JP4335656B2 - Sample preparation method for microscope - Google Patents

Description

  The present invention relates to an observation sample preparation technique capable of cutting out a fine sample such as an observation sample of an electron microscope or a probe microscope with high accuracy from a desired location using a focused ion beam (FIB).

  In recent years, various devices such as a semiconductor device and a display device have a fine and complicated structure in order to realize functional improvement. In particular, elements and wirings forming various devices have a laminated structure in which thin films of several atomic layers are stacked, and there is a strong demand for observing the structure with a microscope. In that case, a desired portion of a sample such as a wafer on which a large number of devices are formed is cut out as a small sample, and the side wall or bottom surface of the small sample is observed with a scanning electron microscope (SEM) or a scanning probe microscope (SPM). Has been made.

Further, since a sample of a transmission electron microscope (TEM) is for observing an electron transmission image, the sample is required to be processed extremely thinly. It is well known that a cross-sectional sample such as a wafer is manufactured by thinning using a focused ion beam (FIB) apparatus, and a small piece is mechanically cut out from a wafer-like sample as shown in FIG. 6 and processed. There are known a method and a method of taking out a thinned sample by etching with a FIB apparatus directly on a wafer as shown in FIG. The former processing method requires a machining process in which a small block having a width of 500 μm to 2 mm and a length of 3 mm is first cut from a sample wafer, and the upper part is further cut to 50 μm or less as shown in FIG. As shown in B, aromatic gas or W (CO) ₆ is sprayed onto the central processed portion of the small block by a gas gun to form a protective deposition film. After that, as shown in FIG. 6C, FIB is irradiated to make a thin piece, and the processed sample is transmitted as an electron beam as shown in FIG. 6D and used as a cross-sectional sample for TEM observation. In this sample, the unprocessed portion of the small block also serves as the sample stage. In this method, a sample such as a wafer must be broken in order to cut out a small block, and many elements are wasted. In the latter processing method, a desired portion is cut out directly from the wafer by FIB processing with an FIB apparatus without machining. Therefore, this method has an advantage that only the target portion can be cut out without breaking a sample such as a wafer, so that elements are not wasted. First, a protective film is formed on the processed portion with a gas gun, and FIB is irradiated from above the surface of the sample. As can be seen from FIG. 7A, both sides of the observation cross section are scraped off by sputtering, and squares are formed on both sides of the observation cross section thin section. Make a hole. Further, FIB is irradiated to thin the observation part. After the thinning process is performed, as shown in FIGS. 7B and 7C, the periphery of the sample is cut off, the microsection is gripped by a probe operated by a manipulator, and is carried to a fixing base such as an organic thin film, as shown in D Secure to. However, there is a demerit that it is difficult to find out once the sample section is lost, and the work must be started again.

In the latter method, without slicing before cutting, the both sides and bottom of the sample piece are separated in a thickness of 0.1 to 1 μm, and a thick sample section is sampled with a probe operated by a manipulator. A method has also been proposed in which the sample is carried on a fixed base and firmly fixed, and after making the sample easy to handle, finishing processing such as thinning of the observation part is performed. (Non-patent document 2) This is an improvement of the demerit while ensuring the latter merit.
M, W, Xuet.a1, “Three-dimensional carrier profiling of InP-based devices using scanning spreading resistsnce microscopy” Appl. Phys. Lett, 81 (1) P177 (2002) Daisuke Sakata, “FIB lift-out method for additional machining”, Abstracts of the 58th Annual Meeting of the Electron Microscopy Society of Japan, Vol.37, P247 (2002)

  As described above, due to miniaturization of semiconductor devices and the like, the size of the observation part of the microscope sample has been reduced. Therefore, a high-precision processing means is required to produce the observation sample. Often FIB is used. However, when this FIB is irradiated to the sample, ion elements such as gallium are implanted and remain in the sample, and the element arrangement (crystallinity) on the sample surface is disturbed by collision with the implanted high energy ions, and ions are This is accompanied by a phenomenon that a damaged layer is formed. In FIB irradiation using gallium, the depth of the ion damage layer in the sample increases as the acceleration voltage increases. The graph of FIG. 5 shows the relationship, but it is about 20 nm when the acceleration voltage is 5 kV, but it is contaminated with gallium element up to about 60 nm when the acceleration voltage is 30 kV. When other elements are mixed in the sample or the crystallinity of the sample element is disturbed, it is no other than changing the original composition and physical properties of the sample, which causes a serious problem in sample preparation.

