Patent Publication Number: US-2018053631-A1

Title: Low Electron Temperature Etch Chamber with Independent Control Over Plasma Density, Radical Composition Ion Energy for Atomic Precision Etching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 62/247,949, filed Oct. 29, 2015 entitled LOW ELECTRON TEMPERATURE ETCH CHAMBER WITH INDEPENDENT CONTROL OVER PLASMA DENSITY, RADICAL COMPOSITION AND ION ENERGY FOR ATOMIC PRECISION ETCHING, by Leonid Dorf, et al. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosure concerns a low electron temperature etch chamber with independent control over plasma density, radical composition and ion energy for atomic precision etching. 
     Background Discussion 
     Diminishing scale and increasing complexity of microfabrication processes necessitate the use of novel ultra-sensitive materials, which in turn requires low-damage plasma etching with atomic layer precision. This imposes progressively stringent demands for accurate control over ion energy and radical composition during plasma processing. 
     SUMMARY 
     A method of processing a workpiece in a processing chamber, comprises: limiting plasma electron temperature by generating a plasma in the processing chamber with a sheet electron beam parallel to a surface of the workpiece; controlling workpiece potential with respect to plasma in a range between 0 and 25 volts by applying bias power to a workpiece support in the chamber; and independently controlling radical population in the plasma by controlling production rate of a remote plasma source feeding the processing chamber. 
     In one embodiment, the limiting of the plasma electron temperature is performed so as to limit workpiece potential with respect to the plasma to not more than a few volts in absence of an applied bias power. 
     In one embodiment, the electron beam energy is limited to a range (such as from sub-keV to a few keV) so as to limit dissociation or radical production by the electron beam. 
     In one embodiment, the bias power controls the plasma ion energy to be on an order of or near a bonding energy of a malarial in the workpiece being etched. 
     A related method of processing a workpiece in a processing chamber comprises: generating a plasma in the processing chamber while limiting plasma electron temperature by propagating an electron beam in the processing chamber; controlling a level of bias power coupled to a workpiece support so as to set plasma ion energy to be on an order of or near a bonding energy of a material on the workpiece being etched; and controlling radical population in the plasma by controlling production rate of a remote plasma source coupled to the processing chamber. In one optional embodiment, the electron beam energy is limited to a range (such as from sub-keV to a few keV) so as to limit dissociation or radical production by the electron beam. 
     A plasma reactor for processing a workpiece comprises: an electron beam gun enclosure having a beam outlet opening at one end of the enclosure and enclosing an electron emission electrode at an opposite end of the enclosure, the electron emission electrode having an electron emission surface facing the beam outlet, the beam outlet and the electron emission electrode defining a beam propagation path between them; an RF power source and an RF power conductor coupled between the RF power source and the electron emission electrode; and a processing chamber having a beam entry port aligned with the beam outlet, a workpiece support in the processing chamber for supporting a workpiece in a plane parallel with the beam propagation path, and a gas distributor coupled to the processing chamber. 
     In one embodiment, the RF power source comprises a first RF power generator and an impedance match coupled between the first RF power generator and the electron emission electrode. In a further embodiment, the impedance match comprises a dual frequency impedance match, the power source further comprising a second RF power generator having a frequency different from a frequency of the first RF power generator. In one embodiment, the first RF power generator produces a low frequency and second RF power generator produces a high frequency. 
     In one embodiment, the plasma reactor further comprises a gas supply having a feed path to the electron beam gun enclosure. In one embodiment, the plasma reactor further comprises an ion-blocking filter in the beam outlet opening, the ion-blocking filter permitting flow of electrons through the beam outlet. 
     In one embodiment, the plasma reactor further comprises: a backing plate insulated from the electron gun enclosure and contacting a back face of the electron emitting electrode; a chiller plate contacting the backing plate; and the RF power conductor is connected to the chiller plate. In one embodiment, the plasma reactor further comprises an insulator surrounding an edge of the electron emitting electrode and disposed between the electron emitting electrode and the electron gun enclosure. 
     In one embodiment, the plasma reactor further comprises a process gas supply coupled to the gas distributor. 
     In one embodiment, the plasma reactor further comprises a remote plasma source coupled to the processing chamber. 
     In one embodiment, the plasma reactor further comprises a bias power generator coupled to the workpiece support. 
     In one embodiment, the first RF power generator, the second RF power generator, the bias power generator and the remote plasma source are independently controllable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  illustrates a low damage reactor in accordance with a first embodiment. 
