JP5066258B2 - Film thickness evaluation apparatus and film thickness evaluation method - Google Patents

Description

  The present invention relates to a film thickness evaluation apparatus and a film thickness evaluation method for evaluating a film thickness of a dielectric by detecting an electrostatic force between a sample and a probe.

  As a conventional film thickness evaluation apparatus, an optical method mainly using interference of light beams is most common. In the light beam interference method, light incident on the sample surface is reflected by the film surface and the film-underlying interface, and interference fringes are observed due to the relationship between the optical path difference between them and the wavelength. Since interference fringes appear at every film thickness that is an integral multiple of the value determined by the wavelength of light and the refractive index of the film, the relative distribution of the film thickness can be obtained while using the wavelength of light as the reference length. In contrast to the interference of a plurality of light beams having a wavefront divided by the light beam interference method, there is an ellipsometry (elliptical ellipsometry) as a vibration surface division type interference photometry method. This method is a method for evaluating optical characteristics of a sample by analyzing elliptically polarized light reflected from the sample. If the sample is a thin film, its optical characteristics are determined by the refractive index and film thickness of the film and the base, and if the characteristics are known, the film thickness can be measured conversely.

  Next, as a method for evaluating a film thickness of about 1 nm or less, which is difficult with ellipsometry, there is a method using photoelectron spectroscopy. Since the photoelectrons emitted from the substrate when the sample surface is irradiated with ultraviolet light are attenuated in the surface film, the number of electrons counted decreases as the film thickness increases. By obtaining the relationship between the film thickness and the number of emitted electrons in advance, the film thickness can be evaluated from the number of electrons. This technique is used when the work function of the base is lower than the work function of the film, and can be applied to a film thickness sample of about 1 nm or less to about several tens of nm.

  Furthermore, the film thickness can also be measured by measuring the height from the substrate surface to the film surface using a stylus method that scans the sample surface with a thin probe and measures the surface roughness from the vertical movement of the probe. Can be evaluated. Since this is a direct method performed only by mechanical operation, there is a feature that measurement can be performed quickly and easily. As a technique using a probe, an example using an atomic force microscope has been reported. The film thickness is evaluated by measuring the tunnel current flowing between the underlying substrate and the probe through the film using a conductive probe (e.g., Alexander Olbrich et al., Applied Physics Letter 78). Volume 2934, 2001). Since the tunnel current decreases exponentially as the distance increases, the film thickness can be determined from the tunnel current by obtaining the relationship between the film thickness and the current in advance. By scanning the probe in a two-dimensional direction with respect to the sample, a film thickness distribution with an in-plane resolution of about 1 nm and a film thickness resolution of about 0.1 nm can be obtained.

  In a film thickness evaluation apparatus using a conventional optical method, it is possible to measure the in-plane film thickness distribution by scanning with the diameter of the incident light narrowed down, but the in-plane resolution is the light used. The limit is about 1 μm. Similar problems arise in photoelectron spectroscopy because of the use of excitation light. In the stylus method, it is necessary to peel off the film on the sample surface, which is not suitable for obtaining a film thickness distribution. In the method using an atomic force microscope, the attenuation of the tunnel current is exponential with respect to the distance, so that when the film thickness is about several nanometers or more, the current becomes very small and measurement is difficult. Furthermore, since the tunnel current is greatly affected by the state of the tip of the probe, there is a problem that the current value is likely to become unstable due to wear of the tip of the probe, attached matter, or the like. Thus, it is difficult to measure the film thickness distribution over a wide film thickness range with high spatial resolution and high accuracy.

  The present invention focuses on the fact that the electrostatic force acting between a conductive probe and a conductive base substrate in an atomic force microscope depends on the insulator film thickness. When an atomic force microscope is operated in an intermittent contact method or a non-contact method and an AC voltage is applied between the probe and the sample with the probe approaching the sample surface, the probe vibrates with electrostatic force. When lock-in detection of the displacement vibration of the probe is performed at a double frequency, a second derivative signal relating to the voltage of the electrostatic force is obtained. Since the electrostatic force is proportional to the square of the applied voltage, the obtained second derivative is a constant. The second derivative is proportional to the reciprocal of the square of the probe-substrate distance, and by calculating the reciprocal of the square root of this coefficient, the probe-substrate distance, ie, the film thickness and the probe-surface distance. A signal proportional to the sum of the distances is obtained. Furthermore, the absolute value of the film thickness is obtained by using the signal obtained by changing the probe-surface distance, the absolute value of the probe-surface distance, and the dielectric constant of the sample. By performing this operation for each measurement point while scanning the probe, the film thickness distribution of the sample can be observed.

