Diffraction-based overlay metrology from visible to infrared wavelengths using a single sensor

Abstract

Background: Integrated circuits are fabricated layer by layer. It is crucial to their performance that these layers are well aligned to each other, and any undesired translation of a layer is called overlay. Thus far, overlay measurements have been limited to visible wavelengths, but the use of materials that are opaque to visible wavelengths necessitates measurements using infrared light.Aim: We set out to demonstrate that an overlay sensor based on digital holographic microscopy can perform such overlay measurement at infrared wavelengths, while maintaining functionality at visible wavelengths.Approach: This was done by constructing a breadboard setup that is capable of measuring overlay at wavelengths ranging from 400 to 1100 nm.Results: Using the setup, we demonstrated good linearity between an applied amount of overlay and the measured amount. In addition, we demonstrated that the setup is only sensitive to structures at the top of the wafer. Measurements are therefore unaffected by the fact that Si is transparent at 1100 nm.Conclusions: These results demonstrate the viability of an overlay sensor that is sensitive to visible and infrared light, allowing more freedom in choice of materials for integrated circuits.

Process window improvement for fin cut layer in self-aligned double-patterning process based on backscattered electron imaging

“…As fin damage is obviously caused by the insufficient coverage of the SOC to the closest covered fin, the OV, defined as the pattern positional accuracy between the bottom and current layers, 19 – 21 and PW, which determines the CD uniformity of the spaces between neighboring fins, 22 should also be considered. To characterizing the PW performance, the etch process is suspended at the step of SOC etch [Fig.…”

Pupil apodization in digital holographic microscopy for reduction of coherent imaging effects

“…It is worth mentioning that the coherent mixing of the diffracted orders with their respective reference beams offers noiseless optical amplification which lifts the image above the noise floor of the image sensor and boosts the detection sensitivity of very weak metrology targets. This has already been demonstrated in [8], where a weak measured signal of a silicon wafer at 1030 nm wavelength is coherently mixed with a reference beam resulting in a "noise clean" amplified holographic image. Finally, the retrieved complex field in DHM allows us to computationally correct for aberrations in the imaging lens [12].…”

“…DHM acquires the amplitude and phase of an image which offers various unique opportunities like (3D) imaging of low-contrast objects and digital aberration correction [5,6]. Recently we have reported the use of dark-field DHM (df-DHM) as a potential solution for the growing challenges of overlay (OV) metrology in semiconductor industry [7,8].…”

Robust semiconductor overlay metrology with non-uniform illumination beams using digital holographic microscopy

“…With two 2D Fourier transforms, the complex amplitude and phase of the diffracted light are reconstructed. 3,4 As metrology demands get higher, measurements are easily susceptible to any imperfections in the measurement system, and access to the complex field of the targets gives us many opportunities for computational image correction techniques. 5, 6…”