JP4888765B2 - Film thickness meter and film thickness measurement method - Google Patents

Description

  The present invention relates to an eddy current film thickness meter that measures the thickness of a conductive film such as a metal thin film in a non-contact manner, and more particularly to a film thickness meter that performs phase detection and a method for measuring the film thickness.

  An eddy current film thickness meter is used as a conductive film inspection apparatus for nondestructively inspecting a film formation state of a conductive thin film formed on a semiconductor substrate or a glass substrate for a liquid crystal display.

  In an eddy current film thickness meter, when a conductor is placed in an alternating magnetic field, eddy current flows in the direction to cancel the magnetic field in the conductor, and the size and distribution of this eddy current depends on the shape of the conductor, conductivity, internal defects, etc. It is a measuring device based on the principle of measuring the electrical resistance value of the film by utilizing the change due to the above. This is because the magnetic field generated by the eddy current changes the impedance of the detection coil due to the mutual inductive action. By detecting this change in impedance as a change in voltage value or phase, the conductor that is the object to be inspected is detected. It is a method of knowing the state of (see Patent Document 1).

  Here, the film thickness measuring device will be described with reference to FIGS. FIG. 10 shows a schematic overall configuration of the film thickness measuring device 91. As shown in FIG. 10, the film thickness measuring device 91 is, for example, a measuring unit disposed above a substrate such as a silicon wafer supported on a substrate stage 93 a driven by a driving system (moving mechanism) 93. 92.

  The drive system 93 is configured to operate in response to a command from the computer 94. By moving the substrate stage 93a in the vertical and horizontal directions, the relative positions of the measurement unit 92 and the substrate 50 are changed. It is supposed to change.

  The measurement unit 92 is provided with a support portion 92a made of, for example, an insulating material such as plastic. The support portion 92a includes an eddy current coil sensor (hereinafter referred to as “eddy current sensor”) 20 and a laser displacement sensor (hereinafter referred to as “displacement sensor”). A "laser sensor") 30 is attached.

  Here, the eddy current sensor 20 is disposed in the vicinity of the substrate 50 and is close to a conductive film (measurement target film) 51 formed on the substrate 50.

  The eddy current sensor 20 is configured by embedding a detection coil 3 and a reference coil 4 described later in a main body 2 made of an insulating material (FIG. 12). Further, the detection coil 3 and the reference coil 4 are connected to an inductance meter 95.

  The laser sensor 30 is attached to a predetermined position above the eddy current sensor.

  The laser sensor 30 is controlled by a laser sensor controller 96. By irradiating a predetermined position on the conductive film 51 on the substrate 50, the distance to the surface of the conductive film 51 can be set with high accuracy. Measure.

  Further, the inductance meter 95 and the laser sensor controller 96 are connected to a computer 94, and the computer 94 performs data analysis.

  FIG. 11 is a circuit diagram showing the configuration of the eddy current sensor 20, and FIG. 12 is a diagram for explaining the relative positional relationship between the detection coil 3 and the reference coil 4 of the eddy current sensor 20. The detection coil 3 is close to the conductive film 51 on the substrate 50.

  As shown in FIG. 11, the eddy current sensor 20 has a bridge circuit 10 called a Maxwell bridge. The state where the detection coil 3 and the reference coil 4 of the eddy current sensor 20 are connected in series is shown. A thick arrow indicates that only the detection coil 3 influences the substrate 50.

  In the eddy current sensor 20, the voltage generated by the mutual influence of the substrate 50 is included at both ends of the detection coil 3 in the voltage at both ends of the coil generated by the AC voltage applied to the bridge circuit 10. The difference in voltage between the terminals of the reference coil 4 is obtained by the bridge circuit 10, and the film thickness is obtained by comparing with the data stored in advance.

  A specific method for obtaining the film thickness will be described in detail with reference to FIGS.

  A configuration example of the eddy current film thickness meter 20 is shown in FIG. The detection coil (pickup coil) 3 is disposed in the vicinity of the thin film conductive film 51 to be measured in the vicinity of about 0.3 mm, and the reference coil 4 is disposed in the place about several mm from the detection coil 3. An AC bridge type circuit as shown in FIG. 2 is formed, a voltage generated when an AC voltage is applied is measured, and the film thickness can be known from the relationship between the voltage and the film thickness obtained in advance.

