Temperature interaction identification method for silicon carbide power MOSFET threshold voltage degradation

Abstract

The invention discloses a temperature interaction identification method for threshold voltage degradation of a silicon carbide power MOSFET, and belongs to the technical field of reliability identification of electronic components. The method aims at solving the problem that in the prior art, a temperature interaction analysis method for silicon carbide power MOSFET threshold voltage degradation does not exist. The method comprises the following steps: firstly, carrying out an accelerated degradation test to obtain threshold voltage degradation data for temperature interaction identification, and then eliminating measurement abnormal fluctuation and measurement errors; fitting each group of sample data with the temperatures of Ti and Tj in the constant-temperature accelerated degradation test and each group of sample data at the second stage of the two-stage temperature accelerated degradation test respectively; and then statistical test is performed on the degradation path data, and whether degradation is influenced by temperature interaction is judged according to a statistical test result. The method is used for identifying the temperature interaction of MOSFET threshold voltage degradation.

Description

Technical Field

[0001] The invention belongs to the technical field of electronic component reliability identification, and in particular relates to a temperature interaction identification method for voltage degradation of an electronic semiconductor device. Background Art

[0002] As a key semiconductor device in modern electronic devices, silicon carbide power MOSFET (insulated gate field effect transistor) plays a vital role. Its superior switching characteristics and low on-resistance make it an ideal choice for high-efficiency power management and signal processing, and it is widely used in power conversion, motor drive, lighting control and electric vehicles. With the development of intelligent and automation technology, the application of silicon carbide power MOSFET in power electronic systems is becoming more and more important, and its performance degradation directly affects the reliable operation of equipment and systems.

[0003] At present, the research on the impact of threshold voltage degradation on SiC power MOSFET is not comprehensive, resulting in poor prediction of MOSFET threshold voltage degradation. Summary of the invention

[0004] The present invention aims to solve the problem that there is no temperature interaction analysis method for threshold voltage degradation of silicon carbide power MOSFET in the prior art.

[0005] A method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET, comprising:

[0006] Step 1: Conduct accelerated degradation tests on SiC power MOSFETs to obtain threshold voltage degradation data for temperature interaction identification. The specific process includes the following steps:

[0007] Step 11: Determine the turn-on signal voltage, turn-off signal voltage, and signal frequency of the MOSFET according to the actual drive signal parameters of the MOSFET in the circuit; determine the accelerated degradation test temperature range;

[0008] Step 12: Set n groups of constant temperature accelerated degradation tests; according to step 11, set the voltage and frequency of the MOSFET on / off signals in each group to be the same; in the test, the drain and source of the MOSFET are short-circuited, and the drive signal is applied only to the gate; select n temperatures in the accelerated degradation test temperature range, which are recorded as T1 to T n ;

[0009] Step 13: Design the duration and test frequency of the constant temperature accelerated degradation test and conduct the test;

[0010] Step 14: At T1 to T n Select two temperatures T i With T j;

[0011] Step 15: Set up two sets of two-stage temperature accelerated degradation tests; in the two-stage temperature accelerated degradation test set 1, T i , then apply T j ; In the two-stage temperature accelerated degradation test group 2, T j , then apply T i ;Other test settings are the same as S12;

[0012] Step 16: Design the duration and test frequency of the two-stage temperature accelerated degradation test, and carry out the two-stage temperature accelerated degradation test;

[0013] Step 2: Processing the threshold voltage degradation data obtained in step 1 by fitting and smoothing methods;

[0014] The temperature in the constant temperature accelerated degradation test is T i With T j The sample data of each group and the sample data of each group in the second stage of the two-stage temperature accelerated degradation test are fitted respectively:

[0015] V th (t)=a1·exp(a2·t)+a3·exp(a4·t) (1)

[0016] Among them, V th (t) is the value of the threshold voltage at time t, a1, a2, a3, a4 are the parameters that need to be determined by fitting;

[0017] Step 3: Perform statistical tests on the processed degradation path data and determine whether the degradation is affected by temperature interaction based on the statistical test results.

