JP3806482B2 - Current value change measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for irradiating a sample used as an inspection apparatus for a sample such as a semiconductor integrated circuit with a laser beam and measuring a change in the current of the sample associated therewith.
[0002]
[Prior art]
As an apparatus for inspecting a defect of a sample such as a semiconductor integrated circuit, there is an apparatus described in JP-A-56-45045. A block diagram of this device is shown in FIG.
[0003]
A laser scanning unit 2 that raster scans the laser beam in a two-dimensional direction orthogonal to the incident direction on the optical path of the laser beam emitted from the laser generating unit 1 that is a light source, and the scanned laser beam is focused on a minute spot diameter. A microscope 3 is arranged. A measured sample 4 such as a semiconductor integrated circuit is disposed at the focal position of the microscope 3. A predetermined voltage is applied to the sample 4 by the voltage source 5. The sample 4 to be measured is connected to a current / voltage conversion amplifier 7 including an operational amplifier 8 and a feedback resistor 9. The current / voltage conversion amplifier 7 is connected to a signal processing unit 10, and a monitor 11 is further connected to the signal processing unit 10. The signal processing unit 10 is also connected to the laser scanning unit 2.
[0004]
The laser beam emitted from the laser generator 1 is raster-scanned in a two-dimensional direction orthogonal to the optical path by the laser scanning unit 2, condensed by the microscope unit 3, and irradiated on a fine portion of the surface of the sample 4 to be measured. . This scanning is controlled by the signal processing unit 10. A predetermined voltage is previously applied to the sample 4 by the voltage source 5, and a predetermined current flows in the circuit. At the portion of the sample 4 irradiated with the laser beam, the laser beam is absorbed and the temperature rises and the resistivity changes, so that the value of the current flowing through the sample 4 also changes. Since heat conduction is poor at places with defects such as voids, when such places are irradiated with laser, the heat does not easily escape to the surroundings, resulting in a large temperature rise and a large change in resistivity, resulting in a current value. Change will also increase.
[0005]
The current value is converted into a voltage value by the current / voltage conversion amplifier 7 and transferred to the signal processing unit 10. The signal processing unit 10 converts the difference between the voltage values into luminance information, generates image information arranged in accordance with the laser beam irradiation position, and displays the image information on the monitor 11.
[0006]
Thereby, the defect location of a sample can be confirmed on a screen.
[0007]
[Problems to be solved by the invention]
The detection accuracy of the sample defect in this apparatus depends on the detection accuracy of the voltage value change in the current / voltage conversion amplifier 7. The signal-to-noise (S / N) ratio S / N and the amplification factor G at the time of signal detection of the current / voltage conversion amplifier 7 are:
S / N = ΔI (R _f / 4kTB) ^1/2 (1)
G = R _f / R _i (2)
It is shown. Here, ΔI is the current fluctuation, k is the Boltzmann constant, T is the temperature, B is the frequency band of the operational amplifier 8, R _f is the resistance value of the feedback resistor 9, and R _i is the negative voltage of the operational amplifier 8 from the voltage source 5. Internal resistance value up to the input terminal.
[0008]
Since the signal detection S / N ratio is determined by the resistance value R _f of the feedback resistor 9 as shown in the equation (1), the resistance value of the feedback resistor 9 is required to improve the S / N ratio. It is preferable to increase R _f . However, the internal resistance of the sample 4 to be measured varies greatly from several Ω to several tens of kΩ depending on the measurement target. If the resistance value R _f of the feedback resistor 9 is increased to several MΩ in order to accurately measure the current value change of the sample having a large internal resistance of the sample 4, conversely, if the internal resistance of the sample 4 is as small as several Ω, The amplification factor G is a very large value of about 1 million times from equation (2). When the amplification factor is large, the low frequency noise of the voltage source 5 and the current / voltage conversion amplifier 7 becomes significantly larger than the fluctuation amount ΔI of the current flowing through the sample 4, and the S / N ratio of the output signal is also lowered. Further, in general, when the amplification factor increases, the frequency band of the current / voltage conversion amplifier 7 shifts to a low frequency band, and thus there is a problem that it becomes impossible to follow the resistance value change speed of the sample 4 due to laser light irradiation. Therefore, it is difficult to measure a sample having a small internal resistance of about several Ω.
[0009]
Further, when measuring a sample that performs AC operation, the applied voltage itself may fluctuate. Japanese Patent Laid-Open No. 56-45045 further discloses a technique for adjusting the voltage fluctuation. This is an improvement of the apparatus shown in FIG. 4. As shown in FIG. 5, an upper limit voltage limiter circuit 15 and a lower limit voltage limiter circuit 16 are connected to the voltage source 5. It is connected.
