WO2002095787A2

WO2002095787A2 - Anti-charging layer for beam lithography and mask fabrication

Info

Publication number: WO2002095787A2
Application number: PCT/US2002/003885
Authority: WO
Inventors: Elizabeth Dobisz; Walter J. Dressick; Susan L. Brandow; Mu-San Chen
Original assignee: The United States Of America, As Represented By The Secretary Of The Navy Naval Research Laboratory
Priority date: 2001-05-25
Filing date: 2002-02-08
Publication date: 2002-11-28
Also published as: US6586158B2; US20030203311A1; US6773865B2; US20020177083A1; WO2002095787A3; AU2002321990A1

Abstract

This invention discloses an anti-charging layer for beam lithography and mask fabrication. This invention reduces beam displacement and increases pattern placement accuracy. The process will be used in the beam fabrication of high-resolution lithographic masks as well as beam direct write lithography of electronic devices. The anti-charging layer is formed by the use of metal films bound to metal ligating self-ssembled monolayers (SAMs) as discharge layers.

Description

ANTI-CHARGING LAYER FOR BEAM LITHOGRAPHY AND MASK FABRICATION

Technical Field

The present invention relates to beam lithography More specifically, the present invention relates to procedures and materials for the reduction of charging during e-beam, ion beam, and X-ray lithography

Background Art

Electron-beam lithography is a process whereby a pattern is written or printed into an imaging layer on a substrate using an electron exposure source The substrate may be a surface on which a device or material will be directly patterned or it may be a surface which will eventually be used as a mask or mold in a subsequent patterning procedure The susceptibility of the imaging layer to electron irradiation allows the chemistry and/or surface energy of the layer to be modified upon electron exposure Selective exposure of the imaging layer by scanning the e-beam can be used to create a patterned chemical template on the surface Typically the imaging layer is a polymer, known as a resist, whose chemistry is altered by electron exposure The chemical change leads to a change in solubility of the polymer in a particular solution, known as the developer This allows for selective removal of the irradiated (or unirradiated) regions of the layer The pattern may be transferred into the underlying substrate by a variety of processes such as, but not limited to, selective etching or materials deposition on or in the imaging layer pattern This process is routinely used to fabricate electronic devices and lithographic masks Other U S applications include, for example, the fabrication of encryption messages on surfaces The treasury department has investigated the possibility of wπting e-beam patterns on currency to reduce the ease of counterfeiting It is intended that the invention cover these novel applications as well as the more common applications in mask making, semiconductor device manufacture, electro-optic device manufacturing, micro-electromechanical (MEMs) device manufacturing, system on a chip (SOC), and data storage

In e-beam lithography, charging of the substrate-resist system can occur The resulting electric field can deflect the e-beam causing pattern placement and critical dimension (CD) error The international Semiconductor Industry Association (SIA) has constructed a roadmap for projected specifications for semiconductor devices, mandated by economic drivers By the end of the next decade a pattern placement accuracy of <20 nm and a CD tolerance of < 5 nm will be required Other industries, such as data storage, are on similar roadmaps Consequently, pattern placement errors due to charging in e-beam lithography are becoming an increasingly important problem

Charging, in e-beam lithography, originates when the number of electrons entering a region of a material does not equal the number of electrons exiting that region of material Conditions encountered in e-beam lithography typically fall into one of the following three categories I) lithography on an insulating substrate, II) lithography of an insulating layer on a conducting substrate with no or only partial penetration of the beam to the substrate, and πi) lithography of an insulating layer on a conducting substrate, with full penetration of the beam to the conducting substrate These cases are illustrated in FIGS la - lc, respectively As shown in FIG la, under electron irradiation 10 ot an insulator 12, a charge density builds until eventually the resulting voltage exceeds the dielectric breakdown of the material and media between the material and ground At this point, discharge occurs via conduction to the ground This form of charge buildup and dissipation in e-beam lithography is highly undesirable Very large electric fields are attained and the fields change unpredictably with time For example, under these conditions we have measured voltages in excess of 100 V In reality, the dielectric breakdown is determined by poorly defined and uncontrolled defects in the materials system In Case III, the beam 10 penetrates the insulating layer 12 to the conducting substrate 16 Here, a small surface potential can develop due to the emission of secondary electrons 18 from the surface Under these conditions we have measured surface potentials of magnitudes 2-10 V In between the two extremes is Case II, that of an insulating layer on a conducting substrate with no or partial penetration of the insulating layer by the electron beam If there is no penetration of the beam to the conducting substrate then the results are equivalent to those of Case I If there is partial penetration of the beam we have found that electron-beam induced current (EBIC) effects are substantial Penetration of as little as 25% of the beam to the conductor can lead to sufficient conductivity to dissipate any building charge, in which case the results are equivalent to Case III

