Consommation
`et Corporations Canada
`
`Consumer and
`Corporate Affairs Canada
`
`Bureau des brevets
`
`Patent Office
`
`Ottawa, Canada
`K1A oce
`
`(11)
`
`(C)
`
`1 3275,8116
`
`(21)
`
`(22)
`
`(45)
`
`493,257
`
`1985/10/18
`
`1990/ll/O6
`
`96-256
`
`5
`
`(51)
`
`INTL.CL. H05K—3/O6
`
`u9)um) CANADUWJPATENT(1m
`
`(54) Process of Forming a Negative Pattern in a Photoresist
`Layer
`
`(72) Roland, Bruno , Belgium
`Vrancken, August
`, Belgium
`
`(73) U C B Electronics S.A.
`
`, Belgium
`
`(30)
`
`(GB) U.K.
`
`84/27149 1984/l0/26
`
`(57) 15 Claims
`
`CCA 3254 (10-89) 41
`
`Page 1 of 30
`
`11> Bridge Exhibit 2014
`
`TSMC V. IP Bridge
`IPR2016-01246
`
`Page 1 of 30
`
`IP Bridge Exhibit 2014
`TSMC v. IP Bridge
`IPR2016-01246
`
`

`

`ABSTRACT OF THE DISCLOSURE:
`
`A process of
`
`forming high resolution negative
`
`patterns in a photoresist layer, comprises the steps of (a)
`coating a substrate with a layer of a photosensitive resin
`comprising a polymer, preferably a phenolic polymer, mixed
`or bound to a photoactive compound such as a diazoquinone,
`(b) exposing the layer to ultraviolet or visible light
`through a mask,
`(c)
`treating the layer with a silicon
`compound (e.g. hexamethyldisilazane) and (d) dry developing
`by plasma etching (e.g. an oxygen plasma)
`to remove the non-
`irradiated portions of
`the layer.
`The silicon compound is
`able to diffuse selectively into the irradiated portions of
`
`a
`By dry etching,
`the layer and fix in these portion.
`silicon oxide etch mask is formed which protects these
`
`irradiated portions efficiently throughout
`
`the process.
`
`The
`
`process is useful
`
`in the manufacture of semiconductor
`
`devices,
`
`including integrated circuits.
`
`Page 2 of 30
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`

`1275848
`
`The present
`
`invention relates to a new microlithographic process.
`
`More particularly,
`
`it relates to a process of forming a negative pattern
`
`in a photoresist layer which enables the production at
`
`industrial scale, with
`
`high yield, of very large scale integrated microcircuits (VLSI),
`
`the linewidth
`
`of which can go down to the submicrometer level. The invention also relates to
`
`the use of
`circuits.
`
`this new process in the manufacture of
`
`integrated semiconductor
`
`The continuing trend towards miniaturization in the field of integrated
`
`semi—conductor circuits gives rise to the need to accomodate more active
`
`10
`
`circuits per unit area on the surface of a semiconductor crystal.
`
`As an illustration of this, reference is made to the commercial
`
`development of metal oxide semiconductor
`
`(MOS)
`
`random access memory (RAM)
`
`devices from 1 Kbyte in 1975, via 16 Kbyte in 1977 and 64 Kbyte in 1979 to
`
`256 Kbyte in 1982. It is expected that this trend will continue over the next
`
`two decades. As a result,
`
`the minimum feature size of semiconductor devices is
`
`expected to continue to decrease from 8 micrometers for 1 Kbyte MOS RAM devices,
`
`over 2 micrometers for 256 Kbyte MOS RAM devices in l982 to below 1 micrometer
`
`before the end of the l98OFs.
`
`Microcircuit fabrication requires the selective diffusion of controlled,
`
`20
`
`small quantities of
`
`impurities into specific regions of the semiconductor
`
`surface to produce the desired electrical characteristics of the circuit, such
`,
`and other
`.
`.
`.
`.
`.
`as transistor/elements of which a large scale integrated circuit contains
`
`several
`
`tens of thousands of
`
`individual units that are interconnected in
`
`complex ways by conductors, such as aluminium or highly doped polycrystalline
`silicon.
`
`25
`
`The technique used in the commercial production of integrated circuits
`
`to obtain specific patterns is called photolithography or microlithography. The
`
`optionally oxidized silicon substrate (wafer),
`
`is coated with a photosensitive
`
`layer (also called photoresist) and exposed through a mask to ultraviolet light
`
`30
`
`The chemical and physical properties of the irradiated regions are different
`
`from the non-irradiated regions and create the possibility,
`
`in a development‘
`
`step, of removing the exposed (in a positive resist) or unexposed part (in a
`
`negative resist).
`
`.,q:4s§:*i
`
`Page 3 of 30
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`Page 3 of 30
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`

