2.1.

Repair and Verification

The accuracy of the repair and hence the EPE can be determined by the position of the repaired edge relative to the target position as measured with the ZEISS AIMS^® tools, an industry standard for defect disposition and repair verification of DUV as well as EUV masks.³³ Within a mask shop the repair process and the subsequent printability verification process with an actinic imaging tool such as the AIMS^® are closely connected since a mask repair is only deemed successful if the aerial image of the defective site on the mask is within specification.³⁴ AIMS^® is an at-wavelength (actinic) metrology system specifically designed to emulate the aerial image formation as being done on exposure tools (e.g., ASML scanners). Figure 2 shows the core functionalities of the AIMS^® EUV system in comparison to a wafer scanner. The illumination of the mask within the AIMS^® system is generated in an equivalent way compared to a scanner tool. While the optical design and components of the AIMS^® tools may differ from the modules building up a scanner, they must be able to reproduce the same angular and spatial light intensity distribution locally within a limited light spot on the mask surface, assuring that the features on the mask experience (locally) the same light distribution they would experience on the scanner. After being collected by the numerical aperture (NA) on the mask side, the light is propagated in a projector lens (or mirror optics in the case of EUV) and the aerial image is magnified to be recorded with a CCD camera for post-evaluation. Not only the illumination of the mask must be emulated to achieve a complete and fully reliable printability statement, but the collection of diffraction orders as transmitted (DUV) or reflected (EUV) off the mask must be equivalent as on the scanner, to reproduce all relevant imaging effects which are significant for a thorough mask qualification, e.g., in the qualification of a repair. Thanks to this scanner equivalent aerial image generation process, and ability to measure through focus, AIMS^® offers capabilities which go well beyond a printability statement, but allows to break down the components of an EPE and qualify different contributions deriving from the mask. This is one major advantage of such powerful metrology system, i.e., the capabilities to provide a full mask qualification, and therefore gaining tight control over an extensive metrology budget, without the need of printing one single wafer.

Fig. 2

AIMS^® EUV core functionalities for actinic defect disposition and repair verification of EUV masks.

One of the challenges still open for having a seamless EUV production line is an inherent property of EUV light: each photon transports a considerable amount of energy when compared to 193 nm photons of DUV. As a consequence, the number of photons needed to develop a resist is much lower than for DUV lithography, giving rise to so-called stochastics failures on wafer, which to date are still a matter of concern in the community. In normal measurement mode, the AIMS^® EUV system itself uses an absolute intensity of light to locally illuminate a mask feature much larger compared to the ones in the scanner. However, being able to tune this light to match the flux seen by the single masks features locally on the mask, the AIMS^® EUV is also able to capture the aerial image of a mask feature emulating the same amount of photon noise as produced in the scanner aerial image, before this gets absorbed into the resist and developed. The novel AIMS^® EUV scanner stochastics emulation mode can be further employed in studying the statistical impact of mask error sources contributing to defectivity and/or EPE, being able to qualify defects or defect repairs from a statistical point of view, providing more insights into the effects such defects and different repairs have on the final wafer product.³³ AIMS^® Auto Analysis (AAA)³⁵ is a powerful tool that automatically evaluates these light intensity maps and subsequently assesses the success of the repair without much overall need of user input.

3.1.

Overview Photomask Repair Tool

As detailed in a previous section, scaling trends in the semiconductor industry towards smaller technology nodes and feature sizes are continuing and first consumer products manufactured with EUV technology are already on the market. These developments lead to more strict technological requirements especially for the corresponding EUV photomasks in terms of repair accuracy, i.e., in particular MRS and EPE (see previous section). The current industry standard for high-end photomask repair tools is the MeRiT^® neXT system. The next generation repair tool targeting the high-end photomasks of the upcoming technology nodes is the MeRiT^® LE system. The MeRiT^® LE offers a 35% improved repair resolution compared to its predecessor and features a MRS of 10 nm. It is designed to repair photomasks of the 5-nm technology node and beyond.³⁶ With the MeRiT^® LE transparent and opaque defects of many different geometries on DUV and EUV photomasks can be repaired successfully. The tool uses FEBIP technology and provides highest repair resolution. The MeRiT^® LE makes use of a new e-beam column, which provides a decreased e-beam spot size at a given voltage compared to e-beam columns employed in predecessor MeRiT^® tools. Furthermore, it is operated at a lower e-beam voltage of 400 V, which reduces the area exposed to SE and shows a reduced jitter by improved tool damping. All these points together enable the repair of photomasks for future technology nodes with shrinking feature sizes (s. previous chapter). In Fig. 3, an image of the tool is depicted.

