Charakterisierung von Nanostrukturen auf EUV-Masken mit CD-AFM und AFM mit geneigtem Messkopf

Photomask is a key component in lithography systems. During the lithography process, the structure patterns on the photomask will be transferred and demagnified by the ratio 4:1 to the silicon wafer by the imaging system of the lithography tools. The measurement of the structure pattern on the photomask is an important task, since its quality will directly impact that of end products. It is particularly important for the EUV photomask, where more complicated materials are applied in its fabrication. Besides the pattern displacement parameters such as overlay, the structure parameters such as critical dimension (CD), sidewall angle, height, line edge roughness (LER) need to be measured accurately.

Critical dimension and sidewall profile of EUV photomask structures were measured by using two kinds of AFM techniques, CD-AFM and tilting-AFM. The CD-AFM applies flared tip, which is capable of measuring such as CD, feature height, left and right sidewall angle, LER and structure position deviation. However, the large and complex geometry of the flared tip limits its spatial resolution and the measurement capability of corner rounding and footing. In contrast, the tilting-AFM may apply super sharp tips, offering high spatial resolution. However, the tilting-AFM needs the rotation of the structure/tip to make the other sidewall measurable, leading to difficulties in measuring the CD. Both techniques are applied complementarily in this study, adding their strengths and overcoming their limitations.

Traceability is a fundamental issue for AFM nano metrology. As the calibration of the scaling factors of scanners can been well satisfied by metrological AFMs today, the calibration of the effective tip geometry remains a challenging issue. In this study a PTB reference structure, whose real geometry was accurately and traceably calibrated by transmission electron microscopy (TEM) using either the silicon crystal lattice or the pitch of structures as an internal scale, is applied for this purpose.

Figure 1 demonstrates the repeatability of the tilting-AFM measurements, where edge profiles obtained from 32 repeated measurements are plotted, with part of their sidewall profiles detailed in the inset. Because of the small tip radius (nominal value of 2 nm) of the applied super sharp tip, the obtained the edge profile should be very close to that of the real structure. Dimensions such as sidewall angle, feature height and edge profile can be evaluated from the measured image.

Figure 2 shows a typical image of a grating structure of the EUV-Photomask measured with CD-AFM using an AFM tip type CDR-70, which had a nominal tip width of 70 nm. The tip geometries was characterised and tip wear was monitored by applying a PTB reference CD structure before, after and during the measurements. Dimensions such as CD, left and right sidewall angle, feature height and line edge roughness (LER) can be obtained from the measured image.

Experimental results indicate the high measurement stability of the developed instrument, for instance, the long term CD stability is better than 1 nm over 500 successive measurements over 55 hours. In addition, the AFM results are compared to that of a PTB EUV scatterometer. The comparison of the middle CD yields a linear relation within a spread of only about +/- 2 nm and an offset of the absolute values below 3 nm. For the sidewall angle, both methods yield consistent results within a range of about 2 °. Currently the expanded uncertainty (k=2) of the CD measurements of EUV photomask structures is preliminary estimated as 2.5 nm.


Fig.1: (a) measured edge profiles of 32 repeated measurement by the tilting-AFM. The inset figure shows the zoom-in details of the sidewall at the marked area; (b) a typical CD-AFM image measured from a grating feature of the EUV-Photomask