MASK REPAIR USING AN OPTIMIZED FOCUSED ION BEAM SYSTEM

Background of the Invention

1. Field Of The Invention

The present invention relates to focused ion beam systems. In addition, the present invention also relates to chemical vapor deposition and use of focused ion beams to enhance chemical vapor deposition rates and sputter etch rates of substrate materials. More particularly, the present invention relates to the use of the above technologies for repair of photomasks and reticles. 2. The Prior Art

Prior art is represented by the KLA/Micrion 808,

Seiko SIR1000, and Ion Beam Systems' MicroTrim. All three of these systems deliver only one type of focused ion beam to the mask under repair, namely a Gallium beam of energy between 20 and 50 keV. Such a beam produces significant mask damage during imaging due to the high sputter rate of the Gallium ion beam. In addition, significant amounts of Gallium are implanted into the mask substrate during imaging and opaque defect repair, resulting in an effect called "Gallium staining" which causes local reductions in the substrate's transparency which are prone to later identification as opaque defects by standard industry mask inspection equipment. While these systems attempt clear defect repair differently, none deposits into clear defects a material compatible with the pre-existing metallic film, usually chrome, normally covering portions of the mask. Brief Description of the Invention

The invention involves a new technique for the repair of semiconductor masks and reticles, utilizing a focused ion beam system which is capable of delivering, from a single ion beam column, several types of ion

beams, each of which is individually optimized to meet the differing requirements of the major functions to be performed in mask repair. These major functions are: mask imaging (the creation of an image of the microscopic structure of a region of the surface of a mask) ; opaque defect repair (the removal of optically-opaque material from regions on the surface of the mask substrate where such material should be absent) ; and clear defect repair (the use of the focused ion beam to catalyze the deposition of a chromium-alloy material into undesired voids on the surface of the mask substrate) .

Features of this invention include: the use in mask repair of multiple ion beams produced by a single ion beam column, mask imaging with greatly reduced damage levels and contamination, repair of opaque defects by either sputter etching or chemically-enhanced sputter etching, and repair of clear defects by deposition of metallic materials closely matched in optical properties to the metallic materials normally used for opaque areas of the mask.

This invention allows the mask to be imaged with high resolution and reduced damage, allows opaque defects to be removed more efficiently with reduced damage to the mask substrate, and allows clear defects to be filled in at high rates using the beam to directly catalyze deposition of metallic material compatible with the native mask materials.

The present invention has five major components. An ion beam column is utilized which can produce a precisely focused ion beam with a beam diameter at the target which is typically less than 0.5 microns, and can be operated at a voltage in the range 4 to 120 kV with a current density from 1 to 5 A/cm ² . A liquid metal alloy source for the ion beam column contains appropriate atomic species including: a low atomic mass element which is compatible with the mask substrate material (example:

silicon) ; a high atomic mass element which will produce a high sputter-etching rate of the optically-opaque metallic film for fast repair of opaque defects (example: gold) ; an element to be used for the ion-beam-enhanced deposition of a metallic material compatible with the metallic material normally used to cover opaque areas of the mask (example: gold, chromium, or silicon, used to enhance deposition of chromium from chromium hexacarbonyl vapor or other compounds) . A mass filter provides rapid selection of the ion species desired for the beam at the mask. Either the low atomic mass element or the high atomic mass element may be selected in accordance with whether the sputter etching or deposition is selected. A gas feed system is utilized in the present invention. It provides a flow of metal-containing vapor near the intersection of the primary ion beam and the target surface during clear defect repair.

The gas feed system may also provide a flow of etch-rate enhancing gas near the intersection of the primary ion beam and the target surface during opaque defect repair. This system is optional, and may consist partially of the same gas feed system used for the delivery of metal containing vapor as described above. Brief Description of Drawings

FIG. 1 - Focused ion beam mask repair system FIG. 2 - Mask imaging FIG. 3 - Opaque defect repair FIG. 4 - Clear defect repair FIG. 5 - Flow chart for the clear defect repair process FIG. 6 - Flow chart for the opaque defect repair process Detailed Description of a Preferred Embodiment

To image the photomask or reticle to be repaired, the mass filter is set to select a beam of the low atomic weight element, the ion column is set to produce a finely-focused beam of the ions at the target surface.

and the beam is scanned in a raster pattern to produce an image of the mask structure. The low atomic mass element may be silicon, compatible with the mask substrate material. In this operating mode, the ion beam causes minimal damage to the mask surface (clear and opaque areas) due to its low sputter-erosion rate of the mask surface. In addition, because the ion species is chosen to be compatible with the mask substrate, it also causes minimal "staining" of clear areas of the mask. To repair an opaque defect, a first beam of the high atomic weight element is produced which efficiently removes the bulk of the undesired optically-opaque material. This first beam is produced by setting the mass filter to select a beam of the high atomic weight element, setting the ion column to produce a finely-focused ion beam, and scanning the ion beam only in areas covered by undesired optically-opaque material. The first ion beam rapidly removes this undesired material by a process of sputter-etching, which may optionally be accelerated by the use of an etch-rate enhancing gas such as chlorine.

