Patent Publication Number: US-2023149976-A1

Title: Anti-stiction bottom cavity surface for micromachined ultrasonic transducer devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 16/683,652, filed Nov. 14, 2019, which is hereby incorporated by reference in its entirety. 
     U.S. patent application Ser. No. 16/683,652 claims the benefit under 35 U.S.C. §119(e) of U.S. Patent Application Ser. No. 62/768,048, filed Nov. 15, 2018, which is hereby incorporated herein by reference in its entirety. 
     U.S. patent application Ser. No. 16/683,652 further claims the benefit under 35 U.S.C. §119(e) of U.S. Patent Application Ser. No. 62/810,358, filed Feb. 25, 2019, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to micromachined ultrasonic transducers and, more specifically, to an anti-stiction, bottom cavity surface for micromachined ultrasonic transducer cavities and transducer manufacturing techniques. 
     BACKGROUND 
     Ultrasound devices may be used to perform diagnostic imaging and/or treatment, using sound waves with frequencies that are higher than those audible to humans When pulses of ultrasound are transmitted into tissue, 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 ultrasonic 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, generates an output electrical signal. 
     SUMMARY 
     In one aspect, a method of forming an ultrasonic transducer devices involves depositing a first layer on a substrate; depositing a second layer on the first layer; patterning the second layer at a region corresponding to a location of a transducer cavity; depositing a third layer that refills regions created by patterning the second layer; planarizing the third layer to a top surface of the second layer; removing the second layer; conformally depositing a fourth layer over the first layer and the third layer; defining the transducer cavity in a support layer formed over the fourth layer; and bonding a membrane to the support layer. 
     In one aspect, a method of forming an ultrasonic transducer device involves depositing a first layer on a substrate; patterning the first layer at a region corresponding to a location of a transducer cavity; conformally depositing an additional layer of a same type as the first layer on the first layer and exposed portions of the substrate; conformally depositing a second layer on the additional layer of the same type as the first layer; conformally depositing a third layer on the second layer; conformally depositing a fourth layer on the third layer; planarizing the fourth layer to a top surface of the third layer; depositing a membrane support layer; defining the transducer cavity in the support layer, wherein exposed portions of the third layer and the fourth layer are removed; and bonding a membrane to the support layer. 
    
    
     
       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 an exemplary micromachined ultrasonic transducer device. 
         FIG.  2    is a cross-sectional view of an exemplary micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to an embodiment. 
         FIG.  3    is top view of an anti-stiction bottom cavity surface according to one embodiment. 
         FIG.  4    is top view of an anti-stiction bottom cavity surface according to another embodiment. 
         FIG.  5    is top view of an anti-stiction bottom cavity surface according to another embodiment. 
         FIG.  6    is top view of an anti-stiction bottom cavity surface according to still another embodiment. 
         FIG.  7    is an enlarged view of a portion of  FIG.  5   . 
         FIG.  8    is a cross-sectional view of the anti-stiction bottom cavity surface of  FIG.  7   , taken along the line  8 - 8 . 
         FIGS.  9 - 1  through  9 - 7    are a series of cross-sectional views illustrating an exemplary process flow for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to an embodiment. 
         FIG.  10    is a flow diagram describing the exemplary process flow of  FIGS.  9 - 1  through  9 - 7   . 
         FIGS.  11 - 1  through  11 - 9    are a series of cross-sectional views illustrating an exemplary process flow for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to another embodiment. 
         FIG.  12    is a flow diagram describing the exemplary process flow of  FIGS.  11 - 1  through  11 - 9   . 
         FIG.  13    is a top view of an example ultrasonic transducer device formed that may be formed using the process flow of  FIGS.  9 - 1  through  9 - 7    and  FIG.  10   , or using the process flow of  FIGS.  11 - 1  through  11 - 9    and  FIG.  12   . 
     
    
    
     DETAILED DESCRIPTION 
     The techniques described herein relate to an anti-stiction, bottom cavity surface for micromachined ultrasonic transducer cavities. 
