1. Field of the Invention
The present invention relates to a semiconductor device which operates by a light.
2. Description of the Related Art
Recently, semiconductor devices which operate by an optical signal as an input signal - monolithic integration of elements for converting light into a current, such as photodiodes, phototransistors, amplifiers, waveform shapers, etc. has advanced, and the application fields of these semiconductor devices has increased. However, their available environments have been gradually deteriorating. Numerous electromagnetic waves are radiated in the air, and surge voltages generated upon opening and closing power sources are always imparted to power lines. Heretofore, on-chip shields for covering semiconductor chips through insulating films with conductive films have been formed as a means of preventing semiconductor devices from erroneously operating due to electromagnetic waves and surge voltages. In the semiconductor devices which operate by inputting light as input signal, a shielding film has to have transparency and conductivity. Thus, optical transmissible conductive films made of tin oxide, indium oxide or polysilicon are used. FIG. 1 is a sectional view showing this conventional semiconductor device. Reference numeral 1 denotes a semiconductor substrate; 2, a photodetector for forming a photodiode; 3, a semiconductor element; 4, an optically transmissible conductive film; 5, metal wirings; and 61, 62, insulating films.
In semiconductor devices in which elements for converting light into a current, such as photodiodes, phototransistors, amplifiers, and waveform shapers are monolithically integrated, problems such as erroneous operations of elements by light irradiated towards the elements other than those elements for converting light into a current, frequently occur. Thus, as shown in FIG. 2, optical shielding materials 9 for shielding light, made of aluminum films or aluminum silicon films are formed on elements other than the elements for converting an optical signal into an electric signal.
The shield of optical transmissible conductive film 4 shown in FIG. 1 results in a selective shielding effect. However, light as an input signal is irradiated over the whole chip surface. Thus, an optical signal is converted into an electric signal even by the element other than the elements intended to convert the optical signal into the electric signal, such as photodiodes, phototransistors thereby causing an erroneous operation to occur.
Optical shielding materials 9 of elements other than the elements for converting an optical signal into an electric signal, to prevent an erroneous operation give a misleading breakdown strength against electromagnetic waves and/or surge voltages, such as the breakdown strength against a noise contained in a momentary electromagnetic wave like 10000 V/microsecond to eliminate an erroneous operation, i.e., shielding effect. This is considered because the photodiodes and phototransistors are the most sensitive among the chips.