JP4639341B2 - Etching method by cluster ion bombardment and mass spectrometric method using the same - Google Patents

Description

  The present invention relates to an ionization method and apparatus capable of imaging by cluster ion bombardment, and in particular, environmental science-related and life science-related samples (biological samples) such as drugs, plasticizers, pigments, fullerenes, amino acids, peptides, protein molecules, and DNA molecules. The present invention relates to an ionization method and apparatus suitable for mass spectrometry of polymer samples (including polymer samples) and capable of acquiring (imaging) molecular level spatial information (particularly two-dimensional distribution information) of a sample. The present invention further relates to a fine etching method and apparatus for material surfaces (including metals and semiconductor materials) by cluster ion bombardment.

Conventional imaging of biological samples mainly uses matrix-assisted laser desorption ionization (MALDI) and ion bombardment (FAB / SIMS). For example, "Imaging mass spectrometry: a new tool to investigate the spatial organization of peptides and proteins in mammalian tissue sections" Current Opinion in Chemical Biology 2002,6,676-681, "Direct molecular imaging of Lymnaea stagnalis nervous tissue at subcellular spatial resolution by mass spectrometry "Anal. Chem. 2005, 77, 735-741.
In MALDI, a sample is irradiated with a laser, and the matrix is ablated by the energy generated thereby, and the sample is desorbed and ionized for analysis. For this reason, it is necessary to apply to the sample a matrix (for example, sinapinic acid) that absorbs laser light (generally a wavelength of 337 nm). Since the matrix agent is applied to the sample surface, there is a problem that chemical noise is generated during imaging measurement of a biological sample. In addition, there is also a problem that the sample is damaged by laser irradiation, resulting in destructive analysis. Furthermore, there is a limit to the laser beam condensing (spot size) (Bragg diffraction condition), and furthermore, since the irradiated part is sputtered violently, it is essentially difficult to obtain a high-resolution imaging image. Since many matrices and biological samples themselves are scattered by a single laser irradiation, this method is not suitable for depth analysis and has a problem of low detection sensitivity.
In FAB / SIMS, since fast atoms and ions are used, there is a problem that primary impact particles penetrate deeply into the sample and chemically destroy the sample.
The inventor has already proposed "electrosprayed droplet impact ionization" (EDI) (WO 2005/083415 A1). According to this EDI method, desorption and ionization can be performed extremely softly without destroying biological samples, drugs, and the like. This is because, since the energy per unit mass of the multiply charged ion cluster used in EDI is 1 eV / u or less, only the surface layer number molecules are desorbed and ionized only by shock wave heating.
On the other hand, the miniaturization of silicon-based semiconductors has progressed to a film thickness of several hundred angstroms. Such a thin film is manufactured using plasma CVD, MBE (molecular beam epitaxy), or the like. The technology for depositing the film has shown great progress, but the technology for etching and removing the film once formed at the atomic level with good reproducibility has been delayed. In plasma etching, sputtering by ion bombardment is the main process, and damage to the ultrathin film is inevitable. There is also a technique (SIMS) that uses an argon atom (or ion) beam, but only a few atomic layers of the thin film cannot be etched, and the internal structure of the thin film is damaged, leading to the development of ultrafine semiconductor technology. Application is difficult. In order to overcome this industry deadlock, it is essential to develop breakthrough technology that etches only a few atomic (molecular) layers without damaging the internal structure at all.

The present invention provides a method and apparatus in which the above ionization method and apparatus utilizing cluster ion bombardment are improved so that imaging is possible.
Furthermore, the present invention provides an etching method and apparatus using the above-described ionization method and apparatus by cluster ion bombardment.
In the ionization method by cluster ion bombardment according to the present invention, charged droplets of a volatile liquid are generated by an electrospray method under atmospheric pressure, the generated charged droplets are guided to a vacuum chamber, and an electric field formed in the vacuum chamber is used. The charged droplet is accelerated toward the position of the sample, and the spread (beam diameter) of the beam of the charged droplet that collides with the sample is limited by the fine hole of the mask arranged in front of the position of the sample. The ionization method according to the present invention can be used in a mass spectrometry method. In this case, the generated ions are guided to a mass spectrometer.