FIG. 3 shows a calculation model. R1, L1 is detected and the reference coil resistance, self-inductance, a change in impedance of the detection coil 3 by the induced eddy currents in the conductive film 51 R _e, L _e farther To becomes as follows.

Amplitude │V _x │ of V _x now becomes a following.

Since | V _x | usually depends on the sheet resistance of the conductive film 51, the film thickness of the conductive film 51 can be obtained by measuring | V _x |.
The change in impedance of the detection coil 3 due to the eddy current in the conductive film 51 can be easily understood when the thin film of the conductive film 51 is considered as one loop as shown in FIG. When the mutual inductance is M _CL , the resistance of the virtual loop, and the self-inductance are R _L and L _L , the impedance Z _C of the detection coil 3 is as follows.

When R _L changes in proportion to the sheet resistance of the conductive film 51, R _e and L _e change, and the output voltage V _x and its amplitude | V _x | in equation (1) change. Therefore, the relationship between the sheet resistance and V _x or │V _x │ exists, by using the "relation between the film thickness and the sheet resistance" in accordance with the material of the conductive film 51, the measurement results of V _x or │V _x │ Thus, the film thickness of the conductive film 51 is obtained.

3 shows R _e, L _e, a calculation example of │V _x │. The detection coil 3 is a solenoid having a length of 1.71 mm, an outer diameter of 2.4 mm, an inner diameter of 0.4 mm, 10 windings per layer, 6 layers, a total of 60 windings, a distance h from the lower end of the coil to the conductive film 51 = In this example, the coil self-inductance L _C = 1.75 x 10 ^-6 H, the coil resistance R _C = 0.66Ω, the radius of the virtual loop is 1.5 mm, and the width is 0.5 mm. The self-inductance L _L and the mutual inductance M _CL of the coil are shown in the figure. (2), (3) and | V _x | are plotted against the reciprocal of the resistance R _L of the loop.

The results at excitation frequencies of 1 MHz and 10 MHz are shown. In the calculation of │V _x │, the voltage amplitude E ₀ applied to the bridge was set to 1V. R _L is proportional to the sheet resistance. Now, in the region sheet resistance is large, contribution to │V _x │ is a R _e is dominant, │V _x │ is proportional to the "inverse of sheet resistance".

  In addition, there is a sensor that measures the film thickness using phase detection (see Patent Document 2). However, in consideration of application to an apparatus for detecting the end of polishing when the film is removed by the polishing apparatus, the film thickness is that when the sheet resistance is very high in the angstrom order.

Furthermore, a sensor using planar coils for the detection coil 3 and the reference coil 4 has been developed (see Patent Document 3). Spatial resolution is improved without degrading the sensitivity of the eddy current film thickness meter with the detection coil 3 and the reference coil 4 wound in opposite phases. Further, a method for obtaining a change in impedance by regarding the thin film as five loops having different radii has been disclosed.
However, the film thickness is obtained from the amount of change in the inductance component.

JP-A-5-149927 (page 3, FIG. 1) Japanese Patent Laying-Open No. 2005-121616 (page 12, FIG. 9) JP-A-2005-227256 (page 6, FIG. 10)

In the above ideal calculation, when the sheet resistance increases, | V _x | should decrease in proportion to the reciprocal of the sheet resistance. However, when actually measured, it hardly changes at a certain sheet resistance value or more. Therefore, the sheet resistance and film thickness cannot be calculated from the measured value of | V _x |.

  In consideration of the use of the characteristics of the formed film, it is desirable that the film thickness is more suitable for measuring the film thickness of a frequently used micrometer order film.

  The above-mentioned problem is that “a circuit in which a reference coil and a detection coil are connected in series and an inductance bridge in which two reference resistors are connected in series, and a predetermined area near the conductive film formed on the surface of the object to be measured are used. The detection coil can be disposed closer to the conductive film than the reference coil, and a predetermined eddy current is generated in the conductive film and a magnetic field generated by the eddy current is generated. An eddy current coil sensor for detecting a phase of a reference signal with respect to an applied voltage applied to the series circuit for generating the predetermined eddy current when detecting the magnetic field from the conductive film. The output of the inductance bridge so that the sheet resistance value of the conductive film is a high resistance value corresponding to the film thickness of the micrometer order. It is solved by an eddy current film thickness meter, "which is characterized in that so as to measure only the anti component.

