METHOD OF FORMING A LIFT-OFF MASK WITH IMPROVED OXYGEN BARRIER LAYER

Abstract: Pinhole-free thin films are deposited by glow discharge or plasma polymerization of organosilanes, organosilazones and organosiloxanes for use as reactive ion etch oxygen barriers in multilayer resist structures, of lift-off masks, for fabrication of semiconductor devices, such as integrated circuits. The process includes deposition of thin plasma polymerized organosilicon barrier film over a radiation insensitive polymeric base layer previously coated on a substrate, followed by thermal annealing of the plasma polymerized barrier layer, over which is then coated a radiation sensitive resist layer. After definition of the desired resist pattern by imagewise exposure and development, the image is etch transferred into the barrier layer by reactive sputter etching in a fluorine containing ambient, and subse­ quently transferred into the base layer, down to the subs­ trate, in an oxygen plasma, during which time the plasma deposited film functions as an oxygen barrier. Final metal patterns are formed by metallization and lift-off steps.

Description: This invention relates to the fabrication of semiconductor components such as integrated circuits, and more particularly to the use of thin films deposited by glow discharge or plasma polymerization of organosilicon monomers as oxygen reactive ion etch barriers during pattern delineation in high resolution lithography processes for large scale integrated circuitry. One object of this invention is to provide a method for forming defect-free polyorganosilicon films. Another object of this invention is to provide pinhole-free oxygen RIE barrier for lift-off masks employed in the fabrication of semiconductor components. Another object of this invention is to provide novel and improved oxygen RIE barrier films for lift-off masks employed for forming interconnecting metallurgy systems for semiconductor components. Another object of this invention is to provide a lift-off mask with improved oxygen barrier layers which eliminate interlevel shorts caused by pinholing occasioned in conventional oxygen RIE barrier films, such as silicon oxide and silicon nitride. Another object of this invention is to provide a pinhole free plasma polymerized oxygen barrier film formed from non-pyrophoric organosilicon monomers which are liquids at room temperature as an improved replacement for silicon oxide and nitride barrier films which are normally formed from hazardous silane (SiH₄) and disilane (Si₂H₆) gases. The films thus prepared are useful as oxygen etch barrier with good dielectric properties. With continued miniaturization of semiconductor integrated circuits to achieve greater component density and smaller units, as in large scale integrated circuits, it is required that high resolution lithographic processes be developed for dense metallization networks. A major factor limiting resolution of micron and submicron resist images in electron beam lithography using single layer thick resist films, is the proximity effect due to electron scattering. As a result, multilayer resist approaches that employ thin imaging layers to eliminate or minimize the problems of proximity effect and having increased resist sensitivity are receiving an increased focus of attention by the research and development areas for submicron lithography. Similarly, in optical lithography using refractive projection mask aligners, multilayer resist processes overcome the problems of resolution and aspect ratio because of depth of field limitations in the case of single layer resist images. A trilevel resist process for integrated circuit devices, typically, comprises a radiation insensitive polymer base layer, a thin O₂/RIE resistant barrier layer and a thin E-beam, UV or X-ray resist layer. The polymeric base layer is generally applied by spin-coating on a substrate followed by a curing step and deposition of the O₂/RIE barrier film which is overcoated with a thin coating of an imaging (resist) layer. The O₂/RIE barrier films conventionally employed in the prior art are mostly inorganic in nature, such as plasma deposited silicon oxide, vacuum evaporated SiO, sputtered quartz and chemical vapor deposited (CVD) silicon nitride. Further information can be obtained on reference to U.S. Patent 4 202 914; J. Moran, Pol. Eng. Sci. 20 , 1097 (1980); L. Rothman, IBM Technical Disclosure Bulletin 