EP3221064A1 - Vertical gap actuator for ultrasonic transducers and fabrication of the same - Google Patents

Vertical gap actuator for ultrasonic transducers and fabrication of the same

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Publication number
EP3221064A1
EP3221064A1 EP15784792.2A EP15784792A EP3221064A1 EP 3221064 A1 EP3221064 A1 EP 3221064A1 EP 15784792 A EP15784792 A EP 15784792A EP 3221064 A1 EP3221064 A1 EP 3221064A1
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European Patent Office
Prior art keywords
membrane
substrate
layer
central part
poly
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Granted
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EP15784792.2A
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German (de)
French (fr)
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EP3221064B1 (en
Inventor
Goksen Goksenin YARALIOGLU
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Ozyegin Universitesi
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Ozyegin Universitesi
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/0292—Electrostatic transducers, e.g. electret-type

Definitions

  • the present invention relates generally to the field of ultrasonic transducers. More specifically it is related to electrostatically driven capacitive micromachined ultrasonic transducers (CMUTs). STATE OF THE ART
  • CMUTs capacitive micromachined ultrasonic transducers
  • a CMUT transducer is composed of many movable membranes which are actuated by electrostatic forces. These membranes are manufactured in such a way that the cavities at the rear side of them constitute vacuum. The cavity height or the distance between the bottom and the top electrode determines the performance parameters such as the sensitivity, output pressure of the transducer.
  • the membranes are built using micromachining techniques on a highly doped silicon substrate which forms the bottom electrode. There is usually a metal electrode on the membrane. The top metal electrode and the bottom electrode constitute a parallel plate actuator for actuating the membrane. During operation, a high DC electrical voltage is applied between the substrate and the top electrode. Due to the electrostatic force, the membrane is attracted towards the bottom electrode. The induced stress inside the membrane resists the electrostatic force.
  • the membrane moves up and down at the same frequency as the AC voltage. This movement of the membrane generates ultrasound in the transducer immersion medium. If the biased membranes are subject to an incoming ultrasound field below the DC voltage, the membrane vibrates at the frequency of the incoming field and an electrical current at the frequency of the incoming field is generated.
  • the mechanical impedance of the CMUT transducer consisting of membranes is much smaller than that of piezoelectric materials. This enables operation of CMUT transducers with much larger bandwidth.
  • the lithography defines the membrane shape and the geometry of array elements, much complex array geometries such as rings, annular arrays can be fabricated.
  • CMUTs have also been demonstrated for high frequency applications such as intravascular imaging [18] and 3D volumetric imaging [19].
  • the literature on the CMUTs is immense, only a few of them have been cited above to show the variety of the work that has been performed up to date.
  • CMUT transducers output pressure and receive sensitivity have conflicting requirements over the cavity height (gap).
  • the output pressure is fundamentally limited by the cavity height (distance between the bottom surface of the membrane and the top surface of the substrate).
  • the membrane cannot travel more than the cavity height. Therefore, to be able to generate high output pressure levels one needs to build transducers with large cavity heights.
  • high receive sensitivity requires high e-fields at small cavity heights. This can be achieved by reducing the cavity height. Therefore, high output pressure and high receive sensitivity cannot be obtained at the same time from the same transducer geometry.
  • CMUT transducers also generate high level of non-linearity due to the parallel plate actuation. As the membrane displaces when the electrical field inside the gap changes, this results in spring softening effect and shifts the resonant frequency. As a result, high levels of non-linearity are generated.
  • a thick part in form of a piston is added to the middle part of the membrane. This part will move within a cavity opened on the substrate.
  • the movement of the membrane is achieved by an electric field created on the gap between the piston-shaped part of the membrane and the side walls of the cavity in the substrate.
  • the motion of the membrane does not affect the height of the gap where the actuation forces are built.
  • the gaps between the thin part of the membrane and the bottom part of the piston and the substrate are selected too large, these do not interfere with the membrane motion. This enables membrane to move without any hard limits.
  • the proposed geometry can be fabricated using micromachining techniques and transducers can be manufactured by repeating the cellular structure.
  • an electromechanical micromachined transducer comprising
  • the cellular structure comprising:
  • a free movable membrane consisting of a thin peripheral part and a thick central part
  • the membrane consists of single crystal silicon, poly-silicon layer or both.
  • the insulating layer consists of silicon dioxide.
  • the substrate consists of single crystal silicon, glass or quartz.
  • the membranes have an electrically connected structure to form ID, 2D or annular arrays.
  • an electromechanical micromachined transducer production method comprising the following processing steps:
  • FIGURE 1 shows the proposed geometry for the vertical gap CMUT cell.
  • FIGURE 2 shows the electrical field lines between the side walls of the first silicon wafer gap and the piston part of the membrane.
  • FIGURE 3 shows the geometry of a standard CMUT cell.
  • FIGURE 4 shows the parallel plate modeling of a standard CMUT cell.
  • FIGURE 5 shows the equivalent circuit modeling of a standard CMUT cell.
  • FIGURE 6 shows the simplified modeling of the vertical gap CMUT.
  • FIGURE 7 shows the equivalent circuit modeling of the vertical gap CMUT.
  • FIGURE 8.8, FIGURE 8.9, FIGURE 8.10, FIGURE 8.11, FIGURE 8.12, show a method to fabricate vertical gap CMUTs.
  • FIGURE 9.1, FIGURE 9.2, FIGURE 9.3, FIGURE 9.4, FIGURE 9.5, FIGURE 9.6, FIGURE 9.7, FIGURE 9.8, FIGURE 9.9, show another method to fabricate vertical gap CMUTs.
  • FIGURE 10.1, FIGURE 10.2 AND FIGURE 10.3 show ID, 2D and annular transducer arrays.
  • the main objective of this invention is to develop novel cell geometry for
  • CMUT transducers where the sensitivity and the maximum output pressure do not have conflicting requirements over the gap.
  • electrodes instead of parallel plate actuation, electrodes will be positioned such that the cavity height does not change with the membrane (100) motion.
