MEMS chip wafer level packaging vacuum degree detection method and system

Abstract

The invention relates to an MEMS chip wafer level packaging vacuum degree detection method and system, and the method comprises the steps: 1, placing a wafer level packaging MEMS device on a vacuum probe table for calibration, adjusting the vacuum degree of a cavity, and recording the vacuum degree Pc at the moment; recording thermal conductance Gth under different vacuum conditions, and performing linear fitting according to Gth = APc + B to obtain A and B values; (2) placing the MEMS device on a vacuum probe table, and carrying out thermal conductivity pre-test in the same way in the step (1); (3) the resistance value Ri and the resistance value Rf at the maximum current and the minimum current are observed in the test data, and if (Ri-Rf) / Ri is larger than or equal to 8% and smaller than or equal to 12%, the thermal conductance Gth given by a test data table is recorded; otherwise, the current in the thermal conductivity test program is increased or decreased, so that the value is within the range of 8%-12%; recording the thermal conductance Gth given by the final test data table; and (4) calculating the vacuum degree Pd of the device by using the calibrated A and B and # imgabs0 #. The method is suitable for detecting the vacuum degree of all MEMS and other types of chip packaging cavities which can be integrated with the micro-bridge structure micro vacuum gauge.

Description

Technical Field

[0001] The present invention belongs to the field of chip packaging, and particularly relates to a method and system for detecting the vacuum degree of a MEMS chip wafer-level packaging, especially a method and system for detecting the vacuum degree of a pixel structure of an uncooled infrared detector chip integrated with a microbridge structure microbolometer at the wafer level. Background Art

[0002] Wafer-level packaging (WLP) has become an important part of advanced packaging technologies in the semiconductor industry. It can meet many advantages such as small size, light weight, portability, high efficiency, and low price. The significant reduction in its packaging volume leads to a large increase in the pressure of the packaging cavity under the condition of a small gas leakage rate, resulting in a decrease in device performance. In addition, gas leakage will cause the device to be exposed to the air atmosphere, causing corrosion, oxidation, etc. to the device, affecting its normal operation. Therefore, it is particularly important to detect the vacuum degree in the packaging process to ensure that it meets the design requirements.

[0003] The continuous miniaturization of microelectromechanical (MEMS) devices has promoted the rapid development of micro vacuum pressure sensors. So far, a large number of related sensors based on different working principles have been reported, such as thin-film capacitance gauges based on the mechanical properties of thin films, Pirani resistance gauges and thermocouple gauges based on aerodynamics, and cathode ionization gauges based on charged particles. However, devices and detection methods that can simultaneously meet the requirements of small volume, high sensitivity, and high reliability for wafer-level packaging are relatively rare. Summary of the Invention

[0004] The purpose of the present invention is to provide a method and system for detecting the vacuum degree of a MEMS wafer-level packaging chip, and this method can detect and evaluate the packaging vacuum degree of a wafer-level packaging device that can integrate a microbridge structure microbolometer.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for detecting the vacuum degree of a MEMS wafer-level packaging chip includes the following steps:

[0007] ① Place the wafer-level packaged uncooled infrared detector chip with a microbridge structure on a vacuum probe table, lower the glass cover, start pumping vacuum, and pump the vacuum to below 2 mTorr;

[0008] ② Adjust the cavity vacuum degree by inflating. After the reading of the cavity vacuum gauge is stable, record the vacuum degree P c ;

[0009] ③ Move the probe above the microvacuum gauge pad through a microscope and a horizontal knob, and then lower the probe height through a vertical knob until the probe stops when it touches the pad;

[0010] ④Pre - test the thermal conductivity, call the thermal conductivity test program in the system, and test the thermal conductivity under default conditions (the default conditions include: measuring the resistance of the thermistor layer using a current of 0.1 - 0.8 μA with a step size of 0.1 μA).

[0011] ⑤Observe the resistance values R i and R f at the maximum current and the minimum current in the test data table. If 8% ≤ (R i -R f ) / R i ≤ 12%, then record the thermal conductivity G th given in the test data table. If (R i -R f ) / R i is too small or too large, then return to step ④ to increase or decrease the current in the thermal conductivity test program so that this value falls within the range of 8% - 12%. Record the thermal conductivity G th given in the final test data table;

[0012] ⑥Rotate the vertical knob in the reverse direction to raise the probe and leave the micro - vacuum gauge pad;

[0013] ⑦Repeat steps ② - ⑥ so that each of the three ranges of 1 - 10 mTorr, 10 - 100 mTorr, and 100 - 1000 mTorr has at least three data points; perform a linear fit on the series of data (G th1 , P c1 ), (G th2 , P c2 )...(G thn , P c2n ) according to G th = APc + B.

