WO2000072368A9

WO2000072368A9 - Process for etching a silicon wafer

Info

Publication number: WO2000072368A9
Application number: PCT/US2000/012481
Authority: WO
Inventors: Anca Stefanescu; Judy Schmidt; Henry F Erk; Tom Doane
Original assignee: Memc Electronic Materials
Priority date: 1999-05-21
Filing date: 2000-05-05
Publication date: 2002-04-18
Also published as: TW476114B; WO2000072368A1

Abstract

A process for etching a silicon wafer is disclosed in which the oxidizing agent of the aqueous etching solution is reconditioned or regenerated by re-oxidation, rather than simply adding fresh reagent. The present process affords the means by which to more consistently obtain a silicon wafer having improved gloss or smoothness, while minimizing the amount of silicon removed from the wafer surface.

Description

PROCESS FOR ETCHING A SILICON WAFER

BACKGROUND OF THE INVENTION

The process of the present invention generally relates to the etching of semiconductor wafers. More particularly, the present invention relates to a process for etching a silicon wafer in a solution which may be reconditioning or regenerating, such that a silicon wafer surface having improved gloss or smoothness may be more consistently obtained. Semiconductor wafers, such as silicon wafers, are typically obtained from single crystal silicon ingots by a process which includes a number of steps. First, the single crystal silicon ingot is sliced in a direction normal to the axis of the ingot to produce thin wafers. These wafers are then subjected to a lapping process to planarize the front and back surfaces of the wafer and to ensure uniform thickness. Following the lapping process, the surfaces of the wafers may be ground to further reduce surface roughness . The wafers are then etched to remove the mechanical damage created by the sawing, lapping and grinding steps, and to remove any embedded lapping grit. Finally, the etched surfaces of the wafer are polished.

To-date, both acidic and caustic chemical formulations have been utilized for purposes of etching the surface of a silicon wafer. One of the most common acidic etchant formulations comprises a solution of hydrofluoric acid (HF) , nitric acid (HN0₃) , and water (hereinafter "HN0,-based etchants"). Caustic solutions typically comprising one or more alkaline hydroxides, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH) , and water (hereinafter "OH-based etchants").

Such formulations, however, have disadvantages which compromise their effectiveness or limit their utilization in commercial wafer manufacturing processes . For example, while HN0₃-based etchants are preferred in some instances because they yield a somewhat smooth wafer surface, they are also problematic because they are prone to the formation of unwanted solid-phase chemical species on the etched surface of the wafer, which create stains that inhibit further reaction and produce inconsistent etching results. (See, e.g., D.G. Schimmel et al . , "An Examination of the Chemical Staining of Silicon", J. Electrochem. Soc . , Vol. 125, p. 152-155 (1978).) HN0₃- based etchants also react during the etching process to produce toxic gases containing oxides of nitrogen (NO_x) , necessitating the use of safety controls and special disposal procedures. Finally, in order to obtain a sufficiently smooth surface using these etchants, a relatively large amount of silicon must be removed from the wafer surfaces, typically about 10-15 μm from each surface.

Generally speaking, limiting the amount of silicon removed from the wafer surfaces is preferred in order to limit the variation in wafer thickness. Stated another way, Global Backside-referenced Indicated Range (GBIR) typically increases as the amount of silicon removed from the wafer surfaces increases. In this respect, caustic etching solutions are preferred because the amount of silicon removed from the wafer surfaces during downstream processes, relative to the acidic solutions, is much less. For example, caustic etchants typically yield wafers having a ΔGBIR (i.e., change in GBIR, as determined by comparing the GBIR of the wafer before and after etching) value of about 0.1 μm, whereas acidic etchants typically yield wafers having ΔGBIR values ranging from about 0.5-1.5 μm. A low ΔGBIR value is important because, as this value increases, more silicon must be removed from the wafer surfaces during the subsequent polishing step in order to ensure the final GBIR is acceptable.

Notwithstanding the foregoing advantages, the hydroxide-based etchants have traditionally not been widely utilized in conventional manufacturing processes because these etchants produce a rougher wafer surface than acid etchants. This rougher surface is visible, having a fish scale appearance due to preferential etching along crystallographic planes.

