Semiconductor wafer assemblies comprising photoresist over silicon nitride materials

[0002] This patent resulted from a divisional application of U.S. patent application Ser. No. 09/057,155, which was filed on Apr. 7, 1998.

[0003] The invention pertains to methods of forming and patterning photoresist over silicon nitride materials, and to semiconductor wafer assemblies comprising photoresist over silicon nitride materials.

[0004] Silicon nitride is frequently utilized in modern semiconductor fabrication methods. For instance, silicon nitride is an insulative material, and can be utilized to electrically isolate conductive components from one another. Also, silicon nitride is selectively etchable relative to other materials utilized in semiconductor fabrication processes, such as, for example, silicon dioxide, and is can thus be utilized as an etch stop material. Another example use of silicon nitride is for LOCOS (LOCal Oxidation of Silicon). LOCOS comprises growing oxide over field regions of a semiconductor substrate, while not growing the oxide over other regions of the substrate. The other regions of the substrate are typically protected by a thin layer of silicon nitride during the oxide growth.

[0005] In many applications of silicon nitride, a silicon nitride layer is patterned into a specific shape. An example prior art patterning process is described with reference to FIGS. 1-2. Referring to FIG. 1, a semiconductor wafer fragment 10 comprises a substrate 12 covered by a pad oxide layer 14, a silicon nitride layer 16, an antireflective coating 18, and a photoresist layer 20.

[0006] Substrate 12 can comprise, for example, monocrystalline silicon lightly doped with a p-type dopant. To aid in interpretation of the claims that follow, the term “semiconductive substrate” is 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), so 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.

[0007] Pad oxide 14 is a thin layer (from about 40 to about 50 nanometers thick) of silicon dioxide, and is provided to alleviate stresses that can be caused by silicon nitride layer 16. Pad oxide 14 can be formed by exposing a silicon-comprising substrate 12 to an oxidizing atmosphere.

[0008] Silicon nitride layer 16 can be formed over pad oxide 14 by, for example, chemical vapor deposition. A thickness of silicon nitride layer 16 will vary depending on the application of the silicon nitride layer. In LOCOS fabrication processes, silicon nitride layer 16 will typically be provided to a thickness of from about 100 nanometers to about 200 nanometers.

[0009] Antireflective coating 18 is a polymer film provided over silicon nitride layer 16 for two purposes. First, antireflective coating 18 absorbs light during photolithographic patterning of photoresist layer 20. Such absorption can prevent light that has passed through photoresist layer 20 from reflecting back into the layer to constructively or destructively interfere with other light passing through layer 20. Second, antireflective coating 18 functions as a barrier to prevent diffusion of nitrogen atoms from silicon nitride layer 16 into photoresist layer 20. It is found that if nitrogen atoms diffuse into photoresist 20, they can alter its sensitivity to light (so-called “poisoning” of the photoresist).

[0010] Photoresist layer 20 is provided to form a pattern over silicon nitride layer 16. Photoresist layer 20 comprises a polymer composition which becomes selectively soluble in a solvent upon exposure to light. If photoresist 20 comprises a negative photoresist, it is rendered insoluble in a solvent upon exposure to light, and if it comprises a positive photoresist, it is rendered soluble in solvent upon exposure to light.

[0011] Referring to FIG. 2, photoresist layer 20 is exposed to a patterned beam of light to selectively render portions of the photoresist soluble in a solvent, while leaving other portions insoluble. After such exposure, the solvent is utilized to selectively remove portions of photoresist layer 20 and thereby convert photoresist layer 20 into the pattern shown.

[0012] Referring to FIG. 3, the pattern from layer 20 is transferred to underlying layers 18, 16 and 14 by an appropriate etch. A suitable etch can comprise, for example, a plasma-enhanced etch utilizing NF₃and HBr. In subsequent processing which is not shown, antireflective coating layer 18 and photoresist layer 20 can be removed to leave stacks comprising pad oxide 14 and silicon nitride 16 over substrate 12. The stacks can then be utilized for subsequent fabrication processes. For instance, the stacks can be utilized for LOCOS by subsequently exposing wafer fragment 10 to oxidizing conditions to grow field oxide between the stacks. As another example, conductive metal layers may be provided between the stacks, and the stacks utilized for electrical isolation of such metal layers.

