Conventional optical microscopes produce an image by recording the intensity of the light reflected from the sample. In an integrated circuit, an alternative method for obtaining an image is to map the photocurrent produced by the device as a laser beam is scanned across its entire area. This technique is known as OBIC (Optical Beam Induced Current) imaging and the method can be implemented using ultrafast lasers to induce a current by means of two-photon absorption.

Silicon can absorb laser light in either a one-photon or two-photon excitation mode. In one-photon absorption (right) a single photon carries enough energy to excite an electron from a low to a high energy state. The energy of the photon is related to its wavelength by the equation E=hc/l, where h is Planck's constant, c is the speed of light and l is the wavelength of the light. This formula shows that shorter (bluer) wavelengths correspond to greater photon energies.

When the energy of a single photon is less than the energy gap (but bigger than half the gap) two photons can coherently excite the electron to the upper level (above, right) if they arrive at the same time (and place) on the sample.

The strength of the two-photon absorption effect depends on the light intensity squared and the effect is often known as nonlinear absorption. As an example, consider the silicon photodiode opposite being illuminated by intense light pulses from an ultrafast laser. For two-photon absorption to occur the wavelength of the laser must be long enough so that the photon energy E is less than the bandgap energy but greater than half the bandgap energy of silicon. This condition implies wavelengths from roughly 1050-2100 nm. When the light is not tightly focused onto the photodiode (a) there is a negligible photocurrent. Only when the light is focused exactly into the junction of the photodiode (b) is a significant photocurrent observed. When OBIC imaging is carrried out using 2-photon absorption it is known as TOBIC (two-photon OBIC) imaging.

Silicon is transparent at wavelength longer than around 1050 nm and conventional OBIC imaging of integrated circuits uses a Nd:YAG laser operating at 1064 nm to penetrate through the silicon substrate and excite carriers in the circuit and so create a photocurrent (image, right).

This approach however has a significant problem because the beam must be sufficiently transmitted by the silicon to travel through a substrate hundreds of microns thick but must experience enough absorption to create carriers in the circuit and so produce an image.

In TOBIC imaging the typical wavelength used is 1300-1550 nm and the beam is therefore easily transmitted by silicon because the wavelength is far away from the bandgap wavelength of 1050 nm. The only absorption occurs at the beam focus where the intensity is high enough to cause two-photon absorption.

By mapping the photocurrent as a function of beam position a TOBIC image can be obtained.

The image on the right was recorded using a super-solid-immersion-lens and a 1530nm femtosecond Er:fibre laser.

The device imaged is a component on a silicon flip-chip similar to the kinds used in modern computer proecessors.

The resolution in this work was 170nm, but later work has achieved resolutions of only 100nm, depending on the polarisation state of the light used.

Two-photon absorption is characterised by an intensity-dependent absorption coefficient, meaning that the laser light is only strongly absorbed at the focus of the microscope objective lens (image, right). In a chip the electrical signal is the combined result of carrrier generation in the vicinity of a junction and subsequent carrier collection and removal to the external circuit by contacts to the junction. The photocurrent signal which forms the image is localised to a small region of the sample in the focal plane of the lens. The TOBIC method therefore is able to produce images which are depth sensitive and therefore 3D imaging is possible using the effect.

The image opposite was obtained by measuring the photocurrent at each pixel position and then scanning the focus to find the depth at which the photocurrent was maximised.

Because of the diffusion process by which pn-junctions are created, different types of junctions exist at slightly different depths. The TOBIC imaging technique is sensitive to this and can discriminate between depth differences of much less than one micron (see cross-sectional profiles opposite). Using a SIL this gives resolutions approaching 100nm in all 3 dimensions.

Because of its high resolution the technique may even be able to resolve different thicknesses of semiconductors in a "sandwich-like" heterostructure.