Abstract:
An antenna adapted to apply uniform electromagnetic fields to a volume of gas and including radiating elements connected in parallel with evenly distributed input terminals for receiving electromagnetic energy into the antenna and output terminals for grounding. In the illustrative embodiment, the antenna has three radiating elements connected in parallel. Each radiating element is a conductor wound in a circular shape with the same diameter. Each radiating element is connected to the input terminal on one end and an output terminal on the other. The input terminal of the second element is 120° rotated counterclockwise from the first and the input terminal of the third is rotated by 120° counterclockwise from the second. The ground terminals of each radiating elements are located in the same manner as the input terminals. Each element is feed by a feeder coil. While the antenna elements are disposed around a chamber, the feeder coils are disposed above the chamber to improved the distribution of electromagnetic energy within the chamber.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuation-in-part of INDUCTIVELY COUPLED PLASMA GENERATION SYSTEM WITH A PARALLEL ANTENNA ARRAY HAVING EVENLY DISTRIBUTED POWER INPUT AND GROUND NODES Ser. No. 10/391,383, filed Mar. 18, 2003 by Harqkyun Kim et al. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to plasma processing systems. More specifically, the present invention relates to plasma sources used for plasma etching, chemical vapor deposition, photo-resist stripping and other applications relating to semiconductor, flat panel display, printed circuit board and other fabrication processes.  
         [0004]     2. Description of the Related Art  
         [0005]     Plasmas sources capable of uniform coupling of electromagnetic energy to plasmas are needed for many plasma processes such as plasma etching, plasma enhanced chemical vapor deposition, physical vapor deposition, photo-resist stripping and surface treatments for many applications. Illustrative applications include silicon and compound semiconductor fabrication, flat panel display fabrication including active matrix liquid crystal display, plasma display panels, field emission displays etc. Additional applications include hard disk drive head and media manufacturing, microelectromechanical system manufacturing and printed wiring board fabrication.  
         [0006]     A plasma source typically includes a radio frequency antenna, a dielectric window and a volume of gas. An electric field from an impedance matched power supply is applied to the gas by the antenna through the dielectric tube. The application of the electric field to the gas generates two fields of interest with respect to plasma processing: time varying electromagnetic fields and capacitive electric fields. Free electrons gain energy by these electromagnetic fields and generate ions by collision with neutral gases, thereby generating plasmas. The inductive technique using a time varying electromagnetic field is known to be more efficient in the production of plasma than the capacitive coupling technique using a capacitive electrostatic field. A typical plasma etcher uses an additional electric field capacitively coupled to the substrate to increase ion energy.  
         [0007]     Inductively coupled plasma sources typically use an antenna wound in circular spiral shape with an input terminal for receiving electromagnetic power at one end of the antenna and an output terminal for grounding at the other. This type of antenna induces a large potential difference between input and output terminals resulting in strong electric fields. Ions and electrons gaining energy through the interaction with these fields cause non-uniformity in the spatial energy distribution of plasmas which adversely impacts process results.  
         [0008]     Hence, there is a need in the art for a system or technique for uniformly coupling electromagnetic energy for inductive generation of uniform plasmas. This need is addressed by the above-identified parent application of which this application is a continuation-in-part. However, a need remains for further improvements in the distribution of electromagnetic energy for inductive generation of uniform plasmas.  
       SUMMARY OF THE INVENTION  
       [0009]     The need in the art is addressed by the antenna of the present invention. The inventive antenna is adapted to apply a uniform electromagnetic field to a volume of gas and includes an array of radiating elements with input terminals for receiving electrical energy into each radiating element and output terminal for grounding. The inventive antenna includes an array of radiating elements in the shape of circular, semicircular or rectangular loops connected in parallel.  
