Abstract:
The invention includes a method of forming an insulating material between components of an integrated circuit. A pair of spaced electrical components are provided over a substrate. Polysilicon is chemical vapor deposited over, between, and against the pair of electrical components. Cavities are formed within the polysilicon to enhance porosity of the polysilicon. After the cavities are formed, at least some of the polysilicon is transformed into porous silicon dioxide.

Description:
RELATED PATENT DATA 
     This patent resulted from a divisional application of U.S. patent application Ser. No. 08/948,372, which was filed on Oct. 9, 1997. 
    
    
     TECHNICAL FIELD 
     The invention pertains to methods of forming insulating material, such as for example, methods of forming insulating material between components of integrated circuits. 
     BACKGROUND OF THE INVENTION 
     In methods of forming integrated circuits, it is frequently desired to isolate components of the integrated circuits from one another with insulative material. Such insulative material may comprise a number of materials, including, for example, silicon dioxide, silicon nitride, and undoped semiconductive material, such as silicon. Although such materials have acceptable insulative properties in many applications, the materials disadvantageously have high dielectric constants which can lead to capacitive coupling between proximate conductive elements. For instance, silicon dioxide has a dielectric constant of about 4, silicon nitride has a dielectric constant of about 8, and undoped silicon has a dielectric constant of about 12. 
     It would be desirable to develop alternative methods for insulating conductive elements from one another with low-dielectric-constant materials. 
     SUMMARY OF THE INVENTION 
     The invention encompasses methods of forming insulating materials proximate conductive elements. 
     In one aspect, the invention encompasses a method of forming an insulating material proximate a substrate in which a first material is chemical vapor deposited proximate the substrate. Cavities are formed within the first material, and, after forming the cavities, at least some of the first material is transformed into an insulative second material. 
     In another aspect, the invention encompasses a method of forming an insulating material proximate a substrate in which porous polysilicon is formed proximate the substrate and at least some of the porous polysilicon is transformed into porous silicon dioxide. 
     In yet another aspect, the invention encompasses a method of forming an insulating material between components of an integrated circuit. Polysilicon is chemical vapor deposited between two components and electrochemically anodized to convert the polysilicon into a porous mass having a first volume. The first volume comprises a polysilicon volume and a cavity volume, with the cavity volume comprising greater than or equal to about 75% of the first volume. The porous polysilicon mass is oxidized to transform the polysilicon into porous silicon dioxide having a second volume. The second volume comprises a silicon dioxide volume and a cavity volume, with the cavity volume comprising less than or equal to about 50% of said second volume. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
     FIG. 1 is a diagrammatic cross-sectional view of a semiconductor wafer fragment at a preliminary step of a processing method of the present invention. 
     FIG. 2 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG. 1. 
     FIG. 3 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG. 2. 
     FIG. 4 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG. 3. 
     FIG. 5 is a view of the FIG. 4 wafer fragment shown at a processing step subsequent to that of FIG. 4. 
    