When this undesired material is largely removed, the system may optionally switch to a second beam using the low atomic weight element. This second ion beam has a sputter-etching rate which is substantially lower than that of the first ion beam but still high enough to remove in an acceptable time a very thin layer (about 0.01 micrometers) of material which is contaminated by implanted atoms from the first (high atomic weight) ion beam previously scanned over the opaque defect area. Implanted atoms from this second ion beam will not contaminate or "stain" the mask substrate since the second ion beam species is compatible with the mask substrate material. To repair a clear defect, the mass filter is set to select an ion species which is able to stimulate ion beam

enhanced deposition of a metallic material from a metal-containing vapor. This requires that the ion beam has sufficient energy to enhance the decomposition of the vapor at only those areas of the mask surface which are scanned by the ion beam to leave an optically-opaque metallic deposit. At the same time, the ion beam cannot have a sputter-etching rate which exceeds the deposition rate. The mass filter is set to select this desired ion species for the beam, the ion column is set to produce a finely-focused ion beam at the target surface, and the beam is scanned over the clear defect area. The metallic element in the metal-containing vapor is chosen to be compatible with the metallic film normally covering portions of the mask. FIG. 1 shows an apparatus 8 for producing focused ion beams according to the present invention. Ion source 10 may be the source for mixed ion beam 12, which is a mixture of several ion species in the present invention. Mixed ion beam 12 first passes through extraction aperture 14. A voltage is applied between ion source 10 and this extraction aperture 14 to induce a high electric field at the emission tip of ion source 10. This electric field causes ions to be emitted down the optical column to form mixed ion beam 12. Mixed ion beam 12 next encounters beam-defining aperture 16, containing a small hole which defines the half-angle of ion beam 12 which passes through into the remainder of the optical column. That portion of mixed ion beam 12 which passes through beam-defining aperture 16 is focused into the plane of mass separation aperture 24 by upper electrostatic lens 18. After passing through upper electrostatic lens 18, mixed ion beam 12 then passes through mass filter 20, whose function is to separate the various ion beams in mixed ion beam 12. In the figure, two separated ion beams are shown, mass separated beam 26 and ion beam 39. Mass separated beam 26 represents an ion species

contained in mixed ion beam 12 which is not desired in ion beam 39. For example, during imaging, ion beam 39 would consist of a light ion species, for example, silicon, while mass separated beam 26 would consist of the high atomic weight ion species not desired for imaging. After passing through mass filter 20, ion beam 12 (which has split into mass separated beam 26 and ion beam 39 as shown), passes through beam blanker 22. The function of beam blanker 22 is to "blank" ion beam 39 at the target, i.e., turn it on and off. Beam blanker 22 turns off ion beam 39 by deflecting it away from the axis of the column so that it no longer passes through mass separation aperture 24. Ion beam 39 is then turned back on by inactivating beam blanker 22. After passing through mass separation aperture 24, ion beam 39 passes through upper deflector 28, which is used for alignment of ion beam 39 with lower electrostatic lens 30. The beam then passes through lower electrostatic lens 30, which focuses ion beam 39 onto the surface of the mask 32. Main deflector 34 is an electrostatic octopole, which has variable dipole electrostatic voltages in the X and Y directions to deflect ion beam 39 across the surface of mask 32. A channel electron multiplier 36

(CEM or channeltron) is shown as a means for collecting secondary electrons or ions from the mask surface for imaging.

