Method for structure investigation in a semiconductor substrate

The invention relates to a method for investigation of structures on a semiconductor substrate. The structures can be particularly serviceable Integrated and non-integrated electronic circuits in the micro and be nanometer or micromechanical components in micro and Nanometer range.

The structures of semiconductor devices such as memory chips, Microprocessors, logic devices are on a semiconductor substrate, a so-called "wafer" manufactured which is generally made of a silicon single crystal there is, and most have a thickness between 200 microns and 600 microns having. In addition, there are also special wafer with very thin semiconductor substrates, whose thicknesses are far below 100 microns.

be by means of lithography, coating on such wafers and manufactured doping process semiconductor devices, which in itself very oberfächennahen layers of a few microns thickness are and whose structures lateral extents of currently about 200 nm or may have less. To a usable semiconductor chip therefrom finished, the so-fabricated structures with electrical lines must are provided, which in turn are produced by means of lithography can. The lines consist in general of metals such as copper or aluminum. Also tungsten and tantalum are used. Thus, may cross such interconnects without electrical shorting, be in Intermediate steps mounted electrically insulating intermediate layers, their lateral extent and form again carefully using the Lithography can be dimensioned. It must then later turn electrical connections by various methods in vertical direction are generated, which leads or doped zones the one plane, the lines of the other plane through the isolator through contact. In result thus created a three-dimensional Structure of semiconducting, conducting and insulating structures in their spatial position must be accurately matched.

If errors occur in such structures, so they need to be investigated. This example is done using microscopes. are used already Microscopes, scanning electron microscopes, atomic force microscopes (AFM) and acoustic microscopes.

It is disadvantageous for optical microscopes that these microscopes close their resolution limit work because the size to be examined the Structures and the wavelength of light, the physical resolution a microscope limited, are approximately equal.

Attempts partially, for example the adhesion of electronic structures to investigate the substrate by means of acoustic microscopes by the differential reflectivity of the sound waves at such Defects is used. However, it is again the limited Resolution of acoustic microscopes hindrance in the way.

Other tests may be carried out in the electron microscope when the are exposed to be examined structures previously by the the Structures supporting substrate as is chemically removed. such a The method is not so destructive and requires careful Preparation of multilayer structures to be exposed. In addition, with the Electron only very thin layers are irradiated, a three-dimensional tomographic reconstruction in depth more extended structures is not possible.

In modern computer chips is a plurality of transistors, the fine wires called interconnects of eg aluminum or Copper are connected. If you look at such a chip in considers sufficient magnification under a light microscope, so you can also in recognize these interconnects grain boundaries.

The interconnects in modern microelectronic devices can without excessive heating very high current densities of 10⁶ A / cm² and more wear because they or by embedding into the surrounding silicon Dielectrics layers can be effectively cooled. The current densities are such large number of electrons that hit directly on the ions, and this in Current direction can move locally - this effect is can called electromigration and damage or destroy the Conductor tracks lead.

Since the atoms very easily in grain boundaries or interfaces Moving between the metal of the conductor tracks and surrounding materials can, it may happen that due to the high material handling make holes in certain places and at other points from the material Conductor is pushed out. Both can lead to failure of the interconnect and thus lead to failure of the entire chip. This effect could in foreseeable future limit the further miniaturization of computer chips and therefore work materials scientists around the world very hard on this problem.

Electromigration in interconnects is one of the main causes of failure of integrated Circuits and due to ever higher integration of circuits remains a major problem. For the lifetime of investigation Leiterbahnmetallisierungen be temporally highly accelerated tests in increased stress conditions, such as increased current density and increased Temperature. Often, to investigate the Migration processes unpassivated interconnects used because they a better microscopic examination of the failure sites with particularly high allow spatial resolution. The omission of the above Conductor tracks applied passivation, ie the conductor tracks on the applied protective insulating layers as SiO₂, Si₃N₄ or Plastic, generally results in an altered sequence of the Electromigration processes. These are namely, material handling and so with <i>local volume changes</i> , whereby the pressure and Temperature conditions in the vicinity of the conductive path through the Passivation layers are affected. Show Therefore unpassivated Conductor tracks, in which the overlying insulating layers is missing, in the Generally, a slightly different behavior for example in the electromigration than the used in practice passivated modules.

Therefore, an imaging method is required, the high spatial resolution also allowed if the structures to be examined dielectrics or insulating layers of several microns thickness are. The microscopes achievable visible light resolution is often too low. With transmission electron can only layers up to a maximum 1 micron thickness are displayed in high resolution. Sensitive surface Methods such as AFM (Atomic Force Microscope) or SEM (secondary electron microscope) require either a destructive sample preparation or reach because of electron scattering in thick passivation only poor resolution. Atomic force microscopes have the additional Disadvantage of the scanning systems relatively long examination times need and that with them no real-time recording of a continuous monitoring is possible.

In principle, it is possible to thin metal structures with a X-ray microscope to investigate. X-ray microscopes Own Wavelength range below a maximum of 20 nm. Since the maximum possible resolution of a microscope the size of half the wavelength is, can be an X-ray microscope is a much higher resolution achieve than with a microscope to visible light or UV radiation. Here, the penetrating power of X-rays by a Sample is generally greater, the shorter the wavelength of the used Radiation, and the thinner the sample.

