Improvements in process for planarizing an integrated circuit structure using low melting inorganic material

0001This invention relates to a method for planarizing an integrated circuit structure. More particularly, this invention relates to a method for planarizing an integrated circuit structure using a low melting glass which is deposited over a layer of insulating material and then etched back. In a preferred embodiment, the etching step is carried out after the deposition step without an intervening exposure of the structure to ambient atmosphere, thus permitting the use of hygroscopic low melting glasses as a planarizing material.

0002As prior art to the present invention, reference can be made to a number of documents.

0003EP 0 386 337 which is an application Falling within the terms of Art 54(3) EPC describes the planarization of an integrated circuit structure using a doped and flowable layer of a low melting planarization material applied over a conformal layer of insulating material previously deposited by an ECR method of deposition. EP 0 249 173 is concerned with the planarization of an integrated circuit structure using a spin-on-glass which requires: <ul id="ul0001" list-style="none" compact="compact"><li>(a) removal cf the structure from a vacuum chamber to permit application of the coating material;</li><li>(b) evaporation of the solvent used to apply the locating; and</li><li>(c) curing in an oven to convert the organic coating material into an incrganic material.</li></ul>

0004In addition, Research Disclosure No. 308 dated December 1989, Abstract 30826, teaches the use of a low melting boron oxide glass as a planarizing material which must be completely removed after the planarizing step because of the hygroscopic nature of the boron oxide material.

0005In the formation of integrated circuit structures, patterning of layers to permit formation on a substrate of active devices such as transistors, passive devices such as resistors, and metal lines to interconnect devices, can result in the formation of uneven surfaces.

0006When a layer of insulating material such as silicon oxide is applied over such uneven surfaces, to permit the formation of further patterned layers thereover, the silicon oxide tends to conform to the underlying topography resulting in the creation of a nonplanar or stepped surface. It is very difficult to pattern further layers over such an uneven surface using standard lithography techniques.

0007It has, therefore, become the customary practice to apply planarizing layers of either photoresist or organic-based glass materials, such as "SOG" (Spin On Glass) which will etch at about the same rate as the underlying silicon oxide insulating layer. The structure is then anisotropically etched to remove the planarizing layer, as well as raised portions of the underlying silicon oxide layer.

0008However, both photoresist and SOG have what is called as a loading effect. This means that the etch rate of these materials depends upon how much of the insulating layer, e.g., the silicon oxide layer, is exposed. Thus, achieving an equal etch rate of both insulating material (silicon oxide) and the sacrificial or planarizing material is very difficult and the etch rate is, therefore, dependent upon the geometry of the structure. Furthermore, when the spaces between raised portions are less than about 1.5 micrometers, the spinning process of applying either of these two planarizing materials is not effective.

0009The above described planarizing materials are limited with respect to the total amount or thickness of these materials which can be deposited. Furthermore, since these planarizing materials are dispersed in organic binders and solvents, prior to application of such planarizing materials, the integrated circuit structure must be removed from a vacuum chamber in which the insulating layer such as silicon oxide is deposited, e.g., by CVD methods, in order to coat the structure with the planarizing layer. After such coating, the solvent in the planarizing coating must be allowed to evaporate and the planarizing coating must then be baked to remove further solvents and to harden the coating prior to the etching step, which is conventionally a dry etching process which is also usually carried out in a vacuum chamber.

0010Thus, the present planarizing processes not only yield unsatisfactory results, but also result in the need for a number of additional and time consuming intermediate steps outside of the vacuum apparatus which is normally used for the preceding CVD deposition of the underlying insulating layer as well as for the subsequent dry etching step which normally follows the formation of such a planarizing layer. Such additional steps not only add expense to the process, but also risk the possible introduction of undesirable contaminants to the surface of the integrated circuit structure by exposure of the integrated circuit structure to the atmosphere.

0011It would, therefore, be highly desirable to be able to planarize an integrated circuit structure without the use of such organic-based planarizing materials which require removal of the integrated circuit structure from the vacuum system for application, drying, and baking of a planarizing layer.

0012In our parent application, we described and claimed a process for planarizing an integrated circuit structure using a low melting inorganic planarizing material, such as a low melting glass, which included the steps of depositing a layer of an insulating material over an integrated circuit structure, then depositing a layer of a low melting inorganic planarizing material over the coated integrated circuit structure, and then dry etching the inorganic planarizing layer to remove it as well as the high portions of the underlying insulating layer. In that application we indicated that the steps could be carried out in the same vacuum apparatus to avoid exposure of the semiconductor wafer to ambient conditions between steps. We also taught that the etching step was carried out to remove substantially all of the low melting glass layer, due to the hygroscopic nature of the planarizing material.

Summary of the Invention

0013It is, therefore, an object of this invention to provide a process for planarizing an integrated circuit structure using a low melting inorganic planarizing material. The object of the invention is solved by a method including the features of claim 1.

0014Further embodiments of the invention are disclosed in claims 2 to 11.

0015The following is a description of some specific embodiments of the invention reference being made to the accompanying drawings in which:

0016Figures 1-3 are fragmentary vertical cross-sectional views which sequentially illustrate the process of the invention.

0017Figures 4-5 are fragmentary vertical cross-sectional views which sequentially illustrate another embodiment of the process of the invention.

0018Figure 6 is a flow sheet illustrating the process of the invention.

0019Figures 7-11 are fragmentary vertical cross-sectional views sequentially illustrating another embodiment of the process of the invention.

0020In its broadest aspects, the invention provides an improved planarization process for integrated circuit structures wherein a low melting inorganic planarizing material is deposited on a conformal insulating layer such as silicon oxide followed by etching of the planarizing layer. By using a low melting inorganic planarizing material such as a low melting glass, no intermediate deposition or coating, solvent evaporation, or baking steps need be carried out outside of the CVD apparatus and the same apparatus may be used to deposit and etch the low melting inorganic planarizing material as is used to deposit the underlying insulating layer.

0021Turning now to Figure 1, portions of a typical integrated circuit structure are generally shown as layer 10 which may include a substrate such as silicon, one or more doped or undoped buried layers, one or more doped or undoped epitaxial layers, one or more doped or undoped polysilicon layers, oxide isolation and insulation portions or layers, etc. Shown formed on integrated circuit structure 10, by way of illustration and not of limitation, are two spaced apart metal lines 14 and 16. An insulating layer 20 of a material such as silicon oxide is deposited over integrated circuit structure 10 and metal lines 14 and 16 in preparation for the formation thereon of further patterned layers such as, for example, a metal wiring harness which will interconnect metal lines 14 and/or 16 with other portions of structure 10.

0022Insulation material 20 may comprise an oxide of silicon or a silicate such as phosphorus silicate when the underlying integrated circuit structure comprises silicon. A nitride or oxynitride of silicon may also be used when the underlying structure comprises silicon. The insulation material may be either a doped or undoped material. Other insulation materials may, of course, be used as well and may even be preferred over those just named when the underlying structure comprises some other material than silicon, e.g., germanium, gallium arsenide, etc.

0023In this regard, it should be noted that the process may be used whenever it is desired to planarize an integrated circuit structure having raised portions with respect to the portion of the substrate structure therebetween. Thus, the process may be used for front end application such as dielectric planarization, for filling trenches or slots, or for top side planarization as well as the illustrated intermetal planarization. For example, when using the process in a front end application, the use of boron phosphorus silicate glass (BPSG) may be eliminated and a phosphorus silicate glass (PSG) may be substituted for the BPSG as the insulating material to be planarized. The process may also be used to planarize an integrated circuit structure prior to a blanket deposit of another metal layer such as tungsten.

0024By use of the term "raised portions" is meant portions of an integrated circuit structure raised with respect to the height of the surface therebetween and thus may include not only structures raised with respect to the entire surface but also the raised sidewalls, for example, of a trench or slot with respect to the bottom of the trench.

0025By way of example, when insulation layer 20 comprises silicon oxide, it may be deposited over integrated circuit structure 10 and on metal lines 14 and 16 within a temperature range of from about 20°C to about 350°C in a non-ECR plasma CVD apparatus to an appropriate thickness which may range from about 100 nm (1000 Angstroms) to about 3 micrometers, typically about 1 micrometer.

0026It will be readily seen in Figure 1 that the application of an insulation material such as silicon oxide as insulating layer 20 results in the formation of a very conformal layer having steps or shoulders 24 interconnecting low regions 22 with raised regions 26 in conformity with the raised metal lines 14 and 16 thereunder.

0027As a result of this conformity of insulating layer 20 to the underlying raised portions of the integrated circuit structure and the resulting uneven or stepped geometry of insulating layer 20, patterning of a subsequently applied layer by photo-lithography would be very difficult.

0028Therefore, in accordance with this embodiment of the invention, a planarizing layer 30 of a low melting inorganic material, such as a low melting glass, is first applied on insulating layer 20, and then the coated structure is subjected to a planarizing etch step to remove planarizing layer 30 as well as the higher regions 24 and 26 of underlying insulating layer 20.

0029Low melting inorganic planarizing material 30 may comprise any inorganic material which: a) may be deposited on the surface of insulating layer 20 without the use of a solvent; b) which does not need to be subsequently cured or baked to harden the deposited material sufficiently to permit etching thereof; and c) is capable of being etched, preferably dry etched, at approximately the same rate as the underlying insulation layer.

0030In a preferred embodiment, low melting inorganic planarizing material 30 comprises a material which may be deposited on insulating layer 20 using the same chemical vapor deposition apparatus as used to deposit insulating layer 20 on integrated circuit structure 10.

0031By "low melting" is meant a material which has a melting point of about 575°C or lower, and which will flow at about 500°C or lower, i.e., from about 100 to about 500°C. In a preferred embodiment where the process will be used over low melting materials already present in the integrated circuit structure such as aluminum, e.g., over aluminum lines or in topside applications, the melting point should not exceed about 480°C with a flow of 390°C or lower. The use of a material which will flow at about 390°C or lower will result in the flowing of the material over the underlying surface without risk of any harm to the underlying integrated circuit structure.

0032By way of example, the low melting inorganic planarizing material comprises a low melting glass. Examples of such low melting glasses include B₂O₃, B₂S₆, B₂O₃/SiO₂ mixtures, As₂O₃, As₂S₃, P₂O₅ or any combinations of the above.

0033By using a low melting planarizing material such as a low melting glass, planarizing material 30 may be deposited using, for example, the same CVD methods and apparatus used to deposit the insulating material 20 such as silicon oxide. Thus, a deposition of a low melting glass such as, for example, B₂O₃ at from about 390°C to about 480°C, at which temperature the planarizing material will flow on the stepped surface of insulating layer 20 on integrated circuit structure 10, will result in the generally planar surface 32 on layer 30 shown in Figure 2.

0034A lower deposition temperature may, of course, be used if the material is subsequently heated sufficiently to cause the planarizing material to flow over the surface. However, usually such an additional heating step will be avoided if possible. A lower deposition temperature may also be used provided that the low melting planarizing material has a flow point temperature at least as low as the deposition temperature so that the planarization material will flow as it is deposited.

0035The use of the same apparatus for deposition of both layers 20 and 30, together with the selection of a low melting inorganic material as the planarizing material which does not use solvents which must be removed, and which does not require further baking or curing prior to etching, permits the preferential carrying out of the two deposition steps sequentially in the same deposition apparatus without intermediate removal of the integrated circuit structure from the vacuum deposition apparatus. This not only reduces the total number of process steps, compared to the prior art planarizing processes, but additionally protects the integrated circuit structure from the risk of possible contamination which may occur whenever the integrated circuit structure is removed from the vacuum apparatus and exposed to the atmosphere.

0036It should be noted that both depositions may be carried out in the same deposition chamber or in separate chambers within the same apparatus which are interconnected in a manner which permits transfer of the integrated circuit structure from one chamber to another without exposure to the atmosphere and particularly to moisture and other contaminants in the atmosphere.

0037The low melting inorganic planarizing material is deposited onto the surface of insulating layer 20 within a temperature range of from about 100°C to about 700°C, preferably about 300°C to about 500°C, and under a pressure of from about 1.33 Pa (10 millitorr) to about atmospheric pressure, preferably from about 0.266 to 4 kPa (2 to 30 torr), to a thickness of from about 20 nm (200 Angstroms), at its thinnest point, up to about 2 micrometers in its thickess regions, i.e., overlying the low areas of the insulation layer beneath it. In a typical plasma CVD deposition of a B₂O₃ glass, the deposition temperature ranges from about 390°C to about 440°C at a pressure of about 1.2 to 1.33 kPa (9-10 torr) with an rf plasma power of about 400-500 watts.

0038As discussed above, the application of a low melting inorganic planarizing material, using a deposition step similar to that used to deposit the underlying insulating layer, makes possible the use of the same deposition apparatus for both deposition steps. This has the dual advantage of reducing the number of processing steps as well as reducing the risk of contamination of the integrated circuit structure by unnecessary exposure of the structure outside of the vacuum apparatus.

0039After deposition of the low melting inorganic planarizing material, in this embodiment, the coated structure is then etched until substantially all of planarizing layer 30 has been removed, i.e., about 99.9% or more, as well as the high areas 26 and the stepped sides 24 of insulating layer 20, as shown at the top surface indicated by solid line 28 of Figure 3, leaving a planarized portion 20′ of insulating layer 20 approximately conforming in height to about the height of the lowest portions 22 of layer 20.

