Abstract:
The disclosure concerns a plasma-enhanced etch process in which chamber pressure and/or RF power level is ramped throughout the etch process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/824,006, filed May 16, 2013 entitled ETCH PROCESS HAVING ADAPTIVE CONTROL WITH ETCH DEPTH OF PRESSURE AND POWER, by Kenny Linh Doan, et al. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The disclosure concerns plasma-enhanced etch processes for fabrication of microelectronic devices such as semiconductor integrated circuits. 
         [0004]    2. Background Discussion 
         [0005]    Fabrication of semiconductor integrated circuits has progressed toward smaller device sizes. Typically, contact holes are etched through insulating material between conductor structures in different layers. The contact holes may be nominally 50 nm in diameter and in excess of 2000 nm in length, so that the holes have an extremely high aspect ratio of about 40:1. Such a high aspect ratio makes the process vulnerable to failure due instability or interruption in plasma generation. One problem is that the etch process must be sufficiently selective so that it does not attack an underlying etch stop layer at the bottom of the hole. The etch stop layer may be a material different from the insulating material through which the hole is etched. The term etch selectivity may be defined as the etch rate at which the process etches the insulating material divided by the etch rate at which the process etches the underlying barrier layer. The problem of etch selectivity to the etch stop layer was conventionally addressed by setting the chamber to a lower pressure before the etch process reached the etch stop layer. For example, in one etch processes, the chamber pressure is set to about 20 milliTorr (mT) at the start. Then, after the etch depth reaches about 33% of the ultimate depth (e.g., of 2200 nm), the chamber pressure is set to 15 mT. Then, after the etch depth reaches about 66% of the ultimate depth, the chamber pressure is set to 10 mT and held there until the ultimate depth is reached. These abrupt changes in chamber pressure create plasma instabilities (such as arcing that can damage devices) to which the high aspect ratio opening is particularly vulnerable. 
         [0006]    This problem was addressed by introducing transition steps during which the step-wise changes in chamber pressure are carried out. Each transition step consists of reducing (or turning off) plasma source power, setting the chamber pressure to the lower value, and then restoring the plasma source power to its former level. During each transition step, the plasma power is sufficiently low to prevent significant plasma instability during the abrupt change in chamber pressure. However, this approach creates yet another problem, namely that the plasma ion density or energy is insufficient during the transition step to avoid polymer build-up inside each hole due to continued presence of carbon-rich process gases within the process chamber. Such build-up of polymer inside the holes leads to etch stop and device failure. Therefore, there is a need to improve etch selectivity to the barrier layer without risking device damage. 
         [0007]    Another problem encountered in plasma etching of holes with a 40:1 aspect ratio is bending, in which the bottom portion of each hole is not coaxial with the top portion of the hole. One approach to this problem is to select a high RF power level to increase plasma ion energy. While this approach can reduce bending, it increases the rate at which plasma sputters the photolithographic mask defining the holes. For etching holes with a 40:1 aspect ratio, if the RF power is set to a level sufficient to prevent bending, then the product of the mask sputtering rate and the required etch time exceeds the initial thickness of the mask, so that the mask is completely removed prior to completion of the etch process. This leads to device failure. Therefore, there is a need to prevent bending without risking loss of the photolithographic mask during processing. 
         [0008]    There are other problems encountered with etching holes with an aspect ratio of at least 40:1. For example, the top and bottom of the hole may be non-concentric. Specifically, the hole shape defined by the mask at the top of the hole is circular, while the final hole shape at the bottom is non-circular (e.g., elliptical, eccentric, star-shaped or the like). Another problem is bowing, in which the hole diameter is larger in a zone between the top and bottom of the hole. A related problem is that non-concentricity, bending, bowing and loss of the mask reduce control over critical dimension in the fabricated structure. 
       SUMMARY 
       [0009]    A method of processing a workpiece in a plasma reactor chamber comprises: (a) defining a starting chamber pressure and an ending chamber pressure at which a desired etch selectivity of an etch process to an underlying barrier layer is realized, (b) defining an etch time and computing a pressure ramping rate as a difference between the starting and ending chamber pressures divided by the etch time, (c) providing in the chamber a plasma containing etchant species, (d) initializing a chamber pressure in the chamber at the starting chamber pressure, and ramping the chamber pressure at the pressure ramping rate. 
         [0010]    The method may further comprise providing a user interface configured to receive values of the starting and ending chamber pressures, and controlling the chamber pressure with a digital control system in response to the user interface. In one embodiment, the ramping comprises generating successive chamber pressure commands in the digital control system representing successive microsteps of decreasing pressure levels. In one embodiment, the pressure difference between successive microsteps is sufficiently small to cause the chamber pressure to decrease in a continuous ramp. In a related embodiment, each of the successive microsteps corresponds to a digital quantization size of the digital control system. 
