JP2005535131A - Removal of plasma deposited surface layer by sputtering of dilution gas - Google Patents

Abstract

A method for limiting the formation of a deposited surface layer on an arc piece, such as a semiconductor wafer, between plasma implants in a plasma doping chamber for ionization to form dopant gas ions and diluent gas ions. Introducing dopant gas and diluent gas, and accelerating the dopant gas ion and diluent gas ion toward the workpiece. Dopant gas ions are implanted into the workpiece and dilution gas ions remove the deposited surface layer from the workpiece. The atomic mass of the dopant gas and diluent gas is similar to achieve effective removal of the deposited surface layer.

Description

  The present invention relates to a plasma doping system used for ion implantation into a workpiece, and more particularly to a method and apparatus for removing a plasma deposited surface layer by dilute gas sputtering.

Plasma doping systems have been studied to form shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen (acting as a cathode and located in a plasma doping chamber). An ionizable dopant gas is introduced into the chamber and a voltage pulse is applied between the platen and the anode or chamber wall to form a plasma containing ions of the dopant gas. The plasma has a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to cross the plasma sheath and be injected into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low energy is achieved. For example, Patent Document 1 (inventor is Sheng, US patent specification issued on October 11, 1994), 2 (inventor is Liebert et al., US issued on February 1, 2000, for example). (Patent specification) and 3 (US patent specification issued on February 6, 2001 by Goeckner et al.).
U.S. Pat.No. 5,354,381 U.S. Patent No. 6,020,592 U.S. Patent No. 6,182,604

  In the plasma doping system described above, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In another type of plasma system known as an immersion plasma system, a continuous plasma is formed, for example, by RF power inductively coupled from an antenna located inside or outside the plasma doping chamber. The antenna is connected to an RF power source. At intervals, a voltage pulse is applied between the platen and the anode, which accelerates positive ions in the plasma toward the wafer.

The dopant gas species used for plasma implantation is decomposed during the implantation process to form a deposited surface layer on the wafer. Examples of dopant gas species include AsH ₃ , PH ₃ and B ₂ H ₆ . For example, hydrogen arsenide (AsH ₃ ) decomposes into As, AsH, and AsH ₂ that deposit on the surface of the wafer to be implanted. The deposited surface layer deteriorates the dose repeatability, reduces the dose uniformity, and causes measurement problems. Accordingly, there is a need for a method and apparatus that prevents or limits the formation of these layers in order to provide sufficient process control for plasma implantation.

  In accordance with an aspect of the present invention, a method is provided for limiting deposition surface layers from being formed on a workpiece during plasma implantation. The method includes introducing a dopant gas and a diluent gas into a plasma doping chamber for ionization to form dopant gas ions and diluent gas ions, and directing the dopant gas ions and diluent gas ions to the workpiece. Accelerating the process. Dopant gas ions are implanted into the workpiece and the diluent gas ions remove the deposited surface layer from the workpiece. The workpiece may be a semiconductor wafer.

The atomic masses of the dopant gas and diluent gas are similar to achieve effective removal of the deposited surface layer. The ratio of diluent gas to dopant gas is selected to remove the deposited surface layer as it is formed. In some embodiments, a high ratio of diluent gas is utilized. In some embodiments, an inert diluent gas is utilized. As an example, the dopant gas may be hydrogen arsenide and the diluent gas may be krypton or xenon. In this example, the ratio of diluent gas to dopant gas is about 50. In other embodiments, the dopant gas may be phosphine and the diluent gas may be argon. Furthermore, in other embodiments, the dopant gas may be B ₂ H ₆ and the diluent gas may be neon. In some embodiments, the diluent gas may include a chemically active component (such as a halogen). In other embodiments, the chemically active component is fluorine or chlorine. As an example, the diluent gas may be an inert gas and fluorine.

  In certain embodiments, the dopant gas and diluent gas are introduced separately into the plasma doping chamber. In other embodiments, the dopant gas and diluent gas are premixed prior to introduction into the plasma doping chamber.

  In accordance with another aspect of the present invention, a plasma doping apparatus is provided. A plasma doping apparatus includes a plasma doping chamber, a platen that supports the workpiece, a platen located in the plasma doping chamber, an anode spaced from the platen in the plasma doping chamber, a process gas source coupled to the plasma doping chamber, and a pulse Including sources. A processing gas source introduces a dopant gas and a dilution gas into the plasma doping chamber, where a plasma containing dopant gas ions and dilution gas ions is formed in the plasma discharge region between the anode and the platen. The pulse source applies a pulse between the platen and the anode to accelerate the ions toward the workpiece. Dopant gas ions are implanted into the workpiece and the diluent gas ions remove the deposited surface layer from the workpiece.

