APPARATUS AND METHOD FOR REDUCING CHARGE ACCUMULATION ON A SUBSTRATE

Abstract: A system for reducing surface charge on a target surface in charged particle beam lithography or microscopy, using an apparatus including: a beam column that outputs a charged particle beam towards the surface; and a charge reducing device positioned between the surface and the beam column, where the charge reducing device emits charged particles to neutralize charge on the surface. The charge reducing device can include a MOS device and a voltage source, where the voltage source is coupled to provide a voltage across the MOS device to cause the MOS device to emit the charged particles. The charge reducing device can include multiple MOS devices mounted on a mechanical mount and a voltage source, where the voltage source is coupled to provide a voltage across the MOS devices to cause the MOS devices to emit the charged particles. The associated method for reducing surface charge on a surface includes outputting the charged particle beam towards the target surface and emitting charged particles to neutralize the resulting charge on the surface.

Description: APPARATUS AND METHOD FOR REDUCING CHARGE ACCUMULATION ON A SUBSTRATE BACKGROUND 1. Field of the Invention This invention relates to charged particle beam columns, and more specifically to techniques for reducing surface charge on a target substrate. 2. Description of The Related Art Charged particle beam columns and microcolumns are well known in the arts of lithography and electron microscopy imaging, i.e., using a charged particle beam (e.g. of electrons or ions) to measure feature dimensions and view a surface sensitive to a charged particle beam. See, e.g., "Electron-Beam Microcolumns for Lithography and Related Applications," by T.H.P. Chang et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3774-81, Nov. /Dec. 1996, incorporated herein by reference in its entirety. FIG. 1 depicts a conventional charged particle beam column 100 that is well known in the art for, e.g., electron beam lithography. A conventional charged particle beam column 100 includes, e.g., a charge particle (electron) source 102 that outputs a charged particle beam 114; a limiting aperture 104 positioned downstream with respect to the direction of charged particle beam 114 from charged particle source 102 (hereafter "downstream" means downstream with regard to a charged particle beam direction from charged particle source) ; a transfer lens 106 positioned downstream from limiting aperture 104, where the transfer lens 106 controls the focal point of the charged particle beam 114; a blanking system 108, positioned downstream from transfer lens 106, that includes blanking deflectors 116 and blanking aperture 118, where blanking deflectors 116 cause charged particle beam 114 to intersect blanking aperture 118; a deflection system 110 positioned downstream from blanking system 108, where deflection system 110 controls the location that charged particle beam 114 intersects surface 120; and an objective lens 112 positioned downstream from deflection system 110 that focuses and controls the cross section size of charged particle beam 114 on surface 120. FIG. 2 depicts a conventional microcolumn 200 that is well known in the prior art. Microcolumn 200 includes, e.g., a beam emitter 202 which emits a charged particle beam 204; a source lens 206 positioned downstream from beam emitter 202; a deflection system 208 positioned downstream from source lens 206, where deflection system 208 controls a location that charged particle beam 204 hits surface 212; and an einzel lens 210 positioned downstream from deflection system 208. When a primary charged particle (electron) beam from a column, e.g., charged particle beam column 100 or microcolumn 200, is incident on a substrate, e.g., surface 120 or surface 212, that is constructed of an insulative or semiconductive material, a variety of charged particles are generated, e.g., secondary electrons, backscattered electrons, and so-called Auger electrons. The primary electrons create electron-hole pairs in the substrate material. Electrons created within a few nanometers of the surface escape and leave behind a positive charge, resulting in a positive surface potential. On a local scale, a significant level of charging can be detected, although on a global scale, charge is balanced. Such charging effects, both local and global, present a significant problem for both lithography and imaging. In particular, charging effects interfere with accurate placement of the charged particle beam on the substrate. Therefore what is needed is a method and apparatus for controlling the undesirable charging effect in such columns. SUMMARY An embodiment of the