Surface potential control in plasma processing of materials

Abstract: An object (30) is plasma processed by placing an electrically conducting grid (34) over all or a portion of the surface (32) of the object (30) so that the grid (34) generally follows the contours of the surface (32) but is displaced outwardly from the surface (32). Ions or electrons from a plasma surrounding the object (30) are accelerated into the surface (32) of the object (30) using as a processing driving force an electrical potential (40) applied to the electrically conducting grid (34). The use of a contoured conducting grid (34) allows plasma processing of large, electrically nonconducting objects (30) and objects (30) having sharp surface features (31) or recesses (33).

Description: This invention relates to a method for plasma processing an object according to the preamble of claim 1. The invention further relates to an object which is prepared by such a method. A method of the kind mentioned at the outset is known from US-A-5 078 847. The useful life of some types of objects is determined by the wear of the surface of the object. For example, the tooling used in metal forming operations such as stamping or forming must retain its close dimensional tolerances in order to be useful. When even a few thousandths of an inch of the surface is worn away, the tooling becomes ineffective and must be discarded or refurbished. The cost of short runs of parts or pieces made with tooling is often high, due to the fixed cost of preparing acceptable tooling. One approach to reducing the tooling cost is to form the tool from a polymeric material such as an epoxy that is more readily made with the required shape and dimensions than is a comparable metal tool. The epoxy is much softer than metal, and can therefore be easily machined to the required shape and dimensional tolerances. The surface softness that permits easy machining results in a short useful life of the tool. For production applications, an epoxy tool cannot be used without a treatment to make the surface more wear resistant. It has been found that several types of ion and electron treatments of the surfaces of polymers such as epoxies and the surfaces of metals can significantly increase their useful lives. Such treatments include, for example, ion implantation, ion deposition and ion mixing, and electron bombardment. Ion implantation is a process wherein ions are accelerated by an electrostatic potential to impact a surface. The energy of the ions causes them to be imbedded beneath the surface. A sufficient concentration of implanted ions can significantly increase the hardness of the surface layer. In ion mixing, a thin (about 3-6x10 -8 m [300-600 Angstrom]) material layer is first deposited onto a surface to be treated, and then the composite surface is ion implanted to mix the deposited material atoms into the surface. Ion deposition is similar to ion mixing except that the accelerating energies of the ions are lower, with the result that the ions are deposited simultaneously with the deposited material layer upon the surface being treated rather than imbedded beneath its surface. In electron bombardment of polymeric surfaces, energetic electrons (100-20,000 volts or more) are accelerated by an electrostatic potential to cause them to impact into the surface to be treated. Such treatments are traditionally accomplished by accelerating a beam of electrical charge carriers (ions or electrons) using electrostatic acceleration electrodes. This approach, while effective, has the drawback that it is difficult to uniformly treat a large, three-dimensional, irregular object such as a typical automotive tool that may measure 0.9m [3 feet] by 1.5m [5 feet] by 0.3m [1 foot] in size. The beam of charge carriers must be moved slowly over the entire surface, including those areas that may be rather inaccessible such as deep holes or recesses, or for protrusions that stick out from the surface. In that case, the object must be manipulated to uniformly treat the area with a beam. For large tools or for large numbers of smaller objects having such surface features, surface treatment using ion or electron beams becomes prohibitively slow and expensive. An alternative approach that has promise for ion implanting, ion depositing, or ion mixing of the surfaces of large objects is plasma ion implantation (PII), described in US Patent 4,764,394. A plasma of ions is created adjacent to the surface of the object to be implanted, and the object is electrostatically charged to a potential opposite to that of the ions. For example, if positively charged nitrogen ions are to be implanted, the object is negatively charged using repetitive voltage pulses of typically about 100,000 volts or more. The nitrogen ions are attracted to the surface of the object by this accelerating potential and driven into the surface and sub-surface regions of the substrate. Plasma ion implantation has the advantage that the plasma of ions provides a source that is distributed around the entire surface area of the object, and uniform implantation over the entire surface area is simultaneously achieved. Plasma ion mixing may be performed in a similar manner, except that first a thin layer of material is deposited into the surface prior to implantation. Plasma ion deposition is also performed in a similar manner, except that the accelerating potential is typically lower so that the ions and material layer deposit upon the surface instead of being driven into the sub-surface layers. It is possible to achieve electron bombardment of a surface using the same plasma techniques used in ion implantation. Plasma ion implantation, deposition and mixing, and electron bombardment work well for metallic objects with smooth, flat faces. However, where the object is electrically conducting or nonconducting with sharp surface contours or deep recesses, the electric field distribution at the surface of the substrate is distorted and therefore nonuniform. The result is a spatially nonuniform and uneven surface treatment, or a discontinuance of the surface treatment due to a space charge buildup with ensuing arcing. Conventional plasma ion implantation is also ineffective for uniformly surface hardening either nonconducting or conducting objects that have sharp surface features or deep recesses. The document US-A-5 078 847 mentioned at the outset discloses an ion plating process, wherein a wire mesh grid is optionally provided for more complete control of the plasma, and to direct the plasma flow to certain locations on or around the substrate to be plated. Plating irregular shapes is achieved by applying certain alternating and direct voltages to the substrate. The grid has the shape of an rectangular box. JP-A-03079768 discloses a method for depositing diamond-like thin films using a flat grid disposed between the ion source and the object to accelerate the hydro-carbon ions to be deposited on the surface of the object. The grid is positioned at right angles to the hydrocarbon beam. By this method a crack-free deposit shall be achieved through reduced thickness of the deposit towards the edge. US-A-3 069 281 discloses a method for metallizing the surface of perfluoro-olefin polymers. In this known method the object to be coated is placed between two screens, which are electrodes consisting of mesh metal screens. The object may be supported from our may rest on one of the two screens. The specimens treated by this known method are very small in size. Both the screens have a flat surface. FR-A-2 608 634 discloses a method and an apparatus for plasma-enhanced chemical vapor deposition for producing a uniform coating of a dielectric, semiconducting or conducting material onto a semiconducting pellet. A grid having a flat surface is used which is positioned adjacent to the surface of the pellet and is spaced therefrom. An electromagnetic energy is applied to the grid which plays the role of confining the plasma and of preventing this from coming into contact with the exposed surface of the pellets to be coated. US-A-4 982 696 discloses an apparatus for forming thin films on a flat substrate, which comprises a grid, which has a flat or curved surface. The area of the opening per unit area is varied in the plane of the grid, so that the density of the evaporated particles which pass through the grid can be adjusted. It is, therefore, an object of the invention to improve the method mentioned at the outset for use with electrically conducting or nonconducting objects having sharp surface features or deep recesses. According to the present invention, this object is achieved for the method mentioned at the outset in that the grid is shaped to generally follow the irregularly shaped contours of the surface of the object to be processed. The present invention provides an improved plasma processing technique for use with objects with sharp features. It is also useful for surface hardening any type of object, either electrically conducting or electrically nonconducting, with sharp features, such as drill bits, sharp-edged tooling, and the like. The approach achieves surface processing results on such objects that are comparable with the results achieved for conventional flat, electrically conducting objects. As used herein, the term "plasma processing" is a term of art that encompasses plasma ion implantation, plasma ion deposition and mixing, electron bombardment, and related techniques. These processes are distinct but related. Plasma ion implantation and mixing uses a high accelerating voltage that results in a distribution of ions below the surface of the object, while plasma ion deposition uses a lower accelerating voltage that results in most of the ions being deposited upon the surface of the object. Electron bombardment uses a high accelerating voltage, opposite in polarity to that used in ion implantation, that results in a distribution of electrons beneath the surface of the object. The apparatus used to practice these various techniques is similar, the principal difference being the polarity and magnitude of the accelerating voltage used. Thus, "charge carriers" as used herein includes both ionic charge carriers and electrons. A key to the present invention is that the object be placed in a constant-potential volume, which is achieved by the placement of an electrically conducting grid over the surface of the object being plasma processed. The grid can be used to cover the entire surface of the object that is being plasma processed, or it can be used to cover a portion of the surface of the object. In the latter case, the remainder of the surface of the object must be covered with a metal foil or other electrical conductor. In either case, the grid is displaced outwardly from the surface of the object. The grid is maintained at an electrical potential opposite to that of the charge carriers, and at a sufficiently high voltage to accelerate the charge carriers into the surface. The grid is preferably made of a mesh of thin wires about 0.25 mm [0.010 inches] thick and arranged such that at least 70 percent of the area of the grid piece is open. It is preferably spaced at least about 1.25 cm [1/2 inch] from the surface of the object in order to ensure uniform plasma processing. The grid material is an electrical conductor selected, in most cases, so as to minimize sputtering of material from the grid onto the object being treated. In the case where the material is intentionally desired to be sputtered or deposited onto the surface of the object, the grid material can likewise be chosen appropriately. When placed in proximity to, but not touching, the surface of the object being plasma processed, the grid aids in accelerating the ion or electron charge carriers toward the surface and conducting away charge that would otherwise accumulate at the surface. The grid is shaped to make the electric potential field more uniform near the surface of the object, effectively "smoothing" the electric potential field while still achieving uniform processing of the surface, which is typically three-dimensional and has surface irregularities. The arcing that prevents plasma processing of surfaces having sharp features is thereby avoided, so that both electrically conducting objects and electrically nonconducting objects having sharp surface features may be plasma processed. Placement of the grid at a distance above the surface avoids a "shadow" effect of the grid wires on the surface of the object being plasma processed. The charge carriers from the plasma are accelerated toward the surface of the object at a variety of angles that reach the regions directly below the wires of the mesh as well as the regions below the openings in the mesh. The grid may also be moved relative to the surface of the object in a manner that eliminates the shadowing effect of the grid. The present invention thus provides a method for plasma processing all objects that have sharp surface features which would otherwise prevent uniform plasma ion processing due to electric field intensification leading to arcing effects, or recesses. In this manner, the present invention achieves surface potential control of objects being plasma processed. Other features and advantages of the invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. Figure 1 is a process flow chart for the plasma ion processing method of the invention; Figure 2 is a sectional view of an object with a grid formed over its entire surface; Figure 3 is a section view of an object with a grid formed over a part of its surface; and Figure 4 is a schematic sectional view of a plasma ion processing apparatus. The preferred embodiments of the invention relate to plasma ion processing of objects, and are described in relation to this process. Similar approaches are used for plasma electron processing, with the principal differences being, as previously discussed, the polarity and magnitude of the applied voltages. Thus, Figure 1 is a flow chart of the method for preparing a plasma ion processed object. Figures 2-3 depict two embodiments of the object during its preparation, and Figure 4 shows the plasma ion processing apparatus. Referring to Figure 1, in a first step, numeral 20, an object 30 (Figures 2 and 3) to be plasma ion processed is provided. In one preferred embodiment, the object 30 is a polymeric material such as an epoxy polymer. The object 30 can be made of other electrically nonconducting objects such as ceramics or glass. The object 30 has sharp features, such as shown at numeral 31, or recesses, such as shown at numeral 33, in Figure 2. In this embodiment, the object can be either electrically conducting, such as a metal, or electrically nonconducting, as previously described. Although the various embodiments are discussed in reference to a single set of drawings Figures 1-4, it is understood that these embodiments are separate and distinct. The invention may be practiced in reference to an electrically nonconducting object having sharp surface features or deep recesses, or an electrically conducting object having sharp surface features or deep recesses. A surface 32 of the object 30 is covered by an electrically conducting metallic grid 34, numeral 22 of Figure 1. The grid 34 is made of an electrically conducting wire such as copper, stainless steel, or titanium. The wire is of small size, preferably about 0.25 mm [0.010 inches] in diameter. The grid 34 is woven from the wire in the manner of a window screen with an open mesh, preferably having at least 70 percent open space. The resulting grid is readily formed into approximately the shape of the surface 32 of the object 30. The terminology "generally following the contours of the surface" and related expressions, as used herein, are meant to convey the sense that the grid need not conform precisely to the surface of the object, with precisely the same outward displacement from the surface at every location, for the invention to be operable. A virtue of the present approach is that it tolerates variations in shape and spacing of the grid 34 as compared with the surface contours of the object 30. The grid 34 thus need not conform precisely to the shape of the surface 32, but should generally follows its contours. The entire surface 32 of the object 30 to be plasma ion processed is covered by the grid 34, as shown in the embodiment of Figure 2. In an alternative approach as shown in Figure 3, a portion of the surface 32 to be ion processed is covered by the grid 34, and the remainder of the surface 32 is covered by a solid electrically conducting mask 35, such as aluminum foil. The mask 35 prevents plasma processing in the covered areas, but permitting implantation, deposition, or mixing to occur through the grid 34 in the manner previously discussed. In both embodiments, an electrically conducting shell is formed over the entire surface of the object that is to be treated. The grid 34 does not touch the surface 32 of the object 30. Instead, the grid 34 is spaced outwardly from the surface 32 by a distance D. The distance D should be at least about 1.25 cm [1/2 inch], and may be greater. Studies have shown that if the grid 34 is placed directly against the surface 32 or spaced less than about 1.25 cm [1/2 inch] from the surface, the subsequently implanted ions do not reach the areas of the object 30 that are directly under the wires that form the grid 34 due to a shadowing effect of the grid wires. Thus, there would be an alternating pattern of plasma ion processed and non-processed regions according to the pattern of the grid 34. Spacing the grid 34 by a distance of at least about 1.25 cm [1/2 inch] has been found to be sufficient to produce a substantially spatially uniform