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FIELD OF INVENTION
The present invention relates to the fabrication and packaging of optical
microelectro-electromechanical devices (MEMS or MOEMS), carrying tiltable
mirrors integrated on substrates; and improved MEMS structures and devices
provided thereby, being more particularly directed to the precise
alignment or tiling of such devices or dies on single packaging
transparent optical substrates and the like, without restriction on the
size of, or the layout upon, the substrate, and with ready adaptability
for large scaling.
BACKGROUND
The present invention, as above stated, generally relates to the packaging
of electronic integrated circuits, and more specifically to the packaging
of MEMS devices with optical components, such as tiltable or orientable
mirrors; being primarily concerned with the means by which optical and
electrical inputs and outputs are made, utilizing packaging substrates,
and how such packaging can monolithically employ general-purpose optical
components.
Recent attention, however, has been paid to making multichip modules (MCM),
systems-on-chip (SOC) and microscale optomechanical devices for a variety
of applications. The MCMs are devoted to miniaturization of electronic
systems into one packaged module where many hybrid technologies may be
employed; while the SOC has focused on the integration of many electronic
functions, (analog, digital and RF, etc.), monolithically onto a single
VLSI die. Optoelectronic devices are beginning to follow along the MCM
route where many optical components, such as lenses, beams splitters,
lasers, detectors, and fiber optics and the like, are integrated onto
MCM-like carrier substrates.
The present invention falls under the particular purview of the so-called
flip-chip (FC) bonded multichip modules (MCM) that employ substrates that
act not only as a mechanical attachment and electrical wiring point, but
also as an optical interface. Typical MCMs incorporate an insulating or
non-conductive substrate resembling a printed circuit board (PCB) where
metalization is placed for the creation of interconnection circuitry. The
substrates have regions where VLSI dies are attached, face-up to the
substrate; and, following attachment to the substrate, are then wirebonded
to complete the electrical connection. An example of such structures is
disclosed in U.S. Pat. No. 6,147,876, creating a special substrate for die
potting. Other forms of substrates, interconnection methodologies,
materials, and architectures have also been proposed for face-up VLSI
MCMs.
More recently, the previously mentioned flip-chip bonding of dies to
substrates and of dies-to-dies for face-to-face solder attachments have
also been proposed as in, for example, U.S. Pat. No. 6,150,724,
illustrating die-on die/die-on-wafer flip-chip bonding. In such cases,
solder is used to attach, align and electrically connect the VLSI to
another die or a sub-wafer package. By utilizing surface tension while the
solder is in its liquid state, the floating die placed face down onto the
target substrate is drawn laterally until minimal misalignment between the
target substrate and the die is achieved. Such techniques are shown, as a
further example, in U.S. Pat. No. 6,151,173, employing solder microballs
to achieve 1 micron alignment. In this case, the solder microballs are
utilized to control the solder coating thickness, which plays an important
role in alignment accuracy. In the field of MEMS or MOEMS, the use of such
flip-chip bonding has been employed to mix differently processed die
substrates in order to achieve hybrid integration of MEMS components for
an optomechanical device, such as an optical scanner of Xerox Corporation,
employing flip chip process for MEMS applications in silicon optical bench
integration. In addition, flip-chip bonding has also been used for the
self-alignment of optical fiber arrays to substrates that have waveguide
components monolithically integrated, as described at
http://www.rereth.ethz.ch/phys/ quantenelectomik/ melchior/pj.17.html. In
this case, the surface tension of the solder bond draws the fiber arrays
into alignment relative to the substrate.
In much of the prior art, the attach substrate has been opaque or not at
all considered for optical interconnection functions or its optical
properties. Recently, however, some consideration has been given to the
use of the attach substrates as an optical path. VLSI dies with detectors
or transmitters, for example, have been bonded to an optical substrate
that provides an optical path for interconnection, as illustrated in U.S.