     One type of transducer suitable for use in ultrasound imaging devices is a micromachined ultrasonic transducer (MUT), which can be fabricated from, for example, silicon and configured to transmit and receive ultrasound energy. MUTs may include capacitive micromachined ultrasonic transducers (CMUTs) and piezoelectric micromachined ultrasonic transducers (PMUTs), both of which can offer several advantages over more conventional 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), a CMUT may be directly integrated on an integrated circuit that controls the operation of the transducer. One way of manufacturing a CMUT is to bond a membrane substrate to an integrated circuit substrate, such a complementary metal oxide semiconductor (CMOS) substrate. This may be performed at temperatures sufficiently low enough to prevent damage to the devices of the integrated circuit. 
     Referring initially now to  FIG.  1   , there is shown a cross-sectional view of an exemplary micromachined ultrasonic transducer device  100 , such as a CMUT. The transducer device  100  includes a substrate, generally designated by  102 , (e.g., a complementary metal oxide semiconductor (CMOS) substrate, such as silicon) having one or more layers such as for example: CMOS circuits, wiring layers, redistribution layers, insulation/passivation layers and one or more metal electrode layers  103 . Such metal electrode layer(s)  103  of the substrate  102  may serve as a transducer bottom electrode. As specific substrate and transducer bottom electrode patterns are not the focus of the present disclosure, only a single example is presented in the figures. It will be appreciated, however, that the present embodiments may also be implemented in conjunction with several other transducer electrode structures including (but not limited to), for example: donut shaped electrodes (e.g., interior metal removed), multiple segment or ring electrodes, and additional metal patterns used for other purposes besides bottom electrodes (e.g., cavity getter during bonding). 
     Still referring to  FIG.  1   , it will be seen that the substrate  102  (including bottom electrode) may have one or more insulating layers formed thereon, generally designated by stack  104 . A transducer cavity  105  is defined by lithographic patterning and etching of a support layer  106  that is formed on stack  104 . The support layer  106  may be an insulating layer, such as SiO 2  for example, the remaining portions of which provide a support surface to which a flexible transducer membrane  108  (e.g., highly doped silicon at a concentration of about 1×10 18  atoms/cm 3  to about 1×10 19  atoms/cm 3 ) is bonded. 
     During operation of the transducer device  100 , whether in a transmit mode and/or a receive mode, the transducer membrane  108  may come into physical contact with the top of the stack  104  (i.e., the bottom surface of the cavity  105 ), as indicated by the dashed position of the membrane  108 ′ in  FIG.  1   . This position may be indicative of the so-called “collapse mode” of operation as known in the art. A “collapse mode” (also used interchangeably with the term “collapsed mode”) refers to a mode of transducer operation in which at least one portion of a micromachined ultrasonic transducer membrane is mechanically fixed and at least one portion of the membrane is free to vibrate based on a changing voltage differential between the bottom electrode and the top membrane. On one hand, operating a micromachined ultrasonic transducer in collapse mode may optimize the acoustic power (output pressure) produced by the transducer. On the other hand, however, one side effect of a micromachined ultrasonic transducer operating in collapse mode may be charge retention (also known as “charge trapping”), wherein the membrane and/or the bottom cavity surface undesirably stores charge or conducts leakage current. As a result, this charge retention may in turn undesirably alter an applied voltage at which membrane collapse is induced during device operation. Another possible side effect of collapse mode for the transducer  100  of  FIG.  1    may be stiction, which is associated with the amount of surface area of contact between the membrane  108  and the bottom surface of the cavity  105  wherein the two surfaces inadvertently stick to one another. Stiction generally will lead to device reliability failure, and hence should be addressed to make sure the device can perform properly through its operation lifetime. 
     Accordingly,  FIG.  2    illustrates a cross-sectional view of an exemplary micromachined ultrasonic transducer device  200  having an anti-stiction bottom cavity surface according to an embodiment. For ease of illustration, like elements among the various figures are designated with the same reference numbers. In the embodiment of  FIG.  2   , the substrate  102  (including bottom electrode) may have one or more insulating layers formed thereon, generally designated by stack  204 . In contrast to the embodiment of  FIG.  1   , the uppermost layer of stack  204  is characterized by having topographic features, so as to reduce the amount of direct surface contact area between the collapsed membrane  108 ′ and the top layer of the stack  204 . 
