The end of the millennium has seen the IC industry make the transition to the sub-wavelength arena. All indications are that the sub-wavelength environment is here to stay until the next generation lithography becomes available (see figure ).
This paper presents a comparative analysis of binary `chrome-on-glass,' attenuated, biased rim, and phase edge shifted DUV lithography solutions for advanced circuitry in the sub-250 nm image size regime. Lithography techniques are compared based on design complexity, ground rule impact, process latitude, and cost. Data are presented from aerial image simulations (SPLAT), aerial image measurements (AIMSR), and SEM measurements. Phase edge shifted designs clearly exhibit the largest process window for 200 nm linewidths exposed on a 0.5 NA 248 nm DUV stepper. The complexity of the mask engineering (design as well as manufacture) and exposure process for this `hard' phase shifting technique warrants the study of less powerful but also less restrictive phase shifting options. This paper investigates the tradeoffs associated with various applicable phase shift mask (PSM) techniques and presents recommendations based on specific program requirements.
The newly developed Aerial Image Measurement System (AIMSTM*) was used to quantify the lithographic benefits of several resolution enhancement techniques as compared to standard binary mask imaging. This system, a microscope based stepper emulator, permits rapid characterization of mask images from both binary and phase shifted mask (PSM) patterns at multiple focal planes. The resultant images are captured digitally with a CCD camera and analyzed using an exposure-defocus tree technique to quantify the depth-of-focus as a function of exposure latitude. The AIMS is used to extract both phase and transmission errors from captured aerial images of all the masks evaluated. AIMS results are compared to wafer electrical linewidth data. A 0.5 numerical aperture (NA) DUV stepper was used with a partial coherence of 0.6 combined with IBM APEX-E resist process. Collected data were analyzed using techniques identical to the AIMS analysis, allowing for a high level of consistency. Comparative data focused on binary mask imaging for the verification of the AIMS results. Trends associated with feature sizes and types are discussed.
Application of an Aerial Image Measurement System (AIMSTM) to binary and phase- shift mask fabrication and evaluation is described. The AIMS tool, an optical system which measures the aerial image directly from a mask, provides rapid feedback on lithographic performance for a variety of stepper configurations through modifications of the wavelength, numerical aperture, and illuminator design. The AIMS tool has been applied during the implementation of an alternating phase-shift mask (PSM) fabrication process in order to understand the impact of the etched-quartz sidewall on lithographic performance. AIMS measurements were used to extract the effective phase and transmission as a function of phase- etch depth as well as post-etch treatment condition. A set of basic test structures are proposed which can be used in conjunction with the AIMS tool to automate the extraction of transmission, phase, and second-level overlay for phase-shifting processes such as alternating and attenuating PSM.
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