Today reticle costs become one of the main contributors in the cost of manufacturing of advanced logic products.
Especially for low volume projects as of product sampling as well as design and product verifications the saving of
reticle costs becomes worthwhile. Multi level reticles are the combination of more then one lithographical layer on one
physical reticle. Due to this approach the physical amount of reticles per tape out will be reduced and thereby also the
costs for reticles will be significantly decreased. The multi level reticle approach is implemented as standard option in
the INFINEON Technologies tape out flow for advanced logic products. This means dependent on forecasted volume
and chip size it could be decided to tape out a project on multi level- or single level reticle. Technical setup, reticle
layout, specification, CAD flow and experience in daily work using multi level reticles in different design nodes will be
shown. Reticle cost advantage versus reduced throughput will be discussed.
Gerd Scheuring, Alexander Petrashenko, Stefan Doebereiner, Frank Hillmann, Hans-Jurgen Bruck, Andrew Hourd, Anthony Grimshaw, Gordon Hughes, Shiuh-Bin Chen, Parkson Chen, Thomas Schatz, Thomas Struck, Paul van Adrichem, Herman Boerland, Sigrid Lehnigk
Besides the metrology performance of a CD measurement tool, its close integration into a manufacturing environment becomes more and more important. This is extremely driven by the ever increasing complexity of masks and their tightening specifications. Hence, this calls for the capability of fully automated CD measurements on a large number of dense and isolated lines and 2-dimensional features under production conditions. In this paper we report on such a highly automated measurement system for CD measurements from MueTec. Either an ASCII software interface or a specially developed software interface to connect the MueTec with the CATSTM mask data fracturing software handles the large amount of co-ordinates and other information like design images from the measurement sites and their surrounding, which are necessary for fully automated CD measurements. Because the latter is the standard in mask-making and data-formats, this level of automation guarantees a good industrial integration of the MueTec system. Fully automated and reliable CD measurements are based on very stable tool hardware and especially on a positioning stage with best possible positioning accuracy (range better 0.5 ?m), significantly improved possibilities of software controlled positioning and an automated job set up and execution. The time gain in relation to existing measurement programs in the extent of supply has turned out to be dramatically large. The User Interfaces and their applications will be described.
From detailed comparisons of stencil mask distortion measurements with Finite Element (FE) analyses the parameters of influence are well known. Most of them are under control of the mask manufacturer, such as the membrane stress level and the etching process. By means of FE analysis the different contributions may be classified. Some of the errors can be reduced if more stringent specifications of the SOI wafer are fulfilled, some of them may be reduced after pre-calculation. Reduction of the remaining placement errors can be achieved if specific means of an Ion Projection Lithography (IPL) tool are applied. These are mainly magnification and anamorphic corrections removing so-called global distortions. The remaining local distortions can be further reduced by applying the concept of thermal mask adjustment (THEMA).
Stencil masks, based on 150mm Si-wafers, with large diameter membrane fields have been fabricated for use in an ion projection lithography (IPL) tool. With a current membrane diameter of 126mm, the control of pattern placement is one of the major challenges. As the masks are produced by a wafer flow process, pattern distortions after membrane etch, caused by stiffness changes, have to be controlled. Additionally, stress inhomogenity resulting from SOI wafer blank fabrication, boron implantation and other process steps has to be addressed. These parameters will be discussed on a global and local scale. Results by both, experiments and FE modeling simulations are presented.
A short review of the current status of IPL stencil mask development is presented in this paper. Stencil masks based on 6' Si-wafer have been fabricated with a membrane diameter of 126 mm. With a typical membrane thickness of 3 micrometers , mechanical stability is a critical issue. The resulting placement errors have been measured using an LMS IPRO measurement tool and have been compared to Finite Element (FE) calculations simulating the fabrication process. Process-induced distortions can be predicted by FE calculations with an accuracy of up to 24 mm 3(sigma) . In addition to large circular membranes, an alternative geometry has been considered. Masks with a quadratic membrane area of 60 X 60 mm2 show IPDs of 3(sigma) equals 39 nm which are about 4 times smaller than those of large circular membranes. This result agrees well with predictions of FE calculations. In order to protect the Si-mask against ion bombardment, a protective carbon layer is deposited onto the membrane, thus preventing stress changes due to ion implantation. The current status of the carbon deposition process will also be addressed briefly.
As part of the European MEDEA project on Ion Projection Lithography (IPL) a Process Development Tool (PDT [1, 2]) has been designed and integrated. The ion-optics and the mask handling system are already assembled and integrated in the PDT. In order to test the ion-optics system (PDT-IOS), an ion beam pattern lock system as well as a metrology stage (in-situ array of faraday cups to measure beam uniformity, in-situ energy spread analyzer, in-situ distortion measurement unit) have been realized and are being integrated. The current status of the tool will be reported. In parallel to testing the ion-optics a test bench for a vertical vacuum wafer stage has been realized. Operation of magnetic bearing supported stage movement has already been demonstrated. An ASML vacuum compatible optical wafer alignment system has been integrated to the wafer test bench system recently. In air an X/Y alignment repeatability of less than 3nm (3sigma) has already been demonstrated. In order to minimize the mechanical influences of the mask clamping and the therefrom arising critical in plane distortion (IPD)during exposure, a mask frame was developed. On the basis of Pattern IPD measurements on a LMS IPRO the influence was determined. Parallel to the IPL tool development, intensive development of IPL stencil masks is ongoing with success in producing 150mm [3] and 200mm [1,2] stencil masks. An overview of the stencil mask development will be reported. The dependence between boron doping and Si membrane stress will be discussed.
