Patent Publication Number: US-6666986-B1

Title: Supercritical etching compositions and method of using same

Description:
This is a division of application Ser. No. 08/841,886, filed May 5, 1997 now U.S. Pat. No. 6,949,828, which incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to supercritical fluids and, in particular, to supercritical etching compositions and methods for using supercritical etching compositions to etch inorganic material of semiconductor-based substrates. 
     BACKGROUND OF THE INVENTION 
     In the fabrication of semiconductor integrated circuits (ICs), various layers of inorganic material form semiconductor-based substrates. Some of the inorganic layers are patterned in desired shapes. The resulting inorganic layers form individual devices and interconnect structures within the IC. 
     Patterning conventionally includes masking an underlying layer with an organic resist material, such as photoresist, exposing the resist, and removing exposed areas of the mask to form a patterned mask layer. The exposed inorganic layer underlying the patterned mask layer is then removed using an appropriate etchant. The patterned mask layer is then removed. 
     Etching is a process for removing unwanted material (i.e. partial or complete layers of material) from a surface (e.g., the surface of a semiconductor-based substrate). Organic or inorganic material, which may be patterned or unpatterned, of a substrate surface can be removed using an etching technique. Ideally, etching should precisely remove material that is not covered by a patterned mask layer (i.e., material that is “exposed” when a patterned mask layer is used). 
     The etchant is typically chemically varied according to the type of material being etched. Etchants are characterized as isotropic or anisotropic. Isotropic etchants remove material in all directions at the same rate. Anisotropic etchants do not remove material in all directions at the same rate. Etchants are further characterized as being selective or non-selective, depending on their ability to differentiate between material that they effectively etch. Selective etchants remove different types of material at different rates. 
     Etching can occur in a wet or dry processing environment. Wet etching refers to the contact of a substrate surface with a liquid chemical etchant. Material is removed as an agitated liquid or spray, for example, passes over the substrate surface. Dry etching refers to the contact of a substrate surface with a gaseous plasma etchant. Wet etching is preferred over dry etching due to its ability to provide high, reliable throughput with excellent selectivity. One problem associated with conventional wet etching, however, is that etched material constituents may move within etched or partially etched openings on the substrate surface. Thus, all of the etched material constituents are not removed from the substrate surface. Furthermore, oftentimes etchants do not completely wet a substrate surface, resulting in incomplete or nonuniform etching. Another problem associated with wet etching is the isotropic nature of most wet etchants, which is problematic when the minimum pattern dimension of the substrate is comparable to the thickness of the material being etched. In that situation, it is hard to achieve precise pattern dimensions on a substrate. 
     Overall, the most commonly used wet etchants are hot alkaline etchants or acidic hydrogen peroxide (H 2 O 2 ) etchants. Typically, alkaline solutions remove organic films, while acidic solutions remove alkali ions, alkali compounds, and other metallic contaminants. For wet etching silicon (Si), mixtures of nitric acid (HNO 3 ) and hydrofluoric acid (HF) are typically used. For wet etching silicon dioxide (SiO 2 ), various HF solutions are typically used, usually further containing a buffering agent to prevent depletion of fluoride ions from the etchant during etching. Silicon nitride (Si 3 N 4 ) is typically wet-etched using a hot phosphoric acid (H 3 PO 4 ) solution. Aluminum (Al) is typically wet-etched using a mixture of phosphoric acid (H 3 PO 4 ), acetic acid (CH 3 COOH), nitric acid (HNO 3 ), and water (H 2 O). 
     While wet etchants have many preferable characteristics as compared to dry etchants, dry etchants can be used in semiconductor fabrication without the need to dry the substrate being processed after an etching step. The added step of drying the substrate that is required when using a conventional wet etchant adds to the cost of semiconductor device fabrication. A lack of full process automation also results from the added step of drying the substrate. Another advantage of etching with a dry etchant is that it often decreases the safety hazards associated with wet etchants due to the relatively small amount of chemicals utilized in the dry etchant. 
