GB2079481A - Method for the formation of surface relief patterns using deep ultraviolet radiation exposure of resist composition - Google Patents

Abstract

A negative acting resist composition is used which comprises a novolak resin which has a window of low optical absorption in the deep ultraviolet range and an aromatic azide sensitizer which has a complementary peak of high optical absorption in the deep ultraviolet range. The resist composition is exposed with deep ultraviolet radiation in the window/peak region, for example, at about 250 to 265 nm, and is thereafter developed with an aqueous alkaline solvent to provide a negative surface relief pattern.

Description

SPECIFICATION Method for the formation of surface relief patterns using deep ultraviolet radiation of resist composition This invention relates to a method for the manufacture of surface relief patterns by exposure of a suitable resist with deep ultraviolet radiation (200-300 nanometer wavelength). More particularly, this invention is concerned with deep ultraviolet radiation sensitive resists which are both negative acting and can be developed with alkaline aqueous solvents.

Microlithography is a widely used industrial process for the manufacture of extremely small and precise surface relief patterns. Microlithography is employed in the microelectronics industry in the manufacture of integrated circuits and the like.

In microlithography materials are utilized as a recording medium which are generally referred to as resists.

Most of the well-known resists are organic polymeric compositions which when exposed to radiation either depolymerize to a lower molecular weight or cross-link to a higher molecular weight. When the molecular weight is reduced as a result of exposure to radiation the solubility of the exposed material will increase and vise versa. Those materials which become more soluble after exposure to radiation are referred to as positive resists. Those materials which become less soluble after exposure to radiation are referred to as negative resists.

It is possible to obtain a surface relief pattern in resists by selectively exposing predetermined areas of a film of a resist on a substrate with radiation and thereafter by treating the exposed film of resist with a solvent which removes the resulting more soluble portion of the resist. The resulting surface relief patterns can then be subjected to various known treatments such as etching or sputtering to form the desired elements on the substrate.

Negative working resists are more commonly used commercially in the microelectronics industry because of the greater variety and the general better quality of the resists which are available. The negative resists generally have wider processing latitudes, that is, exposure times, development times, and the like than the positive resists. Negative resists in addition are also generally more sensitive than most positive resists and in some cases are also more etch resistant. However, most negative resists which have been available up to now have the disadvantage that the solvents which are required for development of the exposed resist are organic materials which cause swelling of the resists during development.The swelling of minute elements of the surface relief image formed in the resist distorts their shape, may bridge or close small openings and can also lead to lifting or adhesion failure between the surface relief image and the substrate on which it is formed.

Attempts have been made to use positive working resists which can be developed with aqueous, non-swelling developers. The positive resists, however, require processing parameters which are much more stringent than those required for negative resists. In addition, positive resists must be exposed everywhere except where the pattern elements are to remain. this generally means that more of the resist must be exposed and thereafter removed in development than is required for negative resists. The removal of substantially greater amounts of the resist increases the problems associated with the exposure and development processes in that larger areas of the sample must be exposed, examined, and given supplemental treatments as required in order to assure complete delineation between the exposed and unexposed portions.

The demands for high resolution and faithful pattern delineation of the industries which employ microlithographic processes and in particular the microelectronic industry have increased substantially over the last few years. There is a constantly increasing demand for even more miniaturization of the surface relief patterns coupled with the requirement that there be no loss of resolution. These increasing demands have resulted in a shift away from the use of optical exposure to higher resolution forms of exposure such as x-rays and electron beam exposure. This is because the practical limits of image resolution with conventional near ultraviolet (350 mm) optical exposure is about 1.5-2 Fm.

