Abstract:
An apparatus and method for maximizing ac energy delivered to a load by minimizing energy reflected from a load, such as an RF power source coupled to a plasma load for substrate processing chambers, including a matching network, wherein the matching network couples an ac power source and load. The matching network having two transmissions lines that are inductively coupled for a fixed portion of their length, such length being at least one wavelength of the ac energy generated by the ac power source. The matching circuit providing continuously variable impedance matching through the use of fixed components.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a matching circuit that reduces power reflected from a plasma formed in a substrate processing chamber. The invention is useful in a variety of technologies but is particularly useful in the manufacture of integrated circuits. 
   Plasma processing is a common step in the manufacture of integrated circuits. Common plasma processing steps include plasma enhanced chemical vapor deposition, reactive ion etching, and sputter etching among others. In such plasma processing steps, precise control of the plasma can be important in order to meet the manufacturing requirements of today&#39;s integrated circuits. 
   Typically a plasma is generated by applying RF energy to a coil or plates of a capacitor (inductive vs. capacitive coupling). To efficiently couple RF energy into a substrate processing chamber matching networks have been used to minimize the energy reflected from a plasma back into the RF generator. The source impedance of an RF generator is constant, typically 50 ohms resistive and zero ohms reactive, while the load of the plasma is transient and variable. The matching network matches the impedance of the load to an RF source from the perspective of the source. Thus, matching networks maximize RF power supplied to the load by minimizing the RF energy reflected from the load. 
   A variety of matching networks have been developed and successfully used in substrate processing.  FIG. 1  is a block diagram of a previously known ac energy delivery system  10 . As shown in  FIG. 1 , energy delivery system  10  includes a matching network  20   a  coupled by transmission lines  30   a-b  between an ac power source  40  and a plasma load  50 . The matching network is comprised of tuning elements  60   a-b  that include capacitors, or inductors, or both. The matching network of  FIG. 1 , having tuning element  60   a  in parallel with the ac power source and the plasma load and having tuning element  60   b  in series with the source and load is commonly referred to as an “L network.” 
     FIGS. 2 and 3  are block diagrams of energy delivery systems  12  and  14  having other previously known matching networks  20   b  and  20   c,  respectively. Matching network  20   b  of  FIG. 2  is commonly referred to as a “T network.” T networks typically have one tuning element  60   a  coupled in parallel with the ac power source  40  and plasma load  50  and have two tuning elements  60   c  and  60   b  in series with the ac power source and plasma load. Matching network  20   c  shown in  FIG. 3  is commonly referred to as a “π network.” Typically π networks have two tuning elements  60   a  and  60   d  coupled in parallel with ac power source  40  and plasma load  50  while having three tuning elements  60   b,    60   c  and  60   e  in series with the ac power source and plasma load. 
   The tuning range of a matching network is a measure of the range of impedance for which disparate load and source impedances can be effectively matched. For example, if the impedance of an ac power source is 50 ohms resistive and a load is 100 ohms resistive and 10 ohms reactive but varies by +/−10 ohms resistive and +/−5 ohms reactive, a matching network tuning range would be sufficiently broad to effectively match these impedances. The tuning range of a matching network is typically related to the number of tuning elements in the network. Thus, a π network typically has a broader tuning range than a T network and a T network typically has a broader tuning range than an L network. However, matching networks having a relatively large number of tuning elements have a relatively higher resistance than matching networks having fewer tuning elements. Thus, total ac energy transfer is typically lower in matching networks with a relatively large number of tuning elements. 
   Matching networks such as networks  20   a - 20   c  shown in  FIGS. 1-3  can include tuning elements that are fixed or variable. Variable tuning elements, which include variable capacitors, and/or variable inductors, provide a matching network with continuously adjustable impedance matching. Such continuous adjustability provides the benefit of continuously matching the impedance of an ac power source to a load that has transient and variable impedance. Thus, a controllable amount of energy may be transferred to a load. For example, if the load is a plasma having a transient and variable impedance, by supplying a controllable amount of energy to the plasma through impedance matching, the plasma can be maintained in a relatively stable state. 
   The cost of variable components is considerably higher than the cost of fixed components. Thus, matching networks that use fixed components are generally less expensive than matching networks that use variable components. Such fixed-element matching networks have limited impedance matching capability, however. Thus, optimal impedance matching is not always achieved with fixed components. To partially overcome the lack of continuous adjustability using fixed components, some previously known matching networks include parallel banks of fixed component tuning elements to provide step wise adjustability.  FIG. 4  is a block diagram of an energy delivery system  16  having a step wise adjustable matching network  20   d.  Step wise adjustability is achieved by switching banks of tuning elements  70   a . . . n  into or out of connection with ac power source  40 . Each bank of tuning elements  70   a . . . n  can be any of the previous described configurations of tuning elements, “L network”, “T network” or “π network.” While step wise adjustable matching networks provide improved impedance matching capabilities, such matching networks have regions for which optimal energy coupling to a plasma load cannot occur. 
