Patent Publication Number: US-7906359-B2

Title: Method of forming a surface micromachined MEMS device

Description:
PRIORITY 
     This patent application is a continuation-in-part and thus, claims priority from U.S. patent application Ser. No. 10/308,688, filed Dec. 3, 2002, entitled, “MEMS DEVICE WITH ALTERNATIVE ELECTRICAL CONNECTIONS,” and naming Bruce Wachtmann, David Collins, and Thomas Chen as inventors, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates microelectromechanical systems and, more particularly, the invention relates to electrical connections on microelectromechanical systems. 
     BACKGROUND OF THE INVENTION 
     Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration). 
     Many types of MEMS sensors, such as those discussed above, are manufactured by means of conventional surface micromachining (“SMM”) techniques. As known by those skilled in the art, surface micromachining techniques build material layers on top of a substrate using additive and subtractive processes. Unlike processes that use silicon-on-insulator (“SOI”) wafers, these processes do not require wafer to wafer bonding. 
     U.S. Pat. No. 5,326,726 (“Tsang”) and U.S. Pat. No. 5,828,115 (“Core”) discuss two conventional SMM processes. The sensors shown by both patents have a conductive layer that connects structure to circuitry. For example,  FIGS. 1 and 2  of the Core patent show a sensor having a ground plane  50  that electrically connects a mass  12  to a contact  68  via a diffused runner  66  embedded in its substrate  36 . As noted at lines  62 - 64  of column 2 of the patent, this arrangement creates a “large” parasitic capacitance between the ground plane  50  and the substrate  36 . In fact, this parasitic capacitance can corrupt electronic signals transmitted through the ground plane  50 . 
     A semiconductor junction within the Core sensor significantly contributes to this parasitic capacitance. More specifically, the point where the diffused runner  66  meets the substrate  36  appears to create a semiconductor junction similar to that in a diode. Under certain conditions, that junction undesirably can begin conducting. For example, if the sensor is near a heat source (e.g., under the hood of an automobile), the increased temperatures can cause the junction to leak, thus causing unintended conduction. As a further example, relatively high voltages applied to the sensor also can cause the junction to begin conducting. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, junctions between a sensor substrate and a conductive path are eliminated, thus eliminating one source for parasitic capacitances. To those ends, a method of forming a surface micromachined MEMS device applies an insulator to a substrate, and then deposits a conductive path on the insulator. The conductive path is capable of transmitting an electronic signal between two points on the MEMS device. The insulator electrically isolates the conductive path from the substrate. The MEMS device illustratively is free of semiconductor junctions formed by the substrate and the conductive path (i.e., the substrate and conductive path do not together form a semiconductor junction). 
     The method also may form circuitry and structure, which serve the function of the two above noted points, and connect the conductive path between the circuitry and the structure. In some embodiments, the structure is electrically isolated from the substrate. Moreover, among other things, the substrate may be produced from substantially intrinsic silicon, while the conductive path may be formed from polysilicon. 
     The method may apply multiple layers of conductive paths. To that end, the method may apply an additional insulator above the conductive path, and then deposit an additional conductive path to the additional insulator. The conductive path and additional conductive path illustratively are in different planes of the MEMS device. As noted above, embodiments of the MEMS device are free of semiconductor junctions between the substrate and the conductive path. The substrate also may be free of embedded electrodes. 
     In accordance with another aspect of the invention, a MEMS device has a substrate, movable structure and circuitry supported by the substrate, and a conductive path between the movable structure and the circuitry. The conductive path is electrically isolated from the substrate and formed within the MEMS device by surface micromachined processes. The MEMS device is free of semiconductor junctions between the substrate and the conductive path. 
     In accordance with another aspect of the invention, a method of forming a sensor forms an insulator on a substantially intrinsic substrate and forms a conductive path on the insulator. The insulator is deposited (or grown) and the conductive layer is formed by surface micromachining processes. The insulator electrically isolates the conductive path from the substrate. The method also forms circuitry and structure, and connects the conductive path between the circuitry and the structure. The conductive path is capable of transmitting an electronic signal between the circuitry and the structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein: 
         FIG. 1  schematically shows a surface micromachined MEMS sensor produced in accordance with illustrative embodiments of the invention. 
         FIG. 2  shows a SMM process of forming the sensor shown in  FIG. 1  in accordance with illustrative embodiments of the invention. 
         FIG. 3  schematically shows a partially formed sensor illustrating step  200  of the process of  FIG. 2 . 
         FIG. 4  schematically shows a partially formed sensor illustrating steps  200  and  202  of the process of  FIG. 2 . 
         FIG. 5  schematically shows a partially formed sensor illustrating steps  204  and  206  of the process of  FIG. 2 . 
         FIG. 6  schematically shows a partially formed sensor illustrating step  208  of the process of  FIG. 2 . 
         FIG. 7  schematically shows a partially formed sensor illustrating step  210  of the process of  FIG. 2 . 