A large number of charged droplets of volatile liquid are continuously generated as a group, accelerated to form a kind of beam, and proceed toward the sample. The spread of the charged droplet beam (beam diameter) is limited by the mask arranged in front of the sample, and a part of the charged droplets that have passed through the fine holes of the mask collide with the sample. In this way, the position and range of the charged droplet that collides with the sample can be controlled. If the collision position of the charged droplets is fixed, information in the depth direction of the sample can be obtained, and if the collision position is scanned two-dimensionally with respect to the sample surface, two molecules of the sample surface layer are detected. Dimensional distribution information can be obtained.
In particular, the present invention increases its utility value by determining the position where the charged droplet should collide with the sample by the position of the micropore.
In order to obtain two-dimensional distribution information, at least one of the fine holes of the mask and the sample may be scanned relatively two-dimensionally. In this case, the fine holes of the mask and the sample are not necessarily displaced in parallel, and may be inclined. In a special form, the fine holes of the mask are scanned two-dimensionally with respect to the sample. In addition, the charged droplets may be scanned by an electric or magnetic field.
The fine hole may be one fine hole formed in the mask, or may be formed by two intersecting fine slits formed in two mask members.
A mass spectrometry method can also be realized by using the ionization method described above. In this case, ions of the sample that are desorbed and ionized by the collision of charged droplets are introduced into the mass spectrometer.
The volatile liquid is preferably water, but may be methanol or ethanol. Water includes water added with weakly acidic solution and weakly alkaline solution. In order to facilitate the generation of charged droplets, an electrolyte is dissolved in water to form an aqueous solution, which is used as a liquid for charged droplets.
An ionization apparatus according to the present invention is provided outside an ion inlet of a mass spectrometer, communicates with the inside of the mass spectrometer through the ion inlet, and includes an acceleration electrode, a mask, and a sample stage. And a charged droplet generation chamber communicating with the vacuum acceleration chamber through a droplet inlet of the vacuum acceleration chamber, and generating charged droplets of a volatile liquid in the charged droplet generation chamber With a charged droplet generator, the mask has fine holes and is placed in front of the sample stage, and the sample stage and the fine holes in the mask are related to the direction of the surface of the sample stage ("in relation" Means that the fine holes of the mask and the surface of the sample stage are not necessarily displaced in parallel but may be displaced in an oblique direction). Bands produced by a drop generator A droplet is guided from the charged droplet generation chamber to the vacuum acceleration chamber through the droplet introduction port, accelerated by the acceleration electrode to which a high voltage is applied, and limited in the spread by the fine holes of the mask, Charged droplets that have passed through the fine holes collide with the sample on the sample stage, and ions thus desorbed and ionized are introduced into the mass spectrometer through the ion inlet. It is.
The vacuum accelerating chamber and the droplet generating chamber do not need to be particularly separated from each other, and may be simply divided (regions may be partially overlapped).
The ionization method according to the present invention can be realized using the above ionization apparatus.
An electrospray method is preferably used for generating charged droplets. When cold controlled nitrogen (N ₂ ) gas is used in combination, cooling, generation of charged droplets (spraying), and transfer to a vacuum chamber (vacuum acceleration chamber) can be performed efficiently. The generation of charged droplets can be performed under atmospheric pressure (including a reduced pressure state). When generating charged droplets under atmospheric pressure, nitrogen gas (which may be cooled) may be used as the nebulizer gas for the purpose of efficiently generating charged droplets by electrospraying. It is desirable that the charged droplet generation chamber (ion source) under atmospheric pressure is a space (closed space) isolated from the atmospheric pressure outside air. This is because there is a possibility that the gas of the ion source including the electrospray device may be contaminated by atmospheric pressure outside air (laboratory air). By using clean nitrogen nebulizer gas, the atmospheric pressure gas in the ion source is not contaminated.
According to the present invention (particularly according to the electrospray method), it is possible to generate a beam by a charged droplet population of the order of microns. The charged droplet beam is sampled in a vacuum chamber (vacuum acceleration chamber) while maintaining the droplet size (beam spread or beam diameter) of this micron order. Further, the beam diameter is limited by the mask before colliding with the sample, and the charged droplet beam diameter on the order of submicron is obtained by passing through the fine hole.
Such cluster ions are accelerated by an electric field in a vacuum chamber (vacuum acceleration chamber), thereby imparting kinetic energy and impacting a sample (for example, a biological sample thin film). A shock wave is generated at the collision interface, and the sample is desorbed and ionized in picosecond order.
