Patent Abstract:
A plasma apparatus capable of adaptive impedance matching comprises a plasma reactor which can produce plasma to proceed with CVD (chemical vapor deposition) process, a bi-polar electrostatic chuck which locates inside the plasma reactor and is used to support and secure a wafer, an alternating current bias power supply which connects to the bi-polar electrostatic chuck supplies the voltage potential bias for ion-bombardment from plasma, and an impedance-matching circuit which connects the alternating current bias power supply to the bi-polar electrostatic chuck is used to balance the inner electrode power output and the outer electrode power output of the bi-polar electrostatic chuck.

Full Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus of the fabrication of integrated circuits, and the present invention especially relates to an apparatus of the fabrication of integrated circuits which is capable of reducing gate oxide damage in a high density plasma chemical vapor deposition process to which a bi-polar electrostatic chuck is applied. 
   2. Description of the Prior Art 
   After the fabrication of the active device of a MOS (metal-oxide semiconductor) device is accomplished, the following work is to proceed the fabrication of the multilevel interconnects above the MOS device. As the process technology progresses and scales of MOS devices get more and more smaller, gaps between metal conductors also become more and more narrower. Accordingly, gaps of high aspect ratio between metal conductors are formed. The gaps of high aspect ration will let the deposition of dielectric layers become incomplete and form voids in the dielectric layers. These voids in the dielectric layers will damage electric properties of MOS devices and lead to scraped wafers. 
   In order to solve the problem of the incomplete deposition of dielectric layers, a HDPCVD (high density plasma chemical vapor deposition) process is proposed to deposit dielectric layers between metal conductors in U.S. Pat. No. 6,239,018 and U.S. Pat. No. 6,218,284. The detailed description of a HDPCVD process is also contained in U.S. Pat. No. 6,117,345. The main reason a HDPCVD process can solve the problem of the incomplete deposition of dielectric layers is that a HDPCVD process is capable of both proceeding chemical vapor deposition process and anisotropic etching process. As shown in  FIG. 1A , the etching function results from the following steps: an AC (alternating current) plasma generating source  12  of the HDPCVD equipment  10  generates plasma  16 , the voltage potential difference between the plasma  16  and the electrostatic chuck  20  attracts the ions of the plasma  16  to bombard the wafer  18 . The ions of the plasma  16  will anisotropically etch the excess dielectric layers above the metal conductors of MOS devices to deposit void-free dielectric layers. Because the voltage potential difference between the plasma  16  and the electrostatic chuck  20  distributes non-uniformly in the process of ion-bombardment on the wafer  18 , currents are produced on the wafer  18  surface. If the voltage potential difference between the plasma  16  and the electrostatic chuck  20  distributes extremely non-uniformly, the produced currents will damage gate oxides of MOS devices. 
   The non-uniform distribution of the voltage potential difference between the plasma  16  and the electrostatic chuck  20  possibly results from the non-uniform distribution of the voltage potential on the electrostatic chuck  20 . In HDPCVD equipment, the type of an electrostatic chuck  20  includes mono-polar and bi-polar. The electrostatic chuck  20  secures the wafer  18  by means of the electrostatic force. If the electrostatic chuck  20  has only one electrode, the distribution of the voltage potential on the electrostatic chuck  20  can be deemed uniform distribution and will not cause the aforementioned non-uniform distribution of the voltage potential difference between the plasma  16  and the electrostatic chuck  20 . However, the electrostatic chuck  20  of the mono-polar type does not have a discharging circuit. When the HDPCVD process is over, the wafer  18  cannot be moved until the electric particles in the plasma  16  neutralize the inductive electric particles on the wafer  18 . The neutralization process delays the throughput of mass production and if the wafer  18  is moved before the neutralization process is completed, the wafer  18  may be broken into fragments. 
   A bi-polar electrostatic chuck  201  is as shown in  FIG. 1B . If a bi-polar electrostatic chuck  201  is used in the HDPCVD process, after the HDPCVD process is over, the electrostatic force on the wafer  18  will be removed more rapidly by virtue of the discharging circuit created by the double electrodes. The discharging circuit created by the double electrodes can avoid delaying the throughput of mass production and prevent the wafer  18  from being broken into fragments. However, as shown in  FIG. 1C , the AC bias source  22  of producing ion-bombardment also connects to the inner electrode  28  and outer electrode  30  of the bi-polar electrostatic chuck  201  to produce the voltage potential difference between the plasma  16  and the bi-polar electrostatic chuck  201 . Accordingly in the process of the transmission of high frequency AC currents, the inner electrode  28  power output always differs from the outer electrode  30  power output because of the impedance difference of the transmitting lines between the inner electrode  28  and the outer electrode  30  causing the non-uniform voltage potential distribution on the bi-polar electrostatic chuck  201 . As shown in  FIG. 2A , after ions bombard the inner side and outer side of the wafer  18 , different voltage potential on the wafer  18  will be generated to produce surface currents on the wafer  18 . As shown in  FIG. 2B , the surface currents on the wafer  18  will cause the accumulated electric particles on the conductive polysilicon layer  36 . The gate oxide  38  on the silicon substrate  40  will be damaged by the accumulated electric particles passing through the gate oxide  38 . 
   In order to solve the problem of the gate oxide damage, process steps are added to change the structure of the MOS device in the U.S. Pat. No. 5,913,140 and U.S. Pat. No. 5,843,827. However, the problem of the non-uniform voltage potential difference between the plasma  16  and the bi-polar electrostatic chuck  201  is not mentioned in these patents. Accordingly, how to avoid the non-uniform voltage potential difference between the plasma  16  and the bi-polar electrostatic chuck  201  is an important issue to be solved. 
   SUMMARY OF THE INVENTION 
   The main purpose of the present invention is to solve the aforementioned problem of the non-uniform voltage potential difference between the plasma and the bi-polar electrostatic chuck. The present invention provides a solution which adds an impedance-matching circuit between the AC bias power for generating the ion-bombardment and the bi-polar electrostatic chuck. The impedance-matching circuit can regulate the impedance of the inner electrode and the outer electrode of the bi-polar electrostatic chuck to balance the inner electrode power output and the outer electrode power output. Furthermore, the impedance-matching circuit can lead to a uniform voltage potential difference between the plasma and the bi-polar electrostatic chuck to avoid the gate oxide damage caused by plasma. According to the experiment results, the fail rate of dies of a wafer decreases from 30%˜60% without adding the impedance-matching circuit to 0%˜2% with adding the impedance-matching circuit. 
   The impedance-matching circuit provided by the present invention includes a power-measuring device which can measure the voltage and the current of both the inner electrode and the outer electrode of the bi-polar electrostatic chuck and transform the voltage values and the current values into power output values of both the inner electrode and the outer electrode of the bi-polar electrostatic chuck, a power comparator which can compare the power value of the inner electrode and the power value of the outer electrode to get a control signal, and an automatic impedance-regulator which can receive the control signal to drive the logic drive motors to regulate the impedance values of the adjustable impedance-elements to let the inner electrode and the outer electrode of the bi-polar electrostatic chuck have the same power output. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1A  shows an illustrative chart of HDPCVD equipment of the prior art; 
       FIG. 1B  shows a top view of distribution of electric particles on a bi-polar electrostatic chuck of the prior art; 
       FIG. 1C  shows an equivalent circuit of the connection between an AC bias power for generating ion-bombardment and a DC (direct current) power for generating an electrostatic force on a bi-polar electrostatic chuck of the prior art; 
       FIG. 2A  shows surface currents on a wafer resulting from the power difference between the inner electrode and the outer electrode of a bi-polar electrostatic chuck of the prior art; 
       FIG. 2B  shows accumulated electric particles resulting from ion-bombardment on a conductive poly-silicon layer passing through a gate oxide to damage the gate oxide of the prior art; 
       FIG. 3A  shows an illustrative chart of an inductively-coupled plasma reactor; 
       FIG. 3B  shows an equivalent circuit of the connection between an AC bias power for generating ion-bombardment and a DC (direct current) power for generating an electrostatic force on a bi-polar electrostatic chuck after adding an impedance-matching circuit; 
       FIG. 3C  shows the equivalent circuit of an impedance-matching circuit; and 
       FIG. 4  shows a flow chart of adjusting impedance values of both the inner electrode and the outer electrode of a bi-polar electrostatic chuck. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Some embodiments of the invention will be described exquisitely as below. Besides, the invention can also be practiced extensively in other embodiments. That is to say, the scope of the invention should not be restricted by the proposed embodiments. The scope of the invention should be based on the claims proposed later. 
   As shown in  FIG. 3A , the apparatus capable of adaptive impedance matching of the present invention mainly applies to HDPCVD equipment. The HDPCVD equipment of the present invention is an inductively-coupled plasma reactor  101 . The plasma  16  of high density and low energy is generated by an electromagnetic field produced by AC currents from an AC plasma generating power  12  passing through the inductive coils  14 . In the preferred embodiment, the operating frequency of the AC plasma generating power  12  is nearly between 200 KHz and 350 KHz. 
   In HDPCVD process (such as the deposition of inter-metal dielectrics), before proceeding with the deposition of dielectrics, a wafer  18  desired to be deposited has to be secured onto a bi-polar electrostatic chuck  201 . The bi-polar electrostatic chuck  201  is connected to a DC power  24  to produce an electrostatic force to secure the wafer  18  onto the bi-polar electrostatic chuck  201 . After the HDPCVD process is over, the bi-polar electrostatic chuck  201  can provide a discharging circuit to remove the electrostatic force more rapidly than a mono-polar electrostatic chuck does to move the wafer  18  out of the inductively-coupled plasma reactor  101 . 
