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Patent Issued for Forming Curved Features Using a Shadow Mask

January 29, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors De Brabander, Gregory (San Jose, CA); Nepomnishy, Mark (San Jose, CA), filed on March 31, 2011, was published online on January 14, 2014.

The patent's assignee for patent number 8628677 is FUJIFILM Corporation (Tokyo, JP).

News editors obtained the following quote from the background information supplied by the inventors: "Many MEMS devices include piezoelectric actuators that deflect under applied electric voltages. Examples of such devices include fluid ejection systems that eject fluid in response to the actuation of a piezoelectric actuator connected to a fluid path. A printhead module in an ink jet printer is an example of a fluid ejection system. A printhead module typically has a line or an array of nozzles with a corresponding array of ink paths and associated actuators, and drop ejection from each nozzle can be independently controlled by one or more controllers.

"A printhead module can include a semiconductor printhead die that is etched to define a fluid flow path that includes a pumping chamber. A piezoelectric actuator can be formed on one side of the pumping chamber, and in operation, the piezoelectric actuator can flex in response to a driving voltage signal to drive fluid along the ink path. The piezoelectric actuator includes a layer of piezoelectric material that changes geometry (i.e., actuates) in response to the driving voltage applied across the piezoelectric layer by a pair of opposing electrodes.

"A piezoelectric element that is curved, such as a dome-shaped or dent-shaped piezoelectric membrane, can produce a larger displacement under a given driving voltage as compared to a flat piezoelectric element of similar lateral dimensions. Since the magnitude of the piezoelectric displacement affects the driving voltage that is required to eject fluid droplets of a desired drop volume, and hence, affects the power efficiency of the printhead module, piezoelectric actuators having curved piezoelectric membranes have been proposed. Various fabrication methods have been proposed to produce piezoelectric membranes that are curved or have curved features."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "This specification describes technologies related to MEMS fabrication processes for producing a substrate surface and membranes having curved features.

"When a thin layer of material is uniformly deposited on a profile-transferring substrate surface, the layer of material assumes a shape that conforms to the profile of the profile-transferring substrate surface. To form a membrane having curved features (e.g., a piezoelectric membrane having an array of concave recesses or dents formed therein), a profile-transferring substrate surface having curved surface features (e.g., an array of concave recesses or dents) is first prepared. A shadow mask having through-holes formed therein is used in an isotropic etching process for etching recesses in a substrate surface. The through-holes in the shadow mask each includes a constricted portion above a widened bottom opening. The constricted portion partially blocks the impinging plasma over the peripheral portion of the substrate area enclosed by the bottom opening of each through-hole, such that a recess with a concave profile can be formed in the central portion of the area enclosed by the bottom opening of the through-hole.

"In general, in one aspect, a process for forming dents in a substrate includes the actions of attaching a bottom surface of a shadow mask to a top surface of the substrate, the shadow mask including a plurality of through-holes, each through-hole leading from a top surface of the shadow mask to the bottom surface of the shadow mask and having a respective lower portion adjacent to the bottom surface and a respective upper portion closer to the top surface of the shadow mask than the respective lower portion, the respective lower portion of the through-hole having a respective bottom opening in the bottom surface of the shadow mask, and a vertical projection of the respective upper portion of the through-hole on the bottom surface of the shadow mask falling entirely within the respective bottom opening of the through-hole; exposing the top surface of the substrate to isotropic plasma etching through the through-holes of the shadow mask; and removing the shadow mask after the dents have been formed to a predetermined size in the top surface of the substrate.

"In some implementations, the respective upper and lower portions of each through-hole are co-axial cylindrical holes.

"In some implementations, the respective upper portion of each through-hole is adjacent to the top surface.

"In some implementations, attaching the bottom surface of the shadow mask to the top surface of the substrate further includes: applying an RCA 1 cleaning solution to the bottom surface of the shadow mask and the top surface of the substrate; and after the applying, pressing the bottom surface of the shadow mask against the top surface of the substrate to form a temporary bond.

"In some implementations, the method further includes reusing the shadow mask for forming dents in a second substrate.

"In some implementations, the shadow mask has a total thickness between 50-700 microns.

"In some implementations, the vertical projection has a width between 10-300 microns.

"In some implementations, for each through-hole, a ratio between a width of the vertical projection and a depth of the through-hole is between 1:30 to 1:50.

