“Today, photothermal detectors are widely used in various functions such as motion detection, temperature measurement, people counting, and smoke detection, covering multiple markets such as construction, security, home appliances, industrial, and consumer.
Today, photothermal detectors are widely used in various functions such as motion detection, temperature measurement, people counting, and smoke detection, covering multiple markets such as construction, security, home appliances, industrial, and consumer.
There are five growth points for the photothermal detector market in the future: portable point temperature measurement, somatosensory detection, intelligent building, temperature measurement of heating, ventilation and air conditioning (HVAC) and other media, and people counting.
Every object produces thermal radiation, the intensity of which is related to its own temperature. According to the Stephen-Boltzmann law, the relationship between the temperature of an object and the radiant energy is fixed. As the temperature increases, the wavelength of the radiation peak begins to shorten: the radiation peak of 300K (room temperature) light is 10 um wavelength, The radiation peak of sunlight (6000K) is 500nm wavelength, which belongs to the visible light frequency domain.
After absorbing incident infrared radiation, photothermal detectors utilize pyroelectric mechanisms to convert electromagnetic wave energy into electrical signals, such as pyroelectric voltage, Seebeck pyroelectric effect3, electrical resistance or pyroelectric voltage).
Modern semiconductor technology, especially MEMS manufacturing technology, can produce very efficient uncooled infrared detectors, because thermal isolation can be achieved, so the sensitivity of the sensor is very high, and the size is small, the response time is very fast, and the mass production of semiconductors Ways 5 and 6 can reduce the price of MEMS sensors. In order to improve the efficiency of the sensor system, it is necessary to match the MEMS sensor with similar packaging and optical units.
Certain physical components of the sensor, such as the encapsulation housing and light windows that allow infrared radiation to reach the sensor, also serve to protect surrounding circuitry and interconnects. In some cases, filter windows can improve the response spectrum of the sensor, preventing visible radiation from affecting sensor performance. The filter window material is usually a silicon-based interference filter.
The physical location of this optical interface is on the top surface of the package, opposite the surface where the leads that connect the sensor to the PCB are located.
This article describes a system-in-package (SIP) that integrates an infrared sensor and an ASIC chip in a filter-enabled package, focusing on package-related characteristics, including material properties, optical performance, and overall system sensitivity. This is a cavity grid array (LGA) package concept with an integrated infrared filter window, and we have designed, produced prototypes, and performed characterization tests. The sensor field of view ranges from 80° to 110°, depending on the geometry of the light window. Finally, we also investigate the effect of packaging on sensor sensitivity.
This innovative package is designed for MEMS infrared sensors based on micro-machined thermopile and is capable of encapsulating different types of infrared sensors. When the photosensitive area of the sensor is different, it is only necessary to recalculate the geometric dimensions of the package without modifying the package design and materials.
The thermopile is composed of N thermocouples in series, and the output voltage of the sensor is the voltage of a single thermocouple multiplied by N. A thermocouple is a temperature sensing element formed by interconnecting the two ends of conductors of two different materials. These two connection ends are called the hot end and the cold end. According to the Seebeck thermoelectric effect3, when the temperature of the cold and hot ends is different, a voltage difference ΔV will be generated between the two conductors. Here is the expression for this voltage difference:
ΔV = N ΔV (1)
where ΔT is the temperature difference between the hot and cold ends, and the size of the Seebeck coefficient a is related to the conductor material.
In a micromachined thermopile, the thermocouple legs are embedded in a dielectric film: the hot end is in the suspended film, and the cold end is in the suspended film of the silicon substrate. This is designed to optimize the temperature difference between the hot and cold ends, maximizing the Increase the output voltage. The output voltage is usually in the range of a few hundred microvolts, up to a few millivolts: therefore, the output signal needs to be properly amplified so that the back-end circuits can process the signal properly.
The micro-micromachined thermopile sensor proposed in this paper is composed of p/n polysilicon thermocouples connected in series. The central aluminum plate is coated with dielectric material and used as a radiation absorbing film, and the photosensitive area of the sensor is 600mmX600mm. Figure 1 is a schematic diagram of the sensor layout. There is also an area on the physical package for integrating test sensors to measure sensor parameters during characterization testing. In order to reduce the chip size and optimize the optical window position, the advanced version will remove the test sensor.
Figure 1: Infrared sensor body and thermopile infrared sensor photosensitive area and test sensor integration area
MEMS infrared sensors are usually electrically connected to an application-specific integrated circuit (ASIC) to control the sensor and amplify the output signal, so we evaluated a system-in-package infrared sensor. To ensure that incident IR radiation reaches the sensor’s light-sensitive area and avoid radiative noise caused by visible flash lamps, we integrate an IR wavelength-selectable long-pass filter with l>5.5µm on the system-in-package for selected applications.
In the wavelength range required by the presence detection sensor system, the total loss due to the infrared long pass filter is controlled to within about 20%, for some major applications such as mounting a presence detection sensor on a device PCB board or infrared temperature measurement For sensors, the energy loss of this magnitude is considered to be very limited. For other potential applications in the future, the interference filters in question will be exchanged for filters with different transmission spectra.
