2.1.

Preparation of a Programmed Line Type Phase Defect EUV Blank

A programmed line type phase defect EUV blank (with no absorber) consists of a Ru-capped Mo/Si ML deposited on a quartz substrate with seeds on quartz for the creation of line type phase defects. The ML consisted of a 2.5-nm-thick Ru film on a 40-bilayer Mo (2.2 nm thick)/Si (4.8 nm thick) film. To serve as seeds for the phase defects, lines of 100 nm in width, arrayed in $7\times 7$ with a pitch of 22 mm, were fabricated on the quartz substrate. As a first step, a photoresist layer was coated on the chrome layer on a quartz substrate. Next, a set of line patterns were drawn on the photoresist layer by an electron beam writing system (EBM-8000, NuFlare Technology Inc., Yokohama, Japan). Then after developing the photoresist layer, the chrome layer was etched. As a last step, the quartz substrate was etched and the chrome layer was removed. After running through a cleaning process, the patterned substrate was then coated with a Ru-capped ML. The nonflatness of the blank before and after the ML coating was below 20 and 800 nm, respectively, and the ML surface was higher in the center of the EUV blank than in its corners.

2.2.

Preparation of a Programmed Dot Type Phase Defect EUV Mask

To investigate the influence of the phase defect on EUV microscope images, a programmed phase defect EUV mask was prepared. Figure 1(a) shows the mask design. Dots on quartz substrate and the absorber patterns of half-pitch (hp) 88-nm L/S were arrayed in block A (at the center of the mask) and block B (66 mm away from the block A). The fabrication processes of the dot type phase defects are same as the line type phase defects. The positions of the phase defects relative to the absorber lines were designed to be shifted in a prescribed fashion as shown in Fig. 1(b). The position of the phase defect was defined as 0 nm when the center of the phase defect coincided with the center of the absorber line. The right side of the absorber line was defined as plus direction as shown in Fig. 1(c). The dimensions and the positions of the phase defects at the mask surface were measured using an SPM (L-Trace II, Hitachi High-Tech Science Corp., Tokyo, Japan).

Fig. 1

(a) Arrangement of the test pattern. (b) Illustrations of the hp 88-nm L/S pattern with dot type phase defect. (c) Definition of the phase defect position.

2.3.

Imaging Conditions and Data Analysis

Figure 2 illustrates the imaging optics of the EUV microscope developed by Tohoku University²⁷ that was utilized in this study. The EUV light was sourced from a beam line BL3 of the NewSUBARU synchrotron facility at the University of Hyogo. The outer and inner numerical apertures of the illumination optics were 0.25 and 0.14, respectively. The coherence factor of the illumination light was about 0.01 because the synchrotron source size is very low. The incident angle of the EUV light to the EUV mask was 11 deg, and the angle between the plane of incidence and line pattern direction was approximately 20 deg. The exposure time to take an image was set at 30 s. To reduce the influence of the speckle noise on the EUV microscope image, nine images of the same pattern were taken and superimposed.

Fig. 2

Schematic model of the extreme ultraviolet (EUV) microscope optics.

Figures 3(a) and 3(b) show the schematic models of an EUV mask and an EUV microscope image of the mask, respectively. When a phase defect is not adequately covered by the absorber line, image intensity of the L/S pattern varies depending on the phase defect position. Figure 3(c) shows the intensity profile of the EUV microscope image along a line A-A′ drawn across the L/S pattern containing the defect. Here, $I_{\mathrm{ML}}$, $I_{\mathrm{ABS}}$, and $I_{\mathrm{DEF}}$ stand for the intensities of the reflected lights from the space pattern without the phase defect, line pattern, and space pattern with the phase defect, respectively. The normalized intensity was calculated by the following equations:

Fig. 3

Illustrations of (a) the phase defect in L/S pattern and (b) the EUV microscope image. (c) Intensity profiles along the line AA′.

3.1.

Analysis of the Phase Defect Structure

According to our previous work, the path of a pit type phase defect propagating from the bottom of an ML and ending at the top is inclined toward the center of the mask.²⁸ Furthermore, the inclination angle increases in proportion to the distance from the center of the mask. In this section, to determine the increasing angle of inclination, and to determine the angles of the line type phase defects, a set of line type phase defects in blocks 1, 2, 3, 4, and 5 [Fig. 4(a)] were prepared for TEM observation. Figure 4(b) describes the schematic view of the TEM observation direction. Each TEM observation sample was cut perpendicularly to the line type phase defect using a focused ion beam. The TEM images and the inclination angles of the phase defects are shown in Fig. 4(c). The inclination angle was defined as the angle between a vertical line from the quartz substrate surface and a line connecting the center of the line pattern on the quartz substrate with the center of the raised surface of the ML. These TEM images clearly show that the phase defect’s propagation through the ML is inclined toward the center of the EUV blank. The relationship between the inclination angles of the phase defects and the distances of the phase defects from the center of the EUV blank is summarized in Fig. 5. Then from this information, the inclination angles of the phase defects in block B [Fig. 1(a)] were estimated to be 4 deg.

