Covering for reducing carbon contamination in plasma doping chambers

Abstract

1. A plasma doping system comprising: a plasma doping chamber; a platen mounted within the plasma doping chamber to support a workpiece; a source of ionizable gas coupled to the chamber, the ionizable gas including a desired dopant for implantation into the workpiece; a plasma source for generating a plasma having a plasma sheath in the vicinity of the workpiece, the plasma including positive ions of the ionizable gas and accelerating the positive ions across the plasma sheath toward the platen for implantation into the workpiece; a shield ring adapted to surround the platen and extend the plasma sheath beyond an edge of the workpiece; and a cover ring disposed on top of the shield ring and adapted to mitigate sputtering of the shield ring, the cover ring including a crystalline base layer and an amorphous top layer.

Description

[Technical field]

[0001] Related Applications

[0001] This application claims priority to U.S. non-provisional patent application Ser. No. 17 / 687,620, entitled "COVER RING TO MITIGATE CARBON CONTAMINATION IN PLASMA DOPING CHAMBER," filed Mar. 5, 2002, the entire contents of which are incorporated herein by reference.

[0002] Field

[0002] Embodiments of the present disclosure relate to plasma doping systems, and more particularly, to devices for mitigating sputter contamination in plasma doping systems. [Background technology]

[0003]

[0003] Ion implantation is a standard technique for introducing electrical conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, the desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of a given energy, and the ion beam is directed toward the surface of the wafer. The energetic ions in the beam penetrate the bulk of the semiconductor material and become embedded in the crystal lattice of the semiconductor material to form regions of the desired electrical conductivity.

[0004]

[0004] In some applications, it is necessary to form shallow junctions in semiconductor wafers, where the impurity material is confined to regions close to the surface of the wafer. In these applications, the high energy acceleration and associated beam forming hardware of conventional ion implanters are not required. Thus, plasma doping systems are used to form shallow junctions in semiconductor wafers. In a plasma doping system, the semiconductor wafer is placed on a platen in a chamber, the platen adapted to function as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a high voltage pulse is applied between the platen and an inductively coupled plasma source, which forms a plasma with a plasma sheath adjacent to the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and become implanted in the wafer. The depth of implantation is related to the voltage applied to the wafer.

[0005]

[0005] In plasma doping systems such as those described above, an annular "shield ring" is typically provided that surrounds the platen. The shield ring, typically made of silicon carbide, is positioned in close radial clearance relationship with a wafer disposed on the platen and is biased to extend the plasma sheath beyond the edge of the wafer. While this configuration is generally effective for its intended purposes, it does suffer from certain drawbacks. For example, ion bombardment of the shield ring during the implant process can cause carbon atoms to be sputtered into the chamber, resulting in undesirable carbon contamination of the wafer being processed. Thus, there is a need to mitigate such contamination while maintaining the functionality of the shield ring.

[0006]

[0006] It is with respect to these and other considerations that the present disclosure is provided. Summary of the Invention

[0007]

[0007] This Summary is provided to introduce in a simplified form selected concepts detailed below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter.

[0008]

[0008] One embodiment of a plasma doping system according to the present disclosure may include a plasma doping chamber, a platen mounted within the plasma doping chamber to support a workpiece, a source of ionizable gas coupled to the plasma doping chamber, the source of ionizable gas including a desired dopant for implantation into the workpiece, a plasma source for generating a plasma having a plasma sheath in the vicinity of the workpiece, the plasma including positive ions of the ionizable gas and accelerating the positive ions across the plasma sheath toward the platen for implantation into the workpiece, a shield ring surrounding the platen and adapted to extend the plasma sheath beyond an edge of the workpiece, and a cover ring positioned on top of the shield ring and adapted to mitigate sputtering of the shield ring, the cover ring including a crystalline base layer and an amorphous top layer.

[0009]

[0009] Another embodiment of a plasma doping system according to the present disclosure may include a plasma doping chamber, a platen mounted within the plasma doping chamber to support a workpiece, a source of ionizable gas coupled to the plasma doping chamber, the source of ionizable gas including a desired dopant for implantation into the workpiece, a plasma source for generating a plasma having a plasma sheath in the vicinity of the workpiece, the plasma including positive ions of the ionizable gas and accelerating the positive ions across the plasma sheath toward the platen for implantation into the workpiece, a shield ring surrounding the platen and adapted to extend the plasma sheath beyond an edge of the workpiece, and a cover ring disposed on top of the shield ring and adapted to mitigate sputtering of the shield ring, the cover ring including a crystalline base layer and an amorphous top layer.