Figure 2: Transmission spectrum of the long-pass infrared filter integrated on the top surface of the package
The package discussed in this article utilizes a silicon-based filter that typically integrates interference layers on both sides, with the option to mount different types of filters to suit different application requirements, such as NDIR spectrometers.
Figure 3: Package layout of MEMS infrared sensor and ASIC
The IR sensor package was designed and developed using a common side-by-side layout, where the sensor and ASIC are placed side-by-side within the package (Figure 3).
An optical window is integrated on the top surface of the package to select the wavelength content of the infrared radiation. This optical window solution prevents ambient light radiation from reaching the photosensitive area of the detector, thereby reducing overall system noise. The polymers that make up the top surface and cavity walls of the package can be considered completely opaque to visible-infrared radiation and can be classified as LCP materials (Liquid Crystal Polymers). Different filters can be installed for different applications, for example, NDIR spectrometers. As shown in Figure 3, the structural elements include two dies and bond wires, the sensor and signal processing circuitry are interconnected, and then connected to the package substrate.
Figure 4: Renderings of the “Small IR Window” package and the “Integral IR Filter Cap” package
Experimental setup and measurements
To characterize the photoelectric characteristics of the MEMS infrared sensor, the target object to be measured is a calibrated blackbody radiation source from -20°C to 160°C. The blackbody radiation source used was an SR-800R 4D/A from CI Systems, which has an area of 4 x 4 square inches and an emissivity of 0.99. During the characterization experiments, the sensor was placed 5.0 cm from the black body surface so as to completely cover the sensor field of view.
Figure 5: Experimental Setup
Data were collected once with and without the filter, and signal-to-noise ratios of 1.6 and 2.36 were observed, respectively. When the filter is used, the sampling SNR is reduced, which is due to the optical attenuation of the filter, and fits perfectly with the spectrum of Figure 2.
Figure 6: Sensitivity characterization of ceramic packaged sensors with and without IR filters.
The system output is a digital signal, and under infrared radiation, the digital change in the least significant bit (lsb) represents the system output change. The overall sensitivity of the sensor under test was approximately 2000 lsb/°C, with noise found at 150 lsb, with the package geometry determined and ensuring that the black body completely covers the field of view of the light window. Infrared long pass filters can be selected primarily to match the desired detection selectivity and the nature and size of the detectable object in front of the optical window.
Figure 7: 3D-X-ray tomography image of a package with an infrared silicon-based filter with M1 and M2 two-layer metal reflectors
As shown in Figure 7, two layers of metal infrared filter films, M1 and M2, are placed on the MEMS infrared sensor to filter the incident radiation on the package surface. Also visible in the 3D image are wire bond structures and package substrate metal traces for sensor and ASIC interconnects.
Field of View (FOV) Angle Calculation
We usually define a field of view (FOV) parameter for the optical system, which is used to evaluate the size of the geometric space that the sensing system can detect. Any optical device can be defined as a half field of view (HFOV) with FOV = ±θ or a full field of view (FFOV) with FOV = θ. This paper adopts the definition of half field of view with FOV = ±θ. In the geometric space evaluation, it is assumed that the refractive index of silicon is n = 3.44; the refractive index of air and vacuum is n = 1. The figure below shows the FOV calculation method for the cross-sectional structure of the package in question.
Figure 8: Cross-sectional view of FOV calculation principle
When calculating the field of view angle, you need to take into account the refraction (or bending) that occurs when light rays pass through the window.
Using the basic relations of trigonometry, we find:
WO = WA + 2 (Wt1+Wh1) (eq. 1)
where WO is the width of the encapsulated light window, WA is the width of the photosensitive area of the sensor, and Wt1+Wh1 is the width of the light path in air and silicon. The calculation method is shown in the following equation group:
Wt1 = t1×tgqS; (eq. 2a)
Wh1 = h1×tgqA; (eq. 2b)
Among them, t1 and h1 are the geometric vertical parameters of the package and the device itself, and qA and qS are the propagation angles of infrared rays in air and silicon, respectively. According to Snell’s Law, the following equation gives the relationship between the two angles:
n1.sin(θ1) = n2.sin(θ2) (eq. 3)
n1 and n2 denote the refractive index of each material, θ1 and θ2 are the angles (counterclockwise) that the light rays propagate in each material with respect to the surface normal, and assuming that the refractive index of silicon is n = 3.44, that of air/vacuum Refractive index n = 1. Based on the geometric assumptions above, the expected field of view angle FFOV = 80° – 82°. The preliminary design of the cavity package then began, and two batches of prototypes were fabricated in the package pilot line laboratory. To obtain different FFOVs, we propose two different window designs. In order to verify the “T% = 0” condition of the package cavity wall material in the wavelength range of 1.0um -13.0um, the infrared transmittance value test of the molding resin material was done. The package structure is a system-in-package, in which the ASIC die is placed side by side with the MEMS infrared sensor, and the die is connected by wire bonds (WB), as shown in the figure below.