Fig. 3

View of the next generation photomask repair tool MeRiT^® LE.

3.2.

High-End EUV Photomask Repair Results

In this section, we present EUV mask repairs of transparent and opaque defects on a programmed defect mask (PDM) to verify the repair capabilities of the MeRiT^® LE. The PDM is based on an industry standard EUV blank, i.e., using the same Mo-Si multi-layer and capping as EUV production masks and a binary Ta based absorber of 60 nm thickness. Please note that, unless stated otherwise, all dimensions are in mask dimensions, and not in wafer dimensions (Factor 4x for current EUV scanners). To demonstrate repair of opaque pattern defects, compact extrusions and bridge defects have been chosen. Broken-line defect types have been selected for clear defect repairs. All repairs have been verified by using AIMS^® EUV actinic measurements, which provide a full emulation of the scanner imaging conditions. To quantify the measurement results, the AAA software has been used. Simply speaking, AAA automatically extracts edges from the recorded aerial images through focus thus allowing for quantitative Edge Placement and CD evaluations. Selected pattern sizes and defect types on the mask reflect decreased device structures on high-end photomasks and increased complexity of pattern defects of the upcoming 5 nm technology node and beyond: Defects on EUV masks with feature sizes down to 60-nm half-pitch and extrusion widths of 9 nm in size have been successfully repaired.

For repair demonstration and verification, SEM Pre-Repair, SEM Post-Repair, and corresponding AIMS^® EUV aerial images have been recorded. For all shown AIMS^® EUV measurements, NXE:3300 scanner Dipole aperture settings have been used. All results have been analyzed at best focus and through focus for various focal planes (FP). Here an FP of $\mathrm{FP}=-1$ describes a FP with a shift of $-1\mu \mathrm{m}$ compared to the best focus plane (FP = 0).

To illustrate what effect even small defects can have on the wafer print of high-end photomasks with shrinking feature sizes a small extrusion type defect with 500 nm in length, 9 nm in width, and a half-pitch size of 88 nm on mask has been analyzed. Figure 4(a) show the SEM image of the programmed defect and Fig. 4(b) the corresponding aerial image, acquired by AIMS^® EUV.

Fig. 4

(a) SEM image of a small extrusion type defect (9-nm width, 500-nm length) with 88-nm half-pitch on mask. (b) Aerial image by AIMS^® EUV of the small extrusion defect. The defect can be clearly recognized in the center.

From the widened blue line in the center of Fig. 4(b), it becomes clear that the aerial image is significantly affected even by this tiny extrusion defect. To repair such small defects, the repair tool needs to provide an excellent MRS capability of better than 10 nm and a highly accurate repair edge placement. Subsequently, it is shown how this kind of small defects can be addressed by the MeRiT^® LE system.

First investigated defect type on the EUV PDM represents the same as just discussed in the previous section, i.e. a small extrusion type defect with 500 nm in length, 9 nm in width, and a half-pitch size of 88 nm on mask. The repair is thus subtractive. Figures 5(a) and 5(b) show the SEM image of the defect before and after the repair, respectively, measured with the MeRiT^® LE system.

Fig. 5

(a) SEM Pre-Repair image of a small extrusion (9-nm width, 500-nm length) type defect with 88-nm half-pitch on mask. The dashed line indicates the desired absorber edge. (b) SEM Post-Repair image of small extrusion defect. (c) Post-repair aerial image by AIMS^® EUV of small extrusion defect. The repair is located in the center and is surrounded by four markers visible as red vertical lines in the very edges of the aerial image. (d) Post-repair aerial image analysis of the CD with AAA at best focus. The black line shows the CD along the repaired space. Gray lines show nearby reference structures, reflecting mask noise. (e) Aerial image analysis of the CDs at the reference sites in the vicinity of the repair for various FP. Blue points indicate the average values, and the bars represents the $3\sigma$ deviations. (f) Post-repair aerial image analysis of the CDs of the repaired extrusion for various FP. Blue points indicate the average values, and the bars represents the $3\sigma$ deviations.

In Figs. 5(c) and 5(d) the post repair aerial image, acquired by AIMS^® EUV and a corresponding detailed analysis of the CD after the repair carried out with the software AAA at best focus are shown. The black line in Fig. 5(d) represents the CD along the center of the defect site, i.e., in “horizontal” direction indicated by the coordinate “$y$.” The gray lines represent the corresponding CDs along the “unrepaired” neighboring reference “spaces,” i.e., along the clear regions. From Fig. 5(d), it becomes evident that the CD variations along the center of the defect (black line) are below the CD variations of the reference, i.e., unrepaired regions (gray lines). A detailed analysis of the aerial image revealed for the repair a maximum CD deviation of $\mathrm{\Delta }{\mathrm{CD}}_{\mathrm{MAX}}=3.3\mathrm{nm}$, whereas for the unrepaired reference regions a standard variation, representing $3\sigma$-values, of $\mathrm{\Delta }{\mathrm{CD}}_{\mathrm{REF}}=3.6\mathrm{nm}$ to the average CD reference value of ${\mathrm{CD}}_{\mathrm{REF}}=88$ has been determined.

Figures 5(e) and 5(f) show the results of the through-focus analysis of the CD of the post-repair aerial image at various FP. In Fig. 5(e), the CDs for the reference structures, i.e., the unrepaired regions, in the vicinity of the repair, and in Fig. 5(f), the post-repair CD values are shown. Depicted CD values represent average values and corresponding deviations ($3\sigma$-values). From Figs. 5(e) and 5(f), it is evident that also for the other analyzed FP the CD deviations of the extrusion repair are about the variations of the reference CDs. This demonstrates that the MRS of the repair tool is better than 10 nm. Furthermore, the average CD in reference site and repaired site are both well within $88\mathrm{nm}+/-1\mathrm{nm}$ demonstrating the Edge Placement capability of the repair tool. In total, these data verify the successful repair of the 9-nm extrusion with the MeRiT^® LE.

The second investigated opaque defect type on the EUV PDM is a bridge type defect with 500 nm in length and a half-pitch size of 60 nm on mask. The repair is thus subtractive. Figures 6(a) and 6(b) show the SEM image of the defect before the repair and the defect site after repair with the MeRiT^® LE system. In Figs. 6(c) and 6(d) the post-repair aerial image, acquired by AIMS^® EUV and a corresponding detailed analysis of the CD after the repair with the software AAA at best focus are depicted. The black line in Fig. 6(d) represents the CD along the center of the defect site, i.e., in horizontal direction indicated by the coordinate $y$. The gray lines represent the corresponding CDs along the unrepaired neighboring reference “lines,” i.e., along the clear regions.

Fig. 6

(a) SEM Pre-repair image of a bridge (500 nm length) type defect with 60-nm half-pitch on mask. (b) SEM Post-repair image of bridge type defect. (c) Post-repair aerial image by AIMS^® EUV of bridge type defect with 60-nm half-pitch on mask. (d) Post-repair aerial image analysis of the CD with AAA at best focus. The black line shows the CD along the repaired space. Gray lines show nearby reference structures, reflecting mask noise. (e) Aerial image analysis of the CDs at the reference sites in the vicinity of the repair for various FP. Blue points indicate the average values, and the bars represents the $3\sigma$ deviations. (f) Post-repair aerial image analysis of the CDs of the repaired bridge defect for various FP. Blue points indicate the average values, and the bars represents the $3\sigma$ deviations.

