Patent Publication Number: US-2016243588-A1

Title: Method for manufacturing capacitive micromachined ultrasonic transducer and apparatus configured to obtain subject information using the capacitive micromachined ultrasonic transducer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/682,640, filed on Nov. 20, 2012, which claims priority from Japanese Patent Application No. 2011-259273, filed Nov. 28, 2011, all of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a capacitive micromachined ultrasonic transducer that can be used in ultrasonic probes and other applications, and also to a method for manufacturing this capacitive micromachined ultrasonic transducer. 
     2. Description of the Related Art 
     Ultrasonic diagnosis has recently been appreciated as a technology for early detection of diseases. In this field of diagnosis, one of the promising ultrasonic transmitting and receiving technologies under research is capacitive micromachined ultrasonic transducers (CMUTs), replacing piezoelectric elements. CMUTs are small and lightweight devices and are fabricated by the rapidly advancing micromachining technology. They have an acoustic impedance similar to that of the human body and thus offer better acoustic impedance matching than known piezoelectric devices. They are also advantageous in many other ways, for example, a broad frequency band in liquids. 
     PCT Japanese Translation Patent Publication No. 2006-516368 discloses a method for fabricating a CMUT, and this method involves the use of a single-crystal silicon vibrating membrane formed on a silicon substrate by bonding them or other suitable means. More specifically, an oxide film is formed on the silicon substrate by thermal oxidation, the resulting thermal oxide film is partially removed, and then the remaining portion of the thermal oxide film and a piece of single-crystal silicon are bonded. This piece of single-crystal silicon is used as the vibrating membrane, with the space formed by the partial removal of the thermal oxide film as a cavity. 
     The above patent publication mentions that the manufacturing method disclosed therein may include a second thermal oxidation step. After the removal of the thermal oxide film to expose a portion of the silicon substrate and thereby to form the cavity, the exposed surface of the silicon substrate is thermally oxidized again and thereby coated with an insulating oxide film. 
     SUMMARY OF THE INVENTION 
     Aspects of the present disclosure provide CMUTs with improved uniformity in device characteristics and a method for manufacturing such CMUTs. 
     In the manufacturing method according to an aspect disclosed herein, a first insulating layer and a vibrating membrane are bonded by heat treatment and a second insulating layer is formed by thermal oxidation in a single heating step, with a cavity provided in the CMUT communicating with an outside of the CMUT through a communication portion. 
     As a result, there are provided CMUTs with improved uniformity in device characteristics and a method for manufacturing such CMUTs. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1F  illustrate a manufacturing process of a CMUT according to Embodiment 1. 
         FIGS. 2A to 2C  are a plan view and cross-sectional views of the CMUT illustrated in  FIG. 1D . 
         FIG. 3  illustrates a cross-section of another CMUT according to Embodiment 1. 
         FIGS. 4A and 4B  are plan views of the other CMUT according to Embodiment 1. 
         FIG. 5  is a plan view of a first insulating layer configured in accordance with Embodiment 2. 
         FIGS. 6A to 6C  illustrate a manufacturing process of a CMUT according to Embodiment 2. 
         FIGS. 7A to 7G  illustrate the manufacturing process of the CMUT described later herein as Example 1. 
         FIGS. 8A and 8B  illustrate the composition of apparatuses configured to obtain subject information (hereinafter also referred to as analyzers). These analyzers contain CMUTs as a necessary component and are described later herein as Example 2. 
         FIGS. 9A to 9D  illustrate a known manufacturing process of a CMUT. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The method for manufacturing a CMUT described in PCT Japanese Translation Patent Publication No. 2006-516368 results in poor bonding between the thermal oxide film and single-crystal silicon because the second heating step causes protrusions of the thermal oxide film to form in the bonding interface, although this is not mentioned in the publication. The following describes this situation with reference to  FIGS. 9A to 9D . 
     First, as illustrated in  FIG. 9A , a silicon (Si) substrate  402  is thermally oxidized (a first heating step) and thereby a base  400  having a silicon oxide film  404  is prepared. Then, as illustrated in  FIG. 9B , the silicon oxide film  404  is partially removed by etching and the Si substrate  402  is partially exposed, and thereby a cavity  406  is formed. The surface of the Si substrate  402  exposed to the cavity  406  is thermally oxidized (a second heating step) and thereby a silicon oxide film  408 , which serves as an insulator, is formed on the exposed surface of the Si substrate  402 . During this second heating step, silicon oxide also accumulates on the exposed bottom portions of the silicon oxide film  404  because these portions are near the cavity  406  and large amounts of oxygen are supplied there. As a result, protrusions  410  are formed on the top of the silicon oxide film  404  as illustrated in  FIG. 9C . These protrusions  410  prevent a vibrating membrane  412  from coming into intimate contact as illustrated in  FIG. 9D . 
