An imaging plane flatness measurement device and method for a detector package

Abstract

The application relates to an imaging surface flatness measuring device and method for a detector package, which comprises a carrier system, a measuring system and a control system; the carrier system carries the packaged detector and an optical flat; the measuring system collects height data of the detector based on an oblique triangulation laser measuring method, the optical flat serves as a reference measuring object, differential measurement of the imaging surface of the detector under low-temperature vacuum packaging is realized, and the flatness change of the imaging surface of the detector under low-temperature is transmitted through the window; the oblique triangulation laser measuring method adopts two opposite triangular laser probes which are obliquely installed to collect the height data of the detector, so that errors caused by the window glass in the detector packaging are avoided; the control system is used for controlling the height data collection and height data processing of the detector in the measuring system, and finally the imaging surface flatness data of the detector package are obtained; in the data processing, an improved gradual consistent sampling algorithm is adopted to remove error points. The application realizes the packaging of the low-temperature detector, reduces the system error by one order of magnitude, and reaches the repeated precision of the um level.

Description

Technical Field

[0001] This invention relates to the field of high-precision non-contact optical measurement, specifically to an imaging surface flatness measurement device and method for detector packaging, which can perform high-precision non-contact optical measurement of the flatness of a target object through a sealing glass. Background Technology

[0002] In the fields of modern precision instruments and precision measurement, many precision instruments or parts require specific flatness specifications during processing and assembly. These specifications can be determined through measurement using high-precision flatness measuring devices. Particularly in modern large-scale photoelectric detection systems, a large target area detector focal plane is required. To ensure image quality or meet the high-precision flatness requirements of the focal plane in photoelectric detection, high-precision non-contact measurement of the detector focal plane is necessary. This is especially crucial during detector operation, as detectors often require cryogenic vacuum sealing. Therefore, the measurement of the detector focal plane necessitates non-contact, high-precision measurement through the sealing glass. Taking a modern large telescope imaging system as an example, its focal plane area is much larger than a single detector chip, requiring detector stitching to create the imaging focal plane. To ensure image quality, the imaging focal plane must maintain high flatness under cryogenic vacuum sealing conditions. Therefore, high-precision measurement under cryogenic vacuum sealing conditions is required, with measurement accuracy reaching the micrometer level. Typical optical telescopes use CCD (Charge-coupled Device) detectors and CMOS image sensors in the visible light band, InGaAs, InSb, and MCT detectors in the infrared band, and similar semiconductor sensors in the ultraviolet and X-ray bands. To reduce dark current, these sensors require cryogenic packaging. The need for large target surface splicing necessitates measuring the flatness of the imaging surface under cryogenic packaging conditions.

[0003] In stitched cameras, the flatness of the focal plane needs to be measured after each installation during sensor stitching to ensure that stress changes during installation do not affect the focal plane's flatness. This is especially important after stitching, when the entire focal plane needs to be vacuum-sealed, and the detector also requires cooling during use. Vacuum sealing introduces assembly and adjustment errors, potentially causing deformation of the focal plane under stress. More importantly, the detector is stitched at room temperature and then used at low temperature. The temperature changes and uneven temperature distribution after cooling can cause deformation of the imaging surface due to temperature variations, including deformation of the detector chip package caused by cooling, and deformation of the substrate due to temperature gradients. Therefore, measuring the flatness of the focal plane after low-temperature vacuum sealing of the sensor is particularly crucial.

[0004] To obtain the focal plane flatness parameters of the stitched image, various methods have been adopted internationally. The Japanese HyperSuprime-Cam (HSC) camera used a contact measuring instrument to obtain the flatness and tilt of the focal plane substrate, and a laser interferometer to measure the flatness and height differences of each CCD, calculating the final focal plane flatness together. The Pan-STARRSGigapixelcamera#1 used a laser interferometer to measure through anti-reflective glass. The J-PASCryoCam (JPCam) used Cyber's CT1000 non-contact 3D measuring instrument to measure the focal plane; this measurement system used a color confocal sensor. The Darkenergycamera (DECam) used a color confocal method. However, none of these methods considered focal plane flatness measurement under vacuum packaging.

