WO2006042065A2 - Method to form microptips - Google Patents

Method to form microptips

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Publication number
WO2006042065A2
WO2006042065A2 PCT/US2005/036075 US2005036075W WO2006042065A2 WO 2006042065 A2 WO2006042065 A2 WO 2006042065A2 US 2005036075 W US2005036075 W US 2005036075W WO 2006042065 A2 WO2006042065 A2 WO 2006042065A2
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WO
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Patent type
Prior art keywords
specimen
microtip
study
posts
post
Prior art date
Application number
PCT/US2005/036075
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French (fr)
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WO2006042065A3 (en )
Inventor
Keith Joseph Thompson
Robert Matthew Ulfig
Scott Albert Wiener
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Imago Scientific Instruments Corporation
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01QSCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
    • G01Q70/00General aspects of SPM probes, their manufacture or their related instrumentation, insofar as they are not specially adapted to a single SPM technique covered by group G01Q60/00
    • G01Q70/16Probe manufacture

Abstract

A method of forming microtips is described. The method includes a microtip with a flattened upper surface, a ring structure (30) that has been deposited on layer (20). The ring structure can be on the perimeter of the flat top of the microtip or located at a point inward from the peripheral edge of the top. The method further includes forming one or more posts in a sample (10), each post having an elongated, acicular body terminating in a tip; and subjecting at least the body of the posts to an etchant, wherein the etchant removes material from the posts. .

Description

METHOD TO FORM MICROTIPS Keith Joseph Thompson Robert Matthew Ulfig Scott Albert Wiener

CROSS-REFERENCE TO RELATED APPLICATIONS Priority is hereby claimed to provisional application Serial No. 60/703,356, filed July 28, 2005, and to provisional application Serial No. 60/617,270, filed October 8, 2004, both of which are incorporated herein.

FIELD OF THE INVENTION

The invention is direct to a method of making of specimens for microscopic analysis. More specifically, the invention is directed to a method of making microtip specimens for analysis by devices such as atom probe microscopes, and to a method of making microtip probes for use in devices such as atomic force microscopes and scanning tunneling microscopes.

BACKGROUND In the manufacture of many modern devices containing microscopically thin layers of different materials or zones of different materials segregated on a microscopic scale, it is important to be able to study the different layers and zones with analytical equipment during and after fabrication of the device. The ability to do so greatly increases the value of quality assurance and quality control protocols. As examples, it is quite valuable to be able to analyze, on a microscopic scale, the structures of semiconductor microelectronic devices, magnetic thin film memory storage devices (such as read/write hard disk heads and platters), thin film-based optical devices, multilayered polymeric-, organic- and/or biochemical-based thin film devices (as used in medicine), composites of inorganic materials, organic materials and/or biological materials (such as biological microelectromechanical systems [bioMEMs], biosensors, bioarray chips, and integrated labs on chips), and other devices wherein nanoscale structures are critical to the functionality of the device.

Common equipment used for such analysis (hereinafter referred to as "microanalysis") includes electron microscopes (including Transmission Electron Microscopes [TEMs] and Scanning Electron Microscopes [SEMs]); spectrometers

(including Raman spectrometers and Auger spectrometers); photoelectron spectrometry (XPS); Secondary Ion Mass Spectrometry (SIMS); and more recently, the atom probe microscope, as described in U.S. Pat. Nos. 5,061,850 and 5,440, 124. Of course, other microanalysis equipment is readily available in the commercial marketplace, and new equipment having different principles of operation is expected to become available over time.

An atom probe microscrope is a device that allows a specimen to be analyzed at the atomic level. A typical atom probe device includes a specimen mount, an electrode, and a detector. A specimen is mounted on the specimen mount. The detector is then disposed a short distance from the tip of the probe. A positive baseline voltage is applied to the specimen. The electrode, which is located between the specimen and the detector, is either grounded or negatively charged. A net positive electrical pulse (either a positive pulse to the specimen, or a negative pulse to the electrode) and/ or a laser pulse is intermittently applied to the specimen. With each pulse, one or more atom(s) on the specimen surface is ionzed and subsequently detected by the detector. The identity of an ionized atom can be determined by measuring its time of flight from the surface of the specimen to the surface of the detector, a quantity that varies based on the mass-to-charge ratio of the ionized atom. The location of the ionized atom on the surface of the specimen can be determined by measuring the location of the atom's impact on the detector. Accordingly, as the specimen is evaporated, a three-dimensional map of the atomic structure of the specimen can be constructed.

Selected specimens taken from portions of the device are studied. Ideally, the specimen of the device under study is formed from the actual material that is intended to perform a function in the device. Accordingly, testing methods are known wherein study specimens are "biopsied" from the objects being studied, and are then subjected to microanalysis. As an example, Focused Ion Beam (FIB) milling processes are often used to excise study specimens from study objects. A good background discussion of FIB processes is set forth in U.S. Pat. No. 6,042,736, to Chung. The Chung patent is of interest for its discussion of a method of cutting a study specimen from a study object by FIB milling, with the study specimen then being removed by electrostatic attraction using micromanipulator. The study specimen is then subjected to TEM microanalysis. The remainder of the Chung patent is directed to a micromanipulator suitable for performing this operation.

