Patent Application
20080283944
Microelectromechanical (MEMs) devices are experiencing greater interest to provide a variety of functions in a variety of applications. For example, many wireless devices rely on film bulk acoustic resonators (FBARs) to realize a variety of circuits. Illustratively, FBARs are used in filter circuits, transformers and microphones.
One type of FBAR includes a piezoelectric material disposed between two electrodes and disposed over a cavity in a substrate. The FBAR is enclosed by a cap structure, which is often referred to as a microcap structure. Vias are provided in the substrate, or the microcap, or both to provide electrical connections to the FBAR.
In many known FBAR structures the microcap and the substrate are made from a semiconductor such as silicon by etching features in the semiconductor. One etching technique useful in MEMS fabrication is known as deep reactive ion etching (DRIE). Among other benefits, DRIE provides high-aspect ratio features. While etching semiconductor materials is a comparatively mature technology, there are drawbacks to certain known methods, especially in MEMs applications. For instance, fabricating comparatively high aspect ratio features and comparatively deep features in material such as silicon often requires costly and time-consuming processes. In addition to requiring specialty tools to etch features, the DRIE and other reactive ion etching methods are generally not amenable to large scale or batch processing. Moreover, semiconductor materials such as silicon may interact with passive MEMS devices.
What is needed, therefore, are MEMs devices and methods of MEMs devices that overcomes at least the shortcomings described.
In accordance with an illustrative embodiment, a method of fabricating a microelectromechanical (MEM) device includes: selectively exposing at least a portion of a photostructurable glass substrate to radiation; heating the substrate to at least partially crystallize at least a portion of the exposed portion of the substrate; selectively etching at least a portion of the substrate in a solution to provide features in the substrate. The etching of the at least partially crystallized portions of the substrate proceeds at a significantly greater rate than the unexposed portions of the substrate.
In accordance with another illustrative embodiment, a film bulk acoustic structure (FBA) includes a photostructurable glass substrate; a cavity provided in a surface of the substrate; and an FBA disposed at least partially over the cavity.
In accordance with yet another illustrative embodiment, a microcap structure includes a photostructurable glass substrate; cavity provided in a surface of the substrate; and a glass gasket extending from the substrate.
Representative embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
The terms ‘a’ or ‘an’, as used herein are defined as one or more than one.
The term ‘plurality’ as used herein is defined as two or more than two.
In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of example embodiments according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of apparati, devices, materials and methods known to one of ordinary skill in the art may be omitted so as to not obscure the description of the example embodiments. Such apparati, devices, methods and materials are clearly within the scope of the present teachings. Furthermore, although described respect to a FBA device, the present teachings may be applied to other devices and structures. Generally, the present teachings may be applied to a variety of MEMs and packaging technologies.
The microcap 102 is usefully bonded to the substrate 101 via an adhesive layer 105 formed over a gasket as shown. The layer 105 is illustratively a metal such as gold and bonds to pads 106 formed over the substrate 101 and made of similar or identical material as the layer 105. Upon bonding of the microcap 102 to the substrate 101, the FBA device 103 is substantially packaged between the substrate 101 and the microcap 102. In certain embodiments, this bonding sequence provides hermetic packaging of the FBA device 103.
In representative embodiments, vias are usefully formed in microcap 102, or the substrate 101, or both. For example, vias 107 are formed by etching features in the microcap 102 and providing a conductive material therein. Notably, the vias 107 include an unexposed portion of photostucturable glass 107′, which allows for the selective etching of the vias 107 by methods described herein. The vias 107 provide an electrical connection between contacts 108 of the device 103 disposed over the substrate 101 and contacts 109 disposed over the microcap 102. As will be appreciated, contacts 108, 109 may be signal contacts for providing electrical signals to and retrieving electrical signals from the device 103.
In representative embodiments, the photostructurable glass may be glass material having the tradename Foturan or Foturan Glass-Ceramic manufactured by Schott AG, Germany, and distributed by Invenios/Mikroglas Chemtech, GmbH, Germany; or glass material having the tradename Fotoform-Fotoceram manufactured by Corning Incorporated, Corning, N.Y. Notably, these glass materials have slightly different physical properties, but have significant common properties. As such, the selection of one over the other is user specific. The photostructurable glasses useful in the representative structures and methods have the property that when exposed by the proper intensity and wavelength of UV radiation silver atoms disassociate from the glass compound.
