High-speed, high-accuracy optical measurements of polycrystalline silicon for process control

Abstract

Highly accurate non-contact polycrystalline silicon (poly-Si) film thickness measurements are important for both real-time feedback control and run-to-run control. Both spectroscopic ellipsometry (SE) and normal-incidence spectral reflectometry (SR) are complicated by poly-Si surface effects and variable bulk poly-Si refractive indices. In this paper we will describe an empirical modification of Beckmann/Kirchhoff scattering theory to account for the effects of rough layers in SR. We will present results from in situ monitoring of reactive ion etching of polySi/SiO 2 /Si, and ex situ comparisons to TEM and AFM. Preliminary conclusions on the applications of this model to SE analysis of poly-Si will also be presented. IntroductionHighly accurate non-contact polycrystalline silicon (poly-Si) film thickness measurements are important for both real-time feedback control and run-to-run control. Both spectroscopic ellipsometry (SE) and normal-incidence spectral reflectometry (SR) are complicated by poly-Si surface roughness effects 1,2,3,4 and variable bulk poly-Si refractive indices. 5,6 The accuracy of poly-Si thickness measurements is usually more strongly limited by the models employed than raw measurement noise or instrument calibration errors. These model-related systematic errors directly limit the minimum time required for extraction of in situ etch rates because the thickness change over this minimum time must be larger than the systematic and random errors. 7 We will present a 2 simple scalar scattering theory method for modeling the poly-Si surface roughness effects and will present results using this model in high-speed poly-Si reactive ion etch (RIE) rate monitoring. We will present quantitative comparisons between in situ SR data and ex situ TEM and AFM measurements that show significant improvements in the accuracy of both bulk film and surface roughness thicknesses obtained using this model vs. the commonly employed Bruggemann effective media approximation (BEMA) model for surface roughness. Improvements in the dynamic response of the etch rate monitor will be shown. We will also discuss the use of the B/K model for SE measurements of poly-Si and improvements in the models for the bulk poly-Si dielectric function. Experimental DetailsWe use a low-cost Ocean-Optics™ CCD spectrometer/W-halogen lamp system to perform high-speed, in situ normal incidence SR measurements over the 400-800nm wavelength range on poly-Si/SiO 2 /Si samples during reactive ion etching . The details of this system, accuracy comparisons to a commercially available spectrophotometer, thickness accuracy tests vs. SE on SiO 2 /Si samples, and its initial applications to high-speed in situ monitoring of poly-Si RIE have been previously presented. 7,8 SR data with 5nm resolution can be collected in 40 ms with good raw signal-to-noise ratios. Ex situ spectroscopic ellipsometry data was collected using a Sopra GESP-5 variable-angle rotating polarizer ellipsometer run in the tracking analyzer mode.Analysis of both SR and SE data wa...

Insituthickness measurements of ultra-thin multilayer polymer films by atomic force microscopy

“…Normal-incidence spectral reflectometry (SR) in combination with AFM roughness measurements were used to determine film thickness and the results were compared with transmission electron microscopy (TEM) measurements. The SR data can yield very accurate surface roughness and total film thickness values [6]. TEM measurements have the inconvenience of requiring pre-treatment of the samples, in addition to the difficulties in determining the film boundary.…”