Patent Publication Number: US-2006006484-A1

Title: Functional material for micro-mechanical systems

Description:
PRIORITY INFORMATION  
      This application claims priority to U.S. provisional patent application Ser. No. 60/585,647 filed Jul. 6, 2004, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      The invention relates to the field of micro-electro-mechanical systems, and in particular using titanium nitride (TiN) as a key active electromechanical component in micro-electro-mechanical (MEMS) devices.  
      In the prior art, there have been occasions in which TiN has been incorporated into MEMS devices. One such structure uses a silicon layer that is coated with highly reflective materials (aluminum, gold etc.) on both sides and wafer bonded to another wafer. The coated silicon is a tiltable mirror used for free-space optical switching. Due to interdiffusion of silicon and materials used for the highly reflective layer, a TiN film is used in between the aluminum and silicon. The use of TiN in this case, therefore, is solely as a diffusion barrier.  
      Another such structure uses a deformable electromechanical structure (beams) that is used to switch an RF signal channel on and off by moving from its steady-state position to its deformed state (snap down/pull-in) by the application of a voltage on the bottom actuation electrode. The deformable structure is made of silicon nitride (SiN). As a refinement to this device, a static top actuation electrode, made of materials which could include TiN (e.g. tungsten, tantalum, tantalum nitride etc.), is used to assist in releasing the deformed structure by pulling up on the beams. Hence, the use of TiN in this MEMS device is as a non-moving, fixed electrode.  
      However, none of the conventional structures involve the use of TiN as an active MEMS element. TiN&#39;s unique combination of mechanical, electrical and chemical properties make it a preferable material for electromechanical devices in MEMS structures.  
     SUMMARY OF THE INVENTION  
      According to one aspect of the invention, there is provided a micro-electro-mechanical (MEMS) device. The MEMS device includes a first material structure. A second material structure includes TiN. The second material structure is moveable relative to the first material structure.  
      According to another aspect of the invention, there is a provided a method of forming a MEMS structure. The method includes forming a first material structure. Also, the method includes forming a second material structure includes TiN. The second material structure is moveable relative to the first material structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a table demonstrating the stiffness to density ratio of TiN relative to other materials used to form MEMS devices;  
       FIG. 2  is a schematic diagram illustrating a MEMS parallel plate actuator having a moveable electrode formed using TiN for displacement control and switching applications;  
       FIG. 3  is a schematic diagram of another embodiment of a MEMS parallel plate actuator formed using TiN for displacement control and switching applications;  
       FIGS. 4A-4D  are schematic block diagrams illustrating one example of a fabrication process for the structures shown in  FIGS. 3-4 ;  
       FIGS. 5A-5B  are schematic diagrams of a MEMS piezoelectric actuator using TiN for displacement control and switching applications;  
       FIG. 6  is a schematic diagram of a MEMS thermal actuator structure formed from TiN for displacement control and switching applications;  
       FIG. 7  is a schematic diagram of another implementation of a MEMS thermal actuator structure formed from TiN for displacement control and switching applications;  
       FIGS. 8A-8C  are schematic diagrams of another implementation of a MEMS thermal actuator structure formed from TiN for displacement control and switching applications;  
       FIGS. 9A-9B  are schematic diagrams of a MEMS magnetic displacement/switch actuator formed using TiN for displacement control or switching applications;  
       FIG. 10  is a schematic diagram of a MEMS parallel plate electrostatic resonator formed from TiN for electrical filtering and clocking applications;  
       FIGS. 11A-11B  are schematic diagrams of a MEMS piezoelectric resonator fabricated using TiN for electrical filtering and clocking applications;  
       FIG. 12  is a schematic diagram of a MEMS accelerometer formed with TiN that uses electrostatic comb drives for sensing and excitation;  
       FIG. 13  is a schematic diagram of a MEMS deformable membrane structure composed of TiN for acoustic sensing (microphone) applications;  
       FIG. 14  is a schematic diagram of a MEMS parallel plate structure used for capacitive energy harvesting formed using the TiN;  
