JPS6240697B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method and material for enhancing the contrast of images of objects, such as masks for photolithographic processes in the manufacture of integrated circuits. Photolithographic processes in the manufacture of integrated circuits are primarily carried out by optical means. In the drive to reduce circuit size, improve performance, and increase yield, the required resolution associated with each generation of circuit technology has been achieved by optical devices. Recently, the resolution of projection photolithographic apparatus has begun to approach physical limits imposed by practical constraints on numerical aperture and wavelength. Although further advances in photolithographic technology are expected, dramatic improvements in inherent lens resolution are not expected. In order to further reduce the minimum detail size achievable by optical techniques, it is necessary to change some other factor in the photolithography process to achieve further improvements. One area where further improvement is possible is photoresist operation. Each photoresist is characterized in that a certain degree of incidence contrast is required to produce a pattern that can be used for subsequent processing.
This minimum required illumination contrast is called the photoresist's contrast threshold. Depending on the nature of the substrate, desired pattern thickness, and photoresist edge geometry, conventionally used positive-tone photoresists have contrast thresholds of 85-90%. Currently, most production is done under 90% or higher contrast. However, since the contrast of an image decreases with increasing spatial frequency in the image, lowering the contrast threshold of the photoresist increases the resolution that can be achieved with a given optical device. The present invention now relates to photoresist operations in which the contrast of the aerial image used in the photoresist operation is enhanced prior to application to the photoresist. One of the objectives of the present invention is to reduce the minimum contrast required to produce a usable image in a photoresist. It is also an object of the present invention to provide novel photobleachable compounds and substances. In accordance with one embodiment of the invention, a photoresist layer is provided having a first thickness and a predetermined contrast threshold. Also, an object or mask is provided that has opaque and transparent areas. By projecting light of a predetermined wavelength through the object, an image of the object is formed on the photoresist layer with a contrast less than a predetermined contrast threshold of the photoresist. Between the object and the photoresist layer, a layer of photobleachable material containing a photobleachable compound is disposed adjacent to the surface of the photoresist layer. Such a photobleachable compound is sensitive to light of the above wavelength, and its extinction coefficient/molecular weight ratio in the non-bleached state is l/
Greater than 10 in g/cm. Furthermore, the ratio of the extinction coefficient in the non-bleached state to the extinction coefficient in the bleached state is also greater than 10. Projecting light of the above wavelength and predetermined intensity through the object onto the photobleachable material layer for a certain period of time causes a decrease in the optical density of the photobleachable material layer that is directly proportional to the amount of incident light of the above wavelength. As a result, the integral contrast of the image transmitted by the photobleachable material layer increases with the amount of light transmitted by it and reaches a maximum value,
Then it decreases. The parameters of the photobleachable material layer are set such that the maximum value of the integrated contrast is greater than a predetermined contrast threshold of the photoresist layer. The sensitivity and thickness of the photoresist layer are such that the amount of light transmitted by the photobleachable material layer within a predetermined range is such that the photoresist layer is fully exposed and is greater than a predetermined contrast threshold of the photoresist layer in the transmitted image. Settings are made to obtain integral contrast. The time for projecting the light of the wavelength through the subject is sufficient for the amount of light within the predetermined range to be transmitted through the layer of material that can be mixed and bleached. After removing the photobleachable material layer, developing the photoresist layer produces a pattern in the photoresist layer that has enhanced contrast compared to the low contrast image of the object. The invention also relates to rotary castable mixtures capable of forming photobleachable layers having an absorption maximum in the range 300-450 nm. Such a mixture comprises (A) 100 parts by weight of an organic solvent, (B) 1 to 30 parts by weight, preferably 5 to 30 parts by weight.
15 parts by weight of an inert organic polymeric binder, and (C) 1
30 parts by weight, preferably 5 to 15 parts by weight of arylnitrone. The novel features believed to be inherent to the invention are pointed out with particularity in the appended claims. The structure and manner of carrying out the invention, as well as other objects and advantages, may, however, be best understood from the following description, taken in conjunction with the accompanying drawings. Currently, most photolithographic operations are performed by projection techniques that expose photoresist using an aerial image of a mask. For low-contrast aerial images, even image parts corresponding to dark areas of the mask have significant intensity. As the contrast decreases, it becomes increasingly difficult to distinguish between dark and bright areas. In accordance with the present invention, a method is provided for enhancing the contrast of an image incident on a photoresist, thereby facilitating such identification. Such contrast enhancement is based on the use of photobleachable materials that are initially relatively opaque but become relatively transparent after exposure to a certain amount of light. The light transmittance of an idealized decolorizable layer is shown in FIG. When an aerial image of the mask is incident on such a layer, the parts of the bleachable layer exposed to the highest intensity will be completely bleached first, while the parts exposed to the lowest intensity will be bleached later. be done. The progress of such a decolorization process is shown in FIGS. 2A to 2F. FIG. 2A shows the relative transmittance of an object such as a mask consisting of opaque areas giving 0% transmission 11 and open or transparent areas giving 100% transmission 12. I ₀ is the intensity of the incident light, I is the intensity of the transmitted light, and Eb is the amount of light required to cause bleaching of the layer. FIG. 2B shows a graph 13 plotting the relative intensity of an aerial image as a function of position when a mask is placed in an optical projection device to produce an aerial image useful, for example, for sensitizing a photoresist layer. There is. It is assumed that the wavelength of the light used and the dimensions of each region of the mask are such as to provide the contrast shown. Imax is the maximum intensity of the image, and Imin is the minimum intensity of the image. FIG. 2C shows a cross section of an idealized photobleachable material layer 15. FIG. FIG. 2D shows the layer 15 at the first point in time.
The decolorized state of the sample is represented by a dotted line 16. FIG. 2E shows layer 1 at a later point in time.
The decolorized state of No. 5 is represented by a dotted line 17. In FIG. 2F, the bleached state of the layer 15 is represented by dotted lines 18 and 19 when the exposure is completed and the bleaching has stopped. If exposure is stopped at a point corresponding to Figure 2F, layer 1
The transmittance of 5 corresponds to the transmittance of the original mask. By coating a conventional photoresist layer with such a material, the resulting composite can have a contrast threshold that is lower than that of the photoresist layer alone. This is possible if the sensitivity of the photoresist is sufficiently high and the exposure time is shorter than the bleaching time. Such a photobleachable material layer essentially constitutes a mask formed directly on the photoresist layer. The real effect of such a directly formed mask is to increase the contrast upon incidence on the photoresist layer over that of the aerial image. In applying contrast enhancement techniques to submicron photolithographic operations, several physical and chemical constraints exist regarding the contrast enhancement layer itself. That is, the contrast enhancement layer must be thin and at the same time have high optical density. The thickness limitation is due to the small depth of focus of high resolution optical devices. For this reason,
Thickness is limited to about 1 micron or less. Since the contrast-enhancing layer must have a high optical density, it is necessary that the photochemical component of the layer has strong light absorption. Since the light transmittance after decolorization is determined by the absorbance of the photoproduct, the photoproduct must have a much smaller extinction coefficient than the parent molecule. By the way, the extinction coefficient is defined by the following equation. ε=A/bc...(1) In the formula, A is the absorbance, b is the optical path length (cm), and c
is the concentration (mol/l). Moreover, absorbance is defined by the following formula. A=log ₁₀ I ₀ /I (2) where I ₀ is the intensity of incident light, and I is the intensity of transmitted light. The extinction coefficient ε of a substance is determined by determining parameters A, b, and c in equation (1). Parameter A is obtained from equation (2).
First, the concentration c of the substance in the solution is determined by dissolving a known weight of the substance in a known volume of solvent.
This solution is injected into a cell of known dimensions, placed in the optical path of a spectrophotometer, and light of known intensity I ₀ is incident on the cell. Next, the intensity of transmitted light from the cell is measured. Similarly, only the solvent is injected into another cell of the same known dimensions and then placed in the optical path of the spectrophotometer, and light of known intensity I ₀ is incident on the cell. Next, the intensity of transmitted light from the cell is measured. The solvent-only intensity measurements are used to correct the transmitted intensity I for absorption by the cell and solvent. By substituting the values of incident intensity I ₀ and transmitted intensity I obtained in this way into equation (2), the absorbance A
is required. The optical path length b is determined from the known dimensions of the cell. By substituting the values of b, c, and A obtained in this way into formula (I), the extinction coefficient of this substance can be determined. The method for measuring the extinction coefficient of a substance is conducted by Hold, Rinehart & Winston, Inc., New York City, New York, USA.
Fundamentals of Analysis by Douglas A. Skoog & Donald M. West, published by Winston, lnc.
Fundamentals of Analytic
Chemistry), 2nd edition (1969), pages 644-652. Continuing again, the quantum yield of the decolorization reaction must be as high as possible in order to minimize the increase in required exposure time. Also, since photoresist layers are typically applied by spin coating techniques, it would be advantageous if the contrast enhancement layer could also be applied by a similar method. Note that the solvent that dissolves the photobleachable substance must be compatible with the photoresist layer. Furthermore, it must be possible to form a contrast-enhancing layer with good optical properties by spin coating.
Finally, the wavelength range in which these photobleachable materials work must be the same as the wavelength range in which the optical projection device operates. Many direct wafer processing equipment operate at wavelengths of 405 nm or 436 nm. For use in the Optimetrix 10:1 DSW instrument available from Optimetrix Co., Mountain View, Calif., USA.
A wavelength of 405 nm was chosen. A search for a substance that can be photobleached was conducted while taking these constraints into account. In particular, a search was conducted for substances that can be photobleached, focusing on the fact that they have an absorption maximum within the wavelength range of 300 to 450 nm. Additionally, a model of the bleaching process was devised for use in evaluating photobleachable materials suitable for use in contrast-enhancing layers. The parameters of such a model are shown in Table 1 below. 1. Surface quantity theory Light ε _A Extinction coefficient of non-bleached molecules ε _B Extinction coefficient of bleached molecules Φ Quantum yield of bleaching reaction N ₀ Initial density of non-bleached molecules F ₀ Flux density of photons incident on contrast enhancement layer t ₀ Contrast Thickness of the enhancement layer n _fRefractive index of the bleached layer _n sRefractive index of the glass substrate on which the contrast enhancement layer is installed Based on the above analytical results and the model of the bleaching process, three criteria regarding the parameters of the material were established. However, they are shown in Table 2 below. 2nd table Weight value (1) ε/molecular weight 〓100/g・cm (2) Φ 〓0.2 (3) ε when not bleached/ε when bleached 〓30 ε/ε when bleached The first criterion is optical It is based on the requirement for high-density films and is essentially related to the packing density of absorption centers in the contrast-enhancing layer. The second criterion is based on the requirement of a transition from the unbleached state to the bleached state as rapidly as possible. It should be noted that whether a given quantum yield qualifies or not depends to some extent on the first criterion. This is because improvements in the first criterion can compensate for deficiencies in the second criterion. The third criterion is based on the requirement of transparency of the contrast enhancement layer after bleaching. Using these criteria, we first searched for substances that can be photobleached. When selecting an appropriate photobleaching compound, we evaluate the model of the bleaching process, and by testing layered compounds, we can calculate the relative transmittance over time or light amount when the light intensity is kept constant. was calculated as a function of When various photobleachable compounds exhibiting decolorizing properties based on different decolorizing mechanisms were evaluated, it was found that photobleachable compounds by photoisomerization are particularly suitable. Among those compounds, the formula Arylnitrones of the formula have been found to be particularly suitable. In formula (I), Z is formula (R ³ )a-
Q—a monovalent group represented by R ⁴ — or R ⁵ —,
Z' is a monovalent group represented by the formula -R ⁶ (X) _b , and R, R ¹ and R ³ are a hydrogen atom, a C(1-8) alkyl group, a C(1-8) substituted alkyl group, C (6~
13) A monovalent group selected from the group consisting of an aryl hydrocarbon group and a C(6-13) substituted aryl hydrocarbon group. Q is monovalent, divalent or trivalent selected from the group consisting of F, C, Br, I, O, S and N.
is a valence atom and a has a value of O, 1 or 2. R ⁴ is a C(6-13) aryl hydrocarbon group or a C(6-13) substituted aryl hydrocarbon group. R ⁵ is 1 selected from O, N and S
Substituted or unsubstituted C (6 to
20) Can be selected from the group consisting of aromatic heterocyclic compounds. ^R6 is a C(6-20) aromatic hydrocarbon group, and X is a halogen atom, a cyano group, an alkylcarbonyl group, a C(1-8) alkyl group, a C(1-8) substituted alkyl group, C (6-13)
selected from the group consisting of an aryl hydrocarbon group, a C(6-13) substituted aryl hydrocarbon group, and an alkoxycarbonyl group, where b is 0, 1, 2, or 3
These groups can be used in any combination depending on the value of . n has a value of 0, 1, 2, 3 or 4. The above compounds are described in ``Methoden'', edited by Houben-Weyl.
Der Olga-Nitsien Hemy (Methoden)
der Organischen Chemie, Vol. 10, Part 4 (1968), pages 315-416, or Jan Hamer & Anthony in Chemical Reviews, Vol. 1 (64). It can be prepared according to the method described in "Nitrones" by J. Macaluso, pp. 476-483. A variety of aryl ring systems can be constructed with various substituents to suit the particular requirements of optical equipment used in photolithographic operations. Such arylnitrones are 2 to 5×10 ⁴ l/
It has an extinction coefficient of mol·cm and is decolorized with a quantum yield within the range of 0.1 to 0.5. For wafer direct processing equipment capable of projection at 405 nm, the general formula It has been found that nitrones of the formula (wherein n has a value of 0 or 1) are particularly useful. Among this class of p-dialkylaminoaryl nitrones, for example, the formula It also includes heterocyclic compounds such as those represented by Suitable binders useful in preparing rotary castable mixtures for forming photobleachable material layers containing arylnitrones of formula (I) include vinyl acetate polymers (homopolymers and copolymers). copolymers of styrene or its derivatives, monopolymers and copolymers of acrylic acid or methacrylic acid esters, acetal resins, acrylonitrile-butadiene copolymers, Included are ethylcellulose and other hydrocarbon soluble cellulose ethers, cellulose propionate and other hydrocarbon soluble cellulose esters, polychloroprene, polyethylene oxide and polyvinylpyrrolidone. Suitable solvents useful in preparing rotocastable mixtures for forming photobleachable material layers containing arylnitrones of formula (I) include aromatic hydrocarbons (e.g. toluene, xylene, ethylbenzene, (e.g. chlorobenzene) or with aliphatic hydrocarbons (e.g. cyclohexane)
halogenated aliphatic compounds (eg, trichloroethylene, methylchloroform, etc.), and alcohols (eg, propanol, butanol, etc.). Diarylnitrones of formula () in which R ³ is CH ₃ CH ₂ - and n is O have been found to be particularly suitable. This nitrone, called α-(4-diethylaminophenyl)-N-phenylnitrone, strongly absorbs light at a wavelength of 405 nm and is decolorized with high efficiency at the same wavelength. Such decolorization occurs as a result of the compound undergoing monomolecular cyclization to oxaziridine. This nitrone is a relatively small polar solvent (e.g. toluene, ethylbenzene, etc.)
It is extremely soluble in polystyrene, poly(hydroxyethyl methacrylate), poly-α-
Methylstyrene, poly(methyl methacrylate), polyvinylpyrrolidone, vinylpyridine
Forms good films with various polymers such as styrene copolymers and allyl alcohol-styrene copolymers at high loadings. α-(4-
Diethylaminophenyl)-N-phenylnitrone has an extinction coefficient/molecular weight ratio of 130 l/g·cm at 405 nm. In order to form a contrast-enhancing layer from this compound, first, α-(4-diethylaminophenyl)-N
- Phenylnitrone and a styrene-allylic alcohol copolymer binder in a proportion of 5% (by weight) were dissolved in toluene. Glass substrates were spin coated with the solution thus obtained to a thickness of 250 nm.