  According to the present invention, it is possible to measure a film thickness distribution of an insulator film having a film thickness of about sub-nanometers to several μm with a film thickness accuracy of about 0.01 nm and a spatial resolution of nanometer level.

It is a block diagram which shows embodiment of the film thickness evaluation apparatus which concerns on this invention. It is a figure which shows a film thickness distribution image. It is a figure which shows the film thickness profile of the A-A 'part of FIG. 2A.

  Embodiments of the invention relating to the film thickness evaluation apparatus will be described below.

  First, the configuration of the film thickness evaluation apparatus will be described. FIG. 1 is a configuration diagram showing an embodiment of a film thickness evaluation apparatus of the present invention. The measurement sample 1 includes a conductive base substrate 2 and an insulator layer 3 formed on the surface thereof. A cantilever 4 is disposed opposite to the surface of the measurement sample 1, and a probe 5 is provided at the tip thereof. The cantilever 4 and the probe 5 are vibrated in the direction perpendicular to the surface of the measurement sample 1 by the oscillation unit 6 at the natural frequency or a frequency in the vicinity thereof. The measurement sample 1 is fixed on the XYZ scanning mechanism 7 and the coarse movement mechanism 8, and can be moved in the three-dimensional azimuth direction with respect to the probe 5 by the XYZ scanning mechanism 7, and measured by the coarse movement mechanism 8. The distance between the sample 1 and the probe 5 can be changed greatly.

  At the time of measurement, first, the control unit 9 drives the coarse movement mechanism 8 using the coarse movement unit 10 to bring the surface of the measurement sample 1 closer to the probe 5. When the measurement sample 1 and the probe 5 are sufficiently close, the vibration state of the cantilever 4 changes due to the interaction with the surface of the measurement sample 1. The displacement of the cantilever 4 is detected using the displacement detector 11, and the vibration amplitude or frequency of the cantilever 4 is detected by the amplitude / frequency detector 12. The feedback control unit 13 drives the XYZ scanning mechanism 7 in the Z direction by the Z drive unit 14 so that the vibration amplitude or frequency of the cantilever 4 becomes a constant value set by the control unit 9, and the probe 5 and the measurement sample 1 Keep the distance to the surface constant. In this state, the control unit 9 scans the XYZ scanning mechanism 7 in the XY plane using the XY scanning unit 15 to map the height information from the surface of the measurement sample 1 of the probe 5 to the surface of the measurement sample 1. The shape can be observed.

An alternating voltage is applied to the probe 5 by the alternating voltage application unit 16 and the base substrate 2 of the measurement sample 1 is grounded, so an electrostatic force acts between the probe 5 and the measurement sample 1, and the cantilever 4 It vibrates in synchronism with the AC voltage applied to the probe 5. The voltage between measurement sample 1 and probe 5 is V, the distance between the surface of measurement sample 1 and the tip of probe 5 is g, the thickness of insulator layer 3 immediately below probe 5 is t, and the insulator layer When the relative dielectric constant of 3 is e and a is a constant, the electrostatic force acting between the measurement sample 1 and the probe 5 is
F = a (V / (t / e + g)) ² ... (1)
It becomes. By using the vibration signal of the cantilever 4 detected by the displacement detection unit 11 and the reference signal from the AC voltage application unit 16, the lock-in amplifier (A) 17 detects the double frequency component of the reference signal, thereby generating electrostatic force. Differential signal with respect to voltage V of
d ² F / dV ² = 2a / (t / e + g) ² (2)
Is obtained. A new signal is obtained by taking the square root of the reciprocal number of this signal using the reciprocal square root conversion unit 18.
S = b (t / e + g) (3)
Is obtained. Here, b is a constant. Further, the signal S is also changed by vibrating the XYZ scanning mechanism 7 in the Z direction by using the sample vibrating section 19 to vibrate the distance g between the surface of the measurement sample 1 and the tip of the probe 5. By using this signal and the reference signal from the sample vibrating section 19, the lock-in amplifier (B) 20 detects the same frequency component as the reference signal, whereby a differential signal related to the distance g of the signal S,
D = dS / dg = b (4)
That is, the proportionality constant b is obtained. In the film thickness calculator 21, the output signal S of the reciprocal square root converter 18, the output signal D of the lock-in amplifier (B) 20, and the known values g and e are used.
t = e (S / Dg) (5)
Is calculated to obtain the film thickness t. By moving the probe 5 relative to the measurement sample 1 and mapping the value of the film thickness obtained at each position on the surface of the measurement sample 1, the film thickness distribution can be obtained simultaneously with the surface shape.

  The invention is explained in detail using the following examples. The film thickness distribution of the thermal oxide film formed on the silicon substrate surface was measured using the film thickness evaluation apparatus of the present invention. A cantilever made of silicon having a length of 100 μm, a width of 35 μm, and a force constant of 11.5 N / m was vibrated at 255 kHz, which is the natural frequency of the cantilever, by a piezo element provided in the cantilever holder.

  The vibration amplitude was 0.5 nm. A tip having a length of 10 μm was provided at the tip of the cantilever, and the surface thereof was coated with a platinum film having a thickness of 20 nm to impart conductivity, and grounded to a ground potential. Here, a platinum film is used as the material of the cantilever, but other materials may be used as long as they are conductive.

  The displacement of the cantilever was detected by irradiating the surface of the cantilever with laser light from the laser diode, detecting the reflected light at that time with a two-divided photodiode, and taking the difference between the two outputs. The sample substrate is fixed on an XYZ scanning stage using a piezo element, the entire stage is moved up and down by a coarse movement mechanism using a stepping motor and screws, and the sample is brought close to a cantilever mounted facing the sample surface. I let you. When the sample surface approaches the tip of the probe to about 1 nm, an attractive force acts between them, so that the natural frequency of the cantilever decreases.

  By detecting the vibration frequency of the cantilever from the displacement signal with a frequency detector using a phase-locked loop, and adjusting the position of the sample in the Z direction so that the change in vibration frequency is constant, the distance between the probe and the sample surface Was kept constant.

  An AC voltage with a frequency of 254 kHz and an amplitude of 1 V was applied to the probe in the vicinity of the natural frequency of the cantilever. By detecting a 508 kHz component, which is twice the frequency of the AC voltage, from the displacement signal of the cantilever using a lock-in amplifier, a differential signal twice regarding the voltage of the electrostatic force acting between the probe and the sample was obtained. Furthermore, this signal was converted into the square root of the reciprocal by a reciprocal circuit using a divider and a square root circuit using a multiplier to obtain a new signal A. Also, by applying a minute alternating voltage to the Z direction drive signal of the XYZ scanning stage, the sample was vibrated in the Z direction with a frequency of 10 kHz and an amplitude of 0.1 nm, and the distance between the probe tip and the sample surface was changed. By detecting a 10 kHz component that is the vibration frequency of the sample from the signal A using a lock-in amplifier, a differential signal B relating to the distance between the probe tip and the sample surface of the signal A was obtained. The values of signal A and signal B were taken into a control computer by an AD converter, and the film thickness was calculated using the tip-sample distance and the relative dielectric constant of the silicon oxide film measured in advance by a force curve. To obtain a film thickness distribution image by mapping the results of measuring the film thickness at each position on the sample surface by scanning the sample in the XY direction while maintaining a constant probe-sample distance using an XYZ scanning stage Successful. Measurements were performed on samples having a film thickness of 0.3 nm to 3 μm, and a clear film thickness distribution was obtained. FIG. 2A shows a film thickness distribution image, and FIG. 2B shows a film thickness profile of the A-A ′ portion of FIG. 2A. The film thickness resolution and the in-plane spatial resolution depend on the film thickness, and were 0.01 nm and 0.1 nm when the film thickness was 0.3 nm, and 10 nm and 100 nm when the film thickness was 3 μm, respectively.

  Similar results were obtained when the probe-sample distance was controlled by keeping the change in the vibration amplitude of the cantilever constant. The frequency of the voltage applied to the probe is preferably in the vicinity of the natural frequency at which the sensitivity to the cantilever force is highest, but may be separated from the natural frequency. If the vibration frequency of the sample is not the same as the natural frequency of the cantilever and the frequency of the probe voltage, frequencies from several tens Hz to the upper limit frequency of the response of the XYZ scanning stage can be used.

  The present invention can be applied to the evaluation of the film thickness distribution of the dielectric film formed on the semiconductor and metal surfaces in the semiconductor process and the evaluation of the uniformity of the film thickness.

1 ... Measurement sample,
2 ... underlying substrate,
3 ... insulator layer,
4 ... cantilever,
5 ... probe,
6 ... oscillator,
7 ... XYZ scanning mechanism,
8 ... Coarse motion mechanism,
9 ... Control part,
10 ... Coarse moving part,
11 ... displacement detector,
12… Amplitude / frequency detector,
13 ... Feedback control unit,
14… Z drive,
15 ... XY scanning section,
16: AC voltage application unit,
17… Lock-in amplifier (A),
18… Reciprocal / square root conversion part,
19 ... Sample vibration part,
20 ... Lock-in amplifier (B),
21: Thickness calculation section.

