As IC density shrinks based on Moore’s law, optical lithography continually is scaled to print ever-smaller features by using resolution enhancement techniques such as phase shift mask, optical proximity correction (OPC), off-axis illumination and sub-resolution assistant features. OPC has been playing a key role to maximize the overlapping process window through pitch in the sub-wavelength optical lithography. As an important cost control measure, one general OPC model is applied to the full exposure field across multiple scanners. To implement this technique, optical proximity matching of line width across the field and across multiple tools turns out to be very crucial particularly at gate pattern. In addition, it is very important to obtain reliable critical dimension (CD) data sets with low noise level and high accuracy from the metrology tool. Otherwise, extracting the real scanner fingerprint in term of CD can not be achieved with precision in the order of 1nm~2nm. Scatterometry CD measurements have demonstrated excellent results to overcome this problem. The methodology of Scatterometry is emerging as one of the best metrology tool candidates in terms of gate line width control for technology nodes beyond 130nm.
This paper investigates the sources of error that consume the CD budget of optical proximity matching for line through pitch (LTP). The study focuses on the 130nm technology node and uses experimental data and Prolith resist vector model based simulations. Scatterometer CD measurements of LTP are used for the first time and effectively correlated to lens aberrations and effective partial coherence (EPC) measurements which were extracted by Litel In-situ Interferometer (ISI) and Source Metrology Instrument (SMI). Implications of optical proximity matching are also discussed for future technology nodes. From the results, the paper also demonstrates the efficacy of scatterometer line through pitch measurements for OPC characterization.
Conventional and annular illumination modes for a 248 nm DUV scanner will be discussed in this paper for their advantage and drawbacks in critical dimension (CD) control. This includes proximity of line width through pitch size, marginality of resist profile measured as sidewall angle, depth of focus (DOF) in line width variation across field/wafer, and isolated space resolution, supported by SEM and scatterometer metrology. Both illumination modes have been applied in the current technology node with sub-wavelength CD, variable pitch sizes, optical proximity correction (OPC) for resolution enhancement and process control optimization. Each illumination defines process margin in exposure, focus and CD uniformity, to gain capability with improved CD control.
Gate critical dimension (CD) uniformity across field is a key parameter in total gate CD control; it is especially important for highly integrated microprocessor chip with large die size and high speed. Intensive study has been conducted to reveal the impact of scanner leveling tilt, defocus and illumination distribution on CD uniformity across field. Correspondingly CD in die range, vertical-horizontal CD bias, resist side wall angle and profile have all been characterized and monitored for each individual scanner. The monitoring methodology we have established enables us to maintain these CD parameters within fairly tight control range, and also provided efficient and accurate data on tool capability and marginality for running production.
Process improvements attributed to the use of bottom anti- reflective coatings (B.A.R.C.s) are well documented. As our experience with these materials improves, so does our understanding of additional optimization. Recent supplier experiments suggest an increase in the thickness of AZR BARLiTM (bottom anti-reflective layer i-line) solution to reduce photoresist swing curve ratios. Also, changes in thin film stack on common substrates can adversely affect the degree of photoresist reflective notching. It is therefore of extreme importance to determine optimum thickness(es) of a B.A.R.C. material to ensure maximum process potential. We document several process effects in the conversion of a SRAM test device (0.38 - 0.45 micrometers) from a 650 angstrom to a 2000 angstrom BARLiTM film thickness using conventional i-line photolithography. Critical dimension (CD) uniformity and depth of focus (DOF) are evaluated. Defect density between the two processes are compared before and after etch employing optical metrology and electrical test structures. Sensitivity of overlay as a function of BARLiTM film thickness is investigated as well.