Ion source 10 is a standard liquid metal type such as those available from FEI Co., Hillsboro, Oregon; V.G. Instruments, Inc., Stamford, Connecticut? or MicroBea , Inc., Newbury Park, California. Ion source 10 is mounted in an ion gun structure which also supports extraction aperture 14. This gun structure is also a standard type available from the above suppliers. The diameter of beam-defining aperture 16 determines the total current in beam 12 as it enters the upper lens 18 and mass filter 20. This aperture will generally define a beam

half-angle from 1 to 4 illiradians, giving beam currents from 100 to 1000 pA. Apertures are standard products available from Advanced Laser Systems, Waltham, Massachusetts and National Aperture, Lantana, Florida. Upper lens 18 forms an image of ion source 10 at mass separation aperture 24. When mass filter 20 is activated, ion beam 12 is split into several beams at the mass separation aperture 24. Mass separated beam 26 shows one of these beams. With proper choice of the mass filter setting, the desired species in mixed ion beam 12 will pass through mass separation aperture 24 to form ion beam 39. Those of ordinary skill in the art will understand that one preferred embodiment of mass filter 20 is a Wien (ExB) filter, utilizing crossed electric and magnetic fields to separate beams of differing velocities. The proper balancing of the electric and field strengths in this ExB mass filter to select the desired ion species for ion beam 39 is readily understood to those of ordinary skill in the art.

Generally, lenses, mass filters, deflectors and blankers are not standard products. However, they are integral components of commercially available ion columns, such as those manufactured by MicroBeam, Inc., Newbury Park, California; JEOL, Boston, Massachusetts; and Ion Beam Systems, Beverly, Massachusetts.

When ion beam 39 is not required for any of the mask repair processes described in FIGs. 2-4, it is turned of by beam blanker 22. This prevents undesirable exposure of the mask to ion beam 39.

Lower lens 28 is used to image the beam crossover formed at mass separation aperture 24 onto the surface of mask 32. Using main deflector 34, the ion beam is generally scanned in a raster pattern for imaging as is known in the art. For defect repair, main deflector 34 scans the beam only in the defect areas within the

imaging raster pattern. Channel electron multiplier 36 collects either secondary electrons or secondary ions emitted from the mask 32 due to the impact of the focused ion beam 39. It may be a standard product supplied by Galileo Electo-Optics in Sturbridge, Massachusetts; or Detector Technology, Brookfield, Massachusetts.

Mask 32 is shown mounted on a movable stage 38, providing precise positioning of mask 32 under the ion beam 12. FIG. 1 illustrates one means known in the art for implementing this mask positioning function, an X-Y target stage, sufficient to position the mask to an accuracy within 1 micron in both X and Y of the known defect location. The center of the field of view for the imaging process described in FIG. 2 is the optical axis of the ion column, while its size is determined by the total acceleration voltage of ion beam 39 (voltage difference from ion source 10 to mask 32) , and the field strength and length of main deflector 34, as is well understood by those of ordinary skill in the art. In a preferred embodiment, ion source 10 may be a conventional ion source containing an alloy of gold and silicon. An ion source containing an alloy of chromium, gold and silicon may also be used. Such ion sources will produce a mixed ion beam from which a single species may be selected by use of a mass filter 20 as is known by those skilled in the art.

In a preferred embodiment the proportion of the alloy constituents of the gold/silicon ion source may be from 15 to 25 percent silicon, with the remainder gold. A typical alloy for the source might be a gold-silicon eutectic alloy consisting of 18.6 atomic % silicon and 81.4 atomic % gold (melting point approx. 363 degrees centigrade) . If a ternary chromium/gold/silicon alloy is used for ion source 10, the preferred consituents may be from 20 to 45 percent chromium, 20 to 55 percent gold, with the remainder silicon. Fabrication of these alloys

is familiar to those skilled in the art. Those of ordinary skill in the art will also readily realize that two binary alloy ion sources may also be used. For example, a first binary alloy ion source for use in imaging (FIG. 2) and opaque defect repair (FIG. 3) can be used in combination with a second binary alloy ion source for use in imaging (FIG. 2) and clear defect repair (FIG. 4) . Choice of suitable binary alloys for the first and second ion sources is readily understood by those of ordinary skill in the art.

The function of mass filter 20 is to select between the imaging (FIG. 2) and repair (FIGs. 3,4) ion beam species, as will be appreciated by those skilled in the art. Imaging of the defect area before performing a defect repair is necessary in order to assure that the ion beams used for clear or opaque defect repair will be scanned across only the actual area of the defect.

The apparatus of the present invention may be used to repair opaque defects in masks by use of a sputtering process. Optionally, the sputtering rate may be enhanced by use of a gas which reacts at the mask surface with the ions in the primary ion. beam 39. A focused ion beam of the sputtering ion species is used at an energy greater than 10 keV with a current density in the range from 1-5 K/cm ² . Gas feed tube 13 conducts an etch rate enhancing gas, such as chlorine, to the vicinity of the target surface. The design of this gas feed tube will be known to those skilled in the art. Pre-repair imaging (FIG. 2) is also performed at the same voltage level for accurate imaging.