Pat. No. 6,097,857, describing optical and electrical interconnections
using such a substrate with integrated holograms, wherein VLSI chips are
flip-chip bonded to the optical substrate. As another illustration,
transmission through a VLSI substrate has also been considered for optical
interconnection as in U.S. Pat. No. 6,052,498.
Up until the present invention, however, it does not appear that the prior
art has taken into full account the problem of integrating many MEMS dies
with high alignment accuracy onto an optically transmissive substrate that
provides not only electrical connectivity but also simultaneously provides
means to integrate passive or active optical components (as later
discussed). MCM and flip-chip approaches heretofore only covered the many
die-to-single substrate attachments. One of the purposes of this
invention, on the other hand, is to create a substrate that provides both
electrical and optical interconnection to optical MEMS-integrated circuits
and components requiring critical alignment, say as low as +/-1 micron.
The present state-of-the-art of VLSI processing, unfortunately, does not
pragmatically provide a mechanism for creating die sizes beyond 20 mm on a
side without defects. Sizes exceeding 20 mm on a side, indeed, require
stitching of stepper repeated masks--a process that encompasses more
defects per area that often result in defective mirrors or electronics,
increasing the risk of unacceptable dies and producing wafers with very
poor yield.
As a result, very large arrays of MEMS devices, sizes exceeding 40 mm on a
side, have not heretofore been possible with the stitched stepper mask
approach, for example, on a single silicon die, particularly where the die
is approaching the wafer sizes. In addition, the scalability of the MEMS
devices, typically on the order of 1 square mm in area, to thousands of
devices, is currently seriously limited. To combat these problems, known
good - die approaches have been the industry standard; that is, VLSI dies
and correspondingly MEMS dies, are tested, and only known good parts are
selected out for packaging. To effectively use known good dies in the
creation of a larger optical MEMS array, however, a precise alignment of
MEMS die-to-MEMS die is necessary in order to maintain beam integrity,
requiring a high-accuracy tiling approach. In addition, while lens arrays
can be used over the MEMS array to reduce the overall size of the MEMS and
increase the amount of real estate available for integrated electronics
and interconnections, a critically tight alignment of the lens arrays to
the center of the MEMS mirror is required to avoid misfocusing of the
optical beams.
In accordance with the present invention, these and other problems of
tiling many MEMS have now been successfully addressed by using a
custom-fabricated optically (for example, visible or near-infrared band)
transmissive substrate. This substrate may have monolithically integrated
optical components, such as lenses, diffractive gratings, optical
absorbers, and transmission filters, and the like; and its MEMS chips are
flip-chip-bonded onto support pillars or posts that act as the electrical
and mechanical connections and also provide the mechanism for
self-alignment. Instead of creating MCMs with standard VLSI dies and an
optical substrate, or MCMs on non-optical substrates, the technique of the
present invention rather builds an optical MCM (OMCM) with MEMS devices.
This invention allows for physically integrated means to set the optical
path, as for a lens which focuses light onto the MEMS mirror. By using
lenses to optically address the MEMS arrays, smaller mirrors are then
possible, enabling greater area for monolithic electronics integration. By
using a self-aligned flip-chip approach in this manner, moreover, the MEMS
are accurately aligned to these passive optical components. The invention,
furthermore, does eliminate the need for attaching monolithic lens arrays
to the MEMS device following the packaging process; but it does require
careful handling of the released MEMS dies during the bonding process, and
careful control of the solder bonding process to assure 100% yield of the
bonded dies, as later more fully explained.
OBJECTS OF INVENTION
A primary object of the invention, accordingly, is to provide new and
improved critically aligned optical MEMS dies particularly suitable for
large packaged substrate arrays, and that shall not be subject to the
above-described prior art limitations and difficulties; but that, to the
contrary, enable large packaged array constructions through the integrated
packaging of MEMS devices with optical components, such as lenses, wherein
electrical and optical inputs and outputs are integrally provided upon an
optical substrate monolithically embodying such optical components.
