor device in U.S. Pat. No. 6,423,566 B1
was, in fact, a microwave device and it was being packaged on a
circuit board, it would still require the solution for non-hermetic
packaging taught herein, i.e., providing thick low-k layer, ionic
sealing layers and EMI blocking layers or absorbing layers as no
provisions are made to solve EMI coming from the device itself or
from a circuit to which it is attached.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates a cross section of a microwave assembly,
wherein a microwave device is mounted on a circuit board, in
accordance with an embodiment disclosed and claimed herein.
[0010] FIG. 2 illustrates a cross section of a microwave assembly,
in accordance with an embodiment disclosed and claimed herein.
DETAILED DESCRIPTION OF THE DRAWINGS
[0011] In accordance with one aspect of the invention, FIG. 1 shows
a cross section of an example microwave circuit assembly of a
chip-in-board construction using a low-k material, wherein an
active microwave device, 10, is thermally coupled to a heat
spreader or CTE matching layer, 15. For small devices and in
certain constructions 15, may be omitted. In this embodiment, the
heat spreader and devices are mounted on a circuit board or
substrate, 5. 5 may be, for example, a multi-layer circuit board, a
Au/Ni plated copper substrate, a brass or Aluminum substrate, a
ceramic such as Alumina or glass, a semiconductor or all or part of
a wafer, or LTCC or HTCC. The thickness of 5 may typically be on
the order of 0.1 mm to several mm. Also shown in this embodiment,
in schematic form, are various functional layers of the microwave
assembly, including layers designed to carry radio frequency (RF)
signals, 20, layers designed to carry direct current (DC), 25, for
example to power the active devices and other layers such as
insulating layers and/or a laminated frame to surround the
microwave device(s), 30. The active microwave device may be
connected to the RF layer by one or more wirebonds, 50, wherein a
plurality of the wires, 50, may carry the same or different
signals, and to the DC layer by wires 45, wherein a plurality of
wires, 45, may carry the same or different voltages. While
wirebonds are shown, the device may alternatively be mounted in the
recess in a flip-chip configuration onto layer 20 or 25 and the
functions of 20 or 25 may be combined into one layer. In this
embodiment, the active microwave device is shown covered by a layer
of a dielectric layer having a low dielectric constant, 55. Such
material, in the case of a syntactic foam, may be vibrationally
compacted and, in the case of a fluid precursor, may be dispensed
into the recess. Disposed atop the dielectric layer having a low
dielectric constant may be several layers represented by 35, each
of which may comprise one or more layers chosen from an
electromagnetic interference (EMI) absorbing, an EMI blocking layer
being electrically conductive, such as a conductive coating or
metal cover, 1, one or more of an adhesive layer between layers to
promote adhesion, and a thin secondary barrier such as a ALD
dielectric layer. In addition, 35 will contain a sealing layer that
operates as an ionic barrier layer and both the ionic barrier and
the low-k material, 55, will themselves have low levels of ionic
contamination. Also shown in this embodiment is a representation of
surface mount components, 40, which may be resistors, capacitors,
surface mount active devices, integrated circuits and/or the like.
Such support devices may also be located in the low-k recess with
the device, 10. Not shown are connectors, wirebondable interfaces,
BGA mounting regions, and other I/O that may be present on the
assembly to interconnect the assembly to a higher level
circuit.
[0012] In accordance with another aspect of the invention, FIG. 2
shows a cross section of a microwave assembly, which includes a
circuit board or substrate as described in FIG. 1, 75, optionally
coated with a dielectric layer or multiple dielectric layers by
atomic layer deposition (ALD) or other suitable coating method, 80.