         FIG. 8  schematically illustrates step  212  of the process of  FIG. 2 . 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In illustrative embodiments, a MEMS sensor produced by surface micromachining (“SMM”) processes has no semiconductor junctions between its substrate and conductive paths. Accordingly, such embodiments eliminate parasitic capacitances produced by these junctions. Other embodiments space the conducive paths from the substrate in a manner that significantly reduces their potential parasitic capacitance. Details of these and other embodiments are discussed below. 
       FIG. 1  schematically shows a SMM MEMS sensor  10  produced in accordance with illustrative embodiments of the invention. The sensor  10  includes a substrate  12  and a plurality of additional components supported by the substrate  12 . Among other things, such additional components include a movable mass  14  (shown schematically as a single mass), a plurality of flexures  16  coupling the mass  14  to the substrate  12 , and circuitry  18  to detect and/or control movement of the mass  14 . A plurality of metal contacts  20  on the top surface electrically connect the movable mass  14  with the circuitry  18  and/or external devices (e.g., computers). 
     The sensor  10  may be any conventionally known MEMS inertial sensor. For example, the sensor  10  may be a gyroscope or an accelerometer. Exemplary MEMS gyroscopes are discussed in greater detail in U.S. Pat. No. 6,505,511, which is assigned to Analog Devices, Inc. of Norwood, Mass. Exemplary MEMS accelerometers are discussed in greater detail in U.S. Pat. No. 5,939,633, which also is assigned to Analog Devices, Inc. of Norwood, Mass. The disclosures of U.S. Pat. Nos. 5,939,633 and 6,505,511 are incorporated herein, in their entireties, by reference. 
     In a manner similar to those shown in these two patents, the mass  14  shown schematically in  FIG. 1  may have one, two or more individual masses that cooperate to provide movement data. Instead of a sensor, however, alternative embodiments may implement other types of devices formed by SMM techniques. Accordingly, discussion of sensors and specific types of sensors is not intended to limit the scope of all embodiments. 
     As noted above, SMM techniques build material layers on a substrate using additive and subtractive processes.  FIG. 2  shows a SMM process of forming the sensor  10  shown in  FIG. 1 .  FIGS. 3-8  show the progression of the SMM process discussed in  FIG. 2 . The process begins at step  200 , in which a base oxide layer  22  is grown on the substrate  12 . To that end, conventional local oxidation of silicon processes (known in the art as “LOCOS”) may be used to grow a base oxide layer  22  on a substantially intrinsic (i.e., undoped) silicon substrate (see  FIG. 3 ). 
     For example, the base oxide layer  22  may be formed from a thermal silicon dioxide having a thickness of between about 0.15 to 1.5 microns. The thickness of the base oxide layer  22  is selected as a function of the material properties of the oxide (e.g., its dielectric constant). More specifically, that thickness is selected (based on the oxide properties) to mitigate parasitic capacitances formed between the substrate  12  and other components above the substrate  12 . These parameters thus are used to ensure that such parasitic capacitances do not significantly impact sensor performance. Further details are discussed below. 
     After it is formed, the base oxide layer  22  may be thinned to conform with the requirements of the sensor  10  (see  FIG. 4 ). For example, the base oxide layer  22  may be thinned at some points to a thickness of about 400 to 500 nanometers. Alternatively, the base oxide layer  22  is not thinned. 
     The process then continues to step  202 , in which base conductors  24  are added to the top surface of the base oxide layer  22  (see  FIG. 4 ). In illustrative embodiments, the base conductors  24  are formed from polysilicon to provide any function required of a conductor or element of a semiconductor device (in the sensor  10 ). Those skilled in the art should appreciate that the base oxide layer  22  both electrically isolates and spaces the base conductors  24  from the substrate  12 . Consequently, there is no semiconductor junction formed by (or between) the substrate  12  and base conductors  24 , thus eliminating one significant source of parasitic capacitance and leakage in many prior art SMM MEMS sensors. Moreover, as noted above, the spacing can be calculated to substantially mitigate parasitic capacitance produced merely by positioning the substrate  12  near the conductors  24 . Accordingly, the base oxide layer  22  mitigates two sources of parasitic capacitance in the sensor  10 ; namely, parasitic capacitances produced by 1) semiconductor junctions and 2) closely positioned substrates and conductors. 
     The base conductors  24  may be patterned in a conventional manner as circuitry  18  (e.g., sense or actuation circuitry), interconnects (also referred to as ground planes or conductive paths), or other conventional semiconductor components. See, for example, U.S. Pat. No. 5,326,726 (hereinafter “Tsang”) and U.S. Pat. No. 5,828,115 (hereinafter “Core”) for exemplary patterning processes, the disclosures of which are incorporated herein, in their entireties, by reference. Both patents discuss conventional methods of performing various steps of  FIG. 2 . For example, the Tsang patent discusses a conventional chemical vapor deposition technique that may be used for depositing polysilicon base conductors  24  on the base oxide layer  22 . 
     The process then adds an additional oxide layer  26  over the base conductors  24  (step  204 ,  FIG. 5 ). In illustrative embodiments, the additional oxide layer  26  may be formed from borophosphosilicate glass (“BPSG”) having a top surface about 550 nanometers from the top surface of the base oxide layer  22 . Those in the art may consider the BPSG to “planarize” the base conductors  24 . Alternative embodiments use a different insulation material, such as that used for the base oxide layer  22 . 
