Abstract:
An automated process for performing MEMS packaging including automatically attaching a die to a chip carrier, resulting in a chip carrier assembly, automatically moving the chip carrier assembly into a vacuum chamber, wherein the vacuum chamber includes one or more lids therein, automatically securing a lid to the chip carrier assembly within the vacuum chamber, thereby forming a packaged die, and automatically removing the packaged die from the vacuum chamber. Unique vacuum chambers suitable for MEMS packaging are also disclosed.

Description:
GOVERNMENT SUPPORT  
       [0001]     This invention was made with government support under Air Force Contract Number F33615-01-2-5705. The government may have certain rights in the invention. 
     
    
     FIELD  
       [0002]     The present invention is related to the field of micro-electro-mechanical systems (MEMS) packaging, and more particularly, to methods and systems for micro-electro-mechanical systems (MEMS) vacuum packaging.  
       BACKGROUND  
       [0003]     An important aspect of many MEMS (or many other electronic chip) devices is the development and execution of efficient, robust, and protective packaging strategies. Typically a wafer containing many MEMS devices is cut into a number of MEMS die, which are then individually packaged using relatively complex and labor intensive processes. For many MEMS devices, particularly those using moving features such as cantilever beams or other suspended structures, there is a desire to create a well-contained and controlled environment such as a vacuum environment. Such packaging often includes a cavity formed by a chip carrier and a lid. A getter is sometimes provided in the cavity to capture gasses that enter the cavity through out gassing and/or small leakage paths.  
         [0004]     Typically, each MEMS device is secured to a chip carrier via solder, adhesive, or in some other way. This mechanically secures the MEMS device to the chip carrier, and is often performed using a conventional die bonding machine. One such die bonding machine is the Automatic Component Assembly Cell 3500-II, available from Palomar Technologies, Inc., located in Vista, Calif. Once the MEMS device is secured to the chip carrier, the I/O pads on the MEMS device are electrically wired to pads on the chip carrier. This is often performed using a conventional wire bonding machine. One such wire bonding machine is the Model 2460-V Automatic Wire Bonder, also available from Palomar Technologies. When flip-chip bonding is used, ball bonds or the like may be used to both secure the MEMS device to the package and to electrically connect the I/O pads of the MEMS device to pads on the chip carrier, as is known in the art.  
         [0005]     Next, a solder ring or the like is often placed on the chip carrier. The solder ring will eventually be used to solder the lid to the chip carrier to create a vacuum seal for the enclosed MEMS device. In many cases, the chip carrier assembly is baked in a vacuum environment for some time to cause the materials used in the chip carrier assembly to outgas as much as possible.  
         [0006]     A getter may be spot welded or otherwise attached to the package lid. The chip carrier assembly and the lid are then typically placed in a vacuum chamber, and heat is applied to activate the getter. Once the getter is activated, the lid is brought into alignment with the chip carrier assembly, and heat and pressure are applied so that the solder ring secures and seals the lid to the chip carrier assembly, thereby providing a vacuum environment for the enclosed MEMS device.  
         [0007]     In many cases, the above process steps are performed in a manual or semi-automated way. Some of the steps may be performed using some batch process techniques and/or some automated equipment. However, significant manual handling of the parts is still often required. What would be desirable, therefore, is a more automated process for packaging MEMS devices.  
       SUMMARY  
       [0008]     The present invention relates to MEMS packaging, and more particularly, to a more automated method for packaging MEMS device. In one illustrative embodiment, the present invention automatically attaches a die to a chip carrier, resulting in a chip carrier assembly. The chip carrier assembly is then automatically moved into a vacuum chamber, wherein the vacuum chamber includes one or more lids therein. The lids are automatically secured to the chip carrier assembly within the vacuum chamber, thereby forming a packaged die. The packaged die is then automatically removed from the vacuum chamber. Unique vacuum chambers suitable for MEMS packaging are also contemplated.  