`

`fi2’?581l8
`
`Development processes are mainly based on solubility differences and are
`
`carried out by a wet
`
`technique. After removal of part of the resist layer,
`
`the uncovered substrate surface can be treated (etched, doped, oxidized,
`
`nitride treated, plated, etc.).
`
`This photolithography process is repeated (to more than l0 times) before
`
`the three-dimensional circuit geometries necessary for a completed metal oxide
`
`semiconductor (M05) or bipolar device are achieved. The structure of an
`
`integrated circuit is complex, both in the topography of its surface and in
`
`its internal composition. Each element of this device has an intricate three-
`
`dimensional structure that must be reproduced exactly in every circuit. The
`
`structure is made up of many layers, each of which is a detailed pattern. Some
`
`of the layers lie within the silicon wafer and others are stacked on the top.
`
`The process is described in detail in the book of L.F. THOMESON, C.G.hHLLSON
`
`and M.J. BOWDEN "Introduction to Microlithography", American Chemical Society
`
`Symposium Series 219, Amer.Chem.Soc., Washington D.C., 1983.
`
`As a result of the increasing density of very large scale integrated
`
`circuits,
`
`the minimum feature size of semiconductor devices decreases and the
`
`production processes become more difficult. Achieving a high resolution with
`
`good linewidth control on substrates with surface topography is a serious
`
`problem. As a result of optical reflections and variations of resist thickness
`
`over high steps,
`
`linewidth control becomes very difficult and relatively thick
`
`resist layers are necessary. Since, for these small features, lateral dimensions
`
`shrink more rapidly than vertical dimensions, higher height-to—width aspect
`
`ratios of resist
`
`images are required.
`
`In addition, dry etch techniques require
`
`relative thick and stable resist patterns. However,
`
`thick resist layers limit
`
`resolution and give depth of focus problems for projection printing. Furthermore
`
`swelling of negative resists in organic solvent developers also makes them
`
`unsuitable for high resolution microlithography and restricts the choice to the
`
`positive-acting diazoquinone resists. However, even these high contrast and
`
`high resolution resists become insufficient when linewidths decrease to the
`
`micrometer and submicrometer regime.
`
`Several approaches to obtain higher resolution and better linewidth
`
`control have been studied during the past few years. New exposure techniques
`
`are being investigated, such as electron beam writing, X-ray and deep UV
`
`exposure. Electron beam lithography requires very costly equipment, suffers
`
`Page 4 of 30
`
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`

