JP6050571B2 - Polishing monitoring method and polishing apparatus - Google Patents

Description

  The present invention relates to a polishing monitoring method and a polishing apparatus for monitoring a change in the thickness of a conductive film formed on the surface of a substrate such as a semiconductor wafer during polishing.

  In recent years, with higher integration and higher density of semiconductor devices, circuit wiring has become increasingly finer and the number of layers of multilayer wiring has increased. When trying to realize multilayer wiring while miniaturizing the circuit, the step becomes larger while following the surface unevenness of the lower layer, so as the number of wiring layers increases, the film coverage to the step shape in thin film formation (Step coverage) deteriorates. Therefore, in order to carry out multilayer wiring, it is necessary to improve the step coverage and perform a flattening process in an appropriate process. Further, since the depth of focus becomes shallower as the optical lithography becomes finer, it is necessary to planarize the surface of the semiconductor device so that the uneven steps on the surface of the semiconductor device are kept below the depth of focus.

Accordingly, in the semiconductor device manufacturing process, a planarization technique for the surface of the semiconductor device is becoming increasingly important. Among the planarization techniques, the most important technique is chemical mechanical polishing (CMP). This chemical mechanical polishing is performed by using a polishing apparatus to supply a polishing solution containing abrasive grains such as ceria (CeO ₂ ) to the polishing pad while sliding a substrate such as a semiconductor wafer against the polishing pad. Is what you do.

  A polishing apparatus that performs the above-described CMP process includes a polishing table having a polishing pad, and a substrate holding device called a top ring or a polishing head for holding a semiconductor wafer (substrate). Such a polishing apparatus is widely used in a polishing process for polishing a conductive film such as a barrier film or a metal film formed on the surface of a semiconductor wafer (substrate). Since the end point detection of the polishing process and the change of the polishing conditions during polishing are determined based on the thickness of the conductive film, the polishing apparatus generally detects the thickness of the conductive film during polishing. It has. An eddy current sensor is a typical apparatus for the film thickness detector.

The eddy current sensor is disposed in the polishing table. During polishing of the substrate, the eddy current sensor induces eddy current in the conductive film on the substrate while the eddy current sensor passes under the substrate as the polishing table rotates. The thickness of the conductive film is detected from a change in impedance caused by the eddy current magnetic field.
FIG. 17 is a diagram showing the relationship between the polishing time (t) from the start of polishing of the semiconductor wafer (substrate) until the conductive film on the semiconductor wafer is cleared (removed) and the signal value of the eddy current sensor. is there. As shown in FIG. 17, since the conductive film is thick immediately after the start of polishing of the semiconductor wafer, the output of the eddy current sensor becomes high. However, as the polishing proceeds, the conductive film becomes thin, so that the signal value of the eddy current sensor is It goes down. Then, when the conductive film is cleared (lost), the signal value of the eddy current sensor becomes constant. By detecting the time point (singular point) when the signal value becomes constant, it can be determined that the polishing end point has been reached.

JP 2005-121616 A JP 2009-99842 A

  However, the value of the output signal of the eddy current sensor may drift (translate) due to changes in the usage environment such as the ambient temperature of the eddy current sensor, water stains on the polishing pad, and changes over time of the eddy current sensor itself. is there. When the value of the output signal of the eddy current sensor drifts in this way, as shown in FIG. 17, the graph itself moves upward in a substantially parallel manner from a solid line to a dotted line. Even in this case, since the singular point moves in the same manner, it is possible to detect the polishing end point. However, when polishing is stopped at a predetermined thickness while leaving a part of the conductive film, or when switching to a different polishing condition such as low pressure and low rotation speed, the signal value (Z2) is used as a feature point to detect. There is a need. In this way, when the feature point is detected according to the signal value, the correspondence between the value of the output signal of the eddy current sensor and the film thickness is shifted due to drift, so that an error occurs in the polishing time to be detected. There is a problem.

  The present invention has been made in view of the above circumstances, a polishing monitoring method and a polishing capable of calibrating an eddy current sensor without reducing the operating rate of the polishing apparatus and enabling highly accurate film thickness monitoring. An object is to provide an apparatus.

To achieve the above object, the polishing monitoring how the present invention presses the substrate to be polished on the polishing surface on the polishing table which rotates to polish the conductive film on the substrate, placed on the polishing table during polishing A method for monitoring the thickness of a conductive film using an eddy current sensor, wherein an output signal of the eddy current sensor is acquired during polishing of the conductive film on the substrate with a polishing liquid containing abrasive grains, An output adjustment amount of the eddy current sensor is calculated using an output signal when the conductive film is being polished and the substrate is not present above the eddy current sensor, and the conductive film on the substrate is calculated using the output adjustment amount The thickness of the conductive film on the substrate is monitored by correcting the output signal when the substrate exists above the eddy current sensor.

  According to the present invention, the polishing process of pressing the substrate to be polished against the polishing surface on the rotating polishing table to start polishing the conductive film on the substrate is started, and the output signal of the eddy current sensor during the polishing is acquired. . Then, an output adjustment amount of the eddy current sensor is calculated using an output signal when no substrate exists above the eddy current sensor. The output signal of the eddy current sensor may drift (translate) due to changes in the usage environment or aging of the eddy current sensor itself, but there is a substrate above the eddy current sensor using the output adjustment amount. By correcting the output signal at the time, an amount corresponding to the drift amount can be removed from the output signal.

