WO1999031715A1

WO1999031715A1 - PROCESS FOR THE CONTROL OF NOx GENERATED BY ETCHING OF SEMICONDUCTOR WAFERS

Info

Publication number: WO1999031715A1
Application number: PCT/US1998/025919
Authority: WO
Inventors: Eugene C. Davis; William L. Smith
Original assignee: Memc Electronic Materials, Inc.
Priority date: 1997-12-16
Filing date: 1998-12-07
Publication date: 1999-06-24
Also published as: TW459067B

Abstract

A process for controlling the evolution of NOx during the etching of a silicon wafer, wherein the wafer is immersed in an etching solution containing nitric acid in order to etch silicon from the wafer surface. The process comprises forming an aqueous etching solution comprising nitric acid and an oxidizing agent which inhibits the evolution of NOx when a silicon wafer surface is etched in the solution, and immersing the wafer in the etching solution to etch the surface.

Description

PROCESS FOR THE CONTROL OF NO_x GENERATED BY ETCHING OF SEMICONDUCTOR WAFERS

BACKGROUND OF THE INVENTION

The process of the present invention relates generally to the control of vapors resulting from the etching of a silicon wafer. More particularly, the present invention relates to a process for controlling the evolution of N0_X, generated by the etching of a single crystal silicon wafer in an etching solution containing nitric acid, by the addition of an oxidizing agent to the etching solution.

Semiconductor wafers, such as silicon wafers, are obtained from a single crystal silicon ingot by a process which includes the steps of: (i) slicing the ingot in a direction normal to the axis of the ingot to produce a thin wafer, (ii) lapping the wafer to planarize its front and back surfaces, (iii) etching the wafer to remove any work-damage created by the sawing and lapping processes, and to remove any embedded lapping grit, and (iv) polishing the etched surface. In a typical etching process, a plurality of semiconductor wafers are placed in an etch rack or cassette and then the entire rack is immersed in an etch solution. Etch solutions typically contain a strong oxidizing agent, such as nitric acid (HN0₃) , a dissolving agent, such as hydrofluoric acid

(HF) , to dissolve the oxidation product, and a diluent, such as acetic acid (HOAc) . Chemical etching with this system proceeds by the following two reactions:

Si + 4HN0₃ -→ Si0₂ + 4N0₂ + 2H₂0 (1) Si0₂ + 4HF -→ SiF₄ + 2H₂0 (2)

It is generally believed that the reaction product, silicon tetrafluoride, escapes the solution as a colorless gas due to its low boiling point, which is about -86°C. Alternatively, it has been suggested that silicon tetrafluoride reacts with water to form hydrofluoric acid and silicon dioxide, an insoluble solid which must be filtered and removed from the etch solution. (See, e.g., Schnegg et al . , U.S. Patent No. 4,971,654.)

During the oxidation of silicon in reaction (1) , gaseous oxides of nitrogen are produced, including the colorless nitric oxide (NO) and the reddish-brown nitrogen dioxide (N0₂) . However, nitric oxide generally reacts instantly and almost quantitatively with atmospheric oxygen to produce nitrogen dioxide. As a result, these two oxides are typically considered a single pollutant. The two may therefore generally be referred to as oxides of nitrogen, or nitrogen oxide, N0_X. Nitrogen dioxide raises concerns about pollution and safety risks due to its ability to cause delayed chemical pneumonitis and pulmonary edema, even upon being exposed to concentrations as low as 100 ppm. Some form of fume or vapor control is therefore needed in order to prevent the nitrogen dioxide which is generated from becoming a serious health hazard. Currently, the most common approach to controlling such fumes is to exhaust them from the immediate wafer etching area, and then subject them to a water scrubbing process in an attempt to prevent their discharge into the surrounding environment . However, the control of nitrogen dioxide in this way is costly and, at best, only marginally effective. Water scrubbing is of limited effectiveness because, as reaction (3) given below indicates, water reacts with the nitrogen dioxide to produce nitric acid and nitric oxide. As previously noted, nitric oxide can then react with oxygen in the atmosphere to produce additional nitrogen dioxide .