ion implantation or deposition pattern in the surface 32 of the object 30 after plasma ion processing is complete. It is believed that the outward spacing of the grid 34 from the surface 32 allows the ions in the plasma to approach the surface 32 from a range of directions, sufficient to ensure ion implantation or deposition at all locations on the surface. The grid 34 may be formed over the surface in a freestanding manner, as shown in Figure 2 or Figure 3, or may be supported by an insulated clamp or the like. Preferably, spacers between the object 30 and the grid 34 are not used, because the spacers would mask a region of the surface from ion implantation or deposition. The grid 34 is made of an electrically conducting material, which is electrically connected, as by a weld 36, to an electrically conducting base plate 38. The base plate 38 is, in turn, electrically connected to a power supply 40 or pulse modulator that biases the grid 34 in a desired manner. In the preferred embodiment, the implanted or deposited ions are positively charged nitrogen ions. The grid is therefore biased negatively relatively to the processing chamber walls. In the absence of a grid covering the surface being treated, when a flat piece of an electrically conducting metal is plasma ion implanted or deposited, the electric field lines induced by the applied voltage all terminate uniformly on the surface of the metal piece and perpendicular to the surface to achieve uniform processing. Because a metal presents an equipotential surface, the applied voltage develops at every point on the surface of the metal. Ions are therefore implanted with the full applied voltage. However, when the object is an electrical nonconductor, the electric field lines pass through the object to the base plate. There is a voltage drop inside the object that increases with increasing thickness of the object. The voltage at the surface of the object therefore is less than the applied voltage and is likely to be nonuniform. Ions are therefore not implanted with the full applied voltage, because a nonconductor does not present an equipotential surface. The electric field lines are not perpendicular to the surface of the object which adds to the nonuniformity of the plasma ion processing. Because electrons cannot flow to the surface of the nonconducting object to cancel the charge carried by the implanted ions, a space charge can develop that inhibits further implantation and there may be surface arcing that can damage the surface of the object. Where the object has sharp features such as those shown at numeral 31, the electric potential field lines are distorted, leading to large gradients and arcing from the sharp feature. Although applicant does not wish to be limited to this explanation, and the operability of the invention does not depend upon any particular explanation of the effect of the grid 34, it is believed that the grid creates a constant potential (equipotential) volume around the object that is devoid of electric field gradients. In this manner, the grid "smooths" the surface electric potential of the object as compared to the case where no grid is present. The constant potential volume aids in achieving a uniform deposition at all points around the surface, regardless of whether there are sharp features on the surface. The grid also aids in either neutralizing or conducting away charge carried to the surface of the object with the electrically charged ions flowing from the plasma to the surface for deposition or implantation, an important consideration when an electrically nonconducting object is being implanted. The grid-covered object 30 is plasma ion processed, numeral 24 of Figure 1, utilizing a plasma ion processing apparatus 50, Figure 4. The apparatus 50 includes a vacuum chamber 52 pumped by a vacuum pump 54 through a gate valve 56. The gas to be ionized and plasma ion implanted or deposited is backfilled to the interior of the chamber 52 through a gas line 58 having a valve 60. The object 30 is supported in the vacuum chamber 52 in the manner previously discussed in relation to Figure 2. The metallic grid 34 is electrically connected to the base plate 38, which in turn is biased to a potential by the power supply 40 or pulse modulator. In the case of the preferred nitrogen plasma ion processing, the grid 34 is pulsed to a negative potential. The walls of the vacuum chamber 52 are maintained at ground relative to the pulsed negative voltage. To conduct plasma ion processing with this apparatus 50, the chamber 52 is first pumped out through the valve 56 to a vacuum of about 1.3 x 10 -6 mbar (10 -6 Torr). Nitrogen (or other gas to be ionized) is backfilled through the valve 60 to a steady state pressure of about 1.3-13.3 x 10 -5 mbar [1-10 x 10 -5 Torr]. The plasma ion processing voltage -Vo is applied in a pulsed fashion by the power supply 40 or voltage modulator. Typical operating parameters are a repetition rate of the applied voltage pulses of 100-1000 cycles per second, with a pulse duration for the applied voltage of 10-30 microseconds. The plasma is usually created using a working gas such as nitrogen, and using a discharge voltage of 100 volts and a discharge current of 1-10 amperes. For nitrogen plasma ion implantation, the negative applied potential is preferably minus 100,000 volts or more, but can be varied as necessary. For plasma ion deposition using nitrogen to form titanium nitride by reacting with titanium provided to the vacuum chamber, the negative potential is preferably minus about 100 to minus about 1000 volts. Typical implantation times under these conditions are 4-8 hours to implant a dose of 1-10 x 10 17 per square centimeter. Typical deposition times under these conditions are 4-8 hours to deposit a surface layer about 1-5 micrometers thick. Further details of plasma ion processing apparatus and procedures are described in US Patent 4,764,394.

