Abstract:
Disclosed are methods for forming semiconductor devices and the semiconductor devices thus obtained. In one embodiment, the method may include providing a semiconductor wafer comprising a surface, forming on the surface at least one device, forming a release layer at least in an area of the surface that encircles the at least one device, forming on the release layer at least one wall structure around the at least one device, and forming at least one cap on the at least one wall structure. In one embodiment, the device may include a substrate comprising a surface, at least one device formed on the surface, a release layer formed at least in an area of the surface that encircles the at least one device, at least one wall structure formed around the at least one device, and at least one removable cap formed on the at least one wall structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a nonprovisional of U.S. Provisional Patent Application Ser. No. 61/415,680 filed Nov. 19, 2010, the contents of which are hereby incorporated by reference. Further, this application claims priority to European Patent Application Serial No. 10191819.1 filed Nov. 19, 2010, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Microelectromechanical system (MEMS) devices are typically very sensitive, such that it is difficult to dice a wafer or substrate comprising MEMS devices once the MEMS devices have been released. Further, even if the MEMS devices are not released until after the dicing, it is still difficult to handle the diced MEMS devices. As a result, many MEMS devices are often damaged during handling. 
     In an effort to protect MEMS devices, many commercial MEMS devices are packaged by a permanent cap (e.g., capping chip or capping layer) prior to dicing. In many cases, however, such a cap is not feasible. For example, micro-opto-electromechanical system (MOEMS) devices cannot function with a permanent cap, as the permanent cap blocks light from reaching the MOEMS. 
     In some cases, rather than using a cap to protect MEMS devices, ultraviolet dicing tape may be used. However, ultraviolet dicing tape may damage the MEMS devices during removal and/or may leave behind a residue. It is possible to pattern the ultraviolet dicing tape around the devices (e.g., in a ring), but this requires non-standard tools, such as a programmable cutting table. 
     SUMMARY 
     Disclosed are methods for fabricating semiconductor devices and the semiconductor devices thus obtained. In particular, microelectromechanical systems (MEMS) devices are considered. The disclosed methods and devices involve a cap formed to protect the MEMS devices. 
     In one aspect, a method is disclosed. The method may include providing a semiconductor wafer comprising a surface, forming on the surface at least one device, forming a release layer at least in an area of the surface that encircles the at least one device, forming on the release layer at least one wall structure around the at least one device, and forming at least one cap on the at least one wall structure. 
     In some embodiments, forming the release layer comprises forming the release layer while forming the at least one device. In other embodiments, forming the release layer comprises forming the release layer after forming the at least one device. 
     In some embodiments, the semiconductor wafer comprises a silicon wafer. In these embodiments, forming the release layer may comprise depositing a sequence of silicon carbide layers using plasma enhanced chemical vapour deposition. 
     In some embodiments, the release layer has a first thermal expansion coefficient and at least a portion of the wall structure that is in contact with the release layer has a second thermal expansion coefficient that differs from the first thermal expansion coefficient. 
     In some embodiments, the at least one wall structure comprises at least one first metal layer, the at least one cap comprises at least on second metal layer, and forming the at least one cap on the at least one wall structure comprises contacting the at least one first metal layer and the at least one second metal layer and heating the at least one first metal layer and the at least one second metal layer, thereby forming a welded connection between the at least one wall structure and the at least one cap. In these embodiments, the at least one first metal layer may comprise copper, the at least one second metal layer may comprise tin, and the welded connection may comprise a copper-tin bond. 
     In some embodiments, the method may further comprise heating the release layer and at least a portion of the at least one wall structure, thereby releasing the at least one cap and the at least one wall structure from the wafer. 
     In some embodiments, the method may further comprise heating the release layer and at least a portion of the at least one wall structure and applying a mechanical force to the at least one wall structure and the at least one cap, thereby releasing the at least one cap and the at least one wall structure from the wafer. 
     In some embodiments, the method may further comprise applying a mechanical force to the at least one wall structure and the at least one cap, thereby releasing the at least one cap and the at least one wall structure from the wafer. 
     In some embodiments, the at least one wall structure comprises a plurality of wall structures and the at least one cap structure comprises one cap structure. In these embodiments, the method may further comprise dicing the wafer, thereby separating the plurality of wall structures from one another. 
     In another aspect, a device is disclosed. The device may comprise a substrate comprising a surface, at least one device formed on the surface, a release layer formed at least in an area of the surface that encircles the at least one device, at least one wall structure formed around the at least one device, and at least one removable cap formed on the at least one wall structure. 
     In some embodiments, the substrate comprises a silicon substrate. 
     In some embodiments, the release layer comprises a stack of silicon carbide layers. 
     In some embodiments, the at least one wall structure comprises at least one first metal layer, the at least one cap comprises at least on second metal layer, and a welded connection is formed between the at least one wall structure and the at least one cap. In these embodiments, the at least one first metal layer may comprise copper, the at least one second metal layer may comprise tin, and the welded connection may comprise a copper-tin bond. 