3NO, + H,0 -→ 2HN0, + NO (3) In U.S. Patent No. 3,945,865, Kamperman suggests the addition of hydrogen peroxide to a solution containing nitric acid and hydrofluoric acid in order to control the evolution of N0_X. The acidic solution is used to remove metallic surface layers from titanium and zirconium metals and metal alloys. The presence of hydrogen peroxide is said either to act to re-oxidize nitrogen dioxide or to assist nitric acid in the oxidation of metals, and thus suppress the evolution of N0_X. In order to achieve the desired result, Kamperman requires that the concentration of hydrogen peroxide in the working solution be maintained above 1 gram per liter, or 0.1 percent by weight of the solution. An excess of hydrogen peroxide, i.e., a concentration above 1 gram per liter, is required because hydrogen peroxide is consumed as the reaction proceeds. If the concentration falls below this level, N0_X will again be evolved from the solution.

Although Kamperman generally addresses the control of NO_x for solutions used to dissolve metals, he does not address surface etching of a single crystal silicon wafer or the problems which are particularly associated with it. For example, while a hydrogen peroxide concentration of at least 0.1 percent by weight is needed in order to effectively strip metallic surface layers from metals, such a high concentration of hydrogen peroxide in the etch solution would interfere with the wafer etching process by lowering the etching rate. In addition, the wafer etching process is also sensitive to the concentration of water in solution. The presence of too much water in the etching solution can reduce the rate at which silicon and metallic impurities are removed from the wafer surface. The presence of water has also been linked to the degree of residual roughness of the surface of the wafer, the roughness increasing as the water content of the etch solution increases, as well as the ultimate gloss of the wafer surface.

In view of the foregoing, a need continues to exist for a single crystal silicon wafer etching process which effectively controls the generation of hazardous nitrogen oxide (N0_X) without causing a decrease in the etching rate or an increase in the roughness of the wafer surface.

SUMMARY OF THE INVENTION Among the objects of the present invention, therefore, is the provision of a process for controlling the evolution of NO_x during the etching of a silicon wafer in a solution containing nitric acid; the provision of a process in which the etching solution comprises nitric acid and an oxidizing agent, the oxidizing agent acting to inhibit the evolution of NO_x when silicon wafer surface is etched; and, the provision of a process in which the oxidizing agent is capable of oxidizing NO_x to nitric acid. Also among the objects of the present invention is the provision of an aqueous etching solution for a silicon wafer, the solution comprising nitric acid and an oxidizing agent capable of oxidizing N0_X to nitric acid; and, the provision of an etching solution in which the oxidizing agent is present in a concentration which is less than about 0.4 grams per liter of etching solution. Briefly, therefore, the present invention is directed to a process for controlling the evolution of NO_x during the etching of a silicon wafer, wherein the wafer is immersed in an etching solution containing nitric acid in order to etch silicon from the wafer surface. The process comprises forming an aqueous etching solution comprising nitric acid and an oxidizing agent which inhibits the evolution of NO_x when a silicon wafer surface is etched in the solution, and immersing the wafer in the etching solution to etch the surface. The present invention is further directed to an aqueous etching solution for a single crystal silicon wafer. The etching solution comprises nitric acid and an oxidizing agent capable of oxidizing N0_X to nitric acid. The etching solution has an oxidizing agent concentration which is less than about 0.4 grams per liter of etching solution.

Other objects will be in part apparent and in part pointed out hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the process of the present invention, controlling the evolution of N0_X, formed during the etching of single crystal silicon wafers in an etching solution containing nitric acid, can be achieved by adding a quantity of an oxidizing agent to the etching solution. The oxidizing agent, such as hydrogen peroxide or ozone, is capable of oxidizing NO_x to nitric acid.

It is to be noted that nitric acid etching of silicon wafers may result in the formation of either nitrogen dioxide (N0₂) or nitric oxide (NO) . Nitric oxide may then react with oxygen to form nitrogen dioxide . As a result, it is to be understood that, as used herein, the term "N0_X" shall mean oxides of nitrogen, or nitrogen oxide, and shall be meant to include both nitrogen dioxide and nitric oxide. It is also to be understood that, as used herein, the phrase "controlling the evolution of N0_X, " and derivations thereof, shall mean reducing the concentration of nitrogen dioxide below a level at which it would otherwise be produced and, preferably, to a level which is safe as determined by regulatory means common in the art .

