Ultrasonic transducer manufacturing method
By forming different extended conductive and dielectric layers on the film of the CMUT transducer, adjusting the initial deflection and mechanical stress of the film, it is solved in the prior art that it is difficult to simultaneously manufacture CMUT transducer at different ultrasonic frequencies on the same substrate in the prior art. The problem of the device is to achieve simplification and stability of forming a multi-frequency CMUT transducer on the same substrate.
Patent Information
- Application Number
- CN202080058716.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Priority Date
- 2019-08-20
- Filing Date
- 2020-08-14
- Publication Date
- 2024-12-31
- Estimated Expiration
- 2040-08-14
AI Technical Summary
It is difficult for existing CMUT transducers to manufacture transducers at different ultrasonic frequencies on the same substrate at the same time, resulting in the problem of frequency mismatch.
By forming different extended conductive and dielectric layers on the film of the CMUT transducer, the initial deflection and mechanical stress of the film are adjusted, thereby forming first and second CMUT transducers with different resonant frequencies on the same substrate.
It is realized that CMUT transducers at different ultrasonic frequencies are manufactured on the same substrate at the same time, which simplifies the electronic control circuit and improves the frequency matching and stability of the transducer.
Smart Images
Figure CN114269684B_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates generally to the field of ultrasonic transducers, and more particularly to the field of capacitive micromachined ultrasonic transducers, also referred to as CMUT transducers. Background Art
[0002] Typically, a CMUT transducer includes a flexible membrane suspended above a cavity, a first electrode (referred to as a lower electrode) located on the side of the cavity opposite to the membrane, and a second electrode (referred to as an upper electrode) located on the side of the cavity opposite to the first electrode and firmly attached to the flexible membrane. In operation, a DC bias voltage is applied between the lower and upper electrodes of the transducer. When an appropriate AC excitation voltage superimposed on the DC bias voltage is applied between the lower and upper electrodes of the transducer, the flexible membrane begins to vibrate under the action of the static electricity applied between the lower and upper electrodes, causing the emission of ultrasonic waves. Conversely, when the transducer receives an ultrasonic wave, the flexible membrane begins to vibrate under the action of a change in mechanical pressure, resulting in an AC voltage (due to a change in capacitance between the electrodes) superimposed on the DC bias voltage between the upper and lower electrodes of the transducer.
[0003] The CMUT transducer is typically coupled to an electronic control circuit configured to apply an excitation voltage superimposed on a DC bias voltage between the transducer electrodes during a transmission phase to cause ultrasound waves to be transmitted by the transducer, and to apply the DC bias voltage between the transducer electrodes and read an AC voltage generated between the electrodes under the action of the received ultrasound waves during a reception phase.
[0004] The transmission frequency of a CMUT transducer generally corresponds to its resonance frequency, which depends on various parameters and especially the geometrical and mechanical properties of the membrane and the cavity.
[0005] For certain applications, it may be desirable to be able to simultaneously form transducers designed to operate at different ultrasonic frequencies on the same substrate. Summary of the invention
[0006] To achieve this, one embodiment provides a method of simultaneously manufacturing at least one first CMUT transducer having a first resonant frequency and at least one second CMUT transducer having a second resonant frequency different from the first frequency from the same substrate, the method comprising the following steps:
[0007] a) for each transducer, forming a cavity on the upper surface side of the substrate, and forming a flexible membrane suspended above the cavity;
[0008] b) forming a first layer that extends only over a portion of the upper surface of the membrane of the first transducer and does not extend over the membrane of the second transducer; and
[0009] c) forming a second layer extending over the entire upper surface of the membrane of the first transducer and over the entire upper surface of the membrane of the second transducer.
[0010] According to one embodiment, the first layer and the second layer are conductive layers.
[0011] According to one embodiment, the first layer and the second layer are metal layers.
[0012] According to one embodiment, the first layer and the second layer are a dielectric layer and a conductive layer, respectively.
[0013] According to one embodiment, in step a), each cavity is formed in a third dielectric layer previously formed on the upper surface of the substrate.
[0014] According to one embodiment, the film is formed in a fourth semiconductor layer disposed on an upper surface of the third layer.
[0015] According to one embodiment, the substrate is a semiconductor substrate.
[0016] According to one embodiment, the pattern formed by the first layer in the first transducer is arranged over a central portion of the transducer cavity.
[0017] According to one embodiment, the cavities of the first transducer and the second transducer formed in step a) have the same depth in order to produce a dispersion.