`

`‘;l2’?E584S
`
`from low throughput and is unable to produce high aspect ratios as a result
`
`of backscattering of electrons,
`
`i.e.
`
`the so-called proximity effect. X-ray
`
`exposure has very high resolution capabilities but equipment and resist
`
`materials are still in the developmental stage. The manufacturing of X-ray
`
`masks is also a difficult operation. The use of deep UV light decreases diff-
`
`raction effects, but resolution is still limited by swelling of the negative
`
`resists and positive deep UV resists with sufficient sensitivity are not yet
`available.
`
`In order to eliminate the adverse effects of solvents during wet
`
`developing, plasma development of photoresists has been considered (cf.
`
`J.N. SMITH, H.G. HUGHES, J.V. KELLER, W.R. GOODNER and T.E. WOOD,
`
`in
`
`Semiconductor International,g,(l979,n°lO, December),p.4l-AD. The dry
`
`development of resists is an important step towards achieving a totally dry
`
`microfabrication process, offering better process control, reproducibility
`
`and cleanliness.
`
`In addition, dry development solves problems such as
`
`resolution loss by swelling (particularly in the case of negative resists) and
`
`handling of large amounts of inorganic or organic solvents. It also has a much
`
`better adaptability to automatic line fabrication processes. However, most dry
`
`development resist systems have a low contrast and undergo a serious reduction
`
`of resist thickness during development.
`
`Another approach to obtain high resolution and high aspect ratio consists
`
`in the use of the multi*layer resist systems.
`
`In these processes, excellent
`
`step coverage and dry etch resistance of a thick planarizing bottom layer are
`
`combined with the high resolution of a thin top imaging layer. Both layers can
`
`be optimized for their specific requirements. After exposure and development
`
`of the thin top resist,
`
`the patterns obtained are transferred almost vertically
`
`into the thick underlying layer either by deep UV exposure and development or
`
`by oxygen-reactive ion etching. For
`
`the latter method, a third intermediate
`
`layer is sandwiched between the imaging and the planarizing layer. Usually, a
`
`thin layer of plasma-deposited or spin-on silicon oxide is applied on top of
`
`the planarizing layer. The exposed and developed patterns in the top resist
`
`layer are first transferred into the intermediate (silicon oxide) layer by
`
`etching techniques. After removal of
`
`the resist,
`
`the thin oxide patterns act
`
`as a very effective mask for oxygen-reactive ion etching of the thick bottom
`
`layer, resulting in profiles with almost vertical sidewalls. Aluminium or
`
`Page 5 of 30
`
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`
`

`

`i'i2‘?5848
`
`silicon nitride have also been used as intermediate layer.
`
`with these techniques, high resolution and high aspect ratios can be
`
`realized on substrates with surface topography (cf. J.M. MORAN and D. HAYDAN
`
`in The Bell System Technical Journal,§§,(l979,n°5,May-June, p.1027-1036).
`
`However,
`
`the multi-layer resist systems have also several serious draw-
`
`backs. Thus,
`
`formation of interfacial
`
`layers occurs when one polymer material
`
`is spun on another. Hard bake steps may somewhat reduce this phenomenon but
`
`even then additional
`
`treatments are required to remove these layers before
`
`pattern transfer. However, baking may induce film stress. Spun-on silicon oxide
`
`films are very susceptible to cracking when the baking temperature is too high.
`
`On the other hand, a sufficiently high baking temperature is required to
`
`avoid interfacial mixing between the oxide and the imaging layer.
`
`Another problem is the optical
`
`interference effects. Even with a highly
`
`absorptive bottom layer, reflections can occur at the interfaces, resulting in
`
`standing waves. A post-exposure bake of the imaging layer can reduce this
`
`effect but only at
`
`the expense of a contrast loss.
`
`Finally,
`
`the thin top resist layers which are used in multi-layer systems
`
`to produce better images make the system more susceptible to pinhole problems.
`
`Multi—1ayer resist systems are discussed in depth by B.J. LIN in the above-
`
`mentioned book "introduction to Microlithography", p.287~3SO.
`
`Another major disadvantage of mu1ti~1ayer systems is process complexity.
`
`Several
`
`layers have to be applied and baked and each of them has to be exposed,
`
`developed and/or etched. Therefore, efforts have recently been made to
`
`simplify these multi-layer systems, without losing their advantages. One
`
`example is the two-layer reactive ion etching (RIE) process in which the top
`
`imaging layer is a resist containing both organic and inorganic components (for
`example, poly(dimethy1 siloxane) and poly(viny1-methyl siloxane) (cf.
`
`M. HATZAKIS §£‘al.,
`
`in Proceedings of the International Conference on Microlitm
`
`graphy, 1981, p.386~396), copolymers of chloromethylstyrene and trimethylsilyl“
`
`styrene (cf. M. SUZUKI eE_al.,
`
`in J.Electrochem.Soc.l30,(l983).P~1962-1966)
`
`and copolymers of vinyltrimethylsilane and sulfur dioxide (U.S.Patent
`
`Specification No. 4,357,369). The thin top resist layer is exposed and
`
`developed,
`
`followed by etching of the thick bottom layer by oxygen RIE. During
`
`this process,
`
`the inorganic components in the resist form refractive oxides
`
`which act as an in situ~formed etch mask.
`
`In this way,
`
`the top imaging layer
`
`Page 6 of 30
`
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`
`