  In accordance with another aspect of the present invention, a plasma injection method is provided. The method includes introducing a dopant gas into a first chamber for ionization to form dopant gas ions, accelerating the dopant gas ions toward the workpiece (where the dopant gas is introduced into the workpiece). Injecting) a diluent gas into the second chamber and accelerating the diluent gas ions toward the workpiece, the diluent gas ions removing the deposited surface layer from the workpiece. When the first and second chambers are the same chamber, introduction of dopant gas into the chamber and introduction of dilution gas into the chamber can be accomplished simultaneously or sequentially.

  For the best understanding of the present invention, reference is made to the drawings appended hereto.

  An example of a plasma doping system suitable for practicing the present invention is shown schematically in FIG. A plasma doping chamber 10 defines an enclosed volume. A platen 14 disposed within the chamber 10 provides a surface for holding a workpiece, such as a semiconductor wafer 20. For example, the wafer 20 is fastened around the flat surface of the platen 14. In one embodiment, the platen has an electrically conductive surface for supporting the wafer 20. In other embodiments, the platen includes conductive pins (not shown) for connection to the wafer 20. Wafer 20 and platen 14 function as cathodes in the plasma doping system.

  An anode 24 is disposed in the chamber 10 spaced from the platen 14. The anode 24 is movable in a direction perpendicular to the platen 14 as indicated by arrow 25. The anode is typically connected to the electrically conductive wall of the chamber 10 (both may be grounded). In another example, the platen 14 is grounded and a pulse is applied to the anode 24.

  When anode 24 is grounded, wear 20 (via platen 14) is connected to high voltage pulse source 30. The pulse source 30 typically provides pulses with an amplitude in the range of about 100 to 5,000 volts, a duration in the range of about 1 to 50 milliseconds, and a pulse repetition rate in the range of 100 Hz to 2 kHz. . It will be appreciated that the pulse parameter values are exemplary and other values may be used.

The enclosed volume 12 of the chamber 10 is connected to a vacuum pump 34 by a controllable valve 32. A process gas source 36 is connected to the chamber 10 via a mass flow controller 38. A pressure sensor 44 disposed in the chamber 10 provides a signal to the controller 46 indicating the chamber pressure. Controller 46 compares the sensed chamber pressure with the desired pressure input and provides a control signal to valve 32. The control signal controls the pressure 32 so as to minimize the difference between the chamber pressure and the desired pressure. Vacuum pump 34, valve 32, pressure sensor 44 and controller 46 constitute a closed loop pressure control system. The pressure is typically controlled in the range of about 1 millitorr to 500 millitorr, but is not limited to this range. A gas source 36 provides an ionizable gas containing the desired dopant for implantation into the workpiece. Examples of ionizable gases include BF ₃ , N ₂ , PH ₃ , AsH ₃ , B ₂ H ₆ , Ne, Ar, Kr and Xe. The mass flow controller 38 adjusts the ratio at which gas is supplied to the chamber 10. The configuration illustrated in FIG. 1 provides a continuous flow of process gas having a constant gas flow rate and a constant pressure. The pressure and flow rate are preferably adjusted to give repeatable results.

The plasma doping system can include a hollow cathode 54 connected to a hollow cathode pulse source 56. In one embodiment, the hollow cathode 54 comprises a conductive hollow cylinder that surrounds the gap between the anode 24 and the platen 14. Other details regarding the use of hollow cathodes are given in US Pat.
U.S. Patent No. 6,182,604

  One or more Faraday cups can be placed in the vicinity of the platen 14 to measure the ion dose implanted into the wafer 20. In the embodiment of FIG. 1, annular Faraday cups 50 and 52 are disposed around the wafer 20. The Faraday cup is electrically connected to a dose processor 70 or a dose monitor circuit. The plasma doping system may have a guard ring 66 that surrounds the platen 14. The guard ring 66 can be biased to improve the uniformity of the ion distribution implanted near the edge of the wafer 60.

  During operation, the wafer 20 is placed on the platen 14. The pressure control system, mass flow controller 38 and gas source 36 form the desired pressure and gas flow rate in the chamber 10. The pulse source 30 applies a high voltage pulse continuously to the wafer 20 to form a plasma 40 in the plasma discharge region 44 between the wafer 20 and the anode 24. In the prior art, the plasma 40 contains positive ions of an ionizable gas from a gas source 36. The plasma 40 includes a plasma sheath 42 in the vicinity of the wafer 20, typically on the surface thereof. During the high voltage pulse, the electric field between the anode 24 and the platen 14 accelerates positive ions from the plasma 40 through the plasma sheath 42 to the platen 14. The accelerated ions are implanted into the wafer 20 to form impurity regions. The pulse voltage is selected to inject positive ions into the wafer 20 to a desired depth. The number of pulses and the pulse duration are selected so that a desired dose of impurities is formed on the wafer 20. The current per pulse is a function of the pulse voltage, gas pressure, seed and various positions of the electrode.

