Chromeless Phase Lithography Offers Simpler RET

Crunching down to the lower k₁ values that lithography is destined to endure, players throughout the lithography chain are forced to come up with some pretty creative solutions. We hear the discussion about such resolution enhancement techniques (RETs) as optical proximity correction (OPC) and phase-shift masks (PSMs) without perhaps realizing the complexity behind some of those solutions. Chromeless phase lithography (CPL) is a new RET that promises to enable enhanced CD uniformity, simplicity and improved cycle time.

CPL was developed recently at ASML MaskTools (Santa Clara, Calif.). Researchers there, along with others from ASML Technology Development Center (Santa Clara), Motorola Inc. (Austin, Texas) and maskmaker Photronics Inc. (Allen, Texas) detailed CPL's use in ArF (193 nm) lithography at the latest Photomask Technology conference in Monterey, Calif. In their paper, the researchers describe CPL as a form of alternating aperture PSM (AAPSM), likening it to a 100% attenuated PSM.

The AAPSM is a group of solutions for increased exposure latitude and enhanced depth of focus. The mask has trenches etched into its quartz substrate that generate a 180° phase shift relative to the unetched areas. The phase differences cause interference, which improves image contrast. Two-beam imaging then improves resolution. The complementary PSM (cPSM), the most common type of AAPSM, uses two masks. The first has the phase-shifted geometry, while the second is a standard binary mask used to remove unwanted phase edges caused by the first exposure.

In contrast, CPL is a technique that uses only one mask, this factor alone being a plus because it inherently improves stepper throughput over dual-mask approaches. CPL offers lithographers simpler designs and verifications than other AAPSM approaches, the authors note, for several reasons: There is no requirement for phase assignments, there are no phase conflicts, and CPL is less sensitive to phase errors. CPL uses three-beam imaging, which still lets it take advantage of off-axis illumination (OAI) — more tricks for enhancing existing lithographic equipment.

This compares a typical reticle process flow of chromeless phase lithography (CPL) with complementary phase-shift mask (cPSM). (Source: Photronics; image originally appeared in BACUS Symposium on Photomask Technology 2002, Vol. 4889)

For their work, the researchers considered only one variation of CPL — the hybrid-mesa design — which they compared with cPSM. Like the cPSM process flow, the process flow for the hybrid-mesa design of CPL requires two writes to achieve the 180° phase shift (Figure). CPL requires an extra step — a blanket quartz etch that comes before second-level printing. The target (in this case, 300 nm) is represented as a mesa in quartz or by an OPC structure in chrome. The final mask pattern defines 0° areas and the chrome shielding structures that will curb optical proximity effects as k1 drops lower.

The research team experimented with several different focus, exposure and illumination conditions on an ASML PAS 5500/1100 lithography system. Resist thicknesses were 1600 Å for line/space patterns and 2200 Å for contact holes, each over an 800 Å antireflective coating. Targets were 100, 75 and 67 nm for line/space patterns, and 100 nm for contact holes.

Although CPL has benefited from the work that has gone into fabricating AAPSMs, it is not without its challenges. Some issues include etch uniformity and the loss of antireflective coating. The removal of the antireflective coating during the quartz etch presents challenges in inspection, depth metrology and second-level processing, and impacts process steps in reticle and wafer manufacturing lines. As an example of the problems posed by loss of the antireflective coating, the presenters detailed the results if reflectivity varies significantly between 0° and 180° areas — a challenge that current inspection algorithms are not able to address, they say.

Overall, however, CPL does not present any technical showstoppers, according to the researchers. Future work will focus on characterizing optimization and tolerance of phase angle, and evaluating the detectability and printability of chrome and phase defects.

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