U.S. patent application number 12/185094 was filed with the patent office on 2008-11-27 for mems passivation with phosphonate surfactants.
This patent application is currently assigned to Texas Instruments Incorporated. Invention is credited to Simon Joshua Jacobs, Seth Adrian Miller.
Application Number | 20080290325 12/185094 |
Document ID | / |
Family ID | 34713257 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080290325 |
Kind Code |
A1 |
Jacobs; Simon Joshua ; et
al. |
November 27, 2008 |
MEMS Passivation with Phosphonate Surfactants
Abstract
Phosphonate surfactants are employed to passivate the surfaces
of MEMS devices, such as digital micromirror devices. The
surfactants are adsorbed from vapor or solution to form
self-assembled monolayers at the device surface. The higher binding
energy of the phosphonate end groups (as compared to carboxylate
surfactants) improves the thermal stability of the resulting
layer.
Inventors: |
Jacobs; Simon Joshua;
(Lucas, TX) ; Miller; Seth Adrian; (Englewood,
CO) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
Texas Instruments
Incorporated
Dallas
TX
|
Family ID: |
34713257 |
Appl. No.: |
12/185094 |
Filed: |
August 3, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11031655 |
Jan 5, 2005 |
7410820 |
|
|
12185094 |
|
|
|
|
60534337 |
Jan 5, 2004 |
|
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Current U.S.
Class: |
252/500 |
Current CPC
Class: |
B81C 2201/112 20130101;
B81B 2201/042 20130101; B82Y 30/00 20130101; B81B 3/0005
20130101 |
Class at
Publication: |
252/500 |
International
Class: |
H01B 1/00 20060101
H01B001/00 |
Claims
1. A MEMS device having at least one surface, said surface coated
with a passivation layer comprising a phosphonate surfactant.
2. The device of claim 1, wherein said phosphonate surfactant
comprises a compound of the formula RPO(OH).sub.2, or salts or
esters of the same, or mixtures of the same, where R is a
hydrocarbon straight chain of 4-18 carbon atoms; saturated or
unsaturated; non-, partially-, or fully-fluorinated; and may
include one or more linear hetero atoms.
3. The device of claim 1, wherein said phosphonate surfactant
comprises n-octylphosphonic acid, octadecylphosphonic acid, salts
or esters of the same, or mixtures of the same.
4. The device of claim 1, wherein said MEMS device is a digital
micromirror device.
5. The device of claim 4, wherein said phosphonate surfactant
comprises a compound of the formula RPO(OH).sub.2, or salts or
esters of the same, or mixtures of the same, where R is a
hydrocarbon straight chain of 4-18 carbon atoms; saturated or
unsaturated; non-, partially-, or fully-fluorinated; and may
include one or more linear hetero atoms.
6. The device of claim 4, wherein said phosphonate surfactant
comprises n-octylphosphonic acid, octadecylphosphonic acid, salts
or esters of the same, or mixtures of the same.
7-20. (canceled)
Description
RELATED APPLICATIONS
[0001] This application claims priority to commonly-owned U.S.
provisional patent application Ser. No. 60/534,3337, filed Jan. 5,
2004.
FIELD OF THE INVENTION
[0002] This invention relates to MEMS devices and the control of
stiction, friction, and related processes through the application
to such a devices of a passivation layer formed from a phosphonate
surfactant.
BACKGROUND
[0003] MicroElectro Mechanical Systems (MEMS) are semiconductor
chips that support a top layer of small mechanical devices, such as
fluid sensors or mirrors. These devices are built onto chips
through growth and etching processes similar to those used to
define the topography of an integrated circuit. These processes are
capable of creating devices with micron dimensions. The MEMS itself
typically packs multiple elements on a single chip.
[0004] A MEMS device, specifically a Digital Micromirror Device
(DMD), is the basis for Digital Light Processing.TM. technology.
The DMD microchip functions as a fast, reflective digital light
switch. The switching is accomplished through the rotation of
multiple small mirrors in response to an electric potential. In a
mirror's "on" state of rotation, light from a projection source is
directed to the pupil of a projection lens and a bright pixel
appears on the projection screen. In the "off" state, light is
directed out of the pupil and the pixel appears dark. Thus the DMD
provides a digital basis for constructing a projected image.
Digital Light Processing.TM. has been employed commercially in
televisions, cinemagraphic projection systems, and business-related
projectors.
