Patent Publication Number: US-7898081-B2

Title: MEMS device and method of making the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a micro-electro-mechanical systems (MEMS) device and a method of making the same, and, more particularly, to a MEMS device with a lateral vent hole and a method of making the same. 
     2. Description of the Prior Art 
     MEMS devices include micromachines integrated with electronic microcircuits on substrates. Such devices may form, for example, microsensors or microactuators which operate based on, for example, electromagnetic, electrostrictive, thermoelectric, piezoelectric, or piezoresistive effects. MEMS devices have been formed on insulators or other substrates using micro-electronic techniques such as photolithography, vapor deposition, and etching. Recently, MEMS is fabricated using the same types of steps (such as the deposition of layers of material and the selective removal of the layers of material) that are used to fabricate conventional analog and digital complementary metal oxide semiconductor (CMOS) circuits. 
     The recent ability to seal micro-machined meshes has lead to the fabrication of microphones and microspeakers. A sealed mesh can function as a movable plate of a variable capacitor, and therefore can operate as a microspeaker or microphone. For a sealed mesh to operate as a microspeaker or microphone, the device needs to be able to push air to create a soundwave just as its larger counterparts must push air to create soundwaves. In the case of a microspeaker or microphone, if the chamber beneath the sealed mesh does not have a vent or other opening to ambient, movement of the sealed mesh inward is inhibited by the inability to compress the air in the chamber while movement of the mesh outward is inhibited by formation of a vacuum. Thus it is necessary to form a vent in the chamber. 
     Currently, such vents are formed by boring through the silicon substrate from the rear. For example, a method of making a MEMS device is disclosed in U.S. Pat. No. 6,936,524 that comprises some steps as shown in  FIGS. 1 and 2 .  FIG. 1  shows the formation of vent holes after the mesh has been released and the pilot openings expanded. As shown in  FIG. 1 , a first dielectric layer  14 , a first metal layer  16 , a second dielectric layer  20 , a second metal layer  22 , a third dielectric layer  26 , a third metal layer  28 , a top dielectric layer  32 , and a photo-resist layer  38  are stacked on the right surface of the silicon substrate  12 . The first metal layer  16  is patterned to allow a portion thereof to form a structure of micro-machined mesh metal  18 . The second and the third metal layers  22  and  28  each have an opening above the micro-machined mesh metal  18  to expose the micro-machined mesh metal  18 . The photo-resist layer  38  covers the above of the third metal layer  28  to protect the portion not to be etched. The reverse surface of the silicon substrate  12  is adhered to a first carrier wafer  36  through an adhesive  34 . Thus, a deep reactive-ion etching (DRIE) process, and subsequently reactive-ion etching (RIE) process, inductively coupled plasma reactive ion etching process, or XeF2 etching process  24  are performed on the right surface of the silicon substrate  12  to partially etch the silicon substrate  12  and to release the micro-machined mesh metal  18  to form vent holes  40 .  FIG. 2  shows another example. After the right surface of the silicon substrate  12  is protected by a protection layer  45  and adhered to a second carrier wafer  44  through an adhesive  42 , an RIE or a DRIE process  48  is performed on the silicon substrate  12  using a photo-resist mask  46  from the reverse surface of the silicon substrate  12 . However, the silicon substrate typically has a thickness of about 700 microns, and it may still remain more than 300 microns even after certain polishing steps are carried out during the manufacturing process. It would be a tedious process to etch though the substrate no matter from the rear or the front. 
     Therefore, there is still a need for a novel MEMS device structure and the making method to conveniently making such devices. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a novel MEMS device and a method of making the same for conveniently making such devices. 
     In an aspect of the present invention, the MEMS device comprises a substrate, a MEMS structure disposed on the substrate, and a vent hole structure disposed on a same side of the substrate on which the MEMS structure is disposed. The vent hole structure adjoins the MEMS structure with a first etch stop structure therebetween. The MEMS structure comprises at least an electrode disposed within or on the substrate, and a micro-machined metal mesh disposed over the substrate. Accordingly, a first chamber is formed between the micro-machined metal mesh and the substrate. The vent hole structure comprises a metal layer disposed over the substrate, a second chamber formed between the metal layer and the substrate and communicating with the first chamber through beneath the first etch stop structure, and a plurality of vent holes throughout the metal layer to communicate with the second chamber. 
