Patent Publication Number: US-2022227620-A1

Title: System For Protecting MEMS Product Under ESD Event

Description:
FIELD OF THE PRESENT DISCLOSURE 
     The present invention relates generally to a system. More particularly, this invention relates to a system for protecting MEMS product under electrostatic discharge event. 
     DESCRIPTION OF RELATED ART 
     A major reliability for many integrated circuits (ICs) is electrostatic discharge (ESD), especially, for MEMS product, such as RF MEMS switch. During an ESD event, a large amount of charge is transferred from one object to another in a relatively short period of time, which results in a peak current that can cause significant damage to the IC. 
     Therefore, it is necessary to provide a new electrostatic discharge protection system for solving above mentioned problem. 
     SUMMARY 
     In one aspect of the present disclosure, a system for protecting a MEMS product from an ESD event, comprises, a control circuit; a MEMS product, electrically connected with the control circuit; an ESD protection device, electrically connected with the control circuit, and electrically connected with the MEMS product in parallel; wherein, the ESD protection device comprises: a top electrode assembly electrically connected with the control circuit; a flexible beam comprising a first electrode layer electrically connected with the control circuit, a second electrode layer electrically connected with the MEMS product, and a moving metal contact electrically connected with the second electrode layer; a bottom electrode assembly having a bottom electrode layer electrically connected with the MEMS product and a fixed metal contact electrically connected with the bottom electrode layer and facing the moving metal contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematically block diagram of a system for protecting a MEMS product from an ESD event according to the present disclosure. 
         FIG. 2  is a schematically block diagram of the system shown in  FIG. 1  under the ESD event. 
         FIG. 3  is a schematically cut-away view of the ESD protection device of the system in a protection state according to one embodiment. 
         FIG. 4  is the schematically cut-away view of the ESD protection device of the system in an isolation state. 
         FIG. 5  is the schematically cut-away view of the ESD protection device of the system under the ESD event. 
         FIG. 6  is a schematically cut-away view of the ESD protection device of the system according to another embodiment. 
         FIG. 7  shows a switching time of the ESD protection device, MEMS product and the difference between them as a function of ESD voltage. 
         FIG. 8  shows a minimum contact resistance needed to dissipate the ESD voltage considering the minimum dissipation time shown in  FIG. 6 . 
         FIG. 9  shows switching time of both the MEMS product and the ESD protection device as function of the area ratio between them. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Hereinafter, the present disclosure will be further described with reference to the accompanying drawings and embodiments. 
     As shown in the  FIG. 1  and  FIG. 2 , a system  100  for protecting a MEMS product under an ESD event according to the present disclosure, comprises: a control circuit  10 , an ESD protection device  20  and a MEMS product  30 , both the MEMS product  30  and the ESD protection device  20  electrically connected with the control circuit  10 , and the MEMS product  30  is electrically connected with the ESD protection device  20  in parallel. The ESD protection device  20  has a faster response time than that of the MEMS product  30  while under an ESD event. Therefore, the ESD protection device  20  dissipates voltage/charge fast under the ESD event, and thus, the MEMS product  30  is protected from electrostatic discharge. 
     As shown in  FIG. 3  through  FIG. 6 , the ESD protection device  20  includes a housing  21  having a cavity  200 , a top electrode assembly  22  fixed to the housing  21 , a bottom electrode assembly  23  fixed to the housing  21  and spaced apart from the top electrode assembly  22 , and a beam  24  disposed between the top electrode assembly  22  and the bottom electrode assembly  23  and elastically supported by the housing  21 . 
     The housing  21  includes a top wall  211 , a bottom wall  212 , and a side wall  213  connected between the top wall  211  and the bottom wall  212 . The top wall  211 , the bottom wall  212  and the side wall  213  cooperatively form the cavity  200 . The top electrode assembly  22  is fixed to the top wall  211 . The bottom electrode assembly  23  is fixed to the bottom wall  212 . The beam  24  is flexible and elastically connected with the side wall  213 . The cavity  200  is sealed under a known pressure. 
     The top electrode assembly  22  includes a plurality of upper electrodes  221  electrically connected with the control circuit  10 , an upper oxide layer  222  encapsulating the electrodes  221  and at least one bump  223  protruding from the oxide layer  222  toward the beam  24  for contacting the beam  24 . 
     The bottom electrode assembly  23  includes a bottom electrode layer  231  fixed to the bottom wall  212 , a bottom oxide layer  232  encapsulating the bottom electrode layer  231 , and a fixed metal contact  233  extending outward from the bottom oxide layer  232 . One end of the fixed metal contact  233  is electrically connected with the bottom electrode layer  231 , and another end thereof is grounded. 
     The beam  24  includes a first electrode layer  241 , a second electrode layer  242 , a first oxide layer  243  encapsulating the first electrode layer  241 , a second oxide layer  244  encapsulating the second electrode layer  242 , and a moving metal contact  245  electrically connected with second electrode layer  242  and facing the fixed metal contact  233 . The moving metal contact  245  is exposed out of the second oxide layer  244 . The moving metal contact  245  may be a single one metal contact, or may comprise several metal contacts and alloys. 
     The control circuit  10  includes a CMOS circuit  11  and high voltage input  12 . The high voltage input  12  is electrically connected with the CMOS circuit  11 . The top electrodes  211  include first top electrodes  2211  and second top electrodes  2212  spaced apart from the first top electrodes  2211 . The high voltage input  12  is electrically connected with the first top electrodes  2211  and the second top electrodes  2212 . The first electrode layer  241  of the beam  24  is also electrically connected with the high voltage input  12 . The CMOS circuit  11  is configured to apply the high voltage to the MEMS product  30  or the ESD protection device  20 . The second electrode layer  242  and the bottom electrode layer  231  are electrically connected with the MEMS product  30  in parallel. For example, the MEMS product  30  may be a RF MEMS product having a RF beam, the second electrode layer  242  and the bottom electrode layer  231  may be electrically connected with the RF beam of the RF MEMS product in parallel. 
     When the MEMS product  30  is in normal operation, the control circuit  10  input a current or voltage into the top electrode assembly  21  and the first electrode layer  241 , respectively, then, an attractive force is formed between the top electrode assembly  21  and the first electrode layer  241 , so that the beam  24  is pulled up, and a gap between the beam  24  and the bottom electrode assembly  23  increased, and thus, an isolation state is created. In the isolation state, the first oxide layer  243  of the beam  24  contacts the bumps  223 . With such configuration, since the top electrodes create a perfect isolation and minimal parasitic capacitance, with minimal influence into the product capacitance. A minimum capacitor is formed between the moving metal contact  245  and the fixed metal contact  233 . 
     When the product is not in operation or not electrically connected, the system is in a protection state, the beam  24  is left free standing, in a release station. 
     When the system is under an ESD event, an electrostatic force applied to the second electrode layer  244  pulls the beam  24  down, so that the moving metal contact  245  contacts the fixed metal contact  234  to form an ohmic contact, and then, the ESD voltage/charge is dissipating via the ohmic contact. The beam  24  collapses much faster than the MEMS product  30 . The ESD charge is grounded via the fixed metal contact  234 . 
     When the ESD charge is dissipated through  233  or  242 , the electrostatic force is reduced, the beam  24  may be released and back to the protection state. 
     In other embodiment, the ESD protection device  20  further includes a flexible member  25  for elastically connected the beam  24  with side wall  213 . For example, the flexible member  25  may be a loop spring. 
     In other embodiment, an area of the beam  24  of the ESD protection device  20  is larger than that of the MEMS product  30 . When the area increased, the time to dissipate the ESD charge by the ESD protection device  20  will be faster. As shown in Table 1, the ESD protection device  20  has a ESD MEMS beam, which has an area at least twice to the beam of RF MEMS. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 RF MEMS product 
                 ESD protection device 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Gap Free (μm) 
                 1 
                 1 
               
               
                 Pull-up gap(μm) 
                 2.5 
                 2.5 
               
               
                 Area 
                 3.00E−09 
                 6.00E−09 
               
               
                 Contact resistance(ohm) 
                 — 
                 1000 
               
               
                 Number of beams 
                 14 
                 — 
               
               
                 Cmin pull-up 
                 172.8 
                 — 
               
               
                 Cmax 
                 3510.84 
                 — 
               
               
                 VSA 
                 13.66 
                 9.66 
               
               
                 Resonant(KHz) 
                 72 
                 72 
               
               
                 ESD Voltage (V) 
                 1500 
                 1500 
               
               
                 ESD Capacitance (pF) 
                 100 
                 100 
               
               
                 ESD Resistance (ohm) 
                 1500 
                 15000 
               
               
                 Max ESD peak 
                 1 
                 1 
               
               
                 Switch time(μs) 
                 0.0739 
                 0.0522 
               
               
                 RF MEMS-ESD time 
                   
                 0.0216 
               
               
                 Charge dissipation (μs) 
                   
                 0.100432 
               
               
                 Contact current (A) 
                   
                 1.5 
               
               
                   
               
            
           
         
       
     
     As shown in the table 1, when the system is unpowered and under a 1500V ESD event, the RF MEMS switching time is about 0.074 B and the switching time of the ESD protection device is about 0.054 s. The beam of the ESD protection device will react first than the RF MEMS product, and the time for dissipating the ESD charge is 0.0216 μs. If the contact resistance of the beam is 1000 ohm, the dissipating time may be around 0.1 μs. Thus, if a faster dissipating time is required, the lower contact resistance or higher area of the ESD protection device is required. As shown in  FIG. 6 , the switching time of the ESD protection device, MEMS product and the difference between them as a function of ESD voltage. As shown in  FIG. 6 , the switching time reduced as the ESD voltage increased. 
     As shown in  FIG. 8 , the minimum contact resistance needed to dissipate the ESD voltage considering the minimum dissipation time show in  FIG. 5 . For example, under a 1500V ESD event, a contact resistance of 200 ohm is required to dissipate the voltage by the ESD protecting device. 
       FIG. 9  shows a switching time of both the MEMS product and the ESD protection device as function of the area ratio between them. As shown in  FIG. 9 , the switching time of the MEMS product is consistent as the area ratio increase, and the switching time of the ESD Protection device is reduced as the area ratio increase. 
     The above is only the embodiment of the present invention, but not limit to the patent scope of the present disclosure, and the equivalent structures or equivalent process transformations made by utilizing the present disclosure and the contents of the drawings, or directly or indirectly applied to other related technology fields, are all included in the scope of the patent protection of the present disclosure.