There is a limit to the minimum feature size that can be printed using current lithographic techniques. For that reason,
engineers often employ various shrink methods in production to reduce the size of features generated by lithography.
One such technique is the application of a shrink assisted film for enhanced resolution (SAFIER). In such a process, a
SAFIER chemical is coated onto a patterned photoresist and baked. During the bake, the resist expands, and hence, the
patterned spaces in the resist shrink. The shrink process, however, does not necessarily occur uniformly across the
wafer, and some critical dimension (CD) non-uniformity can be introduced during this step. This study investigates the
efficacy of using an intentionally biased SAFIER bake temperature profile to compensate for some of the CD nonuniformities
introduced during the SAFIER process. In the baseline case, patterned wafers underwent a standard
SAFIER process using a thermally uniform bake. The bake temperature of the SAFIER bake was then biased to cancel
out some of the shrink induced CD non-uniformity. Wafers processed through the biased temperature SAFIER bake
showed a 20% improvement in post-SAFIER (critical dimension uniformity) CDU. For comparison, a biased post
exposure bake (PEB) temperature was used to create wafers with a non-uniform starting CD distribution designed to
cancel out some of the CD non-uniformity from the SAFIER process. When these wafers were processed through a
uniform temperature SAFIER process, a 54% improvement in post SAFIER CDU was observed over the baseline case.
A pattern-recognition and encoding system has been developed for a biochip platform using shaped hydrogel sensors batch produced via photolithography. Each sensor shape is fashioned with a unique pattern of dots that makes it identifiable to a pattern recognition system. By linking the sensor's function to its shape, "random" arrays can be created (i.e., arrays that do not require sensors to be located at specific positions). Random arraying can be quickly and cost-effectively achieved via self-assembly methods. Pattern-recognition software was written to perform automated recognition of micrographs exhibiting fluorescing sensors. As a test of the recognition process, an array of shape-encoded DNA sensors was fabricated using lithography. Fluorescent micrographs were taken of a DNA-sensing experiment, and then processed with the pattern-recognition software. The results show that this process is quite viable with 98% recognition accuracy of the nondefective sensors in both images.
This paper investigates the feasibility of using an electrostatic chuck (ESC) on a post exposure bake (PEB) plate in the
track to improve the critical dimension uniformity (CDU) for bowed wafers. Although it is more conventional to
consider vacuum chucking during PEB, electrostatic chucking offers some potential advantages, chief among which is
the fact that electrostatic chucking does not require any type of a seal between the wafer and the PEB plate whereas
vacuum chucking does. Such a seal requires contact and therefore has the potential to generate backside particles on the
wafer. Electrostatic chucking therefore has the potential for a cleaner overall process. Three different PEB plates were
tested in the course of this investigation, a non-chucking PEB plate (SRHP), a PEB plate equipped with a vacuum chuck
(VRHP), and a PEB plate equipped with an ESC (eBHP). It was found that CD uniformities were up to 84 percent lower
for bowed wafers that were chucked during PEB relative to wafers that were not chucked. In every case tested, wafers
processed through chucking PEB plates showed lower CDUs than wafers processed through the non-chucking plate.
CDU results were similar between vacuum chucked wafers and electrostatic chucked wafers. Based on the results
presented in this paper, it can be concluded that electrostatic chucking during PEB is a feasible method for controlling
CD uniformities on bowed wafers.
Line edge roughness (LER) and intrinsic bias of 193-nm photoresist based on two methacrylate polymers are evaluated over a range of base concentration. Roughness is characterized as a function of the image log slope of the aerial image, the gradient in photoacid concentration, and the gradient in polymer protecting groups. Use of the polymer protection gradient as a characteristic roughness metric accounts for the effects of base concentration. Results demonstrate that a methacrylate terpolymer exhibits an advantage over the copolymer resist by achieving lower roughness at smaller values for the polymer protection gradient, resulting in lower LER for patterning. Intrinsic bias is found to be a function of the concentration of base. Process window analysis demonstrates that a greater depth of focus can be achieved for resists with low intrinsic bias. However, a tradeoff in depth of focus with LER is found. Spectral analysis indicates resists with greater intrinsic bias exhibit greater correlation lengths. Systems with greater intrinsic bias demonstrate lesser roughness for patterned features, with a minimum roughness achieved at maximum intrinsic bias. Kinetics of deprotection are modeled to calculate the chemical contrast of each resist. Resists exhibiting the greatest chemical contrast are identified as materials that generate the least roughness.