         FIG. 2  depicts a method of operating the reactor of  FIG. 1 . 
         FIG. 3  illustrates a plasma reactor having an electron beam source including an RF-driven electron emission electrode. 
         FIG. 4  depicts a modification of the embodiment of  FIG. 1  in which the e-beam source is the electron beam source of  FIG. 3  that includes an RF-driven electron emission electrode. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale. 
     DETAILED DESCRIPTION 
     Introduction: 
     Using an electron sheet beam (e-beam) parallel to the workpiece surface to produce plasma in a processing chamber provides an order of magnitude reduction in plasma electron temperature Te (˜0.3 eV) and plasma ion energy (Ei less than a few eV in the absence of applied bias power) compared to conventional plasma technologies, thus making an electron beam-generated plasma an ideal candidate for processing features at 5 nm and below. Furthermore, since dissociation is performed only by high-energy beam electrons and not plasma electrons, and since the dissociation cross-section drops off considerably at or below electron beam energies of about 2 keV, the chemical composition of an electron beam-created plasma can be made radical-poor by limiting the electron beam energy in accordance with one option. This allows for independent control over plasma radical composition by an external radical source, which is another advantage of using electron beam technology to create plasma. 
     Low Damage Reactor: 
     In a first embodiment depicted in  FIG. 1 , a low-damage reactor is provided that enables atomic precision processing (as in atomic layer etching) and independent control of plasma ion energy and radical composition of the plasma. The low-damage reactor includes: a processing chamber  50  including an electrostatic chuck  52  holding a workpiece  54 , an electron beam (e-beam) source  56  for creating a radical-poor, low-electron temperature (Te) plasma in the processing chamber  50 , a remote plasma source  58  for producing and supplying radicals through an outlet  58   a  to plasma in the processing chamber  50 , and a bias power generator  60  for creating a voltage drop (with fine control in 0-50 V range) between the workpiece  54  and the plasma to accelerate ions over etch-threshold energies. The outlet  58   a  may include an ion-blocking grid  90 . A beam outlet  56   a  of the e-beam source  56  is covered by a filtering grid  170  that admits electrons forming the electron beam but blocks ions and other plasma by-products produced within the e-beam source  56 . 
     The bias power generator  60  may have a bias voltage control input  60   a  that provides the fine control in a 0-50 V range. In one embodiment, the range is 0-25V. The electron beam source  56  includes a beam acceleration voltage control input  62  that controls the electron energy of the electron beam source  56 . The remote plasma source  58  has a control input  59  for controlling the rate at which the remote plasma source  58  supplies radicals into the processing chamber  50 . The control input  59  is independent of the beam acceleration voltage control input  62 . The rate at which the remote plasma source  58  supplies radicals into the processing chamber  50  and the energy of the electron beam are controlled independently of one another. The control input  59  may be implemented in various ways. For example, the control input  59  may control the power level of an RF generator driving a plasma source power applicator (not shown) in the remote plasma source  58 . As another possibility, the control input  59  may control a valve at the outlet  58   a  between the remote plasma source  58  and the processing chamber  50 . A vacuum pump  66  may be provided for evacuating the processing chamber  50 . 
     Because of the ultra-low electron temperature in the electron-beam generated plasma, the workpiece potential with respect to the plasma is very low, just a few volts, without an applied bias. This is much lower than in conventional plasma etch tools, where it is typically confined to a range above or exceeding about 15 V. Thus, unlike conventional tools, the low-damage reactor of  FIG. 1  enables precise control of ion energy in the range of 0-25 V by limiting the applied bias power accordingly. In this very important range, the plasma ion energy is near (e.g., within 10% of) or on the order of the bond energy of the etched material, which enables performance of an ultra-low damage etch process. The etch rate is likewise quite low at such ion energies—just a few Angstroms per minute—which makes the low damage reactor also uniquely suitable for atomic precision etching or atomic layer etching. Another critical advantage enabling precise control over the etch process is achieved through independent control over the radical composition governed by the radical production rate of the remote plasma source  58 . As a result, true atomic precision etching with ultra-low damage and only one to a few atomic layers per minute etch rates is carried out in the low damage reactor. 