       FIG. 15  is a schematic diagram of a MEMS piezoelectric structure using TiN for energy harvesting;  
       FIG. 16  is a schematic diagram of a MEMS gear train fabricated using TiN;  
       FIGS. 17A-17C  are schematic diagrams of a MEMS tunable optical grating formed from TiN that uses parallel plate electrostatic actuators modifying the grating;  
       FIG. 18  is a schematic diagram of a MEMS tunable optical grating formed from TiN that uses electrostatic comb drive actuators for analog tuning of the grating period;  
       FIG. 19  is a schematic diagram of a MEMS tiltable micromirror formed from TiN that uses torsional electrostatic actuators to tilt the mirror back and forth;  
       FIGS. 20A-20B  are schematic diagrams of a MEMS tunable optical grating formed using TiN that uses a piezoelectric actuator for tuning the period of the grating;  
       FIG. 21  is a schematic diagram of a MEMS tunable optical grating formed using TiN that uses thermal actuators for tuning the period of the grating;  
       FIGS. 22A-22B  are schematic diagrams of a MEMS bistable switch structure formed from TiN;  
       FIG. 23  is a schematic diagram of a MEMS tiltable micromirror formed from TiN that uses a piezoelectric actuator to controllable tilt the mirror to different angles; and  
       FIGS. 24A-24B  are schematic diagrams of a MEMS microfluidic valve formed from TiN that uses a parallel plate actuator to control the valve. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      TiN is an unusual material in exhibiting metallic-like electrical properties while possessing ceramic-like mechanical properties. It is currently commonly used in integrated circuit fabrication as a diffusion barrier between the metallization layer and the active (semiconductor) devices. Its other common use is as a coating for machine tools to reduce wear of the machine tools. Because of its utility as a diffusion barrier in the IC industry, TiN is readily available in microfabrication facilities and much is known about deposition and etching techniques for TiN.  
      TiN has a number of properties that make it a very good material for MEMS devices. It is electrically conductive. It has a high modulus and melting point, but moderate density. Compared to other MEMS materials such as poly-silicon, silicon nitride, or the like, TiN has the highest stiffness to density ratio, as shown in  FIG. 1 . Due to its low surface adhesion energy, TiN naturally displays anti-stiction qualities. TiN also displays high strength, high chemical stability, high wear resistance, and high surface hardness. In addition, TiN acts as a diffusion barrier. These characteristics are all highly desirable in MEMS devices.  
      TiN&#39;s superior stiffness to density ratio compared to other standard MEMS materials, such as silicon, poly-silicon, aluminum, silicon nitride etc. is particularly attractive for use in MEMS. The high stiffness to density ratio translates directly into higher resonant frequencies and, therefore, faster response (i.e. switching or actuation) times than can be achieved with other materials. In addition, the high stiffness of TiN allows for a reduction in the geometrical dimensions of a given device while still maintaining the same compliance. This reduction in feature size leads to both potentially new functionality as well as lower production costs because more devices can be fabricated per wafer.  
      TiN also has a very low susceptibility to creep in free-standing structures and is thus very desirable for use in MEMS devices. Creep causes performance of devices to drift with time and usage. This reduces the useful lifetime of products, if the products are acceptable at all. Creep does occur with MEMS fabricated out of current MEMS materials, all the more so, as the operation temperature is increased. Creep would be largely eliminated in MEMS devices through the use of TiN.  
      The non-stick nature of TiN is beneficial in electromechanical structures that are designed to pull-in and subsequently revert back to their original position as in micro-relays. It would also be beneficial in fabrication of MEMS devices, potentially allowing the removal of sacrificial layers using techniques (such as wet etching) that would otherwise lead to stiction thus simplifying the fabrication process. Having a non-stick surface prevents stiction when structures come into contact with other materials and surfaces. Thus, using TiN in such cases may eliminate the need for additional steps commonly taken to prevent stiction such as complex or uncommon release etch processes, the addition of anti-stiction bumps, or the deposition of anti-stiction films and monolayers.  