   Accordingly, it is desirable to develop matching networks that have low cost fixed components while providing improved impedance matching over a broad range of RF wavelengths and high energy transfer. 
   SUMMARY OF THE INVENTION 
   The previously identified needs as well as other needs are solved by embodiments of the present invention, which provide an apparatus and method for matching the impedance of a load to an ac power source. The apparatus includes a matching network coupled between an ac power source and a load. The matching network provides an increased tuning range for matching the impedance of the ac power source to the load. More specifically, an ac power source having a fixed source impedance can be matched to a load having a transient and variable impedance while the matching network effectively minimizes ac energy reflected from the load by improving power delivered to the load. 
   Embodiments of the present invention provide the above recited features through the use of two transmission lines that inductively couple an ac power source to a load. To both maximize ac energy transferred to the load and to minimize reflected energy, a fixed length of two transmission lines are placed in close proximity for at least one wavelength of the ac energy produced by the ac power source. 
   The apparatus and methods of use of the present invention are important to the manufacture of integrated circuit devices in which an RF source having a fixed impedance is coupled to a plasma load having a transient and variable impedance. The present invention is applicable to an ac energy delivery system in which ac energy delivered to a load needs to be maximized through the minimization of reflected energy. In integrated circuit manufacture plasma processes, both deposition and removal processes are less reliable when reflected energy is not minimized. Embodiments of the invention can be used to minimize reflected energy for plasma processes through the inductive coupling of an RF source to a plasma load by inductively coupling two transmission lines for at least one wavelength of RF energy, thus stabilizing plasmas used in the manufacture of integrated circuits. 
   These and other embodiments of the present invention, as well as its advantages and feature are described in more detail in conjunction with the text below and attached figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art ac energy delivery system that includes an L-network, matching network; 
       FIG. 2  is a block diagram of a prior art ac energy delivery system that includes a T-network, matching network; 
       FIG. 3  is a block diagram of a prior art ac energy delivery system that includes π-network, matching network; 
       FIG. 4  is a block diagram of a prior art ac energy delivery system that includes a matching network having banks of parallel tuning elements coupled to connecting switches; 
       FIG. 5  is a block diagram of an ac energy delivery system that includes one embodiment of a matching network of the present invention; 
       FIG. 6  is a block diagram of an ac energy delivery system that includes another embodiment of a matching network of the present invention; 
       FIGS. 7A-7D  are diagrams of insulators within ground shields according to embodiments of the present invention; 
       FIG. 8  is a block diagram of an ac energy delivery system that includes another embodiment of a matching network of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 5  is a block diagram of an ac energy delivery system  100  according to one embodiment of the present invention. AC energy delivery system  100  includes a matching network  110   a  that inductively couples an ac power source  120  to a load  130 , such as a plasma processing chamber. The ac power source is coupled to a transmission line  140   a  to deliver ac energy to the transmission line. Transmission line  140   a  is inductively coupled to a second transmission line  140   b.  Transmission line  140   b  is further coupled to the load  130 . Transmission line  140   b  inductively receives ac energy from transmission line  140   a  and further delivers the ac energy to the load  130 . The mutual inductance of two transmission lines is proportional to the length of both of the transmission lines and inversely proportional to the distance between them. To effectively control the mutual inductance of the two transmission lines  140   a-b  they are placed within close proximity of each other and are enclosed within a single ground shield  150  for a limited portion of their overall length. Typical spacing between the transmission is in the range of about 5 cm to 0.5 cm. The portion of the two transmission lines not inside ground shield  150  may be enclosed in separate ground shields that limit the mutual inductance of the transmission lines. The length that the two transmission lines are inductively coupled is herein after referred to as an “inductive length.”  FIG. 5  shows the two transmission lines to be parallel within the inductive length. An insulator  160  holds the transmission lines  140   a-b  in a fixed parallel position.  FIGS. 7A and 7B  show varying views of insulator  160  inside ground shield  150 .  FIG. 7A  shows insulator  160  (having a dashed outline) inside ground shield  150  (having a solid outline) from an end perspective; central circles  165  represent openings into which the first and second transmission lines are placed.  FIG. 7B  shows a side view of insulator  160  inside of ground shield  150 . Central openings  165  are parallel to fix the first and second transmission lines in a parallel position as previously described. 
