US 20040060904 A1
A micro-tool and corresponding method are disclosed herein for working a very small surface of a substrate. The micro-tool has a tip of diameter on the order of 1 mm or less for placement in close proximity to a location on a substrate to be worked, and at least two open electrodes located near an end of the tip, such that the gap between the open electrodes is on the order of a few microns or less. The micro-tool further includes a housing which holds the tip and wiring extending from the open electrodes to permit connection to a voltage source. When the electrodes of the micro-tool are connected by such wiring to a voltage, an electric field and electron emission arises between the open electrodes such that electron emission currents are established. In the corresponding method, a localized electric field is generated in close proximity to a substrate using a tool having at least two open electrodes with a gap between them on the order of a few microns or less, by applying a voltage to the open electrodes. The localized electric field and emitted electrons aid in at least one of etching and depositing a feature on the substrate surface.
1. A micro-tool for use in finely working a surface of a substrate, comprising:
a tip having a diameter on the order of 1 mm or less for placement in close proximity to a location on a substrate to be worked;
at least two open electrodes located near an end of said tip, said open electrodes having a gap between them on the order of a few microns or less; and
a housing for holding said tip, said housing including wiring from said open electrodes to permit connection of said micro-tool to a voltage source;
wherein when said electrodes are connected by said wiring to a voltage, an electric field and electron emission arises between said open electrodes.
2. An apparatus including the micro-tool of
3. The apparatus of
4. The micro-tool according to
5. The micro-tool according to
6. The micro-tool according to
7. The micro-tool according to
8. The micro-tool according to
9. An plurality of micro-tools according to
10. The plurality of micro-tools according to
11. A method of finely working a surface of a substrate, comprising:
generating a localized electron emission current in close proximity to a substrate using a tool having at least two open electrodes having a gap between them on the order of a few microns or less, by applying a voltage to said open electrodes,
wherein said localized electric electron field emission current aids in at least one of etching and depositing a feature on a surface of said substrate.
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 This application is one of several related applications, all filed on even date herewith and all owned by the same Assignee, International Business Machines Corporation, these being: Attorney Docket No. FIS9200201xx entitled: “Micro-Tool Having a Plurality of Electrodes and Method of Use Thereof; Attorney Docket No. FIS9200201xx entitled: “Second One; etc.
 The present invention relates to the working of micro-scale surfaces, and more specifically to a tool and corresponding method for working a micro-scale surface by etching or depositing, as assisted by an localized electric field with electron emission produced by the tool.
 Current repair processes for integrated circuit (IC) chips and lithographic reticles (masks) rely primarily on the use of focused beams (ion, electron, and photons) to induce localized reactions for etching or deposition of materials for when editing patterns. Focused Ion Beam (FIB) tools have played a dominant role for most repair applications as well as in failure analysis methods, due to their superior spatial process confinement and reaction rates (relative to scanning electron or photon beams). However, concerns about ion beam induced damage and contamination have severely limited the applicability of FIB tools inside the clean room and lithographic mask production facility. The more recent use of copper scanning metallization and low-K dielectric (polymer) materials in IC fabrication has raised doubts about the extendibility of FIB tools for these applications. FIG. 1 illustrates such example, which is background to the invention, but is not admitted to be prior art. As shown in FIG. 1, a copper feature 10 of a substrate 20 lies under a plurality of layers 12 of interlevel dielectric material. Specifically, the editing (i.e. cutting) of low metallization level IC copper features 10 by FIB tools has proved troublesome due to the tendency of the copper removed by the tool to be redeposited on surfaces 14 of the entry hole 16 made by the tool (FIG. 1). Places 11 where the copper remains or is redeposited are conductive and thus, the desired degree of electrical isolation (i.e. the reason for cutting the line) is not achieved. In addition, when the interlevel dielectric 12 low-K polymer (in appropriate contexts, known as “SiLK”), it becomes conductive in places 14 which are exposed to the ion beam.
 In addition, Changes in the optical properties of lithographic masks, known as staining, caused by gallium ions (the source of ions in FIB tools is Ga+) and edge streaking (river-bedding) are examples of problems being encountered with FIB-based mask repair. Thus, a critical need exists for a new tool and method for the working of micro-scale surfaces, for example, for the repair of IC's and masks. At the same time, the failure of existing in-line metrology techniques to provide accurate three dimensional data for the development and control of IC fabrication processes has highlighted the need for a tool capable of sectioning a surface without the process threatening to contaminate clean room equipment and materials.
