Abstract:
Apparatus, methods, and systems for bonding a cover wafer to a MEMS threshold sensors located on a silicon disc. The cover wafer is trenched to form a region when bonded to the silicon wafer that produces a gap over the contact bond pads of the MEMS threshold sensor. The method includes a series of cuts that remove part of the cover wafer over the trenches to permit additional cuts that may avoid the contact bond pads of the MEMS threshold sensor. In addition the glass frit provides for isolation of the sensor with a hermetic seal. The cavity between the MEMS threshold sensor and the cover wafer may be injected with a gas such as nitrogen to influence the properties of the MEMS threshold sensor. The MEMS threshold sensor may be utilized to sense a threshold for pressure, temperature or acceleration.

Description:
BACKGROUND 
       [0001]    Micro-electro-mechanical systems (MEMS) are well known in the art. The technology is of the very small, and merges at the nano-scale into nano-electro-mechanical systems (NEMS) and Nanotechnology. MEMS are also referred to as micro machines, or Micro Systems Technology (MST). MEMS generally range in size from a micrometer to a millimeter. 
         [0002]    MEMS technology is finding its way into sensors and is utilized in a number of ways each and every day by electronic and mechanical systems. These systems may determine location, speed, vibration, stress, acceleration, temperature as well as a number of other characteristics. Many applications in consumer electronics, automotive electronics, audio/video, camcorder, camera, cell phone, games/toys, watches, PDA, GPS handhelds, medical devices, power supply on off system, navigation system and other electronic devices may utilize multiple sensors to meet their design objectives. 
         [0003]    In many cases, automated decision making is done when a certain threshold value of a physical parameter is higher or lower than a set point. MEMS based threshold sensors provide a low cost fabrication solution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is block diagram of a MEMS sensor operating as a suspended gate MOS transistor. 
           [0005]      FIG. 2  illustrates the MAFET sensor, in the activated position. 
           [0006]      FIG. 3A  is a block diagram of a cover wafer for a MAFET sensor according to an example embodiment. 
           [0007]      FIG. 3B  is a cover wafer for a MAFET sensor with a glass frit applied according to an example embodiment. 
           [0008]      FIG. 4A  illustrates a top view of the sensor with the cover mounted to it, as a part of entire wafer, according to an example embodiment. 
           [0009]      FIG. 4B  illustrates a cross sectional view of the sensor with the cover mounted to it, as a part of the entire wafer, according to an example embodiment. 
           [0010]      FIG. 5  illustrates a method for bonding a cover wafer to a sensor wafer and cutting the die into individual sensors according to an example embodiment. 
           [0011]      FIG. 6  illustrates a top view of the sensor with the cover, as a part of the entire wafer and the cut patterns for the die separation according to an example embodiment. 
           [0012]      FIG. 7  illustrates a block cross sectional view of a pressure sensor according to an example embodiment. 
           [0013]      FIG. 8  illustrates a system incorporating an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The inventors have determined that there is a need for a low cost, high performance, zero level, wafer level packaging technology for MEMS threshold sensors.  FIG. 1  is a MEMS mechanically actuated field effect transistor (MAFET) switch performed in a semiconductor substrate. The threshold sensor  100  is based on a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) with suspended, elastic gate  110 , which is separated by an air/vacuum gap  120  from the thin gate dielectric  130 . This type of threshold sensor is called a Mechanically Actuated Field Effect Transistor (MAFET) switch, where the suspended gate snaps “UP” or “DOWN” as a function of external measurand (temperature, pressure or acceleration) decreasing or increasing above a threshold value of that measurand, and thus driving the MOSFET transistor to the OFF state (no electric currents are flowing through the transistor) or ON state (an electric current is flowing through the transistor), respectively. The threshold sensor  100  comprises a MOSFET a source  140  and a drain  150  both of which may be n-type doped regions of the semiconductor substrate  160 . The threshold sensor also has a gate  110 , that can be made of highly doped polysilicon, metal or bimetal. To elevate the elastic gate  110 , above the dielectric layer ( 130 ) two silicon dioxide posts  170  and  175  are provided. 
         [0015]      FIG. 2  illustrates the MAFET threshold sensor  200 , in the activated position. The beam  210  is snapped down due to pre-biased gate voltage V G  and either the threshold temperature, pressure or shock. The silicon dioxide posts  270  and  275  hold the ends of the beam, however the air gap  120  of  FIG. 1  is significantly reduced if not eliminated. An n-type channel  245  is formed between source  240  and drain  250  when the beam  210  is in contact with the dielectric layer  230 . 
