JPH09178800A - Method for evaluating insulation layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate dielectric breakdown with the same degree as that of the TDDB(time dependent dielectric breakdown) method quickly in any case. SOLUTION: A dielectric breakdown field strength is obtained for all insulation layers 2 to be evaluated, each dielectric breakdown field strength is compared with a threshold field strength, for example, 8MV/cm, a ratio (conforming ratio) where the dielectric breakdown field strength is equal to or more than a threshold field strength is calculated, and the ratio is determined as a primary conforming ratio. When the calculation of the primary conforming ratio is completed, it is judged whether the calculation of the conforming rate is performed for a specific number of times or not, the operation until the calculation of the conforming ratio is repeated until it is judged that the calculation of the conforming ratio has been performed for a specific number of times, and a secondary conforming ratio, a third conforming ratio,... are determined. Then, an insulation layer 2 is evaluated based on the primary conforming ratio, the secondary conforming ratio,....

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 the withstand voltage and durability of an insulating layer provided on a semiconductor element, or the quality of an insulating layer.

[0002]

2. Description of the Related Art An insulating layer provided on a semiconductor element such as an oxide layer of a MOS capacitor or a gate oxide layer of a transistor formed on a semiconductor substrate is reduced in thickness as the density of the semiconductor element increases. Since it is difficult to reduce the power supply voltage, the insulating layer is used under high electric field strength. Therefore, it is important to evaluate the insulating layer in detail in order to secure the reliability of the semiconductor element.

As such an evaluation method, there is a dielectric breakdown voltage measuring method. In this method, the voltage applied to each of the insulating layers of the plurality of semiconductor elements formed on the semiconductor substrate is stepwise increased, and the voltage when a predetermined current is detected is used as the dielectric breakdown voltage. The breakdown electric field strength represented by (dielectric breakdown voltage / thickness of insulating layer) is not less than a predetermined value, for example, 8 MV / cm.
The above insulating layers are regarded as good, and those not so are regarded as defective. Then, the superiority or inferiority of the insulating layer is evaluated based on the ratio of the good number to the total applied voltage. However, although such a dielectric breakdown voltage measuring method can perform evaluation in a short time, it has a problem that it cannot perform evaluation according to a usage state of a semiconductor element, that is, evaluation with time.

Therefore, a constant voltage is continuously applied to the insulating layer, a current is detected at a predetermined time interval to obtain a change over time, and the time until the dielectric breakdown and its progress are described in detail. TDDB (Time Dependent Dilect)
The ric breakdown method has been developed, and as an improved method of the TDDB method, a method of stepwise boosting the applied voltage and applying the Chen-Holland-Hu model is disclosed in Japanese Patent Laid-Open No. 6-34704. There is.

FIG. 6 is a block diagram showing the structure of an apparatus used for carrying out the evaluation method, and 11 in the drawing is a conductive substrate. An insulating layer 12 is formed on the substrate 11, and an electrode 13 is formed on the insulating layer 12. The electrode 13 is connected to one terminal of a variable power source 15 that can change the magnitude of the applied voltage stepwise, and the other terminal of the variable power source 15 is connected to the substrate 11 via an ammeter 14. Then, the variable power source 15 is stepwise boosted from a predetermined voltage near 0 V and is applied to the insulating layer 12 a plurality of times for a predetermined time until dielectric breakdown occurs.

The detection current I of the ammeter 14 is supplied to the comparator 17.
It is. If the value exceeds that value, the comparator 17
Threshold current generation that generates a threshold current that determines that breakdown has occurred
The electric field strength applied from the raw device 18 is 6 MV / cm or less.
If the threshold current J_CR1However, the applied electric field strength is 6M
Threshold current J when V / cm is exceeded_CR2(J_CR1<J
_CR2) Is given, the comparator 17
Current J_CR1Or threshold current J_CR2And the detection current I
You. As a result, the applied voltage was 6 MV / cm or less, and
_CR1<I, the breakdown is in the insulating layer 12
Initial defects such as pinholes or defects such as unevenness of the insulating layer 12
It is judged that it was caused by. On the other hand, the applied voltage is 6M
V / cm is exceeded, J_CR2<If I
Edge breakdown exceeded withstand voltage in a defect-free insulating layer
It is judged that it was caused by.