  The problem to be solved by the present invention is to solve the problem of ions that are implanted by ion irradiation and remain in the vicinity of the sample surface when an observation sample is manufactured using FIB, and the original sample is not affected by the problem. The object is to present a method for preparing a microscope sample that can be observed.

  In the present invention, after the surface of the sample for microscopic observation cut out from the substrate sample is processed by FIB, the vicinity of the surface of the sample is polished using a probe microscope probe to remove the ion element implanted by ion irradiation. The processing to perform.

  Polishing the sample surface with a probe microscope is a method of cutting mechanically by pressing a firm and sharpened probe like a diamond probe in a pressed state, in a solution or gas atmosphere using a conductive probe In this method, a voltage is applied between the probe and the sample surface to oxidize the surface by an electrochemical reaction, and then the oxide layer is removed by chemical etching. Light is emitted using a sharpened light guide probe. A method of removing the oxide layer by chemical etching after scanning the surface while irradiating, and a surface of the surface by annealing or ablation by scanning with laser light using a sharpened light guide probe. A method of removing the contaminated layer can be employed.

  In the method for preparing a microscope sample according to the present invention, the surface of a sample for microscope observation cut out from a substrate sample is processed by FIB, and then the vicinity of the surface of the sample is polished using a probe of a probe microscope that is extremely sharpened. Therefore, the contaminated layer into which ions are implanted can be locally removed.

  The microscope sample preparation method of the present invention, in which polishing performed using a probe microscope probe scans and mechanically cuts a firm and sharpened probe like a diamond probe, is compared The contaminated layer can be reliably removed with a simple operation.

  After conducting polishing using a probe microscope probe, the surface is oxidized by an electrochemical reaction by applying a voltage between the probe and the sample surface in a solution or in a gas atmosphere using a conductive probe, The microscope sample preparation method of the present invention, which removes the oxide layer by chemical etching, can carry out a smooth surface treatment as compared with mechanical polishing. This method takes two steps, but can be executed as a series of operations in the same apparatus by introducing an etching solution or etching gas during the chemical etching process.

  In addition, a photoconductor probe sharpened with a probe microscope probe is used to scan the surface while irradiating light to oxidize the surface, and then remove the oxide layer by chemical etching. The microscope sample preparation method of the present invention and the polishing performed using a probe microscope probe are performed by using a sharpened light guide probe, scanning while irradiating a high-energy laser beam, and annealing or ablation to contaminate the surface. The microscope sample preparation method of the present invention for removing the layer can perform a smooth surface treatment as compared with mechanical polishing.

  The structure of the microscope sample presented by the present invention is a structure in which a reinforcing rim having a thickness is connected to the side of the sample having an ultrathin plate structure and the two are connected by a slope structure. Therefore, when the probe of the probe microscope is brought into contact with the sample surface during the contamination layer removal operation or when observing the sample, the probe is first brought into contact with the strong rim, and then the slope is slid and observed. By moving to the area, the observation area is not scratched or damaged.

  As described above, when fine processing by FIB processing is performed during processing of a microscope sample, an ionic element is shot into the sample. Due to the contamination of the foreign matter, the accuracy of the sample is lowered, and in some cases it becomes a fatal defect. Therefore, the present invention has been conceived to polish and remove the surface layer of several tens of nanometers contaminated with the foreign matter. As a polishing method, there is a method called FIB or FIB gas assist etching with a reduced acceleration voltage, but it goes without saying that these methods using FIB cannot be adopted. Therefore, the present invention has been conceived to apply the technique of a scanning probe microscope as a technique for polishing a limited narrow region and a surface layer of several tens of nanometers.