A further object is to provide such novel structures and devices wherein an
integrated physical optical path is provided for the lenses which focus
light onto the MEMS mirrors and optically address the MEMS array, thereby
enabling the use of small mirrors and providing greater area for
monolithic electronics integration in the substrate.
An additional object is to provide a new and improved method of
manufacturing such novel devices.
Other and further objects will be explained hereinafter and are more
particularly delineated in the appended claims.
SUMMARY
In summary, the invention embraces an assembled array of optically and
electrically interacting optical MEMS dies physically and electrically
integrally attached upon a light-transmissive substrate carrying a pattern
of printed electrical circuit interconnections, whether transmissive,
opaque or a combination of both properties, for operating the dies, the
light-transmissive substrate being integrated monolithically with optical
components (passive and/or active) to provide accurate and fixed optical
alignment of the MEMS and the optical components interacting therewith.
In its fabrication aspects, the invention provides a method for enabling
the precision assembly of optical MEMS arrays upon a single substrate
without substantial restriction on the size or layout of the substrate,
that comprises, custom-forming a plurality of MEMS dies each carrying
electrical signal-controllable mirrors; and forming a light-transmissive
substrate of desired size to accommodate the plurality of MEMS dies, while
monolithically integrating into the light-transmissive substrate, optical
components useful for light-path interfacing with the MEMS dies. Integral
printed electrical circuit interconnections are provided on the substrate
for operation of the mirrors of the dies and the dies are physically and
electrically integrally attached along and upon the single optically
transmissive substrate, and with electrical connection to the printed
circuit, thereby to provide also for the accurate and fixed optical
alignment of the MEMS dies and the optical components optically
interacting therewith, and enabling the focusing of light onto the MEMS
mirrors along fixed optical paths for optically addressing the array
without requiring adjustments.
A scalable approach is thus provided for packaging the optical
microelectro-electromechanical system devices onto the light-transmitting,
(preferably optically transparent) printed wiring or circuit substrate.
This approach allows for the creation of custom-defined optical paths, by
a lens array, antireflecting and/or absorbent surfaces, optical grating
surfaces, wavelength specific filters, etc. The substrate contains
photolithographically defined metalization or conductors that represent
the desired electrical printed circuit interconnections of the many MEMS
dies, once integrally bonded to the substrate. These printed circuit
conductors are defined with well-known high-resolution lithography tools
allowing for inter-MEMS die conductor placement accuracy of less than 1
micron. The mounting substrate accepts separately custom-manufactured MEMS
dies that are bonded physically and simultaneously electrically connected
to the substrate using the before-mentioned present state-of-the-art
flip-chip solder attach tools. Solder reflow techniques are preferably
employed precisely to align MEMS dies to the transparent optical
substrate. This precise alignment process maintains tight alignment of the
MEMS devices relative to the substrate, and, indeed, provides for MEMS die
alignment as low as +/-1 micron, and similar tight alignment for the
optical components monolithically integrated into the packaging substrate.
The overall device package of the invention thus allows for the precise
placement and tiling of many MEMS dies onto a single substrate without
restriction on the size or layout of the substrate.
Preferred and best mode designs, techniques and configurations are
hereinafter more fully explained.