Then, the number of application times until dielectric breakdown,
Using the applied time and the applied electric field strength, Chen-Hol
The insulation layer breakdown life is obtained based on the land-Hu model, and the Weibull distribution is applied to the obtained insulation layer breakdown life to obtain TDDB.
A curve is calculated, and an insulating layer is calculated based on the calculated TDDB curve.
Evaluate 12 breakdowns over time. Since the so-called acceleration test is carried out and the TDDB curve is calculated by increasing the applied voltage stepwise as described above, the TDDB curve can be obtained in a shorter time than the conventional TDDB method described above.

[0008]

However, in the conventional evaluation method, the TDDB curve is obtained by calculation based on the Chen-Holland-Hu model.
An accurate TDDB curve cannot be obtained in the case of dielectric breakdown that does not conform to the model, especially in the case where an initial defective region exists.
There was a problem that the reliability of evaluation was low. The present invention has been made in view of the above circumstances, and an object thereof is to perform an operation of stepwise increasing an applied voltage to detect a dielectric breakdown electric field a plurality of times with respect to an insulating layer of the same semiconductor element. By doing, TDD can be done in a short time in any case.
An object of the present invention is to provide a method for evaluating an insulating layer, which can evaluate dielectric breakdown to the same extent as the method B.

[0009]

According to a first aspect of the present invention, there is provided an insulating layer evaluation method, wherein a voltage applied to an insulating layer provided on a semiconductor element is stepwise increased to detect a dielectric breakdown electric field strength. In the method of evaluating an insulating layer based on the breakdown electric field strength, the detection of the breakdown electric field strength is performed a plurality of times for the insulating layer of the same semiconductor element.

According to a second aspect of the present invention, there is provided an insulating layer evaluation method in which a voltage applied to an insulating layer provided on a plurality of semiconductor elements formed on a semiconductor substrate is stepwise increased to detect a dielectric breakdown electric field strength. In the method for evaluating an insulation layer based on the insulation breakdown electric field strength, the step of respectively detecting the insulation breakdown electric field strength with respect to the insulation layers of a plurality of semiconductor elements, each detected insulation breakdown electric field strength and a predetermined threshold value. And a step of calculating the ratio of the number of semiconductor elements whose dielectric breakdown field strength exceeds a threshold value with respect to the number of semiconductor elements, and repeating each of these steps a plurality of times It is characterized in that the insulating layer is evaluated based on the ratio of.

In the present invention, the operation of stepwise increasing the applied voltage and detecting the dielectric breakdown electric field strength is performed a plurality of times on the insulating layer of the same semiconductor element, so that an excessive amount of voltage is applied to the insulating layer. Repeat stress. As a result, when the insulating layer has a slight distortion or a non-uniform portion that causes dielectric breakdown when a voltage is applied for a long time, the first load has a predetermined dielectric breakdown electric field strength. However, due to repeated overstressing, they are expanded to defects and detected as a sharp drop in the dielectric breakdown field strength. Since the evaluation is performed based on the actual measurement value instead of the calculation as described above, the evaluation equivalent to the TDDB method can be performed in a short time in any case.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of an apparatus used for carrying out the evaluation method according to the present invention,
In the figure, reference numeral 1 denotes a substrate. A plurality of semiconductor elements having insulating layers 2, 2, ... And electrodes 3, 3, ... Are formed in a predetermined pattern on the substrate 1. The lower end of a probe 7 supported so as to be movable back and forth and right and left is in contact with the electrode 3, and the movement of the probe 7 is controlled by a controller 8.

The upper end of the probe 7 is connected to one terminal of the variable power source 5 which can change the magnitude of the applied voltage stepwise, and the other terminal of the variable power source 5 is connected to the substrate 1. The control device 8 controls the on / off of the variable power source 5. Further, a voltmeter 6 for measuring the applied voltage of the variable power source 5 is connected in parallel, and an ammeter 4 is interposed between the probe 7 and the variable power source 5. The measured values of the ammeter 4 and the voltmeter 6 are given to the controller 8.