23 , 2293 (1980); and P. Grabbe et al., J. Vac. Sci. and Tech. 21 , 33 (1982). One method of employing these multilevel resist (MLR) techniques for forming metallization patterns on devices is commonly denoted by the terms "expendable mask method", "lift-off method" or "stencil method", and was initially described in U.S. Patent 2 559 389. Improvements to the basic "lift-off method" have been made, as for example, in U.S. Patents 3 849 136; 3 873 361; 4 004 044 and 4 202 914. In conformance with the foregoing, this "lift-off" method includes the use of organic polymeric material as a base or sub-layer on an integrated circuit substrate and an overlying oxygen RIE resistant layer of diverse material, such as a metal layer, silicon dioxide, silicon nitride or a polydimethylsiloxane, having openings in a selected pattern. Where polydimethylsiloxanes have been proposed as oxygen RIE barrier films, they have been solution coated or cast on the polymeric base layer. See the above noted U.S. Patent 4 004 044, and L. Fried in IBM J. of Res. and Dev. 26 , 362 (1982). Further, in Proceedings on the International ELECTRON DEVICES Meeting, San Francisco, Cal., US, 13th-15th Dec 1982, pp. 395-398, IEEE, N.Y., US, S. Matsui et al., NEW TRI-LEVEL STRUCTURES FOR SUBMICRON PHOTOLITHOGRAPHY, 0.1 to 0.3 µm thick spin-coated organosilica intermediate layers with a dye incorporated therein are described. The processing sequence of these tri-level structures comprises the following steps: 1. Spin-coating of a 1 - 3 µm thick organic bottom layer on a substrate, followed by baking at 250 °C for one hour; 2. Spin-coating on the bottom layer of an intermediate layer of 0.1 - 0.3 µm thick organosilica with dye incorporated in it, followed by baking at 200 °C for 30 min.; and 3. Spin-coating on the organosilica layer a top imaging layer of 0.4 µm thick photoresist, followed by baking at 80 °C for 30 min. The resulting tri-level resist structures differ from those of the present invention in that all layers, specifically the intermediate organosilica layer, are applied by spin-coating, and that the intermediate organosilica layer contains dyes in various concentrations which prevent standing waves in top layer and linewidth variation on substrate steps. Specific polymers forming the organosilica intermediate layer are not described. US-A-3 822 928 describes a thin film light guide circuit for use with electromagnetic radiation including a substrate member having at least one guiding element in contact therewith, said element and said substrate having differing refractive indices at a wavelength of electromagnetic radiation, and said element consisting of essentially homogeneous, clear, smooth, pinhole-free, plasma-deposited polymer. Amongst many monomeric reactants vinyltrimethylsilane and hexamethylsiloxane or combinations thereof are mentioned. The plasma-deposited films are heated at 140 °C in an oxygen ambient for time periods ranging up to 180 min. The deposited films have a thickness of about 3 µm. After deposition of the oxygen barrier mask in tri-level structures, corresponding openings are conventionally formed or transferred in the polymeric base layer by reactive ion etching (RIE) in an oxygen ambient utilizing the overlying barrier film as a dry mask. The desired thin film, e.g. metal, to be deposited, is then blanket coated over the structure and on the surface exposed by the openings in the polymeric base layer. When the polymeric base layer is removed by application of a solvent selective for it, the overlying barrier layer and thin film, above the barrier layer, are "lifted-off" to leave the thin film deposited in the selected pattern on the substrate. Although, the conventional inorganic and solution coated polydimethylsiloxane films have been effective masking layers in O₂/RIE processes and have been employed in multilevel process for fabrication of integrated circuit devices, there are some problems encountered with these materials which detract from their consistency of function and thus renders that application less attractive in lithographic processes designed for submicron patterning. For example, coherent thin films of silicon oxide or silicon nitride are formed at relatively high deposition temperatures, e.g. 200°-275°C, which results in debris generation at the organic-inorganic interface, and thus problems of reticulation and shriveling are often encountered. A