     Although it is possible to ensure good bonding by removing the protrusions  410  from the interface, this may cause the silicon oxide film  404  to be partially removed together with the protrusions  410 . This affects the uniformity of the silicon oxide film  404  and causes the distribution of electric field intensity in the cavity  406  generated upon application of voltage to be uneven. This uneven distribution of electric field intensity leads to varying dielectric breakdown voltages of the resulting CMUT devices and thereby affects the reliability of them. 
     In light of this, embodiments of the present invention provide methods for manufacturing CMUTs with improved uniformity in device characteristics. 
     The following describes some embodiments of a method for manufacturing a CMUT according to an aspect of the present disclosure. 
     Embodiment 1 
       FIGS. 1A to 1F  illustrate a manufacturing process of a CMUT according to this embodiment. 
     First, as illustrated in  FIG. 1A , a silicon substrate  102  is coated with a first insulating layer  104 , which is made of silicon oxide or similar materials, and thereby a base  100  is prepared. The first insulating layer  104  can be formed by thermal oxidation, chemical vapor deposition (CVD), or other suitable techniques. Thermal oxidation can ensure the characteristics of the resulting first insulating layer  104  such as thickness controllability, uniformity, film density, and adhesion to the silicon substrate  102 . 
     Then, as illustrated in  FIG. 1B , the first insulating layer  104  is partially removed and the silicon substrate  102  is partially exposed in a way that the first insulating layer  104  should have a pattern allowing it to support the vibrating membrane and a space that will later serve as a cavity  108  should be formed. A communication passage  106  may also be formed during this step that will later serve as a component of a communication portion that allows the cavity  108  to communicate with the outside. 
     The height of the space that will later serve as the cavity  108  determines the capacitance and thus should be precisely controlled. The technique used for the partial removal of the first insulating layer  104  should therefore be highly controllable. One example is wet etching with an etchant based on hydrofluoric acid, such as buffered hydrofluoric acid, and this can be used when the first insulating layer  104  is made of silicon oxide. Wet etching with buffered hydrofluoric acid or other hydrofluoric-acid-based etchants, which ensures that the selection ratio between silicon oxide and silicon is almost infinity, allows the height of the cavity  108  to be determined by the thickness of the silicon oxide layer. Reactive ion etching (RIE) and other dry etching techniques can also be used as long as adequate controllability is ensured. 
     The patterned first insulating layer  104  can also be obtained by using a mask layer so that the first insulating layer  104  can be formed having a predefined pattern. In this case, the mask layer is formed on the silicon substrate  102 , the first insulating layer  104  is formed, and then the mask layer is removed, and the step illustrated in  FIG. 1A  is unnecessary. 
     Then, as illustrated in  FIG. 1C , a vibrating membrane  206  and the first insulating layer  104  are bonded. The vibrating membrane  206  can be made of a lightweight material with a high Young&#39;s modulus, such as single-crystal silicon or silicon nitride. For example, silicon contained in the active layer of a silicon-on-insulator (SOI) substrate can be used for this purpose. 
     Fusion and other direct bonding techniques ensure sufficient strength of the junction. For example, the vibrating membrane  206  and the first insulating layer  104  can be bonded by overlaying the former on the latter and then heating them. 
     Then, as illustrated in  FIG. 1D , communication holes  112  are formed through the vibrating membrane  206  to allow the cavity  108  to communicate with the outside of the transducer. This establishes a communication portion (the communication passage  106  and the communication holes  112 ) through which the cavity  108  is open to the outside of the CMUT. Although in this embodiment the communication holes  112  are formed through the vibrating membrane  206 , the communication holes  112  may be formed through the silicon substrate  102  instead. Furthermore, although in this embodiment the vibrating membrane  206  and the first insulating layer  104  are bonded first and then the communication holes  112  are formed, it is also allowed to form the communication holes  112  through the vibrating membrane  206  first and then bond the vibrating membrane  206  and the first insulating layer  104 . 
       FIG. 2A  is a plan view of the CMUT illustrated in  FIG. 1D , and  FIGS. 2B and 2C  are cross-sectional views of the CMUT illustrated in  FIG. 2A  taken along lines IIB-IIB and IIC-IIC, respectively. As can be seen from  FIG. 2C , the cavity  108  can communicate with the outside of the CMUT through the communication portion (the communication passage  106  and the communication holes  112 ). 
