JP3147758B2 - Evaluation method of insulation layer - 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 the withstand voltage and durability of an insulating layer provided on a semiconductor device, and 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 has been reduced in thickness as the density of the semiconductor element has increased. Since it is difficult to lower the power supply voltage, the insulating layer is used under a 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.

[0003] One such evaluation method is a dielectric breakdown voltage measurement method. In this method, the voltage applied to each of the insulating layers of a plurality of semiconductor elements formed on the semiconductor substrate is stepped up, and the voltage when a predetermined current is detected is defined as the breakdown voltage. The dielectric 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-mentioned insulating layer is regarded as good, and the others are regarded as defective. Then, the quality of the insulating layer is evaluated based on the ratio of the good number to the total number of applied layers. However, such a dielectric breakdown voltage measurement method has a problem that although it can be evaluated in a short time, it cannot perform an evaluation according to a use state of a semiconductor element, that is, an evaluation over time.

Therefore, a constant voltage is continuously applied to the insulating layer, a current is detected at predetermined time intervals, a change with time is obtained, and the time until the dielectric breakdown, its progress, and the like are described in detail. TDDB (Time Dependent Dietect)
A method of increasing the applied voltage stepwise and applying a Chen-Holland-Hu model is described in JP-A-6-34704 as an improved method of the TDDB method. I have.

FIG. 6 is a block diagram showing a configuration of an apparatus used for carrying out the evaluation method. In the figure, reference numeral 11 denotes a conductive substrate. An insulating layer 12 is formed on a 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 supply 15 capable of changing the magnitude of the applied voltage stepwise, and the other terminal of the variable power supply 15 is connected to the substrate 11 via an ammeter 14. Then, the variable power supply 15 is stepped up from a predetermined voltage near 0 V in a stepwise manner 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 a comparator 17.
It is. In addition, the comparator 17 is insulated when the value is exceeded.
A threshold current generator that generates a threshold current that determines that breakdown has occurred
The electric field intensity applied from the raw device 18 is 6 MV / cm or less.
If the threshold current J_CR1But the applied electric field strength is 6M
If it exceeds V / cm, the threshold current J_CR2(J_CR1<J
_CR2) Is given, and 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 J
_CR1If <I, the dielectric breakdown exists in the insulating layer 12
Defects such as initial defects such as pinholes or unevenness of the insulating layer 12
Is determined to have occurred. On the other hand, when the applied voltage is 6M
V / cm, J_CR2If <I, the absolute
Edge breakdown exceeds breakdown voltage in defect-free insulating layer
Is determined to have occurred.

Then, the number of times of application until dielectric breakdown occurs,
Using the application time and the applied electric field strength, Chen-Hol
Based on the land-Hu model, the insulation layer breakdown life was obtained, and the Weibull distribution was applied to the obtained insulation layer breakdown life to obtain TDDB.
Calculating an insulating layer based on the calculated TDDB curve;
Evaluate the 12 dielectric breakdowns over time. Since the so-called acceleration test is performed and the TDDB curve is obtained by calculation by gradually increasing the applied voltage in this manner, the TDDB curve can be obtained in a shorter time than in the above-described conventional TDDB method.

[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 for a dielectric breakdown that does not fit the model, especially when there is an initial failure region,
There was a problem that the reliability of evaluation was low. The present invention has been made in view of such circumstances, and a purpose thereof is to perform an operation of detecting a dielectric breakdown electric field by stepwise increasing an applied voltage a plurality of times with respect to an insulating layer of the same semiconductor element. By doing so, in any case, TDD in a short time
An object of the present invention is to provide a method for evaluating an insulating layer, which can evaluate dielectric breakdown to the same degree as the method B.

[0009]

According to the method for evaluating an insulating layer according to the present invention , a voltage applied to an insulating layer provided on a plurality of semiconductor elements formed on a semiconductor substrate is stepwise increased to increase a breakdown field strength. A plurality of semiconductor elements in a method of detecting an insulation layer based on the detected breakdown electric field strength.
The breakdown electric field strength of each of the
And the detected breakdown electric field strength
Comparing each with a determined threshold value;
Breakdown electric field strength exceeds the threshold for the number of body elements
Calculating a ratio of the number of semiconductor elements.
Each of these steps is repeated multiple times,
It is characterized in that the insulating layer is evaluated based on the ratio .

[0010]

In the present invention, the operation of detecting the breakdown electric field strength by stepwise increasing the applied voltage is performed on the insulating layer of the same semiconductor element a plurality of times, so that an excessively large voltage is applied to the insulating layer. Apply stress repeatedly. As a result, when a slight distortion or non-uniform portion or the like that causes dielectric breakdown when a voltage is applied for a long time is present in the insulating layer, the first load has a predetermined dielectric breakdown electric field strength. However, due to the repetitive load of excessive stress, they are extended to defects and detected as a sharp decrease in the breakdown electric field strength. As described above, since the evaluation is performed not based on the calculation but on the basis of the actually measured value, the evaluation equivalent to the TDDB method can be performed in a short time in any case.

[0012]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a schematic diagram showing a configuration of an apparatus used for performing an evaluation method according to the present invention.
In the figure, reference numeral 1 denotes a substrate. On the substrate 1, a plurality of semiconductor elements having insulating layers 2, 2,... And electrodes 3, 3,. The lower end of a probe 7 supported movably back and forth and left and right is in contact with the electrode 3, and the movement of the probe 7 is controlled by a control device 8.

