Chusuke Munakata

Observation of p-n Junctions with a Flying-Spot Scanner Using a Chopped Photon Beam

A flying-spot scanner that employs a chopped photon beam emitted from a cathode ray tube is reported. The photon beam scanns a planar p-n junction put on a metal electrode through a In2O3-coated transparent electrode and a 15 µm-thick mylar spacer. The ac photovoltage is picked up with electrodes through a condensor formed by the spacer. The photovoltage signal modulates brightness of another cathode ray tube to form a scanning image. Mean wavelength and chopping frequency of the photon beam are 507 nm and 2 kHz. Scanning image analysis is done using photocurrent density equations based on a step-like junction model. Three kinds of junctions, a solar cell, a partly-deep junction and a non-uniformly ion implanted one, have been evaluated to show the validity of the present method.

Calculation of three-dimensional optical transfer function for a confocal scanning fluorescent microscope

A confocal scanning fluorescent microscope is suitable for 3-dimensional (3-D) imaging. The 3-D optical transfer functions (OTF’s) for such a microscope are calculated to show their dependence on the wavelength of the fluorescence. These calculations reveal that when the wavelength of the fluorescence is equivalent to that of the excitation light, the 3-D OTF has no missing-cone region. However, as the wavelength becomes longer, the 3-D OTF approaches that of an incoherent conventional microscope at the wavelength of the excitation light.

Ac Surface Photovoltages in Strongly-Inverted Oxidized p-Type Silicon Wafers*

Surface photovoltages in Si wafers excited with a chopped 559 nm-wavelength photon beam are analysed using a new half-sided junction model. In this model, the wafer surface with the depletion layer is considered to be one half of the p-n junction. Chopping frequency ranges from 2 Hz through 20 kHz. Four 76 mm-diameter p-type Si wafers having resistivities of 260, 92, 17 and 1.0 mΩ m are used after forming 360 nm-thick wet-oxide layer on their front surfaces. In these wafers, photovoltage increases with resistivity. In three high-resistivity wafers with strongly-inverted surfaces, the inversion capacitances and conductances limit the photovoltages at low frequencies. The obtained inversion time-constant is 7 s for the 17 mΩ m wafer.

Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer

The majority carrier conductance due to the hole flow towards the surface of a wafer from the bulk has been formulated following the half-sided junction model previously reported. An empirical equation for the carrier drift velocities in very high electric fields has been proposed for the formulation. Besides the conductance, the depletion layer capacitance, interface trap capacitance and conductance are found to be responsible for ac surface photovoltages in the depletion case. The majority carrier conductance can explain the formerly observed conductance of 27 S/m2 in a 76-mm-diameter 1.0-mΩm oxidized p-type Si wafer. Analysis of the formerly reported surface photovoltages reveals a surface potential, fixed oxide charge density, hole capture cross section and interface trap density of 0.32 V, 1.7 mC/m2, 2.0×10-20 m2 and 2.0×1016 m-2 eV1 respectively.

Dependence of 3-D optical transfer functions on the pinhole radius in a fluorescent confocal optical microscope.

The resolution of a fluorescent confocal scanning optical microscope (CSOM) is superior to that of a conventional fluorescent optical microscope. To attain this superiority, the fluorescent CSOM uses a pinhole in front of the detector. Thus, the resolution of the CSOM is dependent on the pinhole radius. Three-dimensional optical transfer functions are calculated for the various radii to elucidate this dependence. The results show that a CSOM with a radius smaller than ~1 optical unit has a bandwidth comparable with that of an infinitely small radius.

Nondestructive diagnostic method using ac surface photovoltage for detecting metallic contaminants in silicon wafers

Effects of residual metal impurities after RCA (Radio Corporation of America) standard cleaning (alkaline and acid rinses) on the generation of ac surface photovoltages (SPVs) are investigated using n‐type silicon wafers. Aluminum (Al) and iron (Fe) in the native oxide induce a negative charge, causing high ac SPVs in n‐type wafers. The ac SPV dependency on the concentration of Al and Fe is determined. Nickel and zinc, however, have little effect on the generation of ac SPVs. In commonly employed cleaning processes, Al is the major impurity in the native oxide, and thus the ac SPV technique is applicable to nondestructive diagnostics for quality control in cleaning processes.

A scanning photon microscope for non-destructive observations of crystal defect and interface trap distributions in silicon wafers

A sophisticated flying spot scanner in which 2 kHz AC surface photovoltages induced in a Si wafer due to the chopped irradiation of a photon beam 0.4 mm in diameter are non-destructively measured through a 25 mu m air gap between the wafer surface and a transparent electrode is developed. Photovoltage distributions on a wafer 50 to 150 mm in diameter are measured in 5 min, and displayed as a 512*480 pixel black and white image. With the use of near infrared (peaking at 896 and 1076 nm) and blue (peaking at 448 nm) photon sources (cathode ray tubes), it is possible to measure crystal defect distribution in the bulk and interface trap distribution at the wafer surface separately. The apparatus is successfully applied to observations of swirl patterns and radiation damage induced by an electron beam in thermally oxidised p-type Si wafers.

Depth resolution of the fluorescent confocal scanning optical microscope

The fluorescent confocal scanning optical microscope is capable of 3-D observation, for which depth discrimination is vital. To characterize the depth discrimination capability of this microscope, intensity variations depending on fluorescence wavelength are simulated for fluorescent films being moved in the depth direction. Also, the resolution between two films is calculated for a range of fluorescence wavelengths. Results of experiments using photoresist films are compared to simulations. As a practical application, through-holes opened in a thick photoresist film are observed by slice images in the depth direction.

Analysis of ac Surface Photovoltages in Accumulation Region

Equations for ac surface photovoltages (SPVs) excited with a chopped photon beam (PB) in the accumulation region are proposed for such semiconductors as silicon and germanium. Following the previously reported half-sided junction model for the depleted or inverted region, equations for photocurrent density and surface impedance per unit area have been newly deduced. When the surface potential is highly negative in p-type semiconductors, the maximum ac SPV in the accumulation region is limited by the conductance due to majority carrier diffusion flow. This is compared with the strong inversion region, where the mathematically maximum SPV depends upon the minority carrier diffusion flow. The voltage ratio between the two maximum ac SPVs is the same as that previously reported using the different models for ac SPVs excited with a continuous PB.

Observation of ring-distributed microdefects in Czochralski-grown silicon wafers with a scanning photon microscope and its diagnostic application to device processing

A scanning photon microscope (SPM) based on ac surface photovoltage imaging is applied to observe oxygen-related microdefects which are distributed in a ring in oxidized Czochralski-grown silicon wafers, and morphological and microstructural characteristics of the microdefects are then analyzed. The overall distribution of the ring-shaped region revealed by the SPM correspond well to that observed with X-ray topography. The SPM is able to differentiate deteriorated regions as different image contrasts, where stacking faults or oxide precipitates accompanying punched-out dislocation loops exist.