Abstract:
A microelectromechanical system (MEMS) device is created by forming mechanical structures supported by a substrate having a bond ring area laterally spaced from the mechanical structures and having a sacrificial layer surrounding the mechanical structures. A bond ring material is formed on top of the sacrificial layer and the bond ring area. Some of the bond ring material is then removed to create a bond ring.

Description:
RELATED APPLICATION 
   This application is related to U.S. application entitled: “Cover Secured to Bond Ring” application Ser. No. 10/683,910, filed on the same date herewith, which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to micro-electromechanical system (MEMS) devices, and in particular to a bond ring for MEMS devices. 
   BACKGROUND OF THE INVENTION 
   Micro-electromechanical system (MEMS) devices are very small and fragile. They need to be protected from physical harm and contamination. Some MEMS devices require a special environment, such as a gas or liquid fluid, in which to operate. Prior attempts to provide such protection involve the use of a cover, such as a window or plate fixed over the MEMS device to protect it. Such windows or plates may be fixed on an annular ring of a polymer extending above and around the MEMS device. It may be difficult to form the ring with desired materials due to thermal budget constraints. The ring may also need to be formed in such a manner to facilitate sufficient sealing of the cover. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9  and  10  are side elevation representations illustrating formation of a device having a bond ring area according to an embodiment of the invention. 
       FIG. 11  is a side elevation representation of a device having a bond ring area coupled to a cover according to an embodiment of the invention. 
       FIGS. 12 ,  13 ,  14 ,  15 ,  16  and  17  are side elevation representations illustrating alternative formation of a device having a bond ring area according to an embodiment of the invention. 
       FIG. 18  is a partial top view of an example display device constructed of an array of devices according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. In the example embodiments, the methods include elements that are arranged serially. However, in other embodiments, the elements may be performed in a different order, or simultaneously. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     FIG. 1  shows a cross sectional representation taken during formation of a device  100 . Device  100  is formed on a substrate  110 , on which a micro-electromechanical system (MEMS) device will be formed on a MEMS area portion  112  of the substrate  110 . The substrate  110  extends laterally to a bond ring area  120 , which is also part of substrate  110  in one embodiment. The bond ring area will form an annular platform, surrounding the MEMS device, and supporting a cover, such as a plate, or transparent glass sheet as desired based on operating characteristics of the MEMS device. The bond ring area need not be circular, but may be shaped to effectively cover the MEMS device. 
     FIG. 1  also illustrates other structures that are used in the formation of a MEMS device. Many different types of MEMS device may be formed, and for example purposes, an optical MEMS including a mirror for use as a light modulator for micro-mirror displays is shown. Non-optical MEMS may also be formed. Such structures are formed using photolithographic processes in one embodiment. A layer of tetraethylorthosilicate (TEOS)  130  or similar material contains a bond pad  135 , and metal layers  140 ,  145  and  150 . Bond pad  135  is exposed with a via through TEOS  130 , and metal layers  140 ,  145  and  150  are formed within the layer of TEOS  130 . A pair of electrodes  155  and  160  are formed on top of the TEOS  130 . The electrodes  155  and  160  may be formed of Au (gold), Titanium nitride, Al (aluminum), or other conductive materials. 
   In  FIG. 2 , a sacrificial layer  210  is formed on top of the electrodes,  155  and  160 , with respective vias  215  and  220  formed through the sacrificial layer. In one embodiment, the sacrificial layer  210  is formed of SiO 2 , photoresist, amorphous silicon, or any other material that can be removed later in the process without adversely affecting the MEMS structure. A via  230  is formed in the sacrificial layer  210  to a bond ring area  120  of TEOS  130 . 
     FIG. 3  shows the device  100  following formation of MEMS structures such as a hinge layer  310  and yoke layer  315 . In one embodiment, such structures are formed of Al for reflective purposes. The materials used for such structures may vary greatly depending on the desired type of MEMS to be produced. The hinge layer  310  is in electrical contact with electrodes  155  and  160  in this embodiment. Yoke layer  315  is formed with two openings  317  and  318  to hinge layer  310 . The openings  317  and  318  have edges, which may contain TEOS strips  320  that may remain from a hardmask and plasma etch process used to form the hinge layer  310  and yoke layer  315 . 
   In  FIG. 4 , another layer of sacrificial material  410 , usually the same type of material as sacrificial layer  210 , is deposited and patterned to form a via  415  to yoke  315 , and a via  420  to bond ring area  130 . A mirror  510  is then patterned and etched as shown in  FIG. 5  in the MEMS area  110 . Mirror  510  extends down via  415  and over the exposed hinge area to form a pivot  520 , allowing the mirror  510  to pivot when released from the sacrificial layers. A further sacrificial layer  610  is formed on top of mirror  510  and is patterned to provide a via  620  to the bond ring area  120  TEOS  130 . When photoresist is used for a sacrificial layer, or MEMS protection layer, it is cured at up to 250° C. for a few minutes to an hour. In one embodiment, the photoresist should not be burned, which would make it harder to remove later in the process. 