  According to a preferred aspect of the present invention, the output signal of the eddy current sensor has the thickness of the conductive film when the resistance component and reactance component of the impedance of the electric circuit including the coil of the eddy current sensor are defined as coordinates. The coordinates are expressed as coordinates obtained by rotating and moving the coordinates to a position where the distance between the origin of the coordinate system and the coordinates becomes shorter as the distance becomes smaller.

According to a preferred aspect of the present invention, the correction of the output signal is performed by moving the origin of the coordinate system.
According to the present invention, the amount corresponding to the drift amount can be removed from the output signal by translating the origin of the coordinate system representing the resistance component and reactance component of the impedance of the electric circuit including the coil of the eddy current sensor. .

According to a preferred embodiment of the present invention, it characterized by monitoring the thickness of the conductive film from the distance between the coordinate origin and the impedance of the coordinate system is moved by the correction.

According to a preferred aspect of the present invention, the correction of the output signal uses an average value of N rotation speeds of the output signal when no substrate exists above the eddy current sensor.

According to a preferred aspect of the present invention, only output signals from a region where no substrate holding top ring exists above the eddy current sensor out of output signals when the substrate does not exist above the eddy current sensor. It is characterized by using.
According to a preferred embodiment of the present invention, the of the output signal when a substrate above the eddy current sensor is not present, the output from the region Dore' service is not present for dressing Ken Migakumen above the eddy current sensor Only the signal is used.
According to the present invention, among the output signals when the substrate is not present above the eddy current sensor, only the output signal when nothing is present on or above the polishing surface is used. As a result, only signals in a range that does not affect the eddy current sensor can be used.

The polishing apparatus of the present invention comprises a polishing table having a polishing surface and rotating, a polishing solution supply means for supplying a polishing solution containing abrasive grains to the polishing surface, and a substrate to be polished being pressed against the polishing surface. A top ring for polishing the conductive film on the substrate, an eddy current sensor installed in the polishing table, and a monitoring device for monitoring the thickness of the conductive film based on an output signal of the eddy current sensor, The monitoring device acquires an output signal of the eddy current sensor during polishing of the conductive film on the substrate with a polishing liquid containing abrasive grains , and the substrate is being polished above the eddy current sensor while the conductive film on the substrate is being polished. The output adjustment amount of the eddy current sensor is calculated using the output signal when there is no current, and the substrate exists above the eddy current sensor while the conductive film on the substrate is being polished using the output adjustment amount. Corrects the output signal during Characterized by monitoring the thickness of.
According to a preferred aspect of the present invention, the output signal of the eddy current sensor has the thickness of the conductive film when the resistance component and reactance component of the impedance of the electric circuit including the coil of the eddy current sensor are defined as coordinates. The coordinates are expressed as coordinates obtained by rotating and moving the coordinates to a position where the distance between the origin of the coordinate system and the coordinates becomes shorter as the distance becomes smaller.
According to a preferred aspect of the present invention, the correction of the output signal is performed by moving the origin of the coordinate system.
According to a preferred aspect of the present invention, the thickness of the conductive film is monitored from the distance between the origin of the coordinate system moved by the correction and the coordinates of the impedance.

According to a preferred aspect of the present invention, the correction of the output signal uses an average value of N rotation speeds of the output signal when no substrate exists above the eddy current sensor.
According to a preferred aspect of the present invention, only output signals from a region where no substrate holding top ring exists above the eddy current sensor out of output signals when the substrate does not exist above the eddy current sensor. It is characterized by using.
According to a preferred embodiment of the present invention, the of the output signal when a substrate above the eddy current sensor is not present, the output from the region Dore' service is not present for dressing Ken Migakumen above the eddy current sensor Only the signal is used.

  ADVANTAGE OF THE INVENTION According to this invention, based on the output signal value of an eddy current sensor in the grinding | polishing process of the electrically conductive film on board | substrates, such as a semiconductor wafer, an eddy current sensor can be calibrated on software. Therefore, highly accurate film thickness monitoring can be continuously performed without reducing the operating rate of the polishing apparatus.

