KR20050034731A - Removal of plasma deposited surface layers by dilution gas sputtering - Google Patents

Abstract

A method for limiting the formation of a deposited surface layer on a workpiece, such as a semiconductor wafer, during plasma implantation includes introducing a dopant gas and a dilution gas into a plasma doping chamber for ionization to form dopant gas ions and dilution gas ions, and accelerating the dopant gas ions and the dilution gas ions toward the workpiece. The dopant gas ions are implanted into the workpiece, and the dilution gas ions remove a deposited surface layer from the workpiece. The atomic masses of the dopant gas and the dilution gas may be similar to achieve efficient removal of the deposited surface layer.

Description

Removal of Plasma Deposited Surface Layers by Dilution Gas Sputtering

The present invention relates to a plasma doping system used for ion implantation of a workpiece, and more particularly, to a method and apparatus for removing a plasma deposition surface layer by diluting 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 positioned 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 comprising ions of the dopant gas. The plasma has a plasma envelope around the wafer. The applied pulse causes ions in the plasma to be injected into the wafer across the plasma envelope. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implantation energy is achieved. For example, plasma doping systems include U.S. Patent No. 5,354,381 to Seng, Oct. 11, 1994, U.S. Patent No. 6,020,592 to Libert, et al. US Patent No. 6,182,604 to et al.

In the plasma doping system described above, an applied voltage pulse generates a plasma and accelerates cations from the plasma to the wafer. In another form of plasma system known as a plasma immersion system, continuous plasma is generated by, for example, inductively coupled RF power from an antenna located inside or outside the plasma doping chamber. The antenna is connected to an RF power source. Sometimes a voltage pulse is applied between the platen and the anode, causing ions in the plasma to be accelerated to the wafer.

Dopant gas species used for plasma implantation may decompose during the implantation process to form a surface layer deposited on the wafer. Examples of such dopant gas species include AsH ₃ , PH ₃ and B ₂ H ₆ . For example, arsine gas (AsH ₃ ) can be broken down into As, AsH and AsH ₂ and deposited on the surface of the wafer being implanted. Such deposited surface layers create non-repeatability, poor dose uniformity and metrology problems. Thus, what is needed is a method and apparatus for preventing or limiting the formation of such layers to achieve proper process control during plasma injection.

To better understand the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference.

1 is a simplified schematic block diagram of a plasma doping system.

2 is a block diagram of a gas supply system according to the first embodiment.

3 is a block diagram of a gas supply system according to a second embodiment.

In accordance with one aspect of the present invention, a method is provided for limiting the formation of a surface layer deposited on a workpiece during plasma injection. The method includes introducing a dopant gas and a dilution gas into the plasma doping chamber to ionize to form dopant gas ions and dilution gas ions, and accelerating the dopant gas ions and the dilution gas ions toward the workpiece. Dopant gas ions are implanted into the workpiece, and diluent gas ions remove the deposited surface layer from the workpiece. The workpiece can be a semiconductor wafer.

The atomic mass of dopant gas and diluent gas may be similar to achieve effective removal of the deposited surface layer. The ratio of diluent gas to dopant gas is selected when they are formed to remove the deposited surface layer. In some embodiments, a high proportion of diluent gas is used. In some embodiments, an inert diluent gas is used. In one embodiment, the dopant gas is arycin and the diluent gas is krypton or xenon. In this embodiment, the ratio of diluent gas to dopant gas is approximately 50. In another embodiment, the dopant gas is phosphine and the diluent gas is argon. In a further embodiment, the dopant gas is B ₂ H ₆ and the diluent gas is neon. In certain embodiments, the diluent gas comprises a chemically active component such as halogen. In certain embodiments, the chemically active ingredient consists of fluorine or chlorine. In one embodiment, the diluent gas is an inert gas and fluorine.

In some embodiments, the dopant gas and the diluent gas are introduced separately into the plasma doping chamber. In another embodiment, the dopant gas and the diluent gas are premixed before they are introduced into the plasma doping chamber.

According to another aspect of the invention, a plasma doping apparatus is provided. The plasma doping apparatus includes a plasma doping chamber, a platen located in the plasma doping chamber to support the workpiece, an anode spaced apart from the platen in the plasma doping chamber, a process gas source connected to the plasma doping chamber, and a pulse source. The process gas source introduces dopant gas and diluent gas into the plasma doping chamber, and a plasma comprising dopant gas ions and diluent gas ions is generated 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 ions into the workpiece. Dopant gas ions are implanted into the workpiece, and diluent gas ions remove the deposited surface layer from the workpiece.

