Methods and apparatus for determining an etch endpoint in a plasma processing system

Methods and apparatus for ascertaining the end of an etch process while etching through a target layer on a substrate in a plasma processing system which employs an electrostatic chuck. The end of the etch process is ascertained by monitoring the electric potential of the substrate to detect a pattern indicative of the end of the etch process. By the way of example, changes to this potential may be observed by monitoring the current flowing to the pole of the electrostatic chuck. Upon ascertaining the pattern indicative of the end of the etch process, for example by monitoring the current signal, a control signal is produced to terminate the etch. If a bias compensation power supply is provided to keep the currents flowing to the poles of the electrostatic chuck substantially equal but opposite in sign throughout the etch, the compensation voltage output by the bias compensation power supply may be monitored for the aforementioned pattern indicative of the end of the etch process in order to terminate the etch.

BACKGROUND OF THE INVENTION
 The present invention relates to the manufacture of semiconductor devices.
 More particularly, the present invention relates to improved techniques
 for ascertaining the end of an etch process for endpointing purposes while
 etching through a selected layer on a substrate.
 In the manufacture of semiconductor devices, such as integrated circuits or
 flat panel displays, the substrate (e.g., the wafer or the glass panel)
 may be processed in a plasma processing chamber. Processing may include
 the deposition of layers of materials on the substrate and the selective
 etching of the deposited layer(s). To prepare a layer for etching, the
 substrate surface is typically masked with an appropriate photoresist or
 hard mask. During etching, a plasma is formed from the appropriate etchant
 source gas to etch through regions unprotected by the mask. The etching is
 terminated once it is determined that the target layer is etched through.
 This termination of the etch is typically referred to as the etch
 "endpoint."
 To determine when to terminate an etch, many techniques have been employed
 in the art. By way of example, the etch may be terminated upon the
 expiration of a predefined period of time. The predefined period of time
 may be empirically determined in advance by etching a few sample
 substrates prior to the production run. However, there is no allowance
 made for substrate-to-substrate variations as there is no feedback
 control.
 More commonly, the end of an etch process may be dynamically ascertained by
 monitoring the optical emission of the plasma. When the target layer is
 etched through, the optical emission of the plasma may change due to the
 reduced concentration of the etch byproducts, the increased concentration
 of the etchants, the increased concentration of the byproducts formed by
 reaction with the material(s) of the underlayer, and/or due to the change
 in the impedance of the plasma itself.
 It has been found, however, that the optical emission-based technique has
 some disadvantages. By way of example, the use of some etchants and/or
 additive gases interferes with the optical emission endpoint technique,
 giving rise to inaccurate readings. As a further example, as the feature
 sizes decrease, the amount of film exposed to the plasma through openings
 in the mask is also reduced. Accordingly, the amount of byproduct gases
 that is formed from reactions with the exposed film reduces, rendering
 signals that rely on plasma optical emission less reliable.
 It has been found that, as the target layer etch is completed and the
 underlayer is exposed to the plasma, the self-induced bias of the
 substrate may change. By way of example, for the etch of a dielectric
 target layer, the self-induced bias of the substrate is observed to change
 as a conductive underlayer is exposed to the plasma. As a further example,
 for the etch of a conductive target layer, the self-induced bias of the
 substrate is observed to change when a dielectric underlayer is exposed to
 the plasma. By monitoring the change in the self-induced bias of the
 substrate, the end of the etch process may be ascertained for endpointing
 purposes.
 To facilitate discussion, FIG. 1 illustrates a typical endpointing
 arrangement wherein the self-induced bias on the wafer is monitored to
 determine when the target layer is etched through for the purpose of
 endpointing the etch. As shown in FIG. 1, a wafer 102 is shown disposed on
 an electrode 104, which is typically made of a metallic material.
 Electrode 104, which functions as a chuck in this example, is energized by
 an RF power source 106 through a capacitor 108. During etching, the
 self-induced bias on wafer 102 is detected at a node 110 through a
 monitoring circuit 112. Monitoring circuit 112 include a low pass filter
 114, which blocks the RF component of the signal and allows only the DC
 component to pass through. Since the self-induced bias on the wafer tends
 to be in the hundreds of volts, the signal that is passed through low pass
 filter 114 is typically stepped down through a voltage divider circuit to
 allow the monitoring electronics (not shown to simplify the discussion) to
 monitor the change in the self-induced bias on wafer 102. This information
 pertaining to changes in the self-induced bias on the wafer allows the
 endpointing electronics to determine when the etch should be terminated.