The shift to higher resolution forms of radiation has increased the capability to produce even smaller patterns. However, the use of these newer techniques have presented certain problems. The apparatus which is required to expose a resist with an electron beam is very expensive as compared to the apparatus used for exposure with optical radiation. In addition, the processing conditions employed in electron beam exposure are much more stringent. With various forms of the electron beam or x-ray exposure techniques currently under consideration, it is necessary to conduct the exposure in a vacuum or in a protective atmosphere such as an inert gas. In addition, the rate of exposure, for example with an electron beam where the sample is serially scanned, is relatively slow, especially as compared to flood exposure.As a result of the higher equipment costs, higher costs involved in the processing and a lower production rate, the cost of these higher resolution exposure techniques per unit area exposed is substantially greater than that of the older exposure techniques using optical radiation. The higher cost factor is extremely important especially in the microelectronics industry as there is constant competition in the industry to reduce the cost of each device produced while maintaining or even improving the quality of the devices.

As a result of the various limitations and problems encountered with the newer techniques as noted above, the industry has recently reconsidered a certain aspect of ultraviolet exposure for use in microlithographic processes. It has been suggested to use, for example, deep ultraviolet radiation, i.e., radiation in the wavelength range of 200 to 300 nm, to expose microlithographic resists. The use of these shorter wavelengths permits the delineation of higher resolution patterns. Deep ultraviolet radiation has been found to offer certain substantially important advantages. Existing equipment currently in place for exposure with visible and near ultraviolet radiation can be readily retrofitted to be employed for exposure with deep ultraviolet radiation and the cost involved is relatively modest as compared to purchasing of electronic beam exposure devices or other similar devices.The exposure of the resist with deep ultraviolet light can be conducted at ambient conditions which substantially reduces production costs. In addition, the exposure can be conducted rapidly which further reduces the cost per unit area exposed.

To obtain results similar to those obtained with shorter wavelength forms of radiation such as x-ray or electron beam exposure and the like with deep ultraviolet radiation, it is necessary that process parameters in the microlithographic process be improved. The improvements which are required to be made in order to utilize deep ultraviolet radiation to manufacture patterned resists having high resolution including small pattern elements include providing resists which are exceptionally sensitive to deep ultraviolet radiation so that the precise patterns can be exposed and also development techniques which will enable the exposed resist to be developed into surface relief patterns having a high degree of resolution.

Accordingly, it would be highly advantageous if a resist composition could be provided which was highly sensitive in the deep ultraviolet exposure range. It would also be especially advantageous for the reasons noted if resist compositions could be exposed to provide negative surface relief patterns which could thereafter be developed with a nonswelling solvent so as to maintain high accuracy and resolution in the surface relief pattern.

The present invention uses a negative acting resist composition comprised of a novolak resin which has a window of low optical absorption in the deep ultraviolet range and an aromatic azide sensitizer which has a peak of high optical absorption in the deep ultraviolet range is exposed with deep ultraviolet radiation at, for example, about 250 to 265 nm and the exposed resist is thereafter developed with an aqueous alkaline solvent to provide a negative surface relief pattern.

In the accompanying drawings: Figure 7 is a graphic representation of the absorption spectrum of the resist composition used in the method of this invention and of the novolak resin and azide sensitizer used in the preparation of the resist composition.

Figures2-5are photomicrographs of surface relief patterns obtained using the method of this invention.

Figure 6 is a graphic representation showing ultraviolet sensitivity of certain compositions of this invention.

In accordance with the present invention a resist composition is employed which is especially sensitive in the deep ultraviolet range. The composition is comprised of an alkali soluble novolak resin which has a window of low absorption in the deep ultraviolet range. An aromatic azide sensitizer is included in the composition which has a peak of high absorption in the deep ultraviolet range. The novolak resin and the azide sensitizer are selected so that the window of low absorption and the peak of high absorption are complementary to each other, that is, the window of low absorption should be at approximately the same wavelength as the peak of high absorption. In the method of this invention the resist composition is exposed with deep ultraviolet radiation at a wavelength wherein the window of low absorption and the peak of high absorption occur.In this manner the energy of the ultraviolet radiation is selectively absorbed by the azide sensitizer. The deep ultraviolet radiation, however, readily penetrates through the entire thickness of the resist composition because of the low absorption of the novolak resin.

The importance of the selection of the particular novolak resin and the particular azide sensitizer to be used in combination can be seen from an examination of Figure 1. In Figure 1 the optical absorption spectrum of a novolak resin is shown with a dashed line. The novolak resin, which is illustrated, is a cresol formaldehyde resin having a molecular weight from about 5,000 to 6,000. The optical absorption spectrum of the azide sensitizer, 1 ,2-di(p-azidophenyl) ethane, in chloroform is shown with a dotted line. The optical density of a 0.33 micrometer thick film resist composition formed with a combination of the novolak resin and the azide sensitizer is shown by the solid line on the graph of Figure 1. As can be seen from Figure 1, the optical density of the novolak resin changes rapidly with wavelength.The novolak resin changes from having an extremely high optical density at about 240 nm or so to a point of low optical density at about 255 nm and then again rapidly increases in optical density at about 285 nm to again decrease in optical density at about 300 nm. The azide sensitizer, on the other hand, increases in optical density to a point of about 255 nm then gradually decreases so that at about 295 nm it hits a low point with regard to its optical density. It should be carefully noted that the entire graph shown in Figure 1 is included within the deep ultraviolet spectrum 200-300 nm. However, it is only in a selected portion of the deeop ultraviolet range, that is, from a wavelength at about 250 to 265, labeled on the graph as the "Deep Ultraviolet Exposure Zone", that the particular combination of the selected novolak resin and the selected azide sensitizer have the required complementary relationship of low absorption and high absorption to provide the highest sensitivity required in accordance with the teachings of this invention. By way of comparison it should be noted that a wavelength of about 285 nm, which likewise is included within the deep ultraviolet range, would be considerably less advantageous for use with the particular composition illustrated in Figure 1 in that it exhibits both high optical density for the novolak resin and a relatively low optical density for the azide sensitizer.

As noted above the novolak resin must have a window of low absorption in the deep ultraviolet portion of the spectrum. In addition the novolak resins are also characterized by being readily soluble in the unexposed condition in aqueous alkaline solvents and, after exposure to deep ultraviolet radiation in the presence of a suitable azide sensitizer, becoming substantially insoluble in aqueous alkaline solvents.

The novolak resins are phenol-aldehyde resins such as novolak resins from monomers such as phenol or substituted phenols such as cresol, butylphenol or hexylphenol. One especially useful novolak resin is 1:1 cresol-formaldehyde resin having a molecular weight of about 5,000 to 6,000 such as the resin used in the example whose spectrum is illustrated in Figure 1. The novolak resin can also be prepared from a mixture of substituted phenols. A satisfactory novolak resin for use in this invention was prepared from formaldehyde and a mixture consisting of 85 parts of o-cresol and 15 parts of 2-t-butylphenol. Afurther example of a suitable novolak resin is a reaction product of formaldehyde with the mixture comprised of 73 parts of o-cresol, 13.5 parts m-cresol and 13.5 parts 2-t-butylphenol.

The azide sensitizers which are used in the resist compositions of this invention are selected from a known class of compounds which have been suggested for use with resist systems that are exposed with various energy sources such as visible or near ultraviolet radiation, electron beam radiation, etc. The azide sensitizers which are employed in this invention are decomposed by the action of heat and light to form nitrene radicals. The nitrene radical in turn is effective as an agentforthe cross-linking reaction of phenolic novolak resins. As a result of the condensation reaction, the novolak resins are polymerized or cross-linked to a relatively high molecular weight at which they become relatively insoluble in aqueous alkaline solutions as compared to the unexposed portion of the novolak resin.