According to a further aspect of the invention, a method for plasma injection is provided. The method includes introducing a dopant gas into the first chamber to ionize to form dopant gas ions, and accelerating the dopant gas ions toward the workpiece, wherein the dopant gas ions are implanted into the workpiece to form a second Diluent gas is introduced into the chamber and the dilution gas ions are accelerated toward the workpiece, which dilute gas ions remove the deposited surface layer from the workpiece. The first and second chambers may be the same chamber or different chambers. When the first and second chambers are the same chamber, the dopant gas injection step into the chamber and the dilution gas injection step into the chamber may be performed simultaneously or sequentially.

An example of a suitable plasma doping system for the operation of the present invention is schematically illustrated in FIG. The plasma doping chamber 10 forms a shielded volume 12. The platen 14 located in the chamber 10 provides a surface for holding a workpiece such as the semiconductor wafer 20. For example, the wafer 20 may be secured to the outer periphery of the plane of the platen 14. In one embodiment, the platen has a conductive face for supporting the wafer 20. In another embodiment, the platen includes conductive pins (not shown) for connecting to the wafer 20. Wafer 20 and platen 14 act as cathodes in the plasma doping system.

The anode 24 is located in the chamber 10 spaced apart from the platen 14. The anode 24 is movable in a direction perpendicular to the platen 14 indicated by the arrow 26. The anode is typically connected to the conductive wall of the chamber 10, both of which may be connected to ground. In another structure, the platen 14 is connected to ground, and the anode 24 is vibrated.

In the structure in which the anode 24 is connected to ground, the wafer 20 (via the platen 14) is connected to the high voltage pulse source 30. Pulse source 30 typically provides pulses within a range of amplitudes of about 100 to 5,000 volts, time of about 1 to 50 microseconds, and pulse frequency of about 100 Hz to 2 kHz. It is to be understood that these pulse parameter values are given by way of example only and other values may be utilized.

The enclosed volume 12 of the chamber 10 is connected to the vacuum pump 34 via a controllable valve 32. Process gas source 36 is connected to chamber 10 via mass flow controller 38. The pressure sensor 44 located in the chamber 10 provides a signal indication of the chamber pressure to the controller 46. The controller 46 compares the required pressure input with the sensed chamber pressure and provides a control signal to the valve 32. The control signal controls the valve 32 to minimize the difference between the chamber pressure and the required pressure. The vacuum pump 34, the valve 32, the pressure sensor 44 and the controller 46 constitute a closed loop pressure control system. The pressure is typically controlled in the range of about 1 to about 500 millitorr, but is not limited to this range. Gas source 36 provides an ionizing gas, including the dopant, required to inject into the workpiece. Examples of ionizing gases include BF ₃ , N ₂ , PH ₃ , AsH ₃ , B ₂ H ₆ , F, Cl, Ne, Ar, Kr and Xe. Mass flow controller 38 regulates the rate at which gas is supplied to chamber 10. The structure shown in Figure 1 provides a continuous flow of process gas at a constant gas flow rate and constant pressure. Pressure and gas flow rates can be adjusted to provide repeatable results.

The plasma doping system includes a hollow cathode 54 connected to the hollow cathode pulse source 56. In one embodiment, hollow electrode 54 includes a conductive hollow cylinder that encloses the space between anode 24 and platen 14. Further detailed considerations regarding the use of hollow cathodes are provided in the aforementioned US Pat. No. 6,182,604, which is incorporated herein by reference.

One or more Faraday cups may be positioned proximate to the platen 14 to measure the ion dose implanted into the wafer 20. In the embodiment of Figure 1, annular Faraday cups 50, 52 are located around the outer circumference of the wafer 20. Faraday cups 50, 52 are electrically connected to dose processor 70 or other dose monitoring circuitry. The plasma doping system may include a protective ring 66 surrounding the platen 14. Protective ring 66 may be deflected to enhance uniformity of implanted ion distribution near the edge of wafer 20.