         [0010]     In the illustrative embodiment, the antenna has three radiating elements. Each radiating element includes a conductor wound in a single turn circle around a dielectric tube with the same diameter. In accordance with the invention, each element of the inventive has a one-half turn coil disposed over said tube to further improve the distribution of the plasma. The input terminal of the second element is rotated 120° counterclockwise from the first and the input terminal of the third is rotated counterclockwise by 120° from the second. The ground terminal of each radiating element is located in the same manner as the input terminal.  
         [0011]     The inventive antenna is adapted for use in a plasma processing system comprising a vacuum chamber, a gas disposed within the vacuum chamber, a dielectric tube disposed on the vacuum chamber, and a power circuit. The power circuit includes a source of radio frequency (RF) power, a switch and an impedance matching circuit. The impedance matching circuit efficiently couples power from the RF supply to the antenna.  
         [0012]     The inventive antenna provides uniform coupling of electromagnetic power by spreading out high potential input terminals and ground terminals evenly along the circumference of a process tube, therefore resulting in uniform plasma density and energy across the entire substrate surface under a wide range of processing conditions.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic sectional view of an illustrative embodiment of an inductively coupled plasma reactor implemented in accordance with the teachings of the present invention.  
         [0014]      FIG. 2  is an illustrative embodiment of an antenna configuration adapted for use with the plasma reactor of  FIG. 1  in accordance with the teachings of the present invention.  
         [0015]      FIG. 3  is a schematic diagram of the antenna coils illustrating even distribution of input/output nodes in accordance with the teachings of the present invention.  
         [0016]      FIG. 4  is a top view of the antenna of  FIG. 3 . 
     
    
     DESCRIPTION OF THE INVENTION  
       [0017]     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.  
         [0018]     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.  
         [0019]      FIG. 1  is a schematic sectional view of an inductively coupled plasma reactor in accordance with an illustrative embodiment of the present invention. The system  10  is designed for plasma processes such as etch and plasma CVD. As shown in  FIG. 1 , the system consists of a plasma source  18  and a process chamber  12  for plasma generation and wafer processing. Plasma source  18  is made of a dielectric tube  11 , an antenna  20 , and a gas injector  10 . Process gases are injected through the gas injector  13  to the process chamber  12 . In the best mode, the gas injector  13  is made of dielectric material to facilitate the creation of an electromagnetic field within the chamber  12  by half-turns of the antenna  20  disposed on top of the chamber  12  as discussed more fully below.  
         [0020]     The system operates at low pressure typically around mTorr range by using a vacuum pump  17 . RF bias power is provided by a supply  50 . This allows for RF bias power to be applied to control the ion energy in the plasma independently from an RF power source  40 . Plasma  18  is generated by an inductively coupled electric field produced by an antenna  20 .  
         [0021]      FIG. 2  shows an illustrative embodiment of the antenna of the plasma reactor in accordance with the teachings of the present invention. The antenna  20  consists of multiple inductive parallel coils  21 ,  22  and  23  wrapped around a cylindrical dielectric tube  11 . The present invention is not limited to the shape of the dielectric tube. Other shapes may be used without departing from the scope of the present teachings.  
         [0022]     In a conventional inductive antenna, a spiral coil with multiple turns is wrapped around a plasma generation chamber. Typically, one end of the inductive coil is connected to an RF power source and another end of the coil is connected to system ground. Therefore, there is only one RF input and one ground output in the continuous spiral coil. In that case, because the total coil length is large, transmission line properties of the induction coil result in voltage and current standing waves along its length. The variations in current with position along the coil lead to asymmetries in the induced electromagnetic fields, which, in turn, can lead to asymmetries in the power density of plasma and non-uniformity in the processing of components.  
         [0023]     The inventive antenna  20  includes an array of radiating elements (coils) in the shape of circular, semicircular or rectangular loops connected in parallel. In the illustrative embodiment, the antenna has three radiating elements. Each radiating element includes a conductor wound in a single turn circle with the same diameter and preferrably, each inductive coil has only a single turn or less in order to reduce the transmission line effect. That is, in the best mode, each coil has a single turn or less to reduce the total inductance of the coil. Nonetheless, those skilled in the art will appreciate that the present teachings are not limited to the number of coils or the number of turns thereof.  