DRAWINGS
The invention will now be described in connection with the accompanying
drawings in which FIG. 1 is a top view of an illustrative transparent
substrate of preferred format and a preferred method of fabrication or
manufacture, showing an exemplary optical path and electrical connection
integrated package;
FIG. 2 is a process flow diagram illustrating five successive manufacturing
steps in transverse section for the creation of the preferred type of
optically transmissive substrate for the MEMS die attachments in
accordance with the present invention;
FIG. 3 is a transverse section upon an enlarged scale showing the MEMS
mirror array after dicing from the fabricated wafer;
FIG. 4 illustrates the process of flip-chip attaching the MEMS die array of
FIG. 3 to the optically transparent substrate of FIGS. 2 and 1;
FIG. 5 is a similar diagram illustrating the completed flip-chip assembled
dies of FIG. 4 integrally attached on the transparent optical path
substrate, OMCM, of both FIGS. 1 and 2;
FIG. 6 is an assembly showing of the OMEM of FIG. 5 placed into a modified
PGA package;
FIG. 7 shows the completed packaging process for the tiled optical MEMS
dies with the transparent substrate, including illustrative monolithically
integrated lens arrays;
FIG. 8 illustrates the application of the packages of FIG. 7 to an
exemplary optical cross-connect architecture; and
FIGS. 9-12 show the integration of active optical elements into the
structures of the invention, illustrating the integration of photodiodes
with the MEMS substrate and onto the OMCM substrate, illustrating the
integration of photodiodes with the MEMS substrate and onto the OMCM
substrate, illustrating the integration of photodiodes with the MEMS
substrate and onto the OMCM substrate, and VCSELs onto the surface of the
MEMS device wafer, and onto the OMCM substrate, respectively.
DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION
To achieve the previously described feature of tight alignment of
known-good MEMS dies, a mechanical substrate 1 is employed, FIG. 1, that
is optically transparent in the wavelength range of interest and has
electrical interconnections 7 patterned onto or into it. The substrate
thickness is selected appropriately for the mechanical stability over the
desired operating temperature range and packaging housing, being very
similar to an insulated printed circuit board (PCB) with the exception
that the substrate is light transmissive. It has been patterned
photolithographically, as shown, with 5-micron conductor line widths and
pattern placement to at least 2-micron accuracy across the printed-circuit
patterned substrate. The pattern is created with standard VLSI or
high-resolution PCB tools, as is well-known. At manufacturing time, the
substrate can also be coated with an anti-reflection coating and an
absorbing layer 4 to block stray light that may impinge upon the substrate
or scatter off the MEMS mirrors, later described, or off the electronics.
FIG. 2 illustrates a preferred method of manufacture of the substrate, as a
combined structural and process flow diagram. This process is
representative of a 2-layer metalization, front and backside process, but
multi-layer metalization is possible with increased complexity. As shown
in FIG. 2, the initial substrate is simply a flat optically transparent
plate or blank 1.sup.1, processed preferably also to include lens arrays L
or other optical elements or components machined into and thus integrated
with the substrate, ("Step 1"), either on its frontside, backside, as
illustrated, or both.
When the later-described MEMS device is read out in reflection, as shown in
FIG. 7, optical transmission through the carrier substrate 1 should be
minimally lossy to assure maximum optical power after passing through the
completed and packaged assembly. This substrate may be of gallium
phosphide (GaP), silicon (Si) or other semi-conductor surfaces, sapphire,
a glass composite or quartz, or any other material system that is
transparent to the optical spectrum of interest and well-matched to the
coefficient of thermal expansion of the silicon-based MEMS die. For
example, the substrate selected may be transparent to infrared radiation
in the 1.3 to 1.55 micron wavelength band. The near matching of the
coefficient of thermal expansion between the MEMS die and that of the
substrate 1 is critical to ensure reliable die attach over time, and to
minimize the likelihood of damage to the MEMS die or substrate over time
and temperature cycling. To minimize the interface reflection, the
substrate may be coated with appropriate anti-reflection materials prior
to the subsequent metalization processes.
As illustrated in "Step 2" in FIG. 2, the electrical circuit pattern 7' is
initiated by coating both surfaces with appropriate adhesion layers and
conductors, such as (optically opaque) gold. This initial gold layer is
thin but sufficiently thick for post-assembly wire bonding. The backside
of the substrate is metalized at 7" to form a solid annular region at the
periphery of the substrate. This annular region is included for solder
attachment of the optical substrate to the package housing as illustrated
in FIG. 7. This solder attach methodology facilitates hermetic sealing of
the package. If desired, an optical blocking layer 8 may be applied
following the initial metal layer coating, shown as "Step 3" in FIG. 2,
and in areas around the metalization. To facilitate the attachment and
self-alignment of the MEMS die, posts or pillars 7'" are electroplated
from the initial gold seed layer 7' ("Step 4") through an apertured
plating mask. The gold or an alternative metal is plated to a height H
sufficient for the clearance of the out-of-plane moving components
(mirrors M) of the MEMS die. An example of sufficient clearance for moving
MEMS mirrors M is shown in FIG. 7. Following the post or pillar plating
process, the mask is stripped, and the substrate is prepared for solder
application, "Step 5". The solder is applied to both sides S,S.sup.1 of
the substrate in preparation for the die attach process and the subsequent
bonding of the flip-chip assembly into the package housing.
