Patent Application
20070069157
This invention relates to systems and methods for plasma ion implantation of substrates and, more particularly, to methods and apparatus for plasma ion implantation of substrates with improved dopant profiles.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional beamline ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. Energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Beamline ion implanters are typically designed for efficient operation at relatively high implant energies and may not function efficiently at the low energies required for shallow junction implantation.
Plasma doping systems, also known as plasma ion implantation systems, have been studied for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen, which functions as a cathode and which is located in a process chamber. An ionizable process gas containing the desired dopant material is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implant energies can be achieved.
In the plasma doping systems described above, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, known as plasma immersion systems, continuous or pulsed RF energy is applied to the process chamber, thus producing a continuous or pulsed plasma. At intervals, negative voltage pulses, which may be synchronized with the RF pulses, are applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer.
The plasma doping system produces in the wafer being implanted a dopant profile, which may be defined as the dopant concentration as a function of depth from the surface of the wafer. It is desirable that the dopant profile have a peak at a selected implant depth, with reduced dopant concentration at the surface of the wafer. In practice plasma doping systems may exhibit surface deposition and implantation at depths less than the selected implant depth, resulting in undesired dopant profiles that are peaked at or near the surface of the wafer. Accordingly, there is a need for methods and apparatus for plasma ion implantation with improved dopant profiles.
According to a first aspect of the invention, a plasma ion implantation system is provided. The plasma ion implantation system comprises a process chamber, a plasma source to generate a plasma in the process chamber, a platen to hold the substrate in the process chamber, and a pulse source to generate implant pulses to accelerate ions from the plasma into the substrate. The pulse source generates implant pulses having pulse widths that are sufficiently long to limit plasma ion implantation during a transient period at the start of each implant pulse to a small fraction of the total implanted dose.
According to a second aspect of the invention, a method is provided for plasma ion implantation of a substrate in a plasma ion implantation system including a process chamber. The method comprises generating a plasma in the process chamber, holding a substrate in the process chamber, and accelerating ions from the plasma into the substrate with implant pulses having pulse widths that are sufficiently long to limit plasma ion implantation during a transient period at the start of each implant pulse to a small fraction of the total implanted dose.
According to a third aspect of the invention, a plasma ion implantation system is provided. The plasma ion implantation system comprises a process chamber, a plasma source to generate a plasma in a region of the process chamber near a reference potential, a platen to hold a substrate in the process chamber, and a pulse source to generate implant pulses to accelerate ions from the region of plasma generation into the substrate.
According to a fourth aspect of the invention, a method is provided for plasma ion implantation of a substrate in a plasma ion implantation system including a process chamber. The method comprises generating a plasma in a region of the process chamber near a reference potential, holding a substrate in the process chamber, and accelerating ions with implant pulses from the region of plasma generation into the substrate.
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
An example of a plasma ion implantation system suitable for implementation of the present invention is shown schematically in
A process chamber 10 defines an enclosed volume 12. A platen 14 positioned within chamber 10 provides a surface for holding a substrate, such as a semiconductor wafer 20. The wafer 20 may, for example, be clamped at its periphery to a flat surface of platen 14. In one embodiment, the platen has an electrically conductive surface for supporting wafer 20. In another embodiment, the platen includes conductive pins (not shown) for connection to wafer 20. In a further embodiment, the platen includes a conductive element and a dielectric layer which separates the wafer from the conductive element.
An anode 24 is positioned within chamber 10 in spaced relation to platen 14. Anode 24 may be movable in a direction, indicated by arrow 26, perpendicular to platen 14. The anode is typically connected to electrically conductive walls of chamber 10, both of which may be connected to ground. In another embodiment, platen 14 is connected to ground, and anode 24 is pulsed to a positive voltage. In further embodiments, both anode 24 and platen 14 may be biased with respect to ground.
The wafer 20 (via platen 14) and the anode 24 are connected to a high voltage pulse source 30, so that wafer 20 functions as a cathode. The pulse source 30 typically provides pulses in a range of about 20 to 20,000 volts in amplitude and a pulse repetition rate of about 100 Hz to 20 kHz. Suitable pulse widths are described below. It will be understood that these pulse parameter values are given by way of example only and that other values may be utilized within the scope of the invention. In the embodiment where the platen includes a dielectric layer, the pulses are capacitively coupled to the wafer.