  • an electrostatic gap is formed vertically in the capacitor formed by a piston like part of the membrane (100) with the side walls of the gap.
  • Fig. l This new geometry is shown in Fig. l.
  • the actuation of the membrane (100) is achieved through a vertical gap (101).
  • the membrane (100) is composed of two parts: thick central part (piston part) (103) and thin peripheral part (108).
  • the thin peripheral part (108) is more flexible than the thick central part (103).
  • the thick central part (the piston part) (103) has a vertical side wall (1001) as shown in the Fig. l.
  • the membrane (100) could be in circular, square or rectangular shape. It could be made of a highly conducting material, preferably highly doped silicon. To increase the electrical conductivity, a metal electrode (not shown) could be deposited on top of the membrane (100).
  • the substrate (102) which can be preferably silicon wafer is etched to form a cavity (105) with several steps as shown in Fig.l.
  • Cavity (105) is composed of three different gaps: horizontal gap 1 (106) under the thin peripheral (108) part of the membrane (100), horizontal gap 2 (107) under the thick central part (the piston part) (103) of the membrane (100) and vertical gap (101) between the vertical side walls (104) of the substrate (102) and the thick central part side walls (1001).
  • First silicon wafer namely substrate (102) is also made of a highly conducting material, preferably highly doped silicon.
  • Membrane (100) is attached to the substrate (102) through an insulating material (109) whose electrical impedance is very high.
  • the insulating material is preferably made of silicon dioxide.
  • the thick central part (103) can move freely in the cavity (105) of the substrate (102). When a voltage is applied between the membrane (100) and the substrate (102), electrical field is formed between the membrane (100) and the substrate (102) over the cavity (105).
  • Fig. 2 shows electric field formed in the gap when there is voltage applied between the membrane (100) and the substrate (102).
  • the vertical gap (101) is substantially smaller than the horizontal gap 1 (106) and the horizontal gap 2 (107). Therefore there is a stronger electrostatic force between the side walls (104) of the substrate (102) and the thick central part sidewalls (1001).
  • This electric field has two components as shown in Fig 2: electric field horizontal component (110) and fringing fields (111).
  • the horizontal component does not move the membrane (100) because it is balanced around the perimeter of the piston.
  • fringing fields (111) between the thick central part (103) and the side walls (104) of the substrate (102) in horizontal gaps as shown in Fig. 2 can apply a net force on the thick central part (103), moving it towards the substrate (102).
  • the gaps between the membrane (100) thin peripheral part (108) and the thick central part (103) and the substrate (102) can be chosen substantially larger than the maximum membrane (100) stroke such that they these two gaps do not interfere with the membrane (100) motion.
  • Horizontal gap 1 (106) and horizontal gap 2 (107) can be different from each other and horizontal gap 2 (107) can be chosen much larger than horizontal gap 1 (106).
  • Fig.3 shows the schematic of a standard CMUT cell.
  • the CMUT is composed of a membrane (100), substrate (102) and an insulator layer (109) which electrically isolates the membrane (100) and the substrate (102).
  • the cavity (105) is obtained by etching into the substrate (102).
  • the bottom of the cavity and the membrane (100) forms a parallel plate actuator.
  • the gap distance (117) changes with the applied voltage.
  • the main difference between the geometry constituting the object of the invention disclosed in this patent application and the geometry shown in Fig 3 is that the vertical gap (101) of Fig. l can be chosen as small as possible for increased transmit and receive sensitivity.
  • the important feature of this design is that the selection of the gap does not impose any limits on the maximum output pressure.
  • Fig. 4 shows the simplified model for a standard CMUT cell.
  • the membrane (100) is modelled by a spring (119) and a parallel plate electrode (118).
  • the electrode area is assumed to be A.
  • the gap distance is denoted by g.
  • the details of the calculation of the parameters for the equivalent circuit have been given in [15, 22] for a standard CMUT cell.
  • Fig. 5 shows the equivalent circuit model for the standard CMUT cell.
  • the transformer (121) connects electrical domain to mechanical domain.
  • electrical capacitor (120) represents the electrical parallel capacitance of the CMUT.
  • the membrane (100) impedance is modelled by a series (array) combination of a capacitor (123) and an inductor (124).
  • the spring softening is modelled by another capacitor (122).
  • the radiation losses are modelled by a resistor (125) which is terminated the acoustic port of the transducer.
  • Fig 6 shows the simplified model for vertical gap CMUT.
  • the membrane (100) is modelled by a spring (119) and a piston (126). This time the electrodes are on the side of the piston.
  • the piston radius, namely the membrane radius is denoted by r p .
  • h denotes the length of the electrodes.
  • the equivalent circuit is composed of an electrical capacitor (120) in the electrical part, a transformer (121), a series capacitor (123) and an inductor (124).
  • the output port is again terminated by a resistor (125).
  • the main difference between this circuit and the circuit shown in Fig 5 is that this circuit does not have any spring softening capacitor.
  • the main reason for the lack of spring softening is that the motion of the membrane (100) does not change the gap distance.
  • the calculation of the circuit parameters are given in Table 1. Assuming electrical capacitance and the membrane impedance can be made the same for both types of CMUTs, the main difference between the two circuits is the transformer ratio and the spring softening capacitance.
  • the transformer ratio represents the conversion efficiency between electrical and mechanical domains.
  • the electrode radius (r p ) of the standard CMUT is the same as the piston radius of vertical gap CMUT.
  • g st and g v represents the gap height for standard and vertical gap CMUTs, respectively.
  • 10 micro-meter radius and 0.5 micro-meter standard gap CMUT if one can build a vertical gap device with 50 nm gap, it is possible to get similar transformation ratios. Based on this first order approach, one can achieve similar sensitivities using both devices.
  • Fig. 8.1, Fig. 8.2, Fig. 8.3, Fig. 8.4, Fig. 8.5, Fig. 8.6, Fig. 8.7, Fig. 8.8, Fig. 8.9, Fig. 8.10, Fig. 8.11, Fig. 8.12 show a process to build vertical gap CMUT where the vertical gap height can be made substantially small.