In a standard silicon wafer etching process, single crystal silicon wafers are placed in a wafer holder, or cassette, and immersed in an etch solution which contains nitric acid, hydrofluoric acid, and a diluent. The diluent typically is acetic acid and water, but it may also include phosphoric acid, sulfuric acid, or combinations thereof. As previously noted, the nitric acid oxidizes the silicon on the surface of the wafer to form silicon dioxide, nitrogen dioxide and water.

Hydrogen fluoride then acts to dissolve the silicon dioxide from the wafer surface by reacting with it to form silicon tetrafluoride and water. As shown by reaction (4) , this standard process can be expressed as follows:

Si + 4HN0₃ + 4HF -→ SiF₄ + 4N0₂ + 4H₂0 (4)

Without being held to any particular theory, it is believed the addition of an oxidizing agent, such as ozone or hydrogen peroxide, controls the evolution of NO_x by reacting with, or oxidizing, the nitrogen dioxide to form nitric acid, as indicated by reaction (5) below. It is to be noted that if ozone is used, it is believed that it may first breakdown and react in the presence of water to form hydrogen peroxide in situ, which then reacts with the nitrogen dioxide present as shown.

4N0₂ + 2H₂0₂ -→ 4HN0₃ (5)

By combining reactions (4) and (5) , the overall reaction (6) , given below, is obtained for the silicon wafer etching process of the present invention. Experimental results to-date indicate that the visible signs associated with nitrogen dioxide may be generally eliminated if a silicon wafer is etched in a solution comprising nitric acid and an oxidizing agent, such as hydrogen peroxide or ozone. For example, the presence of hydrogen peroxide in the etching solution in a sufficiently high quantity results in the elimination of yellowish discolorations associated with nitrogen dioxide dissolved in solution, as well as reddish-brown vapors associated with nitrogen dioxide gas in the ambient .

Si + 4HN0₃ + 4HF + 2H₂0₂ -→ SiF₄ + 4HN0₃ + 4H₂0 (6)

The visible signs associated with the presence of nitrogen dioxide are effectively inhibited until the hydrogen peroxide present in solution is consumed, at which time nitrogen dioxide may again be visibly detected. As a result, a sufficient quantity of hydrogen peroxide will preferably be added, or generated in situ by the addition of ozone, such that all of the hydrogen peroxide is not consumed before etching is complete.

It is to be noted, however, that the presence of a large excess of hydrogen peroxide in solution is not preferred because it can interfere with the rate of etching. Without being held to any particular theory, it is believed that this interference occurs because the etching process is continuous, in that as silicon dioxide is formed at the wafer surface it is dissolved and removed. It has been observed that as the concentration of an oxidizing agent, such as nitric acid, increases relative to a dissolving agent, such as hydrofluoric acid, the rate of oxidation increases as compared to the rate of dissolution. A "film," or layer, of oxidized silicon therefore accumulates on the surface of the wafer and slows the rate of etching.

Due to the fact that hydrogen peroxide is also an oxidizing agent, the presence of hydrogen peroxide in the standard etching solution results in an increase in the concentration of the oxidizing agents relative to the dissolving agent. Accordingly, as the concentration of hydrogen peroxide increases relative to the concentration of other reagents in solution, the etching rate decreases. It is therefore generally preferred that a quantity of hydrogen peroxide be present in solution such that, upon completion of the etching process, all or nearly all has been consumed. Stated another way, preferably a quantity of hydrogen peroxide is present in solution such that little or no hydrogen peroxide remains after all of the nitrogen dioxide produced by the etching process has been converted to nitric acid.

In general, more than one wafer will be etched at a time. For example, standard conditions often involved immersing a cassette which holds 50 wafers in the etch solution for a specified period of time until a given quantity of silicon is etched from the surfaces of the wafers. Typically, the cassette of 50 wafers will be immersed in about 400 liters of etch solution which has been heated to a temperature ranging from about 25 °C to about 35°C for about 40 to about 60 minutes. This process results in the removal of about 30 to about 40 microns of silicon, which generally equates to about 25 cm³ to about 35 cm³. Given that the density of the silicon material being etched is about 2.3 grams/cm³, the amount of silicon typically removed, or consumed, by a standard etching process for a single batch of 50 silicon wafers ranges from about 55 to about 80 grams. As can be seen by referring again to reaction (6) , above, the quantity of oxidizing agent, such as hydrogen peroxide, needed in the etch solution is directly related to the quantity of silicon to be etched from the surfaces of the wafers. More specifically, the molar ratio of hydrogen peroxide in the etch solution to silicon etched from the wafer surface is about 2:1. Therefore, the quantity of hydrogen peroxide to be added, or generated in situ, for a given etch solution may be determined once the quantity of silicon to be etched from the wafer surface is known. Preferably, in view of the fact that a large excess of hydrogen peroxide is not desirable, the ratio of hydrogen peroxide to silicon etched from the wafer surface will range from about 2:1 to about 2.2:1. In view of the foregoing, for a standard wafer etch process in which 50 wafers are etched at one time, the hydrogen peroxide content of the etch solution will range from about 0.5 to about 0.2 grams per liter, or about 0.05 weight percent of solution to about 0.02 weight percent. Preferably, the hydrogen peroxide content will range from about 0.4 to about 0.3 grams per liter, or about 0.04 weight percent of solution to about 0.03 weight percent. It is to be noted, however, that the precise quantity of hydrogen peroxide in the etch solution for a given etching process may vary depending upon the amount of silicon to be etched, the amount of time the wafers are immersed in the solution, and the quantity of nitric acid in solution. For example, as long as nitric acid and silicon are present in solution, the oxidation reaction may continue, resulting in the formation of nitrogen dioxide. If all of the hydrogen peroxide is consumed, nitrogen dioxide will once again be evolved from the solution. As a result, additional hydrogen peroxide may need to be added to the existing etch solution. If this is the case, a higher concentration of hydrogen peroxide will preferably be used for all subsequent etchings which are to be performed under similar process conditions.

Typically, the etch solution will be used to treat multiple cassettes of wafers before it is discarded and replaced with a fresh solution. However, a portion of the solution will preferably be removed and replaced by an equal portion of fresh etch solution after each cassette of wafers is treated. For example, in a standard etching process, after each cassette of 50 wafers has been immersed in the etching solution and etched, about 8 liters will be withdrawn from the 400 liter etch solution and replaces with an equal portion of fresh etch solution.

The act of withdrawing a portion of the etch solution and replenishing it with an equal quantity of fresh solution helps to minimize concentration changes in the solution by replacing reagents which have been consumed by the etching process. As a result, the useful life of the solution is prolonged and the rate of etching is maintained. The volume of solution that is withdrawn from the etch solution and replenished is typically a function of the quantity of reagents that were consumed by the etching process, as well as the concentration of the replenishing solution being used. In addition to monitoring the concentration of reagents in the etch solution, in order to ensure the rate of etching is maintained, it is also important to monitor the concentration of water in solution. The concentration of water in the etch solution is critical to the resulting gloss of the wafer surface. Wafer gloss refers to the reflectivity of the wafer surface. The gloss of a wafer is typically measured using a glossmeter and involves a process whereby an incandescent light source is used to direct a beam of visible light onto the surface of the wafer at a specified angle (see, e.g.,

ASTM D-2457) . A portion of the light is reflected by the wafer surface and is then detected by a photosensitive receptor and measured. The fraction of the original light which is reflected is the "gloss" of the sample wafer, as expressed in specular gloss units.

Although preferred absolute gloss values may vary, a degree of uniformity is required in order for an etching process to be deemed acceptable. As a result, for a series of wafers etched in the solution, the gloss values may typically vary by no more than about 20 specular gloss units over the useful lifetime of the etching solution. This applies when etching either one wafer at a time (i.e., compare one etched wafer to the next), or one cassette at a time (i.e., compare a wafer from one cassette to a wafer from another cassette) . Preferably, however, the gloss will vary by less than about 15 specular gloss units. Most preferably, the gloss values will vary by less than about 10 specular gloss units. As a result, not only must the initial concentration of water in the etch solution be controlled, but also the change in the concentration over the lifetime of the etch solution. If the concentration of water in the etch solution increases to a level which causes variation in wafer gloss to become too great, a fresh solution may need to be prepared. The initial concentration of water (i.e., the concentration of water in etch solution before the first wafer or cassette of wafers is immersed in the solution) may vary with the type of etching solution being used. However, typically the initial water concentration will range from about 25 percent to about 30 percent by weight of the etching solution, and preferably from about 26 percent to about 28 percent by weight. Generally, the water concentration will increase from the initial level over the lifetime of the solution because water is produced by the etching process, and because water is typically added during the withdraw and replenish step.