         [0011]    In accordance with another aspect, a method is provided for processing a workpiece in a plasma reactor chamber comprising an RF power applicator. The method comprises: (a) defining a starting RF power level and an ending RF power level sufficient to prevent bowing at the bottom of an etched opening, (b) defining an etch time, (c) computing an RF power ramping rate as a difference between the starting and ending RF power levels divided by the etch time, (d) providing in the chamber a plasma containing etchant species, (e) initializing an RF power level for the RF power applicator at the starting RF power level, and (f) ramping the RF power level pressure at the RF power ramping rate. 
         [0012]    The method may further comprise providing a user interface configured to receive values of the starting and ending RF power levels, and controlling the RF power level with a digital control system in response to the user interface. In one embodiment, the ramping comprises generating successive RF power level commands in the digital control system representing successive microsteps of increasing RF power levels. In one embodiment, the pressure difference between successive microsteps is sufficiently small to cause the RF power level to increase in a continuous ramp. In an embodiment, each of the successive microsteps corresponds to a digital quantization size of the digital control system. 
         [0013]    In one embodiment, the starting RF power level is one a minimum for sustaining an etch process. In one embodiment, the ending RF power level is sufficient to prevent bowing near a bottom of an etched opening having an aspect ratio on the order of approximately 40:1. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    So that the manner in which the exemplary embodiments of the present invention are attained can be understood in detail, a more detailed description of the invention, briefly summarized above, may be obtained by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0015]      FIGS. 1A-1C  depict the problem of non-concentricity, of which  FIG. 1A  depicts a structure in which an opening is etched,  FIG. 1B  depicts a cross-sectional view taken along lines  1 B- 1 B of  FIG. 1A ,  FIG. 1C  depicts a cross-sectional view taken along lines  1 C- 1 C of  FIG. 1A  in which the eccentricity is elliptical. 
           [0016]      FIG. 2  is an elevational cross-sectional view of a portion of a semiconductor wafer depicting bowing in a high aspect ratio opening. 
           [0017]      FIG. 3  is an elevational cross-sectional view of a portion of a semiconductor wafer depicting bending of a high aspect ratio opening. 
           [0018]      FIG. 4  is a simplified block diagram depicting a plasma reactor in accordance with certain embodiments. 
           [0019]      FIGS. 5A-5D  are contemporaneous graphs illustrating operation of the plasma reactor of  FIG. 4  in accordance with a first embodiment, of which  FIG. 5A  depicts digital pressure command value as a function of time,  FIG. 5B  (solid line) depicts actual chamber pressure as a function of time,  FIG. 5C  depicts etch depth as a function of time and  FIG. 5D  is an enlarged view of a superposition of a portions of  FIGS. 5A and 5B . 
           [0020]      FIG. 6  is a simplified block diagram of a method corresponding to  FIGS. 5A-5D . 
           [0021]      FIGS. 7A-7D  are contemporaneous graphs illustrating operation of the plasma reactor of  FIG. 4  in accordance with a second embodiment, of which  FIG. 7A  depicts a digital RF power command value as a function of time,  FIG. 7B  (solid line) depicts actual RF power or plasma ion energy as a function of time,  FIG. 7C  depicts etch depth as a function of time and  FIG. 7D  depicts mask thickness as a function of time. 
           [0022]      FIG. 8  is a simplified block diagram of a method corresponding to  FIGS. 7A-7D . 
       
    
    
       [0023]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary 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. 
       DETAILED DESCRIPTION 
       [0024]      FIG. 1A  depicts etching of a high aspect ratio opening or hole  105  in a thin film structure on a workpiece such as a semiconductor wafer  110 . A photolithographic mask  115  overlies the workpiece surface and has a circular opening  115   a  defining the hole  105 . Ideally, the hole  105  stops at the top surface of an underlying etch stop layer  120 . The hole shape ( FIG. 1B ) at the top of the hole  105  conforms with the circular shape of the mask opening  115   a.  The hole shape ( FIG. 1C ) at the hole bottom may be non-circular or non-concentric.  FIG. 2  depicts how the hole  105  may have bowing in which one zone  105   a  of the hole  105  has a diameter greater than the rest of the hole  105 .  FIG. 3  illustrates bending, in which the axis of hole  105  near the hole bottom bends away from the axis at the top of the hole. 
