U.S. patent application number 10/159470 was filed with the patent office on 2002-10-10 for methods of manufacturing microfabricated substrates.
This patent application is currently assigned to Caliper Technologies Corp.. Invention is credited to McReynolds, Richard J..
Application Number | 20020144774 10/159470 |
Document ID | / |
Family ID | 26936729 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020144774 |
Kind Code |
A1 |
McReynolds, Richard J. |
October 10, 2002 |
Methods of manufacturing microfabricated substrates
Abstract
The present invention is directed to improved methods and
apparatuses for manufacturing microfabricated devices, and
particularly, microfluidic devices. In general the methods and
apparatuses of the invention provide improved methods of bonding
substrates together by applying a vacuum to the space between the
substrates during the bonding process.
Inventors: |
McReynolds, Richard J.; (San
Jose, CA) |
Correspondence
Address: |
CALIPER TECHNOLOGIES CORP
605 FAIRCHILD DRIVE
MOUNTAIN VIEW
CA
94043
US
|
Assignee: |
Caliper Technologies Corp.
Mountain View
CA
|
Family ID: |
26936729 |
Appl. No.: |
10/159470 |
Filed: |
May 31, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10159470 |
May 31, 2002 |
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09244703 |
Feb 4, 1999 |
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6425972 |
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09244703 |
Feb 4, 1999 |
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08877843 |
Jun 18, 1997 |
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5882465 |
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Current U.S.
Class: |
156/285 ;
156/286; 156/308.2; 156/309.9; 438/113 |
Current CPC
Class: |
B29C 65/02 20130101;
B29C 65/10 20130101; B32B 37/1018 20130101; B29C 65/48 20130101;
B29C 65/08 20130101; B81B 2201/058 20130101; B29L 2031/756
20130101; B01L 3/502707 20130101; B81C 3/001 20130101; B29C 65/4895
20130101; B29C 65/4845 20130101; B81C 2203/036 20130101; B29C
66/82661 20130101; B29C 65/7847 20130101; B29C 66/1122 20130101;
B32B 38/1841 20130101; B81C 2201/019 20130101; B32B 2038/1891
20130101; B81C 1/00119 20130101; B29C 66/71 20130101; B29C 66/54
20130101; B29C 66/71 20130101; B29K 2027/06 20130101; B29C 66/71
20130101; B29K 2027/18 20130101; B29C 66/71 20130101; B29K 2033/12
20130101; B29C 66/71 20130101; B29K 2069/00 20130101; B29C 66/71
20130101; B29K 2081/06 20130101; B29C 66/71 20130101; B29K 2083/00
20130101 |
Class at
Publication: |
156/285 ;
156/286; 156/308.2; 156/309.9; 438/113 |
International
Class: |
B29C 065/00; H01L
021/50 |
Claims
What is claimed is:
1. A method of fabricating microfluidic devices comprising:
providing a first substrate and a second substrate, wherein the
second substrate has a plurality of apertures; applying a vacuum to
the apertures to hold the first substrate in contact with the
second substrate; and bonding the first substrate to the second
substrate.
2. The method of claim 1, wherein the bonding step comprises
heating the first and second substrates to bond a first surface of
the first substrate to a first surface of the second substrate.
3. The method of claim 2, wherein the step of heating the
substrates comprises heating the first and second substrates to a
temperature between about 80.degree. C. and 200.degree. C.
4. The method of claim 3, wherein the first and second substrates
comprise polymeric substrates.
5. The method of claim 2, wherein the first and second substrates
comprise silica-based substrates, and wherein the bonding step
comprises heating the first and second substrates to between about
90 and 200.degree. C., followed by the step of heating the first
and second substrates to a temperature between about 500.degree. C.
and 1400.degree. C.
6. The method of claim 1, wherein the bonding step comprises
applying an adhesive to at least one of a first surface of the
first substrate or a first surface of the second substrate prior to
applying a vacuum to the apertures of the second substrate.
7. The method of claim 1, wherein the first and second substrates
comprise glass.
8. The method of claim 1, wherein the first and second substrates
are comprised of polymeric materials.
9. The method of claim 1, wherein the first substrate includes a
plurality of discrete microscale channel networks disposed on a
first surface of the first substrate.
