# Patent

## Title
Semiconductor processing methods of transferring patterns from patterned photoresists to materials, and structures comprising silicon nitride

## Abstract

The invention includes a semiconductor processing method. A first material comprising silicon and nitrogen is formed. A second material is formed over the first material, and the second material comprises silicon and less nitrogen, by atom percent, than the first material. An imagable material is formed on the second material, and patterned. A pattern is then transferred from the patterned imagable material to the first and second materials. The invention also includes a structure comprising a first layer of silicon nitride over a substrate, and a second layer on the first layer. The second layer comprises silicon and is free of nitrogen. The structure further comprises a third layer consisting essentially of imagable material on the second layer.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. patent application Ser. No. 09/488,947 now U.S. Pat. No. 6,440,860, which was filed on Jan. 18, 2000 and which is incorporated by reference herein.


TECHNICAL FIELD
The invention pertains to methods of transferring patterns from photoresists to materials, and also pertains to structures comprising silicon nitride.
BACKGROUND OF THE INVENTION
A commonly utilized process for patterning structures utilized for integrated circuitry is photolithographic processing. An imagable material (typically photoresist) is provided over a mass which is ultimately to be patterned. Portions of the imagable material are then exposed to radiation, while other portions remain unexposed (in the case of photoresist, the radiation is light). After the exposure, the material is subjected to conditions which selectively remove either the portions of the exposed to radiation, or the portions which were not exposed to radiation. If the imagable material comprises photoresist and the portions exposed to radiation are removed, the photoresist is referred to as a positive photoresist, whereas if the portions which are not exposed to radiation are removed the photoresist is referred to as a negative photoresist. Once the imagable material is patterned, it is utilized as a masking layer for patterning the underlying mass. Specifically, the patterned imagable material covers some portions of the mass, while leaving other portions exposed to an etch which removes the exposed portions. Accordingly, the mass remaining after the etch is in approximately the same pattern as the patterned imagable material formed over the mass.
Photolithographic processing is utilized for patterning numerous materials, including silicon nitride. However, problems can occur during the utilization of photolithographic processing for patterning silicon nitride. Specifically, the pattern formed in silicon nitride is frequently not the same as the pattern which was intended to be formed in the photoresist. Such problem can be particularly severe when utilizing photoresist patterned with deep UV light processing, wherein deep UV light is defined as ultraviolet light having a wavelength of less than or equal to 248 nanometers. It would be desirable to develop methods for avoiding the above-discussed problems.
SUMMARY OF THE INVENTION
In one aspect, the invention includes a semiconductor processing method. A first material comprising silicon and nitrogen is formed. A second material is formed over the first material, and the second material comprises silicon and less nitrogen (by atom percent) than the first material. An imagable material is formed on the second material, and patterned. A pattern is then transferred from the patterned imagable material to the first and second materials.
In another aspect, the invention encompasses a method of forming a patterned structure. A first layer comprising silicon and nitrogen is formed over a substrate. A sacrificial layer is formed on the first layer, and comprises less nitrogen (by atom percent) than the first layer. A layer of imagable material is formed on the sacrificial layer and patterned. The patterned structure has a pair of opposing sidewalls extending upwardly from the substrate. A pair of opposing corners are defined where the sidewalls join the substrate. The opposing corners are closer to one another than they would be if the sacrificial layer was absent and the imagable material was on the first layer during the patterning of the imagable material. The sacrificial layer is removed from the patterned structure.
In yet another aspect, the invention encompasses a structure comprising a first layer of silicon nitride over a substrate, and a second layer on the first layer. The second layer comprises silicon and is free of nitrogen. The structure further comprises a third layer consisting essentially of imagable material on the second layer.



BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
FIG. 1 is a fragmentary, diagrammatic, cross-sectional view of a semiconductor wafer fragment.
FIG. 2 is a view of the FIG. 1 fragment shown at a processing step subsequent to that of FIG. 1.
FIG. 3 is a view of the FIG. 1 fragment shown at a processing step subsequent to that of FIG. 2.
FIG. 4 is a diagrammatic, fragmentary, cross-sectional view of a semiconductor wafer fragment.
FIG. 5 is a view of the FIG. 4 fragment shown at a processing step subsequent to that of FIG. 4.
FIG. 6 is a view of the FIG. 4 fragment shown at a processing step subsequent to that of FIG. 5.
FIG. 7 is a view of the FIG. 4 fragment shown at a processing step subsequent to that of FIG. 6.
FIG. 8 is a view of the FIG. 4 fragment shown at a processing step subsequent to that of FIG. 7.
FIG. 9 is a view of the FIG. 4 fragment shown at a processing step subsequent to that of FIG. 8 in accordance with an embodiment of the present invention.
FIG. 10 is a view of the FIG. 4 fragment shown at a processing step subsequent to that of FIG. 8 in accordance with another embodiment of the present invention.
FIG. 11 is a photograph of a semiconductor wafer fragment having structures formed thereover by a particular patterning method.
FIG. 12 is a view of a semiconductor wafer fragment having structures formed thereover by a processing method different than that utilized for forming the structures of FIG. 11.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).
A method of utilizing photoresist for patterning a silicon nitride material is described with reference to FIGS. 1-3. Referring to FIG. 1, a semiconductor wafer fragment 10 is illustrated at a preliminary step of the method. Fragment 10 comprises a substrate 12 having an upper surface 15. Substrate 12 can comprise, for example, monocrystalline silicon. To aid in interpretation of the claims that follow, the terms “semiconductive substrate” and “semiconductor substrate” are defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Layers 16, 18, 20, 22 and 24 are formed over upper surface 15, and are ultimately to be patterned into a wordline. Accordingly, layer 16 comprises silicon dioxide, layer 18 comprises conductively doped silicon (i.e., silicon doped to a concentration of at least about 1018 atoms/cm3 with a conductivity enhancing dopant), layer 20 comprises a metal (such as, for example, tungsten or titanium), and layer 22 comprises silicon nitride. Layer 22 has an upper surface 23, and layer 24 is formed on (i.e., against) such upper surface. Layer 24 comprises an imagable material, and is described herein to comprise photoresist. It is to be understood, however, that the term “imagable material” can encompasses materials patterned by radiation (or energy) other than light, and can accordingly encompass materials other than photoresist.
Referring to FIG. 2, photoresist 24 is patterned to form blocks 26. Such patterning can comprise, for example, exposing portions of the photoresist to radiation while leaving other portions unexposed, and subsequently selectively removing either the exposed or unexposed portions with a solvent.
Blocks 26 comprise sidewalls 28 which are preferably substantially perpendicular to upper surface 23 of silicon nitride layer 22. However, a problem which occurs during the patterning of photoresist 24 is that photoresist adjacent blocks 26 does not remove as well as photoresist which is further removed from blocks 26. Such results in the formation of foot portions 30 at locations where sidewalls 28 join upper surface 23 of silicon nitride layer 22.
Referring to FIG. 3, blocks 26 are utilized as a mask during an etch of underlying layers 16, 18, 20 and 22 to form wordline stacks 40 from layers 16, 18, 20 and 22. Wordline stacks 40 comprise sidewalls 41 which are substantially perpendicular to upper surface 15 of substrate 12.
As shown, foot portions 30 (FIG. 2) are variabily eroded during formation of wordline stacks 40 so that the stacks have laterally extending portions 42 where the stacks join with substrate 12. Foot portions 30 cause laterally extending portions 42 because the photoresist of foot portions 30 is etched by the conditions which etch layers 16, 18, 20 and 22, and is ultimately removed to allow portions of layers 16, 18, 20 and 22 beneath foot regions 30 to be removed. However, the portions of layers 16, 18, 20 and 22 beneath foot regions 30 are exposed to etching conditions for less time than are portions of layers 16, 18, 20 and 22 that are not beneath foot portions 30. Accordingly, the portions beneath foot portions 30 are etched less than are portions of layers 16, 18, 20 and 22 not beneath foot portions 30, causing formation of laterally extending portions 42. The laterally extending portions 42 extend into a gap between adjacent wordline stacks 40, and thus can affect a critical dimension of a structure (such as a conductive plug or capacitor) subsequently formed between stacks 40.
Sidewalls 41 join upper surface 15 of substrate 12 at a pair of opposing corners 43 relative to one of stacks 40, and a pair of opposing corners 45 relative to another of stacks 40. In many applications it would be desirable if the opposing corners relative to a particular stack were as close together as possible after the patterning of layers 16, 18, 20 and 22. However, laterally extending portions 42 extend a distance between the opposing corners 43, and likewise extend a distance between opposing corners 45.
An aspect of the present invention is a recognition that foot portions 30 of FIG. 2 are due primarily to the formation of imagable material directly on silicon nitride layer 22, and accordingly can be alleviated (or even eliminated) by forming another material between silicon nitride layer 22 and imagable material 24. An embodiment of the present invention is described with reference to a wafer fragment 10a of FIG. 4. In referring to FIG. 4, similar numbering will be used as was used above in describing FIGS. 1-3, with differences indicated by the suffix “a”, or by different numerals.