         [0025]      FIG. 4  depicts a plasma reactor capable of preventing bowing, bending and hole no concentricity (eccentricity) while providing improved etch selectivity to the etch stop layer and superior critical dimension control. The reactor of  FIG. 4  includes a vacuum chamber  400  enclosed by a cylindrical side wall  402 , a floor  404  and a ceiling  406 . The ceiling  406  includes a gas distribution showerhead  408  and a gas manifold  410  coupled to the showerhead  408 . The showerhead  408  in a first implementation operates as an RF electrode and may be coupled to an RF source power generator  412  through an impedance match  414 . In another implementation, a coil such as a side coil  416  or an overhead coil  418  may be coupled to the RF source power generator  412  through the impedance match  414 . A workpiece support pedestal  420  beneath the showerhead  408  includes a workpiece support electrode  422  beneath a workpiece support surface  424  coupled to an RF bias power generator  426  through an bias RF impedance match  427 . 
         [0026]    A vacuum pump  428  evacuates the chamber  400  through an exhaust valve  430 . Gas is supplied to the gas distribution showerhead  408  through a gas flow rate valve system  440  from a process gas supply  442 . The supply  442  may provide different process gas species to the valve system  440 , which the valve system  440  may separately control. The exhaust valve  430  is controlled by an exhaust valve controller  444 , which may include actuators to adjust the opening size of the valve  430 . The vacuum pump  428  is controlled by a pump controller  446  which controls the pumping rate of the pump  428 . A system controller  450  governs the valve controller  444 , the pump controller  446 , the gas valve system  440 , the power level of the RF bias power generator  426  and the power level of the RF source power generator  412 . A programmable computer  455  governs the system controller  450  and includes a memory  455 - 1  storing executable instructions. The memory  455 - 1  may be implemented as computer-readable media storing instructions for carrying out any of the methods disclosed herein, such the method of  FIG. 6  or  FIG. 8  or both, for example. A user interface  460  is coupled to the computer  455 . 
         [0027]    In accordance with a first embodiment, the user interface  460  provides the computer  455  with the following information entered by a user (or by an unillustrated superior control system): starting chamber pressure, ending chamber pressure and time (duration) of etch process. Referring to  FIG. 5A , the computer  455  is programmed to command the system controller  450  to set the chamber pressure the starting pressure and commence the etch process, and continuously decreasing the chamber pressure at a computed rate. The rate is the difference between the starting and ending chamber pressures divided by the etch time.  FIG. 5A  shows the commanded chamber pressure being ramped down in micro steps, each microstep corresponding to a digital control sample size of the system controller  450 . Typically, the system controller is implemented as a digital control system, the amplitude change of an individual microstep corresponding to the digital quantization of the digital control system implemented by the system controller  450 . The duration of each microstep is preferably less than the time required for the chamber pressure to fully respond to a commanded pressure change represented by a single microstep. The response of the measured chamber pressure is too slow to follow the microsteps of  FIG. 5A , and therefore follows the smooth ramp of  FIG. 5B .  FIG. 5C  depicts how the etch depth increases during the duration of the etch process. 
         [0028]    The solid line of  FIG. 5D  is a portion of the graph of pressure command microsteps of  FIG. 5A . The dashed line of  FIG. 5D  is a contemporaneous portion of the graph of actual chamber pressure of  FIG. 5B . The actual chamber pressure (dashed line of  FIG. 5D ) is continually changing to meet the latest microstep in the commanded pressure (solid line of  FIG. 5D ), and therefore follows a smooth continuous ramp trajectory as shown in the graph of  FIG. 5D . 
         [0029]      FIG. 6  depicts operation defined by the executable instructions of the memory  455 - 1  in accordance with the first embodiment corresponding to  FIGS. 5A-5D . A first step is to determine the rate RP at which the pressure is to be ramped downwardly (block  610  of  FIG. 6 ). The rate RP is computed as the difference between the starting and ending pressures divided by the etch time. The starting pressure, the ending pressure and the etch time are received from the user interface  460 . The next step is to order the system controller  450  to initialize the pressure to the starting pressure (block  620  of  FIG. 6 ). The controller  450  may accomplish this by controlling any of the pressure-determinative components, such as the exhaust valve controller  444  or the pump controller  446  or the valve system  440 . Next, the computer outputs a succession of pressure commands in decreasing sequence of pressure values as depicted in  FIG. 5A  (block  630  of  FIG. 6 ). The operation is halted at the end of the etch time (block  640  of  FIG. 6 ). 
         [0030]    The starting pressure is selected to optimize the etch rate or other process parameter, and may be as high as needed. The ending pressure is selected to provide sufficient etch selectivity to avoid punch through of the underlying etch stop layer  120  of  FIG. 1 , and therefore may be as low as needed. In one example, the starting pressure inside the chamber  100  was 120 mT and the ending pressure was 110 mT. 