FIG. 2 is a block diagram showing a main configuration of the control device 8 shown in FIG. In the figure, 82 is the probe 7 described above.
(See FIG. 1) A probe control unit that controls movement, 83 a detection unit that detects the dielectric breakdown electric field strength, and 84 a power supply control unit that controls on / off of a variable power supply. These operations are performed by a central control unit. Controlled by 81. Also, the central control unit
In 81, a first memory 85 that stores the dielectric breakdown electric field strength detected by the detection unit 83 is compared with the dielectric breakdown electric field strength stored in the first memory 85 and a preset threshold electric field strength, The calculation unit 86 for calculating the ratio (non-defective product rate) of the electric field strength equal to or higher than the threshold electric field strength and the second memory 87 for storing the non-defective product ratio calculated by the calculation unit 86 are connected to each other, and the central control unit 81 Also controls these actions.

In the probe control unit 82, the position and the detection order of the semiconductor element for detecting the dielectric breakdown electric field strength among the insulating layers 2, 2, ... Of the semiconductor element formed in a predetermined pattern are set, In accordance with a command from the central controller 81, the probe controller 82 moves the probe 7 to the next site of the insulating layer 2 when the detection of the dielectric breakdown field strength of the insulating layer 2 is completed,
The electrode 3 formed on the insulating layer 2 is contacted. When the probe 7 is brought into contact with the electrode 3, the power supply control unit 84 turns on the variable power supply 5 to apply a voltage from the low voltage side to the high voltage side for a predetermined time.

The thickness of the insulating layers 2, 2, ... And the threshold current are preset in the detection unit 83. The detection unit 83 compares the detection current of the ammeter 4 with the threshold current, and the former exceeds the latter. At this time, it is determined that the dielectric breakdown has occurred, the dielectric breakdown electric field strength is obtained from the detected voltage of the voltmeter 6 and the insulating layers 2, 2, ... At that time, and stored in the first memory 85. Also, the detection unit
When it is determined by 83 that the dielectric breakdown has occurred, the power supply control unit 84 turns off the variable power supply 5. Such an operation is performed for all the insulating layers 2, 2, ... Then, the same operation as before is repeated a predetermined number of times.

Each time the above-mentioned operation is performed once, the calculation unit 86 calculates each dielectric breakdown electric field strength stored in the first memory 85,
The non-defective rate is calculated by comparing with a preset threshold electric field strength and stored in the second memory 87. And the output section
Reference numeral 88 outputs the plurality of non-defective product rates stored in the second memory 87 to a printer, a monitor, or the like.

FIG. 3 is a flow chart showing an insulating layer evaluation procedure according to the evaluation method of the present invention. The probe 7 is brought into contact with the predetermined electrode 3 (step S1), and the variable power source 5
To start applying voltage (step S2),
The value of the current flowing through the insulating layer 2 is detected by the ammeter 4 (step S3). The detected current detected while the voltage is applied for a predetermined time is compared with a preset threshold current, for example, 1 μA (step S4). If the detected current is less than or equal to the threshold current, the process returns to step S2 and the voltage is increased by one step. Is applied.
Then, when the detected current exceeds the threshold current, the voltage application is stopped (step S5), and the breakdown electric field strength is obtained from the voltage at that time and the thickness of the insulating layer 2 measured in advance (step S6).

When the breakdown electric field strength is obtained, the electrode 3
Is the electrode related to the last insulating layer 2 to be detected (step S7), and the operations from step S1 to step S7 are repeated until it is determined that this is the case. Then, when the dielectric breakdown electric field strength is obtained for all the insulating layers 2 to be evaluated, each dielectric breakdown electric field strength is compared with the threshold electric field strength, for example, 8 MV / cm (step S8), and the non-defective rate is calculated ( In step S9), it is set as the primary non-defective rate.

