Patent Publication Number: US-2003232513-A1

Title: Plasma method and apparatus for processing a substrate

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1). Field of the Invention  
       [0002] This invention relates to a plasma reactor and a method of processing a substrate by creating a plasma.  
       [0003] 2). Discussion of Related Art  
       [0004] The manufacture of integrated circuits involves the manufacture of field effect transistors in and on silicon or other semiconductor substrates. The manufacture of a field effect transistor includes the formation of a gate dielectric layer. The dielectric layer is typically grown by exposing silicon of the substrate to oxygen, thereby forming silicon dioxide gate dielectric layers.  
       [0005] As logic devices have become smaller, it has become advantageous to include nitrogen into the silicon dioxide gate dielectric layers. Nitrogen is often incorporated by creating a plasma of nitrogen ions within a chamber and implanting the nitrogen ions into the gate dielectric layer. The plasma is typically created utilizing a radio frequency (RF) source, with either an electrode plate (capacitative coupling) or a coil (inductive coupling). The RF source creates an RF field within a gas in the chamber, and this coupling creates the plasma.  
       [0006] Independent of the type of RF source (plate or coil), there can be significant capacitative coupling from the source to the plasma, which creates a relatively large plasma potential, on the order of tens of volts. Such a large plasma potential may cause excessive bombardment of the silicon dioxide layer with nitrogen ions, which can cause damage to the silicon dioxide layer and even incorporation of nitrogen into the underlying silicon. Damage to the silicon dioxide layer or incorporation of nitrogen into the underlying silicon diminishes the advantages of nitrogen incorporation.  
       SUMMARY OF THE INVENTION  
       [0007] According to one aspect of the invention, a method is provided of processing a substrate, including locating the substrate in a processing chamber, creating a nitrogen plasma in the chamber, the plasma having an ion density of at least 10 10  cm −3 , and a potential of less than 20 V, and exposing a layer on the substrate to the plasma to incorporate nitrogen from the plasma into the layer.  
       [0008] According to another aspect of the invention, a method of processing a substrate is provided, wherein the substrate is located in a plasma processing chamber, a nitrogen-containing gas flows into the chamber, an RF current is provided through a coil to generate an RF field in the chamber, the RF field creating a nitrogen-containing RF plasma out of the gas, the RF current being pulsed, and incorporating nitrogen ions and excited neutrals from the plasma into a gate dielectric layer formed on the substrate.  
       [0009] According to a further aspect of the invention, a plasma reactor is provided, including a chamber having an opening to transfer a substrate into an internal volume of the chamber, a substrate holder in the chamber for holding the substrate, an RF coil externally and adjacent to a wall of the chamber, and a grounded electrode plate between the wall and the RF coil. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The invention is further described by way of examples with reference to the accompanying drawings, wherein:  
     [0011]FIG. 1 is a perspective view of a plasma reactor according to an embodiment of the invention;  
     [0012]FIG. 2 is a cross-sectional side view of upper components of the plasma reactor;  
     [0013]FIG. 3 is a cross-sectional side view illustrating nitrogen ion incorporation into a silicon dioxide gate dielectric layer;  
     [0014]FIG. 4 is a graph illustrating plasma potential as a function of pressure for various RF source powers and electrode plate configuration as measured with a Langmuir probe;  
     [0015]FIG. 5 is a graph illustrating the floating voltages as a function of pressure for the electrode plate configuration as measured with a Langmuir probe;  
     [0016]FIG. 6 is a graph illustrating electron density as a function of pressure for the electrode plate configuration as measured with a Langmuir probe;  
     [0017]FIG. 7 is a graph illustrating ion density as a function of pressure for the electrode plate configuration as measured with a Langmuir probe;  
     [0018]FIG. 8 is a graph illustrating electron temperature as a function of pressure for the electrode plate configuration as measured with a Langmuir probe;  
     [0019]FIG. 9 is a bottom view of laminate, including an electrode plate, according to an embodiment of the invention.  