[0018] One embodiment provides a device comprising at least one first CMUT transducer having a first resonance frequency and at least one second CMUT transducer having a second resonance frequency different from the first frequency inside and on top of a same substrate, wherein:
[0019] - Each transducer comprises a cavity located on the upper surface side of the substrate and a flexible membrane suspended above the cavity;
[0020] - the first layer extends over only a portion of the upper surface of the membrane of the first transducer and not over the membrane of the second transducer; and
[0021] The second layer extends over the entire upper surface of the membrane of the first transducer and over the entire upper surface of the membrane of the second transducer. BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The above features and advantages as well as other features and advantages will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which:
[0023] Figure 1A , Figure 1B and Figure 1C are cross-sectional views showing sequential steps of an example of a method for manufacturing an ultrasonic transducer according to an embodiment;
[0024] Figure 2A and Figure 2B Is displayed through Figures 1A to 1C a graph of initial deflections of first and second types of ultrasonic transducers formed by the method; and
[0025] Figure 3A and Figure 3B It is shown by Figures 1A to 1C The method forms a graph of pressure frequency responses of first type and second type ultrasonic transducers. DETAILED DESCRIPTION
[0026] In the various drawings, similar features are represented by similar reference numerals. In particular, common structural and / or functional features in various embodiments may have the same reference numerals and may be arranged with the same structure, dimensions, and material properties.
[0027] For the sake of clarity, only the steps and elements that are useful for understanding the embodiments described herein are detailed and described. In particular, various possible applications of the transducer are not described in detail, and the embodiments are compatible with common applications of ultrasonic transducers, especially in ultrasonic imaging devices. In addition, the circuits for controlling the transducer are not described in detail, and the embodiments are compatible with all or most known CMUT transducer control circuits.
[0028] Unless otherwise specified, when two elements are referred to as being connected together, this means a direct connection without any intermediate elements except conductors, and when two elements are referred to as being coupled together, this means the two elements may be connected or they may be coupled through one or more other elements.
[0029] In the following disclosure, unless otherwise stated, when referring to absolute position qualifiers, such as terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as terms "above", "below", "higher", "lower", etc., or direction qualifiers, such as "horizontal", "vertical", etc., reference is made to the direction shown in the accompanying drawings.
[0030] Unless otherwise specified, the expressions "about," "approximately," "substantially," and "approximately" mean within 10%, and preferably within 5%.
[0031] Figure 1A , Figure 1B and Figure 1C 1 is a cross-sectional view showing sequential steps of an example of a method for manufacturing a CMUT transducer according to an embodiment. The method can simultaneously form a first type (I) CMUT transducer and a second type (II) CMUT transducer inside and / or on top of the same substrate 101, capable of operating at different corresponding ultrasonic frequencies. Figures 1A to 1C Only the formation of a single Type I transducer (on the left side of the drawing) and a single Type II transducer (on the right side of the drawing) is shown. In practice, for example, multiple Type I transducers and multiple Type II transducers identical to those in the fabrication dispersion can be formed simultaneously from the same substrate.
[0032] Figure 1A The structure obtained at the end of the step of forming, for each transducer, a cavity 103 on the upper surface side of the substrate 101 and a flexible membrane 105 suspended above the cavity 103 is shown.
[0033] The substrate 101 is, for example, a doped semiconductor substrate 101 (eg made of silicon), which defines the lower electrode E1 of the transducer.
[0034] In this example, the cavities 103 are formed in a dielectric layer 107, for example made of silicon oxide, which was previously formed on the upper surface of the substrate 101. The layer 107 is in direct contact with the upper surface of the substrate 101, for example, through its lower surface. Each cavity 103 extends vertically from the upper surface of the dielectric layer 107 to the upper surface of the substrate 101. In the example shown, the cavities 103 are non-through, that is, they do not appear on the lower surface side of the layer 107. In other words, the lower part of the thickness of the layer 107 coats the upper surface of the substrate 101 at the bottom of each cavity 103. However, the described embodiments are not limited to this particular case.
[0035] For example, dielectric layer 107 initially extends continuously and across a substantially uniform thickness across the upper surface of substrate 101. As an example, dielectric layer 107 may be formed by oxidizing an upper portion of substrate 101 (e.g., according to a dry oxidation process), or by depositing a dielectric material on the upper surface of substrate 101.
[0036] The cavity 103 may be formed by partially etching the dielectric layer 107 from its upper surface.
[0037] Preferably, the cavities 103 of the type I and type II transducers are formed simultaneously and have the same depth, so as to be within the manufacturing dispersion. As an example, the depth of the cavities 103 is in the range from 10 nm to 5 μm, for example approximately 500 nm.
