Scanning Photon Microscope Research Articles

Nondestructive Characterization of Surface Contaminants in Silicon Wafers Using AC Surface Photovoltage Method

A scanning photon microscope (SPM), based on the imaging of ac surface photovoltages (SPVs), has been developed and used for nondestructive detection of impurities in silicon (Si) wafer surfaces rinsed with various solutions. When aluminum (Al), iron (Fe) and copper are incorporated into native oxide on Si surfaces, they induce a negative charge by forming a network of (AlOSi) − or (FeOSi) − . For n-type Si, this negative charge gives rise to an appreciable ac SPV, while a positive charge due to phosphorus in the form of (POSi) + causes an ac SPV in p-type Si. Thus, once contaminated ions are known on the basis of chemical analysis, the resulting ac SPV features can provide nondestructive information on the content and distribution of the contaminants on the Si wafer surfaces. This is what we demonstrate in the present paper

AC Photovoltaic Images of Thermally Oxidized P-Type Silicon Wafers Contaminated with Metals

Upon oxidation, traces of some metal impurities intentionally introduced are identified qualitatively in p-type silicon (Si) wafers as dark photovoltaic images, using a scanning photon microscope (SPM) based on ac surface photovoltages (SPVs). Metal impurities in the Si bulk act as recombination centers for excess carriers, eventually reducing SPVs at the contaminated region. The SPM was successfully applied to eliminate such deteriorated wafers nondestructively in semiconductor device processing.

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.

Application of a scanning photon microscope to non-destructive detection of resistivity striations in a silicon wafer

Resistivity striations in a strongly inverted FZ grown n-type silicon wafer are non-destructively detected by photovoltage imaging, in which a 2 kHz AC surface photovoltage, induced by the irradiation of a chopped blue (peaking at 448 nm) photon beam with 30 mu W, is mapped as a 512*480 pixel black and white image. The image contrast is due to the interface trap distribution associated with inhomogeneous dopant concentration.

Effects of Dipping in an Aqueous Hydrofluoric Acid Solution before Oxidation on Minority Carrier Lifetimes in p-Type Silicon Wafers

A scanning photon microscope is successfully applied to measure ac surface photovoltages which correspond to apparent minority carrier lifetimes (τPCD) measured by the microwave-detected photoconductive decay (µ-PCD) method, using oxidized p-type silicon wafers pre-treated with or without dipping in an aqueous hydrofluoric acid (HF) solution. The dipping in HF solution causes a larger fixed oxide charge, resulting in a strongly inverted layer beneath the oxide. This situation gives longer τPCD than the bulk lifetime (τPV) by an ac photovoltaic method, while if the fixed oxide charge is small, τPCD gives an apparently smaller lifetime than τPV, implying that τPCD is influenced by the surface charge state or the surface potential. Hence, the process diagnosis by lifetime requires the measurements by both surface photovoltages and the µ-PCD method.

Nondestructive Observations of Surface Flaws and Contaminations in Silicon Wafers by Means of a Scanning Photon Microscope

A scanning photon microscope based on ac surface photovoltage imaging was applied for nondestructive observations of surface scratches and surface contaminations by SO42- in 125 mm diameter n-type silicon wafers. The scratches and contaminations create bulky and interface traps, resulting in lower ac surface photovoltages. The ac surface photovoltages attained between 35 through 150 µV have a relationship with the concentration of SO42- analyzed by ion chromatography ranging between 102 and 2×104 ng/wafer, implying that the present method is useful and simple for evaluation of the contaminated level of wafer surfaces in a nondestructive and noncontact manner.

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.