     In another aspect of the present invention, the MEMS device comprises a substrate; a MEMS structure disposed on the substrate; and a vent hole structure disposed on a same side of the substrate on which the MEMS structure is disposed. The vent hole structure adjoins the MEMS structure with a first etch stop structure therebetween. The MEMS structure comprises at least an electrode disposed within or on the substrate, and a micro-machined metal mesh disposed over the substrate such that a chamber is formed between the micro-machined metal mesh and the substrate. The vent hole structure comprises at least a vent hole surrounded by the first etch stop structure or a second etch stop structure, wherein the at least a vent hole communicates with the chamber through beneath the first and the second etch stop structures. 
     In further another aspect of the present invention, the method of making a MEMS device comprises steps as follow. A substrate comprising a MEMS region having an electrode disposed within or on the substrate and a vent hole region adjoining the MEMS region is provided. A plurality of interlayer dielectrics are formed on the substrate. A micro-machined metal mesh is formed in one, other than the bottom layer and the top layer, of the interlayer dielectrics, over the electrode in the MEMS region. A metal hard mask is formed in one of the interlayer dielectrics, over and corresponding to the micro-machined metal mesh. A first etch stop structure is formed by alternately stacking a plurality of metal layers and a plurality of trench-shaped vias in a lower one of the interlayer dielectrics between the vent hole region and the MEMS region upwardly to the top one of the interlayer dielectrics and allowing the bottom of the first etch stop structure to be higher than the bottom layer of the interlayer dielectrics and not higher than the micro-machined metal mesh. A second etch stop structure is formed by alternately stacking a plurality of metal layers and a plurality of trench-shaped vias in a lower one of the interlayer dielectrics in the vent hole region upwardly to an upper one of the interlayer dielectrics and allowing the bottom of the second etch stop structure to be higher than the bottom layer of the interlayer dielectrics, the second etch stop structure is in a grid shape. A release process is performed to remove the interlayer dielectrics in the MEMS region and the vent hole region, thereby to form a hollowed-out micro-machined metal mesh, to form at least a vent hole in the grid of the first and the second etch stop structures in the vent hole region, and to hollow out a space beneath the first and the second etch stop structures. A vibration film is coated on the micro-machined metal mesh. 
     Compared with the conventional techniques, the MEMS device according to the present invention has a lateral vent hole, and, furthermore, since the vent hole structure is disposed on the same side of the substrate on which the MEMS structure is disposed, in the manufacturing process, the release process for the vent hole and the release process for the micro-machined metal mesh of the MEMS structure can be performed simultaneously on the same side of the substrate. Moreover, since the material to be etched away is dielectric, such as, silicon oxide, the time needed for etching is short with respect to silicon etching. Accordingly, the manufacturing process is convenient, and it is easily integrated with the manufacturing process of the logic structure, such as MOS device. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are schematic cross-section views illustrating two examples of making a micro-machined metal mesh of MEMS devices in the prior art; 
         FIG. 3  is a schematic plan view illustrating an embodiment of the MEMS device according to the present invention; 
         FIG. 4  is a schematic cross-section view along the line AA′ shown in  FIG. 3 . 
         FIGS. 5 and 6  are schematic graphs illustrating two examples of the arrangement of the MEMS structure, the vent hole structure, and the logic structure according to the present invention; 
         FIG. 7  is a schematic cross-section view illustrating another embodiment of the MEMS device according to the present invention; 
         FIG. 8  is a schematic plan view illustrating an embodiment of the MEMS device according to the present invention; 
         FIG. 9  is a schematic cross-section view along the line BB′ shown in  FIG. 7 ; 
         FIG. 10  is a schematic cross-section view illustrating still another embodiment of the MEMS device according to the present invention; 
         FIGS. 11-14  are schematic cross-section views illustrating an embodiment of the method of making a MEMS device according to the present invention; and 
         FIGS. 15-16  are schematic cross-section views illustrating another embodiment of the method of making a MEMS device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following embodiments are described for illustrating the present invention. 