[0038] The cavities 103 of the type I and type II transducers may have the same lateral dimensions (within manufacturing dispersion), or different lateral dimensions. The lateral dimensions of the cavities 103 are, for example, in the range from 5 to 500 μm.
[0039] In each transducer, the membrane 105 extends above the cavity 103 and is attached by its lower surface to the upper surface of the dielectric layer 107 at the periphery of the cavity 103. As an example, the lower surface of the membrane 105 is in direct contact with the upper surface of the dielectric layer 107 at the periphery of the cavity 103.
[0040] After forming the cavity 103, the film 105 can be placed on the upper surface of the dielectric layer 107. For example, the film 105 is made of a doped or undoped semiconductor material (e.g., silicon). For example, the film 105 is attached to the upper surface of the dielectric layer 107 by direct bonding or molecular bonding of its lower surface. Preferably, the film 105 of the type I and type II transducers is formed and arranged at the same time and has the same thickness to be within the manufacturing dispersion. As an example, the film 105 may correspond to a portion of a semiconductor layer of an SOI type (semiconductor on insulator) stack. For example, the SOI stack can be attached to the upper surface of the dielectric layer 107 by direct bonding or molecular bonding, and then thinned from its surface opposite to the dielectric layer 107 to retain only the semiconductor layer forming the film 105. For example, the thickness of the film 105 is in the range from 10nm to 10μm.
[0041] Figure 1B The structure obtained at the end of the step of forming a first layer 109 of a first material is shown, this first layer 109 extending only over a portion of the upper surface of the membrane of each type I transducer and it does not extend over the membrane of the type II transducers.
[0042] As an example, first Figure 1A The layer 109 is continuously deposited with a substantially uniform thickness over the entire upper surface of the structure and then locally removed (e.g., by photolithography and etching) to form a pattern that extends only over a portion of the membrane of each Type I transducer and does not extend over the membrane of the Type II transducer. In other words, the layer 109 is completely removed over the cavity 103 of each Type II transducer and is removed only over a portion of the surface of the cavity 103 of each Type I transducer. In each Type I transducer, for example, the layer 109 is in direct contact with the upper surface of the transducer's membrane 105 by its lower surface over the entire portion of the membrane 105 covered by the layer 109.
[0043] Figure 1C Shown in Figure 1BThe structure obtained at the end of the step of forming a second layer 111 of a second material on the upper surface of the structure of the present invention. The layer 111 may be made of the same material as the layer 109 or of a different material. The layer 111 extends continuously over the entire upper surface of the membrane 105 of each transducer with a substantially uniform thickness. In a type I transducer, the layer 111 is in contact, for example, through its lower surface, on the one hand, with the upper surface of the layer 109 on the portion of the membrane 105 covered by the layer 109, and on the other hand, with the upper surface of the membrane 105 on the portion of the membrane 105 not covered by the layer 109. In a type II transducer, the layer 111 is in contact, for example, through its lower surface, with the upper surface of the membrane 105 over substantially the entire surface of the membrane 105.
[0044] Layers 109 and 111 are preferably conductive layers, for example metal layers. In each transducer, layer 111 defines in particular the upper contact electrode E2 of the transducer. As an example, each of layers 109 and 111 is made of a material from the group consisting of aluminum, copper, gold, tungsten, titanium, nickel or an alloy of one or more of these materials. As a variant, layer 109 may be made of a dielectric material, for example silicon oxide or silicon nitride. Layers 109 and 111 may have the same thickness (within the manufacturing dispersion) or different thicknesses. For example, each of layers 109 and 111 has a thickness in the range from 1 nm to 5 μm.
[0045] The layers 109 and 111 coating the transducer membrane 105 generate mechanical stresses which, in the absence of any electrical bias to the transducer, can result in a deflection of the membrane ( Figure 1C This mechanical stress and the resulting initial deflection depend in particular on the material thickness, properties, and deposition conditions (in particular deposition temperature) of layers 109 and 105.
[0046] In a Type I transducer, since layer 109 is located only on a portion of the surface of the membrane 105, layer 109 creates a discontinuity in the laminate coating membrane 105. This results in a discontinuity in the neutral (non-mechanical stress) plane of the membrane. At the level of the discontinuity, this creates additional mechanical stress, resulting in a significant increase in the initial deflection of the membrane relative to a Type II transducer without such a discontinuity.