U.S. Pat. No. 6, 188,068 to Shaapur et al. appears to describe a similar method, and the Background section of U.S. Pat. No. 5,270,552 also appears to describe similar methods for preparing study specimens using FDB milling and mechanical cutting/polishing steps. U.S. Pat. No. 6, 194,720 to Li et al. describes a method wherein a study object is milled by FIB and other processes to produce a thin cross-sectional study specimen suitable for microanalysis by a TEM. One aspect of the method described in the Li et al. patent involves milling a pair of parallel trenches in the top surface of the study object to define a plate-like first study region between the trenches (see FIGS. 2A-2C of Li et al.). The trenches are then filled in with a filler material (see FIG. 2D).

Portions of the study object are then cut away along planes parallel to the first study region and intersecting the filled trenches (FIG. 3B), or being spaced a short distance away from the filled trenches (FIG. 3C). As a result, the study object is formed into a plate-like shape wherein the first study region defines an area of decreased thickness. The plate-like study object is then milled into a wedge-like form (FIGS. 4A and 4B of

Li et al.) wherein the thinner side(s) of the study object define a second study region. The first and second study regions thereby define thin plate-like areas on the study object wherein the various deposited layers of the study object are displayed. A somewhat similar arrangement is described in U.S. Pat. No. 5,656,811, which is more directly devoted to methods of controlling the FIB milling process.

U.S. Pat. No. 5,270,552 describes a process wherein a study specimen is partially severed from a study object using FEB milling (with the study specimen remaining attached to the study object by a thin bridge of material). A probe is then connected to the partially-disconnected study specimen (as by "soldering" it to the specimen via FIB deposition). The study specimen is then fully removed by cutting away the bridge with FIB milling so that the probe may carry the study specimen to a desired location for study. By using an electrically conductive probe, the voltage between the probe, the study specimen, and the bridge provides a measure of whether the study specimen is intact. The probe may also serve as a support structure for further preparation of the study specimen, or for use during microanalysis of the study specimen. Use of the process to obtain multiple study specimens from points spaced about a semiconductor wafer is illustrated. The '552 patent also discusses using the underlying process steps to separate elements from one chip, to transport the elements to another chip by use of the probe, and then to sever the probe and to solder the elements to the second chip by use of FIB deposition.

Other patents note that study specimens can be formed from a study object by use of material removal processes other than FIB processing. U. S. Pat. No. 6, 140,652 to Shlepr et al. describes forming study specimens from a study object for

TEM microanalysis using photolithography and chemical etching processes. Trenches are etched in the study object to form a circular plug-like study specimen, which then has its base cut free from the study object by further chemical etching techniques. The study specimen can then be microanalyzed using TEM techniques. Regarding specimen probes fabricated specifically for atom probe analysis, see

U.S. Patent No. 6,700, 121. In the '121 Patent, a study specimen is formed in a larger study object of interest, such as a semiconductor wafer. Once the area of interest on the study object is identified, a study specimen is formed in the study object in the chosen area. This is accomplished using known cutting techniques such as FIB milling, etching, and so forth. By way of example, a study specimen is formed in a surface of a study object by using FIB milling to form two adjacent parallel trenches in the study object surface. The two trenches are then joined at their ends so that the study specimen is defined by a freestanding cantilevered wall. A portion of the base of the wall is then removed so that the study specimen is connected to the study object by a thin tether. See Fig. 1 of the '121 Patent.

In many instances, destructive testing (as in the foregoing methods) is undesirable because it renders the study object inoperable. Thus, in some cases a "proxy" or "qualifier" study object is used. A proxy or qualifier is an object which is not the true study object of interest, but which has been subjected to the same processes as the true object of interest. In this fashion, the proxy serves as a reasonable representation of the bona fide study object generated by these processes. As an example, in the field of semiconductors, many thin film deposition systems are designed to deposit layers over an area greater than the size of a typical semiconductor wafer. Qualifier wafers are often processed alongside actual wafers so that the qualifiers receive the same deposited layers as the production wafer. The qualifier wafer is then destructively tested in place of the actual wafer. However, testing of a qualifier wafer assumes that the qualifier wafer receives the same treatment as the actual wafer within the deposition system. This assumption is not always valid because the deposited coatings may vary in time or location within the deposition system.

One significant problem encountered with all known methods of microanalysis is the time and expense of subsequent testing. Often, individual study specimens (after having been obtained in accordance with the foregoing methods) must then be individually prepared for subsequent microanalysis. These preparatory manipuations can include steps such as polishing, mounting, applying protective layers or other coatings, situating the study specimen in a vacuum environment, and so on. Because of the disadvantages of destructive test methods, and because of the time and expense involved in the microanalysis of individual study specimens, there is a need for new methods of microanalysis which are minimally destructive, and which are better suited for rapid processing of multiple study specimens.

SUMMARY OF THE INVENTION In the present invention, tall specimens or posts (preferably greater than about

100 μm) are created on a sample, such as a flat silcon wafer, by a variety of means (such as dicing, anisotropic etching, etc.). (Throughout the remaining description, the term "sample" is used to designate the bulk material from which the "post" or "specimen" is formed.) These specimens are then sharpened by dipping, spraying, coating, or otherwise treating the sample with a liquid etchant (a gas or a fluid) such as a solution of nitric and hydrofluoric acid. In the preferred version of the invention, the etching process removes material in a substantially isotropic manner from all available surfaces. That is, the post is "eroded" at the same rate in all directions. Because this isotropic removal tends to attack the top surface of each post at substantially the same rate as it attacks the edges of the post, sharp tips are thereby formed on the posts. The sharpened tip of each post is located slightly below the original (pre-etching) surface of the sample.