  • the fabrication process starts with a highly doped silicon wafer, namely a substrate (102).
  • the conductivity of the starting substrate (102) should be high because this wafer will provide the side wall electrodes required to actuate the membrane (100).
  • the first silicon wafer (102) is etched as shown in Fig 8.2. For this etch first the substrate (102) is coated by a photoresist.
  • the photoresist is patterned to define the gaps as shown in Fig 8.2.
  • a thin sacrificial layer of dry oxide (134) is grown over the wafer surface. This oxide layer will be later etched to define the vertical gap height.
  • the gap height is substantially small, between 50 nm to 200 nm. To define such a small gap height a well-controlled dry oxide deposition is preferred.
  • Fig 8.4 demonstrates poly-silicon layer (135) deposition. LPCVD method can be used to deposit poly-silicon.
  • poly-silicon should be doped to achieve high conductivity.
  • the poly-silicon is etched back down to oxide layer, leaving poly-silicon only in the cavities etched in Fig 8.2.
  • Poly- silicon can be etched using dry etching or chemical wafer polishing (CMP).
  • CMP chemical wafer polishing
  • more poly-silicon is deposited and is patterned. This part of the poly-silicon will be part of the piston.
  • a thick conformal oxide layer (136) is grown over the wafer surface. This oxide layer will provide the insulation layer between the membrane (100) and the substrate (102).
  • the thick oxide layer is patterned as shown in the figure. This also defines the membrane diameter.
  • the wafer surface is planarized using CMP process (Fig. 8.9). After planarization, an SOI wafer is bonded to the top surface of the wafer (Fig. 8.10). After removing the handle and the box oxide, a membrane (100) is formed.
  • the back side of the wafer is etched using deep reactive ion etching down to the oxide layer as shown in Fig 8.11. This step exposes the oxide layer that is filling the vertical gap. At step 12, the vertical side wall is etched until it is completely cleared of oxide.
  • the oxide etching can be done using vapor phase HF or wet BOE (buffered oxide etch).
  • step 12 the main structure of the membrane (100) is completed.
  • the bottom side of the wafer can be bonded to another wafer at low pressure to evacuate the air in the back side of the membrane cavity (the reason for naming this wafer second wafer is the evaluation of the substrate (102) as the first wafer).
  • Further metallization steps can be found in CMUT fabrication references [11, 12] and same methods can be employed here.
  • Fig 9.1, Fig. 9.2, Fig. 9.3, Fig. 9.4, Fig. 9.5, Fig. 9.6, Fig. 9.7, Fig. 9.8 and Fig. 9.9 shows an alternative approach which also starts with a highly doped silicon wafer (102).
  • the surface of the wafer is etched to form a trench as shown in the figure. This trench will be filled with poly-silicon to form the piston part of the membrane (100) in the subsequent processing steps.
  • step 3 a thin sacrificial layer of dry oxide (134) is grown on the silicon wafer surface. Dry oxidation is the method of choice for this step due to its high uniformity and well- controlled thickness.
  • a thick layer of poly-silicon layer (135) is deposited.
  • the poly-silicon layer is patterned and etched using reactive ion etching.
  • the wafer surface is covered by a thin layer of oxide (136).
  • this oxide layer is patterned and etched. The oxide pattern at this step determines the diameter of the membrane (100).
  • the wafer is polished using CMP process. The purpose of the polishing is to planarize the wafer surface by bringing the oxide part and the poly- silicon islands to the same height for the subsequent bonding process.
  • an SOI wafer is bonded on top of the wafer.
  • the device layer (142) is attached to the oxide layer (136) and the poly-silicon part (135).
  • step 8 the back side of the wafer is patterned and etched until the thin dry oxide sacrificial layer (134) is exposed using deep reactive ion etching.
  • the thin oxide layer covering the bottom part of the poly- silicon layer (135) and the vertical gap is etched and the membrane (100) is released.
  • the back side of the wafer is bonded to another wafer at low pressure to seal the gaps and electrodes are fabricated to provide electrical interface to the mechanical structure.
  • Figure 10.1, Figure 10.2 and Figure 10.3 shows various transducer array geometries where the cells of the transducers are fabricated as explained above.
  • the array geometries consist of electrodes providing electrical connections to the membrane (100) and the substrate (102).
  • the array geometries are not limited to the ones that are shown in this figure.
  • array elements (143) belonging to a linear ID array are formed by electrically isolating the membranes (100) on the substrate (102).
  • the boundary of the isolation layer defines the membranes (100) which are shown by dotted lines in the figure.
  • Fig 10.2 shows array elements (146) belonging to a 2D array where the membrane layer is patterned to form a 2D array of transducer elements.
  • Fig.10.3 shows an array formed by the array elements (147) belonging to an annular array. In all these configurations, the array geometry is defined by patterning the conducting membrane layer.
  • PROC IEEE ULTRASON. SYMP., 1994, PP. 1241-1244.
  • ORALKAN O. ; BAYRAM, B . ; YARALIOGLU, G.G. ; ERGUN, A.S. ; KUPNIK, M. ; YEH, D.T. ; WYGANT, I.O. ; KHURI- YAKUB , B .T., "EXPERIMENTAL CHARACTERIZATION OF COLLAPSE-MODE CMUT OPERATION" ULTRASONICS, FERROELECTRICS AND FREQUENCY CONTROL, IEEE TRANSACTIONS ON, VOLUME: 53 , ISSUE: 8, PAGE(S): 1513 - 1523, 2006.

Abstract

The invention is related to the capacitive micromachined ultrasonic transducers (CMUT) actuated electrostatically.

