JP2000146811A - Scanning capacitance microscope and method using same for discriminating conduction-type of semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the discrimination between P type and N type by separating semiconductor samples having an extremely high carrier density from those depleted, when they are analyzed. SOLUTION: This device has a means 16 for causing a conductive probe 12 to scan over a specimen 11, a means 13 for placing a bias voltage between the specimen 11 and the conductive probe 12, a means 14 for converting a change of capacitance between the specimen 11 and the conductive probe 12 into a change of voltage, and a means for detecting a phase relation between the bias voltage and the changed voltage signal. By providing this device with the means for detecting a phase relation between a change of capacitance and a bias voltage, a scanning capacitance microscope can be realized considerably facilitating the discrimination between P type and N type and the determination of a depletion layer domain when a semiconductor device is analyzed. This can be used as a very powerful tool for development of sub-micron devices.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a technique for analyzing an electrical structure of a material using a scanning capacitance microscope.

[0002]

2. Description of the Related Art FIG. 4 shows a configuration of a conventional scanning capacitance microscope. A voltage applying device 33 for applying a bias voltage to a sample 31 to be measured, a conductive probe 32 caused by this voltage, and a device to be measured. A capacitance-voltage converter 34 for converting a capacitance change between the sample 31 and the sample into a voltage change; a lock-in amplifier 35 for detecting the amplitude of a component of the voltage signal equal to the frequency of the sample bias voltage;
The probe driving device 36 scans the sample while keeping a constant distance from the sample surface, and the control device 37 controls the entire system. Reference numeral 38 denotes an amplitude output of the lock-in amplifier.

Here, when the sample 31 to be measured is a semiconductor device, the carrier concentration of the semiconductor and the lock-in amplifier 35
The relationship between the output and will be briefly described. First, the carrier inside the semiconductor in contact with the conductive probe 32 is attracted to or displaced from the surface by the AC component of the sample bias voltage. As a result, the region in which no carrier is generated inside the semiconductor, that is, the width of the depletion layer is temporally reduced. Changes to Assuming that the sample bias voltage is constant, the change in the width of the depletion layer is large when the carrier concentration is low, and the change in the depletion layer width is small when the carrier concentration is high.
The capacitance between the conductive probe and the sample is determined by the width of the depletion layer. The change in capacitance with respect to the amplitude of the bias voltage decreases as the carrier concentration increases, and increases as the carrier concentration decreases. At this time, if a lock-in amplifier is used, a bias frequency component of the capacitance change converted into a voltage can be detected with a high S / N ratio. In this way, information relating to the carrier density is obtained from the output of the lock-in amplifier 35.

[0004] Since the spatial resolution of a scanning probe is extremely high and is on the order of nanometers, it has attracted attention as a technique for analyzing the electrical structure of a semiconductor device having a submicron structure.

[0005]

However, in the configuration of the conventional scanning capacitance microscope, the capacitance change between the conductive probe and the sample caused by the forcibly applied sample bias voltage is measured by the capacitance-voltage converter. Since the voltage signal is converted into a voltage change and only the magnitude of the component equal to the frequency of the sample bias voltage in the voltage signal is detected, the following problems are encountered particularly when the semiconductor device is analyzed.

First, since the P-type and N-type regions are adjacent to each other on the sample surface, when the depleted portion is measured, the absolute value of the capacitance itself becomes a small value. Signal strength is also reduced. This is apparently indistinguishable from a decrease in signal intensity when the carrier concentration is high. A general semiconductor device has a structure in which P-type and N-type regions are intricately intricate, and it is extremely difficult to analyze a sample whose detailed structure is unknown.

An object of the present invention is to facilitate determination of P-type and N-type and depletion layer regions when a semiconductor device is analyzed with a scanning capacitance microscope.

[0008]

According to the present invention, there is provided a scanning capacitance microscope comprising: means for scanning a conductive probe on a sample; means for applying a bias voltage between the sample and the conductive probe; And a means for converting a capacitance change between the probe and the conductive probe into a voltage change, and a means for detecting a phase relationship between a bias voltage and a signal of the voltage change. As a result, when analyzing a semiconductor device, P
The type, the N type, and the region of the depletion layer can be determined.