`

`12"?ES8§8
`
`and the intermediate silicon oxide layer of
`
`the three—layer resist system
`
`are combined into one organo~metallic resist layer. Processing of
`
`these
`
`systems is easier than the three-layer resists but
`is still more Complex than
`single layer resist systems. They have also only been developed so far for
`
`electron beam and deep UV exposure and still require wet development
`
`in
`
`organic solvents. Another example of a simplified multi-layer system consists
`in the use of a contrast enhancement
`layer.
`In this process, a thin photo-
`bleachable layer is coated over a standard positive photoresist. During
`exposure,
`the dye in the coating bleaches,
`forming a new mask in intimate
`
`Contact with the resist surface. After exposure,
`
`the layer is removed in a
`
`solvent system and the resist is further processed by standard techniques.
`Although this method improves contrast and adds little to the process complexity,
`it does not overcome the standing waves problem and linewidth variations over
`
`steps.
`
`In addition, extended exposure times are required.
`
`The ideal would obviously be to have at one's disposal a single layer
`resist system affording the same technical advantages as multi-layer resist
`systems, but which does not have their drawbacks.
`In this way,
`the formation
`of
`interfacial layers, optical
`interference effects and film stress problems
`would disappear and process complexity would be drastically reduced.
`
`However,
`the difficulty is to find a single layer resist system, which can
`be dry developed and which is characterized at
`the same time by high resolution,
`high aspect ratios and excellent linewidth control on substrates with surface
`
`topography,
`
`in order to obtain high quality resist patterns which enable the
`
`production with a good t'epr.‘OduCib.’l_l.'l.'Cy
`circuits.
`
`of very large scale semiconductor
`
`That is the reason why various attempts have been made
`to
`improve the single layer resist systems. As an example,
`in Japanese patent
`application n° 23937/Sifi a process is described in which a positive photo-
`resist, coated on a substrate,
`is exposed to an atmosphere containing an
`organic silicon compound, more particularly hexamethyldisilazane. After
`
`exposure to ultraviolet light,
`the photoresist is developed in a conventional
`manner by dissolving the exposed areas with an alkaline liquid developer. The
`treatment with the hexamethyldisilazane vapors has as effect
`to reduce the
`
`,
`
`developing rate of
`
`the photoresist in the alkaline developing solution,
`
`thus
`
`* published 8th February 1982
`
`Page 7 of 30
`
`Page 7 of 30
`
`

`

`fi2*?i':S848
`
`permitting the formation of a resist pattern having a higher contrast
`vertical edges).
`
`(more
`
`It must be emphasized that, apart from the treatment with the hexamethyl-
`
`disilazane vapor,
`
`this process does not differ in any way from the conventional
`
`photolithographic process. The image formation is still based on solubility
`
`differences produced by the creation of carboxylic groups in the photoresist
`
`under the effect of the ultraviolet light and development is exclusively
`
`carried out by wet development
`
`in an alkaline developer. Even if the contrast
`
`is enhanced by this process,
`
`the well known drawbacks of wet development are
`
`not suppressed. The light exposure has to be effected in depth in the resist
`
`layer, with the consequence that effects such as reflections and standing
`
`waves cannot be suppressed. This can give rise to linewidth variations
`
`(especially on a reflective topography).
`
`D. FOLLET e£_gl.
`
`("Polarity reversal of PMMA by treatment with chlorosi1a-
`
`H
`
`- The Electrochemical Society Extended Abstracts,§g-2 (1982, October l7-
`nes
`2l), p.321~322) describe a process in which a poly(methyl methacrylate) resin
`
`(hereafter called PMMA)
`is electron beam irradiated,
`then sequentially treated
`with dichlorodimethylsilane and water vapor,
`to form polysiloxanes, and finally,
`
`developed in an oxygen plasma. According to these authors,
`
`there would be
`
`selective diffusion of
`
`the dichlorodimethylsilane in the irradiated and
`
`partially degraded areas of the resist.
`
`followed by polymerization of
`
`the
`
`chlorosilane upon exposure to the water vapor. They find a polarity reversal
`
`of
`
`the resist Erom positive into negative:
`
`the irradiated and treated areas of
`
`indeed a higher resistance to the oxygen plasma. A pattern is
`the PMMA 0EEer
`obtained consisting of 2 pm lines and 4 mm spaces and abrupt edge profiles.
`This process has nevertheless several
`important drawbacks. Electron beam
`
`exposure, even if it allows a high resolution, requires a very expensive
`equipment, and the irradiation times per silicon wafer are too long. Because
`of these limitations, up to the present electron beam lithography is only
`used for direct writing of patterns for devices intended for research. Moreover,
`PMA is a material which offers a very weak resistance to dry etching operations
`because it degrades very rapidly in the plasmas used to this end (5 times faster
`
`than the aromatic polymers). Finally, owing to the fact that dichlorodimethyl-
`
`Page 8 of 30
`
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`
`