The enclosed volume 12 of chamber 10 is coupled through a controllable valve 32 to a vacuum pump 34. A process gas source 36 is coupled through a mass flow controller 38 to chamber 10. A pressure sensor 48 located within chamber 10 provides a signal indicative of chamber pressure to a controller 46. The controller 46 compares the sensed chamber pressure with a desired pressure input and provides a control signal to valve 32 or mass flow controller 38. The control signal controls valve 32 or mass flow controller 38 so as to minimize the difference between the chamber pressure and the desired pressure. Vacuum pump 34, valve 32, mass flow controller 38, pressure sensor 48 and controller 46 constitute a closed loop pressure control system. The pressure is typically controlled in a range of about 1 millitorr to about 500 millitorr, but is not limited to this range. Gas source 36 supplies an ionizable gas containing a desired dopant for implantation into the workpiece. Examples of ionizable gas include BF3, N2, Ar, PH3, AsH3 and B2H6. Mass flow controller 38 regulates the rate at which gas is supplied to chamber 10. The configuration shown in
The plasma doping system may 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 space between anode 24 and platen 14. The hollow cathode may be utilized in applications which require very low ion energies. In particular, hollow cathode pulse source 56 provides a pulse voltage that is sufficient to form a plasma within chamber 12, and pulse source 30 establishes a desired implant voltage. Additional details regarding the use of a hollow cathode are provided in the aforementioned U.S. Pat. No. 6,182,604, which is hereby incorporated by reference.
One or more Faraday cups may be positioned adjacent to platen 14 for measuring the ion dose implanted into wafer 20. In the embodiment of
The Faraday cups are electrically connected to a dose processor 70 or other dose monitoring circuit. Positive ions entering each Faraday cup through entrance 60 produce in the electrical circuit connected to the Faraday cup a current that is representative of ion current. The dose processor 70 may process the electrical current to determine ion dose.
The plasma ion implantation system may include a guard ring 66 that surrounds platen 14. The guard ring 66 may be biased to improve the uniformity of implanted ion distribution near the edge of wafer 20. The Faraday cups 50, 52 may be positioned within guard ring 66 near the periphery of wafer 20 and platen 14.
The plasma ion implantation system may include additional components, depending on the configuration of the system. Systems which utilize continuous or pulsed RF energy include an RF source coupled to an antenna or an induction coil. The system may include magnetic elements which provide magnetic fields that confine electrons and control plasma density and spatial distribution. The use of magnetic elements in plasma ion implantation systems is described, for example, in WO 03/049142, published 12 Jun. 2003, which is hereby incorporated by reference.
In operation, wafer 20 is positioned on platen 14. The pressure control system, mass flow controller 38 and gas source 36 produce the desired pressure and gas flow rate within chamber 10. By way of example, the chamber 10 may operate with BF3 gas at a pressure of 10 millitorr. The pulse source 30 applies a series of high voltage pulses to platen 14, causing formation of plasma 40 in a plasma discharge region 44 between wafer 20 and anode 24. As known in the art, plasma 40 contains positive ions of the ionizable gas from gas source 36. Plasma 40 includes a plasma sheath 42 in the vicinity, typically at the surface, of wafer 20. The electric field that is present between anode 24 and platen 14 during the high voltage pulse accelerates positive ions from plasma 40 across plasma sheath 42 toward platen 14. The accelerated ions are implanted into wafer 20 to form regions of impurity material. The pulse voltage is selected to implant the positive ions to a desired depth in wafer 20. The number of pulses and the pulse duration are selected to provide a desired dose of impurity material in wafer 20. The current per pulse is a function of pulse voltage, gas pressure and species and any variable position of the electrodes. For example, the cathode-to-anode spacing may be adjusted for different voltages.
Prior art plasma ion implantation systems may produce a dose profile that is peaked at or near the wafer surface, as indicated by curve 100 in
In embodiments of the invention, techniques are provided which reduce the low energy component of the implanted ions. The low energy component refers to ions at energies less than the desired implant energy. One source of the low energy component of the ions implanted into the wafer is the transient conditions at the beginning and end of each implant pulse in a pulsed plasma ion implantation system. In a pulsed ion implantation system, implant pulses, or bias pulses, are applied to the platen 14 which supports the semiconductor wafer. The implant pulses are usually negative pulses having the amplitude of the desired implant voltage. When the implant pulse applied to platen 14 switches from ground potential to the desired implant voltage, a plasma sheath at the surface of wafer 20 changes from a relatively thin plasma sheath to a thicker plasma sheath that corresponds to the applied voltage. The plasma sheath after a transient period is a region adjacent to wafer 20 that contains no charged particles except for ions crossing the sheath from the plasma into wafer 20. During a steady state condition after application of the implant pulse, ions are implanted at a more or less uniform energy from the plasma through the plasma sheath into wafer 20.