[0005] Due to the high cost of sensors, only non-contact measurement methods can be used. However, for focal plane flatness measurement under vacuum encapsulation, since the sensor is encapsulated in a vacuum cavity, all non-contact measurement methods require penetration through a sealing glass. This sealing glass, however, can affect the measurement. Commonly used interferometers and color confocal spectral probes cannot achieve measurements through the sealing glass, as they are affected by phase differences, dispersion, and spherical aberration introduced by the glass. Furthermore, existing commercial non-contact measuring instruments mostly use color confocal spectral probes, which cannot guarantee the measurement accuracy under low-temperature vacuum encapsulation of the detector. Summary of the Invention

[0006] The technical problem to be solved by the present invention is measurement under low temperature vacuum packaging of detectors. It overcomes the limitation of existing technology that it is impossible to complete the measurement through the sealing glass of the vacuum cavity. It provides a device and method for high-precision non-contact measurement of the detector imaging surface under the condition of low temperature vacuum packaging. It achieves repeatability measurement accuracy at the μm level at low cost, and the error after passing through the sealing glass is less than 0.5%.

[0007] The specific solution of this invention is as follows:

[0008] An imaging surface flatness measurement device for detector packaging includes: a loading system, a measurement system, and a control system;

[0009] The carrier system carries the packaged detector and optical flat.

[0010] The measurement system uses the oblique triangular laser measurement method to acquire the height data of the detector. An optical flat is used as a reference measurement object to achieve differential measurement of the flatness change of the detector's imaging surface caused by low temperature under vacuum packaging through the sealing window. The oblique triangular laser measurement method uses two opposing triangular laser probes installed at an angle to acquire the height data of the detector, avoiding the error caused by the sealing window glass in the detector packaging.

[0011] The control system is used to control the acquisition and processing of altitude data of the detector in the measurement system to obtain the final flatness data of the imaging surface of the detector package; wherein the data processing adopts an improved Progressive Sample Consensus (PROSAC) algorithm, that is, an improved PROSAC algorithm to remove error points.

[0012] Furthermore, when the triangular laser probe is installed at an angle, the installation angle corresponds to the laser reflection angle.

[0013] Furthermore, when the two triangular laser probes are installed at an angle, the measuring base for the measuring cantilever used to install the triangular laser measuring probes is designed to be lightweight, that is, it is made of lightweight titanium alloy material and has a hollowed-out design to reduce the weight of the parts; at the same time, the measuring cantilever is made of carbon fiber material and adopts a double cantilever structure to further enhance the strength of the cantilever.

[0014] Furthermore, the measuring device achieves repeatability accuracy on the order of micrometers, and reduces system error by an order of magnitude through differential measurement.

[0015] Furthermore, in the control system, the improved PROSAC algorithm for error point removal is as follows: calculate the distribution of the height difference between adjacent points in all data points, i.e., the data points with the highest frequency in the distribution are selected as the data subset. The smallest sample is selected from the data subset, and the fitting parameters are calculated. The fitting parameters are then brought back to the full sample data to obtain the number of inliers within a set threshold. By repeatedly selecting the smallest sample, calculating the fitting parameters, and calculating the number of inliers, the fitting parameters with the maximum number of inliers are obtained as the final fitting datum parameters. Then, the distance from each inlier to the final fitting datum is calculated, i.e., the final flatness.

[0016] A measurement method of the present invention using the above-described device is implemented as follows:

[0017] Step 1: Install the triangular laser probe at an angle. Adjust the installation angle of the probe according to the laser reflection angle of the triangular laser probe itself, so that the laser light reflected from the surface being measured by the triangular laser probe can return to the laser receiver of the triangular laser probe, and no longer receive diffuse reflection light.

[0018] Step 2: Install the packaged detector on the test object tray of the test object system, and install the reference optical flat on the reference flat object tray of the test object system. After completion, adjust the test object and the reference flat object to be horizontal. The horizontal adjustment process uses an electronic level as a reference to continuously adjust the test object system, correct the horizontality and parallelism between the detector and the reference flat object. The detector and the reference flat object form a differential measurement relationship to reduce measurement error.

[0019] Step 3: Scanning Measurement. The control system controls the linear motor to move along the X and Y axes respectively. The displacement platform drives the triangular laser probe assembly to move, measuring the height data of the measured surface and the reference measurement flat along the X and Y directions respectively. The height data of the two are added together to offset the error caused by probe jitter. Then, the improved PROSAC algorithm is performed, that is, the distribution of the height difference between two adjacent points in the height data is calculated, the data point with the highest frequency in the distribution is selected as the data subset, the smallest sample in the data subset is selected to calculate the possible fitting parameters, and then the fitting parameters are brought back to the full sample data to obtain the number of inliers within the specified threshold. By repeating the operation, the fitting parameters with the maximum number of inliers are obtained as the final fitting base plane parameters. Then, the distance from each point to the final fitting base plane is calculated, that is, the final flatness.

