JP2009294235A - Atom probe apparatus and atom probe analysis method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To present a technique for preliminarily working a sample, which makes a place, that is desired to be analyzed, of a device into a needle-like protrusion by locally cutting out the place, and to provide a technique for making an SAP analysis at the atomic level possible by ensuring stabilized ion evaporation sequentially even in the case of a sample of multilayer structure including an element layer of small evaporation electric field in the following circumstances. <P>SOLUTION: A preliminary processing method for a sample used on atom probe apparatus includes the steps of: cutting the desired observing part of the sample into a block using an FIB equipment; transferring the sample block onto a sample substrate and fixing the sample block in place; and processing the sample block fixed onto the sample substrate into a needle-point shape by FIB etching. The sample processed into a needle-point shape is shaped such that the layer direction of the multilayer structure becomes parallel to the longitudinal direction of the needle. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a sample arrangement in an atom probe apparatus equipped with a hollow conical ion extraction electrode and a pre-processing method for a needle-like sample.

  The first high-resolution microscope using the electron tunneling phenomenon is a field emission microscope (FEM) that emits electrons from a sharp needle tip and observes an enlarged projected radiated electron image as shown in FIG. It was. This microscope uses a field emission phenomenon in which electrons are emitted from the surface of a metal conductor across the surface potential barrier by a quantum mechanical tunnel effect when a strong electric field is applied under vacuum conditions. In addition, an electron beam is emitted from the tip surface of the metal toward the screen coated with the phosphor by the action of a strong electric field, so that an enlarged image of the surface of the emitted metal is displayed on the phosphor screen. Since the resolution of FEM is as low as about 1 nm, atoms cannot be seen, but the work function of a fine crystal plane on the hemisphere of the needle tip can be obtained from the negative voltage applied to the needle and the IV characteristics of the radiation current. When the voltage applied to the needle is switched from negative to positive and a low-pressure inert gas is introduced into the lens body, the FEM operates as a field ion microscope FIM (Field Ion Microscope) so that the atomic arrangement of the needle tip can be directly observed. Become. FIM has a characteristic that the surface atoms of the needle tip can be desorbed in order as cations by the electric field evaporation phenomenon. This phenomenon is also used for atomic operations using a scanning tunneling microscope (STM). By detecting and identifying the desorbed ions one by one, the composition of the needle tip can be analyzed at the atomic level. Based on this idea, a composite atom probe AP (Atom Probe) of a mass analyzer capable of detecting a single ion and FIM was developed. The AP is the only device that can analyze the electronic state, atomic arrangement, and composition distribution of the needle tip. Since field evaporation proceeds in order from the first surface layer to each atomic layer, the composition of each layer, the composition distribution of the interface, and the change in the electronic state can be examined by AP.

  However, this AP has severe restrictions on the preparation and shape of the sample, and the field in which the characteristics can be utilized is limited. A scanning atom probe (SAP) has been devised to overcome this limitation. In order to select a specific needle from closely packed needles and examine its tip, an electric field must be localized at the needle tip. Therefore, a grounded fine funnel-shaped extraction electrode is attached to the AP body, and a positive voltage is applied to a planar sample in which fine needles are closely arranged. Then, a high electric field is generated at a single needle tip just below a hole with a diameter of several μm to several tens of μm at the tip of the extraction electrode, and the electric field is localized in a very narrow space between the hole and the needle tip. To do. According to the electric field distribution calculation by a computer, even if the apex angle of the needle tip is 90 ° and the tip radius of curvature is 50 nm, a high electric field required for electric field radiation and field evaporation is generated at the needle tip. This indicates that the tip of the protrusion can be analyzed if there are irregularities of about several μm on the flat sample surface. Surfaces that have not been smoothed, corroded surfaces, highly efficient catalyst surfaces, and the like are usually rich in irregularities, so these surfaces can be left as they are. FIG. 4 shows the basic structure of the SAP. The leftmost sample schematically shows a densely arranged field emission electron source. When the hole at the tip of the funnel-type extraction electrode approaches the tip of the needle tip or protrusion on the sample surface, a high electric field is generated in a very narrow region between the tip and the electrode, and electrons emitted from the tip of the needle are FEM on the screen. Project an image. Further, when an inert video gas such as helium is introduced into the lens body and a positive voltage is applied to the sample, a high-resolution FIM image is displayed on the screen. Furthermore, when a pulse voltage is added to the steady voltage or the surface of the specimen is irradiated with pulse laser light and the surface atoms are field evaporated, the surface atoms evaporated as cations pass through the search hole in the center of the screen and pass through the mass spectrometer. A certain reflectron is entered and detected one by one. The region to be analyzed is a region with a diameter of several nanometers to several tens of nanometers at the tip of the protrusion corresponding to the exploration hole. If the analysis is continued, the compositional change in the depth direction of this region can be investigated with the resolution of one atomic layer.