FIG. 1 is a schematic diagram showing the overall configuration of a polishing apparatus according to the present invention. FIG. 2 is a plan view showing the relationship among the polishing table, the eddy current sensor, and the semiconductor wafer. FIG. 3 is a diagram showing the configuration of the eddy current sensor, FIG. 3 (a) is a block diagram showing the configuration of the eddy current sensor, and FIG. 3 (b) is an equivalent circuit diagram of the eddy current sensor. FIG. 4 is a schematic diagram showing a configuration example of a sensor coil used in the eddy current sensor of the present invention. FIG. 5 is a schematic diagram showing a detailed configuration of the eddy current sensor. FIG. 6 is a diagram showing a main configuration of a polishing apparatus provided with an eddy current sensor. FIG. 6A is a diagram showing an overall configuration including a control unit of the eddy current sensor, and FIG. It is an expanded sectional view of an eddy current sensor part. FIG. 7A is a diagram showing the relationship between the locus when the eddy current sensor scans the surface (surface to be polished) of the semiconductor wafer and the output of the eddy current sensor, and FIG. FIG. 5 is a diagram showing the relationship between the rotation of the polishing table and the output of the eddy current sensor. FIG. 8 is a diagram illustrating a graph drawn by plotting the signal X as the resistance component of the impedance changing with the polishing time and the signal Y as the inductive reactance component on the XY coordinate system. FIG. 9 is a diagram showing a graph obtained by rotating the graph figure of FIG. 8 by 90 degrees counterclockwise and further translating it. FIG. 10 is a diagram illustrating a case where the value of the output signal of the eddy current sensor drifts (translates). FIG. 11 is a diagram showing an aspect of a processing flow for monitoring the change in the film thickness of the conductive film on the semiconductor wafer while calibrating the output signal of the eddy current sensor. FIG. 12 is a diagram for explaining the step of calculating the drift amount (correction amount). FIG. 13 is a diagram for explaining a step of translating the origin O of the XY coordinate system by an amount corresponding to the drift amount (correction amount). FIG. 14 is a diagram showing another aspect of the processing flow for monitoring the change in the film thickness of the conductive film on the semiconductor wafer while calibrating the output signal of the eddy current sensor 50. FIG. 15 is a diagram illustrating a case where the value of the output signal of the eddy current sensor drifts (translates). FIG. 16 is a diagram for explaining a step of shifting the reference point by the correction amount. FIG. 17 is a diagram showing the relationship between the polishing time (t) from the start of polishing of the semiconductor wafer (substrate) until the conductive film on the semiconductor wafer is cleared (removed) and the signal value of the eddy current sensor. is there.

  Hereinafter, embodiments of a polishing monitoring method and a polishing apparatus according to the present invention will be described in detail with reference to FIGS. 1 to 16, the same or corresponding components are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a schematic diagram showing the overall configuration of a polishing apparatus according to the present invention. As shown in FIG. 1, the polishing apparatus includes a polishing table 1 and a top ring 10 that holds a semiconductor wafer W that is an object to be polished and presses it against a polishing pad on the polishing table. The polishing table 1 is connected via a table shaft 1a to a polishing table rotation motor (not shown) disposed below the table 1a, and is rotatable around the table shaft 1a. A polishing pad 2 is affixed to the upper surface of the polishing table 1, and the surface of the polishing pad 2 constitutes a polishing surface 2 a for polishing the semiconductor wafer W. A polishing liquid supply nozzle 3 is installed above the polishing table 1, and the polishing liquid (slurry) is supplied to the polishing pad 2 on the polishing table 1 by the polishing liquid supply nozzle 3. As shown in FIG. 1, an eddy current sensor 50 is embedded in the polishing table 1.

  The top ring 10 is connected to a top ring shaft 11, and the top ring shaft 11 moves up and down with respect to the top ring head 12. By moving the top ring shaft 11 up and down, the entire top ring 10 is moved up and down relative to the top ring head 12 for positioning. The top ring shaft 11 is rotated by driving a top ring rotation motor (not shown). The top ring 10 rotates about the top ring shaft 11 by the rotation of the top ring shaft 11.

The top ring 10 can hold a semiconductor wafer W such as a semiconductor wafer on its lower surface. The top ring head 12 is configured to be pivotable about the top ring head shaft 13, and the top ring 10 holding the semiconductor wafer W on the lower surface thereof is moved from the substrate receiving position to the polishing table 1 by the rotation of the top ring head 12. It can move upward. The top ring 10 holds the semiconductor wafer W on the lower surface and presses the semiconductor wafer W against the surface (polishing surface) of the polishing pad 2. At this time, the polishing table 1 and the top ring 10 are rotated, and the polishing liquid is supplied onto the polishing pad 2 from the polishing liquid supply nozzle 3 provided above the polishing table 1. For the polishing liquid, a polishing liquid containing ceria (CeO ₂ ) or silica (SiO ₂ ) as abrasive grains is used. In this way, while supplying the polishing liquid onto the polishing pad 2, the semiconductor wafer W is pressed against the polishing pad 2 to move the semiconductor wafer W and the polishing pad 2 relative to each other, thereby forming a conductive film such as a metal film on the wafer. Grind. Examples of the metal film include a Cu film, a W film, a Ta film, and a Ti film.

  As shown in FIG. 1, the polishing apparatus includes a dressing apparatus 20 that dresses the polishing pad 2. The dressing device 20 includes a dresser arm 21, a dresser 22 rotatably attached to the tip of the dresser arm 21, a swing shaft 23 connected to the other end of the dresser arm 21, and a dresser centered on the swing shaft 23. A motor (not shown) is provided as a drive mechanism for swinging the arm 21. The lower part of the dresser 22 is constituted by a dressing member 22a. The dressing member 22a has a circular dressing surface, and hard particles are fixed to the dressing surface by electrodeposition or the like. Examples of the hard particles include diamond particles and ceramic particles. A motor (not shown) is built in the dresser arm 21, and the dresser 22 is rotated by this motor. The swing shaft 23 is connected to a lifting mechanism (not shown), and the dressing member 22a presses the polishing surface 2a of the polishing pad 2 to perform dressing when the dresser arm 21 is lowered by the lifting mechanism. The dressing apparatus 20 can dress the polishing pad 2 when the wafer is not being polished, and can also dress the polishing pad 2 while the wafer is being polished.