     After the additional oxide layer  26  is formed, the process continues to step  206  by adding additional conductors  28  on the additional oxide layer  26 . In a manner similar to step  202 , the additional conductors  28  may include polysilicon conventionally patterned as circuitry, interconnects, or other components as discussed above. In some embodiments, more layers of oxide (i.e., insulator) and conductors are added above the additional conductors  28  and additional oxide layer  26 . Accordingly, although the sensor  10  discussed with reference to  FIGS. 3-8  has only two planes of conductors (i.e., conductors  24  and  28 ), some sensors implementing various embodiments of the invention can have more than two planes of conductors. 
     As shown in  FIGS. 6 and 7 , after the additional conductors  28  are formed, conventional processes add a spacing oxide  30  over the top of both the additional conductors  28  and additional oxide layer  26  (step  208 ), and then form the necessary structure  32  (step  210 ). To those ends, conventional processes etch trenches  31  through the spacing oxide  30 . After the trenches  31  are formed, conventional processes form the relevant structure  32  (see above noted incorporated patents for exemplary conventional processes). 
     The structure  32  may be any conventional structure, such as the noted movable mass  14 , which is suspended above the substrate  12  by the flexure system  16 . For example, if the sensor  10  is an accelerometer, the structure  32  may be similar to that disclosed in above noted U.S. Pat. No. 5,939,633. If the sensor  10  is a gyroscope, then the structure  32  may be similar to that disclosed in above noted U.S. Pat. No. 6,505,511. 
     The structure  32  (e.g., the staple) shown in  FIG. 7  electrically connects one base conductor  24  with an additional conductor  28  on a different plane of the sensor  10 . Alternatively, the additional conductor  28  could contact the base conductor  24  directly by forming a trench in the additional oxide layer  26  and forming a via. The base conductor  24  may include circuitry  18 , while the additional conductor  28  (on the higher sensor plane) may act as a ground plane (i.e., an interconnect). Alternatively, the base conductor  24  may act as a ground plane, while the additional conductor  28  may be utilized for circuitry. Of course, both conductors  24  and  28  may be interconnects, or they both may be utilized for circuitry. Those skilled in the art may use the different conductors for yet other functions. 
     As an example, when a conductor acts as an interconnect or ground plane, it can transmit electronic signals between two points in the sensor  10 . Such a signal may have information relating to the movement of the sensor  10 . Specifically, such signals may have information relating to actuating or detecting mass movement. 
     After the structure  32  is formed, the process continues to step  212 , in which the structure  32  is released as shown in  FIG. 8 . More specifically, a conventional aqueous HF release process may be used to remove the spacer oxide, which causes the structure  32  to be suspended above the substrate  12 . Illustrative embodiments use a buffered hydrofluoric acid to execute this step.  FIG. 8  thus schematically shows the sensor as having a structure region formed by the process of  FIG. 2 , and a circuitry side formed by other processes. 
     Additional process steps may be performed to supplement the process discussed above. For example, nitride layers may be added to the top surfaces of the insulator layers, conductors, or both before additional insulator layers are added. As known by those in the art, the nitride layers are substantially impermeable to the aqueous HF release process, thus protecting the underlying layers. Moreover, among other things, the metal contacts  20  ( FIGS. 1 and 8 ) may be added, thus enabling the sensor  10  to electrically connect with external devices. Other conventional processes may be performed to produce the final sensor  10 . 
     Accordingly, as noted above, illustrative embodiments of the invention do not form a semiconductor junction by or between the substrate  12  and any other component on the noted structure region. Parasitic capacitances caused by such junctions thus are eliminated, consequently eliminating the potential for such capacitance to distort signals within the sensor  10 . Eliminating the junctions also 1) permits relatively high voltages to drive the movable mass  14  and 2) eliminates some noise caused by prior art sensors. The sensor  10  also can be used in high temperature applications (e.g., near the motor in an automobile) without the concern that a junction (formed at least in part by the substrate  12 ) undesirably will begin to conduct. Moreover, as noted above, parasitic capacitances potentially formed by the proximity of the substrate  12  and conductors  24  and  28  (and between conductors  24  and  28  on different planes) also are mitigated by appropriately selecting the thickness and type of the various insulators. 
     Of course, each of these advantages is subject to other design limitations and components of the sensor  10 . It should be noted, however, that these advantages relate to the specific improvement provided by illustrative embodiments with regard to 1) the noted parasitic capacitance issues caused by the junction and 2) the relatively close spacing of conductors and substrates of prior art sensors. 
     Some embodiments of the sensor  10  may have semiconductor junctions (such as those used in forming conventional circuit devices) while maintaining the noted spaced and/or electrically isolated configuration between the substrate  12  and noted conductors  24  and  28 . For example, some of the junctions may be formed by contact between other components in the circuitry  18  and the substrate  12 . Other embodiments, however, may eliminate semiconductor junctions, while still having the parasitic capacitance produced by not sufficiently spacing the substrate  12  from the conductors  24 . 
     Although the discussion above discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention.