HK1035577B - Method of molecular-scale pattern imprinting at surfaces - Google Patents

Description

Method for imprinting molecular-level pattern on surface

Technical Field

The invention relates to a method for carrying out atomic or molecular pattern imprinting (patternizing) on a solid surface by initiating a local chemical reaction between adsorbent molecules and the solid surface,

background

Some of the advances in the multi-billion dollar semiconductor industry rely on the ability to mark (i.e., write, dope, or etch) a surface with very small features at controlled intervals. The limits of prior art markings are separated by tenths of a nanometer (typically 0.3 microns, 3000A, a distance of about a thousand atoms). These sized patterns form the minimum achievable using conventional marking methods. Conventional methods include covering the surface of the part with a patterned mask to block reagents (electronic, optical or chemical) used to mark the surface. It has proven impossible to make patterned masks with features smaller than a few tenths of a micron and masks with such small features are difficult to reproduce.

U.S. patent No.5,645,897 discloses a method of surface modification by Andra by ion bombardment of the surface or region directly in front of the portion of the surface to be etched or coated. The ion source is selected to produce ions that are highly charged and have a high enough kinetic energy to bring the ions close to the surface, but a low enough energy to prevent penetration of the surface. One advantage of the process described in this patent is that the high charge state and low kinetic energy of the ions results in very localized energy deposition, resulting in improved spatial resolution when etching or coating a surface for imprinting with a patterned mask. This patent also discloses a method of making a fine etch pattern combining conventional lithographic masking techniques with features deposited using ion beam directed energy.

U.S. patent No.5,405,481 to Licoppe et al discloses a method of making nano-scale surface patterns using a gas photonanograph device. The device comprises a head with a fibre optic cable terminating in a needle tip, and a microcapillary channel at its tip, which delivers a reactive gas from a gas reservoir. This tip is at a distance from the surface of the support that is to be optically excited. By scanning the device, a nanopattern can be obtained as if a person were writing with a pen, where the pen tip is the focused light source.

U.S. patent No.4,701,347 to Higashiteb specifically mentions a method of forming a patterned metal layer on a semiconductor by photolyzing molecules adsorbed on the surface. However, as with the earlier mentioned patents herein: U.S. patent No.3,271,180 issued on 9/6 in 1966 is consistent with: the pattern of photolysis and thermal reactions induced by irradiation of the adsorbent is obtained by the presence of a mask between the light source and the adsorbent layer.

U.S. patent No.5,322,988 uses laser irradiation to produce a photochemical and thermal reaction between the adsorbed layer and the underlying substrate, as in No.4,701,347, but the reaction is not as writing as etching (etching is defined as "texturing"). The reaction described here only occurs when the laser impinges with sufficient fluence that pattern irradiation (like the "mask" below) is the source of the pattern etch.

A method for the maskless etching of semiconductors based on uv photolysis of gaseous methyl halides is described by d.j.ehrlich et al, appl.phys.lett.36, page 698 (1980). The position of the pattern mask is replaced by an interference pattern, i.e. it is again generated by pattern irradiation of the surface.

U.S. patent nos. 4,608,117 and 4,615,904 to ehrlich et al disclose maskless growth methods for forming patterned film layers. This method describes two steps. In the first step, a focused beam or beam of electrons is used as a pen to record a pattern on the surface and photodecomposed as a recording agent (ink). Once the 1-2 monolayer patterns of metal or semiconductor have been recorded in this manner, the second step is to equalize the radiation with gaseous reactants and to concentrate the surface material in "pre-nucleated areas", i.e., in the vicinity of the pattern formed by the first deposition step. Thus, the second growth stage is maskless. In the maskless film growth stage "atoms are mainly provided … by direct photolysis of organometallic molecules in the gas phase" (U.S. Pat. No.4,608,117, col. 2, lines 12 and 13). The growth of the film layer occurs selectively in the pre-nucleated region where the impinging atoms generated in the gas phase have a high adhesion constant at the surface.

U.S. patent No.5,129,991 to Gilton discloses another approach to maskless etching. An adsorbed etching gas (chlorine or fluorine) is present on a substrate having macroscopic regions made of different materials having different light emission thresholds for released electrons. Irradiation of the substrate with a light wave can cause electron emission in some regions but not others. The emitted electrons cause etching to occur only in those areas of the substrate that are defined by a light emission threshold with sufficiently low emitted electrons, i.e. where the reaction is localized, but is visible to the naked eye.

Yan et al, j.phys. chem.99, 6084 (1995) have disclosed a method of halogenating a substrate, preferably, on a surface at adjacent surface silicon atomic sites (70% adjacent, 30% non-adjacent), by reacting molecular chlorine impinging as a molecular energy beam (0.11eV) directly with the substrate as a gas on a silicon (111)7 x 7 substrate. Although the reproduction of chlorinated pairs at these sites is arbitrary over the entire surface, they form a small range of order, i.e., a simple form of molecular scale pattern.

It is known to create a pattern close to the surface by adsorbing a weakly bonded molecular layer patterned in its most stable configuration. Scanning Tunneling Microscopy (STM) has shown undeniably the presence of such an adsorbent pattern, revealing the position and spacing of the adsorbent molecules (see references 1-4). The origin of these spontaneously formed molecular patterns has been the subject of theoretical analysis (e.g., references 5 to 7). These patterns are governed by the effect of the regular arrangement of atoms in the underlying crystal (called the "matrix") on the adsorbent, by the size and shape of the adsorbed molecules themselves, which can only be caused to interact to form the adsorbent pattern, and by the overlayer which determines the distance between the adsorbents and thus the optimum pattern for adsorption. By choosing these variables (substrate, adsorbent and cover) different patterns can be obtained. The patterns of these adsorbents can be repeated at intervals as small as a few atomic diameters.

Typically, so-called "sorbent layers" have a thermal energy of adsorption of 0-1 eV. At the lower end of the range, they are called "physisorbed" and at the upper end of the range are "chemisorbed". They are adsorbent layers because by heating the surface, they can be desorbed with the heat of desorption corresponding to the above heat of adsorption. It would be advantageous to develop such molecular level adsorption patterns as a means of marking the underlying surface. This would provide a way to mark maskless marks, and more importantly, at intervals that are at least 100 times smaller than the minimum distance achievable in prior methods using masks.

Disclosure of Invention

The invention aims to provide a method for marking on a solid surface in a pattern with molecular scale dimensions. In the present invention, a molecular scale pattern is imprinted on the surface, the pattern being derived from a pre-existing molecular scale pattern in the adsorption layer. If the adsorbent is found to adhere to preferred locations on the surface, which locations recur at intervals across the surface, we refer to the adsorbent as "patterned".

The present invention provides a method of forming a regular pattern on a solid surface, the pattern repeating at a pitch on the order of less than or at 0.3 microns (3,000A). The material to be marked spontaneously forms an ordered monolayer upon contact with the adsorbed gas or liquid. This arrangement is generally most complete if the adsorbent is "annealed", i.e., heated sufficiently to cause it to move. In some cases (such as the examples given herein), the adsorbent can be sufficiently mobile without annealing.

As one aspect of the present invention, a method of forming a molecular scale pattern on a solid surface is provided. The method includes providing a reactive solid having a surface and providing preselected molecules capable of forming an adsorption pattern on the surface of the reactive solid, and forming a pattern of adsorbed molecules on the surface of the reactive solid by exposing the surface to the preselected molecules; and forming an imprinted pattern on said surface by irradiating the surface and the pattern of adsorbed molecules with effective excitation energy to form a chemical bond between at least one of the constituents of each individual adsorbed molecule and the reactive solid, said chemical bond being located on said surface adjacent to the adsorption site of the respective adsorbed molecule.

In this aspect of the invention, the reactive solid may be a crystalline solid, or an amorphous solid. The reactive solid may be a semiconductor. The semiconductor may be a crystalline semiconductor selected from silicon, gallium arsenide, and indium phosphide.

As another aspect of the invention, there is provided a method of forming a pattern on a surface of a solid at the molecular level, the method comprising exposing the surface of a reactive crystalline solid to an effective imprinting agent to form a pattern of adsorbed molecules on at least a portion of the surface; and, by applying an effective excitation energy, causing the adsorbed molecules to react with the surface in a localized manner in the vicinity of the site to be adsorbed, thereby imprinting a pattern on the surface.

Some factors controlling the ordered pattern of the adsorption layer (matrix crystal structure, shape of the adsorbent molecules, and covering) have been given in the previous section. We also note that the use of different crystal planes or intentional introduction of pattern defects (patterned defects) can also control the adsorption pattern. The pattern defect is generally an intended pattern in which the crystal of the substrate is cut from a known angle to form an atomic-scale step (see reference 8). Another method for obtaining pattern defects is well established by applying a voltage or current pulse to the tip of the STM, which removes or adds atoms to the site where the tip is applied (see reference 9). Regardless of which of these methods is used to introduce pattern defects into the substrate surface, a corresponding pattern will be obtained in the subsequently deposited adsorption layer.

The ordered adsorbed layer near the surface can be activated by light (photon) radiation or bombardment with charged particles, causing a reaction between the patterned adsorbed layer and the substrate. The result is that a pattern of adsorbent is imprinted onto the surface. Since the adsorbent pattern is repeatedly arranged at a molecular pitch, the same is true of the imprinted pattern.

In one aspect of the invention, the surface is imprinted with intact (intact) adsorbent molecules that are excited by irradiating the adsorbent and the surface.

In another aspect of the invention, the surface is imprinted by chemically reactive fragments formed from the adsorbent molecules by simultaneously irradiating the adsorbent and the substrate.

In one aspect of the invention, the agent that excites the adsorbent is ultraviolet radiation that is directly absorbed by the adsorbent molecules.

In another aspect of the invention, the agent that excites the adsorbent is an electron impact on the matrix at the site of the adsorbed molecule, which is itself excited by an impact photon, electron or ion that causes a portion of the matrix electrons to gain energy.

In the excitation of adsorbents mediated by various substrates, electron excitation of the substrates results in energy transfer to the adsorbent without electron transfer, a mechanism referred to as "electron-to-electron energy transfer".

The embodiments disclosed herein embody several aspects of the above, with adsorbed chlorobenzene on the silicon wafer being irradiated by ultraviolet light or electrons as described herein. The novel and unexpected result of the present invention is neither in the presence of patterned sorbents nor in the direct photoexcitation and photolysis of sorbents, nor in the excitation and decomposition of sorbents by electron impact of matrix electrons, nor in the use of external electron beams to cause sorbent excitation and reaction, nor in the use of other charged particle beams, i.e. ions, to cause sorbent excitation, but rather in the localization of subsequent reactions, which is related to the transfer of the pattern of sorbents to the matrix.

Another different invention disclosed herein is that electrons from an external source can replace light as the primary agent to initiate the patterned adsorbent and imprint it on its underlying surface with the pattern. It is well known that electron irradiation and light irradiation have comparable effects. Thus other charged particles, such as charged molecules-ions, can also be used for irradiation.

The reaction of the irradiated adsorbent with the substrate may be equivalent to the reaction of the excited intact adsorbent molecule, or a molecular fragment of the molecule, or an atomic fragment thereof. Subsequent reaction products may chemically bond to the surface (localized "writing"), or embed into the surface (localized "doping"), or themselves localize atoms from the surface as a result of the localized reaction (localized "etching"), or the adsorbant itself preferentially forms atomic scale pits on the surface at the sites to which the chemically bonded species are attached during subsequent irradiation. The last case translates the patterned attachment ("writing") of the described chemical groups into a pattern of similar "pits" (etched "in a concave shape, the diameter of one or more atoms), thus extending the range of applications.

Another aspect of the invention is some "enhanced" performance. Successive adsorption-irradiation cycles cause the annealed adsorbed layer (prior to irradiation) to find out, or at some point avoid, the sites where the pattern had been previously imprinted. Thus, as by way of example, the adsorbent is preferably patterned in the region of "pits" in the surface. Thus, when the second layer of adsorbent is applied after irradiation, the reaction will preferably take place again at the previously imprinted sites (typically at the sites of the second adsorption). Thus the second and subsequent pattern adsorption plus radiation imprinting may augment or chemically alter the first imprinting. First imprint is understood to be formed by the present method or by an alternative method such as atomic writing/etching of the tip of a Scanning Tunneling Microscope (STM) (see reference 9). This "enhancement" effect is important in allowing the use of this method of adsorption plus irradiation to enlarge the size of the first "pits" in the second and subsequent "etches" and to write or dope with the selected chemical agent in the vicinity of the previous mark.

Since each process of reactivity or etching is excited by the arrival of photons, electrons or ions at the adsorbent plus surface, and since such photons or electrons are easily calculated and controlled, the first and subsequent reactions, doping, or etching can be controlled to the number of atoms involved. Thus, described herein is a method for pattern recording, etching and doping with digital control.

Brief description of the drawings

A method of molecular scale marking or patterning a surface forming the subject of the present invention will be described with reference to the following drawings, in which:

fig. 1(a) to 1(c) show photographs of a Scanning Tunneling Microscope (STM) from which distributions of individual ClBz (chlorobenzene) molecules and Cl (chlorine bonded to silicon adsorption atoms) on each half of Si (111)7 × 7 units F (defective) and U (non-defective) can be obtained. Fig. 1(a) is an STM image of pure Si (111)7 × 7. One side of the cell was 0.00269 microns (26.9A). FIG. 1(b) is a similar surface to FIG. 1(a) using a 1 liter ClBz gauge. The dark shaded portions are ClBz molecules; indicating that they are preferentially adsorbed at the F site. Fig. 1(c) shows two scans of the partially chlorinated area of the Si (111)7 × 7 surface (for this figure, the area previously exposed to chlorine). The dark shading of-1V shows itself to be bonded to chlorine, since they are "lit up" (i.e. current is passed) when the bias current of the STM tip is changed to-3V. Chlorobenzene (ClBz) cannot be "lit" in the range of-1 to-3V.

Fig. 2(a) is a bar graph obtained by counting Cl atoms on F and U that have been formed by irradiating ClBz when the average coverage of individual ClBz molecules before irradiation is 48% on F or U. The irradiation is 193nm ultraviolet light from an excimer laser. Within experimental (counting) error, the ratio of the total number of F/U for parent ClBz (labeled 'P') is the same as for daughter Cl (labeled 'D'), which is characterized as a "positional" response. ClBz and Cl (P and D, respectively) had a 27% higher probability of covering U at the F site than at ClBz

Fig. 2(b) shows a bar graph obtained by counting the average coverage of the individual ClBz molecules on F and U before irradiation at 38%, and counting the chlorine atoms bonded to the silicon adsorption atoms on F and U after electron irradiation.

Fig. 3(a) shows the distribution of parent (ClBz) and daughter (Cl) after light irradiation at atomic sites near the atom labeled (M) called "middle atoms" and near the atoms around the atom labeled (C).

Fig. 3(b) shows the distribution of the parent (ClBz) and daughter (Cl) after electron irradiation, near the atomic site labeled (M) referred to as the "central atom" and near the atoms labeled (C) at the four weeks.

Fig. 4(a) shows a pattern of Cl atoms generated by a Scanning Tunneling Microscope (STM) tip adsorbed to Si (111)7 × 7 ClBz irradiated with electrons. The white point forming the broken line is recorded with a-3V tip at which the Cl atoms are "lit" to form a white point. The spacing of the STM tip movements at the-4V pulse was an equal spacing of about 59 angstroms.

Fig. 4(b) shows a pattern of Cl atoms generated by a Scanning Tunneling Microscope (STM) tip adsorbed to Si (111)7 × 7 ClBz irradiated with electrons. The white dots forming the continuous lines were recorded with a-3V tip. For this experiment, a-4V pulse (approximately every 29 angstroms) was applied to the tip of the STM at a very fast timing as the tip of the STM was moved from top to bottom in the pattern, and the radiation-induced reaction increased the formation of continuous lines of chlorine atoms. The white point forming the continuous line was recorded with a-3 volt tip. At this voltage the chlorine atoms are "lit" to form white dots.

Detailed description of the invention

The present invention will describe a method for atomic or molecular scale marking of surfaces and is illustrated by an illustrative example, not limited thereto, in which chlorobenzene is used as an adsorbed molecule for marking on crystalline silicon wafers. However, to those skilled in the art, the present invention is not limited to this system, and the chlorobenzene-silicon system is only used as an example to illustrate the principles of the present invention. The phrase "imprinting agent" as used herein refers to a material (liquids or gases) that, when exposed to a reactive solid surface, forms a pattern of adsorbent on the exposed surface.

To illustrate the principle of the marking method forming the present invention, as a starting point for the described study, a Scanning Tunneling Microscope (STM) showed a clean Si (111)7 × 7 wafer at atomic level free of contamination at room temperature. The silicon wafer was placed in an ultra-high vacuum chamber at about 1 Langmuir (1L; 10)^-7Torr 10 seconds) in chlorobenzene vapor. The vessel was then evacuated again to Ultra High Vacuum (UHV). All experiments were performed in UHV. Examination of the surface again with STM showed that the surface portion was covered with chlorobenzene (ClBz), which has been confirmed by a black spot of about molecular size, i.e. when a reduced current from the negatively charged STM tip (-1V) localization was applied to the crystal. As expected, the chlorobenzene molecules adsorbed on silicon were stable.

FIGS. 1(a), 1(b) and 1(c) illustrate the types of scanning tunneling microscope data obtained from the basic findings disclosed below in FIG. 2. STM images show pure silicon (Si (111) 7X 7), ClBz-capped silicon, and chlorine-capped silicon. Increasing the negatively charged tip bias to-2V or-3V distinguishes atomic chlorine from chlorobenzene, where the black spot of chlorobenzene is unaffected, but the black spot corresponding to chlorine brightens. It should be noted that the points correspond to individual ClBz molecules or Cl atoms.

Black spots corresponding to chlorobenzene molecules are repeatedly distributed on the silicon surface to form a pattern. These points are mainly located in the defective (F) half of each cell, not the non-defective (U) half (fig. 1). This selectivity decreases with increasing coverage. This indicates that there is more tendency to label F than U, due to the stronger bonding of the adsorbent to the matrix (chlorobenzene to silicon) at F than at U, rather than the (chlorobenzene-chlorobenzene) interaction between the adsorbents.

The regular arrangement of the adsorbents is not in any case formed by the bonding of the adsorbents to the matrix, but also by the adsorbent-adsorbent interaction, known as "SAMs" (e.g. self-assembled monolayers formed of long-chain molecules), which even favour the formation of geometric patterns on amorphous solids. But in this case the regular arrangement of the adsorbent will increase with increasing coverage. The SAM type behaves similarly to brominated long chain hydrocarbons, with about 1ML of brominated long chain hydrocarbon coating on graphite to form a highly regular adsorption layer. (see reference 4)

Here, the silicon is covered with chlorobenzene and distributed over F and U in a repeating pattern. The triangular F and U parts are alternately distributed on the surface of the whole silicon wafer. The center distance of F and U was 0.00155 microns (15.5A). The triangles (F and U forming each unit in pairs) are equilateral triangles with a side length of 0.0027 microns. Regular adsorption on F and U can be achieved without annealing at room temperature, and analogy to benzene (see reference 2) suggests that the monochlorinated benzenes are first weakly adsorbed into a mobile "precursor" state, followed by chemisorption at the preferred location (heat of adsorption of about 1 eV). We believe that the linkage in the chemisorbed state is such that a benzene ring (or in this case a mono-chlorinated benzene ring) lies almost flat on the surface, overlapping the dangling bonds located on the upper surface of the silicon adatom via its delocalized pi-bond.

It will be appreciated by those skilled in the art that this experimental finding is not dependent on whether the above explanation of the bonding properties is correct or not. Moreover, this theory does not explain its qualitative nature to explain that (benzene and) chlorobenzene adsorb more readily on F than on U. It is a known phenomenon that the more preferred bonding of the adsorbent at certain locations on the surface (i.e., for certain specific atoms, or arrangements of atoms in defective portions defined as F). Since the preferred sites repeat at equal intervals on the surface, the adsorbent molecules also repeat, forming a pattern.

The tests mentioned here show not only adsorption patterns (required for the disclosed invention) but also effective activation reactions, for example patterning of reactive solid surfaces upon irradiation (second requirement of the process). An example of photo-imaging is also disclosed in which atoms are deposited in a chemically bonded state on the surface upon irradiation with ultraviolet rays or electron irradiation. The focus of this photoimaging method is that the pattern of the adsorbate or nearly the relevant pattern is imprinted onto the surface by irradiation inducing a reaction with the surface, which reaction is localized near the location of the adsorbed molecules. Complete localization can limit the reaction to the site of adsorption, although in the examples (as we said) the preferential reaction of chlorobenzene is with adjacent atoms (see figure 3). The adsorption pattern can only be transferred to the surface if the reaction is localized.

Light fragments (photofragments) obtained by molecular "scout scanning" of a substrate are known, see references 17, 18. Surprisingly, it is unexpected that the novel finding disclosed herein is that "positional reaction" replaces "positional diffusion" (due to such factors as energy of impact to the diffusion region, angle of impact, and nature of the impacting atoms) where the positional diffusion is preempted by positional perturbation of the surface, resulting in the formation of bonds at the impact site. Thus, under the experimental conditions described above, molecules or atoms will react with the substrate surface to be "diffused" rather than chemically bonding with the substrate surface to form non-volatile or (when etched) volatile products. The well-known reactive silicon used here was replaced by an inert halogenated matrix to study the dispersion "reacting" rather than "diffusing". It must be recognized that even "inert" matrices can react if the impinging species itself is sufficiently reactive and reactive.

The tests disclosed herein do not show the presence of a localized reaction, as will be disclosed later, in view of the mobility of many species at the surface, particularly the energetic species formed by irradiation.

We have proposed that the present invention is not the arrangement of adsorbents described above, but rather consists of developing a method of imprinting such patterns. When the adsorbent is chlorobenzene and the chlorobenzene adsorption layer does not occupy a pattern formed by attaching chlorine atoms to silicon dangling bonds, imprinting the silicon surface by irradiating the adsorbent. On the basis of the extensive work previously done (see, for example, reference 19), the reaction can be caused to occur by irradiation with Ultraviolet (UV) or visible light (using some materials of low workability) or external electron beam (see, for example, references 13, 14). Such a reaction may be initiated by photo-desorption of the layer, or by interaction of electrically excitable adsorbed layer molecules with the matrix. Both types of reactions can be assisted by irradiating the heated surface. The mechanism of the radiation-induced reaction will be briefly reviewed.

However, our disclosed findings are the focus of the claimed method. The finding in question is that the radiation-induced reaction takes place locally, imprinting a pattern of the adsorbent, or a very relevant pattern, on the underlying substrate. Examples given here: the above findings have been illustrated by comparing the pattern of atomic scale adsorbents determined by STM with the pattern of products obtained by surface reactions determined by the same STM.

Fig. 2 is a bar graph of the work of this study showing the pattern of molecules of the adsorbent (average coverage of chlorobenzene monolayer is about 0.4) at the F and U sites of silicon (111)7 × 7 converted to a similar pattern imprinted with chlorine, fig. 2(a) using 193nm excimer laser irradiation, and fig. 2(b) using electrons generated when a-4V pulse is applied to the tip of the STM (while maintaining a typical-1V distance from the surface). 193nm (typically 10)³90mJ/cm²Pulse) or after irradiation with a separate electron pulse (a few millinanoamperes of current), and only after irradiation, does STM show the presence of chlorine at the surface. As shown in fig. 1, when the tip voltage becomes a negative voltage of-2V or more, an extra black area of surface-bonded Cl, i.e., Cl — Si, shown by-1V is opposite to the black area of the chlorobenzene adsorbent, and instead brightens. The areas that brightened at-2V and black at-1V were the result of irradiation, indicating the formation of Cl-Si upon irradiation of chlorobenzene adsorbed on the surface.

The main difference between fig. 2(b) and fig. 2(a) is that fig. 2(b) is electron irradiation. Electrons were generated when-4V pulses were applied to the tip of the STM and were located on the 7 x 7 surface of silicon (111) covered with chlorobenzene. The tip was locked at a fixed distance from the surface while applying the-4V pulse; the tunneling microscope has a current of 0.09nA at a distance of-1V. To resolve a representative number of chlorobenzene molecules using a sufficient area of the surface as a standard, the tip was scanned along a distance of 1180 angstroms while repeatedly applying a-4V pulse. (the partial line formed by the chlorine atom obtained in this test is shown in FIG. 4(b) below). The ratio of the total number of F/U of parent chlorobenzene (P) and daughter chlorine (D) measured by electron irradiation in the "localization" reaction is the same as that measured by light irradiation.

The F/U ratio of the parent molecule P, i.e. adsorbed chlorobenzene, to the corresponding daughter D, i.e. chlorine-silicon ratio, given in FIG. 2 is the same within the expected measurement error. The ratio F/U can be obtained by STM recording the position of the individual chlorobenzene molecules and the chlorine atom (the latter in the form of Cl-Si).

As mentioned above, all experiments were carried out in UHV, so that irradiation-induced gas phase decomposition was negligible. One of ordinary skill in the art will appreciate that the process is not limited to UHV, as long as it allows the ordered adsorbent to be contacted with the substrate.

The mechanism by which the cleavage of the chlorine-carbon bond in chlorobenzene can be initiated by irradiation and the chlorine-silicon bond is formed on the surface will be discussed later. It is described beyond the essential part of the invention. Referring again to FIGS. 2(a) and 2(b), it was unexpectedly found that the distribution of chlorine at F and U sites on the silicon crystal surface was indistinguishable from chlorobenzene distribution within the limits allowed by experimental error. (note that this is not the case if atomic chlorine formed in the gas phase is imprinted on the surface, as this would cause a disordered distribution of Cl).

The distribution of daughter fragment chlorine at the F and U sites was consistent with that of the parent molecular chlorobenzene at the same sites, indicating that irradiation of the silicon surface results in a localization reaction without doubt. This surprising phenomenon of localized reaction was found to confirm the mechanism of the disclosed irradiation pattern formation process at the molecular level.

Above we compared the distribution patterns of the two regions, the F and U, of the 7X 7 surface of silicon (111) for the parent and daughter (P and D). In fig. 3 (again recording a monolayer coverage of about 0.4) we have analyzed the pattern of the distribution of P and D at the atomic sites more closely.

Fig. 3(a) and (b) show a different arrangement of more elaborate types, concurrent with the types of fig. 2(a) and (b). In view of these facts, fig. 2 shows that when the original pattern spontaneously covers more chlorobenzene (parent molecule, P) at a specific position on the surface of silicon (111)7 × 7, the result after irradiation is more chlorine (daughter molecule, D). In contrast, fig. 3(a) and (b) show the observed distribution of "parent" and "daughter" (P and D) at adjacent atomic sites (as is customary), "middle atom" is labeled "M" and the atom at the corner is labeled "C". (three sides of the triangular region of F or U on silicon (111) 7X 7 are composed of three silicon adatoms, namely a middle atom M and two corner adatoms, C on the sides). Fig. 3(a) shows that (47% chlorobenzene coverage) the central atom (M) chlorobenzene (P) has a strong adsorption tendency. It is also shown that corner silicon atom (C) daughter chlorine (D) has a comparably strong adsorption tendency after photon irradiation. This is determined by counting the number of chlorobenzene and chlorine located in the site of the M and C-Si atoms. FIG. 3(b) shows a graph of the substantial identity of one atomic site when irradiated with electrons. (the average coverage of FIG. 3(b) is 38%; the case of electron irradiation has been described above for FIG. 2 (b)). One explanation is that the parent molecule tends to react at sites adjacent to C in the silicon atomic region of M, and thus the adsorbed molecular pattern that tends to be M sites after irradiation ends up as an atomic pattern that tends to be C sites. (elaborate experiments showed that approximately 5% of the parent chlorobenzene molecules on the M site were irradiated close to their daughter chlorine in the vicinity of the C site rather than the 'R' site, R representing a static atom on the second layer of silicon because both C and R are adjacent to M, we have included a smaller percentage of these labeled C in the column.)

The pattern formed spontaneously by adsorption of the precursor is such that: the parent material P tends to be an intermediate atom (M) due to the two adsorbed atoms (C) at the corners. Daughter D was more inclined to C than to M, showing the opposite inclination. The same is true for light irradiation (at 193nm) or electron irradiation (pulsed by an STM tip applying-4V). The result is shown to be the formation of a pattern of 'P' (chlorobenzene) adsorption at the M site, and the delineation of the daughter atom "D" (chlorine) adjacent to the C site. The reaction in this case is "localized" in the spacing of one atom of the Si-Si adatom. Surprisingly enough, figure 3, listed below, is the 3 rd proof of localization response.

FIG. 4 gives direct evidence that the spatial "positional response" is the conversion of the parent adsorption pattern (whether in the F and U regions or near the atoms M and C) to a daughter pattern due to irradiation. The discontinuous line shown by the white dot in fig. 4(a) and the continuous line shown by the white dot in fig. 4(b) (both recorded with the tip at-3V at which the chlorine atom is "lit" to form a white dot) indicate that the electron impact initiates the reaction of chlorobenzene adsorbed at Si (111)7 × 7 to form a chlorine atom. (see the headings of FIGS. 2(b) and 3(b) under the test conditions of electron impact). In the case of FIG. 4(a), the-4V pulses applied to the STM tip are equally spaced at about 59 angstroms, while the-4V pulses in the case of FIG. 4(b) are applied at too short a distance to be resolved (about 29.5 angstroms). The radiation-induced reaction of FIG. 4(b) produces continuous lines of chlorine atoms at the surface, and FIG. 4(a) produces discrete lines of approximately 59 angstroms equal spacing corresponding to the spacing of the electron pulses. The width of the continuous lines is about + -15 * and the width of the discontinuous lines is about + -10 *. In each case, the localization of chlorine (one-dimensional pattern of fig. 4(b) and two-dimensional pattern of fig. 4 (a)) is apparently the result of localized electron impact induced "localization reaction". This is therefore the reason for the conversion of the "parent" adsorption pattern into the pattern of the "daughter" reaction product, i.e. the reason for the graphic imprinting that forms the subject of the present patent.

The most prominent pattern of chlorine atoms (the light spots are individual chlorine atoms) imprinted on the Si (111)7 × 7 surface in fig. 4 was formed by electron irradiation of the chlorobenzene-adsorbed layer. In fig. 4(a), the electron pulses are vertically spaced apart by a fixed distance, about 60 *. It can be seen that the areas of chlorine imprinted by irradiation are equally spaced, indicating that the daughter, D (i.e. chlorine), is bonded to the surface in the region where the parent, P (i.e. chlorobenzene) molecule is decomposed. If the daughter, chlorine, floats on the surface, no pattern of dots is produced, nor do these dots (as they appear) form a line. The same conclusions regarding the chlorine localization reaction can be drawn from FIG. 4 (b). the-4V pulse was applied to the tip very quickly in succession (approximately every 29 *) as the tip of the test STM was moved from top to bottom over the image. In contrast to the discontinuous lines of FIG. 4(a), the result is that the chlorine atoms form continuous lines. The localization of the reaction is confirmed by the narrow width of the line, which is fixed within a range of the ceiling of + -15 * from the decomposition site of the parent P for the daughter D.

Since all electron irradiation along the length of the line indicates the pattern represented by the adsorbent, the data of FIGS. 3(b) and 4(b) are derived from the distribution of chlorine atoms in the F and U regions, and in the atomic sites of M and C in FIG. 4 (b). (for illustration, only a portion of the lines used in FIGS. 2(b) and 3(b) are shown in FIG. 4 (b)).

As in the case of the dots in fig. 4(a), when the localization reaction is initiated by highly localized electron irradiation, the irradiated chlorobenzene no longer represents all the chlorobenzene. By repeatedly irradiating the U site in preference to the F site, the test is recorded in fig. 4(a), showing that the extent of the localized reaction is 42/58F/U; inverting the usual reaction pattern gives a corresponding F/U > 1 (see FIG. 2(a) or 2 (b)).

Needless to say, FIGS. 2-4 demonstrate the localization reaction.

The fact that irradiation of the adsorbent + substrate induces a localized reaction whereby the adsorbent pattern is transferred to the substrate has not been previously mentioned, nor has this surprising result been considered to be an unexpected situation not experimentally confirmed. As shown above, the precursor adsorbed molecular chlorobenzene, when adsorbed at room temperature, was initially highly mobile at the surface, even without excess energy. Corresponding excited electron-excited sorbents, ClBz^*Or adsorbed negative ions, ClBz^-Or the chlorine atom decomposed therefrom, may be confirmed to be similarly mobile when the surface is first formed, thereby losing the "memory" of the pattern arrangement of the parent ClBz, and as a result, being unable to react to form a pattern on the surface.

In accordance with this plausible situation, irradiation of the adsorption pattern may induce imprinting of the surface irregularities. This result (but not the one found by the present invention) is more advantageously justified by the fact that: after energetic irradiation, such as UV, or electron impact, chlorine (unlike chlorobenzene) is released from the surface for the most part and has a substantial initial conversion energy in the range of one volt of electrons. As shown in fig. 2-4, because chlorine reacts in a highly localized manner at the surface from the distribution of the parent molecule chlorobenzene or where the chlorobenzene molecule is localized to the irradiation excitation, it is shown that excessive conversion can carry atoms from some loosely bound mobile precursors, a hypothetical chlorobenzene state, directly to a strong chlorine-silicon chemical bonding. Chloro-silicon has a basic bonding energy in the range of 4-5eV (see, for example, reference 15).

A more likely rationale for the observed localization results is that the excited converted chlorine never exists as a free atom, but rather rapidly undergoes an exchange reaction, irradiating the excited ClBz state^*Or ClBz^-The chlorine-carbon bond is broken while forming a chlorine-silicon bond of a considerable strength to the surface. This exchange reaction can occur on a timescale of tens of millions of seconds and thus has no opportunity to move over a greater range on a surface. We have in early work (see, for example, references 10, 11) obtained indirect evidence of the mechanism by which this radiation initiates the halogenation of the active substrate, in this case silver. However, this early work did not give any results of the nature of whether or not the reaction was localized or not, since there was no means to localize it (e.g., STM).

Those skilled in the art will appreciate that the molecular mechanism of the exchange reaction as postulated above does not form an element of the present invention. As will be discussed below, the irradiation is effective to initiate the reaction.

The mechanism by which radiation induces the sorbent reaction is (a) the formation of reactive electron excited molecules at the adsorption surface interface (see references 7, 19) by direct absorption of radiation by the adsorption layer, and (b) the generation of a neutral active excited state of the sorbent by electron transfer of electrons to electrons after electron excitation at the substrate (which may include the formation of electron-hole pairs at or near the surface for semiconductors) (see reference 12). (c) The transfer of charge or transfer of reactive adsorbed ions from the adsorbent by electron excitation (e.g., (b)) on the substrate (see references 10, 11), and finally (d) bombardment with an external electron source can result in (a), (b) or (c) with the result that the impinging electrons further contact the adsorbent in the subsequent reaction of the adsorbed anions (negative ions) or their decomposition products (see references 13, 14). The 193nm irradiation of chlorobenzene adsorbed on a silicon wafer described herein is believed to involve a certain amount of (a) (b) and (c) (direct excitation, energy conversion excitation and electron conversion excitation) processes, the relative amounts are unknown, and need not be known for photoimaging to be effective. It is also possible that the irradiation of chlorobenzene adsorbed on the silicon wafer described here clearly includes mode (d), but also includes contributions (a) to (c).

Although light irradiation or electron beam bombardment up to the point of preference for exciting the adsorbent or substrate, it will be appreciated that particle bombardment with positively or negatively charged ions may also be preferred in some circumstances.

Those skilled in the art will appreciate that there are many applications for such methods of optical, electronic, or ionic impact patterning of surfaces. They are broadly "writing", "doping" and "etching" which are initiated by irradiation under digital control, since the number of photons, electrons or ions "coming in" in each case determines the number of atoms "coming out". "writing" relates to deposition on a surface; in this case, the resulting output (referred to as "coming out" above) is the material that is attached to the surface. Here, examples of chlorination for forming a pattern on a silicon surface are given. When the written material is masked, it is the result of "doping", either because of the recoil momentum it gets away from the activated parent molecule to drive it into the surface, or after it is deposited on the surface, it is masked by one or more additional layers grown on the surface. In both cases, patterned "doping" and patterned "writing" represent only a minor difference, the important step in each case being the same, i.e. irradiation induces a localized reaction. The result of "etching" relates to a well comparable localized removal of surface atoms. Removal reactions can occur from surfaces "directly" on certain substrates, including energetic halogen atoms, for example. Preferred localizing reagents are fluorine-containing adsorbed molecules, such as fluorobenzene, rather than chlorobenzene, with minor differences. The most essential requirement for direct etching is that the product of the pattern reaction be gaseous or capable of being vaporized by heating. The localized etching can be accomplished by the two-step process already illustrated and described. The first step is the disclosed patterned photochlorination and the second step is further irradiation with 193nm UV, which has shown that preferably optical emission occurs at the chlorinated sites. Both of these steps are "digitally" controlled processes (see reference 15).

The successive adsorption-irradiation cycles will anneal the adsorbed layer (prior to irradiation) to the point where the first imprint is located, thereby "enhancing" the first imprint. Such adsorbents preferably aggregate to form a pattern near the "pits" of the surface (see reference 16). Thus, when the adsorbent is applied a second time after irradiation, the reaction will occur again at the previously imprinted site (which is the site of the second adsorption). The second and subsequent pattern adsorption plus radiation imprinting thus enables the first imprinting to be amplified or chemically altered. The first imprint can be done by existing methods or alternatively with an atomic writing/etching instrument such as the tip of an STM (see the results described in figure 4, or see reference 9). This "enhanced" approach is important in that it can be used to increase the size of the first "pit" at the second and subsequent "etches" and to write or dope with a selective adsorbent in the vicinity of the previous mark.

The "blocking agents" we define is a useful aid in the imprinting of molecular patterns on surfaces. As used herein, "blocking agent" refers only to adsorbed molecules that preferentially block some of the adsorption sites, and do not directly participate in the imprinting of molecules. We have distinguished the four "blocking" modes (a) that the parent molecule P may exist at a sufficiently high coverage that decomposed P has its daughter fragments D captured by a structure surrounded by the undecomposed matrix P. This ensures that P does not move during irradiation and that D reacts near its parent P. (b) A comparable mode of blocking is the adsorption of an inert blocker B on surface vacancies after P has been adsorbed with the intended coverage. For this purpose we used benzene. Benzene does not imprint on the Si (111)7 x 7 surface when 193nm laser pulses are used because benzene does not undergo photoreaction or photodecomposition at this wavelength or energy (90 millijoules per pulse). However, benzene does show blocking effect. (c) (a) and (b) are capable of affecting the imprinted pattern not only by preventing movement of P, but also by deflecting the plasma D to a location near the previously inaccessible surface of P decomposition. Thus when benzene acts as a blocker of chlorobenzene, we have found that the chlorine reacts with (localizes) the silicon to form silicon "dimer pairs" arranged along the periphery of each unit. The "dimer pair" is located at a lower level on the surface than the adatom. It is speculated that the blocking agent, benzene, deflects the chlorine down to these new sites. The reaction is still localized, but is a novel localized reaction. (d) Finally, it is worth noting that it is advantageous to use the blocking agent B when the adsorption takes place before the deposition of the labeling agent (labeling agent). The presence of B blocks those sites by occupying those sites that would otherwise be occupied by P, changing the pattern of P. The imprinted pattern of D may change considerably after irradiation.

It is recognized that one aspect of "blocking" is the locking of the parent molecule at certain sites by trapping the parent molecule in a two-dimensional (or three-dimensional) matrix. The parent molecule is then decomposed and imprinted on the surface by irradiation. We have found that a variation of this process is that we can use light to "bond" the precursor P to the surface to some extent, even in the absence of the blocking agent. We have therefore found that chlorobenzene has a high probability of becoming chemically bonded "sticky" to Si (111)7 x 7 substrates upon 193nm irradiation. This bond is evidenced by the greatly increased energy required for desorption of the parent ClBz after irradiation. (mainly referred to herein as P that is not photolyzed). If P is initially bonded to the surface in a pattern by 193nm radiation, any radiation that can decompose P is used and the imprinted pattern of D is produced without shifting P. No inhibitor is required. Fig. 2-4 demonstrate that "blocking" (by P) and "bonding" (i.e., radiation-induced bonding) may be helpful in successfully forming a patterned imprint.

Since sorbents (particularly after annealing to slightly below the desorption temperature) generally exhibit ordering for one or both of the reasons given above (i.e., patterned bonding to the ordered substrate surface or ordering due to intermolecular forces of adsorption), a wide variety of sorbents can be used in this molecular patterning approach. The further requirement of the adsorbent to react with the solid below it after irradiation with photons, electrons or ions can also be met when the energetic highly reactive radicals containing reactive atoms, generated by the cleavage of the chemical bonds of the adsorbed molecules, are able to react with the solid surface. We have found (see reference 10, as we have found a photoinitiated reaction with a surface) that the irradiated adsorbent need not decompose prior to reacting with the surface. More generally, the excited or charged sorbent reacts while decomposing; reactive surfaces (e.g., Si (111) 7X 7) are effective as adsorbents (e.g., ClBz) for chemical excitation^-) And (4) pulling apart. The choice of imprinting agent to form the adsorbent can be made from a wide variety of gases used for radiation-induced writing and doping (e.g., halides, hydrides and oxides, metalorganics, compounds of Si, Ga, As and In, all of which are familiar to those skilled In the art of chemical vapor deposition CVD) and gases used for plasma etching (the most effective agent In this case being fluoride).

The three modes used in molecular imprinting-writing, doping, and etching-are now used in a broad class of devices known as microcircuitry. The pattern imprinting method disclosed herein provides a way to miniaturize these devices to a nanometer state, which has not been a goal of microcircuit technology to date. Other methods for finding patterns with nanometer scale patterns have not yet been developed using the principles of the method used in the present invention.

For example, the process disclosed in Gliton, U.S. Pat. No.5,129,991, has several important differences from the process disclosed herein: (i) the adsorbed gas does not need to be patterned in the Gliton method but is uniformly distributed, (ii) the etching pattern is formed because the macroscopic regions on the substrate have different light emission thresholds; and (3) the nature of the "localization" of the emission-induced reaction is localized reaction with large regions of the substrate, rather than at the molecular level.

The present invention differs from the Yan et al invention in several important respects. For example, the patterns disclosed herein that are imprinted onto the surface, or very relevant patterns, are pre-existing in the adsorbent layer, and the imprinting process of the present invention is radiation-induced. Yan et al do not disclose an adsorbent layer, nor a patterned layer, nor irradiation of the substrate surface under their experimental conditions.

The foregoing has described preferred embodiments for marking or imprinting a pattern on a surface to illustrate the principles of the present invention, but is not intended to limit the invention to these embodiments. The scope of the invention is to be defined by all embodiments encompassed within the following claims and their equivalents.

Reference to the literature

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10 "photochemistry of adsorbed molecules, charge transfer photodecomposition and photochemical reactions of methyl chloride on Ag (111)" by XI, St.J. Dixon-Warren et al, J. chem. Phys.98, p 5983 (1993).

11 "photochemical adsorption of the molecule, XII. in CH₃Metal surface photo-initiated ion-molecule reactions "of X/RCl/Ag (111) (X ═ Br; I), st.j.dixon-Warren et al, j.chem.phys.98, p.5984 (1993).

Photochemical of the adsorbed molecules, UV decomposition of OCS on V.LiF (001), "K.Leggett et al J. chem.Phys.93, p 3645 (1990).

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14 "electronically driven kinetics of gas-solid interface-decomposition, desorption and reaction of adsorbed molecules", R.J. Guest et al, Faraday Discuss.96 vol., p.117 (1993).

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"kinetics of surface alignment photochemistry (theory)," position scanning with hydrogen atoms "in the Hbr (001)/LiF (001) + hv system", V.J.Barclay et al, J.chem.Phys.97 Vol.9458 (1992).

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Claims (25)

1. A method of forming a molecular scale pattern on a solid surface, comprising:

providing a reactive solid having a surface and providing preselected molecules that can form an adsorption pattern on the surface of the reactive solid;

forming a pattern of adsorbed molecules at said surface of said reactive solid by exposing said surface to said preselected molecules; and

an imprinted pattern on the surface is formed by irradiating the surface and a pattern of adsorbed molecules with effective excitation energy to form a chemical bond between at least one of the constituents of each individual adsorbed molecule and the reactive solid, the chemical bond being located on the surface in close proximity to the adsorption site of the individual adsorbed molecule.

2. The method of claim 1, wherein at least one of the compositions of each individual adsorbed molecule chemically bonded to the reactive solid is the molecule itself, or a molecule or atomic fragment of the molecule, such that the retained imprint-forming species is the molecule itself or a molecule or atomic fragment of the molecule.

3. The method of claim 1, wherein the reactive solid is selected from the group consisting of crystalline solids and amorphous solids.

4. The method of claim 1, 2 or 3 wherein irradiating with effective excitation energy comprises irradiating with energy sufficient to initiate said formation of chemical bonds between at least one constituent of each of said adsorbed molecules and said reactive solid.

5. The method of claim 1, 2 or 3 wherein irradiating with effective excitation energy comprises exciting electrons in said reactive solid by radiation of an energy level sufficient to initiate formation of said chemical bond between at least one component of each of said adsorbed molecules and said reactive solid.

6. The method of claim 1, 2 or 3 wherein irradiating with effective excitation energy comprises bombarding said adsorbed molecules and said reactive solid with particles having energy sufficient to initiate said bond formation between at least one composition of each of said adsorbed molecules and said reactive solid.

7. The method of claim 6, wherein said particles are electron beams.

8. The method of claim 6, wherein said particles are positive or negative ion beams.

9. The method of claim 1, 2 or 3, wherein the reactive solid and preselected molecule are selected based on: and applying excitation energy to said imprinted pattern with radiation to cause atoms to be emitted from said surface, thereby generating a pattern of surface defects substantially identical to said imprinted pattern.

10. The method of claim 1, 2 or 3, wherein the excitation energy is selected to produce an electron-to-electron energy transfer from the reactive solid to the adsorbed molecules to initiate formation of the bond between at least one constituent of each of the adsorbed molecules and the reactive solid.

11. The method of claim 1, 2 or 3 wherein said preselected molecule is gaseous when said reactive solid is exposed to said molecule.

12. The method of claim 1, 2 or 3 wherein said preselected molecule is in a liquid state when said reactive solid is exposed to said molecule.

13. A method as claimed in claim 1, 2 or 3, wherein after exposure of said surface to said preselected molecule, the surface is annealed before being subjected to said excitation energy.

14. A method as claimed in claim 1, 2 or 3, wherein the reactive solid is a semiconductor.

15. The method of claim 14, wherein said semiconductor is a crystalline semiconductor selected from the group consisting of silicon, gallium arsenide, and indium phosphide.

16. The method of claim 1, 2 or 3 wherein said preselected molecule is selected from the group consisting of halides, hydrides, oxides, organometallic compounds, and compounds of Si, Ga, As and In.

17. The method of claim 1, 2 or 3, wherein the selection of the reactive solid and the preselected molecule is based on: the pattern of adsorbed molecules is formed by forces between the adsorbed molecules and the surface.

18. The method of claim 1, 2 or 3 wherein the reactive solid and preselected molecules are selected to form a self-assembled monolayer wherein the adsorbed molecule pattern is formed by forces between said adsorbed molecules.

19. A method according to claim 1, 2 or 3, wherein the step of forming a pattern of adsorbed molecules comprises forming an array of a pre-selected pattern of defects on at least a portion of said surface, thereby to obtain a corresponding pattern of adsorption.

20. The method of claim 19, wherein said pre-selected patterned array of defects is formed by removing atoms from or adding atoms to said surface using a scanning tunneling microscope.

21. The method of claim 1, 2 or 3, comprising exposing said surface to an effective inhibitor to affect diffusion of said adsorbed molecules at said surface after exposing said surface to said preselected molecules, wherein said inhibitor does not imprint said surface upon irradiation.

22. The method of claim 1, 2 or 3 comprising exposing said surface to an effective inhibitor to isolate preselected surface sites from said preselected molecules prior to exposing said surface to said preselected molecules, wherein said inhibitor does not imprint said surface upon irradiation.

23. The method of claim 4, wherein said adsorbed molecules are fully chemically bonded to said surface after irradiating the surface and pattern of adsorbed molecules with effective excitation energy, and wherein further irradiation causes decomposition of said adsorbed molecules leaving molecular or atomic fragments of said adsorbed molecules chemically bonded to said reactive solid to produce said pattern imprint.

24. A method as claimed in claim 1, 2 or 3, wherein the imprinted pattern is substantially the same as the pattern of adsorbed molecules.

25. A method as claimed in claim 1, 2 or 3, wherein the imprinted pattern is different from, but closely related to, the pattern of adsorbed molecules.