Infrared spectroscopic ellipsometry in semiconductor manufacturing

IR-SE metrology is an emerging technology in semiconductor production environment. Infineon Technologies SC300 started the first worldwide development activities for production applications. One application part of our IRSE development roadmap is shallow trench isolation (STI) monitoring. Depth below silicon, film thickness and STI profile are the parameters of interest. A set of wafers with different lines/spaces structures (1D gratings) was prepared. These structures were characterized with atomic force microscope (AFM) for reference data. Extensive IRSE mapping measurements were taken, for 2 different wafers orientations, 0/spl deg/ and 90/spl deg/. Preliminary outcomes are: A clear form birefringency and sensitivity to trench depth and lines density. To take advantage of this sensitivity to gratings depth, 2 concepts based on effective medium theory (EMT) and rigorous coupled wave analysis (RCWA) method were implemented. Excellent correlation results to AFM are presented for EMT for high lines density and for RCWA for low and high lines density.

Introduction

As of today, infrared spectroscopic ellipsometry IRSE [1]–[2] is mainly used for material analysis in research and development. While FTIR based reflection and transmission measurements are widely adopted for process control in semiconductor manufacturing, conventional IRSE does not provide a significant advantage over FTIR for typical applications, like concentration and epitaxial Silicon thickness measurements. On the other hand, a special optical configuration for IRSE provides a small measurement spot, and, at the same time, suppresses IR light reflection from the backside of a wafer. The backside reflection suppression allows to use model based analysis of IRSE spectra, opening a wide range for new and advanced semiconductor process control applications. The IR ellipsometer operates in the mid infrared range and is able to handle 200mm and 300mm wafers automatically. Using model based spectral analysis, new applications have been implemented and existing FTIR applications performances have been extended on both, patterned and blank wafers. The modeling capability is required for fast development and implementation of new applications in production. Model based IRSE analysis combines IR measurements with the advantage of ellipsometry based films characterization. Using IR light for patterned wafers reduces scattering effects significantly compared to light in the visible wavelength range. For advanced semiconductor technologies with lateral feature sizes of 100nm and below, the ratio between the wavelength of IR light and lateral feature size exceeds the factor of 10 and effective medium theory (EMT) describe the optical properties of structured areas. [3]–[4] give significant insights in EMT theory. Complex regular 3D structures can be described in a 1D film stack model for thickness and depth measurements. 1D models were developed and successfully applied to DRAM trench capacitor structures, on real devices. We demonstrate here the potential of IRSE for STI depth. The proposed solutions for 1D grating are robust, uses low CPU resources and can be set-up very quickly in production environment. IR-SE is a statistical metrology method, which gives an average measurement across the spot area [5].