[0014] ⑧Place the wafer - level packaged device integrated with the micro - bridge structure on the vacuum probe stage, perform the thermal conductivity test using step ⑤, and record G th ;

[0015] ⑨According to the A and B values obtained in step ⑦, combined with the thermal conductivity G th recorded in ⑧, calculate the device vacuum degree P from d .

[0016] The beneficial effects of the present invention are:

[0017] (1) It can meet the requirements of small volume, high sensitivity, and high reliability for wafer - level packaging;

[0018] (2) It is applicable to the detection of the vacuum degree of the packaging cavities of all MEMS and other types of chips that can integrate the micro - bridge structure micro - vacuum gauge. Description of the Drawings

[0019] Figure 1 : Schematic diagram of the microbridge structure for thermal conductivity calibration and vacuum degree testing.

[0020] Figure 2 : Schematic diagram of the thermal conductivity - vacuum degree calibration process of the microbridge structure.

[0021] Figure 3 : Schematic diagram of the vacuum degree detection process of the present invention. Specific implementation manner

[0022] The method of the present invention is implemented in a clean room environment with humidity: 40 - 60%RH, temperature: 23 ± 5°C, and the temperature change rate not exceeding 0.5°C / 10min.

[0023] The method of the present invention uses a probe station with a vacuum system, and the vacuum degree in the cavity where the probe station is located can reach 1mTorr. The vacuum system needs to have a valve or flow meter to adjust the vacuum degree in the cavity.

[0024] The pad size of the microbridge structure chip used in the method of the present invention is ≥70×70μm.

[0025] In the method of the present invention, the vacuum degree testing principle is as follows: in the vacuum degree range of 1 - 1000mTorr, the thermal conductivity G th of the micro - vacuum gauge and the device vacuum degree P d are linearly related:

[0026] G th = AP d + B

[0027] By measuring the thermal conductivity G th of the microbridge structure, the device vacuum degree P d can be obtained:

[0028]

[0029] The two constants A and B can be obtained through the thermal conductivity calibration of the microbridge structure.

[0030] In the method of the present invention, the thermal conductivity - vacuum degree calibration principle of the microbridge structure is as follows: at different cavity vacuum degrees P c1 , P c2 ... P cn , measure the thermal conductivity G th1 , G th2 ... G thn . For the data (P c1 , G th1 ), (P c2 , G th2 )... (P cn , G thn)According to formula G th = AP c + B, perform least squares linear fitting to obtain calibration constants A and B.

[0031] In the method of the present invention, the principle of thermal conductivity measurement is as follows: Apply current I to the micro-vacuum gauge, and its resistance value R can be measured. At different currents I1, I 12 , ……, I n , measure the resistance values R1, R2, …… R n . For the data According to the formula lnR = kI 2 R + m, perform least squares fitting to obtain fitting constants k and m.

[0032] Set the resistance temperature coefficient TCR = -0.022 (TCR is set according to the specific test results of the VOx thin film in the microbolometer. For the microbridge structure used in the present invention, its TCR is -0.022). Calculate the thermal conductivity G according to the following formula th :

[0033]

[0034] Refer to Figure 1 As shown, in the present invention, the microbridge structure of the microbolometer used, in addition to including conventional structures such as two metal pillars, a first support layer, a thermistor layer, an electrical connection structure, a first passivation layer, a second support layer, and an absorption layer, also has an additional metal reflection layer (Al) on the topmost layer of the structure. The reason is that when performing thermal conductivity measurement, we apply a constant current I to the microbridge structure. Since VOx has a negative temperature coefficient, the process of the pixel reaching thermal equilibrium can be approximated as: Joule heat is equal to the heat transferred away by thermal conductivity, that is: G th (T - T0) = I 2 R. During this process, the heat generated by pixel absorption is ignored. Therefore, adding a metal reflection layer can reduce the interference of pixel absorption on thermal conductivity calculation.

[0035] As Figure 2 shown, the method of the present invention first needs to calibrate the thermal conductivity - vacuum degree of the microbridge structure, and the steps are as follows:

[0036] ① Place the wafer - level packaged MEMS device with the microbridge structure on the vacuum probe stage, lower the glass cover, start pumping vacuum, and pump the vacuum to below 2 mTorr;

[0037] ② Adjust the cavity vacuum degree by inflating. After the reading of the cavity vacuum gauge is stable, record the vacuum degree P c at this time;

[0038] ③ Move the probe above the micro - vacuum gauge pad through the microscope and the horizontal knob, and then lower the probe height through the vertical knob. Stop when the probe touches the pad;

[0039] ④ Pre - test the thermal conductivity. Call the thermal conductivity test program in the system and test the thermal conductivity using the default conditions.

