Propagation of surface topography of extreme ultraviolet blank substrate through multilayer and impact of phase defect structure on wafer image

Abstract. Our recent study reveals that the propagation of a phase defect (PD) from an extreme ultraviolet mask substrate surface through a multilayer does not always propagate in a vertical direction. To fully understand the propagation model of PDs, two types of defects on a quartz (Qz) substrate are prepared. One is space patterns fabricated by a mask patterning process followed by an etching giving a cross-sectional angle of 90 deg. The others are atomic force microscopy scratched space patterns with their cross-sectional angles as 30 deg and 60 deg. After coating a patterned Qz substrate with a multilayer, propagation of PDs through the multilayer was observed by a transmit electron microscope (TEM). As a result, the TEM images clearly exhibit a tendency that originates from the Qz substrate while the PDs propagate through the multilayer and their propagation path is inclined toward the center of the mask. The impacts of the inclination angles on the printed images on a wafer are calculated using a simulator. A PD with an inclination angle of 1 deg corresponds to a positional shift of 1 nm on a printed wafer image.

Propagation of surface topography of extreme ultraviolet blank substrate through multilayer and impact of phase defect structure on wafer image Tsuyoshi Amano Tsuneo Terasawa EUVL Infrastructure Development Center, Inc. 16-1 Onogawa, Tsukuba-shi Ibaraki-ken 305-8569, Japan E-mail: tsuyoshi.amano@eidec.co.jp Abstract. Our recent study reveals that the propagation of a phase defect (PD) from an extreme ultraviolet mask substrate surface through a multilayer does not always propagate in a vertical direction. To fully understand the propagation model of PDs, two types of defects on a quartz (Qz) substrate are prepared. One is space patterns fabricated by a mask patterning process followed by an etching giving a cross-sectional angle of 90 deg. The others are atomic force microscopy scratched space patterns with their cross-sectional angles as 30 deg and 60 deg. After coating a patterned Qz substrate with a multilayer, propagation of PDs through the multilayer was observed by a transmit electron microscope (TEM). As a result, the TEM images clearly exhibit a tendency that originates from the Qz substrate while the PDs propagate through the multilayer and their propagation path is inclined toward the center of the mask. The impacts of the inclination angles on the printed images on a wafer are calculated using a simulator. A PD with an inclination angle of 1 deg corresponds to a positional shift of 1 nm on a printed wafer image. © The Authors.

Introduction
The influence of phase defect (PD) embedded in extreme ultraviolet (EUV) mask blanks on wafer printing has always been an area of major interest among the device and mask makers. [1][2][3][4][5][6] There have been a number of techniques reported to minimize the printability of PDs. One proposed method is to cover the PDs beneath the absorber layer by shifting the location of the device pattern during mask patterning. [7][8][9][10][11][12] The other one is to eliminate the influence of the phase error by removing the absorber from the close proximity of the PDs after fabricating the device pattern. 13 Regarding these elimination methods, it is necessary to fully understand the effects of multilayer geometry of the PDs and size of PDs on wafer printing. 14,15 As for the growth model of a PD resulting from some bumps or pits on a substrate, a number of transmit electron microscope (TEM) images of PDs have indicated that the growth of PD originating from the substrate surface does not always propagate in a vertical direction. [16][17][18] This means that the multilayer surface geometry measurement of PDs using atomic force microscopy (AFM) or nonactinic system may not identify a defect position correctly and thus may not be the right tools for defect mitigation strategy of covering or compensating for the PDs by absorber pattern. Therefore, to efficiently cover the PDs by an absorber layer, it is necessary to take into account the cross-sectional shape of the PDs and analyze their impacts using high resolution actinic imaging tool like an actinic blank inspection tool or an aerial image measurement system. 19 In this article, to fully understand the defect propagation from the surface of a substrate through the multilayer, several types of defects were fabricated on the substrate. Some such defects were then observed by using TEM after coating the substrate with a multilayer. Using a lithography simulator, the impact of the PD structure on wafer images was also calculated.