The X-rays are also to air under normal condition ever Wavelength significantly weakened part. Since usually a be achieved by a few meters X-ray microscope lengths, is to not to unnecessarily lose radiation by absorption, the radiation always in evacuated chambers led to an area close to the object itself is. There, the radiation is passed through a thin window of, eg, a thin but pressure-resistant foil, in air at normal condition passed, in which the sample is located. Behind the sample may usually returning a window to another evacuated chamber be located, in which the imaging X-ray lens is and in the the X-ray radiation is guided to the X-ray detector. The sample may be but also surrounded by a sealable chamber with a Inert gas is filled or is evacuated.

For wavelengths less than 20 nm are used as high-resolution X-ray lenses in X-ray microscopes nowadays only zone plates in question, since only these provide sufficiently high resolution. However, this must with monochromatic radiation are operated, since the focal length is inversely proportional to the wavelength.

X-ray sources for x-ray microscopes are for example Bending magnets, wigglers or undulators from electron storage rings. The radiation from undulators is quasi-monochromatic (The wavelength λ based on the bandwidth Δλ, ie λ / ▵λ is about 100) and is thus directly suitable for X-ray microscopes, as X lenses zone plates with very low zone numbers (typically 100 use zones); when using zone plates higher zone numbers or if wiggler or deflecting the radiation sources, the X-rays are also monochromatic definitely.

In the wavelength range below 20 nm X-ray microscopes can in are amplitude contrast and operated in phase contrast. Below 2 nm wavelength is phase contrast in a special way, because he significantly higher than the contrasts provides amplitude contrast. The reason for that located in the optical constants of refractive index for X-ray radiation, which in this wavelength range, the phase-contrast favor.

There are two major types of X-ray microscopes, imaging and scanning-ray microscopes. The imaging X-ray microscopes produce a real image that can be captured with a camera. Around when using an imaging X-ray microscope operation in to achieve phase contrast, have in the back focal plane of the Figure used zone plate a phase plate with an appropriate Phase shift are arranged. More details are in the Literature already described in detail.

Locking function X-ray microscopes generate an image by a serial Method in which the object irradiated point by point and the image on a monitor is generated. Because of this serial image recording, the is made point by point, are long examination times needed so that a continuous monitoring is not an object is possible. The image refresh rates are in a time range of 100 - 1000 Seconds. Such rasterndes X-ray microscope for three-dimensional tomographic reconstruction of a metallic compound in an integrated circuit is known from the article by H. Zachary Levine, Andrew R. Kakulin, Sean P. Frigo, lan McNulty and Markus Kuhn: "Tomographic reconstruction of an integrated circuit connnect" Applied Physics Letters, Vol. 74, No.1, page 150-152, January 4, 1999. The thickness of Substrate was thinned for the investigation to a few around.

It is the object of the invention to provide a method and a device for Investigation of structures, in particular functional structures specify semiconductor substrates with which the structures zerstörungsarm, with short exposure times, continuously, even during their operation and with an opposite optical microscopes better be observed resolution and the structures are not from the substrate must be separated.

This object is achieved by the features of claim 1 (Method claim) or by the features of claim 8 (Apparatus claim) dissolved.

give Advantageous developments of the method or device themselves. from the corresponding dependent claims

According to the invention for the study of the structures on a Semiconductor substrate used an imaging X-ray microscope. there the structures are irradiated with x-rays and a position-sensitive detector ready. Appropriate wavelengths of X-ray radiation in function of the thickness of the semiconductor substrate in such a way set such that both a specific for detecting the X-radiation sufficient transmission of the X-ray radiation by the semiconductor substrate as well as a high contrast image is assured. For this, at many thicknesses of the substrates samples already sufficiently low. In all other cases have the thicknesses of the substrates in a suitable manner, , Be reduced accordingly for example by etching.

Is, in particular, the substrate of silicon, as are used for the X-ray wavelengths preferably between 2 nm and 0.1 nm set, wherein the thickness of the silicon wafer ( "wafer") is preferably should be between 10 microns and 30 microns.

For larger substrate thickness, especially at thicknesses greater than 100 microns is the Radiation absorption by the silicon substrate so high that the radiation is almost completely absorbed in it, which leads to very long would lead exposure times for the X-ray microscopic image. That's why Wafers are thinned in accordance with such substrate thicknesses, with the functioning of the structures are not changed. this being the Structures only on one side on the silicon substrate and the other is Page structure free (which is the case for most substrates), may from the structure free backside substrate are thinned.

The structures are in wafers, on which electrical circuits integrated are active interconnect layer systems and doped regions. Make the electronic components is on the wafer. Their electrical Operability is not caused by a potential thinning limited because the electronic components only to depths of extending a few micrometers, and still in the thinning of the substrate sufficient substrate thickness remains.

In addition, on substrates, and particularly on silicon wafers also micromechanical structures on or in the surface of the substrate are introduced. Such micromechanical structures such as the Membranes of a pressure sensor, structures of a movable Acceleration sensor or micromechanical gears or pumps be. Such structures can also with the invention Method to be investigated.