Description

VERTICAL GAP ACTUATOR FOR ULTRASONIC TRANSDUCERS AND
FABRICATION OF THE SAME
TECHNICAL FIELD
The present invention relates generally to the field of ultrasonic transducers. More specifically it is related to electrostatically driven capacitive micromachined ultrasonic transducers (CMUTs). STATE OF THE ART
Ultrasound was first used for medical diagnosis in 1942 [1, 2] and pulse-echo measurements were first demonstrated in 1950s for tissue characterization [3-5]. Since 1970s, the advances in microelectronics, composite piezoelectric materials [6], and piezoelectric array fabrication technologies resulted in high-resolution ultrasound imaging devices, which have become a widely used tool for medical diagnostics. Piezoelectric materials have dominated the medical ultrasound field. However, despite the wide use, piezoelectric transducers have two important limitations: (1) Bandwidth is limited due to high acoustic impedances of piezoelectric materials. Reduced bandwidth results in poor temporal resolution (therefore poor spatial resolution). (2) The manufacturing process is not compatible with monolithic CMOS integration, which increases the cost and limits the array size and building of complex array geometries. Because the manufacturing technique is limited to cutting (dicing) the piezocomposite block in linear directions, only array elements with rectangular shapes can be produced. To improve the bandwidth of the piezoelectric transducers complex acoustic matching materials are used. This increases the cost of the transducers.
In order to address the limitations of piezoelectric transducers, capacitive micromachined ultrasonic transducers (CMUTs) have been proposed [7]. CMUTs have generated large interest from the MEMS community and offered key performance benefits in terms of bandwidth, scaling to large array sizes, and integration with CMOS compared to its piezoelectric counterparts.
A CMUT transducer is composed of many movable membranes which are actuated by electrostatic forces. These membranes are manufactured in such a way that the cavities at the rear side of them constitute vacuum. The cavity height or the distance between the bottom and the top electrode determines the performance parameters such as the sensitivity, output pressure of the transducer. The membranes are built using micromachining techniques on a highly doped silicon substrate which forms the bottom electrode. There is usually a metal electrode on the membrane. The top metal electrode and the bottom electrode constitute a parallel plate actuator for actuating the membrane. During operation, a high DC electrical voltage is applied between the substrate and the top electrode. Due to the electrostatic force, the membrane is attracted towards the bottom electrode. The induced stress inside the membrane resists the electrostatic force. If an AC voltage is applied on top of the DC voltage, the membrane moves up and down at the same frequency as the AC voltage. This movement of the membrane generates ultrasound in the transducer immersion medium. If the biased membranes are subject to an incoming ultrasound field below the DC voltage, the membrane vibrates at the frequency of the incoming field and an electrical current at the frequency of the incoming field is generated. The mechanical impedance of the CMUT transducer consisting of membranes is much smaller than that of piezoelectric materials. This enables operation of CMUT transducers with much larger bandwidth. In addition, since the lithography defines the membrane shape and the geometry of array elements, much complex array geometries such as rings, annular arrays can be fabricated. Furthermore, potential of integration with CMOS electronics enables new imaging modalities as well as cost reduction. Since the first introduction of CMUTs, extensive research has been performed on fabrication, modeling and applications. Initially sacrificial release process has been used for fabrication [8, 9]. Later, wafer bonding technique has been introduced [10]. Variations of these two techniques and other fabrication approaches have been reviewed in [11]. An equivalent circuit based approach has been used for modeling of device parameters [12-14]. FEM methods resulted in more accurate device characterization and revealed sources of cross talk [15-17]. CMUTs have also been demonstrated for high frequency applications such as intravascular imaging [18] and 3D volumetric imaging [19]. The literature on the CMUTs is immense, only a few of them have been cited above to show the variety of the work that has been performed up to date.
However, number of commercial ultrasound imaging systems based on CMUTs has been limited despite the large research effort in this area during the last 20 years. Main challenges of CMUTs can be traced to lack of high sound pressure generation, low receive sensitivity and highly nonlinear behavior of the parallel plate actuation.
In CMUT transducers output pressure and receive sensitivity have conflicting requirements over the cavity height (gap). The output pressure is fundamentally limited by the cavity height (distance between the bottom surface of the membrane and the top surface of the substrate). The membrane cannot travel more than the cavity height. Therefore, to be able to generate high output pressure levels one needs to build transducers with large cavity heights. On the other hand, high receive sensitivity requires high e-fields at small cavity heights. This can be achieved by reducing the cavity height. Therefore, high output pressure and high receive sensitivity cannot be obtained at the same time from the same transducer geometry.
CMUT transducers also generate high level of non-linearity due to the parallel plate actuation. As the membrane displaces when the electrical field inside the gap changes, this results in spring softening effect and shifts the resonant frequency. As a result, high levels of non-linearity are generated.
To solve above problems various solutions have been proposed in the literature. Using piston-shaped membranes have been proposed to achieve piston like membrane motion to increase output pressure [23, 24]. Collapse-snap back operation [25] and collapse mode [26, 27] have been introduced also to increase the output pressure. However, these approaches had marginally increased the output pressure and they also suffered from instabilities due to electric breakdown (break-down) issues. Double electrode configuration has been also proposed to separate transmit and receive signal paths [28]. With this approach, two electrodes were placed onto the membrane. From these, the electrode provided at the center of the membrane is used to collect ultrasonic signals. The electrode around the central electrode, due to a bias voltage applied on it, bends the membrane toward the substrate and reduces the gap between the central electrode and the substrate. This helped improving receive sensitivity, but high electrical cross talk between the receive and the transmit paths and increased number of connections resulted in more complex electronics limiting the usability of the approach. In another approach, Ying proposed adding a flat electrode which moves like a piston [29] in the CMUT cavity. In their publication they did not report the output pressure but they reported only 70% increase in the receive voltage. The disadvantage of this method is the increased complexity of the fabrication methods. Huang proposed using a comb drive [30]. This structure can potentially increase the output pressure but the method does not address building very small vertical gaps and does not propose a solution for separating the requirements for maximum output pressure and increased sensitivity. In addition, building multiple fingers as described in Huang's invention [30] is difficult to achieve.
