Manufacturing - Lithography T
here are a small number of IDM’s and Foundries that order hundreds and even thousands of photomasks every year. Such users are pretty knowledgeable about the criteria used to select the right photomask. However,there are thousands of other mask users who may only order a few photomasks a year and don’t have the time or resource to figure it all out. This article is aimed at them. They will want to order a photomask which is not over specified and thus unnecessarily expensive. Most are using 1x projection lithography systems. Once they are ready to order their photomasks they should consider the substrate choice options. The article examines the pros and cons of these options, setting out the technical differences between them. Then the question arises of how tightly the dimensions on the photomask need to be controlled. The article will give guidance on what sort of cd control is easy, medium and hard, with an explanation of why tight CD control is inevitably more expensive. The article will cover the same ground for overlay control and defectivity. Furthermore there is the question of who to
order this photomask from. The article will provide an overview of the options, ranging from global semiconductor manufacturing companies to university run R&D departments, sometimes used to make masks. There are many benefits in going to dedicated photomask producers and this article will explain why. As the larger photomask manufacturers are already geared up to provide masks to a specified standard and quality, it need not be a more expensive option. A photomask represents a complex piece of engineering, the result of a manufacturing process with many process steps and options. As a result, when a customer orders their first photomask they are presented with a very large number of questions often using terminology which the user will not be familiar with. This article will attempt to demystify the process. There are three significant components which make up the complete photomask order. The first of these is a file that defines the geometrical data which is to be exposed on the photomask. In the mainstream semiconductor industry this will usually be one of three data formats, Calma GDSII Stream (often abbreviated to any one of these three names) , OASIS (a modern, more compact, data representation) or MEBES (the machine specific format used by the Etec photomask writing systems). Smaller customers have a large number of possible
formats for defining their geometry; two common ones are Caltech Intermediate Format (CIF) and AutoCAD DXF. If you are not using one of the main-stream formats, an important aspect to bear in mind is the need to define geometry which defines areas, rather than constructs such as Line and Arc which produce nice pictures, but are not always easy to convert to exposed regions of a photomask. It is preferable to use one of the three main-stream data formats if possible, as these are less ambiguous about what the geometry represents than some of the other formats.
Technical description The final aspect of a photomask order is the commercial aspects of the order. A typical flow for smaller customers is to provide either sample or actual geometrical data to allow a quotation to be generated, which is then followed up a purchase order.
Within the technical requirements, there are a number of different categories of information required. In summary these are: The physical materials required – size and type of glass
The size and acceptable tolerances of the minimum features
Any requirements for registration and overlay
The inspection criteria for acceptable defectivity levels
The information provided in these categories will dictate which of a range of photomask writing and inspection tools will be used, and dictate the cost. As a general rule, the larger the size of glass, the smaller the features, the tighter the tolerances and the more critical the defect levels, the more advanced tools must be used and the greater the cost.
The most common sizes of glass are 4”, 5”, 6” 7”, 9” and 14” square photomasks designed for use with 3”, 4”, 5”, 6”, 8” and 12” wafer sizes. As the size gets larger the thickness also tends to get larger, ranging from 0.06” to 0.25”. For historical reasons, photomask sizes are normally specified in inches, even though the features on them are normally expressed in microns/nanometres. The size required is dictated by the capabilities of the equipment that the photomask will be used on. Most photomasks ordered by smaller customers are for 1X contact or alignment systems, so the photomask needs to be slightly larger than the
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www.euroasiasemiconductor.com Issue V 2011
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