[0047] This increased initial deflection results in a reduction in the breakdown voltage or pull-in voltage of the transducer, ie the threshold voltage between electrodes E1 and E2 from which the flexible membrane 105 contacts the bottom of the cavity 103 through its lower surface.
[0048] Another role of the additional layer 109 is to increase the stiffness of the membrane and then increase the resonant frequency of the transducer.
[0049] According to one aspect of the embodiment, the structural layer ( Figures 1A to 1CThe introduction of layer 109 in the example into the stack coating film 105 takes advantage of the fact that an additional degree of freedom is obtained, enabling transducers capable of operating at different ultrasonic frequencies to be formed simultaneously by a common method and from the same substrate.
[0050] Specifically, combined Figures 1A to 1C The method described is capable of forming a type I transducer having a resonance frequency greater than that of a type II transducer, and applying the same DC bias voltage between their respective electrodes El and E2.
[0051] Preferably, in order to increase the resonant frequency difference between the type I transducer and the type II transducer, the structural layer 109 is deposited under conditions capable of generating relatively high mechanical stress on the membrane 105 of the type I transducer. As an example, the layer 109 is deposited at a relatively high temperature, for example, in the range of from 100 to 450° C. In addition, a heat treatment may be applied after the deposition of the layer 109 or the layer 111 to increase the stress. For example, after the deposition of the layer 109 or the layer 111, a relatively significant temperature change may be applied to the structure, for example, a temperature change in the range of from 300 to 800° C. More generally, those skilled in the art will understand how to adjust the process to obtain a desired resonant frequency by capturing constraints on thermal expansion in the stacked layers.
[0052] The pattern of the portion of the 109 layer arranged on the membrane of the I-type transducer helps to define the initial deflection of the I-type transducer and indirectly defines the resonant frequency. As an example, in a top view (not shown), the portion of the layer 109 held above the membrane 105 of each I-type transducer can have a roughly square or circular shape arranged on the central portion of the membrane. As a variant, the portion of the layer 109 can have an annular shape arranged on the peripheral portion of the membrane. In another variant, multiple separate portions of the layer 109 can be arranged above different areas of the membrane 105 of each I-type transducer.
[0053] Figure 2A and Figure 2B They are shown by Figures 1A to 1C Graphs of the initial deflection (ordinate, in nm) of the membrane 105 of the transducer along the transverse dimension of the membrane (abscissa, in μm) for a Type I transducer and a Type II transducer formed by a method of the type described.
[0054] In this example, a type I transducer and a type II transducer having a membrane with the same lateral dimensions (within the manufacturing dispersion), which is approximately 50 μm, have been considered. In this example, the only difference between the type I transducer and the type II transducer is the presence of a layer 109 on a portion of the membrane 105 surface of the type I transducer, while the type II transducer has no layer 109 on the membrane.
[0055] For the Type I transducer, an initial deflection of approximately 225 nm can be observed at the center of the membrane, and for the Type II transducer, an initial deflection of approximately 50 nm can be observed.
[0056] Figure 3A and Figure 3B Shows Figure 2A and Figure 2B The respective frequency responses of the Type I and Type II transducers in the example. More specifically, Figure 3A and Figure 3B Each of the graphs represents, for the corresponding transducer, the relative sound pressure emitted by the transducer (vertical axis, in dB) as measured on the output direction axis extending from the center of the transducer and at the far-field distance from the transducer during the transmission phase, depending on the frequency of the AC excitation voltage applied between the transducer electrodes E1 and E2 (abscissa, in MHz).
[0057] Figure 3A shows the broadband response of the Type I transducer at center frequencies from approximately 6 to 7 MHz, and Figure 3B The Type II transducer has a broadband response with a center frequency from approximately 2 to 3 MHz.
[0058] In the example shown, for each transducer, the frequency response has been simulated for a DC bias voltage set to 80% of the transducer breakdown voltage. It should be noted that in the example considered, the breakdown voltage of the type I transducer is of the same order of magnitude as the breakdown voltage of the type II transducer (about 135 V in both cases). In practice, similar resonant frequency shifts can be observed when the DC bias voltage applied to the type I transducer is the same as the DC bias voltage applied to the type II transducer.
[0059] The method provided is particularly advantageous for the formation of multi-frequency ultrasound imaging devices, since it enables monolithic formation of transducers capable of operating at different frequencies on the same chip. The fact that the formed transducers can operate at different frequencies with the same DC bias voltage is particularly advantageous in such devices, since this can greatly simplify the electronic control circuitry. Preferably, the DC bias voltage shared by the different transducers is less than the breakdown voltage of the transducer with the smallest breakdown voltage, i.e., the type I transducer in the above example.