None of the above approaches addressed the fundamental problem which is the conflicting requirement of receive sensitivity and maximum output pressure on the cavity height, constituting the reason for the low performance of the CMUTS. There is a need for a new actuation method for the CMUT transducers that will separate the dependence of the output pressure and receive sensitivity on the gap.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a new geometry that will solve the low output pressure problem of CMUTs. In this geometry, a thick part in form of a piston is added to the middle part of the membrane. This part will move within a cavity opened on the substrate. The movement of the membrane is achieved by an electric field created on the gap between the piston-shaped part of the membrane and the side walls of the cavity in the substrate. In this geometry, the motion of the membrane does not affect the height of the gap where the actuation forces are built. Also, as the gaps between the thin part of the membrane and the bottom part of the piston and the substrate are selected too large, these do not interfere with the membrane motion. This enables membrane to move without any hard limits. The proposed geometry can be fabricated using micromachining techniques and transducers can be manufactured by repeating the cellular structure.
In order to achieve the aforementioned object, an electromechanical micromachined transducer has been developed, comprising
a cellular structure and
multiple elements each comprising at least one of said cellular structures, the cellular structure comprising:
a free movable membrane consisting of a thin peripheral part and a thick central part,
an insulating layer supporting the membrane,
an incised substrate bound to the membrane by the insulating layer, second wafer bound to the substrate,
vertical gaps formed between the side walls of the piston part of the membrane and the incised substrate, and
horizontal gaps formed between the membrane thin peripheral part and the bottom part of the thick central part and the incised substrate and being larger than the vertical gap.
In a preferred embodiment of said electromechanical micromachined transducer, the membrane consists of single crystal silicon, poly-silicon layer or both.
In a preferred embodiment of said electromechanical micromachined transducer, the insulating layer consists of silicon dioxide.
In a preferred embodiment of said electromechanical micromachined transducer, the substrate consists of single crystal silicon, glass or quartz.
In a preferred embodiment of said electromechanical micromachined transducer, the membranes have an electrically connected structure to form ID, 2D or annular arrays. In order to achieve the aforementioned object, also an electromechanical micromachined transducer production method has been developed, comprising the following processing steps:
etching two cavities on the substrate to form the thick central part of the membrane,
depositing substantially a thin dry oxide sacrificial layer on the substrate to form a smaller vertical gap with respect to the horizontal gaps, filling the cavities to form the thick central part of the membrane by depositing the poly-silicon layer,
depositing a second poly-silicon layer on the substrate,
depositing a second thick oxide insulating layer and patterning it to form a support to the membrane,
polishing the surface to planarize the second oxide insulating layer and the second poly-silicon layer,
bonding an SOI wafer having a device layer on it to the substrate, etching the handle layer of the SOI wafer and the dry oxide layer, drilling a hole at the rear side of the substrate to enable the thick central part of the membrane to move freely,
etching the dry oxide sacrificial layer from the opened hole,
bonding the second wafer to the rear side of the substrate in vacuum environment to form a vacuum sealed cavity at the rear side of the membrane,
providing electrical contacts to the substrate and the membrane. In order to achieve the aforementioned object, also an electromechanical micromachined transducer production method has been developed, comprising the following processing steps:
etching a single cavity on the substrate to form the thick central part of the membrane,
depositing substantially a thin dry oxide sacrificial layer on the substrate to form a smaller vertical gap with respect to the horizontal gaps, filling the cavities to form the thick central part of the membrane by depositing the poly-silicon layer,
- depositing a second poly-silicon layer on the substrate,
depositing a second thick oxide insulating layer and patterning it to form a support to the membrane,
polishing the surface to planarize the second oxide insulating layer and the second poly-silicon layer,
- bonding an SOI wafer having a device layer on it to the substrate,
etching the handle layer of the SOI wafer and the dry oxide layer, drilling a hole at the rear side of the substrate to enable the thick central part of the membrane to move freely,
etching the dry oxide sacrificial layer from the opened hole,
- bonding the second wafer to the rear side of the substrate in vacuum environment to form a vacuum sealed cavity at the rear side of the membrane,
providing electrical contacts to the substrate and the membrane. Structural and characteristic properties and all advantages of the invention will be understood more clearly due to the following drawings and the detailed disclosure written by referring to these drawings and therefore the evaluation has to be made by taking into account these drawings and the detailed disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 shows the proposed geometry for the vertical gap CMUT cell. FIGURE 2 shows the electrical field lines between the side walls of the first silicon wafer gap and the piston part of the membrane.
FIGURE 3 shows the geometry of a standard CMUT cell.
FIGURE 4 shows the parallel plate modeling of a standard CMUT cell.
FIGURE 5 shows the equivalent circuit modeling of a standard CMUT cell.
FIGURE 6 shows the simplified modeling of the vertical gap CMUT.
FIGURE 7 shows the equivalent circuit modeling of the vertical gap CMUT.
FIGURE 8.1, FIGURE 8.2, FIGURE 8.3, FIGURE 8.4, FIGURE 8.5, FIGURE 8.6, FIGURE 8.7,
FIGURE 8.8, FIGURE 8.9, FIGURE 8.10, FIGURE 8.11, FIGURE 8.12, show a method to fabricate vertical gap CMUTs.
FIGURE 9.1, FIGURE 9.2, FIGURE 9.3, FIGURE 9.4, FIGURE 9.5, FIGURE 9.6, FIGURE 9.7, FIGURE 9.8, FIGURE 9.9, show another method to fabricate vertical gap CMUTs.
FIGURE 10.1, FIGURE 10.2 AND FIGURE 10.3 show ID, 2D and annular transducer arrays.
It is not necessary to scale the drawings and the details which are not necessary for understanding the present invention can be neglected. Beside this, at least substantially identical elements or at least substantially identical functionalized elements are shown with the same numbers.