 Scanning electron microscopy (SEM) has been used in the past to induce spatially localized surface reactions. Inelastic scattering between the electrons (either tunneling or field-emission regimes) and reactant gas molecules, activates the reactant species so that a desired reaction is induced at or near the beam spot. In SEM, a far-field image of the electron source is formed on the sample surface by using apertures to spatially filter (block) the initial flux and then focus the remaining electron beam with electromagnetic lenses down to a fine spot, 210, as shown in FIG. 2a. In the Z, i.e. vertical direction at the XY location of the spot, no confinement or control of the current density is possible, aside from image focusing. As a result, direct surface height sensing and measurement is not possible. Only lateral electron scattering intensity l(x,y), as a function of lateral position Intensity l(x,y) images are generated by the SEM detector.
 SEM instrumentation is somewhat complex in that it requires a high vacuum environment for beam formation, sample interaction, and signal collection. Delivery of gas to the beam-sample region necessitates sophisticated differential pumping aperture additions to the column and special delivery nozzle assemblies which include positioning to inject a gas flux onto the surface. If too much gas is delivered to the sample, the electron beam source and column detection are adversely effected.
 Combined SEM/FIB systems are commercially available, at great expense, mainly for FIB milling while doing SEM imaging. Attempts to utilize the electron beam for processing have been made in a few rare cases, where ion beams are too invasive to the particular sample material at hand. It was generally found that lateral confinement and anisotropy were degraded, because low energy secondary electrons emerging from the beam-to-surface interaction volume tended to induce reactions that spread beyond the primary beam spot region. Reaction rates of SEM based processes were also noticeably lower than those achieved by the FIB. This is mainly due to the cross-section of electron scattering being much smaller than the cross-section of Ga+ion scattering and the lack of mechanical milling to remove the reaction products.
 However, the cross-section of electron scattering does increase with decreasing beam energy, but in such circumstance, the spot size of the electron beam widens undesirably, due to the dependence of column performance on acceleration energy. Even the basic formation of a beam and its current density per solid angle becomes compromised at lower energies of E less than 200 eV, 200 eV or below being about the ideal electron landing energy range for maximizing scattering probability. Maximum electron current density is necessary for increasing the scattering or interaction probabilities.
 Ironically, the use of apertures to create smaller final spot sizes results in most of the original source current being blocked (or thrown away) initially. Future experiments are planned for increasing the electron induced reaction rates through ultra-low voltage acceleration with retardation electron optics and the use of large apertures just for patterning. Small apertures and higher beam energies would then be used during subsequent imaging, after the reactions have occurred.
 SPM tips have also been employed previously as sources of electrons to tunnel from between tip to and sample, mainly for use in direct-write lithographic applications. In these applications, electron field emission occurs between the tip 250 and sample 260 for exposing an electron sensitive resist 270 prior to patterning (FIG. 2b). A difficulty with this approach is that the electron current is a function of both tip-to-sample separation (i.e. topography) and conductivity of material the surface area 270 between interacting with the tip 250. The fact that electrons traverse between the tip 250 and substrate surface of the sample 260 also means there cannot be a direct confinement of electrons, nor such confinement in the z-direction (normal to the plane of the sample 260) of the induced chemical changereaction volume. Furthermore, these techniques have not gained widespread acceptance or use in lithography because the throughput of serial writing with mechanically scanned probes does not compare favorably to electronic scanning of focused electron and/or photon beams. Basically, the feasibility of SPM as a fabrication tool is profoundly limited by the throughput as it relies on a serial type of operation with mechanical scanning of the tip or sample.
 The following describes some references which are background to the invention claimed in this application or applications identified above as being related.
 A 1999 article “Fountain Pen Nanochemistry: Atomic Force Control of Chrome Etching,” Applied Physics Letters, v. 75, no. 17, Oct. 25, 1999, describes using a single channel “micropipette” probe tool in an experiment to deliver etchant through the probe to a chrome-coated glass coverslip, to thereby etch lines in the chrome coating.