Suitable substrates are all materials in question, where and with whom desired structures can be produced. In particular, silicon is for microelectronic circuits and also for micromechanical Components suitable, but also all other, for semiconductor devices be doped crystals such as gallium arsenide used.

With the optionally thinned semiconductor substrates and the use of imaging X-ray microscopes and with the Subtratdicke set wavelengths of X-rays are exposures in reaches the order of 1 second. This means that real-time recording the structures on the substrates and can be continuously while its operation can be observed.

An advantage of the method according to the invention that the structures do not need to be destroyed, as required in many other processes is, where the upper layers of the structures mechanically or by Etching must be removed to expose the underlying structures visible to make and investigate this example with an optical microscope to can. With the help of the selected X-ray radiation and the imaging X-ray microscope according to the invention may be full, operative structures are examined, in particular its size and mutual position. In addition, the operational structures are due to continuous image capture in real time even during its operation observable, whereby movements of the structures or any Structural changes in the micromechanical or electronic structures can be tracked over time.

Generally, with the X-rays is always a certain Radiation damage of materials connected. Therefore, it is advantageous, the structure-free side of the substrate of the incident X-ray first suspend, so that first the substrate absorbs a portion of radiation. The exposure time and thus, indirectly, the radiation damage of on the substrate contained functional elements is through a careful Choice of wavelength under calculation of X-ray optical transmission the substrate and the scattering power located thereon to examined structures minimized.

The method allows fully functional circuits on wafers - also during their operation - higher than light optical resolution to investigate. This may be different parameters and outer Influences happen as are migration processes in passivated Interconnects studyable and also is by tomographic reconstruction of X-ray microscopic images a three-dimensional representation possible. For this it is necessary, the object of different among a plurality map tilt angle.

Changes in distributions of material in electric-migration processes can be studied. This makes it possible, this is also the most technologically relevant <i>passivated</i> Interconnects perform as in commercial integrated circuits are commonly used, and the Examination results compare with model calculations. Thereby obtained better data for simulation and development of integrated Circuits.

More generally, for the study of structures all Radiation sources suited the intense X-ray radiation provide the wavelength range between 2 and 0.1 nm wavelength, of such sources is best not only a particular, in the wavelength should start fixed line radiation to as many of the hereinafter described applications allow.

X-ray microscopes with zone plates must monochromatic Radiation operate as zone plates chromatic aberration show. For the method described in more detail below, this is but not a fundamental disadvantage, since for some of the methods described in principle monochromatic X-ray radiation must be used.

The monochromatisation can be done for example with a monochromator can also be part of the condenser in the microscope. The X-ray sources may also be linearly or circularly polarized deliver radiation, which for the study of "circular magnetic dichroism "ferromagnetic materials is already being used.

Since X-rays emit Undulators quasimonochromatisch and Wavelengths are variable, it is possible to any X-centroid wavelength select the wavelength range mentioned. The monochromaticity of the radiation may then by means of suitable Monochromators be further increased if it for the tests is necessary (see below), so that in measurements of wavelengths the absorption edge whose slight edge shifts to Generating an image contrast can be used.

At 1 nm wavelength have zone plates for X-rays Typically, a focal length of several mm, the object distance then after the imaging equation depending on X magnification something greater. The resolution of the currently available zone plates is for this Wavelength range at 50 nm; it is significantly better than that of a Microscope for visible light or UV radiation. At 0.1 nm wavelength is the focal length for a given zone plate lens 10 times as large as the focal length increases the reciprocal of the wavelength.

Essential for an investigation of semiconductor chips, it is that such a possess sufficiently high X-ray transmission. Will X-ray microscope with x-ray wavelengths in the range of less than 2 operated nm, so layers can already irradiated by several microns thickness will. The exact value depends on the subject to be irradiated material. For silicon, the typical substrate material for semiconductor chips, is the 1 / e thickness which is still permeated with 37% transmission, 4.8 microns in an X-ray wavelength of 1 nm and 33.3 microns at 0.2 nm wavelength. Such thin layers of substrate can be at normal commercial semiconductor devices produced subsequently by the under examination chip from his Rear side is treated with a caustic substance, for example with a chemically active liquid such as ethylene diamine, or mixtures of KOH of hydrofluoric acid, acetic acid and nitric acid, or by heating the substrate by reactive dry etching in an etching system in which reactive ions present are, or by etching with neutral or charged rare gas atoms is thinned.

Here, the side provided with electronic structures before etching protected by being the easiest coated with wax, which is removed again after the etching. There are also other organic Materials as a protective layer possible if it is with solvents be solved again without the electronic circuitry to damage. It is also possible for the action of the means used for etching on the to prevent electronic structures by the too thin substrate as sealing lid is put into a pot, so that the too thin side comes to lie outwardly or inwardly when the etching of the outside or inside is done. Furthermore, it is possible to produce such layers by chemically assisted mechanical grinding to thin.

When a semiconductor chip is thinned such, so can control the remaining remaining thickness easiest done so that the semiconductor chip one side is illuminated with visible light and on the other side Transmission of the light is controlled for example by eye or a photosensitive electric meter is measured.

Once a thickness of about 10 microns is reached, silicon is remarkably optically transparent and the thinning must be stopped in order to examined structures and their functioning not even to damage. This thickness is sufficiently small to X-radiation of wavelengths less than 2 nm with very short exposure times be irradiated can.

