Question

Metal Ta and TaN has used as the barrier materials for copper interconnects. A non-contact, non-intrusive method and tool such as MetaPULSE® G has been used to measure the thickness of metal films.
Please describe its fundamental theory, measurement and data examples from literature, resolution/sensitivity, and limitations
- Non-intrusive method
- Fundamental Theory
(what physical properties should be known as the input data?)
- Example from literature
- Any limitation on the film thickness, sample status, and other

Answer 1

MetaPULSE® G System is used for Thin Film Metrology or measurement tool optimized specifically for copper interconnects and characterization at the 32, 28 and 22nm technology nodes for Front-End-Of-Line, Back-End-Of-Line and back-end Wafer-Layer Packaging applications and process control.

Fundamental theory:

Unlike optical and x-ray techniques, this technology measures film thickness using a time-resolved acoustic signal that can be used in active die in the absence of special test pads. A photoacoustic film thickness system measures thin film thicknesses on wafers. Such a system forms two optical beams: an excitation beam used to excite the surface of the film sample periodically, and a probe beam used to sense the reflectivity of the surface of the sample following each pulse from the excitation beam. The time interval between excitation of the film surface and the measurement of the reflectivity is varied in a controlled manner to obtain a measurement of the surface reflectance as a function of time. This data is analyzed using the acoustical impedance of each layer in the thin film stack on the wafer and a software program to calculate the thickness of each layer in the film stack. (Source: US Patent 7372584 B2)

This system is developed by Rudolph Technologies—the industry-standard for thickness measurements on opaque films. The system delivers superior performance on all metal films, and is optimized for thin single and multilayer applications that are critical in advanced logic, memory and 3D packaging processes.

Specification (Source: Rudolph Technologies Inc.)

• Single wavelength 515nm PULSE laser optimized for copper applications; delivers higher signal-to-noise ratios and higher measurement repeatability
• 10x10 micron spot size assures measurement capability on product wafers in 30x30 micron or smaller test sites
• Measure up to 60 Wafers Per Hour (WPH) with gauge-capable precision
• High repeatibility measurements of metal films from 50Å to 8µm
• In addition to thickness, can determine RMS roughness, material density, adhesion, material phase and interlayer
• capable of handling 150, 200, 300 and 450mm wafers
• Fabwide metal metrology with low CoO

Limittations: Used for opaque films only,

                   Film Thickness = 50Å to 8µm

                   measurement capability in 30x30 micron or smaller test sites