Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2002-11-20
2004-12-14
Coleman, W. David (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S477000, C438S706000, C438S216000, C438S002000
Reexamination Certificate
active
06830950
ABSTRACT:
FIELD OF THE INVENTION
In general, the present invention is an integrated method for release and passivation of MEMS (micro-electro-mechanical systems) structures. In particular, the invention pertains to a method of improving the adhesion of a hydrophobic self-assembled monolayer (SAM) coating to a surface of a MEMS structure, for the purpose of preventing stiction.
BRIEF DESCRIPTION OF THE BACKGROUND ART
Micromachining technology compatible with semiconductor processes is used to produce a number of devices such as piezoelectric motors containing cantilever beams, hinges, accelerometers, reflector antennae, microsensors, microactuators, and micromirrors, for example. One of the most popular microactuators is an electrostatic comb driver, due to its simplicity in fabrication and low power consumption. Surface micromachining fabrication processes for the electrostatic comb driver, as well as other beams and lever arms, have problems with stiction of such beams and lever arms to an underlying layer over which the beam or arm extends. The lever arm becomes deformed from its intended position, so that it does not extend out as desired. In the case of a membrane or diaphragm, the membrane or diaphragm becomes deformed from its intended position and may become stuck to an adjacent surface. Stiction is the number one yield-limiting problem in the production of the kinds of devices described above.
FIGS. 1A through 1C
are simple schematics showing a cross-sectional side view of a starting structure for surface machining of a lever arm, the desired machined lever arm, and a lever arm which has been rendered non-functional due to stiction, respectively.
The
FIG. 1A
structure shows a substrate layer
102
(typically single crystal silicon), a portion of which is covered with a sacrificial layer
104
(typically silicon oxide), and a lever arm layer
106
(typically polysilicon) which is in contact with and adhered to substrate layer
102
at one end of lever arm layer
106
.
FIG. 1B
shows the
FIG. 1A
structure after the removal of sacrificial layer
104
to produce the desired free-moving lever arm
107
. The height “h” of gap
108
between lever arm
107
and substrate
102
, the length “l”, and the cross-sectional thickness “t” of the lever arm
107
depend on the particular device in which the structure is employed. In many instances the relative nominal values of “h”, “l”, and “t” are such that capillary action during the fabrication process; or contaminants formed as byproducts of the fabrication process; or van der Waals forces; or electrostatic charges on the upper surface
110
of substrate layer
102
and/or on the undersurface
112
of lever arm layer
106
, may cause lever arm
106
to become stuck to the upper surface
110
of substrate layer
102
. This problem is referred to as “stiction”, and is illustrated in FIG.
1
C. Stiction may occur during formation of the lever arm
107
, or may occur subsequent to fabrication of the device and during packaging, shipment, or use (in-use stiction) of the device. A single crystal silicon or polysilicon surface of the kind which is frequently used to fabricate a lever arm, beam, membrane, or diaphragm is hydrophilic in nature, attracting moisture, which may cause stiction.
Stiction, which is the primary cause of low yield in the fabrication of MEMS devices, is believed to result from a number of sources, some of the most significant being capillary forces, surface contaminants, van der Waals forces, and electrostatic attraction. Factors which may contribute to stiction include: warpage due to residual stresses induced from materials; liquid-to-solid surface tension which induces collapse; drying conditions during processing; adverse and harsh forces from wet baths; aggressive designs (i.e., long and thin beams); surface-to surface attractions; inadequate cleaning techniques; aggressive cleaning techniques; and environments subsequent to fabrication, including packaging, handling, transportation, and device operation.
Various processes have been developed in an attempt to prevent stiction from occurring during fabrication of micromachined arms and beams. To reduce the possibility of stiction subsequent to release of a beam, lever arm, membrane, or diaphragm (so that it extends over open space), a surface treatment may be applied and/or a coating may be applied over freestanding and adjacent surfaces. For example, in U.S. Pat. No. 6,069,149, to Hetrick et al, issued Aug. 1, 2000, the inventors disclose a method for fabricating an adhesion-resistant microelectromechanical device. Amorphous hydrogenated carbon is used as a coating or structural material to prevent adhesive failures during the formation and operation of a microelectromechanical device. (Abstract) The amorphous hydrogenated carbon (AHC) coating is applied on the micromachined device after removal of the sacrificial layer and release of the structure. The sacrificial layer is removed in a wet etching solution such as hydrofluoric acid or buffered HF acid. (Col. 7, lines 26-32.) The method is said to reduce adhesive forces between microstructure surfaces by altering their surface properties. The AHC is said to create a hydrophobic surface, which results in lower capillary forces and an associated reduction in stiction. (Col. 2, lines 66-67, continuing at Col. 3, lines 1-4.)
U.S. Pat. No. 5,403,665, issued Apr. 4, 1995, to Alley et al., discloses a method of applying a self-assembled alkyltrichlorosilane monolayer lubricant to micromachines. Octadecyltrichlorosilane (OTS; C
18
H
37
SiCl
3
) is provided as an example of an alkyltrichlorosilane. In a dilute, non-polar, non-aqueous solution, OTS will deposit on silicon, polysilicon, and silicon nitride surfaces that have been previously treated to form a hydrophilic chemical oxide. Treatment of the silicon, polysilicon, or silicon nitride surfaces may be accomplished with an approximately 5 to 15 minute exposure to a hydrophilic chemical oxide promoter such as Piranha (H
2
O
2
:H
2
SO
4
), RCA SC-1, or room temperature H
2
O
2
. This treatment changes silicon and polysilicon surfaces from hydrophobic to hydrophilic. Thus, the surface will have a thin layer of adsorbed water. The OTS reacts with the thin adsorbed water layer that is present on the treated surface to form a single layer of molecules that are chemically bonded to the surface. (Col. 3, lines 23-40; Col. 4, lines 19-30)
SUMMARY OF THE INVENTION
The present invention pertains to the application of a hydrophobic, self-assembled monolayer (SAM) coating on a surface of a MEMS (micro-electro-mechanical systems) structure, for the purpose of preventing stiction. In particular, the invention pertains to a method of improving the adhesion of a hydrophobic SAM coating to a surface of a MEMS structure.
Self-assembled monolayer (SAM) coatings are known in the art. Self-assembly is a process in which a single, densely packed molecular layer of a material is selectively deposited on a fresh reactive surface. The process self-terminates after single layer coverage is achieved. SAM coatings are typically deposited from precursor long-chain hydrocarbons or fluorocarbons with a chlorosilane-based head, such as alkylchlorosilanes. Effective alkylchlorosilanes include OTS (octadecyltrichlorosilane; C
18
H
37
SiCl
3
), FDTS (perfluorodecyltrichlorosilane; C
10
H
4
F
17
SiCl
3
), and DMDS (dimethyldichlorosilane; (CH
3
)
2
SiCl
2
), by way of example, and not by way of limitation. The chemical structures of OTS and FDTS are shown in
FIG. 2A
(respectively indicated by reference numerals 200 and 210).
To improve the adhesion, prior to the application of a SAM coating, surfaces of a MEMS structure are treated with a plasma which was generated from a source gas comprising oxygen and, optionally, a source of hydrogen. The treatment oxidizes the surfaces, which are then reacted with hydrogen to form bonded OH groups on the surfaces. The hydrogen source may be present as part of the plasma source gas, so that the bonded OH groups are created during treatment of the surfaces with the p
Chinn Jeffrey D.
Cooper James A.
Guenther Rolf A.
Leung Toi Yue Becky
Rattner Michael B.
Applied Materials Inc.
Bean Kathi
Church Shirley L.
Coleman W. David
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