Preparation of the Qz Substrate with Various Shapes of Space Patterns
The cross-sectional angles of the space patterns on the quartz (Qz) substrate were designed to be 90 deg, 60 deg, and 30 deg. The 90 deg space pattern was fabricated by applying a standard resist patterning on Qz, followed by a dry etching process. The space patterns with 60 deg and 30 deg angles were fabricated by applying scratch markers on the Qz substrate using an AFM tip with the tip angles of 60 deg and 30 deg as shown in Fig. 1(a). By pressing the tip against the Qz surface, five scratch lines were drawn as shown in Fig. 1(b). A cross-sectional profile of Fig. 1(b) along a line between two illustrated arrows is shown in Fig. 1(c).  To confirm the tendency of the defect propagation direction, the block H located in the upper-right corner of the mask was prepared for TEM observation. The centerdirected space pattern and another space pattern normal to the center-directed space pattern, were both cut perpendicular to the space patterns [ Fig. 5(a)]. Figure 5(b) shows the TEM images of the space patterns. The terms TEM-1 and -2 describe the observation directions 1 and 2 depicted in Fig. 5(a). When the space pattern was observed along the direction 1, the inclination angle of the PD was almost 0 deg. However, the space pattern with its direction was normal to the radial direction and was apparently inclined toward the center of the mask (TEM-2). These results indicate that the inclination angle of the PD through the multilayer is not random, but there is a tendency of its propagation direction to lean toward the mask center. Figure 6 represents a relationship between the inclination angles of the PDs [Figs. 4(b) and 5(b)] and the distances of the space patterns on Qz substrate from the mask center. The inclination angle increased in proportion to the distance from the mask center. The maximum inclination angle was 5.9 deg (93.3 mm away from the mask center).   Furthermore, the observation results of block A, which was located 66 mm from the mask center, also showed that not only the propagation direction but also the inclination angles were independent of the shapes of the seeds on the Qz substrate.

Inclination Angle Versus Positional Shift of the Minimum Intensity of PD Image on Wafer
The impact of the inclination angel of the PD on the wafer printed image was calculated by a lithography simulator. The simulated mask models were the Ru-capped multilayer with several shapes and sizes of pit-type PDs as described in Sec. 2.2. Figure 8(a) shows the schematic cross-sectional mask structures with vertical-type and inclined-type pit PDs. The defects were 6-nm deep and 50-nm wide with their inclination angles of 0 deg and 8 deg. The calculated wafer printed images are shown in Fig. 8(b). Compared with the wafer printed image of the vertical-type PD, the inclinedtype PD image shifted about 8 nm on the mask scale, which corresponded to 2 nm on the wafer scale. The calculated positional shift of the PD image on the wafer due to the inclination angles are summarized in     9. The plus or minus of the inclination angles are defined as shown in Fig. 9(a) with a definition of EUV incident direction and angle. Figure 9(b) shows the positional shifts of the PD images on the wafer as a function of the inclination angle of the PDs. The results show that the image shift values are independent of the depths of the pit-type defects. The image of the inclined PD on wafer shifts about 1 nm∕ deg to the inclined direction compared with the vertical-types one.

Summary and Conclusion
Various shapes of the space patterns were fabricated on the surface of a Qz substrate using an AFM scratching process and a mask patterning process. The Qz substrate was coated with a multilayer for defect propagation analysis from the surface of the Qz substrate to the surface of the multilayer. Propagations of the surface topography of the Qz through the multilayer were observed by a TEM. An incident angle of 5.9 deg was observed at a distance of 93 mm away from the mask center. The inclination angle of the defect was independent of the shape of the defect seed on the Qz substrate. A PD with an inclination angle of 1 deg corresponded to a positional shift of image of 1 nm on a printed wafer image.