FIG. 2 is a plan view showing the relationship among the polishing table 1, the eddy current sensor 50, and the semiconductor wafer W. As shown in FIG. As shown in FIG. 2, the eddy current sensor 50 is installed at a position passing through the center Cw of the semiconductor wafer W being polished held by the top ring 1. Reference symbol _CT denotes the center of rotation of the polishing table 1. For example, the eddy current sensor 50 can continuously detect the thickness of the conductive film of the semiconductor wafer W on the trajectory (scanning line) while passing under the semiconductor wafer W.

Next, the eddy current sensor 50 provided in the polishing apparatus according to the present invention will be described in more detail with reference to FIGS.
FIG. 3 is a diagram illustrating the configuration of the eddy current sensor 50, FIG. 3A is a block diagram illustrating the configuration of the eddy current sensor 50, and FIG. 3B is an equivalent circuit diagram of the eddy current sensor 50. is there.
As shown in FIG. 3A, in the eddy current sensor 50, a sensor coil 60 is disposed in the vicinity of a conductive film mf such as a metal film to be detected, and an AC signal source 52 is connected to the coil. Here, the conductive film mf to be detected is, for example, a thin film such as Cu, Al, Au, W formed on the semiconductor wafer W. The sensor coil 60 is a coil for detection, and is disposed, for example, in the vicinity of about 0.5 to 5.0 mm with respect to the conductive film to be detected.

In the eddy current sensor, when an eddy current is generated in the conductive film mf, the oscillation frequency changes. The frequency type for detecting the conductive film from this frequency change and the impedance change. The conductive film is detected from this impedance change. There is an impedance type. That is, in the frequency type, in the equivalent circuit shown in FIG. 3B, when the eddy current I ₂ changes, the impedance Z changes, and the oscillation frequency of the signal source (variable frequency oscillator) 52 changes. The change of the oscillation frequency can be detected at 54, and the change of the conductive film can be detected. The impedance type, in the equivalent circuit shown in FIG. 3 (b), by eddy current I ₂ is changed, the impedance Z is changed, the impedance Z is changed as viewed from the signal source (fixed frequency oscillator) 52, a detection circuit The change of the impedance Z can be detected at 54, and the change of the conductive film can be detected.

  In the impedance type eddy current sensor, signal outputs X, Y, phase, and combined impedance Z are taken out as will be described later. Measurement information of the conductive film can be obtained from the frequency F or the impedances X and Y. As shown in FIG. 1, the eddy current sensor 50 can be built in a position near the inner surface of the polishing table 1 and is positioned so as to face a semiconductor wafer to be polished through a polishing pad. A change in the conductive film can be detected from the eddy current flowing in the conductive film on the wafer.

The impedance type eddy current sensor will be specifically described below. The AC signal source 52 is an oscillator having a fixed frequency of about 1 to 50 MHz, and for example, a crystal oscillator is used. The current I ₁ flows through the sensor coil 60 due to the AC voltage supplied from the AC signal source 52. When a current flows through the sensor coil 60 disposed in the vicinity of the conductive film mf, this magnetic flux is linked to the conductive film mf, thereby forming a mutual inductance M therebetween. An eddy current I ₂ is generated in the conductive film mf. Flowing. Where R1 is the resistance of the primary side including the sensor coil, L ₁ is self inductance of the primary side including the sensor coil as well. The conductive film mf side, R2 is the resistance corresponding to eddy current loss, L ₂ is its self-inductance. Terminal a, the impedance Z viewed sensor coil side from b of the AC signal source 52 is changed by the influence of the magnetic force lines generated by the eddy current I _2.

  FIG. 4 is a schematic diagram showing a configuration example of a sensor coil used in the eddy current sensor of the present invention. As shown in FIG. 4, the sensor coil 60 of the eddy current sensor is obtained by separating a coil for forming eddy current in the conductive film and a coil for detecting eddy current in the conductive film. It is composed of three wound coils 62, 63, 64. Here, the center coil 62 is an oscillation coil connected to the AC signal source 52. The oscillation coil 62 forms an eddy current in the conductive film mf on the semiconductor wafer W disposed in the vicinity by a magnetic field generated by the voltage supplied from the AC signal source 52. A detection coil 63 is disposed on the conductive film side of the bobbin 71 to detect a magnetic field generated by an eddy current formed in the conductive film. A balance coil 64 is arranged on the opposite side of the detection coil 63 across the oscillation coil 62.

  The coils 62, 63, and 64 are formed of coils having the same number of turns, and the detection coil 63 and the balance coil 64 are connected in opposite phases. When the conductive film exists in the vicinity of the detection coil 63, the magnetic flux generated by the eddy current formed in the conductive film is linked to the detection coil 63 and the balance coil 64. At this time, since the detection coil 63 is arranged at a position closer to the conductive film, the balance of the induced voltages generated in the coils 63 and 64 is lost, thereby detecting the interlinkage magnetic flux formed by the eddy current of the conductive film. can do.

  FIG. 5 is a schematic diagram showing a detailed configuration of the eddy current sensor. The AC signal source 52 has a fixed-frequency oscillator made of a crystal oscillator, and supplies, for example, an AC current having a fixed frequency of 1 to 50 MHz to the sensor coil 60. The alternating current formed by the alternating signal source 52 is supplied to the sensor coil 60 through the band pass filter 120. The signal output from the terminal of the sensor coil 60 is sent to the synchronous detection unit 105 including the cos synchronous detection circuit 125 and the sin synchronous detection circuit 126 via the bridge circuit 121 and the high frequency amplifier 123. Then, the synchronous detection unit 105 extracts the impedance resistance component and the inductive reactance component.

  From the resistance component and the inductive reactance component output from the synchronous detection unit 105, unnecessary high frequency components (for example, a high frequency component of 5 KHz or more) are removed by the low-pass filters 127 and 128, and the signal X as the impedance resistance component and the inductive reactance are removed. A signal Y as a component is output. The monitoring device (see FIG. 6) processes the output signals X and Y of the eddy current sensor 50 by a rotation process, a parallel movement process, etc., and calculates a distance Z (described later) as a monitoring signal. Then, the change in the film thickness is monitored based on the change in the distance Z. Note that predetermined processing such as rotation processing and parallel processing for the output signals X and Y of the eddy current sensor may be performed electrically by the eddy current sensor 50 or may be performed by calculation using a monitoring device.

  FIG. 6 is a diagram showing a main configuration of a polishing apparatus provided with an eddy current sensor 50, and FIG. 6 (a) is a diagram showing an overall configuration including a control unit of the eddy current sensor 50, and FIG. ) Is an enlarged sectional view of the eddy current sensor portion. As shown in FIG. 6A, the polishing table 1 of the polishing apparatus is rotatable about its axis as indicated by an arrow. A sensor coil 60 in the eddy current sensor 50 is embedded in the polishing table 1. The sensor coil 60 includes a preamplifier integrated sensor coil including an AC signal source and a synchronous detection circuit. The connection cable of the sensor coil 60 passes through the table shaft 1a of the polishing table 1 and is connected to the monitoring device 55 via a rotary joint 150 provided at the shaft end of the table shaft 1a. The monitoring device 55 is connected to a control device (controller) 56.

  As shown in FIG. 6B, the polishing pad is peeled off by having a coating C of fluororesin such as tetrafluoroethylene resin on the end surface of the eddy current sensor 50 embedded in the polishing table 1 on the polishing pad side. In some cases, both the polishing pad and the eddy current sensor can be prevented from coming off. Further, the end surface on the polishing pad side of the eddy current sensor is installed at a position recessed from 0 to 0.05 mm from the surface of the polishing table 1 (surface on the polishing pad side) made of a material such as SiC in the vicinity of the polishing pad 2, This prevents contact with the wafer during polishing. The difference in position between the polishing table surface and the eddy current sensor surface is preferably as small as possible, but in an actual apparatus, it is often set to about 0.02 mm. For this position adjustment, adjustment by a shim (thin plate) 151m or adjustment means by a screw is taken.

Next, a method for monitoring the film thickness of the conductive film on the semiconductor wafer being polished in the polishing apparatus having the eddy current sensor configured as shown in FIGS. 1 to 6 will be described.
FIG. 7A shows the relationship between the locus when the eddy current sensor 50 scans the surface (surface to be polished) of the semiconductor wafer W and the output of the eddy current sensor 50. As shown in FIG. 7A, the eddy current sensor 50 reacts with the conductive film mf of the semiconductor wafer W while passing through the lower side of the semiconductor wafer W as the polishing table 1 rotates, and has a predetermined signal value. Is output.
FIG. 7B is a diagram showing the relationship between the rotation of the polishing table 1 and the output of the eddy current sensor 50. In FIG. 7B, the horizontal axis is the polishing time (t), and the vertical axis is the output value of the eddy current sensor 50. As shown in FIG. 7B, when the eddy current sensor 50 is in the region (A) in the wafer, a substantially square pulse-like output is generated in response to the conductive film mf on the semiconductor wafer. When in the outer region (B), the output is low at a certain level.

Next, the principle of detecting the film thickness of the conductive film on the semiconductor wafer W based on the output of the eddy current sensor 50 shown in FIG. 7 will be described. In the circuit shown in FIG. 3 (b), the flow from the AC signal source 52 an alternating current I ₁ of the high frequency sensor coil 60, the induced magnetic field lines passing through the conductive film to the sensor coil 60. Thus, mutual inductance occurs between the sensor-side circuit and the conductive film-side circuit, an eddy current I ₂ flows through the conductive film. The eddy current I ₂ generates magnetic lines of force, which change the impedance of the sensor side circuit. The eddy current sensor detects the film thickness of the conductive film from the change in impedance of the sensor side circuit.

In the sensor side circuit and the conductive film side circuit shown in FIG.
R ₁ I ₁ + L ₁ dI ₁ / dt + MdI ₂ / dt = E (1)
R ₂ I ₂ + L ₂ dI ₂ / dt + MdI ₁ / dt = 0 (2)
Here, M is a mutual inductance, R ₁ is an equivalent resistance of the sensor side circuit including the coil 1, and L ₁ is a self inductance of the sensor side circuit including the coil 1. R ₂ is an equivalent resistance corresponding to eddy current loss, and L ₂ is a self-inductance of the conductive film through which the eddy current flows.