Further, according to the method of the present invention for determining the conductivity type of a semiconductor using a scanning capacitance microscope, a step of obtaining a phase image, a step of selecting a specific region in the phase image, and a step of designating the conductivity type of the region Calculating the representative phase of the region; calculating the corrected phase image by subtracting the value of the representative phase from the phase image; calculating the conductivity type images of the P-type and N-type regions using the variable parameter as a threshold value And a process. This makes it possible to efficiently determine the regions of the P-type, the N-type, and the depletion layer.

[0010] In the method for determining the conductivity type of a semiconductor using a scanning capacitance microscope according to the present invention, the step of calculating a representative phase includes:
Desirably, the process is a process of calculating a combined vector by combining the phase vectors of the respective pixels of the region with each other, and using the phase of the combined vector as a representative phase.

Further, in the method for determining the conductivity type of a semiconductor using a scanning capacitance microscope according to the present invention, the step of calculating a representative phase includes:
It is desirable to take a phase histogram of the region and set the phase corresponding to the most frequent section from the histogram as the representative phase.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a scanning capacitance microscope according to the present invention.
Voltage applying device 13 for applying a bias voltage to the conductive probe 12 and the sample 1 to be measured caused by the voltage.
A phase-output type lock-in amplifier 15 for detecting the amplitude and phase of a component of the voltage signal equal to the frequency of the sample bias voltage; It comprises a probe driving device 16 for scanning the conductive probe 12 while keeping a constant distance from the sample surface, and a control device 17 for controlling the entire system. Reference numeral 18 denotes an amplitude output of the lock-in amplifier, and reference numeral 19 denotes a phase output of the lock-in amplifier. The controller 17 has a function of storing the phase output value from the phase output type lock-in amplifier 15 as an image and determining the conductivity type of the image obtained from the amplitude signal.

FIG. 2 shows a method of determining the conductivity type of a semiconductor using the scanning capacitance microscope of the present invention. After obtaining an image (phase image) 21, selecting a reference region 22 from the image 22. And input 23 of the conductivity type of the reference area is performed. Next, a calculation 24 of a representative phase of the selected region is performed, and a phase image correction 25 is performed based on the obtained value. Finally, the calculation 26 of the conductivity type image is performed based on an appropriate threshold value, and the output 27 of the result is performed.

The operation and procedure of the scanning capacitance microscope configured as described above and the method of determining the conductivity type of a semiconductor using the same will be described in more detail below.

The conductive probe is driven horizontally by a piezo drive element or a probe drive device using a stylus pressure detection using reflected light of a laser while maintaining a distance from the sample surface with sub-nanometer accuracy. Scanning is performed with the accuracy of the meter.

Next, how the capacitance between the sample tips when a bias voltage is applied to the sample differs depending on the conductivity type will be described. First, the case where the probe is in contact with the surface of the P-type semiconductor will be described. Since the carrier of the P-type semiconductor is a hole and has a positive charge, when the potential of the probe changes to positive by external control, a repulsive force is received from the probe and the depletion layer expands. Therefore, when the probe potential has the positive maximum value, the capacitance decreases, and when the probe potential has the negative maximum value, the capacitance increases. When the probe is in contact with the surface of the N-type semiconductor, the opposite phenomenon occurs because carriers are electrons and have negative charges. That is, when the probe potential has a positive maximum value, the capacitance increases, and when the probe potential has a negative maximum value, the capacitance decreases.

FIG. 3 schematically shows the above contents, and FIG. 3 (a) shows the change over time of the potential of the probe with respect to the sample. The case of change is shown. FIG. 3B shows a time change of a depletion layer generated on the sample surface due to an electric field between the probe and the sample caused by the bias voltage in the case of the P-type and N-type semiconductors, and FIG. The time change of the probe-to-sample capacitance at that time is shown for P-type and N-type semiconductors. Comparing the time change waveforms of FIG. 3A and FIG. 3C, it can be understood that it is possible to determine whether it is P-type or N-type based on the phase of the capacitance change based on the sample bias voltage.