         [0009]     The above summary is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures, Detailed Description and Examples which follow more particularly exemplify these embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a block diagram of a system in accordance with an illustrative embodiment of the present invention;  
         [0011]      FIG. 2  is a block diagram of an illustrative method in accordance with the present invention;  
         [0012]      FIG. 3  is a block diagram of a system in accordance with another illustrative embodiment of the present invention; and  
         [0013]      FIG. 4  is a block diagram of another illustrative method in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0014]     The following detailed description should be read with reference to the drawings. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention.  
         [0015]      FIG. 1  is a block diagram of an in-line packaging system in accordance with an illustrative embodiment of the present invention. The in-line packing system includes a first subsystem  30  and a second subsystem  32 , with a queue device  34  controlling component movement therebetween. The queue device  34  may be, for example, a magazine handler having modes such as first-in-first-out, last-in-first-out, pass-through, etc.  
         [0016]     In the illustrative embodiment, the first subsystem  30  includes a component assembly device  36  and a bonding/deposition device  38 . The component assembly device  36  may be, for example, a Model 3500-II Automatic Component Assembly Cell, and the bonding/deposition device  38  may be a Model 2460 Automatic Wire Bonder, both available from Palomar Technologies, Vista, Calif. These are illustrative devices, and it should be recognized that any suitable component assembly and bonding/deposition devices may be used, as desired.  
         [0017]     In an illustrative operation, a number of empty boat carriers  40  are sent into the component assembly device  36  (for example, from another magazine handler, which is not shown). The boat carriers may be, for example, Auer boat carriers, and may come in a variety of sizes and configurations. An illustrative boat carrier is shown in U.S. Pat. No. 6,367,635 to Auer, the disclosure of which is incorporated herein by reference.  
         [0018]     The component assembly device  36  is provided with a number of chip carriers  42  that, in some cases, may be Leadless Chip Carriers (LCCs). The component assembly device  36  places the chip carriers  42  into the various receiving recesses in each of the boat carriers  40 . The boat carriers  40 , with the chip carriers  42  placed thereon, are then passed to the bonding/deposition device  38 , where in some embodiments, ball bumps are placed on the back surface of each of the chip carriers. In some cases, the bonding/deposition device  38  provides an adhesive on the back surface of each of the chip carriers rather than, or in addition to, the ball bumps.  
         [0019]     From the bonding/deposition device  38 , the boat carriers  40  pass a first time to the queue device  34 , which may route the boat carriers  40  back to the component assembly device  36  in the first subsystem  30 . Alternatively, two (or more) first subsystems  30   a  and  30   b  may be provided in series, with the boat carriers  40  passing from the bonding/deposition device  38   a  of the first subsystem  30   a  to the component assembly device  36   b  in the first subsystem  30   b , if desired.  
         [0020]     At the component assembly device  36  a second time, die  44  are placed on the ball bumps (and/or adhesive) of each of the chip carriers  42 . In some cases, heat and sometimes pressure are applied to secure the die  44  to the back surface of each of the chip carriers to form a thermo-compression bond. However, it is contemplated that the die  44  may be secured to each of the chip carriers  42  using any suitable method, including using a solder, an adhesive, or in any other suitable technique, as desired. Next, the boat carriers  40  are passed to the bonding/deposition device  38 , where wire bonds may be placed that connect the I/O pads on the die with corresponding I/O pads on the chip carriers  42 .  
         [0021]     In some embodiments, the die  44  may be flip-chip type devices having pads on their front side. With such die  44 , attachment of the die to the ball bumps from the first pass may provide the desired electrical connections between pads on the die  44  and corresponding pads on the chip carriers  42 . For such devices, wire bonding may not be needed during the second pass through the first subsystem.  
         [0022]     From the bonding/deposition device  38 , the boat carriers  40  are passed a second time to the queue device  34 , which in the illustrative embodiment, again routes the boat carriers  40  back to the component assembly device  36  in the first subsystem  30 . At the component assembly device  36  a third time, seal rings  46  are placed or tacked onto the chip carriers  42 . The boat carriers  40  are then passed to the bonding/deposition device  38 , which in the illustrative embodiment, merely passes the boat carriers  40  therethrough.  