Since the sample is bombarded with cluster ions, electron and vibration excitation of the target molecule occurs at the time of collision, and at the same time, the kinetic energy of the molecule in the sample thin film is efficiently excited. In this way, since the sample is softly impacted by the cluster ions, even molecules having a molecular weight exceeding tens of thousands are ionized without being damaged.
In addition, since the sample is vaporized and ionized within a short period of picoseconds shorter than the recombination lifetime between the positive and negative ions, recombination between the positive and negative ions is suppressed, and the generated ions are efficiently massed. It can be led to an analysis device.
As a biological sample to be used, a frozen sample may be used in order to prevent its drying, but it goes without saying that the present invention can be applied to a dried sample.
The present invention also provides an etching method using charged droplets.
In the etching method according to the present invention, charged droplets of an etching solution are generated and guided to a vacuum chamber, and the charged droplets are accelerated toward the position of an object by an electric field formed in the vacuum chamber and collide with the surface of the object. Thus, the surface of the object is etched.
An etching apparatus according to the present invention includes a charged droplet generator that generates charged droplets of an etching solution under atmospheric pressure, and an accelerating electrode and a holding base that holds an object disposed therein, and the charged droplets. It has a vacuum accelerating chamber with fine introduction holes for charged droplets generated by the generator, and a high voltage is applied to the charged droplets generated by the charged droplet generator and introduced through the fine introduction holes. And an accelerating device for accelerating and colliding with the surface of the object held on the holding table by etching with the acceleration electrode formed, and etching the surface of the object.
Charged droplets can be generated by electrospray at atmospheric pressure. Hydrofluoric acid, dilute nitric acid, acetic acid, or the like can be used as a solution for generating electrospray (etching solution, mainly aqueous solution). Atomic level etching of silicon or silicon oxide can be performed by impact of charged droplets of hydrofluoric acid. Atomic level etching of a metal thin film can be performed by charged droplet impact of dilute nitric acid. Atomized and droplet level etching of organic thin film materials and inorganic materials (metals, semiconductors, insulators, etc.) can also be performed by charged droplet impact of an acetic acid aqueous solution.
The etching solution need not be one that causes a chemical reaction with the object. The etching solution may be an aqueous solution, an organic solvent (for example, methanol), or an aqueous solution containing an organic solvent. In short, the etching solution only needs to be charged in a multivalent manner. For example, water (pure water) can be used as the etching solution. However, charged droplets are more likely to be generated by the electrospray method when an electrolyte such as acetic acid or ammonia is dissolved in water to form an aqueous solution. The charged droplet is preferably a multivalent charged droplet having a charge of several tens or hundreds of valences.
It is preferable that the generated charged droplets are passed through an ion guide, and only the droplets having a relatively large charged charge are selected and guided to an acceleration device for acceleration.
Since charged droplets are charged in a multivalent manner as described above, they can be accelerated at an extremely high speed (for example, about 10 times the speed of sound) by an electric field.
The present invention accelerates charged droplets of a multivalent number (for example, a mass of 10 million (u), a valence of 100, or a larger size and a higher valence) at high speed to accelerate the surface of an object (material). Based on technology that makes it collide. When a charged droplet having a multivalent (for example, 100 valent) charge collides with a solid surface at a high speed, a shock wave is generated at the interface. The interface gas generated by this shock wave is a supercritical fluid gas that far exceeds the critical limit of water supercritical fluid (water). Since collisions at the interface are coherent (all colliding water molecules collide with the substrate in the same direction), the reproducibility of the phenomenon is extremely high. Moreover, this supercritical state is relaxed (cooled) in about picoseconds by a high-speed energy dissipation process, so that the inside of the film is not damaged. Furthermore, since the film thickness desorbed from the surface is limited to a few atomic (molecular) layers, it is an optimum ultra-fine processing technology. By selecting the type of etching solution to be electrosprayed, various semiconductors (Si, Ge, etc.), silicon oxide (semiconductor oxide), insulators, organic material thin films, etc., and metals (stainless steel, nickel, etc.) as described above , Iron, copper, tin, gold, silver, etc.) can be etched in a layer-by-layer at the atomic level and in a state close to this. Since the ion source (charged droplet source) is an atmospheric pressure electrospray, it can be easily mounted on a semiconductor manufacturing apparatus and the principle is simple, so that no skill is required. It is a simple, highly efficient and inexpensive technology.