  In addition, the above problem is that "a circuit in which a reference coil and a detection coil are connected in series and an inductance bridge in which two reference resistors are connected in series, and the vicinity of a conductive film formed on the surface of an object to be measured. The detection coil can be disposed at a position closer to the conductive film than the reference coil, and a predetermined eddy current is generated in the conductive film and the eddy current is generated. An eddy current coil sensor for detecting a magnetic field by a reference signal with respect to an applied voltage applied to the series circuit for generating the predetermined eddy current when detecting the magnetic field from the conductive film. Means for phase matching, and the inductance bridge so that the sheet resistance value of the conductive film is a high resistance value corresponding to the film thickness of the micrometer order. It is solved by the measurement method "in thickness, characterized in that so as to measure only the resistance component of the output.

  Specifically, the change Re of the resistance component of the detection coil 3 is measured. Since Re is proportional to the “reciprocal of sheet resistance” of the conductive film 51, that is, changes with respect to the sheet resistance, the film thickness of the conductive film 51 can be obtained by measuring Re.

The relationship between Re and sheet resistance was confirmed by experiments. When the sheet resistance increases, becomes constant value remains large variation Le in self-inductance component of the detection coil 3, experimentally confirmed that │V _x │ becomes constant in order to make the main part of │V _x │ It was.

Specifically, in the measurement of the output signal of the AC bridge 10, only the resistance component change Re is measured by setting the phase difference between the reference signal and the excitation signal at the time of phase detection to the following equation (4). Here, L ₁ and R ₁ are the self-inductance and resistance of the detection coil 3.

φ = tan ^-1 (ω L ₁ / R ₁ ) (4)

  By measuring, as an output signal, “the change Re of the resistance component of the detection coil” proportional to the “reciprocal of the sheet resistance” of the conductive film 51, the conventional “the signal becomes a constant value as the sheet resistance increases”. Therefore, even in the conductive film 51 having a large sheet resistance, the film thickness can be obtained from the signal measurement result.

Since it has been found that the above-mentioned “change amount Re of the resistance component of the detection coil Re” can be obtained by using the phase difference between the reference signal and the excitation signal at the time of phase detection as Equation (4), known L ₁ , R ₁ The phase difference to be set can be calculated from the above, and a desired signal can be obtained without complicated phase adjustment. Here, L ₁ and R ₁ are the self-inductance and resistance of the detection coil.

  Hereinafter, embodiments to which the present invention is applied will be described in detail with reference to the drawings.

  The eddy current sensor used in the embodiment of the present invention has the same configuration as that shown in FIGS.

  In the measurement of the output signal of the AC bridge 10 according to the embodiment of the present invention, only the change Re of the resistance component is measured by using the phase difference between the reference signal and the excitation signal at the time of phase detection as Equation (4).

In order to specifically show the above effects, the calculation model shown in FIG. 4 was used for estimation.
The conductive film 51 is calculated assuming that it is a loop of five concentric circles (radius 1.5, 3, 4.5, 6, 7.5 mm).

  A comparison between the calculation results of Re, Le, │Vx│ and the measurement results is shown below. The coil is a planar coil that prioritizes sensitivity over spatial resolution and has a pitch of 0.23 mm, 19 turns, and an outer diameter of 9 mm. Re and Le in FIG. 7 are calculated values, and the change in coil impedance was calculated by regarding the conductive film 51 as five concentric loops (radius 1.5, 3, 4.5, 6, and 7.5 mm).

The distance between the conductive film 51 and the coil was 0.3 mm, the self-inductance of the coil was 1.4e-6H, and the resistance was 2Ω. In FIG. 6, I ₀ R _e / 2, I ₀ · L _e / 2, and | V _x | {Equation (5)} corresponding to the contribution to the bridge output voltage are plotted.

│V _x │ = [(I ₀ R _e / 2) ² + (I ₀ ωL _e / 2) ² ] ^0.5 (5)

The voltage amplitude applied to the bridge is 1V and the frequency is 10MHz. As the sheet resistance Rs increases, the contribution of Re is proportional to 1 / Rs, and the contribution of Le is proportional to (1 / Rs) ² . Therefore, as can be seen from FIG. 6, it is predicted from the calculation that the contribution of Re is dominant and | V _x | is also proportional to 1 / Rs in the region where Rs is large.