Ion Projection Lithography (IPL) requires stencil masks. These masks are manufactured in a SOI wafer flow process. This means that e-beam patterning and the pattern transfer in silicon is done on the bulk mask-wafer blank before the membrane is formed. The last steps are deposition of a protective carbonic layer and removal of carbon from the stencil openings by etching. The internal stress control of the finally remaining silicon and carbon layers is decisive for the pattern placement accuracy of the stencil mask. The surface geometry and pattern placement are measured with a LEICA LMS IPRO system at different process steps. The initial bow and warp of the SOI mask-wafer blank is measured. Then, the pattern placement is measured after e-beam writing. After membrane formation the samples are measured a third time followed by a final measurement after carbon layer deposition and etch. These results are to be compared with FE (Fenite Elements) modeling calculations. Compared to previous investigations the effect of wafer warp will be included. Furthermore, LMS IPRO measurements will be done with improved tool accuracy on stencil mask membranes as achieved recently. Thus, the claimed functional dependence between stress and pattern distortion is to be verified experimentally.
Stencil masks for Ion Projection Lithography (IPL) are manufactured in a SOI wafer flow process. They consist of a 3 micrometer thick stencil membrane coated by a 0.5 micrometer thick carbonic protection layer. For mask manufacturing, the key parameters which have to be kept under tight control in order to have a high yield are critical dimensions (CD), image placement and defect density. In order to control critical dimensions, the parameters determining CD have to be known in detail. E-beam writing, resist processing, silicon and carbon etching are main contributors. Their impact will be discussed. For CD measurement, different alternatives of tools, optical CD microscopes, AFM and SEM are discussed. Image placement is one of the most critical parameters for IPL stencil masks, as process-induced distortions occur and are to be corrected by a software using FE calculations. Masks usually are specified to 0 defects. Defect inspection results of IPL stencil masks of optical tools are presented, as well as results from e-beam inspection. In addition, defect management for stencil masks in general and cleaning techniques are discussed.
Ion Projection Lithography is one of the major competitors for sub 100 nm-lithography. Within the MEDEA ion projection lithography project and other activities related to it, new results in mask and tool technology have been obtained. The exposure tool is in process of being assembled, so that information of the components as the multi-cusp ion source can be given. Results from the field-composable lens electrode manufacturing and of the off-axis alignment system are to be presented. Mask process technology has been improved by introduction of a multi-step trench etch technique. A stencil mask based on a 200 mm wafer has been produced. In addition, the repeatability values of placement and CD measurements have been decreased. Defect inspection with optical KLA tool results give information on the current limits for stencil mask applications.
Distortion control is one of the key issues to solve for IPL stencil mask development. Placement is measured by a LEICA LMS IPRO system. Registration as well as overlay results and the error contributions of the measurement will be presented. The production flow of IPL stencil masks is marked by the fact, that e-beam patterning is done on the bulk wafer, whereas the removal of the bulk silicon and the creation of the free membrane takes place at the end of the process, after silicon trench etching. Therefore, distortions appear at the release of the membrane after bulk silicon etching and oxide removal. At e-beam patterning, the mask wafer blank is pre-stressed by the sum of the stresses of the different layers as bulk silicon, silicon oxide, the silicon of the latter membrane and resist. Additionally, the initial warp and bow of the mask wafer blank have to be considered. The analysis of the finite element modeling compares the placement at e-beam writing to the situation after membrane completion. With this information, the efficiency of a FE-supported software correction before mask patterning can be improved. Measurements of masks with different stress values are to be discussed in order to deduce the optimum stress values for IPL stencil masks.
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.
One condition for efficient work with more than one metrology system, in one or several facilities, is the matching of the pattern placement metrology tools. The Siemens mask house uses tow pattern placement metrology systems, an LMS 2000 and an LMS IPRO system. Both system are correlated to the PTB standard and match in between. The different measurement performance of the two systems leads to different applications of their use. Both metrology system have ben matched with external facilities. Data will be presented on metrology correlation with Siemens' two 'second source' sites.
Ion beam lithography is one of the most promising future lithography technologies. A helium or hydrogen ion beam illuminates a stencil membrane mask and projects the image with 4X reduction to the wafer. The development of stencil masks is considered to be critical for the success of the new technology. Since 1997, within the European Ion Projection Lithography MEDEA (Microelectronic Devices for European Applications) project silicon stencil masks based on a wafer- flow process are developed. They are produced in a conventional wafer line. Six inch SOI (silicon-on-insulator) wafers are patterned with an e-beam wafer writing tool, then trenches are etched by plasma etching. Afterwards, the membrane is etched by wet etch using the SOI-oxide layer as an etch stop. The last step is to add a coating layer, which is sputtered onto the membrane. It protects the mask against ion irradiation damage. For metrology and inspection, methods used for conventional chromium masks as well as new techniques are investigated. Results from placement measurements on the Leica LMS IPRO tool will be presented. Finally, methods for CD measurement, defect inspection, repair and in-situ-cleaning in the stepper will be discussed, including experimental information of first tests.
We have investigated the pellicle induced distortions on mask of different sizes. The pattern placement on the masks was measured prior to the pelliclization and after pelliclization. The performance of the automatic pelliclization process was compared to the distortions induced by manual pellicle mounting. All investigations were performed using the LEICA LMS IPRO at Leica's demo center in Wetzlar and at Siemens Mask Shop in Munich. The LMS IPRO was equipped with the new long working distance lens having a free working distance of 8mm.
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