PHOTOLITHOGRAPHY PROCESS USING POSITIVE PHOTORESIST CONTAINING UNBLEACHABLE LIGHT ABSORBING AGENT

Abstract: An improved photolithographic process for manufacturing an electrical device, such as an integrated circuit device, which comprises coating a material (30) with a positive photosensitive coating layer (40) containing an unbleachable absorbing agent such as a dye capable of absorbing electromagnetic radiation of predetermined wavelength. The dye is used in an amount sufficient to absorb enough radiation to prevent any substantial reflection back through the photosensitive coating layer (40) of radiation initially passing through the layer from an external radiation source to the material (30) beneath said photosensitive coating layer (40). The photosensitive coating layer (40) is exposed to a light image to pattern said photosensitive layer (40) and then developed to selectively remove portions of said photosensitive coating (40) whereby the substantial absence of reflected light will permit the formation of a sharply defined image conforming to the selective light pattern transmitted to the photosensitive coating (40).

Description: "PHOTOLITHOGRAPHY PROCESS USING POSITIVE PHOTORESIST CONTAINING UNBLEACHABLE LIGHT ABSORBING AGENT" 1. Field of the Invention This invention relates to the production of elec¬ trical devices using photolithography. More parti- 5 cularly, this invention relates to improvements in photolithography utilizing positive photoresist to provide sharper resolution by reducing reflected radiation. 1. Description of the Prior Art 10 In the patterning of conductor levels on an elec¬ trical device, such as, for example, a semiconduc¬ tor wafer coated with a photosensitive layer using an optical projection aligner operating with mono¬ chromatic light, very severe 'thin-film interference 15 and scattering effects can badly degrade the final patterned image af.ter development. This is the result of the facts that the photosensitive layer is partially transparent and that the underlying substrate (e.g., polysilicon, aluminum, or sili- 20 cides) is, in most cases, highly reflective at the operating wavelength of the patterning apparatus. Such light reflected back from the underlying mate¬ rial into the photosensitive coating serves only to distort the final patterned image with undesired 25 features. Reflections parallel to the incident light angle lead to vertical standing wave varia¬ tions in effective exposure while reflections at other angles causes nominally unexposed regions .to receive undesired exposure and subsequently to lose 30 resolution and sharpness of the desired pattern, e.g., for lines to lose significant thickness or width upon development. This set of problems has been attacked by a number of different potential solutions advanced by vari¬ ous workers. B.J. Lin, in an article entitled "Portable Conformable Mask - A Hybrid Near-Ultra- 5 violet and Deep-Ultraviolet Patterning Technique" published in SPIE, Volume 174, Developments in Semiconductor Microlithography IV (1979) at pages 114-121, introduced the portable conformable mask (PCM) concept. In this approach, a positive resist 10 layer is separated from the substrate by an inter¬ mediate resist layer which is chemically dissimilar so that neither layer is soluble in the other's developer. Near-UV radiation is used to expose the top layer while deep-UV is used to expose the 15 bottom layer of photoresist. Van den Berg et al in an article entitled "Antire- flection Coatings on Metal Layers for Photolitho¬ graphic Purposes" published in the Journal of Ap¬ plied Physics 50 (3) in March, 1979, at pages 1212- 20 1214, suggested that the problem of reflectance could be solved by the use of a spacer layer be¬ tween the substrate and the photoresist coating which might comprise a semiconductor or dielectric. Brewer et al in "The Reduction of the Standing-Wave 25 Effect in Positive Photoresists" published in the ' _ ' .. " Journal of Applied Photographic Engineering, Volume. 7, No. 6, December, 1981, at pages 184-186, sug¬ gested the use of a polyimide coating containing a dye between the photoresist coating and the sub- 30 strate.to reduce reflection back through the photo¬ resist from the underlying substrate. Chen et al U.S. Patent 4,362,809 and O'Toole et al U.S. Patent 4,370,405 suggested improvements in the portable conformable mask disclosed by Linn by placing a light absorbing dye in the lower photore- sist layer to thereby reduce reflectance during the initial exposure of the upper photoresist layer. Quinn U.S. Patent 4,123,272 also described a multi¬ layer process comprising photosensitive materials where the upper layer contained radiation absorbers in the actinic light region sensitivity for the lower layer. The purpose of the absorber in the top layer was to shield the lower layer from the light used to expose the top layer. Specht et al U.S. Patent 4,289,844 is directed to a photopolymerizable composition and a method of using the composition. The composition is de¬ scribed as having utility in microelectronics pho- tofabrication wherein an underlying foil is etched into desired configurations. In such a use the photopolymerizable composition, together with a compatible binder acts as a negative resist which is exposed and developed to form, for example, annular-shaped spacers for beam-leads which are etched out of the metal foil using a positive- working resist for the beam-lead portions of the metal. Neureuther et al in an article entitled "Factors Affecting Line Width Control Including Multiple Wave Length Exposure and Chromatic Aberration" published at SPIE, Volume 275, Semiconductor Micro- lithography VI (1981), at pages 110-116, discussed problems involved in the potential use of increased nonbleachable absorption in a resist. The in¬ creased absorption, it was postulated, based upon computer simulation, would attenuate the standing wave, causing the standing wave nulls to be less severe near the top of the resist and also reduce the variation in the energy coupling with resist thickness, e.g., as the thickness changes as it crosses a step on the wafer. The authors felt the improvement could be quite significant for thick resists. However, the addition of absorption also was deemed to have a detrimental effect in that it would be more difficult for the light to penetrate through the substrate necessitating an increase in the dosage. Unfortunately, such a dose compensa¬ tion would reduce the line width of the thin resist at the top of a step, and it would not be possible to reduce the maximum line width at the bottom of the step without seriously degrading the minimum line width which would actually increases the net line width variation according to the authors. This effect of adding unbleachable absorbance to the resist and the subsequent degradation of the wall profile was also commented on by G. Willson in "Organic Resist Materials", Chapter 3 in "Introduc¬ tion To Microlithography" , American Chemical Socie¬ ty Symposium Series #219, Washington (1983). We have found that the use of certain unbleachable absorbing agents such as dyes in positive photore- sist will markedly increase the effective resolu¬ tion of the pattern image by reducing reflection, including the reflection from scattered waves as well as the vertical standing waves. Apparently some conclusions reached to the contrary with re¬ gard to the possible use of unbleachable absorbents in photoresist were based solely on computer model- ing work which failed to consider resist notching due to. laterally scattered light. Furthermore, we have found that by carefully controlling the amount of dye added, the addition of absorbing dye, while decreasing the final resist wall profile angle, only results in small changes in the profile angle. SUMMARY OF THE INVENTION It is therefore an object of this, invention to provide an improved photolithographic process to reduce the amount of light reflected back through a positive photoresist from the substrate by provid¬ ing an unbleachable absorbing agent in the photore¬ sist. It is another object of this invention to provide an improved photolithographic process to reduce the amount of light reflected back through a positive photoresist by providing an unbleachable absorbing agent in the photoresist in an amount sufficient to absorb a preselected amount of the light passing through the photoresist layer. These and other objects of the invention will be apparent from the drawings and description. In accordance with the invention, an improved pho¬ tolithographic process is disclosed for manufac¬ turing an electrical device, such as an integrated circuit device comprises coating a material with a positive photosensitive coating layer containing an unbleachable absorbing agent capable of absorbing electromagnetic radiation of predetermined wave- length. The absorbing agent is used in an amount sufficient to absorb enough radiation to prevent any substantial reflection back through the photo¬ sensitive coating layer of radiation initially passing through the layer from an external radia- tion source to the material beneath said photosen¬ sitive coating layer. - - BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary cross-sectional * view of the first step of the process of the invention. Figure 2 is a fragmentary cross-sectional view of a subsequent stage of the process. Figure 3 is a fragmentary cross-sectional view of a further step of the process. Figure 4 is a photomicrographic top view in section illustrating the prior art practice. Figure 5 is a photomicrographic top view in section showing the same pattern made using the process of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention comprises an improved photolitho¬ graphy process which results in the reduction of reflection damage in microlithographic imaging by increasing the optical density or absorbance of a positive photosensitive layer or photoresist through the addition to the positive photosensitive layer of a suitably chosen unbleachable light ab¬ sorbing agent. The principal requirements of the unbleachable absorbing agent are: 1) to absorb a predetermined amount of light in a selective band of wavelengths corresponding to the exposure wave- length range of the light source used in patterning the photosensitive layer and to be transparent in the wavelength range used for alignment of the wafer or substrate beneath the photosensitive lay¬ er; 2 ) to be fully soluble in the solvents used in the particular photosensitive material (typically Cellosolve Acetate, n-butyl acetate and xylene) " , i.e., to not recrystallize or volatilize out of solution even after extended periods, and to be fully chemically compatible with the resist includ- ing leaving its shelf life unaffected; and 3) to not affect any the resist salient lithographic parameters, such as adhesion to various surfaces, glass transition temperature, chemical activity and developer, etc. , except those associated with light absorption. It is also desirable that the absorb¬ ing agent not be highly toxic. The beneficial effects of the invention include the reduction of thickness or line width loss in nomi¬ nally unexposed regions due to laterally scattered light (known as resist notching) and reduction in the variations of bulk light energy absorption in the resist as thickness changes over variable wafer surface topography (at least for topographical deviations less than 0.5 microns). The concentration of the unbleachable absorbing agent, when added as a dye to the photoresist, may vary from 0.1 to 1.0% by weight of the total photo- sensitive material. Preferably the dye is used in a concentration of from 0.25 to 0.5 weight per cent. The amount of dye used is very important because a minimum amount must be used to realize the beneficial effects of the invention, i.e., to provide sufficient absorption of reflected light. However, if too much dye is used, the absorption of the light will be so great as to interfere with exposure of the photoresist by not permitting light to pass through the entire depth or thickness of the layer. Ideally, the amount of dye used will permit the passage of enough light through the layer to properly expose the photoresist but will be present in an amount sufficient to absorb light reflected from the substrate beneath the photore- sist back into the photoresist layer. The amount of -dye used should, therefore, add about 0.225 to 0.45 micron " to the absorption coefficient to provide sufficient additional absorbance to inhibit reflec¬ tion of light back through the positive photore- sist. In this connection, it will be noted that because of the partial absorption of the light passing through the photoresist layer, using the process of the invention will probably result in a need for longer exposure times, ranging from about 50 to 150% greater, depending on the dye concentration used. In a preferred embodiment, the dye is preselected to have a high absorption at the wavelength used to pattern the photosensitive coating, e.g., 436 nm, while exhibiting high transmission of light at another preselected wavelength used to align the mask used in patterning with previous patterns in layers underlying the photosensitive coating, e.g., 633 nm. , or, in some cases, a broad band of from 500 -700 nm. In a particularly preferred embodiment, the absorbing dye used will be one which also will fluoresce upon exposure to radiation thereby pro¬ viding, as an additional benefit, the potential ability to measure and inspect