JP2900764B2 - Evaluation method of semiconductor surface thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating a semiconductor surface thin film, and more particularly, to a semiconductor surface thin film for evaluating an electrical characteristic of an insulating film formed on a semiconductor surface such as a silicon oxide film or a silicon nitride film. The evaluation method.

[0002]

2. Description of the Related Art Heretofore, evaluation of electrical characteristics of an insulating film formed on a semiconductor surface such as a silicon oxide film and a silicon nitride film has been performed by a TDDB test at a constant current or a constant voltage. TDDB (Time Dependent Dielect
The ric Breakdown) test is a test method for evaluating the reliability of the insulating film by utilizing a phenomenon that when a voltage lower than the breakdown voltage is continuously applied to a material for a long time, a dielectric breakdown occurs depending on an elapsed time. . For example, a constant current TDDB test method is a test method in which a constant current is applied to the insulating film, and a time-dependent change in a voltage required to flow the constant current is measured to evaluate the reliability of the insulating film. In this case, when the insulating film causes dielectric breakdown, the voltage sharply decreases. Therefore, the time from the start of the supply of the constant current to the time when the voltage sharply decreases is the time required for the insulating film to break down under the constant current. Such a constant current T
For example, when observing the presence or absence of a defect in an insulating film having a MOS structure using the DDB test method, a specific region of several μm square to several mm square can be observed and evaluated.

[0003]

The size of the specific region is determined by the size of the electrode area formed on the insulating film to allow a constant current to flow. If the current fine processing technology is used, it is possible to produce an electrode having a diameter of the order of 0.1 μm smaller than the above numerical value. Scanning Tunneling Microscope (STM)
) Is used for the evaluation of the insulating film, the STM can apply a current to a very small area of several nm ² and measure the current-voltage characteristic in the very small area. It is possible to measure the fracture properties (eg, Y. FUKANO et al, Jpn. J. Appl. Phys. 32 (199
3) 290).

However, in the normal constant current TDDB test method, a constant current of 10 μA / cm ^{2 to} 1 A / cm ² flows, but the current flows directly to the electrode having a diameter of the order of 0.1 μm or to the STM. The electrode or the STM
A very small current must be passed through the probe, and the dielectric breakdown characteristics can be observed and evaluated by the ordinary constant current TDDB test method only in the range of several μm square to several mm square described above. It is. For example, the STM pro-
Assuming that the contact area of the probe is 1 nm ² , 10 ⁻¹⁹ A to 10
A very small current of ^-14 A flows through the STM, which is equivalent to injecting about 0.6 to 60,000 electrons per second. It is impossible to control such an extremely small current with current electronic circuit technology.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a method of evaluating a semiconductor surface thin film capable of measuring and observing a dielectric breakdown characteristic in a minute area which cannot be achieved by a conventional constant current TDDB test method. It is intended to provide.

[0006]

In order to achieve the above object, a method for evaluating a thin film on a semiconductor surface according to the present invention comprises: contacting a conductive probe with a thin film on a semiconductor surface; It is characterized by measurement with an atomic force microscope.

[0007]

According to the method for evaluating a semiconductor surface thin film according to the present invention, an insulating film is charged by contacting a conductive probe, and a decrease in the charged charge ΔQ (due to dissipation) is measured by an atomic force microscope (AFM: Atomic Force Microscope) with electrostatic force ΔF
This is a method in which the change is measured and the change with time is observed. In this case, when the dielectric breakdown occurs, the electrostatic force ΔF sharply decreases.

The principle of the method for evaluating a semiconductor thin film according to the present invention will be described below. First, assume the following. -The charged charge ΔQ is defined as a point charge. The influence of the electric charge induced on the lever side or the semiconductor substrate side of the AFM by the charged electric charge ΔQ is negligible compared to the bias voltage Vb applied between the AFM and the semiconductor substrate. The point charge ΔQ receives an electrostatic force ΔF (= ΔQ × Eb) due to an electric field Eb generated by the bias voltage Vb, and the electrostatic charge ΔF
On the lever side of the AFM. The magnitude of the electric field Eb created by the bias voltage Vb depends only on the point charge ΔQ and the distance Z between the tip of the AFM and the lever just above the point charge ΔQ at which the electrostatic force ΔF is maximized. Therefore, if the distance Z is constant, the electric field Eb is also constant.