According to this embodiment, by providing a lock-in amplifier for detecting the magnitude of the component equal to the frequency of the sample bias voltage and the phase difference in the output signal of the capacitance-voltage converter, Information related to the phase difference can be obtained. Further, the measurement is performed while scanning the probe, and the two-dimensional amplitude image and the phase image are obtained by storing the amplitude and the phase difference for each spatially subdivided individual region (pixel). be able to.

Since the obtained phase image is actually affected by the impedance of the measuring circuit, the phase of the voltage applied to the sample is not normally used as a reference. Next, a method for avoiding this problem will be described.

First, a place where the conductivity type is known or tentatively determined in the obtained phase image is selected.
The representative value of the phase representing all the pixels in the selected area is calculated. However, the phase has the property of returning to the original in a 360-degree cycle. For example, a phase advanced by 180 degrees and a phase delayed by 180 degrees can be regarded as the same. Problems such as the phase being reduced due to mutual cancellation occur. Therefore, the phases are combined as a vector, and the phase of the combined vector is calculated.
The following equation (1) indicates that the phase is X (1), X (2),.
One of the methods for calculating the representative phase Y when X (N) is set is shown.

(I) Σcos (X (k))>0; Y = Arctan (Σsin (X (k)) / Σcos (X (k))), (ii) Σcos (X (k)) = 0 and Σsin (X (k))>0; Y = 90, (iii) Σcos (X (k)) = 0 and Σsin (X (k)) <0; Y = −90, (iv) Σcos (X (k )) <0 and Σsin (X (k))>0; Y = 180 + Arctan (Σsin (X (k)) / Σcos (X (k))), (v) Σcos (X (k)) <0 and Σsin (X (k)) <0; Y = −180 + Arctan (Σsin (X (k)) / Σcos (X (k))). (1) where N is the number of pixels in the region, Σ is the sum of k = 1 to N, and the domain of the arctangent function Arctan is from −90 degrees to 90 degrees.

This representative value is subtracted from the original phase image,
Obtain a corrected phase image. Since the result differs depending on whether the region selected as the reference is P-type or N-type, if the region selected as the reference is N-type, add 180 degrees to the phase of each pixel, and if the value exceeds 180 degrees, Subtract another 360 degrees from that value. By doing so, a phase image based on the P-type region can be obtained. In the example shown here, the phase definition range is from -180 degrees to 180 degrees.

Based on the phase image calculated as described above,
To obtain a conductivity type image, the following is performed. If the value of the phase of each pixel is greater than -90 degrees and less than 90 degrees, the pixel is determined to be P-type,
No definition if 90 degrees or 90 degrees, otherwise N
By obtaining a mold, a conductive image is obtained. These phase and conductivity images can be output together with the amplitude image that is the output of a normal scanning capacitance microscope, and by using normal image processing such as line profiles and composite images, more abundant information can be obtained. Can be obtained.

As described above, according to the embodiment of the present invention, when analyzing a semiconductor device, it becomes extremely easy to determine the P-type and N-type regions. Further, since the depletion layer is generated at the boundary between the P-type and the N-type, it is easy to distinguish between the depletion layer and the high carrier concentration portion.

In the embodiment of the present invention, the phase output type lock-in amplifier 15 having two outputs of the amplitude and the phase difference is used for determining the conductivity type. A lock-in amplifier may be used in which the amplitude of the product of the input signals with respect to two reference signals having the same frequency but different phases by 90 degrees such as a cosine waveform is two outputs. At this time, the amplitude is the magnitude of a vector when these two signals are plotted on a complex plane as coordinates, and the phase is given in the direction of this vector.

In the phase image correction step 25, instead of calculating the correction value of the phase value itself, the inner product of the reference direction, for example, the representative phase vector of the reference area is calculated and corrected. It may be used as a substitute for a phase image. Further, here, the phase image correcting step 25 is performed with reference to the P-type, but may be performed with reference to the N-type.

Further, in the image obtaining step 21, the use of noise removal processing such as spike noise and low-frequency noise as necessary enables more accurate determination of the conductivity type. This noise removal processing may be performed at any time during the above-described series of data processing.