The particular azide compounds which are used in the compositions of this invention must have a peak of high absorption in the deep ultraviolet area of the absorption spectrum and in particular it is advantageous to have the absorption occur in the range of 255 to 265 nm. The azide sensitizers which are used in the composition of this invention are aromatic diazides. The aromatic diazides which have been found to be useful can be represented by the general formula: N3 - Ar - X - Ar - N3 wherein Ar is an aromatic ring and X is a connecting linkage. It has been found that the diaromatic azides wherein the linking group X is relatively short and therefore somewhat inflexible are generally more suitable as sensitizers as they generally have a peak of high absorption in the deep ultraviolet portion of the spectrum.It is preferable, for example, if the linking group is

Certain selected aromatic azides which have been found to be especially useful in that they have peaks of high absorption in the preferred 250 to 265 nm portion of the absorption spectrum are the following compounds:

While the aromatic diazides having the short linking groups X are generally preferable as sensitizer compounds, those aromatic diazides having longer linking groups which also have a high absorption in the deep ultraviolet portion of the absorption spectrum can also be used.An example of such a compound having a peak of high absorption in a deep ultraviolet range is a compound of the formula:

The ratio of the diazide sensitizer to the novolak resin will vary depending on the required difference in solubility between exposed and unexposed regions of the resist, the compatability of the particular diazide with the novolak resin and the optical density of the sensitizer. In general, from about 5 to about 25 percent by weight of the sensitizer based on the combined weight of the sensitizer and alkali soluble novolak resin is useful. The resist may also contain minor amounts of conventional additives such as an adhesion promotor, etc.

The resist is generally dissolved in a suitable solvent to apply it to a substrate as a thin film. Suitable solvents are conventional and include 2-methoxyethylacetate, xylene, chlorobenzene, cyclohexanone, cyclopentanone and the like. The concentration of the resist will be dependent on the thickness of the resist layer to be applied. The resist can be applied to a desired substrate in conventional manner, as by dipping, spinning, spraying and the like. The solvent is then removed by air drying or baking in conventional manner.

Baking prior to exposure also serves to anneal the resist film. The prepared resist film is exposed with deep ultraviolet radiation.

The selection of the particular wavelength of deep ultraviolet radiation used to expose the selected resist composition of this invention is extremely important as can be seen from Figure 1. With the particular composition shown in Figure 1 the exposure source should have a wavelength of from 250 to 265 nm to obtain the benefits of this invention. There are various sources which can be utilized to provide energy in this particular wavelength such as high pressure mercury vapor lamp which has a peak of high emission at approximately 254 which falls well within the desired range and gives extremely satisfactory results with the composition graphically represented in Figure 1. Other sources which have been found to be especially useful are xenon arc lamp and xenon mercury lamps.In addition to being flood exposed, the resists can be scan exposed with a deep ultraviolet laser or holographicly exposed with a deep ultraviolet laser.

The resist film is exposed with deep ultraviolet radiation at the selected wavelength wherein the novolak resin has its low absorption window and the azide sensitizer has a peak of high absorption. The resist film is flood exposed with deep ultraviolet radiation through a mask which has delineated on it the desired surface relief pattern. When exposing the resist composition in the deep ultraviolet range, it is preferable to use a material for the mask which does not itself preferentially absorb in the exposure area. Conventional glass is generally not particularly suitable for deep ultraviolet exposure masks as it tends to absorb substantial amounts of ultraviolet radiation and therefore requires relatively long exposure time in order to have adequate exposure of the resist.It is preferable when exposing in the deep ultraviolet range to use a mask which is formed of a material which does not absorb in the ultraviolet range such as quartz and the like.

After exposure the resist film is developed with an alkaline aqueous developer solvent which selectively removes the more soluble unexposed resist composition to provide a surface relief pattern having a high degree of resolution. The developer solvent is generally a water solution of an inorganic alkali metal compound including sodium or potassium hydroxide, phosphate, silicate, borate and the like, but can be an aqueous solution of organic water soluble base such as a tetraalkylammonium hydroxide or can be an alcohol solution. The strength of the developer solvent will vary depending on the development rate desired and the solubility difference between the exposed and unexposed regions in the resist layer. When alkali metal hydroxide such as sodium hydroxide is used, a 0.1-0.2 normal solution is generally adequate.For certain applications a minimum of erosion of the exposed resist may be required, which will necessitate employing a more dilute developer solution.