Wafer fragment 10a of FIG. 4, like wafer fragment 10 of FIG. 1, comprises a substrate 12, a silicon dioxide layer 16, a conductively-doped silicon layer 18, a metal layer 20, and a silicon nitride layer 22. However, fragment 10a of FIG. 4 differs from fragment 10 of FIG. 1 in that a imagable-material-supporting mass (or layer) 50 is provided over silicon nitride layer 22. Layer 50 comprises a different material than silicon nitride layer 22. In particular embodiments, layer 50 comprises less nitrogen (by atom percent) than silicon nitride layer 22. For instance, layer 50 can consist essentially of either silicon or conductively doped silicon, and can accordingly be substantially free of nitrogen (with the term “substantially free” understood to mean that layer 50 comprises less than about 10% of the atom percentage of nitrogen of layer 22, and can comprise no nitrogen). Alternatively, layer 50 can consist entirely of silicon or conductively doped silicon, and accordingly be entirely free of nitrogen.
If layer 50 is to comprise, consist of, or consist essentially of either silicon or conductively doped silicon, such layer can be formed by chemical vapor deposition of silicon or polysilicon over layer 22. For instance, the silicon can be deposited utilizing silane, dichlorosilane, or gases of the general formula SixH(2x+2). Preferably, if layer 50 comprises a conductive material, such layer is formed to be less than 150 Angstroms thick, and more preferably less than 100 Angstroms thick, to enable the layer to be easily removed in subsequent processing. Procedures which can be utilized to form such thin silicon layers are atomic layer deposition (ALD), or low pressure chemical vapor deposition (LPCVD) utilizing a pressure of less than 100 mTorr, at a temperature of less than 550° C. Alternative procedures which could be used for forming thin silicon layers include chemical vapor deposition utilizing a pressure of less than or equal to about 1 Torr, and a temperature of less than or equal to about 650° C.
In an alternative embodiment of the invention, layer 50 can comprise oxygen, and can, for example, comprise, consist of, or consist essentially of silicon dioxide. If layer 50 is to consist of, or consist essentially of silicon dioxide, such layer can be formed by depositing silicon dioxide over layer 22. Alternatively, if layer 50 is to comprise silicon dioxide, such layer can be formed by subjecting an upper surface of layer 22 to oxidizing conditions. The oxidation of silicon nitride layer 22 can comprise, for example, exposing such layer to an oxygen-containing gas, such as, for example, O2, O3, N2O, NO, etc.
If layer 50 is formed by oxidizing an upper portion of silicon nitride layer 22, the resulting structure can be considered to comprise a silicon nitride material which includes both layer 50 and layer 22, with layer 50 being considered an oxidized portion of the silicon nitride material and layer 22 being considered a non-oxidized portion of the material. Further, the oxidized portion defined by layer 50 can be considered to be an oxide cap over the non-oxidized portion.
One method of improving the oxidation of an outer portion of a silicon nitride layer relative to an inner portion is to form the outer portion to have a higher relative concentration of silicon to nitrogen than does the inner portion. A silicon nitride material having a different relative concentration of silicon to nitrogen at one portion than at another portion can be formed by a chemical vapor deposition (CVD) process utilizing a silicon precursor gas (for example, SiH2Cl2 (dichlorosilane)) and a nitrogen precursor gas (for example, NH3 (ammonia)). In an exemplary process, a substrate is provided within a CVD reaction chamber together with a first ratio of a silicon precursor gas to a nitrogen precursor gas. One portion of silicon nitride layer 22 is then deposited. Subsequently, the ratio of the silicon precursor gas to the nitrogen precursor gas is increased and the other portion of the silicon nitride layer is deposited. Exemplary processing conditions for the CVD process include a pressure of from about 100 mTorr to about 1 Torr, and a temperature of from about 700° C. to about 800° C.
In yet another embodiment, layer 50 can comprise silicon, oxygen, and nitrogen, but comprises less nitrogen (by atom percent) than does layer 22. Layer 50 can be formed by, for example, depositing SixOyNz utilizing dichlorosilane and N2O, wherein x is greater than 0 and less than 1, y is greater than 0 and less than 1, and z is greater than 0 and less than 1. Alternatively, layer 50 can be formed from bis-(tertiary butyl amino)-silane (BTBAS).
Referring to FIG. 5, an imagable material layer 24 is formed over imagable-material-supporting layer 50. Imagable material layer 24 is referred to below as comprising photoresist, but it is to be understood that layer 24 can comprise other imagable materials besides photoresist.
Referring to FIG. 6, photoresist 24 is patterned by exposing some portions of resist 24 to radiation while leaving other portions unexposed, and then utilizing a solvent to selectively remove either the exposed or unexposed portions of the photoresist. The patterning forms photoresist 24 into blocks 26a. Blocks 26a comprise sidewalls 28a. Blocks 26a differ from blocks 26 of FIG. 4 in that foot portions 30 (FIG. 4) are missing from blocks 26a. Accordingly, sidewalls 28a of blocks 26a extend substantially perpendicularly from an upper surface of material 50.