         [0023]     From the bonding/deposition device  38  the third time, the boat carriers  40  again go to the queue device  34 , and are this time passed to the second subsystem  32 . The second subsystem  32  is designed for bake-out and sealing of the packaged chips. Getter activation (if provided) may also be performed in the second subsystem. If desired, the second subsystem  32  may include a single vacuum furnace. Alternatively, and as shown in  FIG. 1 , the second subsystem  32  may include a first vacuum furnace  50  and a second vacuum furnace  52 .  
         [0024]     In some embodiments, a robotic device  48  may be provided for passing the devices to the first vacuum furnace  50  and/or the second vacuum furnace  52 . The robotic device may be an automated furnace loader configured to place boat carriers  40  into one of the vacuum furnaces  50 ,  52 , and may include a shuttle arm for loading boat carriers  40  into each furnace  50 ,  52 . In an illustrative embodiment, the robotic device  48  receives boat carriers  40  from the queue device  34  and loads a first vacuum furnace  50  until the first vacuum furnace  50  is at capacity, and then begins loading the second vacuum furnace  52 . In some embodiments, the vacuum furnaces may be single or dual chamber vacuum furnaces, as further described below. Once loaded, the first vacuum furnace  50  may go into a bake out sequence, followed by lid placement and sealing, with optional getter activation.  
         [0025]     When the first and second vacuum furnaces  50  and  52  are dual chamber furnaces, the first vacuum furnace  50  may bake the loaded boat carriers  40  for a first time period at a predetermined bake out temperature using a first vacuum level in a first furnace chamber. While the first chamber is performing the bake out sequence, a second chamber may be depressurized to a vacuum level that approximates the first vacuum level. The second chamber may include a lid placement system for mechanically placing lids  54 ,  56  onto the individual chip carriers  42  on the boat carriers  40 .  
         [0026]     When the lids  54 ,  56  include a getter requiring activation, a getter activation sequence may take place in the second chamber, in parallel with the bake out sequence in the first chamber, in which the second chamber is taken to a second vacuum level at a second temperature to activate the getters. The second temperature may be higher than the first temperature. For example, getter activation may occur in the range of 350-500 degrees C., while the bake out sequence may be at or below the eutectic temperature of the solder ring, which may be in the range of 200-350 degrees C. These are only illustrative temperatures, and may vary depending on the materials used.  
         [0027]     Once the bake out and getter activation are complete, the vacuum levels between the chambers may be equalized, and a divider between the chambers may be opened. The boats may then be passed to the second chamber for lid attach. The boats may be passed to the second chamber using a robot, a moving belt or in any other suitable way. The temperature in the second chamber may be brought to a third temperature (i.e. at or above the eutectic temperature of the solder ring), and the vacuum level may be brought to at a third vacuum level. In some cases, one or more inert gasses may be added to the second chamber to create the desired environment for the packaged die. The lids may then be brought into engagement with the solder rings, in some cases with the assistance of a lid handler. The lids may be temporarily secured to the lid handler, which may be suspended above the boats. The lid handler may include a separately controlled heater that heats the lids separate from the boats. For example, the lid handler may include a resistive heating element that heats the lid handler and thus the lids. This may allow the lids, and any accompanying getter, to be heated to the getter activation temperature (e.g. 450 degrees C.). In some cases, heat and/or pressure may be applied between each lid and chip carrier to cause the solder ring to reflow, thereby helping to create a hermetic seal between the chip carrier and the lid.  
         [0028]     In some cases, solder rings may not be used. For example, the lids may be secured to the chip carrier using other methods such as resistance welding. When resistance welding is used, for example, it may not be necessary to artificially increase the temperature in the vacuum chamber to at or near a solder reflow temperature.  
         [0029]     In any case, the temperature may then be allowed to fall toward ambient, and the vacuum may be removed. The vacuum levels and temperatures are described herein as changing from one step to another, but it should be recognized that these levels need not vary widely, if at all, and are dependent upon the materials and devices used.  