"In some implementations, for each through-hole, the respective bottom opening of the through-hole has a width three-times larger than a width of the vertical projection of the respective upper portion of the through-hole.

"In some implementations, a mixture of CF4, CHF3, and He is used to plasma-etch the substrate.

"In some implementations, a mixture of SF6 and Ar or He is used to plasma-etch the substrate.

"In some implementations, the method further includes: estimating an etch rate associated with the isotropic plasma etching in the substrate; and stopping the isotropic plasma etching after a time period determine based on the etch rate.

"In some implementations, the method further includes polishing the top surface of the substrate after removal of the shadow mask.

"In some implementations, the method further includes: prior to the attaching, forming a protective layer over exposed surfaces of the shadow mask to protect the shadow mask from being altered during the isotropic plasma etching.

"In some implementations, forming the protective layer over the exposed surfaces of the shadow mask further includes: oxidizing the exposed surfaces of the shadow mask to form an oxide layer.

"In some implementations, the shadow mask is made of silicon, glass, aluminum, or graphite.

"In some implementations, attaching the bottom surface of the shadow mask to the top surface of the substrate further includes: aligning the plurality of through-holes relative to predetermined positions on the top surface of the substrate; and pressing the bottom surface of the shadow mask to the top surface of the substrate to form a temporary bond.

"In some implementations, removing the shadow mask after the dents have been formed to a predetermined size further includes: separating the shadow mask from the substrate without damaging the shadow mask or the substrate.

"In some implementations, attaching the bottom surface of the shadow mask to the top surface of the substrate further includes: annealing the temporary bond to form a permanent bond between the bottom surface of the shadow mask to the top surface of the substrate.

"In some implementations, removing the shadow mask after the dents of a predetermined size have been formed further includes: gradually removing materials of the shadow mask to re-expose the top surface of the substrate.

"Particular implementations of the subject matter described in this specification can be implemented to realize one or more of the following advantages.

"A profile-transferring substrate surface having curved surface features can be formed via a series of MEMS fabrication processes. The sizes, shapes, and locations of the curved features formed in the profile-transferring substrate surface are uniform and controllable. In addition, the densities of the curved features in a profile-transferring substrate surface can be higher than those achievable by injection molding or mechanical means. By using the profile-transferring substrate surface produced according to the methods disclosed in this specification, membranes of various materials can be formed over the profile-transferring substrate surface, where each membrane assumes curved features conforming to the curved surface features existing in the profile-transferring substrate surface, and the curved features in the membranes also have well controlled sizes, shapes, locations, and high densities.

"In addition, the grain structures of a membrane formed by material deposition over the profile-transferring substrate surface, such as a piezoelectric membrane deposited by sputtering, can be more uniform in size, shape, and distribution, and have more uniform alignment than those achievable by injection molding or mechanical means. The more uniform and aligned grain structures can help improve the lifetime of the membrane during repeated actuations.

"In addition, micro-defects (e.g., both preexisting micro-defects and those created during etching) can exist near the bottom edges of the through-holes in a mask. These micro-defects can cause irregularities in the substrate surface near the bottom edges of the through-holes. By using a shadow mask to create the dents in the substrate surface, the ill effects of the micro-defects can be reduced as compared to the case where a regular mask with through-holes having a constant width throughout is used. The curved surface features formed using a shadow mask can have smoother edges than the curved surface features formed using a regular mask and undercutting below the bottom edges of the through-holes can be effectively avoided. As a result, the quality and mechanical properties of the membranes subsequently formed over the profile-transferring substrate surface can also be improved.

"The processes described in this specification can be used to form a durable, efficient, compact, and high resolution integrated piezoelectric actuator assembly or piezoelectric transducer array that include curved piezoelectric elements.

"The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims."

For additional information on this patent, see: De Brabander, Gregory; Nepomnishy, Mark. Forming Curved Features Using a Shadow Mask. U.S. Patent Number 8628677, filed March 31, 2011, and published online on January 14, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=65&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=3241&f=G&l=50&co1=AND&d=PTXT&s1=20140114.PD.&OS=ISD/20140114&RS=ISD/20140114

Keywords for this news article include: Nanotechnology, Plasma Etching, FUJIFILM Corporation, Emerging Technologies.

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Source: Journal of Engineering


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