 U.S. Pat. NO. 4,880,496 describes “generating precision micropatterns . . . of very small dimensions by means of electromagnetic radiation, electron beams, chemicals, sound waves, etc., based on the application of the desired energy or chemical via a tapered tube having an outlet of extremely small dimensions, and which can be brought to a very small distance from the substrate by the device.” (Col. 2, lines 19-24) However, there is no teaching or suggestion of a plurality of open electrodes on the tube outlet with which to maintain self-contained electronic emission within the probe body itself.
 IBM Research Report, Martin, Hamann, Wickramasinghe “Strength of Electric Field in Apertureless Near-Field Optical Microscopy,” Nov. 9, 2000, (page 6) describes enhancement of the electric field by focusing a beam of light (far-field) externally onto a gold tip of a probe (See page 6).
 U.S. Pat. NO. 6,078,055 describes patterning a photoresist coating by laser illuminating the tip of a probe in close proximity to the coated substrate. There is no teaching or suggestion of chemical delivery through the probe or of a probe having multiple electrodes, such that a desirable electric field and electron emission are is produced within the probe body or tip aperture in controlled manner regardless of the surface material properties (metal or otherwiseconductivity, refractive index, . . . etc.) extant on the wafer surface.
 Accordingly, a micro-tool and corresponding method are provided herein for working a very small surface of a substrate. The micro-tool has a tip of diameter on the order of 1 mm or less for placement in close proximity to a location on a substrate to be worked, and at least two open electrodes located near an end of the tip, such that the gap between the open electrodes is on the order of a few microns or less. The micro-tool further includes a housing which holds the tip and wiring extending from the open electrodes to permit connection to a voltage source. When the electrodes of the micro-tool are connected by such wiring to a voltage, an electric field and electron emission arises between the electrically open electrodes.
 In the corresponding method, a localized electric field is generated in close proximity to a substrate using a tool having at least two open electrodes with a gap between them on the order of a few microns or less, by applying a voltage to the open electrodes. The localized electric field with electron emission between electrodes aids in at least one of etching and depositing a feature on the substrate surface.
FIG. 1 illustrates a background method of etching a substrate using a focused ion beam (FIB) tool.
FIGS. 2a, 2 b illustrate prior art methods of working a substrate using either a scanning electron microscope (SEM), or scanned probe microscope (SPM) tool.
FIGS. 3a-4 c illustrate embodiments of micro-tools described herein.
FIG. 4d illustrates a micro-tool array embodiment described herein.
FIG. 5 illustrates a system including a micro-tool for working a substrate.
 Other patent applications, identified as related above, disclose and claim localized chemical delivery probes (LCDP) and corresponding methods involving dispensing chemicals directly to a micro-scale surface where reactions are desired. The related applications are filed on the same date and are also owned by the present Assignee; therefore, they do not constitute prior art hereto. Such LCDP methods do not have an active energy source for stimulating the reaction. Process confinement is achieved by the very small aperture of the tool (on the order of 0.1 um or smaller). Chemicals are dispensed from the tip of the tool in passive fashion, without supplying energy for the reaction. Therefore, reaction mechanisms associated with the LCDP technique are primarily chemical. Embodiments described in the present application are directed to a micro-tool and corresponding method which actively supplies energy, in a highly localized volume, to stimulate the reactions via electron scattering. Improved spatial confinement of the induced chemical reactions is possible with the tool disclosed herein, because the activated chemical species can have a much shorter lifetime than the diffusion time of a gas or liquid, as well as the separation between the electric field generating electrodes of the tool can be made smaller than the LCDP aperture of a channel which delivers fluid through the tool to the desired location.
 As will further be described, a single-tipped tool embodiment of the invention can find use in the fabrication, repair and metrology of a specific, very small site on a substrate, i.e. a micro-scale surface. In contrast to other SPM tools, such as the STM (scanning tunneling microscope), which have a single electrode located on the tool for either emitting or collecting electrons, and therefore, rely on transferring electrons to or from the substrate, the tools described in the following embodiments have a plurality of electrodes located withinin their tips, therefore enabling the flow of electrons from one electrode to another, while containing the electric field to the volume near inside the tip. When the tool is brought very close to the substrate using available positioning and control systems, the location of the electric field relative to the substrate can be precisely controlled because the electrodes which control its location are located within the tip of the tool and should not depend on the sample properties.