When the relative transmittance of such a sample was tested at 405 nm, it was found that it had a relative transmittance-time characteristic as shown in FIG. By using a model of the bleaching process, the third
The contrast improvement for the contrast-enhancing layer in the figure was calculated as a function of exposure time. To do this, it is sufficient to calculate the degree of decolorization at two representative points in the predetermined pattern. In this example, two incident intensities were chosen that correspond to the maximum and minimum values of the grid pattern of lines and gaps. The contrast C corresponding to these two intensity levels
can be calculated from the following contrast definition formula. C=Imax-Imin/Imax+Imin...(3
) By using a model of the bleaching process, the transmitted intensity at the maximum and minimum levels is determined as a function of the amount of incident light. From these quantities, instantaneous contrast and integrated contrast can be calculated as a function of the amount of incident light. FIG. 4 shows a set of graphs representing measurement and calculation results for the case where a 30% contrast pattern is incident on the sample as described above. Graph 2
1 shows the relative transmittance at the relative maximum level as a function of the amount of incident light in Joules. Graph 22 shows the relative transmittance at the relative minimum level as a function of the amount of incident light. Graph 23
shows the instantaneous contrast determined from graphs 21 and 22 using equation (3) as a function of the amount of incident light. Graph 24 shows the integral contrast determined from equation (3) using integral values instead of instantaneous values of Imax and Imin. Since bleaching is a dynamic process, the contrast resulting from transmitted light is also a function of the amount of light. A more useful graph of the corresponding data as a function of transmitted light is shown in FIG. 5 below. Using these graphs, it is possible to evaluate the degree of contrast enhancement as a function of the sensitivity of the photoresist layer to be used with the contrast enhancement layer. Graph 26 shows the relative transmittance at the relative maximum level as a function of the amount of transmitted light. Graph 27
shows the relative transmittance at the relative minimum level as a function of the amount of transmitted light. Graph 28 is expressed by formula (3)
The instantaneous contrast determined from graphs 26 and 27 using the above graphs is shown as a function of the amount of transmitted light. Graph 29 shows the integral contrast determined from equation (3) using the integral values of Imax and Imin as a function of the amount of transmitted light. Next, a method for utilizing such a contrast-enhancing layer will be described and the results obtained with that method will then be compared with those obtained under the same conditions without the use of a contrast-enhancing layer. The following discussion will be made with reference to FIGS. 6A-6E, which illustrate steps of a method of forming a pattern of photoresist on a suitable substrate. FIG. 6A shows the placement of a suitable layer 32 of photoresist on substrate 31. FIG. Suitable photoresists include, for example:
Shipley Company, located in Newton, Massachusetts, United States.
For example, the Shipley 1400 series positive photoresists are available from the Company. Such positive photoresists are comprised of a novolac resin or poly(vinylphenol), a diazonaphthoquinone ester and a solvent (eg cellosolve acetate or xylene). A layer of desired thickness is formed by depositing liquid photoresist on the surface of the substrate and then spinning. After heating this photoresist layer to remove the solvent, a contrast enhancement layer 33 is placed on the surface of the photoresist layer. The contrast enhancement layer 33 is formed from a solution of 5% (by weight) styrene-allylic alcohol copolymer binder and 5% (by weight) α-(4-diethylaminophenyl)-N-phenylnitrone in a toluene solvent. It is completed. This solution is placed on the surface of the photoresist layer 32 and then rotated.
Then, by heating to remove the solvent, the sixth
A layer 33 of desired thickness as shown in Figure B is formed. The structure thus obtained is exposed to a light pattern consisting of illuminated areas 35 and non-illuminated areas 36 below the arrow 37 representing illumination, as shown in FIG. 6C. Such operations are carried out within a range of light levels to which the photoresist layer is sensitive and for a time sufficient to produce a contrast-enhanced image such that the photoresist layer is fully exposed. Contrast enhancement layer 33 is then removed using a suitable stripping solvent (eg, trichlorethylene) that removes the contrast enhancement layer without affecting the photoresist layer, as shown in FIG. 6D. The exposed areas of the photoresist layer are then removed, leaving unexposed or incompletely exposed areas 3, as shown in FIG. 6E.
