| Summary: | 博士 === 國立交通大學 === 電子工程系 === 89 === The lithographic technology faces a challenge when the semiconductor moves into the submicron or deep-submicron era. The principal issue is the exposure wavelength should be lower to quarter-micron or even smaller, i.e., the conventional halogen lamp should be replaced by the laser light source. However, the drawback of the laser light source is its relative lower output power. To conquer the lower power output, the original ultra-violet (UV) photoresist should be replaced by the chemically amplified deep UV photoresist. For a positive DUV resist, a radiation sensitive acid generator is decomposed during exposure, and the subsequent acid-catalyzed thermal reaction at an elevated temperature, i.e. post exposure bake (PEB), makes the resist soluble. As well known, chemically amplified (CA) resist based on acid catalysis for DUV lithography is a promising technology for patterns of 0.18 μm or less. Previously, the main problems for DUV resists were airborne contamination and linewidth change with different delay times. For the positive DUV resist, the generation of “T-top” at the resist-air interface is attributed to neutralization of the photogenerated acid by airborne organic bases, such as ammonia, during post exposure delay (PED). Fortunately, a resist system comprising of a CA resist and an organic base not only prevents a T-top formation, but also suppresses acid diffusion reaction within resist film (the linewidth change is mainly induced by the acid diffusion). In this work, a model was established to describe the linewidth according to the PED time, based on the mechanism of neutralizing organic base and photogenerated acid. It has been found that the linewidth broadened immediately after exposure and eventually became saturated. The resist pattern undergo a long term PED can not only obtain a reliable linewidth control, but also receive a larger process window and smaller optical proximity effect (OPE). If the stepper and track should be running under an in-line configuration, the linewidth variation induced by PED can be prevented by employing our model and automatic control system.
Printing smaller linewidth is achievable by either reducing the exposure wavelength or increasing the numerical aperture (NA). However, the Rayleigh depth of focus, DOF = /(2NA2), has become smaller with technology advances. Although 0.8μm or larger DOF is normally a minimum requirement for mass production, the DOF for a specific pattern is generally smaller than 0.6μm. This is owing to that the pattern smaller than the machine's capability is always drawn on the reticle to lower the user’s cost of ownership (COO). In light of the decreasing DOF of modern small wavelength and high NA lithographic tools, the position of best focus must be determined accurately and efficiently. This work presents a novel bar-in-bar (BIB) pattern to monitor the focus and tilting of production wafers. The inner and outer bars contain various hole sizes. When defocused, the shrinkage of the smaller patterns is more significant than that of the larger ones, thus causing the center of gravity to shift. Through the organization of the bar patterns, the centers of inner and outer bars shift in opposite directions when defocused. An overlay measurement tool can be used to easily measure the shift between the centers of inner and outer bars. Therefore, a second order polynomial equation can precisely fit the measured BIB shift. In addition, an accurate and reliable focus value can be obtained with a maximum error less than 0.05μm by simply differentiating the fitting equation. By adding the unique BIB to the scribe lanes of the production wafers, the best focus and tilting of the lithographic tools can be acquired when measuring a layer-to-layer overlay shift and, then, can be fed back to the exposure tool as a valuable reference for following processing wafers. This BIB can also be extended to other useful information, such as lens heating correction, edge die leveling adjustment, and wafer chuck flatness.
In summary, this work focused on two of the main problems, PED effect and best focus determination, in the submicron and deep-submicron era. Of course, there are still many urgent issues need to be solved, such as, laser power output improvement, lens manufacturing, photoresist development, reticle manufacturing, defect reduction, etc. This work is just hoping to give some advises and helps to the lithographic field.
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