[0005] In a typical DMD design, metal, e.g., aluminum, is deposited
to form support posts, a hinge, the mirror itself, and structures
(such as yokes or landing tips) to contain its rotation. The
processes used to define these structures on a DMD (or any other
MEMS device) are known in the art and are not the subject of this
invention. These processes may include growth of a passivation
layer on the mechanical device.
[0006] Passivation layers are added to address several problems
with device operation. One such problem is static friction
(stiction), the static adhesion force between resting bodies in
contact (such as two surfaces of a DMD pixel). Another problem is
dynamic friction, which arises from the contact of moving elements
in the device. Effective passivation layers reduce stiction and
friction by reducing the surface energy of the device. For rotating
devices (such as the hinge in a DMD), repeated movement and
deformation displace molecules and permanently bias the zero state
of the rotation. Passivation layers may reduce this hinge memory
accumulation by stabilizing certain states of the surface.
[0007] Passivation layers are typically formed from surfactants.
Effective surfactants are believed to function by forming
self-assembled monolayers at the device surface. These monolayers
are ordered molecular assemblies formed by the adsorption of a
surfactant on a solid surface. Zhu, et. al., "Self-Assembled
Monolayer used in Micro-motors," report the use of such monolayers,
formed from an octadecyltrichlorosilane precursor, as a passivation
layer for a silicon micromotor. Hornbeck, "Low Surface Energy
Passivation Layer for Micromechanical Devices" (U.S. Pat. No.
5,602,671) has described the use of self-assembled monolayers as
passivation for MEMS devices including DMDs. Suitable
self-assembling carboxylates may be introduced as a vapor under
conditions designed to facilitate the growth of self-assembled
monolayers, as disclosed by Robbins, "Surface Treatment Material
Deposition and Recapture," (U.S. Pat. No. 6,365,229).
[0008] Self-assembled monolayers have been studied outside the
device context. Much of the early research in this field concerned
the interaction of surfactants with gold surfaces; but work has
been published relating to other metals (and metalloids), including
silicon and aluminum. Work pertaining to phosphonate/phosphonic
acid surfactants includes: Gawalt, et. al, "Self-Assembly and
Bonding of Alkanephosphonic Acids on the Native Oxide Surface of
Titanium," Langmuir 2001, 17, 5736-38; Hanson, et. al, "Bonding
Self-Assembled, Compact Organophosphonate Monolayers to the Native
oxide Surface of Silicon," J. Am. Chem. Soc. 2003, 125, 16074-80;
and Nitowski, G., "Topographic and Surface Chemical Aspects of the
Adhesion of Structural Epoxy Resins to Phosphorus Oxo Acid Treated
Aluminum Adherents."
[0009] Within the device context, the passivation layer should be
stable under the intended operating conditions of the MEMS. While
carboxylate surfactants have functioned adequately in commercial
DMD products, the resulting monolayers may thermally desorb under
foreseeable conditions of operation. Such desorption, and the
resulting increase in stiction, friction, and hinge memory
accumulation, would adversely impact the operation of the device.
It is therefore desirable to form passivation layers from
surfactants that bind more tightly with the surface of interest.
That is one objective of the present invention. In addition,
because of their reduced acidity (as compared to some commonly
employed carboxylates), phosphonic acid surfactants may provide
compatibility advantages with common packaging materials, such as
Kovar.
SUMMARY OF THE INVENTION
[0010] The invention provides a MEMS device having an improved
passivation layer formed from a phosphonate surfactant. In certain
embodiments of the invention, the passivation layer is applied to
an aluminum surface. In other embodiments, the MEMS device is a
Digital Micromirror Device, and the mechanical elements coated with
the passivation layer may include the hinges that rotate the
mirrors. The phosphonate surfactant may be introduced either as an
alkylphosphonic acid or as esters of the same.
[0011] The invention also provides methods for assembling a layer
of phosphonate surfactant on the surface of a MEMS device. Where
the phosphonate is introduced as an ester of an alkylphosphonic
acid, that method may include a step of hydroxylating the surface.
In specific embodiments, the methods for assembling layers of these
materials include vapor phase deposition and deposition from
solution.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The mechanical structures of a MEMS device are grown on a
semiconductor surface through any of a variety of methods that are
known in the art. These methods may include conventional
semiconductor processing techniques like sputter metal deposition,
lithography, and plasma etching.