     Supercritical fluids have been used to etch residue from a variety of surfaces or extract substances from various materials. A gas is determined to be in a supercritical state (and is referred to as a supercritical fluid) when it is subjected to a combination of pressure and temperature so that its density approaches that of a liquid (i.e., the liquid and gas state coexist). Supercritical fluids have been used to clean contact lenses by etching residue from lense surfaces, as disclosed by Bawa et al. in PCT Application Publication Number WO 95/20476. Supercritical fluids, namely carbon dioxide (CO 2 ), have also been used to remove exposed organic photoresist films, as disclosed by Nishikawa et al. in U.S. Pat. No. 4,944,837, to form a patterned photoresist film. As further disclosed in Nishikawa et al., once an underlying layer is patterned by conventional methods, supercritical fluids are used to remove the patterned resist film. 
     Deionized water at elevated temperatures and pressures, but not supercritical temperatures and pressures, has also been used to etch thermally grown silicon dioxide (SiO 2 ) on a silicon wafer. This was discussed in “Removal of Thermally Grown Silicon Dioxide Films Using Water at Elevated Temperature and Pressure,” by Bakker et al.,  J. Electrochem. Soc.,  142, 3940-44 (1995). The paper also discussed the solubility of quartz silicon dioxide (SiO 2 ) in supercritical deionized water. Although Bakker et al. noted the use of supercritical water to promote solubility of quartz (i.e., a large-scale; uncontrolled process, as compared to etching in semiconductor fabrication as discussed in “A Portion of the System Silica-Water,” by Kennedy,  Econ. Geol.,  45, 629, (1950)), Bakker et al. found that the preferred temperature and pressure for etching thermally grown silicon dioxide (SiO 2 ) was below the critical point. Processing steps involved in the fabrication of semiconductor ICs need to be very controlled and precise in order to obtain the optimal density and electrical performance of the resulting IC. Thus, the processing steps utilized to solubilize quartz (SiO 2 ), as compared to thermally grown silicon dioxide (SiO 2 ), are not known to be readily interchangeable to one of ordinary skill in the art due to the differing process criteria. The quartz plates studied by Kennedy involved two contiguous square centimeters in solubilized surface area, while typical etched surface areas on semiconductor-based substrates are about one contiguous square micron or less. Furthermore, Kennedy noted that rough-ground or sawed crystals require a shorter time to solubilize as compared to smooth-lapped or well-polished crystals. Thus, due to the microscopically smooth nature of most layers utilized in semiconductor IC fabrication, it would not have been obvious to one of skill in the art to utilize supercritical water to etch inorganic layers within a semiconductor IC when increasing throughput is a prime fabrication concern. 
     An etching composition and method of etching various inorganic films in the fabrication of semiconductor ICs is needed that achieves an effective, highly uniform removal of inorganic material across a substrate. It is further desired that the etching composition and method of etching inorganic films be cost-effective and contribute to full process automation during semiconductor fabrication. 
     SUMMARY OF THE INVENTION 
     A method for etching an inorganic material of a substrate includes providing a semiconductor-based substrate having an exposed inorganic material and exposing the substrate to a supercritical etching composition to remove at least a portion of the inorganic material from the substrate. The inorganic material preferably includes a material selected from the group of silicon (Si), thermally grown silicon dioxide (SiO 2 ), chemical vapor deposited SiO 2 , silicon nitride (Si 3 N 4 ), aluminum (Al), copper (Cu), borophosphosilicate glass (BPSG), titanium nitride (TiN), titanium (Ti), high dielectric constant (HDC) materials, and combinations thereof. 
     The method of the present invention is particularly effective at removing HDC materials. HDC materials preferably include tantalum pentoxide (Ta 2 O 5 ), yttria (Y 2 O 3 ), titanium dioxide (TiO 2 ), strontium bismuth titanate (SBTi), lead zirconate titanate (PZT), lanthanum-doped lead zirconate titanate (PLZT), barium strontium titanate (BST), bismuth titanate (BiTO), strontium titanate (STO), barium titanate (BaTO), strontium bismuth tantalate (SBTa), and combinations thereof. 
     The substrate can have a patterned mask layer thereon, wherein the inorganic material removed from the substrate includes exposed inorganic material under the patterned mask layer. The inorganic material can also be removed from the substrate in an unpatterned manner (i.e., without the presence of a patterned mask layer on the substrate). 
