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Laser Technique Analyzes Microelectronic Thin Films

Photonics Spectra
May 1999
Michael D. Wheeler

A Massachusetts Institute of Technology researcher has developed a nondestructive, noncontact, laser-based acoustic technique called impulsive stimulated thermal scattering that quickly measures the thickness of thin films used in microelectronic components.

A uniform thickness in layers of tungsten, copper, tantalum and other metals used to coat silicon wafers is crucial to the proper function of an integrated circuit. Conventional methods to ensure uniform thicknesses include electron microscopy, x-ray fluorescence and electrical sheet resistance. Although these techniques are effective, they have one or more serious drawbacks, including sample contact or damage, long measurement times and poor spatial resolution.

Philips Analytical introduced the Impulse 300, which enables full water characterization based on impulsive stimulated thermal scattering.

The answer may lie in the novel technique pioneered by a research group led by Keith A. Nelson of MIT's chemistry department. It operates on the principle that the width and the elasticity of materials can be quantified by measuring the ultrasonic waves from metal after it has been irradiated by laser light.

Two subnanosecond excitation laser pulses spatially and temporally overlap at the surface of a sample to form an optical interference or "grating" pattern of alternating light and dark regions. The sample absorbs the light, leading to sudden, spatially periodic heating and thermal expansion of the thin film. This "impulsive" heating generates coherent acoustic waves, whose wavelength and orientation match those of the grating pattern. Time-dependent acoustic oscillations ripple the surface of the thin film, where they are monitored through diffraction of a probe beam. The frequency and velocity of these acoustic oscillations correlate with the thickness of a thin film, yielding precise measurements down to several angstroms.

Because metals vary in stiffness and density, an acoustic wave travels through a layer of copper at a rate different from that of a layer of tungsten. To get accurate readings, the user must factor in the type of metal that is tested and its properties in bulk form.

Commercially available

In the commercial version of the device, available from Philips Analytical NV of Waterloo, Ontario, Canada, a self-Q-switched microchip Nd:YAG laser developed at MIT's Lincoln Laboratory is the excitation source. The microchip laser emits at 1 µm with a power level of 10 µJ and a pulse duration of several hundred picoseconds. A continuous-wave laser diode acts as the probing beam, and a 1-GHz photodetector collects data.

Philips recently delivered one unit to the semiconductor organization Sematech for evaluation and has received the first orders from fabrication facilities.

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