ALD may optionally be used in the example of FIG. 1. The ALD layer
may serve as a secondary barrier layer to protect against corrosion
and ionic contamination and/or moisture penetration in the event of
permeation of the low-k and other coating layers and specifically
to protect the devices from a local failure of the primary barrier
such as a pinhole or a delamination. An example of such an ALD
layer would be 20 alternating layers of zirconia and silica of 2 to
20 nm thickness applied by vapor depostion. Further illustrated in
FIG. 2 are components mounted on the board, 85, a material having a
low dielectric constant 65, an optional RF absorbing layer, 60, an
optional conductive layer 55, and an optional ionic barrier
encapsulant, 115. The circuit board may further have a metal or
metallized plastic over mold, 70, around the edges or sides or
around the top surface of the board and over any edge coaxial or DC
connectors, 90 or around any edge connectors or interconnect
structure formed of the boards metallization. Such overmolding or
laminated frame can be used to create a tub structure or structures
to contain one or more of the coating materials and provide
containment for its dispensing. Further, the circuit board may have
other passive microwave components such as microstrip electrodes,
95, disposed over a ground plane, 100, or stripline electrodes,
105, isolated between two ground planes, 110. Optional layers not
shown are phenolic adhesion layers on either side of the conductive
EMI blocking layer, 55, which may improve adhesion and corrosion
resistance, for example when a silver-filled paint is used. Other
optional layers not shown are an additional ionic sealing layer,
for example an electronics grade silicone applied directly to the
low-k layer, 65. The use of such layer being dependent on the
materials chosen for the low-k material and EMI blocking layer.
DETAILED DESCRIPTION
[0013] As used herein, the conjunction "and" is intended to be
inclusive and the conjunction "or" is not intended to be exclusive
unless otherwise indicated. For example, the phrase "or,
alternatively" is intended to be exclusive.
[0014] As used herein, the term "dielectric" refers to an
electrically insulating material having an electrical conductivity
less than about 10.sup.-10 Siemens per meter. Further
characterizing a dielectric is the dielectric constant, defined by
the permittivity of the substance divided by the permittivity of
free space.
[0015] As used herein, when referring to a coating "on" a specific
layer, it includes a coating directly onto the specified layer and
may include one or more additional layers between the coating and
the specified layer.
[0016] As used herein, when referring to microwave devices and
microwave frequencies, we intend for the term "microwave" to be
used in its broadest sense to include all devices and frequencies
from RF to sub-millimeter wave.
[0017] Disclosed and claimed herein is a method for
non-hermetically protecting a device or assembly containing a
microwave transmission line structure. The protected apparatus may
include a substrate with a microwave device or transmission line
and/or may include a die. The non-hermetic protection is created by
forming a low-k dielectric layer, having a dielectric constant
between 1.01 and 2 and preferably between 1.2 and 1.6, over the
device or assembly and subsequently forming one or more ionic
barrier layers over the dielectric layer. The dielectric layer is
applied to a thickness of between about 0.05 mm and about 5 mm. The
thickness will be based on the distance from the transmission lines
that the fields produce interactions and such that the low-k layer
is thick enough to contain said fields to an extent that any
subsequent layer does not substantially degrade the performance of
the device or assembly. The ionic barrier layer, for example
electronics grade silicone, is disposed on the surface of the low-k
material and is of a thickness from 0.025 mm to several mm and
sufficiently thick to ensure there are no holes in the material.
The ionic barrier layer(s) function to substantially prevent the
penetration of water vapor such that the permeation does not
interfere with the operation of the device or assembly within its
operating parameters over its operating lifetime and to
substantially slow the ingress of ions such as sodium, potassium
and the like. Optional layers which can provide additional
protection and serve other functions depending on the device or
assembly being protected include: one or more EMI blocking layers,
one or more EMI absorbing layers, and/or one or more layers of a
thin dielectric applied on the device or assembly by aerosol,
atomic layer deposition, ionic self-assembly monolayer or another
thin film deposition process. The coating of this secondary thin
dielectric layer must be relatively conformal and be applied in a
sufficiently thin layer to not impact the device operation in a
deleterious manner. Functions of layers may be combined. For
example, the ionic barrier layer(s) may also have properties that
allow it to block or absorb EMI, as may be the case for conductive
or resistive particles in a silicone binder.