Reference List
100. Membrane
1001. Side walls of the thick central part
101. Vertical gap
102. Substrate
103. Thick central part
104. Vertical side walls of the substrate
105. Cavity
106. Horizontal gap 1
107. Horizontal gap 2
108. Thin peripheral part
109. Insulating layer
110. Horizontal component of the electric field
111. Fringing field
117. Gap distance
118. Parallel plate electrode
119. Spring
120. Electrical capacitor
121. Transformer
122. Negative capacitor
123. Series capacitor 124. Inductor
125. Resistor
126. Piston
134. Dry oxide sacrificial layer
135. Poly- silicon layer
136. Oxide layer
142. Device layer
143. Array elements belonging to ID array
Array elements belonging to 2D array
Array elements belonging to annular array o: Device capacitance
VDC: Voltage
V: DC Voltage
£Q: Dielectric permittivity of vacuum
A: Electrode area
-cs Capacitance of the softening of spring constant
cm: Capacitance of spring constant
cso Capacitance corresponding to spring constant together with softening
„,; Inductance
zm Current r. Resistance corresponding to losses
n: Transformer ratio
h: Piston height
g: Gap distance
k: Spring constant
rp: Membrane radius
x: Distance
DETAILED DESCRIPTION OF THE INVENTION
The main objective of this invention is to develop novel cell geometry for
CMUT transducers where the sensitivity and the maximum output pressure do not have conflicting requirements over the gap. To achieve this, instead of parallel plate actuation, electrodes will be positioned such that the cavity height does not change with the membrane (100) motion. In this completely novel geometry, an electrostatic gap is formed vertically in the capacitor formed by a piston like part of the membrane (100) with the side walls of the gap. This new geometry is shown in Fig. l. In this Figure, the actuation of the membrane (100) is achieved through a vertical gap (101). The membrane (100) is composed of two parts: thick central part (piston part) (103) and thin peripheral part (108). The thin peripheral part (108) is more flexible than the thick central part (103). The thick central part (the piston part) (103) has a vertical side wall (1001) as shown in the Fig. l. When a force applied along the axis of the membrane (100), bending is observed at the thin peripheral part (108) of the membrane (100). The membrane (100) could be in circular, square or rectangular shape. It could be made of a highly conducting material, preferably highly doped silicon. To increase the electrical conductivity, a metal electrode (not shown) could be deposited on top of the membrane (100). The substrate (102) which can be preferably silicon wafer is etched to form a cavity (105) with several steps as shown in Fig.l. Cavity (105) is composed of three different gaps: horizontal gap 1 (106) under the thin peripheral (108) part of the membrane (100), horizontal gap 2 (107) under the thick central part (the piston part) (103) of the membrane (100) and vertical gap (101) between the vertical side walls (104) of the substrate (102) and the thick central part side walls (1001). First silicon wafer , namely substrate (102) is also made of a highly conducting material, preferably highly doped silicon. Membrane (100) is attached to the substrate (102) through an insulating material (109) whose electrical impedance is very high. The insulating material is preferably made of silicon dioxide. The thick central part (103) can move freely in the cavity (105) of the substrate (102). When a voltage is applied between the membrane (100) and the substrate (102), electrical field is formed between the membrane (100) and the substrate (102) over the cavity (105).
Fig. 2 shows electric field formed in the gap when there is voltage applied between the membrane (100) and the substrate (102). The vertical gap (101) is substantially smaller than the horizontal gap 1 (106) and the horizontal gap 2 (107). Therefore there is a stronger electrostatic force between the side walls (104) of the substrate (102) and the thick central part sidewalls (1001). This electric field has two components as shown in Fig 2: electric field horizontal component (110) and fringing fields (111). The horizontal component does not move the membrane (100) because it is balanced around the perimeter of the piston. However, fringing fields (111) between the thick central part (103) and the side walls (104) of the substrate (102) in horizontal gaps as shown in Fig. 2 can apply a net force on the thick central part (103), moving it towards the substrate (102). The gaps between the membrane (100) thin peripheral part (108) and the thick central part (103) and the substrate (102) can be chosen substantially larger than the maximum membrane (100) stroke such that they these two gaps do not interfere with the membrane (100) motion. Horizontal gap 1 (106) and horizontal gap 2 (107) can be different from each other and horizontal gap 2 (107) can be chosen much larger than horizontal gap 1 (106).
Fig.3 shows the schematic of a standard CMUT cell. The CMUT is composed of a membrane (100), substrate (102) and an insulator layer (109) which electrically isolates the membrane (100) and the substrate (102). The cavity (105) is obtained by etching into the substrate (102). The bottom of the cavity and the membrane (100) forms a parallel plate actuator. When the membrane (100) is actuated by an applied voltage between the membrane (100) and the substrate (102), the gap distance (117) changes with the applied voltage. The main difference between the geometry constituting the object of the invention disclosed in this patent application and the geometry shown in Fig 3 is that the vertical gap (101) of Fig. l can be chosen as small as possible for increased transmit and receive sensitivity. The important feature of this design is that the selection of the gap does not impose any limits on the maximum output pressure.
To the first order, the proposed geometry can be analysed using parallel plate theory and equivalent circuit approach. Fig. 4 shows the simplified model for a standard CMUT cell. In Fig. 4 the membrane (100) is modelled by a spring (119) and a parallel plate electrode (118). The electrode area is assumed to be A. The gap distance is denoted by g. The details of the calculation of the parameters for the equivalent circuit have been given in [15, 22] for a standard CMUT cell.
Fig. 5 shows the equivalent circuit model for the standard CMUT cell. The transformer (121) connects electrical domain to mechanical domain. On the electrical side electrical capacitor (120) represents the electrical parallel capacitance of the CMUT. On the mechanical side the membrane (100) impedance is modelled by a series (array) combination of a capacitor (123) and an inductor (124). The spring softening is modelled by another capacitor (122). The radiation losses are modelled by a resistor (125) which is terminated the acoustic port of the transducer.
Fig 6 shows the simplified model for vertical gap CMUT. The membrane (100) is modelled by a spring (119) and a piston (126). This time the electrodes are on the side of the piston. The piston radius, namely the membrane radius is denoted by rp. h denotes the length of the electrodes. Following the same technique, one can build an equivalent circuit for the vertical gap CMUTs. One such circuit is shown in Fig. 7. Similar to the circuit shown in Fig. 5, the equivalent circuit is composed of an electrical capacitor (120) in the electrical part, a transformer (121), a series capacitor (123) and an inductor (124). The output port is again terminated by a resistor (125). The main difference between this circuit and the circuit shown in Fig 5 is that this circuit does not have any spring softening capacitor. The main reason for the lack of spring softening is that the motion of the membrane (100) does not change the gap distance.