present invention reduces surface charge on a substrate surface that is the target of a charged particle beam using an apparatus including a beam column that outputs a charged particle beam towards the substrate surface; and a charge reducing device positioned between the surface and the beam column, where the charge reducing device emits charged particles to neutralize charge on the surface induced by the particles. In one embodiment, the charge reducing device includes: a MOS device and a voltage source, where the voltage source is coupled to provide a voltage across the MOS device to cause the MOS device to emit the charged particles (electrons). In another embodiment, the charge reducing device includes multiple MOS devices mounted on a mechanical mount and a voltage source, where the voltage source is coupled to provide a voltage across the MOS devices to cause the MOS devices to emit the charged particles. Thereby, an associated method for reducing surface charge includes the outputting the charged particle beam towards the target surface and emitting charged particles to neutralize the resulting charge on the surface. An embodiment of the present invention provides an associated method for reducing the surface charge on a surface, including: outputting a charged particle beam towards the surface and emitting charged particles to neutralize the resulting charge on the surface. In an embodiment, an additional act includes repelling stray charged particles towards a central region that the charged particle beam intersects on the surface. Various embodiments of the present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 depicts a conventional charged particle beam column 100 that is well known in the prior art. FIG. 2 depicts a conventional microcolumn 200 that is well known in the prior art. FIG. 3 depicts schematically system 300 that includes beam column 302 and charge reducing device 304, in accordance with an embodiment of the present invention. FIG. 4A depicts a top plan view of charge reducing device 304A in accordance with an embodiment of the present invention. FIG. 4B depicts a cross sectional view of charge reducing device 304A of FIG. 4A along line A-A in accordance with an embodiment of the present invention. FIG. 4C depicts a top plan view of charge reducing device 304B in accordance with an embodiment of the present invention. FIG. 4D depicts a cross sectional view of charge reducing device 304B of FIG. 4C along line B-B in accordance with an embodiment of the present invention. FIG. 5 illustrates emission of charged particles from a MOS device when a voltage V b is applied. FIGs. 6A and 6B each depict a side view of an implementation of system 300 respectively having charge reducing device 304A and charge reducing device 304B, each in accordance with an embodiment of the present invention. FIG. 7 depicts schematically an implementation of system 700 that includes microcolumn 200 and modified charge reducing device 702, in accordance with an embodiment of the present invention. FIG. 8A depicts modified charge reducing device 702 in more detail, in accordance with an embodiment of the present invention. FIG. 8B depicts a plan view of modified charge reducing device 702, in accordance with an embodiment of the present invention. DETAILED DESCRIPTION FIG. 3 depicts schematically system 300 that includes beam column 302 and charge reducing device 304. Beam column 302 can include, for example, either a conventional charged particle beam column 100 or a conventional microcolumn 200, both described above. Beam column 302 outputs a charged particle beam 308, being, e.g., charged particle beam 114 or charged particle beam 204, towards surface 306. Surface 306 is, for example, a substrate that beam column 302 writes onto (lithography) or examines (electron microscopy) . Charge reducing device 304 is positioned between beam column 302 and surface 306 and is coaxial with charged particle beam 308. Charge reducing device 304 controls the charging effect on surface 306. FIG. 4A depicts a top plan view of charge reducing device 304A, an embodiment of charge reducing device 304. In this embodiment, charge reducing device 304A includes a metal oxide semiconductor (MOS) device having an opening 402 that charged particle beam 308 passes through. In this embodiment, opening 402 is circular although it can be other shapes, such as a square. Suitable dimensions X and Y of charge reducing device 304A are respectively 10 mm and 10 mm. FIG. 4B depicts a cross sectional view of charge reducing device 304A of FIG. 4A along line A-A. As shown in FIG. 4B, charge reducing device 304 includes three layers: silicon substrate layer 404, silicon dioxide layer 406, and metal layer 408. In one