 However, the sensitivity and accuracy of the monitoring technique discussed
 in FIG. 1 may degrade as the percentage of the target film exposed to the
 plasma decreases and/or if the DC conductivity between the plasma and the
 electrode is decreased (e.g., due to the presence of a dielectric layer
 underlying the target layer to be etched). Furthermore, the monitoring
 technique of FIG. 1 is typically ineffective when electrostatic chucks are
 employed. This is because electrostatic chucks typically employ a
 dielectric layer between the conductive chuck body and the substrate. The
 presence of this dielectric layer interferes with the current path between
 the plasma and the chuck, rendering it very difficult to accurately
 determine the self-induced bias on the wafer at node 110. Furthermore, the
 relationship between the voltage detected at node 110 and the self-induced
 bias on wafer 102 is not linear. By way of example, the resistance of the
 electrostatic chuck depends, in part, on the voltage existing on the
 chuck. Accordingly even if a signal can be detected at node 110, it is
 difficult to correlate the signal detected with the self-induced bias on
 the substrate for endpointing purposes.
 In view of the foregoing, there are desired improved techniques for
 detecting the end of a plasma etch process for endpointing purposes.
 SUMMARY OF THE INVENTION
 The invention relates to methods and apparatus for ascertaining the end of
 an etch process while etching through a target layer on a substrate in a
 plasma processing system. This invention exploits the change in the
 electric potential of the substrate which, for many different etch
 applications, corresponds to the end of the etch process. In one
 embodiment, the endpointing arrangement includes a current monitoring
 circuit configured to monitor the current flowing to a pole of the
 electrostatic chuck to detect a pattern indicative of the end of the etch
 process. Upon ascertaining the pattern indicative of the end of the etch
 process in the current signal, a control signal is produced to terminate
 the etch.
 In another embodiment, the chuck represents a bipolar electrostatic chuck
 and currents flowing to both poles of the electrostatic chucks are
 monitored for the aforementioned pattern indicative of the end of the etch
 process in order to terminate the etch. In yet another embodiment, the
 differential of the currents supplied to the poles of the electrostatic
 chuck is monitored for the aforementioned pattern indicative of the end of
 the etch process in order to terminate the etch.
 In yet another embodiment, the electrostatic power supply includes a bias
 compensation power supply, which monitors currents supplied to the
 electrostatic chuck poles and outputs a compensation voltage responsive
 thereto. The compensation voltage is then input into the chuck power
 supply in order to keep the currents supplied to the poles substantially
 equal but opposite in sign throughout the etch. In this embodiment, the
 compensation voltage is monitored for the aforementioned pattern
 indicative of the end of the etch process in order to terminate the etch.
 These and other advantages of the present invention will become apparent
 upon reading the following detailed descriptions and studying the various
 drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will now be described in detail with reference to a
 few preferred embodiments thereof as illustrated in the accompanying
 drawings. In the following description, numerous specific details are set
 forth in order to provide a thorough understanding of the present
 invention. It will be apparent, however, to one skilled in the art, that
 the present invention may be practiced without some or all of these
 specific details. In other instances, well known process steps and/or
 structures have not been described in detail in order to avoid
 unnecessarily obscuring the present invention.
 It is appreciated by the inventors herein that as the etch progresses
 through a target layer, and particularly as the target layer is etched
 through to the underlayer, the electric potential of the substrate
 changes. The change in the substrate potential is particularly pronounced
 at the end of the etch. While not wishing to be bound by theory, it is
 believed that, as the target layer is etched through, the capacitive and
 resistive coupling between the substrate and the plasma changes. As one
 possible explanation, the self-induced bias on the substrate may change
 due to the increased current leakage between the plasma and the substrate
 as the etch features (such as vias or trenches) are etched down to a stop
 layer. It is also possible that the properties of the plasma itself are
 changed as the target layer is etched through. This change brings about a
 change in the plasma impedance, which in tun changes the self-induced bias
 on the substrate.
 When an electrostatic chuck is employed in the plasma processing system,
 direct measurement of the substrate electric potential is difficult,
 because the dielectric layer of the ESC introduces a large resistance
 between the substrate and the electrical measurement circuitry. The
 present invention overcomes these difficulties.
 It is appreciated by the inventors herein that changes in the substrate
 electric potential cause variations in the current flowing from the ESC
 power supply to the poles of the electrostatic chuck. In one of the
 embodiments of the present invention, the currents flowing to the poles of
 the electrostatic chuck are monitored. In this manner, the change of
 substrate potential associated with the end of the etch process may be
 ascertained, and the information derived therefrom may be employed to
 endpoint the etch.