     Then, the material(s) for a second insulating layer  114  is introduced in a gaseous form through the communication portion (the communication passage  106  and the communication holes  112 ), and the second insulating layer  114  is formed in an atmosphere containing the gas in a way that the surface of the silicon substrate  102  exposed to the cavity  108  should be coated as illustrated in  FIG. 1E . The second insulating layer  114  can be formed by thermal oxidation, CVD, or other suitable techniques. The formation of the second insulating layer  114  by thermal oxidation includes introduction of oxygen through the communication portion and subsequent heating in the oxygen-containing atmosphere and thus ensures that silicon oxide is incorporated in the resulting insulating layer. CVD allows the second insulating layer  114  to be formed of materials such as silicon nitride. In the case of silicon nitride, the gas introduced as a silicon source can be SiH 4 , SiH 2 Cl 2 , or the like, and the gas introduced as a nitrogen source can be N 2 , NH 3 , or the like. 
     When the vibrating membrane  206  is made of silicon and thermal oxidation is chosen or when CVD is chosen, another second insulating layer  115  is formed on the surface of the vibrating membrane  206  exposed to the cavity  108 . Forming two second insulating layers  114  and  115  on the silicon substrate  102  and the vibrating membrane  206  to coat their surfaces exposed to the cavity  108  in this way can lead to improved insulation between the silicon substrate  102  and the vibrating membrane  206 . 
     After that, the communication between the cavity  108  and outside the transducer can be blocked; this allows the transducer to be used in liquid or under similar conditions. For example, it is allowed to form a blocking layer  106  configured to seal the communication holes  112  as illustrated in  FIG. 1F . The blocking layer  116 , if it is used, can be formed from materials such as silicon nitride by CVD or other suitable techniques. In addition to blocking the communication, the blocking layer  116  can be used to modify the mechanical properties of the vibrating membrane  206  as its thickness, stress properties, and other relevant physical characteristics can be appropriately adjusted. It is also allowed to form a blocking layer  116  only to cover the communication holes  112  as illustrated in  FIG. 3 . 
     When the vibrating membrane  206  is made of silicon, the communication holes  112  can also be closed by forming an insulating layer on the vibrating membrane  206  to seal the communication holes  112  in parallel with the formation of the second insulating layer  114  in one thermal oxidation step. In this case, the temperature, time, and other conditions of thermal oxidation and the size of the communication holes  112  are selected so as to ensure that the second insulating layer  114  reaches a sufficient thickness before the communication holes  112  are closed. 
     After the vibrating membrane  206  is placed on the first insulating layer  104 , it is allowed to bond the vibrating membrane  206  and the first insulating layer  104  by heat treatment and form the second insulating layer  114  by thermal oxidation in one heating step. A possible way to do this is as follows: the laminate of the first insulating layer  104  and the vibrating layer  206  is heated in a nitrogen-containing atmosphere at 1050° C. for 3 hours and in an oxygen-containing atmosphere at 1050° C. for additional 1 hour. Completing heat bonding and thermal oxidation in one heating step in this way can lead to a simplified manufacturing process of a CMUT with a reduced number of operations. 
     Completing heat bonding and thermal oxidation in one heating step can also lead to reduced thermal hysteresis of the resulting CMUT. In general, devices with small hysteresis are highly reliable. Completing heat bonding and thermal oxidation in one heating step therefore means improving the manufacturing yield of CMUTs. 
     Importantly, the communication portion should be open during this heating step for simultaneous heat bonding and thermal oxidation so that the cavity  108  can communicate with the outside of the CMUT. 
     As illustrated in  FIG. 4A , the communication holes  112  may be formed in the portion of the vibrating membrane  206  located above the cavity  108 . This arrangement eliminates the need to form the communication passage  106  because the cavity  108  serves as the communication passage  106 . In this case, only the communication holes  112  are regarded as the communication portion. 
     Furthermore, as illustrated in  FIG. 4B , it is allowed to form several cavities  108  linked with several communication passages  106 . This arrangement leads to a smaller number of communication holes  112  required than is necessary when the cavities  108  are isolated. 
     In this way, this manufacturing method makes it possible to produce CMUTs with highly uniform device characteristics. 
     Embodiment 2 
       FIG. 5  is a plan view of a patterned first insulating layer  104  configured in accordance with this embodiment. The difference between this embodiment and Embodiment 1 is that in this embodiment no communication holes are formed and the communication portion consists only of communication passages  106  formed during the patterning of the first insulating layer  104 . Each communication passage  106  extends from one lateral side of the CMUT to the other side through some cavities  108 , allowing the cavities  108  to communicate with the outside of the CMUT. 
