A laser pattern generator for mask making or direct writing of features on a wafer or other substrate uses a pulsed laser source to achieve high power and short wavelength (e.g. 263 nm or less) radiation, for writing very small-sized features. The laser pulse frequency is either synchronous or asynchronous to the writing grid of the features being written.
To form devices having a feature size of 300 nm or less requires high resolution imaging, only provided by short wavelength radiation about 250 nm or less. There is direct ratio between laser wavelength and feature size; it is difficult to fabricate a feature size substantially smaller than the wavelength of the exposure radiation.
However, a laser pattern generator using a pulsed laser, operating at e.g. 355 nm or less instead of a conventional continuous wave laser, allows use of short wavelength laser radiation and hence fabrication of small feature sizes.
This laser pattern generator employs in one embodiment a laser outputting pulsed coherent light at a wavelength of e.g. 263 nm. The laser is a frequency quadrupled solid state laser. This laser is pulsed at high frequency, providing a stream of laser pulses rather than a continuous output beam. The present laser pattern generator may operate in a synchronous mode, where the pulse rate of the laser is synchronous to the beam placement on the writing grid, i.e. the data which is used to operate the modulator is synchronous to the laser beam pulse rate. In this case, the modulator rise time can be much shorter than the laser pulse.
In asynchronous mode, in which the pulse rate of the laser is at least as fast as the pixel rate of the data used for writing the pattern, the phase of the pulse rate of the laser is independent of the phase of the external modulator. The modulation rise time in this case is comparable to or greater than the pixel period of the data being written.
Synchronous and Asynchronous Modes
In the synchronous embodiment of this technology, a mode locked laser is used in an otherwise conventional laser scanner of the type commercially available. However, the data defining the pattern to be written, i.e. on, off or gray level data, is delivered synchronously with the laser pulses so that the pixels are either exposed or non-exposed on a fixed writing grid. In asynchronous mode, the pulse rate is comparable to or faster than the modulation rate and hence not synchronous to the data defining the pattern.
In order to have an efficient operating laser pattern generator, the laser typically has a pulse rate greater than one MHz in either embodiment. Moreover, the spot size to pixel grid separation is large enough to ensure smooth image intensity. The laser itself is a mode-locked solid state laser outputting light at e.g. 263 nm. Commercially available laser pattern and generators already operate in the ultraviolet. Hence, their optics which are optimized for somewhat higher e.g. 350 nm wavelengths, are readily reconfigured by routine optical engineering to deal with the shorter present 263 nm wavelength. The present technology is not limited to a 263 nm laser and is applicable to longer wavelengths and also to shorter wavelengths, to the extent that suitable laser sources are available operating at shorter wavelengths.