Patent Publication Number: US-11655141-B2

Title: Fabrication techniques and structures for gettering materials in ultrasonic transducer cavities

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 62/738,502, filed Sep. 28, 2018 under No. B1348.70103US00, and entitled “FABRICATION TECHNIQUES AND STRUCTURES FOR GETTERING MATERIALS IN ULTRASONIC TRANSDUCER CAVITIES,” which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to micromachined ultrasound transducers and, more specifically, to fabrication techniques and associated structures for gettering materials present in ultrasound transducer cavities during manufacture. 
     BACKGROUND 
     Ultrasound devices may be used to perform diagnostic imaging and/or treatment, using sound waves with frequencies that are higher with respect to those audible to humans. Ultrasound imaging may be used to see internal soft tissue body structures, for example to find a source of disease or to exclude any pathology. When pulses of ultrasound are transmitted into tissue (e.g., by using a probe), sound waves are reflected off the tissue with different tissues reflecting varying degrees of sound. These reflected sound waves may then be recorded and displayed as an ultrasound image to the operator. The strength (amplitude) of the sound signal and the time it takes for the wave to travel through the body provide information used to produce the ultrasound images. 
     Some ultrasound imaging devices may be fabricated using micromachined ultrasound transducers, including a flexible membrane suspended above a substrate. A cavity is located between part of the substrate and the membrane, such that the combination of the substrate, cavity and membrane form a variable capacitor. When actuated by an appropriate electrical signal, the membrane generates an ultrasound signal by vibration. In response to receiving an ultrasound signal, the membrane is caused to vibrate and, as a result, an output electrical signal can be generated. 
     SUMMARY 
     In one aspect, a method of forming an ultrasound transducer device includes bonding a membrane to a substrate so as to form a sealed cavity therebetween, wherein an exposed surface located within the sealed cavity comprises a getter material, the getter material being electrically isolated from a bottom electrode of the cavity. 
     In another aspect, an ultrasound transducer device includes a membrane bonded to a substrate with a sealed cavity therebetween. An exposed surface located within the sealed cavity includes a getter material, the getter material being electrically isolated from a bottom electrode of the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear. 
         FIG.  1    is a cross-sectional view of a micromachined ultrasound transducer having a cavity getter material, in accordance with an embodiment. 
         FIG.  2    is a top view of the ultrasound transducer of  FIG.  1   , taken along the arrows  2 - 2 . 
         FIG.  3    is a top view of the ultrasound transducer of  FIG.  1   , taken along the arrows  3 - 3 . 
         FIG.  4    is a cross-sectional view of a micromachined ultrasound transducer having a cavity getter material, in accordance with another embodiment. 
         FIG.  5    is a top view of the ultrasound transducer of  FIG.  4   , taken along the arrows  5 - 5 . 
         FIG.  6    is a top view of the ultrasound transducer of  FIG.  4   , taken along the arrows  6 - 6 . 
         FIG.  7    is a cross-sectional view of a micromachined ultrasound transducer having a cavity getter material, in accordance with another embodiment. 
         FIG.  8    is a top view of the ultrasound transducer of  FIG.  7   , taken along the arrows  8 - 8 . 
         FIG.  9    is a top view of the ultrasound transducer of  FIG.  7   , taken along the arrows  9 - 9 . 
         FIGS.  10 - 19    illustrate a process flow sequence for forming the ultrasound transducer embodiments of  FIGS.  1 - 9   , in which: 
         FIG.  10    illustrates the formation of an electrode layer over a CMOS substrate. 
         FIG.  11 - 1    illustrates patterning of the electrode layer of  FIG.  10    according to the embodiment of  FIG.  1   . 
         FIG.  11 - 2    illustrates patterning of the electrode layer of  FIG.  10    according to the embodiment of  FIG.  4   . 
         FIG.  11 - 3    illustrates patterning of the electrode layer of  FIG.  10    according to the embodiment of  FIG.  7   . 