10. The method of claim 9, wherein the first substrate includes at
least four discrete microscale channel networks disposed on the
first surface of the first substrate.
11. The method of claim 9 wherein the first substrate includes at
least ten discrete microscale channel networks disposed on the
first surface of the first substrate.
12. The method of claim 9 wherein the first and second bonded
substrates form a unitary bonded substrate, the method further
comprising separating a first portion of the bonded substrate
containing at least a first discrete channel network from a second
portion of the bonded substrate containing at least a second
discrete channel network.
13. The method of claim 12, wherein the bonded substrate comprises
a thinned region between at least the first and second discrete
channel networks, and the separating step comprises breaking the
first discrete channel network from at least the second discrete
channel network along the thinned region.
14. The method of claim 12, wherein the bonded substrate comprises
a perforated region between at least the first and second discrete
channel networks, and the separating step comprises breaking the
first discrete channel network from at least the second discrete
channel network along the perforated region.
15. The method of claim 9 wherein the plurality of apertures are
positioned in fluidic communication with the plurality of discrete
microscale channel networks prior to said bonding step.
16. The method of claim 1 further comprising aligning the first
substrate with the second substrate prior to said applying
vacuum.
17. The method of claim 1 wherein said applying vacuum is performed
by placing the second substrate upon a platform surface which
includes a plurality of grooves fabricated therein which extend
laterally from one or more vacuum ports in the platform surface,
and applying a vacuum to the one or more vacuum ports.
18. The method of claim 1 wherein said applying vacuum is performed
by placing the second substrate upon a platform surface which
includes a plurality of vacuum ports fabricated therein, and
applying a vacuum to the plurality of vacuum ports.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/244,703, filed Feb. 4, 1999, which is a
continuation-in-part of U.S. patent application Ser. No.
08/877,843, filed Jun. 18, 1997, now U.S. Pat. No. 5,882,465, and
incorporated herein by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] Microfabricated devices are used in a wide variety of
industries, ranging from the integrated circuits and
microprocessors of the electronics industry to, in more recent
applications, microfluidic devices and systems used in the
pharmaceutical, chemical and biotechnology industries.
[0003] Because of the extreme small scale of these devices, as well
as the highly precise nature of the operations which they perform,
the manufacturing of these microfabricated devices requires
extremely high levels of precision in all aspects of fabrication,
in order to accurately and reliably produce the various microscale
features of the devices.
[0004] In a number of these disciplines, the manufacturing of these
microfabricated devices requires the layering or laminating of two
or more layers of substrates, in order to produce the ultimate
device. For example, in microfluidic devices, the microfluidic
elements of the device are typically produced by etching or
otherwise fabricating features into the surface of a first
substrate. A second substrate is then laminated or bonded to the
surface of the first to seal these features and provide the fluidic
elements of the device, e.g., the fluid passages, chambers and the
like.
[0005] While a number of bonding techniques are routinely utilized
in mating or laminating multiple substrates together, these methods
all suffer from a number of deficiencies. For example, silica-based
substrates are often bonded together using thermal bonding
techniques. However, in these thermal bonding methods, substrate
yields can often be extremely low, as a result of uneven mating or
inadequate contact between the substrate layers prior to the
thermal bonding process. Similarly, in bonding semi-malleable
substrates, variations in the contact between substrate layers,
e.g., resulting from uneven application of pressure to the
substrates, may adversely affect the dimensions of the features
within the interior portion of the device, e.g., flattening
channels of a microfluidic device, as well as their integrity.
[0006] Due to the cost of substrate material, and the more precise
requirements for microfabricated devices generally, and
microfluidic devices, specifically, it would generally be desirable
to provide an improved method of manufacturing such devices to
achieve improved product yields, and enhanced manufacturing
precision. The present invention meets these and a variety of other
needs.