The same charging concerns are present for ion beam and X-ray lithography

An important technological example of Case I is encountered in the manufacture of phase shift masks and optical devices on glass or quartz substrates An example of Case II is encountered in low voltage e-beam lithography, such as with the low voltage micro-machined e-beam column arrays currently under development at ETEC, Inc Most often, one uses a bilayer resist scheme for patterning in which the beam penetrates the top resist layer and the developed pattern is etched into an underlying polymer or dielectric layer Case III is typically encountered with the low voltage e-beam lithography of thin, single layer resist systems, and higher e- beam voltage machines The deflection of the e-beam due to sample charging will be most pronounced with the lower voltage and or greater working distance e-beam instruments

Disclosure of Invention

Accordingly, it is an object of the present invention to provide a method of making an anti-charging layer for beam lithography (including electron beam, ion beam, and X-ray lithography) and mask fabrication

Additional objects and advantages of the invention will be set forth in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention Objects of the present invention are achieved by applying a metal ligating self-assembled monolayer to a substrate, binding a metal to the ligand site of the metal ligating self-assembled monolayer and applying a resist to the metal

According to the present invention, there is also provided a method for forming anti-charging layers for electron beam lithography by applying a resist to a substrate, applying a metal ligating self-assembled monolayer to the resist, and binding a metal to the ligand site of the self-assembled monolayer

According to the present invention, there is also provided a method for forming anti-charging layers for electron beam lithography by applying a metal ligating self-assembled monolayer (SAM) to a substrate, binding a metal to this SAM, applying a second SAM to this metal, and binding a second metal to this second SAM

According to the present invention, there is also provided an improved method for conducting electron lithography by (a) forming one or more anti-charging layers above or below a resist by binding a metal to a SAM, (b) grounding the anti-charging layers, (c) exposing the resist with an electron beam, and (d) developing the exposed resist

The technique can be carried out on a non-patterned surface, a patterned surface, or a planarizer The resist can be a self-assembled monolayer film or a composite self-assembled monolayer film

Bπef Description of Drawings

FIGS la through lc are a collection of schematic diagrams of conditions that produce charging problems in e- bea lithography

FIG 2 is a collection of schematic diagrams of the use of a S AM-metal layer to reduce charging in e-beam lithography

FIGS 3-4 are schematics showing the successive stages of using a SAM-metal layer to reduce charging in e- beam lithography using electroless plating of metal on SAM on bare substrate or polymer prior to photoresist coating

FIGS 5-6 are schematics showing the successive stages of using a SAM-metal layer to reduce charging in e- beam lithography using a substrate with SAM/metal discharge layer on top of the resist

FIG 7 is a graph showing the relationship between copper film thickness and plating and the relationship between resistivity and plating time

Modes for Carrying Out the Invention

The invention is based upon the use of ultrathin grounded conducting layers to act as both discharge layers and as a shield from electric fields that may exist beneath the imaging layer within the sample, since the presence of an electric field in a conductor is forbidden by Maxwell's equations The invention uses ultrathin metal films 24 bound to metal ligating self-assembled monolayers 26 (SAMs) as discharge layers The invention includes both materials and processes useful for the fabrication of the discharge layers A key to the invention is the use of bifunctional molecules, in which one functional group is capable of binding to the resist or substrate and a second functional group is capable of binding metal or metal ions The molecule can bind to the substrate via either physisorption or chemisorption The bifunctional molecules typically are organosilanes possessing alkoxysilane or halosilane groups capable of binding to surface hydroxyl sites on the substrate 20 or resist 22 and an organofunctional group containing a ligand, such as a primary amine or pyπdyl group, capable of binding a metal 24 or metal ion That is, they are usually materials of the general formula RSιR'_nX_{3 n} (where R is an organofunctional group having a metal ligating site, R' is typically a small alkyl group such as the methyl or ethyl group or a simple aromatic group such as the phenyl group, X is an alkoxide such as methoxide or a hahde group such as chloride or bromide, and n is zero, 1, or 2) The organosiloxane SAM 26 can be deposited onto a variety of surfaces by either wet chemical dipping or spincasting from organosilane solution Subsequent binding of an electroless Pd/Sn catalyst species to the ligand site of the SAM, followed by immersion of the substrate in an electroless metal bath, deposits an ultrathin layer of conductive metal 24 on the substrate 20 Various conducting metals, such as Cu, Co, Au, and Ni, among others, can be deposited in this manner with thicknesses controlled by the metallization conditions (i e , concentration, time, and temperature) Other methods that are currently employed to reduce or eliminate charging involve the vacuum deposition of metal films or spincasting of conducting polymers The advantages of SAM assisted metal attachment over the current schemes is discussed below

Several schemes by which metal-bindmg SAMs 26 can be used to reduce or eliminate charging effects in e- beam lithography are shown in FIGS 2a-2d The substrate can be a non-patterned 19 surface (FIG 2a), a patterned surface 20 (FIGS 2b-2d), or a surface with a planarizer 23 (FIGS 2e-2g) In all cases the metal layers are grounded during e-beam lithography The SAM/metal layer 26 can be placed directly on an insulating substrate 19 (FIG 2a) or on an insulating substrate containing a pattern 20 (FIG 2b), e g patterned Cr on a quartz mask The scheme of FIGS 2a-2b reduces the severe Case I charging to the minimal charging of case III In FIG 2c, the SAM/metal layer 26 is placed on top of the resist 22 Because the electπc field must be zero in the conducting layer, no electric field extends from the sample surface to deflect the electron beam In FIG 2d, two SAM/metal layers 26 are employed, one at the resist 22 top surface and one on the substrate 20 surface The layer 26 that is on the resist 22 surface once again prohibits the formation of an electπc field arising from substrate 20 effects. The layer 26 at the substrate facilitates imaging the substrate 20 surface for alignment purposes. In all cases the SAM-metal layer 26 follows the topography of the surface. Note that a SAM film or a composite SAM/physisorbed ligand film can also be used as a resist, provided that the film's chemical functionality can be modified by exposure (in this case, by an e-beam). This is illustrated in FIG. 2g, where a second SAM 30 is coated over the first metal layer 24 and lithographically exposed. The second metal 32 binds to the top SAM 30 selectively in the exposed pattern. In this case, the second metal 32 is chosen to have different etch characteristics than the first to facilitate etching and pattern transfer. An example of such a bilayer scheme would be a patterned Ni, Au, or Cr coated SAM layer on top of a Cu-SAM layer.

The invention described herein offers several advantages compared to the current state of the art. Currently used methods for reducing charging include the vacuum deposition of a metal film on the surface or the spincoating of a conducting polymer, such as polyaniline, on top of the resist. The first technique adds a process step that is too time intensive for a viable manufacturing process. The process requires that the workpiece be placed in a vacuum system, the system evacuated, and metal deposited. The process is slow, requires a major piece of equipment in the clean room. In addition, it is difficult to consistently deposit thin, continuous metal films using vacuum deposition methods. The techniques described in this disclosure utilize low-cost wet chemical processing involving simple dipcoating or spincoating steps for application of the charge dissipation layer. The use of electroless plating without the metal-binding organosiloxane film offers similar cost advantages to our method. However, metal films produced in this manner are not sufficiently continuous to function as effective charge dissipaters unless their thickness exceeds -100 nm. Metal films of this thickness are of limited use as charge dissipaters because they facilitate excessive scattering of the e-beam, compromising patterning and measurement of alignment marks.

Spincast polyaniline films also require thick films to achieve sufficient conductivity to dissipate charge and are therefore similarly undesirable. The requirement that thick conducting polymer films be used to sufficiently dissipate charge also limits the ability of an operator to view substrate alignment marks during patterning. In addition, outgassing of the dopant required to maintain conductivity of the polyaniline in the vacuum system during patterning severely limits the effectiveness of this material. Finally, polyaniline exhibits sufficient water solubility such that, if used as a layer under the photoresist, it could promote photoresist delamination and liftoff during pattern development. Consequently, the disclosed invention offers a unique, low-cost means for controlling workpiece charging that is unmatched by current methods and meets the needs of lithography practitioners.

Persons of ordinary skill in the art know that SAMs may by deposited by either chemisorption or physisorption. See generally, Brandow et al, copending application serial number 09/339,917. In some instances, a surface to which a SAM is to be deposited will require pretreatment to have the prefeπed surface chemistry for SAM deposition Typically, one may create surface OH groups on the substrate surface by 0₂ plasma etching the surface briefly Most resists and planaπzers do not have a sufficient level of surface OH to chemisorb a good ligand organosiloxane film The ligand organosiloxane can then be chemisorbed onto a substrate to create a ligand SAM (self-assembled monolayer) film A Pd catalyst can then be bound to the ligand The binding is covalent The Pd catalyst is capable of initiating and sustaining electroless metal deposition The catalyzed SAM can then be treated with an electroless metal bath to deposit an ultrathin metal film onto the surface as the anti-charging layer

A prefeπed method for the chemisorption of the ligand SAM follows First, the nature of the precursor monomer for the ligand SAM is RSιR'_nX_{3 n}, where R is a ligand group, R' is an alkyl or aromatic hydrocarbon group, X is an alkoxide or hahde group, and n is 0, 1, or 2 The ligand group R is a group that can covalently bind the catalyst Pd species R is therefore alkylamine, pyπdine, diphenylphosphino, etc

A SAM need not be an organosilane It can also be a polymer film having pyridyl, etc ligand groups as part of the polymer structure In such a case, the substrate can be coated directly with a ligand polymer film The ligand can also be a polymer ligand such as pyπdine, dimethylamine, etc that is physisorbed onto/into the substrate such that the physisorbed ligand retains its ability to covalently bind the Pd catalyst species

The method of applying the metal ligating SAM to a substrate can vary For example, spincoating can be used (as with an organosilane or ligand polymer) Dipcoating can also be used (as for an organosilane or a monomeπc ligand for physisorption)

The ligand surface can be applied to or contacted with Pd" salts or Pd° mateπals It is usually important that sufficient Pd" or Pd° is bound to initiate and sustain electroless metal deposition and that Pd" or Pd° is bound in a sufficiently homogenous fashion to ensure deposition of an ultrathin conductive metal film

The preferred catalyst systems are commercial Pd/Sn colloids Such colloids usually require several processing steps prior to metallization Specifically for Shipley Co Cataposit 44 Pd/Sn catalyst treat the substrate with Pd/Sn catalyst for about 5 mins, rinse with 0 1M HC1 three times, apply Shipley accelerator 19 solution for 90 sees (about 10% strength), rinse once with H₂0, and then plate with a metal bath

For the metal deposition, any of several metals can be used including Co, Cu, Ni, or Au Copper metal is preferred due to the combination of low cost, good electrical conductivity, and ease of removal by acid etching Prefeπed m combination with the Shipley Co Cuposit 328 metal bath is the Pd/Sn catalyst Metallization using physisorbed hgands is slightly different than using chemisorbed ligand organosilanes as films for catalyst binding There are two prefeπed pathways In the first, organosiloxane SAMs are used, however, the SAM is not a ligand Its R group is a simple alkyl or aromatic species A solvent is chosen that is geometrically matched to the R group of the silane for deposition For aromatic R groups, aromatic solvents like toluene are used For aliphatic R groups, a long chain alkyl solvent like dodecane is used It is known that use of geometrically matched solvents leads to less dense packing of the SAM R groups during the chemisorption process The "spaces" or "voids" between adjacent R groups in the SAM can physisorb similarly matched hgands For example, voids in cast octadecystloxane SAMs prepared using hexadecane solvent can bind n-hexadecylamine ligand via ligand insertion The amine group of the bound ligand can bind catalyst and give metal deposition A similar situation occurs using aromatic organosiloxane SAMs with pyπdine or other aromatic hgands

A second path uses spincoated polymers for physisorption This is similar as for non-hgand organosiloxane SAMs For example, spincoating the non-hgand polymer polyvinylbenzyl chloride from the aromatically matched solvent, toluene, creates loosely packed polymer film having voids at the surface (templated by toluene) The aromatic ligand, pyπdine, can be physisorbed into these voids and thereby metallize the polymer surface

Once the ligand surface is prepared, catalysis and metallization proceed as per the steps described above for binding a Pd catalyst and treating the catalyzed SAM with an electroless metal bath

If a polymer contains Pd-binding hgands like pyπdine, it can be spincoated as a planaπzing layer or conformal layer onto the substrate Because the coated surface now has hgands on it (because the ligand groups are part of the polymer structure) and direct catalysis / metallization can occur

A prefeπed feature for the metal deposit is that it can carry a cuπent from anywhere on the metal deposit to a ground connect, which will typically be at or near the edge of the deposit

For metal deposits over a resist layer, it is desired to have the metal deposit be sufficiently transparent to the exposing beam so that the beam can successfully expose the resist through the metal deposit Also, it is desired to have metal deposits that are sufficiently transparent, so that alignment marks and other features are visible through the deposit Typical metal deposits will be between 15 and 20 nm thick

Examples

Having described the invention, the following examples are given to illustrate specific applications of the invention, including the best mode(s) now known to perform the invention. These specific examples are given with the understanding that they are not intended to limit the scope of the invention described in this disclosure.

Example 1 : This example illustrates the fabrication and processing of a metal-coated substrate as an anti-charging layer for e-beam lithography. Figures 3 and 4 show diagramatically a proven means to fabricate an ultrathin metal layer useful for charge dissipation on an insulating substrate surface. Although FIG. 3 shows a substrate of patterned Cr on glass, the process can be used for any substrate that is compatible with the e-beam tool. The process flow is shown for a substrate with a planarizer or etch-stop layer. The process can be applied to a substrate without the planarizer layer by omitting steps 1, 8, and 11. Step 2, a brief 0₂ etch, can be omitted if the surface is hydroxylated and clean or if the SAM binds to the surface without the 0₂ etch. If the planarizer option is not selected the substrate surface should be cleaned with normal degreasing methods. Surfaces of oxide (native or ceramic) possess the hydroxylated groups for organosilane attachment. Following the surface or planarizer preparation, the ligand SAM organosilane layer is deposited from a 1 % (by volume) aqueous solution of (2- aminoethyl)-3-aminopropyltrimethoxysilane (EDA) containing 0.001 M acetic acid (pH 5). A puddle of the EDA solution is placed on the substrate surface for -15 sec. Excess solution is removed by spincasting at 3000 rpm for 15 sec while rinsing the substrate in a stream of water, followed by spinning an additional 15 sec to remove the water. The substrate is dried by baking at 120^CC for 4 min to complete the chemisorption of the EDA SAM. A Pd/Sn catalyst (Shipley Cataposit^®44) 27 is next applied for 5 min and the sample is gently rinsed three times in 0.1 M HC1 (aq) as directed by the manufacturer. Next, an accelerator (Shipley Cuposit Accelerator 19) is applied for 90 sec and removed as directed by the manufacturer. Cu is then deposited by immersion of the substrate in Shipley 328 Series EL Cu bath for 30 sec at 25°C. As shown in FIG. 4, the resist is then spun onto the Cu coated workpiece and processed, exposed, and developed under standard conditions. The developed pattern is wet-etched into the -15 nm thick Cu layer by a 5-10 sec immersion in a 50% HN0₃ solution (FIG. 4, Step 7). The HN0₃ process was not found to etch Cr on a mask. Following the Cu etch, the pattern is etched into the planarizer (if present) (FIG. 4, Step 8) and then into the substrate (FIG. 4, Step 9) using standard wet or plasma etches, as applicable. In FIG.4, Step 10, the resist is removed by standard processes such as an 0₂ plasma etch, followed by a 5-10 sec immersion in 50% HN0₃ to remove the remaining Cu discharge layer. In FIG. 4, Step 11, the planarizer (optional) is removed by standard techniques.

Example 2: This example illustrates the fabrication and processing of a metal-coated resist as an anti-charging layer for e-beam lithography. Shown in FIGS .5 and 6 is the process flow for e-beam lithography with the S AM-Cu layer on top of the resist. (Approach of FIG. 2(c)). Although FIG. 5 shows a substrate of patterned Cr on glass, the process can be used for any substrate that is compatible with the e-beam tool. In step 1, the resist is deposited onto the substrate using standard techniques. A SAM is deposited onto the resist in step 2 though the spincoating process as described in Example 1 above or alternatively by dipcoating the substrate in the SAM solution. Yet another method to apply a metal binding SAM layer is through physisorption. The metal film is then formed on the S AM-coated resist surface following catalysis (step 3) and electroless metal deposition (step 4) using conditions described in example 1. After patterned e-beam exposure, the metal is removed using the HN0₃ wet etch of example 1. Subsequent resist development and pattern transfer then proceed according to standard routes, as shown in FIG. 6.

Example 3: This example demonstrates the fabrication of a highly conductive ultrathin metal layer according to the method of the invention. The procedure described in example 1 above for the fabrication of a substrate coated by an ultrathin metal film, as illustrated in FIG. 3, was repeated using an unpatterned substrate. Specifically, planar Si wafers not bearing Cr patterns were used. Electroless Cu depositions (step 5 of FIG. 3) were carried out for different times ranging from 30-120 sec. Following metal deposition, metal thicknesses were determined using surface profilometry and electrical conductivities were measured using a standard 4-point probe approach. Shown in Figure 7 are curves of Cu film thickness vs. plating time and Cu film resistivity vs. plating time. The curves are for Cu film deposition on EDA on glass via the deposition process described above in Example 1. The results indicate that a reasonably uniform 15 nm thick metal film can be deposited with a resistivity of 40 μΩ-cm. A novel feature of the process is the use of the metal-binding SAM, which allows the attachment of a very thin (15 nm) continuous low-resistivity Cu film. Without the EDA SAM layer, metallization quality and completeness were compromised. For some substrates, no Cu deposition was observed. In other cases, although Cu was deposited, continuous Cu films were not obtained for film thicknesses < 100 nm.

Example 4: This example demonstrates the efficacy of the ultrathin metal layer described in examples 1 and 3 in dissipating charge buildup on an insulating substrate during e-beam exposure. Shown in figure 8 is a SEM micrograph of a Cr on glass mask partially coated by Cu using the process of FIG. 3 according to the conditions of example 1. In the leftmost region, where the Cu/S AM layer is absent, charging is observed on the glass, as noted by the white areas indicative of charged surfaces. The charging is not observed in the rightmost region, which is covered by the Cu SAM layer. Note that, due to the ultrathin nature of the Cu/SAM layer, observation of the underlying Cr/glass pattern is preserved. Consequentiy, the Cu/SAM coated regions satisfy both the anti-charging and transparency conditions required for use as charge dissipation layers in e-beam lithography. This allows imaging for alignment as well as potential use in the preparation of routine SEM samples for high-resolution imaging.

Example 5 : This example demonstrates the use of an ultrathin metal film at the substrate-resist interface as a charge dissipation layer under e-beam iπadiation. Shown in FIG. 9 are: (a) a cross-sectional diagram of the sample shown in (c) and (d); (b) a graph of % transmission (calculated via Monte Carlo simulation) of an electron beam through a resist vs. resist thickness for different beam energies; and (c) and (d) SEM micrographs at 3 kV and 5 kV, respectively, of a resist on a glass substrate sequentially coated with a 15 nm thick Cu/EDA layer and 500 nm of UVII resist. Severe charging in the SEM image is seen in FIG. 9(c) as bright areas denoting previously irradiated areas The Monte Carlo simulation predicts zero beam penetration of a 3 kV electron beam through the 500 nm thick resist, a condition under which we expect and observe severe charging Similarly, the Monte Carlo code predicts 25% transmission of a 5 kV electron beam through 500 nm of resist In this case essentially no charging is observed as seen in FIG 9(d) The 25% beam transmission is clearly sufficient for electron beam induced cuπent (EBIC) effects to dissipate any charge build up through the Cu EDA layer underlying the resist Additional observations of the dissipation of charge through EBIC effects were also noted for 100 nm, 500 nm, and 1 μm thick resist on Cr on glass via SEM and/or Auger electron spectroscopy Figure 9(d) also again confirms that the 15 nm Cu/EDA layer is completely sufficient to eliminate the severe charging

Example 6 This example illustrates the use of a 15 nm thick Cu/EDA layer on a planarizer, as described in FIGS 3 and 4 and Example 1, to dissipate charge and maintain feature continuity across a chrome to glass boundary Shown in FIG 10 is a developed e-beam lithographic pattern in UVII positive chemically amplified resist on a 15 nm thick Cu/EDA layer, which coats a cross-linked SAL-601 resist (Shipley) etch-stop layer on a patterned Cr on glass mask substrate Only the 15 nm Cu/EDA layer is grounded A cross-sectional schematic of the sample is shown in FIG 10(a) and a top view of the sample following exposure and development is shown in figure 10(b) The continuity of the lines represented by the vertical channels, as they cross from the chrome to glass regions of the underlying substrate, is maintained within < 1 nm lateral variation (^* e , the limit of our ability to measure line displacement in our system) The absence of line displacement is again consistent with effective charge dissipation dunng patterning by the Cu SAM underlayer

Example 7 This example illustrates, via FIG 11, the improvement in pattern placement and subfield stitching in e-beam lithography by the inclusion of a Cu-EDA layer The pattern written consisted of a 1 μm period grating of single pass lines, exposed with a dose of 2 nC/cm In FIG 11(a) the sample consisted of 100 nm of UVII resist (Shipley Corp ) on glass In FIG 11(b) the sample was 100 nm of UVII resist (Shipley Corp ) coating a 15 nm Cu/EDA layer on patterned Cr on glass The Cr pattern was isolated in a region near the center of the glass and not electrically grounded except by contact to the 15 nm Cu layer The patterns were written in a JEOL JBX-5DII operated at 50 kV The JEOL writes the pattern with 80 μm fields between stage movements Once the stage is stationary for a given field the JEOL writes the pattern in 10 μm square sub-fields within the 80 μm field Shown in FIG 11(a) is the interaction of the sub-fields due to charging The charging causes deflections of the e-beam resulting in 0 1-0 4 μm placement eπors in adjacent fields In figure 11(b) the subfield stitching eπor is not observed Thus the 15 nm thick Cu layer is a sufficient grounding layer to circumvent beam deflection due to charging and to reduce pattern placement eπor A similar lack of observation of sub-field stitching eπor was found on samples in which the Cu layer was deposited on top of the resist (as described in FIG 2(c) and FIGS 5 and 6) Following resist exposure and a post exposure bake, we were able to remove the Cu using the HN0₃ wet-etch and develop the resist using standard procedures to complete the patterning process The above examples show that a 15 nm thick Cu/SAM layer is sufficient to eliminate charging effects under e-beam lithography conditions, while maintaining sufficient transparency to permit observation of alignment marks and underlying topographies for efficient patterning. The metal/S AM layer is an efficient discharge layer when placed directly on a Cr on glass mask or on top of a resist (in a resist bilayer scheme). With regard to the latter, the Cu/SAM film can be deposited on top of a resist without materially affecting its exposure properties or normal processing. It can be deduced from the above examples that a 15 nm layer of Cu/SAM on a substrate surface or on the top surface of a resist coating a substrate can function as an effective discharge layer in e-beam lithography.

Claims

What is claimed is

1 A method for forming anti-charging layers for beam lithography comprising applying a self-assembled monolayer having metal ligand sites to a substrate,

binding a metal deposit to said ligand sites of said metal ligating self-assembled monolayer, and

applying a resist to said metal deposit

2 The method as in claim 1 , wherein the substrate is non-patterned

3 The method as in claim 1, wherein the substrate is patterned

4 The method as in claim 1 , wherein the substrate is a surface with a planarizer

5 The method as in claim 1, further comprising applying a second self-assembled monolayer having metal ligand sites to said resist, and

binding a second metal deposit to said ligand sites of said second self-assembled monolayer having metal hgand sites

6 The method as in claim 1 , wherein said resist is a self-assembled monolayer film

7 The method as in claim 1 , wherein said resist is a composite self-assembled monolayer film

8 The method as in claim 1 , wherein said self-assembled monolayer having metal ligand sites is applied by physisorption

9 The method as in claim 1 , wherein said self-assembled monolayer having metal ligand sites is applied by chemisorption

10 The method as in claim 1, wherein said self-assembled monolayer having metal ligand sites is applied by wet chemical dipping or spincasting

11. The method as in claim 1, wherein the self-assembled monolayer having metal ligand sites is assembled from monomers having the formula RSiR'_nX₃._n, wherein R is a ligand group, R' is an alkyl or aryl group, X is a halide or alkoxide group, and n is 0, 1, or 2.

12. The method as in claim 1, wherein said metal is selected from the group consisting of copper, cobalt, nickel, and gold.

13. The method as in claim 1, wherein said metal deposit forms a conductive pathway.

14. The method as in claim 1, wherein said step of binding said metal deposit to said ligand sites comprises binding by electroless metallization.

15. A method for forming an anti-charging layer for beam lithography comprising: applying a resist to a substrate;

applying a self-assembled monolayer having metal ligand sites to said resist; and

binding a metal deposit to said ligand sites of said self-assembled monolayer having metal ligand sites.

16. The method as in claim 15, wherein said resist is a self-assembled monolayer film.

17. The method as in claim 15, wherein said resist is a composite self-assembled monolayer film.

18. The method as in claim 15, wherein said self-assembled monolayer having metal ligand sites is applied by physisorption.

19. The method as in claim 15, wherein said self-assembled monolayer having metal ligand sites is applied by chemisorption.

20. The method as in claim 15, wherein said self-assembled monolayer having metal ligand sites is assembled from monomers having the formula RSiR'_nX₃._n, wherein R is a ligand group, R' is an alkyl or aryl group, X is a halide or alkoxide group, and n is 0, 1, or 2.

21. The method as in claim 15, wherein said self-assembled monolayer having metal ligand sites is applied by wet chemical dipping or spincasting.

22. A method for forming an anti-charging layer for beam lithography comprising: applying a self-assembled monolayer having metal ligand sites to a substrate;

binding a metal deposit to said ligand sites of said self-assembled monolayer having metal ligand sites;

applying a second self-assembled monolayer having metal ligand sites to said metal; and

binding a second metal deposit to said second self-assembled monolayer having metal ligand sites.

23. The method as in claim 22, wherein said resist is a self-assembled monolayer film.

24. The method as in claim 22, wherein said resist is a composite self-assembled monolayer film.

25. The method as in claim 22, wherein said self-assembled monolayer having metal ligand sites is applied by physisorption.

26. The method as in claim 22, wherein the self-assembled monolayer having metal ligand sites is applied by chemisorption.

27. The method as in claim 22, wherein said self-assembled monolayer having metal ligand sites is assembled from monomers having the formula RSiR'_nX₃._n, wherein R is a ligand group, R' is an alkyl or aryl group, X is a halide or alkoxide group, and n is 0, 1, or 2.

28. The method as in claim 22, wherein said self-assembled monolayer having metal ligand sites is applied by wet chemical dipping or spincasting.

29. A method for conducting beam lithography comprising: applying a self-assembled monolayer having metal ligand sites to a resist;

binding a metal deposit to said self-assembled monolayer having metal ligand sites;

forming an anti-charging layer;

exposing said resist with a beam; and

developing said resist.

30. The method as in claim 29, wherein said beam is selected from the group consisting of ion beams, electron beams, and X-ray beams.

31. The method as in claim 29, further comprising: removing said metal deposit.

32. An improved resist layer for beam lithography, comprising: a layer of polymer resist, disposed over a wafer for lithographic patterning;

a metal deposit for conductively dissipating charge, in contact with said polymer resist layer, either directly or through a self-assembled monolayer; and

a ground connect, for connecting said metal deposit to ground.