         FIG.  12 - 1    illustrates forming an insulation layer over the structure of  FIG.  11 - 1   . 
         FIG.  12 - 2    illustrates forming an insulation layer over the structure of  FIG.  11 - 2   . 
         FIG.  12 - 3    illustrates forming an insulation layer over the structure of  FIG.  11 - 3   . 
         FIG.  13 - 1    illustrates planarizing the insulation layer of the structure of  FIG.  12 - 1   . 
         FIG.  13 - 2    illustrates planarizing the insulation layer of the structure of  FIG.  12 - 2   . 
         FIG.  13 - 3    illustrates planarizing the insulation layer of the structure of  FIG.  12 - 3   . 
         FIG.  14    illustrates forming an insulation stack over the structure of  FIG.  13 - 1   . 
         FIG.  15    illustrates forming a cavity in the insulation stack. 
         FIG.  16    illustrates removing a portion of the lower insulation layers of the insulation stack to expose adjacent electrode material that serves as a getter material. 
         FIG.  17    illustrates a silicon-on-insulator (SOI) wafer to be bonded to the structure of  FIG.  16   . 
         FIG.  18    illustrates the SOI wafer bonded to the structure of  FIG.  16   . 
         FIG.  19    illustrates removing a portion of the SOI wafer to define a membrane of a micromachined ultrasound transducer. 
         FIG.  20    illustrates a top view of an example ultrasound transducer device formed using any of the process flow sequences described herein. 
     
    
    
     DETAILED DESCRIPTION 
     One type of transducer suitable for use in ultrasound imaging devices is a micromachined ultrasound transducer (MUT), which can be fabricated from, for example, silicon and configured to transmit and receive ultrasound energy. MUTs may include capacitive micromachined ultrasound transducers (CMUTs) and piezoelectric micromachined ultrasound transducers (PMUTs), which can offer several advantages over more conventional ultrasound transducer designs such as, for example, lower manufacturing costs and fabrication times and/or increased frequency bandwidth. With respect to the CMUT device, the basic structure is a parallel plate capacitor with a rigid bottom electrode and a top electrode residing on or within a flexible membrane. Thus, a cavity is defined between the bottom and top electrodes. In some designs (such as those produced by the assignee of the present application for example), the CMUT transducer may be directly integrated on an integrated circuit that controls the operation of the transducer. One way of manufacturing a CMUT ultrasound device is to bond a membrane substrate to an integrated circuit substrate, such as a complementary metal oxide semiconductor (CMOS) substrate. This may be performed at temperatures sufficiently low to prevent damage to the devices of the integrated circuit. 
     However, during bonding of the membrane substrate to the CMOS substrate, there may be a difference in cavity pressures across the die and wafer due to the water vapor and other gaseous byproducts and the propagation of the bond. This in turn may result in undesired variability of certain CMUT-based operating parameters such as for example, collapse voltage, as well as transmit/receive pressure sensitivity. Accordingly, it is desirable to be able to control cavity pressure within such a transducer device during the manufacturing process, as well as over the lifetime of the device. 
     Referring now to  FIG.  1   , there is shown a cross-sectional view of a micromachined ultrasound transducer  100  having a cavity getter material, in accordance with an embodiment. As is shown, the ultrasound transducer  100  includes a lower electrode  102  formed over a substrate  104  (e.g., a CMOS substrate, such as silicon). The CMOS substrate  104  may include, but is not necessarily limited to, CMOS circuits, wiring layers, redistribution layers, and insulation/passivation layers. In an exemplary embodiment, suitable materials for the lower electrode  102  include one or more of titanium (Ti), zirconium (Zr), vanadium (V), cobalt (Co), nickel (Ni), as well as alloys thereof. 