Etchants containing Cr⁶⁺ (such as chromate (Cr04^2~) , dichromate (Cr₂0₇ ^2~) or chromium trioxide (Cr0₃) ) , hydrofluoric acid and water have been proposed as an alternative to the above-noted hydroxide and HN0₃-based etchants. Significantly, these chromium oxide-based etchants can achieve a smoothness equivalent to that of the HN0₃-based etchants, while yielding a ΔGBIR similar to that of the caustic etchants (removing only about 2-5 μm from each side of the wafer surfaces) .

Although the chromium oxide-based etchants can produce a smooth surface while removing substantially less silicon than the HN0₃-based etchants, it also has a number of disadvantages. For example, due to the hazardous nature of chromium, precautions must be taken to limit environmental and human exposure. More importantly, however, is that unlike the HN0₃-based etchants, chromium oxide-based etchants cannot be reconditioned by the simple addition of fresh reagents. The continuous addition of a chromium oxidizing agent to the etchant solution results in the gradual build-up of chromium salts in the etching bath, which ultimately reduce the oxidant capacity in the bath. Acid etchants utilizing the permanganate ion (Mn0₄ ^~) as an oxidizer have previously been reported. (See, e.g., U.S. Pat. Nos. 2,847,287 and 5,445,706.) Experience to-date suggests these etchants are preferred over the noted chromium oxide-based etchants because they provide similar results in terms of the amount of silicon removed and the resulting surface smoothness, while being much safer to handle and utilize in a production environment .

Permanganate-based etchants, however, have not been widely utilized in conventional manufacturing processes for a number of reasons. First, such etchants can form stains on the surface of the wafer under certain conditions (see K.S. Nahm et al . , "Formation mechanism of stains during Si etching reaction in HF-oxidizing agent- H₂0", J. Appl. Phys., Vol. 81, No. 5, March 1, 1997, p.

2418-2424) , although stain formation is still less likely than when HN0₃-based etchants are used. The primary disadvantage however is that, much like the chromium oxide-based etchants and unlike the HN0₃-based etchants, a permanganate-based etchant bath cannot be reconditioned by simple addition. The continuous addition of permanganate results in the gradual build-up of manganese and potassium reaction byproducts and, ultimately, reduces the oxidation capacity in the bath. In addition, the manganese and potassium reaction byproducts can produce inconsistent etching results by precipitating out of solution and depositing on the wafer surface. The precipitated compounds mask the wafer surface from the etching action and results in a non-uniform surface.

Masking of the wafer surface can also be caused by the adherence of bubbles which are byproducts of the etching reaction (e.g., H₂ or N0_X in the case of HN0₃- based etchants) . Furthermore, such bubbles are also believed to have an effect on the kinetic resistance and the mass transfer resistance of the etching reaction. (See, e.g., M.S. Kulkarni and E.F. Erk, "Wet Etching of Silicon Wafers: Transport and Kinetic Effects," Paper 124f, AIChE Conference, Los Angeles (1997).)

In view of the forgoing, a need continues to exist for a chemical etchant that can be safely used in the commercial production of silicon wafers, that consistently produces a substantially smooth wafer surface without excessive removal of silicon or staining of the wafer surface, and that can be reconditioned to ensure prolonged periods of use .

SUMMARY OF THE INVENTION

Among the objects of the present invention may be noted the provision of a process for etching the surface of a silicon wafer; the provision of such a process wherein the etchant solution employed provides improved safety; the provision of such a process wherein the etching solution has improved stability; the provision of such a process wherein the etching solution may be reconditioned to ensure prolonged periods of use, thus avoiding the build-up of unwanted salts and other contaminants; the provision of such a process wherein the surfaces of the silicon wafer are not stained; the provision of such a process wherein micro-etching of the wafer surface may be achieved; and, the provision of a process wherein such a solution may be utilized to consistently obtain a wafer surface with improved gloss and smoothness .

Briefly, therefore, the present invention is directed to a process for removing silicon from a surface of a silicon wafer. The process comprises contacting the surface of the silicon wafer with an aqueous etch solution comprising hydrofluoric acid and an oxidizing agent selected from the group consisting of chromium trioxide or a compound capable of forming permanganate ions, chromate ions or dichromate ions in solution, as well as mixtures thereof, the oxidizing agent being reduced during removal of silicon from the wafer surface, and reconditioning the aqueous etch solution by re- oxidizing the reduced agent. Other objects and features will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a photograph of a Nomarski image of a silicon wafer surface etched in a permanganate-based solution to which a surfactant was added in accordance with the process of the present invention.

Figure 2 is a photograph of a Nomarski image of a silicon wafer surface etched in a permanganate-based solution having the same composition as that used to etch the wafer of Figure 1, without the surfactant being added.