Given the thickness and X-ray wavelength, each material another transmission. Since the X-ray transmission generally decreasing atomic number of the chemical element increases has, Carbon in the wavelength range between 2 nm and 0.1 nm significantly higher transmission than for example: silicon. If therefore as airtight protective coating to comply with the electrical structures page (Passivation of the structures) uses a plastic so affected this the transmission in the wavelength range below 2 nm hardly if its Thickness is around or below 10 microns. A thinning of the plastic coatings is therefore not necessary in general.

Some are the semiconductor chips in the commercial version already thinned before (eg memory chips). Such blocks can (After removal of an optional shell) directly without further thinning with short exposure times for the X-ray microscopic image are examined.

The (possibly thinned) semiconductor chip is in the specimen holder of a brought X-ray microscope and with a carefully be selected Wavelength observed in transmission. The wavelengths used in Range between 0.1 and 2 nm high-resolution zone plates lenses Depth of field in the range of typically 10 microns or more, which is substantially greater than the thickness of the conductor tracks and doped regions of semiconductor chips can be larger (and possibly as the Substrate thickness of the semiconductor chip).

Under these conditions, subject widths result of up to about one centimeter, which allow one to test sample also to tilt to radiographic images at different angles to receive, eg to obtain images for stereoscopic viewing. If a whole series of images at different angles with X-ray lenses high S chärfentiefe added, so can the numeric means of tomographic reconstruction of the three-dimensional structure of the semiconductor chip structures be constructed.

Since metals such as aluminum, copper, tungsten, tantalum in the wavelength range show absorption edges between 0.1 and 2 nm, changes the Image contrast strongly when x-ray images at wavelengths before and after the corresponding absorption edge occur. This is possible because at the absorption edge of an element whose absorption and to change phase shift almost abruptly. Thus it is possible, by means of the pictures near the absorption edge specifically the spatial register distribution of the appropriate material.

The study of functional semiconductor chips in X-ray microscope also offers other advantages. Since the radiation all Structures penetrates and makes their overlaps visible creates a visual impression of the conventional medical Radiograph corresponds. If the substrate untilted - the Substrate surface is therefore perpendicular to the X-rays - ready, so can a single shot relatively short exposure time the X-ray microscopic image to evaluate how the structures on the cover semiconductor chip and whether they are correctly adjusted laterally to each other. Further, the quality of the structuring with respect to line width, Edge roughness, uniformity of film thickness can be checked directly. Likewise, contaminants can be detected.

The semiconductor chip may be in air, under protective gas or under vacuum . are Various investigations, in particular Electronic checks during the X-ray microscopic observation be performed. Thus, it is possible, for example the transport of Conductor material under current flow, the so-called electromigration to observe. In principle it is possible, over a wide temperature range away - from temperatures of more than 100 ° Celsius down to the Liquid helium temperature - to carry out measurements, wherein in Temperatures below room temperature, the environment of the sample with an anhydrous gas (eg N₂) Must be purged to the freezing to prevent water on the sample, or the sample chamber with sample is evacuated.

allow measurements on semiconductor chip structures at an elevated temperature the acceleration of the aging process on, for example a few hours, by a substrate having semiconductor chip structures to high temperatures is heated. Relevant observable changes, such as Open circuits then can within seconds play. With the inventive method and apparatus, it is possible this happening timely and accurately observed. It is known by the diffusion or electromigration processes Material distribution can change on the substrate. This change in Material distribution resulting in a change of the image contrast and, in the imaging X-ray microscope can be observed.

There may X-ray microscopic images done during the Current flow to be made by functional structures. If higher than the allowable current flows, can be observed, at which Make the structures are greatly changed or destroyed, so that electrical interruptions (English: voids) or shorts be caused. This process is due to the modified Material Distribution as a change in the X-ray contrast of the image visible.

There may X-ray microscopic images carried on which Voids, dilutions of the traces, dust grains and Interruptions can be searched in interconnects.

There may X-ray microscopic pictures under energization of Semiconductor circuit done in real time a change in the Image contrast detected. The change explained by the fact that by applied electric voltage, the electron density distribution is changed, which in turn has repercussions on the Inner shell electrons, thereby an energetic shift of the absorption edge and a Change the fine structure results. For this it is necessary, the microscope with operate highly monochromatic X-rays, which, for example by Upstream of Kristallgittermonochromators can be achieved. It is possible radiograph those areas in the electronic to locate structures on which the current flow in the semiconductor chip concentrated.

All these observations are basically with a rasterized X-ray microscope possible, both in the pure amplitude contrast, in the pure Phase contrast - ie at a phase shift of the zero diffraction order the object X-ray light by + 90 ° (or -90 °) in the phase plate - and in "Optimized phase contrast". In "optimized phase contrast" is the thickness selected the phase plate so that a given object for a given ready signal -to-noise ratio with minimal number of photons may be, resulting in very short exposure times, what for the continuous monitoring of temporal processes in semiconductor chips is important so that the shortest possible exposure times are achieved. there generally results a phase plate whose phase shift not equal 0 °, 90 °, 180 ° and 270 °.

