# Patent

## Title
Generation of compact alumina passivation layers on aluminum plasma equipment components

## Abstract

A process for generating a compact alumina passivation layer on an aluminum component includes rinsing the component in deionized water for at least one minute, drying it for at least one minute, and exposing it to concentrated nitric acid, at a temperature below 10° C., for one to 30 minutes. The process also includes rinsing the component in deionized water for at least one minute, drying it for at least one minute, and exposing it to NH4OH for one second to one minute. The process further includes rinsing the component in deionized water for at least one minute and drying it for at least one minute. A component for use in a plasma processing system includes an aluminum component coated with an AlxOy film having a thickness of 4 to 8 nm and a surface roughness less than 0.05 μm greater than a surface roughness of the component without the AlxOy film.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/973,077, filed 31 Mar. 2014, the entire contents of which are incorporated by reference herein for all purposes.


TECHNICAL FIELD
The present disclosure is in the field of semiconductor process engineering. More specifically, embodiments are disclosed that generate compact alumina passivation layers on aluminum plasma equipment components, for quick stabilization of etch rates in plasma processing equipment that uses the equipment components.
BACKGROUND
Semiconductor processing often utilizes plasma processing to etch or clean semiconductor wafers. Predictable and reproducible wafer processing is facilitated by plasma processing parameters that are stable and well controlled. Certain changes to equipment and/or materials involved in plasma processing can temporarily disrupt stability of plasma processing. For example, introducing a material to a plasma chamber that is unstable in the plasma processing environment, switching among plasma processes performed in the plasma chamber, exposing the chamber to different gases or plasmas than usual, and/or replacing components that are part of or within the plasma chamber, may disrupt process stability. In such cases, initially, the process may change substantially, but may stabilize over time, for example as an introduced material gradually clears from the process chamber or as surface coatings within the process chamber come into equilibrium with the plasma process conditions.
SUMMARY
In an embodiment, a process for generating a compact alumina passivation layer on an aluminum component includes exposing the aluminum component to nitric acid (HNO3) having a concentration of at least 30 percent, at a temperature below 10° C., for between one minute and 30 minutes.
In an embodiment, a component for use in a plasma processing system includes an aluminum component coated with an AlxOy film having a thickness of 4 to 8 nm and a surface roughness less than 0.05 μm greater than a surface roughness of the aluminum component without the AlxOy film.
In an embodiment, a process for generating a compact alumina passivation layer on an aluminum component includes rinsing the aluminum component in deionized water for at least one minute, drying the aluminum component for at least one minute, and exposing the aluminum component to nitric acid (HNO3) having a concentration of at least 30 percent, at a temperature below 10° C., for between one and 30 minutes. The process also includes rinsing the aluminum component in deionized water for at least one minute, drying the aluminum component for at least one minute, and exposing the aluminum component to NH4OH for between one second and one minute. The process further includes rinsing the aluminum component in deionized water for at least one minute and drying the aluminum component for at least one minute.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification.



BRIEF DESCRIPTION OF THE DRAWINGS
The present disclosure may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below, wherein like reference numerals are used throughout the several drawings to refer to similar components. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. In instances where multiple instances of an item are shown, only some of the instances may be labeled, for clarity of illustration.
FIG. 1 schematically illustrates major elements of a plasma processing system, according to an embodiment.
FIG. 2 describes an exemplary process for producing a compact alumina passivation layer on aluminum plasma equipment components.
FIGS. 3A and 3B are scanning electron microscopy (SEM) photographs at similar magnifications, showing Al surfaces treated under similar process conditions utilizing dilute and concentrated HNO3 respectively, in an embodiment.
FIGS. 4A and 4B are transmission electron microscopy (TEM) photographs at similar magnifications of cross-sectional slices of Al surfaces cleaned with a previously best known method including treatment with dilute HNO3, and concentrated HNO3 at room temperature, in an embodiment.
FIGS. 5A and 5B each show a series of SEM photographs showing Al surfaces treated with highly concentrated (69%) HNO3 for different temperatures and different times respectively, in an embodiment.
FIGS. 6A through 6H show SEM photographs at identical magnification, showing Al surfaces treated with concentrated or highly concentrated HNO3, at room temperature (RT) or a low temperature, and for short or long times, in embodiments.
FIG. 7 is a bar chart showing results of surface roughness of treated and untreated Al samples, measured using laser microscopy, in an embodiment.
FIGS. 8A and 8B are graphs showing process stability results for plasma components treated with dilute and concentrated HNO3, respectively, in embodiments.