[0018] Further disclosed and claimed herein is a non-hermetic
package for protecting a device or assembly containing a microwave
transmission line structure constructed with the method outlined
above.
[0019] Further disclosed to the above methods and structures are
adhesion promoting layers or barrier layers which may be used
between layers if the layers themselves do not adhere well or may
be reactive between them. For example, silver containing conductive
paints serving as EMI blocking layers may not adhere well to
silicone layers serving as moisture and ionic blocking layers. An
adhesion promoting layer such as a phenolic resin may be applied
between the layers. All layers in this case may be applied for
example by brushing, spraying, and/or casting.
[0020] As will become evident, various modifications and
enhancements of the above embodiments are within the scope of the
subject matter disclosed and claimed herein.
[0021] In accordance with the above, microwave devices can be
active or passive. Active microwave devices are designed to operate
at frequencies between about 300 MHz and about 300 GHz or more and
include, without limitation, monolithic microwave integrated
circuits (MMICs) and devices comprising discrete transistors or
diodes. Such devices may be formed in silicon or other
semiconductor materials such as Gallium Arsenide, Germanium,
Silicon/Germanium, Indium Phosphide, Gallium Nitride or other
semiconductor materials. Functionally, active microwave devices
include, without limitation amplifers, transistors, equalizers,
integrated circuits, rectifiers, and similar. Passive microwave
devices include, without limitation, isolated electrodes such as
microstrip waveguides, coplanar waveguides, as well as hollow
waveguides, resonators, filters, delay lines but do not comprise
layers between antennas. Moreover, the microwave assembly may
contain non microwave components for power conditioning,
interfacing and the like. In addition, microwave components can be
made with normal metals or with superconductors. While normal metal
technology is more mature, superconductive components often exhibit
advantages in size, speed, and signal fidelity.
[0022] Materials having low dielectric constants of between 1 and
about 2.0, referred to generally herein as "low-k materials"
include aerogels, syntactic foams, expanded foams, materials formed
from the decomposition of porogens and the like. The dielectric
constant of a porous low-k material depends on the degree of
porosity. Generally, the more porous the material, the lower the
dielectric constant. However, other factors may affect the
dielectric constant such as the presence of solvent impurities,
moisture, and ionic materials such as salts, acids and bases and
the nonuniformity of the porous low-k material. Accordingly, in one
example, dielectric constants may be between 1 and about 1.7. In
another example, dielectric constants may be between 1 and about
1.5. In still another example the dielectric constant of a given
porous low-k material may be between 1 and about 1.46.
[0023] Aerogels describe a class of material based upon a low
density, open cell structure with large surface areas (often 900
m.sup.2/g or higher) and nanometer scale pore sizes of about 5 nm
to about 50 nm. A variety of different aerogel compositions are
known. These may be inorganic, organic and inorganic/organic
hybrids (see N. Husing and U Schubert, Angew. Chem. Int. Ed. 1998,
37, 22-45). Inorganic aerogels are generally based upon metal
alkoxides and include materials such as silica, carbides, and
alumina. Organic aerogels include, but are not limited to, urethane
aerogels, resorcinol formaldehyde aerogels, and polyimide aerogels.
Organic/inorganic hybrid aerogel were mainly organically modified
silicate (organically modified silica or "ormosil"). In these
materials, the organic and inorganic phases are chemically bonded
to each other. Methods of making silica aerogels having low
dielectric constants are described infra.
[0024] Silica aerogels are formed from sols. Silica (SiO.sub.2) sol
may be prepared by a two-step process involving acid and base
catalysts with tetraethoxysilane (TEOS) as a precursor and
isopropyl alcohol (IPA) as a solvent with the sol composition of;
TEOS:H.sub.2O:NH.sub.4OH:HCl:IPA=1.0:4.0:8.2.times.10.sup.-3:1.8.times.10-
.sup.-4:3.0 in molar ratio. This sol is spin-deposited on the
desired substrate using a commercial photoresist spinner in the
optimized viscosity range at a spin rate of 2000 to 7000 rpm,
depending on the desired thickness. Spin deposition is conducted
under an atmosphere saturated with IPA to minimize solvent
evaporation from the SiO.sub.2 gel. The substrate with the spun-on
film is then immersed in IPA and placed in an autoclave at
450.degree. C. and annealed under vacuum for 2 hours.