       FIGS. 3 and 4  show an embodiment of the MEMS according to the present invention.  FIG. 3  is a schematic plan view thereof. As shown in  FIG. 3 , the MEMS device  50  may include a MEMS region  102  and a vent hole region  104 , and may further include a logic region  106 . A MEMS structure  52 , such as, a microphone, a microspeaker, or the like, is disposed in the MEMS region  102 . A vent hole structure  54  is disposed on the vent hole region  104 . The vent hole structure  54  includes, for example, vent holes  74  formed through a stack of a plurality of patterned metal layers (such as the fifth metal layer M- 5  shown) and a plurality of trench-shaped vias (such as the fifth trench-shaped via V- 5  shown). A logic structure  62 , such as a MOS device or a metal interconnect, is disposed in the logic region  106 . The MEMS structure  52  and the logic structure  62  may be further surrounded by an etch stop structure respectively for protection during the release process. The etch stop structure, or may referred to as “protection ring”, is a wall-shaped protection structure made up by a plurality of metal layers and trench-shaped vias alternately stacked up and down. The etch stop structure may be disposed so as to surround the MEMS region to protect structures not in the MEMS region during the release process. Such etch stop structure is explicitly described in the specification of co-pending U.S. patent application Ser. No. 12/056,286 invented by some same inventors and assigned to the same assignee, which is entirely incorporated by reference. 
       FIG. 4  is a schematic cross-section view along the line AA′ shown in  FIG. 3 . As shown in  FIG. 4 , the MEMS device  50  of the present invention includes a substrate  64 , a MEMS structure  52  disposed on the substrate  64  in the MEMS region  102 , and a vent hole structure  54  disposed on the substrate  64  in the vent hole region  104 . The vent hole structure  54  is disposed on the same side of the substrate  64  on which the MEMS structure  52  is disposed. The vent hole structure  54  adjoins the MEMS structure  52  with an etch stop structure  66  therebetween. The MEMS structure  52  includes at least an electrode structure  68  and a micro-machined metal mesh  70 . The electrode structures  68  are disposed on the substrate  64 . In other embodiments, the electrode structure may be disposed within the substrate. The micro-machined metal mesh  70  is disposed over the substrate  64 , so as to have a distance from the substrate  64 . An empty chamber  72  is formed between the micro-machined metal mesh  70  and the substrate  64 . The micro-machined metal mesh  70  may be coated with a vibration film  78 . The vent hole structure  54  includes at least a vent hole  74  surrounded by the etch stop structure  66  or an etch stop structure  76 . The bottom of the etch stop structure  66  and the bottom of the etch stop structure  76  have a distance from the substrate  64 , such that the vent holes  74  communicate with the chamber  72 . That is, the air in the chamber  72  can be compressed and then laterally flows to vent holes  74  to exhaust when the vibration film  78  vibrates. 
     It should be noticed that the etch stop structures  66  and  76  shown in  FIG. 4  each include a plurality of metal layers (such as the first metal layer M- 1 , the second metal layer M- 2 , the third metal layer M- 3 , the fourth metal layer M- 4 , the fifth metal layer M- 5 , and the top metal layer M-Top) and a plurality of trench-shaped vias (such as the first, second, third, fourth, and fifth trench-shaped vias V- 1 , V- 2 , V- 3 , V- 4 , and V- 5 ) alternately stacked together. The bottoms of the etch stop structures  66  and  76  are not higher than both the micro-machined metal mesh  70  and the vent holes  74 . Furthermore, the bottom of the etch stop structure  66  does not connect the substrate  64 . For example, as shown in  FIG. 4 , the etch stop structures  66  and  76  are not formed from the contact via to connect the substrate but from the first metal layer M- 1 . In such way, the chamber  72  and the vent hole  74  are allowed to communicate with each other through the etch stop structures  66  and  76  after the release process performed on the interlayer dielectrics. 
     Furthermore, another etch stop structure, such as the etch stop structure  82  or  80 , can be formed to surround the entire MEMS structure  52  and the vent hole structure  54 , serving as a protection ring, to prevent the structures other than the MEMS structure  52  and the vent hole structure  54  from damage during the release process. The etch stop structures  80  and  82  are formed by starting forming one or more trench-shaped contact vias in the firs interlayer dielectrics ILD- 1  and then alternately stacking the metal layers and the trench-shaped vias upwardly. 
     The MEMS device  50  may further include a logic structure disposed on the substrate  64  in the logic region  106 . The logic structure is on the same side of the substrate  64  on which the MEMS structure  52  is disposed. One of the vent hole structure  54  and the MEMS structure  52  adjoins the logic structure with the etch stop structure  80  therebetween.  FIGS. 5 and 6  show two examples of the arrangement. The vent hole structure  54  may be disposed on only one or more sides of the MEMS structure  52  or to entirely surround the MEMS structure  52 . The logic structure  62  may adjoin the vent hole structure  54  only and not the MEMS structure  52 , or the logic structure  62  may adjoin the MEMS structure  52 . The logic structure may include a metal interconnect structure including a plurality of metal layers, vias, and interlayer dielectrics. 