  In this document, a sample with an uneven surface is used as an analysis target. In particular, a convex portion is searched for and opposed to an extraction electrode, and a sample projection is sequentially evaporated from an upper layer atom and extracted as an ion, and behind the extraction electrode. Elemental analysis can be performed by measuring the time of flight of each ion when it is detected by the installed ion detector (two-dimensional detection type). Since position information is also obtained, it is shown that three-dimensional composition analysis at the atomic level is possible.

  On the other hand, when a sample to be analyzed such as a semiconductor wafer having a strong analysis need or a thin film magnetic head wafer called GMR or TMR is used as a sample, it is often a multi-layer structure in which complicated patterns are stacked. The structure of the parts is diverse. In order to analyze such an object to be analyzed using AP, it is necessary to locally cut out the portion to be analyzed and cut out and fix it as a fine section at the tip of the needle-like projection that becomes an electrode. Only the conventional method of forming a sample of a metal material or the like into a needle shape existed, and it was very difficult to analyze a fine specific site by AP. Therefore, as an alternative method, it is essential to develop a pre-processing technique for processing the sample itself into a needle shape. Because of the analysis at the atomic level, the size of the analysis target is about 100 nm cubic, so the technology for producing the analysis target portion into a needle-like sample is extremely important.

JP 2004-361139 A

Seizo Morita, “Scanning Probe Microscopy Fundamentals and Future Prediction” published by Maruzen on February 10, 2000 2.7 Scanning Atom Probe (SAP) 70-73 Edited by Osamu Nishikawa “From Scanning Probe Microscope STM to SPM” March 30, 1998, published by Maruzen Page 8 and Table 1-2 Evaporation field strength of various elements

  The problem to be solved by the present invention is that the sample to be analyzed is not limited to a protrusion having a micro unevenness, and the sample preliminary processing of the sample to be processed into a needle-like protrusion by locally cutting out the portion of the device to be analyzed In addition to presenting the technology, a technology that enables sequential and stable ion evaporation and enables atomic level SAP analysis even for multi-layered samples including element layers with greatly different evaporation fields in the circumstances described later. It is to provide.

  An atom probe apparatus according to the present invention includes an extraction electrode that applies an extraction electric field to the end of each layer of a sample having a multilayer structure, a storage unit that stores the strength of the evaporation electric field of each layer of the multilayer structure, and an ion detection device. Means for detecting an arrival position on the screen, and has a function of specifying the end position of the layer specified from the evaporation electric field strength and the arrival position of each layer of the multilayer structure.

  In addition, the atom probe analysis method of the present invention is configured so that ions evaporated by applying an electric field to the end of each layer of a needle-tip-shaped sample in which the interface direction of each layer of the multilayer structure is parallel to the longitudinal direction of the needle are applied to the screen. In the atom probe analysis method for detecting the position reached and analyzing the element constituting the sample, the step of gradually increasing the electric field strength applied to the sample, the step of storing the evaporation electric field strength of each layer, and the evaporation electric field strength of each layer And correcting the position of the constituent element of the sample from the position where the ions have reached the screen.

  The method for pre-processing a sample for an atom probe device of the present invention includes a step of cutting a sample desired analysis site into a block shape using an FIB device, a step of transferring and fixing the block-shaped cut sample on a sample substrate, Since the block-shaped sample fixed on the sample substrate includes a step of processing into a needle tip shape by FIB etching processing, a desired analysis point is cut out and a sharp point suitable as a sample for an atom probe apparatus is obtained. It can be formed into a needle tip shape.