     Although there are several possible topographic patterns that may be used for the stack  204 , some specific examples are illustrated in  FIGS.  3 - 6   , each of which depicts a top view of an anti-stiction bottom cavity surface according to various embodiments. As indicated by the legends in  FIGS.  3 - 6   , the darker and lighter shades represent different elevations of a same anti-stiction material layer (as opposed to the shades representing different materials located at a same elevation). In the embodiment of  FIG.  3   , the topographic (e.g., raised) regions  302   a  may have a generally rectangular shape, disposed in generally concentric circular patterns from a cavity center. The topographic regions  302   a  (as well as the additional darker shaded region  302   b  of the stack  204  proximate the outer perimeter of the cavity) are disposed at a higher elevation than the lighter shaded regions  304 . As will be described in further detail, the topmost layer of the stack  204  may include a thin film, anti-stiction layer conformally deposited over previously defined topography located in a lower layer(s) of the stack  204 . In the embodiment of  FIG.  4   , the topographic regions  402   a  may be generally circular and/or ring shaped, and optionally segmented. Again, as is with the case of the  FIG.  3    embodiment, topographic regions  402   a  (as well as the additional darker shaded region  402   b  of the stack  204  proximate the outer perimeter of the cavity) are at a higher elevation than lighter shaded regions  404 . 
     Still another embodiment for the topographic patterns that may be used for the stack  204  is illustrated in  FIG.  5   . In this embodiment, the topographic regions  502   a  may be defined by an array of circular posts, generally arranged in rows and columns. As compared to an entire cavity region, the lighter shaded, lower elevation region  504  may have a relatively small area with to that of the remaining outer regions  502   b  of the stack  204  at the higher elevation. This embodiment may be advantageous, for example, with transducer operating modes where only a relatively small area of the transducer membrane comes into contact with the bottom cavity surface during collapse mode. On the other hand,  FIG.  6    is an alternative embodiment for the stack  201 , similar to  FIG.  5   , but with a greater number of topographic regions  502   a  (posts). The  FIG.  6    embodiment may be advantageous, for example, with transducer operating modes where a relatively large area of the transducer membrane comes into contact with the bottom cavity surface during collapse mode. 
     An enlarged view of the dashed square region of  FIG.  5    is shown in  FIG.  7   , and  FIG.  8    is a corresponding cross-sectional view of the stack  204  taken along the line  8 - 8  of  FIG.  7   . By way of illustration only, an exemplary topographic configuration may have the topographic regions  502   a  (posts) dimensioned with a first diameter, d 1 , on the order of about 3 microns (μm) and spaced at a pitch of about 3 μm. Correspondingly, a second diameter, d 2  of the lower elevation region  504  may be on the order of about 40 μm. Again, for the  FIG.  5    embodiment, d 2  may be relatively small as compared to an exemplary diameter of the entire transducer cavity (e.g., about 200 μm). Alternatively, for an embodiment such as in  FIG.  6   , d 2  may be relatively larger (e.g., about 140 μm) as compared to the diameter of the entire transducer cavity (e.g., about 200 μm). It will be appreciated that these dimensions are provided for the sake of illustration only, and it is contemplated that other geometric dimensions and configurations are also possible. 
     Referring generally now to  FIGS.  9 - 1  through  9 - 7   , there is shown a series of cross-sectional views illustrating an exemplary process for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to an embodiment. In addition,  FIG.  10    is an accompanying flow diagram describing the exemplary processing cross-sectional views of  FIGS.  9 - 1  through  9 - 7   . It will be appreciated that the exemplary process flow may be used to form a device such as shown in any of  FIGS.  2 - 8   , as well as other topographic structures not specifically depicted in the previously described drawings. As such, it should be understood that the topographic features depicted in the process flow of  FIGS.  9 - 1  through  9 - 7    are for illustrative purposes only and should not otherwise be construed in any limiting sense. 
