1.  Field of the Invention
The present invention relates to a piezoelectric transducer, an ultrasonic transducer which includes the piezoelectric transducer and is used in an ultrasonic scan, an ultrasonic probe including the ultrasonic transducer, and an ultrasound imaging apparatus including such an ultrasonic probe.  The present invention further relates to a method of making a plurality of the piezoelectric transducers. 
2.  Discussion of the Background
An ultrasound imaging apparatus is well known to be used for medical purposes as an ultrasound diagnosis apparatus.  The ultrasound diagnosis apparatus scans a patient's body by transmitting ultrasound pulses from an ultrasonic transducer and prepares ultrasound images of the inside of the patient's body based on echo signals caused by acoustic impedance mismatching inside the patient's body. 
An ultrasonic transducer typically includes a plurality of transducer elements which are arrayed along a scan direction of the above scan.  The transducer elements vibrate and generate the ultrasound pulses which are transmitted to the patient's body.  The transducer elements also receive the echo signals from the body.  The transducer elements have a flat strength along a direction perpendicular to the scan direction.  Such a direction perpendicular to the scan direction is hereinafter referred to as a slice direction whether the scan is conducted at a fixed position or at various positions along the slice direction.  The transducer elements also form a focal point at a certain depth inside the patient's body by that a delay difference is given to the generated ultrasound pulses by an acoustic lens provided in the ultrasonic transducer. 
However, there is a limit to improve a behavior of an ultrasonic pulse beam convergence provided by the acoustic lens.  Therefore, ultrasound acoustic pressure is weighted along the slice direction so as to improve the convergence behavior. 
For example, Japanese Patent Application Publication No.  PH11-146492 discloses an ultrasonic transducer in which an acoustic matching material attached to a piezoelectric transducer is provided with a plurality of gutters along a scan direction so as to give weightings along the slice direction. 
Also for example, Japanese Patent Application Publication No.  PH05-23331 discloses an ultrasonic transducer in which a piezoelectric transducer and one of electrode plates are divided into plural portions in the slice direction.  Voltages to be applied to the electrode plate are weighted differently among the divided plural portions of the electrode plate. 
In the above first example, there is a problem that the piezoelectric transducer cannot transmit ultrasound pulses and receive echo signals in some parts, which results in high side lobes.  In addition, an ultrasonic transducer and an ultrasonic probe become complicated in their structures.  Such structures lead to an increase in manufacturing processes and accordingly to increased manufacturing cost. 
In the above second example, there is a problem that an electric circuitry scale becomes large because it is necessary to apply different voltages to divided electrode portions.  As a result, manufacturing cost of the ultrasonic transducer increases.  In addition, manufacturing processes of the ultrasonic transducer increase for the above reason.