In Fig. 6(d), the CD variations along the center of the defect (black line) are below the CD variations of the reference regions (gray lines). A detailed analysis of the aerial image revealed for the repair a maximum CD deviation of $\mathrm{\Delta }{\mathrm{CD}}_{\mathrm{MAX}}=2.6\mathrm{nm}$, whereas for the unrepaired reference regions a standard variation, representing 3σ-values, of $\mathrm{\Delta }{\mathrm{CD}}_{\mathrm{REF}}=4.1\mathrm{nm}$ to the average CD reference value of ${\mathrm{CD}}_{\mathrm{REF}}=59.4\mathrm{nm}$ has been found. This indicates that at best focus the repaired regions appear even smoother than the original structures where the noise of the structures is determined by the mask manufacturing process. Figures 6(e) and 6(f) show the results of the through-focus analysis of the CD of the Post-repair aerial image at various FP. From Figs. 6(e) and 6(f), it is evident that the CD deviations of the repair are comparable to or even below the CD deviations of the reference sites.

In addition to the extrusion and the bridge defect types, we have investigated clear defects on the EUV PDM using a broken-line type defect with 500 nm in length and a half-pitch size of 60 nm on mask, i.e. the repair in this case is additive. Figures 7(a) and 7(b) show the SEM image of the defect before the repair and a corresponding SEM image of the defect site after the repair with the MeRiT^® LE system. In Figs. 7(c) and 7(d), the corresponding post-repair aerial image and the detailed analysis of the CD after repair carried out with AAA at best focus are shown. The black line in Fig. 7(d) shows the CD along the center of the defect site, i.e., in horizontal direction indicated by the coordinate $y$. The gray lines represent the corresponding CDs along the unrepaired neighboring reference lines, i.e., along the opaque regions.

Fig. 7

(a) SEM Pre-Repair image of broken-line type defect (500-nm length) with 60 nm half-pitch on mask. (b) SEM Post-Repair image of broken-line defect. (c) Post-repair aerial image by AIMS^® EUV of broken-line defect. (d) Post-repair aerial image analysis of the CD with AAA at best focus. The black line shows the CD along the repaired line. Gray lines show nearby reference structures, reflecting mask noise. (e) Aerial image analysis of the reference CDs in the vicinity of the repair for various FP. Average values and deviations ($3\sigma$ values) are given. (f) Post-repair aerial image analysis of the CDs of the repaired broken-line defect for various FP. Average values and deviations ($3\sigma$ values) are given.

In Fig. 7(d), the CD variations along the center of the defect (black line) are about the CD variations of the reference regions (gray lines).

Since the repair processes are still under optimization on the new tool, the edges of the deposited line do not yet fully close up on the mask feature line [Fig. 7(b)]. However, since the gaps are very small and the imaging process of the scanner acts as a low-pass filter, the post repair AIMS^® EUV aerial image is only marginally affected [Fig. 7(c)]. Nevertheless, at the edges of the repair site (around $y=\pm 250\mathrm{nm}$) slightly higher CD deviations for the repair occur. Consequently, the maximum extracted CD deviation in that case is a bit higher compared to opaque defect types. A detailed analysis of the aerial image revealed a maximum CD deviation of $\mathrm{\Delta }{\mathrm{CD}}_{\mathrm{MAX}}=3.8\mathrm{nm}$ for the repair, whereas for the unrepaired reference regions a standard variation, representing $3\sigma$-values, of $\mathrm{\Delta }{\mathrm{CD}}_{\mathrm{REF}}=1.9\mathrm{nm}$ to the average CD reference value of ${\mathrm{CD}}_{\mathrm{REF}}=58.5\mathrm{nm}$ was determined.

Figures 7(e) and 7(f) show the results of the through-focus analysis of the CD of the Post-repair aerial image at various FP. From Figs. 7(e) and 7(f), it appears that for the analyzed FP, the CD deviations of the repair are even lower than at best focus. Overall, the absolute $3\sigma$-values of the repair CD are for all FP at a very low level with a highest $3\sigma$-value of only $\mathrm{\Delta }{\mathrm{CD}}_{3\sigma }=2.4\mathrm{nm}$. It is worth mentioning that these CD deviations are the to our knowledge lowest values reported so far for a deposition repair at a half-pitch size of 60 nm on mask.