0040It should be noted, in this regard, that while new surface line 28 is shown as substantially flat in Figure 3, the planarized surface may still have somewhat raised portions adjacent the underlying raised parts of the integrated circuit structure, e.g., above lines 14 and 16. However the 45° or higher slopes of the steps of the unplanarized insulation layer will be reduced down to about 10 to 15° or even lower after the planarization process of this invention.

0041It should also be noted that the final slope is controllable by varying the film thickness and/or deposition temperature used to deposit the planarizing material. Raising the deposition temperature reduces the slope because the planarizing material will flow better. Increasing the thickness of the planarizing material will also cause the film to flow more evenly across the underlying integrated circuit structure.

0042The etch step may comprise any etch system capable of etching both the planarizing layer 30 and underlying insulating layer 20 at approximately the same rate. The etchant may comprise any dry etch such as a conventional anisotropic etch. Preferably the dry etch will comprise a plasma etch using CHF₃ or CF₄ or argon. Examples of other dry etching systems useful in the practice of the invention include a sputter etching system or an RIE system.

0043In a preferred embodiment of the invention, the integrated circuit structure, after having both the insulation layer and the planarizing layer deposited in the same deposition apparatus, is etched in another zone in the same apparatus while still maintaining the integrated circuit structure under vacuum. In a particularly preferred embodiment, the two deposition steps and the etching step are all carried out in the same chamber in the same vacuum apparatus. Thus, as shown in the flow chart of Figure 6, the integrated circuit structure may be coated with both insulation layer 20 and planarizing layer 30 in a deposition zone, which may comprise the same or different deposition chambers in a common deposition apparatus, and then the coated structure may be either etched in the same chamber used for one or both of the depositions or it may be moved to or through an interlock or intermediate chamber from which the coated structure may be moved to a different etching zone without, in either case, removing the coated structure from the vacuum apparatus.

0044Turning now to the embodiment shown in Figures 4 and 5, after deposition of the low melting inorganic planarizing material, the coated structure is etched to remove a portion of planarizing layer 30 to expose at least the high areas 26 of insulating layer 20.

0045It will be noted, however, in accordance with this embodiment of the planarizing process of the invention, that not all of planarizing layer 30 need be removed. As shown in the embodiment of Figure 4, portions 34 of planarizing layer 30 may remain on low regions 22 and shoulders 24 of insulating layer 20. As shown by dotted line 36, it is also within the scope of this embodiment of the process of the invention to even leave a thin portion of layer 30 over raised portions 26 of insulating layer 20, although this would not be preferable.

0046In accordance with this embodiment of the invention, portions of low melting glass planarizing layer 30 may be left on insulating layer 20 without damage to the underlying integrated circuit structure (even when the low melting glass is hygroscopic in nature) by providing a further layer of insulating glass which serves to encapsulate such remaining portions of low melting glass planarizing layer 30. By such encapsulation, any deleterious effects which might, otherwise, be experienced by the presence of such hygroscopic materials are, in effect, neutralized or nullified.

0047Insulating layer 40 may comprise a material identical to that used for insulating layer 20 and may be applied in an identical manner. In accordance with a preferred aspect of this embodiment of the invention, layer 40 may also be applied over the structure using the same vacuum apparatus used for the deposition of layers 20 and 30 and the etching of layer 30, so that wafer or integrated circuit structure 10 will not be exposed to the atmosphere outside of the vacuum apparatus during any of the steps of the process.

0048When the etching step is carried out in the same chamber in the vacuum apparatus as used for the deposition of layers 20 and/or 30, layer 40 may also be deposited on the structure in the same chamber, thus permitting the deposition of layers 20 and 30, the etching of layer 30, and the deposition of layer 40, all to be carried out without removing the semiconductor wafer containing integrated circuit structure 10 from the particular vacuum chamber.

0049Turning now to Figures 7-11, the steps of another embodiment of the invention are sequentially illustrated for the case where the close horizontal spacing between several adjacent raised shapes or structures on the surface of an integrated circuit structure, such as metal lines, may result not only in the formation of steps in an insulating layer deposited thereon, but also in the formation of voids in the overlying insulation material. This void formation may occur when the raised portions are closely spaced apart.

0050The term "closely spaced" may be defined as when, in an integrated circuit structure having raised portions or structures, the ratio of the height of the raised portions to the spacing between the raised portions is 0.5 or greater, e.g., where the height is 1 micrometer and the spacing is 2 micrometers or less. It may also be defined as the case whenever the spacing between the raised portions is less than about 1 micrometer.