     As shown in  FIG.  9 - 1    and indicated in block  1002  of  FIG.  10   , a first type layer  902  and a second type sacrificial layer  904  are deposited on a transducer lower electrode layer. More specifically, a first type layer  902  is deposited on substrate  102 , and a second type sacrificial layer  904  is deposited on the first type layer  902 . Again, because specific transducer bottom electrode patterns are not the focus of the present disclosure, such metal electrode layer(s)  103  should be understood to be generally incorporated into an upper portion of the substrate  102  as represented in the figures. In an exemplary embodiment, the first type layer  902  may be a thin film layer of SiO 2 , formed at a thickness of about 10-30 nanometers (nm), and the second type sacrificial layer  904  may be a thin film layer of SiN, formed at a thickness of about 30-70 nm. As shown in  FIG.  9 - 2    and indicated in block  1004  of  FIG.  10   , the second type sacrificial layer  904  is patterned (e.g., by photolithography and etching) at a region corresponding to the location of the transducer cavity. 
     Following patterning of the second type sacrificial layer  904 , a third type layer  906  may be deposited as shown in  FIG.  9 - 3    and indicated in block  1006  of  FIG.  10   . The third type layer  906  may be the same type material as the first type layer  902  (e.g., SiO 2 ) and formed at a thickness (e.g., 400-700 nm) that is sufficient to refill the regions created by patterning and removal of portions of the second type sacrificial layer  904 . Then, as shown in  FIG.  9 - 4    and indicated in block  1008  of  FIG.  10   , the third type layer  906  is planarized to the level of the top surface of the patterned second type sacrificial layer  904 , such as by chemical mechanical polishing (CMP) for example. The second type sacrificial layer  904  may then be selectively removed (block  1010  of  FIG.  10   ), such as by etching for example, to result in the structure shown in  FIG.  9 - 5   . 
     As shown in  FIG.  9 - 6    and indicated in block  1012  of  FIG.  10   , the insulating stack  204  is defined by conformal deposition of a fourth type thin film layer  1008  over the topography of layers  902  and  906 , such as by atomic layer deposition (ALD). Since the exemplary embodiment may use a same type material for layer  902  and  906  (e.g., SiO 2 ), the combination of layers  902 / 906  are depicted in subsequent figures as a single layer  910  for ease of illustration. The fourth type thin film layer  908  is selected to be an anti-stiction material, such as aluminum oxide formed at a thickness of about 20-40 nm. By conformally depositing the thin film aluminum oxide layer  908  over the topography defined by layer  910  (i.e., layers  902  and  906 ), a relatively uniform, continuous thin film of anti-stiction material is produced (as opposed to, for example, etching through a thin, planar layer of anti-stiction material which may result in a non-smooth, discontinuous top layer of stack  204 ). This in turn may be beneficial for a subsequent bonding process by, for example, providing a superior bonding interface with minimal process induced defects. Once the thin film layer  908  is formed in  FIG.  9 - 6   , additional transducer processing may continue as known in the art, such as shown in  FIG.  9 - 7    and indicated in block  1014  of  FIG.  10   . This may include, for example, defining a transducer cavity in an insulating (support) layer  106  and bonding the flexible membrane  108  to patterned support portions of layer  106  as previously described. 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 generally now to  FIGS.  11 - 1  through  11 - 9   , there is shown a series of cross-sectional views illustrating an exemplary process for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to another embodiment. In addition,  FIG.  12    is an accompanying flow diagram describing the exemplary processing cross-sectional views of  11 - 1  through  11 - 9 . It will be appreciated that the exemplary process flow may be used to form a device such as shown in any of  FIGS.  2 - 8   , as well as other topographic structures not specifically depicted in the previously described drawings. As such, it should again be understood that the topographic features depicted in the process flow of  FIGS.  11 - 1  through  11 - 9    are for illustrative purposes only and should not otherwise be construed in any limiting sense. 