Here, if I _n = A _n e ^jωt (sinusoidal wave), the above equations (1) and (2) are expressed as follows.
(R ₁ + jωL ₁ ) I ₁ + jωMI ₂ = E (3)
(R ₂ + jωL ₂ ) I ₂ + jωMI ₁ = 0 (4)
From these equations (3) and (4), the following equation is derived.
I ₁ = E (R ₂ + jωL ₂ ) / {(R ₁ + jωL ₁ ) (R ₂ + jωL ₂ ) + ω ² M ² }
= E / {(R ₁ + jωL ₁ ) + ω ² M ² / (R ₂ + jωL ₂ )} (5)

Therefore, the impedance Φ of the sensor side circuit is expressed by the following equation.
Φ = E / I ₁
= {R ₁ + ω ² M ² R ₂ / (R ₂ ² + ω ² L ₂ ² )
+ Jω {L ₁ −ω ² L ₂ M ² / (R ₂ ² + ω ² L ₂ ² )} (6)
Here, when the real part (resistance component) and the imaginary part (inductive reactance component) of Φ are set as X and Y, respectively, the above equation (6) becomes as follows.
Φ = X + jωY (7)

FIG. 8 is a diagram showing a graph drawn by plotting X and Y that change with the polishing time on the XY coordinate system. The coordinate system of FIG. 8 is a coordinate system in which the Y axis is the vertical axis and the X axis is the horizontal axis. The coordinates of the point T∞ are the values of X and Y when the film thickness is infinite, that is, when R ₂ is 0, and the coordinates of the point T0 are assumed that the conductivity of the substrate can be ignored. The values of X and Y when the film thickness is 0, that is, when R ₂ is infinite. The point Tn positioned from the values of X and Y advances toward the point T0 while drawing an arc-shaped locus as the film thickness decreases. In addition, the symbol k shown in FIG. 8 is a coupling coefficient, and the following relational expression is established.
M = k (L ₁ L ₂ ) ^1/2 (8)

FIG. 9 is a diagram showing a graph obtained by rotating the graph figure of FIG. 8 by 90 degrees counterclockwise and further translating it. That is, the point represented by the coordinates X and Y is rotated counterclockwise around the origin O on the XY coordinates, and the rotated coordinates are moved so that the distance between the origin O and the coordinates X and Y is Generate a graph that shortens with decreasing film thickness. Note that the graph shown in FIG. 9 may be further subjected to processing such as amplification. Although FIG. 9 shows a case where the graph of FIG. 8 is rotated 90 ° counterclockwise, the rotation angle is not limited to 90 °. For example, the rotation angle is adjusted so that the Y coordinate with respect to the upper limit of the film thickness to be monitored is equal to the Y coordinate of the point of film thickness 0. As shown in FIG. 9, as the film thickness decreases, the point Tn positioned from the X and Y values advances toward the point T0 while drawing an arcuate locus. At this time, the distance Z (= (X ² + Y ² ) ^1/2 ) from the origin O to the point Tn in the XY coordinate system becomes smaller as the film thickness decreases except for the vicinity of the point T∞. Therefore, by sending the output signal of the eddy current sensor 50 to the monitoring device 55, the monitoring device 55 calculates the distance Z as a monitoring signal that changes according to the thickness of the conductive film. If the monitoring device 55 previously knows the relationship between the distance Z and the film thickness through experience and tests, the change in the film thickness during polishing can be understood by monitoring the distance Z.

  FIG. 10 is a diagram illustrating a case where the value of the output signal of the eddy current sensor drifts (translates). The value of the output signal of the eddy current sensor 50 may drift (translate) due to changes in the usage environment such as the ambient temperature of the eddy current sensor, water stain on the polishing pad, and changes with time of the eddy current sensor itself. . That is, as shown in FIG. 10, the value of the output signal of the eddy current sensor 50 may drift from an arcuate curve indicated by a solid line to an arcuate curve indicated by a dotted line. When the value of the output signal of the eddy current sensor drifts in this way, the distance Z from the origin O of the XY coordinate system changes. As a result, the correspondence between the value of the output signal of the eddy current sensor and the film thickness is shifted.

Therefore, in this embodiment, the output signal of the eddy current sensor 50 is calibrated by the monitoring device 55, and an accurate change in film thickness is monitored.
Next, a method for monitoring the change in the film thickness of the conductive film on the semiconductor wafer while calibrating the output signal of the eddy current sensor 50 during polishing will be described.
FIG. 11 is a diagram showing an aspect of a processing flow for monitoring the change in the film thickness of the conductive film on the semiconductor wafer while calibrating the output signal of the eddy current sensor 50. As shown in FIG. 11, in step 1, the semiconductor wafer W is held by the top ring 10, the polishing table 1 and the top ring 10 are respectively rotated, and the semiconductor wafer W is pressed against the polishing pad 2 to conduct the conductive film on the wafer. A polishing step for polishing the substrate is started. At this time, the eddy current sensor 50 passes through the region (A) in the wafer and the region (B) outside the wafer as the polishing table 1 rotates as shown in FIG. Acquires data when the eddy current sensor 50 is in the region (B) outside the wafer. In this case, after the polishing is started, the data of the area (B) outside the wafer after the polishing table 1 has been rotated one or more times is acquired, and thereafter the polishing table 1 continues to acquire data until N rotations (N is an integer). . In step 3, the drift amount (correction amount) is calculated based on the average value of the output values of the region (B) outside the wafer of the eddy current sensor 50 up to N rotation speeds.
The area outside the wafer is an area other than the area of the top ring, the area of the dresser, the area of the atomizer or the like, and is an area where nothing exists on the polishing table (polishing pad).