     In some embodiments, the release layer has a first thermal expansion coefficient and at least a portion of the wall structure that is in contact with the release layer has a second thermal expansion coefficient that differs from the first thermal expansion coefficient. 
     In some embodiments, the at least one device comprises a microelectromechanical systems device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate example devices, in accordance with an embodiment. 
         FIGS. 2A to 2F  illustrate an example method for forming a device, in accordance with an embodiment. 
         FIG. 3  shows an example composition of wall structures in a device, in accordance with an embodiment. 
     
    
    
     It is to be understood that the figures are merely illustrative and are not meant to be limiting. In particular, the relative dimensions of the components shown in the drawings may differ from those shown. 
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  illustrate example devices, in accordance with an embodiment. A wafer  100  is shown. At least one MEMS device  200  is formed on its surface using, for example, typical MEMS fabrication processes. As shown, a release layer  300  is produced on the wafer  100 . The release layer  300  is formed at least in an area that encircles at least one of the at least one MEMS devices  200 . Further, a support wall structure  400  is formed on and in contact with the release layer  300 . Thereafter, a cap  500  is formed on the support wall structure  400 . The composition and mechanical characteristics of the release layer  300  and of the wall structure  400  are such that the cap  500  and the support wall structure  400  are removable from the wafer  100  through application of a first removing force to the support wall structure  400  and/or the cap  500 . As a result of the first removing force, the release layer  300  and/or an interface between the release layer  300  and the support wall structure  400  and/or an interface between the wafer  100  and the release layer  300  will break more easily than the support wall structure  400 , such that the release layer  300  may be removed, as shown in  FIG. 1B . As a result, the cap  500  and the support wall structure  400  may be released from the wafer  100 , as further shown in  FIG. 1B . 
     The cap  500  may additionally be removed. To this end, a second removing force may be applied to the cap  500  and/or the support wall structure  400 . In some embodiments, the release layer  300  may have a different thermal expansion coefficient than a portion or all of the support wall structure  400 . In these embodiments, applying the second removing force may comprise heating the release layer  300  and the portion or all of the support wall structure  400 , leading to a heat-induced stress that generates the second removing force. The second removing force may be oriented substantially along the plane of the release layer  300 . In other embodiments, applying the second removing force may alternatively or additionally involve applying a mechanical removing force. 
     In some embodiments, the release layer  300  may be formed along with the at least one MEMS device  200 . For example, the release layer  300  may be a functional layer of the MEMS fabrication process. This is illustrated in  FIGS. 2A-F . It is to be understood, however, that, in other embodiments, the release layer  300  may be formed in a separate process. 
       FIG. 2A  shows an MEMS device  2  formed on a silicon wafer  1 . Metallic connection structures  3  are visible, as well as a sacrificial silicon oxide layer  4  and a silicon carbide passivation layer  5 , which may be applied during MEMS processing to protect the underlying layers. The passivation layer  5  also acts as the release layer of the invention. In some embodiments, the passivation layer  5  may be produced as a sequence of thin SiC layers through a series of Plasma Enhanced Chemical Vapour Deposition (PECVD) steps. For example, a sequence of 4 layers of SiC each having a thickness of 80 nm may be produced. The passivation layer  5  may be produced in other manners as well. 
     A support wall structure  6  is formed on the passivation layer  5 , as shown in  FIG. 2B . The support wall structure  6  may be formed using, for example, lithography or etch techniques in combination with PECVD and Electrochemical Deposition (ECD). In particular, in order to form the support wall structure  6 , a trench may be etched in the sacrificial oxide layer  4 , thereby exposing the release layer  5  in the area encircling the MEMS device  2 . Thereafter, using PVD and ECD, the support wall structure  6  may be formed on the exposed portion of the release layer  5 . 
     The support wall structure  6  may comprise, for example, Cu. As shown, the support wall structure  6  substantially encircles the MEMS device  2 . In some embodiments, the support wall structure  6  may comprise a number of layers, such as a diffusion barrier layer, a thin PVD Cu layer, and a thicker ECD Cu layer. Further, in some embodiments, the support wall structure  6  may be taller than the MEMS device  2 , as shown. The release layer  5  may have a thermal expansion coefficient that is different from a thermal expansion coefficient of the support wall structure  6 , or different from a thermal expansion coefficient of a portion of the wall structure that is in direct contact with the release layer  5 . 
     As shown in  FIG. 2C , the sacrificial oxide layer  4  may be removed using, for example, a vapour HF etch. Thereafter, a cap  10  may be formed, as shown in  FIG. 2D . The cap may comprise any suitable materials, such as silicon, glass, or metal, and may have any suitable shape, such as a disc shape. Other materials and shapes are possible as well. 
