Dry etch release of MEMS structures

Details Dry etch release of MEMS structures Dry etch release of MEMS structures

2001-10-29

2003-12-23

Powell, William A. (Department: 1765)

Etching a substrate: processes

Etching of semiconductor material to produce an article...

C134S003000, C134S031000, C216S058000, C216S079000, C438S706000, C438S739000, C438S745000, C438S756000

Reexamination Certificate

active

06666979

ABSTRACT:

FIELD OF THE INVENTION
In general, the present invention relates to a method of dry, plasmaless etch of MEMS (micro-electro-mechanical systems) structures, which includes a release step in which gaps are opened (etched) between various structural surfaces. For example, surface machined cantilever beams and lever arms may be fabricated by etching away sacrificial layers within a structure; or, thin membranes or diaphragms for sensors or pumps may be created by hollowing out areas of a structure.
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.
Various processes have been developed in an attempt to prevent stiction from occurring during fabrication of micromachined arms and beams. For example, in U.S. Pat. No. 6,027,571 to Kikuyama et al., issued Feb. 22, 2000, the inventors describe a wet etching process for micromachining, where the wet etchant preferably includes a surfactant. (Abstract) The surfactant is said to improve wetability during etching so that etching uniformity of a silicon oxide film is improved; in addition, if a silicon surface is exposed during the etching, the roughness of the surface can be suppressed by the surfactant (Col. 2, lines 58-64). Crystalline particles which are byproducts produced during etching can be prevented from adhering to the wafer surface by adding surfactant to the surface treatment. (Col. 3, lines 29-32). The surfactant is evidently used in an attempt to reduce some of the factors which contribute to stiction.
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.)
An article in IEEE Electron Devices Magazine, IEDM 96-761 (1996 IEEE), entitled “Fabrication of Surface Micromachined Polysilicon Actuators Using Dry Release Process of HF Gas-Phase Etching” by Jong Hyun Lee et al. describes a process developed for the dry-release of sacrificial oxide in polysilicon surface micro-machining. Using anhydrous HF gas with a CH
3
OH vapor catalyst (Page 30.1.1, last paragraph, first column), the authors successfully fabricated a vibrating micro-gyroscope structure with “virtually no process-induced stiction”. (Abstract). The authors describe how one of the major issues in surface micromachining is process-induced stiction of highly compliant microstructures to an underlying layer. The stiction of the microstructures is attributed mainly to capillary forces developed during a drying step which follows wet etching of the sacrificial layer. The Lee et al. article references reports by other researchers which describe other methods which have been used in an attempt to solve the stiction problem. For example, Lee et al. mention the use of a micromechanical temporary support, sublimation of the final liquid (present after etching of the sacrificial layer) by freeze-dry, temporary photoresist support with subsequent plasma ashing, removing the final liquid through supercritical state, or using low surface tension liquids. The supercritical dry method is said to generate excellent results, but this method requires the use of high pressure equipment which is not desirable in a fabrication process, due to equipment cost and safety issues.
Applicants' review of the background art in general has indicated that stiction is the primary cause of low yield in the fabrication of MEMS devices. As mentioned above, stiction 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.
To reduce the probability 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. However, in some instances stiction occurs during the release process and prior to application of a stiction-preventing coating.
The present invention relates to a m

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