       FIGS. 6A to 6C  illustrate a manufacturing process of a CMUT according to this embodiment. 
     First, as illustrated in  FIG. 6A , a patterned first insulating layer  104  is formed on a silicon substrate  102 . The first insulating layer  104  is patterned in a way that the pattern will later provide the communication passages  106  that ensure communication between the cavities  108  and outside the CMUT. A possible approach to the patterning of the first insulating layer  104  in this way is to form the first insulating layer  104  by thermal oxidation, CVD, or other suitable techniques followed by etching. In another possible approach, the first insulating layer  104  is formed using a mask layer prepared in advance, and then the mask layer is removed. 
     Then, as illustrated in  FIG. 6B , a vibrating membrane  206  and the first insulating layer  104  are bonded. 
     Through the thus-formed communication portion (the communication passages  106 ), the material(s) for second insulating layers  114  is introduced in a gaseous form. The second insulating layers  114  are formed in an atmosphere containing the gas in a way that the surfaces exposed to the cavities  108  should be coated as illustrated in  FIG. 6C . The second insulating layers  114  can be formed by thermal oxidation, CVD, or other suitable techniques. 
     After that, the communication between the cavity  108  and outside the transducer can be blocked so that the transducer can be used in liquid or under similar conditions. 
     In this way, this manufacturing method makes it possible to produce CMUTs with highly uniform device characteristics. 
     Furthermore, the manufacturing method according to this embodiment, in which no communication holes are formed and the communication portion consists only of communication passages  106  formed during the patterning of the first insulating layer  104 , includes a smaller number of operations and is simpler than that according to Embodiment 1. 
     Forming communication holes as a component of the communication portion through the vibrating membrane  206  as in Embodiment 1 may also cause damage to the vibrating membrane  206 . Damage to the vibrating membrane  206  may affect the characteristics of the resulting CMUT. 
     The method according to this embodiment, however, by which the communication portion can be formed without making communication holes in the vibrating membrane  206 , poses reduction of risk for damage to the vibrating membrane  206  associated with the formation of the communication portion. 
     Example 1 
     The following describes Example 1 of an aspect of the present disclosure with reference to  FIGS. 7A to 7G . 
     First, as illustrated in  FIG. 7A , a silicon oxide film  104  is formed on a silicon substrate  102  and thereby a base  100  is prepared. The silicon substrate  102  has a thickness of 300 μm, and its resistance is low (specific resistance ≦0.02 Ω·cm) so that it can be used as the lower electrode. This silicon substrate  102  is thermally oxidized at 1100° C. and thereby the silicon oxide film  104  is formed with a thickness of 200 nm. By this operation, the surface of the silicon substrate  102  opposite to the silicon oxide film  104  is also coated with a silicon oxide film  110 . 
     Then, a resist pattern is formed on the silicon oxide film  104 . This resist pattern will later provide a cavity  108  and a communication passage  106  that allows the cavity  108  to communicate with the outside of the transducer. The silicon oxide film  104  is then partially removed by etching with buffered hydrofluoric acid and the silicon substrate  102  is partially exposed in a way that the portions corresponding to the cavity  108  and the communication passage  106  are etched away. After removing the resist pattern, the silicon oxide film  104  is patterned as illustrated in  FIG. 7B , with the silicon oxide film  110  covered with a resist so as not to be etched. 
     Separately, as illustrated in  FIG. 7C , another silicon substrate  202  is coated with a silicon oxide layer  204  and then with a silicon film  206  and thereby a SOI base  200  is prepared. The silicon substrate  202  is a 725-μm thick handle layer, the silicon oxide layer  204  is a 400-nm thick buried oxide (BOX) layer, and the silicon film  206  is a 1-μm thick active layer. 
     Then, as illustrated in  FIG. 7D , the silicon film  206  of the SOI base  200  is bonded to the silicon oxide film  104  of the base  100 , which has the cavity  108  and the communication passage  106  formed thereon. More specifically, the base  100  and the SOI base  200  are thoroughly washed with sulfuric acid, hydrogen peroxide, hydrochloric acid, and ultrapure water so that their surfaces should be made hydrophilic, and the laminate obtained by simply placing one on the other is heated at 1000° C. for 4 hours. After the completion of this heat bonding step at 1000° C., the silicon handle layer  202  and the silicon oxide BOX layer  204  need to be removed so that the silicon film  206  of the SOI base  200  can be used as the vibrating membrane. 
