DE19733194B4 - Laser Scanning Microscope - Google Patents

Abstract

Semiconductor- Inspection microscope for generating non-optical evidence of defects during the material examination according to the OBIC or LIVA procedure, consisting of a confocal laser scanning microscope with a short pulse laser for multiphoton excitation, the excitation light having a wavelength greater than 1000 nm generated.

Description

2-photon excitation (as a special case of multi-photon excitation) is the excitation of a transition in the excitation structure (termschma) of a gas, a liquid or a solid (such as electronic, vibrational, rotational transitions or fine structures) by the quasi-simultaneous absorption of two photons of the longer wavelengths λ ₁ and λ ₂ (where λ ₁ and λ _{2 may be the} same or different), which would otherwise require a single photon of the shorter wavelength (λ ₁ + λ ₂ ) / 4. For example, two photons in the 'longwave' (eg in the red) can excite a UV-absorbing transition, which usually (ie in the conventional 1-photon excitation case) absorbs in the 'shortwave' (eg in the blue) ( 1a and 1b ). Since 2 photons are required to excite a 2-photon transition, the transition rate for a given transition depends on the square of the excitation intensity. Intensive pulsed laser sources are therefore generally used for 2-photon excitation, with 2-photon transition probability increasing at constant average light output when shorter, but more intense, light pulses are used.

The first experimental observation of 2-photon absorption by Kaiser and Garrett in 1961 describes the excitation of an Eu ²⁺ -doped CaF ₂ crystal in the optical domain, which became possible only after the development of high-performance monochromatic ruby lasers. Theoretically, the possibility of 2-photon absorption or 2-photon stimulated emission was already described in 1931 by Maria Göpper-Mayer. The use of the 2-photon technique in laser scanning microscopy was first proposed by Denk, Strickler and Webb (1990).

Out WO 91/07651 A1 discloses a two-photon laser scanning microscope, with excitation by Subpicosecond laser pulses at excitation wavelengths in the red or infrared area.

EP 666473 A1 , WO 95/30166 A1, DE 4414940 A1 describe excitations in the picosecond range and above, with pulsed or continuous radiation.

A method for optically exciting a sample by means of a two-photon excitation is in DE 4331570 C2 described.

DE 29609850 U1 The applicant describes the coupling of the radiation of short-pulse lasers in a microscopic beam path via optical fibers.

State of the art:

In the detection of lattice defects today prober techniques are used, such as OBIC and LIVA. For OBIC (Optical Beam Induced Current), see US 5493236 A , is the generation of electron-hole pairs by means of sufficiently energetic laser radiation, ie photons that can skip the band gap of the examined semiconductor (ie the energy of the irradiated photons is greater than the bandgap energy E _{G of} the semiconductor; 2 ). The location-dependent carrier current generated in this way by the scanning laser beam can be used to localize lattice defects in the crystal. For this purpose, either the wafer to be examined is contacted (prober station) or the wafer is 'packed' and the technology applied to the finished integrated circuit. After amplification, this carrier current forms the 'video signal' as a function of the scanning position (non-optical detection signal). Disadvantage of this method is that the generation of electron-hole pairs in this way is not z-selective. Thus, to provide the z-information, after localization of a defect by the 2D technique, the wafer is laboriously polished off layer by layer and inspected after each polishing step by electron microscopy to locate the defect in the z-coordinate , LIVA (Light Induced Voltage Alteration) is a technique related to the OBIC technique whereby a constant voltage is applied to the prober electrodes (or the IC pins) and voltage changes are detected as a function of the scanning laser beam.

to Investigation of silicon wafers using 1-photon laser scanning Microscopy is used i.a. a scanning near-infrared laser beam (e.g., Nd: YAG laser at a wavelength ~ 1064 nm), which is sufficient well also by doped silicon is transmitted and thus deep can penetrate into the silicon wafer. In particular, it gets away with it possible, with optically impenetrable metal coating of the IC top, with the laser beam the whole silicon substrate (a few mm thickness) optically penetrate from behind (backside imaging or backside OBIC) to the to reach structured top.

3D OBIC

The invention describes the use of multi-photon laser scanning microscopy in material analysis, in particular the investigation of structured silicon wafers by means of non-optical detection techniques, such as OBIC or LIVA. Due to the high localization of the multi-photon excitation in all three spatial coordinates when using high-aperture microscope objectives, the non-destructive three-dimensional localization of crystal defects in the semiconductor structures is possible. This technique advantageously avoids the detection of lattice defects by means of 2D techniques (eg laser scanning microscopy, non-confocal or detection of non-optical detection signals) and the subsequent successive mechanical removal of the crystal structure, coupled with electron microscopy for detection of the defect as well in the third dimension.