[0018] Furthermore, in step 1, it is necessary to determine the accelerated degradation test temperature range, during which the temperature operating range specified in the MOSFET parameter manual is determined as the accelerated degradation test temperature range.

[0019] Furthermore, the test duration designed in step 13 should ensure that the mean value of the threshold voltage of the test samples in each group can be degraded by more than 10% of the initial value mean value.

[0020] Furthermore, the test frequency designed in step 13 should ensure that the number of tests for each group during the test is greater than 10 times.

[0021] Furthermore, the process of designing the duration and test frequency of the two-stage temperature accelerated degradation test and conducting the two-stage temperature accelerated degradation test in step 16 includes the following steps:

[0022] In the two-stage temperature accelerated degradation test group 1, T i, when the mean value of the threshold voltage of the test sample degrades by P% of the initial mean value, the application duration is t s1 T j ;

[0023] In the two-stage temperature accelerated degradation test group 2, T j , when the mean value of the threshold voltage of the test sample degrades by P% of the initial mean value, the application duration is t s2 T j .

[0024] Furthermore, the P% is set to 5%.

[0025] Furthermore, the application time of the second stage temperature of the two-stage temperature accelerated degradation test group 1 and the two-stage temperature accelerated degradation test group 2 should ensure that at least 10 test points can be obtained in each group.

[0026] Furthermore, the specific process of step 3 includes the following steps:

[0027] Step 31: Calculate the average threshold voltage degradation V of the test samples in Group 1 and Group 2 at the end of the first stage of the two-stage temperature accelerated degradation test th1 and V th2 .

[0028] Step 32: Determine the average degradation of samples in G1 as V th1 The time is recorded as t1, and the average degradation of samples in G2 is determined as V th2 The time is recorded as t2;

[0029] Step 33: After time t1 in G1, the time length is t s1 The degradation curve of each sample is recorded as S1; after time t2 in G2, the time length is t s2 The degradation curve of each sample is recorded as S2;

[0030] Among them, G1 is the temperature T in the constant temperature accelerated degradation test in step 2. j G2 is the degradation curve of each sample obtained by group, and the temperature is T in the constant temperature accelerated degradation test in step 2. i The degradation curves of each sample obtained by the group;

[0031] Step 34: Perform Mann-Whitney U test on the degradation data of groups S1 and C1 at the same time, and perform Mann-Whitney U test on the degradation data of groups S2 and C2 at the same time, and obtain two p-value curves. Determine the distribution difference of the corresponding two groups of samples based on p in the curve, and then determine the interaction between temperature levels.

[0032] Wherein, C1 is the degradation curve of each sample obtained by fitting the two-stage temperature accelerated degradation test group 1 in step 2, and C2 is the degradation curve of each sample obtained by fitting the two-stage temperature accelerated degradation test group 2 in step 2.

[0033] Furthermore, the distribution difference between the two groups of samples is determined according to p in the curve, and in the process of determining the interaction between the temperature levels, the distribution difference between the two groups of samples corresponding to the part of the curve where p < the discrimination threshold exists, that is, there is an interaction between the temperature levels.

[0034] Furthermore, the discrimination threshold is 0.05.

[0035] Effect of the invention:

[0036] The present invention provides an effective solution for determining whether a silicon carbide power MOSFET is affected by temperature interaction and thus degrades at a dynamic temperature, and can accurately identify potential temperature interactions. The present invention can serve the development of new degradation accumulation calculation methods and improve the accuracy of reliability prediction. The present invention can quantitatively describe the degree to which threshold voltage degradation is affected by temperature interaction, and can be extended to a wide range of electronic component types. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is the flow chart of temperature interaction identification of MOSFET threshold voltage degradation;

[0038] Figure 2 is a constant temperature accelerated degradation test data diagram in the embodiment;

[0039] Figure 3 It is a two-stage temperature accelerated degradation test data diagram in the embodiment;

[0040] Figure 4 is a temperature interaction identification diagram of two-stage temperature group 1 in the embodiment;

[0041] Figure 5 2 is a temperature interaction identification diagram of the two-stage temperature group 2 in the embodiment. DETAILED DESCRIPTION

[0042] In the process of studying the reliability of silicon carbide power MOSFET, the present invention found that silicon carbide power MOSFET usually works under complex and changeable temperature conditions. The premise of accurately predicting the degradation amount is to determine whether there is interaction between different temperature levels, which will have a serious impact on the degradation path. Therefore, a temperature interaction identification method for threshold voltage degradation of silicon carbide power MOSFET is proposed. It is described in detail below in conjunction with specific embodiments.