As described above, the dopant gas species used for plasma implantation decomposes during the implantation process to form a surface layer deposited on the wafer 20. Examples of dopant gas species include AsH ₃ (hydrogen arsenide), PH ₃ (phosphine), and B ₂ H ₆ . For example, hydrogen arsenide decomposes to As, AsH, and AsH ₂ (which can be deposited on the surface of the wafer 20). These deposited surface layers degrade the dose repeatability, reduce the dose uniformity, and cause measurement problems.

  The formation of the deposition surface layer can be reduced or eliminated by introducing a dilution gas into the plasma chamber 10 along with the dopant gas. The dilution gas molecules are ionized and collide with the surface of the wafer, sputtering the deposited surface layer formed.

  An embodiment of a gas supply system for introducing dopant gas and dilution gas into the plasma doping chamber 10 is shown in FIGS. In FIG. 2, a dopant gas source 100 is connected to the plasma doping chamber 10 via a mass flow controller 102, and a dilution gas application 110 is connected to the plasma doping chamber 10 via a mass flow controller 112. Therefore, the supply of dopant gas and the supply of dilution gas are controlled separately. In the embodiment of FIG. 3, a premixed gas source 120 is connected to the plasma doping chamber 10 via a mass flow controller 122. The premixed gas source 120 includes a dopant gas and a dilution gas in a desired ratio.

  In general, the dilution gas parameter is selected to limit the formation of a deposition surface layer on the wafer 20. The atomic mass of the dopant gas and diluent gas can be similar to achieve effective removal of the deposited surface layer. Further, the ratio of diluent gas is selected such that the ratio of diluent gas to dopant gas removes the deposited surface layer upon formation of that layer. In other embodiments, a dilution gas with a high ratio is utilized. A high ratio is utilized because the dilution gas is not ionized more efficiently than the dopant gas, and a high ratio of dilution gas is required to remove the deposited surface layer as desired. In other embodiments, an inert diluent gas is used.

As an example, the dopant gas may be arsenic gas and the diluent gas may be krypton or xenon. In this example, the ratio of diluent gas to dopant gas is about 50. As a result, the gas in the plasma doping chamber 10 becomes 98% dilution gas and 2% dopant gas. In another embodiment, the dopant gas is phosphine gas and the diluent gas is argon. In this example, the ratio of diluent gas to dopant gas is also about 50. As another example, the dopant gas is B ₂ H ₆ and the diluent gas is neon.

  In the above embodiment, the diluent gas is an inert gas and the deposited surface layer is removed by physical sputtering. In other embodiments, the diluent gas component may have chemically active properties. In particular, the diluent gas component is selected to weaken the bonding of the deposited surface layer, reduce the energy required for sputtering, and perform chemically enhanced sputtering. In some embodiments, a halogen gas is used that includes a gas having fluorine or chlorine atoms. As one example, the dilution gas is a mixture of the above-described inert gas and fluorine. The mixture of inert gas and fluorine achieves a combination of physical sputtering and chemically enhanced sputtering. The chemically active component of the dilution gas should be a species that does not form a stable solid surface layer on the substrate.

  The process of reducing or removing the deposited surface layer is to introduce a diluent gas into the plasma dopant chamber simultaneously with the plasma injection, as described above, and remove the deposited surface layer during the formation of that layer. As another example, the deposited surface layer can be removed following plasma implantation. For example, plasma implantation can be performed by using a desired dopant gas without using a dilution gas. Following completion of the plasma implantation, the plasma doping chamber is evacuated with dopant gas and a diluent gas is introduced into the plasma doping chamber. The dilution gas is ionized for plasma formation, and the dilution gas is accelerated towards the wafer to remove the deposited surface layer. In this embodiment, the deposited surface layer is removed after its formation. The above dilution gas can also be used in this example. In this example, a “dilution gas” is a gas that does not dilute the dopant gas but is used to remove the deposited surface layer. In another embodiment, plasma implantation and removal of the deposited surface layer by dilution gas sputtering are performed in different processing chambers.