DETAILED DESCRIPTION
FIG. 1 schematically illustrates major elements of a plasma processing system 100, according to an embodiment. System 100 is depicted as a single wafer, semiconductor wafer plasma processing system, but it will be apparent to one skilled in the art that the techniques and principles herein are applicable to processing systems for any type of workpiece (e.g., items that are not necessarily wafers or semiconductors). Processing system 100 includes a housing 110 for a wafer interface 115, a user interface 120, a process chamber 130, a controller 140 and one or more power supplies 150. Process chamber 130 includes one or more wafer pedestals 135, upon which wafer interface 115 can place a workpiece 50 (e.g., a semiconductor wafer, but could be a different type of workpiece) for processing. Gas(es) 155 may be introduced into process chamber 130 through a plenum 139 and a diffuser plate 137, and a radio frequency generator (RF Gen) 165 supplies power to ignite a plasma within process chamber 130. Surfaces of wafer pedestal 135, walls and floor of chamber 130, and diffuser plate 137 are all surfaces that can significantly affect processing characteristics of system 100. Diffuser plate 137, in particular, forms many small holes therethrough to distribute gas and/or plasma uniformly in process chamber 130, and surface chemistry effects of walls of these holes may be significant.
The elements shown as part of system 100 are listed by way of example and are not exhaustive. Many other possible elements, such as: pressure and/or flow controllers; gas or plasma manifolds or distribution apparatus; ion suppression plates; electrodes, magnetic cores and/or other electromagnetic apparatus; mechanical, pressure, temperature, chemical, optical and/or electronic sensors; wafer or other workpiece handling mechanisms; viewing and/or other access ports; and the like may also be included, but are not shown for clarity of illustration. Internal connections and cooperation of the elements shown within system 100 are also not shown for clarity of illustration. In addition to RF generator 165 and gases 155, other representative utilities such as vacuum pumps 160 and/or general purpose electrical power 170 may connect with system 100. Like the elements shown in system 100, the utilities shown as connected with system 100 are intended as illustrative rather than exhaustive; other types of utilities such as heating or cooling fluids, pressurized air, network capabilities, waste disposal systems and the like may also be connected with system 100, but are not shown for clarity of illustration. Similarly, while the above description mentions that plasma is ignited within process chamber 130, the principles discussed below are equally applicable to so-called “downstream” or “remote” plasma systems that create a plasma in a first location and cause the plasma and/or its reaction products to move to a second location for processing.
Certain plasma processes are sensitive to surface conditions in a plasma chamber. In the case of semiconductor processing, process stability and uniformity requirements are exacerbated as device geometries shrink and wafer sizes increase. New equipment (or equipment that has had any chamber components replaced) may require significant downtime to condition the chamber through simulated processing—that is, performing typical plasma processes without exposing actual workpieces—until acceptable process stability is reached.
One plasma process that is very sensitive to chamber surface conditioning is etching of thin silicon nitride (Si3N4) layers with a plasma formed from nitrogen trifluoride (NF3) and nitrous oxide (N2O) gases. Plasma chamber components such as wafer pedestal 135, walls and floor of chamber 130, and diffuser plate 137, FIG. 1, may be made of aluminum and may be coated with a thin layer of alumina (generally AlxOy, and often approximately Al2O3, but variations in the alumina stoichiometry are contemplated and are considered within the scope of this disclosure). New aluminum components may be cleaned and subjected to a dilute nitric acid (HNO3) mixture to generate the alumina layer; this may take the form of placing the aluminum components in contact with a pad soaked in HNO3. When HNO3 is used in any type of processing, it is often utilized in a dilute form because it may be considered safer to handle.
In embodiments herein, concentrated HNO3 is used, instead of dilute HNO3, to generate an alumina layer on plasma chamber components. “Highly concentrated” HNO3 is used herein to denote HNO3 having a concentration of 60% to 100% HNO3 by weight, and “concentrated” HNO3 (including “highly concentrated” HNO3) is used herein to denote HNO3 having a concentration of 30% to 100% by weight. Although care is required when handling concentrated HNO3, embodiments herein utilize concentrated HNO3 to provide a denser and less porous AlxOy layer on aluminum components than is provided by dilute HNO3, thus minimizing conditioning time required in a nitride plasma etch environment. It is also believed that soaking the aluminum components in the concentrated HNO3 instead of placing HNO3-soaked pads in contact with the components is advantageous in that it produces a compact, smooth and uniform AlxOy layer on exposed Al surfaces, including in crevices, holes and the like. Concentrated HNO3 has also been found to provide a more compact and smoother alumina layer than other acids and/or oxidizers such as H2O2, HCl, HF, HNO3+HF, H2SO4, HCl+HNO3 and NH4OH.
