Abstract:
The present invention discloses a Micro-Electro-Mechanical System (MEMS) pressure sensor device and a manufacturing method thereof. The MEMS pressure sensor device includes: a substrate having at least one recess formed on an upper surface thereof, the recess defining a boss; a membrane, which is bonded to at least a part of the upper surface and at least a part of the boss, so that the at least one recess forms a cavity; at least one sensing unit, which is coupled to the membrane, for sensing deflection of the membrane; and an opening, which is formed on a lower surface of the substrate, and connects to the cavity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a Micro-Electro-Mechanical System (MEMS) pressure sensor device and a manufacturing method thereof; particularly, it relates to such MEMS pressure sensor device and manufacturing method capable of sensing a lower pressure. 
     2. Description of Related Art 
       FIGS. 1A-1E  show cross-section views of a manufacturing flow of a MEMS pressure sensor device disclosed by U.S. Pat. No. 6,093,579. As shown in  FIG. 1A , by a first lithography process, a region  104  is defined on a substrate  100  by a mask  102  and a mask  106 , wherein the mask  102  covers the rest of the substrate  100  and the mask  106  has a specific pattern to define the pattern of the region  104 .  FIG. 1B  is a schematic cross-section view showing the MEMS pressure sensor device after the first lithography process and a first etching process. As shown in  FIG. 1B , after the first etching process, a membrane (the substrate) of the MEMS pressure sensor device includes a boss  108  with the specific pattern in the center area and deep trenches  110  around the boss  108 , wherein the boss  108  has a thickness d. Next, as shown in  FIG. 1C , a second lithography process is performed wherein the boss  108  is no longer covered by the mask  106 , and a second etching process is applied to obtain a desired thickness of the membrane, wherein the thickness d of the boss  108  is relatively maintained. In the second etching process, the trapezoid boss  108  is rounded at edges as shown by the cross-section view in  FIG. 1D . At last the mask  102  outside the region  104  is removed, such that a thinner membrane with the boss  108  is obtained as shown in  FIG. 1E . 
     A membrane with the boss  108  and having a thinner thickness causes the MEMS pressure sensor device to have a relatively higher sensitivity and a relatively wider linear operation region. However, although this prior art can generate a MEMS pressure sensor device with such a membrane, it requires two lithography processes and the second lithography process must be aligned with the first lithography process. After the trenches  110  are formed, the topography of the substrate  100  is rugged and this is very disadvantageous to the second lithography process. The alignment may fail, and specific process and material maybe required. Therefore, this prior art needs relatively high manufacturing cost. 
     Besides, the second etching process is an anisotropic etching process. The boss  108  will be squared from a top view (not shown) after the anisotropic etching process; this is due to the characteristics of the anisotropic etching and the lattice of the substrate  100 . Therefore, this prior art limits the shape of the boss  108  and therefore limits the design and application of the MEMS pressure sensor device. 
     In view of above, to overcome the drawbacks in the prior art, the present invention proposes a MEMS pressure sensor device and a manufacturing method thereof, which can sense a lower pressure with a lower manufacturing cost so as to provide wider applications. 
     SUMMARY OF THE INVENTION 
     The first objective of the present invention is to provide a MEMS pressure sensor device. 
     The second objective of the present invention is to provide a manufacturing method of MEMS pressure sensor device. 
     To achieve the objectives mentioned above, from one perspective, the present invention provides a MEMS pressure sensor device, including: a substrate having at least one recess formed on an upper surface thereof, the recess defining a boss; a membrane, which is bonded to at least a part of the upper surface of the substrate and at least apart of the boss, so that the at least one recess forms a cavity; at least one sensing unit, which is coupled to the membrane for sensing deflection of the membrane; and a first opening, which is formed on a lower surface of the substrate, and connects to the recess. 