From these assumptions, the maximum electrostatic force ΔF _max (t) = ΔQ (t) × Eb (1). Here, t is time. Dissipation of the point charge ΔQ is caused by charge inflow and outflow (current i
(T) when due to), ΔQ (t) = ΔQ (0) -∫i (t) dt ... (2) ΔF max (t) = Eb × [ΔQ (0) -∫i (t) dt] ... (3) Therefore, from the equation (3), if the change over time of the electrostatic force ΔF _max (t) due to the charged electric charge ΔQ is measured,
The current i (t) generated by the point charge ΔQ under the bias voltage Vb
Can be measured over time.

Here, assuming that the insulation resistance R (t) is constant without a change with time due to insulation breakdown, in this case, i (t) = _ic (constant) and ΔF _max (t) = Eb × [ΔQ (0) -i c × t] ... is (4). Conversely, if the change with time of ΔF _max (t) occurs at a constant speed, the flowing current can be considered to be constant. Therefore, it can be considered that the dielectric breakdown has occurred when the change with time of ΔF _max (t) is abrupt.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for evaluating a semiconductor surface thin film according to the present invention will be described below with reference to the drawings. FIG. 1 shows an AFM apparatus 10 used in a method for evaluating a semiconductor surface thin film according to an embodiment.
FIG. 2 is a block diagram schematically showing

In FIG. 1, reference numeral 3 denotes a silicon substrate, on which a silicon oxide film 2 grown by a thermal oxidation method is formed. The silicon substrate 3 is mounted on a PZT tube type scanning system 4 for moving the silicon substrate 3 in the X, Y, and Z directions. ) 5). The control drive system 6 is connected to the interferometer 7 and drives the PZT tube type scanning system 4 based on information from the interferometer 7, and the interferometer 7 is electrically conductive via an optical fiber-8. The amount of displacement of the cantilever-1 can be measured. The tip of the conductive cantilever 1 is provided with a conductive probe 1a, and the rear end of the conductive cantilever 1 is connected to a bias power supply 9. In the AFM device 10, the distance Z between the tip of the conductive probe 1a and the silicon oxide film 2 at the time of measurement is kept constant by the control drive system 6.

The evaluation of the silicon oxide film 2 by the method for evaluating a semiconductor surface thin film according to the embodiment is performed as follows using the AFM apparatus 10 configured as described above. First, a silicon oxide film 2 is applied by a conductive probe 1a to which a voltage is applied.
After contacting and charging the above point to be measured, the conductive probe
The blade 1a is separated. Next, a bias voltage Vb is applied to the conductive probe 1a. Next, the conductive probe is moved to a distance (several nm to several tens nm) where the electrostatic force can be measured between the conductive probe 1a and the electric charge ΔQ on the surface of the silicon oxide film 2.
The tip of the valve 1a is brought closer. Then, the contact-charged area is scanned, and the distribution of the electrostatic force ΔF is repeatedly measured. From the change with time of the maximum value ΔF _max of the measured electrostatic force (3)
The value of the current flowing from the point charge ΔQ is obtained by the equation.

When contact charging is performed, the actual contact between the atom at the tip of the conductive probe 1a and the silicon oxide film 2 is made.
Because of the atoms on the surface, the contact area is the conductive probe 1a.
It becomes smaller than the radius of curvature at the tip. For example, even if the radius of curvature of the tip of the conductive probe 1a is 1 μm, the contact area is about several nm ² . In addition, the spatial resolution of the electrostatic force ΔF when measuring the electrostatic force ΔF is 0. 5 in the half width of the distribution of the electrostatic force ΔF due to the spatial spread of the electrostatic force ΔF. It becomes several μm.

FIG. 2 shows that the method for evaluating a semiconductor surface thin film according to the embodiment is applied to a 5 nm thick silicon oxide film 2 grown on a P-type silicon substrate 3 (plane orientation (100)) by a thermal oxidation method. It is the graph which showed the measurement result in the case schematically. The vertical axis represents electrostatic force (nN), and the horizontal axis represents time (second).

In the graph shown in FIG. 2, a conductive probe 1a having a tip radius of curvature of about 0.1 μm was used, and a voltage of -4 V was first applied to the conductive probe 1a for contact charging. Thereafter, a bias voltage of +4 V is applied to the conductive probe 1a and scanning is performed at a distance of about 35 nm from the surface of the silicon oxide film 2, and the maximum value ΔF of the electrostatic force applied to the conductive probe 1a.
_It shows the result of measuring the change over time of _max . As can be seen from the graph of FIG. 2, the electrostatic force decreases almost linearly with time. This indicates that the expression (4) shown in the “action” holds. Calculating using equation (4), the value of the current flowing using the charge ΔQ due to contact charging as a current source can be estimated to be about 4 × 10 ⁻¹⁶ A. This indicates that an extremely small current, which cannot be directly controlled by the prior art, can be flowed at a substantially constant value by the method according to the embodiment.

FIG. 3 is a graph showing the result of evaluation of another semiconductor surface thin film using the method for evaluating a semiconductor surface thin film according to the embodiment, and specifically, the same measurement as the measurement in the above embodiment. Is a graph showing the measurement results when the measurement is performed on another portion of the same sample. The difference between the present measurement method and the previous measurement method is that in this case, the distance between the surface of the silicon oxide film 2 and the tip of the conductive probe 1a is about 90 nm, which is smaller than the previous case (about 35 nm). Only those points that are far apart.

As can be seen from FIG. 3, in the present measurement, the electrostatic force ΔF _max rapidly decreases from around the measurement time of 85 seconds, and a sharp increase in current occurs. This means that there was a defect in the silicon oxide film 2 at the place where the conductive probe 1a contact-charged the silicon oxide film 2 and dielectric breakdown occurred at the place, and the current value increased sharply.
As described above, a minute current is caused to flow by using the charge ΔQ due to contact charging as a supply source, and a change in the current value is measured by the AFM device 10 as a change in the electrostatic force ΔF _max to evaluate a small area in the silicon oxide film 2. Can be performed.

As described above, in the method for evaluating a semiconductor surface thin film according to the embodiment, since the current source is the dissipation of the charge from the point charge ΔQ due to the contact charging, it cannot be controlled by the conventional technology. The constant current TDDB can be evaluated with a very small current. Further, since contact charging is performed by the conductive probe 1a, charging in an extremely small area is possible, and constant current TDDB evaluation in the extremely small area can be performed. For example, the spatial distribution of the charge ΔQ given by the contact charging in the above embodiment is about 400 nm as the half width of the measured value of the distribution of the electrostatic force ΔF. This is a measurement of a minute area which cannot be achieved by a conventional evaluation method using a MOS capacitor or the like. In the embodiment, the value of the flowing current can be controlled by the contact time and the voltage at the time of first applying the contact charging.

Further, if the method for evaluating a semiconductor surface thin film according to the embodiment is used, it is possible to evaluate and measure a minute region in the semiconductor surface thin film regardless of the thickness of the insulating film such as the silicon oxide film 2.

[0021]

As described in detail above, when the method for evaluating a semiconductor surface thin film according to the present invention is used, an extremely small current which cannot be controlled by the conventional technique can be replaced by an extremely small current which cannot be measured by the conventional technique. Constant current TD of semiconductor surface thin film such as silicon oxide film with unprecedented spatial resolution
DB evaluation can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram schematically showing an AFM apparatus used when performing a method for evaluating a semiconductor surface thin film according to an embodiment of the present invention.

FIG. 2 is a graph showing a result of evaluating a silicon oxide film by using a method for evaluating a semiconductor surface thin film according to an example.

FIG. 3 is a graph showing the results of evaluation of another silicon oxide film using the semiconductor surface thin film evaluation method according to the example.

[Description of Signs] 1 conductive cantilever 1a conductive probe 2 silicon oxide film 3 silicon substrate 5 AFM (topography) 9 bias power supply 10 AFM device

Continuation of the front page (56) References JP-A-3-277903 (JP, A) JP-A-4-270903 (JP, A) JP-A-4-12547 (JP, A) JP-A-4-330752 (JP JP-A-4-137644 (JP, A) JP-A-6-26855 (JP, A) JP-A-6-273155 (JP, A) JP-A-6-201373 (JP, A) 2-212702 (JP, A) JP-A-6-273159 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01L 21/66 G01B 7/34 102 G01B 21/30

Claims (1)

(57) [Claims] 1. A method for evaluating a thin film on a semiconductor surface, comprising: contacting a conductive probe with a thin film on a semiconductor surface, and then measuring a change over time in a contact charge amount with an atomic force microscope.