DETAILED DESCRIPTION OF THE INVENTION

Mechanical damage to the surfaces of a semiconductor wafer, such as a single crystal silicon wafer, resulting from slicing, lapping and grinding is typically removed by chemical etching. In accordance with the process of the present invention, the surface of the wafer is etched with an aqueous solution comprising hydrofluoric acid (HF) and an oxidizing agent which consistently yields a wafer surface having improved gloss and smoothness. Generally speaking, suitable oxidizing agents for the present invention are manganese and chromium based; more specifically, suitable oxidizing agents include those capable of forming permanganate ions (Mn0₄ ^") , chromate ions (Cr0₄ ^2~) , and dichromate ions (Cr₂0₄ ^2~) in solution, as well chromium trioxide (Cr0₃) and mixtures thereof. Oxidizing agents capable of forming permanganate ions in solution, such as potassium permanganate (KMn0₄) or sodium permanganate (NaMn0₄) , are preferred, in part due to their less hazardous nature. The aqueous etching solution of the present invention typically comprises between about 10 and about 50 weight percent hydrofluoric acid, and between about 0.2 and about 6 weight percent of an oxidizing agent. Preferably, the aqueous solution comprises between about 30 and about 40 weight percent hydrofluoric acid, and between about 1 and about 2 weight percent of an oxidizing agent. Most preferably, however, the aqueous solution comprises between about 30 and about 35 weight percent hydrofluoric acid and between about 1 and about 1.5 weight percent of an oxidizing agent. In one preferred embodiment, the aqueous solution comprises about 33 weight percent hydrofluoric acid and about 1 weight percent potassium permanganate. Usually, the hydrofluoric acid and oxidizing agent are dissolved in water forming an aqueous HF solution and an aqueous oxidizing agent solution and then the solutions are mixed together to produce an etching solution with the desired composition. For example, a typical aqueous HF solution will consist essentially of water and about 50 weight percent hydrofluoric acid, whereas the oxidizing agent is generally added to the etching solution as a 1 N aqueous solution. The two solutions are then mixed to form an etching solution wherein the weight ratio of oxidizing agent to hydrofluoric acid is from about 0.01 to about 0.1. Preferably, the weight ratio of oxidizing agent to hydrofluoric acid is from about 0.03 to about 0.05. The ratio of oxidizing agent to hydrofluoric acid determines the etch rate of the solution, the gloss and roughness of an etched wafer. However, it is to be understood that the concentration of hydrofluoric acid and the oxidizing agent in the present aqueous etching solution may be other than that herein described without departing from the scope of the present invention. Referring now to Equations (1) and (2) , the process of the present invention will be further described, wherein potassium permanganate is utilized as the oxidizing agent. Without being held to any particular theory, it is generally believed that etching proceeds with the potassium permanganate (KMn0₄) , or rather permanganate ion (Mn0₄ ^") , oxidation of silicon (Si) on the wafer surface to form silicon dioxide (Si0₂) . The silicon dioxide is then dissolved by the hydrofluoric acid (HF) .

12HF + 4KMn0₄ + 5Si « 5Si0₂ + 4MnF₂ + 6H₂0 + 4KF (1) Si0₂ + 6HF - H₂SiF₆ + 2H₂0 (2)

The etchant solution of the present invention may be employed in a number of different techniques common in the art in order to etch the wafer surface. For example, one technique, referred to as spin etching, is disclosed in U.S. Patent No. 4,903,717. The spin etching technique comprises rotating the wafer while a continuous stream of etchant is applied to the top of the wafer. Another technique is spray etching, wherein a continuous spray of etchant is applied to the wafer surface. Preferably, however, the etching process of the present invention comprises partially, and more preferably fully, immersing the wafer into a bath of the etchant solution. (See, e.g., U.S. Patent No. 5,340,437.) Although one wafer at a time may be immersed in the solution, preferably a number of wafers (e.g., 25 or more) will be assembled in a cassette, or wafer carrier, and immersed at the same time in the solution. When such a carrier is used, however, certain portions of each stationary wafer will be in constant contact with the carrier, resulting in nonuniform etching across the surface of each wafer. To eliminate this problem and provide a more uniform result over the entire wafer surface, the wafers are preferably rotated while immersed in the etchant solution.