FIG. 12 is a diagram for explaining the step of calculating the drift amount (correction amount). As shown by the arcuate curve in FIG. 12, the value of the output signal of the eddy current sensor 50 may drift from a solid line to a dotted line. This drift amount (correction amount) is calculated by the following equation. .
ΔXa = X11−X1, ΔYa = Y11−Y1
Here, X11 and Y11 are average values of output values of the area (B) outside the wafer of the eddy current sensor 50, and X1 and Y1 are correction reference signal values. The correction reference signal value is a similar value when the film thickness of the conductive film becomes zero.
Next, in step 4, the drift amount (correction amount) calculated in step 3 is registered (saved). In step 5, the origin O of the XY coordinate system is translated by an amount corresponding to the registered drift amount (correction amount).

  FIG. 13 is a diagram for explaining a step of translating the origin O of the XY coordinate system by an amount corresponding to the drift amount (correction amount). As shown in FIG. 13, the origin O of the XY coordinate system is translated from a solid line to a dotted line. That is, the X axis and Y axis indicated by solid lines are translated by ΔXa and ΔYa to obtain X and Y axes indicated by dotted lines. Then, the distance Z from the origin O of the XY coordinate system indicated by the dotted line is calculated. Then, by monitoring the distance Z by the monitoring device 55, the change in film thickness during polishing can be known.

  FIG. 14 is a diagram showing another aspect of the processing flow for monitoring the change in the film thickness of the conductive film on the semiconductor wafer while calibrating the output signal of the eddy current sensor 50. The drift of the output value of the eddy current sensor also affects other methods other than the film thickness monitoring method based on the distance Z as described above. For example, FIG. 13 of Patent Document 1 shows an angle between a reference line passing through the reference point (center point) and a line connecting the output signal (X component, Y component) of the eddy current sensor and the reference point (center point). A method of monitoring a change in film thickness during polishing from a change in the thickness is shown. In this method, the resistance component (X component) and reactance component (Y component) of the impedance acquired under different conditions between the sensor coil end and the conductive film are displayed on the Cartesian coordinate axis. A reference point (center point) that is an intersection of preliminary measurement straight lines connecting the coordinates of the resistance component and the reactance component for each thickness of the conductive film is obtained, and the impedance coordinates and the reference point (center point) are obtained. The film thickness of the conductive film is detected from the angle formed by the actual measurement straight line connecting the coordinates. This method has an advantage that the change in the film thickness can be accurately monitored regardless of the change in the thickness of the polishing pad. However, even in this method, the angle changes due to a change with time of the output value of the eddy current sensor, and the correspondence between the value of the output signal of the eddy current sensor and the film thickness shifts.

  FIG. 15 is a diagram illustrating a case where the value of the output signal of the eddy current sensor drifts (translates). As indicated by the arcuate curve in FIG. 15, the value of the output signal of the eddy current sensor 50 may drift from a solid line to a dotted line due to changes with time of the eddy current sensor itself. When the value of the output signal of the eddy current sensor drifts in this way, the reference line passing through the reference point (center point), the output signal (X component, Y component) of the eddy current sensor and the reference point (center point) are connected. The angle with the line (Angle) will change from Angle1 to Angle2. Here, the reference point refers to the impedance resistance component (X component) and reactance component (Y component) obtained under different distances between the sensor coil end and the conductive film on the orthogonal coordinate axis. It is a reference point that is an intersection where the preliminary measurement straight lines connecting and displaying the coordinates composed of the resistance component and the reactance component for each film thickness of the conductive film are displayed.

  Therefore, in this embodiment, as shown in FIG. 14, the process from the start of the polishing process in step 1 to the registration of the drift amount (correction amount) in step 4 is performed. The process from step 1 to step 4 is the same as the process flow shown in FIG. In the processing flow shown in FIG. 14, in step 5, the reference point is shifted by an amount corresponding to the registered drift amount (correction amount).

  FIG. 16 is a diagram for explaining a step of shifting the reference point by the correction amount. As shown in FIG. 16, the reference point is shifted by the correction amount (ΔXa, ΔYa) as indicated by an arrow. Next, in step 6, the angle (Angle) of the impedance curve is calculated using the corrected reference point. That is, the angle (Angle) between the reference line passing through the corrected reference point (center point) and the line connecting the output signal (X component, Y component) of the eddy current sensor and the reference point (center point) is calculated. Thus, the film thickness of the conductive film can be detected. Thus, by detecting the drift amount of the value of the output signal of the eddy current sensor and shifting the reference point by the amount corresponding to the drift amount, the angle before and after the drift (Angle) can be kept at the same value. .