In many cases is one at the spatial (x-y-z) resolution the silicon structures to be examined interested in 3D. By using excitation light in the NIR (?> 1100 nm), i. beyond the band edge of silicon is achieved that the radiation with less Absorption by the i.a. thick (i.a. doped) silicon substrate is transmitted. Only at the place of the i.a. high-aperture microscope lens Shaped focus is then reached sufficiently high intensities, that electron hole Pairs generated by the non-linear multi-photon excitation process become. With the help of 2-photon microscopy can be so with radiation in the wavelength range of the 'optical window' of silicon with high z-discrimination induce electron-hole pairs.

pictures:

1a ) Representation of the propagation of the focused by means of a high-aperture microscope objective laser beam. In the 1-photon excitation case, excitation results along the entire laser beam cone. By using a confocal aperture, however, the light coming out of focus can be discriminated against the extra-focal light.

1b ) Representation of the propagation of the focused by means of a high-aperture microscope objective laser beam. In the 2-photon excitation case excitation results only in the region of highest intensity, ie in the laser beam focus. Thus, this technique is deeply discriminatory even without the use of a confocal diaphragm.

2 ) If the bandgap energy E _{G is} smaller than the photon energy E of the incident light, pairs (in a semiconductor electron-hole) are formed ( 1 ). In the homogeneous semiconductor, they usually recombine very quickly. If this occurs near a blocked pn junction, the holes and electrons are separated ( 2 ). As the electrons diffuse from the p-doped region into the n-doped region, a light-induced current flows through an amplifier 3 is detected. At corresponding contact points, this current is registered as a function of the position, ie synchronously with the scanning of the scanning laser spot, and used to construct an electronic image.

3 ) Combination of confocal laser scanning microscopy and multi-photon excitation in a device system (using the example of an inverted microscope system).

3 shows by way of example the combination of a confocal laser scanning microscope with a system for multiphoton excitation.

Here are a short pulse laser 1 and another laser 2 in a common housing 3 provided as part of a scan head of a laser scanning microscope or as a separate unit which is prepared in a known manner (US Ser. DE 29609850 U1 ) are connected via optical fibers with a scanning unit.

The laser light of the laser 1 and 2 passes through a beam splitter 4 , another dichroic beam splitter 5 on a two-dimensional deflection unit 6 and from this via a scanning lens 7 and a tube lens 8th and another beam splitter 9 and the objective lens 10 on the object 11 , which is defined adjustable at least in the vertical direction.

The object 11 coming light passes through the beam splitter 9 on a direct detector 12 with upstream filter 13 as well as an imaging optics 14 in order to enable detection without the passage of the object light through the scanning beam path, which is currently of importance for the multiphoton application.

About the beam splitter 9 continues to be an LSM standard detection beam path in the direction of a detector 15 with upstream pinhole 17 as well as filters 16 hidden. Furthermore, a non-optical detection 18 according to 2 provided synchronously to the laser scanning (Lit.).

Optical processes directly on the sample, without the imaging beam path of the microscope, are performed by another detector 19 or an image acquisition unit additionally detected.

Claims (12)

Semiconductor inspection microscope for the generation of non-optical evidence of defects in the material examination according to the OBIC or LIVA method, consisting of a confocal laser scanning microscope with a short-pulse laser for Multiphoton excitation of the excitation light with a wavelength greater than 1000 nm generated. Semiconductor inspection microscope according to claim 1, for the inspection of structured silicon wafers Semiconductor inspection microscope according to claim 1 or 2, non-destructive three-dimensional localization of crystal defects Semiconductor inspection microscope after at least one of the preceding claims, with pulse durations of the laser in the picosecond or subpicosecond range Semiconductor inspection microscope after at least one of the preceding claims, wherein optical detection means for detecting the of the sample coming radiation are provided Semiconductor inspection microscope after at least one of the preceding claims, wherein in addition to the short pulse laser at least one further laser in the Scan beam path is coupled Semiconductor inspection microscope after at least one of the preceding claims, wherein a detection of currents in synchronization with the scan. Semiconductor inspection microscope after at least one of the preceding claims, with additional optical detection means for detecting the sample coming from the sample Radiation directly, i. before the return over the Scanning device decoupled. Semiconductor inspection microscope after at least one of the preceding claims, wherein for the optical detection of the semiconductor, an infrared microscope or emission microscope is provided. Semiconductor inspection microscope after at least one of the preceding claims, wherein a detection of a reflection optical signal in a device system is provided with the semiconductor inspection microscope. Semiconductor inspection microscope after at least one of the preceding claims, as a combination of multi-photon laser scanning microscopy and / or Confocal laser scanning microscopy and / or infrared microscopy and / or emission microscopy (EMIC) in a device system. Using a Confocal Laser Scanning Microscope with a short pulse laser for multiphoton excitation, the excitation light in the NIR with one wavelength greater than 1000 nm generated to generate non-optical evidence of defects Material examination for semiconductor inspection according to the OBIC or LIVA procedure.