[0013] The above-described processing sequence requires formation of four distinct layers (14, 16, 18, and 20), each of which is formed by processing conditions significantly different than those utilized for formation of the other three layers. For instance, antireflective coating 18 is commonly formed by a spin-on process, followed by a bake to remove solvent from the layer. In contrast, silicon nitride layer 16 is typically formed by a chemical vapor deposition process. The spin-on and baking of layer 18 will typically not occur in a common chamber as the chemical vapor deposition of layer 16, as processing chambers are generally not suited for such diverse tasks. Accordingly, after formation of silicon nitride layer 16, semiconductor wafer fragment 10 is transferred to a separate processing chamber for formation of antireflective coating 18. The semiconductive wafer fragment 10 may then be transferred to yet another chamber for formation of photoresist layer 20.

[0014] A continuing goal in semiconductive wafer fabrication processes is to minimize processing steps, and particularly to minimize transfers of semiconductive wafers between separate processing chambers. Accordingly, it would be desirable to develop alternative fabrication processes wherein fabrication steps could be eliminated.

[0015] It has been attempted to pattern silicon nitride layers without utilizing an antireflective coating over the layers. However, such creates complications, such as those illustrated in FIG. 4. Identical numbering is utilized in FIG. 4 as was utilized with reference to FIGS. 1-3, with differences indicated by the suffix “a”. A difference between the semiconductive wafer fragment 10aof FIG. 4 and the wafer fragment 10 of FIGS. 1-3 is that antireflective coating 18 is eliminated from the 16 wafer fragment 10aconstruction. Wafer fragment 10aof FIG. 4 is shown at a processing step analogous to the processing step shown in FIG. 2. Elimination of antireflective coating layer 18 has enabled nitrogen atoms to diffuse from silicon nitride layer 16 into a lower portion of photoresist layer 20a. The nitrogen atoms have altered the photoresist such that regions which should be removed by exposure to a solvent are no longer removable by the solvent. This can render semiconductive wafer fragment 10aunsuitable for the further processing described above with reference to FIG. 3. It would be desirable to develop alternative methods of forming photoresist over silicon nitride which avoid the adverse effects illustrated in FIG. 4. Traditional silicon nitride layers have stoichiometries of about Si₃N₄. Silicon enriched silicon nitride layers (i.e., silicon nitride layers having a greater concentration of silicon than Si₃N₄, such as, for example, Si₄N₄) have occasionally been used in semiconductor fabrication processes. The silicon enriched silicon nitride was utilized as a layer having a substantially homogenous composition throughout its thickness, although occasionally a small portion of the layer (1% or less of a thickness of the layer) was less enriched with silicon than the remainder of the layer due to inherent deposition problems.

[0016] When the silicon enriched silicon nitride layers were utilized in a process such as that shown in FIG. 4 (i.e., a process wherein no antireflective coating layer is provided between the silicon nitride and the photoresist), it was found that good photolithographic patterning of the photoresist could be obtained. However, it was also found that the silicon nitride was difficult to pattern due to a resistance of the silicon nitride to etching. Accordingly, it would be desired to develop alternative methods of utilizing silicon nitride in wafer fabrication processes.

[0017] In one aspect, the invention encompasses a semiconductor processing method of forming a photoresist over a silicon nitride material. The silicon nitride material has a surface. A barrier layer comprising silicon and nitrogen is formed over the surface. The photoresist is formed over and against the barrier layer.

[0018] In another aspect, the invention encompasses a semiconductor processing method of forming and patterning a photoresist layer over a silicon nitride material. The silicon nitride material has a surface. A barrier layer comprising silicon and nitrogen is formed over the surface. The photoresist is formed over and against the barrier layer. The photoresist is exposed to a patterned beam of light to render at least one portion of the photoresist more soluble in a solvent than an other portion. The barrier layer is an antireflective surface that absorbs light passing through the photoresist. The photoresist is exposed to the solvent to remove the at least one portion while leaving the other portion over the barrier layer.

[0019] In yet another aspect, the invention encompasses a semiconductor wafer assembly. The assembly includes a silicon nitride material, a barrier layer over a surface of the material, and a photoresist over and against the barrier layer. The barrier layer comprises silicon and nitrogen.

[0020] Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

[0021] FIG. 1 is a diagrammatic, fragmentary, cross-sectional view of a semiconductor wafer fragment at a preliminary step of a prior art processing sequence.