The upper end of the probe 7 is connected to one terminal of a variable power supply 5 capable of changing the magnitude of the applied voltage stepwise, and the other terminal of the variable power supply 5 is connected to the substrate 1. ON / OFF of the variable power supply 5 is controlled by the control device 8. A voltmeter 6 for measuring the applied voltage of the variable power supply 5 is connected in parallel, and an ammeter 4 is interposed between the probe 7 and the variable power supply 5. The measured values of the ammeter 4 and the voltmeter 6 are given to the control device 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 the movement of the probe, 83 is a detection unit that detects the breakdown electric field strength, and 84 is a power supply control unit that controls ON / OFF of the variable power supply. It is controlled by 81. Also, the central control unit
The first memory 85 stores the breakdown electric field strength detected by the detection unit 83, and compares the breakdown electric field strength stored in the first memory 85 with a preset threshold field strength. A calculating unit 86 for calculating the ratio (non-defective product ratio) of the electric field intensity being equal to or higher than the threshold electric field intensity and a second memory 87 for storing the non-defective product ratio calculated by the calculating unit 86 are connected. Also controls these actions.

In the probe control section 82, of the insulating layers 2, 2,... Of the semiconductor elements formed in a predetermined pattern, the portions of the semiconductor elements for detecting the breakdown electric field strength and the detection order are set. The probe control unit 82 moves the probe 7 to the next part of the insulating layer 2 when the detection of the breakdown electric field strength of the insulating layer 2 ends according to a command from the central control device 81,
It is brought into contact with the electrode 3 formed on the insulating layer 2. When the probe 7 comes into contact with the electrode 3, the power supply control unit 84 turns on the variable power supply 5 and applies a voltage from a low voltage side to a high voltage side for a predetermined time.

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

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

FIG. 3 is a flowchart showing a procedure for evaluating an insulating layer by the evaluation method according to 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 during application of the voltage for a predetermined time is compared with a preset threshold current, for example, 1 μA (step S4). If the detected current is equal to or less than the threshold current, the process returns to step S2 to increase the voltage by one step. Is applied.
When the detected current exceeds the threshold current, the application of the voltage 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 determined, the electrode 3
Is determined to be the electrode relating 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 breakdown electric field strengths are obtained for all the insulating layers 2 to be evaluated, each breakdown electric field strength is compared with a threshold electric field strength, for example, 8 MV / cm (step S8), and a non-defective rate is calculated (step S8). Step S9), and set it as the primary non-defective rate.

When the calculation of the primary non-defective rate is completed, it is determined whether the non-defective rate has been calculated a predetermined number of times (step S).
10) Repeat steps S1 to S10 until it is determined to be so.
Tertiary 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). As described above, the operation of detecting a breakdown electric field by applying a high voltage stepwise is performed on the same insulating layer 2 a plurality of times, so that excessive stress is repeatedly applied to the insulating layer 2 and the voltage is increased. When a slight distortion or a non-uniform portion or the like that causes dielectric breakdown when applied for a long time is present in the insulating layer, these are expanded to defects and detected as dielectric breakdown. Therefore, the same evaluation as the TDDB method described above can be performed in a short time.

[0021]

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

In the method of the present invention shown in FIG.
The S capacitors were divided into three groups of 200 each, and the secondary non-defective rate was detected for each group.
○ indicates the primary non-defective rate, and ● indicates the secondary non-defective rate. In the conventional TDDB method shown in FIG.
Voltage is applied to the 00 MOS capacitors so as to be 11 MV / cm, and the cumulative destruction rate is plotted against logarithmic time.

As is clear from FIG. 5, the evaluation result by the conventional TDDB method shows that EＣC ≒ D> for the CZ wafer.
A>B> F, and A <F for the Epi wafer. In comparison of wafers, CZ wafer≪Epi wafer. On the other hand, as is clear from FIG. 3, the evaluation result according to the present invention indicates that C ≒ D ≒ E> A for the CZ wafer.
>B> F, and A <F for Epi wafers. In comparison of wafers, CZ wafer≪Epi wafer. Thus, it can be seen that the same evaluation as the TDDB method can be performed by the method of the present invention.

[0024]

As described in detail above, in the method for evaluating an insulating layer according to the present invention, a detailed evaluation can be performed in a short time as well as the TDDB method, and a calculation model is not used. Therefore, the present invention has an excellent effect such that any defect of 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 performing an evaluation method according to the present invention.

FIG. 2 is a block diagram illustrating a main configuration of a control device illustrated in FIG. 1;

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

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

FIG. 5 is a graph showing a measurement result 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]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Insulating layer 3 Electrode 4 Ammeter 5 Variable power supply 7 Probe

Claims (1)

(57) [Claims] A voltage applied to an insulating layer provided on a plurality of semiconductor elements formed on a semiconductor substrate is increased stepwise to detect a breakdown electric field strength, and based on the detected breakdown electric field strength, the insulating layer is provided. Evaluating the dielectric breakdown field strength against the insulating layers of a plurality of semiconductor elements.
Detecting degrees, respectively, and a predetermined threshold and the dielectric breakdown field intensity detected it
And a step of comparing the breakdown electric field strength with respect to the number of the semiconductor elements.
Calculating a ratio of the number of semiconductor elements beyond the the steps
Multiple comprising, where each of these steps a plurality of times repeatedly, calculated
A method for evaluating an insulating layer based on the ratio of the insulating layer .