SUMMARY OF THE INVENTION
[0007] The present invention is generally directed to improved
methods of manufacturing microfabricated devices, and particularly,
microfluidic devices. In particular, in a first aspect, the present
invention provides methods and apparatuses for bonding
microfabricated substrates together. In accordance with the methods
of the present invention, a first substrate is provided which has
at least a first planar surface, a second surface opposite the
planar surface, and a plurality of apertures disposed through the
first substrate from the first surface to the second surface. A
vacuum is applied to the apertures, while the first planar surface
of the first substrate is mated with a first planar surface of the
second substrate. The mating of these substrates is carried out
under conditions wherein the first surface of the first substrate
is bonded to the first surface of the second substrate. Such
conditions can include, e.g., heating the substrates, or applying
an adhesive to one of the planar surfaces of the first or second
substrate.
[0008] In a related aspect, the present invention also provides an
apparatus for manufacturing microfluidic devices in accordance with
the methods described above. Specifically, such apparatus typically
comprises a platform surface for holding a first substrate, the
first substrate having at least a first planar surface and a
plurality of holes disposed therethrough, and wherein the platform
surface comprises a vacuum port connected to a vacuum source, for
applying a vacuum to the plurality of holes. The apparatus also
comprises a bonding system for bonding the first surface of the
first substrate to a first surface of a second substrate.
BRIEF DESCRIPTION OF THE FIGURES
[0009] FIG. 1 illustrates the layered fabrication of a typical
microfluidic device, from at least two separate substrates, which
substrates are mated together to define the microfluidic elements
of the device.
[0010] FIG. 2 illustrates a mounting table and vacuum chuck for
bonding substrates together according to the methods of the present
invention.
[0011] FIG. 3 illustrates an apparatus for mounting and thermally
bonding substrates together.
[0012] FIG. 4 illustrates a bonded substrate that includes multiple
discrete channel networks to be separated into individual
microfluidic devices.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The present invention is generally directed to improved
methods of manufacturing microfabricated substrates, and
particularly, to improved methods of bonding together
microfabricated substrates in the manufacture of microfluidic
devices. These improved methods of bonding substrates are generally
applicable to a number of microfabrication processes, and are
particularly well suited to the manufacture of microfluidic
devices.
[0014] As used herein, the term "microscale" or "microfabricated"
generally refers to structural elements or features of a device
which have at least one fabricated dimension in the range of from
about 0.1 .mu.m to about 500 .mu.m. Thus, a device referred to as
being microfabricated or microscale will include at least one
structural element or feature having such a dimension. When used to
describe a fluidic element, such as a passage, chamber or conduit,
the terms "microscale," "microfabricated" or "microfluidic"
generally refer to one or more fluid passages, chambers or conduits
which have at least one internal cross-sectional dimension, e.g.,
depth, width, length, diameter, etc., that is less than 500 .mu.m,
and typically between about 0.1 .mu.m and about 500 .mu.m. In the
devices of the present invention, the microscale channels or
chambers preferably have at least one cross-sectional dimension
between about 0.1 .mu.m and 200 .mu.m, more preferably between
about 0.1 .mu.m and 100 .mu.m, and often between about 0.1 .mu.m
and 20 .mu.m. Accordingly, the microfluidic devices or systems
prepared in accordance with the present invention typically include
at least one microscale channel, usually at least two intersecting
microscale channels, and often, three or more intersecting channels
disposed within a single body structure. Channel intersections may
exist in a number of formats, including cross intersections, "T"
intersections, or any number of other structures whereby two
channels are in fluid communication.
[0015] The body structure of the microfluidic devices described
herein typically comprises an aggregation of two or more separate
layers which when appropriately mated or joined together, form the
microfluidic device of the invention, e.g., containing the channels
and/or chambers described herein. Typically, the microfluidic
devices described herein will comprise a top portion, a bottom
portion, and an interior portion, wherein the interior portion
substantially defines the channels and chambers of the device.