[0010]
During the measurement, the voltage applied to the sample 4 by the signal processing unit 10 is compared with the upper limit voltage and the lower limit voltage set by the upper limit voltage limiter circuit 15 and the lower limit voltage voltage limiter circuit 16, respectively. When the upper limit voltage is exceeded, the voltage applied by the voltage source 5 is lower than the upper limit voltage, and when the applied voltage is lower than the lower limit voltage, the voltage applied by the voltage source 5 is higher than the lower limit voltage. Is to control. However, this technique has a problem that the applied voltage varies between the upper limit voltage and the lower limit voltage and is not constant, and accordingly, the current flowing through the sample to be measured also fluctuates and the measurement accuracy deteriorates.
[0011]
It is an object of the present invention to provide a current value change measuring apparatus capable of measuring a current value change when a sample such as a semiconductor integrated circuit is irradiated with a laser with high accuracy.
[0012]
[Means for Solving the Problems]
The present invention relates to a current value change measuring apparatus for inspecting a defective portion of a sample by irradiating a sample such as a semiconductor integrated circuit that is energized while scanning with a laser beam and measuring a change in the current value of the sample accompanying this irradiation. A variable resistance means connected in series with the sample and adjusting the total resistance value of the series circuit so that the total resistance value of the series circuit with the sample when not irradiating the laser beam is equal to or higher than a predetermined resistance value; and laser beam irradiation Set the voltage value to be applied to the sample so that the current flowing through the sample becomes a predetermined current value before, and keep the voltage applied to the sample at the set voltage value while scanning the laser beam for measurement And means.
[0013]
As a result, since the total resistance value of the circuit in which the sample and the variable resistor are connected in series is maintained at a predetermined resistance value or higher, the current value flowing through the sample is suppressed to a predetermined current value or lower. Further, the voltage applied to the sample is kept at a voltage value suitable for the measurement set in advance during the measurement.
[0014]
[Means for Solving the Problems]
The present invention relates to a current value change measuring apparatus for inspecting a defective portion of a sample by irradiating a sample such as a semiconductor integrated circuit that is energized while scanning with a laser beam and measuring a change in the current value of the sample accompanying this irradiation. a variable resistance means that will be connected to the sample series, the variable resistor means, connected in series with the sample, a current / voltage conversion amplifier for converting the current flowing through the sample into a voltage signal, the variable resistance means, the sample A first voltage source for applying a voltage proportional to the input voltage to a series circuit composed of a current / voltage conversion amplifier, and a first voltage source connected to the first voltage source, and the input voltage value is directly output to the first voltage source. Voltage holding means for switching between two types of operation, holding the input voltage value and outputting the held voltage value to the first voltage source, and a second voltage source capable of adjusting the output voltage value And the voltage holding hand An operational amplifier for comparing the second voltage source and the output voltage of the current / voltage conversion amplifier, and a feedback control of the voltage holding means before irradiation of the laser beam to maintain the current value flowing through the sample constant. The internal resistance of the sample is determined, the resistance of the variable resistance means is set so that the resistance value of the series circuit is equal to or higher than the predetermined value, and the voltage holding means is kept in the holding operation during the laser beam irradiation, and the current value changes. And a signal processing unit that performs measurement .
[0015]
First, the configuration of this apparatus will be described. On the optical path of the laser beam emitted from the laser generator 1 which is a light source, a laser scanning section 2 for raster scanning the laser beam in a two-dimensional direction orthogonal to the optical path, and the scanned laser beam is focused on a minute spot diameter. A microscope 3 is arranged. A measured sample 4 such as a semiconductor integrated circuit is disposed at the focal position of the microscope 3. The sample 4 is connected in series with variable resistance means 6 capable of setting its resistance value. A predetermined voltage is applied to one end of the series circuit by a first voltage source 5 including, for example, a buffer amplifier that outputs a voltage proportional to the input voltage.
[0016]
The other end of the series circuit is connected to the negative input end of the first operational amplifier 8, and the negative input end and the output end of the first operational amplifier 9 are connected to the feedback resistor 9 in parallel. Yes. In addition, the positive input terminal of the first operational amplifier 8 is grounded, so that the first operational amplifier 8 and the feedback resistor 9 constitute a current / voltage conversion amplifier 7. The current / voltage conversion amplifier 7 is connected to a signal processing unit 10, and a monitor 11 is further connected to the signal processing unit 10. The signal processing unit 10 also controls the laser scanning unit 2 and the variable resistance means 6.
