Present status of development of F2 (157nm) exposure tool in Nikon is described. Key points of F2 exposure tool are reported; low aberration projection optics, CaF2 quality, coating durability and gas purging of the pellicle space. We also report the measurement of refractive index inhomogeneity inside CaF2 crystals, which is suspected as the cause of local flare. Characteristics of high NA optics over 0.9 are investigated by imaging simulations for both 193nm and 157nm wavelengths, which are compared NA=0.85 imaging.
Detecting position of the wafers such as after CMP process is critical theme of current and forthcoming IC manufacturing. The alignment system must be with high accuracy for any process. To satisfy such requirements, we have studied and analyzed factors that have made alignment difficult. From the result of the studies, we have developed new optical alignment techniques which improve the accuracy of FIA (alignment sensor of Nikon's NSR series) and examined them. The approaches are optimizing the focus position, developing an advanced algorithm for position detection, and selecting a suitable mark design. For experiment, we have developed the special wafers that make it possible to evaluate the influence of CMP processes. The experimental results show that the overlay errors decrease dramatically with the new alignment techniques. FIA with these new techniques will be much accurate and suitable alignment sensor for CMP and other processes of future generation ULSI production.
Progress of Nikon's F2 tool development is described. Intrinsic birefringence of CaF2 reported in the middle of last year by NIST had large impact on F2 optics designing. However, we believe Nikon has already overcome it, and the imaging performance of our newest design is almost the same level to the performance without the intrinsic birefringence. Several methods to correct the intrinsic birefringence are discussed in this paper. Evaluation software for the intrinsic birefringence is also developed, and simulated performances of the newest optical designs, which correct for the intrinsic birefringence, are shown. Among them, simulated CD uniformity of 35nm width gate is a good measure to evaluate the optical design performance. We have also made a steady progress on gas purging. Purging of 02 and H20 concentration less than O.lppm and lppm respectively has been attained.
Current status of Nikon's F2 tool development is reported. At first, the required otpical performance of F2 tools will be discussed. Image simulation results show that the required NA for 70 nm is 0.8 or more. Then Nikon's approaches to realize the optics and tools are presented. For the F2 tools, the most important elements are the projection optics and gas purging of the light path. As for the projection optics, conventional lens type may not be applicable for F2 wavelength, and new type catadioptric optics may have to be developed. In this paper, designs of some catadioptric types and some all refractive types are shown and compared. AR-coatings are very important to obtain enough illumination power. New data on Nikon's AR-coatings are presented. For the gas purging, Nikon has already achieved oxygen concentration less than 1 ppm and further improvements are now in development. Results of gas purging are also presented.
In this paper, Dry & F doped fused silica (modified fused silica) expected as material of photo-mask substrate in the 157 nm (F2 laser) lithography has been evaluated with respect to the optical properties of transmittance, internal loss, F2 laser durability, index homogeneity and stress birefringence, etc. Obtained internal loss coefficient at 157.6 nm was approximately 0.015/cm (base 10), which was measured by several samples with different thickness. From this coefficient, internal loss was calculated as approximately 3% per 1 cm, 2% per quarter-inch. The uniformity in transmittance inside the sample diameter (120 mm) was nearly less than plus or minus 0.5% per 1 cm, plus or minus 0.3% per quarter-inch. Laser durability test was made by MIT/LL. The samples (3 mm, 20 mm and 40 mm thickness) were exposed to F2 laser light up to 80 - 520 million pulses with energy density of 0.1 to 1.4 mJ/cm2/pulse. No significant transmittance change was observed (change was less than 1.0% per quarter-inch at 0.1 to 1.4 mJ/cm2/pulse, 520 million pulses). We measured the index homogeneity by using interferometer and the stress birefringence by using phase modulation method. Inside the sample diameter (120 mm), index homogeneity was 150 ppm at 632.8 nm, the distribution configuration of relative refractive index has a central symmetry property. The stress birefringence was less than 5 nm per quarter-inch at 632.8 nm. As the result, we concluded that this new material had enough capability for the mask substrate of 157 nm lithography.
The faculties of current alignment sensors for CMP processes are studied. The alignment sensors are LSA, LIA and FIA, and all of them are currently equipped on Nikon's NSR series. The results showed that those sensors have enough faculties for present CMP processes. For much progressed CMP technologies, we have experimentally made a new alignment sensor and examined it. The new sensor is based on the `Phase Contrast Microscope' and is made by modifying the FIA optics. By using the new sensor, a mark with only 20 nm step height can be clearly detected with higher accuracy than conventional FIA, and the sufficient possibilities of the new concept alignment sensor for the `progressed CMP process' are confirmed.
Two types of innovative imaging systems are studied. One is the system with non-incoherent effective source and the other is the system with non-coherent pupil function. When the optics are configured in such a way that a certain pattern would be illuminated by a group of waves diffracted by the pattern itself or an exactly the same pattern, the effective source can be made no longer incoherent. Then the coherence of the effective source can be a new parameter to improve image quality. The simulation shows that the contrast of L/S pattern imaging under the off-axis illumination, for instance, can be boosted up to 99.5% with this system, while to only 90.6% with conventional optics. A new pupil filter is proposed; the area of which is divided into some annular zones to be made mutually incoherent. Then the phase difference becomes less crucial and large DOF can be achieved. The simulation shows that this filter is effective for the exposure of contact hole patterns.
We have established a new photolithographic technique called SHRINC ( Super High Resolution by I I tumi-Nation Control ) which is based on an innovative illumination system. SHRINC improves the resolution and depth-of-focus ( DOF ) by optimum arrangement of the illumination system in respect of the angle of the Ist-order of diffraction generated by the reticle pitch.
The capabilities of SHRINC have been studied by computer simulation. Results from phase shift, annular illumination, and conventional illumination are compared with those of SHRINC. The results show that using SHRINC with 0.35? m line and space patterns, the DOF, defined as the distance over which the aerial image contrast exceeds 60%, is 2. 5x larger than that obtained with conventional illumination, and almost the same as that with phase shift techniques.
In our experiments we have obtained a critical resolution of 0. 275 ? m and more than 2.8 ? m DOF with 0.35? m L/S patterns, using an i-line stepper and SHRINC illumination.
Moreover SHRINC is effective not only for simple line and space patterns, but also for complicated patterns with 0.30 or 0.35?m design rules, such as memory cell patterns or peripheral circuit patterns in the DRAM.
From these results we conclude that i-line steppers with SHRINC will make possible mass production of 64M-DRAMs with single layer resist.
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