Furthermore, because the wafers are closely spaced, typically between about 4 mm to about 7 mm apart, rotation of the wafers tends to produce a rigid-body rotation of the liquid between the wafers. As a result, stagnation of the etchant solution between the wafers typically occurs. Stagnation is a concern because acid etching of silicon is believed to be at least in part dependent upon the mass transfer rate at the silicon- etchant interface. As the etching reaction proceeds, the concentrations of acid and oxidizing agent decrease at the interface and the concentration of reaction products increases. Accordingly, nonuniform etching results may be obtained not only across the surface of a given wafer, but also from one wafer surface to the next within the set of wafers in the wafer carrier.

In order to produce uniformly etched wafers and to ensure consistent results from one set of wafers to the next, it is preferred that the etchant solution be continuously mixed or agitated for the duration of the etching process. Bath agitation or mixing may be achieved by means known in the art, such as by employing ultrasonic agitation, stirring devices and pumps. Preferable, however, agitation is achieved by passing or "bubbling" a gas through the etchant solution (see, e.g., U.S. Patent No. 5,340,437). Generally, any gas which will not react with the wafer surface may be employed, including elemental gases (e.g., hydrogen, nitrogen, oxygen), noble gases (e.g., helium or argon) or compound gases (e.g., carbon dioxide).

It is to be noted that, in addition to the gas bubbles introduced into the etchant solution as a result of gas agitation, gas bubbles may also be formed via the etching reaction itself. More specifically, as the etchants of the present process react with the wafer surface, hydrogen gas evolves, creating hydrogen bubbles in the etching bath. These bubbles tend to adhere to the wafer surface and may interfere with the action of the etchant, resulting in nonuniform etching and possibly surface staining. (See, e.g., Kulkarni et al . , AIChE Conference, Paper 124f, AIChE Conference, Los Angeles (1997) .)

The effects of these bubbles can be minimized, however, by the addition of a surfactant to the etchant solution. For example, referring now to Figs. 1 and 2 (which are photographs of portions of the surfaces of two wafer, reproduced by Nomarski imaging by means standard in the art) , it can be observed that the surface of the wafer in Fig. 2 (which was etched in a solution without a surfactant) clearly has circular or semi-circular imprints, left behind by bubbles present in the solution. In contrast, it can be observed that the surface of the wafer in Fig. 1 (which was etched in a solution comprising a surfactant) does not have these imprints. Without being held to any particular theory, it is generally believed that the surfactant acts as a wetting agent, reducing the surface tension of the aqueous solution on the surface of the wafer and thus preventing the gas bubbles from adhering thereto. Furthermore, it is believed that the surfactant stabilizes the size of the bubbles in the bath, which also helps to produce a smoother and more uniform surface and thus provide more consistent etching results.

Any surfactant that is stable in the presence of the oxidizing agent of this invention can be added to the etching solution. For example, a potassium fluoroalkyl carboxylate surfactant sold under the trade designation FC-129 (commercially available from 3M Corporation; St. Paul, MN) , or sodium dodecyl sulfate can be added to the etchant solution. Experience to-date, however, suggests that a smoother, more uniformly etched surface may be obtained if fluoroalkyl sulfonate surfactants, such as ammonium perfluoroalkyl sulfonate and potassium perfluoroalkyl sulfonate (sold under the respective trade designations FC-93 and FC-95; commercially available from 3M Corporation; St. Paul, MN) are added to the solution. When added to the etching solution, generally speaking a quantity of surfactant will be used which is sufficient to prevent the adherence of gas bubbles on the surfaces of the wafer. As further described in the Examples, below, wafers may be analyzed in a way which allows for the clear detection of imprints left by bubbles which adhere to the wafer surfaces. Typically, the aqueous etch solution comprises about 0.05 to about 1 weight percent of the surfactant. Preferably, the etch solution comprises about 0.1 to about 0.5 weight percent, and more preferably from about 0.15 to about 0.25 weight percent of the surfactant. In one preferred embodiment, the aqueous etch solution comprises about 0.2 weight percent of a fluoroalkyl sulfonate surfactant. It is to be understood, however, that the concentration of surfactant in the present aqueous etching solution may be other than that herein described without departing from the scope of the present invention.

As previously noted, a heretofore known limitation of etchant solutions employing the oxidizing agents of the present invention is the inability to regenerate or recondition these solutions. The introduction of additional reagents results in the build-up of salts in the etch bath which interfere with the etching process. This interference may be due to the salts becoming deposited on the wafer surface, thus acting as a mask and causing nonuniform results, or the salts may act to reduce the oxidizing capacity of the reagents.