     As further shown in  FIG.  1   , the lower electrode  102  is electrically isolated from adjacent metal regions  106  that are also formed on the substrate  104 . Exposed portions of the adjacent metal regions  106  may thus serve as a getter material during cavity formation. The adjacent metal regions  106  may be formed from a same metal material as the lower electrode  102 , and are electrically isolated from the lower electrode  102  by an insulator material  108  (e.g., silicon oxide). By way of further illustration,  FIG.  2    is a top down view of the lower electrode, adjacent metal regions  106  and insulator material  108 , taken along the arrows  2 - 2  in  FIG.  1   . It should be appreciated that although the exemplary geometric structure of this portion of the ultrasound transducer  100  is generally circular in shape, other configurations are also contemplated such as for example, rectangular, hexagonal, octagonal, and other multi-sides shapes, etc. 
     Referring again to  FIG.  1   , an insulator layer (e.g., one or more individual insulator layers, such as an insulator stack  110 ) is formed over the lower electrode  102  and portions of adjacent metal regions  106 . Portions of the stack  110  provide support for a moveable membrane  112  (e.g., an SOI wafer having a doped silicon device layer with an oxidized surface) bonded to the stack  110 . In the illustrated embodiment, the insulator stack  110  includes a first oxide layer  114  (e.g., chemical vapor deposition (CVD) silicon oxide), a second oxide layer  116  (e.g., atomic layer deposition (ALD) aluminum oxide) and a third oxide layer  118  (e.g., sputter deposited silicon oxide). By suitable lithographic patterning and etching of the third oxide layer  118 , a cavity  120  may be defined for the ultrasound transducer  100 . Further, in embodiments where the second oxide layer  116  is chosen from a material having an etch selectivity with respect to the third oxide layer  118 , the second oxide layer  116  may serve as an etch stop for removing portions of the third oxide layer  118  in order to define the cavity  120 . 
     In addition to the etch of the third oxide layer that defines the cavity  120 , another etch is used to define openings  122  through the second oxide layer  116  and first oxide layer  114 , thereby exposing a top surface of a portion of metal regions  106 . A top down view of the cavity  120 , illustrating remaining portions of the second oxide layer  116  and the exposed portions of metal regions is illustrated in  FIG.  3   , taken along the arrows  3 - 3  in  FIG.  1   . The exposed portions of metal regions  106  may advantageously serve as a getter material of one or more gases present during a bonding operation of the membrane  112  to seal the cavity  120 . Additional exemplary processing operations used in forming the ultrasound transducer  100  are discussed hereinafter. 
     It will be appreciated that the gettering technique and associated removal of a portion of oxide layers  116  and  114  to expose a portion of metal regions  106  can be implemented in conjunction with any of a number of transducer electrode structures. For example,  FIGS.  4 - 6    illustrate a cross-sectional and top down views of a micromachined ultrasound transducer  400  having a cavity getter material, in accordance with another embodiment. For ease of description, like elements are designated by like reference numerals in the various embodiments. As particularly illustrated in  FIG.  4    and in  FIG.  5    (taken along the lines  5 - 5  in  FIG.  4   ) the lower electrode  102  is formed so as to have a “donut” pattern; that is, a region corresponding to the innermost radius of the electrode structure of the earlier described embodiment is instead formed from an insulating material (e.g., oxide  108 ) rather than the conductive electrode material. It will be noted that since oxide layers  116  and  114  are only removed from the outer region of the cavity geometry, the top down view of  FIG.  6    is substantially similar to that of the embodiment of  FIG.  3   . 
