Precise Determination of Thin Metal Film Thickness With Laser-Induced Acoustic Grating Technique
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INTRODUCTION With ever increasing speed and packing density of chips, fabrication of metal interconnects is becoming an increasingly critical step in the integrated circuit (IC) technology [1]. The current stage of the metal interconnect technology is characterized by an increase in the aspect ratio of wires, complex multilevel design and introduction of new materials such as copper. At the same time, metal layers should be controlled with greater precision as resistivity and capacity of wires are becoming main limiting factors of the electrical performance of a chip. The needs of developing technology result in requirements to metal film thickness metrology tools that can no longer be met by traditional techniques such as sheet resistance measurements. There is a growing demand for a fast, noncontact and nondestructive metrology technique that would be capable of performing metal thickness measurements on small structures on productionwafers with angstrom-level reproducibility [2]. Optical techniques such as
ellipsometry meet these requirements and are used with great success to control transparent dielectric layers. Unfortunately, ellipsometry cannot be used for opaque metal layers (with the exception of ultrathin ones). For metals and other opaque materials, all-optical metrology is made possible by virtue of acoustic waves generated by a laser. In transient laser-induced grating technique also called Impulsive Stimulated Thermal Scattering (ISTS) [3], two short laser pulses overlapped at the surface of a sample generate surface acoustic waves (SAWs) whose propagation is monitored via diffraction of the third (probe) laser beam. Surface waves penetrate into the sample over the distance of the order of the acoustic wavelength. The wavelength is controlled by the excitation geometry and ranges typically from several microns to tens of microns. Thus the mediation of acoustic waves allows probing subsurface layers of the sample much deeper than the optical penetration length. Recently, compact and robust optical design enabling the use of the ISTS technique in the industrial environment has been developed [4,5]. The current paper describes applications of a metal film metrology tool based on ISTS to film thickness measurements for metal interconnect process control.
195 Mat. Res. Soc. Symp. Proc. Vol. 591 © 2000 Materials Research Society
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Excitation Laser m Phase Masks
Laser
Wafer Positioning Stage Fig.1 Optical set-up MEASUREMENT TECHNIQUE Fig. I shows a schematic drawing of the optical set-up of the ISTS measurement system. A subnanosecond excitation pulse emitted by a miniature diode-pumped YAG laser is transmitted trough the phase grating designed to effectively split the incident beam into two diffracted ones. The beams are reconciled at the sample surface yielding spatially periodic intensity pattern. Absorption of the optical radiation at the sample surface results in impulsive thermal expansion which launches counter-propagating surface acoustic waves at the wavelength equal to the period of t
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