When the calculation of the primary non-defective rate is completed, it is judged whether or not the non-defective rate has been calculated a predetermined number of times (step S
10), the operations from step S1 to step S10 are repeated until it is determined that they are, and the secondary non-defective rate,
The third-order non-defective rate, ... Then, the insulating layer 2 is evaluated based on the primary non-defective rate, the secondary non-defective rate, ... (Step S1
1). In this way, the operation of applying a high voltage stepwise to detect the dielectric breakdown electric field is performed a plurality of times on the same insulating layer 2, thereby repeatedly applying excessive stress to the insulating layer 2 to apply a voltage. When there are slight strains or non-uniform portions in the insulating layer that cause dielectric breakdown when applied for a long time, these are expanded to defects and detected as dielectric breakdown. Therefore, the same evaluation as the above-mentioned TDDB method can be performed in a short time.

[0021]

EXAMPLES Next, the results of comparison tests will be described. FIG. 4 is a graph showing the measurement result by the method of the present invention, and FIG. 5 is a graph showing the measurement result by the conventional TDDB method. For the sample, the surface of the wafer obtained by the Czochralski (CZ) method was washed with solutions having different compositions of A to E, and then 600 MOS capacitors were formed. MOS
What formed the capacitor (F) was used. In addition, the surface of an epitaxially grown wafer was cleaned with the composition A and then 600 MOS capacitors were formed, and as a control, the same number of MO capacitors was used without cleaning.
The thing which formed the S capacitor was also used.

In the method of the present invention shown in FIG.
The S capacitors are divided into three groups of 200 each, and the secondary non-defective rate is detected for each group.
○ indicates the primary non-defective product rate, and ● indicates the secondary non-defective product ratio. In the conventional TDDB method shown in FIG.
A voltage was applied so that 00 MV capacitors had a voltage of 11 MV / cm, and the cumulative breakdown rate was plotted against logarithmic time.

As is apparent from FIG. 5, the evaluation result by the conventional TDDB method shows that for a CZ wafer, E≈C≈D>
A>B> F, and A <F for the Epi wafer. In comparison of wafers, CZ wafer << Epi wafer. On the other hand, as is apparent from FIG. 3, the evaluation results according to the present invention show that CZ wafers have C≈D≈E> A.
>B> F, A <F for the Epi wafer, and a CZ wafer << Epi wafer in the wafer comparison. As described above, it is understood that the method of the present invention can perform the same evaluation as the TDDB method.

[0024]

As described above in detail, in the method for evaluating an insulating layer according to the present invention, it is possible to perform a detailed evaluation in the same time as the TDDB method in a short time, and it does not depend on a calculation model. Therefore, the present invention has excellent effects such that any defect in the insulating layer can be accurately evaluated.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of an apparatus used for carrying out an evaluation method according to the present invention.

FIG. 2 is a block diagram showing a main configuration of a control device shown in FIG.

FIG. 3 is a flowchart showing an evaluation procedure according to the present invention.

FIG. 4 is a graph showing the measurement results by the method of the present invention.

FIG. 5 is a graph showing measurement results by a conventional TDDB method.

FIG. 6 is a block diagram showing a configuration of an apparatus used for implementing a conventional evaluation method.

[Explanation of symbols]

 1 substrate 2 insulating layer 3 electrode 4 ammeter 5 variable power supply 7 probe

Claims (2)

[Claims] 1. A voltage applied to an insulating layer provided in a semiconductor element is stepwise increased to detect a dielectric breakdown electric field strength,
A method for evaluating an insulating layer based on the detected dielectric breakdown field strength, wherein the dielectric breakdown field strength is detected a plurality of times with respect to the insulating layer of the same semiconductor element. 2. A dielectric breakdown electric field strength is detected by stepwise increasing a voltage applied to an insulating layer provided on a plurality of semiconductor elements formed on a semiconductor substrate, and the insulating layer is based on the detected dielectric breakdown electric field strength. In the method for evaluating the above, a step of detecting the dielectric breakdown electric field strengths for insulating layers of a plurality of semiconductor elements, a step of comparing the respective detected dielectric breakdown electric field strengths with a predetermined threshold value, and the semiconductor A step of calculating a ratio of the number of semiconductor elements whose dielectric breakdown electric field intensity exceeds a threshold value to the number of elements, repeating each of these steps a plurality of times, and evaluating the insulating layer based on the calculated plurality of ratios. A method for evaluating an insulating layer, comprising: