WO1986002581A1

WO1986002581A1 - Focused substrate alteration

Info

Publication number: WO1986002581A1
Application number: PCT/US1985/002108
Authority: WO
Inventors: Edwin M. Kellogg
Original assignee: Ion Beam Systems, Inc.
Priority date: 1984-10-26
Filing date: 1985-10-25
Publication date: 1986-05-09
Also published as: EP0198908A1; EP0198907A1; WO1986002774A1; EP0198908A4

Abstract

Accurately altering a precisely localized site on a substrate (35) by scanning the site with a focused ion beam (24) in the presence of a gas (hydrocarbon or hydrogen) at the site to form an opaque deposit at the site.

Description

FOCUSED SUBSTRATE ALTERATION

Background of the Invention

This invention relates to alteration of a precisely localized site on a substrate, for example, creating an opaque deposit to correct a transparent defect in a photolithographic mask.

Photolithographic masks are used to pass light (usually ultraviolet light) to a workpiece in a specified pattern. Such masks often consist of a clear substrate such as glass or quartz onto which a pattern of an opaque material such as chrome has been deposited. For various reasons, a mask may develop or be manufactured with small defects or imperfections, for example pinholes in the chrome layer, that allow exposure of the workpiece in undesired locations. Also, it may be desirable to alter the pattern on the mask, by rendering a previously transparent site opaque. Mask alteration should be effective and durable without complex processing steps that can introduce contaminants or cause further defects.

One way to repair mask imperfections involves a lift-off procedure using a positive photoresist. The resist is applied to the affected area, exposed, and developed, after which any resist that was applied to a transparent imperfection is removed. An opaque layer, e.g., of aluminum, is deposited over the area, and any of the deposited layer that overlies photoresist is lifted off using a solvent that dissolves the resist.

Laser beams are also used to repair photolithographic mask defects, particularly opaque defects, by removing the undesired opaque material. Gamo et al. (1984) Intern. Conf. on Solid State Devices and Materials (Kobe 9/1/84) and Gamo et al. (1984) Japanese J. Appl. Physics 23_(5.) :L293-L295 disclose the use of a focused or broad argon or gold ion beam in an atmosphere of tri ethyl aluminum. The resulting deposited film contains oxygen, carbon, and aluminum in varying ratios. The technique is reported to be promising for mask repair or mask fabrication for optical, ion, or X-ray lithography. It is further reported that inclusion of C and 0 in the film may be decreased by using other metallo-compounds (tungsten hexafluoride is suggested) and reducing the background pressure. Finally, the rate of film deposition reportedly is increased if the molecules resulting from the bombardment are not volatile.

Osias et al. (1984) Kodak Microelectronics Seminar, San Diego, October 29-30, Abstract, discloses mask repair using an ion flood beam to convert a previously applied layer of photoresist into a vitreous carbon film. "The resultant material is an excellent hard mask fabrication or repair material having scratch resistance and UV optical density comparable to that of chrome and having chemical resistance and substrate adhesion superior to that of chrome." The disclosed method involves wet processing of a negative photoresist to create a layer of resist at the site of transparent imperfections.

Kellogg, Ph.D. thesis. University of Pennsylvania, November 1965, discloses a method of making a self-supporting carbon target for helium bombardment by heating a nickel foil substrate in an atmosphere of methyl iodide to deposit a carbon film on the nickel. Hydrogen is released as a gas, and iodine is deposited as an amorphous layer on the walls of the chamber. The nickel is dissolved later to leave a carbon target.

Moller et al. (1981) Nuclear Instruments and Methods Vol. 182/183, pp. 297-302 discloses ion-induced carbon buildup*on a nickel surface in various

—6 hydrocarbon atmospheres at pressures in the 10

__7 millibar (7.5 x 10 torr) range; for example, unwanted buildup from vacuum pump oil occurs when performing ion implantation, ion beam analysis, or experimental nuclear physics. Methane, benzene, rough pump oil, and squalene were bombarded with ion beams of hydrogen, helium, and lithium ions at between 100 and

400 keV. The gases were provided as controlled atmospheres (10 -7 to 10-5 -mbar) by means of a needle ' valve or by means of a small liquid container installed in a vacuum chamber; the container temperature could be varied between -30°C and 40°C, depending on the gas.

The residual gas consists mainly of water, nitrogen, carbon dioxide, and argon. The amount of deposited material increases with increasing molecular weight of the gas and with dose rate (between 1.5 x 10 13 and 1 x 10 15.cm-2 seconds-1) . The authors conclude that deposition is controlled by ion-induced polymerization in an adsorbed layer.