The sharp tips formed in this fashion are substantially uniform across the sample (provided the posts have the same pre-etching geometry). The tips so formed are also remarkably uniform from sample-to-sample for any given etchant and etching procedure utilized. Where the microtips are generated for the purpose of atom probe analysis, the tips can be removed from the bulk material for individual mounting and analysis. Alternatively, the entire sample with its multiple sharpened tips can be affixed to a conductive mount to allow any number of tips from the same sample to be analyzed. The microtips can also be used as tips and/or probes for scanning probe microscopy and other microscopy techniques.

Throughout this document, it is understood that while microtips are (at the time of this writing) generally formed with diameters of from about 1 to about 10 μm and lengths of from about 50 to about 150 μm, their sizes are expected to vary with future advances in microanalysis methods. For example, such "microtips" may significantly grow in size as field ion microscopy data acquisition speeds increase, and/ or they may shrink as new methods and materials are developed in scanning probe microscopy. Thus, the term "microtip" as used herein refers to articles used as field ion microscopy specimens or as probes for a scanning probe microscope, and should not be construed as connoting a particular size.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a vertical cross-sectional view of a square pyramidal microtip fabricated according to the present invention.

Fig. 2 is a schematic representation of cross-sectional view of a tapered shank microtip fabricated according to the present invention.

Figs. 3A, 3B, 3C, and 3D are exemplary mask patterns that can be used in the present invention. Fig. 3A depicts a circular mask; Fig. 3B depicts a square mask; Fig. 3 C depicts a cruciform mask; Fig 3D depicts a grid mask.

Figs. 4A and 4B are schematic representations of vertical cross-sectional views of microtips according to the present invention having material deposited across the entire upper surface of each microtip. Fig. 4A depicts a square pyramidal microtip. Fig 4B depicts a square pyramidal microtip having a rounded top. Figs. 5A and 5B are schematic representations of vertical cross-sectional views of microtips according to the present invention having material deposited in selected areas on the tip of each microtip. Fig. 5A depicts a square pyramidal microtip having material deposited at the edges of the upper surface of the microtip. Fig 5B depicts a square pyramidal microtip having material deposited more toward the central area of the upper surface of the microtip.

Figs. 6 is schematic representation of a vertical cross-sectional view of a different version of microtips according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

FORMATION OF POSTS ON THE SAMPLE:

In the first step of the invention, posts are formed from an item to be analyzed (or a section thereof). The posts are preferably at least about 100 μm tall, and more preferably still on the order of about 200 μm or more, so as to define an elongated microtip after the subsequent etching step is performed. Posts shorter than 100 μm and taller than 200 μm are explicitly within the scope of the present invention. Because silicon wafers are commonly analyzed samples in atom probe microscopy, the remainder of this document will generally assume that a silicon wafer is the sample to be analyzed. This is for purposes of brevity alone. The invention may also be modified to prepare other (non-silicon) materials and samples for analysis.

One easy and inexpensive technique of forming posts in silicon is via cross-dicing, a well-known technique. Suitable dicing equipment and dicing services can be obtained commercially, e.g., Dicing Blade Technology (Longwood. Florida) and American Precision Dicing, Inc. (San Jose, California), respectively. In cross- dicing, the sample is placed flat on a dicing saw chuck. The cutting depth is then set to the desired dimension (e.g., preferably from about 100 μm to 500 μm) and a first series of parallel cuts are made in one direction. The spacing between the cuts is preferably set such that elevated lanes/plateaus about 10 to about 15 μm wide are formed in the sample. For example, if the thickness of the dicing blade is 75 μm, the cut spacing may be set to roughly 85 to 90 μm. The sample is then rotated (preferably 90 degrees) and a second series of parallel cuts (intersecting the first cuts) are made. If the sample is rotated exactly 90° between the first series of cuts and the second series of cuts, the result is a series of raised square posts approximately 10 to 15 μm wide, and as tall as the depth of the cuts (that is, from about 100 μm to 500 μm tall). The quality of the diced posts is improved by using a thicker blade (> 200 μm) and a small diamond grit (< 5 μm, preferably 1 μm). If the sample is rotated less than or greater than 90° between the first series of cuts and the second series of cuts, the result is a series of raised posts having a parallelogram-shaped cross-section.

Another more preferred technique for forming posts in silicon is via Deep Reactive Ion Etching (DRIE). DRIE is a known technique. DRIE equipment and DRIE services can be obtained commercially, e.g., Oxford Instruments (Concord,

Massachusetts) and Silex Microsystems AB (Jarfalla, Sweden), respectively. DRIE can generate posts of any desired cross-sectional geometry, (e.g., circular, oval, triangular, rectangular, etc.), and which are sized on the order of 5 μm in diameter (for a circular post) and > 150 μm tall. When using DRIE, the etch process is compromised if the posts are too far apart, whereas the photolithography pattern becomes more complex if the posts are too close together; thus, a preferred post spacing is from about 10 to about 15 μm, post-to-post. An exemplary DRIE post formation process is summarized as follows:

(1) The sample is coated with a light-sensitive photoresist (PR) mask and patterned with an array of circles having a diameter of from about 5 to about 10 μm.