         [0030]     When the first and second vacuum furnaces only have a single vacuum chamber, the above describes steps may all take place in the single vacuum chamber, and the boats need not be shuttled between first and second vacuum chambers. In such an embodiment, the single vacuum chamber may also include a lid handler. The lids may be temporarily secured to the lid handler, which may be suspended above the boats. The lid handler may include a separately controlled heater that heats the lids separate from the boats. For example, the lid handler may include a resistive heating element that heats the lid handler and thus the lids. This may allow the lids, and any accompanying getter, to be heated to the getter activation temperature (e.g. 450 degrees C.) separately from the boats, which may remain below the eutectic solder temperature (e.g. 300 degrees C.) during the bake out process. Thus, getter activation may occur in the same chamber and sometimes at or near the same time as the bake out process, if desired. Once the bake out process is complete, and the getter is activated, heat and/or pressure may be applied between each lid and chip carrier to cause the solder ring to reflow, thereby helping to create a hermetic seal between the chip carrier and the lid. The lid handler may be adapted to move the lids until they engage the chip carriers, if desired. When resistance welding is used, the chamber may not need to be heated to the solder reflow temperature. Instead, pressure and a rather large voltage may be applied between the lid and the chip carrier, often assisted by the lid handler, to form the desired resistance weld.  
         [0031]     In the illustrative embodiment, the second vacuum furnace  52  may operate similar to, and in parallel with, the first vacuum furnace  50 . Thus, in the illustrative embodiment, the second vacuum furnace  52  may increase the throughput of the packaging process. Also, it is contemplated that more than two vacuum furnaces may be provided to further increase the throughput of the packaging process, if desired.  
         [0032]     In some cases, the vacuum furnace(s) or chambers may be adapted to accept only a single or few (e.g. two, four, etc.) chip carriers. This may help reduce the vacuum pump down time required to achieve a desired vacuum level in the vacuum furnace(s) or chambers.  
         [0033]     After the getter activation and cap seal sequence is performed in the vacuum furnaces  50 ,  52 , the vacuum furnaces  50 ,  52  are unloaded. Unloading can occur by the use of the robotic device  48 , or the furnaces  50 ,  52  may be configured to unload themselves. The result is a finished product  58  that includes a sealed chip including the chip carrier  42 , a packaged die  44 , and a lid  54 ,  56 , optionally including an activated getter therein. If desired, the boat carriers  40  may then be loaded onto another magazine handler until the chip carriers  42  can be unloaded.  
         [0034]     Also illustrated in  FIG. 2  is a controller  59 , which may optionally be included to control the functioning of the various system elements as described with reference to  FIG. 2 . For example, the controller  59  may be provided with a readable memory having an instruction set executable for performing the methods described with reference to  FIG. 2  and/or the method described with respect to  FIG. 3 . The controller  59  may take any suitable form, including a microcontroller, computer, PDA, or the like. While the controller  59  is shown as having connections to several system elements, a wireless communication system/protocol may also be used for delivering commands from the controller to the system elements.  
         [0035]      FIG. 2  is a block diagram of an illustrative method in accordance with the present invention. The illustrative method begins by queuing empty boats, as noted at  60 . Next, chip carriers are placed in the boats as shown at  62 , and ball bumps are placed on the chip carriers as shown at  64 . In some embodiments, these two steps  62 ,  64 , as indicated by the box, may be performed during a first pass through the first subsystem  30  of  FIG. 1 .  
         [0036]     The method is continued by queuing the boats with the carriers, as shown at  66 . Next, die are placed and secured to the chip carriers  68 , and in some cases, wire bonds are provided between the chips and carriers, as shown at  70 . In some cases, the die may be secured to the chip carriers  68  via solder, adhesive, thermo-compression bonding, or in any other suitable way, as desired.  
         [0037]     Again, as indicated by the dashed box, steps  68  and  70  may be performed during a second pass through the first subsystem  30  of  FIG. 1 . As noted above, with some die, for example certain flip chip type die, the placement of wire bonds during the second pass may not be necessarily, and thus may be omitted, if desired.  
         [0038]     Next, the boats with the die and chip carriers are queued again as shown at  72 . Seal rings are then placed on the chip carriers as shown at  74 . This step  74 , as indicated by the box, may be performed during a third pass through the first subsystem  30 . The chip carriers loaded with die and seal rings are, while respectively in their boats, queued again as shown at  76 .  
