Discussion of Mechanical Shock Test Stress for Ultra-large-scale CMOS Image Sensors

The ultra-large-scale CMOS image sensors are significantly different from the traditional CMOS image sensors in terms of pixel size, chip size and structure. CMOS image sensors generally come with an optical window structure that is sealed to the ceramic housing by means of adhesive. The optical window material is generally sapphire, and the larger the image element size, the larger the required glass optical window area. Ultra-large size CMOS image sensors in the package before the general optical window thickness, parallelism, light window average transmittance assessment. The object of mechanical impact test assessment is the CMOS image sensor optical window structure and capping process. The paper mainly discusses the typical failure cases of ultra-large-scale CMOS image sensors on the basis of the applicability of mechanical shock test standards, and investigates how to define the mechanical shock test stress from simulation, limit test and the requirements by users. This paper provides the qualification assessment basis of mechanical shock for newly developed products.


Introduction
CMOS image sensors are optical sensors manufactured using the CMOS process.The package is characterized by a cavity structure inside the package, and an optical glass cover is used to seal the device on top of the cavity [1].The glass cover not only prevents contaminants from entering the chip and protects the chip's internal circuitry, but also facilitates the entry of light into the chip's surface light-sensitive unit through the cavity.According to the package form, CMOS image sensors are divided into plastic package and ceramic package [2].The structure of the two package forms is basically the same, including optical glass cover, package shell, chip and internal leads, pins.The package schematic of the image sensor is shown in figure 1 [3].Vertically, the chip of a CMOS image sensor includes an on-chip microlens, a color filter array, a metal wiring layer, and a photodiode.The top layer of the sensor is the on-chip microlens, where each pixel is covered with a microlens.It has a spherical mesh surface, which allows more light to converge on the pixel and makes subsequent light reception more efficient by improving the angle of incidence [4][5].Below the microlens is a color filter array, and the current mainstream design scheme is the RGGB arrangement, i.e., the Bayer arrangement.The color filter array allows filtering different wavelengths of light and obtaining the value of each color component (RGB) in the light to invert the color of the actual scene.The filtered light eventually hits a photodiode at the bottom of the pixel well, which captures the light signal and converts it into an electrical signal.The metal wiring layer is used to transmit the signal.Since image sensors are typically DRAM-like row-select/column-select principle components, each row and column requires corresponding readout and control circuitry to connect to the pixels, and these interwoven circuits are collectively referred to as the metal wiring layer [6].
Currently, the main production companies of CMOS sensor products are E2V, Dalsa, CMOSIS, etc., whose products have reached the highest resolution of billion pixels [7][8].It is developing in the direction of higher resolution, higher sensitivity, wider dynamic range, more miniaturization and digitalization.At present, the CMOS sensor products are mainly divided into two categories: the surface array type and TDI type.For example, the 8K x 8K CMOS image sensor, with a pixel scale of 10 million and a chip size of 50mm x 55mm, is an ultra-large CMOS image sensor [9].
CMOS image sensors generally come with a light window structure, which is sealed to the ceramic housing by means of adhesive.The light window material is generally sapphire, and the larger the image element size, the larger the required glass light window area.Ultra-large-scale CMOS image sensors are generally evaluated for light window thickness, parallelism, and average light window transmittance before packaging.However, the fracture toughness of sapphire glass is poor and prone to cracking and breakage, and there is a risk of broken windows and cracks in the mechanical shock test.In this paper, we discuss how to determine the mechanical impact test stress values for ultralarge-scale CMOS image sensors.It provides a reference for conducting mechanical shock tests in the qualification of ultra-large-scale products.

Analysis of the Applicability of Mechanical Shock Test Standards for Cmos Image Sensors
The mechanical shock test is a test in the quality assurance of CMOS image sensors and is generally carried out in the qualification 1 subgroup and C2 subgroup.Mechanical shock test conditions will be specified in the detailed product specifications, which are generally formulated with reference to the requirements of the general specifications.For CMOS image sensors, domestic general specifications related to CMOS image sensors have not been developed.At the same time, considering that CMOS sensors are consistent with integrated circuit products in terms of process and structure, GJB 597B-2012 should be used as the basis for determining the general technical requirements for CMOS image sensors.At present, foreign countries have developed CMOS sensor-related standards, including ESCC developed ESCC 9020, the standard is mostly for domestic CMOS sensor manufacturers to refer to [10].
The comparison reveals that the conventional CMOS image sensor mechanical shock test is assessed according to condition B (peak acceleration 1500g).This is not a problem for products with small size structure.But with the exponential increase in image element size, the chip size multiplier increment, 12-inch wafer may be distributed on a CMOS chip.The traditional assessment conditions are far beyond the limit of stress that the product can withstand.

Research of Typical Ultra-Large Scale Cmos Image Sensor Failure Cases under Mechanical Shock
Research of typical ultra-large scale CMOS image sensor failure cases under mechanical shock.
• Chip shattering Ultra-large-scale chips are not uniformly stressed during the test, and cracks are likely to occur at the stress concentration location, and may even shatter.When an ultra-large-scale CMOS image sensor was tested, two product chips were cracked when a test stress of 1500g peak acceleration was applied, and one chip was cracked when the peak acceleration was reduced to 1000g.
• Cracks in the shell of CMOS image sensor There are two main types of ultra-large-scale CMOS image sensor ceramic shells: ones with a sealing ring and ones without a sealing ring.The structure with a sealing ring is a tight bond between the sealing ring and the ceramic housing using silver-copper solder brazing.The structure without sealing ring is to fuse the cover plate with gold-tin solder ring to the shell directly in the metallization area of the shell.Ultra-large-scale CMOS image sensing is generally designed with a light window on the front side and an array of pin grids on the back side.It cannot be fixed on the platform in the mechanical shock test, and specific fixtures are usually designed according to the product size.Therefore, it is easy to drop porcelain at the position where the edges of both sides of the tube housing touch the edges of the fixture notch.The weld between the shell and the cover plate is prone to small cracks.
• Excessive light window leakage rate Light windows are usually made of sapphire material.Due to the poor rupture toughness of sapphire glass, cracks and breakage easily occur when the glass size increases.This leads to excessive leakage rate of the light window.7100Q light window had excessive leakage rate problem during the mechanical shock probing test.