[0017]
On the other hand, the output terminal of the second operational amplifier 13 is connected to the input side of the first voltage source 5 via the voltage holding means 12 controlled by the signal processing unit 12. The voltage holding means 12 performs two types of operations, that is, an operation for outputting the input voltage value as it is and an operation for outputting the voltage held and holding the input voltage value. Can be used. A second voltage source 14 controlled by the signal processing unit 10 is connected to the negative input terminal of the second operational amplifier 13, while the positive input terminal is connected to the output terminal of the current / voltage conversion amplifier 7. It is connected.
[0018]
The variable resistance means 6 can be installed between the sample 4 and the current / voltage conversion amplifier 7. However, since the sample 4 is mounted close to the ground potential microscope 3 or the like, in order to suppress induction noise. The arrangement of FIG. 1 in which the potential on one side can be the ground potential is preferable.
[0019]
In the present embodiment, the variable resistance means 6 uses means for selecting and connecting either 0Ω (short circuit) or 50Ω resistance. Here, the reason why a 50Ω resistor is selected as the resistor added to the sample 4 will be described. FIG. 2 shows measurement results of output noise of the current / voltage amplifier 7 when the resistance value of the series circuit of FIG. 1 is changed. The resistance value of the feedback resistor 9 at this time was 1 MΩ. When the internal resistance value is 10Ω, the low frequency noise of 100 Hz to 1 kHz is 10 dB or more higher than the base noise of the device. However, when the internal resistance value is set to 50Ω, noise of 100 Hz or more is reduced to almost the same level as the base noise. Even if the internal resistance value is set to a larger value, the noise characteristics cannot be improved.
[0020]
On the other hand, the fluctuation ΔI of the current flowing through the sample 4 is
ΔI = − (ΔR / R) I (3)
Given in. Here, ΔR is an increase in resistance of the sample 4 due to laser irradiation, R is an internal resistance value of the sample 4, and I is a current value passed through the sample 4. From equations (1) and (3), it can be seen that the S / N ratio increases as the internal resistance value R of the sample 4 decreases. Therefore, the internal resistance value R of the sample 4 is preferably as small as possible within a range in which the low frequency noise component can be suppressed. Therefore, in this embodiment, the resistance value added by the variable resistance means 6 is set to 50Ω so that the resistance value of the series circuit of the variable resistance means 6 and the sample 4 is 50Ω or more. In this case, the maximum amplification factor is 20000 times. Note that the variable resistance means 6 may be one that keeps the resistance of the series circuit of the sample 4 and the variable resistance means 6 at 50Ω or more by using a resistance whose resistance value can be continuously changed from 0 to 50Ω.
[0021]
Next, the operation of this embodiment will be described with reference to FIGS. FIG. 3 is a timing chart of the present embodiment.
[0022]
After the power supply to the apparatus shown in FIG. 1 is turned on, the resistance value of the variable resistance means 6 is set to 50Ω, and feedback control of the voltage holding means 12 is performed (at the time of FIG. 3A). Specifically, the output value of the first operational amplifier 8 and the second voltage source 14 are compared by the second operational amplifier 13, and the output is input to the first voltage source 5. Therefore, feedback control is performed so that the current flowing through the sample 4 is always constant even when the resistance value of the variable resistance means 6 changes. The second voltage source 14 sets a current value that flows through the sample, and the current that flows through the sample can be increased by increasing the output voltage value.
[0023]
After the voltage of the first voltage source 5 is stabilized, the calculation process of the internal resistance value of the sample 4 is performed (at the time of FIG. 5B). Specifically, the current flowing through the series circuit of the sample 4 and the variable resistance means 6 is converted into a voltage signal by the current / voltage conversion amplifier 7 and transmitted to the signal processing unit 10. In the signal processing unit 10, the current flowing through the series circuit is calculated from the transmitted voltage signal, and based on this current value, the output voltage of the first voltage source 5, and the additional resistance value of the variable resistance means 6, the sample 4 An internal resistance value is calculated. When the calculated internal resistance value is 50Ω or more, the variable resistance means 6 is switched to a short-circuited state (at the time point (c) in the figure). When the internal resistance value is less than 50Ω, the variable resistance means 6 adds a 50Ω resistor as it is. As a result, the resistance value of the series circuit can be maintained at 50Ω or more, the S / N ratio is sufficiently high, and the measurement can be performed under a large amplification factor.