8, 39 and 40 will remain. In order to demonstrate the contrast threshold enhancement obtained for the composite of a contrast-enhancing layer and a photoresist layer according to the present invention, the above-mentioned compound comprising α-(4-diethylaminophenyl)-N-phenylnitrone and a styrene-allylic alcohol copolymer was prepared. Various patterns were formed using a photobleachable material with a Shipley 1400 series positive photoresist. That is, a first silicon wafer was coated with only a 1.6 micron thick photoresist layer, while a second silicon wafer was coated with a 1.6 micron thick layer of photoresist and a 0.25 micron thick layer of photobleachable material. Then a 2 micron wide opaque line, a 2 micron wide transparent gap, a width
Optimetric 10:1 for objects consisting of 0.8 micron opaque lines and 0.8 micron wide transparent gaps.
A projection device projected light with a wavelength of 405 nm. By using a range of light intensities, patterns were projected into the photoresist layer of a wafer having only a photoresist layer, and patterns were also projected into the photoresist layer of a wafer having a photoresist layer and a contrast enhancement layer. .
The photoresist layer of each wafer was developed, resulting in a plurality of patterns of lines and gaps formed in the photoresist layer. By using a minimum light intensity that produces a gap of 0.8 microns at the interface between the photoresist layer and the substrate, the pattern obtained on a wafer with only a photoresist layer is 0.8 microns at the interface between the photoresist layer and the substrate.
A comparison was made with the pattern obtained on a wafer with a photoresist layer and a contrast-enhancing layer by using a minimum light intensity that produced micron gaps. The 2.0 micron wide lines and 2.0 micron wide gaps were about right for both the wafers with only the photoresist layer and the wafers with the photoresist layer and contrast enhancement layer. For wafers with only a photoresist layer, 0.8 micron wide lines were generally overexposed when using light doses that produced 0.8 micron wide gaps in the photoresist layer. However, in a wafer with a photoresist layer and a contrast enhancement layer, a 0.8 micron wide line is moderately exposed when using a light dose that produces a 0.8 micron wide gap in the photoresist layer, and a 0.8 micron wide line in the photoresist layer is This resulted in a line of Furthermore, the sidewalls of the contrast-enhanced pattern were nearly vertical. It should be noted that the poor quality of the pattern obtained in the photoresist layer without a contrast-enhancing layer is due to the high spatial frequency in the aerial image obtained from the optometric projection device, and therefore the low contrast of the aerial image. . Although the invention has been described above in connection with a particular positive-working photoresist, other positive-working and negative-working photoresists can also be used. Additionally, although a specific thickness of photoresist layer and a specific thickness of contrast enhancement layer were used in the above examples, it will be appreciated that other thicknesses of photoresist layer and contrast enhancement layer may also be used. . It is noted that the photoresist layer preferably has a thickness of about 3 microns or less, and the contrast enhancement layer preferably has a thickness of about 1 micron or less. Although equal weight proportions of photobleachable compound and binder were used in the composition of the contrast-enhancing layer described above, other proportions can be used if desired. Although we mentioned above that photoresists have a property called contrast threshold, it should be noted that this property depends not only on the photoresist itself but also on the conditions of use of the photoresist (for example, the type of substrate used and its reflectivity). Should. Regarding the photobleaching compound, it is said that it is preferable that the extinction coefficient/molecular weight ratio in the non-bleached state is larger than about 100 and the ratio of the extinction coefficient in the non-bleached state to the extinction coefficient in the bleached state is larger than about 30. Satisfactory results should be obtained even with a value of the ratio of about 10. Although a specific class of photobleachable compounds based on monomolecular cyclization (i.e., aryl nitrones) was used above, other classes of photobleaching compounds based on monomolecular cyclization and other bleaching mechanisms (e.g., photocleavage) were used. It goes without saying that chemical compounds can also be used according to the invention. The above α-(4-
To prepare N-phenylnitrone (diethylaminophenyl), the condensation of 18.5 g (0.1 mol) p-diethylaminobenzaldehyde and 11.4 g (0.1 mol) fresh phenylhydroxylamine in 40 ml absolute ethanol at room temperature. It was conducted for 18 hours. The red oil obtained by evaporation of the solvent was recrystallized twice from toluene/petroleum ether and had a melting point of 103-105 °C.
13.0 g (0.05 mol) of the above nitrone was obtained.
When the sample for analysis was further recrystallized, the melting point was 110.