     From another perspective, the present invention provides a manufacturing method of Micro-Electro-Mechanical System (MEMS) pressure sensor device, including: f forming at least one recess on an upper surface of a substrate, the at least one recess defining a boss; bonding a membrane to at least a part of the upper surface of the substrate and at least apart of the boss, so that the at least one recess forms a cavity; coupling at least one sensing unit to the membrane for sensing deflection of the membrane; and forming a first opening on a lower surface of the substrate by an etching process or a polishing process, and connecting the first opening to the cavity. 
     In one embodiment of the MEMS pressure sensor device, the first opening is preferably formed by a deep reactive ion etching (DRIE) process. 
     In another embodiment, the sensing unit preferably senses the deflection of the membrane and generates a resistance difference signal, a capacitance difference signal, or a voltage difference signal. 
     In yet another embodiment, the MEMS pressure sensor device preferably further includes a bonding glass, which is bonded to the lower surface of the substrate, and the bonding glass preferably includes a second opening which connects to the first opening. 
     In another embodiment, the bonding step preferably includes one of: a direct bonding process, an anodic bonding process, an eutectic bonding process, an adhesive bonding process, and a glass frit bonding process. 
     In the aforementioned MEMS pressure sensor device and the manufacturing method thereof, the boss has a thickness which is preferably adjusted by the etching process or the polishing process. 
     The aforementioned manufacturing method of the MEMS pressure sensor device preferably further includes: forming a stop layer between the cavity and the substrate, for stopping the etching process or the polishing process. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1E  show cross-section views of a manufacturing flow of a MEMS pressure sensor device disclosed by U.S. Pat. No. 6,093,579. 
         FIGS. 2A-2F  show a first embodiment of the present invention. 
         FIGS. 3A-3D  are examples explaining how a resistance difference signal is generated by a sensing unit  13  according to piezoresistive sensing mechanism. 
         FIG. 4  shows a second embodiment of the present invention. 
         FIG. 5  shows a third embodiment of the present invention. 
         FIG. 6  shows that the bonding glass  30  may include no opening to form an absolute pressure sensor device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawings as referred to throughout the description of the present invention are for illustration only, to show the interrelations between the regions and the process steps, but not drawn according to actual scale. 
       FIGS. 2A-2F  show a first embodiment of the present invention. This embodiment shows an example of a MEMS pressure sensor device manufacturing flow of the present invention.  FIGS. 2A-2B  and  2 D- 2 F are cross-section views, and  FIG. 2C  is a top view. Referring to  FIG. 2A , a substrate  10  is provided first, and the substrate  10  is for example but not limited to a silicon substrate. Next, a recess/recesses  11  is/are formed and a boss  12  is defined inside/between the recess/recesses  11  by a lithography process and an etching process as shown in  FIG. 2B .  FIG. 2C  for example shows two embodiments of the recess/recesses  11  from top views. The number of the recess/recesses  11  may be single or plural. The multiple recesses  11  may connect to each other or separate from each other, and they may be regular shape or irregular shape. If the recesses  11  are multiple, sizes of the recesses  11  maybe same or different, and the arrangement of the multiple recesses  11  may be regular or irregular as long as the boss  12  is defined by the arrangement of the multiple recesses  11 . The boss  12  is used to increase the sensitivity of the MEMS pressure sensor device, and the shape and the location of the boss  12  are not limited. Referring to  FIG. 2D , a membrane  20 , for example but not limited to a silicon substrate with an oxide layer  24  on its surface, is bonded to an upper surface of the substrate  10 . The recess/recesses  11  thus becomes/become one or more cavities  11 . The upper surface of the substrate  10  for example may be an oxide layer  14 , and the bonding method may be but not limited to: a direct bonding process, an anodic bonding process, an eutectic bonding process, an adhesive bonding process, or a glass frit bonding process. The process steps shown in  FIGS. 2A-2D  may be but not limited to a manufacturing method of cavity silicon on insulator (cavity SOI).  FIG. 2E  shows that an electronic unit, for example sensing units  13  and related wiring, is formed in the membrane  20 . The electronic unit does not have to be formed in the membrane  20  and it may be formed outside the membrane  20 , as long as it is coupled to the membrane  20  to sense the deflection of the membrane  20 . The sensing units  13  maybe formed for example by a combination of selected ones of the followings: a lithography process, an etching process, a deposition process, a diffusion process, and an ion implantation process, etc. 