 More preferably, some electrostatic chuck power supplies employ a
 compensation circuit to keep the currents flowing to the poles of the
 electrostatic chucks substantially equal in magnitude but opposite in
 sign. Compensation circuits are employed since if electrostatic forces
 between the chuck poles and the overlying substrate regions vary during an
 etch, inconsistent chucking, inconsistent heat transfer, and undesirable
 etch results may occur. In some systems, however, the compensation circuit
 may be employed to keep the currents flowing to the poles of the
 electrostatic chuck substantially constant (i.e., relatively unchanging
 even if they are unequal throughout the etch).
 In general, the compensation circuit typically monitors the currents
 flowing to the poles of the electrostatic chuck and provides a control
 signal to a variable bias compensation power supply. When the currents
 flowing to the poles of the electrostatic chuck poles change, the changing
 control signal varies the voltage output by a bias compensation power
 supply. The voltage output by the bias compensation power supply, referred
 to herein as the compensation voltage, is then employed to offset the
 voltages supplied to the chuck poles in order to keep the currents flowing
 to the electrostatic chuck poles substantially equal in magnitude but
 opposite in sign (or substantially constant in other systems as mentioned
 earlier).
 It is discovered by the inventors that the compensation voltage changes as
 the etch progresses and typically changes dramatically as the target layer
 is cleared, i.e., etched through. In accordance with one embodiment of the
 present invention, information regarding end of the etch process may be
 obtained by monitoring the compensation voltage in order to endpoint the
 etch.
 To facilitate discussion, FIG. 2 is a simplified illustration of a
 compensation arrangement for keeping the currents supplied to the chuck
 poles substantially equal but opposite in sign as the etch progresses. It
 should be kept in mind, however, that while the compensation arrangement
 of the exemplary embodiment functions to keep the currents supplied to the
 chuck poles substantially equal but opposite in sign, the concepts
 disclosed herein also apply equally to compensation arrangements that keep
 the currents flowing to the poles substantially unchanging (i.e.,
 relatively unchanging even if they are unequal throughout the etch). The
 adaptation of the exemplary arrangement to work with such a compensation
 circuit is well within the skills of one of ordinary skills in the art
 given this disclosure.
 With reference to FIG. 2, the object to be processed 200, e.g. a wafer or
 glass panel, includes the target layer to be etched, and is represented in
 a simplified manner by a photoresist mask layer 202, a target layer 204,
 underlayer film or films 206, and the substrate 207. Target layer 204 may
 represent any layer to be etched through. In one example, target layer 204
 represents a silicon dioxide-containing layer such as a doped CVD
 (chemical vapor deposition) or PECVD (plasma-enhanced chemical vapor
 deposition) glass layer. In another example, target layer 204 may
 represent a low dielectric constant (low-k dielectric) layer. In yet
 another example, target layer 204 represents a metal layer or polysilicon
 (doped or undoped) to be etched. Underlayer film or films 206 may include
 any and all layers and/or structures that underlie target layer 204.
 Underlayer film or films 206 may include, for example, one or more
 conductive (metallic or doped polysilicon) layers and/or one or more
 dielectric layers. By way of example, an etch stop layer may be disposed
 immediately below target layer 204 and may be formed of, for example,
 silicon nitride, titanium silicide, or titanium nitride material.
 Substrate 207 represents the supporting material of the object to be
 etched, for example, a wafer or glass panel. For the sake of discussion in
 the present example, substrate 207 does not include the layers and/or
 device structures which may be present on its surface, which are instead
 represented by the aforementioned layers 202, 204, and 206. In some cases,
 the underlayer film or films 206 may be absent, and the target layer 204
 is disposed directly on the substrate 207.
 In the example of FIG. 2 a Johnsen-Rahbek chuck is employed although the
 invention is believed to work with any type of electrostatic chuck such as
 monopole ESC chucks, multipole ESC chucks of any configuration, or the
 like. The construction of a Johnsen-Rahbek chuck is well known in the art
 and will not be discussed in detail here for brevity's sake. Further,
 although the chuck poles are of a concentric configuration in the example
 of FIG. 2, the poles of the electrostatic chuck may assume any
 configuration and/or geometry (e.g., inter-digitated). For the concentric
 Johnsen Rahbek chuck of the example of FIG. 2, an outer pole 208 and an
 inner pole 210 are embedded in a slightly conductive layer 212, which may
 be formed of, for example, a ceramic material that is lightly doped for
 conductivity. An RF electrode 214, which is disposed below slightly
 conductive layer 212, is typically formed of a metallic material and is
 coupled to an RF power supply 216 through a capacitor 218. To facilitate
 chucking, the poles of chuck 220 are coupled to an electrostatic power
 supply 222.