Fabrication of a Digital Micromirror Device
[0013] In one example, a DMD superstructure is grown on an SRAM
address circuit employing standard CMOS technology. A thick oxide
is deposited over Metal-2 of the CMOS and planarized through
chemical mechanical polishing to yield a flat substrate for DMD
fabrication. Construction of the DMD superstructure begins with
deposition and patterning of aluminum for a metal layer. An organic
sacrificial layer (spacer) is then spin-coated, lithographically
patterned and hardened. Holes patterned in the spacer will form
metal support posts after the yoke metal covers their sidewalls.
These posts will support the hinges and the mirror address
electrodes.
[0014] A second metal layer is sputter-deposited and patterned to
form the hinges and other elements, such as springs, supports,
electrodes, or mechanical stops that may be desirable for control
of micromirror motion.
[0015] A second organic sacrificial layer is spin-coated,
patterned, then hardened. The holes patterned in this spacer form
the support posts that secure the mirrors to the underlying yokes.
An aluminum layer is sputter-deposited and patterned over this
spacer to form the mirrors. A final coating of photoresist
completes the wafer.
[0016] Through standard semiconductor processes, the wafers are
singulated, and the individual die are mounted in ceramic headers.
A plasma etching step is then used to remove the photoresist from
the MEMS structures, thereby freeing the superstructure.
Device Passivation
[0017] After the device superstructure has been fabricated, a
passivation layer is applied to it. The passivation layer comprises
a phosphonate surfactant, which may be introduced either as an
alkylphosphonic acid (RPO(OH).sub.2) or esters of the same. For
preferred surfactants, the alkyl group is a hydrocarbon straight
chain having between four and eighteen carbon atoms. It may be
saturated or unsaturated. It may be partially or fully fluorinated.
It may include linear hetero atoms, such as oxygen. Methods for
synthesizing alkyl phosphonic acids and esters are disclosed in,
e.g., U.S. Pat. Nos. 4,108,889; 4,393,011; and 4,655,883. Suitable
phosphonates include materials sold commercially as lubricants. For
reasons of availability, n-octylphosphonic acid (NOPA) and
octadecylphosphonic acid (NOPA) are especially preferred
surfactants.
[0018] The phosphonate surfactant may be introduced as a salt or
ester of the alkylphosphonic acid. For reasons of reactivity and
availability, preferred esters include the methyl ester
(RPO(CH.sub.3).sub.2), ethyl ester (RPO(CH.sub.2CH.sub.3).sub.2)
and trimethylsilyl ester (RPO(Si(CH.sub.3).sub.3).sub.2). Before
the ester is used, it may be desirable to first hydroxylate the
surface to be coated. This can be done by exposing the surface to a
solution of sulfuric acid and hydrogen peroxide, or by exposing the
device to a plasma formed from one or more of the following:
hydrogen, water, ammonia and oxygen. As used herein, the term
"phosphonate surfactant" encompasses surfactants introduced both as
an alkylphosphonic acid and as salts or esters of the same.
[0019] The phosphonate surfactant is contacted with the surface to
be coated under conditions selected to facilitate the formation and
adsorption of a self-assembled monolayer. The surface may be
exposed to a vapor of the phosphonate surfactant--typically at or
near the native surfactant vapor pressure, under vacuum, at
temperatures below 150.degree. C. Alternatively, the surfactant may
be adsorbed from solution. Suitable solution-based methods include
the THF/aerosol method disclosed in Gawalt, et. al, and the
THF/evaporation method disclosed in Hanson, et. al. Water,
isopropyl alcohol, and supercritical CO.sub.2 are other solvents
that may be particularly useful in the adsorption of phosphonate
surfactant monolayers on the surfaces of interest.
[0020] The surface to be coated should be exposed to the
phosphonate surfactant for a time sufficient for the self-assembled
monolayer to form. For vapor-based adsorption, that time is
typically in the range of minutes. For solution-based adsorption,
that time is typically in the range of several hours. For any
process, monolayer formation is conveniently verified by measuring
liquid contact angles on a test surface. For aluminum, the process
is substantially complete when the contact angle for water exceeds
100.degree. or when the contact angle for methylene iodide exceeds
70.degree..
EXAMPLE 1
[0021] Aluminum-coated silicon substrates were cut into
.about.1.4.times.1.4 cm coupons. Sample coupons were pre-washed
with either isopropyl alcohol (IPA) or sodium carbonate solution.
The sodium carbonate-washed substrates were prepared by dipping the
substrates into a 0.1 molal solution (pH=11.47) for 15 seconds
under ambient conditions. The substrates were then rinsed with
deionized, distilled water and air-dried under ambient conditions.