     [0020]FIG. 10 is a cross-sectional side view illustrating the laminate in an installed position;  
     [0021]FIG. 11 is a graph illustrating pulsing of RF power to an RF coil with a 30% duty cycle;  
     [0022]FIG. 12 is a graph similar to FIG. 11 at a 50% duty cycle;  
     [0023]FIG. 13 is a graph illustrating thickness change before and after nitrogen plasma treatment with pulsed RF power, and provides a measure of incorporated nitrogen;  
     [0024]FIG. 14 is a graph illustrating thickness change for different samples processed at different continuous RF power settings;  
     [0025]FIG. 15 is a graph illustrating thickness change as a function of RF source peak power for two pulsing frequencies;  
     [0026]FIG. 16 is a graph illustrating thickness change as a function of duty cycles for two pulsing frequencies;  
     [0027]FIG. 17 is a graph illustrating optical emissions spectra for 500 W peak power at various pulsing frequencies and duty cycles; and  
     [0028]FIG. 18 illustrates optical emission spectra for a 50% duty cycle at various pulsing frequencies and peak powers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0029]FIGS. 1 and 2 illustrate a plasma reactor  10 , according to an embodiment of the invention, including a chamber  12 , a substrate holder  14 , an RF coil  16 , and an electrode plate  18 . The electrode plate  18  is connected through a body of the chamber  12  to ground  20 . By grounding the electrode plate  18 , a capacitive coupling between the RF coil  16  and a plasma  22  in an internal volume  24  of the chamber  12  is eliminated. The elimination of the capacitive couple reduces the potential of the plasma  22  without dramatically altering other properties of the plasma  22 , such as ion density and electron density. The inductive coupling from the RF coil  16  is not eliminated, and this coupling creates and maintains the plasma  22 .  
     [0030] Referring specifically to FIG. 1, the plasma reactor  10  further includes a lower transfer chamber  26  and a transfer mechanism  28 . The chamber  12  is positioned on top of the transfer chamber  26 . An internal volume  30  of the transfer chamber  26  is placed in communication with the internal volume  24  of the chamber  12  through a circular opening  32  in a base of the chamber  12 . The substrate holder  14  is secured on top of the transfer mechanism  28 , and the transfer mechanism  28  can be used to elevate or lower the substrate holder  14 .  
     [0031] In use, the transfer mechanism  28  is operated so that the substrate holder  14  is lowered into the internal volume  30  of the transfer chamber  26 . A wafer substrate, positioned on a blade attached to a robot arm, is then transferred through a slit-valve opening in a wall of the transfer chamber  26  into the internal volume  30 . The transfer mechanism  28  is then operated to elevate the substrate holder  14  so that the substrate holder  14  contacts a lower surface of the wafer substrate and elevates the wafer substrate off the blade. The blade is then removed from the transfer chamber  26 , whereafter the transfer mechanism  28  is again operated to elevate the substrate holder  14  into the opening  32 . The wafer substrate, located on the substrate holder  14 , then has an upper surface which is exposed to the internal volume  24  of the chamber  12 .  
     [0032] The chamber  12  includes primarily a conductive body  36  and a dielectric dielectric quartz upper wall  38 . The conductive body  36  forms a lower portion of the chamber  12 , and the upper wall  38  forms an upper portion of the chamber  12 . The conductive body  36  and the upper wall  38  jointly define the internal volume  24 .  
     [0033] Four gas nozzle ports  40  are formed through the conductive body  36  into the internal volume  24 . The gas nozzle ports  40  are positioned at 90° intervals around the substrate holder  14 . The conductive body  36  also defines a vacuum pumping channel  42  on one side thereof. The gas nozzle ports  40  are connected through valves to a gas manifold, and the vacuum pumping channel  42  is connected to a pump. When the pump is operated, gases are extracted from the internal volume  24  through the vacuum pumping channel  42  to reduce a pressure within the internal volume  24 . The valves can be operated to allow gases from the manifold through the valves and the gas nozzle ports  40  into the internal volume  24 .  
     [0034] Referring more specifically to FIG. 2, the upper wall  38  has a dome shape, and the electrode plate  18  has a dome shape that conforms to an outer surface of the upper wall  38 . The electrode plate  18  is in fact located directly on the upper wall  38 . The electrode plate  18  defines a circular opening  44  over a center of the upper wall  38 . The upper wall  38  and the electrode plate  18  are symmetrical around a vertical axis  46 .  
     [0035] The coil  16  spirals around the vertical axis  46  and the opening  44 . The coil  16  is positioned on and conforms to the dome shape of the electrode plate  18 . One end of the coil  16  is connected to an RF source  50 , and an opposing end of the coil  16  is connected to ground  52 .  
     [0036] Reference is now made to FIGS. 2 and 3 in combination. An epitaxial silicon layer  54  is formed on an upper surface of a wafer substrate before the wafer substrate is inserted into the plasma reactor  10  positioned on an upper surface of the substrate holder  14 . A thin silicon dioxide layer  58  is grown on the silicon layer  54 , also before the wafer substrate is inserted into the plasma reactor  10 . The silicon dioxide layer  58  is on the order of a few angstroms (e.g., 40 Å) thick, and is later used as a gate dielectric layer in a finally manufactured transistor. The purpose of inserting the wafer substrate into the plasma reactor  10  is to incorporate nitrogen (N) into the silicon dioxide layer  58  for purposes of modifying or improving its dielectric properties. The plasma  22  of nitrogen ions (N 2   + ) is created within the internal volume  24 . The nitrogen ions have energies defined by the properties of the plasma which leads to their being incorporated into the silicon dioxide layer  58 .  