embodiment, silicon substrate layer 404 is approximately 2 to 300 μm thick, silicon dioxide (Si0 2 ) layer 406 is approximately 2 to 10 nm thick, and metal layer 408 is approximately 2 to 20 nm thick. A suitable process to fabricate charge reducing device 304A of FIG. 4A follows. To form silicon substrate layer 404, a surface of an approximately 300 μm thick crystalline silicon substrate wafer is implanted with n-type donor ions so that the wafer becomes n+ doped or n++ doped. A suitable resulting implant level of the wafer is lxl0 19 /cm 3 . Next, silicon dioxide layer 406 is formed by, e.g., thermal growth, over silicon substrate layer 404, thereby having a thickness of 5 to 10 nm. Next, metal layer 408 being, e.g., aluminum, palladium, chromium, or platinum, is formed over silicon dioxide layer 406 by, e.g., a conventional thermal evaporation or electron beam sputtering process to have a thickness of 3 to 20 nm. For the charge reducing device 304A of FIG. 4A, a circular opening, corresponding to opening 402, with a diameter of 1 to 3 mm is next etched through the combination of metal layer 408, silicon dioxide layer 406, and silicon substrate layer 404. FIG. 4C depicts a top plan view of charge reducing device 304B, another embodiment of charge reducing device 304. Charge reducing device 304B includes four distinct, MOS devices 410A-410D mounted on a mechanical support 420 by, for example, clamping or glue. The mechanical support 420 includes an opening 430, through which charged particle beam 308 passes. A suitable shape of opening 430 is a circle, although other shapes such as a square are suitable. A suitable diameter of opening 430 is approximately 100 μm, where beam column 302 includes microcolumn 200, or approximately 1 to 2 mm, where beam column 302 includes charged particle beam column 100. A structure of each of MOS devices 410A-410D is similar to charge reducing device 304A. A suitable process for fabricating each of MOS devices 410A-410D is described earlier with respect to charge reducing device 304A, except no opening is formed through a MOS device. A suitable shape of each of MOS devices 410A-410D is square having a side length S of approximately 1 to 10 mm. The shape of each of MOS devices 410A-410D can be varied to be, for example, circular or rectangular. A suitable thickness of each of MOS devices 410A-410D is approximately 300 μm. A suitable distance D (FIG. 4C) between each MOS device is approximately 0.5 to 2 mm. The MOS devices 410A-410D should be mounted as close as possible to the opening 430, so that any neutralizing charge 312, discussed in more detail below, emits close to the area on surface 306 that charged particle beam 308 intersects. A suitable material of mechanical support 420 is, for example, aluminum, or a metal. The dimensions M and N of the mechanical support 420 are respectively 30 mm and 30 mm. FIG. 4D depicts a cross sectional view of charge reducing device 304B of FIG. 4C along line B-B. By comparison, the MOS devices of charge reducing device 304B may operate more reliably than charge reducing device 304A because charge reducing device 304A may suffer from defects incurred from the formation of opening 402. Referring to FIG. 5, when a voltage V b is applied between metal layer 408 and silicon substrate layer 404 so that silicon substrate layer 404 is biased more negatively than metal layer 408, an electric field forms that forces electrons 502 from silicon substrate layer 404 into silicon dioxide layer 406 by Fowler-Nordheim tunneling. The majority of the tunneling electrons scatter inelastically in the metal layer 408, although a small fraction of the electrons, approximately 10 "3 to 10 ~7 , tunnel through and out of metal layer 408 (electrons 504). For example, if metal layer 408 and surface 306 are both biased at ground potential and the silicon substrate layer 404 is biased to approximately -5 to -10 V, where metal layer 408 faces surface 306, emission of low energy electrons from metal layer 408 towards surface 306 is likely. Referring to FIG. 3, the emitted low energy electrons correspond to neutralizing charge 312 and are injected into the region between charge reducing device 304 and surface 306. The neutralization of charge on surface 306 can be achieved by at least two different mechanisms. When surface 306 charges positively, the accumulation of positive charge on surface 306 creates an electric field which attracts neutralizing charge 312, i.e., the low energy electrons from charge reducing device 304. Absorption of these low energy electrons into surface 306 eliminates or minimizes the positive charge buildup. Alternatively, the cloud of low energy electrons establishes surface 306 as the potential of the source of low energy