[0043] Specific implementation method 1: Combination Figure 1 To explain this embodiment,

[0044] This embodiment is a method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET, which specifically includes the following steps:

[0045] Step 1: Conduct accelerated degradation tests on SiC power MOSFETs to obtain threshold voltage degradation data for temperature interaction identification. The specific process includes the following steps:

[0046] Step 11: Determine the turn-on signal voltage, turn-off signal voltage, and signal frequency of the MOSFET according to the actual drive signal parameters of the MOSFET in the circuit; determine the temperature operating range specified in the MOSFET parameter manual as the accelerated degradation test temperature range.

[0047] Step 12: Set n groups (n≥2) of constant temperature accelerated degradation tests. According to step 11, set the voltage and frequency of the MOSFET on / off signals in each group to be the same, as shown in Table 1. Select n temperatures (T1, ..., T2) in the accelerated degradation test temperature range determined in step S11. n ). In the experiment, the drain and source of the MOSFET are short-circuited, and the driving signal is only applied to the gate.

[0048] Table 1 Constant temperature accelerated degradation test settings

[0049] Experimental Group Accelerated test stress setting Constant temperature group 1 <![CDATA[Turn-on signal voltage V1, turn-off signal voltage V2, signal frequency f, temperature T1]]> Constant temperature group 2 <![CDATA[Turn-on signal voltage V1, turn-off signal voltage V2, signal frequency f, temperature T2]]> …… …… Constant temperature group n <![CDATA[Turn-on signal voltage V1, turn-off signal voltage V2, signal frequency f, temperature T n >

[0050] Step 13: Design the duration and test frequency of the constant temperature accelerated degradation test and conduct the test.

[0051] The test duration should ensure that the mean threshold voltage of the test samples in each group can degrade by more than 10% of the initial mean value.

[0052] The test frequency should ensure that the number of tests per group during the test is greater than 10 times.

[0053] Step 14: At T1, …, T n Select two temperatures T i With T j , T i With T j The greater the difference in the threshold voltage degradation paths of the constant temperature accelerated degradation test, the more conducive it is to the identification of temperature interaction.

[0054] Step 15: Set up two sets of two-stage temperature accelerated degradation tests. In the two-stage temperature accelerated degradation test set 1, T i , then apply T j In the two-stage temperature accelerated degradation test group 2, T j , then apply T i The other test settings were the same as those of S12.

[0055] Step 16: Design the duration and test frequency of the two-stage temperature accelerated degradation test and conduct the test. i When the threshold voltage mean of the test sample degrades by about 5% of the initial mean, the application duration is t s1 T j In the two-stage temperature accelerated degradation test group 2, T j When the threshold voltage mean of the test sample degrades by about 5% of the initial mean, the application duration is t s2 T j The application time of the second stage temperature should ensure that at least 10 test points can be obtained in each group.

[0056] Step 2: Process the threshold voltage degradation data obtained in step 1 by fitting and smoothing methods to eliminate abnormal measurement fluctuations and measurement errors;

[0057] The temperature in the constant temperature accelerated degradation test is T i With T j The sample data of each group and the sample data of each group in the second stage of the two-stage temperature accelerated degradation test are fitted respectively:

[0058] V th (t)=a1·exp(a2·t)+a3·exp(a4·t) (1)

[0059] Among them, V th (t) is the value of the threshold voltage at time t, and a1, a2, a3, and a4 are parameters that need to be determined through fitting.