 Further, in contrast to the previously mentioned SPM tools, no particular conduction is required between the tool and the substrate, no beam formation or focusing of a beam is required, and energies can be set as low as one desires since the electron source of the tool can operate in either tunneling or field emission regimes. High current density can be maintained even at low energies, since confinement the volume of the electron scattering region is confined by between the electrodes located on the tip of the tool. Once an electron current density is established by the tool in within the volume tip body in close proximity to the surface of the substrate to be worked, reactant gas molecules are introduced to that the volume surrounding the tip apex. Inelastic scattering with the electrons then causes the gas molecules to become activated (in many cases by dissociation into reactive species). This results in the desired reaction being confined to the region of the substrate where the gas is electron-activated by the tool due to the short lifetime; that is, the reaction is spatially confined by the region of electron activation to a volume which is smaller than the volume where the chemical is distributed. Among some of the advantages which may be brought forth from working a micro-scale surface of a substrate by a multiple open electrode tool are the following:
 1. Beam energetic damage, contamination, and/or undesired byproduct redeposition are avoided.
 2. Chemical modifications based on these low energy electrons can be spatially confined to a significantly smaller scale than with high energy electron or ion beams, for which backscattering and secondary electron generation is known to delocalize the induced reactions.
 3. Reactions are spatially confined in all three dimensions, as needed for the particular application, to manipulate sample features on the order of a few microns, and less, to those of nanometer scale, This is a significant advantage over beam-based processing techniques (e.g. FIB, electron-beam and SEM), which are not confined in the vertical direction, i.e. along the beam axis in the direction of propagation, wherein the thickness of material removed is controlled only by the dose delivered to the substrate surface.
 4. Activation with an electron source also permits better spatial resolution in lateral directions (in the plane of the substrate surface), than use of localized delivery techniques alone, such as LCDP alone or Nanojet techniques. The Nanojet involves guiding plasma from an upstream source down to the probe body, where the flux to the sample is constricted laterally by an aperture at the tip apex. The problem is that the aperture is worn by the ion flux, due to mechanical milling. One also finds a trade-off between plasma intensity at the sample and lateral resolution of the aperture size. A multiple electrode tipped tool, as disclosed herein can provide good or even better spatial resolution than such techniques, especially when applied to reactions activated by tunneling electron emission.
 5. The direction of electron emission is between electrodes in the tool tip, thus minimizing dependence of the electron current on the sample or ambient properties. One of the disadvantages encountered by prior techniques was the “stealing” of electron current due to changes which occurred in topographic features.
 7. Tip degradation is avoided, because reactive species are not formed inside the probe until they exit the final aperture.
 8. Etch anisotropy can be increased by vertical extension or retraction of the multiple electrode tipped tool as the surface material is removed. This reduces etching of adjacent areas in lateral directions. If a bias is applied to the substrate, further confinement may be possible by directing the activated species and/or electrons towards the desired area to be worked.
 9. In a preferred implementation, the removal of reaction products can also be mechanically assisted by the tool, either by intermittent contact of the tip with the substrate or by electrostatic force exerted between tip and sample surface. Doing so increases the rate and anisotropy of the etch in a similar fashion to the mechanical sputter mill component of FIB GAE processes.
 A first micro-tool embodiment is illustrated in FIG. 3a. As shown in FIG. 3a, the tip 300 of the tool has a diameter on the order of 1 mm or less, and includes a first electrode 310 and a second electrode 320, which are open, i.e. not in conductive contact with each other or any other conductor at the apex 330. An insulator 340 separates the first electrode 310 from the second electrode 320. In this embodiment, the first and second electrodes 310, 320 are coaxial, a first electrode 310 having a smaller diameter and arranged concentrically inside a second electrode 320. The first and second electrodes 310, 320 have a gap between them on the order of a few microns or less. As shown in FIG. 3b, this spatial arrangement produces an electron current which is radially symmetric. As further shown in FIG. 3b, the apex 330 of the tip 300 has the first open electrode 310 extended from the tip 300. Such arrangement produces an electric field having lines of electric flux extending into the volume near the tip 300 but outside of the tip 300. FIG. 3c illustrates a variation of this first embodiment, in which the apex 332 of the tip 300 is recessed within the tip 300. In this variation, more of the lines of electric flux remain within the cavity 334 within tip 300, and less remain outside of the tip 300. First and second electrodes 310, 320 are configured in such manner that lines of the electric flux are oriented in generally lateral direction, i.e not at a high angle thereto, thereby assisting vertical confinement of the reaction, i.e. confinement in a direction perpendicular to the substrate plane. Preferably, the apex 332 of the tip 300 is coated with one or more protective layers, used to help preventing damage to the tip 300, if particularly aggressive chemical processes are to be induced.