`

`?l2’?/58118
`
`silane is not
`
`immediately fixed in the irradiated areas of the PMMA layer
`
`(since water vapor is necessary for the conversion in polysiloxanes),
`
`it
`
`is conceivable that
`
`this compound can easily go out again from the layer by
`
`diffusion. This will necessarily have a detrimental effect on the reproducibi-
`
`lity of the characteristics of the obtained patterns, since the concentration
`
`of
`
`the dichlorodimethylsilane in the layer will be a function of the time
`
`which elapses between the treatment with this compound and the treatment with
`
`the water vapor.
`
`T.M. WOLF 35 al.,
`
`in J.E1ectrochem.Soc.131,(1984,n°7),p.l664-1670,
`
`propose still another process intended to improve the single layer resist
`
`systems. The photoresists used by these authors are negative photoresists
`
`conventionally used in photolithography. They consist of a partially cyclized
`
`polyisoprene containing a bis~azide as photosensitizer (commercial WAYCOAT IC-
`
`&3*and SELECTILUX N-60*).The proposed process comprises the steps of
`
`ultraviolet irradiation or electron beam exposure,
`
`treatment with a volatile
`
`inorganic halide,
`
`followed by development by oxygen-reactive ion etching.
`
`In
`
`the described experiments,
`
`these authors use silicon tetrachloride (SiCl4),
`
`tin tetrachloride (SnCl4) and dichlorodimethylsilane ((CH3)2SiCl2) as volatile
`inorganic halides, since these compounds are able to react with the secondary
`
`amines created during the photolysis of the resist. They hoped indeed that
`
`this reaction could be used to incorporate the inorganic halides selectively
`
`into the exposed areas of
`
`the resist,
`
`thus allowing the formation by oxidation
`
`of a thick inorganic oxide protecting layer in said exposed areas during the
`
`following development step by oxygen~reactive ion etching. Only the unexposed
`
`areas of the photoresist would thus be removed selectively by oxygen—reactive
`
`ion etching.
`
`Nevertheless,
`
`they find that the inorganic compounds are sorbed quickly,
`
`not only in the exposed areas, but also in the unexposed areas of the resist.
`
`Moreover,
`
`they note that, contrary to all expectations, by oxygen-
`
`reactive ion etching,
`
`the unexposed areas of the photoresist are protected
`
`by an oxide layer and etched at a significantly lower rate than the exposed
`
`areas, whereas these latter are selectively removed during the dry etching
`
`development step.
`
`In other words,
`
`the photoresist behaves as a positive tone
`
`resist. The authors explain this phenomenon by the fact that,
`
`in the
`
`unexposed areas, complexes between the inorganic halide and the azide group
`
`of the photosensitizer are also formed, while in the exposed areas,
`
`the
`
`* trade mark
`
`Page 9 of 30
`
`Page 9 of 30
`
`