  In addition, the preliminary processing method of the sample for the atom probe apparatus of the present invention includes a step of temporarily adhering the block-shaped cut-out sample on the sample substrate by FIB-CVD, a base of the block-shaped sample, and the sample substrate. Since the FIB-CVD was performed on the cut portion and the cut portion was subjected to FIB-CVD, the sample substrate and the block sample could be firmly bonded and fixed. As a result, the test using the atom probe apparatus can be performed stably.

  In the finishing process in which a block-shaped sample is processed into a needle tip shape by FIB etching, the damage of irradiation ions remaining in the sample can be suppressed as much as possible by performing the acceleration voltage at 10 kV or less.

  In addition, after the finishing process is performed at an acceleration voltage of 10 kV or less, the damaged layer can be removed by low acceleration Ar ion milling or the like.

  In the sample for the atom probe apparatus of the present invention, the sample processed into the shape of the needle tip is formed so that the layer direction of the multilayer structure is parallel to the longitudinal direction of the needle. Even if there is a layer, there will be no separation at the boundary surface, and the test using the atom probe apparatus can be performed stably.

  The sample for the atom probe apparatus of the present invention comprises a storage means for accumulating information such as which element layer evaporates first, and a means for detecting the arrival position on the screen serving as an ion detector. The end position of the element layer to be identified can be determined based on the information.

  The atom probe apparatus according to the present invention can separately analyze different elements that co-mix and ionize.

It is a figure explaining the method of this invention which pre-processes the needle-shaped sample for SAP. It is a figure explaining the method of this invention which pre-processes the needle-shaped sample for SAP by making the layer of a multilayer structure sample into the axial direction of a needle | hook. It is a figure explaining the basic composition of field emission microscope FEM. It is a figure which shows the basic structure of the scanning atom probe SAP.

  The inventors of the present invention tried to perform preliminary processing of a sample and analysis using the sample in order to analyze a device having a multi-layer structure in which complicated patterns are stacked for each atomic layer by using the AP technique. First, there is a processing method in which a part of a device is cut out in a block shape using a focused ion beam (FIB) device as a preliminary processing of a sample, transferred and fixed on a sample substrate, and further processed into a needle-like sample using an FIB device. I took it. For example, a desired observation position of the device is specified from a large wafer-like sample by using a scanning ion microscope (SIM) function of the FIB apparatus, and a protective film is formed on the surface by FIB-CVD. Here, when a region to be analyzed by AP exists on the sample surface, a protective film is attached before FIB irradiation in order to avoid damage such as Ga ion implantation due to FIB irradiation. The protective film can be formed by a method using a vacuum deposition apparatus or a sputtering film forming apparatus. Note that a method suitable for the point that a protective film can be applied at a desired position after the placement is described below. Using a device equipped with an SEM arranged so as to irradiate the same point as the FIB, after roughly positioning by this SEM, a protective film of about 50 nm thicker than at least the depth of penetration of the FIB is attached by EB-CVD. If a thicker film is required, additional film formation may be performed by FIB-CVD, which allows high-speed film formation. Subsequently, as shown in FIG. 1A, the periphery of the desired observation point is drilled by FIB etching, and the sample stage is tilted so that FIB irradiation can be performed from the direction of the large hole, and the bottom cut by FIB etching is performed. Disconnect from the device. The separated block-shaped observation sample piece 1 is transferred onto a fixed substrate 2 by a fine probe 3 operated by a manipulator as shown in FIG. 1B and temporarily fixed by FIB-CVD. Since the sample piece 1 cut out at this time is bottom-cut by FIB from the tilt angle direction, the bottom portion has an inclination angle with respect to the surface, and the fixation to the fixed sample stage 2 is inclined as shown in the figure. Fix it to fill the part. The above-described process is a description of a "sample with a mark indicating the vertical direction and a vertical positioning method of a pickup sample" relating to a transmission electron microscope sample developed by the present inventors group and filed as Japanese Patent Application No. 2003-157120 ( This is in accordance with the method disclosed in Patent Document 1).

