Optical Beam-Induced Current (OBIC) Analysis


Optical Beam-Induced Current (OBIC) is a semiconductor analysis technique that employs a scanning laser beam to induce a current flow within a semiconductor sample which may be collected and analyzed to generate images that represent the sample's properties.  It is a useful imaging technique for detecting or locating various defects or anomalies on a semiconductor sample.


Conventional OBIC scans an ultrafast laser beam over the surface of the sample, exciting some electrons into the conduction band through what is known as 'single-photon absorption'. As its name implies, single-photon absorption involves just a single photon to excite the electron into conduction. This can only occur if that single photon carries enough energy to overcome the band gap of the semiconductor (1.2 eV for Si) and provide the electron with enough energy to make it jump into the conduction band.


The energy of a photon is given by the equation E = hc/λ, where h is Planck's constant, c is the speed of light, and λ is the wavelength of the photon.  Thus, photons with shorter wavelengths possess higher energies than those with longer wavelengths.


The creation of carriers by exciting the semiconductor with optical beam results in a current flow that can be collected and used for imaging.  The variations in currents produced by the laser beam as it scans the sample is converted into variations in contrast to form the OBIC image.   


One limitation of conventional OBIC operating on single-photon absorption is its difficulty with transmitting light uniformly through the top surface of modern IC's to the semiconductor itself.  This non-uniform transmission of light through the top surface is caused by the presence of several layers of metal lines and other materials. In such instances, one solution is to perform the OBIC imaging from the IC's backside through the substrate.


Figure 1.  OBIC Image of a p-n Junction with

a recombination center


The spatial resolution of single-photon backside OBIC imaging, however, is limited by the compromise between being able to transmit the beam through the substrate and at the same time allowing the beam to be absorbed by the semiconductor for the generation of electron-hole pairs that are measurable as a current.  The range of wavelengths that can meet both of these requirements does not give single-photon OBIC enough resolution for effective use in analyzing IC's with submicron IC features.


This limitation of single photon OBIC in backside analysis may be overcome by 'two-photon absorption', which involves two photons arriving at the same time on the sample to coherently 'free' the electron. The photons used for this purpose must have energies that are less than the band gap of the semiconductor, but greater than half the band gap.


Two-photon absorption is a non-linear form of absorption, with the success of generating carriers depending on the square of the light intensity. This quadratic intensity dependence makes two-photon carrier generation very inefficient outside the area of focus. OBIC that operates on two-photon absorption is also known as 'TOBIC', which stands for 'Two-photon OBIC.' 


Applications of OBIC include: 1) detection of recombination or generation centers in a p-n junction; 2) detection of inter-level shorts; 3) confirmation of the 'on' and 'off' states of a transistor; 4) detection of a latch-up mechanism within a circuit; 5) location of weak points within a MOS transistor; etc. 


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