A conventional ultrasonic sensor includes an ultrasonic transducer disposed on a substrate made from material such as metal and resin. The ultrasonic sensor is mounted to, for example, a vehicle. The ultrasonic sensor sends an ultrasonic wave to an object and receives the ultrasonic wave reflected by the object. The sending and receiving of the ultrasonic wave is done by an element capable of sending and receiving the ultrasonic wave. The wave sent and received provides information on an object around the vehicle, such as a location of the object, a distance from the object, a two-dimensional shape of the object and a three dimensional shape of the object.
Some of the ultrasonic sensors include an acoustic matching layer that controls acoustic impedance. The acoustic matching layer improves efficiency of the sending and receiving of the ultrasonic wave. An example of the ultrasonic sensor having the acoustic matching layer is shown in JP-A-H10-224895 where an acoustic matching layer, which is made from synthetic resin including glass balloons, is fixed to one side of the ultrasonic sensor.
Since the ultrasonic sensor is attached to an exposed place of an apparatus such as the vehicle, the downsizing of the ultrasonic sensor is required in order to avoid spoil of the beauty of the apparatus. Specifically, diminish in the apparent area of the acoustic matching layer is needed. Diminishing in the size of the acoustic matching layer requires the downsizing of an ultrasonic detection element that is attached to the acoustic matching layer. When bulky piezoelectric material is used for the ultrasonic detection element, it is difficult for the ultrasonic detection element to resonate with the vibration of the acoustic matching layer with increasing the reception sensitivity and improving the detection sensitivity to the ultrasonic wave. The problem is that it has been difficult to balance two goals for the downsizing of the acoustic matching layer and the improving of the detection sensitivity to the ultrasonic wave. Thus it is required for the ultrasonic sensor to improve the detection sensitivity and to minimize its dimension.