      In structures that come into contact with other surfaces, it is desirable to use a material with a higher hardness and abrasion resistance so that such structures do not become damaged from periodic contact. TiN has very high hardness and abrasion resistance. It is commonly used as a coating on machine tools to prevent wear. These qualities should translate into longer lifetimes and better reliability for MEMS devices where there is periodic contact between surfaces.  
      The ultimate (fracture) strength of TiN is very high. This property is important for a number of reasons. First, because of this property TiN can be used in applications that require materials that can handle high stress. Second, the high strength of TiN allows structures to be able to withstand unusual stress situations that do not occur in normal operation (i.e. drop tests). Finally, the high strength provides device robustness during the fabrication process, allowing more stressful process steps to be used with success.  
      The chemical stability of TiN allows devices fabricated out of TiN to be used in environments that may not be feasible for other MEMS materials. This is important for applications such as pressure sensors, micro-valves, and micro-motors where the MEMS material comes into direct contact with a variety of chemicals. This property also minimizes the effects of aging for virtually all applications.  
      The melting point of TiN is very high. This allows the use of TiN at much higher temperatures than many other MEMS materials. The high melting temperature allows TiN to maintain many of its important characteristics, such as high stiffness and strength as well as creep resistance, at elevated temperatures. This is a good property to have in general but this is specifically important for micro-motors, pressure sensors, and micro-reactors.  
      In addition to these desirable mechanical and chemical properties, TiN is electrically conductive. This allows structures fabricated out of TiN to be used for various actuation methods that require electrical functionality, such as electrostatic, thermal, piezoelectric, and magnetic actuation. To achieve this combination of mechanical performance and electrical behavior, MEMS designers in the past have used structures composed of two or more materials. For instance, silicon nitride and aluminum bilayers have been used to provide high stiffness and conductive structures. One drawback to this approach is that stress induced bending of the bilayers commonly arises due to the thermal-mechanical mismatch between the different materials. A structure with comparable or better capabilities would be much more easily obtained by using only TiN. The electrical conductivity of TiN is a key benefit of using TiN.  
      Fabrication techniques for TiN are similar to other materials used in microfabrication. It can be deposited in a variety of ways including both high and low temperature processes. It can be annealed to relieve residual stress. For sputter deposited TiN films, annealing can be achieved at temperatures as low as 300° C. For other deposition techniques, annealing does not begin until approximately 1300° C. Etching can be accomplished with both wet etching techniques and reactive ion etching (RIE) techniques. These techniques are commonly used in the integrated circuit and machine tool coating industries.  
      TiN would be useful in a wide range of MEMS devices including, but not limited to, electrostatic switches (optical, electrical (DC through RF), etc.), piezoelectric switches (optical, electrical (DC through RF), etc.), thermally actuated switches (optical, electrical (DC through RF), etc.), magnetically actuated switches (optical, electrical (DC through RF), etc.), electrostatic resonators, piezoelectric resonators, accelerometers, microphones, energy harvesting devices (mechanical (i.e. vibrational) energy to electrical energy), energy conversion devices (motors—chemical to mechanical energy), gear trains, electrostatically actuated optical gratings, piezoelectrically actuated optical gratings, thermally actuated optical gratings, bistable mechanisms, electrostatically actuated micromirrors, piezoelectrically actuated micromirrors, and valve structures for microfluidic systems.  
      Two possible implementations  20 ,  40  of a parallel plate electrostatic actuator formed from TiN for displacement control and switching applications are depicted in  FIGS. 2 and 3 . TiN can form both the fixed  26 ,  46  and moving electrodes  22 ,  42  for these actuators  20 ,  40 . Also, the actuators  20 ,  40  include voltage sources  32 ,  50  to provide the necessary voltage for proper operation. The fixed electrodes  26 ,  46  are fabricated on top of the substrate  24 ,  44  (usually silicon with a thin dielectric film). The moving electrodes  22 ,  42  are suspended above the fixed electrodes  26 ,  46  by anchors  28 ,  48  that provide electrical isolation between the moving and fixed electrodes. Electrical leads, optical waveguides, or other structures can be added to this structure to provide switch functionality in various physical domains.  
