05898172

Page 1

Inline Control of an Ultra Low-k ILD layer using Broadband Spectroscopic Ellipsometry Ronny Haupt, Jiang Zhiming, Leander Haensel

Ulf Peter Mueller, Ulrich Mayer

KLA-Tencor Corporation One Technology Drive, Milpitas 95035, CA

GLOBALFOUNDRIES Module One Limited Liability Company & Co. KG Wilschdorfer Landstrasse 101, 01109 Dresden

Abstract— As chip dimensions are scaling down new challenges develop in the back-end-of-line. In order to keep the capacitance small while decreasing the volume of the inter-layer dielectric (ILD), new materials and processes have been introduced over the past years to lower the dielectric constant of the ILD layers. For design rules of 45nm and below porous Ultra low-k materials are widely used in today’s semiconductor process flows. Beside process challenges this introduces stringent requirements for metrology not only to monitor film thickness but other properties of the material as well. This paper discusses the development and implementation of a Broadband Spectroscopic Ellipsometer for inline process control of a SiCOH based porous ultra low-k film. After deposition the material is cured with UV light to introduce the porosity. The challenge for the metrology is to measure both the thickness and an adequate metric for the chemical properties which do no longer correlate with optical properties. In addition the chemical properties vary as a gradient from top to bottom of the film. We discuss the methodology to develop a metrology recipe resulting in the thickness of a metric layer and the percentaged thickness shrink being the best parameters to sense and track the process adequately. Results demonstrate the sensitivity of the technique to process variations. Short term precision, long term stability and tool-to-tool matching results prove that the technique enables routine process monitoring in a high volume automated semiconductor fab.

Inter-layer dielectric, ILD, ultra low-k, ULK, Metrology, Spectroscopic Ellipsometry, UV cure

I.

INTRODUCTION

Semiconductor manufacturers are driven to an aggressive roadmap to improve device performance in order to remain competitive and provide technology that enables complex computations to be performed in reasonable time with high reliability. In the back-end-of-line (BEOL) of semiconductor processing one of the keys to stay competitive is the inter-layer dielectric (ILD). In order to avoid unwanted effects like capacitive charging or cross-talk between layers the capacitance of the ILD has to be kept small while its volume is decreasing due to shrinking chip dimensions. This led to the introduction of several new materials and processes to lower the dielectric constant of the ILD layers [1]. One of the major

978-1-61284-409-1/11/$26.00 ©2011 IEEE

innovations in the recent past has been the introduction of porous ultra low-k (ULK) materials in the BEOL process flow. These materials with a typical dielectric constant of 2.6 and below are SiCOH-based and cured with UV light after deposition to generate the pores [2]. Besides the challenge to handle the process itself these new ILD films generate new metrology requirements in order to appropriately monitor and control the process conditions. This paper discusses the capabilities of optical film metrology to measure both the thickness and an adequate metric for the chemical properties which do no longer correlate with optical properties of the ILD material necessarily and vary as a gradient in the film. The existing metrology regime on another spectroscopic ellipsometer (SE) system only monitors the thickness after SiCOH deposition and secondly thickness and refractive index of the total film after UV cure. II.

ELLIPSOMETRY METHODOLOGY

This study uses the Aleris 8350 Broadband Spectroscopic Ellipsometer (BBSE) from KLA-Tencor. The signal captured by the ellipsometer (tanΨ and cosΔ) is sensitive to the refractive index and thickness of the film being measured. The chemical composition of the film is correlated with the optical properties i.e. the refractive index of the film. This is the basis for sensitivity of the ellipsometer to composition changes in the film. The Aleris 8350 is a broadband SE with effective wavelength range from 220nm to 800nm. The system has an effective measurement box size of 50 x 50 μm which enables in-line measurements on product wafers. The tool is further equipped with software for model based analysis of collected spectra. A homogenous model representing the film or stack being measured is used as a starting point with reasonable space where the thickness and refractive index can vary. A mathematical regression is performed to match the measured spectra to the modeled spectra resulting in an estimation of the thickness and refractive index. Advanced models such as Harmonic Oscillator (HO), Bruggeman Effective Medium Approximation (BEMA) or parametric gradient models can be implemented. In the case of the SiCOH-based ULK film a two layer approach with HO models consisting of two oscillators each was used as described in the next chapter.


Turn static files into dynamic content formats.

Create a flipbook
Issuu converts static files into: digital portfolios, online yearbooks, online catalogs, digital photo albums and more. Sign up and create your flipbook.