Since the charged droplets are charged fine particles, the beam can be swept in two directions (X and Y directions) orthogonal to each other by the deflection electrode. It is also possible to realize a large area processing technique by sweeping or scanning the charged droplet beam or the object (its holding table) in a wide range in the XY direction (two-dimensionally). Precision processing of large-area high-definition semiconductor materials, metals, organic thin films, insulator films, etc. becomes possible. In order to form a fine pattern, only a necessary portion may be etched using a mask.
In the etching method according to the present invention, ions desorbed and ionized from the surface of the object by collision of charged droplets can be introduced into the mass spectrometer. That is, the etching apparatus and the mass spectrometer according to the present invention can realize a composite etching surface analyzer, a composite mass spectrometer, or various composite devices (scanning tunnel microscope, photoelectron spectrometer, etc.).

FIG. 1 is a block diagram of an ionization apparatus.
FIG. 2 is a perspective view showing the mask and its driving device.
FIG. 3 is a perspective view showing a modification of the mask and its driving device.
FIG. 4 is a block diagram showing an embodiment of the etching apparatus.
FIGS. 5A to 5C schematically show the phenomenon that the surface of the object is etched.
FIG. 6 is a block diagram showing another embodiment of the etching apparatus.
FIG. 7 shows the configuration of the etching apparatus used in the experiment.
8 and 9 show the results of mass analysis of ions generated by etching.

First, an embodiment of an ionization method and apparatus will be described.
In FIG. 1, an ionizer 20 is installed in a portion including the ion inlet of the mass spectrometer 10.
A skimmer 11 having a hole 11a is attached to a portion of an ion introduction port of a mass spectrometer (for example, a time-of-flight mass spectrometer) 10. Ions having the same direction are introduced into the mass spectrometer through the holes 11a (ion introduction ports). The inside of the mass spectrometer 10 is kept at a high vacuum by an exhaust device (not shown).
The ionization device 20 includes a charged droplet generation device 30 including a charged droplet generation chamber (ion source chamber; a chamber equipped with a general electrospray, nanoelectrospray, or cold electrospray) 31, and charged droplet generation. It is comprised from the acceleration apparatus 40 provided with the vacuum acceleration chamber 41 connected with the chamber 31 in a straight line form.
The charged droplet generating device 30 includes the above-described electrospray device 32. The electrospray device 32 includes a metal (conductive) capillary tube (capillary) 33 to which a high voltage is applied, and an enclosure that covers the periphery with a space therebetween. Tube 34. The tips of the metal thin tube 33 and the surrounding tube 34 protrude into the charged droplet generation chamber 31. The thin metal tube 33 is supplied with a cooled or room temperature volatile liquid (solution) that becomes charged droplets. A cooling medium, for example, nitrogen (N ₂ ) gas, is supplied as a nebulizer gas in the space between the metal thin tube 33 and the surrounding tube 34 (atmospheric pressure or pressure in the vicinity thereof: the atmospheric pressure including these). Nitrogen gas is supplied from a nitrogen production machine or a nitrogen cylinder, or is generated from liquid nitrogen, and is introduced into the surrounding tube 34 under temperature control. Cooling with nitrogen gas makes it possible to generate larger-sized charged droplets (cluster ions). Further, even if cooling with nitrogen gas is not performed, the tip of the capillary 33 is sufficiently close to a small hole 34a of a charged droplet sampling orifice 34, which will be described later, to prevent drying of the charged droplet, and a large-sized charged liquid. Drops can be taken into the vacuum. Since the temperature of the droplets taken into the vacuum is drastically decreased due to the vaporization of the solvent, and the vapor pressure is lowered as a result, the size reduction of the droplets under vacuum is slight. Fly in a vacuum with almost the same volume.
From the tip of the metal thin tube 33 to which a high voltage is applied, a highly charged fine droplet group D is sprayed into the charged droplet generating chamber 31 in a beam shape (about several millimeters in diameter). Nitrogen gas is injected into the charged droplet generation chamber 31 from the tip of the surrounding tube 34 around the tip of the metal thin tube 33. The nitrogen gas helps spray the charged droplets and cools the charged droplets, and further transfers the charged droplet beam D toward the vacuum acceleration chamber 41 in the cooled state. Nitrogen gas is discharged to the outside from the charged droplet generation chamber 31 through the exhaust port. Depending on the experimental conditions, it is not necessary to cool the charged droplets, but it is preferable to cool them.