   Depositing dielectric materials into gaps of high aspect ratio in a dielectric layer always leads to incomplete deposition and voids in the dielectric layer. The problem can be solved by means of both chemical vapor deposition process and anisotropic etching process of ion-bombardment of the HDPCVD process. In the present invention, the ion-bombardment results from an AC bias power  22 . The AC bias power  22  connects to the bi-polar electrostatic chuck  201  for supporting the wafer  18 . Then a DC self bias will be generated because of the surface area difference between the bi-polar electrostatic chuck  201  and the inductively-coupled plasma reactor  101 . The generated DC self bias will attract ions in the plasma  16  to bombard onto the surface of the wafer  18  to etch excess deposited materials which stop following deposition. Further, a void-free deposition layer will be formed. The operating frequency of the AC bias power is about 13.56 MHz. 
   As shown in  FIG. 3B , the bi-polar electrostatic chuck has an inner electrode  28  and an outer electrode  30 . The inner electrode  28  and the outer electrode  30  both connects to the DC power  24  and the AC bias power  22 . The capacitive impedance  321  for isolation of the inner electrode  28  and the capacitive impedance  322  for isolation of the outer electrode  30  are used to prevent direct currents from entering the AC bias power  22  because capacitors to direct currents are open-circuit (capactive impedance Zc=1/jωC, the frequency of direct current ω=0, Zc=∞). The inductive impedance  341  for isolation of the inner electrode  28  and the inductive impedance  342  for isolation of the outer electrode  30  are used to prevent alternating currents from entering the DC power  24 . 
   The capacitive impedance  321  for isolation of the inner electrode  28  and the capacitive impedance  322  for isolation of the outer electrode  30  have different impedance values because of different transmitting lines. The different impedance values will lead to the power outputs difference between the inner electrode  28  and the outer electrode  30  (the power of AC bias power  22  P generator  minus the power of the consumption of the impedance P impedance  equals to power output P out ). In this situation when proceeding with ion-bombardment onto the wafer  18 , surface currents will be generated. Surface currents on the wafer  18  will damage the gate oxides of the devices on the wafer  18 . To avoid damaging the gate oxides of the devices on the wafer  18 , the inner electrode  28  power output and the outer electrode  30  of the bi-polar electrostatic chuck  201  power output should be the same. That is to say the impedance of the inner electrode  28  and the impedance of the inner electrode  30  have the same impedance values. To achieve this goal, an impedance matching circuit  42  is added in the present invention connecting the AC bias power  22  to the capacitive impedance  321  for isolation of the inner electrode  28  and the capacitive impedance  322  for isolation of the outer electrode  30  to let the capacitive impedance  321  for isolation of the inner electrode  28  and the capacitive impedance  322  for isolation of the outer electrode  30  have the same impedance values. And then the inner electrode power output will be the same with the outer electrode power output to avoid damaging gate oxides of the devices on the wafer  18 . 
   One preferred embodiment of the present invention is as shown in  FIG. 3C , the impedance matching circuit mainly includes an adjustable capacitor  441  of the inner electrode  28 , an adjustable capacitor  442  of the outer electrode  30 , an adjustable inductor  461  of the inner electrode  28 , an adjustable inductor  462  of the outer electrode  30 , a power-measuring device  50 , a power comparator  51 , and an automatic impedance-regulator  52 . One terminal of the impedance matching circuit  42  connects to the AC bias power  22  and the other terminal of the impedance matching circuit  42  connects to both the capacitive impedance  321  for isolation of the inner electrode  28  and the capacitive impedance  322  for isolation of the outer electrode  30 . 
   The way of impedance-matching provided by the present invention includes the following steps: as shown by the power-measuring block  500  in  FIG. 4  a power-measuring device  50  which can measure the voltage and the current of both the inner electrode  28  and the outer electrode  30  of the bi-polar electrostatic chuck  201  and transform the voltage values and the current values into power output values of the inner electrode  28  and the outer electrode  30  of the bi-polar electrostatic chuck  201 , as shown by the power-comparing block  510  a power comparator  51  which can compare the power value of the inner electrode  28  and the power value of the outer electrode  30  to get a control signal, as shown by the automatic impedance-matching block  520 , and an automatic impedance-regulator  52  which can receive the control signal to drive the logic drive motors to regulate the impedance values of the adjustable impedance-elements to let the inner electrode  28  and the outer electrode  30  of the bi-polar electrostatic chuck  201  have the same power output. Proceeding with the HDPCVD process at this time will not produce the voltage potential difference between the inner portion and the outer portion of the wafer  18  when the wafer  18  are bombarded by a plurality of ions because the inner electrode  28  power output is the same with the outer electrode  30  power output. 
   What is said above is only a preferred embodiment of the invention, which is not to be used to limit the claims of the invention; any change of equal effect or modifications that do not depart from the essence displayed by the invention should be limited in what is claimed in the following.

Technology Classification (CPC): 7