single major disadvantage of inorganic oxygen etch barrier films is a too frequent occurrence of high defect densities that are above the acceptable limits for high resolution lithographic applications. On the other hand, the solution coated or cast thin films of polydimethylsiloxanes and "glass resins", described as for example in Proceedings on the International ELECTRON DEVICES Meeting, Dec 1982, pp. 395-398, IEEE, N.Y., US, S. Matsui et al., NEW TRI-LEVEL STRUCTURES FOR SUBMICRON PHOTOLITHOGRAPHY, have the problems of non-uniformity, thickness control and shelf life of polymer solutions due to the presence of a certain amount of reactive groups on the polymer chain. Object of the invention is a method for forming a lift-off mask with an improved oxygen barrier layer which does not possess the disadvantages of the above described process. The object of the invention is achieved by a process according to claim 1. In the accompanying drawings forming a material part of this disclosure: Figs. 1A to 1H are diagrammatic cross-sectional views illustrating a portion of a substrate on various stages of processing during the practice of this invention; and Figs. 2A and 2B are photomicrographs illustrating transfers of images into a polysulfone layer from a pattern formed in an overcoating of a plasma polymerized organosilicon film. For further comprehension of the invention, one of the objects and advantages thereof, reference will be made to the following description and accompanying drawings, and to the appended claims in which the various features of the invention are more particularly set forth. Briefly, the improved multilayer resists or lift-off process of this invention employs glow discharge or plasma polymerized organosilicon oxygen etch resistant films as improved replacement or substitutes for conventional inorganic silicon oxide or silicon nitride film in multilevel or lift-off processes for fine line lithography. The materials of this invention also provide novel and improved oxygen RIE barrier film over solution deposited films of polyorganosiloxanes such as "glass resins" or poly(dimethylsiloxane) resin. Thin films deposited by glow discharge or plasma polymerization of organosilicon monomers are known to be useful as protective coatings for passivation and encapsulation, antireflection coatings for conventional optics, dielectrics in microelectronics, thin light wave guides, and moisture resistant barriers of alkali halide optics. Discussion and description can be found in U.S. Patents 3 847 652; 4 013 532; 4 018 945; 4 137 365; 4 212 719; 4 226 896; 4 260 647; and 4 279 723; as well as in the publications of M. Maisonneune et al., Thin Solid Films 33 , 35 (1975); Y. Segui et al. ibid 50 , 321 (1978); M. Aktik et al. J. Appl. Physics 51 , 5055 (1980); M. P. Schrieber et al. Thin Solid Films 72 , 487 (1980); and T. Hirotsu J. Applied Poly. Sci. 24 , 1957 (1979). The organosilicon polymers can be formed by glow discharge or plasma polymerization of monomers, of organosilicons including organosilanes, organosilazanes, organosiloxanes and mixtures thereof having the general configuration: where X is zero or oxygen, NH, NR₁, CH₂ or group; R is CH₃, -CH=CH₂, OR₁, and R₁ is an alkyl group, preferably having from 1 to 3 carbon atoms; or the configuration where Y is R or OR; Z is -CH=CH₂, -C≡CH, -OR, phenyl or aminoalkene group; and R is an alkyl group. Illustrative of the material contemplated are: These materials can be used singly or in admixture with each other. The composition can be readily varied to in accordance with desired film properties in terms of hydrophobicity and thermal stability, mechanical strength, adhesion, compatibility with contacting surfaces and oxygen etch rates. The plasma polymerized organosilicon films obtained are thermally stable when heated up to 450°C, they are resistant to most development and lift-off solvents, have good adhesion to contacting organic layers and have essentially zero etch rate in O₂ plasma or RIE, but can be etched by RIE in ambients containing CF₄, CHF₃, or CF₄/O₂. These films can also be employed as interlayer between organic or organic-inorganic polymer films without requiring application of adhesion promoters or other surface modifications. The MLR or "lift-off" mask is completed by overcoating the plasma polymerized organosilicon barrier layer, with a blanket layer of a photo, UV, X-ray or electron beam (E-beam) resist, which is patterned, using standard photo, UV, X-ray or E-beam lithographic techniques, to form a pattern of openings corresponding to a desired conductor pattern which concurrently exposes a corresponding pattern of the plasma polymerized barrier layer. Using the patterned resist as a masking layer, conformal openings can be suitably formed in the barrier layer by reactive ion etching in a suitable ambient which, as indicated above, can comprise a CF₄ ambient. This is normally followed by suitable etching of conformal opening in the lift-off mask base layer, again, by reactive ion etching, normally, in the same sputtering chamber in which, for a polysulfone base, the ambient can be switched, with purging, to an oxygen containing ambient to which the plasma polymerized organosilicon is essentially resistant. As will be appreciated, the formation of the indicated conformal openings in the lift-off base layer exposes a corresponding pattern of the substrate. It is noted, that the resist layer is removed coincidently by O₂/RIE etching of the lift-off base layer (e.g. polysulfone). After removal of the resist layer (which may be concurrently with O₂/RIE etching of the base layer), a blanket layer or coating of a conductive metal, for example, aluminum based alloys such as a 96/4 aluminum/copper alloy, is deposited (e.g. by sputtering or vacuum deposition) over the MLR or lift-off mask exposed plasma polymerized barrier layer as well as over the exposed portions of the substrate. This is followed by exposing the substrate to a solvent that is selective to the lift-off base layer (e.g. polysulfone). On solvation of the base layer component, all superimposed films and layers will lift off to expose the remaining portions of the substrate complementary to the retained metallization pattern. This invention also comprehends the deposition of modified plasma polymerized polyorganosilicon by the inclusion of organoborons (as for example trimethyl-boroxine, in the plasma polymerizing ambient. The resultant plasma polymerized organometallic polymer film can then be treated in an oxygen plasma (as for example a conventional oxygen plasma resist asher), which will convert the surface of the film as well as a surface adjacent portion thereof into a mixture of metal oxides which also function as an oxygen RIE barrier. The composite or modified film remains amenable to CF₄ and CHF₃ RIE etching. Also, this modification not only permits the deposition of plasma polymerized organometallic polymer film, but also a further modification into completely or partially oxidized (O₂ plasma) materials, as desired. The C and H content of the film can be decreased simply by injecting oxygen into the plasma system during deposition and increasing the pressure as required. It may be noted that deposition of silicon oxide over moisture sensitive organic substrates or surfaces is known to suffer occasionally from the problem of reticulations, particularly when humidity levels are high. In the present modification, no such problems are encountered. The organosilicon polymers are deposited by plasma polymerization at ambient or at elevated temperatures, whereas the silicon oxide or nitride films are in general deposited at 270° to 300°C. Also, the present modification of the invention allows tailoring of expansion coefficients at the interfaces through varying of organic content of the plasma polymerized film by selection of an appropriate monomer. Referring to the drawings, and to Figs. 1A to 1H in particular, there is shown a substrate 1 which is typically monocrystalline silicon or other semiconductor material (as for example, germanium, III-V compounds and the like), with an overlying layer 2 of a dielectric coating, as for example, silicon oxide, Si₃N₄, or composites thereof, and the like. It may be noted that plasma polymerized organosilicon films described in this invention are also useful as dielectric coatings on conductor and semiconductor materials. The substrate 1, in this embodiment of the invention is comprehended as an integrated circuit having active and passive devices fabricated therein (not shown, but illustrated by a diffused region 3) and means for electrically isolating the devices from each other. In such application, the insulating layer 2 is normally provided with contact openings (illustrated by 4) for making contact to active and passive devices. However, the