[0060] The degradation curves of each sample obtained by fitting the two-stage temperature accelerated degradation test group 1 are recorded as C1, and the degradation curves of each sample obtained by fitting the two-stage temperature accelerated degradation test group 2 are recorded as C2. The temperature in the constant temperature accelerated degradation test is T j The degradation curves of each sample obtained by the group are recorded as G1, and the temperature in the constant temperature accelerated degradation test is T i The degradation curve of each sample obtained by the group is recorded as G2.

[0061] Step 3: Perform statistical tests on the degradation path data after S2 processing, and determine whether the degradation is affected by temperature interaction based on the statistical test results; the specific process includes the following steps:

[0062] Step 31: Calculate the average threshold voltage degradation amount V of the test samples in the two-stage temperature accelerated degradation test group 1 and the two-stage temperature accelerated degradation test group 2 at the end of the first stage of the two-stage temperature accelerated degradation test. th1 and V th2 .

[0063] Step 32: Determine the average degradation of samples in G1 as V th1 The time is recorded as t1, and the average degradation of samples in G2 is determined as V th2 The time is recorded as t2.

[0064] Step 33: After time t1 in G1, the time length is t s1 The degradation curve of each sample is recorded as S1; after time t2 in G2, the time length is t s2 The degradation curve of each sample is recorded as S2.

[0065] Step 34: Perform Mann-Whitney U test on the degradation data of groups S1 and C1 at the same time, and perform Mann-Whitney U test on the degradation data of groups S2 and C2 at the same time, and obtain two p-value curves. The part of the curve where p < the discrimination threshold (0.05 in this embodiment) corresponds to the distribution of the two groups of samples with significant differences, that is, there is a temperature level interaction.

[0066] It should be noted that: the present embodiment is for silicon carbide power MOSFET, and the determined identification threshold is 0.05. The present invention can actually also be used for temperature interaction identification of threshold voltage degradation of other components, and the specific identification threshold is determined according to actual conditions.

[0067] Example:

[0068] This embodiment uses a certain type of silicon carbide NMOSFET as an object for temperature interaction identification, and its operating temperature is -55°C to +150°C.

[0069] Step 1: Conduct accelerated degradation tests to obtain threshold voltage degradation data for temperature interaction identification;

[0070] Step 11: Determine the turn-on signal voltage, turn-off signal voltage, and signal frequency of the MOSFET according to the actual drive signal parameters of the MOSFET in the circuit, which are 20V, -5V, and 100KHz respectively; determine the temperature operating range specified in the MOSFET parameter manual as the accelerated degradation test temperature range, which is -55℃~+150℃.

[0071] Step 12: Set up two groups of constant temperature accelerated degradation tests, and the specific stress settings are shown in Table 2. In the test, the drain and source of the MOSFET are short-circuited, and the driving signal is only applied to the gate. There are 10 MOSFETs in each group.

[0072] Table 2 Constant temperature accelerated degradation test settings

[0073]

[0074] Step 13: Design a constant temperature accelerated degradation test for 3000 hours. The average of the two threshold voltages can degrade by more than 10% of the initial average. The test is measured every 20 hours. The test results are as follows: Figure 2 shown.

[0075] Step 14: T i =25℃, T j =120℃.

[0076] Step 15: Set up two sets of two-stage temperature accelerated degradation tests. In the two-stage temperature accelerated degradation test set 1, T i , then apply T j In the two-stage temperature accelerated degradation test group 2, T j , then apply T i The other test settings are the same as S12, with 10 MOSFETs in each group.

[0077] Step 16: Also with 20 hours as the test interval, Group 1 first applies 25°C. After 1120 hours, the mean threshold voltage degradation of the test samples reaches 5% of the initial mean value. Then the duration of the application is t s1 = 120°C for 880 hours. In group 2, 120°C was first applied, and the mean threshold voltage degradation of the test samples reached 5% of the initial mean value after 200 hours. Then the application duration was t s2 = 800 hours at 25°C, the results are as follows Figure 3 shown.

[0078] Step 2: The threshold voltage degradation data obtained in step 1 is processed by fitting and smoothing methods to eliminate abnormal measurement fluctuations and measurement errors.