A limited increase in the initial water concentration is acceptable and will not unduly interfere wafer etching. For example, typically the water content of the etch solution may increase by up to about 7 percent by weight of the solution without resulting in any significant reduction in wafer gloss or etch rate. Preferably, however, the water concentration will not increase by more than about 2 percent to about 5 percent by weight of the etch solution. The addition of hydrogen peroxide to the etch solution complicates the etching process because, due to safety concerns, hydrogen peroxide is generally not available in commercial quantities at concentration greater than about 50 percent. As a result, while the addition of hydrogen peroxide means controlling the evolution of nitrogen dioxide can be achieved, it also means additional water will be introduction into the etching solution. The additional water becomes a concern as the concentration of water in the etch solution reaches a threshold level at which problems with wafer gloss begins to appear.

As previously noted, an excess of hydrogen peroxide may also detrimentally effect the etching process by decreasing the etching rate. Therefore, after etching of the first cassette of wafers is complete, all or nearly all of the hydrogen peroxide initially present will preferably have been consumed by the process. As a result, before another cassette of wafers can be safely etched, additional hydrogen peroxide will be needed. The hydrogen peroxide is preferably included with the fresh solution of reagents added during the withdrawn and replenish step. However, due to the fact that a dilute solution of hydrogen peroxide is used, the replenish solution will contain more water, which means the total volume of the replenish solution will be greater than the total volume of the standard solution which is typically added. Therefore, in order to maintain the water concentration of the etch solution at an acceptable level, as well as maintain approximately the same total volume of etch solution, more of the etch solution must be withdrawn as compared to a standard process .

As the quantity withdrawn from the etch solution increases, the per wafer cost associated with the etching process also increases. This increase in cost is due to the fact that, overall, the amount of reagents being used in the process has increased. As more of the etch solution is withdrawn to make room for the hydrogen peroxide-containing replenishing solution, more of the reagents are being withdrawn as compared to the standard process. As a result, if the solution of hydrogen peroxide being used becomes too dilute, the volume of the etch solution being withdrawn becomes so great that this may unfavorably affect the overall cost of the etching process. Therefore, typically the concentration of a hydrogen peroxide solution included in the replenishing solution will be greater than about 40 percent by weight. Preferably, the concentration will be greater than about 50 percent by weight. Most preferably, the hydrogen peroxide solution will be as concentrated as wafer etching conditions and safety precautions will allow. It is to be noted, however, that as safety precautions and other issues dictate, a hydrogen peroxide solution concentration of less than about 40 percent by weight may be needed. If this is the case, other options are available that, used singularly or in combination, will still allow for the use of hydrogen peroxide to control and inhibit the evolution of nitrogen dioxide. For example, typically a hydrofluoric acid solution is used which has a concentration of less than about 50 percent by weight. As the concentration of the hydrogen peroxide solution falls below about 40 percent, a correspondingly more concentrated hydrofluoric acid solution, up to and including anhydrous hydrogen fluoride, (i.e., ranging from about 50 percent up to about 100 percent by weight) may be used to compensate for water included in the hydrogen peroxide solution. Likewise, typically a nitric acid solution is used which has a concentration of about 70 percent by weight. An increasingly more concentrated nitric acid solution, up to and including fuming nitric acid which has a concentration of about 90 percent by weight, (i.e., ranging from about 70 percent to about 90 percent by weight) may be used as the concentration of the hydrogen peroxide solution decreases. Lastly, phosphorus pentoxide may be added to react with the water present in solution to form phosphoric acid, thus reducing the overall water content in solution. In addition, it has been reported that the presence of phosphoric acid in the etch solution may act to increase the resulting smoothness of the wafer surface. (See, e.g., Stadler et al., U.S. Patent No. 5,587,046.) However, if phosphorus pentoxide is added, the addition will preferably be performed in such a way as to ensure it does not directly contact the wafer surface before being dissolved into solution. In situations where the particular etching process is unusually sensitive to the introduction of additional water into the etch solution, ozone will preferably be used to generate hydrogen peroxide in situ. Ozone may be introduced into the etch solution as a gas by means which are standard in the art, such as an ozone generator.

