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 pin oles 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 chirable 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 trimethyl 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 hydrocarbon atmospheres at pressures in the 10 —6

-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

The invention features apparatus and methods for alteration of a precisely located site on a substrate. The apparatus comprises: a) a focusable ion source; b) a lens positioned to focus ions emitted by the source into an ion beam; c) a vacuum chamber for containing ^" the substrate site in the path of the ion beam; and d) a directed gas inlet positioned to provide a localized supply of a substance at the site whereby the beam interacts with the substance to cause the alteration localized at the site.

In preferred embodiments, the substance is supplied as a gas that is adsorbed at the substrate site; the substance comprises a hydrocarbon, and the resulting deposit is a coherent carbonaceous deposit.

Alternatively, the substance comprises an organometallic compound (e.g. Al (CH ₃ ) ₃ o.r WF _g ) . Also alternatively, the substance comprises an agent (e.g.

H ₂ ) that reduces the substrate material (e.g. Si0 ₂ ) opacifying the site. The directed gas inlet is preferably a jet formed of small diameter tubing that is angled with respect to vertical and ends a predetermined distance above the substrate, e.g. a distance on the order of the tubing diameter. The equivalent gas pressure at the site during deposition is about 3 x

-2 10 torr; the beam energy is preferably between 25 and 70 keV (most preferably 30-50 keV) ; the beam spot

size is on the order of 1 micron or less; an 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; 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 substance.

The invention is particularly useful for repairing a defect site in a photolithographic mask, preferably by opacifying a transparent defect. The invention is also useful for dual function substrate alteration--!.e. , apparatus that also can remove material, e.g. by sputtering, when an opaque defect is detected. This is so because localization of the gas used for opaσification enables faster gas removal necessary for the material removal reqime. When repairing a defect in a photolithographic mask, the substrate can be imaged and the defect positioned in the beam path by scanning the substrate with the beam without the presence of the gas, and detecting the resulting charged particle emissions using a particle detector.

The use of the directed gas inlet provides local gas pressure high enough to enable deposition or creation of a thin film adsorbed at the substrate site, without excessively restricting the area that can be swept by the ion beam to scan the workpiece, e.g. by scanning the workpiece in a vacuum and detecting charged particle emissions. In short, by using the jet the rate of deposit of material from the gas can be high enough (due to the enhanced local gas concentration) to overcome the rate of loss of material from ion beam induced sputtering.

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 partially sectional, diagrammatic view of the jet and adjacent portions of the 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. Wrthin' 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 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. A range of 30 keV - 50 keV is most preferable. 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 below-described jet feature that enhances gas concentration at the defect site is 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 leading to a jet described below in connection with Fig. 3.

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 -5 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, 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, and one possible gas (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 particles beam 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 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 Fig. 3.

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.

Fig. 3 shows 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 spread angle of 30°. Inlet 55 is spaced a distance f above mask 35 on the order of (most preferably about equal to) the diameter of the opening of inlet 55 which has a

diameter of between 125 y 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 at the sample is about 3 x

_2 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 those listed above can be used in the featured apparatus. The gas can be hydrogen to reduce silicon oxide in the substrate to opaque, metallic silicon.