     As shown in  FIG. 2D , an additional wall structure  11  may be formed on a surface of the cap  10 . The additional wall structure  11  may correspond (e.g., in circumference and/or wall thickness) to the wall structure  6  formed on the wafer  1 . As illustrated, the cap  10  may be lowered onto the wafer  1 , and the additional wall structure  11  may be welded to the wall structure  6 . To this end, a gripping tool used to lower the cap  10  onto the wafer  1  may be heated. The welding may take other forms as well. In some embodiments, the release layer  5  may not be heated. 
     The additional wall structure  11  may comprise a layer  12  of Sn at the top, so that the weld is obtained by formation of a Sn—Cu bond. In some embodiments, a cap  10  comprising a single wall structure may be applied to a wafer  1  comprising a single MEMS device  2 , while in other embodiments a single cap  10  comprising a plurality of additional wall structures may be applied to a wafer  1  comprising a plurality of MEMS devices and a plurality of wall structures. In the latter embodiments, the wafer may be diced to separate the MEMS devices following the welding. 
     Following the welding, the wall structure  6  and the additional wall structure  11  may together form a welded wall structure  13 , as shown in  FIG. 2E . The welded wall structure  13  may serve to protect the MEMS device  2 . It is to be understood that in some embodiments, the additional wall structure  11  may not be present, and the cap  10  may be welded directly to the wall structure  6 . 
     The structure shown in  FIG. 2E  may exhibit improved robustness as compared to typical MEMS devices. Accordingly, the structure may be transported and handled more easily. Once the structure is mounted as desired for a particular application, the cap  10  and/or the welded wall structure  13  may be removed, as illustrated in  FIG. 2F . To this end, the release layer  5  may be heated. For example, the release layer  5  may be heated through the wafer  1  and/or the cap  10 . The release layer  5  may be heated in other manners as well. 
     Due to a difference in expansion coefficients between the release layer  5  and the welded wall structure  13 , a transverse force may be exerted on the release layer  5 . In some embodiments, this heating may be sufficient to cause breaking of the release layer  5 . In other embodiments, a mechanical force may be applied along with the heating. For example, the cap  10  may be gripped and twisted around a central axis of the welded wall structure  13 . In still other embodiments, only a mechanical force may be applied. The release layer  5  may break along a place in the release layer  5 , along an interface between the release layer  5  and the welded wall structure  13 , and/or along an interface between the release layer  5  and the wafer  1 . 
     In an example embodiment, the release layer  5  may comprise SiC sublayers. In this embodiment, the release layer  5  may break along one or more interfaces of the sublayers. In another example embodiment, the dimensions of the welded wall structure (e.g., height, width, and shape) may be designed such that stress resulting from heating and/or a mechanical force is highest in the release layer  5 , so as to ensure that the release layer  5  will break before the welded wall structure  13 . 
     Referring again to  FIGS. 1A-B , it is to be understood that, in some embodiments, the wall structure  400  may be a continuous wall structure encircling the MEMS device  200 , such as a wall with a circular cross-section so that a cylindrical enclosure is formed around the MEMS device  200 . In other embodiments, the wall structure may not be continuous, but rather may be formed of arcs places around the MEMS device  200 . In embodiments where the release layer  300  is formed as a process separate from the MEMS fabrication process (e.g., after the MEMS fabrication process), the release layer  300  may be produced at least in at least a portion of an area encircling the MEMS device  200 , and the wall structure  400  may be formed on the release layer  300 . 
       FIG. 3  shows an example composition of wall structure  6  and additional wall structure  11  in a device, in accordance with an embodiment. As shown, wall structure  6  may comprise, for example, a number of layers produced onto a 400 nm SiC layer (e.g., the release layer  5 ). The release layer  5  may comprise four SiC layers of 100 nm thick deposited at 350° C. with interface hardening by He plasma in between each layer deposition. The layers of the wall structure  6  may include a 30 nm TaN diffusion barrier, applied by PVD; a 150 nm Cu layer applied by PVD; and a 15 μm Cu layer applied by ECD. Further, the additional wall structure  6  may comprise a 30 nm Ti layer; a 150 nm Cu layer applied by PVD; a 5 μm Cu layer applied by ECD; and a 10 μm Sn layer applied by ECD. As described above, the wall structure  6  and the additional wall structure  11  may be welded together. During welding, the 15 μm Cu layer and 10 μm Sn layer may be contacted and welded together by heating only the cap  10  at 250° C. and by applying a mechanical pressure of 0.1 MPa. 
     As noted above, the release layer  5  may be released along an interface place of the release layer. As shown, the release layer  5  breaks along an interface between SiC sublayers of the release layer  5 , as indicated by dotted line  101 . In this manner, the cap  10  may be released. 
     In some embodiments, the welded wall structure  13  may have a height greater than, for example, 20 μm. Further, in some embodiments, the welded wall structure  13  may have a width of about, for example, 100 μm. Still further, in some embodiments, releasing the release layer  5  may involve heating the full assembly to 250° C. and applying an upward mechanical force to the cap  10  with a gripping tool. 
     Other release layers, wall structures, additional wall structures, caps, welding parameters, dimensions, and release mechanisms are possible as well.