     The electrode geometry of  FIG.  4    may be employed in various operating modes, including in conjunction with a collapse mode of operation of the ultrasound transducer  400 , where at least a part of the membrane  112  comes into physical contact with a bottom surface of the cavity  120  (for example, second oxide layer  116 ). In this case, substituting a central portion of the lower electrode material with an insulator material can help to reduce parasitic capacitance of the ultrasound transducer  400  without significantly compromising performance, as the central portion of the electrode in physical contact with the bottom of the ultrasound transducer cavity has a minimum contribution to the production of ultrasonic signals. Still a further benefit to such an electrode structure may be the reduction of charging on the membrane  112  otherwise caused by repeated collapsing. Additional information regarding a donut shaped lower electrode may be found in U.S. Patent Application Ser. No. 62/666,643, filed May 3, 2018 and assigned to the assignee of the present application, and in co-pending U.S. patent application Ser. No. 16/401,630, filed May 2, 2019 and assigned to the assignee of the present application, the contents of both of which are incorporated by reference herein in their entireties. 
     Referring now to  FIGS.  7 - 9   , there is illustrated a cross-sectional and top down views of a micromachined ultrasound transducer  700  having a cavity getter material, in accordance with still another embodiment. Again, for ease of description, like elements are designated by like reference numerals in the various embodiments. As particularly illustrated in  FIG.  7    and in  FIG.  8    (taken along the lines  8 - 8  in  FIG.  7   ) the lower electrode  102  is still formed so as to have a “donut” pattern. However, in contrast to the embodiment of  FIGS.  4 - 6   , an additional electrode  702  is formed at the central portion of the cavity region. The electrode  702  is patterned to be electrically isolated from lower electrode  102 , such as by being insulated therefrom by oxide  108  for example. As is the case with the other embodiments, the top down view of  FIG.  9    is substantially similar to that of the embodiments of  FIG.  3    and  FIG.  6   . 
     An electrode geometry of the type shown in  FIG.  7   , in addition to the benefits described with respect to the previous embodiment, can also contribute to a bypass capacitance between the membrane  112  and ground, which in turn may contribute to noise reduction on the voltage of the membrane  112 . Another benefit of electrode  702  may be to help provide for a lower collapse voltage for the ultrasound transducer  700  by way of the electrode  702  attracting the membrane  112  toward the bottom of the cavity  120 . Additional information regarding this electrode design may also be found in the aforementioned co-pending U.S. Patent Application Ser. No. 62/666,643 and Ser. No. 16/401,630. 
     Referring generally now to  FIGS.  10 - 19   , there is shown an exemplary process flow sequence for forming the ultrasound transducer embodiments described above. In  FIG.  10   , an electrode layer  1000  is formed over CMOS substrate  104 , such as a silicon substrate for example. Again, the CMOS substrate  104  may include, but is not necessarily limited to, CMOS circuits, wiring layers, redistribution layers, and insulation/passivation layers. As also mentioned previously, suitable materials for the electrode layer  1000  include one more of titanium (Ti), zirconium (Zr), vanadium (V), cobalt (Co), nickel (Ni), as well as alloys thereof. 
     Depending on the specific lower electrode pattern desired, the process may proceed to any of  FIG.  11 - 1 ,  11 - 2  or  11 - 3   . For example, in  FIG.  11 - 1   , a photolithographic process is used to pattern and etch openings into the electrode layer  1000  so as to define the electrode pattern of  FIG.  1   , namely a lower electrode  102  and adjacent metal regions  106 . In this particular embodiment, a center region of the lower electrode  102  remains intact.  FIG.  11 - 2    illustrates the patterning of the electrode layer  1000  of the  FIG.  4    embodiment (i.e., center portion of electrode removed to define a “donut” pattern), and  FIG.  11 - 3    illustrates the patterning of the electrode layer  1000  of the  FIG.  7    embodiment (i.e., formation of the additional electrode  702  at the center portion of the donut pattern). 
     Once the electrode pattern is defined, the process may then proceed to an insulation fill operation as illustrated in  FIGS.  12 - 1 ,  12 - 2  and  12 - 3   . As shown, an insulation layer  1200  (e.g., silicon oxide) is formed over the patterned electrode material. The insulation layer  1200  is then planarized as respectively shown in  FIGS.  13 - 1 ,  13 - 2  and  13 - 3    to form the insulator material  108  described above. From this point, processing for each of the illustrated electrode design embodiments is substantially the same. Accordingly, the remaining figures are illustrated in the context of the first embodiment only (i.e., from  FIG.  13 - 1   ) for conciseness, although it should be understood that the subsequent processes are equally applicable to the other embodiments. 