Venkatesan et al., (1983) J. Applied Phys.

Letters 5_4_:3150-3153 disclose irradiating polymer films

(e.g., positive photoresists) with high electron or ion beam doses causing the resist layer to become conductive and to behave as a negative resist as a result of carbonization--i.e. , creation of a highly cross-linked carbon structure. The Raman spectra of such films resemble, but are consistently different from, the spectra of amorphous carbon films. Calcagno et al. , (1984) J. Applied Phys. Letters _4_4:761-763 disclose ion bombardment of frozen benzene to produce a stable polymeric film. The resulting film has a carbon-to-hydrogen ratio of between 1:2 and 1:3 as compared to 1:1 for benzene. ^! Summary of the Invention

In a first aspect, the invention features, generally, a method of accurately altering a precisely localized site on a substrate. The method comprises: (1) placing the substrate in a vacuum chamber and forming an image of the site; (2) by means of a liquid metal ion source and associated focusing optics, providing a focused ion beam having a spot size smaller than the site; (3) on the basis of that image, scanning the focused beam over the site in the presence of hydrocarbon gas in a manner to react the hydrocarbon and form a coherent carbonaceous opaque deposit of predetermined desired form, the deposit being adherent to the substrate at the site to render the site opaque.

In a second aspect, the invention features a method as described above in which hydrogen gas is used rather than a hydrocarbon gas in order to reduce a silicon compound in the substrate to form a coherent opaque silicon deposit of predetermined desired form, adherent to the substrate at the site.

In a third aspect, the invention features, generally, apparatus for accurately altering a site on a substrate, comprising: (1) a focusable ion source; (2) a lens positioned to focus ions emitted by the source, thus forming a beam; (3) a chamber adapted to receive the substrate and position it in the focused ion beam, the chamber comprising at least two baffles defining a series of adjacent chamber regions along the path of the ion beam beginning at the substrate, each baffle defining an opening positioned along the beam path to allow the beam to pass therethrough; (4) a gas inlet adapted to introduce into a first chamber region a gas for deposition;at the site, the first chamber region being the chamber region containing the site, and (5) an outlet from a second chamber region, the outlet communicating with a vacuum pump, the second chamber region being spaced upwardly along the beam path from the first chamber region, so that the pressure in the first chamber region is greater than the pressure in the second chamber region.

In preferred embodiments of the methods: the site is a transparent defect on a photolithographic mask requiring alteration and the scanning is performed to provide a deposit which renders the defect opaque; the gas is adsorbed at the substrate surface where the beam causes it to react; the mask substrate is comprised predominantly of compounds of silicon and oxygen and the method is conducted to form at the surface of the mask substrate an adherent, hard deposit comprised primarily of carbon; the hydrocarbon gas may be directed at the site in the form of a beam of negatively charged hydrocarbon molecules; the above-described baffles are included to maintain differential pressures of the hydrocarbon gas in the resulting chamber regions, the greatest pressure being maintained in the region containing the site; the ion beam energy is between about 25 and 70 keV (most preferably 30-50 keV) ; the image is formed by scanning the focused ion beam over the site without the presence of the hydrocarbon gas and by sensing charged particles such as ions or electrons emitted from the site; and the ion beam spot size is on the order of 1 micron or less; and to repair the defect, the ion beam is repeatedly scanned over the defect site to overcome localized depletion of the hydrocarbon. In preferred embodiments of the apparatus: the apparatus includes a means for forming an image of the site to be altered (e.g., a detector that provides a signal representative of charged particles such as electrons or ions emitted in response to bombardment of the mask) , means for positioning the site in the ion beam path, and a beam deflector for repeatedly scanning the beam in- a scan pattern over the site area on the basis of that image; the ion source is a liquid metal ion source; a source of hydrocarbon gas communicates with the gas inlet; each of the baffles comprises a horizontal member defining the opening for the beam; and the horizontal member of the first baffle is spaced below the horizontal member of the second baffle a distance at least two (most preferably four) times the diameter of either of the openings in the baffles.