    
     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 &#34;to promote the progress of science and useful arts&#34; (Article 1, Section 8). 
     FIG. 1 shows a semiconductive wafer fragment 10 at a preliminary processing step of the present invention. Wafer fragment 10 comprises a substrate 12 and conductive elements 14 and 16 overlying substrate 12. Substrate 12 may comprise, for example, a monocrystalline silicon wafer. To aid in interpretation of the claims that follow, the term &#34;semiconductive substrate&#34; 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), and semiconductive material layers (either alone or in assemblies comprising other materials). The term &#34;substrate&#34; refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. 
     Conductive elements 14 and 16 may comprise, for example, conductive lines. Conductive elements 14 and 16 might be part of an integrated circuit, for example. Although conductive elements 14 and 16 are illustrated as being horizontally displaced, such elements could also be displaced along a non-horizontal axis. For example, such elements could be vertically displaced from one another. 
     An insulative material 18 is formed between substrate 12 and conductive elements 14 and 16. Insulative material 18 can comprise a number of materials known to persons of ordinary skill in the art, such as, for example, silicon nitride and silicon dioxide. Insulative material 18 is provided to electrically isolate conductive elements 14 and 16 from substrate 12. Such electrical isolation might be desired, for example, if substrate 12 is conductive or semiconductive. 
     Referring to FIG. 2, a first material 20 is deposited proximate substrate 12 and between conductive elements 14 and 16. First material 20 preferably comprises polysilicon, and is preferably formed by chemical vapor depositing. Methods for chemical vapor depositing polysilicon are known to persons of ordinary skill in the art, and include, for example, methods comprising thermal decomposition of silane. 
     Referring to FIG. 3, cavities 22 are formed within first material 20. The formation of cavities 22 within first material 20 converts first material 20 into a porous first material. In a preferred example in which first material 20 comprises polysilicon, cavities 22 may be formed by, for example, either electrochemical anodization or by subjecting the polysilicon to a chemical etch. An example method of electrochemical anodization comprises doping preferred polysilicon layer 20 and making wafer fragment 10 an anode in an aqueous hydrofluoric acid solution. The hydrofluoric acid solution can comprise, for example, 20 wt. % HF, and the amount of current applied with wafer fragment 10 as anode can comprise, for example, about 10 mA for a 100 mm diameter wafer. An example method of chemical etching comprises doping preferred polysilicon layer 20 with a p-type conductivity-enhancing dopant and subsequently chemically etching layer 20 with a phosphoric acid solution. 
     Preferably, greater than about 50% of a volume of layer 20 will be removed in forming cavities 22. More preferably, at least about 75% of a volume of layer 20 will be removed in forming cavities 22. In other words, the formation of cavities 22 converts the first material of layer 20 into a porous mass having a first volume which comprises a polysilicon volume and a cavity volume, wherein the cavity volume is most preferably greater than or equal to about 75% of the first volume. 
     Referring to FIG. 4, first material 20 (shown in FIG. 3) is transformed into an insulative second material 30. Where the first material 20 comprises polysilicon, such transformation can occur, for example, by oxidizing polysilicon layer 20 to transform such polysilicon layer to a silicon dioxide layer 30. Methods for oxidizing a polysilicon layer are known to persons of ordinary skill in the art, and include, for example, thermal oxidation utilizing one or more of the oxygen-containing containing compounds O 2 , O 3  and H 2  O. In the shown embodiment, substantially all of first material 20 is transformed into insulative second material 30. However, it is to be understood that the invention also encompasses embodiments in which only some of first material 20 is transformed into insulative second material 30. In the shown preferred embodiment, oxidation of polysilicon layer 20 (shown in FIG. 3) having a first volume swells the layer into a silicon dioxide layer 30 having a second volume which is larger than the first volume. The increase in volume of layer 30 relative to layer 20 changes the relative volume occupied by cavities 22. For instance, in an example embodiment in which cavities 22 comprise a cavity volume greater than or equal to about 75% of a first volume of porous polysilicon layer 20 (shown in FIG. 3), the cavity volume can comprise less than or equal to about 50% of a volume of porous silicon dioxide layer 30 formed by oxidizing such layer 20. 
     The cavities 22 within second material layer 30 lower a dielectric constant of the material relative to what the dielectric constant would be in the absence of cavities 22. Cavities 22 will preferably be filled with some gas. Gases typically have a dielectric constant of about 1, which is less than a dielectric constant of most commonly used insulative materials. For instance, if the insulative solid material of layer 30 comprises silicon dioxide, the silicon dioxide will typically have a dielectric constant of about 4. The addition of cavities 22 within material layer 30 decreases the dielectric constant of the material 30 to less than 4. In the above-described embodiment in which cavities 22 comprise about 50% of the total volume of layer 30, and in which layer 30 comprises silicon dioxide, layer 30 can have a dielectric constant of about 1.6. Accordingly, the method of the present invention can form a porous silicon dioxide insulative layer having a dielectric constant of less than or equal to about 1.6. 
     As shown in FIG. 5, layer 30 can be utilized to support additional circuitry formed over conductive elements 14 and 16. In the shown embodiment, a filling layer 32 is provided over layer 30. Filling layer 32 can comprises any of a number of materials known to persons of ordinary skill in the art, including, for example, insulative materials such as silicon dioxide or silicon nitride. Filling layer 32 can be provided by, for example, chemical vapor deposition. Filling layer 32 is planarized, such as, for example, by chemical-mechanical polishing, to form a substantially planar upper surface 34. 
     After forming a planar upper surface 34 over layer 30, circuit elements 40, 42 and 44 are formed over the upper surface. Circuit elements 40, 42 and 44 can be formed by conventional methods. 
     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.