[0024]
When the calculation process of the internal resistance value is completed and the resistance value of the variable resistance means 6 is switched, the voltage of the first voltage source 5 is adjusted by the feedback control described above. (Time of (c) in the figure). After the voltage of the first voltage source 5 is stabilized and the value of the current flowing through the sample 4 is stabilized, the voltage holding unit 12 is switched to the voltage holding operation (at the time of FIG. 4D). Thereafter, the voltage of the first voltage source 5 is held at the voltage set at the end of the feedback control operation (at the time point (c) in the figure). The reason for switching to the holding operation is that if the feedback control operation is continued, the fluctuation in the current value accompanying the laser irradiation is canceled by the feedback control, and measurement cannot be performed. On the other hand, since the voltage is always kept constant, the voltage does not fluctuate during measurement, and fluctuations in the current flowing through the sample 4 due to factors other than defects inside the sample 4 can be suppressed and detection with high accuracy is possible. become.
[0025]
Thereafter, the measurement operation is started (at the time point (e) in the figure). The laser beam emitted from the laser generation unit 1 is raster-scanned in a two-dimensional direction by the laser scanning unit 2, collected by the microscope 3, and irradiated onto the sample 4. When the laser beam is irradiated to the defective part of the sample 4 (at the time of (f) in the figure), the temperature of the irradiated part rises due to poor heat conduction to the surroundings, and the internal resistance value increases. The current flowing through the sample is reduced. In a portion having no defect, the temperature of the irradiated portion does not increase due to heat conduction to the surroundings, and the resistance value does not change, so the current value does not change. By processing this with the information of the scanning position irradiated with the laser, the signal processing unit 10 can display the defect position of the sample 4 on the monitor 11 as a luminance change.
[0026]
【The invention's effect】
As described above, according to the present invention, by adding a resistor having an arbitrary resistance value in series to the sample, even if the internal resistance of the sample is small, the operational amplifier for detecting the current value flowing through the sample is provided. The amplification factor can always be set to an appropriate range. For this reason, it has become possible to perform a measurement with a good S / N ratio even for a sample having an internal resistance of about several Ω, which has conventionally been too high. In addition, it has a function to automatically determine the appropriate voltage value to be applied to the sample during measurement and maintain the applied voltage during measurement at this voltage value, so there is little unnecessary fluctuation in the current flowing through the sample. Highly accurate measurement is possible.
[Brief description of the drawings]
FIG. 1 is a block diagram of one embodiment of the present invention.
FIG. 2 is a diagram showing noise characteristics obtained by comparing and measuring samples having different internal resistance values in the apparatus according to FIG. 1;
FIG. 3 is an operation timing chart of the apparatus according to FIG. 1;
FIG. 4 is a block diagram of a first conventional example.
FIG. 5 is a block diagram of a second conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Laser generating part, 2 ... Laser scanning part, 3 ... Microscope, 4 ... Sample to be measured, 5 ... Voltage source, 6 ... Variable resistance means, 7 ... Current / voltage conversion amplifier, 8 ... Operational amplifier, 9 ... Feedback resistance DESCRIPTION OF SYMBOLS 10 ... Signal processing part, 11 ... Monitor, 12 ... Voltage holding means, 13 ... Operational amplifier, 14 ... Voltage source, 15 ... Upper limit voltage limiter circuit, 16 ... Lower limit voltage limiter circuit

Claims (2)

In a current value change measuring apparatus for inspecting a defective portion of the sample by irradiating a sample such as a semiconductor integrated circuit energized while scanning with a laser beam and measuring a change in the current value of the sample accompanying the irradiation,
A variable resistance means that will be connected to the sample series,
The variable resistance means, a current / voltage conversion amplifier connected in series to the sample, and converting the current flowing in the sample into a voltage signal and outputting the voltage signal;
A first voltage source for applying a voltage proportional to an input voltage to a series circuit comprising the variable resistance means, the sample, and the current / voltage conversion amplifier;
The first voltage source is connected to the first voltage source and outputs the input voltage value to the first voltage source as it is, and the voltage value held while holding the input voltage value is the first voltage source. Voltage holding means for switching between two types of operations to be output to
A second voltage source capable of adjusting the output voltage value;
An operational amplifier connected to the voltage holding means for comparing the second voltage source and the output voltage of the current / voltage conversion amplifier;
Before the laser beam is irradiated, the voltage holding means is feedback-controlled to determine the internal resistance of the sample while maintaining the current value flowing through the sample constant, so that the resistance value of the series circuit becomes a predetermined value or more. A signal processing unit that sets the resistance of the variable resistance means and keeps the voltage holding means in a holding operation when the laser beam is irradiated, and measures a change in current value,
A current value change measuring apparatus comprising:   2. The current value change measuring device according to claim 1, wherein the variable resistance means has a resistance value which can be continuously changed from 0 to 50 [Omega].

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