      A possible fabrication approach for a parallel plate actuator formed from TiN is shown in  FIGS. 4A-4D . Standard CMOS (microelectronic) processing methods can be used to fabricate MEMS devices having TiN. For example, the fabrication of the structures  20 ,  40  would involve depositing an insulating layer  60 , e.g. silicon dioxide (SiO 2 ) that was formed on a substrate  62 , as shown in  FIG. 4A . Moreover, a poly-silicon layer  64  is deposited on the insulating layer  60 . A second insulating layer  66  is deposited on the poly-silicon layer  64 . Afterwards, a sacrificial layer  68  is deposited and encompasses the layers  66 ,  68  and a portion of the insulating layer  60 , as shown in  FIG. 4B . A TiN layer  70  is deposited on the sacrificial layer  68 . In depositing the TiN layer  70 , electron-beam evaporation, sputtering and chemical vapor deposition can be used, as shown in  FIG. 4C . The TiN layer  70  is released by removing the sacrificial layer  68 , as shown in  FIG. 4D .  
      One possible implementation of a piezoelectric actuator  72  is shown in  FIG. 5A . This actuator  72  could be used in displacement control or switching applications. In this actuator  72 , the electrodes  74  are formed with TiN, and are on either side of the piezoelectric material  76 . In addition to acting as electrodes, the TiN  74  also acts as an adhesion layer and diffusion barrier (depending on the type of piezoelectric material). The TiN also provides the necessary mechanical structure for the actuator  72 .  FIG. 5B  shows the actuator  72  being displaced after the application of a voltage from a voltage source  78 . Electrical leads, optical waveguides, or other structures can be added to this switching structure  72  to give the switch functionality in various physical domains.  
      Three possible implementations of a thermal actuator are shown in  FIGS. 6-8 . These actuators can be used for displacement control or switching applications. For the implementation  80  shown in  FIG. 6 , the entire thermal actuator is formed with TiN. The actuator  80  includes two leg beams  82 ,  84  that create an input and return path for a current source  85 . Note the leg beams  82 ,  84  have different cross-sectional areas, and therefore different electrical resistance. The smaller beam  84  therefore heats up and expands more than the thicker beam  82 , which creates motion.  
      The thermal actuator implementation  86  shown in  FIG. 7  uses a bi-material structure. One of the materials used in the bi-material actuator is TiN  88 . The second material  90  is selected to be a material with a much different coefficient of thermal expansion. An electrical current source  98  flows an electric current through the TiN  88  which provides heat for the actuator due to the electrical resistance in the TiN. The thermal actuator  86  is fabricated on and anchored to a substrate  92  (usually silicon).  
      The thermal actuator implementation  100  shown in  FIG. 8A  also uses a bi-material structure. TiN can be used for one of the materials  108  in the structure  100  ( FIG. 8B ). The second material  102  is a material with a coefficient of thermal expansion that is different than TiN. The resistive heater  104  on top of the structure can also be made of TiN. When an electrical current source  106  flows a current through the resistive heater  104 , the temperature of the bi-material structure  100  is raised which leads to a deflection of the structure  100  as in  FIG. 8C . The structure is fabricated on and anchored to a substrate  110  (usually silicon). Electrical leads, optical waveguides, or other structures can be added to these thermal actuators to provide switch functionality in various physical domains.  
      One possible implementation of a magnetic actuator  118  is shown in  FIGS. 9A-9B . This structure  118  can be used for displacement control and switch applications. The actuator  118  includes a bridge  119  that is fabricated on a substrate  122  that includes an insulating layer  123 . Also, a magnetic field  121  is applied that is directed out of the plane where actuator  118  is positioned.  FIG. 9B  shows a current source  120  that supplies current to the bridge  119 , which causes displacement as shown.  
