The push to achieve higher density devices continues to place tremendous demands on optical lithography. Several techniques have been used to achieve 0.35 micrometers feature sizes. This paper presents data on the practical application of numerical aperture (NA) and partial (sigma) for 0.35 micrometers imaging. A number of conventional photoresist systems are characterized at various NA/(sigma) . Important differences in the response of photoresists have been observed. These are quantified with respect to various types of structures. Of particular interest are the affects on dense lines and contact structures. The affect of bias also is quantified by using a special reticle where pitch is held constant and the chrome linewidth is varied to determine optimal process latitude. After examining the imaging performance of a few high-contrast photoresists, the use of normalized image log-slope (NILS) is applied for two stepper conditions. Finally, response curves are generated to show optimal exposure conditions for resolution and depth of focus versus NA/(sigma) performance for a number of different NILS photoresists.
Device manufacturers and lithographic equipment makers are presently preparing for circuit production at design rules of 0.35 micrometers . It is evident that optical steppers will be the production tools, but the choice between i-line and deep UV (DUV) steppers (248 nm) is still a matter of debate. This paper reports the progress made in the development of DUV steppers for production purposes. As a successor to the earlier DUV machine, discussed at the SPIE conference in 1990, which uses a TTL alignment system and automatic excimer laser wavelength control, a new DUV stepper has recently been developed with a new lens at 248 nm wavelength, a 29.7 mm diameter field and a numerical aperture (NA) of 0.5. The stepper body is similar to that of the wide field, i-line systems which have been in production since 1991. The key design parameters and results are reported, including imaging performance down to 0.25 micrometers in both negative and positive resists and a high overlay accuracy based on the TTL alignment system. The capability of matching with i-line systems also is reported.
Many lithographic approaches to achieving 0.35 micron IC design rules have been proposed. Several years ago, the primary candidate was x-ray lithography. Today it is generally acknowledged that an optical approach will be used for such design rules. Both deep UV and i-line stepper technologies have progressed with capability to achieve 0.35pm design rules. High NA, wide-field lenses now exist for both deep UV and i-line [1], With the renewed interest in phase shift technology, i-line capability at 0.35pm design rules is comparable to deep UV technology.
The development of a stepper architecture that allows both wide-field i-line and deep UV lenses to be accommodated in the same body and using thru-the-lens, direct-reticle-referenced alignment method [2] is reported. Common improvements in the areas of stage, die-by-die leveling and environmental control allow exceptional overlay performance to be achieved for both i-line and deep UV. The use of common architecture and the same alignment method facilitates the optimum mix and match combination of i-line and deep UV at
0. 35?m design rules
Experimental investigation of stepper performance is reported in comparison to criteria established for design rules at 0.35pm. Overlay is evaluated on substrates typical of CMOS IC manufacturing. Lithographic performance is investigated using conventional techniques as well as more advanced techniques including phase shift reticles.
Results indicate that overlay performance on tested substrates meets the requirements for 0.35?m design rules. Lithographic results indicate that 0.35pm lines/spaces are achievable using both conventional i-line and deep UV techniques, however, the implementation of phase shift reticles enhances the process latitudes for i-line at 0.35?m.
The possibilities of in-process blue image sensing by using only the implemented darkfield TTL alignment system of a stepper are investigated. It is shown, that all overlay related parameters of a PAS5000/50 and /70 stepper such as red-blue, magnification, rotation and translation can be measured from an enhanced latent image in a special dyed resist but also with some reduction in accuracy in a pure resist only. Results of overlay measurements on typical technological layers and of similar experiments based on latent image sensing in dyed and in pure resist are given, indicating the capability of in-situ overlay control and correction on process wafers. A new alignment target sensing (ATS) technique of focus measurement using the alignment detection system of the stepper for both daily focus control and correction as well as for focus setting on process wafers with typical technological layers is introduced. The method uses special designed alignment markers containing lines and spaces at the working resolution. Results are given for sensing the developed resist pattern as well as the latent image for an i-line and a DUV stepper. The applicability is demonstrated by measuring the influence of varying resist layer thickness and of varying oxide layer thickness of various film stacks of technological layers on optimum focus. The validity of these results is proven by comparisons to other focus measurement techniques such as chessboard and SEM linewidth measurements. A mathematical model based on the diffraction theory of thin gratings has been developed to support the marker design as well as to calculate the Image Quality Signal (IQS) vs. focus/exposure curves of developed images. The model has been verified by experiments.
i-line wafer steppers are evolving as established production tools, and it is evident that they will be used to realize features in the sub-half-micron region. Consequently, i-line steppers can be expected to be the equipment of choice for volume production of 16 MBit DRAMs and possibly the first generation of 64 Mbit devices, before the introduction of DUV lithography. However, for this sub-half-micron resolution, lenses with higher apertures and large field sizes will be required. In this paper a new family of wafer steppers is introduced, with a new mechanical frame design and modular architecture which can accommodate a family of large field i-line and deep UV lenses. Results from the first lens type with NAequals0.54 and a field diameter of 25.5 mm are described. To overcome the anticipated depth of focus problems on production wafers, a field-by-field leveling system is introduced, ensuring optimum focus over the whole image field. A challenging problem of wafer steppers using this option is the alignment accuracy during stepping of stage and active leveling of the wafer chuck. The stepper concept introduced here is able to realize the field-by-field leveling without the need for the throughput consuming field-by-field alignment. For that purpose a wafer stage with a new metrology system and improved accuracy has been designed, resulting in an overlay accuracy better than 85 nm in the global alignment mode. Simultaneously a throughput of more than 80 150 mm wafers per hour is realized. Along with the new lens and metrology concept, the stepper contains local environmental control systems performing better than class 1, to ensure clean handling of 8-inch wafers without the need for space consuming environmental enclosures. This paper reports practical results from the new stepper, including resist features below 0.4 micrometers , overlay measurements, particle control, as well as a general description of the new stepper architecture.
A new excimer laser stepper at 248 nm wavelength Is described wilti an all quartz 5x reduction lens with NA 0. 42 and 21 . 2 mm field size. Design aspects and experimental data are reported. A key feature of the system is a ilL alignment system with direct referencing of reticle to wafer operating at 633 nm The problem of the large focal plane difference beiween exposure and alignment wavelength is solved by special correcflon oplics in the lens. The introducflon of exclmer lasers as new light sources for produclion optical lithography has been facilitated by a flexible optical interface beiween laser and stepper and by an on line calibrafion system to control the laser wavelength. Experimental results for resolution and overlay performance are gtven. 1 .
This paper introduces a new wafer stepper on-line calibration sensor, the Image Sensor, which refers directly to the
aerial reticle image at the exposure wavelength. This sensor system is integrated with other stepper metrology
systems by a so-called Image Sensor fiducial plate, which interacts simultaneously with the aerial reticle image, the
Image Sensor, the TTL alignment system and the focus sensor of the stepper. An integrated software package
ensures on-line regular stepper calibration, eliminating dependance on environmental, process and time
parameters. Unique in this concept is the direct measurement of the consequence of wavelength drift in excimer
laser steppers by measuring the aerial image deformation at the exposure wavelength. This information is used as
a direct feedback to the wavelength control of the laser. Initial results of this system are presented for both UV (365
nm) and DUV (248 nm) steppers.
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