Fig. 4

(a) Arrangement of the test pattern. (b) Direction of the phase defect observation. (c) Cross-sectional TEM images and calculated inclined angles of the phase defects.

Fig. 5

Inclination angle of the phase defect as a function of the distance of the phase defects from the center of the blank.

3.2.

Measurement of the Phase Defects Using SPM

The profiles and sizes of the phase defects evaluated in this study were measured using SPM. Figures 6(a) and 6(b) represent the SPM image and cross-sectional profiles of the dot type phase defect on the ML. The measured dimension of the phase defect on the ML was 80.0 nm in full-width at half-maximum and 2.4 nm in height.

Fig. 6

(a) Scanning probe microscope (SPM) image of the dot type phase defect on the ML. (b) Cross-sectional profile of (a).

The phase defect positions relative to the absorber lines were also measured using SPM. To evaluate the influence of the inclination angles of the phase defects on the EUV microscope images, the same position of the phase defect relative to the absorber line needed to be identified in both block A and block B. Calculating from the total thickness of the ML and the inclination angle of the phase defect in block B, the phase defect position shifts about 20 nm toward the center of the mask in relation to the position of the dot on the quartz substrate. Figure 7(a) represents a test pattern which, by the help of SPM, enables us to measure the positional shift of the phase defect relative to the absorber pattern. The test pattern consists of a square-shaped absorber hole $1\times 1\mu \text{m}$ in size and a phase defect in the center of the hole. On the mask pattern data, the center of the square hole was aligned with the center of the phase defect. After running through the fabrication process of the programmed phase defect mask, the test patterns in the blocks A and B were measured using SPM. Figure 7(b) shows the cross-sectional profiles of the test pattern in blocks A and B. The measured positional shift of the phase defect in block B was 19.9 nm, which corresponded to 4 deg of the inclination angle. This result agreed well with the result described in Sec. 3.1.

Fig. 7

(a) Design of the test pattern. (b) Cross-sectional profiles along the line X–X′.

The SPM measurement sites in block B were selected by taking into consideration the positional shift of the phase defects. Figure 8 represents the SPM profiles of the hp 88-nm L/S pattern with dot type phase defect 80 nm in size and 2.4 nm in height prepared for EUV microscope observation. In both blocks A and B, the measured phase defect positions were 0, 8, 56, 88, and 160 nm.

Fig. 8

SPM images of the programmed phase defect EUV mask and the measured defect positions.

3.3.

Observation of the Phase Defects Using EUV Microscope

The L/S patterns with various positions of the phase defect shown in Fig. 8 were observed using the EUV microscope. Figure 9(a) shows the EUV microscope images and their intensity profiles. The intensity fluctuations observed in the images are due to speckle pattern effect occurring with the utilization of a synchrotron EUV source that has a high spatial coherence. These fluctuations are not mask defect.²⁷ However, the captured images clearly indicate the intensity loss of the EUV light from the space pattern due to the presence of the phase defects. In these cases, the locations of the phase defects were defined as 56, 88, and 160 nm in position. The influence of the phase defect position on the intensity loss of the EUV light was calculated in accordance with Eq. (1). Figure 9(b) represents the normalized light intensity of the space pattern affected by the phase defect as a function of the phase defect position. For both blocks A and B, the maximum drop value of 0.4 was observed at the position of 88 nm; and the intensity differences between blocks A and B were negligibly small at the position of 56 and 160 nm. On the other hand, the impact of the inclination angle on the intensity loss of the EUV light was significant when the phase defect location was 8 nm in position. This phenomenon is the effect of the positional shift of the effective defect position caused by the inclination angle. The effective defect position shifts about $1\mathrm{nm}/\mathrm{deg}$ to the inclined direction compared with the vertical grown one.²⁸

Fig. 9

(a) EUV microscope images and intensity profiles of the programmed phase defect EUV mask. (b) Normalized light intensity of the space pattern image as a function of the defect position.

After running through the mask patterning process, removing of the absorber pattern away from the proximity of the phase defect is recommended as an effective way to eliminate the influence of the phase defect on wafer printing image. The decision on the removing area is based on the information of SPM image. However, this study showed that SPM is not a perfect method to pinpoint the actual phase defect position because the phase defect does not always propagate vertically through the ML. On the other hand, a EUV microscope has potential to pinpoint the actual position and affected area by the phase defect.