         [0039]     From this fourth queue shown at  76 , the method continues with the step of going to a second subsystem, as indicated by the box around steps  78 - 86 . In the second subsystem, the boats are placed in a vacuum furnace, as shown at  78 . While in the vacuum furnace, a first bake out is performed, as shown at  80 . Next, as shown at  82 , lids are placed on the individual chip carriers. If desired, getters are activated as shown at  84 , and then the devices undergo cap, seal and, if desired, a second bake out step, as shown at  86 . In some embodiments, the getters may be activated prior to placing the lids on the individual chip carriers, if desired. That is, step  84  may occur prior to step  82  in  FIG. 2 . Also, it is contemplated that getters may not be provided at all, and in this case, the getter activation step  84  may be eliminated altogether. The result is a hermetically sealed chip carrier containing a die and an optional activated getter, the chip carrier being carried by a boat carrier.  
         [0040]     In an illustrative embodiment, some lids may be provided without getters. The devices packaged using lids without getters can be used to observe a number of process and device characteristics. For example, a completed device lacking a getter may help provide an indication of the level of seal and vacuum achieved using the packaging process since, without the getter provided, any residual substances or leaks are not masked by the effects of the getter. In some cases, a getter-less packaged device can provide an indication of the effectiveness of the getters that are provided.  
         [0041]     In an illustrative embodiment, the cap and seal process occurs as follows. Once a furnace is loaded, the interior of a vacuum chamber is sealed and pumped down to a desired vacuum. Next, the chamber is heated to a temperature (e.g. 280 degrees Celsius) while hydrogen levels in the furnace are monitored and the vacuum level is observed. This bake out step continues until a desired hydrogen/vacuum level is reached, or a time limit expires.  
         [0042]     If a two-chamber furnace is used, the lids having getters may be situated in the second chamber, and the second chamber may be pumped down to a desired vacuum level. The getters may be heated to a getter activation temperature (e.g. 450 degrees Celsius) for a predetermined time period such as 15 minutes. Once bake out is complete in the first chamber, the vacuum levels in the first and second chambers may be equalized, and a divider may be opened. The boats may then be shuttled into the second chamber. The second chamber may then be heated to a temperature that is at or above the eutectic solder temperature of the solder ring, and a desired vacuum may be provided. The lids may then be brought into engagement with the chip carriers to create a seal therebetween. In some embodiments, ibis is accomplished with a lid handler, as described above. The chamber may then be allowed to cool, and the vacuum may be released. The temperatures, times, and vacuum levels may all vary depending on the characteristics of the getters and solder, for example, as well as the desired end product characteristics.  
         [0043]     When a single vacuum chamber is used, the above described steps may all take place in the single vacuum chamber, and the boats need not be shuttled between first and second vacuum chambers. Like above, the single vacuum chamber may include a lid handler. The lids may be temporarily secured to the lid handler, which may be suspended above the boats. The lid handler may include a separately controlled heater that heats the lids separate from the boats. For example, the lid handler may include a resistive heating element that heats the lid handler and thus the lids. This may allow the lids, and any accompanying getter, to be heated to the getter activation temperature (e.g. 350-500 degrees C.) separately from the boats, which may remain below the eutectic solder temperature (e.g. 200-350 degrees C.) during the bake out process. Thus, getter activation may occur in the same chamber and sometimes at or near the same time as the bake out process, if desired.  
         [0044]      FIG. 3  and  FIG. 4  show another illustrative embodiment of the present invention. First, second and third vacuum devices are used in these embodiments. Referring to  FIG. 4 , a first subsystem  100  and a second subsystem  102  are shown, with a queue device  104  controlling component movement therebetween. The first subsystem works in similar fashion to that explained above with reference to  FIG. 2 .  
         [0045]     More particularly, boat carriers  110  may pass through a component assembly device  106  a first time and may be populated with chip carriers. The boat carriers  110  may then go to the bonding/deposition device  108 , where solder bumps may be placed thereon. From the bonding/deposition device  108 , the boat carriers  110  may go to the queue device  104  a first time and get routed back to the component assembly device  106  a second time. Alternatively, two (or more) first subsystems  100   a  and  100   b  may be provided in series, with the boat carriers  110  passing from the bonding/deposition device  108   a  of the first subsystem  100   a  to the component assembly device  106   b  in the first subsystem  100   b , if desired.  