[0020] The advantages of this invention compared to the prior art are:

[0021] (1) Measurement System: The oblique triangular laser method is used for detector packaging. This method allows for measurement of detectors under cryogenic packaging through a planar sealing window. Since cryogenic refrigeration requires vacuum packaging of the sensor, existing non-contact measurement technologies are affected by the sealing window glass of the vacuum packaging. The oblique triangular laser method can avoid the influence of the sealing window glass.

[0022] (2) In the object-carrying system: The differential measurement method, which simultaneously measures the object under test and the reference flat crystal, can reduce the system error by an order of magnitude, achieving repeatability accuracy on the order of μm. Currently, the motion error caused by straightness and other factors in commonly used linear motors is often around 100 μm or even larger, far exceeding the flatness of the detector itself. Improving the motion error of linear motors by further improving the manufacturing precision is usually constrained by high costs. Therefore, this part of the error is offset by measuring the standard optical flat component, i.e., the reference flat crystal. Since the surface of the flat crystal is a standard plane with a flatness on the order of 10 nm, the measurement result of it is the motion jitter error of the measurement system, which can be subtracted from the flatness result of the detector.

[0023] (3) In the control system: the data processing section includes an improved PROSAC algorithm, which can be used for error point removal. During the measurement process, due to dust, rust, etc. on the measured surface, error points often exist in the measurement results. Currently, existing flatness measurement techniques usually remove error points manually. With this improved algorithm, the computer can automatically find and remove error points. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the overall measuring device of the present invention;

[0025] Figure 2 This is a schematic diagram of the oblique triangulation laser measurement method of the present invention;

[0026] Figure 3 This is a schematic diagram of the differential measurement method of the present invention;

[0027] Figure 4 This is a schematic diagram of the measurement system in the measuring device of the present invention;

[0028] Figure 5 This is a schematic diagram of the laser probe assembly in the measurement system of the present invention;

[0029] Figure 6 This is a schematic diagram of the loading system in the measuring device of the present invention;

[0030] Figure 7 This is a schematic diagram of the data processing flow in the measuring device of the present invention. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described examples are only a part of the examples of the present invention, and not all of them. All other examples obtained by those skilled in the art based on the examples in this invention without inventive effort are within the scope of protection of this invention.

[0032] like Figure 1The diagram shows the overall design of the measuring device of this invention. The measuring device mainly consists of a control system, a measuring system, and a load system. The control cabinet in the control system includes a controller for the displacement platform motor, a controller for the laser probe assembly, a programmable logic controller (PLC) responsible for managing each controller, a main control computer, and a power supply for each component. The displacement platform controller can achieve high-precision control of the motor movement, with a control accuracy of 0.01 mm. This controller communicates with the PLC. The laser probe assembly controller is responsible for controlling the probe. The main control computer connects to the PLC and the laser probe controller in the control cabinet. The control program in the control computer uses the PLC to control the motion of the displacement platform and acquire position information, and uses the laser probe controller to read data from the laser probe assembly. The measuring system is responsible for moving the triangular laser measuring probe to measure flatness. To achieve measurement under the detector enclosure, the triangular laser measuring probe needs to be installed at an angle corresponding to the laser reflection angle. The load system is responsible for carrying the object being measured and the reference measuring flat, and also provides functions such as distance adjustment and horizontal adjustment for the object being measured and the reference measuring flat as needed. The measurement system is controlled by the control system. After the object to be measured is mounted on the carrier system, the flatness of the object on the carrier system can be measured by the measurement system. The carrier system not only carries the object to be measured, but also has a standard reference measurement surface with a flatness on the order of 10 nm to provide differential measurement function.

[0033] like Figure 2 The diagram illustrates an implementation example of the oblique triangular laser measurement method of the present invention. A monochromatic triangular laser displacement probe is used. Because the probe uses monochromatic light, it is not affected by dispersion after passing through the glass. The probe works by emitting a monochromatic laser to the measurement point, which is then reflected back to the lens and imaging sensor used to receive the laser. For original measurement points 1 and 2 at different heights, the reflected laser returns to the imaging sensor at different positions, thus obtaining height information. Due to the use of an oblique measurement mode, the oblique angle α corresponds to the laser reflection angle and is 12°. This measurement method ensures that the incident angle α does not change with the height of the measurement point. Figure 2 As shown, after passing through a glass of thickness d, the height change from measurement point 1 to the original measurement point 1 is consistent with the height change from measurement point 2 to the original measurement point 2, i.e., Δh1 = Δh2, thus ensuring that the glass does not affect the flatness measurement. This oblique triangular laser measurement method solves the problem of the influence of the sealing glass in the low-temperature vacuum sealing of the detector.