It is further believed that performing the HNO3 processing at a low temperature and for a relatively short amount of time limits dissociation of the HNO3 (e.g., 4HNO3=>2H2O+4NO2+O2), further promoting a compact (e.g., dense) and nonporous AlxOy layer by inhibiting attack of the original Al surface by H2O. While thickness of an AlxOy layer achieved within a reasonable process time does not change much (5-6 nm of AlxOy), the Al surface remains about as smooth as its initial condition with concentrated HNO3, instead of rougher, as observed with dilute HNO3. Minimizing surface roughness is believed to be key to rapid stabilization of a plasma process that the aluminum component is exposed to, because surface roughening presents variations in the AlxOy layer that interact with the plasma processing until the variations are smoothed out. For example, initial local thin spots and/or voids in the AlxOy at surface projections or indentations may interact with the plasma until the AlxOy layer reaches at least several nm in thickness. It is believed that embodiments herein are capable of producing a surface finish previously not found on Al parts, namely, a compact AlxOy film with a net surface roughness less than 0.05 nm greater than the Al part on which the film exists. Embodiments that utilize concentrated HNO3 to generate a compact AlxOy layer, examples of processing results and passivated components generated thereby, and rapid process stabilization effects of the passivated components, are now disclosed.
Processing with Concentrated HNO3 to Generate Compact ALxOy Layer
FIG. 2 describes an exemplary process 200 for producing a compact alumina passivation layer on aluminum plasma equipment components. Process 200 is used, for example on an aluminum part that is new or has been treated to remove previous coatings. Certain portions of process 200 may be performed differently than those shown in exemplary process 200, as described further below.
Process 200 begins with a deionized (DI) water flush 210 of the aluminum part for 5 minutes, followed by drying it in clean dry air (CDA) 215 for 5 minutes. While steps 210 and 215 are taking place, a bath of concentrated or highly concentrated HNO3 may be cooled to a low temperature (e.g., below 10° C.) in an optional step 220. In embodiments, the bath is advantageously at least 60% HNO3 to minimize effects of H2O on the AlxOy layer being formed. In certain embodiments, the bath is advantageously cooled to below 5° C., to minimize surface roughening of the AlxOy layer, however in other embodiments the HNO3 bath may be at room temperature, to minimize equipment and power requirements for cooling the bath. The aluminum part then receives an HNO3 treatment 225 for one to 30 minutes, advantageously about one minute to 15 minutes, followed by another DI water flush 230 for one to 30 minutes, advantageously about 5 minutes, and a CDA dry 235 of one to 30 minutes, advantageously about 5 minutes. The HNO3 treatment grows about 4 to 8 nm of AlxOy, typically about 5 to 6 nm, while not increasing surface roughness of the aluminum part more than 0.05 μm more than its original roughness. Next, the aluminum part is exposed to ammonium hydroxide (NH4OH) 240 for one second to one minute, advantageously about one second to 5 seconds, to neutralize any remaining HNO3. The exposure to NH4OH is followed by a final DI water flush 245 for one to 30 minutes, advantageously about 5 minutes and a CDA dry 250 of one to 30 minutes, advantageously about 5 minutes.
Numerous substitutions and rearrangements of process 200 will be apparent to one skilled in the art, and all such substitutions and rearrangements are considered to be within the scope of the present disclosure. A few examples of such substitutions and rearrangements are to omit the initial DI water flush and CDA drying steps 210 and 215; to perform any of the CDA drying steps 215, 235, 250 with nitrogen (N2) or other relatively inert gas instead of CDA; to utilize heated CDA (or other relatively inert gas) to promote drying; to omit CDA drying steps 215 and/or 235, instead going directly from the preceding DI water flush to the following chemical steps 225 or 240, and/or to shorten or lengthen the DI water flush or CDA drying steps.
Examples of Compact ALA Layer Generated by Processing with Concentrated HNO3 
Examples of aluminum plasma equipment components and/or aluminum coupons processed with various dilutions, temperatures and times of HNO3 are now shown.
FIGS. 3A and 3B are scanning electron microscopy (SEM) photographs at identical magnifications, showing Al surfaces treated under similar process conditions utilizing dilute and concentrated HNO3 respectively. FIG. 3A shows the Al surface treated with dilute HNO3 as being significantly rougher than the Al surface treated with concentrated HNO3 (FIG. 3B).
FIGS. 4A and 4B are transmission electron microscopy (TEM) photographs at similar magnifications of cross-sectional slices of Al surfaces cleaned with a previously best known method including treatment with dilute HNO3(FIG. 4A), and concentrated HNO3 at room temperature (FIG. 4B). As shown in FIGS. 4A and 4B, respective layers 40A and 40B are the underlying Al, layers 42A and 42B are AlxOy formed by the respective HNO3 treatments. Further layers 44A and 44B are iridium and 46A and 46B are carbon layers utilized in TEM sample preparation. The AlxOy layers were measured at just over 5 nm thickness in each of layers 42A and 42B.