Without being held to any particular theory, experience to-date suggests that the etchants of the present invention may be regenerated or reconditioned by restoring the oxidation state of the reagents, or more specifically the ions, responsible for oxidizing the surface of the silicon wafer as part of the etching process. For example, referring specifically to permanganate-based etchants, it is to be noted that a freshly prepared etchant solution is typically transparent with a deep purple hue. This purple hue is believed to be attributable to the presence of permanganate ions in solution. With the passage of time, and as the number of wafers etched in the solution increases, the color of this solution changes, typically becoming purple/brown. It is believed that this color change reflects the degradation of the etchant solution due to a consumption of permanganate ions in solution. The consumption of permanganate ions in solution, and thus the degradation of the etchant, is generally believed to be attributable to three potential causes. First, as mentioned above, a portion of the etching reaction entails oxidizing the silicon on the wafer surface with the permanganate-based agent or, more specifically, the permanganate ions (Mn0₄ ^~) , to form silicon dioxide. The formation of silicon dioxide may be explained by the Equations (3) , (4) and (5) , below:

6H⁺ + 4Mn0₄ ^" + 5Si •* 5Si0₂ + 4Mn²⁺ + 60H^" (3 )

Mn⁷⁺ → Mn ⁺ (4 )

Si⁰ → Si⁴⁺ ( 5 )

As can be seen from these Equations, during the oxidation of the silicon at the wafer surface, the oxidation state of the solute manganese is reduced from +7 to +2 and the oxidation state of the silicon is increased from 0 to +4. As the etching reaction proceeds, the concentration of Mn⁷⁺ ions in the bath decreases resulting in the diminished oxidation/etching ability of the bath. However, while the oxidation state of manganese will typically be reduced directly from +7 to +2 , a portion of the manganese may alternatively be reduced from +7 to +4. It is believed that manganese in the +4 oxidation state in the etching solution tend to form manganese dioxide (Mn0₂) . Referring now to Equations (6) , (7) and (8) , the formation of manganese dioxide in the etching process may be explained as follows :

4KMn0₄ + 3Si +4HF ** 3Si0₂ + 4Mn0₂ + 2H₂0 + 4KF (6)

Mn⁷⁺ → Mn⁴⁺ ( 7 )

S i⁰ → Si⁴⁺ ( 8 )

The formation of manganese dioxide is generally believed to degrade the etching bath because it can precipitate out of solution and prevent the manganese from further oxidizing silicon. Additionally, precipitated manganese dioxide can be deposited on the wafer and mask the surface from the etching action.

A second potential cause of etchant degradation is the strong ionizing power of water. It is generally believed that water molecules can slowly break down the permanganate ion (Mn0₄ ^~ ) into Mn0₃ ^2" and O^2" ions, or into Mn0₂ and 0₂. This phenomenon usually occurs in dilute acidic solutions, such as the etchant solutions of the present invention.

Lastly, the third potential cause of etchant degradation is believed to be the tension exerted by water molecules upon the permanganate ions. It is generally believed that this tension can cause the breakdown of these ions. In dilute solutions, such as the etchant solutions of the present invention, permanganate ions can be hydrolyzed and decomposed into a colloidal solution of manganic hydroxide (MnO(OH)₂) and free oxygen. This action occurs under all conditions, but exposure to ultraviolet light and sunlight has been reported to dramatically increase the rate of hydrolyzation .

In accordance with the process of the present invention, the etchant solution may be reconditioned or regenerated by restoring the oxidizing ability of agents in the solution. For example, the permanganate-based etchants are reconditioned by increasing the oxidation state of degraded (i.e., reduced oxidation state) manganese to the +7 oxidation state, which in turn results in the reformation of permanganate ion (Mn0₄ ^") . Therefore, the addition of fresh reagents may be avoided, along with the related build up of salts and other unwanted contaminants . Generally speaking, the etch solution may be reconditioned or regenerated by contacting the spent or reduced oxidizing agent or solute with any agent capable of returning the oxidation state of the solute to its original level. For example, theoretically the spent agent may be re-oxidized simply by contacting the etch solution with oxygen. Preferably, however, the spent agents, and thus the etching solution itself, is reconditioned in one embodiment of the present invention by contacting the etchant solution with ozone. Typically, the dose of ozone introduced into the solution is greater than the dose which is the stoichiometric equivalent of the spent oxidizing agent or solute; that is, reconditioning of the etchant solution is achieved by introducing into the solution a quantity of ozone greater than the stoichiometric equivalent quantity, relative to the spent oxidizing agent or solute. In fact, it is preferred than a dose of about twice the stoichiometric amount or more be added to the etchant solution. It should be noted, however, that higher levels of the agent added to regenerate the spent oxidizing agent of the present invention may be required if the etchant solution contains other compounds or reagents that may also be oxidized by the agent being added.