`

`fl2”?E$8£l£
`
`anticipated reaction with the photolysis products of the resist takes place.
`
`Moreover,
`
`in the unexposed areas,
`
`the formed complexes are readily hydrolyzed
`
`by the water vapor present
`
`in the ambient atmosphere and converted to
`
`refractory inorganic oxides,
`
`thus forming an in situ protective masking layer,
`
`whereas the reaction products formed in the exposed areas are more slowly
`
`hydrolyzed and therefore readily removed as volatile compounds during oxygen-
`
`reactive ion etching.
`
`Nevertheless,
`
`this process still has important drawbacks.
`
`First, according to the statements made by the authors themselves,
`
`this
`
`process is satisfactory only provided very strict light exposure times and
`
`at the same time very strict treatment
`
`times with the inorganic halide are
`
`observed.
`
`Indeed,
`
`in order to obtain a satisfactory result, it is necessary
`
`that
`
`the exposure of the photoresist
`
`to light
`
`takes place during 16 seconds
`
`and that the treatment with the inorganic halide takes place during 7 seconds.
`
`Shorter or longer exposure and/or treatment
`
`times give unsatisfactory results.
`
`Thus, for example, after a treatment with the inorganic halide for 10 seconds,
`
`it is no longer possible to develop the resist by etching, regardless of exposur
`
`time.
`
`In other words,
`
`the operating conditions are very critical, which can
`
`only be detrimental
`
`to the reproducibility of the results.
`
`3eC0nd1Y.ewen under the optimal oxrfitkxs curd hereinbefore, Only 70% Of
`
`the initial thickness of the photoresist remains after development by dry
`
`etching.
`
`Thirdly, as shown in figure 11 of page 1669 of the publication,there
`.
`.
`.
`.
`at as
`.
`remains after development, an important residue in the uncoveredfi w ich residue
`
`is very difficult to remove (this fact shows at
`
`the same time that
`
`the
`
`selectivity of the process is insufficient).
`
`To conclude, it can be seen that till now, a microlithographic process
`
`has not yet been developed in which a single layer resist system is used which
`
`is entirely satisfactory.
`
`It is for these reasons that we have carried out research work to develop
`
`a single layer microlithographic process, which would be free from the
`
`drawbacks of the hitherto known processes, particularly of those described in
`
`Japanese patent application n° 23937/82 and in the above mentioned publications
`
`of D. FOLLET 3c_§_1., and of 17.24. WOLF 35 31;
`
`This object is fully achieved by the process described hereinafter,
`
`which has all the advantages and the simplicity of single layer microlitho-
`
`_ 3 _
`
`Page 10 of 30
`
`Page 10 of 30
`
`

