We report on the traceable calibration of linewidth (CD) photomask standards which are used as reference standards for
production masks of the 65 nm node. Two different types of masks with identical layout were produced and calibrated,
namely a binary mask (CoG) and a half-tone phase shifting mask (193MoSi PSM). We will in particular describe the
applied calibration procedures and cross-correlate the results from different high resolution metrology tools, like SEM,
UV microscopy and AFM. The layout of the CD photomask standard contains isolated as well as dense line features in
both tones with nominal CD down to 100 nm. Calibration of the standards was performed at PTB by UV microscopy and
LV-SEM, supported by additional AFM measurements. For analysis of the measured high resolution microscopy images
and the deduced profiles appropriate signal modeling was applied for every metrology tool, which allows a meaningful
comparison of geometrical parameters of the measured calibration structures. By this approach, e.g. the deduced feature
widths at the top of the structures and the widths at 50% height of the structures can be related to the measured edge angles.
The linearity e. g. of the measured top CD on different type of structures on the CoG CD standard was determined
to be below 5 nm down to line feature dimensions well below 200 nm.
In the framework of the European EXTUMASK project, the Advanced Mask Technology Center in Dresden (AMTC) has established in close collaboration with the Institute of Microelectronics in Stuttgart (IMS-Chips) an integrated mask process suited to manufacture EUV masks for the first full field EUV scanner, the ASML α-demo tool. The first product resulting from this process is the ASML set-up mask, an EUV mask designed to realize the tool set-up.
The integrated process was developed based on dummy EUV blank material received from Schott Lithotec in Meiningen (Germany). These blanks have a TaN-based absorber layer and a SiO<sub>2</sub> buffer layer. During process development the e-beam lithographic behaviour as well as the patterning performance of the material were studied and tuned to meet first EUV mask specifications.
For production of the ASML set-up mask the new process was applied to a high performance EUV blank from Schott Lithotec. This blank has absorber and buffer layers identical to the dummy blanks but a multilayer is embedded which is deposited on an LTEM substrate. The actinic behaviour of the multilayer and the flatness of the substrate were tuned to match the required mask specifications. In this article we report on the development of the mask manufacturing process and show performance data of produced EUV full field scanner masks. Thereby, special attention is given to the ASML set-up mask.
Different type of CD metrology instrumentation is in use today for production control of photomasks, namely SEM,
AFM as well as optical microscopy and optical scatterometry is emerging, too. One of the challenges in CD metrology is
to develop a system of cross calibration which allows a meaningful comparison of the measurement results of the different
systems operated within a production environment. Here it is of special importance to understand and also to be able
to simulate the response of different metrology instrumentation to variations in sidewall profile of features on photomasks.
We will report on the preparation of a special COG test mask with an intended variation of sidewall features
and the subsequent metrological characterization of this mask in different type of CD instrumentation. The discussion of
the measurement results will be accompanied by a discussion of the simulation of instrument response to feature sidewall
The precise targeting of critical dimension (CD) features on photolithographic masks is an essential part of the mask production process. It is straight forward that the usual decrease of specification numbers can only be achieved using cutting edge CD Metrology tools. That also implies that the most advanced CD tool might change from node to node and over time mask houses accommodate a small variety of different tools. Therefore, it is an important task of current mask metrology to ensure accurate matching and calibration and also to transfer these standards precisely over time.
Here, we investigate the influences of the photolithographic mask material and the resist type on critical dimension measurements utilizing one Atomic Force Microscope (AFM), two CD-Scanning electrical microscopes (CD-SEM) by different suppliers and one optical CD tool. Simulating usual mask house strategies we defined one CD tool as golden tool and measured a 700 A chrome mask on it. This reference measurement was then repeated on all other tools and each of them was matched to the golden tool using standard procedures. Once matching was achieved 5 other masks were measured on all tools with exactly the same settings as the reference measurements. In all we varied the material COG, Mosi193, Mosi248, Chrome thickness 700A and 1000A and different resists. We do observe that calibration within the CD SEM tool class works very well for linearity, but with detectable offset in the range of a couple of nm for different reference masks used. Cross-calibration on the other hand from optical CD to CD SEM tools shows significant differences for process variations, layer thickness and different materials. These findings strongly point out that first of all cross calibration is extremely difficult with current metrology tools and can not be utilized for high end products with the necessary precision. And secondly, even matching within tool classes is material dependent which has to be considered for accurate tool to tool matching.
Proc. SPIE. 6152, Metrology, Inspection, and Process Control for Microlithography XX
KEYWORDS: Semiconductors, Lithography, Electron beams, Electron microscopes, Scanning electron microscopy, Photomasks, Critical dimension metrology, Line edge roughness, Semiconducting wafers, Standards development
Line edge roughness (LER) has become a standard topic in the semiconductor industry for its possible yield impact on wafer production. Recently a number of studies address its measurement process to increase the reliability of results. Here, we investigated roughness on photo lithographic masks, which is the blue print for wafer production. It is shown that LER influences feature uniformity and thus has a considerable impact on overall mask production yield. To determine the roughness parameters we varied measurement parameters on a scanning electron microscope to optimize reproducibility and repeatability of the obtained values. Two parameters dominate the LER values. The first parameter is the length used to average the signal from the scanning electron microscope and to obtain the position of a single edge point. Good results for this so called summing length were obtained for values above 100 nm. The second important parameter is the total length of the investigated line that is used to calculate the LER. Here, we found that the increase of LER values with increasing investigation length is similar to the well-established behavior on wafers. It was found that the average LER value calculated from various sites saturates at investigation lengths larger than 10 μm, whereas single LER results show no scattering within measurement precision for investigation lengths larger than 30 μm independently of mask position. In comparison to similar investigation on wafer both the summing length as well as the investigation length have to be chosen about one magnitude larger. It is suggested that the multi exposure process of mask creates roughness on length scales of the order of several micrometers.