By nature of its high speed, the GLV offers an additional modulation mode not typically found in other spatial light modulators: pulse-width modulation or “PWM.” This feature allows the duration of the pulse to be controlled with extremely high precision. Many high-throughput direct-write systems operate with continuous media motion (as opposed to step and expose mode). In continuous print systems, the PWM capability of the GLV can be used to achieve superior spatial resolution while simultaneously maintaining high throughput.
Dynamic PWM functionality in GLV modules allows the optical transition to be placed anywhere within one column period, as shown in Figure 1. The blue line can represent either GLV drive voltage or optical intensity. External timing inputs delivered to the GLV controller are (a) column strobe which divides elapsed time into discrete time periods or columns and (b) delay clock, a higher-frequency signal for controlling transition timing. These signals are typically derived from the stage or motion controller on the lithography tool. For full 8-bit delay resolution, the delay clock frequency is adjusted to 28 = 256x the column strobe frequency. An 8-bit digital delay value instructs when the amplitude update occurs. Amplitude and delay values can be updated for all pixels independently and at every column period.
But how can PWM be used to enhance feature resolution direct-write systems? In Figure 2a, a diagonal line composed of minimum features is printed on a grid of equivalent pitch. As shown in the figure, the line is not rendered with sufficient resolution. Resolution can be improved by re-exposing the minimum features at a sub-grid resolution (1/4 sub-grid shown in Figure 2b), as might occur in systems using 2D spatial light modulators. However, while this improves resolution, it requires 4x the number of exposures, thereby degrading throughput. Figure 3c illustrates printing with a GLV using PWM. Programmable delay is used to place edges onto the sub-grid in the time axis, and amplitude control is used to place edges onto the sub-grid of the orthogonal (pixel) axis. This allows the UVDI module to achieve sub-grid resolution in a single pass. Figure 3 illustrates the dynamic PWM capability of the G8192 module. In this case, optical pulses of various duration were created by adjusting the delay of the falling edge. The delay can be swept over the 4us column at a resolution of 20ns. As shown in the inset, the digital delay capability delivers excellent delay linearity.