In a preferred system of the invention, the before-mentioned MEMS mirror
array (MOEMS) is prepared having electrical signal-controlled orientable
or tiltable gimbaled mirrors M that, as is well-known, can move out of
plane as shown in FIG. 3. The MEMS die is prepared for solder attach prior
to dicing by applying adhesion and gold diffusion block layers below the
gold metalization, which will interface the solder from the carrier
substrate, as later discussed in connection with FIG. 4. After the MEMS
device is diced as shown in FIG. 3, the mechanical structures are
released, and care must be taken when handling the die substrate with the
subsequently used flip-chip bonding tool.
To assemble the optical MCM (OMCM), the MEMS die, shown face-up in FIG. 3,
is flipped upside-down to face the optically transparent substrate 1 shown
in FIG. 4. To achieve the tight alignment refinements of the invention, a
commercially available flip-chip bonding tool T is used to pick and place
the known-good MEMS die accurately on the grid defined by the substrate 1.
This machine places each die in close proximity of the substrate pads 3,
FIG. 1, and solders them into place to within +/-1 micron accuracy. The
flip-chip bonder thus tiles the MEMS dies sequentially until the carrier
substrate assembly is complete. Once the dies are bonded to the substrate,
the entire assembly is either complete or may be annealed to complete the
die attach process.
The completed assembly on the transparent substrate 1 is shown in FIG. 5.
In this attach process, solder materials are selected such that all reflow
and attach temperatures are mutually compatible.
Once complete, FIG. 5, the OMCM is placed into a pin grid array (PGA) or
ball grid array (BGA) package, so labeled, specifically modified with an
aperture or window W in the bottom of the package. This PGA, BGA or other
suitable package is modified so that light can pass through the bottom of
the package as by machining a hole into the central area of the package
backside, as shown in cross-section in FIG. 6. This hole may be fitted
with a window W and hermetically sealed into the package; or the aperture
may be left open, and the OMCM substrate used to sealed the package
cavity. In the case where a window W is utilized, this window would
preferentially be antireflection-coated on both sides to minimize spurious
reflections. In the open aperture case, the OMCM substrate can act as a
window into the package; or, in the separate window case, as shown in FIG.
6, with the OMCM substrate acting purely as a transparent carrier. The
substrate may also include a solder ring on its backside to create the
lower hermetic sealing ring to the package; or, if desired, the OMCM
substrate may be bonded in place with a low outgassing epoxy. The package
is ultimately sealed with a metallic lid that preferably includes a
desiccant and getters on the surface; or, alternatively, the package may
be sealed with a controlled environment gas such as dry nitrogen or dry
air.
Once the OMCM is bonded into the package (PGA or BGA), the substrate is
electrically connected to the annular package pads P' to complete the
electrical connection between the OMCM and PGA, BGA or other package. The
preferred method of interconnect is wire bonding of the OMCM pads to the
annular package pads P.sup.1 at B, as by means of an automated wirebonder.
An alternate electrical connection methodology to such wire bonding may
include solder ball attachment from the backside of the OMCM to the
corresponding package pads. This alternate method, however, is of
increased complexity, requiring electrical connections through the OMCM
and additional solder joints on the backside of the OMCM substrate. The
package assembly process is completed by soldering the cover metal cap
plate C in place, as indicated in FIG. 6, the completely assembled package
being shown in FIG. 7. The cover C is soldered inplace in a controlled
ambient environment E to minimize the inclusion of water vapor, and/or
getters may be included for residual vapor absorption.