  Although the embodiments of the present invention have been described so far, it is needless to say that the present invention is not limited to the above-described embodiments and may be implemented in various forms within the scope of the technical idea.

DESCRIPTION OF SYMBOLS 1 Polishing table 1a Table axis | shaft 2 Polishing pad 2a Polishing surface 3 Polishing liquid supply nozzle 10 Top ring 11 Top ring shaft 12 Top ring head 20 Dressing device 21 Dresser arm 22 Dresser 22a Dressing member 23 Oscillating shaft 50 Eddy current sensor 52 AC signal Source 55 Monitoring device 56 Control device (controller)
60 Sensor Coil 62 Oscillation Coil 63 Detection Coil 64 Balance Coil 71 Bobbin 105 Synchronous Detection Unit 120 Band Pass Filter 121 Bridge Circuit 123 High Frequency Amplifier 125 Cos Synchronous Detection Circuit 126 Sin Synchronous Detection Circuit 127, 128 Low Pass Filter 150 Rotary Joint 151m Shim (Thin Plate) )
mf conductive film W semiconductor wafer

Claims (14)

Polishing monitoring method of pressing a substrate to be polished against a polishing surface on a rotating polishing table to polish the conductive film on the substrate, and monitoring the thickness of the conductive film by an eddy current sensor installed on the polishing table during polishing Because
When the output signal of the eddy current sensor is acquired while polishing the conductive film on the substrate with the polishing liquid containing abrasive grains, and the substrate is not present above the eddy current sensor while the conductive film on the substrate is being polished. The output adjustment amount of the eddy current sensor is calculated using the output signal, and the output signal when the conductive film on the substrate is being polished and the substrate exists above the eddy current sensor using the output adjustment amount. A polishing monitoring method characterized by correcting the thickness and monitoring the thickness of the conductive film on the substrate.   The output signal of the eddy current sensor includes the origin of the coordinate system and the origin of the coordinate system as the thickness of the conductive film decreases when the resistance component and reactance component of the impedance of the electric circuit including the coil of the eddy current sensor are defined as coordinates. The polishing monitoring method according to claim 1, wherein the polishing monitoring method is expressed as coordinates obtained by rotating and moving the coordinates to a position where a distance from the coordinates becomes short.   The polishing monitoring method according to claim 2, wherein the correction of the output signal is performed by moving an origin of the coordinate system.   4. The polishing monitoring method according to claim 3, wherein the thickness of the conductive film is monitored from the distance between the origin of the coordinate system moved by the correction and the coordinates of the impedance.   The polishing monitoring method according to claim 1, wherein the correction of the output signal uses an average value of N rotation speeds of the output signal when no substrate exists above the eddy current sensor.   2. The output signal when the substrate is not present above the eddy current sensor, only the output signal from a region where the top ring for holding the substrate does not exist above the eddy current sensor is used. The polishing monitoring method described.   The output signal when the substrate is not present above the eddy current sensor, only the output signal from the region where the dressing dresser for the polishing surface does not exist above the eddy current sensor is used. 6. The polishing monitoring method according to 6. A polishing table having a polishing surface and rotating;
Polishing liquid supply means for supplying a polishing liquid containing abrasive grains to the polishing surface;
A top ring for pressing the substrate to be polished against the polishing surface and polishing the conductive film on the substrate;
An eddy current sensor installed in the polishing table;
A monitoring device for monitoring the thickness of the conductive film based on the output signal of the eddy current sensor,
The monitoring device acquires an output signal of the eddy current sensor while the conductive film on the substrate is being polished with a polishing liquid containing abrasive grains, and is being polished above the eddy current sensor while the conductive film on the substrate is being polished. The output adjustment amount of the eddy current sensor is calculated using the output signal when the substrate does not exist, and the conductive film on the substrate is being polished using the output adjustment amount, and the substrate exists above the eddy current sensor. A polishing apparatus, wherein the thickness of a conductive film on a substrate is monitored by correcting an output signal at the time of performing.   The output signal of the eddy current sensor includes the origin of the coordinate system and the origin of the coordinate system as the thickness of the conductive film decreases when the resistance component and reactance component of the impedance of the electric circuit including the coil of the eddy current sensor are defined as coordinates. The polishing apparatus according to claim 8, wherein the polishing apparatus is expressed as coordinates obtained by rotating and moving the coordinates to a position where a distance from the coordinates becomes short.   The polishing apparatus according to claim 9, wherein the correction of the output signal is performed by moving an origin of the coordinate system.   11. The polishing apparatus according to claim 10, wherein the thickness of the conductive film is monitored from the distance between the origin of the coordinate system moved by the correction and the coordinates of the impedance.   9. The polishing apparatus according to claim 8, wherein the correction of the output signal uses an average value of N rotation speeds of the output signal when no substrate is present above the eddy current sensor.   9. The output signal when the substrate is not present above the eddy current sensor, only the output signal from a region where the top ring for holding the substrate does not exist above the eddy current sensor is used. The polishing apparatus as described.   The output signal when the substrate is not present above the eddy current sensor, only the output signal from the region where the dressing dresser for the polishing surface does not exist above the eddy current sensor is used. 14. The polishing apparatus according to 13.

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