The calculation of the circuit parameters are given in Table 1. Assuming electrical capacitance and the membrane impedance can be made the same for both types of CMUTs, the main difference between the two circuits is the transformer ratio and the spring softening capacitance. The transformer ratio represents the conversion efficiency between electrical and mechanical domains. By using the equations shown in Table 1, one can calculate the ratio of the transformer n's, as following:
9 o 2
nvertical _ ° t
nst r P8v In the above equation, the electrode radius (rp) of the standard CMUT is the same as the piston radius of vertical gap CMUT. gst and gv represents the gap height for standard and vertical gap CMUTs, respectively. For 10 micro-meter radius and 0.5 micro-meter standard gap CMUT, if one can build a vertical gap device with 50 nm gap, it is possible to get similar transformation ratios. Based on this first order approach, one can achieve similar sensitivities using both devices.
Table 1. First order modeling for the vertical gap CMUT cell. Another, interesting feature of the vertical gap device is that there is no collapse phenomena and spring softening effect. For standard CMUT cells, as the bias voltage increases, the membrane (100) gets closer to the substrate (102). Beyond certain voltage, the membrane (100) collapses onto the substrate (102). The voltage where the membrane (100) collapses is called collapse voltage. This is due to the fact that the electrostatic force gradient increases as the bias voltage increases and after certain voltage, the force gradient exceeds the spring constant of the membrane (100) and the membrane (100) is pulled towards the substrate. For vertical gap devices, because the electrostatic force is independent of membrane displacement x), the force gradient is always zero. Therefore there is no collapse voltage associated with these devices. For standard CMUT cells, there is also spring softening effect which is due to the fact that electrostatic force gradient is not zero. When the membrane (100) moves in an electric field, the apparent spring constant of the membrane (100) reduces. This is called spring softening effect. For vertical gap CMUTs, because the electric force gradient is zero, there is no spring softening effect. Lack of spring softening effect increases the linearity of the vertical gap device. The relation between the membrane (100) motion and the applied voltage is quadratic; therefore a simple algorithm can be designed to reduce the harmonic content of output pressure from vertical gap CMUTs.
In the following, it is explained how to fabricate the membrane (100) geometry array disclosed in this patent by using two exemplary silicon micromachining methods. The main purpose of both methods is to fabricate a very thin vertical gap (101) between the walls of the gap at the rear side of the membrane (100) and the thick central part (piston part) (103) of the membrane (100). This small vertical gap (101) is achieved by growing a thin dry oxide sacrificial layer (134) and etching this layer from the back side of the wafer.
Fig. 8.1, Fig. 8.2, Fig. 8.3, Fig. 8.4, Fig. 8.5, Fig. 8.6, Fig. 8.7, Fig. 8.8, Fig. 8.9, Fig. 8.10, Fig. 8.11, Fig. 8.12 show a process to build vertical gap CMUT where the vertical gap height can be made substantially small. The fabrication process starts with a highly doped silicon wafer, namely a substrate (102). The conductivity of the starting substrate (102) should be high because this wafer will provide the side wall electrodes required to actuate the membrane (100). The first silicon wafer (102) is etched as shown in Fig 8.2. For this etch first the substrate (102) is coated by a photoresist. Then the photoresist is patterned to define the gaps as shown in Fig 8.2. One can also start with an SOI wafer where the gap depth is determined by the device layer of the SOI (Silicon On Insulator) wafer. This provides more uniform gap depth across the wafer. In the next step (Fig 8.3), a thin sacrificial layer of dry oxide (134) is grown over the wafer surface. This oxide layer will be later etched to define the vertical gap height. The gap height is substantially small, between 50 nm to 200 nm. To define such a small gap height a well-controlled dry oxide deposition is preferred. Fig 8.4 demonstrates poly-silicon layer (135) deposition. LPCVD method can be used to deposit poly-silicon. During poly-silicon deposition, poly-silicon should be doped to achieve high conductivity. In the next step (Fig 8.5), the poly-silicon is etched back down to oxide layer, leaving poly-silicon only in the cavities etched in Fig 8.2. Poly- silicon can be etched using dry etching or chemical wafer polishing (CMP). In the next step, as shown in Fig 8.6, more poly-silicon is deposited and is patterned. This part of the poly-silicon will be part of the piston. In the next step (Fig. 8.7), a thick conformal oxide layer (136) is grown over the wafer surface. This oxide layer will provide the insulation layer between the membrane (100) and the substrate (102). In the subsequent step (Fig. 8.8), the thick oxide layer is patterned as shown in the figure. This also defines the membrane diameter. Before bonding an SOI wafer, the wafer surface is planarized using CMP process (Fig. 8.9). After planarization, an SOI wafer is bonded to the top surface of the wafer (Fig. 8.10). After removing the handle and the box oxide, a membrane (100) is formed. In the next step (Fig. 8.11), the back side of the wafer is etched using deep reactive ion etching down to the oxide layer as shown in Fig 8.11. This step exposes the oxide layer that is filling the vertical gap. At step 12, the vertical side wall is etched until it is completely cleared of oxide. The oxide etching can be done using vapor phase HF or wet BOE (buffered oxide etch). After step 12, the main structure of the membrane (100) is completed. In the subsequent step the bottom side of the wafer can be bonded to another wafer at low pressure to evacuate the air in the back side of the membrane cavity (the reason for naming this wafer second wafer is the evaluation of the substrate (102) as the first wafer). Further metallization steps can be found in CMUT fabrication references [11, 12] and same methods can be employed here.
