AN IMAGE REVERSAL TECHNIQUE FOR FORMING SMALL STRUCTURES IN INTEGRATED CIRCUITS

Abstract: The present invention provides a method for fabricating small structures to be employed in integrated circuits formed on a semiconductor substrate (12). Examples of such small structures include contacts, vias, and metal lines. The method of the present invention employs an image reversal technique to obtain improved feature definition. In forming a feature (28) in a layer of material (16), a clear field reticle is used to form patterned segments of photoresist (18) each having a size, a shape, and a location substantially identical to the size, the shape, and the location of one of the features (28) intended to be formed in the layer of material (16). This method is employed instead of using a dark field reticle which forms windows in a photoresist each having a size, a shape, and a location substantially identical to the size, the shape, and the location of one of the features (28) intended to be formed in the layer of material (16). For small structures, the openings or windows in a photoresist are harder to form than the patterned segments of photoresist (18). With the method of the present invention which employs a clear field reticle to form a mask comprising patterned segments of photoresist (18), the limitations of patterning small windows in a photoresist with the use of a dark field reticle are avoided. The accuracy of forming the small structures is thus improved.

Description: AN IMAGE REVERSAL TECHNIQUE FOR FORMING SMALL STRUCTURES IN INTEGRATED CIRCUITS TECHNICAL FIELD The present invention relates generally to fabπcating small structures (≤O 5 μm in size) such as con¬ tacts, vias, and metal lines to be employed in integrated circuits formed on a semiconductor substrate, and, more panicularly, to a method which enables a clear field reticle to be used in the formation of such small structures BACKGROUND ART The present integrated circuit devices are formed on a silicon substrate and compπse doped regions in the substrate to which source or drain connections are made, separated by a gate control region Electncal connections to the source, gate, and drain electrodes are made by interconnects, e g , metal interconnects, which are supported over the substrate by an interlayer dielectric Conventional integrated circuits employ multiple levels of interconnects For example, a first level patterned interconnect contacts the source and drain contacts and the gate electrode in a desired pattern The formation of the first level patterned interconnect is conventionally followed by deposition of an oxide over the semiconductor substrate This oxide forms the interlayer dielectric The oxide is sequentially patterned and etched to form contact/via openings therein These contact/via openings in the interlayer dielectric are filled with metal plugs using blanket metal deposition followed by etchback or polishing A layer of patterned metal lines compπsing the second level patterned interconnect is formed on the interlayer dielectπc The metal plugs form electncal contact between the first le\el patterned interconnect and the second level patterned in¬ terconnect Additional le\els of patterned metal lines or interconnects mav be formed as well each level of interconnects is also separated b\ an ιnterla>er dielectric laver Electrical connection between the different levels of interconnects is provided b\ addiuonal metal plugs To etch contactΛ'ia openings into the dielectric a photoresist is formed thereon The pho¬ toresist is exposed to light and developed out. leaung designated regions of the oxide exposed, I e . having no photoresist formed thereon The exposed regions of the are etched to create contact/via openings therein In general terms, an opening or small window is formed in the photoresist which exposes the under¬ lying layer of mateπal (e.g., oxide) to subsequent treatment, (e.g . etching). Such a small window m the pho¬ toresist, which exposes the underlying layer of mateπal. is formed by exposing the photoresist to light and de¬ veloping out the photoresist. In pπnciple. the photoresist may compπse either positive photoresist or negative photoresist How¬ ever, for patterning small structures, 1 e.. ≤0.5 μm in size, positive photoresist is conventionally employed. For a positive photoresist, the portion of the photoresist exposed to light is developed out The portion of the photoresist which is not exposed remains intact, thereby providing a mask compπsing patterned pho¬ toresist which protects the underlying material foπned directly beneath dunng subsequent processing In contrast, for a negaUve photoresist, the portion of the photoresist which is not exposed to light is developed out. The portion of the photoresist which is exposed remains intact, thereby providing a mask com¬ prising patterned photoresist which protects the underlying matenal formed directly beneath during subse¬ quent processing. Negative photoresist for use in defining small structures, i.e., ≤0.5 μm in size, however, is not avail- able. The mimmum feature size which can be patterned using currently available negative photoresist is not smaller than 1.0 μm. In contrast, currently available positive photoresist can be used to pattern structures as small as 0.25 to 0.35 μm in