[0016] FIG. 1 illustrates a two layer body structure 10, for a
microfluidic device. In preferred aspects, the bottom portion of
the device 12 comprises a solid substrate that is substantially
planar in structure, and which has at least one substantially flat
upper surface 14. A variety of substrate materials may be employed
as the bottom portion. Typically, because the devices are
microfabricated, substrate materials will be selected based upon
their compatibility with known microfabrication techniques, e.g.,
photolithography, wet chemical etching, laser ablation, air
abrasion techniques, injection molding, embossing, and other
techniques. The substrate materials are also generally selected for
their compatibility with the full range of conditions to which the
microfluidic devices may be exposed, including extremes of pH,
temperature, salt concentration, and application of electric
fields. Accordingly, in some preferred aspects, the substrate
material may include materials normally associated with the
semiconductor industry in which such microfabrication techniques
are regularly employed, including, e.g., silica based substrates,
such as glass, quartz, silicon or polysilicon, as well as other
substrate materials, such as gallium arsenide and the like. In the
case of semiconductive materials, it will often be desirable to
provide an insulating coating or layer, e.g., silicon oxide, over
the substrate material, and particularly in those applications
where electric fields are to be applied to the device or its
contents.
[0017] In additional preferred aspects, the substrate materials
will comprise polymeric materials, e.g., plastics, such as
polymethylmethacrylate (PMMA), polycarbonate,
polytetrafluoroethylene (TEFLON.TM.), polyvinylchloride (PVC),
polydimethylsiloxane (PDMS), polysulfone, and the like. Such
polymeric substrates are readily manufactured using available
microfabrication techniques, as described above, or from
microfabricated masters, using well known molding techniques, such
as injection molding, embossing or stamping, or by polymerizing the
polymeric precursor material within the mold (See U.S. Pat. No.
5,512,131). Such polymeric substrate materials are preferred for
their ease of manufacture, low cost and disposability, as well as
their general inertness to most extreme reaction conditions. Again,
these polymeric materials may include treated surfaces, e.g.,
derivatized or coated surfaces, to enhance their utility in the
microfluidic system, e.g., provide enhanced fluid direction, e.g.,
as described in U.S. Pat. No. 5,885,470, and which is incorporated
herein by reference in its entirety for all purposes.
[0018] The channels and/or chambers of the microfluidic devices are
typically fabricated into the upper surface of the bottom substrate
or portion 12, as microscale grooves or indentations 16, using the
above described microfabrication techniques. The top portion or
substrate 18 also comprises a first planar surface 20, and a second
surface 22 opposite the first planar surface 20. In the
microfluidic devices prepared in accordance with the methods
described herein, the top portion also includes a plurality of
apertures, holes or ports 24 disposed therethrough, e.g., from the
first planar surface 20 to the second surface 22 opposite the first
planar surface.
[0019] The first planar surface 20 of the top substrate 18 is then
mated, e.g., placed into contact with, and bonded to the planar
surface 14 of the bottom substrate 12, covering and sealing the
grooves and/or indentations 16 in the surface of the bottom
substrate, to form the channels and/or chambers (i.e., the interior
portion) of the device at the interface of these two components.
The holes 24 in the top portion of the device are oriented such
that they are in communication with at least one of the channels
and/or chambers formed in the interior portion of the device from
the grooves or indentations in the bottom substrate. In the
completed device, these holes function as reservoirs for
facilitating fluid or material introduction into the channels or
chambers of the interior portion of the device, as well as
providing ports at which electrodes may be placed into contact with
fluids within the device, allowing application of electric fields
along the channels of the device to control and direct fluid
transport within the device.
[0020] Conditions under which substrates may be bonded together are
generally widely understood, and such bonding of substrates is
generally carried out by any of a number of methods, which may vary
depending upon the nature of the substrate materials used. For
example, thermal bonding of substrates may be applied to a number
of substrate materials, including, e.g., glass or silica based
substrates, as well as polymer based substrates. Such thermal
bonding typically comprises mating together the substrates that are
to be bonded, under conditions of elevated temperature and, in some
cases, application of external pressure. The precise temperatures
and pressures will generally vary depending upon the nature of the
substrate materials used.
[0021] For example, for silica-based substrate materials, i.e.,
glass (borosilicate glass, Pyrex.TM., soda lime glass, etc.),
quartz, and the like, thermal bonding of substrates is typically
carried out at temperatures ranging from about 500.degree. C. to
about 1400.degree. C., and preferably, from about 500.degree. C. to
about 1200.degree. C. For example, soda lime glass is typically
bonded at temperatures around 550.degree. C., whereas borosilicate
glass typically is thermally bonded at or near 800.degree. C.