Claims (6)

An XYZ scanning mechanism having an XY scanning unit for placing a measurement sample having an insulator film on a conductive substrate and moving the measurement sample in the XY direction and a Z driving unit for moving the measurement sample in the vertical direction;
A cantilever having a conductive probe at its tip, and movable relative to the surface of the measurement sample in an in-plane direction and in a vertical direction;
An oscillating unit that vibrates the cantilever in a direction perpendicular to the surface of the measurement sample at a natural frequency or a frequency in the vicinity thereof; and
Electrostatic force (a (V / (t / e + g)) ² applied to the probe when the probe is brought close to the measurement sample, where a is a constant, and e is a value of the insulator film. The relative dielectric constant, t: film thickness of the insulator film, g: distance between the probe and the measurement sample surface, V: voltage applied to the probe)) A force detector for detecting
An amplitude / frequency detector that inputs a signal from the force detector and detects a vibration amplitude or a frequency that is a vibration signal of the cantilever;
Based on the vibration signal, the Z driving unit drives the XYZ scanning mechanism in the Z direction so that the distance between the probe and the surface of the measurement sample becomes a constant value set by the control unit. A feedback control unit to control;
By using the vibration signal and a reference signal from an AC voltage application unit that applies an AC voltage to the probe, a double frequency component of the reference signal is detected, so that a second derivative with respect to the voltage of the electrostatic force is obtained. A first amplitude detector for obtaining a signal (2a (1 / (t / e + g)) ² );
A reciprocal / square root conversion unit that obtains a first signal (S = b (t / e + g)) by performing a conversion that is a square root of the reciprocal of the double differential signal;
A measurement sample vibrating section that changes the first signal to obtain a second signal by vibrating the XYZ scanning mechanism in the Z direction and vibrating the distance between the surface of the measurement sample and the tip of the probe. When,
By detecting the same frequency component as the second signal and the reference signal from the measurement sample vibrating section, the first signal related to the distance (g) between the surface of the measurement sample and the tip of the probe is detected. A second amplitude detector for obtaining a differential signal (D = b),
The first signal (S), the differential signal (D) of the first signal, the distance (g) between the surface of the measurement sample and the tip of the probe, and the relative dielectric constant of the insulator film (E) and the thickness distribution of the insulator film on the conductive substrate by repeating the determination of the thickness (t = e (S / Dg)) of the insulator film on the conductive substrate. The film thickness evaluation apparatus characterized by evaluating.   The film thickness evaluation apparatus according to claim 1, wherein a displacement detector that detects a displacement of a cantilever having the probe at one end is used as the force detector.   The film thickness evaluation apparatus according to claim 1, wherein a lock-in amplifier is used as the first amplitude detector and the second amplitude detector. In a film thickness evaluation method for measuring a film thickness distribution of a sample in which an insulator film is provided on a conductive substrate using a conductive probe,
Means for moving the probe relative to the surface of the sample in an in-plane direction and a vertical direction; control means for maintaining a constant distance between the probe and the sample; and applied to the probe A force detector for detecting a force; means for applying an AC voltage to the probe; a first amplitude detector for detecting a double frequency component of the AC voltage from an output signal of the force detector; A signal converter for performing reciprocal conversion and square root conversion on the output signal of the amplitude detector, means for applying AC vibration to the distance between the probe and the sample, and a frequency component of the AC vibration from the output signal of the signal converter And the output signal of the signal converter is divided by the output signal of the second amplitude detector, the distance between the probe and the sample is subtracted, and the insulator film A film thickness evaluation apparatus comprising a calculation means for multiplying a relative dielectric constant Stomach,
Applied to the probe, expressed by a constant a, a dielectric constant e of the insulator film, a film thickness t of the insulator film, a distance g between the probe and the sample, and a voltage V of the probe Electrostatic force a (V / (t / e + g)) ²
A second differential signal 2a (1 / (t / e + g)) ² relating to the voltage V of the electrostatic force is detected by the first amplitude detector;
A first signal S = b (t / e + g) expressed using a constant b is obtained by performing reciprocal transformation and square root transformation with the signal converter on the double differential signal,
With respect to the first signal, the second amplitude detector detects a second signal D = b that is a derivative of the first signal with respect to the distance g,
By calculating t = e (S / Dg) by the calculation means, the film thickness t is obtained,
A film thickness evaluation method, wherein the film thickness distribution is evaluated by moving the probe on the sample surface and measuring the film thickness at each position on the sample surface.   5. The film thickness evaluation method according to claim 4, wherein means for detecting displacement of a cantilever having the probe at one end is used as the force detector.   5. The film thickness evaluation method according to claim 4, wherein a lock-in amplifier is used as the first amplitude detector and the second amplitude detector.