Keith A. Serrels

Solid immersion lens applications for nanophotonic devices

Solid immersion lens (SIL) microscopy combines the advantages of conventional microscopy with those of near-field techniques, and is being increasingly adopted across a diverse range of technologies and applications. A comprehensive overview of the state-of-the-art in this rapidly expanding subject is therefore increasingly relevant. Important benefits are enabled by SIL-focusing, including an improved lateral and axial spatial profiling resolution when a SIL is used in laser-scanning microscopy or excitation, and an improved collection efficiency when a SIL is used in a light-collection mode, for example in fluorescence micro-spectroscopy. These advantages arise from the increase in numerical aperture (NA) that is provided by a SIL. Other SIL-enhanced improvements, for example spherical-aberration-free sub-surface imaging, are a fundamental consequence of the aplanatic imaging condition that results from the spherical geometry of the SIL. Beginning with an introduction to the theory of SIL imaging, the unique properties of SILs are exposed to provide advantages in applications involving the interrogation of photonic and electronic nanostructures. Such applications range from the sub-surface examination of the complex three-dimensional microstructures fabricated in silicon integrated circuits, to quantum photoluminescence and transmission measurements in semiconductor quantum dot nanostructures.

Three-dimensional nanoscale subsurface optical imaging of silicon circuits

Three-dimensional subsurface imaging through the back side of a silicon flip chip is reported with a diffraction-limited lateral resolution of 166nm and an axial performance capable of resolving features only 100nm deep. This performance was achieved by implementing sample-scanned two-photon optical beam induced current microscopy using a silicon solid immersion lens and a peak detection algorithm. The excitation source was a 1530nm erbium:fiber laser, and the lateral optical resolution obtained corresponds to 11% of the free-space wavelength.

70 nm resolution in subsurface optical imaging of silicon integrated-circuits using pupil-function engineering

We present experimental evidence for the resolution-enhancing effect of an annular pupil-plane aperture when performing nonlinear imaging in the vectorial-focusing regime through manipulation of the focal spot geometry. By acquiring two-photon optical beam-induced current images of a silicon integrated-circuit using solid-immersion-lens microscopy at 1550 nm we achieved 70 nm resolution. This result demonstrates a reduction in the minimum effective focal spot diameter of 36%. In addition, the annular-aperture-induced extension of the depth-of-focus causes an observable decrease in the depth contrast of the resulting image and we explain the origins of this using a simulation of the imaging process.

Three-dimensional nanometric sub-surface imaging of a silicon flip-chip using the two-photon optical beam induced current method

Two- and three-dimensional sub-surface optical beam induced current imaging of a silicon flip-chip is described and is illustrated by results corresponding to 166 nm lateral resolution and an axial performance capable of localising feature depths to around 100 nm accuracy. The experimental results are compared with theoretically modelled performance based on analytic expressions for the system point spread functions valid for high numerical apertures, and are interpreted using numerical geometric ray tracing calculations. Examples of depth-resolved feature profiling are presented and include depth cross-sections through a matrix of tungsten vias and a depth-resolved image of part of a poly-silicon wire.

Two-photon laser-assisted device alteration in silicon integrated-circuits

By inducing two-photon absorption to perturb the switching characteristics of sensitive transistors located within the active layer of a proprietary 28-nm silicon test chip, we demonstrate time-resolved nonlinear laser-assisted device alteration.

Optical super-resolution with aperture-function engineering

We demonstrate optical super-resolution (resolution better than conventional diffraction-limited resolution) in a simple optical configuration by using annular apertures to manipulate the pupil function of the system. The theoretical basis of the technique is described, and it is shown how good agreement between theory and experiment can be achieved by suitable selection of the principal system parameters. The ready implementation and suitability for theoretical interpretation makes the demonstration a good candidate for an undergraduate laboratory experiment in classical optics.

Solid-immersion-lens-enhanced nonlinear frequency-variation mapping of a silicon integrated-circuit

By inducing two-photon absorption within the active layer of a proprietary silicon test chip, we demonstrate solid-immersion-lens-enhanced nonlinear frequency-variation mapping of a 500-MHz ring oscillator circuit.

Optical coherence tomography for non-destructive investigation of silicon integrated-circuits

We present the development of an ultra-high-resolution high-dynamic-range infrared optical coherence tomography imaging system for the novel purpose of sub-surface inspection of silicon integrated-circuits. Examples of substrate thickness profiling and device feature inspection are demonstrated.

Optical super-resolution through aperture-function engineering and vectorial-focusing effects

We demonstrate optical super-resolution by using custom obscuration apertures and polarization effects to manipulate the pupil-function and focal-plane point-spread-function in a two-photon microscope used for semiconductor flip-chip imaging. Experimental results suggest sub-100 nm performance.

Nanometric three-dimensional sub-surface imaging of a silicon flip-chip

In an attempt to improve the resolution of optical microscopy of semiconductor devices, it is possible to use a Weierstrass solid immersion lens (SIL). A suitable SIL eliminates spherical aberration due to the refractive-index mismatch at the silicomair interface and increases the numerical aperture (NA) of the system, decreasing the focal spot size. An increase in the spatial sampling frequency also results because physical translations of the SIL and sample result in a smaller movement of the focused spot. It is known that in the lateral direction the reduction factor equals the square of the refractive index, 12.1 in silicon. In the axial direction the scaling factor is around 75. We report here 2D imaging with an improved resolution of 166nm and 3D sub-surface imaging with approximately 100nm resolution.