However, when we actually measured Re and Le in the area of the micrometer order often used, the result shown in FIG. 7 was obtained (plotted by multiplying the bridge by I ₀ when 1 V was applied). . Cu, Ta, and TiN were used for the conductive film 51. The straight line in FIG. 7 is the calculation result shown in FIG. │V _x │ is calculated by using the measured values of Re and Le.

As can be seen from FIG. 7, the Re component matches the calculation result well, while the Le component is much larger than the calculated value and is constant with respect to the sheet resistance Rs. Then, Rs ingredients of Le is dominant in │V _x │ is 50Ω or more, does not change with respect to │V _x │ also Rs.

According to the formula (5), only the contribution of Re changes | V _x |, but it reads “small change in large value”, which is not good as a measuring instrument from the viewpoint of measurement accuracy.

From the above, the reason for the above behavior of Le is unknown at present, but the measurement accuracy of film thickness is low when measuring conventional │V _x │. Since it changes sensitively to the sheet resistance Rs, it can be said that it becomes an eddy current film thickness meter capable of highly accurate film thickness measurement.

Next, a method for selectively measuring Re will be described. From equation (1), are orthogonal is Re component and Le component, the phase setting of the reference signal at the phase detection in the measurement of the alternating voltage signal V _x of the bridge output, the R _e, L _e can be measured independently I understand.

  FIG. 6 is a calculation example of the following equation (6) at the above-described solenoid, 1 MHz.

V _x = I ₀ [R _e cos (ωt−φ) + ωL _e sin (ωt−φ)] / 2 (6)

The applied voltage amplitude was set to 1V, and plotted in FIG. Applied voltage, Re, components of Le, shows the phase relationship of V _x. The phase differences φ and α are as shown below (1). The Re component is delayed in phase by φ with respect to the applied voltage. If the phase of the reference signal is also delayed by φ, only the Re component is detected.

  When the sheet resistance of the conductive film 51 is large, Re and Le are sufficiently smaller than R1 and L1, so φ in Table 1 can be approximated by equation (4), and φ is set from the values of R1 and L1 of known detection coils. it can.

  Therefore, as shown in FIG. 9, in the measurement of the AC voltage signal at the bridge output, if the phase of the reference signal at the time of phase detection is delayed by this φ from the excitation signal, it changes in proportion to the “reciprocal of sheet resistance”. Thus, only the “Re component” is detected, and the film thickness of the conductive film 51 having a large sheet resistance can be measured with high accuracy.

  The reference signal is generated in synchronization with the output of the AC voltage source 26 by an oscillation circuit (not shown) in the inductance meter 95 and can be phase-matched with the Re component by a phase shift circuit (not shown). It has become.

  As the phase shift circuit, a time constant circuit using a combination of a shift register using a digital circuit, a delay line of an analog circuit, a resistor, a capacitor, and the like can be used.

  In addition, the present invention is particularly suitable for film thickness measurement in the area of the micrometer order which is generally used well. That is, as shown in FIG. 7, it is desirable that good measurement is possible in the range of Rs of 20 to 110Ω, and then in the range of 1Ω to 1KΩ. Le is a range in which Rs is constant, and if only Re is measured, the film thickness can be measured with high accuracy.

  As mentioned above, although embodiment of invention was described, of course, this invention is not limited to these, Based on the technical idea of this invention, a various deformation | transformation is possible.