     In one embodiment, the supercritical etching composition and method of removing an inorganic material using the composition includes at least one supercritical component and at least one nonsupercritical component. Preferably, the supercritical component is selected from the group of ammonia (NH 3 ), amines (NR 3 ), alcohols (ROH), water (H 2 O), carbon dioxide (CO 2 ), nitrous oxide N 2 O), inert gases (e.g., He, Ne, Ar), hydrogen halides (e.g., hydrofluoric acid (HF), hydrochloric acid (HCl), hydrobromic acid (HBr)), boron trichloride (BCl 3 ), chlorine (Cl 2 ), fluorine (F 2 ), hydrocarbons (e.g., dimethyl carbonate (CO(OCH 3 ) 2 ), methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), etc.), fluorocarbons (e.g., CF 4 , C 2 F 4 , CH 3 F, etc.), hexafluoroacetylacetone (C 5 H 2 F 6 O 2 ), and combinations thereof. These compounds may be capable of etching without an additional component, depending on the material being etched. If, however, they are not capable of etching in and of themselves, a nonsupercritical component that is capable of etching (i.e., a nonsupercritical etching component) is used. Preferably, the nonsupercritical etching component is selected from the group of ammonia (NH 3 ), hydrofluoric acid (HF), phosphoric acid (H 3 PO 4 ), nitric acid (HNO 3 ), acetic acid (CH 3 COOH), hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ), hydrogen peroxide (H 2 O 2 ), nitrous oxide (N 2 O), nitrogen trifluoride (NF 3 ), sulfur hexafluoride (SF 6 ), ozone (O 3 ), sulfur dioxide (SO 2 ), sulfur trioxide (SO 3 ), amines (NR 3 ), ammonium salts, hexafluoroacetylacetone (C 5 H 2 F 6 O 2 ), and combinations thereof. 
     In another embodiment, the supercritical etching composition and method of removing an inorganic material using the composition includes a supercritical component that is capable of etching the desired inorganic material in and of itself. Preferably, for such embodiments., the supercritical component is selected from the group of ammonia (NH 3 ), hydrofluoric acid (HF), acetic acid (CH 3 COOH), nitrous oxide (N 2 O), nitrogen trifluorde (NF 3 ), hydrochloric acid (HCl), hydrobromic acid (HBr), water (H 2 O), and combinations thereof. In any of these embodiments, the supercritical etching composition can include a component selected from the group of oxidizers, buffering agents, surfactants, selectivity enhancers, and ligands. These components may be in the supercritical or nonsupercritical state. Thus, they may be present in the supercritical etching composition as supercritical components or nonsupercritical components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic representation of a pressure vessel for use in etching in accordance with the present invention. 
     FIG. 2 is a schematic representation of another pressure vessel for use in etching in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Supercritical etching compositions and methods for using such compositions are disclosed herein. The following description is illustrative of various embodiments of the invention. It is to be understood that the term substrate, as used herein, includes a wide variety of semiconductor-based structures that can be etched or have inorganic layers therein that can be etched. A substrate can be a single layer of material, such as a silicon (Si) wafer. Substrate is also to be understood as including silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI) technology, doped and undoped semiconductors, epitaxial layers of silicon (Si) supported by a base semiconductor, as well as other semiconductor-based structures, including any number of layers as is well known to one skilled in the art. Furthermore, when reference is made to a substrate in the following description, previous process steps may have been utilized to form regions/junctions in a base semiconductor structure. It is also important to note that the substrate being etched can have a patterned mask layer thereon, such as, for example, a patterned resist layer, but it is not required. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     A gas enters the supercritical state when the combination of pressure and temperature of the environment in which the gas is contained is above a critical state. For example, the critical temperature of carbon dioxide (CO 2 ) is 31° C. The critical pressure of CO 2  is 7.38 MPa (72.8 atm). When CO 2  is subjected to temperatures and pressures above 31° C. and 7.38 MPa (72.8 atm), respectively, it is in the supercritical state. The critical temperature and pressure of other components is readily calculated or experimentally determined by one skilled in the art. A gas in the supercritical state is referred to as a supercritical fluid. Supercritical fluids have high solvating capabilities that are typically associated with compositions in the liquid state. Supercritical fluids also have a low viscosity that is characteristic of compositions in the gaseous state. Furthermore, supercritical fluids are able to penetrate (i.e., effectively contact) surfaces better than compositions in the liquid state. 
