Patent Application
20060234412
This invention relates generally to microfabrication methods and, more particularly, to methods for making micro-electro-mechanical (MEMS) devices and to the MEMS devices made.
In fabrication of microelectromechanical system (MEMS), deflectable or movable structures are typically produced by etching features into a device layer, using silicon processing techniques common to the semiconductor industry to form the structure's form. The deflectable structures are often held immobile initially by a layer of sacrificial material. Typically, the layer of sacrificial material underlies the deflectable or movable structure. The underlying sacrificial layer is subsequently removed (e.g., by preferential etching) in a release process to produce a suspended deflectable structure or, in some cases, a free element. Often the structural device layer is silicon, silicon compound, a metal, or an alloy. Various sacrificial materials, such as silicon dioxide, photoresist, polyimide, epoxy, wax, polysilicon, and amorphous silicon, have been used for the sacrificial layer. Some MEMS devices are made by using two or more sacrificial materials for support, immobilization, and/or release of different structures of the MEMS device, which may have more than one structural device layer. The various sacrificial materials may be removed by the same etch process or by different selective etch processes. For example, a first sacrificial material or a portion of it may be removed by a wet etch and a second sacrificial material and/or a remaining portion of the first sacrificial material may be removed by a plasma etch.
Some specific sacrificial materials and etchants that have been used with the sacrificial materials include silicon oxide, removed, e.g., by hydrofluoric acid (HF) or buffered HF etching; amorphous silicon, removed, e.g., by xenon difluoride (XeF2) etching; and organic materials such as photoresist removed by oxygen plasma ashing.
After release by removal of the sacrificial material(s), the MEMS structures may be subject to ambient conditions which can lead to particulate and chemical contamination while the MEMS wafer is being stored, being inspected, or being prepared for packaging. Standard practice in MEMS fabrication often includes enclosing the MEMS devices within a package that protects the MEMS devices from environmental effects after MEMS release. The package may be hermetic, and the MEMS fabrication process may include bonding.
It has been reported that the greatest single cause of yield problems in fabrication of MEMS structures is “stiction,” unwanted adhesion of a MEMS structural element to another surface. Various coating materials have been employed to help prevent stiction. Such anti-stiction coatings are commonly applied after release of the MEMS device structures. Some anti-stiction coatings that have been used include amorphous hydrogenated carbon, perfluoropolyethers, perfluorodecanoic acid, polytetrafluoroethylene (PTFE), diamond-like carbon, and an alkyltrichlorosilane monolayer lubricant. Dessicants are also sometimes used in MEMS packages to help keep moisture away from device structures.
When bonding of a package seal occurs after MEMS release, packaging processes, including desiccant introduction or anti-stiction coating, can lead to particulate generation and chemical contaminants on the MEMS devices.
Other steps of many packaging procedures may require processes that can also adversely affect the MEMS structures if they are in a fully released state. For example, soldering or anodic bonding can lead to thermally or electrically induced strain and/or bending in the MEMS structures. Radiation, e.g., ultraviolet (UV) radiation used for curing epoxies, has the potential to damage fragile circuits through solid-state interactions with high-energy photons and can indirectly lead to heating, causing problems as described with reference to soldering or anodic bonding. High electric fields, such as the fields that may occur in anodic bonding, can damage MEMS by causing “snap-down,” charge-trapping, and other unwanted electrical phenomena. Outgassing of organic materials, e.g., in adhesive curing, can lead to surface adsorbed contamination of sensitive MEMS areas causing corrosion, stiction, charge-trapping, or other dielectric-related phenomena. Deposition of an anti-stiction coating after MEMS release, but before plasma-assisted bonding, may lead to fouling of the bonding surfaces. Conversely, high-temperature bonding processes may adversely affect the anti-stiction coating. Thus, if the anti-stiction coating is placed in or on the MEMS device after release, but before package seal bonding, process integration problems may arise, such as surfaces that will no longer bond, or, an anti-stiction coating that loses functionality for the MEMS due to thermally induced chemical changes.