Claims: A method for plasma processing an object (30), comprising the steps of: providing (20) a three-dimensional irregularly shaped object (30) having a contoured surface (32); placing (22) an electrically conducting grid (34) over the surface (32) to be processed of the object (30), the grid (34) being displaced outwardly (D) from the surface (32); and plasma processing (24) the surface (32) of the object (30) with charge carriers produced in a plasma, the plasma being generated around and adjacent to the surface (32) of the object (30), and using as a processing driving force an electrical potential (40) applied to the electrically conducting grid (34), characterized in that the grid (34) is shaped to generally follow the irregular shaped contours of the surface (32) of the object (30) to be processed. The method of claim 1, characterized in that the charge carriers are ions or electrons. The method of claim 1 or claim 2, characterized in that the grid (34) covers the entire surface (32) of the object (30). The method of any of claims 1 - 3, characterized in that the grid (34) provides a constant potential volume that encloses the object (30). The method of claim 1 or claim 2, characterized in that the grid (34) covers a portion of the surface (32) of the object (30), a conducting foil (35) covers the remainder of the surface (32) of the object (30), and the grid (34) and conducting foil (35) are electrically conducting. The method of claim 5, characterized in that the grid (34) and conducting foil (35) together provide a constant potential volume that encloses the object (30). The method of any of claims 1 - 6, characterized in that the step (24) of plasma processing achieves ion deposition on the surface (32) of the object (30), and the step (24) of plasma processing includes the step of plasma ion depositing ions onto the surface (32) of the object (30), using as a depositing driving force an electrical potential applied to the electrically conducting grid (34) sufficient to deposit ions onto the surface but not implant ions into the surface (32). The method of any of claims 1 - 6, characterized in that the step (24) of plasma processing achieves ion implantation into the surface (32) of the object (30), and the step (24) of plasma processing includes the step of plasma ion implanting ions into the surface (32) of the object (30), using as an implanting driving force an electrical potential (40) applied to the electrically conducting grid (34) sufficient to implant ions into the surface (32). The method of any of claims 1 - 6, characterized in that the step (24) of plasma processing achieves ion mixing into the surface (32) of the object (30). The method of any of claims 1 - 6, characterized in that the step (24) of plasma processing achieves electron bombardment into the surface (32) of the object (30). The method of any of claims 1 - 10, characterized in that the object (30) is electrically conducting. The method of any of claims 1 - 10, characterized in that the object (30) is electrically nonconducting. The method of any of claims 1 - 10, characterized in that the object (30) is made of a polymeric material. The method of any of claims 1 - 13, characterized in that the electrically conducting grid (34) has an open area fraction of at least about 70 percent. The method of any of claims 1 - 14, characterized in that the electrically conducting grid (34) is made of a material that has substantially no sputter deposition onto the surface (32) of the object (30). The method of any of claims 1 - 14, characterized in that the electrically conducting grid (34) is made of a material that sputter deposits onto the surface (32) of the object (30). The method of any of claims 1 - 16, characterized in that the electrically conducting grid (34) is displaced outwardly from the surface (32) of the object (30) by at least about 1.25 cm [1/2 inch]. The method of any of claims 1 - 17, characterized in that the surface (32) of the object (30) has a sharply shaped feature (31) thereon, and the grid (34) is shaped to generally conform to the feature (31). The method of any of claims 1 - 18, characterized in that the surface (32) of the object (30) has a recess (33) thereon, and the grid (34) is shaped to generally conform to the recess (33). The method of any of claims 1 - 19, characterized in that an equipotential surface is created adjacent to the surface (32) of the object by the use of the grid (34). The method of claim 20, characterized in that the plasma is created outside of the equipotential surface (34). A plasma processed three-dimensional irregularly shaped object (30) having a contoured surface, characterized in that it is prepared by the method of any of claims 1 - 21.

Inventors: MATOSSIAN JESSE N, PEPPER DAVID M, SCHUMACHER ROBERT W, MATOSSIAN, JESSE N., PEPPER, DAVID M., SCHUMACHER, ROBERT W.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1993
Main classification: C23C14
IPC codes: H01J37, B29C59, C23C8, C23C14, B29C35, H01L21