[0079] The temperature in the constant temperature accelerated degradation test is T i With T j The data of each group of samples and each group of samples in the second stage of the two-stage temperature accelerated degradation test are fitted:

[0080] V th (t)=a1·exp(a2·t)+a3·exp(a4·t) (2)

[0081] Among them, V th (t) is the value of the threshold voltage at time t, and a1, a2, a3, and a4 are parameters that need to be determined by fitting. The degradation curves of each sample obtained by fitting the two-stage temperature accelerated degradation test group 1 are recorded as C1, and the degradation curves of each sample obtained by fitting the two-stage temperature accelerated degradation test group 2 are recorded as C2. The temperature in the constant temperature accelerated degradation test is T j=120℃ group degradation curves of each sample are recorded as G1, and the temperature in the constant temperature accelerated degradation test is T i The degradation curves of each sample obtained in the group with temperature = 25°C are recorded as G2.

[0082] Step 3: Perform statistical tests on the processed degradation path data and determine whether the degradation is affected by temperature interaction based on the statistical test results:

[0083] Step 31: Calculate the average threshold voltage degradation V of the test samples in Group 1 and Group 2 at the end of the first stage of the two-stage temperature accelerated degradation test th1 =4.59V and V th2 =4.49V.

[0084] Step 32: The average degradation of samples in G1 is V th1 The time is recorded as t1 = 149 hours, and the average degradation of samples in G2 is V th2 The time is recorded as t2 = 301 hours.

[0085] Step 33: After time t1 = 149 hours in G1, the time length is t s1 = 880 hours of degradation curves of each sample is recorded as S1; after time t2 = 301 in G2, the time length is t s2 The degradation curve of each sample with a time of =800 hours is recorded as S2.

[0086] Step 34: Perform Mann-Whitney U test on the degradation data of S1 and C1 groups at the same time, and perform Mann-Whitney U test on the degradation data of S2 and C2 groups at the same time, and obtain two p value curves. The part of the curve with p<0.05 corresponds to the distribution of the two groups of samples with significant differences. Figure 4 As shown in the figure, the Mann-Whitney U test results of the degradation paths of the two groups S1 and C1 were less than 0.05 after about 580 hours, indicating that there were significant differences in the degradation paths, which means that temperature interaction existed. Compared with the accelerated degradation test group (S1) that was kept at 120°C, the temperature interaction was introduced by first applying 25°C and then 120°C (C1), which affected the degradation, so that the degradation rate was different at the same temperature of 120°C, starting from the average degradation amount of the same sample. Figure 5 As shown in the figure, the Mann-Whitney U test results of the degradation paths of the S2 and C2 groups were less than 0.05 after about 380 hours, indicating that there were significant differences in the degradation paths, which means that there was a temperature interaction. Compared with the accelerated degradation test group (S2) that was kept at 25°C, the temperature interaction was introduced by applying 120°C first and then 25°C, which affected the degradation, so that the degradation rate was different at the same temperature of 25°C, starting from the average degradation amount of the same sample.

[0087] The above calculation examples of the present invention are only used to explain the calculation model and calculation process of the present invention in detail, and are not intended to limit the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET, characterized in that: include: Step 1: Conduct accelerated degradation tests on SiC power MOSFETs to obtain threshold voltage degradation data for temperature interaction identification. The specific process includes the following steps: Step 11: Determine the turn-on signal voltage, turn-off signal voltage, and signal frequency of the MOSFET according to the actual drive signal parameters of the MOSFET in the circuit; determine the accelerated degradation test temperature range; Step 12: Set n groups of constant temperature accelerated degradation tests; according to step 11, set the voltage and frequency of the MOSFET on / off signals in each group to be the same; in the test, the drain and source of the MOSFET are short-circuited, and the drive signal is applied only to the gate; select n temperatures in the accelerated degradation test temperature range, which are recorded as T1 to T n ; Step 13: Design the duration and test frequency of the constant temperature accelerated degradation test and conduct the test; Step 14: At T1 to T n Select two temperatures T i With T j ; Step 15: Set up two sets of two-stage temperature accelerated degradation tests; in the two-stage temperature accelerated degradation test set 1, T i , then apply T j ; In the two-stage temperature accelerated degradation test group 2, T j , then apply T i ;Other test settings are the same as S12; Step 16: Design the duration and test frequency of the two-stage temperature accelerated degradation test, and carry out the two-stage temperature accelerated degradation test; Step 2: Processing the threshold voltage degradation data obtained in step 1 by fitting and smoothing methods; The temperature in the constant temperature accelerated degradation test is T i With T j The sample data of each group and the sample data of each group in the second stage of the two-stage temperature accelerated degradation test are fitted respectively: V th (t)=a1·exp(a2·t)+a3·exp(a4·t) (1) Among them, V th (t) is the value of the threshold voltage at time t, a1, a2, a3, a4 are the parameters that need to be determined by fitting; Step 3: Perform statistical tests on the processed degradation path data and determine whether the degradation is affected by temperature interaction based on the statistical test results.

2. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 1, characterized in that: In step 1, the accelerated degradation test temperature range needs to be determined. In this process, the temperature operating range specified in the MOSFET parameter manual is determined as the accelerated degradation test temperature range.

3. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 1, characterized in that: The test duration designed in step 13 should ensure that the mean value of the threshold voltage of the test samples in each group can be degraded by more than 10% of the initial value mean value.

4. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 3, characterized in that: The test frequency designed in step 13 should ensure that the number of tests for each group during the test period is greater than 10 times.

5. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 1, characterized in that: The process of designing the duration and test frequency of the two-stage temperature accelerated degradation test and conducting the two-stage temperature accelerated degradation test in step 16 includes the following steps: In the two-stage temperature accelerated degradation test group 1, T i , when the mean value of the threshold voltage of the test sample degrades by P% of the initial mean value, the application duration is t s1 T j ; In the two-stage temperature accelerated degradation test group 2, T j , when the mean value of the threshold voltage of the test sample degrades by P% of the initial mean value, the application duration is t s2 T j .

6. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 5, characterized in that: The P% is set to 5%.

7. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 5, characterized in that: The application time of the second stage temperature of the two-stage temperature accelerated degradation test group 1 and the two-stage temperature accelerated degradation test group 2 should ensure that at least 10 test points can be obtained in each group.

8. A method for identifying temperature interaction of threshold voltage degradation of a silicon carbide power MOSFET according to any one of claims 1 to 7, characterized in that: The specific process of step 3 includes the following steps: Step 31: Calculate the average threshold voltage degradation V of the test samples in Group 1 and Group 2 at the end of the first stage of the two-stage temperature accelerated degradation test th1 and V th2 . Step 32: Determine the average degradation of samples in G1 as V th1 The time is recorded as t1, and the average degradation of samples in G2 is determined as V th2 The time is recorded as t2; Step 33: After time t1 in G1, the time length is t s1 The degradation curve of each sample is recorded as S1; after time t2 in G2, the time length is t s2 The degradation curve of each sample is recorded as S2; Among them, G1 is the temperature T in the constant temperature accelerated degradation test in step 2. j G2 is the degradation curve of each sample obtained by group, and the temperature is T in the constant temperature accelerated degradation test in step 2. i The degradation curves of each sample obtained by the group; Step 34: Perform Mann-Whitney U test on the degradation data of groups S1 and C1 at the same time, and perform Mann-Whitney U test on the degradation data of groups S2 and C2 at the same time, and obtain two p-value curves. Determine the distribution difference of the corresponding two groups of samples based on p in the curve, and then determine the interaction between temperature levels. Wherein, C1 is the degradation curve of each sample obtained by fitting the two-stage temperature accelerated degradation test group 1 in step 2, and C2 is the degradation curve of each sample obtained by fitting the two-stage temperature accelerated degradation test group 2 in step 2.

9. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 8, characterized in that: The distribution difference of the two groups of samples is determined according to p in the curve, and then in the process of judging the interaction of temperature levels, the distribution difference of the two groups of samples corresponding to the part of the curve where p < the discrimination threshold exists, that is, there is an interaction of temperature levels.

10. The method for identifying temperature interaction of threshold voltage degradation of silicon carbide power MOSFET according to claim 9, characterized in that: The discrimination threshold is 0.05.

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