     At least one supercritical component is used in the compositions of the invention to effectuate the etching process. Supercritical component, as used herein, encompasses all compounds or elements that are present as supercritical fluids. The supercritical component may, or may not, be able to effectively etch a material by itself. If the supercritical component is not an etchant in and of itself for the particular inorganic material to be etched, at least one additive that is capable of etching can be added to the supercritical component. In this embodiment, these additives are nonsupercritical etching components. The addition of a nonsupercritical etching component ensures that the material to be etched will be solubilized, for example, in the supercritical etching composition. Thus, conventional etchants can be utilized in the compositions of this invention to promote the removal of the targeted inorganic material when a supercritical component is used that is not an etchant for the particular material to be removed. 
     In the following embodiments of the invention, a supercritical component is supplied. The supercritical component can be, for example, ammonia (NH 3 ), amines (NR 3 ), alcohols (ROH), water (H 2 O), carbon dioxide (CO 2 ), nitrous oxide (N 2 O), inert gases (e.g., helium (He), neon (Ne), argon (Ar), etc.), hydrogen halides (e.g., hydrofluoric acid (HF), hydrochloric acid (HCl), and hydrobromic acid (HBr)), boron trichloride (BCl 3 ), chlorine (Cl 2 ), fluorine (F 2 ), hydrocarbons (e.g., dimethyl carbonate (CO(OCH 3 ) 2 ), methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), etc.), fluorocarbons (e.g., CF 4 , C 2 F 4 , CH 3 F, etc.), hexafluoroacetylacetone (C 5 H 2 F 6 O 2 ), and similar compounds, or combinations of the above. As stated above, there may be more than one supercritical component in the supercritical etching composition. Such components may or may not be known etchants for a particular inorganic material. 
     In one embodiment, a supercritical component is not capable of etching in and of itself. At least one nonsupercritical component is then added to the supercritical component to form a supercritical etching composition. The nonsupercritical component may be added to the supercritical component either before it is brought to the supercritical state, or after it is brought to the supercritical state. The nonsupercritical component may also be injected into an etching chamber while the substrate is being exposed to the supercritical component. By varying combinations of pressure and temperature, variable etching characteristics can be obtained. For example, increasing pressure on the supercritical etching composition causes its density to increase, thus increasing its solvent strength. Increasing the temperature of the supercritical etching composition normally increases the etching rate of the composition due to increasing the kinetic energy of the etching species. 
     The nonsupercritical etching component is selected according to known criteria in the art, some of which are discussed above, to provide effective removal of material from a substrate. Preferably, the nonsupercritical etching component is selected from the group of ammonia (NH 3 ), hydrofluoric acid (HF), phosphoric acid (H 3 PO 4 ), nitric acid (HNO 3 ), acetic acid (CH 3 COOH), hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ), hydrogen peroxide (H 2 O 2 ), nitrous oxide (N 2 O), nitrogen trifluoride (NF 3 ), sulfur hexafluoride (SF 6 ), ozone (O 3 ), sulfur dioxide (SO 2 ), sulfur trioxide (SO 3 ), amines (NR 3 ), ammonium salts (e.g., tetramethyl ammonium hydroxide (TMAH), ammonium fluoride (NH 4 F), etc.), hexafluoroacetylacetone (C 5 H 2 F 6 O 2 ), and combinations thereof. Chemical constituents in the nonsupercritical etching component vary. For example, ammonia (NH 3 ) can be used when it is desired to remove titanium nitride (TiN), aluminum (Al), and aluminum/copper (Al/Cu) alloys from a substrate. Hydrofluoric acid (HF) can be used when it is desired to remove silicon (Si), thermally grown silicon dioxide (SiO 2 ), chemical vapor deposited SiO 2 , or metals, such as titanium (Ti) or Al/Cu alloys from a substrate. Phosphoric acid (H 3 PO 4 ) can be used when it is desired to remove silicon nitride (SiN) or Al from a substrate. Nitric acid (HNO 3 ) can be used when it is desired to remove Si or Al from a substrate. Acetic acid (CH 3 COOH) can be used when it is desired to remove Al from a substrate. 
     It is desirable to be able to selectively control the material being etched. For example, although CO 2  is known to remove photoresist material when used in a supercritical state, the supercritical etching composition of the present invention can be tailored using components other than CO 2  to etch exposed material beneath a patterned photoresist layer, without simultaneously etching the photoresist layer. Likewise, when more than one inorganic material is present at an exposed surface of a substrate, the supercritical etching composition can be tailored to effectively etch only one, or only selected layers, of the materials. 