In an illustrative embodiment, the regions 102, 202 are formed by exposing the substrate 101 under mask to light in the range of approximately 290 nm to approximately 330 nm and having a suitable intensity to expose the photostructurable glass. The substrate 101 is then subject to a heat treatment of approximately 600° C., which causes the glass to crystallize around the silver atoms (cerimization). These crystallized areas etch at a rate of more than approximately 20 times greater than the etching rate in the unexposed vitreous regions thereabout. For example, the exposed/heat treated regions 102, 202 typically having an etch rate of about 25 μm per minute in 10:1 HF.
As will be readily appreciated by one of ordinary skill in the art, the disparity in the etch rates between the exposed and unexposed regions of the substrate 101 allows complex features to be exposed and etched into the exposed regions and etched. The combined advantages of the glass' insulating properties and ease of selective etching fundamentally provide other advantages as well. For example, a glass wafer which has been photostructured is less expensive than a similar silicon wafer with etched vias. The coefficient of thermal expansion can also be somewhat tailored by the vendor by comparatively minor variations in its chemical composition. In addition, the glass substrate 101 can be further heat treated at approximately 800° C. to form a higher temperature ceramic material which can be heated up to 700° C. if high temperature applications is required. Furthermore, the photostructurable glass material has a much lower dielectric constant (∈r=6.5) than silicon (∈r=12) resulting in lower electric loss.
The piezoelectric element 205 may be AlN, ZnO, lead zirconium titanate (PZT) or combinations thereof; the electrodes 204, 206 may be metal such as Mo, Pt, or W. Moreover, and as will be appreciated by one of ordinary skill in the art, mass loading layers of dielectric, ceramic and piezoelectric materials, and metals may be included. It is emphasized that the noted materials are merely illustrative.
The fabrication of the device 103 and the metallization (contacts, bond pads, etc.) are effected by known methods. For example, the methods of fabricating the device 103 and materials therefore may be as described, for example, in U.S. Pat. No. 6,384,697 entitled “Cavity Spanning Bottom Electrode of Substrate Mounted Bulk Wave Acoustic Resonator” to Ruby, et al. and assigned to the present assignee. The disclosure of this patent is specifically incorporated herein by reference. The metallization may be fabricated by one or more methods known to one of ordinary skill in the art, such as standard lift-off methods.
While the FBA device 103 may be fabricated directly on the substrate 101, alternatively the device 103 may be fabricated on another substrate and transferred to the substrate 101 by known methods. Notably, the thermal constraints on certain types of photostructurable glass materials may prohibit or curtail the use of known methods of the piezoelectric element 204. Therefore, it may be useful, depending on the photostructurable glass material selected, to transfer the FBA device 103 after fabrication on a substrate more tolerant of temperatures of fabrication.
In the representative embodiment, the FBA structure 500 includes a cavity 501 through the substrate 101. Such a structure may be useful in devices such as microphones of the type described in U.S. patent application Ser. Nos. 11/588,752, entitled “Piezoelectric Microphones”, filed Oct. 27, 2006; 11/604,478, entitled “Transducers with Annular Contacts” filed on Nov. 26, 2007; and 11/727,735, entitled Multi-Layer Transducers with Annular Contacts, filed on Apr. 19, 2007 all to R. Shane Fazzio, et al. The inventions disclosed in these applications are assigned to the present assignee and are specifically incorporated herein by reference.
In the present embodiment, the exposed/heat treated region 208 is removed to reveal the cavity 501. This may be carried out by foregoing the mask used in revealing the vias 401 as described previously. Otherwise, the fabrication sequence of the FBA structure 500 and features thereof is substantially identical to one or more of the sequences described in connection with the embodiments of
In the representative embodiments described to this point, the exposure of the glass material to UV radiation is generally a blanket exposure, which provides suitable intensity to a depth of approximately 200 μm, to expose the glass so that cerimitization can be achieved as described. In other embodiments, more than one source of radiation for exposure regions with particularity is contemplated. For example, in one representative embodiment two or more UV radiation sources (e.g., UV lasers), each of requisite wavelength but not of sufficient intensity to expose the glass, are incident on a region of the glass so that their beams overlap in the region. If the combined intensity is greater than that required to expose the glass, then the glass will be selectively exposed. This allows one to select with significant precision a region of the glass to be exposed as only the overlapping regions, while not exposing all regions in the path of the individual beams. As will be appreciated, this allows for comparatively precise 3D features to be formed and, as applicable, the exposure of regions of the glass without the need for a mask.
In connection with illustrative embodiments, MEMs devices and methods of manufacture are described. One of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. These and other variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.