As an example of the foregoing, to increase the oxidation state of the solute manganese, typically an amount of ozone greater than about 0.9 mg per mg of manganese is added to the etchant solution. Preferably, however, an amount greater than about 2 mg, and more preferably about 2.5 mg to about 10 mg, of ozone will be added. In this regard it is to be noted that the oxidation of Mn²⁺ to Mn⁷⁺ may be visually detected because this oxidation results in the reformation of Mn0₄ ^", which results in a color change in the bath from purple-brown to purple. It is also to be noted that if an insufficient quantity of ozone is added to the solution, solute Mn²⁺ ions may be oxidized to an oxidation state less than Mn⁷⁺. These lesser oxidized manganese ions may eventually hydrolyze and form manganese dioxide, which can precipitate out of solution. Furthermore, if ozone- depleting substances such as nitrites or sulfides are present in the etchant solution, additional ozone may be required.

The etchant may be contacted with ozone in one of several ways by means known in the art, including: 1) injecting gaseous ozone directly into the etching bath, similar to nitrogen gas injection, or 2) by using a packed tower or hollow fiber gas-liquid contactor (commercially available from Hoechst Celanese and W.L. Gore & Associates) . The first approach is preferred if the reconditioning ozone is also being used to agitate the bath. However, the second approach may be preferred if a surfactant is present in the etch bath because the bubbling action caused by gas injection may result in excessive foaming, which is detrimental to the etching process .

It is to be noted that chemical oscillators, (see, e.g., A. Nagy "Design of a Permanganate Chemical

Oscillator with Hydrogen Peroxide", J. Phys . Chem. , vol. 93, pp. 2807-28 (1989)) may be added to the etchant solution as a means by which to prevent the formation of manganese dioxide precipitates . Without being held to any particular theory, it is generally believed that chemical oscillator agents are capable of complexing with or binding to colloidal manganese dioxide to prevent its precipitation, thus allowing all manganese oxidation states to remain in solution. Such agents could be utilized in conjunction with, for example, ozone to further prolong the lifetime of the etchant solution.

As an alternative to the use of chemical oscillators, phosphoric acid (H₃P0₄) may be added to the etchant solution of the present invention. More specifically, phosphoric acid may be added to the solution to prevent manganese dioxide from precipitating in much the same way as the above-noted chemical oscillators. Accordingly, a quantity of phosphoric acid will be added which is sufficient to complex with the manganese dioxide that is formed and maintain it in solution. More specifically, the originally prepared etchant will comprise about 1 to about 10 weight percent phosphoric acid. Preferably, however, the amount of phosphoric acid added is about 1 to about 5 weight percent. The phosphoric acid is generally added to the etchant as an aqueous solution, such as an 85 weight percent H₃P0₄ solution.

It is believed that, as an alternative embodiment for reconditioning the oxidizing agents of the present invention, potassium persulfate and sulfuric acid may be added to the etchant solution. Without being held to a particular theory, the proposed regeneration mechanism of the permanganate-based etchant solution is represented by Equations (9) through (12), below.

K₂S₂0_a + H₂S0₄ → H₂S₂0₈ + K₂S0₄ ( 9 )

4H₂0 + 3H₂S₂0₈ + 2Mn0₂ → 2HMn0₄ + 6H₂S0₄ ( 10 )

( S₂ ⁷⁺0₈ ) ^{2 "} → 2 ( S⁶⁺0₄ ) ^{2 "} ( 11 )

(Mn⁴⁺0₂ ) → (Mn⁷⁺0₄ ) ^" ( 12 )

Referring to Equation (9) , the potassium persulfate and the sulfuric acid react to form peroxydisulfuric acid and potassium sulfate. In Equation (10), the peroxydisulfuric acid oxidizes the manganese dioxide to form hydrogen permanganate and sulfuric acid. The oxidation of Mn²⁺ to Mn⁷⁺ results in the reformation of Mn0₄ ^" which is visible by the color change in the bath from purple-brown to purple.

