Capacitive micromachined ultrasonic transducer(CMUT) with varying thickness membrane

A capacitive micro-machined ultrasonic transducer as comprising:

• a substrate;
• a support having a base in contact with the substrate, and a wall extending away from the base to a top;
• and
• a membrane having an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;
• the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of:
• (i) a thickening on the upper surface of the membrane outside of the cavity, (ii) a thickening on the lower surface of the membrane inside of the cavity, (iii) a trench on an upper surface of the membrane, (iv) a trench on a lower surface of the membrane, and (v) any combination of two or more of these;
• the membrane thickness, the membrane lateral dimensions (or diameter or radius), and the membrane material properties determine the stiffness and the mass of the membrane and therefore determine transducer device parameters;
• the device parameters include at least one of capacitance, collapse voltage, and operation frequency;
• and
• membrane stiffness and the membrane mass increase as the lateral dimensions and thickness increase, and a non-uniform membrane thickness is used so that the capacitance, collapse voltage, and operation frequency parameters can be adjusted substantially individually, separately, and independently.

A capacitive micro-machined ultrasonic transducer (CMUT) comprising:

• a substrate;
• a support having a base in contact with the substrate, and a wall extending away from the base to a top;
• and
• a membrane having a membrane thickness and including an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;
• an electrode different from the membrane disposed on the upper surface of the membrane and having a separate electrode thickness;
• the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of:
• (i) a thickening of the membrane on the upper surface of the membrane outside of the cavity, (ii) a thickening of the membrane on the lower surface of the membrane inside of the cavity, (iii) a trench formed into the membrane on an upper surface of the membrane, (iv) a trench formed into the membrane on a lower surface of the membrane, and (v) any combination of two or more of these;
• and the membrane includes at least one trenched portion that has a membrane thickness that is thinner than the membrane thickness in untrenched portions of the membrane, and at least one trenched portion comprises an annular trench having an inner annular radius and an outer annular radius.

A capacitive micro-machined ultrasonic transducer (CMUT) comprising:

• a substrate;
• a support having a base in contact with the substrate, and a wall extending away from the base to a top;
• a membrane having a membrane thickness and including an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;
• and
• an electrode different from the membrane disposed on the upper surface of the membrane and having a separate electrode thickness;
• the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of:
• (i) a thickening of the membrane on the upper surface of the membrane outside of the cavity, (ii) a thickening of the membrane on the lower surface of the membrane inside of the cavity, (iii) a trench formed into the membrane on an upper surface of the membrane, (iv) a trench formed into the membrane on a lower surface of the membrane, and (v) any combination of two or more of these;
• and
• the membrane includes at least one trenched portion that has a membrane thickness that is thinner than the membrane thickness in untrenched portions of the membrane;
• the trenches are formed into the membrane on at least one of:
• (i) the upper surfaces of the membrane outside of the cavity, (ii) the lower surfaces of the membrane within the cavity, and (iii) both the upper surfaces of the membrane and on the lower surfaces of the membrane within a cavity in the transducer device;
• and
• for transducers having trenches on both upper and lower portions of the membrane, (i) the trenches are formed into the membrane at the same or substantially the same locations on the upper and lower portions of the membrane so that they overlie or underlie each other, and/or (ii) the trenches are formed into the membrane at different locations on the upper and lower portions of the membrane so that they do not overlie or underlie each other.

A capacitive micro-machined ultrasonic transducer (CMUT) comprising:

• a substrate;
• a support having a base in contact with the substrate, and a wall extending away from the base to a top;
• a membrane having a membrane thickness and including an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;
• and
• an electrode different from the membrane disposed on the upper surface of the membrane and having a separate electrode thickness;
• the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of:
• (i) a thickening of the membrane on the upper surface of the membrane outside of the cavity, (ii) a thickening of the membrane on the lower surface of the membrane inside of the cavity, (iii) a trench formed into the membrane on an upper surface of the membrane, (iv) a trench formed into the membrane on a lower surface of the membrane, and (v) any combination of two or more of these;
• and
• the membrane includes at least one trenched portion that has a membrane thickness that is thinner than the membrane thickness in untrenched portions of the membrane;
• the trenches are formed into the membrane on at least one of:
• (i) the upper surfaces of the membrane outside of the cavity, (ii) the lower surfaces of the membrane within the cavity, and (iii) both the upper surfaces of the membrane and on the lower surfaces of the membrane within a cavity in the transducer device;
• and
• the trenches comprises annular trenches formed in a circular membrane, and the different locations include locations at different radii from the center of a circular membrane.

A capacitive micro-machined ultrasonic transducer (CMUT) comprising:

• a substrate;
• a support having a base in contact with the substrate, and a wall extending away from the base to a top;
• a membrane having a membrane thickness and including an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;
• and
• an electrode different from the membrane disposed on the upper surface of the membrane and having a separate electrode thickness;
• the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of:
• (i) a thickening of the membrane on the upper surface of the membrane outside of the cavity, (ii) a thickening of the membrane on the lower surface of the membrane inside of the cavity, (iii) a trench formed into the membrane on an upper surface of the membrane, (iv) a trench formed into the membrane on a lower surface of the membrane, and (v) any combination of two or more of these;
• and
• the membrane is a substantially circular membrane, and comprises a plurality of substantially concentric annular or circular trenches disposed near the outer circular edge of the membrane;
• and the substantially concentric annular or circular trenches disposed near the outer edge of the membrane are disposed within an annular region that has a radius of between about 50% and 100% of the radius of the membrane.

A capacitive micro-machined ultrasonic transducer device comprising:

• a membrane supported by support structures or posts, the membrane having a nominal uniform thickness extending between the support structures or posts except for first and second trenches wherein some membrane material is absent from or has been removed from the upper surface of the membrane outside of a cavity formed between the membrane and the substrate;
• and
• at least one of:
• (i) a first thickened portion formed on or integral with the membrane on the inner surface of the membrane within the transducer cavity and having a first lateral dimension and a first thickness, and (ii) a second thickened portion formed on or integral with the membrane on the outer surface of the membrane outside of the transducer cavity and having a second lateral dimension and a second thickness.

A method for designing a capacitive micro-machined ultrasonic transducer (CMUT) comprising:

• specifying a CMUT membrane mass characteristic to achieve a first performance metric;
• specifying a CMUT membrane stiffness characteristic to achieve a second performance metric, the specification of the membrane mass and thickness being independent of each other;
• the specifying of the mass characteristic and the stiffness characteristic including specifying that the membrane exclusive of any electrode thickness has a non-uniform membrane thickness resulting from at least one of:
• (i) a thickening of the membrane on the upper surface of the membrane outside of a CMUT cavity, (ii) a thickening of the membrane on the lower surface of the membrane inside of the CMUT cavity, (iii) a trench formed into the membrane on an upper surface of the membrane, and (iv) a trench formed into the membrane on a lower surface of the membrane;
• and
• determining a CMUT membrane structure having the non-uniform membrane thickness using the specified membrane mass and membrane stiffness characteristics.