Is a scanning X-ray microscope operated with energy radiation, so can X-ray fluorescence radiation of all those chemical elements are produced, have the absorption edge whose energy is less than the excitation radiation are. <i>simultaneously</i> , the transmission signal be observed in scanning X-ray operation. This can be by means one mounted behind the sample photon energy-selective detector in Depending on the location of the rasterized ray spot on the sample simultaneously different elements quantitatively and at a sufficiently high demonstrate selectivity, for example, p- and n-doped regions of a Semiconductor to make visible. This is also important for example in the Observing the previously mentioned artificial Aging processes and the above-mentioned electromigration.

Furthermore, with both types of microscope, the imaging and the rasterized ray microscope, all X-ray micro piss tests on rehearsals possible that the spread of X-rays by the Sample space - ie the absence of air pressure, inert gas or vacuum located Volume, in which the sample is located - not prevent, for example by about strong absorption.

In particular, the sample may be external electric and magnetic be exposed to fields; can any light or radiation Wavelength are irradiated. You can ion and electron beams and elevated pressure or mechanical stresses are exposed.

The tests referred to the X-ray microscope are naturally also comparable assembled nanostructures possible eg to micromechanical components or photonic crystals.

The invention will be illustrated in the drawing on the basis of Embodiments explained in more detail. The drawing shows schematically in:<dl tsize="6"><dt>Fig.1</dt><dd>an imaging X-ray microscope with an X-ray lens (but without condenser) to record one with a detector device captured with camera real image of a to be examined Sample,</dd><dt>2a</dt><dd>a zone plate,</dd><dt>2b</dt><dd>a condenser zone with outer segments, which are free of Zone structures</dd><dt>2c</dt><dd>a segmented phase ring</dd><dt>2 d</dt><dd>a segmented aperture, </dd><dt>Fig.3</dt><dd>an X-ray microscope with an X-condenser, X lens and a detector device with camera,</dd><dt>3a</dt><dd>an X-ray microscope according to Figure 3, in addition to a phase plate and a diaphragm,</dd><dt>Fig.4</dt><dd>an X-ray microscope in which an X-ray lens, a real image of the Designs to be examined sample, with a detector device is recorded with a camera and in which the illumination of the sample a rotating condenser is used and</dd><dt>Fig.5</dt><dd>a scanning X-ray microscope that the sample with a small X-spot pointwise irradiated and at the same time X-ray fluorescence radiation excites.</dd></dl>

The Figure 1 shows schematically an X-ray microscope 100, without the a condenser operates. This means that one of an X-ray source 1a (eg electron storage ring) derived X-rays 1 without more focus on an object to be examined 7 hits. The object 7 is a semiconductor substrate to be examined or micromechanical electronic structures having. According to Figure 1 takes the X-rays 1 by a vacuum-tight with X windows closed 5 Vacuum vessel 2 through it, in the vacuum to prevent absorption the X-ray radiation prevails. The rectilinear radiation 1 illuminates the object located in an object chamber 6 7, which contained air is.

An imaging lens 8 forms a portion of the object 7 to a Ray converter 9 from which the X-ray optical radiation in a 10 converts, which is usually in the visible wavelength range. The optical radiation 10 is a lens 11 of a camera 12 added. The X-ray converter 9, the lens 11 and the camera 12 therefore form detection means for X-ray radiation. Such but detector device can also only a single directly illuminated detector consist, for example, a PN-CCD detector or a MOS-CCD-detector in place of the x-ray converter 9 X-ray microscope 100 is disposed. These different types of detector for the X-rays are all shown in the figures X-ray microscopes 100 usable.

If the imaging lens 8 of a diffractive optics such as a Zone plate (and not from a refractive optics as shown below described is), so can not the image field lying in straight alignment be used, as there is a strong, the image interfering radiation 21 zero Rules would be superimposed on the image and reduce the contrast. In this case, a picture with oblique illumination can be made, so that the image and the interfering radiation 21 zero-order juxtaposed. For this purpose, the optical axis 28 of the X-ray microscope 100 against the radiation direction of the illuminating X shaft inclined so that oblique illumination arises.

It may be necessary - especially if elements with similar Atomic number are distinguished from each other in the X-ray image to a Minimizing the need for imaging photon density at the object 7 make as side by side or superimposed structures Elements similar atomic numbers particularly low image contrasts can produce. In this case, the objective is, the wavelength out in the required for imaging photon density a reaches minimum, and thus, minimized in photon density maximum Signal-to-noise ratio in the image can be obtained.

The minimum photon density can be determined by first the scattering cross section according to Rayleigh-Gans is calculated (see. eg Dissertation G. Schneider, Universität Göttingen, 1992, and Gerd Schneider, "Cryo X-ray microscopy with high spatial resolution in amplitude and phase contrast ", in: Ultramicroscopy 75, (1998), pages 85-104, the minimum Photon density is calculated there by formula 31). Is this Scattering cross section is zero, the object can not be imaged. This Scattering cross section is generally a measure of the radiation scattering power of fine structures and in the wavelength range of X-rays a Function of the atomic scattering factors f1 and f2, the wavelength and the Object size. The atomic scattering factors f1 and f2 are in BL Henke et. al. tabulated (BL Henke, EM Gullikson, and JC Davis: X-ray interactions, photoabsortion, scattering, transmission and reflection. E = 50-3000 eV, Z = 1-92, Atomic Data and Nuclear Data Tables, 27, S.1-144, (1982)).