0051In this embodiment of the planarization process of the invention, an insulation layer 20a is deposited over integrated circuit structure 10 in similar fashion to the deposition shown in Figure 1. However, due to the close spacing between the underlying structures, such as metal lines 14 and 15, a void 25 may be formed in the portion of insulation layer 20a deposited between the facing sidewalls of metal lines 14 and 15, as shown in Figure 7.

0052Since subsequent planarization may open up the top of void 25, it is important that void 25 be removed prior to final planarization of the structure.

0053Therefore, as shown in Figure 8, the structure is subjected to an etch step prior to deposition of the low melting inorganic planarization layer.

0054This etch step is preferably an isotropicetch which will preferentially etch the less dense sidewall insulating material, e.g., silicon oxide material, leaving a portion of layer 20a designated as 20b in Figure 8. This etching step is carried out until about 90% of the sidewall thickness has been removed.

0055While a wet etch, such as an HF or NH₄F etch may be used here, it is preferred to use a dry etch since this again permits transfer of the coated structure from the deposition zone to an etching zone of the same vacuum apparatus without removal of the coated structure from the vacuum apparatus so that the risk of contamination is again minimized.

0056Examples of dry etchants which may be utilized as isotropic etchants for this step of the process include a C₂F₆ or NF₃ plasma etch within a temperature range of from about 80°C to about 500°C, preferably about 350°C to about 450°C, at a vacuum of about 13,3 Pa to about 4 kPa (about 100 millitorr to about 30 torr), preferably about 0.666 to 1,33 kPa (about 5 to 10 torr).

0057After the etching step is completed, a further layer of insulating material 20c may be deposited on etched layer 20b, as shown in Figure 9, followed by deposition of the low melting inorganic planarizing material 30a, as shown in Figure 10. Alternatively, planarizing material 30a may be deposited directly on the etched insulating layer 20b.

0058After the deposition of low melting inorganic planarizing layer 30a, either on second insulation layer 20c, as shown in Figure 10, or directly on etched insulation layer 20b, the structure is subjected to an etch step, as described in the previous embodiments of Figures 1-3 and 4-5, to either remove substantially all of planarizing layer 30a as well as the raised portions of the underlying insulation layer resulting in the structure shown in Figure 11 with a planarized layer 20d of insulation material over integrated circuit structure 10 and on metal lines 14 and 15, or, as described in the embodiment of Figures 4 and 5, to remove at least a portion of planarizing layer 30a followed by deposition of a further insulating layer 40 which will encapsulate any portions of planarizing layer 30a remaining on the structure after the etching step.

0059In a variation on this embodiment when silicon oxide comprises insulation layer 20a, the vapor being deposited to form layer 20a may be mixed with an etchant such as a fluorine species, e.g., a CF₄, C₂F₆, or NF₃ gas, so that an in situ etch of the less dense silicon oxide sidewalls of layer 20a will occur during the deposition. The gaseous etchants may be mixed, for example, with the gaseous constituents used to deposit the silicon oxide in a ratio of about 1 to about 20 volume % etchant to provide a deposition vapor mixture which will for a layer of silicon oxide over integrated circuit structure 10 without forming voids between closely spaced together metal lines 14 and 15.

0060Thus, the invention provides a planarization process for removing the steps formed in an insulation layer deposited over an integrated circuit structure in which a low melting inorganic planarizing material is deposited on the insulation layer, preferably using the same deposition apparatus used to deposit the insulation layer, and most preferably the same chamber, to minimize the risk of contamination of the integrated circuit structure. The use of such a low melting inorganic planarization material, such as a low melting glass, eliminates the prior art steps of evaporating solvents, drying the planarization coating, and then baking the coating sufficiently to harden it.

0061The integrated circuit structure, having the low melting inorganic planarizing layer deposited thereon in accordance with the invention, is then etched to remove at least a portion of the planarization layer and optionally the high or stepped portions of the underlying insulation layer leaving a substantially planarized structure. Preferably the etch step is a dry etch step which is also carried out in the same vacuum apparatus used in the deposition steps, and most preferably in the same chamber as used for one or both depositions, to again lower the risk of contamination of the integrated circuit structure.

0062When some of the low melting glass planarizing layer is not removed in the etching step, a further layer of insulating material may be formed over the structure to encapsulate remaining portions of the low melting glass which may be hygroscopic. This latter deposition step is also preferably carried out in the same apparatus, and most preferably in the same chamber as used in one or more of the previous steps to minimize contamination or other damage to the structure due to movement from one process station to another.