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Dense 0.10 µm hole pattern formation is achieved by optical lithography with a KrF excimer laser. A Double exposure utilizing two alternating phase shift masks (PSMs) of the line-and-space (L/S) pattern laid out in different directions produces a dense and small hole image in bright field with large focus and exposure latitude. Applying this method with a KrF excimer laser stepper and a chemically amplified negative-tone resist, a two-dimensional (2-D) 0.10 µm hole array with 0.40 µm pitch is resolved with a 0.6 µm depth of focus (DOF). The hole diameter and pitch in the resolution limit seem to be less than 0.10 µm and 0.28 µm, respectively. Also, 2-D hole arrays with different pitches in the x and y directions are easily formed using masks with different pitches in each direction. The hole pattern of an actual DRAM cell is successfully formed by this method. Because of the excellent patterning performance, this method will enable the fabrication of multi giga bit DRAMs by KrF excimer laser lithography.

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An attenuated phase-shifting mask is one of the most useful technologies for sub-half-micron lithography. However, it is necessary to control new parameters such as phase or transmittance when a phase-shifting mask is applied to practical use. We investigated the effect of phase error on lithographic characteristics for a hole pattern using an attenuated phase shifting mask. It is found that a phase error causes a decrease of depth of focus (DOF) and shift of best focus position. It is indicated that this effect depends on several optical parameters such as hole size and wavelength. In the case of the 0.4 µ m hole pattern with i-line stepper, the phase tolerance must be less than 2.7° to control loss of DOF within 0.1 µ m. It is also found that the edge slope effect of the shifter is rather small and the side wall angle of 70° is acceptable.

A super resolution technique has been developed in order to support next-generation devices. We found that the super resolution technique was effective for smaller patterns but not for larger ones, leading to the restriction of its applicability. In order to obtain wider applicability, we propose a new optical system which has been improved and is applicable to all kinds of mask patterns. According to the proposed optical system, the mask pattern produces a specific source shape preferable for the mask pattern itself. Then the mask pattern is illuminated by this self-optimized illumination source. Thus optimal improvements can be obtained under the same illumination conditions, even if conventional patterns and phase-shift patterns of spatial frequency modulation types and diffracted amplitude modulation types are on the same photomask. Consequently, the super resolution technique will be brought into full use by this optical system.