(See, e.g., Effizon Ozone System, commercially available from PCI Wedeco Environmental Technologies, located in West Caldwell, N.J.) Without being held to any particular theory, it is believed that, upon being introduced into the etch solution, ozone gas may decompose into diatomic oxygen (0₂) and molecular oxygen

(O) . Molecular oxygen may then react with water to form hydrogen peroxide. This process is advantageous because additional water is not introduced into the etching solution.

Referring again to reactions (5) and (6) , above, it is to be noted that the presence of hydrogen peroxide not only acts to control and inhibit the evolution of harmful nitrogen dioxide, or NO_x, but it also results in the regeneration of nitric acid in solution. Complete conversion of all of the nitrogen dioxide which is generated may result in a zero net consumption of nitric acid by the etching process. The addition of hydrogen peroxide or ozone to the etching solution therefore has the added advantage of reducing nitric acid consumption and, accordingly, reducing the per wafer cost otherwise associated with the etching process .

The following Examples set forth one set of conditions that may be used to achieve the results of the process of the present invention. Accordingly, these Examples should not be interpreted in a limiting sense.

Example 1

Standard Etch Solution Composition and Wafer Etching Process

A standard solution which may be used to etch the surfaces of single crystal silicon wafers has the following composition:

In a typical etching process, a cassette holding 50 single crystal silicon wafers is immersed for about 45 seconds in the above solution which has been heated to a temperature of about 30°C. Using a contact thickness gage standard in the art (see, e.g., model number DH-50, commercially available from Tokyo Seimitsu) to measure the thickness of the wafers before and after etching, the approximate amount of silicon removed by the etching process is determined. For the present example, 50 etched wafers are analyzed, each having a diameter of 150 mm. A total of about 36 microns of silicon are etched from the surfaces of the wafers, which corresponds to a volume of about 32 cm³ of silicon. Using the density of the silicon material removed, which is about 2.3 grams/cm³, the total amount of silicon etched from the surfaces of the wafers, or consumed by the etching process, is about 74 grams.

In accordance with the reaction for the standard etch process, Si + 4 HN0₃ + 4 HF -→ SiF₄ + 4 N0₂ + 4 H₂0, the following observations can be made regarding the consumption of reagents and formation of products.

From this data, it can be noted that the consumption of a single mole of silicon results in the formation of 4 moles of nitrogen dioxide which, as a function of its solubility in the etch solution, may remain dissolved in solution or escape the solution as harmful vapors .

Example 2

Addition of H₂0₂ to Control the Evolution of NO_x

In accordance with the overall reaction in which hydrogen peroxide is added to a standard etch solution,

Si + 4 HN0₃ + 4 HF + 2 H₂0₂ -→ SiF₄ + 4 HN0₃ + 4 H₂0, it can be seen that for each mole of silicon consumed about two moles of hydrogen peroxide are needed in order to control the evolution of NO_x. Based on the data from Example 1 as to the amount of silicon removed in a standard etching process of a single cassette of 50 wafers, as well as the amount of nitrogen dioxide which is produced, a weight of hydrogen peroxide needed initially in the 400 liters of etch solution can be calculated.

The resulting solution comprises nitric acid and hydrogen peroxide, and the molar ratio of hydrogen peroxide present in solution to silicon to be etched from the wafer surface is about 2:1. A 400 liter solution containing the stated amount of hydrogen peroxide will thus have a concentration of hydrogen peroxide in solution of about 0.3 grams per liter.

It is to be noted that the presence of hydrogen peroxide in the etch solution acts to control the evolution of nitrogen dioxide, or N0_X. In addition, the reaction between the hydrogen peroxide and the nitrogen dioxide results in the formation of nitric acid.

It is also to be noted that, after a first cassette of 50 wafers is etched, about the same quantity of hydrogen peroxide will be needed in solution in order to successful etch a subsequent cassette of 50 wafers.

In view of the foregoing, 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 and examples be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is :

1. A process for controlling the evolution of NO_x during the etching of a silicon wafer, wherein the wafer is immersed in an etching solution containing nitric acid in order to etch silicon from the wafer surface, the process comprising: forming an aqueous etching solution comprising nitric acid and an oxidizing agent which inhibits the evolution of NO_x when a silicon wafer surface is etched in the solution; and, immersing the wafer in the etching solution to etch the surface.