[0060] As a variant, the method described herein can be used to form on the same substrate transducers capable of operating at different frequencies and intended to be used in different devices after the substrate has been diced into individual chips (multi-project approach).
[0061] Various embodiments and variations have been described. Those skilled in the art will appreciate that certain features of these different embodiments and variations can be combined, and those skilled in the art will think of other variations. Specifically, the described embodiments are not limited to the examples of dimensions and materials mentioned in this disclosure.
[0062] Furthermore, the method provided can be easily adapted to simultaneously form more than two different transducer types capable of operating at different frequencies from the same substrate. To this end, different patterns of the structural layer 109 and / or the lateral dimensions of the cavities in the different types of transducers can be provided in particular. Furthermore, additional structural layers covering the transducer membrane can be provided to increase the number of available degrees of freedom. Preferably, all transducers of the same type are identical within the manufacturing dispersion.
[0063] Finally, based on the functional indications given above, the actual implementation of the described embodiments and variants is within the capabilities of a person skilled in the art. Specifically, through simulations or routine testing, a person skilled in the art is capable of defining the structural pattern of layer 109 or layer 111 to be applied to the type I transducer to obtain the desired operating frequency.
Claims
1. A method for simultaneously manufacturing at least one first CMUT transducer I having a first resonance frequency and at least one second CMUT transducer II having a second resonance frequency different from the first resonance frequency from the same substrate (101), the method comprising the following consecutive steps: a) for each transducer, forming a cavity (103) on the upper surface side of the substrate, and forming a flexible membrane (105) suspended above the cavity (103); b) forming a first layer (109), the first layer (109) extending only on a portion of the upper surface of the flexible membrane (105) of the first CMUT transducer I, and not extending on the flexible membrane (105) of the second CMUT transducer II; and c) forming a second layer (111) extending over the entire upper surface of the flexible membrane (105) of the first CMUT transducer I and the entire upper surface of the flexible membrane of the second CMUT transducer II, in, When the DC bias voltage applied to the first CMUT transducer I and the DC bias voltage applied to the second CMUT transducer II are the same, a similar resonant frequency shift can be observed.
2. The method according to claim 1, wherein: The first layer (109) and the second layer (111) are conductive layers.
3. The method according to claim 1 or 2, wherein: The first layer (109) and the second layer (111) are metal layers.
4. The method according to claim 1, wherein: The first layer (109) and the second layer (111) are a dielectric layer and a conductive layer, respectively.
5. The method according to claim 1 or 2, wherein: In step a), each cavity (103) is formed in a third dielectric layer (107) previously formed on the upper surface of the substrate (101).
6. The method according to claim 5, wherein: The flexible film (105) is formed in a fourth semiconductor layer placed on an upper surface of the third dielectric layer (107).
7. The method according to claim 1 or 2, wherein: The substrate (101) is a semiconductor substrate.
8. The method according to claim 1 or 2, wherein: The pattern formed by the first layer (109) in the first CMUT transducer 1 is arranged over a central portion of a cavity (103) of the CMUT transducer.
9. The method according to claim 1 or 2, wherein: The cavities (103) of the first CMUT transducer I and the second CMUT transducer II formed in step a) have the same depth so as to be within the manufacturing dispersion.
10. A method for simultaneously manufacturing at least one first CMUT transducer I having a first resonance frequency and at least one second CMUT transducer II having a second resonance frequency different from the first resonance frequency from the same substrate (101), the method comprising the following consecutive steps: a) for each transducer, forming a cavity (103) by locally etching a third dielectric layer (107) previously deposited on the upper surface of the substrate, wherein the cavities of the first and second CMUT transducers have the same depth; b) after step a), affixing a flexible film (105) above each cavity (103) by directly bonding the lower surface of the flexible film (105) to the upper surface of the third dielectric layer (107), c) after step b, forming a first layer (109), the first layer (109) extending only on a portion of the upper surface of the flexible membrane (105) of the first CMUT transducer I, and not extending on the flexible membrane (105) of the second CMUT transducer II; and d) after step c), forming a second layer (111), the second layer (111) extending over the entire upper surface of the flexible membrane (105) of the first CMUT transducer I and the entire upper surface of the flexible membrane of the second CMUT transducer II, in, When the DC bias voltage applied to the first CMUT transducer I and the DC bias voltage applied to the second CMUT transducer II are the same, a similar resonant frequency shift can be observed.