     In another embodiment, the supercritical component is capable of etching in and of itself. For example, the supercritical component is an etchant for the particular inorganic material in the supercritical state, such as ammonia (NH 3 ), hydrofluoric acid (HF), acetic acid (CH 3 COOH), nitrous oxide (N 2 O), nitrogen trifluoride (NF 3 ), hydrochloric acid (HCl), hydrobromic acid (HBr), water (H 2 O), and combinations thereof. Chemical components in the supercritical component of this embodiment are selected according to criteria well known in the art, as described above, to effectively etch the particular inorganic material. Numerous wet etchants are convertible into the supercritical state by subjecting them to a temperature or pressure above their critical temperature or critical pressure, respectively. By varying combinations of pressure and temperature used in converting etchants to the supercritical state, variable etching characteristics can be obtained, as described previously. Supercritical etching compositions according to this embodiment can also include nonsupercritical components. For example, nonsupercritical etching components can also be added to the supercritical component. 
     In a further embodiment of the invention, the supercritical etching composition can further include components, such as oxidizers (e.g., hydrogen peroxide (H 2 O 2 ), nitrogen trifluoride (NF 3 ), ozone (O 3 ), oxygen (O 2 ), halogens (e.g., F 2 , Cl 2 , Br 2 , I 2 ), sulfur dioxide (SO 2 ), sulfur trioxide (SO 3 )), buffering agents (e.g., ammonium fluoride (NH 4 F), tetramethyl ammonium fluoride (Me 4 NF)), surfactants, selectivity enhancers (e.g., tetramethyl ammonium hydroxide (Me 4 NOH), tetramethyl ammonium fluoride (Me 4 NF), and ammonium fluoride (NH 4 F)), or ligands (e.g., beta-diketones, fluorinated-diketones, organic acids, and the like) to enhance the etching effectiveness. In particular, the addition of ligands promotes removal of metal from a substrate. The addition of ligands in the supercritical etching composition is particularly beneficial for etching HDC materials. These components, as well as many others, can be added to the supercritical etching composition. These components may or may not be in the supercritical state. Thus, they may be supercritical components or nonsupercritical components. 
     According to the present invention, a substrate having material that is to be removed (e.g., a semiconductor-based substrate with an inorganic material of an exposed surface) is exposed to the supercritical etching composition. A preferred process sequence utilizes an apparatus preferably customized for the particular supercritical etching composition. For example, the pressure vessel  14  of FIG. 1 is a useful tool in which to expose the substrate  16  to the supercritical etching composition. The pressure vessel  14  includes a chuck  18  for supporting and/or rotating the substrate  16 . Mass or liquid flow controllers  20  control the input of each component of the supercritical etching composition. For example, a supercritical component (prior to being brought to the supercritical state) and a nonsupercritical component (if used) each enter the pressure vessel  14  directly through a separate controller  20 . It should be understood, however, that an apparatus can be used that allows for the mixing of the components prior to their entering the pressure vessel  14 . 
     As illustrated in FIG. 1, the components can be pre-mixed in a mixing manifold  22 . The components can also be brought to the supercritical state in the mixing manifold  22  by including heating coils or vanes in the manifold  22  walls to increase heat transfer to the components before passing through an optional circulation heater  24 , such as a circulation heater supplied by Watlow Co. of Saint Louis, Mo. The supercritical etching composition then enters the pressure vessel  14  through a dispensing device  26 , such as a shower-head. The circulation heater  24  is preferably used to increase the efficiency of the system when creating the supercritical etching composition in the mixing manifold  22  prior to entering the pressure vessel  14 . The supercritical etching composition passes over the substrate  16  from the dispensing device  26 . 
     It is also to be understood that the supercritical etching composition can optionally pass directly over the substrate  16  from a heated transfer line without using a dispensing device  26 . Alternatively, each component of the supercritical etching composition can enter the pressure vessel through plumbed supply lines, where the components can then be brought to the supercritical state. The supercritical etching composition is passed over the substrate  16  until the desired thickness has been etched away. 