In view of the above, the potassium persulfate and sulfuric acid would typically be added in about equal molar amounts. Furthermore, each of these compounds would typically be added in about a 1:1 to about 1.5:1 molar ratio, relative to the amount of manganese dioxide in solution. Accordingly, to sufficiently recondition the permanganate-based etchant, about 3 to about 10 weight percent potassium persulfate and about 1 to about 5 weight percent sulfuric acid would be added to the bath. Preferably, however, about 3 to about 5 weight percent potassium persulfate and about 1 to about 3 weight percent sulfuric acid would be added. The potassium persulfate would generally be added to the etchant in powder form (the purity of KS0₄ powder is typically about 99% or more) , while the sulfuric acid would typically be added as about a 95 weight percent H₂S0₄ aqueous solution. It is to be noted that the potassium persulfate and sulfuric acid method of reconditioning in theory would be a preferred embodiment in situations where the introduction of gaseous ozone to a permanganate-based etchant would cause excessive foaming, such as when a surfactant is present in solution.

It is to be further noted that the previous reconditioning reaction can be accomplished by directly adding peroxydisulfuric acid, which is commercially available, in place of the potassium persulfate and sulfuric acid combination. To sufficiently recondition the permanganate-based etchant in this way, typically about 2 to about 10 weight percent peroxydisulfuric acid is added. Preferably, however, about 2 to about 5 weight percent peroxydisulfuric acid is added.

The timing of the regeneration step, or rather the timing of the addition of reagents to re-oxidize the spent oxidizing agent or solute, is at least in part a function of the type of regeneration process employed. For example, generally the addition of these reagents may be timed based on the visual appearance of the etch solution, the addition occurring as the solution color changes from purple in color to a purple-brown. Alternatively, to ensure a more efficient process and prevent unnecessary delays, the addition may be continuous (such as when ozone is used to agitate the solution) or it may be performed after each cassette of wafers is removed from the solution. In addition, by closely monitoring wafer thickness before and after the etching process, the amount of silicon being removed may be determined. Using the above Equations, the amount of oxidizing agent being consumed may be determined, which in turn can be used to calculate the precise amount of "re-oxidizing" agent to be added at any given time.

The process of the present invention can be used to treat a wide variety of incoming semiconductor wafers. However, the etching process of this invention is preferably performed after mechanical operations upon the wafer, such as lapping and grinding, are complete. Lapping operations are performed after slicing to further flatten the wafer surface. Preferably an incoming wafer will have a GBIR value of about 1 μm. Grinding is generally performed to reduce the roughness of the lapped wafer surface. Typically, a ground wafer has a surface roughness of about 0.02 to about 0.05 μm Ra . It is to be noted, however, that the process of the present invention may be performed on wafers having other than the GBIR and roughness values as herein described without departing from the scope of the present invention.

Prior to etching the incoming wafer, it is preferred that the wafer be pre-treated, ensuring that one or more surfaces of the wafer is clean, passivated, and free of lapping and grinding residue. This pre-treatment can be accomplished by any means known in the art (see, e.g., U.S. Pat. No. 5,593,505) . Without being held to any particular theory, it is generally believed that the present invention enables a level of surface roughness to be obtained which is at least equivalent to that of nitric acid-based etchants, while removing less silicon from the wafer surface, due to the slower etching rate. More specifically, generally speaking, it is believed that the present process removes silicon from the wafer surface at a rate which is about three times slower than standard nitric acid-based etchants. Accordingly, therefore, the process of the present invention typically involves contacting the wafer surface with the aqueous etchant solution for about 1 to about 10 minutes, and preferably for about 2 to about 5 minutes. However, while a standard nitric acid-based etchant typically removes about 10-15 μm of silicon from each side of a wafer and produces a surface roughness of about 0.08 to about 0.13 μm RA, the present process achieve generally the same roughness by removing less than about 8 μm, and preferably only about 2 μm to about 5 μm, per side. The removal of less silicon from the surface is advantageous because it allows for a more uniformly flat wafer to be obtained.

