NONDESTRUCTIVE TESTING

A noncontact, nondestructive technique that simultaneously measures the individual thicknesses of all layers in multilayer metal-film stacks will debut at Semicon/West (San Francisco, CA) in July. The picosecond ultrasonic laser sonar (PULSE) technique was originally developed by Humphrey Maris, Jan Tauc, and associates at Brown University (Providence, RI) and will be introduced in a commercial instrument by Rudolph Technologies (Flanders, NJ).

Jun 1st, 1997

NONDESTRUCTIVE TESTING

Laser sonar measures multiple-layer thin films

A noncontact, nondestructive technique that simultaneously measures the individual thicknesses of all layers in multilayer metal-film stacks will debut at Semicon/West (San Francisco, CA) in July. The picosecond ultrasonic laser sonar (PULSE) technique was originally developed by Humphrey Maris, Jan Tauc, and associates at Brown University (Providence, RI) and will be introduced in a commercial instrument by Rudolph Technologies (Flanders, NJ).

"This is a revolutionary technology," says George Collins, of Rudolph Technologies. "No other method can perform metrology on the three- to five-layer metal film stacks used to interconnect transistors in semiconductor chips. We can measure u¥to eight or nine layers in a single experiment . . . but the real advantage of this is that one can `see` into optically opaque materials in a nondestructive way and quantify interlayer adhesion and roughness or detect misprocessing, such as a missing layer" (see Fig. 1 on p. 16).

The PULSE process is fairly simple, Collins continues. A diode-pumped Ti:sapphire laser is focused to a 10-µm spot on a semiconductor wafer. The wafer surface is rapidly heated between 5°C and 10°C by 100-fs, 800-nm laser pulses (average power of 200 mW). Localized expansion generates an ultrasonic sound wave that separates from the thermal event and propagates through the film at the speed of sound in that material.

When the sound wave encounters the interface from one thin-film layer to another, a portion of the wave reflects back to the surface as an echo; the rest transmits through the interface. As the echo, or reflected wave, returns to the surface, it changes the reflectivity of the surface, says Collins.

This change in reflectivity indicates the arrival time of the echo. The change is optically detected using light diverted from the pum¥laser pulse by a beamsplitter and delayed by guiding the light over a longer path to the sample (see Fig. 2). Accurately measuring the times between sound generation and the returning echoes determines the thickness of each layer in the stack with angstrom accuracy.

Multilayer stacks return a sequence of echoes, each echo being separated in time from the echoes of lower layers. "The echo from the layers of thin film is exactly like the echo from shouting hello across a canyon," Collins says. "We `hear` a series of responses as the sound wave reflects off each layer."

In addition to measuring thickness of opaque layers, the shape and amplitude of the echo indicates properties of the interface between layers. In this way, the system can quantitatively measure interlayer roughness, interlayer adhesion, interface contamination, and reaction between layers.

The pulse technique can resolve features approaching the atomic scale and measure thicknesses u¥to 5 µm, says Collins. Changes in the echo can identify the chemical composition of silicides and other materials in processed layers.

Two advances have made the technique possible, says Collins. First is the reduction in size of ultrafast lasers. Second, he adds, "we are standing on the shoulders of 50 years of development of sonar." The algorithms for interpreting sonar echoes are very sophisticated, allowing even buried features to be measured in three dimensions.

The PULSE technique has been under development for the past 10 to 12 years at Brown University. Rudolph Technologies has worked with Brown to implement the commercial instrument specifically for the semiconductor industry. The technology may be useful in the optics and other industries as well, says Collins.

The first commercial system was shipped by Rudolph to a semiconductor company in mid-May. Full production will begin later this year.

Laurie Ann Peach

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