William J. Adair

Lithographic evaluation of a DUV carbon attenuated phase shift mask

Abstract A DUV (248 nm) attenuated phase shift mask (APSM) using a hydrogenated amorphous carbon film was fabricated and its lithographic performance was evaluated against a binary Cr mask. The attenuated mask transmittance was ∼6.4% with a phase angle of ∼180±3°. Exposures on DUV photoresist were performed using an excimer laser stepper on contact vias arranged in both isolated and dense patterns. The depth-of-focus (DOF) process window of the APSM was found to increase significantly with decreasing partial coherence (σ=0.3) for isolated contact vias. The top down SEM measurements show an approximately 30% improvement in DOF over the full range when the APSM is used instead of the binary mask. For smaller contacts around 200 nm, DOF process window improvement can be as high as 100%. The smallest vias printed were ∼150 nm in diameter. At higher exposure dose, sidelobes printing was clearly visible showing an asymmetric “3-leaf clover” pattern behaviour. This non-ideal behaviour is attributed to lens aberrations of the stepper. For dense contact via patterns, a significantly greater process window was found at a higher partial coherence (σ=0.6) when compared to a lower σ. This occurs because in a dense pattern the sidelobes, which tend to print more at a lower partial coherence, overlap between the vias enhancing their printing in the DUV resist.

Impact of attenuated mask topography on lithographic performance

Experimental evaluations were used in conjunction with rigorous electromagnetic simulations to evaluate the affect of attenuated phase-shifting mask (PSM) fabrication processes on lithographic performance. Three attenuated PSMs were fabricated including a normal leaky- chrome reticle and two novel approaches: a recessed leaky-chrome reticle for reduction of edge scattering and a single-layer reticle employing a hydrogenated amorphous carbon film. Direct aerial image measurements with the Aerial Image Measurement System (AIMSTM), exposures on an SVGL Micrascan 92 deep-UV stepper, and TEMPEST simulations were used to explore the effects of edge-scattering phenomena for the different mask topographies. For each reticle, the process window at a feature size of 0.25 micrometers was evaluated for four basic feature types: nested lines, isolated lines, isolated spaces, and contact holes. Further evaluation of the sidewall profiles and the image size on the mask are required to address these discrepancies.

High-density plasma dry etch for DUV attenuated phase-shifting masks

Attenuated phase-shifting masks have gained wide acceptance in the manufacturing environment during the last few years. Etching attenuated films remains a challenging process step that affects several critical mask parameters including critical dimension (CD) and phase angle. This paper reports the result of etching MoSi attenuated phase-shifting materials using an inductively coupled plasma system. CD and phase-control performance is presented as well as a performance comparison between ICP and reactive ion etching. Attenuated PSMs have typically been used primarily for contact-type patterns. However, recent lithographic simulation result show significant benefits of attenuated PSM with off-axis illumination for gate-type patterns. Fabrication of gate-type attenuated PSMs introduces new challenges for the etch process. Initial etch performance result are also presented.