         [0031]    An advantage is that there are no abrupt changes in pressure. The pressure is changed beginning from a high pressure ideal at start of the etch process to a low pressure that is ideal for etch selectivity to the underlying etch stop layer, without requiring any interruption or discontinuity in RF power or plasma generation. In addition, we have discovered that the foregoing process of ramping the pressure solves the problems of non-concentricity, bending and bowing. It is a surprising result that the pressure ramping method of the first embodiment achieves the following: concentric hole shapes at the top and bottom of each hole, elimination of bending and elimination of bowing. 
         [0032]    In accordance with a second embodiment, RF power ramping solves the problem of bending without damaging the photolithographic mask. In the second embodiment, the user interface  460  provides the computer  455  with a starting RF power level, an ending RF power level and an etch time. 
         [0033]    Referring to  FIG. 7A , the computer  455  is programmed to command the system controller  450  to set the RF power (e.g., of the RF power generator  412  or  426 ) to the starting RF power level and continuously increase (ramp up) the RF power level at a computed rate. The rate is the difference between the starting and ending RF power levels divided by the etch time.  FIG. 7A  shows the commanded RF power level being ramped up in micro steps, each microstep corresponding to a digital control sample size of the system controller  450 . The duration of each microstep is preferably less than the time required for the RF power to fully respond to a commanded power level change represented by a single microstep. The response of the measured ion energy level or actual RF power level is too slow to follow the individual microsteps of  FIG. 7A , and therefore follows the smooth ramp of  FIG. 7B .  FIG. 7C  depicts how the etch depth increases during the duration of the etch process.  FIG. 7D  depicts how the thickness of the photolithographic mask decreases at a sufficiently slow rate to leave a finite thickness at the end of the etch process. This finite thickness remains because the RF power level was kept low during the beginning of etch process, to conserve mask thickness, and did not reach a high level until the etch depth had increased so that a high RF power level was needed to prevent bending. 
         [0034]    The RF power level or ion energy ( FIG. 7B ) is continually increasing to meet the latest microstep in the commanded RF power level pressure ( FIG. 7A ), and therefore follows a smooth continuous ramp trajectory as shown in the graph of  FIG. 7B . 
         [0035]      FIG. 8  depicts operation defined by the executable instructions of the memory  455 - 1  in accordance with the second embodiment corresponding to  FIGS. 7A-7D . A first step is to determine the rate Rrf at which the pressure is to be ramped downwardly (block  810  of  FIG. 8 ). The rate Rrf is computed as the difference between the starting and ending RF power levels divided by the etch time. The starting RF power level, the ending RF power level and the etch time are received from the user interface  460 . The next step is to order the system controller  450  to initialize the RF power level to the starting RF power level (block  820  of  FIG. 8 ). The controller  450  may accomplish this by controlling any of the RF power generators  412 ,  426 . Next, the computer  455  outputs a succession of RF power level commands in increasing sequence of RF power levels as depicted in  FIG. 7A  (block  830  of  FIG. 8 ). The operation is halted at the end of the etch time (block  840  of  FIG. 8 ). 
         [0036]    The starting RF power level may be the minimum required to perform etch while the hole depth is relatively shallow. The level is minimize to reduce or minimize sputtering of the photolithographic mask  115 . The ending RF power level sufficient to prevent bending at the extreme depth (e.g., 2200 nm) of the hole, and may be as high as needed. In one example, the starting RF power level of the RF bias generator  426  was 1 kW and the ending RF power level was 7 kW. 
         [0037]    The RF power ramping method of the second embodiment solves the problem of preventing bending without removing the photolithographic mask, due to the reduction in RF power level during the initial stage of the etch process, as described above. In addition, we have discovered that the foregoing process of ramping the RF power level solves the problems of non-concentricity, bending and bowing. It is a surprising result that the RF power level ramping method of the second embodiment achieves the following: concentric hole shapes at the top and bottom of each hole, elimination of bending and elimination of bowing. 
         [0038]    In accordance with a third embodiment, the method of the first embodiment ( FIGS. 5A-5D  and  6 ) and the method of the second embodiment ( FIGS. 7A-7D  and  8 ) are performed simultaneously during processing of the same workpiece or wafer. In this third embodiment, the chamber pressure is ramped downwardly while simultaneously the RF power level is ramped upwardly. The third embodiment can provide the advantage of solving all the problems of etch selectivity, non-concentricity, bending and bowing in the same etch process. 
         [0039]    While the foregoing is directed to embodiments of the present 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.