size. Consequently, for patterning small structures, positive photoresist is conven¬ tionally employed. Accordingly, the term "photoresist" will hereinafter be equated with "positive photoresist" unless specified otherwise. To expose the photoresist, ultra-violet light is passed through a reticle mask having the desired pat¬ tern formed thereon. The pattern in the reticle mask comprises opaque and transparent areas, i.e., dark areas and clear areas. Photoresist beneath the clear areas of the reticle are exposed to the UV light, while the pho¬ toresist under the dark areas of the reticle mask remain unexposed. Accordingly, a single window is formed in a photoresist, i.e., a positive photoresist, by transmitting UV light through a clear area in the reticle mask which otherwise comprises dark areas. The clear area is sur¬ rounded by the dark areas. Consequently, this reticle mask is conventionally referred to as a dark field reticle. The photoresist directly beneath the clear areas of the dark field reticle is exposed and developed out. Accordingly, the layer of material lying under the photoresist can be exposed to subsequent treatment to form features such as openings therein. For example, to elch a contact opening in an oxide formed beneath a posi- Uve photoresist, a dark field reticle is employed to develop out an opening or window in the photoresist which allows subsequent etching of the oxide. The photoresist having the window formed therein serves as a mask. The window in the photoresist has a size, a shape, and a location substantially identical to the size, the shape, and the location ofthe contact opening intended to be formed in the oxide. In contrast, a photoresist can be patterned such that a patterned segment of the photoresist (instead of a window in the photoresist), has a size, a shape, and a location substanually identical to the size, the shape, and the location of the feature intended to be formed in the underlying layer of matenal The patterned seg¬ ment of photoresist is formed to protect the underlying layer of material, (e.g., oxide) from subsequent treat¬ ment, (e.g., etching). The patterned segment of photoresist is delineated by a dark area in the reticle mask which otherwise compπse clear areas This dark area in the reticle mask is suπounded by the clear areas in the reticle mask As such, this reticle mask is conventionally refened to as a clear field reticle Photoresist. 1 e , positive photoresist, directly beneath the dark areas of the reticle mask is not ex¬ posed to ultra-violet light and thus not developed out Accordingly, the layer of mateπal lying under the pho- toresist is protected dunng subsequent treatment, enabling a feature to be formed in the underlying mateπal For example, to form a feature in a layer of polysilicon formed beneath a positive photoresist, a clear field reticle is employed The dark areas in the clear field reticle (i e , the reticle mask) prevent the photoresist di¬ rectly beneath from being developed out, thereby forming a patterned segment of photoresist which protects the layer of material lying under the photoresist from subsequent treatment As device geometry becomes smaller, the fabπcation of windows in the photoresist becomes increas¬ ing difficult due to the physical limitations of the small clear areas in the dark field reticle which are used to form the windows For small windows, the width of the clear areas in the dark field reucle is accordingly small Consequently, an insufficient amount of light is passed through these small clear areas in the reticle mask Fidelity between the pattern in the dark field reticle and the resultant pattern formed in the photoresist is thus degraded For example, a dark field reticle intended to create square contact/via openings may instead produce rounded contact/via openings The pnor art solution to the problem of having an insufficient amount of light transmitted through the dark field reucle is to over-expose the photoresist To over-expose the photoresist, either the intensity of the UV source is increased or the photoresist is exposed for a longer penod of Ume. Convenuonally, to over- expose the photoresist, the photoresist is exposed for a longer penod of Ume. Over-exposing the photoresist is an acceptable solution when employing a dark field reucle to pattern contact/via openings wherein each of the contact/via openings is the same size However, this soluuon is in¬ adequate if both small windows and large windows are to be formed on the same photoresist mask; e.g., if a single photoresist mask is intended to pattern both contact/via openings which are small, i e , < 0 5 μm, as well as to form metal lines which can be any length, width, and shape (As used herein, the term "metal lines" refers to conductive lines used for electncally connecting devices on the same level ) The clear areas in the - dark field reucle which delineate the contact/via openings that are small will limit light throughput In con¬ trast, the clear areas in the dark field reticle which delineate the metal lines, which are larger, will not ad¬ versely restπct throughput Consequently, over-exposing the photoresist will cause too much light to be passed through the larger clear areas in the dark field reucle resulting in pattern distortion Fidelity between the pat¬ tern in the dark field