Thus, an improved MEMS fabrication method is needed to minimize or avoid these shortcomings of the prior art.
The features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawings, wherein:
For clarity of the description, the drawings are not drawn to a uniform scale. In particular, vertical and horizontal scales may differ from each other and may vary from one drawing to another. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the drawing figure(s) being described. Because components of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting.
The terms “microfabrication” and “MEMS” as used herein are not meant to exclude structures characterized by nanoscale dimensions, i.e., a scale corresponding generally to the scale in the definition of U.S. Patent Class 977, generally less than about 100 nanometers (nm). Nor are these terms meant to exclude methods for making such nanoscale structures.
As illustrated by
If a sealed environment is required for the MEMS device, the release port or ports may be sealed.
Thus, specific embodiments of such methods include removing a first portion of the sacrificial material before performing the step of bonding the second substrate to the first substrate, while leaving a second portion of the sacrificial material to be removed through the release port. Thus, a portion of the sacrificial material may be removed (e.g., by partial etching, which may be a wet etch) without releasing the immobilized element(s), leaving an amount of sacrificial material sufficient to hold the element(s) immobile until they are released later (after bonding the substrates) as described above.
Both the first and second substrates may be planar or have substantially planar portions where the two substrates are bonded. The first substrate may comprise a material such as silicon, an oxide of silicon, an oxynitride of silicon, a nitride of silicon, a metal, an oxide of a metal, an oxynitride of a metal, a nitride of a metal, or combinations of these materials, for example. The second substrate may comprise a material such as glass, quartz, alumina, silicon, an oxide of silicon, an oxynitride of silicon, a nitride of silicon, a metal, an oxide of a metal, an oxynitride of a metal, a nitride of a metal, or combinations of these materials, for example.
The release port or ports may extend through either (or both) of the first or second substrates, or may be disposed in the bond formed between the substrates. Where the MEMS device might be damaged due to proximity of a release port, the release port(s) may be disposed to avoid alignment with the MEMS device(s). Thus, an axis of the release port may be disposed to be laterally spaced apart from the MEMS device.
In a particular embodiment of a method for fabricating a packaged MEMS device, the steps may include providing a first substrate; forming the MEMS device on the first substrate, the MEMS device including at least one element initially held immobile by a sacrificial material; removing a first portion of the sacrificial material without releasing the element initially held immobile, while leaving a second portion of the sacrificial material to be removed later; providing a second substrate; forming at least one release port; bonding the second substrate to the first substrate; and completely removing the second portion of the sacrificial material through the release port to release the element that was initially held immobile. In this embodiment, again, if a sealed environment is required for the MEMS device, the release port or ports may be sealed.
Such embodiments of methods are illustrated in
In step S30, the sacrificial material may be partially removed without releasing the element that was initially held immobile (by removing only a portion of the sacrificial material while leaving a second portion of the sacrificial material to be removed later). This partial removal of sacrificial material may be performed by a suitable wet etch, for example, timed to leave a portion of sacrificial material in place. Those skilled in the art will recognize that the etchant should be chosen which is suitable to selectively etch the sacrificial material used, with an etch-rate ratio or ratios suitable to avoid affecting functionality of the MEMS device.
In step S40, a second substrate is provided. The second substrate may provide a protective cover to cover the MEMS device. For example, the second substrate may be a glass substrate if a transparent cover is required, such as for optical applications of the MEMS device. A planar second substrate may have one or more seal rings formed on it for bonding to the first substrate. In step S50, one or more release ports are formed.