[0034] like Figure 3The diagram shown illustrates a differential measurement method according to an embodiment of the present invention. It employs a differential compensation measurement method, where probe 1 and probe 2 measure the measured surface and the reference surface respectively, performing differential compensation measurements to compensate for jitter errors generated during probe movement. By eliminating jitter measurement errors caused by the movement of the displacement platform in the measurement system, the overall measurement error of the measurement system can be reduced by an order of magnitude, achieving repeatability accuracy at the μm level.

[0035] like Figure 4 The diagram shows a measurement system according to an embodiment of the present invention, comprising an X-axis linear motor 1, a Y-axis linear motor 2, a cantilever counterweight 3, a measuring base 4, a measuring cantilever 5, and a laser probe assembly 6. The X-axis linear motor 1 and the Y-axis linear motor 2 together form a displacement platform. The measuring base 4 is mounted on the displacement platform and is responsible for moving the laser probe assembly 6. The measuring cantilever 5 connects the measuring base 4 and the laser probe assembly 6, allowing the probe to measure surfaces outside the range of motion of the displacement platform. The cantilever counterweight 3 maintains the balance of the measuring base, keeping the center of gravity at the center of the measuring base 4, ensuring the stability of the displacement platform during operation. The laser probe assembly 6 is mounted at the end of the measuring cantilever 5 and enters the loading system for measurement under the movement of the measuring cantilever 5.

[0036] like Figure 5 The diagram shows a schematic of the laser probe assembly 6 according to an embodiment of the present invention, including a probe connector 601, a measured surface triangular laser displacement probe 602, a reference surface triangular laser displacement probe 603, and a probe measurement angle adjustment device 604. The measured surface triangular laser displacement probe 602 is mainly responsible for measuring the object under test, while the reference surface triangular laser displacement probe 603 is mainly responsible for measuring the reference optical flat. By measuring the optical flat, systematic errors generated during probe movement can be offset.

[0037] like Figure 6 The diagram shows a schematic of the loading system according to an embodiment of the present invention, including a reducer 7, a reference standard optical disc 8, a reference optical disc loading tray 9, a test object loading tray 10, a connecting bracket 11, a loading ring 12, a bearing 13, and adjustable height casters 14. The reducer 7 and bearing 13 allow adjustment of the level of the loading trays 9 and 10. The test object is fixed on the test object loading tray 10, and the reference optical disc loading tray 9 is mounted below it via the connecting bracket 11. The reference standard optical disc 8 is fixed on the reference optical disc loading tray 9. Simultaneous measurement of the reference standard optical disc 9 compensates for errors caused by displacement platform vibration.

[0038] In actual operation, the measuring cantilever 5 is bonded to the measuring base 4 with epoxy resin, and the measuring cantilever 5 is also bonded to the laser probe assembly 6 with epoxy resin. After installation, the measured surface triangular laser displacement probe 602 and the reference surface triangular laser displacement probe 603 need to be leveled to ensure that the probe measurement angle is perpendicular to the horizontal direction. During actual measurement, the X-axis linear motor 1 and Y-axis linear motor 2 are first controlled to move. The displacement platform composed of the linear motors drives the measuring base 4 and the measuring cantilever 5 to move, which in turn drives the laser probe assembly 6 to move, thus scanning the entire plane. The measuring base 4 is designed for lightweight use, employing lightweight, high-strength titanium alloy material and a perforated design to reduce the weight of the parts. This reduces the load on the linear motors, preventing excessive load from causing motor instability.

[0039] The probe connector 601 is bonded to the measuring cantilever 5, which is responsible for fixing the entire laser probe. The measured surface triangular laser displacement probe 602 and the reference surface triangular laser displacement probe 603 are fixed on its two sides respectively. Before measurement, the probe needs to be calibrated using the probe measuring angle adjustment device 604. Simultaneously, the reducer 7 and the connecting bracket 11 are used to level the measured object and the reference standard flat crystal 8 respectively. Depending on the measured object, the height of the object is adjusted via the connecting bracket 11 to meet the required working distance of the laser probe. The measuring cantilever 5 connects the measuring base 4 and the laser probe assembly 6. The cantilever structure allows the probe to measure objects outside the movement range of the displacement platform. The cantilever is made of lightweight carbon fiber material. Since cantilever swaying can cause significant errors, a double cantilever beam structure is designed in this invention to effectively reduce cantilever swaying without excessively increasing the load weight. The cantilever counterweight is responsible for maintaining the balance of the measuring base 4, keeping the center of gravity at the center of the measuring base, and ensuring the stability of the displacement platform during operation.