  The reduction of the deposited surface layer by the dilution gas was measured by several techniques. X-ray photoelectron spectroscopy (XRS) has been used with an argon diluent gas to quantify the surface concentration of phosphorus without this gas. Measurements were made at two depths in the wafer and showed preferential removal of surface phosphorus without significantly changing the amount of deeper phosphorus in the silicon (implanted part). This was also confirmed by secondary ion mass spectroscopy (SIMS) showing a phosphorus implantation depth profile with and without argon dilution gas. This profile is almost identical except for the wafer surface (implantation with argon dilution gas showed a relatively high surface phosphorus concentration). Similar SIMS was performed for arsenic injection with or without xenon diluent gas, with similar results to phosphorus. Furthermore, dilution with hydrogen gas did not achieve this effect, and switching from argon to xenon showed an increased deposition effect on phosphorus injection. This indicates the mass dependent efficiency of the dilution gas species.

  While the preferred embodiment of the invention has been illustrated and described, those skilled in the art will recognize that various changes and modifications can be made without departing from the spirit of the invention as defined by the claims. .

FIG. 1 is a simplified schematic block diagram of a plasma doping system. FIG. 2 is a block diagram of a gas supply system according to the first embodiment. FIG. 3 is a block diagram of a gas supply system according to the second embodiment.

Claims (25)

A method for limiting the formation of a deposited surface layer on a workpiece during plasma implantation comprising:
Introducing dopant gas and diluent gas into a plasma doping chamber for ionization to form dopant gas ions and diluent gas ions;
Accelerating dopant gas ions and diluent gas ions toward the workpiece;
Wherein dopant gas ions are implanted into the workpiece and the diluent gas ions remove the deposited surface layer from the workpiece.   The method of claim 1, wherein the atomic masses of the dopant gas and the diluent gas are approximately equal.   The method of claim 1, wherein the ratio of diluent gas to dopant gas is selected to remove the deposited surface layer upon formation thereof.   The method of claim 1, wherein the ratio of diluent gas to dopant gas is relatively high.   The method of claim 1, wherein the ratio of diluent gas to dopant gas is about 50.   The method of claim 1, wherein introducing the dopant gas and diluent gas comprises introducing an inert diluent gas.   The method of claim 1, wherein the dopant gas is hydrogen arsenide and the diluent gas is krypton or xenon.   The method of claim 1, wherein the dopant gas is phosphine and the diluent gas is argon. The method of claim 1, wherein the dopant gas is B ₂ H ₆ and the diluent gas is neon.   The method of claim 1, wherein the step of introducing a dopant gas and a dilution gas comprises separately supplying the dopant gas and the dilution gas to the plasma doping chamber.   The method of claim 1, wherein the step of introducing a dopant gas and a diluent gas comprises premixing the dopant gas and the diluent gas prior to the introduction.   The method of claim 1, wherein the diluent gas comprises a chemically active component.   The method of claim 1, wherein the diluent gas comprises a halogen atom.   The method of claim 1, wherein the diluent gas comprises fluorine or chlorine.   The method of claim 1, wherein the diluent gas comprises fluorine.   The method of claim 1, wherein the diluent gas comprises an inert gas and fluorine. A plasma dopant device comprising:
A plasma doping chamber;
A platen located in the plasma doping chamber for supporting a workpiece;
An anode spaced from the platen in the plasma dopant chamber;
A process gas source coupled to the plasma doping chamber for introducing a dopant gas and a dilution gas into the plasma doping chamber;
A pulse source that applies a pulse between the platen and the anode to accelerate ions from the plasma toward the workpiece;
Including
A plasma comprising dopant gas ions and diluent gas ions is formed in a plasma discharge region between the anode and the platen;
Dopant gas ions are implanted into the workpiece, and dilution gas ions remove the deposited surface layer from the workpiece;
But the device.   The apparatus of claim 17, wherein the process gas source includes a dopant gas source coupled to the plasma doping chamber and a dilution gas source coupled to the plasma doping chamber.   The process gas source comprises a premixed gas source coupled to the plasma doping chamber, the premixed gas source comprising a dopant gas and a dilution gas in a desired ratio. Equipment. A method for plasma injection comprising:
Introducing a dopant gas into a first chamber for ionization to form dopant gas ions;
Accelerating dopant gas ions toward the workpiece such that dopant gas is injected into the workpiece;
Introducing diluent gas into a second chamber for ionization to form diluent gas ions;
Accelerating dilution gas ions toward the workpiece such that the dilution gas ions remove the deposited surface layer from the workpiece;
Including methods. The first and second chambers are the same chamber,
21. The method of claim 20, wherein introducing the dopant gas into the chamber and introducing the diluent gas into the chamber are performed simultaneously. The first and second chambers are the same chamber,
21. The method of claim 20, wherein introducing the diluent gas into the chamber is performed after introducing the dopant gas into the chamber.   21. The method of claim 20, wherein introducing the diluent gas into the second chamber is performed after introducing the dopant gas into the first chamber.   21. The method of claim 20, wherein the first and second chambers are the same chamber.   21. The method of claim 20, wherein the first and second chambers are different chambers.

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