     As described above, the etch stop structure  66  is disposed such that its bottom is not higher than both of the micro-machined metal mesh  70  and the vent holes  74  and it does not contact the substrate  64 . Specifically, if the micro-machined metal mesh  70  is formed from the third metal layer M- 3 , the manufacturing of the etch stop structure  66  may be started with the first metal layer M- 1 , the second metal layer M- 2 , or the third metal layer M- 3 . Accordingly, the vent holes  74  may be disposed at the third metal layer M- 3 , the fourth metal layer M- 4 , the fifth metal layer M- 5 , or the top metal layer M-Top. 
     The vent holes  74  are located in a metal layer not lower than the micro-machined metal mesh  70  and not lower than the bottom of the etch stop structure  66 . Thus, the vent holes may be formed utilizing the space surrounded by the etch stop structure  66  and the etch stop structure  76 , or the etch stop structure  76  and the etch stop structure  80 . For example, as shown in  FIG. 4 , the vent holes  74  are disposed in the top metal layer M-Top and throughout the etch stop structure  76  composed of the top metal layer M-Top, the metal layers M- 1  to M- 5 , and the trench-shaped vias V- 1  to V- 5  to form the openings.  FIG. 7  shows another embodiment. In the MEMS device  55 , the vent holes  74  are disposed in the fourth metal layer M- 4  and throughout the etch stop structure  76  composed of the third metal layer M- 3 , the fourth metal layer M- 4 , and the trench-shaped via V- 3  to form the openings. 
     The vent hole of the vent hole structure may be also a mesh having a plurality of openings.  FIG. 8  shows an embodiment. Compared with the vent hole  74 , the vent hole  86  of the vent hole structure  84  of the MEMS device  85  further has a mesh-typed hole in a shape of 3×3 grid.  FIG. 9  shows a schematic cross-section view of the MEMS device  85  along the line BB′ in  FIG. 7 . The mesh diameter of the vent hole  86  is relatively much small. 
     Furthermore, the number of the vent holes of the vent hole structure is not particularly limited. The MEMS device  90  as shown in  FIG. 10  includes a vent hole structure  88  having only one mesh-typed vent hole  86 , which is formed from a metal layer. The mesh-typed vent hole  86  has some small openings. There is not an etch stop structure further disposed beneath the mesh-typed vent hole  86 . The mesh-typed vent hole  86  has a distance from the substrate  64 , and thereby an empty chamber  92  is formed between the mesh-typed vent hole  86  and the substrate  64  and communicates with the chamber  72 , allowing the chamber  72  to let out air to or get in air from the ambient environment. 
     The MEMS device according to the present invention can be made by individually forming the MEMS structure, the vent hole structure, and the logic structure, but it is more convenient and economical to forming those structures simultaneously correspondingly from a same metal layer using the metal interconnect process in the semiconductor technology.  FIGS. 11-14  illustrate an embodiment of the method of making a MEMS device according to the present invention. As shown in  FIG. 11 , a substrate  64 , such as a semiconductor substrate, such as silicon substrate, is provided. The substrate  64  includes a MEMS region  102  and a vent hole region  104  adjoining the MEMS region  102 , and may further include a logic region  106 . There are electrode structures  68  composed of a gate  69  and a gate dielectric layer  71  between the gate  69  and the substrate  64  disposed in the MEMS region  102 . 