The photon density needed at pictures in the X-ray microscope 100 behaves in a very good approximation to the inverse scattering cross section, ie, Photon density reaches a minimum at the wavelength at which the Scattering cross section reaches a maximum. In addition, the transmission of the to consider substrate, ie ultimately the necessary Photon density and hence the optimum wavelength for imaging a To determine structure, the inverse of the scattering cross-section must be the reciprocal of the substrate transmission to be multiplied. After this The method can then the wavelength range for imaging certain Structures are optimized within a layer system with regard to the necessary photon density.

Starting from the calculation of the photon density can be also at the calculate a wavelength in the structures deposited radiation dose and minimize to possible radiation effects on the structures applicable to suppress as possible. The minimum dose of consideration here Structures is proportional to the linear absorption coefficient of the The material, the quantum energy of the X-ray photons, the inverse of the Scattering cross section and the reciprocal of the density of the interest Material. It is presupposed for the validity of that proportionality, that the thickness of the structures used in the investigation to X-ray wavelength is less than the 1 / e thickness at which the structures 37% have X-ray transmission. This condition is for the here considered materials and their thickness given in interconnects. Out For the sake of minimizing the dose of interest in the structures landfilled is, the thinned side of the substrate should always in Source direction be installed showing the X-ray microscope. On the Towards the structures of interest, the X-rays on her Way then already weakened by the wafer crystal and irradiating the Structures with less intensity than would be the case if the Structures on the side subject, which is directed towards the radiation source.

To the photon density or minimum dose for imaging in a) Amplitude contrast, b) pure phase contrast, and c) a combined case of amplitude and phase contrast (which to image formation, both the Phase shift and the absorption in the object exploit) to calculate, these three cases can be simulated, characterized in that a) the real part of complex atomic scattering factor is set to zero b) the imaginary part of complex atomic scattering factor is zeroed and c) neither of the two atomic scattering factors is changed, ie, the tabulated values ​​of BL Henke et al. from "atomic data and nuclear data tables" (see above) be used.

Said bending magnets wigglers and undulators a Electron storage ring are sources of intense X-rays and deliver a high flux of X-rays. Nonetheless, the per second available number of photons always a the Image recording time limiting factor. Therefore, it makes sense that to operate X-ray microscopes in phase contrast. The phase shift the phase plate should be such that the microscope to Video recording, amplitude attenuation and phase shift evaluates. The number of required for imaging photons can be minimize, if the phase plate for the particular wavelength and the Material to be examined structure a very specific, individually determining transmission and phase rotation has. In general phase rotations are required of non + 90 ° and -90 °. criterion thereby, the best signal to noise ratio with minimal number of photons to to accomplish. Programs have the potential literature at Gerd Schneider, "Cryo X-ray microscopy with high spatial resolution in amplitude and phase contrast ", in: Ultramicroscopy 75, (1998), pages 85-104 to find.

In the case of imaging X-ray microscope is the Phase contrast imaging by a phase ring in the rear Focal plane of the X-ray lens is placed, reaches (G. Schmahl et al.). In the case of the scanning X-ray microscope studies are of the Structures on semiconductor substrates with phase contrast assemblies possible, which are described here for the first time, even with Arrangements without real phase rings.

The 2a shows in plan view a zone plate a radially symmetric Circle, Grid represents a decreasing outwardly lattice constants. The bright zone rings are transparent to the x-ray. The in 2a shown in black areas are made of either a material the X-rays absorbed - this is called absorption or Amplitude zone plates - or a material containing the X-rays particularly strong pushes in their phase and only a small has absorption - then one speaks of phase zone plates. zone plates are used for X-ray radiation having diameters of less than 1 mm Microscope zone plates designated. are zone plates for X-rays used among other things as X-ray lenses 8 and condensers. 3

Figures 2b to 2d are in connection with the following described descriptions of the other figures.

According to Figure 3, the object 7 is measured using a 3 Röntgenkondensors illuminated. A spring disposed in the center of the emerging radiation cone radiopaque diaphragm 4 causes a hollow conical Illumination of the object 7. with an imaging microscope of FIG. 3 with hollow cone illumination quickly from amplitude contrast to being able to switch phase contrast or by the phase angle of the making phase contrast assembly adjustable, can in the rear Focal plane of the x-ray objective 8 a segmented phase ring 20 to be ordered. This arrangement is shown in 3a. The segmented Phase ring 20 consists of many phase-shifting and not phase-shifting portions 26, 27. The portions 26, 27 are preferably all the same size and equidistant to the area of Phase ring 20 distributed (Figure 2c). If an as Röntgenkondensor 3 annular condenser zone uses 19 (2b), then <i>simultaneously</i> on Phase contrast and an amplitude contrast image on the detector 9, 12 generated. Used as Röntgenkondensor 3 a rotating condenser 13 after Fig.4 used, then in the image plane during the rotational movement<i>alternately</i> a phase contrast and an amplitude contrast image and generates sequentially recorded in the image plane of the detector 9.12.