Quartz substrates, on the other hand, are typically thermally
bonded at temperatures at or near 1200.degree. C. These bonding
temperatures are typically achieved by placing the substrates to be
bonded into high temperature annealing ovens. These ovens are
generally commercially available from, e.g., Fischer Scientific,
Inc., and LabLine, Inc.
[0022] Polymeric substrates that are thermally bonded, on the other
hand, will typically utilize lower temperatures and/or pressures
than silica-based substrates, in order to prevent excessive melting
of the substrates and/or distortion, e.g., flattening of the
interior portion of the device, i.e., channels or chambers.
Generally, such elevated temperatures for bonding polymeric
substrates will vary from about 80.degree. C. to about 200.degree.
C., depending upon the polymeric material used, and will preferably
be between about 90.degree. C. and 150.degree. C. Because of the
significantly reduced temperatures required for bonding polymeric
substrates, such bonding may typically be carried out without the
need for high temperature ovens, as used in the bonding of
silica-based substrates. This allows incorporation of a heat source
within a single integrated bonding system, as described in greater
detail below.
[0023] Adhesives may also be used to bond substrates together
according to well known methods, which typically comprise applying
a layer of adhesive between the substrates that are to be bonded
and pressing them together until the adhesive sets. A variety of
adhesives may be used in accordance with these methods, including,
e.g., UV curable adhesives, that are commercially available.
Alternative methods may also be used to bond substrates together in
accordance with the present invention, including e.g., acoustic or
ultrasonic welding and/or solvent welding of polymeric parts.
[0024] Typically, a number of microfabricated devices will be
manufactured at a time. For example, polymeric substrates may be
stamped or molded in large separable sheets which can be mated and
bonded together. Individual devices or discrete channel networks
may then be separated from the larger bonded substrate sheet.
Similarly, for silica-based substrates, individual devices can be
fabricated from larger substrate wafers or plates, allowing higher
throughput of the manufacturing process. Specifically, a number of
discrete channel networks, e.g., where each separate channel
network includes at least two intersecting channels, can be
manufactured into a first substrate wafer or plate. A second wafer
or plate is then provided that includes a plurality of holes
disposed through it, which holes align with the unintersected
termini of the various channel networks. The two substrate wafers
are first bonded together such that multiple channel networks are
created in the integrated substrate. The resulting multiple devices
are then segmented from the larger substrates using known methods,
such as sawing (See, e.g., U.S. Pat. No. 4,016,855 to Mimata,
incorporated herein by reference), scribing and breaking (See
Published PCT Application No. WO 95/33846), and the like. In
particular, a large bonded substrate including multiple separate
and discrete channel networks is separated into individual devices
by, e.g., sawing them apart, scribing between the channel networks
and breaking them apart. In the case of polymeric substrates these
methods are also as applicable, however, discrete devices may be
cut or melted apart. In some cases, where the fabrication process
has included perforations or thinner areas of the bonded
substrates, the discrete devices may be simply snapped or broken
apart. An example of this fabrication method is schematically
illustrated in FIG. 5. As shown, the bonded substrate (seen only
from above) includes the apertures 24 as described with reference
to FIG. 1. These apertures are in fluid communication with discrete
channel networks (not shown) in the interior portion of the bonded
substrate 50. The discrete channel networks or individual
microfluidic devices are then separated from the larger sheet
along, e.g., dashed lines 52. Depending upon the method employed,
these dashed lines may be the lines along which sawing or scribing
and breaking take place, or they can include perforated regions or
thinned substrate regions which may be easily broken apart.
[0025] Typically, these larger wafer techniques may be used to
simultaneously fabricate at least 4 separate microfluidic devices,
e.g., as discrete channel networks in the larger wafer, typically
at least 8 separate devices, preferably at least than 10 separate
devices, more preferably at least 20 separate devices and still
more preferably, at least 40 separate devices from a single bonded
substrate.
[0026] As noted above, the top or second substrate is overlaid upon
the bottom or first substrate to seal the various channels and
chambers. In carrying out the bonding process according to the
methods of the present invention, the mating of the first and
second substrates is carried out using vacuum to maintain the two
substrate surfaces in optimal contact. In particular, the bottom
substrate may be maintained in optimal contact with the top
substrate by mating the planar surface of the bottom substrate with
the planar surface of the top substrate, and applying a vacuum
through the holes that are disposed through the top substrate.