     Referring now to  FIG.  14   , an insulator stack  110  as described above is formed over the lower electrode layer, such as the lower electrode  102  and adjacent metal regions  106  illustrated in  FIG.  13 - 1   . In the illustrated embodiment, the insulator stack  110  includes a first oxide layer  114  (e.g., CVD silicon oxide having a thickness of about 1-100 nm) formed over the lower electrode  102  and adjacent metal regions  106 , a second oxide layer  116  (e.g., ALD aluminum oxide having a thickness of about 5-100 nm) formed over the first oxide layer  114 , and a third oxide layer  118  (e.g., sputter deposited silicon oxide having a thickness of about 1-300 nm) formed over the second oxide layer  116 . 
     In  FIG.  15   , a first lithographic patterning and etch process is performed to define the cavity  120  by removing a portion of the third oxide layer  118 , using the second oxide layer  116  as an etch stop. As an added benefit, aluminum oxide material of the second oxide layer  116  present at the bottom of the cavity  1500  may also help to reduce charging of a (subsequently formed) top membrane in the event the top membrane comes into contact with the second oxide layer  116  during device operation (e.g., such as during a collapse mode of transducer operation). Optionally, a thin layer of aluminum oxide (not shown) and also a thin self-assembled monolayer (SAM) with a heptadecafluoro tetrahydrodecyl trichlorosilane or dodecyltrichlorosilane precursor (not shown) may be formed on the second oxide layer  116  after patterning and before photoresist removal. A SAM formed at the bottom of the cavity  120  may help to reduce any stiction of the top membrane to the bottom of the cavity  120  in the aforementioned collapse mode of operation or other mode where the top membrane comes in physical contact with the bottom of the cavity  120 . It should further be appreciated at this point that although the illustrated embodiments depict a single cavity, any suitable number of cavities and corresponding electrode structures may be formed (e.g., hundreds, thousands, tens of thousands, etc.) 
     Referring now to  FIG.  16   , a second lithographic patterning and etch process is performed to expose the adjacent metal regions  106  at the outer perimeter of the cavity  120 . The second etch removes a portion of the second oxide layer  116  and first oxide layer  114 , stopping on the adjacent metal regions  106 , which will serve as a getter material. At this point, the device as depicted in  FIG.  16    is prepared for membrane bonding. A particular size or size range of the opening(s) formed through the second oxide layer  116  and first oxide layer  114  (and thus the resulting amount of adjacent metal region material to be exposed) may be chosen based on one or more calculations on how much gas needs to be consumed/gettered during the bonding process. In addition, a determination of how much getter material area to be formed in each cavity may depend on factors such as for example, how much gas is released during the bonding process, the device lifetime, and the desirable cavity pressure after the getter is activated. Final pressure may be adjusted by getter activation, which in turn may be accomplished by annealing at elevated temperatures. In embodiments, it is generally preferable to have more exposed getter material than a targeted amount, rather than less exposed getter material than the target amount, since the annealing time could be shortened with the extra getter material. By way of example only, for a transducer cavity having a cross sectional area of about 0.030 cm 2 , an example range of getter material area within this cavity for effective gettering maybe about 1×10 −4  cm 2  to about 2.5×10 −4  cm 2 . In embodiments, it may be desirable to build in a further margin for getter material exposure to allow for process variations and calculated getter efficiency. Thus, for this specific example shown in  FIG.  16   , an inner radius r 1  of the getter material (metal regions  106 ) is about 83 microns (μm), an outer radius r 2  of the getter material is about 96 microns μm, and a transducer cavity radius r 3  is about 98 μm. With a circular cavity configuration, the resulting getter area is about 7.3×10 −3  cm 2 , which is roughly an order of magnitude more than an exemplary calculated range needed for a desired gettering capability. Again, it should be appreciated that these values are exemplary only and the disclosure is not limited to such values and ranges. 
