In recent years, Si-waveguide became widely used for high data-rate communication devices and CMOS-compatible process enables high-volume/low-cost manufacturing (F.Boeuf, IEDM2013). The behavior of optical devices depends on the materials quality, doping, junctions and interfaces. The intrinsic dimension variations due to patterning techniques might induce a significant signal loss, especially on the outer edges of Mutlimode waveguides where light can diffuse at the interface (A.Fay, EMLC2019).

Recently, annealing techniques were implemented in order to reduce the effect of roughness on optoelectronic transmission efficiency (Q.Wilmart, JLT2020). To better understand the geometrical effects and since the optical characterization is performed on a final device, a variety of metrology techniques can be used to assess the waveguide morphology during its fabrication process, mostly with SEM as they are reliable, fast, and widely available in the microelectronics industry.

SEM review allows the extraction of line-width-roughness (LWR), and algorithms can compute the line-edge-roughness (LER). However, LER and the actual sidewall roughness will be different since top-view images from SEM might not reflect the exact topology of the side-surface, because it integrates the full height as a single line. At the opposite, a mechanical probe acquires data on its full vertical dimension, with a resolution limited by its physical size. AFM technique is well-known for flat surface roughness analysis, but specific systems have been developed to assess non-planar surfaces. A typical example involves the use of an inclined probe and tilted samples, but this setup makes it too difficult for multiple devices/wafers scanning at high rate, and requires a 2-pass acquisition to gather data on each side of a device, with a positive or negative probing angle. On the other hand, specific probing methods in addition to AFM boot-shape tip designs are commercially available to easily interact with a vertical structure and allow for the sidewall roughness calculation and sidewall angle computation, in addition to traditional line width calculation and the associated LWR.

We propose to compare the SEM images analyses and AFM CD-mode available on our 300mm platform to describe the techniques limitations and methods to improve their accuracy. An extensive evaluation of both techniques will be presented to compare the morphological information we can reliably retrieve from both techniques.

Specifically, probing protocols and methods will be discussed, and a statistical comparison will be performed in order to mitigate the AFM limitations related to the probes size and give support to the SEM technique measurements limited by its intrinsic single-line data acquisition.

Keywords : Si Photonics, Roughness, Sidewall, 3D-structure morphology, AFM, SEM, PSD, LER, LWR