[0040] During the measurement process, the movement of motors 1 and 2 drives the movement of the measured surface triangular laser displacement probe 602 and the reference surface triangular laser displacement probe 603, allowing the triangular laser displacement probes to scan the measured surface and the reference standard flat crystal 8 on the measured object carrier plate 10. Data from the measured surface triangular laser displacement probe 602 and the reference surface triangular laser displacement probe 603 are then obtained. This data includes data from the measured surface and the reference flat crystal. The data are added together to compensate for systematic errors generated during probe movement, yielding the final height data Z. Simultaneously, the position information of the X-axis linear motor 1 and the Y-axis linear motor 2 is acquired to obtain the XYZ three-dimensional coordinate data for each point. After data acquisition, the height data is processed. Since the measured surface and the reference standard flat crystal 8 are rigidly fixed by the low-temperature drift connecting bracket 11, their distance is considered constant. This dual-probe differential compensation measurement method, which measures the high-precision, high-flatness reference standard flat crystal to obtain the jitter error of the displacement platform, effectively improves the accuracy of the measuring device. Finally, the data was processed, and the errors caused by the jitter of the displacement platform were offset by adding the data to obtain the height data.

[0041] Through such Figure 7 The data processing flow shown describes the processing of height data. The improved PROSAC algorithm first calculates the height difference distribution between non-boundary adjacent data points, selects the data points with the highest frequency in the distribution as a subset, and then calculates the possible fitting parameters from the smallest sample within this subset. These fitting parameters are then applied back to the full sample data to obtain the number of inliers within a specified threshold. This process of repeatedly selecting the smallest sample, calculating the fitting parameters, and counting the number of inliers is repeated until the fitting parameters with the maximum number of inliers are obtained, which are then used as the final fitting datum parameters. Finally, the distance from each point to the final fitting datum is calculated, representing the final flatness. Three-dimensional data visualization is then performed, thus achieving high-precision flatness measurement.

[0042] To ensure the reliability of the flatness measurement of the imaging surface of the detector package, the measurement device underwent accuracy testing and calibration. A standard flat crystal was used instead of the object being measured, and a dual-flat crystal measurement was performed, yielding a system repeatability accuracy on the order of μm. Testing on a high-precision standard step block showed that the system error after passing through the sealing glass was less than 0.5%.

[0043] The above embodiments are provided merely for the purpose of describing the present invention and are not intended to limit the scope of the invention. The scope of the invention is defined by the appended claims. Various equivalent substitutions and modifications made without departing from the spirit and principles of the invention should be covered within the scope of the invention.

Claims

1. A device for measuring the flatness of an imaging surface for detector packaging, characterized in that, The measuring device achieves repeatability accuracy at the μm level, with an error of less than 0.5% when viewed through the sealed window glass. The measuring device includes: Cargo loading system, measurement system, and control system; The carrier system carries the packaged detector and optical flat. The measurement system acquires the detector's height data based on the oblique triangular laser measurement method. An optical flat is used as a reference measurement object, i.e., a reference optical flat, to achieve differential measurement of the flatness changes of the detector's imaging surface caused by low temperature under vacuum packaging, which are transmitted through the sealing glass. The oblique triangular laser measurement method uses two triangular laser probes installed at an angle to measure the measured surface and the reference optical flat along the X and Y directions respectively, obtaining their respective height data, thus avoiding the measurement errors caused by the sealing glass in the detector packaging. The control system is used to control the acquisition and processing of altitude data of the detector in the measurement system to obtain the final flatness data of the imaging surface of the detector package; wherein the improved PROSAC algorithm is used to remove error points in the data processing. In the control system, the improved PROSAC algorithm for error point removal is as follows: Calculate the distribution of the height difference between adjacent points in all data points, i.e., the data points with the highest frequency in the full sample data. Select the data points as the data subset. Randomly select the smallest sample in the data subset and calculate the fitting parameters. Then, substitute the fitting parameters back into the full sample data to obtain the number of inliers within a set threshold. By repeatedly selecting the smallest sample, calculating the fitting parameters, and calculating the number of inliers, the fitting parameters with the maximum number of inliers are obtained as the final fitting datum parameters. Then, calculate the distance from each inlier to the final fitting datum, i.e., the final flatness. When the triangular laser probe is installed at an angle, the installation angle is equal to the laser reflection angle.

2. The imaging surface flatness measuring device for detector packaging according to claim 1, characterized in that: When the two triangular laser probes are installed at an angle, the measuring base of the measuring cantilever in the triangular laser measuring probe is designed to be lightweight, that is, it is made of lightweight titanium alloy material and has a hollow design to reduce the weight of the parts; at the same time, the measuring cantilever is made of carbon fiber material and adopts a double cantilever structure to further enhance the strength of the cantilever.

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