reticle and the resultant pattem formed in the photoresist is degraded The windows in the photoresist will be enlarged while the remaining photoresist will be smaller than intended Rounding of the edges or comers ofthe photoresist may also occur The throughput of UV light is less of a problem when small structures are delineated b> a clear field reticle used to produced patterned segments of photoresist having a size, a shape, and a locauon substantialh identical lo the size, the shape, and the location of the features intended to be formed in the underlying layer of matenal With a mask compnsmg such patterned segments of photoresist, the small structures are deline¬ ated b> small dark areas in the clear field reucle as opposed to small clear areas in the dark field reticle To foπn the mask compnsmg such patterned segments of photoresist, the UV light is transmitted through the wide clear regions that suπound the small dark areas in the clear field reticle Accordingly, sufficient light levels are achieved to properly expose the photoresist without relying on over-exposure techniques It can be concluded that window s in a photoresist, i.e., a positive photoresist, are more difficult to fabricate than pat¬ terned segments of photoresist when the size of the windows or patterned segments of photoresist is small (<0.5 μm) Consequenth . the formation of contact/via openings and metal lines is better achieved with a mask comprising patterned segments of photoresist than with a mask comprising windows formed in a photoresist (In pπnciple, a negative photoresist can be used to form contact/via openings and metal lines with a clear field reucle. However, as described above, negative photoresist for use in defining small structures, i.e.. <0.5 μm in size, is not available.) A method for forming small structures such as contact/via openings and metal lines which employs patterned segments of positive photoresist having a size, a shape, and a location substanually idenucal to the size, the shape, and the location of the contact/via openings and metal lines has been disclosed in prior art. For example, U.S. Patent 5,283,208 issued February 1, 1994, discloses one method whereby an insulating layer is formed around a mandrel or sacrificial stud, i.e., a block of material with substanually vertical side- walls. The size and position of the mandrel are delineated by patterned segments of photoresist. The mandrel is removed, thereby leaving a cavity in the insulaung layer. The cavity is filled with material to form the de¬ sired structure. This cavity in the insulating layer may be filled with a metal plug or a metal line. As such, the mandrel defines the size and position of the metal plug or metal line which, as stated above, is delineated by the patterned segments of photoresist. Accordingly, the metal plug or metal line is delineated by a clear field reticle. This method, however, is complicated by the requirement for a layer of sacrificial material to form the mandrel. Thus, there remains a need for an alternative method for forming contact/via openings and metal lines using a clear field reticle which avoids most, if not all, the foregoing problems. DISCLOSURE OF INVENTION In accordance with the invention, a method is provided for using a clear field reticle to form a mask comprising patterned segments of photoresist formed from a first layer of photoresist to pattern openings each having a size, a shape, and a location in a layer of material foπned over a semiconductor substrate. The method comprises the steps of (a) forming the first layer of photoresist on the layer of mateπal foπned over the semicon¬ ductor substrate, (b) exposing the first layer of photoresist by transmitting light through a clear field reucle having clear areas which allow light to be directed onto portions of the first layer of photoresist to become ex- posed regions and having dark areas which prevent light from exposing portions of ihe first layer of photore¬ sist to remain unexposed regions, the unexposed regions having a size, a shape, and a locauon substanually identical to the size, tiie shape, and the location of the openings intended to be formed in the layer of mateπal; (c) developing the first layer of photoresist, thereby removing the exposed regions of the first layer of photoresist, the unexposed regions remaining intact, the unexposed regions forming the patterned segments of photoresist, each patterned segment of photoresist having a size, a shape, and a location substan¬ ually identical to the size the shape and the location of the openings intended to be formed in the laver of material (d) etching the laver of material, thercbv fonning a step therein directK beneath each of the patterned segments of photoresist which serv e as a mask during etching each step hav ing a size, a shape, and a location substantialh identical to the size, the shape, and the location of one of the openings intended to be foπned in the layer of material. (e) removing the first layer of photoresist. (f) depositing a layer of planaπzable mateπal over the layer of material (g) removing a portion of the layer of plananzable mateπal. thereby exposing each step in the layer of matenal, and (h) etching the layer of mateπal, thereby forming the openings therein, the layer of planaπz¬ able matenal