Micromachining and Image Recording on Thin Films by Laser Beams

Images can be recorded on an opaque metallic film on a transparent substrate by using an amplitude-modulated laser beam to machine the metallic film. The modulated laser beam is scanned in raster fashion over the surface of the film, and the thermal energy dissipated in the film causes a local displacement or removal of the metallic material. In this way a permanent image suitable for immediate display or storage is created. Experiments along this line have been reported 1,2 but reveal a number of shortcomings from the standpoint of practical application. Generally, there is no gray scale, only negative line images are reproduced, resolution is limited, and the required laser power is large. A new laser machining technique is described in this paper which overcomes these shortcomings and permits the generation of highquality, continuous-tone, permanent images using low-power gas lasers. The significant departure from previous work rests in the use of very short laser pulses to record the image in the form of an array of discrete holes in the metal film and in the modulation of the intensity of the laser pulses so as to vary the size of the holes. Each laser pulse serves to machine a single, nearly circular hole in the metal film by displacing and removing metal from the transparent substrate. In this way a transparency is created, and incandescent light can be directed through the film to display the image on a screen. The delivery of the laser energy to the metal film in short (20-30 nanosecond) pulses reduces the energy density required to raise the metal film to a given temperature by at least an order of magnitude over that which would be required for the same writing speed if a CW laser were used.