     [0037] The plasma is created by first reducing the pressure within the internal volume  24  to a predetermined level. A nitrogen-containing gas is then introduced into the internal volume  24 . The nitrogen-containing gas may, for example, be pure nitrogen (N 2 ), a mixture of nitrogen and helium gases (N 2 /He), a mixture of nitrogen and neon gases (N 2 /Ne), or a mixture of nitrogen and argon gases (N 2 /Ar). For purposes of further discussion, examples are given where the gas is pure nitrogen gas.  
     [0038] The RF source  50  is then operated to provide RF current to the coil  16  at a frequency of 13.56 MHz. The RF coil  16  generates an RF field which is spread by the electrode plate  18  across the upper wall  38 . The circular opening  44  permits the RF field to enter through the upper wall  38  into the internal volume  24 . The RF field then couples with the nitrogen gas in the internal volume  24 . The RF field initially excites a small number of free electrons. The free electrons then collide with other atoms to release more electrons from these atoms. The process is continued until a steady-state condition is achieved, where the plasma  22  has a steady amount of free electrons and free ions, a steady electron temperature, and a constant voltage relative to ground. A “reservoir” of ions is so created within the internal volume  24 , and the voltage potential of the plasma  22  assists in incorporating ions from this reservoir into the silicon dioxide layer  58 . The potential of the substrate and the substrate holder  14  floats freely during the entire process, but there is a difference in the voltage of the plasma  22  and that of the substrate holder  14 , the difference driving the incorporation of the ions.  
     [0039] Without grounding the electrode plate  18 , the RF coil  16  couples capacitively to the plasma  22 . Such a capacitive couple between the RF coil  16  and the plasma  22  increases the voltage of the plasma  22 . Conversely, by grounding the electrode plate  18 , the capacitive coupling is substantially reduced, and the voltage of the plasma  22  is reduced. The plasma potential and the electron temperature are reduced, but ion density remains relatively high. To prevent excessive incorporation of nitrogen through the SiO 2  and into the silicon substrate, plasma potential is preferably less than 10 V. Electron temperatures are preferably near or less than 2 eV. Ion density is preferably at least 10 10  cm −3 .  
     [0040]FIG. 4 illustrates experimental results utilizing no electrode plate, a regular ungrounded electrode plate, and a grounded electrode plate, respectively. In each case, experimental results were obtained when applying 300 W, 500 W, and 900 W of power to the RF coil  16 . Larger blocks or triangles indicate larger power magnitudes. At a given power provided to the RF coil  16 , the plasma voltage (Vp) is the smallest for a grounded electrode plate, higher for an ungrounded electrode plate, and even higher when there is no electrode plate. In other examples, effective RF power supplied to the RF coil  16  may be between 160 and 3000 W. Potentials below 10 V are not achievable without the grounded electrode plate. What should also be noted is that the potentials do not substantially increase with an increase in power provided to the RF coil. Even very large power magnitudes above 1000 W (e.g., 1400 W), crease plasma voltages below 20 V at pressures above 5 milliTorr (mT), and plasma voltages below 10 V at pressures above 40 mT.  
     [0041]FIG. 5 illustrates the floating voltage of the plasma for the condition of FIG. 4. The potential at which the wafer resides is at or near Vf. Again, it can be seen that the substrate voltage (Vs) is the smallest for a grounded electrode plate, higher for an ungrounded electrode plate, and even higher when there is no electrode plate.  
     [0042]FIGS. 6 and 7 illustrate electron density and ion density, respectively. For a given magnitude of power applied to the RF coil  16 , there is very little difference between the electron density (or the ion density), when using a grounded electrode plate and when using an ungrounded electrode plate. Although not slow, ion densities above 50×10 9  are achievable when RF power above 1000 W is provided to the RF coil.  
     [0043]FIG. 8 illustrates electron temperature. It can be seen that at lower pressures there is relatively little difference in electron temperatures when using a grounded, ungrounded, or no electrode plate. However, at higher pressures, typically above 40 mT, it can be seen that electron temperature is much higher where an ungrounded electrode plate is used, or when no electrode plate is used, than when a grounded electrode plate is used.  