     In one embodiment, a method of operating the low-damage reactor chamber is provided, in which the plasma ion energy and the radical composition of the plasma are independently controlled. The method is depicted in  FIG. 2  and proceeds as follows: 
     First, limit plasma electron temperature to not exceed 0.3 eV and plasma ion energy to not exceed a few eV in absence of applied bias power. This is done by generating in the e-beam source  56  a sheet electron beam parallel to the workpiece surface (block  310  of  FIG. 2 ). This beam creates plasma in the processing chamber  50 . Such limiting of the plasma electron temperature helps to minimize workpiece potential relative to the plasma (i.e., sheath voltage) to not more than about a few volts without applied bias. 
     Second, control workpiece potential with reference to the plasma inside the processing chamber  50  by controlling the bias power generator  60  to set the workpiece potential to a range between 0 and 25 volts (block  320  of  FIG. 2 ). Alternatively or equivalently, set the plasma ion energy to near the bonding energy of the material being etched by controlling the bias power generator  60 . 
     Third, as one option that is not necessarily required, limit electron beam energy to a range between several hundred volts and a few kilovolts (block  330  of  FIG. 2 ). This has the effect of minimizing dissociation or radical production by the electron beam. 
     Fourth, independently control radical population in the plasma by controlling production rate of the remote plasma source feeding the processing chamber (block  340  of  FIG. 2 ). 
     E-Beam Source with RF-Driven Electrode 
     The challenges of developing an industry-worthy electron beam plasma source include meeting the following requirements: 
     1. Process-chemistry compatibility: chemically aggressive and/or depositing process gas should not affect e-beam source (gun) operation or render it impossible, as with DC electron beam sources; conversely, sputtering of the e-beam gun parts should not adversely affect the process.
 
2. Capability for operation over a wide range of process gas chamber pressures.
 
3. Robustness, i.e. ability to operate for a long time between the preventive maintenance events involving parts replacement.
 
4. High operational stability and reproducibility.
 
5. Independent control over density and energy of beam electrons.
 
     What is needed is an electron beam source that satisfies the foregoing criteria. 
       FIG. 3  depicts an embodiment of a plasma reactor having an electron beam (e-beam) plasma source that satisfies the criteria discussed above. Referring to  FIG. 3 , an emitting electrode  110  is mounted on a backing plate  120 . The backing plate  120  is mounted on a chill plate  130 . A ceramic spacer  140  and an insulator  150  hold the emitting electrode  110  in place relative to an electron gun body  160 . The electron gun body  160  may be formed of an electrically conductive material and be connected to a return potential or to ground. In the illustrated embodiment, the electron gun body  160  extends along an e-beam propagation path P and has a beam outlet opening  160   a  at a distal end opposite the emitting electrode  110 . A filtering grid  170  is positioned within the beam outlet opening  160   a.  A backfill gas feed  180  conducts gas suitable as an electron source (e.g., Argon) from a gas supply  182  into the interior of the electron gun body  160 . A coolant liquid feed or conduit  190  conducts coolant from a coolant source  192  to the chill plate  130 . An RF feed  200  conducts RF power to the emitting electrode  110  through the chill plate  130  and through the backing plate  120 . An insulator  210  surrounds a portion of the RF feed  200 . The electron gun body  160 , the emitting electrode  110 , the backing plate  120 , the chill plate  130 , the ceramic spacer  140 , the insulator  150  and the RF feed  200  together form an e-beam source assembly  212 , which is contained within an RF shield  220 . The RF feed  200  receives RF power through a dual frequency impedance match  230  from RF power generators  242  and  244 . In one embodiment, the RF power generator  242  produces low frequency RF power and the RF power generator  244  produces high frequency RF power. 
     In one modification, the e-beam source assembly  212  of the embodiment of  FIG. 3  may be used as the e-beam source assembly  212  of the embodiment of  FIG. 1 . Such a modification is depicted in  FIG. 4 . 
     A process chamber  260  is coupled to the electron gun body  160  through the opening  160   a,  and has a ceiling gas distributor  270  coupled to a process gas supply  272 . An electrostatic chuck  280  within the process chamber  260  supports a workpiece  290  in a plane parallel to the beam propagation path P. 
     An RF plasma discharge is ignited between the emitting electrode  110  and the electron gun body  160  that serves as an RF return. Two RF frequencies can be supplied by the RF power generators  242 ,  244  including a low frequency such as 2 MHz, and a HF or VHF frequency such as 60 MHz. This provides independent control over: (1) the density of plasma (controlled by the level of the HF or VHF power), which determines the density of the beam electrons, and (2) the DC self-bias at the emitting electrode  110  (controlled by the level of the low frequency power), which determines the energy of the beam electrons. Generally, the energy of the beam electrons may be controlled by controlling the output power level of the low frequency bias power generator  242 . Independent control over beam electron density can also be achieved by adding an inductively coupled plasma source to the e-beam source assembly  212 . 