electrons, e.g., approximately 0 V, and locks the surface potential within the range of the energy spread of the low energy electrons, e.g., 0.2 eV to 1 eV. When a high energy primary electron beam impacts surface 306 locked to a uniform potential of approximately 0 V, any placement or imaging errors are minimized, because no electric field, which could distort the path of incident charged particle beam 308, can be created at surface 306. FIGs. 6A and 6B each depict in side view implementations of system 300 respectively including charge reducing device 304A and charge reducing device 304B. In the embodiments depicted in FIGs. 6A and 6B, the metal layers 408 of both charge reducing device 304A and MOS devices 410A-410D of charge reducing device 304B face surface 306. A voltage V b is applied between each metal layer 408 and silicon substrate layer 404 so that silicon substrate layer 404 is biased more negatively than metal layer 408 to cause either charge reducing device 304A or 304B to emit neutralizing charge 312. Surface 306 is biased to the same voltage as metal layer 408. In an embodiment of the present invention, charge reducing device 304 and an electrode layer of einzel lens 210 of conventional microcolumn 200 are combined. Specifically, in this embodiment, an electrode layer of the einzel lens acts as the silicon substrate layer of the charge reducing device 304. FIG. 7 depicts schematically system 700, in accordance with this embodiment, that includes a conventional microcolumn, described in more detail earlier with respect to FIG. 2, having a beam emitter 202 which emits a charged particle beam 308, a source lens 206, a deflection system 208, and an einzel lens 210, having electrode layers 704, 706, and 802, positioned downstream from deflection system 208; and modified charge reducing device 702. In this embodiment, electrode layer 802 is the substrate layer of modified charge reducing device 702. FIG. 8A depicts a cross sectional view of modified charge reducing device 702. As discussed in "Electron-Beam Microcolumns for Lithography and Related Applications," a suitable method to construct an einzel lens is to fabricate each electrode layer of the einzel lens separately and then assemble the electrode layers. In accordance with this embodiment, a suitable implementation of electrode layer 802 is either an n-t- or n++ doped silicon substrate, where electrode layer 802 is approximately 0.2 to 10 μm thick. A suitable implant level of electrode layer 802 is 10 19 /cm 3 . Electrode layer 802 includes a circular opening 710 formed, e.g., by etching. A suitable diameter of opening 710 is approximately 100 μm. In this embodiment, each of electrode layers 704 and 706 includes a circular opening having the same diameter as circular opening 710 and similarly located so that the openings align when the electrode layers 704, 706, and 802 are assembled. Electrode layer 802, used as the bottom electrode of the einzel lens, i.e., closest to the surface 306, acts as the substrate layer of modified charge reducing device 702. Next, silicon dioxide layer 804 is formed by, e.g., thermal growth, to a thickness of 5 to 10 nm over the bottom electrode layer 802. Next, a metal layer 706 such as aluminum, palladium, chromium, or platinum is formed over silicon dioxide layer 804 by a conventional thermal evaporation process so that metal layer 806 is 3 to 20 nm thick. A circular opening 808 with a diameter of approximately 100 to 300 μm is next etched through only the silicon dioxide layer 804 and metal layer 806. The diameter of opening 808 defined in the silicon dioxide layer 804 and metal layer 806 is larger than the diameter of opening 710 defined in electrode layer 802. Further, opening 808 is coaxial with opening 710. In other embodiments, opening 808 can be other shapes, such as a square. FIG. 8B depicts a bottom plan view of modified charge reducing device 702 shown in FIG. 8A from a direction indicated by the arrow from C. FIG. 8B illustrates the relationship between opening 710 in electrode layer 802 and opening 808 in a combination of silicon dioxide layer 804 and metal layer 806. Subsequently, electrode layers 704 and 706 and the electrode layer 802 of modified charge reducing device 702 are combined to form a modified einzel lens. For example, a Pyrex™ insulator can separate each electrode of the einzel lens. In a microcolumn, metal layer 806 of modified charge reducing device 702 is an outer surface. In system 700, metal layer 806 faces surface 306. To reduce the charging effect, neutralizing charge 312 must be concentrated near an area on surface 306 that the charged particle beam 308 intersects (hereafter "central region") . In an embodiment of the present invention, a negatively