     Temperature and pressure within the pressure vessel  14  should be at or above the critical temperature and pressure for the supercritical component. The etching component is then dissolved in the supercritical composition as a nonsupercritical component or is itself in the supercritical state as part of the supercritical component, depending on the temperature and pressure within the pressure vessel  14 . A thermocouple  28  monitors temperature within the pressure vessel  14  and relays an appropriate signal to a temperature control unit  30 . The temperature control unit  30  sends appropriate signals to heater inputs  32 , which provide heat to the pressure vessel  14 . A pressure gauge  34  monitors pressure within the pressure vessel  14  and sends appropriate signals to a pressure control unit  36 , which pressurizes/depressurizes the pressure vessel  14 . Excess composition is vented or pumped through a vent or pumping system in the direction indicated by arrow  38 . The supercritical etching composition passes from the pressure vessel  14  to the vent or pumping system  38  by operation of a pressure differential between adjacent chambers. The pressure in the vent or pumping system  38  is lower than the pressure within the pressure vessel  14  causing this transfer. 
     Alternatively, a tool as illustrated in FIG. 2 can be useful for removing material from a substrate  16  using supercritical etching compositions of the invention. The tool illustrated in FIG. 2 is similar to the tool illustrated in FIG. 1, but the supercritical component (prior to being brought to the supercritical state) first flows from mass or liquid flow controller  40  through circulation heater  24  prior to entering the mixing manifold  22 , where any nonsupercritical components enter through mass or liquid flow controllers  42 . When the supercritical component is capable of etching a particular inorganic material in and of itself, and when using the tool illustrated in FIG. 2, a nonsupercritical etching component need not be added to the mixing manifold  22  from mass or liquid flow controllers  42 , although the supercritical component is supplied from mass or liquid flow controller  40 . 
     A wide variety of inorganic materials can be effectively etched using the compositions and methods of the present invention. For example, silicon (Si), thermally grown silicon dioxide (SiO 2 ), chemical vapor deposited SiO 2 , silicon nitride (Si 3 N 4 ), borophosphosilicate glass (BPSG), titanium nitride (TiN), titanium (Ti), aluminum (Al), copper (Cu), and similar materials used in semiconductor processing, and combinations thereof, can be removed in accordance with the present invention. Also, evolving high dielectric constant (HDC) materials (i.e., those having a dielectric constant of greater than about 7) are advantageously etched in accordance with the present invention. Known HDC materials include: tantalum pentoxide (Ta 2 O 5 ), yttria (Y 2 O 3 ), titanium dioxide (TiO 2 ), strontium bismuth titanate (SBTi), lead zirconate titanate (PZT), lanthanum-doped lead zirconate titanate (PLZT), barium strontium titanate (BST), bismuth titanate (BiTO); strontium titanate (STO), barium titanate (BaTO), and tantalates, such as strontium bismuth tantalate (SBTa). For example, Ta 2 O 5  potentially has a dielectric constant (∈) more than 20 times greater than conventional silicon oxide, which has a dielectric constant (∈) of 3.9. BST has a dielectric constant (∈) about 100 times as large as that of a conventional thermally grown silicon dioxide (SiO 2 ) film or a silicon nitride (Si 3 N 4 , having a ∈ of about 7) film. 
     As consumer demand for smaller computing devices and increased memory storage therein increases, it is important to effectively adapt HDC materials for use in fully automated fabrication of capacitive memory cells. Previously, for example, such HDC materials were difficult to remove at relatively low temperatures. Furthermore, such HDC materials could only be removed using a wet etchant solvated in strong organic solvents. It is desirable to decrease the use of relatively high temperature processing steps for relatively long periods of time. A process sequence in the fabrication of semiconductor integrated circuits (ICs) ideally maintains a relatively low thermal budget. The thermal budget for an IC process sequence is that combination of time and temperature for heat treatments utilized in the fabrication of the IC, above which electrical performance of the IC is possibly degraded. An IC can only be subjected to a limited number of thermal steps for a limited amount of time before its electrical performance is potentially detrimentally affected. For example, thermal steps often cause dopant gradients at junctions between two regions in an IC to diffuse, such that the potential barrier between the two regions is altered. Furthermore, thermal steps often cause dopants to migrate into undesired regions, altering device characteristics. Because access transistors are formed prior to the capacitor in many dynamic random access memory (DRAM) devices, it is desirable to use relatively low temperatures to form the capacitor because higher temperatures cause dopants to diffuse, forming dopant gradients, across the many junctions therein. 