The invention provides a coherent, adherent, and durable opaque coating layer specifically and accurately positioned at the site that is to be altered, e.g., the transparent defect, without the need for wet processing steps that can create particulates and damage mask surfaces. The focused ion beam provides accuracy and resolution to control the deposition of the layer. The layer provides good adherence to both the mask substrate and the mask opaque layer surrounding the site. The layer is sufficiently hard or resistant to withstand abrasion encountered in the course of ordinary photolithographic processing. Potentially troublesome materials that could contaminate extremely pure semiconductor workpieces are avoided because non-carbonaceous products preferably are removed as a gas. Dangerous or poisonous materials can also be avoided; or, if they are used, such materials can be better controlled. Using the apparatus and method, the rate of. eposit of the material from the gas can be high enough to overcome the rate of loss of substrate from ion beam induced sputtering; thus, the beam does not cause a net loss of mask material, in part because loss of material, if any, from sputtering can be more than compensated by a high deposition rate achieved, e.g., by locally increased hydrocarbon gas concentrations or by use of a relatively high molecular weight gas.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiment thereof and from the claims.

Description of the Preferred Embodiment

Fig. 1 is a highly diagrammatic sectional representation of apparatus for repairing a mask.

Fig. 2 is a greatly enlarged diagrammatic representation of the mask repair process.

Fig. 3 is a sectional, diagrammatic view of a portion of one embodiment of apparatus for repairing a mask.

Fig. 4 is a sectional, diagrammatic view of another embodiment of apparatus for repairing a mask.

In Fig. 1, apparatus 5 for repairing a photolithographic mask includes, in general, ion beam source 10 and beam focusing column 20 arranged to deliver a focused ion beam 24 to vacuum chamber 30 through opening 32. ithin chamber 30 is an X-Y stage table 40 adapted to hold mask 35 in the path of ion beam 24. A gas delivery system includes an inlet tube 50 (broken away in Fig. 1) extending through the wall of chamber 30. A vacuum pump 60 communicates with chamber 30 through port 62 to evacuate the chamber. These various components of the mask repair apparatus and the functioning of that apparatus are discussed in greater detail below. Ion source 10 is a liquid metal ion source for generating a stable focusable ion beam. For example, suitable ion sources are disclosed in Clampitt et al. , U.S. Patent 4,088,919, Jergenson, U.S. Patent 4,318,029, and Jergenson, U.S.^' Patent 4,318,030, each of which is hereby incorporated by reference. The source includes a tip 12 from which metal ions are emitted under the action of extractor electrode 14 in a continuous and stable beam that can be focused by column 20. Electrode 14 is insulated from tip 12 by electrical insulation 13.

Focusing -column 20 includes lens 22 to deliver a focused^•ion beam through opening 32 in chamber 30. Specifically, an axial electric field between conductive elements 28 and 29 creates, an electrostatic immersion lens 22. Insulation 85 electrically insulates chamber 30 from focusing column 20. A suitable focusing column is described in Wang et al. J. Vac. Sci. Tech. 19(4) :1158-1163 (1981) which is hereby incorporated by reference. Ion source 10 and column 20 are capable of delivering a high-energy focused liquid-metal ion beam

24, e.g., a gallium or gold ion beam of densities of at least 10 -3 amp/cm2 and up to about 1 amp/cm2 over a spot size that provides the desired accuracy and resolution to control the carbon deposited, e.g. a spot size of 0.1 to 1.0 microns (micrometers). This is in contrast to non-liquid-metal ion sources which generally do not provide dose rates in excess of 3 x 10 -5 amps

_2 cm . Ion beam energy should be great enough to enable focusing (at least 20 keV) . A preferred range of operation is 25 to 70 keV, most preferably 25 - 50 keV. Advantageously, such energies are satisfactory for removing opaque defects, so that both types of defects can be repaired by the same piece of apparatus. By providing sufficiently high local gas concentrations, it is possible to achieve a deposition rate that exceeds the rate of material loss due to ion beam sputtering, and thus to provide a coherent opaque deposit that is carefully limited to the site to be altered, e.g., the defect site. The use of a focused liquid metal ion beam, in combination with the hydrocarbon selection and the chamber design as described below are particularly advantageous in this regard.

An important aspect of the repair apparatus 5 is the ability to control the point on the mask that is bombarded by the ion beam and to ensure that the resulting deposit completely covers the mask defect. To achieve this, the apparatus includes an X-Y stage table 40 controlled by stage drive motor 42 connected thereto by a transmission shaft 44 through a sealed bearing in the wall of chamber 30. Alternatively, the motor may be positioned inside the vacuum chamber. Motor 42 is controlled by stage drive electronics 45, which in turn are connected to computer pattern generator and image display 46. Display 46 is supplied by electron detector 48 with a signal representative of electrons generated by the collision of beam 24 with the mask to indicate the position of the beam on the mask as described in more detail below. Alternatively, detector 48 can be used to detect ions emitted from the mask during beam bombardment. An electron source (not shown) also can be provided to neutralize charge build-up during ion bombardment for imaging.

