U.S. patent application number 09/885226 was filed with the patent office on 2002-04-18 for solid-phase welded optical element attach process.
This patent application is currently assigned to AXSUN Technologies, Inc.. Invention is credited to Payer, Robert L., Racz, Livia M..
Application Number | 20020043551 09/885226 |
Document ID | / |
Family ID | 27095361 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020043551 |
Kind Code |
A1 |
Payer, Robert L. ; et
al. |
April 18, 2002 |
Solid-phase welded optical element attach process
Abstract
A micro-optical component comprises an optical element for
interacting with an optical beam and a mounting structure for
attaching the optical element to an optical bench. This optical
element is solid phase welded to the mounting structure. Solid
phase welding has advantages in that it can be performed at lower
temperatures than most soldering, even some eutectic soldering.
Solid-phase welding, however, is much more robust during subsequent
temperature cycling. This is especially important when the optical
components undergo subsequent high temperature cycling.
Inventors: |
Payer, Robert L.;
(Pepperell, MA) ; Racz, Livia M.; (Belmont,
MA) |
Correspondence
Address: |
J GRANT HOUSTON
AXSUN TECHNOLOGIES INC
1 FORTUNE DRIVE
BILLERICA
MA
01821
US
|
Assignee: |
AXSUN Technologies, Inc.
|
Family ID: |
27095361 |
Appl. No.: |
09/885226 |
Filed: |
June 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09885226 |
Jun 20, 2001 |
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09707721 |
Nov 7, 2000 |
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09707721 |
Nov 7, 2000 |
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09648348 |
Aug 25, 2000 |
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Current U.S.
Class: |
228/254 ;
228/110.1; 228/253 |
Current CPC
Class: |
G02B 7/003 20130101;
G02B 27/62 20130101; G02B 6/4226 20130101; G02B 6/4239 20130101;
G02B 6/3692 20130101; G02B 6/4238 20130101; G02B 6/3636 20130101;
G02B 6/3656 20130101; G02B 6/4237 20130101 |
Class at
Publication: |
228/254 ;
228/110.1; 228/253 |
International
Class: |
B23K 001/06; B23K
005/20; B23K 020/10; B23K 035/12 |
Claims
What is claimed is:
1. A micro-optical component, comprising: an optical element for
interacting with an optical beam; and a mounting structure for
attaching the optical element to an optical bench; wherein the
optical element is solid-phase welded to the mounting
structure.
2. A micro-optical component as claimed in claim 1, wherein the
optical element is thermocompression bonded to the mounting
structure.
3. A micro-optical component as claimed in claim 1, wherein the
optical element is thermosonically bonded to the mounting
structure.
4. A micro-optical component as claimed in claim 1, wherein the
optical element is ultrasonically welded to the mounting
structure.
5. A micro-optical component as claimed in claim 1, wherein the
optical element comprises a lens substrate.
6. A micro-optical component as claimed in claim 1, wherein the
optical element comprises a microelectromechanical device.
7. A micro-optical component as claimed in claim 1, wherein the
optical element comprises a Fabry-Perot tunable filter.
8. A micro-optical component as claimed in claim 1, wherein the
mounting structure is fabricated from a metal.
9. A micro-optical component as claimed in claim 1, wherein the
mounting structure is metal coated.
10. A micro-optical component as claimed in claim 1, wherein the
mounting structure is coated with a thermocompression bond
metal.
11. A micro-optical component as claimed in claim 10, wherein the
bond metal comprises gold.
12. A micro-optical component as claimed in claim 1, further
comprising depositing bond metal bumps on the mounting
structure.
13. A micro-optical component as claimed in claim 1, further
comprising depositing bond metal bumps on the optical element.
14. A micro-optical system, comprising: an optical element for
interacting with an optical beam; a mounting structure, the optical
element being solid-phase welded to the mounting structure; and an
optical bench, the mounting structure being solder bonded to the
optical bench.
15. A micro-optical system as claimed in claim 14, wherein the
optical element is thermocompression bonded to the mounting
structure.
16. A micro-optical system as claimed in claim 14, wherein the
optical element is thermosonically bonded to the mounting
structure.
17. A micro-optical system as claimed in claim 14, wherein the
optical element is ultrasonically welded to the mounting
structure.
18. A micro-optical system as claimed in claim 14, wherein the
optical element comprises a lens substrate.
19. A micro-optical system as claimed in claim 14, wherein the
optical element comprises a microelectromechanical device.
20. A micro-optical system as claimed in claim 14, wherein the
optical element comprises a Fabry-Perot tunable filter.
21. A micro-optical system as claimed in claim 14, wherein the
mounting structure is fabricated from a metal.