For clear defect repair, the apparatus of the present invention deposits material on the mask 32 (FIG. 4) . A focused ion beam is used at an energy greater than 10 keV -with a current density in the range from 1-5 K/cm ² . Gas feed tube 13, conducts a metal containing vapor, such as chromium hexacarbonyl, to the vicinity of

the target surface. Pre-repair imaging (FIG. 2) is also performed at the same voltage, again to insure accuracy.

As those of ordinary skill in the art will be aware, it is preferable to neutralize the accumulated charge on the surface of mask 32 before it reaches a level which affects the optical performance of the beam (position, size, current density, energy) . A presently preferred method of neutralizing the ion beam is by using a flood gun 37 as is known in the art, to flood the surface of mask 32 with electrons. This flooding process may occur simultaneously with the imaging (FIG. 2) or repair (FIGs. 3,4) processes, or alternately with them.

Imaging is performed using a type of scanning microscopy employing either secondary electrons or secondary ions. A collector 36 collects secondary particles of either type for use in imaging, as is well known in the art. Techniques for imaging are well known and beyond the scope of the invention.

FIG. 2a shows mask imaging as typically performed in the prior art. Mask substrate 40 consists of atoms 42 of a first atomic species, usually silicon. Opaque regions 44 consist of atoms 46 of a second atomic species, typically chromium. Imaging species ions 48 are used, which implant into the clear regions of the mask, resulting in contamination or "staining" of mask substrate 40 as shown. In addition, the impact of primary ions 48 results in the sputter-removal from the mask of substrate atoms 40 and opaque region atoms 46 as shown. Both the "staining" and sputter-etching result in undesirable damage to the mask.

FIG. 2b shows mask imaging as performed by the present invention. The details of mask substrate 40, substrate atomic species 42, opaque regions 44, and opaque region atomic species 46 are the same as for FIG 2a. The present invention is distinguished from the prior art by the use of imaging species atoms 40a, which.

when implanted into the mask substrate, cause very little "staining". In addition, the impact of primary ions 40a results in much less sputter-removal from the mask of substrate atoms .40 and opaque region atoms 46. The net result is substantially reduced damage to both the clear and opaque regions of the mask.

FIG. 3a shows the repair of opaque defects as typically performed in the prior art. Mask substrate 40 is comprised of atoms of a first atomic species 42, typically silicon. Opaque area 44 is comprised of atoms of a second atomic species 46, typically chromium.

Abutted with desired opaque region 44 is opaque defect region 50, comprised of atomic species 52. Atomic species 52 is shown here as identical to opaque area atomic species 46. It is possible that opaque defect region 50 might be comprised wholly or in part of a third atomic species. In addition it is also possible that opaque defect region 50 might be isolated and not abut a desired opaque region. Gallium ions 48 are used to sputter-etch region 50, resulting in Gallium atoms 48a becoming implanted in substrate 40, causing "staining" and thus reduced transmission of light through mask substrate 40 in the areas of Gallium implantation.

FIG. 3b shows the repair of opaque defects as performed by the present invention. The details of the substrate 40, substrate atomic species 42, opaque area

44, opaque area atomic species 46, opaque defect area 50, and opaque defect atomic species 52 are the same as for

FIG 3a. The present invention differs from the prior art in the use of high-atomic weight ion species 54, shown here sputter-etching opaque region 50. Atomic species 54 is chosen to maximize the efficiency of the sputter-etching process. Optionally, the rate of sputter-etching may be further increased by the use of an etch-rate enhancing gas 56. This system is optional, and may consist partially of the same gas geed system used

for the delivery of metal-containing vapor for deposition. the pressure of the etch-rate enhancing gas is in the range of 1 to 100 microTorr. This range is chosen to provide sufficient gas to appreciable enhance the etch rate while still allowing the primary ion beam to remain focused on the target.

FIG. 4a shows the repair of clear defects as typically performed in the prior art. Two alternative procedures are commonly used. At the top, a process of beam-enhanced carbon deposition is shown. Gallium ions 80 cause the decomposition of carbon-containing gas molecules 82 at the surface of mask substrate 40, comprised of atomic species 42, typically silicon. While this process does deposit an opaque patch over the clear defect, this patch differs greatly from the optical properties of the native opaque mask material 44, containing ^* atomic species 46, typically chromium. For example-, the opaque patch is black in reflected light, compared with the relatively shiny chromium opaque regions of the mask. It also has differing behavior during the opaque defect repair process described in FIG. 3.