Typical PR mask thicknesses are from about 1 to about 2.5 μm, depending on the etch depth. The etch selectivity of silicon-to-photoresist is roughly 75-to-l. Thus, to generate silicon posts approximately 150 μm tall, the sample would be coated with a photoresist mask approximately 2 μm thick. Other masking agents, such as silicon oxide, silicon nitride, gold, platinum, and the like, can also be used.

The mask can be of any desired geometry that results in one or more acicular (L e. , high aspect ratio) posts in the sample. Exemplary mask patterns are shown in Figs. 3A, 3B, 3C, and 3D. Fig. 3A depicts a circular mask that yields a single post having a circular cross-section. Fig. 3B depicts a square mask that yields a single post having a square cross-section. Fig. 3C depicts a cruciform mask that yields four posts having square cross-sections. Fig 3D depicts a grid mask that yields 25 posts, each having a square cross-section. In each drawing figure, the shaded portion 100 depicts the mask, while the unshaded portion 10 represents the surface of the sample that will be etched away in the DRIE process. (2) Alternatively, a SiO2 hard mask can be used to improve the masking protection of the underlying silicon. If it is not necessary to preserve the surface of the sample on each microtip for subsequent microanalysis, a thermal oxide mask can be grown on the surface by baking the sample at 900-1100 0C. When a near-surface region of interest is to be preserved, a coating such as a spin coating (e.g., spin-on glass) or a dip coating can be applied and cured at 300-400 0C. A thermal oxide mask/coating will provide a selectivity of approximately 150-to-l over the silicon, whereas a spin-on glass mask/coating will generally provide a selectivity of approximately 75-to-l over silicon. (3) The mask is then cleared by etching, e.g., by wet or plasma etching.

Any suitable etchant, now known or developed in the future, can be used. HF diluted in H2O, typically 6-to-l, is generally a suitable wet etch mask clearing agent. This etchant etches thermally grown SiO2 at a rate of approximately 65 nm/minute (and spin-on glass at a rate of approximately 125 nm/min). The selectivity of PR is infinite, but the PR coating is undercut at a rate similar to that of the vertical etch, i.e., about 65 nm/minute for thermal oxide and 125 nm/min for spin-on glass. The total undercut generally amounts to about 0.5 μm. A suitable plasma etch mask clearing agent is a CHF3 (or CF4) and O2 reactive ion etch (RIE) plasma, although any suitable plasma etch mask clearing agent may be used. Here, the selectivity between the PR and SiO2 is about 5-to-l, and undercutting is insignificant

(4) The actual DRIE process comprises an alternating etch and passivation sequence, known as Bosch processing (named after Robert Bosch GmbH, the German company that pioneered the process). Two different gas compositions are alternated in the reactor. The first gas composition creates a polymer on the surface of the substrate, and the second gas composition etches the substrate. The polymer is immediately sputtered away by the physical part of the etching, but only on the horizontal surfaces and not the sidewalls. Because the polymer only dissolves very slowly in the chemical part of the etching, the polymer builds up on the sidewalls of the etched surface and protects them from etching. As a result, etching aspect ratios of 50-to-l can be achieved. Thus, for example, the silicon sample can be etched with an SF6 plasma for about 5 to 12 seconds while the wafer is biased. The sample is then passivated with a C4F8 plasma for about 4 to 12 seconds with no chuck bias. The bias during the etch removes any polymer on the silicon surface, but the polymer remains on the sidewalls of the features. In this alternating fashion, the net etch proceeds at a rate of about 2.5 μm/minute with excellent anisotropy and selectivity over both PR and SiO2. The etch rate will vary if the polymerization and etch cycles are varied. More polymerization leads to a lower etch rate, and vice-versa.

(5) After the etching process is complete, the PR and SiO2 are stripped from the sample.

The foregoing process works well for creating dense arrays of posts. In an alternative version of the method, spaced/isolated arrays of posts are fashioned from this dense array by adding an etch step. Such spaced arrays, where each array corresponds to a particular region of interest on the sample, can enhance the ease of microanalysis. For example, arrays of posts might be left only at gate areas on a chip, with surrounding "blank" areas on the chip being post-free. A preferred method of forming such spaced arrays via the foregoing DRIE process is to form ordered arrays of both smaller-sized posts (as described in paragraphs 1-4 above) and posts having a relatively larger cross-section (e.g., from about 8 to about 16 μm in diameter) in an initial processing step. The sample is then exposed to an isotropic etching step, for example a plasma etch using SF6 or NF3, or a wet etch. (The same type of isotropic etching may also be used to "sharpen" the posts as discussed below). The isotropic etching is timed so that the smaller posts are completely eliminated, while at the same time the diameter of the larger posts is reduced. The end result is an array of posts separated by regions that are completely devoid of any posts. In short, the big posts

(initially about 8 to 16 μm in diameter) have been reduced to small posts (about 4 to 8 μm in diameter), and open regions are created where the small posts have been etched away entirely. In this fashion, dense arrays of relatively tall posts (> 100 μm), of relatively small diameter (< 8 μm) can be created in a predictable fashion. Alternatively, the smaller posts can be removed by processes other than etching. They can be knocked down with a micromanipulator, diced away, or otherwise mechanically removed. The sample may be oxidized until all of the small posts are oxidized to SiO2, followed by an HF strip to remove the small posts. The small posts are used solely to fill in the structure of the dense array so that the etching steps of the DRIE process proceed in a well-defined and repeatable manner. After these steps, the small posts are removed, leaving the desired isolated collections of posts. If the aforementioned dicing process is used to form posts rather than the DRIE process, the dicing saw can be used to eliminate (/. e. , dice out) any regions where posts are not desired.