The completed and packaged MEMS OMCM shown in FIG. 7 may be mounted onto a
standard PCB or flexible cable connector avoiding optical access
occlusion. The electrical connections are in this instance on the bottom
side of the package. The MOEMS devices are shown in FIG. 7 as optically
readout in reflection from the backside surface.
FIG. 8 shows an exemplary application of the packages of FIG. 7, being
illustrated as applied to critically aligned optical MEMS dies in an
optical cross-connect architecture (OXC) for telecommunications. To move
flight from one fiber to another in an optically dense fashion,
three-dimensionally steering MEMS mirrors have been employed in a common
OXC architecture (see, for example, WO 00/20899 Xros, Inc. published
patent application). The many channels of collimated input light propagate
from ingress fiber bundle 11 to the first packaged OMCM array 9. This OMCM
is interfaced electrically to control electronics via a printed circuit
board 10 which is preferably soldered to the OMCM. The OMCM is suitably
mechanically attached to the OXC housing via an external PCB electrical
interface and suitable mechanical attach 10, or tooled lugs in 9 (not
shown). Light reflects off the MOEMS in the packaged OMCM 9 and reflects
off the mirror surface 13 to be redirected towards the second OMCM array
9.sup.1. The light is further directed toward the egress fiber bundle 12
by reflection off the MOEMS mirrors packaged in OMCM 9.sup.1. By
appropriately controlling the MOEMS mirrors, light can be selectively
switched from one fiber in the ingress fiber bundle 11 to a fiber in the
egress fiber bundle 12.
The OMCM approach of the invention, moreover, allows for the integration
not only of passive optical elements, but interacting active optical
elements, as well. Passive optical elements include, for example, the
before-described lenses, diffractive optical elements, arbitrary phase
elements, optical masks, mirrors, polarizers, etc. In addition to such and
other passive elements, active optical components can well be integrated
onto or into either the MEMS substrate or the OMCM substrate. The active
optical components may be lasers, photodetectors, modulators, switches,
and filters, as examples. MEMS devices are typically integrated on silicon
substrates that readily allow for the cointegration of MEMS and
photodetectors onto a single MEMS substrate, as illustrated in FIG. 9. In
this figure, a photodetector 14 is integrated into the MEMS substrate
either laterally placed with respect to the MEMS device M or as part of
the MEMS device. In this case, the OMCM substrate acts as a masked window
8. An alternative to this configuration is where the photodetector is
integrated onto or within the OMCM substrate, as illustrated in FIG. 10.
In this case, the OMCM substrate may act as a window for the
photodetector, or it positions the photodetector relative to the MEMS
device M to collect light reflected from the MEMS device.
Other active devices useful with the invention include lasers, such as
vertical cavity surface emitting lasers, VCSELs. VCSELs may be mounted to
the MEMS substrate as in FIG. 11, or to the OMCM substrate, as in FIG. 12,
with the before-described well-known bonding processes. As illustrated in
FIG. 11, the VCSEL 15 is attached to the MEMS substrate. The light from
the laser 15 propagates to the OMCM substrate 1.sup.1 and may either pass
directly through or interact with an optical element 16. In this case, the
optical element 16 is a blazed mirror grating to reflect light to the MEMS
device M. This example creates a compact optical scanner. An alternative
means for the integration of such VCSELs with the OMCM is illustrated in
FIG. 12. In this case, the VCSELs are integrated onto the OMCM substrate
1.sup.1. The light from the VCSEL can either propagate through the OMCM
substrate 1.sup.1 or towards the MEMS device M and reflects off M under
its control.
Further modifications in the manufacturing techniques and in the structural
details will occur to those skilled in this art, including alternatives
above suggested--and such are considered to fall within the spirit and
scope of the invention as defined in the appended claims.
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