     Next, as shown in  FIG. 12 , a metal interconnect process is performed. Specifically, a first interlayer dielectric ILD- 1  is formed on the substrate  64 , a patterned first metal layer M- 1  is formed on the interlayer dielectric ILD- 1  to form a portion of an etch stop structure. A second interlayer dielectric ILD- 2  is formed on the first metal layer M- 1 . A plurality of trenches are formed in the second interlayer dielectric ILD- 2  and via material is filled to the trenches to form the first trench-shaped vias V- 1  with the bottom contacting the metal layer M- 1 . The interlayer dielectric may comprise silicon oxide and may be formed by deposition as used in the conventional technology. The metal layer may be formed using a conventional copper process or aluminum process, as well as a damascene or double-damascene process. Likewise, the second metal layer M- 2  is formed to stack on the first trench-shaped vias V- 1 , and then the third interlayer dielectric ILD- 3  and the second trench-shaped vias V- 2  are formed in the order. Thereafter, the patterned third metal layer M- 3  is formed. The third metal layer M- 3  further includes a pattern of micro-machined metal mesh in the MEMS region  102 . Likewise, the fourth interlayer dielectric ILD- 4 , the third trench-shaped vias V- 3  only located on the third metal layer M- 3  of the etch stop structure, the patterned fourth metal layer M- 4  stacking on the third trench-shaped vias V- 3 , the fifth interlayer dielectric ILD- 5 , and the fourth trench-shaped vias V- 4  are formed in the order. Thereafter, the patterned fifth metal layer M- 5  is formed and further includes a portion as a metal hard mask  77  located above the micro-machined metal mesh  70  and corresponding to the micro-machined metal mesh  70  for used in the subsequent etching process. Thereafter, the sixth interlayer dielectric ILD- 6  is formed and a plurality of the fifth trench-shaped vias V- 5  are formed in the six interlayer dielectric ILD- 6  with the bottom contacting the fifth metal layer M- 5 . Thereafter, the patterned top metal layer M-Top is formed on the sixth interlayer dielectric ILD- 6  and to contact the fifth trench-shaped vias V- 5 . 
     It should be noticed that the bottom of the etch stop structure between the MEMS structure and the vent hole structure has at least a distance from the substrate; however, it is not necessary to form the etch stop structure by starting with the upper surface of the first interlayer dielectric. It is optional to form the etch stop structure by starting with the upper surface of any interlayer dielectric ILD, as long as the bottom of the etch stop structure is not higher than the micro-machined metal mesh and not higher than the vent holes. 
     The MOS device or the metal interconnect structure in the logic region  106  may be formed simultaneously in the process described above. Accordingly, in the present invention, the thickness and material of the metal layer and the interlayer dielectric may be the same as or similar to the metal layer and the interlayer dielectric of conventional metal interconnect structures. In addition, it should be noticed that in case that the etch stop structure for surrounding the MEMS region and the vent hole structure is formed, trench-shaped contacts should be first formed in the first interlayer dielectric ILD- 1  and then the metal layer and trench-shaped vias stack is formed upwardly, to form an entire etch stop structure for protection. 
     Thereafter, referring to  FIG. 13 , a release process is performed. First, an anisotropic deep reactive-ion etching (DRIE) process for dry-etching silicon oxide is performed on the MEMS region  102  and the vent hole region  104 , respectively using the metal hard mask  77  of the fifth metal layer M- 5  and the top metal layer M-Top as a mask to etch through the interlayer dielectrics. The etching stops on the substrate  64  and the openings  79  and  81  are formed. Then, the residual metal hard mask  77  is removed using a metal stripping process. Thereafter, referring to  FIG. 14 , an isotropic wet etching or vapor etching process is performed to etch away each interlayer dielectric in the MEMS region  102  and the vent hole region  104  using, for example, an etchant containing HF (fluorohydric acid), but the interlayer dielectrics within each of the etch stop structures  66  and  76  are not etched. Thus, a hollowed-out micro-machined metal mesh  70  is formed in the MEMS region  102  and has a distance from the substrate  64  to form a chamber  72 , and as well as vent holes  74  are formed in the vent hole region  104  and communicate the chamber  72 . 
     Finally, a vibration film  78  is formed and coated on the micro-machined metal mesh  70 , to obtain the MEMS device  50  according to the present invention, as shown in  FIG. 4 . When the vibration film is silicon, it may be formed right after the patterned metal layer for the micro-machined metal mesh is formed. Thus, since the vibration film is silicon, it is not etched away as the silicon oxide is in the subsequent release process. 
       FIGS. 15-16  illustrate another embodiment of the method of making a MEMS device according to the present invention. When the vent hole is the type of the mesh-typed vent hole  86 , the vibration film can be conformally formed on the MEMS structure  52  and the vent hole structure  54 . The vent hole is mesh-typed to have, for example, an opening width d 1  of the vent hole being 6 microns and the mesh diameter d 2  being 1.2 microns. Accordingly, melted vibration film material will not fall into the openings of the mesh-typed vent hole. Thereafter, referring to  FIG. 16 , a patterned photo-resist layer  93  is formed on the surface to expose the vibration film on the mesh-typed vent holes  86 . The exposed vibration film is etched away, and then the photo-resist layer  93  is stripped, to form the MEMS device according to the present invention. 
     All combinations and sub-combinations of the above-described features also belong to the present invention. Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.