`

`'132’?58¢‘li§
`
`graphy, which allows the use of dry etching techniques, which can be used
`
`on standard projection printing equipment and wafer stepper equipment, and
`
`yet gives equal or better submicrometer resolution than the above-mentioned
`
`multi-layer processes.
`
`Thus,
`
`the present invention provides a new improved microlithographic
`
`process. More specifically,
`
`the present
`
`invention provides a process of
`
`forming
`
`a negative pattern in a photoresist layer comprising the following steps:
`
`(a) coating a substrate with a layer of a photosensitive resin comprising a
`
`polymer mixed or bound to a photoactive compound, said layer
`
`enabling
`
`a silicon compound to diffuse selectively into irradiated portions
`
`thereof, on exposure of said portions to visible or ultraviolet
`radiation;
`
`(b) exposing the photosensitive resin layer to ultraviolet or visible light
`
`through a mask to expose only selected portions of the layer;
`
`(c)
`
`treating the photosensitive resin layer with a silicon compound, whereby
`
`said compound is selectively absorbed into the irradiated portions of
`
`the coating and is caused to react with said irradiated portions; and
`
`(d) dry developing the thus treated photosensitive resin layer by plasma
`
`etching to remove selectively the non-irradiated portions thereof in order
`
`to obtain the desired negative pattern.
`
`According to a preferred embodiment of the present invention,
`
`the
`
`substrate is a silicon water,
`
`the photosensitive resin used comprises a phenolic
`
`polymer and the photoactive
`
`compound which is mixed or bound to it is a
`
`diazoquinone, whereas the silicon compound is an easily vaporizable silylating
`agent.
`
`According to a particularly preferred embodiment of the present invention,
`
`the phenolic polymer is chemically bound to the diazoquinone.
`
`On the other hand, according to the present invention,
`
`the treatment with
`
`the silicon compound is preferably carried out after exposure of the photo~
`
`sensitive resin layer to ultraviolet or visible light. However,
`
`those skilled
`
`in the art will understand that, for the purpose of simplicity and readiness
`
`of execution, it is possible to carry out
`
`the treatment with the silicon
`
`compound already during the exposure of the photosensitive resin layer to
`
`light.
`
`In other words, it is possible to conduct steps (b)
`
`and U3)oE the
`
`process simultaneously.
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`Page]l_0f30
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`Page 11 of 30
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`12758413
`
`According to a particularly preferred embodiment,
`
`the silicon compound is
`
`volatilized and brought
`
`into contact
`
`in vapor form with the photosensitive
`
`resin layer.
`
`According to a particularly advantageous embodiment of the invention, dry
`
`development
`
`is carried out by oxygen~reactive ion or oxygen plasma etching.
`
`The invention further comprises the use of
`
`the new microlithographic
`
`process in the manufacture of
`
`integrated semiconductor circuits.
`
`The process according to the invention is based on the surprising
`
`discovery that positive acting photosensitive resins, comprising a polymer
`
`associated with a photoactive
`
`compound such as a diazoquinone, undergo
`
`substantial modifications of their properties under the influence of a visible
`
`or ultraviolet radiation. We have found indeed that
`
`the exposure to the
`
`radiation modifies to a considerable extent
`
`the permeability properties of
`
`these resins, and we have taken advantage of this particularity in order to
`
`better differentiate the irradiated portions from the nonrirradiated portions
`
`of a layer of these resins coated on a substrate, and in consequence,
`
`to
`
`develop an improved single layer microlitnographic process. To this end,
`
`according to the invention,
`
`the irradiated photosensitive resin layer is
`
`subjected to a treatment with a silicon compound,
`to the particularity just mentioned hereinbefore,
`
`in order to allow,
`thanks
`this compound to penetrate
`
`selectively in the irradiated portions of the layer, preferably in the top
`
`part thereof, and consequently fix selectively in these irradiated portions
`
`by reaction with the functional groups of the photosensitive resin.
`
`Experience has shown that
`
`the silicon compounds actually diffuse
`
`selectively in the irradiated portions of the layer, whereas such a diffusion
`
`practically does not occur-in the non-irradiated portions, or only to a
`minor extent.
`
`Thus, unlike the process of T.M. WOLF et_al. above described,
`
`the silicon
`
`compounds do not diffuse in all the regions of the photoresist layer, but only
`
`in the irradiated portions of
`
`this layer.
`
`The process of the invention is thus based on selective diffusion of the
`
`silicon compounds in the irradiated portions of the photoresist, unlike the
`
`process of
`
`the state of the art, which is exclusively based on a selective
`
`Page 12 of 30
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`Page 12 of 30
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`i’i.2’?ii§84£
`
`reaction of the silicon compounds in either the exposed or the unexposed
`
`r€8i0flS
`
`(the silicon compounds being incorporated in every region of the
`
`photoresist).
`
`From the technical point of view,
`
`this difference has considerable
`
`repercussions on the results because,
`
`in the process according to the
`
`invention, after development by dry plasma etching, superior quality patterns
`
`are obtained. Moreover,
`
`the above-mentioned disadvantages of
`
`the known
`
`single layer processes are definitively suppressed. By dry etching,
`
`the non-
`
`irradiated portions of the photoresist are completely removed and,
`
`in the
`
`irradiated portions, a silicon oxide mask is very quickly formed in situ,
`
`which remains and protects very efficiently these portions throughout
`
`the
`
`etching process. After development,
`
`the thickness of the obtained pattern is
`
`practically identical
`
`to the initial thickness of the photoresist;
`
`the
`
`residual
`
`thickness generally represents more than 952 of the initial thickness
`
`of the resin layer coated on the substrate.
`
`In the uncovered parts, no residue
`
`remains. Moreover,
`
`the reproducibility is considerably improved since the
`
`light exposure times and the treatment
`
`times with the silicon compound do
`
`practically not have any effect on the results (contrary to the process of
`
`T.M. WOLF et al.
`
`above described).
`
`Thus,
`
`the present
`
`invention provides a single layer resist system which
`
`can be dry developed and which has all the advantages of the multi-layer
`
`resists: planarization, high resolution, high aspect ratios with excellent
`
`retention of the initial thickness of the photosensitive resin layer coated
`
`on the substrate, good linewidth control over the steps, good reproductibility
`
`of the pattern characteristics and suppression of the interference with light
`
`reflected at substrate topography.
`
`In addition,
`
`the described system has
`
`several advantages over the multi-layer resist systems. Formation of
`
`interfacial layers does not occur,
`
`interference with reflected light does not
`
`occur, film stress problems do not exist and process complexity is drastically
`reduced.
`
`Even if the process of
`
`the invention provides a negative acting resist
`
`system, it does not need wet development, which definitively suppresses the
`
`problems of resolution losses caused by the swelling in development solvents.
`
`Moreover, since no solvents are used in the development step, adhesion is no
`
`longer a critical parameter. The completely dry processing of the resist
`
`Page 13 of 30
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`Page 13 of 30
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`