     Because the area of the electron gun body  160  is larger than the area of the emitting electrode  110 , the RF-induced DC self-bias will be much larger at the smaller emitting electrode  110 , and can reach a level appropriate for the electron beam technology. For example, the self-bias can reach 1-1.5 kV at about 1.5 kW of 2 MHz power with about 600 W of 60 MHz power, at an internal pressure within the electron gun body  160  of about 20 mT. The ions accelerated in the sheath at the emitting electrode  110  bombard the electrode surface and cause ion-induced secondary electron emission. These emitted secondary electrons are in turn accelerated in the same sheath voltage drop as they move away from the electrode surface, resulting in formation of the electron beam. Thus, the secondary electron emission coefficient of the emitting surface of the emitting electrode  110  plays a very significant role in determining the density of the beam electrons. 
     A significant portion of the applied RF power is deposited into the emitting electrode  110  in the form of heat, due to constant bombardment by high-energy ions. The chiller plate  130  has non-conductive cooling fluid running through it, and is coupled through the backing plate  120  to the emitting electrode  110 . The RF feed  200  is coupled through the chill plate  130  and the backing plate  120  to the emitting electrode  110 . The backing plate  120  serves as an RF plate distributing applied RF power evenly over the emitting electrode  110 . 
     The filtering grid  170  has high aspect ratio openings and prevents leakage of the RF plasma ions and radicals created inside the electron gun body  160  into the process chamber  260 . Further, the chemically aggressive process gas inside the process chamber  260  is blocked from entering the interior of the electron gun body  160 . This gas separation is achieved using the back fill gas feed  180  by backfilling the interior of the electron gun body  160  with inert gas such as Argon, supplied at a sufficiently high flow rate to create a considerable gas pressure drop (for example, about 30 mT) across the filtering grid  170 . In turn, high-energy electrons can go through the high aspect ratio openings of the filtering grid  170 , due to high directionality of their velocity distribution. 
     Backfilling the interior of the electron gun body  160  with process-independent gas also allows modification of the electrode emitting surface of the emitting electrode  110  to control secondary electron emission coefficient by forming, for example, a Silicon Nitride on the surface. Due to the nature of the plasma discharge, practically any material (silicon, ceramic, quartz) can be used to form the emitting surface of the emitting electrode  110  without affecting general operation of e-beam source assembly  212 . 
     The material sputtered by the ions off of the emitting electrode  110  and re-deposited on the other parts of the e-beam source assembly  212  can be cleaned in-situ by running HF or ICP plasma only (at much lower self-bias) with appropriate chemistry, if the emitting surface material is adequately selected. Likewise, the grounded surface of the electron gun body  160  can be coated with any process-compatible and not necessarily conductive material, as long as the capacitance of the coating layer is sufficiently small. Penetration of the sputtered material into the process chamber  260  is also considerably restricted by the filtering grid  170 . 
     Advantages: 
     An advantage of using an RF-driven electrode (i.e., the electrode  110 ) rather than a DC discharge to create the electron beam is that electron beam density and electron beam energy are independently controlled by high frequency power and low frequency power, respectively, applied to the electrode  110 . Further, use of metals or other conductive materials may be minimized in the construction of the e-beam source assembly  212 , which makes penetration of any sputtered material through the filtering grid  170  into the process chamber  260  generally less damaging for the wafer processing. 
     Using an electron sheet beam (e-beam) parallel to the workpiece surface to produce plasma in a processing chamber provides an order of magnitude reduction in plasma electron temperature Te (˜0.3 eV) and plasma ion energy Ei (&lt;2 eV in the absence of applied bias power) compared to conventional plasma technologies. This enables the plasma ion energy to be reduced to near or below the binding energy of the material being etched (e.g., silicon, silicon oxide, silicon nitride). Furthermore, since dissociation is performed only by high-energy beam electrons and not plasma electrons, and since the dissociation cross-section drops off considerably at or below electron beam energies of about 2 keV, the chemical composition of an electron beam-created plasma can be made radical-poor. This allows for independent control over plasma radical composition by the remote radical source  58 . 
     While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.