Figure 9: Package with IR window (left) and integrated IR filter package (right), soldered on DIL 24 test board by surface mount technology (SMT)
Characterization experiments were performed on the two system packages described above at 22 cm from the top of the package using the aforementioned blackbody radiation source.
Figure 10: Sensitivity comparison of MEMS infrared sensors between a package with a small light window on the cap and a package with an infrared filter as a whole
After the experiment, at 22 cm, no difference in sensitivity measurements was observed between the small light window and the integrated infrared filter cap, and the response time was the same. This distance is chosen so that the beam direction is close to the infrared plane incident wave on the upper surface of the sensor. For the FOV characterization experiments, the sensor was mounted on a rotating stage from -90° to +90°, given the normal condition where the sensor photosensitive area is placed in front of a black body.
Figure 11: Experimental results of FOV characterization of infrared sensor with small infrared light window package, integrated infrared filter package and large ceramic package
In the large ceramic package, the FFOV angle of the infrared sensor is 109°±2°, which is smaller than the theoretical value of Lambertian distribution (theoretically 120°), which may be due to the silicon embedded structure of MEMS. The FFOV angle of the small light window package is 88°. Using the same package rotation method, the FFOV of the one-piece IR filter molded package is 100°. In the last case, an asymmetric effect was observed due to the proximity of the molded package cavity walls to the sensor photosensitive area.
Package Stress Simulation
For a specific absorbed power, the high thermal isolation ensures that the temperature difference between the hot and cold ends is maximized, which is an important factor in obtaining a large output voltage from a thermopile. Using the MEMS package, the gas in the cavity can be selected, and the pressure can be selected in the range of 100Bar to 100mBar. Gas thermal conductivity affects the rate of temperature conduction, as well as the temperature difference between the hot and cold ends of the thermopile, which in turn affects output voltage variation and sensor efficiency.
MEMS packaging is achieved by wire bonding technology between wafers. The MEMS sensor system is mainly composed of a silicon microstructure fabricated by surface micromachining process, usually two or more wafers (die) are stacked and welded in a silicon-based package with a glass material compound solder. .
There is a silicon protective cap with a thickness of about 150um on the sensor, which itself has a natural infrared wavelength filtering function for the radiation incident on the surface of the sensor. Of course, the infrared transmission spectrum of the silicon protective cap degrades the sensor optical performance in the 1-13um wavelength infrared region12, depending on the silicon properties.
Sensor development requires the integration of the MEMS silicon cap on the sensor wafer. We simulated the entire sensor system consisting of the infrared sensor, silicon cap, ASIC and package. Because the die stack is mounted on the package substrate, the sensor microstructure is integrated with the package structure, and thus, the package has an impact on the sensor signal performance. In addition to the stresses experienced during operation, critical situations can also occur during the manufacturing process, especially the cooling process after the package is soldered to the PCB. Since the packages are made of materials with different coefficients of thermal expansion (CTE), thermal gradients can cause warpage, which results in the transfer of stress to the sensor microstructure, which affects the sensing performance.
A finite element 3D model was built with SolidWorks Simulation software to simulate the warpage that occurs on the silicon substrate carrying the sensor microstructure. The post-solder cooling simulation takes into account soldering the package on a reference PCB. Table 3 summarizes the thermal loads and boundary conditions. Figure 12 is a finite element model.
Table 2 lists the properties of the materials used for the simulation.
While knowing that the simulation results are highly dependent on the material model and the properties of the materials used, we assumed the purpose of the analytical comparison, the available material data, and the static nature of the simulations performed, given the usual practice in the packaging simulation literature. , the isotropic elasticity of the material.
To reduce computation time, we consider creating a simplified model. However, due to the asymmetric placement of the ASIC inside the package, with a light window on the cap, the entire model needs to be simulated. For the top and bottom substrate layers of the package, the equivalent mechanical properties are calculated as follows14:
where Eeff is the effective Young’s modulus and αeff is the effective thermal expansion coefficient, which are the Young’s modulus Ei, αi, Vi and CTE and the volume or area percentage of the constituent material, respectively. Fig. 12 is the finite element model and Fig. 13 is the warpage simulation result on the sensor, ASIC and substrate. The warpage w of the substrate carrying the sensor microstructures is defined as the difference between the maximum and minimum values of the displacement z along the frame itself.
Table 2. Material Properties
Figure 12: Thermomechanical simulation finite element model. a,b) CAD model, c,d) finite element model with and without caps. There is no PCB board for post-solder simulation in the picture.
Table 3. Thermomechanical FEA Boundary Conditions and Loads
Figure 13: Package substrate, ASIC and MEMS (waferless on top) warpage (w).
This paper introduces the package design of an infrared sensor. The prototype characterization test results are satisfactory. The measured FFOV angle is between 80° and 110°, depending on the size of the light window. In order to reduce the influence of flash light and ambient noise, a silicon-based infrared filter is installed on the top of the package, and a characterization experiment is done. The stress simulation did not find critical conditions at the material interface. The package reliability has preliminarily met the environmental stress requirements of JEDEC L3.