`

`results in improved process control and makes this system particularly
`
`suitable for automatic line fabrication processes.
`
`The invention will now be further described with reference to the
`
`accompanying drawing wherein like reference numerals refer to same parts
`
`throughout
`
`the several views, wherein the layers are greatly exaggerated.
`
`Figure 1
`
`is a partial cross-section of a silicon wafer coated with a
`
`photosensitive resin layer before the exposure step.
`
`Figure 2 is a partial cross~section of a silicon wafer coated with a
`
`photosensitive resin layer during its exposure to light through a mask.
`
`Figure 3 is a partial cross-section of a silicon wafer coated with a
`
`photosensitive resin layer after the treatment with the silicon compound.
`
`Figure 4 is a partial cross-section of a negative pattern obtained after
`
`development by oxygen-reactive ion or oxygen plasma etching.
`
`A wide range of polymers can be used in order to prepare the
`photosensitive resin used in the process according to the invention. However,
`
`preferably a phenolic polymer is used, which is selected from:
`
`~ the condensation products of a phenol, a naphthol or a derivative thereof
`
`ring-substituted by an alkyl or aryl radical or by a halogen atom, with an
`aliphatic or aromatic aldehyde, which can be substituted by a halogen atom,
`
`the po1y(viny1pheno1s),
`
`the phenolic group of which can be substituted by an
`
`alkyl or aryl radical or a halogen atom,
`
`the copolymers of a vinylphenol with an ethylenically unsaturated compound,
`and
`
`mixtures of the aforesaid polymers between them or with other aromatic
`
`polymers, such as polystyrene or poly(N—vinylcarbazole).
`
`As illustrative but not limitative examples of phenolic polymers,
`
`there
`
`may be mentioned: phenolsnovolacs, cresol-novolacs, condensation products of
`
`formaldehyde with alkylphenols (p-tert~butylphenol; p-n-propylphenol; p-
`
`ethylphenol;
`
`octylpheuol and the like), condensation products of benzaldehyde
`
`with cresols or naphthols (e.g.
`
`l-naphthol), poly(p-vinylphenols), copolymers‘
`
`of p-vinylphenols with p—ch1orostyrene, and the like.
`
`The photoactive compound mixed or bound to the polymer is
`
`preferably a diazoquinone such as those used in traditional positive photo-
`
`_ 12 _
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`Page 14 of 30
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`Page 14 of 30
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`

`

`’.l.2’?58-5&8
`
`resists (see British Patent Specification No. 711,626). As non-limitative
`
`examples of these diazoquinones,
`
`there may be mentioned: 5-diazo-5,6-dihydro-
`
`6-oxo-1-naphthalenesulfonic acid, 6-diazo—5,6-dihydro-S-oxo-1-naphthalene-
`
`sulfonic acid, 3-diazo-3,A-dihydro-&*oxo-lrnaphthalenesulfonic acid, 4-
`
`diazo-3,4-dihydro-3-oxo-l-naphthalenesulfonic acid, 3-diazo-3,4—dihyaro—
`
`4-oxo-1-benzenesulfonic acid,
`
`the corresponding carboxylic acids, derivatives
`
`thereof and mixtures of at least
`
`two of
`
`the aforesaid compounds.
`
`As an example of a derivative of the aforesaid acids,
`
`the condensation
`
`product of 3 moles of 6-diazo-S,6-dihydro-S~oxo-l-naphthalenesulfonyl chloride
`
`with 1 mole of 2,3,4-trihydroxybenzophenone may be mentioned.
`
`The photoactive compounds such as diazoquinones can easily be chemically
`
`bound to the afores