Use of silicon-versus-layout verification (SiVL) in process control of wafer lithography and mask-making metrology

Abstract

The latest generations of CMOS are increasingly being patterned closer to the resolution limits of optical lithography, which is one of the reasons that their process windows are decreasing. Hence, control of the process gets more and more important and in-die critical-dimension (CD) measurements are gradually being introduced for the monitoring of the in-line lithographic process performance. Because an increasingly large portion of the CD-error-budget is already being consumed by the mask-making, there is also a strong tendency toward improving process control of the mask, which in turn leads to a rapid increase in the number of mask-CD measurements even within the die.The main two reasons for the larger contribution of the mask to the error budget at wafer level are 1) the statistical process variations of the mask-making process, and 2) the mask-error enhancement factor (MEEF), the magnification factor of reticle-to-wafer error. The latter, and its lithographic process dependence, are very much depending on the shape of a feature and its local vicinity in the design. For example a narrow dense binary line has a larger MEEF value than an isolated line and its partial derivative to defocus is much larger.In this work we use a commercial IC design to demonstrate a workflow that enables mask qualification on the basis of flexible specifications that satisfy manufacturing needs. This flow selects smart qualification sites combining two software options integrated in CATS mask-data transcription. The first retrieves MEEF information from Siliconversus-layout (SiVL) scripting and has various capabilities such as gathering MEEF-defocus behavior. The second is selection of relevant functional sites by using logic operations in the so-called CATS Marking software. The acquired information can then be used to generate realistic mask specifications and forms a cost-method to control both mask quality and price.