   Energy transfer from the ac power source and first transmission line to the second transmission line and load is improved if the inductive length is at least one wavelength of the ac energy. Thus, in order to ensure energy reflected from the load back to the ac power source is effectively minimized, the inductive length should be at least one wavelength of the ac energy. AC energy traveling in transmission line  140   a  not inductively coupled to transmission line  140   b  is prevented from reflecting from ground by a trimming element  170 . Trimming element  170  is typically a resistor used to match the transmission line impedance to ground. 
   One application of the matching circuit of the present invention is to couple an RF source to a gaseous species within a substrate processing chamber to generate a plasma. For a substrate processing system, typical ac energy delivered by an ac power source ranges from radio frequencies to microwave frequencies, approximately 100 kHz to 2.45 GHz. Typical RF energy used for plasma generation is in the range of about 350 kHz to 400 MHz. Thus, the inductive length of the transmission lines of the present invention is in the range of 3000 meters to about 0.12 meters and more typically between about 857 meters to about 0.75 meters. These inductive coupling lengths are quite long with respect to other equipment used in substrate processing. To make the embodiments of the present invention practical for use, the transmission lines and ground shield can be bent into various shapes to reduce their overall dimensions. For example, the transmission lines and ground shield can be bent into spirals, coils, or serpentines as well as other shapes (see FIG.  7 D). Such shapes can be less than a meter across in any direction, thus, making the dimensions of the transmission lines practical for use. In embodiments of the present invention discussed below, these dimensional issues are further addressed. 
     FIG. 6  is a block diagram of an AC energy deliver system  100  according to a second embodiment of the present invention. As similarly shown in FIG.  5  and similar to previously described embodiments, AC energy system  100  includes a matching network  110   b  that inductively couples an AC power source  120  to a load  130 , such as a plasma processing chamber. The ac power source is coupled to transmission line  140   a  to deliver ac energy to the transmission line. Transmission line  140   a  is inductively coupled to a second transmission line  140   b.  Transmission line  140   b  is further coupled to the load  130 . Transmission line  140   b  inductively receives ac energy from transmission line  140   a  and further delivers the ac energy to the load  130 . First and second transmissions lines  140   a  and  140   b  are inductively coupled over an inductive length of at least one wavelength of incident ac energy. The spacing between the first and second transmission lines as shown in  FIG. 6  varies along the length of the lines. Similarly described, the angle between the two transmission lines is non-zero. The transmission lines are held fixed within insulator  160 .  FIG. 7C  shows a side view of insulator  160  inside ground shield  150 . Openings  165  in insulator  160  are positioned such that the transmission lines are fixed as previously described. 
   The variable spacing between the transmission lines as shown in  FIG. 6  minimizes ac energy reflected from the load to the ac power source if the inductive length is at least one wavelength of the ac energy generated by the source. AC energy traveling in transmission line  140   a  not inductively coupled to transmission line  140   b  is prevented from reflecting from ground by a trimming element  170 . 
     FIG. 8  is a block diagram of AC energy delivery system  100  incorporating a third embodiment of a matching network  110   c  of the present invention. The matching network inductively couples an ac power source  120  to a load  130 , such as a plasma processing chamber. The ac power source is coupled to a transmission line  140   a  to deliver ac energy to the transmission line. Transmission line  140   a  is inductively coupled to a second transmission line  140   b.  Transmission line  140   b  is further coupled to the load  130 . Transmission line  140   b  inductively receives ac energy from transmission line  140   a  and further delivers the ac energy to the load  130 . In the third embodiment transmission lines  140   a-b  are both coiled. The coils of transmission line  140   a  have a constant radius of curvature, while the coils of transmission line  140   b  have a radius of curvature that increases from one end of the coils to the other end. The coils of transmission line  140   a  surround the coils of transmission line  140   b.  Similar to the previously discussed embodiments, to minimize reflected ac energy, both transmission lines are inductively coupled for at least one wavelength of ac energy generated by the ac power source. Similarly described, the transmission lines have an inductive length of at least one wavelength of ac energy generated by the ac power source. This embodiment is relatively physically small as compared to the previously described embodiments and further spiraling, coiling, or serpentineing of the ground shield and enclosed transmission lines is generally not necessary. 
   A number of different embodiments of matching networks have been described above, as well as methods for use. As will be appreciated by one of ordinary skill in the art, the embodiments described above are exemplary only. The present invention has application for other ac power delivery systems when power transfer needs to me maximized through minimal reflection for an ac power source coupled to a load, when both source and load have either real or complex impedances that are mismatched. 
   Although the present invention has been described and illustrated in detail, it is to be clearly understood that the above descriptions and illustrations are by way of example only and are not to be taken as limiting the invention, the spirit and scope of the present invention being limited only by the terms of the appended claims.