serving as a mask dunng etching The layer of planaπzable material can be removed after etching the layer of matenal to foπn each of the openings therein The openings can be filled with a metal to form contacts, vias, or metal lines In the case where the openings are filled with metal, the layer of mateπal is a dielectπc such as oxide, nitπde, or poly¬ imide The present invention provides a method for fabπcating small structures to be employed in integrated circuits formed on a semiconductor substrate (By small structures is meant structures ≤O 5 μm in size, such as contacts, vias, and metal lines ) The method of the present invention improves the definition of features, e g., contacts or vias, pat¬ terned in the layer of mateπal As descnbed above, for small structures, openings or windows in a photoresist are harder to form than patterned segments of photoresist In the method of the present invention, a clear field reucle is used to form patterned segments of photoresist each having a size, a shape, and a locauon substanually idenucal to the size, the shape, and the locauon of one of the features intended to be formed in the layer of matenal This method is employed instead of using a dark field reticle which forms windows in a photoresist each having a size, a shape, and a location substantially identical to the size, the shape, and the locauon of one of the fea¬ tures intended to be formed in the layer of matenal Consequently, the limitations of patterning small windows in a photoresist are avoided The method of the present invention thus improves the accuracy in forming such small structures Other objects, features, and advantages of the present invention will become apparent upon consid¬ eration of the following detailed description and accompanving drawings, in which like reference designations represent like features throughout the Figures BRIEF DESCRIPTION OF THE DRAWINGS The drawings refened to in this description should be understood as not being drawn to scale except if specifically noted. Moreover, the drawings are intended to illustrate only one portion of an integrated circuit fabncated in accordance with the present invention FIGS 1-8 are cross-secuonal views at vaπous stages in the processing of a semiconductor substrate in accordance with the present invention, in which FIG I depicts a conventional semiconductor structure prior to performing tiie first masking and etching, FIG. 2 depicts the structure after exposing and developing the first layer of photoresist. FIG. 3 depicts the step in the layer of oxide formed by etching the oxide, FIG. 4 depicts the structure after removing the first layer of photoresist; FIG. 5 depicts the second layer of photoresist formed on the layer of oxide; FIG. 6 depicts the second layer of photoresist after etching to expose the step; FIG. 7 depicts the opening formed in the layer of oxide by etching the oxide; and FIG. 8 depicts the structure after removing the second layer of photoresist and filling the opening with metal BEST MODES FOR CARRYING OUT THE INVENTION Reference is now made in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventor for practicing the invention. Alternative embodiments are also briefly described as applicable. The present invention provides a method for fabricating small structures (structures ≤0.5 μm in size, such as contacts, vias, and metal lines) to be employed in integrated circuits formed on a semiconductor sub¬ strate. As is conventional for defining small structures, positive photoresist (hereinafter refened to as pho¬ toresist) is used. The method of the present invention employs an image reversal technique to obtain improved feature definiuon. To form features in a layer of mateπal, a clear field reticle is used to form patterned segments of photoresist, each having a size, a shape, and a location substantially identical to the size, the shape, and the location of one of the features intended to be formed m the layer of mateπal. This method is employed instead of using a dark field reucle which forms windows in a photoresist, each having a size, a shape, and a location substanually idenucal to the size, the shape, and the location of one of the features intended to be formed in the layer of matenal. The clear field reticle produces an image or pattern on the surface of the substrate that is the negauve (i.e.. reverse) of the image produced bv a dark field reticle. By image reversal is meant that a clear field reucle is used instead of a dark field reticle to form a pattern with the reverse image With a mask comprising patterned segments of photoresist each having a size, a shape, and a loca¬ tion substanually identical to the size, the shape, and the location of one of the features intended to be formed, small structures such as small contact/via openings are delineated by small opaque or dark areas in a clear field reticle as opposed to small transparent or clear areas in the dark field reucle To form the patterned seg¬ ments of photoresist. UV light is transmitted through the wide transparent regions thai sunound ihe small dark areas in the clear field reticle Accordingly , sufficient light lev els are achieved to properly expose the photoresist without reiving on over-exposure techniques In particular, die method of the present invention emplovs a clear field