Thomas Schaetz

Reliable Sub-Nanometer Repeatability for CD Metrology in a Reticle Production Environment

The new MueTec , an advanced CD metrology and review station operating at DUV (248nm) wavelength, has been extensively characterised in a reticle production environment. Performance data including resolution, measurement repeatability and throughput will be discussed. The system has demonstrated the ability to image 100nm Cr lines and sub-nanometer (3-sigma) long-term repeatability on lines and spaces down to 200nm in size. Metrology capability on contact hole and serif structures will also be discussed. The paper will also introduce the application of a long working distance DUV objective compatible with pelliclised masks. With a 9% EAPSM reticle for 193nm wavelength a very appropriate image contrast was obtained with both objective types, allowing reliable automated linearity measurements on this type of reticle also. In addition to the metrology performance of the tool, its integration into a manufacturing environment will also be described. This will show how the availability of networked co-ordinate data (either in the form of ASCII files or CATS data) and the high-accuracy stage of the tool enable efficient, automated measurement of large numbers of dense features under production conditions.

CD metrology on OPC features using light optical and electron optical tools

TO enable lithography at low k1 factors, OPC on photo masks is a method of strong and growing importance. But CD metrology of OPC features is suffering form several drawbacks: (1) OPC structures often do not have two parallel edges, which are required for usual CD metrology methods; (2) There are no commonly agreed standard definitions how and where to measure some types of OPC structures; (3) The CDs are very small; (4) There is no automated software for the measurement of OPC structures on operator level. A metrology method and a fully automated software for metrology of OPC structures based on LWM optical CD metrology tools is shown in this paper. It automatically distinguishes between hammerheads and serifs in various orientations. The method works analog to pitch measurements and requires only pitch calibration and no additional CD calibration. The results of light optical metrology tools with i-line and DUV wavelength is compared to CD SEM results.

Through-pellicle-capable DUV-based CD metrology on reticles for wafer fab and R&D environment

A comparison has been made in terms of mask CD linearity measurements between the 2 tool versions of a 248nm based optical CD metrology tool for photomasks, i.e., the high-NA M5k-SWD and the through-pellicle M5k-LWD, as well as to a reticle SEM, i.e., the KLA-T 8250-XR. The measured pattern consists of lines and dots (dark features), and spaces arid contact holes (clear features), both in equal-lines-and-spaces and as isolated feature. Two masks have been measured with the same test pattern, i.e., a binary and a 9%-attPSM for 193nm lithography. The latter was especially challenging because typically such embedded phase shift masks are much more transparent at higher wavelengths than those for which they are optimized. All measurements on the M5k were made intentionally before calibration (apart fmm pitch calibration). The resolution performance of the M5k-LWD and the measurement offsets found between M5k and SEM, as well as between the two M5k-versions is discussed. In addition, two-dimensional metrology based on feature contour extraction from optical or from SEM images has been compared. Although its resolution is inherently lower than that of the high-NA M5k-SWD and a reticle SEM, the M5k-LWD offers a possibility to extend such assessment to pelliclized reticles, which is not possible on the alternative tools.

Definition of new quality criteria and assessment means for masks at 150-nm design rules and beyond

Low-k1 lithography requires enhancement techniques like phase shift and OPC. These techniques impose new and challenging specifications on photomasks. A development to establish means and methods to verify corner rounding, line end shortening, defect printability and the size of jogs, serifs and assist lines in a production worthy manner is based on the assessment of mask production data through a new cluster software tool which combines the output data of a mask defect inspection system, a CD metrology system, an AIMS based mask review station and printing simulation results. Possible definitions of new type photomask quality criteria are discussed and measurement procedures are proposed. As a key application the review of critical features on reticles (OPC, classical defects, contact printability, etc.) at changing stepper conditions ((lambda) , N.A., (sigma) ) is discussed. The concept and the development status of a Photomask Qualification Cluster is presented and early performance results are examined against the target values which are a defect detection sensitivity of 125 nm, optical resolution of 200 nm lines for assist line assessment, CD measurement on lines, contacts and OPC structures with 5 nm repeatability and mask pattern fidelity assessment at printing conditions down to 500 nm lines at reticle level.

Major improvements in mask CD metrology: enhanced performance on attenuated phase-shift masks, corner rounding measurements, and improved measurement automation

With continuously shrinking design rules enhanced techniques are required in mask manufacture which requires more sophisticated procedures for their characterization. As Phase Shift Masks (PSM) are of growing importance a new CD algorithm had to be developed to achieve the same or even higher level of CD accuracy and repeatability as on chrome masks. Major improvements in measurement performance on attenuated PSM have been achieved resulting from improving the PSM CD algorithm based on the experiences reported earlier. With shrinking feature sizes and masks layouts with denser patterns the quantification of corner rounding effects on contacts and line ends is of growing importance. Based on the algorithm developed for the effect of corner rounding on line end shortening a measurement procedure has been developed for contact holes. Measurement results have been shown. To further improving CD measurement automation and to enable easy measurement job modifications a highly flexible device has been developed to import measurement parameters into a macro template.

CD measurements of IPL stencil masks with optical microscopes

CD measurement of photomasks is currently mostly done with microscopes using white light. In the development phase of ion projection lithography, it is useful to evaluate the limitations of this approach. Here, the focus is on measurements with a Muetec2010/Leica LWM200 optical microscope which operates with white light. Stencil masks for ion projection lithography consist of a 3 micrometer thick silicon membrane with a top carbonic protection layer of about 0.5 micrometer. For a feature size of 0.4 micrometer, which corresponds to a wafer technology of 0.10 micrometer (IPL uses a 4:1 reduction), an aspect ratio of about 10:1 results. So, it is questionable if transmissive measurements are appropriate. Effects of reflections on the sidewalls and diffraction effects influence the repeatability. The absolute intensity of the transmitted light is reduced compared to thin mask layers. Results of transmissive measurements are presented and compared to reflective measurements on the same tools. But as the stencil openings are retrograde, the CD which is critical for the use in the ion exposure tool is defined on the top of the Si layer. This can be measured by a reflective method. For stencil openings, it resulted a repeatability of less than 16 nm 3s for a whole range of structure sizes down to 0.5 micrometer (smaller sizes were not evaluated for this purpose yet). This is comparable to the repeatability of chromium-on-glass masks, so a good perspective for future use results assuming an evolutive development of optical CD microscopes.