X-Ray Fluorescence Determination of Low Metal Concentration in Surface Masking Layers on Organic Polymers
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X-RAY FLUORESCENCE DETERMINATION OF LOW METAL CONCENTRATION IN SURFACE MASKING LAYERS ON ORGANIC POLYMERS
RICHARD S. HUTTON AND GARY N. TAYLOR AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT A highly sensitive x-ray fluorescence (XRF) technique has been developed for the determination of low metal concentrations in the etch resistant layer of surface-imaged photoresists. The samples may be analyzed quickly and the method is applicable to a wide variety of surfaceimaging processes. For each metal of interest the XRF spectrometer is easily calibrated using polymer films containing known concentrations of a metal-containing reagent. Examples of the application of this technique to the analysis of Ti, Si and Zr-containing films are described. INTRODUCTION The essential step in the process of surface imaging '2 is the reaction of the photochemically modified resist surface with a reagent to form a barrier to the plasma etching of the resist. Typically, this reagent contains metal atoms which are incorporated into the etch resistant layer. This lithographic method has been implemented using a variety of materials and processes. The commercially available Plasmask® resist and the DESIRE process 3 are based on the reaction of volatile silicon-containing reagents to form a near-surface layer having Si-O bonds while the reaction of TiCI 4 with water sorbed on hydrophilic polymers forms TiO 2 which also is an excellent etch mask 4 ' 5 . Recently, we found that inorganic layers containing zirconium on photoresists can have 02 reactive ion etching (RIE) selectivities as high as 50:1 . In all of these approaches the etching resistance of the film is directly related to the metal content and film thickness. Consequently, in developing an optimum process for each of these approaches it is important to have a means for determining a low metal concentration which is precise and facile. X-ray fluorescence spectroscopy (XRF) provides a means for monitoring the metal content of the polymer films during processing. It has high sensitivity to a wide range of elements and data can be obtained in a matter of minutes. By calibrating the instrument with polymer films having known metal concentrations, the metal content of the processed resists can be determined. In this paper we describe the use of calibration curves for determining the concentrations of metals used in surface imaging and show how XRF can be used to optimize processing. EXPERIMENTAL X-ray fluorescence spectra were obtained using a Princeton Gamma Tech System 4 X-ray Fluorescence Spectrometer. This employed a Cr target with a 50 keV source operating at 3 mA and a 0.75 mm aperture. The sample compartment was continuously flushed with helium. The counting time and beam stop were optimized for each metal; these values and the spectral regions monitored are listed in Table 1. Reagents and polymers were used as received from the following suppliers: Aldrich Chemical Co.: titanocene dichloride (TiCp 2 Cl 2 ), zirconocene dichloride (ZrCp 2 CI 2 ); Alfa Chemical Co.: zircon
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