During the transient period after application of the implant pulse, the thin plasma sheath changes to a thicker plasma sheath. At the instant when the implant pulse is applied, a region above wafer 20 contains both electrons and positive ions. The electrons are quickly removed by the applied voltage, leaving the positive ions. The positive ions are attracted to the platen 14 by the applied negative voltage. However, the positive ions leave the plasma sheath region over a longer period, for example, over about 2 microseconds. The sheath, during this transient period, is known as a matrix sheath. The length of the transient period during which the matrix sheath exists depends on pulse risetime, ion mass, sheath thickness and the voltage across the sheath, and may be longer than the pulse risetime. The positive ions that are accelerated from the sheath region to the wafer 20 during the transient period may not be accelerated by the full voltage of the implant pulse applied to platen 14. The actual acceleration of these ions depends on their positions in the sheath region when the implant pulse is applied. The ions in the sheath region are accelerated by less than the full voltage of the implant pulse and, thus, are implanted nearer to the surface of the wafer than the desired implant depth, D.
A matrix sheath is not associated with the falltime of the implant pulse. However, a transient period during which ions are not accelerated by the full voltage of the implant pulse is associated with the end of each implant pulse. The transient period at the end of each implant pulse may correspond to the falltime of the implant pulse.
According to an embodiment of the invention, the effect of the low energy ion component that results from the transient period at the beginning of each implant pulse and the falltime at the end of each implant pulse is reduced by utilizing relatively long implant pulses. Current practice typically utilizes implant pulses of about 1 to 50 microseconds. For an implant pulse having a pulse width of 10 microseconds, a transient period of 2 microseconds at the beginning of each pulse has a significant contribution to the low energy component of the implant. However, the transient period remains fixed in length as the pulse width is varied, and the transient period makes a negligible contribution to the overall implant in the case of a relatively long implant pulse, such as 500 microseconds, for example.
Implant pulse widths in a range of greater than 100 microseconds to 5 milliseconds may be utilized to improve the dose profile produced by plasma ion implantation of semiconductor wafers. More preferably, implant pulse widths in a range of greater than 200 microseconds to 5 milliseconds may be utilized. The maximum pulse width may be limited by charging considerations in the case of plasma ion implantation of semiconductor wafers. In other applications, the maximum pulse width may not be limited.
The pulse width of the implant pulse should be much greater, preferably 100 or more times greater, than the sum of the transient period at the start of the implant pulse and the falltime at the end of the implant pulse. Thus for an example of a transient period of 2 microseconds and a falltime of 1 microsecond, an implant pulse of 300 microseconds or greater is used. This approach reduces the number of ions implanted at less than full energy to less than 1% of the total implanted dose. Thus, referring to
According to additional embodiments of the invention, the plasma ion implantation system is configured such that the plasma is generated in a region of the process chamber that is near ground potential or other reference potential and is accelerated to the wafer for implantation. This configuration ensures that the ions in the plasma are accelerated from near ground potential to the full energy produced by the implant pulse. As a result, the number of ions implanted at less than full energy is reduced in comparison with prior art configurations. A further feature of these embodiments is that the plasma generator can be enabled or turned on after the implant pulse reaches full voltage and can be disabled or turned off before the implant pulse ends. This further reduces the number of ions accelerated at less than full energy.
Simplified schematic block diagrams of plasma ion implantation systems in accordance with embodiments of the invention are shown in
As shown in
A plasma source 200 is positioned within chamber 10 and is spaced from platen 14. Plasma source 20 may be energized by a plasma power source 210. A plasma may be generated by any suitable technique, including but not limited to RF, microwave, glow discharge and the like. An electrically conductive grid 220 is positioned in chamber 10 between plasma source 200 and platen 14. Grid 220 may be electrically connected to chamber 10 and thus is at ground potential or other reference potential. Grid 220 defines a first region 230 of chamber 10 that contains plasma source 200 and a second region 232 of chamber 10 that contains platen 14. Grid 220 is provided with openings 222 so that a plasma generated by plasma source 200 may move from region 230 to region 232.
The plasma ion implantation system of
In the embodiment of
According to a further aspect of the invention, the low energy component of the implanted ions may be further reduced by controlling the timing of the plasma generation pulse relative to the implant pulse. An implant pulse 400 and a plasma generation pulse 410 are shown schematically in
Having described several embodiments and an example of the invention in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and the scope of the invention. Furthermore, those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the system of the present invention is used. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined by the following claims and their equivalents.
