Advanced Applications of Picosecond Ultrasonic Technology in Semiconductor Manufacturing

Learn more about the CSTIC's Symposium VI: Metrology, Reliability and Testing session.

Abstract

Non-destructive Picosecond Ultrasonic (PULSE™) technology enjoys widespread adoption and has been a workhorse for more than two decades for measuring opaque films. This talk will highlight some of the recent developments in PULSE technology to meet the ever-growing need for more stringent process control in multiple end-market segments. We will focus on the advanced applications in two key segments: 3D NAND and automotive.

For 3D NAND, one of the key control steps is the hard mask process as it is critical for 3D NAND high aspect ratio (HAR) etch. Amorphous carbon (a-C) is the commonly used hard mask material for HAR etch. However, a-C is a complex material. Its strength, electrical conductivity, thermal conductivity and optical properties are very process dependent. With each successive node, material is increasingly graphitic and thick, and has lower thermal conductivity; this makes a-C one of the most challenging materials to measure. Signal amplitude from the measurement depends upon both pump power delivered to a sample and probe power reflected from a sample. We have identified that in the case of a-C, there is stronger absorption for p-polarized light and stronger reflectivity for s-polarized light. By making design changes to the optics, we can realize at least a 3X improvement in the signal-to-noise ratio (SNR). However, the lower thermal conductivity of the material also requires that we optimize the measurement power to strike a balance between the signal to noise  needed to meet precision requirements at production-worthy throughputs. We will report results from a study on increasing the spot size for in-cell measurements for a-C. Additionally, we tested the feasibility of this configuration to study other thermally sensitive films, such as implant monitoring.

For automotive devices, measurement of the multilayer rough metal films used in backside metallization is always challenging. The rough surface scatters the measurement beams excessively, thus increasing the noise and reducing the overall SNR. Results from a dual-modulation setup where the pump and probe beam are modulated, demonstrating the improvements over a standard single-modulation configuration, will be presented.