[0025] Other methods of making aerogels are known in the art. For
example, in U.S. Pat. No. 6,380,105, Smith et al. describe the
following method: In a 500 ml flask are combined, 61.0 ml (0.275
mol) tetraethoxysilane (TEOS), 61.0 ml (0.835 mol) glycerol, 4.87
ml (0.27 mol) water, and 0.2 ml (2.04.times.10.sup.-4 mol) of 1 M
HNO.sub.3. The mixture is then refluxed for 1.5 hours at 60.degree.
C. to form a stock solution. After cooling, the solution may be
diluted with ethanol to reduce its viscosity. One suitable stock
solution: solvent volume ratio is 1:8. However, this ratio will
depend upon desired film thickness, spin speed, and substrate. This
is mixed vigorously and typically stored in a refrigerator at
7.degree. C. to maintain stability until use. The solution is
warmed to room temperature prior to film deposition.
[0026] To produce a film, 3-5 ml of the above precursor sol are
dispensed at room temperature onto the substrate, which is then
spun between 1500 and 7000 rpm (depending on desired film
thickness) for about 5-10 seconds to form sol thin film. During and
after this deposition and spinning, the ethanol (from the
decomposition of TEOS), water, and the nitric acid are evaporating
from film, but due to glycerol's low volatility, no substantial
evaporation of the glycerol occurs. This evaporation also shrinks
thin film and concentrates the silica content of the sol, forming a
film of reduced thickness film shows a reduced thickness sol film
obtained after substantially all (about 95% or more) of the ethanol
has been removed.
[0027] The ratio of silica to fluid can be derived approximately
from the TEOS to glycerol ratio in the as-deposited sol (with minor
changes due to remaining water, continued reactions and incidental
evaporation). As this method largely prevents the gel from
permanently collapsing, this ratio determines the density of the
aerogel film that will be produced from the sol thin film. Aging is
accomplished by allowing the substrate and gel to sit for
approximately 24 hours at about 25.degree. C. or by heating it to
130-150.degree. C. for about 1 minute in a closed container.
[0028] To form a film of lower dielectric constant, the aged film
may be dried without substantial densification by using solvent
exchange to replace the aging fluid (glycerin) with a drying fluid
and then air drying the film. The first step replaces the aging
fluid with an intermediate by dispensing approximately 3-8 mL of
ethanol at room temperature onto aged thin film and spinning the
wafer between approximately 50 and 500 rpm for about 5-10 seconds.
This step is repeated several times as required to eliminate the
aging fluid. The second step replaces ethanol with a drying fluid
such as heptane. This step comprises dispensing approximately 3-8
mL of heptane at room temperature onto aged thin film. The wafer is
spun at approximately 50 to 500 rpm for about 5 to about 10
seconds. This step is repeated several times as required to
eliminate the ethanol. The heptane is allowed to evaporate from the
wet gel, forming a dry nanoporous dielectric. Alternatively, the
wet gel can be dried directly from ethanol. After room temperature
drying to remove most of the ethanol (or heptane), the film is
baked in a convection oven at 300.degree. C. for 15 to 60 minutes
to remove any residual materials. The theoretical dielectric
constant (before surface modification) of this embodiment is
estimated to be about 1.3.