Typically, application of a vacuum to the holes in the top
substrate is carried out by placing the top substrate on a vacuum
chuck, which typically comprises a mounting table or surface,
having an integrated vacuum source. In the case of silica-based
substrates, the mated substrates are subjected to elevated
temperatures, e.g., in the range of from about 100IC to about
200.degree. C., in order to create an initial bond, so that the
mated substrates may then be transferred to the annealing oven,
without any shifting relative to each other.
[0027] One example of an apparatus for use in accordance with the
methods described herein is shown in FIG. 2. As shown, the
apparatus includes a mounting table 30, which comprises a platform
surface 32, having a vacuum port 34 disposed therethrough. In
operation, the top substrate, e.g., having the plurality of holes
disposed therethrough, is placed upon the platform surface and
maintained in contact with that surface by virtue of the
application of a vacuum through vacuum port 34. Although FIG. 2
shows the platform surface as being the upper surface of the
mounting table, it will be appreciated that such a device would
also function in an inverted orientation, relying upon the applied
vacuum to maintain the substrate in contact with the platform
surface. The platform may also comprise one or more alignment
structures for maintaining the substrate in a set, predefined
position. These alignment structures may take a variety of forms,
including, e.g., alignment pins 36, alignment ridges, walls, or
wells disposed upon the mounting surface, whereupon placement of
the substrates in accordance with such structures ensures alignment
of the substrates in the appropriate position, e.g., over the
vacuum port, as well as aligning the individual substrate portions
with other substrate portions, as described in greater detail
below. In addition to such structures, alignment may also be
facilitated by providing the platform at an appropriate angle, such
that gravity will maintain the substrate in contact with the
alignment structures. Vacuum port 34 is disposed through the
platform surface and mounting table, and is connected via a vacuum
line 38 to a vacuum source (not shown), e.g., a vacuum pump.
[0028] The first substrate is placed upon the platform surface such
that the planar surface of the top substrate faces away from the
platform surface of the mounting table, and such that the holes in
the substrate are in communication with the vacuum port in the
platform surface of the mounting table. Alignment of the holes over
the vacuum port is typically accomplished through the incorporation
of alignment structure or structures upon the mounting table
platform surface, as described above. In order to apply vacuum
simultaneously at a plurality of the holes in the top substrate, a
series of vacuum ports may be provided through the platform
surface. Preferably, however, the platform surface comprises a
series of grooves 40 fabricated therein, and extending outward from
a single vacuum port, such that each of the plurality of holes in
the top substrate will be in communication with the vacuum port via
at least one of these grooves or "vacuum passages," when the top
substrate is placed upon the platform surface.
[0029] The bottom substrate, also having a first planar surface, is
then placed on the top substrate such that the first planar surface
of the bottom substrate mates with that of the top substrate.
Again, the alignment structures present upon the platform surface
will typically operate to align the bottom substrate with the top
substrate as well as maintain the substrates over the vacuum
port(s). The alignment of the various substrate portions relative
to each other is particularly important in the manufacture of
microfluidic devices, wherein each substrate portion may include
microfabricated elements which must be in fluid communication with
other microfabricated elements on another substrate portion.
[0030] A vacuum is then applied through the vacuum passages on the
platform surface, and to the holes through the top substrate. This
acts to pull the two substrates together by evacuating the air
between their planar surfaces. This method is particularly useful
where the top and bottom substrates are elements of microfluidic
devices, as described above. Specifically, upon mating the top
substrate with the bottom substrate, the holes disposed through the
top substrate will generally be in communication with the
intersecting channel structures fabricated into the planar surface
of the bottom substrate. In these methods, the channel networks
enhance the efficiency of the bonding process. For example, these
channel networks typically cover large areas of the surface of the
bottom substrate, or the space between the two substrates. As such,
they can enhance the efficiency with which air is evacuated from
this space between the two substrates, ensuring sufficient contact
between the substrates over most of the planar surfaces of the two
substrates for bonding. This is particularly the case for those
areas between the substrates that are immediately adjacent the
channel structures, where complete bonding is more critical, in
order to properly seal these channels.
