Steven A. Scheer

Finding the right way: DFM versus area efficiency for 65 nm gate layer lithography

DFM (Design for Manufacturing) has become a buzzword for lithography since the 90nm node. Implementing DFM intelligently can boost yield rates and reliability in semiconductor manufacturing significantly. However, any restriction on the design space will always result in an area loss, thus diminishing the effective shrink factor for a given technology. For a lithographer, the key task is to develop a manufacturable process, while not sacrificing too much area. We have developed a high performing lithography process for attenuated gate level lithography that is based on aggressive illumination and a newly optimized SRAF placement schemes. In this paper we present our methodology and results for this optimization, using an anchored simulation model. The wafer results largely confirm the predictions of the simulations. The use of aggressive SRAF (Sub Resolution Assist Features) strategy leads to reduction of forbidden pitch regions without any SRAF printing. The data show that our OPC is capable of correcting the PC tip to tip distance without bridging between the tips in dense SRAM cells. SRAF strategy for various 2D cases has also been verified on wafer. We have shown that aggressive illumination schemes yielding a high performing lithography process can be employed without sacrificing area. By carefully choosing processing conditions, we were able develop a process that has very little restrictions for design. In our approach, the remaining issues can be addressed by DFM, partly in data prep procedures, which are largely area neutral and transparent to the designers. Hence, we have shown successfully, that DFM and effective technology shrinks are not mutually exclusive.

Design of a cost-effective multiwavelength development rate monitoring tool

Understanding the development rate of resists is critical for the characterization of photoresist formulations and accurate modeling of the photolithographic process. Most commercial development rate monitors (DRMs) are based on the optical interference of a single wavelength of light. (Perkin-Elmer DRM5800; Litho-tech Japan RDA-790). DRMs based on the interference across a broad spectrum of wavelengths, known as multi wavelength DRMs (MW-DRM), were first reported by Konnerth1,2 and have also been used for photolithographic research3,4. This technique has been applied to commercial DRMs (SC Technology Inspector), but the high cost of these tools has made them inaccessible to most research and development facilities. This paper describes the development of a new cost-effective, scaleable, multi-channel DRM that allows collection and calculation of multiple development rate curves using MW-DRM technology. Techniques are presented for collection of multi wavelength data at rates exceeding 80 Hz, which in turn allows the study of photoresists that develop at rates in excess of 5 microns per second. The algorithms necessary to analyze this data are presented. The use of these algorithms for the extraction of development rate curves is demonstrated with resists that exhibit surface inhibition and standing waves. The use of multi-layer algorithms to collect development rate information in films between 0 and 200 nm thick is also shown. Finally, the use of these techniques for characterization of deprotection in chemically amplified photoresists, is presented.

Novel polychromatic measurement technique for determining the dissolution rate of very thin resist films

Conventional optical development rate measurement techniques are generally unsuitable for monitoring the dissolution very tin resist films. Monochromatic systems have inadequate thickness resolution to capture the details of surface and standing wave effects, while traditional polychromatic techniques are generally unable to measure thicknesses below 250 nm. The failure of polychromatic analysis methods occurs when there is an absence of turning points int eh relative reflection spectrum. The exact thickness at which this happens is a function of the wavelength range utilized and the resist materials optical characteristics. A novel measurement method is introduced which allows a polychromatic DRM system to measure any resist thickness. Rather than placing the film under analysis directly on a reflecting substrate, it is spun on a wafer that has a relatively thick transparent film on its surface. The transparent film induces turning points in the relative reflection spectrum. The position of these turning points is modified by the presence of thin resist films in a predictable way, allowing accurate measurement of the resist film, providing the optical and thickness details of the intermediate film are known. Experimental results are presented demonstrating the capability of the technique to measure the dissolution rates of films with initial thickness ranging from 56 nm to 4400 nm. The ability of the method to resolve fine dissolution detail, such as standing waves and surface effects is also presented.