2. The process as set forth in claim 1 wherein the oxidizing agent is capable of oxidizing NO_x to nitric acid.

3. The process as set forth in claim 1 wherein the oxidizing agent is hydrogen peroxide.

. The process as set forth in claim 3 wherein hydrogen peroxide in the etching solution has a concentration which is less than about 0.

4 grams per liter of etching solution.

5. The process as set forth in claim 3 wherein silicon is etched from the wafer surface, and the hydrogen peroxide in the etching solution has a ratio to the silicon etched from the wafer surface ranging from about 2 : 1 to about 2.2:1.

6. The process as set forth in claim 3 wherein an aqueous hydrogen peroxide solution having a concentration of greater than about 40 percent by weight is added to the etching solution.

7. The process as set forth in claim 3 wherein an aqueous hydrogen peroxide solution having a concentration of less than about 40 percent by weight is added to the etching solution, the aqueous hydrogen peroxide solution being added in the presence of hydrofluoric acid having a concentration ranging from about 50 percent up to about 100 percent by weight, the concentration of hydrofluoric acid increasing as the concentration of the hydrogen peroxide solution decreases.

8. The process as set forth in claim 3 wherein an aqueous hydrogen peroxide solution having a concentration of less than about 40 percent by weight is added to the etching solution, the aqueous hydrogen peroxide solution being added in the presence of a nitric acid solution having a concentration ranging from about 70 percent to about 90 percent by weight, the concentration of the nitric acid solution increasing as the concentration of hydrogen peroxide solution decreases.

9. The process as set forth in claim 3 wherein an aqueous hydrogen peroxide solution having a concentration of less than about 40 percent by weight is added to the etching solution, the aqueous hydrogen peroxide solution being added in the presence of phosphorous pentoxide.

10. The process as set forth in claim 1 wherein the oxidizing agent is ozone.

11. The process as set forth in claim 1 wherein the etching solution may be used to treat multiple wafers or multiples cassettes of wafers in succession.

12. The process as set forth in claim 11 wherein the etching solution has a water concentration ranging from about 26 percent to about 28 percent by weight of the etching solution prior to immersing a first wafer, or a first cassette of wafers, in the solution.

13. The process as set forth in claim 12 wherein the water concentration does not increase by more than about 2 to about 5 percent by weight of the etching solution.

14. The process as set forth in claim 12 wherein wafer gloss does not vary by more than about 15 specular gloss units, upon comparing the gloss of successively etched wafers .

15. An aqueous etching solution for a single crystal silicon wafer, the etching solution comprising nitric acid and an oxidizing agent capable of oxidizing N0_X to nitric acid, the etching solution having an oxidizing agent concentration which is less than about 0.4 grams per liter of etching solution.

16. The etching solution as set forth in claim 15 wherein the oxidizing agent is ozone.

17. The etching solution as set forth in claim 15 wherein the oxidizing agent is hydrogen peroxide.

18. The etching solution as set forth in claim 17 wherein the hydrogen peroxide is added as an aqueous solution having a hydrogen peroxide concentration of greater than about 40 percent by weight of the aqueous solution.

19. The etching solution as set forth in claim 17 wherein the hydrogen peroxide is added as an aqueous solution having a hydrogen peroxide concentration of less than about 40 percent by weight of the aqueous etching solution, the aqueous hydrogen peroxide solution being added in the presence of hydrofluoric acid having a concentration ranging from about 50 percent up to about 100 percent by weight, the concentration of hydrofluoric acid increasing as the concentration of the hydrogen peroxide solution decreases.

20. The etching solution as set forth in claim 17 wherein the hydrogen peroxide is added as an aqueous solution having a hydrogen peroxide concentration of less than about 40 percent by weight is added to the etching solution, the aqueous hydrogen peroxide solution being added in the presence of a nitric acid solution having a concentration ranging from about 70 percent to about 90 percent by weight, the concentration of the nitric acid solution increasing as the concentration of hydrogen peroxide solution decreases.

21. The etching solution as set forth in claim 17 wherein the hydrogen peroxide is added as an aqueous solution having a hydrogen peroxide concentration of less than about 40 percent by weight is added to the etching solution, the aqueous hydrogen peroxide solution being added in the presence of phosphorous pentoxide .