substrate can also be a body of insulating material when this invention is employed to form metallurgical patterns on a dielectric module or carrier (e.g. alumina ceramics, glass-ceramics, etc.) for supporting integrated circuit chips or devices, and appropriate associated circuitry. Next, as shown in Fig. 1A, a blanket layer 6 of a solvent soluble organic polymer, illustratively in the range of 1 to 15 µm and preferably 1 to 3 µm, is deposited over substrate 1. Various suitable solvent soluble polymers can be used, as for example the polysulfone polymerized resin, many of which are described in U.S. Patent 4 045 318. A typical polysulfone is sold under the trademark "ASTREL 360" by the 3M Company. Another effective polymer which can be used for the base layer 6 is a conventional photoresist such as the AZ-1350 distributed by the Shipley Corporation, and which comprises a novolak-type phenol-formaldehyde resin and an o-quinonediazide photosensitive cross-linking agent. When this photoresist material (e.g. AZ-1350) is baked at about 210°C to improve adhesion, to the substrate, the photoresist is rendered thermally stable as well as desensitized. The resist can be applied by any well known technique such as spinning, brushing, dipping and the like. For purpose of illustration the polymeric base coating 6 can comprise of a polysulfone layer comprised of ICI polysulphone which was applied on silicon wafers by spin coating a 20 wt.% solution in diglycol methyl ether/N-methyl-pyrrolidone solvent mixture and baked to form a coating thickness of 2 µm. Next, a blanket barrier layer 7 of the above-described plasma polymerized organosilicon films is deposited over the solvent soluble thermally cured base layer 6. As indicated above the barrier film 7 is resistant to reactive ion etching in an oxygen containing ambient, and can be applied in a thickness range of about 50 to about 300 nm, and preferably from about 80 to 150 nm. For the specific embodiment, a 100 nm thick barrier film 7 was deposited by plasma polymerization of divinyl-tetramethyldisiloxane or hexamethyldisilazane which will serve as a barrier to oxygen RIE through in-situ oxidation to SiO x like material. Thus, when this plasma polymerized film 7 was coated on a 2 µm thick layer of polysulfone and the composite subjected to O₂/RIE, no measurable thickness loss was observed. Also, a plasma polymerized organosilicon film 7 was formed from 1,3-divinyl-tetramethyldisiloxane or hexamethyldisilazane to a thickness of 150 nm on the above noted ICI polysulfone base layer 6, followed immediately by an annealing schedule of 85°C for 5 minutes, 200°C for 10 minutes and 300°C for 10 minutes. Alternatively, the deposition was carried out directly on a substrate heated to 100-250°C. Plasma polymerization of organosilicon monomers was carried out by radiofrequency discharge using an inductively coupled RF generator operated at 13.54 MHz. The substrates were positioned downstream from the glow in a chamber equipped with a rotating stage and a Sloan quartz thickness monitor. Prior to introduction of the monomer vapor at a pressure of 53.3-66.6 mbar through the inlet tube, the deposition chamber is evacuated up to 6.6 mbar. Under these conditions, with a power input of 5-20 watts, a deposition rate of 4-6 nm/minute was achieved, as measured by the quartz crystal. The plasma coatings obtained at ambient temperatures were subsequently annealed up to 300°C on a hot plate or static oven. Various substrates employed were silicon, oxidized silicon (Si/SiO₂) wafers, cured polyimide films coated on silicon substrates, polysulfones and baked AZ-type coatings. Plasma films deposited on silicon were analyzed by IR transmission spectra on Nicolet FTIR instrument (Fourier Transform Infra Red Spectroscopy) and ESCA (Electron Spectroscopy for Chemical Analysis), before and after annealing. These IR spectra are characteristic of organosilicon polymers except for broadening of various bands indicating complexity of polymer structure. The annealing step was found to cause increase in the Si-O-Si band intensity and decrease in the Si-C bands. Measurements of pinhole density using Navonic Electrolysis Defect Detector, showed that the post-deposition annealing conditioned the plasma polymerized film to be consistently defect free, even after subjecting them to oxygen plasma or reactive ion etching in an oxygen ambient. Under the same conditions, 100 