  The simplest technique is to mechanically polish the surface of the observation sample using a probe having high mechanical strength such as diamond. The specified region is scanned under the condition that the probe of the probe microscope is pressed against the sample surface with a predetermined pressure. For example, it is a technique in which the contaminated surface layer is scraped off by scanning the surface like a raster. FIG. 1 shows a conceptual diagram of the present invention. A scanning dielectric microscope (SNDM), one of the probe microscopes, is an inspection method suitable for observing the distribution of electrical characteristics in a sample. If it remains, correct characteristics of the original sample cannot be obtained. Therefore, the sample preparation method according to the present invention is applied. As shown in A, the sample is first processed by the conventional method. In the example shown in the drawing, a sample section is cut out by the FIB method shown in FIG. 7 and the part to be observed such as a semiconductor wafer is picked up by a manipulator probe and fixed to an appropriate sample stage. In the vicinity of the observed sample surface, the irradiated ion element is implanted and contaminated. The contaminated layer is removed by polishing. First, as shown in B, the first method is to scan the observation region with the contaminated observation surface as a table and with the probe of the diamond chip pressed. . The surface is scraped off by a hardened sharp diamond tip. When an observation sample from which the contaminated layer has been removed is used and observation by SNDM is performed, the electrical characteristics of the original sample without the influence of ionic elements can be observed.

  Although the technique of processing the sample surface using a diamond needle has been introduced in Non-Patent Document 1, this technique performs dopant profiling observation of a sample with a pressing resistance microscope (SSRM) using a diamond needle. After that, the diamond needle is used to etch the sample surface thinly, and the exposed layer is observed again. This observation and etching operation is repeated to perform three-dimensional dopant profiling with information in the depth direction, which is a concept different from the present invention in recognition of the problem.

  Next, the second method will be described. In this method, a conductive probe is used for the probe microscope, and the sample surface is electrochemically oxidized (anodized) by applying a voltage between the probe and the sample, and then the oxide film is selectively etched and removed. That's it. Etching of the oxide film mainly uses chemical etching using a solution or gas. A specimen section cut out by a conventional FIB technique as shown in FIG. 1A and picked up by a probe of a manipulator and fixed on an appropriate specimen stage is observed by an electrochemical reaction as shown in FIG. Oxidizes the surface of the region. That is, the sample section is placed in a gas containing oxygen or a processing solution, and the surface of the observation region is scanned, for example, in a raster shape while a voltage is applied between the sample and the conductive probe. Since an electrochemical reaction occurs between the sample surface facing the probe tip and the portion is oxidized, the surface of the scanned region is oxidized. Next, the sample section is immersed in an etching solution, or the layer oxidized in the etching gas is chemically dissolved to remove the contaminated layer. Incidentally, when the sample is a silicon device, a halogen-based etching material is generally used. When an observation sample from which the contaminated layer has been removed is used and observation by SNDM is performed, the electrical characteristics of the original sample without the influence of ionic elements can be observed.

  The third method uses an optical probe of a scanning near-field optical microscope (SNOM) as a probe microscope. This method is a method in which a probe formed of a photoconductive material with a sharpened tip is irradiated with light such as laser to a local region of the sample surface, the surface is oxidized, and then the oxide layer is etched and removed. The high energy laser irradiation causes the phenomenon of annealing and ablation, and the contaminants are lifted or volatilized and removed. The configuration of this probe is as shown in FIG. 2. In this example, the tip of the probe is a minute opening, and light such as a laser beam is transmitted from the light source through the optical fiber to the light propagation part inside the probe. Will be sent. Then, light is spot-irradiated on the sample surface from the minute aperture.

  In the surface oxidation method using this optical probe, oxidation treatment is generally performed by irradiating an ultraviolet laser in an oxygen atmosphere. After the vicinity of the sample surface contaminated with the ionic element is oxidized, the oxide layer is removed by chemical etching using a dilute hydrogen fluoride HF solution. When an observation sample from which the contaminated layer has been removed is used and observation by SNDM is performed, the electrical characteristics of the original sample without the influence of ionic elements can be observed.