If now - as shown in the 3a - to improve X-ray microscopes 100 in front of or behind the plane of Röntgenkondensors 3 (Figure 3) or the rotating condenser 13 (Figure 4) an aperture 29 (2 d) of transparent segments 23 and absorbing segments 22 arranged that after their appearance in the segmented phase ring 20 2c corresponds so by suitable rotation of the aperture 29 or the Phase ring 20 about the optical axis 28 in one or the other position to ensure that X-rays of the zero diffraction order of the object 7 either through the transparent portion 27 of segmented Phase ring 20 occurs, or only by the phase-shifting region 26: In Phase contrast are the aperture 29 and the phase ring 20 such each other such that the light through the segments 23 of the aperture 29 in the 0th Rules of the object 7 only by the phase-shifting segments 26 of Phase ring 20 falls. In contrast, the radiation amplitude of the 0th order falls 20 only in the segments 27 of the phase ring in all other Intermediate positions of segmented phase ring 20 and the diaphragm 29 with transparent and absorbing segments 23, can another 22 any intermediate values ​​of the phase shift can be adjusted.

In another improvement, the segmented aperture 29 may be omitted, by as Röntgenkondensor 3 an annular condenser zone 19 (2b) is used in which only the phase corresponding ring Segments 25 with zone sections and zone-free segments 24 are structured. If such segmented condenser zone 19 or the segmented phase ring 20 (2c) rotated suitable, then the Figure in phase contrast for imaging in amplitude contrast image switch: In the phase contrast condenser 3 is such that the Light of the 0th order of the object 7 only by the phase-shifting Segments 26 of the phase ring 20 falls while the amplitude contrast the Radiation of the 0th order only intervening in the segments 27 falls. In all other intermediate positions of segmented phase ring 20 and a Condenser zone 19 with zone-free segments 24 and Zone sections 25 to one another any intermediate values ​​of Phase shift can be adjusted.

A further improvement can be the critical in so-called " Lighting "illuminate illuminated object plane homogeneous by the condenser zone 3 by means of adjusting elements in two dimensions is scanned over the object field.

To record a pure phase contrast or Amplitude contrast image in the X-ray microscope 100 under after Fig.4 Using a rotating condenser 13 can also be a Chopper disc with at least one transmissive segment somewhere be placed in the beam path. The chopper disc with the Rotational movement of the rotating condenser 13 synchronized. Depending on Phase angle between chopper disc and rotating condenser 13 is the recording of the phase contrast image or Amplitude contrast image on the detector permitted 9.12 by the Radiation, leading to the generation of unwanted image contrast would, drops to the chopper disc and this hides characterized. This can also happen outside the X-ray passage when a mechanical or electro-optical shutter between Ray converter 9 and 12 camera is brought or by the camera 12 by electronic means, the recording during the times turns, in which the desired image to be recorded is present.

Instead of the known arrangement of a phase plate or phase ring after Fig.2 c with the supporting film, it is possible to use these elements phasenenschiebenden cantilevered to structure. In a cantilevered structure, the Segments at its edge by a series of fine supporting webs, partly run like the spokes or randomly distributed, maintained that the Segments with each other and with an outer retaining ring connect.

In imaging X-ray microscopes 100 of Fig. 3, two optics Needed: a condenser 3 that the incident X-ray 1 the object to be examined 7 and focuses an X-ray lens 8, the X-ray radiation fields, which has penetrated the object 7th The X lens 8 forms a real image of the object 7 on a resolving detector device after an image integration time Image signal output to a monitor.

As already mentioned, the position-resolving detector device in imaging X-ray microscopes 100 from the X-ray converter 9, the Lens 11 and the camera 12. The camera 12 records the image. Because of the possible damage to the camera 12 in direct irradiation with very short wavelengths but from the real radiograph best initially produces visible radiation, for example by means of a fluorescent Substance of the x-ray converter 9, as a phosphor. The visible Radiation 10 is then one of the camera 12 for visible light, for example, CCD detector ready by means of the optics 11, for example, a deflecting mirror may contain - to the camera 12 from the direct X-ray, and thus any possible radiation damage by remove X-rays. Such radiation damage to the CCD detector occur when using wavelengths below 2 nm, since these X-rays at a noticeable transmission in some microns deep in X-CCD image sensor can penetrate. These depths are but the electrical conductor tracks, insulating layers and semiconductive Functional elements of a CCD image sensor, the through-running Irradiation with relatively high-energy X-ray damaged permanently can be, for example, by the insulating layers between the conductor tracks be disrupted and / or destroyed. can instead of the x-ray converter 9 but also directly in the X-ray microscope 100, a camera with PN-CCD are arranged, which is insensitive to radiation damage.

If such an imaging X-ray microscope 100 in phase contrast operated, it must, as described in the literature, a phase plate in the back focal plane of the X-ray lens can be arranged. For a imaging X-ray microscope 100 is this phase plate normally annular configuration, so as the phase ring. It is also described that when Using zone plates 18 as X-ray lens 8 is always required , the object 7 to illuminate a hollow cone in order to avoid that the Image in the central area of ​​radiation without information that in radiation zero order of the zone plate 18 (micro zone plate) can be made, is superimposed.

