David S. O'Grady

Alternating phase-shifted mask for logic gate levels, design, and mask manufacturing

While the benefits of alternating phase shifted masks in improving lithographic process windows at increased resolution are well known throughout the lithography community, broad implementation of this potentially powerful technique has been slow due to the inherent complexity of the layout design and mask manufacturing process. This paper will review a project undertaken at IBMs Semiconductor Research and Development Center and Mask Manufacturing and Development facility to understand the technical and logistical issues associated with the application of alternating phase shifted mask technology to the gate level of a full microprocessor chip. The work presented here depicts an important milestone toward integration of alternating phase shifted masks into the manufacturing process by demonstrating an automated design solution and yielding a functional alternating phase shifted mask. The design conversion of the microprocessor gate level to a conjugate twin shifter alternating phase shift layout was accomplished with IBMs internal design system that automatically scaled the design, added required phase regions, and resolved phase conflicts. The subsequent fabrication of a nearly defect free phase shifted mask, as verified by SEM based die to die inspection, highlights the maturity of the alternating phase shifted mask manufacturing process in IBMs internal mask facility. Well defined and recognized challenges in mask inspection and repair remain and the layout of alternating phase shifted masks present a design and data preparation overhead, but the data presented here demonstrate the feasibility of designing and building manufacturing quality alternating phase shifted masks for the gate level of a microprocessor.

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.

Attenuating phase-shift mask by thermal oxidation of chrome

A novel approach to fabricating an embedded attenuating DUV phase-shift mask by thermally oxidizing chrome is examined. Sputtered films of various chrome oxide composition have been proposed for i-line phase-shift masks, but the films to data have lacked a high enough transmission at the DUV wavelength. Thermally oxidized chrome is shown to produce a transmission usable at the DUV wavelength. The effects of oxidation time and temperature are examined, along with transmission and phase-shift as a function of chrome oxide thickness. While the thermal oxidation of chrome is not fully workable to fabricate a complete mask, it demonstrates the feasibility of using chrome-based materials in fabricating a DUV-embedded attenuating phase-shift mask.

Self-aligned rim-type phase-shift mask fabrication by backside exposure

A novel technique to fabricate a rim-type phase-shifting mask has been developed by exposing the back side of the mask with light. Such a mask requires no significant modification to conventional design layouts and improves aerial image contrast and depth of focus over that of a conventional reticle. The technique is self-aligning, and requires only one data writing level. Results for the simple and highly controllable process are presented, characterizing rim size versus dose and rim image uniformity, linearity, fidelity, and controllability.