Determination of Mechanical Shock Test Stress for Ultra-Large-Scale Cmos Image Sensors
Ultra-large-scale CMOS image sensors have significant differences in structure, size, and weight from traditional small-size products.The existing assessment conditions exceed the actual affordability of the product materials.The assessment requirements should be adjusted accordingly.This paper discusses how to determine the shock test stress value.

Determination of Impact Resistance Based on Ansys Workbench 1) 3D modeling of 8K×8K type CMOS image sensor
ANSYS Workbench simulation is used to build a three-dimensional model of CMOS image sensor and calculate the theoretical value of stress-strain under different test conditions.The advantages of the simulation method are simple operation and low cost.However, since the simulation model is simplified in structure and the theoretical material parameters differ from the actual parameters of the product, the theoretical values calculated by simulation may differ from the actual values.Through the simulation method, the impact resistance of the product can be predicted.Take a CMOS image sensor with an image element size of 8K×8K as an example.Building a 3D model is shown in figure 2.   The simulation results are analyzed as follows.
• Under different stresses, the product displacement deformation, equivalent strain, and equivalent force show nearly multiplicative relationships. • The main deformation location is the glass cover plate, and the rest of the place is almost no deformation.The location of the maximum stress is concentrated in the contact between the glass cover plate and the casing.
• The tensile strength of sapphire optical glass is around 80 MPa.According to the test simulation results, the maximum equivalent force of the product occurs at the long edge of the glass cover.Under the test stress of 500g, the maximum equivalent force of the product is 41Mpa, under the test stress of 1000g, the maximum equivalent force of the product is 82Mpa, and under the test stress of 1500g, the maximum equivalent force of the product reaches 123Mpa.According to the results of the simulation, the 8K×8K type CMOS image sensor may have a glass cover shattering problem when the mechanical test stress reaches 1000g.

Determination of Shock Resistance Based on Limit Test Methods
In order to find out the shock resistance of oversized image sensors, step limit tests can be carried out.Before each stress is applied, the functional performance parameters of the product are tested.After the test is passed, the next group stress test can be carried out.Until the product has a functional, performance failure or appearance cracks.Limit test can truly and objectively reflect the product's shock resistance.But for CMOS chips, the cost is very high, and the test sample is small.The test process may be due to fixtures, personnel and other external factors affect the test results.As an example, the 8K × 8K type CMOS image sensor to carry out mechanical impact limit test.The 8K×8K type CMOS image sensor was set up with different steps of test stress as shown in the table below.From the test data, it can be seen that the product can pass the test when the peak acceleration is 500g.When the peak acceleration is 1000g and 1500g, the product cannot pass the test.Some of the sample circuit chips were shattered and cracks were produced in the casing.This indicates that the applied stress is too large and exceeds the maximum withstand capacity of the device.The test results are consistent with the simulation results.

Determination of Shock Resistance Based on User Requirements
Mechanical shock test is a test in the quality assurance process.The purpose of quality assurance is to meet the user's requirements for product functional performance, quality and reliability throughout the life cycle.For ultra-large-scale structure of the product, can be comprehensive consideration of user needs and the inherent capacity of the product, set the appropriate test stress.Failure to meet the user requirements, the user can also be based on the structural characteristics of domestic products, and actively take positive design.From the system level to the product to take protective measures to reduce the shock stress applied to the product.

Conclusion and Suggestions
In this paper, the following suggestions are made for how to determine the shock test stresses for ultralarge-scale CMOS image sensors: • Mechanical shock test should consider the actual limit capacity of the product structure and materials.The method of mechanical shock test for traditional CMOS image sensor is method 2002 in GJB548B, test conditions B (peak acceleration 1500g).Compared with the traditional small-sized products, ultra-large size CMOS image sensors have significant differences in structure and process.The test requirements also need to be accordingly adjusted.
• The simulation and limit test could be used in order to find out the impact resistance of the product.The results of ANSYS Workbench simulation show that the maximum equivalent force of the product occurs at the long edge of the glass cover plate when the stress of the 8K×8K type CMOS image sensor is 1000g.This has reached the tensile strength limit value of the sapphire optical glass, which may lead to the problem of glass cover shattering.And the limit test results show that some products have chip breakage when the test stress is 1000g.This is consistent with the simulation results.

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Different CMOS image sensors with the same structure and process, the test stress should be consistent.For example, the image sensor has similar metric scale, chip size and structure with 8K × 8K type CMOS image sensor, the mechanical impact test stress can be set to 500g.
• The stress of the mechanical shock test should be based on the inherent ability of the product as well as user requirements.In order to find out the inherent capacity of the product, simulation tests and limit tests can be used.For ultra-large-scale newly developed products, the test stress should be set up on the basis of simulation results and carry out the limit test if the similar size product test data is lacked.
• Users can also actively adopt positive design based on the structural characteristics of ultralarge-scale CMOS image sensors.Protective measures are taken for the product at the system level to reduce the impact stresses applied to the product during work.

Figure 1 .
Figure 1.Schematic diagram of image sensor chip package.

Table 1 .
Mechanical shock limit test of 8K×8K type CMOS image sensor.