While the present etching process may be utilized to obtain a surface roughness which is essentially the same as a nitric acid-based etchants, experimental evidence to-date suggest the present process may be optimized to obtained a surface roughness of less than about 0.08 μm Ra, preferably less than about 0.05 μm Ra, and most preferably less than about 0.02 μm RA. In comparison, a typical mechanochemical polishing process, wherein a polishing pad and polishing slurry are involved (see, e.g., U.S. Pat. No. 5,377,451), produces a wafer with a surface roughness of about 0.001 μm Ra. However, such standard polish processes remove about 10-15 μm silicon from the wafer surface. Accordingly, it is believed that the present "micro-etching" process (i.e., a process which provides a smooth wafer surface with minimal silicon removal) may be a potential alternative to standard mechanochemical polishing processes. More specifically, it is generally believed that a mechanochemical process utilizing a slurry comprising the permanganate-based etchants of the present invention and standard particulate matter could be employed to produce a finished wafer in less time and with less silicon removed than when standard acid etching and polishing operations are performed separately. The permanganate- based etchants could be applied as a slurry to a polishing pad in accordance with standard polishing processes. This integration of acid etching and mechanical polishing would attain a low degree of surface roughness through the combined chemical effect of the present etchants and mechanical effect of the particulate/polishing pad.

However, it is to be noted that if the present process were to be utilized as a replacement for standard polishing techniques, improvements in existing polishing pads would likely be required. Such improvements would be needed if the present process were to be so utilized for commercially practical periods of time because standard acid-resistant pads will not typically withstand the particulate abrasion which occurs on the pad surfaces for a period of time sufficient to make the process economically feasible. Likewise, pads capable of withstanding the abrasion which occurs typically cannot resist the extremely corrosive hydrofluoric acid environment. Accordingly, until polishing pad technology can produce pads with sufficient acid and abrasion resistance, the benefits of integrating the etchants of the present invention with the polishing step cannot be fully realized.

The process of the present invention is typically performed at room temperature (i.e., about 20°C to about 25°C) ; that is, typically the etching process of the present invention is carried out at room temperature and without the application of heat. Although temperatures in the range of about 25°C to about 5°C may be employed, it is to be noted that experience to-date suggests temperature generally does not plays a significant role in the present etching process.

In view of the above, it will be seen that the several objects of the invention are achieved. As various changes could be made in the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. An etching process for removing silicon from a surface of a silicon wafer, the process comprising: contacting the surface of the silicon wafer with an aqueous etch solution comprising hydrofluoric acid and an oxidizing agent selected from the group consisting of chromium trioxide or a compound capable of forming permanganate ions, chromate ions or dichromate ions in solution, as well as mixtures thereof, the oxidizing agent being reduced during removal of silicon from the wafer surface; and, reconditioning the aqueous etch solution by re- oxidizing the reduced agent.

2. The process as set forth in claim 1 wherein the oxidizing agent is a compound capable of forming permanganate ions in solution.

3. The process as set forth in claim 2 wherein the oxidizing agent is potassium permanganate or sodium permanganate .

4. The process as set forth in claim 1 wherein the etch solution is reconditioned by contacting the etch solution with ozone to re-oxidize the reduced agent.

5. The process as set forth in claim 1 wherein the etch solution additionally comprises a surfactant.

6. The process as set forth in claim 5 wherein the etch solution comprises about 0.1 to about 0.5 weight percent of a surfactant .

7. The process as set forth in claim 5 wherein the surfactant is ammonium perfluoroalky sulfonate or potassium perfluoroalkyl sulfonate.

8. The process as set forth in claim 1 wherein the etch solution comprises about 30 to about 35 weight percent hydrofluoric acid.

9. The process as set forth in claim 8 wherein the etch solution comprises about 1 to about 1.5 weight percent of the oxidizing agent.

10. The process as set forth in claim 1 wherein less than about 8 microns of silicon are removed the surface of the wafer.

11. The process as set forth in claim 1 wherein the resulting etched surface of the wafer has a surface roughness less than about 0.02 μm RA.

12. The process as set forth in claim 1 wherein the oxidizing agent is a compound capable of forming chromate ions in solution.

13. The process as set forth in claim 1 wherein the oxidizing agent is a compound capable of forming dichromate ions in solution.

14. The process as set forth in claim 1 wherein the oxidizing agent is chromium trioxide.

15. An etching process for removing silicon from a surface of a silicon wafer, the process comprising: contacting the surface of the silicon wafer with an aqueous etch solution comprising hydrofluoric acid and potassium permanganate or sodium permanganate as an oxidizing agent, the oxidizing agent being reduced during removal of silicon from the wafer surface; and, reconditioning the aqueous etch solution by contacting the solution with ozone to re-oxidize the reduced agent to its initial oxidation state.