reticle to form a mask com¬ pπsing patterned segments of photoresist formed from a first layer of photoresist to pattern openings in a layer of matenal formed over a semiconductor substrate. The patterned segments of photoresist protect the underly¬ ing layer of material, (e.g., oxide) from subsequent treatment, (e.g , etching). The patterned segments of pho¬ toresist are delineated by dark areas in the clear field reticle, which prevent portions of the photoresist from being exposed to ultra-violet light and developed out. In accordance with the present invention, each of the openings in the layer of material has a size, a shape, and a location substantially identical to one of the pat¬ terned segments of photoresist. The method ofthe present invention comprises the steps of. (a) forming the first layer of photoresist on the layer of material formed over the semicon- ductor substrate; (b) exposing the first layer of photoresist by transmitung light through a clear field reticle having clear areas which allow light to be directed onto to portions of the first layer of photoresist to become exposed regions and having dark areas which prevent light from exposing portions of the first layer of pho¬ toresist to remain unexposed regions, the unexposed regions having a size, a shape, and a location substan- tially identical to the size, the shape, and the location of the openings intended to be formed in the layer of material, (c) developing the first layer of photoresist, thereby removing the exposed regions ofthe first layer of photoresist, the unexposed regions remaining intact, the unexposed regions forming the patterned segments of photoresist, each patterned segment of photoresist having a size, a shape, and a location substan- tially idenucal to the size, the sliape, and the location of the openings intended to be formed in the layer of material; (d) etching the layer of matenal, thereby forming a step therein directly beneath each of the patterned segments of photoresist which serve as a mask during etching, each step having a size, a shape, and a location substantially identical to the size, the sliape, and the locauon of one of the features intended to be formed in the layer of material; (e) removing the first layer of photoresist; (f) depositing a layer of planaπzable matenal. e.g., a second layer of photoresist, over the layer of material; (g) removing a portion of the layer of planaπzable material, thereby exposing each step in the layer of mateπal: and (h) etching the layer of material, thereby forming the openings therein, the laver of planaπz¬ able matenal serving as a mask during etching For small structures, windows in a photoresist are harder to form than patterned segments of pho¬ toresist Consequently . the foπnation of contactΛia openings and metal lines is better achieved with a mask comprising patterned segments of photoresist each having a size, a shape, and a locauon substanually identi¬ cal to the size, the shape, and the locauon of ihe contact/via openings and metal lines intended to be formed than with a mask compπsing windows in a photoresist each having a size, a shape, and a location substan¬ tially identical to the size, the shape, and the location of the contact/via openings and metal lines intended to be formed With the use of a clear field reucle to fomi a mask comprising patterned segments of photoresist, the limitations of patterning small windows in a photoresist with the use of a dark field reucle are avoided Accordingly, the method of the present invention, which employs a clear field reticle to form a mask compπs¬ ing patterned segments of photoresist, improves the accuracy of forming such small structures In the fabπcauon of semiconductor integrated circuits, doped regions 10 are formed in a major sur- face of a semiconductor substrate 12. as shown in FIG. 1. or m an epitaxial layer (not shown) formed thereon Oxide regions such as field oxide 14 may be formed to isolate one device from another. Once the devices have been defined by the various steps of masking, etching, doping, oxide forma¬ tion, etc., it is necessary to form interconnects, e.g., metal interconnects, to electrically connect the devices with each other and provide a conductive path to external circuitry As is conventional, the interconnect formation is preceded by formation of a layer of dielectric, or pattemable material, 16 which covers the field oxide 14 and all underlying conductive regions in order to pre¬ vent shorts to the interconnects. These underlying conductive regions may include polysilicon, which may be employed, for example, in the formation of gate control regions, and doped regions 10 in the semiconductor substrate, such as source/drain regions. The layer of dielectric 16 is referred to as the first interlayer dielectric. This layer of dielectric 16 electrically isolates the underlying conductive regions. The method ofthe present invention begins with the formation of this layer of dielectric 16. The layer of dielectric 16 may comprise oxide, and consequently, is referred to herein as a layer of oxide. This layer of oxide 16 is formed by conventional processes to the typical thickness employed in this technology. Preferably, the layer of oxide 16 is planarized. Such planarization can be accomplished by etchback or polishing. Polish- ing