A charged droplet is a volatile liquid. When charged droplets vaporize (dry), the droplet size decreases. In order to suppress the vaporization of the charged droplets, it is nitrogen gas that cools the charged droplets in the generation of the charged droplets and until the charged droplets reach the vacuum acceleration chamber 41. The cooling temperature is preferably about just before the charged droplets solidify. However, it may not always be necessary to cool the charged droplets. If the distance between the tip of the metal thin tube 33 and the small hole 34a of the charged droplet sampling orifice 34 is shortened (closed), drying (vaporization) of the charged droplet can be prevented.
Typical examples of the volatile liquid that becomes a charged droplet include an aqueous solution containing an electrolyte (for example, water added with a weakly acidic or weakly alkaline solution such as acetic acid or ammonia), water, or the like. . Methanol or acetonitrile may be mixed with water, or only methanol may be used.
Another example of the charged droplet generator is an ultrasonic vibration device. The inside of the charged droplet generation chamber 31 is about atmospheric pressure, but may be kept in a reduced pressure state. The charged droplet generation chamber 31 may be opened to the atmosphere (for example, a peripheral wall or the like is omitted).
An orifice 34 is provided at the boundary between the charged droplet generation chamber 31 and the vacuum acceleration chamber 41, and a small hole 34 a is formed in the orifice 34. This small hole 34a is a charged droplet introduction port. The charged droplet generation chamber 31 and the vacuum charged droplet acceleration chamber 41 communicate with each other through the charged droplet introduction port 34a.
The charged droplet group D sprayed from the tip of the metal thin tube 33 and formed into a beam shape moves in the charged droplet generation chamber 31 in the direction of the vacuum acceleration chamber 41 together with the cooling and transporting nitrogen gas, and a small hole in the orifice 34 It is introduced into the vacuum acceleration chamber 41 through 34a.
In the vacuum acceleration chamber 41, an acceleration electrode 42, a sample stage 43, and a mask 50 are provided. A positive or negative high voltage (for example, 10 KV) opposite to the polarity of the charged droplet is applied to the acceleration electrode 42. The charged droplet D introduced into the vacuum acceleration chamber 41 is accelerated and converged (focused) by the accelerating electrode 42, and obliquely collides with the sample S provided on the sample stage 43, and the ionized molecules are desorbed from the sample. Release. The inside of the mass spectrometer 10 and the vacuum accelerating chamber 41 communicate with each other through the ion introduction port 11a opened in the skimmer 11, and is generated by the collision of the charged droplet and jumps out from the surface of the sample S (sample stage 43). Ion molecules (or atoms) are introduced into the mass spectrometer 10 through the ion introduction port 11a.
As described above, the diameter (spread) of the charged droplet beam (charged droplet group) generated by the charged droplet generator 30 is in the millimeter order. The diameter of the charged droplets (cluster ions) ranges from a few microns to a few nanometers. This is called a giant cluster-ion. This huge cluster-ion beam is introduced from the charged droplet generation chamber 31 to the vacuum acceleration chamber 41 while maintaining the spread of millimeter order, and is accelerated by the electric field of the acceleration electrode 42. For example, a kinetic energy of about 1 million eV (106 eV) is given to a large cluster ion.
The sample stage 43 holds, for example, a frozen biological sample thin film S to prevent drying. The accelerated cluster ions strike the biological sample thin film S (for example, a biological tissue sample placed on a substrate attached to a low-temperature refrigerator). As a result, the thin film sample is vaporized within a short time of picoseconds. Equal amounts of positive and negative ions exist in the desorbed ions, but ions are generated in a time period shorter than their recombination lifetime, preventing recombination (neutralization reaction) of the generated ions. , Many ions are supplied from the vacuum acceleration chamber 41 into the mass spectrometer 10 through the ion inlet 11a. This enables highly sensitive mass spectrometry.
In addition, when a sample is bombarded by cluster ions, a shock wave is generated at the collision interface. Due to the action of the shock wave, even a molecule having a molecular weight exceeding tens of thousands such as a protein is desorbed and ionized without being damaged. That is, mass spectrometry (for example, orthogonal (orthogonal) time-of-flight mass spectrometry) of biomolecules including proteins becomes possible.