         [0016]    While MAFETs have been used before, the inventors have created a method for low cost packaging of these MAFET threshold sensors. The packaging should meet the requirements of the MOSFET technology in terms of contamination, humidity free and low thermal budget of packaging technology that preserves the performances of the already fabricated chip. 
         [0017]      FIG. 3A  is a cover wafer for a MAFET threshold sensor  100  and  200  of  FIGS. 1 and 2  respectively according to an example embodiment. Cover  300  incorporates a series of trenches  320 . The trenches  320  may be formed in the cover material  330  by any number of processes including but not limited to etching, drilling or other means. The trenches are laid out to align with the outer edges of the sensors and may be located over the drain, source, gate and substrate bond pads of the sensor. 
         [0018]      FIG. 3B  is a cover wafer for a MAFET threshold sensor with a glass frit  310  applied according to an example embodiment. Cover  300  has a glass frit  310  applied to the cover  330  in the selected areas between the trenches  320 . The glass frit  310  may be applied either utilizing a screen print or by direct printing. 
         [0019]      FIG. 4A  illustrates a top view of the threshold sensor with the cover mounted to it according to an example embodiment. The sensor  400  comprises a suspended gate  440  located within the glass frit  435 . The glass frit  435  surrounds the suspended gate  440  at a distance such that it does not interfere with its movement. The source contact bond pad  445  extends beyond the glass frit, as does the drain contact bond pad  470 , the gate contact bond pad  475 , and the substrate pad  480 . A cross sectional cut from A to A′ is shown across the glass frit  435 , the suspended gate  440  and the source and drain contact bond pads  445  and  470 . The cross sectional view is shown in  FIG. 4B . 
         [0020]      FIG. 4B  illustrates a cross sectional view of the threshold sensor  400 , as a part of the wafer with the cover mounted to it according to an example embodiment. The cover  410  is mounted onto the sensor  420  with glass frits  435 . The glass frits  435  are positioned between the trench  430  and the suspended gate  440  such that the glass frit  435  provides a gap between the cover  410  and the threshold sensor  420 . The glass frits  435  may be in contact with both the source contact bond pad  445  and the drain contact bond pad  470 . The dielectric layer  455  is located from the source  450  to the drain  460  as illustrated in  FIG. 1  and  FIG. 2 . 
         [0021]    The process  500  may be a method for bonding the cover wafer onto a temperature sensor MAFET (TMAFET) and acceleration threshold sensor g-MAFET. The process also cuts the die and may comprise the following activities.  FIG. 5  is a method for bonding a cover wafer to a sensor wafer and cutting the die into individual sensors according to an example embodiment. At  510  a cover wafer is selected. The cover wafer may be either silicon wafer or glass wafer with the same diameter and similar thickness as sensor wafer. The silicon or the glass wafer should have the same coefficient of thermal expansion (CTE) as the flit glass to be used for wafer bonding. Both the frit glass and the cover wafer should be metal ion free, (concentration of Na + , Mg + , Mn, Cr, ions below 50 parts per billion (ppb)) to prevent contamination of the gate dielectric with ionic charges. The ionic charges which will generate an electric field altering in an unpredictable way the threshold voltage of the field effect metal oxide semiconductor (MOS) transistor. The inventors have determined for an example embodiment that the chemical formulation of the frit glass should not contain metal ions above 50 ppb. This is due to the situation that during thermal densification of glass some vapors containing those metal ions may go outside the frit glass and be adsorbed on the gate dielectric surface and from there be absorbed into the gate dielectric. Once absorbed, the ionic charges generate electric fields that may interfere with the electric field applied from outside. In addition, the ionic charges may have a thermal diffusion inside the dielectric as a function of the applied voltage, so the ionic charges move up and down depending on the absence or presence of the applied potential on the gate. This may result in the existence of random threshold voltage on the sensor. In further embodiments, the existence of such a random threshold voltage may not be a problem, or may not result from higher metal ion concentrations. 
         [0022]    At  515  the cover wafer is trenched to a depth of approximately 50 micrometers with the diamond disc. The trenches are to be aligned above the metal bond pads of the sensor wafer such as bond pads  445 ,  470 ,  475 , and  480  of  FIG. 4 . 
         [0023]    At  520  a low temperature glass powder is prepared of a MOS compatible composition (concentration of metal ions like Na, Mg, Mn, Cr, etc., below 50 ppb in one embodiment). The glass powder is formed into a paste, containing base glass, refractory filler for decreasing the CTE of base glass and organic solvent for binding the components of the glass powder with the cover wafer. 
         [0024]    At  525  the paste is selectively applied to the cover wafer by either traditional screen printing or direct printing by additive, mask-less paste application. The direct printing is computer controlled dispensing and no mask is needed. 