[0029] Syntactic foams are composite sparse materials synthesized
by filling a metal, polymer or ceramic matrix with hollow particles
called microballoons. The presence of hollow particles results in
low density and a low dielectric constant. These materials can be
tailored to a given application by selecting from almost any metal,
polymer or ceramic. A wide variety of microballoons are known,
including cenospheres, glass microspheres, and carbon and polymer
microballoons. The most widely used and studied foams are glass
microballoon-epoxy, glass microballoon-phenolic, glass
microballoon-polystyrene, glass microballoon-polyurethane glass
microballoon-polysilsesquioxane and the like. Glass microballoons
can be formulated into solutions of virtually any polymer binder
material. Glass microballoons suitable for this purpose include
ECCOSPHERES.RTM. available from Trelleborg Offshore Boston,
Inc.
[0030] Expanded foams can comprise various polymers, including,
without limitation, polystyrene, polyethylene, polyurethane,
polypropylene polysilsesquioxane and the like. Several formulations
capable of producing cured expanded foams are commercially
available, for example TAP X-30.RTM., available from Tap Plastics
incorporated.
[0031] Matrix materials such as organic and silicon containing
polymers such as silsesquioxanes may have their dielectric
constants lowered by using porogens. Usually, porogens are
nanometer-sized polymer materials, formed as particles, that
decompose upon heating, leaving a pore in the matrix material.
Commercially available porogen containing materials have dielectric
constants of 3 or less. However, one commercially available
material is LKD-6103.RTM., available from JSR-Micro Inc., with a
dielectric constant of about 1.91.
[0032] Ionic barriers may be comprised for example of one or more
of an electronics grade silicone, a parylene, a polyimide, and/or a
curable BCB resin. The low-k layers may be comprised of one or more
of a syntactic foam, an expanded foam, and aerogel, and/or a highly
porous material formed from a composite material and optically
containing or comprising a porogen.
[0033] EMI blocking layers are differentiated from EMI absorbing
layers in the blocking layers having a conductive property where as
the absorbing layers have a attenuating property through a
thickness and do not substantially produce a reflection at their
surface in comparison to a blocking layer. EMI blocking layers may
be a metal lid or cover, a metallized plastic lid or cover, a
conductive paint, a sputtered or evaporated coating, or a
conductive coating deposited from a liquid such as a electroless
metal coating. Such conductive coatings or paints may contain one
or more of silver, gold, copper, palladium or platinum or may be
comprised of a conductive polymer or plastic material. EMI
attenuating layers serve as absorbers to prevent resonances and
reflections inside the cavity formed by the low-k layer and may be
comprised of one or more of a resistive film, a resistive coating,
a ferrite and/or carbon filled silicone, an iron containing
compound, a carbon nanotube composite, or any electric field and/or
magnetic field attenuating material which may be cohesively held in
a binder or support matrix as a silicone, urethane, or epoxy.
[0034] Other layers may be applied on or over the low-k material.
For example, a metal lid or metallized plastic cover or case may be
used to cover portions of the circuit. Further, a metallic or
conductive coating, such as a silver filled paint can be used to
introduce electromagnetic interference (EMI) shielding to the
circuit. In some cases, such a metal lid or coating may produce a
resonant cavity when combined with the low-k coating. Further, a
microwave absorbing layer may be used either in conjunction with
the EMI shield between it and the low-k layer to reduce resonance
or in place of the EMI shield in circumstances where interference
with the outside environment or adjacent circuits is negligible.