On the flight path of the beam D by the charged droplet group accelerated in the vacuum accelerating chamber 41 and just before the sample stage 41, the mask 50 has its surface in the flight direction (Z direction) of the charged droplet beam. ) And substantially vertical (referred to as XY direction). The mask 50 is driven by the driving device 60 so as to be slightly displaced two-dimensionally in the direction along the surface.
Specifically, as shown in FIG. 2, the mask 50 has a thin plate 50 (preferably an insulating plate) formed with fine holes 50a. It is supported on a stage (not shown) that is movable in the Y direction. The displacement of the mask 50 in the X direction is driven by the piezo element 61, and the displacement in the Y direction is driven by the piezo element 62. The driving device 60 includes piezo elements 61 and 62. For driving in the X and Y directions, a mechanical device other than the piezo element may be used. In conjunction with the driving of the mask 50, the cluster ion beam can be deflected by an electric field to adjust the ion beam having the maximum intensity.
The surface of the mask 50 is perpendicular to the flight path of the charged droplet group. Most of the spread of the beam-like charged droplet group D in the millimeter order is limited by the mask 50 and does not reach the sample S. A part of the charged droplet passes through the fine hole 50a of the mask 50, collides with the sample as described above, and ionizes the sample. For example, a charged droplet beam having a diameter of 3 mm (with a beam diameter) is limited to a beam having a diameter of several μm to 0.1 μm and collides with the sample S.
In this way, the position of the sample where the charged droplet collides is limited to a small range. By moving the mask 50 in the XY directions, the position of the sample where the charged droplet collides can be changed. Therefore, it is possible to obtain the two-dimensional molecular level information by two-dimensionally scanning the collision position of the charged droplet on the surface of the sample.
The mask 50 (its fine hole 50a) determines the position where the charged droplet on the sample surface collides. If the collision position of the charged droplet is fixed, distribution information in the depth direction at the molecular level can be obtained. As described above, if the collision position of the charged droplet is displaced one-dimensionally or two-dimensionally, one-dimensional or two-dimensional information on the molecular level of the sample surface layer can be obtained. Instead of displacing (scanning) the mask 50, the sample stage 43 may be displaced (scanned). If possible, the charged droplets can be deflected by an electric or magnetic field and scanned.
As shown in FIG. 3, the mask 50 is formed by superposing two mask plates 51 and 52 having micro slits 51a and 52a intersecting with each other, and the crossing position of the micro slits 51a and 52a is a micro hole. Good. The mask plate 51 is supported so as to be displaceable in the X direction and is driven in the X direction by the piezo element 61, and the mask plate 52 is supported so as to be displaceable in the Y direction and is driven in the Y direction by the piezo element 62.
Next, examples of the etching method and apparatus will be described.
FIG. 4 shows an etching apparatus 20A combined with a mass spectrometer. In this figure, the same components as those shown in FIG.
A difference from the ionization apparatus shown in FIG. 1 is that, firstly, an etching solution (including an aqueous solution) is used as a solution of charged droplets generated by electrospray.
In the etching apparatus and method, charged droplets of an etching solution generated by electrospray under atmospheric pressure are guided into a vacuum, and this is accelerated by a potential difference of about 10 kV, and impacts the object Su. As an etching solution for generating electrospray, hydrofluoric acid, diluted nitric acid, acetic acid, or the like can be used. Atomic level etching of silicon or silicon oxide can be performed by impact of charged droplets of hydrofluoric acid. Atomic level etching of a metal thin film can be performed by charged droplet impact of dilute nitric acid. Atomic and molecular level etching of organic thin film material can be performed by charged droplet impact of acetic acid. The etching solution may be simple water or an organic solvent.
The etching solution need not be one that causes a chemical reaction with the object. The etching solution may be an aqueous solution, an organic solvent (for example, methanol), or an aqueous solution containing an organic solvent. In short, the etching solution only needs to be charged in a multivalent manner. For example, water (pure water) can be used as the etching solution. However, charged droplets are more likely to be generated by the electrospray method when an electrolyte such as acetic acid or ammonia is dissolved in water to form an aqueous solution. The charged droplet is preferably a multivalent charged droplet having a charge of several tens or hundreds of valences. Etching with water is possible because various reactive species are generated in the shock wave region generated at the collision interface between the charged droplet and the sample, and these etch metal, semiconductor, insulator, etc. . Such etching action is reactive etching, and enables layer-by-layer etching at the atomic level. In etching using such cluster ions, no crater is generated on the surface, so that only the surface can be etched without damage inside the sample.