[0040] ⑤ Observe the resistance values R i and R f at the maximum current and the minimum current in the test data table. If 8% ≤ (R i -R f ) / R i ≤ 12%, then record the thermal conductivity G th given in the test data table. If (R i -R f ) / R i is too small or too large, then increase or decrease the current in the thermal conductivity test program so that this value falls within the range of 8% - 12%. Record the thermal conductivity G th ;

[0041] ⑥ Rotate the vertical knob in the reverse direction to raise the probe and leave the micro - vacuum gauge pad;

[0042] ⑦ Repeat steps ② - ⑥ so that there are at least three data points in each of the three ranges of 1 - 10 mTorr, 10 - 100 mTorr, and 100 - 1000 mTorr; perform a linear fit on the series of data (G th1 , P c1 ), (G th2 , P c2 )…(G thn , P c2n ) according to G th =AP c +B to obtain the values of A and B.

[0043] As Figure 3 shown, the vacuum degree test steps of the packaged device are as follows:

[0044] ① Place the wafer - level packaged MEMS device with a micro - bridge structure on the vacuum probe stage;

[0045] ② Move the probe above the micro - vacuum gauge pad through the microscope and the horizontal knob, and then lower the probe height through the vertical knob. Stop when the probe touches the pad;

[0046] ③ Pre - test the thermal conductivity. Call the thermal conductivity test program in the system and test the thermal conductivity using the default conditions;

[0047] ④ Observe the resistance values R i and R fIf 8% ≤ (R i - R f ) / R i ≤ 12%, record the thermal conductivity G given in the test data table th . If (R i - R f ) / R i is too small or too large, increase or decrease the current in the thermal conductivity test program so that the value falls within the range of 8% - 12%. Record the thermal conductivity G given in the final test data table th ;

[0048] According to the A and B values obtained from the thermal conductivity calibration and the G th obtained from the vacuum degree test, from the formula calculate the vacuum degree P of the packaged device d .

[0049] The above are only embodiments of the present invention and do not impose any form of limitation on the present invention; any person skilled in the art can, without departing from the scope of the technical solution of the present invention, make possible changes and modifications to the technical solution of the present invention using the methods and technical contents disclosed above, or modify it into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for detecting the vacuum degree of a wafer-level package of a MEMS chip, characterized in that, It includes the following steps: ① In a clean room environment, place the MEMS chip on a vacuum probe station, lower the glass cover, and evacuate the air. ②Adjust and record the cavity vacuum degree P c ; ③ Move the probe above the micro-vacuum gauge pad through a microscope and a horizontal knob, and lower the probe height until it touches the pad. ④ Use default conditions to pre-test the thermal conductivity, record the test data, and form a test data table. ⑤ Observe the resistance values R at the maximum current and the minimum current in the test data table i and R f , if 8% ≤ (R i - R f ) / R i ≤ 12%, record the thermal conductivity G given in the test data table th ; if (R i - R f ) / R i is too small or too large, return to step ④ to increase or decrease the current in the pre - test thermal conductivity under the default conditions so that the value falls within the range of 8% - 12%; ⑥ Raise the probe to leave the micro-vacuum gauge pad. ⑦ Repeat steps ② - ⑥ to make the vacuum degree P c There are at least three data points in each of the ranges of 1 - 10 mTorr, 10 - 100 mTorr, and 100 - 1000 mTorr; for the series of data, perform a linear fit according to G th = APc + B to obtain two constants A and B; ⑧Place the wafer-level packaged device on a vacuum probe station, perform the thermal conductivity test using step ⑤, and record G th ; ⑨Based on the A and B values obtained in step ⑦, combined with the thermal conductivity G recorded in ⑧ th , from calculate the device vacuum degree P d .

2. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to claim 1, characterized in that: In step ①, evacuate the air to below 2 mTorr.

3. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to claim 1, characterized in that: The default conditions include: measuring the resistance of the thermistor layer using a current of 0.1 - 0.8 μA with a step size of 0.1 μA.

4. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to claim 1, characterized in that: The MEMS chip is a non-cooled infrared detector chip integrated with a micro-bridge structure micro-bolometer.

5. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to claim 4, characterized in that: The non-cooled infrared detector chip includes a micro-bolometer body, and the circuit substrate includes a silicon substrate, a readout circuit, a reflective layer, and a circuit protection layer.

6. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to claim 3, characterized in that: The micro-bolometer body includes two metal struts, a first support layer, a thermistor layer, an electrical connection structure, a first passivation layer, and a metal reflective layer.

7. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to any one of claims 1 - 6, characterized in that: The clean room environment includes a humidity of 40 - 60% RH, a temperature of 23 ± 5°C, and a temperature change rate not exceeding 0.5°C / 10 min.

8. A method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to claim 4, characterized in that: The pad size of the non-cooled infrared detector chip integrated with a micro-bridge structure micro-bolometer is ≥ 70 × 70 μm.

9. A MEMS chip wafer-level packaging vacuum degree detection system for implementing the method for detecting the vacuum degree of wafer-level packaging of a MEMS chip according to any one of claims 1 - 8.

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