The reference area selecting step 22 may select only one pixel, and the representative phase calculating step 24 of the selected area at that time may use the phase of the one pixel.
The unit of the phase may be degrees or radians.
Other units may be used if necessary. Furthermore, the phase is 36
Since the values are equal at 0 degrees or 2π radian periods, there is no need to limit the expression in the output of the result or during the calculation.

In the calculation step 26 of the conductivity type image,
If the value of the phase of each pixel is greater than -90 degrees and less than 90 degrees, it is assumed to be P-type, and if -90 degrees or 90 degrees, there is no definition.
In other cases, instead of the N-type, an appropriate angle Z of 0 degree or more and less than 90 degrees is used, and the value of the phase of each pixel is (-
In the range of 90 + Z degrees to (90-Z) degrees, the P-type is set, and from (-90-Z) degrees to (-90 + Z) degrees and (9
There is no definition in the case of 0-Z) degrees to (90 + Z) degrees, otherwise it may be N-type. Note that these boundaries (−9
0 + Z) degree, (90-Z) degree, (90-Z) degree, (90
If the phase value matches the (+ Z) degree, it may be undefined, or (-90 + Z) degree and (90-Z) degree may be P-type.
The (90-Z) degree and the (90 + Z) degree may be N-type or the like.

In the embodiment of the present invention, the representative phase is calculated by taking the phase of the composite vector of the phase vectors of the entire selected area. , It is also possible to select a representative value of the phase in the most frequent section. The representative value of the section can be, for example, a central value. Here, the number of sections of the histogram may be variable, but in the case of automatic calculation, for example, the same number of sections as the number of pixels in the area selected in the reference area selecting step 22 is selected. ,
A section is determined by equal division.

[0031]

As described above, according to the present invention, the means for detecting the phase relationship between the capacitance change and the bias voltage is provided.
When analyzing a semiconductor device, it is possible to realize a scanning capacitance microscope in which it is extremely easy to determine P-type and N-type and depletion layer regions. This can provide a very powerful tool in submicron device development.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a scanning capacitance microscope according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a method for determining the conductivity type of a semiconductor using a scanning capacitance microscope according to an embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the capacitance between sample tips and the conductivity type when a bias voltage is applied to the sample in the embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of a conventional scanning capacitance microscope.

[Explanation of symbols]

 REFERENCE SIGNS LIST 11 Sample to be measured 12 Conductive probe 13 Voltage application device 14 Capacitance voltage conversion device 15 Phase output type lock-in amplifier 16 Probe drive device 17 Control device 18 Amplitude output of lock-in amplifier 19 Phase output of lock-in amplifier 21 Image Acquisition step 22 reference area selection step 23 reference area conductivity type input step 24 selected area representative phase calculation step 25 phase image correction step 26 conductivity type image calculation step 27 result output step

Claims (4)

[Claims] Means for scanning a conductive probe over a sample;
Means for applying a bias voltage between the sample and the conductive probe; means for converting a change in capacitance between the sample and the conductive probe into a voltage change; and means for changing the bias voltage and the voltage change. Means for detecting a phase relationship between the signals. A step of obtaining a phase image; a step of selecting a specific region in the phase image; a step of designating a conductivity type of the region; a step of calculating a representative phase of the region; A step of calculating a correction phase image by subtracting the value of the representative phase from the image, and a step of calculating conductivity type images of P-type and N-type regions using a variable parameter as a threshold value. A method for determining the conductivity type of a semiconductor using a microscope. 3. The step of calculating the representative phase includes calculating a composite vector obtained by synthesizing the phase vectors of the respective pixels of the region with each other, and setting the phase of the composite vector as the representative phase. 3. A method for determining the conductivity type of a semiconductor using the scanning capacitance microscope according to 2. 4. The method according to claim 2, wherein in the step of calculating the representative phase, a phase histogram of the area is obtained, and a phase corresponding to a section having the highest frequency is set as the representative phase. Of determining the conductivity type of a semiconductor using a scanning capacitance microscope.