In operation, the wafer 20 is positioned on the platen 14. The pressure control system, mass flow controller 38 and gas source 36 provide the pressure and gas flow rates required within the chamber 10. The pulse source 30 applies a series of high voltage pulses to the wafer 20 to form a plasma 40 in the plasma discharge region 48 between the wafer 20 and the anode 24. As can be seen in the art, the plasma 40 includes cations of ionizing gas from the gas source 36. The plasma 40 typically includes a plasma envelope 42 near the surface of the wafer 20. The electric field provided between the anode 24 and the platen 14 during the high voltage pulse accelerates positive ions across the plasma envelope 42 from the plasma 40 to the platen 14. Accelerated ions are implanted into the wafer 20 to form an impurity material region. The pulse voltage for injecting cations to the required depth in the wafer 20 is selected. Multiple pulses and pulse durations are selected to provide the desired dose of impurity material into the wafer 20. The current per pulse is a function of pulse voltage, gas pressure, gas type and any variable position of the electrode.

As can be seen above, the dopant gas species used for plasma implantation can decompose during the implantation process to form the surface layer deposited on the wafer 20. Examples of dopant gas species may include AsH ₃ (arcin), PH ₃ (phosphine) and B ₂ H ₆ . For example, arsine gas may be decomposed into As, AsH and AsH ₂ and deposited on the surface of the wafer 20. Such deposited surface layers cause dose non-repeatability, poor dose uniformity and metrology problems.

The formation of the deposited surface layer can be reduced or eliminated by introducing diluent gas into the plasma doping chamber 10 with the dopant gas. Dilution gas molecules are ionized to impact the surface of the wafer, which occurs when sputtering of the deposited surface layer occurs.

Examples of gas supply systems for introducing dopant gas and diluent gas into the plasma doping chamber 10 are shown in FIGS. 2 and 3. In FIG. 2, the dopant gas source 100 is connected to the plasma doping chamber 10 via a mass flow controller 102, and the dilution gas source 110 is connected to the plasma doping chamber 10 through a mass flow controller 112. Is connected to. In addition, the dopant gas source and the diluent gas source are individually controlled. In the embodiment of FIG. 3, the premixed gas source 120 is connected to the plasma doping chamber 10 via a mass flow controller 122. Premixed gas source 120 includes dopant gas and diluent gas in the required proportions.

In general, the dilution gas parameter is selected to limit the formation of the surface layer deposited on the wafer 20. The atomic mass of dopant gas and diluent gas may be similar to achieve effective removal of the deposited surface layer. In addition, the ratio of diluent gas to dopant gas is selected to remove the deposited surface layer formed. In some embodiments, a high proportion of diluent gas is used. Since a diluent gas is ionized less effectively than a dopant gas, a high ratio can be used, and a higher ratio of diluent gas is necessary to achieve the required removal of the deposited surface layer. In some embodiments, an inert diluent gas is utilized.

In one embodiment, the dopant is lysine gas and the diluent gas may be krypton or xenon. In such embodiments, the ratio of diluent gas to dopant gas may be about 50. As a result, the gas in the plasma doping chamber 10 is 98% diluent gas and 2% dopant gas. In another embodiment, the dopant gas is phosphine and the diluent gas is argon. The ratio of diluent gas to dopant gas in this embodiment may also be about 50. In a further embodiment, the dopant gas is B ₂ H ₆ and the diluent gas is neon.

In the embodiment described above, the diluent gas is an inert gas and the deposited surface layer is removed by physical sputtering. In other embodiments, the components of the diluent gas may have chemical activity. In particular, the components of the diluent gas may be selected to weaken the bonding of the deposited surface layer and reduce the energy required for sputtering, resulting in chemically enhanced sputtering. In certain embodiments, halogens containing gases such as fluorine or chlorine atoms are used. In one particular embodiment, the diluent gas is a mixture of inert gas and fluorine as described above. The mixture of inert gas and fluorine produces a combination of physical sputtering and chemically enhanced sputtering. The chemically active component of the diluent gas should be a species that does not form a stable solid surface layer on the substrate.

The process of reducing or eliminating the formation of the deposited surface layer has been described above including the introduction of diluent gas into the plasma doping chamber simultaneously with plasma injection to remove the deposited surface layer formed. In another embodiment, the deposited surface layer may be removed by subsequent plasma injection. For example, plasma injection can be performed by using the required dopant gas and undiluted gas. After completion of the plasma injection, the plasma doping chamber may be washed with the dopant gas and the dilution gas may be introduced into the plasma doping chamber. Dilution gas is ionized to form a plasma, and dilution gas ions are accelerated toward the wafer to remove the deposited surface layer. In this embodiment, the deposited surface layer is removed after it is formed. The diluent gas described above can be used in this embodiment. In this embodiment, although the gas does not dilute the dopant gas, "diluent gas" refers to the gas used for removal of the deposited surface layer. In further embodiments, removal of the deposited surface layer by plasma injection and dilution gas sputtering may be performed in a different process chamber.