charged, cylindrically shaped metallic barrier 800 surrounds but does not contact charge reducing device 304 and extends towards but does not contact surface 306. The metallic barrier 800 is aligned so that charged particle beam 308 passes through the opening. FIGs. 6A, 6B, and 7, each depict barrier 800, in side view, in broken lines. In this embodiment, barrier 800 is made of a metal such as aluminum, copper, or stainless nonmagnetic steel. When a bias voltage being more negative than the voltage of metal layer 408 and surface 306, is applied to barrier 800, barrier 800 charges negatively relative to the surface 306. The negatively charged barrier 800 forces stray low energy electrons 314 (FIGs. 6A, 6B, and 7) towards the central region. The above-described embodiments are illustrative and not limiting. All parameters and dimensions herein are illustrative. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects For example, the shape and dimensions of the MOS devices 410A-410D and charge reducing device 304A, the materials of the MOS devices 410A-410D and charge reducing device 304A, the number of MOS devices 410A-410D, the range of bias voltages can be varied. For example, the charge reducing device 304 can be used to counter negative charge accumulation on surface 306. Therefore, the appended claims encompass all such changes and modifications as fall within the scope of this invention.

Claims: CLAIMS What is claimed is: 1. A system for reducing surface charge on a surface comprising: a beam column that directs a charged particle beam towards said surface; and a charge reducing device positioned between said surface and said beam column, wherein said charge reducing device emits charged particles to neutralize resulting charge on said surface. 2. The system of Claim 1, wherein said charge reducing device comprises a MOS device; and a voltage source coupled to provide a voltage across said MOS device thereby to cause said MOS device to emit said neutralizing charged particles. 3. The system of Claim 2, wherein said system further comprises a barrier that repels said charged particles, wherein said barrier laterally surrounds said charge reducing device . 4. The system of Claim 3, wherein said beam column comprises a charged particle beam column. 5. The system of Claim 3, wherein said beam column comprises a microcolumn. 6. The system of Claim 1, wherein said charge reducing device comprises a mechanical support, wherein said mechanical support includes an opening that allows said charged particle beam to pass towards said surface; a plurality of semiconductor devices attached to said mechanical support; and a voltage source coupled to provide a voltage across each of said plurality of semiconductor devices thereby to cause said semiconductor devices to emit said charged particles. 7. The system of Claim 6, wherein said system further comprises a barrier that repels said charged particles, wherein said barrier laterally surrounds said charge reducing device . 8. The system of Claim 7, wherein said beam column comprises a charged particle beam column. 9. The system of Claim 7, wherein said beam column comprises a microcolumn. 10. The system of Claim 1, wherein said charge on said surface is positive. 11. A method for reducing surface charge on a surface, comprising the acts of: directing a charged particle beam towards said surface; and emitting charged particles to neutralize said charge on said surface. 12. The method of Claim 11 further comprising repelling stray charged particles towards a central region that said charged particle beam intersects on said surface. 13. An einzel lens for use with a charged particle beam column, wherein the einzel lens controls charge buildup on a surface exposed to charged particles from said column, comprising: a first electrode; a second electrode separated from said first electrode; and a third electrode separated from said second electrode, wherein a charge reducing device is coupled to said third electrode and faces said surface. 14. The einzel lens of Claim 13, wherein said charge reducing device and said third electrode together comprise a MOS device. 15. The einzel lens of Claim 14, wherein said einzel lens further comprises: a voltage source coupled to provide a voltage across said MOS device thereby to cause said MOS device to emit charged particles. 16. The einzel lens of Claim 15, wherein said einzel lens further comprises: a barrier that repels said charged particles, wherein said barrier laterally surrounds said einzel lens .

Inventors: CHANG TAI-HON P, MANKOS MARIAN, CHANG, TAI-HON, P., MANKOS, MARIAN
Applicants: ETEC SYSTEMS INC, ETEC SYSTEMS, INC.
Application year: 2000
Main classification: H01J37
IPC codes: G21K5, H01J37, H01L21