Other methods to form posts can also be used in addition to the (preferred) DRIE method and the cross-dicing method. Some examples include:

(1) Abrasion: As an example, diamond-coated tubes can be ultrasonically vibrated to create a core from the substrate. (2) Electrical-Discharge Manufacturing (EDM): EDM can be used to selectively erode portions of the substrate.

(3) LASER: A laser can be used to remove portions of the sample selectively. Selective laser annealing may improve sample preparation and increase specimen survival rates. (4) Focused Ion Beam milling (FIB, as discussed earlier).

(5) Wet or other chemical etching.

(6) Plasma etching.

CHEMICAL ETCHING OF POSTS: An etching fluid (preferably a liquid, but gaseous etchants can also be used) is then used to shape (sharpen) the posts as desired, with such shaping taking place via oxidation of the posts coupled with stripping of the oxide. Any etchant suitable for the workpiece being manipulated may be used, without limitation of any sort. A host of suitable etchants are known in the art and can be obtained from commercial sources such as Chesetech Ltd. (Warwickshire, England) and Transene Company, Inc. (Danvers, Massachusetts). Explicitly included within the term "etchant" are the strong acids and bases commonly used as etchants, including hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid, acetic acid, potassium hydroxide, sulfur hexafluoride, tetramethylammonium hydroxide, ammonium fluoride, ammonium hydroxide, hydrogen peroxide, orthophosphoric acid, and the like. Suitable combinations of etchants are also encompassed within the term "etchant." The term also explicitly includes isotropic etchants and anisotropic etchants. Isotropic etchants are preferred. As a first example, a liquid "HNA" mixture - HF:HNO3:H2O (hydrofluoric acid, nitric acid, and water) - can be used as a wet etching solution for a silicon sample. (Other chemicals such as acetic acid could be used instead of water if desired.) Using this etchant, the nitric acid will oxidize the silicon to form SiO2, and the HF will then strip the newly formed SiO2. This reaction attacks the weakest bonds first, thereby better ensuring that the remaining silicon is strongly bonded together.

The maximum etch rate (generally about 500 μm/minute) occurs when the oxidation rate exactly equals the oxide stripping rate. When the HF concentration significantly exceeds the HNO3 concentration, the oxide stripping rate exceeds the oxide growth rate (a regime which is useful for rapid, bulk material removal). When the HNO3 concentration exceeds the HF concentration, the oxide growth rate exceeds the oxide stripping rate, and a consistent oxide layer (of about 3 to 5 nm thickness) will remain on the surface of each post throughout the etch process. Because it takes longer for each post to oxidize under these circumstances (that is, it takes longer for the oxygen radicals to diffuse through the thin oxide layer), the oxidation step self-limits the silicon removal step and the etch proceeds at a slower overall rate. This slower rate, however, is useful for "fine polishing" (i.e., controlled shaping) of the posts. A ratio of l-to-2, HF-to-HNO3 yields an etch rate of about 25 μm/minute. Diluting this 1-to- 2 solution with 50% H2O drops the etch rate to about 2.5μm/minute. At 75% H2O, the etch rate further drops to about 1 μm/minute). The etchant is preferably applied to the posts simply by dipping the posts into the etchant solution until an atomically sharp tip is formed. A high etch rate is preferably utilized when the post diameter is above about 10 μm. Once the tip diameter is between about 5 to about 10 μm, the etch solution is preferably diluted to an etch rate of about 1 to 2 μm/minute for more controlled etching. The isotropic nature of the etching process assures that an atomically sharp tip is formed: the chemical etchants attack the top and the sides of each post simultaneously, and by nature the tip (attacked from all the sides and from the top) will etch more rapidly than the bulk of the post (which is attacked only from the sides). In some cases, the surface of the specimen (the surface of each post) may be of interest for analysis, in which case it may be protected from etching by applying a protective mask or cap (preferably prior to forming the posts). If the surface is protected, the post will not be sharpened, and instead the entire body of the post will be eroded at the same rate. As an example, a silicon nitride (Si3N4) masking layer can be used. Note that this material is chosen for its 5-to-l selectivity over silicon during the etching process with the aforementioned HNA solution. Other masking materials are dissolved or delaminated during the etching process.

