covering the exposed through conductor in the embodiment shown in fig3 a and 3b an etching time is prolonged because the whole thickness of the si substrate is etched however it is easy to form thin film circuit elements on the bottom surface of the si substrate an embodiment of forming additional circuit elements will be described in the following as shown in fig4 a on the top surface of an si substrate 11 formed with silicon oxide layers 12 and 12 b thin film circuit elements tf 1 are formed and on the bottom surface thereof thin film circuit elements tf 2 are formed as shown in fig4 b by using a photoresist pattern pr 5 as a mask holes or trenches are etched from the top surface of the si substrate in this etching the insulating layer 12 b on the bottom surface of the si substrate functions as an etching stopper layer as shown in fig4 c through conductors 29 t are formed as shown in fig4 d the etching stopper layer 12 b is partially etched to expose the bottoms of the through conductors 29 t and pads 35 are formed with these processes a circuit board can be formed which has thin film circuit elements on the top and bottom surfaces of the si substrate methods of forming a circuit board has been described by using an si substrate a substrate other than an si substrate may be used fig5 a to 5c illustrate a method of forming a circuit board by using a glass substrate as an insulating substrate as shown in fig5 a thin film circuit elements 42 are formed on the surface of a glass substrate 41 after a hole.