  When annealing is performed using this optical probe, a high energy laser is used and the vicinity of the surface is heated by the laser. As a result, the crystal bonding state of the sample is loosened, and gallium, which is an implanted impurity, is deposited. Thereafter, the crystal structure is recovered in the process of cooling and recrystallization. The deposited gallium is cleaned with a chelating agent or the like and removed. In addition, since this annealing also eliminates a trace amount of components contained in the original sample and may change the dopant distribution of the sample itself, a pulse laser or the like may be used for a short time if the influence is considered. It is necessary to do in.

  An example in which a sample for observing a dopant profile at a P / N junction in an element by SNDM is prepared by an electrochemical technique which is the second technique of the present invention will be described with reference to FIG. As shown in FIG. A, an observation target portion of the silicon-based device wafer is specified, and holes are formed by FIB sputter etching before and after that portion. The machining at this time is so-called rough machining, and the acceleration voltage of the FIB is set as high as about 30 kV. Therefore, ions are implanted deeply into the sample. The observation portion is further processed to a required thickness. In the processing, intermediate processing and finishing processing are performed while reducing the FIB beam current so that the sample surface is smoothed and finished. Although the surface is smoothed, an ionic element is implanted in the sample. Since this sample is a sample for SNDM, it is not necessary to make it thin like a TEM sample, and the processing thickness may be about 1 to several μm. Both sides and bottom of the sample processed into a thin plate shape are cut by FIB sputtering, the manipulator is operated as shown in FIG. B, the sample piece 1 is picked up by the probe 5, and carried to the sample fixing base 2 and fixed. This sample is set in the processing cell 6 in the probe microscope. Then, as shown in C, a voltage source is connected between a conductive probe such as tungsten and the sample, and a voltage of about 10 V is applied. In this state, an electrochemical reaction occurs between the conductive probe and the sample facing the tip of the probe, and the sample is anodized. In this state of application of voltage, the probe tip is scanned over the entire observation region, so that the vicinity of the surface of the observation region is oxidized. In this state, diluted hydrogen fluoride aqueous solution as an etching solution is put into the processing cell 6. Then, the oxide film is dissolved by the action of the etching solution, and is removed from the sample piece to expose the original sample surface that is not contaminated by the ionic element. By scanning and observing the surface of the sample prepared through the above steps with an SNDM probe, the electrical characteristics of the original sample without the influence of the ionic element can be observed. Since this sample is used for observing a dopant profile at the P / N junction, a trace amount component for the semiconductor to be P-type and N-type is targeted. For this reason, it is extremely inconvenient for gallium and the like to be mixed especially by FIB irradiation, but this problem can be solved by the method of the present invention.

  Microscopic sample preparation by this method can be performed in two steps by exchanging the etching solution in the processing cell 6 after the oxidation process is completed, so it is necessary to carry out continuous in-situ processing within the same electrochemical probe microscope. I can do it. In this embodiment, the oxide film is removed by chemical etching. However, when anodization is performed using a tungsten probe harder than silicon, the observation region is scanned in a constant tunnel current mode thereafter. As in the first method, it is possible to adopt a modified form in which the oxide layer is scraped off.

  Next, an example of producing a TEM sample by the first method of the present invention will be described with reference to FIG. First, the sample piece specifies the observation target portion of the wafer, and a hole is formed before and after that portion by FIB sputter etching, as in the previous example. The thickness of the observation cross section is processed to about 10 μm, both sides and bottom of the sample processed into a plate shape are cut by FIB sputtering, the manipulator is operated to pick up the sample piece 1 with the probe 5, and A As shown in FIG. Next, the sample fixing table 2 is placed on the sample table of the FIB apparatus, and the thinning process is performed again to process to the submicron order necessary for the TEM sample. Subsequently, the sample is transferred to a probe microscope, and as shown in B, the observation region is scanned with the probe of the diamond probe 3 pressed against the sample surface with a predetermined pressure, and the contaminated layer near the surface is scraped off. For a TEM sample that requires a thickness of about 0.1 μm, as shown in C, a thick reinforcing rim 1a is continuously provided on the side of the observation region during the thinning process, and the reinforcing rim is also provided. Processing is performed so that 1a and the observation region are connected by a slope 1b. This is to prevent the sample piece from being damaged or scratched by the impact when the probe probe 3 comes into contact with the sample piece 1 in the next processing of the contaminated layer with the probe microscope. At the time of contact, as shown in D, the probe probe 3 is made to come to the thick reinforcing rim 1a, and then moved to the observation region so as not to damage the sample. Such a sample structure not only ensures safety during polishing, but also ensures safe and reliable handling of the reinforced rim 1a by observing and moving as a TEM sample. it can.