may include chemical mechanical polishing. In accordance with the present invention, a mask compπsing patterned segments of photoresist is provided which delineates the desired pattem to be transferred to the layer of oxide 16. Specifically, a first layer of photoresist 18 is formed on the layer of oxide 16. The first layer of photoresist 18 is exposed and de¬ veloped to form a pattern therein. In particular, UV light is transmitted through a reticle mask, i.e., a clear field reticle, having clear areas and dark areas which delineate the pattern to be formed in the first layer of photoresist 18. The clear areas are transparent while dark areas are opaque. Accordingly, the portions of the first layer of photoresist 18 below the clear areas are exposed and become exposed regions such as region 20 shown in FIG. 1. The portions of the first layer of photoresist below the dark areas are not exposed and remain unexposed regions such as region 22 shown in FIG 1 In accordance with the present mvenuon. the unex- posed regions 22 have a size, a shape, and a location substantialh identical to the size, the shape, and the lo¬ cation of the features intended to be formed m the laver of oxide 16 The exposed regions 20 of the photoresist layer 18 are developed out The unexposed regions. 1 e . region 22. remain intact The first layer of photoresist 18. once patterned, (hereinafter refened to as the patterned segments of photoresist), serves as a mask As described above, the patterned segments of photoresist 18 protect ihe under- SUBSTrrUTE SHEET (RULE 26) h ing laver of matenal i e the laver of oxide 16 from subsequent treatment which in this case conesponds lo etching of the laver of oxide As described abov e the patterned segments of photoresist 18 are delineated bv dark areas in the clear field reticle To form Uie patterned segments of photoresist 18 the UV light is transmitted through the clear areas that sunound the dark areas in the clear field reucle In the case w here the structures to be patterned in the laver of oxide 16 are small the dark areas in the clear field reticle are small in comparison to the clear area Accordingly, sufficient light levels are achieved to properlv expose Ihe photoresist without reiving on over-exposure techniques FIG 2 depicts one such patterned segment of photoresist 18 which is thereby formed While one such patterned segment of photoresist 18 is shown, it will be readily apparent to those skilled in this art that in fact a plurality of such patterned segments of photoresist are formed The patterned segments of photoresist 18, such as the patterned segment of photoresist shown in FIG 2, serve as a mask for the subsequent patterning of the layer of oxide 16 In general, the patterned segments of photoresist 18 reside above regions in the layer of oxide 16 where the interconnects or other small structures are intended to be formed The size of a patterned segment of photoresist 18 may be less than about 0 5 μm without requinng over-exposure of the photoresist The layer of oxide 16 is etched to form steps therein such as the step 24 shown in FIG 3 While one such step 24 is shown formed in the layer of oxide 16, it will be readily apparent to those skilled in this art that in fact a plurality of such steps are formed The step is formed beneath the patterned segment of photore¬ sist 18 which protects the underlying portion of the layer of oxide 16 from etching Consequently, the pat- temed segments of photoresist 18 serve as a mask dunng etching In accordance with the present invention, the step 24 has a size, a shape, and a location substantially identical to the size, the shape, and the location of the feature intended to be formed tn the layer of oxide 16 The patterned segments of photoresist 18 are then removed FIG 4 shows the step 24 in the layer of oxide 16 after the patterned first layer of photoresist 18 is removed A layer of piananzable matenal 26 is formed over the layer of oxide 16 In one embodiment of the present invention, the layer of piananzable mateπal 26 comprises a second layer of photoresist The thickness of Ihe second layer of photoresist 26, is preferably sufficient to cover the step 24 as depicted in FIG 5 Pref¬ erably, the second layer of photoresist 26 has a surface which is substantially flat or planar as may be achieved with spin-on photoresist Spin-on photoresist, after deposition, dries and flattens out The previous step, i e , removing the patterned segments of photoresist 18 subsequent to etching the layer of oxide 16, aids in providing a plananzed second layer of photoresist 26 Depositing the second layer of photoresist 26 over the step 24 having the patterned segment of photoresist 18 formed thereon prevents the second laver of photoresist from being uniformly smooth over the step The second laver of photoresist 26 is plananzed bv etchback such that step 24 in the laver of oxide 16 is exposed as shown in FIG 6 The exposed oxide is etched therebv forming an opening 28 in the laver of oxide 16 as depicted in FIG 7 While one such opening 28 is shown foπned in the layer of oxide 16 it will be readilv apparent to those skilled in this art that in fact a pluralitv of such openings are foπned The second laver of photoresist 26 serves as an etch mask during the etching of the laver of oxide 16 