[0010]

[0010] An embodiment of a shield ring assembly for a plasma doping system according to the present disclosure may include a shield ring adapted to surround a platen of the plasma doping system and extend a plasma sheath beyond the edge of the workpiece, and a cover ring disposed on top of the shield ring and adapted to mitigate sputtering of the shield ring, the cover ring including a crystalline base layer and an amorphous top layer.

[0011]

[0011] The accompanying drawings illustrate exemplary approaches of the present disclosure, including practical applications of the principles of the present disclosure, as follows: [Brief description of the drawings]

[0012] [Figure 1] 1 is a cross-sectional side view illustrating a plasma doping system according to an exemplary embodiment of the present disclosure. [Diagram 2]

[0013] 2 is a detailed cutaway view of a portion of the shield ring and cover ring of the plasma doping system shown in FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013]

[0014] The drawings are not necessarily to scale. The drawings are merely representational and are not intended to portray specific parameters of the present disclosure. The drawings are intended to illustrate example embodiments of the present disclosure and therefore are not to be considered limiting of the scope. In the drawings, like numbers represent like elements.

[0014]

[0015] The embodiments will now be described more fully with reference to the accompanying drawings, which show several embodiments. The subject matter of the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art. In the drawings, like numbers refer to like elements throughout.

[0015]

[0016] As used herein, an element or operation referred to in the singular and followed by the word "a" or "an" should be understood to include a plurality of elements or operations unless otherwise indicated. Additionally, various embodiments herein are described in the context of one or more elements or components. An element or component may comprise any structure configured to perform a particular operation. Although an embodiment may be described with a limited number of elements in a topology as an example, an embodiment may include more or fewer elements in an alternative topology as desired for a given implementation. It should be noted that "one embodiment" or "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the invention. The appearance of the phrases "in one embodiment" or "in some embodiments" in various places in this specification does not necessarily refer to the same embodiment.

[0016]

[0017] Referring to FIG. 1, an exemplary embodiment of a plasma doping system according to the present disclosure is shown. The system may include a plasma doping chamber 10 defining an enclosed space 12. A platen 14 may be positioned within the chamber 10 to provide a surface for holding a workpiece, such as a semiconductor wafer 20. The wafer 20 may be clamped, for example, at its periphery to a flat surface of the platen 14. The platen 14 may support the wafer 20 and provide electrical connections to the wafer 20. In another embodiment, the platen 14 may include conductive pins for providing electrical connections to the wafer 20. The disclosure is not limited in this respect.

[0017]

[0018] The wafer 20 may be electrically connected to a first high voltage pulse generator 30 through the platen 14, and the wafer 20 may function as a cathode. The pulse generator 30 may typically provide pulses ranging from about 20 to 20,000 volts, with durations of about 5 to 200 microseconds, and pulse repetition frequencies of about 500 Hz to 20 KHz. The values ​​of the pulse parameters are exemplary and other values ​​and ranges are contemplated within the scope of the invention.

[0018]

[0019] The enclosed space 12 of the chamber 10 may be coupled to a vacuum pump 34 via a controllable valve 32. A gas source 36 may be coupled to the chamber 10 via a mass flow controller 38. A pressure sensor 44 installed in the chamber 10 may provide a signal indicative of the chamber pressure to a controller 46. The controller 46 may compare the sensed chamber pressure to a desired pressure input and provide a control signal to the valve 32. The control signal may control the valve 32 to minimize the difference between the chamber pressure and the desired pressure. The vacuum pump 34, the valve 32, the pressure sensor 44, and the controller 46 may comprise a closed loop pressure control system. The pressure is typically controlled in a range of about 1 millitorr to about 500 millitorr. The disclosure is not limited in this respect. The gas source 36 may provide an ionizable gas containing a desired dopant for implantation into the wafer 20. Examples of ionizable gases include BF3, N2, Xe, Ne, He, H2, D2, Ar, PF5, PH3, AsH3, AsF5, AsF3, and B2H6. The disclosure is not limited in this respect. Mass flow controller 38 may adjust the rate at which gas is supplied to chamber 10. The configuration shown in FIG. 1 may provide a continuous flow of process gas at a constant gas flow rate and constant pressure. The pressure and gas flow rate are preferably adjusted to provide reproducible results.