The second difference from the method and apparatus shown in FIG. 1 is that a holding table 43A is provided in the vacuum acceleration chamber 41 in place of the sample table. The object Su to be etched is fixedly held on the holding table 43A.
Nitrogen gas is not necessarily used. It is not always necessary to cool the charged droplets. By introducing or filling the charged droplet generation chamber 31 with high-purity nitrogen gas at a temperature close to room temperature, mixing of impurities can be suppressed. It is not always necessary to provide the mask 50. Since charged droplets are charged in a multivalent number (several tens to hundreds), they can be accelerated by an electric field at a very high speed (for example, about 10 times the speed of sound). Further, the charged droplet beam can be swept in two directions (X and Y directions) orthogonal to each other by the deflection electrode. Large area etching can be performed by sweeping (two-dimensionally scanning) the charged droplet beam or the holding table 43A.
In this etching method, as an example, charged droplets having a mass of 10 million (u) and a valence of 100 are collided with the surface of the object Su at high speed. As shown in FIG. 5A, when a charged droplet having a valence of 100 collides with the solid surface at a speed about 10 times the speed of sound, a shock wave is generated at the interface as shown in FIG. 5B. The interface gas generated by this shock wave is a supercritical fluid gas that far exceeds the critical limit of water supercritical fluid (water). Since collisions at the interface are coherent (all colliding water molecules collide with the substrate in the same direction), the reproducibility of the phenomenon is extremely high. In addition, the supercritical state relaxes (cools) in about picoseconds, so that the inside of the film is not damaged. Furthermore, as shown in FIG. 5C, since the film thickness desorbed from the surface is limited to a few atoms (molecules) layer, it is optimal for ultra-fine processing technology and surface analysis technology using layer-by-layer. Is the law.
By selecting the type of liquid to be electrosprayed, various semiconductors (Si, Ge, etc.), silicon oxide (semiconductor oxide), insulators, organic material thin films, etc. Even layer-by-layer etching techniques are possible. Since the ion source (charged droplet source) is an atmospheric pressure electrospray, it can be easily mounted on a semiconductor manufacturing apparatus and the principle is simple, so that no skill is required. It is a simple, highly efficient and inexpensive technology. For the formation of the fine pattern, only a necessary portion may be etched using the mask 50. By sweeping the charged droplets or the holding table 43A in the xy direction, precision processing of a large-area high-definition semiconductor material, metal, organic thin film, insulator film, or the like is possible.
According to experiments, it was confirmed that only a few molecular layers on the surface of the organic material were detached by a single charged droplet impact, and the underlying sample was not damaged at all. This is because the desorption efficiency is high and the thickness of the desorbed film is limited to a few molecular layers.
Ions desorbed and ionized from the surface of the object Su by collision of charged droplets (that is, by etching) are introduced into the mass spectrometer 10 through the introduction path (ion introduction port 11a) and subjected to mass analysis.
FIG. 6 shows another embodiment of the etching method and apparatus.
An electrospray apparatus 32B having a metal thin tube 33B to which a high voltage is applied is provided outside the main body of the etching apparatus 20B and is exposed to the atmosphere. Of course, the electrospray device 32B may be provided in the charged droplet generation chamber. Charged droplets of the etching solution are generated by the electrospray device 32B.
The main body of the etching apparatus 20B includes a roughing chamber 71, an adjustment chamber 72, and a vacuum acceleration chamber 41B of the acceleration device 40B, and these chambers 71, 72, and 41B are connected in a straight line.
The roughing chamber 71 has a slightly low degree of vacuum exhausted by a rotary pump or the like. The adjustment chamber 72 and the vacuum acceleration chamber 41B are kept at a high vacuum. A quadrupole ion guide 73 is provided in the adjustment chamber 72. The charged droplet of the etching solution generated by the electrospray device 32B enters the roughing chamber 71 through the fine hole 71a of the orifice of the roughing chamber 71, and is further guided to the adjustment chamber 72 through the fine hole 72a. The spread is suppressed by 73, and a charged charged droplet beam is formed. In addition, a high frequency electric field is applied to the ion guide 73, and only charged droplets having a relatively large charge and mass are selected. The selected charged droplet beam is introduced into the vacuum acceleration chamber 41B through the fine hole 41a.