22. A micro-optical system as claimed in claim 14, wherein the
mounting structure is metal coated.
23. A micro-optical system as claimed in claim 14, wherein the
mounting structure is coated with a thermocompression bond
metal.
24. A micro-optical system as claimed in claim 23, wherein the bond
metal comprises gold.
25. A micro-optical system as claimed in claim 14, further
comprising depositing bond metal bumps on the mounting
structure.
26. A micro-optical system as claimed in claim 14, further
comprising depositing bond metal bumps on the optical element.
27. A process for assembling an optical system, the process
comprising: solid-phase welding an optical element to a mounting
structure; and then attaching the mounting structure to an optical
bench.
28. A process as claimed in claim 27, wherein the step of
solid-phase welding the optical element to the mounting structure
comprises thermocompression bonding the mounting structure and the
optical element.
29. A process as claimed in claim 27, wherein the step of
solid-phase welding the optical element to the mounting structure
comprises thermosonically bonding the mounting structure and the
optical element.
30. A process as claimed in claim 27, wherein the step of
solid-phase welding the optical element to the mounting structure
comprises ultrasonically bonding the mounting structure and the
optical element.
31. A process as claimed in claim 27, wherein the step of attaching
the mounting structure to the optical bench comprises solder
bonding the mounting structure to the optical bench.
32. A process as claimed in claim 27, wherein the step of attaching
the mounting structure to the optical bench comprises: depositing
solder material on solder mating surfaces of the mounting structure
and the optical bench; reflowing the solder material to join the
mating surfaces.
33. A process as claimed in claim 27, wherein the step of
solid-phase welding the optical element to the mounting structure
comprises coating weld mating surfaces of the optical element and
the mounting structure with bond material.
34. A process as claimed in claim 27, wherein the step of
solid-phase welding the optical element to the mounting structure
comprises coating weld mating surfaces of the optical element and
the mounting structure with gold.
Description
BACKGROUND OF THE INVENTION
[0001] The packaging of fiber optic systems typically requires the
installation of a free space optical element within a hermetic
package. This is even required for integrated waveguide systems
since light must be coupled across a free space link between the
optical fiber endface and the waveguide "chip". In a typical
configuration, the optical element is connected to an optical
bench, sometimes also referred to as a submount.
[0002] In the packaging of these systems, especially within the
hermetic package, metal-based bonding techniques are desirable and
often required. Epoxy-based bonds generally are not robust across
temperature and time. Moreover, out-gassed organics can damage
optoelectronic components, such as semiconductor lasers. As a
result, laser welding and solder bonding are the preferred optical
element bonding techniques in fiber optic system manufacture.
[0003] A different class of metal bonding is referred to as
solid-phase welding. Two metallic materials are brought into
contact with heat, pressure, and/or ultrasonic energy. This results
in electron sharing or interdiffusion of atoms, which forms a
mechanical bond.
[0004] One type of solid phase welding is referred to as
thermocompression bonding. The process involves two clean, ductile
metals that are forced together and heated to a temperature less
than their melting points. Gold is common bond metal, with
aluminum, copper, or nickel being less common due to oxidation
concerns. Temperatures of between 300.degree.-500.degree. C. are
common for gold welding. Welding to silicon is also possible with
higher heat and pressures.
[0005] In order to achieve tight contact between the bonding
surfaces in thermocompression bonding, a very small mechanical
scrubbing action can be used. Generally, the process takes place in
a nitrogen environment to prevent oxidation.
[0006] Ultrasonic and thermosonic are two other examples of
solid-phase welding. These welding processes can be performed at
lower pressures and temperatures than thermocompression bonding,
but require the addition of ultrasonic energy. Ultrasonic bonding
can be performed at room temperature, whereas temperatures of
100-150.degree. C. are common for thermosonic gold bonding.
SUMMARY OF THE INVENTION
[0007] Laser welding can be difficult to implement in fiber optic
packaging production lines. A clear path to weld site is required.
The required lasers are expensive and complex alignment/focusing
tolerances require constant calibration. Moreover, the technique
can be difficult to integrate with some optical elements because of
the large thermal gradients associated with the process, and the
concomitant residual material stresses can impact production
yields. For example, laser welding is commonly used to attach
optical elements such as optical fibers to clip mounting structures
and the clips to a submount or bench. This process, however,
requires that the optical fiber first be inserted through and
bonded to a tube to which the clip is welded since the silica fiber
could be damaged if directly irradiated by the laser welder.