In step S60, the two substrates are bonded together. For example, the cover and the silicon wafer may be plasma treated and bonded to form an oxide-to-oxide bond. Many other methods for bonding two substrates together, such as anodic bonding and adhesive bonding, are known in the art. Unlike the standard MEMS processing which includes release etching before bonding, the MEMS structures of the present invention are not exposed to ambient conditions which can lead to particulate and chemical contamination before they are packaged. The present packaging process can reduce and possibly eliminate particulate exposure on the MEMS devices. Thermal excursions of the bonding process cannot greatly strain the MEMS devices because they are still held immobile by sacrificial material such as amorphous silicon. Ultraviolet (UV) adhesives can be utilized for non-hermetic packaging, if desired, since the MEMS are protected by the encapsulating sacrificial material from outgassing or UV radiation. Anodic bonding can be utilized, if desired, since the MEMS devices are held firmly in place and cannot “snap-down” from electrostatic forces. Anti-stiction coating can be applied at an appropriate time through the release ports if desired, e.g. by a chemical vapor deposition (CVD) process.
In step S70, the sacrificial material is completely removed through the release port(s), releasing the element or elements that were initially held immobile by the sacrificial material. For example, if amorphous silicon is used as the sacrificial material, the whole assembly may be placed in an etching chamber (an XeF2 etcher, for example). The etchant attacks sacrificial material, and etching proceeds until all the required sacrificial material is etched from the MEMS array, i.e., until the MEMS structures are released.
In step S80, the release port may be sealed (if required) after releasing the immobilized element(s). A number of such lateral release ports may be used for each MEMS device.
An array of MEMS devices may be fabricated on a single substrate such as a silicon wafer, each MEMS device having one or more lateral release ports. The methods disclosed herein may be practiced at a wafer level of processing, i.e., before dicing or singulation of the wafer.
Another aspect of the invention provides embodiments of a packaged MEMS device 10 comprising a first substrate carrying the MEMS device, the MEMS device including at least one element initially held immobile by a sacrificial material, a second substrate having at least one release port for removing the sacrificial material, and a bond joining the second substrate to the first substrate. Some embodiments of the packaged MEMS device may further comprise a seal for closing the release port(s) after removing the sacrificial material to release the element initially held immobile.
As shown in
As shown in
In the embodiment shown in
For some embodiments, release port 80 may be formed in the first substrate 20 instead of the second substrate 70.
The packaged MEMS device may be an integrated circuit or may form part of an integrated circuit.
Another aspect of the invention is a method of using a MEMS device requiring release of an element initially held immobile by a sacrificial material. This method includes carrying the MEMS device on a first substrate. A portion of the sacrificial material may be removed (e.g., by partial etching) without releasing the immobilized element(s), leaving an amount of sacrificial material sufficient to hold the element(s) immobile until they are released later. The MEMS device is covered with a cover formed by a second substrate. One or more post-bond release ports are provided, e.g., in either the first or second substrate. The second substrate is bonded to the first substrate without blocking any of the post-bond release ports, before removing all of the sacrificial material. The sacrificial material is completely removed through the post-bond release port(s) after bonding of the two substrates together to fully release the immobile element. This full release is performed before sealing the post-bond release port(s) if such sealing is required.
Methods performed in accordance with the invention are useful in fabrication of many kinds of MEMS devices. The methods may be practiced on a wafer scale (i.e., before any dicing or singulation). Such MEMS devices may include high frequency switches, high Q capacitors, electromechanical motors, pressure transducers, accelerometers, and displays, for example. MEMS devices made in accordance with the invention are useful in many other sensor, actuator, and display applications, for example. MEMS devices made in accordance with the invention may be used in integrated circuits.
Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims. For example, functionally equivalent materials may be substituted for the specific materials described in the embodiments, and the order of steps may be varied somewhat. For another example, although various embodiments are shown with one or more release ports through the first or second substrate, other locations for the release port(s) may be used in other embodiments. For some applications, various elements may be released at different times in the fabrication process; some may be released before bonding of the two substrates together, and some may be released after bonding, e.g., by using different sacrificial materials.
This application is related to co-pending and commonly assigned application Ser. No. ______, filed on Apr. 11, 2005 (attorney docket no. 200501403-1).