In the vacuum acceleration chamber 41B, an acceleration electrode 42B, a deflection electrode 44, and a holding base 43B are arranged in this order. An object Su to be etched is fixed on the holding table 43B.
The charged droplet beam is accelerated by the acceleration electrode 42B to which a high voltage is applied, collides with the object Su, and the object Su is etched as described above. In this embodiment, the charged droplet beam collides perpendicularly with the surface of the object Su. When the charged droplet beam passes between the deflection electrodes 44, it is swept according to the sweep voltage applied to the deflection electrode 44. Therefore, the charged droplet scans the surface of the object.
Instead of the deflection electrode 44, the charged droplets may be swept by a magnetic field by energizing the coil. Instead of sweeping the charged droplet beam, the holding table 43B may be scanned two-dimensionally. A mask can also be provided.
FIG. 7 shows an etching apparatus 20C used in the experiment. The etching apparatus 20C is combined with the time-of-flight mass spectrometer 10, but the basic structure is the same as the etching apparatus 20B shown in FIG. Avoid duplication.
The roughing chamber 71 is provided with a ring lens 71b, and the adjustment chamber 72 is divided into two chambers 72A and 72B that are evacuated separately, and a high-frequency quadrupole ion guide 73 is provided over both the chambers 72A and 72B. It has been.
Ions released from the surface of the object Su are collected by the extraction electrode 19, arranged in the introduction chamber 14 of the mass spectrometer 10, and converged and reduced through the high-frequency quadrupole ion guide 16 into which N ₂ gas has been introduced. The velocity is increased (collision damping) and guided into the analysis chamber 17. The vacuum acceleration chamber 41B is also provided with an entrance / exit 18 for the object Su.
A 1M aqueous acetic acid solution is used as the etchant, and as shown in FIG. 5A, about 100-valent water molecules collide with the object Su at a speed of about 10 times the speed of sound (12 Km / s).
FIG. 8 shows mass spectrometry results obtained when a 10 ML (monolayer) gramicidin layer is formed on a 100 ML (monolayer) arginine (Arg) layer as an object (sample). Is shown. The horizontal axis represents time (minutes) and the vertical axis represents relative intensity. It can be seen that the upper gramicidin is etched first, and then the lower arginine is gradually etched. That is, it can be seen that etching is performed in a layer-by-layer or a state close thereto.
FIG. 9 shows experimental results obtained when using an object (Sn (1 nm) on Si) in which a tin (Sn) foil having a thickness of 1 nm is formed on silicon. Etching is predominant for tin (Sn) (Sn ⁺ , H ⁺ (SnO), H ⁺ (SnO) ₂ ) and gradually silicon (H ⁺ (SiO ₂ ) ₂ , H ⁺ (SiO ₂ ) ₃ ). It can be seen that is etched. Tin (Sn), which is one of the metals, is etched by layer-by-layer or in a state close thereto.
From the above, it can be seen that etching of organic material thin films, semiconductors, insulators, metals, and the like is performed by layer-by-layer (or in a state close to this). Etching of oxide semiconductor, graphite, diamond, etc. is also possible.

Claims (8)

Using water, an aqueous solution or an organic solvent as the etching solution, polyvalent charged droplets of this etching solution are generated by electrospray at atmospheric pressure and guided to a vacuum chamber.
Multi-charged droplets are accelerated toward the position of the object by the electric field formed in the vacuum chamber and collide with the object surface, thereby etching the layer structure surface of the object layer-by-layer.
Etching method by cluster ion bombardment. The etching method according to claim 1 , wherein the multivalent charged droplet has a charge of several tens to hundreds. The etching method according to claim 1 , wherein at least one of the multivalent charged droplet and the target is scanned two-dimensionally. The etching method according to claim 1 , wherein a mask is disposed in front of the position of the object, and the spread of the multivalent charged droplet beam that collides with the object is limited by the fine holes of the mask. The etching method according to claim 4 , wherein a position at which the multivalent charged droplet is to collide with the object is determined by the position of the fine hole. The etching method according to claim 4 or 5 , wherein at least one of the mask and the object is relatively two-dimensionally scanned. The etching method according to claim 1 , wherein the object is a semiconductor, a semiconductor oxide, an insulator, a metal, or an organic material. A mass spectrometric method for introducing ions desorbed and ionized from the surface of a layer structure of an object by collision of multivalent charged droplets in the etching method according to claim 1 into a mass spectrometer.

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