Fig 9.1, Fig. 9.2, Fig. 9.3, Fig. 9.4, Fig. 9.5, Fig. 9.6, Fig. 9.7, Fig. 9.8 and Fig. 9.9 shows an alternative approach which also starts with a highly doped silicon wafer (102). At the first step (Fig 9.2), the surface of the wafer is etched to form a trench as shown in the figure. This trench will be filled with poly-silicon to form the piston part of the membrane (100) in the subsequent processing steps. At step 3 (Fig 9.3), a thin sacrificial layer of dry oxide (134) is grown on the silicon wafer surface. Dry oxidation is the method of choice for this step due to its high uniformity and well- controlled thickness. At step 4 (Fig 9.4), a thick layer of poly-silicon layer (135) is deposited. At step 5 (Fig 9.5), the poly-silicon layer is patterned and etched using reactive ion etching. In the subsequent step, the wafer surface is covered by a thin layer of oxide (136). At step 6, (Fig 9.6) this oxide layer is patterned and etched. The oxide pattern at this step determines the diameter of the membrane (100). After oxide etch, the wafer is polished using CMP process. The purpose of the polishing is to planarize the wafer surface by bringing the oxide part and the poly- silicon islands to the same height for the subsequent bonding process. At step 7 (Fig 9.7), an SOI wafer is bonded on top of the wafer. The device layer (142) is attached to the oxide layer (136) and the poly-silicon part (135). At step 8 (Fig 9.8), the back side of the wafer is patterned and etched until the thin dry oxide sacrificial layer (134) is exposed using deep reactive ion etching. In the next step (Fig 9.9), the thin oxide layer covering the bottom part of the poly- silicon layer (135) and the vertical gap is etched and the membrane (100) is released. In the subsequent processing steps (not shown) the back side of the wafer is bonded to another wafer at low pressure to seal the gaps and electrodes are fabricated to provide electrical interface to the mechanical structure.
Figure 10.1, Figure 10.2 and Figure 10.3 shows various transducer array geometries where the cells of the transducers are fabricated as explained above. The array geometries consist of electrodes providing electrical connections to the membrane (100) and the substrate (102). The array geometries are not limited to the ones that are shown in this figure. In Fig 10.1, array elements (143) belonging to a linear ID array are formed by electrically isolating the membranes (100) on the substrate (102). The boundary of the isolation layer defines the membranes (100) which are shown by dotted lines in the figure. Fig 10.2 shows array elements (146) belonging to a 2D array where the membrane layer is patterned to form a 2D array of transducer elements. Fig.10.3 shows an array formed by the array elements (147) belonging to an annular array. In all these configurations, the array geometry is defined by patterning the conducting membrane layer.
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[30]US PATENT APPLICATION 2009/0152980 Al, HUANG, ELECTROSTATIC COMB DRIVER ACTUATOR TRANSDUCER AND FABRICATION OF THE SAME.

Claims

1. An electromechanical micromachined transducer, characterised in that it comprises a cellular structure and multiple elements each comprising at least one of said cellular structures, the cellular structure comprising:
a free movable membrane (100) consisting of a thin peripheral part (108) and a thick central part (103),
an insulating layer (109) supporting the membrane (100),
an incised substrate (102) bound to the membrane (100) by the insulating layer (109),
second wafer bound to the substrate (102),
vertical gaps formed between the side walls (1001) of the thick central part of the membrane (100) and the incised substrate (102), and
horizontal gaps 1 and 2 (106, 107) formed between the membrane thin peripheral part (108) and the bottom part of the thick central part (103) and the incised substrate (102) and being larger than the vertical gap.
2. A micromachined transducer according to claim 1, characterised in that the membrane (100) consists of single crystal silicon, poly-silicon layer (135) or both.
3. A micromachined transducer according to claim 1, characterised in that the insulating layer (109) consists of silicon dioxide.
4. A micromachined transducer according to claim 1, characterised in that the substrate (102) consists of single crystal silicon, glass or quartz.
5. A micromachined transducer according to claim 1, characterised in that the membranes have an electrically connected structure to form array elements (143) belonging to ID array, array elements (146) belonging to 2D array or array elements (147) belonging to annular array.
6. An electromechanical micromachined transducer production method, characterised in that it comprises the following processing steps:
etching two cavities on the substrate (102) to form the thick central part (103) of the membrane (100),
depositing substantially a thin dry oxide sacrificial layer (134) on the substrate
(102) to form a smaller vertical gap with respect to the horizontal gaps 1 and 2 (106, 107),
filling the cavities to form the thick central part (103) of the membrane (100) by depositing the poly-silicon layer (135),
depositing a second poly-silicon layer on the substrate (102),
depositing a second thick oxide insulating layer and patterning it to form a support to the membrane (100),
polishing the surface to planarize the second oxide insulating layer and the second poly- silicon layer,
bonding an SOI wafer having a device layer (142) on it to the substrate (102), etching the handle layer of the SOI wafer and the dry oxide layer,
drilling a hole at the rear side of the substrate (102) to enable the thick central part
(103) of the membrane (100) to move freely,
etching the dry oxide sacrificial layer (134) from the opened hole,
bonding the second wafer to the rear side of the substrate (102) in vacuum to form a vacuum sealed cavity at the rear side of the membrane (100),
providing electrical contacts to the substrate (102) and the membrane (100).
7. An electromechanical micromachined transducer production method, characterised in that it comprises the following processing steps:
etching a single cavity on the substrate (102) to form the thick central part (103) of the membrane (100),
depositing substantially a thin dry oxide sacrificial layer (134) on the substrate
(102) to form a smaller vertical gap with respect to the horizontal gaps, filling the cavities to form the thick central part (103) of the membrane (100) by depositing the poly-silicon layer (135),
depositing a second poly-silicon layer on the substrate (102),
depositing a second thick oxide insulating layer and patterning it to form a support to the membrane (100),
polishing the surface to planarize the second oxide insulating layer and the second poly-silicon layer,
bonding an SOI wafer having a device layer (142) on it to the substrate (102), etching the handle layer of the SOI wafer and the dry oxide layer,
drilling a hole at the rear side of the substrate (102) to enable the thick central part
(103) of the membrane (100) to move freely,
etching the dry oxide sacrificial layer (134) from the opened hole,
bonding the second wafer to the rear side of the substrate (102) in vacuo to form a vacuum sealed cavity at the rear side of the membrane (100),
providing electrical contacts to the substrate (102) and the membrane (100).
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