      For the actuator  118 , the TiN provides both the structural element  119  as well as the electrical pathway for the current that causes the actuation in the magnetic field via Lorentz forces. Electrical leads, optical waveguides, or other structures can be added to this structure to provide switch functionality in various physical domains.  
       FIG. 10  shows one possible implementation of an electrostatic resonator  124  using the invention. Note the resonator structure is similar to the structure described in  FIG. 3 . The fixed electrode  127  and the moving (resonating) electrode  126  can both be fabricated out of TiN. The fixed electrode  127  is fabricated directly on the substrate  128 . The moving electrode  126  is suspended above the substrate by an anchoring material  129 . The excitation of the resonator is provided by an electrical signal source  125 . This device would be used in electrical filtering and clocking applications.  
       FIG. 11A  shows a piezoelectric resonator  130  that is formed using TiN. In this implementation, the resonator  130  includes an electromechanical structure  134  that includes electrodes  132 , which are formed with TiN. The electrodes  132  surround the piezoelectric material  131 . The electrodes  132  provide for electrical excitation and sensing of the resonator  130  as well as act as structural elements and a diffusion barrier for the piezoelectric material  131 .  FIG. 11B  shows the structure  134  under mechanical motion when a voltage is applied by a voltage source  136 . Uses for the structure  130  include electrical filtering and clocking applications.  
       FIG. 12  shows an accelerometer  140  formed in accordance with the invention. In this implementation, the TiN is used to create the springs or flexures  142 , the mass  144 , and the comb electrodes  146 ,  148  for sensing and excitation. When the mass  144  is subjected to an acceleration, the springs  142  allow the mass  144  to displace. The displacement is sensed by one of the two sets of comb electrodes  146 ,  148 . The second set of electrodes  148  is used to maintain the position of the mass and to provide a self test of the functionality of the device.  
       FIG. 13  shows a microphone (vibration sensor)  150  formed in accordance with the invention. In this implementation, TiN is used to create the membrane  151  that is excited by the acoustic vibrations. The TiN membrane  151  is suspended over a second, fixed electrode  152 , which also could be made of TiN, for sensing of the vibrations in the membrane. The fixed electrode  152  is fabricated on top of a substrate  154  while the membrane  151  is suspended by non-conducting anchors  153 .  
      Two possible implementations of energy harvesting devices are shown in  FIGS. 14 and 15 A- 15 B. In the implementation  155  shown in  FIG. 14  the energy is harvested by creating a capacitor out of two electrodes  156  and  157 . On electrode  157  is fixed to the substrate  158  while the second electrode  156  is suspended above the fixed electrode by anchors  159 . The suspended electrode  156  vibrates as a result of external disturbances  161 . Electrical energy is created by applying a voltage  160  to the two electrodes  156 ,  157  and extracting the current that flows due to the mechanical vibrations of the suspended electrode  156 . Either or both of the electrodes  156 ,  157  could be beneficially composed of TiN, providing the necessary mechanical and electrical capabilities.  
      In the second implementation shown in  FIGS. 15A-15B , the energy harvester  164  is a piezoelectric cantilever  170  that includes electrodes  166  that are formed with TiN and piezoelectric material  168  in between the electrodes  166 . The cantilever  170  vibrates as a result of external disturbances  171  which produces a voltage  172  and current between the electrodes  166  as shown in  FIG. 15B . From the current, energy is extracted from the device  164 .  
      One implementation of a gear train  173  is shown in  FIG. 16 . In this implementation, both the gears  174  and the axels  176  around which the gears  174  turn could be fabricated out of TiN. The anti-stiction and wear resistance of TiN would prove very important in this application. This gear train  173  is driven by a linear motion actuator (such as an electrostatic comb drive actuator) through the links  175 . Both the mechanical links and the electrostatic comb drive actuator can be fabricated out of TiN.  
      Two implementations of electrostatically actuated optical gratings are shown in  FIGS. 17A-17C  and  18 . In the implementation shown in  FIG. 17A , the grating  177  is created by long and narrow suspended electrodes  178  made of TiN. These electrodes  178  are formed over fixed electrodes  180  that could also be TiN. The suspended electrodes  178  are pulled to different heights by applying different voltages to create adaptive optical gratings.  FIG. 17B  shows a cross-section view of the grating  177  along the A-A direction, and  FIG. 17C  show a cross-section view of the grating  177  along the B-B direction.  