FIGS. 5A and 5B each show a series of SEM photographs originally taken at 10,000× magnification, showing Al surfaces treated with highly concentrated (69%) HNO3 for different temperatures and different times respectively. FIG. 5A shows SEM photographs of Al surfaces treated at temperatures ranging from about 5° C. to about 60° C., which were visually evaluated as having less compact/rougher AlxOy layers with increasing temperature. The SEM photographs are arranged according to the visual evaluation and according to HNO3 processing temperature, although the positioning along the directions of temperature and compactness are not to scale. FIG. 5B shows SEM photographs of Al surfaces treated for times within the range of 5 minutes to 400 minutes, which were visually evaluated as having less compact/rougher AlxOy layers with increasing temperature. The SEM photographs are arranged according to the visual evaluation of compactness and according to HNO3 processing time; again, the positioning along the directions of directions of time and compactness are not to scale.
FIGS. 6A through 6H show SEM photographs at identical magnification, showing Al surfaces treated with concentrated or highly concentrated HNO3, at room temperature or a low temperature (e.g., less than 10° C.), and for a short time (e.g., 5-25 minutes) or a long time (e.g., 90-150 minutes). Among the surface morphologies shown in FIGS. 6A through 6H, the samples treated for the long times are notably rougher than those treated under the same conditions for the short times, the samples treated at room temperature are rougher than those treated under the same conditions at low temperatures, and the samples treated with concentrated HNO3 are rougher than those treated under the same conditions with highly concentrated HNO3.
FIG. 7 is a bar chart showing results of surface roughness of treated and untreated Al samples, measured using laser microscopy. All of the treated samples were treated with HNO3 at a low temperature (e.g., less than 10° C.). Surface roughness of an Al sample treated with highly concentrated HNO3 was very close to that of untreated Al, while surface roughness of the Al samples treated with concentrated and dilute HNO3 were progressively higher. The surface roughness of the Al samples treated with concentrated and highly concentrated HNO3 were less than 0.05 μm greater than that of the untreated sample; it is believed that Al components having AlxOy layers of about 5 nm with surface roughness less than 0.05 μm greater than that of the untreated component have not been previously produced.
FIGS. 8A and 8B are graphs showing process stability results for plasma components treated with dilute and concentrated HNO3, respectively. For FIG. 8A, Al components that were treated according to previous processes were installed within two process chambers (two “sides”) of a two-chamber plasma processing system. The chambers were then cycled through process cycles in which a typical plasma processing recipe was run. An amount of silicon nitride etched under standard process conditions was measured at intervals, resulting in the data shown in FIG. 8A. The etched amount varied significantly until about 3000 process cycles, then stabilized around the target etch amount, although one chamber continued to etch somewhat more than the other. For FIG. 8B, Al components treated according to process 200, FIG. 3, were installed within two process chambers of a two-chamber plasma processing system. The chambers were then cycled through the same process cycles as for the data shown in FIG. 8A. An amount of silicon nitride etched under the same standard process conditions was measured at intervals, resulting in the data shown in FIG. 8B. The etched amount is seen to be relatively consistent around the target etch amount after only about 25 process cycles, with better side-to-side matching.
Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.
As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the electrode” includes reference to one or more electrodes and equivalents thereof known to those skilled in the art, and so forth. Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.

## Claims

1. A process for generating a compact alumina passivation layer on an aluminum component, comprising:
rinsing the aluminum component in deionized water for at least one minute;
drying the aluminum component for at least one minute;
exposing the aluminum component to nitric acid (HNO3) having a concentration of at least 30 percent, at a temperature below 10° C., for between one and 30 minutes;
rinsing the aluminum component in deionized water for at least one minute;
drying the aluminum component for at least one minute;
exposing the aluminum component to NH4OH for between one second and one minute;
rinsing the aluminum component in deionized water for at least one minute; and
drying the aluminum component for at least one minute.

2. The process of claim 1, wherein the HNO3 has a concentration of at least 60%.

3. The process of claim 1, wherein the HNO3 has a temperature of 5° C. or below.

4. The process of claim 1, wherein exposing comprises soaking the aluminum component in the HNO3 for between one minute and 15 minutes.

5. The process of claim 1, wherein exposing the aluminum component to the NH4OH comprises dipping the aluminum component in the NH4OH for between one and ten seconds.