This absorbing layer may be a resistive film or coating such as a
resistive metal layer, a composite material such as a carbon filled
foam or carbon filled polymer. For example, a carbon or
ferrite-filled silicone disposed over the low-k layer may serve as
an absorber to suppress cavity modes in the low-k cavity region. It
may be comprised of a magnetic material or ferrite or ferrite
powder. It may be comprised of a combination of electrically and
magnetically lossy materials at the frequencies that need to be
attenuated in the circuit. The absorber coating may be used in
conjunction with a conductive or resistive coating or metal case to
further suppress EMI. Other microwave absorbing materials may be
formulated with a binder polymer or foam material, such ground
ferrites absorbing ceramics or other resistive materials. Such
materials may be integrated into a silicone to provide a ionic
barrier. Suppressing EMI may further be accomplished by mechanical
design of the encapsulant, for example, by selecting a geometry
that eliminates or modifies resonant effects to ensure they occur
at frequencies that do not interfere with circuit operation. For
example, forming, imprinting, molding, or stamping a pattern into
the low-k material, the absorber material or the conductive or
resistive coating may be used to diminish resonance in a given
interfering frequency range or change the resonant frequency so
that it is outside the interfering frequency range. As a particular
example, stamping a series of pyramids into the surface of the
low-k material, and then applying the absorber material may be used
to reduce the Q-factor, or reciprocal of the loss tangent,
characteristic of the interfering frequency, or move it out of
band. Yet another design approach is to partition a particular area
containing multiple circuits into a series of smaller areas, or
vice versa, to avoid interfering frequencies or move resonances out
of the bands of interest.
[0035] Protection layers for the low-k materials and for the
microwave devices may be used to improve the robustness of the
overall package. Atomic layer deposition can be used to deposit
materials or nano-layer composites such as TiO2, Al2O3, SiO2, ZrO2
and combinations thereof or other materials may be used to protect
active and passive microwave devices from moisture or other
chemical attack. These coatings may be used in the approach
disclosed herein. Such coatings may be applied to sensitive
components before mounting or may be applied to the circuits after
assembly. However, these coatings alone may be relatively thin or
may not substantially prevent the field interaction issues
presented supra. Condensation of moisture or contaminants on an
active or passive microwave device, for example, can reduce the
desired circuit performance in a non-hermetic package. Thus such
protection layers are complementary to other approaches described
herein. The low-k material can also be protected with overcoatings
such as the absorber, conducting layers, or by sealing the low-k
material in vapor deposited materials such as a paralene vapor
deposited layer.
[0036] Before application of the coatings, a cleaning process can
be used to create a clean ionic free environment beneath the ionic
barrier layer disposed over the low-k material ensuring the low-k
filled cavity produced under the barrier layer is free of
contaminants. This cleaning process typically will involve a rinse
in 18 mega-ohm resistance DI water and performed until the rinse
water achieves an equivalently high 18 mega-ohm resistivity
endpoint as can be monitored using a conductivity meter in the
rinse water path. This or a similar cleaning process to ensure all
salts and other ionic contaminates are removed before sealing the
components under an ionic blocking layers such as electronics grade
silicone is necessary to ensure such contaminants don't interfere
with device performance or reliability though changes in
conductivity between transmission lines or device corrosion.
Additional cleaning steps may be utilized such as plasma cleaning
in an argon oxygen plasma, where such plasma cleaning can promote
adhesion of the layers being applied by eliminating hydrocarbons
and activating the exposed surfaces.
[0037] In a preferred embodiment, the device or assembly may be
coated in a secondary barrier applied directly on the device,
assembly, circuit-board or housing and may consist of a ALD (atomic
layer deposition) coating of 20 alterating zirconia and alumina
nanolayers of 5 nm each which serves as a secondary protection.
This may be followed by layer of Eccostock FFP syntactic foam made
by Emerson and Cumming and applied to a thickness of 0.5 to 7 mm
depending on the frequency of operation and structures being coated
and for example may be 2 mm thick on a 10 GHz circuit containing
PWB, MMICs and wirebonds. The foam may be compacted using a
vibration table and then baked at 120 C for an hour in air or as
necessary to fuse the coatings on the syntactic foam. Next a 0.5 mm
thick electronics grade silicone may be applied directly to the
syntactic foam and baked. This can be followed by an acid resistant
solder mask applied to approximately 25 microns thickness and baked
to provide both a layer of protection and adhesion for a subsequent
conductive silver paint, also applied by a painting process to
approximately 25 microns thickness and which serves as a EMI
blocking or shielding layer. The coating is baked to dry and cure
it. The EMI shielding layer is prepared in a way to ensure it is in
electrical contact with the ground plane of the circuits or device
at one or more locations around the perimeter of the syntactic
foam. A second adhesion layer (the acid resistant solder mask) is
next applied and cured by baking and then a final additional ionic
and moisture barrier, such as silicone, is applied at a thickness
of 0.5 mm thickness and baked until cured at 120 C for an hour or
according to the manufacturer's directions.