2 is a perspective view of an eddy current sensor 20. FIG. It is a figure which shows arrangement | positioning of the component of the eddy current type sensor 20 used as the basis of a calculation model. The schematic positions of the conductive film 51, the detection coil 3, and the reference coil 4 that are measurement targets are shown. It is a circuit diagram of an eddy current type sensor. L ₁ and R ₁ correspond to the reference coil 4. L ₂ and R ₂ correspond to the detection coil 3. 3 is a calculation model of the eddy current sensor 1. The relationship between the detection coil 3 and the conductive film 51 was modeled as a calculation model. A loop corresponds to the conductive film 51. The mutual inductance M _CL affects the detection coil 3 and the conductive film 51. This is a calculation model in which the conductive film 51 is regarded as five loops. It is a graph which shows the relationship between │V _x │ and Re, Le. It is a graph of the calculation result of Re and Le. It is a graph which shows the measured value of Re and Le. It can be seen that the measured value Le component has not changed. On the other hand, the Re component is changing. An example of the phase of the bridge output component is shown. It can be seen that the Re component is delayed by φ from the applied voltage. It is a phase of embodiment of this invention. The reference signal is delayed by φ so that only the Re component can be extracted. 1 is a schematic overall configuration diagram of an eddy current film thickness measuring device 91. FIG. It has a measuring unit 92 disposed above a substrate 50 such as a silicon wafer supported on a substrate stage 93a driven by a drive system (moving mechanism) 93. The drive system 93 is configured to operate in accordance with a command from the computer 94, and changes the relative position between the measurement unit 92 and the substrate 50 by moving the substrate stage 93a in the vertical and horizontal directions. It is like that. 1 is a circuit diagram of an eddy current film thickness meter (eddy current sensor) 20. FIG. A difference between outputs of the reference coil 4 and the detection coil 3 is obtained by the bridge 10 and received by the measurement circuit 27. FIG. 4 is a diagram for explaining a relative positional relationship between a detection coil 3 and a reference coil 4 of the eddy current sensor 20. The detection coil 3 is at the lower end of the main body 2 near the conductive film 51 of the substrate 50. On the other hand, the reference coil is separated from the conductive film 51 of the substrate 50.

Explanation of symbols

2 ... main body, 3 ... detection coil, 4 ... reference coil, 5 ... heat shield,
10 ... Inductance bridge, 14 ... Reference resistance, 15 ... Reference resistance,
20 ... Eddy current sensor (eddy current type film thickness meter), 21 ... Parallel connection point, 22 ... Parallel connection point, 23 ... Connection midpoint, 24 ... Connection midpoint,
30 ... Laser displacement sensor,
50 ... substrate, 51 ... conductive film,
DESCRIPTION OF SYMBOLS 91 ... Film thickness measuring apparatus, 92 ... Measuring part, 92a ... Supporting part, 93 ... Drive system (moving mechanism), 93a ... Substrate stage, 94 ... Computer 95 ... Inductance meter, 96 ... Laser sensor controller,

Claims (6)

Using a circuit in which a reference coil and a detection coil are connected in series and an inductance bridge in which two reference resistors are connected in series, it can be placed at a predetermined position near the conductive film formed on the surface of the measurement object An eddy current that can be arranged at a position closer to the conductive film than the reference coil and that generates a predetermined eddy current to the conductive film and detects a magnetic field due to the eddy current A film thickness meter ,
And means for phase alignment of the reference signal to the applied voltage applied to the series circuit for generating the predetermined eddy currents when detecting the magnetic field from the conductive film,
The means for performing phase adjustment is an arithmetic means for obtaining a phase difference between the applied voltage and the reference signal by approximating the self-inductance value and resistance value of the detection coil, and advancing the reference signal by the phase difference or And a phase adjusting means for delaying,
An eddy current film thickness meter characterized in that only the resistance component of the output of the inductance bridge is measured.   2. The eddy current film thickness meter according to claim 1, wherein the sheet resistance value of the conductive film is 1Ω to 1 KΩ.   The eddy current film thickness meter according to claim 2, wherein the sheet resistance value of the conductive film is 20 to 110Ω. Using a circuit in which a reference coil and a detection coil are connected in series and an inductance bridge in which two reference resistors are connected in series, it can be placed at a predetermined position near the conductive film formed on the surface of the measurement object The detection coil can be arranged at a position closer to the conductive film than the reference coil, and a film thickness that generates a predetermined eddy current to the conductive film and detects a magnetic field due to the eddy current. Measuring method ,
And means for phase alignment of the reference signal to the applied voltage applied to the series circuit for generating the predetermined eddy currents when detecting the magnetic field from the conductive film,
In order to perform the phase alignment, the phase difference between the applied voltage and the reference signal is approximated from the self-inductance value and the resistance value of the detection coil,
Only the resistance component of the output of the inductance bridge is measured. The film thickness measuring method according to claim 4 , wherein the sheet resistance value of the conductive film is 1Ω to 1 KΩ. 6. The film thickness measuring method according to claim 5 , wherein the sheet resistance value of the conductive film is 20 to 110 [Omega].

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