         [0024]     Each coil  21 ,  22  and  23  is fed by a half-turn coil feeder segment  31 ,  32  and  33  respectively. The half-turn segments are disposed on top of the chamber along a plane generally transverse to a longitudinal axis coaxial to the coils  21 ,  22  and  23 . Those skilled in the art will appreciate that the feeder coils  31 ,  32  and  33  could be embedded in the dielectric  13  without departing from the scope of the present teachings. In addition, the feeder coils may have additional turns without departing from the scope of the present teachings.  
         [0025]     The coils  21 ,  22 , and  23  are arranged in parallel around the plasma generation reactor  11 . Three different input nodes  24 ,  26 , and  28  are provided for receiving RF energy and three different output nodes  25 ,  27 , and  29  are provided for ground, respectively.  
         [0026]      FIG. 3  is a schematic diagram of the antenna coils illustrating the even distribution of input/output nodes in accordance with the teachings of the present invention. Each antenna coil of single-turn loop  21 ,  22  and  23  is provided with a common RF power generator  40  connection via its own RF input nodes  24 ,  26 , and  28  and a connection to ground via output nodes  25 ,  27 , and  29 , respectively. An RF impedance match network  41  is connected in series between the RF power generator  40  and a common RF input node  30  for the three antenna coils  21 ,  22 , and  23 .  
         [0027]     The first coil  21  has an input node  24  for receiving RF energy at one end and an output node  25  connected to ground at the other end. Since it has a circular turn, the two nodes  24  and  25  are close to each other. The second  22  and third  23  coils are configured by 120 deg and 240 deg rotation from the position of the input and output nodes  24  and  25  of the first coil  21  for even distribution. The feeder coils serve to create an electric field near the top of the chamber  12  thereby facilitating an improved distribution of the plasma  18 . Therefore, the present invention improves the power deposition symmetry as well as the ion flux uniformity on the surface of wafer  16  ( FIG. 1 ).  
         [0028]     In the best mode, the antenna rotation is equal to 360 divided by the number of turns thereof. Nonethesless, those skilled in the art will appreciate that the present invention is not limited thereto. Other antenna rotations and turns ratios may be used without departing from the scope of the present teachings.  
         [0029]     The RF current direction along the antenna coils  20  is same for all three loops  21 ,  22 , and  23 . The feeder coils  31 ,  32 , and  33  should have the same length from the common RF input point  30  to each RF input node  24 ,  26 , and  28  to avoid destructive interference among the coil currents. This can be implemented by wiring three feeder coils  31 ,  32 , and  33  of same length with a half-turn to make the length from the common input point  30  to each three different RF input nodes  24 ,  26 , and  28  equal.  
         [0030]      FIG. 4  is a top view showing the inventive feeder coils  31 ,  32 , and  33  of the antenna  20  with 120 degree angle differences for the input/output nodes thereof. The advantage of this method of evenly distributing the RF input nodes  24 ,  26 , and  28  and ground nodes  25 ,  27 , and  29  of each coil loops  20  is that it minimizes the potential unbalance along the antenna coil  20 . Usually, the potential difference between the RF input and output nodes is smaller in the coil with shorter length.  
         [0031]     However, some potential difference between the input/output nodes may not be avoided even though a single-turn coil is used. Thus, if the antenna nodes are mounted using a common input and a common output arrangement such as superposition, the potential difference between each input/output nodes will be overlapped and enhanced. This typically results in non-uniform plasma in the wafer process.  
         [0032]     However, by the inventive method of using even distribution, if the three pairs of nodes  24 / 25 ,  26 / 27 , and  28 / 29  of each of the coil loops  20  are distributed evenly, the unbalance of total potential can be minimized to get uniform plasma and process results across the entire wafer  16  surface.  