 Electrostatic chuck power supply 222 includes a main power supply 224,
 which supplies the DC chucking voltages to the poles of chuck 220. Low
 pass filters to 230 and 232 are interposed between poles 208 and 210 and
 electrostatic chuck power supply 222 to couple main power supply 224 to
 poles 208 and 210 of chuck 220 and to isolate RF power 216 from power
 supply 222. Current monitoring circuits 234 and 236 are coupled in series
 with the current paths between the poles of the electrostatic chuck and
 ESC power supply 222 to monitor the currents in these legs.
 Each of current monitor circuits 234 and 236 may be implemented by a simple
 resistive arrangement, and the potential difference across each may be
 ascertained to determine the current flowing to each of poles 208 and 210.
 The outputs of current monitor circuits 234 and 236 are input into a
 comparator circuit 238, which may represent, for example, a differential
 amplifier circuit. Comparator circuit 238 outputs a control signal 240 for
 controlling a variable bias compensation power supply 242. Bias
 compensation power supply 242 changes its output responsive to control
 signal 240. The output of bias compensation power supply 242 is employed
 to bias main power supply 224 to keep the currents flowing to poles 208
 and 210 substantially equal in magnitude and opposite in sign. The
 arrangement of FIG. 2, including the bias compensation arrangement in
 electrostatic chuck power supply 222, is well known in the art.
 As target layer 204 is etched through, the compensation voltage at node 250
 changes as the compensation circuit attempts to keep the currents flowing
 to poles 208 and 210 substantially equal. It is appreciated by the
 inventors herein that the information contained in the compensation
 voltage, which is found either in control signal 240 or at node 250 at the
 output of bias compensation power supply 242, includes information
 pertaining the progress of the etch and particularly pertaining when the
 end of the etch occurs. This is because, as explained earlier, the
 electric potential of the substrate 207 changes as the etch progresses,
 and causes the currents flowing to each of the poles 208 and 210 to
 change. These changes are detected by current monitor circuits 234 and 236
 to produce a control signal 240, which serves as the feedback signal to
 bias compensation power supply 242, whose job it is to bias main power
 supply 224 to keep the currents flowing to poles 208 and 210 substantially
 equal.
 FIG. 3 illustrates a typical compensation voltage as the etch progresses
 through the target layer. At point 302, the etch begins on compensation
 voltage plot 300. As the etch progress, the compensation voltage changes.
 Although the change is illustrated in FIG. 3 by an increasing compensation
 voltage, the compensation voltage may change in other ways, such as
 decreasing, as the etch progresses in other substrates. As the etch clears
 the target layer, a significant change in the compensation voltage is
 typically observed. Although the end of the etch is evidenced by a steep
 upward slope in the vicinity of region 304 in FIG. 3, the end of the etch
 may also be evidenced (in other etch processes) by a sharp downward slope,
 a spike or a sudden dip in the signal. Irrespective of the exact shape of
 the compensation voltage plot at the time the etch ends, the end of the
 etch is typically evidenced by a clearly discernible change in the
 compensation voltage. The specific characteristic shape of the
 compensation voltage plot at the time the etch ends may be ascertained by
 performing sample etches on sample wafers. Thereafter, the monitoring
 circuitry may be instructed to look for the ascertained characteristic
 shape in the compensation plot that signals the end of the etch for
 endpointing purposes.
 FIG. 4 illustrates, in accordance with one embodiment of the present
 invention, a simplified arrangement for monitoring the compensation
 voltage for the purpose of endpointing the etch. In FIG. 4, the voltage at
 node 250 is input into endpoint monitoring circuitry 402, which outputs an
 endpoint signal 404 when the characteristic change indicative of the end
 of the etch process is ascertained. Monitoring circuitry 402 may
 represent, for example, programmable digital circuitry that has been
 programmed to analyze the input compensation voltage signal and to output
 a control signal 404 for endpointing the etch process. In one example,
 monitoring circuitry 402 represents a general purpose digital computer
 (e.g., a microcomputer) or a digital signal processor that has been
 programmed to analyze the digitized compensation voltage signal for
 changes indicative of the end of the etch process.
 In accordance with another embodiment of the present invention, it is also
 possible to monitor control signal 240 itself for changes characteristic
 of the end of the etch for endpointing purposes. In accordance with yet
 another embodiment of the present invention, the currents through the legs
 themselves may be monitored (by, for example, monitoring the outputs of
 current monitor circuits 234 and 236) for changes in the current(s) that
 are indicative of the end of the etch process. This latter embodiment is
 particularly useful for chucks which do not employ compensation circuitry.