[0008] Solder-based joinery avoids some of the problems associated
with laser welding. The joints, however, can have high residual
material stresses and have exhibited poor performance in situations
of high thermal expansion coefficient (TEC) mismatch between the
optical element and the mounting structure. Moreover, eutectic
solder joinery requires careful control over alloy compositions and
typically depends on some diffusion to raise melting point so that
the joint is robust against subsequent reflow cycles.
[0009] In contrast, solid phase welding has advantages in that it
can be performed at lower temperatures than most soldering, even
some eutectic soldering. Solid-phase welding, however, is much more
robust during subsequent temperature cycling. This is especially
important when the optical components undergo subsequent high
temperature cycling.
[0010] In general, according to one aspect, the invention features
a micro-optical component, which comprises an optical element for
interacting with an optical beam and a mounting structure for
attaching the optical element to an optical bench. This optical
element, according to the invention, is solid phase welded to the
mounting structure.
[0011] In the current embodiment, the element is thermocompression
bonded to the mounting structure. In other implementations,
thermosonic or ultrasonic welding are used, however.
[0012] In one example, the optical element comprises a lens
substrate having an antireflection (AR) coated convex or concave
lens formed in the substrate. In other examples, the optical
element comprises a microelectromechanical device, such as a
Fabry-Perot tunable filter, for example.
[0013] In the current implementation, the mounting structure
includes a metal. Generally, these mounting structures are small.
Currently, the LIGA fabrication process is preferred because of its
ability to yield high aspect ratio structures. LIGA is a German
acronym that stands for lithography, plating, and molding
(lithographic, galvanoformung, abformung). Alternatively, other
microforming fabrication processes are used to make the mounting
structures. Laser machining, spark erosion, and deep reactive ion
etching are alternatives.
[0014] In order to enable the solid phase welding, such as
thermocompression bonding, the mounting structures are typically
coated with a bond metal, if not directly fabricated in the metal.
Currently gold is used. It can be sputtered, plated, or sonically
deposited on the mounting structure. Alternatively, bond metal
bumps are placed at discrete locations on the mounting structure
and/or the optical element.
[0015] In general, according to another aspect, the invention also
features a micro-optical system. The system comprises an optical
element and a mounting structure. The optical element is
solid-phase welded to the mounting structure. Thereafter, the
mounting structure is solder bonded to the optical bench. This
invention is important in this application, since the solid-phase
welding can withstand the subsequent high temperature cycling
associated with the solder bonding process.
[0016] In general, according to still another aspect, the invention
features a process for assembling an optical system. This process
comprises solid-phase welding an optical element to a mounting
structure and then attaching the mounting structure to an optical
bench by solder bonding, for example.
[0017] The above and other features of the invention including
various novel details of construction and combinations of parts,
and other advantages, will now be more particularly described with
reference to the accompanying drawings and pointed out in the
claims. It will be understood that the particular method and device
embodying the invention are shown by way of illustration and not as
a limitation of the invention. The principles and features of this
invention may be employed in various and numerous embodiments
without departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In the accompanying drawings, reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale; emphasis has instead been placed upon
illustrating the principles of the invention. Of the drawings:
[0019] FIG. 1 is a perspective view of an exemplary mounting
structure with an exemplary optical element, which have been
thermocompression bonded to each other, according to the present
invention;
[0020] FIG. 2 is a top plan view of the welded mounting structure
and optical element showing the bond metal bumps;
[0021] FIG. 3 is a perspective view of another exemplary mounting
structure and optical element, which have been welded according to
the present invention;
[0022] FIGS. 4A and 4B are perspective, exploded views of a MEMS
tunable filter optical component;
[0023] FIG. 5 is an elevation illustrating the attachment of a MEMS
optical component to a bench using a bonder; and
[0024] FIG. 6 is a process diagram illustrating a process for
thermocompression bonding the optical element to the mounting
structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] FIG. 1 shows an exemplary mounting structure with an
exemplary optical element, which have been solid-phase welded to
each other according to the present invention.
[0026] In the illustrated example, the mounting structure comprises
a micro-fabricated metal frame. Currently, these are fabricated
using the LIGA process. The bulk or plate metal is currently a
nickel or nickel alloy, such as nickel iron.
[0027] Additional description of the mounting and alignment
structures, and associated optoelectronic packaging technology is
found in U.S. Pat. application Nos. 09/707,721, filed on Nov. 7,
2000 by Flanders, et al., and 09/648,348, filed on Aug. 25, 2000 by
Masghati, et al., the teachings of these applications being
incorporated herein by this reference in their entirety.
[0028] The exemplary optical element 52 comprises a substrate 54.
In an example, the substrate 54 is a spectral filter substrate. In
a more common example, a lens is formed in the substrate, such as a
convex or concave lens. These lenses are currently manufactured
using an etch process such as gray-scaling or a mass transport
process. The optical element 52 is connected to the mounting
structure 50 via bond metal bumps 110. Generally, a gold bond metal
is used.