Computer simulators are ideal tools to study complex process spaces,
but current lithography simulators are based on empirically-derived
continuum approximations and thus are unsuited for investigating
properties like line edge roughness (LER) because they do not incorporate molecular level details. A "mesoscale" simulation is
described that enables molecular level effects to be captured. This
technique is a compromise between accurate, but slow, atomic-level
simulations and the less accurate, but fast, continuum models. The
modeling of stochastic processes that lead to LER is enabled via use
of Monte Carlo techniques. Mesoscale simulation was used to study
the effects of added base quencher to overall photoresist performance. Simulations of acid/base kinetics with quencher loadings ranging from 0 to 20% show good qualitative agreement with
experimental data. Results show that decreasing aerial image quality
increases the root-mean-square (RMS) roughness, whereas increasing
base quencher loading improves LER, up to approximately 50% base. A
mechanism that explains line edge roughness stemming from acid gradients is proposed. This mechanism is supported by simulations
showing that the catalytic chain length varies inversely with acid
concentration. Simulation results show that base effectively limits
the influence of acid in low concentration regions. A critical drawback of using base additives is significantly reduced photospeed.
Previous work has demonstrated the dependence of photoresist line edge roughness (LER) on the image-log-slope of the aerial image over a wide range of conditions; however, this relationship does not describe the influence of other factors such as photoresist composition or processing conditions on LER. This work introduces the concept of chemical gradients in the photoresist film rather than gradients in aerial image intensity as being a governing factor in the formation of photoresist LER. This concept is used to explain how differences in acid and base concentration in the photoresist lead directly to differences in observed LER. Numerous photoresist formulations were made over a wide range of compositions using 193 nanometer photoresist polymers as the basis. Experimental results coupled with results from simulation show that increasing the gradient of photoacid and hence increasing the gradient of protected polymer and the overall chemical contrast of the system reduces printed LER.
As critical dimensions in microlithography become ever smaller and the importance of line edge roughness becomes
more pronounced, it is becoming increasingly important to gain a fundamental understanding of how the chemical
composition of modern photoresists influences resist performance. Modern resists contain four basic components:
polymer, photoacid generator, dissolution inhibitor, and base quencher. Of these four components, the one that is least
understood is the base quencher. This paper examines the influence of base additives on line edge roughness, contrast,
photospeed, and isofocal critical dimension (CD). A mathematical model describing the tradeoff between contrast and
photospeed is developed, line edge roughness values for different base types and loadings are reported, and isofocal CD
is shown for various photoacid types as well as for different base types and loadings.
In a chemically amplified resist the exposure energy is used to generate a catalytic species, which promotes a solubility-switching reaction during a post exposure processing step. Using an absorbed photon to generate a catalyst, instead of using it to directly cause a solubility-switching photochemical reaction, allows for much lower exposure doses to be used for patterning since the catalytic species can eventually promote multiple solubility-switching events instead of just one. Some level of catalyst mobility is necessary to achieve the amplification effect as the catalyst must move from reaction site to reaction site, but any catalyst mobility creates the possibility of movement from exposed regions into unexposed regions causing image blur or line width spreading. As the catalyst diffuses in the resist, it promotes chemical reactions; these chemical reactions complicate analyses of catalyst diffusion by changing the chemical environment of the diffusant. Thus, the material properties of the surrounding resin are changing, sometimes drastically, as the catalyst diffuses. In addition to simple changes in material type, the chemical reaction also generates a transient material state as reaction by-products either remain in the resist film or desorb. The variation in lifetime of this transient state is another factor that must be considered in a full analysis. This work reports a method to separate reaction effects from catalyst diffusion effects. Acid diffusion in polymers which are close structural analogues to poly(4-t-butyloxycarbonyloxystrene) (TBOCST), while being unreactive to diffusing acidic molecules, was studied. Specifically, the diffusion properties of photogenerated perfluorobutanesulfonic acid in the unreactive TBOCST analogues poly(4-isopropyloxycarbonyloxystyrene) and poly(4-neopentyloxycarbonyloxysytrene) are reported. Measuring and understanding diffusion in these analogue polymers provides insight into the more complicated, and more important, reaction-diffusion processes of TBOCST.
An alternative approach to lithography is being developed based on a dual-layer imprint scheme. This process has the potential to become a high-throughput means of producing high aspect ratio, high-resolution patterns without projection optics. In this process, a template is created on a standard mask blank by using the patterned chromium as an etch mask to produce high-resolution relief images in the quartz. The etched template and a substrate that has been coated with an organic planarization layer are brought into close proximity. A low-viscosity, photopolymerizable formulation containing organosilicon precursors is introduced into the gap between the two surfaces. The template is then brought into contact with the substrate. The solution that is trapped in the relief structures of the template is photopolymerized by exposure through the backside of the quartz template. The template is separated from the substrate, leaving a UV-curved replica of the relief structure on the planarization layer. Features smaller than 60 nm in size have been reliably produced using this imprinting process. The resolution silicon polymer images are transferred through the planarization layer by anisotropic oxygen reactive ion etching. This paper provides a progress report on our efforts to evaluate the potential of this process.