FIG. 3d illustrates a second micro-tool embodiment. In this embodiment, the tip 350, of diameter on the order of 1 mm or less, has two parallel electrodes 352 and 354, respectively. As in the first embodiment, electrodes 352 and 254 are open, that is, not in conductive contact with each other or any other conductor at the apex 356. Insulative material 357 helps mechanically support electrodes 352, 354 while insulating them from each other and unwanted contact with an external conductor. As in the first embodiment, the first and second electrodes 310, 320 have a gap between them on the order of a few microns or less. Electric flux lines 358 extend between the electrodes 352, 354, as shown in FIG. 3e.
FIG. 3f illustrates yet another micro-tool embodiment, having a tip 360, of diameter on the order of 1 mm or less, in which first and second electrodes 362 and 364 are arranged in parallel, but also arranged in arc fashion over an underlying insulator 366 of the tip 360. The gap between the first and second electrodes is on the order of a few microns or less. FIG. 3g illustrates a top down view of the apex 368 of tip 360. Fabrication of tip 360 may be simplified. For example, an insulator 366 can be coated with a conductive film, and the conductive film then etched in two lines extending down the tip 360 to the apex 368 form the two open electrodes 362, 364.
FIG. 4a illustrates yet another micro-tool embodiment having a tip 400 which includes not only first and second open electrodes 410, 420, but also one or more channels 430 for delivering a fluid or gas to the apex 402. An insulator 404 separates the open electrodes 410, 420 from each other, while also helping to mechanically support them and hold them in relation to each other. The channel or channels 430 deliver a fluid reactant to the volume near the apex 402 of the tip 400, the fluid which may contain one or more chemicals helpful in assisting the reaction or controlling it.
 In another embodiment (FIG. 4d) a plurality of micro-tools constructed according to one or more of the foregoing described embodiments are arranged in an array 450, the array brought in close proximity to a substrate surface 452, such that a larger area of the substrate can be worked simultaneously.
 In any of the above tip configurations, emission of electrons is established between electrodes inside the tip while the gas or liquid is introduced and activated via inelastic scattering. Etching or deposition then occurs if the source is brought to sufficient proximity of the appropriate surface material. Protective coatings can be added to the active region of the tip to insure immunity to the particular chemical process being induced. The farthest protruding electrode should be grounded, so that incidental contact with the sample will not result in electrical shorting on conductive surface regions. However, shorting could be used to intentionally terminate the process on conductive features, if desired. The electrodes are configured such that the current density only has lateral components, therefore achieving extreme vertical confinement (i.e. along the probe axis direction). At small enough electrode separations, tunneling electrons can be used for inducing reactions, instead of field emission. Utilization of tunneling electrons further improves spatial confinement of the chemical reaction in three dimensions, and, under appropriate conditions, can provide better spatial resolution than the upper limit of that which is provided by field emission alone or LCDP techniques.
 The multiple open electrode tipped micro-tool can be moved into close proximity to a surface to be worked using apparatus that is available currently for the positioning of a scanned probe microscope (SPM). Existing SPMs are able to operate in a wide range of ambients without modification. They are relatively more simple and inexpensive (by a factor of at least ten times) to construct and operate than their beam-based counterparts, such as FIB or SEM. FIG. 5 illustrates the micro-tool as placed in a system for controlling movement thereof in close proximity to a surface to be worked. A substrate 510 containing the surface to be worked rests on a movable stage 512 for initial coarse positioning of the substrate 510 and optical navigation under a high-NA (numerical aperture) objective lens microscope 514 to the surface thereon to be worked. High NA optical microscope viewing/ imaging allows one to see where the tip 516 of the micro-tool 518 is relative to the feature of interest on the substrate, even if the feature of interest is below the top surface 520 of the substrate 510 (cases when the substrate includes one or more optically transparent layers above the feature of interest). The separation between the tip 516 and the surface to be worked are then actively regulated via surface force feedback (from transducer 522) and control electronics 524, as shown in FIG. 5. Wiring 525 from the open electrodes in the tip 516 of the micro-tool 518 is coupled to a voltage source 526 for producing electric effects at the tip 516. In a preferred embodiment, a reservoir source of fluid 528 is coupled through one or more ducts 530 for supplying the fluid to the surface to be worked on the substrate 510. Preferably the duct(s) 530 provide fluid into a channel of micro-tool 518 such that the location of fluid delivery to the substrate 510 is controlled in connection with the above-described method for positioning the tip 516 in proximity to the surface to be worked.