     As shown in  FIG.  17   , a substrate  1700  (e.g., a silicon-on-insulator (SOI) substrate) includes a handle layer  1702  (e.g., a silicon layer), a buried oxide (BOX) layer  1704 , and a silicon device layer  1706 . An oxide layer  1708  may optionally be provided on a backside of the handle layer  1702 . The silicon device layer  1706  may be formed from single crystal silicon and may be doped in some embodiments. Such doping may be highly doped P-type or, alternatively N-type, and may be uniform through the silicon device layer  1706  or patterned by implantation in certain regions. In addition, an oxide layer  1710  (e.g., a thermal silicon oxide) is formed on the silicon device layer  1706 . 
     As shown in  FIG.  18   , the substrate  1700  is bonded to the substrate  104  and the aforementioned structures formed on the substrate  104 . In the embodiment depicted, the oxide material of layer  1710  is bonded to the oxide material  118  by low temperature oxide bonding methods (e.g., below 450° C.), which may prevent damage to circuitry of the substrate  104 . As stated above, there may be a difference in cavity pressures across the die and wafer due to H 2 O based byproducts and the propagation of the bond. Because the metal surface of the adjacent metal regions  106  is exposed during bonding of the substrate  1700 , the metal is able to consume gases such as oxygen, nitrogen, argon, water vapor, etc., resulting in a more uniform pressure across the various cavities  120  of the ultrasound device. 
     After bonding, the oxide layer  1708  and the handle layer  1702  are removed by a suitable technique (e.g., etching, grinding, etc.), thereby defining the membrane  112  discussed above and as illustrated in  FIG.  19   . Optionally, the BOX layer  1704  may also be removed prior to additional processing, which may include suitable steps to complete final wiring, interconnect and/or packaging steps used to produce an ultrasound device. 
       FIG.  20    illustrates a top view of an example ultrasound transducer device  2000  formed using any of the process flow sequences described herein. As illustrated, the transducer device includes an array of individual transducers  100 , such as those described above. The specific number of transducers  100  shown in  FIG.  20    should not be construed in any limiting sense, and may include any number suitable for a desired imaging application, which may be for example on the order of tens, hundreds, thousands, tens of thousands or more. It will be appreciated the above described gettering techniques are particularly beneficial with an increasing larger array, given the ability to provide a uniform cavity pressure across a wafer or die.  FIG.  20    further illustrates an example location of metal  2002  that may distribute an electrical signal to the membranes (upper electrodes) of the transducers  100 . 
     It should be appreciated that although the exemplary embodiments illustrate and describe a same bottom electrode metal material used as a getter material during membrane bonding, other non-metallic or non-metallic alloy getter materials may also be used in a similar manner. For example, graphite, phosphorous and/or certain salts may serve as a cavity getter material. Moreover, with respect to the specific placement of a getter material, other locations in addition to a cavity bottom are also contemplated. For example, with additional processing operations, layers may be formed in a manner so as to have getter material disposed on cavity sidewalls and/or the membrane itself (top of sealed cavity) for gettering during membrane bonding. Such additional getter layer(s) may also be formed at a different level than the bottom electrode material. 
     The above-described embodiments can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor (e.g., a microprocessor) or collection of processors, whether provided in a single computing device or distributed among multiple computing devices. It should be appreciated that any component or collection of components that perform the functions described above can be generically considered as one or more controllers that control the above-discussed functions. The one or more controllers can be implemented in numerous ways, such as with dedicated hardware, or with general purpose hardware (e.g., one or more processors) that is programmed using microcode or software to perform the functions recited above. 
     Various aspects of the present application may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, features described in one embodiment may be combined in any manner with features described in other embodiments. 
     Also, certain aspects may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value.