  If the block-shaped sample piece 1 cut out on the fixed substrate by FIB etching after specifying the location to be observed is temporarily fixed (5 temporarily fixed portions), the sample stage is tilted to form the base of the block-shaped sample piece 1 And the fixed substrate 2 are cut by FIB etching. Then, FIB-CVD is applied to the cut portion, and the sample substrate and the block-shaped sample are bonded and fixed (six fixed portions). This state is shown in FIG. The block sample piece 1 is firmly fixed to the fixed substrate 2 by performing the main fixing at a plurality of positions. Finally, the block-shaped sample piece 1 is formed into a needle shape by FIB etching. The needle-like sample 1a preliminarily processed in this way is shown in FIG. Here, as shown in FIG. 1B, the fixed substrate 2 is a flat plate larger than the cut sample piece 1, but in some cases, the fixed substrate 2 has a needle shape with a flat tip as shown in FIG. 1C. Protrusions may be provided, and the sample piece 1 cut out thereon may be fixed and processed so as to have a large needle shape as a whole. This is because the length of the needle needs to be several times the diameter of the ion extraction electrode in order to concentrate the electric field on the sample needle tip. Providing the protrusions on the fixed substrate as described above makes it possible to cope with the case where the diameter of the AP ion extraction electrode is large. The depth of damage caused by irradiation of irradiated ions such as gallium into the sample by FIB processing can be made shallower if the acceleration voltage is lowered. You can: Since the tip diameter of the sample needle is about 200 nm, it is possible to analyze most of the region excluding a part of the outer periphery. It is also effective to include a step of removing the damaged layer by low acceleration Ar ion milling or the like.

  The sample processed in this way has a form in which different materials are stacked in layers from the tip of the needle to the base. This sample is set on the SAP, a voltage is applied between the sample substrate and the hollow conical extraction electrode, and ions are evaporated for each atomic layer from the tip of the sample, and elemental analysis is performed with an ion detector. It is a known technical matter that the electric field strength required for the field evaporation differs greatly depending on the element. Non-Patent Document 2 shows a list of evaporation electric fields and evaporation electric field experimental values obtained from theoretical formulas for each element. Therefore, in the AP analysis of a multilayer thin film sample, it is necessary to quickly switch the applied electric field strength to an appropriate value for each layer and proceed with the analysis. When there is a layer with low interfacial adhesion strength under a layer of an element with a particularly large evaporation field, the adhesion force is defeated by electrostatic attraction, causing separation at the weak interface and causing a sample above that layer to fly. The problem occurs.

  Therefore, the present inventors have made it possible to perform stable ion evaporation without peeling off at the element layer even if there is an easily peelable interface in the multilayer structure of the sample, and at the atomic level The research was advanced as a further problem to provide an atom probe device that can analyze the above.

  As a first method for solving this problem, in a thin film sample, when there are a layer with a large evaporation electric field and a layer with a small adhesion strength across the interface, the layer with a small evaporation electric field is on the surface side. The sample direction is cut out and bonded onto the sample substrate. By arranging in this way, atoms in a layer with a small evaporation electric field are initially ion-evaporated by applying a low voltage, and after the atomic layer has evaporated, the electric field is strengthened so that atoms in a layer with a large evaporation electric field are ion-evaporated. By doing so, analysis at the atomic level becomes possible.

  This method cannot be used when there is a layer with low interface adhesion strength in the intermediate layer in a multilayer thin film sample. In the present invention, the sample structure is processed so that the interface direction of each layer is the longitudinal direction of the sample needle. I came up with it. The preliminary processing of the sample in this case proceeds as shown in FIG. Block cutting by FIB etching is performed so that the laminated surface comes in the longitudinal direction of the needle shape as a sample processing finish. In general, since the laminated surface of the semiconductor element has a layer surface parallel to the surface, a rectangular area is cut out shallowly as shown in FIG. 2A, but before that, an analysis point is located and damage caused by FIB. The first protective film 4a is formed on the surface of the region so as not to receive. For this side, it is preferable to use an acceleration voltage of 10 kV or less and a shallow damage layer. Next, the cut sample piece 1 including the observation region is cut out by FIB etching, and the manipulator is operated as shown in FIG. 2B to transfer the cut sample piece 1 onto the substrate 2 on which the cut sample piece 1 is fixed. Temporarily fixed (5 temporarily fixed parts) by FIB-CVD. At this time, since the direction of the sample piece 1 forms a needle shape, the longitudinal direction is fixed in a direction orthogonal to the sample substrate 2. When temporary fixing has been completed, the main fixing (six fixing parts) is performed. At that time, as shown in FIG. 2C, in order to prevent damage due to the FIB irradiation, the second end of the sample is subjected to FIB-CVD. A protective film 4b is formed. Before forming the protective film 4a, it is necessary to detect the position information of the multilayer structure portion serving as the observation region by the SIM. The method of permanently fixing the block-shaped cut sample piece 1 on the sample fixing substrate 2 is to tilt the sample stage and irradiate the FIB from the diagonally upper part to the base of the block-shaped sample piece 1 and the sample substrate 2 and perform FIB etching. An incision is made, and while a source gas such as phenanthrene is ejected from a gas gun, FIB is irradiated to the incision portion to perform CVD, and the sample substrate and the block-like sample are bonded and fixed. Such adhesion processing is applied to a plurality of locations, and the block sample is firmly fixed to the sample substrate.