The resist film can then be postbaked, although this is an optional step. Postbaking may improve results, probably by increasing the crosslinking that has occurred in the exposed areas, or by improving the adhesion between the resist and the substrate prior to subsequent processing.

The invention will be further described in the following Examples, but the claims of the invention are not to be limited to the specific details described therein. In the Examples parts and percentages are by weight.

Example 1 A resist was prepared by mixing 15 parts of 4,4'-diazidodiphenylmethane sensitizer of the formula:

which has an absorption spectrum peak at 250 nm and 85 parts of a cresol formaldehyde novolak resin having a molecular weight of 4,000 to 5,000 and a low optical absorption at about 255 nm. The mixture was dissolved in 2-methoxyethylacetate to form a 24 percent solids solution.

The resist was spin coated at 4200 rpm onto a wafer of sapphire coated with a layer of silicon about 6,000 angstroms thick and a second layer of thermally grown silicon dioxide about 500 angstroms thick to form a layer of resist about 0.8 micron thick. The coated wafer was then baked for 20 minutes at 750C and cooled.

The resist layer was contacted with a mask having 2.5 micron wide chromium lines separated by 22.5 micron wide spaces and exposed to a 1200 watt pulsed xenon arc lamp. The resist layer was developed by immersing in an aqueous alkaline solution (the Shipley Company's AZ developer) for 3 minutes, rinsed with water and blown dry.

The wafer was then immersed in buffered HF solution to etch the silicon dioxide layer away in the regions from which the resist was etched away, rinsed with water and dried. The remaining resist was stripped with acetone, the wafer was rinsed with isopropanol and dried.

The pattern replication was excellent, producing 2.5 micron wide troughs with straight walls,22.5 microns apart.

Example 2 PART A - The resist of Example 1 was coated onto a glass substrate having a thin layer of chromium thereon (800 angstroms) and exposed through a mask as in Example 1. The mask bore chromium lines separated by spaces of widths varying from 1 to 9 microns. Figure 2 is a photomicrograph of the resulting developed resist pattern magnified 200 times, showing excellent resolution of the resist.

PART B - The procedure as above was repeated except employing a mask having 0.5 micron lines and 0.5 micron spaces. Figure 3 is a photomicrograph of the resulting developed resist pattern magnified 1010 times.

Example 3 The procedure of Example 1 was followed except that a 15 percent solids solution of resist was prepared which was applied at 600 rpm to a silicon wafer having a thermally grown oxide layer about 1500 angstroms thick.

Excellent pattern replication in the oxide layer was achieved forming straight-walled 1 micron wide oxide bars separated by 1 and 2 micron wide spaces. Figure 4 is a photomicrograph of the resulting pattern, magnified 1000 timees.

Example 4 The procedure of Example 3 was followed using a glass substrate having a layer of chromium about 800 angstroms thick thereon. A mask having 0.5 micron wide openings and 2 micron widths was employed.

Figure 5 is a photomicrograph of the resulting pattern, following stripping of the resist, after etching with ceric ammonium nitrate in dilute acetic acid solution, magnified about 10,000 times.

Example 5 Three "deep-UV" resists were formulated as 24% total solids in 2-methoxyethyl acetate using 15/85 ratios of 1,2-di-(p-azidophenyl)ethane sensitizer/novolak resin. Three resins were used, as follows: A. A m-cresol/formaldehyde novolak.