micron-level- resist features through the use of fluorescence microscopy as opposed to conventional bright-field microscopy. A particularly preferred dye which has been found to be useful in the practice of the invention is a dye distributed under the trademark COUMARIN 504 by Exciton Corporation of Dayton, Ohio, or COUMARIN 314 by Eastman Kodak Company. The chemical name of COUMARIN 504 or COUMARIN 314 is 1 ,2 ,4,5 ,3H,6H,10H- tetrahydro-9-carbethoxy(l)benzopyrano(9,9a, 1-gh) quinolizin-10-one. Other dyes which may also be useful in the invention include photosensitive compounds such as described in Specht et al U.S. Patent 4,289,844. It should be noted that the unbleachable absorbing agent need not necessarily be in the form of an added dye, but may rather, for example, comprise a modification of the photosensitive material itself to increase its absorbance sufficiently to inhibit reflection at the normal range of photoresist coat¬ ing thicknesses used. Referring now to the drawings, in Figure 1 a micro- electronic structure is shown having a previously applied metal interconnect or substrate layer 10 and a previously patterned oxide layer 20 thereon. A metal interconnect layer 30 is placed over oxide layer 20 and a photoresist layer 40, having incor- porated therein a light absorbing dye in accordance with the invention, is placed over metal intercon¬ nect layer 30. As shown in figure 1, the dye-containing photo¬ resist layer 40 is exposed to a pattern of light at a preselected wavelength 436 nm in this instance. Turning now to Figure 2, the resist layer 40 is shown after development to remove the exposed por¬ tions of photoresist layer 40 leaving portions 42. In Figure 3, underlying metal interconnect layer 30 has been selectively etched through the openings provided by patterned photoresist layer 40 to se¬ lectively etch out portions of layer 30 leaving only connections 32. Referring now to Figures 4 and 5, Figure 4 illus- : trates a prior art structure wherein the line width has been severely eroded as shown by the arrow at 70 indicative of poor resolution of the patterning due to reflectance and scattering of the light used to expose the photoresist used in forming the lines. In contrast. Figure 5 shows the identical pattern produced, however, using the process of the invention wherein the photoresist material had incorporated therein a dye which is selectively absorbent of the wavelength of light used to expose the photoresist. It will be seen that at the same point 70' as in Figure 4, the line width resolution is markedly improved. The following example will serve to further illus- trate the process of the invention. A 1 micron film of aluminum containing 1% silicon and 0.5% copper was sputter deposited on a sub¬ strate. A positive photoresist coating containing 0.3% by weight COUMARIN 504 was deposited on the aluminum film. to a thickness of 1.6 micron. The dye-containing photoresist was soft baked at 100°C for 300 seconds in an infrared convection track oven. The baked resist was then exposed to a pattern of 436 nm light for about 0.50 seconds at an intensity of roughly 300 milliwatts per square centimeter. The exposed resist was then developed with an AZ351 developer in a ratio of 1 part devel¬ oper to 5 parts water at 21°C for 180 seconds. The resist was then rinsed with deionized water and allowed to dry. The developed photoresist was then exposed to a deep-UV cure of 100 millijoules per square centimeter exposure at 254 nm wavelength and then baked at 200°C for 60 minutes. The pattern of exposed aluminum was then plasma etched in a reac- tive ion etch with carbon tetrachloride gas mixed with chloroform. Finally, the resist was removed by stripping in an oxygen plasma. The resultant product was examined and found to exhibit high resolution with very little variation in line width indicating that very little damage was done to the resolution by scattering and standing waves from reflected light. Thus, the invention provides an improved process wherein a' photoresist material is provided with a selected amount of a light absorbing dye capable of absorbing a sufficient amount of light at the wave length of exposure whereby the radiation used to expose the photoresist will be absorbed sufficient¬ ly by the dye therein to prevent or inhibit the reflectance back into the photoresist layer of light from the underlying* substrate thereby reduc- ing or eliminating standing waves as well as scat¬ tered light which would otherwise result from such reflection.