Apparatus 5 also includes a means for controlling the beam direction. Specifically, an electronic beam deflector 26 is positioned inside chamber 30 at opening 32 to enable deflection of the beam according to a desired pattern as described below. The gas delivery system includes a pressurized gas bottle 52, a regulator 54 and micro-leak valve 56 to deliver extremely low gas pressures (e.g. between about

—2 —6 10 and 10 torr) to inlet tube 50. Specifically, this gas pressure should be low enough to avoid destabilizing the ion source or dispersing the beam as it travels through the vacuum chamber; at the same time, the pressure should be high enough to provide a deposition rate that overcomes sputtering.

To repair a mask such as chrome/glass mask 35, the mask is placed on X-Y table 40 and chamber 30 is pumped down e.g., to 3 x 10 torr. Source 10 and column 20 are activated to direct a focused gallium ion beam 24 at mask 35. When'beam 24 strikes mask 35, electrons are emitted and detected by detector 48 (for example, a channel electron multiplier such as one manufactured by Galileo Electrooptics Corp., Sturbridge Mass.), generating a signal to computer display 46 to generate a display in much the same manner as occurs in an electron microscope display. The signal generated will depend upon the characteristics of the specific region of mask 35 under bombardment by beam 24. Thus it is possible, by means of feedback from computer display 46 and knowledge of the desired mask pattern, to use electronics 45 to move table 40 to position a mask defect in the path of beam 24, with an accuracy of + 0.05 microns.

Once beam 24 is generally centered in the mask defect, hydrocarbon gas from bottle 52 is introduced into the defect region in a manner described in more detail below. Since the size of a pin-hole defect may be upwards from about 0.25 microns to about 100 microns or more, which is much greater (e.g. over 10 times greater) than the beam spot size, beam deflector 26 is activated by computer display 46 to scan the beam to cover the entire defect. For example, a raster scan can be generated to cover a polygon that covers the entire site with some overlap of the chrome layer. When repairing the- defect, carbon is locally deposited as described elsewhere herein, momentarily depleating the hydrocarbon supply at the beam spot. By repeatedly scanning the beam over a defect site, the beam spot is continually being moved to a location that has a replenished hydrocarbon supply, and the beam can be repeatedly scanned over the defect at a rate that allows hydrocarbon to replenish before the beam spot returns to a given location in the defect site.

Fig. 2 depicts in a very diagrammatic way the interaction between beam 24, the hydrocarbon gas, and mask 35. Specifically, at the left side of the figure, the chrome layer 81 on mask 35 ends at the perimeter of a transparent defect that is being repaired. The far side of that defect is not shown. Beam 24, represented by broken lines, is scanning from left to right. Beam 24 is between 0.1 and 10 microns in diameter. As a result of beam particle energy, hydrogen-carbon bonds are broken. Hydrogen gas is formed and carbon atoms combine either with each other and preferably with atoms or compounds in the mask. Specifically, the carbon atoms may combine with the silicon of the silicon dioxide in the glass or quartz, releasing oxygen gas and forming a silicon carbide transition layer. On top of that transition layer, a tenacious, hard, opaque carbon layer is deposited, probably in amorphous (vitreous) form, and hydrogen gas is released.

In order to maintain a relatively high pressure of hydrocarbon gas while at the same time avoiding unstable operation of the focussing column, or beam dispersal as the beam travels through chamber 30, it is desirable to maintain a very low gas pressure in chamber 30 while maintaining a relatively higher gas pressure in the region immediately adjacent the defect. Apparatus for accomplishing these goals is shown in Figs. 3 and 4 which depict features that can be combined with each other, or can be used individually, with the apparatus diagrammed in Fig. 1.