[0038] In the context of mass transport, permeability is defined as
the product of solubility and the coefficient of diffusion. Hence,
polymer coatings are known that are hydrophobic and thus
impermeable to water but permeable to ionic species. Further,
polymer materials are known that are both hydrophobic and ion
impermeable. Ionic species comprise cations and anions. Cations
include, without limitation, lithium, sodium, potassium, cesium,
magnesium, calcium, cupric, cuprous, ferric, and ferrous ions.
Anions include, without limitation, the halides such as fluorides,
chlorides, bromides and iodides as well as hydroxides, nitrates,
sulfates, acetates and the like.
[0039] Examples of water impermeable but ion permeable polymer
materials include ion porous polytetrafluoroethylene film
(PTFE-commonly known as Teflon.RTM. or Fluon.RTM., marketed as
Tetratex.RTM. or Gore Fabric.RTM.) as well as water swellable
polyurethane or polyurethane/polyol copolymer, for example,
Scotch-Seal Chemical Grout 5610, manufactured by 3M Corporation, or
a water soluble polyacrylamide Cyanagel 2000, available from CYTEC
Technology Corporation. Examples of polymers that are both water
impermeable and ion impermeable include, without limitation,
Amorphous copolymers of PTFE and perfluoro
(2,2-dimethyl-1,3-dioxaole), available from DuPont,
polychlorofluoropolymers, available from Aclon, olynorbornenes,
polyphenylenes, silicones, and parylenes. Thus, to enable a
reliable non-hermetic package using a low-k materials a sealing
layer containing both properties of being an ionic barrier and a
moisture barrier are important to providing a primary protective
coating to the circuits.
[0040] Application of these approaches may be used for packaging
chips where a microwave capable lead-frame or a package such as a
quad-flat-no lead (QFN) package is used. The methods described
herein may advantageously be used, instead of using the more
expensive and difficult hermetic or semi-hermetic packages, because
it is difficult to package a device in such a way that one obtains
a hermetic bond between different materials such as plastics and
metals that prevents diffusion of gasses or moisture through seals,
adhesive layers, and plastics.
[0041] Circuit boards used in microwave assemblies may include
several features such as recessed topographies on the upper and
lower surfaces, embedded passive devices, ground planes, metal
cladding and heat spreaders mounted on the board to accommodate
heat generating devices, particularly active microwave devices.
[0042] It may be desirable to employ heat spreaders to dissipate
heat generated by various microwave devices. Heat spreaders
comprise thermally conductive materials. Such thermally conductive
materials may include, without limitation, silver filled, metal
filled, ceramic filled materials or thermally conductive epoxies
and thermally reworkable adhesives, such as those available from
AIT Technology, Masterbond, or Diemat. For example, Diemat has a
thermal conductivity of 17 W/mK in a reworkable
thermoplastic/thermoset material 4130HT and has indicated that
materials of 25 W/mK are available for sampling as well as 6030 H,
which has a thermal conductivity of 75 W/mK. This approach allows
the use of heat-spreaders having thermal expansion coefficients
that are more closely matched to the die, for example Cu/Mo, Cu W,
synthetic diamond, and laminates thereof.
[0043] Advantages of having a metal clad board may also include
enabling the bonding of the board directly to a secondary heat
sink, which for high power applications may be passively air
cooled, forced air cooled, liquid cooled, or cooled by refrigerant.
Such a heat sink may be mechanically clamped or permanently bonded
with a thermally conductive adhesive or solder, or cooling channels
may be directly created in the metal of the board if it is suitably
thick.
[0044] Portions of the circuit board upper and lower surfaces may
be recessed, for example as illustrated on the top surface in FIG.