     As shown in  FIG.  11 - 1    and indicated in block  1202  of  FIG.  12   , a first type layer  1102  is deposited on substrate  102  (e.g., a thin film layer of SiO 2 , formed at a thickness of about  10 - 30  nm) and patterned (e.g., by photolithography and etching) at a region corresponding to the location of the transducer cavity. Again, because specific transducer bottom electrode patterns are not the focus of the present disclosure, such metal electrode layer(s)  103  should be understood to be generally incorporated into an upper portion of the substrate  102  as represented in the figures. As shown in  FIG.  11 - 2    and indicated in block  1204  of  FIG.  12   , an additional layer  1104  of the first type material (e.g., a thin film layer of SiO 2 , formed at a thickness of about 10-30 nm) is conformally deposited over the patterned layer  1102  and the exposed portions of the metal electrode layer(s)  103 . The resulting layer  1106  (shown as a single layer in subsequent figures) has a desired bottom cavity layer topography, such as for example one corresponding to any of the patterns discussed above or other patterns not explicitly described above. It will be noted that an alternative way to form the intermediate structure shown in  FIG.  11 - 2    could be to form a thicker layer of first type material (e.g., a thin film layer of SiO 2 , formed at a thickness of about 20-60 nm), followed by a timed etch (i.e., one that does not go completely through the SiO 2  material down to the metal electrode layer(s)  103 ). However, the latter method as shown in the  FIGS.  11 - 1  and  11 - 2    may provide a more reliable way to control topographic features such as step heights of subsequently formed layers. 
     Following the formation of the first type material topographical layer  1106  and as indicated in block  1206  of  FIG.  12   , a second type material layer  1108  is conformally deposited over layer  1106  as shown in  FIG.  11 - 3   , followed by a third type material layer  1110  conformally deposited over layer  1108  as shown in  FIG.  11 - 4   . The second type material layer  1108  may be an anti-stiction material, such as aluminum oxide formed by ALD at a thickness of about 20-40 nm. The third type material layer  1110  is selected to as to act as a CMP stop layer, and may be for example a SiN layer formed at an initial thickness of about 20-50 nm. 
     As shown in  FIG.  11 - 5    and indicated in block  1208  of  FIG.  12   , a fourth type layer  1112  (e.g., thin film SiO 2 ) is conformally deposited over the third type material/CMP stop layer  1110  at a thickness (e.g., about 400-700 nm), and may act as a CMP buffer layer. Then, in  FIG.  11 - 6    and as indicated in block  1210  of  FIG.  12   , the structure is planarized so as to substantially remove the CMP buffer layer  1112 , stopping on the CMP stop layer  1110 , the thickness of which may be reduced as a result of the CMP operation. Some portions of the CMP buffer layer  1112  might still remain over the lower topographic regions of the CMP stop layer  1110 , as also shown in  FIG.  11 - 6   . An advantage of the above described combination of the CMP buffer layer  1112  and CMP stop layer  1110  may be to provide an anti-stiction surface  1108  integrated into a transducer device while the processing operations are still compatible with a CMUT planarization process, thereby providing a superior bonding interface with minimal process induced defects. 
     Following planarization, additional transducer processing operations may continue as known in the art. As shown in  FIG.  11 - 7    and indicated in block  1212  of  FIG.  12   , the membrane support layer  106  (e.g., SiO 2 ) is formed. Then, as shown in  FIG.  11 - 8    and indicated in block  1214  of  FIG.  12   , the membrane support layer  106  is lithographically patterned and etched to define the transducer cavity  105 , where the exposed portions of the membrane support layer  106 , the CMP polish stop layer  1110  and remaining CMP buffer layer  1112  are removed so as to expose the topographic anti-stiction layer  1108 . The flexible membrane  108  may then be bonded to the remaining portions of the membrane support layer  106  as shown in  FIG.  11 - 9    and indicated in block  1216  of  FIG.  12   . Again, 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.) 
       FIG.  13    illustrates a top view of an example ultrasonic transducer device  1300  formed using any of the exemplary process flow embodiments 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.  13    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.  FIG.  13    further illustrates an example location of metal  1302  that may distribute an electrical signal to the membranes (upper electrodes) of the transducers  100 . 
     It should be appreciated that although the exemplary geometric structure of this portion of the ultrasonic 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. 
     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 invention 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, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. 
     Also, some aspects of the technology 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. 
     Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively.