Digital heterodyne interference fringe control system

Abstract

In traditional interference lithography, interference fringes are typically phase locked to a stationary substrate using analog homodyne photodiode signals that are fed back to control a phase-shifting device such as an electro-optic modulator or a piezoelectrically transduced mirror. Commercially available fringe-locking systems based on this approach often achieve stability of the interference fringes to within a small fraction of the fringe period p ͑typically Ϯ p/20 peak-to-peak͒. We describe the performance of a heterodyne fringe control system utilizing acousto-optic phase shifters and digital controls that is designed to satisfy the much more stringent fringe control requirements for scanning beam interference lithography. We demonstrate locking to Ϯ p/100, and expect further significant improvements. This versatile system can also be used to lock the phase of moving fringes in almost arbitrary fashion at fringe velocities up to 2.5ϫ10 7 periods/s and to measure the phase of gratings.

Doppler writing and linewidth control for scanning beam interference lithography

“…2 obtains the phase difference ⌬⌽ = ⌽ L − ⌽ R between the left and the right arms. 3 In order to implement Doppler writing, this phase must be synchronized with the stage motion. This requires significant frequency differences ⌬f between the arms, where ⌬f = ͑1/2͒͑d / dt͒⌬⌽.…”

“…In both methods the interference fringes are held stationary with respect to the substrate using fringe locking electronics and acousto-optic modulators. 3 The first method ͓Fig. 1͑b͔͒ consists of a parallel scan, in which the stage scans along a direction parallel to the interference fringes.…”

Determination of two-dimensional phase shifts in three-beam laser interference patterns

“…The phase shifters that are often used for the introduction of phase shifts are opto-electronic and piezo-mounted mirrors [10]. In this work, a piezoelectric transducer (PZT) on which the reference mirror is mounted is used to vary the phase difference between the interfering beams by changing the voltage applied to the PZT [11,12].…”

Phase control in multiexposure spatial frequency multiplication

“…2͑b͔͒, 11 the left interference arm is modulated by 90 MHz and the right interference arm by 110 MHz. The phase difference between the incident left arm and the incident right arm is detected by phase meter 3.…”