Accordmgh, the etching process must se- leciively etch the layer of oxide 16 and not the second layer of photoresist 26. In accordance with the present invention, the opening 28 has a size, a shape, and a location delineated bv the patterned segment of photore¬ sist 18 which is delineated by the dark area of ihe clear field reticle At this stage of processing, the second layer of photoresist 26 may be removed and the opening 28 filled with material such as metal to form an interconnect 30 as shown in FIG 8 Examples of metals suitably employed in the practice of the present invention include tungsten and aluminum Titanium and titanium- tungsten may also be employed to form ihe interconnect 30. However, the metalization employed for the inter¬ connect 30 is conventional and forms no part of this invention It will be appreciated that the method of the present invention is not limited to etching an opening 28 in a layer of oxide 16. The method of the present invention will find use in patterning features in materials other than oxide, and consequently, the layer of oxide 16 is herein refened to broadly as a layer of material. Other dielectrics as well as other materials such as metal and semiconductor may be patterned as well. In an alternative embodiment, a second layer of photoresist 26 is not employed to cover the step 24. Rather, a layer of planarizable material 26 other than photoresist, such as described below, is employed. The process flow is identical to the steps described above all the way through etching the step 24 in the layer of oxide 16 and removing the patterned segment of photoresist 18. FIG. 4 shows the step 24 in the layer of oxide 16 at this stage in the processing. In this alternative embodiment, a layer of planarizable mate¬ rial 26 is formed on the layer of oxide 16. As with the second layer of photoresist 26, the thickness of the layer of planarizable material 26 is preferably sufficient to cover the step 24 as depicted in FIG. 5. The layer of planarizable material 26 is planarized by etchback or polishing such that the step 24 in the layer of oxide 16 is exposed as shown in FIG. 6. Preferably, the layer of planarizable material 26 is plananzed by polishing. Spin-on photoresist, for example, which is not polished, will be flat only over local regions upon drying. Additionally, spin-on pho¬ toresist will not flatten if formed over holes and/or features which are too large. In contrast, polishing enables the layer of planarizable material 26 to be unifomiiy smooth over larger holes and/or features as well as over wider regions. Accordingly, polishing enables the layer of planarizable mateπal 26 to be uniformly smooth over the step 24. It will be appreciated that polishing may compnse chemical mechanical polishing. As described above, the exposed oxide is etched thereby forming an opening 28 in the layer of oxide 16 as depicted in FIG 7. The layer of planarizable material 26 serves as a mask dunng the etching of the layer of oxide 16. Consequently, the etching process must selectively etch the layer of oxide 16 and not the layer of planarizable material 26 Accordingly, any material that is both planarizable by either etchback or polishing and that is resistant to the etchant employed to etch the layer of material 16, may be employed as the layer of piananzable mateπal 26. In this case, the layer of oxide 16 is the layer of matenal. Accordingly, the layer of plananzing material 26 must be resistant to the etchant used lo etch the layer of material 16 (layer of oxide) Consequently, the layer of material 16 (layer of oxide) is selectively etched with respect lo the layer of pia¬ nanzable material 26 Alternatively, ihe layer of planarizable mateπal 26 may comprise oxide, which can be polished, and the layer of mateπal 16 may compπse a nitride, for example, BN (boron nitπde) Boron nitπde can be selec¬ tively etched with respect to oxide. Other suimbly employed piananzable mateπals may be used in the method of the present invention An example of another planarizable material suitably employed in the method of the present invention includes polysilicon Polysilicon can be also used as the layer of planaπzable mateπal 26 when the layer of mateπal 16 comprises oxide Polysilicon is also planarized by polishing In addition, polyimides may be may be used as the layer of planarizable material 26 when the layer of mateπal 16 comprises oxide. Polyimides are flat upon deposition and can be etched back to expose the step 24 Converse]) . oxide may also be used as the layer of planarizable material 26 and the layer of mateπal 16 may compπses a polyimide As described above, oxide is planarized with polishing At this stage of processing, the layer of planarizable material 24 may be removed and the opening 28 filled with material such as metal to form an interconnect 30 as shown in FIG 8. As descnbed above, exam- pies of metals suitably employed in the practice of the present invention include tungsten and aluminum. Ti¬ tanium and titanium-tungsten may also be employed to form the interconnect 30. The metalizauon employed for the interconnect 30 is conventional and forms no part of this invention. As described above, it will be appreciated that Ihe method of the present invention is not limited to etching an opening 28 in a layer of dielectric 16, and in