      In the second implementation  181  shown in  FIG. 18  is fabricated completely out of TiN. The grating  182  is a structure that moves in plane by being stretched apart by electrostatic comb drives  184  on either side of the grating  182 . This causes the period of the grating  182  to change, allowing the grating  182  to be adaptive. The flexures  188  that connect the beams that form the grating  182  allow large displacements with the relatively small force provided by the comb drive actuators. A thin layer of aluminum or gold could be deposited on the surface of either device  177 ,  181  to enhance the optical reflectivity of the structures.  
       FIG. 19A  shows an electrostatically actuated micromirror  190 . In this instance, the TiN provides the mechanical material for the mirror  192  which is also the moving electrode. There are an additional two fixed electrodes  194 ,  196  which can be fabricated out of TiN and are fixed to the substrate  200 . A voltage  198  is applied between the movable mirror electrode  192  and one or the other of the two fixed electrodes  194 ,  196  to cause the mirror to tip in one direction or the other, as in  FIGS. 19B and 19C . A thin layer of aluminum or some other reflective material is coated on top of the TiN to provide the necessary reflectivity.  
       FIGS. 20A-20B  shows a piezoelectrically actuated optical grating structure  202 . In this case, the piezoelectric actuator  207  is created by piezoelectric material  208  that is sandwiched between TiN layers  204 ,  210  that form the electrodes to cause the piezoelectric material to deform. These actuators  207  stretch a TiN membrane  210  that has the optical grating  206  in it. This grating  206  could also be TiN with a thin layer of aluminum to enhance reflectivity.  FIG. 20B  shows a cross-section view of the structure  202  along the A-A direction.  
       FIG. 21  shows a thermally actuated optical grating structure  212 . The thermal actuation is due to the TiN structure  218  conducting a current, using current sources  214 , that causes the structure  212  to heat up and expand. The thermal expansion is enhanced in the direction perpendicular to the grating by the shape of the TiN structure  218  such that the period of the grating becomes larger. The structure of the grating  216  could also be TiN coated with aluminum or another reflective material. Electrical isolation would need to be provided for between the thermal actuators on either side of the grating  212 .  
       FIG. 22A  shows a bistable mechanism  220 . This mechanism  220  can be almost completely created out of TiN. For the electrostatic actuation shown, a thin layer of a dielectric material would be needed between the movable electrode  222  and the fixed electrodes  230 . The movable electrode is created by a central, solid link  226 , with a double flexure structure  224  connecting it to the anchor points  228 . The double flexures  224  allow the bistable mechanism to move between the two stable positions as shown in  FIG. 22B .  
       FIG. 23  shows a piezoelectrically actuated micromirror structure  232 . The structure  232  includes electrodes  242  created out of TiN that sandwich the piezoelectric layer  234 . Moreover, the structure  232  includes a mirror substructure  236  that is formed with TiN. The mirror substructure  236  is coated with aluminum or some other reflective material to achieve the necessary reflectivity. The entire structure is anchored to a substrate  240 . A voltage source  238  provides voltage to the TiN electrodes  242 . The electrodes  242  in turn create an electric field in the piezoelectric layer  234  which causes a mechanical deformation to occur, leading to a displacement of the micromirror.  
       FIGS. 24A-24B  shows a valve structure  244  for microfluidic systems. The valve  244  is actuated with electrostatic actuation, as shown in  FIG. 24A . The top movable electrode  246  is formed from TiN. The bottom electrodes  248  are fixed to the substrate  252 . The substrate  252  has microfluidic channels  254  to conduct the fluid to and from the valve. When a voltage  250  is applied between the movable electrode  246  and the fixed electrodes  248 , the valve is close, as in  FIG. 24B , and the fluid is no longer able to flow through the valve.  
      Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.