At the bottom of FIG. 4a, an alternative procedure used in the prior art for clear defect repair is shown. Gallium ions 80 are used to sputter special shapes 82 in the surface of mask substrate 40, comprised of atomic species 42, typically silicon. These shapes are designed to deflect light away from the clear defect area, thus making it appear dark. This process has the disadvantages of being very slow, irreversible, and restricted to the particular illuminating light wavelength for which the special shapes 82 were designed.

FIG. 4b shows the repair of clear defects as performed by the present invention. Primary ions 100 scan the area of the clear defect 102, typically an undesired gap in opaque area 44, comprised of atomic

species 46, typically chromium. While primary ions 100 are scanning the clear defect area, a flow of metal-containing gas molecules 104 is directed at the surface of the mask near the area where primary ions 100 impact with the surface of mask substrate 40, comprised of atomic species 42, typically silicon. Primary ions 100 induce the decomposition of metal-containing molecules 104 only in the region directly scanned by the beam, thus causing a deposition of metal atoms 106 within the defect area. The mass flow rate of the metal- containing vapor is adjusted to provide a pressure in the vicinity of the target surface in the range of 1 to 100 icroTorr. This range is chosen to allow sufficient vapor for efficient beam deposition while still allowing the primary ion beam to remain focused on the target. Since the metal atoms deposited are compatible with the native opaque region 44, the repair is nearly indistinguishable from these native opaque regions as far as appearance and in its behavior during the opaque defect repair process of FIG. 3.

At the bottom in FIG. 4b the completed clear defect repair with the present invention is shown schematically.

FIG. 5 shows a preferred embodiment for the clear defect repair process. First, in block 200, the area on the mask containing the defect is imaged as described in FIG.

2b. After the defect area has been imaged, the beam is blanked (turned off at the mask). In block 202, the exact area of the clear defect is then determined within the overall image area. In block 204, the flow of metal-containing gas is activated. In block 206, during the flow of this metal-containing gas at the mask surface in the vicinity of the intersection of the ion beam and the mask substrate, the ion beam is unblanked and scanned only over the exact area of the clear defect. Under the process shown in FIG. 4b, a deposition of metallic material compatible with the native opaque mask material

is formed. After a time determined to deposit a sufficient thickness of material, block 208 is entered, in which the ion beam is blanked again and the metal-containing gas supply is then inactivated. Block 210 then images the defect area again as was done in block 200, using the procedure described in FIG. 2b. Using the image displayed in block 210, the operator (or system computer) , makes a decision in block 212 whether the clear defect has been completely repaired. If the mask image indicates that the defect is not yet fully repaired, branch 214 is taken out of block 212, leading back to block 202. In block 202, the remaining defect area is again defined, as described above. If the image resulting from block 210 indicates that the defect has been fully repaired, branch 216 is taken out of block 212, leading to the process completion block 218.

FIG. 6 shows a preferred embodiment for the opaque defect repair process.- First, in block 250, the area on the mask containing the defect is imaged as described in FIG. 2b. After the defect area has been imaged, the beam is blanked (turned off at the mask). In block 252, the exact area of the opaque defect is then determined within the overall image area. In block 254, a supply of etch-rate enhancing gas may optionally be activated at this point. In block 256, the desired ion species for sputter-etching of the opaque defect is then selected using the mass filter and the beam is unblanked and scanned only over the defect area. After the opaque defect has been removed, block 258 is entered, in which the beam is blanked again and the supply of etch-rate enhancing gas is inactivated if it was activated in block 254, above. Block 260 then images the mask again as was done in block 250, using the procedure described in FIG. 2b. Using the image displayed in block 260, the operator (or system computer) , makes a decision in block 262 whether the opaque defect has been completely repaired.

If the mask image indicates that the defect is not yet fully repaired, branch 264 is taken out of block 262, leading back to block 252. In block 252, the remaining defect area is again defined, as described above;. If the image resulting from block 260 indicates that the defect has been fully repaired, branch 266 is taken out of block 262, leading to block 268.

After the defect has been removed, some residual "staining" of the mask substrate may remain. In block 268, an optional scan using an ion species compatible with the substrate may be used to clean-up the mask. After the completion of block 268, process completion block 270 is entered.