     [0044] Referring to FIGS. 9 and 10, the electrode plate  18  is laminated between two dielectric sheets  60  and  62 . The electrode plate  18  and the dielectric sheets  60  and  62  are formed in strips  64  that, when folded toward one another, collectively define a dome shape. The dielectric sheet  60  is positioned at the top between the electrode plate  18  and the RF coil  16 . The dielectric sheet  62  is located between the electrode plate  18  and the upper wall  38 . Ends of the electrode plate are not covered by the dielectric sheet  62 , to leave exposed lands  66 . The exposed lands  66  contact a conductive portion of the conductive body  36 , to ground the electrode plate  18  to the conductive body  36 . The lands  66  are disposed on a perimeter of the electrode plate  18 , so that the electrode plate  18  is peripherally grounded. Peripheral grounding of the electrode plate  18  ensures that the entire electrode plate  18  is as close to zero volts as possible.  
     [0045] The plasma voltage can also be reduced by pulsing the RF power provided to the RF coil  16 . In the examples that are now provided, the electrode plate  18  was not grounded, although it should be understood that the electrode plate  18  may be grounded in addition to pulsing of the RF power provided to the RF coil  16 .  
     [0046] As illustrated in FIGS. 11 and 12, RF power having a frequency of 13.56 MHz and a predetermined peak power is provided to the RF coil  16 . The RF power may be automatically switched on and off, i.e., “pulsed.” In the examples that are provided, the RF power is automatically pulsed at a frequency of 10 kHz. In other examples, the RF power may be pulsed at frequencies between 1 kHz and 100 kHz. The composition of the nitrogen plasma is continuously varied by varying the RF current between high and low states. In FIG. 11, the duty cycle, i.e., the total amount of time that the RF power is on, is 30%, and in FIG. 12, the duty cycle is 50%. The RF source  50  is pulsing-enabled, and both the pulsing frequency and duty cycle are manually adjustable. The effective delivered power is the peak power times the duty cycle. In other examples, the duty cycle may be between 10% and 90%. In the given example, the amplitude of the RF power is continually altered between 0% and 100%, but in another example, the amplitude may, for example, be altered between 10% and 100%.  
     [0047] One way to measure incorporation of nitrogen is by measuring the thickness change (“optical delta”) before and after a nitrogen plasma treatment. A larger thickness change indicates more nitrogen incorporation. As shown in FIG. 13, the amount of incorporated nitrogen using continuous power can also be achieved using pulsed power, with the amount of incorporated nitrogen scaling approximately with the effective delivered power. The change in optical thickness is relatively insensitive to pulsing frequency.  
     [0048]FIG. 14 illustrates optical delta for samples prepared with continuous RF source power; the saturation in incorporated nitrogen with power is observed for both pulsed and continuous power.  
     [0049]FIGS. 15 and 16 show the same data as in FIG. 13, plotted against source power and duty cycle, showing the same trends as FIG. 13.  
     [0050] In FIGS. 17 and 18, optical emission spectra are captured with an optical emission spectrometer. As one increases the duty cycle at fixed-peak RF power (500 W), the spectra approach the 500 W continuous power spectra (top line), as can be seen in FIG. 17. Pulsing frequency has a small effect on the observed intensity. FIG. 18 shows that the pulsed RF emission level can be restored to the continuous-power emission level (top line) by increasing peak RF power. Again, the emission is relatively insensitive to pulsing frequency.  
     [0051] FIGS.  13  to  16  indicate that on-wafer nitrogen incorporation similar to the incorporation of continuous RF power is possible with pulsed-RF plasmas. FIGS. 17 and 18 indicate that plasmas of similar ion density to continuous-RF power plasmas can be achieved with pulsed-RF power. These data, coupled with the effect of pulsed-RF power to reduce the electron temperature and plasma potential relative to continuous power, indicate that the pulsing of RF power provides a method for incorporation of nitrogen into gate dielectric oxides at lower energy levels. While incorporating the same amount of nitrogen in the oxide, nitrogen ions in the pulsed plasmas are accelerated into the wafer less than ions in the continuous-power plasmas because of the lower plasma potentials of the pulsed plasmas. Because of this reduced acceleration, the nitrogen will not penetrate as far into the oxide and the underlying silicon.  
     [0052] The simulation of ion implantation into silicon, specifically into Si(100), at various ion energies (10 eV to 30 eV), through a thin oxide layer shows less penetration for lower energy, as can be readily expected. Achieving nitrogen incorporation in such a low-energy fashion with the pulsed-nitrogen plasmas may provide for an improved dielectric that will lead directly to improvements in transistor performance.  
     [0053] It should be noted that although nitrogen incorporation into a thin gate silicon dioxide has been described, the described processes may have applications for nitrogen incorporation in other gate dielectric materials.  
     [0054] While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art.