nm to 150 nm thick films of SiO x showed 10 - 12 defects /cm², while 120 nm thick films of silicon nitride showed 6 defects/cm². Also, as shown in Fig. 1C, a layer 8 of a photo, X-ray or E-beam resist, as for example an E-beam sensitive positive novolak/o-quinonediazide resist; of about 0.5 µm to 0.8 µm thickness is blanket coated over the barrier layer 7, by spin-coating. Openings 9A and 9B are then formed in resist layer 8 by conventional lithographic techniques well known in the integrated circuit fabrication art. The exposed and patterned resist layer 8 is then used as a mask for reactive ion etching of the plasma polymerized barrier layer 7 to replicate in it the opening pattern of resist layer 8. Thus, openings 9A and 9B (Fig. 1D) of resist layer 8 are effectively transferred into the plasma polymerized barrier layer 7 as respective openings 10A and 10B (Fig. 1E). In reactive ion etching, the structure of Fig. 1D is placed in a diode type system having an ambient of at least CHF₃ at 25 sccm (standard cubic centimeters/min.) flow; at a pressure of 33.33 mbar; and 200 watts RF-power. An illustrative R.F. tool which can also be employed in etching the plasma polymerized barrier layer 7 is described in U.S. Patent 3 598 710. At this point, if desired, the resist layer 8 can be removed by a suitable solvent selective for the resist but not for the base polymeric layer 6. However, as indicated above, the resist layer 8 can conveniently and preferably be removed concurrently with reactive ion etching of the polymeric base layer 6 in an oxygen ambient. Thus, utilizing the composite barrier and resist layers 7/8, with the openings therein, as a mask, the polymeric base layer 6, is then apertured by reactive ion etching in an oxygen containing ambient. For a polysulfone base layer 6 the etching can be accomplished over 5-8 minutes at a pressure of 26.66-39.99 mbar at a power density of 200 watts/cm², whereas for a desensitized Shipley AZ-1350 (a novolak/o-quinonediazide resist) base layer 6, the etching can be effective at about 0.133 bar of O₂ at a power density of about 0.25 watts/cm². Alternately, the ambient could consist of O₂ in combination with an inert gas such as argon, nitrogen and the like. In this manner, the openings 10A and 10B in barrier layer 7 are effectively transferred into the polymeric base layer 6 as respective openings 11A and 11B. As indicated above, the remaining portions of resist layer 8 are coincidently removed by O₂/RIE etching of the polysulfone base layer 6. It is noted that the successive reactive ion etching steps of Figs. 1E and 1F can be conveniently accomplished in the same reactive ion etching chamber by first utilizing a fluorine containing gas ambient, for the barrier layer 7, followed by purging and substitution of an oxygen containing ambient for the polymeric base layer 6. As shown in Fig. 1F, the O₂/RIE etching of the polymeric base layer 6 can be continued to extend the etching (overetch) of the base layer 6 under the barrier layer 7 to form the overhangs shown in Fig. 1F. These overhangs function to insure complete removal of the base layer 6 in openings 11A and 11B, provide for control of pattern dimensioning and insure of tearing of any overlaid coatings during lift-off of the mask, as more fully discussed in the above noted U.S. Patent 4 004 044. Next, using the lift-off composite 6A/7A of Fig. 1F with openings 10A/11A and 10B/11B, as a mask, a functional metal film 15 is blanket coated over the structure as shown in Fig. 1G. This metallic film 15 may be any metal conventionally used for metallization of devices in the semiconductor technology, e.g., aluminum, aluminum-copper alloys, platinum, palladium, chromium, etc. The metallization can be blanket coated by evaporation or sputtering over the surface of the structure (e.g. on the barrier layer 7A) and on the portions of the substrate exposed by the openings 10A/11A and 10B/11B in the composite lift-off mask 7A/6A. In the next operation, as shown in Fig. 1H, the lift-off mask (7A/6A) and all overlying layers of metallization 15, are then removed by exposing the substrate to a solvent specific to the polymeric base layer 6A of the lift-off mask. In the case of polysulfone base layer 6A, the solvent can be n-methylpyrrolidone at 60°C enhanced by ultrasonic agitation. As shown in Fig. 1H, the resultant structure is provided with a metallization pattern comprised of