  The technical meaning of polishing the sample surface in the present invention is to remove the damaged layer in which the ion element is implanted. How to detect the end of the polishing process will be described. Since the damaged layer into which the ion element is implanted is amorphized, if there is a difference in hardness from the crystal layer (original material of the sample), the friction force between the sample surface and the probe is measured with a processing probe. However, the end point of the processing can be determined when the processing is performed and the frictional force becomes the value of the original sample.

  If there is no difference in crystal layer hardness, repeat the processing / observation by providing a processing probe and an observation probe, and confirm the completion of the polishing process when the desired characteristic image is obtained. And processing can be stopped.

  From the above description, it is obvious that the microscope sample preparation method of the present invention can be industrially used as an effective technique for solving the problem of ion element injection that is inevitably caused by FIB irradiation. Regarding the structure of the sample, it is effective not only when polishing the probe microscope at the time of preparing the TEM sample as described above, but also an effective sample for a thin plate-like sample even for a probe microscope sample. The present invention can be applied as it is.

It is a figure which shows the technique method of grinding | polishing by the probe microscope employ | adopted in this invention. It is a figure explaining the probe used with an optical method. It is a figure explaining the Example which produces the sample for SNDM observation. It is a figure explaining the Example which produces the sample for TEM observation. It is a graph which shows the relationship between the acceleration voltage of FIB using gallium, and ion implantation depth. It is a figure explaining 1 method of producing a TEM sample from a semiconductor wafer. It is a figure explaining the other method of producing a TEM sample from a semiconductor wafer.

Explanation of symbols

1 Sample piece 2 Probe
1a Reinforcing rim 4 Probe
1b Slope 5 Manipulator probe 3 Sample holder 6 Processing cell

Claims (7)

A thinning process using FIB is performed on a sample for microscopic observation cut out from a substrate sample, and an ion element implanted by ion irradiation by performing a polishing process with a probe microscope probe near the surface of the thinned sample and In a method for preparing a microscope sample for removing an ion damage layer,
In the thinning process, a slope structure is formed between a microscope observation region of the sample and a reinforcing rim that reinforces the microscope observation region of the sample.   2. The method for preparing a microscope sample according to claim 1, wherein the polishing performed using the probe microscope probe is performed by scanning and mechanically cutting a firm and sharpened probe such as a diamond probe while being pressed. .   Polishing using a probe microscope probe uses a conductive probe, applies a voltage between the probe and the sample surface, oxidizes the surface by an electrochemical reaction, and then chemically etches the oxide layer. The method of preparing a microscope sample according to claim 1, wherein   Polishing using a probe microscope probe is performed by using a sharpened light guide probe, scanning while irradiating light to oxidize the surface, and then removing the oxide layer by chemical etching. Item 2. A method for preparing a microscope sample according to Item 1.   2. Polishing using a probe microscope probe is performed by using a sharpened light guide probe, scanning while irradiating a high-energy laser beam, and removing a contamination layer on the surface by annealing or ablation. A method for preparing a microscope sample as described in 1. above.   The method for preparing a microscope sample according to claim 2, wherein the end point of the processing is determined by performing polishing while measuring the frictional force with a processing probe.   3. The microscope sample preparation method according to claim 2, further comprising an observation probe in addition to the processing probe, and processing / observation is repeated, and the processing is stopped when a desired characteristic image is obtained.

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