Referring to FIG. 7, a pattern is transferred from blocks 26a to underlying materials 16, 18, 20, 22 and 50 to form patterned structures 60 comprising the materials of layers 16, 18, 20, 22 and 50. Patterned structures 60 comprise sidewalls 61 which are coextensive with sidewalls 28a of blocks 26a, and which extend perpendicularly relative to an upper surface of substrate 12. A difference between sidewalls 61 of FIG. 7 and sidewalls 41 of FIG. 3 is that sidewalls 61 lack laterally extending portions (such as the laterally extending portions 42 shown in FIG. 3). Sidewalls 61 join substrate 12 to form opposing corners 63 relative to one of the stacks 60, and opposing corners 65 relative to another of the stacks 60. Opposing corners 63 are closer to one another than opposing corners 43 (FIG. 3), due to the lack of lateral extending portions 42 (FIG. 3) in the FIG. 7 structure. Likewise, opposing corners 65 are closer to one another than opposing corners 45 (FIG. 3). The structure shown in FIG. 7 can be considered to comprise a first layer 22 of silicon nitride over a substrate 20. Such structure can further comprise a second layer 50 which comprises silicon and is free of nitrogen on first layer 22. Additionally, the structure can comprise a third layer 24 consisting essentially of imagable material on second layer 50. Third layer 24 can be, for example, photoresist, and second layer 50 can consist essentially of silicon, conductively doped silicon, or silicon dioxide.
Referring to FIG. 8, photoresist blocks 26a (FIG. 7) are removed and a material 66 is formed over patterned stacks 60, as well as over substrate 12. Material 66 can comprise, for example, an inorganic and electrically insulative material, such as, for example, silicon dioxide or silicon nitride. Material 66 can be formed by, for example, chemical vapor deposition.
The structure of FIG. 8 can be considered to comprise a layer of silicon nitride 22 over a substrate (with the substrate understood to comprise material 12 and layers 16, 18, and 20). The structure further comprises layer 50 over silicon nitride layer 22, and a layer 66 formed on (i.e., against) layer 50. Layer 66 can consist essentially of inorganic material, such as, for example, silicon nitride, silicon dioxide, or SixOyNz (wherein x, y and z are greater than 0), and can comprise a different chemical composition than layer 50. In the structure of FIG. 8, layers 22 and 50 are part of a stack 60 comprising a pair of substantially planar opposing sidewalls 61. Further in the structure of FIG. 8, layer 66 is over the stack 60 comprising layers 50 and 22, as well as along sidewalls 61 of the stack.
FIGS. 9 and 10 illustrate alternative processing which can occur relative to the FIG. 8 structure. Referring first to FIG. 9, material 66 is subjected to anisotropic etching conditions which forms material 66 into spacers 70 extending along sidewalls 61 of stack 60. Such anisotropic etching is conducted for a sufficient period of time to entirely remove material 50 (FIG. 8) from over silicon nitride material 22. The processing of FIG. 9 can be preferred in embodiments in which material 50 comprises a conductive material, such as, for example, conductively doped silicon. If material 50 were not removed in such embodiments, it could short conductive components across an upper surface of stacks 60. The processing of FIG. 9 can be easier to utilize if material 50 is kept thin (i.e., less than 150 Angstroms thick, and more preferably less than 100 Angstroms thick), as the material can then be removed with less etching than could a thicker material. It is noted that substrate 12 may be etched during the removal of material 50. Such etching into substrate 12 is shown in FIG. 8 as trenches 72 formed within regions of substrate 12 that are not covered by spacers 70 or stacks 60.
Material 50 can be considered a sacrificial material relative to the method of FIGS. 4-9. Specifically, the material is provided in the processing of FIGS. 4-6 to improve patterning of a photoresist material, and subsequently removed in the processing of FIG. 9.
The processing of FIG. 10 is similar to that of FIG. 9 in that material 66 of FIG. 8 is etched to form spacers 70. However, the processing of FIG. 10 differs from that of FIG. 9 in that material 50 remains after the etch of material 66. The processing of FIG. 10 can be preferred in embodiments in which material 50 consists of an electrically insulative material, such as, for example, silicon dioxide, or undoped silicon. If the processing of FIG. 10 is utilized, and if material 50 comprises an insulative material, there can be less preference to keeping the material to a thickness of less than 150 Angstroms relative to the advantages of keeping material 50 to a thickness below 150 Angstroms if the material is electrically conductive and to be removed by the processing of FIG. 9.
An improvement which can be obtained utilizing photoresist-supporting mask 50 between a layer of photoresist and a layer of silicon nitride during patterning of the photoresist is evidenced by the photographs of FIGS. 11 and 12. Specifically, FIG. 11 shows a structure wherein photoresist is patterned while on silicon nitride, and FIG. 12 shows a structure wherein photoresist is patterned while on a layer of amorphous silicon that is conductively doped to concentration of about 1020 atoms/cm3 with phosphorus. The structure of FIG. 11 shows photoresist blocks which join an underlying substrate at corners which are less than 90° (and which specifically comprise foot portions at the locations where the sidewalls join the underlying substrate), whereas the structure of FIG. 12 shows photoresist blocks which join an underlying substrate at corners which are about 90°.
EXAMPLES
Example 1
A silicon nitride layer is formed by chemical vapor deposition with dichlorosilane and ammonia at a temperature of from about 600° C. to about 800° C. Subsequently, a layer of silicon is formed on the silicon nitride by chemical vapor deposition utilizing silane at a temperature of from about 500° C. to about 700° C. The silicon can then be utilized to support a layer of photoresist formed over the silicon nitride.
Example 2
A silicon nitride layer is formed by chemical vapor deposition with dichlorosilane and ammonia at a temperature of from about 600° C. to about 800° C. Subsequently, a layer of silicon is formed on the silicon nitride by chemical vapor deposition utilizing silane at a temperature of from about 500° C. to about 700° C. Finally, the silicon is oxidized by exposure to one or more of N2O, NO, O2, O3, at a temperature of from 500° C. to about 800° C. Such forms a layer of silicon dioxide on the silicon nitride. The silicon dioxide can then be utilized to support a layer of photoresist formed over the silicon nitride.
In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.