The reduction of the deposited surface layer due to the diluent gas has been measured by various techniques. X-ray photoelectron spectroscopy (XPS) is used to measure the amount of phosphorus surface concentration with and without argon diluent gas. Measurements were made on the depth of both cases in the wafer and showed preferential removal of surface phosphorus without significantly altering the amount of phosphorus deeper into the silicon (ie implanted portion). Evidence was confirmed by a second ion mass spectrometry (SIMS) measurement with a phosphorus depth profile with and without argon diluent gas. The profile is almost the same except for the wafer surface, and the implants made with argon diluent gas showed very low relative surface phosphorus concentrations. Similar SIMS measurements were made for arsine injection with and without xenon diluent gas, with similar results as for phosphorus. In addition, dilution with hydrogen gas did not provide this effect, and in the case of phosphorus implantation, switching argon to xenon showed an increased deposition effect. This shows the mass dependent effect of the diluting gas species.

While preferred embodiments of the invention have been shown and described herein, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope of the invention as defined by the appended claims. something to do.

Claims (25)

A method of limiting the formation of a surface layer deposited on a workpiece during plasma injection, Introducing a dopant gas and a dilution gas into the plasma doping chamber to ionize to form dopant gas ions and dilution gas ions, Accelerating the dopant gas and the diluent gas toward the workpiece, Dopant gas ions are implanted into the workpiece, and dilute 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 the same. The method of claim 1, wherein the ratio of diluent gas to dopant gas is selected to remove the deposited surface layer when the deposited surface layer is formed. 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 the diluent gas comprises introducing an inert diluent gas. The method of claim 1, wherein the dopant gas comprises arsine and the diluent gas comprises krypton or xenon. The method of claim 1 wherein the dopant gas comprises phosphine and the diluent gas comprises argon. The method of claim 1, wherein the dopant gas comprises B ₂ H ₆ and the diluent gas comprises neon. The method of claim 1, wherein introducing the dopant gas and the dilution gas comprises separately supplying the dopant gas and the diluent gas to the plasma doping chamber. The method of claim 1, wherein introducing the dopant gas and the diluent gas comprises premixing the dopant gas and the diluent gas before introducing into the plasma doping chamber. The method of claim 1 wherein the diluent gas comprises a chemically active ingredient. 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 doping chamber, A platen located in the plasma doping chamber to support a workpiece; An anode spaced from the platen of the plasma doping chamber; A process gas source connected to the plasma doping chamber for introducing dopant gas and diluent gas into the plasma doping chamber; A pulse source for providing a pulse between the platen and the anode to accelerate ions from a plasma to a workpiece, A plasma comprising ions of a dopant gas and a diluent gas is generated 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 stacked surface layer from the workpiece. 18. The plasma doping apparatus of claim 17, wherein the process gas source comprises a dopant gas source connected to the plasma doping chamber and a diluent gas source connected to the plasma doping chamber. 18. The plasma doping apparatus of claim 17, wherein the process gas source comprises a premixed gas source coupled to a plasma doping chamber, wherein the premixed gas source comprises a dopant gas and a diluent gas in a desired ratio. Method for plasma injection, Introducing a dopant gas into the first chamber to ionize to form dopant gas ions, Accelerating the dopant gas ions toward the workpiece such that dopant gas ions are injected into the workpiece, Introducing a dilution gas into the second chamber to ionize to form dilution gas ions, Accelerating the dilution gas ions toward the workpiece such that the dilution gas ions remove the deposited surface layer from the workpiece. The method of claim 20, wherein the first and second chambers are the same chamber, Introducing a dopant gas into the chamber and introducing a diluent gas into the chamber at the same time. The method of claim 20, wherein the first and second chambers are the same chamber, 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 diluent gas into the second chamber is performed after introducing dopant gas into the first chamber. The method of claim 20, wherein the first and second chambers are the same chamber. The method of claim 20, wherein the first and second chambers are different chambers.