[0019]

[0020] During operation of the system, the wafer 20 may be positioned on the platen 14. The pressure control system, mass flow controllers 38, and gas source 36 may then generate a desired pressure and gas flow rate within the chamber 10. By way of example, the chamber 10 may operate with He gas at a pressure of 10 millitorr. The pulse generator 30 applies a series of high voltage pulses to the wafer 20, which may cause a plasma 40 to form between the wafer 20 and a radio frequency inductively coupled plasma (RF ICP) source 24 above the wafer 20. As is known in the art, the plasma 40 includes positive ions of an ionizable gas from the gas source 36. The plasma 40 further includes a plasma sheath 42 in the vicinity of the platen 14. During the high voltage pulse, an electric field existing between the RF ICP source 24 and the platen 14 accelerates the positive ions from the plasma 40 across the plasma sheath 42 toward the platen 14. The accelerated ions are implanted into the wafer 20 to form a region of impurity material. The pulse voltage is selected to implant the positive ions to a desired depth in the wafer 20. The number of pulses and pulse duration are selected to provide a desired dose of the impurity material to the wafer 20 .

[0020]

[0021] The system may further include an annular Faraday cup 80 positioned around the wafer 20 and platen 14. The Faraday cup 80 may be positioned within an annular shield ring 82 (described in more detail below) and may be located as close as practical to the wafer 20 and platen 14 to intercept a sample of the positive ions accelerated from the plasma 40 toward the platen 14. In various alternative embodiments, the Faraday cup 80 may have other positions relative to the wafer 20 to provide a representative measurement of the ion current implanted in the wafer 20. The Faraday cup 80 may include a conductive housing having an inlet 60 facing the plasma 40.

[0021]

[0022] The Faraday cup 80 may be electrically connected to the dose processor 70 or other dose monitoring circuitry. Positive ions entering the Faraday cup 80 through the inlet 60 may generate a current in an electrical circuit connected to the Faraday cup 80. The current is indicative of the number of positive ions received per unit time, i.e., the ion current density. The ion current density received by the Faraday cup 80 may have a fixed relationship to the number of ions implanted into the wafer 20 per unit time. Depending on the uniformity of the plasma 40 and the uniformity of the ions accelerated toward the platen 14, the ion current per unit area received by the Faraday cup 80 may be equal to or a fixed percentage of the ion current per unit area implanted into the wafer 20. Because the current output of the Faraday cup 80 is representative of the ion current implanted into the wafer 20, the Faraday cup 80 may provide a measurement of the ion dose implanted into the wafer 20.

[0022]

[0023] The shield ring 82 may surround the platen 14 and may be biased to extend the plasma sheath beyond the edge of the wafer 20. As previously described, the Faraday cup 80 may be positioned within the shield ring 82 near the periphery of the wafer 20 and platen 14. The sidewall 84 of the shield ring 82 may include a horizontal protrusion 86 that extends beyond the entrance 60 of the Faraday cup 80 to provide a narrow opening 87 (best shown in FIG. 2 ) above the Faraday cup 80. Secondary electrons may be generated by ions colliding with the sidewall 88 of the Faraday cup 80, causing an electrical discharge inside the Faraday cup 80. The narrow opening 87 may reduce the potential drop within the Faraday cup 80 and improve electron confinement within the Faraday cup 80.

[0023]

[0024] The system may further include an annular cover ring 90 disposed over and concentrically positioned with the shield ring 82. The shield ring 82 and the cover ring 90 may be collectively referred to herein as a "shield ring assembly." The cover ring 90 may have inner and outer diameters equal or approximately equal to the inner and outer diameters of the shield ring 82. The cover ring 90 may further have an annular opening 92 (best shown in FIG. 2 ) formed therethrough. The annular opening 92 may match (or be aligned with) the narrow opening 87 defined by the shield ring 82, and its width may be equal or approximately equal to the width of the opening 87. Thus, the cover ring 90 may block accelerated ions emitted from the plasma 40 from impacting the shield ring 82 while allowing the ions to enter the Faraday cup 80.

[0024]

[0025] 2, a detailed cutaway view is shown showing a segment of the cover ring 90 and the top of the shield ring 82. The shield ring 82 may be formed of silicon carbide (SiC), a material well known to those skilled in the art as a standard material used in the construction of shield rings. The cover ring 90 may be formed primarily of silicon or another material known to have a low sputter yield (described in further detail below). As discussed above, the cover ring 90 may prevent accelerated ions ejected from the plasma 40 (see FIG. 1) from impacting the shield ring 82, and therefore may prevent carbon atoms from being sputtered therefrom. As a result, carbon contamination in the chamber 10 is mitigated or prevented, improving the quality of the implantation process performed in the chamber 10.

[0025]

[0026] In some cases, crystalline silicon (c-Si) may be susceptible to "bubbling" or other forms of degradation when subjected to ion bombardment. Thus, the cover ring 90 may be formed of a crystalline base layer 96 and may have an insulating, amorphous top layer 98 adapted to resist such bubbling or degradation. For example, in one non-limiting embodiment of the present disclosure, the top layer 98 may be formed of an oxide. Such an oxide may be thermally grown on the top surface of the silicon base layer 96, sprayed on the top surface of the crystalline base layer 96, or deposited on the top surface of the crystalline base layer 96 using chemical vapor deposition (CVD) or atomic layer deposition (ALD). The present disclosure is not limited in this respect. In another embodiment of the present disclosure, the insulating top layer 98 may be formed by amorphizing the top of the crystalline base layer 96 using any suitable amorphization process. The amorphization process may be performed on the cover ring prior to attachment to the shield ring 82, or may be performed in situ after attachment to the shield ring 82. The present disclosure is not limited in this respect. In yet another embodiment of the present disclosure, the top layer 98 is silicon nitride (SiN x ). Silicon nitride may be sprayed onto the top surface of the crystalline base layer 96 or may be deposited on the top surface of the crystalline base layer 96 using chemical vapor deposition (CVD) or atomic layer deposition (ALD). The disclosure is not limited in this respect. Thus, the insulating amorphous top layer 98 may be formed by applying a separate layer of material over the base layer 96 (e.g., as in the case of applying an oxide or silicon nitride via spraying) or by changing the composition of the top portion of the base layer 96 (e.g., as in the case of amorphization).

[0026]

[0027] In various non-limiting embodiments of the present disclosure, the top layer 98 of the covering ring 90 may have a thickness in the range of 1000 Angstroms to 2000 Angstroms as measured along the Y-axis of the illustrated Cartesian coordinate system. In particular, if the top layer 98 is not thick enough, the underlying silicon substrate 96 may not be adequately protected from ion bombardment and may be prone to bubbling or prematurely degrade. Conversely, if the top layer 98 is too thick, it may result in a number of undesirable effects, including, but not limited to, voltage drops / drops in the chamber 10, sheath bending, charging and arcing, etc. Thus, the specific thickness of the top layer 98 of the covering ring 90 may be tailored to the energy of the implantation process being performed (i.e., the higher the energy of the implantation, the greater the thickness of the top layer 98).

[0027]

[0028] The covering ring 90 may be conveniently removed and replaced, such as when the covering ring 90 is "used" (i.e., etched or degraded beyond further usefulness) or when a covering ring formed of a different material or having a top (insulating) layer of a different thickness is desired. The covering ring 90 was previously described as being formed primarily of silicon. The disclosure is not limited in this respect. In alternative embodiments, the covering ring 90 may be formed of various metallic materials that are known to have low sputter yields. Such metallic materials include, but are not limited to, titanium (Ti), yttrium (Y), yttrium oxide (Y2O3), and the like. For example, if the wafer 20 (FIG. 1) is implanted with titanium nitride, the covering ring 90 may be formed of titanium, since the wafer 20 will be less susceptible to contamination by titanium sputtered from the covering ring 90 during the subsequent implantation process.

[0028]

[0029] In summary, the embodiments described herein provide at least the following technical advantages. First, the cover ring 90 of the embodiments of the present invention reduces the generation of carbon that is sputter etched from conventional shield rings made of silicon carbide during ion implantation processes, thereby reducing carbon contamination of the processed wafers. Second, the cover ring 90 is easily replaceable and can be implemented at a relatively low cost.

[0029]

[0030] The present disclosure should not be limited in scope by the specific embodiments described herein. Indeed, in addition to the embodiments described herein, various other embodiments of the present disclosure and modifications of the present disclosure will be apparent to those skilled in the art from the above description and the accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, the present disclosure has been described herein in the context of specific implementations in specific environments for specific purposes. Those skilled in the art will recognize that the usefulness is not limited thereto, and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Therefore, the claims set forth below should be construed in light of the full scope and spirit of the present disclosure as described herein.

Claims

1. 1. A plasma doping system, comprising: a plasma doping chamber; a platen mounted within the plasma doping chamber for supporting a workpiece; a source of ionizable gas coupled to the plasma doping chamber, the source of ionizable gas including a desired dopant for implantation into the workpiece; a plasma source for generating a plasma having a plasma sheath adjacent to the workpiece, the plasma including positive ions of the ionizable gas, the plasma source accelerating the positive ions across the plasma sheath toward the platen for implantation into the workpiece; a shield ring adapted to surround the platen and extend the plasma sheath beyond an edge of the workpiece; a cover ring disposed on top of the shield ring and adapted to mitigate sputtering of the shield ring; 1. A plasma doping system comprising:

2. The plasma doping system of claim 1 , wherein the cover ring is formed of silicon.

3. The plasma doping system of claim 1 , wherein the covering comprises a crystalline underlayer and an amorphous top layer.

4. The plasma doping system of claim 3 , wherein the amorphous top layer is formed of an oxide.

5. The plasma doping system of claim 3 , wherein the amorphous top layer is an amorphized portion of the crystalline underlayer.

6. The plasma doping system of claim 3 , wherein the amorphous top layer has a thickness in the range of 1000 Angstroms to 2000 Angstroms.

7. The plasma doping system of claim 1 further comprising a Faraday cup positioned around the periphery of the platen.

8. 8. The plasma doping system of claim 7, wherein the Faraday cup is disposed within the shield ring, the shield ring having an opening formed therein to allow ions to enter the Faraday cup.

9. The plasma doping system of claim 8 , wherein the cover ring has an annular opening formed therein, the annular opening mating with the opening in the shield ring.

10. a plasma doping chamber; a platen mounted within the plasma doping chamber for supporting a workpiece; a source of ionizable gas coupled to the plasma doping chamber, the source of ionizable gas including a desired dopant for implantation into the workpiece; a plasma source for generating a plasma having a plasma sheath adjacent to the workpiece, the plasma including positive ions of the ionizable gas, the plasma source accelerating the positive ions across the plasma sheath toward the platen for implantation into the workpiece; a shield ring adapted to surround the platen and extend the plasma sheath beyond an edge of the workpiece; a cover ring disposed on top of the shield ring and adapted to mitigate sputtering of the shield ring, the cover ring including a crystalline underlayer and an amorphous top layer; 1. A plasma doping system comprising:

11. The plasma doping system of claim 10 , wherein the cover ring is formed of silicon.

12. The plasma doping system of claim 10 , wherein the amorphous top layer is formed of an oxide.

13. The plasma doping system of claim 10 , wherein the amorphous top layer is an amorphized portion of the crystalline underlayer.

14. The plasma doping system of claim 10, wherein the amorphous top layer has a thickness in the range of 1000 Angstroms to 2000 Angstroms.

15. 11. The plasma doping system of claim 10, further comprising a Faraday cup positioned about the platen and disposed within the shield ring, the shield ring having an opening formed therein to allow ions to enter the Faraday cup.

16. 16. The plasma doping system of claim 15, wherein the cover ring has an annular opening formed therein, the annular opening mating with the opening in the shield ring.

17. 1. A shield ring assembly for a plasma doping system, comprising: a shield ring adapted to surround a platen of the plasma doping system and extend a plasma sheath beyond an edge of a workpiece; a cover ring disposed on top of the shield ring and adapted to mitigate sputtering of the shield ring, the cover ring including a crystalline underlayer and an amorphous top layer; a shield ring assembly.

18. The plasma doping system of claim 17 , wherein the cover ring is formed of silicon.

19. 20. The plasma doping system of claim 17, wherein the amorphous top layer is formed of an oxide.

20. 20. The plasma doping system of claim 17, wherein the amorphous top layer is an amorphized portion of the crystalline underlayer.

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