         [0033]     The coils are conductive materials such as copper or other suitable conductor. Those of ordinary skill in the art may choose the diameters and number of turns of the coils to suit a particular application. In the best mode, cooling water is flowed through the antenna coils  20 . This should allow the coils to deliver up to 2500 W of RF power to a plasma  18 .  
         [0034]     Returning to  FIG. 1 , the output of the RF generator  40  is connected to the antenna coils  20  via the matching network  41 . In the illustrative embodiment, the matching network includes two series capacitors  42  and  43  (C 1  and C 2 ) and a capacitor  44  (C 3 ) connected to ground. Two capacitors  42  and  44  are variable capacitors and the third capacitor  43  is a shunt capacitor with a fixed capacitive value.  
         [0035]     In conventional RF systems, ion energy and flux are linked, and cannot be controlled independently. However, the mean ion bombarding energy and its energy distribution should be controlled independently of the ion and neutral fluxes to tailor the film properties such as stress, composition, refractive index, crystallinity, and topography. In the plasma etching, ion energy also needs to be controlled to control etch rate, optimize selectivity, and minimize the device damage. Therefore, it is very important for the plasma process to offer better control of ion energy and couple it from ion flux control.  
         [0036]     As shown in  FIG. 1 , the plasma reactor of this invention contains two auxiliary power sources  50  and  55 . Each bias can be selected using switches  52  and  59 . The substrate holder, chuck  13 , has an anodized surface and it is mounted on chamber bottom  15 . Insulation  14  is supplied between chuck  13  and the chamber bottom  15  to isolate the chuck  13  from the chamber ground. The first bias is the “RF bias” power  50  which is applied to the chuck  13  to control ion bombardment energies onto the substrate  16  surface. Independent control over ion bombarding energy can be achieved by biasing a second RF source  50  on the substrate chuck  13 . The RF bias power  50  is applied to the substrate chuck  13  to control the energy of ions bombarding the substrate  16  surface. Thus, the present invention provides a wider process window for etching such as etch rate, etch profile, and selectivity.  
         [0037]     The second bias is “positive DC voltage”. This bias can be used for film deposition by plasma chemical vapor deposition (CVD). A “Low Pass Filter”  56  is connected in series between the DC power supply  55  and the chuck  13  to supply DC potential without interference with RF energy. Positive DC potential is applied to the anodized chuck  13  to control the flux of positive ions from the plasma  18  to the wafer  16 . This positive DC bias modifies the chuck  13  potential near the wafer  16 , and the electric field generated by the biased chuck  13 . The DC bias has a strong influence on the charged particles impinging on the wafer  16  surface. Applying positive bias to the chuck  13  leads to a decrease of ion flux, which improves the quality and surface roughness of the film by CVD process.  
         [0038]     In general, the plasma consists of electrons, ions, neutral radicals, and neutral atoms or molecules. A decrease of ion flux implies that the contribution of neutral radicals of plasma become larger than that of energetic ions. Therefore, the proper control of ion energy and flux can be an effective way to suppress plasma induced damage and film stress.  
         [0039]     Also, since the ion energy can be minimized, the reactor of this invention can be applied to the plasma process for damage-sensitive devices such as GaAs or InP compound semiconductor devices. One advantage of this invention is that high plasma densities can be produced with low or controlled ion energy. An RF bias  50  on the substrate  16  is used to draw ions to the substrate  16  at the desired impact energy. Thus, optimum ion energy can be selected—one that is high enough to produce anisotropic etching, but not too high as to cause lattice damage or impurity implantation. Therefore, the plasma reactor  11 ,  12  of this invention can be applied to the plasma processing of fabrication of III-V semiconductor (GaAs- or InP-based HBT&#39;s and HEMT&#39;s) and photonic devices (nitride based photonic devices and quantum well lasers). Low ion energy as well as controllability of the ion energy is an essential requirement in the fabrication of these devices.  
         [0040]     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.  
         [0041]     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.  
         [0042]     Accordingly,