Speckle In Interferometric Scattering Optical Microscopy:Generation,Suppression And Its Metrology Implication

Since the advent of optical microscopy in the late 16-th century,it has provided humanity with the capability to explore the microscopic world.To facilitate this exploration,a substrate,such as a coverslip or wafer,is a necessary component of any modern optical microscope.The interaction between the substrate and sample inherently imprints the substrate’s characteristics into the observations.Notably,interferometric scattering microscopy,a recently developed ultrasensitive imaging modality,depends directly on the interference between the reflected(or transmitted)field from the substrate surface and scattering of the sample.The random scattering from the rough substrate surface would introduce speckle background into the microscopic imaging and thus limit the detection sensitivity.Consequently,typical substrates employed in high sensitivity microscopy,such as glass coverslips or silicon wafers,have been engineered to be extremely flat,oftentimes with surface roughness below 0.5 nm.However,even through the utilization of such flat substrates,images obtained by far field microscopy still exhibit seemingly mysterious "random" speckle patterns,which can draw the optical signal from a nanoparticle with a diameter of tens of nanometers and have been speculated to arise from the combined effects of optical system defects,non-uniform illumination,and substrate surface fluctuations,although no definitive conclusion has been reached.In our prior work,via the correlation of morphology and optical measurements for the same area of a glass coverslip,we demonstrated that the speckle in interferometric scattering microscopy imaging for the coverslip is predominantly due to the sub-nanometer roughness of the substrate surface and the speckle background is greater than the signal from a 15 nm gold nanoparticle.It provides a breakthrough for comprehending the mechanism of speckle generation,speckle suppression and speckle utilization for metrology.Therefore,this thesis consists of the following components:1.We present a comprehensive theoretical framework of multiscale modeling for interferometric scattering microscopy,which encompasses near-field numerical simulation,near-far-field transformation,ray tracing,and Fourier optics over a range of nine orders of magnitude from the sub-nanoscale to the decimetre-scale.The multiscale analysis is applicable in principle for samples of any shape under any illumination and detection schemes.2.By means of the multiscale modeling for interferometric scattering microscopy,we elucidated the light scattering mechanism of the substrate surface with sub-nanometer roughness: Due to the diffraction limit,the surface undulation can be considered as a large number of scatterers with a height of sub-nanometer and a lateral size of hundreds of nanometers on the perfectly flat substrate.Therefore,the volume of these scatterers is equivalent to that of a sphere with a diameter of tens of nanometers.The scattering intensity of a scatterer is proportional to the square of its volume,thus the sub-nanometer substrate surface undulation can produce a considerable speckle.3.Based on the determinacy,stability,and uniqueness of the source for speckle patterns,the concept of speckle optical fingerprint was proposed.By pre-recording the speckle optical fingerprint of a specific area on a substrate surface,the repeatability recognition of the area was realized,and then the 5 nm-diameter gold nanoparticles,which were "subsequently added" in the multi-process micro-nano processing,were correctly identified.The signals of the particles are only 1/30 of the speckle background intensity.Additionally,based on the speckle optical fingerprint,the lateral displacement detection with an accuracy of 0.22 nm for label-free substrate was also accomplished.4.Based on the geometric distinction between the surface undulation and the nanoparticles,an optical system combining p-polarized obliquely incident illumination and orthogonal polarization detection was designed to suppress the scattering of the silicon wafer surface undulation and thus the signal-to-noise ratio of nanoparticle detection was significantly improved.The individual 10 nm-diameter polystyrene nanoparticles immobilized to silicon wafer could be discerned by the proposed method under wide field illumination with 100 μm × 100 μm field of view.By comparison,the reported advanced optical methods can only detect polystyrene particles with a minimum diameter of 30 ～ 40 nm.According to the Mie scattering theorem,the light scattering intensity from 10 nm particles is only 1/1000 of that from 30 ～ 40 nm particles.The theoretical tools,research methods,and the experimental results of the thesis possess considerable scientific and engineering value.Firstly,the multiscale modeling and the understanding of the light scattering mechanism of the substrate surface undulation will stimulate the advancement in suppressing speckle and exploiting the utilization of speckle for various microscopy.Secondly,the proposed speckle optical fingerprint concept has exceptionally important value in super-resolution optical localization technology and semiconductor chip multi-process fabrication.Lastly,the 10 nm polystyrene particles wide field detection scheme on silicon wafer is of immense importance for the advancement of high-throughput wafer pre-inspection equipment for integrated circuit related processes.