If other etchants are used, other masking materials may be utilized. To illustrate, the following process has been found suitable to preserve the surface of the specimen when forming microtips therein. First, a layer of Si3N4 (about 500 run thick) is deposited on the silicon specimen. Posts are then formed using a dicing saw or via DRIE as described earlier. A Focused Ion Beam (FIB) device is then used to mill the post until a tip of 1 μm radius remains. (This is done to create the optimal tip shape; a smaller tip radius is usually desirable for microanalysis.) The specimen is then etched/polished in the HNA solution until the tip radius is about 100 nm. Finally, the remaining Si3N4 is stripped in concentrated HF. Because the etching process is isotropic, the original (FIB milled) tip shape will be maintained throughout the process. The result is a chemically polished/sharpened microtip. As shown in Figs. 1 and 2, the sharpened microtip preferably has a tip whose width is less than about 150 nm. The resulting tips are thus acicular (needle-shaped). The microtips preferably have an aspect ratio (the ratio of the height of the microtip to the width at the peak of the microtip) of at least about 500, more preferably at least about 1,000, and more preferably still at least about 2,000. Microtips having aspect ratios smaller than 500 and greater than 2,000 are included within the scope of the invention. By way of example, a microtip having a height of 200 μm and a peak width of 100 nm has an aspect ratio of 2,000 (i.e., 200 x 10"6m ÷ 100 x 10"9 m = 2,000). A microtip having a height of 100 μm and a peak width of 150 nm has an aspect ratio of 666.67.

Other masking methods include (but are not limited to) utilizing transmission electron microscope (TEM) grids. Masks can be fixed by a variety of techniques, including thermally annealing protective layers over substrates, electrostatically placing protective masking material over the substrate, and the like. While a liquid etchant is used in the foregoing example, a gaseous etchant may be used instead. As an example, XeF2 crystals actively decompose into atomic fluorine when subjected to pressures below 3 Torr. The atomic fluorine actively and isotropically etches silicon, but does not etch basic masking materials (SiO2, Al, photoresist, etc.). Thus, an XeF2 etch acts much like the liquid etch discussed above. One method of using XeF2 is first to mask the sample with one or more materials such as SiO2, Ni, Al, photoresist, etc. Posts are then be formed using DRIE, a dicing saw, or other methods. If needed or desired, the posts can then be FIB milled to a desired tip radius (preferably about 1 μm). The specimen is then be exposed to fluorine generated from the decomposition Of XeF2 to etch the post(s). Other fluorine sources such as SF6, NF3, plasmas, etc. may be used, but XeF2 is the preferred source of gaseous fluorine. Beneficially, the specimen can be exposed to the etching fluorine in situ in the FIB chamber. The masking layer can then be stripped to leave the ready- to-be-analyzed microtip. The chemical etching of the various posts can be varied in regions, or even omitted from one or more posts, so that certain unsharpened posts and/or sharpened microtips can be used as fiducials. That is, the present invention can be used to create one or more fiducials within an array of otherwise homogeneous microtips. A fiducial is a reference point, a marker to which other objects (e.g. other microtips) on the array can be related in space. Thus, the present invention can be used to make an array of mircotips having at least one tip that functions as a fiducial, that is, as a reference point for creating an x,y grid (a Cartesian coordinate system) that reflects the positioning of the other microtips (or other objects) in the array. In short, by being able to designate a particular mircotip or post as a fiducial, an addressed array can be fabricated.

Additionally, in certain some microanlysis protocols, an extremely sharp tipped probe is not ideal. For example, where the material to be analyzed is deposited onto the microtip, the material may deposit preferentially on the shank of the microtip, rather than uniformly across top end of the tip. In this circumstance, the non-uniform deposition of the material to be analyzed yields a microtip whose cross-sectional composition does not reflect the typical morphology of the material when deposited on a planar surface.

To address this problem, the present invention can also be used to fabricate acicular microtips having a flat, crystalline, mesa-like top, as shown in Figs. 1 and 2.

In Fig. 1, the microtip was fabricated using an anisotropic etchant, thus yielding a microtip whose vertical cross-section, as shown in the figure, has planar side-walls. In Fig. 2, the microtip was fabricated using an isotropic etchant, thus yield a microtip whose vertical cross-section has smoothly curved, tapering side walls. In each of Figs. 1 and 2, the microtip has a flat, planar, plateau-like upper surface. The width

(or diameter in the case of a circular microtip) of this upper surface is generally less than about 150 nm.

The flat top of the microtip can be created in a number of different ways, and the material to be analyzed may be incorporated into the sample prior to creating the microtip or after creating the microtip. Specifically, the flat-top microtip can be created by using a judicious choice of mask geometry, etchants and mask-strippers, to yield a microtip whose upper surface retains (or is etched to form) a planar, crystalline surface. Alternatively, the microtips may be fashioned into atomically sharp points, as discussed previously, and then milled, via FIB milling or any other suitable method, to flatten the sharpened point of each tip.

The flat-topped microtips are also useful where material for analysis is to be deposited upon the microtip. It is simply easier to affix the material to the flattened top as compared to an atomically sharpened tip. Material can be glued to the microtip (e.g. , using silver epoxy) or "welded" onto the tip using FDB, more specifically via platinum deposition in the focused ion beam.