particular, to a layer of oxide 16. Other dielectrics, as well as other materials such as metal and semiconductor, may be patterned as well. In any case, however, the layer of planarizable matenal 26 which may be a second layer of photoresist 26, must be resistant to the etchant employed to etch the layer of material 16. The method of the present invention will find immediate applicability in defining small structures such as contact/via openings and metal lines. The method of the present invention may be applied to future 0.25 μm technology. INDUSTRIAL APPLICABILITY The method of the invention for forming small structures ≤0.5 μm in size such as contacts, vias, and metal lines is expected to find use in the fabπcation of all deep sub-micrometer IC technology. The foregoing description of the prefened embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit ihe invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to practiuoners skilled in this art. Many variations of films and materials are possible. It is possible that tiie invention may be practiced in other fabrication technologies in MOS or bipolar processes. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and de¬ scribed in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the an to understand the invention for various embodiments and with vanous modifications as are suited to the particular use contemplated It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents

Claims: CLAIMS What Is Claimed Is 1 A method for using a clear field reticle to form a mask comprising patterned segments of photore¬ sist (18) formed from a first layer of photoresist (18) to pattem openings (28) each having a size, a shape, and a locauon in a layer of matenal (16) formed over a semiconductor substrate (12), said method characteπzed by the steps of: (a) forming said first layer of photoresist (18) on said layer of matenal (16) foπned over said semiconductor substrate (12), (b) exposing said first layer of photoresist (18) by transmitting light through a clear field reticle having clear areas which allow light to be directed onto portions of said first layer of photoresist (18) to become exposed regions (20) and having dark areas which prevent light from exposing portions of said first layer of photoresist (18) to remain unexposed regions (22), said unexposed regions (22) having a size, a shape, and a location substanually identical to said size, said shape, and said location of said opemngs (28) intended to be formed in said layer of material (16), (c) developing said first layer of photoresist (18), thereby removing said exposed regions (20) of said first layer of photoresist (18), said unexposed regions (22) remaining intact thereby forming said patterned segments of photoresist (18), each said patterned segment of photoresist (18) having a size, a shape, and a location substantially identical to said size, said shape, and said locauon of said opemngs (28) intended to be foπned in said layer of material (16); (d) etching said layer of material (16), thereby forming a step (24) therein directly beneath each of said patterned segments of photoresist (18) which serve as a mask during etching, each said step (24) having a size, a shape, and a location substantially idenucal to said size, said shape, and said location of one of said openings (28) intended to be formed in said layer of matenal (16); (e) removing said first layer of photoresist (18), (f) depositing a layer of planaπzable material (26) over said layer of matenal (16); (g) removing a portion of said layer of piananzable material (26), thereby exposing each said step (24) in said layer of material (16); and (h) etching said layer of mateπal (16), thereby forming said openings (28) therein, said layer of piananzable mateπal (26) serving as a mask dunng etching 2 The method of Claim 1 wherein removing a portion of said layer of piananzable mateπal (26) is accomplished by etchback or polishing 3 The method of Claim 2 wherein removing a portion of said layer of planaπzable matenal (26) causes said layer of planaπzable material (26) to become plananzed 4. The method of Claim 1 wherein said layer of planarizable material (26) is removed after etching said layer of material (16) to foπn said openings (28) therein. 5. The method of Claim 1 wherein said layer of material (16) is selected from the group of materials consisting of oxide, boron nitride, and polyimide. 6. The method of Claim 1 wherein said layer of planarizable material (26) is selected from the group of materials consisting of photoresist, polysilicon, polyimide, and oxide. 7. The method of Claim 6 wherein said layer of planarizable material (26) comprises a second layer of photoresist (26). 8. The method of Claim 7 wherein removing a portion of said second layer of photoresist (26) is ac¬ complished by etchback. 9. The method of Claim 1 wherein at least one of said openings (28) is filled with a metal. 10. The method of Claim 1 wherein said layer of planarizable material (26) is deposited on said layer of material (16) in step (0 such that said layer of planarizable material (26) covers each said step (24) in said layer of material (16).

Inventors: STOLMEIJER ANDRE, STOLMEIJER, ANDRE
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1997
Main classification: H01L21
IPC codes: H01L21