## Claims

1. A method of forming a patterned structure, comprising:
forming a first layer comprising silicone and nitrogen over a substrate; 
depositing a silicon dioxide layer on an upper surface the first layer; 
forming a layer of imagable material on the silicon dioxide layer; 
patterning the imagable material; 
transferring a pattern from the patterned imagable material to the underlying silicon dioxide layer and first layer; 
removing the imagable material from on the silicon dioxide layer; 
after removal of the imagable material, forming a layer of third material, a portion and less than an entirety of the third material being over the silicon dioxide layer; and 
removing the portion of the third material from over the silicon dioxide layer, removing the silicon dioxide layer to expose the upper surface of the first layer and retaining the first layer.

2. The method of claim 1 wherein the imagable material is photoresist.

3. A method of forming a patterned structure, comprising:
forming a first layer comprising silicon and nitrogen over a substrate, the first layer having an exposed outer portion; 
oxidizing the exposed outer portion of the first layer to form an oxide cap over an underlying non-oxidized portion of the first layer; 
forming a layer of imagable material on the oxide cap; 
patterning the imagable material; 
transferring a pattern from the patterned imagable material to the underlying oxide cap and a non-oxidized portion of the first layer; 
removing the imagable material; and 
after removing the imagable material, forming a insulative material against the oxide cap and subsequently removing the entire oxide cap.

4. The method of claim 3 wherein the imagable material is photoresist.

5. The method of claim 3 wherein the exposed outer portion of the first layer comprises a higher relative concentration of silicon to nitrogen then a portion of the first layer under the exposed outer portion.