Figs. 4A and 4B, for example, depict vertical cross-sectional schematics of a flat-tip microtip 10 according to the present invention having disposed thereon two layers of material (20 and 30 in Fig. 4A; 20' and 20' in Fig. 4B). In the LEAP -brand method of atom probe spectroscopy (LEAP is a registered trademark of Imago

Scientific Instruments Corporation, Madison, Wisconsin), after an initial portion of the specimen has been ionized from the surface, the specimen generally has a rounded tip geometry, as depicted in Fig. 4B. For purposes of data reconstruction in the LEAP process, a rounded tip geometry of the specimen probe is often assumed. If the specimen has a flat-top geometry, as shown in Fig. 4A, the sharp edges of the top may yield data that is not reflective of the 3-dimensional structure of the specimen itself (because the atoms are not ionized in the same fashion from the edges of the planar top, as compared to its surface). Thus, to arrive at a more suitable rounded tip geometry, the layer 30 as shown in Fig. 4A can be a sacrificial layer that is deposited upon the true specimen 20 that is to be microanalyzed. An initial ionization step then removes the sharp edges of the sacrificial layer to yield the rounded tip geometry 30' as shown in Fig. 4B. The LEAP analysis would then proceed in conventional fashion. By ionizing the sacrificial layer 30 as shown in Fig. 4A to yield a preferred rounded tip as shown in Fig. 4B, the LEAP analysis data subsequently gathered is generally of higher quality. Note also that layer 30 need not be a sacrificial layer, but could be simply another layer of the specimen to be analyzed. The formation of the rounded tip as shown in Fig. 4B is simply a means to optimize the data gathered in the LEAP analysis. The LEAP analysis may also proceed directly from the flat-topped tip depicted in Fig. 4A, although this route is not the preferred embodiment. For the LEAP analysis, a probe having a rounded apex is preferred because it provides a well-defined local field maximum from which the ionization of the tip starts. With a flattened microtip, ionization may start from one of the edges or corners, which is not ideal. Thus, when using a flat-topped microtip, the preferred embodiment for LEAP analysis is to deposit a material to be analyzed onto the flat surface of the probe, and then to sharpen the resulting composite microtip (using an etchant or via FIB) to yield a sharpened microtip as is illustrated in Fig. 4B. The sharpened composite microtip is then microanalyzed. A microtip with a flattened upper surface also provides an opportunity to fabricate more complex layers on the upper surface of the microtip, as is depicted in Figs. 5A and 5B (both of which are vertical cross-sectional schematics). For example, in Figs. 5A and 5B a ring structure 30 has been deposited on layer 20. The ring structure 30 can be on the perimeter of the flat top of the microtip (as shown in Fig. 5A) or located at a point inward from the peripheral edge of the top, as shown in Fig.

5B. With this ring structure in place, the center of the ring (in the case of Fig. 5A) or the center and the portions outside the ring (in the case of Fig. 5B) may be filled in with other materials, via any means, such as chemical vapor deposition, magnetron sputtering, ion implantation, and the like. Microanalysis of the specimen can then proceed in any fashion desired. The ring structure described and shown in Figs. 5 A and 5B is for exemplary purposes only. Any desired pattern can be deposited or otherwise disposed on the top surface of the microtip. Alternatively, microtips can be formed via a three-step process comprising a first step of depositing a mask on the sample and etching or ion milling the sample to yield tip 30 (see Fig. 6) of from about 0.5 to 5 μm in diameter and from about 1 to 10 μm in height, followed by a mechanical dicing step to yield an underlying post 20 at least about 50 μm tall, and more preferably still on the order of about 100 μm. The mask is removed (via any suitable means, preferably via chemical etching or ion milling, either before or after the dicing step) and the post is sharpened using a chemical etchant or via FIB, as noted in the immediately prior section.

USE OF THE MICROTIPS:

After etching and rinsing, the posts are ready for analysis by atom probing or other analysis methods. The present invention also allows for easy and inexpensive manufacture of microtips for scanning probe microscopy applications (e.g., atomic force microscopy, scanning tunneling microscopy, etc.). Additionally, the foregoing methods generate superior microtips for microanalysis methods using ionization (e.g., atom probe analysis, field ion analysis, field emission analysis) because the tips are atomically sharp and highly aligned about their central axis.

The microtip may itself be the primary object of interest for microanalysis, for example, where the microtip is formed from a gate region of a chip (an example noted earlier).

Alternatively, the microtips may not themselves be of significant interest for microanalysis. Rather, the microtips may be used as carriers or substrates for other matter to be microanalyzed. As an example, where it is desirable to determine the results of a thin film deposition process, one or more layers of other materials (e.g. metals, ceramics, polymers, etc.) can be deposited on a microtip made according to the present invention. The deposited layers can then be microanalyzed by a selected field ion microanalysis method (e.g., atom probe analysis, field ion analysis, field emission analysis) without particular attention being paid to the structure of the microtip itself.