     Referring to  FIG. 2F , an opening  15  is formed on a lower surface of the substrate  10 . The opening  15  is formed by for example but not limited to a deep reactive ion etching (DRIE) process, or a chemical mechanical polishing (CMP) process. Different from the prior art, the opening  15  is formed on the flat lower surface of the substrate  10  in this embodiment. Therefore, there is no misalignment problem as in the prior art, and no specific process or materials are required, so the manufacturing cost is decreased. This is one of the advantageous features of the present invention over the prior art. The opening  15  is connected to the cavity/cavities  11 , such that the cavity/cavities  11  can sense pressure (that is, the pressure can exert on the membrane  20 ). The thickness h of the boss  12  may be adjusted by the etching process or the polishing process. Note that, referring to  FIGS. 2E and 2F , the oxide layer  14  between the cavity  11  and the substrate  10  may be used as a stop layer for stopping the etching process or the polishing process which forms the opening  15 . 
       FIGS. 3A-3D  are examples explaining how a resistance difference signal is generated by a sensing unit  13  according to piezoresistive sensing mechanism. As shown in  FIG. 3A , when no pressure is applied to the membrane  20 , the membrane  20  is flat as shown in the figure for example. As shown in  FIG. 3B , when a pressure to be sensed is applied to the membrane  20 , the membrane  20  will deflect with different degrees according to the sensed pressure (indicated by the arrow shown in the figure). The boss  12  increases the sensitivity of the membrane  20  to the sensed pressure. The sensing unit  13  for example may be a bridge circuit as shown in  FIG. 3C , by arranging resistors R 1 , R 2 , R 3 , and R 4  at proper positions in the device. When the membrane  20  is deflected because of the sensed pressure, resistances of the resistors in the sensing unit  13  change accordingly, for example R 1 +αR 1 , R 2 −αR 2 , R 3 −αR 3 , and R 4 +αR 4  as shown in  FIG. 3D . The pressure may be calculated from the changes in an output voltage Vout for a given input voltage Vin. Certainly, the sensing unit of the MEMS pressure sensor device of the present invention is not limited to operating according to the piezoresistive sensing mechanism. The MEMS pressure sensor device of the present invention can be applied to other types of applications such as for generating a capacitance difference signal by capacitive sensing mechanism, or for generating a voltage difference signal by piezoelectric sensing mechanism, etc.; i.e., the sensing unit  13  can be designed to sense the deflection of the membrane  20  and generate a resistance difference signal, a capacitance difference signal, or a voltage difference signal. 
       FIG. 4  shows a second embodiment of the present invention. This embodiment is different from the first embodiment in that, the opening  15  of this embodiment is for example formed by a CMP process, such that the lower surface of the substrate  10  is globally planarized. 
       FIG. 5  shows a third embodiment of the present invention. This embodiment is different from the first embodiment in that the MEMS pressure sensor device further includes a bonding glass  30 . The bonding glass  30  is bonded to the lower surface of the substrate  10 . The bonding glass  30  includes an opening  31 , which is connected to the opening  15 , such that the cavities  11  can sense pressure to form a relative pressure sensor.  FIG. 6  shows an embodiment which includes the bonding glass  30  without opening to form an absolute pressure sensor device. 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, the opening  15  may be formed by a combination of two or more processes, not limited to only one of the etching process and the polishing process. For another example, the substrate  10  and the membrane  20  are not limited to the silicon substrate, but instead may be other types of substrates such as semiconductor substrates, metal substrates, or insulator substrates. For yet another example, the stop layer is not limited to the oxide layer  15  as shown in the embodiments, but may be a nitride layer or other types of stop layers made of other materials which can stop the etching process or the polishing layer. For yet another example, the shapes of the structural parts of the present invention are not limited to those shown in the embodiments, but they may be changed; such as, the shape of the boss  12  is not limited to a square, etc. In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.