deposited segments 15A and 15B. In studies of plasma polymerized organosiloxane film, the following results were obtained. Thin plasma polymerized films from divinyltetramethyldisiloxane (DVS) hexamethyl-disilazane (HMDS), hexamethyldisiloxane (HMS), vinyltrimethoxysilane (VTS), and γ-aminotriethoxypropyl-silane (A-1100) were deposited on silicon wafers placed downstream from the glow in an inductively coupled RF plasma system at 13.56 MHz, and tested for defects, thermal stability and solvent resistance. Defect density evaluation was by the bubble trail method using Navonic Electrolysis Defect Detector. These plasma films were deposited at 700 nm, 120 nm, 150 nm and 200 nm thickness on silicon wafer which were then annealed at 300°C and examined for defects at 10⁶V/cm in methanol. On 2.8 cm² areas screened for each thickness, no pinholes were observed as indicated by the absence of bubble trails. On treatment in oxygen plasma or RIE the films were found to be consistently defect-free. Upon O₂ ashing (plasma treatment) for 5-10 minutes at 200 watts/1.07 mbar O₂ pressure in a Tegal Plasma System, these films remained mostly defect-free with one defect/cm² detected in some cases. When tested under the same conditions, a 166 nm SiO₂ film, on silicon wafers, showed 10-15 defects per cm². Plasma polymerization of DVS was also carried out in the presence of argon at 80:20 DVS to argon ratio. A 150 nm thickness of deposited film showed no detectable pinholes when tested as above. When tested for thermal stability, the plasma polymerized DVS, HMDS, HMS and VTS films are found to be stable up to 400°C. Thermally annealed plasma polymerized films were found to be insoluble in common organic solvents, including NMP (n-methyl pyrrolidone). With respect to contact angles, the as-deposited plasma polymerized films on silicon substrates, have contact angles ϑ H₂O of 80-82° indicating a hydrophobic surface. When annealed up to 300°C under nitrogen, the decreased contact angle of ϑ H₂O 76-78° was obtained. The contact angle is reduced to ϑ H₂O 10-15° when the film surface is modified by a brief oxygen plasma ashing (treatment) in the above noted Tegal plasma ashing system. FTIR analysis of the plasma polymerized films is typified by DVS deposited on KBr discs, showed characteristic vibration frequencies for C-H (2955 cm⁻¹), Si-O (1032 cm⁻¹), Si-C (839 cm⁻¹) and Si-(CH₃)₃(839 cm⁻¹). ESCA data for some of the plasma polymerized organosilicon films is given in Table I. The spectral measurements were made with films deposited on gold coated substrates. A 150 nm thick plasma polymerized DVS film (annealed up to 300°C followed by a 30 second oxygen ashing in the Tegal Plasma System) was employed as an oxygen barrier in a tri-level resist structure composed of 1.1 µm thick polysulfone as the base layer and 0.85 µm thick phenol-formaldehyde/o-quinonediazide positive imaging (resist) layer. E-beam exposure at 10 µm C/cm² and conventional development gave clearly resolved 1 µm and 1.5 µm geometries. Reactive ion etching with CHF₃ followed by O₂ allowed image transfer through the barrier into the polysulfone base layer, as shown in Figs. 2A and 2B. In a modification of this invention, as discussed above, a 100 nm thick barrier film 7 was deposited on a 1.8 µm thick layer 8 of polysulfone by plasma polymerization of a 1:1 mixture of divinyltetramethyldisiloxane and trimethoxyboroxine to provide an effective oxygen/RIE barrier for the etching of the polysulfone. When a 800 nm film of a novolak/o-quinonediazide resist was used on top as an imaging layer 8 at 10 µm C/cm² E-beam dose, after usual development, the image could be transferred cleanly to the barrier layer 7 by CHF₃-RIE, and 1 µm and submicron lines could be opened. The image was then etched (O₂-RIE) through the polysulfone base layer 6. Here, the organometallic plasma polymerized film 7, upon exposure to O₂-RIE (e.g. etching of base layer 6) is converted in-situ, at top surface adjacent portions, to SiO x BO y mixtures, which serve as an oxygen etch mask. Also, the incorporation of boron into the plasma polymerized organosilicon polymers, enhances its CHF₃-RIE rate. The CHF₃ etch rate for plasma polymerized layers of divinyltetramethyldisiloxane is 5 nm/min, while a plasma polymerized film from a monomer mixture of trimethoxyboroxine with divinyltetramethyldisiloxane gave a film with an etch rate of 14 nm/min under the same conditions.