The temperature rose to ~112℃. Also shown in Table 3 are other nitrones having absorption maxima in the wavelength range of 300-450 nm. In Table 3, λmax (nm) represents the position of absorption maximum in nanometers, εmax represents the extinction coefficient at the wavelength of absorption maximum, and
mp represents the melting point in degrees Celsius. As with the nitrones described above, these nitrones were prepared by condensation of the appropriate aldehyde and phenylhydroxylamine in a polar solvent.

【table】

【table】

In the method of the invention, the photobleachable material layer forms a mask formed directly on the photoresist layer. Forming such composite structures has several advantages. Since the photobleachable material layer follows the surface topology of the photoresist layer, the formation of voids between the high points of the surfaces of the photobleachable material layer and the photoresist layer is avoided. Such voids would occur if the two layers were formed on separate supports and then brought into contact. Such voids are extremely detrimental to the resolution of the image formed in the photoresist layer, especially when the dimensions of the details to be formed are on the order of a few microns and the depth of focus of the projection device is on the order of a few microns. That is remarkable. Also, if a layer of photobleachable material is brought into contact with a photoresist layer and then pulled away, there is a risk that fragments of one layer will adhere to the other layer, thereby damaging the photoresist layer. Although in describing embodiments of the invention it has been stated that a contrast-enhancing layer is disposed in contact with the photoresist layer, if desired, contrast-enhancing may be provided, for example, by a thin intervening layer of neutral material formed in situ. The layer can also be isolated from the photoresist layer. Although the invention has been described in connection with specific embodiments, it will be obvious to those skilled in the art that modifications and changes, such as those described above, may be made. Therefore,
It is to be understood that the appended claims are intended to cover all such variations insofar as they do not depart from the spirit and scope of the invention.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relative transmittance of an idealized bleachable layer as a function of the amount of light incident on its entrance surface; Figures 2A-2F illustrate the evolution of the bleaching process used in the present invention; Diagrams useful for
FIG. 3 is a graph showing the relative transmittance of a particular contrast-enhancing layer as a function of time exposed to a particular amount of incident light; FIG. FIG. 5 is a set of graphs showing the resulting instantaneous contrast and integrated contrast as a function of the amount of incident light; FIG. Figures 6A-6E are cross-sectional views of a structure illustrating successive steps in one method of practicing the invention. In the figure, 31 is a substrate, 32 is a photoresist layer, 3
3 is a contrast enhancement layer, 35 is an irradiation area, 36
represents the non-irradiated area and 38, 39 and 40 represent the remaining portions of the photoresist layer.

Claims (1)

[Claims] 1. A method for manufacturing an integrated circuit comprising the following steps: a. Providing a photoresist layer exhibiting a contrast threshold of a predetermined level for light of a predetermined wavelength; b. Photobleaching that is sensitive to the light. A photobleachable layer coating is formed on the photoresist layer to obtain a composite structure consisting of the photobleachable layer and the photoresist layer, and the photobleachable layer has an extinction coefficient/molecular weight ratio in a non-bleached state. Approximately 10l/g・cm
a photobleaching compound having a ratio of the extinction coefficient in the unbleached state to the extinction coefficient in the bleached state of greater than about 10; c. exposing the photobleachable layer to a radiation image of the pattern exhibiting a lower level of contrast than the predetermined level of contrast threshold due to the high spatial frequency of the pattern, thereby causing the pattern to appear within the photobleachable layer; exposing the photoresist layer through the mask in the photobleachable layer while a pattern of the radiation image is formed in the photoresist layer that is enhanced over the low level contrast while reproducing a mask of the photoresist layer; forming the pattern in the photoresist layer. 2. A patent in which the extinction coefficient/molecular weight ratio of the photobleachable compound in the non-bleached state is greater than about 100, and the ratio of the extinction coefficient of the photobleachable compound in the non-bleached state to the extinction coefficient in the bleached state is greater than about 30. The manufacturing method according to claim 1. 3. The method according to claim 1, wherein the photobleaching compound is a compound selected from arylnitrone compounds. 4 The arylnitrone compound is α-(4-
4. The method according to claim 3, wherein the predetermined wavelength is about 405 nm. 5. The method of claim 1, wherein the photoresist layer has a thickness of about 3 microns or less and the photobleachable layer has a thickness of about 1 micron or less.