B. A novolak resin prepared from formaldehyde and a mixture of 85 parts of o-cresol plus 15 parts of 2-t-butylphenol C. A novolak resin prepared from formaldehyde and a mixture of 73 parts of o-cresol, 13.5 parts of m-cresol, and 13.5 parts of 2-t-butylphenol Chrome-on-glass substrates were spin-coated with the three resists. Resists A and B were spun at 3,000 rpm to give films 1,034 Fm thick. Resist C was spun at 2,700 rpm to give films 1.004 Rm thick.

The coated substrates were baked at 75"C for 20 minutes. The thickness measured for the baked films were: A: 0.89cm B: 0.87#m C: 0.86i#m These samples were then given a series of exposures to filtered ultraviolet light, wavelengths between 230-270 nm, corresponding to incident doses ranging from 1 mj/cm2 to 500 mj/cm2. Unexposed samples were immersed in Shipley AZ developer to determine the development time required to remove the unexposed resist to the substrate.These "clearing" times were: A: 70 seconds B: 345 seconds C: 57 seconds The corresponding development rates for the unexposed resist are: A: 128aztec B: 26alec C: 153A1sec The samples of each resist, exposed at a number of doses, were then developed in AZ developer for the "clearing" time listed above for each resist. The samples were rinsed with water and dried, and the thickness of the film remaining was measured. The resulting data were plotted as the normalized fraction of the original film thickness remaining after development vs log exposure, as shown on the attached curve, Figure 6.

A value that can be used to characterize the sensitivity of the resists is D: that dose necessary such that half the original film thickness remains after the unexposed material has been removed. D for the 3 resists are: A: 3 mj/cm2 B: 1.9 mjlcm2 C: 1.9 mjlcm2 The contrast of the resist, that is the measures of the steepness of the straight-line portion of the curves of normalized remaining film thickness vs log dose. The ability of a resist to define sharply-delineated line edges is related to its constrast. The constrasts measured for these resists are: A: 0.93 B: 0.89 C: 0.89

Claims (9)

1. The method for the formation of surface relief patterns of a predetermined configuration comprising the steps of: (a) providing a recording medium comprised of a substrate and a film of a negative resist on said substrate, said resist being comprised of an aqueous alkaline soluble novolak resin having a window of low optical absorption in the deep ultraviolet portion of the spectrum and an aromatic azide sensitizer having a peak of high optical absorption in the deep ultraviolet portion of the spectrum, said window of low absorption and peak of high absorption being complementary to each other in a given wavelength area of the deep ultraviolet portion of the spectrum, said diaromatic azide sensitizer being presented in said resist in an amount of from about 5 to about 25 percent by weight based on the combined weight of novolak resin and aromatic azide;; (b) exposing said recording medium imagewise to define a pattern corresponding to a predetermined configuration with an energy source having emission in said given wavelength area of the deep ultraviolet spectrum; and (c) developing the exposed resist with an aqueous alkaline solvent to selectively remove the unexposed portion of the resist; whereby said surface relief pattern is formed in said film of resist.

2. The method according to claim 1 wherein the window of low absorption and the peak of high absorption are in the wavelength range of from about 250 to 265 nm.

3. The method according to claim 2 wherein the exposure is conducted with a high pressure, mercury vapor lamp.

4. The method according to claim 2 wherein the novolak resin is a phenol-aldehyde resin prepared from phenol or a substituted phenol.

5. The method according to claim 2 wherein the novolak resin is a copolymer of an aldehyde and a monomer selected from cresol, butylphenol or hexyiphenol.

6. The method according to claim 2 wherein the novolak resin is a cresol-formaldehyde resin having a molecular weight of from about 5,000 to 6,000.

7. The method according to claim 2 wherein the aromatic azide is represented by the formula N3-Ar-X-Ar- N3 wherein Ar is an aromatic ring and X is a linking member selected from the group consisting of

8. The method according to claim 2 wherein the aromatic azide sensitizer is selected from the group consisting of a compound of the formula:

9. A method for the formation of surface relief patterns substantially as hereinbefore described.