1. Such topography may enable a controlled volume of low-k material
to be used and for it to be confined on its sides to provide
mechanical forming and structural support Further, a recessed
topography can reduce profile, or provide a direct thermal path for
dissipating heat generated by a device. Notwithstanding the
foregoing, mounting on a top surface of a board may also be used,
for example, when there is limited heat generated, such as in a
diode or low noise amplifier or when fabrication simplicity is
desired. Alternatively thermal vias may be used to heat-sink
devices mounted on dielectric layers on a boards surface. In such
cases where a low-k material is to be used on upper board surface
or where a low-k material is used to mold around a device or
circuit on one or more surfaces a die or mold may be used to
compress or form the low-k material into a desired shape. The
material may be deposited and then molded or may be injected into
the die or mold at the time the mold or die is placed on or over or
around the circuit to be coated.
[0045] The methods described herein enable non-hermetic
circuit-boards to replace hermetic modules. For example, in FIG. 1,
a microwave device such as a GaN microwave die may be bonded to a
heat-spreader, mounted in a stamped or cut or machined pocket on a
microwave circuit board. The use of metal backed boards may further
provide a heat sinking function as well as mechanical rigidity and
strength. Non-hermetic connectors may be attached to such a board
and/or interconnect may be fed under one or more layer to a second
recessed periphery area of the board for wirebonding. Board
substrates with copper, brass, or aluminum backings are available
from, for example, Taconic and Rodgers Corporation. Where minimized
weight or cost is desired, one or more metal slugs or thermal vias
may be used in a non-metal clad board. Microwave devices may be
interconnected on a circuit board using wire-bonds, surface
mounting, microwave jumpers or beam-leads in design configurations
known to reduce parasitic effects.
[0046] According to the instant approaches, substantially all
traditional board mounting options are preserved. These include
mounting technologies for multi-layer boards, surface mount, and
through-hole techniques, where the metal backing is removed or is
not present. Such boards may use thin film or laminated sheet
resistor or capacitor materials, such as those available from
DuPont.
[0047] Rework of boards after low-k material and one or more ionic
barrier sealing layers is applied can be accomplished by any
process that suitably removes coatings. For example, some materials
can be removed mechanically while other polymer materials,
particularly those that have not been cross-linked, can be removed
by solvent dissolution. Certain cross-linked materials may be
removed using solvents that swell and delaminate the coating.
However, coatings such as densely cross-linked epoxy materials may
require an oxygen plasma strip. Moreover, inorganic filled
materials may require further stripping with an etch chemistry that
attacks the inorganic substance. For example, silica microballoon
composites may require a fluorine plasma or an HF wet strip either
alone or in conjunction with any of the foregoing. Alternatively a
oxygen containing plasma etch may be used to remove the binders
from a syntactic foam and the device may be placed in an inverted
manner to accelerate removal of glass microballons. Alternatively,
brief contact with buffered hydrofluoric acid can be used to remove
silica based aerogel. Re-routing of the pocket mechanically may be
accomplished to provide a fresh surface of the heat-sink layer. The
applicable techniques may be used according to materials in use. In
a preferred rework approach, one or more primary barriers may be
cut away with a razor and peeled away and the syntactic foam may be
oxygen plasma ashed to remove its binder. A secondary protection
layer such as an ALD coating may typically be thin enough to allow
wirebonding through it and reworkable conductive adhesvies or
solders can allow chip or mounted devices to be detached and
re-attached. The non-hermetic sealing steps can then be applied a
second time.
[0048] In addition to reducing production costs, the approach
disclosed herein also enables the production of relatively large
sheets containing an array of devices to be produced or assembled
at one time. These can later be divided into separate devices, or
can be used in strips, or 2-D arrays for applications such as
phased arrays.
[0049] Although the present invention has been shown and described
with reference to particular examples, various changes and
modifications which are obvious to persons skilled in the art to
which the invention pertains are deemed to lie within the spirit,
scope and contemplation of the invention.
* * * * *