The coupons were exposed to n-octylphosphonic acid (NOPA) or
octadecylphosphonic acid (NOPA) in liquid solution. The coupons
were also exposed, for purposes of comparison, to lauric acid (LA)
in liquid solution. Surfactant solutions were prepared at a 0.0128
molal concentration, and the samples were soaked under ambient
conditions for one hour. The samples were post-washed (with either
water or IPA) and air-dried for a period of at least four hours.
Static water contact angles were measured using a Gardco Model PG-1
Goniometer. After heating for 12 hours at 150.degree. C., the
static water contact angles were measured again.
TABLE-US-00001 Initial Contact Angle Contact After 12 h at Sample
Prep History Angle 150.degree. C. 1 Water pre-wash 69 +/- 5 59 +/-
3 2 Sodium carbonate pre-wash 56 +/- 4 58 +/- 4 3 NOPA, water
carrier, sodium 108 +/- 4 72 +/- 3 carbonate pre-wash, water post-
wash 4 NOPA, IPA carrier, water pre- 102 +/- 5 74 +/- 7 wash, water
post-wash 5 OPA, IPA carrier, IPA pre-wash, 114 +/- 6 110 +/- 2
water post-wash 6 LA, IPA carrier, IPA pre-wash, 110 +/- 4 66 +/- 2
IPA post-wash 7 LA, water carrier, IPA pre-wash, 113 +/- 3 68 +/- 5
IPA post-wash
[0022] This example demonstrates the improved thermal stability (as
compared to carboxylates) of monolayers formed from phosphonate
surfactants.
EXAMPLE 2
[0023] Aluminum-coated silicon substrates were cut into
.about.1.4.times.1.4 cm coupons. Sample coupons were either treated
"as received" or washed with sodium carbonate. Sodium
carbonate-washed substrates were prepared by dipping the substrates
into a 0.1 molal solution (pH=11.47) for 15 seconds under ambient
conditions. The substrates were then rinsed with deionized,
distilled water and air-dried under ambient conditions. The coupons
were exposed to n-octylphosphonic acid (NOPA) or
octadecylphosphonic acid (NOPA) in liquid solution. The coupons
were also exposed to lauric acid (LA), stearic acid (ST) and
various surfactant mixtures in liquid solution. Surfactant
solutions were prepared at a 0.0128 molal concentration, and the
samples were soaked under ambient conditions for one hour. The
samples were post-washed with water and air-dried for a period of
at least four hours. Static water contact angles were measured
(using a Gardco Model PG-1 Goniometer) before and after a 12-hour,
150.degree. C. thermal exposure test, and before and after a
24-hour ambient soak test.
TABLE-US-00002 Contact Contact Angle: 12 Angle: 24 Cleaning Initial
Contact hours at hour water Sample Surfactant Solvent Method Angle
150.degree. C. soak 1 NOPA water as received 111 +/- 2 65 +/- 2 55
+/- 4 2 NOPA water carbonate 115 +/- 4 88 +/- 9 62 +/- 27 3 LA IPA
as received 113 +/- 2 55 +/- 2 59 +/- 3 4 LA IPA carbonate 112 +/-
3 66 +/- 4 38 +/- 15 5 OPA IPA as received 115 +/- 2 111 +/- 4 109
+/- 3 6 OPA IPA carbonate 115 +/- 3 104 +/- 5 107 +/- 6 7 ST IPA as
received 116 +/- 3 77 +/- 3 106 +/- 4 8 ST IPA carbonate 115 +/- 3
80 +/- 3 70 +/- 20 9 OPA/ST IPA as received 112 +/- 2 107 +/- 5 100
+/- 4 (80/20) 10 OPA/ST IPA carbonate 119 +/- 5 102 +/- 6 105 +/- 4
(80/20) 11 OPA/ST IPA as received 115 +/- 2 112 +/- 4 105 +/- 2
(50/50) 12 OPA/ST IPA carbonate 115 +/- 2 102 +/- 6 97 +/- 4
(50/50) 13 OPA/ST IPA as received 118 +/- 2 91 +/- 2 108 +/- 5
(20/80) 14 OPA/ST IPA carbonate 113 +/- 3 77 +/- 6 90 +/- 3
(20/80)
[0024] This example demonstrates the improved thermal stability (as
compared to carboxylates) of monolayers formed from phosphonate
surfactants.
* * * * *