Therefore, is always selected a condenser 3 in practice, the from among its members no light on the object throws. This can for example with an annular Condenser zone happened 19 or it is by Lens 4 behind Condenser, the central radiation disappears. It is also in the described literature and shown in Fig. 4, that an imaging X-ray microscope 100 as a rotating condenser condenser 13 contains, from the obliquely illuminates the radiation 14 to the object. 7 then by rotating the condenser 13 the necessary produces hollow cone illumination for the object. 7 The remaining structure corresponds the in Figure 3.

The rotating condenser 13 may also include rotating mirrors with Multilayers are occupied. Such multilayer films have a limited range of wavelengths and angles of incidence, which can be greater than the critical angle for total reflection, even a high reflectivity, the is typically well above 10%.

This can occur at wavelengths below 1 nm with reflection angles work which 10 are typically times greater than the incidence angle at where simple mirror are used by utilizing the total reflection can. Typical angles of incidence for multilayer mirror of 1 ° can be realized. Since this a required projection of the mirror in a plane perpendicular to the optical axis 28 of the X-ray microscope 100 with much shorter mirror lengths can be reached, simplifying the use of Multilayer mirrors the structure of a rotating condenser 13. There are only small mirror lengths necessary. In addition, the simplified Angular adjustment of the mirror, because the absolute tolerances to be observed the Angle are much larger than when using simply mirroring with Total reflection.

In scanning X-ray microscope according to FIG. 5 is an X-ray lens with 8 an X-ray wave 1 illuminated. The X-ray lens 8 focuses the X-shaft 1 in the plane of the object 7 to a small X-spot. The object 7 by radiating X-rays is with a Röntgendektor 16 measured with or without spatial resolution. The object 7 thereby moves in raster of exercise by means of a Raster mechanism 17 includes the translators. these can Piezo elements, pneumatics, moving coil, Mechanical spindles, lever or other displacement devices contain. Detector 16 registers the Object 7 penetrating X-rays. The structure of the picture takes place point for point on a monitor, so serially. The screening is usually carried out, by either the object 7 in two dimensions or the X-ray lens 3 two-dimensionally or two elements in mutually perpendicular moving directions. Behind the X-ray lens 8 is still a X-ray absorbing shutter 4, which appears in shadow X-ray light spot is produced. Such scanning microscopes are described in the literature described in detail.

If a scanning X-ray microscope operated in phase contrast, so is it is necessary, before an annular portion of the x-ray objective 8 an annular attach phase plate in the incident on her X-wave suitably rotates (Details are described in the literature). The annular phase plate consists of a material of suitable optical Constant, which on a transparent X-ray film is applied and which are concentrically aligned with respect to x-ray objective 8 have to be.

Another possibility described in the literature to be rasterized Phase contrast X-ray microscopy is given if, instead of a Phase ring a normal X-ray lens is used, and if for obtaining the phase information of a segmented in the surface, extended detector is used, read their segments individually can be. The phase information is then passed through the links individual signals of the segments obtained, eg by difference of Signals of opposite segments.

Is a scanning X-ray microscope operated with energy radiation, so can X-ray fluorescence radiation of all those chemical elements are produced which have less than the absorption edge at energies the exciting radiation. Thus, a transmission grid image and a be recorded fluorescent image simultaneously.

<u>LIST OF REFERENCE NUMBERS</u>

<dl tsize="3" compact="compact"><dt>1a</dt><dd>X-ray source</dd><dt>1</dt><dd>incident X-ray</dd><dt>2</dt><dd>vacuum vessel</dd><dt>3</dt><dd>Röntgenkondensor</dd><dt>4</dt><dd>opaque visor</dd><dt>5</dt><dd>röntgenstrahlung transparent window</dd><dt>6</dt><dd>Property chamber</dd><dt>7</dt><dd>to be examined object: semiconductor substrate</dd><dt>8th</dt><dd>X lens</dd><dt>9</dt><dd>X-ray converter, for example a phosphorus</dd><dt>10</dt><dd>visible light</dd><dt>11</dt><dd>lens</dd><dt>12</dt><dd>camera</dd><dt>13</dt><dd>rotating condenser</dd><dt>14</dt><dd>from condenser focused X-ray</dd><dt>15</dt><dd>Detector for X-ray fluorescence</dd><dt>16</dt><dd>Detector for transmitted X-rays</dd><dt>17</dt><dd>raster mechanism</dd><dt>18</dt><dd>zone plate</dd><dt>19</dt><dd>annular Kondensorplatte</dd><dt>20</dt><dd>segmented phase ring</dd><dt>21</dt><dd>Direct light in the zero-order</dd><dt>22</dt><dd>absorbing segment</dd><dt>23</dt><dd>non-absorbing segment </dd><dt>24</dt><dd>transmitting segments without zone structures</dd><dt>25</dt><dd>radially undisplaced zone segments</dd><dt>26</dt><dd>phase-shifting a phase segment ring</dd><dt>27</dt><dd>not phase-shifting a phase segment ring</dd><dt>28</dt><dd>optical axis of the X-ray microscope</dd><dt>29</dt><dd>Lens with absorbent and transparent segments (22,23)</dd><dt>100</dt><dd>imaging X-ray microscope</dd></dl>