[0029] FIG. 2 better shows the profile of the bumps 110. In one
implementation, the bumps are placed on the mounting structure
using a wire bonder and subsequently tamped. During the
thermocompression process, additional flattening occurs to ensure a
robust bond between the optical element 52 and the mounting
structure 50.
[0030] Returning to FIG. 1, the mounting structure 50 is in turn
attached to an optical bench 10, sometimes referred to as a
submount. In the current implementation, the preferred mode of
attachment is a solder bond between the bench 10 and mounting
structure 50.
[0031] FIG. 3 illustrates another example of a mounting structure
and thermocompression bond. In this example, the mounting structure
is not designed to be easily plastically deformable for a post
installation alignment, for example. Further, in this example,
discrete bond metal bumps are not placed on the optical element 52
or the mounting structure 50. Instead, a thick coating of
compression bond metal 114 is placed at least on an optical element
interface surface 58 of the mounting structure 50. A corresponding
bond metal layer 60 is also selectively deposited on the optical
element 52 to provide a robust metal-to-metal thermocompression
bond interface. This bond metal layer on the optical element is
preferably patterned to avoid obstructing an optical axis 12 of the
optical element to thereby avoid interference between the bond
metal and the optical signals that interact with the optical
element 52.
[0032] FIGS. 4A and 4B illustrate still another example of a
mounting structure 50. Geometry of this mounting structure is
somewhat different to accommodate a relatively large MEMS optical
element 52. In the illustrated example, this MEMS device 52 is an
optical membrane or Fabry-Perot tunable filter as described in U.S.
Pat. application Serial No. 09/797,529, filed on Mar. 1 2001,
entitled "Integrated Tunable Fabry-Perot Filter and Method of
Making Same", this application being incorporated herein by this
reference.
[0033] As described previously, bond metal bumps 110 are deposited
on the weld mating surfaces of the component interface of the
mounting structure 50. Corresponding bond metal pads 116 are
deposited on the weld mating surfaces of the MEMS optical element
52. The MEMS optical element is then solid-phase welded to the
mounting structure using a combination of heat, pressure, and/or
ultrasonic energy as illustrated by arrows 118. Currently, gold is
the bond metal.
[0034] FIG. 5 also illustrates installation of the optical
component mounting structure 50 onto the bench 10. Specifically, a
chuck 202 of a pick and place bonder engages and picks-up the
mounting structure 52, then places it on the bench 10. Solder
preforms or predeposited solder pads 120 are located at the point
of attachment or solder mating surfaces between the feet 150A, 150B
of the mounting structure 50 and the bench 10. Heating element in
the bonder chuck 202, for example, then raises the temperature of
the solder pads 120, so that the solder is reflowed to thereby
solder attach the mounting structure 50 to the bench 10.
[0035] In another example, the mounting structure 50 is located on
the bench manually and/or using a templating system. The solder
pads 120 are then reflowed in a solder reflow oven in a batch
process.
[0036] Shown in phantom are the thermocompression metal bumps 110
that connect the substrate 56 of the MEMS optical element 52 to the
mounting structure 50.
[0037] FIG. 6 illustrates a solid-phase welding process utilizing
thermocompression bonding between the mounting structures 52 and
the optical elements 52.
[0038] Gold 5-8 .mu.m thick is deposited on the mounting structures
50 in step 610, using a plating process for example.
[0039] Alternatively, if a ball bumping process is used, a
thermosonic ball bonder with a gold wire feed is used to form the
bond metal bumps 110 on the mounting structure 50 or the optical
element 52 in step 612. Currently, 1.0 MIL gold wire is used with
0.5-3% elongation. The bumps 110 are two to three times the wire
diameter in size. The stage temperature of the thermosonic ball
bonder is 130.degree. C..+-.5.degree. C.
[0040] In step 614, the bumps are tamped to a uniform height.
Currently, a tamping tool with a head polished with diameter of 0.9
MIL and a tool angle 20-30.degree. is used. The diameter of the
bumps after tampling is 100 micrometers .+-.20 micrometers. The
bump heights are 25 micrometers.+-.5 micrometers.
[0041] In step 616, the bond metal layer is deposited on the
optical element. Currently, a tri-metal system of 500 Angstroms Ti,
500 Angstroms Pt, and 5000 Angstroms Au is used for good
adhesion.
[0042] Finally, in step 618, the element is bonded to the
structure. Currently, temperature of 320.degree. C..+-.2.degree. C.
for 3 minutes is used. The force is between 200 and 350 grams.
[0043] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *