Method for producing surface-treated substrate

Abstract

Provided is a method that allows selective modification of a substrate surface using an environmentally friendly component. This method for producing a surface-treated substrate comprises: a vaporizing step for vaporizing a nitrogen-containing compound; and a step for selectively modifying the surface of the substrate by causing contact between the substrate and the nitrogen-containing compound, the surface of the substrate including a first region and a second region made from a material different from the first region.

Description

Method for manufacturing surface-treated substrate

[0001] The present invention relates to a method for producing a surface-treated substrate.

[0002] In the semiconductor manufacturing process, in order to cover and protect a portion of the surface of a substrate, a protective film is formed on that portion, and various surface treatments such as forming a metal layer or etching are performed on the substrate after the protective film is formed. As a method for forming a protective film on the surface of a substrate to protect a portion of the surface of the substrate, a technique for introducing a region-selective raw material gas to form a film on the substrate has been proposed (WO 2021 / 161830).

[0003] International Publication No. 2021 / 161830

[0004] With growing environmental awareness, there is a demand for alternative materials to perfluorinated compounds as region-selective source gases. In addition, with the further miniaturization of semiconductor devices, there is a demand for the ability to selectively form films on the substrate surface in areas that need to be protected.

[0005] The present invention has been made in view of the above problems, and aims to provide a method capable of selectively modifying the surface of a substrate with components that have a low environmental impact.

[0006] As a result of extensive research into solving the above problems, the present inventors have found that the above object can be achieved by employing the following configuration, and have thus completed the present invention.

[0007] In one embodiment, the present invention relates to a method for producing a surface-treated substrate, the method comprising: a vaporization step of vaporizing a nitrogen-containing compound (hereinafter also referred to as "compound [A]"); and a step of contacting a substrate, the substrate including a first region on a surface thereof and a second region made of a material different from that of the first region, with the nitrogen-containing compound to selectively modify the surface of the substrate.

[0008] According to the method for producing a surface-treated substrate, the surface of the substrate can be selectively modified with a nitrogen-containing compound.

[0009] The method for producing a surface-treated substrate according to each embodiment of the present invention will be described in detail below. Combinations of preferred aspects of the embodiments are also preferred.

[0010] <<Method for Producing Surface-Treated Substrate>> A method for producing a surface-treated substrate includes a vaporization step of vaporizing a nitrogen-containing compound, and a step (hereinafter also referred to as a “contact step”) of contacting a substrate, which includes a first region on a surface thereof and a second region made of a material different from that of the first region, with the nitrogen-containing compound to selectively modify the surface of the substrate.

[0011] <Vaporization Step> In this step, the compound (A) is vaporized. Note that the compound (A) may be liquid or solid at room temperature and normal pressure.

[0012] (Compound [A]) The compound [A] is not particularly limited as long as it is an organic compound that has a nitrogen atom and is vaporizable.

[0013] Examples of the compound [A] include a compound in which at least one carbon atom constituting a hydrocarbon having 3 to 30 carbon atoms is substituted with a nitrogen atom (hereinafter also referred to as "compound (α1)"), a compound in which some or all of the hydrogen atoms in the hydrocarbon are substituted with -NH ２ a group in which some or all of the hydrogen atoms in the compound (α1), compound (α2), or compound (α3) have been substituted with a monovalent or divalent heteroatom-containing substituent (hereinafter also referred to as "compound (α4)"); and a compound in which at least two selected from the group consisting of the compound (α1), compound (α2), compound (α3), and compound (α4) are combined (hereinafter also referred to as "compound (α5)").

[0014] Examples of hydrocarbons having 3 to 30 carbon atoms include chain hydrocarbons having 3 to 30 carbon atoms, alicyclic hydrocarbons having 3 to 30 carbon atoms, aromatic hydrocarbons having 6 to 30 carbon atoms, and combinations thereof.

[0015] Examples of chain aliphatic hydrocarbons having 3 to 30 carbon atoms include chain aliphatic saturated hydrocarbons having 3 to 30 carbon atoms and chain aliphatic unsaturated hydrocarbons having 3 to 30 carbon atoms. Examples of chain aliphatic saturated hydrocarbons having 3 to 30 carbon atoms include alkanes such as propane, 2-methylpropane, butane, 2-methylbutane, pentane, and hexane. Examples of chain aliphatic unsaturated hydrocarbons having 3 to 30 carbon atoms include alkenes such as propene, butene, and pentene; and alkynes such as propyne, butyne, and pentyne.

[0016] Examples of the alicyclic hydrocarbon having 3 to 30 carbon atoms include monocyclic saturated alicyclic hydrocarbons such as cyclopentane and cyclohexane; polycyclic saturated alicyclic hydrocarbons such as norbornane, adamantane, tricyclodecane and tetracyclododecane; monocyclic unsaturated alicyclic hydrocarbons such as cyclopentene, cyclopentadiene and cyclohexene; and polycyclic unsaturated alicyclic hydrocarbons such as norbornene, tricyclodecene and tetracyclododecene.

[0017] Examples of aromatic hydrocarbons having 6 to 30 carbon atoms include benzene, naphthalene, anthracene, phenanthrene, and pyrene.

[0018] Examples of heteroatoms constituting the divalent heteroatom-containing linking group and the monovalent or divalent heteroatom-containing substituent include an oxygen atom, a sulfur atom, a phosphorus atom, a silicon atom, a halogen atom, etc. Examples of halogen atoms include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.

[0019] Examples of the divalent heteroatom-containing linking group include -O-, -C(=O)-, -S-, -C(=S)-, and -SO ２ -, and groups in which two or more of these are combined.

[0020] Examples of the monovalent or divalent heteroatom-containing substituent include a halogen atom, a hydroxy group, a carboxy group, a cyano group, a sulfanyl group, and an oxo group (=O).

[0021] The compound (A) preferably has a nitrogen-containing aromatic heterocycle. Examples of the nitrogen-containing aromatic heterocycle include compounds in which at least one carbon atom constituting the above-mentioned aromatic hydrocarbon having 6 to 30 carbon atoms is substituted with a nitrogen atom, as well as five-membered nitrogen-containing aromatic heterocycles having 2 to 4 carbon atoms that exhibit aromaticity due to the inclusion of a nitrogen atom, and rings formed by combining such five-membered nitrogen-containing aromatic heterocycles with the above-mentioned aromatic hydrocarbons. Examples of the nitrogen-containing aromatic heterocycle include aromatic heterocycles containing only nitrogen atoms, such as pyrrole rings, pyrazole rings, imidazole rings, triazole rings, pyridine rings, pyridazine rings, pyrimidine rings, pyrazine rings, triazine rings, indole rings, benzimidazole rings, and quinoline rings, and aromatic heterocycles containing heteroatoms other than nitrogen atoms, such as oxazole rings and thiazole rings. Among these, imidazole rings, pyridine rings, and pyrimidine rings are preferred as the nitrogen-containing aromatic heterocycle.

[0022] The lower limit of the molecular weight of the compound (A) is preferably 80, more preferably 90, and even more preferably 100. The upper limit of the molecular weight is preferably 400, more preferably 380, and even more preferably 360.

[0023] The compound (A) has one or more nitrogen atoms. The number of nitrogen atoms in the compound (A) is preferably 1 to 4, more preferably 1 to 3, and even more preferably 1 or 2.

[0024] The compound [A] preferably has 6 or more carbon atoms. The lower limit of the number of carbon atoms in the compound [A] is preferably 7, more preferably 8, and even more preferably 9. The upper limit of the number of carbon atoms is preferably 30, more preferably 28, and even more preferably 26.

[0025] By setting the molecular weight, the number of nitrogen atoms, and the number of carbon atoms of the compound [A] within the above ranges, the compound [A] can be easily vaporized, and the selectivity of the substrate surface with the compound [A] and the chemical and physical stability of the modified region after selective modification (typically, a film of the compound [A]) can be improved.

[0026] The compound [A] preferably has a crosslinkable group. This can improve the heat resistance of the region (e.g., a film) modified by the compound [A]. Examples of the crosslinkable group include a group having a carbon-carbon unsaturated bond, a group having a cyclic ether structure such as an oxirane structure or an oxetane structure, a group having a cyclic carbonate structure, a cyano group, and a group having a benzocyclobutene structure. Examples of the group having a carbon-carbon unsaturated bond include a vinyl group, an ethynyl group, an ethenediyl group, and an ethynediyl group. Examples of the group having a cyclic ether structure include an oxiranyl group, an oxetanyl group, and an epoxycyclohexyl group. Among these, a group having a carbon-carbon unsaturated bond is preferred as the crosslinkable group.

[0027] When the compound (A) has a crosslinkable group, the number of crosslinkable groups is not particularly limited, but is preferably 1 to 5, more preferably 1 to 4, even more preferably 1 to 3, and particularly preferably 1 or 2.

[0028] Specific examples of the compound [A] include compounds represented by the following formulas (1-1) to (1-40).

[0029]

[0030]

[0031]

[0032] (Vaporization of Compound [A]) The method for vaporizing Compound [A] is not particularly limited, and examples thereof include heating a raw material storage container in which Compound [A] is stored, reducing the pressure inside the raw material storage container in which Compound [A] is stored, or a combination thereof. Compound [A] may be vaporized using a vaporization chamber instead of a raw material storage container. The size, material, and structure of the raw material storage container and the vaporization chamber are not particularly limited, and may be determined appropriately taking into account the heating temperature and the degree of reduced pressure.

[0033] The heating temperature is not particularly limited. The lower limit of the heating temperature is preferably 30° C., more preferably 35° C., and even more preferably 40° C. The upper limit of the heating temperature is preferably 150° C., more preferably 100° C., and even more preferably 70° C.

[0034] The pressure at which the pressure is reduced is not particularly limited.

[0035] When vaporizing the compound [A], the compound [A] itself may be vaporized, a plurality of compounds [A] may be combined, or a solution of the compound [A] dissolved in an organic solvent may be vaporized. The organic solvent is not particularly limited, and examples thereof include ester solvents, ether solvents, and hydrocarbon solvents.

[0036] Examples of ester-based solvents include carbonate-based solvents such as diethyl carbonate, acetate monoester-based solvents such as methyl acetate and ethyl acetate, lactone-based solvents such as γ-butyrolactone, polyhydric alcohol partial ether carboxylate-based solvents such as diethylene glycol monomethyl ether acetate and propylene glycol monomethyl ether acetate, and lactate-based solvents such as methyl lactate and ethyl lactate.

[0037] Examples of ether solvents include chain ether solvents such as n-butyl ether, polyhydric alcohol ether solvents such as cyclic ether solvents such as tetrahydrofuran, and polyhydric alcohol partial ether solvents such as diethylene glycol monomethyl ether and propylene glycol monomethyl ether.

[0038] Examples of hydrocarbon solvents include aliphatic hydrocarbon solvents such as n-pentane, n-hexane, and cyclohexane, and aromatic hydrocarbon solvents such as benzene, toluene, and xylene.

[0039] The vaporized compound [A] is introduced into, for example, a film formation chamber for the contacting step. The method for introducing the vaporized compound [A] into the film formation chamber is not particularly limited, and examples thereof include a method in which the vaporized compound [A] is directly circulated into the film formation chamber, and a method in which the compound [A] is circulated into the film formation chamber together with a carrier gas such as argon, nitrogen, or helium.

[0040] <Contacting Step> In this step, the nitrogen-containing compound is brought into contact with a substrate having a first region on its surface and a second region made of a material different from that of the first region, thereby selectively modifying the surface of the substrate. The substrate is placed, for example, in a film-forming chamber. By contacting the substrate with the compound [A] that has been circulating in the film-forming chamber, a thin film of the compound [A] can be selectively formed on the surface of the substrate. In particular, the selective modification in the contacting step is preferably a step of forming a film of the nitrogen-containing compound on the first region. The obtained film of the nitrogen-containing compound is typically a monomolecular film, but may also be a laminated film in which the nitrogen-containing compound is further adsorbed to part or all of the monomolecular film.

[0041] (Substrate) As the substrate, a substrate having a first region on its surface and a second region made of a material different from that of the first region can be preferably used. The first region is preferably made of a different material from the second region, thereby making the adsorption of the compound [A] different from that of the second region. As an example, the first region is a region having a higher adsorption of the compound [A] than the second region. From the viewpoint of selectively and densely modifying the surface of the substrate with the compound [A], it is preferable that the first region be formed of a conductive material and the second region be formed of an insulating material. Furthermore, the substrate to be selectively modified with the compound [A] preferably has a first region formed of a conductive material and a second region formed of an insulating material.

[0042] When the first region is a region formed of a conductive material, the first region contains a metal element. Examples of the metal element include at least one selected from the group consisting of copper, iron, zinc, cobalt, aluminum, titanium, tin, tungsten, zirconium, tantalum, germanium, molybdenum, ruthenium, gold, silver, platinum, palladium, and nickel. The metal element contained in the first region may be a simple metal, an alloy (nickel-copper alloy, cobalt-nickel alloy, gold-silver alloy, etc.), a conductive nitride (tantalum nitride, titanium nitride, etc.), or the like. The first region preferably contains a simple metal, an alloy, or a conductive nitride, more preferably at least one selected from the group consisting of copper, cobalt, tungsten, tantalum, titanium, molybdenum, and ruthenium, and even more preferably at least one selected from the group consisting of copper, tungsten, tantalum, titanium, molybdenum, and ruthenium.

[0043] When the second region is formed of an insulating material, the second region may be formed of a silicon-containing compound such as silicon oxide, silicon nitride, silicon oxynitride, or silicide (e.g., SiO ２ , SiOC, Si ３ N ４ The second region is preferably formed of a silicon oxide, a silicon nitride, a silicon oxynitride, a silicide, or a metal oxide (e.g., silicon nitride, silicon oxynitride, silicide, or metal oxide).

[0044] In particular, when the substrate to be selectively modified with the compound [A] has, as the first region, a region containing at least one selected from the group consisting of copper, tungsten, tantalum, titanium, molybdenum, and ruthenium, and has, as the second region, a region containing silicon oxide, silicon nitride, silicon oxynitride, or a metal oxide, this is preferable in that the compound [A] can be highly selectively and densely adsorbed to the first region out of the first and second regions.

[0045] The shape of the contact surface of the nitrogen-containing compound on the substrate is not particularly limited, and may be, for example, planar (either flush with or parallel to the substrate surface), concave, protruding, or a combination thereof relative to the substrate surface. Furthermore, when the contact surface has a first region and a second region, the arrangement of the first region and the second region in a planar view is also not particularly limited. For example, the first region and the second region may be adjacent to each other and disposed in predetermined regions on a plane. Alternatively, one of the first region and the second region may be disposed in a dot-like manner within the other region (in a sea-island structure). Furthermore, one of the first region and the second region may form the bottom surface of a hole or groove formed in the substrate, and the other may form its side surface. One of the first region and the second region may form the inner surface of a hole or groove formed in the substrate, and the other may form the substrate surface other than the hole or groove. One of the first region and the second region may form the surface of a protrusion formed in the substrate, and the other may form the substrate surface other than the protrusion surface.

[0046] The surface of the substrate to be brought into contact with the nitrogen-containing compound may be, for example, H ２ , N ２ and H ２ Mixture of gas with O ２ Pretreatment such as plasma treatment using gas or the like, cleaning treatment of the substrate surface, or wet modification treatment may be performed.

[0047] (Contact Conditions) When the vaporized compound [A] is brought into contact with the surface of the substrate, the inside of the film formation chamber is preferably heated in order to promote the formation of a thin film. The lower limit of the temperature in the film formation chamber is preferably 50°C, more preferably 70°C, and even more preferably 80°C. The upper limit of the temperature is preferably 150°C, more preferably 130°C, and even more preferably 120°C.

[0048] When reducing the pressure inside the film formation chamber in the contact step, the lower limit of the pressure is 1.0 × 10 -３ Pa is preferred, 4.0 × 10 -３ Pa is more preferable, and 8.0 × 10 -３ The upper limit of the pressure is 1.0×10 -１ Pa is preferred, 6.0 × 10-２ Pa is more preferable, and 2.0 × 10 -２ Pa is more preferred.

[0049] The contact time between the substrate and the compound (A) may be appropriately set in consideration of the thickness of the target thin film. The lower limit of the contact time is preferably 10 seconds, more preferably 20 seconds, and even more preferably 30 seconds. The upper limit of the contact time is preferably 20 minutes, more preferably 10 minutes, and even more preferably 5 minutes.

[0050] The contacting step may be repeated multiple times to obtain the desired thin film thickness.

[0051] <Heating Step> After the contacting step, a step of heating the substrate that has been subjected to the selective modification may be performed. This can improve the heat resistance and strength of the film of the compound [A]. In particular, when the compound [A] has a crosslinkable group, crosslinking between the compounds [A] progresses, thereby further improving the heat resistance and strength of the film.

[0052] The heating temperature and heating time are not particularly limited. The lower limit of the heating temperature is preferably 100°C, more preferably 150°C, and more preferably 200°C. The upper limit of the heating temperature is preferably 500°C, more preferably 450°C, and even more preferably 400°C. The lower limit of the heating time is preferably 30 seconds, more preferably 1 minute, and more preferably 3 minutes. The upper limit of the heating time is preferably 30 minutes, more preferably 20 minutes, and even more preferably 10 minutes.

[0053] <Metal Layer Forming Step> After forming the film, a metal layer forming step may be performed to form a metal layer on the second region. The first region of the substrate surface is covered with a film (typically a monomolecular film) formed from the compound [A]. Therefore, when forming a metal layer in the metal layer forming step, the metal layer can be selectively formed on the second region out of the first region and the second region.

[0054] The method for forming the metal layer is not particularly limited. From the viewpoint of highly selective pattern formation in a fine region, it is preferable to form the metal pattern by chemical vapor deposition (CVD) or atomic layer deposition (ALD). The metal layer formed on the second region may be a barrier layer for separating the metal portion from the insulating portion or for suppressing metal diffusion from the metal portion. The thickness of the metal layer can be appropriately set depending on the application of the substrate, and is, for example, 1 to 30 nm.

[0055] (Removal Step) After the formation of the metal layer in the metal layer forming step, the method may further include a step of removing the film of the nitrogen-containing compound formed on the first region. The film can be removed by, for example, etching. Examples of etching methods include known methods such as reactive ion etching (RIE) and physical etching. Examples of reactive ion etching include CF ４ Ya O ２ Examples of the etching method include chemical dry etching, which uses a gas or the like and utilizes the difference in etching rate between layers; and chemical wet etching (wet development), which uses a liquid etching solution such as an organic solvent or hydrofluoric acid. Examples of physical etching include sputter etching and ion beam etching. Of these, reactive ion etching is preferred, and chemical dry etching or chemical wet etching is more preferred.

[0056] Examples of solvents used in chemical wet etching include acetic acid, APM (a mixture of ammonia, hydrogen peroxide, and water), SPM (a mixture of sulfuric acid and hydrogen peroxide), a 2.38% by mass aqueous solution of tetramethylammonium hydroxide (TMAH), dilute hydrofluoric acid, and water.

[0057] By carrying out the above steps, a metal layer is selectively formed on the second region out of the first and second regions, and a substrate can be produced in which the surface of the first region (e.g., the substrate surface) is exposed in the first region. The obtained substrate may further undergo various treatments, such as plating. When plating is performed, the plating method is not particularly limited, and known electroplating, electroless plating, hot-dip plating, vacuum plating, vapor-phase plating, etc. can be used. When the method of the present disclosure is applied to a semiconductor manufacturing process, electroplating or electroless plating is preferred. After plating, if necessary, the substrate surface may be planarized (CMP treatment, etc.).

[0058] The present invention will be specifically described below based on examples. Note that the examples shown below are representative examples of the present invention, and the scope of the present invention should not be construed as being narrow.

[0059] <Preparation of Surface-Treated Substrate> The following nitrogen-containing compounds were used as raw materials.

[0060] [[A] Compound] A-1: ​​A compound represented by the following formula (A-1) A-2: A compound represented by the following formula (A-2) A-3: A compound represented by the following formula (A-3) A-4: A compound represented by the following formula (A-4) A-5: A compound represented by the following formula (A-5) A-6: A compound represented by the following formula (A-6) A-7: A compound represented by the following formula (A-7) A-8: A compound represented by the following formula (A-8) A-9: A compound represented by the following formula (A-9) A-10: A compound represented by the following formula (A-10) A-11: A compound represented by the following formula (A-11) A-12: A compound represented by the following formula (A-12) A-13: A compound represented by the following formula (A-13) A-14: A compound represented by the following formula (A-14) A-15: A compound represented by the following formula (A-15) A-16: A compound represented by the following formula (A-16) A-17: A compound represented by the following formula (A-17) A-18: A compound represented by the following formula (A-18) A-19: A compound represented by the following formula (A-19) A-20: A compound represented by the following formula (A-20) A-21: A compound represented by the following formula (A-21) A-22: A compound represented by the following formula (A-22) A-23: A compound represented by the following formula (A-23) A-24: A compound represented by the following formula (A-24) A-25: A compound represented by the following formula (A-25) A-26: A compound represented by the following formula (A-26) A-27: A compound represented by the following formula (A-27) A-28: A compound represented by the following formula (A-28)

[0061]

[0062]

[0063] [Example 1-1] A substrate (S1) on which a copper film was formed was placed in a chamber. The pressure in the chamber was increased to 1.0 × 10 -２The pressure was reduced to 100 Pa, and the temperature inside the chamber was heated to 50°C. The temperature of the vaporization vessel containing compound (A-1) was maintained at 30°C, and the vaporized compound (A-1) was introduced into the chamber and brought into contact with the substrate for 10 minutes. The substrate (S1) was removed from the chamber, washed with ethanol, and dried to obtain a surface-treated substrate for evaluation (S1-1). Except for using a substrate (S2) having a silicon oxide film formed thereon instead of the substrate (S1) having a copper film formed thereon, the substrate was treated under the same conditions as for the surface-treated substrate for evaluation (S1-1), to obtain a surface-treated substrate for evaluation (S2-1).

[0064] Examples 1-2 to 1-32 Surface-treated substrates for evaluation (S1-1) to (S1-32) and surface-treated substrates for evaluation (S2-1) to (S2-32) were prepared in the same manner as in Example 1, except that the types of [A] compounds, chamber temperatures, and vaporization vessel temperatures shown in the following Table 1 were used. In the following Table 1, the "(1 / 1)" symbol for the [A] compounds in Examples 1-29 to 1-32 indicates that two types of [A] compounds were used in a volume ratio of 1:1.

[0065] <Evaluation of Selective Modification> The contact angle of water on the surface of the surface-treated substrate for evaluation was measured using a contact angle meter ("DropMaster DMo-701SA" manufactured by Kyowa Interface Science Co., Ltd.). The contact angle of the substrate before surface treatment was measured using a substrate washed with ethanol. A value obtained by subtracting the contact angle of the substrate before surface treatment from the contact angle of the substrate after surface treatment of 20 to 90 was evaluated as A, and a value less than 20 was evaluated as B. If the static contact angle of the substrate after surface treatment with the compound [A] is significantly increased compared to the static contact angle of the substrate before surface treatment, it can be determined that a film by the compound [A] has been formed. On the other hand, if the static contact angle of the substrate after surface treatment with the compound [A] is almost unchanged compared to the static contact angle of the substrate before surface treatment, it can be determined that a film by the compound [A] has not been formed. From the evaluation of these changes in static contact angle, it can be determined that the compound [A] has selectively modified the substrate (S1).

[0066]

[0067] According to the method for producing a surface-treated substrate of the present invention, the surface of the substrate can be selectively modified with a nitrogen-containing compound.

Claims

1. A method for producing a surface-treated substrate, comprising: a vaporization step of vaporizing a nitrogen-containing compound; and a step of contacting a substrate having a first region on its surface and a second region made of a material different from that of the first region with the nitrogen-containing compound to selectively modify the surface of the substrate.

2. The method for producing a surface-treated substrate according to claim 1, wherein the selective modification is a step of forming a film of the nitrogen-containing compound on the first region.

3. The method for producing a surface-treated substrate according to claim 2, further comprising the step of forming a metal layer on the second region after forming the film.

4. A method for manufacturing a surface-treated substrate according to any one of claims 1 to 3, wherein the first region is formed of a conductive material, and the second region is formed of an insulating material.

5. The method for producing a surface-treated substrate according to claim 3, further comprising the step of removing the film of the nitrogen-containing compound after forming the metal layer.

6. The method for producing a surface-treated substrate according to claim 1, wherein the nitrogen-containing compound has a molecular weight of 80 or more and 400 or less, has one or more nitrogen atoms, and has six or more carbon atoms.

7. The method for producing a surface-treated substrate according to claim 1, wherein the nitrogen-containing compound has a crosslinkable group.

8. The method for producing a surface-treated substrate according to claim 1, wherein the nitrogen-containing compound has a nitrogen-containing aromatic heterocycle.