  The columnar sample piece 1 fixed to the fixed substrate 2 is etched by FIB irradiation from above to form a needle shape. At this time, which part of the columnar sample piece 1 has the observation region is detected and etched based on the stored positional information. The state machined in this way is shown in FIG. The tip diameter is processed to about 0.2 μmφ.

  When a multilayer sample having a layer thickness of nm or sub-nano order is produced in such a needle shape, the end of each layer is exposed to the extraction electric field at the same time, so atoms in each layer start ionization evaporation at the same time. In particular, it ionizes and jumps out from the element layer with a small evaporation field. Then, since a layer having a relatively large evaporation electric field is left behind, the layer becomes convex and the electric field concentrates, while the evaporated layer becomes concave, and the tip portion is caused by the difference in the element's evaporation electric field as shown in FIG. A step will be produced at the position of. The end of the layer with a low evaporation electric field is lowered, but when this is done, the electric field strength formed between the extraction electrodes is lowered. As a result, if the applied voltage is constant, the ionization of the layer with a small evaporation field is slowed down. Therefore, if the extraction electric field is set so that the ionization rate is constant within the range that can be analyzed, it will be balanced even if there is a time lag, and eventually all the layers will be evaporated. As a result, the problem that a specific layer evaporates at a high speed and analysis of each layer cannot be performed can be solved.

  When analyzing this sample by SAP, there is a difference in the value of the evaporation electric field of each layer of abcd. As the electric field strength is gradually increased, the lowest value element can be detected on the screen first, and the value of the evaporation electric field is sequentially increased. Can be detected. If this is traced in time series, even if the position of the tip changes due to the difference in evaporation speed, the correspondence can be taken and the position can be corrected. In other words, if the evaporation surface is uneven, the flight direction of ions will change from the case of a simple needle tip, but from the positional relationship between the end of each layer and the extraction electrode, the ion generation location and detector Since it corresponds to the arrival position on the screen, position correction is possible. These various analyzes have made it possible to perform atomic level composition analysis of ultra-thin multi-layered samples, which has been difficult in the past.

DESCRIPTION OF SYMBOLS 1 ... Cut-out sample piece 1a ... Needle-shaped sample 2 ... Sample fixing board 3 ... Probe 4, 4a, 4b ... Protective film 5 ... Temporary fixing part 6 ... Main fixing part

Claims (2)

An extraction electrode for applying an extraction electric field to the end of each layer of the sample having a multilayer structure;
Storage means for storing the intensity of the evaporation electric field of each layer of the multilayer structure;
Means for detecting the arrival position on the screen for detecting ions,
An atom probe apparatus having a function of specifying an end portion position of a layer specified from an evaporation electric field strength of each layer of the multilayer structure and the arrival position. The sample is formed by detecting the position where the ions that have evaporated by reaching the screen by applying an electric field to the end of each layer of the sample of the needle tip shape in which the interface direction of each layer of the multilayer structure is parallel to the longitudinal direction of the needle In the atom probe analysis method for analyzing the element to be
Gradually increasing the electric field strength applied to the sample;
Storing the evaporation electric field strength of each of the layers;
Correcting the position of the constituent element of the sample from the position where ions have reached the screen using the evaporation electric field strength of each layer;
Atom probe analysis method comprising:

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