JP2012042213A - Film thickness evaluation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the spatial distribution of film thickness of an insulation thin film inside a measurement area subjected to film thickness measurement.SOLUTION: The present invention relates to a film thickness evaluation method for measuring the distribution of film thickness of an insulation thin film by using an atomic force microscope which applies a variable DC voltage between a sample and a probe held by a cantilever. The method includes: applying the voltage to a measurement area of the sample while varying the voltage; measuring a generated current and a dielectric breakdown voltage; and determining the distribution of a relative film thickness within the measurement area on the basis of a relationship between the dielectric breakdown voltage at each measuring point and an expression (1) which is film thickness=dielectric breakdown voltage/dielectric breakdown field intensity, while defining the film thickness at any one arbitrary point in the measurement area as a criterion value.

Description

  The present invention relates to a film thickness evaluation method for obtaining a spatial distribution of the film thickness of an insulating thin film included in a magnetic recording medium, a thin film solar cell, and a semiconductor element.

  Currently, a manufacturing technique of a device such as a semiconductor element used for a magnetic recording medium, a thin film solar cell, a transistor, or the like is rapidly developing. In order to improve the performance of these devices, the thickness of each layer of the multilayer thin film constituting the device has been reduced, and thin films of several nm to several tens of nm have been formed.

  In these devices, since the film thickness of each thin film of the multilayer thin film is directly related to the device performance, accurate film thickness evaluation and film thickness management are important issues, and the insulating thin film is no exception.

  Up to now, TEM cross-section analysis, X-ray fluorescence analysis, X-ray reflectometry, X-ray photoelectron spectroscopy, infrared absorption spectroscopy, spectroscopic ellipsometry, etc. have been developed as methods for evaluating the thickness of insulating thin films. It has been. Further, Patent Document 1 discloses a method for detecting a minute current as a method for evaluating the thickness of a dielectric thin film.

  1A and 1B show measurement examples of TEM cross-sectional analysis. FIG. 1A shows an image of a TEM cross-sectional view, and FIG. 1B shows an image of an actual TEM measurement sample. The TEM cross-sectional analysis is to measure only the local portion of the measurement sample 3 in which the conductive substrate 2 and the insulating thin film 1 are laminated, and the measurement result is the sample thickness in the electron beam transmission direction of the measurement sample. The information contained in is superimposed. In addition, it takes 10 hours or more to produce one measurement sample, and a great deal of labor is required to perform the measurement.

  FIG. 2 shows a measurement example of the X-ray reflectance measurement method. This method gives the absolute value of the film thickness by analyzing the incident angle dependence of the interference of the reflected X-ray 5 with respect to the incident X-ray 4, but gives the average film thickness of the entire X-ray irradiation region. Further, when the film thickness is evaluated by this method, since the fitting is performed by simulation, it is difficult to evaluate the film thickness of the insulating thin film formed on the multilayer thin film. This is the same in the spectroscopic ellipsometry method.

  The film thickness evaluation method using X-ray fluorescence, X-ray photoelectron spectroscopy, infrared absorption spectroscopy, and the method for detecting a minute current are to be compared with the TEM cross-sectional analysis results and evaluated using a calibration curve. Therefore, it similarly has the problems included in the TEM cross-sectional analysis.

  As described above, by using the above method, it is possible to evaluate the film thickness of the insulating thin film or the dielectric thin film. A spatial distribution of film thickness in the sample area cannot be obtained.

  However, in order to maintain, improve, and stabilize the device performance described at the beginning, it is important to grasp the spatial distribution of the film thickness of the insulating thin film.

JP 2002-22639 A

  The above-mentioned prior art evaluates the film thickness of the thin film using electron beam, X-ray, light, and current. However, the prior art is limited to obtaining average information of the measurement region, and the film in the measurement region A spatial distribution of thickness cannot be obtained.

The problems to be solved by the present invention are as follows.
• Evaluate the spatial distribution of the thickness of the insulating thin film in the measurement area.
-The spatial distribution of the film thickness of the insulating thin film is evaluated without using a calibration curve based on the TEM cross-sectional analysis result.

    ・ Evaluate the spatial distribution of the thickness of the insulating thin film formed on the multilayer thin film.

In order to solve the above problems, according to the present invention,
The spatial distribution of the film thickness of the insulating thin film is measured using an atomic force microscope that applies a variable DC voltage between the sample and the probe held by the cantilever.

  Here, the voltage is applied to the measurement region of the sample while being changed, the generated current and the breakdown voltage are measured, and the dielectric breakdown at each measurement point is determined using the film thickness at any one point in the measurement region as a reference value. Based on the relationship between the voltage and the formula (1), the distribution of the relative film thickness in the measurement region is obtained.

Film thickness = breakdown voltage ÷ breakdown field strength (1)
Further, the absolute film thickness distribution in the measurement region is obtained from the absolute film thickness at one arbitrary point and the relative film thickness distribution obtained from the equation (1) and the known breakdown electric field strength. In this case, the absolute distribution of the film thickness is obtained from the reference value of the absolute film thickness obtained through experiments and the relative distribution of the film thickness.

  Alternatively, the voltage is applied to the measurement region of the sample while being applied, and the generated current and the breakdown voltage are measured. From the relationship of equation (1) and the known breakdown field strength, the absolute film thickness in the measurement region is The distribution is to be obtained.

Film thickness = breakdown voltage ÷ breakdown field strength (1)
Here, it is preferable to repeat the measurement by changing the applied voltage by a predetermined value until the entire thickness measurement region causes dielectric breakdown.

In addition, it is preferable that the insulating thin film is electrically joined to the conductive substance.
Furthermore, it is preferable that the insulating thin film electrically bonded to the conductive substance is formed on the multilayer thin film.

The atomic force microscope used in the present invention has a known structure as the atomic force microscope itself.
In the present invention, the breakdown voltage value and the film thickness are in a proportional relationship with the breakdown electric field strength as a constant. That is, there is a relationship of “dielectric breakdown field strength = dielectric breakdown voltage / film thickness”.

  The interval between the applied voltage values varies depending on the target film. That is, if the dielectric breakdown electric field strength of the target film is E, it depends on a change in the film thickness distribution in the system, that is, the film thickness unevenness (the peaks and valleys of the film thickness unevenness).

For example, when E is 1 × 10 ⁴ V / cm (in the case of a protective film of some magnetic recording media), for example, if measurement is performed every 1 mV, a difference in film thickness per 1 nm can be detected. Accordingly, if the system has this E and the film thickness unevenness is 1 nm or more, the predetermined interval may be 1 mV. However, when the unevenness is likely to be 0.5 nm or less like a medium, it is necessary to narrow the applied voltage interval accordingly.

When E is 1 × 10 ⁵ / cm (in the case of a medium protective film of some magnetic recording media), it is basically the same as described above. By measuring this material at 1 mV, 0 The difference in film thickness every 1 nm can be obtained.

When E is 1 × 10 ⁶ V / cm (silicon oxide film or the like), if the measurement is performed for every 1 mV, the difference for every 0.01 nm can be detected. Since it is not realistic, it is sufficient to measure every 10 mV or more.

  The current flowing through the insulating thin film varies depending on the sample. For example, in a protective film of a magnetic recording medium, a current of several tens of pA flows at an applied voltage of −5 mV. Such a current value depends on the IV characteristics of the target system (film material, film composition).

  According to the present invention, it is possible to relatively or absolutely measure the film thickness distribution of an insulating thin film that has been difficult to measure.

It is a figure which shows the example of a measurement of a TEM cross section analysis, (a) is a figure which shows the image of a TEM cross section, (b) is a figure which shows the image of an actual TEM measurement sample. It is a block diagram which shows the example of a measurement of a X-ray reflectivity measuring method. It is a figure which shows the apparatus structure for implementing this invention. It is the figure which expanded the sample part which shows the sample structure of this invention. It is a top view which shows typically the image of the film thickness distribution obtained by this invention.

3 and 4 show an apparatus configuration and a sample configuration for carrying out the present invention.
In FIG. 3, an atomic force microscope having a known configuration is used. It consists of a sample stage 7 attached to a piezo element 6 that performs scanning in the x, y, and z directions, a sample 3 attached to the sample stage 7, and a cantilever 8 that holds a probe that scans the sample surface.

  A variable DC voltage source 9 is connected to the sample stage 7, and an IV amplifier 10 and an ammeter 11 are connected to the cantilever 8. The variable DC voltage source 9, the sample stage 7, the sample 3, the cantilever 8, and the IV amplifier 10 A series circuit is formed by the ammeter 11.

  In addition, an “optical lever method” in which the cantilever 8 is irradiated with the laser light 12, the reflected light 13 is received by the photodetector 14, and the distortion of the cantilever 8 is detected as a deviation of the reflected light irradiation position on the photodetector 14. The surface of the sample 3 can be scanned by controlling the piezo element so that the force applied to the cantilever 8 is constant.

  In the present invention, by applying a voltage between the cantilever 8 and the sample 3, the current flowing through the sample 3 is detected by the ammeter 11, and at the same time, the piezo element 6 is controlled by the control system 15. By scanning the surface of the sample 3, the spatial distribution of the current flowing through the sample 3 is taken, dielectric breakdown is detected from the current value, and the relative distribution of film thickness is evaluated using this dielectric breakdown phenomenon.

  In addition, since the atomic force microscope outputs unevenness on the surface of the sample by surface scanning, the film thickness cannot be evaluated even if the unevenness can be evaluated only by scanning. Therefore, the method of the present invention is applied.

  FIG. 4 shows an example of the sample shape. The sample 3 includes a conductive substrate 2 and an insulating thin film 1 formed on the conductive substrate 2. However, an insulating thin film formed on a multilayer thin film is also an object to be measured, and the substrate is not limited to a conductive substrate, and even an insulator such as a glass substrate is electrically connected to a conductive substance. Insulating thin films that can be applied with voltage are all subject of the present invention.

FIG. 5 is a plan view schematically showing the film thickness distribution obtained by the evaluation method of the present invention.
This result can be obtained as follows.
First, in a state where the sample 3 and the cantilever 8 are equipotential, the region to be measured is scanned, and after the scanning is completed, a predetermined voltage between the cantilever 8 and the sample 3 is applied to scan the measurement region and the current is Get a statue. This measurement is repeated until the entire measurement region causes dielectric breakdown. Note that the interval between the applied voltage values when performing repeated measurement is determined as described in the above section.

  After the measurement is completed, an arbitrary measurement point in the measurement area is selected, and that measurement point is set as a reference point. The film thickness at the reference point is set to 1, and the breakdown voltage value at the reference point is compared with the breakdown voltage values at other measurement points. If the film quality of the insulating thin film is uniform within the measurement area, the breakdown voltage value and the film thickness are proportional to each other via the breakdown electric field strength. By normalizing with the dielectric breakdown voltage value, it is possible to obtain a relative film thickness distribution with the reference point film thickness being 1.

  Here, if the dielectric breakdown electric field strength and dielectric breakdown voltage value of the insulating thin film to be measured are known, the absolute value of the reference point is calculated using the proportional relationship between the dielectric breakdown voltage value and the film thickness via the dielectric breakdown electric field strength. The film thickness is evaluated (however, since the applied voltage is obtained by experiment, if only the breakdown electric field strength is known, the absolute film thickness of the reference point can be calculated). By combining the measurement result of the relative film thickness distribution with this result, the absolute film thickness distribution can be known.

In FIG. 5, the size of the outer frame in the figure is 500 nm × 500 nm. The thickness of the thick film portion indicated by 16 and the thickness of the thin film portion indicated by 17 are both about several tens of nm in diameter.
Normally, the tip diameter of the probe used in the atomic force microscope is about 20 nm, and a voltage is applied between the probe and the conductive substrate, so that scanning is performed with an interval equal to the probe diameter. For example, the film thickness can be evaluated without being affected by dielectric breakdown at adjacent measurement points.

  As described above, the present invention can be applied in a wide range such as a protective film for magnetic recording media, a silicon oxide film for thin film solar cells, and a gate oxide film for silicon devices and silicon carbide devices.

DESCRIPTION OF SYMBOLS 1 Insulating thin film 2 Conductive substrate 3 Sample 4 Incident X-ray 5 Reflected X-ray 6 Piezo element 7 Sample stand 8 Cantilever 9 Variable DC voltage source 10 IV amplifier 11 Ammeter 12 Laser light 13 Reflected light 14 Photo detector 15 Control system 16 Thick film part 17 Thick film part

Claims (7)

  A film thickness evaluation method for measuring a film thickness distribution of an insulating thin film using an atomic force microscope that applies a variable DC voltage between a sample and a probe held by a cantilever. While changing and applying a voltage to the measurement region of the sample, the generated current and the breakdown voltage are measured, and the dielectric breakdown voltage at each measurement point and ( 2. The film thickness evaluation method according to claim 1, wherein a distribution of a relative film thickness in the measurement region is obtained based on the relationship of the expression (1).
Film thickness = breakdown voltage ÷ breakdown field strength (1)   The absolute film thickness distribution in the measurement region is obtained from the absolute film thickness and the relative film thickness distribution at any one point obtained from the equation (1) and the known breakdown electric field strength. 2. The film thickness evaluation method according to 2. While changing the voltage to the measurement area of the sample and applying it, the generated current and the breakdown voltage are measured, and the distribution of the absolute film thickness in the measurement area is calculated from the relationship of equation (1) and the known breakdown electric field strength. The film thickness evaluation method according to claim 1, wherein the film thickness evaluation method is obtained.
Film thickness = breakdown voltage ÷ breakdown field strength (1)   5. The film thickness evaluation according to claim 2, wherein the measurement is repeated by changing the applied voltage by a predetermined value until the entire measurement region causes dielectric breakdown. 6. Method.   The film thickness evaluation method according to any one of claims 1 to 5, wherein the insulating thin film is electrically joined to the conductive substance.   The film thickness evaluation according to any one of claims 1 to 6, wherein the insulating thin film electrically bonded to the conductive substance is formed on a multilayer thin film. Method.