   The sacrificial layers indicated at  610  effectively encapsulate and provide protection of the MEMS structures for deposition of a bond ring material  710  in  FIG. 7 . The bond ring material covers the sacrificial layer  610  as well as the bond ring area  120  of TEOS  130 . In one embodiment, the bond ring material  710  is formed such that the height of it extending up from the bond ring area is higher than a height of the MEMS structures, allowing such structures to operate properly when a cover is fixed to the bond ring material later in processing. The bond ring material  710  is metal, such as Au in one embodiment, an AuSn alloy, or other material as desired. Other materials may also be used for the bond ring. 
   The bond ring material  710  is formed by a process selected from the group consisting of physical vapor deposition, sputtering, evaporation, plating, or chemical vapor deposition. Such processes should be controlled to not adversely affect the sacrificial layers. When the sacrificial layers are formed of resist, lower temperature processes should be used to avoid burning the sacrificial layers. When the sacrificial layers are formed of other materials, processes at higher temperatures may be employed. 
   In further embodiments, the cover may contain a mating ring, providing sufficient height for proper operation of the MEMS structures. The bond ring material  710  is coated with a resist or material that is the same or compatible with the sacrificial layers. The resist is patterned to leave resist as indicated at  810  in  FIG. 8  to define the shape of the bond ring. 
   The bond ring material is removed via etching, leaving a bond ring  910  in the bond ring area  120 . In  FIG. 10 , the sacrificial layers and resist or sacrificial layer  810  are removed, resulting in a released MEMS device as indicated at  1010  along with a bond ring  910  for attaching a cover for the MEMS device. 
   The bond ring  910  may alternatively be formed with a metal floor to promote adhesion of Au or Au alloys that form the bond ring in one embodiment. Further, Ta, or TaN may be used as an adhesion layer in some embodiments. 
     FIG. 11  shows the device  100  with a cover  1110  coupled to a bond ring  1120  that may be formed higher than the MEMS device  1010 . The cover  1110  may also have a mating ring  1130  that mates with the bond ring  1120  and provides additional height of the cover  1110  over the MEMS device  1010  allowing desired operation of MEMS device  1010 . The cover  1110  may be formed of glass or other transparent material for optical type MEMS devices, or may be opaque. The characteristics of the cover  1110  should be compatible with any operating environment required for the MEMS device  1010 . In one embodiment, the cover  1110  is coupled to the bond ring in accordance with U.S. application entitled: “Cover Secured to Bond Ring” application Ser. No. 10/683,910, filed on the same date herewith, which is incorporated herein by reference. In one embodiment, the cover mating ring  1130  is provided with a bonding layer  1140 , and a tacking layer  1150  that provides a quick tacking type function allowing placement of the cover on the ring. The device  100  with cover  1110  is then heated at a later time in processing to provide a eutectic bond between the cover and the bond ring. The tacking layer  1150  provide sufficient adhesion to allow proper permanent bonding. This bond in one embodiment provides a hermetic seal for MEMS device  1010 , allowing selection of an operating environment within the sealed area that contains the MEMS. Any type of fluid, such as liquid or gas may be used within the sealed area to obtain a desired operating environment. 
   An alternative method of forming the bond ring is illustrated in  FIGS. 12 ,  13 ,  14 ,  15 ,  16  and  17 . In  FIG. 12 , the first sacrificial layer  210  extends beyond the MEMS area and also covers the bond ring area  120  as indicated at  1210 . The yoke  310  and hinge  315  are then formed as shown in  FIG. 13 , without a via being formed to the bond ring area  120 . A further sacrificial layer  410  in  FIG. 14  is formed as before, but also extends over the bond ring area as indicated at  1410 , without a via. The mirror  510  is then formed as shown in  FIG. 15 , followed by a protection layer  610  that protects the MEMS device as seen in  FIG. 16 . In one embodiment, the protection layer  610  is photoresist, and is exposed to provide an opening above the bond ring area  120  as indicated at  1610 . 
   In  FIG. 17 , an etch of the sacrificial layers is performed through opening  1610  to the TEOS layer  130 , and the bond ring is then formed in a manner similar or identical to that previously described. 
   A partial cut away block representation of a micro-mirror display device incorporating an array of optical MEMS including a mirror used as a light modulator is shown in  FIG. 18  generally at  1810 . The display comprises an array of such optical MEMS, one of which is shown at  1815 . 
   Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will appreciate that any arrangement calculated to achieve the same purpose can be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments of the invention. It is to be understood that the above description has been made in an illustrative fashion, and not a restrictive one. Combinations of the above embodiments, and other embodiments not specifically described herein will be apparent to those of skill in the art upon reviewing the above description. The scope of various embodiments of the invention includes any other applications in which the above structures and methods are used. Therefore, the scope of various embodiments of the invention should be determined with reference to the appended claims, along with the full range of equivalents to which such claims are entitled.