We selected alternating phase shift technology to image 90nm dense lines with a pitch of 200nm for volume production purposes. We simulated which settings of illumination were needed to achieve these pitches taking into account the boundary condition of the exposure tool with the wavelength of 248nm and its maximum NA of 0.68. The simulations showed, that normalized image log slope (NILS) is above 2 for a focus range of at least 600nm, if an alternating phase shift mask is used at the very low σ of 0.2 Typical manufacturing conditions with process variations, lens errors and mask deviations were included in the simulations. Based on these results, on the one hand the mask was specified and manufactured; on the other hand the tool was adaptated to the low σ requirement and the specific lens error sensitivities. Shipley's UV212 on BARC AR7 was used at a resist thickness of 250nm. The resist process was optimized by reducing the concentration of the developer. Finally, experimental verification of this entire system with wafer exposures shows that 90nm lines with a pitch of 200nm could be printed with a focus window of more than 600nm.
In this paper we will examine some of the fundamental imaging effects that must be considered with the intended implementation of Extreme Ultraviolet Lithography (EUVL). The paper will show how simulation can be used to examine issues such as image placement and the effect of mask dimension errors. We will show how the exact structure of an EUV mask need not be simulated, but the use of Fourier boundary conditions may be used as an accurate substitute, considerably speeding up computation time. Further, this technique is used to show the positional error that is inherent in an off-axis reflective optic design such as that proposed for EUV exposure tools. Any dimension error that is produced on the mask will not linearly transfer to the printed wafer, this is known as Mask Error Factor (MEF). We will present simulation data showing that the off-axis nature of the incident light leads to different rates of change of printed CD, at defocus, for features orientated perpendicular to each other. These effects must then be taken into account when we consider reticle error budget for EUVL technology.
We are now at a major junction in lithography where non-optical lithographies, such as Electron Projection Lithography (EPL) [1], are being introduced. The mask used in EPL is a non-transparent silicon substrate with a thin silicon (~2m) membrane with openings for electrons to pass through acting as a scatterer. This must be inspected as defects may cause printable defects. Initial mask inspection work has used SEM inspection to find these defects. However, we have historically used optical mask inspection tools, utilising wavelengths at or above what we are using for imaging, to qualify masks. This technology has been increasingly difficult to sustain as we have moved from imaging using mercury lamp based sources to pulsed excimer laser based sources that are not very suited to the inspection imaging. Indeed, review of defects found has moved from optical microscopes to SEM based tools. Inspection tools have also evolved, with the first SEM based mask inspection tools being developed to find the smallest defects, however these have the penalty of very low throughput. We will show the potential of using optical systems for the transmissive inspection of these EPL masks. The high potential of existing tools will be shown together with the need for a next generation of inspection tools. We will show that simulations indicate that an inspection source with 193nm wavelength would be required for the detection of 50nm defects on a mask used to print 70nm dense lines. It will also be shown how the position of the defect within the membrane greatly influences detection as well as the implications of moving to a thinner silicon membrane.
The rapidly emerging Optoelectronics market is bringing new challenges to the lithographer. The production of very large devices, such as waveguides, has historically utilised contact printers to manufacture many devices with both large field size and relatively large dimensions. However, the devices being manufactured today have dimension requirements well below that possible on either contact or projection aligners. As a result of this, i-line lithography is now seen as almost standard, with some leading edge companies producing devices with dimensions that require DUV lithography. An obvious outcome of this is the need to stitch fields together in order to print these relatively large devices, further complicated where i-line and DUV lithography solutions are used, due to the majority of i-line tools being 5X reduction steppers whereas most DUV tools are 4X reduction scanners. In this paper we show results from stitching of multiple reticle fields on i-line steppers and DUV scannners as well as the interdependence between scanner and stepper fields. Experimental results will show that, through various techniques such as field rotation or field magnification, the stitch accuracy can be adjusted. We will also show that this can be applied not only to the complete length of the stitched field, but also to a specific part that may encompass only the device being stitched. Data will show the ability to stitch successfully 5X to 5X, 4X to 4X, and 5X to 4X. In order to measure the accuracy of the stitched field, an overlay tool using box in box structures is normally used. We will show that an exposure tool can use its self-metrology to measure the accuracy of the stitch. The accuracy and repeatability of these measurements as well as a comparison to a typical overlay tool will be given.
As we introduce alternating phase-shifting masks (altPSM) to production we have been very aware of our inability to detect quartz defects in the phase-shifted well. Much work has been completed to establish the printability of various defects. This paper will show, using rigorous three dimensional electro-magnetic simulation, that it is the effect upon the aerial image of the surrounding features and the resultant CD variation that is most critical, not the actual printing of the defects. Data will be presented showing that smaller defects in the center of the shifted space than at the edge of the space cause the allowable CD variation to be reached. It will also be shown that the most sensitive position for a quartz defect in a shifted space is not at the edge or center of the space, but rather at a point between these two. We will further present work showing that asymmetrical aberrations enhance the effects of these defects, especially at defocus, while symmetric aberrations reduce the effect. We will then use advanced techniques, that take into account the extra non-planar incident waves we must consider in 1X imaging, to establish how these defects are 'seen' by 1X actinic mask inspection systems. In 1X imaging we will show again how the transmitted aerial images of these defects are effected by the aberrations present.
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