[0022] FIG. 2 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG. 1.

[0023] FIG. 3 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG. 2.

[0024] FIG. 4 is a diagrammatic, fragmentary, cross-sectional view of a prior art wafer fragment processed according to a second prior art processing sequence.

[0025] FIG. 5 is a diagrammatic, fragmentary, cross-sectional view of a 1S semiconductor wafer fragment at a preliminary processing step of a method of the present invention.

[0026] FIG. 6 is a view of the FIG. 5 wafer fragment shown at a is processing step subsequent to that of FIG. 5.

[0027] FIG. 7 is a view of the FIG. 5 wafer fragment shown at a processing step subsequent to that of FIG. 6.

[0028] FIG. 8 is a view of the FIG. 5 wafer fragment shown at a processing step subsequent to that of FIG. 7.

[0029] 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).

[0030] A semiconductor processing method of the present invention is described with reference to FIGS. 5-8. Referring first to FIG. 5, a semiconductor wafer fragment 30 comprises a substrate 32, a pad oxide layer 34 and a silicon nitride layer 36. Substrate 32 and pad oxide layer 34 can comprise identical compositions to those discussed above in the “background” section regarding prior art substrate 12 and prior art pad oxide 14. Silicon nitride layer 36 differs from the prior art silicon nitride layer 16 (discussed in the “background” section) in that silicon nitride layer 36 comprises two distinct portions 38 and 40, having different compositions. A dashed line 39 is utilized to indicate an interface between portions 38 and 40. Portion 40 is a nitrogen barrier layer formed over portion 38, and is preferably thinner than portion 38 so that the majority of silicon nitride layer 36 has the properties of portion 38. Portion 40 can comprise, for example, a thickness of less than or equal to about 5 nanometers, and portion 38 can comprise, for example, a thickness of greater than 95 nanometers.

[0031] Each of portions 38 and 40 preferably comprises silicon and nitrogen, but portion 40 preferably comprises a lower percentage of nitrogen than portion 38. Portion 38 can comprise, for example, a traditional silicon nitride composition, such as a composition having a stoichiometry of about Si₃N₄. Such traditional silicon nitride material can be formed by, for example, chemical vapor deposition. Portion 40 can comprise, for example, a silicon nitride material enriched in silicon relative to the material of portion 38. For example, portion 40 can comprise a stoichiometry of Si_xN_y, where in x is greater than or equal to y. Example stoichiometries of portion 40 are Si₄N₄, Si₇N₄and Si₁₀N₁.

[0032] Portion 40 is preferably formed from a silicon precursor gas and a nitrogen precursor gas in a common and uninterrupted deposition process with portion 38. By “common deposition process” it is meant a deposition process wherein a wafer is not removed from a reaction chamber between the time that an initial portion of a silicon nitride layer is formed and the time that a final portion of the silicon nitride layer is formed. By “uninterrupted deposition process” it is meant a process wherein the flow of at least one of the silicon precursor gas and the nitrogen precursor gas does not stop during the deposition process.

[0033] An example deposition process for forming silicon nitride layer 36 is a chemical vapor deposition (CVD) process utilizing SiH₂Cl₂(dichlorosilane) as a silicon precursor gas, and NH₃(ammonia) as a nitrogen precursor gas. Substrate 32 is provided within a CVD reaction chamber, together with the dichlorosilane and ammonia. A pressure within the chamber is, for example, from about 100 mTorr to about 1 Torr, and a temperature within the chamber is, for example, from about 700° C. to about 800° C.

[0034] The dichlorosilane and ammonia are provided in the chamber to a first ratio, and such first ratio is utilized to deposit portion 38. The first ratio can be, for example, 0.33 to form a portion 38 have a stoichiometry of about Si₃N₄. After portion 38 is formed, the ratio of dichlorosilane to ammonia is altered to a second ratio having an increased relative amount of dichlorosilane. Such second ratio of dichlorosilane to ammonia is utilized to form second portion 40. An example second ratio is about 6, which forms a silicon nitride portion 40 having a stoichiometry of Si_xN_y, wherein the ratio of x to y is greater than 1.