The choice of hydrocarbon gas for any particular application depends inter alia upon the particular deposition apparatus employed, and upon the requirements for thin layer adsorption of hydrocarbon monolayers at the substrate surface site. To provide sufficient hydrocarbon concentration at the surface, it is generally desirable to use a hydrocarbon whose vapor pressure is high enough to condense upon the substrate under the temperature and pressure conditions prevailing at the surface. In certain cases, it is further desirable to provide relatively high gas pressure in the gas delivery tube.. In such cases, the hydrocarbon should be selected to have a vapor pressure high enough to avoid condensation under delivery tube conditions. Heat may be used to avoid such condensation; if heat is used, in certain instances a means of cooling may be employed to enhance condensation at the site. For example, in certain instances a cooling heat exchanger is employed in the region of the directed gas outlet of the delivery tube. In another case, the flow system and directed gas outlet are constructed and arranged to provide substantial cooling by adiabatic expansion. In still other cases, the substrate is cooled by supporting it on a cooled surface, with a gas confined at the interface between the substrate and support, at a subatmospheric pressure greater than that of the surrounding vacuum chamber to provide a conductive medium that enables effective heat transfer from the substrate to the cooled support.

In Fig. 3, mask 35 is surrounded by a plurality of rectangular ^'baffles 37, 38, and 39, each of which has a central opening 33 and is aligned with respect to beam 24 and mask 35 to allow the beam to bombard the mask. The outer baffles 37 and 38 define respectively, two chambers 70 and 72, each communicating with an outlet 64 to a vacuum pump. The inner baffle defines a chamber 74 over the operative site of mask 35. Gas inlet 53 provides hydrocarbon gas to chamber 74. At the junction between the baffles and the mask 35 are graded differential pressure seals 84 that allow horizontal movement of the mask relative to the nest of baffles, while maintaining a pressure seal.

The gas pressure is greatest in inner chamber 74 and lowest in outer chamber 70 so that some gas flow occurs from chamber 74, through chambers 72 and 70, to chamber 30. This gas flow should be minimized to maintain a relatively high pressure in the region immediately adjacent the mask without causing excessive beam scattering from gas molecules in the beam path in chamber 30.

Due to the low pressures involved, the flow from chamber to chamber will be, in effect, close to a molecular straight-line flow so that the angular spread alpha at an orifice will be approximately 30°. By arranging the spacing between baffles to be at least 3-4 times the diameter d of the central openings 33, the predominant gas flow through an orifice will be deflected by the next baffle. Thus it is possible to maintain pressures in chamber 30 which are low enough (about 3 x 10 -5 torr) to avoid excessive beam scattering and unstable column operation, while the

_2 pressure in chamber 74 is high enough (about 3 x 10 torr) to allow significant carbon deposition. The diameter of openings 33 should be established to allow satisfactory scanning of the defect site. If the defect encountered is 'likely to be about 20 microns, for example, openings 33 should be about 2 1/2 times that distance or 50 microns.

Fig. 4 shows an alternate embodiment that uses a carefully positioned gas inlet 55 (e.g. a hypodermic needle or other small diameter tubing) angled slightly to direct the gas flow to the spot of impact of beam 24 on mask 35. As noted above, the molecular gas flow will exit inlet 55 at a sprea 'angle of 30°. Inlet 55 is spaced a distance f above mask 35 on the order of (most preferably approximately equal to) the diameter of the opening of inlet 55, which has a diameter of between

125 u and 250 μ. Distance f may be adjusted to optimize deposition, and it is preferably between 0.02 and 0.2 mm. The distance can be up to about 1 mm. Gas pressure

-2 at the sample is about 3 x 10 torr. In that way, the gas concentration at the surface of the mask site is enhanced to allow deposition to overcome sputtering; at the same time the gas is localized so that it does not cause excessive scattering of the ion beam. To further contain the gas, the substrate may be contained in a differentially pumped chamber having an opening designed to transmit the ion beam.

Other embodiments are within the following claims. Gases other than hydrocarbons can be used in the featured apparatus. For example, dangerous or poisonous gases can be used with the apparatus of Fig. 3 and such gases will be trapped largely in the outflow of outlets 64, thus allowing improved control of the gas. The hydrocarbon gas may be introduced as negatively charged hydrocarbon molecules. Hydrogen gas may be used to reduce the silicon dioxide in the mask substrate, forming an opaque silicon deposit and water vapor. The differentially -pumped chambers may be arranged in other ways; for example, all of the chambers may enclose the substrate entirely, so that there is no seal between the substrate and the chamber walls.

Claims

1. A method of accurately altering a precisely localized site on a substrate (35) characterized in that said method comprises placing said substrate in a vacuum chamber and forming an image of said site, by means of a liquid metal ion source (10) and associated focusing optics (20) , providing a focused ion beam (24) having a spot size smaller than said defect, on the basis of said image, scanning said focused beam over said site in the presence of hydrocarbon gas in a manner to react hydrocarbon and form a coherent carbonaceous opaque deposit of predetermined desired form, adherent to said substrate at said site to render said site opaque.