[0031] In addition to more efficiently removing air from between
the substrates, the application of vacuum at each of the plurality
of holes in the top substrate, as well as through the intersecting
channel structures between the two substrates results in a more
even application of the pressure forcing the substrates together.
Specifically, unevenly applied pressures in bonding methods can
have substantial adverse effects on the bonding process. For
example, uneven application of pressures on the two substrates
during the bonding process can result in uneven contact between the
two surfaces of the two substrates, which, as described above, can
reduce the efficiency and quality, as well as the effective product
yield of the bonding process.
[0032] Further, even where substrates are completely bonded under
such uneven pressure, e.g., for thermally bonded polymeric
substrates or substrates bonded with adhesives, such uneven
pressures can result in variations in the dimensions of the
internal structures of the device from one location in a
microfabricated device to another. Again, the channel networks
extending across wide areas of the interior portion of the two
substrates, e.g., fabricated into the surface of the second
substrate, allows application of vacuum across a substantially
larger, and more evenly distributed area of the substrates interior
portion.
[0033] In addition to the vacuum chuck, the bonding system shown in
FIG. 3 also includes a heat source, e.g., a controllable heat
source such as heat gun 42, for elevating the temperature of the
substrates 12 and 18 while they are mounted on the platform
surface/mounting table 30. For bonding silica based substrates,
this heat source applies an elevated temperature to the two
substrates to create a preliminary bond between the substrates, so
that they can be readily transferred to an annealing oven without
the substrates shifting substantially relative to each other. This
is generally accomplished by heating the two substrates to between
about 90.degree. C. and about 200.degree. C. In the case of
polymeric substrates, this heat source can take the place of the
annealing oven by elevating the temperature of the polymeric
substrates to appropriate bonding temperatures, e.g., between about
80.degree. C. and 200.degree. C. Further, this can be done while
the substrates are mounted upon the mounting table, and while a
vacuum is being applied to the substrates. This has the effect of
maintaining an even, constant pressure on the substrates throughout
the bonding process. Following such initial bonding, the substrates
are transferred to an annealing oven, e.g., as described above,
where they are subjected to bonding temperatures between about
500.degree. C. and 1400.degree. C., again, as described above.
[0034] Although illustrated in FIG. 3 as a heat gun, it will be
readily appreciated that the heat source portion of the apparatus
may include multiple heat sources, i.e., heat guns, or may include
heating elements integrated into the apparatus itself. For example,
a thermoelectric heater may be fabricated into or placed in thermal
contact with the platform surface/mounting table 30, which itself,
may be fabricated from a thermally conductive material. Such
thermal bonding systems are equally applicable to both polymeric
substrates and silica based substrates, e.g., for overall bonding
of polymeric substrates, or for producing the initial, preliminary
bonding of the silica-based substrates.
[0035] Alternate bonding systems for incorporation with the
apparatus described herein include, e.g., adhesive dispensing
systems, for applying adhesive layers between the two planar
surfaces of the substrates. This may be done by applying the
adhesive layer prior to mating the substrates, or by placing an
amount of the adhesive at one edge of the adjoining substrates, and
allowing the wicking action of the two mated substrates to draw the
adhesive across the space between the two substrates.
[0036] In certain embodiments, the overall bonding system can
include automatable systems for placing the top and bottom
substrates on the mounting surface and aligning them for subsequent
bonding. Typically, such systems include translation systems for
moving either the mounting surface or one or more of the top and
bottom substrates relative to each other. For example, robotic
systems may be used to lift, translate and place each of the top
and bottom substrates upon the mounting table, and within the
alignment structures, in turn. Following the bonding process, such
systems also can remove the finished product from the mounting
surface and transfer these mated substrates to a subsequent
operation, e.g., separation operation, annealing oven for
silica-based substrates, etc., prior to placing additional
substrates thereon for bonding.
[0037] Although the present invention has been described in some
detail by way of illustration and example for purposes of clarity
and understanding, it will be apparent that certain changes and
modifications may be practiced within the scope of the appended
claims. All publications, patents and patent applications
referenced herein are hereby incorporated by reference in their
entirety for all purposes as if each such publication, patent or
patent application had been individually indicated to be
incorporated by reference.
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