     The first step is to make the silicon handle layer  202  as thin as 50 μm by backgrinding. The remaining portion of the silicon handle layer  202  is then removed by etching with a solution of tetramethylammonium hydroxide, with the silicon oxide BOX layer  204  as the etching stop layer and with the silicon oxide film  110  as the etching mask layer for the silicon substrate  102 . Subsequently, the silicon oxide BOX layer  204  is removed by etching with buffered hydrofluoric acid. During this operation to remove the silicon oxide BOX layer  204  by etching, the silicon active layer  206  serves as the etching stop layer. The silicon oxide film  110  is also etched away by this operation. 
     The silicon active layer  206 , which will later serve as a vibrating membrane, is exposed in this way. Then, as illustrated in  FIG. 7E , communication holes  112  are formed in a way that the cavity  108  can communicate with the outside of the transducer. The communication holes  112  are 6-μm diameter holes formed by patterning using a photoresist and subsequent RIE with a tetrafluoromethane gas and extend through to the communication passage  106 . 
     After removing the unnecessary portion of the photoresist, the entire structure is washed, placed into a thermal oxidation furnace, and thermally oxidized in an oxygen atmosphere at 1000° C. for 2 hours. As a result, the surface of the silicon substrate  102  exposed to the cavity  108  is coated with a thermal oxide film  114  having a thickness of 50 nm as illustrated in  FIG. 7F . By this operation, the exposed inner surface of the silicon film  206  is also coated with a silicon oxide layer  115  of a similar thickness. 
     Then, as illustrated in  FIG. 7G , a silicon nitride film  116  is formed on the silicon film  206  by plasma CVD with a thickness of 700 nm to seal the communication holes  112  and to isolate the cavity  108  from outside the transducer. 
     Example 2 
     The CMUT described in the above example can be applied to apparatuses configured to obtain subject information by means of acoustic waves (hereinafter referred to as acoustic wave analyzers). The acoustic wave analyzer uses CMUTs to receive acoustic waves from a subject and converts the electric signals generated by the CMUTs into pieces of information that indicate the coefficient of optical absorption and other optical properties of the subject, the distribution of acoustic impedance in the subject, and so forth. 
       FIG. 8A  illustrates the composition of an analyzer that operates on the photoacoustic effect. A light source  2010  generates light pulses, and these light pulses travel through an optical member  2012  composed of lenses, mirrors, fiber optics, and other components and illuminate a subject  2014 . The light-absorbing material  2016  existing in the subject  2014  absorbs the energy of the light pulses and emits photoacoustic waves  2018  that behave as acoustic waves. A probe composed of CMUTs  2020  and a housing  2022  receives the photoacoustic waves  2018 , transforms them into electric signals, and sends the signals to a signal processor  2024 . After processing by the signal processor  2024  including analog-to-digital (A/D) conversion, amplification, and so forth, the electric signals are transmitted to a data processor  2026 . The data processor  2026  processes the signals and collects pieces of information about the subject (those indicating the coefficient of optical absorption and other optical properties) in the form of image data. In this example, the signal processor  2024  and the data processor  2026  together operate as a processing unit. A display unit  2028  creates an image from the image data provided by the data processor  2026  and shows the image. 
       FIG. 8B  illustrates the composition of an analyzer that analyzes subjects by means of reflected acoustic waves, e.g., an ultrasonic diagnosis system. 
     A probe composed of CMUTs  2120  and a housing  2122  emits acoustic waves into a subject  2114 , and a portion of the acoustic waves is reflected by a reflective material  2116 . The probe in turn receives the reflected acoustic waves  2118  (reflected waves), transforms them into electric signals, and sends the signals to a signal processor  2124 . After processing by the signal processor  2124  including A/D conversion, amplification, and so forth, the electric signals are transmitted to a data processor  2126 . The data processor  2126  processes the signals and collects pieces of information about the subject (those indicating the distribution of acoustic impedance in the subject) in the form of image data. In this example, the signal processor  2124  and the data processor  2126  together operate as a processing unit. A display unit  2128  creates an image from the image data provided by the data processor  2126  and shows the image. 
     For both the analyzers illustrated in  FIGS. 8A and 8B , the probe may be a scanning unit of mechanical equipment or a hand-held scanner that can be moved manually by the user (a physician, a technician, or the like) on or above the subject. In cases where reflected waves are utilized as in  FIG. 8B , the analyzer may incorporate two separate components that serve as an emitter of acoustic waves and a receiver of reflected waves instead of the probe. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.