         [0046]     In the component assembly device  106  a second time, the chip carriers  110  may be populated with die  114  and passed to the bonding/deposition device  108 . While in the bonding/deposition device  108  a second time, the die  114  may be secured to the chip carriers  112  and, if needed, wire bonds may be provided. From the bonding/deposition device  108  the second time, the boat carriers  110  may return to the component assembly device  106  for placement of sealing rings  116  on the chip carriers  112 , if desired. From the component assembly device  106 , and in the illustrative embodiment, the bonding/deposition device  108  simply passes the boat carriers  110  through to the queue device  104 .  
         [0047]     After the third pass to the queue device  104 , the boat carriers  110  may be passed to the second subsystem  102 . In the second subsystem  102 , the boat carriers  110  first go to a first vacuum furnace  118 . While in the first vacuum furnace  118 , the chip carriers  112 , die  114 , and seal rings  116  are baked out by subjecting them to a vacuum of a first level at a first elevated temperature for a first time period. Like above, hydrogen levels may be monitored to determine if/when the devices are sufficiently baked out.  
         [0048]     From the first vacuum furnace  118 , the boat carriers  110  may pass to a robotic device  120  that loads each of second and third vacuum furnaces  122 ,  124  to capacity. Once so loaded, the vacuum furnaces  122  may be used to place and seal lids  126 ,  128  on the chip carriers  112 , after any getters on the chip carriers  112 , die  114 , or lids  126 ,  128  are activated. The finished product  130  may include a batch of hermetically sealed, packaged die  114 .  
         [0049]     As with  FIG. 1 ,  FIG. 3  also illustrates a controller  132 , which may optionally be included to control the functioning of the various system elements as described with reference to  FIG. 3 . For example, a controller  132  may be provided with a readable memory having an instruction set executable for performing the methods described with reference to  FIG. 3  and/or the method described with respect to  FIG. 4 . The controller  132  may take any suitable form, including a microcontroller, computer, PDA, or the like. While the controller  132  is shown as having connections to several system elements, a wireless communication system/protocol may also be used for delivering commands from the controller to the system elements.  
         [0050]     Turning now to  FIG. 4 , a method is illustrated for using three vacuum furnaces as described above with respect to  FIG. 3 . First, a number of empty chip boats may be queued, as shown at  150 . Next, chip carriers may be placed in the boats, as shown at  152 , and ball bumps may be placed on the chip carriers, as shown at  154 . In some embodiments, the steps  152 ,  154  may take place in a first subsystem, as indicated by the dashed line box.  
         [0051]     Next, the boats with carriers may be queued, as shown at  156 . The boats with carriers can then be returned to the first subsystem (as indicated by the box), where die are placed and secured in the chip carriers as shown at  158 , and wire bonds are optionally provided, as shown at  160 . In some cases, the die may be secured to the chip carriers via solder, adhesive, thermo-compression bonding, or in any other suitable way, as desired. The boats with carriers and die may then be queued again, as shown at  162 , and returned to the first subsystem (as indicated by the box) for placement of seal rings on the chip carriers, as shown at  164 .  
         [0052]     The prepared boats may again be queued, as shown at  166 . The next steps may occur in a second subsystem, as indicated by the box enclosing steps  168 - 176 . While in the second subsystem, a bake out of the chip boats, chip carriers, and die may be performed, as indicated at  168 . The boats may then be placed in the second and/or third vacuum furnaces, depending on which of the second and/or third vacuum furnaces is available at the time, as indicated at  170 . While in either the second or third vacuum furnace, lids are placed, as indicated at  172 , on the chip carriers. Getters, if provided, may then be activated, as indicated at  174 . Alternatively, the getters may be activated prior to loading the boats into the vacuum furnaces. A final bale out and seal may be performed, as indicated at  176 , in the same vacuum furnace as was used for lid placement  172  and/or optional getter activation  174 .  
         [0053]     Those skilled in the art will recognize that the present invention may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departures in form and detail may be made without departing from the scope and spirit of the present invention as described in the appended claims.