 The multiple open electrode tipped tool embodiments described above can be utilized in any of several ways. First, the wiring leads to such tools can be connected to a DC or low frequency AC voltage source such that electron tunneling and/or an electric field emission arises between the open electrodes in the tip. The tool is moved into position such that the tip is very close to the surface to be worked on the substrate. A fluid (gas or liquid) is delivered to the location of electron tunneling and/or electric field emission field induced by the tool, such that a reaction involving molecules from the fluid is aided by the electron tunneling and/or electric field current. The fluid can be delivered either by introduction to the substrate surface by means separate from the tool, or in a preferred embodiment, by a channel present within the tool itself. Further, a bias can be applied to the substrate, if desired, to further contain or direct electrons to the surface to be worked.
 Etching or deposition can then occur on the substrate surface as the electron source is brought within sufficient proximity to the micro-scale surface by precisely controlled movement of the tool. It may be desirable to ground the substrate and wiring connected to the farthest protruding open electrode of the tool, so that contact between the tool and the substrate, accidental or otherwise, does not result in an electrical short to conductive surface features. On the other hand, if desired, an electrode of the tool can be intentionally contacted with the substrate to short the same, in order to intentionally end the reaction when a conductive feature of the substrate is reached.
 In another example, the multiple open electrode tipped tool can be used to generate a plasma in the vicinity of the surface of the substrate to be worked. In such case, the wiring leads to the tool are connected to a DC voltage source or an AC voltage source (typically an AC voltage source having a radio frequency). A fluid, being a gas or liquid, containing molecules capable of being dissociated by the excitation, is delivered to the location of the DC or AC electric field induced by the open electrodes of the tool, such that a plasma results. As in the above described embodiment, the fluid can be delivered either by introduction to the substrate surface by means separate from the tool, or in a preferred embodiment, by a channel present within the tool itself. The fluid can contain molecules of either inert or reactive substances, or both, so long as some component of the fluid can be dissociated into a plasma. Reactive species are generated by interaction of the plasma with the fluid, which is either present in the volume already, from having been introduced by means external to the tool, or, in a preferred embodiment, as introduced through a channel in the tool.
 Further, a bias voltage can be applied to the substrate, if desired, to further contain or direct electrons to the surface to be worked. If the electrodes in a particular tip of a tool are made very close together, a low voltage can be used to create the micro-plasma. Operation in this low-voltage regime also improves spatial confinement of the chemical reaction in three dimensions.
 In such manner, a highly localized and confined “micro-plasma” is generated in close proximity to a surface of a substrate to be worked, wherein the location of the micro-plasma is determined by the placement of the tool. An advantage of working a substrate with such micro-plasma is that it does not require use of a high-energy beam, as some FIB methods do, such that unintentional sputtering and substrate damage can be reduced, if not avoided. Another advantage over FIB methods is that typically no gallium ions are necessary to promote the desired reaction, and therefore, use of the micro-plasma method avoids concerns over gallium contamination.
 Use of the micro-tool and the method disclosed herein is not limited to merely altering a point feature on a substrate. It may be desirable instead to perform etching or deposition over a certain surface area of a substrate. In such case, the micro-tool can be drawn over the surface to be worked in a scanned fashion, while etching or depositing, to define linear features and/or features of any other shape. It may also be desirable to scan successive lines of the small surface area, while modulating the voltage provided to the electrodes through the micro-tool. By modulating the electrode voltage during scans, electronic image data can be transferred as images to the surface. Then, if further desired, the micro-tool can be scanned over a surface already once scanned to further etch or deposit over the features defined by the previous scan. In such manner, an etch of a certain surface area can be made deeper, or a deposited layer on a surface area can be made thicker, through successively scanning over the same surface area.
 While the invention has been described herein in accordance with certain preferred embodiments thereof, those skilled in the art will recognize the many modifications and enhancements which can be made without departing from the true scope and spirit of the present invention, limited only by the claims appended below.