Claims: What is claimed is; 1. An improved photolithographic process for use in manufacturing an electrical device wherein por¬ tions of a material are removed by photoetching which comprises: 5 a) coating said material with a positive photosensitive coating layer containing an unbleachable absorbing agent capable of ab¬ sorbing electromagnetic radiation of predeter¬ mined wavelength, said agent being present in 10 an amount sufficient to absorb enough radia¬ tion to prevent any substantial reflection back through said photosensitive coating layer of radiation passing through said layer from an external radiation source to said material 15 beneath said photosensitive coating layer; b) exposing said photosensitive coating layer to a light image of said predetermined wavelength to pattern said photosensitive layer; and 20 c) developing said exposed photosensitive material to selectively remove portions of said photosensitive coating; whereby the substantial absence of reflected light will permit the formation of a sharply defined 25 image conforming to the selective light pattern transmitted to said photosensitive coating. 2. The process of claim 1 wherein said unbleach¬ able absorbing agent is preselected to absorb rad¬ iation at a wavelength used for patterning said photosensitive layer and to be transparent at a wavelength used for alignment of underlying por¬ tions of said electrical device prior to said pat¬ terning. 3. The process of claim 1 wherein said positive photoresist contains an unbleachable absorbing agent in an amount sufficient to increase the absorption coefficient from about 0.225 to 0.45 micron . 4. The process of claim 3 wherein said unbleach¬ able absorbing agent is capable of fluorescing upon exposure to radiation to facilitate examination of micron-level resist features. 5. The process of claim 4 wherein said unbleach¬ able absorbing agent is a cou arin based dye. 6. The process of claim 5 wherein said dye is 1,2,4,5,3H,6H,10H-tetrahydro-9-carbethoxy(1)benzo- pyrano(9 ,9a,l-gh)quinolizin-10-one. 7. The process of claim 6 wherein said photoresist contains from 0.1 to 1.0 wt% of said dye. 8. The process of claim 6 wherein said photoresist contains from 0.25 to 0.5 wt% of said dye. 9. The process of claim 5 wherein said dye is preselected to absorb light of 436 nm wavelength. 10. The process of claim 9 wherein said dye pre¬ selected to absorb light at 436 nm wavelength will substantially transmit light at a preselected wave¬ length to permit alignment of a wafer containing said material with the mask used to pattern said photosensitive coating on said material. 11. In a photolithographic process for patterning a layer in the construction of an integrated cir¬ cuit device wherein a positive photosensitive coating is placed over a material, patterned by exposure to light at a preselected wavelength passing through a mask, and then developed to provide a pattern for selective etching of the underlying material, the improvement which comprises: ' coating said material with a positive photosensitive coating containing a dye capable of absorbing light at the frequency used to pattern said photosensitive coating, said dye being present in an amount sufficient to inhi¬ bit reflectance of said light from said material back through said photosensitive coating while permitting sufficient light to pass through said coating to expose the entire thickness of said photosensitive coating whereby the resolution of said exposure and resultant pattern will be en¬ hanced by the elimination or inhibition of re- fleeted light back through said photosensitive coating. 12. The process of claim 11 wherein said dye is capable of fluorescing to facilitate examination of said patterned photosensitive coating. 13. The process of claim 12 wherein said dye will selectively absorb light at a wavelength used to pattern said photosensitive coating while permit¬ ting transmission of light at other wavelengths used for alignment. 14. An improved photolithographic process for use in manufacturing an integrated circuit device on a substrate wherein portions of a photosensitive coating on said substrate are removed by photo- etching which comprises: a) coating said substrate with a positive photosensitive coating layer containing an unbleachable absorbing agent capable of ab¬ sorbing e'le.ctro agnetic radiation at a wave- length at which said photosensitive layer is normally exposed to pattern said photosensi¬ tive layer, said agent being present in an amount sufficient to absorb enough radiation to prevent any substantial reflection back through said photosensitive layer of radiation passing through said layer from an external radiation source; b) exposing said photosensitive coating layer to a light image to pattern said photo- sensitive layer; and c) developing said exposed photosensitive coating layer to selectively remove portions of said photosensitive layer; whereby the substantial absorption of reflected light by said unbleachable absorption agent will permit , the formation of a sharply defined image conforming to the selective light pattern trans¬ mitted to said photosensitive coating layer.

Inventors: ARNOLD WILLIAM H III, BROWN ANDREW VICTOR, KNIGHT COLIN W T, ARNOLD, WILLIAM, H., III, BROWN, ANDREW, VICTOR, KNIGHT, COLIN, W., T.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1985
Main classification: G03C5
IPC codes: G03C1, G03F7, H01L21