Claims: Method of forming a lift-off mask with improved oxygen barrier layer of an organosilicon polymer comprising the steps of: A) blanket depositing a first layer (6) of a solvent soluble organic polymer on substrate (1); B) depositing on said first layer a second oxygen barrier layer (7); C) overcoating said second layer (7) with a blanket layer (8) of a radiation sensitive resist and patterning said resist layer using standard lithographic techniques; D) forming conformal openings (10A, 10B) in said barrier layer (7) by reactive ion etching in a fluorine containing ambient, using the patterned resist (9A, 9B) as a mask; E) reactive ion etching in an oxygen containing ambient the exposed portions of said first layer (6) to form openings (11A, 11B) therein and extending therethrough to expose corresponding portions of said substrate characterized in that said second oxygen barrier layer (7) of an organosilicon polymer is applied by plasma polymerization on said first layer (6) from an ambient containing monomers selected from the group consisting of organosilicones, including organosilanes, organosilazanes, organosiloxanes, and mixtures thereof; and heat treating said layer (7) for 10 to 30 minutes at temperatures in the range of 250 to 400 °C. Method of claim 1 including depositing a blanket coating of a conductive metal layer (15) on said substrate inclusive of said openings (11A, 11B) onto said exposed portions of said substrate; and subjecting the composite structure to said solvent to solvate said polymeric first layer (6) and concurrently lifting-off all superimposed layers thereon so as to retain said metallization pattern (15A, 15B) on said exposed portions of said substrate. Method of claim 1 including an oxygen plasma treatment of plasma polymerized organosilicon layer (7) to form a barrier layer through in-situ oxidation to SiO x like material. Method of claim 1 wherein said plasma polymerizing ambient includes organoboron monomers whereby said formed layer comprises an organoborosilicone polymer barrier layer, and an oxygen plasma treatment of the opening patterned barrier layer forms a borosilicate surface layer on said barrier layer. Method of claim 4 wherein said organoboron monomers comprise trimethoxy boroxine. Method of claim 1 wherein said substrate (1) is an oxidized monocrystalline silicon substrate having at least one via opening (4) in the oxide film (2) in register with a corresponding one of said openings to expose a surface adjacent portion of said silicon substrate, with said substrate having at least one integrated circuit (3) formed thereon, and said metal film (15) is in contact with the exposed portion of said substrate (1). Method of claim 1 wherein said substrate is heated to between 100 to 300 °C. Method of claim 1 wherein said monomers are represented by the formula: where X is zero or oxygen, NH, NR₁, CH₂, R is CH₃, -CH=CH₂, -OR₁, -O-Si-R₁, or -O-Si-CH=CH₂; and R₁ is an alkyl group. Method of claim 1 wherein said monomers are represented by the formula: where Y is R, or OR; Z is -CH=CH₂, -C≡CH, -OR, phenyl or aminoalkene group; and R is an alkyl.

Inventors: SACHDEV, HARBANS SINGH, SACHDEV, KRISHNA GANDHI
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1983
Main classification: G03F7
IPC codes: H01L21, G03F7