As previously noted, the method may be used to generate microtips on materials other than silicon. As examples, microtips can be formed on germanium (Ge), silicon germanium (SiGe), silicon carbide (SiC), carbon, and metallic samples

(provided, of course, that appropriate etchants and masks are selected for the material/sample in question). As with the HNA etchant used for silicon, appropriate etchants for other materials may be formed from a mixture of one or more oxide- forming agents, one or more oxide-stripping agents, and any desired neutral agents or "carriers. "

Different masks may be used to preserve areas on the specimen during etching, including oxide masks grown thermally on the specimen. Such thermal oxide masks are preferably cleared during formation of the microtips so that the thermal oxide layers are no longer present in the final product. However, it has been found that for some specimens/microtips, the retention (or regrowth) of a thermal oxide layer can enhance the conductivity of the microtips. This increased conductivity can greatly enhance the utility of the microtips in field ion microanalysis methods. For example, in an atom probe microscope, the electrical resistivity of a microtip is a key parameter affecting the speed and quality of data collection from the microtip. Microtips with lower resistivity generally provide a higher signal-to-noise ratio and higher mass resolution, thereby resulting in a better understanding of the identity and position of every atom collected from the microtip. With certain specimens - in particular, those formed of semiconductors with certain dopants (such as As, Sb, P, B, etc.) - the growth of a thermal oxide causes dopant to migrate from the growing oxide layer toward the interface between the oxide layer and the remainder of the specimen. This migration results in a higher concentration of dopant in the semiconductor shortly beneath the oxide layer (and higher conductivity in the highly-doped region). Thus, if a thermal oxide layer is grown on a microtip generated by the foregoing methods (e.g. , by baking), or if a thermal oxide layer is otherwise retained in a microtip during its formation (as by masking the oxide layer when protection is needed, and later stripping it), the microtip may yield superior performance during subsequent microanalysis. This is particularly true where the microtip is merely used as a substrate/carrier for some other matter to be microanalyzed, and such other matter is substantially nonconductive - for example, where a microtip is coated with a thin ceramic or polymeric film which is the subject of interest for microanalysis.

The description set out above is merely of exemplary preferred versions of the invention. It is contemplated that numerous additions and modifications can be made. These examples should not be construed as describing the only possible versions of the invention, and the true scope of the invention is defined by the claims contained herein.

Claims

CLAIMS What is claimed is:
1. A method of forming microtips comprising:
(a) forming at least one specimen in a sample, the specimen having an elongated, acicular body terminating in a tip; and
(b) subjecting at least the body of the specimen to an etchant, wherein the etchant removes material from the specimen, and wherein a microtip is formed on the tip of the specimen.
2. The method of claim 1 further comprising the step of microanalyzing the specimen after it is etched.
3. The method of claim 2 wherein microanalyzing the specimen includes subjecting the specimen to an electrical field sufficient to cause ionization from the microtip of the specimen.
4. The method of claim 2 wherein microanalyzing the specimen includes at least one of:
(a) atom probe microanalysis;
(b) field ion microanalysis; and
(c) field emission microanalysis.
5. The method of claim 1 further comprising the step of contacting the specimen with an object to be microanalyzed.
6. The method of claim 5 wherein the step of contacting the specimen with the object is performed in a scanning probe microscope. 7. The method of claim 1 wherein the step of forming at least one specimen in the sample includes at least one of:
(a) mechanically removing material from the sample about the specimen; and
(b) deep reactive ion etching.
8. The method of claim 1 wherein the etchant includes an oxide-forming agent and an oxide-stripping agent.
9. The method of claim 1 wherein the etchant is gaseous.
10. The method of claim 1 further comprising the step of applying a mask to the tip of the specimen prior to subjecting the body of the specimen to an etchant.
11. The method of claim 1 wherein a mask is applied to a surface of the sample prior to forming the post.
12. The method of claim 1 further comprising, after step (b), the steps of:
(a) oxidizing the post to yield an oxidized post; and then
(b) performing microanalysis on the oxidized post.
13. The method of claim 1 further comprising attaching additional material to the sample, either before or after forming the post in the sample.
14. A method of forming microtips comprising:
(a) forming at least one post in a sample by mechanically removing material from the sample about the post or by deep reactive ion etching, thereby yielding at least one post having an elongated, acicular body terminating in a tip; and then
(b) subjecting at least the body of the post to an etchant, wherein the etchant removes material from the post, and wherein a microtip is formed on the tip of the post.
15. The method of claim 14 further comprising, after step (b)
(c) microanalyzing the post.
16. The method of claim 15 wherein microanalyzing the post includes subjecting the post to an electrical field sufficient to cause ionization from the tip of the post.
17. The method of claim 15 wherein microanalyzing the post includes at least one of:
(a) atom probe microanalysis;
(b) field ion microanalysis; and
(c) field emission microanalysis.
18. The method of claim 14, further comprising attaching additional material to the sample, either before or after forming the post in the sample.
19. The method of claim 18 further comprising, after step (b) (c) microanalyzing the post. (a) forming at least one post in a sample by mechanically or chemically removing material from the sample about the post or by deep reactive ion etching, thereby yielding at least one post having an elongated, acicular body terminating in a tip; and
(b) attaching additional material to the tip of the post, either before or after forming the post in the sample; and then
(c) subjecting at least the body of the post to an etchant, wherein the etchant removes material from the post and wherein a microtip comprising the additional material from step (b) is formed.
21. A method of forming microtips comprising:
(a) depositing a mask on a sample;
(b) mechanically, chemically, or ionically removing material from the sample, thereby yielding an initial post having an elongated, acicular body terminating in a tip; and then
(c) dicing the sample in registration with the initial post to yield an underlying post.
22. The method of claim 21, further comprising, after step ®);
(d) sharpening the initial post with a focused ion beam wherein a microtip is formed.
PCT/US2005/036075 2004-10-08 2005-10-07 Method to form microptips WO2006042065A3 (en)

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