[0035] An alternative method of forming a nitrogen barrier portion 40 of silicon nitride layer 36 is to form the portion 40 from silicon, oxygen and nitrogen. For instance, portion 40 can comprise silicon oxynitride having a stoichiometry of Si_xN_yO_z, wherein x, y and z are greater than or equal to 1 and less than or equal to 5. An example composition of the silicon oxynitride is Si₃N₄O₂.

[0036] The silicon oxynitride can be formed by exposing silicon nitride portion 38 to an atmosphere comprising oxygen. The oxygen can be in the form of, for example, one or more of ozone, NO or N₂O. Methods for utilizing an atmosphere comprising oxygen to form silicon oxynitride over silicon nitride portion 30 include, for example, plasma-enhanced chemical vapor deposition, rapid thermal processing, high pressure oxidation and low pressure oxidation. For the purposes of interpreting this disclosure and the claims that follow, high pressure oxidation is defined as oxidation occurring at pressures of 1 atmosphere and above, and low pressure oxidation is defined as oxidation occurring at pressures of less than 1 atmosphere. Example temperatures for forming silicon oxynitride by high pressure oxidation are from about 600° C. to about 900° C., and example temperatures forming silicon oxynitride by low pressure oxidation are from about 700° C. to about 1000° C. Example conditions for forming silicon oxynitride by rapid thermal processing comprise a temperature of from about 700° C. to about 1000° C. and a ramp rate of from about 20° C./second to about 100° C./second.

[0037] If the silicon oxynitride is formed by chemical vapor deposition of silicon, oxygen, and nitrogen, it can be formed in a common and uninterrupted chemical vapor deposition process with portion 38. For example, portion 38 can be formed in a CVD reaction chamber from a first ratio of a silicon precursor gas and a nitrogen precursor gas. Subsequently, an oxygen precursor gas can be introduced into the reaction chamber. The oxygen precursor gas, silicon precursor gas and nitrogen precursor gas can, in combination, form a silicon oxynitride portion 40 over silicon nitride portion 38.

[0038] Referring to FIG. 6, a photoresist 42 is formed over silicon nitride layer 36. In the shown preferred embodiment, there is no antireflective coating formed between silicon nitride layer 36 and photoresist 42. Instead, photoresist 42 is formed directly against upper portion 40 of silicon nitride layer 36.

[0039] Referring to FIG. 7, photoresist 42 is exposed to a patterned beam of light to render portions of photoresist 42 more soluble in a solvent than other portions. The more soluble portions are then removed with the solvent to pattern photoresist 42.

[0040] During the exposure of photoresist 42 to the beam of light, portion 40 of silicon nitride layer 36 can be an antireflective surface. It is found that a refractive index of a silicon nitride material increases as the stoichiometric amount of silicon within the material is increased. Silicon nitride materials having stoichiometries of Si_xN_y, wherein x is greater than or equal to y, have refractive indices of greater than or equal to about 2.2. Such silicon nitride materials can effectively function as antireflective coatings. In contrast, traditional silicon nitride materials (i.e., silicon nitride materials having stoichiometries of Si₃N₄) have refractive indices of less than 2.0, and do not function as effective antireflective surfaces.

[0041] Referring to FIG. 8, the pattern of photoresist 42 is transferred to silicon nitride layer 36 and pad oxide 34 to form stacks 44 comprising pad oxide 34, silicon nitride 36 and patterned photoresist 42. The transferring of a pattern from photoresist 42 to layers 34 and 36 can comprise, for example, an etch utilizing NF₃and HBr. The susceptibility of a silicon nitride layer to etching generally decreases as the silicon nitride layer becomes more enriched with silicon. Thus, the removability of a silicon nitride layer can be adjusted by adjusting the relative proportions of silicon enriched portions to non-silicon enriched portions. Specifically, silicon nitride layers having a higher proportion of less silicon enriched portions to heavily silicon enriched portions can be easier to remove in subsequent wafer processing. Example etch conditions which proceed slower with respect to an silicon enriched silicon nitride relative to a silicon nitride having a lower stoichiometric amount of silicon are etching in hot phosphoric acid, or etching in dilute hydrofluoric acid (wherein “dilute” refers to a hydrofluoric acid solution comprising less than 50% hydrofluoric acid (by weight)).

[0042] In subsequent processing which is not shown, photoresist 42 can be removed from over stacks 44. Additional processing can be utilized to form field oxide between stacks 44, or to form conductive materials electrically isolated by the insulative stacks 44.

[0043] 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.