     Thus, due to the etching temperatures of this invention being relatively low, such as slightly above 31° C. when using a CO 2  supercritical component, the thermal budget is conserved. Other examples of relatively low critical temperatures and their respective critical pressures for various supercritical components include: N 2 O (36.5° C., 7.26 MPa), C 2 H 6  (32.2° C., 4.88 MPa), CH 3 F (44.6° C., 5.88 MPa), HBr (90° C., 8.56 MPa), and HCl (51.4° C., 8.32 MPa). A supercritical component is selected accordingly. For example, when substrates are not as thermally sensitive, supercritical components having higher critical temperatures can be used. For example, supercritical components having higher critical temperatures include: H 2 O (374.1° C., 22.11 MPa), methanol (240° C., 10.23 MPa), ammonia (132.5° C., 11.40 MPa), and dimethyl amine (164.6° C. 5.31 MPa). 
     Supercritical etching compositions can be environmentally advantageous over current chemistries, particularly where CO 2  is the supercritical component. CO 2  is much less toxic than standard wafer cleaning strategies. Supercritical fluids offer high purity where etchants are converted to a supercritical state without the need for a non-etching supercritical component. Supercritical fluids are also capable of being relatively easy to recycle. Due to the high rate of penetration that is characteristic of supercritical fluids as compared to wet etchants, etching in accordance with the present invention is more uniform. Uniformity is promoted due to the superior wetting abilities of supercritical etching compositions, along with their ability to break away etched material from within etched or partially etched openings on the substrate surface. Another desirable advantage of some of the supercritical etching compositions and methods of the present invention is that aqueous solvent systems and alcohols can be eliminated from the etching step. By eliminating such wet solvents, a drying step is not needed in the IC processing sequence subsequent to etching steps in accordance with the present invention. Elimination of such drying steps contributes to optimizing efficiency in the fabrication of ICs. 
     The following examples are meant to be illustrative of numerous embodiments of practicing the invention. They are meant to be understood as examples only, and not limiting on the scope of the invention. 
     EXAMPLES 
     Example 1 
     In this example, the supercritical component is capable of etching in and of itself. The tool of FIG. 1 is heated to above the critical temperature (188° C.) of HF. The pressure vessel  14  is then charged with electronic grade HF at a pressure of 6.58 MPa. A desired flow of the supercritical etching composition, HF, flows into the pressure vessel  14  from a mass flow controller  20 . A desired pressure is maintained in the pressure vessel  14  using the pressure controller  36  with a feedback loop coupled to the venting outlet valves  38  of the pressure vessel  14 . 
     Example 2 
     A substrate  16  having a surface layer of copper is placed on the chuck  18  in the tool of FIG.  2 . The chuck  18 , pressure vessel  14  wall, and circulation heater  24  are ail heated to 60° C. The supercritical component is CO 2 . A flow of 100 sccm CO 2  is established through the system from mass flow controller  40 , with pressure controlled to 10.13 MPa (100 atm) using pressure controller  36 . A nonsupercritical etching component hexafluoroacetylacetone (C 5 H 2 F 6 O 2 ), is introduced into the mixing manifold  22  using a syringe pump, at a rate of 0.90 mL/minute to form the supercritical etching composition. Thus, the supercritical etching composition comprises dissolved C 5 H 2 F 6 O 2 . The supercritical etching composition is passed over the substrate  16  from a shower-head  26  until a desired thickness of copper is removed. 
     Example 3 
     A substrate  16  having a surface layer of thermally grown silicon dioxide (SiO 2 ) is placed on the chuck  18  in the tool of FIG.  2 . The chuck  18 , pressure vessel  14  wall, and circulation heater  24  are all heated to 375° C. The supercritical component in this example, deionized water, is capable of etching in and of itself. Deionized water is provided to the circulation heater  24  from liquid flow controller  40  at a flow rate of 10 mL/minute, with an established pressure of 22.29 MPa (220 atm), to form the supercritical etching composition. The supercritical etching composition is passed over the substrate  16  from shower-head  26 . Flow of the supercritical etching composition under these conditions is maintained until a desired thickness of the oxide has been removed. 
     All patents, patent applications, and publications disclosed herein are incorporated by reference in their entirety, as if individually incorporated. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for variations obvious to one skilled in the art will be included within the invention defined by the claims.