 In accordance with another embodiment of the present invention, the
 difference in currents through the pole legs may be monitored indirectly
 by the current monitoring circuit 248, even in the absence of power supply
 242.
 As can be appreciated from the foregoing, many embodiments of the invention
 take advantage of existing signals in the electrostatic chuck power supply
 for the purpose of ascertaining when the end of the etch occurs in order
 to terminate the etch. In an indirect manner, changes in the currents
 supplied to the poles of the electrostatic chuck are employed to ascertain
 the etch progress for endpointing purposes. Unlike prior art techniques,
 the endpointing technique of the present invention does not require
 directly monitoring the self-induced bias of the substrate through the
 electrode (as was done in the case of FIG. 1). Accordingly, the technique
 works even with electrostatic chucks, which has a nonconductive dielectric
 layer disposed between the wafer and the body of the chuck.
 In fact, the accurate determination of when the etch ends is possible even
 if there is a nonconductive layer disposed between the chuck's metallic
 body and the target layer. The presence of the nonconductive dielectric
 layer, either as part of the electrostatic chuck or within the substrate,
 would presumably have caused the prior art endpointing circuitry of FIG. 1
 to fail to accurately provide an endpoint signal since the prior art
 technique depends on the direct measurement of the self-induced bias on
 the substrate through the electrode for endpointing purposes.
 Additionally, one of ordinary skills in the art would have assumed that
 the presence of a dielectric layer on the surface of the electrode and/or
 under the target layer would block the electrical path, rendering the
 direct monitoring of the self-induced bias on the substrate impossible
 and/or very difficult. Since the present invention does not rely on direct
 contact between the substrate and the electrode, the presence of a such a
 dielectric layer does not prevent the ascertaining of the end of the etch
 in the present invention.
 It is also observed that the inventive endpointing technique is highly
 sensitive and is capable of accurately providing endpointing information
 even when etching substrates having a small fraction (or percentage) of
 the target layer exposed to the etching plasma. The sensitivity appears to
 increase if a conductive layer, e.g., a conductive metal or doped
 polysilicon interconnect layer, is disposed below the target layer to be
 etched. As alluded to earlier, the sensitivity of the present technique is
 such that the end of the etch process may be ascertained even if there is
 a dielectric layer disposed under the target layer. Furthermore, since
 endpointing does not depend on monitoring the optical emission of the
 plasma, the inventive technique also works irrespective of the etchant
 and/or additive gas employed.
 While this invention has been described in terms of several preferred
 embodiments, there are alterations, permutations, and equivalents which
 fall within the scope of this invention. In general, it is proposed that
 the endpoint data can be derived from the changes in the substrate
 potential, which can in turn be obtained by looking at various signals at
 various points in the system. Thus, although the endpoint data can be
 ascertained by monitoring the changes in the current(s) flowing to the
 pole(s) of the ESC chuck (which reflect the changes in the substrate
 potential), there are other ways of obtaining this substrate
 potential-based endpoint data when an ESC chuck is involved. By way of
 example, a probe which contacts the backside of the substrate or some
 appropriate place on the substrate may be employed to measure the
 substrate potential directly throughout the etch, and the probe signal may
 be analyzed for changes indicative of the etch termination for endpointing
 purposes.
 As another example, the leakage flow rate of coolant gas from the edges of
 the ESC chuck may be monitored during the etch, as an indirect measure of
 the substrate electric potential. This flow rate is dependent upon the
 clamping force of the ESC, which is, in turn, dependent upon the potential
 difference(s) between the ESC and the substrate. As the etch proceeds,
 detectable changes in the flow rate may arise due to changes in the
 substrate potential. In one embodiment, the leakage flow rate may be
 monitored in conjunction with or as part of a pressure control arrangement
 which supplies the coolant gas to the interface between the substrate and
 the ESC. The flow rate signal may be analyzed for changes indicative of
 the etch termination, for endpointing purposes.
 In fact, given this disclosure, one of ordinary skills in the art will
 readily recognize that changes in the substrate potential impact other
 signals at various points in the plasma processing system. With the
 knowledge imparted by this disclosure, the identification of the possible
 signals and locations in a specific plasma processing system that may be
 monitored to ascertain the changes in the substrate potential is well
 within the skills of one familiar with plasma processing equipment. It
 should also be noted that there are many alternative ways of implementing
 the methods and apparatuses of the present invention. It is therefore
 intended that the following appended claims be interpreted as including
 all such alterations, permutations, and equivalents as fall within the
 true spirit and scope of the present invention.