Resonant waveguide gratings (RWGs) are thin-film structures, where coupled modes interfere with the diffracted incoming wave and produce strong angular and spectral filtering. The combination of two finite-length and impedance matched RWGs allows the creation of a passive beam steering element, which is compatible with up-scalable fabrication processes. Here, we propose a design method to create large patterns of such elements able to filter, steer, and focus the light from one point source to another. The method is based on ellipsoidal mirrors to choose a system of confocal prolate spheroids where the two focal points are the source point and observation point, respectively. It allows finding the proper orientation and position of each RWG element of the pattern, such that the phase is constructively preserved at the observation point. The design techniques presented here could be implemented in a variety of systems, where large-scale patterns are needed, such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays.
Many recent publications have highlighted pattern density effects as a problem in both electron-beam and optical
lithography. These effects are manifested as a systematic variation in critical dimension as a function of position on the
wafer. It is becoming an increasing problem as the pattern density and diminishing critical dimensions are needed for
production nodes 32nm and beyond.
One potential source of pattern density effects is acid volatility, where acid is presumed to redeposit during exposure or
bake; here we refer to this effect as chemical flare. Another source of density effects is develop loading which refers to
the impact of local depletion of developer in highly exposed regions. Both develop loading and chemical flare can cause
deviations in feature size that may be difficult to correct for by adjustment of the exposure process.
Here we describe a method that allows the detrimental effects of chemical flare and develop loading to be separately characterized. The method makes use of arrays of 248 nm exposure sites and a controlled develop process within a custom liquid flowcell; this combination enables a systematic study of these effects.
Several issues, including resolution, etch resistance, chromium-resist interface adhesion,
and sensitivity with post coat delay, complicate the selection of photoresists for 32nm
photomask development from the broad pool of candidates. These issues and others are
addressed after an initial screening of critical resist characteristics to reduce the number
of contenders. A balanced initial screening of photoresists for 32nm photomasks is
presented including global and local critical dimension uniformity, line edge roughness,
and resolution of low and high sensitivity positive and negative tone photoresists, relative
to exposure duration. The multi-dimensional assessment of candidate resists for
photomask applications was summarized with emphasis on the process of selection.
With mask critical dimension (CD) uniformity requirements becoming tighter with each new technology node, mask
manufacturing must deploy a wide range of corrections to meet the CD specifications. These corrections compensate for
e-beam proximity effects, fogging effects, etch loading effect, and other global process non-idealities. In this paper, we
present data demonstrating that the current capability of universal e-beam dose corrections meets 32nm CD uniformity
requirements in the presence of various systematic CD errors. Given that the resist process demonstrates enough
latitude to accommodate the required dose variations, it is the stability and repeatability of the process itself that limits
the ability to meet CD requirements. Substrates, resist coating, post-coat delay, develop variations, and etch stability all
contribute to CD variations. Rather than simply focusing on reducing systematic errors, the process stability must be
As critical dimension uniformity requirements tighten for advanced technology nodes, it becomes increasingly important
to characterize and correct for systematic sources of critical dimension error in mask manufacturing. A long range
proximity effect has been previously reported in the industry to occur in chemically amplified resists that appears to be
related to the develop process and we call this phenomenon chemical flare. Several attempts to modulate this effect have
been characterized and at least one develop nozzle modification has been found to reduce chemical flare by ~50%. In
addition, develop time, develop and rinse processes, and top anti-reflective coatings have been evaluated as methods of
minimizing chemical flare effects in e-beam lithography applications. Positive and negative chemically amplified ebeam
resists have been evaluated and characterized for this effect.
Resist heating has been known to be one of the main contributors to local CD variation in mask patterning using variable shape e-beam tools. Increasingly complex mask patterns require increased number of shapes which drives the need for higher electron beam current densities to maintain reasonable write times. As beam current density is increased, CD error resulting from resist heating may become a dominating contributor to local CD variations.
In this experimental study, the IBM EL4+ mask writer with high voltage and high current density has been used to quantitatively investigate the effect of resist heating on the local CD uniformity. ZEP 7000 and several chemically amplified resists have been evaluated under various exposure conditions (single-pass, multi-pass, variable spot size) and pattern densities. Patterns were designed specifically to allow easy measurement of local CD variations with write strategies designed to maximize the effect of resist heating. Local CD variations as high as 15 nm in 18.75 × 18.75 μm sub-field size have been observed for ZEP 7000 in a single-pass writing with full 1000 nm spots at 50% pattern density. This number can be reduced by increasing the number of passes or by decreasing the maximum spot size. The local CD variation has been reduced to as low as 2 nm for ZEP 7000 for the same pattern under modified exposure conditions. The effectiveness of various writing strategies is discussed as well as their possible deficiencies. Minimal or no resist heating effects have been observed for the chemically amplified resists studied. The results suggest that the resist heating effect can be well controlled by careful selection of the resist/process system and/or writing strategy and that resist heating does not have to pose a problem for high throughput e-beam mask making that requires high voltage and high current densities.