2. The method of claim 1 characterized in that said gas is adsorbed at the substrate where said beam causes it to react.

3. The method of claim 1 characterized in that said site requiring alteration is a transparent defect in a pattern of a photolithographic mask and said scanning is performed to provide a deposit which renders said defect opaque to photolithographic light.

4. The method of claim 1 characterized in that said mask substrate is comprised predominantly of compounds of silicon and oxygen and said method is conducted to form at the surface of said mask substrate an adherent, hard deposit comprised primarily of carbon.

5. The method of claim 1 characterized in that said hydrocarbon gas is directed at said site in the form of a beam of negatively charged hydrocarbon molecules. ' ^*

6. The method of claim 1 characterized in that said method includes providing baffles (37, 38, 39) defining a series of adjacent chamber regions (70, 72, 74) along the path of said focused ion beam beginning at said mask, and said method comprises maintaining differential pressures of said hydrocarbon gas in said chamber regions, the greatest pressure being maintained in the region containing said site.

7. The method of claim 1 characterized in that said ion beam energy is between about 25 and 70 keV.

8. The method of claim 1 characterized in that said image is formed by scanning said focused ion beam over said site without the presence of said hydrocarbon gas and sensing charged particles emitted from said site.

9. The method of claim 1 characterized in that said ion beam spot size is on the order of 1 micron or less.

10. The method of claim 1 wherein said scanning of said beam over said site is performed repeatedly to overcome localized depletion of said hydrocarbon.

11. • Apparatus for accurately altering a site on a substrate (35) , characterized in that said apparatus comprises a focusable ion source (10) a lens (22) positioned to focus ions emitted by said source, said focused ions forming a beam (24) , a chamber (30) adapted to receive said mask and position it in said focused ion beam, said chamber comprising at least two baffles (38, 39) defining a series of adjacent chamber regions (72, 74) along the path of said ion beam beginning at said mask, each said baffle defining an opening (33) positioned along said beam path to allow said beam to pass therethrough, a gas inlet (53) adapted to introduce into a first chamber region (74) a gas for deposition at said site, said first chamber region containing said site, an outlet (64) from a second chamber region (72) , said outlet communicating with a vacuum pump, said second chamber region being spaced upwardly along said beam path from said first chamber region, the pressure in said first chamber region thereby being relatively greater than the pressure in said second chamber region.

12. The apparatus of claim 11 characterized in that said first baffle (39) and said second baffle (38) each comprises a horizontal member defining said opening (33), said horizontal member of said first baffle being spaced a distance below said horizontal member of said second baffle, said distance being at least two times the diameter of either of said openings in said baffles.

13. The apparatus of claim 11 characterized in that said apparatus comprises means (46) for forming an image of said site and means (40) for positioning said site in said beam path, and a beam deflector (26) for scanning said beam in a scan pattern over the area of said site on the basis of said image.

14. The apparatus of claim 11 characterized in that said beam deflector is adapted to scan said defect site repeatedly to overcome localized depletion of the gas.

15. The apparatus of claim 11 characterized in that said apparatus further comprises a source (52) of hydrocarbon gas communicating with said gas inlet (53, 55).

16. The apparatus of claim 11 characterized in that said apparatus further comprises a means (46) for imaging said site comprising a detector (48) providing a signal representative of charged particles emitted in response to bombardment of said mask by said beam.

17. The apparatus of claim 11 characterized in that said ion source (10) is a liquid metal ion source.

18. The apparatus of claim 11 characterized in that said apparatus comprises a source (32) of hydrogen gas, and said substrate comprises a silicon oxide compound, whereby said focused beam interacts with said gas and said substrate to form a deposit of metallic silicon at said site.

19. The apparatus of claim 11 wherein said substrate is a photolithographic mask, and said site is a transparent defect on said mask.

20. A method of altering a precisely localized site on a substrate (35) comprising a silicon oxide, said method comprising placing said substrate in a vacuum chamber and forming an image of said site, by means of a liquid metal ion source (10) and associated focusing optics (20) , providing a focused ion beam (24) having a spot size smaller than said defect, on the basis of said image, scanning the focused ion beam in the presence of hydrogen gas in a manner to reduce said silicon oxide and form an opaque silicon deposit adherent to said substrate at said site to render said site opaque.