         [0025]    At  530  the cover wafer and glass frit are dried for solvent evaporation and pre-consolidation of the glass paste or glass flit. 
         [0026]    At  535  the cover wafer is aligned with the sensor wafer so that trenches in the cover wafer overlap over the metal bond pads of the threshold sensor. In  FIG. 6  a top view the wafers is shown with the alignment of the trench after frit glass bonding. The trench  610  will align such that the end of the source contact bond pad  615  and the drain contact bond pad  620  will be located under the trench  610 . 
         [0027]    At  537  for a better thermal response by a temperature sensor (TMAFET) a dry nitrogen (N 2 ) or other inert gas at a pressure of 1 bar may be injected into the cavity  485  of  FIG. 4B . The gas should be dry and as pure as possible to avoid contamination. The gas pressure influences the bending/vibration of pressure/acceleration diaphragm. By setting the pressure in cavity  485  it may be possible to modify the dynamic response properties of the threshold sensor  400  both of  FIG. 4B . For example, the damping behavior of suspended gate  440  of  FIG. 4  may be influenced. For a thermal threshold sensor TMAFET, the suspended gate  440  is fully immersed in N 2 . For an acceleration threshold sensor or g-MAFET, the N 2  pressure may be characterized as “pushing” on the external side of suspended gate  440  which may affect the damping of suspended gate  440 . 
         [0028]    At  540  the cover wafer and sensor wafer with the flit glass are subject to thermal compression in an oven at temperature below  400 C preserving the sensor metallization from deterioration. The temperature used for the frit glass consolidation should be low enough so that the temperature does not affect the metallurgy of the metal-semiconductor contacts  615 ,  620 ,  625 , or  630  of  FIG. 6 . In the case of gold metallization for bond pads, chromium/nickel, titanium nitride (/TiN), or chromium/nickel/tantalum nitride (Cr/Ni/TaN) barrier layers or others should be used below the gold layer to stop the gold from diffusing toward the silicon wafer. The final height of the flit glass spacer may be in the range of 10-40 micrometers, high enough to allow the snapping up and down of the suspended diaphragm of MAFET. 
         [0029]    At  545  the bonded cover wafer is cut along the trenches using a diamond saw. As shown in  FIG. 6 , CUT # 1  and CUT # 2  are along the trench lines, so that a stripe corresponding to the trenches  610  above the bond pads  615 ,  620 ,  625  and  630  may be removed. CUT # 1  and CUT# 2  only cut through the cover wafer and do not cut or touch the bond pads  615  or  620 ,  625  and  630 . 
         [0030]    At  550  CUT # 3  and CUT # 4  are completed as seen in  FIG. 6 . Cut lines three and four will fully perforate the cover wafer and partially cut the sensor wafer. The sensor wafer may be cut approximately half way through. 
         [0031]    At  555  with the strip above trenches  610  are removed and CUT # 5  may be to cut the sensor wafer through the chip between two wafers, without touching the bonding pads. The cutting sequence is repeated across the wafer with a well defined repetition step. However, at the end of cutting sequence, the bonded wafer is still in one piece. 
         [0032]    At  560  a heavy cylinder mass is rolled over the partially cut bonded wafer tandem. At  565  the sensors are separated. 
         [0033]    The frit glass wafer level packaging of an acceleration MAFET sensor (g-MAFET) should be done exactly in the same way, with the advantage that the vacuum level above the suspended diaphragm of the acceleration threshold sensor can be flexible designed. 
         [0034]      FIG. 7  illustrates a cross sectional view of a pressure threshold sensor according to an example embodiment. The frit glass packaging of the pressure MAFET (P-MAFET) is different with respect to above bonding technology for temperature and acceleration sensors, by the fact that after activity  540  a selective perforation of the cover wafer  710  is done so that each chip will have a hole  715  above the cavity  720  of  FIG. 7 . 
         [0035]      FIG. 8  illustrates a system incorporating an embodiment of the invention. System  800  may be utilized in a moving vehicle such as an automobile. MAFET  810  may provide information to one or more of the following: an airbag control  820 , or an anti-lock brake control  830 . The MAFET  810  may indicate that a threshold such an acceleration or deceleration has been exceeded. For example, if the car should come to a sudden stop an acceleration threshold sensor may indicate that threshold has been exceeded to the airbag control  820  indicating when the airbag should deploy. In addition, the MAFET  810  may provide information that a temperature is below a threshold permitting ice to form. This information may be provided to anti-lock brake system  830 . 
         [0036]    The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b) requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. The above description and figures illustrate embodiments of the invention to enable those skilled in the art to practice the embodiments of the invention. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.